Ruili Xie - US grants

DESCRIPTION (provided by applicant): Noise-induced hearing loss (NIHL) and age-related hearing loss (AHL) are two major hearing impairments, and affected individuals often show compromised ability in processing fine temporal structures of sound. While most studies of NIHL and AHL focus on the peripheral defects, much less is known about accompanying alterations in the central auditory system. This proposal will explore the changes of synaptic transmission and the underlying cellular mechanisms following NIHL and AHL at the endbulb of Held synapses. These synapses are the first synaptic connection of the central auditory system and are well known to be important in fine temporal processing. Preliminary data shows that synaptic transmission at mouse endbulbs of Held is compromised with NIHL and AHL, likely due to impaired calcium homeostasis in the endbulb terminals. The first aim of this proposal will test the hypothesis that elevation of calcium concentration at the presynaptic terminal underlines the aberrant synaptic release during sustained activity following NIHL. Synaptic transmission will be evaluated in both normal and NIHL mice utilizing manipulations that modify calcium buffering in the microdomain and nanodomains near the active zone. The source of the calcium elevation will be identified by assessing synaptic transmission while disrupting specific pathways including external calcium influx via voltage-gated calcium channels, Ca-calmodulin dependent calcium channel inactivation, and calcium induced calcium release from internal calcium stores via ryanodine receptors and IP3 receptors. The second aim of this proposal will explore the functional impact of the changes in synaptic transmission following NIHL by examining the firing properties of postsynaptic bushy cells. This aim will test the hypothesis that increase in asynchronous release in NIHL mice leads to decreased synaptic efficacy and compromised temporal precision. Particularly, the study will also explore manipulations that can acutely restore the normal firing properties of bushy cells in slice. The third aim of this proposal will tst the hypothesis that impaired synaptic transmission at the endbulbs during AHL share the same impaired calcium homeostasis mechanisms as observed in NIHL. As in Aims 1 and 2, Aim 3 will examine the synaptic transmission and test the signaling pathways in aged mice with AHL. The long-term goal of this study is to identify the common cellular mechanisms that underlie synaptic modifications of the central auditory system following NIHL and AHL, and to identify manipulations that can acutely rescue normal synaptic function in a preclinical model. Ultimately, these studies could suggest approaches for future clinical treatments of NIHL and AHL.

PROJECT SUMMARY/ABSTRACT Cochlear nucleus (CN) is the first neural station of the central auditory system that processes all sound information from the auditory nerve (AN). Principal neurons of CN encode different aspects of sound, including information about the temporal fine structure (TFS) that is essential for auditory tasks like sound localization and speech detection in noisy environment. During age related hearing loss (ARHL), the central processing of TFS information is compromised, leads to perceptual deficits. The overall hypothesis is that modifications in CN neurons and neural circuits during aging contribute to the malfunction of auditory temporal processing that underlies ARHL. The project investigates the cellular mechanisms of ARHL in CN bushy neurons, which are specialized in processing TFS information, as well as their excitatory inputs from AN and inhibitory inputs from CN interneurons during aging. Our previous studies showed that synaptopathy occurs at AN central terminals during ARHL, specifically the endbulb of Held synapses, which show age related degradation in transmitting auditory information to postsynaptic bushy neurons. The decrease in endbulb function is due to compromised synaptic transmission that is associated with dysregulated calcium signaling at the synaptic terminal. In Aim1, the project investigates the mechanisms of different calcium signaling pathways during aging at the endbulb of Held synapse, including calcium uptake and removal, calcium influx via voltage gated calcium channels, synaptic vesicle replenishment, as well as the expression of different calcium sensors. Our prior study also found that bushy neurons are depolarized and more excitable during aging. In Aim2, the project will test the hypothesis that auditory system enhances central gain in bushy neurons to compensate for the weakened AN input during ARHL. Mechanisms of voltage-gated ion channels during ARHL will be studied by quantifying membrane conductances that underlie neural excitability in bushy neurons during aging. In Aim3, the project will elucidate the mechanisms of inhibition during ARHL by investigating the effect of inhibition on firing property of bushy neurons, assessing synaptic strength of glycinergic inputs, and evaluating the neural excitability of CN interneurons as well as their AN inputs during aging. To achieve these goals, the project utilizes techniques including behavioral hearing test (auditory brainstem response), whole-cell recording under current or voltage clamp mode using acute brain slices, pharmacological manipulation, as well as immunohistochemistry, using CBA/CaJ mice as the animal model for ARHL at ages up to 30 months. These studies will have a significant impact on our understanding of synaptic and cellular mechanisms underlying ARHL, which is fundamental and essential for developing therapeutic approaches to restore neural processing in the central auditory system and eventually reinstate sound perception in patients with hearing impairments.

PROJECT SUMMARY/ABSTRACT Sound information is transmitted from the peripheral cochlea to the central auditory system via two key synapses, including the cochlear synapse and auditory nerve (AN) synapse, which are the peripheral and central endings of the spiral ganglion neurons (SGNs), respectively. Selective cochlear synaptopathy among different subtypes of SGNs has been recognized as a main mechanism of hearing loss, one of the most common forms of which is noise-induced hearing loss (NIHL). Cochlear synapses from SGNs of the low spontaneous rate/high threshold subtype are especially vulnerable and can be preferentially damaged by even moderate noise insult, which are likely the primary cause of the perceptual deficit, especially during hidden hearing loss. At the SGN central endings, however, the mechanisms of NIHL remain largely unclear. Little is known about how AN synapses from different subtypes of SGNs change in morphology and physiology during NIHL, and how such central synaptopathy contributes to the central processing deficits in target neurons in the cochlear nucleus (CN). The long-term goal of this project is to elucidate the mechanisms of NIHL at different subtypes of AN synapses and their target CN neurons. Our overall hypothesis is that AN synapses from different subtypes of SGNs have unique synaptic properties, target distinct populations of CN neurons that are dedicated to processing different aspects of sound information, and subject to different levels of synaptopathy during NIHL that lead to distinctive central auditory processing deficits among associated CN neurons. Combining electrophysiology with immunohistochemistry using transgenic mice, this project investigates selective synaptopathy at different subtypes of AN synapses during NIHL, as well as the associated central processing deficits in their target CN neurons. In Aim1, we will identify AN central synaptopathy both morphologically and physiologically at the giant endbulb of Held synapses from three subtypes of type I SGNs after two different levels of noise damage. In Aim 2, we will elucidate the mechanisms of central processing deficits in CN bushy neurons following selective AN synaptopathy during NIHL. In Aim 3, we will elucidate the mechanisms of NIHL in CN inhibitory neural network by characterizing synaptopathy of AN bouton synapses onto D-stellate neurons and clarifying the changes of their output inhibition onto CN bushy neurons during NIHL, especially hidden hearing loss. The outcome of the project will fill our knowledge gap on AN central synaptopathy as well as the mechanisms of central processing deficits in target CN neurons during NIHL.