Ascending inhibition of nociceptive neurons in the nucleus ventralis posterolateralis following conditioning stimulation of the nucleus raphe magnus

Recordings were made from neurons in the nucleus ventralis posterolateralis (VPL) of urethane-chloralose-anesthetized cats, following both noxious mechanical stimulation of the integument and electrical stimulation of the greater splanchnic nerve (SPL). The effects of stimulating the nucleus raphe magnus (NRM) on responses obtained from these units were investigated. Units responding to noxious mechanical stimulation of the integument with SPL input were found in the posterior shell region of the VPL. Responses elicited from these units by electrical stimulation of the SPL were inhibited following conditioning stimulation in or near the NRM. Inhibition could still be demonstrated after bilateral section of the dorsolateral funiculi at the level of C₃−C₄. Responses of these units to electrical stimulation of the ventrolateral funiculus (VLF) of the cervical cord were also inhibited following conditioning stimulation in or near the NRM. These results suggest that inhibition of these units produced by conditioning NRM stimulation may be partially mediated by an ascending pathway, in addition to the well-known descending spinal pathways. Glutamate stimulation of the NRM inhibited responses of nociceptive VPL units to SPL stimulation, but responses of the same units to VLF stimulation were little affected by the glutamate stimulation of the NRM. Inhibition of responses of nociceptive VPL units to SPL stimulation may be due to anti-dromic excitation of brainstem neurons having efferent connection with the NRM.