Effects of cocaine and d-amphetamine on behavior maintained under various schedules of food presentation in squirrel monkeys.

Dose-response functions were determined for the effects of cocaine and d-amphetamine on lever-pressing responses maintained under three schedules of food presentation: 1) a simple fixed-ratio schedule, 2) a multiple fixed-ratio, fixed-interval schedule and 3) a second-order fixed-interval schedule with fixed-ratio components. These schedules generated a variety of rates and patterns of responding. The effects of the drugs on response rate depended on the control rates of responding. The highest overall rates of responding were maintained under the fixed-ratio schedule and under the fixed-ratio component of the multiple schedule. Both cocaine and d-amphetamine produced dose-related decreases in these high response rates. Lower overall rates of responding were maintained under the second-order schedule and under the fixed-interval component of the multiple schedule. Intermediate doses of cocaine or d-amphetamine (0.1-1.0 mg/kg) increased these lower rates of responding. Under the second-order schedule and the multiple schedule, local response rates during successive segments of the fixed-intervals were differentially affected by the drugs. Cocaine and d-amphetamine markedly increased low response rates that occurred during initial segments of the fixed intervals, but either had little effect on or decreased higher responses rates that occurred during later segments of the fixed intervals. Cocaine and d-amphetamine had similar qualitative and quantitative effects on responding, but d-amphetamine was longer lasting than cocaine.