Single-Pixel Optical Fluctuation Analysis of Calcium Channel Function in Active Zones of Motor Nerve Terminals

摘要

We used high-resolution fluorescence imaging and single-pixel optical fluctuation analysis to estimate the opening probability of individual voltage-gated calcium (Ca²⁺) channels during an action potential and the number of such Ca²⁺ channels within active zones of frog neuromuscular junctions. Analysis revealed ∼36 Ca²⁺ channels within each active zone, similar to the number of docked synaptic vesicles but far less than the total number of transmembrane particles reported based on freeze-fracture analysis (∼200–250). The probability that each channel opened during an action potential was only ∼0.2. These results suggest why each active zone averages only one quantal release event during every other action potential, despite a substantial number of docked vesicles. With sparse Ca²⁺ channels and low opening probability, triggering of fusion for each vesicle is primarily controlled by Ca²⁺ influx through individual Ca²⁺ channels. In contrast, the entire synapse is highly reliable because it contains hundreds of active zones.