Effects of conditioning periaqueductal gray stimulation on responses of thalamic nociceptive neurons to tooth pulp stimulation.

Nociceptive neurons receiving afferent input from the tooth pulp (TP) were recorded from the nucleus ventralis posteromedialis proper (VPM) and intralaminar nuclei of the thalamus in cats anesthetized with urethane and chloralose. The effects of stimulating the periaqueductal gray (PAG), or the nucleus raphe dorsalis (NRD) on responses of thalamic nociceptive neurons were investigated. Eight tooth pulp specific (TPS) and 7 wide dynamic range (WDR) neurons with TP input were observed around the periphery (shell region) of the posterior half of VPM. Following electrical stimulation of either the ventral PAG or the NRD, responses to TP stimulation were inhibited in all TPS and WDR neurons tested. Responses of these neurons to electrical stimulation of trigeminothalamic tract (TTT) fibers in the trigeminal medial lemniscus were also inhibited following PAG/NRD stimulation. These results suggest that PAG/NRD stimulation-produced inhibition of both TPS and WDR neurons may be partially mediated by an ascending antinociceptive mechanism. Intralaminar nociceptive neurons with TP input were observed in the nucleus centralis lateralis (CL), and parafascicularis (Pf). The effects of conditioning electrical stimulation of either the ventral PAG or the NRD on responses of intralaminar nociceptive nerons were studied. Of 15 intralaminar nociceptive neurons tested, 6 neurons were inhibited, 5 neurons were excited and 4 were unaffected following the conditioning stimulus. In neurons in which responses to TP stimulation were inhibited, responses elicited by electrical stimulation of the mesencephalic reticular formation (MRF) were also inhibited. These data suggest that although there is an ascending inhibitory pathway from PAG/NRD to intralaminar nuclei, this system is far less potent compared with the ascending inhibitory system acting upon the VPM.