Vestibular afferent nerve fibers convey head position and motion signals from the inner ear to the brain, where they drive reflexes controlling gaze, posture and balance and contribute to perceptions of orientation and self-motion. We propose to study how electrical signals are generated in primary vestibular afferents, focusing on the contributions of intrinsic ion channels and calcium binding proteins. We will examine the origins of specific patterns of activity (firing patterns of action potentials, also called spikes) in vestibular afferents. In recordings made in vivo, vestibular afferents vary greatly in the regularity of inter-spike intervals;this variation is highly systematic, co-varying with many morphological and physiological properties. Thus, highly regular afferents tend to carry tonic signals, to have small diameters and extended dendritic arbors, and to innervate peripheral zones of the sensory epithelia. In contrast, highly irregular afferents tend to have phasic signals, to have large diameters and compact dendritic arbors with large, calyceal afferent terminals, to innervate central zones and to express particular Ca2+ binding proteins (CBPs). High regularity may enhance the information content and temporal encoding for head movement frequencies below ~20 Hz;high irregularity may reflect high sensitivity to synaptic currents. Over two decades ago, investigators exploring possible factors in setting firing patterns suggested that intrinsic ion channels in the afferent fibers were likely to be important. We now know that ion channel complements in vestibular ganglion neurons (VGNs), as in many brain neurons, are much more complex than previously imagined, and we have better tools to study this issue. We propose to use the whole-cell patch clamp method on VGNs from rodents to characterize firing patterns and the underlying voltage- and calcium-gated ion channels. VGNs will be studied in vitro as isolated somata and also within a semi-intact preparation that includes sensory epithelia and the ganglion. The two preparations have different advantages: isolated VGNs provide superior voltage clamp, while VGNs in the semi-intact preparation receive synaptic input from hair cells. Preliminary data suggest that VGNs express specific complements of ion channels, giving rise to distinct endogenous firing patterns in response to injected currents. We will test the hypothesis that the endogenous ion channels help establish different patterns of spike regularity in vivo, and the involvement of specific ion channels in setting firing patterns. We will use RT-PCR to screen for candidate ion channels and immunocytochemistry to localize channels according to afferent type, zone within the sensory epithelium, and cellular location (soma vs. afferent terminal). We will investigate whether specific CBPs help set firing patterns by modulating calcium-dependent potassium channels. We will develop computational models to test our comprehensive understanding of how ion channels contribute to firing patterns.
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