Synapto-pHluorin is a genetically encoded optical indicator of vesicle release and recycling.
When vesicles get released, synapto-pHluorin is exposed to the neutral extracellular space and the presynaptic terminal becomes brightly fluorescent.
Synapto-pHluorin sometimes consists of yellow fluorescent protein (YFP) to monitor the cytoplasm because its pKa is higher than GFP (7.1 versus 6.0).
[3] In 2013, a two-color release sensor (ratio-sypHy) was introduced to determine the size of the recycling pool at individual synapses.
[4] Synapto-pHluorin is mainly used by neurobiologists to study transmitter release and recycling at presynaptic terminals.