Ca2+ modulation of the isolated ligand-gated currents through the native nicotinic ACh receptor in developing rat cochlear inner hair cells

GOMEZ CASATI ME; FUCHS P; ELGOYHEN AB; KATZ E

Daytona Beach, USA

Congreso; 27th Midwinter Meeting, Association for Research in Otolaryngology; 2004

Association for Research in Otolaryngology

In the developing mammalian cochlea, inner hair cells (IHCs) are transiently innervated by efferent fibers of the olivocochlear (OC) system. The OC synapse to IHCs is fast and inhibitory and mediated by a nicotinic acetylcholine receptor (nAChR) (Glowatzki & Fuchs 2000). Electrophysiological and pharmacological data support the hypothesis that fast inhibition is brought about by Ca2+ flowing in through the a9a10-containing nAChR and the subsequent activation of an apamin sensitive Ca2+-dependent K+(SK) channel. The biophysical and functional properties of the native nAChR were studied using whole-cell recordings of IHCs in acutely excised apical turns of the rat organ of Corti (P8-11). To isolate the nAChR currents from SK channel currents, experiments were performed either in KCl or CsCl, plus 10 mM of the fast Ca2+ chelator BAPTA in the pipette solution and 1 nM apamin in the external solution. Given the key role Ca2+ plays at the OC synapse, the effects of Ca2+ on the nAChR were studied. ACh-evoked currents were potentiated by low Ca2+ concentrations up to 0.5 mM and were blocked by higher concentrations of this cation. Concentration-response curves show that the receptor had an apparent affinity for ACh of 60.7 ± 2.8 mM in an external saline containing 0.5mM Ca2+. In the absence of Ca2+, the EC50 for ACh was increased, thus suggesting that potentiation by Ca2+ involves changes in the apparent affinity of the receptor for ACh. ACh-evoked responses were markedly reduced after the continuous application of 1mM ACh. At a holding potential of –90 mV, the current remaining after 20 sec was 54.2 ± 1.5% of its peak value. The biophysical characteristics of the IHC nAChR closely resemble those of the recombinant a9a10 nAChR (Elgoyhen et al., 2001, Weisstaub et al., 2002), thus reinforcing the hypothesis that the nAChR mediating neurotransmission at the OC-IHC synapse is composed of both the a9 and a10 subunits