Functional alteration of ribbon synapses in inner hair cells by noise exposure causing hidden hearing loss

For decades, studies on noise-induced hearing loss have been focusing on the loss of sensory hair cells and/or auditory afferent fibers following severe noise exposure. Recently, a condition of hidden hearing loss was characterized, in which moderate noise exposure that causes only temporary threshold elevation could induce persistent reduction in auditory brainstem response (ABR) amplitudes and loss of ribbon synapses in inner hair cells (IHCs). However, it is not clear whether and how moderate noise exposure alters the functionality of surviving and/or recovering ribbon synapses in IHCs. To address this issue, we applied moderate noise exposure to mice and combined auditory systems physiology, whole-mount immunofluorescence staining and patch-clamp electrophysiology to characterize changes of ribbon synapse functions in IHCs. After the noise exposure, the ABR threshold was elevated and then recovered, while the ABR Wave I amplitude was reduced but did not recover. Coincidently, whole-mount cochlea staining revealed the loss and recovery of ribbon synapses in IHCs. We then performed whole-cell patch-clamp recording in IHCs and we found that the Ca²⁺ current, the sustained exocytosis of synaptic vesicles, and the replenishment of synaptic vesicles were all significantly reduced one day after the noise exposure. Fourteen days after the noise exposure, however, only the sustained exocytosis failed to recover, and further examination revealed that this persistent reduction is due to a decrease in the Ca²⁺ efficiency of triggering exocytosis. In conclusion, our results suggest temporary and persistent alterations of ribbon synapse functions in IHCs contribute to the hidden hearing loss.