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 li ght) for 5 s (CS5) or the other lever (L20) after a conditioned stimul us for 20 s (CS20). Various drugs were administered before experimenta l 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 d oses of haloperidol or of scopolamine increased the percentage of resp onses made on L5 after the presentation of CS20 and/or CS12. These eff ects could be due to a reduction of the speed of an internal clock. Hi gh 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 p ercentage of responses made on L5 after the presentation of CS5 and/or CS12, an effect that could reflect an acceleration in the speed of th e internal clock. Physostigmine, buspirone, mianserin and piracetam di d not consistently alter performance, suggesting that these drugs do n ot affect timing processes.