title: A highly Ca2+-sensitive pool of vesicles contributes to linearity at the rod photoreceptor ribbon synapse.
abstract: Studies of the properties of synaptic transmission have been carried out at only a few synapses. We analyzed exocytosis from rod photoreceptors with a combination of physiological and ultrastructural techniques. As at other ribbon synapses, we found that rods exhibited rapid kinetics of release, and the number of vesicles in the releasable pool is comparable to the number of vesicles tethered at ribbon-style active zones. However, unlike other previously studied neurons, we identified a highly Ca(2+)-sensitive pool of releasable vesicles with a relatively shallow relationship between the rate of exocytosis and [Ca(2+)](i) that is nearly linear over a presumed physiological range of intraterminal [Ca(2+)]. The low-order [Ca(2+)] dependence of release promotes a linear relationship between Ca(2+) entry and exocytosis that permits rods to relay information about small changes in illumination with high fidelity at the first synapse in vision.
authors.affiliation: Department of Ophthalmology, University of Nebraska Medical Center, Omaha, NE 68198, USA. wbthores@unmc.edu
authors.forename: Wallace B
authors.initials: WB
authors.lastname: Thoreson
authors.forename: Katalin
authors.initials: K
authors.lastname: Rabl
authors.forename: Ellen
authors.initials: E
authors.lastname: Townes-Anderson
authors.forename: Ruth
authors.initials: R
authors.lastname: Heidelberger
mesh_terms: D000557:Ambystoma; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D004533:Egtazic Acid / Q000031:analogs & derivatives*; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008854:Microscopy, Electron; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D013569:Synapses / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology; D013572:Synaptic Vesicles / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D014785:Vision, Ocular / Q000187:drug effects / Q000502:physiology*
chemical_list: C085112:2-nitrophenyl-EGTA; D004533:Egtazic Acid; D002118:Calcium