Avaliação do metabolismo energético em cérebro de ratos infantes após a administração aguda de l-tirosina

Dissertação de Mestrado apresentada ao Programa de Pós-Graduação em Ciências da Saúde da Universidade do Extremo Sul Catarinense – UNESC, para obtenção do título de Mestre em Ciências da Saúde.Mutações no gene tirosina aminotransferase (TAT) causam tirosinemia tipo II (síndrome de Richner-Hanhart) é uma doença autossômica recessiva causada pela atividade reduzida ou inexistente da enzima tirosina aminotransferase, o que leva a um aumento dos níveis sanguíneos de tirosina e manifestações clínicas como ceratite, lesões palmo plantares dolorosas e hiperqueratose e inflamações nos olhos. As manifestações neurológicas são variáveis e alguns pacientes tem desenvolvimento normal, enquanto outros mostram diferentes graus de retardo no desenvolvimento. Considerando que aumento dos níveis de tirosina no sangue, causam manifestações neurológicas e que os mecanismos responsáveis por estas alterações são mal compreendidos. No presente estudo, foram avaliados os efeitos da administração de L-tirosina sobre a atividade das enzimas citrato sintase, malato desidrogenase, succinato desidrogenase, e complexos I, II, II-III, e IV da cadeia respiratória mitocondrial no córtex, hipocampo e estriado de ratos com 10 dias de idade. Ratos Wistar foram mortos uma hora após uma única injeção intraperitoneal de tirosina (500mg/kg) ou solução salina. As atividades das enzimas do metabolismo energético foram avaliadas. Os nossos resultados demonstram que a administração aguda de L-tirosina em ratos com 10 dias de idade, inibiu a enzima citrato sintase e a atividade dos complexos I e II da cadeia respiratória mitocondrial no estriado. A enzima succinato desidrogenase e atividade do complexo II-III foram aumentadas no hipocampo. Além disso, a malato-desidrogenase e a atividade do complexo IV não foram alteradas pela administração aguda de L-tirosina. Em particular, diversos grupos relataram que a tirosina administrada sistemicamente é diferencialmente distribuída entre as regiões do cérebro. Em conclusão, os nossos resultados indicam que uma alteração no metabolismo energético do hipocampo e corpo estriado pode levar a danos no desenvolvimento na aquisição, recuperação, consolidação e no armazenamento de memória, e os processos cognitivos após a administração aguda de L-tirosina em ratos infantes. Assim, nossos resultados contribuem para um melhor conhecimento da fisiopatologia da hipertirosinemia.Mutations in the TAT gene cause tyrosinaemia type II (Richner–Hanhart syndrome). It is an autosomal recessive disorder caused by, producing a TAT enzyme with decreased or inexistent activity, which leads to increased blood tyrosine levels and clinical manifestations such as keratitis and/or painful palmoplantar hyperkeratotic lesions and inflammations in the eyes. Neurological manifestations are variable and some patients are developmentally normal, while others show different degrees of developmental retardation. Considering that increased blood tyrosine levels cause neurological manifestations and that the mechanisms responsible by this alterations are poorly understood, In the present study, we evaluated the effects of L-tyrosine administration on the activities of enzymes citrate synthase, malate dehydrogenase, succinate dehydrogenase, and complexes I, II, II-III, and IV of mitochondrial respiratory chain in the posterior cortex, hippocampus and striatum of rats with 10-day-old. Wistar rats were killed one hour after a single intraperitoneal injection of either tyrosine (500 mg/kg) or saline. The activities of energy metabolism enzymes were evaluated. Our results demonstrated that the acute administration of L-tyrosine in rats with 10-day-old inhibited the citrate synthase enzyme and the complex I and II activity of the mitochondrial respiratory chain in the striatum. The succinate dehydrogenase enzyme and the complex II-III activity were increased in the hippocampus. Furthermore, the malate dehydrogenase and the complex IV activity were not altered by an acute administration of L-tyrosine. In particular, several groups report that systemically administered tyrosine is differentially distributed across brain regions. In conclusion, our results indicate that an alteration in the energetic metabolism of the hippocampus and striatum can leads to damage in the involved in acquisition, retrieval, consolidation and storage of memory, and cognitive processes after the acute administration of L-tyrosine in young rats. Thus our results contribute to a better knowledge of the pathophysiology of hypertyrosinemia

Assessing motor and cognitive function to detect shifts in brain function in two models of Parkinson\u27s disease

Cognitive changes accompany and often precede the onset of classic motor deficits typical of Parkinson&;#8217;s disease. A current focus of Parkinson&;#8217;s research has become understanding the development and progression of pre-motor cognitive changes. Based on previous research showing that hippocampus-sensitive spatial learning can be enhanced at the cost of impaired striatum-sensitive response learning, we hypothesized that changes in the balance between these two cognitive systems could be used as a proxy for the relative strength or health of their associated brain regions. Because non-motor symptoms of Parkinson&;#8217;s disease can precede the onset of the diagnostic motor dysfunction, changes in the balance between distinct learning strategies may represent an early marker of Parkinson’s-related neurodegeneration. Two rat models were used to assess the relationship between Parkinson’s disease-related motor dysfunction and changes in cognition. In the first study, a 6-OHDA rat model of Parkinson’s disease was used to generate a partial lesion of dopaminergic neurons in the nigrostriatal pathway. Despite the probable depletion of dopamine in the nigrostriatal pathway of lesioned rats that presented as impairment in two motor tasks, rats showed enhanced performance on the cognitive spontaneous alternation task, a test of spatial working memory. However, recent data suggest that multiple brain regions, including both the hippocampus and striatum, are activated during performance of the spontaneous alternation task; Parkinson’s-induced enhancements on this task may not be due solely to a shift in cognitive balance. Previous data show that inactivation of the hippocampus can enhance striatum-sensitive learning; however, it is unclear if inactivation of the striatum enhances hippocampus-sensitive functions. Prior to determining the effect of a 6-OHDA-induced lesion on hippocampus-sensitive learning, we wanted first to assess how impairing striatum function modulated place learning to determine cognitive shifts in rats with an intact brain. The second study uses two single-solution cognitive tasks that may link more closely to activation of separate neural systems. Temporary inhibition of the dorsal striatum by the GABAA receptor agonist, muscimol, produced deficits in motor function similar to those seen in the 6-OHDA model of Parkinson’s. Intrastriatal muscimol also impaired learning on a striatum-sensitive response learning task, suggesting that striatum-sensitive motor processes may overlap with striatum-sensitive cognitive processes. However, muscimol-induced striatum dysregulation did not produce enhancements on a hippocampus-sensitive spatial learning task. It is possible that the cognitive enhancements in hippocampus-sensitive processes are maximized when only specific neurotransmitter systems are dampened, such as the loss of dopaminergic signaling seen in Parkinson’s disease. Unlike 6-OHDA, which targets dopaminergic neurons, muscimol activates GABAA receptors, leading to the opening of Cl- channels, altering membrane potentials, and changing the likelihood of neurotransmitter release. Thus, activation of GABAA receptors by muscimol will alter neuron activity regardless of neurotransmitter system while 6-OHDA must initially affect dopaminergic neurons. Consequently, it is possible that muscimol decreases activity in neurotransmitter systems that play a compensatory role following 6-OHDA-induced dopaminergic degeneration. As such, a generalized inhibitor of neural activity like muscimol, may disrupt neural processes that are integral for seeing the Parkinson’s disease-related cognitive enhancements

The Neural Substrates of Deterministic Decision-making

When making a decision, we draw upon multiple mnemonic resources to inform our behavior and to ideally produce a good outcome. Multiple memory systems guide this process, including a medial temporal lobe (MTL) system and a striatal system. The MTL provides episodic details about specific instances of prior experience, whereas the striatum provides a prediction about possible outcomes based upon a fusion of many prior experiences. While both of these systems are assumed to support decision behavior, extricating their discrete contributions has been challenging. Using neuroimaging and computational reinforcement learning, this study investigated the extent to which the MTL and striatal systems are co-active during single-exposure learning and how these systems each support subsequent behavior. This was done in the context of a single-exposure deterministic decision-making task that separated encoding processes from subsequent decision-making processes. Human subjects learned to associate words with monetary feedback in a single decision experience. They then used that information to make better choices in a subsequent round without feedback. Activity in MTL regions predicted episodic memory accuracy and correlated with subsequent decision accuracy and response times. Additionally, the MTL supported a model-based reinforcement learning process wherein initial decision experiences were used to build a model of the environment that was then used to prospectively formulate future decision outcome predictions. Activity in striatal regions also correlated with subsequent decision accuracy and response times, but did not relate to memory accuracy. The striatum supported a model-free reinforcement learning process wherein predictions about decision outcomes were generated from a retrospective accumulation of prior decision experiences. Together, these results implicate both the MTL and striatum as essential substrates to single-exposure learning, but underscore that these systems operate in fundamentally different ways. The MTL is associated with prospective learning, wherein single instances of prior experience can be leveraged to inform subsequent choice. The striatum, in contrast, is associated with retrospective learning, wherein a history of experience is required to build reliable predictions about subsequent choices. In combination, the MTL system seems to support decision behavior until the striatal system has had enough experience to refine predictions about outcomes

The prospective power of memory on recall and attentional orienting

Memories are built from past experiences, but they continue to carry meaning and weight through to the present and future. The way in which we learn impacts not only how we remember but also how we perceive in the present moment. It is well established that memory is important for attentional guidance, as is attention for directing selective retrieval, yet these two abilities have primarily been studied separately. Therefore, our understanding of how memory guides recall compared to attentional orienting is lacking. In this thesis, I measured the formation of associative memories between target objects and scenes during visual search tasks and then probed how variations in learning parameters impacted subsequent performance. The search tasks differed based in the number of targets associated with each scene (Chapter 2), the type of spatial association (Chapter 3), or the spatial predictability of targets and distractors (Chapter 4). Recall and/or attentional-orienting tasks tested the relative influence of these different learning manipulations on future performance. Contrary to my hypotheses, competition among items associated with the same scene had little impact on memory recall. The type of association and predictability of items associated with scenes had significant impacts on both subsequent recall and attentional-orienting tasks. Interestingly, the patterns of influence were functionally dissociable. Memory recall was versatile: different types of associations and spatial expectations facilitated memory recall depending on the congruency between learning and recall demands. Instead, the pattern of memory-guided attentional orienting was more consistent and strongly dependent on spatial expectations pertaining to target objects. The thesis opens with a literature review on the relationship between visual search, memory recall, and attentional orienting (Chapter 1). The empirical chapters follow (Chapters 2 through 4). The final chapter discusses the implications of the studies and how they can facilitate future research

Learning and memory systems supporting decision making in the human brain

We successfully navigate the world by making decisions based on what we have learned. In the brain, two prominent learning systems have been identified and each is likely to guide decisions in different ways. Research on decision making has primarily focused on a reward learning system in the striatum. These studies have illuminated the how repeated choices and rewards build representations that guide choices and actions when encountering the same situation again. However, in a constantly changing environment, choices may not repeat themselves. Further, the environment may have more structure than simple reward learning can navigate. In these situations, decisions may be guided by a different learning system, namely a flexible learning system in the hippocampus which encodes episodes, or more broadly, relations between stimuli. However, investigations into the role of a reward learning system and a relational learning system in decision making have developed largely independently of each other. In the studies described below, I explore the function of these learning systems in value-guided decision making. Complementarily, I also explore how ongoing reward learning may modulate memory formation in the hippocampal system. In these studies, I demonstrate that reward learning and decision making is influenced by relational learning, and that these effects are predicted by hippocampal-striatal connectivity during learning. Separately, I establish that episodic memory is, in turn, influenced by ongoing reward learning. Successful memory is predicted by modulations of reward and memory regions including the striatum and hippocampus. Overall, these results provide novel insights into the learning systems encoding memories for future adaptive behavior

Hippocampal regulation of encoding and exploration under the influence of contextual reward and anxiety

Hippocampal researchers have recently turned their attention to the computations that may be implemented by the hippocampal circuit (e.g. pattern separation and pattern completion). This focus on the representational and information-processing capabilities of the hippocampus is likely to be important in resolving on-going debates regarding the nature of hippocampal contributions to perception, anxiety and exploration. A first aim of my research was to examine how context representations interact with reward to influence memory for embedded events. In my first experiment, I show that recollection for neutral objects is improved by sharing a context with other rewarding events. To further examine contextual influences on memory, I conducted a second experiment that examined the effect of contextual reward itself on object memory. Additionally, I manipulated the extent to which disambiguation should rely on hippocampal computations, by varying the perceptual similarity between the rewarding and neutral contexts. Improved object memory was only observed when the rewarding and neutral contexts were perceptually similar, and this contextual memory effect was further linked to co-activation of the hippocampal CA3/dentate gyrus and substantia nigra/ventral tegmental area. A second major aim of my work was to characterize hippocampal contributions to anxiety. In my third experiment, I combine a novel experiment with fMRI to show that hippocampal activation is associated with behavioural inhibition rather than exploratory risk assessment. This insight is important because a major theoretical perspective in the literature conflates these two psychological processes. In my final experiment, I employ this novel experimental paradigm to examine the effect of exploration on memory, and find that the propensity to explore (rather than the act of exploring per se) leads to better memory at subsequent recall

Cognitive anatomy of the temporal lobe: Effect of personality in population with mild cognitive impairment & Functional specialization for memory systems in healthy individuals.

Résumé: L'impact de la maladie d'Alzheimer (MA) est dévastateur pour la vie quotidienne de la personne affectée, avec perte progressive de la mémoire et d'autres facultés cognitives jusqu'à la démence. Il n'existe toujours pas de traitement contre cette maladie et il y a aussi une grande incertitude sur le diagnostic des premiers stades de la MA. La signature anatomique de la MA, en particulier l'atrophie du lobe temporal moyen (LTM) mesurée avec la neuroimagerie, peut être utilisée comme un biomarqueur précoce, in vivo, des premiers stades de la MA. Toutefois, malgré le rôle évident du LMT dans les processus de la mémoire, nous savons que les modèles anatomiques prédictifs de la MA basés seulement sur des mesures d'atrophie du LTM n'expliquent pas tous les cas cliniques. Au cours de ma thèse, j'ai conduit trois projets pour comprendre l'anatomie et le fonctionnement du LMT dans (1) les processus de la maladie et dans (2) les processus de mémoire ainsi que (3) ceux de l'apprentissage. Je me suis intéressée à une population avec déficit cognitif léger (« Mild Cognitive Impairment », MCI), à risque pour la MA. Le but du premier projet était de tester l'hypothèse que des facteurs, autres que ceux cognitifs, tels que les traits de personnalité peuvent expliquer les différences interindividuelles dans le LTM. De plus, la diversité phénotypique des manifestations précliniques de la MA provient aussi d'une connaissance limitée des processus de mémoire et d'apprentissage dans le cerveau sain. L'objectif du deuxième projet porte sur l'investigation des sous-régions du LTM, et plus particulièrement de leur contribution dans différentes composantes de la mémoire de reconnaissance chez le sujet sain. Pour étudier cela, j'ai utilisé une nouvelle méthode multivariée ainsi que l'IRM à haute résolution pour tester la contribution de ces sous-régions dans les processus de familiarité (« ou Know ») et de remémoration (ou « Recollection »). Finalement, l'objectif du troisième projet était de tester la contribution du LTM en tant que système de mémoire dans l'apprentissage et l'interaction dynamique entre différents systèmes de mémoire durant l'apprentissage. Les résultats du premier projet montrent que, en plus du déficit cognitif observé dans une population avec MCI, les traits de personnalité peuvent expliquer les différences interindividuelles du LTM ; notamment avec une plus grande contribution du neuroticisme liée à une vulnérabilité au stress et à la dépression. Mon étude a permis d'identifier un pattern d'anormalité anatomique dans le LTM associé à la personnalité avec des mesures de volume et de diffusion moyenne du tissu. Ce pattern est caractérisé par une asymétrie droite-gauche du LTM et un gradient antéro-postérieur dans le LTM. J'ai interprété ce résultat par des propriétés tissulaires et neurochimiques différemment sensibles au stress. Les résultats de mon deuxième projet ont contribué au débat actuel sur la contribution des sous-régions du LTM dans les processus de familiarité et de remémoration. Utilisant une nouvelle méthode multivariée, les résultats supportent premièrement une dissociation des sous-régions associées aux différentes composantes de la mémoire. L'hippocampe est le plus associé à la mémoire de type remémoration et le cortex parahippocampique, à la mémoire de type familiarité. Deuxièmement, l'activation correspondant à la trace mnésique pour chaque type de mémoire est caractérisée par une distribution spatiale distincte. La représentation neuronale spécifique, « sparse-distributed», associée à la mémoire de remémoration dans l'hippocampe serait la meilleure manière d'encoder rapidement des souvenirs détaillés sans interférer les souvenirs précédemment stockés. Dans mon troisième projet, j'ai mis en place une tâche d'apprentissage en IRM fonctionnelle pour étudier les processus d'apprentissage d'associations probabilistes basé sur le feedback/récompense. Cette étude m'a permis de mettre en évidence le rôle du LTM dans l'apprentissage et l'interaction entre différents systèmes de mémoire comme la mémoire procédurale, perceptuelle ou d'amorçage et la mémoire de travail. Nous avons trouvé des activations dans le LTM correspondant à un processus de mémoire épisodique; les ganglions de la base (GB), à la mémoire procédurale et la récompense; le cortex occipito-temporal (OT), à la mémoire de représentation perceptive ou l'amorçage et le cortex préfrontal, à la mémoire de travail. Nous avons également observé que ces régions peuvent interagir; le type de relation entre le LTM et les GB a été interprété comme une compétition, ce qui a déjà été reporté dans des études récentes. De plus, avec un modèle dynamique causal, j'ai démontré l'existence d'une connectivité effective entre des régions. Elle se caractérise par une influence causale de type « top-down » venant de régions corticales associées avec des processus de plus haut niveau venant du cortex préfrontal sur des régions corticales plus primaires comme le OT cortex. Cette influence diminue au cours du de l'apprentissage; cela pourrait correspondre à un mécanisme de diminution de l'erreur de prédiction. Mon interprétation est que cela est à l'origine de la connaissance sémantique. J'ai également montré que les choix du sujet et l'activation cérébrale associée sont influencés par les traits de personnalité et des états affectifs négatifs. Les résultats de cette thèse m'ont amenée à proposer (1) un modèle expliquant les mécanismes possibles liés à l'influence de la personnalité sur le LTM dans une population avec MCI, (2) une dissociation des sous-régions du LTM dans différents types de mémoire et une représentation neuronale spécifique à ces régions. Cela pourrait être une piste pour résoudre les débats actuels sur la mémoire de reconnaissance. Finalement, (3) le LTM est aussi un système de mémoire impliqué dans l'apprentissage et qui peut interagir avec les GB par une compétition. Nous avons aussi mis en évidence une interaction dynamique de type « top -down » et « bottom-up » entre le cortex préfrontal et le cortex OT. En conclusion, les résultats peuvent donner des indices afin de mieux comprendre certains dysfonctionnements de la mémoire liés à l'âge et la maladie d'Alzheimer ainsi qu'à améliorer le développement de traitement. Abstract: The impact of Alzheimer's disease is devastating for the daily life of the affected patients, with progressive loss of memory and other cognitive skills until dementia. We still lack disease modifying treatment and there is also a great amount of uncertainty regarding the accuracy of diagnostic classification in the early stages of AD. The anatomical signature of AD, in particular the medial temporal lobe (MTL) atrophy measured with neuroimaging, can be used as an early in vivo biomarker in early stages of AD. However, despite the evident role of MTL in memory, we know that the derived predictive anatomical model based only on measures of brain atrophy in MTL does not explain all clinical cases. Throughout my thesis, I have conducted three projects to understand the anatomy and the functioning of MTL on (1) disease's progression, (2) memory process and (3) learning process. I was interested in a population with mild cognitive impairment (MCI), at risk for AD. The objective of the first project was to test the hypothesis that factors, other than the cognitive ones, such as the personality traits, can explain inter-individual differences in the MTL. Moreover, the phenotypic diversity in the manifestations of preclinical AD arises also from the limited knowledge of memory and learning processes in healthy brain. The objective of the second project concerns the investigation of sub-regions of the MTL, and more particularly their contributions in the different components of recognition memory in healthy subjects. To study that, I have used a new multivariate method as well as MRI at high resolution to test the contribution of those sub-regions in the processes of familiarity and recollection. Finally, the objective of the third project was to test the contribution of the MTL as a memory system in learning and the dynamic interaction between memory systems during learning. The results of the first project show that, beyond cognitive state of impairment observed in the population with MCI, the personality traits can explain the inter-individual differences in the MTL; notably with a higher contribution of neuroticism linked to proneness to stress and depression. My study has allowed identifying a pattern of anatomical abnormality in the MTL related to personality with measures of volume and mean diffusion of the tissue. That pattern is characterized by right-left asymmetry in MTL and an anterior to posterior gradient within MTL. I have interpreted that result by tissue and neurochemical properties differently sensitive to stress. Results of my second project have contributed to the actual debate on the contribution of MTL sub-regions in the processes of familiarity and recollection. Using a new multivariate method, the results support firstly a dissociation of the subregions associated with different memory components. The hippocampus was mostly associated with recollection and the surrounding parahippocampal cortex, with familiarity type of memory. Secondly, the activation corresponding to the mensic trace for each type of memory is characterized by a distinct spatial distribution. The specific neuronal representation, "sparse-distributed", associated with recollection in the hippocampus would be the best way to rapidly encode detailed memories without overwriting previously stored memories. In the third project, I have created a learning task with functional MRI to sudy the processes of learning of probabilistic associations based on feedback/reward. That study allowed me to highlight the role of the MTL in learning and the interaction between different memory systems such as the procedural memory, the perceptual memory or priming and the working memory. We have found activations in the MTL corresponding to a process of episodic memory; the basal ganglia (BG), to a procedural memory and reward; the occipito-temporal (OT) cortex, to a perceptive memory or priming and the prefrontal cortex, to working memory. We have also observed that those regions can interact; the relation type between the MTL and the BG has been interpreted as a competition. In addition, with a dynamic causal model, I have demonstrated a "top-down" influence from cortical regions associated with high level cortical area such as the prefrontal cortex on lower level cortical regions such as the OT cortex. That influence decreases during learning; that could correspond to a mechanism linked to a diminution of prediction error. My interpretation is that this is at the origin of the semantic knowledge. I have also shown that the subject's choice and the associated brain activation are influenced by personality traits and negative affects. Overall results of this thesis have brought me to propose (1) a model explaining the possible mechanism linked to the influence of personality on the MTL in a population with MCI, (2) a dissociation of MTL sub-regions in different memory types and a neuronal representation specific to each region. This could be a cue to resolve the actual debates on recognition memory. Finally, (3) the MTL is also a system involved in learning and that can interact with the BG by a competition. We have also shown a dynamic interaction of « top -down » and « bottom-up » types between the pre-frontal cortex and the OT cortex. In conclusion, the results could give cues to better understand some memory dysfunctions in aging and Alzheimer's disease and to improve development of treatment