IDENTIFICATION OF THE SUBUNITS OF THE NICOTINIC CHOLINERGIC RECEPTORSIN THE RAT COCHLEA USING RT-PCR AND IN-SITU HYBRIDIZATION

There are two tissues in the adult mammalian cochlea that are post-syn aptic to cholinergic efferent fibers: The outer hair cells (OHCs) and the dendrites of the afferent fibers of the type I spiral ganglion cel ls. The unusual nicotinic-like pharmacology of cochlear cholinergic re sponses and the unique embryonic development of cochlear tissues sugge st that the inner-ear nicotinic cholinergic receptor (nAChR) may be di fferent from nAChRs described previously at synapses in the mammalian brain, autonomic ganglia, or skeletal muscle. In this study, we determ ined the mRNA expression of the alpha 2-7, alpha 9, and beta 2-4 subun its of the nicotinic acetylcholine receptor (nAChR) family in the rat cochlea. In micro-dissected tissue from the organ of Corti, spiral gan glion, and the membranous lateral wall, we found mRNA expression of th e alpha 7 and alpha 9 subunits in the organ of Corti and alpha 5-7, an d 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 t he spiral ganglion. No evidence of nAChR subunit mRNA expression was f ound in supporting cells, but beta 2 was expressed in type II spiral g anglion cells, which are neither cholinergic nor cholinoceptive. (C) 1 998 Elsevier Science B.V.