Alzheimer’s disease (AD) is a degenerative disorder where the distribution of pathology throughout the brain is not random but follows a predictive pattern used for pathological staging. While the involvement of defined functional systems is fairly well established for more advanced stages, the initial sites of degeneration are still ill defined. The prevailing concept suggests an origin within the transentorhinal and entorhinal cortex (EC) from where pathology spreads to other areas. Still, this concept has been challenged recently suggesting a potential origin of degeneration in nonthalamic subcortical nuclei giving rise to cortical innervation such as locus coeruleus (LC) and nucleus basalis of Meynert (NbM). To contribute to the identification of the early site of degeneration, here, we address the question whether cortical or subcortical degeneration occurs more early and develops more quickly during progression of AD. To this end, we stereologically assesses neurone counts in the NbM, LC and EC layer-II in the same AD patients ranging from preclinical stages to severe dementia. In all three areas, neurone loss becomes detectable already at preclinical stages and is clearly manifest at prodromal AD/MCI. At more advanced AD, cell loss is most pronounced in the NbM > LC > layer-II EC. During early AD, however, the extent of cell loss is fairly balanced between all three areas without clear indications for a preference of one area. We can thus not rule out that there is more than one way of spreading from its site of origin or that degeneration even occurs independently at several sites in parallel.

The brain processes information sensed from the environment and guides behaviour. A fundamental component in this process is the storage and retrieval of past experiences as memories, which relies on the hippocampal formation. Although there has been a great progress in understanding the underlying neural code by which neurons communicate information, there are still open questions. Neural activity can be measured extracellularly as either spikes or field potentials. Isolated spikes and bursts of high-frequency spikes followed by silent periods can transmit messages to distant networks. The local field potential (LFP) reflects synaptic activity within a local network. The interplay between the two has been linked to cognitive functions, such as memory, attention and decision making. However, the code by which this neural communication is achieved is not well understood. We investigated a mechanism by which local network information contained in LFP rhythms can be transmitted to distant networks in the formof spike patterns fired by bursting neurons. Since rhythms within different frequency bands are prevalent during behavioural states, we studied this encoding during different states within the hippocampal formation. In the first paper, using a computational model we show that bursts of different size preferentially lock to the phase of the dominant rhythm within the LFP.We also present examples showing that bursting activity in the subiculum of an anaesthetised rat was phase-locked to delta or theta rhythms as predicted by the model. In the second paper, we explored possible neural codes by which bursting neurons can encode features of the LFP.We used the computational model reported in the first paper and analysed recordings from the subiculum of anaesthetised rats and the medial entorhinal cortex of an awake behaving rat. We show that bursting neurons encoded information about the instantaneous voltage, phase, slope and/or amplitude of the dominant LFP rhythm (delta or theta) in their firing rate. In addition, some neurons encoded about 10-15% of this information in intra-burst spike counts. We subsequently studied how the interactions between delta or theta rhythms can transfer information between different areas within the hippocampal formation. In the third paper, we show that delta and theta rhythms can act as separate routes for simultaneously transferring segregate information between the hippocampus and the subiculum of anaesthetised mice. We found that the phase of the rhythms conveyed more information than amplitude. We next investigated whether neurodegenerative pathology affects this information exchange. We compared information transfer within the hippocampal formation of young transgenic mice exhibiting Alzheimer’s disease-like pathology and healthy aged-matched control mice and show that at early stages of the disease the information transmission by LFP rhythm interactions appears to be intact but with some differences. The outcome of this project supports a burst code for relaying information about local network activity to downstream neurons and underscores the importance of LFP phase, which provides a reference time frame for coordinating neural activity, in information exchange between neural networks.

Les réseaux neuronaux, pendant leur période de développement, génèrent des patrons d’activité immatures qui sont supposés participer à la formation des circuits neuronaux. Ces activités synchronisées créent des conditions favorables pour la plasticité hebbienne qui soutient la formation des circuits locaux. Les travaux menés notamment sur les systèmes sensoriels ont montré que les circuits pauci-neuronaux locaux sont capables de présenter une activité synchrone tout en étant isolés du reste des structures cérébrales. La moelle épinière isolée produit des bursts qui sont à l’origine des secousses musculaires, la rétine insensible à la lumière génère des ondes d’activité, d’autres régions cérébrales génèrent des activités synchrones avant de remplir la fonction à laquelle ils sont destinés. De manière similaire, l’hippocampe du rat nouveau-né ou primate prématuré in vitro, ainsi que les néocortex immature in vitro, génèrent une activité neuronale synchronisée, appelée giant depolarising potentials (GDPs). En se basant uniquement sur ces études et en prenant en compte la maturation tardive de certaines projections neuronales à distance, il serait tentant de conclure que le cerveau immature fonctionne comme un ensemble de petits modules fonctionnels qui auto-entretiennent leur activité intrinsèque avant de se connecter entre eux pour créer un cerveau fonctionnel adulte. Cependant, certaines connexions à longue distance sont formées très tôt pendant le développement et permettent la propagation des oscillations immatures entre les structures connectées. En effet, les ondes rétinales se propagent au noyau géniculé latéral et ensuite jusqu’au cortex visuel ; les GDPs hippocampiques se propagent à l’hippocampe controlatéral, septum et cortex entorhinal et finalement, les secousses musculaires, en créant un feed-back sensoriel, déclenchent des oscillations gamma immatures ainsi que les spindle bursts dans le réseau thalamo-cortical. Un fonctionnement similaire est décrit chez le nouveau-né prématuré. Il paraît donc plus probable, que le cerveau soit, dès les stades précoces du développement, organisé en sous-systèmes fonctionnels reliés entre eux anatomiquement et fonctionnellement. Au sein des unités fonctionnelles sont générés des patrons d’activité immatures synchrones afin de créer des connexions organisées topographiquement qui serviront de base anatomique de la fonction finale. Si ces étapes développementales sont perturbées pendant les périodes critiques, le système ne pourra pas assurer sa fonction de manière adéquate au stade mature. L’hippocampe mature, ou plus exactement les circuits cortico-hippocampiques, jouant un rôle primordial dans la mémoire déclarative, l’orientation spatiale et l’inhibition du comportement. L’établissement de ces fonctions est progressif au cours du développement. Par exemple les adultes humains n’ont que rarement des souvenirs personnels datant avant l’âge de trois ans. Or, nous savons aujourd'hui que le bébé humain est capable de garder des souvenirs dans la mémoire déclarative (dépendante de l’hippocampe) au cours de la première année de vie avec une efficacité croissante, mais il ne se rappellera pas ces souvenirs à l’âge adulte (Bauer, 2006). Nous ne savons pas s’il s’agit d’un encodage différent d’emblée ou d’un processus secondaire supprimant l’accès à ces souvenirs précoces. Nous pouvons présumer qu’il existe des modifications des activités électrophysiologiques pendant le développement qui soutiennent la modification de ces fonctions. Au cours de ce travail de thèse, nous voulions savoir comment et à partir de quand l’hippocampe, qui reçoit des informations convergentes de nombreuses régions néocorticales, acquiert son mode de fonctionnement adulte. Afin de répondre à cette question nous avons étudié le système cortex entorhinal – hippocampe, le cortex entorhinal étant la principale entrée excitatrice de l’hippocampe et recevant des afférences de nombreuses régions du néocortex. (...) / Neuronal networks spontaneously generate “immature” patterns of activity during development, which are thought to participate on the formation of neural circuits. Local neocortical as well as hippocampal circuits generate synchronised neuronal discharges providing support for Hebbian plasticity. Studies of sensory systems showed that local pauci-neuronal circuits were able to generate synchronous activity while isolated from other brain structures. Isolated spinal cord produces bursts evoking muscle twitching, light insensitive retina generates waves of activity, as well as other brain regions generate synchronous activities before fulfilling the function for which they are intended. Similarly, the hippocampus of newborn rat or premature primate in vitro, as well as immature neocortex in vitro, generates synchronised neuronal activity called giant depolarising potentials (GDPs). Based solely on these studies and taking into account the delayed maturation of certain long-distance neuronal projections, it would be tempting to conclude that the immature brain functions as a set of small functional modules that self-maintain their intrinsic activity before connecting together to create a functional adult brain. However, some long-distance connections are formed very early during development and allow the propagation of oscillations between immature connected structures. Indeed, retinal waves propagate to the lateral geniculate nucleus and then to the visual cortex, hippocampal GDPs propagate to the contralateral hippocampus, septum and entorhinal cortex, and finally, twitching, creating a sensory feedback, triggers immature gamma oscillations and spindle bursts in the thalamo-cortical network. A similar functioning is described in the premature newborn. It therefore seems more likely that the brain is, during the early stages of development, organised into functional subsystems interconnected anatomically and functionally. Within functional units are generated immature patterns of synchronous activity to create topographically organised connections that serve as anatomical basis of the final function. If these developmental stages are disturbed during critical periods, the system cannot perform its function adequately in mature stage. The mature hippocampus, or more precisely the cortico-hippocampal circuits, plays a key role in declarative memory, spatial organisation and behavioural inhibition. The establishment of these functions is progressive during development. For example, human adults rarely have personal memories dating before the age of three years. However, we now know that the human baby is able to keep memories in declarative memory (hippocampus-dependent) during the first year of life with increasing efficiency, but will not remember them in the adulthood. We do not know if the encoding of the memories is different or a secondary process inhibits the access to the early memories. We can assume that changes in electrophysiological activity during development support modification of these functions. In this thesis, we wanted to know how and from when the hippocampus, which receives convergent information from many cortical areas, acquires his adult mode of functioning. To answer this question we studied the entorhinal cortex-hippocampus system, entorhinal cortex being the main excitatory input to the hippocampus and receiving afferents from many parts of the neocortex. We were able to distinguish several periods in the development of the immature hippocampus: Period from P1 till P12 characterised by the sole presence of immature sharp waves triggered by the entorhinal cortex. Period from P13, when two types of sharp waves coexisted: the immature sharp waves and sharp waves as described in the adult animals newly emerged. The mature sharp waves, unlike the immature, can be accompanied by ripples. (...)

Schicht 2 des mediale entorhinale Kortex (MEK) beinhaltet die größte Anzahl von Gitterzellen, welche durch ein hexagonales Aktivitätsmuster während räumlicher Exploration gekennzeichnet sind. In dieser Arbeit wurde gezeigt, dass spezielle Pyramidenzellen, die das Protein Calbindin exprimieren, in einem hexagonalen Gitter im Gehirn der Ratte angeordnet sind und cholinerg innerviert werden. Es ist bekannt, dass die cholinerge Innervation wichtig für die Aktivität von Gitterzellen ist. Weiterhin ergaben neuronale Ableitungen und Methoden zur Identifikaktion einzelner Neurone in frei verhaltenden Ratten, dass Calbindin-positive Pyramidenzellen (Calbindin+) eine große Anzahl von Gitterzellen beinhalten. Reelin-positive Sternzellen (Reelin+) im MEK, zeigten keine anatomische Periodizität und ihre Aktivität orientierte sich an den Begrenzungen der Umgebung. Eine weitere Studie untersucht die Architektur des MEK in verschiedenen Säugetieren, die von der Etrusker Spitzmaus, bis hin zum Menschen ~100 Millionen Jahre evolutionäre Vielfalt und ~20,000 fache Variation der Gehirngröße umfassen. Alle Arten zeigten jeweils eine periodische Anhäufung der Calbindin+ Zellen, was deren evolutive Bedeutung unterstreicht. Eine Studie zur Ontogenese der Calbindin Anhäufungen ergab, dass die periodische Struktur der Calbindin+ Zellen, sowie die verstreute Anordnung der Reelin+ Sternzellen schon zum Zeitpunkt der Geburt erkennbar war. Weitere Ergebnisse zeigen, dass Calbindin+ Zellen strukturell später ausreifen als Reelin+ Sternzellen - passend zu der Erkenntnis, dass Gitterzellen funktionell später reifen als Grenzzellen. Eine Untersuchung des Parasubiculums ergab, dass Verbindungen zum MEK präferiert in die Calbindin Anhäufungen in Schicht 2 projizieren. Zusammenfassend beschreibt diese Doktorarbeit eine Dichotomie von Struktur und Funktion in Schicht 2 des MEK, welche fundamental für das Verständnis von Gedächtnisbildung und deren zugrundeliegenden Mikroschaltkreisen ist. / The medial entorhinal cortex (MEC) is an important hub in the memory circuit in the brain. This thesis comprises of a group of studies which explores the architecture and microcircuits of the MEC. Layer 2 of MEC is home to grid cells, neurons which exhibit a hexagonal firing pattern during exploration of an open environment. The first study found that a group of pyramidal cells in layer 2 of the MEC, expressing the protein calbindin, were clustered in the rat brain. These patches were physically arranged in a hexagonal grid in the MEC and received preferential cholinergic-inputs which are known to be important for grid-cell activity. A combination of identified single-cell and extracellular recordings in freely behaving rats revealed that grid cells were mostly calbindin-positive pyramidal cells. Reelin-positive stellate cells in MEC were scattered throughout layer 2 and contributed mainly to the border cell population– neurons which fire at the borders of an environment. The next study explored the architecture of the MEC across evolution. Five mammalian species, spanning ~100 million years of evolutionary diversity and ~20,000 fold variation in brain size exhibited a conserved periodic layout of calbindin-patches in the MEC, underscoring their importance. An investigation of the ontogeny of the MEC in rats revealed that the periodic structure of the calbindin-patches and scattered layout of reelin-positive stellate cells was present around birth. Further, calbindin-positive pyramidal cells matured later in comparison to reelin-positive stellate cells mirroring the difference in functional maturation profiles of grid and border cells respectively. Inputs from the parasubiculum, selectively targeted calbindin-patches in the MEC indicating its role in shaping grid-cell function. In summary, the thesis uncovered a structure-function dichotomy of neurons in layer 2 of the MEC which is a fundamental aspect of understanding the microcircuits involved in memory formation.

<p>In order to understand how the brain functions, under normal as well as pathological conditions, it is important to study the mechanisms underlying information integration. Depending on the nature of an input arriving at a synapse, different strategies may be used by the neuron to integrate and respond to the input. Naturally, if a short train of high-frequency synaptic input arrives, it may be beneficial for the neuron to be equipped with a fast mechanism that is highly sensitive to inputs on a short time scale. If, on the contrary, inputs arriving with low frequency are to be processed, it may be necessary for the neuron to possess slow mechanisms of integration. For example, in certain working memory tasks (e. g. delay-match-to-sample), sensory inputs may arrive separated by silent intervals in the range of seconds, and the subject should respond if the current input is identical to the preceeding input. It has been suggested that single neurons, due to intrinsic mechanisms outlasting the duration of input, may be able to perform such calculations. In this work, I have studied a mechanism thought to be particularly important in supporting the integration of low-frequency synaptic inputs. It is mediated by a cascade of events that starts with activation of group I metabotropic glutamate receptors (mGlu1/5), and ends with a membrane depolarization caused by a current that is mediated by canonical transient receptor potential (TRPC) ion channels. This current, denoted I_TRPC, is the focus of this thesis.</p><p>A specific objective of this thesis is to study the role of I_TRPC in the integration of synaptic inputs arriving at a low frequency, < 10 Hz. Our hypothesis is that, in contrast to the well-studied, rapidly decaying AMPA and NMDA currents, I_TRPC is well-suited for supporting temporal summation of such synaptic input. The reason for choosing this range of frequencies is that neurons often communicate with signals (spikes) around 8 Hz, as shown by single-unit recordings in behaving animals. This is true for several regions of the brain, including the entorhinal cortex (EC) which is known to play a key role in producing working memory function and enabling long-term memory formation in the hippocampus.</p><p>Although there is strong evidence suggesting that I_TRPC is important for neuronal communication, I have not encountered a systematic study of how this current contributes to synaptic integration. Since it is difficult to directly measure the electrical activity in dendritic branches using experimental techniques, I use computational modeling for this purpose. I implemented the components necessary for studying I_TRPC, including a detailed model of extrasynaptic glutamate concentration, mGlu1/5 dynamics and the TRPC channel itself. I tuned the model to replicate electrophysiological in vitro data from pyramidal neurons of the rodent EC, provided by our experimental collaborator. Since we were interested in the role of I_TRPC in temporal summation, a specific aim was to study how its decay time constant (τ_decay) is affected by synaptic stimulus parameters.</p><p>The hypothesis described above is supported by our simulation results, as we show that synaptic inputs arriving at frequencies as low as 3 - 4 Hz can be effectively summed. We also show that τ_decay increases with increasing stimulus duration and frequency, and that it is linearly dependent on the maximal glutamate concentration. Under some circumstances it was problematic to directly measure τ_decay, and we then used a pair-pulse paradigm to get an indirect estimate of τ_decay.</p><p>I am not aware of any computational model work taking into account the synaptically evoked I_TRPC current, prior to the current study, and believe that it is the first of its kind. We suggest that I_TRPC is important for slow synaptic integration, not only in the EC, but in several cortical and subcortical regions that contain mGlu1/5 and TRPC subunits, such as the prefrontal cortex. I will argue that this is further supported by studies using pharmacological blockers as well as studies on genetically modified animals.</p> / QC 20101005

No condicionamento clássico de medo, os animais são treinados associando-se um estímulo neutro, por exemplo, som, contexto ou luz a um estímulo aversivo incondicionado, como um choque elétrico nas patas. Apos repetidos pareamentos, a presença do estímulo que inicialmente era neutro passa a eliciar uma resposta condicionada de medo no animal. O congelamento é a resposta mais proeminente dos animais expostos aos estímulos condicionados previamente pareados com choques nas patas, sendo freqüentemente utilizado como medida de medo condicionado (MC). Circuitos cerebrais independentes subjacentes a diferentes formas de memória, e, dentro de um determinado domínio de memória, o envolvimento de estruturas específicas pode depender do tipo de condicionamento se utilizando contexto ou explícito tais sinais leves ou som. Diversos relatos clínicos têm implicado o prejuízo do lobo temporal medial (LTM) com amnésia retrógrada. Embora muito tenha sido feito para desvendar os circuitos neurais subjacentes ao medo condicionado, utilizando contexto, som ou luz como estímulo condicionado (EC) o envolvimento do LTM nessas formas de condicionamento ainda não está claro. Para abordar esta questão foi avaliada a distribuição de Fos no LTM de ratos após a exposição a um contexto, um som ou luz, previamente emparelhado com choques nas patas. Vinte e quatro horas após as sessões de condicionamento, os animais foram colocados na mesma caixa experimental ou a um contexto distinto ou foram expostos ao som e luz sem receber choques nas patas. Diferença significativa na expressão de Fos foi determinada por análise de regiões do lobo temporal medial (córtex ectorrinal, perirrinal e entorrinal) e do hipocampo ventral. Os resultados comportamentais mostraram que houve congelamento nos três tipos de medo condicionado, mas o padrão de distribuição Fos foi diferente em ratos expostos a estímulos específicos ou contexto previamente emparelhado com choques nas patas. Apesar da saliente aquisição da resposta do medo se simular nas três condições, o achado mais saliente foi uma distribuição selectiva de Fos no córtex ectorrinal, perirrinal e entorrinal do grupo. Surpreendentemente, esses animais não mostraram significativa expressão Fos no hipocampo ventral. Isto sugere que o contexto e estímulos aversivos explícitos apresentam propriedades distintas de mapeamento ao de distribuição de Fos no circuito cortico-hipocampal cerebral. Estes resultados indicam que regiões corticais no LTM parecem ser críticas no armazenamento de informações contextuais, mas não de informações associadas a estímulos explícitos previamente pareados a choques nas patas. / Conditioned fear (CF) is one of the most frequently used animal models of associative memory to background or foreground stimuli. Independent brain circuits underlie different forms of memory, and, within a particular memory domain, the involvement of specific structures may depend upon the type of conditioning whether using context or explicit cues such light or tone. Several clinical reports have implicated the damage to the medial temporal lobe (MTL) with retrograde amnesia. Although much has been done to disclose the neural circuits underlying CF using context, tone or light as conditioned stimuli (CS) the involvemet of the MTL in these forms of conditioning is still unclear. To address this issue we assessed the Fos distribution in the MTL of rats following exposure to a context, a tone or a light previously paired with footshocks. Twenty-four hours later the conditioning sessions they were placed to the same chamber or to a distinct context and presented with tone or light only without any footshocks. Significant group differences in regional Fos expression were determined by analysis in regions of the medial temporal lobe (ectorhinal, perirhinal and entorhinal cortices) and the ventral hippocampus. The behavioral results showed comparable freezing in the three types of CF but the pattern of Fos distribution was distinct in rats exposed to specific cues or context previously paired with footshocks. Despite comparable acquisition of the conditioned fear response, the most remarkable finding was a selective distribution of Fos in the entorhinal, perirhinal and ectorhinal cortices of the MTL for context-CS groups. Remarkably, these animals did not show significant Fos expression in the ventral hippocampus. It is suggested that context and explicit stimuli endowed with aversive properties through conditioning cause distinct Fos brain mapping in the corticohippocampal circuitry. These results indicate that tasks requiring the association between context and an aversive stimulus depend on subregions of the MTL. Such findings suggested that cortical regions of the MTL appears to be critical for storing context but not explicit cue footshock associations.

No condicionamento clássico de medo, os animais são treinados associando-se um estímulo neutro, por exemplo, som, contexto ou luz a um estímulo aversivo incondicionado, como um choque elétrico nas patas. Apos repetidos pareamentos, a presença do estímulo que inicialmente era neutro passa a eliciar uma resposta condicionada de medo no animal. O congelamento é a resposta mais proeminente dos animais expostos aos estímulos condicionados previamente pareados com choques nas patas, sendo freqüentemente utilizado como medida de medo condicionado (MC). Circuitos cerebrais independentes subjacentes a diferentes formas de memória, e, dentro de um determinado domínio de memória, o envolvimento de estruturas específicas pode depender do tipo de condicionamento se utilizando contexto ou explícito tais sinais leves ou som. Diversos relatos clínicos têm implicado o prejuízo do lobo temporal medial (LTM) com amnésia retrógrada. Embora muito tenha sido feito para desvendar os circuitos neurais subjacentes ao medo condicionado, utilizando contexto, som ou luz como estímulo condicionado (EC) o envolvimento do LTM nessas formas de condicionamento ainda não está claro. Para abordar esta questão foi avaliada a distribuição de Fos no LTM de ratos após a exposição a um contexto, um som ou luz, previamente emparelhado com choques nas patas. Vinte e quatro horas após as sessões de condicionamento, os animais foram colocados na mesma caixa experimental ou a um contexto distinto ou foram expostos ao som e luz sem receber choques nas patas. Diferença significativa na expressão de Fos foi determinada por análise de regiões do lobo temporal medial (córtex ectorrinal, perirrinal e entorrinal) e do hipocampo ventral. Os resultados comportamentais mostraram que houve congelamento nos três tipos de medo condicionado, mas o padrão de distribuição Fos foi diferente em ratos expostos a estímulos específicos ou contexto previamente emparelhado com choques nas patas. Apesar da saliente aquisição da resposta do medo se simular nas três condições, o achado mais saliente foi uma distribuição selectiva de Fos no córtex ectorrinal, perirrinal e entorrinal do grupo. Surpreendentemente, esses animais não mostraram significativa expressão Fos no hipocampo ventral. Isto sugere que o contexto e estímulos aversivos explícitos apresentam propriedades distintas de mapeamento ao de distribuição de Fos no circuito cortico-hipocampal cerebral. Estes resultados indicam que regiões corticais no LTM parecem ser críticas no armazenamento de informações contextuais, mas não de informações associadas a estímulos explícitos previamente pareados a choques nas patas. / Conditioned fear (CF) is one of the most frequently used animal models of associative memory to background or foreground stimuli. Independent brain circuits underlie different forms of memory, and, within a particular memory domain, the involvement of specific structures may depend upon the type of conditioning whether using context or explicit cues such light or tone. Several clinical reports have implicated the damage to the medial temporal lobe (MTL) with retrograde amnesia. Although much has been done to disclose the neural circuits underlying CF using context, tone or light as conditioned stimuli (CS) the involvemet of the MTL in these forms of conditioning is still unclear. To address this issue we assessed the Fos distribution in the MTL of rats following exposure to a context, a tone or a light previously paired with footshocks. Twenty-four hours later the conditioning sessions they were placed to the same chamber or to a distinct context and presented with tone or light only without any footshocks. Significant group differences in regional Fos expression were determined by analysis in regions of the medial temporal lobe (ectorhinal, perirhinal and entorhinal cortices) and the ventral hippocampus. The behavioral results showed comparable freezing in the three types of CF but the pattern of Fos distribution was distinct in rats exposed to specific cues or context previously paired with footshocks. Despite comparable acquisition of the conditioned fear response, the most remarkable finding was a selective distribution of Fos in the entorhinal, perirhinal and ectorhinal cortices of the MTL for context-CS groups. Remarkably, these animals did not show significant Fos expression in the ventral hippocampus. It is suggested that context and explicit stimuli endowed with aversive properties through conditioning cause distinct Fos brain mapping in the corticohippocampal circuitry. These results indicate that tasks requiring the association between context and an aversive stimulus depend on subregions of the MTL. Such findings suggested that cortical regions of the MTL appears to be critical for storing context but not explicit cue footshock associations.

Alzheimer’s disease (AD) is a degenerative disorder where the distribution of pathology throughout the brain is not random but follows a predictive pattern used for pathological staging. While the involvement of defined functional systems is fairly well established for more advanced stages, the initial sites of degeneration are still ill defined. The prevailing concept suggests an origin within the transentorhinal and entorhinal cortex (EC) from where pathology spreads to other areas. Still, this concept has been challenged recently suggesting a potential origin of degeneration in nonthalamic subcortical nuclei giving rise to cortical innervation such as locus coeruleus (LC) and nucleus basalis of Meynert (NbM). To contribute to the identification of the early site of degeneration, here, we address the question whether cortical or subcortical degeneration occurs more early and develops more quickly during progression of AD. To this end, we stereologically assesses neurone counts in the NbM, LC and EC layer-II in the same AD patients ranging from preclinical stages to severe dementia. In all three areas, neurone loss becomes detectable already at preclinical stages and is clearly manifest at prodromal AD/MCI. At more advanced AD, cell loss is most pronounced in the NbM > LC > layer-II EC. During early AD, however, the extent of cell loss is fairly balanced between all three areas without clear indications for a preference of one area. We can thus not rule out that there is more than one way of spreading from its site of origin or that degeneration even occurs independently at several sites in parallel.

In order to understand how the brain functions, under normal as well as pathological conditions, it is important to study the mechanisms underlying information integration. Depending on the nature of an input arriving at a synapse, different strategies may be used by the neuron to integrate and respond to the input. Naturally, if a short train of high-frequency synaptic input arrives, it may be beneficial for the neuron to be equipped with a fast mechanism that is highly sensitive to inputs on a short time scale. If, on the contrary, inputs arriving with low frequency are to be processed, it may be necessary for the neuron to possess slow mechanisms of integration. For example, in certain working memory tasks (e. g. delay-match-to-sample), sensory inputs may arrive separated by silent intervals in the range of seconds, and the subject should respond if the current input is identical to the preceeding input. It has been suggested that single neurons, due to intrinsic mechanisms outlasting the duration of input, may be able to perform such calculations. In this work, I have studied a mechanism thought to be particularly important in supporting the integration of low-frequency synaptic inputs. It is mediated by a cascade of events that starts with activation of group I metabotropic glutamate receptors (mGlu1/5), and ends with a membrane depolarization caused by a current that is mediated by canonical transient receptor potential (TRPC) ion channels. This current, denoted ITRPC, is the focus of this thesis. A specific objective of this thesis is to study the role of ITRPC in the integration of synaptic inputs arriving at a low frequency, &lt; 10 Hz. Our hypothesis is that, in contrast to the well-studied, rapidly decaying AMPA and NMDA currents, ITRPC is well-suited for supporting temporal summation of such synaptic input. The reason for choosing this range of frequencies is that neurons often communicate with signals (spikes) around 8 Hz, as shown by single-unit recordings in behaving animals. This is true for several regions of the brain, including the entorhinal cortex (EC) which is known to play a key role in producing working memory function and enabling long-term memory formation in the hippocampus. Although there is strong evidence suggesting that ITRPC is important for neuronal communication, I have not encountered a systematic study of how this current contributes to synaptic integration. Since it is difficult to directly measure the electrical activity in dendritic branches using experimental techniques, I use computational modeling for this purpose. I implemented the components necessary for studying ITRPC, including a detailed model of extrasynaptic glutamate concentration, mGlu1/5 dynamics and the TRPC channel itself. I tuned the model to replicate electrophysiological in vitro data from pyramidal neurons of the rodent EC, provided by our experimental collaborator. Since we were interested in the role of ITRPC in temporal summation, a specific aim was to study how its decay time constant (τdecay) is affected by synaptic stimulus parameters. The hypothesis described above is supported by our simulation results, as we show that synaptic inputs arriving at frequencies as low as 3 - 4 Hz can be effectively summed. We also show that τdecay increases with increasing stimulus duration and frequency, and that it is linearly dependent on the maximal glutamate concentration. Under some circumstances it was problematic to directly measure τdecay, and we then used a pair-pulse paradigm to get an indirect estimate of τdecay. I am not aware of any computational model work taking into account the synaptically evoked ITRPC current, prior to the current study, and believe that it is the first of its kind. We suggest that ITRPC is important for slow synaptic integration, not only in the EC, but in several cortical and subcortical regions that contain mGlu1/5 and TRPC subunits, such as the prefrontal cortex. I will argue that this is further supported by studies using pharmacological blockers as well as studies on genetically modified animals. / QC 20101005

Encoding and consolidation of memories are two processes within the hippocampus, and connected cortical networks, that recruit different circuit level dynamics to effectively process and pass information from brain region to brain region. In the hippocampal CA1 pyramidal layer local field potential (LFP), these processes take the form of theta and sharp wave ripples (SPW-Rs) for encoding and consolidation, respectively. As an animal runs through an environment, neurons become active at specific locations in the environment (place cells) increasing their firing rate, functionally representing these specific locations. These firing rate increases are organized within the local theta oscillations and sequential activation of many place cells creates a map of the environment. Once the animal stops moving and begins consummatory behaviors, such as eating, drinking, or grooming, theta activity diminishes, and large irregular activity (LIA) begins to dominate the LFP. Spontaneously, with the LIA, the place cells active during the experience are replayed during SPW-Rs in the same spatial order they were encountered in the environment. Both theta and SPW-R oscillations and their associated neuronal firing are necessary for effective place recognition as well as learning and memory. As such, interruption or termination of SPW-R events results in decreased learning performance over days. During exploration, the associated theta and sequential place cell activity is thought to encode the experience. During quiet restfulness or slow wave sleep (SWS), SPW-R events, that replay experience specific place sequences, are thought to be the signal by which systems consolidation progresses and the hippocampus guides cortical synaptic reorganization. The granular retrosplenial cortex (gRSC) is an associational area that exhibits high frequency oscillations (HFOs) during both hippocampal theta and SPW-Rs, and is potentially a period when the gRSC interprets incoming content from the hippocampus during encoding and systems consolidation. However, the precise laminar organization of synaptic currents supporting HFOs, whether the local gRSC circuitry can support HFOs without patterned input, and the precise coupling of hippocmapla oscillations to gRSC HFOs across brain states remains unknown. We aimed to answer these questions using in vivo, awake electrophysiological recordings in head-fixed mice that were trained to run for water rewards in a 1D virtual environment. We show that gRSC synaptic currents supporting HFOs, across all awake brain states, are exclusively localized to layer 2/3 (L2/3), even when events are detected within layer 5 (L5). Using focal optogenetics, both L2/3 and L5 can generate induced HFOs given a strong enough broad stimulation. Spontaneous gRSC HFOs occurring outside of SPW-Rs are highly comodulated with medial entorhinal cortex (MEC) generated high gamma in hippocampal stratum lacunosum moleculare. gRSC HFOs may serve a necessary role in communication between the hippocampus during SPW-Rs states and between the hippocampus, gRSC, and MEC during theta states to support memory consolidation and memory encoding, respectively. / Doctor of Philosophy / As an animal moves through an environment, individual neurons in the hippocampus, known as place cells, increase and decrease their firing rate as the animal enters and exits specific locations in the environment. Within an environment, multiple neurons become active in different locations, this cooperation of spiking in various locations creates a place map of the environment. Now let's say when the animal moved from one corner of the environment to another, place cells 'A', 'C', 'B', 'E', and 'D' became active in that order. This means, at any given point in the environment, the animal is standing in a venn-diagram-esque overlap of place fields, or locations individual place cells represent. A key question that entranced researchers for many years was how do these neurons know when to be active to not impinge on their neighbor's locations? The answer to this question rested with population electrical activity, known as the local field potential (LFP), that place cell activity is paced to. During active navigation through an environment, place cells activity is coupled to the phase of a slow ~8 hertz (Hz) theta oscillation. Within one theta cycle, or peak to peak, multiple place cells are active, representing the venn diagram of location the animal is in. Importantly, this theta activity and encoding of place cell activity is largely seen during active running or rapid eye movement (REM) sleep. During slow wave sleep (SWS), after an animal has experienced a specific environment and has created a place map, place cells are reactivated in the same order the animal experienced them in. From our previous example, the content of this reactivation would be the place cells 'A', 'C', 'B', 'E', and 'D' which all would be reactivated in that same order. These reactivations or replays occur during highly synchronous and fast LFP oscillations known as sharp wave-ripples (SPW-Rs). SPW-Rs are thought to be a key LFP event that drives memory consolidation and the eventual conversion of short-term memory into long-term memory. However, for consolidation to occur, connected cortical regions need to be able to receive and interpret the information within SPW-Rs. The granular retrosplenial cortex (gRSC) is one proposed region that serves this role. During SPW-Rs the superficial gRSC has been shown to exhibit high frequency oscillations (HFOs), which potentially serve the purpose for interpreting SPW-R content. However, HFOs have been reported during hippocampal theta, suggesting HFOs serve multiple purposes in interregional communication across different states. In this study, we found that naturally occurring gRSC HFOs occur exclusively in layer 2/3 across all awake brain states. Using focal optogenetic excitation we were able to evoke HFOs in both layer 2/3 and 5. Spontaneous gRSC HFOs occurring without SPW-Rs were highly comodulated with medial entorhinal cortex (MEC) generated high gamma in hippocampal stratum lacunosum moleculare. gRSC HFOs may serve a general role in supporting hippocampo-cortical dialogue during SPW-R and theta brain states to support memory consolidation and encoding, respectively.