The effects of (+)-amphetamine and apomorphine on responding for a conditioned reinforcer

Two psychomotor stimulants, (+)-amphetamine and apomorphine, were examined for effects on associative learning and responding for a conditioned reinforcer. The experimental phases included: preexposure to an operant test chamber with two levers, each of which produced a neutral stimulus when pressed; pairings of one stimulus with food; and a subsequent test of lever pressing for the two stimuli. Groups of food deprived rats (n = 8-12) were given IP injections of one stimulant prior to each pairing or testing session. Given during pairings, (+)-amphetamine produced a dose-related attenuation of responding for the conditioned stimulus in the test; doses of 0.5, 1.0 and 2.0, but not 0.25 or 4.0 mg/kg, given during the test enhanced responding, as did 0.5 mg/kg given in both phases. Apomorphine did not significantly alter responding during testing when administered in either the pairing (0 0.75 mg/kg) or test (0.5 1.0 mg/kg) phase. The results suggest that the modulation of conditioned reinforcement by psychomotor stimulants may occur through a presynaptic influence. Furthermore, the results with (+)-amphetamine suggest that this drug differentially affects the learning of an association between a conditioned and unconditioned stimulus versus the acquisition of responding for that conditioned stimulus.