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

• Published in: Journal of neurophysiology Vol. 90; no. 5; pp. 3295 - 3303
• Main Authors: Lang, P. M, Burgstahler, R, Sippel, W, Irnich, D, Schlotter-Weigel, B, Grafe, P
• Format: Journal Article
• Language: English
• Published: United States Am Phys Soc 01-11-2003
• Subjects: Action Potentials - drug effects; Action Potentials - physiology; Adult; Aged; Aged, 80 and over; Axons - drug effects; Axons - physiology; Axons - ultrastructure; Cell Membrane - drug effects; Cell Membrane - physiology; Female; Humans; Male; Middle Aged; Nerve Fibers, Unmyelinated - drug effects; Nerve Fibers, Unmyelinated - physiology; Nerve Fibers, Unmyelinated - ultrastructure; Neurons - drug effects; Neurons - physiology; Neurons - ultrastructure; Nicotine - pharmacology; Receptors, Nicotinic - physiology
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.00512.2003

Departments of 1 Physiology and 2 Anesthesiology and 3 Friedrich-Baur-Institute, Ludwig-Maximilians University, 80336 Munich, Germany
Submitted 28 May 2003;
accepted in final form 18 July 2003
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 Ca 2+ 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 Ca 2+ 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.
Address for reprint requests and other correspondence: P. Grafe, Dept. of Physiology, University of Munich, Pettenkoferstr. 12, 80336 Munich, Germany (E-mail: P.Grafe{at}lrz.uni-muenchen.de ).