In 1973 John Heuser and Tom Reese demonstrated that neurotransmitter was released from neurons via the fusion of neurotransmitter-filled vesicles with the cell membrane. But at the same time, these experiments launched a controversy that is unresolved today – do vesicles collapse into the membrane and are then recycled slowly on the order of 20 seconds? Or do they retain their existence – and reverse the pore in just 1 second, as proposed in ‘kiss and run’ endocytosis? Since then, molecular pathways for fusion and recycling have been put forward, but the field remains divided. We have used channelrhodopsin to stimulate neurons in intact nematodes and in cultured hippocampal neurons. The specimen is then frozen 15 ms to 20 seconds after the stimulus. To our surprise, we observed a different form of vesicle recycling that is ultrafast, in which membrane is endocytosed at lateral edges of active zones between 30-100 ms after stimulation. The large endocytic vesicles then fuse to form an endosome and are resolved by clathrin into synaptic vesicles. Although rapid, several molecules coordinately mediate ultrafast endocytosis. I will discuss the findings from the original studies and our current work on molecular mechanisms underlying ultrafast endocytosis.
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