Regulation of Synaptic Vesicle Dynamics in the Auditory System

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Project Summary/Abstract The ability to localize sound sources and detect temporal features of sound is fundamental to hearing. Encoding this information within the first few auditory processing stations requires reliable and precise synaptic transmission in response to rapid and large fluctuations of upwards to the kilohertz range in action potential (AP) firing rates. However, the number of synaptic vesicles (SVs) available for AP-evoked release is limited. Many auditory brainstems synapses must sustain fast and repetitive SV release to encode sound information. Therefore, sound encoding places great demands on the temporal dynamics of SV release and replenishment. A key step regulating AP evoked SV release is priming, the process that creates fusion competent SVs in close proximity to voltage-gated CaV2 channels (CaV). The rate of priming and SV replenishment is highly dependent on the magnitude of presynaptic Ca2+ through CaV2 channels. Human mutations in the molecules regulating priming result in dysregulation of SV release which is the cause of many auditory and neurological disorders. In mammals, the pathway between the globular bushy cells (GBCs) and the medial nucleus of the trapezoid body neurons (MNTB) is critical for encoding sound localization and temporal features of sound in music and communication found in animal vocalizations to human speech. The GBC axon forms the calyx of Held, a glutamatergic presynaptic terminal, that is the sole input that drives AP spiking in the MNTB. The calyx uses fast SV release kinetics to relay the patterns of afferent AP spikes in the cochlear nucleus to the MNTB. This, in turn, results in rapid and precise inhibition of key mono- and binaural cell groups. It is emerging that aberrant MNTB signaling underlies sound localization and speech perception defects in the aging population and can contribute to central auditory defects found in neurological disorders. Therefore, our goal is to delineate the molecular mechanisms regulating the temporal dynamics of SV release and replenishment required for proper auditory information processing. Given the importance of priming in synaptic transmission, as well as the pathological consequences of aberrant SV release, our findings will provide fundamental insights into how information is encoded by the nervous system and are expected to facilitate the development of treatments for a wide range of neurological and neuropsychiatric disorders.