Abstract
In 2 experiments, we investigated the utility of a progressive-duration schedule as an assay for measuring drug effects. In both experiments, rats responded on a schedule of reinforcement in which food delivery was contingent upon response duration. Response requirements increased after each reinforcer delivery in a fashion similar to that of progressive-ratio schedules. Naloxone (1.0, 3.0, and 10.0 mg/kg) produced dose-dependent decreases in breaking points, a finding consistent with those of previous studies demonstrating that opioid antagonists may reduce the reinforcing efficacy of highly palatable foods in sated organisms. In a second experiment, caffeine (3.0, 6.25, and 12.5 mg/kg) sometimes produced increases in breaking points but reduced efficiency by increasing the proportions of lever presses that were too short to satisfy reinforcer requirements. Food deprivation had similar effects. The progressive-duration schedule shows promise as an assay sensitive to motivational aspects of behavior; however, distinguishing the rate-altering effects of drugs from their effects on motivating operations may pose interpretation challenges.