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C. Schietroma, G. A. Zampighi, N. C. Brecha; Synaptic Vesicle Release at Non Ribbon-Associated Sites in Cone Pedicles. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4123.
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At the cone ribbon synapse, fusion-competent vesicles are thought to be ribbon associated, and Ca++ dependent vesicle exocytosis is commonly assumed to occur near the base of the ribbon. The aim of this study was to evaluate the contribution of ribbon- and non ribbon-associated vesicle populations to synaptic transmission in the mouse cone pedicle.
Retinas from light- and dark-adapted C57Bl/6J mice were collected between 12:00-3:00, fixed by a mixture of aldehydes, embedded and prepared for thin sectioning and conical electron tomography. 3D-maps were reconstructed using the weighted back projection algorithm and analyzed by volume rendering and density segmentation.
Docked (hemi-fused) and fused (omega figures) synaptic vesicles were distributed near the ribbon and along the pedicle surfaces, adjacent to HC processes; the occurrence of hemi-fused and fused synaptic vesicles was as frequent near the ribbon (<350nm) as farther away from the ribbon. The comparison between light- and dark-adapted retinas showed a significant increase in the density of ribbon- and non ribbon-associated omega figures in the dark (150±50 Ω/µm2 of invaginating membrane) compared to mesopic levels (55±20 Ω/µm2 of invaginating membrane).
In the mouse cone pedicle, association to the ribbon is not a requirement for synaptic vesicles fusion. Both ribbon and non-ribbon transmitter release contribute to the shaping of cone signaling to post-synaptic neurons.
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