Endolymphatic Potential Measured From Developing and Adult Mouse Inner Ear.

Yi Li,Huizhan Liu,David Z He,Xiaochang Zhao

doi:10.3389/fncel.2020.584928

Abstract

The mammalian inner ear has two major parts, the cochlea is responsible for hearing and the vestibular organ is responsible for balance. The cochlea and vestibular organs are connected by a series of canals in the temporal bone and two distinct extracellular fluids, endolymph and perilymph, fill different compartments of the inner ear. Stereocilia of mechanosensitive hair cells in the cochlea and vestibular end organs are bathed in the endolymph, which contains high K+ ions and possesses a positive potential termed endolymphatic potential (ELP). Compartmentalization of the fluids provides an electrochemical gradient for hair cell mechanotransduction. In this study, we measured ELP from adult and neonatal C57BL/6J mice to determine how ELP varies and develops in the cochlear and vestibular endolymph. We measured ELP and vestibular microphonic response from saccules of neonatal mice to determine when vestibular function is mature. We show that ELP varies considerably in the cochlear and vestibular endolymph of adult mice, ranging from +95 mV in the basal turn to +87 mV in the apical turn of the cochlea, +9 mV in the saccule and utricle, and +3 mV in the semicircular canal. This suggests that ELP is indeed a local potential, despite the fact that endolymph composition is similar. We further show that vestibular ELP reaches adult-like magnitude around post-natal day 6, ~12 days earlier than maturation of cochlear ELP (i.e., endocochlear potential). Maturation of vestibular ELP coincides with the maturation of vestibular microphonic response recorded from the saccular macula, suggesting that maturation of vestibular function occurs much earlier than maturation of hearing in mice.

Highlights

The inner ear of mammals has two major parts, the cochlea is responsible for hearing and the vestibular organ is responsible for balance
Endolymph is the fluid contained within the scala media of the membranous labyrinth of the inner ear; endolymph is high in K+ ions and resembles intracellular fluid in its composition
We further show that vestibular ELP (vELP) and vestibular microphonic reach adult-like responses around post-natal day 6 (P6), suggesting that maturation of vELP and vestibular function occurs much earlier than maturation of endocochlear potential (EP) and auditory function in mice

Summary

Introduction

The inner ear of mammals has two major parts, the cochlea is responsible for hearing and the vestibular organ is responsible for balance. The cochlea and vestibular organ are connected by a series of canals in the temporal bone referred to as the bony labyrinth. Endolymph and perilymph, separated by a membranous labyrinth, fill different compartments of the inner ear. Endolymph is the fluid contained within the scala media of the membranous labyrinth of the inner ear; endolymph is high in K+ ions and resembles intracellular fluid in its composition. The endolymph in the scala media continues beyond the base of the cochlea into the vestibular portion of the inner ear. The stria vascularis and vestibular dark cells and transitional cells are responsible for endolymph secretion These structures and cells are the foundations for regulation of inner-ear homeostasis. Impaired function of these structures due to mutations, damage, and aging leads to hearing loss (Steel and Barkway, 1989; Wu and Marcus, 2003; Gow et al, 2004; Wangemann et al, 2004; Ohlemiller et al, 2006; CohenSalmon et al, 2007; Sha et al, 2008; Ni et al, 2013; Zhang et al, 2014; Liu et al, 2016)

Methods

Results

Conclusion