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 t he caudate putamen may be involved in food reward-related learning. Th e purpose of the present study was to evaluate the contribution of DA terminals in the dorsal caudate putamen to food-rewarded operant respo nding. Experiment 1, a study measuring circling behaviour in 18 rats r eceiving systemic amphetamine (1.5 mg/kg) and unilateral intracaudate putamen injections of cis-flupenthixol (0.0, 1.0, 10.0, and 25.0 mug i n 0.5 mul), a DA receptor antagonist, or its pharmacologically inactiv e isomer trans-flupenthixol (25.0 mug in 0.5 mul), determined a behavi ourally effective dose of cis-flupenthixol. Results showed that cis-fl upenthixol dose dependently increased ipsiversive turning and trans-fl upenthixol did not. In Experiment 2, an operant study, 36 rats were tr ained to press a lever for food on a variable interval 30-s schedule. Rats were then randomly assigned to four groups, three of which receiv ed one of the following bilateral intracaudate injections prior to thr ee 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 bi lateral 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, bu t not trans-flupenthixol or saline, produced a time-dependent intrases sion decline in operant responding. This pattern resembled that seen i n 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 t he cis-flupenthixol group, suggesting that the extinction-like pattern of responding in the cis-flupenthixol group was not a result of repea ted central injections per se. These results provide support for the h ypothesis that DA transmission may be involved in incentive learning a nd, further, that dopaminergic projections to the dorsal caudate putam en may play a role in food reward-related incentive learning,