Vasodilator flare due to activation of superficial cutaneous afferents in humans: heat-sensitive versus histamine-sensitive fibers.

Histamine-sensitive nerve endings are assumed to terminate in the superficial epidermis. Heat-sensitive nociceptors that are excited by brief carbondioxide-laser pulses must also terminate within the epidermis, because this infrared radiation has an extinction length of about 10 microns. We now compared laser heat stimuli (10 W, 50 ms, 20 mm2) with intradermal injections of histamine (10(-10) to 10(-8) mol) in their capacity to cause cutaneous vasodilatation (flare) in awake human subjects. Cutaneous blood flow was measured with a two-channel laser-Doppler device. The radiant heat pulses caused a transient cutaneous vasodilatation that spread at least 15 mm from the stimulus site. There was no visible flare and vasodilatation could only be detected by laser Doppler measurements. Although the heat pulses elicited enough nociceptor activity to be perceived as moderately painful, the magnitude of the vasodilatation was smaller and its duration shorter than after the smallest dose of histamine. In contrast, nociceptor activation by heat is usually stronger than by histamine. These data indicate that flare and pain are two different aspects of cutaneous small fiber function.