No abstract

We report the cloning and characterization of rat ␣10, a previously unidentified member of the nicotinic acetylcholine receptor (nAChR) subunit gene family. The protein encoded by the ␣10 nAChR subunit gene is most similar to the rat ␣9 nAChR, and both ␣9 and ␣10 subunit genes are transcribed in adult rat mechanosensory hair cells. Injection of Xenopus laevis oocytes with ␣10 cRNA alone or in pairwise combinations with either ␣2-␣6 or ␤2-␤4 subunit cRNAs yielded no detectable ACh-gated currents. However, coinjection of ␣9 and ␣10 cRNAs resulted in the appearance of an unusual nAChR subtype. Compared with homomeric ␣9 channels, the ␣9␣10 nAChR subtype displays faster and more extensive agonist-mediated desensitization, a distinct currentvoltage relationship, and a biphasic response to changes in extracellular Ca 2؉ ions. The pharmacological profiles of homomeric ␣9 and heteromeric ␣9␣10 nAChRs are essentially indistinguishable and closely resemble those reported for endogenous cholinergic eceptors found in vertebrate hair cells. Our data suggest that efferent modulation of hair cell function occurs, at least in part, through heteromeric nAChRs assembled from both ␣9 and ␣10 subunits.

SUMMARY Patterned spontaneous activity is a hallmark of developing sensory systems. In the auditory system, rhythmic bursts of spontaneous activity are generated in cochlear hair cells and propagated along central auditory pathways. The role of these activity patterns in the development of central auditory circuits has remained speculative. Here we demonstrate that blocking efferent cholinergic neurotransmission to developing hair cells in mice that lack the α9 subunit of nicotinic acetylcholine receptors (α9 KO mice) altered the temporal fine-structure of spontaneous activity without changing activity levels. KO mice showed a severe impairment in the functional and structural sharpening of an inhibitory tonotopic map, as evidenced by deficits in synaptic strengthening and silencing of connections and an absence in axonal pruning. These results provide evidence that the precise temporal pattern of spontaneous activity before hearing onset is crucial for the establishment of precise tonotopy, the major organizing principle of central auditory pathways.

Cochlear outer hair cells (OHCs) express alpha9 nACh receptors and are contacted by descending, predominately cholinergic, efferent fibers originating in the CNS. Mice carrying a null mutation for the nACh alpha9 gene were produced to investigate its role(s) in auditory processing and development of hair cell innervation. In alpha9 knockout mice, most OHCs were innervated by one large terminal instead of multiple smaller terminals as in wild types, suggesting a role for the nACh alpha9 subunit in development of mature synaptic connections. Alpha9 knockout mice also failed to show suppression of cochlear responses (compound action potentials, distortion product otoacoustic emissions) during efferent fiber activation, demonstrating the key role alpha9 receptors play in mediating the only known effects of the olivocochlear system.

The isk gene is expressed in many tissues. Pharmacological evidence from the inner ear suggests that isk mediates potassium secretion into the endolymph. To examine the consequences of IsK null mutation on inner ear function, and to produce a system useful for examining the role(s) IsK plays elsewhere, we have produced a mouse strain that carries a disrupted isk locus. Knockout mice exhibit classic shaker/waltzer behavior. Hair cells degenerate, but those of different inner ear organs degenerate at different times. Functionally, we show that in mice lacking isk, the strial marginal cells and the vestibular dark cells of the inner ear are unable to generate an equivalent short circuit current in vitro, indicating a lack of transepithelial potassium secretion.