Rate-dependent effects of drugs on the variable-interval behavior of rats

Three rats were exposed to a variable-interval (VI) schedule of food reinforcement and three were exposed to the same VI schedule but with the added constraint that reinforcement could follow only a response which occurred at least 5 seconds after the preceding response. The rats exposed to this pacing requirement responded at considerably lower rates, but obtained only slightly fewer reinforcers than those exposed to the simple VI schedule. The effects of d-amphetamine were found to be dependent on the schedule which maintained behavior. This drug produced a dose-related decrease in rates of responding maintained by the unpaced VI schedule but a dose-related increase in the much lower rates maintained by the paced VI schedule. The effects of chlordiazepoxide were not so clearly schedule-dependent although there were some differences between the effects of this drug on responding maintained by the two schedules. These results support the view that the effects of d-amphetamine depend critically on the rate of the response under investigation, but this does not appear to be the case for chlordiazepoxide.