Changes in the rate-increasing effects of d-amphetamine and pentobarbital by response consequences

Keypecking in one group of pigeons was maintained under schedules in which food was presented only when a specified number of responses was followed by a 30-s pause without a response. d-Amphetamine and pentobarbital increased low rates of responding (and, thus, decreased food presentation) only after initial injections or when, during drug sessions, responses during the 30-s period did not reset the period. When responses during the pause-interval postponed food delivery, the rate-increasing effects of both drugs diminished over succeeding administrations. Thus, immediate effects of response consequences were as influential as the actual presence of a drug in determining the reproducibility of the behavioral effects of that drug. In a second experiment, keypecking in another group of pigeons was maintained under a 10-min fixed-interval schedule of food presentation but suppressed by a 100-response fixed-ratio schedule of shock delivery (punishment). d-Amphetamine and pentobarbital increased low rates of punished responding when shock delivery was eliminated during drug sessions. Pentobarbital, but not d-amphetamine, also increased punished responding when shock delivery was present. Rate-increasing effects of these drugs were determined by not only predrug patterns of responding but also effects of reinforcers and punishers that occurred during exposure to the drug.