Inhibition of nociceptive neurons in the shell region of nucleus ventralis posterolateralis following conditioning stimulation of the periaqueductal grey of the cat. Evidence for an ascending inhibitory pathway.

Recordings were made from neurons in the ventroposterior lateral nucleus (VPL) of urethane chloralose-anaesthetised cats, following both noxious mechanical stimulation of the integument, and electrical stimulation of the inferior cardiac nerve. The effects of stimulating the periaqueductal grey (PAG), or the nucleus raphe dorsalis (NRD) on the responses obtained from these units were also investigated. Units responding to noxious mechanical stimulation of the integument, with inferior cardiac nerve input, were found only around the periphery of the posterior half of VPL. Responses elicited from these units by electrical stimulation of the inferior cardiac nerve were inhibited following electrical stimulation of either the ventral PAG or the NRD. Inhibition following PAG or NRD stimulation could still be demonstrated after bilateral section of the dorsolateral funiculi at the junction between C3 and C4, although the degree of inhibition decreased. Responses elicited from these units by electrical stimulation of the anterolateral funiculus were also inhibited following PAG or NRD stimulation. These data suggest that PAG or NRD stimulation-produced inhibition of these units may be partially mediated by an ascending pathway, in addition to the well-known descending spinal pathways in the dorsolateral funiculi of the spinal cord.