Constitutive expression of the alpha10 nicotinic acetylcholine receptor subunit fails to maintain cholinergic responses in inner hair cells after the onset of hearing

JIMENA A. BALLESTERO; JULIAN TARANDA; HAKIM HIEL; FLAVIO S. J. DE SOUZA; CAROLINA WEDEMEYER; M. EUGENIA GOMEZ-CASATI; MARCELA LIPOVSEK; DOUGLAS E. VETTER; PAUL A. FUCHS; ELEONORA KATZ; A. BELEN ELGOYHEN

JARO-JOURNAL OF THE ASSOCIATION FOR RESEARCH IN OTOLARYNGOLOGY

SPRINGER

Año: 2009 vol. 10 p. 397 - 406

1525-3961

Efferent inhibition of cochlear hair cells is mediated by á9á10 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 á10 into adulthood by expressing the á10 cDNA under thecontrol of the Pou4f3 gene promoter. In situ hybridization showed that the á10 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 á10 transgenic mice backcrossed to a Chrna10−/− background (whose IHCshave  no  cholinergic  function)  displayed  normal synaptic and acetylcholine (ACh)-evoked currents in patch-clamp recordings. Thus, the á10 transgene restored nAChR function. However, in the á10 transgenic mice,  no  synaptic  or  ACh-evoked  currents  were  observed in P16-18 IHCs, indicating developmental downregulation  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.