Imaging of an Early Memory Trace in theDrosophilaMushroom Body

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Abstract
Extensive molecular, genetic, and anatomical analyses have suggested that olfactory memory is stored in the mushroom body (MB), a higher-order olfactory center in the insect brain. The MB comprises three subtypes of neurons with axons that extend into different lobes. A recent functional imaging study has revealed a long-term memory trace manifested as an increase in the Ca2+activity in an axonal branch of a subtype of MB neurons. However, early memory traces in the MB remain elusive. We report here learning-induced changes in Ca2+activities during early memory formation in a different subtype of MB neurons. We used three independentin vivoandin vitropreparations, and all of them showed that Ca2+activities in the axonal branches of α′/β′ neurons in response to a conditioned olfactory stimulus became larger compared with one that was not conditioned. The changes were dependent on proper G-protein signaling in the MB. The importance of these changes in the Ca2+activity of α′/β′ neurons during early memory formation was further tested behaviorally by disrupting G-protein signaling in these neurons or blocking their synaptic outputs during the learning and memory process. Our results suggest that increased Ca2+activity in response to a conditioned olfactory stimulus may be a neural correlate of early memory in the MB.