Kinetics and regulation of fast endocytosis at hippocampal synapses

Presynaptic nerve terminals often contain as few as a hundred vesicles1,2 and so must recycle them soon after exocytosis to preserve synaptic transmission and presynaptic morphology3,4 during repetitive firing. The kinetics3,4 and mechanisms5 of vesicular endocytosis and repriming have therefore been studied. Vesicles in hippocampal nerve terminals can become available to release their contents within ∼40 s of the previous round of exocytosis6,7. Studies using the styryl dye FM1-43 (ref. 3) have estimated the time constant for endocytosis as ∼20–30 s (refs 4, 8), at least half of the total recycling time, which is much slower than endocytosis in other secretory systems9,10,11. It seems paradoxical that the neurosecretory terminals that could benefit themost from rapid endocytosis do not use such a mechanism. Here we demonstrate the existence of fast endocytosis in hippocampal nerve terminals and derive its kinetics from fluorescence measurements using dyes with varying rates of membrane departitioning. The rapid mode of vesicular retrieval was much faster after exposure to staurosporine or elevated extracellular calcium. Thus hippocampal synapses take advantage of efficient mechanisms for endocytosis, and their vesicular retrieval is subject to modulatory control.