The effects of d-amphetamine on food competition in male rats.

Male Wistar rats were tested in 3 competitive situations to determine the effects of d-amphetamine (AMP) on competitive behavior. Subjects were initially tested in a fixed-pair food competition paradigm. Based on the percentage of pellets obtained, a winner and loser were defined for each pair. AMP (2.0 and 4.0 mg/kg) was then administered to one or both members of pair, and the effects of the drug on the defined winners and losers were determined. AMP did not significantly alter the percentage of pellets obtained by losers except when only the winners were administered AMP (4.0 mg/kg). Subjects were then tested in a round-robin competition paradigm in which each subject was paired with every other subject. The animals were ranked according to the percentage of pellets obtained out of a possible 450 pellets. AMP (2.0 mg/kg) decreased pellet acquisition in higher ranking subjects, whereas pellet acquisition was increased in lower ranking subjects. The effects of AMP were also evaluated in a "worker-parasite" paradigm. Subjects were individually conditioned to press a lever using an FI schedule for presentation of a food pellet. When the lever was located on the wall opposite the food hopper, a worker and parasite were defined for each pair. The worker was the subject that made the most responses on the lever while obtaining fewer reinforcements. Subjects that were defined as winners in the fixed pair food competition emerged as the workers in this situation. AMP (0.8 mg/kg) given concurrently to both subjects resulted in a reversal of roles; the parasites now emitted the majority of responses and obtained less pellets. The data indicate that AMP has a differential effect on participants in competitive situations when all subjects in the situation are treated.