UAS regulatory sequences drive expression of a ratiometric pH sensor ("pHusion") that consists of pHluorinSE (pH-sensitive) fused (via a 20 residue RGSASGGGGGLVPRGSASGA linker) to FusionRed (pH-insensitive). The pHusion sensor is targeted to the synaptic cleft due to the presence of an N-terminal secretion signal sequence from IgGκ and a C-terminal transmembrane domain from PDGFR (Tag:TM(hPDGFRB)), which anchors the protein to the membrane and displays it on the extracellular side.