Sr²⁺ has low efficiency in regulating spontaneous release at the Calyx of Held synapses

It has been known that Ca²⁺ 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 Ca²⁺ sensors for synchronous release and synaptotagmin 7 and Doc-2 were proposed as the Ca²⁺ sensors for asynchronous release. Comparatively, the Ca²⁺ sensor for spontaneous release remains a mystery. At the Calyx of Held synapse, the Ca²⁺ sensor for spontaneous release was found not identical to the sensor for synchronous release, synaptotagmin 2. As Ca²⁺ sensors have different sensitivity to Sr²⁺ and Ca²⁺ and induce significantly different rate of vesicle release, Sr²⁺ is traditionally used as a tool to examine the intrinsic properties of different Ca²⁺ 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 Sr²⁺ and Ca²⁺ influx into the nerve terminal at resting potential and observed the effects of Ca²⁺ and Sr²⁺ on spontaneous neurotransmitter release. We found that the dwell time of single voltage gated Ca²⁺ channel opening increased around threefold for Sr²⁺ than Ca²⁺ with the channel conductance unchanged; the divalent cation sensing machinery in regulating spontaneous release has much lower sensitivity to Sr²⁺ than Ca²⁺ . Thus, our study reveals some of the intrinsic properties of Ca²⁺ sensor(s) of spontaneous transmitter release and provided an insight into the underlying mechanisms.