Constitutive Expression of the alpha10 Nicotinic Acetylcholine Receptor Subunit Fails to Maintain Cholinergic Responses in Inner Hair Cells After the Onset of Hearing

JULIAN TARANDA; JIMENA BALLESTERO; HAKIM HIEL; FLAVIO SJ DE SOUZA; CAROLINA WEDEMEYER; MARÍA E. GÓMEZ-CASATI; MARCELA LIPOVSEK; DOUGLAS VETTER; PAUL A. FUCHS; ELEONORA KATZ; ANA BELÉN ELGOYHEN

Journal of the Association for Research in Otolaryngology

Springer

Año: 2009 vol. 10 p. 397 - 406

1438-7573

Efferent inhibition of cochlear hair cells is mediated byalpha9alpha10 nicotinic cholinergic receptors (nAChRs) functionally  coupled  to  calcium-activated,  small  conductance (SK2) potassium channels. Before the onset ofhearing,  efferent  fibers  transiently  make  functional cholinergic synapses with inner hair cells (IHCs). The retraction  of  these  fibers  after  the  onset  of  hearingcorrelates  with  the  cessation  of  transcription  of  the Chrna10 (but not the Chrna9) gene in IHCs. To further analyze  this  developmental  change,  we  generated  atransgenic mice whose IHCs constitutively express alpha10 into adulthood by expressing the alpha10 cDNA under the control of the Pou4f3 gene promoter. In situ hybridiza-tion showed that the alpha10 mRNA is expressed in IHCs of 8-week-old transgenic mice, but not in wild-type mice. Moreover, this mRNA is translated into a functionalprotein, since IHCs from P8-P10 alpha10 transgenic micebackcrossed to a Chrna10-/- background (whose IHCshave  no  cholinergic  function)  displayed  normalsynaptic and acetylcholine (ACh)-evoked currents inpatch-clamp recordings. Thus, the alpha10 transgene restored nAChR function. However, in the alpha10 transgenic mice,  no  synaptic  or  ACh-evoked  currents  were  observed in P16-18 IHCs, indicating developmental down-regulation  of  functional  nAChRs  after  the  onset  ofhearing, 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 insynchrony after the onset of hearing, leading to lack of responses to ACh.