The kinetics of synaptic vesicle recycling measured at single presynaptic boutons

We used the fluorescent membrane probe FM 1-43 to label recycling synaptic vesicles within the presynaptic boutons of dissociated hippocampal neurons in culture. Quantitative time-lapse fluorescence imaging was employed in combination with rapid superfusion techniques to study the dynamics of synaptic vesicles within single boutons. This approach enabled us to measure exocytosis and to analyze the kinetics of endocytosis and the preparation of endocytosed vesicles for re-release (repriming). Our measurements indicate that under sustained membrane depolarization, endocytosis persists much longer than exocytosis, with a t _{1 2} = 60 s (∼ 24°C); once internalized, vesicles become reavailable for exocytosis in ∼ 30 s. Furthermore, we have shown that endocytosis is not dependent on membrane potential and, unlike exocytosis, that it is independent of extracellular Ca²⁺.