Effect of vagal afferent conditioning on trigeminal motor neuron activity associated with tooth-pulp-evoked jaw-opening reflex

To clarify the vagal afferent modifying effect on the neurons constituting the nociceptive jaw-opening reflex (JOR), we conducted extracellular recording of trigeminal motor nucleus (TMN) neuron activity in pentobarbital-anesthestized rats. Of 12 TMN neuron responses evoked by tooth pulp (TP) stimulation, 10 were suppressed by vagal afferent conditioning (83%). The mean time of the suppressive effect on the TMN neurons paralleled that found with the digastric electromyogram (dEMG), and maximal inhibition of both TMN neuronal spikes and the dEMG amplitude were observed at a 50-ms conditioning-test interval. The ratio of inhibition was approximately 38%. Seven of 12 units were activated antidromically by digastric muscle stimulation (1–3 mA, 0.1 ms, 2 Hz) and fulfilled the criteria for antidromic activation. Vagal afferent conditioning stimulation had no effect on the antidromic TMN neuronal responses to digastric muscle stimulation. These results suggest that suppression of the TP-evoked JOR in response to vagal afferent stimulation in rats is the result of an inhibitory effect on the sensory neurons rather than on the motor neurons.