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 alpha beta, alpha'beta', and gamma neurons. Previously we demonstrated that two parallel pathways mediated ARM consolidation: the serotonergic dorsal paired medial (DPM)-alpha beta neurons and the octopaminergic anterior paired lateral (APL)-alpha'beta' neurons. This finding prompted us to ask how this composite ARM is retrieved. Here, we showed that blocking the output of alpha beta neurons and that of alpha'beta' neurons each impaired ARM retrieval, and blocking both simultaneously had an additive effect. Knockdown of radish and oct beta 2R in alpha beta and alpha'beta' neurons, respectively, impaired ARM. A combinatorial assay of radish mutant background rsh(1) and neurotransmission blockade confirmed that ARM retrieved from alpha'beta' neuron output is independent of radish. We identified MBON-beta 2 beta'2a and MBON-beta'2mp as the MB output neurons downstream of alpha beta and alpha'beta' neurons, respectively, whose glutamatergic transmissions also additively contribute to ARM retrieval. Finally, we showed that alpha'beta' neurons could be functionally subdivided into alpha'beta'm neurons required for ARM retrieval, and alpha'beta'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.