The effects of repeated dermal application of capsaicin to the human skin on pain and vasodilatation induced by intradermal injection of acid and hypertonic solutions.

1. The effect of repeated capsaicin application on the skin of the volar surface of the forearm on the pain sensation and on the increase in blood flow induced by intradermal injection of low pH media or hypertonic solutions was investigated in 13 healthy volunteers. 2. Low pH media (4, and 2.5) were obtained by adding HCl to 0.9% saline. Hypertonic solutions (300 and 600 mM) were obtained by adding NaCl to pH 7.4, 0.9% saline. Capsaicin (1% in 50% ethanol) was painted on the volar skin of one forearm, chosen at random, for 7 days. The contralateral forearm was treated with the capsaicin vehicle. Pain was assessed by a visual analogue scale and skin blood flow by a laser doppler flowmeter. 3. Pain sensation and increase in blood flow (both peak and area under the curve) induced by low pH media were markedly reduced in the capsaicin pretreated side. Capsaicin pretreatment also reduced the increase in blood flow, but did not affect the pain response induced by hypertonic saline solutions. 4. Repeated application of capsaicin to the human skin inhibits both the sensory (pain) and 'efferent' (vasodilatation) responses induced by low pH media, whereas it reduces the vasodilatation, but not the pain caused by hypertonic media. 5. Repeated application of capsaicin to the human skin, a therapy used in various diseases, discriminates between sensory, but not 'efferent' responses induced by different stimuli.