Inhibitory effects of vagal afferent inputs on nociceptive neurons responding to tooth pulp stimulation in the nucleus ventralis posteromedialis of the cat thalamus

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Thalamic nociceptive neurons receiving afferent inputs from the tooth pulp (TP) were recorded from the nucleus ventralis posteromedialis proper (VPM) of cats anesthetized with urethane and chloralose. We investigated the effects of cervical vagus nerve (CVN) stimulation on the responses of TP neurons in the VPM and observed 31 tooth pulp specific (TPS) neurons and 15 wide dynamic range (WDR) neurons with TP inputs around the periphery (shell region) of the posterior half of the VPM. Of these neurons, 22 TPS and 11 WDR neurons were excited by electrical stimulation of trigeminothalamic tract (TTT) fibers in the trigeminal medial lemniscus. We used a conditioning-test paradigm to examine the effects of CVN stimulation on the responses of VPM neurons to electrical stimulation of TP and of TTT. Inhibition of the responses was observed in 17 TPS and 8 WDR neurons. Local anesthetic block of the mesencephalic periaqueductal gray (PAG) and/or nucleus raphe dorsalis (NRD) markedly eliminated the inhibitory effects of CVN stimulation on the responses of TPS and WDR neurons to TTT stimulation, and had a limited inhibitory effect on the response of these neurons to TP stimulation. These results suggest that vagal afferents were able to activate the ascending antinociceptive pathway from the PAG/NRD to the VPM, as well as activating the descending antinociceptive system that acts on the lower brain stem. In addition, vagal afferent inputs to the PAG/NRD, which may be relayed through the nucleus of the solitary tract, acted as an afferent mechanism that triggered this ascending antinociceptive system.

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