Additive Expression of Consolidated Memory through Drosophila Mushroom Body Subsets.

Associative olfactory memory in Drosophila has two components called labile anesthesia-sensitive memory and consolidated anesthesia-resistant memory (ARM). Mushroom body (MB) is a brain region critical for the olfactory memory and comprised of 2000 neurons that can be classified into αß, α'ß', and γ neurons. Previously we demonstrated that two parallel pathways mediated ARM consolidation: the serotonergic dorsal paired medial (DPM)-αß neurons and the octopaminergic anterior paired lateral (APL)-α'ß' neurons. This finding prompted us to ask how this composite ARM is retrieved. Here, we showed that blocking the output of αß neurons and that of α'ß' neurons each impaired ARM retrieval, and blocking both simultaneously had an additive effect. Knockdown of radish and octß2R in αß and α'ß' neurons, respectively, impaired ARM. A combinatorial assay of radish mutant background rsh1 and neurotransmission blockade confirmed that ARM retrieved from α'ß' neuron output is independent of radish. We identified MBON-ß2ß'2a and MBON-ß'2mp as the MB output neurons downstream of αß and α'ß' neurons, respectively, whose glutamatergic transmissions also additively contribute to ARM retrieval. Finally, we showed that α'ß' neurons could be functionally subdivided into α'ß'm neurons required for ARM retrieval, and α'ß'ap neurons required for ARM consolidation. Our work demonstrated that two parallel neural pathways mediating ARM consolidation in Drosophila MB additively contribute to ARM expression during retrieval.

Mushroom body glycolysis is required for olfactory memory in Drosophila.

Glucose catabolism, also known as glycolysis, is important for energy generation and involves a sequence of enzymatic reactions that convert a glucose molecule into two pyruvate molecules. The glycolysis process generates adenosine triphosphate as a byproduct. In this study, we investigated whether glycolysis plays a role in maintaining neuronal functions in the Drosophila mushroom bodies (MBs), which are generally accepted to be an olfactory learning and memory center. Our data showed that individual knockdown of glycolytic enzymes in the MBs, including hexokinase (HexA), phosphofructokinase (Pfk), or pyruvate kinase (PyK), disrupts olfactory memory. Whole-mount brain immunostaining indicated that pyruvate kinase is strongly expressed in the MB αß, α'ß', and γ neuron subsets. We conclude that HexA, Pfk, and PyK are required in each MB neuron subset for olfactory memory formation. Our data therefore indicates that glucose catabolism in the MBs is important for olfactory memory formation in Drosophila.