Gentamicin blocks ACh‐evoked K⁺ current in guinea‐pig outer hair cells by impairing Ca²⁺ entry at the cholinergic receptor

Abstract

Aminoglycoside antibiotics such as gentamicin are known to block the medial olivocochlear efferent system. In order to determine whether this inhibition takes place at the postsynaptic cholinergic receptors in outer hair cells (OHCs), we studied the effects of these polycationic molecules on cholinergic currents evoked in isolated guinea‐pig OHCs. The cholinergic response of OHCs involves nicotinic‐like receptors (nAChRs) permeable to Ca2+ ions that activate nearby Ca2+‐sensitive K+ channels (KCa(ACh) channels). The extracellular application of gentamicin and neomycin reversibly blocked ACh‐evoked K+ current (IK(ACh)) with IC50 values of 5.5 and 3.2 μm, respectively. The results showed that the blocking mechanism of IK(ACh) was due to inhibition of Ca2+ influx via nAChRs. Our study also provides interesting insights into the functional coupling between nAChRs and KCa(ACh) channels in OHCs. By directly recording the cation current flowing through nAChRs (In(ACh)) using an intracellular solution containing 10 mm BAPTA, we measured an EC50 near 110 μm for ACh‐evoked In(ACh). This EC50 for ACh is one order of magnitude higher than that measured indirectly on IK(ACh). This reveals a rather low affinity of ACh for its receptor but a very efficient coupling between nAChRs and KCa(ACh) channels. We also show that a high external Ca2+ concentration reverts the gentamicin inhibition of IK(ACh) and that gentamicin directly alters the cation current flowing through the nAChRs of OHCs. We propose that gentamicin acts as a non‐competitive cholinergic blocker by displacing Ca2+ from specific binding sites at the nAChRs. This block of the nAChRs at the level of the postsynaptic membrane in OHCs could explain the inhibitory effect of gentamicin reported on the crossed medial olivocochlear efferent system in vivo.

Efferent Pathways Modulate Hyperactivity in Inferior Colliculus

“…In addition to modulating the electro-mechanical gain of the cochlea (Dallos et al, 1997), medial system activation also results in a reduction of spontaneous firing of auditory afferents (Wiederhold and Kiang, 1970;Guinan and Gifford, 1988). This is most likely caused by a decrease in the endocochlear potential, arising from a shunt in current through the OHC when the medial system is activated (Housley and Ashmore, 1991;Blanchet et al, 2000). The reduced endocochlear potential causes hyperpolarization of the inner hair cells and a reduction of the spontaneous neurotransmitter release onto the auditory afferent fibers, reducing spontaneous firing of the afferents (Brown and Nuttall, 1984;Sewell, 1984).…”

Pharmacology of Acetylcholine-Mediated Cell Signaling in the Lateral Line Organ Following Efferent Stimulation

“…The ability to observe specific effects of many cell-signal specific agents, such as ryanodine, sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) inhibitors, and ion channel blockers is compromised by the myriad of cellular processes that must function unimpeded to produce a synchronized compound action potential in response to shaped tone bursts. Pharmacological agents can begin to have more global effects on cochlear function at concentrations that are just beginning to affect efferent function (Bobbin 2002;Hendricson and Guth 2002;Lelli et al 2003;Sridhar et al 1997).…”

TRPA1-Mediated Accumulation of Aminoglycosides in Mouse Cochlear Outer Hair Cells

“…In hair cells of the inner ear, aminoglycosides can block mechanotransduction channels (Ohmori 1985;Kroese et al 1989;Kros et al 1992;Kimitsuki and Ohmori 1993;Ricci 2002;Marcotti et al 2005) and nicotinic acetylcholine receptors (Blanchet et al 2000). However, aminoglycoside ototoxicity develops following the entry of these molecules into the hair cells (Hiel et al 1993), most likely due to their intracellular cytotoxic effects (Forge and Schacht 2000; Rizzi and Hirose 2007).…”