title: Different roles of ribbon-associated and ribbon-free active zones in retinal bipolar cells.
abstract: Synaptic ribbons with a halo of synaptic vesicles are seen at the active zones of sensory neurons that release transmitter tonically. Thus, ribbons are assumed to be a prerequisite for sustained exocytosis. By applying total internal reflection fluorescence microscopy to goldfish retinal bipolar cell terminals, we visualized Ca2+ entry sites, ribbons, and vesicle fusion events. Here we show that the main Ca2+ entry sites were located at ribbons, and that activation of the Ca2+ current induced immediate and delayed vesicle fusion events at ribbon-associated and ribbon-free 'hot spots', respectively. The activation of protein kinase C (PKC) specifically potentiated vesicle fusion at ribbon-free sites. Electron microscopy showed that PKC activation selectively increased the number of docked vesicles at ribbon-free sites, which faced neuronal processes with the postsynaptic density. Retinal bipolar cells have both ribbon-associated and ribbon-free active zones in their terminals and might send functionally distinct signals through ribbon-associated and ribbon-free synapses to postsynaptic neurons.
authors.affiliation: Department of Psychology, Graduate School of Humanities and Sociology, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033 Japan.
authors.forename: Mitsuharu
authors.initials: M
authors.lastname: Midorikawa
authors.forename: Yoshihiko
authors.initials: Y
authors.lastname: Tsukamoto
authors.forename: Ken
authors.initials: K
authors.lastname: Berglund
authors.forename: Masaaki
authors.initials: M
authors.lastname: Ishii
authors.forename: Masao
authors.initials: M
authors.lastname: Tachibana
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D004307:Dose-Response Relationship, Radiation; D004558:Electric Stimulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D006054:Goldfish; D021621:Imaging, Three-Dimensional / Q000379:methods; D008564:Membrane Potentials / Q000502:physiology / Q000528:radiation effects; D008854:Microscopy, Electron / Q000379:methods; D018408:Patch-Clamp Techniques; D051571:Protein Kinase C-alpha / Q000378:metabolism; D012160:Retina / Q000166:cytology*; D051245:Retinal Bipolar Cells / Q000166:cytology*; D013569:Synapses / Q000145:classification / Q000187:drug effects / Q000648:ultrastructure*; D013572:Synaptic Vesicles / Q000648:ultrastructure*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013997:Time Factors