Functional Significance of Medial Olivocochlear System Morphology in the Mouse Cochlea

Abstract

Objectives Baso-apical gradients exist in various cochlear structures including medial olivocochlear (MOC) efferent system. This study investigated the cochlear regional differentials in the function and morphology of the MOC system, and addressed the functional implications of regional MOC efferent terminals (ETs) in the mouse cochlea.Methods In CBA/J mice, MOC reflex (MOCR) was assessed based on the distortion product otoacoustic emission in the absence and presence of contralateral acoustic stimulation. High, middle, and low frequencies were grouped according to a mouse place-frequency map. Cochlear whole mounts were immunostained for ETs with anti-α-synuclein and examined using confocal laser scanning microscopy. The diameters of ETs and the number of ETs per outer hair cell were measured from the z-stack images of the basal, middle and apical regions, respectively.Results The middle cochlear region expressed large, clustered MOC ETs with strong MOCR, the base expressed small, less clustered ETs with strong MOCR, and the apex expressed large, but less clustered ETs with weak MOCR.Conclusion The mouse cochlea demonstrated regional differentials in the function and morphology of the MOC system. Strong MOCR along with superior MOC morphology in the middle region may contribute to ‘signal detection in noise,’ the primary efferent function, in the best hearing frequencies. Strong MOCR in spite of inferior MOC morphology in the base may reflect the importance of ‘protection from noise trauma’ in the high frequencies.

Avocado Oil Extract Modulates Auditory Hair Cell Function through the Regulation of Amino Acid Biosynthesis Genes

“…We focused in the mid-apex section of the cochlea. According to our ABR test, we evaluated the DKB122 effect at 8 and 16 kHz, which corresponds to that region of the cochlea [13,14,15]. That is the reason for us evaluating morphological changes specifically in that region.…”

Noise-induced hearing loss vulnerability in type III intermediate filament peripherin gene knockout mice

“…Reduction of OHC electro-mechanical transduction by the MOC efferent pathway is otoprotective ( 14 , 35 ). For example, surgical ablation of the COCB at the floor of the fourth ventricle in cats increased noise-induced threshold shifts ( 36 ) and over-expression of the OHC α9 nicotinic acetylcholine receptor subunit in mice confers protection to noise-induced hearing loss (NIHL) ( 37 ).…”

“…This position is primarily founded on studies in guinea pig and cat where single MOC efferent fibers show recruitment from low sound levels (20 dB SPL), with sharp tuning curves and characteristic frequencies similar to those of adjacent type I SGN afferents ( 12 , 13 ). However, evidence is emerging that at high sound levels, where MOC efferent feedback confers protection from noise-induced hearing loss ( 14 ), input from the OHC–type II SGN pathway may complement the IHC - type I SGN drive of the MOC efferent feedback circuit. For example, contralateral suppression was maintained following ouabain treatment of the cochlea that selectively ablated type I SGN (reducing ABR amplitude), while leaving the type II SGN innervation of OHCs intact ( 15 ).…”

Association of cochlear outer hair cell - type II spiral ganglion afferents with protection from noise-induced hearing loss

“…Reduction of OHC electro-mechanical transduction by the MOC efferent pathway is otoprotective (Park et al, 2017, Housley et al, 2020. For example, surgical ablation of the COCB at the floor of the fourth ventricle in cats increased noise-induced threshold shifts (Rajan, 2000) and overexpression of the OHC α9 nicotinic acetylcholine receptor subunit in mice confers protection to NIHL (Maison et al, 2002).…”

“…This position is primarily founded on studies in guinea pig and cat where single MOC efferent fibres show recruitment from low sound levels (20 dB SPL), with sharp tuning curves and characteristic frequencies similar to those of adjacent type I SGN afferents (Robertson andGummer, 1985, Liberman andBrown, 1986). However, evidence is emerging that at high sound levels, where MOC efferent feedback confers protection from noise-induced hearing loss (Park et al, 2017), input from the OHC -type II SGN pathway may complement the IHC -type I SGN drive of the MOC efferent feedback circuit. Contralateral suppression was maintained following ouabain treatment of the cochlea that selectively ablated type I SGN, while leaving the type II SGN innervation of OHCs intact (Li et al, 2018).…”