Characterization of Neuronal Nicotinic Acetylcholine Receptors in the Membrane of Unmyelinated Human C-Fiber Axons by In Vitro Studies

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Abstract
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 ≤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-β-erythroidine. Many C-fiber axons were labeled by an antibody that recognizes the α5 subunit of nicotinic acetylcholine receptors. In summary, electrophysiological and immunohistochemical data indicate the functional expression of nicotinic acetylcholine receptors composed of α3, α5, and β4 but not of α4/β2 or of α7 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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