A preliminary examination of some effects of cocaine on within-session patterns of responding

Two experiments examined within-session changes in operant responding when cocaine or cocaine plus food served as the reinforcer. In Experiment 1, male rats self-administered intravenous cocaine according to several fixed interval schedules. The within-session patterns of responding differed for the different schedules early in the session, but they converged by 50 minutes into the session. Because this convergence occurred regardless of the amount of cocaine consumed, it questions the response-stereotypy and dopamine-loading explanations for within-session changes in responding for cocaine reinforcers. In Experiment 2, rats responded for sweetened condensed milk during baseline sessions. During experimental sessions, responding produced cocaine in addition to the condensed milk. The addition of cocaine altered the pattern of responding during the early (first 15 minutes), but not the later (last 45 minutes), parts of the session. These results suggest that different factors produce the early-session increases and late-session decreases in responding. They are also consistent with the idea that the early-session changes in responding represent changes in 'arousal'.