Inhibition of the tooth pulp-evoked jaw opening reflex by stimulation of raphe nuclei in the rat

Effects of conditioning electrical stimuli applied to nucleus raphe dorsalis (NRD), nucleus raphe medialis (NRMed), and nucleus raphe magnus (NRMag) on the jaw opening reflex (JOR) were compared using Wistar rats. Activity of the digastric musle after tooth-pulp stimulation was used as an index of the magnitude of the JOR which was considered a noxious response. The JOR was suppressed to about 30 to 40% of the control after a single-pulse stimulation of the NRD, NRMed, or NRMag. None of these raphe-induced suppressions was reversed by naloxone, a specific opiate antagonist, suggesting that these effects are not related to the endogenous opiate system. By contrast, 50-Hz electrical pulse trains of 0.1-ms duration for 1 min applied to the NRD and NRMed suppressed the reflex to about 30% of the control, an effect sustained for 10 to 20 min, whereas this stimulation of the NRMag suppressed the reflex to about 20% of the control and the suppressive effects were maintained for more than 30 min. In these cases, NRMed- and NRd-induced suppression was partially antagonized and the NRMag-induced suppression was almost completely antagonized by naloxone, suggesting that this long-term suppression, especially that induced by NRMag, is closely related to endogenous opiates. These results show that raphe-induced suppression may be produced by complex actions of a short-term neural mechanism and a long-term opiate-related mechanism.