Fixed ratio schedules of food presentation and stimulus shock termination: effects of d-amphetamine, morphine, and clozapine.

Squirrel monkeys responded under a multiple schedule in which 30 responses during a specified time limit resulted in either food presentation or termination of visual stimuli associated with impending shoch delivery. Schedule components were associated with different colored lights and were separated by 60-s time-out periods in which all lights were extinguished. If the response requirement was not met within the time limit, either the time-out period alone (food components) was presented or a single shock was delivered coincident with onset of time-out. In experiments with d-amphetamine, different control rates of responding were engendered by varying the time limit. When the time limit was 60s, all monkeys responded at higher overall rates during food presentation components. When the time limit was reduced to 15 s, rates of responding in both components increased and became more similar than under the 60-s limit. When control rates in the two components differed under the 60-s time limit, d-amphetamine sulfate (0.01-1.0mg/kg) increased the normally lower rates under the shock schedule at intermediate doses, but generally only decreased the higher rates under the food schedule. With more comparable control rates under the 15-s time limit, the effects of amphetamine were also more comparable. In most cases low and moderate doses either had little effect or slightly increased responding in both schedule components and higher doses decreased responding. Morphine sulfate (0.03-1.7mg/kg) and clozapine (0.1-3.0mg/kg) decreased responding comparably under both food and shoch schedules with the 15-s time limit.