Heterogeneity of responses in vestibular primary afferents

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Project Summary Approximately 8 million adults in the US suffer from balance impairment due to damage to the peripheral vestibular system, but effective treatments for balance dysfunction are lacking. Vestibular hair cells within vestibular canal and otolith organs convert motion into receptor potentials and sensory information is relayed to the brain by action potentials in vestibular afferent nerves. Afferents in central zones (CZ) of vestibular neuroepithelia exhibit different responses to vestibular stimuli than afferents in peripheral zones (PZ). The nature of the neural code conveying vestibular information in distinct afferent types is poorly understood. There are 3 types of vestibular afferents: calyx-only afferents innervate one or more type I hair cells, bouton dendrites innervate type II hair cells and dimorphic afferents contact both hair cell types. Our goal is to elucidate distinct action potential firing mechanisms in afferents with calyx terminals to better understand vestibular coding. Calyx-only afferents are present solely in CZ and have irregular firing patterns, whereas dimorphic afferents exist in both CZ and PZ and have regular firing patterns. To achieve our goal we will refine novel preparations of vestibular cristae and utricles, developed by our laboratory, as tools to study calyx-bearing afferents in CZ and PZ of rodent neuroepithelia. We will employ electrophysiological, hair bundle stimulation, immuno- histochemical and pharmacological approaches to characterize ion channels in CZ and PZ afferent fibers in developing and mature epithelia. In Aim 1 we will determine the contributions of K+ channels to action potential firing in CZ and PZ afferents. Aim 2 will test the hypotheses that tetrodotoxin (TTX)-sensitive Nav1.6 Na+ channels contribute uniquely to action potential firing in mature PZ dimorphs, and that TTX-insensitive Na+ channels are transiently expressed during development. In Aim 3 we will incorporate ion channel data from Aims 1 and 2 into a novel, custom-written three dimensional mathematical model of the calyx to provide insight into our zonally-driven experimental findings.To determine how channel localization directly impacts action potential firing, identified channel types will be strategically placed on the inner and outer faces of the calyx terminal and associated axon and channel density varied. Our results will clarify how sensory information is conveyed and how zonal encoding is generated within segregated vestibular afferents. Our data will inform development of vestibular neurotherapeutics targeting specific groups of ion channels in afferent nerves. Existing vestibular prosthetic implants attempt to restore normal vestibular function by direct electrical stimulation of vestibular afferents, but implants that restore function to both otolith and semicircular canal afferent neurons do not yet exist. Our results will provide important new information on vestibular afferent coding that could inform development of and drive new paradigms in vestibular implants.