Calcium dependence of exocytosis and endocytosis at the cochlear inner hair cell afferent synapse |
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Authors: | Beutner D Voets T Neher E Moser T |
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Affiliation: | Department of Membrane Biophysics, Max-Planck-Institute for Biophysical Chemistry, Am Fassberg, 37077, G?ttingen, Germany. dbeutne@gwdg.de |
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Abstract: | Release of neurotransmitter at the inner hair cell (IHC) afferent synapse is a fundamental step in translating sound into auditory nerve excitation. To study the Ca2+ dependence of the underlying vesicle fusion and subsequent endocytosis, we combined Ca2+ uncaging with membrane capacitance measurements in mouse IHCs. Rapid elevations in [Ca2+]i above 8 microM caused a biphasic capacitance increase corresponding to the fusion of approximately 40,000 vesicles. The kinetics of exocytosis displayed a fifth-order Ca2+ dependence reaching maximal rates of >3 x 10(7) vesicle/s. Exocytosis was always followed by slow, compensatory endocytosis (tau congruent with 15 s). Higher [Ca2+]i increased the contribution of a faster mode of endocytosis with a Ca2+ independent time constant of approximately 300 ms. These properties provide for rapid and sustained transmitter release from this large presynaptic terminal. |
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