Biophysical and pharmacological characterization of nicotinic cholinergic receptors in cochlear inner hair cells

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

J Physiology (Lond)

Año: 2005 vol. 566 p. 103 - 118

Before the onset of hearing, a transient efferent innervation is found on inner hair cells (IHCs). This synapse is inhibitory and mediated by a nicotinic cholinergic receptor (nAChR) probably formed by the a9 and a10 subunits. We analyzed the pharmacological and biophysical characteristics of the native nAChR using whole-cell recordings from IHCs in acutely excised apical turns of the rat organ of Corti. Nicotine did not activate but it blocked the acetylcholine (ACh)-evoked currents with an IC50 of 1 ¡Ó 0.1 mM. Antagonists of non-cholinergic receptors such as strychnine, bicuculline and ICS-205,930 blocked ACh-evoked responses with an IC50 of 8.6 ¡Ó 0.8 nM, 59 ¡Ó 4 nM and 0.30 ¡Ó 0.02 mM, respectively. The IHC nAChR was both permeable to (PCa/PNa = 8 ¡Ó 0.9) and modulated by external Ca2+. ACh-evoked currents were potentiated by Ca2+ up to 500 mM but were reduced by higher concentrations of this cation. Ba2+ mimicked the effects of Ca2+ whereas Mg2+ only blocked these currents. In addition, elevation of extracellular Ca2+ reduced the amplitude of spontaneous synaptic currents without affecting their time course. The receptor had an EC50 for ACh of 60.7 ¡Ó 2.8 mM in 0.5 mM Ca2+. In the absence of Ca2+, the EC50 for ACh increased, suggesting that potentiation by Ca2+ƒninvolves changes in the apparent affinity for the agonist. These pharmacological and biophysical characteristics of the IHC nAChR closely resemble those of the recombinant ƒna9a10 nAChR, reinforcing the hypothesis that the functional nAChR at the olivocochlear-IHC synapse is composed of both the a9 and a10 subunits.