Fetal Alcohol Spectrum Disorders (FASD) are the most common cause of cognitive impairment in the United States (Sokol et al., 2003). In young school children in North America and some Western European countries, recent reports have estimated the prevalence of FASD to be as high as 2-5% (May et al., 2009). Currently there are no widely accepted treatment options for FASD, mainly due to the fact that the underlying neurological deficits that occur with prenatal ethanol exposure (PNEE) are still largely unknown. This thesis examines the long-lasting changes that occur in the hippocampus following PNEE using biochemical and electrophysiological techniques. We find that PNEE produces a reduction of the endogenous antioxidant glutathione (GSH), resulting in an increase in oxidative stress that is accompanied by long-lasting reductions in long-term potentiation (LTP) of synaptic efficacy. Interestingly, males exhibited greater deficits in synaptic plasticity than females, despite similar reductions in GSH in both sexes. By depleting GSH in control animals we determined that LTP in the DG of female animals is more resistant to changes in GSH, which may explain the sexual dichotomy observed in these studies of PNEE. Based on these findings, ethanol-exposed animals received postnatal dietary supplementation with either a precursor of GSH, N-Acetylcysteine (NAC) or Omega-3 fatty acids. These supplements helped to counteract the effects of PNEE and improved hippocampal function. The findings in this thesis support the hypothesis that increasing antioxidant capacity can enhance hippocampal function, which in turn may improve learning and memory in FASD, providing a therapeutic avenue for children suffering with these disorders. / Graduate / 0570 Nutrition / 0317 Neuroscience / anna.r.patten@gmail.com

Adrenalectomy (ADX) has been shown to cause selective degeneration of granule cells in the dentate gyrus (DG). This occurs due to the reduction of corticosterone (CORT) and behavioural deficits are associated with the loss of these neurons. Dentate lesions and cell loss associated with ADX have been shown to effect behaviour in a number of spatial tasks. In contras, it has been shown granule cell loss does not affect the specificity of place cells in CA3 and CA1. We used the ADX model to examine the role of DG granule cells plays in representing space using immediate early gene (IEG) activation in the principal hippocampal subfields after exploration of novel environments. Rats were allowed to free explore multiple novel environments and then the mRNA for the IEG Homer 1a (H1a) was used as a marker of neural activity. After degeneration of approximately half of the DG granule cells we found a significant increase in number of active cells in the DG, CA3 and CA1 in ADX animals. The results indicate a reduction in granule cells causes a dramatic increase in the proportion of remaining DG granule cells in response to exploration. The change in DG activation disrupts the representations in CA3 and CA1 and thereby affects behaviour. / vii, 60 leaves : ill. (some col.) ; 29 cm

En plus des crises, les patients atteints d'épilepsie du lobe temporale (ELT) souffrent de déficits cognitifs tels que des troubles de l'apprentissage et de la mémoire épisodique. La formation de la mémoire épisodique nécessite des interactions entre le cortex et l'hippocampe pendant le sommeil. Ces interactions sont orchestrées par l'oscillation lente qui est générée dans le réseau thalamocortical. L'oscillation lente se propage dans d'autres structures sous corticales mais l'hippocampe semble être moins influencé. Cela pourrait être du à la fonction de filtre du gyrus denté. Dans l'ELT, le gyrus denté subit une réorganisation structurelle et fonctionnelle qui pourrait altérer sa fonction de filtre et aussi modifier la propagation d'activités épileptiformes du cortex vers l'hippocampe. Cependant, la propagation de rythmes physiologiques du cortex vers le réseau hippocampique pendant l'épileptogenèse a été peu étudié. Ce travail de thèse a eu pour but d'étudier l'influence des oscillations lentes corticales sur le potentiel de membrane et la décharge des cellules granulaires du gyrus denté dans un modèle d'ELT sous anesthésie. Nos résultats montrent une augmentation de la modulation du potentiel de membrane et ainsi que de la décharge des cellules granulaires du gyrus par l'oscillation lente corticale pendant l'épileptogenèse. Les changements qui s'opèrent dans le gyrus denté pendant l'épileptogenèse le rendraient plus permissif aux informations en provenance du cortex facilitant ainsi la propagation des oscillations lentes du cortex vers l'hippocampe. / In addition to seizures, patients with temporal Lobe Epilepsy (TLE) suffer from cognitive deficits such as learning and episodic memory impairment. The functional interactions between the cortex and the hippocampus notably during sleep are thought to be important for episodic memory formation. These interactions are orchestrated by the slow oscillation which is generated in thalamo-cortical networks. The slow oscillation is not confined to thalamo-neocortical networks but propagates to other subcortical structures but the hippocampus seems however less strongly influenced by the widespread propagation of the slow oscillation. This could result from the gate function of the dentate gyrus. In TLE, the dentate gyrus is associated with profound structural and functional network alterations which can alter the propagation of pathological activities such as epileptiform discharges from the cortex to the hippocampus. However, whether and how epilepsy modifies the impact of physiological activities on hippocampal networks remains to be investigated. This work was designed to study the influence of slow cortical oscillations on the membrane potential and discharge of granule cells in the dentate gyrus in an animal model of TLE. Our results show an increase in the modulation of membrane potential and as well as the discharge of granule cells in the dentate gyrus by the cortical slow oscillation during epileptogenesis. The changes that occur in the dentate gyrus during epileptogenesis would make it more permissive facilitating the spread of slow oscillations from the cortex to the hippocampus.

No description available.

Early life adversity (ELA) produces long lasting developmental changes to the postnatal brain, increasing predisposition to a number of physical and psychiatric disorders. The mechanisms through which ELA is able to create lasting detrimental changes to neuronal development remains unclear. This thesis tested the hypothesis that increases in fear generalization, a common symptom in psychiatric disorders, follows ELA exposure in age dependent and sexually dimorphic ways in alignment with the findings of clinical studies. The effects of ELA often impact fear circuitry and we confirmed, using electrophysiology and tissue imaging, that 5-HT circuitry from the median raphe nucleus (MRN), integral to fear response, was impaired following ELA. Using a transgenic mouse model that allows for modulation of serotonergic release, we showed that circumventing serotonergic pathways disrupted by ELA and increasing whole brain 5-HT release was enough to rescue hippocampal dependent fear responses and fear generalization. Involvement of the hippocampus in ELA effects, particularly the ventral dentate gyrus (vDG), in fear overgeneralization was confirmed as hyperactivity in thevDG following exposure to novel contexts was rescued by increased 5-HT release. In addition to ELA-induced hyperactivity of the vDG, known to potentiate stress susceptibility, I demonstrated that ELA resulted in an increase in passive coping strategies, HPA axis dysfunction and elevated stress hormone release. These effects were seen predominantly in adult females and rescued in those with increased 5-HT release. Together these data suggest that increased predisposition to psychiatric disorders following ELA exposure involves the disruption of fear circuitry regulated by 5-HT activity. Identifying the underlying circuits altered by ELA not only provides insight about disrupted postnatal brain development, but also increases our knowledge of the timeline, trajectory and factors affecting healthy postnatal brain development.

No description available.

Animals learn from past experience to guide future behavior and improve survival. This ability relies in part on specific episodic memories of past events encoded by neuronal activity and stored by updated connectivity between neurons. The unique architecture and activity of the hippocampus and related cortical regions are crucial for supporting these episodic memories. Hippocampal models propose the need for a pattern separation function to disambiguate similar memories and a pattern completion function to recall the full breadth of an experience from a partial cue. Past work suggests that neuronal activity in the dentate gyrus (DG) of hippocampus contributes to memory-guided navigation and plays a role in pattern separation. We tested the role of specific DG neuronal ensembles (i.e. engrams) in supporting the pattern separation function and altering downstream neural activity and, ultimately, behavior. To that end, we used an activity-dependent labeling paradigm to identify and manipulate engram ensembles during navigational and contextual fear conditioning (CFC) tasks. The results of our first experiment revealed that the DG partially disambiguates specific maze trajectories while still exhibiting greater overlap than chance levels. These findings suggest that the DG contributes to memory-guided navigation by both pattern separation and completion. Our second experiment manipulated nonspecific memory-related DG populations to assess the functional role of these cells in task generalization across contexts and ongoing spatial working memory. Optogenetic activation of these ensembles disrupted performance accuracy and exhibited a time-dependent impairment effect suggesting a role of the DG in task generalization between contexts. The final experiments investigated the physiological ramifications of artificial memory ensemble reactivation during ongoing navigation behavior. We recorded local field potential (LFP) and single unit responses in mouse DG and CA1 during artificial reactivation of a DG-mediated CFC memory engram. Stimulation of the DG entrained LFP and individual cell spiking in a subpopulation of CA1 pyramidal cells. Their spatial information was disrupted by stimulation despite stable navigational representation before and after the manipulation. Further, the presence of stimulation could be reliably decoded by the firing rate of the network, suggesting that engram reactivation forced the CA1 to adopt a repeatable state, perhaps to support behavioral expression of memories. In summary, my dissertation work presents empirical and theoretical evidence for the role of the dentate gyrus as a single node of an extended separation/completion circuit distributed anatomically and temporally as a neural mechanism supporting episodic memory.

Die Erforschung der Zusammenhänge zwischen Gehirnplastizität und individuellem Verhalten gestaltet sich aufgrund ihrer Komplexität im Tiermodell schwierig. Die vorliegende Studie wurde im mit dem Ziel konzipiert, die Individualitätsentwicklung bei Mäusen mit den gleichen physiologischen und genetischen Voraussetzungen in einer komplexen räumlichen und sozialen Umgebung zu beschreiben. Ich untersuchte die Korrelation dieser Entwicklung mit der Neurogenese im adulten Hippokampus als Maß für Gehirnplastizität. Zu diesem Zweck wurden zwei je mit einem automatisierten RFID-Tracking-System ausgestattete Großgehege mit jeweils 40 Tieren besiedelt. Die Bewegungen der Tiere wurden kontinuierlich aufgezeichnet und es wurden zudem direkte Verhaltensbeobachtungen durchgeführt. Die Tiere zeigten eine normale physiologische Entwicklung. Die Roaming Entropy (RE), ein Maß für die Gleichmäßigkeit, mit der die Tiere ihr Gehege nutzten, beschreibt das Erkundungsverhalten der einzelnen Mäuse. Die kumulativ erworbenen RE-Werte (cRE) in jedem der beiden Gehege wurden mit der Zeit zunehmend verschieden. Es war nicht möglich, aufgrund kleiner anfänglicher Unterschiede die Endwerte zu berechnen. Das bedeutet, dass die Tiere erst durch die andauernde Interaktion mit ihrer Umwelt und den Artgenossen unterschiedlicher wurden. Darüber hinaus sind die cRE-Werte am Endpunkt positiv mit den Neurogenesewerten korreliert. Dies beweist, dass während der Entwicklung auftretende Faktoren die Individualitätsentwicklung beeinflussen. Dieser Prozess benötigt plastische Hirnstrukturen und formt diese wiederum. Die Verhaltensanalysen zeigten, dass Tiere, die viele Antennenkontakte sammelten („most active“, MA) nicht zwangsläufig auch hohe cRE-Werte hatten. MA-Mäuse waren häufiger an sozialen Interaktionen beteiligt als Tiere mit wenigen Antennenkontakten („least active“, LA), akkumulierten über die Zeit niedrigere cRE-Werte und standen vermutlich weiter unten in der sozialen Hierarchie. Zusammenfassend kann man sagen, dass das Ausmaß der räumlichen Exploration und die allmähliche Erweiterung der Erfahrung mit einer gesteigerten Plastizität des Gehirns in Form von adulter Neurogenese verbunden war. Die Daten zeigen, dass Tiere mit den gleichen Voraussetzungen sich dennoch auf zunehmend divergierende, individuelle Art entwickeln. Dies ist zumindest teilweise durch leicht unterschiedliche epigenetische Voraussetzungen zu erklären, die durch das Wechselspiel mit dem komplexen Umfeld weiter auseinanderdriften. Auch scheint es, dass Individuation lebenden Organismen inhärent und Voraussetzung für evolutionäre Prozesse ist. Die Studie zeigt, dass die Unterschiede in individuellem Verhalten und Gehirnstruktur nicht allein durch Genen und Umweltbedingungen festgelegt sind, sondern auch durch Faktoren, die sich während der ontogenetischen Entwicklung entfalten, beeinflusst werden. Der beschriebene Versuchsaufbau stellt darüber hinaus ein Tiermodell für die Untersuchung von Mechanismen und modulierenden Faktoren auf die strukturellen Grundlagen der Plastizität als individuelle Reaktion auf die gemeinsam genutzte Umgebung dar. / The complex nature of the relationship between brain plasticity and individual behaviour renders its investigation using animal models difficult. The present study was designed to describe the emergence of individuality in mice with the same physiological, environmental and genetic preconditions in response to complex environmental and social cues. I investigated the correlation of this development to brain plasticity, namely neurogenesis in the adult hippocampus. To this end, two large, multi-level enclosures fitted with and automated RFID tracking system were populated with 40 animals to each. The mice were continuously tracked and live behaviour observations were done. The animals showed normal physiological development. The Roaming Entropy (RE), a measure for the evenness of their usage of the enclosure, describes the exploration behaviour of each animal. Cumulatively acquired RE scores (cRE) within an enclosure increasingly diverged with time. Small differences at the beginning were not predictive of the end values. Thus, the animals became different through the continued interaction with environment and conspecifics. Moreover, the cRE values at the end point positively correlated with the amount of hippocampal neurogenesis. This proves that factors emerging during development contribute to individuality development. These factors at the same time shape and rely on plastic brain structures. Behavioural analyses showed that animals with a high amount of antenna contacts (most active, MA mice) were not necessarily those with high cRE values. MA mice were more often involved in social interactions than the least active mice (least active, LA), accumulated lower cRE scores over time and seemed to be lower in the social hierarchy. In conclusion, the amount of spatial exploration and gradual broadening of experience was linked to brain plasticity in the form of elevated levels of hippocampal neurogenesis. The data shows that animals with same preconditions still develop along increasingly divergent, individual paths. This is probably partly given through slightly different epigenetic preconditions, drifting further apart by interaction with the complex environment. Also, individuation seems to be inherent in living organisms and necessary for evolutionary processes. The study shows firstly that differences in individual behaviour and brain structure are defined not only by genes and the environment but also modulated by factors unfolding or emerging during ontogenetic development. The present paradigm moreover introduces an animal model for studying mechanisms and influences on the structural basis of plasticity as an individual response to the nonshared environment.

Following traumatic brain injury (TBI), the hippocampus is particularly vulnerable to damage, and BDNF, an endogenous neurotrophin that activates the TrkB receptor, has been shown to play a key role in the brain’s neuroprotective response. Activation of the TrkB signaling pathway by BDNF in the CNS promotes cell survival and aids in cell growth. However, due to its inability to cross the blood brain barrier (BBB), the therapeutic advantages of BDNF treatment following TBI are limited. 7,8-Dihydroxyflavone (7,8-DHF) is a flavonoid that mimics the effects of BDNF, is a potent TrkB receptor agonist, and can successfully cross the BBB. Our lab has previously demonstrated that administration of 7,8-DHF post-TBI results in improved cognitive functional recovery, increased neuronal survival, and reduced lesion volume. The current study examined the effects of 7,8-DHF on neurogenesis and neuronal migration in the dentate gyrus following TBI. In this study, adult male Sprague-Dawley rats were subjected to moderate controlled cortical impact injury (CCI) or sham surgery. Injured animals received 5 daily single doses of 7,8-DHF treatment (i.p) or vehicle starting either 60 mins after injury or 2 days after injury. BrdU was administered in 3 doses at 2 days post-injury for animals sacrificed at day 15, and single daily doses at days 1-7 post-injury for animals sacrificed at day 28 to label cell proliferation. Animals were sacrificed at 15 days or 28 days post-injury to examine cell proliferation, generation of new neurons, and differentiation of newly generated cells using proliferation marker Ki67, immature neuronal marker DCX, and BrdU double-labeling with markers for mature neurons (NeuN), astrocytes (GFAP) and microglia (Iba1). We found that administration of 5 doses (5mg/kg) of 7,8-DHF beginning two days post-injury had the strongest effect on neurogenesis and migration, but did not have a significant prolonged effect on cell proliferation at 15 days post-injury. We also found that 7,8-DHF treatment given early or 2 days post-TBI did not affect the neuronal differentiation in the granule cell layer. However, a higher percentage of BrdU/GFAP+ and BrdU/IBa1+ cells were found in the hilus regions in 7,8-DHF treated animals, suggesting newly generated cells in this region are mostly glial cell types. Our results suggest that 7,8-DHF has neurotrophic-like therapeutic effects following injury, and due to increased neurogenesis (compared to injured animals treated with vehicle), may effectively contribute to greater cell survival long-term. Additionally, potential long-term survival coupled with increased outward migration from the subgranular zone may result in increased integration of newly formed neurons into existing hippocampal circuitry, further contributing to cognitive recovery.

Les épilepsies mésiales du lobe temporal (ELTM) sont parmi les formes les plus fréquentes d’épilepsies partielles pharmaco-résistantes de l’adulte et l’enfant. Dans ces épilepsies les études électrocliniques et expérimentales indiquent que la zone épileptogène, qui désigne un ensemble de neurones nécessaire et suffisant à l’organisation d’une décharge anormale, ne peut être réduite à la seule formation hippocampique (FH) et impliquerait une réorganisation mettant en jeu plusieurs structures au sein du système limbique. Dans ce travail de thèse, nous nous sommes intéressés à la connectivité structurale entre le noyau supramammillaire (SuM) et la FH chez le rat dans le but de déterminer l’identité neurochimique de la voie de projection supramammillaro-hippocampique qui n’avait pas été clairement identifiée et, vérifier l’hypothèse d’une éventuelle réorganisation de cette voie de projection dans le modèle d’ELTM induit par l’injection intrapéritonéale de pilocarpine chez le rat. Chez les rats naïfs, nous mettons en évidence deux voies de projection distinctes. La première a pour origine les neurones localisés dans la partie latérale du SUM (SuML) qui innervent le champ CA2-CA3a et principalement la couche supragranulaire du gyrus dentelé dorsal. Cette voie est essentiellement ipsi-latérale et a la caractéristique de présenter un profil neurochimique unique, à la fois GABAergique et glutamatergique. La seconde voie de projection a pour origine les neurones localisés dans la partie plus postérieure et médiane du SuM (SuMM) qui innervent la région CA2-CA3a et la région ventrale du gyrus dentelé exclusivement ; cette voie est purement glutamatergique. Chez les rats traités à la pilocarpine, nos résultats montrent une réorganisation structurale des afférences des noyaux SuML et SuMM qui innervent le gyrus dentelé. Cette réorganisation est caractérisée par une distribution aberrante et une augmentation du nombre de fibres et terminaisons axonales en provenance des noyaux SuML et SuMM dans la couche moléculaire interne du gyrus dentelé. Cette réorganisation commence à la fin de la période de latence, et évolue pendant l’épilepsie induite par la pilocarpine. Avec ce travail, nous montrons pour la première fois : 1) l’hétérogénéité à la fois anatomique et neurochimique des voies de projection supramammillaro-hippocampiques ; 2) dans le gyrus dentelé des animaux traités à la pilocarpine, une réorganisation structurale d’origine extra-hippocampique, en provenance des noyaux SuML et SuMM. Cette connectivité aberrante pourrait contribuer avec la réorganisation des circuits intrinsèques de l’hippocampe à l’émergence des premières crises spontanées et à l’installation de l’épilepsie. / Mesial temporal lobe epilepsies (MTLE) are among the most common forms of pharmacoresistant partial epilepsies in adults and children. In these epilepsies, spontaneous seizures likely originate from a multi-structural epileptogenic zone including several structures of the limbic system connected to the hippocampal formation (HF). In this thesis, we investigate the structural connectivity between the supramammillary nucleus (SuM) and the HF in rat, in order to determine the not yet known neurochemical identity of the supramammillaro-hippocampal pathway and, to test the hypothesis of a potential reorganization of this pathway in the rat pilocarpine model of MTLE. In naïve rats, our results highlight two distinct pathways. The first pathway originates in the lateral part of the SuM (SuML) and innervates the supragranular layer of the dorsal dentate gyrus mainly, and the CA2-CA3a pyramidal cell layer of the hippocampus. This pathway is mainly ipsilateral and displays a unique dual phenotype for GABAergic and glutamatergic neurotransmission. The second pathway originates in the most posterior and medial part of the SuM (SuMM) and innervates exclusively the inner molecular layer of the ventral dentate gyrus and the CA2-CA3a subfield and is glutamatergic only.In pilocarpine-treated animals, our findings demonstrate a structural reorganization of dentate gyrus afferents originating from the SuM nuclei. Such reorganization is characterized by an aberrant distribution and an increased number of fibers and axon terminals from neurons of the both lateral and medial regions of the SuM, invading the entire inner molecular layer of the dentate gyrus. It starts at the end of the latent period and evolves during the epilepsy induced by pilocarpine. Our findings demonstrate for the first time: 1) the anatomical and neurochemical heterogeneity of the supramammillaro-hippocampal pathways; 2) in pilocarpine-treated animals, a marked reorganization of dentate gyrus afferents originating from the SuM nuclei. This aberrant connectivity could contribute along with the reorganization of hippocampal intrinsic circuitry to the emergence of the first spontaneous seizures and epilepsy installation.