Modulation of the glutamatergic transmission by Dopamine: a focus on Parkinson, Huntington and Addiction diseases.

Dopamine (DA) plays a major role in motor and cognitive functions as well as in reward processing by regulating glutamatergic inputs. In particular in the striatum the release of DA rapidly influences synaptic transmission modulating both AMPA and NMDA receptors. Several neurodegenerative and neuropsychiatric disorders, including Parkinson, Huntington and addiction-related diseases, manifest a dysregulation of glutamate and DA signaling. Here, we will focus our attention on the mechanisms underlying the modulation of the glutamatergic transmission by DA in striatal circuits

The impact of psychostimulant administration during development on adult brain functions controlling motivation, impulsivity and cognition.

ADHD pharmacotherapy uses methylphenidate (MPH), D-amphetamine (D- amph), two psychostimulants targeting dopamine transporters, or atomoxetine (ATX), specifically targeting norepinephrine transporters. We have assessed the pharmacological mechanisms of these three drugs on the in vitro efflux of neurotransmitters in rat prefrontal cortex (PFC) and striatal slices as well as on the in vivo electrical activities of PFC pyramidal neurons, striatal medium spiny neurons, ventral tegmental area dopamine neurons or dorsal raphe nucleus serotonin neurons, using single cell extracellular electrophysiological recording techniques. We have also tested whether chronic methylphenidate treatment, during either adolescence or adulthood, could have long-lasting consequences on body growth, depression and neuronal functions. Release experiments showed that all ADHD drugs induce dose-dependent dopamine efflux in both the PFC and striatum, with different efficacies, while only D- amph induced cortical norepinephrine efflux. Atomoxetine induced an unexpected massive dopamine outflow in striatal regions, by mechanisms that depend on physiological parameters. Our electrophysiological studies indicate that all three drugs equally stimulate the excitability of PFC pyramidal neurons, in basal and NMDA-evoked conditions, when administered acutely (3 mg/kg). While the electrophysiological effects elicited by psychostimulants may be dependent on D1 receptor activation, those induced by atomoxetine relied on different mechanisms. In the ventral tegmental area (VTA), methylphenidate (2 mg/kg), but not atomoxetine, induced firing and burst activity reductions, through dopamine D2 autoreceptor activation. Reversal of such effects (eticlopride 0.2 mg/kg) revealed an excitatory effect of methylphenidate on midbrain dopamine neurons that appear to be dependent on glutamate pathways and the combination of D1 and alpha-1 receptors. Finally, acute intraperitoneal psychostimulant injections increased vertical locomotor activity as well as NMDA2B protein expression in the striatum. Some animals chronically treated with intraperitoneal administrations (methylphenidate 4 mg/kg/day or saline 1.2 ml/kg/day) showed decreased body weight gain. Voluntary oral methylphenidate intake induces desensitisation to subsequent intravenous methylphenidate challenges, without altering dopamine D2 receptor plasticity. Significant decreases in striatal NMDA2B protein expression were observed in animals chronically treated. After adolescent MPH treatment, midbrain dopaminergic neurons do not display either desensitisation or sensitisation to intravenous methylphenidate re-challenges. However, partial dopamine D2 receptor desensitisation was observed in midbrain dopamine neurons. Using behavioural experiments, cross-sensitisation between adolescent methylphenidate exposure and later-life D-amphetamine challenge was observed. Significant decreases in striatal NMDA2B protein expression were observed in animals chronically treated, while striatal medium spiny neurons showed decreased sensitivities to locally applied NMDA and dopamine. While caffeine is devoid of action on baseline spike generation and burst activity of dopamine neurons, nicotine induces either firing rate enhancement, firing rate reduction, or has no consequences. Adolescent methylphenidate treatment leads to decreased neuronal sensitivities to the combination of nicotine, MPH and eticlopride, compared to controls. Finally, nicotine partially prevented D-amphetamine-induced increase of rearing activities. Our results show that increases in the excitability of PFC neurons in basal conditions and via NMDA receptor activation may be involved in the therapeutic response to ADHD drugs. Long-term consequences were observed after psychostimulant exposure. Such novel findings strengthen the mixed hypothesis in ADHD, whereby both dopamine and glutamate neurotransmissions are dysregulated. Therefore, ADHD therapy may now focus on adequate balancing between glutamate and dopamine

Post-Translational Modification Biology of Glutamate Receptors and Drug Addiction

Post-translational covalent modifications of glutamate receptors remain a hot topic. Early studies have established that this family of receptors, including almost all ionotropic and metabotropic glutamate receptor subtypes, undergoes active phosphorylation at serine, threonine, or tyrosine residues in their intracellular domains. Recent evidence identifies several glutamate receptor subtypes to be direct substrates for palmitoylation at cysteine residues. Other modifications such as ubiquitination and sumoylation at lysine residues also occur to certain glutamate receptors. These modifications are dynamic and reversible in nature and are regulatable by changing synaptic inputs. The regulated modifications significantly impact the receptor in many ways, including interrelated changes in biochemistry (synthesis, subunit assembling, and protein–protein interactions), subcellular redistribution (trafficking, endocytosis, synaptic delivery, and clustering), and physiology, usually associated with changes in synaptic plasticity. Glutamate receptors are enriched in the striatum and cooperate closely with dopamine to regulate striatal signaling. Emerging evidence shows that modification processes of striatal glutamate receptors are sensitive to addictive drugs, such as psychostimulants (cocaine and amphetamine). Altered modifications are believed to be directly linked to enduring receptor/synaptic plasticity and drug-seeking. This review summarizes several major types of modifications of glutamate receptors and analyzes the role of these modifications in striatal signaling and in the pathogenesis of psychostimulant addiction

Rôle de la protéine Arc (Activity-regulated cytoskeleton-associated protein) dans les adaptations moléculaires et comportementales induites par la cocaïne

Molecular and cellular adaptations induced by drugs of abuse in the reward system play a key role in long-term behavioral alterations encountered in addiction. This work falls within an approach of understanding the cellular processes rapidly engaged by cocaine that could underlie the persistent alteration of neuronal physiology and behaviors. Arc protein is a major player in neuronal plasticity. Arc is induced in many behavioral paradigms and is essential for long-term synaptic plasticity and memory consolidation. The aim of this study was to characterize the profile and modality of Arc induction within the mouse striatum in response to cocaine administration. Our study shows that Arc expression is rapidly and transiently increased in the striatum after acute cocaine in an ERK-dependent fashion. This work revealed that cocaine-induced Arc protein rapidly and transiently accumulates in the nucleus of striatal neurons. In the nucleus, Arc is preferentially expressed in active transcription regions and localizes at the vicinity of phosphorylated histones H3. In vitro Arc overexpression decreased glutamate-induced Histones H3 phosphorylation showing that Arc interferes with activity-dependent chromatin remodeling. In vivo genetic invalidation of Arc expression in a transgenic mouse model was associated with a decreased chromatin compaction and increased RNA Polymerase II activity suggesting a repressive role of Arc on transcriptional mechanisms. Total Arc loss of expression leads to increased sensitivity to cocaine and promotes long-term behavioral alterations induced by low doses of cocaine.Les adaptations cellulaires et moléculaires induites par les drogues jouent un rôle central dans les altérations comportementales à long terme observées dans l’addiction. Cette étude s’inscrit dans une démarche de compréhension des processus cellulaires rapidement mis en jeu par la cocaïne et susceptibles d’impacter durablement le fonctionnement neuronal et les comportements. La protéine Arc joue un rôle clé dans l’établissement de la plasticité synaptique à long-terme et la consolidation de la mémoire. Cette étude visait à caractériser l’induction de Arc dans le striatum en réponse à la cocaïne et d’analyser son rôle dans les réponses moléculaires et comportementales qu’elle induit. Notre étude a montré que l’expression de Arc est augmentée rapidement et transitoirement dans le striatum après une injection de cocaïne sous la dépendance de l’activation de la voie ERK. Nous montrons que la cocaïne induit une forte accumulation de la protéine Arc dans le noyau des neurones striataux où Arc se localise dans des zones actives de transcription, à proximité des histones H3 phosphorylées. In vitro, la surexpression de Arc diminue la phosphorylation des histones H3 induite par le glutamate indiquant qu’elle altère le remodelage de la chromatine. L’invalidation génétique de la protéine in vivo dans un modèle de souris transgénique conduit à une décompaction de la chromatine associée à une augmentation de l’activité de la RNA Polymerase II démontrant que Arc exerce un effet répresseur sur les mécanismes transcriptionnels. La perte totale d’expression de Arc favorise le développement d’altérations comportementales à long terme chez des animaux exposés à de faibles doses de cocaïne

Behavioral Pharmacology of Alcohol and Legal Psychostimulants

Substance abuse, including alcohol and psychostimulant abuse, is a widespread and dangerous public health issue. In the United States, 8-10% of people 12 years of age or older (accounting for 20-22 million persons) are addicted to alcohol or other drugs, and the results of substance abuse are costly at both the individual and society level. Despite the large financial burden of substance abuse to society, efficacious psychosocial and pharmacologic treatment options are lacking. For example, in the pharmacologic treatment of alcohol use disorders (AUD), only three drugs have been approved by the Food and Drug Administration, and each have their own limitations that restrict efficacy and recovery outcomes. Here, the behavioral pharmacology of alcohol and psychostimulants is investigated using a variety of in vitro and in vivo techniques to better develop treatment options for AUD and to further our basic understanding of adolescent psychostimulant use. Overall, these studies provide significant progress towards the development of novel, functionally selective delta-opioid therapeutics for alcohol use disorder and also help elucidate the potential aversive behavioral outcomes of adolescent psychostimulant use

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms

Arc and homer 1a expression following intravenous administration of heroin and cocaine: a novel application of the catFISH technique

This thesis applies catFISH, a variant of the standard fluorescence in situ hybridisation technique, to study the neuronal ensembles activated by heroin and cocaine across brain structures involved in motivated behaviour, in the Sprague-Dawley rat. The first chapter reviews the pharmacology of heroin and cocaine, rodent models of drug-related behaviours, and heroin and cocaine’s ability to trigger immediate-early gene expression when administered acutely or chronically. It is suggested that the empirical evidence points towards a significant separation between the neuronal systems engaged by the two drugs. The main goal of this thesis was to test whether this separation can be seen within brain areas playing a key role in motivation and reward (e.g. the nucleus accumbens). Since immediate-early genes serve as markers of neuronal activity, and catFISH is a technique which can detect the expression of such genes in response to two separate stimuli, the technique was chosen as the best method to test if heroin and cocaine activate the same neuronal ensembles when administered acutely. The second chapter summarises the methods used across experiments described in following chapters. The third chapter presents an experiment addressing the methodological issues associated with using catFISH to study pharmacological stimuli. The technique was originally used to study the hippocampus and brain activity triggered by stimuli with well-controlled on- and offset (e.g. exposure to a novel environment or discrete cues). Arguably, acute drug injections comprise a stimulus with an on- and offset which can only be controlled indirectly – they depend on the drug dose and route of administration, among other factors. It was shown that acute intravenous injections of heroin and cocaine at doses usually self-administered by animals are suitable stimuli to use with catFISH. Chapter four describes an experiment showing that, across the striatum, the neuronal ensembles activated by an injection of cocaine followed by an injection of heroin overlap significantly less than the neuronal ensembles activated by two consecutive injections of cocaine. This is interpreted as direct evidence for a significant separation between the neuronal pathways activated by heroin vs. cocaine, even in brain areas often considered a common neurobiological substrate for the effects of the two drugs. It must be noted that the magnitude and the nature of this separation depends on the particular part of the striatum and the order in which drugs are administered. Chapter five describes a pilot experiment attempting to incorporate the logic of the catFISH technique into a rodent drug self-administration paradigm. It was found that the rats preferred self-administering heroin over cocaine, and that there was no detectable expression of the homer 1a gene across the striatum in response to acute heroin and cocaine after extended experience with the two drugs. There was also no change from baseline expression of the homer 1a and arc genes in areas of the prefrontal cortex. Finally, chapter six summarises the main findings and the key methodological considerations from all three experiments. As a whole, it is suggested that the experiments in this thesis provide a proof of concept that heroin and cocaine are processed differently by the brain, even within brain areas considered to be common substrates for the reinforcing and addictive properties of the two drugs. Future studies should take this separation into account, as it may have important implications for understanding drug addiction as a whole. The appendices contain representative fluorescence microscopy images of brain tissue analysed for catFISH

Rôle de Narp dans le développement des dyskinésies induites par la L-DOPA

Dopaminergic replacement therapy in Parkinson’s disease is hampered by the occurrence of L-DOPA-induced dyskinesia (LID). One major hypothesis is that LID result from L-DOPA-induced aberrant plasticity in the striatum due to modifications of the transcriptional program. Using a microarray-based approach, we identified Narp as a putative candidate implicated in LID induction. Thus, we investigated Narp involvement in LID by examining abnormal involuntary movements (AIM) development in Narp genetically-ablated mice or upon intrastriatal injection of a dominant negative form of Narp. Interestingly, the total AIM score was greatly reduced in these two models of impaired Narp expression. Hence, my results highlight Narp as an important actor in LID development. Then, I further examined Narp regulatory mechanisms in the striatum and I demonstrated that dopamine stimulation leads to increased Narp expression both at the transcriptional level and at the protein level through its accumulation within the synaptic compartment. These findings advance knowledge about mechanisms underlying dyskinesia with the hope of delaying their appearance in patients.Le traitement substitutif par la L-DOPA, indiqué dans la maladie de Parkinson, induit à terme des complications motrices appelées les dyskinésies induites par la L-DOPA. L'apparition des dyskinésies est due, au moins en partie, à la mise en place d'une plasticité aberrante dans le striatum, qui fait suite à des modifications transcriptionnelles induites par la L-DOPA. Une analyse du transcriptome nous a permis d'identifier le gène Nptx2, codant pour la neuropentraxine Narp, comme étant un candidat potentiellement impliqué dans l'apparition des dyskinésies. L'objectif de ce travail était d'étudier la régulation et le rôle de Narp dans l'apparition des dyskinésies, dans un modèle de souris lésée à la 6-hydroxydopamine. Nous avons montré que les dyskinésies induites par la L-DOPA sont diminuées chez des souris invalidées pour Nptx2 (Narp-KO). Par ailleurs, l'injection dans le striatum dorsal d'un adénovirus exprimant une forme dominante négative de Narp, induit une réduction importante des scores de dyskinésies. Dans le striatum, Narp est exprimé par les neurones épineux de taille moyenne et par les interneurones à parvalbumine. Après une stimulation dopaminergique, l'augmentation de l'expression de Nptx2 s'accompagne d'un enrichissement de Narp au niveau synaptique. Nos travaux montrent donc que Narp joue un rôle important dans le développement des dyskinésies et suggèrent qu'il pourrait être impliqué dans la plasticité synaptique des neurones du striatum, comme cela a été montré dans l'hippocampe. Ces résultats permettent d'ouvrir de nouvelles perspectives thérapeutiques pour retarder l'apparition de ces complications motrices chez les patients parkinsoniens