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In rodents, massive initial synapses are formed in the auditory peripheral nervous system at the early postnatal stage, and one of the major phenomena is that the number of afferent synapses in the cochlea is significantly reduced in the duration of development. This raises the hypothesis that the number of cochlear ribbon synapses are dramatically changed with hearing development and maturation. In this study, several tracers identifying activities of autophagy were applied to estimate the level of autophagy activity in the process of ribbon synapse development in mice; further, changes in the synaptic number and spiral ganglion nerve (SGN) fibers were quantitatively measured. We found robust expression of LC3B and lysosomal-associated membrane protein 1 as well as LysoTracker in or near inner hair cells and cochlear ribbon synapses in the early stage of postnatal development. Moreover, we found a significant loss in the intensity of SGN fibers at ribbon synaptic development and hearing onset. Thus, this study demonstrates that ribbon synaptic refinement and SGN fibers pruning are closely associated with the morphological and functional maturation of ribbon synapses and that synaptic refinement and SGN fiber pruning are regulated by the robust activities of autophagy in the earlier stages of auditory development. [ABSTRACT FROM AUTHOR]

Conclusion: Ototoxic gentamicin exposure does not disrupt the expression of myosin VIIa in the inner hair cells (IHCs) of mice, whereas gentamicin ototoxicity causes altered expression of otoferlin in IHCs, as well as parallel hearing threshold shifts. Objective: To explore whether myosin VIIa and otoferlin in IHCs have different responses to gentamicin ototoxicity. Methods: Lower dose treatment (100 mg/kg): adult C57 mice were continuously injected intraperitoneally with gentamicin once a day for 14 consecutive days. Dose-dependent gentamicin treatment: mice were injected intraperitoneally with differing doses (100, 200, and 300 mg/kg) once a day for 2 consecutive days. The hearing thresholds were detected by auditory brainstem response (ABR). Immunostaining and Western blotting were utilized to measure the manner of expression of myosin VIIa and otoferlin in IHCs. Results: Lower dose treatment: There were no significant differences among the control (day 0), and 4, 7, and 14 days after the ototoxicity exposure ( p > 0.05). Dose-dependent gentamicin treatment: There were no significant differences among the control, 100, 200, and 300 mg/kg groups after the ototoxicity exposure ( p > 0.05). In contrast, we found an altered expression of otoferlin in IHCs among the control (day 0), and 4, 7, and 14 days of exposure, when the mice were exposed to gentamicin ototoxicity ( p > 0.05). [ABSTRACT FROM AUTHOR]

Conclusion: Dynamic distribution of ototoxic gentamicin entry into inner hair cells (IHCs) may cause an interruption of cochlear ribbon synapse. Objective: To explore the pattern of uptake of gentamicin into IHCs, as well as its possible contribution to hearing loss. Methods: Adult C57 mice were injected intraperitoneally with gentamicin (100 mg/kg, conjugated with Texas Red ester) continuously for 14 days. The hearing thresholds were detected by auditory brainstem response (ABR) examinations. Immunostaining and confocal microscopy were utilized to trace gentamicin distribution, as well as the expression of RIBEYE/CtBP2 in mouse IHCs. Scanning electron microscopy (SEM) observation was used to find possible alterations of stereocilia. Results: The distribution of gentamicin in IHCs was first found on the 4th day and the accumulation of gentamicin was increased significantly on the 7th day after treatment, corresponding to the maximal elevation of the hearing threshold. However, the accumulation of gentamicin on the 14th day did not show significant differences compared with the level found on the 7th day. In addition, the uptake of gentamicin was excessively identified at or near the synaptic ribbons of IHCs throughout the 4th, 7th, and 14th days after the treatment, suggesting that cochlear ribbon synapses could be affected by ototoxic gentamicin exposure. [ABSTRACT FROM AUTHOR]