Voluntary and forced exercises prevent the development of tolerance to analgesic effects of morphine in rats

Objective(s):Morphine is widely used to treat chronic pain. However, its utility is hindered by the development of tolerance to its analgesic effects. Despite the renowned beneficial effects of physical exercise on cognitive functions and signs of morphine withdrawal in morphine-dependent rats, little is known about the roles of voluntary and forced exercises in tolerance to analgesic effect of morphine in rats. Materials and Methods: In this study, rats were injected with 10 mg/kg of morphine, once daily, SC over a period of 8 days of either voluntary or treadmill exercise. Following these injections, the percent of maximum possible effect (%MPE) of morphine was measured on the 1st, 4th, and 8th days by hot plate test. Results: Both voluntary and forced exercises significantly increased pain threshold compared to the sedentary group (P