The incorporation of double-labelled acetate into glutamate ad related amino acids from adult mouse brain: compartmentation of amino acid metabolism in brain

Central Role of Glutamate Metabolism in the Maintenance of Nitrogen Homeostasis in Normal and Hyperammonemic Brain

Glutamate is present in the brain at an average concentration—typically 10–12 mM—far in excess of those of other amino acids. In glutamate-containing vesicles in the brain, the concentration of glutamate may even exceed 100 mM. Yet because glutamate is a major excitatory neurotransmitter, the concentration of this amino acid in the cerebral extracellular fluid must be kept low—typically µM. The remarkable gradient of glutamate in the different cerebral compartments: vesicles > cytosol/mitochondria > extracellular fluid attests to the extraordinary effectiveness of glutamate transporters and the strict control of enzymes of glutamate catabolism and synthesis in well-defined cellular and subcellular compartments in the brain. A major route for glutamate and ammonia removal is via the glutamine synthetase (glutamate ammonia ligase) reaction. Glutamate is also removed by conversion to the inhibitory neurotransmitter γ-aminobutyrate (GABA) via the action of glutamate decarboxylase. On the other hand, cerebral glutamate levels are maintained by the action of glutaminase and by various α-ketoglutarate-linked aminotransferases (especially aspartate aminotransferase and the mitochondrial and cytosolic forms of the branched-chain aminotransferases). Although the glutamate dehydrogenase reaction is freely reversible, owing to rapid removal of ammonia as glutamine amide, the direction of the glutamate dehydrogenase reaction in the brain in vivo is mainly toward glutamate catabolism rather than toward the net synthesis of glutamate, even under hyperammonemia conditions. During hyperammonemia, there is a large increase in cerebral glutamine content, but only small changes in the levels of glutamate and α-ketoglutarate. Thus, the channeling of glutamate toward glutamine during hyperammonemia results in the net synthesis of 5-carbon units. This increase in 5-carbon units is accomplished in part by the ammonia-induced stimulation of the anaplerotic enzyme pyruvate carboxylase. Here, we suggest that glutamate may constitute a buffer or bulwark against changes in cerebral amine and ammonia nitrogen. Although the glutamate transporters are briefly discussed, the major emphasis of the present review is on the enzymology contributing to the maintenance of glutamate levels under normal and hyperammonemic conditions. Emphasis will also be placed on the central role of glutamate in the glutamine-glutamate and glutamine-GABA neurotransmitter cycles between neurons and astrocytes. Finally, we provide a brief and selective discussion of neuropathology associated with altered cerebral glutamate levels.

Kinetic study of benzyl [1-14C]acetate as a potential probe for astrocytic energy metabolism in the rat brain: Comparison with benzyl [2-14C]acetate

Brain uptake of [(14)C]acetate has been reported to be a useful marker of astrocytic energy metabolism. In addition to uptake values, the rate of radiolabeled acetate washout from the brain appears to reflect CO2 exhaustion and oxygen consumption in astrocytes. We measured the time-radioactivity curves of benzyl [1-(14)C]acetate ([1-(14)C]BA), a lipophilic probe of [1-(14)C]acetate, and compared it with that of benzyl [2-(14)C]acetate ([2-(14)C]BA) in rat brains. The highest brain uptake was observed immediately after injecting either [1-(14)C]BA or [2-(14)C]BA, and both subsequently disappeared from the brain in a single-exponential manner. Estimated [1-(14)C]BA washout rates in the cerebral cortex and cerebellum were higher than those of [2-(14)C]BA. These results suggested that [1-(14)C]BA could be a useful probe for estimating the astrocytic oxidative metabolism. The [1-(14)C]BA washout rate in the cerebral cortex of immature rats was lower than that of mature rats. An autoradiographic study showed that the washout rates of [1-(14)C]BA from the rat brains of a lithium-pilocarpine-induced status epilepticus model were not significantly different from the values in control rat brains except for the medial septal nucleus. These results implied that the enhancement of amino acid turnover rate rather than astrocytic oxidative metabolism was increased in status epilepticus.

Labeled acetate as a marker of astrocytic metabolism

Astrocytes have various important roles in brain physiology. To further elucidate the details of astrocytic functions under normal and pathological states, astrocyte-specific measurements are mandatory. For studying brain energy metabolism, the use of the astrocyte-specific energy substrate acetate has proven to be of great value. Since the first applications of labeled acetate for brain studies about 50 years ago, numerous methodologies have been developed and employed in compartment-specific investigations of brain metabolism. Here, we provide an overview of these different methodological approaches and review studies employing acetate labeled with the most commonly used carbon isotopes.

Stimulation-induced increases of astrocytic oxidative metabolism in rats and humans investigated with 1-11C-acetate

The purpose of this study was to investigate astrocytic oxidative metabolism using 1-(11)C-acetate. 1-(11)C-acetate kinetics were evaluated in the rat somatosensory cortex using a beta-scintillator during different manipulations (test-retest, infraorbital nerve stimulation, and administration of acetazolamide or dichloroacetate). In humans a visual activation paradigm was used and kinetics were measured with positron emission tomography. Data were analyzed using a one-tissue compartment model. The following features supported the hypothesis that washout of radiolabel (k(2)) is because of (11)C-CO(2) and therefore related to oxygen consumption (CMRO(2)): (1) the onset of (11)C washout was delayed; (2)k(2) was not affected by acetazolamide-induced blood flow increase; (3)k(2) demonstrated a significant increase during stimulation in rats (from 0.014+/-0.007 to 0.027+/-0.006 per minute) and humans (from 0.016+/-0.010 to 0.026+/-0.006 per minute); and (4) dichloroacetate led to a substantial decrease of k(2). In the test-retest experiments K(1) and k(2) were very stable. In summary, 1-(11)C-acetate seems a promising tracer to investigate astrocytic oxidative metabolism in vivo. If the washout rate indeed represents the production of (11)C-CO(2), then its increase during stimulation would point to a substantially higher astrocytic oxidative metabolism during brain activation. However, the quantitative relationship between k(2) and CMRO(2) needs to be determined in future experiments.

Astrocyte activation in vivo during graded photic stimulation

Astrocytes have important roles in control of extracellular environment, de novo synthesis of neurotransmitters, and regulation of neurotransmission and blood flow. All of these functions require energy, suggesting that astrocytic metabolism should rise and fall with changes in neuronal activity and that brain imaging can be used to visualize and quantify astrocytic activation in vivo. A unilateral photic stimulation paradigm was used to test the hypothesis that graded sensory stimuli cause progressive increases in the uptake coefficient of [2-(14)C]acetate, a substrate preferentially oxidized by astrocytes. The acetate uptake coefficient fell in deafferented visual structures and it rose in intact tissue during photic stimulation of conscious rats; the increase was highest in structures with monosynaptic input from the eye and was much smaller in magnitude than the change in glucose utilization (CMR(glc)) by all cells. The acetate uptake coefficient was not proportional to stimulus rate and did not correlate with CMR(glc) in resting or activated structures. Simulation studies support the conclusions that acetate uptake coefficients represent mainly metabolism and respond to changes in metabolism rate, with a lower response at high rates. A model portraying regulation of acetate oxidation illustrates complex relationships among functional activation, cation levels, and astrocytic metabolism.

Activation of astrocytes in brain of conscious rats during acoustic stimulation: acetate utilization in working brain

To evaluate the response of astrocytes in the auditory pathway to increased neuronal signaling elicited by acoustic stimulation, conscious rats were presented with a unilateral broadband click stimulus and functional activation was assessed by quantitative autoradiography using three tracers to pulse label different metabolic pools in brain: [2-14C]acetate labels the 'small' (astrocytic) glutamate pool, [1-14C]hydroxybutyrate labels the 'large' glutamate pool, and [14C]deoxyglucose, reflects overall glucose utilization (CMR(glc)) in all brain cells. CMR(glc) rose during brain activation, and increased activity of the oxidative pathway in working astrocytes during acoustic stimulation was registered with [2-14C]acetate. In contrast, the stimulation-induced increase in metabolic activity was not reflected by greater trapping of products of [1-14C]hydroxybutyrate. The [2-14C]acetate uptake coefficient in the inferior colliculus and lateral lemniscus during acoustic stimulation was 15% and 18% (p < 0.01) higher in the activated compared to contralateral hemisphere, whereas CMR(glc) in these structures rose by 66% (p < 0.01) and 42% (p < 0.05), respectively. Calculated rates of brain utilization of blood-borne acetate (CMR(acetate)) are about 15-25% of total CMR(glc) in non-stimulated tissue and 10-20% of CMR(glc) in acoustically activated structures; they range from 28 to 115% of estimated rates of glucose oxidation in astrocytes. The rise in acetate utilization during acoustic stimulation is modest compared to total CMR(glc), but astrocytic oxidative metabolism of 'minor' substrates present in blood can make a significant contribution to the overall energetics of astrocytes and astrocyte-neuron interactions in working brain.

Cellular heterogeneity in primary cultures of brain cells revealed by immunocytochemical localization of glutamine synthetase

The co-localization of glutamine synthetase, glial fibrillary acidic protein, galactocerebroside and fibronectin was investigated by immunofluorescence double staining in primary cultures of dissociated brain cells from newborn mice. In cultures, grown in serum-free medium, containing dibutyryl-cAMP, glutamine synthetase was found in about half of the glial fibrillary acidic protein containing astroblasts. After addition of dexamethasone to the cultures, glutamine synthetase appeared also in another cell type, in which no glial fibrillary acidic protein, but fibronectin was detectable. This demonstrates that these cells, which are present in substantial amounts, are not of glial nature. In cultures treated with dibutyryl-cAMP no other cell type was found positive for fibronectin. For cultures grown in serum-containing medium lacking dibutyryl-cAMP, evidence was obtained that glutamine synthetase was induced by dexamethasone only in part of all cells staining for fibronectin. This suggests the presence of two different populations of fibronectin-positive cells. Oligodendrocytes, revealed by staining for galactocerebroside, never contained detectable amounts of glutamine synthetase, irrespective of the presence of dexamethasone. This also holds for cultures grown in serum-free medium containing dibutyryl-cAMP. However, under these conditions, oligodendroblasts are seen only very rarely. Our results demonstrate an unexpected heterogeneity in the cellular composition of primary cultures from newborn mouse brain.

Decreased labeling of amino acids by inhibition of the utilization of [3H, 14C]glucose via the hexosemonophosphate shunt in rat brain in vivo

Treatment of rats with 6-aminonicotinamide showed a small but significant decrease in the labeling of amino acids in the brain after injection of [3H]acetate. The results of these experiments also gave evidence of the presence of [3H]glucose and [3H]acetate, and an increase in [3H]glucose content in the brain of 6-aminonicotinamide treated rats. To apportion the contribution of [3H]glucose formed by gluconeogenesis from [3H]acetate to the labeling of amino acids a method was formulated based on the measurement of radioactivity of amino acids, lactate and free sugars in brain after injection of [6-3H]glucose or [1-3H]glucose relative to that after co-injection of [U-14C]glucose or [2-14C]glucose. In contrast to the expected formation of [1,6-3H]glucose by gluconeogenesis from [3H]acetate, 3H-labeled glucose isolated from brain, blood and liver showed the presence of [6-3H]glucose only. The values corrected for the presence of [6-3H]glucose showed that treatment with 6-aminonicotinamide had no effect on the labeling of amino acids by oxidation of [3H]acetate. These findings indicated that a significant decrease in the labeling of amino acids from [U-14C]glucose reported previously and again confirmed using [1-3H], [6-3H], [2-14C] or [U-14C]glucose in the present investigation was not due to the inhibition of the activities of enzymes of the citric acid cycle. These results support the postulated role of the hexosemonophosphate shunt for the utilization of glucose in providing neurotransmitter amino acids glutamate and gamma-aminobutyrate.

Changes in medium radioactivity and composition accompany high-affinity uptake of glutamate and aspartate by mouse brain slices

In measurements of high affinity transport in tissue slices, the incubation medium is often treated as an "infinitely large pool". External substrate concentrations, even at the micromolar level, are assumed to be constant and metabolic interactions between tissue and medium are neglected. In the present report we describe experiments in which glutamic and aspartic acid uptake by mouse brain slices were studied using techniques that could test these assumptions. Cerebral hemispheres were cut into 0.1 mm sections and about 90 mg of tissue incubated in 10 ml of oxygenated medium. After 45 minutes of equilibration, radioactive substrates were added and the concentrations and specific activities of the amino acids and their metabolites in the medium were determined. During the first 10 min following substrate addition, rapid decreases in glutamic and aspartic acid concentrations in the medium were accompanied by large decreases in specific activity caused by the continuous release of these amino acids from the tissue. In addition, extensive conversion of both substrates to glutamine and the preferential accumulation of this metabolite, in the medium, was found. These results demonstrate that metabolism and release occur simultaneously with uptake during transport experiments in vitro and that these processes can take place in specific tissue compartments. It is therefore necessary to measure the tissue and medium concentration levels of amino acids along with their radioactivity in such experiments, since all three processes (transport, metabolism, and compartmentation) are interrelated in the clearance of amino acids from the incubation medium and probably from the extracellular spaces in vivo as well.

Selective inhibition of glial versus neuronal uptake of L-glutamic acid by SITS

SITS, an inhibitor of anion exchange, was found to be a potent and selective inhibitor of L-glutamic acid uptake by cultured LRM55 glioma cells and rat brain astrocytes. Synaptosomal uptake of glutamate was relatively insensitive to inhibition by SITS. This differential effect indicates that the glutamate transport system in glia differs from that in neurons and that SITS may provide a tool for investigating the exclusive neuronal transport and metabolism of L-glutamic acid.

Cellular localization and regulation of glutamine synthetase in primary cultures of brain cells from newborn mice

The cellular distribution of glutamine synthetase was determined by indirect immunofluorescence in cultures of dissociated brain cells from newborn mice. The enzyme could be detected in about 40% of all cells, among which cells with astrocytic morphology were clearly identified. Treatment with the glucocorticoid dexamethasone led to a strong increase in the number of positivity stained cells. Enzyme induction by dexamethasone was maximal after 36 h and at a concentration of 0.1 micrometer. Under these conditions glutamine synthetase specific activity was elevated about six fold. Steroid hormones other than corticosteroids had no effects. The basal activity in these cultures was near that found in brains of newborn mice, but far below the activity in adult brains, showing that in culture the normal development of these cells is disturbed. A comparison of glial and neuronal cell lines showed that glutamine synthetase is present in both types of cell lines at a very low specific activity. Inducibility of this enzyme by dexamethasone was found in glial but not in neuronal cell lines.

Glutamate metabolism in ageing rat brain

The metabolism of glutamate, taken as an index of the metabolic state of the brain, was studied in brains of 3-, 12- and 30-month-old rats. Following the injection of a mixture of [3H]acetate and d-[2-14C]glucose, the brain levels of glutamate and aspartate were decreased in 30-month-old rats when compared with those of 3-month-old rats. No significant age-related differences were found in glutamine levels. Neither the protein levels nor the incorporation of the radioactivity in brain proteins differed among the three age groups, suggesting that there are no age-related differences in protein synthesis. The incorporation of d-[2-14C]glucose into aspartate and glutamine, expressed as the respective relative specific activities (RSA: specific activity of amino acid/specific activity of glutamate), did not change with age. Since glucose is the precursor of the large glutamate pool in brain, it can be concluded that no age-related changes occur in the metabolism of glutamate in the large compartment. The incorporation of [3H]acetate into aspartate, expressed as the RSA, did not differ among the age groups. The RSA of 3H-labelled glutamine, however, was significantly decreased 10 minutes after injection of the precursor mixture in brains of 30-month-old rats when compared with those of 3-month-rats. This difference had disappeared 20 minutes after injection, suggesting a somewhat delayed metabolism of glutamate in the small compartment, for which acetate is a precursor. These results and all the other parameters measured indicate that no large age-related metabolic changes in rat brain occur.

Utilization of acetate by chick brain during postnatal maturation

1. The study of the compartmentation of glutamate metabolism has been performed in the chick brain in vivo and in vitro in the presence of [U-14C]acetate between day 1 and day 30 of postnatal maturation. 2. The compartmentation of glutamate metabolism in vivo appears between day 1 and day 4 after hatching in the cerebral hemispheres and optic lobes. It is however more precocious in the optic lobes. In the cerebellum, it appears later, at about day 4 after hatching. The compartmentation of glutamate metabolism appears at the same time as the rapid incorporation of glucose into amino acids takes place in the cerebral hemispheres and the optic lobes. 3. In the chick telencephalon in vitro, the compartmentation of glutamate metabolism is visible from day 1 after hatching onwards. This difference is undoubtedly linked to the absence of an interference of glucose metabolism with acetate metabolism in vitro, and to the presence of a third compartment in the cerebral slices.