Suppressive effect of vagal afferents on cervical dorsal horn neurons responding to tooth pulp electrical stimulation in the rat

The aim of the present study was to test the hypothesis that vagal afferent (VA) inputs modify the tooth pulp (TP) stimulation-evoked activity of the first cervical dorsal horn (C1) neurons via the activation of endogenous noradrenergic and serotonergic systems. In 30 anesthetized rats, the activity of 56 C1 spinal neurons and the amplitude in a digastric muscle electromyogram (dEMG, n=30) increased proportionally during TP stimulation at an intensity of 1-3.5 times the threshold for the jaw-opening reflex (JOR). The activity in 46 of these C1 neurons (82.1%) was suppressed by VA stimulation (1.0 mAx0.1 ms, 50 Hz for 30 s) of the right vagus nerve. The suppressive effects of VA stimulation on C1 spinal neuron activity were significantly reduced after intravenous administration of either the alpha-adrenergic receptor antagonist phenoxybenzamine (POB, 2.0 mg/kg and 4.0 mg/kg) or the 5-hydroxytryptamine-3 (5-HT3) receptor antagonist ICS 205-930 (1.0 mg/kg and 2.0 mg/kg). But the 5-HT1/2 receptor antagonist methysergide (1.0 mg/kg and 2.0 mg/kg) had no significant effect on VA stimulation-induced inhibition of the C1 spinal neuron activity. These results suggest that VA stimulation inhibits nociceptive transmission in the C1 spinal neuron activity via the activation of both noradrenergic and serotonergic (5-HT3) descending inhibitory systems.