Capsaicin-induced central facilitation of a sympathetic vasoconstrictor response to painful stimulation in humans

The effect of capsaicin, a compound selectively activating nociceptive primary afferent fibers, on a centrally mediated autonomic (sympathetic) vasoconstriction response to painful peripheral stimulation was studied in healthy human volunteers. Capsaicin (1%) was applied topically to the dorsal forearm and the threshold for eliciting a vasoconstriction response in the contralateral forefinger to painful electrical or thermal stimulation of the forearm skin adjacent to or remote from the capsaicin-treated region was determined with Laser Doppler flowmetry. Capsaicin produced a significant decrease of the threshold for the vasoconstriction response to painful electrical stimulation of the area of central allodynia; i.e., the skin area located adjacent to the capsaicin-treated region in which a light touch evoked an unpleasant sensation. Capsaicin did not change the threshold for the vasoconstriction response to painful heat stimuli applied to the area of central allodynia or to painful electric stimulation applied outside the borders of the allodynic area. Heart rate, blood pressure and heart vagal tone were not modified by capsaicin. It is concluded that a selective activation of nociceptive primary afferent fibers of the skin by capsaicin produces a central submodality-dependent facilitation of an autonomic vasoconstriction response to noxious stimulation in humans. This central facilitation can be explained by segmental excitability changes of afferent interneurons at the spinal cord level.