Efficient stimulus-secretion coupling at ribbon synapses requires RIM-binding protein tethering of L-type Ca channels.

Fast neurotransmitter release from ribbon synapses via Ca-triggered exocytosis requires tight coupling of L-type Ca channels to release-ready synaptic vesicles at the presynaptic active zone, which is localized at the base of the ribbon. Here, we used genetic, electrophysiological, and ultrastructural analyses to probe the architecture of ribbon synapses by perturbing the function of RIM-binding proteins (RBPs) as central active-zone scaffolding molecules. We found that genetic deletion of RBP1 and RBP2 did not impair synapse ultrastructure of ribbon-type synapses formed between rod bipolar cells (RBCs) and amacrine type-2 (AII) cells in the mouse retina but dramatically reduced the density of presynaptic Ca channels, decreased and desynchronized evoked neurotransmitter release, and rendered evoked and spontaneous neurotransmitter release sensitive to the slow Ca buffer EGTA. These findings suggest that RBPs tether L-type Ca channels to the active zones of ribbon synapses, thereby synchronizing vesicle exocytosis and promoting high-fidelity information transfer in retinal circuits.