Histochemical evidence for sympathetic innervation of hair receptor afferents in cat skin.

The goal of the present study was to demonstrate histochemically fine catecholamine-containing sympathetic nerve fibers which might be associated with hair receptor afferent fibers in cat skin. Positive results would be consistent with the hypothesis that the sympathetically mediated desensitization of hair afferents, observed in this an earlier studies, is mediated by direct neurotransmitter release onto the afferent fibers. Activity in single primary afferent fibers was recorded electrophysiologically during mechanical stimulation of hairs and electrical stimulation of the lumbar sympathetic trunk in anesthetized cats. The mechanical threshold for activation was seen to increase markedly during low-frequency sympathetic stimulation. Fresh-frozen sections of skin from cat hindleg were treated with glyoxylic acid for detection of fluorescent catecholamines. Fluorescent fibers were observed in association with hair follicles as well as with arterioles and arrector-pili muscles. Subsequent silver staining of the identical skin sections revealed that the fine catecholamine-containing fibers located near hair follicles were closely associated with hair receptor afferent fibers. The results of hair receptor sensitivity is mediated by direct neurotransmitter release onto the afferent fibers.