Effects of phentermine on responding maintained by progressive-ratio schedules of cocaine and food delivery in rhesus monkeys

Previous reports indicate that intravenous pretreatment with phentermine ca n decrease cocaine-maintained responding without affecting food-reinforced responding under fixed-ratio schedules, The present experiments were design ed to explore the generality of this effect using progressive-ratio schedul es of reinforcement and different routes of phentermine administration. Uni t doses of cocaine and food-pellet magnitudes mere identified that maintain ed similar breaking points, and the effects of acute exposure to phentermin e were assessed. In Experiment 1, a 'conventional' (one trial) progressive- ratio schedule was used, in which response requirements increased after eac h reinforcer delivery; in Experiment 2, a 'modified' (five-trial) progressi ve-ratio schedule was used, in which response requirements increased after every five reinforcer deliveries. In one group of monkeys, responding was m aintained by food; in another, cocaine infusions maintained responding. Phe ntermine (0.1-5.6 mg/kg, intramuscularly (i.m.)) dose-dependently decreased breakpoints on both progressive-ratio schedules. There mere no differences in phentermine's effects on cocaine- and food-maintained behavior. In Expe riment 3, intravenous administration of phentermine had largely similar eff ects. Taken together with results from previous reports, these data suggest that the effects of phentermine pretreatment are influenced by the behavio ral procedure used to maintain responding and/or by the efficacy of the foo d and cocaine reinforcers. (C) 1999 Lippincott Williams & Wilkins.