FLIPR(R) membrane potential dye measures changes of charges across the cell membrane, upon activation of SLC6A9. The assay allows the detection of ion channel and transporter modulation by increasing or decreasing the fluorescent signal as cellular membrane potential changes. When cells are depolarized dye enters the cells, causing an increase in fluorescent signal, conversely, cells hyperpolarization results in dye exit and decreased fluorescence. The transport of glycine in glutamatergic synapse is carried out by SLC6A9, a Na+/Cl--dependent carrier molecule. The primary role of SLC6A9 is to maintain glycine concentrations below saturation level at postsynaptic NMDA receptors. It operates bidirectionally: it decreases synaptic glycine concentration when operates in normal mode and releases glycine from glial cells as operates in a reverse mode.