Acetylcholine-induced potassium current of guinea pig outer hair cells: its dependence on a calcium influx through nicotinic-like receptors

Abstract

The cholinergic efferent inhibition of mammalian outer hair cells (OHCs) is mediated by a hyperpolarizing K+ current. We have made whole-cell tight-seal recordings from single OHCs isolated from the guinea pig cochlea to characterize the mechanism by which acetylcholine (ACh) activates K+ channels. After ACh application, OHCs exhibited a biphasic response: an early depolarizing current preceding the predominant hyperpolarizing K+ current. The current-voltage (I-V) relationship of the ACh-induced response displayed an N-shape, suggesting the involvement of Ca2+ influx. When whole-cell recording was combined with confocal calcium imaging, we simultaneously observed the ACh-induced K+ current (IK(ACh)) and a Ca2+ response restricted to the synaptic area of the cell. This IK(ACh) could be prevented by loading OHCs with 10 mM of the fast Ca2+ buffer bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (or BAPTA), therefore allowing the observation of the ACh-induced early current in isolation. This early current revealed nicotinic features because it activated with an intrinsic delay in the millisecond range, reversed nearly in between potassium and sodium equilibrium potentials, and was blocked by curare. However, it was strongly reduced in the absence of external Ca2+, and its I-V relationship displayed an unusual outward rectification at positive membrane potentials and an inward rectification below -60 mV. The results indicate that the cholinergic response of mammalian OHCs involves a "nicotinic-like" nonspecific cation channel through which Ca2+ enters and triggers activation of nearby Ca2+-dependent K+ channels.

Linopirdine Blocks α9α10-Containing Nicotinic Cholinergic Receptors of Cochlear Hair Cells

“…The striking similarities found so far between the pharmacology of recombinant nAChRs assembled from a9 and a10 subunits (Elgoyhen et al 1994(Elgoyhen et al , 2001) and that of native nAChRs present in isolated OHCs from different species (Housley and Ashmore 1991; Fuchs and Murrow 1992;Kakehata et al 1993;Erostegui et al 1994;Blanchet et al 1996;Chen et al 1996;Dulon and Lenoir 1996;Evans 1996;McNiven et al 1996) suggest that the native cholinergic hair cell receptor is assembled from both a9 and a10 subunits. Moreover, this notion is further reinforced by in situ hybridization studies carried out in rat cochlear OHCs and IHCs at the same postnatal ages as those employed in the present study (Elgoyhen et al 2001;FIG.…”

“…Acetylcholine (ACh) is the principal neurotransmitter released by medial olivocochlear efferent axons (Eybalin 1993), and existing pharmacological and electrophysiological data suggest a central role for an atypical, nicotinic ACh receptor (nAChR) located at the synapse between efferent fibers and vertebrate OHCs (Fuchs 1996). Current data support a model in which inhibition can be attributed to a transient, ACh-gated depolarization followed by activation of small-conductance, calcium-activated potassium channels (SK2) and subsequent hair cell hyperpolarization (Art et al 1984; Housley and Ashmore 1991; Fuchs and Murrow 1992;Evans 1996;Blanchet et al 1996).…”

“…The cholinergic hair cell receptor displays an unusual pharmacological profile which does not resemble the one described for muscle or neuronal nAChRs (Housley and Ashmore 1991; Fuchs and Murrow 1992;Kakehata et al 1993;Erostegui et al 1994;Blanchet et al 1996;Chen et al 1996;Dulon and Lenoir 1996;Evans 1996;McNiven et al 1996), thus suggesting the presence of a new receptor subtype (Fuchs 1996). The more recently cloned a9 and a10 gene subunits are distant members of the nAChR family and, when expressed in Xenopus laevis oocytes, form a heteromeric receptor-channel complex that displays a pharmacological profile distinctly different from that of other cholinergic receptors, with sensitivity to glycinergic, gabaergic, serotonergic, as well as nicotinic and muscarinic antagonists (Elgoyhen et al 1994(Elgoyhen et al , 2001).…”

ATP activates P2x receptors and requires extracellular Ca++ participation to modify outer hair cell nonlinear capacitance

“…Also, there is a dense distribution of IP 3 receptors along the OHC lateral wall just beneath the plasma membrane [33,35]. It has been proposed that the application of acetylcholine (Ach) can induce the release of Ca ++ from intracellular stores located near the lateral plasma membrane to influence OHC function [33,36,37]. Currently, the subcellular mechanism underlying the regulation of OHC electromotility is little known.…”

Slow motility in hair cells of the frog amphibian papilla: Myosin light chain-mediated shape change

“…Slow motility in OHCs can be induced by a number of biochemical agents, including the presumed efferent nerve neurotransmitter, acetylcholine (ACh). ACh binds to and activates the ionotropic nicotonic cholinergic receptor in the OHCs, and induces an inward cationic current predominantly carried by Ca 2+ (Blanchet et al, 1996). Strategically-located Ca 2+ -activated K + channels translate the Ca 2+ entry into K + exit, leading to the cell membrane hyperpolarization, in the micro-to millisecond time scale (Housley et al, 1992), changing the operating point of the prestin motor.…”