The influence of topical capsaicin on the local thermal control of skin blood flow in humans.

To test whether heat-sensitive receptors participate in the cutaneous vascular responses to direct heating, we monitored skin blood flow (SkBF; laser Doppler flowmetry) where the sensation of heat was induced either by local warming (T(Loc); Peltier cooling/heating unit) or by both direct warming and chemical stimulation of heat-sensitive nociceptors (capsaicin). In part I, topical capsaicin (0.075 or 0.025%) was applied to 12 cm(2) of skin 1 h before stepwise local warming of untreated and capsaicin-treated forearm skin. Pretreatment with 0.075% capsaicin cream shifted the SkBF/T(Loc) relationship to lower temperatures by an average of 6 +/- 0.8 degrees C (P < 0.05). In part II, we used a combination of topical capsaicin (0.025%) and local warming to evoke thermal sensation at one site and only local warming to evoke thermal sensation at a separate site. Cutaneous vasomotor responses were compared when the temperatures at these two sites were perceived to be the same. SkBF differed significantly between capsaicin and control sites when compared on the basis of actual temperatures, but that difference became insignificant when compared on the basis of the perceived temperatures. These data suggest heat-sensitive nociceptors are important in the cutaneous vasodilator response to local skin warming.