Distinct roles for two synaptotagmin isoforms in synchronous and asynchronous transmitter release at zebrafish neuromuscular junction.

An obligatory role for the calcium sensor synaptotagmins in stimulus-coupled release of neurotransmitter is well established, but a role for synaptotagmin isoform involvement in asynchronous release remains conjecture. We show, at the zebrafish neuromuscular synapse, that two separate synaptotagmins underlie these processes. Specifically, knockdown of synaptotagmin 2 (syt2) reduces synchronous release, whereas knockdown of synaptotagmin 7 (syt7) reduces the asynchronous component of release. The zebrafish neuromuscular junction is unique in having a very small quantal content and a high release probability under conditions of either low-frequency stimulation or high-frequency augmentation. Through these features, we further determined that during the height of shared synchronous and asynchronous transmission these two modes compete for the same release sites.