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Na⁺ Currents in Vestibular Type I and Type II Hair Cells of the Embryo and Adult Chicken

¹ Dipartimento di Scienze Fisiologiche-Farmacologiche Cellulari-Molecolari—Sez. di Fisiologia Generale e Biofisica Cellulare, Università di Pavia, 27100 Pavia, Italy; ² Departments of Otolaryngology, Physiology and Biophysics, and Anatomy and Neurosciences, The University of Texas Medical Branch, Galveston, Texas 77555; ³ Centre for Molecular Biology and Neuroscience and Department of Anatomy, Institute of Basic Medical Sciences, University of Oslo, N-0317 Oslo, Norway

Submitted 23 December 2002; accepted in final form 4 April 2003

In birds, type I and type II hair cells differentiate before birth. Here we describe that chick hair cells, from the semicircular canals, begin expressing a voltage-dependent Na current (I_Na) from embryonic day 14 (E14) and continue to express the current up to hatching (E21). During this period, I_Na was present in most (31/43) type I hair cells irrespective of their position in the crista, in most type II hair cells located far from the planum semilunatum (48/63), but only occasionally in type II hair cells close to the planum semilunatum (2/35). I_Na activated close to –60 mV, showed fast time- and voltage-dependent activation and inactivation, and was completely, and reversibly, blocked by submicromolar concentrations of tetrodotoxin (K_d = 17 nM). One peculiar property of I_Na concerns its steady-state inactivation, which is complete at –60 mV (half-inactivating voltage = –96 mV). I_Na was found in type I and type II hair cells from the adult chicken as well, where it had similar, although possibly not identical, properties and regional distribution. Current-clamp experiments showed that I_Na could contribute to the voltage response provided that the cell membrane was depolarized from holding potentials more negative than –80 mV. When recruited, I_Na produced a significant acceleration of the cell membrane depolarization, which occasionally elicited a large rapid depolarization followed by a rapid repolarization (action-potential-like response). Possible physiological roles for I_Na in the embryo and adult chicken are discussed.

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