Social anxiety disorder (SAD) is one of the most common anxiety disorders and understanding its symptoms and risk factors is vital for developing treatments and prevention strategies. Atypical physiological responses have been observed in anxious individuals and their consequences present a human and economic burden. This dissertation includes two studies that explore the subjective and physiological responses to an acute stress in the context of treatment and risk factors for SAD. The goal of the first study was to examine whether cognitive behavioural group therapy (CBGT) and a mindfulness-based stress reduction (MBSR) program differentially influenced the subjective and physiological response to a speech task. Participants in the treatment groups performed two speech tasks, before and after treatment, while a healthy control group completed it only once. Results indicated significant differences for the subjective, but not the physiological measures of stress. Patients with SAD reported higher subjective anxiety than the healthy control group and these scores were significantly reduced following treatment. Greater improvements were noted in the CBGT group; nonetheless, the study did indicate promising results for MBSR. The second study aimed to explore the effects of behavioural inhibition (BI), parental bonding variables and their interaction on the subjective and physiological responses to a similar speech task in healthy children. BI was related to subjective anxiety in a predictive manner, but was generally unrelated to the physiological measures. Parental bonding variables were not related to any of the stress responses and no interaction between BI and parental bonding was observed. These studies contribute to the literature by demonstrating treatment differences and their subjective and physiological consequences on stress reactions and exploring the extent to which risk factors for SAD affect the stress response in healthy children.

In fish, maternal stress prior to spawn has been reported to have effects on offspring phenotype. Cortisol, the main glucocorticoid (GC) stress hormone, has been proposed as a potential mediator of such effects because of its organizational role in early teleost development. The present thesis tested whether maternal social stress or treatment with cortisol (as a proxy for maternal stress) prior to spawn affects the cortisol response to stress and anxiety-related behaviours in offspring. In zebrafish (Danio rerio), offspring of dominant females exhibited greater boldness at 6 days post-fertilization (DPF). Interestingly, offspring of females that engaged in social interactions, regardless of the resulting social status of the two females, exhibited greater survival at 1 DPF, a greater fear-related decrease in activity in response to bright light at 6 DPF, and decreased baseline whole-body cortisol content at 0 and 30 DPF. A field experiment with wild largemouth bass (Micropterus salmoides) revealed that maternal cortisol treatment prior to spawn also affected offspring phenotype; offspring of cortisol-treated females had higher masses right after hatch, had greater fear responses, were less bold and less anxious, and exhibited an attenuated cortisol response to an acute stressor. Together, the results of the present thesis suggest that effects of maternal stress prior to spawn on offspring survival, growth, responses to stress, and anxiety-related behaviours are mediated, at least in part, by elevated maternal cortisol but not likely via increased deposition of maternal cortisol into eggs. The effects of maternal stress and cortisol treatment on offspring reported in the present thesis also suggest that maternal stress may prime offspring with adaptive traits to better survive in a stressful environment.

L’addiction à la nicotine est une pathologie qui concerne un tiers de la population adulte mondiale et qui est souvent associée avec d’autres troubles psychiatriques tels que la dépression, la schizophrénie ou encore les troubles liés au stress. Chaque année, près de 8 millions de personnes décèdent des conséquences de la consommation de tabac. Cette pathologie constitue la première cause de morts évitables dans le monde. Ce phénomène de dépendance au tabac est induit par la nicotine, principale substance addictive et psychoactive du tabac, qui va agir sur les récepteurs nicotiniques de l’acétylcholine (nAChR) et ainsi détourner le fonctionnement normal de différents circuits neuronaux. De manière aigüe, la nicotine agit directement sur les nAChR ce qui va globalement activer les réseaux neuronaux. A plus long terme, elle va induire une plasticité synaptique et perturber la transmission nicotinique endogène. La nicotine va notamment détourner le système dopaminergique, acteur majeur de l’apprentissage par renforcement, de la motivation et de l’évaluation de la récompense. Ces modifications neuronales conduisent non seulement au renforcement mais entrainent aussi une perturbation de différents traits comportementaux (prise de décision, exploration, vulnérabilité au stress, etc.). Ces relations entre symptômes et traits pourraient expliquer les fortes comorbidités observées entre la dépendance aux drogues d’abus, et particulièrement au tabac, et d’autres manifestations pathologiques telles que les troubles liés au stress. Au cours de cette thèse j’ai tout d’abord abordé les bases neurophysiologiques qui sous-tendent ces comorbidités, en proposant la dopamine comme un substrat commun aux effets du stress social, de la nicotine et des perturbations de la prise de décision associées (impulsivité, sensibilité à la récompense, évaluation du risque, etc.). J’ai pu montrer que l’augmentation de l’activité des neurones dopaminergiques observée après une exposition à la nicotine ou à un stress social est responsable des perturbations des comportements de choix chez la souris. En effet, nous avons pu reproduire ces altérations comportementales en élevant artificiellement le niveau d’activité des neurones dopaminergiques à l’aide de stimulations optogénétiques. La dissection des mécanismes par lesquels la nicotine détourne les circuits neuronaux se heurte aujourd’hui à un manque d’outils permettant une manipulation sélective, réversible et avec une résolution spatio-temporelle suffisante des acteurs moléculaires impliqués. Une deuxième partie de mon travail de thèse a consisté en l’implémentation in vivo chez la souris, de la pharmacologie optogénétique pour les nAChR. La photo-inhibition des nAChR contenant la sous-unité beta2 nous a permis de mettre en évidence l’impact de la modulation cholinergique endogène sur l’activité des neurones dopaminergiques. Nous avons pu, en outre, inhiber la réponse de ces mêmes neurones à l’injection intraveineuse aiguë de nicotine et le renforcement associé dans une tâche de préférence de place conditionnée pour la nicotine. / Nicotine addiction is a condition that affects one third of the world's adult population and is often associated with other psychiatric disorders such as schizophrenia, mood- and stress-related disorders. Every year, nearly 8 million people die from the consequences of tobacco use. This pathology is the leading cause of preventable death in the world. This phenomenon of tobacco dependence is induced by nicotine, the main addictive and psychoactive substance in tobacco, which acts on nicotinic acetylcholine receptors (nAChRs) and thus hijacks the normal functioning of various neuronal circuits. Acute nicotine directly acts on nAChRs and activates neural networks. In the longer term, it will induce synaptic plasticity and disrupt endogenous nicotinic transmission. In particular, nicotine disrupts the dopaminergic system, a key player in reinforcement learning, motivation and reward evaluation. These neural changes not only lead to reinforcement but also to a disruption of different behavioral traits such as decision-making, exploration, vulnerability to stress, etc. These relationships between symptoms and features could explain the strong comorbidities observed between substance abuse, and particularly tobacco addiction, and other pathologies such as stress-related disorders. During this thesis, I first addressed the neurophysiological bases underlying these comorbidities, by proposing dopamine as a common substrate for the effects of social stress, nicotine and associated decision-making disorders (impulsivity, reward sensitivity, risk assessment, etc.). I have shown that the increase in dopamine neuron activity observed after exposure to nicotine or social stress is responsible for disrupting choice behavior in mice. Indeed, we could reproduce these behavioral maladaptations by artificially increasing the activity level of dopaminergic neurons using optogenetic stimuli. The dissection of the mechanisms by which nicotine diverts neuronal circuits is currently hampered by a lack of tools for selective, reversible, spatially and temporally precise manipulation of the molecular players involved. A second part of my thesis work consisted in the in vivo implementation in mice of optogenetic pharmacology for nAChR. The photoinhibition of beta2-containing nAChRs revealed the impact of endogenous cholinergic modulation on the activity of dopaminergic neurons. We could optically inhibit the response of these same neurons to acute intravenous injection of nicotine and the associated reinforcement in a task of conditioned place preference for nicotine.

The ability to learn and change future behaviour based on past experiences is crucial for the life and survival of animals. For various behaviours exhibited by animals it is clear that in a seemingly homogeneous population not all individuals behave the same way, even in invertebrates. In crickets (Gryllus bimaculatus), a model system for the mechanisms of intra-specific aggression, agonistic experiences with the underlying impact of neuromodulators have been identified as a cause of inter-individual differences. For mammals and humans, the experience of adversity and stress can have detrimental effects on cognitive abilities and chronic defeat stress is used as a model for depression. In crickets the equivalent, the chronic social defeat stress paradigm, has been established. This thesis first sets out to construct a new model for measuring a conditioned response from multiple behavioural aspects and quantify learning in individual crickets. Video tracking of responses revealed behavioural variables that were included in a binary logistic regression analysis, whereas the resulting multi-variable model proves to be superior to other models constructed and can give the probability of an individual exhibiting a conditioned response. With this, learning indices can be calculated for each individual trained in a differential appetitive olfactory paradigm. With the method at hand, this thesis reveals that the experience of chronic social stress impairs learning in crickets, susceptible and resilient to defeat stress alike. The experience of multiple wins, however, does neither improve nor decrease learning abilities, but a long-term winner effect on aggression could be shown. Although inter-individual differences in learning are present, the aggressive state of crickets is not correlated to the learning indices. The application of serotonergic drugs that block receptors or act as re-uptake inhibitors reveal the influence of serotonin on learning within this paradigm. In addition to maintaining reduced aggressiveness, serotonin promotes the impairment of learning after the experience of chronic social defeat stress.:1 INTRODUCTION.....................................................................................................1 2 MATERIALS AND METHODS............................................................................... 8 2.1 Experimental animals...................................................................................... 8 2.2 Appetitive olfactory conditioning..................................................................... 8 2.2.1 Odour application and rewarding....................................................... 8 2.2.2 Absolute conditioning paradigm........................................................ 10 2.2.3 Differential conditioning paradigm.................................................... 11 2.3 Experimental setup for video-tracking.............................................................. 12 2.4 Binary logistic regression model....................................................................... 13 2.4.1 Binary groups for model building...................................................... 13 2.4.2 Variables of a behavioural response................................................... 14 2.4.3 Calculating a conditioned odour response probability (Presp) ............ 15 2.5 Evaluation of learning with the binary logistic model...................................... 17 2.6 Evaluation of aggression with a standardised fight.......................................... 18 2.7 Multiple agonistic experiences......................................................................... 19 2.7.1 Chronic social defeat stress................................................................ 19 2.7.2 Multiple wins..................................................................................... 20 2.8 Serotonin......................................................................................................... 20 2.8.1 Pharmacological treatments............................................................... 20 2.8.2 Methiothepin and ketanserin.............................................................. 21 2.8.3 Fluoxetine with non-chronic defeat................................................... 21 2.9 Additional data analysis and statistic................................................................ 22 3 RESULTS............................................................................................................ 23 3.1 Binary logistic regression model for quantifying learning............................... 23 3.1.1 Behavioural variables of a conditioned odour response.................... 23 3.1.2 Model building and selection............................................................. 29 3.1.3 Odour response probabilities (Presp)................................................... 31 3.1.4 Application of the regression model to assess the quantification of learning.................................................................................................... 34 3.2 The influence of agonistic experiences on aggression and learning................. 39 3.2.1 Chronic social defeat stress................................................................ 39 3.2.2 Multiple experiences of winning........................................................ 46 3.2.3 Correlation of aggression and learning............................................... 48 3.2.4 Summary of learning capacities – multiple experiences.................... 50 3.3 The influence of serotonergic drugs on learning after chronic defeat.............. 51 3.3.1 Methiothepin and ketanserin.............................................................. 51 3.3.2 Fluoxetine........................................................................................... 57 3.3.3 Summary of learning capacities – chronic defeat and serotonin........ 60 4 DISCUSSION....................................................................................................... 63 4.1 The semi-automated measurement of olfactory learning in individually assayed crickets................................................................................................................... 64 4.2 The influence of multiple agonistic experiences on learning........................... 71 4.3 The role of serotonin in chronic social defeat influenced learning................... 77 4.4 Overall conclusion and outlook........................................................................ 80 5 SUMMARY........................................................................................................... 82 6 ZUSAMMENFASSUNG........................................................................................ 87 7 REFERENCES.................................................................................................... 93 8 APPENDIX................................................................................................... 106 8.1 Figures and tables................................................................................... 106 8.2 Publications and published abstracts....................................................... 108 8.3 Curriculum vitae....................................................................................... 109 8.4 Acknowledgements.................................................................................. 111

No description available.

The purpose of this study was to disentangle the psychobiological mechanisms and social-evaluative conditions that mediate the process by which the Autonomic Nervous System reacts in male and female humans. We used the original Trier Social Stress Test, as well as two modifications to this original social stressor: a punishment modification and a reward modification. We obtained measures of autonomic (heart rate and respiratory sinus arrhythmia; HR and SA respectively) reactivity before, during and after the stress test. To distinguish the contribution of the different modifications and any additional difference in reactivity due to gender, the participants were randomly separated into the three modifications, where N=35 (17 male) for the no modification group, N=12 (7 male) for the punishment condition, and N=13 (8 male) for the reward condition. All participants exhibited ANS reactivity to the stressor; females exhibited the most magnified response to all modifications. Overall, the most ANS reactivity was found within the reward condition, with the no modification group exhibiting the least amount of reactivity. This suggests that the reward paradigm was the most salient of all the stressors. Evidence indicated that the ANS stress response system is highly sensitive to potential for gain and reward, especially in females.

Over the years stress has been a term lacking one clear and specific definition. In general, the term stress has been used mostly as an explanation of a response or reaction to a stressor. A stressor can be of both physiological and behavioral character. The experience of stress can occur both due to a real or a perceived stressor. In this literature review, the concept of stress is viewed with insights from biological, psychological, and social perspectives. The stress response is described biologically with the central nervous system (CNS), the brain, and the hypothalamic-pituitary-adrenal (HPA) axis. Social and psychological stress are concepts related to how stress is perceived by the mind and due to social surroundings which is described in relation to social support, self-efficacy, the locus of control and cognitive appraisal. Dealing with stress can be done through coping which refers to the individual capacity to handle a stressor and has generally been divided into two categories, active/passive coping and problem-focused/emotion-focused coping. Depending on the individual resources to cope with a stressor and the ability to decrease the stress response when needed, the long-term effects of stress can therefore vary between individuals. It has been found that positive coping (known as reducing stress) can increase the anterior cingulate cortex (ACC) volume and decrease anxiety and depression. The prefrontal cortex (PFC), the hippocampus, and the amygdala are closely linked to the ACC and affect emotions, learning, and memory related to the stress response.

Transtornos de humor tais como a depressão e a ansiedade estão entre as desordens psiquiátricas mais comuns na atualidade, com tendência de aumento do número de casos, assim como vêm ocorrendo nas últimas décadas. O Transtorno Depressivo Maior (TDM), uma desordem psiquiátrica dispendiosa e ameaçadora da vida, afeta profunda e negativamente a qualidade de vida dos indivíduos afetados e irá atingir até 20% da população em algum momento ao longo de sua existência. Porém, a descrição de sua patofisiologia segue incompleta e o principal pré-requisito para controlar a doença é entender de forma detalhada as alterações moleculares e comportamentais que a acompanham. O estresse social, como um dos principais indutores da depressão, tem sido alvo de estudos tanto clínicos quanto pré-clínicos, servindo também como um mecanismo laboratorial que auxilia pesquisadores a rastrear alterações moleculares e comportamentais dessa doença. Recentemente, sem deixar de lado as demais hipóteses, o sistema imunológico através de respostas inflamatórias, tem recebido atenção crescente e é investigado por ser potencialmente um indutor e/ou facilitador de estados depressivos, contribuindo para a patofisiologia da depressão. Além disso, considerando que o estresse não afeta a todos os indivíduos da mesma maneira, a compreensão das diferenças individuais que podem resultar em resiliência pode auxiliar pesquisadores quanto aos fatores que afetam o desenvolvimento ou não do transtorno na população. Esta tese é composta por dois artigos, e visa investigar em um modelo pré-clínico, a contribuição do estresse social por subordinação (do inglês, social defeat – SD) ao comportamento tipo-depressivo, relacionando-o com inflamação. No primeiro artigo foi revisada a literatura mais recente sobre a contribuição do SD para a ativação microglial, o principal elemento neuroinflamatório do sistema nervoso central (SNC), e sua relação com o desenvolvimento dos comportamentos tipo-ansioso e tipo-depressivo. No segundo artigo investigou-se o papel de um estressor social contínuo (21 dias consecutivos de derrota social crônica) em um grupo de ratos Wistar adultos, relacionando respostas comportamentais a alterações de marcadores imunológicos periféricos e a duas estratégias de coping. O modelo de estresse por derrota social tem sido utilizado em diversos estudos de transtornos psiquiátricos e é uma das principais formas de indução de estados tipo-depressivos em animais de laboratório. Ademais, vêm demonstrando ser útil na indução de alterações do sistema 11 imunológico, tanto centrais quanto periféricas. Exposição ao estresse por derrota social induz em células microgliais um estado de hiperativação que, dependendo do tempo de exposição, pode levar ao desenvolvimento de desordens psiquiátricas como a ansiedade e a depressão. Além disso, o protocolo de 21 dias de derrota social contínua revelou dois estilos comportamentais em ratos Wistar. As estratégias de coping passivo e ativo observadas estão relacionadas a vulnerabilidade e a resiliência, respectivamente, e foram correlacionadas com distintos perfis imunológicos periféricos. Animais resilientes apresentam comportamentos e perfil imunológico periférico que os protegem do desenvolvimento de psicopatologias associadas ao estresse. / Humor disorders such as depression and anxiety are among the currently most common disorders, with a trend to an increasing number of cases, like in the past few decades. Major Depressive Disorder (MDD) is an expensive and life-threatening psychiatric disorder that profoundly and negatively affects individual´s quality of life and will affect up to 20% of the population at some point throughout life. However, the pathophysiology of MDD remains incompletely described, and a detailed description of its molecular and behavioral alterations is one of the core prerequisites for disease control. Social stress, one of the main inducers of depression, has been the subject of both clinical and preclinical studies, and has been used as a laboratory tool to help researchers track molecular and behavioral changes of this disease. Recently, without leaving other hypotheses aside, the immune system through inflammatory responses has received increasing attention and is investigated as a potential contributor in the pathophysiology of depression. Furthermore, considering that stress does not affect all individuals to de same extend, understanding individual differences that can turn into resilience may help researchers unravel the factors that influence the development of this disorder in the population. The present dissertation is composed of two articles, aiming to investigate in a preclinical model the contribution of social defeat stress (SD) to depressive-like behavior and correlate it to inflammation. In the first article, we reviewed the most recent literature on the contribution of SD to microglial activation, the main neuroinflammatory element of the central nervous system (CNS), and its relation to the development of anxiety and depressive-like behaviors. In the second paper, we investigated the role of a continuous social stressor (21 consecutive days of chronic social defeat) in a group of adult male Wistar rats, relating behavioral responses and two different coping strategies to changes in peripheral immune markers. The social defeat stress model has been used in several studies of psychiatric disorders and is one of the main forms to induce depressive-like states in laboratory animals. Additionally, it has been shown to be useful in inducing central and peripheral immune alterations. Exposure to stress due to social defeat induces microglial cells into a state of hyperactivation that, depending on the time of exposure, can lead to the development of psychiatric disorders such as anxiety and depression. Furthermore, the 21-day protocol of continuous social defeat revealed two behavioral 13 styles in Wistar rats. The observed passive and active coping strategies are related to vulnerability and resilience, respectively, and have been correlated with different peripheral immunological profiles. Resilient animals present behaviors and a peripheral immune profile that protect them from the development of stress-associated psychopathologies.

Transtornos de humor tais como a depressão e a ansiedade estão entre as desordens psiquiátricas mais comuns na atualidade, com tendência de aumento do número de casos, assim como vêm ocorrendo nas últimas décadas. O Transtorno Depressivo Maior (TDM), uma desordem psiquiátrica dispendiosa e ameaçadora da vida, afeta profunda e negativamente a qualidade de vida dos indivíduos afetados e irá atingir até 20% da população em algum momento ao longo de sua existência. Porém, a descrição de sua patofisiologia segue incompleta e o principal pré-requisito para controlar a doença é entender de forma detalhada as alterações moleculares e comportamentais que a acompanham. O estresse social, como um dos principais indutores da depressão, tem sido alvo de estudos tanto clínicos quanto pré-clínicos, servindo também como um mecanismo laboratorial que auxilia pesquisadores a rastrear alterações moleculares e comportamentais dessa doença. Recentemente, sem deixar de lado as demais hipóteses, o sistema imunológico através de respostas inflamatórias, tem recebido atenção crescente e é investigado por ser potencialmente um indutor e/ou facilitador de estados depressivos, contribuindo para a patofisiologia da depressão. Além disso, considerando que o estresse não afeta a todos os indivíduos da mesma maneira, a compreensão das diferenças individuais que podem resultar em resiliência pode auxiliar pesquisadores quanto aos fatores que afetam o desenvolvimento ou não do transtorno na população. Esta tese é composta por dois artigos, e visa investigar em um modelo pré-clínico, a contribuição do estresse social por subordinação (do inglês, social defeat – SD) ao comportamento tipo-depressivo, relacionando-o com inflamação. No primeiro artigo foi revisada a literatura mais recente sobre a contribuição do SD para a ativação microglial, o principal elemento neuroinflamatório do sistema nervoso central (SNC), e sua relação com o desenvolvimento dos comportamentos tipo-ansioso e tipo-depressivo. No segundo artigo investigou-se o papel de um estressor social contínuo (21 dias consecutivos de derrota social crônica) em um grupo de ratos Wistar adultos, relacionando respostas comportamentais a alterações de marcadores imunológicos periféricos e a duas estratégias de coping. O modelo de estresse por derrota social tem sido utilizado em diversos estudos de transtornos psiquiátricos e é uma das principais formas de indução de estados tipo-depressivos em animais de laboratório. Ademais, vêm demonstrando ser útil na indução de alterações do sistema 11 imunológico, tanto centrais quanto periféricas. Exposição ao estresse por derrota social induz em células microgliais um estado de hiperativação que, dependendo do tempo de exposição, pode levar ao desenvolvimento de desordens psiquiátricas como a ansiedade e a depressão. Além disso, o protocolo de 21 dias de derrota social contínua revelou dois estilos comportamentais em ratos Wistar. As estratégias de coping passivo e ativo observadas estão relacionadas a vulnerabilidade e a resiliência, respectivamente, e foram correlacionadas com distintos perfis imunológicos periféricos. Animais resilientes apresentam comportamentos e perfil imunológico periférico que os protegem do desenvolvimento de psicopatologias associadas ao estresse. / Humor disorders such as depression and anxiety are among the currently most common disorders, with a trend to an increasing number of cases, like in the past few decades. Major Depressive Disorder (MDD) is an expensive and life-threatening psychiatric disorder that profoundly and negatively affects individual´s quality of life and will affect up to 20% of the population at some point throughout life. However, the pathophysiology of MDD remains incompletely described, and a detailed description of its molecular and behavioral alterations is one of the core prerequisites for disease control. Social stress, one of the main inducers of depression, has been the subject of both clinical and preclinical studies, and has been used as a laboratory tool to help researchers track molecular and behavioral changes of this disease. Recently, without leaving other hypotheses aside, the immune system through inflammatory responses has received increasing attention and is investigated as a potential contributor in the pathophysiology of depression. Furthermore, considering that stress does not affect all individuals to de same extend, understanding individual differences that can turn into resilience may help researchers unravel the factors that influence the development of this disorder in the population. The present dissertation is composed of two articles, aiming to investigate in a preclinical model the contribution of social defeat stress (SD) to depressive-like behavior and correlate it to inflammation. In the first article, we reviewed the most recent literature on the contribution of SD to microglial activation, the main neuroinflammatory element of the central nervous system (CNS), and its relation to the development of anxiety and depressive-like behaviors. In the second paper, we investigated the role of a continuous social stressor (21 consecutive days of chronic social defeat) in a group of adult male Wistar rats, relating behavioral responses and two different coping strategies to changes in peripheral immune markers. The social defeat stress model has been used in several studies of psychiatric disorders and is one of the main forms to induce depressive-like states in laboratory animals. Additionally, it has been shown to be useful in inducing central and peripheral immune alterations. Exposure to stress due to social defeat induces microglial cells into a state of hyperactivation that, depending on the time of exposure, can lead to the development of psychiatric disorders such as anxiety and depression. Furthermore, the 21-day protocol of continuous social defeat revealed two behavioral 13 styles in Wistar rats. The observed passive and active coping strategies are related to vulnerability and resilience, respectively, and have been correlated with different peripheral immunological profiles. Resilient animals present behaviors and a peripheral immune profile that protect them from the development of stress-associated psychopathologies.

Transtornos de humor tais como a depressão e a ansiedade estão entre as desordens psiquiátricas mais comuns na atualidade, com tendência de aumento do número de casos, assim como vêm ocorrendo nas últimas décadas. O Transtorno Depressivo Maior (TDM), uma desordem psiquiátrica dispendiosa e ameaçadora da vida, afeta profunda e negativamente a qualidade de vida dos indivíduos afetados e irá atingir até 20% da população em algum momento ao longo de sua existência. Porém, a descrição de sua patofisiologia segue incompleta e o principal pré-requisito para controlar a doença é entender de forma detalhada as alterações moleculares e comportamentais que a acompanham. O estresse social, como um dos principais indutores da depressão, tem sido alvo de estudos tanto clínicos quanto pré-clínicos, servindo também como um mecanismo laboratorial que auxilia pesquisadores a rastrear alterações moleculares e comportamentais dessa doença. Recentemente, sem deixar de lado as demais hipóteses, o sistema imunológico através de respostas inflamatórias, tem recebido atenção crescente e é investigado por ser potencialmente um indutor e/ou facilitador de estados depressivos, contribuindo para a patofisiologia da depressão. Além disso, considerando que o estresse não afeta a todos os indivíduos da mesma maneira, a compreensão das diferenças individuais que podem resultar em resiliência pode auxiliar pesquisadores quanto aos fatores que afetam o desenvolvimento ou não do transtorno na população. Esta tese é composta por dois artigos, e visa investigar em um modelo pré-clínico, a contribuição do estresse social por subordinação (do inglês, social defeat – SD) ao comportamento tipo-depressivo, relacionando-o com inflamação. No primeiro artigo foi revisada a literatura mais recente sobre a contribuição do SD para a ativação microglial, o principal elemento neuroinflamatório do sistema nervoso central (SNC), e sua relação com o desenvolvimento dos comportamentos tipo-ansioso e tipo-depressivo. No segundo artigo investigou-se o papel de um estressor social contínuo (21 dias consecutivos de derrota social crônica) em um grupo de ratos Wistar adultos, relacionando respostas comportamentais a alterações de marcadores imunológicos periféricos e a duas estratégias de coping. O modelo de estresse por derrota social tem sido utilizado em diversos estudos de transtornos psiquiátricos e é uma das principais formas de indução de estados tipo-depressivos em animais de laboratório. Ademais, vêm demonstrando ser útil na indução de alterações do sistema 11 imunológico, tanto centrais quanto periféricas. Exposição ao estresse por derrota social induz em células microgliais um estado de hiperativação que, dependendo do tempo de exposição, pode levar ao desenvolvimento de desordens psiquiátricas como a ansiedade e a depressão. Além disso, o protocolo de 21 dias de derrota social contínua revelou dois estilos comportamentais em ratos Wistar. As estratégias de coping passivo e ativo observadas estão relacionadas a vulnerabilidade e a resiliência, respectivamente, e foram correlacionadas com distintos perfis imunológicos periféricos. Animais resilientes apresentam comportamentos e perfil imunológico periférico que os protegem do desenvolvimento de psicopatologias associadas ao estresse. / Humor disorders such as depression and anxiety are among the currently most common disorders, with a trend to an increasing number of cases, like in the past few decades. Major Depressive Disorder (MDD) is an expensive and life-threatening psychiatric disorder that profoundly and negatively affects individual´s quality of life and will affect up to 20% of the population at some point throughout life. However, the pathophysiology of MDD remains incompletely described, and a detailed description of its molecular and behavioral alterations is one of the core prerequisites for disease control. Social stress, one of the main inducers of depression, has been the subject of both clinical and preclinical studies, and has been used as a laboratory tool to help researchers track molecular and behavioral changes of this disease. Recently, without leaving other hypotheses aside, the immune system through inflammatory responses has received increasing attention and is investigated as a potential contributor in the pathophysiology of depression. Furthermore, considering that stress does not affect all individuals to de same extend, understanding individual differences that can turn into resilience may help researchers unravel the factors that influence the development of this disorder in the population. The present dissertation is composed of two articles, aiming to investigate in a preclinical model the contribution of social defeat stress (SD) to depressive-like behavior and correlate it to inflammation. In the first article, we reviewed the most recent literature on the contribution of SD to microglial activation, the main neuroinflammatory element of the central nervous system (CNS), and its relation to the development of anxiety and depressive-like behaviors. In the second paper, we investigated the role of a continuous social stressor (21 consecutive days of chronic social defeat) in a group of adult male Wistar rats, relating behavioral responses and two different coping strategies to changes in peripheral immune markers. The social defeat stress model has been used in several studies of psychiatric disorders and is one of the main forms to induce depressive-like states in laboratory animals. Additionally, it has been shown to be useful in inducing central and peripheral immune alterations. Exposure to stress due to social defeat induces microglial cells into a state of hyperactivation that, depending on the time of exposure, can lead to the development of psychiatric disorders such as anxiety and depression. Furthermore, the 21-day protocol of continuous social defeat revealed two behavioral 13 styles in Wistar rats. The observed passive and active coping strategies are related to vulnerability and resilience, respectively, and have been correlated with different peripheral immunological profiles. Resilient animals present behaviors and a peripheral immune profile that protect them from the development of stress-associated psychopathologies.