alpha-latrotoxin, acting via two Ca2+-dependent pathways, triggers exocytosis of two pools of synaptic vesicles

alpha -Latrotoxin stimulates three types of [H-3]gamma -aminobutyric acid a nd [C-14]glutamate release from synaptosomes. The Ca2+-independent componen t (i) is insensitive to SNAP-25 cleavage or depletion of vesicle contents b y bafilomycin A1 and represents transmitter efflux mediated by alpha -latro toxin pores. Two other components of release are Ca2+-dependent and vesicul ar but rely on distinct mechanisms. The fast receptor-mediated pathway (ii) involves intracellular Ca2+ stores and acts upon sucrose-sensitive readily releasable vesicles; this mechanism is insensitive to inhibition of phosph atidylinositol 4-kinase (PI 4-kinase). The delayed pore-dependent exocytoti c component (iii) is stimulated by Ca2+ entering through alpha -latrotoxin pores; it requires PI 4-kinase and occurs mainly from depot vesicles. Lanth anum perturbs alpha -latrotoxin pores and blocks the two pore-mediated comp onents (i, iii) but not the receptor-mediated release (ii). alpha -Latrotox in mutant (LTXN4C) cannot form pores and stimulates only the Ca2+-dependent receptor-mediated amino acid exocytosis (ii) (detectable biochemically and electrophysiologically). These findings explain experimental data obtained by different laboratories and implicate the toxin receptors in the regulat ion of the readily releasable pool of synaptic vesicles. Our results also s uggest that, similar to noradrenergic vesicles, amino acid-containing vesic les at some point in their cycle require PI 4-kinase.