MICROINJECTIONS OF FLUPENTIXOL INTO THE CAUDATE-PUTAMEN OF RATS PRODUCE INTRASESSION DECLINES IN FOOD-REWARDED OPERANT RESPONDING

Results of recent studies suggest that dopamine (DA) transmission in the caudate putamen may be involved in food reward-related learning. The purpose of the present study was to evaluate the contribution of DA terminals in the dorsal caudate putamen to food-rewarded operant responding. Experiment 1, a study measuring circling behaviour in 18 rats receiving systemic amphetamine (1.5 mg/kg) and unilateral intracaudate putamen injections of cis-flupenthixol (0.0, 1.0, 10.0, and 25.0 mug in 0.5 mul), a DA receptor antagonist, or its pharmacologically inactive isomer trans-flupenthixol (25.0 mug in 0.5 mul), determined a behaviourally effective dose of cis-flupenthixol. Results showed that cis-flupenthixol dose dependently increased ipsiversive turning and trans-flupenthixol did not. In Experiment 2, an operant study, 36 rats were trained to press a lever for food on a variable interval 30-s schedule. Rats were then randomly assigned to four groups, three of which received one of the following bilateral intracaudate injections prior to three subsequent test sessions: saline (n = 6; 0.5 mul), cis-flupenthixol (n = 10; 25.0 mug/0.5 mul), and trans-flupenthixol (n = 10; 25.0 mug/0.5 mul). Rats in the home cage control group (n = 10) received two bilateral intracaudate putamen injections of cis-flupenthixol (25.0 mug/0.5 mul) in their home cages and a final injection of cis-flupenthixol prior to a test session. The results showed that cis-flupenthixol, but not trans-flupenthixol or saline, produced a time-dependent intrasession decline in operant responding. This pattern resembled that seen in extinction. The intrasession pattern of responding in the home cage control group did not differ significantly from that of the first test day but did differ significantly from that of the third test day of the cis-flupenthixol group, suggesting that the extinction-like pattern of responding in the cis-flupenthixol group was not a result of repeated central injections per se. These results provide support for the hypothesis that DA transmission may be involved in incentive learning and, further, that dopaminergic projections to the dorsal caudate putamen may play a role in food reward-related incentive learning,