Developmental Changes in Two Voltage-Dependent Sodium Currents in Utricular Hair Cells

1 Department of Neuroscience and 2 Bobby R. Alford Department of Otorhinolaryngology—Head and Neck Surgery, Baylor College of Medicine, Houston, Texas; and 3 Department 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 V 1.5 subunits based on biophysical and pharmacological properties, mRNA expression, and immunoreactivity. Na V 1.5 was also localized to calyx endings on type I hair cells. Several TTX-sensitive subunits are candidates for I Na,2 . Present address and address for reprint requests and other correspondence: R. A. Eatock, Eaton-Peabody Lab., Massachusetts Eye and Ear Infirmary, 243 Charles St., Boston, MA 02114 (E-mail: eatock{at}meei.harvard.edu )