A labor-supply analysis of cocaine self-administration under progressive-ratio schedules: antecedents, methodologies, and perspectives

Rationale: Under a progressive-ratio (PR) schedule, a subject must complete increasing fixed-ratio (FR) response requirements to obtain reinforcers. R esponse requirements are increased until responding stops; the final ratio completed being the "break point" and providing an index of the relative ef fectiveness, or value, of the reinforcer to maintain behavior. Objectives: This review examines the historical and conceptual framework underlying the PR procedure and examines the concept of relative reinforcer value. Pharma cological analysis (based on receptor theory), and behavior analysis (based on microeconomic theory) are reviewed. Methods: Using a microeconomic adap tation of the reinforcement model referred to as conservation, a mathematic al model of PR performance is proposed based on the curvilinear relationshi p between economic supply and labor. Drug consumption and instrumental resp onding were assumed to reflect deviations from a balance point, defined as the levels of consumption and responding under no scheduled restraint. Data sets were re-analyzed in which several response sequences were examined in rhesus monkeys maintained on PR schedules of intravenous cocaine delivery. Results: The modified conservation equation fitted the PR data accurately, and results consistent with both linear and concave labor-supply functions were obtained. These results suggest that cocaine self-administration unde r PR schedules conforms to labor-supply relationships characterized as inel astic (consumption is resistant to increases in schedule requirements) and unit elastic (at high response costs, consumption declines with no correspo nding increase or decrease in total responding). Conclusions: The labor-sup ply methodology allows for a definition of the relative value of a drug rei nforcer in PR studies based on changes in consumption across response costs . Specifically, relative reinforcer value is defined in terms of changes in behavior from a balance point, rather than as a property that determines t he strength of the instrumental response.