Anxiety is an affective state defined by heightened arousal and unease in the absence of a clear and present fear-inducing stimulus. Chronic and inappropriate anxiety leads to anxiety disorders, the most common class of human mental disorder. Recent work suggests projections to the ventral tegmental area (VTA), are critical for anxiety behavior expression. However, the relationship between efferent VTA projections and anxiety is unclear. This thesis resolves anxiety circuitry connecting the dopaminergic (DAergic) VTA to the interpeduncular nucleus (IPN), coined the mesointerpeduncular circuit. I hypothesize the mesointerpeduncular circuit affects anxiety through the release of anxiogenic corticotropin releasing factor (CRF) during nicotine withdrawal and anxiolytic dopamine (DA) during drug naïve behavior. Electrophysiological and pharmacological data suggest CRF release from the DAergic VTA during nicotine withdrawal activates CRF receptor 1 (CRFR1) potentiating the glutamatergic activation of “Type 2” neurons and anxiety-like behavior in mice. However, in nicotine naïve conditions CRF production is negligible. Instead, in vivo DA release is anticorrelated with anxiety-like behaviors. Optogenetic stimulation and inhibition drives decreased and increased anxiety-like behaviors, respectively. Electrophysiological experiments reveal a complex interpeduncular microcircuit where D1-like DA receptor expressing “Type C” neurons in the caudal IPN (cIPN) regulate glutamatergic release in the ventral IPN (vIPN) through presynaptic GABA receptors. The result is propagation of the signal to excite “Type A” and inhibit “Type B” vIPN neurons. Finally, pharmacological activation or inhibition of interpeduncular D1-like DA receptors is sufficient to decrease and increase anxiety-like behaviors respectively. Thus, this circuit is important for modulating anxiety-like behavior.

No description available.

The purpose of this thesis project was to examine the effects of chronic alcohol on the excitability and molecular adaptation of GABA neurons of the ventral tegmental area (VTA). GABA neurons are of interest with regards to ethanol intoxication, reinforcement, and dependence due to their widespread distribution and connectivity to mesocorticolimbic dopamine (DA) neurons implicated in alcohol reward and addiction. Since we have previously shown adaptation of VTA GABA neuron firing rate to chronic ethanol (Gallegos, Criado et al. 1999) and suppression of gap-junction (GJ) mediated coupling between these neurons by acute ethanol (Stobbs, Ohran et al. 2004), we wanted to further characterize the effects of chronic ethanol on VTA GABA neuron excitability, electrical coupling and molecular adaptation. In particular, we analyzed the GJ mediated coupling and protein regulation of VTA GABA neurons following a three week period of continuous ethanol exposure via liquid diet. Although some animals showed tolerance, there was no significant tolerance to ethanol inhibition of GJ-mediated electrical coupling. In addition, we were able to characterize differences in mRNA expression levels for the DA synthesizing enzyme tyrosine hydroxylase (TH), the DA D2 receptor and the NMDAR2B receptor subunit in DA versus GABA neurons, all three of which were expressed at higher levels in DA neurons. We also determined the effects of chronic ethanol on mRNA levels of these same proteins as well as μ-opioid receptors (μORs) and connexin-36 (Cx36) GJs. Most significantly, we found a down-regulation of the DA D2 receptor, confirming that molecular modification occurs in these VTA GABA neurons with chronic alcohol. While we reject our hypothesis that acute ethanol inhibition of VTA GABA neuron electrical coupling would undergo tolerance to chronic ethanol in these non-dependent rats, which was the focus of this thesis, it remains to be determined if tolerance to chronic ethanol might be obtained in ethanol-dependent rats.

The therapeutic benefits attributed to activation of peripheral mechanoreceptors are poorly understood. There is growing evidence that mechanical stimulation modulates substrates in the supraspinal central nervous system (CNS) that are outside the canonical somatosensory circuits. This work demonstrates that activation of peripheral mechnoreceptors via mechanical stimulation (MStim) is sufficient to increase dopamine release in the nucleus accumbens (NAc), alter neuron firing rate in the ventral tegmental area (VTA) and increase membrane translocation of delta opioid receptors (DORs) in the NAc. Further, we demonstrate that these effects are dependent on DORs and acetylcholine receptors. Additionally, MStim can block neuronal markers of chronic ethanol dependence including ethanol-induced changes to VTA GABA neuron firing during withdrawal, and DA release profiles after reinstatement ethanol during withdrawal. These are presented in tandem with evidence that MStim also ameliorates behavioral indices of ethanol withdrawal. Finally, exercise, a modality that includes a mechanosensory component, is shown to alter expression of kappa opioid receptors (KORs) in the NAc. This change substantively depresses KORs influence over evoked DA release in direct contraversion to the effects of chronic ethanol. These changes translate into reduced drinking behavior.

Les neurones dopaminergiques (DA) de l’aire tegmentale ventrale (VTA) sont influencés par différents stimuli comme des récompenses naturelles et d’autres stimuli moins physiologiques tels que les drogues d’abus. Ces drogues agissent en détournant les mécanismes d’apprentissage qui sous-tendent normalement la motivation pour des renforçateurs naturels. Les neurones DA, en conditions physiologiques, sont subtilement régulés par une balance entre tonus GABA et glutamatergique. Ils sont soumis à de multiples sources inhibitrices dont le noyau accumbens, les interneurones locaux ou les neurones GABA de la queue de la VTA (tVTA). Le glutamate est également important dans leur modulation. Il contrôle leur activité en bursts, qui est le mode de décharge le plus efficace pour libérer de la dopamine et coder des informations associées à la récompense. Il permet des adaptations synaptiques à long terme qui se sont révélées importantes dans la prise de drogue. La connaissance des facteurs endogènes qui contrôlent l’excitabilité des cellules DA de la VTA est essentielle à la compréhension des processus physiologiques (recherche de plaisir…) mais aussi pathologiques (addiction…). L’objectif de mon travail a été de comprendre les circuits de régulation des neurones DA en conditions physiologiques et lors de l’exposition à la morphine. Dans un premier temps, nous avons étudié les mécanismes de régulation des neurones DA par la formation hippocampique ventrale incluant le subiculum ventral et l’aire CA1 ventrale (vSUB/CA1). Grâce à l’utilisation d’approches d’électrophysiologie in vivo chez le rat anesthésié, nous avons montré que le vSUB/CA1 exerce un contrôle excitateur glutamatergique des neurones DA. Nous avons mis en évidence que cette voie vSUB/CA1-VTA est polysynaptique, faisant intervenir le BNST comme relais. J’ai aussi pu confirmer le rôle fonctionnel de la tVTA en tant que nouvelle structure GABA modulant l’activité des neurones DA, renforçant ainsi l’idée d’une balance entre tonus GABA et glutamatergique régulant les neurones DA in vivo.La deuxième partie de ma thèse a consisté en l’étude des circuits neuronaux à l’origine des effets excitateurs de la morphine sur les neurones DA de la VTA in vivo. L’hypothèse actuelle est que la morphine excite les neurones DA par un mécanisme de désinhibition en inhibant les neurones GABA de la VTA. Grâce à l’utilisation d’approches multiples, nous avons proposé un nouveau circuit expliquant les effets de la morphine. Ces effets sont la conséquence d’une modification de la balance GABA/glutamate par la morphine. Elle se traduit par une diminution du tonus GABA et d’une augmentation du tonus glutamatergique. Enfin, nous avons pu démontrer qu’une seule exposition à la cocaïne augmente l’activité de base des neurones DA. Chez ces animaux, les effets excitateurs de la morphine sont potentialisés confirmant ainsi l’hypothèse que l’amplitude de l’activation des neurones DA par la morphine dépend de leur état d’excitabilité. / Dopaminergic (DA) neurons of the ventral tegmental area (VTA) are influenced by several stimuli such as natural rewards or drugs of abuse. Drugs shunt learning mechanisms which underlie motivation for natural reinforcers. Under physiological conditions, DA neurons are regulated by a balance between GABA and glutamatergic inputs. They receive several inhibitory inputs especially from the nucleus accumbens, VTA local interneurons and GABA neurons of the tail of the VTA (tVTA). Glutamate is also important in modulating DA neuron activity. It controls their bursting activity which is the most efficient way to release dopamine and to encode reward-associated informations. It allows long term synaptic adaptations important for addiction. Knowing how these endogenous factors control VTA DA neuron excitability is essential to understand physiological (search for pleasure…) and pathological (drug addiction…) processes.In the first part of my thesis, we studied the regulation of the VTA by the hippocampal formation including the ventral subiculum and the ventral CA1 area (vSUB/CA1). Using electrophysiological approaches in anesthetized animal, we showed that the vSUB/CA1 controls VTA DA neurons and that this input is glutamatergic. We also demonstrated that the vSUB/CA1-VTA pathway is polysynaptic implicating the BNST as a relay. I also confirmed the inhibitory control of the VTA by tVTA, new GABA input to DA neurons. Thus, in vivo, DA neurons are regulated by a balance between GABA and glutamatergic inputs. The second part of my research consisted in studying the neuronal circuits underlying excitatory effects of morphine on VTA DA neurons in vivo. The actual hypothesis is that morphine excites DA neurons by a disinhibition mechanism inhibiting VTA GABA neurons. Using several approaches (electrophysiological approaches in anesthetized animal, tract-tracing methods), we proposed a new circuitry explaining morphine effects. These excitatory effects result from a modification of the balance between GABA and glutamatergic inputs with a decrease of the GABA tone and an increase of the glutamatergic tone. Finally, we demonstrated that an acute cocaine exposure increases DA neuron activity. In animals exposed to cocaine, morphine excitatory effects are potentiated. This last experiment confirms the hypothesis that the amplitude of morphine-induced activation of VTA DA neurons depends on their excitability state.

The basal ganglia are the key circuitry controlling movement and reward behavior. Both locomotion and reward-related behavior are also modified by dopaminergic input from the substantia nigra and the ventral tegmental area (VTA). If the basal ganglia are severed by lesion or in disease, such as in Parkinson’s disease, the affected individuals suffer from severe motor impairments and often of affective and reward-related symptoms. The subthalamic nucleus (STN) is a glutamatergic key area of the basal ganglia and a common target for deep brain stimulation in Parkinson’s disease to alleviate motor symptoms. The STN serves not only motoric, but also limbic and cognitive functions, which is often attributed to a tripartite anatomical subdivision. However, the functional output of both VTA and STN may rely more on intermingled subpopulations than on a strictly anatomical subdivision. In this doctoral thesis, the role of subpopulations within and associated with the basal ganglia is addressed from both a genetic and a behavioral angle. The identification of a genetically defined subpopulation within the STN, co-expressing Paired-like homeodomain transcription factor 2 (Pitx2) and Vesicular glutamate transport 2 (Vglut2), made it possible to conditionally reduce glutamatergic transmission from this subgroup of neurons and to investigate its influence on locomotion and motivational behavior, giving interesting insights into the mechanisms possibly underlying deep brain stimulation therapy and its side-effects. We address the strong influence of the Pitx2-Vglut2 subpopulation on movement, as well as the more subtle changes in reward-related behavior and the impact of the alterations on the reward-related dopaminergic circuitry. We also further elucidate the genetic composition of the STN by finding new markers for putative STN subpopulations, thereby opening up new possibilities to target those cells genetically and optogenetically. This will help in future to examine both STN development, function in the adult central nervous system and defects caused by specific deletion. Eventually identifying and characterizing subpopulations of the STN can contribute to the optimization of deep brain stimulation and help to reduce its side-effects, or even open up possibilities for genetic or optogenetic therapy approaches.

Les comportements motivés dont l‟addiction aux drogues d‟abus, mettent en jeu le système dopaminergique mésolimbique. Aussi connu sous le nom de système de la récompense, celui-ci comprend les neurones à dopamine de l‟aire tegmentale ventrale qui projettent, entre autres, vers le noyau accumbens. Tout comme les neurones de l‟hypothalamus, les neurones à dopamine de l‟aire tegmentale ventrale répondent aux hormones telles que la leptine, l‟insuline et la ghréline pour modifier la prise alimentaire, la motivation ou encore le tonus dopaminergique. Ceci indique que le système dopaminergique mésolimbique est sensible aux signaux hormonaux circulants et suggère que les neurones de l‟aire tegmentale ventrale pourraient percevoir les signaux métaboliques comme le glucose ou les acides gras. De plus, plusieurs études chez les humains et les rongeurs démontrent que l‟obésité et les diètes riches en gras affectent négativement la fonction dopaminergique mésolimbique. Étant donné les lacunes qui demeurent quant aux mécanismes impliqués dans la dysfonction du système dopaminergique mésolimbique induite par la nourriture riche en gras, nous avons cherché à évaluer les effets de l‟acide oléique et de l‟acide palmitique, deux des acides gras les plus abondants dans l‟organisme et l‟alimentation contemporaine, sur le système de la récompense. Ces deux acides gras, l‟un saturé (acide palmitique) et l‟autre mono-insaturé (acide oléique), se distinguent par leurs effets différentiels sur la prise alimentaire, la signalisation hormonale ou encore leur métabolisme intracellulaire mais aussi sur la santé cardiovasculaire et mentale. Nous avons dans un premier temps évalué la capacité du système dopaminergique mésolimbique à détecter les acides gras. Nous avons comparé les effets de l‟injection d‟acide oléique ou d‟acide palmitique dans l‟aire tegmentale ventrale sur la prise alimentaire, la motivation et l‟activité électrique des neurones à dopamine de l‟aire tegmentale ventrale. Nos résultats montrent que l‟acide oléique, mais pas l‟acide palmitique, diminue la prise alimentaire et le comportement motivé. L‟acide oléique inhibe également l‟activité électrique des neurones à dopamine, ces effets semblent dépendre de son entrée dans la cellule. De plus, nous montrons que les neurones à dopamine de l‟aire tegmentale ventrale expriment plusieurs 3 gènes de protéines importantes pour le transport et le métabolisme des acides gras et qu‟ils sont capables de d‟incorporer les acides gras. Nous avons dans un second temps évalué les effets de l‟acide oléique et de l‟acide palmitique dérivés de l‟alimentation. Nous avons soumis des rats à l‟une de ces trois diètes : une riche en gras enrichie en acide oléique, une riche en gras enrichie en acide palmitique ou une contrôle faible en gras. Après huit semaines, et en l‟absence d‟obésité ou d‟altérations métaboliques majeures, la diète enrichie en acide palmitique, mais pas la diète isocalorique enrichie en acide oléique, induit une hyposensibilité aux effets récompensants et locomoteurs de l‟amphétamine, associée, entre autres, à la diminution de la signalisation du récepteur à la dopamine D1R et de l‟expression du transporteur de la dopamine. Nous avons finalement exploré l‟impact de ces diètes sur l‟activité de l‟axe hypothalamo-hypophysaire-surrénalien. Les résultats montrent que la diète enrichie en acide palmitique altère aussi la fonction de l‟axe et l‟expression de plusieurs gènes cibles des corticostéroïdes, sans toutefois modifier le comportement anxieux. Ce travail de doctorat vient compléter les connaissances sur les dysfonctions du système dopaminergique mésolimbique induites par la nourriture riche en gras. Il met en lumière les effets différentiels des classes d‟acides gras et les mécanismes par lesquels ils modulent les comportements motivés et alimentaires. De façon chronique, avant l‟apparition d‟obésité et d‟altérations métaboliques, les acides gras saturés, et non les acides gras mono-insaturés, issus de l‟alimentation perturbent le fonctionnement de l‟axe hypothalamo-hypophysaire-surrénalien et réduisent la fonction dopaminergique. Ceci pourrait contribuer à perpétuer la recherche et la prise de ce type d‟acides gras afin de compenser ce déficit. / The mesolimbic dopamine system, also known as the reward system, is well recognized for its role in motivated reward-related behaviours such as drug addiction. It consists of dopamine neurons originating in the ventral tegmental area that project, among others, to the nucleus accumbens. Similar to neurons in the hypothalamus, dopamine neurons in the ventral tegmental area can detect circulating hormones such as leptin, insulin and ghrelin to adjust food intake, motivation and dopamine tone. This suggests that they could also perceive nutritional signals like glucose and fatty acids. Moreover, several lines of evidence exist showing that palatable food enriched in fat and obesity reduce mesolimbic dopamine function. Given the many unknowns regarding the mechanisms of obesity-induced dopamine dysfunction, and given that fatty acids differentially influence cardiovascular and mental health according to their class, we sought to determine the effects of the monounsaturated fatty acid oleic acid and the saturated fatty acid palmitic acid, two of the most abundant fatty acids in the body and foods, on mesolimbic dopamine function. Notably palmitic acid and oleic acid differ in their intracellular metabolic fate as well as in their effects on food intake and leptin and insulin signaling at the level of the hypothalamus. We first evaluated the fatty acid sensing properties of the mesolimbic dopamine system. We looked at the effects of the injection of oleic acid or palmitic acid in the ventral tegmental area on food intake, motivation and dopamine neurons activity. Our results demonstrate that oleic acid, but not palmitic acid, reduces basal and motivated feeding behavior and neuronal activity. Those effects seem to be dependent on its entry into the cell. Moreover, using a neurons culture system we show that dopamine neurons can uptake fatty acids. We then examined the effect of food-derived oleic and palmitic acid on mesolimbic dopamine function. We assigned rats to a low-fat control diet or to one or the other of a high-fat diet: one enriched in oleic acid or one enriched in palmitic acid. The two high-fat diets are isocaloric and differed only in the fat source. Following eight weeks of feeding, the palmitic 5 acid-enriched high-fat diet, but not the oleic acid-enriched diet, decreased the sensitivity to the rewarding and locomotor-sensitizing effects of amphetamine. This was associated with a reduction of dopamine receptor D1R signaling and dopamine transporter expression. Importantly this occured independently of weight gain and hormonal changes. Lastly, we explored the impact of those diets on the activity of the hypothalamus-pituitary-adrenal axis. Results show that the saturated fat diet alters the function of the axis as well as the expression of several keys genes targeted by glucocorticoids in the hypothalamus but without affecting anxiety-related behavior. This work provides further insight into how the mesolimbic dopamine system is altered by high-fat food consumption. It brings light to the differential effects of two classes of fatty acids and the mechanisms by which they modulate food intake and motivation. The prolonged intake of saturated fat, but not mono-unsaturated fat, disrupts the hypothalamus-pituitary-adrenal axis and decreases mesolimbic dopamine function prior to the onset of obesity and major metabolic alterations. Dysfunction of dopaminergic systems induced by saturated fat consumption could promote further intake of such palatable food as a means to compensate for reward hyposensitivity.

D-amino acid oxidase (DAO) is a flavin-dependent enzyme that is expressed in the mammalian brain. It is the metabolising enzyme of several D-amino acids, including D serine, which is an endogenous agonist at the glycine co-agonist site of the glutamatergic NMDA receptor. As such, regulation of D serine levels in the brain by DAO may indirectly modulate the activity of NMDA receptors. The expression and activity of DAO have been reported to be increased in schizophrenia. It has been identified as a putative susceptibility gene for the disorder, and as a potential therapeutic target. This thesis explored three aspects of the interface between DAO and the DA system. First, the expression of DA was investigated in the ventral tegmental area (VTA), the source of the dopaminergic mesocortical pathway. Traditionally, DAO was considered to be an enzyme confined to the hindbrain and to glia, but more recent studies have reported its expression in additional brain regions, and also in neurons. DAO mRNA and protein was found to be expressed in the VTA, and was present in both neurons and glia in this region, whereas in the cerebellum, DAO expression appeared solely glial. DA output from the VTA is regulated by NMDA receptors, and hence expression of DAO in the VTA suggests that it may serve a role in modulating cortical DA via regulation of D serine levels and NMDA receptor function. The second part of this thesis investigated the effects of DAO inhibition and D serine administration on DA levels in the prefrontal cortex (PFC) using in vivo microdialysis. Systemic DAO inhibition and D serine administration resulted in increases in extracellular levels of DA metabolites in the PFC, despite no detectable change in DA. Similarly, DA metabolites in the PFC increased after local application of D serine to the VTA, but no change was detected in DA. However, local DAO inhibition in the VTA resulted in increased levels of both DA and its metabolites, and DAO inhibition combined with D serine administration also produced increases in DA. This suggested that DAO and its regulation of D-serine levels may serve to indirectly modulate mesocortical DA function, and this may be mediated via the VTA. This notion was supported in the final section of this thesis, in which the expression of three DA genes was measured in the PFC of a novel line of DAO knockout mice. In this pilot study, there was evidence for an increase in Comt and Drd2 mRNAs in the knockout mice. As such, constitutive abolition of DAO activity may also alter mesocortical DA function. These studies provide new insights into the presence and role of DAO beyond the hindbrain, and point to a potentially important physiological function in modulating the activity of the mesocortical DA system via the VTA. This could be therapeutically relevant in the context of elevating cortical DA in the treatment of schizophrenia, and may provide supporting evidence for the clinical use of DAO inhibitors.

No description available.

A amígdala expandida central (EAc) inclui os núcleos central da amígdala (CeA), intersticial lateral da estria terminal (BSTl), intersticial do ramo posterior da comissura anterior (IPAC) e amígdala expandida sublenticular (SLEA). A EAc e a concha do acumbens possuem densa inervação dopaminérgica, implicada em processos motivacionais, e cuja origem foi estudada com a técnica de dupla marcação celular, combinando-se imunofluorescência para o traçador retrógrado Fluoro-Gold e para a tirosina hidroxilase. Nossos resultados indicam que a inervação dopaminérgica do CeA e BSTl é semelhante, se originando em igual proporção da área tegmental ventral (A10) e do núcleo dorsal da rafe/substância cinzenta periaquedutal (A10dc). A inervação dopaminérgica da SLEA, IPAC e concha do acumbens se origina principalmente do grupo A10. Com um anticorpo específico para dopamina vimos que parte da projeção do A10dc para o CeA é de fato dopaminérgica. Os grupos dopaminérgicos diencefálicos não inervam a EAc e a concha do acumbens. / The central extended amygdala (EAc) includes the central amygdaloid nucleus (CeA), lateral bed nucleus of the stria terminalis (BSTl), interstitial nucleus of the posterior limb of the anterior commissure (IPAC) and sublenticular extended amygdala (SLEA). The dopaminergic innervation of the EAc and nucleus accumbens shell is functionally related to motivational processes. Its origin was studied by combining immunofluorescence to tyrosine hydroxylase and Fluoro-Gold, used as retrograde tracer. Our results show that dopaminergic fibers to the CeA and BSTl derive in equal proportion from neurons in ventral tegmental area (A10) and in dorsal raphe nucleus/periaqueductal gray (A10dc). Dopaminergic inputs to SLEA, IPAC and accumbens shell arise mainly from A10 neurons. Using a dopamine antibody, we confirmed that A10dc projections to CeA are in part dopaminergic. Futhermore, the present data indicate that the diencephalic dopaminergic groups do not project to EAc and accumbens shell.