CHARACTERIZATION OF FLUO-3 LABELING OF DENSE BODIES AT THE HAIR-CELLSPRESYNAPTIC ACTIVE ZONE

The presynaptic active zone is the critical region of a chemical synap se at which Ca2+ entry provokes neurotransmitter release by exocytotic fusion of synaptic vesicles. To facilitate investigations of synaptic function, we have identified a group of fluorescent substances that l abel individual active zones in living hair cells. The Ca2+ indicator fluo-3, the compound studied in most detail, binds to the presynaptic dense bodies that are characteristic of active zones in hair cells and other cells that tonically release transmitter. The indicator's bindi ng is reversible, with a dissociation constant of approximately 350 mu M. Because fluo-3 that is bound to a presynaptic dense body continues to detect Ca2+ with an unaltered dissociation constant, the binding o f this substance provides a valuable tool for exploration of the Ca2concentration at the site of vesicle fusion.