Two types of exocytosis at retinal ribbon synapses and their functions in visual information processing

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Synaptic ribbons with a halo of synaptic vesicles are observed at the terminal of sensory neurons that release transmitter tonically. Thus, ribbons are assumed to be prerequisite to sustained exocytosis. Retinal bipolar cells have plenty of ribbons in their terminal and send outputs to ganglion and/or amacrine cells. In the present study, applying total internal reflection fluorescence microscopy to goldfish retinal Mb1 bipolar cell terminals under whole-cell voltage clamp, we visualized Ca^2+ entry sites, ribbons, and vesicle fusion events. We found that main Ca^2+ entry sites were located at ribbons, and that activation of the presynaptic L-type Ca^2+ current induced a transient high-rate vesicle fusion followed by a low-rate fusion inside the ribbon region and a delayed sustained fusion outside the ribbon region. Activation of protein kinase C (PKC) by phorbol ester specifically increased vesicle fusion events outside the ribbon region. Test for spatial randomness of vesicle fusion events revealed that vesicle fusion occurred not randomly but at "hot spots" in both regions. Electron microscopic examination of goldfish Mb1 bipolar cell terminals revealed that PKC activation selectively increased the number of docked vesicles at ribbon-free sites, which faced neuronal processes with the postsynaptic density (PSD). Reconstruction of the whole terminal of an Mb1 bipolar cell from serial ultra-thin sections showed that the number of ribbon-associated synapses with dyad processes was almost comparable to that of ribbon-free synapses with single processes. These results indicate that exocytosis may occur at both ribbon-associated and ribbon-free active zones. Dichotomy of transient and sustained photoresponses in retinal ganglion cells may be partly ascribed to two types of active zones in bipolar cell terminals.