Sympathetically induced changes in the responses of slowly adapting type I receptors in cat skin

The effects of sympathetic efferent activity on slowly adapting Type I receptors in the hairy skin of cats were studied by recording from single afferent units in the saphenous nerve. Stimulation of the sympathetic trunk at 10 Hz had predominantly excitatory effects, which were seen in some units as the development of a background discharge in the absence of overt mechanical stimulation, or in most other units as a reduction in the threshold for mechanical activation. These effects generally persisted throughout the 3-min period of sympathetic stimulation (SS). The percentage of afferent units that began to discharge during SS was significantly greater in female cats than in males (53% vs. 19%). An increase in the force exerted by the skin on the stimulus probe was also observed during SS. Several tests were conducted to assess possible neurochemical and mechanical mechanisms of action. Administration of the alpha-adrenergic blocking agent phentolamine produced a marked reduction in the sympathetic effects. However, histochemical analysis of sections from the touch domes showed no catecholamine fluorescence near the sensory fibers. Cessation of local blood flow just prior to SS, produced by occlusion of the descending aorta, had no apparent effect on the sympathetically induced changes in afferent activity. It was concluded that sympathetic activity has an excitatory action on most Type I afferents in the cat. Because this sympathetic action is neither replicated nor altered by aortic occlusion, it appears not to be mediated by changes in blood flow. It also appears not to be mediated by direct neurotransmitter action on the sensory receptor, because no catecholamine fluorescence was observed, yet the action was blocked by an alpha-adrenergic blocker. It is likely, therefore, that this sympathetic action is mediated by some unidentified mechanical response within the skin.