Dopamine receptors belong to the G protein-coupled receptor (GPCR) superfamily and are classified into two families: D1-like (D1R and D5R) and D2-like (D2R, D3R and D4R), based on their ability to stimulate or inhibit adenylyl cyclase (AC). Classically, GPCRs (including D2R and D3R) are desensitized by the activation of the serine/threonine protein kinase C (PKC) upon phorbol-12-myristate-13-acetate (PMA) treatment. Previous studies demonstrate that while human D5R (hD5R) is also strongly desensitized upon PMA treatment, the human D1R (hD1R) undergo a robust PMA-induced sensitization. The aim of this PhD thesis was to explore how the canonical PKC- or phorbol ester-linked pathway can control the responsiveness of two similar GPCRs like hD1R and hD5R in an opposite fashion. Our data indicate that hD1R sensitization and hD5R desensitization are not mediated by a direct modulation of AC activity by PKC. Using a chimeric approach, we identified the third intracellular loop (IL3) as the key structural determinant controlling in an opposite manner the PMA-mediated regulation of hD1R and hD5R. To delineate the potential PKC phosphorylation sites, a series of mutation of serine (Ser) and threonine (Thr) located into IL3 of hD1R and hD5R were used. No hD1R mutation decreased the PMA-mediated sensitization. This suggests that hD1R phosphorylation is not required for PMA-induced sensitization. In contrast, our results indicate that PMA-mediated hD5R desensitization occurs through a hierarchical phosphorylation of Ser260, Ser261, Ser271 and Ser274. Notably, these hD5R mutants exhibited a PMA-induced sensitization, reminiscent of the PMA-induced hD1R sensitization. Additionally, using short hairpin RNAs (shRNAs), we showed that PKCε is the potentiating PKC while the desensitizing isoform is δ. Overall, our work suggests the presence or absence of specific Ser residues on IL3 of hD1-like receptors dictate if phosphorylation-dependent desensitization (through PKCδ) or phosphorylation-independent potentiation (via PKCε) will occur.

Dopamine receptors belong to the G protein-coupled receptor (GPCR) superfamily and are classified into two families: D1-like (D1R and D5R) and D2-like (D2R, D3R and D4R), based on their ability to stimulate or inhibit adenylyl cyclase (AC). Classically, GPCRs (including D2R and D3R) are desensitized by the activation of the serine/threonine protein kinase C (PKC) upon phorbol-12-myristate-13-acetate (PMA) treatment. Previous studies demonstrate that while human D5R (hD5R) is also strongly desensitized upon PMA treatment, the human D1R (hD1R) undergo a robust PMA-induced sensitization. The aim of this PhD thesis was to explore how the canonical PKC- or phorbol ester-linked pathway can control the responsiveness of two similar GPCRs like hD1R and hD5R in an opposite fashion. Our data indicate that hD1R sensitization and hD5R desensitization are not mediated by a direct modulation of AC activity by PKC. Using a chimeric approach, we identified the third intracellular loop (IL3) as the key structural determinant controlling in an opposite manner the PMA-mediated regulation of hD1R and hD5R. To delineate the potential PKC phosphorylation sites, a series of mutation of serine (Ser) and threonine (Thr) located into IL3 of hD1R and hD5R were used. No hD1R mutation decreased the PMA-mediated sensitization. This suggests that hD1R phosphorylation is not required for PMA-induced sensitization. In contrast, our results indicate that PMA-mediated hD5R desensitization occurs through a hierarchical phosphorylation of Ser260, Ser261, Ser271 and Ser274. Notably, these hD5R mutants exhibited a PMA-induced sensitization, reminiscent of the PMA-induced hD1R sensitization. Additionally, using short hairpin RNAs (shRNAs), we showed that PKCε is the potentiating PKC while the desensitizing isoform is δ. Overall, our work suggests the presence or absence of specific Ser residues on IL3 of hD1-like receptors dictate if phosphorylation-dependent desensitization (through PKCδ) or phosphorylation-independent potentiation (via PKCε) will occur.

L’une des caractéristiques électrophysiologiques majeure de la maladie de Parkinson est l’émergence de bouffées de potentiels d’action au niveau du noyau sous-thalamique (NST). Des travaux récents menés in vitro ont soulevé l’hypothèse de l’implication des récepteurs dopaminergiques D5 (RD5) dans la genèse de cette activité pathologique. Nous avons mis en évidence que les RD5, seuls récepteurs D1-like exprimés au niveau du NST, présentent une activité constitutive in vivo, et que celle-ci est bloquée par l’injection locale d’a-flupentixol. Le blocage de cette activité intrinsèque d’une part, améliore le comportement locomoteur d’animaux rendu hémiparkinsonien, et d’autre part réduit la tendance des neurones du NST à décharger en bouffées in vivo et in vitro, signature physiopathologique de la maladie de Parkinson. L’ensemble de ces données souligne le rôle clé des RD5 dans la physiopathologie de la maladie de Parkinson, ce qui ouvre la voie à de nouvelles approches thérapeutiques… Outre cette propriété d’agoniste inverse des RD5, l’a-flupentixol est connu pour ses propriétés antipsychotiques en tant qu’antagoniste des RD2. C’est pourquoi lorsqu’il est injecté de façon systémique il induit des troubles moteurs et une catalepsie caractérisés de syndrome extrapyramidaux, même chez les animaux contrôles. Les mécanismes électrophysiologiques qui sous-tendent cet état cataleptique n’ayant jamais été étudié auparavant, nous avons mis en évidence que l’administration, par voie intra-péritonéale, d’a- flupentixol induit des changements drastiques de l’activité électrique au sein du réseau des ganglions de la base. En effet, nous avons observé une augmentation de la fréquence de décharge des neurones du globus pallidus et une diminution de celle-ci au niveau du NST et de la substance noire pars reticulata, accompagnée d’une désorganisation de l’activité électrique au niveau de ces deux noyaux. Cette étude offre une vue d’ensemble sur les mécanismes électrophysiologiques à l’origine des effets secondaires extrapyramidaux induits par les antipsychotiques, et souligne le caractère fondamental de la désorganisation de l’activité électrique des ganglions de la base dans les troubles moteurs. / Burst-firing in the subthalamic nucleus (STN) is a hallmark of Parkinson’s disease. Previous in vitro studies have raised the hypothesis of the involvement of dopamine D5 receptor (D5R) in the genesis of this pathological activity. Here we have shown that D5R exert a constitutive activity in vivo, which can be blocked by local application of a-flupentixol. Blockade of this intrinsic activity improved locomotor behaviour in an animal model of Parkinson’s disease and alleviate burst-firing of STN neurons both in vitro and in vivo. Taken together, these results highlight the key role play by local D5R in the pathophysiology of Parkinson’s disease and open the way to new pharmacological treatment of the disease… In addition to this property D5R inverse agonist, a-flupentixol is known for its antipsychotic properties as a D2R antagonist. Therefore, when injected systemically, it induced motor disturbances and catalepsy characterized as extrapyramidal motor side-effects. The electrophysiological mechanisms underlying this cataleptic state had never been studied before. Here we have demonstrated that the intra-peritoneal administration of a-flupentixol induced dramatic changes in the electrical activity of the basal ganglia network. Indeed, we observed an increase in firing rate of globus pallidus neurons and a decrease in both STN and substantia nigra pars reticulata, accompanied by a disorganisation of the electrical activity of these two nuclei. This study provides an overview of the electrophysiological mechanisms underlying extrapyramidal motor side-effects induced by antipsychotics, and stresses the fundamental nature of the disorganisation of the electrical activity in the basal ganglia network as a source of movement disorders.