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

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 alpha 9 and alpha 10 subunits. We analysed 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 rather blocked the acetylcholine (ACh)-evoked currents with an IC50 of 1 +/- 0.1 mu m. Antagonists of non-cholinergic receptors such as strychnine, bicuculline and ICS-205930 blocked ACh-evoked responses with an IC50 of 8.6 +/- 0.8 nm, 59 +/- 4 nm and 0.30 +/- 0.02 mu m, respectively. The IHC nAChR was both permeable to (P-Ca/P-Na = 8 +/- 0.9) and modulated by external Ca2+. ACh-evoked currents were potentiated by Ca2+ up to 500 mu m 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 +/- 2.8 mu M in 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. These pharmacological and biophysical characteristics of the IHC nAChR closely resemble those of the recombinant alpha 9 alpha 10 nAChR, reinforcing the hypothesis that the functional nAChR at the olivocochlear efferent-IHC synapse is composed of both the alpha 9 and alpha 10 subunits.