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1 Otolaryngology and Physiology & Biophysics, University of Colorado Health Sciences Center, Denver, CO, USA
* To whom correspondence should be addressed. E-mail: katie.rennie{at}uchsc.edu.
Na+ currents were studied by whole cell patch clamp of chalice-shaped afferent terminals attached to type I hair cells isolated from the gerbil semicircular canal and utricle. Outward K+ currents were blocked with intracellular Cs+, or with extracellularly applied 20 µM linopirdine and 2.5 mM 4-aminopyridine (4-AP). With K+ currents blocked, inward currents activated and inactivated rapidly, had a maximum mean peak amplitude of 0.92 ± 0.60 nA (mean ± SD, n = 24) and activated positive to -60 mV from holding potentials of -70 mV and more negative. The transient inward currents were blocked almost completely by 100 nM tetrodotoxin, confirming their identity as Na+ currents. Half-inactivation of Na+ currents occurred at -82.6 ± 0.9 mV, with a slope factor of 9.2 ± 0.8 (mean ± SD, n = 7) at room temperature. In current clamp, large overshooting action potential-like events were observed only following prior hyperpolarizing current injections. However, spontaneous currents consistent with quantal release from the hair cell were observed at holding potentials close to the zero-current potential. This is the first report of ionic conductances in calyx terminals postsynaptic to type I hair cells in the mammalian vestibular system.
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