Assessment of Pimozide's Motor and Hedonic Effects on Operant Behavior in Rats

The present study examined the effects of the neuroleptic pimozide on several measures of motor capacity and reinforcement efficacy in rats trained to respond according to a multiple random interval (RI) food reinforcement schedule (mean interreinforcement intervals of 10, 20, 40, 80, and 160 sec). Pimozide (0.125, 0.25, 0.5, and 1.0 mg/kg) produced a dose-dependent suppression of response rates for all five RI schedules and a dose-dependent increase in response duration. An independent measure of motor activity in photocell activity chambers also was decreased by pimozide in a dose-dependent manner. Photocell activity was significantly correlated with response duration and with the Matching Equation parameter k. Thus, all three measures of motor performance revealed similar decreases in motor capacity at the high dose of pimozide. Reinforcement efficacy also was reduced by the 1.0 mg/kg dose of pimozide as indicated by an increase in the Matching Equation parameter Re. The parameters k and Re were not significantly correlated, suggesting that these two Matching Equation parameters do provide independent measures of motor capacity and reinforcement efficacy, respectively. The present results demonstrate the importance of obtaining measures other than simple response rates in order to assess drug effects on operant behavior.