The activation of N-methyl-D-aspartate glutamate receptors (NMDARs) is required for the long term potentiation (LTP) and long term depression (LTD) of excitatory synaptic transmission at hippocampal CA1 synapses, and plays an important role in learning and memory. In addition, it is accepted that the over-activation of NMDARs leads to the neurotoxicity associated with stroke and other neurodegenerative disorders. Thus, the NMDAR provides a logical starting point to investigate a possible relationship between synaptic plasticity and the cell-signalling pathways which ultimately determine neuronal fate. Research in our lab has indicated that NR2A-containing NMDARs are essential for LTP induction whereas NR2B-containing NMDARs are crucial for the production of LTD in vitro, and the results of this study support these findings in the anaesthetized rat. Furthermore, using the kainic acid (KA) model of neurotoxicity, this research has explored the opposing roles that activity-dependent synaptic plasticity, through different NMDAR subtypes, can play in determining neuronal outcome in an excitotoxic environment. In these experiments, it is shown that (1) the induction of LTP using high-frequency stimulation (HFS) promotes the phosphorylation of Akt, which plays a critical role in controlling cell survival and apoptosis, (2) the induction of LTP using HFS attenuates kainic acid (KA) induced neurodegeneration while the induction of LTD using low-frequency stimulation (LFS) has no incremental effect on the degree of cell death resulting from exposure to KA, (3) the blockade of NR2B-containing NMDARs using Ro25-6981 attenuates KA-induced neurodegeneration while the blockade of NR2A-containing NMDARs using NVP-AAM077 does not influence KA-induced neurotoxicity, (4) pre-treatment with NR2A antagonists blocks both the induction of LTP and its neuroprotective effect against KA while NR2B antagonists neither block the induction of LTP nor the neuroprotection that this can provide against KA, (5) the administration of NR2A antagonists after the induction of LTP has no effect on the expression of LTP or its neuroprotective effect against KA, and (6) pre-treatment with a high dose (2.4mg/kg) of NVP-AAM077 leads to the induction of LTD rather than LTP as a result of HFS. Altogether this research supports the hypothesis that the production of LTP via the activation of NR2A-containing NMDARs protects neurons against excitotoxic neuronal death by promoting cell survival signalling. Furthermore, because NR2A antagonists applied after the production of LTP do not block neuroprotection, it can be concluded that LTP itself, and not NR2A activation, is responsible for this neuroprotective effect.

Synaptic plasticity plays a key role in processes of learning and memory. Long-term potentiation (LTP) is a relatively stable enhancement of synaptic transmission following specific patterns of electrical stimulation. Some types of learning (e.g. motor learning, fear conditioning) result in LTP-like changes at synapses. However, no studies have examined LTP-like plasticity in the visual cortex as a result of visual discrimination learning. A visual discrimination task was used to examine changes in LTP in the primary visual cortex (V1) of adult rats. Rats were placed in a Y-shaped water maze and required to swim to one choice arm containing a hidden platform. Distinct visual cues indicated the presence (CS+) and absence (CS-) of the platform. Rats learned to reliably discriminate the visual cues to successfully navigate the maze. Control rats received the same procedure, but the visual cues did not have a predictive relation with the platform. Following training, trained, control, and task-naïve rats were anesthetized and visual evoked potentials (VEPs) in V1 were recorded in response to CS+, CS-, and novel stimuli. Results indicate that, in both task-naïve and control animals, all visual stimuli elicit VEPs of similar (p > 0.05) amplitude. In contrast, trained animals show significantly larger amplitude VEPs to stimuli encountered during training relative to novel stimuli, regardless of whether stimuli act as CS+ or CS-. In addition, trained animals show 71% and controls 47% potentiation (p < .05) that was induced by electrical (theta-burst) stimulation of the lateral geniculate nucleus (LGN) indicating greater plasticity of thalamocortical synapses following training. There were no differences between controls and task-naïve animals. This facilitation of LTP was shown to allow past visual experience to influence the efficiency of encoding novel visual features. These experiments demonstrate that visual discrimination learning might involve stimulus-selective facilitation of neuronal responses at early stages of visual processing (LGN, V1). The effect requires that stimuli carry some significance to the animal, while exposure to stimuli with no significance does not result in the same level of neuronal enhancement. Further, visual experience alters the plasticity properties of V1 (metaplasticity) by facilitating LTP along thalamocortical sensory fibers. / Thesis (Master, Psychology) -- Queen's University, 2008-09-25 12:28:27.468

The activation of N-methyl-D-aspartate glutamate receptors (NMDARs) is required for the long term potentiation (LTP) and long term depression (LTD) of excitatory synaptic transmission at hippocampal CA1 synapses, and plays an important role in learning and memory. In addition, it is accepted that the over-activation of NMDARs leads to the neurotoxicity associated with stroke and other neurodegenerative disorders. Thus, the NMDAR provides a logical starting point to investigate a possible relationship between synaptic plasticity and the cell-signalling pathways which ultimately determine neuronal fate. Research in our lab has indicated that NR2A-containing NMDARs are essential for LTP induction whereas NR2B-containing NMDARs are crucial for the production of LTD in vitro, and the results of this study support these findings in the anaesthetized rat. Furthermore, using the kainic acid (KA) model of neurotoxicity, this research has explored the opposing roles that activity-dependent synaptic plasticity, through different NMDAR subtypes, can play in determining neuronal outcome in an excitotoxic environment. In these experiments, it is shown that (1) the induction of LTP using high-frequency stimulation (HFS) promotes the phosphorylation of Akt, which plays a critical role in controlling cell survival and apoptosis, (2) the induction of LTP using HFS attenuates kainic acid (KA) induced neurodegeneration while the induction of LTD using low-frequency stimulation (LFS) has no incremental effect on the degree of cell death resulting from exposure to KA, (3) the blockade of NR2B-containing NMDARs using Ro25-6981 attenuates KA-induced neurodegeneration while the blockade of NR2A-containing NMDARs using NVP-AAM077 does not influence KA-induced neurotoxicity, (4) pre-treatment with NR2A antagonists blocks both the induction of LTP and its neuroprotective effect against KA while NR2B antagonists neither block the induction of LTP nor the neuroprotection that this can provide against KA, (5) the administration of NR2A antagonists after the induction of LTP has no effect on the expression of LTP or its neuroprotective effect against KA, and (6) pre-treatment with a high dose (2.4mg/kg) of NVP-AAM077 leads to the induction of LTD rather than LTP as a result of HFS. Altogether this research supports the hypothesis that the production of LTP via the activation of NR2A-containing NMDARs protects neurons against excitotoxic neuronal death by promoting cell survival signalling. Furthermore, because NR2A antagonists applied after the production of LTP do not block neuroprotection, it can be concluded that LTP itself, and not NR2A activation, is responsible for this neuroprotective effect.

The activation of N-methyl-D-aspartate glutamate receptors (NMDARs) is required for the long term potentiation (LTP) and long term depression (LTD) of excitatory synaptic transmission at hippocampal CA1 synapses, and plays an important role in learning and memory. In addition, it is accepted that the over-activation of NMDARs leads to the neurotoxicity associated with stroke and other neurodegenerative disorders. Thus, the NMDAR provides a logical starting point to investigate a possible relationship between synaptic plasticity and the cell-signalling pathways which ultimately determine neuronal fate. Research in our lab has indicated that NR2A-containing NMDARs are essential for LTP induction whereas NR2B-containing NMDARs are crucial for the production of LTD in vitro, and the results of this study support these findings in the anaesthetized rat. Furthermore, using the kainic acid (KA) model of neurotoxicity, this research has explored the opposing roles that activity-dependent synaptic plasticity, through different NMDAR subtypes, can play in determining neuronal outcome in an excitotoxic environment. In these experiments, it is shown that (1) the induction of LTP using high-frequency stimulation (HFS) promotes the phosphorylation of Akt, which plays a critical role in controlling cell survival and apoptosis, (2) the induction of LTP using HFS attenuates kainic acid (KA) induced neurodegeneration while the induction of LTD using low-frequency stimulation (LFS) has no incremental effect on the degree of cell death resulting from exposure to KA, (3) the blockade of NR2B-containing NMDARs using Ro25-6981 attenuates KA-induced neurodegeneration while the blockade of NR2A-containing NMDARs using NVP-AAM077 does not influence KA-induced neurotoxicity, (4) pre-treatment with NR2A antagonists blocks both the induction of LTP and its neuroprotective effect against KA while NR2B antagonists neither block the induction of LTP nor the neuroprotection that this can provide against KA, (5) the administration of NR2A antagonists after the induction of LTP has no effect on the expression of LTP or its neuroprotective effect against KA, and (6) pre-treatment with a high dose (2.4mg/kg) of NVP-AAM077 leads to the induction of LTD rather than LTP as a result of HFS. Altogether this research supports the hypothesis that the production of LTP via the activation of NR2A-containing NMDARs protects neurons against excitotoxic neuronal death by promoting cell survival signalling. Furthermore, because NR2A antagonists applied after the production of LTP do not block neuroprotection, it can be concluded that LTP itself, and not NR2A activation, is responsible for this neuroprotective effect. / Medicine, Faculty of / Graduate

No description available.

Long-term potentiation (LTP) is a widely studied cellular mechanism of learning and memory. LTP occurs at excitatory synapses on dendritic spines. Two hours after LTP induction in mature rat hippocampal slices, a reduction in spine number that is perfectly balanced by enlargement of the remaining synapses was previously observed. The sequence of events by which mature synapses enlarge is not well understood, but potential pre- and postsynaptic ultrastructural correlates of synapse growth have been identified. Vesicle-free transition zones (VFTZs) are postsynaptic thickenings contiguous with the PSD that have no apposing presynaptic vesicles perpendicular to the presynaptic membrane. VFTZs could be regions where synapses have expanded postsynaptically, but to which presynaptic vesicles have not yet been recruited. Presynaptic 80-nm dense core vesicles (DCVs) transport active zone proteins to the synapse during synaptogenesis, and may perform the same function during synaptic plasticity. 3-D reconstructions from ssTEM were used to investigate changes in VFTZs, DCVs, and presynaptic vesicles following LTP induction. By 30 minutes, VFTZ area and docked vesicle counts decreased, suggesting mobilization of additional vesicles to the synapse and enhanced release or delayed recycling. By two hours, VFTZs enlarged, suggesting VFTZ assembly contributes to synapse enlargement. DCV counts at 2 hours decreased relative to that at 30 minutes in both control and LTP conditions, suggesting DCVs were inserted at existing synapses to enlarge potentiated synapses in the LTP condition and to support ongoing spinogenesis in the control condition. The overall vesicle count in presynaptic boutons decreased at 2 hours following LTP induction, but docked vesicle count did not. Docked vesicle count was elevated at 2 hours relative to 30 minutes, suggesting that the depletion of docked vesicles observed at 30 minutes was followed by a replenishment and enhancement by 2 hours supplied by the non-docked vesicle pool. That the largest spines had more and larger VFTZs and recruited more DCVs and docked vesicles, and that the ratio of the sum of VFTZ area to the sum of PSD area is constant, provide further evidence that dendritic segments serve as functional units that manage resources in a coordinated and homeostatic way. / text

Severas altera¸c~oes no metabolismo energ´etico, no consumo de glicose e na sinaliza¸c~ao de insulina cerebral est~ao presentes na doen¸ca de Alzheimer (DA). O modelo animal da DA obtido pela administra¸c~ao intracerebroventricular de estreptozotocina (STZ-icv) em ratos induz um estado de resist^encia `a insulina no c´erebro associado `a disfun¸c~oes colin´ergicas e a d´eficits cognitivos, tornando-o um dos poucos modelos experimentais da forma espor´adica da DA. Este trabalho tem como objetivo caracterizar, neste modelo, as disfun¸c~oes sin´apticas na via hipocampo - c´ortex pr´e-frontal medial (CA1-CPFm) e testar se o tratamento com nicotina ´e capaz de prevenir as disfun¸c~oes sin´apticas e reverter os preju´?zos cognitivos induzido pelo STZ-icv. Para isso, ratos Wistar receberam STZ e foram submetidos a 20 dias de tratamento com nicotina. Dois dias depois, foram realizados nos animais teste de campo aberto e de reconhecimento de objeto. Em seguida os animais foram anestesiados com uretana para que os registros eletrofisiol´ogicos fossem realizados. Um eletrodo foi utilizado para estimular CA1 com pulso pareado e potenciais de campo p´os-sin´apticos (fPSP1) e sua facilita¸c~ao (fPSP2) foram registradas por um eletrodo no CPFm. Ap´os 30 minutos de linha de base, uma estimula¸c~ao em alta frequ^encia foi aplicada para induzir a potencia¸c~ao de longa dura¸c~ao (LTP), seguido de mais quatro horas de registro. Outro grupo experimental foi realizado para avaliar o efeito de longo prazo da STZ-icv e do tratamento com nicotina. Neste grupo, testes comportamentais e eletrofisiol´ogicos foram realizados 60 dias ap´os o fim do tratamento. Independentemente do tempo, os resultados indicam que a STZ produziu uma redu¸c~ao na indu¸c~ao e na manuten¸c~ao da LTP, mas a facilita¸c~ao por pulso pareado (PPF = fPSP2 / fPSP1) mostra que a STZ prejudica a plasticidade pr´e-sin´aptica apenas a curto prazo. O tratamento com nicotina atenua a disfun¸c~ao na LTP induzida pela STZ. Al´em disso, apenas o tratamento de nicotina tamb´em ´e capaz de reduzir a plasticidade pr´e-sin´aptica no grupo controle dois dias ap´os o fim do tratamento. Estes resultados tamb´em est~ao associados com os dados comportamentais, uma vez que a nicotina reverteu os d´eficits de mem´oria de reconhecimento nos animais STZ mas manteve o comportamento explorat´orio reduzido. Sugerimos com isso que o sistema colin´ergico, que desempenha um papel importante em fun¸c~oes cognitivas e na LTP, est´a afetado nos animais injetados com STZ e o tratamento cr^onico com nicotina consegue reduzir os danos na plasticidade sin´aptica e comportamentais, induzidos pela STZ. / Severe abnormalities in brain glucose/energy metabolism and insulin signaling have been documented to play an important role in early stage of alzheimer disease (AD) pathology. Intracerebroventricular administration (icv) of streptozotocin (STZ) in rats can induce an insulin-resistant brain state associated with cholinergic dysfunctions and memory impairments, which make it a suitable experimental model of the sporadic form of AD. The present work aimed to extend the characterization of this model by probing synaptic plasticity dysfunctions in the medial prefrontal cortex (mPFC)- hippocampal (CA1) pathway and test if nicotine can prevent synaptic dysfunction and revert cognitive impairment induced by icv STZ. Here, Wistar rats received bilateral microinjection of STZ and were submitted to 20 days of nicotine treatment. After 2 days of withdrawing the subjects were submitted to open field and object recognition tests. After that, animals were anesthetized with urethane for electrophysiological tests. A twisted bipolar electrode was used to stimulate posterior-dorsal hippocampus (CA1/subiculum) with paired-pulse. Basal field post-synaptic potentials (fPSP1) and facilitated responses (fPSP2) were recorded by a monopolar electrode in the medial mPFC. After 30min of baseline, high frequency stimulation was applied to induce long-term potentiation (LTP) and additional four hours of electrophysiological recordings was performed. Another experimental group was performed to evaluate the long term effect of both icv STZ and nicotine treatment. In this group behavioral and electrophysiological tests were performed with 60 days after chronic treatment. Independently of time, our results indicate that STZ produced a significant decrease in the induction and maintenance of LTP, but paired pulse facilitation (PPF = fPSP2/fPSP1) shows that only the short-term pre-synaptic plasticity was impaired after STZ injection. The nicotine treatment attenuates the STZ-induced LTP dysfunction in the CA1-mPFC pathway. However, just the nicotine treatment (in control group) can reduce pre-synaptic plasticity two days after chronic treatment. These results are also associated with behavioral data, since nicotine treatment reversed the deficits in recognition memory of STZ animals but maintained the reduced exploratory behavior. We suggest that the brain cholinergic system, which plays a role in cognition function and LTP, is affected in STZ injected animals and chronic treatment with nicotine can attenuate the STZ-induced synaptic plasticity and behavioral dysfunctions.

Severas altera¸c~oes no metabolismo energ´etico, no consumo de glicose e na sinaliza¸c~ao de insulina cerebral est~ao presentes na doen¸ca de Alzheimer (DA). O modelo animal da DA obtido pela administra¸c~ao intracerebroventricular de estreptozotocina (STZ-icv) em ratos induz um estado de resist^encia `a insulina no c´erebro associado `a disfun¸c~oes colin´ergicas e a d´eficits cognitivos, tornando-o um dos poucos modelos experimentais da forma espor´adica da DA. Este trabalho tem como objetivo caracterizar, neste modelo, as disfun¸c~oes sin´apticas na via hipocampo - c´ortex pr´e-frontal medial (CA1-CPFm) e testar se o tratamento com nicotina ´e capaz de prevenir as disfun¸c~oes sin´apticas e reverter os preju´?zos cognitivos induzido pelo STZ-icv. Para isso, ratos Wistar receberam STZ e foram submetidos a 20 dias de tratamento com nicotina. Dois dias depois, foram realizados nos animais teste de campo aberto e de reconhecimento de objeto. Em seguida os animais foram anestesiados com uretana para que os registros eletrofisiol´ogicos fossem realizados. Um eletrodo foi utilizado para estimular CA1 com pulso pareado e potenciais de campo p´os-sin´apticos (fPSP1) e sua facilita¸c~ao (fPSP2) foram registradas por um eletrodo no CPFm. Ap´os 30 minutos de linha de base, uma estimula¸c~ao em alta frequ^encia foi aplicada para induzir a potencia¸c~ao de longa dura¸c~ao (LTP), seguido de mais quatro horas de registro. Outro grupo experimental foi realizado para avaliar o efeito de longo prazo da STZ-icv e do tratamento com nicotina. Neste grupo, testes comportamentais e eletrofisiol´ogicos foram realizados 60 dias ap´os o fim do tratamento. Independentemente do tempo, os resultados indicam que a STZ produziu uma redu¸c~ao na indu¸c~ao e na manuten¸c~ao da LTP, mas a facilita¸c~ao por pulso pareado (PPF = fPSP2 / fPSP1) mostra que a STZ prejudica a plasticidade pr´e-sin´aptica apenas a curto prazo. O tratamento com nicotina atenua a disfun¸c~ao na LTP induzida pela STZ. Al´em disso, apenas o tratamento de nicotina tamb´em ´e capaz de reduzir a plasticidade pr´e-sin´aptica no grupo controle dois dias ap´os o fim do tratamento. Estes resultados tamb´em est~ao associados com os dados comportamentais, uma vez que a nicotina reverteu os d´eficits de mem´oria de reconhecimento nos animais STZ mas manteve o comportamento explorat´orio reduzido. Sugerimos com isso que o sistema colin´ergico, que desempenha um papel importante em fun¸c~oes cognitivas e na LTP, est´a afetado nos animais injetados com STZ e o tratamento cr^onico com nicotina consegue reduzir os danos na plasticidade sin´aptica e comportamentais, induzidos pela STZ. / Severe abnormalities in brain glucose/energy metabolism and insulin signaling have been documented to play an important role in early stage of alzheimer disease (AD) pathology. Intracerebroventricular administration (icv) of streptozotocin (STZ) in rats can induce an insulin-resistant brain state associated with cholinergic dysfunctions and memory impairments, which make it a suitable experimental model of the sporadic form of AD. The present work aimed to extend the characterization of this model by probing synaptic plasticity dysfunctions in the medial prefrontal cortex (mPFC)- hippocampal (CA1) pathway and test if nicotine can prevent synaptic dysfunction and revert cognitive impairment induced by icv STZ. Here, Wistar rats received bilateral microinjection of STZ and were submitted to 20 days of nicotine treatment. After 2 days of withdrawing the subjects were submitted to open field and object recognition tests. After that, animals were anesthetized with urethane for electrophysiological tests. A twisted bipolar electrode was used to stimulate posterior-dorsal hippocampus (CA1/subiculum) with paired-pulse. Basal field post-synaptic potentials (fPSP1) and facilitated responses (fPSP2) were recorded by a monopolar electrode in the medial mPFC. After 30min of baseline, high frequency stimulation was applied to induce long-term potentiation (LTP) and additional four hours of electrophysiological recordings was performed. Another experimental group was performed to evaluate the long term effect of both icv STZ and nicotine treatment. In this group behavioral and electrophysiological tests were performed with 60 days after chronic treatment. Independently of time, our results indicate that STZ produced a significant decrease in the induction and maintenance of LTP, but paired pulse facilitation (PPF = fPSP2/fPSP1) shows that only the short-term pre-synaptic plasticity was impaired after STZ injection. The nicotine treatment attenuates the STZ-induced LTP dysfunction in the CA1-mPFC pathway. However, just the nicotine treatment (in control group) can reduce pre-synaptic plasticity two days after chronic treatment. These results are also associated with behavioral data, since nicotine treatment reversed the deficits in recognition memory of STZ animals but maintained the reduced exploratory behavior. We suggest that the brain cholinergic system, which plays a role in cognition function and LTP, is affected in STZ injected animals and chronic treatment with nicotine can attenuate the STZ-induced synaptic plasticity and behavioral dysfunctions.

Les astrocytes sont des partenaires clés des neurones. Dans l’hippocampe, et tout particulièrement au niveau des synapses CA3-CA1, en libérant la D-sérine, ces cellules gliales régulent l’activité des récepteurs glutamatergiques de type N-methyl-D-aspartate (NMDA) et de ce fait la mémoire synaptique, aussi connue sous le nom de plasticité synaptique à long terme. Cependant, le signal synaptique à l’origine de la libération de la D-sérine par les astrocytes reste à ce jour méconnu. De par des données rapportées dans la littérature nous nous sommes tout particulièrement intéressés aux récepteurs astrocytaires aux ephrins de type B3 (EphB3) et aux endocannabinoïdes de type 1 (CB1). Pour ce faire nous avons principalement utilisé une approche électrophysiologique sur des tranches aiguës d’hippocampe de souris adulte. Dans une première étude, nos données indiquent que l’activation des récepteurs EphB3 augmente la présence de D-sérine synaptique et en conséquence l’activité des récepteurs NMDA synaptiques. A l’inverse, leur inhibition diminue à la fois l’activité des récepteurs NMDA synaptiques et la potentialisation à long-terme qui en dépend (LTP ; une forme de plasticité synaptique à long terme). L’interaction EphB3-ephrinB3 contrôle donc la LTP en contrôlant la disponibilité en D-sérine synaptique. Dans une seconde étude, nous avons utilisé un modèle transgénique permettant d’inhiber l’expression des récepteurs CB1 astrocytaires (souris GFAP-CB1-KO). Nous avons découvert que la suppression de ces récepteurs diminue la disponibilité en D-sérine synaptique. De plus, nos travaux montrent que les récepteurs CB1 astrocytaires sont nécessaires à l’induction de la LTP via la D-serine. En conclusion, ces travaux de Thèse révèlent que les récepteurs astrocytaires EphB3 et CB1 régulent les fonctions dépendantes des récepteurs NMDA via le contrôle qu’ils exercent sur la disponibilité en D-sérine. / Astrocytes are key partners of neurons. In the hippocampus, and more particularly at CA3-CA1 synapses, by releasing D-serine, these glial cells regulate the activity of synaptic Nmethyl-D-aspartate (NMDA) receptors and thus synaptic memory, also known as long-term synaptic plasticity. Yet, the synaptic signal inducing D-serine release by astrocytes is still unknown. Based on interesting data from the literature we have investigated the role of the astrocytic receptors for ephrinB3 (EphB3) and endocannabinoids (CB1). To this end we used electrophysiological approaches on acute hippocampal slices of adult mice. In a first study, our data indicate on one hand that the activation of EphB3 receptors increases synaptic D-serine availability and in consequences the activity of synaptic NMDA receptor activity. On the other hand, inhibition of EphB3 receptors induces a decrease of synaptic NMDA receptor activity as well as the induction of the long-term potentiation (LTP; a form of long-term plasticity). Thus, EphB3-ephrinB3 interaction controls LTP induction through the availability of synaptic D-serine. In a second study, we used a transgenic model allowing the inhibition of CB1 receptors expression in astrocytes (GFAP-CB1-KO mice). We discovered that their deletion reduced synaptic D-serine availability. Our work shows that astrocytic CB1 receptors are necessary for LTP induction via this D-serine. All together, this PhD work reveals that astrocytic EphB3 and CB1 receptors regulate synaptic NMDA receptor functions through the control of D-serine availability.

No description available.