Voltage-dependent currents in isolated vestibular afferent calyx terminals

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 mu M 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.