Optogenetic Control of Serotonin and Dopamine Releasein Drosophila Larvae

摘要

Optogenetic control of neurotransmitter release is an elegant method to investigate neurobiological mechanisms with millisecond precision and cell type-specific resolution. Channelrhodopsin-2 (ChR2) can be expressed in specific neurons, and blue light used to activate those neurons. Previously, in Drosophila, neurotransmitter release and uptake have been studied after continuous optical illumination. In this study, we investigated the effects of pulsed optical stimulation trains on serotonin or dopamine release in larval ventral nerve cords. In larvae with ChR2 expressed in serotonergic neurons, low-frequency stimulations produced a distinct, steady-state response while high-frequency patterns were peak shaped. Evoked serotonin release increased with increasing stimulation frequency and then plateaued. The steady-state response and the frequency dependence disappeared after administering the uptake inhibitor fluoxetine, indicating that uptake plays a significant role in regulating the extracellular serotonin concentration. Pulsed stimulations were also used to evoke dopaminerelease in flies expressing ChR2 in dopaminergic neurons and similarfrequency dependence was observed. Release due to pulsed optical stimulationswas modeled to determine the uptake kinetics. For serotonin, Vmax was 0.54 ± 0.07 μM/s and Km was 0.61 ± 0.04 μM; and for dopamine, Vmax was 0.12 ± 0.03 μM/s and Km was 0.45 ± 0.13 μM. The amountof serotonin released per stimulation pulse was 4.4 ± 1.0 nM,and the amount of dopamine was 1.6 ± 0.3 nM. Thus, pulsed opticalstimulations can be used to mimic neuronal firing patterns and willallow Drosophila to be used as a modelsystem for studying mechanisms underlying neurotransmission.