Calcium Source Alters Synaptic Vesicle Recycling

Abstract

Two pools of synaptic vesicles are identifiable in synapses: the readily releasable pool (the so-called ECP) that is located at the periphery of the neuron that undergoes rapid exocytosis and endocytosis upon depolarization, and the reserve pool (RP) that is located more centrally and that is released upon high-frequency tetanic stimulation. Kuromi and Kidokoro used uptake of the fluorescent dye FM1-43 into neuromuscular junction synaptic boutons under different stimulation conditions and at different times to analyze how calcium controls the replenishment of the ECP and RP. The ECP was selectively labeled by simultaneous incubation with FM1-43 during depolarization by high K+ or during tetanic stimulation, whereas the RP was selectively labeled if FM1-43 was added after a tetanic stimulation. Altering external calcium altered the labeling of the ECP and RP, with labeling of the ECP decreasing with decreasing [Ca2+]o and the RP labeling decreasing when [Ca2+]o was high. Several experiments using different stimulation conditions and changing external calcium concentrations suggested that the ECP was selectively affected by changes in [Ca2+]o. When internal calcium stores in the endoplasmic reticulum were depleted by application of either cyclopiazonic acid or thapsigargin, the labeling of the RP during the late phase of tetanic stimulation was inhibited, suggesting that replenishment of the RP requires calcium released from internal stores. Conditions that promoted release of calcium from internal stores or photorelease of caged calcium inside the boutons promoted the redistribution of peripheral FM1-43-labeled structures to the center of the bouton, presumably representing a movement of ECP vesicles into the RP. H. Kuromi, Y. Kidokoror, Selective replenishment of two vesicle pools depends on the source of Ca2+ at the Drosophila synapse. Neuron 35, 333-343 (2002).[Online Journal]