In forebrain neurons, knockout of synaptotagmin-1 blocks fast Ca2+-triggered synchronous neurotransmitter release but enables manifestation of slow Ca2+-triggered asynchronous release. Here, we show using single-cell PCR that individual hippocampal neurons abundantly coexpress two Ca2+-binding synaptotagmin isoforms, synaptotagmin-1 and synaptotagmin-7. In synaptotagmin-1-deficient synapses of excitatory and inhibitory neurons, loss of function of synaptotagmin-7 suppressed asynchronous release. This phenotype was rescued by wild-type but not mutant synaptotagmin-7 lacking functional Ca2+-binding sites. Even in synaptotagmin-1-containing neurons, synaptotagmin-7 ablation partly impaired asynchronous release induced by extended high-frequency stimulus trains. Synaptotagmins bind Ca2+ via two C2 domains, the C2A and C2B domains. Surprisingly, synaptotagmin-7 function selectively required its C2A domain Ca2+-binding sites, whereas synaptotagmin-1 function required its C2B domain Ca2+-binding sites. Our data show that nearly all Ca2+-triggered release at a synapse is due to synaptotagmins, with synaptotagmin-7 mediating a slower form of Ca2+-triggered release that is normally occluded by faster synaptotagmin-1-induced release but becomes manifest upon synaptotagmin-1 deletion.
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