The effects of D-amphetamine on responding for candy and fruit drink usinga fixed ratio and a progressive ratio schedule of reinforcer delivery

The first purpose of this study was to compare the effects of D-amphetamine (AMPH) on operant responding reinforced under fixed ratio (FR) or progress ive ratio (PR) schedules of reinforcement, testing the hypothesis that resp onding reinforced under a PR operant schedule would be disrupted by lower d oses of AMPH than responding reinforced under a FR operant schedule. The se cond purpose of this study was to test the generalizability of the first hy pothesis by comparing the effects of AMPH on responding reinforced by two d ifferent reinforcers under both FR and PR operant schedules. Rhesus monkeys had five to six candy and five to six fruit-drink sessions per day, and co uld receive two reinforcers per session. Responding was initially reinforce d under a PR procedure, such that the ratio size increased with each subseq uent session. The parameters of the PR schedule were individually selected so that monkeys consumed a similar number of candy and fruit-drink reinforc ers each day. The effects of oral AMPH (0.5, 0.75, 1.0 mg/kg) on responding were assessed. Responding was then stabilized using a FR schedule with par ameters individually selected so that monkeys consumed a similar number of candy and fruit-drink reinforcers each day, and the effects of oral AMPH we re again assessed. The PR breakpoint was significantly greater fur candy th an fruit-drink. AMPH produced dose-related decreases in both candy and frui t-drink intake, but each AMPH dose decreased the number of fruit-drink deli veries to a greater extent than the number of candy deliveries. The results failed to support the hypothesis that responding under PR schedules of rei nforcement would be disrupted by lower doses of AMPH. (C) 2001 Elsevier Sci ence Inc. All rights reserved.