Figure 6.

Synaptic vesicle replenishment at the calyx of Held synapse. A. Ca uncaging was used to elevate Ca uniformly in the presynaptic terminal. Top, the cumulative number of fusion (disappearance) events (measured using TIRF microscopy) increased rapidly upon flash photolysis, and the sustained component followed afterwards. The rapid component corresponds to depletion of the RRP, whereas the sustained component corresponds to the fusion of newly replenished vesicles. Newly replenished vesicles are not mediated by newly tethered vesicles, because the tethering rate is too slow to explain the sustained component (bottom). In addition, newly tethered vesicles cannot be fusion competent during the period of our measurements. Rather, the sustained component is mediated by the release of already tethered vesicles. B. Schematic view of the experiment. Vesicles are docked with a time constant of several seconds, which cannot be release competent immediately. Vesicles have to mature to become release competent with a time constant of at least 300 ms. A and B are reproduced from Midorikawa and Sakaba (2015), with permission from Elsevier.⁵¹⁾