AN J J i VIVO ELECTROCHEMICAL ANALYSIS OF THE ROLE OF DOPAMINE I N FEEDING BEHAVIORS By LORINDA JEAN HOLMES B.Sc, University of Victoria, 1988 A THESIS SUBMITTED I N P A R T I A L FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Neuroscience We a c c e p t t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF B R I T I S H COLUMBIA A u g u s t 1990 ®Lorinda J e a n H o l m e s , 1990 In presenting this degree at the thesis in University of partial fulfilment of of department this thesis for or by his or requirements British Columbia, I agree that the freely available for reference and study. I further copying the representatives. an advanced Library shall make it agree that permission for extensive scholarly purposes may be her for It is granted by the understood that head of copying my or publication of this thesis for financial gain shall not be allowed without my written permission. Department of T \ o XK^O^cXfi^t The University of British Columbia Vancouver, Canada Date DE-6 (2/88) QuKj^h , t^fj p Abstract The involvement of dopamine i n a n t i c i p a t o r y and consummatory aspects of feeding behaviors was i n the present activity thesis. A l l measurements of dopaminergic were taken by i n vivo e l e c t r o c h e m i c a l In Experiment 1, dopamine e f f l u x investigated i n the nucleus techniques. accumbens and caudate of male r a t s was monitored during s e s s i o n s in which a s m a l l , u n s i g n a l l e d l i q u i d meal was consumed. Increases i n the e l e c t r o c h e m i c a l activity, measure of dopamine which were of s i m i l a r temporal p a t t e r n and magnitude, were observed i n both the nucleus accumbens and s t r i a t u m f o l l o w i n g meal consumption. possible postingestional structures. These data suggest a r o l e of dopamine i n these two b r a i n In Experiment 2, a c o n d i t i o n e d feeding paradigm was u t i l i z e d to study the r o l e of dopamine during a d i s c r e t e a n t i c i p a t o r y phase of feeding. Rats were c o n d i t i o n e d d i s c r i m i n a t e between a p o s i t i v e c o n d i t i o n e d stimulus p r e d i c t i v e of meal d e l i v e r y , stimulus electrochemical (CS+) and a negative c o n d i t i o n e d (CS-) that was not a s s o c i a t e d i n dopamine a c t i v i t y , to with food. Increases as determined by changes i n o x i d a t i o n c u r r e n t s , were found to be d u r i n g the CS+ than during the C S - i n both the accumbens and caudate. greater nucleus In a d d i t i o n , the magnitude of increase was greater i n the nucleus accumbens than the caudate, suggesting that the accumbens may be p r e f e r e n t i a l l y iii involved i n the p r o c e s s i n g of external incentive s t i m u l i . The r e s u l t s support a r o l e f o r dopamine i n both the nucleus accumbens and caudate during a p p e t i t i v e or a n t i c i p a t o r y responding f o r food i n the male r a t . iv TABLE OF CONTENTS Abstract i i L i s t of Figures vi Acknowledgements viii INTRODUCTION Experimental Background 1 T h e o r e t i c a l Background 8 Rationale f o r the a p p l i c a t i o n o f e l e c t r o c h e m i s t r y to the study o f feeding i n the r a t 14 GENERAL METHODS Electrode preparation 17 Surgery 17 Recording procedure 18 Histology 19 EXPERIMENT I E f f e c t s o f an u n s i g n a l l e d l i q u i d electrochemical meal on measures o f DA a c t i v i t y 22 Methods 23 Results 25 Discussion 42 V EXPERIMENT I I E l e c t r o c h e m i c a l measures o f dopamine a c t i v i t y d u r i n g a n t i c i p a t o r y and consummatory phases o f f e e d i n g b e h a v i o r s i n the r a t 44 Methods 45 Results 52 Discussion 81 GENERAL DISCUSSION 85 REFERENCES 92 vi L i s t of Figures Figure Figure Figure Figure Figure Figure Figure Figure Figure 1 2 3 4 5 6 7 8 9 F i g u r e 10 F i g u r e 11 Example of a t y p i c a l voltammetric ramped at 10 mv/s sweep, 20 Chronoamperometric s i g n a l recorded from the caudate and nucleus accumbens o f r a t #1 before and a f t e r the d e l i v e r y of a lOcc S u s t a c a l meal 26 Chronoamperometric s i g n a l recorded from the caudate and nucleus accumbens of r a t #2 before and a f t e r the d e l i v e r y of a lOcc S u s t a c a l meal 28 Chronoamperometric s i g n a l recorded from the caudate and nucleus accumbens of r a t #3 before and a f t e r the d e l i v e r y of a lOcc S u s t a c a l meal 30 Chronoamperometric s i g n a l recorded from the caudate and nucleus accumbens of r a t #4 before and a f t e r the d e l i v e r y of a lOcc S u s t a c a l meal 32 R e p r e s e n t a t i v e chronoamperometric r e c o r d from the caudate and nucleus accumbens of a s i n g l e r a t on the f i r s t day of testing 34 Average chronoamperometric s i g n a l recorded from the caudate and nucleus accumbens of r a t s 1, 2, 3, and 4 before and a f t e r the d e l i v e r y of a lOcc Sustacal meal . . 36 E l e c t r o d e placements d i r e c t e d at the l e f t nucleus accumbens and r i g h t caudate i n Experiment 1 39 Mean amount of time spent i n the f e e d i n g niche d u r i n g the CS+ p e r i o d p r i o r to the onset o f food d e l i v e r y 50 Mean l a t e n c y to enter niche onset 55 f o l l o w i n g CS Mean changes i n DA o x i d a t i o n c u r r e n t i n the nucleus accumbens and caudate d u r i n g CS+, CS- and pre-CS b a s e l i n e p e r i o d s . . 57 vii F i g u r e 12 F i g u r e 13 F i g u r e 14 F i g u r e 15 F i g u r e 16 F i g u r e 17 F i g u r e 18 R e p r e s e n t a t i v e chronoamperometric r e c o r d s o b t a i n e d from four d i f f e r e n t s u b j e c t s d u r i n g c o n s e c u t i v e CS+ and C S - t r i a l s . . 59 B e h a v i o r a l and e l e c t r o c h e m i c a l from e x t i n c t i o n s u b j e c t #1 results 64 B e h a v i o r a l and e l e c t r o c h e m i c a l from e x t i n c t i o n s u b j e c t #2 results B e h a v i o r a l and e l e c t r o c h e m i c a l from e x t i n c t i o n s u b j e c t #3 results R e p r e s e n t a t i v e CS+ sweep t r i a l subject from one 67 70 73 Mean peak, h e i g h t r a t i o s from 10 CS+ and 10 C S - voltammetric sweep t r i a l s recorded from the nucleus accumbens of a single rat 75 E l e c t r o d e placements d i r e c t e d at the nucleus accumbens and r i g h t caudate i n Experiment 2 77 left Acknowledgements I would l i k e to thank Chuck Blaha, Mike Jung, Fred LePiane, Jim Pfaus, and Tony P h i l l i p s for the generous a s s i s t a n c e they provided d u r i n g the p r e p a r a t i o n of t h i s thesis. 1 INTRODUCTION Numerous s t u d i e s have i m p l i c a t e d dopaminergic systems i n feeding behaviour, but c u r r e n t l y , there consensus as to the exact nature of t h i s first section of t h i s that have a s s o c i a t e d i s no c l e a r involvement. i n t r o d u c t i o n reviews The experimental dopaminergic a c t i v i t y with feeding. T h e o r e t i c a l models t h a t have been proposed to e x p l a i n r o l e of dopamine i n feeding behaviors are d i s c u s s e d The i n t r o d u c t i o n concludes rationale for the use o f , measurements different with a b r i e f i n vivo data the next. d e s c r i p t i o n of, and electrochemical of e x t r a c e l l u l a r dopamine l e v e l s d u r i n g phases o f feeding i n the r a t . Experimental Background Selective d e s t r u c t i o n of n i g r o s t r i a t a l dopamine (DA) neurons has been shown to produce a syndrome of aphagia and adipsia ( F i b i g e r , Z i s , & McGeer, Similarly, Ungerstedt, pharmacological blockade of dopamine has r e s u l t e d and 1973; receptors i n a d i s r u p t i o n of feeding b e h a v i o u r s . S c l a f a n i (1981) found that the DA a n t a g o n i s t suppressed consumption of a p a l a t a b l e solution in a dose-related deprivation period. decreased food intake manner a f t e r The DA antagonist 1971). Xenakis pimozide saccharin-glucose a 4-hr food spiroperidol also i n r a t s r e s t r i c t e d to a 4-hr feeding 2 period (Heffner, Zigmond, and S t r i e k e r , 1977). Because reduced dopaminergic a c t i v i t y has been l i n k e d with the a t t e n u a t i o n of feeding, i n c r e a s e d DA a c t i v i t y might be expected to f a c i l i t a t e feeding. (1987) observed s i g n i f i c a n t Evans and i n c r e a s e s i n food intake meal d u r a t i o n when they administered bromocriptine and the DA agonists behaviours s e l e c t i v e to e t a l . (1977) have shown t h a t the DA amphetamine and apomorphine reduce food deprived r a t s . Electrical tegmental area (VTA) and & Phillips, agonist intake i n food- the s u b s t a n t i a n i g r a (SN) 1987; and also neurons ( F i b i g e r , LePiane, P h i l l i p s & F i b i g e r , 1979). a d d i t i o n , feeding has been e l i c i t e d by s t i m u l a t i o n of the VTA However, s t i m u l a t i o n of the v e n t r a l i n c r e a s e s s y n a p t i c a c t i v i t y of DA Jakubovic, food (grooming, d r i n k i n g , a c t i v i t y ) were not a f f e c t e d s i g n i f i c a n t l y by the drugs. Heffner and d-amphetamine i n t r a c e r e b r o v e n t r i c u l a r l y . They a l s o noted that the a c t i v a t i o n was i n t a k e , as other Eikelboom SN, In electrical both o r i g i n s of ascending dopaminergic pathways ( P h i l l i p s Se F i b i g e r , 1973). The evidence r o l e of DA unnatural either presented so f a r d e a l s s o l e l y with systems i n feeding behaviours conditions. the of the r a t under That i s , a l l s u b j e c t s were t e s t e d i n a s t a t e of reduced or e l e v a t e d DA produced by the experimenter. activity More b i o l o g i c a l l y r e l e v a n t data have emerged from s t u d i e s that have attempted to c o r r e l a t e dopaminergic a c t i v i t y and n a t u r a l circumstances, without f e e d i n g under more producing pharmacological or 3 physical a l t e r a t i o n s of brain a c t i v i t y . conducted One such study was by H e f f n e r , Hartman, and Seiden (1980). Rats maintained on a f e e d i n g schedule c o n s i s t i n g o f 20 hours o f food d e p r i v a t i o n f o l l o w e d by 4 hours o f access to food p e l l e t s were d e c a p i t a t e d e i t h e r before food d e l i v e r y or a f t e r one hour o f access to food. acid Dihydroxyphenylacetic (DOPAO/DA r a t i o s were determined by radioenzymatic assay to p r o v i d e i n d i c e s o f dopaminergic r e s e a r c h e r s found e l e v a t e d r a t i o s activity. These i n the nucleus accumbens, hypothalamus, and amygdala o f d e p r i v e d r a t s g i v e n access to food compared to animals that were not f e d or to nondeprived c o n t r o l s . R a t i o s remained unchanged i n the s t r i a t u m , o l f a c t o r y t u b e r c l e , f r o n t a l c o r t e x , and septum. Chance, F o l e y - N e l s o n , Nelson, and F i s c h e r (1987) r e p o r t e d e l e v a t e d l e v e l s o f the DA m e t a b o l i t e s DOPAC and HVA i n the nucleus accumbens and s t r i a t u m o f c h r o n i c a l l y deprived r a t s sacrificed f o l l o w i n g one hour of access to food, as compared to values from c o n t r o l r a t s s a c r i f i c e d i n a deprived s t a t e . In t h i s l a t e r study, n e u r o t r a n s m i t t e r and metabolite measurements were conducted using high performance on r e g i o n a l b r a i n homogenates l i q u i d chromatography with e l e c t r o c h e m i c a l d e t e c t i o n (HPLC/ED). and Smith Simansky, (1985) a l s o employed HPLC/ED to determine r a t i o s i n r a t s under v a r i o u s feeding c o n d i t i o n s . observed Bourbonais, DOPAC/DA They that DOPAC/DA r a t i o s were i n c r e a s e d i n the hypothalamus o f r a t s maintained on a d a i l y 4-hr food-access schedule when they were measured a f t e r one hour of access to 4 food or simply to f o o d - r e l a t e d s t i m u l i alone, as compared to nondeprived c o n t r o l s or deprived r a t s not exposed to food or food-related stimuli. They d i d not observe any similar r a t i o changes i n the nucleus accumbens, s t r i a t u m , o l f a c t o r y t u b e r c l e , or amygdala. A s i g n i f i c a n t advance i n the a n a l y s i s of a c t i v i t y and feeding behaviours o c c u r r e d with a p p l i c a t i o n of c h r o n i c i n vivo techniques dopaminergic the to measure e x t r a c e l l u l a r l e v e l s of n e u r o t r a n s m i t t e r s i n f r e e l y moving animals. One HPLC/ED, was of these techniques, m i c r o d i a l y s i s with employed by Church, J u s t i c e , and N e i l l These r e s e a r c h e r s found that r a t s t r a i n e d on a 1-min interval feeding schedule striatal DA the 15-min feeding p e r i o d s . DA Hoebel 15 to 20 min after Using the same technique, (1988) observed increased e x t r a c e l l u l a r l e v e l s i n the nucleus accumbens of food-deprived during feeding s e s s i o n s . fixed showed i n c r e a s e d e x t r a c e l l u l a r l e v e l s d u r i n g and extending Hernandez and (1987). rats E x t r a c e l l u l a r l e v e l s of DA i n the nucleus accumbens a l s o were e l e v a t e d , but no changes were seen i n the s t r i a t u m of deprived r a t s b a r - p r e s s i n g for food. In another study employing m i c r o d i a l y s i s and HPLC/ED, Radhakishun, van Ree, and Westerink (1988) observed i n c r e a s e i n DA e f f l u x i n the nucleus accumbens that c o r r e l a t e d with the onset of a scheduled an was feeding s e s s i o n . L e v e l s of DA remained e l e v a t e d d u r i n g the e n t i r e food intake period. Another i n v i v o technique, voltammetry, has been used 5 to study the r o l e o f DA i n f e e d i n g behaviours. Keller, S t r i e k e r , and Zigmond (1983) found a v a r i e t y o f exteroreceptive stimuli including t a i l shock, ice-water bath, and food a f t e r a 24 hour d e p r i v a t i o n p e r i o d , to i n c r e a s e voltammetric s i g n a l s i n the r a t s t r i a t u m . the e l e c t r o d e s used Although i n t h i s study were not s e l e c t i v e f o r DA ( i . e . , DOPAC, AA, and DA may have a l l c o n t r i b u t e d to the s i g n a l o b t a i n e d ) , the r e s u l t s o f pharmacological manipulations suggested that the changes i n the e l e c t r o c h e m i c a l s i g n a l were due mainly to i n c r e a s e d DA efflux. In v i v o voltammetric a n a l y s e s o f e x t r a c e l l u l a r l e v e l s o f the DA m e t a b o l i t e HVA (used as an index o f DA a c t i v i t y ) were observed r e c e n t l y i n the r a t caudate during l e v e r p r e s s i n g f o r food reward Gray, 1989). exposure nucleus (Joseph, Hodges, and D'Angio and S c a t t o n (1989) found feeding or to food odors alone, both r e s u l t e d i n increased e x t r a c e l l u l a r DOPAC l e v e l s i n the anteromedial p r e f r o n t a l c o r t e x as determined by i n v i v o voltammetric measurements. In v i v o voltammetric data obtained by L o u i l o t , Le Moal, and Simon (198S), argued a g a i n s t the h y p o t h e s i s that the observed i n c r e a s e s i n dopaminergic to the i n d u c t i o n o f motor a c t i v i t y . a c t i v i t y are due simply Exposure of a test s u b j e c t to an a g g r e s s i v e male caused an i n c r e a s e i n the DA a c t i v i t y o f the t e s t s u b j e c t , even though i t typically assumed a f r o z e n stance. As i n d i c a t e d by the evidence reviewed so f a r , appears t h a t dopaminergic it systems a r e i n v o l v e d i n feeding 6 behaviors. However, there i s s t i l l u n c e r t a i n t y as to which aspects o f feeding are a s s o c i a t e d with dopaminergic and which b r a i n s t r u c t u r e s are i n v o l v e d . P h i l l i p s , and F i b i g e r activity Blackburn, (1987) s t r e s s e d the u t i l i t y o f f a c t o r i n g motivated behaviors i n t o t h e i r component p a r t s and a n a l y z i n g the r o l e o f DA w i t h i n each component. These authors emphasize the major d i s t i n c t i o n between consummatory and p r e p a r a t o r y behaviours. With r e s p e c t to f e e d i n g , they d e f i n e consummatory behaviours as "those that occur after the animal has made c o n t a c t with food and that r e s u l t in its i n g e s t i o n " and give " b i t i n g , chewing, and swallowing" as examples o f t h i s category. Preparatory behaviours are d e f i n e d as " a p p e t i t i v e acts that t y p i c a l l y lead t o , and make p o s s i b l e , consummatory behaviours". S t u d i e s by these and other r e s e a r c h e r s suggest that dopaminergic a c t i v i t y may p l a y a g r e a t e r r o l e i n preparatory than consummatory feeding behaviours. U n l i k e the r e s u l t s of e a r l i e r s t u d i e s with DA a n t a g o n i s t s , Blackburn e t a l . (1987) found that pimozide d i d not s i g n i f i c a n t l y attenuate food i n t a k e , but d i d a f f e c t p r e p a r a t o r y responses. was The procedure followed i n t h i s based on a c o n d i t i o n e d feeding d e s i g n by (1984). Using t h i s experimental procedure, study Weingarten the r e s e a r c h e r s found t h a t a p p e t i t i v e responses to a c o n d i t i o n e d s t i m u l u s (CS+) onset o f food d e l i v e r y were s i g n a l l i n g the subsequent a t t e n u a t e d by the a d m i n i s t r a t i o n o f pimozide, whereas consummatory behaviours appeared normal when the food was 7 delivered. In a r e l a t e d study, the DA a n t a g o n i s t s metoclopramide and t h i o r i d a z i n e had d i f f e r e n t i a l e f f e c t s on p r e p a r a t o r y behaviour 1989a). (Blackburn, P h i l l i p s , & Fibiger, L i k e pimozide, metoclopramide attenuated p r e p a r a t o r y responses to the c o n d i t i o n e d s t i m u l u s , whereas t h i o r i d a z i n e d i d not. I t was suggested that the e f f e c t s o f metoclopramide r e f l e c t a p r e f e r e n t i a l a c t i o n of t h i s drug i n the caudate nucleus. conditioned feeding paradigm, i n c r e a s e d DOPAC/DA and HVA/DA r a t i o s were observed (though s t r i a t a l In another study employing the same i n the nucleus accumbens and s t r i a t u m values d i d not reach s i g n i f i c a n c e ) a f t e r exposure statistical to the CS+ alone (Blackburn, P h i l l i p s , Jakubovic, & F i b i g e r , 1989b). No i n c r e a s e s i n e i t h e r r a t i o were found i n r a t s allowed to ingest an u n s i g n a l l e d meal. A l l measurements were made on postmortem t i s s u e by HPLC/EC a n a l y s i s . Further support f o r the h y p o t h e t i c a l r o l e of DA i n p r e p a r a t o r y behaviours comes from the a t t e n u a t i o n o f hoarding, a form of p r e p a r a t o r y f e e d i n g behaviour, the d i s r u p t i o n o f mesolimbic ( K e l l e y & S t i n u s , 1985). related stimuli following DA neurons by 6-OHDA l e s i o n s Others have found that food- (odors, s i g h t s and sounds a s s o c i a t e d with scheduled feeding) i n the absence o f access to food, can elicit i n c r e a s e d DA turnover (D'Angio & Scatton, 1989; Simansky e t a l . , 1985). electrophysiological dopaminergic S c h u l t z (1986) used techniques to monitor activity i n midbrain neurons o f monkeys during an 8 a p p e t i t i v e task. their The m a j o r i t y of these neurons i n c r e a s e d f i r i n g r a t e when the monkeys were presented with v i s u a l and a u d i t o r y cues that s i g n a l l e d the a v a i l a b i l i t y of food and a l s o when the monkeys reached out toward the food. However, the f i r i n g r a t e s of very few neurons i n c r e a s e d above b a s e l i n e l e v e l s when food was a c t u a l l y placed into the mouth. T h e o r e t i c a l Background Several hypotheses have been proposed to account f o r the experimental data r e l a t i n g changes i n dopaminergic activity to feeding behaviors. h y p o t h e s i s " , was developed The "sensorimotor i n response to o b s e r v a t i o n s f o l l o w i n g dopamine d e p l e t i o n caused by 6-OHDA l e s i o n s of the n i g r o s t r i a t a l pathway. system involvement The hypothesis r e l a t e s ascending i n sensorimotor Richardson, & Teitelbaum, 1974; DA f a c i l i t a t i o n (Marshall, White, 1986). That i s , DA a c t i v i t y can modulate the a b i l i t y of sensory s t i m u l i to i n i t i a t e motor responses. Numerous s t u d i e s have shown that d e s t r u c t i o n of t h i s pathway r e s u l t s i n locomotor and p o s t u r a l d e f i c i t s , an i n a b i l i t y to o r i e n t to e x t e r n a l s t i m u l i and a d i s r u p t i o n of spontaneously initiated behaviors ( c . f . B e r r i d g e , V e n i e r , & Robinson, 1989). natural With r e s p e c t to feeding b e h a v i o r s , the sensorimotor hypothesis p r e d i c t s that DA a c t i v i t y would be r e q u i r e d f o r the 9 p r o d u c t i o n o f a p p e t i t i v e and c o n s u m m a t o r y r e s p o n s e s external to f o o d - r e l a t e d cues such as food o d o r s . An a l t e r n a t i v e h y p o t h e s i s , t h e " r e s p o n s e h y p o t h e s i s " , proposes production t h a t DA is e s s e n t i a l ( F i b i g e r , Zis, & P h i l l i p s , initiation for response 1975; P o s l u n s , 1962). T h i s h y p o t h e s i s proposes t h a t an a n i m a l t r e a t e d w i t h a n e u r o l e p t i c i s c a p a b l e o f u n d e r s t a n d i n g the s i g n i f i c a n c e of a b i o l o g i c a l l y r e l e v a n t e n v i r o n m e n t a l cue, but i s unable to initiate a motor r e s p o n s e developed t o t h i s cue. This hypothesis to account f o r the o b s e r v a t i o n that was neuroleptic- t r e a t e d animals could not perform avoidance responses to a stimulus that predicted l a t e n c y escape f o o t s h o c k , but could perform s h o r t r e s p o n s e s o n c e t h e s h o c k began ( H u n t , During p r e s e n t a t i o n of the p r e d i c t i v e s q u e a l e d and d e f e c a t e d , i n d i c a t i n g s t i m u l u s , the a n i m a l s t h a t they r e a l i z e d r e l e v a n c e o f t h e c u e , y e t were u n a b l e t o p e r f o r m a to the avoid the impending shock. shock began d e m o n s t r a t e s physical The ability c a p a c i t y to perform the response. (1975) e x p l a i n t h i s p r e s e r v e d a b i l i t y Fibiger et a l . by s u g g e s t i n g t h a t t o p e r f o r m an avoidance c a u s e d by t h e itself an e s c a p e be s u f f i c i e n t t o i n i t i a t e B l a c k b u r n e t a l . (1987) s u g g e s t e d a n d a p p e t i t i v e b e h a v i o r s may of fall shock response. that both avoidance under the g e n e r a l c a t e g o r y p r e p a r a t o r y r e s p o n s e s , whereas escape be c l a s s i f i e d once the r e s p o n s e , i n v o l u n t a r y motor a c t i v i t y may the response to escape t h a t t h e a n i m a l had e v e n though an a n i m a l i s u n a b l e 1956). as consummatory r e s p o n s e s . and ingestion Given this may 10 d i s t i n c t i o n , the response i n i t i a t i o n d e f i c i t h y p o t h e s i s d e r i v e d from shock experiments could be a p p l i e d to feeding behaviors. DA a c t i v i t y may be r e q u i r e d f o r the initiation of any preparatory response, whether t h i s response i s made to avoid shock or to approach In 1978, food. Wise, S p i n d l e r , DeWit, and Gerber proposed the "anhedonia h y p o t h e s i s " to e x p l a i n the a c t i o n o f n e u r o l e p t i c drugs and p o s s i b l y the r o l e o f ascending dopamine systems. T h i s hypothesis suggested that dopaminergic neurons the rewarding or "hedonic" impact o f p o s i t i v e such as food (Wise, 1982, 1985; Wise et a l . , mediate reinforcers, 1978a). A c c o r d i n g l y , d i s r u p t i o n o f ascending dopaminergic pathways attenuates the hedonic p r o p e r t i e s o f v a r i o u s p o s i t i v e reinforcers. The anhedonia hypothesis i s c o n s i s t e n t with the r e d u c t i o n i n food-rewarded responses observed i n animals t r e a t e d with the DA a n t a g o n i s t pimozide 1984; (Geary & Smith, Wise, S p i n d l e r , & L e g a u l t , 1978b; Xenakis & S c l a f a n i , 1981). Therefore, i f the r e i n f o r c e r l o s e s i t s hedonic impact, the response a s s o c i a t e d with i t w i l l extinguished. resulting be However, t h i s apparent " e x t i n c t i o n " effect from DA r e c e p t o r blockade i s not e q u i v a l e n t to the t y p i c a l e x t i n c t i o n seen f o l l o w i n g a p e r i o d of nonreinforcement. S p e c i f i c a l l y , more r a p i d e x t i n c t i o n i s observed as the e f f e c t s o f n e u r o l e p t i c s sum with those of e x t i n c t i o n by nonreinforcement (Gray & Wise, 1980; & F i b i g e r , 1979; Tombaugh et a l . , Phillips 1980). R e s u l t s obtained by Blackburn, P h i l l i p s , Jakubovic, and 11 Fibiger (1986) a l s o do not support the anhedonia hypothesis. In t h i s study, i n c r e a s e d DA a c t i v i t y was observed following i n g e s t i o n of a n u t r i t i v e l i q u i d d i e t or l a b chow, but not after ingestion of a p a l a t a b l e saccharin solution. These r e s u l t s suggest that s i n c e s a c c h a r i n s o l u t i o n i s a v i d l y consumed by r a t s and has been shown to maintain high r a t e s of operant responding, the apparent "reward" a s s o c i a t e d with s a c c h a r i n i n g e s t i o n i s not s u f f i c i e n t to increase DA activity. As mentioned above, Blackburn e t a l . (1987) noted pimozide d i s r u p t e d responses signalling argued to a c o n d i t i o n e d stimulus food, but not responses t h a t the anhedonia e x p l a i n why the response that to the food i t s e l f . They h y p o t h e s i s does not adequately to a secondary r e i n f o r c e r (the c o n d i t i o n e d stimulus) should be more s u s c e p t i b l e to d i s r u p t i o n by pimozide than a primary r e i n f o r c e r a d d i t i o n , the hypothesis cannot account obtained from a second experiment (food). In f o r the r e s u l t s i n which pimozide-treated animals were given u n l i m i t e d q u a n t i t i e s o f food i n t h e i r home cages. Under t h i s c o n d i t i o n , the anhedonia would p r e d i c t that f o l l o w i n g pimozide would be l e s s rewarding, r e s u l t i n g hypothesis treatment, the food i n decreased consumption. However, t h i s p r e d i c t i o n was not confirmed. In a recent study, B e r r i d g e e t a l . (1989) provided evidence that argues a g a i n s t both the anhedonia sensorimotor hypotheses. and the These r e s e a r c h e r s employed a t a s t e r e a c t i v i t y paradigm to examine the e f f e c t s of m e s o s t r i a t a l 12 6-OHDA l e s i o n s . Rats emit p o s i t i v e and a v e r s i v e responses n a t u r a l l y , depending substance. on the p a l a t a b i l i t y of an i n g e s t e d By s t u d y i n g the r e a c t i o n of both l e s i o n e d c o n t r o l r a t s to s o l u t i o n s over a range of t a s t e and palatability and i n t e n s i t y , the r e s e a r c h e r s sought to d i s c r i m i n a t e between the sensorimotor and anhedonia proposed hypotheses. that a decrease i n sensorimotor a r o u s a l would l e a d to a general r e d u c t i o n i n t a s t e r e a c t i v i t y whereas would s e l e c t i v e l y reduce p o s i t i v e r e a c t i o n s . r e a c t i v i t y remained anhedonia However, t a s t e unchanged a f t e r the l e s i o n s , a r e s u l t that supported n e i t h e r h y p o t h e s i s . argued They B e r r i d g e et a l . (1989) that t h e i r study "provides evidence that the c a p a c i t y for hedonics can be n e u r o l o g i c a l l y d i s s o c i a t e d from motivated a p p e t i t i v e behavior". an They proposed a l t e r n a t i v e theory based on i n c e n t i v e attribution. The concept of " i n c e n t i v e m o t i v a t i o n " emphasizes the a b i l i t y of environmental s t i m u l i a s s o c i a t e d with b i o l o g i c a l l y s i g n i f i c a n t events to e l i c i t b e h a v i o r a l responses (Bindra, 1978; B o l l e s , 1972; Toates, 1981). Preparatory behavior, i n c l u d i n g both a p p e t i t i v e and a v e r s i v e responses, may be i n i t i a t e d by formerly n e u t r a l s t i m u l i that have become i n c e n t i v e s through experience. The s t r e n g t h of an i n c e n t i v e stimulus i s determined by the degree of a s s o c i a t i o n between t h i s stimulus and a b i o l o g i c a l l y r e l e v a n t stimulus (e.g., food, shock), and a l s o by the s a l i e n c e of the b i o l o g i c a l stimulus. That i s , although an i n t e r n a l c o n d i t i o n such as "hunger" cannot initiate activity 13 itself, i t can p o t e n t i a t e the e f f e c t i v e n e s s of a g i v e n r e l a t e d environmental stimulus responses (Wise, 1987). Bindra in initiating future (1978) argued that p h y s i o l o g i c a l c o n d i t i o n s are important as food- because they "serve 'gates' or l i m i t s w i t h i n which c e r t a i n p a r t i c u l a r i n c e n t i v e s t i m u l i become e f f e c t i v e " , but without an i n c e n t i v e or a s t i m u l u s p r e d i c t i v e of an i n c e n t i v e , there i s no m o t i v a t i o n or response initiation. I t has been suggested that i n c e n t i v e s t i m u l i not p r e d i c t hedonic events, only but a l s o acquire some of the hedonic p r o p e r t i e s of the b i o l o g i c a l l y s i g n i f i c a n t events they a s s o c i a t e d with 1989; (Berridge & S c h u l k i n , 1989; Stewart, de Wit, & Eikelboom, 1984). are Berridge e t a l . , In a study cited by Stewart et a l . (1984), o p i a t e - l i k e e f f e c t s were r e p o r t e d by former a d d i c t s s e l f - i n j e c t i n g s a l i n e under expectancy c o n d i t i o n s . Berridge and high-drug- S c h u l k i n (1989) found that p o s i t i v e p a l a t a b i l i t y - d e p e n d e n t r e a c t i o n s to a t a s t e p r e v i o u s l y p a i r e d with s a l t , were enhanced only when a r a t was deprived of sodium. This enhancement of p a l a t a b i l i t y occurred even though the c o n d i t i o n e d t a s t e became a s s o c i a t e d with s a l t during "sodium-balanced" t r i a l s , a c o n d i t i o n under which r a t s show a v e r s i v e r e a c t i o n s to h y p e r t o n i c s a l t . shift i n p a l a t a b i l i t y of the c o n d i t i o n e d taste^ resembled s h i f t that would have occurred with s a l t itself the during sodium d e p l e t i o n , i n d i c a t i n g t h a t the c o n d i t i o n e d t a s t e a c q u i r e d some of the hedonic q u a l i t i e s of the The salt. I t has been proposed t h a t the neural s u b s t r a t e of had 14 i n c e n t i v e m o t i v a t i o n i n v o l v e s ascending (Blackburn et a l . , 1987; and P h i l l i p s , dopaminergic 1976). activity DA systems Fibiger & P h i l l i p s , According to t h i s 1986; Mogenson hypothesis, i s r e q u i r e d when m o t i v a t i o n a l i n c e n t i v e s i g n i f i c a n c e i s assigned to n e u t r a l s t i m u l i through a s s o c i a t i o n with b i o l o g i c a l l y r e l e v a n t events. D i s r u p t i o n of the neural system i n v o l v e d i n the of m o t i v a t i o n a l s a l i e n c e would d i s r u p t motivated attribution behavior even i f motor, sensory, and a f f e c t i v e systems were i n t a c t . The i n c e n t i v e m o t i v a t i o n model of DA function asserts that exposure to an i n c e n t i v e stimulus can l e a d to an i n c r e a s e i n the r e l e a s e of DA i n the f o r e b r a i n , which i n turn r e s u l t s i n a p r e p a r a t o r y response 1987). This hypothesis i n c r e a s e d dopaminergic ( S c h u l t z , 1986; i s supported (Blackburn et a l . , by the o b s e r v a t i o n of a c t i v i t y during a p p e t i t i v e behaviors Blackburn et a l . , 1989b). It i s also c o n s i s t e n t with the s e l e c t i v e d i s r u p t i o n of p r e p a r a t o r y , not consummatory feeding behaviors observed pimozide Rationale treatment but following (Blackburn et a l . , 1987). for the a p p l i c a t i o n of e l e c t r o c h e m i s t r y to the study of feeding i n the r a t The purpose of the present t h e s i s was of DA i n feeding behaviors to study the i n v i v o by employing role the technique of e l e c t r o c h e m i s t r y with s t e a r a t e - m o d i f i e d carbon 15 paste e l e c t r o d e s . T h i s technique o f f e r s s e v e r a l advantages. E l e c t r o c h e m i s t r y does not depend on postmortem t i s s u e a n a l y s i s , but can be performed moving s u b j e c t s . on u n a n e s t h e t i z e d , f r e e l y - E l e c t r o c h e m i c a l s i g n a l s can be o b t a i n e d from one animal over an extended p e r i o d of time (i.e., d u r i n g both b a s e l i n e and experimental phases), a l l o w i n g i t to serve as i t s own control. This a l s o enables the r e s e a r c h e r to observe the time course of an observed neurochemical response. In a d d i t i o n , the technique can be used to study neurochemical and b e h a v i o r a l c o r r e l a t i o n s without the need f o r e x p e r i m e n t a l l y manipulated DA obtained by e l e c t r i c a l levels s t i m u l a t i o n , l e s i o n s , or pharmacology. Although a l l of the p o i n t s mentioned so f a r c o u l d apply e q u a l l y to both e l e c t r o c h e m i s t r y and m i c r o d i a l y s i s , technique has i t s own unique advantages. The each temporal r e s o l u t i o n of chronoamperometry i s s u p e r i o r to m i c r o d i a l y s i s because to samples can be taken every few seconds (usually 15 20 s) as opposed to the 5 to 20 min sampling time i n microdialysis. Also, c h r o n i c a l l y implanted e l e c t r o c h e m i c a l e l e c t r o d e s tend to remain v i a b l e f o r much longer p e r i o d s than m i c r o d i a l y s i s probes ( o f t e n , weeks as opposed to days), thus f a c i l i t a t i n g r e p l i c a t i o n . Finally, electrochemical e l e c t r o d e s are g e n e r a l l y s m a l l e r than d i a l y s i s probes and as a r e s u l t , cause l e s s damage d u r i n g i m p l a n t a t i o n . However, the exact nature of the substance c o n t r i b u t i n g to s i g n a l s obtained by e l e c t r o c h e m i c a l methods i s not firmly e s t a b l i s h e d , whereas the r e s u l t s o b t a i n e d by m i c r o d i a l y s i s with HPLC/ED a n a l y s i s are unequivocal. carbon-paste e l e c t r o d e s have been developed s e l e c t i v e monitoring of e x t r a c e l l u l a r DA brain regions Stearate-modified (Blaha & Lane, 1983). for the l e v e l s i n "DA-rich T h i s g i v e s them an advantage over other e l e c t r o c h e m i c a l e l e c t r o d e s that are e i t h e r n o n s e l e c t i v e , or that monitor DA i n d i c e s of DA turnover. metabolites as 17 GENERAL METHODS Electrode preparation A three-electrode e l e c t r o c h e m i c a l d e t e c t i o n system was used i n a l l experiments. from T e f l o n - c o a t e d coated). Recording e l e c t r o d e s were made s t a i n l e s s s t e e l wire (0.008" bare, 0.011" The s t a i n l e s s s t e e l was withdrawn approximately 0.5mm through the T e f l o n c o a t i n g l e a v i n g a w e l l a t one end. This w e l l was packed with a g r a p h i t e paste mixture c o n s i s t i n g o f g r a p h i t e powder, s t e a r i c a c i d , and l i q u i d paraffin (Nujol). Teflon-coated Reference e l e c t r o d e s were made from s i l v e r wires. These e l e c t r o d e s had the T e f l o n c o a t i n g s t r i p p e d f o r l-2mm a t one end, and the exposed s i l v e r wire was s i l v e r - c h l o r i d e d . The t h i r d type o f e l e c t r o d e used, was the a u x i l i a r y e l e c t r o d e , c o n s i s t i n g of a bare s t a i n l e s s s t e e l wire anchored to a s k u l l screw. free ends o f each type o f e l e c t r o d e were soldered The to gold connector p i n s . Surgery All r a t s used i n the f o l l o w i n g experiments had two cranial electrodes anesthesia implanted (Somnitol, was implanted under sodium p e n t o b a r b i t a l 60mg/kg i . p . ) One r e c o r d i n g electrode s t e r e o t a x i c a l l y i n the nucleus accumbens, 18 1.2mm a n t e r i o r t o bregma, 1.2mm l a t e r a l (left and cortex with the 6.5mm v e n t r a l t o t h e surface o f the hemisphere), s incisor b a r s e t a t -3.3. implanted i n t o the lateral The s e c o n d r e c o r d i n g e l e c t r o d e was s t r i a t u m , 1.0mm a n t e r i o r t o bregma, 1.7mm ( r i g h t h e m i s p h e r e ) , a n d 4.0mm v e n t r a l t o t h e cortical surface. For each r a t , a s i n g l e r e f e r e n c e implanted approximately 2-3mm p o s t e r i o r a n d l a t e r a l t o bregma i n t o t h e r i g h t h e m i s p h e r e . was lowered u n t i l cortex. The r e f e r e n c e the c h l o r i d e d t i p d i s a p p e a r e d A separate auxiliary t i m e s a r o u n d one s k u l l e l e c t r o d e was s n a p p e d screw. recover c o u l d be s c r e w e d on. This whole with dental acrylic. c o n t a i n i n g the g o l d so t h a t a l e a d used f o r from surgery several The g o l d p i n f r o m e a c h into a p l a s t i c holder. u p p e r edge o f t h e p l a s t i c h o l d e r recording into the Three t o four a d d i t i o n a l a s s e m b l y was c e m e n t e d t o g e t h e r threaded electrode e l e c t r o d e was wrapped s c r e w s were a n c h o r e d t o t h e s k u l l . was e l e c t r o d e was The pins electrochemical The r a t s were a l l o w e d t o f o r a t l e a s t 2 days before being used i n an experiment. Recording procedure Chronoamperometric r e c o r d i n g s square-wave p o t e n t i a l p u l s e s 20-s (Expt. 1) o r 30-s ( E x p t . were t a k e n by a p p l y i n g t o each r e c o r d i n g e l e c t r o d e a t 2) i n t e r v a l s a n d m e a s u r i n g t h e 19 c u r r e n t a f t e r 1 s. The voltage parameters f o r the pulses were determined by f i r s t o b t a i n i n g voltammetric sweep records. In a sweep, the a p p l i e d v o l t a g e was ramped a t +10mV/s, and the r e s u l t i n g c u r r e n t was recorded. When the • r e c o r d was s e m i d i f f e r e n t i a t e d , any c u r r e n t due to the o x i d a t i o n o f a g i v e n substance a t the e l e c t r o d e t i p showed up as an exaggerated peak superimposed upon a background curve ( F i g u r e 1). The pulse parameters f o r chronoamperometry were s e t by determining which the peak o f i n t e r e s t l a y (e.g., the window w i t h i n -lOOmV to +175mV f o r the proposed DA peak i n F i g u r e 1). When voltammetric sweep r e c o r d s themselves were used to r e c o r d changes i n DA l e v e l s d u r i n g a t e s t s e s s i o n ( i n Expt. 2 o n l y ) , a sweep i n t e r v a l o f 10 min was chosen to allow s u f f i c i e n t time f o r e q u i l i b r a t i o n o f DA a t the e l e c t r o d e tip. Peak height was used as an i n d i c a t i o n o f e x t r a c e l l u l a r DA l e v e l s . Histology At the c o n c l u s i o n of an experiment, each r a t was sacrificed stored i n a C02 chamber and i t s b r a i n was removed and i n a 10% f o r m a l i n s o l u t i o n f o r s e v e r a l days. Later, each b r a i n was f r o z e n , s e c t i o n e d , and mounted i n order to allow h i s t o l o g i c a l c o n f i r m a t i o n o f e l e c t r o d e placements i n accordance with the Paxinos and Watson a t l a s (1982). 20 Figure 1 Example o f a t y p i c a l lOmV/s (-150 approximately peak. v o l t a m m e t r i c sweep, ramped a t t o +450mV). The peak, o c c u r r i n g a t lOOmV r e p r e s e n t s t h e p u t a t i v e DA 21 (yu) ^uojjno uoftDpjxo EXPERIMENT 1 E f f e c t s of an u n s i q n a l l e d l i q u i d meal on measures of DA electrochemical activity Experiment 1 was designed to determine whether changes i n the dopaminergic e l e c t r o c h e m i c a l detected i n the r a t caudate and d u r i n g , and signal could nucleus accumbens a f t e r the d e l i v e r y and any before, consumption of a small u n s i g n a l l e d meal of a p a l a t a b l e l i q u i d d i e t . The procedure followed observation i n t h i s experiment allowed the time course of any DA levels. One f r e e access resulting advantage of t h i s experiment over DA not a f a c t o r here. o testing. past that Each r a t was i n a more normal p h y s i o l o g i c a l , and n e u r o p h y s i o l o g i c a l , s t a t e during increases r e l e a s e was to food except during the a c t u a l t e s t r e s u l t s of other simple e f f e c t s of feeding on e x t r a c e l l u l a r s t u d i e s c o r r e l a t i n g feeding and d e p r i v a t i o n was be food allowed sessions, possibly Based on the feeding s t u d i e s mentioned e a r l i e r , i n DA a c t i v i t y would be p r e d i c t e d to occur response to the meal presented i n the c u r r e n t study. in Methods Subjects \ Four male hooded Long-Evans r a t s from Charles Laboratories were used i n t h i s study. the experiment, the r a t s weighed River At the beginning o f between 350g and 450g. Apparatus During the d a i l y r e c o r d i n g s e s s i o n s , i n d i v i d u a l r a t s were p l a c e d i n s i d e a P l e x i g l a s t e s t i n g chamber l o c a t e d w i t h i n a l a r g e r Faraday cage to reduce e l e c t r i c a l The bottom of the chamber c o n s i s t e d of a wire g r i d with a bed of S a n - i - c e l beneath it. noise. floor At one side o f the compartment, a hole was d r i l l e d and a brace attached accommodate a removable d i e t , Sustacal to R i c h t e r tube c o n t a i n i n g the l i q u i d (Mead Johnson), used i n the study. commutator f o r e l e c t r o c h e m i c a l A r e c o r d i n g extended down from the r o o f o f the Faraday cage to j u s t above the r o o f o f the P l e x i g l a s compartment. A r e c o r d i n g lead was attached to t h i s commutator a t one end and to the r a t ' s e l e c t r o d e assembly a t the other. originating Outside of the Faraday cage, leads from the commutator were connected to an electrochemical r e c o r d i n g box. Recorded electrochemical 24 s i g n a l s were t r a n s l a t e d by, and viewed on, a micro-computer. On each t r i a l , the r a t s ' behavior was observed video camera i n f r o n t o f the t e s t chamber. via a The room was l i t by a bulb turned away from the r a t s and dimmed as much as p o s s i b l e while s t i l l a l l o w i n g an adequate video r e c o r d i n g of the r a t s ' behavior. Procedure In order to prevent neophobia during t e s t s e s s i o n s , a l l r a t s were preexposed i n t h e i r home cages to 10-20ml o f the chocolate-flavored l i q u i d diet i n a d d i t i o n to t h e i r usual d i e t of l a b chow p e l l e t s and water. liquid Rats that consumed the d i e t o v e r n i g h t or upon a second exposure, were used i n the experiment. During a given experimental was connected chamber. s e s s i o n , an i n d i v i d u a l r a t to a r e c o r d i n g lead and placed i n s i d e the t e s t At the beginning o f a t e s t s e s s i o n , the r a t had no access to food nor water. r e c o r d s were f i r s t Each day, voltammetric sweep obtained and chronoamperometric parameters were then determined from these sweep data. A f t e r a s t a b l e b a s e l i n e ( l e s s than 1 nA peak, to peak f l u c t u a t i o n ) of a t l e a s t 10 min was recorded a t a 20 s pulse i n t e r v a l r a t e , the t e s t box was opened and a 10ml p o r t i o n of the l i q u i d d i e t was p l a c e d i n s i d e . signal The e l e c t r o c h e m i c a l from the r a t was recorded f o r another 18-30 min. During a l l phases of the experiment, the rat remained undisturbed in a closed room except during the b r i e f food delivery period. Each session was videotaped and the tapes were used to analyze the time spent on feeding behavior. the conclusion of a test session, A the rat was returned to i t s home cage and given free access to lab chow and water u n t i l the following session. Each rat was tested in this manner, once per day, for seven consecutive days. Results Each of the 4 rats used in this experiment showed an increase in chronoamperometric oxidation current of approximately 0.5 to 2.0 nA over baseline levels from both the n. accumbens and caudate in the period following the introduction of Sustacal inside the test chamber (Figures 2 3, 4, and 5). Each figure represents data obtained from 1 of the 4 single rats averaged over test days 2 through 7. Data from the f i r s t test day for each rat were not included in this average because the feeding response was t y p i c a l l y not immediate nor as vigorous on this day as on the following sessions, when the rat was more familiar with the test chamber and procedure. In fact, none of the four subjects showed any recorded increases in either the n. accumbens or caudate on the f i r s t day of testing (Figure 6) Aside from the differences observed between day 1 and days 26 Figure 2 Chronoamperometric s i g n a l s recorded from the caudate (squares) and nucleus accumbens ( c r o s s e s ) before and a f t e r the d e l i v e r y of a lOcc S u s t a c a l meal. The obtained time f i g u r e r e p r e s e n t s averaged from r a t #1 over days 2-7. zero was adjusted to OnA to data The p o i n t at facilitate comparison of o x i d a t i o n c u r r e n t changes between the two r e c o r d i n g channels. d u r a t i o n from days 2-7 The average meal i s bounded by time (the p o i n t of food d e l i v e r y ) and the arrow i n d i c a t i n g the end of the f e e d i n g bout. zero 27 (yu) }uexino uoftDpjxo 28 Figure 3 Chronoamperometric caudate ( s q u a r e s ) and before and after meal. The figure obtained time from z e r o was comparison the ' two signal recorded n u c l e u s accumbens the d e l i v e r y r a t #2 over days a d j u s t e d t o OnA d u r a t i o n from days (the food d e l i v e r y ) t h e end 2-7 of 2-7. to The the feeding (crosses) data The point at facilitate between average i s b o u n d e d by and the lOcc Sustacal c u r r e n t changes recording channels. indicating of a r e p r e s e n t s averaged of oxidation point of from the meal time arrow bout. zero 29 (Vu) *u©xmo uoftDpixo 30 Figure 4 Chronoamperometric s i g n a l recorded caudate (squares) and nucleus from the accumbens (crosses) before and a f t e r the d e l i v e r y of a lOcc S u s t a c a l meal. The obtained figure represents averaged from r a t #3 over days 2-7. time zero was adjusted to OnA to data The p o i n t at facilitate comparison of o x i d a t i o n c u r r e n t changes between the two ' r e c o r d i n g channels. d u r a t i o n from days 2-7 The i s bounded by time (the p o i n t of food d e l i v e r y ) and i n d i c a t i n g the end average meal the arrow of the feeding bout. zero (yu) *u©xino uoftDpjxo 32 Figure 5 Chronoamperometric caudate ( s q u a r e s ) and before and after meal. The figure obtained time from z e r o was comparison the ' two signal of recorded nucleus r a t #4 over days a d j u s t e d t o OnA oxidation 2-7 (the p o i n t of food d e l i v e r y ) of 2-7. to The the feeding (crosses) data The point at facilitate between average i s b o u n d e d by and the lOcc Sustacal c u r r e n t changes days end of a represents averaged d u r a t i o n from the accumbens the d e l i v e r y recording channels. indicating from the meal time arrow bout. zero 34 Figure 6 Representative caudate chronoamperometric r e c o r d (squares) o f r a t #2 on indicates the meal and the feeding bout. the and first nucleus day accumbens of t e s t i n g . point of d e l i v e r y of a arrow i n d i c a t e s the end from (crosses) Time lOcc of the zero Sustacal the first oxidation current (nA) 36 Figure 7 Chronoamperometric s i g n a l recorded before and a f t e r the d e l i v e r y of a lOcc S u s t a c a l meal. f i g u r e represents, averaged 1, 2, 3, and zero was data obtained from r a t s 4 over days 2-7. a d j u s t e d to OnA The The p o i n t at time to f a c i l i t a t e comparisons of o x i d a t i o n c u r r e n t changes between the 2 r e c o r d i n g channels. The average meal d u r a t i o n i s bounded by time zero (the p o i n t of food d e l i v e r y ) and the arrow i n d i c a t i n g the end of the feeding bout. squares The symbols (crosses = n. accumbens, = caudate) r e p r e s e n t averaged the 4 r a t s , and the s o l i d e r r o r of the mean. data from l i n e s represent a) n. accumbens standard b) caudate TIME (min) 38 39 Figure 8 E l e c t r o d e placements d i r e c t e d a t the l e f t n u c l e u s accumbens (n=4) and r i g h t caudate (n=4) i n Experiment 1. 40 +L2mm +0.7mm 41 2-7, no f u r t h e r developments a c r o s s s e s s i o n s were observed. For a g i v e n r a t , the time course the r e c o r d e d and the magnitude of i n c r e a s e s i n chronoamperometric c u r r e n t were v e r y s i m i l a r i n both the n. accumbens and caudate. In a d d i t i o n , the p a t t e r n o f change seen i n each o f the four r a t s was comparable. The average change i n r e c o r d e d o x i d a t i o n c u r r e n t a c r o s s a l l 4 s u b j e c t s i s presented Figure The in 7. time f o r a r a t to complete i t s f i r s t f e e d i n g bout was intense d e f i n e d as ending when the r a t had l e a s t 30s away from the feeder spent a t ( g e n e r a l l y , the r a t had consumed most or a l l o f the meal a t t h i s p o i n t ) . of the f o u r f i g u r e s ( F i g u r e s 2-4), the recorded In three current appears to be r e t u r n i n g to b a s e l i n e l e v e l s a p p r o x i m a t e l y 30 minutes a f t e r food d e l i v e r y . Although 25- F i g u r e 5 does not i n d i c a t e a complete r e t u r n to b a s e l i n e , t h i s f i g u r e r e p r e s e n t s data c o l l e c t e d f o r l e s s than 20 minutes, which may not have been s u f f i c i e n t to observe a r e t u r n to b a s e l i n e in t h i s subject. A n a l y s i s of the h i s t o l o g i c a l d a t a r e v e a l e d t h a t a l l of the e l e c t r o d e t i p s were l o c a t e d e i t h e r i n the dorsomedial 8) . s t r i a t u m or the l e f t n u c l e u s right accumbens ( F i g u r e 42 Discuss ion The r e s u l t s of Experiment 1 i n d i c a t e t h a t f e e d i n g an e f f e c t on dopaminergic a c t i v i t y i n both the accumbens and s t r i a t u m of the r a t . has nucleus I n both c a s e s , estimates of e x t r a c e l l u l a r DA l e v e l s d e r i v e d from chronoamperometric records i n c r e a s e d a f t e r consumption o f a l i q u i d meal. the magnitude and time course o f the r e c o r d e d s i m i l a r i n the n. accumbens and the two r e c o r d i n g s i t e s may Both i n c r e a s e s were s t r i a t u m , suggesting that share a s i m i l a r r o l e i n f e e d i n g behaviors. Two may f a c t o r s i n t h i s study i n d i c a t e t h a t the DA response be a s s o c i a t e d w i t h a p o s t i n g e s t i v e stage o f f e e d i n g . F i r s t , t h e r e appeared to be a l a t e n c y between the initiation of food comsumption and the o b s e r v a t i o n o f a n o t a b l e i n c r e a s e i n the chronoamperometric s i g n a l . l e v e l s remained e l e v a t e d between 20 and Second, the 30 min a f t e r DA the meal had been completed. One p o t e n t i a l l y confounding f a c t o r i n the present study a r i s e s from the p o s s i b i l i t y t h a t the " b a s e l i n e " v a l u e s recorded a t the b e g i n n i n g of each t e s t s e s s i o n r e p r e s e n t an e x t r a c e l l u l a r DA c o n c e n t r a t i o n t h a t has a l r e a d y been e l e v a t e d s i m p l y by p l a c i n g the r a t i n t o the t e s t chamber. The c o m b i n a t i o n of h a n d l i n g , exposure to a novel environment complete w i t h odors from p r e v i o u s f e e d i n g experiments i n the same t e s t chamber, and p o s s i b l y , a n t i c i p a t i o n of a p a l a t a b l e meal, c o u l d have r e s u l t e d i n an i n c r e a s e i n DA activity. 43 This e l e v a t i o n c o u l d have i n f l u e n c e d the o b s e r v a t i o n of any f u r t h e r i n c r e a s e i n DA a c t i v i t y due and to the p r e s e n t a t i o n consumption of food. That i s , the changes from t r u e " b a s e l i n e " l e v e l s may have been even l a r g e r than those that were a c t u a l l y observed. One a s p e c t of f e e d i n g t h a t t h i s study does not i s the r o l e of DA explore i n a n t i c i p a t o r y s t a g e s of f e e d i n g . In Experiment 1, the r a t s began to consume the u n s i g n a l l e d meal w i t h i n seconds of d e l i v e r y . As noted i n the I n t r o d u c t i o n , e a r l i e r s t u d i e s have i n d i c a t e d t h a t DA systems may s e l e c t i v e l y i n v o l v e d i n p r e p a r a t o r y a s p e c t s of behaviors (Blackburn et a l . , 1987, 1989a). be feeding This distinction between p r e p a r a t o r y and consummatory f e e d i n g s t a g e s i n v e s t i g a t e d i n Experiment 2. was EXPERIMENT 2 E l e c t r o c h e m i c a l measures of dopamine a c t i v i t y d u r i n g a n t i c i p a t o r y and consummatory phases o f f e e d i n g b e h a v i o r s i n the r a t T h i s experiment was designed to study changes i n e l e c t r o c h e m i c a l measurements o f e x t r a c e l l u l a r DA d u r i n g d i f f e r e n t phases o f a c o n d i t i o n e d f e e d i n g paradigm by Weingarten (1983, 1984). developed As r e v i e w e d i n the I n t r o d u c t i o n , DA has been a s s o c i a t e d w i t h a p p e t i t i v e r e s p o n d i n g f o r food ( B l a c k b u r n e t a l . , 1986). 1987, 1989b; S c h u l t z , I n o r d e r to study t h i s proposed c o r r e l a t i o n , i t was n e c e s s a r y to observe the a p p e t i t i v e phase i n d e p e n d e n t l y o f the consummatory response. accomplished Weingarten's t h i s s e p a r a t i o n by c o n d i t i o n i n g the s u b j e c t s t o an e x t e r n a l s t i m u l u s cue (CS+) o f a meal. paradigm t h a t p r e d i c t e d the d e l i v e r y T h i s cue, c o n s i s t e d o f a buzzer and a b r i g h t l i g h t , preceded, and then c o n t i n u e d throughout the food delivery period. I n the p r e s e n t experiment, d u r i n g the CS+ p e r i o d p r e c e d i n g food d e l i v e r y , the r a t ' s b e h a v i o r (nosepokes i n t o the f e e d i n g n i c h e ) was monitored as an i n d i c a t i o n of a p p e t i t i v e responding. chronoamperometric Simultaneously, r e c o r d s were o b t a i n e d . As i n Experiment 1, the r a t ' s e l e c t r o c h e m i c a l s i g n a l from both the caudate and n u c l e u s accumbens was r e c o r d e d d u r i n g a l l stages o f the f e e d i n g bout, i n c l u d i n g a d i s t i n c t " a p p e t i t i v e " phase. One i n t e r p r e t a t i o n o f the i n c e n t i v e m o t i v a t i o n hypotheses of f u n c t i o n p r e d i c t s t h a t the CS+ alone s h o u l d a c t as an i n c e n t i v e s t i m u l u s r e s u l t i n g i n i n c r e a s e d DA r e l e a s e subsequently, al., an approach to the f e e d i n g n i c h e DA and (Blackburn et 1989a). Methods Subj e c t s A l l 40 s u b j e c t s used i n t h i s study were male hooded Long-Evans r a t s w e i g h i n g 350 to 450g a t the b e g i n n i n g of the exper iment. Apparatus Rats were housed i n d i v i d u a l l y w i t h i n four c o n d i t i o n i n g chambers. separate Each chamber c o n s i s t e d of a sound a t t e n u a t i n g m o d i f i e d Coleman c o o l e r ( S n o - L i t e Low Boy model) c o n t a i n i n g a s m a l l e r P l e x i g l a s compartment i n which the r a t was housed. The f l o o r of the i n n e r compartment was made of a wire g r i d w i t h a bed of absorbent S a n - i - c e l below i t . Each compartment had s i t u a t e d a t one two h o l e s and two R i c h t e r tube braces end w a l l . Water was f r e e l y a v a i l a b l e to the 46 r a t at a l l opposite times through one of these two h o l e s . wall c o n t a i n e d a recessed feeding niche with a 1 cm l i p enabling i t to hold the l i q u i d d i e t . d r i l l e d at the back of the feeding end of a length of S i l a s t i c through t h i s tubing. with a dim h o u s e l i g h t pm) and a fan (always o n ) . S u s t a c a l was d e l i v e r e d The with a p h o t o c e l l . beam i n d i c a t e d a r a t ' s entry compartment, each c o o l e r (on a t 11:00 am, o f f at mounted near the c e i l i n g of the c o o l e r . photocells was 11:00 For c o n d i t i o n i n g purposes, buzzer, b r i g h t cue l i g h t and tone generator a were a l s o The cues, pump, and were c o n t r o l l e d i n i t i a l l y by a Nova and subsequently by an INI computer using Manx Two of the electrochemical four chambers were set recording. software. up for In these chambers, a Faraday cage was placed between the c o o l e r and the smaller compartment. Each cage was f i t t e d r e c o r d i n g lead was attached between t h i s r a t ' s head apparatus. Plexiglas with a commutator extending down to the r o o f of the P l e x i g l a s chamber. A commutator and the Cables were extended from the end of the commutator to an e l e c t r o c h e m i c a l r e c o r d i n g box o u t s i d e of the c o o l e r . Recorded s i g n a l s t r a n s l a t e d by and viewed on an IBM compatible personal computer. the niche. Outside of the P l e x i g l a s opposite was t u b i n g by a Cole-Parmer p e r i s t a l t i c pump. I n t e r r u p t i o n of the p h o t o c e l l fitted A small hole niche to accommodate s i d e s of the feeding niche were f i t t e d into the The were 47 Procedure The procedure followed i n Experiment conditioned feeding p a r a d i g m d e v e l o p e d by 1984). present procedure The h o u s e d r a t s t o an meals per day, between 90% average of e i g h t 100% the buzzer-light stimulus When the conditioned i n t o the interrupted and second stimulus paired with from 43 several the to 105 The niche considered Subjects two c u e s by stimulus of a 9ml was minute o f a (CS+). a t one The as animals Presentation 330 second A l l m e a l s were end by a pure intertrial a mean o f o f the 90 cage. was computer. A t o n e , was not interval ranged minutes. After an a l t e r n a t i n g CS+/CSthe ability time time and i n the the CS+ schedule, to d i s c r i m i n a t e the CS+ niche number o f p r e s e n t a t i o n of (1983, liquid a p h o t o c e l l beam recorded a individually t o keep the s p e n d i n g much o f o r no amount o f t o be on displayed little niche presented training during anticipation event minutes with two and signalled final niche, delivery. rats reliably niche, the (CS-), days of period. the this food between the the feeding r a t entered Weingarten f r e e - f e e d i n g body w e i g h t . o f a meal c o i n c i d e d w i t h delivered b a s e d on involved exposing a volume s u f f i c i e n t and 2 was period during the "nosepokes" stimulus i n d i c a t i o n s of preparatory in CS- into were responding in meal. were c o n d i t i o n e d cues e i t h e r before t o d i s c r i m i n a t e between or a f t e r electrode the 48 implantation. In both cases, c o n d i t i o n e d r a t s were monitored chronoamperometrically and b e h a v i o r a l l y nosepoke data) continuously more e x t e n s i v e weekly, period. except d u r i n g a b r i e f d a i l y , and maintenance and data collection Once a week, the r a t s were removed t e m p o r a r i l y from the c o n d i t i o n e d feeding t h e i r cages c l e a n e d . apparatus, lead, (via boxes to be weighed and to have After i n i t i a l hook-up to the r e c o r d i n g a 2-day p e r i o d was allowed the novel t e s t box, implantation after conditioned feeding to the colony for adaptation to the and for those r a t s removed for t r a i n i n g , to become reacquainted with schedule. for any reason, a d a p t a t i o n was allowed If the s u b j e c t the was returned an a d d i t i o n a l day of following reconnection. Event-marked chronomperometric r e c o r d s were scored blindly for any changes i n s i g n a l , and l a t e r , matched with computer data i n d i c a t i n g whether represented event markers CS+, C S - , h o u s e l i g h t s on (11:00 p . m . ) , houselights off (11:00 a . m . ) . For each t r i a l , chronoamperometric c u r r e n t value by determining the change the b a s e l i n e they were level (nosepoke data) a was obtained in oxidation current r e l a t i v e preceding the For one s u b j e c t , ( i n nanoamps) or to event. which was showing good b e h a v i o r a l and chronoamperometric CS d i s c r i m i n a t i o n , a d d i t i o n a l voltammetric sweep r e c o r d s were obtained during a s e r i e s of c o n d i t i o n e d feeding trials. C S - t r i a l s conducted, the sweep i n t e r v a l was set with a v o l t a g e ramp of +10mV/s. In the Each t r i a l 10 CS+ and 10 at 10 min was timed such 49 that the end of one sweep occurred approximately 270 s the onset of a CS. after The a c t u a l sweeps l a s t e d between 40 and 60 s depending on the s e l e c t e d voltage parameters. This timing was chosen so t h a t one sweep c o u l d be obtained as l a t e as p o s s i b l e i n the "CS alone" p e r i o d which ended with the onset of food d e l i v e r y , 270 s a f t e r the CS+ began. a d d i t i o n to the sweep recorded during the CS p e r i o d , In at l e a s t three sweeps were obtained both before and a f t e r this per i o d . Three s u b j e c t s from which s e v e r a l days of good b e h a v i o r a l and chronoamperometric data had been c o l l e c t e d , were p l a c e d on an e x t i n c t i o n schedule. condition, two changes were made. tubing was disconnected During First, this the food d e l i v e r y from the back, of the feeding n i c h e , and second, a R i c h t e r tube with a d a i l y supply of S u s t a c a l was f i t t e d niche. into the e x t r a brace i n the wall opposite B e h a v i o r a l and e l e c t r o c h e m i c a l measurements continued throughout the e x t i n c t i o n p e r i o d . three s u b j e c t s , the were In one of the these measurements were continued i n t o an a d d i t i o n a l r e i n i t i a t i o n p e r i o d i n which the ad l i b food supply was removed and the food d e l i v e r y tubing reconnected. Of the t o t a l o f 40 r a t s that began t h i s experiment, r e c o r d s from 10 r a t s the s t a t i s t i c a l (3 with 2 good channels) analysis. were used i n Electrochemical recording channels were e l i m i n a t e d from the a n a l y s i s i f any one of three f o l l o w i n g c o n d i t i o n s occurred before s u f f i c i e n t were c o l l e c t e d : the the data n o i s y chronoamperometric r e c o r d s , small 50 (<l/2 nA) or absent v o l t a m m e t r i c peaks, o r a p u l l e d e l e c t r o d e cap. Of the r a t s t h a t developed n o i s y s i g n a l s or p u l l e d caps a f t e r o n l y a few r e c o r d i n g s e s s i o n s , s e v e r a l showed p r o m i s i n g r e s u l t s up u n t i l the s i g n a l was l o s t . The t h r e e c r i t e r i a d i s c u s s e d above accounted f o r the r e c o r d s o f a l l b u t 2 o f the d i s c a r d e d s u b j e c t s . had r e a s o n a b l e voltammetric Although these 2 r a t s peak a m p l i t u d e s and chronoamperometric n o i s e l e v e l s , showed b e h a v i o r a l CS+/CSd i s c r i m i n a t i o n , and h a d . h i s t o l o g i c a l l y confirmed p l a c e m e n t s , f o r unknown reasons, electrode they d i d not appear t o e x h i b i t chronoamperometric responses t o the CS s t i m u l i . Because no chronoamperometric responses were ever from these obtained 2 r a t s , they were not i n c l u d e d i n the f o l l o w i n g statistical analyses. Statistical Analyses For each v i a b l e e l e c t r o c h e m i c a l r e c o r d i n g site (accumbens, n=8; caudate, n=5), a s e r i e s o f 20 c o n s e c u t i v e " s c o r a b l e " CS+ and 20 CS- t r i a l s were a n a l y z e d . not s c o r e d i f i n the 10 minutes p r e c e d i n g A t r i a l was the event, there was a t r a n s i e n t i n c r e a s e i n the n o i s e o f the s i g n a l , a rising b a s e l i n e , o r a s p i k e i n the r e c o r d . E x c l u d i n g the a d a p t a t i o n days, the f i r s t 20 s c o r a b l e t r i a l s were used i n the s t a t i s t i c a l analysis. The mean change from b a s e l i n e was c a l c u l a t e d f o r each s e t o f 20 CS t r i a l s . 51 The chronoamperometric measures were a n a l y z e d u s i n g a two-way w i t h i n s u b j e c t s ANOVA f o r repeated measures, w i t h i m p l a n t l o c a t i o n (accumbens vs. caudate) and event CS-) as the two (CS+ vs. factors. In o r d e r to e s t i m a t e b a s e l i n e v a r i a b i l i t y , a b a s e l i n e d i f f e r e n c e s c o r e was c a l c u l a t e d f o r each r e c o r d i n g s i t e . D i f f e r e n c e s c o r e s were determined p r e c e d i n g the f i r s t CS+ by u s i n g b a s e l i n e data and CS- t r i a l s i n a s e r i e s . mean b a s e l i n e v a l u e over f i v e p o i n t s b e g i n n i n g 540 i n t e r v a l s ) p r i o r to the CS onset was The s (30 s s u b t r a c t e d from the mean v a l u e b e g i n n i n g 270 s b e f o r e the CS. These b a s e l i n e d i f f e r e n c e s c o r e s were then averaged f o r each b r a i n site/event combination. B e h a v i o r a l d i s c r i m i n a t i o n was two measures: a n a l y z e d w i t h r e s p e c t to t o t a l time spent i n n i c h e d u r i n g the first 270 s of a CS and l a t e n c y to e n t e r n i c h e f o l l o w i n g CS Values onset. f o r these measures were o b t a i n e d from the same t r i a l s t h a t were used i n the chronoamperometric a n a l y s i s . Average v a l u e s w i t h i n each s e t o f 20 t r i a l s were c a l c u l a t e d and a n a l y z e d i n two s e p a r a t e two-way ANOVAs; one then f o r the amount o f time spent i n n i c h e , and the other f o r entrance l a t e n c y . For both measures, the two f a c t o r s analyzed were i m p l a n t l o c a t i o n (accumbens vs. caudate) and event Voltammetric (CS+ vs. CS-). sweep r e c o r d s from a s i n g l e s u b j e c t were a n a l y z e d w i t h r e s p e c t t o peak, h e i g h t r a t i o s . a peak was determined The h e i g h t of by f i r s t drawing a l i n e between the two minimum v a l u e s on e i t h e r s i d e of the proposed DA peak. 52 The peak, h e i g h t v a l u e was subsequently o b t a i n e d by measuring the v e r t i c a l d i s t a n c e from the peak t i p t o t h i s R a t i o s were determined line. between peak h e i g h t measured d u r i n g the CS p e r i o d and peak h e i g h t i n the p r e v i o u s 10 min p e r i o d . The peak h e i g h t r a t i o s f o r CS+ and CS- (10 t r i a l s each) were a n a l y z e d u s i n g an independent v a r i a b l e s t - t e s t . Results B e h a v i o r a l d i s c r i m i n a t i o n between the two CS c o n d i t i o n s i s i n d i c a t e d by the r e s u l t s from the two b e h a v i o r a l measures shown i n F i g u r e s 9 and 10. Whereas d u r i n g the CS+, t h e average l a t e n c y t o e n t e r the f e e d i n g n i c h e was r e l a t i v e l y s h o r t , w i t h a s u b s t a n t i a l amount o f time spent i n the n i c h e , the r e v e r s e was t r u e d u r i n g the CS-. supported statistically. These r e s u l t s were The ANOVA f o r the amount o f time spent i n the f e e d i n g n i c h e showed no s i g n i f i c a n t e f f e c t o f i m p l a n t l o c a t i o n (accumbens vs. caudate) and no s i g n i f i c a n t l o c a t i o n by event interaction. However, there was a s i g n i f i c a n t w i t h i n s u b j e c t s e f f e c t ( F ( l , l l ) = 56.55, p<.00001). The amount o f time spent i n the n i c h e was s i g n i f i c a n t l y g r e a t e r d u r i n g the CS+ than the CS- f o r both groups. The ANOVA f o r l a t e n c y t o e n t e r the niche a l s o showed no s i g n i f i c a n t e f f e c t o f i m p l a n t l o c a t i o n or l o c a t i o n by event interaction. A g a i n , t h e r e was a s i g n i f i c a n t w i t h i n s u b j e c t s e f f e c t ( F ( l , l l ) = 152.19, p<.00001). Here, t h e 53 Figure 9 Mean amount o f time spent i n the feeding niche during the CS+ and C S - periods p r i o r to the of food d e l i v e r y . Data are the average of onset 20 c o n s e c u t i v e t r i a l s on which a good chronoamperometric record was o b t a i n e d . bars r e p r e s e n t *p<.0001 Error standard e r r o r of the mean. 60 caudate 50 H 40 H n. accumbens 30 20 H 1(H CS- 55 F i g u r e 10 Mean l a t e n c y to enter niche f o l l o w i n g CS+ or C S onset on the same t r i a l s as shown i n Figure 8. E r r o r bars r e p r e s e n t standard e r r o r of the mean. *p<.0001 57 Figure 11 Mean changes i n DA o x i d a t i o n c u r r e n t i n the n u c l e u s accumbens (n=8) and caudate (n=5) d u r i n g CS+, CS- and pre-CS b a s e l i n e p e r i o d s (bCS+, bCS-). S i g n i f i c a n t d i f f e r e n c e s were found between v a l u e s obtained from the n u c l e u s accumbens and caudate (p<.03) and between the CS+ and CS- (p<.0004). The vertical l i n e d i v i d i n g b a s e l i n e and event r e s u l t s i n d i c a t e s t h a t no s t a t i s t i c a l comparison was made betwen the two c o n d i t i o n s , due t o l a r g e sample s i z e d i s c r e p a n c i e s . standard e r r o r o f the mean. E r r o r bars represent 59 Figure 12 Representative chronoamperometric records from four d i f f e r e n t CS+ and C S - t r i a l s . meal onset. a),b) subjects during obtained consecutive Arrows i n d i c a t e CS+, C S - , or n. accumbens c),d) caudate TIME (min.) 61 (yu) }u©xino uop,opjxo (yu) }u©jjno uoftDpjxo 64 Figure 13 B e h a v i o r a l and e l e c t r o c h e m i c a l r e s u l t s e x t i n c t i o n s u b j e c t #1. extinction E = first from day o f a ) mean amount o f t i m e s p e n t i n feeding niche d u r i n g CS+ p e r i o d o x i d a t i o n current recorded accumbens d u r i n g CS+ b) mean c h a n g e i n from t h e n u c l e u s 65 (s) oijom U J euni i 1 LO O CVJ C\J 1 LO 1 O t— r LO o O o (VU) j u e j j n o uoijepjxo in e6ueqo 67 Figure 14 B e h a v i o r a l and e l e c t r o c h e m i c a l r e s u l t s from e x t i n c t i o n s u b j e c t #2. extinction E = f i r s t day o f a) mean amount o f time spent i n feeding niche during CS+ p e r i o d o x i d a t i o n c u r r e n t recorded b) mean change i n from the nucleus accumbens and caudate d u r i n g CS+. time in niche (s) 89 69 ( V U ) j u e j j n o uojiepjxo UJ e6uei|o 70 F i g u r e 15 B e h a v i o r a l and e l e c t r o c h e m i c a l extinction subject #3. results from E = f i r s t day of extinction, R = f i r s t day of r e i n i t i a t i o n conditioned feeding i n feeding a) mean amount of time spent niche d u r i n g CS+ p e r i o d b) mean change i n o x i d a t i o n c u r r e n t recorded from the accumbens and caudate during CS+. signal to from the caudate nucleus Electrochemical was l o s t on day 8. (S) 9 i p j U Uj 9UJI1 (VU) j u e j j n o uoji&pixo m e6ueqo 73 F i g u r e 16 Representative CS+ sweep t r i a l The f i g u r e presents from one nine consecutive at lOmV/s with a 10-min inter-sweep arrow i n d i c a t e s subject. sweeps ramped interval. the DA peak that was obtained during the CS+ p e r i o d p r i o r to food d e l i v e r y . The oxidation current (nA) 75 F i g u r e 17 Mean peak h e i g h t r a t i o s from 10 CS+ and 10 CSvoltammetric sweep t r i a l s recorded from the nucleus accumbens o f a s i n g l e r a t . Error represent standard e r r o r of the mean. *p=.001 bars 76 os- cs* 77 Figure 18 E l e c t r o d e placements d i r e c t e d at the l e f t nucleus accumbens (n=8) and r i g h t caudate (n=5) i n Experiment 2. 78 + 1.2mm +0.7mm l a t e n c y to e n t e r the n i c h e f o l l o w i n g CS onset was s i g n i f i c a n t l y g r e a t e r d u r i n g the CS- than the CS+ f o r both groups. F i g u r e 11 shows the r e s u l t s from the e l e c t r o c h e m i c a l measurements. t h a t the CS+ than the CS-. corresponding I t i s e v i d e n t from the figure has a g r e a t e r e f f e c t on DA o x i d a t i o n c u r r e n t s I n a d d i t i o n , the chronoamperometric response i n the nucleus accumbens appears to be g r e a t e r than i n the caudate. C o n s i s t e n t w i t h t h i s , the ANOVA d e t e c t e d a s i g n i f i c a n t e f f e c t of i m p l a n t l o c a t i o n ( F ( l , l l ) = p<.03; accumbens > c a u d a t e ) , and o f event p<.0004; CS+ interaction. 6.82, ( F ( l , l l ) = 30.78, > CS-), but no s i g n i f i c a n t l o c a t i o n by event F i g u r e 11 a l s o p r e s e n t s average pre-CS baseline d i f f e r e n c e scores. Although were not compared s t a t i s t i c a l l y these b a s e l i n e v a l u e s to event s c o r e s because of the d i s c r e p a n c y i n sample s i z e , they are i n c l u d e d to g i v e an i n d i c a t i o n of b a s e l i n e f l u c t u a t i o n . The average magnitude o f change i n o x i d a t i o n c u r r e n t observed d i f f e r e n c e s c o r e s was f o r these b a s e l i n e s i m i l a r to the average s c o r e s i n the CS- c o n d i t i o n , both o f which were s u b s t a n t i a l l y s m a l l e r than the v a l u e s o b t a i n e d f o r the CS+. F i g u r e 12 d e p i c t s r e p r e s e n t a t i v e chronoamperometric r e c o r d s d u r i n g CS+ and CS- t r i a l s from f o u r d i f f e r e n t s u b j e c t s (2 n. accumbens, 2 c a u d a t e ) . These r e c o r d s i n d i c a t e an immediate i n c r e a s e i n DA o x i d a t i o n c u r r e n t a t CS+ o n s e t , w i t h no apparent change i n response to the I n d i v i d u a l data from s u c c e s s i v e d a i l y t e s t s are CS-. 80 presented Figures 13, 14, and 15. dramatic the decreases niche first 13a, 14a, and 15a, i n t h e a v e r a g e amount o f time s p e n t i n t h e CS+ a l o n e Although found 13b, during " e x t i n c t i o n " subjects i n In Figures day o f , and c o n t i n u i n g period. are f o r each o f the three less period, c a n be s e e n on t h e throughout, the e x t i n c t i o n pronounced, c o r r e s p o n d i n g i n the average o x i d a t i o n c u r r e n t 14b, and 1 5 b ) . Figure 15 i l l u s t r a t e s decreases measures (Figures t h a t when t h e CS+/food c o n d i t i o n was r e - e s t a b l i s h e d , t h e amount o f time spent i n the niche immediately even h i g h e r than those extinction. However, there is i s an apparent detected 16. records and in records trial that were t h e 2 days p r i o r t o viable channel, any n o t a b l e increase i n oxidation current. i n which l i n e a r were o b t a i n e d , sweep i s presented i n Figure c a n be s e e n f o l l o w i n g CS + f o r two sweeps. A v e r a g e peak height were c a l c u l a t e d from a s e r i e s o f 10 CS+ and 10 CSof this type are presented Figure 17. significant with to levels i n t h e one r e m a i n i n g i n peak, h e i g h t and c o n t i n u i n g ratios during f o r t h e a v e r a g e change An i n c r e a s e onset observed 2-day l a g b e f o r e A representative voltammetric increased (CS+ r a t i o i n Figure The t - t e s t = 1.09, CS- r a t i o 17. The r e s u l t s p e r f o r m e d on t h i s =0.95) are presented data detected a d i f f e r e n c e between t h e CS+ and CS- c o n d i t i o n s , t h e CS+ peak h e i g h t t h a n t h e CS- r a t i o s Histological ratios (t(18)=3.95, confirmation accumbens e l e c t r o d e being placements significantly greater p=.001). o f t h e c a u d a t e and i s presented nucleus i n F i g u r e 18. 81 Discussion The b e h a v i o r a l r e s u l t s from t h i s experiment provide a c l e a r demonstration of the s u b j e c t s ' a b i l i t y to d i s c r i m i n a t e between CS+ and CS- c o n d i t i o n s . The average amount of time spent i n the niche d u r i n g the CS alone p e r i o d s u b s t a n t i a l l y g r e a t e r d u r i n g the CS+ enter the niche upon CS onset was CS- t r i a l s . all was t r i a l s and latency to much g r e a t e r during the In f a c t , most r a t s r a r e l y entered the niche at d u r i n g the CS- p e r i o d . The r e s u l t s of both measures suggest that the r a t s were responding i n a n t i c i p a t i o n of a meal p r e d i c t e d by the CS+, but not the CS-. The corresponding e l e c t r o c h e m i c a l r e s u l t s that changes i n the neurochemical d i s c r i m i n a t i o n between CS+ f o l l o w i n g CS+ onset was indicated signal also reflected and CS-. Increased DA the release i n d i c a t e d by changes i n the chronoamperometric r e c o r d s from both the nucleus accumbens and caudate. G e n e r a l l y , the i n c r e a s e d o x i d a t i o n c u r r e n t s i n the caudate p a r a l l e l l e d those i n the n. accumbens, although the magnitude of these i n c r e a s e s were lower i n the caudate. The r e s u l t s provide evidence that e x t r a c e l l u l a r l e v e l s of DA may be a f f e c t e d s e l e c t i v e l y by s a l i e n t environmental external cues. Although the magnitude of change f o r a given t r i a l v a r i e d both w i t h i n and between s u b j e c t s , a c h a r a c t e r i s t i c p a t t e r n c o u l d be c l e a r l y seen i n the chronoamperometric 82 record. I n t r i a l s d u r i n g which an i n c r e a s e c o u l d be o b s e r v e d unambiguously, the r e c o r d t y p i c a l l y showed an immediate and r a p i d a s c e n t onset. The to peak l e v e l s f o l l o w i n g CS+ l a t e n c y from CS+ onset to peak c u r r e n t approximated 5 min. A more g r a d u a l decrease to b a s e l i n e o b s e r v e d , u s u a l l y w i t h i n 20 to 30 min a f t e r s t i m u l u s The was onset. e x t i n c t i o n experiment c o n s i s t e d of p r o v i d i n g ad 1 i b i t u r n access to the l i q u i d d i e t i n a R i c h t e r tube, c o i n c i d e n t w i t h s t o p p i n g the d e l i v e r y o f food on a l l CS+ trials. Three r a t s were t e s t e d i n e x t i n c t i o n and on each e x t i n c t i o n day, both the time spent i n the n i c h e and the average change i n the o x i d a t i o n c u r r e n t decreased to l e v e l s below those o b t a i n e d expected, f o r the 2 p r e c e d i n g b a s e l i n e days. on the f i r s t day of e x t i n c t i o n , the amount of time spent i n the n i c h e d u r i n g the CS+ d e c r e a s e d over trials. day, As However, even d u r i n g the f i r s t successive few t r i a l s on this b e h a v i o r a l responses were s u b s t a n t i a l l y reduced r e l a t i v e to the p r e - e x t i n c t i o n p e r i o d . One f a c t o r that c o u l d account f o r t h i s o b s e r v a t i o n i s s a t i e t y . r a t s were g i v e n f r e e access to the l i q u i d d i e t , they have f e d to s a t i a t i o n and as a r e s u l t , may responsive Since the may have been l e s s to e x t e r n a l cues s i g n a l l i n g food. changes were observed i n both the b e h a v i o r a l Although and e l e c t r o c h e m i c a l measures, the b e h a v i o r a l changes were much more pronounced. I f DA p l a y s a r o l e i n the p r o c e s s i n g of s a l i e n t e x t e r n a l cues and the i n i t i a t i o n of g o a l - d i r e c t e d behaviors, i t i s p o s s i b l e t h a t some degree o f p r o c e s s i n g was 83 still o c c u r r i n g i n response to the CS+ d u r i n g the e x t i n c t i o n phase. The r a t s t h a t were p l a c e d under e x t i n c t i o n had been exposed t o the c o n d i t i o n e d f e e d i n g program f o r s e v e r a l weeks, a l l o w i n g f o r the development o f a s t r o n g a s s o c i a t i o n between t h e CS+ and food. During e x t i n c t i o n , the n e u r a l response t o the CS+ may have p e r s i s t e d t o some e x t e n t , although i t may n o t have been s u f f i c i e n t t o generate a b e h a v i o r a l approach to the n i c h e . The i n d i v i d u a l sweep t r i a l s showed r e l a t i v e l y changes i n peak h e i g h t f o l l o w i n g CS+ o n s e t . small However, these changes were c o n s i s t e n t w i t h the chronoamperometric i n t h a t the t r e n d s were i n the same d i r e c t i o n . records That i s , i n c r e a s e s were o b s e r v e d d u r i n g CS+, but not CS- t r i a l s . advantage o f a sweep t r i a l One i s t h a t i t a l l o w s the o b s e r v a t i o n of p o t e n t i a l changes i n the s i z e , shape, and p o s i t i o n , o f the proposed DA peak. disadvantages However, t h e r e a r e a l s o d e f i n i t e t o t h e use o f t h i s t e c h n i q u e . temporal r e s o l u t i o n i s l i m i t e d . F i r s t , the I t i s p o s s i b l e t h a t any change i n DA r e l e a s e c o r r e l a t e d w i t h the CS+ may be masked by subsequent r e - u p t a k e and d e g r a d a t i o n min i n t e r v a l between sweeps. o c c u r i n g i n the 10 F o r t u n a t e l y , the n e u r a l changes examined i n t h i s experiment tended to l a s t between 20 and 30 min, as mentioned above, a l l o w i n g the c o l l e c t i o n of 2 o r 3 sweeps d u r i n g t h i s p e r i o d . trial I n the sample sweep shown i n F i g u r e 15, the two c o n s e c u t i v e sweeps f o l l o w i n g CS+ o n s e t showed i n c r e a s e d peak h e i g h t s , whereas, the t h i r d peak was c o n s i d e r a b l y d i m i n i s h e d . This temporal 84 p a t t e r n o f change i n peak c u r r e n t i s c o n s i s t e n t w i t h the changes observed i n the chronoamperometric r e c o r d s . A second d i s a d v a n t a g e of l i n e a r sweep voltammetry a t the sweep r a t e of 10 mV/s employed here, i n v o l v e s the i n f l u e n c e o f a s c o r b i c a c i d (AA) on DA o x i d a t i o n . the o x i d a t i o n of AA Although i t s e l f does not c o n t r i b u t e to the DA peak, AA a m p l i f i e s the DA peak by c a t a l y z i n g a r e a c t i o n t h a t a l l o w s repeated and regeneration of DA from the o r t h o q u i n o n e , subsequent r e o x i d a t i o n at the e l e c t r o d e ( S t a m f o r d , 1986). surface T h i s e f f e c t i s pronounced at the slow sweep r a t e , l e a d i n g to an a m p l i f i c a t i o n of the peak. Therefore, the c o n t r i b u t i o n of AA d u r i n g slow sweeps may mask d e t e c t i o n of changes i n e x t r a c e l l u l a r levels. voltammetric DA E c h i z e n and Freed (1986) p r o v i d e evidence t h a t r e l a t i v e c o n c e n t r a t i o n of c a t e c h o l a m i n e ( i n t h i s c a s e , the DA) to AA d e t e r m i n e s the amount of a m p l i f i c a t i o n t a k i n g p l a c e . They found t h a t the a m p l i f i c a t i o n f a c t o r decreased as catechol concentration concentration. i n c r e a s e d r e l a t i v e to the As a r e s u l t , they proposed t h a t changes i n c a t e c h o l a m i n e c o n c e n t r a t i o n s the AA recorded were a c t u a l l y much lower t h a n the r e a l changes t h a t were o c c u r r i n g . During square-wave p u l s e chronoamperometry, c a t a l y t i c a m p l i f i c a t i o n i s m i n i m i z e d because of the s h o r t d u r a t i o n of e l e c t r o c h e m i c a l measurement (Blaha & Jung, i n p r e p a r a t i o n ; S t a m f o r d , 1986). 85 GENERAL DISCUSSION The experiments presented i n t h i s t h e s i s provide f u r t h e r e v i d e n c e f o r the involvement of dopamine i n f e e d i n g . The combined b e h a v i o r a l and e l e c t r o c h e m i c a l analyses r e v e a l e d d i f f e r e n t i a l changes i n DA a c t i v i t y i n the and n u c l e u s accumbens d u r i n g a p p e t i t i v e and phases o f striatum consummatory feeding. T h i s work r e p r e s e n t s an e x t e n s i o n of the conditioned f e e d i n g s t u d i e s conducted p r e v i o u s l y by B l a c k b u r n (1987, 1989a, 1989b). While the b e h a v i o r a l paradigm f o l l o w e d i n both t h i s t h e s i s and the B l a c k b u r n s i m i l a r , the approach used to a n a l y z e i s very d i f f e r e n t . pharmacological studies i s dopaminergic a c t i v i t y B l a c k b u r n ' s experiments i n v o l v e d manipulations whereas the p r e s e n t et a l . and ex. v i v o t i s s u e a n a l y s e s , i n v e s t i g a t i o n employed the technique of i n v i v o e l e c t r o c h e m i s t r y w i t h c h r o n i c a l l y implanted s t e a r a t e - m o d i f i e d carbon paste e l e c t r o d e s . permitted DA This technique the r e c o r d i n g of ongoing changes i n e x t r a c e l l u l a r l e v e l s o f f r e e l y moving s u b j e c t s . To b e g i n t h i s d i s c u s s i o n , a summary of the major r e s u l t s obtained i n the c u r r e n t study i s presented. In Experiment I , i n c r e a s e s i n DA a c t i v i t y were observed i n both the n u c l e u s accumbens and the caudate f o l l o w i n g the p r e s e n t a t i o n of a l i q u i d meal. Changes observed f o r each of the f o u r s u b j e c t s were very s i m i l a r i n magnitude and temporal p a t t e r n f o r both b r a i n s t r u c t u r e s . The similarity 86 of r e s p o n s e s a t both r e c o r d i n g s i t e s might i n d i c a t e a common r o l e f o r the caudate and accumbens i n f e e d i n g . The f a c t t h a t t h e i n c r e a s e s i n the chronoamperometric r e c o r d behind lagged the i n i t i a t i o n o f meal consumption suggests t h a t the o b s e r v e d e f f e c t s may be due t o p o s t i n g e s t i v e f a c t o r s . Experiment I I a l l o w e d a more d e t a i l e d a n a l y s i s o f d o p a m i n e r g i c a c t i v i t y d u r i n g f e e d i n g by f a c t o r i n g the behaviors phases. i n t o separate a n t i c i p a t o r y and consummatory With t h i s approach, i t was d i s c o v e r e d t h a t changes i n e l e c t r o c h e m i c a l e s t i m a t e s o f e x t r a c e l l u l a r DA l e v e l s c o r r e s p o n d e d t o the p r e s e n t a t i o n o f an e x t e r n a l cue (CS+) which, through p r e v i o u s e x p e r i e n c e , p r e d i c t o r o f meal d e l i v e r y . had become a r e l i a b l e S i g n i f i c a n t i n c r e a s e s i n DA o x i d a t i o n c u r r e n t s were observed i n both the nucleus accumbens and the caudate, but the magnitude o f change observed i n the nucleus accumbens d u r i n g stimuli p r e s e n t a t i o n s was s i g n i f i c a n t l y g r e a t e r than i n the caudate. When the b e h a v i o r a l paradigm was a l t e r e d such t h a t the CS+ was no l o n g e r p r e d i c t i v e o f a meal ( t h e e x t i n c t i o n p h a s e ) , the DA response t o the e x t e r n a l s t i m u l u s diminished accordingly. The r e s u l t s from Experiment I I were c o n s i s t e n t w i t h hypotheses a s s o c i a t i n g DA a c t i v i t y and " i n c e n t i v e m o t i v a t i o n " as d i s c u s s e d i n the I n t r o d u c t i o n . As p r e d i c t e d by i n c e n t i v e m o t i v a t i o n a l t h e o r i e s o f DA f u n c t i o n , i n c r e a s e s i n DA a c t i v i t y were observed f o l l o w i n g onset o f an e x t e r n a l s t i m u l u s p r e d i c t i v e o f a meal. In a d d i t i o n , increased 87 entries into appetitive stimulus" theories the feeding responding, (CS+) and C S - o n s e t . Also f i n d i n g that between the neutral s t i m u l i , CS+ w i t h food, caudate CS- that were f o l l o w i n g CS+ CS+ and t h e C S - p r e s u m a b l y presentation of became a s s o c i a t e d from the significantly greater the with accumbens and than those following onset. compared, change it results from E x p e r i m e n t s I and I I c a n been i n DA l e v e l s , seen that as factors First, feeding subjects handling, for from a b s o l u t e was g r e a t e r procedure. levels feeding palatable of with this DA b e f o r e into a test experiments, meal. elevated as the or to the levels. subjects A second e x p l a n a t i o n m a g n i t u d e may be t h a t the the of elevated meal d e l i v e r y due t o box t h a t for a n t i c i p a t i o n of This the daily had p r e v i o u s l y a blunted lived in difference. T h e r e f o r e , any o b s e r v e d baseline of changes There are a v a r i e t y meal d e l i v e r y may r e p r e s e n t i n Experiment II chambers. of are magnitude i n E x p e r i m e n t I may have had placement following because average w h i c h c o u l d have c o n t r i b u t e d t o extracellular highly the estimated chronoamperometric s i g n a l , conditioned used incentive observed repeated in oxidation currents When the the significant were of "incentive there after CS+ a l o n e this with A l t h o u g h b o t h the the be a measure consistent DA r e s p o n s e s began as increases assumed t o were o b s e r v e d d u r i n g period. was t h e differences niche, been a increase response was n o t a factor i n the r e c o r d i n g difference two e x p e r i m e n t s were in investigating the r o l e of DA feeding. i n r e l a t i o n to two d i f f e r e n t a s p e c t s In Experiment I , s u b j e c t s were p r o v i d e d s m a l l l i q u i d meal t h a t was with a s u f f i c i e n t l y p a l a t a b l e to i n i t i a t e immediate i n g e s t i o n even i n non-deprived Small i n c r e a s e s i n DA animals. a c t i v i t y , which tended to occur f o l l o w i n g meal consumption, may ingestional factors. of have been due to post- I n Experiment I I , a d i s t i n c t a n t i c i p a t o r y phase was added to the f e e d i n g s e s s i o n s . The r e l a t i v e l y l a r g e i n c r e a s e s observed d u r i n g t h i s phase suggest t h a t DA may important p l a y a d i f f e r e n t , and p o s s i b l y more r o l e d u r i n g a p p e t i t i v e r e s p o n d i n g than d u r i n g subsequent consummatory and p o s t - i n g e s t i v e stages of feeding. The preceding d i s c u s s i o n of magnitudes of change i n DA o x i d a t i o n c u r r e n t s must be approached w i t h c a u t i o n . e x a c t l y does the magnitude r e p r e s e n t ? What Although i t may r e f l e c t actual quantitative differences in extracellular l e v e l s , i t may a l s o be a f f e c t e d by f a c t o r s such as q u a l i t y of the r e c o r d i n g e l e c t r o d e s u r f a c e , and of s c a r t i s s u e s u r r o u n d i n g an i m p l a n t . DA the the amount A l s o , the placement of the e l e c t r o d e would have an i n f l u e n c e . I t i s well e s t a b l i s h e d t h a t the s t r i a t u m , f o r example, i s not a homogeneous system o p e r a t i n g i n synchrony. The proximity of an e l e c t r o d e to an a c t i v e s i t e of t r a n s m i t t e r r e l e a s e would be expected to a f f e c t the magnitude of any recorded I n a d d i t i o n , a s m a l l change i n one s t r u c t u r e may changes. have g r e a t e r f u n c t i o n a l s i g n i f i c a n c e than a l a r g e change a t a 89 d i f f e r e n t neural site. These f a c t o r s s h o u l d when g e n e r a l i z a t i o n s a r e made c o n c e r n i n g be k e p t i n mind relative magnitudes o f c h a n g e , e s p e c i a l l y when t h e y a r e b a s e d on s m a l l sample sizes. C o m p a r i s o n o f t h e c a u d a t e and n u c l e u s accumbens across both experiments r e v e a l s further interesting results. Whereas t h e p a t t e r n a n d m a g n i t u d e o f i n c r e a s e f o l l o w i n g meal d e l i v e r y a r e s i m i l a r Experiment I , there two brain sites records i n DA release f o r both s t r u c t u r e s i n i s a s i g n i f i c a n t d i f f e r e n c e between the i n Experiment I I . This r e s u l t again s u g g e s t s t h a t we may be i n v e s t i g a t i n g two d i f f e r e n t a s p e c t s of feeding behaviors. Despite the aforementioned problems with comparisons o f magnitude, i t i s tempting to present few speculations. obtained S u p p o s e we assume t h a t t h e i n c r e a s e s i n E x p e r i m e n t I a r e p o s t - i n g e s t i o n a l and E x p e r i m e n t II are, at least i n i t i a l l y , appetitive. c a u d a t e and n. accumbens s h a r e a s i m i l a r ingestive stages of feeding, is preferentially DA t e r m i n a l s processing small involved I t may be t h a t t h e role during post- b u t t h a t t h e n u c l e u s accumbens i n appetitive responding. i n t h e n. accumbens may be i n v o l v e d i n the o f , and t h e i n i t i a t i o n o f motor r e s p o n s e s t o , e x t e r n a l cues. reflect a Could t h e magnitude o f increase the s a l i e n c e o f the cue? increases T h i s m i g h t e x p l a i n why ( l . l n A ) were o b s e r v e d accumbens i n r e s p o n s e t o t h e CS-. anticipatory arousal i n DA l e v e l s i n the nucleus Whereas some d e g r e e o f i n v o l v i n g DA c o u l d have o c c u r r e d i n r e s p o n s e t o t h i s c u e , i t may n o t have b e e n s u f f i c i e n t t o 90 m e r i t an a p p r o a c h t o w a r d t h e n i c h e . would account f o r the A similar p e r s i s t e n t , though d o p a m i n e r g i c r e s p o n s e s t o t h e CS+ explanation diminished, under e x t i n c t i o n conditions. The obtained p a t t e r n o f change i n e x t r a c e l l u l a r i n these a c t i v e during a p p e t i t i v e responding, past of a feeding i n t e r p r e t a t i o n of the the r e s u l t s present session. results during a n t i c i p a t o r y stages s t a t e of a r o u s a l sensitivity, and biologically relevant also could result increased specific Results o f DA t o any i s that increased by incentive i n an in different p a r t i c u l a r , Pfaus biological types activity i n c r e a s e d DA incentive stimuli? involvement of a n t i c i p a t o r y responding. shown t h a t DA activity of sexual i n v e s t i g a t i o n by t o be signalling activity animal c u e s , o r w o u l d i t be s t u d i e s have d e m o n s t r a t e d t h e ( 1 9 9 0 ) has r e s p o n s e s t o a cue to cues. In a d d i t i o n , an ( 1 9 8 9 ) showed DA an stimuli. l i k e l i h o o d of response, increased during a n t i c i p a t o r y stages male r a t s . DA enhanced t o the e v e n t a s s o c i a t e d w i t h t h e from o t h e r even Another Would the e n h a n c e d m o t i v a t i o n a l s t a t e make an more r e s p o n s i v e be i s associated with "aroused" m o t i v a t i o n a l s t a t e induced This t e r m i n a l s may l e v e l s r e m a i n e l e v a t e d t h r o u g h o u t and the c o m p l e t i o n activity DA experiments deserves f u r t h e r mention. Whereas E x p e r i m e n t I I s u g g e s t s t h a t DA i n d i c a t e t h a t DA l e v e l s of results i n a general is behavior in Blackburn associated with impending shock. In preparatory Possibly, s t a t e of arousal 91 that i s common t o s u b j e c t s predicting different exposed consummatory to incentive cues goals. Summary This behaviors the t h e s i s examined i n the the male r a t . i n v e s t i g a t i o n o f DA r o l e o f DA The in a p p r o a c h used has involvement the in different feeding. 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(1986) I n v i v o voltammetry: some m e t h o d o l o g i c a l c o n s i d e r a t i o n s . J o u r n a l of Neuroscience Methods, 17, 1-29. 96 S t e w a r t , J . , de W i t , H., and Eikelboom, R. (1984). Role o f u n c o n d i t i o n e d and c o n d i t i o n e d drug e f f e c t s i n s e l f a d m i n i s t r a t i o n o f 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, 9 1 ( 2 ) , 251-268. Toates, F.M. (1981). The c o n t r o l o f i n g e s t i v e behaviour by i n t e r n a l and e x t e r n a l s t i m u l i - a t h e o r e t i c a l r e v i e w . A p p e t i t e , 2, 35-50. Tombaugh, T.N., Anisman, H., and Tombaugh, J . (1980). E x t i n c t i o n and dopamine r e c e p t o r blockade a f t e r intermittent reinforcement t r a i n i n g : f a i l u r e to observe f u n c t i o n a l e q u i v a l e n c e . Psychopharmacology, 70, 19-28. U n g e r s t e d t , U. (1971). 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An in vivo electrochemical analysis of the role of dopamine in feeding behaviors Holmes, Lorinda Jean 1990
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Title | An in vivo electrochemical analysis of the role of dopamine in feeding behaviors |
Creator |
Holmes, Lorinda Jean |
Publisher | University of British Columbia |
Date Issued | 1990 |
Description | The involvement of dopamine in anticipatory and consummatory aspects of feeding behaviors was investigated in the present thesis. All measurements of dopaminergic activity were taken by in vivo electrochemical techniques. In Experiment 1, dopamine efflux in the nucleus accumbens and caudate of male rats was monitored during sessions in which a small, unsignalled liquid meal was consumed. Increases in the electrochemical measure of dopamine activity, which were of similar temporal pattern and magnitude, were observed in both the nucleus accumbens and striatum following meal consumption. These data suggest a possible postingestional role of dopamine in these two brain structures. In Experiment 2, a conditioned feeding paradigm was utilized to study the role of dopamine during a discrete anticipatory phase of feeding. Rats were conditioned to discriminate between a positive conditioned stimulus (CS+) predictive of meal delivery, and a negative conditioned stimulus (CS-) that was not associated with food. Increases in dopamine activity, as determined by changes in electrochemical oxidation currents, were found to be greater during the CS+ than during the CS- in both the nucleus accumbens and caudate. In addition, the magnitude of increase was greater in the nucleus accumbens than the caudate, suggesting that the accumbens may be preferentially involved in the processing of external incentive stimuli. The results support a role for dopamine in both the nucleus accumbens and caudate during appetitive or anticipatory responding for food in the male rat. |
Subject |
Dopamine -- Physiological effect Feeds Animals -- Food Dopamine -- physiology Feeding Behavior -- drug effects Feeding Behavior -- physiology Animal feeding |
Genre |
Thesis/Dissertation |
Type |
Text |
Language | eng |
Date Available | 2010-10-04 |
Provider | Vancouver : University of British Columbia Library |
Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
IsShownAt | 10.14288/1.0098090 |
URI | http://hdl.handle.net/2429/28908 |
Degree |
Master of Science - MSc |
Program |
Neuroscience |
Affiliation |
Medicine, Faculty of |
Degree Grantor | University of British Columbia |
Campus |
UBCV |
Scholarly Level | Graduate |
AggregatedSourceRepository | DSpace |
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