Search

The neocortex is the most distinctive feature of the mammalian brain and it is considered to be the substrate of high-order cognitive functions. The nature and the arrangement of the diverse neuronal elements constituting its multiple areas have received longstanding attention. Progressively such anatomical and functional investigations are undertaken in the context of an intact living being. It offers the possibility of examining the role of particular areas, networks or even individual neurons during various behavioral states. Synaptic connectivity and synaptic transmission have been traditionally investigated in reduced preparations. Typically, electrophysiological and optical techniques have been used to control and record the propagation of electrical activity between two or more neurons in acute brain slices in vitro. The purpose of this thesis is the investigation of synaptic connectivity and synaptic transmission within the intact neocortex of the living mouse. Here I took advantage of the recent development of optogenetics, in combination with electrophysiology and two-photon microscopy to systematically and directly record synaptic transmission between a single excitatory neuron and two main types of GABAergic neurons in layer 2/3 of the mouse barrel cortex in vivo. Overall, I discovered stronger excitatory connections onto GABAergic neurons than onto excitatory neurons, irrespective of the absolute or relative locations of the pre- and postsynaptic neurons somas. I further revealed that parvalbumin-expressing (PV) and somatostatin-expressing (Sst) GABAergic neurons received excitatory inputs that were similar in magnitude, but were more reliable and faster in PV neurons than in Sst neurons. Exploring postsynaptic responses to multiple presynaptic action potentials elicited at high frequency, I found a strong short-term facilitation accompanied by significant input summation in Sst neurons, but little short-term dynamics with no summation in PV neurons. Lastly, I compared the amplitude of single action potential-evoked postsynaptic responses as a function of neocortical activity level and found that it was unchanged in both neuron types. Overall, the results of this thesis provide new insights into the functioning of microcircuits in vivo while confirming many findings from reduced preparations. In the future, it will be interesting to extend these initial in vivo measurements to other neuron and synapse types, particularly in awake animals engaged in different behavioral states.

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.