Étude du système dopaminergique pré- et
postsynaptique : régulation de l'autorécepteur D2 par la neurotensine et formation des synapses excitatrices dans
le striatum

Dopamine (DA) is a neurotransmitter involved in the modulation of essential brain functions such as control of voluntary movements, the reward system and certain aspects of cognition. Since its discovery, DA has attracted a lot of attention, in part because of the severe pathologies associated with dysfunctions in the DAergic system such as Parkinson's disease, schizophrenia and substance abuse. In the brain, the majority of DAergic neurons are found in the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA), two nuclei of the ventral mesencephalon. These neurons project theirs axons in a dense network of DAergic fibers that form three major pathways: the nigrostriatal, mesolimbic and mesocortical pathways. DAergic transmission is accomplished by the activation of DAergic receptors that are part of the G protein-coupled receptor family. These receptors are abundantly expressed by DAergic target neurons and DAergic neurons themselves, which implicates that the comprehension of DAergic signaling and specific receptor function pre- and postsynaptically is a crucial aspect in the study of DA. This thesis is thus divided into two distinct parts: the first part addresses the regulation of presynaptic D2 receptor function by neurotensin, a neuropeptide intimately associated with the modulation of the DAergic system; the second part addresses the postsynaptic influences of DA, looking specifically at the segregation of DAergic receptor expression in the developing striatum and the function of these receptors in the establishment of neuronal excitatory connections in this region. In the first part of this thesis, we demonstrate that the activation of the high affinity neurotensin receptor, NTR1, induces heterologous D2 receptor internalization, with some particular differences in the amplitude and kinetics between the two D2 isoforms. This internalization is dependent on protein kinase C activity (PKC), and we demonstrate that overexpression of a D2 receptor with mutations on PKC phosphorylation sites and the knockdown of β-arrestin1 by interfering RNAs in cultured DAergic neurons completely abrogates the functional interaction between the NTR1 and the D2. iv In the second part of this thesis, we first demonstrate that D1 and D2 DAergic receptor segregation is already well established in the striatum by embryonic day 18, even if it still progresses significantly through postnatal days 0 and 14. Our results also show complete maintenance of this segregation in cultured MSNs either with or without cortical and/or mesencephalic neurons. Next, we demonstrate that the presence of mesencephalic neurons stimulates excitatory synapse formation on D2-expressing striatal medium spiny neurons (D2-MSNs). The conditional genetic deletion of the glutamatergic co-phenotype of DA neurons prevents much of this effect, and thus glutamate corelease by DA neurons seems to play an important role in excitatory synapse formation in the striatum. We also find that the establishment of excitatory terminals by cortical and mesencephalic neurons is regulated by a dynamic equilibrium. Finally, we show that chronic DA receptor blockade is not required for D2-MSN synaptogenesis, but chronic glutamatergic receptor blockade decreases dendritic spine formation and regulates cortical and mesencephalic glutamatergic synapses in an opposite manner. Globally, this work represents a significant contribution to a better understanding of the normal functioning of the DAergic system. These discoveries could prove useful to better understand the dysfunctions of this system in the context of brain pathologies such as Parkinson’s disease.

Keyphrases: Striatum, Dopamine, Neurotensin, Receptors, Desensitization, Synaptogenesis