Pk. Hitchcott et Gd. Phillips, Effects of intra-amygdala R(+) 7-OH-DPAT on intra-accumbens d-amphetamine-associated learning - II. Instrumental conditioning, PSYCHOPHAR, 140(3), 1998, pp. 310-318
Rats were trained to associate an initially neutral conditioned stimulus (C
S) with a response-independent, intra-accumbens infusion of d-amphetamine (
the unconditioned stimulus; US). Elsewhere, we have reported that as a resu
lt of this training. presentations of the CS alone elicited a conditioned r
esponse consisting of increased locomotor activity and that acquisition of
this conditioned response was enhanced by post-session, intra-amygdala infu
sion of the dopamine D-3 receptor preferring agonist, R(+) 7-OH-DPAT. Here,
in this same group of animals, we have examined the conditioned rewarding
properties of the drug-associated CS by determining its ability to support
the acquisition of a novel instrumental response in the absence of drug rew
ard. Thus, rats were presented with two novel levers. Presentation of the d
rug-associated CS was made continent upon depression of one of the levers (
CR lever), while responding upon the other lever (NCR lever) had no program
med consequences. Preferential responding upon the lever delivering the dru
g-associated CS was observed despite a 6-week interval between CS-US traini
ng and the conditioned reward test. Intra-accumbens administration of d-amp
hetamine (0-20 mu g) increased the control over behaviour exerted by the CS
. increasing CR, but not NCR lever responding. In contrast, rats that recei
ved three pairings of an intra-accumbens infusion of d-amphetamine in combi
nation with intra-amygdala infusion of R(+) 7-OH-DPAT, 3 weeks prior to tes
ting, displayed similar rates of response upon both levers and were insensi
tive to the potentiation of responding for conditioned reward following int
raaccumbens d-amphetamine. However, intra-accumbens IZ-amphetamine stimulat
ed locomotor activity in a similar dose-related manner in both groups. In t
his way, rats that had received intra-accumbens infusion of d-amphetamine i
n combination with intra-amygdala infusion of R(+) 7-OH-DPAT appeared exact
ly like control group rats, for which the CS had been paired with intra-acc
umbens d-amphetamine on a negative basis only. A locomotor activity test in
dicated that one behavioural consequence of intra-amygdala administration o
f R(+) 7-OH-DPAT was the reduction of the unconditioned locomotor response
resulting from intra-accumbens administration of d-amphetamine. Hence, the
present data demonstrate that the conditioned rewarding properties of a dru
g-associated CS are specific to the CS-US association and are relatively in
sensitive to decay over time. However, the rewarding properties of a drug-a
ssociated CS were selectively abolished following activation of amygdala D-
3 receptors during presentation of the drug reward. Potential explanations
for this effect are discussed, including the possibility that intra-amygdal
a R(+) 7-OH-DPAT reduced the incentive value of the US.