Reflex control of the cutaneous circulation after acute and chronic local capsaicin

To investigate whether local activity of capsaicin-sensitive sensory afferents in the skin has a modulatory role in the reflex cutaneous vasodilator response to hyperthermia in humans, experiments were conducted in two parts. First, low-dose topical capsaicin (0.025%) was administered acutely to stimulate local activity of these afferents. Second, we temporarily desensitized these nerves in a small area of skin using chronic capsaicin treatment (0.075% for 7 days). Each intervention was followed by whole body heating using water-perfused suits and then by local warming to 42 degreesC for assessment of maximum cutaneous vascular conductance. Skin blood flow was measured by laser-Doppler flowmetry and divided by mean arterial pressure (Finapres) for assessment of cutaneous vascular conductance. Maximum vascular conductance was not influenced by either acute or chronic capsaicin treatment (P > 0.10). After acute capsaicin, baseline cutaneous vascular conductance was elevated above that at control sites (25.34 +/- 6.25 vs. 10.57 +/- 2.42% max; P < 0.05). However, internal temperature thresholds for vasodilation were not affected by either acute or chronic capsaicin (P > 0.10). Furthermore, neither acute (control: 112.74 +/- 36.83 vs. acute capsaicin: 96.92 +/- 28.92% max/degreesC; P > 0.10) nor chronic (control: 142.45 +/- 61.89 vs. chronic capsaicin: 132.12 +/- 52.60% max/degreesC; P > 0.10) capsaicin administration influenced the sensitivity of the reflex cutaneous vasodilator response. We conclude that local activity of capsaicin-sensitive afferents in the skin does not modify reflex cutaneous vasodilation during hyperthermia.