Nanophysiology Approach Reveals Diversity in Calcium Microdomains in Zebrafish Retinal Bipolar Ribbon Synapses

Rapid and high local calcium (Ca ²⁺ ) signals are essential for triggering neurotransmitter release from presynaptic terminals. In specialized bipolar ribbon synapses in the retina, these local Ca ²⁺ signals control multiple processes, including the priming, docking, and translocation of vesicles on the ribbon before exocytosis and the replenishment of release-ready vesicles to the fusion sites to sustain neurotransmission. However, our knowledge about Ca ²⁺ signals along the axis of the ribbon active zone is limited. Here, we used fast confocal quantitative dual-color ratiometric line-scan imaging of a fluorescently labeled ribbon binding peptide and Ca ²⁺ indicators to monitor the spatial and temporal aspects of Ca ²⁺ transients of individual ribbon active zones in zebrafish retinal rod bipolar cells. We observed that a Ca ²⁺ transient elicited a much greater fluorescence amplitude when the Ca ²⁺ indicator was conjugated to a ribeye-binding peptide than when using a soluble Ca ²⁺ indicator, and the estimated Ca ²⁺ levels at the ribbon active zone exceeded 26 μM in response to a 10-millisecond stimulus, as measured by a ribbon-bound low-affinity Ca ²⁺ indicator. Our quantitative modeling of Ca ²⁺ diffusion and buffering is consistent with this estimate and provides a detailed view of the spatiotemporal [Ca ²⁺ ] dynamics near the ribbon. Importantly, our data demonstrates that the local Ca ²⁺ levels may vary between ribbons of different rod bipolar cells and within the same cells. The variation in local Ca ²⁺ signals is likely due to heterogeneity in local Ca ²⁺ channels clustered at the ribbon active zone, as serial electron microscopy provides new information about the heterogeneity in ribbon size, shape, and area of the ribbon in contact with the plasma membrane.