Backgrounds: The amnesic effect of morphine is well known in the laboratory animals. But, it is unclear that morphine at what times can exactly affect different phases of memory, including acquisition, consolidation, and retrieval. Therefore, we investigated the time profile of morphine’s amnesic effect on passive (inhibitory) avoidance learning and memory in male Wistar rats. Methods: In order to evaluate the outcomes of pre and post-training administration of morphine, the animals were trained in a step-through type of passive avoidance task at various time points, and were tested 24 h after training to measure memory retrieval. Results: The results showed that acquisition of memory was impaired in the animals that received a dose of 7.5 mg/kg morphine injection (intraperitoneally) at 0, 30 min and 1 h before training as evidenced by a decrease in step-through latency on the test day. Post-training administrations of morphine (30 min and 1h -4h), except for the time immediately after training, did not impair memory consolidation. The results also showed that pre-test administration of morphine, at 0 and 30 min before the test, impaired retrieval of inhibitory avoidance memory. Conclusion: Taken together, the results suggest that morphine, when injected at different time points that is before training, after training or before testing, affects different phases of inhibitory avoidance memory. With regard to the time of injections related to each phase, other experiments can be designed to investigate molecular mechanisms involved in the impairing effect of morphine in each phase.
