Combination of low doses of Naltrexone and physical exercise in an animal fibromyalgia model

Abstract

Our aim was to evaluate if a single session of physical activity until fatigue promotes analgesic effects in a fibromyalgia model, and if this effect is enhanced by low dose of naltrexone. 40 male Wistar rats were used, between 55 and 65 days-old (≥ 250g). The fibromyalgia model was developed using the reserpine model (1mg/kg, 3 consecutive days). The rats were divided into five groups: control (reserpine vehicle+naltrexone vehicle); 2nd group: fibromyalgia (reserpine+naltrexone+physical exercise); 3rd group: fibromyalgia (reserpine+naltrexone vehicle +physical exercise); 4th group: fibromyalgia (reserpine+ naltrexone + no exercise), 5th group: fibromyalgia (reserpine+naltrexone vehicle+no exercise). Interventions were: low dose of naltrexone (LDN, 0.5 mg/kg by gavage) or vehicle, and treadmill exercise or sedentary. Mechanical hyperalgesia was measured using Von-Frey test. Anxiety-like behavior was assessed using Plus-Maze test. BDNF levels were measured by ELISA. The results reserpine induced mechanical hyperalgesia in rats, and LDN associated or not to exercise presented an analgesic effect at short-term; however only LDN plus exercise was able to maintain this effect 24h after. Anxiety-like behavior and BDNF levels were modulated by LDN and exercise. LDN and exercise are promising tools to treat chronic pain as fibromyalgia.