Acoustic overexposures causing only temporary threshold shifts can cause permanent loss of auditory nerve (AN) fibers, despite no loss of cochlear hair cells. This primary neuropathy, seen as an immediate retraction of synaptic terminals on inner hair cells (IHC), followed by slow death of neuronal cell bodies, is selective for the subgroup of AN fibers with high thresholds and low spontaneous rates (SRs). This explains why ABR thresholds can recover despite loss of ~40% of AN fibers. Preliminary results suggest this neuropathy is elicited even by moderate-level exposure (84 dB SPL), especially in the absence of olivocochlear (OC) feedback, and that selective low-SR fiber loss may lead to hyperacusis, tinnitus and hyperactivity in central circuits. Recent human studies also suggest that low-SR neuropathy may be associated with tinnitus;and AN masking studies in animals suggest it should contribute to difficulties hearing in a noisy background. We pursue these clinically important issues, from cochlea to colliculus and from mouse to human, in five Specific Aims:
Aim 1 uses the confocal to examine AN/IHC synapses in humans to ask whether the low/high SR dichotomy is present in our ears and to quantify primary neuropathy in the aging ear.
Aim 2 is a neurophysiological study of masking in the AN of noise-damaged ears designed to assess the impact of low-SR neuropathy on coding of signals in noise and to develop an ABR-based assay to diagnose the loss of low-SR fibers.
Aim 3 uses tract-tracing techniques to examine the central projections of low-SR fibers, to test the hypothesis that they represent the major ascending input to OC reflex circuitry.
Aim 4 uses selective OC lesions to test the hypothesis that a major role of the OC system is to minimize primary neuropathy in everyday acoustic environments.
Aim 5 combines neurophysiological studies of the inferior colliculus with behavioral measures based on the acoustic startle responses to test the hypothesis that low-SR neuropathy leads to central hyperactivity, hyperacusis and tinnitus.

Public Health Relevance

Damaging effects of intense noise are assessed by measuring threshold shifts;however there can be permanent nerve loss in the inner ear, even if thresholds fully recover. Our recent discovery of this noise-induced neurodegeneration has inspired new hypotheses as to the mechanisms underlying tinnitus (ringing in the ears), hyperacusis (reduced sound-level tolerance) and problems hearing in a noisy environment. This proposal takes a multidisciplinary approach to the testing of these new hypotheses in animal models and human autopsy material.

National Institute of Health (NIH)
National Institute on Deafness and Other Communication Disorders (NIDCD)
Research Project (R01)
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Special Emphasis Panel (ZRG1-IFCN-B (02))
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Cyr, Janet
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Massachusetts Eye and Ear Infirmary
United States
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Street, Valerie A; Kujawa, Sharon G; Manichaikul, Ani et al. (2014) Resistance to noise-induced hearing loss in 129S6 and MOLF mice: identification of independent, overlapping, and interacting chromosomal regions. J Assoc Res Otolaryngol 15:721-38
Liberman, M Charles; Liberman, Leslie D; Maison, St├ęphane F (2014) Efferent feedback slows cochlear aging. J Neurosci 34:4599-607
Hickox, Ann E; Liberman, M Charles (2014) Is noise-induced cochlear neuropathy key to the generation of hyperacusis or tinnitus? J Neurophysiol 111:552-64
Yuan, Yasheng; Shi, Fuxin; Yin, Yanbo et al. (2014) Ouabain-induced cochlear nerve degeneration: synaptic loss and plasticity in a mouse model of auditory neuropathy. J Assoc Res Otolaryngol 15:31-43
Maison, Stephane F; Usubuchi, Hajime; Liberman, M Charles (2013) Efferent feedback minimizes cochlear neuropathy from moderate noise exposure. J Neurosci 33:5542-52
Sergeyenko, Yevgeniya; Lall, Kumud; Liberman, M Charles et al. (2013) Age-related cochlear synaptopathy: an early-onset contributor to auditory functional decline. J Neurosci 33:13686-94
Huang, Mingqian; Kantardzhieva, Albena; Scheffer, Deborah et al. (2013) Hair cell overexpression of Islet1 reduces age-related and noise-induced hearing loss. J Neurosci 33:15086-94
Furman, Adam C; Kujawa, Sharon G; Liberman, M Charles (2013) Noise-induced cochlear neuropathy is selective for fibers with low spontaneous rates. J Neurophysiol 110:577-86
Maison, Stephane F; Liu, Xiao-Ping; Eatock, Ruth Anne et al. (2012) Dopaminergic signaling in the cochlea: receptor expression patterns and deletion phenotypes. J Neurosci 32:344-55
Zilberstein, Yael; Liberman, M Charles; Corfas, Gabriel (2012) Inner hair cells are not required for survival of spiral ganglion neurons in the adult cochlea. J Neurosci 32:405-10

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