Clin Exp Otorhinolaryngol.  2017 Jun;10(2):137-142. 10.21053/ceo.2016.00444.

Functional Significance of Medial Olivocochlear System Morphology in the Mouse Cochlea

• Park SY
• Park JM
• Back SA
• Yeo SW
• Park SN

• KMID: 2380430
• DOI: http://doi.org/10.21053/ceo.2016.00444

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.

Keyword

Baso-Apical Gradient; Contralateral Suppression; Distortion Product Otoacoustic Emission; Efferent Terminal; Medial Olivocochlear Reflex

MeSH Terms

Acoustic Stimulation
Animals
Cochlea*
Hair
Hearing
Mice*
Microscopy, Confocal
Noise
Reflex

Figure

• 

  Fig. 1. Individual DP-grams of 16 ears at 65 dB sound pressure level (SPL) primary tone level (A) in quiet, and (B) with contralateral acoustic stimulation (CAS) showing alternating peaks and dips. DPOAE, distortion-product otoacoustic emission.

• 

  Fig. 2. (A) Plotting of distortion-product otoacoustic emission (DPOAE) changes by contralateral MOC reflex at all valid data points. Negative values indicate suppression and positive values, enhancement. (B) Comparison of DPOAE suppression ratios (suppression amplitude/baseline DPOAE) across three frequency ranges. The low frequencies (LF) covers 6,573–10,779 Hz; middle frequencies (MF), 11,904–23,592 Hz; high frequencies (HF), 26,048–35,056 Hz. Error bars indicate standard error of mean. SPL, sound pressure level.

• 

  Fig. 3. (A) Representative confocal z-stacks from three cochlear regions of 8, 16, and 32 kHz, which reveal the clusters of medial olivocochlear (MOC) efferent terminals immunolabeled for α-synuclein (arrow) under the outer hair cell (OHC) nuclei stained with DAPI (arrowhead; Vectashield) (×1,000). (B) The diameters of efferent terminals and (C) the number of terminals per OHC were compared among three cochlear regions. Apex, middle, and base correspond to 8, 16, and 32 kHz regions, respectively (n=16). Error bars indicate standard error of mean.

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