The antinociceptive effect of vaginal stimulation in the rat is reduced by naloxone

Mechanical stimulation of the vagina produces a powerful antinociceptive effect against a variety of noxious stimuli. In rats tested in either the radiant heat tail flick or the warm water tail immersion test the antinociceptive effect of vaginal stimulation was found to be significantly reduced, but not abolished, following the administration of 10 mg/kg of naloxone. These results are in contrast to those of an earlier study in which naloxone was ineffective against the antinociceptive action of vaginal stimulation in a tailshock vocalization paradigm. It therefore appears that the nature of the noxious stimulus used may influence the type of antinociceptive mechanism triggered by vaginal stimulation.