Modified Trans-synaptic Tracing Viruses Reveal Novel Connectivity in the Rat Olfactory Bulb

Olfaction and the olfactory pathway is a good system to study sensory processing in general, mainly because of the simplified organization of the cells constituting the olfactory bulb, the anatomical structure responsible for early odor encoding and processing. Olfaction is also a good model system, because the connections between olfactory neurons directly influence the functions performed by the network they are part of. In this thesis, we choose a particular kind of virus, the pseudorabies virus (PRV) as trans-synaptic tracer to understand the synaptic connections made within the olfactory bulb (OB) as well as between the OB and the olfactory cortex. By using PRV strains differing in their directions of propagation across the synapses and by genetically modifying them, we are able to highlight different anatomical and functional characteristics of the OB circuit. First, we confirm the columnar organization of the granule cells (GCs) present in the granule cell layer (GCL), previously demonstrated by our research group. Then, we show the extent in the GCL of the axon collateral network of the OB principal cells, mitral and tufted cells (MCs and TCs) and its targets: GCs columns but also neurons morphologically different than GCs known as short-axon cells, that we further try to characterize. Based on morphological information as well as electrical and protein expression profiles we conclude to a high level of heterogeneity amongst the short-axon population. Finally, we confirm the presence of projections from anterior piriform cortex (aPCX) pyramidal cells onto GCs columns as well as onto a short-axon cells population.