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Glutamate/GABA Synthesis and Metabolism | Sigma …

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Glutamine and glutamate as vital metabolites - SciELO

Given this basic neuroscience framework, early studies of the Fmr1 knockout model examined a possible deficit in hippocampal synaptic plasticity. A potential breakthrough came with the discovery that a novel form of long-term synaptic depression in the hippocampus3 was exaggerated in the Fmr1 knockout mouse4. Unlike the N-methyl D-aspartate receptor dependent forms of plasticity examined previously5,6, this synaptic depression is induced by activation of group I metabotropic glutamate receptors (Gp I mGluRs), and is normally protein synthesis dependent. Meanwhile, a number of studies showed that FMRP is an mRNA binding protein, is enriched postsynaptically at glutamatergic synapses and functions as a repressor of protein synthesis.

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So, the synthesis of asparagine is intrinsically tied to that of glutamine,and it turns out that glutamine is the amino group donor in the formation ofnumerous biosynthetic products, as well as being a storage form of NH3. Therefore, one would expect that glutamine synthetase, the enzyme responsiblefor the amidation of glutamate, plays a central role in the regulation ofnitrogen metabolism. We will now look into this control in more detail, beforeproceeding to the biosynthesis of the remaining nonessential amino acids.

GABA and glutamate in the human brain.

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After termination of increased glutamatergic activity, excess glutamate is degraded, predominantly by oxidative metabolism of glutamate in astrocytes (pathway 3). Pronounced oxidative deamination of glutamate by glutamate dehydrogenase has been shown in cultured astrocytes (see ), and astrocytes express a mitochondrial glutamate/hydroxyl carrier (). Nevertheless, the predominant, but possibly not the only, reaction in the brain in vivo seems to be the formation of mitochondrial α-ketoglutarate by transamination (), coupled with the conversion of OAA to aspartate, and following glutamate/aspartate exchange by AGC1 activity as shown in pathway 3. The ‘excess' molecule of mitochondrial OAA created in pathway 1 now provides the OAA for the transamination as indicated by pathway 4, and the cytosolic aspartate generated in pathway 3 can provide the aspartate required in pathway 1. The formation of cytosolic aspartate would be absent in aralar−/− animals, and, in both wild-type and aralar-knockout cells, glutamate/glutamine synthesis can be predicted to be stimulated by addition of extracellular aspartate, because the aspartate may be normally delivered to pathway 1 with considerable delay, representing the time period between glutamate synthesis and glutamate oxidation, thus creating an apparent dependence on exogenous aspartate.

After termination of increased glutamatergic activity, excess glutamate is degraded, predominantly by oxidative metabolism of glutamate in astrocytes (pathway 3). Pronounced oxidative deamination of glutamate by glutamate dehydrogenase has been shown in cultured astrocytes (see ), and astrocytes express a mitochondrial glutamate/hydroxyl carrier (). Nevertheless, the predominant, but possibly not the only, reaction in the brain in vivo seems to be the formation of mitochondrial α-ketoglutarate by transamination (), coupled with the conversion of OAA to aspartate, and following glutamate/aspartate exchange by AGC1 activity as shown in pathway 3. The ‘excess' molecule of mitochondrial OAA created in pathway 1 now provides the OAA for the transamination as indicated by pathway 4, and the cytosolic aspartate generated in pathway 3 can provide the aspartate required in pathway 1. The formation of cytosolic aspartate would be absent in aralar−/− animals, and, in both wild-type and aralar-knockout cells, glutamate/glutamine synthesis can be predicted to be stimulated by addition of extracellular aspartate, because the aspartate may be normally delivered to pathway 1 with considerable delay, representing the time period between glutamate synthesis and glutamate oxidation, thus creating an apparent dependence on exogenous aspartate.

01/11/2017 · GABA and glutamate in the human brain

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After formation in the astrocytic mitochondria, α-ketoglutarate is oxidized in the TCA cycle by conversion to malate that exits the TCA cycle and the mitochondria (pathway 3), followed by conversion to pyruvate in the astrocytic cytosol and reentry of pyruvate into the TCA cycle. If malate stays in the cycle, α-ketoglutarate will, after introduction of another molecule acetyl coenzyme A (and NADH formation) serve as the source for another molecule of glutamate (pathway 3, ). The relatively low activity of aralar (or that of another redox shuttle) may suffice for transfer of reducing equivalents for these reactions and for generation of the second pyruvate molecule required for glutamate synthesis (left part of ), because they do not need to occur very rapidly. However, as shown in , aralar activity in astrocytes corresponding to 7% of that in the brain is approximately twice of what is required for glutamate production and degradation and would be sufficient for the entire in vivo oxidation of glucose in astrocytes.

After formation in the astrocytic mitochondria, α-ketoglutarate is oxidized in the TCA cycle by conversion to malate that exits the TCA cycle and the mitochondria (pathway 3), followed by conversion to pyruvate in the astrocytic cytosol and reentry of pyruvate into the TCA cycle. If malate stays in the cycle, α-ketoglutarate will, after introduction of another molecule acetyl coenzyme A (and NADH formation) serve as the source for another molecule of glutamate (pathway 3, ). The relatively low activity of aralar (or that of another redox shuttle) may suffice for transfer of reducing equivalents for these reactions and for generation of the second pyruvate molecule required for glutamate synthesis (left part of ), because they do not need to occur very rapidly. However, as shown in , aralar activity in astrocytes corresponding to 7% of that in the brain is approximately twice of what is required for glutamate production and degradation and would be sufficient for the entire in vivo oxidation of glucose in astrocytes.

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    13/01/2018 · Glutamine and glutamate as vital metabolites

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Glutamate as a Neurotransmitter in the Brain: Review …

Glutamate and GABA metabolic pathways are closely related and represent key steps in the regulation of synaptic transmission . However, neurons are not the unique players in this phenomenon since glia, and in particular astrocytes, are part of the synaptic unit and modulate neurotransmitter availability through recycling and transport. For instance, astrocytes express excitatory amino acids transporters such as GLAST and GLT-1 and carry out the majority of the glutamate transport in the CNS . In humans and rodents astrocytes GAD1, GABA transaminase and GABA receptors are expressed , . These glial cells can also terminate the GABA neurotransmission by removing it from the extracellular milieu using the GAT1, GAT2 and BGT1 transporters .

Brain serotonin, dopamine, epinephrine, and …

Earlier studies suggested that RTT was due exclusively to the loss of Mecp2 function in neurons. Since then, it has been shown that the re-expression of Mecp2 in astrocytes of Mecp2-deficient mice raised in vitro and in vivo parameters to normal level, and largely extended their lifespan compared to the Mecp2-deficient mice . Therefore, from our data, we cannot exclude the influence of glia as all our samples contained both neuronal and glial cells. This information is of particular interest since Mecp2 has been found to affect astroglial genes expression in vitro and Mecp2 deletion in astrocytes leads to an abnormal clearance of glutamate . Moreover, it was reported that Mecp2-deficient microglia released a fivefold higher level of glutamate and that the blockade of microglial glutamate synthesis and release decrease the neurotoxicity in cell culture . Further experiments will have to tease out the contribution of each cell compartment (neuronal/glial) in the deregulation of the amino acids metabolism in the context of Mecp2-deficiency.

Newer aspects of glutamine/glutamate metabolism: the …

Alternatively, we do the hypothesis that the hippocampus circuitry and its cellular diversity (, ) can also influence the level of expression of specific GABAergic genes. Depending on the cells considered (astrocytes, interneurons) and their correct integration in the hippocampus they could respond differently according to the level of neuronal activation or their position in an LTP/LTD-competent pathway. For instance GAD exists as two isoforms, GAD65 and GAD67, which are the products of two different genes (Gad1 and Gad2) (). Both Gad genes are coexpressed in the vast majority of GABA-positive neurons () but GAD65 seems to represent the most abundant GAD protein isoform in brain areas such as the dentate gyrus and the CA1 field in the rodent hippocampus (). Immunohistochemichal studies with the same antibody (GAD6) we used for western blots experiments have shown that the GAD65 is localized in the nerve terminals, while GAD67, probed with the K2 antibody is expressed in mammalian neurons without specific subcellular enrichment (). Indeed, data from the literature suggest that GAD65 is somatic and then transported along the axons toward the terminals to allow GABA synthesis and packaging into synaptic vesicles at the presynaptic level (). Based on these observations and the limitations inherent to the methods we used (sampling and quantification), it is difficult to argue that any decrease/increase of GABA refers to any hippocampal field or is exclusively neuronal. As discussed previously we can also hardly relate those specific changes to intra- vs extracellular GABA contents.

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