First, we cultured two types of cells: PC12 and PC12 with an overexpression of tryptophan hydroxylase 2 (TPH2). TPH is the rate limiting enzyme in the synthesis of serotonin from Trp and is localized intracellularly. While TPH1 is expressed primarily in the pineal gland and the periphery, TPH2 is expressed mainly in serotonergic neurons in the brain . We positioned a CFM in the extracellular media surrounding the cells (with a pseudo-Ag/AgCl in the same well). We then exogenously added Trp We observed a large range of maximum current densities in the individual recordings even within the same well plate of cells. This can be rationalized by the fact that the current densities depend on: 1) how close the CFMs are positioned near the cell membranes; and 2) how close the Trp is applied near the CFMs. In order to quantitatively compare between the different recordings, we normalized each current density vs. time profile to the maximum signal from the exogenously added Trp (excluding higher currents that were due to mechanical noise of adding the Trp itself, usually present at the time of addition). As seen in Figure , the normalized data revealed two distinct profiles. The general shape of the time profiles is similar for both types of PC-12 cells: there is an initial increase due to the application of the exogenous Trp and then a subsequent decay. Both decay curves should include contribution from mass transport (i.e. diffusion) and possibly from the activity of any membrane-bound transporters. Our data shows a slower decay curve for those Trp measurements done with PC-12 cells with TPH2(+) than their wildtype counterpart. Assuming that the Trp decay curves in both should experience similar contributions from mass transport, then the discrepancy between the two could differences in transport activity that internalize amino acids such as Trp into the cells. In fact, these data are incredibly reproducible, as indicated by the relatively small error bars (±SEM) at each time point (pale blue for PC-12 with TPH2(+), pale green for PC-12 wildtype). Wilcoxon t-test results revealed a significant difference between the two decay profiles (****p<0.0001). A speculative hypothesis to explain these data is that the PC-12 cells with TPH2(+) may have lower expression of amino acid transporters . This hypothesis is supported by the following evidence. First, PC-12 cells are suspended cells and are presumably more rapidly dividing than PC-12 with TPH2(+), which are adherent cells. LAT1, L-type amino acid transporter 1, is the transporter protein responsible for shuttling Trp across the membrane and has been linked to cell proliferation . It is possible that indeed PC12 with TPH2(+) has lower LAT1 activity than PC-12 wildtype. More experiments will be needed to evaluate and test this hypothesis. Nevertheless, these recordings demonstrate the utility of our optimized method for measuring and differentiating Trp dynamics (from an exogenous source) between two cell lines with good spatiotemporal resolution.