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J Neurophysiol 97: 1684-1704, 2007. First published October 25, 2006; doi:10.1152/jn.00649.2006
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Developmental Changes in Two Voltage-Dependent Sodium Currents in Utricular Hair Cells

¹Department of Neuroscience and ²Bobby R. Alford Department of Otorhinolaryngology—Head and Neck Surgery, Baylor College of Medicine, Houston, Texas; and ³Department of Anatomy and Cell Biology, University of Illinois College of Medicine, Chicago, Illinois

Submitted 21 June 2006; accepted in final form 19 October 2006

Two kinds of sodium current (I_Na) have been separately reported in hair cells of the immature rodent utricle, a vestibular organ. We show that rat utricular hair cells express one or the other current depending on age (between postnatal days 0 and 22, P0—P22), hair cell type (I, II, or immature), and epithelial zone (striola vs. extrastriola). The properties of these two currents, or a mix, can account for descriptions of I_Na in hair cells from other reports. The patterns of Na channel expression during development suggest a role in establishing the distinct synapses of vestibular hair cells of different type and epithelial zone. All type I hair cells expressed I_Na,1, a TTX-insensitive current with a very negative voltage range of inactivation (midpoint: –94 mV). I_Na,2 was TTX sensitive and had less negative voltage ranges of activation and inactivation (inactivation midpoint: –72 mV). I_Na,1 dominated in the striola at all ages, but current density fell by two-thirds after the first postnatal week. I_Na,2 was expressed by 60% of hair cells in the extrastriola in the first week, then disappeared. In the third week, all type I cells and about half of type II cells had I_Na,1; the remaining cells lacked sodium current. I_Na,1 is probably carried by Na_V1.5 subunits based on biophysical and pharmacological properties, mRNA expression, and immunoreactivity. Na_V1.5 was also localized to calyx endings on type I hair cells. Several TTX-sensitive subunits are candidates for I_Na,2.

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