Acquisition of cocaine reinforcement using fixed-ratio and concurrent choice schedules in socially housed female and male monkeys

Rationale Previous studies in socially housed monkeys examining acquisition of cocaine self-administration under fixed-ratio (FR) schedules of reinforcement found that subordinate males and dominant females were more vulnerable than their counterparts. Objectives The present studies extended these findings in two ways: (1) to replicate the earlier study, in which female monkeys were studied after a relatively short period of social housing (similar to 3 months) using cocaine-naive female monkeys (n = 9; 4 dominant and 5 subordinate) living in well-established social groups (similar to 18 months); and (2) in male monkeys (n = 3/social rank), we studied cocaine acquisition under a concurrent schedule, with an alternative, non-drug reinforcer available. Results In contrast to earlier findings, subordinate female monkeys acquired cocaine reinforcement (i.e., > saline reinforcement) at significantly lower cocaine doses compared with dominant monkeys. In the socially housed males, no dominant monkey acquired a cocaine preference (i.e., > 80% cocaine choice) over food, while two of three subordinate monkeys acquired cocaine reinforcement. In monkeys that did not acquire, the conditions were changed to an FR schedule with only cocaine available and after acquisition, returned to the concurrent schedule. In all monkeys, high doses of cocaine were chosen over food reinforcement. Conclusions The behavioral data in females suggests that duration of social enrichment and stress can differentially impact vulnerability to cocaine reinforcement. The findings in socially housed male monkeys, using concurrent food vs. cocaine choice schedules of reinforcement, confirmed earlier social-rank differences using an FR schedule and showed that vulnerability could be modified by exposure to cocaine.