The effects of amphetamine and scopolamine on adjunctive drinking and wheel-running in rats.

Two groups of rats were exposed to a fixed-interval 90 s schedule of food reinforcement. One group had access to a drinking tube containing water and the second had access to a running wheel. Amphetamine (0.3-10.0 mg/kg) and scopolamine (0.1-3.0 mg/kg) were assessed for their effects on lever-pressing, adjunctive drinking and adjunctive wheel-running. Low to moderate doses of amphetamine increased overall rates of lever-pressing, whereas the highest dose decreased them. Scopolamine decreased overall lever-pressing rates in a dose-dependent manner. Both drugs changed the within-interval pattern of lever-pressing from one of increasing probability through the interval to almost constant probability throughout. Overall rates of adjunctive drinking and adjunctive wheel-running were decreased by amphetamine and scopolamine. Amphetamine failed to alter the within-interval patterns of either drinking or wheel-running in any substantial manner. The effect of scopolamine was to make the probabilities of each adjunctive behaviour more even through the interval. Although the two drugs had different actions, there was little difference in the way drinking and wheel-running were affected by each.