Abstract
After acoustic trauma, there can be loss of synaptic connections between inner hair cells and auditory neurons in the cochlea, which may lead to hearing abnormalities including speech-in-noise difficulties, tinnitus, and hyperacusis. We have previously studied mice with blast-induced cochlear synaptopathy and found that they also developed a build-up of endolymph, termed endolymphatic hydrops. In this study, we used optical coherence tomography to measure endolymph volume in live CBA/CaJ mice exposed to various noise intensities. We quantified the number of synaptic ribbons and postsynaptic densities under the inner hair cells 1 week after noise exposure to determine if they correlated with acute changes in endolymph volume measured in the hours after the noise exposure. After 2 h of noise at an intensity of 95 dB SPL or below, both endolymph volume and synaptic counts remained normal. After exposure to 2 h of 100 dB SPL noise, mice developed endolymphatic hydrops and had reduced synaptic counts in the basal and middle regions of the cochlea. Furthermore, round-window application of hypertonic saline reduced the degree of endolymphatic hydrops that developed after 100 dB SPL noise exposure and partially prevented the reduction in synaptic counts in the cochlear base. Taken together, these results indicate that endolymphatic hydrops correlates with noise-induced cochlear synaptopathy, suggesting that these two pathologic findings have a common mechanistic basis.
Highlights
Acoustic trauma is the most common preventable cause of hearing loss, and it has been suggested that 12% or more of the world population is at risk for noise-induced loss of hearing (Alberti et al, 1979; Le et al, 2017)
We examined the relationship between noise intensity, endolymph volume, and synapse loss, and found that endolymphatic hydrops correlates with the loss of inner hair cells (IHCs) ribbons and postsynaptic densities (PSDs)
This finding suggests that endolymphatic hydrops and cochlear synaptopathy may derive through a common mechanism
Summary
Acoustic trauma is the most common preventable cause of hearing loss, and it has been suggested that 12% or more of the world population is at risk for noise-induced loss of hearing (Alberti et al, 1979; Le et al, 2017). This phenomenon has been called hidden hearing loss, since it would not be detected on traditional hearing tests (Liberman et al, 2016). Noise-induced damage to auditory nerve dendrites is caused by excess release of glutamate, the neurotransmitter responsible for afferent signaling between hair cells and auditory neurons (Spoendlin, 1971; Robertson, 1983; Choi and Rothman, 1990; Pujol et al, 1993; Puel et al, 1998; Kim et al, 2019a). Treating the endolymphatic hydrops with hypertonic saline reduced cochlear synaptopathy. These data do not prove that endolymphatic hydrops causes cochlear synaptopathy, they suggest that they may be related
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