Nerve terminal probes are a series of fluorescent cationic styryl dyes developed to follow synaptic activity. These dyes have a lipophilic tail at one end and a hydrophilic cationically charged head group at the other end. The dyes vary in the number of carbons in the lipophilic tail (m) and the number of double bonds linking the two aromatic rings in the dye (n). AM dyes are similar to FM dyes, but contain an additional formaldehyde-fixable amino group in the cationic head.

A frequent problem encountered with nerve terminal dyes is background fluorescence due to non-vesicle membrane staining. While most background fluorescence can be removed with repeated washes, background can be a problem with more lipophilic dyes. ADVASEP-7 is a sulfonated beta-cyclodextrin that facilites removal of dye from the cell surface during washing. SCAS reduces background fluorescence without additional washes. Sulforhodamine 101 has been used to quench SynaptoGreen background by fluorescence resonance energy transfer (FRET).

Styryl dyes stain synaptic vesicles in an activity-dependent fashion. The dyes are virtually non-fluorescent in aqueous solution, but become brightly fluorescent upon insertion into the plasma membrane. During endocytosis following neuronal stimulation, the dyes become trapped in vesicles. After free dye is removed from the cell surface by washing, the remaining fluorescence is proportional to the number of newly formed endocytic vesicles. During exocytosis, the dyes are released to the aqueous medium, causing a decrease in fluorescent signal. As a result, the change in fluorescence intensity reflects the amount of endocytosis and exocytosis occuring during synaptic activity. The rate of fluorescence increase during endocytosis (on-rate) and the rate of fluorescence decrease during exocytosis (off-rate) varies from dye to dye. In general, dyes with longer lipophilic tails and more double bonds have a faster on-rate and slower off-rate. AM dyes have an additional amine group and therefore tend to be more water soluble than their FM dye counterparts, resulting in a slower on-rate and faster off-rate.