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P. Datta, L. Curtis, R. Janz, R. Heidelberger; Synaptic Vesicle Pools Associated With SNARE Complexes in Bipolar Cells. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4124.
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Neurotransmitter release occurs via SNARE (Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor) complex-mediated exocytosis. In bipolar cells, three kinetic components of release have been described that are attributed to the fusion of vesicles that are docked and ribbon-associated (the rapidly-releasing pool; RRP), followed by the recruitment and fusion of other ribbon-associated vesicles (the releasable pool; RP) and those in the cytoplasm. To better understand the organization of vesicle pools at a ribbon synapse, we asked which components are sensitive to a SNARE-competing peptide.
Membrane capacitance and calcium current were monitored in isolated goldfish bipolar cell synaptic terminals. Terminals were dialyzed with a peptide derived from the N-terminal part of the SNARE domain of the goldfish syntaxin 3B (synt3B) via the patch pipette. A scrambled peptide served as control. A stimulus train designed to capture the three components of release was given every 60s.
One minute after break-in, synt3B terminals exhibited a decrease in the total extent of exocytosis evoked by the train compared to controls. Closer examination of synt3B terminals revealed that the first two components of release were intact, while the third was inhibited (p<0.05). With subsequent trains, there was progressive decrease in the first two components of release in synt3B terminals relative to controls. These changes could not be attributed to a decrease in calcium current. By the third stimulus train, 180s after break-in, the RRP and RP were significantly reduced relative to controls (p<0.002). However, when the first stimulus was given at 180s, there was no decrease in the size of the RRP and RP, indicating that the block by the synt3B peptide required activity-dependent vesicle turnover.
In contrast to vesicles in the cytoplasmic reserve, vesicles in both the RRP and RP are initially resistant to the synt3B peptide. However, the SNARE-competing peptide inhibits exocytosis from these two pools in an activity-dependent manner. This raises the question of how vesicles presumed to be in a ribbon-associated pool have preformed SNARE complexes.
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