HEROIN SELF-ADMINISTRATION IN RATS UNDER A PROGRESSIVE RATIO SCHEDULEOF REINFORCEMENT

Heroin self-administration behavior under a progressive ratio (PR) sch edule of reinforcement was evaluated in rats. The schedule was designe d to restrict drug intake, minimize opiate dependency, and quantify th e number of responses emitted (final response ratio) in order to recei ve a limited number of heroin infusions. Final ratios were found to be stable and did not increase with chronic (31 days) PR reinforcement. The ability of the PR schedule to detect changes in heroin reinforceme nt was demonstrated by evaluating the effect of naltrexone pretreatmen t and unit dose alteration on final ratios. Naltrexone (0.4 mg/kg) red uced final ratios and an inverted U dose-response relationship was est ablished for the unit heroin doses 12.5-100 mug/injection. Maximal fin al ratios occurred with 50 mug/injection heroin reinforcement. This PR schedule may provide a useful method for evaluating the effects of ph armacological manipulations or lesions on opiate reinforcement.