Synapses are a fundamental unit of computation in the brain. Far from being passive connections between spiking neurons, synapses display striking short-term dynamics, undergo long-term changes in strength, and sculpt network-level processes in a complex manner. These synaptic dynamics, both in time and across space, may be a fundamental determinant of population-level computations and behavioral output of the brain, yet their role in neuromodulatory circuits is relatively under-explored. First, I developed and validated a set of likelihood-based inference tools to quantify the dynamics of synaptic ensemble composition throughout development. Second, I examined network computations in the serotonergic dorsal raphe nucleus through a dynamical lens, exploring the role of short-term synaptic dynamics at sparse recurrent connections, and of distinct long-range synaptic inputs, in shaping the output of spiking populations. 1. Simulation-based inference of synaptic ensembles. Functional features of synapses are typically inferred by sampling small ensembles of synapses, yet it is unclear if such subsamples exhibit biases. I developed a statistical framework to address this question, using it to demonstrate that common bulk electrical stimulation methods for characterizing the fraction of silent synapses exhibit high bias and variance, and using typical sample sizes, possess insufficient statistical power for accurate inference. I developed and validated a novel synthetic likelihood-based inference approach based on a simulator of the underlying experimental methodology. This new estimator, made available in an object-oriented Python toolbox, reduces bias and variance compared to previously reported methods, and provides a scalable method for examining synaptic dynamics throughout development. These tools were validated by targeted recording from hippocampal CA1 neurons in juvenile mice, where they reveal fundamental tradeoffs between release probability, number of synapses sampled, and statistical power. 2. Synaptic dynamics and population computations in the serotonin system. This part is comprised of two manuscripts. First, in the dorsal raphe nucleus, I uncovered slow, inhibitory recurrent interactions between serotonin neurons that are generated by local serotonin release. These connections were probabilistic, displayed striking short-term facilitation, gated the spiking output of serotonin neurons, and could be activated by long-range excitatory input from lateral habenula, representing threat signals. Targeted physiology and modeling revealed that these recurrent short-term facilitation features generated paradoxical excitation-driven inhibition in response to high-frequency habenula input. These facilitation rules additionally supported winner-take-all dynamics at the population level, providing a contrastive operation between functionally distinct serotonergic ensembles. Behaviorally, activating long-range lateral habenula input to dorsal raphe nucleus generated a transient, frequency-dependent suppression of reward anticipation consistent with these recurrent dynamics, without modulating the underlying reward association itself. These dynamics, we suggest, support sharp behavioral state transitions in changing environments. In a second manuscript, I explored the multiplexing of distinct long-range inputs in serotonergic circuits through spike synchrony. I demonstrated that a population of serotonergic neurons receives input from both lateral habenula and prefrontal cortex. These inputs produced similar subthreshold events, but prefrontal cortex triggered spikes with much higher latencies, supporting a population synchrony code for input identity. These input-specific spike timing patterns could be read out by simple linear decoders with high accuracy, suggesting they could be demultiplexed by downstream circuits receiving sparse innervation by serotonergic axons. We uncovered a novel intracellular calcium conductance in serotonergic neurons that altered the spectral characteristics of membrane voltage in a manner sufficient to generate long-latency, power law-distributed spike times, suggesting a simple dynamical origin for the production of synchronous or asynchronous spiking. This work indicates that serotonergic circuits can multiplex distinct informational streams through population spike synchrony mechanisms. Together, these investigations reveal that the dynamics of short-term facilitation and synaptic ensemble composition can act as the fundamental substrate for flexible computation by spiking networks across the brain.

The relatively few serotonin (5-HT) neurons located in the Dorsal Raphe Nucleus (DRN) give rise to an extensive axonal network modulating a wide-range of brain functions and behaviors. In turn, the DRN receives inputs from several brain regions and therefore exhibits the characteristics of a hub network. While recent technological advancements have provided an unprecedented look at the neurobiology of the DRN, important knowledge gaps remain in understanding how the constellation of synaptic inputs to this region confers 5-HT neurons their unique coding features. As a first step towards characterizing the DRN's input processing strategy, we set out to explore the landscape of dendritic operation operating in DRN 5-HT neurons. Using multi-photon microscopy and in vitro electrophysical recordings, we conducted a morphological and electrophysiological survey of 5-HT neurons where we identified two structurally and morphologically distinct types of glutamatergic synapses both expressing small NMDAR-mediated conductance. Our initial findings provide valuable insights on local rules that govern how synaptic inputs to the DRN are being processed to ultimately confer 5-HT neurons their unique coding features.

The nine amino acid peptide vasopressin acts as a neurohormone in the periphery and a neurotransmitter/neuromodulator in the central nervous system. Historically, research on vasopressin neurons and their projections to the pituitary has helped lay the groundwork for our understanding of peptidergic neurotransmission. Currently, our research on central vasopressin projections is driving a revolution in our understanding of social behavior. Vasopressin affects a number of social behaviors from social memory to aggression to affiliative behavior, such as pair-bonding. Further, with the addition of more and more transgenic mouse models of disease states, anxiety and depression related disorders, and social behavior dysfunction, it is important now more than ever to have a clear knowledge of the mouse vasopressin system, which derives from a number of distinct nuclei within the brain. Here, I map out vasopressin immunoreactivity in the mouse brain, and delineate the subset of brain regions with gonadal steroid hormone-dependent vasopressin immunoreactivity. Such projections are thought to derive from the bed nuclei of the stria terminalis and medial amygdala in the telencephalon. Finally, based on data from mice with lesions of the suprachiasmatic nucleus, I outline the subset of regions that likely receive vasopressin from this source. Our research on the anatomy of the vasopressin system of mice and our attempts to delineate the site of origin of the many vasopressin fibers found throughout the brain suggest that a significant amount of the vasopressin innervation deriving from cells in the bed nuclei of the stria terminalis and medial amygdala project to areas in the midbrain involved in serotonin and dopamine transmission, such as the dorsal raphe and ventral tegmental area. These transmitter systems play a crucial role in the control of anxiety and depression levels as well as motivated behavior and emotional regulation. Our results strongly suggest that a direct link exists between these systems, and future plans include an examination of this possibility. It is our hope that this work will further our understanding of the role of vasopressin and other transmitter systems in the regulation of social behaviors.

No description available.

A depressão se constitui um transtorno de alta frequência mundial, apresentando como um dos fatores etiológicos, a exposição a eventos estressores. Em termos neurobiológicos, vários trabalhos da literatura sugerem a importância das projeções serotoninérgicas que partem do Núcleo Mediano da Rafe (NMnR) para o Hipocampo Dorsal (HD) na adaptação ao estresse, a partir da desconexão das associações aversivas previamente aprendidas de suas consequências comportamentais. Assim, o presente trabalho apresenta como objetivo investigar o envolvimento dos receptores tipo 5-HT1a localizados no Núcleo Mediano da Rafe no processo de evocação de memórias aversivas. Para isso, ratos Wistar foram submetidos a cirurgia estereotáxica para a implantação de uma cânula-guia direcionada ao Núcleo Mediano da Rafe (NMnR). Passado o período de recuperação, os animais foram expostos a 40 choques inescapáveis (0,6mA; 10) (condição experimental) em uma caixa de vai-e-vem. Após 24 h, os animais receberam duas injeções intracerebrais de Salina (Sal), 8OHDPAT (DPAT; 3nmoles/0,2µl; agonista de 5-HT1aR) e/ou WAY100635 (WAY; 0,3nmoles/0,2µl; antagonista de 5-HT1aR) combinadas de forma a compor os seguintes grupos experimentais: Sal+Sal, WAY+Sal, Sal+DPAT e WAY+DPAT. Cinco minutos após as injeções, todos animais foram submetidos a uma sessão de teste, na qual foram apresentados 30 choques elétricos escapáveis (0,4mA; 10) nas patas, sinalizados por uma luz (15W; 20) que permanecia acesa durante a apresentação dos choques. Durante a sessão teste foram registrados a latência, o número de esquivas, fugas, falhas e o total de cruzamentos. Um grupo controle também foi realizado, sendo que os animais foram apenas colocados na caixa, sem exposição a choques inescapáveis, sendo testados 24 h após. Os resultados foram analisados pela ANOVA de medidas repetidas. Também foi calculada porcentagem de animais desamparados (%DA) em cada condição, analisada pelo teste do Qui-quadrado. A pré-exposição a choques inescapáveis levou ao desenvolvimento de desamparo aprendido, efeito esse não atenuado pelo tratamento com DPAT, o que pode ser observado na elevada %DA em todos os grupos. Por outro lado, na condição controle, observou-se uma reduzida %DA para todos os tratamentos. Nossos resultados sugerem que a ativação de receptores de tipo 5-HT1a do NMnR não está envolvida na evocação de memórias aversivas. / The depression is a high-frequency disorder in the world, having as one of the etiological causes the exposure to stressful events. In neurobiological terms, several studies of literature suggest the importance of serononinergic projections from the Median Raphe Nucleus for dorsal hipocampus in adaptation to stress, from the disconnection of aversive associations previously learnead of its behavioral consequences. Thus, the aim of this work is to investigate the involvement of 5-HT1a receptors located in the Median Raphe Nucleus in the aversive memory recall process. For this, Wistar rats were submitted to sterioc surgery for the implantation of a guided cannula aimed to MnRN. After the recovery period, the animals were exposed to 40 inescapable shocks (0,6mA; 10 \") (experimental condition) in a shuttled box. After 24 h, the animals received two intracerebral injections of saline (Sal), 8OHDPAT (DPAT; 3nmoles / 0,2l; 5-HT1aR agonist) and / or WAY100635 (WAY; 0,3nmoles / 0,2l; antagonist 5 -HT1aR) combined to compose the following groups: Salt + Salt + WAY Sal, Sal + DPAT and WAY + DPAT. Five minutes after the injections, all animals were submitted to a test session in which were presented 30 escapable eletric footshocks (0,4mA; 10 \") sinilized by a light (15W, 20\") which remained on during the presentation of shock. During the test session were recorded latency for these responses, the number of avoidance, escape, failure and the total crossings. A control group was performed, and the animals just placed in the shuttle box, without exposing inescapable shocks. The results were analyzed by repeated measures ANOVA. It was also calculated percentage of helpless animals (%HA) in each condition analyzed by chi-square test. The pre-exposure to inescapable shocks led to the development of learned helplessness, this effect does not attenuated by treatment with DPAT, which can be observed in high % of all groups. On the other hand, in the control condition, there is a reduced % of all treatments. Our results suggest that activation of 5-HT 1a of NMnR is not involved in the evocation of aversive memories.

A depressão se constitui um transtorno de alta frequência mundial, apresentando como um dos fatores etiológicos, a exposição a eventos estressores. Em termos neurobiológicos, vários trabalhos da literatura sugerem a importância das projeções serotoninérgicas que partem do Núcleo Mediano da Rafe (NMnR) para o Hipocampo Dorsal (HD) na adaptação ao estresse, a partir da desconexão das associações aversivas previamente aprendidas de suas consequências comportamentais. Assim, o presente trabalho apresenta como objetivo investigar o envolvimento dos receptores tipo 5-HT1a localizados no Núcleo Mediano da Rafe no processo de evocação de memórias aversivas. Para isso, ratos Wistar foram submetidos a cirurgia estereotáxica para a implantação de uma cânula-guia direcionada ao Núcleo Mediano da Rafe (NMnR). Passado o período de recuperação, os animais foram expostos a 40 choques inescapáveis (0,6mA; 10) (condição experimental) em uma caixa de vai-e-vem. Após 24 h, os animais receberam duas injeções intracerebrais de Salina (Sal), 8OHDPAT (DPAT; 3nmoles/0,2µl; agonista de 5-HT1aR) e/ou WAY100635 (WAY; 0,3nmoles/0,2µl; antagonista de 5-HT1aR) combinadas de forma a compor os seguintes grupos experimentais: Sal+Sal, WAY+Sal, Sal+DPAT e WAY+DPAT. Cinco minutos após as injeções, todos animais foram submetidos a uma sessão de teste, na qual foram apresentados 30 choques elétricos escapáveis (0,4mA; 10) nas patas, sinalizados por uma luz (15W; 20) que permanecia acesa durante a apresentação dos choques. Durante a sessão teste foram registrados a latência, o número de esquivas, fugas, falhas e o total de cruzamentos. Um grupo controle também foi realizado, sendo que os animais foram apenas colocados na caixa, sem exposição a choques inescapáveis, sendo testados 24 h após. Os resultados foram analisados pela ANOVA de medidas repetidas. Também foi calculada porcentagem de animais desamparados (%DA) em cada condição, analisada pelo teste do Qui-quadrado. A pré-exposição a choques inescapáveis levou ao desenvolvimento de desamparo aprendido, efeito esse não atenuado pelo tratamento com DPAT, o que pode ser observado na elevada %DA em todos os grupos. Por outro lado, na condição controle, observou-se uma reduzida %DA para todos os tratamentos. Nossos resultados sugerem que a ativação de receptores de tipo 5-HT1a do NMnR não está envolvida na evocação de memórias aversivas. / The depression is a high-frequency disorder in the world, having as one of the etiological causes the exposure to stressful events. In neurobiological terms, several studies of literature suggest the importance of serononinergic projections from the Median Raphe Nucleus for dorsal hipocampus in adaptation to stress, from the disconnection of aversive associations previously learnead of its behavioral consequences. Thus, the aim of this work is to investigate the involvement of 5-HT1a receptors located in the Median Raphe Nucleus in the aversive memory recall process. For this, Wistar rats were submitted to sterioc surgery for the implantation of a guided cannula aimed to MnRN. After the recovery period, the animals were exposed to 40 inescapable shocks (0,6mA; 10 \") (experimental condition) in a shuttled box. After 24 h, the animals received two intracerebral injections of saline (Sal), 8OHDPAT (DPAT; 3nmoles / 0,2l; 5-HT1aR agonist) and / or WAY100635 (WAY; 0,3nmoles / 0,2l; antagonist 5 -HT1aR) combined to compose the following groups: Salt + Salt + WAY Sal, Sal + DPAT and WAY + DPAT. Five minutes after the injections, all animals were submitted to a test session in which were presented 30 escapable eletric footshocks (0,4mA; 10 \") sinilized by a light (15W, 20\") which remained on during the presentation of shock. During the test session were recorded latency for these responses, the number of avoidance, escape, failure and the total crossings. A control group was performed, and the animals just placed in the shuttle box, without exposing inescapable shocks. The results were analyzed by repeated measures ANOVA. It was also calculated percentage of helpless animals (%HA) in each condition analyzed by chi-square test. The pre-exposure to inescapable shocks led to the development of learned helplessness, this effect does not attenuated by treatment with DPAT, which can be observed in high % of all groups. On the other hand, in the control condition, there is a reduced % of all treatments. Our results suggest that activation of 5-HT 1a of NMnR is not involved in the evocation of aversive memories.

Il y a longtemps qu’on a attribué à l’hippocampe un rôle central dans la mémoire, mais ce n’est pas son unique rôle. Un nombre grandissant d’études attestent que l’hippocampe peut être séparé en deux régions, dorsale et ventrale, qui sont fonctionnellement différentes. La partie dorsale de l’hippocampe est responsable du rôle classique dans la mémoire spatiale et contextuelle, alors que la région ventrale de l’hippocampe est importante dans l’expression de l’anxiété et de la motivation, entre autres. Les projections des noyaux du raphé, l’unique source d’afférences sérotoninergiques de l’hippocampe, auraient un rôle régulateur sur ses fonctions, dont le comportement anxieux. Toutefois, les fonctions de la projection sérotoninergique raphé-hippocampe ventral ne sont pas entièrement caractérisées et les différents rôles des sous-populations de neurones sérotoninergiques au sein même de la projection raphé-hippocampe ventral sont peu connus. Dans ce projet de recherche, nous avons utilisé des tests comportementaux et des outils optogénétiques, afin de déterminer le rôle de la projection sérotoninergique raphé-hippocampe ventral dans le comportement d’aversion. Notre hypothèse est que la sérotonine régule l’anxiété en agissant sur l’hippocampe ventral via cette projection. Nous démontrons entre autres que l’activation de la projection sérotoninergique raphé-hippocampe ventral induit une hausse de l’anxiété, mais spécifiquement chez les femelles. Nous démontrons aussi que l’activation de la projection réduit la locomotion. Nos données offrent un nouveau point de vue sur le rôle du raphé médian dans l’anxiété ainsi que sur l’importance du sexe dans l’expression du comportement anxieux. / It has been known for a long time that the hippocampus has a central role in memory, but it isn’t its sole function. A growing number of studies are showing that the hippocampus can be split in two regions, dorsal and ventral, that are functionally different. The dorsal part is responsible for the classic and well-known role of the hippocampus in spatial and contextual memory, while the ventral region is important for the expression of anxiety and motivation, among other roles. The only serotonergic input of the hippocampus are the raphe nuclei and it has been suggested that it has a regulatory effect over its functions, such as anxiety. Nonetheless, the functions of the raphe-ventral hippocampus serotonergic projection are not fully characterized and sub-populations of serotonergic neurons inside the projection itself aren’t known. In this research project, we used behavioral tests and optogenetic tools to determine whether the raphe to ventral hippocampus serotonergic projection is able to influence aversive behaviors. Our hypothesis is that serotonin regulates anxiety through its influence on the ventral hippocampus via the raphe-ventral hippocampus serotonergic projection. We found that optogenetic activation of the projection induces heightened anxiety, but only in female mice. Our data offer new insight as to how the median raphe regulates anxiety and the importance of sex in the expression of anxiety.

Les troubles anxieux comptent parmi les troubles psychiatriques les plus courants dans le monde, les femmes étant presque deux fois plus susceptibles que les hommes de recevoir un diagnostic de trouble d’anxiété au cours de leur vie. Les neurones sérotoninergiques (5-HT) du raphé médian sont fortement impliqués dans la régulation de l’humeur et de l’anxiété, mais les substrats neuronaux sous-tendant les différences liées au sexe dans l’anxiété sont encore largement méconnus. L'hippocampe ventral (HPv), une région qui a été décrite comme un modulateur majeur de l'anxiété, entre autres grâce à ses communications oscillatoires avec d'autres zones cérébrales, reçoit des afférences denses de 5-HT des noyaux du raphé. Des résultats préliminaires obtenus par notre laboratoire montrent que l’activation optogénétique des neurones 5-HT du raphé qui projettent à l’HPv influence le niveau d’anxiété des souris femelles, mais pas des mâles. En se basant sur ces résultats, l’objectif de mon projet est d’explorer les causes de ce dimorphisme sexuel de la voie raphé-HPv dans l’anxiété. J’analyserai l’expression du marqueur d’activation c-Fos après un test d’anxiété, avec ou sans activation optogénétique de la voie 5-HT raphé-HPv. Notre hypothèse est qu’il existe une différence mâle-femelle dans l’excitabilité des neurones 5-HT projetant à l’HPv. Les résultats obtenus permettront de mettre en lumière i) l’expression de c-Fos dans les neurones 5-HT qui projettent à l’HPv en conditions basales (eYFP) chez les mâles et femelles et ii) la différence dans l’expression de c-Fos après activation optogénétique de notre population d’intérêt chez les mâles et femelles. Nous démontrons que l’activation optogénétique de la voie 5-HT du raphé projetant à l’HPv augmente les comportements anxieux, seulement pour les souris femelles. Nous démontrons aussi que l’activation de cette projection diminue l’activité locomotrice. Par ailleurs, les comportements anxieux semblent activer différemment les sous-régions du raphé en fonction du sexe. Ce travail contribue à une meilleure compréhension des mécanismes sous-jacents au rôle de la voie 5-HT du raphé projetant à l’HPv dans la modulation différentielle des comportements d’anxiété selon le sexe. / Anxiety disorders rank among the most common psychiatric conditions worldwide, with women being nearly twice as likely as men to receive a diagnosis of an anxiety disorder during their lifetime. Serotonergic neurons (5-HT) in the median raphe are heavily involved in regulating mood and anxiety, but the neuronal substrates underlying sex-related differences in anxiety are still largely unknown. The ventral hippocampus (vHP), a region described as a major modulator of anxiety, including through oscillatory communication with other brain areas, receives dense inputs of 5-HT from the raphe nuclei. Preliminary results from our laboratory indicate that optogenetic activation of 5-HT neurons projecting to the vHP influences the level of anxiety in female mice but not in males. Building upon these findings, the aim of my project is to explore the causes of this sexual dimorphism in the raphe-vHP pathway related to anxiety. I will analyze the expression of the c-Fos activation marker after an anxiety test, with or without optogenetic activation of the raphe-vHP 5-HT pathway. Our hypothesis is that there is a male-female difference in the excitability of 5-HT neurons projecting to the vHP. The results obtained will shed light on i) c-Fos expression in 5-HT neurons projecting to the vHP under baseline conditions (eYFP) in males and females, and ii) the difference in c-Fos expression after optogenetic activation of our population of interest in males and females. We demonstrate that optogenetic activation of the raphe-vHP 5-HT pathway increases anxiety behaviors, only in female mice. We also show that activation of this projection decreases locomotor activity. Furthermore, anxiety behaviors appear to activate different subregions of the raphe depending on sex. This work contributes to a better understanding of the underlying mechanisms of the role of the raphe-vHP 5-HT pathway in the differential modulation of anxiety behaviors based on sex.

A abstinência ao etanol está associada ao transtorno de ansiedade. Várias estruturas e sistemas de neurotransmissores estão envolvidos na mediação deste efeito, entretanto, tais substratos ainda não foram completamente elucidados. Em modelos animais, estudos mostram aumento significativo da neurotransmissão nitrérgica em resposta à abstinência do consumo crônico de etanol. O Núcleo Dorsal da Rafe (NDR) é uma importante região envolvida na gênese da ansiedade e possui grande quantidade da isoforma neuronial da enzima Sintase do Óxido Nítrico (nNOS). Assim, o objetivo deste estudo foi verificar se a inibição da transmissão nitrérgica no NDR atenua os efeitos ansiogênicos induzidos pela abstinência do etanol. Com essa finalidade, ratos Wistar machos foram submetidos a tratamento crônico (21 dias) ou agudo (1 dia) com etanol 6%(v/v) seguido da suspensão abrupta do tratamento por 48hs. Após as 48hrs os animais receberam uma injeção intra-NDR de L-NAME (100nmoles/0,2µL; inibidor não seletivo da NOS) ou 7-NI (7-Nitro-Indazol; 5 nmoles/0,2µL; inibidor seletivo para nNOS) e 5 min. depois foram testados no Labirinto em Cruz Elevado (LCE). Em resumo, os dados mostraram que a abstinência de 48hs. do tratamento crônico com etanol promoveu efeitos ansiogênicos significativos, evidenciados pela redução do número de entradas e tempo gasto nos braços abertos do LCE, o qual foi atenuado pelo tratamento intra-NDR com L-NAME mas não com 7-NI, sugerindo o envolvimento do NO no NDR na ansiedade induzida pela abstinência do etanol / The ethanol withdrawal is linked to the disorder of anxiety. Several structures and neurotransmitter systems are involved in mediating this effect, however, such substrates have not yet been elucidated. In animal models, studies show significant increase in neurotransmission nitrérgica in response to abstinence from chronic consumption of ethanol. The Dorsal Raphe Nucleus (DNR) is an important region involved in the genesis of anxiety and has large amounts of the enzyme Sintase neuronial isoform of nitric oxide (nNOS). The aim of this study was to verify that the inhibition of transmission in the DNR nitrérgica attenuates the anxiogenic effects induced by ethanol withdrawal. For this purpose, male Wistar rats were subjected to chronic treatment (21 days) or acute (1 day) with 6% ethanol (v / v) followed by the abrupt suspension of treatment for 48 hours. 48hrs after the animals received an intra-DNR L-NAME (100nmoles / 0.2 L, non-selective inhibitor of NOS) or 7-NI (7-Nitro-indazol, 5 nmol / 0,2 L; selective inhibitor for nNOS ) and 5 min. then were tested in the elevated plus-maze (EPM). Data obtained showed that ethanol withdrawal elicited significant \"anxiety-like\" behaviors, as revealed by the decrease in the number of entries into and time spent onto the open arms of the EPM., which was attenuated by treatment with intra-DNR L-NAME but not with 7-NI. These findings are indicative of the involvement of the DNR \"anxiety-like\" behaviors induced by withdrawal of ethanol.

Introdução: A depressão é um transtorno psiquiátrico cujas causas parecem envolver a exposição crônica a estressores. Sabe-se que o hipocampo dorsal (HD) tem participação no mecanismo de adaptação a estímulos aversivos contínuos, pois a facilitação da neurotransmissão serotonérgica nessa estrutura diminui o efeito de um estressor prévio. A principal fonte de inervação serotonérgica para o HD é o núcleo mediano da rafe (NMnR). Objetivo: O objetivo geral deste trabalho foi verificar se a integridade da via NMnR-HD é essencial para os efeitos antidepressivos do tratamento crônico com Imipramina em animas com lesão dos neurônios serotonérgicos do NMnR e submetidos ao estresse crônico repetido. Materias e Métodos: Foram utilizados ratos Wistar, machos, com sete semanas de vida. Estes animais foram submetidos à cirurgia estereotáxica para lesão (ou não - grupo controle) dos neurônios seronotérgicos do NMnR. Novamente foram subdivididos em grupos de acordo com o tratamento farmacológico que iriam receber cronicamente (Imipramina ou Salina). Todos os animais foram submetidos ao estresse de restrição crônico (7dias/duas horas por dia) ou agudo (episódio único de restrição. No oitavo dia os animais foram testados no Labirinto em cruz Elevado (LCE). Resultados: Quando submetidos ao estresse agudo, animais com o NMnR íntegro e tratados com Imipramina apresentaram uma maior porcentagem de tempo despendido nos braços abertos (%TA) e no número de entradas nos braços abertos (%FA), quando comparados com o controle. Ainda, quando submetidos ao estresse agudo e com NMnR lesionados quimicamente, observou-se uma maior exploração dos braços abertos (%TA e %FA) quando comparados com o grupo sem lesão. Em relação aos animais submetidos ao estresse crônico, tanto os animais com NMnR íntegro, quanto os com este núcleo lesionados, a Imipramina foi capaz de atenuar os efeitos do estresse, pois foi observado uma maior %TA e %FA quando comparados com os animais que receberam salina. A lesão do NMnR em animais submetidos ao estresse crônico e tratados com salina apresentaram uma diminuição tanto da %TA quanto da %FA, quando comparados com as animais tratados com salina e com NMnR íntegro. Discussão: Nossos resultados mostraram que a lesão dos neurônios serotonérgicos do NMnR não interferiu no comportamento dos animais no LCE quando submetidos a um episódio único de estresse, mas diminui a exploração dos braços abertos em animais submetidos ao estresse crônico. Este efeito, contudo, foi atenuado pela administração crônica de Imipramina. Conclusão: Estes resultados sugerem que a integridade da via NMnR-HD não é essencial para os efeitos antidepressivos da Imipramina no desenvolvimento da tolerância ao estresse repetido. / Introduction: Depression is a psychiatric disorder whose causes seem to involve chronic exposure to stressors. It is known that the dorsal hippocampus (DH) participates in the adaptation mechanism to aversive stimuli, once the facilitation of serotonergic neurotransmission in this structure reduces the effect of a stressor. The main source of serotonergic innervation to the HD is the median raphe nucleus (MRN). Aim: The aim of this study was to verify if the integrity of the MRN-HD pathway is essential for the antidepressant effects of chronic treatment with imipramine in animals with lesion on serotonergic neurons of MRN submitted to chronic and repeated stress. Materials and Methods: We used male Wistar rats, seven weeks old. The animals were submitted to stereotactic surgery for lesion (or not - control group) on MRN serotonergic neurons. Later, they were divided in groups according to the chronic drug treatment they would receive (Imipramine or Saline). All animals were subjected to chronic (7days/Two hours a day) or acute restraint stress (a single episode of restraint). On the eighth day the animals were tested in the Elevated Plus Maze (EPM). Results: When subjected to acute stress, animals with intact MRN and treated with imipramine showed a higher percentage of time spent in open arms (% TO) and the number of entries in open arms (% FO) when compared to control. Moreover, rats subjected to acute stress and chemically injured showed a greater exploration of open arms (% TO and % FO) when compared with the group without lesion. Imipramine was able to attenuate the effects of stress on animals subjected to chronic stress from both groups, with intact MRN and those with lesion. That was observed on a increase of % TO and % FO when compared with rats treated with saline. The injury on MNR in animals submitted to chronic stress decreased both %TO and % FO. Discussion: Our results showed that the lesion on serotonergic neurons of MNR dont interfere in the behavior of animals in the EPM when subjected to a single stress episode, but decreases the open arm exploration in rats exposed to chronic stress. This effect, however, was attenuated by chronic administration of imipramine. Conclusion: These results suggest that the integrity of the NMnR-HD pathway is not essential for the antidepressant effects Imipramine in the development of tolerance to repeated stress.