The relative roles of the three sodium-dependent transport systems: A, ASC and N in the uptake of
3
H
Gln, and the compatibility of the uptake characteristics with the expression of mRNAs coding for ...the Gln transporting molecules, were examined in primary cultures of astrocytes and neurons derived from mouse cerebellum, a glutaminergic system-enriched structure, and in cerebral cortex. Gln uptake activity (
V
max) was higher in cerebellar astrocytes or neurons than in their cerebral cortical counterparts. The
N-methylamino-isobutyric acid (MeAiB)- and pH-sensitive, system A-mediated component of the uptake, and the uptake of
14
C
MeAiB itself, was much more active in neurons than in astrocytes derived from either region. Also, the expression of mRNA for GlnT (SAT1), a system A isoform specific for Gln, was only expressed in neurons derived from both structures, while an alanine (Ala)-preferring system A transporter, SAT2, was expressed in neurons and astrocytes from either region. System ASC-mediated Gln uptake and expression of ASCT2 mRNA were in both structures more pronounced in astrocytes than in neurons, consistent with the postulated role of ASCT2 in the efflux of de novo synthesized Gln from astrocytes. System N-mediated (threonine+MeAiB-inhibitable) Gln uptake showed comparable activities in all four types of cells, which is compatible with the ubiquitous expression of NAT2 mRNA—a mouse brain-specific N-system isoform.
Paradoxically, glutamate receptor antagonists have neurotoxic and psychotogenic properties in addition to their neuroprotective potential during excessive glutamate release. In the present study the ...non‐competitive N‐methyl‐d‐aspartate (NMDA) receptor antagonist MK801 was used to examine glial–neuronal interactions in NMDA receptor hypofunction. Rats were given a subanesthetic dose of MK801 together with 1‐13Cglucose and 1,2‐13Cacetate, and brains were removed 20 min later. Analyses of extracts from cingulate, retrosplenial plus middle frontal cortices (CRFC) and temporal lobe were performed using HPLC and 13C and 1H nuclear magnetic resonance spectroscopy. Hypofunction of the NMDA receptor induced similar changes in both brain areas investigated; however, the changes were most pronounced in the temporal lobe. Generally, only labeling from 1‐13Cglucose was affected by MK801. In CRFC and temporal lobe amounts of both labeled and unlabeled glutamine were increased, whereas those of aspartate were decreased. In the CRFC the decrease in labeling of aspartate was greater than the decrease in concentration, leading to decreased 13C enrichment. In temporal lobe, not in CRFC, increased concentrations of glutamate, GABA, succinate, glutathione and inositol were detected together with increased labeling of GABA and succinate from 1‐13Cglucose. 13C Enrichment was decreased in glutamate and increased in succinate. The results point towards a disturbance in glutamate–glutamine cycling and thus interaction between neurons and glia, since labeling of glutamate and glutamine from glucose was affected differently.
Abstract Neuronal–astrocytic interactions in 1-month-old Genetic Absence Epilepsy Rats from Strasbourg (GAERS) before the occurrence of seizures are compared to those in non-epileptic rats (NERs) and ...in adult GAERS expressing epilepsy. Animals received 1-13 Cglucose and 1,2-13 Cacetate, preferential substrates of neurons and astrocytes, respectively, and extracts from cerebral cortex, subcortex and cerebellum were analyzed by NMR spectroscopy. Increased mitochondrial metabolism took place in the cortical neurons of immature and adult GAERS and therefore does not seem to be a consequence of the occurrence of absence seizures. Glutamine supply to GABAergic neurons was reduced in cortex and subcortex in young GAERS, as reflected by increased glutamine content and decreased 13C-labeling of GABA. In the brain of immature GAERS, interactions between glutamatergic neurons and astrocytes appeared normal whereas increased astrocytic metabolism took place in adult GAERS, suggesting that astrocytic alterations could possibly be the cause of seizures.
The operation of a glutamine–glutamate/GABA cycle in the brain consisting of the transfer of glutamine from astrocytes to neurons and neurotransmitter glutamate or GABA from neurons to astrocytes is ...a well-known concept. In neurons, glutamine is not only used for energy production and protein synthesis, as in other cells, but is also an essential precursor for biosynthesis of amino acid neurotransmitters. An excellent tool for the study of glutamine transfer from astrocytes to neurons is
14
Cacetate or
13
Cacetate and the glial specific enzyme inhibitors, i.e. the glutamine synthetase inhibitor methionine sulfoximine and the tricarboxylic acid cycle (aconitase) inhibitors fluoro-acetate and -citrate. Acetate is metabolized exclusively by glial cells, and
13
Cacetate is thus capable when used in combination with magnetic resonance spectroscopy or mass spectrometry, to provide information about glutamine transfer. The present review will give information about glutamine trafficking and the tools used to map it as exemplified by discussions of published work employing brain cell cultures as well as intact animals. It will be documented that considerably more glutamine is transferred from astrocytes to glutamatergic than to GABAergic neurons. However, glutamine does have an important role in GABAergic neurons despite their capability of re-utilizing their neurotransmitter by re-uptake.
Regional hypometabolism of glucose in the brain is a hallmark of Alzheimer's disease (AD). However, little is known about the specific alterations of neuronal and astrocytic metabolism involved in ...homeostasis of glutamate and GABA in AD. Here, we investigated the effects of amyloid β (Aβ) pathology on neuronal and astrocytic metabolism and glial-neuronal interactions in amino acid neurotransmitter homeostasis in the transgenic McGill-R-Thyl-APP rat model of AD compared with healthy controls at age 15 months. Rats were injected with 1-13Cglucose and 1,2-13Cacetate, and extracts of the hippocampal formation as well as several cortical regions were analyzed using 1H- and 13C nuclear magnetic resonance spectroscopy and high-performance liquid chromatography. Reduced tricarboxylic acid cycle turnover was evident for glutamatergic and GABAergic neurons in hippocampal formation and frontal cortex, and for astrocytes in frontal cortex. Pyruvate carboxylation, which is necessary for de novo synthesis of amino acids, was decreased and affected the level of glutamine in hippocampal formation and those of glutamate, glutamine, GABA, and aspartate in the retrosplenial/cingulate cortex. Metabolic alterations were also detected in the entorhinal cortex. Overall, perturbations in energy- and neurotransmitter homeostasis, mitochondrial astrocytic and neuronal metabolism, and aspects of the glutamate-glutamine cycle were found in McGill-R-Thy1-APP rats.
•Pyruvate carboxylation in astrocytes is important for glutamate synthesis.•Pyruvate carboxylation has to be matched by a cataplerotic mechanism.•Partial pyruvate recycling (PPR) generates lactate ...released to blood and lymph.•PPR increases with increasing glutamate concentration in astrocytes.•Full PR increases with increasing glutamate concentration in neurons.
Pyruvate carboxylation, the anaplerotic reaction in the brain, has been demonstrated in astrocytes but not neurons. Since anaplerosis cannot proceed without cataplerosis in a closed system such as the brain, there have to be mechanisms to degrade molecules such as glutamate, glutamine, GABA and aspartate which have more carbon atoms than pyruvate. Pyruvate recycling is a cataplerotic process which is very active in liver. It has also been demonstrated in the brain and has been shown to proceed both in astrocytes and neurons. Increasing recycling as a consequence of increasing glutamate concentration in medium has been shown in astrocytes. In the present study cerebellar granule neurons were incubated with medium containing 0.1, 0.25 or 0.5 mM U-13Cglutamate or U-13Caspartate and pyruvate recycling in combination with tricarboxylic acid (TCA) cycle metabolism was analysed in glutamate, aspartate and malate using mass spectrometry. It could be shown that pyruvate recycling of TCA cycle intermediates as seen in glutamate increased with increasing U-13Cglutamate but not U-13Caspartate concentration confirming compartmentation of glutamate metabolism and the importance of glutamate in cataplerosis. Partial pyruvate recycling (lactate production from the TCA cycle) was more active in astrocytes than neurons in line with the astrocytes' greater capacity for glutamate uptake.
Neonatal hypoxia–ischemia (HI) and the delayed injury cascade that follows involve excitotoxicity, oxidative stress and mitochondrial failure. The susceptibility to excitotoxicity of the neonatal ...brain may be related to the capacity of astrocytes for glutamate uptake. Furthermore, the neonatal brain is vulnerable to oxidative stress, and the pentose phosphate pathway (PPP) may be of particular importance for limiting this kind of injury. Also, in the neonatal brain, neurons depend upon de novo synthesis of neurotransmitters via pyruvate carboxylase in astrocytes to increase neurotransmitter pools during normal brain development. Several recent publications describing intermediary brain metabolism following neonatal HI have yielded interesting results: (1) Following HI there is a prolonged depression of mitochondrial metabolism in agreement with emerging evidence of mitochondria as vulnerable targets in the delayed injury cascade. (2) Astrocytes, like neurons, are metabolically impaired following HI, and the degree of astrocytic malfunction may be an indicator of the outcome following hypoxic and hypoxic-ischemic brain injury. (3) Glutamate transfer from neurons to astrocytes is not increased following neonatal HI, which may imply that astrocytes fail to upregulate glutamate uptake in response to the massive glutamate release during HI, thus contributing to excitotoxicity. (4) In the neonatal brain, the activity of the PPP is reduced following HI, which may add to the susceptibility of the neonatal brain to oxidative stress. The present review aims to discuss the metabolic temporal alterations observed in the neonatal brain following HI.
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