Analyse Intégrée de la Dynamique des Phases de Mémoire Olfactive chez la Drosophile – MemoDynam

How are memories encoded in the brain? How are memory centers organized? Recent models propose that anatomically distinct systems coexist, each storing specific aspects of memories. These systems are not independent, but rather interact with each other dynamically. The major challenge faced by neuroscientists studying memory is to define the links between the various levels of nervous system organization, starting with molecules and cells, and then neuronal circuits, and finally the global cognitive functions of the brain. Drosophila has many advantages for the study of brain function: its brain is small (100 000 cells) yet highly structured, and genetic tools allow us to manipulate gene expression in discrete neuronal assemblies while recording behavior. We use this power to perform an integrated analysis of the processes underlying associative learning and memory, studying not only the activity of individual genes and proteins, but also the behavior of cells, neural networks, and of the fly itself. In the past few years our team has made important discoveries as we have identified a long-term memory center (Pascual and Preat, 2001 Science), and shown that the two forms of Drosophila consolidated memory, anesthesia-resistant memory (ARM) and long-term memory (LTM) are exclusive (Isabel et al., 2004 Science). We have also characterized several genes involved in LTM formation and whose expression is regulated after conditioning (Comas et al., 2004 Nature; Didelot et al., 2006 Science). At last, we have shown that the Drosophila brain is asymmetric and that this asymmetry is necessary for LTM (Pascual et al., 2004 Nature). The present project aims at further integrating the various levels involved in memory formation, consolidation and retrieval, using in particular in vivo brain imaging. We have five major goals: 1) We have shown recently that dopamine plays a key role in the transformation of short-term memory into long-term memory (Isabel et al., unpublished). We will further analyze how dopamine regulates the activity of the mushroom bodies, the olfactory memory center. In particular we will analyze how dopamine and the DAMB receptor control the transition between short-term and LTM. A proteic probe has been introduced in Drosophila that will be used to record in vivo the activity of the protein kinase A, a downstream effector of the DAMB receptor. 2) The identification of new ARM mutants should help understanding how the two consolidated memories interact. A recent behavioral screen failed to produce bona fide ARM mutants that are affected only for this memory phase. We will perform a new and original screening to identify new genes involved in ARM formation. The success of this approach would contribute importantly to our understanding of memory dynamics, showing that indeed ARM acts as a gating mechanism that controls the transition between short-term and LTM. 3) We have accomplished a microarray analysis of the transcriptome after LTM conditioning. A short list of 112 genes has been generated. We will pursue the characterization of several pathways involved in LTM. In particular we will quantify gene expression in the brain of naïve and trained flies, and analyze the LTM of flies after adult inhibition of gene expression by RNA interference. 4) We plan to use fluorescent calcium probes to image the changes of brain activity linked to memory formation. Our team has acquired a two-photon microscope that allows precise imaging of brain circuits using genetically-encoded probes. We plan to further study how the different memory networks interact by imaging the activity of one neuronal ensemble while blocking the activity of another neuronal circuit with a thermosensitive inhibitory protein. We will investigate in particular the function of three newly identified neuronal ensembles that connect to the mushroom bodies and that are involved in LTM retrieval. 5) Formation of aversive LTM requires multiple presentations of the odorant paired with electric shocks. On the contrary we have shown recently that appetitive LTM is generated after a single training session. We will further compare the dynamics of aversive and appetitive memory phases, using memory mutants and calcium imaging. All together this program should have a major impact on our understanding of the dynamics of memory phase in Drosophila, and provide a link between the molecular and behavioral levels.