Effects of chronic d-amphetamine administration on the reinforcing strength of cocaine in rhesus monkeys

Agonist medications have been proven effective in treating opioid and nicotine dependence; results from clinical studies suggest that the indirect dopamine agonist d-amphetamine may reduce cocaine abuse. In preclinical studies, chronic d-amphetamine treatment decreased ongoing cocaine self-administration. The present study extended previous results by determining effects of chronic d-amphetamine treatment on the reinforcing strength of cocaine under conditions in which access to cocaine was suspended during d-amphetamine treatment. Daily operant conditioning sessions consisted of morning access to food pellets delivered under a 50-response fixed-ratio schedule and evening access to cocaine (0.005-0.48 mg/kg per injection, i.v.) under a progressive-ratio schedule. After responding maintained by 0.045 mg/kg per injection cocaine stabilized, self-administration sessions were suspended and d-amphetamine (0.01-0.1 mg/kg per hr, i.v.) was administered continuously for 5 days. On the following day, d-amphetamine treatment was discontinued and daily self-administration sessions resumed. Following termination of d-amphetamine treatment, food- and cocaine-maintained responding was decreased in a dose-related manner. Decreases in the reinforcing strength of cocaine were larger and lasted longer than effects on food reinforcement. However, cocaine self-administration was unaltered if 6 days elapsed between discontinuation of d-amphetamine treatment and the next cocaine self-administration session. The necessity of a self-administration session in the presence of d-amphetamine suggests that the protracted decrease in cocaine self-administration may be a manifestation of behavioral tolerance. Regarding treatment of cocaine dependence, data suggest that prolonged d-amphetamine treatment may be necessary to produce a sustained reduction in the reinforcing effects of cocaine.