Abstract
When exposed to continuous high-level noise, cochlear neurons are more susceptible to damage than hair cells (HCs): exposures causing temporary threshold shifts (TTS) without permanent HC damage can destroy ribbon synapses, permanently silencing the cochlear neurons they formerly activated. While this “hidden hearing loss” has little effect on thresholds in quiet, the neural degeneration degrades hearing in noise and may be an important elicitor of tinnitus. Similar sensory pathologies are seen after blast injury, even if permanent threshold shift (PTS) is minimal. We hypothesized that, as for continuous-noise, blasts causing only TTS can also produce cochlear synaptopathy with minimal HC loss. To test this, we customized a shock tube design to generate explosive-like impulses, exposed anesthetized chinchillas to blasts with peak pressures from 160–175 dB SPL, and examined the resultant cochlear dysfunction and histopathology. We found exposures that cause large >40 dB TTS with minimal PTS or HC loss often cause synapse loss of 20–45%. While synaptopathic continuous-noise exposures can affect large areas of the cochlea, blast-induced synaptopathy was more focal, with localized damage foci in midcochlear and basal regions. These results clarify the pathology underlying blast-induced sensory dysfunction, and suggest possible links between blast injury, hidden hearing loss, and tinnitus.
Highlights
When exposed to continuous high-level noise, cochlear neurons are more susceptible to damage than hair cells (HCs): exposures causing temporary threshold shifts (TTS) without permanent HC damage can destroy ribbon synapses, permanently silencing the cochlear neurons they formerly activated
Recent work on continuous-noise exposures in animal models has revealed that many of the synaptic connections between inner hair cells (IHCs) and auditory-nerve fibers can be permanently destroyed in cases where the noise-induced threshold shifts, and hair cell damage, are completely reversible[10]
In animal models of cochlear synaptopathy, noise-exposure intensities are often adjusted to be at the border between temporary and permanent threshold shifts
Summary
When exposed to continuous high-level noise, cochlear neurons are more susceptible to damage than hair cells (HCs): exposures causing temporary threshold shifts (TTS) without permanent HC damage can destroy ribbon synapses, permanently silencing the cochlear neurons they formerly activated. Recent work on continuous-noise exposures in animal models has revealed that many of the synaptic connections between inner hair cells (IHCs) and auditory-nerve fibers can be permanently destroyed in cases where the noise-induced threshold shifts, and hair cell damage, are completely reversible[10]. This type of cochlear synaptopathy has been demonstrated in mice[10], guinea pigs[11], chinchillas[12], monkeys[13], and humans[14]. The answer is relevant to hypotheses about the mechanisms www.nature.com/scientificreports/
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