The effect of thiopental on glutamate metabolism in mouse cerebellar astrocytes in vitro

Intracellular metabolic compartmentation assessed by 13C magnetic resonance spectroscopy

Our understanding of the brain has developed from the theory that it is one continuous cell to the knowledge that there are many brain cells originally termed neurons and, furthermore to the discovery of glial cells and their multiple functions. Thus, an increasing complexity was unraveled and we have not reached a complete understanding of the phenomenon which comprises the compartmentation of metabolic pathways and metabolites. This is an important principle needed to fully understand the metabolic processes of the brain. At the cellular level this concept is well established whereas intracellular compartmentation has yet to be explored. Using magnetic resonance spectroscopy (MRS) for analysis of isotopomer composition combined with quantification of amino acid contents it is possible to construct models that describe intracellular compartmentation. Results of studies of cultures of astrocytes and neurons incubated in media containing [U- 13C]glutamate in the presence or absence of thiopental may be used to propose an intracellular three compartment model of mitochondrial function. Due to the experimental paradigm only certain aspects of metabolism can be described. The present model consists of compartments assigned to CO(2) production, glutamate synthesis from ketoglutarate and finally synthesis of a four-carbon metabolite which is shuttled between compartments. It is likely that metabolism may be far more complex than this and we are only beginning to glimpse some aspects of compartmentation at the cellular level.

GABA alters the metabolic fate of [U-13C]glutamate in cultured cortical astrocytes

The effect of gamma-aminobutyric acid (GABA) on glutamate metabolism was studied by (13)C-nuclear magnetic resonance (NMR) spectroscopy. Cerebral cortical astrocytes were incubated with 0.5 mM [U-(13)C]glutamate and 5 mM glucose in the presence or absence of 0.2 mM GABA for 2 hr. (13)C-labeled glutamate, glutamine, and aspartate were observed in cell extracts, and (13)C-labeled glutamine and lactate were present in the media. Both uniformly labeled glutamate and [1,2,3-(13)C]glutamate derived from the tricarboxylic acid (TCA) cycle were present in the cells. The consumption of [U-(13)C]glutamate and glucose was unchanged in the presence of GABA; however, the formation of [U-(13)C]lactate and [U-(13)C]aspartate from metabolism of [U-(13)C]glutamate was increased in cells incubated with GABA. The total concentration of aspartate was increased to the same extent as the (13)C-labeled aspartate, suggesting increased entry of [U-(13)C]glutamate into the TCA cycle to allow for the transamination of GABA. Although the concentrations of unlabeled glucose and lactate in the media were unchanged in the presence of GABA, the concentration of alanine was decreased, indicating that there was decreased transamination of the unlabeled pyruvate from glucose metabolism. The amount of [U-(13)C]glutamate converted to [U-(13)C]glutamine and [U-(13)C]lactate was increased in the presence of GABA. However, since the overall consumption of [U-(13)C]glutamate was not different, it can be concluded that the amount of [U-(13)C]glutamate used for energy was decreased. This suggests that exogenous GABA could substitute for glutamate as an energy source for astrocytes. The results indicate that the presence of GABA influences the metabolic fate of both glutamate and glucose in astrocytes, suggesting that fluctuations in the concentration of GABA in normal and pathological conditions can alter the compartmentation of glial metabolism in brain.

Pentylenetetrazole affects metabolism of astrocytes in culture

Cortical and cerebellar astrocytes were cultured in medium containing pentylenetetrazole (PTZ), a gamma-aminobutyric acid (GABA)(A) receptor antagonist, for 3 weeks (up to 6 mM) or 2 hr (10 mM). Cells were incubated in medium containing [U-(13)C]glutamate (0.5 mM) and unlabeled glucose (3 mM) for 2 hr and cell extracts and media were analyzed by (13)C magnetic resonance (MR) spectroscopy and high-performance liquid chromatography (HPLC). When cerebellar astrocytes were incubated with PTZ for 2 hr, the amount of glucose removed from the medium and glucose and [U-(13)C]glutamate oxidation were decreased. Metabolism in cortical astrocytes was affected only slightly; amounts of glutathione and aspartate were decreased. When cerebellar and cortical cells were cultured in the presence of PTZ for 3 weeks, the amount of glucose removed from the medium and lactate formed were increased, indicating increased glycolytic activity. Despite the increased intracellular [U-(13)C]glutamate concentration in both types of astrocytes cultured with PTZ, labeled glutamine and glutathione were unchanged, indicating intracellular compartmentation. The amount of cellular protein was decreased at 6 mM PTZ for cerebellar astrocytes and 1 mM for cortical astrocytes, indicating a differential sensitivity to the effects of PTZ. In conclusion, mitochondrial metabolism and glycolysis were decreased by short-term incubation with PTZ in cerebellar astrocytes, whereas long-term incubation affected both types of astrocytes, leading to increased glycolysis.

Effect of methylmercury on glutamate metabolism in cerebellar astrocytes in culture

The effect of methylmercury (MeHg) on [U-13C]glutamate metabolism was studied in cerebellar astrocytes using 13C nuclear magnetic resonance spectroscopy. The cells were preincubated in medium containing 25 or 50 microM MeHg and 10% fetal calf serum for 4h and then in medium with [U-13C]glutamate (0.5mM) for 2h. Labeled glutamate, glutamine and aspartate were observed both in the cell extracts and media, labeled glutathione in the cell extracts and labeled lactate and alanine in the media. The amount of glutamate removed from the media was decreased in the 50 microM MeHg group, furthermore, the levels of both labeled and unlabeled glutamine were decreased. This might indicate a decreased synthesis and/or increased degradation. An increase was observed for glutathione in the 25 microM group, which might be due to an upregulated synthesis of glutathione in response to the toxic effects of MeHg. The percentage of [U-13C]glutamate used for the synthesis of metabolites via the tricarboxylic acid cycle was increased in the presence of 50 microM MeHg. However, the percentage used for energy production was decreased in both groups, indicating selective mitochondrial vulnerability due to the inhibitory effect of MeHg.

Demonstration of pyruvate recycling in primary cultures of neocortical astrocytes but not in neurons

Pyruvate recycling was studied in primary cultures of mouse cerebrocortical astrocytes, GABAergic cerebrocortical interneurons, and co-cultures consisting of both cell types by measuring production of [4-(13)C]glutamate from [3-(13)C]glutamate by aid of nuclear magnetic resonance spectroscopy. This change in the position of the label can only occur by entry of [3-(13)C]glutamate into the tricarboxylic acid (TCA) cycle, conversion of labeled alpha-ketoglutarate to malate or oxaloacetate, malic enzyme-mediated decarboxylation of malate to pyruvate or phosphoenolpyruvate carboxykinase-mediated conversion of oxaloacetate to phosphoenolpyruvate and subsequent hydrolysis of the latter to pyruvate, and introduction of the labeled pyruvate into the TCA cycle, i.e., after exit of the carbon skeleton of pyruvate from the TCA cycle followed by re-entry of the same pyruvate molecules via acetyl CoA. In agreement with earlier observations, pyruvate recycling was demonstrated in astrocytes, indicating the ability of these cells to undertake complete oxidative degradation of glutamate. The recycled [4-(13)C]glutamate was not further converted to glutamine, showing compartmentation of astrocytic metabolism. Thus, absence of recycling into glutamine in the brain in vivo cannot be taken as indication that pyruvate recycling is absent in astrocytes. No recycling could be demonstrated in the cerebrocortical neurons. This is consistent with a previously demonstrated lack of incorporation of label from glutamate into lactate, and it also indicates that mitochondrial malic enzyme is not operational. Nor was there any indication of pyruvate recycling in the co-cultures. Although this may partly be due to more rapid depletion of glutamate in the co-cultures, this observation at the very least indicates that pyruvate recycling is not up-regulated in the neuronal-astrocytic co-cultures.

Pharmacology and toxicology of astrocyte-neuron glutamate transport and cycling

The interaction between astrocytes and neurons is examined from the standpoint of glutamate neurotoxicity. The review details 1. the distribution of glutamate transporters on astrocytes and neurons, provoking a reformulation of the interdependence between these two cell types in removing extracellular glutamate and preventing excitotoxic injury; 2. the potential involvement of aberrant glutamate transporter function in the etiology of neuropathological conditions; 3. the role of astrocyte-neuron interaction in widely divergent aspects of brain energetics; 4. the role of astrocytes in the process of glutamate recycling within the context of anesthetic treatment with pentobarbital and thiopental.

Glutamate decreases pyruvate carboxylase activity and spares glucose as energy substrate in cultured cerebellar astrocytes

The effects of glutamate on [U-(13)C]glucose metabolism were studied in cerebellar astrocytes using (13)C magnetic resonance spectroscopy. Labeled glutamate, glutamine, aspartate, lactate, and alanine were observed both in the cell extracts and in media, and, additionally, labeled glycogen was detected in the cell extracts. However, only labeled lactate and alanine were quantifiable in the medium in addition to [U-(13)C]glucose. In the presence of unlabeled glutamate, the amount of [U-(13)C]glucose removed from the medium was decreased, indicating that glutamate might spare glucose as an energy substrate and thus decrease the uptake of glucose. Labeled glycogen, [4,5-(13)C]glutamate, [3,4,5-(13)C]glutamate, [3,4-(13)C]aspartate, and [U-(13)C]alanine were increased in the presence of glutamate. However, the increase in the amount of [3,4,5-(13)C]glutamate from the second turn in the tricarboxylic acid (TCA) cycle was less pronounced than that of [4,5-(13)C]glutamate from the first turn in the TCA cycle. This indicates the dilution of label, probably resulting from the synthesis of unlabeled oxaloacetate from glutamate in the TCA cycle. Furthermore, exogenous glutamate had an inhibiting effect on pyruvate carboxylation, presumably by formation of oxaloacetate from 2-oxoglutarate derived from glutamate. It could be shown that glucose is a better substrate for energy production than glutamate; it is, however, less efficient in labeling amino acids than glutamate in cerebellar astrocytes.

Effect of glutamine and GABA on [U-(13)C]glutamate metabolism in cerebellar astrocytes and granule neurons

To probe the effect of glutamine and GABA on metabolism of [U-(13)C]glutamate, cerebellar astrocytes were incubated with [U-(13)C]glutamate (0.5 mM) in the presence and absence of glutamine (2.5 mM) or GABA (0.2 mM). It could be shown that consumption of [U-(13)C]glutamate was decreased in the presence of glutamine and release of labeled aspartate and [1,2,3-(13)C]glutamate decreased as well, whereas the concentrations of these metabolites increased inside the cells. Glutamine decreased energy production from [U-(13)C]glutamate presumably by substituting for glutamate as an energy substrate. No additional effect was seen in the presence of both glutamine and GABA. When cerebellar granule neurons were incubated with [U-(13)C]glutamate (0.25 mM) and GABA (0.05 mM), less [U-(13)C]glutamate was used for energy production than in controls. Because the barbiturate thiopental did not elicit such response (Qu et al., 2000, Neurochem Int 37:207-215) it appears that GABA also has a metabolic function in the glutamatergic cerebellar granule neurons in contrast to the astrocytes.