Project Summary The proposed research addresses critical issues of high translational importance concerning the mechanisms and outcomes of partial dysfunction of the vestibular sensory epithelia, referred to as peripheral vestibular hypofunction. The research plan utilizes chemotoxin- induced hypofunction, the foundation for which was identified through recent work from the PI’s laboratory in an animal model enabling precise intraperilymphatic dosing resulting in the production of highly reproducible lesions. This provides the basis for producing lesions of graded magnitudes within the sensory neuroepithelia, documented through histopathologic analyses. The physiologic outcome of these lesions will be evaluated through recordings of single afferent neuron electrophysiology and the vestibulo-ocular reflex, providing the bases for establishing histologic and physiologic correlates to a direct behavioral test of vestibular function. Previous work has demonstrated that the afferent neuron calyx is highly labile to pathologic compromise, and owing to its important contribution to shaping neural dynamics in untreated epithelia it is a focus for assessing pathologic damage. The present research plan will enable the direct correlate of afferent discharge dynamics to critical cellular components of the calyx, including its morphology and expression of KCNQ4 and sodium-potassium ATPase. In addition, we will examine the distribution of synaptic ribbons within hair cells of lesioned epithelia, testing whether a systematic synaptopathy also contributes to the compromised vestibular function. In summary, the present investigation provides critical insight into the histopathologic substrates of vestibular hypofunction and the alterations in sensory coding that underlies the functional compromise. At the same time, however, this investigation will reveal important cellular and physiologic metrics that are required for normal vestibular function, addressing longstanding question in vestibular neurobiology.