Objective: 

Exercise is known to influence pain perception. This phenomenon, termed exercise-induced hypoalgesia (EIH) typically reported during and/or following exercise, but less is known the mechanisms responsible for EIH. Melatonin is a hormone synthesized principally in the pineal gland that has been classically associated with endocrine actions. However, several lines of evidence suggest that melatonin plays a role in pain modulation. In this study, we investigated the nociceptive response before and after exercise in both sedentary and exercise trained rats under light-induced functional pinealectomy.

Methods: 

Male wistar rats were assigned to one of four groups: sedentary control (C), sedentary pinealectomized (Px), exercise trained (T), and trained under pinealectomy (T-Px) groups. Exercise trained groups were subjected to 6 weeks of motor-driven treadmill exercise. C and T groups exposed to normal light/dark cycle (12:12-hour light/ dark cycle) and Px and T-Px rats exposed to continuous light for 6 weeks. Hot plate test was repeated before and after exhaustive exercise.

Results: 

Animals in T goup had significantly higher pain thresholds compared to other groups both before and after exercise. Although Px was found to be ineffective on pain threshold before and after exercise, pain threshold in T-Px group was found to be returned to baseline levels obseved in control animals.

Conclusions: 

Our findings support the hypothesis that endogenous melatonin plays a role in the exercise-induced hypoalgesia.