Autonomous function of synaptotagmin 1 in triggering synchronous release independent of asynchronous release

Ca2+ triggers neurotransmitter release in at least two principal modes, synchronous and asynchronous release. Synaptotagmin 1 functions as a Ca2+ sensor for synchronous release, but its role in asynchronous release remains unclear. We now show that in cultured cortical neurons stimulated at low frequency (<= 0.1 Hz), deletion of synaptotagmin 1 blocks synchronous GABA and glutamate release without significantly increasing asynchronous release. At higher stimulation frequencies (>= 4 Hz), deletion of synaptotagmin 1 also alters only synchronous, not asynchronous, release during the stimulus train, but dramatically enhances delayed asynchronous release following the stimulus train. Thus synaptotagmin 1 functions as an autonomous Ca2+ sensor independent of asynchronous release during isolated action potentials and action potential trains, but restricts asynchronous release induced by residual Ca2+ after action potential trains. We propose that synaptotagmin 1 occupies release slots at the active zone, possibly in a Ca2+- independent complex with SNARE proteins that are freed when action potential-induced Ca2+ influx activates synaptotagmin 1.