Functional and morphological characterization of efferent synapses in cochlear hair cells of nAChR ƒÑ10 ¡¥knockout¡¦ mice

DOUGLAS VETTER; ELEONORA KATZ; STEPHANE MAISON; JULIÁN TARANDA; MARÍA E. GÓMEZ-CASATI; VIDYA MURTHY; CHARLES LIBERMAN; ANA BELÉN ELGOYHEN; JIM BOULTER

Tuebingen, Alemania

Workshop; Inner Ear Biology Meeting; 2005

The postsynaptic cholinergic receptor at the olivocochlear (OC)-hair cell synapse is encoded by the a9 and a10 nAChR subunit genes. While a9 functions as a homomeric receptor in heterologous expression assays, expression of the native hair cell-like physiology requires a10 co-expression. Changes in the level of a10 mRNA during normal postnatal development correlate with increased sensitivity of IHCs to ACh; however, the precise function played by the a10 gene in auditory processing is unknown. We have therefore generated a10-‘knockout’ mice to assess the role of the a10 gene in the development and function of the OC system. Homozygous a10 mutant mice breed normally and show no overt behavioral phenotype. Antibody staining of nerve terminals revealed hypertrophied OC synaptic terminals on OHCs in a10 null mice reminiscent of those described for the a9 knockouts. Whole cell electrophysiological recordings of IHCs obtained from acutely excised P8-9 organs of Corti revealed no ACh-inducible responses or K-induced synaptic activity in IHCs of a10 knockouts. In preliminary experiments, however, ACh-evoked responses in 1 out of 4 OHCs (P11-12). Wildtype and heterozygous littermates showed robust ACh-evoked responses and K-induced synaptic activity in all IHCs and OHCs tested at the same ages. In IHCs from a10 knockouts, no changes with respect to wild type and heterozygous littermates, were observed in the amplitude, voltage sensitivity, reversal potential and activation kinetics of total K-currents before or after the onset of hearing. Relative to wild type littermates, no changes in threshold were observed in the alpha10 null mice as assessed by ABR across a frequency spectrum spanning 8-32 kHz. DPOAE amplitudes during OC stimulation, however, were enhanced compared to the suppression observed in wildtype controls. These results suggest that the lack of the a10 subunit gene has a different impact on auditory physiology than that observed in the a9 null mice.