Data from: Nanophysiology approach reveals diversity in calcium microdomains across Zebrafish retinal bipolar ribbon synapses

Rapid and high local calcium (Ca2+) signals are essential for triggering
neurotransmitter release from presynaptic terminals. In specialized
bipolar ribbon synapses of the retina, these local Ca2+ signals control
multiple processes, including the priming, docking, and translocation of
vesicles on the ribbon before exocytosis, endocytosis, and the
replenishment of release-ready vesicles to the fusion sites for sustained
neurotransmission. However, our knowledge about Ca2+ 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 Ca2+ indicators to monitor the spatial
and temporal aspects of Ca2+ transients of individual ribbon active zones
in zebrafish retinal rod bipolar cells (RBCs). We observed that a Ca2+
transient elicited a much greater fluorescence amplitude when the Ca2+
indicator was conjugated to a ribeye-binding peptide than when using a
soluble Ca2+ indicator, and the estimated Ca2+ levels at the ribbon active
zone exceeded 26 μM in response to a 10-millisecond stimulus, as measured
by a ribbon-bound low-affinity Ca2+ indicator. Our quantitative modeling
of Ca2+ diffusion and buffering is consistent with this estimate and
provides a detailed view of the spatiotemporal [Ca2+] dynamics near the
ribbon. Importantly, our data demonstrates that the local Ca2+ levels may
vary between ribbons of different RBCs and within the same cells. The
variation in local Ca2+ signals is found to correlate with ribbon size and
active zone extent. Our serial electron microscopy results provide new
information about the heterogeneity in ribbon size, shape, and area of the
ribbon in contact with the plasma membrane.