POTASSIUM CURRENTS IN PRESYNAPTIC HAIR-CELLS OF HERMISSENDA

Apart from their primary function as balance sensors, Hermissenda hair cells are presynaptic neurons involved in the Ca2+-dependent neuronal plasticity in postsynaptic B photoreceptors that accompanies classica l conditioning. With a view to beginning to understand presynaptic mec hanisms of plasticity in the vestibule-visual system, a locus for cond itioning-induced neuronal plasticity, outward currents that may govern the excitability of hair cells were recorded by means of a whole-cell patch-clamp technique. Three K+ currents were characterized: a 4-amin opyridine-sensitive transient outward K+ current (I-A), a tetraethyl a mmonium-sensitive delayed rectifier K+ current (I-K,I-V), and a Ca2+-a ctivated K+ current (I-K,I-Ca). I-A activates and decays rapidly; the steady-state activation and inactivation curves of the current reveal a window current close to the apparent resting voltage of the hair cel ls, suggesting that the current is partially activated at rest. By mod ulating firing frequency and perhaps damping membrane oscillations, I- A may regulate synaptic release at baseline. In contrast, I-KV and I-K ,I-Ca have slow onset and exhibit little or no inactivation. These two K+ currents may determine the duration of the repolarization phase of hair-cell action potentials and hence synaptic release via Ca2+ influ x through voltage-gated Ca2+ channels. In addition, I-K,I-Ca may be re sponsible for the afterhyperpolarization of hair cell membrane voltage following prolonged stimulation.