Capsaicin-induced central facilitation of a nociceptive flexion reflex in humans.
The effect of selective activation of nociceptive primary afferent fibers by capsaicin on a nociceptive lower limb flexion reflex was studied in healthy human subjects. Capsaicin (1%) applied topically to the skin produced a burning spontaneous pain sensation and allodynia in the treated region and in its immediate vicinity. Capsaicin applied to the distal innervation area of the sural or saphenous nerve produced a significant decrease of the threshold for the nociceptive limb flexion reflex induced by electric stimulation of the proximal sural nerve trunk, and this threshold decrease was rapidly attenuated by a cool compress concomitantly with the attenuation of the capsaicin-induced spontaneous pain. The latency of the flexion reflex response was not changed by capsaicin. The non-nociceptive H-reflex was not modified by capsaicin. It is concluded that a selective activation of nociceptive primary afferent fibers of the skin by capsaicin produces a central facilitation of a nociceptive flexion reflex in humans. This facilitation is selective on the nociceptive reflex and depends, at least partly, on the on-going afferent barrage in C-fibers.
Grönroos
M
M
Department of Physiology, University of Helsinki, Finland.
Pertovaara
A
A
D000287:Administration, Topical; D000328:Adult; D002211:Capsaicin / Q000008:administration & dosage / Q000494:pharmacology*; D004558:Electric Stimulation; D004594:Electrophysiology; D005260:Female; D006181:H-Reflex / Q000187:drug effects*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009412:Nerve Fibers / Q000187:drug effects; D009475:Neurons, Afferent / Q000187:drug effects; D009619:Nociceptors / Q000187:drug effects*; D010146:Pain / Q000139:chemically induced; D010147:Pain Measurement / Q000187:drug effects; D017288:Pain Threshold / Q000187:drug effects; D013497:Sural Nerve / Q000502:physiology
D002211:Capsaicin