Nicotinic AChR in Subclassified Capsaicin-Sensitive and -Insensitive Nociceptors of the Rat DRG

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Abstract
Nociceptive cells of the dorsal root ganglion (DRG) were subclassified, in vitro, according to patterns of voltage-activated currents. The distribution and form of nicotinic ACh receptors (nAChRs) were determined. nAChRs were present on both capsaicin-sensitive and -insensitive nociceptors but were not universally present in unmyelinated nociceptors. In contrast, all Aδ nociceptors (types 4, 6, and 9) expressed slowly decaying nAChR. Three major forms of nicotinic currents were identified. Specific agonists and antagonists were used to demonstrate the presence of α7in two classes of capsaicin-sensitive, unmyelinated nociceptors (types 2 and 8). In type 2 cells, α7-mediated currents were found in isolation. Whereas α7was co-expressed with other nAChR in type 8 cells. These were the only classes in which α7was identified. Other nociceptive classes expressed slowly decaying currents with β4pharmacology. Based on concentration response curves formed by nicotinic agonists [ACh, nicotine, dimethyl phenyl piperazinium (DMPP), cytisine] evidence emerged of two distinct nAChR differentially expressed in type 4 (α3β4) and types 5 and 8 (α3β4α5). Although identification could not be made with absolute certainty, patterns of potency (type 4: DMPP > cytisine > nicotine = ACh; type 5 and type 8: DMPP = cytisine > nicotine = ACh) and efficacy provided strong support for the presence of two distinct channels based on an α3β4platform. Studies conducted on one nonnociceptive class (type 3) failed to reveal any nAChR. After multiple injections of Di-I (1,1′-dilinoleyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate) into the hairy skin of the hindlimb, we identified cell types 2, 4, 6, 8, and 9 as skin nociceptors that expressed nicotinic receptors. We conclude that at least three nicotinic AChR are diversely distributed into discrete subclasses of nociceptors that innervate hairy skin.