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.
Instead, different dopaminergic neurons convey valence in a context-dependent and flexible manner, reflecting the genetic heterogeneity of the dopaminergic neuronal population. Motivation, attention and value are central for adaptive behavioral choice and, hence, evolutionary fitness.
The literature on dopamine function suggests a substantial and evolutionary conserved role of dopamine neurons DANs in all three processes. The function of DANs in valuation, in terms of reward and punishment, has been especially prominent, in particular with regard to clinical aspects of tolerance and addiction to drugs of abuse.
In vertebrates, midbrain DANs have been reported to encode reward prediction errors in classical conditioning experiments Bayer and Glimcher, ; Schultz, , , ; Schultz et al. Drugs of abuse primarily target this dopaminergic system and promote debilitating loss of behavioral control once an addiction has been established. Also in flies, dopamine neurons have been reported to be involved in valuation processes in classical conditioning experiments.
