The effects of multiple injections of morphine sulfate on shuttle-box behavior in the rat

Abstract

F344 male rats were trained in shuttle-box avoidance to a criterion of over 90% avoidance, and independent groups of subjects (Ss) were then given 13 daily injections of 5, 10, or 20 mg/kg of morphine or saline (s.c.) 30 min prior to testing. In Experiment 1, Ss were injected and tested on each of the 13 days while in Experiment II, Ss were injected daily but tested only on Day 13. Five mg/kg of morphine did not decrease avoidance responding. While 10 mg/kg initially caused a decrease in avoidance responses, responding rapidly returned to the pre-injection level. For the groups receiving either 5 or 10 mg/kg there was an increase in locomotor activity and a decrease in response latencies when compared to saline controls on Day 13. The 20 mg/kg dose initially disrupted avoidance, but not escape, responses but after 13 daily injections avoidance responses were near pre-injection levels even though locomotor activity remained above baseline. Results of the two experiments were identical for drug Day 13 suggesting that Ss injected and run daily had not merely learned to respond under the influence of the drug but had developed tolerance to the drug.