Behavioral effects of caffeine, N6-(L-phenylisopropyl) adenosine and their combination in the squirrel monkey.

The behavioral effects of caffeine, N6-(L-phenylisopropyl)adenosine (PIA) and the two drugs in combination were studied in squirrel monkeys. Monkeys responded by pressing a lever under different schedules of reinforcement involving either food or electric shock. The effects of caffeine depended primarily on the type of schedule that controlled responding and were largely independent of the type of consequent event. When responding was maintained at high rates under fixed-ratio schedules of either food presentation or stimulus-shock termination, caffeine (1-56 mg/kg) typically decreased responding in a dose-dependent manner. When responding was maintained at moderate rates under fixed-interval schedules involving either event, intermediate doses of caffeine (3-10 mg/kg) slightly increased responding, whereas higher doses decreased responding. When responding was maintained under a fixed-interval schedule of stimulus-shock termination and concurrently suppressed by response-produced shock, intermediate doses of caffeine (3-30 mg/kg) markedly increased responding. PIA (0.03-1 mg/kg) only decreased responding under all schedules. Caffeine (1-30 mg/kg) antagonized the rate-decreasing effects of PIA under each schedule. PIA (0.03-1 mg/kg) reduced or eliminated the rate-increasing effects of intermediate doses of caffeine under the fixed-interval schedules but did not attenuate the rate-decreasing of high doses of caffeine under any schedule. Thus, the rate-increasing and rate-decreasing effects of caffeine differed in their susceptibility to modification by PIA.