Effect of Aerobic Exercise on Morphine Self-administration and Pain Modulation in Rats

Background: Exercise reverses retention deficit induced by morphine. The present study investigated the effect of aerobic exercise on tolerance to morphine usage and pain modulation. Materials and Methods: Male Wistar rats were divided into four groups as follows: (1) saline group (S), (2) morphine group (M), (3) saline + exercise (S + E), and (4) morphine + exercise group (M + E). The rats were initially trained to receive small pellets of food by pressing an active lever in the self-administration apparatus. The tail-flick and hot-plate tests were used for pain assessment. To perform the experiment, the jugular vein was exposed and cannulated. After recovery, the animals were placed in the self-administration apparatus and allowed to self-administer morphine in 2 h sessions over 11 consecutive days. Results: The morphine group was found to record a higher number of active lever pressings than did the saline one while this parameter decreased in the morphine + exercise group compared with the morphine one. Moreover, the morphine + exercise exhibited lowered pain sensitivity as evidenced to have reduced morphine use in the hot plate test. Conclusion: The exercise might be suggested to reduce using of morphine and modulate pain probably through the release of endogenous opioid