Cloning and expression of the alpha 9 nicotinic acetylcholine receptor subunit in cochlear hair cells of the chick

Hair cells of the Vertebrate inner ear an subject to efferent control by th e release of acetylcholine (ACh) from brainstem neurons. While ACh ultimate ly causes the hair cen to hyperpolarize through the activation of small con ductance Ca2+-activated K+ channels, the initial effect is to open a ligand -gated cation channel that briefly depolarizes the hair cell. The hair cell 's ligand-gated cation channel has unusual pharmacology that is well matche d to that of the nicotinic subunit alpha 9 expressed in Xenopus oocytes. We used sequence-specific amplification to identify the ortholog of alpha 9 i n the chick's cochlea (basilar papilla). Chick alpha 9 is 73% identical to rat alpha 9 at the amino acid level. A second transcript was identified tha t differed by the loss of 132 base pairs coding for 44 amino acids near the putative ligand-binding site. RT-PCR on whole cochlear ducts suggested tha t this short variant is less abundant than the full length alpha 9 mRNA. In situ hybridization revealed alpha 9 mRNA in sensory hair cells of the chic k cochlea. The pattern of expression was consistent with the efferent inner vation pattern. The alpha 9 label was strongest in shea (outer) hair cells on which large calyciform efferent endings an found. Tall (inner) hair cell s receiving little or no efferent innervation had substantially less label. The cochlear ganglion neurons were not labeled, consistent with the absenc e of axo-dendritic efferent innervation in birds. These findings suggest th at alpha 9 contributes to the ACh receptor of avian hair cells and supports the generality of this hypothesis among all vertebrates. (C) 2000 Publishe d by Elsevier Science B.V. All rights reserved.