Short-term plasticity and modulation of synaptic transmission at mammalian inhibitory cholinergic olivocochlear synapses.

Eleonora Katz,Ana Belã©N Elgoyhen

doi:10.3389/fnsys.2014.00224

Abstract

The organ of Corti, the mammalian sensory epithelium of the inner ear, has two types of mechanoreceptor cells, inner hair cells (IHCs) and outer hair cells (OHCs). In this sensory epithelium, vibrations produced by sound waves are transformed into electrical signals. When depolarized by incoming sounds, IHCs release glutamate and activate auditory nerve fibers innervating them and OHCs, by virtue of their electromotile property, increase the amplification and fine tuning of sound signals. The medial olivocochlear (MOC) system, an efferent feedback system, inhibits OHC activity and thereby reduces the sensitivity and sharp tuning of cochlear afferent fibers. During neonatal development, IHCs fire Ca2+ action potentials which evoke glutamate release promoting activity in the immature auditory system in the absence of sensory stimuli. During this period, MOC fibers also innervate IHCs and are thought to modulate their firing rate. Both the MOC-OHC and the MOC-IHC synapses are cholinergic, fast and inhibitory and mediated by the α9α10 nicotinic cholinergic receptor (nAChR) coupled to the activation of calcium-activated potassium channels that hyperpolarize the hair cells. In this review we discuss the biophysical, functional and molecular data which demonstrate that at the synapses between MOC efferent fibers and cochlear hair cells, modulation of transmitter release as well as short term synaptic plasticity mechanisms, operating both at the presynaptic terminal and at the postsynaptic hair-cell, determine the efficacy of these synapses and shape the hair cell response pattern.

Highlights

The organ of Corti, the mammalian sensory epithelium of the inner ear, has two types of mechanoreceptor cells, inner hair cells (IHCs) and outer hair cells (OHCs)
Notwithstanding, one can argue that the lack of BK channels during development might have altered synaptic strength at the medial olivocochlear (MOC)-OHC synapse by changing the balance between regulatory mechanisms, namely GABA acting through presynaptic GABAB receptors (Wedemeyer et al, 2013), the coupling of L-type voltage-gated Ca2+ channels (VGCC) to BK channels (Zorrilla de San Martín et al, 2010) as well as nitric oxide (NO) released from the postsynaptic cell upon efferent stimulation (Kong et al, 2013)
Synaptic strength is a key variable for transmitting information, synapses, both in the developing and mature nervous system, must be highly regulated in order to adapt to the changing demands of the environment

Summary

SYSTEMS NEUROSCIENCE

Reviewed by: Elisabeth Glowatzki, Johns Hopkins University, USA Sonja Pyott, University of North Carolina Wilmington, USA. The organ of Corti, the mammalian sensory epithelium of the inner ear, has two types of mechanoreceptor cells, inner hair cells (IHCs) and outer hair cells (OHCs). In this sensory epithelium, vibrations produced by sound waves are transformed into electrical signals. IHCs fire Ca2+ action potentials which evoke glutamate release promoting activity in the immature auditory system in the absence of sensory stimuli. During this period, MOC fibers innervate IHCs and are thought to modulate their firing rate.

INTRODUCTION

Katz and Elgoyhen

Findings

CONCLUSIONS