Dorsal Root Ganglion Neurons Express Multiple Nicotinic Acetylcholine Receptor Subtypes

Although nicotinic agonists can modulate sensory transmission, particularly nociceptive signaling, remarkably little is known about the functional expression of nicotinic acetylcholine receptors (nAChRs) on primary sensory neurons. We have utilized molecular and electrophysiological techniques to characterize the functional diversity of nAChR expression on mammalian dorsal root ganglion (DRG) neurons. RT-PCR analysis of subunit mRNA in DRG tissue revealed the presence of nAChR subunits α2–7 and β2–β4. Using whole cell patch-clamp recording and rapid application of nicotinic agonists, four pharmacologically distinct categories of nicotinic responses were identified in cultured DRG neurons. Capacitance measurements were used to divide neurons into populations of large and small cells, and the prevalence of nicotinic responses was compared between groups. Category I (α7-like) responses were seen in 77% of large neurons and 32% of small neurons and were antagonized by 10 nM methyllycaconitine citrate (MLA) or or 50 nM α-bungarotoxin (α-BTX). Category II (α3β4-like) responses were seen in 16% of large neurons and 9% of small neurons and were antagonized by 20 μM mecamylamine but not 10 nM MLA or 1 μM DHβE. Category II responses had a higher sensitivity to cytisine than nicotine. Two other types of responses were identified in a much smaller percentage of neurons and were classified as either category III (α4β2-like) or category IV (subtype unknown) responses. Both the α7-like and α3β4-like responses could be desensitized by prolonged applications of the analgesic epibatidine.