High frequency stimulation of the perforant pathway triggers a prolonged field potential in the dentate gyrus that far outlasts that obtained with single pulses. The late rising component of this field potential has recently been shown to be mediated by N-methyl-D-aspartate (NMDA). In the present thesis, rats were implanted with stimulating electrodes in the perforant pathway and recording electrodes in the dentate gyrus of the hippocampus. Baseline input/output functions of field potentials (or population EPSPs) were established for each rat. Ketamine, an NMDA receptor antagonist, was then administered to confirm its effects on the late component of the EPSP. The late component was measured by subtracting the pulse-evoked from the train-evoked response. Ketamine was shown to significantly attenuate the late component. Diazepam, a GABA agonist, had no significant effect on the late component. Having established an NMDA component in the field potential allows for the monitoring of the levels of NMDA activation over prolonged periods. Hence, the effect of kindling, an animal model of temporal lobe epilepsy, was also determined. Fully kindled rats--defined as those who had experienced four stage 5 seizures--also had significantly attenuated late components. In contrast to decreased late components, kindled rats displayed increased population spike amplitudes and EPSP slopes. Such a decrease in the late component suggests that the NMDA receptor plays a role in kindling. Subjects were also given ketamine and diazepam following kindling, whereby the effects were proportionately the same as those observed prior to kindling. / Thesis / Master of Science (MS)

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.

Sharp-wave ripples (SWR) generated in the CA3 subregion of the hippocampus (HC) during rest and sleep appear to coordinate memory consolidation to the neocortex (NC) by (1) reactivating small subsets of neurons (i.e. cell-assemblies) that encode recent waking experience and (2) propagating this information through the hippocampal formation. Although CA3 self-organizes SWRs in the absence of extrinsic inputs, cortical input to the HC conveyed by perforant path (PP) may influence SWR initiation nevertheless. Still, direct evidence that PP synaptic excitation can elicit SWRs is lacking, and it is unclear how this influence might compete or interact with self-organizing mechanisms. This dissertation tested the hypothesis that CA3's SWR pattern generator would self-organize its activity in the absence of PP input, but readily entrain to such input when present. Spontaneous SWRs (sSWR) occurred in slices prepared from the ventral portion of the mouse HC. Low-intensity electrical stimulation of PP afferents evoked short-latency field EPSPs in CA3 that were often followed by precisely timed evoked SWRs (eSWR). The network and single-cell characteristics of sSWRs and eSWRs were indistinguishable, indicative of a common patter generator. PP stimuli that followed sSWRs too closely usually failed to elicit eSWRs. Using a custom MATLAB/Simulink application to control PP stimulus timing during the ~250 ms sSWR refractory period revealed a statistically significant effect of stimulus delay (25, 50, 100, and 200 ms) on eSWR incidence, reaching a value of 0.72 (95% CI = [0.61, 0.81]) 200 ms after sSWR onset. In contrast, sSWR incidence at this time was much lower (95% CI = [0.015, 0.049]). Lesions targeting the direct PP input to CA3 substantially reduced eSWR incidence. In intact slices, eSWRs were readily evoked by stimulating the medial entorhinal cortex (MEC). In summary, PP input to CA3 from the MEC can initiate SWRs at times when self-organizing mechanisms generally cannot. Assuming sSWRs convey information to the NC, the ensuing refractory period might provide an opportunity for cortical feedback to reinforce the recently engaged cell-assembly. In the absence of such feedback, CA3 could revert to its default mode of self-organized replay.

Layer II neurons of the entorhinal cortex (ECII) are selectively vulnerable to Alzheimer’s disease (AD). Investigations into the molecular mechanisms of this ECII vulnerability provide unique opportunities to better understanding the pathology of AD. Preliminary data has suggested teneurin-3 (Tenm3) to have a role in this vulnerability due to its ECII enrichment, genetic variants associated with AD, and altered electrophysiology in Tenm3-knockout (KO) mice. In this study, the impacts of Tenm3- KO in mice were further explored. Electron tomography and immunofluorescent confocal microscopy were utilized to compare wild-type (WT) and KO mice’s perforant pathway synaptic densities and structures. A slight trend was found for increased synaptic density in Tenm3-KO mice. The structural changes in Tenm3-KO mice were more pronounced and encompassed alterations to active zones, bouton volumes, and synaptic vesicle pools. Overall, this work suggests Tenm3’s involvement in structural remodeling of both axonal boutons and dendritic spines thus providing a hypothesis for its role in ECII’s selective vulnerability to AD.

La maladie d'Azheimer (MA) est une pathologie neurodégénérative qui est liée dans ses stades précoces à un dysfonctionnement synaptique et une perte de synapses. De nombreuses données cliniques obtenues chez des patients mais également des données expérimentales obtenues sur des modèles murins de la MA montrent qu'il existe un dimorphisme sexuel s'exprimant par un dépôt de plaques amyloïdes supérieur et une apparition précoce de troubles mnésiques chez les souris femelles par rapport aux souris mâles. Dans ce travail, nous avons étudié les altérations moléculaires et cellulaires de la MA ainsi que les déficits cognitifs associés chez la souris femelle APP/PS1, un modèle murin double transgénique de la MA. En parallèle nous avons étudié les altérations de la transmission et de la plasticité synaptique dans le stratum moleculare, une couche proche du gyrus dentelé (DG) en raison de la forte densité de plaques amyloïdes dans cette région de l'hippocampe.Nous avons mis en évidence la présence de nombreuses plaques amyloïdes dans le DG en quantité supérieure chez les femelles âgées de 6 mois par rapport aux mâles du même âge ainsi qu'une forte activation des cellules gliales astrocytes et microglie. Ces altérations moléculaires et cellulaires s'accompagnent de déficits mnésiques hippocampo-dépendants (test du comportement de peur conditionné et test de la nouvelle localisation spatiale d'un objet) dès l'âge de 4 mois chez les femelles alors que les mâles ne présentent aucun déficit jusqu'à l'âge de 12 mois.Nous avons alors étudié les propriétés électriques des neurones du gyrus dentelé (DG), la transmission et la plasticité de la synapse voie perforante - neurones du gyrus dentelé (synapse PP-DG) chez la souris femelle âgée de 6 mois en comparant les deux génotypes APP/PS1 vs sauvage.Les neurones du DG présentent deux populations distinctes en terme de résistance d'entrée et de patron de décharge de potentiels d'action (PAs). A l'inverse, le potentiel membranaire de repos, la résistance d'entrée, le seuil d'activation et l'amplitude du potentiel d'action ne sont pas modifiés chez la souris APP/PS1 vs la souris sauvage. La fréquence de décharge des potentiels d'action est augmentée chez la souris APP/PS1 sans que la probabilité de décharge en fonction de la pente du pied du potentiel d'action (courbe E-S) soit différente entre la souris APP/PS1 et la souris sauvage. La transmission basale à la synapse PP-DG est modifiée chez la souris APP/PS1 vs la souris sauvage sans altérations du ratio AMPA/NMDA ni de l'index de rectification AMPA. La fréquence des courants miniatures NMDA est augmentée dans les neurones DG de la souris APP/PS1 vs la souris sauvage ce qui suggère le démasquage de synapses silencieuses qui n'expriment peu ou pas de récepteurs AMPA. La potentialisation à long terme (PLT) de l'amplitude des potentiels d'action synchrone est diminuée d'environ 50% chez la souris APP/PS1. La diminution de la PLT observée chez la souris APP/PS1 est en partie liée à des altérations des propriétés intrinsèques des neurones du DG comme le montre le déplacement des courbes E-S induit par la PLT qui traduit une augmentation d'excitabilité de la souris APP/PS1.En conclusion nos résultats montrent un dimorphisme sexuel important avec un dépôt des plaques amyloïdes et une activation neuroinflammatoire des cellules gliales plus précoce chez la souris femelle vs mâle. En parallèle, des déficits importants de la mémoire hippocampale-dépendante sont observés ainsi que des altérations de la transmission et de la plasticité synaptique à la synapse voie perforante - neurones du gyrus dentelé, une synapse clé de l'intégration des informations mnésiques en provenance du cortex enthorhinal. / Azheimer's disease (AD) is a neurodegenerative disease that is linked in its early stage to synaptic dysfunction and loss of synapses. Numerous clinical data obtained from patients but also experimental data obtained on mouse models of AD show that there is a sexual dimorphism evidenced by a higher amyloid plaque deposition and an early onset of memory disorders in female mice compared to male mice.In this work, we investigated the molecular and cellular alterations of AD as well as the associated cognitive deficits in female APP/PS1 mice, a double transgenic murine model of AD. In parallel we studied the alterations of hippocampal synaptic transmission and plasticity in the stratum moleculare, a layer in the vicinity of the dentate gyrus (DG) which specifically displayed a high density of amyloid plaques. We showed the presence of numerous amyloid plaques in the DG in a larger amount in 6 month old females compared to age-matched males as well as a strong activation of astrocyte and microglia glial cells. These molecular and cellular alterations are accompanied by hippocampo-dependent memory deficits (contextual fear conditioning and novel object place recognition task) from the age of 4 months in females whereas males have no deficit until the age of 12 months. We then studied the electrical properties of DG neurons, the transmission and the plasticity of the perforant pathway - DG neurons (PP-DG synapse) in the 6-month old female mouse by comparing the two genotypes APP/PS1 vs wild type (WT).In both genotypes, DG neurons displayed two distinct populations in terms of input resistance and action potential discharge pattern (APs). In contrast, the resting membrane potential, the input resistance, the activation threshold and the amplitude PAs were not modified in APP/PS1 vs WT. The frequency of discharge of APs was increased in APP/PS1 without shift of E-S curve which relates EPSP-slopes to the associated AP firing probability.Basal transmission at the PP-DG synapse was altered in the APP/PS1 mouse vs WT without alterations in the AMPA/NMDA ratio or the AMPA rectification index. The frequency of the NMDA miniature currents was increased in APP/PS1 DG neurons vs WT which suggests the unmasking of silent synapses that express almost no AMPA receptors. The long term potentiation (LTP) of population spike amplitude was decreased by approximately 50% in APP/PS1 mice. The decrease in LTP observed in APP/PS1 was partly related to alterations in the intrinsic properties of DG neurons as evidenced by LTP-induced shifts of E-S curves, which reflects an increased excitability for APP/PS1 mice.In conclusion our results show a prominent sexual dimorphism with much earlier amyloid plaque deposition, neuroinflammatory glial activation in female vs male APP/PS1. In parallel, significant deficits in hippocampal-dependent memory are observed as well as alterations of synaptic transmission and plasticity at the PP-DG synapse, a key synapse of the integration of mnesic informations originated from the entorhinal cortex