The differential requirement of mushroom body α/β subdivisions in long-term memory retrieval in Drosophila

The mushroom body (MB), a bilateral brain structure possessing about 2000-2500 neurons per hemisphere, plays a central role in olfactory learning and memory in Drosophila melanogaster. Extensive studies have demonstrated that three major types of MB neurons (alpha/beta, alpha'/beta' and I") exhibit distinct functions in memory processing, including the critical role of approximately 1000 MB alpha/beta neurons in retrieving long-term memory. Inspired by recent findings that MB alpha/beta neurons can be further divided into three subdivisions (surface, posterior and core) and wherein the alpha/beta core neurons play an permissive role in long-term memory consolidation, we examined the functional differences of all the three morphological subdivisions of MB alpha/beta by temporally precise manipulation of their synaptic outputs during long-term memory retrieval. We found the normal neurotransmission from a combination of MB alpha/beta surface and posterior neurons is necessary for retrieving both aversive and appetitive long-term memory, whereas output from MB alpha/beta posterior or core subdivision alone is dispensable. These results imply a specific requirement of about 500 MB alpha/beta neurons in supporting long-term memory retrieval and a further functional partitioning for memory processing within the MB alpha/beta region.