Project Summary The overarching goal of this research is basic discovery of fundamental cochlear signaling mechanisms. This knowledge will help to identify new cellular and molecular targets for future therapeutic intervention. The synapse between inner hair cells (IHCs) and type I auditory nerve fibers (ANFs) is the first station in the auditory pathway to code the sound signal. Coding of sound in the auditory nerve depends on the diverse properties of ANFs, described by their different spontaneous firing rates (SRs), acoustic thresholds and dynamic ranges. Here we investigate, how ANF spontaneous and evoked firing is shaped by properties of individual IHC afferent synapses (Aim 1). The mammalian auditory system has a number of systems that operate in parallel to adjust the dynamic ranges of auditory nerve fibers (ANFs) and to prevent damage due to sound exposure. Efferent lateral olivocochlear (LOC) neurons may serve this role at the IHC afferent synapse. LOC neurons originate in the brainstem, and their synapses in the cochlea are strategically located to release a diverse cohort of neurotransmitters onto the unmyelinated ANF dendrites, providing the ability to directly modulate the hair cell induced postsynaptic activity in ANFs. Therefore, LOC fibers are ideal candidates for adjusting the ANF's dynamic range, and for protecting the IHC afferent synapse from damage due to sound exposure. Here we determine, how lateral efferent inputs modulate ANF firing on the cellular level (Aim 2), focusing on dopaminergic and cholinergic transmission. Finally, we investigate, how LOC neurons respond to sound exposure and adjust their modulation of ANF firing (Aim 3). We will use electrophysiological methods, and measure postsynaptic activity, spontaneous and evoked firing of ANFs in excised cochlear tissue of 4-6 week old mice. We will use specific mouse models and optogenetics to stimulate IHCs, ANFs or LOC fibers in excised cochlear tissue. Secondly, we will use sound exposure experiments and immunolabeling techniques to systematically investigate changes in the peripheral and central components of LOC neurons in response to a range of sound exposure conditions. We will specifically design sound exposure conditions that induce measurable changes in LOC fibers and allow us to investigate the underlying mechanisms of such changes with electrophysiological methods.