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

Area of Science:

• Neuroscience
• Cell Biology
• Biophysics

Background:

• Presynaptic calcium (Ca 2+ ) signals are vital for neurotransmitter release.
• Bipolar ribbon synapses in the retina utilize Ca 2+ signals for vesicle dynamics and sustained neurotransmission.
• Limited understanding exists regarding Ca 2+ signal distribution along the ribbon active zone axis.

Purpose of the Study:

• To investigate the spatial and temporal dynamics of Ca 2+ transients at individual ribbon active zones in zebrafish retinal rod bipolar cells (RBCs).
• To quantify local Ca 2+ levels and their relationship with ribbon structure.

Main Methods:

• Fast confocal quantitative dual-color ratiometric line-scan imaging.
• Utilized a fluorescently labeled ribeye-binding peptide and Ca 2+ indicators.
• Employed quantitative modeling of Ca 2+ diffusion and buffering.
• Serial electron microscopy to analyze ribbon morphology.

Main Results:

• Ca 2+ transients showed significantly higher fluorescence amplitude when Ca 2+ indicators were bound to a ribeye-binding peptide.
• Estimated local Ca 2+ levels at the ribbon active zone exceeded 26 μM with a 10-millisecond stimulus.
• Demonstrated variability in local Ca 2+ levels between and within RBCs.
• Correlated variations in Ca 2+ signals with ribbon size, shape, and active zone extent.

Conclusions:

• Local Ca 2+ dynamics at ribbon active zones are heterogeneous and influenced by ribbon morphology.
• Ribbon size and active zone characteristics contribute to the observed variations in Ca 2+ signaling.
• Provides a detailed spatiotemporal view of Ca 2+ dynamics crucial for neurotransmission at ribbon synapses.