L-glutamate is the predominant excitatory amino acid neurotransmitter inthe mammalian central nervous system. Prior aging studies have focusedprimarily on dopaminergic circuitry of the striatum, and data obtained studyingglutamate regulation in the striatum have been largely equivocal. Thesediscrepancies are due in large part to the limitations of microdialysis; while it isextremely sensitive to minute concentrations of analyte, it is lacking in terms ofthe temporal resolution necessary to study a neurotransmitter with rapid releaseand clearance kinetics such as glutamate. In order to address this matter, ourlaboratory has designed a ceramic-based multisite microelectrode with thecapability to detect and analyze fluctuations in extracellular glutamateconcentrations on a sub-second basis. These microelectrodes were utilized tostudy the phasic release and uptake dynamics of potassium-evoked glutamate inthe striatum of young (6 month), late-middle aged (18 month) and aged (24month) Fischer 344 rats. Our results showed a reduced glutamate clearancerate and an attenuated response to potassium depolarization in the corticostriatalprojections of aged animals in comparison to other age groups. In addition,average maximal glutamate release amplitudes were decreased in the striatumof aged animals. Pressure ejection of exogenous glutamate solution furtherconfirmed the decreased glutamate clearance ability of the aged striatum. Thesepotassium and exogenous glutamate data also highlighted a markeddorsoventral gradient in the striatum in terms of glutamate release and clearanceability. We further explored this phenomenon of age-related decreased glutamateuptake by coupling our in vivo technology with classical immunoblotting andbiotinylation techniques in order to investigate glutamate transporter regulation.Decreased glutamate clearance in the aged rats cannot be attributed to areduction in steady-state total transporter protein levels. Rather, our resultsindicate that reduced plasma membrane surface trafficking of GLAST in the agedstriatum may be partially responsible for this effect. Finally, we modified ourmicroelectrodes to study basal glutamate levels in the striatum of the aging,freely moving rat. This approach allowed us to study extracellular glutamateregulation free from the potential confounding variable of anesthesia. Our resultsdemonstrate that there is no significant alteration in basal glutamate levels inaging in the brain regions investigated. More importantly, this study validated theefficacy of the utilization of ceramic-based multisite microelectrodes for the studyof alterations in glutamate neurotransmission in the aging, freely moving rat, andit lays the foundation for future work correlating such changes with age associatedimpairments in motor function.

No description available.

Ce manuscrit résume les travaux entrepris par l'auteur dans le domaine de l'exploration du ciel gamma.

Le Gamma Large Array Space Telescope (GLAST), d es septembre 2007, observera les rayons entre 100 MeV et 300 GeV. Leur direction est mesurée par un trajectographe en couches à pistes de Si, leur énergie par un calorimètre rendu hodoscopique gr^ ace a la disposition de ses barreaux de CsI(Tl). Pesant près de deux tonnes, une structure en fibres de carbone rigidifiée par de l'époxy les soutient pour les protéger durant l'envol. Dans un barreau, un signal crée par un dépôt d'énergie est attenue en fonction de la distance parcourue. Un système de lecture compose d'une double diode a chaque extrémité permet alors de mesurer l'énergie déposée et sa position moyenne. Sur Terre, la calibration s'effectue grâce aux muons cosmiques. Un rayon interagit en créant une gerbe électromagnétique. Les contraintes spatiales sur la conception du calorimètre font que seule une fraction de l'énergie y aboutit, ce en fonction de l'énergie, l'orientation et la proximité aux parois de la gerbe. L'énergie (E) est reconstruite en optimisant, en fonction de ces paramètres, un estimateur base sur l'énergie mesurée dans le calorimètre (Q) et des observables réduisant la variance de Q car anti corrélées a celle-ci a Exe. L'algorithme de reconstruction procède à un maximum de vraisemblance sur cet estimateur. Les rayons extragalactiques peuvent initier des cascades électron-photon sur le fond de photons. Les spectres observes dépendent de l'atténuation des rayons, permettant en retour une mesure du fond infrarouge, et de l'émission de leurs cascades. Une signature spectrale du champ magnétique extragalactique est alors perceptible.

Our previous results showed that oligodendrocyte development is regulated by both nociceptin and its G-protein coupled receptor, the nociceptin/orphanin FQ receptor (NOPR). The present in vitro and in vivo findings show that nociceptin plays a crucial conserved role in both human and rodent brain astrocytes, regulating the levels of the glutamate/aspartate transporter GLAST/EAAT1. This nociceptin-mediated response takes place during a critical developmental window that coincides with astrocyte maturation and synapse formation. GLAST/EAAT1 upregulation by nociceptin is mediated by NOPR and the downstream participation of a complex signaling cascade that involves the interaction of several kinase systems, including PI-3K/AKT, mTOR and JAK. Because GLAST is the main glutamate transporter during brain maturation, these novel findings suggest that nociceptin plays a crucial role in regulating the function of early astrocytes and their capacity to support glutamate homeostasis in the developing brain.

No description available.

Doctor of Philosophy (Medicine) / Glutamate transporters play a role in removing extracellular excitatory neurotransmitter, L-glutamate into the cells. The rate of the uptake depends on the density of the transporters at the membrane. Some studies claimed that glutamate transporters could transit between the cytoplasm and the membrane on a time-scale of minutes. The present study examined the distribution of glutamate transporter GLAST predominantly expressed in rat cortical cultured astrocytes between the membrane and the cytoplasm by using deconvolution microscopy and then analyzing the images. The regulation of the distribution of GLAST was studied in the presence of glutamate transporter substrate (D-aspartate), purinergic receptor activators (α,β-methylene ATP, adenosine), neuroleptic drugs (clozapine, haloperidol), ammonia (hyperammonia) and Na+/K+-ATPase inhibitors (ouabain, digoxin and FCCP). It was demonstrated that the translocation of GLAST towards the plasma membrane was induced by D-aspartate, α,β-methylene ATP, adenosine, clozapine and ammonia (at 100 μM and very high concentrations of 10 mM). However, the inhibition of Na+/K+-ATPase activity had an opposite effect, resulting in redistribution of GLAST away from the membrane. It has previously been claimed that the membrane-cytoplasm trafficking of GLAST was regulated by phosphorylation catalysed by protein kinase C delta (PKC-delta). Involvement of this mechanism has, however, been put to doubt when rottlerin, a PKC-delta inhibitor, used to test the hypothesis showed to inhibit Na+/K+-ATPase-mediated uptake of Rb+, suggesting that rottlerin influenced the activity of Na+/K+-ATPase. As Na+/K+-ATPase converts ATP to energy and pumps Na+, K+ ions, thus helping to maintain normal electrochemical and ionic gradients across the cell membrane. Its inhibition also reduced D-aspartate transport and could impact on the cytoplasm-to-membrane traffic of GLAST molecules. Furthermore, rottlerin decreased the activity of Na+/K+-ATPase by acting as a mitochondrial inhibitor. The present study has focused on the inhibition of Na+/K+-ATPase activity by rottlerin, ouabain and digoxin in homogenates prepared from rat kidney and cultured astrocytes. The activity of Na+/K+-ATPase was measured by the absorption of inorganic phosphate product generated from the hydrolysis of ATP and the fluorescent transition of the dye RH421 induced by the movement of Na+/K+-ATPase. This approach has a potential to test whether the rottlerin effect on Na+/K+-ATPase is a direct inhibition of the enzyme activity. Rottlerin has been found to block the activity of Na+/K+-ATPase in a dose-dependent manner in both rat kidney and astrocyte homogenates. Therefore, rottlerin inhibited the activity of Na+/K+-ATPase directly in a cell-free preparation, thus strongly indicating that the effect was direct on the enzyme. In parallel experiments, ouabain and digoxin produced similar inhibitions of Na+/K+-ATPase activity in rat kidney while digoxin blocked the activity of Na+/K+-ATPase to a greater extent than ouabain in rat cortical cultured astrocytes. In a separate set of experiments, Na+/K+-ATPase in the astrocytic membrane was found to be unsaturated in E1(Na+)3 conformation in the presence of Na+ ions and this could explain the differences between the effects of digoxin and ouabain on the activity of Na+/K+-ATPase in rat astrocytes. In addition, it was found that at low concentrations of rottlerin, the activity of Na+/K+-ATPase was increased rather than inhibited. This effect was further investigated by studying rottlerin interactions with membrane lipids. The activity of Na+/K+-ATPase has been reported to be regulated by membrane lipids. The enzyme activity can be enhanced by increasing fluidity of the lipid membrane. I have, therefore, proposed that rottlerin binds to the membrane lipids and the effects of rottlerin on Na+/K+-ATPase are mediated by changes in the properties (fluidity) of the membrane. The hypothesis was tested by comparing rottlerin and a detergent, DOC (sodium deoxycholate), for their binding to the lipids by using a DMPC (1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine) monolayer technique. DOC has been shown to both increase and inhibit activity of Na+/K+-ATPase in a manner similar to that displayed by rottlerin. The effects of rottlerin and DOC on the DMPC monolayers were studied by measuring the surface pressure of DMPC monolayers and surface area per DMPC molecule. I established that both rottlerin and DOC decreased the surface pressure of DMPC monolayers and increased the surface area per DMPC molecule. This indicates that both rottlerin and DOC penetrated into the DMPC monolayers. If rottlerin can interact with the lipids, changes in fluidity of the lipid membrane cannot be ruled out and should be considered as a possible factor contributing to the effects of rottlerin on the activity of Na+/K+-ATPase. Overall, the study demonstrates that rottlerin is not only a PKC-delta inhibitor but can have additional effects, both on the enzyme activities (Na+/K+-ATPase) and/or on lipid-containing biological structures such as membranes. The findings have implication not only for studies where rottlerin was used as a supposedly specific PKC-delta inhibitor but also for mechanisms of its toxicity.

Doctor of Philosophy (Medicine) / Glutamate transporters play a role in removing extracellular excitatory neurotransmitter, L-glutamate into the cells. The rate of the uptake depends on the density of the transporters at the membrane. Some studies claimed that glutamate transporters could transit between the cytoplasm and the membrane on a time-scale of minutes. The present study examined the distribution of glutamate transporter GLAST predominantly expressed in rat cortical cultured astrocytes between the membrane and the cytoplasm by using deconvolution microscopy and then analyzing the images. The regulation of the distribution of GLAST was studied in the presence of glutamate transporter substrate (D-aspartate), purinergic receptor activators (α,β-methylene ATP, adenosine), neuroleptic drugs (clozapine, haloperidol), ammonia (hyperammonia) and Na+/K+-ATPase inhibitors (ouabain, digoxin and FCCP). It was demonstrated that the translocation of GLAST towards the plasma membrane was induced by D-aspartate, α,β-methylene ATP, adenosine, clozapine and ammonia (at 100 μM and very high concentrations of 10 mM). However, the inhibition of Na+/K+-ATPase activity had an opposite effect, resulting in redistribution of GLAST away from the membrane. It has previously been claimed that the membrane-cytoplasm trafficking of GLAST was regulated by phosphorylation catalysed by protein kinase C delta (PKC-delta). Involvement of this mechanism has, however, been put to doubt when rottlerin, a PKC-delta inhibitor, used to test the hypothesis showed to inhibit Na+/K+-ATPase-mediated uptake of Rb+, suggesting that rottlerin influenced the activity of Na+/K+-ATPase. As Na+/K+-ATPase converts ATP to energy and pumps Na+, K+ ions, thus helping to maintain normal electrochemical and ionic gradients across the cell membrane. Its inhibition also reduced D-aspartate transport and could impact on the cytoplasm-to-membrane traffic of GLAST molecules. Furthermore, rottlerin decreased the activity of Na+/K+-ATPase by acting as a mitochondrial inhibitor. The present study has focused on the inhibition of Na+/K+-ATPase activity by rottlerin, ouabain and digoxin in homogenates prepared from rat kidney and cultured astrocytes. The activity of Na+/K+-ATPase was measured by the absorption of inorganic phosphate product generated from the hydrolysis of ATP and the fluorescent transition of the dye RH421 induced by the movement of Na+/K+-ATPase. This approach has a potential to test whether the rottlerin effect on Na+/K+-ATPase is a direct inhibition of the enzyme activity. Rottlerin has been found to block the activity of Na+/K+-ATPase in a dose-dependent manner in both rat kidney and astrocyte homogenates. Therefore, rottlerin inhibited the activity of Na+/K+-ATPase directly in a cell-free preparation, thus strongly indicating that the effect was direct on the enzyme. In parallel experiments, ouabain and digoxin produced similar inhibitions of Na+/K+-ATPase activity in rat kidney while digoxin blocked the activity of Na+/K+-ATPase to a greater extent than ouabain in rat cortical cultured astrocytes. In a separate set of experiments, Na+/K+-ATPase in the astrocytic membrane was found to be unsaturated in E1(Na+)3 conformation in the presence of Na+ ions and this could explain the differences between the effects of digoxin and ouabain on the activity of Na+/K+-ATPase in rat astrocytes. In addition, it was found that at low concentrations of rottlerin, the activity of Na+/K+-ATPase was increased rather than inhibited. This effect was further investigated by studying rottlerin interactions with membrane lipids. The activity of Na+/K+-ATPase has been reported to be regulated by membrane lipids. The enzyme activity can be enhanced by increasing fluidity of the lipid membrane. I have, therefore, proposed that rottlerin binds to the membrane lipids and the effects of rottlerin on Na+/K+-ATPase are mediated by changes in the properties (fluidity) of the membrane. The hypothesis was tested by comparing rottlerin and a detergent, DOC (sodium deoxycholate), for their binding to the lipids by using a DMPC (1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine) monolayer technique. DOC has been shown to both increase and inhibit activity of Na+/K+-ATPase in a manner similar to that displayed by rottlerin. The effects of rottlerin and DOC on the DMPC monolayers were studied by measuring the surface pressure of DMPC monolayers and surface area per DMPC molecule. I established that both rottlerin and DOC decreased the surface pressure of DMPC monolayers and increased the surface area per DMPC molecule. This indicates that both rottlerin and DOC penetrated into the DMPC monolayers. If rottlerin can interact with the lipids, changes in fluidity of the lipid membrane cannot be ruled out and should be considered as a possible factor contributing to the effects of rottlerin on the activity of Na+/K+-ATPase. Overall, the study demonstrates that rottlerin is not only a PKC-delta inhibitor but can have additional effects, both on the enzyme activities (Na+/K+-ATPase) and/or on lipid-containing biological structures such as membranes. The findings have implication not only for studies where rottlerin was used as a supposedly specific PKC-delta inhibitor but also for mechanisms of its toxicity.

No description available.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.