Exocytosis in small steps

Neuroendocrine cells have two modes of exocytosis. In one mode, the vesicle fuses to the target membrane, and in the other—referred to as kiss-and-run—the vesicle touches the membrane and creates a small temporary pore. On page 929, Richards et al. show that the small synaptic vesicles in hippocampal neurons also use both mechanisms. The two modes of exocytosis differ in their kinetics and amplitude of the release, and may generate different postsynaptic responses.

To determine whether neurons have two pathways for exocytosis, the team loaded only a fraction of the synaptic vesicles with a fluorescent dye, a trick that allows them to resolve the activity of individual vesicles. They saw two types of exocytic events occurring: fast large releases of dye and slow small ones. The researchers conclude that the fast release events occur...