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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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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.  T h i s a p p r o a c h combined  behaviors  i n t o a p p e t i t i v e , consummatory, and  division  ingestional  phases, with  an  technique.  The  r e s o l u t i o n of  records  allowed  responses.  The  temporal  feeding  electrochemical the  of  t h e o r i e s b a s e d on  incentive  this  of  feeding  recording electrochemical  t h e s i s provide  motivation.  aspects  post-  c o r r e l a t i o n of ongoing b e h a v i o r a l r e s u l t s of  allowed  and  support  neural for  92  References B e r r i d g e , K.C., and S c h u l k i n , J . ( 1 9 8 9 ) . Palatability shift of a s a l t - a s s o c i a t e d i n c e n t i v e d u r i n g sodium d e p l e t i o n . The Q u a r t e r l y J o u r n a l o f E x p e r i m e n t a l Psychology, 4 1 B ( 2 ) , 121-138. 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