Some effects of chlordiazepoxide and chlorpromazine on response force in extinction

A total of 52 male Wistar rats were continuously reinforced with food pellets for paw-pressing a silent, isometric, force-sensing manipulandum. Subsequently, extinction was introduced and the effects of chlordiazepoxide (CDP, 2.5, 5.0, 10.0 mg/kg) and chlorpromazine (CPZ, 1.0, 2.0, 4.0 mg/kg) upon response force, response rate, and resistance to extinction were observed. CPZ reduced these three extinction measures in a dose-related manner. In accord with predictions from frustration theory, CDP (5.0 mg/kg) increased resistance to extinction. However, contrary to the theory, CDP did not attenuate the extinction-related force increase. This latter result prompted an analysis of the pattern of force emission during a session of reinforced responding. Force for the first response was found to be very near extinction levels. This result, combined with the observation that the first response of a session is virtually uncued by reinforcement, suggested that high extinction force may result from a generalization decrement and not from unconditioned frustration effects.