Encoding continuous sensory variables requires sustained synaptic signalling. At several
sensory synapses, rapid vesicle supply is achieved via highly mobile vesicles and
specialized ribbon structures, but how this is achieved at central synapses without
ribbons is unclear. Here we examine vesicle mobility at excitatory cerebellar mossy
fibre synapses which sustain transmission over a broad frequency bandwidth. Fluorescent
recovery after photobleaching in slices from VGLUT1(Venus) knock-in mice reveal 75%
of VGLUT1-containing vesicles have a high mobility, comparable to that at ribbon synapses.
Experimentally constrained models establish hydrodynamic interactions and vesicle
collisions are major determinants of vesicle mobility in crowded presynaptic terminals.
Moreover, models incorporating 3D reconstructions of vesicle clouds near active zones
(AZs) predict the measured releasable pool size and replenishment rate from the reserve
pool. They also show that while vesicle reloading at AZs is not diffusion-limited
at the onset of release, diffusion limits vesicle reloading during sustained high-frequency
signalling.
