Differential inhibitory mechanisms in VPL versus intralaminar nociceptive neurons of the cat: I. Effects of periaqueductal gray stimulation.

Nociceptive thalamic units receiving afferent input from the greater splanchnic nerve (SPL) were recorded from the nucleus ventralis posterolatealis (VPL) and intralaminar nuclei in urethane-chloralose anesthetized cats. The effects of stimulating the periaqueductal gray (PAG), or the nucleus raphe dorsalis (NRD) on responses of nociceptive thalamic units were investigated. Forty-eight nociceptive specific (NS) and 20 wide dynamic range (WDR) units with SPL input were found in the shell region of the caudal VPL. Following electrical stimulation of either the ventral PAG or the NRD, responses to SPL input were inhibited in all NS and WDR units tested. Responses of these units to electrical stimulation of spinothalamic tract fibers in the ventrolateral funiculus (VLF) were also inhibited following the PAG/NRD stimulation. These results suggest that PAG/NRD stimulation-produced inhibition of both NS and WDR units may be partially mediated by an ascending antinociceptive mechanism. Intralaminar nociceptive units with SPL input were found in the nuclei centralis lateralis (CL), paracentralis (Pc), and parafascicularis (Pf). The effects of conditioning electrical stimulation of either the ventral PAG or the NRD on responses of intralaminar nociceptive units were studied. Of 113 intralaminar nociceptive units studied, 68 units were unaffected, 23 units were excited and 22 units were inhibited following the conditioning stimulation. In the units in which responses to SPL 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 VPL.