During exocytosis, synaptic vesicles (red circles) fuse with the plasma membrane at active zones (left) to release neurotransmitter molecules (glutamate; pale blue) and protons (red dots) into the synaptic cleft. Synaptic vesicles that have fused to the plasma membrane are then recycled to make new vesicles in a process involving slow (middle) or fast (right) endocytosis. Synaptic vesicles are more acidic than the cytoplasm due to the action of pump proteins (purple) that load protons into the vesicles. Transport proteins (yellow) load glutamate into vesicles in exchange for protons. Slow endocytosis (middle) relies on a protein called clathrin (green), with membrane retrieval and acidification happening at approximately the same time. Fast endocytosis involves the production of large structures called endosomes that slowly become more acidic due to the action of proton pumps. New synaptic vesicles then bud from the endosome in a process that depends on clathrin. Both modes of endocytosis require dynamin (turquoise), a protein that pinches off the membrane.