Activation of ascending antinociceptive system by vagal afferent input as revealed in the nucleus ventralis posteromedialis.

Thalamic nociceptive neurons receiving afferent input from the tooth pulp (TP) were recorded from the nucleus ventralis posteromedialis proper (VPM) in cats anesthetized with urethane and chloralose. Effects of cervical vagus nerve stimulation on responses of TP neurons in the VPM were investigated. Twenty-one tooth pulp specific (TPS) and eight wide dynamic range (WDR) neurons with TP input were obtained from the periphery (shell region) of the posterior half of the VPM. Of these, many were also excited by electrical stimulation of trigeminothalamic tract (TTT) fibers in the trigeminal medial lemniscus. A conditioning-test paradigm was used to examine effects of vagal stimulation on responses of VPM neurons to electrical stimulation of TP and TTT. Inhibition of the responses was observed in 12 TPS and seven WDR neurons. Local anesthetic block of the mesencephalic periaqueductal gray (PAG) and/or nucleus raphe dorsalis (NRD) eliminated the inhibitory effects of vagal stimulation on the responses of both classes of TP neurons to TTT stimulation. In contrast, the inhibitory effects on responses to TP stimulation were insignificantly affected. These data suggest that vagal afferents can activate the ascending antinociceptive pathway from PAG/NRD onto VPM, in addition to activating the descending antinociceptive system acting upon the lower brain stem.