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1 Neuroscience, Baylor College of Medicine, Houston, Texas, United States
2 Anatomy and Cell Biology, University of Illinois School of Medicine, Chicago, Illinois, United States
3 Otorhinolaryngology - Head and Neck Surgery, Baylor College of Medicine, Houston, Texas, United States
4 Otorhinolaryngology - Head and Neck Surgery, Baylor College of Medicine, Houston, Texas, United States; Neuroscience, Baylor College of Medicine, Houston, Texas, United States
* To whom correspondence should be addressed. E-mail: eatock{at}meei.harvard.edu.
Two kinds of sodium current (INa) 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 INa 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 INa,1, a TTX-insensitive current with a very negative voltage range of inactivation (midpoint --94 mV). INa,2 was TTX-sensitive and had less negative voltage ranges of activation and inactivation (inactivation midpoint --72 mV). INa,1 dominated in the striola at all ages, but current density fell by two-thirds after the first postnatal week. INa,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 INa,1; the remaining cells lacked sodium current. INa,1 is probably carried by NaV1.5 subunits, based on biophysical and pharmacological properties, mRNA expression, and immunoreactivity. NaV1.5 was also localized to afferent somata and calyx endings on type I hair cells. Several TTX-sensitive subunits are candidates for INa,2.
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