Abstract
The axon reflex induced by intracutaneous application of capsaicin to the forearm of human subjects and the back of anesthetized rats pretreated with intravenous injection of Evans blue was investigated using sequential subtraction thermography. In the human experiment, thermograms showed an immediate and general temperature decrease after capsaicin injection. Four min after application, several spotty areas with a temperature increase (`hot spots') appeared within and outside of the flare caused by capsaicin-induced axon reflex. A vascular murmur was observed on ultrasonic Doppler flowscopy at the hot spots. In the rat experiment, two hot spots appeared, one cranial to and one caudal to the site of injection, within different dermatomes. Hot spots appeared in rats with the pretreatment of intravenous hexamethonium and surgical removal of the bilateral lumbar paravertebral sympathetic trunks. Postmortem examination of the rats revealed that these hot spots coincided with perforating blood vessels. It was suggested that hot spots in the axon reflex identified by subtraction thermograms are induced by a passive dilatation of perforating vessels which supply blood to the flare.
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