Decreased odor avoidance after electric shock in Drosophila mutants biases learning and memory tests

The Drosophila mutants amnesiac, dunce (dnc), and rutabaga were isolated after associative conditioning tests, during which animals were trained to associate the presence of an odor with that of electric shocks (ES). In the absence of conditioning, the odor avoidance (OA) of these mutants was shown to be normal, indicating that their poor associative conditioning performance was attributable to specific learning or memory deficits. However, I show that the OA of the mutants is greatly decreased after their exposure to ES. This effect can last for hours. These results strongly suggest that part of the defect displayed by these mutants in associative conditioning tests does not correspond to a learning or memory deficit but might arise from abnormal sensitivity to stressful stimuli. I looked at the OA after ES of two previously characterized dnc mutants. Df(1)N79f specifically decreases Dnc expression in the mushroom bodies, leading to a normal level of learning but decreased memory. Df(1)N79f mutants displayed a normal OA after ES. Df(1)N64j15 affects the entire brain expression of Dnc, leading to decreased learning and memory. Df(1)N64j15 animals showed a strong decrease of their OA after ES. Thus, the lack of Dnc "general" expression is most likely responsible for the OA defect, which would be responsible for the apparent learning defect after conditioning. In contrast, the Dnc phosphodiesterase accumulated in the mushroom bodies would be involved specifically in memory formation.