Effects of capsaicin on cutaneous vasodilator responses in humans.

The neurovascular responses to noxious electrically evoked axon reflex (EAR) stimulation and iontophoretic application of endothelium-dependent [acetylcholine (ACh)] and independent [sodium nitrite (NaNO2)] vasodilator substances were examined in human forearm skin using laser Doppler flowmetry, before and after repeated topical applications of the neurotoxin capsaicin (chronic capsaicin pre-treatment). Following 3 or 4 days of capsaicin pre-treatment the normal vasodilator response to acute application of capsaicin was significantly reduced as was the EAR response. There were, however, no significant changes in the vasodilator responses to either ACh or NaNO2, suggesting that the neurogenic axon reflex is in series with the endothelial and microvascular smooth muscle mechanisms of vasodilatation. Recovery of normal EAR occurred within 2-4 weeks of cessation of capsaicin pre-treatment.