Analgesia induced by vaginal stimulation in rats is apparently independent of a morphine-sensitive process

Previous studies have suggested that in rats probing against the vaginal cervix with a glass of is analgesic, for this stimulus elevates the threshold for eliciting vocalization in response to tail shock. In the present studies pretreatment with naloxone HCl (1 or 10 mg/kg), a potent narcotic antagonist did not antagonize this vaginal stimulation-induced analgesia. Furthermore, vaginal stimulation was found to exert its analgesic effect even in rats made tolerant to, and dependent upon, morphine sulfate. These results suggest that the analgesic effect of vaginal stimulation is not necessarily mediated by an opiate-sensitive neural system. However, we hypothesize that even though vaginal stimulation and other analgesic manipulations may act via different neural substrates, they may nevertheless converge onto a final common mechanism for pain suppression.