Probing Vesicles Dynamics in Single Hippocampal Synapses

The classic mode of communication between neurons occurs via chemical synapses. In this process, vesicles dock at the active zone and fuse with the cell membrane, emptying neurotransmitter into the synaptic cleft. This process is stochastic and the efficacy of synaptic communication depends on the availability and movement of vesicles. We use fluorescence correlation spectroscopy (FCS) and fluorescence recovery after photobleaching (FRAP) to study vesicle dynamics inside the synapses of cultured hippocampal neurons labeled with the fluorescent vesicle marker FM 1-43. These studies show that when the cell is electrically at rest, only a small population of vesicles is mobile, taking seconds to traverse the synapse. Applying the phosphatase inhibitor okadaic acid (OA) causes vesicles to diffuse freely, moving 30 times faster than vesicles in control synapses. In contrast, eliminating polymerized synaptic actin does not free vesicles. These results suggest that vesicles move sluggishly due to binding to structural proteins within the synapse, and that this binding is altered by phosphorylation. Motivated by these results, a model is constructed consisting of diffusing vesicles that bind reversibly within the synapse. This stick-and-diffuse model accounts for the FCS and FRAP data, and also predicts the well-known exponential refilling of the readily releasable pool. Our measurements suggest that the movement of vesicles to the active zone is the rate limiting step in this process