Behavior maintained under fixed-interval and second-order schedules of cocaine or pentobarbital in rhesus monkeys

Responding was maintained under a fixed-interval 5 min (FI 5 min) or a second-order FI 5 min schedule of fixed-ratio 10 brief stimuli components [FI 5 min (FR 10:S)] by cocaine (0.0125-1.0 mg/kg/infusion) or pentobarbital (0.05-3.0 mg/kg/infusion) delivery during daily 3-hr sessions. Rates and patterns of responding were similar with both drugs, although cocaine was 10 to 64 times more potent than pentobarbital. As dose was increased, rates of responding increased to a maximum and then decreased under both schedules. For both drugs, rates of responding were variables from session to session as well as over successive intervals, but performance was generally typical of interval schedules. The variability in responding over successive intervals was least for cocaine under the FI schedule and greatest for pentobarbital under the FI (FR) schedule. Drug intake was a direct function of dose and the number of infusions per session decreased as dose was increased. At lower doses of cocaine, rates of responding and intake were similar during each hour of the session under the FI schedule, but were more variable under the second-order schedule. At lower doses of pentobarbital, responding and intake were variable across the session under both schedules. At higher doses of both drugs, rates of responding and intake decreased over the session and the pattern of the performance was disrupted under both schedules. Although the dose-response functions for rates of responding and intake under both schedules were similar, the local pattern of responding under the second-order schedule was under the control of both the brief stimulus presentations as well as drug delivery.