Synaptic vesicle undocking induces low-frequency depression

Synaptic depression is often interpreted as reflecting depletion of the readily releasable pool (RRP) following exocytosis. Such a mechanism predicts little or no depression at low stimulation frequency, as RRP replenishment should then offset the loss of vesicles by exocytosis. Nevertheless, in several types of mammalian central synapses, repetitive presynaptic stimulation at low frequency (<5 Hz) elicits synaptic depression (low-frequency depression or LFD). In the present work, we count the number of synaptic vesicles released at individual active zones to study the RRP and its replenishment during LFD. Contrary to depletion models of synaptic depression, we find that LFD does not depend on previous SV consumption. We find that LFD displays a long recovery time course (tens of seconds) when challenged by isolated stimulations but is immediately reversed by a high-frequency train. We suggest that LFD results from undocking, a shift between two classes of synaptic vesicles organized sequentially inside the RRP (replacement vs. docked vesicles) in favor of the upstream (replacement) state. To explain the dependence of LFD recovery on stimulation frequency, we propose that, while undocking is apparent hundreds of milliseconds after a stimulation, calcium-dependent docking takes only a couple of milliseconds. Consistent with the undocking model, we find that double presynaptic stimulations alleviate LFD as they favor vesicular docking and RRP replenishment. Finally, we expand our model to explain how stimulation frequency shapes short-term synaptic depression, changing from depression at low frequency to a facilitation-depression sequence at medium- or high-frequency trains.