Background & Aims: Many evidences show that regular resistance exercise training can increase activity and release of opioids in the CNS. Also it is known that as the body needs magnesium sulfate antagonist of N-methyl-D-aspartate receptor as a pain reliever, it increases resistance exercise. In this research, the effect of resistance exercise and oral magnesium sulfate on pain threshold of morphine dependent rats following withdrawal syndrome was investigated.

Materials & Methods: In this study male Wistar rats weighting 200±10g, n=35 were randomly divided into five groups (addicted: A, Mg treatment; addicted: Mg-A; exercised addicted: EA, Mg treatment; exercised addicted: Mg-E-A; and Control: C). Morphine sulfate 0.4 grams per liter was added to the animals water and after 21 days they were addicted to morphine. Animals were submitted to resistance exercise training and magnesium sulfate (10 grams/liter) for 9 weeks; they initially were trained climbing on 24 steps ladder with 20 percent of body weight (four sets with three repeated) weighting gradually were increased to 160 percent body weight in the 9th week . At the end of each three weeks of exercise protocol (3rd, 6th,9th), we injected naloxone hydrochloride (3mg/kg.ip). The tail-flick test was used to assess the effects of training on nociceptive threshold at before, 0.5,1,6 and 24 hours after naloxone hydrochloride injection. In addition, pain threshold was measured in animals before Naloxone hydrochloride injection. Analysis of variance with repeated measures (with the software SPSS 20) were used to analyze the data.

Results: Hyperalgesia following withdrawal syndrome decreased significantly by regular resistance exercise and magnesium sulfate in morphine dependent rats. Our results also showed that the effect of exercise was higher than magnesium sulfate on the pain threshold. Concurrent effects of regular resistance exercise and magnesium sulfate on thermal pain threshold were significantly higher than the effect of each on the pain threshold in morphine dependent rats (P<0.05).

Conclusion: Our results showed regular resistance exercise and magnesium sulfate have analgesic property in morphine withdrawal-induced hyperalgesia in rats. Therefore, it can be used for the treatment/management of painful conditions.