The effects of acute and chronic cocaine administration on paced responding in intact and gonadectomized male and female Wistar rats.

Intact and gonadectomized male and female Wistar rats pressed a lever on a multiple (tandem Random-Interval 30-s, Differential Reinforcement of High Rate 0.5-s) (tandem Random-Interval 30-s, Differential Reinforcement of Low-Rate 5.0-s) schedule of reinforcement. The pacing requirements maintained high and low response rates under equal reinforcement frequencies. Low doses of cocaine (1 and 3 mg/kg) either did not affect or slightly increased high and low response rates of intact and gonadectomized female rats, while they did not affect or decrease high and low response rates of intact and castrated male rats. Higher doses of cocaine (up to 30 mg/kg) dose-dependently decreased both high and low response rates for all subjects. Intact male rats were less sensitive to the rate-decreasing effects of these doses of cocaine than castrated male rats or intact and ovariectomized female rats. Chronic cocaine administration consistently resulted in behavioral sensitization only in intact male subjects. The results of this experiment provide further support for the notion that the behavioral effects of psychomotor stimulant drugs are not necessarily rate dependent but may depend upon the extent to which schedule contingencies allow for behavioral variability without negatively affecting behavioral outcomes.