Revisiting the role of cAMP in Drosophila aversive olfactory memory formation

Abstract

In aversive olfactory conditioning in Drosophila melanogaster, an odor (the conditioned stimulus, CS) is paired with an electric shock (the unconditioned stimulus, US). Rutabaga adenylyl cyclase, which synthesizes cAMP in Kenyon cells (KCs), has long been proposed to act as a coincidence detector, synergistically activated by calcium/calmodulin in response to odor-evoked KC activity and by Gs downstream of shock-evoked dopamine signaling. This framework implies that CS+ responsive KCs, which receive both CS and US signals, should be distinguishable from the remaining KCs by elevated cAMP levels, such that elevated cAMP would selectively mark these KCs for memory-related plasticity. However, our live imaging revealed that olfactory stimulation alone activates dopaminergic signaling and elevates cAMP broadly across KC axons, with only weak spatial correspondence to sparse local Ca2+ responses, such that CS+ responsive KCs could not be distinguished from the remaining KCs by bulk cAMP levels. This observation challenges a key prediction of the Rutabaga coincidence-detection model. Our findings instead suggest that cAMP concentrations do not selectively mark CS+ neurons. Rather, we propose that the compartmental population dynamics of cAMP across the {gamma} lobe represent a KC-intrinsic expression of valence, integrated across compartments from multiple dopaminergic inputs. Associative conditioning reshaped dopamine responses to the CS+ odor, with corresponding changes in this compartmental cAMP pattern. Simultaneous imaging of cAMP and acetylcholine (ACh) revealed that conditioning establishes an inverse relationship between this pattern and ACh release for the CS+ but not for the CS- odors; this relationship was absent in rutabaga mutants. Pharmacological elevation of cAMP suppressed ACh release from KCs. These findings suggest that cAMP-dependent suppression of ACh release from KCs may shift the valence of the CS+ odor toward a more aversive state in conditioned flies, thereby contributing to aversive olfactory memory formation.