Individual differences in behavioral tolerance to amphetamine and the economic context of reinforcement loss

Abstract

The economic context (i.e. an enriched vs impoverished environment) affects many drug-induced phenomena. The present study examined whether the 'experienced' economic context of operant responding was associated with the degree of tolerance to the behavioral effects of amphetamine. Eight rats lever pressed for food reinforcement under a multiple schedule consisting of several variable-interval schedules (8, 17, 55, 150, and 250 s). Amphetamine was first administered acutely (0.2, 0.8, 1.6, and 3.2 mg/kg), then chronically (dose tailored for each subject) over 30 consecutive sessions. Baseline saline injections were also administered during the acute regimen. Herrnstein's single-alternative matching equation described the rats' response rate data well under all conditions. A parameter in Herrnstein's equation (re), which has been shown to vary with experimentally-arranged contextual reinforcement, was used as the index of the experienced economic context for each subject under baseline conditions. Differences in the value of re predicted individual differences in the degree of tolerance. Under most variable-interval (VI) schedules, and when all schedules were aggregated, less tolerance accrued if the baseline context was experienced as enriched, and more tolerance accrued if the baseline context was experienced as impoverished. In terms of the reinforcement loss hypothesis, the results suggest that tolerance was not determined by reinforcement loss per se, but by how much the animal lost relative to the economic context in which the operant task was embedded.