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

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 <or=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-beta-erythroidine. Many C-fiber axons were labeled by an antibody that recognizes the alpha5 subunit of nicotinic acetylcholine receptors. In summary, electrophysiological and immunohistochemical data indicate the functional expression of nicotinic acetylcholine receptors composed of alpha3, alpha5, and beta4 but not of alpha4/beta2 or of alpha7 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.

A novel mechanism of inhibition of high-voltage activated calcium channels by α-conotoxins contributes to relief of nerve injury-induced neuropathic pain

“…[34] and unmyelinated nerves in humans [25]. By contrast, there is some evidence that antagonism of α3β2 nAChRs in the spinal cord has pro-nociceptive actions [45], but this effect is probably restricted to spinal neurons as we observed only anti-allodynic actions after peripheral administration of MII.…”

Axon Physiology

“…Nicotinic acetylcholine receptors are encountered on unmyelinated nerve fibers of mammals where they modulate axonal excitability and conduction velocity (18,314).…”

Conus Venom Peptide Pharmacology

“…Bovine adrenal chromaffin cells express ␣3-, ␣5-, ␣7-, and ␤4-nAChR subunits (Sala et al, 2008). Accordingly, the main nAChR subtypes expressed on nociceptive peripheral fibers and their central terminations are composed of ␣3, ␣5, ␤4, and ␤2 subunits (Khan et al, 2003;Lang et al, 2003;Rau et al, 2005). Vc1.1 also inhibited peripheral nerve-mediated vascular responses in rats (Satkunanathan et al, 2005) and, at low concentrations (100 nM), inhibited excitatory responses of Sandall et al, 2003;Satkunanathan et al, 2005;Vincler et al, 2006;Klimis et al, 2011 CyclizedVc1.1 Partially reverses mechanical allodynia in several neuropathic models after oral dosing.…”