Synaptic Processing in the Vestibular System

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Project Summary/Abstract The vestibular inner ear supplies information about head motion and position to the brain, driving powerful reflexes that stabilize gaze and posture during head motions, and contributing to our sense of heading and orientation as we move through the world. Although we are not normally aware of these functions, their loss severely affects mobility by destabilizes vision and causes vertigo. Loss of vestibular function often originates in damage to hair cells and their synapses with the afferent vestibular nerve fibers that project to the brain. These hair cells, synapses, and afferent fibers have striking properties that are only partly understood. The longterm goal of this program of research is to build a comprehensive understanding of how vestibular information is generated and encoded in the inner ear. The current proposal focuses on the synaptic transfer of head motion signals from hair cells to primary vestibular neurons (Aim 1) and the subsequent initiation of action potentials (spikes) (Aim 2) in the mouse utricle, a model preparation for genetic, developmental and physiological studies. Principal methods are whole-cell patch clamping of hair cells and afferent neurons; immunolocalization of voltage-gated ion channels, pumps and synaptic markers; and computational modeling of the hair cells, synapses and afferent nerve fibers, incorporating current information on ion channels, pumps, and morphology. Vestibular afferent neurons make conventional bouton synaptic terminals on type II hair cells and unique calyceal contacts on type I hair cells. At both boutons and calyces, hair cells release vesicles of glutamate (“quantal” synaptic transmission) into the synaptic cleft, activating glutamate receptor-channels in the postsynaptic membrane to produce excitatory postsynaptic potentials and initiate spikes. At calyceal contacts, an additional “non-quantal” transmission mechanism depends not on vesicular release or gap junctions, but rather on flow of ions from the hair cell through ion channels into the synaptic cleft and into the calyx through different ion channels. Postsynaptic responses to controlled stimulation of individual hair bundles show that quantal and non-quantal transmission modes can occur at the same calyceal synapse and that the non-quantal mode provides a fast signal that may be important for high-speed vestibular reflexes. Proposed experiments and modeling will investigate the impact of key hair cell ion channels on non-quantal transmission and delineate how quantal and non-quantal transmission are integrated in individual calyces and afferent nerve fibers. Other experiments will test how specific voltage-gated potassium and sodium channels in calyces and boutons shape the postsynaptic voltage response and spikes in the axonal initial segment. Immunolocalization has revealed remarkable concentrations of ion channels in microdomains of the calyx ending and nearby spike initiation zone. Experiments focus on channels with the potential to shape salient differences in response dynamics and spike timing between afferents of different connectivity (hair cell inputs) and different zones of the sensory epithelium.