Age effects on aerobic exercise‐induced reductions in pain sensitivity in female Sprague‐Dawley rats

Abstract

Age differences appear to exist in the pain experience. Bouts of aerobic or anaerobic exercise are known to reduce pain perception. We examined age effects on exercise‐induced reductions in pain sensitivity in aerobic exercise trained female Sprague‐Dawley rats. In a previous study, we found that running started in youth (age 11 wk) significantly reduced pain sensitivity in middle‐aged (38 wk) rats compared with sedentary rats. To examine whether starting exercise at middle‐age, rather than in youth, also affects pain sensitivity, 10 rats, starting at age 21 wk, ran 11 wk for up to 30 min every other day in motorized running wheels. Ten sedentary rats were handled every other day. A tail flick latency (TFL) test measured pain perception at 5–6 hr post‐exercise. A longer latency indicated reduced pain sensitivity. TFL was significantly longer in exercised rats compared to sedentary rats (p=0.01). After 2 wk, we examined endogenous opioid involvement with the opioid antagonist, naloxone (2 mg/kg, s.c.). Naloxone eliminated the difference, suggesting opioid mediation. No significant difference in pain sensitivity was found between rats that began exercising in youth and those that started running in middle‐age. Our results show that aerobic exercise reduces pain sensitivity in female rats independent of age at implementation of an exercise program and this effect is opioid‐mediated.