Synaptic vesicles in rat hippocampal boutons recycle to different pools in a use-dependent fashion.

Efficient vesicle membrane recycling at presynaptic terminals is pivotal for preventing depletion and maintaining high firing rates in neuronal networks. We used a new approach, based on the combination of spectrally different optical probes, to investigate how stimulation determines the fate of synaptic vesicles after endocytosis. We found that in the small central synapses of rat hippocampal neurones low frequency stimulation (40 action potentials at 2 Hz) targets vesicles preferentially to vesicle pools that were kinetically faster. Vesicles taken up during endocytosis triggered by high frequency stimulation (400 action potentials, 20 Hz) were also placed in the back of the release queue. We performed a spatial analysis of the recycled vesicles in living hippocampal boutons using two spectrally different FM-dyes (FM1-43 and FM5-95). By using these consecutively, vesicles endocytosed by either stimulation protocol were labelled with a different colour. This revealed that the kinetic arrangement was also reflected in the spatial organization of vesicles within the bouton. Next, we identified the postsynaptic site of the active zone by transfecting the neurones with postsynaptic density protein PSD-95-CFP. The data from these triple colour experiments suggest that retrieval after low frequency stimulation keeps vesicles in a more confined region closer to the active zone as identified by PSD-95-CFP expression at the postsynaptic site.