Effects of Naloxone and Caffeine on Responding under a Progressive-Duration Schedule of Food Delivery
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.
KeywordsCaffeine Naloxone Rats Duration Progressive-duration schedule Motivation
We are grateful to three anonymous reviewers who provided valuable suggestions on an earlier version of this manuscript.
Compliance with Ethical Standards
Conflict of Interest
On behalf of all authors, the corresponding author states that there are no conflicts of interest. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. This study was conducted in partial fulfillment of the requirements for Taylor Manning’s senior thesis at the Massachusetts College of Liberal Arts.
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