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1 Physiology, University of Munich, Munich, Germany; Anesthesiology, University of Munich, Munich, Germany
2 Physiology, University of Munich, Munich, Germany
3 Anesthesiology, University of Munich, Munich, Germany
4 Friedrich-Baur-Institute, University of Munich, Munich, Germany
* To whom correspondence should be addressed. E-mail: p.grafe{at}lrz.uni-muenchen.de.
Application of acetylcholine to peripheral nerve terminals in the skin is a widely used test in studies of human small fiber functions. However, a detailed pharmacological profile and the subunit composition of nicotinic acetylcholine receptors in human C-fiber axons are not known. In the present study, we recorded acetylcholine-induced changes of the excitability and of the intracellular Ca2+ concentration in C-fiber axons of isolated human nerve segments. In addition, using immunohistochemistry, an antibody of a subtype of nicotinic acetylcholine receptor was tested. Acetylcholine and agonists reduced the current necessary for the generation of action potentials in C-fibers by up to 30 %. This increase in axonal excitability was accompanied by a rise in the free intracellular Ca2+ concentration. The following rank order of potency for agonists was found: epibatidine >> 5-Iodo-A-85380 > 1,1-dimethyl-4-phenylpiperazinium iodide > nicotine > cytisine > acetylcholine; choline had no effect. The epibatidine-induced increase in axonal excitability was blocked by mecamylamine and, less efficiently, by methyllycacontine and dihydro-
-erythroidine. Many C-fiber axons were labeled by an antibody that recognizes the 
5 subunit of nicotinic acetylcholine receptors. In summary, electrophysiological and immunohistochemical data indicate the functional expression of nicotinic acetylcholine receptors composed of 3, 5, and 4 but not of 4/2 or of 7 subunits in the axonal membrane of unmyelinated human C-fibers. In addition, the observations suggest that the axonal membrane of C-fibers in isolated segments of human sural nerve can be used as a model for presumed cholinergic chemosensitivity of axonal terminals.
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