Effects of cardiac vagal afferent electrostimulation on the responses of trigeminal and trigeminothalamic neurons to noxious orofacial stimulation.

We have previously reported that electrical stimulation of cardiac vagal afferents produces an inhibition of the feline's digastric reflex evoked by tooth-pulp stimulation. In the present study, we evaluated whether cardiac vagal afferent stimulation (CVAS) alters the responses of trigeminal sensory neurons to noxious orofacial stimulation in alpha-chloralose-anesthetized cats. A total of 37 trigeminal and trigeminothalamic neurons were recorded from trigeminal nucleus caudalis and trigeminal nucleus oralis. Thirty-five of these 37 neurons were classified as wide-dynamic-range (WDR) neurons because they had cutaneous receptive fields and responded to both noxious heat and non-noxious tactile stimuli. The effects of continuous CVAS (5 Hz, 3 msec, 2 mA) on heat-evoked responses (6 sec 50 degrees C heat pulse) were examined on 32 WDR neurons. CVAS inhibited (21 of 32 neurons), facilitated (5 of 32 neurons) or did not affect neuronal responses (6 of 32 neurons) to noxious heat. The effects of CVAS on heat-evoked responses of trigeminal and trigeminothalamic neurons were equivalent. The effects of intermittent CVAS (7 pulses at 333 Hz, 5 mA, delivered 200 msec prior to the test stimulus) on the responses to electrical test stimuli delivered to the center of a neuron's cutaneous receptive field or to the tooth pulp were also examined. Intermittent CVAS inhibited (15 of 24 neurons), facilitated (4 of 24 neurons) or had no effect (5 of 24 neurons) on A delta-mediated responses evoked by the electrical stimulation of facial skin. Intermittent CVAS either inhibited (8 of 12 neurons) or had no effect (4 of 12 neurons) on C-fiber-mediated responses evoked by electrical stimulation of the facial skin. Eight cells were recorded that received tooth-pulp input. Six of these 8 cells also received afferent input from facial skin, the remaining 2 cells responded only to tooth-pulp stimulation. Intermittent CVAS either inhibited (7 of 8 neurons) or had no effect (1 of 8 neurons) on A delta-mediated responses evoked by electrical stimulation of the tooth pulp. The modulatory actions of intermittent CVAS on trigeminal and trigeminothalamic neuronal responses to convergent afferent input from both skin and tooth pulp were equivalent. The outcomes of this study provide additional evidence that cardiopulmonary vagal afferent stimulation modulates neuronal responses to noxious stimulation and suggest that alterations in cardiopulmonary dynamics may modulate nociception.