We also cultured pinealocytes, which were harvested from pineal gland of rats. Upon exposure to norepinephrine, these cells produce melatonin from Trp in the surrounding media. In this protocol, baseline recordings were done for 5 min, then either 5 mL water or 5 mL water with 1 mM norepinephrine (NE) were added to the pinealocytes near the electrode. The current densities corresponding to Trp oxidation was then plotted against time for control group (water injection) and experimental group (NE). The binned data with 1 min time resolution is shown in Figure . Since the Trp dynamics in this particular system was slower than our time resolution of 100 ms, we binned the data further (7 min bins) to show the dynamics more clearly (Figure ). We calculated the difference in current density at each time point between the control and NE group and then normalized as % control (water). As seen in Figure , there is a significant decrease in Trp at time points 28 min (p = 0.0153) and 35 min (p = 0.0070) compared to the beginning of the experiment. This makes sense as the cells exposed to NE were stimulated to make melatonin from Trp, thereby decreasing the amount of Trp in the media in the process. It also makes sense that this process takes some time as the Trp would need to transported into the cells via transporters before melatonin production can begin. We performed linear regression to demonstrate these trends in a more quantitative manner. The linear regression for control group (blue) was y = (0.0010± 0.0009)*X + (0.05 ± 0.02), with a positive slope. The linear regression for the NE group (pink) was y = (-0.0007593 +/-0.0009)*X +(0.08 +/-0.02), with an overall negative slope, in agreement with our conclusion that Trp levels were lower in pinealocytes stimulated with NE. Collectively, these results demonstrate that our method was capable of measuring endogenous Trp dynamics as well.