Calcium dependence of neurotransmitter release at a high fidelity synapse

Abstract The Ca2+-dependence of the recruitment, priming, and fusion of synaptic vesicles are fundamental parameters controlling neurotransmitter release and synaptic plasticity. Despite intense efforts, these important steps in the synaptic vesicles’ cycle remain poorly understood because disentangling recruitment, priming, and fusion of vesicles is technically challenging. Here, we investigated the Ca2+-sensitivity of these steps at cerebellar mossy fiber synapses, which are characterized by fast vesicle recruitment mediating high-frequency signaling. We found that the basal free Ca2+ concentration (<200 nM) critically controls action potential-evoked release, indicating a high-affinity Ca2+ sensor for vesicle priming. Ca2+ uncaging experiments revealed a surprisingly shallow and non-saturating relationship between release rate and intracellular Ca2+ concentration up to 50 μM. Sustained vesicle recruitment was Ca2+-independent. Finally, quantitative mechanistic release schemes with five Ca2+ binding steps incorporating rapid vesicle recruitment via parallel or sequential vesicle pools could explain our data. We thus show that co-existing high and low-affinity Ca2+ sensors mediate recruitment, priming, and fusion of synaptic vesicles at a high-fidelity synapse.