Effects of psychoactive drugs on temporal discrimination in rats

Male Wistar rats were trained, in two-lever operant chambers, to press one lever (L5) after the presentation of a conditioned stimulus (a light) for 5 s (CS5) or the other lever (L20) after a conditioned stimulus for 20 s (CS20). Various drugs were administered before experimental sessions, during which CS5, CS20 and a stimulus of the intermediate duration of 12 s (CS12) were randomly presented. Rats pretreated with vehicle made approximately 50% of presses on L5 after the presentation of CS12. Atropine, diazepam, desipramine, clomipramine and moderate doses of haloperidol or of scopolamine increased the percentage of responses made on L5 after the presentation of CS20 and/or CS12. These effects could be due to a reduction of the speed of an internal clock. High doses of either haloperidol or scopolamine decreased the percentage of correct responses, an effect that was interpreted as a disruption of temporal discrimination. Nicotine and d-amphetamine decreased the percentage of responses made on L5 after the presentation of CS5 and/or CS12, an effect that could reflect an acceleration in the speed of the internal clock. Physostigmine, buspirone, mianserin and piracetam did not consistently alter performance, suggesting that these drugs do not affect timing processes.