Les neurones dopaminergiques (DA) de l'aire tegmentale ventrale (VTA) sont au cœur du système de la récompense. Ils sont impliqués dans la transmission de messages liés au contrôle motivationnel et à l'apprentissage par renforcement, permettant de mettre en place des comportements adaptés à l'environnement, mais ont aussi un rôle important dans la mise en place des addictions. Les neurones DA de la VTA projettent majoritairement vers le Noyau Accumbens (NAc) et vers le Cortex Préfrontal (PFC). Ces neurones sont la cible des drogues d'abus qui ont toutes en commun une capacité à augmenter les niveaux de dopamine dans les structures cibles. En effet, les drogues agissent au niveau des neurones DA, ce qui provoque un détournement du système de récompense et mène à l'addiction. La nicotine module le système DAergique à travers ses actions sur les récepteurs nicotiniques à l'acétylcholine (nAChRs), qui sont fortement exprimés dans la VTA. Ainsi la nicotine provoque l'augmentation de la concentration de DA dans les zones de projection, ce qui sous-tendrait le développement de comportements maladaptés liés à une prise de drogue excessive. Néanmoins cette augmentation apparente du relargage de DA dissimule la complexité de la réponse du système DAergique à la nicotine. En effet, les neurones DA forment une population hétérogène qui ne répond pas uniformément ni aux évènements ni aux drogues. Ainsi mon travail s'est focalisé sur la caractérisation de la réponse hétérogène des neurones DA de la VTA à la nicotine. Contrairement à ce qui est communément accepté dans la littérature, la nicotine n'induit pas une augmentation généralisée de l'activité des neurones DA. Ainsi, j'ai pu analyser une sous-population DAergique qui est inhibée par la nicotine. Ces réponses à la nicotine simultanées et opposées sont anatomiquement dépendante, la population inhibée étant localisée de manière plus médiale dans la VTA que la population activée. Nous montrons que cette inhibition requiert l'activation de récepteurs DA inhibiteurs, et pourrait donc être médiée par un relargage de DA intra-VTA. Malgré le fait que nous observons aussi une ségrégation anatomique des réponses DAergiques à des évènements aversifs, ces réponses ne sont pas corrélées à celles induites par la nicotine et sont organisées de manière dorso-ventrale plutôt que médio-latérale, ce qui suggère que le message DAergique transmis en réponse à la nicotine n'est pas le même que celui transmis par l'aversion. De plus, nous avons trouvé que les réponses excitatrices et inhibitrices sont aussi induites pas d'autres drogues d'abus comme l'alcool et les benzodiazépines. Cela suggère que ces réponses peuvent être étendues à d'autres stimuli, mais aussi qu'il existe un principe d'organisation de la VTA qui est anatomiquement défini, par lequel la VTA latérale interagit avec la VTA médiale à travers un effet de réseau local. / Midbrain dopamine (DA) neurons of the ventral tegmental area (VTA) are at the center of the brain's reward system. They are involved in many different brain processes such as motivation, associative learning and reinforcement, and are critically involved in the pathophysiology of addiction. VTA DA neurons mostly project to the Nucleus Accumbens (NAc) and the Prefrontal Cortex (PFC) where they send signals related to motivational control. The common feature of drugs of abuse relates to their ability to increase DA levels in these regions. Indeed, drugs target VTA DA neurons, derailing the system from its natural reward-processing functions, which is thought to be the underlying neurophysiological mechanism leading to addiction. Nicotine is known to modulate the DAergic system through its actions on nicotinic acetylcholine receptors (nAChRs), which are highly expressed in the VTA. This results in an increase of DA concentrations in the areas of projection, which leads to the maladaptive behavior of nicotine addiction. However this apparent increase of DA release conceals the complexity of the system's response to nicotine. Indeed, DA neurons consist of a very heterogenous population, and don't respond uniformly to events or to addictive drugs. Thus my work has focused on the characterization of VTA DA neuron's heterogenous responses to nicotine. In contrast to the commonly acknowledged excitatory effect of nicotine on DA neurons, I've analyzed a subpopulation of DA neurons which are inhibited by nicotine. These concurrent but opposite responses to nicotine occur in an anatomically-dependent manner, with the inhibited population being localized more medially within the VTA compared to the activated population. We show that this inhibition requires the activation of inhibitory dopamine receptors, and could thus be mediated by an intra-VTA DA release. Although we also observe anatomically segregated excitatory and inhibitory responses to painful stimuli, these responses do not correlate to the ones induced by nicotine and are organized in a dorso-ventral fashion rather than a medio-lateral one, suggesting that the DAergic message conveyed by nicotine does not match the one conveyed by aversion. Moreover, we found that concurrent excitatory and inhibitory responses are also induced by other drugs of abuse such as ethanol and benzodiazepines. This suggests that these responses can be extended to other stimuli, but also that there exists an anatomically defined organizing principle of the VTA, in which the lateral VTA interacts with the medial VTA via a local network effect.

Il y a quatre ans, lorsque j’ai commencé ma thèse en Neuropharmacologie, le rôle fonctionnel du récepteur serotoninergique2B (5-HT2B) central n’était guère connu. En effet, compte tenu de la mise en évidence de son expression dans le cerveau relativement récente par rapport aux autres récepteurs à la 5-HT, peu d’études avaient porté sur son impact au sein du système nerveux central. En particulier, il était établi que les récepteurs 5-HT2B, sans effet au niveau de la voie dopaminergique (DA) nigrostriée, sont capables d’exercer un contrôle tonique excitateur sur l’activité de la voie DA méso accumbale. Sur la base de cette régulation différentielle des voies DA sous-corticales, il avait été proposé que les antagonistes du récepteur 5-HT2B pourraient constituer des outils pharmacologiques pertinents pour le traitement des pathologies liées à unedysfonction du système DA et requérant une modulation indépendante des voies DA ascendantes, telles que la schizophrénie. Cependant, l’effet du blocage des récepteurs 5-HT2B au niveau de la voie DA mésocorticale, laquelle joue un rôle pivot dans le bénéfice thérapeutique des antipsychotiques (APs) atypiques,n’avait jamais été exploré. De plus, l’analyse de la littérature avait révélé que le blocage du récepteur 5-HT2B réduit les réponses neurochimiques et comportementales induites par l’amphétamine et la 3,4-méthylènedioxyméthamphétamine, suggérant que ce récepteur pourrait également représenter une cible pharmacologique intéressante pour le traitement de l’addiction. Néanmoins, la possible implication de ce récepteur dans les effets de la cocaïne, l’une des drogues les plus consommées au monde, restait alors inconnue.Ainsi, l’objectif de cette thèse était d’étudier l’influence exercée par le récepteur5-HT2B sur l’activité DA basale et activée par la cocaïne, afin de mieux évaluer le potentiel thérapeutique de ce récepteur dans le traitement de la schizophrénie et de l’addiction. A cette fin, nous avons étudié les effets de deux antagonistes puissants et sélectifs du récepteur 5-HT2B (RS 127445 et LY 266097) sur l’activité DA, en utilisant des approches biochimiques, électrophysiologiques et comportementales chez le rat.Un premier groupe d’expériences a mis en évidence l’existence d’un contrôle tonique inhibiteur exercé par le récepteur 5-HT2B sur la libération de DA dans le cortex préfrontal médian (CPFm). Ce résultat, démontrant que les récepteurs 5-HT2B régulent de manière différentielle les trois voies DA ascendantes, indique que les antagonistes du récepteur 5-HT2B présentent un profil d’action idéale pour restaurer une fonction DA normale chez les patients schizophrènes. En accord avec cette proposition, les antagonistes 5-HT2B se révèlent efficaces dans plusieurs modèles classiquement utilisés pour prédire l’efficacité des APs, alors qu’ils n’ont pas d’effet dans une tâche comportementale prédisant la tendance des APs à induire des effets secondaires moteurs. Un second groupe d’expériences visant à étudier les mécanismes sous-tendant le contrôle différentiel exercé par le récepteur 5-HT2B sur l’activité DA montre que les effets opposés induits par les antagonistes 5-HT2B sur la libération de DA dans le CPFm et le noyau accumbens (NAc) résultent d’une interaction fonctionnelle avec les récepteurs 5-HT1A exprimés dans le CPFm. Enfin, nous avons également démontré que le blocage du récepteur 5-HT2B supprime l’hyperlocomotion provoquée par la cocaïne. Cet effet, qui se produit indépendamment de la libération de DA dans le NAc et le striatum, où l’activité DA est étroitement liée aux effets comportementaux induits par la cocaïne,implique une interaction post-synaptique dans les régions DA sous-corticales. En conclusion, le travail accompli au cours des quatre années passées apporte des informations substantielles quant au rôle fonctionnel du récepteur 5-HT2Bdans la régulation des voies DA ascendantes. / Four years ago, at the beginning of my thesis in Neuropharmacology, the functional role of the central serotonin2B receptor (5-HT2BR) remained poorly investigated. Indeed, in light of the relatively recent discovery of its presence in the mammalian brain, as compared to other 5-HT receptors, only few studies had explored its impact within the central nervous system. Interestingly, it had been shown that 5-HT2BRs, while having no effect at the level of the nigrostriatal dopaminergic (DA) pathway, afford a tonic excitatory control on the activity of the mesoaccumbal DA tract. This differential influence on subcortical DA brain regions had led to the proposal that 5-HT2BR antagonists may be a useful tool for improved treatment of DA-related disorders requiring an independent modulation of the activity of ascending DA pathways, such ass chizophrenia. However, the effect of 5-HT2BR blockade at the level of themesocortical DA pathway, which plays a pivotal role in the the rapeutic benefit of atypical antipsychotic drugs (APDs), had never been studied. In addition,analysis of the literature revealed that 5-HT2BR blockade suppresses amphetamine and 3,4-methylenedioxymethamphetamine-induced neurochemical and behavioral responses, suggesting that this receptor may also be a relevant pharmacological target for treating drug addiction. Nevertheless,its possible implication in the effects induced by cocaine, one of the most worldwide abused drugs, remained unknown.Thus, the aim of the present thesis was to study the regulatory control exerted by the 5-HT2BR on both basal and cocaine-induced stimulation of DA activity,in order to evaluate its therapeutic relevance for improved treatment of schizophrenia and drug abuse and dependence. To this purpose, we assessed the effects of potent and selective 5-HT2BR antagonists (RS 127445 and LY266097) on DA activity, by using biochemical, electrophysiological and behavioral approaches in rats.In a first group of experiments, we found that 5-HT2BRs exert a tonic inhibitory control on DA outflow in the medial prefrontal cortex (mPFC). This finding, by showing that 5-HT2BRs afford differential controls over the three ascending DA pathways, indicates that 5-HT2BR antagonists display an ideal pattern of effects to restore normal DA function in schizophrenia. Accordingly, 5-HT2BRantagonists were efficient in several behavioral models aimed at predicting APD efficacy, and had no effect in a behavioral task reflecting APD propensity to induce motor side effects. In a second group of experiments performed to determine the mechanisms under lying the differential control exerted by 5-HT2BRs on DA activity, we demonstrated that 5-HT2BR antagonist-induced opposite effects on DA ouflow in the mPFC and the nucleus accumbens (NAc)involve a functional interplay with 5-HT1ARs expressed in the mPFC. Finally,we found that 5-HT2BR blockade suppresses cocaine-induced hyperlocomotion.This effect, which occurs independently from changes of DA outflow in theNAc and the striatum, where DA activity is tightly related to cocaine-induced behavioral responses, likely involves a post-synaptic interaction in subcorticalDA brain regions.To conclude, the work accomplished over the past four years provides substantial information with regards to the functional role of 5-HT2BRs in the regulation of the activity of ascending DA pathways. In addition, while improving the understanding of the interaction between DA and 5-HT systems,the present findings altogether highlight the therapeutic potential of 5-HT2BRantagonists for treating schizophrenia and cocaine addiction.

The aim of this thesis was the functional characterization of the zebrafish parl (Presenilin-Associated Rhomboid-Like) genes which code for mitochondrial proteins involved in cell survival. A mutation in PARL has been described in Parkinson’s disease patients. I investigated the role of mitochondrial PD-related proteins using a zebrafish parla and parlb deficiency model. I found that the knockdown of both parl genes is lethal. Parla plays a larger role in patterning of the DA neurons in the ventral diencephalon than Parlb. The human PARL rescued the double morphant phenotype, suggesting function conservation between zebrafish and humans. I was able to rescue the mortality and DA neuron mispatterning observed in double morphants with synthetic pink1 mRNA. This suggests that parl genes are epistatic to pink1 in zebrafish. To visualize mitochondria specifically in dopaminergic neurons of live zebrafish, I established a transgenic line Tg(dat:tom20 MLS-mCherry) where regulatory elements of the dopamine transporter (dat) were used to drive expression of a Tom20-mCherry fusion protein that is targeted to the mitochondria. I characterised the expression of Tom20-mCherry to the mitochondria of the majority of DA neuron groups. In addition, I observed a decrease in mCherry fluorescence following MPTP exposure of live fish. The PD-related mutation in PARL is located in a cleavage site of the mammalian protein, which is necessary for the production of the beta peptide; however, this site is predicted to be absent in the zebrafish Parls. To establish the cleavage patterns of the zebrafish Parls and compare them to those of human PARL, I examined the cleavage of Parl-Flag constructs in cultured cells. I detected one band for Parla-Flag and two bands representing Parlb-Flag. The parla and parlb deficiency model along with the characterization of the cleavage patterns of Parl and the Tg(dat:tom20 MLS-mCherry) transgenic line are tools which will help elucidate the role of mitochondrial proteins in PD research.

La dopamine (DA) et la noradrénaline (NA) jouent un rôle primordial dans de nombreuses fonctions cérébrales. Elles agissent par l’intermédiaire de récepteurs couplés aux protéines G qui régulent la voie de signalisation cAMP/PKA. La NA est libérée dans tout le néocortex par les neurones du locus coeruleus (LC). Les projections DA des neurones de l’aire tegmentale ventrale sont présentes dans des régions restreintes du cortex. Le but de ce projet est de caractériser les effets de la NA et de la DA exogènes mais aussi de la libération des catécholamines endogènes sur la voie AMPc/PKA en utilisant l’imagerie de sondes fluorescentes. J’ai montré que la NA, l’isoprotérénol, la DA ou le SKF 38393 stimulent l’activité PKA dans les cellules pyramidales, notamment dans les régions peu innervées par les fibres DA. Les réponses NA et isoprotérénol sont inhibées par les antagonistes β-adrénergiques et les réponses DA par un antagoniste des récepteurs D1/D5. La contribution inhibitrice des récepteurs D2 dopaminergiques et α2-adrénergiques a été mise en évidence. Les récepteurs NA et DA sont donc fonctionnellement exprimés bien au delà du territoire innervé par les fibres DA. L’expression sélective du canal photosensible channelrhodopsine (ChR2) dans les neurones NA combinée à l’imagerie PKA dans le cortex m’a permis de montré que la libération endogène de NA induite par photostimulation augmente réversiblement l’activité PKA. Ces réponses sont inhibées par les antagonistes β- adrénergiques et leur amplitude augmentée par un bloquant de la recapture de la NA. Cette approche permet donc de caractériser en temps réel la transmission catécholaminergique avec une résolution subcellulaire. / Dopamine (DA) and noradrenalin (NA) are critically involved in multiple brain functions. NA and DA modulate neuronal functions via G protein coupled receptors and cAMP/PKA signaling. NA is released in the entire neocortex by axons of locus coeruleus (LC) neurons. DA projections to the neocortex, which originate in the ventral tegmental area, innervate more restricted areas. The aim of this project is to characterize the effect of exogenous and endogenously released NA and DA on cAMP/PKA signaling by using PKA sensitive fluorescent sensors. I showed PKA activation in pyramidal neurons in response to bath application of NA, isoproterenol, DA and SKF38393. Responses were observed throughout the cortex even in areas poorly innervated by DA fibers. Responses to NA and isoproterenol were inhibited by β-receptors antagonists while responses to DA and SKF38393 were blocked by a D1/D5 receptor antagonist. The negative contribution of D2-like and α2-adrenoceptor to PKA signaling was also demonstrated using specific antagonists. These results show a widespread distribution of functional NA and DA receptors in the neocortex that extends far beyond the territory of DA fiber innervations. The selective expression of the light activated Channelrhodopsin2 in NA neurons combined with cortical PKA imaging allowed demonstrating that release of endogenous NA reversibly increases PKA activity. These responses were blocked by β-receptors antagonists while a NA transporter inhibitor increased responses. These results demonstrate that endogenous NA released from LC fibers activates the cAMP/PKA pathway in cortical neurons, providing a means to characterize catecholaminergic transmission events.

La tétrahydrobioptérine (BH4) est le cofacteur requis pour l’activité des enzymes responsables de la synthèse de la dopamine, la sérotonine et l’oxyde nitrique. Par conséquent, la BH4 est nécessaire au bon déroulement de nombreux processus physiologiques centraux et périphériques dont la neurotransmission, la réponse inflammatoire, la régulation du stress oxydatif, la fonction vasculaire et endothéliale et le métabolisme. Or, de par sa structure chimique, la BH4 est une molécule facilement oxydable et dégradable, et des diminutions des niveaux centraux de BH4 ont été observées dans de nombreuses maladies neuropsychiatriques à composante inflammatoire dont la maladie d’Alzheimer et la dépression majeure. Le déficit de BH4 apparait donc comme un mécanisme pouvant participer à l’étiologie de ces pathologies et en aggraver les symptômes. Malgré ces observations, les effets d’une administration de BH4 sur la neurophysiologie et les comportements en s’y rapportant restent largement inexplorés. L’objectif de cette thèse a donc été de caractériser les effets d’une administration périphérique de BH4 sur la fonction cérébrale et le comportement chez la souris. Nous nous sommes intéressés aux effets de BH4 sur le système dopaminergique mésolimbique et la motivation en condition physiologique puis en situation d’inflammation aigue. Dans un second temps, nous avons exploré le potentiel thérapeutique de la BH4 dans le traitement de la maladie d’Alzheimer. Nous avons tout d’abord démontré que la BH4 traversait la barrière hémato-encéphalique et qu'une injection périphérique permettait d’augmenter les niveaux centraux de BH4. Nos travaux ont montré qu’en condition physiologique, l’administration de BH4 potentialise la libération de dopamine dans le nucleus accumbens et les comportements motivés. En condition de neuroinflammation aiguë induite par le LPS, l’administration de BH4 permet d’atténuer la neuroinflammation. Ces données suggèrent un potentiel bénéfique de la BH4 sur les troubles motivationnels induits par l’inflammation. Dans notre deuxième étude, nous avons démontré que l’administration chronique de BH4 permet de corriger les déficits mnésiques observés dans le modèle murin triple transgénique de la maladie d’Alzheimer. La supplémentation en BH4 induit également une diminution de la neuroinflammation ainsi qu’une amélioration de la tolérance au glucose. Cependant, ces améliorations mnésiques, métaboliques et inflammatoires ne s’accompagnent pas d’une diminution des pathologies amyloïde et tau.Ainsi, l’ensemble de ces travaux a permis une meilleure caractérisation des effets neurobiologiques et comportementaux de la BH4 et renforce son potentiel thérapeutique. / Tetrahydrobiopterin (BH4) is the required cofactor for the activity of the enzymes involved in the synthesis of dopamine, serotonin and nitric oxide. BH4 is therefore necessary for many central and peripheral physiological processes including neurotransmission, inflammatory response, oxidative stress regulation, vascular and endothelial function, and metabolism. However, BH4 can be easily oxidized and degraded and decreased BH4 brain levels has been observed in many neuropsychiatric diseases including Alzheimer's disease and major depression. Consequently, the decrease in BH4 levels in these pathologies could contribute to the onset and aggravation of symptoms. Despite these observations, the effects of BH4 administration on brain function and related behavior remain largely unexplored. The aim of this thesis was to characterize the effects of peripheral BH4 administration on brain function and behavior in mice. We investigated the effects of BH4 on the mesolimbic dopaminergic system and motivation in physiological condition and during acute inflammation. Then, we explored the therapeutic potential of BH4 in the treatment of Alzheimer's disease. We first demonstrated that BH4 crossed the blood-brain barrier and that a peripheral injection of BH4 increased its cerebral levels. Our results also showed that under physiological condition, BH4 administration potentiates dopamine release into the nucleus accumbens and motivated behaviors. In condition of acute LPS-induced neuroinflammation, BH4 administration helps to reduce neuroinflammation. Therefore, BH4 may have beneficial effects on dopaminergic and motivational disturbances induced by inflammation. In our second study, we demonstrated that chronic administration of BH4 reversed memory deficits observed in the transgenic triple murine model of Alzheimer's disease. We also observed a decrease in neuroinflammation and an improvement in glucose tolerance. However, these memory, metabolic and inflammatory improvements are not accompanied by a decrease in amyloid and tau pathologies. This work had contributed to a better characterization of the neurobiological and behavioral effects of the BH4 and reinforces its therapeutic potential.

Previous findings from this laboratory demonstrating changes in dopamine (DA) transporter and D2 receptors in the amygdaloid complex of subjects with major depression indicate that disruption of dopamine neurotransmission to the amygdala may contribute to behavioral symptoms associated with depression. Quantitative real-time RT-PCR was used to investigate the regional distribution of gene expression of DA receptors in the human amygdala. In addition, relative levels of mRNA of DA receptors in the basal amygdaloid nucleus were measured postmortem in subjects with major depression and normal control subjects. All five subtypes of DA receptor mRNA were detected in all amygdaloid subnuclei, although D1, D2, and D4 receptor mRNAs were more abundant than D3 and D5 mRNAs by an order of magnitude. The highest level of D1 mRNA was found in the central nucleus, whereas D2 mRNA was the most abundant in the basal nucleus. Levels of D4 mRNA were highest in the basal and central nuclei. In the basal nucleus, amounts of D4, but not D1 or D2, mRNAs were significantly higher in subjects with major depression as compared to control subjects. These findings demonstrate that the D1, D2 and D4 receptors are the major subtypes of DA receptors in the human amygdala. Elevated DA receptor gene expression in depressive subjects further implicates altered dopaminergic transmission in the amygdala in depression.

Neonatal 6-hydroxydopamine (6-OHDA) treatment of rats is associated with supersensitization of the dopamine (DA) D1 agonist induction of stereotyped and locomotor behaviors. The present study was conducted to determine whether ontogenetic treatments of these rats with the DA D1 receptor agonist, SKF 38393, would produce a maximal DA D1 receptor supersensitivity, as measured by locomotor behavior in adulthood. Rat pups were treated daily with SKF 38393-HCl (3.0 mg/kg per day, i.p.) or saline vehicle for 28 consecutive days from birth. These animals were additionally treated at 3 days after birth with 6-OHDA-HBr (100 μg, in each lateral ventricle, salt form) or its vehicle. Between 6 and 9 weeks locomotor activity or stereotyped behaviors were observed after weekly challenge doses of SKF 38393-HCl (3.0 mg/kg, i.p.). In the neonatal 6-OHDA group, successive SKF 38393 treatments produced progressively greater locomotor activity. In the group of rats treated during postnatal ontogeny with both 6-OHDA and SKF 38393 daily treatments, the first adult challenge dose of SKF 38393 produced an enhanced locomotor response, greater than that seen in other groups (P < 0.01). Subsequent SKF 38393 treatments of this group produced increasingly greater locomotor responses. SKF 38393-induced stereotyped behavioral effects were greater in the 6-OHDA-lesioned groups, whether or not SKF 38393 was administered ontogenetically. Profound reductions (> 99%) of DA and its metabolites were found in the striatum of neonatal 6-OHDA treated rats, regardless of whether SKF 38393 was co-administered ontogenetically. A marked elevation in striatal 5-HT (> 50%) accompanied the DA depletion in the striatum. These findings indicate that neonatal 6-OHDA treatment produces the expected destruction of striatal DA fibers with associated sprouting of 5-HT fibers, while repeated ontogenetic treatments of these rats with a D1 agonist produces partial sensitization of the DA D1 receptors in adulthood.

Repeated treatments with a dopamine (DA) D2 receptor agonist result in the induction of DA D2 receptor supersensitivity, as evidenced by enhanced behavioral responses to subsequent D2 agonist treatments - a phenomenon known as priming of receptors. Priming of D2 receptors has been well-studied in otherwise intact (non-lesioned) rats. In contrast to D2 priming, repeated treatments with a DA D1 agonist are unable to prime D1 receptors unless nigrostriatal DA fibers are largely destroyed in early postnatal ontogeny. In order to determine if D2 receptors could be primed in rats in which nigrostriatal DA fibers were largely destroyed in early postnatal ontogeny, rats were (a) lesioned at 3 days after birth with 6-hydroxydopamine (67 μg in each lateral ventricle; desipramine, 20 mg/kg IP, 1 h; 6-OHDA), (b) treated daily for the first 28 days after birth with the D2 agonist quinpirole HCl (3.0 mg/kg IP), and (c) observed in adulthood for both quinpirole-induced and SKF 38393- (D1 agonist-) induced locomotor activity and stereotyped activities. In 6-OHDA-lesioned rats in which endogenous striatal DA was reduced by 99%, quinpirole did not produce enhanced locomotor or stereotyped activities. However, SKF 38393 produced increased locomotor and stereotyped activities even after the first dose of SKF 38393. These findings demonstrate that D2 receptors are not primed by ontogenetic quinpirole treatments of neonatally 6-OHDA-lesioned rats, although D2 agonist treatments do at least partially prime D1 receptors in 6-OHDA-lesioned rats.

Repeated treatments of rats with the dopamine (DA) D2 receptor agonist quinpirole, consistently produce long-lived DA D2 receptor supersensitization, by the process that has been termed priming. Rats so-primed in ontogeny behaviorally demonstrate adulthood enhancement of low-dose quinpirole-induced yawning. Because 1) dopaminergic neurons originate in midbrain nuclei (substantia nigra and ventral tegmental area), and 2) noradrenergic neurons originate in pontine (locus coeruleus) and medullary areas, it might be presumed that these two monoaminergic systems are independent, not interdependent. However, in the present study we demonstrate that there was an attenuation of quinpirole-enhanced yawning at 8 weeks in rats that were 1) primed by repeated neonatal quinpirole HCl treatments (50 μg/kg per day SC) during the first ten days of postnatal ontogeny, and 2) lesioned at 3 days after birth with DSP-4 (N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride, 50 mg/kg SC). Dose-effect curves indicated a 23-45% reduction in yawning by DSP-4 treatment of quinpirole-primed rats, acutely treated as adults with quinpirole (25, 50, or 100 μg/kg). Effectiveness of DSP-4 is reflected by the 95% and 99% reductions in norepinephrine contents of frontal cortex and hippocampus, respectively (HPLC/ED method). The findings are supportive of a modulatory role of noradrenergic fibers on dopamine receptor priming (supersensitization) in rat brain.

The present study investigated the effects of forced overexpression of Phox2a/2b, two transcription factors, in the locus coeruleus (LC) of aged rats on noradrenergic and dopaminergic phenotypes in brains. Results showed that a significant increase in Phox2a/2b mRNA levels in the LC region was paralleled by marked enhancement in expression of DBH and TH per se. Furthermore, similar increases in TH protein levels were observed in the substantial nigra and striatum, as well as in the hippocampus and frontal cortex. Overexpression of Phox2 genes also significantly increased BrdU-positive cells in the hippocampal dentate gyrus and NE levels in the striatum. Moreover, this manipulation significantly improved the cognition behavior. The in vitro experiments revealed that norepinephrine treatments may increase the transcription of TH gene through the epigenetic action on the TH promoter. The results indicate that Phox2 genes may play an important role in improving the function of the noradrenergic and dopaminergic neurons in aged animals, and regulation of Phox2 gene expression may have therapeutic utility in aging or disorders involving degeneration of noradrenergic neurons.