Highlights d Synaptic microglomeruli linked to a specific odor can be identified in Drosophila d Microglomeruli represent complex microcircuits involving different types of neurons d Long-term memory results in increased microglomeruli in an input-specific manner d Newly formed microglomeruli participate in conditioned odor representation

The capacity of utilizing past experience to guide future action is a fundamental and conserved function of the nervous system. Associative memory formation initiated by the coincident detection of a conditioned stimulus (CS, e.g. odour) and an unconditioned stimulus (US, e.g. sugar reward) can lead to a short-lived memory trace (STM) within distinct circuits. Memories can be consolidated into long-term memories (LTM) through processes that are not fully understood, but depend on de-novo protein synthesis, require structural modifications within the involved neuronal circuits and might lead to the recruitment of additional ones. Compared to modulation of existing connections, the reorganization of circuits affords the unique possibility of sampling for potential new partners. Nonetheless, only few examples of rewiring associated with learning have been established thus far. Here, we report that memory consolidation is associated with the structural and functional reorganization of an identified circuit in the adult fly brain. The formation and retrieval of olfactory associative memories in Drosophila requires the mushroom body (MB). We identified the individual synapses of olfactory projection neurons (PNs) that deliver a conditioned odour to the MB and reconstructed the complexity of the microcircuit they form. Combining behavioural experiments with high-resolution microscopy and functional imaging, we demonstrated that the consolidation of appetitive olfactory memories closely correlates with an increase in the number of synaptic complexes formed by the PNs that deliver the conditioned stimulus and their postsynaptic partners. These structural changes result in additional functional synaptic connections.

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