Divalent cations modulate the nicotinic cholinergic receptor of cochlear hair cells.

MARÍA E. GÓMEZ-CASATI; PAUL A. FUCHS; ANA B. ELGOYHEN; ELEONORA KATZ

San Diego, USA

Congreso; Society For Neuroscience 34th Annual Meeting; 2004

In the neonatal mammalian cochlea, inner hair cells (IHCs) are transiently innervated by efferent fibers of the olivocochlear (OC) system. The OC synapse to IHCs is inhibitory and mediated by a nicotinic acetylcholine receptor (nAChR) composed of the a9 and a10 subunits (Glowatzki & Fuchs 2000). Electrophysiological and pharmacological data support the hypothesis that inhibition is brought about by Ca2+ flowing in through the a9a10-containing nAChR and the subsequent activation of a small conductance, apamin sensitive Ca2+-dependent K+ channel (SK channel). The biophysical and functional properties of native nAChRs were studied using whole-cell recordings from IHCs in acutely excised apical turns of the rat organ of Corti (postnatal days 8-11). To isolate nAChR currents from SK channel currents, we used 10 mM BAPTA in the pipette solution and 1 nM apamin in the extracellular solution. Currents evoked by ACh were potentiated by either Ca2+ or Ba2+ up to 500 mM but were reduced by higher concentrations of these cations, probably by permeation block. Mg2+ did not potentiate but only blocked ACh-evoked currents (IC50 = 0.21 mM). Dose-response curves show that the receptor had an apparent affinity for ACh of 60.7±2.8 mM in saline containing 0.5 mM Ca2+. In the absence of Ca2+, the EC50 for ACh increased, suggesting that potentiation by Ca2+ involves changes in the apparent affinity for the agonist. In addition, we found that ACh-evoked responses were reduced during either continuous or after repeated challenges with the agonist. These biophysical characteristics of the IHC nAChR closely resemble those of the recombinant a9a10 nAChR (Elgoyhen et al., 2001, Weisstaub et al., 2002), reinforcing the hypothesis that the nAChR at the OC-IHC synapse is composed of both the a9 and a10 subunits.