The role of opioids in the analgesic response of rats during the estrous cycle.

The analgesic response was evaluated by the tail immersion test in female rats during each phase of the estrous cycle. The reaction time (mean +/- SEM) for tail withdrawal from the hot water bath was faster during proestrus (5.78 +/- 0.28 s) and decreased significantly during estrus (5.31 +/- 0.30 s) and diestrus 1 (5.40 +/- 0.21 s). Blockade of opiate receptors with naloxone (2 mg/kg, ip) increased the sensitivity to the noxious stimulus only during proestrus (6.46 +/- 0.42 vs 5.02 +/- 0.41 s). These results suggest that the effects of gonadal steroids on nociception may involve an opioid pathway.