Endogenous opioids and feeding in the male rat, a learning approach
by Gregory Lee Burns
A thesis submitted in partial fulfillment of the requirements for the degree . of Master of Science in
Psychology
Montana State University
© Copyright by Gregory Lee Burns (1987)
Abstract:
The role of endogenous opioid peptides (EOP's) in the ingestion of highly palatable solutions in the
male rat was investigated in three experiments. In Experiment 1 acquisition of taste preference for
sucrose or saccharin was blocked by the opiate antagonist naloxone. Following removal of the drug,
preference quickly recovered; with intake levels surpassing that of controls. In Experiment 2 preference
for a sucrose solution was enhanced with a low dose of the kappaopiate receptor agonist U 50,488H.
The high level of intake for this group was maintained for five days following removal of the drug. In
Experiment 3 the release of EOP's in response to actual, or expected, intake of a palatable solution was
assessed with an analgesia test. No decrease in pain sensitivity was observed when a delay was
included between ingestion (or expected ingestion) and the analgesia test. However, testing without
delay demonstrated significant analgesia effects following intake. These effects were reversible by
administration of naloxone, suggesting that the analgesia was the result of EOP release. Taken together,
the results of these experiments suggest that one of the functions of EOP's is the reinforcement of
preference learning. ENDOGENOUS OPIOIDS AND FEEDING IN THE
MALE RAT: A LEARNING APPROACH
by
G re g o ry Lee Burns
A th e s i s subm itted in p a r t i a l f u lf i l l m e n t
of th e requirem ents fo r the degree .
of
M aster o f S cience
in
Psychology
MONTANA STATE UNIVERSITY
Bozeman,Montana
J u n e I 98?
ii
APPROVAL
of a thesis
s u b m i t t e d by-
G re g o ry Lee Burns
T h i s t h e s i s h a s b e e n r e a d by e a c h member o f t h e t h e s i s c o m m i t t e e
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u s a g e , f o r m a t , c i t a t i o n s , b i b l i o g r a p h i c s t y l e , and c o n s i s t e n c y , and
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P e r m i s s i o n f o r e x t e n s i v e q u o t a t i o n from o r r e p r o d u c t i o n o f t h i s
t h e s i s , may be g r a n t e d by my m a j o r p r o f e s s o r ,
t h e De an o f L i b r a r i e s
o r i n h i s a b s e n c e , by
when, i n t h e o p i n i o n o f e i t h e r ,
th e proposed
u s e o f t h e m a t e r i a l i s f o r s c h o l a r l y p u r p o s e s . Any c o p y i n g o r u s e o f
the m a te ria l
in t h i s
t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d
w i t h o u t my w r i t t e n p e r m i s s i o n .
Date
<,
iv
ACKNOWLEDGMENTS
I
would l i k e
been in v o l v e d ,
t o e x p r e s s my s i n c e r e g r a t i t u d e t o e v e r y o n e who h a s
b o th d i r e c t l y and i n d i r e c t l y ,
in th is p ro ject.
In
p a r t i c u l a r , t h e member s o f my c o m m i t t e e h a v e b e e n o f i n v a l u a b l e
assistance
a nd h a v e h e l p e d t o make t h i s
learning process.
Dr. W esley Lynch,
thesis
a most v a lu a b le
t h e c h a i r m a n o f my c o m m i t t e e ,
t h e p e r s o n who s t i m u l a t e d my i n t e r e s t i n t h i s a r e a o f p s y c h o l o g y .
course,
is
Of
he a l s o h a p p e n e d t o be t h e p e r s o n who p r o v i d e d t h e l a b s p a c e ,
anim als,
drugs,
and c o u n t l e s s hours o f c o n s u l t a t i o n so t h a t I might
know a b i t a b o u t t h e p h y s i o l o g y i n v o l v e d .
gratefu l.
To hi m I w i l l a l w a y s be
D r . R o b e r t P a t t e r s o n a r r i v e d l a t e on t h e s c e n e y e t he
a g r e e d t o be a member o f my c o m m i t t e e ;
as s u c h h e w orked d i l i g e n t l y
t o e n s u r e t h a t t h e w r i t i n g a n d f l o w o f i d e a s c o n t a i n e d h e r e i n was o f
an a c c e p t a b l e q u a l i t y .
I am n o t i m p l y i n g t h a t h e i s t o
w r i t i n g i s n o t up t o p a r ;
the
i n s t e a d I am t r y i n g t o t h a n k h i m f o r t h e
t i m e and e f f o r t he i n v e s t e d i n t h i s
H o r s w i l l . .,. . t h a n k s f o r a l l
blame i f
project.
To D r . R i c h a r d
you h a v e s h a r e d w i t h me o v e r t h e l a s t
couple of y e a rs.
F inally,
thanks i s
due t o my o f f i c e p a r t n e r a n d f e l l o w g r a d u a t e
s t u d e n t , R oberta W in ters.
interest,
made i t
caring,
a n d n e v e r e n d i n g s t r e a m o f b ad j o k e s ,
through t h i s
course th a t I did.
W ithout h er u n c o n d itio n a l s u p p o r t,
I may n o t h a v e
p r o g r a m w i t h my s a n i t y i n t a c t . . . a s s u m i n g o f
T h a n k s t o you a l l .
V
TABLE OF CONTENTS
ACKNOWLEDGMENTS
LIST OF TABLES
LIST OF FIGURES
ABSTRACT............
INTRODUCTION . .
EXPERIMENT I
Method . . .
R esults ..,
D iscussion
EXPERIMENT 2
Method . . .
R esults ..
D iscussion
EXPERIMENT
Method . . .
R esults ..
D iscussion
GENERAL DISCUSSION.
REFERENCES CITED
. iv
. vi
vii
v iii
I
. 13
. 13
. 15
. 20
23
23
2-4
27
29
29
32
33
37
43
vi
LIST OF TABLES
Table
Page
1
Treatm ent
C o n d i t i o n s f o r E x p e r i m e n t I ......................................... 13
2
ANOVA f o r
Experim ent I ,
Days 1 - 1 0 .................................................... 1 7
3
ANOVA f o r
Experiment I,
Days 1 0 - 1 5 . . . - . .......................................
18
4
ANOVA f o r
Experiment I ,
Da ys 1 5 = 2 0 .................................................
19
5
T r e a t m e n t C o n d i t i o n s f o r E x p e r i m e n t 2 ............ '.........................
23
6
ANOVA f o r E x p e r i m e n t 2 , Da y s 1 - 1 0 ................................................... 26
7
ANOVA f o r Experiment 2, Days 10-15
27
vii
LIST OF FIGURES
Figure
Page
1
N alo x o n e E f f e c t on Sweet I n t a k e
2
E f f e c t o f Drug H i s t o r y on P r e f e r e n c e
3
A g o n i s t E f f e c t s on S u c r o s e I n t a k e
4
S c h e m a t i c o f T a i l - F l i c k D e v i c e f o r E x p e r i m e n t 3 ...............
5
E f f e c t o f D e l a y a n d NAL o n A n a l g e s i a ............................................. 35
\
.......................................................
16
................* .................... 21
.............................
25
31
,
v iii
ABSTRACT
The r o l e o f e n d o g e n o u s o p i o i d p e p t i d e s ( E O P ' s ) i n t h e i n g e s t i o n
o f h i g h l y p a l a t a b l e s o l u t i o n s i n t h e m a l e r a t was i n v e s t i g a t e d i n
th re e experim ents.
In Experim ent I a c q u i s i t i o n o f t a s t e p r e f e r e n c e
f o r s u c r o s e o r s a c c h a r i n was b l o c k e d by t h e o p i a t e a n t a g o n i s t
naloxone.
F o llo w in g removal of th e d ru g , p r e f e r e n c e q u ic k ly
re c o v e re d , with in ta k e le v e ls s u rp a ssin g th a t of c o n tr o ls .
In
E x p e r i m e n t 2 p r e fe r e n c e f o r a s u c r o s e s o l u t i o n was e n h a n c e d w i t h a
low d o s e o f t h e k a p p a S o p i a t e r e c e p t o r a g o n i s t U«50,488H.
The h i g h
l e v e l o f i n t a k e f o r t h i s g r o u p was m a i n t a i n e d f o r f i v e d a y s f o l l o w i n g
removal of th e drug.
I n E x p e r i m e n t 3 t h e r e l e a s e o f EOP’ s i n
r e s p o n s e t o a c t u a l , o r e x p e c t e d , i n t a k e o f a p a l a t a b l e s o l u t i o n was
a s s e s s e d w i t h an a n a l g e s i a t e s t .
No d e c r e a s e i n p a i n s e n s i t i v i t y was
o b s e r v e d when a d e l a y was i n c l u d e d b e t w e e n i n g e s t i o n ( o r e x p e c t e d
i n g e s t i o n ) and t h e a n a l g e s i a t e s t .
However, t e s t i n g w i t h o u t d e l a y
dem onstrated s i g n i f i c a n t a n a lg e s ia e f f e c ts fo llo w in g in ta k e .
These
e f f e c t s w e r e r e v e r s i b l e by a d m i n i s t r a t i o n o f n a l o x o n e , s u g g e s t i n g
t h a t t h e a n a l g e s i a was t h e r e s u l t o f EOP r e l e a s e .
Taken t o g e t h e r ,
t h e r e s u l t s of t h e s e e x p e rim e n ts s u g g e s t t h a t one of t h e f u n c t i o n s of
EOP's i s t h e r e i n f o r c e m e n t o f p r e f e r e n c e l e a r n i n g .
I
INTRODUCTION
Morphine
( a d e r i v a t i v e o f t h e o p i u m p o p p y ) h a s l o n g b e e n known
t o p o s s e s s a n a l g e s i c and e u p h o r i c p r o p e r t i e s and h a s been u sed
m e d i c i n a l l y f o r more t h a n a c e n t u r y .
A num be r o f o p i o i d compounds
h a v e b e e n d e v e l o p e d t h a t e i t h e r m i mi c o r b l o c k t h e e f f e c t s
m orphine;
of
t h i s has le d to the proposal th a t s p e c if ic o p io id re c e p to rs
e x i s t w ith in th e n ervous system t h a t a r e th e s i t e o f a c t i o n f o r th e
o p ia te drugs
(Reid,
19 8 5 ) .
In a c la s s ic study.
P e r t and Snyder
(1973) d e m o n s t r a t e d t h e e x i s t e n c e o f s p e c i f i c o p i o i d r e ce p to r s i n t h e
nervous system ,
l e a d i n g t o t h e c o n c l u s i o n t h a t t h e r e m i g h t be
n a t u r a l l y produced n e u r o tr a n s m itte r s w i t h i n t h e b r a i n w h i c h b i n d w i t h
these re c e p to rs .
In the l a s t
15 y e a r s e f f o r t s h a v e b e e n d i r e c t e d
t o w a r d s i d e n t i f y i n g t h e s e s u b s ta n c e s ( e . g . , G o l d s t e i n ,
19 76 ) w i t h a
n u m be r o f r e s e a r c h e r s r e p o r t i n g s u c c e s s f u l i d e n t i f i c a t i o n o f
endogenous p e p t i d e s
th at possess o p iate a c tiv ity
(Hughes; S m ith ,
Morgan, & F o t h e r g i l l , 1975; T e r e n i u s , & W a h ls tro m , 1975;
Goodman, & S n y d e r , 19 7 5 ) .
For ex am p le, Hughes, S m i t h , K o s t e r l i t z ,
F o t h e r g i l l , Morgan, and M o r r i s
pentapeptides
activ ity
naloxone.
peptides
P asternak,
( 1 9 7 5 ) i s o l a t e d a n d d e s c r i b e d two
( e n k e p h a l i n s ) which p o s s e s s e d o p i a t e a c t i v i t y ,
b e i n g b l o c k e d by t h e e f f e c t s
such
of the o p ia te an tag o n ist
The d i s c o v e r y o f t h e s e a n d o t h e r e n d o g e n o u s o p i o i d
(EOP’ s ) wa s i m p o r t a n t s i n c e s u c h p e p t i d e s h a v e b e e n
im p lica ted in the re g u la tio n of c e r t a i n behaviors,
such as i n g e s tio n .
2
In the present study,
th e r o l e of th e endogenous o p i o i d p e p t i d e s i n
ingestion is fu rth e r in v estig ated .
One a p p r o a c h f o r s t u d y i n g t h e e f f e c t s o f e n d o g e n o u s o p i o i d s on
in g e stio n is to adm inister opioid an tag o n ists
receptors,
(which occupy t h e
t h u s b l o c k i n g t h e a c t i o n o f EO P's) and t h e n m easu re
consequent in tak e of various substances.
This approach has c l e a r l y
i m p l i c a t e d t h e o p i o i d s y s t e m i n i n g e s t i o n a l t h o u g h t h e mechanism o f
t h e r e g u l a t i o n o f i n g e s t i o n by o p i o i d s i s u n c l e a r
S i n c e t h e p i o n e e r i n g work o f Holtzman
(1975),
(Reid,
19 8 5 ) .
i n which d e c r e a s e s i n
i n g e s t i o n were r e p o r t e d f o l l o w i n g s i n g l e e x p o su re s t o t h e o p i a t e
a n ta g o n is ts naloxone
(NAL) a n d n a l t r e x o n e
(NALT), t h e r e h a v e b e e n
numerous r e p o r t s t h a t o p i o i d a n t a g o n i s t s d e c r e a s e t h e i n t a k e o f f o o d
and w a t e r
( e . g . , Levine, M orley, G o s n e ll, B illin g to n ,
1985; R e i d ,
19 8 5 ) .
example, a n t a g o n i s t s
These e f f e c t s , a r e b o th r o b u s t and r e l i a b l e .
(S iviy,
For
e f f e c t i v e l y i n h i b i t i n t a k e o f food and w ater
when a d m i n i s t e r e d p e r i p h e r a l l y
O lson,
& B artness,
(Hemmer, O l s o n , K a s t i n , McLean,
&
19 8 2 ) , when i n j e c t e d d i r e c t l y i n t o s p e c i f i c b r a i n r e g i o n s
Bermudez-1S R a t t o n i , D a r g i e , & R e i d , . 1 9 8 1 ) ,
c o n d i t i o n s such as m ild d e p r iv a t io n
N a j am, & H e rm an ,
1981).
and u n d e r s t r e s s f u l
( J a l o w i e c , P a n s k e p p , Zolovick.,
M c C a r t h y , D e t t m a r , Lynn a n d S a n g e r ( 1 9 8 1 )
dem onstrated th a t th ese e f f e c ts
are not r e s tr ic t e d to r a ts :
NAL w i l l
a l s o d i s r u p t i n t a k e i n c a t s and r a b b i t s .
Yet th e most i n t r i g u i n g f i n d i n g s p o i n t to t h e a n t a g o n i s t s '
ab ilities
to a f f e c t the in ta k e of p re fe rr e d
R eliable antagonist e ffe c ts
sweetened w ater
( e . g . , sw eet) s u b s ta n c e s .
include the decrease in in ta k e of
(L e v in e , Murray, K n e ip , Grace,
& M orley,
1982) and
3
th e b l o c k i n g o f t h e normal developm ent of p r e f e r e n c e f o r s a c c h a r i n
solutions
(Lynch,
1986).
The s u p p r e s s i o n o f t h e i n g e s t i o n o f a
h i g h l y p a l a t a b l e c a f e t e r i a d i e t has a l s o been r e p o r t e d
M andenoff, 1981).
Interestin g ly ,
n a l o x o n e ' s e f f e c t on t h e i n t a k e o f
a 20% s u c r o s e s o l u t i o n d e c r e a s e s o v e r t i m e
McLean,
P h i l l p o t t , & Olson,
(Apfelbaum &
19 8 5 ) ,
(Olson, D e l a t t e , K a stin ,
an e f f e c t t h a t has been
i n t e r p r e t e d a s due t o t h e d ev e lo p m e n t o f t o l e r a n c e
t o t h e NAL.
S i n c e - t h e e f f e c t s o f t h e E O P ' s a r e more p r o n o u n c e d f o r p r e f e r r e d
ingestables
t h a n f o r s t a n d a r d l a b o r a t o r y chow a n d w a t e r , i t
t h a t E O P ' s may f u n c t i o n t o r e g u l a t e
is,
s e e ms
t a s t e a m otivated behavior.
That
t h e E O P ' s s e e m p a r t i c u l a r l y i n v o l v e d when a n i m a l s a r e f e e d i n g
because they p r e f e r th e t a s t e of a su b sta n c e , r a t h e r than because
they a re hungry.
T h i s i d e a o r i g i n a t e d w i t h a s t u d y by Rockwood a n d
R eid (1 982) i n which r a t s
w ith open g a s t r i c f i s t u l a s
sign ifican tly
r e d u c e d i n t a k e o f a s u c r o s e s o l u t i o n f o l l o w i n g NAL i n j e c t i o n s .
was a v e r y i m p o r t a n t d i s c o v e r y s i n c e t h e f i s t u l a s
sucrose before i t
This
drained o ff the
c o u l d be a b s o r b e d i n t h e d i g e s t i v e t r a c t ,
th u s the
p r i m a r y f e e d b a c k t h e a n i m a l s r e c e i v e d r e g a r d i n g t h e s u c r o s e was
gustatory.
T h is f i n d i n g i m p lie s t h a t th e change i n t h e d r i n k i n g as a
r e s u l t o f NAL a d m i n i s t r a t i o n was d u e t o some m o d i f i c a t i o n o f t h e
t a s t e by t h e d r u g .
The i m p l i c a t i o n o f t h e s e r e s u l t s
is
t h a t NAL may
d i s r u p t e a t i n g o r d r i n k i n g by m o d i f y i n g t h e p a l a t a b i l i t y o f f o o d s .
In support of th is conjecture is
the fin d in g t h a t g e n e tic a lly
o b e s e r a t s who i n c r e a s e t h e i r i n t a k e o f p r e f e r r e d f o o d s t o a g r e a t e r
degree than t h e i r lean c o u n terp arts
(presum ably because th e obese
r a t s f i n d t h e f o o d m or e p a l a t a b l e t h a n t h e l e a n r a t s )
a r e e v e n more
4
sensitive
to the suppressant e f fe c ts
& C arter,
19 8 5 ) .
In a r e la te d study,
o f NAL ( C o o p e r ,
J a c k s o n , Morgan,
Lynch and L ibby (1 9 8 3 ) found
t h a t NAL s u p p r e s s e s i n t a k e o f a w i d e r r a n g e o f s a c c h a r i n s o l u t i o n s
when r a t s
a r e d e p r i v e d t h a n when t h e y a r e s a t e d .
in terp retatio n
is
t h a t s in c e d e p r i v e d r a t s
substances p a la ta b le ,
the an ta g o n is ts
H ere, one
f in d a wider range of
a wider ra n g e of s u b s ta n c e s w i l l
in deprived r a t s
be a f f e c t e d by
than in nondeprived r a t s .
I f t h e EOP’ s a r e i n v o l v e d i n t a s t e
p referen ces then the
a d m i n i s t r a t i o n o f o p i a t e a g o n i s t s s h o u l d i n c r e a s e t h e i n t a k e o f some
substances.
In d e e d , such f a c i l i t a t i o n of in ta k e has been r e p o r te d
when m o r p h i n e i s
Gowdey, 1 9 8 1 ) ,
i n j e c t e d in t o th e hypothalamus
a n d when d y n o r p h i n
(an endogenous kappa a g o n i s t )
in je c te d in to the v e n tr ic le s of r a t s '
These e f f e c t s
antagonists
are le ss robust,
(Levine,
(Xepperman, H i r s t ,
brains
( Morl e y & L e v i n e ,
e t a l . , 1985).
dose,
is
1981).
however, th a n th o s e found w ith th e
R e i d ( 1 9 8 5 ) n o t e d t h a t many
v a r i a b l e s may i n f l u e n c e t h e r e s p o n s e s t o o p i a t e a g o n i s t s ,
type of a g o n is t,
&
tim e of t e s t ,
including
and th e n a t u r e o f th e t e s t
environm ent.
These v a r i a b l e e f f e c t s
c a n be p a r t i a l l y e x p l a i n e d by t h e f a c t
t h a t t h e r e e x i s t a t l e a s t t h r e e t y p e s of o p i o i d r e ce p to r s
Sanger,
(Cooper &
19 8 4) w h i c h p r o b a b l y b i n d d i f f e r e n t o p i o i d p e p t i d e s .
support of t h i s
are s tu d ie s in v e s tig a tin g the e f f e c ts
In
of morphine
p u t a t i v e m u - r e c e p t o r a g o n i s t ) on i n g e s t i o n t h a t h a v e r e p o r t e d
in c o n s i s t e n t changes in in ta k e
( J a l o w i e c e t a l . , 1 98 1; M o r l e y ,
Levine,
a n d t h o s e w h i c h h a v e f o u n d more
Grace,
& K n e i p , 19 8 2 ) ,
c o n s i s t e n t i n c r e a s e s i n i n t a k e when k a p p a - r e c e p t o r a g o n i s t s a r e
(a
5
employed.
For i n s t a n c e , Morley and a s s o c i a t e s
Levine,
1982; M o rley , L e v i n e ,
Levine,
1983) have d e m o n s t r a t e d t h a t
(Morley, E lso n ,
&
G ra c e , K n e ip , & Z e u g n e r , 1983; M orley &
kappa a g o n i s t s ,
such as the
h i g h l y s e l e c t i v e U - 5 0 , 4 8 8 H a nd t h e k a p p a b e n z o d i a z e p i n e ,
tifluadom ,
c o n s i s t e n t l y s t i m u l a t e i n t a k e o f s t a n d a r d l a b o r a t o r y c how.
Thus,
the
k a p p a r e c e p t o r s u b t y p e may be m o re i m p o r t a n t t h a n o t h e r o p i o i d
receptors
Jackson,
i n th e r e g u l a t i o n o f normal fe e d in g p a t t e r n s
& Kirkham,
(C ooper,
1985).
As w i t h t h e a n t a g o n i s t e f f e c t s ,
of i n t e r e s t regarding the e f fe c ts
p re ferred substances.
there is
cu rre n tly a great deal
o f kappa a g o n i s t s on t h e i n t a k e o f
I n one s t u d y , Lynch (1983) r e p o r t e d t h a t t h e
mixed kappa/m u a g o n i s t k e t o c y c l a z o c i n e s t i m u l a t e d i n t a k e o f a
s a c c h a r in s o l u t i o n i n comparison t o a c o n tr o l
nondeprived r a t s ,
(saline)
group in
w h i l e J a c k s o n and Cooper (1985) and Cooper e t a l .
(1985) fo u n d t h a t t h e kappa a g o n i s t U-50,488H s i g n i f i c a n t l y
heightened r a t
studies
H irst,
in ta k e of h ig h ly p a la ta b le s o l i d food m ix tu re s.
and o t h e r s
(e .g .,
Leander & Hynes,
These
1 98 3; K a v a l i e r s , T e s k e y ,
&
1985) s u g g e s t t h a t t h e kappa o p i o i d r e c e p t o r s y s t e m i s
im portant fo r the expression of t a s t e p referen ces in a d d itio n to the
m o re g e n e r a l r e g u l a t i o n o f f e e d i n g p a t t e r n s .
Thus, t h e e f f e c t s o f
b o t h m u . a n d k a p p a a g o n i s t s on t h e i n t a k e o f p r e f e r r e d s w e e t s o l u t i o n s
i n n o n d e p r i v e d a n i m a l s s h o u l d be i n v e s t i g a t e d .
provide evidence concerning th e d i f f e r e n t i a l
Such s t u d i e s would
e f f e c t i v e n e s s of
a g o n i s t s which b ind w ith t h e s e r e c e p t o r s u b ty p e s .
I n o rd e r to u n d e rsta n d th e f u n c ti o n s of the o p i o i d system i n th e
r e g u l a t i o n o f f e e d i n g a nd d r i n k i n g ,
it
is im portant to e lu c id a te the
6
c a u s e and t i m i n g o f t h e r e l e a s e o f t h e o p i o i d s .
One a p p r o a c h f o r
in v e s tig a tin g these
q u e s tio n s has been to a s s e s s a c t u a l
b rain opioid le v e ls
i n re s p o n se to v a r io u s s tim u lu s e v e n ts through
th e use of radioim m unoassay te c h n iq u e s .
Levine,
and S h a fe r
For ex a m p le , M o r l e y , E l s o n ,
(1982) found i n c r e a s e d c o r t i c a l l e v e l s o f
d y n o rp h in -lik e im m unoreactivity in response to s t r e s s f u l
d e p r i v a t i o n and t a i l - p i n c h .
1986) i t
changes in
events l i k e
I n a n o t h e r s t u d y (Vaswani and T e j w a n i ,
was r e p o r t e d t h a t f o o d d e p r i v a t i o n r e s u l t s
a n d /o r decreased l e v e l s of B -endorphin in d i f f e r e n t
which r e t u r n to normal f o l l o w i n g i n t a k e .
in increased
b ra in regions,
A r e l a t e d stu d y looked a t
t h e l o c a t i o n s o f kappa r e c e p t o r s i n n o n s t r e s s e d , n o n d e p r iv e d r a t s
and
f o u n d h i g h c o n c e n t r a t i o n s i n a n u m be r o f g u s t a t o r y a n d f e e d i n g
sites
(Lynch, W a t t , K r a l l , & P a d e n , 1985).
Of p a r t i c u l a r i m p o r t a n c e
i s t h e f i n d i n g by Durn, G r a m s c h , a n d H e r z ( 1 9 8 3 ) t h a t i n g e s t i o n o f
sweet foods r e s u l t s
opioid)
i n h i g h e r l e v e l s o f B - e n d o r p h i n (an endogenous
in th e hypothalam us.
O th er r e s e a r c h e r s have p ro v id e d l e s s d i r e c t
evidence fo r r e le a s e
o f o p i o i d p e p t i d e s by s h o w i n g t h a t i n c r e a s e d f e e d i n g b e h a v i o r
r e s u l t i n g from s t r e s s
antagonists.
that stresso rs
i s b l o c k e d by t h e a d m i n i s t r a t i o n o f o p i a t e
Lo wy , M a i c k e l , a n d Yim ( 1 9 8 0 ) , f o r e x a m p l e , r e p o r te d
such as d e p r i v a t i o n and t a i l - p i n c h i n d u c e d r a t s
a n d s u c h f e e d i n g b e h a v i o r was b l o c k e d by NAL.
to eat
A more r e c e n t s t u d y
( B e r t i e r e , S y , B a i g h t s , M a n d e n o f f , a n d A p f e l b a u m , 19 8 4 ) d e m o n s t r a t e d
th a t stre ss-in d u c e d feeding is
injections
b l o c k e d n o t o n l y by NAL, b u t a l s o by
o f B - e n d o r p h i n ( a n o p i a t e a g o n is t ) i n t o t h e v e n t r i c l e s .
The a u t h o r s e m p l o y e d l e a r n i n g p r i n c i p l e s
as a framework f o r
7
understanding t h i s
apparently co n tra d icto ry finding:
the a n ta g o n ist
i n t e r f e r e d w ith th e rew arding p r o p e r t i e s of th e food w h ile th e
a g o n i s t made i n g e s t i o n u n n e c e s s a r y b e c a u s e i t
A re la ted series
shown t h a t r a t s
produced s a t i a t i o n .
o f s t u d i e s by L i e b l i c h a n d a s s o c i a t e s h a v e
which a r e g e n e t i c a l l y s e l e c t e d f o r h ig h r a t e s
of s e l f -
a d m i n i s t e r e d b r a i n s t i m u l a t i o n a l s o t e n d t o c o n s um e l a r g e a m o u n t s o f
saccharin.
A fte r re p e a te d in g e s tio n of s a c c h a rin they dem onstrate
decreased an a lg e sic responses
(L ieblich,
(i.e .,
Cohen, Ganchrow,' B l a s s ,
& Bergmann,
1984;
tolerance)
& Bergmann,
Bergmann, Cohen, & L i e b l i c h ,
t o morphine
1983;
C o h en , L i e b l i c h ,
19 8 4 ) .
These
r e s e a r c h e r s s u g g e s t t h a t r e p e a te d s a c c h a r in consum ption s t i m u l a t e s
r e l e a s e o f EOP’ s a s t h e r e s u l t o f a c t i v a t i o n o f s w e e t r e c e p t o r s i n
th e g u s t a t o r y system , l e a d i n g to t o l e r a n c e developm ent.
The r e s u l t s
c i t e d above a r e p o o r l y i n t e g r a t e d and i t re m a in s
u n c l e a r what e v e n ts
( e ith e r p h y s io lo g ic a l or environm ental) are
a s s o c ia te d w ith endorphin r e le a s e .
the o p ia te s
i n t h e r e g u l a t i o n o f f e e d i n g and d r i n k i n g i s
drive theory.
m otivates,
m aterials
One e x p l a n a t i o n f o r t h e r o l e o f
T h i s t h e o r y s u g g e s t s t h a t EOP r e l e a s e
or "d riv es",
(L eibow itz,
d e r i v e d from
directly
th e organism to i n g e s t n u t r i e n t - r i c h
19 8 5 ) .
This id e a f a i l s ,
however,
to explain
why a g o n i s t d r u g s e n h a n c e i n g e s t i o n o n l y u n d e r c e r t a i n c i r c u m s t a n c e s
o r how a n t a g o n i s t s r e d u c e i n t a k e w h i c h i s m o t i v a t e d p u r e l y by t a s t e
(Lynch,
1983; J a c k s o n & C o o p e r , 1 9 8 5 ).
An a l t e r n a t i v e e x p l a n a t i o n i s t h a t
c e rta in behaviors are
r e i n f o r c e d by t h e r e l e a s e o f e n d o r p h i n s a n d i t
is
t h i s p r o p e r ty t h a t
accounts f o r th e e stab lish m en t an d /o r m aintenance of t a s t e
8
preferences.
I n s u p p o r t o f t h i s v i e w , Mucha a n d I v e r s e n
(1984) have
shown v i a a c o n d i t i o n e d p l a c e p r e f e r e n c e p a r a d i g m t h a t m o r p h i n e i s
r e w a r d i n g a n d NAL i s a v e r s i v e
aversion).
(since i t leads
to c o n d itio n e d place
A dditional support fo r the idea th a t o p ia te s
are
r e w a r d i n g comes f r o m s t u d i e s d e m o n s t r a t i n g t h a t a n i m a l s w i l l s e l f adm inister o p ia te s
(eg. B e llu z i & S te i n ,
1977).
o p i o i d a g o n i s t s sometimes s t i m u l a t e i n t a k e ,
c e rta in concentrations
( L y n c h a nd L i b b y ,
F inally,
although
t h e y do s o f o r o n l y
1983),
a r e s u l t which
s u g g e s t s t h a t o p i a t e r e c e p t o r s t i m u l a t i o n does n o t d i r e c t l y i n d u c e
intake.
The i n v o l v e m e n t o f t h e o p i o i d r e c e p t o r s y s t e m i n t a s t e
p re fe re n c e s i s p a r t i c u l a r l y w ell s u i t e d to a c o n d itio n in g e x p la n a tio n
s i n c e p r e f e r e n c e c a n be c o n c e p t u a l i z e d a s a l e a r n e d b e h a v i o r .
instance,
if
a n a n i m a l h a s no p h y s i o l o g i c a l n e e d t o i n g e s t ,
so n o n e t h e l e s s ,
i t may be c o n c l u d e d t h a t t h e a n i m a l i s
r e i n f o r c e d i n some m a n n e r f o r e n g a g i n g i n t h e i n g e s t i v e
For
but does
being
behavior.
S i n c e EOP1s a p p e a r t o be i n v o l v e d i n t h e a c q u i s i t i o n o f t a s t e
preferences,
it
seems r e a s o n a b l e t o s u g g e s t t h a t t h e s e
p e p tid e s might
n o r m a l l y a c t as r e i n f o r c e r s f o r t h e l e a r n i n g o f p r e f e r e n c e s .
th is
idea,
To t e s t
t h e E O P ' s s h o u l d be s t u d i e d w i t h i n a n o p e r a n t c o n d i t i o n i n g
p arad ig m so a s t o i n v e s t i g a t e w h e th e r t h e r e l a t i o n s h i p
between t h e s e
endo g en o u s p e p t i d e s and p r e f e r e n c e a c q u i s i t i o n i s on e i n v o l v i n g
d ire c t m otivation,
i n s p i r e d by a d r i v e ,
or in s te a d in v o lv es
reinforcem ent fo r le a rn in g a preference.
S u p p o r t f o r t h i s n o t i o n o f l e a r n e d p r e f e r e n c e s comes f r om a
s t u d y b y Dum a n d H e r z
(1984) which p r o v i d e d e v i d e n c e f o r t h e r e l e a s e
9
o f EOF' s i n r e s p o n s e t o t h e a n t i c i p a t i o n o f r e c e i v i n g h i g h l y
p alata b le substances.
In th is
e x p e r i m e n t , r a t s w hich had been t a u g h t
t o expect a sweet su b sta n c e a p p a re n tly r e le a s e d o p io id s s in c e th ese
anim als dem onstrated a N A L-reversible decrease in s e n s i t i v i t y to
painful
s t i m u l i as compared t o c o n tr o l a n i m a l s .
To s u m m a r i z e ,
the opioid antag o n ists
a g o n i s t s sometimes i n c r e a s e i n t a k e .
d e c r e a s e i n t a k e and
These e f f e c t s ,
h o w e v e r , do n o t
dem onstrate a degree of r e l i a b i l i t y a c ro ss s i t u a t io n s
expected i f
t h e r e were a d i r e c t
s t i m u l a t i o n a nd i n g e s t i o n .
t h a t w o u l d be
c o n n e c tio n between r e c e p t o r
Since a g o n is ts
do n o t d i r e c t l y s t i m u l a t e
i n t a k e i t may be t h a t t h e y a c t i n d i r e c t l y by r e i n f o r c i n g b e h a v i o r s
which a r e co n d u c iv e t o i n g e s t i o n .
I n o t h e r w o r d s , EOP’ s may be
r e l e a s e d i n r e sp o n s e t o e n v i r o n m e n t a l c u e s t h a t i n d i c a t e t h e p r e s e n c e
of a p p ro p riate foods,
increase intake.
a n d t h i s r e l e a s e may s i g n a l a n o r g a n i s m t o
I f o p ioids are in h e re n tly rew arding,
and i f
encourage i n g e s t i o n in th e absence of p h y s io lo g ic a l need,
they
t h e n i t may
be t h a t t h e m e c h a n i s m o f o p i a t e a c t i o n i n v o l v e s o p e r a n t c o n d i t i o n i n g
r a th e r than s t r i c t l y m o tiv atio n
( i . e . , driv e).
In order to in v e s tig a te th a t p o s s ib ility ,
o f o p i o i d r e l e a s e s h o u l d be e x a m i n e d .
th e tim in g and e x t e n t
T h i s c o u l d be a c c o m p l i s h e d by
e x a m i n i n g t h e e v e n t s w h i c h c a u s e t h e r e l e a s e o f EOF' s a n d t h e
tem poral p a t t e r n of such r e l e a s e .
S p ecifically ,
it
w o u l d be
im p o rta n t t o a s s e s s whether g u s t a t o r y s t i m u l a t i o n a n d /o r the
environm ental cues t h a t p r e d ic t such s tim u la tio n
r e s u l t i n m a x i m a l EOF r e l e a s e .
(i.e .,
expectancy)
10
In a d d itio n to the operant c o n d itio n in g id ea o u tlin e d above,
th e re a re se v e ra l s u b s id ia ry is s u e s th a t deserve a t t e n t i o n .
instance,
For
t h e r o l e o f E O P ' s i n r e g u l a t i n g i n g e s t i o n may be d i f f e r e n t
for n u tritiv e
vs n o n - n u t r i t i v e s u b s t a n c e s .
s u p p o r t e d by t h e r e s u l t s
This id e a i s
in d irectly
of s t u d i e s showing t h a t th e b lo ck ad e of
p r e f e r e n c e a c q u i s i t i o n by NAL d o e s n o t d e c r e a s e o v e r t i m e f o r
saccharin
sucrose
(Lynch,
1986) b u t t h a t i t
(Olson e t a l .
1985).
does d e c r e a s e o ver tim e f o r
It is,
therefore,
a lso im portant to
d i r e c t l y i n v e s t i g a t e b o t h s a c c h a r i n a n d s u c r o s e w i t h i n t h e same s t u d y
t o s e e i f su ch a d i f f e r e n c e does e x i s t .
A n o t h e r i m p o r t a n t q u e s t i o n c o n c e r n s t h e p e r m a n e n c e o f t h e NAL
effects.
Lynch, Kra l l ,
Fernandez,
autoradiographic recep to r assays,
and Paden (1 985) showed, u s i n g
t h a t chronic blockade of o pioid
r e c e p t o r s c a u s e d a n i n c r e a s e i n t h e n u m be r o f t h e s e r e c e p t o r s
(upregulation).
An u p r e g u l a t i o n o f r e c e p t o r s c o u l d r e s u l t i n l o n g -
la s tin g a lte ra tio n s in ta ste preference.
in v e s tig a te the e f f e c ts
A lo g ic a l next step is to
o f r e p e a t e d e x p o s u r e t o NAL b y f i r s t
t h e normal p r e f e r e n c e a c q u i s i t i o n ,
blocking
th en m easuring th e re c o v e ry of
p r e f e r e n c e f o l l o w i n g removal of th e a n t a g o n i s t .
This type of study
w o u l d p r o v i d e a d d i t i o n a l d a t a s u g g e s t i n g how t h e E O P ' s may be
i n v o l v e d i n t h e a c q u i s i t i o n and m a in te n a n c e o f t a s t e
preferences.
Th e f o l l o w i n g e x p e r i m e n t s w e r e d e s i g n e d t o p r o v i d e a n s w e r s t o
the q u e stio n s r a i s e d above.
In Experim ent I th e i n t a k e o f anim als
who w e r e d r i n k i n g f o r t a s t e was b l o c k e d by a d m i n i s t r a t i o n o f t h e
o p ia te a n ta g o n is t naloxone.
previously.
Such b l o c k a d e has been r e p o r t e d
The p u r p o s e o f E x p e r i m e n t I was t o e x t e n d t h e s e r e s u l t s
by r e v e r s i n g t h e o r d e r o f t h e c o n d i t i o n s s o a s t o m e a s u r e t h e
r e c o v e r y o f p r e f e r e n c e i n t h e NAL- t r e a t e d a n i m a l s a n d t o i n v e s t i g a t e
t h e e f f e c t NAL w o u l d h a v e on a n i m a l s w h i c h h a d a l r e a d y a c q u i r e d a
taste
preference.
I t was e x p e c t e d t h a t t h e a n t a g o n i s t w o u l d h a v e a
d r a m a t i c e f f e c t on t h e l a t t e r
would n o t be t o t a l l y
learned response.
group,
but th a t t h e i r
elim in a ted because of th e s tr e n g th of th e
In a d d itio n ,
it
was t h o u g h t t h a t t h e a n i m a l s w h i c h
s t a r t e d o u t r e c e i v i n g NAL i n j e c t i o n s w o u l d ,
p r e f e r e n c e s a t an e l e v a t e d r a t e
treatm ents
resu lt
d rin k in g behavior
consequently,
b a s e d on r e p o r t s
acquire
th at antagonist
ca u se r e c e p t o r u p r e g u l a t i o n which would, pre su m a b ly ,
in extrem e s e n s i t i v i t y of the r a t s
t o any s u b s e q u e n t t a s t e -
m o t i v a t e d EOP r e l e a s e .
I n E x p e r im e n t 2,
the d i f f e r e n t i a l
e f f e c t s o f mu a n d k a p p a -
r e c e p t o r a g o n i s t s o n i n t a k e o f a s u c r o s e s o l u t i o n wa s i n v e s t i g a t e d .
B a s e d o n p r e v i o u s r e p o r t s t h a t k a p p a a g o n i s t s g e n e r a l l y y i e l d more
reliab le
effects
than a g o n is ts of o th e r o p io id r e c e p to r s u b ty p e s , i t
was e x p e c t e d t h a t t h e k a p p a a g o n i s t w o u l d i n c r e a s e i n t a k e t o a
g r e a t e r d e g r e e t h a n w o u l d t h e mu a g o n i s t .
What was new i n t h i s
d e s i g n was t h e a d d i t i o n o f a r e c o v e r y p e r i o d d u r i n g w h i c h a l l d r u g s
w ere removed w h i l e t h e a n i m a l s were a l l o w e d t o c o n t i n u e t h e i r
d r in k in g o f th e sweet s o l u t i o n .
g ro u p s showed e l e v a t e d i n t a k e ,
I t was p r e d i c t e d t h a t ,
if
any o f t h e
t h e s e l e v e l s would c o n s e q u e n t l y
d e c r e a s e and a p p r o a c h t h a t o f t h e c o n t r o l group o n ce t h e a g o n i s t d ru g
was r e m o v e d .
I n E x p e r i m e n t 3 , t h e i d e a t h a t e n d o g e n o u s o p i o i d s may s e r v e t o
r e i n f o r c e i n g e s t i v e b e h a v i o r wa s i n v e s t i g a t e d by a t t e m p t i n g t o
12
d e t e r m i n e t h e t i m e c o u r s e o f r e l e a s e o f t h e s e compou nds i n r e s p o n s e
to feeding behavior.
in analgesia
(i.e .,
I n t h i s e x p e r i m e n t , a NAL-r e v e r s i b l e i n c r e a s e
i n c r e a s e d l a t e n c y t o t a i l —f l i c k i n r e s p o n s e t o a
h e a t s t i m u l u s ) was t a k e n a s a n i n d i c a t i o n t h a t a n i n c r e a s e i n t h e
r e l e a s e o f EOP.'s h a d o c c u r r e d w i t h i n t h e b r a i n .
an algesic response is
release.
The m a g n i t u d e o f t h e
p r e s u m a b l y p r o p o r t i o n a l t o t h e a mo u n t o f t h e
I t was e x p e c t e d t h a t b o t h t h e a c t u a l
i n g e s t i o n o f a sweet
s o l u t i o n and t h e l e a r n e d a n t i c i p a t i o n o f s u c h i n t a k e would r e s u l t i n
opioid r e le a s e ,
hypothesis is
a s i n d i c a t e d by i n c r e a s e d t a i l - f l i c k
latency.
This
b a s e d on r e p o r t s i n t h e l i t e r a t u r e w hich i n d i c a t e t h a t
b o th i n g e s t i o n o f p r e f e r r e d s u b s t a n c e s and t h e a n t i c i p a t i o n of
i n g e s t i n g p r e f e r r e d f o o d s c a u s e EOP r e l e a s e
Taken t o g e t h e r ,
(Dum & H e r z ,
1984).
th e se experim ents should provide a d d itio n a l
e v id e n c e c o n c e rn in g t h e r o l e of endogenous o p i o i d s i n t h e r e g u l a t i o n
of ingestion.
One f u n c t i o n o f t h e E O P ' s may be t o r e i n f o r c e
a n t i c i p a t o r y as w e ll as i n g e s t i v e b e h a v io r s .
Since th e se
a n t i c i p a t i o n s w o u l d be l e a r n e d r e s p o n s e s r e i n f o r c e d by t h e r e l e a s e o f
EO P's, a l e a r n i n g ap p ro a c h i s
in v e s tig a tin g the re la tio n s h ip
endogenous compounds.
an a p p r o p r i a t e fram ew ork f o r
b etw e en i n g e s t i v e b e h a v i o r and t h e s e
13
EXPERIMENT I
The p u r p o s e o f E x p e r i m e n t I was t o r e p l i c a t e
a n d e x t e n d t h e work
o f L y n c h ( 1 9 8 6 ) , who f o u n d t h a t NAL i n t e r f e r e s w i t h t h e a c q u i s i t i o n
of t a s t e p rfe re n c e s,
solu tio n s in r a ts
A fter th is
by c o m p a r i n g t h e i n t a k e o f s u c r o s e o r s a c c h a r i n
g i v e n NAL f o r -10 d a y s ' t o r a t s "who r e c e i v e d s a l i n e .
p e r i o d t h e a n t a g o n i s t was r e m o v e d a n d a l l
anim als re c e iv e d
i n j e c t i o n s of s a l i n e in o rd e r to i n v e s t i g a t e the long term e f f e c ts
o p io id r e c e p t o r blockade
(see Table I ) .
w e r e u s e d t o s e e i f NAL h a s d i f f e r e n t i a l
Both s u c r o s e and s a c c h a r i n
e f f e c t s on t h e s e t wo
sw eeteners.
T ab le I . Treatm ent C o n d itio n s f o r Experim ent I
D ru g T r e a t m e n t s
Gr ou p S o l u t i o n •
Da y s 1-1 O
Da ys 1 1 - 1 5 Da ys 1 6 - 2 0
I
sucrose
NAL
SAL
SAL
2
saccharin
NAL
SAL
SAL
sucrose
3
SAL
SAL
NAL
4
saccharin
SAL
SAL
NAL
M et h od
S ubjects
F o r t y m a le H o ltzm an n S p rag u e Dawley a l b i n o r a t s , e a c h w e ig h i n g
a p p r o x i m a t e l y I 5 0 -2 0 0 g upon a r r i v a l , were p u r c h a s e d fr o m S a s c o I n c .
f o r use in t h i s
experim ent.
The a n i m a l s w e r e h o u s e d i n g r o u p s o f
f i v e a nd h a d f r e e a c c e s s t o f o o d a n d w a t e r e x c e p t d u r i n g t e s t i n g .
of
Apparatus
Intake te s ts
was f i t t e d
o c c u r r e d i n t e n i n d i v i d u a l t e s t c a g e s ea ch o f which
w i t h t wo s t a n d a r d 1 00 ml v o l u m e t r i c d r i n k i n g b o t t l e s .
of these b o ttle s
One
c o n t a i n e d w a t e r a n d t h e o th e r e i t h e r 2 0 % s u c r o s e
(w /v) o r 0 . 1% sodium s a c c h a r i n
(w/v) d i s s o l v e d i n w a t e r .
Procedure
The a n i m a l s w e r e r a n d o m l y a s s i g n e d t o t h e f o u r t r e a t m e n t
conditions.
Each day t h e a n i m a l s w ere b ro u g h t i n t o t h e e x p e r i m e n t a l
room a n d g i v e n s u b c u t a n e o u s i n j e c t i o n s
( I m l/kg).
o f e i t h e r NAL ( ! m g / k g ) o r SAL
Twenty m i n u t e s a f t e r i n j e c t i o n ,
i n t o a t e s t cage where i t
e a c h a n i m a l was p l a c e d
was p r e s e n t e d w i t h two b o t t l e s ,
c o n t a i n i n g w a t e r and t h e o t h e r t h e s w e e t s o l u t i o n .
a llo w e d t o d r i n k ad l i b .
f o r a p e r i o d o f 30 m i n u t e s ,
was r e t u r n e d t o t h e home c a g e a n d t h e t o t a l
one
The r a t was t h e n
a f t e r which i t
vol ume o f w a t e r a n d s w e e t
i n g e s t e d was r e c o r d e d .
The e x p e r i m e n t was r u n f o r a t o t a l
in itial
p re fe re n c e a c q u i s i t i o n phase
o f 20 d a y s .
During the
(days T -I0 ) t h e i n t a k e of a l l
a n i m a l s was m o n i t o r e d f o l l o w i n g d a i l y i n j e c t i o n s o f e i t h e r n a l o x o n e
or s a lin e .
T h i s p h a s e was f o l l o w e d by f i v e d a y s o f r e c o v e r y
(days I l ­
l s ) d u r i n g w h i c h t h e NAL was r e m o v e d a n d a l l a n i m a l s r e c e i v e d SAL
in j e c t i o n s w hile s t i l l
above.
The f i n a l
e x p e r i e n c i n g t h e same p r o c e d u r e s o u t l i n e d
f i v e days of the experim en t
crossover of the i n i t i a l
( d a y s 16 - 2 0 ) i n v o l v e d a
d r u g t r e a t m e n t s s u c h t h a t t h e a n i m a l s who
b e g a n t h e e x p e r i m e n t w i t h NAL ( b u t h a d b e e n r e c e i v i n g SAL f o r f i v e
d a y s ) c o n t i n u e d t o be i n j e c t e d w i t h SAL, w h i l e t h e o t h e r s
( w h i c h h ad
15
b e e n i n j e c t e d w i t h SAL up t o t h i s p o i n t ) r e c e i v e d NAL (I m g / k g , s c ) .
R esults
Figure
I illu strates
■
t h e e f f e c t o f NAL on g r o u p mean i n t a k e
v o l u m e o v e r t h e 20 d a y s o f t e s t i n g .
It
i s apparent th a t
the
a n t a g o n i s t had a s t r o n g e f f e c t on t h e i n t a k e l e v e l s o f t h e a n i m a l s ,
i n t h a t n a l o x o n e s u p p r e s s e d i n t a k e o f b o t h s u c r o s e and s a c c h a r i n
r e g a r d l e s s o f w h e t h e r i t was a d m i n i s t e r e d b e f o r e ,
or a f t e r ,
p r e f e r e n c e had been l e a r n e d .
the elev a ted in tak e
Also of i n t e r e s t i s
a taste
o f t h e g r o u p w h i c h o r i g i n a l l y was t r e a t e d w i t h NAL a n d o f f e r e d
sucrose,
at
d a y 20 o f t h e e x p e r i m e n t .
A 2 x 2 x 10 t h r e e - w a y A n a l y s i s o f V a r i a n c e
(ANOVA) f o r s p l i t -
p l o t d e s i g n s was c a l c u l a t e d on t h e d a t a f o r days. 1- 1 0 ' ( t h e
a c q u i s i t i o n p e r i o d , see Table 2 ).
type did r e l i a b l y a f f e c t
NAL r e l i a b l y
intake,
blocked in g e s tio n .
The a n a l y s i s r e v e a l e d t h a t d r u g
F(1,
36) = 7 4 . 2 2 , p < . 0 0 1 .
Here,
The a n a l y s i s a l s o r e v e a l e d t h a t
s o lu tio n type r e l i a b l y a f f e c te d in ta k e ,
F ( 1 , 36) = 7 . 7 5 ,
p < .01.
H e r e s u c r o s e was i n g e s t e d i n l a r g e r a m o u n t s t h a n s a c c h a r i n .
A
s i g n i f i c a n t i n t e r a c t i o n b e t w e e n d r u g a n d s o l u t i o n t y p e was f o u n d t o
be p r e s e n t ,
F( 1, 36) = 7 . 8 8 , p < . 0 1 .
T h i s in d ic a t e d t h a t t h e
a n i m a l s w h i c h w e r e i n j e c t e d w i t h SAL c on s um ed more s u c r o s e t h a n
s a c c h a r in w h ile th e N A L-treated anim als drank very l i t t l e
so lution.
intake,
of e ith e r
The a n a l y s i s a l s o r e v e a le d t h a t d a y s r e l i a b l y a f f e c t e d
F( 9 , 324) =' 1 5 . 4 5 , p < . 0 0 1 .
H e r e , t h e a m ou nt o f s w e e t
i n g e s t e d by t h e SAL g r o u p s i n c r e a s e d o v e r d a y s .
A l s o t h e r e was a
figure
i.
N a lo x o n e E ffe c t o n Siueet I n ta k e
NAL-Suc
TREATMENTS
SAL - ALL
M ea n In ta k e
CROSS
Gt- oxj-p
ONAL-Sacc
>6----«
' 1SAL-Suc
SAL-Sacc
9
10
11
12
13
I !
15
16
17
IO
19 2 0
Daya o f Tes tiny
N A L-Sacc
+
N a l-S tic
O
SA L -S a ec
A
SA L -Sue
17
significant
.001.
i n t e r a c t i o n o f days and d r u g ,
F(9,
3 24) = 1 4 . 7 9 ,
p <
This confirm s t h a t th e S A L -treated anim als in c re a s e d
consum ption ov er days w h ile t h e N A L -treated an im als d id n o t .
There
was a l s o a s i g n i f i c a n t i n t e r a c t i o n o f d a y s a n d s o l u t i o n ,
324) =
2.03,
p < .05.
F(9,
This f in d in g r e v e a ls t h a t th e S A L -treated animals
i n g e s t e d m o re s u c r o s e t h a n s a c c h a r i n o v e r d a y s .
T a b l e 2. A n a l y s i s o f V a r i a n c e f o r E x p e r im e n t I , .D ay s 1 - 1 0 .
Source
Between
Drug
Solution
Drug X S o l u t i o n
error
df
39
I
I
I
36
SS
21 6 7 . 0 8
1278.06
133.40
135.72
619.89
MS
55.57
1278.06
133.40
135.72
17.22
W ithin
360
Days
9
Days X Drug
9
Da ys X S o l u t i o n
9
Days X D ru g X S o l u t i o n
9
error
324
* p < . 05
** p < .01
*** p < .001
556.85
158.08
151 .71
20.77
14.35
370.02
I .55
1 7. 6 1
16.86
2. 3 1
I .59
I .14
A 2 x 2 x 6 t h r e e - w a y ANOVA f o r s p l i t - p l o t
F
—
74.22***
7.75**
7 . 8 8 **
-
15.45***
14.79***
2.03*
I . 39
-
designs
( s e e T a b l e 3)
was p e r f o r m e d o n t h e i n t a k e v o l u m e d a t a f r o m d a y s 1 0 - 1 5
(the recovery
p eriod).
This a n a ly s i s re v e a le d a r e l i a b l e o v e r a ll
between groups due t o drug h i s t o r y ,
difference
F ( I , 36) = 2 8 . 1 5 ,
i n d i c a t e s t h a t t h e g r o u p s w h i c h h a d r e c e i v e d SAL a l l
p < .001.
This
along continued
t o i n g e s t more s w e e t t h a n t h e g ro u p s which began t h e e x p e r i m e n t w i t h
NAL i n j e c t i o n s .
The a n a l y s i s a l s o r e v e a l e d a s i g n i f i c a n t e f f e c t o f
s o lu tio n type, F(1,
36 ) = 1 9 . 4 9 ,
p < 0.001.
H e r e , s u c r o s e was
i n g e s t e d i n g r e a t e r amounts t h a n s a c c h a r i n i n a l l
groups.
Again, as
18
in the previous a n a ly s is ,
F(5,
181) = 2 2 . 3 3 ,
t h e r e wa s a s i g n i f i c a n t e f f e c t o f d a y s ,
p < .001.
T h i s i n d i c a t e s t h a t t h e amount i n g e s t e d
in c re a s e d over the f i v e days.
A s i g n i f i c a n t i n t e r a c t i o n was r e v e a l e d
b etw e en days and d ru g h is to r y ,
F(5,
indicates
1 8 1) = 1 0 . 2 8 ,
t h a t th e NAL-treated anim als
p < .001 .
This
(now t r e a t e d w i t h SAL)
i n c r e a s e d i n t a k e o v e r days w h i l e t h e i n t a k e o f t h e S A L - t r e a t e d
anim als changed v ery l i t t l e .
days,
F in ally ,
d r u g a n d s o l u t i o n was r e v e a l e d ,
a s ig n ific a n t in te ra c tio n of
F ( 5 , .181)-= 6 . 6 8 , p < . 0 0 1 .
\
This in d ic a te s
t h a t t h e g r o u p s w h i c h w e r e c h a n g e d f r o m NAL t o SAL
i n c r e a s e d i n t a k e and d ra n k p r o g r e s s i v e l y more s u c r o s e t h a n s a c c h a r i n
over days.
T a b l e 3- A n a l y s i s o f V a r i a n c e f o r E x p e r i m e n t I ,
Source
Between
Dru g
S olution
Drug X S o l u t i o n
error
df
39
I
I
I
36
SS
MS
53.43
683.44
473.20
53.20
24.28
2 0 8 3 .9 6
683.44
473.20
53.20
874.12
W ithin
201
Days
5
Da y s X D ru g
5
Da ys X S o l u t i o n
5
Da y s X D r u g X S o l u t i o n
5
error
181
Da ys 1 0 - 1 5 .
652.76
265.74
122.34
19.97
79.47
430.98
F
28.1 5 *
1 9 .4 9 *
2.19
-
3.25
53.15
24.47
2 2 .3 3 *
10 .2 8 *
1 .6 8
6 .6 8 *
3 .9 9
15.89
2 .3 8
—
* p < .001
A 2 x 2 x 6 t h r e e - w a y ANOVA f o r s p l i t - p l o t
o n t h e i n t a k e v o l u m e d a t a f o r d a y s 15 - 2 0
d e s i g n s , was c a l c u l a t e d
(the crossover p erio d ).
T h i s a n a l y s i s r e v e a le d t h a t r e v e r s a l o f t h e d r u g t r e a t m e n t s r e v e r s e d
th e d r in k in g p a t t e r n s of th e groups
fo rm er N A L -treate d group
(see Table 4 ).
S p ecifically ,
the
(now t r e a t e d w i t h SAL) d r a n k s i g n i f i c a n t l y
m o r e t h a n t h e f o r m e r SAL g r o u p
(now t r e a t e d w i t h NAL) ,
F(1,
36) =
-/■
19
37.05,
p < .001.
As i n t h e p r e v i o u s a n a l y s e s , t h e r e was a
s ig n ific a n t e ffe c t of so lu tio n ,
all
F(1,
36) = 3 0 . 6 5 ,
p < .001.
g r o u p s i n g e s t e d m o re s u c r o s e t h a n s a c c h a r i n .
revealed a re lia b le
p < .001.
Here,
The a n a l y s i s a l s o
d r u g a nd s o l u t i o n i n t e r a c t i o n ,
F ( I , 36) = 15 . 9 0 ,
T h is r e s u l t i n d i c a t e s t h a t th e enhanced i n t a k e of s u c r o s e ,
a s c o m p a r e d t o s a c c h a r i n , was e v e n m o r e p r o n o u n c e d f o r t h e a n i m a l s
w h i c h h a d b e g u n t h e e x p e r i m e n t w i t h NAL t r e a t m e n t s t h a n i t
was f o r
t h o s e a n i m a l s w h i c h b e g a n w i t h SAL b u t l a t e r w e r e t r e a t e d w i t h NAL.
T a b l e 4 . A n a l y s i s o f V a r i a n c e f o r E x p e r i m e n t I , Days 15 - 2 0 .
Source
Between
Drug
S olution
Dr ug X S o l u t i o n
error
W ithin
Days
Da ys X
Da y s X
Da ys X
error
* p
MS
48.64
589.07
487.35
248.07
15.90
SS
1896.92
■
201
5
D ru g
5
solution
5
Dr ug X S o l u t i o n
5
181
< .001
During t h i s
F(5,
df
39
I
I
I
36
5 8 9 .0 7
487.35
248.07
572.43
543.93
127.40
2 5 1 .08
13.70
I 4.78
264.37
F
-
37.05*
30.6 5 *
15.60*
-
2. 71
25.48
50.22
2.74
2.96
1.46
■
17.45*
34.40*
1 .8 8
-
2.02
-
c r o s s o v e r p e r i o d t h e r e was a r e l i a b l e e f f e c t o f d a y s ,
181) = 1 7 . 4 5 ,
p < .001. T h is i n d i c a t e s t h a t i n t a k e v a r ie d a c r o s s
days.
A s i g n i f i c a n t i n t e r a c t i o n b etw e en days and d ru g F (5,
34.40,
p < .001 was a l s o f o u n d .
This re v e a ls t h a t the anim als t h a t
h a d b e g u n t h e e x p e r i m e n t w i t h SAL t r e a t m e n t s
(now t r e a t e d w i t h NAL)
d e c r e a s e d i n t a k e o v e r d a y s w h i l e t h e NAL a n i m a l s
SAL) i n c r e a s e d i n t a k e .
181 ) =
(now t r e a t e d w i t h
■
To d e t e r m i n e w h e t h e r p r e t r e a t m e n t w i t h NAL r e s u l t s
in d iffe re n t
20
acquisition ra te s,
a p o s t h o c c o m p a r i s o n was made b e t w e e n t h e l e v e l
o f s u c r o s e i n t a k e on t h e f i f t h
groups
d a y o f i n g e s t i o n f o r t h e SAL c o n t r o l
(day 5 o f th e e x p e rim e n t) and t h e f i f t h day o f i n g e s t i o n f o r
t h e NAL g r o u p s
( d a y 15 o f t h e e x p e r i m e n t ) . T h i s a n a l y s i s r e v e a l e d a
s i g n i f i c a n t d i f f e r e n c e betw een c o n t r o l s and t h e a n i m a l s p r e v i o u s l y
t r e a t e d w ith naloxone,
resu lt
t ( 1 8) = 2 . 7 0 2 ,
dem onstrates t h a t ,
p_= . 0 1 5 ,
(see Figure 2).
a f t e r removal of th e a n t a g o n i s t ,
This
the
a n i m a l s p r e v i o u s l y t r e a t e d w i t h NAL d e v e l o p e d a" s t r o n g e r p r e f e r e n c e
f o r s u c r o s e w i t h i n t h e same p e r i o d o f t i m e a s c o n t r o l s .
D iscussion
T h is experim ent d e m o n s tra te s t h a t sweet t a s t e - m o t i v a t e d b e h a v io r
c a n b e d i s r u p t e d by b l o c k a d e o f o p i o i d r e c e p t o r s .
The a n i m a l s w h i c h
r e c e i v e d i n j e c t i o n s o f NAL d r a n k e s s e n t i a l l y no s w e e t s o l u t i o n w h i l e
th e c o n t r o l an im als developed normal t a s t e p r e f e r e n c e s f o r both
s u c r o s e and s a c c h a r i n ,
stronger.
a l t h o u g h t h e s u c r o s e p r e f e r e n c e was c l e a r l y
Removal o f t h e a n t a g o n i s t i n t h e N A L - t r e a t e d a n i m a l s
re su lte d in a recovery of ta s te preference.
These a n im als
c o n s e q u e n tly drank th e sw eet s o l u t i o n s i n l a r g e amounts,
and, as
ex p e cted , th e y q u ic k ly developed p re fe re n c e for both p a l a t a b l e
liq u id s.
The f i n d i n g t h a t ,
o n c e t h e NAL was r e m o v e d , t h e N A L - t r e a t e d
anim als a tta in e d h ig h e r le v e l s
of i n t a k e than th e c o n t r o l anim als
(who h a d r e c e i v e d no a n t a g o n i s t t r e a t m e n t ) may i n d i c a t e t h a t t h e
d a i l y p r e t r e a t m e n t w i t h NAL f o r TO d a y s r e s u l t e d i n f u n d a m e n t a l
changes i n th e o p io id r e c e p to r system .
Interestingly,
th e anim als
FIGURE
2-
E f f e c t o f JDrxig H i s t o r y o r . P r e f e r e n c e
Mea.rx. Volxj-Trxe I n t a k e
I
O
D ays o f D m y - F r e e Inyestimx
t
NM--Stunose
Svi L -S 1I tc r o s e
22
w h i c h w e r e o f f e r e d s u c r o s e d r a n k c o n s i s t e n t l y m o re t h a n d i d t h e i r
saccharin-consum ing c o u n te r p a r ts a c ro ss a l l experimental- c o n d itio n s .
Also of i n t e r e s t i s
t h e f i n d i n g t h a t when NAL was a d m i n i s t e r e d
t o t h o s e a n im a ls which had a l r e a d y a c q u ir e d p r e f e r e n c e s ,
o f i n t a k e was n o t c o m p l e t e l y e l i m i n a t e d .
This im p lie s
E O P ' s a r e somehow i m p o r t a n t d u r i n g t h e l e a r n i n g o f t a s t e
b u t t h a t such p r e f e r e n c e s can l a t e r
the peptides.
th e ir level
th a t the
preferences,
be m a i n t a i n e d i n t h e a b s e n c e o f
This m aintenance of th e
drinking"behavior in the
absence of o p io id r e c e p to r s tim u la tio n argues a g a in s t a d riv e th eo ry
■ e x p l a n a t i o n o f EOP r e g u l a t i o n o f f e e d i n g ,
requires
s i n c e s u c h an e x p l a n a t i o n
t h e d ep e n d en cy o f i n g e s t i o n on a f u l l y f u n c t i o n i n g o p i o i d
r e c e p t o r system.
r
23
■
EXPERIMENT 2
The p u r p o s e o f E x p e r i m e n t 2 was t o t e s t t h e p o s s i b i l i t y t h a t t h e
rew arding e f f e c ts
of o p io id a g o n i s t s might f a c i l i t a t e
the intake of a
2 0 % s u c r o s e s o l u t i o n by r e p e a t e d a s s o c i a t i o n o f i n t a k e w i t h e i t h e r
t h e mu a g o n i s t m o r p h i n e o r t h e k a p p a a g o n i s t U - 5 0 , 4 8 8 H .
measured under fo u r tre a tm e n t c o n d itio n s :
dose o f U-50, m o rp h in e , o r s a l i n e
in ste a d of sac ch arin in t h i s
I n t a k e was
a low d o s e o f U-50, a h i g h
(see Table 5 ).
S u c r o s e was u s e d
s t u d y , f o l l o w i n g a s u g g e s t i o n made by
Cooper ( p e r s o n a l c o m m u n ic a tio n ) t h a t U-50,488H m ig h t m e d i a t e an
i n c r e a s e d i n t a k e o f s u c r o s e b u t may n o t h a v e t h e s ame e f f e c t on
saccharin.
it
is
A l s o , U - 5 0 , iISSH was e m p l o y e d a l o n g w i t h m o r p h i n e b e c a u s e
q u i t e s p e c i f i c i n a f f i n i t y f o r kappa r e c e p t o r s w hich have been
im p lica ted in t a s t e m otivated in tak e
(Lynch,
1 98 3; J a c k s o n & C o o p e r ,
1985).
T a b l e 5.
Group
I
2
3
4
T reatm ent C o n d itio n s f o r Experim ent 2
Days 1 - 1 0
Days 1 1 - 1 5
U-50L
SAL
U-50H
SAL
MOR
SAL
SAL
SAL
Method
S ubjects
F o r ty male r a t s
o f t h e s am e s t r a i n w e r e p u r c h a s e d f r o m t h e s a m e
s u p p l i e r as i n Experim ent I .
As i n t h e f i r s t
experim ent,
the anim als
/
24
w e r e h o u s e d i n g r o u p s o f f i v e a nd a l l o w e d a d l i b .
a c c e s s t o fo o d and
w a te r, except during te s tin g .
A pparatus
The same t e s t c a g e s , a s d e s c r i b e d i n E x p e r i m e n t I ,
f o r p r e s e n t a t i o n o f w a t e r and sw ee t s o l u t i o n s .
w ere employed
Each t e s t
c a g e was
f i t t e d w i t h one b o t t l e c o n t a i n i n g t a p w a t e r and a n o t h e r b o t t l e
c o n t a i n i n g a 20% (w/v) s u c r o s e s o l u t i o n .
Procedure
The a n i m a l s w e r e r a n d o m l y a s s i g n e d t o t h e f o u r t r e a t m e n t
conditions.
Each day t h e a n i m a l s w ere b r o u g h t i n t o t h e e x p e r i m e n t a l
ro om a n d a d m i n i s t e r e d o n e o f f o u r s u b c u t a n e o u s i n j e c t i o n s :
0 . 3 mg/kg
U - 5 0 , 4 8 8 H (U-50L g r o u p ) ; 1 . 0 m g / k g U - 5 0 , 4 8 8 H (U-50H g r o u p ) ; 1 . 0 m g / k g
m o r p h i n e (MOR g r o u p ) ;
and 1.0 m l/kg s a l i n e
(SAL g r o u p ) .
30
m i n u t e s a f t e r r e c e i v i n g t h e i n j e c t i o n e a c h a n i m a l was p l a c e d
i n d i v i d u a l l y i n t o a t e s t c a g e f o r 30 m i n u t e s .
A fter t h i s intake
p e r i o d t h e a n i m a l was r e t u r n e d t o t h e home c a g e a n d t h e i n t a k e v o l u me
was r e c o r d e d .
T h i s o c c u r r e d f o r 10 d a y s a n d was f o l l o w e d by a 5 d a y
r e c o v e r y p e r i o d d u r i n g w h i c h t h e d r u g was r e m o v e d f r o m a l l
and ea c h r e c e i v e d d a i l y i n j e c t i o n s
anim als
o f SAL w h i l e t h e i r i n t a k e v o l u m e s
c o n t i n u e d t o be r e c o r d e d .
R esults
Figure 3 i l l u s t r a t e s
the d i f f e r e n t i a l
e f f e c t s o f a g o n i s t s on
i n t a k e v o l u m e s d u r i n g 10 d a y s o f t r e a t m e n t a n d 5 d a y s o f r e c o v e r y .
I t a p p e a r s t h a t t h e l ow d o s e o f U - 5 0 , 488H e l e v a t e d i n t a k e
figure
3.
A cjoixist E f f e c t s o n S u c r o s e I n t a k e
TREATMENTS
G rovcp M e a n I n t a k e
IU - 5 OL
SAL
U- 5 OH
MOR
—-A-
A ------ j /
1------D a y a o f Teat irt y
MOIt
26
c o n s i s t e n t l y o v e r t h e 10 d a y s o f t r e a t m e n t a s c o m p a r e d t o s a l i n e , a n d
t h i s h i g h e r l e v e l was m a i n t a i n e d 5 d a y s a f t e r t h e d r u g t r e a t m e n t s
ended.
N e i t h e r o f t h e o t h e r d ru g g r o u p s d i f f e r e d from t h e s a l i n e
c o n t r o l g ro u p i n i n t a k e volumes e i t h e r b e f o r e , or a f t e r ,
drug
treatm ent.
A 2 x 2 x 10 t h r e e - w a y ANOVA f o r s p l i t - p l o t
designs
(see Table
6 ) c a l c u l a t e d o n t h e i n t a k e v o l u me d a t a f o r t h e f i r s t
t e n days'
r e v e a l e d a s i g n i f i c a n t main e f f e c t of drug t r e a t m e n t ,
F(3,
7.10,
p < .001 .
This in d ic a te s
t h e a m ou nt o f s u c r o s e i n t a k e . '
significant
indicates
e f f e c t of days,
an aly sis.
t h a t th e trea tm e n t groups d i f f e r e d in
The a n a l y s i s a l s o r e v e a l e d a
F(9,
324) = 4 0 .0 9 ,
t h a t t h e volumes changed o v e r d ay s.
between days and d ru g , F(27,
324),
p_<
p < . 001 .
This r e s u l t
A re lia b le
interaction
. 0 0 1 , was a l s o r e v e a l e d by t h e
T his d e m o n strate s t h a t th e drug e f f e c t s
A p o s t hoc c o m p ar iso n ,
36) =
to determ ine i f
changed over days.
t h e U-50L g r o u p i n g e s t e d m o re
s u c r o s e o n d a y TO t h a n d i d t h e SAL g r o u p , was s i g n i f i c a n t , _t( 1 8 )
=2.963, P = .0083.
T a b l e 6 . A n a l y s i s o f V a r i a n c e f o r E x p e r i m e n t 2 , Days 1 - 1 0 .
Source
Between
Dru g
error
W ithin
Days
Da ys X D ru g
error
* p < .001
df
39
3
36
SS
2469.20
919.03
1550.17
360
1 8 5 6 .9 8
9
27
324
1612.72
408.05
I 448.93
MS
63.31
306.03
43.06
5.16
179.19
15.11
4.47
A 2 x 2 x 6 t h r e e - w a y ANOVA f o r s p l i t - p i o t d e s i g n s
F
—
7 .10*
-
4 0 .0 9 *
3 .3 8 *
_
( s e e T a b l e 7)
27
c a l c u l a t e d on t h e r e c o v e r y d a t a
e f f e c t of drug h i s t o r y ,
F(3,
(days 10-15) r e v e a l e d a s i g n i f i c a n t
36) = 3 . 0 1 ,
p < .05.
This in d ic a te s a
d i f f e r e n c e i n i n t a k e between t h e t r e a t m e n t g r o u p s .
The a n a l y s i s a l s o
r e v e a l e d a s i g n i f i c a n t i n t e r a c t i o n b etw e en days and d ru g h i s t o r y ,
F(15,
I S i ) = 1.91,
p < .05.
This r e s u l t i n d i c a t e s t h a t the d i f f e r e n t
i n t a k e s b e t w e e n g r o u p s c h a n g e d o v e r d a y s , w i t h t h e U-50L g r o u p
m a i n t a i n i n g i n t a k e l e v e l s a b o v e t h e SAL g r o u p .
T a b l e 7 . A n a l y s i s o f V a r i a n c e f o r E x p e r i m e n t 2, Da ys 1 0 - 1 5 .
Source
Between
Dr ug
error
W ithin
Da ys
Da y s X Drug
error
* P < .05
df
39
3
36
SS
1817.96
364.08
1453.88
MS
4 6. 6 1
121.36
40.38
201
5
15
181
837.16
183.67
114.15
723.01
4.16
F
-
3. 0 1 *
—
9.2 1 * *
3 6 .7 3
7. 61
I .9 1 *
3 .9 9
-
** p < .001
D iscussion
T h i s e x p e r i m e n t was d e s i g n e d t o m e a s u r e t h e e f f e c t o f
subcutaneous in j e c t i o n s
sucrose so lu tio n s.
increased intake,
o f o p i o i d a g o n i s t s on i n g e s t i o n o f p r e f e r r e d
As e x p e c t e d ,
the
kappa a g o n i s t U-50,488H
b u t o n l y when t h e d o s e was a r e l a t i v e l y s m a l l o n e .
F iv e days a f t e r removal of th e d r u g s ,
t h e U-50H, MOR, a n d SAL
groups were n o t consuming s i g n i f i c a n t l y d i f f e r e n t volumes of sw ee t,
b u t t h e U- 50 L g r o u p d i d d e m o n s t r a t e a m a r g i n a l l y s i g n i f i c a n t
in crease in intake
indicated
c o m p a r e d t o t h e SAL c o n t r o l g r o u p .
by t h e s e r e s u l t s
What i s
i s t h a t n o n - d e p r i v e d a n i m a l s which a r e
.
28
d r in k in g p u re ly f o r sweet t a s t e w i l l
i n c r e a s e t h e i r i n t a k e i f a low
d o s e o f a kappa a g o n i s t i s p r e s e n t e d c o n c u r r e n t l y w i t h i n g e s t i o n .
I t rem ains u n c le a r i f
th is f a c i l i t a t i o n of preference learning
i s due t o a m o d i f i c a t i o n o f t a s t e
q u a l i t y or to th e s tr e n g t h of the
l e a r n e d r e s p o n s e w h i c h may h a v e b e e n r e i n f o r c e d by t h e r e p e a t e d
a d m i n i s t r a t i o n of exogenous o p io id a g o n i s t s .
I,
A gain,
as i n Experim ent
the m aintenance of the p re fe re n c e in the absence o f the a g o n ist
a r g u e s a g a i n s t a d r i v e mechanism o f o p i o i d r e g u l a t i o n o f f e e d i n g .
29
EXPERIMENT 3
The p u r p o s e o f E x p e r i m e n t 3 was t o i n v e s t i g a t e t h e e f f e c t o f
ingestion,
and th e a n t i c i p a t i o n o f i n g e s t i o n ,
s o l u t i o n on t h e r e l e a s e o f EO P 's.
The r e l e a s e o f EOP' s was a s s e s s e d
by m e a s u r i n g s e n s i t i v i t y t o a p a i n f u l
C oh en e t a l . ,
1984;
of a highly p alatab le
stim ulus,"reasoning
B er gmann e t a l . , 1 9 8 4 ;
(as did
a n d Dum & H e r z ,
19 8 4 ) t h a t
a N A L^suppressible d ecrease in s e n s i t i v i t y to p ain i n d i c a t e s
r e l e a s e of endogenous o p i o i d s .
in th is
The a n a l g e s i c r e s p o n s e m e a s u r e u s e d
e x p e r i m e n t was t h e l a t e n c y t o t a i l - f l i c k
h i g h - i n t e n s i t y beam o f l i g h t
the
in response to a
( s e e D'Amour a n d S m i t h ,
1941).
Method
S ubjects
F o r t y male r a t s
supplier,
o f t h e same s t r a i n ,
a n d p u r c h a s e d f r o m t h e same
a s i n E x p e r im e n ts I and 2 were a l s o u s e d i n t h i s
experim ent.
These were housed i n groups of f i v e ,
a c c e s s t o food
(except during t e s t i n g ) ,
a l l o w e d ad l i b .
and d e p r i v e d o f w a t e r f o r 18
hours each day im m e d ia tely p r i o r t o t e s t i n g .
Apparatus
The s ame t e s t
c a g e s , a s d e s c r i b e d i n E x p e r i m e n t s I a n d 2, w e r e
employed f o r p r e s e n t a t i o n o f t h e w a t e r and s w e e t s o l u t i o n s .
sweet s o l u t i o n used i n t h i s
The
e x p e r i m e n t was a m i x t u r e o f 3 % g l u c o s e
a nd .125% s a c c h a r i n w h i c h h a s b e e n d e m o n s t r a t e d t o i n d u c e l a r g e
30
intakes
in laboratory ra ts
(Smith & F o s t e r ,
1980).
r e s p o n s e was d e f i n e d a s t h e l a t e n c y t o t a i l - f l i c k
a d e v ic e d e s ig n e d and b u i l t i n our l a b
The a n a l g e s i c
a n d was m e a s u r e d by
(see Figure 4 ).
This device
d i r e c t s a s e m i - f o c u s e d beam o f l i g h t f r o m a 1 50 w a t t G. E. p r o j e c t o r
l a m p ( t y p e EKL) o n t o a b l a c k e n e d s p o t 4 cm f r o m t h e b a s e o f e a c h
r a t Ts t a i l .
A d i g i t a l tim er i s a c t i v a t e d sim u lta n e o u sly with the
o n set of the l i g h t
a n d m e a s u r e s t o w i t h i n 10 ms ec t h e t i m e i t
f o r th e animal to f l i c k i t s
tail
p h o t o r e s i s t o r under th e t a i l
away f r o m t h e h e a t s o u r c e .
detects
the l i g h t a f te r
takes
A
th e t a i l has
moved a n d a u t o m a t i c a l l y s t o p s t h e t i m e r a n d t u r n s o f f t h e l i g h t
sou rce.
The a n i m a l s w e r e l i g h t l y r e s t r a i n e d by h a n d by t h e
experim enter w hile in t h i s
d e v ic e which l e f t
the t a i l
f r e e t o move.
Procedure
A l l a n i m a l s w e r e d e p r i v e d o f w a t e r i n t h e i r home c a g e s f o r 18
h o u r s i m m e d i a t e l y p r i o r t o e a c h t r a i n i n g and t e s t i n g s e s s i o n .
Da ys 1 - 1 0 c o m p r i s e d t h e t r a i n i n g
p o rtio n o f the experim ent.
S u b j e c t s i n g ro u p s I and 2 ( t h e s w e e t and w a t e r p r e - i n t a k e g ro u p s ,
r e s p e c t i v e l y ) w e r e b r o u g h t i n t o t h e e x p e r i m e n t a l r o o m, i n j e c t e d
s u b c u t a n e o u s l y w i t h I m l / k g SAL, a n d 15 m i n u t e s l a t e r
in d iv id u a lly in the t a i l - f l i c k
h e a t s tim u lu s bein g engaged.
test
placed
a p p a r a t u s f o r 15 s e c o n d s w i t h o u t t h e
The a n i m a l s i n g r o u p I w e r e p l a c e d i n t o
c a g e s which were f i t t e d w i t h two b o t t l e s ,
one c o n t a i n i n g w ater
a n d t h e o t h e r a h i g h l y p r e f e r r e d m i x t u r e o f 3 % g l u c o s e + .125%
saccharin.
The g r o u p 2 a n i m a l s w e r e p l a c e d i n t e s t c a g e s w h i c h a l s o
c o n t a i n e d t wo b o t t l e s , b o t h o f w h i c h c o n t a i n e d t a p w a t e r . The a n i m a l s
FIGURE 4. S chem atic o f T a i l - F l i c k D ev ice f o r E xperim ent 3.
32
were a l l o w e d t o d r i n k ad l i b .
r e t u r n e d t o t h e home c a g e .
intake , re sp ectiv ely )
for
10 m i n u t e s a t w h i c h t i m e e a c h was
Groups 3 and 4 (s w e et and w a t e r p o s t ­
w e r e e x p o s e d t o t h e same c o n d i t i o n s a s g r o u p s
I and 2 e x c e p t t h a t t h e y had a c c e s s t o t h e s o l u t i o n b o t t l e s
p rio r to
i n j e c t i o n s and a n a l g e s i a t e s t i n g .
On d a y s 11 a n d 12 t h e a c t u a l a n a l g e s i c r e s p o n s e s w e r e m e a s u r e d
w i t h e a c h g r o u p b e i n g s p l i t i n h a l f a n d c o u n t e r b a l a n c e d s u c h t h a t SAL
( I m l / k g ) was i n j e c t e d o n o n e d a y a n d NAL ( I m g / k g ) d e l i v e r e d o n t h e
o th e r day.
F o r t h e preMi n t a k e a n i m a l s
i n j e c t i o n s were s t i l l
(g ro u p s I and 2 ) ,
d e l i v e r e d 15 m i n u t e s b e f o r e t h e r a t s w e r e
placed in the t a i l - f l i c k
d e v i c e f o r 15 s e c o n d s ,
t h e h e a t s o u r c e was e n g a g e d a n d t h e t a i l
recorded.
the
S ubjects in the p o s t-in ta k e
but a f t e r t h i s
flick latencies
groups
time
(TFL's)
( 3 a n d 4) f i r s t
had
a c c e s s t o t h e s o l u t i o n s , t h e n t h e y w e r e i n j e c t e d w i t h SAL o r NAL, a n d
15 m i n u t e s l a t e r t h e i r T F L ' s w e r e r e c o r d e d .
R esults
A 2 x 2 ANOVA f o r s p l i t - p l o t
a n d d a y 12 TFL d a t a .
due t o d r u g ,
addition,
d e s i g n s was p e r f o r m e d on t h e d a y 11
T h i s a n a l y s i s r e v e a l e d no s i g n i f i c a n t e f f e c t s
s o l u t io n ty p e , or tim e of the a n a lg e s ia t e s t .
t h e a n a l y s i s r e v e a l e d no s i g n i f i c a n t i n t e r a c t i o n s .
f a i l u r e t o r e v e a l any s i g n i f i c a n t
of system atic d iffe re n c e s
effects
This
s e e m s d ue t o b o t h t h e l a c k
betw een g ro u p s and t o t h e e x t r e m e
in d iv id u a l v a r i a b i l i t y w ith in the d ata.
In
33
D iscussion
T h i s e x p e r i m e n t was i n t e n d e d t o i n v e s t i g a t e t h e r e l e a s e o f
e n d o g e n o u s o p i o i d s by m e a s u r i n g t a i l - f l i c k
e ith e r the actu a l
la te n c ie s in response to
in g e s tio n or expected in g e s tio n of a h ig h ly
p a l a t a b l e m i x tu r e o f g l u c o s e and s a c c h a r i n .
reasoning of other re se a rc h e rs
Herz,
In accord w ith the
(eg. L i e b l i c h ,
e t a l .,
1 9 8 3 ; Dum &
1 9 8 4 ) a d e c r e a s e i n p a i n s e n s i t i v i t y w h i c h c a n b e r e v e r s e d by
NAL was c o n s i d e r e d t o be i n d i c a t i v e o f EOP r e l e a s e ,
and i t
was
p r e d i c t e d t h a t b o th e x p e c t e d and a c t u a l i n t a k e of a p r e f e r r e d
s u b s t a n c e w o u l d r e s u l t i n some a n a l g e s i a .
U nfortunately,
no e f f e c t
.
o n TFL s c o r e s was f o u n d b e t w e e n t h e v a r i o u s c o n d i t i o n s i n t h i s
experim ent.
The e x t r e m e b e t w e e n s u b j e c t v a r i a b i l i t y may h a v e
accounted fo r th e lack of s t a t i s t i c a l r e l i a b i l i t y ,
but in sp ectio n of
t h e d a t a a l s o r e v e a l s no a p p a r e n t s y s t e m a t i c d i f f e r e n c e s
between
groups.
At t h i s p o i n t o n e may c o n c l u d e t h a t
reliab le,
e i t h e r . t h e p he no m en on i s n o t
o r t h a t t h e b e h a v i o r a l m e a s u r e u s e d i s n o t s e n s i t i v e enough
t o d e t e c t c h a n g e s i n a n a l g e s i a i n d u c e d by g u s t a t o r y e v e n t s .
However,
a n o t h e r p o s s i b i l i t y i s t h a t t h e 15 m i n u te d e l a y b e tw e e n i n j e c t i o n and
analgesia t e s t
( e s p e c i a l l y i n t h e p o s t - i n t a k e g r o u p ) may h a v e a l l o w e d
a n y EOP r e l e a s e t o d i s s i p a t e ,
I n o th e r words,
short-lived.
t h u s l e a d i n g t o no t r e a t m e n t e f f e c t s .
t h e r e l e a s e o f EOP' s i n r e s p o n s e t o i n t a k e may be
T h i s makes i n t u i t i v e s e n s e :
I f a prim ary fu n c tio n of
EOP r e l e a s e i s t o r e i n f o r c e t h e l e a r n i n g o f t a s t e
p re fe re n c e s , then
34
t e m p o r a l c o n t i g u i t y s h o u l d be a n e c e s s a r y c o n d i t i o n f o r l e a r n i n g t o
occur.
To t e s t f u r t h e r t h i s
idea,
s ome a d h o c m a n i p u l a t i o n s w e r e
p erfo rm ed i n which t h e d a t a were c o l l e c t e d under c o n d i t i o n s of
m inimal e x p e r im e n ta l c o n t r o l .
N evertheless,
the r e s u l t s
were
provocative.
F i v e days a f t e r t h e end o f E x p e r im e n t 3 t h e p o s t - i n t a k e a n i m a l s
(n =10 ) w hich had been e x p o s e d t o t h e s w e e t s o l u t i o n w ere b r o u g h t i n t o
t h e e x p e r i m e n t a l room f o l l o w i n g a n T8- h o u r d e p r i v a t i o n p e r i o d .
These
a n i m a l s w e r e a l l o w e d t o d r i n k f o r TO m i n u t e s i m m e d i a t e l y a f t e r w h i c h
t h e y were s u b j e c t e d t o th e p a in t e s t .
f o r t wo c o n s e c u t i v e d a y s .
T h i s p r o c e d u r e was f o l l o w e d
The h y p o t h e s i s t h a t t h e a n a l g e s i a e f f e c t
d i s a p p e a r s q u i c k l y was su p p o rted by t h e f i n d i n g t h a t ,
in th is
m a n i p u l a t i o n , t h e TFLt s f o r e a c h g r o u p w e r e e l e v a t e d a s c o m p a r e d t o
th e c o r re s p o n d in g s c o r e s c o l l e c t e d d u rin g Experiment 3 .
These r e s u l t s were e n c o u ra g in g ,
s o a f u r t h e r m a n i p u l a t i o n was
i m p o s e d u po n t h e s am e a n i m a l s t h e n e x t d a y .
A f t e r a n 18 h o u r
d e p r i v a t i o n p e r i o d , t h e y were a l l o w e d a c c e s s t o t h e s w e e t m i x t u r e f o r
6 m i n u t e s a f t e r w hich h a l f were t e s t e d i m m e d i a t e l y an d t h e o t h e r h a l f
w e r e t e s t e d f o l l o w i n g a 15 m i n u t e d e l a y .
c o n d i t i o n s were r e v e r s e d so t h a t
and w i t h o u t , a d e l a y .
The n e x t d a y t h e s e
e a c h a n i m a l was t e s t e d b o t h w i t h ,
A s i x m i n u t e d r i n k i n g p e r i o d was c h o s e n i n
order to t e s t the anim als during th e f i r s t
were d r i n k i n g a v i d l y .
A gain,
c o n d i t i o n s were f o u n d ,
t(9)
in ta k e bout w hile they
s i z e a b l e d i f f e r e n c e s b e t w e e n t h e two
= 4.493,
P = .0015, w ith th e delay
a p p a r e n t l y c a u s i n g t h e l a r g e d e c r e a s e i n TFL s c o r e s
(see Figure 5).
5.
E ffe c t o f D elay a n d
T clxI Ir Zxck L a t e i x c ' y
figure
Delay
o
Ti A n a Ig e s i a.
36
I n o r d e r t o show t h a t t h i s a n a l g e s i c r e s p o n s e i s o p i o i d m e d i a t e d , t h e f o l l o w i n g day a l l a n i m a l s were g i v e n s u b c u t a n e o u s
i n j e c t i o n s o f 2 . 0 m g / k g NAL a n d f o u r m i n u t e s l a t e r w e r e a l l o w e d t o
d rin k f o r s ix m inutes.
painful
F i g u r e 4 s h o ws t h e i n c r e a s e i n s e n s i t i v i t y t o
s t i m u l i w h i c h was d e m o n s t r a t e d by t h e s e a n i m a l s when t r e a t e d
w i t h NAL.
A c o m p a r i s o n o f t h e d i f f e r e n c e s i n TFL s c o r e s b e t w e e n t h e
NAL c o n d i t i o n a n d t h e p r e v i o u s n o - d r u g c o n d i t i o n w e r e f o u n d t o be
s ig n ific a n t, t(9)
= 2.9 7 , p = .016.
These f i n d i n g s ,
although p re lim in ary ,
suggest th a t opioid
p ep tid es are re le a s e d in response to th e in tak e of p a la ta b le
s u b s t a n c e s a n d t h a t t h i s r e l e a s e o f EOP’ s i s a p h y s i o l o g i c a l e v e n t o f
s h o r t d u r a t i o n which i s u n d e t e c t a b l e as l i t t l e
ingestion.
In a d d itio n ,
a s 15 m i n u t e s a f t e r
i t seems t h a t t h e l a t e n c y t o t a i l - f l i c k
a n a p p r o p r i a t e m e a n s o f a s s e s s i n g t h i s EOP r e l e a s e .
Th e i s s u e o f
w hether or not such r e l e a s e a ls o occ u rs in a n t i c i p a t i o n of in ta k e
rem ains u n re so lv e d .
is
37
GENERAL DISCUSSION
It
i s w e l l e s t a b l i s h e d t h a t t h e EOP's h av e r e g u l a t o r y e f f e c t s on
ingestion
(Reid,
19 8 5 ) a n d t h i s i d e a i s
the experim ents r e p o rte d here.
s u p p o r t e d by t h e r e s u l t s o f
Experim ent I ,
fo r instance,
d e m o n s t r a t e d t h e a l m o s t c o m p l e t e c e s s a t i o n o f d r i n k i n g i n a n i m a l s who
received in je c tio n s
o f t h e o p i o i d a n t a g o n i s t NAL.
c o n t r o l a n i m a l s d e v e l o p e d . n o rm al ' t a s t e
and s a c c h a r i n s o l u t i o n s ,
As e x p e c t e d ,
p re fe re n c e s fo r both sucrose
a l t h o u g h s i g n i f i c a n t l y g r e a t e r amounts of
s u c r o s e were i n g e s t e d th a n s a c c h a r i n .
I t was p r e d i c t e d t h a t r e m o v a l o f t h e a n t a g o n i s t w o u l d a l l o w t h e
consequent developm ent o f p r e f e r e n c e f o r sweet s o l u t i o n s ,
was f o u n d t o b e t h e c a s e .
finding th a t,
What i s
p a r tic u la r ly in tr ig u in g is the
f o l l o w i n g r e m o v a l o f t h e NAL, t h e a n i m a l s d e v e l o p e d
p r e f e r e n c e a t an e x t r e m e l y r a p i d r a t e ,
were o f f e r e d s u c r o s e .
drug h i s t o r y )
e s p e c i a l l y t h o s e a n i m a l s who
The l e v e l o f i n t a k e 5 d a y s a f t e r NAL was
withdrawn exceeded t h a t of t h e s a l i n e
prior
and t h i s
c o n tro l anim als
(who h a d no
a f t e r 5 days of ex p o su re t o th e s u c r o s e s o l u t i o n .
T h i s h i g h e r l e v e l o f i n t a k e may i n d i c a t e t h a t r e p e a t e d a d m i n i s t r a t i o n
o f NAL r e s u l t s
i n changes i n th e o p i o i d system i t s e l f ,
b ec o me s h y p e r s e n s i t i v e t o i n g e s t i v e s t i m u l a t i o n .
This hyper­
s e n s i t i v i t y would c o n c e i v a b l y have t h e o p p o s i t e e f f e c t
b l o c k a d e by a n t a g o n i s t s
(i.e .,
such t h a t i t
of re c e p to r
an i n c r e a s e d e f f e c t i v e n e s s o f EOP's,
and t h u s an i n c r e a s e i n i n t a k e ) .
38
One w o u l d e x p e c t t h e s e n s i t i v i t y o f t h e o p i o i d s y s t e m t o be
enhanced i f
t h e number o f o p i o i d r e c e p t o r s was i n c r e a s e d
(upregulation)
by r e p e a t e d t r e a t m e n t w i t h NAL, a n d s u c h u p r e g u l a t i o n
has been r e p o r t e d i n re sp o n se to c h ro n ic exposure to o p i a t e
antagonists
(Tempel, G a rd n e r, & Z ukin,
1985; P ad en , K r a l l , & Ly n ch ,
in p r e s s ) .
T h e r e f o r e , th e r e s u l t s r e p o r te d here are c o n s is te n t with
t h e v i e w t h a t E O P ' s a r e i n v o l v e d i n t a s t e - m o t i v a t e d b e h a v i o r a nd t h a t
e x t e r n a l l y i n d u c e d c h a n g e s i n t h e o p i o i d s y s t e m may c a u s e
co rresp o n d in g changes, in s e n s i t i v i t y to i n g e s t i v e s t i m u l a t i o n .
R elated to t h i s notion of re c e p to r u p re g u la tio n is
th e high
i n t a k e v o l u m e w h i c h was m a i n t a i n e d by t h e NAL-t r e a t e d a n i m a l s who
were o f f e r e d s u c r o s e
th e drug.
th is
(in experim ent I)
In Figure I i t
is
10 d a y s f o l l o w i n g r e m o v a l o f
e v i d e n t t h a t t h e mean d a i l y i n t a k e f o r
g r o u p wa s h i g h e r t h a n t h e i n t a k e o f t h e c o n t r o l a n i m a l s d u r i n g
th eir f ir s t
10 d a y s o f i n g e s t i o n ,
a n d t h i s may i n d i c a t e e i t h e r t h a t
t h e r e c e p t o r s remained u p re g u la te d f o r t h i s r e l a t i v e l y long p erio d of
t i m e , o r t h a t t h e h y p e r s e n s i t i v i t y o f t h e s y s t e m was i m p o r t a n t , f o r
the i n i t i a l
le a rn in g of the t a s t e
m aintenance of i t .
p re fere n ce but not f o r the
Support i s la c k in g f o r the id e a t h a t the
r e c e p to r s re m a in e d 'u p re g u la te d ,
however,
in l i g h t of a re c e n t study
w h i c h f o u n d t h a t o p i o i d r e c e p t o r d e n s i t i e s w h i c h w e r e i n c r e a s e d by
IongSterm exposure to n a ltre x o n e r e tu r n e d to b a sa l l e v e l s a f t e r only
6 days fo llo w in g t e r m in a tio n o f drug tre a tm e n t
I 985).
Thus,
(Tempel,
e t a l .,
a m o re p l a u s i b l e e x p l a n a t i o n f o r t h e m a i n t e n a n c e o f t h e
heightened in tak e is
t h a t the h y p e r s e n s i t i v i t y of the r e c e p t o r system
is im portant fo r the a c q u is itio n
but not th e m aintenance of
39
preferences.
Another i n t e r e s t i n g f i n d i n g i s
the c o n s is te n tly higher in take
v o l u m e s by t h o s e a n i m a l s d r i n k i n g s u c r o s e c o m p a r e d t o t h o s e d r i n k i n g
saccharin.
This h eld tr u e f o r th e c o n tr o l anim als
(when t r e a t e d w i t h
SAL o r NAL) ,
a n d a l s o f o r t h e NAL-t r e a t e d a n i m a l s o n c e t h e a n t a g o n i s t
was r e m o v e d .
T h i s may s u g g e s t t h a t t h e r e i s a f u n d a m e n t a l d i f f e r e n c e
i n th e o p i a t e sy stem ’s response to n u t r i t i v e
and n o n - n u t r i t i v e
s u b s t a n c e s o r i t may s i m p l y i n d i c a t e t h a t t h e s e t wo s w e e t e n e r s a r e
p re fe rre d to d i f f e r e n t d e g re e s .
The r e s u l t s
a l s o s u p p o r t t h e i d e a t h a t t h e c o n n e c t i o n between
t h e o p i o i d s y s t e m and i n g e s t i v e b e h a v i o r i s n o t m e r e l y one o f
g u s t a t o r y s t i m u l a t i o n ca u sin g th e r e l e a s e o f endogenous o p i a t e s o r,
conversely,
EOP r e l e a s e i n d u c i n g i n t a k e .
This is e v id e n t in the
f i n d i n g t h a t t h e a d m i n i s t r a t i o n o f NAL a f t e r
a p r e f e r e n c e had a l r e a d y
been e s t a b l i s h e d d id not co m p le te ly e l i m i n a t e th e d r i n k i n g beh av io r
(as i t
did p r i o r t o p re fe re n c e l e a r n i n g ) ; complete s u p p re ss io n of
p r e f e r r e d i n t a k e w o u l d be e x p e c t e d i f t h i s
one.
Instead,
the r e s u lts
of acquired t a s t e
c o n n e c t i o n were a d i r e c t
are c o n s is te n t w ith a le a r n in g ex p lan atio n
p r e f e r e n c e s , w hich i s f u r t h e r d i s c u s s e d below.
E x p e r i m e n t 2 i n v e s t i g a t e d t h e r o l e o f EOP’ s i n i n g e s t i o n f r o m
the opposite perspective:
th e f a c i l i t a t i o n of in ta k e fo llo w in g th e
ad m in istra tio n of opioid a g o n ists.
The l i t e r a t u r e
d ea lin g w ith the
u s e o f a g o n i s t s i n d i c a t e s t h a t t h e c o n n e c t i o n b etw e en i n g e s t i o n and
o p i o i d r e c e p t o r a c t i v a t i o n i s n o t a d i r e c t one ( R e i d ,
addition,
is
it
1985).
In
has been r e p o r t e d t h a t th e k a p p a -o p io id r e c e p t o r s u b ty p e
p a r t i c u l a r l y involved in ta s te - m o tiv a te d behavior
( L o c k e , Brown, &
Holtzman,
1982;
Cooper, e t a l .,
1985).
Therefore,
t h e e f f e c t s o f t wo d o s e s o f a s p e c i f i c
in th is
experiment
k a p p a a g o n i s t a n d a mu a g o n i s t
on i n t a k e w ere m e a s u r e d .
In agreement w ith p re v io u sly re p o r te d fin d in g s
(Locke,
et a l.,
1 9 8 2 ) t h e mu a g o n i s t h a d a s l i g h t s u p p r e s s a n t e f f e c t a c r o s s a l l
days of drug tre a tm e n t
significant).
(although t h i s
10
e f f e c t was n o t s t a t i s t i c a l l y
The h i g h d o s e o f t h e k a p p a a g o n i s t U - 5 0 , 4 8 8 H
s t i m u l a t e d i n t a k e above t h e c o n t r o l l e v e l
m iddle of th e tre a tm e n t p e rio d ,
quickly tapered o f f .
but o nly fo r 3 days in the
and a f t e r t h i s
the le v e l of intake
F iv e days a f t e r re m o v a l o f t h e d r u g s t h e r e were
no s i g n i f i c a n t d i f f e r e n c e s i n i n t a k e b e t w e e n t h e m o r p h i n e , U-50H, a n d
saline
groups.
The r e s u l t s a r e d i f f e r e n t ,
however, f o r th e anim als
w h ic h r e c e i v e d a low d o se o f U-50,488H (U -50L).
intakes
T h i s g r o u p h a d mean
c o n s i s t e n t l y above c o n t r o l s o v e r th e e n t i r e
15 d a y s o f t h e
e x p e rim e n t, which in c l u d e s th e p e r io d f o llo w in g drug rem oval
ls).
These r e s u l t s
i n d i c a t e t h a t a low d o se o f t h e kappa a g o n i s t ,
d e liv e r e d in a s s o c ia tio n w ith th e o p p o rtu n ity to
solution,
(days I l ­
c o n s i s t e n t l y enhances in ta k e .
ty p e and dose o f a g o n i s t f a c i l i t a t e s
The f i n d i n g t h a t a s p e c i f i c
intake is
more i n t r i g u i n g i s t h e f i n d i n g t h a t t h i s
d r in k a sweet
encouraging,
but even
enhanced i n t a k e i s
m a in ta in e d f o r f i v e days fo llo w in g removal of th e a g o n i s t .
This
seems t o i n d i c a t e t h a t t h e p r e s e n t a t i o n o f exogenous o p i o i d s
c o n c u rre n tly w ith p r e fe rr e d su b stan ce s g r e a tly f a c i l i t a t e s
learning of preferences,
and f u r t h e r ,
the
once t h i s l e a r n i n g has taken
p l a c e t h e o p i a t e s a r e no l o n g e r n e c e s s a r y f o r m a i n t e n a n c e .
T h e r e was a n u n e x p e c t e d d r o p i n i n t a k e f o r o n e t e s t p e r i o d
41
f o l l o w i n g re m o v a l o f t h i s low d o se o f U-50,488H.
However, th e
p r e v i o u s l e v e l o f i n t a k e was r e g a i n e d i n o n l y f o u r d a y s s u c h t h a t t h e
d r o p may h a v e b e e n d ue t o s t i m u l u s g e n e r a l i z a t i o n d e c r e m e n t , t h a t i s ,
a tem porary d is r u p tio n of behavior i n re sp o n se to experim ental,
change.
A g a i n , t h e f a c t t h a t t h e U- 50L g r o u p m a i n t a i n e d i t s
high
l e v e l of in ta k e even in th e absence of the a g o n is t drug im p lies t h a t
o p i o i d s which a r e p r e s e n t e d c o n t i g u o u s l y w i t h p r e f e r r e d s u b s t a n c e s
facilitate
the le a rn in g of p reference fo r those t a s t e s ,
but these
e le v a te d l e v e l s of o p io id s are not n e c e ssa ry fo r the maintenance of
the preference.
The w o r k i n g h y p o t h e s i s t h r o u g h o u t t h i s r e s e a r c h h a s b e e n t h a t
t h e EOP' s a c t t o r e i n f o r c e t h e l e a r n i n g o f t a s t e
p r e f e r e n c e s , which
may l a t e r be m a i n t a i n e d i n t h e a b s e n c e o f t h e e x o g e n o u s o p i o i d s o r i n
th e presence of o p io id a n ta g o n is ts .
T h i s h y p o t h e s i s w a s s u p p o r t e d by
the r e s u lt s
A lo g ic a l follow -up question,
then,
o f E x p e r i m e n t s I an d 2.
i s u n d e r w h a t s t i m u l u s c o n d i t i o n s a r e t h e EOP' S r e l e a s e d ?
Experim ent 3 a d d r e s s e d t h i s
q u e s t i o n by m eans o f a c o m p l e t e l y
d i f f e r e n t paradigm which used p a i n s e n s i t i v i t y as an i n d i r e c t m easure
o f t h e r e l e a s e o f EOPt s i n r e s p o n s e t o i n g e s t i v e e v e n t s .
Some
s t u d i e s have i n d i c a t e d t h a t such r e l e a s e o c c u rs i n re s p o n s e to
ingestion
(L ieblich,
e t a l .,
p a l a t a b l e foods are expected
not in d ic a te ,
19 83 ) w h i l e o t h e r s r e p o r t r e l e a s e when
(Dum & H e r z ,
1984).
T h i s r e s e a r c h does
h o w e v e r , w h e th e r t h e r e l e a s e i s maximal i n r e s p o n s e t o
in g e s tio n or in response to the ex p e ctatio n of in g e s tio n .
t h i s i s s u e was i n v e s t i g a t e d i n E x p e r i m e n t 3.
Therefore,-
42
The r e l e a s e o f e n d o r p h i n s was a s s e s s e d i n E x p e r i m e n t 3 by
m easu rin g changes i n p a in s e n s i t i v i t y as a r e s u l t
the ex p e c ta tio n of in g e s tio n .
found,
of in g e s tio n or of
I f a n i n c r e a s e i n s e n s i t i v i t y was
a n d t h i s was r e v e r s e d b y n a l o x o n e , t h e n i t
t h a t endogenous o p i o i d s had b een r e l e a s e d .
c o u l d be i n f e r r e d
No s y s t e m a t i c d i f f e r e n c e s
i n a n a l g e s i a were found between t h e g ro u p s .
Subsequent
m a n i p u l a t i o n s , h o w e v e r , d i d p r o d u c e some i n t e r e s t i n g r e s u l t s
suggest an algesia is
a f f e c t e d by i n g e s t i o n .
In t h i s
that
case, the
f i n d i n g t h a t a n a l g e s i a was d e t e c t e d o n l y i n t h o s e a n i m a l s who w e r e
t e s t e d i m m e d i a t e l y a f t e r i n t a k e s u g g e s t s t h a t t h e phenomenon i s s h o r t
lived.
In o t h e r w ords, th e d a t a i n d i c a t e t h a t i n g e s t i o n o f sweet
s o l u t i o n s c a u s e s a r e l e a s e o f EOP’ s , r e s u l t i n g i n i n c r e a s e d a n a l g e s i c
r e s p o n s e s , b u t t h a t t h i s EOP e f f e c t d i s a p p e a r s q u i c k l y .
An i m p o r t a n t d i r e c t i o n f o r f u r t h e r r e s e a r c h t o t a k e ,
t h e m e a s u r e m e n t o f EOP r e l e a s e j u s t p r i o r t o ,
follow ing a d rin k in g bout.
during,
then,
is
and im m ediately
Such s t u d i e s would i n c r e a s e o u r
u n d e r s t a n d i n g o f when ( a n d u n d e r w h a t s t i m u l u s c o n d i t i o n s ) o p i o i d s
are released.
clarified,
it
Onc e t h e t e m p o r a l p a t t e r n s o f EOP r e l e a s e
are
i s a n t i c i p a t e d t h a t such i n f o r m a t i o n would p ro v id e
.support fo r the hypo th esis th a t op io id peptides r e in f o r c e in g e s tiv e
behaviors', th e r e b y e n c o u rag in g th e c o n t i n u a t i o n o f f e e d in g i n
specific situ atio n s.
43
REFERENCES CITED
A p f e l b a u m , M ., & M a n d e n o f f , A. ( 1 9 8 1 ) . N a l t r e x o n e s u p p r e s s e s
h y p e r p h a g i a i n d u c e d i n t h e r a t by a h i g h l y p a l a t a b l e d i e t .
P h a r m a c o l o g y B i o c h e m i s t r y & B e h a v i o r , 1 5 , 8 9- 91 .
B e l l u z z i , J . D . , & S t e i n , L. ( 1 9 7 7 ) . E n k e p h a l i n may m e d i a t e e u p h o r i a
d riv e r -re d u c tio n re w ard . N a tu r e , 266, 556-558.
B e r gm a r m , F . , C o h e n , E . , & L i e b l i c h , I . ( 1 9 8 4 ) . B i p h a s i c e f f e c t s o f
c h r o n i c s a c c h a r i n i n t a k e on p a i n r e s p o n s e s o f h e a l t h y a n d d i a b e t i c
r a t s o f two g e n e t i c a l l y s e l e c t e d s t r a i n s . P s y c h o p h a r m a c o l o g y , 82,
248-251.
B e r t i e r e , M. C . , S y , T. M., B a i g h t s , F . , M a n d e n o f f , A . , & A p f e l b a u m ,
M. ( 1 9 8 4 ) . S t r e s s a n d s u c r o s e h y p e r p h a g i a : R o l e o f e n d o g e n o u s
o p i a t e s . P h arm ac o lo g y B i o c h e m i s t r y & B e h a v i o r , 20, 6 7 5 -6 7 9 .
C o h e n , E . , L i e b l i c h , I . , & B e r g m a n n , F. ( 1 9 8 4 ) . E f f e c t s o f
c h ro n ic a lly elev ated in tak e of d i f f e r e n t c o n c en tratio n s of
s a c c h a r i n on m o rp h in e t o l e r a n c e i n g e n e t i c a l l y s e l e c t e d r a t s .
P h y sio lo g y & B e h a v io r, 32, 1041-1043.
C o o p e r , S . J . , J a c k s o n , A . , & K i r k h a m , T. C . ( 1 9 8 5 ) . E n d o r p h i n s a n d
f o o d i n t a k e : K a p p a o p i o i d r e c e p t o r a g o n i s t s a nd h y p e r p h a g i a .
P h a rm a c o lo g y B i o c h e m i s t r y & B e h a v i o r , 23, 8 8 9 -9 0 1 .
C o o p e r , S . J . , J a c k s o n , A . , M o r g a n , R . , & C a r t e r , R. ( 1 9 8 5 ) . E v i d e n c e
f o r o p i a t e r e c e p t o r involvem ent i n th e consumption of a high'
p a l a t a b i l t y d i e t i n n o n d ep riv ed r a t s . N e u r o p e p t i d e s , 5, 345-348.
Cooper,
food,
S . J . , & S a n g e r , D. J . ( 1 9 8 4 ) . E n d o r p h i n e r g i c m e c h a n i s m s i n
s a l t a n d w a t e r i n t a k e : An o v e r v i e w . A p p e t i t e , 5 , 1 - 6 .
D ' Amour, F. E . , & S m i t h , D. L. ( 1 9 4 1 ) . A m e t h o d f o r d e t e r m i n i n g a
l o s s of p a in s e n s a t i o n . J o u r n a l of Pharmacology & E xp erim e n ta l
T h e r a p e u t i c s , 72, 74-79.
Dum, J . , G r a m s c h , C . , & H e r z , A. ( 1 9 8 3 ) . A c t i v a t i o n o f h y p o r t h a l a m i c
B S e n d o rp h in p o o l s by re w a r d i n d u c e d by h i g h l y p a l a t a b l e fo o d .
Pharm acology B io c h e m is try & B e h a v i o r , 18, 443-447.
Dum, J . , & H e r z , A. ( 1 9 8 4 ) . E n d o r p h i n e r g i c m o d u l a t i o n o f n e u r a l
r e w a r d s y s t e m s i n d i c a t e d by b e h a v i o r a l c h a n g e s . P h a r m a c o l o g y
B i o c h e m i s t r y & B e h a v i o r , 21, 2 5 9 - 2 6 6 .
;■
44
G o l d s t e i n , A. (1 9 7 6 ) . O p i o i d p e p t i d e s
b r a i n . S c i e n c e , 193, 1 081-1086.
( e n d o r p h i n s ) i n p i t u i t a r y and
Hemmer, R. C . , O l s o n , G.. A . , K a s t i n , A. J . , McLean, J . H . , & O l s o n ,
R. D. ( 1 9 8 2 ) . E f f e c t s o f n a l o x o n e a n d i t s q u a t e r n a r y f o r m o n f l u i d
c o n s u m p t i o n i n r a t s . P h a r m a c o l o g y B i o c h e m i s t r y & B e h a v i o r , 17,
1287- 1290.
H o l t z m a n , S . G. ( 1 9 7 5 ) . E f f e c t s o f n a r c o t i c a n t a g o n i s t s o n f l u i d
i n t a k e i n t h e r a t . L i f e S c i e n c e s , I 6 , I 465-1470.
H u g h e s , J . , S m i t h , T. W. , K o s t e r l i t z , H. W., F o t h e r g i l l , L. A . ,
M o r g a n , B. A . , & M o r r i s H. R. ( 1 9 7 5 ) . I d e n t i f i c a t i o n o f two r e l a t e d
p e n t a p e p t i d e s from t h e b r a i n w i t h p o t e n t o p i a t e a g o n i s t a c t i v i t y .
N a t u r e , 258, 577-579.
H u g h e s , J . , S m i t h , I . W., M o r g a n , B . , & F o t h e r g i l l , L, ( 1 9 7 5 ) .
P u r i f i c a t i o n and p r o p e r t i e s o f e n k e p h a l i n ^ t h e p o s s i b l e endogenous
l i g a n d f o r t h e m o r p h i n e r e c e p t o r . L i f e S c i e n c e s , 16 ,' 1 7 5 3 - 1 7 5 8 .
J a c k s o n , A . , & C o o p e r , S. J . ( 1 9 8 5 ) . E f f e c t s o f k a p p a o p i a t e a g o n i s t s
on p a l a t a b l e f o o d c o n s u m p tio n i n n o n ^ d e p r i v e d r a t s , w i t h and
w ith o u t food p r e lo a d s . B ra in R esearch B u l l e t i n , 15 , 391-396.
J a l o w i e c , J . E . , P a n s k e p p , J . , Z o l o v i c k , A. J . , N a j am, N . , & He rman,
B . H. ( 1 9 8 1 ) . O p i o i d m o d u l a t i o n o f i n g e s t i v e b e h a v i o r . P h a r m a c o l o g y
B i o c h e m i s t r y & B e h a v i o r , 15 , 4 7 7 -4 8 4 .
K a v a l i e r s , M ., T e s k e y , G. C . , & H i r s t , M. ( 1 9 8 5 ) . The e f f e c t s o f
a g i n g on d a y S n i g h t rh y th m s o f k a p p a ^ m e d i a t e d f e e d i n g i n t h e mouse.
Psychopharm acology, 87 , 286-291.
L e a n d e r , J . D . , & H y n e s , M. D. ( 1 9 8 3 ) . O p i o i d a n t a g o n i s t s a nd
d r i n k i n g : E v idenc e o f k-Sreceptor in v o lv e m e n t. European J o u r n a l of
P h a r m a c o l g y , 87, 4 8 1 -4 8 4 .
L e i b o w i t z , S. F . , ( 1 9 8 5 ) . B r a i n n e u r o t r a n s m i t t e r s and a p p e t i t e
r e g u l a t i o n . P s y c h o p h a r m a c o l o g y B u l l e t i n , 2MK3), 4 1 2 - 4 1 8 .
L e v i n e , A. S . , M o r l e y , J . E . , G o s n e l l , B. A . , B i l l i n g t o n , C. J . , &
B a r t n e s s , T. J . ( 1 9 8 5 ) . O p i o i d s a n d c o n s u m m a t o r y b e h a v i o r . B r a i n
R e s e a rc h B u l l e t i n , 14 , 663-672.
L e v i n e , A. S . , M u r r a y , S . S . , K n e i p , J . , G r a c e , M ., & M o r l e y , J . E.
(1982). F lav o r enhances the a n tid ip s o g e n ic e f f e c t of n a lo x o n e .
P h y sio lo g y & B e h a v io r, 28, 23-25.
L i e b l i c h , I . , C o h e n , E . , G a n c h r o w , J . R . , B l a s s , E. M ., & B e r g m a n n ,
F. (1 9 8 3 ) . Morphine t o l e r a n c e i n g e n e t i c a l l y s e l e c t e d r a t s in d u ced
by c h r o n i c a l l y e l e v a t e d s a c c h a r i n i n t a k e . S c i e n c e , 2 2 1 , 8 7 1 - 8 7 2 .
45
L o c k e , K. W. , Brown, D. R . , & H o l t z m a n , S . G. ( 1 9 8 2 ) . E f f e c t s o f
o p i a t e a n t a g o n i s t s a nd p u t a t i v e ra u-and k a p p a - a g o n i s t s o n m i l k
i n t a k e i n r a t and s q u i r r e l m onkey. P h arm ac o lo g y B i o c h e m i s t r y &
B e h a v i o r , 1_7, 1 2 7 5 - 1 2 7 9
-- ------------------------------------------Bowy, M. T . , M a i c k e l , R. P . , & Yl m, G. K. W. ( 1 9 8 0 ) , N a l o x o n e
r e d u c t i o n o f s t r e s s - r e l a t e d f e e d i n g . L i f e S c i e n c e s , 26, 2113-2118.
L y n c h , W. C. ( 1 9 8 3 ) . N a l o x o n e i n h i b i t s a n d t h e k a p p a - o p i a t e k e t o c y c l a z o c in e f a c i l i t a t e s i n ta k e o f p r e f e r r e d sweets in non-deprived
r a t s . S o c i e t y f o r N e u r o s c i e n c e , A b s t r a c t s , 9, 1 9 3 .
L y n c h , W. C . ( 1 9 8 6 ) . O p i a t e b l o c k a d e i n h i b i t s s a c c h a r i n i n t a k e a n d
b lo c k s normal p r e f e r e n c e a c q u i s i t i o n . Pharmacology B io c h e m is try &
B e h a v i o r , 24, 833-836.
----------------------------------------- !—
L y n c h , W. C . , K r a l l , S . , F e r n a n d e z , B. Q . , & P a d e n , C. M. ( 1 9 8 5 ) .
G u s t a t o r y i n h i b i t i o n and o p i a t e r e c e p t o r u p r e g u l a t i o n f o l l o w i n g .
ch ro n ic naloxone trea tm e n t in r a t . S o c ie ty fo r N euroscience
A b s t r a c t s , JM , 5 5 8 .
--------------------------- ---------- —
L y n c h , W. C . , & L i b b y , L. ( 1 9 8 3 ) . N a l o x o n e s u p p r e s s e s i n t a k e o f
h i g h l y p r e f e r r e d s a c c h a r i n s o l u t i o n s i n food d e p r iv e d and s a t e d
r a t s . L i f e S c i e n c e s , 3 3 , 1909-1914.
L y n c h , W. C . , W a t t , J . , K r a l l , S . , & P a d e n , C. M. ( 1 9 8 5 ) .
A u t o r a d i o g r a p h i c l o c a l i z a t i o n o f k a p p a o p i a t e r e c e p t o r s , i n CNS
t a s t e a nd f e e d i n g a r e a s . P h a r m a c o l o g y B i o c h e m i s t r y & B e h a v i o r
22
699-705.
--------------------------------- ------- — '
M c C a r t h y , P . S . , D e t t m a r , P . W. , L y n n , A. G . , & S a n g e r , D. J .
A n o re ctic a c tio n s of th e o p ia te a n ta g o n is t naloxone.
N europharm acology, 20, 1347-1349.
(1981).
M o r l e y , J . E . , E l s o n , M. K . , L e v i n e , A. S . , & S h a f e r , R. B. ( 1 9 8 2 ) .
Th e e f f e c t s o f s t r e s s o n c e n t r a l n e r v o u s s y s t e m c o n c e n t r a t i o n s o f
th e o p i o i d p e p t i d e , d y n o rp h in . P e p t i d e s , 3 , 901-906.
M o r l e y , J . E . , & L e v i n e , A. S . ( 1 9 8 1 ) . D y n o r p h i n : ^ I - 1 3 ) i n d u c e s
sp o n tan eo u s f e e d i n g i n r a t s . L i f e S c i e n c e s , 2 9 , 1901-1903.
M o r l e y , J . E . , & L e v i n e , A. S. ( 1 9 8 3 ) . I n v o l v e m e n t o f d y n o r p h i n a n d
t h e k a p p a - o p i o i d r e c e p t o r i n f e e d i n g . P e p t i d e s , _4, 7 9 7 - 8 0 0 .
M o r l e y , J . E . , L e v i n e , A. S . , G r a c e , M . , _& K n e i p , J . ( 1 9 8 2 ) . An
i n v e s tig a tio n o f the r o l e of k appa-opiate re c e p to r a g o n ists in the
i n i t i a t i o n o f f e e d i n g . L i f e S c i e n c e s . 31. 2617-2636.
M o r l e y , J . E . , L e v i n e , A. S . , G race, M . , K n e i p , J . , & Z e u g n e r , H.
( 1 9 8 3 ) . The e f f e c t o f t h e o p i o i d b e n z o d i a z e p i n e , t i f l u a d o m , o n
i n g e s t i v e b e h a v io r s . European J o u r n a l o f Pharmacology, 93, 265-269.
46
M u c h a , R. F . , & I v e r s e n , S. D. ( 1 9 8 4 ) . R e i n f o r c i n g p r o p e r t i e s o f
m o r p h i n e a n d n a l o x o n e r e v e a l e d by c o n d i t i o n e d p l a c e p r e f e r e n c e s : A
p r o c e d u r a l e x a m i n a t i o n . P s y c h o p h a r m a c o lo g y , 82, 2 4 1 -2 4 7 .
O l s o n , G. A . , D e l a t t e , S . W. , K a s t i n , A. J . , McLean, J . H . ,
P h i l l p o t t , D. F . , & O l s o n , R. D. ( 1 9 8 5 ) . N a l o x o n e a n d f l u i d
c o n s u m p t i o n i n r a t s : D o s e ^ r e s p o n s e r e l a t i o n s h i p s f o r 15 d a y s .
Pharmacology B io c h e m is try & B e h a v i o r . 2 3 , 1065-1068.
P a d e n , C . M. , K r a l l , S . , & L y n c h , W. C . ( i n p r e s s ) . H e t e r o g e n e o u s
d i s t r i b u t i o n a n d u p r e g u l a t i o n o f mu, d e l t a a n d k a p p a o p i o i d
r e c e p t o r s i n t h e amygdala. B r a i n R e s e a r c h .
P a s t e r n a k , G. W. , Goodman, R . , & S n y d e r , S . H . ( 1 9 7 5 ) . An e n d o g e n o u s
m o r p h i n e - l i k e f a c t o r i n m am mal i an b r a i n . L i f e S c i e n c e s . 16, 1 7 6 5 -
P e r t , C . B . , & S n y d e r , S . H. ( 1 9 7 3 ) . O p i a t e r e c e p t o r s :
in nervous t i s s u e . S c ie n c e , 179, I 011-1014.
Demonstration
R e i d , L. D. (1 9 8 5 ) . E n d o g e n o u s o p i o i d p e p t i d e s a n d r e g u l a t i o n o f
d r i n k i n g a n d f e e d i n g . The A m e r i c a n J o u r n a l o f C l i n i c a l N u t r i t i o n
42, 1 0 9 9 -1 1 3 2 .
---------------------- --------------------------R oc k w o o d , G. A . , & R e i d , L. D. ( 1 9 8 2 ) . N a l o x o n e m o d i f i e s s u g a r - w a t e r
i n t a k e i n r a t s d r i n k i n g w ith open g a s t r i c f i s t u l a s . P hysiology &
B e h a v i o r , 3 0 , 11 7 5 - 1 1 7 8 .
---------------------- -S i v i y , S . M ., B e r m u d e z - R a t t o n i , F . , R o c k w o o d , G. A . , D a r g i e , C. M. ,
R e i d , L. D . ( 1 9 8 1 ) . I n t r a c e r e b r a l a d m i n i s t r a t i o n o f n a l o x o n e and
d r in k in g i n w a te r - d e p r i v e d r a t s . Pharmacology B io c h e m is tr y &
B eh av io r, 15, 257-262.
-----
&
S m i t h , J . C . , & F o s t e r , D. F . ( 1 9 8 0 ) . Some d e t e r m i n a n t s o f i n t a k e o f
g lu c o s e + s a c c h a r i n s o l u t i o n s . P h y s io lo g y & B e h a v i o r , 25, i 2 7 -1 3 3 .
T e m p e l , A . , G a r d n e r , E. L . , & Z u c k i n , R. S . ( 1 9 8 5 ) . N e u r o c h e m i c a l a n d
f u n c t i o n a l c o r r e l a t e s o f n a l t r e x o n e ' i n d u c e d o p i a t e r e c e p t o r upr?
r e g u l a t i o n . J o u r n a l o f Pharm acology and E x p e r im e n ta l T h e r a p e u t i c s .
--------------------2 3 2 , 439-4447^
T e p p e r m a n , F. S . , H i r s t , M., .& Gowde y, C . W. ( 1 9 8 1 ) . A p r o b a b l e r o l e
fo r n o re p in e p h rin e in fe ed in g a f t e r hypothalam ic i n j e c t i o n of •
m o r p h i n e . P h a r m a c o l o g y B i o c h e m i s t r y & B e h a v i o r , 15, 5 5 5 - 5 5 8 .
T e r e n i u s , L . , & Wa h l s t r o m , A. ( 1 9 7 5 ) . M o r p h i n e ^ l i k e l i g a n d f o r
o p i a t e r e c e p t o r s i n human CSF. L i f e S c i e n c e s , 1_6, 1 7 5 9 - 1 7 6 4 .
V a s w a n i , K. K . , & T e j w a n i , G. A. ( 1 9 8 6 ) . Fo od d e p r i v a t i o n - i n d u c e d
changes i n t h e l e v e l o f o p i o i d p e p t i d e s i n t h e p i t u i t a r y and b r a i n
o f r a t . L i f e S c i e n c e s , 3 8 , 19 7 - 2 0 1 .
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