MeCP2 est une protéine multifonctionnelle agissant à de nombreux niveaux de contrôle des programmes génétiques. Un mauvais dosage de MeCP2 cause un groupe de maladies neurologiques dont le point commun est une déficience intellectuelle sévère. Des mutations ou une délétion de MECP2 causent le syndrome de Rett chez les filles, alors que sa surexpression cause chez les garçons le syndrome de duplication de MECP2. Plusieurs modèles murins de Mecp2-pathies ont été générés qui permettent d’expliciter les mécanismes qui sous-tendent l’apparition des symptômes dans ces différentes maladies. Dans notre laboratoire, deux modèles murins sont utilisés: le modèle Mecp2tm1Bird qui présente une déficience en Mecp2 et le modèle Mecp2Tg1 présentant une surexpression de Mecp2. Ce travail de thèse a permis de caractériser l’évolution postnatale des déficits moteurs et physiologique affectant la souris Mecp2Tg1. Nos résultats montrent que la surexpression de Mecp2 conduit à l’apparition de problèmes moteurs, et des convulsions chez la souris. En parallèle, nous avons étudié les déficits neuronaux affectants la voie GABAergique et glutamatergique chez la souris déficiente en Mecp2. Nous avons montré que la déficience en Mecp2 cause une dérégulation de la transmission synaptique dépendante du ‘territoire’ et de l’âge de la maladie. Ces dérégulations sous-tendent vraisemblablement des différences neurophysiologiques importantes entre les régions du cerveau qu’il nous reste encore à découvrir. Par ailleurs, nous avons constaté que la stimulation pharmacologique du système GABAergique par la Tiagabine, permet d’augmenter la survie des animaux Mecp2-déficients. / MeCP2 is a multifunctional protein acting on many levels of control of genetic programs. Thus, an abnormal dosage of MeCP2 protein causes a group of neurological disorders with a common feature of severe intellectual disability. Mutations or deletions in MECP2 gene cause Rett Syndrome in females, whereas in boys its overexpression causes the MECP2-duplication Syndrome. Several mouse models of MECP2-pathologies were generated. The use of these models is crucial for understanding the mechanisms underlying the onset of symptoms related to the pathology. In our laboratory, two mouse models are under study: The Mecp2tm1Bird model with an Mecp2 deficiency and the transgenic Mecp2Tg1 model with a double expression of Mecp2. My thesis work enabled the characterization of the postnatal physiological and motor deficits affecting Mecp2Tg1 mice. My work led to a better understanding of the gene dosage effect. Our results showed that overexpression of Mecp2 in mice, led to the occurrence of motor problems, and seizures. In parallel, we studied the neural deficits affecting the GABA and the glutamate pathway in several structures of the Mecp2 deficient brain (Mecp2tm1bird). We showed that Mecp2-deficiency causes deregulation of the synaptic transmission, which is dependent on the area, and the age of the study. These deregulations underlie significant neurophysiological differences between the different regions of the brain that we still have to uncover. Furthermore, we found that pharmacological stimulation of the GABA system with Tiagabine, a molecule capable of acting on GABA transporters to prevent its uptake, increases the survival of Mecp2-deficients animals.

The purpose of this paper was to develop a single assay system for the enzymes which regulate GABA and glutamate concentrations in brain and nerve tissue. Since all the enzymes produce L-glutamate, their activities were measured by coupling them to L-glutamate decarboxylase. Enzymatic activity was determined by measuring the release of co2 from radioactive substrates. The glutamate decarboxylase was obtained from a commercial acetone powder by simplifying existing procedures. The glutamate decarboxylase produced was of sufficient purity to be used in the coupled assays, which were checked with commercial preparations of each enzyme, where available, and with crude brain homogenates. All of the assays were shown to be linear with respect to both time and enzyme concentration, thus assuring the feasibility of the technique.

In the field of drug addiction, relapse back to drug seeking and taking is the major unmet clinical need. The rate of relapse back to drug-taking is ~70-80% within a year of drug abstinence. Gaining a better understanding of the prolonged neuronal changes that have taken place during drug addiction may lead to the design of better anti-relapse therapies. It is now widely believed that one component of drug addiction is by aberrant learning and memory processes. To study this, we investigated synaptic changes caused by the development of drug-seeking behaviour in C57BL/6J mice. Mice were treated either with non-contingent morphine or trained to exhibit drug-seeking behaviour following morphine-induced conditioned place preference (CPP) training, hippocampal slices were taken from these animals and synaptic changes examined at the CA3-CA1 synapse using electrophysiological methods. Mice that underwent morphine CPP were demonstrated to exhibit a significant preference for the morphine paired compartment before ex vivo electrophysiological analysis. Using field recordings, both non-contingent morphine and morphine CPP treatments resulted in a reduced ability to undergo stimulus-induced LTP compared to their respective controls. Whole cell patch clamp was then utilised to further investigate these effects. Non-contingent morphine treatment resulted in both pre- and post-synaptic changes with an increased AMPA:NMDA receptor ratio, concurrent increases in cell size, and reductions in the release probability of both glutamate and GABA. Morphine CPP treatment resulted in a more variable increase in AMPA:NMDA receptor ratio (presumably by the same mechanism but in a more specific group of neurones) and GABA release probability was also decreased. There were no detected increases in cell size however, or any detected changes in glutamate release probability. These findings therefore reveal a set of synaptic adaptations in the hippocampus unique to morphine-induced behavioural change, and may provide targets for future intervention in drug addiction.

The basal ganglia (BG) is known for its function not only in motor modulation but also in action selection and reward learning. There are two major anatomical pathways through the BG, the direct and the indirect pathways. The direct pathway starts from the striatum and then directly projects to the globus pallidus, pars interna (GPi) and the substantia nigra, pars reticulata (SNr) respectively, while the indirect pathway starts from the striatum but then indirectly projects to GPi and SNr through the globus pallidus, pars externa and then to the subthalamic nucleus. In addition, the output from GPi not only projects to the thalamus where it has been proposed to function in motor control, but also to the lateral habenula (LHb) where it has been proposed to function in outcome evaluation. Previous studies have found that there are three major genetically distinct neuron groups in the entopeduncular nucleus (EP) (rodent homologue of the primate GPi): 1) purely glutamatergic neurons projecting to LHb neurons expressing parvalbumin (PVLHb); 2) purely GABAergic neurons projecting to motor thalamic neurons expressing parvalbumin (PVThal); 3) GABA/glutamate co-releasing neurons projecting to LHb neurons expressing somatostatin (SstLHb). In this study, we knocked out the vesicular glutamate transporter 2 in SstLHb neurons through an adeno-associated virus in mice to test for the impact on goal-directed behavior using a probabilistic switching, two-armed bandit task (2ABT). Results obtained from the freely moving, water-restricted somatostatin-cre mice with the vesicular glutamate transporter 2 ablated in SstLHb neurons showed that: 1) there was neither improvement nor decline in their performance on the task; 2) they might be more distracted between trials while more concentrated within a trial; 3) they had an increase in the probability of switching between ports on consecutive trials when uncertainty in the location of the highly rewarded port was maximum compared to the control animals with intact glutamate release from SstLHb neurons to LHb. The success of the viral expression was then confirmed through whole-cell voltage-clamp recordings of postsynaptic neurons of the LHb, receiving projections from SstLHb neurons. In conclusion, our study has suggested that the glutamate release from the GABA/glutamate co-releasing neurons of EP projecting to LHb may play a role in reinforcement learning and motivation to obtain rewards, and the loss of glutamate in the GABA/glutamate co-releasing vesicles results in increasing uptake of GABA into these vesicles, leading to possible rebound burst firing of SstLHb neurons that eventually increases the sensitivity towards low rate of reward-delivery dramatically. / 2026-03-13T00:00:00Z