Strychnine antagonizes vaginal stimulation-produced analgesia at the spinal cord.

Vaginal-cervical mechanostimulation (VS) suppresses vocalization and withdrawal responses to noxious stimulation. To determine whether the inhibitory neurotransmitter, glycine, contributes to the action of VS, strychnine, a specific glycine receptor antagonist was administered perispinally via intrathecal catheter in dosages of 1,5,25 and 100 micrograms. Prior to strychnine administration, VS (400 g force) elevated thresholds to elicit vocalization in response to graded intensities of tail shock, and blocked vocalization elicited by stimulation of a skin area, previously sensitized by intradermal injection of a 20% yeast solution. After strychnine administration the analgesic effects of VS were significantly attenuated. These findings suggest that the analgesic action of VS is partially mediated by glycine at the spinal level.