A vesicle superpool spans multiple presynaptic terminals in hippocampal neurons.
Bottom Line: Here, we demonstrate a vesicle pool that is not confined to a synapse but spans multiple terminals.In acute hippocampal slices we show that the mobile vesicle pool is also a feature of native brain tissue.We also demonstrate that superpool vesicles are available to synapses during stimulation, providing an extension of the classical recycling pool.
Affiliation: School of Life Sciences, University of Sussex, Brighton, UK. firstname.lastname@example.orgShow MeSH
Related in: MedlinePlus
Mentions: Mobile populations of extrasynaptic vesicles that are adjacent to stable presynaptic terminals might serve as additional vesicle reservoirs for presynaptic release. While previous work has shown that mobile vesicles enter synaptic terminals and undergo fusion alongside native vesicles (Darcy et al., 2006a), whether incorporation and fusion are sufficiently rapid to contribute to release during sustained transmission has not been considered. We investigated this issue in culture using FM-dye-loaded neurons combined with field stimulation. Mobile vesicles that became newly incorporated into terminals could readily participate in vesicle fusion (Figure 4A). A similar observation was also made in an acute slice preparation (Figure 4B). Complementary to this idea of rapid fusion-competence, we also observed examples of mobile vesicle clusters that underwent FM-dye loss while moving (Figure 4C). Next, we examined the consequence of synaptic incorporation of mobile vesicles during continuous stimulation. A synapse along a process with high vesicle mobility continually received new consignments of fluorescent vesicles that, during stimulation, were released alongside native vesicles (Figure 4D, left). This lateral draining of mobile vesicles into stable synapses can be observed directly in a kymograph plot (Figure 4E), and in this example, resulted in a delayed stimulation-evoked FM-dye loss compared with a synapse on a process that showed low levels of mobile vesicle traffic (Figures 4E and 4F). Quantifying the fate of mobile vesicle packets during activity by measuring stimulus-evoked fluorescence changes in intersynaptic axonal segments (n = 23 from three cultures) revealed a net loss of FM-dye fluorescence signal (39% ± 2.2%: Figure 4G and 4I). This indicates substantial activity-dependent fusion of mobile vesicles originating from axonal regions. To establish what fraction of the packets destained after moving into neighboring terminals (versus those released while moving along the axon), we tracked the fate of the mobile axonal packets prior to their destaining and found that the majority (65% ± 7%, n = 23) of packets entered a flanking synapse during destaining (Figure 4H). Taken together, these findings show that the shared pool of functional vesicles can provide an additional vesicle reserve available to synapses during ongoing transmission.
Affiliation: School of Life Sciences, University of Sussex, Brighton, UK. email@example.com