Changes of Cochlear Nerve Terminals after Temporary Noise-Induced Hearing Loss

Abstract

Background and ObjectivesZZOverexposure to intense sound can cause temporary or permanent hearing loss. Post-exposure recovery of thresholds has been assumed to indicate reversal of damage to the inner ear without persistent consequences for auditory function. However, there was a report that acoustic overexposures causing moderate temporary threshold shift caused acute loss of afferent nerve terminals and delayed degeneration of the cochlear ganglion cells while cochlear sensory cells were intact. The purpose of the study was to evaluate the numerical changes of ribbon synapses and efferents to the outer hair cells in ears with temporary noise-induced threshold shifts. Materials and MethodsZZFour-week old CBA mice with normal Preyer's reflexes were used. Mice were exposed to white noise of 110 dB SPL for one hour. Auditory brainstem response (ABR) and distortion-product otoacoustic emission (DPOAE) were recorded before exposure and at four different post-exposure times, 1, 3, 5, and 7 days after noise exposure. Ribbon synapses and efferents near cochlear nerve terminals were stained and calculated in the control group mice at two post-exposure times, 3 and 5 days after the exposure. ResultsZZIn the noise-exposed ears, there was no loss of hair cells, in either inner hair cells or outer hair cells. ABR and DPOAE showed maximum threshold shifts after noise-exposure; they returned to the normal pre-exposure values by at day 5. The number of ribbon synapses tended to decrease at 3 days after noise-exposure, but the number of efferent fibers was not statistically different from those of the control mice. ConclusionZZOur results suggest that the loss of ribbon synapses could be related with the recovery course of temporary threshold shift, even to the point of full hearing recovery.

Possible Existence of Cochlear Synaptopathy in Patients Completely Recovered from Idiopathic Sudden Sensorineural Hearing Loss

“…It occurs without damage to the hair cells causing permanent hearing loss, and leads to a temporary threshold shift (TTS), which is a reversible change in hearing threshold [7]. Although hearing recovery from TTS has been assumed to indicate the reversal of inner ear damage, the loss of ribbon synapses could be related to the recovery course of temporary threshold shift, even to the point of full hearing recovery [8].…”