Voltage-Dependent Currents in Isolated Vestibular Afferent Calyx Terminals

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Abstract
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 (SD) nA (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 TTX, 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 (n = 7) at room temperature. In current clamp, large overshooting action potential–like events were observed only after 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.