title: Intracellular calcium stores drive slow non-ribbon vesicle release from rod photoreceptors.
abstract: Rods are capable of greater slow release than cones contributing to overall slower release kinetics. Slow release in rods involves Ca(2+)-induced Ca(2+) release (CICR). By impairing release from ribbons, we found that unlike cones where release occurs entirely at ribbon-style active zones, slow release from rods occurs mostly at ectopic, non-ribbon sites. To investigate the role of CICR in ribbon and non-ribbon release from rods, we used total internal reflection fluorescence microscopy as a tool for visualizing terminals of isolated rods loaded with fluorescent Ca(2+) indicator dyes and synaptic vesicles loaded with dextran-conjugated pH-sensitive rhodamine. We found that rather than simply facilitating release, activation of CICR by ryanodine triggered release directly in rods, independent of plasma membrane Ca(2+) channel activation. Ryanodine-evoked release occurred mostly at non-ribbon sites and release evoked by sustained depolarization at non-ribbon sites was mostly due to CICR. Unlike release at ribbon-style active zones, non-ribbon release did not occur at fixed locations. Fluorescence recovery after photobleaching of endoplasmic reticulum (ER)-tracker dye in rod terminals showed that ER extends continuously from synapse to soma. Release of Ca(2+) from terminal ER by lengthy depolarization did not significantly deplete Ca(2+) from ER in the perikaryon. Collectively, these results indicate that CICR-triggered release at non-ribbon sites is a major mechanism for maintaining vesicle release from rods and that CICR in terminals may be sustained by diffusion of Ca(2+) through ER from other parts of the cell.
authors.affiliation: Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center Omaha, NE, USA ; Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center Omaha, NE, USA.
authors.forename: Minghui
authors.initials: M
authors.lastname: Chen
authors.affiliation: Department of Ophthalmology and Visual Sciences, Moran Eye Center, University of Utah School of Medicine Salt Lake City, UT, USA.
authors.forename: David
authors.initials: D
authors.lastname: Križaj
authors.affiliation: Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center Omaha, NE, USA ; Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center Omaha, NE, USA.
authors.forename: Wallace B
authors.initials: WB
authors.lastname: Thoreson
keywords: calcium-induced calcium release; exocytosis; retina; ribbon synapse; ryanodine receptors; synaptic vesicle; total internal reflection fluorescence microscopy