The effects of noxious dental heating on the jaw-opening reflex and trigeminal Fos expression in the ferret.

Previous studies have established that the activation of peripheral nociceptors alters the central processing of nociceptive stimuli. In this study, we examined whether noxious heating of the dental pulp enhances the nociceptive jaw-opening reflex (JOR) and the expression of the immediate early gene c-fos in chloral hydrate/pentobarbital-anesthetized ferrets. We hypothesized that the application of noxious heat to the dental pulp, a procedure that evokes a preferential activation of pulpal C-fibers, will enhance JOR responses to electrical stimulation of the tooth pulp and that this enhanced response will be associated with the expression of Fos protein in discrete regions of the trigeminal nucleus. Consistent with our predictions, we observed that noxious heat conditioning enhanced the JOR as indicated by an increase in the magnitude of the signal averaged digastric electromyogram response evoked by electrical stimuli applied to either a heat-conditioned maxillary canine or the contralateral nonconditioned canine. The enhancement in JOR responses was independent of temporal summation of the electrical stimulus for test stimuli delivered at either 1.0 or 0.1 Hz. Sensitization of the JOR was associated with an increase in the number of immunohistochemically identified Fos-positive nuclei in trigeminal caudalis (Vc) and the transition zone between trigeminal interpolaris and caudalis (Vi/Vc) ipsilateral to the site of stimulation compared with sham stimulated animals. These findings suggest that neuronal populations in Vc and Vi/Vc play a role in the enhanced reflex responses to tooth pulp stimulation and may contribute to the pain and hyperalgesia associated with a symptomatic pulpitis.