Pimozide attenuates free feeding: best scores analysis reveals a motivational deficit

Pimozide treatment (1.0 and 2.0 mg/kg) decreased free feeding in rats. The animals were presented daily with 35 meal segments, each consisting of five 45-mg pellets; pimozide resulted in longer mean latencies to initiate eating, longer mean eating times per segment (duration scores) and more pellets left uneaten. The increase in durations was progressive both within and across test sessions; toward the end of the final session many pellets were left uneaten. Failure to initiate eating of the first pellet of each segment was rare, and was always preceded by failure to eat the fifth pellet of the preceding meal segment. To assess whether either the increase in latencies or the increase in durations reflected an impairment of absolute response capability, 'best scores' in the pimozide and control conditions were compared; the shortest latencies and durations in the pimozide condition were as 'good' as those of the control condition. However, the animals generally produced 'best' scores on fewer trials in the pimozide condition. An exception was on day 1 of testing, when the frequency of 'best' latencies was higher in the pimozide condition. The fact that the 'best' scores under pimozide equalled the 'best' scores under vehicle suggests that the pimozide-treated animals had the motoric capacity to respond normally. The facts that the pimozide-treated animals did not perform to the demonstrated limits of that capacity in a normal percentage of trials and that performance on days 2 and 3 of testing were 'worse' than performance on day 1 of testing suggest that pimozide causes a motivational deficit that has not been widely recognized.