The effects of clonidine and tizanidine on responses of nociceptive neurons in nucleus ventralis posterolateralis of the cat thalamus.

The effects of intravenous clonidine and tizanidine on nociceptive neurons in the nucleus ventralis posterolateralis (VPL) of the thalamus, a key station in the lateral system of ascending pain pathways, were evaluated in urethane-chloralose anesthetized cats. Intravenous clonidine and tizanidine produced a dose-dependent (5 and 10 micrograms/kg, and 25 and 50 micrograms/kg, respectively) suppression of responses of nociceptive specific (NS) and wide dynamic range (WDR) neurons in the VPL to high threshold splanchnic input. In contrast, the responses of both NS and WDR units to electrical stimulation of spinothalamic tract fibers in the ventrolateral funiculus (VLF) were little affected. We conclude that a site of suppressive action of the alpha 2-adrenoceptor agonists, as observed in nociceptive VPL neurons, is at the level of the spinal dorsal horn rather than in the VPL itself.