Comparison of dopamine kinetics in the larval Drosophila ventral nerve cord and protocerebrum with improved optogenetic stimulation

Dopamine release and uptake have been studied in the Drosophila larval ventral nerve cord (VNC) using optogenetics to stimulate endogenous release. However, other areas of the central nervous system remain uncharacterized. Here, we compare dopamine release in the VNC and protocerebrum of larval Drosophila. Stimulations were performed with CsChrimson, a new, improved, red light-activated channelrhodopsin. In both regions, dopamine release was observed after only a single, 4ms duration light pulse. Michaelis-Menten modeling was used to understand release and uptake parameters for dopamine. The amount of dopamine released ([DA](p)) on the first stimulation pulse is higher than the average [DA](p) released from subsequent pulses. The initial and average amount of dopamine released per stimulation pulse is smaller in the protocerebrum than in the VNC. The average V-max of 0.08M/s in the protocerebrum was significantly higher than the V-max of 0.05M/s in the VNC. The average K-m of 0.11M in the protocerebrum was not significantly different from the K-m of 0.10M in the VNC. When the competitive dopamine transporter (DAT) inhibitor nisoxetine was applied, the K-m increased significantly in both regions while V-max stayed the same. This work demonstrates regional differences in dopamine release and uptake kinetics, indicating important variation in the amount of dopamine available for neurotransmission and neuromodulation.