title: Properties of exocytotic response in vertebrate photoreceptors.
abstract: Synaptic transmission at the photoreceptor synapse is characterized by continuous release of glutamate in darkness. Release is regulated by the intracellular calcium concentration ([Ca2+]i). We here examined the physiological properties of exocytosis in tiger salamander (Ambystoma tigrinum) retinal rods and cones. Patch-clamp capacitance measurements were used to monitor exocytosis elicited by a rapid and uniform increase in [Ca2+]i by photolysis of the caged Ca2+ compound NP-EGTA. The amplitude of flash-induced increases in membrane capacitance (Cm) varied monotonically with [Ca2+]i beyond approximately 15 microM. The following two types of kinetic responses in Cm were recorded in both rods and cones: 1) a single exponential rise (39% of cells) or 2) a double-exponential rise (61%). Average rate constants of rapid and slow exocytotic responses were 420 +/- 168 and 7.85 +/- 5.02 s-1, respectively. The rate constant for the single exponential exocytotic response was 17.5 +/- 12.4 s-1, not significantly different from that of the slow exocytotic response. Beyond the threshold [Ca2+]i of approximately 15 microM, the average amplitude of rapid, slow, and single Cm response were 0.84 +/- 0.35, 0.82 +/- 0.20, and 0.70 +/- 0.23 pF, respectively. Antibodies against synaptotagmin I, a vesicle protein associated with fast exocytosis, strongly stained the synaptic terminal of isolated photoreceptors, suggesting the presence of fusion-competent vesicles. Our results confirm that photoreceptors possess a large rapidly releasable pool activated by a low-affinity Ca2+ sensor whose kinetic and calcium-dependent properties are similar to those reported in retinal bipolar cells and cochlear hair cells.
authors.affiliation: Laboratory Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Medical Faculty, Zaloska 4, and Celica Biomedical Sciences Center, Stegne 21, 1000 Ljubljana, Slovenia.
authors.forename: M
authors.initials: M
authors.lastname: Kreft
authors.forename: D
authors.initials: D
authors.lastname: Krizaj
authors.forename: S
authors.initials: S
authors.lastname: Grilc
authors.forename: R
authors.initials: R
authors.lastname: Zorec
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins; D018929:Cell Culture Techniques; D004533:Egtazic Acid / Q000031:analogs & derivatives*; D005089:Exocytosis; D018698:Glutamic Acid / Q000378:metabolism*; D007150:Immunohistochemistry; D008562:Membrane Glycoproteins / Q000032:analysis; D018613:Microscopy, Confocal; D009419:Nerve Tissue Proteins / Q000032:analysis; D018408:Patch-Clamp Techniques; D010782:Photolysis; D017729:Presynaptic Terminals / Q000502:physiology; D012160:Retina / Q000502:physiology; D017949:Retinal Cone Photoreceptor Cells / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D017948:Retinal Rod Photoreceptor Cells / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D009435:Synaptic Transmission; D013572:Synaptic Vesicles / Q000502:physiology; D050863:Synaptotagmin I; D050857:Synaptotagmins; D014562:Caudata
chemical_list: D002135:Calcium-Binding Proteins; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D050863:Synaptotagmin I; D050857:Synaptotagmins; C085112:2-nitrophenyl-EGTA; D018698:Glutamic Acid; D004533:Egtazic Acid; D002118:Calcium