Opioid receptors are not involved in the increase of the nociceptive threshold induced by aerobic exercise.

OBJECTIVE To investigate the involvement of opioid receptors in antinociception induced by different aerobic exercise protocols in rats. METHODS This experimental study, conducted in the Federal University of Minas Gerais, Belo Horizonte, Brazil from November 2011 to May 2012, included 60 female Wistar rats, divided into 10 groups of 6 animals per group. The rats were subjected to different aerobic exercise protocols: acute, cardiac stress, eccentric, and training. The nociceptive threshold was measured by the paw-withdrawal test. To investigate the involvement of the endogenous opioids system, the non-selective opioid receptor antagonist naloxone (5 mg/kg) was administered subcutaneously before the beginning of the exercise. RESULTS All exercise protocols increased the nociceptive threshold for 15 minutes. The naloxone pre-treatment did not alter the antinociception induced by aerobic exercise protocols. CONCLUSION The endogenous opioids system did not participate in the antinociceptive effect produced by the aerobic exercise protocols.