Neuron 76, 511–517 (2012)

Neurotransmitters such as glutamate are released at synapses by exocytosis, followed by endocytosis to retrieve the vesicles and their refilling with neurotransmitter for recycling and continued synaptic transmission. The vesicle refilling speed has been predicted to be on the order of milliseconds. However, when monitored in isolated or reconstructed vesicles, the speed of glutamate uptake is upwards of 10 min. To look at this more closely in a physiological context, Hori and Takahashi monitored vesicle refilling kinetics at living synapses, where intravesicular glutamate could be washed out of nerve terminals, and refilling was monitored by introducing caged glutamate and measuring the recovery of excitatory postsynaptic currents. From these experiments, the authors concluded that uncaged glutamate was taken up via H+-ATPase and the glutamate transporter VGLUT, with a maximal refilling time constant of 15.1 sec—much faster than in isolated or reconstructed vesicles and consistent with the idea that most vesicles are fully filled during recycling. As the refilling rates measured here do not support fast reuse of recycling vesicles consistent with a kiss-and-run (subsecond) mode of endocytosis, the results suggest that this fast endocytosis may contribute to balancing vesicular and terminal membranes rather than to synaptic transmission.