Background: Hippocampus is rich in corticosteroid and opioid receptors and is involved in memory. Both acute stress and morphine have a different effect on different stages of memory. Thus, the effect of acute forced swim stress on formed memory recall was investigated after prescription of morphine different doses. Materials and Methods : In this experimental study, adult male Wistar rats (200-250 g) were classified into three categories (for each group, n=7): 1) repeated morphine groups, 30 minutes before each training, they were intraperitoneally prescribed morphine for 4 days in Morris water maze, (1, 10,100 μg/kg and 1, 10 mg/kg) and the recall test was performed on 5th day. 2) Acute stress group, they were on training for 4 days without morphine and on the fifth day, 30 minutes before the recall test, they were subjected to forced swim stress for 5 min. 3) Repeated morphine plus acute stress groups, they were first treated same as first category and on the 5th day, they were behaved like the second category. On the day 12th, the same test as the 5th day was conducted, but the groups received no treatment. Results: In the repeated morphine groups, apparently only 10 mg/kg of morphine significantly disturbed learning (p < 0.001). But during the 5th and 12th day no significant difference was observed between all groups and the control group in memory recall. In the group treated with morphine plus acute stress, only 10 mg/kg of morphine significantly decreased memory recall during the 5th and 12th day compared to the control group (p<0.05). Conclusion: Repeated prescription of 10 mg/kg morphine plus acute stress intensifies the deleterious effects of stress on memory.

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