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 α2-7, α9, and β2-4 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 α7 and α9 subunits in the organ of Corti and α5-7, and β2 and β3 in the spiral ganglion using RT-PCR. Employing in situ hybridization with ³⁵S-riboprobes, we localized α9 in hair cells regions and α6, α7 and β2 in the type I cells of the spiral ganglion. No evidence of nAChR subunit mRNA expression was found in supporting cells, but β2 was expressed in type II spiral ganglion cells, which are neither cholinergic nor cholinoceptive.