The FM™ 2-10 membrane probe is an excellent reagent both for identifying actively firing neurons and for investigating the mechanisms of activity-dependent vesicle cycling. This water-soluble dye, which is nontoxic to cells and virtually nonfluorescent in aqueous medium, is believed to insert into the outer leaflet of the cell membrane where it becomes intensely fluorescent. In a neuron that is actively releasing neurotransmitters, the dye becomes internalized within the recycled synaptic vesicles and the nerve terminals become brightly stained.