Figure 1

The recycling pool of hippocampal synapses is turned over completely by spontaneous vesicle recycling. A Scheme of experimental setup: Boutons were first labeled with FM1-43 by spontaneous uptake for different time periods. At the end of the first part of the experiment, boutons were completely destained using twice 900 pulses to determine the amount of spontaneous turnover. After a 10 minute recovery synaptic boutons were labeled a second time using 1200 electrically evoked action potentials and again completely destained to determine the recycling pool size. B Difference images for 15 and 120 minutes spontaneous FM1-43 uptake and evoked staining, respectively. Scale bar, 10 μm. C Corresponding mean fluorescence profiles to determine ∆F_spontaneous and ∆F_evoked for each time period. D Time course of ∆F_spontaneous to ∆F_evoked ratio depending on extracellular calcium concentration (t _½ in minutes: 0 mM Ca²⁺ = 43.96, 2.5 mM Ca²⁺ = 20.58, 5 mM Ca²⁺ = 15.73). Arrows mark the exemplary time points. E Correlation of spontaneous and evoked release at individual synaptic boutons (time point at 60 min).