FM1-43 is a lipophilic styryl dye, nontoxic to the cells, virtually nonfluorescent in aqueous media. The dye will become fluorescent once it binds to the plasma membrane. In neurons that are actively releasing neurotransmitters, these dyes become internalized within the recycled synaptic vesicles, and the nerve terminals become brightly stained. This shows an increase in the fluorescence intensity of FM1-43. When the cells undergo another stimulation (through KCL or electrical) the FM1-43 dye containing vesicles will be released through exocytosis. Thus, resulting in a decrease in the FM1-43 fluorescence intensity.