Slow‐decaying presynaptic calcium dynamics gate long‐lasting asynchronous release at the hippocampal mossy fiber to CA3 pyramidal cell synapse

Action potentials trigger two modes of neurotransmitter release, with a fast synchronous component and a temporally delayed asynchronous release. Asynchronous release contributes to information transfer at synapses, including at the hippocampal mossy fiber (MF) to CA3 pyramidal cell synapse where it controls the timing of postsynaptic CA3 pyramidal neuron firing. Here, we identified and characterized the main determinants of asynchronous release at the MF–CA3 synapse. We found that asynchronous release at MF–CA3 synapses can last on the order of seconds following repetitive MF stimulation. Elevating the stimulation frequency or the external Ca(2+) concentration increased the rate of asynchronous release, thus, arguing that presynaptic Ca(2+) dynamics is the major determinant of asynchronous release rate. Direct MF bouton Ca(2+) imaging revealed slow Ca(2+) decay kinetics of action potential (AP) burst‐evoked Ca(2+) transients. Finally, we observed that asynchronous release was preferentially mediated by Ca(2+) influx through P/Q‐type voltage‐gated Ca(2+) channels, while the contribution of N‐type VGCCs was limited. Overall, our results uncover the determinants of long‐lasting asynchronous release from MF terminals and suggest that asynchronous release could influence CA3 pyramidal cell firing up to seconds following termination of granule cell bursting.