Identification of the subunits of the nicotinic cholinergic receptors in the rat cochlea using RT-PCR and in situ hybridization

There are two tissues in the adult mammalian cochlea that are post-synaptic to cholinergic efferent fibers: The outer hair cells (OHCs) and the dendrites of the afferent fibers of the type I spiral ganglion cells. The unusual nicotinic-like pharmacology of cochlear cholinergic responses and the unique embryonic development of cochlear tissues suggest that the inner-ear nicotinic cholinergic receptor (nAChR) may be different from nAChRs described previously at synapses in the mammalian brain, autonomic ganglia, or skeletal muscle. In this study, we determined the mRNA expression of the alpha 2-7, alpha 9, and beta 2-4 subunits of the nicotinic acetylcholine receptor (nAChR) family in the rat cochlea. In micro-dissected tissue from the organ of Corti, spiral ganglion, and the membranous lateral wall, we found mRNA expression of the alpha 7 and alpha 9 subunits in the organ of Corti and alpha 5-7, and beta 2 and beta 3 in the spiral ganglion using RT-PCR. Employing in situ hybridization with S-35-riboprobes, we localized alpha 9 in hair cells regions and alpha 6, alpha 7 and beta 2 in the type I cells of the spiral ganglion. No evidence of nAChR subunit mRNA expression was found in supporting cells, but beta 2 was expressed in type II spiral ganglion cells, which are neither cholinergic nor cholinoceptive. (C) 1998 Elsevier Science B.V.