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Abstract

Nerve cells store neurotransmitters in synaptic vesicles. These vesicles dock to a specialized region of the synaptic plasma membrane, the active zone, where they undergo a maturation or priming process. Upon depolarization and a consequential rise in the intracellular calcium concentration, primed vesicles release their content by exocytosis. Following release, vesicular membrane and protein components are retrieved by endocytosis and recycled through an early endosomal compartment. From there, synaptic vesicles bud off for a new round of regulated exocytosis (Bennett and Scheller, 1994; Südhof, 1995). Obviously, synaptic vesicles play a key role in presynaptic processes. The only known function of these secretory organelles is the accumulation and release of neurotransmitter. Their high abundance, good accessibility, and relatively simple protein composition have made them the focus of research efforts in molecular exocytosis.