High calcium permeability and calcium block of the α9 nicotinic acetylcholine receptor

Abstract

At the synapse between olivocochlear efferent fibers and outer hair cells (OHCs) of the cochlea, a non-classical ionotropic cholinergic receptor allows Ca 2+ entry into the hair cell, thus activating a Ca 2+-sensitive K + current which hyperpolarizes the cell’s membrane. In the mammalian ear, this leads to a reduction in basilar membrane motion, altering auditory nerve fiber activity and reducing the dynamic range of hearing. The α9 nicotinic acetylcholine receptor (nAChR) subunit mediates synaptic transmission between cholinergic olivocochlear fibers and OHCs. Given that Ca 2+ is a key player at this inhibitory synapse, we evaluated the permeability to Ca 2+ of the recombinant α9 receptor expressed in Xenopus laevis oocytes and the modulation of its activity by extracellular Ca 2+. Our results show that the α9 receptor is highly permeable to Ca 2+ and that this cation potently blocks monovalent currents through this channel (IC 50=100 μM, at −70 mV) in a voltage-dependent manner. At a Ca 2+ concentration similar to that found in the perilymph bathing the base of the OHCs, approximately 90% of the Na + current through the α9 receptor is blocked, suggesting that one of the main functions of this channel could be to provide a pathway for Ca 2+ influx.