For the study of exo- and endocytosis in neurons from the Central Nervous System (CNS) we use synaptic proteins tagged with Green Fluorescence Protein (GFP). Because of the difficulties to apply capacitance methods to monitor exo- and endocytosis at the level of single vesicles we use SynaptopHluorin (SpH) (Miesenbock et al, Nature 394: 192-5, 1998). This protein is obtained by tagging GFP to the carboxi terminal of synaptobrevin. This makes the GFP to be sequestred inside synaptic vesicles

This scheme shows the principle behind SpH and its application in the study of synaptic activity.  The acidic interior of the vesicle (pH = 5.5) leads to a quenched SpH molecule. Upon fusion, protons scape to the extracellular medium,  balancing the intravesicular pH to the external pH (7.4). Sph are unquenched and light up. On fusion pore reclosure or endocytosis teaking place somewhere in the terminal, SpH are reacidified by the proton pump, giving rise again to a SpH quenched molecule. The rapid lighting up of SpH contrats to the slower reacidification process. With the method, aspects of endocytosis that remain silent when using cell membrana capacitance measurements, are reveal nicely by this method.