α-Latrotoxin Stimulates a Novel Pathway of Ca2+-Dependent Synaptic Exocytosis Independent of the Classical Synaptic Fusion Machinery

alpha-Latrotoxin induces neurotransmitter release by stimulating synaptic vesicle exocytosis via two mechanisms: (1) A Ca2+-dependent mechanism with neurexins as receptors, in which alpha-latrotoxin acts like a Ca2+ ionophore, and (2) a Ca2+-independent mechanism with CIRL/latrophilins as receptors, in which alpha-latrotoxin directly stimulates the transmitter release machinery. Here, we show that the Ca2+-independent release mechanism by alpha-latrotoxin requires the synaptic SNARE-proteins synaptobrevin/VAMP and SNAP-25, and, at least partly, the synaptic active-zone protein Munc13-1. In contrast, the Ca2+-dependent release mechanism induced by alpha-latrotoxin does not require any of these components of the classical synaptic release machinery. Nevertheless, this type of exocytotic neurotransmitter release appears to fully operate at synapses, and to stimulate exocytosis of the same synaptic vesicles that participate in physiological action potential-triggered release. Thus, synapses contain two parallel and independent pathways of Ca2+-triggered exocytosis, a classical, physiological pathway that operates at the active zone, and a novel reserve pathway that is recruited only when Ca2+ floods the synaptic terminal.