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

Abstract

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.

Cortico-NRM Influences on Trigeminal Neuronal Sensation

“…There is ample evidence that NRM neurons, generally containing 5‐HT, project to the trigeminal nucleus and the dorsal horn of the spinal cord, and sometimes to both, via axon collaterals (77), and make contact there with incoming sensory fibres and with trigeminothalamic projection neurons (55, 78–84). Gao et al.…”

Pulp microenvironment and mechanisms of pain arising from the dental pulp: From an endodontic perspective

“…Nociceptive pain protects the pulp from potential damage and it remains active in the presence of a noxious stimulus. The withdrawal reflexes in the jaw muscles induced by the activation of A fibre nociceptors can modify the masticatory functions and prevent excessive tooth wear or even cracking of teeth .…”

Brain sites involved in the antinociceptive effect of bradykinin in rats

“…Stimuli originated in the dental pulp reach the trigeminal nucleus, an area that contains excitatory interneurons involved in the jaw‐opening reflex (Hu et al , 1986). Furthermore, the oral and interpolar parts of the spinal trigeminal nucleus receive neural input from brain centers involved in the nociceptive control (Matthews et al , 1984; Sessle, 1987).…”