Spontaneous Activity in Development of Auditory Processing Circuitry

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Project Summary Developing auditory neurons assemble into rudimentary circuits that are subsequently modulated through acoustic experience. Altered development of these circuits due to trauma, ototoxicity or congenital deafness can lead to acoustic hypersensitivity, profound hearing loss, or debilitating tinnitus. Intrinsically generated, “spontaneous” neural activity propagates through nascent auditory neural circuits prior to the onset of acoustic input, providing an early training period that is thought to initiate key developmental processes. However, the precise roles for spontaneous pre-hearing activity remain poorly understood. Our laboratory has identified a critical role for inner supporting cells (ISCs) within the developing cochlea, which initiate a cascade of events that trigger action potential firing in inner hair cells during the pre-hearing period. This process critically depends upon opening of calcium-activated chloride channels (TMEM16A/ANO-1) that induce efflux of K+ from ISCs. Genetic deletion of TMEM16A from ISCs dramatically reduces pre-hearing neural activity in the auditory CNS in vivo, but TMEM16A cKO mice have preserved hearing thresholds and peripheral responses to sound after ear canal opening, providing the means to interrogate how spontaneous activity influences the maturation of central sound processing circuits via functional and behavioral assessments. Topographic organization is a defining feature of the sensory CNS. Pioneering experiments in the visual system show that the sensory input an organism receives during restricted developmental periods is critical for establishing, maintaining, and modulating precise topographic maps of the external world. Pre- sensory neural activity is therefore likely critical for refining developmentally coarse topographic organization. To assess if spontaneous pre-hearing activity in the developing auditory system contributes to refinement of central tonotopic maps and neural tuning, I will dramatically reduce spontaneous activity in the auditory CNS through deletion of TMEM16A within ISCs and subsequently image neural Ca2+ responses to sound in the auditory midbrain and cortex just after hearing onset. I will determine if subsequent acoustic input is capable of refining auditory cortical tonotopy without pre-hearing activity. Finally, I will identify if spontaneous activity sharpens auditory circuitry and neural tuning required for tone discrimination through frequency-dependent inhibition of the acoustic startle response and self-motivated operant conditioning behavioral paradigms. These experiments will provide important insight into how early acute injury to the cochlea and congenital deafness lead to long-term changes in the capacity of auditory circuits to process and interpret sounds, potentially leading to new approaches for improving auditory function in hearing impaired patients.