Released fraction and total size of a pool of immediately available transmitter quanta at a calyx synapse.
The size of a pool of readily releasable vesicles at a giant brainstem synapse, the calyx of Held, was probed with three independent approaches. Using simultaneous pre- and postsynaptic whole-cell recordings, two forms of presynaptic Ca2+ stimuli were applied in rapid succession: uncaging of Ca2+ by flash photolysis and the opening of voltage-gated Ca2+ channels. The ensuing transmitter release showed a nearly complete cross-inhibition between the two stimuli, indicating the depletion of a limited pool of about 700 transmitter quanta. The pool size was confirmed in experiments using enhanced extracellular Ca2+ concentrations, as well as short, high-frequency stimulus trains. The results reveal a surprisingly large pool of functionally available vesicles, of which a fraction of about 0.2 is released by a single presynaptic action potential under physiological conditions.
Schneggenburger
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Max-Planck-Institut für biophysikalische Chemie, Abteilung Membranbiophysik, Göttingen, Germany. rschneg@gwdg.de
Meyer
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Neher
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D000200:Action Potentials; D000465:Algorithms; D000818:Animals; D001933:Brain Stem / Q000502:physiology*; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism / Q000502:physiology; D004558:Electric Stimulation; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D066298:In Vitro Techniques; D015640:Ion Channel Gating; D018377:Neurotransmitter Agents / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010782:Photolysis; D051381:Rats; D013569:Synapses / Q000502:physiology*
D015220:Calcium Channels; D018377:Neurotransmitter Agents; D002118:Calcium