EFFECTS OF HIGH-DOSES OF NALTREXONE ON RUNNING AND RESPONDING FOR THEOPPORTUNITY TO RUN IN RATS - A TEST OF THE OPIATE HYPOTHESIS

Previous research shows that low to moderate doses of opiate antagonis ts do not affect running whereas high doses decrease running. This dec rease in running has been interpreted as a motivational effect; howeve r, it may also be an effect of motoric impairment, malaise, or sedatio n. The purpose of the present study was to evaluate the effects of hig h doses of naltrexone on running and responding for the opportunity to run in rats. Seven male Wistar rats, trained to press levers for the opportunity to run for 30 s, were exposed to a series of tandem fixed- ratio 1 variable-interval (VI) schedules of reinforcement where the va lue of the VI schedule was varied. Four rats were exposed to a VI 60-, VI 30-, and VI 5-s schedule order and the remaining three were expose d to VI 5-, VI 30-, and VI 60-s schedule order. After 50 sessions, dos es of 10, 20, or 40 mg/kg of naltrexone were administered prior to a s ession. Results showed that naltrexone significantly decreased rates o f lever pressing and running and increased postreinforcement latency t o respond. Within-session analyses showed that naltrexone significantl y decreased responding and running during the first schedule of a sess ion regardless of schedule value. The lack of interaction of the drug effect with the schedule of reinforcement is inconsistent with a motiv ational effect. Consequently, it was concluded that decreases in runni ng caused by high doses of naltrexone are not motivational and should not be taken as evidence in support of the opiate hypothesis.