Facilitation of hot-plate response learning by pre- and posttraining naltrexone administration

Retention performance for shock-motivated learning is generally enhanced by opiate antagonists. To test the hypothesis that opioid systems mediate learning and memory in nonshock-motivated tasks, the effects of naltrexone on hot-plate response learning were investigated. Naltrexone (0.1 or 1.0 mg/kg IP) 10 min before hot-plate exposure produced hyperalgesia, as measured by decreased latencies of naltrexone-injected mice to escape. Jump latencies, however, were not significantly decreased by naltrexone given 10 min before initial testing. Nevertheless, jump latencies of naltrexone-injected mice, but not saline-injected mice, decreased further on subsequent test trials 1 and 4 days later although treatment was discontinued, suggesting that the opiate antagonist also influenced learning or memory. Mice given naltrexone (0.3-10.0 mg/kg IP) immediately after their first hot-plate exposure also exhibited learning of the jump response, on a test trial 48 h later. The decreases in jump latencies on test trials were smaller after posttraining, than pretraining naltrexone administration. The results indicate that the effect of naltrexone on learning and memory can be at least partially separated from its hyperalgesic activity.