Evidence that probing the vaginal cervix is analgesic in rats, using an operant paradigm.

Probing against the vaginal cervix (CP) suppresses responses to noxious stimulation in rats. The first experiment rules out the possibility that this effect is due to CP-induced immobilization. All rats first learned to press a panel, thus terminating noxious skin shock. Then they either received CP (experimentals) or did not (controls) when they pressed the panel during skin shock that was inescapable. The controls soon showed extinction of the panel-press response, whereas the experimentals continued pressing the panel and obtaining CP, for significantly more trials. The rats thus performed an operant response for CP at a time before CP could have blocked their movement. The second experiment argues against the possiblity that CP exerts its effect by "distracting" the rats from the skin shock. The rats received inescapable skin shock which continued for 7 sec after each panel-press response. During this shock one group received CP, and another received perineal probing ("distraction" control). The panel-press latency was significantly shorter in the CP group than in the perineal group. Latency in a control group, in which shock was terminated as soon as the rats pressed the panel, did not differ significantly from the CP group. Latency in another control group, in which the shock persisted for 7 sec after the rats pressed the panel with no probing being applied, was not significantly different from that of the perineal-probing group but was significantly longer than latency in the CP group. Thus the present studies suggest that CP is indeed analgesic and that this effect of CP is mediated by neither movement inhibition nor distraction.