Effects of nucleus raphe magnus stimulation on jaw-opening reflex and trigeminal brain-stem neurone responses in normal and tooth pulp-deafferented cats.

Since we have recently shown that tooth pulp deafferentation results in changes in the receptive field properties and activity of brain-stem neurones in the adult cat's subnucleus oralis of the trigeminal (V) spinal tract nucleus, we wished to determine if these changes are associated with alterations in the powerful inhibitory influence that the nucleus raphe magnus (NRM) normally exerts on these neurones and on the related digastric jaw-opening reflex. In control cats or in cats that had undergone mandibular or maxillary tooth pulp deafferentation 7-140 days previously, the effects of NRM conditioning stimulation were tested on jaw-opening reflex responses or oralis neuronal responses evoked by stimulation of the maxillary or mandibular tooth pulp, facial skin, or oral mucosa. No statistically significant difference was noted between control and deafferented animals (n = 32) in the incidence, threshold or time course of NRM-induced inhibition of the reflex responses. Likewise, no difference was noted between control and deafferented animals in these features of the inhibition of oralis neuronal responses. In 276 neurones tested, the high incidence (92%), low threshold (0.08-0.15 mA) and prolonged time course (approximately 400 msec) of NRM-induced inhibition of responses evoked by electrical stimulation of the tooth pulp or by low-intensity electrical or mechanical stimulation of facial skin and oral mucosa were comparable in both groups of animals. These findings indicate that the alterations in properties or oralis neurones subsequent to tooth pulp deafferentation may not be associated with changes in the modulatory influence emanating from the NRM.