Auditory nerve synaptopathy and the central mechanisms underlying noise-induced hearing loss

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PROJECT SUMMARY/ABSTRACT Noise-induced hearing loss (NIHL) is one of the most prevalent hearing conditions that affects people of all ages. As a major risk factor, noise insult early in life accelerates auditory dysfunction and exacerbates hearing loss with age. Understanding the mechanisms of NIHL at early stages is crucial for the development of clinical interventions to prevent or ameliorate permanent damage of the auditory system. Pathophysiology of NIHL has been mostly reported in the cochlea, including detrimental changes in the sensory hair cells, the spiral ganglion neurons (SGN), and the cochlear synapses connecting the two. One significant finding was that cochlea synapses of the low spontaneous rate SGNs are vulnerable and can be preferentially damaged by noise, preceding the occurrence of permanent overt hearing loss. It remains unclear how such noise-induced peripheral changes link to structural and functional alterations in the central auditory system in contributing to compromised hearing perception. As the only target for all SGNs and the starting site of central auditory processing, the cochlear nucleus (CN) is expected to alter in morphology and physiology after noise insult in conjunction with selective SGN changes and impact the signal processing of the entire central auditory system. The long-term goal of this project is to elucidate the central mechanisms of NIHL in the CN by identifying noise-induced synaptopathy at the auditory nerve (AN) central synapses from different subtypes of SGNs, and clarifying the impact on the structure and function of the CN circuits. We hypothesize that AN synapses from low spontaneous rate SGNs are subject to more profound synaptopathy upon noise insult, which lead to more dramatic morphological and physiological changes in linked CN neurons with altered neural processing that contribute to NIHL. We further postulate that AN synapses from high spontaneous rate SGNs and linked CN neurons are unchanged during hidden hearing loss caused by moderate noise exposure, but damaged during overt hearing loss after traumatic noise exposure. Combining electrophysiology with immunohistochemistry in genetically modified mice, this project investigates the structure and function of identified AN synapses and neurons in CN circuits after moderate or traumatic noise exposure. In Aim1, we will identify noise-induced AN central synaptopathy both morphologically and physiologically at the giant endbulb of Held synapses from different subtypes of type I SGNs. In Aim 2, we will characterize noise-induced changes in cellular morphology and physiological properties of CN principal bushy neurons, and identify the altered CN output during NIHL. In Aim 3, we will elucidate the mechanisms of NIHL in CN inhibitory neural network by characterizing the noise-induced synaptopathy at AN bouton synapses onto D-stellate neurons and identifying the weakened inhibition onto CN bushy neurons. The outcome of this project will fill our knowledge gap on noise-induced AN central synaptopathy, clarify the linked changes in CN circuits, and ultimately elucidate the central mechanisms of NIHL in the CN.