title: Sharp Ca²⁺ nanodomains beneath the ribbon promote highly synchronous multivesicular release at hair cell synapses.
abstract: Hair cell ribbon synapses exhibit several distinguishing features. Structurally, a dense body, or ribbon, is anchored to the presynaptic membrane and tethers synaptic vesicles; functionally, neurotransmitter release is dominated by large EPSC events produced by seemingly synchronous multivesicular release. However, the specific role of the synaptic ribbon in promoting this form of release remains elusive. Using complete ultrastructural reconstructions and capacitance measurements of bullfrog amphibian papilla hair cells dialyzed with high concentrations of a slow Ca²⁺ buffer (10 mM EGTA), we found that the number of synaptic vesicles at the base of the ribbon correlated closely to those vesicles that released most rapidly and efficiently, while the rest of the ribbon-tethered vesicles correlated to a second, slower pool of vesicles. Combined with the persistence of multivesicular release in extreme Ca²⁺ buffering conditions (10 mM BAPTA), our data argue against the Ca²⁺-dependent compound fusion of ribbon-tethered vesicles at hair cell synapses. Moreover, during hair cell depolarization, our results suggest that elevated Ca²⁺ levels enhance vesicle pool replenishment rates. Finally, using Ca²⁺ diffusion simulations, we propose that the ribbon and its vesicles define a small cytoplasmic volume where Ca²⁺ buffer is saturated, despite 10 mM BAPTA conditions. This local buffer saturation permits fast and large Ca²⁺ rises near release sites beneath the synaptic ribbon that can trigger multiquantal EPSCs. We conclude that, by restricting the available presynaptic volume, the ribbon may be creating conditions for the synchronous release of a small cohort of docked vesicles.
authors.affiliation: National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: Cole W
authors.initials: CW
authors.lastname: Graydon
authors.forename: Soyoun
authors.initials: S
authors.lastname: Cho
authors.forename: Geng-Lin
authors.initials: GL
authors.lastname: Li
authors.forename: Bechara
authors.initials: B
authors.lastname: Kachar
authors.forename: Henrique
authors.initials: H
authors.lastname: von Gersdorff
mesh_terms: D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000818:Animals; D055592:Biophysical Phenomena / Q000187:drug effects / Q000502:physiology*; D001703:Biophysics; D002118:Calcium / Q000378:metabolism*; D002120:Calcium Channel Agonists / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D025802:Electric Capacitance; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D005089:Exocytosis / Q000187:drug effects; D005260:Female; D006198:Hair Cells, Auditory / Q000166:cytology*; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D008854:Microscopy, Electron / Q000379:methods; D008959:Models, Neurological; D009010:Monte Carlo Method; D018408:Patch-Clamp Techniques; D011892:Rana catesbeiana; D013223:Statistics as Topic; D013569:Synapses / Q000187:drug effects / Q000502:physiology / Q000648:ultrastructure*; D013572:Synaptic Vesicles / Q000187:drug effects / Q000378:metabolism*
chemical_list: D002120:Calcium Channel Agonists; D002614:Chelating Agents; D004533:Egtazic Acid; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium
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