Two pathways of synaptic vesicle retrieval revealed by single-vesicle imaging.

Synaptic vesicle recycling is essential for maintaining efficient synaptic transmission. Detailed dissection of single-vesicle recycling still remains a major challenge. We have developed a fluorescent pH reporter that permits us to follow the fate of individual vesicles at hippocampal synapses after exocytosis. Here we show that, during low-frequency stimulation, single-vesicle fusion leads to two distinct vesicle internalizations, instead of one, as in general perception: one by a fast endocytosis pathway ( approximately 3 s), the other by a slow endocytosis pathway (after 10 s). The exocytosed vesicular proteins are preferentially recaptured in both pathways. RNAi knockdown of clathrin inhibits both pathways. As stimulation frequency increases, the number of endocytosed vesicles begins to match antecedent exocytosis. Meanwhile, the slow endocytosis is accelerated and becomes the predominant pathway. These results reveal that two pathways of endocytosis are orchestrated during neuronal activity, establishing a highly efficient endocytosis at central synapses.