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1 Inst. Pathophysiology, Medical Faculty, Lab. Neuroendo-Molecular Cell Physiology, Ljubljana, Slovenia; Celica Biomed. Sci. Center, Ljubljana, Slovenia
2 Depts. of Ophthalmology and Physiology, University of California School of Medicine, San Francisco, California, USA
3 Inst. Pathophysiology, Medical Faculty, Lab. Neuroendo-Molecular Cell Physiology, Ljubljana, Slovenia
* To whom correspondence should be addressed. E-mail: robert.zorec{at}mf.uni-lj.si.
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 ~ 15 µM. Two types of kinetic responses in Cm were recorded in both rods and cones: i) a single exponential rise (39 % of cells) or ii) a double exponential rise (61 %). Average rate constants of rapid and slow exocytotic responses were 420 ± 168 s-1 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 ~ 15 µM, 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 stained strongly the synaptic terminal of isolated photoreceptors, suggesting the presence of fusion competent vesicles. Our results confirm that photoreceptors posses 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.
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