Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

O núcleo tegmental rostromedial (RMTg) é uma estrutura relé GABAérgica entre a habênula lateral (LHb) e a área tegmental ventral (VTA), que também recebe projeções diretas da LHb. Para detalhar a topografia das projeções da LHb para o RMTg e VTA, injeções de um traçador anterógrado foram depositadas na LHb. A origem das projeções da LHb para o RMTg e VTA foi confirmada com injeções de um traçador retrógrado, no RMTg ou VTA. Além disso, comparamos a posição topográfica dos neurônios marcados no RMTg após injeções na VTA, com os axônios marcados, vindas da LHb. Nossos dados revelaram que as projeções da LHb estão organizadas de maneira topográfica, com projeções para o RMTg, se originando da LHb lateral e as projeções para a VTA da LHb medial. No RMTg, axônios oriundos da LHb foram observados pertos de neurônios que se projetam para a VTA, formando aposições com esses neurônios. Os resultados indicam que as projeções da LHb para o RMTg e VTA originam-se de diferentes regiões da LHb e provam a existência de uma via bisináptica que conecta a LHb com a VTA via o RMTg. / The rostromedial segmental nucleus (RMTg) is a GABAergic relay between the lateral habenula (LHb) and the ventral tegmental area (VTA), which also receives direct projections from the LHB. To detail the topography of LHb projections to the RMTg and VTA, injections of an anterograde tracer were made into the LHb. The origin of LHb projections to the RMTg and VTA was then confirmed by injections of a retrograde tracer into the RMTg or VTA. Additionally, we compared the topographic position of RMTg neurons labeled from injections in the VTA with that of axons labeled from the LHb. Our data revealed that LHb projections are organized in a topographic manner, with projections to the RMTg emerging from the lateral LHb and those to the VTA arising from the medial LHb. In the RMTg, LHb axons were found in register with VTA projecting neurons and were frequently apposed to the latter. Our findings indicate that LHb inputs to the RMTg and VTA arise from different divisions of the LHb and provide evidence for a disynaptic pathway that links the LHb to the VTA via the RMTg.

A habênula é uma estrutura epitalâmica diferenciada em dois complexos nucleares, a habênula medial (MHb) e a habênula lateral (LHb). Recentemente, a MHb junto com seu alvo principal, o núcleo interpeduncular (IP), foram identificados como estruturas chaves envolvidas na mediação dos efeitos aversivos da nicotina. Contudo, estruturas intimamente interligadas com o eixo MHb-IP, como o núcleo mediano (MnR), a parte caudal do núcleo dorsal da rafe (DRC), e o núcleo tegmental laterodorsal (LDTg) podem contribuir para os efeitos comportamentais da nicotina. As conexões aferentes e eferentes do IP, até agora, não foram sistematicamente investigadas com traçadores sensíveis. Assim, realizamos injeções de traçadores retrógrados ou anterógrados em diferentes subdivisões do IP, no MnR, ou LDTg e também examinamos a assinatura neuroquímica de algumas das mais proeminentes aferências dessas três estruturas através da combinação de rastreamento retrógrada com métodos de imunofluorescência e hibridização in situ. Além de receber entradas topograficamente organizadas da MHb e também da LHb, observamos que o IP está principalmente interligado de forma recíproca com estruturas da linha média, incluindo o MnR/DRC, o núcleo incerto, o núcleo supramamilar, o septo e o LDTg. As conexões bidirecionais entre o IP e o MnR assim como as entradas do LDTg para o IP provaram de ser principalmente GABAérgicas. Com respeito a uma possível topografia das saídas do IP, todos os subnúcleos do IP deram origem a projeções descendentes, enquanto as suas projeções ascendentes, incluindo projeções focais para o hipocampo ventral, o septo ventrolateral, e a LHb originaram da região dorsocaudal do IP. Nossos resultados indicam que o IP está intimamente associado a uma rede de estruturas da linha média, todos eles considerados moduladores chave da atividade teta do hipocampo. Assim, o IP forma um elo que liga MHb e LHb com esta rede e com o hipocampo. Além disso, as proeminentes interconexões predominantemente GABAérgicas entre IP e MnR, assim como IP e LDTg, suportam um papel chave dessas vias bidirecionais na resposta comportamental à nicotina. / The habenula is an epithalamic structure differentiated into two nuclear complexes, medial (MHb) and lateral habenula (LHb). Recently, MHb together with its primary target, the interpeduncular nucleus (IP), have been identified as major players in mediating the aversive effects of nicotine. However, structures downstream of the MHb-IP axis, including the median (MnR), caudal dorsal raphe nucleus (DRC), and the laterodorsal tegmental nucleus (LDTg), may contribute to the behavioral effects of nicotine. The afferent and efferent connections of the IP have hitherto not been systematically investigated with sensitive tracers. Thus, we placed injections of retrograde or anterograde tracers into different IP subdivisions, the MnR, or LDTg and additionally examined the transmitter phenotype of some major IP and MnR afferents by combining retrograde tract tracing with immunofluorescence and in situ hybridization techniques. Besides receiving topographically organized inputs from MHb and also LHb, we found that the main theme of IP connectivity are strong reciprocal interconnections with midline structures, including the MnR/DRC, nucleus incertus, supramammillary nucleus, septum, and LDTg. The bidirectional connections between IP and MnR and the LDTg inputs to the IP proved to be mostly GABAergic. Regarding a possible topography of IP outputs, all IP subnuclei gave rise to descending projections, whereas ascending projections, including focal projections to the ventral hippocampus, ventrolateral septum, and LHb mostly originated from the dorsocaudal IP. Our findings indicate that IP is closely associated to a distributed network of midline structures, all of them considered key modulators of hippocampal theta activity. Thus, IP forms a node that links MHb and LHb with this network and the hippocampus. Moreover, the rich predominantly GABAergic interconnections between IP and MnR, as well as IP and LDTg, support a cardinal role of these bidirectional pathways in the behavioral response to nicotine.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

O núcleo tegmental rostromedial (RMTg) é uma estrutura relé GABAérgica entre a habênula lateral (LHb) e a área tegmental ventral (VTA), que também recebe projeções diretas da LHb. Para detalhar a topografia das projeções da LHb para o RMTg e VTA, injeções de um traçador anterógrado foram depositadas na LHb. A origem das projeções da LHb para o RMTg e VTA foi confirmada com injeções de um traçador retrógrado, no RMTg ou VTA. Além disso, comparamos a posição topográfica dos neurônios marcados no RMTg após injeções na VTA, com os axônios marcados, vindas da LHb. Nossos dados revelaram que as projeções da LHb estão organizadas de maneira topográfica, com projeções para o RMTg, se originando da LHb lateral e as projeções para a VTA da LHb medial. No RMTg, axônios oriundos da LHb foram observados pertos de neurônios que se projetam para a VTA, formando aposições com esses neurônios. Os resultados indicam que as projeções da LHb para o RMTg e VTA originam-se de diferentes regiões da LHb e provam a existência de uma via bisináptica que conecta a LHb com a VTA via o RMTg. / The rostromedial segmental nucleus (RMTg) is a GABAergic relay between the lateral habenula (LHb) and the ventral tegmental area (VTA), which also receives direct projections from the LHB. To detail the topography of LHb projections to the RMTg and VTA, injections of an anterograde tracer were made into the LHb. The origin of LHb projections to the RMTg and VTA was then confirmed by injections of a retrograde tracer into the RMTg or VTA. Additionally, we compared the topographic position of RMTg neurons labeled from injections in the VTA with that of axons labeled from the LHb. Our data revealed that LHb projections are organized in a topographic manner, with projections to the RMTg emerging from the lateral LHb and those to the VTA arising from the medial LHb. In the RMTg, LHb axons were found in register with VTA projecting neurons and were frequently apposed to the latter. Our findings indicate that LHb inputs to the RMTg and VTA arise from different divisions of the LHb and provide evidence for a disynaptic pathway that links the LHb to the VTA via the RMTg.

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 physiological significance of noradrenergic sympathohabenular ingrowth following medial septal lesions was investigated. Following septal lesions, sympathetic fibers originating in the superior cervical ganglia are known to sprout into the medial habenular nuclei, and into the hippocampal formation. Previous work involving sympathohippocampal ingrowth showed that firing rates in septal animals with no ingrowth showed that firing rates in septal animals with no ingrowth were higher than rates of septal animals with ingrowth and controls. Those results suggested that sympathetic ingrowth in the hippocampus had some functional capability in a modulatory manner. The primary aim of the present study was to determine if the peripheral sympathetic ingrowth into the medial habenular nuclei following a septal lesion is functionally significant. The results showed that firing rates of neurons of the medial habenulae in animals receiving septal lesions were significantly higher than rates of control animals and septal lesioned + ganglionectomized animals.

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Le traitement de la dépression reste insatisfaisant. Avec un tiers des patients ne répondant à aucun traitement proposé, un délai d’action long, et des effets secondaires non négligeables, la nécessité de développer de nouvelles stratégies thérapeutiques devient urgente. La luminothérapie, traitement de choix de la dépression saisonnière, a été montrée depuis une trentaine d’années comme présentant également un intérêt pour le traitement des dépressions non saisonnières, unipolaires comme bipolaires. Cependant, les mécanismes d’actions sous-tendant l’effet antidépresseur de la lumière restent mal connus. L’objectif de ce travail de thèse est de comprendre, à l’aide d’un modèle original de dépression, les mécanismes neurobiologiques à l’origine de l’effet antidépresseur de la lumière. Nous avons pour cela développé un modèle de dépression combinant stress par la nage forcée et isolation sociale. Nos résultats montrent que ce protocole induit chez les animaux des comportements pseudo-dépressifs stables et résistants à des traitements classiques (escitalopram) mais également à la kétamine, utilisée récemment en étude clinique pour traiter certains patients réfractaires. Si la lumière seule à forte irradiance (Bright light stimulation, BLS, 1000 lux, une heure par jour) n’a pas d’effet antidépresseur, nous avons démontré dans notre modèle de dépression résistante que la BLS permettait de potentialiser la réponse antidépressive d’une combinaison de kétamine et de scopolamine (utilisée récemment comme d’antidépresseur potentiel) à des doses sous-efficaces. Cet effet est modulé par la sérotonine. En effet, la déplétion en tryptophane, précurseur de la sérotonine, bloque l’effet antidépresseur de cette combinaison. De manière intéressante, nous avons découvert que l’effet potentialisateur de la lumière met en jeu les astrocytes de l’habénula latérale. Ces données suggèrent que la lumière associée à la kétamine et la scopolamine, ciblerait les astrocytes afin de rétablir une activité normale dans l’habénula latérale, désinhibant les centres monoaminergiques, menant ainsi à une réponse antidépressive. Ce travail a permis de mieux comprendre les mécanismes à l’origine de la potentialisation de l’effet antidépresseur par la lumière et pourrait aider à optimiser les stratégies thérapeutiques chez les patients déprimés résistants aux traitements incluant la kétamine / The treatment of depression remains unsatisfactory. Given that one third of patients does not respond to any of the proposed treatments, the long delay of action, and the significant side effects, there is an urgent need to develop new and effective therapeutic strategies. Light, a treatment of choice for seasonal depression, has been of particular interest since thirty years in the treatment of non-seasonal unipolar and bipolar depressions. However, the mechanisms underlying the antidepressant effect of light therapy remains poorly understood. The aim of this thesis was to understand, using an original model of depression, the neurobiological mechanisms of the antidepressant effect of light stimulation. We developed an original model of depression combining forced swimming stress and social isolation. Our results showed that the latter protocol induced pseudo-depressive behaviors that were stable and resistant to classical treatments (escitalopram), but also ones recently tested in clinical studies to treat refractory patients (ketamine). Although bright light stimulation (BLS, 1000 lux, one hour per day) failed to present an antidepressant effect, we demonstrated in our model of resistant depression that BLS potentiated the antidepressant response of sub-effective doses of ketamine and scopolamine combination. This effect was modulated by serotonin tone. Indeed, this effect was blocked by tryptophan depletion. Remarkably, we unveiled that the potentiating action of light involves lateral habenula astroglia. These results suggest that light stimulation, associated with ketamine and scopolamine combination, modulated astroglia, in order to restore a normal activity in the lateral habenula and to regulate monoaminergic systems, leading to an effective antidepressant response. This work allowed to better understand the mechanisms responsible of the potentiating action of light and will certainly help in optimizing therapeutic strategies in treatment-resistant depression

La survie des individus dépend de leur capacité d’anticiper la survenue d’une récompense ou d’un danger leur permettant ainsi de s’adapter à leur environnement. De considérables efforts ont été réalisés pour identifier les mécanismes cellulaires et synaptiques ayant lieu au niveau du circuit de la récompense afin d’avoir une meilleure compréhension des processus sous tendant des états motivationnels physiologiques et pathologiques tels que l’addiction et la dépression. Pour autant, ce n’est que récemment qu’on commence à comprendre les circuits capables de contrôler les systèmes monoaminergiques mésencéphaliques et leurs contributions aux comportements motivés. Dans les dernières décennies l’habénula latérale (LHb) a émergé comme un acteur majeur capable d’encoder des stimuli de valeur motivationnelle et de contrôler les systèmes monoaminergiques. La connectivité de cette structure épithalamique joue un rôle clé dans différents aspects des comportements motivationnels, comme l’approche et la fuite. Des avancées récentes ont aussi démontré que des altérations de la fonction de la LHb entrainent des états émotionnels négatifs caractéristiques de la dépression et l’addiction. Ces observations suggèrent que la LHb pourrait s’avérer une cible importante pour le traitement de ces pathologies. Au cours de mon travail de thèse, j’ai d’abord cherché à comprendre comment moduler la transmission synaptique au niveau de la LHb pouvait contrôler son activité. Pour répondre à cette question, je me suis focalisée sur le rôle des récepteurs métabotropiques au glutamate (mGluRs). Dans une seconde étude, j’ai examiné les mécanismes par lesquels les drogues d’abus modifient la transmission synaptique des neurones de la LHb. Ces modifications se produisent spécifiquement dans les neurones LHb se projetant vers le noyau tegmental rostral (RMT) et sont nécessaires pour l’émergence des états dépressifs. Dans un premier temps, nous avons démontré qu’au niveau de la LHb les mGluRs de type I sont capables d’induire une dépression à long terme de la transmission synaptique excitatrice (eLTD) et inhibitrice (iLTD). Ces deux formes de plasticité dépendent de la signalisation PKC, mais requièrent des mécanismes d’expression différents. Tandis que eLTD réduit la probabilité de libération du glutamate via l’activation de récepteurs présynaptiques aux endocannabinoides (CB1), iLTD s’exprime par la réduction de la fonction des récepteurs GABAA postsynaptiques contenant la sous-unité β2. De plus, eLTD and iLTD exercent un contrôle bidirectionnel sur la décharge des neurones de la LHb. Dans un second temps, nous avons mis en évidence qu’une exposition chronique à la cocaïne produit une augmentation persistante de la transmission excitatrice au niveau des neurones de la LHb ciblant le RMTg. Cette forme de potentialisation synaptique nécessite l’insertion membranaire de récepteurs contenant la sous-unité GluA1, ainsi que la réduction de conductances potassiques entrainant une hyperexcitabilité neuronale in vitro et in vivo dans la LHb. Ces modifications sont nécessaires pour l’établissement d’états dépressifs émergeant lors de la période de sevrage à la cocaïne. En conclusion, ce travail a contribué à la compréhension de mécanismes de plasticité synaptique ayant lieu au niveau de la LHb et leurs répercussions pour son activité contrôlant ainsi des comportements motivationnels. / The capacity of the brain to anticipate and seek future rewards or alternatively escape aversive events allows individuals to adapt to their environment. A considerable research effort has focused on unraveling the cellular and synaptic mechanisms within the meso-cortico-limbic system underlying motivational processing both in physiological conditions and in pathologies such as addiction and depression. However, only recently we begin to understand the circuit substrates capable to control midbrain monoaminergic nuclei and their contribution to motivated behaviors. The Lateral Habenula (LHb) has emerged in the last decade, as a major player encoding stimuli with motivational value and in controlling monoaminergic systems. The wiring of this epithalamic structure subserves discrete features of motivated behaviors, including preference and avoidance. Recent advances have also demonstrated that aberrant modifications in LHb function trigger negative emotional states in disorders including depression and addiction, highlighting the LHb as an important brain target for therapeutic intervention for these pathological states. In my thesis work I first sought to investigate how modulation of synaptic transmission in the LHb controls neuronal activity, especially focusing on the role of metabotropic glutamate receptors. In a second study, I expanded my work examining how drug experience changes synaptic transmission in a precise habenular circuit that we discovered to be crucial for depressive states during cocaine withdrawal. In an initial data set, we found that, in the LHb, metabotropic glutamate receptor 1 activation drives a PKC-dependent long term depression of excitatory (eLTD) and inhibitory (iLTD) synaptic transmission. Despite the common induction, eLTD and iLTD diverged in their expression mechanism. While eLTD required endocannabinoid-dependent reduction of glutamate release, iLTD expressed postsynaptically through a decrease of β2-containing GABAA receptors function. Further, eLTD and iLTD bidirectionally controlled LHb neuronal output. In a second study, we showed that chronic cocaine exposure leads to a persistent and projection-specific increase of excitatory synaptic transmission onto LHb neurons. This form of synaptic potentiation required membrane insertion of GluA1-containing AMPA receptors and a reduction in potassium channels function ultimately leading to increased LHb neuronal excitability both in vitro and in vivo. These cocaine-driven adaptations within the LHb were instrumental for depressive-like states emerging after drug withdrawal. Altogether this work demonstrates how synaptic plasticity in the LHb affects neuronal output and thereby contributes to behaviors associated with the pathology of motivation.