Functional Significance of Medial Olivocochlear System Morphology in the Mouse Cochlea

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.

Radial Maze Test in a Mouse Model of Noise-Induced Hearing Loss / 대한이비인후과학회지

BACKGROUND AND OBJECTIVES: Hearing loss has been hypothesized to contribute to cognitive decline in the elderly. However, earlier studies yielded conflicting results, and the causal relationship has not been elucidated to date. In the present study, the authors set up a radial maze apparatus for assessment of cognition in mice, and investigated the relationship between cognition and hearing loss that occur immediately after noise exposure. MATERIALS AND METHOD: Spatial learning and memory were assessed in male C57BL/6 mice with or without hearing loss using an unconfined, partially-baited, 8-arm radial maze with different arm length (25 or 50 cm). The performance indicators included total trial time, latency, reference memory error, working memory error, and correct entry ratio. Mice in the hearing loss group were exposed to 60 min of 110 dB white noise for 14 days, and then auditory brainstem response thresholds were measured. RESULTS: Mice showed better performances in a 25 cm-arm radial maze than in a 50 cm-arm maze. The control and the hearing loss groups exhibited similar performance curves, except for the working memory curve in 25 cm-arm radial maze. Hearing-impaired mice were better at working memory tasks than the control mice, probably due to an early cognitive compensatory mechanism. CONCLUSION: A radial maze with 25 cm arm is appropriate for the test of learning and memory in mice. Acute hearing loss after noise exposure was not associated with decreased cognition. Further follow-up research has been planned to find the effect of long-term hearing loss on cognition and the mechanism by which these two domains are connected.

Comparison of Cochlear Morphology and Apoptosis in Mouse Models of Presbycusis

OBJECTIVES: Morphological studies on presbycusis, or age-related hearing loss, have been performed in several different strains of mice that demonstrate hearing loss with auditory pathology. The C57BL/6 (C57) mouse is a known model of early onset presbycusis, while the CBA mouse is characterized by relatively late onset hearing loss. We performed this study to further understand how early onset hearing loss is related with the aging process of the cochlea. METHODS: We compared C57 cochlear pathology and its accompanying apoptotic processes to those in CBA mice. Hearing thresholds and outer hair cell functions have been evaluated by auditory brainstem response (ABR) recordings and distortion product otoacoustic emission (DPOAE). RESULTS: ABR recordings and DPOAE studies demonstrated high frequency hearing loss in C57 mice at P3mo of age. Cochlear morphologic studies of P1mo C57 and CBA mice did not show differences in the organ of Corti, spiral ganglion, or stria vascularis. However, from P3mo and onwards, a predominant early outer hair cell degeneration at the basal turn of the cochlea in C57 mice without definitive degeneration of spiral ganglion cells and stria vascularis/spiral ligament, compared with CBA mice, was observed. Additionally, apoptotic processes in the C57 mice also demonstrated an earlier progression. CONCLUSION: These data suggest that the C57 mouse could be an excellent animal model for early onset 'sensory' presbycusis in their young age until P6mo. Further studies to investigate the intrinsic or extrinsic etiologic factors that lead to the early degeneration of organ of Corti, especially in the high frequency region, in C57 mice may provide a possible pathological mechanism of early onset hearing loss.