Activity-dependent release is the typical action potential–generating mechanism. But occasionally a vesicle leaks its contents without provocation. Most scientists figured these events—which affect synaptic development and inhibit translation in dendrites—reflect the occasional escape of a vesicle primed for activity-dependent release. But the new results reveal that spontaneously released vesicles comprise a pool of their own.
The two pools were distinguished by their filling mechanism: vesicles loaded with dyes by spontaneous endocytosis were then unloaded more rapidly by spontaneous release than by stimulated release. Activity-dependent endocytosis filled vesicles that were more rapidly unloaded by stimulated release. Blocking neurotransmitter refilling into vesicles at rest only affected spontaneous release, suggesting that the pools do not intermix.
Spontaneous vesicles may be defective in fusion yet occasionally fuse where and when they should not. The two pools looked the same by EM and were similarly localized, so the differences probably lie in lipid or protein content. How the differences originate is unclear. “There could be two recycling pathways” such as local and endosomal routes, says Kavalali. “Or [the spontaneous pool] might just be the use-dependent accumulation of defective vesicles over time.”