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Investigating innate valence signals in Drosophila: Probing dopaminergic function of PPM2 neurons with optogenetics

The goal of this study was to investigate the role of dopamine in reward and punishment processing in the fly brain, and to identify specific neuronal populations mediating these functions.
We screened over 50 transgenic Drosophila lines, each labeling distinct subsets of dopaminergic neurons, for their ability to
sustain approach or avoidance of optogenetic activation in three paradigms: the Joystick, T-Maze, and Y-Maze. Only three lines
produced consistent results across all assays, including TH-D′, which labels dopaminergic neurons in the PPL1-FB, PPM2, and PPM3 clusters.
The development of refined split-GAL4 lines allowed us to target individual subpopulations. Using TH-C-AD; TH-D-DBD, which drives expression in the PPM2 cluster, we examined their role in optogenetic preference. Activation of PPM2 neurons produced a response similar to TH-D′: flies initially avoided stimulation but gradually developed a preference during later training trials. This effect was stronger in THD′ flies but followed the same trend in PPM2-specific flies.
In contrast, activation of PPL1-FB or PPM3 neurons did not alter preference (data not shown). To further assess reproducibility,
PPM2-specific flies expressing CsChrimson were also tested in the T-Maze.