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

摘要

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 α9 and α10 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 μm. 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 μm, respectively. The IHC nAChR was both permeable to (PCa/PNa = 8 ± 0.9) and modulated by external Ca²⁺. ACh-evoked currents were potentiated by Ca²⁺ up to 500 μm 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 μm 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 α9α10 nAChR, reinforcing the hypothesis that the functional nAChR at the olivocochlear efferent–IHC synapse is composed of both the α9 and α10 subunits.