Cholinergic relay from punishment- to reward-encoding dopamine neurons signals punishment withdrawal as reward in Drosophila

Animals form and update learned associations between otherwise neutral cues and aversive outcomes to predict and avoid danger in changing environments. When a cue later occurs without punishment, this unexpected withdrawal of aversive outcome is encoded as reward, via activation of reward-encoding dopaminergic neurons. Using real-time in vivo functional imaging, optogenetics, behavioral analysis, and electron-microscopy, we identify the neural mechanism through which Drosophila reward-encoding dopaminergic neurons are activated when an olfactory cue is unexpectedly no longer paired with electric shock punishment. Reduced activation of punishment-encoding dopaminergic neurons relieves depression of synaptic inputs to cholinergic neurons, which in turn synaptically increase odor responses of reward-encoding dopaminergic neurons to decrease odor avoidance. These studies reveal for the first time how an indirect excitatory cholinergic synaptic relay from punishment- to reward-encoding dopaminergic neurons encodes the absence of a negative as a positive, revealing a general circuit motif for unlearning aversive memories that could be present in mammals.