Differential sensitivity of operant behaviors to changes in the concentration of a sucrose reinforcer: effects of pimozide.

The sensitivity of operant response rates to changes in a sucrose reinforcer was examined in well-trained animals maintained on a variable ratio (VR) or variable interval (VI) schedule (experiment 1). Although VR performance showed greater resistance to small reductions in the concentration of the sucrose reinforcer than VI performance, VR performance was more sensitive to large reductions in the sucrose concentration. Despite this differential sensitivity only the smallest dose of pimozide (0.125 mg/kg) differentially affected these behaviors by reducing VI rates without affecting VR rates. These and other results support the view that low doses of pimozide reduce the hedonic impact of the reinforcer. The results also indicate that the attenuation of operant responding by higher doses (0.25, 0.5, and 1.0 mg/kg) cannot be solely a result of the blunting of reward. Experiment 2 demonstrated that when rats drink in daily, brief one-bottle tests they show greater resistance to reductions in the sucrose concentration than when they lever-press for sucrose, and require a higher dose of pimozide (2.0 mg/kg) to attenuate consumption. Together the results of both experiments suggest that the greater the resistance to reductions in the reinforcement value, the greater the dose of pimozide necessary to attenuate performance. We discuss the importance of attaining a more complete understanding of the factors in control of operant performance in order to better assess the effects of neuroleptics on reward.