Sr has low efficiency in regulating spontaneous release at the Calyx of Held synapses.

It has been known that Ca plays an essential role in mediating different modes of neurotransmitter release via different sensing mechanisms. Synaptotagmin 1, 2, and 9 were found to act as the Ca sensors for synchronous release and synaptotagmin 7 and Doc-2 were proposed as the Ca sensors for asynchronous release. Comparatively, the Ca sensor for spontaneous release remains a mystery. At the Calyx of Held synapse, the Ca sensor for spontaneous release was found not identical to the sensor for synchronous release, synaptotagmin 2. As Ca sensors have different sensitivity to Sr and Ca and induce significantly different rate of vesicle release, Sr is traditionally used as a tool to examine the intrinsic properties of different Ca sensors. Here, we employed cell-attached patch recording and presynaptic/postsynaptic whole-cell recording at the Calyx of Held synapses of synaptotagmin 2 knock-out mice to assay the Sr and Ca influx into the nerve terminal at resting potential and observed the effects of Ca and Sr on spontaneous neurotransmitter release. We found that the dwell time of single voltage gated Ca channel opening increased around threefold for Sr than Ca with the channel conductance unchanged; the divalent cation sensing machinery in regulating spontaneous release has much lower sensitivity to Sr than Ca . Thus, our study reveals some of the intrinsic properties of Ca sensor(s) of spontaneous transmitter release and provided an insight into the underlying mechanisms.