Homosensory and heterosensory dishabituation engage distinct circuits in Drosophila

Habituation, the adaptive reduction of responsiveness to repetitive inconsequential stimuli and dishabituation, the reinstatement of the naive reaction after exposure to a potent novel stimulus are conserved fundamental neuroplasticity processes thought to underlie salience filtering and selective attention. Dishabituation is routinely used to differentiate bona fide habituation from fatigue. However, the mechanisms engaged to drive dishabituation remain largely debatable as to whether the novel dishabituating stimulus elicits sensitization of the habituated circuits, or it engages distinct neuronal routes to bypass habituation reinstating the naive response. Using the established olfactory habituation paradigm in Drosophila, we examined whether dishabituators of the same sensory modality as the habituated stimulus (homosensory) or of a different one (heterosensory) engage distinct, converging, or the same neuronal circuits to reinstate the naive avoidance response following habituation to an odorant (3-octanol). We demonstrate that dopaminergic inputs via the PAM and PPL1 clusters differentiate the homosensory and heterosensory dishabituators respectively, converge onto and recruit the Mushroom Bodies (MBs) into a distinct dishabituation circuit than that driving habituation, which is MB-independent. GABAergic neurotransmission to the MB from APL neurons is specifically required for homosensory dishabituation, likely to differentiate the olfactory habituating and dishabituating stimuli. Dishabituator-driven activation of the MBs elicits neurotransmission from their γ neurons onto GABAergic MB-output neurons (MBONS), relaying signals to bypass the inhibition of OCT avoidance established in the Lateral Horn (LH) by habituation. Because distinct circuitries are engaged for habituation and dishabituation, our results strongly suggest that both homosensory and heterosensory stimuli drive dishabituation by recruiting habituation-independent circuitry to modulate the activity of the LH and reinstate the naive response.