Conventional recordings have shown that hair cells in the bullfrog utriculus, a sensor of static gravity and dynamic linear acceleration, differ in their responses to intracellular current and hair bundle displacement. These recordings suggest that utricular hair cells differ in their adaptation kinetics and their complement of basolateral membrane conductances. We will study the cellular mechanisms underlying regional variations in these properties, correlating these variations with the macular location and hair bundle morphology of individual hair cells. We will carry out the proposed in vitro experiments in isolated utricular hair cells and, where necessary, slice preparations of the utricular macula. Using whole-cell patch-clamp techniques, we will characterize the adaptation of the receptor current to hair bundle displacement (Project 1). Using electron microscopy and theoretical models, we will determine if adaptation is limited by geometric factors of hair bundles which limit tip link movement. We will also test the hypothesis that regional variations in the voltage or calcium sensitivity of adaptation exist, providing a mechanism for regulating adaptation kinetics in individual hair cells. If so, we will determine if adaptation kinetics are controlled by the dynamics of calcium sequestration in intracellular organelles or calcium buffering by Ca 2+-binding proteins. In other studies, we will identify the membrane currents of utricular hair cells (Project 2). Using physiological and pharmacological methods, we will isolate these membrane currents, determining their contribution to sensitivity and frequency selectivity and characterizing the size, ionic basis, and gating kinetics of their underlying conductances. Immunocytochemical methods will be used to identify and localize the distribution of Ca 2+-binding and ion channel proteins in hair cells. The proposed studies, by revealing how utricular hair cells process static and dynamic information, will clarify the functional organization of the vestibular otolith organs and, ultimately, reveal the contribution of hair cell transduction mechanisms to the discharge properties of vestibular nerve afferents.
