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No -adrenoreceptor-induced C-fiber activation in healthy human skin

¹Department of Neurology, Johannes Gutenberg Universität Mainz, D-55101 Mainz; and ²Department of Neurology and ³Institute for Physiology and Experimental Pathophysiology, Friedrich Alexander Universität Erlangen, 91054 Erlangen, Germany

Submitted 11 September 2003 ; accepted in final form 3 December 2003

In healthy volunteers, flare responses induced by norepinephrine (NE) iontophoresis have been observed. However, as NE iontophoresis is a combined electrical and chemical stimulus axon, reflexes cannot be directly linked to pharmocological activity of NE. Different concentrations of NE, clonidine (CL), and phenylephrine (PE) (NE: 10^-10-10^-3 M; CL and PE: 10^-8-10^-3 M) were applied via intradermal microdialysis fibers into the skin of healthy volunteers. Simultaneously, skin blood flow was visualized by laser-Doppler imaging scans and quantified in a vasoconstriction skin area directly above the membranes to control drug effects and in expected axon reflex vasodilation areas that were 0.75 cm apart. NE, PE, and CL caused dose-dependent vasoconstriction. However, neither in the presumed axon reflex areas (quantitative analysis) nor on laser-Doppler imaging pictures (qualitative analysis) were any vasodilation observed. Even at concentrations causing maximum vasoconstriction (10^-3 M for any drug), no vasodilation was induced. Our results indicate that, in healthy human skin, exogenously supplied -adrenoreceptor agonists alone do not activate nociceptors sufficiently to induce axon reflex flare.

sympathetic nervous system; axon reflex; laser-Doppler imaging

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