Effects of Tooth Pulp Deafferentation on Brainstem Neurons of the Rat Trigeminal Subnucleus Oralis

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The effects of tooth pulp deafferentation on brainstem neuronal properties were examined in the rat trigeminal (V) subnucleus oralis. Deafferentation was produced by removal of the coronal tooth pulp of all left mandibular molars. Neurons in the subnucleus oralis were then electrophysiologically characterized in chloralose/urethane-anesthetized rats at a single postoperative time. The mechanoreceptive field and response properties of low-threshold mechanoreceptive (LTM) neurons in rats studied at postoperative times of 3-4 days, 7-13 days, and 28-40 days were compared to those in control (unoperated) rats. Functional changes in oralis LTM neurons were observed in all tooth-pulp-deafferented rats, but most statistically significant changes were apparent only at the 7- to 13-day postoperative period. In 7- to 13-day pulp-deafferented rats, there was a significant increase in the incidence of neurons with a two-divisional mechanoreceptive field, accompanied by a significant decrease in the incidence of neurons with a maxillary mechanoreceptive field. This group of rats also showed a significant increase in the incidence of neurons with a mechanoreceptive field involving both mandibular and maxillary divisions. For neurons that could be activated by light mechanical stimulation of one or more mandibular or maxillary teeth, the 7- to 13-day pulp-deafferented rats showed a significant increase in the incidence of neurons with such periodontal mechanosensitive inputs involving both mandibular and maxillary divisions. There was also a significant increase in the incidence of spontaneously active neurons in this group of rats. For neurons with a mechanoreceptive field involving mystacial vibrissae, there was a significant increase in all three groups of pulp-deafferented rats in the maximal width of the vibrissal row (i.e., the number of vibrissae in the longest horizontal row of vibrissae, stimulation of which was effective in activating a given neuron). No significant differences were found between groups in the incidence of neurons with a mandibular mechanoreceptive field or in the proportions of neurons with a mechanoreceptive field located in each of several defined orofacial regions. There was also no significant difference between groups in the mean latency to electrical stimulation of the neuronal mechanoreceptive field, or in the proportions of rapidly adapting (RA) and slowly adapting (SA) neurons. These results show many similarities with the functional changes of oralis LTM neurons in the cat following tooth pulp deafferentation, and indicate that the rat may serve as a very useful model for V brainstem neuroplasticity induced by tooth pulp deafferentation.