Excitatory and inhibitory effects of vagal afferent input on nociceptive neurons in the nucleus ventralis posterolateralis of the cat thalamus

Forty three nociceptive specific (NS) and 36 wide dynamic range (WDR) neurons recorded from the shell region of nucleus ventralis posterolateralis (VPL) of the thalamus were examined for responses to electrical stimulation of the cervical vagus nerve in urethanechloralose anesthetized cats.Each neuron could be excited by manipulation of its cutaneous receptive field and by electrical stimulation of the greater splanchnic nerve (SPL).The vagus nerve stimulation excited 8 NS and 4 WDR neurons, suggesting that vagal afferents can mediate visceral pain.In the remaining 35 NS and 32 WDR units, a conditioning-test paradigm was used to examine effects of the vagus nerve stimulation on responses evoked by electrical stimulation of SPL and/or spinothalamic tract fibers in the ventrolateral funiculus (VLF).The conditioning vagus nerve stimulation inhibited responses to SPL input in 27 NS and 25 WDR units.In 18 NS and 15 WDR units effects of conditioning vagal nerve stimulation on responses to SPL and VLF stimulation were examined.Inhibition of both responses was observed in 12 NS and ll WDR units.Following local anesthetic blockade of the midbrain periaqueductal gray (PAG) and/or nucleus raphe dorsalis (NRD), the inhibitory effect of the vagus nerve stimulation on responses of NS and WDR units to VLF stimulation was eliminated, whereas the inhibitory effect on responses to SPL stimulation was unaffected.The data suggest that vagal afferents can activate ascending antinociceptive pathway from the PAG,/NRD onto the VPL言n addition to descending antinociceptive system acting upon the spinal cord.