Constitutive Expression of the alpha 10 Nicotinic Acetylcholine Receptor Subunit Fails to Maintain Cholinergic Responses in Inner Hair Cells After the Onset of Hearing

Efferent inhibition of cochlear hair cells is mediated by alpha 9 alpha 10 nicotinic cholinergic receptors (nAChRs) functionally coupled to calcium-activated, small conductance (SK2) potassium channels. Before the onset of hearing, efferent fibers transiently make functional cholinergic synapses with inner hair cells (IHCs). The retraction of these fibers after the onset of hearing correlates with the cessation of transcription of the Chrna10 (but not the Chrna9) gene in IHCs. To further analyze this developmental change, we generated a transgenic mice whose IHCs constitutively express alpha 10 into adulthood by expressing the alpha 10 cDNA under the control of the Pou4f3 gene promoter. In situ hybridization showed that the alpha 10 mRNA is expressed in IHCs of 8-week-old transgenic mice, but not in wild-type mice. Moreover, this mRNA is translated into a functional protein, since IHCs from P8-P10 alpha 10 transgenic mice backcrossed to a Chrna10 super( -/- ) background (whose IHCs have no cholinergic function) displayed normal synaptic and acetylcholine (ACh)-evoked currents in patch-clamp recordings. Thus, the alpha 10 transgene restored nAChR function. However, in the alpha 10 transgenic mice, no synaptic or ACh-evoked currents were observed in P16-18 IHCs, indicating developmental down-regulation of functional nAChRs after the onset of hearing, as normally observed in wild-type mice. The lack of functional ACh currents correlated with the lack of SK2 currents. These results indicate that multiple features of the efferent postsynaptic complex to IHCs, in addition to the nAChR subunits, are down-regulated in synchrony after the onset of hearing, leading to lack of responses to ACh.