Structural and functional plasticity associated with memory consolidation in the mushroom body calyx

A major task for the nervous system is to encode and process the flow of information from the environment, which is used to define behaviour and can be stored in lasting memories. A conserved way to filter and extract essential cues involves the generation of sparse representations. The insect mushroom body (MB) is a brain neuropil involved in associative memory and a model for sparse coding. Here, olfactory information is delivered from projection neurons (PNs) to the Kenyon cells (KCs) of the MB. Within the MB input region, the calyx, PN axonal boutons contact multiple claw-like dendrite termini of KCs, forming microglomeruli (MGs). As part of a collaboration, we utilized the reconstruction of an entire adult fly brain at the EM level generated in the lab of Dr. Bock (Janelia research Campus, HHMI), to produce a first detailed 3D reconstruction of a calycal MG, including the identification of all the neurons contributing to it. This high-resolution description of the MG microcircuit stirred a number of questions, relevant to sparse coding and to the consolidation of memories, that we now intend to address in the present proposal. In particular, we will investigate whether MGs represent computational units and whether MGs might be functionally organized in the calyx in larger units. Further, we will investigate the topology of modulatory inputs in the calyx and the detailed role of inhibitory circuits. These experiments will reveal whether and how local microcircuits contribute to sparse representation of olfactory information in the MB. Recently, we have revealed that MGs undergo structural modifications during memory consolidation. We now propose to investigate whether the structural changes that we described correspond to modified functional responses in the MGs. These experiments will provide important insights into the role of the MB input compartment in odour processing during memory consolidation. On the basis of the structural and functional analysis of the MGs, we will then start investigating the cell biological mechanisms that underlie structural MG remodelling in the adult calyx. On the long-term, these latter experiments will potentially provide the tools to ask direct cause-effect relationships between structural modifications and aspects of memory consolidation at identified synapses.