Reinforcing effects of smoked methamphetamine in rhesus monkeys.

Abstract

RATIONALE: The occurrence of methamphetamine (METH) use by the smoking route is increasing. A nonhuman primate model for examining the reinforcing effects of smoked METH would be valuable for testing potential interventions for treating METH abuse disorders. OBJECTIVE: The purpose of the present study was to examine the reinforcing effects of smoked METH in monkeys. MATERIALS AND METHODS: Four rhesus monkeys were trained to smoke cocaine (COC) under a chain fixed-ratio (FR) 64 lever press, FR 5 inhalation schedule of reinforcement. Upon observing stable levels of self-administration, METH was substituted for COC and a dose-response function for METH (0.08-0.8 mg/kg) was determined. Subsequently, the number of deliveries of COC (1 mg/kg), and 0.2 and 0.8 mg/kg METH were examined across increasing response requirements. RESULTS: METH was dose-dependently self-administered. Higher doses of METH (0.2, 0.4, and 0.8 mg/kg) produced asymptotic levels of responding that were slightly lower than those obtained with 1 mg/kg COC. Numbers of deliveries of COC and METH decreased as response requirement increased. METH, however, maintained fewer deliveries than 1 mg/kg COC at most response requirements. CONCLUSIONS: METH is readily self-administered by smoking in rhesus monkeys when substituted for COC. METH may have a lower reinforcing strength than COC, but further research is needed to fully characterize its relative reinforcing strength.