HEAT-EVOKED VASODILATATION IN HUMAN HAIRY SKIN - AXON REFLEXES DUE TOLOW-LEVEL ACTIVITY OF NOCICEPTIVE AFFERENTS

1. Spreading vasodilatation of the axon reflex type was evoked by cont act heat stimulation of the hairy skin in the human forearm (13.3 cm(2 ) stimulus area) and was detected by laser Doppler flowmetry at 8, 19 and 30 mm distance. 2. From a base temperature of 35 degrees C, rapidl y rising short heat stimuli (4 degrees C s(-1), 2 s plateau) elicited vasodilatation at an average threshold of 39.4 degrees C. For slowly r ising sustained heat stimuli (64 s duration) the average threshold was 39.6 degrees C (n.s.). Laser Doppler flowmetry revealed a rapid onset within about 4 s, a long duration of several minutes beyond the end o f the stimulus, and a rapid spread of vasodilatation to remote skin ar eas. These characteristics are typical for vasodilatation by an axon r eflex of nociceptive afferents. 3. Axon reflex thresholds matched the lower range of C fibre nociceptor heat thresholds. Thermal stimuli tha t were adjusted to elicit about half-maximal phasic responses in warm fibres (steps from 30 to 35 degrees C), but mere below the range of C fibre nociceptor thresholds, did not cause any vasodilatation. 4. Pain thresholds were higher than axon reflex thresholds for both rapidly a nd slowly rising heat stimuli and strongly depended on the stimulus pa ttern (40.1 degrees C for rapidly rising stimuli and >43 degrees C for slowly rising stimuli). This observation is consistent with recent re ports that the phasic response of nociceptive afferents is essential t o overcome the summation requirements at central synapses. 5. In concl usion, axon reflex vasodilatation in response to heat stimuli in the h airy skin of humans is elicited by activation of heat-sensitive nocice ptors, even in the absence of a conscious perception of heat pain. The dissociation of pain and vasodilatation thresholds supports the conce pt of two operating ranges of primary nociceptive afferents. Warm fibr es do not contribute to axon reflex vasodilatation in the hairy skin o f the human forearm. Release of vasoactive peptides by nociceptive pri mary afferents may also contribute to local heat-evoked vasodilatation at temperatures above 40 degrees C.