CA1 is the main source of afferents from the hippocampus, but the function of CA1 and its perforant path (PP) input remains unclear. In this thesis, Marr’s model of the hippocampus is used to investigate previously hypothesized functions, and also to investigate some of Marr’s unexplored theoretical ideas. The last part of the thesis explains the excitatory responses to PP activity in vivo, despite inhibitory responses in vitro. Quantitative support for the idea of CA1 as a relay of information from CA3 to the neocortex and subiculum is provided by constraining Marr’s model to experimental data. Using the same approach, the much smaller capacity of the PP input by comparison implies it is not a one-shot learning network. In turn, it is argued that the entorhinal-CA1 connections cannot operate as a short-term memory network through reverberating activity. The PP input to CA1 has been hypothesized to control the activity of CA1 pyramidal cells. Marr suggested an algorithm for self-organising the output activity during pattern storage. Analytic calculations show a greater capacity for self-organised patterns than random patterns for low connectivities and high loads, confirmed in simulations over a broader parameter range. This superior performance is maintained in the absence of complex thresholding mechanisms, normally required to maintain performance levels in the sparsely connected networks. These results provide computational motivation for CA3 to establish patterns of CA1 activity without involvement from the PP input. The recent report of CA1 place cell activity with CA3 lesioned (Brun et al., 2002. Science, 296(5576):2243-6) is investigated using an integrate-and-fire neuron model of the entorhinal-CA1 network. CA1 place field activity is learnt, despite a completely inhibitory response to the stimulation of entorhinal afferents. In the model, this is achieved using N-methyl-D-asparate receptors to mediate a significant proportion of the excitatory response. Place field learning occurs over a broad parameter space. It is proposed that differences between similar contexts are slowly learnt in the PP and as a result are amplified in CA1. This would provide improved spatial memory in similar but different contexts.

Alzheimer’s disease (AD) is a neurodegenerative disorder resulting in the decline of cognitive function, memory formation and retrieval, and abrupt changes in personality. Damage to brain networks occur during prodromal stages of AD, prior to the development of clinical symptoms of dementia. Further characterising this state and identifying reliable biomarkers for early detection are priorities in AD research. I characterised neuronal changes within the dorsal CA1 and subiculum regions of the hippocampal formation (HF) in the well-characterised 3xTgAD mouse model of AD. These regions are well-established sites for early neurodegeneration in both AD patients and AD animal models. We inserted multi-electrode recording arrays into CA1 and subiculum of urethane anaesthetised 3xTgAD mice and recorded spontaneous local field potential activity. Using traditional and novel information theoretic approaches, I determined the information carrying capacity of the CA1- subiculum network during different network rhythms, and how this altered with age and AD-like pathology. A bipolar stimulating electrode was inserted into CA1, allowing the assessment of synaptic integrity between CA1 and subiculum. Results showed that synaptic and network changes occur in CA1 and subiculum during the early stages of AD-like pathology and correlates with the development of intracellular beta-amyloid. There is a progressive breakdown in synaptic facilitation as early as 3 months in the 3xTgAD mouse. These data support an advanced ageing-like phenotype in AD model mice, with an enhanced age/pathology-dependent breakdown in neuronal communication compared to age-matched controls. In agreement with other studies, 3xTgAD mice demonstrate evidence of pathology-related changes in the network rhythms of the HF. 3xTgAD mice show an increase in the power of alpha and beta rhythms, and a concurrent reduction in the power of delta oscillations. Application of novel information theoretic techniques results in a breakdown in the information carrying capacity of the hippocampal system. This deficit manifests as a reduction in information flow during delta-dominant periods of EEG rhythms, with a specific reduction during slow-wave ripple activity. This change in neuronal communication correlates with the onset of memory-retention/consolidation deficits. These network changes are complex, with alterations in the information carrying capacity of the system during theta rhythms at 6 months, and during slow-wave components by 9 months in the 3xTgAD mouse. This study provides the first evidence of an early and progressive decline in neuronal connectivity and communication that correlates with changes in cognition in the 3xTgAD mouse. Application of novel analytical techniques to multi-site EEG recording revealed early and measureable changes in information processing during the onset of AD-like pathology. These are important new biomarkers for early AD characterisation.

GABA is the major inhibitory neurotransmitter in the central nervous system. It binds to two types of receptors –ionotropic GABAA and metabotropic GABAB. The GABAA receptor directly gates a Clionophore that causes hyperpolarization in mature excitatory neurons while GABAB receptor mediates a slower hyperpolarizing response via G-protein coupled receptor (GPCR) activated potassium channels. This signaling mechanism gets further complicated by the heterogeneous GABA receptor subunit composition that influences the response kinetics in the postsynaptic membrane. In this thesis, the focus has been to decipher the role of GABAA receptors in relation to cellular excitability in the subiculum under physiological and pathophysiological conditions. The subiculum, considered as the output structure of hippocampus, modulates information flow from hippocampus to various cortical and sub-cortical areas and has been implicated in learning and memory, rhythm generation and various neurological disorders. It gates hippocampal activity with its well orchestrated and fine tuned intrinsic and local network properties. Over the years many studies have shown the involvement of subiculum in temporal lobe epilepsy where it forms the focal point of epileptiform activities with altered cellular and network properties. The subiculum is characterized by the presence of a significant population of burst firing neurons that lead local epileptiform activity. By virtue of its bursting nature and recurrent connections, it is a potential site for seizure generation and maintenance. Epileptiform activities are dynamic in nature and change temporally and spatially according to the alterations in electrophysiological properties of neurons. Transitions to different electrical activities in neurons following a prolonged challenge with epileptogenic stimulus have been shown in other brain structures, but not in the subiculum. Considering the importance of the subicular burst firing neurons in the propagation of epileptiform activity to the entorhinal cortex, we have explored the phenomenon of electrophysiological phase transitions in the burst firing neurons of the subiculum in an in vitro brain slice model of epileptogenesis. Whole-cell patch clamp and extracellular field recordings revealed a distinct phenomenon in the subiculum wherein an early hyperexcitable phase was followed by a late suppressed phase upon continuous perfusion with epileptogenic 4-amino pyridine and magnesium-free medium. The late suppressed phase was characterized by inhibitory post-synaptic potentials (IPSPs) in pyramidal excitatory neurons and bursting activity in local fast spiking interneurons at a frequency of 0.1-0.8 Hz. The IPSPs were mediated by GABAA receptors that coincided with excitatory synaptic inputs to attenuate action potential discharge. These IPSPs ceased following a cut between the CA1 and subiculum. Our results suggest the importance of feedforward inhibition in the suppression of epileptiform activity in subiculum to mediate a homeostatic response towards the induced hyper-excitability. GABA release from presynaptic nerve endings activates postsynaptic GABAA receptors, which evoke faster phasic inhibitory postsynaptic currents (IPSCs) and non-inactivating inhibitory tonic current, mediated through extrasynaptic GABAA receptors. These receptors are heteropentameric GABA-gated channels assembled from 19 possible subunits (α1-6, β1-3, γ1-3, δ, π, ρ1-3, θ, and ε). The 2 major subunits involved in tonic GABAA currents in the hippocampus are α5 and δ subunits. Tonic GABAA receptor mediated inhibitory current plays an important role in neuronal physiology as well as pathophysiology such as mood disorders, insomnia, epilepsy, autism spectrum disorders and schizophrenia. While the alterations of various electrical properties due to tonic inhibition have been studied in neurons from different regions, its influence on intrinsic subthreshold resonance in pyramidal excitatory neurons having hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is not known. In the present study, we show the involvement of α5βγ GABAA receptors in mediating picrotoxin sensitive tonic current in subicular pyramidal neurons using known pharmacological agents that target specific GABAA receptor subunits. We further investigated the contribution of tonic conductance in regulating subthreshold electrophysiological properties using current clamp and dynamic clamp experiments. Our experiments suggest that tonic GABAergic inhibition can actively modulate subthreshold properties of subicular pyramidal neurons including resonance due to HCNchannels that may potentially alter the response dynamics in an oscillating neuronal network.

Thèse (Ph. D.)--Université Laval, 2009. / Bibliogr.: f. 191-236. Publié aussi en version électronique dans la Collection Mémoires et thèses électroniques.

1. Interaktion zwischen entorhinalem Kortex und Hippokampus Lernen und Gedächtnis sind auf das engste mit dem Hippokampus und dem entorhinalen Kortex (EC) verbunden. Allerdings sind diese Hirnstrukturen auch an einer der häufigsten und medikamentös oftmals nur schwer therapierbaren fokalen Epilepsien beteiligt: der mesialen Temporallappenepilepsie (TLE). Der EC scheint eine wesentliche Bedeutung in der Generierung extrahippokampaler Temporallappenanfälle zu besitzen. Unsere bisherigen Untersuchungen zur Interaktion zwischen dem EC und dem Hippokampus haben gezeigt, daß unter physiologischen Bedingungen die Area dentata eine Filterfunktion übernimmt und die Übertragung epileptiformer Aktivität vom EC zum Hippokampus unterbindet. Im chronisch epileptischen Tier (Kindling-Modell) kommt es allerdings zu einer Aufhebung dieser Filterfunktion und somit zu einer ungehinderten Ausbreitung epileptiformer Aktivität in den Hippokampus. Da der glutamaterge NMDA-Rezeptor eine zentrale Rolle in der Induktion nutzungsabhängiger Plastizität spielt, ist er von wesentlicher Bedeutung in der Epileptogenese. Untersuchungen an Körnerzellen der Area dentata zeigten wenige Stunden nach dem letzten epileptischen Anfall eine Zunahme der über NMDA-Rezeptoren vermittelten Ströme. Diese führte zu einer Faszilitierung hochfrequenter reizevozierter Potentiale. Dieser Befund zeigt, daß im epileptischen Gewebe hochfrequente Entladungen die Area dentata überwinden können und in den Hippokampus weitergeleitet werden. Vier Wochen nach dem letzten Anfallsereignis waren die beschriebenen Veränderungen allerdings nicht mehr nachweisbar. Diese kurzzeitig veränderte synaptische Transmission der NMDA-Rezeptorkanäle scheint demzufolge eher für die Epileptogenese als für die Ictogenese verantwortlich zu sein. Die Bedeutung der Kainat-Rezeptoren im chronisch epileptischen Gewebe ist aufgrund der bis vor wenigen Jahren fehlenden selektiven Agonisten und Antagonisten kaum untersucht worden. Wir haben gezeigt, daß in der Area dentata des chronisch epileptischen Tieres (Kindling-Modell) die Aktivierung von präsynaptischen Kainat-Rezeptoren inhibitorischer Interneurone sowohl die spontane als auch die reizevozierte GABA-Freisetzung reduziert. Über diesen Mechanismus scheint der während eines epileptischen Anfalls vermehrt freigesetzte exzitatorische Neurotransmitter Glutamat die GABAerge Inhibition zu vermindern und somit die Erregbarkeit der Area dentata zu steigern. 2. Die Rolle des Subikulums in der Temporallappenepilepsie Eine wesentliche Aufgabe des Subikulums ist es, hippokampale Informationen zu verarbeiten und in verschiedene kortikale und subkortikale Hirnregionen weiterzuleiten. Zudem scheint es von besonderer Bedeutung für die Generierung und Ausbreitung hippokampaler Anfälle zu sein. Gestützt wird diese Annahme durch folgende Befunde: Zunächst besitzt das Subikulum Netzwerkeigenschaften, die es ihm im in vitro Epilepsiemodell ermöglichen, spontane epileptiforme Aktivität zu generieren. Darüber hinaus verfügt es über einen hohen Anteil sogenannter burst-spiking Zellen. Deren intrinsische Eigenschaften tragen erheblich zu dem epileptogenen Verhalten des Subikulums bei. Weiterhin erhalten subikuläre Pyramidenzellen exzitatorische Eingänge sowohl aus der Area CA1 als auch aus dem EC, welche bereits bei Ruhemembranpotential aktivierbare NMDA-Rezeptorströme zeigen. Schließlich zeigen burst-spiking Zellen im Vergleich zu regular-spiking Zellen eine ausgeprägte über NMDA-Rezeptoren vermittelte synaptische Plastizität (Langzeit-Potenzierung; LTP). Untersuchungen am chronisch epileptischen Tier (Kindling-Modell) ergaben einen unverändert hohen Anteil an burst-spiking Zellen im Subikulum. Wenige Stunden nach dem letzten epileptischen Anfall fällt bei diesen Neuronen eine fehlende, durch Aktionspotentiale induzierte Nachhyperpolarisation auf. Diese supprimierte intrinsische Hemmung ist jedoch 28 Tage nach dem letzten epileptischen Anfall nicht mehr nachzuweisen und spielt demzufolge insbesondere in der Genese, weniger im chronischen Verlauf der Erkrankung eine Rolle. Neben den exzitatorischen und inhibitorischen Neurotransmittern Glutamat und GABA bestimmen auch körpereigene Amine wie Serotonin und Dopamin über subkortikale Afferenzen das funktionelle Gleichgewicht aus Erregung und Hemmung wesentlich mit. Da die TLE nicht selten mit neurologischen und psychiatrischen Erkrankungen einhergeht, die mit in das Dopamin- und Serotoninsystem eingreifenden Pharmaka therapiert werden, haben wir uns in einigen Arbeiten mit deren modulatorischen Wirkungen auf die Membraneigenschaften und die synaptische Transmission befaßt. Die Wirkungen von Dopamin auf die Neurotransmission sind vielfältig, abhängig von den beteiligten Rezeptoren in der entsprechenden Hirnregion. Das Subikulum, das eine ausgeprägte mesenzephale, dopaminerge Projektion vom ventralen Tegmentum erhält, expremiert sowohl D1- als auch D2-Rezeptoren. Wir konnten zeigen, daß Dopamin primär die glutamaterge synaptische Transmission über einen präsynaptisch lokalisierten D1-Dopaminrezeptor unterdrückt und sekundär über die verminderte Erregung inhibitorischer Interneurone die polysynaptische GABAerge Hemmung reduziert. / 1. Interaction between the entorhinal cortex and the hippocampus The hippocampus and the entorhinal cortex are crucially involved in the acquisition, consolidation and retrieval of long-term memory traces. However, both structures play a critical role in pharmacologically intractable temporal lobe epilepsy. The entorhinal cortex provides the main input to the hippocampus. We have shown that kindling facilitates the propagation of epileptiform activity through the dentate gyrus. Our data are consistent with the normal function of the dentate gyrus as a filter limiting the spread of epileptiform activity within the entorhinal-hippocampal complex. This gating mechanism breaks down after chronic epilepsy induced by kindling. In the mammalian brain, the NMDA subclass of glutamate receptors plays a central role in the induction of several forms of use-dependent plasticity. However, synaptic plasticity can potentially underlie pathological situations, notably in animal and human forms of epilepsy. The enhanced excitability of the kindled dentate gyrus several hours after the last seizure, as well as the breakdown of its gating function, appear to result from transiently enhanced NMDA receptor activation that provides significantly slower EPSC kinetics than those observed in control slices and in slices from kindled animals with a four weeks seizure-free interval. Therefore, NMDA receptors seem to play a critical role in the acute throughput of seizure activity and in the induction of the kindled state but not in the persistence of enhanced seizure susceptibility. The functional involvement of kainate receptors in epileptogenesis gets more and more elucidated. We found that in chronic epileptic rats (kindling-model), activation of presynaptic kainate receptors of inhibitory interneurons depresses spontaneous and stimulus-induced GABA release. The kindling-induced sensitivity of GABA release to kainate receptor activation may produce a use-dependent hyperexcitability in the epileptic dentate gyrus facilitating the spread of limbic seizures through the entorhinal-hippocampal complex in temporal lobe epilepsy. 2. The role of the subiculum in temporal lobe epilepsy The subiculum controls most of the entorhinal-hippocampal output. It receives strong excitatory input from area CA1 and the entorhinal cortex and relays information to a variety of distant cortical and subcortical structures. The subiculum seems to be crucially involved in the generation and propagation of hippocampal seizures. The seizure susceptibility of the subiculum relies (a) on a high fraction of burst-firing principle cells that a capable to undergo synaptic plasticity and (b) on an epilepsy-prone network to generate spontaneous seizures. In both, control and kindled preparations the subiculum contains an extensive sub-population of bursting cells expressing amplifying membrane characteristics. Subicular cells showed a transient depression of the fast and slow afterhyperpolarization in the course of kindling that may contribute to the induction but not permanence of the kindled state. Apart from the excitatory and inhibitory neurotransmission physiological amines like 5-HT and dopamine (DA) may offset the frail balance between excitation and inhibition in the hippocampus. As temporal lobe epilepsy is often associated with diseases that are treated with drugs affecting the 5-HT and DA system, we investigated the effect of these transmitters on intrinsic and synaptic properties of subicular principle cells. The subiculum receives a dense mesencepahalic dopaminergic projection from the ventral tegmental area and expresses high levels of D1- and D2-like DA receptors. Our results indicate that DA strongly suppresses glutamatergic hippocampal and entorhinal neurotransmission onto subicuar neurons by activation of presynaptic D1-like DA receptors. In addition, DA decreases polysynaptic inhibition by attenuating the glutamatergic drive onto subicular interneurons.

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.

No description available.

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.

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2008-2009 / Cette thèse a pour objectif d'examiner la contribution de deux composantes de la formation hippocampique à la mémoire spatiale chez le rat. Les trois expériences présentées évaluent l'effet de lésions sélectives de portions de l 'hippocampe et du subiculum dans des tâches de mémorisation de positions dans l'espace, utilisant différents types de stimuli. L'hypothèse principale est que le subiculum et l'hippocampe jouent des rôles différents dans la mémoire. Chacune des expériences compare la performance de groupes de rats avec lésions à celle d'un groupe de rats contrôles avec lésions simulées. Le premier article (chapitre 3) vérifie l 'hypothèse selon laquelle l'hippocampe dorsal (DR), contrairement à l'hippocampe . ventral (VR), est nécessaire à l'apprentissage de tâches de mémoire spatiale et tient compte d' autres facteurs que la lésion hippocampique elle-même, comme le dommage involontaire au subiculum. Les résultats indiquent que la lésion de DR provoque, contrairement à la lésion de VR, un déficit d'apprentissage dans les deux tâches utilisées. Toutefois, les analyses histologiques révèlent que la combinaison de la lésion à DR et d'un dommage involontaire au subiculum dorsal (DS) est responsable d'un des déficits obtenus. Le deuxième article (chapitre 4) examine les rôles respectifs du DS et de DR dans la mémorisation de stimuli visuels et idiothétiques (informations relatives aux mouvements du corps, par exemple l' information du système vestibulaire). Les résultats démontrent que les deux structures contribuent de façon importante à la mémorisation des indices idiothétiques et que la lésion combinée des deux structures affecte probablement le sens de la direction. Par ailleurs, la contribution de DR et du DS à la mémorisation d'indices visuels semble moins cruciale dans les tâches utilisées. Le troisième article (chapitre 5) évalue les contributions spécifiques du DS et de DR dans la mémorisation de stimuli visuels. Il étudie l ' effet de lésions hippocampiques et subiculaires identiques à celles du deuxième article, dans une nouvelle tâche de mémoire en présence de stimuli uniques et en présence de stimuli avec recoupements. Les résultats indiquent que des dommages à DH affectent l'apprentissage peu importe le type de stimuli visuels utilisés, alors que la lésion du DS affecte seulement l' apprentissage lorsque les stimuli possèdent des recoupements entre eux. Dans l'ensemble, la thèse montre que le subiculum et 1 'hippocampe jouent un rôle dans la mémorisation des stimuli idiothétiques et dans la mémorisation des stimuli visuels. Aucune différence n'a été identifiée dans leurs contributions respectives lorsque les stimuli à mémoriser sont idiothétiques ou lorsque les tâches peuvent être résolues à l'aide d' indices idiothétiques. En revanche, les résultats de la thèse appuient l'idée que l'hippocampe et le subiculum ont des fonctions distinctes dans la mémoire visuelle.

Les neurones dopaminergiques (DA) de l’aire tegmentale ventrale (VTA) sont influencés par différents stimuli comme des récompenses naturelles et d’autres stimuli moins physiologiques tels que les drogues d’abus. Ces drogues agissent en détournant les mécanismes d’apprentissage qui sous-tendent normalement la motivation pour des renforçateurs naturels. Les neurones DA, en conditions physiologiques, sont subtilement régulés par une balance entre tonus GABA et glutamatergique. Ils sont soumis à de multiples sources inhibitrices dont le noyau accumbens, les interneurones locaux ou les neurones GABA de la queue de la VTA (tVTA). Le glutamate est également important dans leur modulation. Il contrôle leur activité en bursts, qui est le mode de décharge le plus efficace pour libérer de la dopamine et coder des informations associées à la récompense. Il permet des adaptations synaptiques à long terme qui se sont révélées importantes dans la prise de drogue. La connaissance des facteurs endogènes qui contrôlent l’excitabilité des cellules DA de la VTA est essentielle à la compréhension des processus physiologiques (recherche de plaisir…) mais aussi pathologiques (addiction…). L’objectif de mon travail a été de comprendre les circuits de régulation des neurones DA en conditions physiologiques et lors de l’exposition à la morphine. Dans un premier temps, nous avons étudié les mécanismes de régulation des neurones DA par la formation hippocampique ventrale incluant le subiculum ventral et l’aire CA1 ventrale (vSUB/CA1). Grâce à l’utilisation d’approches d’électrophysiologie in vivo chez le rat anesthésié, nous avons montré que le vSUB/CA1 exerce un contrôle excitateur glutamatergique des neurones DA. Nous avons mis en évidence que cette voie vSUB/CA1-VTA est polysynaptique, faisant intervenir le BNST comme relais. J’ai aussi pu confirmer le rôle fonctionnel de la tVTA en tant que nouvelle structure GABA modulant l’activité des neurones DA, renforçant ainsi l’idée d’une balance entre tonus GABA et glutamatergique régulant les neurones DA in vivo.La deuxième partie de ma thèse a consisté en l’étude des circuits neuronaux à l’origine des effets excitateurs de la morphine sur les neurones DA de la VTA in vivo. L’hypothèse actuelle est que la morphine excite les neurones DA par un mécanisme de désinhibition en inhibant les neurones GABA de la VTA. Grâce à l’utilisation d’approches multiples, nous avons proposé un nouveau circuit expliquant les effets de la morphine. Ces effets sont la conséquence d’une modification de la balance GABA/glutamate par la morphine. Elle se traduit par une diminution du tonus GABA et d’une augmentation du tonus glutamatergique. Enfin, nous avons pu démontrer qu’une seule exposition à la cocaïne augmente l’activité de base des neurones DA. Chez ces animaux, les effets excitateurs de la morphine sont potentialisés confirmant ainsi l’hypothèse que l’amplitude de l’activation des neurones DA par la morphine dépend de leur état d’excitabilité. / Dopaminergic (DA) neurons of the ventral tegmental area (VTA) are influenced by several stimuli such as natural rewards or drugs of abuse. Drugs shunt learning mechanisms which underlie motivation for natural reinforcers. Under physiological conditions, DA neurons are regulated by a balance between GABA and glutamatergic inputs. They receive several inhibitory inputs especially from the nucleus accumbens, VTA local interneurons and GABA neurons of the tail of the VTA (tVTA). Glutamate is also important in modulating DA neuron activity. It controls their bursting activity which is the most efficient way to release dopamine and to encode reward-associated informations. It allows long term synaptic adaptations important for addiction. Knowing how these endogenous factors control VTA DA neuron excitability is essential to understand physiological (search for pleasure…) and pathological (drug addiction…) processes.In the first part of my thesis, we studied the regulation of the VTA by the hippocampal formation including the ventral subiculum and the ventral CA1 area (vSUB/CA1). Using electrophysiological approaches in anesthetized animal, we showed that the vSUB/CA1 controls VTA DA neurons and that this input is glutamatergic. We also demonstrated that the vSUB/CA1-VTA pathway is polysynaptic implicating the BNST as a relay. I also confirmed the inhibitory control of the VTA by tVTA, new GABA input to DA neurons. Thus, in vivo, DA neurons are regulated by a balance between GABA and glutamatergic inputs. The second part of my research consisted in studying the neuronal circuits underlying excitatory effects of morphine on VTA DA neurons in vivo. The actual hypothesis is that morphine excites DA neurons by a disinhibition mechanism inhibiting VTA GABA neurons. Using several approaches (electrophysiological approaches in anesthetized animal, tract-tracing methods), we proposed a new circuitry explaining morphine effects. These excitatory effects result from a modification of the balance between GABA and glutamatergic inputs with a decrease of the GABA tone and an increase of the glutamatergic tone. Finally, we demonstrated that an acute cocaine exposure increases DA neuron activity. In animals exposed to cocaine, morphine excitatory effects are potentiated. This last experiment confirms the hypothesis that the amplitude of morphine-induced activation of VTA DA neurons depends on their excitability state.