Enhancement of α9α10 Nicotinic Acetylcholine Receptor Desensitization by Nimodipine

Abstract

Entry of calcium ions through nicotinic acetylcholine receptors hyperpolarizes outer hair cells (OHCs) by activating colocalized SK potassium channels. This process, termed efferent inhibition, plays a central role in the protective reflex by suppressing the amplification of incoming sound. OHC nicotinic receptors are composed of α9 and α10 subunits that are activated by acetylcholine (ACh) release from efferent neurons originating within the medial olivocochlear nucleus. Fast application of ACh to GH4C1 cells transiently transfected with plasmids encoding mouse α9 and α10 subunits, produced an inward current that displayed a concentration-dependent amplitude, with an EC50 of 41 ± 5.4 μM and Hill slope (n H) of 0.89 ± 0.1 (n=5). This current was antagonized by nicotine, displaying a KB of 9.0 ± 1.0 μM (n=8), consistent with the ACh-mediated current arising from activation of heteromeric α9α10 receptor channels. Inward ACh currents in the presence of the dihydropyridine antagonist nimodipine (10 μM) were reduced in amplitude and displayed faster decay kinetics. Exponential decay rates (τ) increased from 647.5 ± 66.0 ms (n=6) in control, to 160.6 ± 25.8 ms (n=6) in the presence of nimodipine (10 μM) (p 0.05), suggesting that the increased decay rate of ACh currents did not result from a decrease in agonist binding affinity. using a double-pulse protocol, ACh (300 μM)-mediated responses recovered from desensitization with an exponential time-course (τ = 3.81 ± 0.67 s, n=3). Recovery was slowed in the presence of nimodipine (τ = 8.88 ± 2.54 s; n=3), indicating that nimodipine stabilizes the desensitization state of the α9α10 receptor and suggests that these receptors can be modulated to fine-tune efferent inhibition.