FLIPR(R) membrane potential dye measures changes of charges across the cell membrane, upon activation of SLC6A5. 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. SLC6A5 is a transporter protein of the SLC6 family of sodium- and chloride-dependent neurotransmitter transporters. It regulates synaptic glycine concentrations. Glycine transport by SLC6A5 is coupled to the electrochemical movement of three Na+, favoring the maintenance of a high glycine concentration gradient along the presynaptic membrane.