Pain is so close to pleasure: the same dopamine neurons can mediate approach and avoidance in Drosophila

In mammals, dopamine is considered a central neuromodulator involved in all kinds of rewarding experiences (‘common currency’ hypothesis). In insects, the role of do-paminergic neurons in aversive stimuli was discovered before dopaminergic neurons were found to also be involved in processing appetitive stimuli. Here, we screened about 50 transgenic Drosophila lines, representing different subpopulations of dopa-minergic neurons for their ability to sustain approach or avoidance behavior, when activated optogenetically in four different operant self-stimulation paradigms. None of the lines sustain consistent behavioral valence in all experiments. Individual lines sustain approach in one experiment and avoidance in another. One line mediated strong avoidance early in the experiment and weak approach in later stages. The evidence presented here appears to contradict a ‘common currency’ dopamine function in flies. Instead, different dopaminergic neurons convey valence in a context-dependent and flexible manner, reflecting the genetic heterogeneity of the dopaminergic neuronal population.