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Exocytosis of small synaptic vesicles is a very rapid process (<1 ms) by which a rise in intraterminal [Ca2+]i is coupled to the release of neurotransmitter into the synaptic cleft. This process is mediated by a protein complex consisting of a core complex (the SNARE complex) and its accessory proteins, which include synaptotagmin. In response to changes in [Ca2+]i, the complex acts to promote fusion between the synaptic vesicle membrane and the plasma membrane. This is thought to occur via the opening of a fusion pore, which then dilates and potentially allows full fusion of the synaptic vesicle and plasma membrane. While clearly established in the fusion of large dense core vesicles, the extent to which full fusion occurs, as compared to a fusion pore that opens and then closes again (termed “kiss-and-run” exocytosis), remains controversial in the fusion of small synaptic vesicles. Following full fusion, the exocytosed membrane must be retrieved by a method other than fusion pore reversal, involving endocytosis of either individual synaptic vesicle membrane domains, or bulk retrieval of membrane from many vesicles. At hippocampal synapses, two modes of synaptic vesicle exocytosis can be distinguished by virtue of the rate and extent of loss of a fluorescent lipid marker (FM1-43). We have shown that these exocytic modes result in distinct postsynaptic consequences, such that so-called “kiss-and-run” exocytosis results in negligible activation of AMPA-receptors (due to desensitization), compared to the robust postsynaptic responses elicited by apparent full fusion. In contrast NMDA receptors are robustly activated by this form of glutamate delivery. The balance between these two release modes also appears to be regulated by the level of intracellular [Ca2+] within the nerve terminal.
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