Pargyline-induced increases in sensitivity to the effects of drugs on operant behavior in pigeons.

Pigeons responded under a multiple fixed-interval 5-min, 30-response fixed-ratio schedule of food reinforcement. Acute pargyline doses between 10.0 and 50.0 mg/kg (i.m.); given immediately prior to the session, decreased responding. Daily administration of 50 mg/kg pargyline (24 mg/kg, every 12 hr) initially decreased responding. Tolerance developed so that after 4 days of daily pargyline, responding had returned to control values. Chronic pargyline resulted in an enhanced sensitivity to the effects of d-amphetamine, ephedrine, tyramine, and morphine on schedule-controlled responding. Both d-amphetamine and pentobarbital increased fixed-interval responding at relatively low doses, while higher doses decreased responding. Daily pargyline resulted in an increased sensitivity to both the increases and decreases in response rates produced by d-amphetamine. In contrast, sensitivity to pentobarbital was not changed after daily pargyline, Ephedrine, tyramine, and morphine only decreased fixed-interval responding. Chronic pargyline resulted in an increased sensitivity to the response-rate decreasing effects of ephedrine, tyramine, and morphine. In addition to the increased sensitivity of fixed-interval responding to the effects of tyramine, the dose-effect curve for fixed-ratio responding was also a shifted to the left. Daily pargyline did not result in changes in sensitivity of fixed-ratio responding to the effects of the other drugs tested.