Raphe magnus-induced descending inhibition of spinal nociceptive neurons is mediated through contralateral spinal pathways in the cat.

In anesthetized cats, extracellular recordings were made from lumbar spinal dorsal horn neurons, driven by noxious radiant skin heating. Heat-evoked responses were inhibited during electrical stimulation in the medullary nucleus raphe magnus (NRM). To identify the spinal pathways mediating this descending inhibition, reversible blocks in the spinal cord white matter were produced by microinjection of the local anesthetic lidocaine. Descending inhibition from the NRM was significantly reduced during blocks in the dorsal and medial, but not ventral parts of the contralateral lateral funiculus (LF). Blocks at any site in the ipsilateral LF failed to affect NRM-induced descending inhibition. These results indicate that NRM-induced inhibition of nociceptive dorsal horn neurons is conveyed primarily in fibers descending in the contralateral spinal white matter.