抄録
Itch is one of unpleasant sensations but is clearly distinct from pain sensation. It provokes scratching behavior while painful stimulation provokes withdrawal reflexes. Although numerous studies have examined nociceptive signal transduction in primary afferent fibers, the mechanism of itch sensation remains largely unknown. In order to clarify how itch sensation is conveyed in primary afferent fibers, we developed an in vivo intracellular recording technique from dorsal root ganglion (DRG) cells in adult rats. Under urethane-anesthesia, lumbar laminectomy was performed and DRG at lumbar levels was exposed. Microelectrodes were inserted into DRG neurons and action potentials (APs) elicited by cutaneous stimulation were recorded. In behavior analysis, 5-HT applied to the skin provoked scratching. The 5-HT-induced scratching behavior was inhibited by naloxone. DRG neurons examined were classified with Aβ, Aδ and C fibers based on their conduction velocities calculated with latency of APs elicited by cutaneous electric stimulation. 5-HT applied to the skin elicited APs in a subpopulation of C fibers, but not in Aβ and Aδ fibers. The 5-HT-sensitive C fibers also responded to mechanical stimulation applied to the skin. The time course of the 5-HT responses was quite similar to that of 5-HT-induced scratching in behavioral analysis. These results suggest that selective activation of a subpopulation of C fibers by 5-HT may be essential to elicit itch sensation. [J Physiol Sci. 2008;58 Suppl:S162]
