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

Hair cells of the vertebrate inner ear are subject to efferent control by the release of acetylcholine (ACh) from brainstem neurons. While ACh ultimately causes the hair cell to hyperpolarize through the activation of small conductance 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 matched to that of the nicotinic subunit α9 expressed in Xenopus oocytes. We used sequence-specific amplification to identify the ortholog of α9 in the chick's cochlea (basilar papilla). Chick α9 is 73% identical to rat α9 at the amino acid level. A second transcript was identified that 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 that this short variant is less abundant than the full length α9 mRNA. In situ hybridization revealed α9 mRNA in sensory hair cells of the chick cochlea. The pattern of expression was consistent with the efferent innervation pattern. The α9 label was strongest in short (outer) hair cells on which large calyciform efferent endings are found. Tall (inner) hair cells receiving little or no efferent innervation had substantially less label. The cochlear ganglion neurons were not labeled, consistent with the absence of axo-dendritic efferent innervation in birds. These findings suggest that α9 contributes to the ACh receptor of avian hair cells and supports the generality of this hypothesis among all vertebrates.