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 syna pse, the calyx of Held, was probed with three independent approaches. Using simultaneous pre- and postsynaptic whole-cell recordings, two forms of pre synaptic Ca2+ stimuli were applied in rapid succession: uncaging of Ca2+ by flash photolysis and the opening of voltage-gated Ca2+ channels. The ensui ng transmitter release showed a nearly complete cross-inhibition between th e two stimuli, indicating the depletion of a limited pool of about 700 tran smitter quanta. The pool size was confirmed in experiments using enhanced e xtracellular Ca2+ concentrations, as well as short, high-frequency stimulus trains. The results reveal a surprisingly large pool of functionally avail able vesicles, of which a fraction of about 0.2 is released by a single pre synaptic action potential under physiological conditions.