Drug effects upon force and duration of response during fixed-ratio performance in rats

Rats responded on a tandem FR24CRF CRF CRF CRF schedule of water reinforcement by paw-pressing a silent, isometric, force-sensing manipulandum. Oral dose ranges of d-amphetamine, chlordiazepoxide, chlorpromazine, and dantrolene were evaluated for their effects on this schedule-controlled behavior. Peak force, duration and interresponse time (IRT) of individual responses were recorded with a laboratory computer system. Conjoint examination of these three dependent variables revealed that the intensitive measures of response (i.e., peak force and duration) provided information about drug effects that could not be detected with the conventional IRT measure alone. More specifically, peak force was elevated by d-amphetamine at 1.6 mg/kg during the FR component, but IRT was unaffected at this dose. At 3.2 mg/kg, d-amphetamine decreased peak force and lengthened IRT during the FR component. Chlordiazepoxide increased peak force up to the highest dose examined (27.0 mg/kg), whereas dantrolene decreased peak force. Chlorpromazine did not affect peak force but did increase response duration. Higher doses of chlordiazepoxide, chlorpromazine, and dantrolene lengthened IRT during the FR component. For all three dependent variables drug effects were generally less pronounced or altogether undetected in the CRF component. The results are discussed in relation to explanatory principles such as rate-dependency and stereotyped behaviors.