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An assessment of the effects of psychoactive drugs and electrical stimulatin of the ventral tegmental… Druhan, Jonathan Peter 1989

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AN ASSESSMENT OF THE EFFECTS OF PSYCHOACTIVE DRUGS AND ELECTRICAL STIMULATION OF THE VENTRAL TEGMENTAL AREA ON THE STIMULUS PROPERTIES OF AMPHETAMINE By JONATHAN PETER DRUHAN B.Sc, McGill University, 1983 M.A., The University of B r i t i s h Columbia, 1985 A THESIS SUBMITTED IN THE REQUIREMENTS DOCTOR OF PARTIAL FULFILLMENT OF FOR THE DEGREE OF PHILOSOPHY THE FACULTY OF (Department in GRADUATE STUDIES of Psychology) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January 1989 (c) Jonathan Peter Druhan , 1989 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of PSYCHOLOGY The University of British Columbia Vancouver, Canada D a t e JANUARY 20, 1989 DE-6 (2/88) ABSTRACT The discriminative stimulus properties of amphetamine are thought to resu l t from the f a c i l i t a t o r y actions of th i s drug on dopamine neurotransmission within the nucleus accumbens. As such actions within the nucleus accumbens also are hypothesized to be responsible for amphetamine's rewarding e f f e c t s , the stimulus properties of amphetamine may be related to the hedonic e f f e c t s of the drug. If these conclusions are correct, then tests for generalization with the stimulus properties of amphetamine might be useful either to determine the dopaminergic actions of drugs, or to screen newly developed compounds for their abuse po t e n t i a l . In the present thesis rats were trained to discriminate 1.0 mg/kg amphetamine from sal i n e , and then tested for stimulus generalization to a range of amphetamine doses (0.0, 0.25, 0.50 and 1.0 mg/kg) injected either alone or in combination with either cocaine, apomorphine, haloperidol, nicotine, morphine, midazolam, ethanol or e l e c t r i c a l stimulation of the ventral tegmental area (VTA). Comparisons were then made between the amphetamine stimulus generalization functions obtained in the presence and absence of the test s t i m u l i , to determine whether the functions were altered in a manner consistent with the known dopaminergic actions or hedonic effects of the drugs and VTA stimulation. It was predicted that test stimuli that could enhance dopamine neurotransmission or produce positive hedonic effects might augment the stimulus properties of 111 amphetamine and elevate stimulus generalization functions r e l a t i v e to a control curve. Conversely, test stimuli that inhibited dopamine neurotransmission or reduced positive a f f e c t might interfere with the amphetamine stimuli and lower the generalization functions. The results indicated that amphetamine stimulus generalization functions were altered in a manner that generally r e f l e c t e d the known actions of each test stimulus on dopamine neurotransmission. Thus, the generalization functions were elevated by stimuli that enhanced dopamine neurotransmission (cocaine, a dose of apomorphine a f f e c t i n g post-synaptic dopamine receptor s i t e s , nicotine and VTA stimulation) and lowered by stimuli that interfered with dopamine neurotransmission (haloperidol, midazolam, and a dose of apomorphine that acts p r e f e r e n t i a l l y at presynaptic dopamine autoreceptors). Ethanol, which has not been found to consistently a f f e c t dopamine neurotransmission, did not generalize with the stimulus properties of amphetamine. Only morphine was found to a f f e c t amphetamine stimulus generalization functions (a lowering) in a manner that was inconsistent with the drug's f a c i l i t a t o r y actions on dopamine neurotransmission. The amphetamine stimulus generalization functions were not affected in a manner consistent with the hedonic actions of each test stimulus. Certain drugs that could produce positive hedonic effects (morphine, midazolam and ethanol) f a i l e d to elevate the generalization functions. In fact, the iv functions were elevated only by stimuli that appear to produce most of their rewarding effects by enhancing mesoaccumbens dopamine neurotransmission (cocaine, apomorphine, nicotine, and VTA stimulation). Two additional experiments suggested that t h i s property could have been responsible for the a b i l i t y of VTA stimulation to elevate amphetamine stimulus generalization functions. In one experiment, the a b i l i t y of the VTA stimulation to substitute for the stimulus properties of amphetamine was found to be correlated p o s i t i v e l y with i t s rewarding e f f i c a c y measured during ICSS te s t s . A subsequent experiment indicated that dopamine neurons could indeed mediate discriminative stimuli produced by VTA stimulation, as the brain stimulation cues were augmented by amphetamine and attenuated by the dopamine receptor antagonist, haloperidol. Together, the findings of t h i s thesis indicated that amphetamine stimulus generalization paradigms might be useful for detecting the dopaminergic actions of certain psychoactive drugs. However, such procedures may not detect the abuse potential of a l l compounds. This l a t t e r r e s u l t indicates that certain drugs of abuse do not produce amphetamine-like stimulus properties, and that t h i s may be due to differences in the neural mechanisms that mediate their positive hedonic e f f e c t s . V TABLE OF CONTENTS Abstract i i Table of Contents v L i s t of Tables ix L i s t of Figures xi Acknowledgment x i i i Introduction 1 Procedures for Measuring the Rewarding Effects of Drugs 4 Drug Discrimination Procedures: General Overview .. 9 The Relation of Drug Discrimination Performance to the Stimulus Properties of Drugs 11 Relations Between the Stimulus Properties and Hedonic Actions of Drugs 16 Pharmacological S p e c i f i c i t y of Amphetamine Stimuli 18 Neurochemical Substrates for Amphetamine Stimuli .. 19 The Role of Spec i f i c Dopamine Projections in Mediating the Stimulus Properties of Amphetamine .. 21 Common Neurochemical Substrates for the Stimulus Properties and Rewarding Effects of Amphetamine ... 23 Issues to be Addressed in the Present Thesis 24 Discrimination Training Procedures 27 The Amphetamine Stimulus Generalization Paradigm .. 28 Predicted Effects of Different Test Stimuli on Amphetamine Stimulus Generalization Functions 33 v i General Methods 40 Subjects 40 Surgery and Histology 40 Apparatus 41 Drug Discrimination Training 43 Generalization Tests 44 Locomotor A c t i v i t y Tests 45 Drugs 46 S t a t i s t i c a l Analyses 47 Experiment 1: The Effects of Cocaine, Apomorphine, and Haloperidol on Amphetamine Stimulus Generalization Functions 50 Methods 52 Results 53 Discussion 62 Experiment 2: The Effects of Nicotine on Amphetamine Stimulus Generalization Functions 67 Methods 69 Results 69 Discussion 76 Experiment 3: The Effects of Morphine on Amphetamine Stimulus Generalization Functions 81 Methods 83 Results 83 Discussion 89 v i i Experiment 4: The Effects of Midazolam on Amphetamine Stimulus Generalization Functions 94 Methods 95 Results 96 Discussion 101 Experiment 5: The Effects of Ethanol on Amphetamine Stimulus Generalization Functions 105 Methods 106 Results 107 Discussion 113 Experiment 6: The Effects of E l e c t r i c a l Stimulation of the VTA on Amphetamine Stimulus Generalization Functions 117 Methods 118 Results 119 Discussion 128 Experiment 7: The Relationship Between the Rewarding Effects of VTA Stimulation and i t s A b i l i t y to Generalize with the Stimulus Properties of Amphetamine 131 Methods 132 Results 133 Discussion 147 Experiment 8: The Effects of Amphetamine and Haloperidol on Discriminative Stimuli Produced by E l e c t r i c a l Stimulation of the VTA 151 Methods 153 v i i i Results 156 Discussion 162 General Discussion 165 Implications of the Present Findings for a Theory of the Stimulus Properties of Amphetamine 177 The U t i l i t y of Amphetamine Stimulus Generalization Paradigms as Screening Procedures for Assessing the Dopaminergic and Hedonic Properties of Drugs 184 Implications of the Present Findings for Theories of Drug Abuse 188 References 192 i x LIST OF TABLES Table 1: The a c t i o n s on dopamine neur o t r a n s m i s s i o n and the hedonic e f f e c t s of the t e s t s t i m u l i employed f o r Experiments 1 through 6 35 Table 2: Percentages of responses on the i n i t i a l l y s e l e c t e d l e v e r a f t e r c o c aine, apomorphine and h a l o p e r i d o l 59 Table 3: T o t a l number of responses a f t e r c o c a i n e , apomorphine and h a l o p e r i d o l 61 Table 4: Percentages of responses on the i n i t i a l l y s e l e c t e d l e v e r a f t e r n i c o t i n e 73 Table 5: T o t a l number of responses a f t e r n i c o t i n e 75 Table 6: Percentages of responses on the i n i t i a l l y s e l e c t e d l e v e r a f t e r morphine 87 Table 7: T o t a l number of responses a f t e r morphine 88 Table 8: Percentages of responses on the i n i t i a l l y s e l e c t e d l e v e r a f t e r midazolam 99 Table 9: T o t a l number of responses a f t e r midazolam .... 100 Table 10: Percentages of responses on the i n i t i a l l y s e l e c t e d l e v e r a f t e r ethanol 111 Table 11: T o t a l number of responses a f t e r e thanol 112 Table 12: Percentages of responses on the i n i t i a l l y s e l e c t e d l e v e r d u r i n g VTA s t i m u l a t i o n 126 Table 13: T o t a l number of responses durinng VTA s t i m u l a t i o n 127 X Table 14: Percentages of responses on the i n i t i a l l y selected lever during substitution tests with d i f f e r e n t parameters of VTA stimulation 140 Table 15: Total number of responses during substitution tests with d i f f e r e n t parameters of VTA stimulation . 141 LIST OF FIGURES Figure 1: Theoretical outcomes of stimulus generalization experiments 31-32 Figure 2: Effects of cocaine, apomorphine and haloperidol on amphetamine stimulus generalization functions 54-55 Figure 3: Effects of nicotine on amphetamine stimulus generalization functions 71-72 Figure 4: Effects of nicotine on locomotor a c t i v i t y 77-78 Figure 5: Effects of morphine on amphetamine stimulus generalization functions 84-85 Figure 6: Effects of morphine on locomotor a c t i v i t y 90-91 Figure 7: Effects of midazolam on amphetamine stimulus generalization functions 97-98 Figure 8: Effects of midazolam on locomotor a c t i v i t y 102-103 Figure 9: Effects of ethanol on amphetamine stimulus generalization functions 108-109 Figure 10: Effects of ethanol on locomotor a c t i v i t y 114-115 Figure 11: Electrode placements for the rats employed in Experiment 6 120-121 Figure 12: Ef f e c t s of VTA stimulation on amphetamine stimulus generalization functions 123-124 Figure 13: Stimulus generalization between amphetamine and VTA stimulation during drug-free substitution tests 134-135 Figure 14: Relationship between electrode placements and stimulus generalization with amphetamine for the rats employed in Experiment 7 137-138 Figure 15: Scattergrams showing the correlations between stimulus generalization with amphetamine and ICSS rates obtained with VTA stimulation 143-144 Figure 16: Relationship between electrode placements and ICSS rates for the rats employed in Experiment 7 145-146 Figure 17: Electrode placements for the rats employed in Experiment 8 158-159 Figure 18: Effects of amphetamine and haloperidol on stimulus generalization to a range of current i n t e n s i t i e s in rats trained to discriminate high and low i n t e n s i t i e s of VTA stimulation 160-161 x i i i ACKNOWLEDGMENT I would l i k e to extend my gratitude to Dr. Anthony P h i l l i p s for the support and supervision that he provided me throughout my graduate studies. His influence has been c r i t i c a l in keeping my thoughts focussed and my research 'on track'. I also would l i k e to thank Dr. Donald Wilkie for his helpful advice at several stages of my research, from the construction of equipment to the writing of t h i s thesis. I also am grateful to Dr. Peter Graf for his involvement on my departmental thesis committee and his helpful comments concerning the preparation of t h i s thesis. I am grateful to a number of research colleagues who deserve special mention for their contributions to my work. In p a r t i c u l a r , Dr. Chuck Blaha provided me with valuable instruction in the preparation and administration of various drugs, and he also directed my attention to several of the references reported in t h i s t h e s i s . Fred LePiane, Shayne Kardell, Lonn Myronuck and Chris Yamakura also provided th e i r assistance at various stages of thi s research. F i n a l l y , I would l i k e to express my deepest appreciation to my wife, Lorraine for the consistent emotional, i n t e l l e c t u a l , and p r a c t i c a l support that she provided me (and Josh) throughout one very intense year. 1 INTRODUCTION As was the case with many drugs of abuse, the psychomotor stimulant amphetamine was f i r s t used for medical purposes, s p e c i f i c a l l y , for i t s peripheral actions as a bronchodilator and nasal decongestant. Subsequently, amphetamine was shown to be a central nervous system (CNS) stimulant capable of producing behavioral a c t i v a t i o n and subjective feelings of well-being and euphoria. The drug soon became subject to widespread non-medical use, and i t became apparent that amphetamine might have powerful addictive properties (Cox, Jacobs, Leblanc, & Marshman, 1983). Studies on the etiology of amphetamine abuse have played a major role in the development of recent theories of drug addiction. T r a d i t i o n a l theories (Wikler, 1973; Seigel, 1983) often emphasized the importance of physiological dependence factors in maintaining patterns of habitual drug use. Drug-taking behaviors were thought to be maintained primarily by a need to a l l e v i a t e the aversive withdrawal effects associated with drug abstinence. This model of drug addiction was based primarily on evidence that withdrawal from opiates, barbiturates or ethanol could re s u l t in severe abstinence syndromes that could i n t e n s i f y an individual's urge to self-administer these drugs (Cox et a l . , 1983). However, such abstinence syndromes did not appear to be a prime motivator for the continued use of amphetamines or other psychomotor stimulant compounds ( G r i f f i t h , 1977). The 2 symptoms associated with stimulant withdrawal were found to be r e l a t i v e l y mild in comparison with those of the depressant drugs, and the patterns of stimulant s e l f -administration were not consistent with what might be predicted from dependence models. The use of psychomotor stimulant drugs t y p i c a l l y was found to occur in "runs", wherein an individual would repeatedly administer high doses of a stimulant over a period of a few days and then "crash". During t h i s crash phase, the individual might experience withdrawal symptoms characterized by fatigue and depression. However, drug-taking behaviors often did not resume u n t i l well after t h i s withdrawal phase, when the physiological abstinence syndrome had dissipated (Jaffe, 1987; V i l l a r r e a l & Salazar, 1981) . In 1964 the World Health Organization Expert Committee on Addiction-Producing Drugs indicated that the central c h a r a c t e r i s t i c s of amphetamine addiction included: "...(1) a desire or need to continue taking the drug; (2) consumption of increasing amounts to obtain greater excitatory and euphoric effects or to combat more e f f e c t i v e l y depression and fatigue, accompanied in some measure by the development of tolerance; (3) a psychic dependence on the effects of the drug related to a subjective and individual appreciation of the drug's e f f e c t s ; and (4) general absence of physical dependency so that there is no c h a r a c t e r i s t i c abstinence syndrome when the drug is discontinued." ( c f . G r i f f i t h s , 1977). This statement emphasized the importance of d i r e c t subjective effects of amphetamine as the basis for i t s addictive properties, and downplayed the role of physiological dependence factors. Acceptance of t h i s view of amphetamine addiction lead researchers to develop a variety 3 of psychometric scales for quantifying the subjective effects of drugs in humans, and the application of these measures to the study of non-stimulant drugs revealed that a wide range of addictive compounds shared an a b i l i t y to elevate mood and induce euphoria (Bozarth, 1988). In recent years, attempts to provide a unified theory of drug addiction have focused primarily on the positive a f f e c t i v e consequences of drug intake as the fundamental common denominator between drugs of abuse, and physiological dependence factors have been given a secondary role in maintaining pharmacological addictions (Baker, Morse, & Sherman, 1986; Stewart, de Wit, & Eikelboom, 1984; Wise, 1987; Wise & Bozarth, 1987). This change in t h e o r e t i c a l perspective has been accompanied by a s h i f t in focus for empirical investigations into the physiological basis for drug addiction. Instead of investigating the potential mechanisms for drug tolerance and dependence, studies over the past 20 years have attempted to i d e n t i f y the neural processes that give r i s e to the positive a f f e c t i v e properties of various compounds. To f a c i l i t a t e t h i s l a t t e r goal, a number of experimental procedures have been developed to measure the hedonic effects of drugs in laboratory animals. For the most part these procedures have involved measurements of the capacity for drugs to produce rewarding effects in operant conditioning paradigms. 4 Procedures for Measuring the Rewarding Effects of Drugs The most commonly employed techniques for measuring the rewarding effects of drugs have been the s e l f -administration, conditioned place preference and in t r a c r a n i a l s e l f - s t i m u l a t i o n (ICSS) procedures (Bozarth, 1988; P h i l l i p s , Broekkamp, & Fibiger, 1983). With the s e l f -administration procedure, animals are given the opportunity to perform an operant response which results in intravenous or intracerebral infusions of a drug (Brady, G r i f f i t h s , Heinz, Ator, Lukas, & Lamb, 1988). The capacity of the infused drug to exert rewarding effects is re f l e c t e d by select i v e increases in the performance of responses that res u l t in the infusion, but not in the performance of inconsequential responses. To date, there has generally been a strong correspondence between the range of drugs that are self-administered by animals and compounds that are abused by humans (Brady et a l . , 1988). Moreover, the patterns of drug intake observed in animals often p a r a l l e l those seen in human drug addicts. For example, experimental animals given access to amphetamine or other psychomotor stimulant compounds show the same pattern of "runs" and "crash" phases seen with human users (Yokel, 1988). In contrast, animals provided with opiate compounds generally exhibit the steady, continuous patterns of drug intake that are c h a r a c t e r i s t i c of human opiate addictions. These p a r a l l e l s between human and animal drug taking behaviors indicate that the s e l f -administration paradigm may provide a valuable model for 5 investigating the neurobehavioral basis of drug addictions in infrahuman species. The conditioned place-preference procedure measures the tendency for neutral environmental stimuli to acquire conditioned incentive value as a consequence of repeated association with a rewarding drug (Carr, Fibiger, & P h i l l i p s , in press; White, Messier, & Carr, 1988). Animals tend to approach and spend more time in a chamber previously paired with injections of rewarding drugs r e l a t i v e to a chamber paired with saline i n j e c t i o n s . This preference for the previously neutral environment appears to r e f l e c t the development of an association between the environment and the positive hedonic properties of the drug (White et a l . , 1988). The i n t r a c r a n i a l s e l f - s t i m u l a t i o n paradigm represents a th i r d means of assessing the rewarding properties of drugs (Esposito & Kornetsky, 1978; Wise & Bozarth, 1984; P h i l l i p s et a l . , 1983). In th i s paradigm, animals are trained to respond for brain-stimulation reward and the effects of addictive drugs on se l f - s t i m u l a t i o n are determined. Numerous studies have found that drugs which have abuse potential in humans also enhance the rewarding effects of e l e c t r i c a l brain-stimulation (Bozarth, 1988). This enhancement usually is r e f l e c t e d as increases in response rates for the brain-stimulation reward or as decreases in the current i n t e n s i t y required to maintain threshold levels of responding. These effects are attributed to the capacity of addictive drugs to 6 enhance the e x c i t a b i l i t y of neural processes that give r i s e to positive a f f e c t (Esposito & Kornetsky, 1978; Wise & Bozarth, 1984). Two other paradigms that have been employed to assess the rewarding effects of drugs in animals include the conditioned reinforcement and the self-administration reinstatement procedures (Beninger, Hanson, & P h i l l i p s , 1981; Davis & Smith, 1988; Robbins, Watson, Gaskin, & Ennis, 1983; Stewart & de Wit, 1988). Two variations of the conditioned reinforcement procedure e x i s t . In one procedure (Davis & Smith, 1988), animals are given repeated presentations of a neutral external stimulus (e.g. a buzzer) paired with the non-contingent administration of a rewarding drug. After such pairings, the animals w i l l respond for presentations of the drug-paired stimulus in the absence of the drug, suggesting that the stimulus has acquired rewarding e f f e c t s . In the second procedure (Beninger et a l . , 1981; Robbins et a l . , 1983), a neutral stimulus is paired with a natural reward (e.g. food) in the absence of any drug treatment. The animals are then given tests in which they may perform a novel response to receive presentations of the conditioned reinforcer after injections of either a rewarding drug or i t s vehicle solution. Studies involving t h i s procedure have indicated that a few addictive compounds w i l l increase responding for the conditioned rei n f o r c e r , but the paradigm does not provide a r e l i a b l e measure of the abuse potential of drugs. 7 The f i n a l procedure to be discussed in t h i s section is an extension of the drug self-administration paradigm discussed above. In t h i s paradigm, rats are trained to s e l f -administer a rewarding drug intravenously and then placed on extinction for as many t r i a l s as are required for the cessation of operant responding. Following extinction, the rats are given a single non-contingent priming i n j e c t i o n of either the t r a i n i n g drug or another rewarding compound. The priming i n j e c t i o n usually results in the reinstatement of operant responding even when responses do not produce further infusions of the drug. This reinstatement e f f e c t has been attributed to the capacity of the injected drug to activate appetitive motivational processes that usually mediate the drug's rewarding e f f e c t s . As a consequence, the rats again become responsive to drug-related stimuli in the environment ( i . e . the response lever; Stewart & de Wit, 1988; Stewart et a l . , 1984). The procedures outlined above have been extremely useful for investigating the nature of drug addiction. On a the o r e t i c a l l e v e l , studies employing the abovementioned procedures have provided firm evidence that addictive drugs can produce rewarding effects in non-dependent rat s . Furthermore, studies of the reinstatement phenomenon have indicated that rats w i l l r e i n i t i a t e self-administration responding for certain compounds after priming injections of the t r a i n i n g drug, but not after injections of an antagonist to the drug. Thus, i t is the presence of a drug in the body 8 that appears to determine the resumption of drug taking behaviors after a period of abstinence, not the absence of the drug (Stewart et a l . , 1984). These findings provide strong support for theories maintaining that the positive hedonic properties of drugs are important determinants of drug addictions in humans. On a more p r a c t i c a l l e v e l , these procedures have provided both a means of assessing the abuse potential of various compounds and a tool for investigating the neural basis for drug addiction. As noted, there has been a strong co r r e l a t i o n between a drug's potential for abuse by humans and i t s capacity to produce rewarding e f f e c t s in animals (Brady et a l . , 1988; Weeks & C o l l i n s , 1988). With respect to determining the neural basis for drug addiction, many of these procedures have been used to assess the effects of lesions and transmitter manipulations on the rewarding effects of drugs (for reviews, see Carr et a l . , in press; P h i l l i p s et a l . , 1983; Wise, 1983). These procedures also have been employed to assess the potential rewarding effects of drugs administered d i r e c t l y into discrete brain regions (Bozarth & Wise, 1981; Broekkamp, 1988; Carr & White, 1986; P h i l l i p s & Lepiane, 1980; P h i l l i p s , Mora, & Ro l l s , 1981; Stewart & de Wit, 1988). Accordingly, such studies have helped to i d e n t i f y brain regions and neurotransmitter systems involved in mediating the hedonic properties of drugs. Although the procedures outlined above have been 9 extremely useful for investigating the hedonic properties of drugs, there are certain l i m i t a t i o n s associated with each paradigm that sometimes have made interpretation of experimental results d i f f i c u l t (Bozarth, 1988). For example, the self-administration, s e l f - s t i m u l a t i o n , conditioned reinforcement and reinstatement procedures each measure animals' response l e v e l , and t h i s dependent variable may be confounded by drug or lesion effects on response capacity. On the other hand, place preference learning might be influenced by drug or lesion effects on associative learning, or by effects of state-dependent learning. In acknowledging these l i m i t a t i o n s , investigators have gone to great lengths either to control for such confounds, or to develop additional methods for assessing the hedonic effects of drugs. The remainder of t h i s thesis w i l l be devoted to reviewing and assessing one such alternative method which involves measuring the discriminative stimulus properties of drugs. Drug Discrimination Procedures: General Overview Another technique that could be employed to measure the hedonic actions of drugs in animals involves using drug sensations as discriminative stimuli in operant conditioning procedures. Animals may be trained to emit one response to obtain reinforcement (e.g. food) after receiving a s p e c i f i c dose of a drug, and an alternative response after saline in j e c t i o n s . After several t r a i n i n g sessions animals learn to discriminate accurately between the drug and saline states 10 and make the appropriate response at the s t a r t of each session. Thereafter, the i n i t i a l response may be used to index the presence or absence of the drug state following experimental manipulations (see Colpaert, 1977b, 1978b, for reviews of these procedures). The main advantage of employing drug discrimination procedures to investigate the hedonic properties of drugs is that the dependent measure i s largely unaffected by the types of confounds that interfere with measures of the rewarding effects of drugs. For example, discrimination measures may be less affected when lesions or drug manipulations dramatically reduce operant performance capacity. Although such effects might decrease the l e v e l of responding observed during a discrimination t r i a l , they would not be expected to influence animals' choices of which response i s appropriate for the pa r t i c u l a r stimulus conditions. Discrimination measures might be affected by drugs that a l t e r cognitive performance, however t h i s l a t t e r influence can be distinguished from sele c t i v e effects on stimulus perception by employing appropriate control procedures (see pages 28 to 33). The major l i m i t a t i o n of the drug discrimination procedure i s that i t s face v a l i d i t y for the study of drug abuse has not yet been established. Recently, Overton (1988) indicated that the establishment of such face v a l i d i t y requires that three conditions be s a t i s f i e d . F i r s t , drug abuse must be related to the sensory (subjective) effects of 11 abused compounds. Second, operant responses observed i n drug d i s c r i m i n a t i o n experiments must r e f l e c t the c o n t r o l of behavior by sensory p r o p e r t i e s of the drug. T h i r d l y , the sensory e f f e c t s t h a t are d i s c r i m i n a t e d by animals must be the same as those t h a t determine the abuse p o t e n t i a l of the substance. As i n d i c a t e d above (pp. 3), there i s now good reason to b e l i e v e t h a t most drugs of abuse are s e l f - a d m i n i s t e r e d by humans f o r t h e i r euphoria-producing p r o p e r t i e s . Thus, the requirement t h a t drug abuse be r e l a t e d to the s u b j e c t i v e e f f e c t s of drugs appears to be s a t i s f i e d . The f o l l o w i n g two s e c t i o n s w i l l b r i e f l y e v a l u a t e : 1) whether drug d i s c r i m i n a t i o n procedures measure sensory p r o p e r t i e s of drugs; and 2) whether such sensory p r o p e r t i e s are r e l a t e d to the euphoria-producing e f f e c t s of the drugs. The R e l a t i o n of Drug D i s c r i m i n a t i o n Performance to the  Stimulus P r o p e r t i e s of Drugs. Un l i k e e x t e r n a l l y generated s t i m u l i , drug cues g e n e r a l l y do not a r i s e from a c t i o n s on p r e - e s t a b l i s h e d , p e r i p h e r a l sensory pathways (Overton, 1988). Th e r e f o r e , drugs cannot be a t t r i b u t e d s t i m u l u s p r o p e r t i e s by v i r t u e of an a b i l i t y to impinge on normal sensory t r a n s d u c t i o n mechanisms. Instead, the c a p a c i t y of drugs to f u n c t i o n as s t i m u l i must be i n f e r r e d from s i m i l a r i t i e s between drugs and e x t e r n a l l y generated events i n the stimulus c o n t r o l of c o n d i t i o n e d responses. Although e x t e n s i v e comparisons between d i s c r i m i n a t i o n s 12 involving drug or external stimuli have not yet been conducted, cert a i n experimental observations suggest that the control of operant responses by drugs is similar to that obtained when conventional stimuli are employed. For example, the development of stimulus control appears to be similar regardless of whether animals are trained to discriminate drugs or exteroceptive stimuli (Overton, 1988). In either case, operant responses during i n i t i a l t r a i n i n g t r i a l s tend to r e f l e c t the unconditioned biases of the animals. Response accuracy then increases gradually over several t r i a l s so that responses come to r e f l e c t the stimulus conditions for the t r i a l rather than unconditioned biases (Colpaert, 1978b; Overton, 1988). The exact rate of acqu i s i t i o n may increase as a function of both increasing stimulus in t e n s i t y when exteroceptive cues are employed, and increasing drug dosage when interoceptive cues are used. High levels of asymptotic performance may be obtained in either case, depending on the par t i c u l a r q u a l i t a t i v e and quantitative c h a r a c t e r i s t i c s of stimulus or drug employed (for further discussion of these comparisons see Overton, 1988). Interestingly, a recent study has shown that prior learning of a l i g h t versus dark discrimination could p a r t i a l l y block the ac q u i s i t i o n of a superimposed drug discrimination task (Jarbe, Svensson, & Laaksomen, 1983; c.f. Overton, 1988). Conversely, prior learning of a drug discrimination could block a c q u i s i t i o n of discriminative control by d i f f e r i n g illumination conditions. Thus, i t 13 appeared that interoceptive drug stimuli could compete with exteroceptive stimuli for associative processing mechanisms. Animals trained to discriminate a drug from saline may show a graded decrease in drug-appropriate responding when tested with progressively lower doses of the tr a i n i n g compound (Colpaert, Niemegeers, & Janssen, 1979; Jarbe & Swedberg, 1982). Such dose-response gradients are analogous to the stimulus generalization functions obtained when animals are tested with progressively lower i n t e n s i t i e s of an exteroceptive discriminative stimulus (Mackintosh, 1974). Similar functions may be obtained when animals are tested with d i f f e r e n t doses of novel compounds that have pharmacological properties similar to those of the t r a i n i n g drug. For example, animals trained to discriminate a psychomotor stimulant drug w i l l emit drug-appropriate responses when tested with other stimulants (Colpaert et a l . , 1979; Silverman & Ho, 1977), and animals trained with an a n x i o l y t i c compound w i l l emit drug appropriate responses when tested with other a n x i o l y t i c s (Garcha, Rose, & Stolerman, 1985). In contrast, drug-appropriate responses do not occur when animals are tested with compounds that lack pharmacological actions of the tr a i n i n g drug (Colpaert et a l . , 1979; Garcha et a l . , 1985; Silverman & Ho, 1977). In such instances, animals consistently emit responses appropriate for the saline-condition. These findings are consistent with the hypothesis that responses measured in drug discrimination experiments are 14 controlled by the stimulus properties of the t r a i n i n g compound. Animals may learn to discriminate the presence versus absence of s p e c i f i c pharmacological actions exerted by a drug. Injections of other compounds with similar actions may reproduce the stimulus effects and r e s u l t in drug-appropriate responses. In contrast, drugs that lack the necessary pharmacological actions do not reproduce the stimulus e f f e c t s , and under these conditions animals emit responses appropriate for the absence of the t r a i n i n g cue. Although the present thesis is concerned primarily with the use of operant conditioning procedures to study the stimulus properties of drugs, i t is worth mentioning here that drugs also appear capable of functioning as conditional stimuli in Pavlovian learning paradigms. Much of t h i s work has involved analyses of the effects of pairing the stimulus properties of one drug with i l l n e s s produced by a second drug (e.g., lithium c h l o r i d e ) . In these procedures, the f i r s t drug acts as a conditional stimulus whereas the illness-producing drug functions as an aversive unconditional stimulus. Following the drug-drug pairings, subsequent injections of the conditional drug alone re s u l t in a v a r i e t y of apparent conditioned responses. The conditioned responses that have been found to occur include bradycardia (Wilkin, Cunningham, & F i t z g e r a l d , 1982), adipsia (conditioned sickness; Revusky, Taukulis, & Peddle, 1979) and various conditioned opponent processes that counteract the aversive properties of the illness-producing 15 drug (conditioned antisickness response; Lett, 1983; Revusky & Harding, 1986). Such responses were not observed in control groups that received the two drugs either alone or in the reverse i n j e c t i o n order (backward p a i r i n g s ) . These findings suggest that drugs may have conditional stimulus attributes in addition to the discriminative stimulus properties described above. Alternative explanations for drug discrimination learning invoke state-dependent mechanisms to account for the d i f f e r e n t i a t i o n of responses under d i f f e r e n t i n j e c t i o n conditions. During drug discrimination t r a i n i n g , animals are required to perform one response alternative while in a drugged state or another response alternative while in an undrugged state. Such t r a i n i n g could conceivably r e s u l t in the independent learning of two state-dissociated responses, the performance of which would require the operation of state-dependent r e t r i e v a l or response i n i t i a t i o n mechanisms (Bindra & Reichert, 1966; 1967; B l i s s , 1974; Overton, 1978). Indeed, numerous studies have shown that responses learned in a drugged state often are not performed in an undrugged state, whereas responses acquired in the absence of a drug are often not emitted following drug treatments (for a review see Overton, 1988). However, the drug doses usually required to demonstrate such state-dependent phenomena are usually much higher than those employed during drug discrimination t r a i n i n g (Overton, 1988). Furthermore, state-dependency models for drug discrimination learning do not 16 predict the occurrence of saline-appropriate responses when animals are given substitution tests with compounds that d i f f e r pharmacologically from the t r a i n i n g drug. Rather, such models would predict responses to be randomly dis t r i b u t e d and dramatically reduced after injections of these l a t t e r compounds. Relations Between the Stimulus Properties and Hedonic  Actions of Drugs At present, there is l i t t l e evidence to confirm or refute a r e l a t i o n s h i p between the discriminative stimulus properties and hedonic actions of addictive drugs. Evidence in support of a general r e l a t i o n between the cueing and hedonic effects of drugs has been limited primarily to casual observations that a l l drugs of abuse can be discriminated by animals (Overton, 1988). However, animals can also discriminate a number of non-addictive drugs (Overton, 1982; 1988), and there appears to be only a weak cor r e l a t i o n between the d i s c r i m i n a b i l i t y of d i f f e r e n t compounds and their abuse potential (Overton & Batta, 1977). These findings indicate that animals are capable of discriminating non-hedonic actions of drugs, and that such actions may contribute to the discriminative stimulus properties of certain addictive compounds. The r e l a t i v e contributions of hedonic and non-hedonic drug actions to the control of discriminated responding may vary as a function of the p a r t i c u l a r compound employed during t r a i n i n g . For example, animals trained to 17 discriminate narcotic compounds from saline appear to respond primarily to non-hedonic properties of the drugs. In one study, midbrain lesions that blocked the rewarding actions of morphine did not prevent t h i s drug from acting as a discriminative stimulus (Martin, Bechara, & van der Kooy, 1987). Additional studies have indicated that the cueing potencies of several narcotic compounds are p o s i t i v e l y correlated with their analgesic actions (Colpaert, 1978a). Drug discrimination studies involving psychomotor stimulant compounds present a d i f f e r e n t picture. For example, Colpaert et a l . (1979) found that rats trained to discriminate cocaine from saline generalized to a v a r i e t y of compounds that supported intravenous self-administration. In contrast, generalization was not observed when the rats were tested with compounds that lacked such rewarding actions. These results suggested a possible hedonic basis for the discriminative stimulus properties of cocaine. Indeed, other studies have found that the cueing effects of cocaine may be mediated by the same neural processes that give r i s e to the hedonic properties of t h i s compound (for reviews see Ho & Silverman, 1978; Silverman & Ho, 1977). An important goal for future drug discrimination research should be to provide independent assessments of the extent to which the stimulus properties of individual substances r e f l e c t hedonic actions of the drugs. One means of accomplishing t h i s task might be to compare the neural substrates for the cueing and hedonic actions of drugs. A 18 relationship between these actions of a drug would be indicated i f the separate functional properties were found to be derived from the same neuropharmacological actions. To date, only a few compounds have been studied extensively enough that the neural substrates for both the stimulus properties and rewarding effects of the drug are known. Perhaps the most extensively studied compound in this regard is amphetamine. The following four sections w i l l review evidence that the stimulus properties of amphetamine are derived from pharmacological actions on s p e c i f i c neural processes, and that these processes may be the same ones which mediate the hedonic properties of amphetamine. Pharmacological S p e c i f i c i t y of Amphetamine Stimuli In an e a r l i e r section i t was mentioned that drug stimuli tend to be pharmacologically s p e c i f i c , such that stimulus generalization may occur only between drugs with similar pharmacological properties. The stimulus properties of amphetamine are consistent with t h i s r u l e . Thus, animals trained to discriminate amphetamine from saline emit drug appropriate responses following injections of other psychomotor stimulant drugs such as 1-amphetamine, methyl-amphetamine, cocaine, methylphenidate, cathinone and amfonelic acid (Aceto, Rosencrans, Young, & Glennon, 1984; Colpaert, Niemegeers, & Janssen, 1978; Ho & McKenna, 1978; Huang & Ho, 1974a; Huang & Wilson, 1986; Schecter & Rosencrans, 1973). In contrast, amphetamine-trained animals respond primarily on the saline-appropriate lever when 19 tested with compounds that lack psychomotor stimulant actions (Jarbe, 1982; Schecter & Rosencrans, 1973; Silverman & Ho, 1977). Among the more important of these l a t t e r compounds are the peripherally acting stimulant, para-hydroxyamphetamine (Jarbe, 1982; Jones, H i l l , & Harris, 1974), and the general CNS stimulants nikethamide, picrotoxin and strychnine (Huang & Ho, 1974b). Para-hydroxyamphetamine is an amphetamine analogue which does not cross the blood-brain b a r r i e r , therefore i t exerts only peripheral actions. The lack of generalization to thi s drug indicates that the stimulus properties of amphetamine involve actions within the CNS3-. The absence of generalization to the CNS stimulants nikethamide, picrotoxin and strychnine further suggests that animals do not simply discriminate a general increase in CNS e x c i t a b i l i t y or arousal. Rather, animals appear to discriminate s p e c i f i c psychomotor stimulant properties of amphetamine that re s u l t from the CNS actions of thi s drug. Neurochemical Substrates for Amphetamine Stimuli Although amphetamine is capable of enhancing neurotransmission at dopaminergic, noradrenergic, and serotonergic synapses throughout the CNS, i t is the actions at dopaminergic synapses that appear to be responsible for 1. The stimulus properties of low amphetamine doses (0.125 mg/kg) may be mediated by peripheral processes, as they have been found to generalize with parahydroxyamphetamine (Colpaert, Kuyps, Niemegeers, & Janssen, 1976). However, the use of such low t r a i n i n g doses i s rare, and thus the present review w i l l refer only to stimulus properties associated with higher t r a i n i n g doses ( i . e . > 0.5 mg/kg). 20 the drug's discriminative stimulus properties. Rats trained to discriminate amphetamine from saline responded on the drug-appropriate lever when injected with the d i r e c t dopamine receptor agonists apomorphine, p i r i d e b i l , and N-propylnoraporphine (Schecter, 1977). Amphetamine-appropriate responses also were observed following injections of the anti-Parkinson drug amantidine (Schecter, 1977), the antidepressant compounds deprenyl, tranycypromine, amineptine, and bupropion (Porsolt, Pawelec, & J a l f r e , 1982) and as indicated above (pp. 18), the psychomotor stimulants cocaine, methylphenidate, cathinone, nomifensine, and amfonelic acid. A l l of these compounds are capable of enhancing dopamine neurotransmission, either by stimulating the release of dopamine or by blocking i t s reuptake (Wagner, Preston, Ricaurte, Schuster, & Seiden, 1982; Westerink, 1979). Importantly, rats did not emit amphetamine-appropriate responses when injected with compounds that did not a f f e c t dopaminergic neurotransmission (see Silverman and Ho, 1977; Porsolt et a l . , 1982). Studies involving pharmacological interference with monoaminergic neurotransmission also support the hypothesis that amphetamine stimuli r e s u l t from drug actions at dopaminergic synapses. Rats trained to discriminate amphetamine from saline responded primarily on the s a l i n e -appropriate lever when the amphetamine injections were preceded by pretreatments with the DA receptor antagonists haloperidol, pimozide, s p i r o p e r i d o l , c i s ( Z ) - f l u p e n t h i x o l , 21 t r i f l u p e r a z i n e , perphenazine or chlorpromazine (Ho & Huang, 1975; Nielsen & Jepsen, 1985; Schecter & Cook, 1975). Similar r e s u l t s were obtained when dopamine synthesis was impaired temporarily by pretreatment with the tyrosine hydroxylase i n h i b i t o r , alpha-methyl-p-tyrosine (Ho & Huang, 1975; Schecter & Cook, 1975) or when CNS dopamine neurons were destroyed by an intra v e n t r i c u l a r i n j e c t i o n of the neurotoxin 6-hydroxydopamine (6-OHDA; Woolverton & Cervo, 1986). In contrast, pretreatment with the noradrenaline antagonists phentolamine, phenoxybenzamine, and propranolol (Ho & Huang, 1975; Schecter & Cook, 1975) or the serotonin antagonists methysergide and cinanserin (Ho & Huang, 1975) had no ef f e c t on amphetamine s t i m u l i . The drug cues also were not affected when noradrenaline or serotonin synthesis was disrupted by treatment with the dopamine-B-hydroxylase in h i b i t o r d i s u l f i r a m or the tryptophan hydroxylase i n h i b i t o r , p-chloro-phenylalanine (Schecter & Cook, 1975). These l a t t e r results indicate that the amphetamine stimuli do not depend on drug actions at noradrenergic or serotonergic synapses. The Role of Sp e c i f i c Dopamine Projections in Mediating the  Stimulus Properties of Amphetamine Many of the behavioral e f f e c t s of amphetamine appear to be mediated by either the mesostriatal or mesocortical dopamine neurons. Both of these neural projections arise from c e l l bodies within the ventral mesencephalon and terminate either within the s t r i a t a l nuclei or within 22 various limbic and c o r t i c a l regions of the forebrain (Fallon & Moore, 1978). Recent studies have indicated that the mesocortical dopamine neurons may mediate the stimulus properties of amphetamine. In one study, amphetamine stimuli were attenuated by the at y p i c a l dopamine receptor antagonists s u l p i r i d e , clozapine, thioridazine, and molindone (Nielsen & Jepsen, 1985). These antagonists exhibited r e l a t i v e l y s e l e c t i v e binding at mesocortical dopamine receptor s i t e s , and exerted limited a c t i v i t y at s t r i a t a l dopamine receptors (Lane & Blaha, 1986). Although the amphetamine cue also was antagonized by the s t r i a t a l dopamine receptor antagonist metoclopromide, the doses required for t h i s antagonism were quite high (7.5 & 10.0 mg/kg). At these doses metoclopramide may be less selective for s t r i a t a l receptors and the attenuation of the amphetamine cue may have re f l e c t e d additional actions at mesocortical receptors. The component of the mesocortical dopamine pathways that terminates in the nucleus accumbens appears to play a p a r t i c u l a r l y important role in mediating the stimulus properties of amphetamine. Rats trained to discriminate systemic injections of amphetamine from saline emitted drug appropriate responses when amphetamine was injected d i r e c t l y into the nucleus accumbens (Nielsen & Scheel-Kruger, 1986). This generalization between systemic and central nucleus accumbens injections was blocked when the dopamine receptor antagonist, s u l p i r i d e was coinjected into the nucleus 23 accumbens along with amphetamine. Importantly, rats in thi s study did not generalize to injections of amphetamine into the dorsomedial and lateroventral caudate. This l a t t e r r e s u l t indicates that the stimulus properties of amphetamine are not mediated by mesostriatal dopamine neurons. Common Neural Substrates for the Stimulus Properties and  Rewarding Effects of Amphetamine The studies reviewed above indicate that the stimulus properties of amphetamine may be mediated by dopamine projections to the nucleus accumbens. These mesoaccumbens dopamine neurons also appear to mediate the rewarding effects of amphetamine. Rats w i l l self-administer amphetamine d i r e c t l y into the nucleus accumbens (Monaco, Hernandez, & Hoebel, 1980) and show conditioned place preferences for environments that have been associated with intra-accumbens injections of t h i s drug ( A u l i s i & Hoebel, 1983; Carr & White, 1983, 1986). Injections of amphetamine into the nucleus accumbens also can enhance the behavioral control exerted by conditioned reinforcers (Taylor & Robbins, 1984). In contrast, self-administration and place preference conditioning maintained by peripheral injections of amphetamine can be attenuated by 6-OHDA lesions of the mesoaccumbens dopamine projections (Lyness, F r i e d l e , & Moore, 1979; Spyraki, Fibiger, & P h i l l i p s , 1982). Given the evidence that mesoaccumbens dopamine neurons may mediate both the stimulus and rewarding properties of amphetamine, i t i s possible that the stimulus properties may r e f l e c t 24 hedonic actions of th i s drug. Issues to be Addressed in the Present Thesis From the findings reviewed in the previous four sections, i t is possible to formulate two hypotheses about the stimulus properties of amphetamine. One hypothesis is that the stimulus properties of amphetamine r e s u l t from the capacity of t h i s drug to enhance mesocortical dopamine neurotransmission, p a r t i c u l a r l y at synapses within the nucleus accumbens. To the extent that t h i s neurochemical action produces rewarding e f f e c t s , a second hypothesis may be formulated regarding the functional nature of the amphetamine s t i m u l i . S p e c i f i c a l l y , i t is suggested that the stimulus properties of amphetamine may r e f l e c t the hedonic actions of t h i s drug. If these hypotheses are correct, then tests for generalization with the stimulus properties of amphetamine might be useful either as a general behavioral assay for determining the dopaminergic actions of drugs or as a screening procedure to evaluate the abuse potential of newly developed compounds. Generalization with the stimulus properties of amphetamine might indicate that a drug could enhance dopamine neurotransmission or produce positive hedonic e f f e c t s , whereas an absence of generalization might indicate that the compound lacked these actions. To date, these predictions have been s a t i s f i e d consistently by experiments showing that amphetamine stimuli generalize to other psychomotor stimulants, d i r e c t dopamine agonists, and 25 psychomotor stimulant-like antidepressants, but amphetamine cues do not generalize to a v a r i e t y of non-stimulant drugs that lack dopaminergic or positive hedonic actions (Porsolt et a l . , 1982; Silverman & Ho, 1977). Nevertheless, there are potential l i m i t a t i o n s of t h i s screening procedure which should be evaluated before i t i s accepted as a v a l i d paradigm for assessing the neurochemical or a f f e c t i v e properties of drugs. One potential l i m i t a t i o n of the amphetamine generalization paradigm as a general screening procedure is that the stimulus properties of amphetamine may not generalize to drugs other than psychomotor stimulants and d i r e c t dopamine agonists. In general, there appear to be li m i t a t i o n s on the a b i l i t y of drugs to generalize to compounds from d i f f e r e n t pharmacological classes (Colpaert, 1978a; L a i , 1977; Overton, 1988; Silverman & Ho, 1977). These lim i t a t i o n s may vary depending on the p a r t i c u l a r class of t r a i n i n g drug employed. For example, generalization with the stimulus properties of narcotic agonists appears to be highly s e l e c t i v e (Colpaert, 1978a). In contrast, anesthetics, benzodiazepines, muscle relaxants and anticonvulsants r e a d i l y generalize to one another, suggesting that drugs from these individual classes may generate common stimuli related to their depressant actions (Overton, 1988). The stimulus properties of psychomotor stimulant compounds appear to be f a i r l y s p e c i f i c , although cocaine has been found to generalize p a r t i a l l y to the 26 narcotic analgesic fentanyl, the psychotomimetic phencyclidine, the noradrenergic receptor blocker propranalol, and the muscarinic acetylcholine receptor blockers dexetimide and benztropine. A second possible l i m i t a t i o n of the amphetamine generalization paradigm relates s p e c i f i c a l l y to i t s u t i l i t y for detecting the abuse potential of drugs. As yet, i t has not been determined whether the stimulus properties of amphetamine r e f l e c t a general state of positive a f f e c t which could generalize to a l l p o s i t i v e l y hedonic compounds. Stimulus generalization with amphetamine may be r e s t r i c t e d to drugs that produce their hedonic effects by acting on those processes that mediate the rewarding (and cueing) actions of amphetamine. If t h i s l a t t e r case were true, then the screening procedure might only be useful for i d e n t i f y i n g the abuse potential of a limited range of compounds2. The present thesis determined whether these constraints on generalization between drug stimuli would l i m i t the u t i l i t y of an amphetamine stimulus generalization paradigm for screening either the dopaminergic actions or the abuse potential of psychoactive drugs. To accomplish t h i s goal, rats were trained to discriminate amphetamine from saline and then given stimulus generalization tests with drugs from 2. Wise and Bozarth (1987) have suggested that a l l drugs of abuse may produce their hedonic e f f e c t s by acting either on the mesoaccumbens dopamine projection, or processes afferent or efferent to these neurons. If t h i s hypothesis is correct, then the second l i m i t a t i o n presented here would not r e s t r i c t the u t i l i t y of the amphetamine generalization paradigm as a procedure for screening the abuse potential of drugs. 27 diverse pharmacological classes and with a non-pharmacological stimulus produced by e l e c t r i c a l stimulation of the VTA. The drugs and brain-stimulation employed for these experiments were chosen on the basis that their actions on both dopamine neurotransmission and a f f e c t i v e processes were already known. Thus, i t was possible to predict the probable outcomes that should be obtained in the absence of any of the abovement ioned constraints on generalization. Differences between the actual and predicted outcomes would indicate that the generalization paradigm might have l i m i t s to i t s u t i l i t y as a screening procedure. Discrimination Training Procedures A wide var i e t y of procedures have been employed to assess the discriminative stimulus properties of drugs. For example, animals may be trained to discriminate either a fixed dose of a drug from sa l i n e , one dose from a second dose of the same compound or a s p e c i f i c dose of one drug from a s p e c i f i c dose of a d i f f e r e n t compound (Jarbe and Swedberg, 1982). Animals also may be trained to perform a variety of other discriminations that can involve three or more drug stimuli (Overton, 1988). To date, most studies of the stimulus properties of amphetamine have involved discriminations between a set dose of amphetamine (usually between 0.75 and 1.0 mg/kg) and s a l i n e . This type of discrimination procedure also was employed for the present experiments, with rats being trained to discriminate 1.0 mg/kg amphetamine from s a l i n e . 28 Drugs may exert stimulus control over a wide variety of operant responses. These responses may be a p p e t i t i v e l y or aversively motivated, may involve position or go/no-go tasks, and they may be maintained by a range of reinforcement schedules. The par t i c u l a r s of these procedures, as well as their advantages and disadvantages have been reviewed by Colpaert (1977b, 1978b) and by Overton and Hayes (1984). In general, most investigators have come to prefer a two-lever, a p p e t i t i v e l y motivated choice procedure in which animals are required to respond on one of two levers to obtain reinforcement after receiving a drug in j e c t i o n , or on the alternative lever after receiving a saline i n j e c t i o n . Reinforcement is usually delivered on a fixed r a t i o (FR) schedule that requires the animals to emit at least ten responses on the appropriate lever to obtain each reward. The present experiments employed a similar two-lever choice procedure with an FR-32 reinforcement schedule. The Amphetamine Stimulus Generalization Paradigm In most drug discrimination experiments, stimulus generalization between compounds is assessed by giving animals tests in which a novel compound is substituted for the usual t r a i n i n g drug. Generalization is assumed when animals emit responses appropriate for the training-drug state following i n j e c t i o n of the novel compound (Colpaert, 1977b). However, t h i s assumption is v a l i d only when drugs induce strong generalization such that the animals consistently emit responses appropriate for the drug 29 condition. In cases where only p a r t i a l generalization occurs, animals w i l l often alternate their responses between the two available choices. This behavior pattern may be d i f f i c u l t to interpret as the random lever-choices may r e f l e c t either p a r t i a l generalization or a general disruption of performance (Colpaert, 1988). In these circumstances, i t i s important to demonstrate further that stimuli associated with the test compound can summate with the cueing effects produced by low doses of the tr a i n i n g compound (e.g., see R e a v i l l & Stolerman, 1988). Such summation would be re f l e c t e d as a more complete generalization with the combination of the two drugs than is obtained with either drug alone. In order to provide for the unambiguous interpretation of the stimulus generalization experiments 3 in the present thesis, the individual psychoactive drugs and VTA stimulation were given in combination with a range of amphetamine doses (0.0 4, 0.25, 0.50 and 1.0 mg/kg). Under 3. The present experiments represent a departure from t r a d i t i o n a l approaches to generalization t e s t i n g . Generalization tests usually involve the presentation of simple stimuli that d i f f e r q u a n t i t a t i v e l y from the t r a i n i n g stimulus along a pa r t i c u l a r sensory dimension (Mackintosh, 1974). In the present experiments, animals are tested with complex st i m u l i that r e s u l t from the super imposition of drug cues. The tests require that the animals detect the extent of summation of familiar cue properties upon a background of novel stimuli that may d i f f e r q u a l i t a t i v e l y from the tra i n i n g stimulus. Despite these differences, the procedure w i l l continue to be referred to as a generalization test to r e f l e c t the fact that animals are being tested for the extent to which the test stimuli evoke the responses appropriate for the t r a i n i n g cue. 4. The 0.0 mg/kg amphetamine "dose" is a c t u a l l y a saline i n j e c t i o n . Although t h i s i n j e c t i o n normally i s used as the reference stimulus for the amphetamine discrimination, i t 30 baseline conditions, each successive dose of amphetamine was expected to e l i c i t increasing amounts of responding on the drug-appropriate lever, r e s u l t i n g in an orderly stimulus generalization function (Figure 1). The combination of amphetamine with test stimuli that possessed amphetamine-l i k e cueing actions might res u l t in increased drug-lever responding at the low amphetamine doses (eg. 0.0, 0.25 and 0.5 mg/kg in Figure 1) due to a summation of the common stimulus properties. This would be ref l e c t e d as an elevation of the stimulus generalization function r e l a t i v e to the vehicle control curve. In contrast, test stimuli that antagonized the cueing potency of amphetamine might decrease drug-lever responses at most amphetamine doses (0.25, 0.5 and 1.0 mg/kg in Figure 1) and lower th i s function r e l a t i v e to the vehicle curve. Test stimuli that produced general disruptions of discriminative performance would not produce such uniform additive or subtractive effects on the generalization functions. Instead, these stimuli should res u l t in generalization functions that r e f l e c t a more w i l l be described throughout the thesis in terms of i t s position along a continuum of amphetamine doses when the results of generalization tests are being considered. This w i l l f a c i l i t a t e discussions of the data. Nevertheless, the reader should bear in mind that increases in drug-appropriate responding observed when a test stimulus is combined with t h i s amphetamine "dose" a c t u a l l y r e f l e c t generalization with the stimulus properties of amphetamine rather than summation between drug s t i m u l i . 31 Figure 1: Theoretical outcomes of stimulus generalization experiments. The curves r e f l e c t the percentage of responses that rats might emit on the lever appropriate for an amphetamine test (y-axis) as a function of increasing amphetamine doses (x-axis). The s o l i d curve represents the th e o r e t i c a l baseline generalization function that might be obtained following vehicle control t e s t s . Under these conditions, the percentage of responses emitted on the amphetamine (drug) lever should increase as a monotonic function of increasing amphetamine dose. Experimental manipulations (drugs or brain-stimulation) that augment the stimulus properties of amphetamine should r e s u l t in increased responding on the drug-lever at the lower amphetamine doses and an elevation of the amphetamine generalization function r e l a t i v e to the control curve. In contrast, manipulations that interfere with the amphetamine stimuli should reduce drug-lever responses at the higher amphetamine doses and lower the generalization function. Extreme disruptions of discriminative performance would cause responses to be randomly distributed between levers regardless of the amphetamine dose, and thus the generalization function would l i e f l a t around the 50% response l e v e l . 32 • DISRUPTION 0.0 0.25 0.50 . 1.0 AMPHETAMINE DOSE (mg/kg) 33 random d i s t r i b u t i o n of responses. Figure 1 shows a hypothetical example of the most extreme case in which a drug produces random responding at each dose of amphetamine. Other cases may exist where the amount of disruption varies as a function of the amphetamine dose. Under such circumstances the test and control functions would overlap at one end of the amphetamine dose continuum and diverge at the other end, with responses at the divergent end being somewhat random. Predicted Effects of Different Test Stimuli on Amphetamine  Stimulus Generalization Functions In the previous section i t was suggested that test stimuli that possess amphetamine-like stimulus properties might elevate amphetamine stimulus generalization functions due to the summation of common stimulus elements. In contrast, test stimuli that were antagonistic to amphetamine cues might lower the stimulus generalization functions due to subtractive interactions between the s t i m u l i . To the extent that the stimulus properties of amphetamine may be related to either the dopaminergic and or hedonic actions of the drug, i t would be predicted that: 1 ) Test stimuli that can enhance mesocortical dopamine neurotransmission might summate with the stimulus properties of amphetamine and elevate the stimulus generalization functions r e l a t i v e to curves obtained with amphetamine alone. In contrast, test stimuli that interfere with mesocortical dopamine neurotransmission might antagonize the 34 cueing effects of amphetamine and lower the stimulus generalization functions. 2) Test stimuli that exert positive hedonic actions might summate with the stimulus properties of amphetamine and elevate the stimulus generalization functions r e l a t i v e to curves obtained with amphetamine alone. In contrast, test stimuli with anhedonic actions might antagonize the cueing effects of amphetamine and lower the stimulus generalization functions. The p a r t i c u l a r test stimuli to be employed in the present thesis are presented in Table 1. As indicated above (pp. 26) the drugs were chosen from a wide range of pharmacological classes to determine whether the stimulus generalization paradigm could be used to screen a diverse selection of compounds for their dopaminergic actions and hedonic e f f e c t s . Thus, each drug l i s t e d in Table 1 is a representative either of a major class of abused compounds or of a dopamine receptor agonist (apomorphine) or antagonist (haloperidol). E l e c t r i c a l stimulation of the VTA also was employed as a test stimulus, to represent a major class of non-pharmacological hedonic s t i m u l i . The p a r t i c u l a r stimuli selected as representative of each class were chosen on the basis that their dopaminergic actions and hedonic effects have been determined previously. As indicated in Table 1 the range of test stimuli employed includes agents that either can enhance, attenuate or have no effects on dopamine neurotransmission, and that may produce either 35 TABLE 1 The a c t i o n s on dopamine neur o t r a n s m i s s i o n and the hedonic e f f e c t s of the t e s t s t i m u l i employed f o r Experiments 1 through 6. DRUG DOPAMINERGIC ACTION HEDONIC EFFECT Expt 1: Cocaine Apomorphine Enhances Low Dose Attenuates High Dose Mimicks P o s i t i v e P o s i t i v e Haloper i d o l Attenuates Anhedoni Expt 2: N i c o t i n e Enhances Pos i t i v e Expt 3: Morphine Enhances P o s i t i v e Expt 4: Midazolam Attenuates P o s i t i v e Expt 5: Ethanol No E f f e c t s P o s i t i v e Expt 6: Stimulate VTA Enhances P o s i t i v e 36 positive hedonic or anhedonic e f f e c t s . The selection of test s t imuli with these d i f f e r e n t actions allowed an assessment of the capacity for the present generalization paradigm to detect both additive and subtractive interactions with the stimulus properties of amphetamine. The f i r s t experiment assessed the effects of cocaine, apomorphine, and haloperidol on amphetamine stimulus generalization functions. Cocaine and high doses of apomorphine can both augment dopamine neurotransmission and produce rewarding effects in laboratory animals (Anden, Rubensson, Fuxe, & Hokfelt, 1967; R i t z , Lamb, Goldberg, & Kuhar, 1987). Either of these actions might r e s u l t in elevations of the amphetamine stimulus generalization functions r e l a t i v e to control curves. In contrast, the generalization functions might be lowered by haloperidol, which can both interfere with dopamine neurotransmission and attenuate the rewarding effects of various drugs (Anden, Butcher, Corrodi, Fuxe, & Ungerstedt, 1970; Carr et a l . , in press) and by low doses of apomorphine that decrease the impulse-dependent release of dopamine (Gonon & Buda, 1985; Grace & Bunney, 1985; Lane & Blaha, 1986; Zetterstrom & Ungerstedt, 1984). Such results would confirm the s e n s i t i v i t y of the generalization paradigm for detecting the additive or subtractive effects of other psychomotor stimulants and dopaminergic receptor agonists and antagonists. The subsequent four experiments examined the effects on 37 amphetamine stimulus generalization functions of non-psychomotor stimulant drugs including nicotine (a CNS stimulant), morphine (an opiate), midazolam (a benzodiazepine), and ethanol (a sedative hypnotic). Nicotine and morphine both can enhance mesoaccumbens dopamine neurotransmission (Di Chiara & Imperato, 1986; Gysling & Wang, 1983; Imperato, Mulas, & Di Chiara, 1986; Mereu et a l . , 1987; Moleman, van Valkenburg, & van der Krogt, 1984 ) and can exert positive hedonic actions in laboratory animals and humans (Henningfield & Goldberg, 1983; J a s i n s k i , 1977; Martin & J a s i n s k i , 1977). Accordingly, these drugs were predicted to augment the stimulus properties of amphetamine and elevate the generalization functions r e l a t i v e to control curves. Midazolam and ethanol each produce d i f f e r e n t i a l e f f e cts on dopamine neurotransmission and on a f f e c t i v e processes. Both of these drugs can produce rewarding effects in laboratory animals (Ator & G r i f f i t h s , 1987; Mello, 1981; Szostak, Finlay, & Fibiger, 1987). However, midazolam has been found to decrease dopamine neurotransmission (Finlay et a l . , 1987; Haefely, P i e r i , Pole, & Shaffner, 1981) whereas ethanol has been shown to have no e f f e c t on dopamine function (Hellevuo & Kiianmaa, 1988; Kalant, 1975; Nutt & Glue, 1986; Smith, 1977; but see Di Chiara & Imperato, 1986). These differences in the neurochemical and functional actions of midazolam and ethanol necessitate separate predictions for the stimulus generalization experiments. If 38 the dopaminergic actions of a drug determine i t s effects on amphetamine s t i m u l i , then midazolam should lower generalization functions whereas ethanol should have no e f f e c t s . If the hedonic actions of a drug determine i t s effects on amphetamine s t i m u l i , then both drugs should elevate the generalization functions. The outcomes of these experiments may indicate the r e l a t i v e importance of either the hedonic properties or the dopaminergic actions of drugs in determining interactions with amphetamine s t i m u l i . The four abovementioned experiments also examined the effects of nicotine, morphine, midazolam, and ethanol on locomotor a c t i v i t y . Changes in a c t i v i t y levels often r e f l e c t p a r a l l e l changes in dopaminergic neurotransmission within the nucleus accumbens. Thus, drugs which decrease mesoaccumbens dopamine neurotransmission consistently reduce a c t i v i t y l e v e l s , whereas compounds that enhance dopamine function within t h i s region often increase locomotion. Accordingly, the locomotor a c t i v i t y tests provided an independent behavioral means of assessing the dopaminergic actions of the drugs employed in experiments 2 through 5. Experiment 6 evaluated the effects of e l e c t r i c a l stimulation of the VTA on amphetamine stimulus generalization functions. As stimulation of t h i s region can both activate mesoaccumbens dopamine neurons and produce rewarding effects in rats (Blaha, P h i l l i p s , & Fibiger, 1988; Fibiger, Lepiane, Jakubovic, & P h i l l i p s , 1987), t h i s stimulation might augment the cueing effects of amphetamine 39 and e l e v a t e the stimulus g e n e r a l i z a t i o n f u n c t i o n s . Such a r e s u l t would i n d i c a t e t h a t the amphetamine stimulus g e n e r a l i z a t i o n f u n c t i o n c o u l d d e t e c t the dopaminergic or hedonic a c t i o n s of non-pharmacological s t i m u l i . Experiments 7 and 8 were designed to i n v e s t i g a t e the r o l e s of both the hedonic e f f e c t s and dopaminergic a c t i o n s of VTA s t i m u l a t i o n i n determining the i n t e r a c t i o n s of t h i s t e s t stimulus with the s t i m u l u s p r o p e r t i e s of amphetamine observed i n Experiment 6. Experiment 7 assessed whether i n d i v i d u a l d i f f e r e n c e s i n the amount of g e n e r a l i z a t i o n obtained between the stimulus p r o p e r t i e s of amphetamine and VTA s t i m u l a t i o n might be r e l a t e d to d i f f e r e n c e s i n the e f f i c a c y of the s t i m u l a t i o n f o r producing rewarding e f f e c t s d u r i n g ICSS t e s t s . A c o r r e l a t i o n between the extent of s t i m u l u s g e n e r a l i z a t i o n and ICSS r a t e s would suggest t h a t the g e n e r a l i z a t i o n was r e l a t e d to the hedonic e f f e c t s of the s t i m u l a t i o n . Experiment 8 determined whether the s t i m u l u s p r o p e r t i e s of VTA s t i m u l a t i o n c o u l d be modulated by amphetamine and h a l o p e r i d o l , i n an attempt to c o n f i r m the dopaminergic mediation of t h i s non-pharmacological s t i m u l u s . 40 GENERAL METHODS Subjects Male hooded rats (Charles River, Long Evans strain) were used as subjects for these experiments. A l l rats were housed i n d i v i d u a l l y in stainl e s s s t e e l cages with tap water available ad libitum. Access to food was r e s t r i c t e d , and 22 to 24 gms of standard rat chow was provided at the end of each day. A 12 hr light-dark cycle was maintained in the rat colony, and the rats were tested either within the l a s t two hrs of the l i g h t phase or during the f i r s t two hours of the dark phase of thi s cycle. Surgery and Histology Prior to discrimination t r a i n i n g , the rats in Experiments 6 through 8 were anesthetized with 65 mg/kg sodium pentobarbital and a bipolar electrode ( P l a s t i c Products MS303/2) was implanted s t e r e o t a x i c a l l y into the VTA. With the incisor bar set at -3.2 mm below the interaural l i n e , the coordinates from stereotaxic zero were: 2.8 mm anterior, 0.6 mm l a t e r a l (toward the l e f t hemisphere) and 2.1 mm dorsal. Electrodes were anchored to the s k u l l with jewelers screws and dental cement. Upon completion of the brain-stimulation experiments, rats with electrodes were k i l l e d with an overdose of sodium pentobarbital and perfused t r a n s c a r d i a l l y with 0.9% saline followed by 10% formol-saline. The brains were removed and stored for at least one week in 10% f ormol-sal ine before being frozen, s l i c e d in 30 um sections, mounted on 41 microscope s l i d e s and stained with c r e s y l - v i o l e t for v e r i f i c a t i o n of electrode placements. Apparatus Most of the discrimination experiments were conducted in six test chambers (24 X 29 X 30 cm), each having Plexiglas walls and c e i l i n g and a wire grid f l o o r . These chambers contained two levers ( 5 X 8 cm) mounted 3.5 cm above the floor on opposite walls, a 28 v house-light attached to the center of a t h i r d wall and a food-hopper positioned d i r e c t l y below the l i g h t (3 cm above the f l o o r ) . Sound attenuating enclosures (55 X 55 X 60 cm) with v e n t i l a t i o n fans were used to contain each chamber and iso l a t e the rats from extraneous environmental s t i m u l i . For Experiment 8, electrode leads (P l a s t i c Products 303-302) were attached to Mercotac swivel commutators stationed on the c e i l i n g s of the outer enclosures and the wires were passed through openings at the top of each chamber. Constant current e l e c t r i c a l stimulation was delivered by independent channels of a programmable sine-wave stimulator. A Data General Nova 3 computer with MANX software was used to control the experimental events and record responses made by the rat s . Experiments 6 and 7 were conducted in four test chambers (32 X 32 X 40 cm), each having four walls (one aluminum and three P l e x i g l a s s ) , a Plexiglas c e i l i n g and a wire grid f l o o r . A food hopper was positioned 8 cm above the floor in the middle of the aluminum wall of each chamber, with a 28 v house-light stationed 8 cm d i r e c t l y above. A retractable 42 lever (Coulbern Instruments, model E23-05, 4 X 4 cm) was stationed 6.5 cm on either side of each food hopper (8 cm above the f l o o r ) . Sound attenuating enclosures (55 X 55 X 60 cm) with v e n t i l a t i o n fans were used to contain the chambers and mask extraneous noise. During generalization tests, the rats were attached to electrode leads ( P l a s t i c Products, 303-302) that were suspended from Mercotac commutators and passed through openings in the chamber c e i l i n g s . Constant current stimulation was delivered to the rats by a 6 channel, programmable sine-wave stimulator. An INI computer with MANX software was used to control the experimental events and record responses made by the r a t s . Stimulation currents were monitered continuously on a Tektronix T912 storage oscilloscope. Locomotor a c t i v i t y tests were conducted in six chambers (45 X 45 X 30 cm), each having three wooden walls, one Plexiglas wall, and either a wood or wire mesh f l o o r . Each chamber had eight photocell emitters stationed 10 cm apart and 1 or 2.5 cm above the floor on two adjacent walls. The associated photocell detectors were placed opposite the emitters on the remaining walls. Interruptions of the horizontal photocell beams were integrated for each chamber by a multiplexor and relayed to the abovementioned INI computer, which recorded the a c t i v i t y scores. The s e l f stimulation tests in Experiment 7 were conducted in 5 chambers (23 X 29 X 46 cm), each having plexiglass walls and a wire grid f l o o r . A single lever (5 X 43 8 cm) was mounted at the middle of one wall (2.5 cm above the floor) and electrode leads were suspended from Mercotac commutators and passed through the open tops of the chambers. E l e c t r i c a l stimulation was delivered by five independent sine-wave stimulators and responses were recorded on mechanical counters. Drug Discrimination Training A l l rats were i n i t i a l l y trained to lever-press for food (45 mg Bioserve pe l l e t s ) on a continuous reinforcement schedule during four 30 min drug-free sessions. Subsequently, the rats were given d a i l y 30 min discrimination t r i a l s , with either 1.0 mg/kg d-amphetamine sulphate (dissolved in 0.9% saline to a concentration of 1.0 mg/ml) or 1.0 ml/kg saline injected i n t r a p e r i t o n e a l l y (IP) immediately prior to each session. During the i n i t i a l 15 min of the session the house-light was turned off and the p e l l e t dispenser was inactivated. For Experiments 6 and 7, the levers were retracted during t h i s period. Subsequently, the house-light was turned on (and the levers inserted in Experiments 6 and 7) for the remaining 15 minutes and the rats could obtain food by responding s e l e c t i v e l y on one of the two levers following injections of 1.0 mg/kg amphetamine or on the alternative lever after saline i n j e c t i o n s . The lever appropriate for each i n j e c t i o n was counterbalanced among rats and responses on the inappropriate lever were recorded but had no programmed consequences. Throughout t r a i n i n g the amphetamine and saline t r i a l s were intermixed 44 randomly, with the single l i m i t a t i o n that neither solution was administered on more than two consecutive sessions. During the f i r s t four discrimination sessions, each response on the appropriate lever resulted in the delivery of a food p e l l e t . Subsequently, the number of responses required for each food p e l l e t was doubled after every fourth session, u n t i l a fixed-ratio-32 (FR-32) reinforcement schedule was in e f f e c t . Training then continued with the FR-32 schedule u n t i l the rats responded c o r r e c t l y on eight out of 10 consecutive sessions. A correct response was defined as the completion of the f i r s t FR-32 requirement within a session on the lever appropriate for the in j e c t i o n received on that t r i a l . Generalization Tests Rats that acquired the discrimination task were given generalization tests with a range of amphetamine doses (0.00 [ i . e . , s a l i n e ] , 0.25, 0.50 and 1.0 mg/kg) administered either alone ( i . e . with a vehicle solution where appropriate) or in combination with a psychoactive drug or VTA stimulation. These tests were similar to the usual t r a i n i n g sessions except that the rats were injected with a test compound at an appropriate i n t e r v a l before testing or administered VTA stimulation beginning 3 minutes after the st a r t of the session. As usual, the house-light was turned on after 15 min and the responses on both levers were recorded. As the appropriate lever could not be determined a p r i o r i for these tests, the rats were reinforced for 45 continuing to respond on the lever on which the f i r s t FR-32 requirement was completed. The tests were spaced at least two days apart, with the usual saline and amphetamine baseline t r i a l s occuring on the intervening days. The order in which drug doses and stimulation currents were administered was counterbalanced for a l l r a t s . Locomotor A c t i v i t y Tests The a c t i v i t y tests were conducted at least two weeks after completion of the generalization t e s t s . During the interim period the rats remained in their home cages, where they were fed d a i l y . For the four days prior to the st a r t of a c t i v i t y testing, the rats were given injections of amphetamine and saline on alternate days and returned to their home cages. On the f i f t h day the rats were given the f i r s t of three locomotor a c t i v i t y t e s t s . Each rat was injected with either saline or one of the two doses of the drug i t had received during generalization t e s t s . After the appropriate delay (nicotine = 20 min; morphine = 60 min; ethanol = 15 min; midazolam = 5 min), the rats were placed in the darkened a c t i v i t y chambers for 30 min and their a c t i v i t y was measured. A l l rats were tested in t h i s manner on every t h i r d day with a d i f f e r e n t dosage of the drug ( i . e . vehicle, dose 1, or dose 2). Alternating amphetamine and saline injections were given in the home cages on the intervening days. 46 Drugs Amphetamine sulphate (Smith, Kline and French) was dissolved in physiological (0.9%) saline and injected IP at the s t a r t of each session. Apomorphine hydrochloride (Sigma) was dissolved in de-oxygenated physiological saline and injected subcutaneously (SC) 5 min before the s t a r t of a session. Cocaine hydrochloride (BDH Chemicals) was dissolved in saline and injected IP 5 min before the session. Haloperidol (Haldol, McNeil Pharmaceuticals) was diluted in d i s t i l l e d water and injected IP 45 min before a session. L-nicotine b i t a r t r a t e (BDH Chemicals) was dissolved in sa l i n e , brought to a pH of 7.2 with 0.05 M NaOH, and frozen in sealed ampules. On test days, individual ampules were thawed and the nicotine solutions were injected SC 20 min before the session. Morphine sulphate (BDH Chemicals) was dissolved in saline and injected SC 60 min before test sessions, and midazolam maleate (Hoffman-La Roche) was dissolved in d i s t i l l e d water and injected SC 5 min before the sessions. Ethanol was prepared by d i l u t i n g a 95% stock solution down to either 25% or 12.5% with s a l i n e . This yeilded solutions with concentrations of 0.125 and 0.25 g/ml, each which was injected IP 15 min before a session at a volume of 4 ml/kg. With the exception of ethanol, the concentrations of a l l drug solutions were varied to maintain an i n j e c t i o n volume of 1 ml/kg. Fresh drug solutions were prepared for a l l test sessions. Most reported dosages are expressed in terms of the weight of the drug s a l t , except for nicotine which is 47 expressed as the weight of the drug base. S t a t i s t i c a l Analyses From each test session conducted in Experiments 1 through 6, a c a l c u l a t i o n was made of the percentage of responses emitted prior to the f i r s t reinforcement that occurred on the amphetamine-appropriate lever. These percentages were averaged across rats and dose-response functions were constructed which indicated the tendency for rats to respond on the amphetamine-appropriate lever as a function of the amphetamine dose following either saline injections, d i f f e r e n t doses of a psychoactive drug or d i f f e r e n t stimulation currents. These data were then analysed using a two-way repeated measures analysis of variance (ANOVA) with the dose or current i n t e n s i t y of the test stimulus as one factor and the amphetamine dose as the second factor. Occasionally, the disruptive effects of certain dosage combinations prevented rats from completing even the f i r s t FR-32 response requirement of the test session. Under these circumstances, analyses were performed only on the data from rats that completed the f i r s t response requirement. Consequently, the ANOVA1s for these experiments had fewer degrees of freedom. The post-reinforcement responses within a session could not be used as a r e l i a b l e measure of the discriminative stimulus properties of amphetamine, as the d i s t r i b u t i o n of these responses was influenced to a large extent by the delivery of the f i r s t food p e l l e t . The rats usually 48 continued to respond s e l e c t i v e l y (90 to 100% of responses) on the lever that had produced the f i r s t reinforcer within the session ( i . e . the lever on which the f i r s t FR-32 response requirement was completed). Nevertheless, the percentage of post-reinforcement responses emitted on t h i s i n i t i a l l y selected l e v e r a was used as an index of possible disruptive actions of drugs on discriminated responding. If these responses were found to be less s e l e c t i v e for the reinforced lever after a pa r t i c u l a r drug treatment, then thi s might indicate that the drug was exerting a general disruptive action on discriminative and/or reinfor c i n g stimulus control of responding. Accordingly, the percentages of post-reinforcement responses emitted on the selected-lever were analysed using a two-way, repeated measures ANOVA with the dose or current i n t e n s i t y of the test stimulus as one factor and the amphetamine dose as the second factor. A similar ANOVA was performed on the t o t a l number of responses emitted within the l a s t 15 min of each session, to provide an additional measure of response f a c i l i t a t i o n or disruption. A c t i v i t y scores from each locomotion test were recorded separately for each 5 min period of the 30 min test sessions. These scores were analysed with a two-way repeated measures ANOVA with drug dose as one factor and time period as the second factor. 5. Throughout the thesis, the term "selected lever" w i l l be used to refer to the lever on which a rat completed the f i r s t FR-32 response requirement within a pa r t i c u l a r test session. 49 When s i g n i f i c a n t main effects were found with any of the above ANOVA's, post hoc analyses were performed using Newman-Keuls t e s t . Differences revealed with both the ANOVA's and Newman-Keuls tests were considered s i g n i f i c a n t when the p r o b a b i l i t y l e v e l was less than 0.05. 50 EXPERIMENT 1 The Effects of Cocaine, Apomorphine and Haloperidol on  Amphetamine Stimulus Generalization Functions As indicated in the introduction, drug discrimination studies t r a d i t i o n a l l y have employed simple drug-substitution procedures to determine whether generalization can occur between d i f f e r e n t pharmacological compounds. By comparison, the procedures employed in the present thesis represent a r e l a t i v e l y novel approach to the study of drug s t i m u l i . Rats trained to discriminate amphetamine from saline w i l l be given generalization t r i a l s with a range of amphetamine doses (0.0, 0.25, 0.5 and 1.0 mg/kg) administered either alone or in combination with d i f f e r e n t psychoactive drugs or VTA stimulation. If the drugs or the VTA stimulation produce amphetamine-like s t i m u l i , then these stimuli should summate with the cues produced by each amphetamine dose. This would be re f l e c t e d as an increase in the amount of drug-appropriate responses e l i c i t e d at the lower doses of amphetamine, and an elevation of the amphetamine stimulus generalization functions r e l a t i v e to curves obtained in the absence of the drugs or brain-stimulation. Subtractive interactions between amphetamine and a test stimulus would resul t in decreases in drug-appropriate responding over the range of amphetamine doses, and a lowering of the stimulus generalization functions. To assess the r e l i a b i l i t y of the abovementioned procedure, rats in the present experiment were given 51 generalization tests with a range of amphetamine doses after pretreatment with drugs that previously have been found either to generalize or interfere with the stimulus properties of amphetamine. These l a t t e r drugs included the psychomotor stimulant cocaine, the dopamine receptor agonist apomorphine and the dopamine receptor antagonist haloperidol. Stimuli produced by cocaine and apomorphine have been shown to generalize r e l i a b l y with the stimulus properties of amphetamine (Colpaert et a l . , 1978; Huang & Ho, 1974a; Huang & Wilson, 1986; Schecter, 1977; Schecter & Cook, 1975). Accordingly, the cocaine and apomorphine stimuli also might summate with the cueing e f f e c t s of low amphetamine doses and elevate the stimulus generalization functions r e l a t i v e to the respective control curves. Haloperidol has been shown to interfere with the stimulus properties of amphetamine (Colpaert et a l . , 1978; Nielsen & Jepsen, 1985; Schecter & Cook, 1975), and thus should lower the amphetamine generalization functions r e l a t i v e to the control curve. Such results would both confirm the r e l i a b i l i t y of the procedures employed in the present thesis, and i l l u s t r a t e how drugs that either generalize or interfere with the stimulus properties of amphetamine might a f f e c t amphetamine generalization functions. The present experiment also might confirm previous reports that the stimulus properties of amphetamine r e s u l t from the actions of t h i s drug at dopaminergic synapses (Ho & Silverman, 1978; Silverman & Ho, 1977). Elevations of the 52 amphetamine stimulus generalization functions after cocaine and apomorphine, and a lowering of the function following haloperidol would be consistent with the effects of these drugs on dopamine neurotransmission. Cocaine enhances dopamine neurotransmission by blocking the reuptake and metabolic degradation of thi s neurotransmitter (Ritz et a l . , 1987), whereas high doses of apomorphine can mimic dopaminergic neurotransmission by exerting agonist actions at post-synaptic dopamine receptors (Anden et a l . , 1967). Haloperidol acts as an antagonist at dopamine receptors and interferes with the function of thi s neurotransmitter (Anden et a l . , 1970). In addition, low doses of apomorphine can act p r e f e r e n t i a l l y at dopamine autoreceptors and decrease the impulse-dependent release of dopamine (Gonon & Buda, 1985; Grace & Bunney, 1985; Lane & Blaha, 1986; Zetterstrom & Ungerstedt, 1984). A lowering of the stimulus generalization functions at t h i s dose of apomorphine would provide further evidence of a dopaminergic substrate for the stimulus properties of amphetamine. Methods Twelve male hooded rats were used as subjects for thi s experiment. These rats were trained to discriminate 1.0 mg/kg amphetamine from saline using the procedures outlined in the general methods section. Rats that acquired the discrimination task (11 out of 12) were tested for generalization to a range of amphetamine doses (0.0 [ i . e . , s a l i n e ] , 0.25, 0.50 and 1.0 mg/kg) following pretreatment 53 with various doses of apomorphine (0.05, 0.15 and 0.20 mg/kg, injected SC 5 min before the session). After these apomorphine experiments, ten of the rats were given a second set of generalization tests with the same range of amphetamine doses being injected after pretreatment with cocaine (2.5 and 5.0 mg/kg, injected IP 5 min before the session). A t h i r d set of tests was then conducted following pretreatment with haloperidol (0.10 and 0.125 mg/kg, given IP 45 min before the session). Separate control sessions were conducted for each set of generalization t e s t s , in which the amphetamine doses were administered after pretreatment with the respective drug vehicles. Results Eleven of the 12 rats trained for thi s experiment learned to discriminate 1.0 mg/kg amphetamine from sal i n e , requiring 20 to 30 days to reach the c r i t e r i o n of eight correct responses across 10 consecutive t r i a l s . Figure 2 shows the results of the generalization tests in which a range of amphetamine doses (0.0, 0.25, 0.5 and 1.0) was administered in combination with either cocaine (Figure 2a), apomorphine (Figure 2b) or haloperidol (Figure 2c). Each of the three graphs reveals that the rats emitted a greater percentage of their pre-reinforcement responses on the amphetamine-appropriate lever as a di r e c t function of increasing amphetamine doses. The significance of these dose-response relations was confirmed by separate ANOVAs performed for the effects of amphetamine-dosage on drug-54 Figure 2: Effects of cocaine (vehicle, 2.5 and 5.0 mg/kg), apomorphine (vehicle, 0.05, 0.15 and 0.20 mg/kg) and haloperidol (vehicle, 0.10 and 0.125 mg/kg) on amphetamine stimulus generalization functions. A) Both doses of cocaine (2.5 and 5.0 mg/kg) produced s i g n i f i c a n t increases in drug-lever responses, causing amphetamine generalization functions to be elevated r e l a t i v e to the control curve. B) Relative to the control curve, generalization functions were lowered by the lowest dose of apomorphine (0.05 mg/kg) and elevated by the highest dose of apomorphine (0.20 mg/kg). The intermediate dose of apomorphine produced drug-appropriate responding when administered alone ( i . e . at the 0.0 mg/kg amphetamine dose), but also decreased the drug-lever responding usually e l i c i t e d at high amphetamine doses. However, these changes were not s i g n i f i c a n t . C) The high dose of haloperidol reduced drug-lever responses s i g n i f i c a n t l y and lowered the amphetamine stimulus generalization functions r e l a t i v e to the control curve. A COCAINE B. APOMORPHINE C HALOPERIDOL AMPHETAMINE DOSE (mg/kg) 56 lever responses measured during the experiments with cocaine (F[3,221 = 15.80; p < .0001), apomorphine (F[3,30] = 27.42; p < .0001) and haloperidol (F[3,27] = 20.67; p < .0001). S t a t i s t i c a l analyses of the influence of test-drug dosage on amphetamine-lever responses revealed s i g n i f i c a n t e f f e cts of cocaine (F[2,16] = 20.78; p < .0001), apomorphine (F[3,27] = 10.16; p < .0001) and haloperidol (F[2,18) = 5.55; p < .025). Post hoc analyses (Newman-Keuls, p < .05) indicated that the rats emitted s i g n i f i c a n t l y more responses on the drug-lever across the range of amphetamine doses after pretreatment with each dose of cocaine r e l a t i v e to tests with the drug vehicle (Figure 2a). In addition, the rats emitted s i g n i f i c a n t l y more drug lever responses during tests with the high dose of cocaine (5.0 mg/kg) r e l a t i v e to tests with the lower dose (2.5 mg/kg). These effects resulted in elevations of the amphetamine dose-response gradients r e l a t i v e to the curves obtained during vehicle t e s t s . After pretreatment with the lowest dose of apomorphine (0.05 mg/kg) the amount of drug-lever responding was s i g n i f i c a n t l y reduced r e l a t i v e to tests with the drug vehicle, so that the amphetamine dose-response gradient was lowered r e l a t i v e to the control curve (Figure 2b). In contrast, drug-lever responses following pretreatment with the highest dose of apomorphine (0.20 mg/kg) were s i g n i f i c a n t l y increased such that the amphetamine response gradient was elevated above the 80% response l e v e l . The 57 intermediate dose of apomorphine (0.15 mg/kg) produced trends towards a r e d u c t i o n of d r u g - l e v e r responses a t higher amphetamine doses (0.5 and 1.0 mg/kg), but t h i s dose a l s o r e s u l t e d i n s u b s t a n t i a l d r u g - l e v e r responding when i t was given alone ( i . e . , with 0.0 mg/kg amphetamine). The net r e s u l t of these opposing e f f e c t s was such t h a t the average l e v e l of drug l e v e r responding a t t h i s apomorphine dose was not s i g n i f i c a n t l y d i f f e r e n t from that obtained d u r i n g t e s t s with the apomorphine v e h i c l e . The s i g n i f i c a n t h a l o p e r i d o l e f f e c t s were due to re d u c t i o n s i n drug l e v e r responding a f t e r pretreatment with the higher dose of t h i s drug (0.125 mg/kg) r e l a t i v e to t e s t s with the lower dose (0.10 mg/kg) or v e h i c l e s o l u t i o n (Figure 2c). T h i s e f f e c t r e s u l t e d i n a lowering of the amphetamine dose-response g r a d i e n t with the high dose of h a l o p e r i d o l . There a l s o appeared to be a lowering of the response g r a d i e n t with the lower dose of h a l o p e r i d o l , however t h i s e f f e c t was not s t a t i s t i c a l l y s i g n i f i c a n t . Analyses of the drug dosage i n t e r a c t i o n s r e v e a l e d s i g n i f i c a n t d i f f e r e n c e s i n the e f f e c t s of apomorphine treatment a t the d i f f e r e n t amphetamine doses (F[9,73] = 4.77; p < .0001). Post hoc analyses i n d i c a t e d t h a t these d i f f e r e n c e s were due i n pa r t t o : a) the i n c r e a s e of drug-l e v e r responses observed a f t e r the combined i n j e c t i o n of 0.20 mg/kg apomorphine with s a l i n e r e l a t i v e to the combination of the apomorphine v e h i c l e with s a l i n e ; and b) the r e d u c t i o n of d r u g - l e v e r responding a f t e r the combined 58 treatment of 0.05 mg/kg apomorphine and 0.25 mg/kg amphetamine r e l a t i v e to tests with t h i s dose of amphetamine injected with either 0.20 mg/kg apomorphine or the apomorphine vehicle. In contrast to these apomorphine e f f e c t s , there were no s i g n i f i c a n t drug dosage interactions during tests with cocaine (F[6,44] = 2.0; p > .05) or haloperidol (F[6,48] = 2.28; p > .05). Discriminated response performance was affected to varying degrees by cocaine, apomorphine, and haloperidol. Analyses of the influences of test-drug dosages on the percentage of post-reinforcement responses emitted on the selected lever revealed s i g n i f i c a n t effects of apomorphine (F[3,30] = 5.875; p < .005), but not cocaine (F[2,16] = 0.80; p > .05) or haloperidol (F[2,48] = 0.84; p > .05). Post hoc analyses of the apomorphine dose factor indicated an o v e r a l l reduction in the percentage of selected-lever responses at the highest dose of apomorphine (0.20 mg/kg; Table 2) r e l a t i v e to tests with the other doses or vehicle solution. However as may be seen in Table 2, t h i s reduction in selected lever responding was r e l a t i v e l y minor. Additional s i g n i f i c a n t differences in selected-lever responses were obtained from the analyses of the cocaine X amphetamine dosage interactions (F[6,44] = 3.91; p < .005), the apomorphine X amphetamine dosage interactions (F[9,73] = 2.34; p < .025) and the main e f f e c t of amphetamine dosage obtained from tests with apomorphine (F[3,30] =3.09; p < .05). The s i g n i f i c a n t cocaine X amphetamine dose interaction 59 TABLE 2 The percentages of a l l responses occurring after the f i r s t presentation of reinforcement within a session that were emitted on the i n i t i a l l y selected lever. The data represent the means for a l l rats at each dose combination during tests with cocaine, apomorphine, and haloperidol. AMPHETAMINE DOSE (mg/kg) 0.0 0.25 0.5 1.0 COCAINE: vehicle 100 2.5 mg/kg 9 8 5.0 mg/kg 91 APOMORPHINE: vehicle 99 0.05 mg/kg 99 0.15 mg/kg 89 0.20 mg/kg 79 HALOPERIDOL: vehicle 100 0.10 mg/kg 100 0.125 mg/kg 98 98 96 99 100 100 100 100 100 100 91 97 100 97 97 100 100 97 92 86 100 100 96 98 100 98 100 100 99 100 92 60 was due to a s l i g h t reduction in the percent of responses emitted on the selected-lever when 5.0 mg/kg cocaine was injected alone ( i . e . , 0.0 mg/kg amphetamine; Table 2). The s i g n i f i c a n t effects obtained during the apomorphine tests were due to s l i g h t reductions in the selected-lever response percentages when apomorphine was injected alone. Analyses of the t o t a l number of responses emitted during the l a s t 15 min of the generalization sessions (Table 3) revealed s i g n i f i c a n t differences between dosages of apomorphine (F[3,30] = 93.4; p < .0001) and haloperidol (F[2,18] = 14.70; p < .0005), but no effects of the cocaine dosages (F[2,16] = 1.58; p > .05). Post hoc analyses indicated that responses were decreased s i g n i f i c a n t l y after pretreatment with each dose of apomorphine and haloperidol r e l a t i v e to responses emitted during vehicle control t e s t s . In addition, the reduction in responding was greater with each successive dose, such that a l l doses were s i g n i f i c a n t l y d i f f e r e n t from each other. S i g n i f i c a n t differences in responding also were revealed by analyses of the amphetamine dosage and drug-dosage interaction effects obtained during tests with haloperidol (F[3,27] = 5.68; p < .005; and F[6,54] = 3.97; p < .005, respe c t i v e l y ) . Post hoc analyses of these effects indicated that response levels were s i g n i f i c a n t l y lower during tests in which haloperidol was injected alone or with 0.25 mg/kg amphetamine r e l a t i v e to tests either with the haloperidol TABLE 3 The t o t a l number of responses occurring during the f i n a l 15 minutes of each session. The data represent the means for a l l rats at each dose combination during tests with cocaine, apomorphine, and haloperidol. 61 AMPHETAMINE DOSE (mq/kq) 0.0 0.25 0.5 1.0 COCAINE; vehicle 2.5 mg/kg 5.0 mg/kg 1255 1275 1324 1401 1359 1323 1249 1032 1101 980 919 865 APOMORPHINE; vehicle 1056 0.05 mg/kg 657 0.15 mg/kg 303 0.20 mg/kg 221 1140 783 404 210 1143 978 605 212 1257 893 549 96 HALOPERIDOL: vehicle 1619 0.10 mg/kg 674 0.125 mg/kg 327 1608 968 760 1380 1457 934 1377 1328 798 62 vehicle or with t h i s drug injected along with the higher doses of amphetamine. The response suppressing effects of apomorphine also appeared to be greater when injected alone or with 0.25 mg/kg amphetamine (see Table 3), however analyses of the amphetamine dosage and drug-dosage interaction e f f ects did not reach s t a t i s t i c a l significance (F[3,30] = 2.54; p = .07; and F[9,90] = 1.90; p = .06, resp e c t i v e l y ) . Discussion In the present experiment rats were trained to discriminate 1.0 mg/kg amphetamine from saline and then tested with a range of amphetamine doses after pretreatment with either cocaine, apomorphine, haloperidol or the respective drug vehicles. The results of the vehicle control tests indicated that responses on the drug-appropriate lever increased as a d i r e c t function of increasing amphetamine dose. To the extent that the drug lever responses were determined by the discriminative stimulus properties of amphetamine, the dose-response gradients l i k e l y r e f l e c t e d varying degrees of stimulus generalization between the tra i n i n g and tests doses of the drug. The results of tests with cocaine, apomorphine, and haloperidol confirmed that these drugs could interact with the stimulus properties of amphetamine. Cocaine resulted in an overal l increase in drug lever responding r e l a t i v e to vehicle control tests and an elevation of the amphetamine stimulus generalization function. Thus, i t appears that 63 cocaine produced stimuli capable of summating with the stimulus properties of amphetamine. In contrast, haloperidol appeared to interfere with the amphetamine stimuli as indicated by the overall decrease in drug lever responses and lowering of the stimulus generalization functions. The effe c t s of apomorphine depended on the dose employed. The low dose of apomorphine (0.05 mg/kg) appeared to interfere with the cueing effects of amphetamine as the generalization function was lowered r e l a t i v e to the vehicle control curve. The high dose (0.2 mg/kg) of apomorphine appeared to generalize with the stimulus properties of amphetamine as the rats responded consistently on the drug lever irrespective of the amphetamine dose. The intermediate dose (0.15 mg/kg) of apomorphine produced both non-significant decreases in drug-lever responding e l i c i t e d by the higher doses (0.5 and 1.0 mg/kg) of amphetamine, and also a substantial amount of drug lever responding when administered alone ( i . e . , with 0.0 mg/kg amphetamine). This l a t t e r pattern of effects could be interpreted as a disruption of stimulus control. However, given the results obtained with the high and low doses of apomorphine i t is reasonable to conclude that the drug effects r e f l e c t e d the capacity for the intermediate dose to produce both p a r t i a l generalization and p a r t i a l interference with the stimulus properties of amphetamine. These effects of cocaine, apomorphine, and haloperidol on the amphetamine stimulus generalization functions are 64 consistent with the known actions of these drugs on dopaminergic neurotransmission. Cocaine enhances dopamine neurotransmission by blocking the reuptake and subsequent degradation of t h i s neurotransmitter (Ritz et a l . , 1987). High doses of apomorphine mimic the effects of dopamine at postsynaptic receptor s i t e s (Anden et a l . , 1967) whereas low doses of t h i s drug decrease dopamine neurotransmission by acting at presynaptic autoreceptors to i n h i b i t impulse dependent release (Gonon & Buda, 1985; Grace & Bunney, 1985; Lane & Blaha, 1986; Zetterstrom & Ungerstedt, 1984). Haloperidol antagonizes dopamine neurotransmission by blocking postsynaptic dopamine receptors (Anden et a l . , 1970). In view of the p a r a l l e l s between the effects of these drugs on dopamine neurotransmission and on amphetamine stimulus generalization functions, i t i s reasonable to conclude that the changes in the generalization functions were related to the dopaminergic actions of the drugs. S p e c i f i c a l l y , the drugs may have modified the cueing e f f i c a c y of amphetamine by acting on dopaminergic substrates that mediate the stimulus properties of t h i s drug. The changes in the amphetamine stimulus generalization functions also were consistent with the known hedonic actions of cocaine, apomorphine, and haloperidol. Cocaine and apomorphine (at doses a f f e c t i n g post-synaptic receptors) can produce rewarding effects in laboratory animals, whereas haloperidol has been shown to attenuate the rewarding effects of various drugs, including amphetamine (for reviews 65 see Carr et a l . , in press; Yokel, 1988). Accordingly, the elevations of the generalization functions by cocaine and high doses of apomorphine may have ref l e c t e d the summation of the hedonic properties of these drugs with those of amphetamine, and the lowering of the functions by haloperidol may have been due to the capacity for t h i s drug to interfere with the hedonic actions of amphetamine. This interpretation would not necessarily supersede an explanation of the results in terms of the dopaminergic actions of the test drugs. In fact, the hedonic actions of cocaine, apomorphine and haloperidol may r e s u l t from their influences on mesocortical dopamine systems (Carr et a l . , in press; Ritz et a l . , 1987; Roberts & Vickers, 1984; 1987; Roberts, Corcoran, & Fibiger, 1977; Zito, Vickers, & Roberts, 1985). Thus, the separate interpretations of the present results in terms of the dopaminergic or the hedonic actions of the test drugs might simply represent d i f f e r e n t functional and neurochemical descriptions of the same neuropharmacological events. In the present experiment, the effects of apomorphine and haloperidol on amphetamine stimulus generalization functions were measured even at doses that strongly inhibited lever-pressing for food, and at a dose of apomorphine (0.2 mg/kg) that s l i g h t l y reduced the consistency with which rats responded on the selected lever. Thus, i t appeared that performance factors did not interfere with the a b i l i t y of these drugs to s e l e c t i v e l y a l t e r the 66 dependent measure of the amphetamine s t i m u l i . This resistence of the discrimination measure to disruptive effects of drugs contrasts with other behavioral paradigms that have been employed to assess the dopaminergic and/or hedonic actions of pharmacological compounds. In many of these l a t t e r paradigms the performance disrupting effects of high drug dosages often confound the dependent variable being measured, and selective effects may only be obtained within a limited dose range (Fibiger, 1978; Wise, 1982). In contrast, the effects of a wide range of drug dosages on amphetamine stimulus generalization functions may be measured without appreciable interference of performance factors with the dependent variable. 67 EXPERIMENT 2  Effects of Nicotine on Amphetamine Stimulus  Generalization Functions Nicotine i s a general CNS stimulant that i s s e l f -administered by both laboratory animals and humans (Henningfield & Goldberg, 1983). Recent studies have indicated that t h i s drug increases both the f i r i n g rate of mesocortical dopamine neurons and the e x t r a c e l l u l a r concentrations of dopamine in the nucleus accumbens (Imperato et a l . , 1986; Mereu et a l . , 1987). Thus, i t appears that nicotine i s capable of enhancing dopamine neurotransmission at synapses within the nucleus accumbens by increasing impulse flow in the mesoaccumbens neurons. This pharmacological action may be the basis for nicotine's rewarding e f f e c t s , as 6-OHDA lesions of the nucleus accumbens block the self-administration of th i s drug by rats (Singer, Wallace, & H a l l , 1982). If the pharmacological enhancement of dopamine neurotransmission or the production of rewarding effects represent s u f f i c i e n t conditions for stimulus generalization with amphetamine, then such generalization should be obtained after nicotine i n j e c t i o n s . However, previous attempts to demonstrate such generalization have reported inconsistent r e s u l t s . For example, Schecter and Rosecrans (1973) reported that rats trained to discriminate 4.0 mg/kg amphetamine from saline emitted saline-appropriate responses when tested for generalization to nicotine. In a separate 68 study (Ho & Huang, 1975), rats trained with a lower dose of amphetamine (0.8 mg/kg) appeared to show p a r t i a l generalization to a nicotine stimulus. However, as these rats emitted only 41% of their responses on the drug lever, i t was not possible to establish whether these results r e f l e c t e d true generalization or a general impairment in discriminative performance. Recently, R e a v i l l and Stolerman (1988) confirmed that nicotine can produce a limited amount of amphetamine-appropriate responding when given alone, and can increase the amount of drug-appropriate responding e l i c i t e d by low amphetamine doses. This r e s u l t suggested that nicotine indeed may have produced a stimulus that could induce both p a r t i a l generalization and summation with the stimulus properties of amphetamine. The present experiment attempted to confirm that nicotine can produce an amphetamine-like stimulus which would elevate stimulus generalization functions. This r e s u l t would v e r i f y both predictions under investigation in the present th e s i s : 1) that stimulus summation w i l l occur between amphetamine and other drugs that can enhance mesoaccumbens dopamine neurotransmission; and 2) that stimulus summation w i l l occur between amphetamine and drugs that can produce rewarding e f f e c t s . The effects of nicotine on locomotor a c t i v i t y also were assessed. As indicated in the introduction (p. 38), changes in locomotor a c t i v i t y levels often r e f l e c t p a r a l l e l changes in mesoaccumbens dopamine neurotransmission. In fact, the 69 locomotor s t i m u l a n t p r o p e r t i e s of n i c o t i n e may be blocked by 6-OHDA l e s i o n s of the nucleus accumbens ( K s i r & K l i n e , 1987). Within the present c o n t e x t , the locomotor a c t i v i t y t e s t s provided independent b e h a v i o r a l c o n f i r m a t i o n of n i c o t i n e ' s f a c i l i t a t o r y e f f e c t s on dopamine neurotransmiss i o n . Methods Twelve e x p e r i m e n t a l l y naive male hooded r a t s were used as s u b j e c t s f o r t h i s experiment. These r a t s were t r a i n e d to d i s c r i m i n a t e 1.0 mg/kg amphetamine from s a l i n e u s i n g the procedures o u t l i n e d i n the gen e r a l methods s e c t i o n . Rats t h a t a c q u i r e d the d i s c r i m i n a t i o n task subsequently were given g e n e r a l i z a t i o n t e s t s with the same doses of amphetamine t h a t were employed i n Experiment 1 (0.0, 0.25, 0.50 and 1.0 mg/kg) a f t e r r e c e i v i n g i n j e c t i o n s of 1 - n i c o t i n e b i t a r t r a t e (0.2 and 0.4 mg/kg) or s a l i n e . The i n j e c t i o n s of n i c o t i n e or i t s v e h i c l e were given SC, 20 min before the s t a r t of each s e s s i o n . Upon completion of the d i s c r i m i n a t i o n experiment, a l l twelve r a t s were t e s t e d f o r the e f f e c t s of n i c o t i n e on locomotor a c t i v i t y . Each r a t was given three 30 min a c t i v i t y t e s t s 20 min a f t e r SC i n j e c t i o n s of n i c o t i n e (0.2 and 0.4 mg/kg) or s a l i n e . R e s u l t s Eleven of the r a t s used f o r t h i s experiment learned to d i s c r i m i n a t e amphetamine from s a l i n e w i t h i n 30 t r i a l s . During subsequent g e n e r a l i z a t i o n t e s t s , pre-reinforcement 70 responses on the drug-appropriate lever were found to increase as a d i r e c t function of the increasing amphetamine dose. Analysis of the ef f e c t of amphetamine dosage on drug-lever responding (collapsed across the nicotine dosage factor) confirmed the s t a t i s t i c a l significance of t h i s trend (F[3,30] = 20.33; p < .0001). As may be seen in Figure 3, the amphetamine dose-response gradients obtained from generalization tests with nicotine were elevated r e l a t i v e to the vehicle control curve. Analyses of the nicotine dosage e f f e c t on discriminated responses revealed s i g n i f i c a n t differences between the drug conditions (F[2,20] = 7.10; p < .005), which post hoc analyses indictated were due to s i g n i f i c a n t increases in drug-lever responding at both doses of nicotine (0.2 and 0.4 mg/kg) r e l a t i v e to tests with the nicotine vehicle. Although these increases were evident primarily at the lower doses of amphetamine (0.0 and 0.25 mg/kg), the interaction e f f e c t of the nicotine and amphetamine dosages did not reach s t a t i s t i c a l s ignificance (F[6,601 = 2.08; p = .07) . Analyses of the selected-lever response percentages (Table 4) revealed s i g n i f i c a n t effects of the nicotine dosage (F[2,20) = 4.11; p < .05) and the nicotine and amphetamine dosage interactions (F[6,59J = 2.23; p = .05), but not of the amphetamine dosage (F[3,30] = 1.46; p > .05). The nicotine dosage e f f e c t r e f l e c t e d a s i g n i f i c a n t increase in the consistency of selected lever responses at the lower 71 Figure 3: Effects of nicotine on amphetamine stimulus generalization functions. Both closes of nicotine (0.20 and 0.40 mg/kg) increased drug-lever responses s i g n i f i c a n t l y and elevated the amphetamine stimulus generalization functions r e l a t i v e to the control curve. 72 AMPHETAMINE DOSE (mg/kg) 73 TABLE 4 Percentages of responses on the i n i t i a l l y s e l e c t e d l e v e r a f t e r n i c o t i n e . AMPHETAMINE DOSE (ma/kg) 0.0 0.25 0.5 1.0 NICOTINE DOSE: v e h i c l e 98 0.2 mg/kg 99 0.4 mg/kg 91 88 99 97 96 100 100 94 99 100 74 dose of nicotine (0.2 mg/kg) r e l a t i v e to tests with s a l i n e . Post hoc analyses of the drug-dosage interactions revealed that selected-lever response percentages were higher when nicotine was injected in combination with certain doses of amphetamine r e l a t i v e to tests with 0.25 mg/kg amphetamine plus the nicotine vehicle. These l a t t e r interaction effects might best be attributed to random error rather than a s p e c i f i c action of nicotine on response accuracy, as the effects do not follow any l o g i c a l order. Analyses of the number of responses during the l a s t 15 min of the generalization sessions revealed s i g n i f i c a n t effects of nicotine dosage (F[2,20] = 4.63; p < .025), amphetamine dosage (F[3,30] = 3.02; p < .05) and the interaction of nicotine and amphetamine dosages (Ft 6,60] = 2.46; p < .05). Post hoc analyses of the nicotine dosage ef f e c t indicated that the rats emitted more responses following injections of the lower dose of nicotine (0.2 mg/kg) than during tests with either the high dose of nicotine (0.4 mg/kg) or the nicotine vehicle (see Table 5). However, post hoc tests of the interaction effects indicated that t h i s increase in responding was s i g n i f i c a n t only when 0.2 mg/kg nicotine was injected alone ( i . e . , with 0.0 mg/kg amphetamine). This increase after i n j e c t i o n of 0.2 mg/kg nicotine alone also may account for the s i g n i f i c a n t amphetamine dosage e f f e c t , which was due to increases in responding during tests with 0.0 mg/kg amphetamine r e l a t i v e to tests with 1.0 mg/kg amphetamine. TABLE 5 Total number of responses after nicotine. AMPHETAMINE DOSE (mg/kg)  0.0 0.25 0.5 1.0 NICOTINE DOSE: vehicle 1199 1079 1200 1101 0.2 mg/kg 1642 1223 1255 1171 0.4 mg/kg 1357 1320 1029 1028 76 The effects of nicotine on locomotor a c t i v i t y are shown in Figure 4. Analyses of the a c t i v i t y scores obtained from these tests revealed s i g n i f i c a n t effects of the nicotine dosage (F[2,22] = 10.86; p < .001), the time period (F[5,55] = 65.77; p < .0001) and the interaction of nicotine dose with the time period (F[10,110] = 3.33; p < .001). Post hoc tests indicated that the time period e f f e c t was due to a greater amount of a c t i v i t y e a r l i e r in the session r e l a t i v e to the later time periods, regardless of the drug condition. Analyses of the nicotine dosage effects revealed that a c t i v i t y scores were increased s i g n i f i c a n t l y after injections of both doses of nicotine (0.2 and 0.4 mg/kg) re l a t i v e to tests with the drug vehicle. The s i g n i f i c a n t interaction between nicotine dosage and time period effects r e f l e c t e d differences in the time course of the separate nicotine doses. The lower dose of nicotine exerted i t s strongest stimulant actions e a r l i e r in the sessions, whereas the higher dose produced greater effects during the later periods in the session. Importantly, the stimulant actions of both doses were present 15 to 20 min into the session (35 to 40 min after i n j e c t i o n ) ; the time in t e r v a l when the rats would usually be making th e i r f i r s t discriminated responses during generalization tests with nicotine. Discussion In the present experiment rats trained to discriminate 1.0 mg/kg amphetamine from saline were given tests with a range of amphetamine doses after receiving injections of 77 Figure 4: Effects of nicotine on locomotor a c t i v i t y . The data represent the t o t a l a c t i v i t y counts measured during each 5-minute block over a 30-minute test period following injections of vehicle, 0.20 or 0.40 mg/kg nicotine. Both doses of nicotine s i g n i f i c a n t l y elevated the a c t i v i t y scores r e l a t i v e to tests with the vehicle. 78 79 either nicotine or i t s vehicle solution. During vehicle control tests the amount of responding on the drug-appropriate lever was found to increase as a d i r e c t function of the increasing amphetamine dose, r e f l e c t i n g an orderly stimulus generalization function. Pretreatments with nicotine resulted in functions that were elevated r e l a t i v e to the control curve. This finding suggests that the stimulus properties of nicotine may have summated with those of amphetamine and augmented the cueing e f f i c a c y of th i s psychomotor stimulant drug. The summation between nicotine and amphetamine stimuli may r e f l e c t a common dopaminergic substrate for the stimulus properties of these drugs. Indeed, the increases in locomotor a c t i v i t y in the present experiment suggested that the doses of nicotine employed were capable of augmenting dopaminergic function within the nucleus accumbens. Furthermore, previous studies have suggested a role for dopamine in mediating some of nicotine's stimulus properties. Rats trained to discriminate nicotine from saline were found to generalize p a r t i a l l y to both amphetamine and to the d i r e c t Dl dopamine receptor agonist SKF 38393 (Chance, Murfin, Krynock, & Rosecrans, 1977; R e a v i l l & Stolerman, 1988). In contrast, the cueing effects of nicotine were attenuated by the dopaminergic antagonists haloperidol, pimozide and Sch 23390 (Reavill & Stolerman, 1988). Although the s p e c i f i c dopaminergic pathways mediating the stimulus properties of nicotine have not been 80 determined, i t i s possible that they may include the same dopaminergic processes that mediate the stimulus properties of amphetamine. The results of the present experiment are consistent with the prediction that stimulus summation w i l l occur between amphetamine and other drugs with f a c i l i t a t o r y actions on dopaminergic neurotransmission. In addition, the results v e r i f y the prediction that summation may occur with drugs that are capable of producing rewarding e f f e c t s . Nicotine i s r e a d i l y self-administered by both humans and laboratory animals (Henningfield & Goldberg, 1983). Injections of t h i s drug also may produce place preference conditioning (Fudala & Iwamoto, 1986; Fudala, Teoh, & Iwamoto, 1985) and can increase response-rates for LH and VTA brain-stimulation rewards (Clarke & Kumar, 1983a; Druhan, Fibiger, & P h i l l i p s , in press; Newman, 1972; Olds & Domino, 1969; Schaefer & Michael, 1987). Recent evidence has indicated that these rewarding effects of nicotine may involve actions on mesoaccumbens dopamine neurons, as 6-OHDA lesions of the nucleus accumbens can block the s e l f -administration of t h i s drug (Singer et a l . , 1982). Thus, the successful prediction of nicotine's effects on amphetamine stimulus generalization functions both from i t s rewarding effects and from i t s dopaminergic actions may r e f l e c t a common neuronal basis for these two c h a r a c t e r i s t i c s of nicotine. 81 EXPERIMENT 3  Effects of Morphine on Amphetamine Stimulus  Generalization Functions Morphine is an opiate compound that i s r e a d i l y s e l f -administered by laboratory animals and humans (Jasinski, 1977; Martin & J a s i n s k i , 1977). Like nicotine, morphine can increase the f i r i n g rate of mesoaccumbens dopamine neurons and e x t r a c e l l u l a r concentrations of dopamine within the nucleus accumbens (Di Chiara & Imperato, 1986; Gysling & Wang, 1983). These actions of morphine on dopamine neurotransmission may be p a r t l y responsible for the hedonic effects of t h i s drug. Rats w i l l self-administer morphine d i r e c t l y into the VTA and develop preferences for environments that have been paired with opiate injections into t h i s region (Bozarth, 1987; Bozarth & Wise, 1981; P h i l l i p s & LePiane, 1980, 1982; P h i l l i p s et a l . , 1983). In view of the abovementioned dopaminergic actions and rewarding effects of morphine, th i s drug might be expected to augment the stimulus properties of amphetamine. Morphine previously has been shown to potentiate the threshold lowering e f f e c t s of amphetamine on l a t e r a l hypothalamic ICSS (Hubner, Bain, & Kornetsky, 1987) and to enhance the l e v e l of euphoria produced by amphetamine in human subjects (Jasinski & Preston, 1986). Nevertheless, attempts to demonstrate generalization between morphine and amphetamine have yeilded c o n f l i c t i n g or ambiguous r e s u l t s . In one study (Jarbe, 1982), pigeons trained to discriminate amphetamine 82 (1.6 mg/kg) from saline f a i l e d to generalize to morphine (1.5 and 3.0 mg/kg). In contrast, morphine (5.0 mg/kg) produced 50% drug-lever responding when administered to rats trained to discriminate 0.75 mg/kg amphetamine from saline (Hernandez, Holohean, & Appel, 1978). However, morphine also decreased drug-lever responding to a l e v e l of 63% when co-injected with the t r a i n i n g dose of amphetamine. It i s unclear whether these results r e f l e c t e d generalization between the stimulus properties of morphine and amphetamine or merely a disruption of discriminative performance. The present experiment assessed the a b i l i t y of morphine to summate with amphetamine s t i m u l i . Rats trained to discriminate 1.0 mg/kg amphetamine from saline were tested for stimulus generalization to lower doses of amphetamine following pretreatment with either saline or low doses of morphine (1.0 and 2.0 mg/kg). If morphine could produce amphetamine-like stimulus properties, then t h i s drug might augment the cueing effects of amphetamine and elevate the stimulus generalization functions r e l a t i v e to a curve obtained in the absence of the drug. In contrast, any disruptive actions of morphine might be re f l e c t e d as concomitant increases and decreases in drug-lever responding at low and high doses of amphetamine, respectively. As in the previous experiment, the stimulant actions of morphine were assessed independently during locomotor a c t i v i t y tests conducted after completion of a l l generalization sessions. 83 Methods Twelve experimentally naive male hooded rats were used as subjects for t h i s experiment. These rats were trained to discriminate 1.0 mg/kg amphetamine from saline using the procedures outlined in the general methods section. A l l of the rats acquired the discrimination task and were given generalization tests with the standard range of amphetamine doses injected 60 min after pretreatment with morphine sulphate (1.0 and 2.0 mg/kg) or sa l i n e . Subsequently, these twelve rats were given three 30 min a c t i v i t y tests 60 min after SC injections of morphine (1.0 and 2.0 mg/kg) or sal i n e . Results A l l 12 rats u t i l i z e d in thi s experiment learned the discrimination task within 20 to 30 t r i a l s . During subsequent generalization t e s t s , these rats increased their responses on the drug lever as a function of increasing amphetamine doses. Analysis of the eff e c t of amphetamine dosage on drug-lever responding confirmed the s t a t i s t i c a l s i g n ificance of thi s trend (F[3,33 = 28.43; p < .0001). The e f f e c t s of morphine on amphetamine discriminated response gradients are shown in Figure 5. Analyses of the effects of morphine on amphetamine-lever responses indicated a s i g n i f i c a n t dosage e f f e c t (F[2,22] = 5.06; p < .025), which was due to decreases in drug lever-responding at the higher dose of morphine (2.0 mg/kg). These decreases resulted in a lowering of the discriminated response 84 F i g u r e 5: E f f e c t s of morphine on amphetamine stimulus g e n e r a l i z a t i o n f u n c t i o n s . Responses on the drug l e v e r were s i g n i f i c a n t l y reduced a t the high dose of morphine (2.0 mg/kg), causing the stimulus g e n e r a l i z a t i o n f u n c t i o n to be lowered r e l a t i v e to the curves obtained f o l l o w i n g pretreatment with e i t h e r the drug v e h i c l e or the low dose of morphine. 85 86 gradient r e l a t i v e to the curves obtained after vehicle or the low dose of morphine (1.0 mg/kg). Analysis of the drug-dosage interactions did not reveal s i g n i f i c a n t differences in the effects of morphine at the separate amphetamine dosages (F[6,61] = 1.28; p > .05). The ANOVA's performed on the percentage of selected lever responses (Table 6) did not reveal s i g n i f i c a n t effects of either morphine dosage (F[2,22] = 1.75; p > .05) or amphetamine dosage (F[3,33J =0.26; p > .05). However, there was a s i g n i f i c a n t interaction e f f e c t (F[6,61] = 2.44; p < .05). Post hoc analysis revealed that t h i s interaction e f f e c t was due to s l i g h t l y more consistant selected-lever responding after injections of 1.0 mg/kg amphetamine plus 1.0 mg/kg morphine r e l a t i v e to tests with 0.25 mg/kg amphetamine alone. These differences l i k e l y r e f l e c t random var i a t i o n rather than a s p e c i f i c f a c i l i t a t o r y action of morphine. Morphine produced s i g n i f i c a n t effects on the amount of responding exhibited during the l a s t 15 min of the generalization sessions (F[2,22] = 19.80; p < .0001; Table 7). Post hoc analyses indicated that responding was s i g n i f i c a n t l y lower at both doses of morphine r e l a t i v e to tests with the morphine vehicle, and that the higher dose of morphine (2.0 mg/kg) produced s i g n i f i c a n t l y greater decrements than the lower dose (1.0 mg/kg). S i g n i f i c a n t e f f e c t s of the amphetamine dosage also were observed (F[3,33] = 14.05; p < .0001), which were due to the lower 87 TABLE 6 Percentages of responses on the i n i t i a l l y selected lever after morphine. AMPHETAMINE DOSE (mo/kg) 0.0 0.25 0.5 1.0 MORPHINE DOSE: vehicle 1.0 mg/kg 2.0 mg/kg 92 81 91 95 91 88 87 85 87 96 90 82 88 TABLE 7 Total number of responses after morphine. AMPHETAMINE DOSE (mg/kg) 0.0 0.25 0.5 1.0 MORPHINE DOSE: vehicle 1370 1351 1362 821 1.0 mg/kg 1180 1204 1157 659 2.0 mg/kg 997 982 983 356 89 levels of responding at the high dose of amphetamine (1.0 mg/kg) r e l a t i v e to tests with the lower doses of t h i s drug (0.0, 0.25 and 0.5 mg/kg). Analyses of the drug-dosage interactions did not reveal s i g n i f i c a n t differences in the effects of morphine at the d i f f e r e n t amphetamine doses (F[6,66] = 0.14; p > .05). As may be seen in Figure 6, morphine increased a c t i v i t y levels during locomotion t e s t s . An ANOVA confirmed the significance of the morphine dosage ef f e c t (F[2,221 = 19.22; p < .0001). Post hoc tests indicated that a c t i v i t y scores were elevated s i g n i f i c a n t l y after both doses of morphine r e l a t i v e to tests with the morphine vehicle, and that a c t i v i t y levels were greater at the high dose (2.0 mg/kg) than at the lower dose (1.0 mg/kg) of t h i s drug. Si g n i f i c a n t interaction effects also were revealed (F[10,110] = 2.28; p < .025). Post hoc analyses indicated that the magnitude of the a c t i v i t y increases varied throughout the session, being non-significant during the f i r s t 5 min and highly s i g n i f i c a n t during later periods in the sessions. Analysis of the effects of time period on a c t i v i t y scores revealed s i g n i f i c a n t differences (F[5,55] = 76.10; p < .0001), which r e f l e c t e d the progressive decline in a c t i v i t y levels observed over the course of each session. Discussion The present experiment assessed the effects of morphine on amphetamine stimulus generalization functions in rats trained to discriminate 1.0 mg/kg amphetamine from s a l i n e . 90 Figure 6: Eff e c t s of morphine on locomotor a c t i v i t y . Both doses of morphine (1.0 and 2.0 mg/kg) increased a c t i v i t y scores s i g n i f i c a n t l y r e l a t i v e to those obtained following vehicle i n j e c t i o n s . 91 92 The doses of morphine employed in t h i s experiment previously have been shown to increase neuronal f i r i n g rates and ext r a c e l l u l a r concentrations of dopamine (Di Chiara & Imperato, 1986; Glysing & Wang, 1983), support place preference conditioning (Mackey & Van der Kooy, 1985; Mucha, Van der Kooy, 0'Shaughnessy, & Bucenieks, 1982) and enhance sel f - s t i m u l a t i o n behavior (Hubner et a l . , 1987; Kornetsky & Esposito, 1979). These individual doses also have served as rewards for intravenous self-administration responding, and rats commonly respond at a rate that maintains an average dosage l e v e l of 2.0 to 3.0 mg/kg/hour (Smith, Guerin, Co, Barr, & Lane, 1985; Smith, Shultz, Co, Goeders, & Dworkin, 1987). Nevertheless, these doses f a i l e d to augment the stimulus properties of amphetamine during generalization t e s t s . Instead, the higher dose of morphine (2.0 mg/kg) appeared to interfere with the cueing effects of amphetamine as indicated by the lowering of the generalization gradient r e l a t i v e to the curves obtained from tests with the vehicle or low dose of morphine. Although morphine also reduced operant response levels during the generalization t e s t s , the interference with the amphetamine stimuli did not appear to r e f l e c t a general loss of stimulus control over discriminated responding. Analyses of the selected-lever response percentages did not reveal differences between tests with morphine or i t s vehicle solution, indicating that the influence of the rei n f o r c i n g stimulus on discriminated responses was maintained after 93 morphine in j e c t i o n s . The effects of morphine on the amphetamine stimuli also did not appear to r e f l e c t a general depressant action of the drug, as both doses of morphine exerted strong stimulant actions during subsequent a c t i v i t y t e s t s . Rather, i t appears that the high dose of morphine may have attenuated the cueing e f f i c a c y of amphetamine by i n t e r f e r i n g with the perception of i t s stimulus properties. The absence of additive interactions between the stimulus properties of morphine and amphetamine would appear to suggest that morphine did not exert f a c i l i t a t o r y actions on neural processes that give r i s e to amphetamine s t i m u l i . However, th i s interpretation would not be consistent with the evidence that morphine can enhance mesoaccumbens dopamine neurotransmission (Di Chiara & Imperato, 1986; Gysling & Wang, 1983). A more viable explanation might be that the detection of such f a c i l i t a t o r y interactions may have been obscured by stronger, antagonistic actions of morphine on the stimulus properties of amphetamine. These l a t t e r i n h i b i t o r y effects may have involved selective actions on processes post-synaptic to the mesoaccumbens dopamine neurons. A l t e r n a t i v e l y , other mesocortical dopamine projections may play a role in mediating the amphetamine st i m u l i , and morphine's i n h i b i t o r y effects may have resulted from actions on processes that are post-synapt ic to these neurons. F i n a l l y , the i n h i b i t i o n may have resulted from a non-specific masking of the amphetamine cues by non-dopaminergic stimulus properties of morphine. 94 EXPERIMENT 4  Effects of Midazolam on Amphetamine Stimulus  Generalization Functions Midazolam is a benzodiazepine compound that is r e a d i l y self-administered by laboratory animals (Ator & G r i f f i t h s , 1987; Falk & Tang, 1985; G r i f f i t h s , Lukas, Bradford, Brady, & Sn e l l , 1981; Szostak et a l . , 1987). However, unlike the compounds employed in Experiments 1 through 3, midazolam has been reported to reduce both the spontaneous f i r i n g rate of mesocortical dopamine neurons (cf. Haefley et a l . , 1981) and ex t r a c e l l u l a r dopamine concentrations in the nucleus accumbens of unanesthetized rats (Finlay et a l . , 1987). These studies suggest that midazolam may i n h i b i t mesoaccumbens dopamine neurotransmission. The abovementioned hedonic and dopaminergic actions of midazolam offer c o n f l i c t i n g predictions with respect to the drug's interactions with the stimulus properties of amphetamine. If such interactions were determined by the rewarding e f f i c a c y of drugs, then the cueing effects of midazolam and amphetamine might summate and elevate the stimulus generalization functions r e l a t i v e to a control curve. In contrast, i f interactions with amphetamine stimuli were related to drug actions on dopamine neurotransmission, then midazolam might reduce the cueing effects of amphetamine and lower the generalization functions. The present experiment assessed the r e l a t i v e importance of the rewarding effects or the dopaminergic actions of midazolam 95 as p r e d i c t o r s of g e n e r a l i z a t i o n with amphetamine, by determining the i n f l u e n c e of t h i s drug on amphetamine stimulus g e n e r a l i z a t i o n f u n c t i o n s . As i n the previous two experiments, the doses employed f o r the d i s c r i m i n a t i o n t e s t s a l s o were administered p r i o r to locomotor a c t i v i t y t e s t s to o b t a i n an independent assessment of the p o s s i b l e dopaminergic a c t i o n s of midazolam. Methods Twelve e x p e r i m e n t a l l y naive male hooded r a t s were used as s u b j e c t s f o r t h i s experiment. These r a t s were t r a i n e d to d i s c r i m i n a t e 1.0 mg/kg amphetamine from s a l i n e u s i n g the procedures o u t l i n e d i n the gen e r a l methods s e c t i o n . Ten of these r a t s a c q u i r e d the d i s c r i m i n a t i o n and were given f u r t h e r t e s t s e s s i o n s with the usual doses of amphetamine, 5 min a f t e r SC i n j e c t i o n s of midazolam maleate (0.1 and 0.2 mg/kg) or d i s t i l l e d water. A l l twelve r a t s were subsequently given three 30 min locomotor a c t i v i t y t e s t s 5 min a f t e r SC i n j e c t i o n s of midazolam (0.2 and 0.4 mg/kg) or d i s t i l l e d water. R e s u l t s Ten of the 12 r a t s employed f o r t h i s experiment learned the d i s c r i m i n a t i o n task w i t h i n 20 to 30 t r i a l s . During subsequent g e n e r a l i z a t i o n t e s t s , these r a t s emitted more responses on the d r u g - l e v e r a t high doses of amphetamine r e l a t i v e to t e s t s with lower doses. A n a l y s i s of the amphetamine dosage e f f e c t f o r the experiment confirmed the s i g n i f i c a n c e of t h i s trend (F[3,27] = 10.15; p < .0001). 96 The results of the generalization tests with midazolam are shown in Figure 7. ANOVA's performed on the percentage of drug-lever responses during these tests revealed a s i g n i f i c a n t e f f e c t of midazolam dosage (F[2,18] = 5.76; p < .025), but no s i g n i f i c a n t interaction between the effects of midazolam and amphetamine dosages (Ft 6,54] = 0.78; p > .05). Post hoc tests indicated that the midazolam dosage ef f e c t was due to s i g n i f i c a n t reductions in drug-lever responding at both doses of t h i s drug (0.1 and 0.2 mg/kg) r e l a t i v e to tests with s a l i n e . These decreases resulted in the amphetamine dose-response gradients being lowered r e l a t i v e to curves obtained from tests with the midazolam vehicle. The downward s h i f t s of the amphetamine discriminated response gradients were not associated with concomitant changes in either the consistency of selected-lever responses (Table 8) or the number of responses emitted during tests with midazolam (Table 9). Analyses of the percentages of responses emitted on the selected lever did not reveal any s i g n i f i c a n t e f f ects related to the midazolam dosage (F[2,18] = 0.14; p > .05), the amphetamine dosage (F[3,27] = 1.39; p > .05) or the interaction of the dosage variables (F[6,52] = 1.84; p > .05). S i m i l a r l y , analyses of the response levels also did not reveal s i g n i f i c a n t e f f ects of the midazolam dosage (F[2,18) = 0.06; p > .05), the amphetamine dosage (Ft 3,27] = 0.76; p > .05) or the interaction of these variables (F[6,54] = 1.17; p > .05). 97 Figure 7: E f f e c t s of midazolam on amphetamine stimulus generalization functions. Drug lever responses were reduced s i g n i f i c a n t l y by both doses of midazolam (0.10 and 0.20 mg/kg) causing the amphetamine stimulus generalization functions to be lowered r e l a t i v e to the vehicle control curve. 98 0.0 0.25 0.50 AMPHETAMINE DOSE (mg/kg) 10 99 TABLE 8 Percentages of responses on the i n i t i a l l y selected lever a f t e r midazolam. AMPHETAMINE DOSE (mq/kq) 0.0 0.25 0.5 1.0 MIDAZOLAM DOSE: vehicle 98 92 99 99 0.1 mg/kg 100 96 98 96 0.2 mg/kg 98 98 97 97 100 TABLE 9 Total number of responses after midazolam. AMPHETAMINE DOSE (mg/kg) 0.0 0.25 0.5 1.0 MIDAZOLAM DOSE: vehicle 1926 1861 1633 1639 0.1 mg/kg 1890 1799 1669 1715 0.2 mg/kg 1832 1798 1879 1475 101 As may be seen in Figure 8, midazolam reduced a c t i v i t y scores obtained during subsequent locomotion t e s t s . Analyses of the midazolam dosage e f f e c t confirmed the significance of these reductions (F[10,110] = 6.13; p < .01), and post hoc tests indicated that both doses of the drug (0.1 and 0.2 mg/kg) were e f f e c t i v e in decreasing a c t i v i t y r e l a t i v e to scores obtained after vehicle inj e c t i o n s . As in previous experiments, the analysis of the time-period e f f e c t revealed s i g n i f i c a n t differences in a c t i v i t y levels over the course of the session (F[5,55] = 38.14; p < .0001), with scores being highest at the s t a r t of each t e s t . Analysis of the interaction of the midazolam dosage and time-period variables did not reveal s i g n i f i c a n t differences in the effects of midazolam as a function of time (F[10,55] = 1.20; p > .05) . Discussion In the present experiment, midazolam was found to attenuate the cueing effects of amphetamine so that stimulus generalization functions obtained from tests with t h i s drug were lower than the vehicle control curve. This res u l t was contrary to the effects predicted from the rewarding effects of midazolam. If interactions with amphetamine stimuli were determined by the hedonic actions of drugs, then the stimulus properties of midazolam should have summated with those of amphetamine and elevated the stimulus generalization functions. The absence of an elevation in the present experiment suggested that the capacity to produce 102 F i g u r e 8: E f f e c t s of midazolam on locomotor a c t i v i t y . Both doses of midazolam (0.10 and 0.20 mg/kg) s i g n i f i c a n t l y reduced locomotor a c t i v i t y counts r e l a t i v e to t e s t s with v e h i c l e i n j e c t i o n s . 103 500 - i • Vehicle 450 ° 0 1 mg/kg • 0.2 mg/kg 400 350 H £ 300 H 250 200 l I i I I 10 15 20 25 30 TIME (minutes) 104 rewarding effects was not a s u f f i c i e n t condition for inducing stimulus summation with amphetamine. The effects of midazolam on amphetamine stimulus generalization functions corresponded with the known actions of t h i s drug on dopamine neurotransmission. Previous studies have found that midazolam can i n h i b i t the f i r i n g rate of mesocortical dopamine neurons (cf. Haefley et a l . , 1981) and reduce e x t r a c e l l u l a r dopamine concentrations in the nucleus accumbens of rats (Finlay et a l . , 1987). The e f f i c a c y of the doses employed in the present experiment for producing such effects was suggested by the findings that midazolam decreased locomotor a c t i v i t y without otherwise a f f e c t i n g discriminated response l e v e l s . However, midazolam's in h i b i t o r y e f f e c t s on locomotion and amphetamine generalization also could have resulted from actions on GABA-ergic processes post-synaptic to the dopamine projections. This p o s s i b i l i t y requires further investigation. Notwithstanding, the present results concur with the prediction that the interactions of a drug with the stimulus properties of amphetamine would r e f l e c t the dopaminergic actions of the compound. 105 EXPERIMENT 5  Ef f e c t s of Ethanol on Amphetamine Stimulus  Generalization Functions The self-administration of ethanol by humans has been well documented (see Mello, 1981; Mello & Mendelson, 1977). In laboratory animals the rewarding effects of ethanol have been less consistent, but numerous studies have demonstrated that self-administration behaviors may be established when animals are given extensive prior exposure to the drug (Numan, 1981), when schedule induction procedures are employed (concomitent fixed i n t e r v a l schedule of food presentation; Falk, Samson, & Winger, 1972) or when operant responding is f i r s t established with a highly rewarding drug (e.g., cocaine or pentobarbital; c f . Mello & Mendelson, 1977). With respect to i t s neurochemical actions, many studies have reported that ethanol does not a f f e c t levels of either dopamine or i t s metabolite, dihydroxyphenylacetic acid within the nucleus accumbens (Ellingboe and Mendelson, 1982; Hellevuo & Kiianmaa, 1988; Kalant, 1975; Nutt & Glue, 1986). However, some reports have indicated that ethanol may increase the f i r i n g rate of VTA dopamine neurons (Gessa, Muntoni, Collu, Vargiu, & Mereu, 1985) and the release of th i s neurotransmitter within the nucleus accumbens (Di Chiara & Imperato, 1986; Imperato & Di Chiara, 1986). The present experiment determined the effects of ethanol on amphetamine stimulus generalization functions. If interactions with the stimulus properties of amphetamine 106 were determined by a drug's capacity to produce rewarding e f f e c t s , then ethanol might augment amphetamine stimuli and elevate generalization functions r e l a t i v e to a vehicle control curve. In contrast, ethanol might not a f f e c t these functions i f f a c i l i t a t o r y actions on dopamine neurotransmission were a prerequisite for summation with amphetamine. As indicated above, most studies of ethanol's neurochemical effects have indicated that t h i s substance has no effects on dopamine neurotransmission. Previous neurochemical studies into ethanol's effects on dopamine neurotransmission have been inconsistent enough to merit an independent assessment of ethanol's effects on a dopaminergically mediated behavior. Therefore, the present experiment also evaluated the effects of ethanol on locomotor a c t i v i t y . As indicated above (pp. 38 & 60), changes in mesoaccumbens dopamine neurotransmission often resu l t in p a r a l l e l a l t e r a t i o n s of a c t i v i t y l e v e l s . Accordingly, locomotor a c t i v i t y may provide a behavioral index of the possible effects of ethanol on dopaminergic function. Comparisons of the results of discrimination and locomotor a c t i v i t y tests might c l a r i f y the relationship between ethanol's effects on amphetamine stimulus generalization functions and i t s actions on neurotransmission at dopaminergic synapses. Methods Twelve experimentally naive male hooded rats were used as subjects for t h i s experiment. These rats were trained to 107 discriminate 1.0 mg/kg amphetamine from saline using the procedures outlined in the general methods section. Eleven of these rats acquired the discrimination task and were given generalization tests with the usual range of amphetamine doses, 15 min after IP injections of ethanol (0.5 and 1.0 g/kg) or s a l i n e . Upon completion of th i s phase of the experiment, 11 of the o r i g i n a l 12 rats were tested for the effects of ethanol on locomotor a c t i v i t y . Each rat was given three 30 min a c t i v i t y tests 15 min after IP injections of ethanol (0.5 and 1.0 g/kg) or s a l i n e . Results Eleven of the 12 rats employed for th i s experiment learned the discrimination task within 20 to 30 t r i a l s . During subsequent generalization tests, these rats showed an orderly increase in drug-lever responding as a d i r e c t function of amphetamine dose. Analysis of the amphetamine dosage e f f e c t for the experiment confirmed the significance of t h i s trend (F[3,30] = 39.22; p < .0001). The effects of ethanol on amphetamine discriminated response functions are shown in Figure 9. Although the response gradients obtained following ethanol injections appear to be elevated r e l a t i v e to the control curve, analyses of the ethanol dosage effects on drug-lever responses revealed that the observed differences f e l l short of s t a t i s t i c a l s ignificance (F[2,20] = 2.87; p = .08). Analysis of the interactions between the ethanol and amphetamine dosage variables also did not reveal s i g n i f i c a n t 108 F i g u r e 9: E f f e c t s of ethanol on amphetamine sti m u l u s g e n e r a l i z a t i o n f u n c t i o n s . Although ethanol produced l a r g e i n c r e a s e s i n drug l e v e r responding at intermediate amphetamine doses, the e l e v a t i o n of the g e n e r a l i z a t i o n f u n c t i o n f a i l e d to reach s t a t i s t i c a l s i g n i f i c a n c e . 109 AMPHETAMINE DOSE (mg/kg) 110 differences in drug lever responses (F[6,54] = 1.38; p > .05) . Analyses of the percentage of responses emitted on the i n i t i a l l y selected lever (Table 10) revealed s i g n i f i c a n t effects of ethanol dosage (F[2,20] = 3.57; p < .05) and the interaction of ethanol and amphetamine dosage variables (F[6,51] = 3.70; p < .005), but no s i g n i f i c a n t main e f f e c t of amphetamine dosage (F[3,30] = 0.55; p > .05). Post hoc analyses of the ethanol dosage ef f e c t indicated that rats responded more consistently on the selected lever at the high dose of ethanol (1.0 g/kg) r e l a t i v e to tests with the ethanol vehicle. Analyses of the interaction e f f e c t revealed that selected-lever responses were s i g n i f i c a n t l y less consistent when 0.5 mg/kg amphetamine was injected alone r e l a t i v e to a l l other test conditions. This l a t t e r e f f e c t was l i k e l y due to random v a r i a t i o n rather than a loss of stimulus control at t h i s dose of amphetamine. Analyses of the amount of responding during the l a s t 15 min of each generalization session (Table 11) revealed s i g n i f i c a n t e f f e cts of ethanol dosage (F[2,20] = 12.81; p < .0005) and amphetamine dosage (F[3,30] = 3.48; p < .05), but no s i g n i f i c a n t interaction effects (F[6,60] = 1.46; p > .05). Post hoc analyses of the main effects indicated that responses were reduced s i g n i f i c a n t l y at the high dose of ethanol (1.0 g/kg) r e l a t i v e to tests with the low dose (0.5 g/kg) or the ethanol vehicle, and responses also were reduced at the high dose of amphetamine r e l a t i v e to tests 111 TABLE 10 Percentages of responses on the i n i t i a l l y selected lever after ethanol. AMPHETAMINE DOSE (mg/kg) 0.0 0.25 0.5 1.0 ETHANOL DOSE: vehicle 99 0.5 g/kg 99 1.0 g/kg 99 96 94 99 88 100 100 97 95 100 112 TABLE 11 Total number of responses after ethanol. AMPHETAMINE DOSE (ma/kg) 0.0 0.25 0.5 1.0 ETHANOL DOSE: vehicle 1203 1188 1282 826 0.5 g/kg 1015 1089 1208 872 1.0 g/kg 862 819 788 671 113 with 0.5 mg/kg of thi s drug. Ethanol did not a l t e r a c t i v i t y scores during subsequent locomotion tests, as indicated by that lack of s i g n i f i c a n t e f f e cts of either the ethanol dosage (Figure 10; F[2,20] = 0.47; p > .05) or the interaction of the dosage and time-period variables (F[10,100] = 0.66; p > .05). However, as in previous experiments there was a s i g n i f i c a n t e f f e c t of the time period (F[5,50] = 49.37; p < .0001), which was due to the steady decline in a c t i v i t y levels over the course of the test sessions. Discussion In the present experiment, rats were trained to discriminate 1.0 mg/kg amphetamine from saline and then given tests with a range of amphetamine doses injected in combination with either ethanol (0.5 or 1.0 g/kg) or s a l i n e . The results of these tests were somewhat ambiguous with respect to ethanol's effects on discriminated responding. Ethanol appeared to produce large increases in drug-lever responses at the 0.25 and 0.50 mg/kg amphetamine doses r e l a t i v e to tests with vehicle inj e c t i o n s . However, ethanol did not r e s u l t in drug lever responding when administered alone (ethanol plus 0.0 mg/kg amphetamine), and the overall differences between the amphetamine stimulus generalization functions obtained with ethanol or vehicle injections f a i l e d to reach s t a t i s t i c a l s i g n i f i c a n c e . The absence of amphetamine-appropriate responding when ethanol was administered alone suggests that the stimulus 114 Figure 10: Effects of ethanol on locomotor a c t i v i t y . Ethanol had no s i g n i f i c a n t effects on locomotor a c t i v i t y scores. 115 500 - i 450 H • Vehicle o 0.5 g/kg ° 1.0 g/kg 400 350 H b- 300 H 250 200 H 10 15 20 TIME (minutes) 25 30 116 properties of th i s drug did not generalize with those of amphetamine. This conclusion would be consistent with previous reports that the stimulus properties of ethanol do not involve dopaminergic components that could generalize with an amphetamine cue (Signs & Schecter, 1988). Although the apparant increases in drug-lever responding at intermediate doses may have indicated stimulus summation between the cueing properties of ethanol and amphetamine, thi s r e s u l t also could have resulted from pharmacokinetic interactions between the two drugs. Ethanol can increase blood levels of amphetamine by i n t e r f e r i n g with i t s metabolic degradation (Ellinwood, Eibergen, & Kilbey, 1976), and such an e f f e c t might have accounted for the pattern of results obtained in the present experiment. The lack of an eff e c t of ethanol on locomotor a c t i v i t y suggests that ethanol may not have altered dopamine neurotransmission in the nucleus accumbens at the doses employed here. If th i s was the case, then the absence of generalization between amphetamine and ethanol would be consistent with the predicted r e l a t i o n between such generalization and the dopaminergic actions of drugs. In contrast, the results of t h i s experiment refute the prediction that amphetamine w i l l generalize to drugs that produce rewarding e f f e c t s . Although ethanol may be s e l f -administered in humans and laboratory animals (Mello, 1981; Mello & Mendelson, 1977), t h i s rewarding capacity was not s u f f i c i e n t to produce generalization with amphetamine. 117 EXPERIMENT 6  Effects of E l e c t r i c a l Stimulation of the VTA on  Amphetamine Stimulus Generalization Functions If either the induction of positive a f f e c t or the enhancement of mesoaccumbens dopamine neurotransmission were s u f f i c i e n t conditions for stimulus summation with amphetamine to occur, then such summation might be obtained with non-pharmacological stimuli that possess these properties. For example, the stimulus properties of amphetamine might summate with those produced by e l e c t r i c a l stimulation of the VTA. Stimulation of t h i s region can serve as a potent reward for operant responding in rats (Fibiger et a l . , 1987), and recent ex-vivo chromatographic and i n -vivo electrochemical measures have suggested that dopamine may be released in the i p s i l a t e r a l nucleus accumbens of rats responding for VTA stimulation (Blaha et a l . , 1988; Fibiger et a l . , 1987). In a previous study, D'Mello (1981) f a i l e d to demonstrate stimulus generalization between amphetamine and VTA stimulation. However, recent in-vivo electrochemical experiments have indicated that the square-wave stimulation parameters employed by D'Mello may have been inappropriate for a c t i v a t i n g dopamine neurons. In these studies, dopamine was released in forebrain regions only when the stimulation involved t r a i n s of either square-wave pulses with long durations ( i . e . , > 0.5 msec; M i l l a r , Stamford, Kruk, & Wightman, 1985) or 60 Hz sine-waves (Blaha et a l . , 1988; 118 Fibiger et a l . , 1987). Accordingly, the present experiment reassessed the effects of VTA stimulation on discriminated responses to a range of amphetamine doses using sine-wave currents that could both increase the release of dopamine in the nucleus accumbens and produce rewarding e f f e c t s . If the dopaminergic or hedonic actions of th i s non-pharmacological stimulus were s u f f i c i e n t to produce amphetamine-like cueing e f f e c t s , then the VTA stimulation should summate with the stimulus properties of amphetamine and elevate generalization functions r e l a t i v e to a control curve. Methods Fifte e n male hooded rats were employed as subjects for th i s experiment. One week after electrode implantation, the rats were given fiv e 30 min ICSS sessions in which they could lever-press for 200 msec trains of 20 uA, 60 Hz sine wave stimulation on a CRF schedule. This i n i t i a l s e l f -stimulation screening ensured that the brain-stimulation was capable of supporting ICSS. Subsequently, the rats were trained to discriminate 1.0 mg/kg amphetamine from saline using the procedure described in the General Methods section. Following a c q u i s i t i o n , the rats were given generalization tests with a range of amphetamine doses (0.0, 0.125, 0.25, 0.5 and 1.0 mg/kg) either alone or in combination with intermittent VTA stimulation. At the s t a r t of the brain-stimulation t r i a l s , the rats were injected with one of the solutions and then immediately attached to 119 electrode leads and placed in the testing chambers. The intermittent delivery of e l e c t r i c a l stimulation then began three minutes later and continued throughout the test session. The stimulation was maintained at a constant in t e n s i t y within each test (15 or 20 uA), and presented every 10 sec. Each presentation consisted of four 200 msec trains of 60 Hz sine-wave stimulation delivered 200 msec apart. During control tests with amphetamine alone, the rats were attached to the electrode leads but no stimulation was delivered. The rats were reinforced (FR-32 schedule) during these tests for continuing to press the lever on which the f i r s t FR-32 response requirement was completed. Results The electrode placements for the 15 rats employed in the present experiment are shown in Figure 11. These rats acquired the discrimination task at a rate comparable to that observed for rats in the previous experiments. During subsequent generalization t e s t s , the rats were found to emit more drug-lever responses at the high amphetamine doses r e l a t i v e to when lower doses were injected. An ANOVA performed on the amphetamine dosage effects for the experiment confirmed the significance of t h i s trend (F[4,56] = 25.93; p < .0001). S t a t i s t i c a l analyses of the influence of VTA stimulation on discriminated responses to amphetamine revealed s i g n i f i c a n t e f f e cts of both the current i n t e n s i t y (F[2,28] = 5.34; p < .025) and the interaction of the current i n t e n s i t y Figure 11: Electrode placements for the 15 rats employed in Experiment 6. A l l electrodes were implanted in the l e f t hemisphere. The f i l l e d t riangles on the l e f t side of the brain represent the electrode placements in rats for which the amphetamine-appropriate lever was contr a l a t e r a l to the stimulating electrode, whereas the triangles on the right represent placements in rats for which the drug-lever was i p s i l a t e r a l to the electrode. The numbers to the right of the diagrams represent the plate numbers corresponding to the coronal sections from the brain atlas of Konig and Kli p p e l (1963). 122 and amphetamine dosage variables (Ft8 /104] = 5.84; p < .0001). Post hoc analyses of the effects of current i n t e n s i t y indicated that the rats emitted more responses on the drug lever during tests with both 15 and 20 uA r e l a t i v e to control sessions without stimulation (Figure 12a). Thus, the amphetamine dose-response functions were elevated in the presence of the VTA stimulation r e l a t i v e to the control curve. The s i g n i f i c a n t interaction r e f l e c t e d the selec t i v e increases in drug-lever responses when each stimulation i n t e n s i t y was delivered in combination with 0.0 and 0.125 mg/kg amphetamine and when 15 uA was given with 0.25 mg/kg, re l a t i v e to tests with these doses in the absence of VTA stimulation. The increased bias towards responding on the drug lever in the present experiment could have been related to a general sensorimotor asymmetry associated with u n i l a t e r a l stimulation of the VTA. The influence of such sensorimotor effects was examined by analysing separately the effects of the stimulation on discriminated responses when the drug-lever was either i p s i l a t e r a l or contra l a t e r a l to the stimulating electrode. These analyses indicated that for the i p s i l a t e r a l group (Figure 12b), there was a s i g n i f i c a n t interaction between the current intensity and amphetamine dosage variables (F[8,43] = 4.74; p < .0005) but no s i g n i f i c a n t main ef f e c t of the current i n t e n s i t y (F[2,12] = 1.43; p > .05). Post hoc tests revealed that the interaction was due to s i g n i f i c a n t increases in drug-lever responding 123 Figure 12: Effects of e l e c t r i c a l stimulation of the VTA on amphetamine stimulus generalization functions. Rats were tested for generalization to amphetamine in the absence of VTA stimulation ( 0 ) , with the delivery of VTA stimulation at a constant in t e n s i t y of 15 uA ( O ) / o r with VTA stimulation delivered at a constant intensity of 20 uA ( Q ) . A) Generalization functions averaged across a l l r a t s . B) generalization functions obtained from rats for which the drug-lever was i p s i l a t e r a l to the stimulating electrode. C) Generalization functions obtained from rats for which the drug-lever was contralateral to the stimulating electrode. S i g n i f i c a n t increases in drug-lever responding were observed under a l l three conditions, causing the generalization functions to be elevated r e l a t i v e to the control curves. 125 when each stimulation intensity was delivered in combination with 0.0 and 0.125 mg/kg amphetamine and when 15 uA was given with 0.25 mg/kg, r e l a t i v e to tests with these doses in the absence of VTA stimulation. Analyses of the data for the cont r a l a t e r a l group (Figure 12c) revealed s i g n i f i c a n t effects of both the current i n t e n s i t y (F[2,28] = 5.34; p < .025) and the interaction of current i n t e n s i t y with amphetamine dose (F[8,53] = 2.81; p < .01). Post hoc analyses indicated that the current i n t e n s i t y e f f e c t was due to a s i g n i f i c a n t increase in drug-lever responding at the 20 uA intensity r e l a t i v e to tests without VTA stimulation. The interaction e f f e c t r e f l e c t e d s i g n i f i c a n t increases in drug-lever responding when each int e n s i t y was presented in combination with saline injections (0.0 mg/kg amphetamine). These results confirmed that VTA stimulation could produce increases in drug-lever responding regardless of whether the lever was i p s i l a t e r a l or contra l a t e r a l to the stimulating electrode. Analyses of the influence of VTA stimulation on the consistency of selected-lever responses (Table 12) did not reveal any s i g n i f i c a n t effects of either the current i n t e n s i t y (F[2,28] = 0.92; p > .05), the amphetamine dosage (F[4,56] = 2.44; p > .05) or the interaction of these variables (F[8 /104] = 1.11; p > .05). In contrast, ANOVAs •i performed on the response levels during the l a s t 15 minutes of the generalization tests (Table 13) revealed s i g n i f i c a n t effects of the current i n t e n s i t y (F[2,28] = 26.31; p < 126 TABLE 12 Percentages of responses on the i n i t i a l l y selected lever during VTA stimulation. AMPHETAMINE DOSE (mq/kq) 0.0 0.125 0.25 0.5 1.0 INTENSITY LEVEL: no stimulation 99 92 96 98 99 15 uA 98 94 92 91 97 20 uA 97 97 92 97 98 127 TABLE 13 Total number of responses during tests with VTA stimulation. AMPHETAMINE DOSE (mg/kg) 0.0 0.125 0.25 0.5 1.0 INTENSITY LEVEL: no stimulation 1978 1660 1788 1978 1498 15 uA 1709 1406 1548 1318 859 20 uA 1257 1071 1235 939 643 128 .0001) and amphetamine dosage variables (F[4,56] = 4.62; p < .005), but no s i g n i f i c a n t interaction e f f e c t (F[8,108] = 1.61; p > .05). Post hoc analyses of the current i n t e n s i t y e f f e c t revealed that responses were reduced s i g n i f i c a n t l y at both stimulation i n t e n s i t i e s r e l a t i v e to tests without the stimulation, with s i g n i f i c a n t l y fewer responses being emitted at the higher i n t e n s i t y (20 uA) r e l a t i v e to tests with the lower intensity (15 uA) . Analyses of the amphetamine dosage ef f e c t indicated that responses were reduced at the high dose of amphetamine (1.0 mg/kg) r e l a t i v e to the lower doses of th i s drug. Discuss ion The present experiment examined the eff e c t s of VTA stimulation on amphetamine stimulus generalization functions in rats trained to discriminate 1.0 mg/kg amphetamine from sa l i n e . The results of t h i s experiment confirmed that amphetamine generalization functions could be elevated by e l e c t r i c a l stimulation of the VTA. Although u n i l a t e r a l stimulation of regions containing dopamine neurons can produce both co n t r a l a t e r a l c i r c l i n g behavior (Gratton & Wise, 1985) and greater responsiveness to stimuli in the contral a t e r a l sensory f i e l d (Bandler & Flynn, 1971; Beagley & Holley, 1977; Nakahara & Ikeda, 1984; Smith, 1972), such sensorimotor e f f e c t s did not appear to be responsible for the observed biasing of responses toward the drug-lever. The rats increased t h e i r responses on the drug-lever during stimulation t r i a l s regardless of whether t h i s lever was 129 i p s i l a t e r a l or contralateral to the stimulating electrode. Admittedly, the effects on amphetamine generalization functions appeared to be strongest when the drug-lever was con t r a l a t e r a l to the electrode, thus the sensorimotor effects may have exaggerated the response biases of these r a t s . However, the s i g n i f i c a n t elevation observed with the i p s i l a t e r a l group suggests that the main e f f e c t of the stimulation was to produce stimulus properties which could interact in an additive manner with the cueing effects of amphetamine. The summation observed between the stimulus properties of VTA stimulation and amphetamine in th i s experiment contrasts with e a r l i e r findings by D'Mello (1981). In t h i s previous study, VTA stimulation appeared to have only disruptive e f f ects on amphetamine discriminated response functions. These incongruent results may have been related to differences in the types of stimulation parameters employed by the two studies. The stimulation parameters employed by D'Mello consisted of trains with short-duration ( i . e . , 0.2 msec) square-wave pulses. Recent in vivo electrochemical measurements have indicated that such parameters may be i n s u f f i c i e n t to activate mesotelencephalic dopamine neurons (Millar et a l . , 1985). In the present experiment, 60 Hz sine-wave currents were used to stimulate the VTA. This type of stimulation appears to be adequate for act i v a t i n g dopamine neurons,, as indicated by the increases in dopamine release and turnover within the nucleus 130 accumbens of rats following sine-wave stimulation of the VTA (Blaha et a l . , 1988; Fibiger et a l . , 1987). Conceivably, t h i s capacity to activate mesoaccumbens dopamine neurons could.have accounted for the summation between the stimulus properties of the brain-stimulation and those of amphetamine in the present experiment. The results of the present experiment were consistent with the prediction that the stimulus properties of amphetamine might summate with those of any test stimulus with f a c i l i t a t o r y actions on dopamine neurotransmission. These findings also were consistent with predictions that summation may occur with rewarding s t i m u l i . A l l of the rats employed for th i s experiment displayed ICSS during preliminary tests, thereby confirming the rewarding e f f i c a c y of the brain-stimulation. Accordingly, the s h i f t s of the discriminated response functions observed in the present experiment could have r e f l e c t e d an additive interaction between stimuli associated with rewarding actions of amphetamine and the e l e c t r i c a l brain-stimulation. However, i t i s noteworthy that the VTA stimulation also was rewarding for many of the rats in the D'Mello (1981) study, yet the stimulation did not generalize or summate with the stimulus properties of amphetamine. Thus, amphetamine does not appear to generalize to a l l of the rewarding effects of VTA stimulation. Instead, such generalization may be related only to the hedonic effects produced by ac t i v a t i o n of the mesoaccumbens dopamine neurons. 131 EXPERIMENT 7 The Role of the Rewarding Effects of VTA Stimulation  in Determining Generalization with the  Stimulus Properties of Amphetamine Experiment 6 revealed that the stimulus properties of amphetamine could be augmented by e l e c t r i c a l stimulation of the VTA. In the discussion of that experiment i t was suggested that the augmentation may have r e f l e c t e d a summation between the stimulus properties of amphetamine and stimuli associated with dopaminergically mediated hedonic properties of the brain-stimulation. The present experiment investigated t h i s issue further, focussing s p e c i f i c a l l y on the putative role of the hedonic properties of the brain-stimulation in determining the interactions with the stimulus properties of amphetamine. In the i n i t i a l phase of t h i s experiment, rats trained to discriminate 1.0 mg/kg amphetamine from saline were given six drug-free generalization tests with d i f f e r e n t parameters of VTA stimulation (15 or 20 uA tr a i n s , presented 10, 5 or 2.5 sec apart). Subsequently, each rat was given a single test wherein ICSS rates were measured at the two current i n t e n s i t i e s employed during the discrimination phase of the experiment (15 and 20 uA) . The results of these tests were then compared to determine whether generalization between amphetamine and VTA stimulation might correlate with the rewarding effects of t h i s stimulation. If rats that displayed the strongest stimulus generalization between the 132 VTA s t i m u l a t i o n and amphetamine a l s o were found to have the h i g h e s t ICSS r a t e s , then t h i s might suggest t h a t the g e n e r a l i z a t i o n was r e l a t e d to the hedonic p r o p e r t i e s of the b r a i n - s t i m u l a t i o n . Methods Twenty male hooded r a t s were used as s u b j e c t s f o r t h i s experiment. F i f t e e n of these r a t s had been used as s u b j e c t s i n Experiment 6, whereas the other f i v e animals were e x p e r i m e n t a l l y n a i v e . The procedures f o r ICSS s c r e e n i n g and d i s c r i m i n a t i o n t r a i n i n g employed f o r the 5 new r a t s were i d e n t i c a l to those d e s c r i b e d i n Experiment 6. For the t e s t s with VTA s t i m u l a t i o n , each r a t was given a s a l i n e i n j e c t i o n and placed i n the chamber with the e l e c t r o d e leads a t t a c h e d . As i n Experiment 6, the i n t e r m i t t e n t p r e s e n t a t i o n s of VTA s t i m u l a t i o n began 3 min a f t e r the s t a r t of the h a l f - h o u r s e s s i o n and continued throughout. The b r a i n - s t i m u l a t i o n remained at a constant i n t e n s i t y (15 or 20 uA) w i t h i n a s e s s i o n and was presented e i t h e r once every 10 sec (.1 Hz), once every 5 sec (.2 Hz) or once every 2.5 sec (.4 Hz). Each p r e s e n t a t i o n c o n s i s t e d of four 200 msec t r a i n s of 60 Hz sine-wave s t i m u l a t i o n , d e l i v e r e d 200 msec a p a r t . Two days a f t e r the f i n a l g e n e r a l i z a t i o n t e s t , each r a t was placed i n a separate s e l f - s t i m u l a t i o n chamber and given 10 f r e e s t i m u l a t i o n s (200 msec t r a i n s of 60 hz sine-wave spaced 1 sec apart) of 15 uA c u r r e n t i n t e n s i t y . The r a t s could then l e v e r - p r e s s d u r i n g a 5 min p e r i o d to r e c e i v e a 133 single t r a i n of VTA stimulation upon each response (CRF reinforcement schedule). The t o t a l number of responses emitted during t h i s period were recorded on mechanical counters. At the end of t h i s i n i t i a l 5 min, the current in t e n s i t y was increased to 20 uA and responses were recorded for a further 5 min. Rats that were not already lever-pressing at the end of the f i r s t 5 min were given 10 free stimulations at the new int e n s i t y . Results The results of generalization tests with VTA stimulation are shown in Figure 13. As indicated by Figure 13a, the rats emitted an average of 48 to 64% of their responses on the drug-lever during tests with d i f f e r e n t parameters of VTA stimulation. ANOVAs performed on the drug-lever responses obtained from the stimulation tests did not reveal s i g n i f i c a n t e f f ects of either the current i n t e n s i t y manipulation (F[l,18] = 0.64; p > .05) or the d i f f e r e n t stimulus presentation rates (F[2,38] = 0.06; p > .05). However, there was a s i g n i f i c a n t interaction between the inte n s i t y and presentation rate variables (F[2,34] = 6.75; p < .005), which post hoc tests indicated was due to s i g n i f i c a n t l y more drug-lever responses when 20 uA stimulation was delivered at a presentation rate of 0.4 Hz r e l a t i v e to tests with the 10 uA current i n t e n s i t y presented at t h i s same rate. Although inspection of the group means from t h i s experiment suggested only intermediate levels of drug-lever 134 F i g u r e 13: Stimulus g e n e r a l i z a t i o n between amphetamine and VTA s t i m u l a t i o n d u r i n g d r u g - f r e e s u b s t i t u t i o n t e s t s . The data are presented as the percent of responses emitted on the drug l e v e r as a f u n c t i o n of d i f f e r e n t r a t e s of s t i m u l u s p r e s e n t a t i o n : once every 10 sec (0.1 Hz), every 5 sec (0.2 Hz) or every 2.5 sec (0.4 Hz). The c u r r e n t i n t e n s i t y was kept at a constant l e v e l of e i t h e r 15 uA (open symbols) or 20 uA ( f i l l e d symbols). A): Average percentage of drug l e v e r responses f o r a l l r a t s d u r i n g t e s t s at d i f f e r e n t c u r r e n t i n t e n s i t i e s and p r e s e n t a t i o n r a t e s . B): Average percentages of drug l e v e r responses f o r 3 sub-groups of r a t s t h a t c o n s i s t e n t l y showed e i t h e r s t r o n g g e n e r a l i z a t i o n ( t r i a n g l e s = > 67%; N=8), intermediate g e n e r a l i z a t i o n (squares = 33 to 67%; N=6) or no g e n e r a l i z a t i o n ( c i r c l e s = < 33%; N = 6) between amphetamine and the VTA s t i m u l a t i o n . 136 responding during stimulation t r i a l s , closer examination of the data revealed that the stimulation could e l i c i t strong biases towards responding on the drug lever in many of the r a t s . In fact, 8 of the rats responded primarily on the drug lever ( > 67% of pre-reinforcement responses) across a l l tests with VTA stimulation. Another six rats showed intermediate levels of drug lever responding (34 to 67% of pre-reinforcement responses), whereas the remaining six responded predominantly on the saline lever ( < 33% of pre-reinf orcement responses). The average levels of drug-lever responding for these three subgroups of rats are shown in Figure 13b. Separate analyses of the effects of the parameter manipulations for these sub-groups did not reveal any s i g n i f i c a n t differences related to either the current intensity, the stimulus presentation rate or the interaction of these variables either for: A) the rats that responded consistently on the drug lever (F[l,7] = 1.55; p > .05; F[2,14] = 0.46 p > .05; and F[2,14] = 3.05; p > .05); B) the rats that showed intermediate amounts of drug-lever responding (F[l,4] = 0.52; p > .05; F[2,10] = 0.03; p > .05; and F[2,8] = 0.55; p > .05); or C) rats that emitted few responses on the drug-lever (F[l,5] = 2.35; p > .05; F[2,10] = 1.70; p > .05; and F[2,8] = 1.48; p > .05). Figure 14 indicates the electrode placements for the rats employed in the present experiment. Inspection of the figure reveals that moderate to high levels of drug-appropriate responding could be obtained from stimulation of 137 Figure 14: Relationship between the electrode placements and stimulus generalization with amphetamine for the 20 rats employed in Experiment 7. A l l electrodes were implanted in the l e f t hemisphere. The d i f f e r e n t symbols r e f l e c t the levels of drug-appropriate responding > 67%; fl = 33 to 67%; 0 = < 33%) when each electrode s i t e was stimulated at a current i n t e n s i t y of either 15 uA ( l e f t sides of the sections) or 20 uA (right sides of the sections). The response percentages for each group of rats were obtained by averaging over the tests with d i f f e r e n t stimulus presentation rates. The numbers to the right of the diagrams represent the plate numbers corresponding to the coronal sections from the brain atlas of Konig and K l i p p e l (1963). 139 regions surrounding the anterior portion of the interpeduncular nucleus (Plate #'s 47b and 48b) and ventromedial placements at a le v e l just anterior to th i s nucleus (Plate # 46b). Placements that produced the least amount of drug-appropriate responding were located primarily in the dorsal part of the region just anterior to the interpeduncular nucleus (Plate #'s 45b and 46b). Two additional placements associated with low levels of drug-appropriate responding were situated at dorsal and ventral extremities in the posterior VTA (Plate # 49b). Analyses of the percentages of selected-lever responses measured during the generalization tests (Table 14) did not reveal any s i g n i f i c a n t effects of either the current i n t e n s i t y (F[l,18] = 0.87; p > .05), the stimulus presentation rate (F[2,38] = 1.91; p > .05) or the interaction of these variables (F[2,33] = 0.24; p > .05). However, there were s i g n i f i c a n t effects of the brain-stimulation on the number of responses emitted during these sessions (Table 15). S t a t i s t i c a l analyses revealed s i g n i f i c a n t e f f e cts of both the current i n t e n s i t y (Ftl,18] = 12.14; p < .005) and the stimulus presentation rate (F[2,38] = 10.76; p < .0005), but there was no interaction between these variables (F[2,36] = 0.82; p > .05). Post hoc tests revealed that the number of responses were reduced during tests with the high current i n t e n s i t y (20 uA) r e l a t i v e to tests with the lower i n t e n s i t y (15 uA) . The number of responses also were reduced as a function of increasing the 140 TABLE 14 Percentages of responses on the i n i t i a l l y s e l e c t e d l e v e r d u r i n g s u b s t i t u t i o n t e s t s with d i f f e r e n t parameters of VTA s t i m u l a t i o n . PRESENTATION RATE (Hz) 0.1 0.2 0.4 INTENSITY LEVEL; 15 uA 94 92 91 20 uA 93 87 88 141 TABLE 15 Total number of responses during substitution tests with d i f f e r e n t parameters of VTA stimulation. PRESENTATION RATE (Hz) 0.1 0.2 0.4 INTENSITY LEVEL: 15 uA 1294 1059 962 20 uA 898 797 499 142 stimulus presentation rate, with response levels measured at each rate being s i g n i f i c a n t l y d i f f e r e n t from response levels observed for the other two rates. During subsequent se l f - s t i m u l a t i o n tests, a l l of the rats responded for the VTA stimulation. However, there were substantial differences in the ICSS rates obtained from the individual r a t s . Importantly, there appeared to be a relationship between the a b i l i t y of the stimulation to e l i c i t amphetamine-appropriate responses during stimulus generalization tests and i t s e f f i c a c y in producing rewarding effects (see Figure 15). In general, the rats that emitted a high percentage of their responses on the amphetamine-appropriate lever during generalization tests also responded at high rates during ICSS sessions. Rats that did not emit drug-appropriate responses during generalization tests tended to respond at low rates for the brain-stimulation reward. To determine the significance of t h i s r e l a t i o n s h i p , the individual percentages of drug-lever responses emitted at each in t e n s i t y (averaged across a l l presentation frequencies) were correlated with the sel f - s t i m u l a t i o n rates measured at these i n t e n s i t i e s using Pearson's c o r r e l a t i o n c o e f f i c i e n t . These analyses revealed s i g n i f i c a n t positive correlations between drug-lever responses and ICSS rates for tests with both the low int e n s i t y (15 uA, Figure 15a; r = 0.46; d.f. = 19; p < .05) and the higher in t e n s i t y (20 uA, Figure 15b; r = 0.71; d.f. = 18; p < .01). Figure 16 again shows the electrode placements of the 143 Figure 15: Scattergrams showing the correlations between stimulus generalization with amphetamine and ICSS rates obtained with VTA stimulation. The data are expressed as the percent of responses emitted on the drug-lever during generalization tests (y-axis) r e l a t i v e to the response rates for the VTA stimulation (x-axis; average responses per min) obtained during ICSS tests at: A) 15 uA (Pearsons r = .46; p < .05); and B) 20 uA (Pearsons r = 0.71; p < .01). The generalization data represent the averaged scores for the three tests at the d i f f e r e n t rates of stimulus presentation. The s o l i d l i n es through the scattergrams represent the lines of best f i t obtained from the regression equations. 144 S3SNOdS3« U3A31 DfiyQ lN30d3d 145 Figure 16: Relationship between electrode placements and ICSS rates for the 20 rats employed in Experiment 7. A l l electrodes were implanted in the l e f t hemisphere. The d i f f e r e n t symbols r e f l e c t the levels of se l f - s t i m u l a t i o n obtained > 80 presses/min; | = 40 to 80 presses/min; 40 presses/min) when each electrode s i t e was stimulated at a current i n t e n s i t y of either 15 uA ( l e f t sides of the sections) or 20 uA (right sides of the sections). The numbers to the right of the diagrams represent the plate numbers corresponding to the coronal sections from the brain atlas of Konig and Kli p p e l (1963). 146 147 rats in the present experiment, with the d i f f e r e n t symbols presented to indicate the le v e l of sel f - s t i m u l a t i o n obtained at each s i t e . Inspection of the l e f t side of the figure reveals that moderate to high s e l f - s t i m u l a t i o n rates were obtained from a l l but six s i t e s when the current i n t e n s i t y was set at 15 uA. Four of these l a t t e r s i x s i t e s were situated at various extremities of the VTA and correspond to the s i t e s which also were associated with low levels of drug-appropriate responding during stimulus generalization tests (compare with Figure 14). The right side of the Figure 16 shows that a l l of the s i t e s yielded moderate to high levels of ICSS when the current intensity was set at 20 uA. Many of the s i t e s that yielded high s e l f - s t i m u l a t i o n rates also were associated with high levels of amphetamine-appropriate responding during generalization t e s t s , whereas s i t e s that yielded only moderate ICSS rates tended to produce only moderate to low levels of drug-appropriate responding. There were also a few s i t e s that yeilded high ICSS rates but low levels of amphetamine-appropriate discriminated responding. Discuss ion The results of the present experiment confirmed that rats trained to discriminate 1.0 mg/kg amphetamine from saline would emit drug-appropriate responses when tested with e l e c t r i c a l stimulation of the VTA. As a group, the rats emitted between 48 and 64% of their responses on the amphetamine-appropriate lever during stimulation t r i a l s . 148 This l e v e l of drug-appropriate responding was similar to that observed when the stimulation was delivered alone ( i . e . , with 0.0 mg/kg amphetamine) in the previous experiment. Given that the average percentage of responses emitted on the drug-appropriate lever did not exceed a maximum of 64% for the group, and the discriminated responses did not vary systematically with changes in the stimulation parameters, the results of the stimulation tests could have refl e c t e d disruptive effects of the VTA stimulation on operant performance. However, the results of Experiment 6 revealed that the stimulation could augment the cueing effects of low amphetamine doses and increase drug-lever responses to levels above that of chance performance. Such augmentation would not occur i f the stimulation was exerting only disruptive actions. Moreover, examination of the data from individual animals in the present experiment revealed that eight of the rats responded predominantly on the amphetamine-appropriate lever ( > 67% of their pre-reinforcement responses) during each of the tests with VTA stimulation. Six other rats exhibited intermediate amounts of responding on the drug-appropriate lever (33 to 67%) during stimulation t r i a l s , whereas the remaining 6 animals responded consistently on the saline-appropriate lever. Thus, the apparent p a r t i a l biasing of responses toward the drug-lever indicated by the group mean was not a r e f l e c t i o n of p a r t i a l biasing within each r a t . In fact, many of the 149 rats responded primarily on the drug lever during stimulation t r i a l s , suggesting that VTA stimulation could generalize with the stimulus properties of amphetamine in certain i n d ividuals. The a b i l i t y of the brain-stimulation to substitute for the stimulus properties of amphetamine appeared to be related to the placement of the stimulating electrodes within the VTA. Examination of Figure 14 revealed that the stimulus generalization with amphetamine was strongest when the electrodes were located within the VTA either in the regions immediately adjacent the anterior interpeduncular nucleus or in a ventromedial location just anterior to t h i s nucleus. These s i t e s correspond with the location of c e l l bodies for the mesocortical dopamine neurons (Fallon & Moore, 1978). Electrode s i t e s that did not produce generalization tended to be located in regions lacking high densities of dopamine perikarya. This r e l a t i o n between stimulus generalization with amphetamine and anatomical proximity of the electrodes to dopamine neurons suggests that the generalization might have been determined by the e f f i c a c y of the stimulation for acti v a t i n g mesocortical dopamine projections. During subsequent ICSS te s t s , a l l of the rats were found to self-stimulate when the brain-stimulation was made available as a reward. However, there were individual differences among the rats, with some exhibiting higher ICSS rates than others. As was the case during stimulus 150 generalization tests, the e f f i c a c y of the stimulation appeared to depend on the electrode placements. The rats with the highest ICSS rates generally were found to have electrodes located within regions containing high densities of dopamine c e l l s (Fallon & Moore, 1978) whereas rats with lower ICSS rates tended to have electrodes outside of these regions. Comparisons of the data obtained during generalization and ICSS tests for each rat revealed a s i g n i f i c a n t positive c o r r e l a t i o n between the amount of drug-appropriate responding e l i c i t e d by the stimulation and the rate of responding for the brain-stimulation reward. Thus, stimulus generalization with amphetamine was strongest when the VTA stimulation was highly rewarding, but was absent when the stimulation had r e l a t i v e l y weak rewarding e f f e c t s . This res u l t suggested that the a b i l i t y of the VTA stimulation to generalize with amphetamine may have been related to i t s capacity to produce rewarding e f f e c t s . In p a r t i c u l a r , the generalization may have r e f l e c t e d the ac t i v a t i o n of dopaminergic substrates for the brain-stimulation reward, as the strongest effects were produced when electrodes were within regions containing high densities of dopamine perikarya. 151 EXPERIMENT 8 Effects of Amphetamine and Haloperidol on Discriminative  Stimuli Produced by E l e c t r i c a l Stimulation of the VTA The results of Experiments 6 and 7 suggested that e l e c t r i c a l stimulation of the VTA could produce stimulus properties similar to those of amphetamine. At a neuronal l e v e l , the s i m i l a r i t y between stimuli may have re f l e c t e d the a b i l i t y of the VTA stimulation to activate dopaminergic processes that mediate the stimulus properties of amphetamine. Indeed, the generalization between amphetamine and VTA stimulation observed in Experiment 7 was strongest when the stimulating electrodes were located within regions containing high densities of mesocortical dopamine c e l l s . Furthermore, previous studies have indicated that the sine-wave currents employed for the generalization tests were capable of increasing the turnover and release of dopamine within various forebrain regions, including the nucleus accumbens (Blaha et a l . , 1988; Fibiger et a l . , 1987). However, d i r e c t attempts to demonstrate dopaminergic substrates for brain-stimulation cues have yielded c o n f l i c t i n g r e s u l t s . Several reports have indicated that the discriminative stimulus properties of LH or VTA brain-stimulation were unaffected by injections of amphetamine, cocaine or haloperidol (Druhan, 1985; Druhan, Martin-Iverson, Wilkie, Fibiger, & P h i l l i p s , 1987a; Kornetsky & Esposito, 1981; Schaefer & Michael, 1985). Instead, the stimulus properties of VTA stimulation appeared to be 152 modulated by cholinergic drugs (Druhan, 1985; Druhan, Martin-Iverson, Wilkie, Fibiger, & P h i l l i p s , 1987b; Druhan, Fibiger & P h i l l i p s , in press). On the other hand, Colpaert, Niemegeers, & Janssen (1977) found that LH brain-stimulation cues could be blocked by haloperidol, and in a subsequent study Colpaert (1977a) demonstrated that detection thresholds for such stimuli could be increased by haloperidol and decreased by cocaine. These l a t t e r e f f e c ts were consistent with the capacity of haloperidol and cocaine to respectively interfere with and enhance neurotransmission at dopaminergic synapses. A plausible explanation for these inconsistent drug effects is that LH and VTA stimulation may produce several neurochemically d i s t i n c t s t i m u l i , each which may be measured under appropriate circumstances. For example, studies suggesting the involvement of non-dopaminergic substrates for the brain-stimulation cues employed massed-trial procedures wherein rats were required to make discrete discriminated responses to brief presentations of brain-stimulation. In contrast, the studies implicating dopaminergic substrates used procedures that were similar to the methods commonly employed in drug-discrimination experiments. Thus, rats received single d a i l y t r i a l s in which intermittent trains of brain-stimulation were delivered for 5 min prior to access to the response levers. The rats then could respond on the appropriate lever for a further 15 min (on an FR-10 schedule of reinforcement) in 153 the continued presence of the brain-stimulation cue. In the present experiment, a discrimination procedure was developed which incorporated certain c h a r a c t e r i s t i c s of t h i s l a t t e r method, and t h i s new procedure was employed to investigate the possible involvement of dopamine neurons in mediating the stimulus properties of VTA stimulation. Rats were trained with a massed-trial procedure to discriminate between high and low i n t e n s i t i e s of VTA stimulation, but with each in t e n s i t y being delivered intermittently over a 2-minute period prior to a c h o i c e - t r i a l . These rats were then given generalization tests with intermediate current i n t e n s i t i e s after pretreatment with amphetamine, haloperidol or the drug vehicles. If dopamine neurons mediated the brain-stimulation cues produced with t h i s new procedure, then these cues would be enhanced by amphetamine and attenuated by haloperidol. Methods Nine male hooded rats were employed as subjects for t h i s experiment. One week after implantation of the electrodes, the rats were given five 30 min ICSS sessions in which they could lever-press for 200 msec trains of 20 uA, 60 Hz sine wave stimulation on a CRF schedule. Subsequently, the rats received eight ICSS sessions in which their response rates were measured over a range of current i n t e n s i t i e s (6 to 26 uA) . For these tests, the current i n t e n s i t y was set i n i t i a l l y at 6 uA and subsequently increased by 2 uA every 5 min u n t i l 26 uA was reached. The same range of currents (26 154 to 6 uA) was then delivered in a descending order of presentation. Each change in the current i n t e n s i t y was signalled by the free delivery of 10 stimulation trains (2 trains/sec) at the new i n t e n s i t y . The ascending and descending r a t e - i n t e n s i t y functions measured over the f i n a l 4 days of t h i s phase were averaged to obtain a single function for each rat r e l a t i n g response rate to current i n t e n s i t y . From this function, one low current i n t e n s i t y (10 or 12 uA) and one high current i n t e n s i t y (10 uA higher than the low intensity) were chosen for use as discriminative s t i m u l i . The low i n t e n s i t y supported threshold ICSS rates (10 to 20 presses per min) and the high i n t e n s i t y supported near assymptotic ICSS rates (approximately 250 presses per min). Discrimination Training After ICSS testing was complete, the rats were trained during two 30 min sessions to lever-press for 45 mg Noyes food p e l l e t s on a CRF schedule. Subsequently, each rat was given d a i l y discrimination sessions with 12 t r i a l s spaced 120 to 180 sec apart (variable i n t e r v a l - 150 sec). The beginning of each t r i a l was signalled by a brief (0.05 sec) f l a s h of the houselight, followed 1 sec later by the delivery of the f i r s t of s ix presentations of either high (20 or 22 uA) or low (10 or 12 uA) i n t e n s i t y VTA stimulation. Each stimulus presentation consisted of four 200 msec trains of 60 Hz sine-wave stimulation delivered 200 msec apart. The stimulation was maintained at a constant 155 high or low in t e n s i t y throughout a given cueing period and was delivered at 20 sec in t e r v a l s . A f i n a l 20 sec int e r v a l followed the sixth stimulus presentation, after which the houselight was turned on to signal the a v a i l a b i l i t y of food. The houselight remained on for 30 sec during which time the rat could respond to obtain food on the lever appropriate for the stimulus i n t e n s i t y presented during the cueing period. Responses on the incorrect lever had no programmed consequence. The lever appropriate for each current intensity was counterbalanced between r a t s . During i n i t i a l t r a i n i n g , each response on the correct lever resulted in the del i v e r y of one food p e l l e t . Subsequently, the response requirement was increased so that a food p e l l e t was delivered after every s i x t h response on the correct lever (FR-6), regardless of whether intervening responses were made on the other lever. The accuracy of the discrimination was assessed by recording the lever on which a rat f i r s t completed s i x responses. Stimulus Generalization After Amphetamine and Haloperidol Rats that acquired the discrimination task (8 of the 9) were given generalization tests 10 min after receiving intraperitoneal injections of either d-amphetamine sulphate (0.5 and 1.0 mg/kg) or s a l i n e . During these generalization tests, rats were presented with four equally spaced intermediate i n t e n s i t i e s (2 uA apart) delivered randomly along with the usual t r a i n i n g currents. Each intermediate in t e n s i t y was delivered once and each t r a i n i n g current was 156 presented four times within a single generalization t e s t . During t r i a l s with intermediate i n t e n s i t i e s , the rats were reinforced for continuing to respond on the lever on which the f i r s t FR-6 response requirement had been completed. Upon completion of the tests with amphetamine, further generalization tests were given 45 min after intraperitoneal injections of either haloperidol (0.10 and 0.125 mg/kg) or d i s t i l l e d water. The order of dose administration was counterbalanced across rats during both the amphetamine and haloperidol test phases. At least two regular t r a i n i n g sessions were interposed between each generalization t e s t . The data obtained from each generalization session were expressed in terms of the percentage of responses emitted prior to the f i r s t reinforcement that occurred on the HS lever after each current i n t e n s i t y . The percentages obtained from the amphetamine and haloperidol tests were analysed separately using two-way repeated measures ANOVA's with the amphetamine or haloperidol dose as one factor and the stimulation i n t e n s i t y as the second factor. As in previous experiments, post hoc analyses were conducted using Newman-Keuls test (p < 0.05) when the ANOVA's revealed differences s i g n i f i c a n t beyond a p r o b a b i l i t y of 0.05. Results The eight rats that learned the discrimination task reached an accuracy l e v e l of over 80% correct choices per session. The electrode placements for these rats are shown in Figure 17. During generalization tests, the rats 157 increased their responses on the lever appropriate for high i n t e n s i t y stimulation (HS lever) as a function of the increasing current i n t e n s i t i e s . This trend was ref l e c t e d in the s i g n i f i c a n t current i n t e n s i t y effects obtained during the experiments with amphetamine (F[5,35] = 11.75; p < .0001) and haloperidol (F[5,35] = 13.33; p < .0001). The results of the tests with amphetamine and haloperidol are shown in Figure 18. Analyses of the amphetamine experiment (Figure 18a) revealed s i g n i f i c a n t e f f e c t s of the amphetamine dosage (F[2,12] = 9.89; p < .005), which were due to increases in HS-lever responding at both dosages of amphetamine r e l a t i v e to tests with saline i n j e c t i o n s . These effects of amphetamine resulted in elevations of the discriminated response gradients for the range of current i n t e n s i t i e s . Analysis of the haloperidol experiment (Figure 18b) revealed a s i g n i f i c a n t e f f e c t of haloperidol dosage (F[2,12] = 4.28, p < .05), which was due to decreases in HS-lever responding at the high dosage of th i s drug (0.125 mg/kg) r e l a t i v e to tests with the vehicle solution. This decrease in HS-lever reponding resulted in a lowering of the discriminated response function at the high dose. Analyses of the interactions between drug dosage and current i n t e n s i t y variables did not reveal s i g n i f i c a n t effects for experiments involving either amphetamine (F[10,60] = 1.46; p > .05) or haloperidol (F[10,51] = 1.56; p > .05). 158 Figure 17: Electrode placements for the 8 rats that acquired the discrimination task in Experiment 8. A l l electrodes were implanted in the l e f t hemisphere. The numbers to the right of the diagrams represent the plate numbers corresponding to the coronal sections from the brain atlas of Konig and Kli p p e l (1963). 159 160 Figure 18: Effects of amphetamine and haloperidol on stimulus generalization to a range of current i n t e n s i t i e s in rats trained to discriminate between high and low i n t e n s i t i e s of VTA stimulation. The data are expressed as the mean percentage of responses emitted on the lever appropriate for high-intensity stimulation (HS) as a function of the average current intensity delivered. A) Amphetamine (0.5 and 1.0 mg/kg) resulted in s i g n i f i c a n t increases in HS lever responding, causing the generalization functions to be elevated r e l a t i v e to those obtained with vehicle i n j e c t i o n s . B) Haloperidol (0.125 mg/kg) resulted in s i g n i f i c a n t decreases in HS lever responses, so that the generalization functions were lowered r e l a t i v e to the vehicle control curve. A AMPHETAMINE B. HALOPERIDOL 162 Discussion In the present experiment, rats were trained to discriminate between high and low i n t e n s i t i e s of VTA stimulation and then given generalization tests with a range of current i n t e n s i t i e s after pretreatment with amphetamine, haloperidol or the respective vehicle solutions. Amphetamine appeared to enhance the cueing effects of the brain-stimulation, as indicated by elevations of the stimulus generalization functions r e l a t i v e to the vehicle control curve after pretreatment with t h i s drug. In contrast, haloperidol appeared to interfere with the stimulus properties of the brain-stimulation as the generalization function was lowered at the high dose of t h i s drug. These drug effects are consistent with the known dopamine agonist properties of amphetamine (Chiueh & Moore, 1975; F e r r i s , Tang, & Maxwell, 1972) and the selective dopamine receptor antagonist actions of haloperidol (Anden et a l . , 1970). Accordingly, the results of the present experiment might be taken as evidence of a dopaminergic substrate for the stimulus properties of VTA brain stimulation. The results of the present experiment contrast with those of e a r l i e r pharmacological studies in which the stimulus properties of VTA stimulation were enhanced by the acetylcholinesterase i n h i b i t e r physostigmine and the d i r e c t muscarinic acetylcholine receptor agonists pilocarpine and RS-86, but not by amphetamine, haloperidol or nicotine (Druhan, 1985; Druhan et a l . , 1987a; Druhan et a l . , in 163 press). These reports suggested that cues produced by VTA stimulation might be mediated by non-dopaminergic processes which involve muscarinic cholinergic receptor mechanisms. However, these studies employed t r a i n i n g procedures that d i f f e r e d in many respects from those used in the present experiment. For the e a r l i e r studies, rats were trained to discriminate b r i e f presentations of VTA stimulation and emit discrete responses on the appropriate one of two levers to receive food reinforcement. In the present experiment rats were trained to discriminate VTA stimulation that was presented intermittently over a 2-minute period prior to the choice t r i a l . The rats could then respond for 30 sec on the appropriate lever to receive food on an FR-10 schedule of reinforcement. It is conceivable that these d i f f e r e n t t r a i n i n g procedures may have resulted in the measurement of neurochemically d i s t i n c t cue properties of VTA stimulation. As indicated above, procedures involving b r i e f presentations of VTA stimulation yielded cues that appeared to involve cholinergic rather than dopaminergic processes. In contrast, the brain-stimulation cues measured with the present procedure appeared to be mediated by independent dopaminergic substrates. The independence of these l a t t e r cues from those measured with brief presentations of stimulation has been substantiated by a recent study in which the cue properties of VTA stimulation measured with the present procedure were not affected by physostigmine (Druhan et a l . , 1987b). 164 In Experiments 6 and 7 i t was suggested that the amphetamine-like cueing effects of sine-wave VTA stimulation might have resulted from the production of dopaminergically mediated stimuli (possibly of a hedonic nature). This suggestion was consistent with previous evidence that sine-wave stimulation of the VTA could increase the release and turnover of dopamine in various forebrain regions (Blaha et a l . , 1988; Fibiger et a l . , 1987). Also, Experiment 7 revealed that the generalization between amphetamine and VTA stimulation was strongest when the stimulating electrodes were within regions of the VTA that contained high densities of dopamine c e l l bodies. The present experiment provided further evidence in support of a dopaminergic basis for the amphetamine-like cueing actions of the brain-stimulation, in demonstrating that some of the stimuli produced by e l e c t r i c a l stimulation of the VTA may be mediated by dopaminergic substrates. 165 GENERAL DISCUSSION The studies reviewed in the Introduction indicated that the discriminative stimulus properties of amphetamine may res u l t from the f a c i l i t a t o r y actions of t h i s drug on mesoaccumbens dopamine neurotransmission. As such actions also give r i s e to amphetamine's rewarding effects ( A u l i s i & Hoebel, 1983; Carr & White, 1983, 1986; Lyness et a l . , 1979; Monaco et a l . , 1980; Spyraki et a l . , 1982), i t is conceivable that amphetamine stimuli may r e f l e c t the hedonic actions of th i s drug. The present series of experiments determined whether amphetamine stimulus generalization functions could be affected by psychoactive drugs from diverse pharmacological classes and VTA stimulation in a manner that was consistent either with the actions of the test s t imuli on dopamine neurotransmission or with th e i r hedonic e f f e c t s . The f i r s t experiment assessed the effects of the psychomotor stimulant cocaine, the dopamine receptor agonist apomorphine and the dopamine receptor antagonist haloperidol on amphetamine stimulus generalization functions. Cocaine elevated amphetamine stimulus generalization functions r e l a t i v e to the vehicle control curve in a dose dependent manner, suggesting that t h i s drug may have augmented the stimulus properties of amphetamine. In contrast, haloperidol lowered the generalization functions in a manner suggestive of an antagonistic interaction with the stimulus properties of amphetamine. The effects of apomorphine varied according 166 to the pa r t i c u l a r dosage employed. At the lowest dose (0.05 mg/kg), apomorphine appeared to antagonize the stimulus properties of amphetamine as the generalization function was lowered r e l a t i v e to the control curve. At the highest dose (0.2 mg/kg), these i n h i b i t o r y actions were superseded by the drug's capacity to generalize with the stimulus properties of amphetamine, and the generalization functions were elevated r e l a t i v e to the control curve. The intermediate dose of apomorphine (0.15 mg/kg) appeared both to substitute p a r t i a l l y for the stimulus properties of amphetamine and to i n h i b i t the stimulus properties of high amphetamine doses. However, these l a t t e r e f f e cts were not s t a t i s t i c a l l y s i g n i f i c a n t and the overall generalization function at th i s dose did not d i f f e r from the curve obtained under control conditions. Experiments 2 through 5 examined whether amphetamine stimulus generalization functions could be influenced by drugs other than psychomotor stimulants or dopamine receptor agonists and antagonists. These compounds included the general CNS stimulant nicotine, the opiate narcotic morphine, the benzodiazepine midazolam and the sedative hypnotic, ethanol. Nicotine was observed to elevate amphetamine stimulus generalization functions r e l a t i v e to the control curve, in a manner suggestive of an additive interaction with the amphetamine s t i m u l i . Morphine and midazolam lowered the generalization functions, suggesting that these drugs may have attenuated the amphetamine 167 s t i m u l i . Ethanol increased the amount of drug-appropriate responding emitted at the intermediate doses of amphetamine, but i t did not e l i c i t drug-appropriate responses when injected alone ( i . e . with 0.0 mg/kg amphetamine). This pattern of results may have been due to an interference with the metabolic degradation of amphetamine by ethanol (Ellinwood et a l . , 1976), rather than a summation of the cueing effects of the two drugs. Experiments 2 through 5 also examined the eff e c t s of nicotine, morphine, midazolam and ethanol on locomotor a c t i v i t y . To the extent that changes in a c t i v i t y levels often r e f l e c t p a r a l l e l changes in dopaminergic function, these tests could provide independent behavioral evaluations of the dopaminergic actions of each drug. The results of these tests appeared to confirm previous neurochemical and physiological reports concerning the effects of the drugs on dopamine neurotransmission. Locomotor a c t i v i t y was increased by nicotine and morphine, decreased by midazolam and unaffected by ethanol. Experiment 6 determined whether the stimulus properties of amphetamine could be augmented by a non-pharmacological stimulus produced by e l e c t r i c a l stimulation of the ventral tegmental area. The VTA contains the c e l l bodies for the mesocortical dopamine projection (Fallon & Moore, 1978), and stimulation of t h i s s i t e can both increase dopamine release and turnover in forebrain regions and produce rewarding ef f e c t s (Blaha et a l . , 1988; Fibiger et a l . , 1987). On the 168 basis of each of these actions, i t was predicted that VTA stimulation might summate with the stimulus properties of amphetamine. Indeed, t h i s expectation was confirmed in Experiment 6, wherein the VTA stimulation was found to elevate amphetamine stimulus generalization functions r e l a t i v e to curves obtained in the absence of the stimulation. Experiment 7 investigated the possible role of hedonic factors in determining the capacity of VTA stimulation to produce amphetamine-like cueing e f f e c t s . The rats employed in Experiment 6 were given additional drug-free tests for generalization to VTA stimulation delivered at two d i f f e r e n t current i n t e n s i t i e s and at three separate rates of stimulus presentation. Subsequently, each rat was given a single ICSS test to determine the rewarding e f f i c a c y of the current i n t e n s i t i e s employed during the generalization t e s t s . The results of these tests revealed that there were consistent individual differences in the degree of generalization produced by the stimulation, and these differences correlated p o s i t i v e l y with the ICSS rates measured in these rats . These individual differences also appeared to r e f l e c t the proximity of the stimulating electrodes to regions of the VTA that contain high densities of dopamine c e l l bodies (Fallon & Moore, 1978). This pattern of results suggested that the VTA stimulation might have produced i t s amphetamine-like cueing effects by a c t i v a t i n g the same dopaminergically-mediated a f f e c t i v e processes that commonly 169 give r i s e to the stimulus properties of amphetamine. Experiment 8 investigated whether the stimulus properties of VTA stimulation could be mediated by dopaminergic substrates. Rats were trained to discriminate between high and low i n t e n s i t i e s of VTA stimulation and then tested for generalization to a range of current i n t e n s i t i e s after injections of amphetamine, haloperidol or the respective vehicle solutions. These tests revealed that amphetamine could augment the stimulus properties of VTA stimulation, as indicated by elevations of the stimulus generalization functions r e l a t i v e to the vehicle control curve. In contrast, haloperidol appeared to attenuate the brain-stimulation cues as the stimulus generalization functions were lowered by th i s drug. Given the known c a p a b i l i t i e s of amphetamine (Chiueh & Moore, 1975; F e r r i s et a l . , 1972) to enhance dopamine neurotransmission and of haloperidol to block dopamine receptors s e l e c t i v e l y at the doses employed here (Anden et a l . , 1970), these effects on stimulus generalization gradients might reasonably be attributed to alte r a t i o n s in the a c t i v i t y of a dopaminergic substrate for the stimulus properties of VTA stimulation. This dopaminergic substrate could conceivably be the neural mechanism by which the VTA stimulation produces i t s amphetamine-like cueing e f f e c t s . The pattern of results obtained in the present series of experiments allows several conclusions to be made about the stimulus properties of amphetamine. On the most basic l e v e l , 170 the findings of Experiment 1 confirmed that the amphetamine stimulus generalization paradigm employed in t h i s thesis could produce results similar to those obtained with simple substitution and antagonism te s t s . For example, the elevations of the amphetamine stimulus generalization functions after cocaine and a high dose of apomorphine (0.2 mg/kg) were consistent with previous reports that these l a t t e r compounds can generalize with the stimulus properties of amphetamine (Colpaert et a l . , 1978; Huang & Ho, 1974a; Huang & Wilson, 1986; Schecter, 1977; Schecter & Cook, 1975). S i m i l a r l y , the lowering of the generalization function after haloperidol was consistent with evidence that th i s drug can attenuate the cueing effects of amphetamine (Colpaert et a l . , 1978; Nielsen & Jepsen, 1985; Schecter & Cook, 1975). Experiment 1 also revealed a previously unreported finding; that low doses of apomorphine (0.05 mg/kg) could attenuate the stimulus properties of amphetamine and lower stimulus generalization functions. This e f f e c t was strongest at the two intermediate doses of amphetamine (0.25 and 0.50 mg/kg), whereas the in h i b i t o r y e f f ects exerted at the tr a i n i n g dose of amphetamine were r e l a t i v e l y weak. Indeed, the in h i b i t o r y e f fects of apomorphine might not have been detected had the present generalization paradigm not been used. This r e s u l t indicated that, in addition to providing results comparable to those obtained with simple substitution and antagonism paradigms, the amphetamine stimulus generalization paradigm employed in 171 the present thesis might have the advantage of being able to detect subtle interactions with the stimulus properties of amphetamine that might otherwise go unnoticed. The effects of cocaine, apomorphine, and haloperidol on amphetamine stimulus generalization functions in Experiment 1 were consistent with the hypothesis that the stimulus properties of amphetamine may be mediated by a dopaminergic substrate. The selective dopamine receptor agonist actions of the high dose of apomorphine (Anden et a l . , 1967) were s u f f i c i e n t to produce amphetamine-like cueing e f f e c t s . Similar e f f e c t s were produced by cocaine, which can increase synaptic concentrations of dopamine by blocking the reuptake and metabolic degradation of t h i s transmitter (Ritz et a l . , 1987). In contrast, the findings that amphetamine stimulus generalization functions were lowered both by a low dose of apomorphine and by haloperidol suggests that dopamine release and receptor a c t i v a t i o n are necessary for the cueing effects of amphetamine. Low doses of apomorphine can decrease dopamine neurotransmission by blocking impulse-dependent release of t h i s transmitter (Gonon & Buda, 1985; Grace & Bunney, 1985; Lane & Blaha, 1986; Zetterstrom & Ungerstedt, 1984), and haloperidol has been shown to act as a s e l e c t i v e dopamine receptor antagonist at the doses employed here (Anden et a l . , 1970). The present thesis also provided some support for the hypothesis that the stimulus properties of amphetamine might, at a functional l e v e l , r e f l e c t the hedonic actions of 172 th i s compound. In Experiment 7, the capacity of VTA stimulation to generalize with the stimulus properties of amphetamine was p o s i t i v e l y correlated with the a b i l i t y of the stimulation to produce rewarding e f f e c t s . Rats that displayed strong generalization between the stimulus properties of amphetamine and VTA stimulation generally were observed to respond at high rates for the stimulation when i t was made available as a reward. Conversely, rats that showed intermediate or low levels of generalization tended to respond at lower rates for the stimulation. In view of thi s c o r r e l a t i o n , i t is conceivable that the rats that generalized between the stimulus properties of amphetamine and VTA stimulation may have done so on the basis of common hedonic elements of the two s t i m u l i . A central issue in t h i s thesis was whether amphetamine stimulus generalization functions could be affected by test stimuli other than psychomotor stimulants or dopamine receptor agonists/antagonists in a manner consistent with either the dopaminergic actions or hedonic e f f e c t s of the s t i m u l i . This issue was addressed in Experiments 2 through 6, wherein the effects of nicotine, morphine, midazolam, ethanol, and VTA stimulation on amphetamine stimulus generalization functions were determined. The results of these experiments revealed that the generalization functions could indeed be influenced by these test stimuli in a manner that generally was consistent with the known effects of the stimuli on mesoaccumbens dopamine neurotransmission. 173 For example, the functions were elevated r e l a t i v e to vehicle control curves by nicotine and VTA stimulation, both of which can increase indices of dopamine release and u t i l i z a t i o n within the nucleus accumbens (Blaha et a l . , 1988; Fibiger et a l . , 1987; Imperato et a l . , 1986; Mereu et a l . , 1987). In contrast, the functions were lowered by midazolam, a compound that has been shown to decrease the a c t i v i t y of dopamine neurons and reduce accumbens dopamine concentrations (Finlay et a l . , 1987; Haefley et a l . , 1981). Although ethanol appeared to increase drug-appropriate responding e l i c i t e d at intermediate amphetamine doses, there was no evidence for stimulus generalization with amphetamine when ethanol was administered alone ( i . e . , with 0.0 mg/kg amphetamine). In interpreting t h i s l a t t e r r e s u l t , i t i s important to note that ethanol has been found to increase blood levels of amphetamine by d i r e c t l y i n t e r f e r i n g with i t s metabolic degradation (Ellinwood et a l . , 1976). This pharmacokinetic action may have been responsible for the increases in drug-appropriate responding observed when ethanol was injected with intermediate amphetamine doses. In contrast, the absence of generalization observed when ethanol was injected alone indicates that t h i s substance may have lacked amphetamine-like stimulus properties. Interpreted as such, the effects of ethanol would be consistent with evidence that ethanol does not a f f e c t mesoaccumbens dopamine neurotransmission (Ellingboe & Mendelson, 1982; Kalant, 1975; Nutt & Glue, 1986). 174 Although the interactions of these test stimuli with amphetamine stimulus generalization functions generally appeared to r e f l e c t their known actions on dopamine neurotransmission, additional evidence is needed to confirm a dopaminergic basis for the e f f e c t s . Some of th i s evidence may be obtained by determining whether animals trained to discriminate the individual test stimuli respond to cues associated with their dopaminergic actions. For example, previous studies have shown that the stimulus properties of nicotine may be mediated in part by dopaminergic substrates, as nicotine stimuli can both generalize p a r t i a l l y to the Dl dopamine receptor agonist SKF 38393 and they can be attenuated p a r t i a l l y by the dopamine receptor antagonists haloperidol, pimozide, and Sch 23390 (Reavill & Stolerman, 1988). Likewise, the results of Experiment 8 revealed that the stimulus properties of VTA stimulation may be mediated in part by dopaminergic substrates, as the cueing effects of the stimulation were augmented by amphetamine and attenuated by haloperidol. In view of these findings, i t i s conceivable that the dopaminergic components of the nicotine and brain-stimulation cues might have provided the basis for the summation observed between these test stimuli and the stimulus properties of amphetamine. Indeed, Experiment 7 revealed that the amphetamine-like cueing effects of VTA stimulation were most pronounced when the stimulating electrodes were placed in regions containing high densities of dopamine c e l l bodies. To date, only a limited number of 175 studies have investigated the stimulus properties o£ midazolam, and as yet there have been no attempts to determine whether the dopaminergic actions of thi s drug give r i s e to i t s stimulus properties. Only one of the test stimuli employed in the present thesis was found to exert an e f f e c t on amphetamine stimulus generalization functions that was inconsistent with i t s actions on dopamine neurotransmission. Although morphine appears to enhance mesoaccumbens dopamine neurotransmission (Di Chiara & Imperato, 1986; Glysing & Wang, 1983; Moleman et a l . , 1984), the amphetamine stimulus generalization function obtained after injections of t h i s drug was lower than the vehicle control curve. This unexpected outcome may have resulted from a non-dopaminergic action of morphine that superseded the drug's f a c i l i t a t o r y actions on dopamine function and attenuated the stimulus properties of amphetamine. The possible mechanisms that may have been responsible for t h i s e f f e c t w i l l be discussed further below (p. 180). The important conclusion to be made here is that amphetamine stimulus generalization functions may be influenced by non-dopaminergic actions of drugs, even to the extent that pharmacological effects on dopamine neurotransmission may be obscurred. A second major issue addressed in the present thesis was whether the stimulus properties of amphetamine might summate with those of any test stimulus that could exert positive hedonic actions, or whether such summation might occur only 176 with a limited range of hedonic s t i m u l i . The results of Experiments 2 through 6 provided a clear answer to t h i s question. Of the wide range of test stimuli examined in these experiments, only nicotine and VTA stimulation were observed to elevate amphetamine stimulus generalization functions in a manner suggestive of an additive interaction with the amphetamine s t i m u l i . Morphine and midazolam appeared to interfere with the stimulus properties of amphetamine, whereas ethanol produced a pattern of results which l i k e l y r e f l e c t e d pharmacokinetic effects on amphetamine metabolism rather than an interaction between the drug s t i m u l i . These results indicated that the stimulus properties of amphetamine could summate with only a limited range of positive hedonic test s t i m u l i . The test stimuli that showed summation with the stimulus properties of amphetamine were those whose hedonic actions are mediated by mesoaccumbens dopamine neurons. The rewarding e f f e c t s of cocaine, apomorphine, nicotine, and VTA stimulation a l l appear to involve agonistic actions on mesoaccumbens dopamine neurotransmission (Fibiger et a l . , 1987; Roberts & Vickers, 1987; 1984; Roberts, Corcoran, & Fibiger, 1977; Singer et a l . , 1982; Zito, Vickers, & Roberts, 1985). In contrast, two of the drugs that did not show stimulus summation with amphetamine (midazolam and ethanol) previously have been suggested to have non-dopaminergic substrates for the i r rewarding effects (Amit & Brown, 1982; Finl a y et a l . , 1987). The t h i r d drug that 177 f a i l e d to show such summation was morphine. Although the rewarding effects of morphine may res u l t in part from f a c i l i t a t o r y actions on dopamine neurons in the VTA ( P h i l l i p s & Lepiane, 1980; P h i l l i p s et a l . , 1983; Stewart, 1984; Wise & Bozarth, 1982; 1984) there is some evidence that these effects also may be produced by morphine's actions in the periaqueductal gray, the l a t e r a l hypothalamus and on non-dopaminergic processes within the nucleus accumbens (Koob et a l . , 1987; Olds, 1979; 1982; van der Kooy, Mucha, 0'Shaughnessy, & Bucenieks, 1982). These l a t t e r actions could have interfered with the perception of amphetamine-like stimuli (including those of amphetamine i t s e l f ) and prevented the measurement of additive interactions between the stimulus properties of amphetamine and morphine. Whereas the effects of morphine w i l l require further investigation, the pattern of results obtained with the other test stimuli would appear to suggest that generalization or summation with the stimulus properties of amphetamine might be limited to drugs that produce th e i r positive hedonic properties by exerting f a c i l i t a t o r y actions on dopamine neurotransmission. Implications of the Present Findings for a Theory of the  Stimulus Properties of Amphetamine As discussed above, the results of the present thesis provided support for the general hypotheses that the stimulus properties of amphetamine involve dopaminergic substrates, and that they r e f l e c t the drug's hedonic 178 e f f e c t s . In fact, the present data allow s p e c i f i c statements to be made about the precise neurochemical mechanisms responsible for the transduction of amphetamine's pharmacological actions into a discriminative stimulus, and the s p e c i f i c i t y of the a f f e c t i v e state that constitutes t h i s stimulus at the perceptual l e v e l . With respect to the transduction mechanisms, i t is important that the effects of the various test stimuli on amphetamine generalization functions were observed regardless of the s p e c i f i c means through which the drugs exerted their dopaminergic actions. Although amphetamine's primary action is to stimulate the non-voltage dependent release of dopamine (Chiueh & Moore, 1975), amphetamine-like cueing a c t i v i t y also could be produced by a dopamine receptor agonist (the high dose of apomorphine; Anden et a l . , 1967), by a dopamine reuptake blocker (cocaine; Ritz et a l . , 1987), and by pharmacological and non-pharmacological agents that can increase the impulse-dependent release of dopamine (nicotine and VTA stimulation; Mereau et a l , 1987; Powell, Carr, & Garner, 1987; Fibiger et a l . , 1987). A l t e r n a t i v e l y , the stimulus properties of amphetamine were attenuated by a dopamine receptor antagonist (haloperidol; Anden et a l . , 1970) and by compounds that can i n h i b i t the a c t i v i t y of dopamine neurons (the low dose of apomorphine and midazolam; Grace & Bunney, 1985; Haefley et a l . , 1981). This capacity of the amphetamine stimuli to be influenced by stimuli with diverse actions on dopamine neurotransmission suggests that the 179 common transducer of amphetamine-like cueing effects may be independent from the s p e c i f i c physiological or pharmacological a c t i v i t i e s of the individual test s t i m u l i . These r e s u l t s , and those of previous studies, might best be accounted for by a transduction theory that describes the stimulus properties of amphetamine in terms of the drug's consequences for postsynaptic dopamine receptor a c t i v i t y . S p e c i f i c a l l y , the cueing actions of amphetamine may re s u l t from the capacity of t h i s drug to increase synaptic concentrations of dopamine and thereby increase the amount of agonistic a c t i v i t y at postsynaptic receptors. This emphasis on the postsynaptic receptor a c t i v i t y would account for the results of generalization and summation tests wherein the cueing actions of amphetamine were mimicked both by dopamine agonists that acted d i r e c t l y on the postsynaptic receptors, and by test stimuli that increased the synaptic concentrations of dopamine by f a c i l i t a t i n g i t s release or by blocking i t s reuptake and metabolic degradation. This theory also would account for evidence that the stimulus properties of amphetamine can be attenuated by dopamine receptor antagonists and by compounds that decrease synaptic dopamine concentrations by i n t e r f e r i n g with transmitter synthesis, storage or release (see Silverman & Ho, 1977). It is now well established that there are two separate subtypes of dopamine receptor that exist in the CNS. These receptor subtypes have been referred to as DI and D2 dopamine receptors (Creese, Sibley, Hamblin, & Leff, 1983). 180 Recently, Smith and Lyness (1988) demonstrated that the stimulus properties of amphetamine generalized to the D2 receptor agonist quinpirole but not the Dl receptor agonist SKF 38393. However, both drugs were capable of increasing drug-appropriate responses e l i c i t e d by a low amphetamine dose, and the Dl receptor antagonist SCH 23390 antagonized the cueing effects of amphetamine. S i m i l a r l y complex patterns of results have been observed in other behavioral studies of dopamine receptor subtypes (Waddington, 1986), suggesting that Dl and D2 dopamine receptors may have an interactive influence in the control of dopaminergically mediated behaviors. The results of Smith and Lyness (1988) suggest a similar interactive involvement of the two receptor subtypes in mediating the stimulus properties of amphetamine. As indicated in previous sections of th i s thesis, the stimulus properties of amphetamine appear to resu l t from the drug's f a c i l i t a t o r y actions at dopaminergic synapses within the nucleus accumbens. By increasing the a v a i l a b i l i t y of dopamine at postsynaptic receptors within t h i s structure, amphetamine may i n i t i a t e a chain of physiological events which ultimately results in a positive change in the a f f e c t i v e state of the animal. It is th i s positive a f f e c t i v e state that appears to be discriminated by animals during operant conditioning t r i a l s . The stimulus properties of amphetamine do not appear to represent a general state of positive a f f e c t , as they summated with only a limited range 181 of hedonically positive test stimuli in the present thesis. In fact, such summation only was observed with test stimuli (apomorphine, cocaine, nicotine, and VTA stimulation) that previously have been shown to produce their rewarding effects by f a c i l i t a t i n g dopamine neurotransmission. When rats were tested with drugs that appear to exert their rewarding effects through non-dopaminergic mechanisms (midazolam and ethanol), there was no evidence for summation with the stimulus properties of amphetamine. These results suggested that rats may be able to discriminate between the positive hedonic consequences of enhanced mesoaccumbens dopamine neurotransmission and the positive a f f e c t i v e states that r e s u l t from the a c t i v a t i o n of non-dopaminergic processes. One anomalous r e s u l t that may have important implications for the theory outlined above was the apparent i n h i b i t o r y actions of morphine on the stimulus properties of amphetamine. As indicated in previous sections of t h i s thesis, morphine has been shown to increase both the f i r i n g rate of mesoaccumbens dopamine neurons and e x t r a c e l l u l a r dopamine concentrations within the nucleus accumbens. In behavioral studies, morphine has been found to enhance both the euphoria-producing effects of amphetamine in humans (Jasinski & Preston, 1986) and threshold lowering effects of amphetamine on l a t e r a l hypothalamic ICSS in rats (Hubner et a l . , 1987). Accordingly, i t was anticipated that morphine also would summate with the stimulus properties of 182 amphetamine and elevate the stimulus generalization functions r e l a t i v e to the vehicle control curve. The f a i l u r e of morphine to produce these effects (and i t s actual interference with the amphetamine stimuli) suggested that amphetamine stimulus generalization functions might be influenced by factors unaccounted for in the present theory. The precise mechanisms responsible for morphine's effects on amphetamine stimulus generalization functions could not be determined from the experiments in the present thesis. However, a number of plausible explanations may be offered to account for the r e s u l t s . For example, the non-dopaminergic stimulus properties of morphine might have acted to mask stimuli associated with the combined dopaminergic actions of morphine and amphetamine. These masking stimuli could conceivably be related to other hedonic properties of morphine that r e s u l t from the actions of t h i s drug in the nucleus accumbens and the periaqueductal gray (Olds, 1982; van der Kooy et a l . , 1982). Indeed, masking stimuli are most e f f e c t i v e when they are from the same sensory modality as the discriminative stimuli (Kahneman & Treisman, 1984; Treisman, 1969). A second explanation for morphine's effects takes into account the p o s s i b i l i t y that the stimulus properties of amphetamine may represent a stimulus complex composed of at least two independent subjective phenomena. Amphetamine can produce a v a r i e t y of subjective effects in humans that p o t e n t i a l l y could control discriminated responses in 183 laboratory animals. The i n h i b i t o r y effects of morphine on amphetamine stimuli might have resulted from antagonistic actions on processes (postsynaptic to dopamine projections) that mediate these non-hedonic stimulus properties of amphetamine, such that the overall i n t e n s i t y of the amphetamine stimuli were reduced. Although there is presently no evidence from animal studies to confirm or dispute t h i s hypothesis, morphine has been reported to attenuate the anxiogenic properties of amphetamine in human drug users (Cox et a l . , 1983). One f i n a l explanation for morphine's i n h i b i t o r y e f fects deserves mention here. Although morphine frequently has been suggested to exert f a c i l i t a t o r y actions on dopamine neurotransmission, there have been some reports that morphine a c t u a l l y may act as a dopamine receptor antagonist. At a behavioral l e v e l , morphine can produce effects similar to those of dopamine antagonists such as the induction of catalepsy and the reduction of stereotyped behaviors produced by dopamine agonists (Lai, 1975). At a biochemical l e v e l , morphine has been found to i n h i b i t the a b i l i t y of dopamine to activate adenylate cyclase systems that are necessary for the functioning of DI dopamine receptors (Neff, Parenti, Gentleman, & Olianas, 1981). Neff et a l . (1981) have suggested that morphine can modulate the functioning of dopamine receptors and attenuate their responsiveness to dopamine and dopaminergic agonists. This hypothesis would not c o n f l i c t with evidence that morphine 184 can increase the f i r i n g rate of dopamine neurons and synaptic dopamine concentrations. These effects on dopamine function are commonly observed after administration of dopamine receptor antagonists (Bunney, Chiodo, Grace, & Schenk, 1985; Blaha & Lane, 1984), presumably as a consequence of their a b i l i t y to act at presynaptic dopamine receptors and interfere with the feedback i n h i b i t i o n of dopamine release. However, th i s hypothesis would not explain the contradiction between morphine's i n h i b i t o r y e f fects on amphetamine stimuli and i t s enhancing effects on the hedonic properties of amphetamine (Jasinski & Preston, 1986; Hubner et a l . , 1987). The U t i l i t y of Amphetamine Stimulus Generalization Paradigms  as Screening Procedures for Assessing the Dopaminergic and  Hedonic Properties of Drugs A major reason for conducting the present series of experiments was to determine whether an amphetamine stimulus generalization paradigm might be useful as a screening procedure to assess either the dopaminergic or hedonic actions of various test s t i m u l i . With the exception of the tests with morphine, the results of these experiments indicated that such a paradigm indeed might be useful for assessing the dopaminergic actions of s t i m u l i . In general, amphetamine stimulus generalization functions were elevated by test stimuli that could enhance dopamine neurotransmission and lowered by test stimuli that could interfere with dopamine neurotransmission. Ethanol, which 185 does not exert consistent actions on dopamine neurotransmission (Ellingboe and Mendelson, 1982; Kalant, 1975; Nutt & Glue, 1986), did not have an e f f e c t on the generalization functions indicative of a stimulus interaction. Importantly, these effects were obtained with drugs from diverse pharmacological classes and with a non-pharmacological stimulus produced by e l e c t r i c a l stimulation of the VTA. Thus, the range of stimuli that could interact with the stimulus properties of amphetamine was not limited necessarily by pharmacological class boundries. However, the results of the tests with morphine indicated that the generalization paradigm sometimes might be limited by factors that have yet to be i d e n t i f i e d . The results of the present experiments c l e a r l y indicated that amphetamine stimulus generalization paradigms would not be useful as a general screening procedure for assessing the hedonic properties of s t i m u l i . Amphetamine stimulus generalization functions were elevated only by test stimuli which produce their rewarding effects by f a c i l i t a t i n g dopamine neurotransmission. Such elevations were not observed following injections of drugs whose rewarding ef f e c t s appear to be mediated by non-dopaminergic mechanisms ( i . e . , midazolam and ethanol), nor were they obtained after pretreatment with morphine, a drug that may produce only some of i t s rewarding effects by f a c i l i t a t i n g dopamine neurotransmission ( P h i l l i p s et a l . , 1983; Stewart et a l . , 1984; Wise and Bozarth, 1982). 186 Although the present amphetamine stimulus generalization procedure does not detect the hedonic actions of a l l test s t i m u l i , the paradigm might be useful for a more limited purpose. S p e c i f i c a l l y , tests for summation with the stimulus properties of amphetamine may indicate whether the hedonic properties of a test stimulus are similar to those of amphetamine. Evidence that a test stimulus could elevate amphetamine stimulus generalization functions might suggest a common neural and perceptual basis for the hedonic properties of the amphetamine and test s t i m u l i . The absence of effects or a lowering of the generalization functions would be less informative. These l a t t e r r esults could indicate either that the test stimulus was devoid of hedonic actions, that i t produced i t s hedonic actions through non-dopaminergic mechanisms (as with midazolam and ethanol) or that i t blocked the hedonic properties of amphetamine (as with haloperidol). Furthermore, the results with morphine indicated that certain drugs may lower amphetamine generalization functions even when some of their hedonic properties involve dopaminergic substrates. Although the present stimulus generalization paradigm may be useful for making q u a l i t a t i v e comparisons between amphetamine and test drug states, one should not attempt to use results from amphetamine stimulus generalization experiments to make quantitative statements regarding the r e l a t i v e abuse potential of d i f f e r e n t drugs. The magnitude of summation between the stimulus properties of amphetamine 187 and those of another drug may be influenced by a variety of factors other than the in t e n s i t y of the a f f e c t i v e state produced by the test drug. For example, i t appeared that the a b i l i t y of morphine to summate with the stimulus properties of amphetamine may have been limited by some non-dopaminergic property of the narcotic. Conceivably, other drugs might possess similar properties that would l i m i t the extent to which they elevated amphetamine stimulus generalization functions. Another factor to consider is the extent to which tolerance or s e n s i t i z a t i o n e f f e c t s may influence the degree of summation observed between amphetamine and other drug s t i m u l i . Repeated injections of amphetamine (which are necessary for discrimination training) commonly resu l t in either increases or decreases in the e f f i c a c y of the drug in exerting various behavioral ef f e c t s (Demellweek & Goudie, 1983; Stewart & Vezina, in press). Such chronic amphetamine treatments also may a l t e r the magnitude of psychomotor stimulant-like effects produced by other drugs. For example, repeated amphetamine injections may enhance the locomotor stimulant actions of cocaine and morphine (Kalivas & Weber, 1987; Stewart & Vezina, 1987). At present, there are no data available to indicate the extent to which cross-tolerance and c r o s s - s e n s i t i z a t i o n might influence the magnitude of stimulus summation observed. However, the p o s s i b i l i t y of these effects occurring should be considered when interpreting the changes in amphetamine stimulus generalization functions produced by various test 188 s t i m u l i . Implications of the Present Findings for Theories of Drug  Abuse As indicated in the Introduction, recent theories of drug addiction have emphasized that the positive a f f e c t i v e consequences of drug intake may represent a primary determinant of substance abuse. The role of positive a f f e c t i v e properties in maintaining drug-taking behavior has been established most convincingly for the case of psychomotor stimulant compounds such as amphetamine and cocaine. These drugs appear to produce their hedonic effects by acting d i r e c t l y on dopaminergic processes that o r d i n a r i l y determine a f f e c t i v e and behavioral responses to exteroceptive appetitive motivational stimuli (Blackburn, P h i l l i p s , & Fibiger, 1987; Blackburn, P h i l l i p s , Jakubovic, & Fibiger, 1986; Stewart et a l . , 1984). Recently, i t has been suggested that addictive drugs from a var i e t y of pharmacological classes may produce their hedonic effects by acting on the same neural systems that mediate the hedonic properties of psychomotor stimulants (Wise & Bozarth, 1987; Wise, 1987). Thus, the abuse potential of d i f f e r e n t drugs may r e s u l t from the capacity of each compound either to f a c i l i t a t e dopaminergic neurotransmission or to influence processes afferent or efferent to the dopamine neurons. Indeed, there is now evidence to suggest that dopamine neurons may play an important role in mediating the hedonic actions of opiates 189 (Bozarth, 1987; Bozarth & Wise, 1981; P h i l l i p s & LePiane, 1980; Smith et a l . , 1985; Stewart, 1984; Stewart et a l . , 1984) and nicotine (Singer, Wallace, & H a l l , 1982). However, few studies have investigated the neural basis for the hedonic properties of addictive drugs from other pharmacological classes. The results of the present thesis may have important implications for the theory of drug addiction proposed by Wise and Bozarth (1987) and Wise (1987). If a l l drugs of abuse produced hedonic effects similar to those of psychomotor stimulants, then each addictive drug employed in the present thesis might have possessed amphetamine-like stimulus properties. However, as noted above, only cocaine, apomorphine and nicotine were found to be capable of exerting amphetamine-like cueing e f f e c t s . The three other hedonically positive compounds (morphine, midazolam, and ethanol) a l l f a i l e d to elevate amphetamine stimulus generalization functions in a manner suggestive of an additive interaction with the stimulus properties of amphetamine. In the case of morphine, the absence of summation with the stimulus properties of amphetamine was contrary to expectations based on previous findings that at least some of the rewarding effects of opiates involve dopaminergic substrates ( P h i l l i p s et a l . , 1983; Stewart et a l . , 1984; Wise & Bozarth, 1984). Thus, i t was concluded that the amphetamine-like stimulus properties of t h i s drug may have 190 been obscurred by non-dopaminergic actions that remain to be i d e n t i f i e d . In contrast, the absence of summation between the stimulus properties of amphetamine and those of midazolam or ethanol could reasonably be attributed to an i n a b i l i t y of these test drugs to exert amphetamine-like cueing actions. Unlike the results obtained with morphine, the absence of summation with midazolam and ethanol was consistent with previous reports that these drugs do not exert f a c i l i t a t o r y actions on dopamine neurotransmission (Ellingboe & Mendelson, 1982; F i n l a y et a l . , 1987; Kalant, 1975; Nutt & Glue, 1986). Furthermore, these drugs also f a i l e d to exert stimulant actions during locomotor a c t i v i t y tests with the same animals. Wise and Bozarth (1987) have suggested that any drug which produces i t s rewarding effects by f a c i l i t a t i n g mesoaccumbens dopamine function (or the afferents or efferents) should stimulate locomotor a c t i v i t y as a consequence of a c t i v a t i n g processes that normally regulate appetitive motivational responses to environmental s t i m u l i . Indeed, locomotor stimulant actions have been reported for compounds such as psychomotor stimulants, opiates and nicotine (Clarke & Kumar, 1983b; Swerdlow, Vaccarino, Amalric, & Koob, 1986; Vezina & Stewart, 1984). The f a i l u r e of midazolam and ethanol to produce either amphetamine-like stimulus properties or locomotor stimulant actions suggests that these drugs may not influence the same appetitive motivational processes that appear to be responsible for generating the positive a f f e c t i v e properties of amphetamine. Although the results of t h i s thesis imply the existence of independent neural substrates for the positive a f f e c t i v e properties of certain drugs, i t is also true that many addictive drugs appear to produce their hedonic effects by acting on the same dopaminergic processes that mediate the hedonic actions of amphetamine. Accordingly, studies of the neural processes underlying amphetamine's a f f e c t i v e properties may lead to a general understanding of the mechanisms responsible for the abuse potential of a wide varie t y of drugs. The drug discrimination paradigm employed in t h i s thesis may offer a unique means of investigating these mechanisms. Whereas most procedures for studying the af f e c t i v e properties of drugs assess the changes in overt appetitive motivational responses associated with a drug's actions, measurements of the stimulus properties of amphetamine may provide a means of indexing changes in the more covert a f f e c t i v e reactions to the drug after s p e c i f i c experimental manipulations. By evaluating the cueing e f f i c a c y of amphetamine after various pharmacological challenges, CNS lesions or intracerebral drug injections, we may come to understand the neural processes through which certain drugs of abuse give r i s e to subjective sensations of pleasure and euphoria. 192 REFERENCES Aceto, M.D., Rosencrans, J.A., Young R., & Glennon R.A. (1984). S i m i l a r i t y between (+)-amphetamine and amfonelic aci d . Pharmacology, Biochemistry and Behavior, 20, 635-637. Amit, Z., & Brown Z.W. (1982). Actions of drugs of abuse on brain reward systems: a reconsideration with s p e c i f i c attention to alchohol. Pharmacology, Biochemistry and  Behavior, 17, 233-238. Anden, N.E., Rubensson, A., Fuxe, K., & Hokfelt, T. (1967). Evidence for dopamine receptor stimulation by apomorphine. Journal of Pharmacy and Pharmacology, 19, 627-629. Anden, N.E., Butcher, S.G., Corrodi, H., Fuxe K., & Ungerstedt U. (1970). Receptor a c t i v i t y and turnover of dopamine and noradrenealine after neuroleptics. European  Journal of Pharmacology, 11, 303-314. Ator, N.A., & G r i f f i t h s , R.R. (1987). Self-administration of barbiturates and benzodiazepines: A review. Pharmacology,  Biochemistry and Behavior, 27, 391-398. A u l i s i , E.F., & Hoebel, B.G. (1983). Rewarding effects of amphetamine and cocaine in the nucleus accumbens and block by alpha-flupenthixol. Society for Neuroscience Abstracts, i / 121. Baker, T.B., Morse E., & Sherman J.E. (1986). The motivation to use drugs: A psychobiological analysis of urges. Nebraska  Symposium on Motivation, 3_4, 2 57-32 3. Bandler, R. & Flynn, J.P. (1971). Visual patterned r e f l e x present during hypothalamically e l i c i t e d attack. Science, 171, 817-818. Beagley, W.K., & Holley, T.L. (1977). Hypothalamic stimulation f a c i l i t a t e s c o n t r a l a t e r a l v i s u a l control of a learned response. Science, 196, 321-322. Beninger, R.J., Hanson, D.R., & P h i l l i p s , A.G. (1981). The a c q u i s i t i o n of responding with conditioned reinforcement: effects of cocaine, (+)-amphetamine and pipradol. B r i t i s h  Journal of Pharmacology, 74., 149-154. Bindra, D., & Reichert, H., (1966). Dissociation of movement i n i t i a t i o n without d i s s o c i a t i o n of response choice. Psychonomic Science, 4., 95-96. Bindra D. & Reichert, H., (1967). The nature of d i s s o c i a t i o n : effects of t r a n s i t i o n s between normal and barbiturate-induced states on reversal learning and habituation. Psychopharmacologia, 10., 330-344 . 193 Blackburn, J.R., P h i l l i p s , A.G., Jakubovic, A., & Fibiger, H.C. (1986). Cues s i g n a l l i n g meal onset produce increases in dopamine turnover. Society for Neuroscience Abstracts, 12, 1139. Blackburn, J.R., P h i l l i p s , A.G., & Fibiger, H.C. (1987). Dopamine and preparatory behavior: I. effects of pimozide. Behavioral Neuroscience, 101, 3 52-36 0. Blaha, CD., & Lane, R.F. (1984 ). Direct in vivo electrochemical monitoring of dopamine release in response to neuroleptic drugs. European Journal of Pharmacology, 9 8, 113-117. Blaha, CD., P h i l l i p s , A.G., & Fibiger, H.C. (1988). S e l f -stimulation of the ventral tegmentum and concurrent release of dopamine measured by in vivo electrochemistry. Society  for Neurosciences Abstracts, 14., 7 42. B l i s s , D.K. (1974). Theoretical explanations of drug-dissociated behaviors. Federation Proceedings, .33., 1787-1796. Bozarth, M.A. (ed.). (1988). Methods of Assessing the  Reinforcing Properties of Abused Drugs. New York: Springer-Verlag. Bozarth, M.A. (1987). Neuroanatomical boundries of the reward-relevant opiate-receptor f i e l d in the ventral tegmental area as mapped by the conditioned place preference method in ra t s . Brain Research, 414, 77-84. Bozarth, M.A., & Wise, R.A. (1981). Intracranial s e l f -administration of morphine into the ventral tegmental area in r a t s . L i f e Sciences, 28, 551-555. Brady, J.V., G r i f f i t h s , R.R., Hienz, R.D., Ator, N.A., Lukas, S.E., & Lamb, R.J. (1988). Assessing drugs for abuse l i a b i l i t y and dependence potential in laboratory primates. In M.A. Bozarth (ed.), Methods of Assessing the Reinforcing  Properties of Abused Drugs (pp. 45-85). New York: Springer-Verlag. Bunney, B.S., Chiodo, L.A., Grace, A.A., & Schenk, J.O. (1985). In Vivo effects of acute and chronic antipsychotic drug administration on midbrain dopaminergic neuron a c t i v i t y . In L.S. Seiden & R.L. Balster (eds.), Behavioral  Pharmacology: The Current Status (pp. 205-220). New York: Alan R. L i s s . Carr, CD., & White, N.M. (1983). Conditioned place preference from intra-accumbens but not intra-caudate amphetamine inj e c t i o n s . L i f e Sciences, 33., 2551-2557. 194 Carr, G.D., & White, N.M. (1986). Anatomical disassociation of amphetamine's rewarding and aversive e f f e c t s : an in t r a c r a n i a l microinjection study. Psychopharmacology, 89, 340-346. Carr, G.D., Fibiger, H.C., & P h i l l i p s , A.G. (In press). Conditioned place preference as a measure of drug reward. In J.M. Liebman, & S.J. Cooper (eds.), The Neuropharmacological  Basis of Reward. New York: Oxford University Press. Chance, W.T., Murfin, D., Krynock, G.M., & Rosecrans, J.A. (1978). A description of the nicotine stimulus and tests of i t s generalization to amphetamine. Psychopharmacology, 55, 19-26. Chiueh, C.C., & Moore, K.E. (1975). D-amphetamine induced release of newly synthesized and stored dopamine from the caudate nucleus in vivo. Journal of Pharmacology and  Experimental Therapeutics, 19 2, 642-653. Clarke, P.B.S., & Kumar R. (1983a). Nicotine does not improve discrimination of brain stimulation reward by rats . Psychopharmacology, 79, 271-277. Clarke, P.B.S., & Kumar R. (1983b). The effects of nicotine on locomotor a c t i v i t y in non-tolerant and tolerant r a t s . B r i t i s h Journal of Pharmacology, 78, 329-337. Colpaert, F.C. (1977a). Se n s i t i z a t i o n and desensitization to l a t e r a l hypothalamic stimulation. Archives of International  Pharmacodynamics, 230, 319-320. Colpaert, F.C. (1977b). Drug-produced cues and states: some t h e o r e t i c a l and methodological inferences. In H. Lai (ed.), Discriminative Stimulus Properties of Drugs (pp. 5-21). New York: Plenum Press. Colpaert, F.C. (1978a). Discriminative stimulus properties of narcotic analgesic drugs. Pharmacology, Biochemistry and  Behavior, 9, 863-887. Colpaert, F.C. (1978b). Some properties of drugs as physiological signals: the FR procedure and signal detection theory. In F.C. Colpaert & J.A. Roscrans (eds.), Stimulus Properties of Drugs: Tens Years of Progress (pp 53-68). Amsterdam: Elsevier/North-Holland Press. Colpaert, F.C. (1988). Drug discrimination: methods of manipulation, measurement, and analysis. In M.A. Bozarth (ed.), Methods of Assessing the Reinforcing Properties of Abused Drugs (pp. 341-372). New York: Springer-Verlag. 195 Colpaert, F.C, Niemegeers, C.J.E., & Janssen, P.A.J. (1977) . Haloperidol blocks the discriminative stimulus properties of l a t e r a l hypothalamic stimulation. European  Journal of Pharmacology, 42, 9 3-9 7. Colpaert, F.C, Niemegeers, C.J.E., & Janssen, P.A.J. (1978) . Discriminative stimulus properties of cocaine and d-amphetamine, and antagonism by haloperidol: a comparative study. Neuropharmacology, 1.7, 937-942. Colpaert, F.C, Niemegeers, C.J.E., & Janssen, P.A.J. (1979) . Discriminative stimulus properties of cocaine: neuropharmacological c h a r a c t e r i s t i c s as derived from stimulus generalization experiments. Pharmacology, Biochemistry and Behavior, 10., 535-546. Colpaert, F.C, Kuyps, J.M.D., Niemegeers, C.J.E., & Janssen, P.A.J. (1976). Discriminative stimulus properties of low dl-amphetamine dose. Archives of International  Pharmacodynamics, 223., 3 4-42 . Corbett, D., & Wise, R.A., (1980). Intracranial s e l f -stimulation in r e l a t i o n to the ascending dopaminergic systems of the midbrain: a moveable electrode mapping study. Brain Research, 185, 1-15. Cox, T.C, Jacobs, M.R., Leblanc, A.E., & Marshman, J.A. (1983). Drugs and Drug Abuse. Toronto: Addiction Research Foundation. Creese, I., Sibley, D.R., Hamblin, M.W., & Leff, S.E. (1983). C l a s s i f i c a t i o n of dopamine receptors: Relationship to radioligand binding. Annual Review of Neurosciences, 6, 43-71. - - - - -Davis, W.M., & Smith, S.G. (1988). Conditioned reinforcement as a measure of the rewarding properties of drugs. In M.A. Bozarth (ed.), Methods of Assessing the Reinforcing Properties of Abused Drugs (pp 199-209). New York: Springer-Verlag. Demellweek, C , & Goudie, A.J. (1983). Behavioral tolerance to amphetamine and other psychostimulants: The case for considering behavioral mechanisms. Psychopharmacology, 80, 287-307. Di Chiara, C , & Imperato, A. (1986). P r e f e r e n t i a l stimulation of dopamine release in the nucleus accumbens by opiates, alcohol, and barbiturates: Studies with transcerebral d i a l y s i s in f r e e l y moving rat s . Annals New  York Academy of Sciences, 473, 50-69. 1 9 6 D ' M e l l o , G . D . ( 1 9 8 1 ) . A C o m p a r i s o n o f s o m e b e h a v i o r a l e f f e c t s o f a m p h e t a m i n e a n d e l e c t r i c a l b r a i n s t i m u l a t i o n o f t h e m e s o l i m b i c d o p a m i n e s y s t e m i n r a t s . P s y c h o p h a r m a c o l o g y , 7 5 , 1 8 4 - 1 9 2 . D r u h a n , J . P . ( 1 9 8 5 ) . P h a r m a c o l o g i c a l a s s e s s m e n t o f t h e r e l a t i o n s h i p b e t w e e n c u e p r o p e r t i e s a n d r e w a r d i n g e f f e c t s o f e l e c t r i c a l s t i m u l a t i o n o f t h e v e n t r a l t e g m e n t a l a r e a . U n p u b l i s h e d M a s t e r ' s t h e s i s . D r u h a n , J . P . , F i b i g e r , H . C . , & P h i l l i p s , A . G . ( I n p r e s s ) . D i f f e r e n t i a l e f f e c t s o f c h o l i n e r g i c d r u g s o n d i s c r i m i n a t i v e c u e s a n d s e l f - s t i m u l a t i o n p r o d u c e d b y v e n t r a l t e g m e n t a l a r e a s t i m u l a t i o n . P s y c h o p h a r m a c o l o g y D r u h a n , J . P . , M a r t i n - I v e r s o n , M . T . , W i l k i e , D . M . , F i b i g e r , H . C . , & P h i l l i p s , A . G . ( 1 9 8 7 a ) . D i s s o c i a t i o n o f d o p a m i n e r g i c a n d n o n - d o p a m i n e r g i c s u b s t r a t e s f o r c u e s p r o d u c e d b y e l e c t r i c a l s t i m u l a t i o n o f t h e v e n t r a l t e g m e n t a l a r e a . P h a r m a c o l o g y , B i o c h e m i s t r y a n d B e h a v i o r , 2 8 , 2 5 1 - 2 5 9 . D r u h a n , J . P . , M a r t i n - I v e r s o n , M . T . , W i l k i e , D . M . , F i b i g e r , H . C . , & P h i l l i p s , A . G . ( 1 9 8 7 b ) . D i f f e r e n t i a l e f f e c t s o f p h y s o s t i g m i n e o n c u e s p r o d u c e d b y e l e c t r i c a l s t i m u l a t i o n o f t h e v e n t r a l t e g m e n t a l a r e a u s i n g t w o d i s c r i m i n a t i o n p r o c e d u r e s . P h a r m a c o l o g y , B i o c h e m i s t r y a n d B e h a v i o r , 2 8 , 2 6 1 - 2 6 5 . E l l i n g b o e , J . , & M e n d e l s o n , J . H . ( 1 9 8 2 ) . B i o c h e m i c a l p h a r m a c o l o g y o f a l c o h o l . I n F . H o f f m e i s t e r a n d G . S t i l l e ( e d s . ) , H a n d b o o k o f E x p e r i m e n t a l P h a r m a c o l o g y , V o l 5 5 / 1 1 1 ( p p 2 0 9 - 2 3 7 ) . B e r l i n : S p r i n g e r - V e r l a g . E l l i n w o o d , E . H . , E i b e r g e n , R . D . , & K i l b e y , M . M . ( 1 9 7 6 ) . S t i m u l a n t s : I n t e r a c t i o n w i t h c l i n i c a l l y r e l e v a n t d r u g s . A n n a l s o f t h e N e w Y o r k A c a d e m y o f S c i e n c e s , 2 8 1 , 3 9 3 - 4 0 8 . E s p o s i t o , R . U . , & K o r n e t s k y , C . ( 1 9 7 8 ) . O p i o d s a n d r e w a r d i n g b r a i n s t i m u l a t i o n . N e u r o s c i e n c e a n d B i o b e h a v i o r a l R e v i e w s , 2 , 1 1 5 - 1 2 2 . F a l k , J . L . , & T a n g , M . ( 1 9 8 5 ) . M i d a z o l a m o r a l s e l f -a d m i n i s t r a t i o n . D r u g a n d A l c o h o l D e p e n d e n c e , 1 5 , 1 5 1 - 1 6 3 . F a l k , J . L . , S a m s o n , H . H . , & W i n g e r , G . ( 1 9 7 2 ) . B e h a v i o r a l m a i n t e n a n c e o f h i g h c o n c e n t r a t i o n s o f b l o o d e t h a n o l a n d p h y s i c a l d e p e n d e n c e i n t h e r a t . S c i e n c e , 1 7 7 , 8 1 1 - 8 1 3 . F a l l o n , J . H . , & M o o r e , R . Y . ( 1 9 7 8 ) . C a t e c h o l a m i n e i n n e r v a t i o n o f t h e b a s a l f o r e b r a i n . I V . T o p o g r a p h y o f t h e d o p a m i n e p r o j e c t i o n t o t h e b a s a l f o r e b r a i n a n d n e o s t r i a t u m . J o u r n a l o f C o m p a r a t i v e N e u r o l o g y , 1 8 0 , 5 4 5 - 5 8 0 . F e r r i s , R . M . , T a n g , F . L . M . , & M a x w e l l , R . A . ( 1 9 7 2 ) . A 197 comparison of the capacities of isomers of amphetamine, deoxypipradrol, and methylphenidate to i n h i b i t the uptake of t r i t i a t e d catecholamines into rat cerebral cortex s l i c e s , synaptosomal preparations of the rat cerebral cortex, hypothalamus and striatum and into adrenergic nerves of rabbit aorta. Journal of Pharmacology and Experimental  Therapeutics, 181, 407-416. Fibi g e r , H.C. (1978). Drugs and reinforcement mechanisms: a c r i t i c a l review of the catecholamine theory. Annual Review  of Pharmacology and Toxicology, 18., 37-56. Fibiger, H.C, LePiane, F.C, Jakubovic, A., & P h i l l i p s , A.G. (1987). The role of dopamine in i n t r a c r a n i a l s e l f -stimulation of the ventral tegmental area. The Journal of  Neuroscience, 7, 3888-3896. Finlay, J.M., Szostak, C , Blaha, CD., Lane, R.F., & Fibiger, H.C (1987). Self-administration of midazolam may be associated with a decrease in dopamine release in the nucleus accumbens. Society for Neurosciences Abstracts, 14, 4 49. Fudala, P.J., & Iwamoto, E.T. (1986). Further studies on nicotine-induced conditioned place preference in the r a t . Pharmacology, Biochemistry and Behavior, 25, 1041-1049. Fudala, P.J., Teoh, K.W., & Iwamoto, E.T. (1985). Pharmacologic characterization of nicotine-induced conditioned place-preference. Pharmacology, Biochemistry and  Behavior, 22, 237-241. Garcha, H.S., Rose, I.C and Stolerman, I.P. (1985). Midazolam cue in rat s : Generalization tests with a n x i o l y t i c and other drugs. Psychopharmacology, 87., 233-237. Gessa, G.L., Muntoni, F., Collu, M., Vargiu, L., & Mereu, G. (1985). Low doses of ethanol activate dopamine neurons in the ventral tegmental area. Brain Research, 348, 201-204. Gonon, F.C, & Buda, M.J. (1985). Regulation of dopamine release by impulse flow and by autoreceptors as studied by in vivo voltammetry in the rat striatum. Neuroscience, 14., 765-774. Goudie, A.J., Atkinson, J., & West, C.R. (1986). Discriminative properties of the psychostimulant d l -cathinone in a two lever operant task: lack of evidence for dopaminergic mediation. Neuropharmacology, 25, 85-94. Grace, A.A., & Bunney, B.S. (1985). Low doses of apomorphine e l i c i t two opposing influences on dopamine c e l l electrophysiology. Brain Research, 333, 285-298. Gratton, A. & Wise, R.A. (1983). Brain stimulation reward 198 in the l a t t e r a l hypothalamic medial forebrain bundle: mapping of boundaries and homogeneity. Brain Research, 274, 25-30. Gratton, A. & Wise, R.A. (1985). Mapping of contraversive and i p s i v e r s i v e c i r c l i n g responses to ventral tegmental and substantia nigra e l e c t r i c a l stimulation. Physiology and  Behavior, 35, 61-65. G r i f f i t h , J.D. (1977). Amphetamine dependence; c l i n i c a l features. In W.R. Martin (ed.), Handbook of Experimental  Pharmacology, Vol 45/11 (pp 277-304). B e r l i n : Springer-Verlag. G r i f f i t h s , R.R., Lukas, S.E., Bradford, L.D., Brady, J.V., & Snel l , J.D. (1981). S e l f - i n j e c t i o n of barbiturates and benzodiazepines in Baboons. Psychopharmacology, 75, 101-109. Gysling, K., & Wang, R.Y. (1983). Morphine induced ac t i v a t i o n of A10 dopamine neurons in the r a t . Brain  Research. 277, 119-127. Haefely, W., P i e r i , L., Pole, P., & Shaffner, R. (1981). General pharmacology and neuropharmacology of benzodiazepine derivatives. In F. Hoffmeister and G. S t i l l e (eds.), Handbook of Experimental Pharmacology, Vol 55/11 (pp 13-211). B e r l i n : Springer-Verlag. Hellevuo, K., & Kiianmaa, K. (1988). Effects of ethanol, b a r b i t a l and lorazepam on brain monoamines in rat lines s e l e c t i v e l y outbred for d i f f e r e n t i a l s e n s i t i v i t y to ethanol. Pharmacology Biochemistry and Behavior, 29, 18 3-18 8. Henningfield, J.E., & Goldberg S.R. (1983). Nicotine as a reinforcer in human subjects and laboratory animals. Pharmacology Biochemistry and Behavior, 19., 989-992. Hernandez, L.L., Holohean, A.M., & Appel, J.B. (1978). Effects of opiates on the discriminative stimulus properties of dopamine agonists. Pharmacology Biochemistry and  Behavior, 9, 459-463. Ho, B.T., & Huang, J.T. (1975). Role of dopamine in d-amphetamine-induced discriminative responding. Pharmacology  Biochemistry and Behavior, 3., 1085-1092. Ho, B.T., & McKenna, M.L. (1978). Discriminative stimulus properties of central stimulants. In: B.T. Ho, D.W Richards, IIJ and D.L. Chute (eds.), Drug Discrimination and State  Dependent Learning (pp 67-77). New York: Academic Press. 199 Ho, B.T., & Silverman, P.B. (1978). Stimulants as discriminative s t i m u l i . In: F.C. Colpaert & J.A. Rosencrans (eds.), Stimulus Properties of Drugs: Ten Years of Progress (pp 53-68). Amsterdam: Elsevier/North-Holland Press. Huang, J.T., & Ho, B.T. (1974a). Discriminative stimulus properties of d-amphetamine and related compounds in rats . Pharmacology, Biochemistry and Behavior, 2, 669-67 3. Huang, J.T., & Ho, B.T. (1974b). Effects of nikethamide, picrotoxin and strychnine on 'amphetamine state'. European  Journal of Pharmacology, 29, 17 5-17 8. Huang, D., & Wilson, M.C. (1986). Comparative discriminative stimulus properties of dl-cathinone, d-amphetamine, and cocaine in ra t s . Pharmacology, Biochemistry and Behavior, 24/ 205-210. Hubner, C.B., Bain, G.T., & Kornetsky, C. (1987). The combined effects of morphine and d-amphetamine on the threshold for brain stimulation reward. Pharmacology, Biochemistry and Behavior, 28, 311-315. Imperato, A., & Di Chiara, G. (1986). P r e f e r e n t i a l stimulation of dopamine release in the nucleus accumbens of fr e e l y moving rats by ethanol. Journal of Pharmacology and  Experimental Therapeutics, 239, 219-228. Imperato, A., Mulas, A., & Di Chiara, G. (1986). Nicotine p r e f e r e n t i a l l y stimulates dopamine release in the limbic system of f r e e l y moving r a t s . European Journal of  Pharmacology, 132, 337-338. Jaf f e , J.H. (1987). Pharmacological agents in the treatment of drug dependence. In H.Y. Meltzer (ed.), Psychopharmacology: The Third Generation of Progress (pp 1605-1616). New York: Raven Press. Jarbe, T.U.C. (1982). Discriminative stimulus properties of d-amphetamine in pigeons. Pharmacology, Biochemistry and  Behavior, 17, 671-675. Jarbe, T.U.C, & Swedberg, M.D.B. (1982). A conceptualization of drug discrimination learning. In F.C. Colpaert & J.L. Slangen (eds.), Drug discrimination: Applications in CNS Pharmacology (pp 327-341). Amsterdam: Elsevier Biomedical Press. Jarbe, T.U.C, Svensson, R., & Laaksonen, T., (1983). Conditioning of a discriminative stimulus: Overshadowing and blocking l i k e procedures. Scandinavian Journal of  Psychology, 24, 325-330. 200 J a s i n s k i , D.R. (1977). Assessment of the abuse p o t e n t i a l i t y of morphinelike drugs (methods used i n man). In W.R. M a r t i n (ed.) Handbook of Experimental Pharmacology, Vol 45/1 (pp 197-258). B e r l i n : S p r i n g e r - V e r l a g . J a s i n s k i , D.R., & P r e s t o n , K.L. (1986). E v a l u a t i o n of mixtures of morphine and d-amphetamine f o r s u b j e c t i v e and p h y s i o l o g i c a l e f f e c t s . Drug and A l c o h o l Dependence, 17., 1-13. Jones, C.N., H i l l , H.F., & H a r r i s , R.T. (1974). D i s c r i m i n a t i v e response c o n t r o l by d-amphetamine and r e l a t e d compounds i n the r a t . Psychopharmacologia, 36, 347-356. Kahneman, D., & Treisman, A. (1984). Changing views of a t t e n t i o n and a u t o m a t i c i t y . In R. Parasuraman & D.R. Davies (eds.) V a r i e t i e s of A t t e n t i o n (pp 29-61). New York: Academic P r e s s . Kalant, H. (1975). D i r e c t e f f e c t s of ethanol on the nervous system. F e d e r a t i o n Proceedings, 34, 1930-1941. K a l i v a s , P.W., & Weber, B. (1987). Role of the A10 dopamine neurons i n s e n s i t i z a t i o n to cocaine and amphetamine. S o c i e t y  f o r Neuroscience A b s t r a c t s , 13., 600. Koob, G.F., V a c c a r i n o , F . J . , Amalric, M., & Swerdlow, N.R. (1987). Neural s u b s t r a t e s f o r cocaine and o p i a t e r e i n f o r c e m e n t. In S. F i s h e r , A. Raskin and E.H. Uhlenhuth (eds.) Cocaine: C l i n i c a l and B i o b e h a v i o r a l Aspects (pp 80-107). London: Oxford U n i v e r s i t y P r e s s . Konig, J.R., & K l i p p e l , R.A. (1963). The Rat B r a i n : A  S t e r e o t a x i c A t l a s . B a l t i m o r e : W i l l i a m s & W i l k i n s . Kornetsky, C , & E s p o s i t o , R.U. (1981). Reward and d e t e c t i o n t h r e s h o l d s f o r b r a i n s t i m u l a t i o n : d i s s o c i a t i v e e f f e c t s of c o c a i n e . B r a i n Research, 209, 496-500. K s i r , C , & K l i n e , E . J . (1987). 6-OHDA l e s i o n s of nucleus accumbens block the s t i m u l a n t e f f e c t s of n i c o t i n e i n r a t s . S o c i e t y f o r Neuroscience A b s t r a c t s , 13., 4 4 7 . L a i , H. (1975). N a r c o t i c dependence, n a r c o t i c a c t i o n and dopamine r e c e p t o r s . L i f e S c i e n c e s , 17, 483-496. L a i , H. (1977). Drug induced d i s c r i m i n a b l e s t i m u l i : past r e s e a r c h and f u t u r e p e r s p e c t i v e s . In H. L a i (ed. ) D i s c r i m i n a t i v e Stimulus P r o p e r t i e s of Drugs (pp 207-231). New York: Plenum P r e s s . 201 Lane, R.F., & Blaha, C D (1986). E l e c t r o c h e m i s t r y i n V i v o : A p p l i c a t i o n to CNS pharmacology. Annals New York Academy of Sc i e n c e s , 4,7,3,, 50-69. L e t t , B.T. (1983). P a v l o v i a n d r ug-sickness p a i r i n g s r e s u l t i n the c o n d i t i o n i n g of an a n t i s i c k n e s s response. B e h a v i o r a l  Neuroscience, 97, 779-784. Lyness, W.H., F r i e d l e , N.M., & Moore, K.E. (1979). D e s t r u c t i o n of dopaminergic nerve t e r m i n a l s i n the nucleus accumbens: E f f e c t on d-amphetamine s e l f - a d m i n i s t r a t i o n . Pharmacology, B i o c h e m i s t r y and Behavior, 11, 553-556. Mackey, W.B, & van der Kooy, D. (1985). N e u r o l e p t i c s block the p o s i t i v e r e i n f o r c i n g e f f e c t s of amphetamine but not morphine as measured by plac e c o n d i t i o n i n g . Pharmacology,  B i o c h e m i s t r y and Behavior, 22, 101-105. Mackintosh, N.J. (1974). The Psychology of Animal L e a r n i n g . London: Academic P r e s s . M a r t i n , CM., Bechara, A., & van der Kooy, D.J. (1987). D i s s o c i a t i o n of morphine's d i s c r i m i n a t i v e s t i m u l u s p r o p e r t i e s from i t s m o t i v a t i o n a l p r o p e r t i e s . S o c i e t y f o r  Neuroscience A b s t r a c t s , 13., 1546. M a r t i n , W.R., & J a s i n s k i , D.R. (1977). Assessment of the abuse p o t e n t i a l of n a r c o t i c a n a l g e s i c s i n animals. In W.R. M a r t i n (ed.), Handbook of Experimental Pharmacology, V o l 4 5/1 (pp 159-196). B e r l i n : S p r i n g e r - V e r l a g . M e l l o , N.K. (1981). B e h a v i o r a l pharmacology of a l c o h o l . In F. Hoffmeister and C S t i l i e ( e d s . ) , Handbook of Experimental Pharmacology, Vol 55/111. (pp 177-208). B e r l i n : S p r i n g e r - V e r l a g . M e l l o , N.K., & Mendelson, J.H. (1977). C l i n i c a l aspects of a l c o h o l dependence. In W.R. Mar t i n (ed.), Handbook of  Experimental Pharmacology, Vol 45/1 (pp 613-666). B e r l i n : Spr i n g e r - V e r l a g . Mereu, C , Yoon, K.P., B o i , B., Gessa, G.L., Naes, L., & W e s t f a l l , T.C. (1987). P r e f e r e n t i a l s t i m u l a t i o n of v e n t r a l tegmental area dopaminergic neurons by n i c o t i n e . European  J o u r n a l of Pharmacology, 14,1, 395-399. M i l l a r , J . , Stamford, J.A., Kruk, Z.L., & Wightman, R.M. (1985). E l e c t r o c h e m i c a l , pharmacological and e l e c t r o p h y s i o l o g i c a l evidence of r a p i d dopamine r e l e a s e and removal i n the r a t caudate nucleus f o l l o w i n g e l e c t r i c a l s t i m u l a t i o n of the median f o r e b r a i n bundle. European J o u r n a l  of Pharmacology, 109, 341-348. 202 Mogenson, G.J., Takigawa, M., Robertson, A., & Wu. M. (1979). Self-stimulation of the nucleus accumbens and ventral tegmental area of Tsai attenuated by microinjections of spiroperidol into the nucleus accumbens. Brain Res, 171, 247-259. Moleman, P., van Valkenburg, C.F.M., & van der Krogt, J.A. (1984). Effects of morphine on dopamine metabolism in rat striatum and limbic structures in r e l a t i o n to the a c t i v i t y of dopaminergic neurons. Archives of Pharmacology, 327, 208-213. Monaco, A.P., Hernandez, L., & Hoebel, B.G. (1980). Nucleus accumbens: Site of amphetamine i n j e c t i o n : comparison with the l a t e r a l v e n t r i c a l . In R.B. Chronister and J.F. DeFrance (eds.), The Neurobiology of the Nucleus Accumbens (pp 338-342). New Brunswick: Haer I n s t i t u t e . Mucha, R.F., van der Kooy, D., 0 1Shaughnessy, M., & Bucenieks, P. (1982). Drug reinforcement studied by the use of place conditioning in the ra t . Brain Research, 243, 91-105. Nakahara, D., & Ikeda, T. (1984). D i f f e r e n t i a l behavioral responsiveness to i p s i l a t e r a l and con t r a l a t e r a l v i s u a l stimuli produced by u n i l a t e r a l rewarding hypothalamic stimulation in the rat. Physiology and Behavior, 32., 1005-1010. Neff, N.H., Parenti, M., Gentleman, S., & Olianas, M.C. (1981). Modulation of dopamine receptors by opiates. In G.L. Gessa and G.U. Corsini (eds.), Apomorphine and Other  Dopaminometics, Vol 1: Basic Pharmacology (pp 193-200). New York: Raven Press. Newman, M.L. (1972). Effects of cholinergic agonists and antagonists on sel f - s t i m u l a t i o n behavior in the ra t . Journal  of Comparative and Physiological Psychology, 79./ 394-413. Nielsen, E.B., & Jepsen, S.A. (1985). Antagonism of the amphetamine cue by both c l a s s i c a l and atyp i c a l antipsychotic drugs. European Journal of Pharmacology, 111, 167-176. Nielsen, E.B., & Scheel-Kruger, J. (1986). Cueing effects of amphetamine and LSD: E l i c i t a t i o n by di r e c t microinjection of the drugs into the nucleus accumbens. European Journal of  Pharmacology, 125, 8 5-92. Numan, R. (1981). Multiple exposures to ethanol f a c i l i t a t e intravenous self-administration of ethanol by ra t s . Pharmacology, Biochemistry and Behavior, 15., 101-10 8. 203 Nutt, D. & Glue, P. (1986). Monoamines and a l c o h o l . B r i t i s h  J o u r n a l of A d d i c t i o n , 81, 3 27-338. Olds, M.E. (1982). R e i n f o r c i n g e f f e c t s of morphine i n the nucleus accumbens. B r a i n Research, 237, 429-440. Olds, M.E. & Domino, E.F. (1969). Comparison of m u s c a r i n i c and n i c o t i n i c c h o l i n e r g i c a g o n i s t s on s e l f - s t i m u l a t i o n behavior. The J o u r n a l of Pharmacology and Experimental T h e r a p e u t i c s , 166, 189-204. Overton, D.A. (1978). Major t h e o r i e s of s t a t e dependent l e a r n i n g . In B.T. Ho, D.W. R i c h a r d s , I I I , & D.L. Chute (ed s . ) , Drug D i s c r i m i n a t i o n and State Dependent L e a r n i n g (pp 283-318). New York: Academic P r e s s . Overton, D.A. (1982). Comparison of the degree of d i s c r i m i n a b i l i t y of v a r i o u s drugs using the T-maze drug d i s c r i m i n a t i o n paradigm. Psychopharmacology, 76, 385-395. Overton, D.A. (1988). A p p l i c a t i o n s and l i m i t a t i o n s of the drug d i s c r i m i n a t i o n method f o r the study of drug abuse. In M.A. Bozarth (ed.), Methods of A s s e s s i n g the R e i n f o r c i n g P r o p e r t i e s , o f Abused Drugs (pp 291-340). New York: S p r i n g e r -V e r l a g . Overton, D.A., & B a t t a , S.K. (1977). R e l a t i o n s h i p between abuse l i a b i l i t y of drugs and t h e i r degree of d i s c r i m i n a b i l i t y i n the r a t . In T. Thompson & K.R. Unna (eds. ), P r e d i c t i n g Dependence L i a b i l i t y of Stimulant and  Depressant Drugs (pp 125-135). B a l t i m o r e , MD: U n i v e r s i t y Park P r e s s . Overton, D.A., & Hayes, M.W. (1984). Optimal t r a i n i n g parameters i n the two-bar f i x e d - r a t i o drug d i s c r i m i n a t i o n t a s k . Pharmacology, B i o c h e m i s t r y and Behavior, 21, 19-25. P h i l l i p s , A.G., & F i b i g e r , H.C. (1978). The r o l e of dopamine i n m a i t a i n i n g i n t r a c r a n i a l s e l f - s t i m u l a t i o n i n the v e n t r a l tegmentum, nucleus accumbens and medial p r e f r o n t a l c o r t e x . Canadian J o u r n a l of Psychology, 32, 58-66. P h i l l i p s , A.G., & LePiane, F.G. (1980). R e i n f o r c i n g e f f e c t s of morphine m i c r o i n j e c t i o n i n t o the v e n t r a l tegmental area. Pharmacology, B i o c h e m i s t r y and Behavior, 12, 965-968. P h i l l i p s , A.G., & LePiane, F.G. (1982). Reward produced by m i c r o i n j e c t i o n of (d-ala)-met enkephalinamide i n t o the v e n t r a l tegmental area. B e h a v i o r a l B r a i n Research, 5, 225-229 . 204 P h i l l i p s , A.G., Broekkamp, C.L., & F i b i g e r , H.C. (1983). S t r a t e g i e s f o r s t u d y i n g the neurochemical s u b s t r a t e s of drug reinforcement i n rodents. Progress i n Neuro-Psychopharmacology and B i o l o g i c a l P s y c h i a t r y , X, 585-590. P h i l l i p s , A.G., Mora, F., & R o l l s , E.T. (1981). I n t r a c e r e b r a l s e l f - a d m i n i s t r a t i o n of amphetamine by rhesus monkeys. Neuroscience L e t t e r s , 24, 81-8 6. P o r s o l t , R.D., Pawelec, C , & J a l f r e , M. (1982). Use of a drug d i s c r i m i n a t i o n procedure to d e t e c t amphetamine-like e f f e c t s of a n t i d e p r e s s a n t s . In F.C. C o l p a e r t and J.L. Slangen (ed s . ) , D r u g D i s c r i m i n a t i o n : A p p l i c a t i o n s i n CNS  Pharmacology (pp 193-202). Amsterdam, E l s e v i e r Biomedical P r e s s . Powell, P.P., Ca r r , L.A., & Garner, A.C. (1987). S t i m u l a t i o n of [3H]dopamine r e l e a s e by n i c o t i n e i n r a t nucleus accumbens. J o u r n a l of Neurochemistry, 49, 1449-1454. R e a v i l l , C , & Stolerman, I.P. (1988). I n t e r a c t i o n of n i c o t i n e with dopaminergic mechanisms assesed through drug d i s c r i m i n a t i o n and r o t a t i o n a l behavior i n r a t s . J o u r n a l of  Psychopharmacoloqy, 1, 264-273. Revusky, S., & Harding, R.K. (1986). P a i r i n g p e n t o b a r b i t a l with one t o x i n causes i t to attenuate t a s t e a v e r s i o n s produced by a d i f f e r e n t t o x i n : i m p l i c a t i o n s f o r c o n d i t i o n e d a n t i s i c k n e s s theory. B e h a v i o r a l Neuroscience, 100, 6 8 5-69 4. Revusky, S., T a u k u l i s , H.K., & Peddle, C. (1979). Learned a s s o c i a t i o n s between drug s t a t e s : attempted a n a l y s i s i n P a v l o v i a n terms. P h y s i o l o g i c a l Psychology, 7., 352-363. R i t z , M.C., Lamb, R.J., Goldberg, S.R., & Kuhar, M.J. (1987). cocaine r e c e p t o r s on dopamine t r a n s p o r t e r s are r e l a t e d to s e l f - a d m i n i s t r a t i o n of cocaine, Science, 237, 1219-1223. Robbins, T.W., Watson, B.A., Gaskin, M., & Ennis, C. (1983). C o n t r a s t i n g i n t e r a c t i o n s of p i p r a d r o l , d-amphetamine, coc a i n e , cocaine analogues, apomorphine and other drugs with c o n d i t i o n e d r e i n f o r c e m e n t . Psychopharmacology, 80, 113-119. Roberts, D.C.S., & V i c k e r s , G. (1984). A t y p i c a l n e u r o l e p t i c s i n c r e a s e s e l f - a d m i n i s t r a t i o n of c o c a i n e : and e v a l u a t i o n of a b e h a v i o r a l screen f o r a n t i p s y c h o t i c a c t i v i t y . Psychopharmacology, 82, 135-139. Roberts, D.C.S., & V i c k e r s , G. (1987). Increased m o t i v a t i o n to s e l f - a d m i n i s t e r apomorphine f o l l o w i n g 6-hydroxydopamine l e s i o n s of the nucleus accumbens. S o c i e t y f o r Neuroscience  A b s t r a c t s , 13., 448. 205 Roberts, D.C.S., Corcoran, M.E., & F i b i g e r H.C. ( 1 9 7 7 ) . On the r o l e of ascending catecholamine systems i n s e l f -a d m i n i s t r a t i o n of c o c a i n e . Pharmacology, B i o c h e m i s t r y and  Behavior, 6, 6 1 5 - 6 2 0 . Roberts, D.C.S., Koob, G.F., K l o n n o f f , P., & F i b i g e r H.C. ( 1 9 8 0 ) . E x t i n c t i o n and r e c o v e r y of cocaine s e l f -a d m i n i s t r a t i o n f o l l o w i n g 6-hydroxydopamine l e s i o n s of the nucleus accumbens. Pharmacology, B i o c h e m i s t r y and Behavior, 12 , 6 8 1 - 6 8 7 . Schaefer, G.J., & M i c h a e l , R.P. ( 1 9 8 5 ) . The d i s c r i m i n a t i v e s timulus p r o p e r t i e s and d e t e c t i o n t h r e s h o l d s of i n t r a c r a n i a l s e l f - s t i m u l a t i o n : E f f e c t s of d-amphetamine, morphine and h a l o p e r i d o l . Psychomarmacology, 85 , 2 8 9 - 2 9 4 . Schaefer, G.J., & M i c h a e l , R.P. ( 1 9 8 7 ) . T a s k - s p e c i f i c e f f e c t s of n i c o t i n e i n r a t s : I n t r a c r a n i a l s e l f - s t i m u l a t i o n and locomotor a c t i v i t y . Neuropharmacology, 2 5 , 1 2 5 - 1 3 1 . Schecter, M.D. ( 1 9 7 7 ) . Amphetamine d i s c r i m i n a t i o n as a t e s t f o r a n t i - P a r k i n s o n i s m drugs. European J o u r n a l of Pharmacology, 4 4, 5 1 - 5 6 . Schecter, M.D., & Cook, P.G. ( 1 9 7 5 ) . Dopaminergic mediation of the i n t e r o c e p t i v e cue produced by d-amphetamine i n r a t s . Psychopharmacologia, 42 , 1 8 5 - 1 9 3 . Schecter, M.D., & Rosecrans, J.A. ( 1 9 7 3 ) . D-amphetamine as a d i s c r i m i n a t i v e cue: drugs with s i m i l a r s t i m u l u s p r o p e r t i e s . European J o u r n a l of Pharmacology, 2JL, 2 1 2 - 2 1 6 . S i e g a l , S. ( 1 9 8 3 ) . C l a s s i c a l c o n d i t i o n i n g , drug t o l e r a n c e , and drug dependence. In Y. I s r a e l , F.B. G l a s e r , H. K a l a n t , R.E. Popham, W. Schmidt, and R.G. Smart (ed s . ) , Research  Advances i n A l c o h o l and drug problems, v o l . 7. New York:Plenum Press Signs, S.A., & Schecter, M.D. (1988 ). The r o l e of dopamine and s e r o t o n i n r e c e p t o r s i n the mediation of the ethanol i n t e r o c e p t i v e cue. Pharmacology, B i o c h e m i s t r y and Behavior, 30 , 5 5 - 6 4 . Silverman, P.B., & Ho, B.T. ( 1 9 7 7 ) . C h a r a c t e r i z a t i o n of d i s c r i m i n a t i v e response c o n t r o l by psychomotor s t i m u l a n t s . In: H. L a i (ed.) D i s c r i m i n a t i v e Stimulus P r o p e r t i e s of Drugs (pp 1 0 7 - 1 1 9 ) . New York: Plenum P r e s s . Singer, G., Wallace, M., & H a l l , R. ( 1 9 8 2 ) . E f f e c t s of dopaminergic nucleus accumbens l e s i o n s on the a c q u i s i t i o n of schedule induced s e l f i n j e c t i o n of n i c o t i n e i n the r a t . Pharmacology, B i o c h e m i s t r y and Behavior, 1 7 , 579 -581 206 Smith, CM. (1977). The pharmacology of s e d a t i v e / h y p n o t i c s , a l c o h o l , and a n e s t h e t i c s : S i t e s and mechanisms of a c t i o n . In W.R. M a r t i n (ed.) Handbook of Experimental Pharmacology, Vol  45/1 (pp 613-666). B e r l i n : S p r i n g e r - V e r l a g . Smith, F.L., & Lyness, W.H. (1988). The r o l e of s p e c i f i c dopamine r e c e p t o r subtypes i n d-amphetamine (AMPH) d i s c r i m i n a t i o n . S o c i e t y f o r Neurosciences A b s t r a c t s , 14., 807. Smith, J.E., Guerin, G.F., Co, C , Barr, T.S., & Lane, J.D. (1985). E f f e c t s of 6-OHDA l e s i o n s of the c e n t r a l medial nucleus accumbens on r a t intravenous morphine s e l f -a d m i n i s t r a t i o n . Pharmacology, B i o c h e m i s t r y and Behavior, 2.3., 843-849. Smith, J.E., S h u l t z , K., Co, C , Goeders, N.E., & Dworkin, S.I. (1987). E f f e c t s of 5,7-dihydroxytryptamine l e s i o n s of the nucleus accumbens on r a t intravenous morphine s e l f -a d m i n i s t r a t i o n . Pharmacology, B i o c h e m i s t r y & Behavior, 26., 607-612. S p y r a k i , C , F i b i g e r , H.C, & P h i l l i p s , A . C (1982). Dopaminergic s u b s t r a t e s of amphetamine-induced plac e preference c o n d i t i o n i n g . B r a i n Research, 253, 185-193. Stewart, J . , (1984). Reinstatement of h e r o i n and cocaine s e l f - a d m i n i s t r a t i o n behavior i n the r a t by i n t r a c e r e b r a l a p p l i c a t i o n of morphine i n the v e n t r a l tegmental a r e a . Pharmacology, B i o c h e m i s t r y and Behavior, 20, 917-9 23. Stewart, J . , & de Wit, H. (1988). Reinstatement of drug-t a k i n g behavior as a method of a s s e s s i n g i n c e n t i v e m o t i v a t i o n a l p r o p e r t i e s of drugs. In M.A. Bozarth (ed.) Methods of A s s e s s i n g the R e i n f o r c i n g P r o p e r t i e s of Abused Drugs (pp 291-34611, New York: S p r i n g e r - V e r l a g . Stewart, J . , & Vezina, P. (1987). E n v i r o n m e n t - s p e c i f i c enhancement of the h y p e r a c t i v i t y induced by systemic or intra-VTA morphine i n j e c t i o n s i n r a t s preexposed to amphetamine. Psychobiology, 15, 144-153. Stewart, J . , & Vezina, P. (In P r e s s ) . C o n d i t i o n i n g and b e h a v i o r a l s e n s i t i z a t i o n . In P.W. K a l i v a s & C D . Barnes (eds.), S e n s i t i z a t i o n i n the Nervous System. CaIdwe11, N.J.: T e l f o r d P r e s s . Stewart, J . , de Wit, H., & Eikelboom, R. (1984). Role of unconditioned and c o n d i t i o n e d drug e f f e c t s i n the s e l f -a d m i n i s t r a t i o n of o p i a t e s and s t i m u l a n t s . P s y c h o l o g i c a l Review, 91, 211-227. 207 Swerdlow, N.R., Va c c a r i n o , F . J . , Amalric, M., & Koob, G.F. (1986). The ne u r a l s u b s t r a t e s f o r the m o t o r - a c t i v a t i n g p r o p e r t i e s of psy c h o s t i m u l a n t s : A review of rec e n t f i n d i n g s . Pharmacology, B i o c h e m i s t r y and Behavior, 25, 233-248. Szostak, C , F i n l a y , J.M., & F i b i g e r , H.C. (1987). Intravenous s e l f - a d m i n i s t r a t i o n of the s h o r t - a c t i n g benzodiazepine midazolam i n the r a t . Neuropharmacology, 26, 1673-1676. T a y l o r , J.R., & Robbins, T.W. (1984). Enhanced b e h a v i o u r a l c o n t r o l by c o n d i t i o n e d r e i n f o r c e r s f o l l o w i n g m i c r o i n j e c t i o n s of d-amphetamine i n t o the nucleus accumbens. Psychopharmacology, 84, 405-412. Treisman, A.M. (1969). S t r a t e g i e s and models of s e l e c t i v e a t t e n t i o n . P s y c h o l o g i c a l Review, 76, 282-299. van der Kooy, D., Mucha, R.F., 0 1Shaughnessy, M., & Bucenieks, P. (1982). R e i n f o r c i n g e f f e c t s of b r a i n m i c r o i n j e c t i o n s of morphine r e v e a l e d by c o n d i t i o n e d p l a c e -p r e f e r e n c e . B r a i n Research, 243, 107-117. Vezina, P., & Stewart, J . (1984). C o n d i t i o n i n g and p l a c e -s p e c i f i c s e n s i t i z a t i o n of i n c r e a s e s i n a c t i v i t y induced by morphine i n the VTA. Pharmacology, B i o c h e m i s t r y and  Behavior, 20, 925-934. V i l l a r r e a l , J.E., & S a l a z a r , L.A. (1981). The dependence producing p r o p e r t i e s of psychomotor s t i m u l a n t s . In F. Hoffmei s t e r and G. S t i l l e ( e d s . ) , Handbook of Experimental  Pharmacology, Vol 55/111 (pp 607-635). B e r l i n : S p r i n g e r -V e r l a g . Waddington, J.L. (1986) B e h a v i o r a l c o r r e l a t e s of the a c t i o n of s e l e c t i v e D - l dopamine r e c e p t o r a n t a g o n i s t s : Impact of SCH 23390 and SKF 83566 and f u n c t i o n a l l y i n t e r a c t i v e D-l:D-2 re c e p t o r systems. Biochemical Pharmacology, 35, 3661-3667. Wagner, G.C., Prest o n , K., R i c a u r t e , G.A., Schuster, C.R., & Seiden, L.S. (1982). Neurochemical s i m i l a r i t i e s between d l -cathinone and d-amphetamine. Drug and A l c h o h o l Dependence, 9, 279-284. Weeks, J.R., & C o l l i n s , R.J. (1988). Screening f o r drug reinforcement u s i n g intravenous s e l f - a d m i n i s t r a t i o n i n the r a t . In M.A. Bozarth (ed.) Methods of A s s e s s i n g the  R e i n f o r c i n g P r o p e r t i e s of Abused Drugs (pp 35-43). New York: Spr i n g e r - V e r l a g . Westerink, B.H.C. (1979). The e f f e c t s of drugs on dopamine b i o s y n t h e s i s and metabolism i n the b r a i n . In A.S. Horn, J . Korf and B.H.C. Westerink (e d s . ) , The Neurobiology of Dopamine (pp 255-291). London: Academic P r e s s . 208 White, N.M., Messier, C , & Carr, G.D. (1988). O p e r a t i o n a l i z i n g and measuring the o r g a n i z i n g i n f l u e n c e of drugs on behavior. In M.A. Bozarth (ed.), Methods of A s s e s s i n g the R e i n f o r c i n g P r o p e r t i e s of Abused Drugs (pp 591-617). New York: S p r i n g e r - V e r l a g . Wikler, A. (1973). C o n d i t i o n i n g of s u c c e s s i v e adaptive responses to the i n i t i a l e f f e c t s of drugs, C o n d i t i o n a l  R e f l e x , 8, 193-210. W i l k i n , L.D., Cunningham, C.L., & F i t z g e r a l d , R.D. (1982). P a v l o v i a n c o n d i t i o n i n g with ethanol and l i t h i u m : e f f e c t s on heart r a t e and t a s t e a v e r s i o n i n r a t s . J o u r n a l of  Comparative and P h y s i o l o g i c a l Psychology, 96, 781-790. Wise, R.A. (1978). Catecholamine t h e o r i e s of reward: A c r i t i c a l review. B r a i n Research, 152, 215-247. Wise, R.A. (1982). N e u r o l e p t i c s and operant behavior: The anhedonia h y p o t h e s i s . B e h a v i o r a l and B r a i n S c i e n c e s , 5, 39-87. Wise, R.A. (1987). The r o l e of reward pathways i n the development of drug dependence. Pharmac Ther, 35, 227-263. Wise, R.A., & Bozarth, M.A. (1984). B r a i n reward c i r c u i t r y : Four c i r c u i t elements "wired" i n apparent s e r i e s . B r a i n  Research B u l l e t i n , 297, 265-273. Wise, R.A., & Bozarth, M.A. (1987). A psychomotor s t i m u l a n t theory of a d d i c t i o n . P s y c h o l o g i c a l Review, 94, 469-492. Woolverton, W.L., & Cervo, L. (1986). E f f e c t s of c e n t r a l dopamine d e p l e t i o n on the d-amphetamine d i s c r i m i n a t i v e stimulus i n r a t s . Psychopharmacology, 88, 196-200. Yokel, R.A. (1988). Intravenous s e l f - a d m i n i s t r a t i o n : response r a t e s , the e f f e c t s of pharmacological c h a l l e n g e s , and drug p r e f e r e n c e s . In M.A. Bozarth (ed.) Methods of A s s e s s i n g the R e i n f o r c i n g P r o p e r t i e s of Abused Drugs (pp 1-33). New York: S p r i n g e r - V e r l a g . Z e t t e r s t r o m , T., & Ungerstedt, U. (1984). E f f e c t s of apomorphine on the i n v i v o r e l e a s e of dopamine and i t s m e t a b o l i t e s , s t u d i e d by b r a i n d i a l y s i s . European J o u r n a l of  Pharmacology, 97, 29-36. Z i t o , K.A., V i c k e r s , G., & Roberts, D.C.S. (1985). D i s r u p t i o n of cocaine and h e r o i n s e l f - a d m i n i s t r a t i o n f o l l o w i n g k a i n i c a c i d l e s i o n s of the nucleus accumbens. Pharmacology, Bio c h e m i s t r y and Behavior, 23., 1029-1036. 

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