RATE OF UTILIZATION OF GLUCOSE AND 'COMPARTMENTATION' OF ALPHA-OXOGLUTARATE AND GLUTAMATE IN RAT BRAIN

A procedure for the quantitative analysis of the sulphur amino acids of rat tissues

1. A method is described for the quantitative separation of the sulphur compounds in a single sample of tissue by passing an extract through a serial assembly of ion-exchange resins in the order: Dowex 2 (Cl(-) form), Dowex 1 (CO(3) (2-) form), Amberlite CG-50 (H(+) form) and Zeo-Karb 225 (H(+) form). 2. Groups of sulphur amino acids were eluted separately from each column; the recovery of sulphur compounds after their labelling with (35)S in vivo by injection of l-[(35)S]-methionine was 91-106%. Individual sulphur compounds were further resolved by one-dimensional or two-dimensional paper chromatography. 3. Evidence is presented on the occurrence of S-adenosylmethionine and S-adenosylhomocysteine in rat liver and brain. Rat liver and brain contained 83.6 and 31.4mmu-moles/g. respectively of S-adenosylmethionine.

Labeling of metabolic pools by [6-14C]glucose during K(+)-induced stimulation of glucose utilization in rat brain

[6-14C]Glucose is the tracer sometimes recommended to assay cerebral glucose utilization (CMRglc) during transient or brief functional activations, but when used to study visual stimulation and seizures in other laboratories, it underestimated CMRglc. The metabolic fate of [6-14C]glucose during functional activation of cerebral metabolism is not known, and increased labeling of diffusible metabolites might explain underestimation of CMRglc and also reveal trafficking of metabolites. In the current studies cerebral cortex in conscious rats was unilaterally activated metabolically by KCl application, and CMRglc was determined in activated and contralateral control cortex with [6-14C]glucose or 2-[14C]deoxy-glucose ([14C]DG) over a 5- to 7-min interval. Local 14C concentrations were determined by quantitative autoradiography. Labeled precursor and products were measured bilaterally in paired cortical samples from funnel-frozen brains. Left-right differences in 14C contents were small with [6-14C]glucose but strikingly obvious in [14C]DG autoradiographs. CMRglc determined with [6-14C]glucose was slightly increased in activated cortex but 40-80% below values obtained with [14C]DG. [14C]Lactate was a major metabolite of [6-14C]glucose in activated but not control cortex and increased proportionately with unlabeled lactate. These results demonstrate significant loss of labeled products of [6-14C]glucose from metabolically activated brain tissue and indicate that [14C]DG is the preferred tracer even during brief functional activations of brain.

Measurement of amino acid metabolism derived from [1-13C]glucose in the rat brain using 13C magnetic resonance spectroscopy

To clarify the unique characteristics of amino acid metabolism derived from glucose in the central nervous system (CNS), we injected [1-13C]glucose intraperitoneally to the rat, and extracted the free amino acids from several kinds of tissues and measured the amount of incorporation of 13C derived from [1-13C]glucose into each amino acid using 13C-magnetic resonance spectroscopy (NMR). In the adult rat brain, the intensities of resonances from 13C-amino acids were observed in the following order: glutamate, glutamine, aspartate, gamma-aminobutyrate (GABA) and alanine. There seemed no regional difference on this labeling pattern in the brain. However, only in the striatum and thalamus, the intensities of resonances from [2-13C]GABA were larger than that from [2,3-13C]aspartate. In the other tissues, such as heart, kidney, liver, spleen, muscle, lung and small intestine, the resonances from GABA were not detected and every intensity of resonances from 13C-amino acids, except 13C-alanine, was much smaller than those in the brain and spinal cord. In the serum, 13C-amino acid was not detected at all. When the rats were decapitated, in the brain, the resonances from [1-13C]glucose greatly reduced and the intensities of resonances from [3-13C]lactate, [3-13C]alanine, [2, 3, 4-13C]GABA and [2-13C]glutamine became larger as compared with those in the case that the rats were sacrificed with microwave. In other tissues, the resonances from [1-13C]glucose were clearly detected even after the decapitation. In the glioma induced by nitrosoethylurea in the spinal cord, the large resonances from glutamine and alanine were observed; however, the intensities of resonances from glutamate were considerably reduced and the resonances from GABA and aspartate were not detected. These results show that the pattern of 13C label incorporation into amino acids is unique in the central nervous tissues and also suggest that the metabolic compartmentalization could exist in the CNS through the metabolic trafficking between neurons and astroglia.

Cerebral metabolic compartmentation as revealed by nuclear magnetic resonance analysis of D-[1-13C]glucose metabolism

Nuclear magnetic resonance (NMR) was used to study the metabolic pathways involved in the conversion of glucose to glutamate, gamma-aminobutyrate (GABA), glutamine, and aspartate. D-[1-13C]Glucose was administered to rats intraperitoneally, and 6, 15, 30, or 45 min later the rats were killed and extracts from the forebrain were prepared for 13C-NMR analysis and amino acid analysis. The absolute amount of 13C present within each carbonatom pool was determined for C-2, C-3, and C-4 of glutamate, glutamine, and GABA, for C-2 and C-3 of aspartate, and for C-3 of lactate. The natural abundance 13C present in extracts from control rats was also determined for each of these compounds and for N-acetylaspartate and taurine. The pattern of labeling within glutamate and GABA indicates that these amino acids were synthesized primarily within compartments in which glucose was metabolized to pyruvate, followed by decarboxylation to acetyl-CoA for entry into the tricarboxylic acid cycle. In contrast, the labeling pattern for glutamine and aspartate indicates that appreciable amounts of these amino acids were synthesized within a compartment in which glucose was metabolized to pyruvate, followed by carboxylation to oxaloacetate. These results are consistent with the concept that pyruvate carboxylase and glutamine synthetase are glia-specific enzymes, and that this partially accounts for the unusual metabolic compartmentation in CNS tissues. The results of our study also support the concept that there are several pools of glutamate, with different metabolic turnover rates.(ABSTRACT TRUNCATED AT 250 WORDS)

NMR determination of the TCA cycle rate and alpha-ketoglutarate/glutamate exchange rate in rat brain

A mathematical model of cerebral glucose metabolism was developed to analyze the isotopic labeling of carbon atoms C4 and C3 of glutamate following an intravenous infusion of [1-13C]glucose. The model consists of a series of coupled metabolic pools representing glucose, glycolytic intermediates, tricarboxylic acid (TCA) cycle intermediates, glutamate, aspartate, and glutamine. Based on the rate of 13C isotopic labeling of glutamate C4 measured in a previous study, the TCA cycle rate in rat brain was determined to be 1.58 +/- 0.41 mumol min-1 g-1 (mean +/- SD, n = 5). Analysis of the difference between the rates of isotopic enrichment of glutamate C4 and C3 permitted the rate of exchange between alpha-ketoglutarate (alpha-KG) and glutamate to be assessed in vivo. In rat brain, the exchange rate between alpha-KG and glutamate is between 89 +/- 35 and 126 +/- 22 times faster than the TCA cycle rate (mean +/- SD, n = 4). The sensitivity of the calculated value of the TCA cycle rate to other metabolic fluxes and to concentrations of glycolytic and TCA cycle intermediates was tested and found to be small.

NMR determination of intracerebral glucose concentration and transport kinetics in rat brain

The concentration of intracerebral glucose as a function of plasma glucose concentration was measured in rats by 13C NMR spectroscopy. Measurements were made in 20-60 min periods during the infusion of [1-13C]D-glucose, when intracerebral and plasma glucose levels were at steady state. Intracerebral glucose was found to vary from 0.7 to 19 mumol g-1 wet weight as the steady-state plasma glucose concentration was varied from 3 to 62 mM. A symmetric Michaelis-Menten model was fit to the brain and plasma glucose data with and without an unsaturable component, yielding the transport parameters Km, Vmax, and Kd. If it is assumed that all transport is saturable (Kd = 0), then Km = 13.9 +/- 2.7 mM and Vmax/Vgly = 5.8 +/- 0.8, where Vgly is the rate of brain glucose consumption. If an unsaturable component of transport is included, the transport parameters are Km = 9.2 +/- 4.7 mM, Vmax/Vgly = 5.3 +/- 1.5, and Kd/Vgly = 0.0088 +/- 0.0075 ml mumol-1. It was not possible to distinguish between the cases of Kd = 0 and Kd greater than 0, because the goodness of fit was similar for both. However, the results in both cases indicate that the unidirectional rate of glucose influx exceeds the glycolytic rate in the basal state by 2.4-fold and as a result should not be rate limiting for normal glucose utilization.

Acetylcholine formation from glucose following acute choline supplementation

The effects of choline administration on acetylcholine metabolism in the central nervous system are controversial. Although choline supplementation may elevate acetylcholine (ACh) content in brain, turnover studies with labelled choline precursors suggest that systemic choline administration either has no effect or actually diminishes brain ACh synthesis. Since choline supplementation elevates brain choline levels, the apparent decreases in previous turnover studies may reflect dilution of the labelled choline precursor pool rather than altered ACh formation. Therefore, brain ACh formation from [U-14C]glucose was determined after choline supplementation. A two to three fold elevation of brain choline did not alter ACh levels or [U-14C]glucose incorporation into ACh in the cortex, hippocampus or striatum. Although atropine stimulated ACh formation from [U-14C]glucose in hippocampus, two to three fold increases in brain choline did not augment ACh synthesis or content in atropine pretreated animals. Atropine depressed brain regional glucose utilization and this effect was not reversed by choline treatment. These results suggest that short-term elevation of brain choline does not enhance ACh formation from [U-14C]glucose, and argue against enhanced presynaptic cholinergic function after acute, systemic choline administration.

The origin of free brain malonate

Rat brain contains substantial concentrations of free malonate (192 nmol/g wet weight) but origin and biological importance of the dicarboxylic acid are poorly understood. A dietary source has been excluded. A recently described malonyl-CoA decarboxylase deficiency is associated with malonic aciduria and clinical manifestations, including mental retardation. In an effort to study the metabolic origin of free malonate, several labeled acetyl-CoA precursors were administered by intracerebral injection. [2-14C]pyruvate or [1,5-14C]citrate produced radioactive glutamate but failed to label malonate. In contrast, [1-14C]acetate, [2-14C]acetate, and [1-14C]butyrate were converted to labeled glutamate and malonate after the same route of administration. The intracerebral injection of [1-14C]-beta-alanine as a precursor of malonic semialdehyde and possibly free malonate did not give rise to radioactivity in the dicarboxylate. The labeling pattern of malonic acid is compatible with the reaction sequence: acetyl-CoA----malonyl-CoA----malonate. The final step is thought to occur by transfer of the CoA-group from malonyl-CoA to succinate and/or acetoacetate. Labeling of malonate from acetate is most effective at the age of 7 days when the net concentration of the dicarboxylic acid in rat brain is still very low. At this age, butyrate was a better precursor of malonate than acetate. It is proposed that fatty acid oxidation provides the acetyl-CoA which functions as the precursor of free brain malonate. Compartmentation of malonate biosynthesis is likely because the acetyl-CoA precursors citrate and pyruvate are ineffective.

Glycolysis-induced discordance between glucose metabolic rates measured with radiolabeled fluorodeoxyglucose and glucose

We have developed an autoradiographic method for estimating the oxidative and glycolytic components of local CMRglc (LCMRglc), using sequentially administered [18F]fluorodeoxyglucose (FDG) and [14C]-6-glucose (GLC). FDG-6-phosphate accumulation is proportional to the rate of glucose phosphorylation, which occurs before the divergence of glycolytic (GMg) and oxidative (GMo) glucose metabolism and is therefore related to total cerebral glucose metabolism GMt: GMg + GMo = GMt. With oxidative metabolism, the 14C label of GLC is temporarily retained in Krebs cycle-related substrate pools. We hypothesize that with glycolytic metabolism, however, a significant fraction of the 14C label is lost from the brain via lactate production and efflux from the brain. Thus, cerebral GLC metabolite concentration may be more closely related to GMo than to GMt. If true, the glycolytic metabolic rate will be related to the difference between FDG- and GLC-derived LCMRglc. Thus far, we have studied normal awake rats, rats with limbic activation induced by kainic acid (KA), and rats visually stimulated with 16-Hz flashes. In KA-treated rats, significant discordance between FDG and GLC accumulation, which we attribute to glycolysis, occurred only in activated limbic structures. In visually stimulated rats, significant discordance occurred only in the optic tectum.

13C nuclear magnetic resonance evidence for gamma-aminobutyric acid formation via pyruvate carboxylase in rat brain: a metabolic basis for compartmentation

The compartmentation of amino acid metabolism is an active and important area of brain research. 13C labeling and 13C nuclear magnetic resonance (NMR) are powerful tools for studying metabolic pathways, because information about the metabolic histories of metabolites can be determined from the appearance and position of the label in products. We have used 13C labeling and 13C NMR in order to investigate the metabolic history of gamma-aminobutyric acid (GABA) and glutamate in rat brain. [1-13C]Glucose was infused into anesthetized rats and the 13C labeling patterns in GABA and glutamate examined in brain tissue extracts obtained at various times after infusion of the label. Five minutes after infusion, most of the 13C label in glutamate appeared at the C4 position; at later times, label was also present at C2 and C3. This 13C labeling pattern occurs when [1-13C]glucose is metabolized to pyruvate by glycolysis and enters the pool of tricarboxylic acid (TCA) intermediates via pyruvate dehydrogenase. The label exchanges into glutamate from the TCA cycle pool through glutamate transaminases or dehydrogenase. After 30 min of infusion, approximately 10% of the total 13C in brain extracts appeared in GABA, primarily (greater than 80%) at the amino carbon (C4), indicating that the GABA detected is labeled through pyruvate carboxylase. The different labeling patterns observed for glutamate and GABA show that the large detectable glutamate pool does not serve as the precursor to GABA. Our NMR data support previous experiments suggesting compartmentation of metabolism in brain, and further demonstrate that GABA is formed from a pool of TCA cycle intermediates derived from an anaplerotic pathway involving pyruvate carboxylase.

Quantitative measurements of regional glucose utilization and rate of valine incorporation into proteins by double-tracer autoradiography in the rat brain tumor model

We examined the rate of glucose utilization and the rate of valine incorporation into proteins using 2-[18F]fluoro-2-deoxyglucose and L-[1-14C]-valine in a rat brain tumor model by quantitative double-tracer autoradiography. We found that in the implanted tumor the rate of valine incorporation into proteins was about 22 times and the rate of glucose utilization was about 1.5 times that in the contralateral cortex. (In the ipsilateral cortex, the tumor had a profound effect on glucose utilization but no effect on the rate of valine incorporation into proteins.) Our findings suggest that it is more useful to measure protein synthesis than glucose utilization to assess the effectiveness of antitumor agents and their toxicity to normal brain tissue. We compared two methods to estimate the rate of valine incorporation: "kinetic" (quantitation done using an operational equation and the average brain rate coefficients) and "washed slices" (unbound labeled valine removed by washing brain slices in 10% trichloroacetic acid). The results were the same using either method. It would seem that the kinetic method can thus be used for quantitative measurement of protein synthesis in brain tumors and normal brain tissue using [11C]-valine with positron emission tomography.

Comparison of [14C]glucose and [14C]deoxyglucose as tracers of brain glucose use

Because glucose metabolism and functional activity in brain regions are normally coupled, knowledge of regional brain glucose use can yield insights into regional functional activity. The deoxyglucose (DG) method is widely used for this purpose in experimental animals and humans but questions have arisen regarding its limits and accuracy. Therefore an experiment was designed to compare the DG method on a structure-by-structure basis with another tracer of glucose use, [6-14C]glucose, in normal rats. The cerebral metabolic rates obtained using the two tracers were similar in the telencephalon, but the results using DG were substantially lower in the midbrain and hindbrain (diencephalon, 18%; mesencephalon, 20%; metencephalon, 29%; and myelencephalon, 35%). The primary DG metabolite, DG 6-phosphate (DG-6-P) was found to disappear in a non-uniform manner from the major brain structures: telencephalon less than diencephalon less than mesencephalon = metencephalon less than myelencephalon. Thus a correlation was found between the rate of DG-6-P loss and the extent to which the DG method gave lower values of glucose use. Thus this may explain, at least in part, the discrepancies between the two methods.

Metabolism of glucose into glutamate via the hexose monophosphate shunt and its inhibition by 6-aminonicotinamide in rat brain in vivo

The treatment of rats for 4 h with 6-aminonicotinamide (60 mg kg-1) resulted in an 180-fold increase in the concentration of 6-phosphogluconate in their brains; glucose increased 2.6-fold and glucose 6-phosphate, 1.7-fold. Moreover, lactate decreased by 20%, glutamate by 8% and gamma-aminobutyrate by 12%, and aspartate increased by 10%. No significant changes were found in glutamine and citrate. In blood, 6-phosphogluconate increased 5-fold; glucose, 1.4-fold and glucose 6-phosphate, 1.8-fold. The metabolism of glucose in the rat brain, via both the Embden-Meyerhof pathway and the hexose monophosphate shunt, was investigated by injecting [U-14C]glucose or [2-14C]glucose, and that via the hexose monophosphate shunt alone by injecting [3,4-14C]glucose. The total radioactive yield of amino acids in the rat brain was 5.63 mumol at 20 min after injection of [U-14C]glucose, or 5.82 mumol after injection of [2-14C]glucose; by contrast, it was 0.62 mumol after injection of [3,4-14C]glucose. The treatment of rats with 6-aminonicotinamide showed significant decreases in these values, owing to decreases in the radioactive yields of glutamate, glutamine, aspartate, gamma-aminobutyrate, and alanine+glycine+serine. Glutamate isolated from the brain contained approximately 43% of its radioactivity in carbon 1 after injection of [3,4-14C]glucose, in contrast to 13% and 18% after injection of [U-14C]glucose and [2-14C]glucose, respectively, in both the control and treated rats. The calculations based on these findings showed that approximately 69% of the 14C-labelled glutamate was formed from [14C]acetyl coenzyme A (acetyl CoA) and the residual 31% by 14CO2 fixation of pyruvate after injection of [3,4-14C]glucose in both control and treated rats. The results gave direct evidence that glutamate and gamma-aminobutyrate in the brain were formed by metabolism of glucose via the hexose monophosphate shunt as well as via the Embden-Meyerhof pathway. From the radioactive yields of glutamate formed via [14C]acetyl CoA it was estimated that approximately 7.8% of the total glucose utilized was channelled via the hexose monophosphate shunt. Assuming that [14C]glutamate formed by carbon-dioxide fixation of pyruvate was also dependent on the metabolism of glucose through the hexose monophosphate shunt, the estimated value was approximately 9.5% of the total glucose converted into glutamate. The results of the present investigation, taken in conjunction with other findings, suggest that the utilization of glucose via the hexose monophosphate shunt is functionally important in the rat brain.

Locomotor activity as a predictor of times and dosages for studies of nicotine's neurochemical actions

Nicotine's action on the central nervous system is complex and likely involves an interaction of neurotransmitters. To determine the time after administration of nicotine and dosage for neurochemical studies, locomotor activity of CD-1 mice was determined at 5 min intervals between 0-60 min. A low nicotine dosage (0.05 mg/kg) did not alter activity 5-15 min after drug injection, but increased activity 28% at 15-25 min post-injection. A high dosage (0.8 mg/kg) reduced total distance 62% and rearing 87% at 5-15 min; at 15-25 minutes total distance declined 56% and rearing 69%; all measures returned to control values after 30 minutes; rearing then increased at 40 min after nicotine. Pretreatment (15 min before nicotine) with mecamylamine (1.0 mg/kg), but not hexamethonium (1.0 mg/kg), prevented the depressant effect of nicotine. Dopamine (DA) and its metabolites as well as acetylcholine (ACh) synthesis were measured at the point of nicotine's maximal depressant action. Striatal levels of dihydroxyphenylacetic acid (DOPAC) were increased and ACh utilization was reduced in striatum (-25%) and cortex (-24%) 10 min after nicotine (0.8 mg/kg). Mecamylamine, while preventing the depressant effect of nicotine on locomotor activity, did not alter its effects on DA metabolism. These results demonstrate that the behavioral outcome of acute nicotine treatment is time and dose-dependent. Nicotine's depressant action appears not to be due to altered DA but may be related to changes in carbohydrate and acetylcholine metabolism.

Concentration of free amino acids in primary cultures of neurones and astrocytes

The cellular distribution of free amino acids was estimated in primary cultures (14 days in vitro) composed principally of cerebellar interneurones or cerebellar and forebrain astrocytes. In cultured neural cells, the overall concentration of amino acids resembled that found in brain at the corresponding age in vivo. In the two neural cell types, there were marked differences in the distribution of amino acids, in particular, those associated with the metabolic compartmentation of glutamate. In neuronal cell cultures, the concentrations of glutamate, aspartate, and gamma-aminobutyric acid were, respectively, about three, four, and seven times greater than in astrocytes. By contrast, the amount of glutamine was approximately 65% greater in astroglial cell cultures than in interneurone cultures. An unexpected finding was a very high concentration of glycine in astrocytes derived from 8-day-old cerebellum, but the concentrations of both serine and glycine were greater in nerve cell cultures than in forebrain astrocytes. The essential amino acids threonine, valine, isoleucine, leucine, tyrosine, phenylalanine, histidine, lysine, and arginine were all present in the growth medium, and small cellular changes in the contents of some of these amino acids may relate to differences in their influx and efflux during culturing and washing procedures. The present results, together with our previous findings, provide further support for the model assigning the "small" compartment of glutamate to glial cells and the "large" compartment to neurones, and also underline the metabolic interaction between these two cell types in the brain.

Patterns of deoxyglucose and glucose labeling in the optic tectum of monocularly stimulated bass

Uniformly labeled deoxyglucose and glucose were used to examine patterns of altered metabolic activity in the optic tectum of largemouth bass. Autoradiographs from fish which viewed moving vertical stripes with one eye show that the metabolites of the two sugars procedure similar patterns of activity-related labeling in the tectum: tangentially arranged bands of increased optical density through the SFGS and the SGC. In addition, aldehyde fixation was found to improve the histological quality of the sections without altering the patterns of labeling.

Glutamine synthetase activity in Huntington's disease

Glutamine synthetase activity was measured in seven brain areas post-mortem from control patients, and those with Huntington's disease. The activity of the enzyme was reduced in the frontal and temporal cortex, putamen and cerebellum, but not in the hippocampus, thalamus or olivary nucleus. The results do not suggest a generalised deficiency of glutamine synthetase in Huntington's disease. However, as this enzyme is localised to astrocytic cells, the reduction in activity in areas of neuronal devastation, where the ration of astrocytes to neurones is increased, may reflect a greater functional deficit. The enzyme plays a crucial role in cerebral ammonia assimilation and its inhibition in laboratory animals is known to produce neuronal toxicity. A reduction in its activity in Huntington's disease may well contribute to the neuronal pathology in certain areas.

The effect of inhibition of hexosemonophosphate shunt on the metabolism of glucose and function in rat brain in vivo

Rats treated 4 hr previously with 6-aminonicotinamide showed a twenty-four fold increase of [14C]phosphogluconate in the adult brain at 30 min after injection of [U-14C]glucose indicating a blockade of the hexosemonophosphate shunt. There was a significant increase in the 14C-content of glucose and glucose-6-phosphate, and a decrease in that of amino acids. [14C]Phosphoglycerate content showed no consistent change after 6-aminonicotinamide treatment. The concentration of glucose and glucose 6-phosphate increased significantly without a significant change in the lactate pool in the brain of 6-aminonicotinamide treated rats. The rate of utilization of glucose in the brain of control rats was 0.73 mumol/min per g of brain. It decreased by 16% in rats treated with 6-aminonicotinamide; the results suggested that both glycolysis and pyruvate oxidation were affected. The amount of glucose utilized in the brain by the hexosemonophosphate shunt was approximately 0.0093 mumol/min per g of brain, i.e. 1.3% of the total rate of utilization of glucose. The observed changes were not due to hypothermia. The rate of glucose utilization was higher in animals exposed to higher ambient temperature and to stress caused by handling. The results were explained by postulating a role for the hexosemonophosphate shunt in providing neurotransmitter amino acids glutamate and gamma-aminobutyrate, and interdependence of brain function and glucose utilization.

Calculation of cerebral glucose phosphorylation from brain uptake of glucose analogs in vivo: a re-examination

The 2-deoxyglucose (2-DG) method of functional neuroanatomical mapping25 was re-examined in order to (1) obtain physical descriptions of the transfer constants K1 and k2, (2) estimate the changes of the 'lumped constant' with the condition of the experimental animals, and (3) examine the use of 3-O-methylglucose (3-O-MG) to estimate the fraction of unphosphorylated 2-DG in the tissue, and the value of the 'lumped constant'. The transfer constants K1 and k2 were shown to be simple exponential forms of the apparent permeability of the cerebral capillary endothelium to glucose and glucose analogs. The 'lumped constant' was shown to be influenced by any reduction of the ratio between glucose transport and glucose phosphorylation in the tissue, e.g. by hypoglycemia and increased glycolysis, while hyperglycemia and decreased glycolysis resulted in very minor changes of the 'lumped constant'. The glucose analog 3-O-MG was shown accurately to trace unphosphorylated 2-DG in brain and to be an index of the brain content of glucose and the regional value of the 'lumped constant'. In addition, 3-O-MG proved to be an accurate tracer of unphosphorylated 2-DG for experimental times as low as 10 min.

Cerebral metabolic responses to electroconvulsive shock and their modification by hypercapnia

Brain glucose metabolism was studied in paralyzed, ventilated rats given electroconvulsive shock (ECS) under normocapnic and hypercapnic conditions. Brains were obtained with a freeze-blowing apparatus. Rates of glucose utilization were determined with [2-14C]glucose and [3H]deoxyglucose as tracers. In normocapnic rats, ECS caused a large increase in the rate of glycolysis to 5--6 mumol/g/min. Brain lactate levels increased three- to fourfold. The stimulation of glucose metabolism was reflected in decreased brain glucose 6-phosphate concentration as early as 2--3 s after ECS. There were significant decreases in brain glucose and glycogen levels at 20 and 30 s after ECS. The decreases in endogenous brain glucose accounted for most of the increases in glucose utilization measured isotopically, implying that influx of glucose from blood into brain did not increase greatly over these time periods. Animals made hypercapnic by respiration with 10% CO2 for 2 min prior to ECS were different in their metabolic responses to ECS in several ways. The increases in glycolytic rate and lactate content of brain were half of those found in normocapnic rats. Brain glycogen and glucose concentrations did not change significantly in the hypercapnic rats during seizure activity. Thus, hypercapnia lessened the stimulation of glycolysis caused by ECS, but increased net influx of glucose from blood to brain. The mechanisms of these effects of hypercapnia are uncertain, but it is postulated that the effect on glycolytic activity is due to the acidosis and that the effect on glucose transport is due to an increase in capillary surface area.

The effect of hypoxia on the metabolism of labeled glucose and acetate in the rat brain

Glucose consumption and utilization of amino acids, lipids and proteins was measured in the rat brain under normoxia and hypoxia (7%O2:93%N2). After 2 h of hypoxia the brain glucose consumption increased by over 60% of control value. In spite of this increase, the radioactivity of amino acid fraction did not increase or parallel changes of glucose radioactivity in the blood. This strongly suggested that glucose flux into amino acids remained unchanged in hypoxia. Incorporation of 14C from glucose into macromolecules was found to decrease. The above changes demonstrated that the metabolic steps which precede synthesis of amino and tricarboxylic acids were inhibited. In some experiments, the incorporation of 14C from [2-14C]-acetate into a macromolecular fraction was also measured. The amounts of radioactivity found in these fractions were unchanged under hypoxic conditions. Possible differences in the influence of hypoxia on macromolecular synthesis in different metabolic compartments of the brain are discussed.

Neurotransmitter and carbohydrate metabolism during aging and mild hypoxia

Alterations in the metabolism of the glucose derived neurotransmitters may underlie some of the deficits in brain function that can accompany aging. We examined the whole brain syntheses of acetylcholine (ACh), alanine, aspartate, glutamate, gamma-aminobutyrate (GABA), glutamine and serine in two strains (C57BL and BALB/c) of aged mice (3, 10 and 30 months). ACh synthesis in C57BL and BALB/c mice declined 41 and 44% at 10 months and 64 and 75% by 30 months. Incorporation of [U-14C]glucose into amino acids generally decreased with aging, but it was not depressed as much as ACh formation. The only significant reductions in the amino acids in the 30 month old mice of both strains were in the syntheses of GABA (46 and 32%) and glutamine (44 and 55%). These changes may make the aged brain more vulnerable to metabolic insults, since mild anemic hypoxia decreased the syntheses of all the neurotransmitters at all ages even further. ACh synthesis in hypoxic 30 month old mice was only 9-11% of the 3 month old nonhypoxic mice, whereas amino acid formation ranged from 18-55% of the 3 month old nonhypoxic mice. Carbohydrate metabolism and its response to metabolic insults was also altered by age in both strains. The 30 month old mice had higher brain lactate concentrations than the 3 month old mice. The combination of hypoxia and aging further depressed oxidative metabolism, since a greater increase in brain lactates occurred in the aged hypoxilism and its response to metabolic insults was also altered by age in both strains. The 30 month old mice had higher brain lactate concentrations than the 3 month old mice. The combination of hypoxia and aging further depressed oxidative metabolism, since a greater increase in brain lactates occurred in the aged hypoxilism and its response to metabolic insults was also altered by age in both strains. The 30 month old mice had higher brain lactate concentrations than the 3 month old mice. The combination of hypoxia and aging further depressed oxidative metabolism, since a greater increase in brain lactates occurred in the aged hypoxic mice than in young hypoxic mice. Thus, aging may reduce the ability of the brain to adapt to metabolic insults.

Brain acetylcholine synthesis declines with senescence

The synthesis of whole brain acetylcholine is reduced in two strains (C57BL and BALB/c) of senescent mice. The incorporation of [U-14C]glucose into acetylcholine decreased in both strains by 40 +/- 4 per cent in 10-month-old mice and by 58 +/- 9 percent in 30-month-old mice compared with mice 3 months old. The incorporation of [2H4]choline into acetylcholine declined 60 and 73 percent in 10- and 30-month-old mice, respectively. Deficits in the cholinergic system may contribute to brain dysfunctions that complicate senescence.

Decreases in amino acids and acetylcholine metabolism during hypoxia

Hypoxia impairs brain function by incompletely defined mechanisms. Mild hypoxia, which impairs memory and judgment, decreases acetylcholine (ACh) synthesis, but not the levels of ATP or the adenylate energy charge. However, the effects of mild hypoxia on the synthesis of the glucose-derived amino acids [alanine, aspartate, gamma-amino butyric acid (GABA), glutamate, glutamine, and serine] have not been characterized. Thus, we examined the incorporation of [U-14C]glucose into these amino acids and ACh during anemic hypoxia (injection of NaNO2), hypoxic hypoxia (15 or 10% O2), and hypoxic hypoxia plus hypercarbia (15 or 10% O2 with 5% CO2). In general, the synthesis of the amino acids and of ACh declined in parallel with each type of hypoxia we studied. For example, anemic hypoxia (75 mg/kg of NaNO2) decreased the incorporation of [U-14C]glucose into the amino acids and into ACh similarly. [Percent inhibition: ACh (57.4), alanine (34.4), aspartate (49.2), GABA (61.9), glutamine (59.2), glutamate (51.0), and serine (36.7)]. A comparison of several levels (37.5, 75, 150, 225 mg/kg of NaNO2) of anemic hypoxia showed a parallel decreased in the flux of glucose into ACh and into the amino acids whose synthesis depends on mitochondrial oxidation: GABA (r = 0.98), glutamate (r = 0.99), aspartate (r = 0.96), and glutamine (r = 0.97). The synthesis of the amino acids not dependent on mitochondrial oxidation did not correlate as well with changes in ACh metabolism: serine (r = 0.68) and alanine (r = 0.76). The decreases in glucose incorporation into ACh and into the amino acids with hypoxic hypoxia (15% or 10% O2) or hypoxic hypoxia with 5% CO2 were very similar to those with the two lowest levels of anemic hypoxic. Thus, and explanation of the brain's sensitivity to a decrease in oxygen availability must include the alterations in the metabolism of the amino acid neurotransmitters as well as ACh.

Brain carbohydrate metabolism in developing rats during hypercapnia

Brain glucose metabolism was studied in developing rats at ages 10 and 20 days postnatal under normal and hypercapnic conditions. Brains were removed and frozen within 1 s with a freeze-blowing apparatus. Glucose utilization was measured with [2-14C]glucose and [3H]deoxyglucose as tracers. Metabolites were determined by standard enzymatic techniques. Data from [3H]deoxyglucose phosphorylation indicated that normal brain glucose utilization increased almost threefold between the 10th and 20th postnatal days. From the relative rates of utilization of the two isotopes in the 20-day-old control group, it appeared that about 25% of 14C label derived from metabolism of [2-14C]glucose was lost from brain (probably as lactate) rather than entering the Krebs cycle. Under hypercapnic conditions (20% CO2-21% O2-59% N2), rates of glucose utilization by brain were decreased by one-half at both ages and there were progressive decreases in the concentrations of many intermediary metabolites. The bases for concluding that these metabolites were used to supplement glucose as a fuel for respiration, rather than being lost by leakage into blood, are discussed. Despite the differences in brain glucose metabolism between 10-day-old and 20-day-old rats, their responses to hypercapnia are remarkably similar: Rates of glucose utilization are reduced to approximately the same proportion of the original rate by 20% CO2, and endogenous metabolites (particularly glutamate and lactate) appear to be oxidized as replacement fuels.

Impaired synthesis of acetylcholine by mild hypoxic hypoxia or nitrous oxide

The effect of mild hypoxic hypoxia on brain metabolism and acetylcholine synthesis was studied in awake, restrained rats. Since many studies of hypoxia are done with animals anesthetized with nitrous oxide (N2O), the effects of N2O were evaluated. N2O (70%) increased the cerebral cortical blood flow by 33% and the cortical metabolic rate of oxygen by 26%. In addition, the synthesis of acetylcholine in N2O-anesthetized animals, measured with [U-14C]glucose and [1-2H2,2-2H2]choline, decreased by 45 and 53%, respectively. Consequently, mild hypoxia was studied in unanesthetized rats. Control rats breathing 30% O2 (partial pressure of oxygen, PaO2 = 120 mm Hg) were compared with rats exposed to 15% O2 (PaO2 = 57 mm Hg) or 10% O2 (PaO2 = 42 mm Hg). The synthesis of acetylcholine, measured with [U-14C]glucose, was decreased by 35 and 54% with 15% O2 and 10% O2, respectively; acetylcholine synthesis, measured with [1-2H2,2-2H2]choline, was decreased by 50 and 68% with 15% O2 and 10% O2, respectively. Animals breathing either 15% or 10% O2 had normal cerebral metabolic rates of oxygen but had increased brain lactates and increased cortical blood flows compared with animals breathing 30% O2. These results show that even mild hypoxic hypoxia impairs acetylcholine synthesis, which in turn may account for the early symptoms of brain dysfunction associated with hypoxia.

Measurement of mouse brain glucose utilization in vivo using [U-14C]glucose

The rate of [2-14c]glucose uptake has been used as an indication of the status of energy consumption by the rat brain, but the cost of this radiolabel can be prohibitive and the surgical manipulation involved in published methods is extensive. A method for measuring glucose utilization in vivo in mouse brain with [U-14C]glucose is described in this article. Glucose consumption in whole mouse brain obtained with [U-14C]glucose or [2-14C]glucose was 0.650 +/- 0.022 -and 0.716 +/- .036 nmol/mg/min, respectively. In all instances the rate obtained with the uniformly labeled isotope was somewhat lower than that found with [2-14C]glucose. The rate of glucose utilization measured with either isotope was significantly depressed in sodium pentobarbital anesthetized mice. The method described here is advantageous because [U-14C]glucose is substantially less expensive than [2-14C]glucose and surgical intervention is avoided.

Measurement of regional brain glucose utilization in vivo using [2(-14)C] glucose

A new technique is described for the autoradiographic determination of regional brain glucose metabolism employing 14C labeled glucose as substrate and measurement principles previously described for whole brain. Regional glucose values correlate closely with those reported for the 14C-deoxyglucose technique. The method has the advantages of 1) a much shorter experimental period, 2) a relatively simple mathematical treatment, and 3) the utilization of the actual, fully metabolizable substance itself, glucose, as the label. In addition to normal rats, regional values are reported for 20 individual brain areas of rats in bicuculline induced status epilepticus, rats intoxicated with ammonium and rats anesthetized with pentobarbital sodium or ketamine.

Drug-induced parkinsonism in the rat- a model for biochemical investigation of the parkinson-syndrome. III. The incorporation of D-glucose-14C(U) in amino acids of brain and liver from rats pretreated with reserpine or with phenothiazines

Following treatment with reserpine or alternatively with a combination of phenothiazines (Randolektil, Majeptil) a drug-induced parkinsonoid reaction was provoked in rats. Twenty min before decapitation, 18 muCi d-glucose-14C(U) was administered intravenously. Concentration and radioactivities of glutamic acid (glu), glutamine (gln), serine (ser), and glycine (gly) were assayed in some regions of brain and in liver. Separation was performed by a combination of paper electrophoresis and chromatography or by an automatic amino acid analyzer. 1 After reserpine, the concentrations of serine and glycine were increased ten-fold while their specific activities decreased by the same factor. The interconversion serine-glycine was not affected. The concentration of glutamic acid was reduced while its specific activity remained constant. 2. After phenothiazines, the concentrations of serine and glycine in brain were also increased but their specific activities were decreased to a different degree. This indicates an additional effect on the serine-synthesis from glucose. The interconversion serine-glycine was also altered. The concentration of glutamic acid was decreased but specific activity was constant except in the thalamus region tested. 3. The influence of both treatments on amino acid turnover in liver differed from the observed impairment of brain metabolism. 4. Possible correlations between the changes in amino acid metabolism, catecholamines, and the neurologic parkinsonian symptoms are discussed.

Glucose consumption in rat cerbral cortex in normoxia, hypoxia and hypercapnia

Glucose consumption was measured in the cerebral cortex of rats, anesthetized with 70% N20, under normoxic conditions, as well as in hypoxia (Pao2 about 25 mmHg) and hypercapnia (Paco2 80-85 mmHg). The method used was that Hawkins et al (1974), modified to allow studies of transients in glycolytic rate. Cortical glucose consumption in normoxia was 0.77 mumol-g(-1)-min(-1). It is concluded that whereas glucose consumption in the whole brain of unanesthetized rat may be close to 0.6 mumol-g(-1)-min(-1), that of the cerebral cortex is close to 0.8 mumol-g(-1)-min(-1). During the first 2 min of hypoxia, glucose consumption was increased to twice the normal, and during the fist 2 min of hypercapnia, the corresponding value was less thane third of the normal. After 15 min of hypoxia, the glucose consumption had returned towards control values. In "steady state" hypercapnia, the glycolytic flux was higher than in the inital phase although still below normocapnic values.

A comparative analysis of compartmentation of metabolism in the dorsal root ganglion and ventral spinal cord gray using [U-14C]glucose, [2-14C]glucose, [6-14C]glucose, [3,4-14C]glucose, NaH14CO3, and [2-14C]pyruvate

A detailed temporal comparison of glucose metabolism, in the production of glutamate and glutamine as well as aspartate and alamine, was conducted in order to further define the uniqueness of the dorsal root ganglion compared to the ventral spinal cord gray. Experiments with injected labeled NaHCO3 and pyruvate were used in an attempt to clarify certain aspects of the above results with different [14C]glucose precursors. The glutamine/glutamate relative specific activity ratio (RSA) was consistently lower in the ganglion than in the ventral spinal cord gray, as was also true for glutamate specific activity from the same amount of injected [14C]glucose. The ganglion is characterized by a high level of alanine production from glucose and pyruvate. The NaH14CO3 experiments suggest that CO2 fixation from [3,4-14C]-glucose in the dorsal rool ganglion resul .ts in a higher glutamine/glutamate RSA when compared to results using either [6-14C] or [2-14C]glucose.

The estimation of rates of utilization of glucose and ketone bodies in the brain of the suckling rat using compartmental analysis of isotopic data

The brains of 18-day-old rats utilize glucose and ketone bodies. The rates of acetyl-CoA formation from these substrates and of glycolysis were determined in vivo from the labelling of intermediary metabolites after intraperitoneal injection of d-[2-(14)C]glucose, l(+)-[3-(14)C]- and l(+)-[U-(14)C]-lactate and d(-)-3-hydroxy[(14)C]butyrate. Compartmental analysis was used in calculating rates to allow for the rapid exchange of blood and brain lactate, the presence in brain of at least two pools each of glucose and lactate, and the incomplete equilibration of oxaloacetate with aspartate and of 2-oxoglutarate with glutamate. Results were as follows. 1. Glucose and ketone bodies labelled identical pools of tricarboxylate-cycle metabolites, and were in every way alternative substrates. 2. The combined rate of oxidation of acetyl-CoA derived from pyruvate (and hence glucose) and ketone bodies was 1.05mumol/min per g. 3. Ketone bodies contributed 0.11-0.53mumol/min per g in proportion to their concentration in blood, with a mean rate of 0.30mumol/min per g at 1.24mm. 4. Pyruvate and ketone bodies were converted into lipid at 0.018 and 0.008mumol/min per g respectively. 5. Glycolysis, at 0.48mumol/min per g, was more rapid in most rats than pyruvate utilization by oxidation and lipid synthesis, resulting in a net output of lactate from brain to blood. 6. Rates of formation of brain glutamate, glutamine and aspartate were also measured. Further information on the derivation of the models has been deposited as Supplementary Publication SUP 50034 (18 pages) at the British Library, Lending Division (formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS23 7QB, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1973) 131, 5.