Author/Editor		Kreft, M; Križaj, D; Grilc, S; Zorec, R
Title		Properties of exocytotic response in vertebrate photoreceptors
Type		članek
Source		J Neurophysiol
Vol. and No.		Letnik 90
Publication year		2003
Volume		str. 218-25
Language		eng
Abstract		Synaptic transmission at the photoreceptor synapsc is charactcrized 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+]; by photolysis of the caged Ca2+ compound NP-EGTA. The amplitude of flash-induced increases in membrane capacitance (Cm) varied monotonically with [Ca2+]; beyond approximately 15 microM. The following two types of kinetic responses in Cm were recorded in both rods and cones: I) 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, respectivcly. 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+]; of approximately 15 microM, the average amplitude oi 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 staincd the synaptic tenninal of isolated photoreceptors, suggesting the prescnce of fusion-competent vesicles. Our results confinn that photoreceptors possess a largc rapidly releasable pool activated by a low-affinity Ca2+ scnsor whosc kinetic and calcium-dependent properties are similar to those reported in retinal bipolar cells and cochlear hair cells.
Descriptors		EXOCYTOSIS PHOTORECEPTORS CALCIUM SALAMANDRA PATCH-CLAMP TECHNIQUES PHOTOLYSIS