Glucose availability to individual cerebral structures is correlated to glucose metabolism

The metabolic costs of cognition

Cognition and behavior are emergent properties of brain systems that seek to maximize complex and adaptive behaviors while minimizing energy utilization. Different species reconcile this trade-off in different ways, but in humans the outcome is biased towards complex behaviors and hence relatively high energy use. However, even in energy-intensive brains, numerous parsimonious processes operate to optimize energy use. We review how this balance manifests in both homeostatic processes and task-associated cognition. We also consider the perturbations and disruptions of metabolism in neurocognitive diseases.

Whole-brain deuterium metabolic imaging via concentric ring trajectory readout enables assessment of regional variations in neuronal glucose metabolism

Deuterium metabolic imaging (DMI) is an emerging magnetic resonance technique, for non-invasive mapping of human brain glucose metabolism following oral or intravenous administration of deuterium-labeled glucose. Regional differences in glucose metabolism can be observed in various brain pathologies, such as Alzheimer's disease, cancer, epilepsy or schizophrenia, but the achievable spatial resolution of conventional phase-encoded DMI methods is limited due to prolonged acquisition times rendering submilliliter isotropic spatial resolution for dynamic whole brain DMI not feasible. The purpose of this study was to implement non-Cartesian spatial-spectral sampling schemes for whole-brain 2H FID-MR Spectroscopic Imaging to assess time-resolved metabolic maps with sufficient spatial resolution to reliably detect metabolic differences between healthy gray and white matter regions. Results were compared with lower-resolution DMI maps, conventionally acquired within the same session. Six healthy volunteers (4 m/2 f) were scanned for ~90 min after administration of 0.8 g/kg oral [6,6']-2H glucose. Time-resolved whole brain 2H FID-DMI maps of glucose (Glc) and glutamate + glutamine (Glx) were acquired with 0.75 and 2 mL isotropic spatial resolution using density-weighted concentric ring trajectory (CRT) and conventional phase encoding (PE) readout, respectively, at 7 T. To minimize the effect of decreased signal-to-noise ratios associated with smaller voxels, low-rank denoising of the spatiotemporal data was performed during reconstruction. Sixty-three minutes after oral tracer uptake three-dimensional (3D) CRT-DMI maps featured 19% higher (p = .006) deuterium-labeled Glc concentrations in GM (1.98 ± 0.43 mM) compared with WM (1.66 ± 0.36 mM) dominated regions, across all volunteers. Similarly, 48% higher (p = .01) 2H-Glx concentrations were observed in GM (2.21 ± 0.44 mM) compared with WM (1.49 ± 0.20 mM). Low-resolution PE-DMI maps acquired 70 min after tracer uptake featured smaller regional differences between GM- and WM-dominated areas for 2H-Glc concentrations with 2.00 ± 0.35 mM and 1.71 ± 0.31 mM, respectively (+16%; p = .045), while no regional differences were observed for 2H-Glx concentrations. In this study, we successfully implemented 3D FID-MRSI with fast CRT encoding for dynamic whole-brain DMI at 7 T with 2.5-fold increased spatial resolution compared with conventional whole-brain phase encoded (PE) DMI to visualize regional metabolic differences. The faster metabolic activity represented by 48% higher Glx concentrations was observed in GM- compared with WM-dominated regions, which could not be reproduced using whole-brain DMI with the low spatial resolution protocol. Improved assessment of regional pathologic alterations using a fully non-invasive imaging method is of high clinical relevance and could push DMI one step toward clinical applications.

High-temporal resolution functional PET/MRI reveals coupling between human metabolic and hemodynamic brain response

PURPOSE

Positron emission tomography (PET) provides precise molecular information on physiological processes, but its low temporal resolution is a major obstacle. Consequently, we characterized the metabolic response of the human brain to working memory performance using an optimized functional PET (fPET) framework at a temporal resolution of 3 s.

METHODS

Thirty-five healthy volunteers underwent fPET with [18F]FDG bolus plus constant infusion, 19 of those at a hybrid PET/MRI scanner. During the scan, an n-back working memory paradigm was completed. fPET data were reconstructed to 3 s temporal resolution and processed with a novel sliding window filter to increase signal to noise ratio. BOLD fMRI signals were acquired at 2 s.

RESULTS

Consistent with simulated kinetic modeling, we observed a constant increase in the [18F]FDG signal during task execution, followed by a rapid return to baseline after stimulation ceased. These task-specific changes were robustly observed in brain regions involved in working memory processing. The simultaneous acquisition of BOLD fMRI revealed that the temporal coupling between hemodynamic and metabolic signals in the primary motor cortex was related to individual behavioral performance during working memory. Furthermore, task-induced BOLD deactivations in the posteromedial default mode network were accompanied by distinct temporal patterns in glucose metabolism, which were dependent on the metabolic demands of the corresponding task-positive networks.

CONCLUSIONS

In sum, the proposed approach enables the advancement from parallel to truly synchronized investigation of metabolic and hemodynamic responses during cognitive processing. This allows to capture unique information in the temporal domain, which is not accessible to conventional PET imaging.

Carotid foramen size in the human skull tracks developmental changes in cerebral blood flow and brain metabolism

OBJECTIVES

In humans, neuronal processes related to brain development elevate the metabolic rate of brain tissue relative to the body during early childhood. This phenomenon has been hypothesized to contribute to slow somatic growth in preadolescent Homo sapiens. The uncoupling of the brain's metabolic rate from brain size during development complicates the study of the evolutionary emergence of these traits in the fossil record. Here, we extend a method previously developed to predict interspecific differences in cerebral blood flow (a correlate of cerebral glucose use) to predict ontogenetic changes in human brain metabolism.

MATERIALS AND METHODS

Radii of the carotid foramen from an ontogenetic series of modern human crania were used to predict blood flow rates through the internal carotid arteries (ICA), which were compared to empirically measured ICA flow and brain metabolism values.

RESULTS

Predictions of both absolute ICA blood flow rates and perfusion (ICA blood flow rates relative to brain size) generally match measured values in infancy and childhood. Maximum predicted ICA blood flow rates and perfusion were found to occur between ages 5 and 8, which roughly correspond to the age of maximum measured ICA blood flow rate and absolute and brain mass-specific rate of whole brain glucose uptake.

DISCUSSION

These findings suggest that, during human growth and development, the size of the carotid foramen corresponds well to blood flow requirements through the ICA, and the method tested here may provide new opportunities for studying developmental changes in brain metabolism using osteological samples, including fossil hominins.

Scaling of bony canals for encephalic vessels in euarchontans: Implications for the role of the vertebral artery and brain metabolism

Supplying the central nervous system with oxygen and glucose for metabolic activities is a critical function for all animals at physiologic, anatomical, and behavioral levels. A relatively proximate challenge to nourishing the brain is maintaining adequate blood flow. Euarchontans (primates, dermopterans and treeshrews) display a diversity of solutions to this challenge. Although the vertebral artery is a major encephalic vessel, previous research has questioned its importance for irrigating the cerebrum. This presents a puzzling scenario for certain strepsirrhine primates (non-cheirogaleid lemuriforms) that have reduced promontorial branches of the internal carotid artery and no apparent alternative encephalic vascular route except for the vertebral artery. Here, we present results of phylogenetic comparative analyses of data on the cross-sectional area of bony canals that transmit the vertebral artery (transverse foramina). These results show that, across primates (and within major primate subgroups), variation in the transverse foramina helps significantly to explain variation in forebrain mass even when variation in promontorial canal cross-sectional areas are also considered. Furthermore, non-cheirogaleid lemuriforms have larger transverse foramina for their endocranial volume than other euarchontans, suggesting that the vertebral arteries compensate for reduced promontorial artery size. We also find that, among internal carotid-reliant euarchontans, species that are more encephalized tend to have a promontorial canal that is larger relative to the transverse foramina. Tentatively, we consider the correlation between arterial canal diameters (as a proxy for blood flow) and brain metabolic demands. The results of this analysis imply that human investment in brain metabolism (∼27% of basal metabolic rate) may not be exceptional among euarchontans.

Scaling of cerebral blood perfusion in primates and marsupials

The evolution of primates involved increasing body size, brain size and presumably cognitive ability. Cognition is related to neural activity, metabolic rate and rate of blood flow to the cerebral cortex. These parameters are difficult to quantify in living animals. This study shows that it is possible to determine the rate of cortical brain perfusion from the size of the internal carotid artery foramina in skulls of certain mammals, including haplorrhine primates and diprotodont marsupials. We quantify combined blood flow rate in both internal carotid arteries as a proxy of brain metabolism in 34 species of haplorrhine primates (0.116-145 kg body mass) and compare it to the same analysis for 19 species of diprotodont marsupials (0.014-46 kg). Brain volume is related to body mass by essentially the same exponent of 0.70 in both groups. Flow rate increases with haplorrhine brain volume to the 0.95 power, which is significantly higher than the exponent (0.75) expected for most organs according to 'Kleiber's Law'. By comparison, the exponent is 0.73 in marsupials. Thus, the brain perfusion rate increases with body size and brain size much faster in primates than in marsupials. The trajectory of cerebral perfusion in primates is set by the phylogenetically older groups (New and Old World monkeys, lesser apes) and the phylogenetically younger groups (great apes, including humans) fall near the line, with the highest perfusion. This may be associated with disproportionate increases in cortical surface area and mental capacity in the highly social, larger primates.

Glucose metabolism in normal aging and Alzheimer's disease: Methodological and physiological considerations for PET studies

Alzheimer's disease (AD) is an age-dependent neurodegenerative disorder associated with progressive loss of cognitive function. 2-[¹⁸F]fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) has long been used to measure resting-state cerebral metabolic rates of glucose, a proxy for neuronal activity. Several FDG PET studies have shown that metabolic reductions occur decades before onset of AD symptoms, suggesting that metabolic deficits may be an upstream event in at least some late-onset AD cases. This review explores this possibility, initially discussing the link between AD pathology, neurodegeneration, oxidative stress and AD, and then discussing findings of FDG PET hypometabolism in AD patients as well as in at-risk individuals, especially those with a first-degree family history of late-onset AD. While the rare early-onset form of AD is due to autosomal dominant genetic mutations, the etiology and pathophysiology of age-dependent, late-onset AD is more complex. Recent FDG PET studies have shown that adult children of AD-affected mothers are more likely than those with AD-fathers to show AD-like brain changes. Given the connection between glucose metabolism and mitochondria, and the fact that mitochondrial DNA is maternally inherited in humans, it is here argued that altered bioenergetics may be an upstream event in those with a maternal history of late-onset AD. Biomarkers of AD have great potential for identifying AD endophenotypes in at-risk individuals, which may help direct investigation of potential susceptibility genes.

Deep thiopental anesthesia alters steady-state glucose homeostasis but not the neurochemical profile of rat cortex

Barbiturates are regularly used as an anesthetic for animal experimentation and clinical procedures and are frequently provided with solubilizing compounds, such as ethanol and propylene glycol, which have been reported to affect brain function and, in the case of (1)H NMR experiments, originate undesired resonances in spectra affecting the quantification. As an alternative, thiopental can be administrated without any solubilizing agents. The aim of the study was to investigate the effect of deep thiopental anesthesia on the neurochemical profile consisting of 19 metabolites and on glucose transport kinetics in vivo in rat cortex compared with alpha-chloralose using localized (1)H NMR spectroscopy. Thiopental was devoid of effects on the neurochemical profile, except for the elevated glucose at a given plasma glucose level resulting from thiopental-induced depression of glucose consumption at isoelectrical condition. Over the entire range of plasma glucose levels, steady-state glucose concentrations were increased on average by 48% +/- 8%, implying that an effect of deep thiopental anesthesia on the transport rate relative to cerebral glucose consumption ratio was increased by 47% +/- 8% compared with light alpha-chloralose-anesthetized rats. We conclude that the thiopental-induced isoelectrical condition in rat cortex significantly affected glucose contents by depressing brain metabolism, which remained substantial at isoelectricity.

Regional differences in the coupling between resting cerebral blood flow and metabolism may indicate action preparedness as a default state

Although most functional neuroimaging studies examine task effects, interest intensifies in the "default" resting brain. Resting conditions show consistent regional activity, yet oxygen extraction fraction constancy across regions. We compared resting cerebral metabolic rates of glucose (CMRgl) measured with 18F-labeled 2-fluoro-2-deoxy-D-glucose to cerebral blood flow (CBF) 15O-H2O measures, using the same positron emission tomography scanner in 2 samples (n = 60 and 30) of healthy right-handed adults. Region to whole-brain ratios were calculated for 35 standard regions of interest, and compared between CBF and CMRgl to determine perfusion relative to metabolism. Primary visual and auditory areas showed coupling between CBF and CMRgl, limbic and subcortical regions--basal ganglia, thalamus and posterior fossa structures--were hyperperfused, whereas association cortices were hypoperfused. Hyperperfusion was higher in left than right hemisphere for most cortical and subcallosal limbic regions, but symmetric in cingulate, basal ganglia and somatomotor regions. Hyperperfused regions are perhaps those where activation is anticipated at short notice, whereas downstream cortical modulatory regions have longer "lead times" for deployment. The novel observation of systematic uncoupling of CBF and CMRgl may help elucidate the potential biological significance of the "default" resting state. Whether greater left hemispheric hyperperfusion reflects lateral dominance needs further examination.

Cellular pathways of energy metabolism in the brain: Is glucose used by neurons or astrocytes?

Most techniques presently available to measure cerebral activity in humans and animals, i.e. positron emission tomography (PET), autoradiography, and functional magnetic resonance imaging, do not record the activity of neurons directly. Furthermore, they do not allow the investigator to discriminate which cell type is using glucose, the predominant fuel provided to the brain by the blood. Here, we review the experimental approaches aimed at determining the percentage of glucose that is taken up by neurons and by astrocytes. This review is integrated in an overview of the current concepts on compartmentation and substrate trafficking between astrocytes and neurons. In the brain in vivo, about half of the glucose leaving the capillaries crosses the extracellular space and directly enters neurons. The other half is taken up by astrocytes. Calculations suggest that neurons consume more energy than do astrocytes, implying that astrocytes transfer an intermediate substrate to neurons. Experimental approaches in vitro on the honeybee drone retina and on the isolated vagus nerve also point to a continuous transfer of intermediate metabolites from glial cells to neurons in these tissues. Solid direct evidence of such transfer in the mammalian brain in vivo is still lacking. PET using [(18)F]fluorodeoxyglucose reflects in part glucose uptake by astrocytes but does not indicate to which step the glucose taken up is metabolized within this cell type. Finally, the sequence of metabolic changes occurring during a transient increase of electrical activity in specific regions of the brain remains to be clarified.

Neurobarrier coupling in the brain: a partner of neurovascular and neurometabolic coupling?

Neurovascular and neurometabolic coupling help the brain to maintain an appropriate energy flow to the neural tissue under conditions of increased neuronal activity. Both coupling phenomena provide us, in addition, with two macroscopically measurable parameters, blood flow and intermediate metabolite fluxes, that are used to dynamically image the functioning brain. The main energy substrate for the brain is glucose, which is metabolized by glycolysis and oxidative breakdown in both astrocytes and neurons. Neuronal activation triggers increased glucose consumption and glucose demand, with new glucose being brought in by stimulated blood flow and glucose transport over the blood-brain barrier. Glucose is shuttled over the barrier by the GLUT-1 transporter, which, like all transporter proteins, has a ceiling above which no further stimulation of the transport is possible. Blood-brain barrier glucose transport is generally accepted as a nonrate-limiting step but to prevent it from becoming rate-limiting under conditions of neuronal activation, it might be necessary for the transport parameters to be adapted to the increased glucose demand. It is proposed that the blood-brain barrier glucose transport parameters are dynamically adapted to the increased glucose needs of the neural tissue after activation according to a neurobarrier coupling scheme. This review presents neurobarrier coupling within the current knowledge on neurovascular and neurometabolic coupling, and considers arguments and evidence in support of this hypothesis.

Localization of the Na+-D-glucose cotransporter SGLT1 in the blood-brain barrier

Immunoreactivity of the Na+-D-glucose cotransporter SGLT1 was demonstrated in intracerebral capillaries of rat and pig. Immunostaining suggested that SGLT1 is located in the luminal membrane of the endothelial cells and in intracellular vesicles. Using in situ hybridization, SGLT1 mRNA was not detectable in intracerebral capillaries of non-treated or sham-operated Wistar rats. However, 1 day after a transient occlusion of the right middle cerebral artery, SGLT1 mRNA was detected in capillaries of both brain hemispheres. Expression of SGLT1 was also demonstrated in primary cultures of capillary endothelial cells from pig using polymerase chain reaction after reverse transcription and western blotting. The data suggest that SGLT1 participates in transport of D-glucose across the blood-brain barrier and is upregulated after brain ischemia and reperfusion.

Causes and consequences of disturbances of cerebral glucose metabolism in sporadic Alzheimer disease: therapeutic implications

Alzheimer disease is not a single disorder. Etiologically, two different types or even diseases exist: inheritance in 5% to 10% of all Alzheimer cases versus 90% to 95% AD cases whith sporadic origin (SAD). Different susceptibility genes along with adult lifestyle risk-factors- in the case of SAD the risk factor aging- may be assumed to cause the latter disorder. There is evidence that a disturbance in the insulin signal transduction pathway may be a central and early pathophysiologic event in SAD. Both, hypercortisolemia and increased adrenergic activity, in both old age and SAD may render the function of the neuronal insulin receptor vulnerable resulting in a diminished production of ATP. The reduced availability of ATP may damage the function of the endoplasmic reticulum/Golgi apparatus/trans Golgi network generating misfolded and malfolded proteins retained in the cell. In SAD, amyloid precursor protein is found to accumulate intracellularly thus not representing the cause but a driving force in the pathogenesis of SAD. Additionally, both disturbed insulin signaling and reduced ATP forward the hyperphosphorylation of tau protein. Thus, abnormalities in oxidative brain metabolism lead to the formation of two main morphologic hallmarks of SAD: senile plaques and neurofibrillary tangles. Therefore, the therapeutic goal in SAD should be the improvement of the neuronal energy state. Findings from both basic and clinical studies showed that Ginkgo biloba extract (EGb 761) may be appropiate to approach that goal.

In vivo measurements of brain glucose transport using the reversible Michaelis-Menten model and simultaneous measurements of cerebral blood flow changes during hypoglycemia

Glucose is the major substrate that sustains normal brain function. When the brain glucose concentration approaches zero, glucose transport across the blood-brain barrier becomes rate limiting for metabolism during, for example, increased metabolic activity and hypoglycemia. Steady-state brain glucose concentrations in alpha-chloralose anesthetized rats were measured noninvasively as a function of plasma glucose. The relation between brain and plasma glucose was linear at 4.5 to 30 mmol/L plasma glucose, which is consistent with the reversible Michaelis-Menten model. When the model was fitted to the brain glucose measurements, the apparent Michaelis-Menten constant, Kt, was 3.3 +/- 1.0 mmol/L, and the ratio of the maximal transport rate relative to CMRglc, Tmax/CMRglc, was 2.7 +/- 0.1. This Kt is comparable to the authors' previous human data, suggesting that glucose transport kinetics in humans and rats are similar. Cerebral blood flow (CBF) was simultaneously assessed and constant above 2 mmol/L plasma glucose at 73 +/- 6 mL 100 g(-1) min(-1). Extrapolation of the reversible Michaelis-Menten model to hypoglycemia correctly predicted the plasma glucose concentration (2.1 +/- 0.6 mmol/L) at which brain glucose concentrations approached zero. At this point, CBF increased sharply by 57% +/- 22%, suggesting that brain glucose concentration is the signal that triggers defense mechanisms aimed at improving glucose delivery to the brain during hypoglycemia.

Co-localization of GLUT1 and GLUT4 in the blood-brain barrier of the rat ventromedial hypothalamus

The ventromedial hypothalamus (VMH) has been proposed to be a glucose sensor within the brain and appears to play a critical role in initiating the counterregulatory response to hypoglycemia. Transport of glucose across the brain capillaries and into neurons in this region is mediated by different isoforms of the sodium-independent glucose transporter gene family. The objective of the present study was to identify the specific glucose transporter isoforms present, as well as their cellular localization, within the VMH. Immunohistochemistry was performed for GLUT1, GLUT2 and GLUT4 in frozen sections of hypothalami from normal rats. GLUT1 was present on the endothelial cells of the blood-brain barrier (BBB) of the VMH. GLUT2 immunoreactivity was seen in the ependymal cells of the third ventricle and in scattered cells in the arcuate and periventricular nuclei. There was no GLUT2 expression in the VMH. The insulin-sensitive GLUT4 isoform was localized to vascular structures within the VMH. Double-labeled immunohistochemistry demonstrated co-localization of GLUT4 with GLUT1 and with the tight junction protein ZO-1 in the VMH and suggested that VMH GLUT4 expression was restricted to the BBB. The role of GLUT4 in the brain and within the VMH is unknown, but given its location on the BBB, it may participate in brain sensing of blood glucose concentrations.

Differentiation of glucose transport in human brain gray and white matter

Localized 1H nuclear magnetic resonance spectroscopy has been applied to determine human brain gray matter and white matter glucose transport kinetics by measuring the steady-state glucose concentration under normoglycemia and two levels of hyperglycemia. Nuclear magnetic resonance spectroscopic measurements were simultaneously performed on three 12-mL volumes, containing predominantly gray or white matter. The exact volume compositions were determined from quantitative T1 relaxation magnetic resonance images. The absolute brain glucose concentration as a function of the plasma glucose level was fitted with two kinetic transport models, based on standard (irreversible) or reversible Michaelis-Menten kinetics. The steady-state brain glucose levels were similar for cerebral gray and white matter, although the white matter levels were consistently 15% to 20% higher. The ratio of the maximum glucose transport rate, V(max), to the cerebral metabolic utilization rate of glucose, CMR(Glc), was 3.2 +/- 0.10 and 3.9 +/- 0.15 for gray matter and white matter using the standard transport model and 1.8 +/- 0.10 and 2.2 +/- 0.12 for gray matter and white matter using the reversible transport model. The Michaelis-Menten constant K(m) was 6.2 +/- 0.85 and 7.3 +/- 1.1 mmol/L for gray matter and white matter in the standard model and 1.1 +/- 0.66 and 1.7 +/- 0.88 mmol/L in the reversible model. Taking into account the threefold lower rate of CMR(Glc) in white matter, this finding suggests that blood--brain barrier glucose transport activity is lower by a similar amount in white matter. The regulation of glucose transport activity at the blood--brain barrier may be an important mechanism for maintaining glucose homeostasis throughout the cerebral cortex.

Maturation-dependent neurotoxicity of 3-hydroxyglutaric and glutaric acids in vitro: a new pathophysiologic approach to glutaryl-CoA dehydrogenase deficiency

Glutaryl-CoA dehydrogenase deficiency is a neurometabolic disorder with a specific age- and region-dependent neuropathology. Between 6 and 18 mo of age, unspecific illnesses trigger acute encephalopathic crises resulting in acute striatal and cortical necrosis. We hypothesized that acute brain damage in glutaryl-CoA dehydrogenase deficiency is caused by the main pathologic metabolites 3-hydroxyglutaric and glutaric acids through an excitotoxic sequence. Therefore, we investigated the effect of 3-hydroxyglutaric acid and glutaric acid on primary neuronal cultures from chick embryo telencephalons and mixed neuronal and glial cell cultures from neonatal rat hippocampi. Exposure to glutaric acid and 3-hydroxyglutaric acid decreased cell viability in a concentration- and time-dependent fashion. This neurotoxic effect could be totally prevented by preincubation with an N-methyl-D-aspartate receptor subunit 2B (NR2B)-specific antagonist, NR2B antibodies, and an unspecific N-methyl-D-aspartate receptor blocker and was partially blocked with an NR2A-specific antagonist but not with NR2A antibodies or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor and metabotropic glutamate receptor antagonists. Furthermore, metabolite toxicity increased in parallel with the increasing expression of the NR2B subunit on cultured neurons from second to sixth day in vitro. We conclude from these results that 3-hydroxyglutaric acid and glutaric acid act as false neurotransmitters, in particular through NR1/2B, and that the extent of induced neurotoxicity is dependent on the temporal and spatial expression of NR1/2B in the CNS during maturation. Beyond favorable implications for treatment and long-term prognosis, glutaryl-CoA dehydrogenase deficiency is the first neurologic disease in which specific neuropathology could be experimentally linked to ontogenetic expression of a particular neurotransmitter receptor subtype.

Measurement of the tricarboxylic acid cycle rate in human grey and white matter in vivo by 1H-[13C] magnetic resonance spectroscopy at 4.1T

13C isotopic labeling data were obtained by 1H-observed/13C-edited magnetic resonance spectroscopy in the human brain in vivo and analyzed using a mathematical model to determine metabolic rates in human grey matter and white matter. 22.5-cc and 56-cc voxels were examined for grey matter and white matter, respectively. When partial volume effects were ignored, the measured tricarboxylic acid cycle rate was 0.72+/-0.22 (mean +/- SD) and 0.29+/-0.09 micromol min(-1) g(-1) (mean +/- SD) in voxels of approximately 70% grey and approximately 70% white matter, respectively. After correction for partial volume effects using a model with two tissue compartments, the tricarboxylic acid cycle rate in pure grey matter was higher (0.80+/-0.10 mol min(-1) g(-1); mean +/- SD) and in white matter was significantly lower (0.17+/-0.01 micromol min(-1) g(-1); mean +/- SD). In 1H-observed/13C-edited magnetic resonance spectroscopy labeling studies, the larger concentrations of labeled metabolites and faster metabolic rates in grey matter biased the measurements heavily toward grey matter, with labeling time courses in 70% grey matter appearing nearly identical to labeling in pure grey matter.

In vivo upregulation of the blood-brain barrier GLUT1 glucose transporter by brain-derived peptides

Glucose is the critical metabolic fluid for the brain, and the transport of this nutrient from blood to brain is limited by the blood-brain barrier (BBB) GLUT1 glucose transporter. The expression of the BBB-GLUT1 gene is augmented in brain endothelial cultured cells incubated with brain-derived trophic factors and the brain-derived peptide preparation Cerebrolysin (C1, EBEWE, Austria). The aim of the present investigation was to determine if C1 induces similar changes in the expression of the BBB-GLUT1 gene following its administration to rats in vivo. The BBB glucose transporter activity was investigated with the intracarotid artery perfusion technique using [3H]diazepam as cerebral blood flow marker. The acute or chronic administration of C1 markedly increased the brain permeability surface area of D-[14C]glucose compared to controls (D-[14C]glucose/[3H]diazepam ratio, 1.6- to 1.9-fold increase in frontal cortex, P < 0.05). Increased activity of the BBB glucose transporter was correlated with a significant rise in the abundance of the BBB-GLUT1 protein measured by both Western blot analysis and immunocytochemistry, and with a decrease in the transcript levels of this transporter. Data presented here demonstrate that the in vivo administration of Cl increases the transport of glucose from blood to brain via BBB-GLUT1 gene expression.

Quantitative immunocytochemical study of blood-brain barrier glucose transporter (GLUT-1) in four regions of mouse brain

Application of immunogold cytochemistry revealed polar (asymmetric) distribution of GLUT-1 in mouse brain microvascular endothelia, representing the anatomic site of the blood-brain barrier (BBB). This polarity was manifested by an approximately threefold higher immunolabeling density of the abluminal than the luminal plasma membrane of the endothelial cells. The immunoreaction for GLUT-1 in nonbarrier continuous (skeletal muscle) or fenestrated (brain circumventricular organs) microvascular endothelial cells was absent. In the choroid plexus, the basolateral plasmalemma of the epithelial cells was labeled more intensely than the vascular fenestrated endothelium. Addition of morphometry to the applied immunogold technique makes it possible for even subtle differences to be revealed in the density of immunolabeling for GLUT-1 in blood microvessels located in four brain regions. We found that the density of immunosignals in the microvessels supplying the cerebral cortex, hippocampus, and cerebellum was essentially similar, whereas in the olfactory bulb it was significantly lower. Asymmetric distribution of GLUT-1 in the endothelial plasma membranes presumably leads to a reduced concentration of glucose molecules in the endothelial cells compared to blood plasma and also secures their more rapid transport across the abluminal plasmalemma to the brain parenchyma.

Cognitive performance and biochemical markers in septum, hippocampus and striatum of rats after an i.c.v. injection of streptozotocin: a correlation analysis

In the present study we evaluated the effects of an intracerebroventricular injection of streptozotocin on cognitive behavior and biochemical markers in the brain of middle-aged Wistar rats. Intracerebroventricular injected streptozotocin has previously been reported to decrease the central metabolism of glucose. We found that streptozotocin-treated rats showed an impaired cognitive performance in the delayed non-matching to position task and the Morris water escape task. Glial fibrillary acidic protein, an indicator of reactive astroglial changes, was measured in three different (soluble, Triton X-100 soluble and crude cytoskeletal) protein fractions and its content in the fractions of the septum, hippocampus and striatum of streptozotocin-treated rats was increased. Furthermore, the glial fibrillary acidic protein response of each protein fraction to streptozotocin treatment appeared to be differently regulated. In streptozotocin-treated rats the choline acetyltransferase activity was decreased in the hippocampus only, which was correlated with the hippocampal glial fibrillary acidic protein contents of all three hippocampal protein fractions, thus suggesting that the cholinergic deficit is a consequence of direct damage to the hippocampus. The cognitive deficits in both tasks were related to the increased glial fibrillary acidic protein contents, especially of the soluble and cytoskeletal fraction, and the decreased choline acetyltransferase activity in the hippocampus. Taken together, these findings indicate that it is important to take into account which protein fraction has been used for measuring the glial fibrillary acidic protein response to a stressor. Furthermore, intracerebroventricular injected streptozotocin may provide a relevant model for studying neurodegenerative changes due to a metabolic insufficiency and testing neuroprotective effects of substances.

Glucose transport in brain and retina: implications in the management and complications of diabetes

Neural tissue is entirely dependent on glucose for normal metabolic activity. Since glucose stores in the brain and retina are negligible compared to glucose demand, metabolism in these tissues is dependent upon adequate glucose delivery from the systemic circulation. In the brain, the critical interface for glucose transport is at the brain capillary endothelial cells which comprise the blood-brain barrier (BBB). In the retina, transport occurs across the retinal capillary endothelial cells of the inner blood-retinal barrier (BRB) and the retinal pigment epithelium of the outer BRB. Because glucose transport across these barriers is mediated exclusively by the sodium-independent glucose transporter GLUT1, changes in endothelial glucose transport and GLUT1 abundance in the barriers of the brain and retina may have profound consequences on glucose delivery to these tissues and major implications in the development of two major diabetic complications, namely insulin-induced hypoglycemia and diabetic retinopathy. This review discusses the regulation of brain and retinal glucose transport and glucose transporter expression and considers the role of changes in glucose transporter expression in the development of two of the most devastating complications of long-standing diabetes mellitus and its management.

Altered brain glucose metabolism in transgenic-PFKL mice with elevated L-phosphofructokinase: in vivo NMR studies

The gene for the liver-type subunit of phosphofructokinase (PFKL) resides on chromosome 21 and is overexpressed in Down syndrome (DS) patients. Transgenic PFKL (Tg-PFKL) mice with elevated levels of PFKL were used to determine whether, as in DS, overexpression of PFKL was also associated with altered sugar metabolism. We found that Tg-PFKL mice had an abnormal glucose metabolism with reduced clearance rate from blood and enhanced metabolic rate in brain. Transgenic-PFKL mice exhibited elevated activity of phosphofructokinase in both blood and brain, as compared to control non-transgenic (ntg) mice. Following glucose infusion, the rate of glucose clearance from the blood of Tg-PFKL mice was significantly slower than that of control ntg mice, although the basal blood glucose levels were similar. However, unlike the slower rate of glucose metabolism in blood, the initial rate of glucose utilization in the brain of the transgenic mice, was 58% faster than in control ntg mice. This was determined by infusion of [1-13C]-glucose followed by in vivo nuclear magnetic resonance (NMR) measurements of brain glucose metabolism. The faster utilization of glucose in Tg-PFKL brain is similar to the increased rate of cerebral glucose metabolism found in the brain of young adult DS patients, which may play a role in the etiology of their cognitive disabilities.

Chronic administration of propionic acid reduces ganglioside N-acetylneuraminic acid concentration in cerebellum of young rats

Elevated levels of propionate comparable to those of human propionic acidaemia were achieved in the blood of young rats by injecting subcutaneously buffered propionic acid (PPA) twice a day at 8-h intervals from the 6th to the 28th day of life. A matched group of animals (controls) was treated with the same volumes of saline. The animals were weighed and sacrificed by decapitation at 28, 35 or 60 days of age. Cerebellum and cerebrum were weighed and their protein and ganglioside N-acetylneuraminic acid (G-NeuAc) contents determined. Body, cerebral and cerebellar weights were similar in both groups, suggesting that PPA per se neither alters the appetite of the rats nor causes malnutrition. Brain protein concentration was also not affected by chronic administration of PPA, in contrast to G-NeuAc concentration which was significantly reduced in the cerebellum. Since ganglioside concentration is closely related to the dendritic surface and indirectly reflects synaptogenesis, our results of an important ganglioside deficit in the brain of PPA-treated animals may be related to the neurologic dysfunction characteristic of propionic acidaemic patients.

Dynamic [18F]fluorodeoxyglucose positron emission tomography and hypometabolic zones in seizures: reduced capillary influx

We performed dynamic [18F]fluorodeoxyglucose ([18F]FDG) positron emission tomographic (PET) analyses in 8 patients. Rate constants of influx (K1*), efflux (k2*), phosphorylation (k3*), and dephosphorylation (k4*) were derived for the regions of interest (ROIs), which included (1) the hypometabolic epileptogenic regions and (2) the homologous regions in the contralateral hemispheres. In addition, the four constants were determined from at least one clearly defined (control) ROI from the same plane and its homologous contralateral ROI. Influx (K1*) in the epileptogenic region was reduced in comparison with the contralateral ROI. Reductions in influx (K1*), which averaged 18 +/- 13% (mean +/- SD), [18F]FDG phosphorylation (k3*) (25 +/- 20%), and brain glucose utilization rates (26 +/- 10%) were observed in the epileptogenic region. Reductions in efflux were not statistically significant (k2* = 13 +/- 28%) but were comparable in magnitude to the average reduction in K1*. No ipsilateral versus contralateral differences were seen for any rate constants measured outside the epileptogenic region. Influx correlated highly with phosphorylation in the epileptogenic region. Our data suggest that the hypometabolic epileptogenic focus seen in [18F]FDG-PET studies is also a region of reduced blood-brain barrier glucose transporter activity and that reductions in phosphorylation are proportional to reductions in [18]FDG influx.

Neurological dysfunction in methylmalonic acidaemia is probably related to the inhibitory effect of methylmalonate on brain energy production

Methylmalonic acidaemia is an inherited metabolic disorder caused by a severe deficiency of the activity of the enzyme L-methylmalonyl-CoA mutase or its cofactor 5'-deoxyadenosylcobalamin, resulting in tissue accumulation of large quantities of methylmalonic acid. Among the various clinical features, neurological symptoms are frequently observed. Patients may present cerebral atrophy and basal ganglia abnormalities are common. In the present report, we update the current knowledge on the influence of methylmalonic acid on brain metabolism in the hope of better understanding the neurological dysfunction characteristic of methylmalonic acidaemia. We present evidence showing that the metabolite inhibits brain energy production by various mechanisms and propose that a fall in cellular ATP generation leading to excitotoxicity is crucial for the occurrence of the neurological damage observed in these patients.

Effect of streptozotocin-induced maternal diabetes on fetal rat brain glucose transporters

Glucose, an essential substrate for brain oxidative metabolism, is transported across the blood-brain barrier and into neuronal and glial cells via Glut 1 and Glut 3 facilitative glucose transporter isoforms. To examine the effect of excessive circulating glucose on fetal brain glucose transporter expression, we investigated the effect of streptozotocin-induced maternal diabetes (SEVERE-D; n = 29) on the 20-d gestation fetal rat brain Glut 1 and Glut 3. We studied the effect of streptozotocin alone (STZ-ND; n = 12) in a nondiabetic state as well, along with vehicle injected controls (C; n = 24). In the presence of fetal hyperglycemia (12.63 +/- 0.82 nM-SEVERE-D versus 2.35 +/- 0.28-STZ-ND and 2.42 +/- 0.16-C; p < 0.001) and hypoinsulinemia (0.38 +/- 0.03 nM-SEVERE-D versus 0.50 +/- 0.07-STZ-ND and 0.55 +/- 0.06-C; p < 0.02), no detectable change in fetal brain Glut 1 and Glut 3 pretranslational expression (transcription/elongation rates and corresponding steady state mRNA levels) was noted when simultaneously compared with the STZ-ND and C groups. In contrast, a trend toward a decline in Glut 1 (approximately 25 to 30%, p = 0.05) and a substantive decrease in Glut 3 (approximately 35 to 50%, p = 0.0006) protein concentrations was present in both the STZ-ND and SEVERE-D groups when compared with the C group. These observations support a chemical effect of streptozotocin independent of maternal diabetes upon the translation or posttranslational processing of fetal brain glucose transporters. Maternal diabetes with fetal hyperglycemia, however, failed to substantively alter fetal brain glucose transporters independent of the streptozotocin effects upon neuroectodermally derived tissues. We conclude that maternal diabetes with associated overt fetal hyperglycemia does not significantly change fetal brain glucose transporter levels.

The supply of glucose to the brain and cognitive functioning

Unlike other organs the energy requirement of the brain is met almost exclusively by aerobic glucose degradation (Siesjo, 1978). The energy requirement of the brain is 20–30% of the whole organism at rest, although its weight is only 2%. The energy stores in the brain are extremely small when compared with the high rate of glucose utilisation: thus the brain is reliant on a continuous glucose supply. Only about 30% of glucose is required for direct energy production; much of the remainder is used for the synthesis of amino acids, peptides, lipids and nucleic acids (Siebert, Gessner & Klasser, 1986). Thus a source of glucose is essential for the synthesis of physiologically active amines such as serotonin, noradrenaline and acetylcholine. Although it is well accepted that hypoglycaemia can result in the disruption of cognitive functioning, this is a rare phenomenon and it has usually been assumed that levels of blood glucose, within the normal range, do not influence intellectual functioning. This assumption is discussed in this paper.

Metabolismo da glicose cerebral no trauma crânio-encefálico: uma avaliação

Os autores apresentam revisão geral da distribuição e metabolização da glicose, com ênfase para os distúrbios que ocorrem no trauma crânio-encefálico, como a hiperglicemia que ocorre na fase aguda. Finalizando, são feitos comentários sobre as possíveis conseqüências desses conhecimentos sobre os procedimentos atuais, que aconselham a restrição na oferta de glicose a pacientes com catabolismo acentuado e que necessitam poupar o contingente de proteína corporal.

Regional brain glutamate transport in rats at normal and raised concentrations of circulating glutamate

The permeability of the blood-brain barrier to glutamate was measured by quantitative autoradiography in brains of control rats (average plasma glutamate concentration of 95 microns) and rats infused with glutamate (average plasma glutamate concentration of 837 microns). Measurements of glutamate permeability were initiated by the injection of [14C]glutamate and stopped at 1 min to avoid the accumulation of [14C]glutamate metabolites. Glutamate entered the brain at a slow rate, with an average permeability-surface area product of 7 microliters.min-g-1, except in those areas known to have fenestrated capillaries. Glutamate accumulated in the choroid plexus of ventricles, but did not seem to enter the cerebrospinal fluid in detectable amounts regardless of the circulating concentration. Glutamate accumulated in circumventricular organs, such as the median eminence, where the radioactivity was localized without detectable spread. Infusion of glutamate to create high plasma concentrations did not result in greater spread of [14C]glutamate beyond the immediate vicinity of the circumventricular organs.

Intracerebroventricular injection of streptozotocin induces discrete local changes in cerebral glucose utilization in rats

The purpose of the present study was to investigate whether or not cerebral glucose utilization is changed locally after damage of the neuronal insulin receptor by means of intracerebroventricular (icv) streptozotocin (STZ) administered in a subdiabetogenic dosage (1.5 mg/kg bw.). STZ was administered at the start of the study, and 2 and 21 days later bilaterally into the cerebral ventricles in rats of a mean age of 18 months. The local distribution of cerebral glucose utilization was analyzed in conscious rats on the 42nd day after the first STZ injection using the quantitative (14C)-2-deoxyglucose method. Of the 35 brain structures investigated from autoradiograms of brain sections, 17 showed a reduction in glucose utilization. Decreases in glucose utilization were observed in the frontal, parietal, sensory motor, auditory and entorhinal cortex and in all hippocampal subfields. In contrast, glucose utilization was increased in two white matter structures. The decrease in cerebral glucose utilization observed in cortical and hippocampal areas in the present study may correspond to changes in morphobiological parameters which have been found in patients with Alzheimer's disease. The present data are in accordance with the hypothesis that an impairment in the control of neuronal glucose metabolism at the insulin receptor site may exist in sporadic dementia of Alzheimer type (DAT), and can be studied by the icv STZ animal model.

Developmental modulation of blood-brain-barrier glucose transport in the rabbit

Blood-brain barrier (BBB) glucose transport rates were measured using the intracarotid injection method in newborn, 14-day-old suckling, 28-day-old weanling and adult rabbits, and compared with membrane transporter density. Light microscope immunochemistry confirmed the presence of the GLUT1 glucose transporter isoform in these rabbits. Quantitative electron microscopic immunogold analyses of GLUT1-immunoreactive sites per micrometer of capillary membrane indicated GLUT1 density increased with age, and correlated with in vivo measurements of Vmax. Maximal transport velocities (Vmax) of glucose transfer (an indicator of the activity and relative number of transporter proteins) increased significantly (P = 0.05) with age: in neonates Vmax = 0.61 mumol.min-1.g-1, in sucklings Vmax = 0.68 mumol.min-1.g-1, in weanlings Vmax = 0.88 mumol.min-1.g-1, and in adults Vmax = 1.01 mumol.min-1 g-1. Cerebral blood flow (CBF) rates, increased with age from 0.19 and 0.26 ml.min-1.g-1 in neonates and sucklings to 0.51 (weanlings) and 0.70 (adults) ml.min-1.g-1. Non-linear regression analyses indicated the half-saturation constant (Km) for glucose transport ranged from 13 mM in adult rabbits to 19 mM in 14-day-old sucklings: differences in Km were not significant. Age-related changes in the Permeability-Surface Area product (PS +/- S.E.) of both water and glucose were also seen. At all ages studied, the diffusion component (Kd) of glucose uptake was not distinguishable from zero. We conclude developmental up-regulation of the rabbit BBB glucose transporter is characterized by no changes in transporter affinity, and provide the first demonstration of increased membrane transporter proteins correlating with an age-related increase (65%) in glucose transporter maximal velocity.

Optimizing the measurement of regional cerebral glucose consumption with [6-14C]glucose

[6-14C]Glucose is used to trace the cerebral metabolic rate of glucose (CMRGlc) in vivo in experiments lasting 5-10 min. Initially 14C is trapped in intermediary metabolite pools. Subsequently 14C is lost as a function of time and metabolic rate, primarily as 14CO2. Experiments were designed to evaluate the rate of 14C lost as 14CO2 or as [14C]lactate from brain labeled with [6-14C]glucose during times up to 15 min. CMRGlc was measured during 5, 7.5, 10 and 15 min in 60 brain areas. At longer times the loss of 14C was reflected by lower apparent values of brain CMRGlc. Arteriovenous measurements across brain revealed no significant loss of [14C]lactate in normal rats or rats with bicuculline-induced seizures. It was concluded that the primary form in which 14C was lost was as 14CO2. As expected, the rate of 14CO2 loss was greater in structures with high metabolic rates. The data were analyzed to determine the parameters necessary to rectify the data so that uniform values of CMRGlc were obtained up to 15 min. Tables were made to predict the degree of 14C loss as well as the 14C-metabolites/[6-14C]glucose ratio as a function of time and metabolic rate. These tables can be used to plan the maximum and minimum experimental times for optimal results.

Regional cerebral blood flow in IDDM patients: effects of diabetes and of recurrent severe hypoglycaemia

Chronic hyperglycaemia and recurrent severe hypoglycaemia have both been implicated as causing cerebral damage in patients with diabetes. Although cognitive dysfunction and intellectual impairment have been demonstrated in patients with recurrent severe hypoglycaemia, structural correlates have not been described, and it is not known whether specific functional changes occur in the brains of affected patients. Regional cerebral blood flow was estimated by SPECT with 99mTechnetium Exametazime in 20 patients with IDDM. Ten patients had never experienced severe hypoglycaemia and 10 had a history of recurrent severe hypoglycaemia. Patient results were compared with 20 age- and sex-matched healthy volunteers. We observed differences between the two patient groups and the control group. Tracer uptake was greater in diabetic patients in the superior pre-frontal cortex. This effect was particularly pronounced in the group who had a history of previous severe hypoglycaemia. Patients with a history of recurrent hypoglycaemia also had a relative reduction in tracer uptake to the calcarine cortex. This suggests an alteration in the pattern of baseline regional cerebral blood flow in diabetic patients with frontal excess and relative posterior reduction in cerebral blood flow.

Brain glucose levels in portacaval-shunted rats with chronic, moderate hyperammonemia: implications for determination of local cerebral glucose utilization

Rates of glucose utilization (lCMRglc) in many structures of the brain of fed, portacaval-shunted rats, when assayed with the [14C]deoxyglucose (DG) method in our laboratory, were previously found to be unchanged (30 of 36 structures) or depressed (6 structures) during the first 4 weeks after shunting, but to rise progressively to higher than normal values in 25 of 36 structures from 4-12 weeks. In contrast, lCMRglc, when assayed with the [14C]glucose method in another laboratory, was depressed in most structures of brains of 4-8-week shunted rats that had relatively high brain ammonia levels. There was a possibility that the increases in lCMRglc obtained with the [14C]DG method may have been artifactual, due, in part, to a change in brain glucose content which could alter the value of the lumped constant of the DG method. Brain glucose levels of shunted rats were, therefore, assayed by both direct chemical measurement in freeze-blown samples and by determination of steady-state brain:plasma distribution ratios for [14C]methylglucose; the methylglucose distribution ratio varies as a function of plasma and tissue glucose contents. Within a week after shunting, ammonia levels in blood and brain rose to 0.25-0.30 mM and 0.35-0.70 mumol/g, respectively, and mean plasma glucose levels fell from 9-10 mM to 7.4-8.5 mM, and then remained nearly constant. Brains of fed-shunted rats had normal glycogen levels and stable but moderately reduced glucose contents between 1 and 12 weeks (i.e., 1.9-2.2 mumol/g). [14C]Methylglucose distribution ratios were essentially the same as those in controls in 22 brain structures at 2 and 8 weeks after shunting. Because brain glucose levels remained stable from 1 to 12 weeks after shunting, there is no evidence to support the hypothesis that the value of the lumped constant would have changed and caused an artifactual rise in lCMRglc.

Blood-brain glucose transfer in the mouse

The intracarotid injection method has been utilized to examine blood-brain barrier (BBB) glucose transport in normal mice, and after a 2-day fast. In anesthetized mice, cerebral blood flow (CBF) rates were reduced from 0.86 ml.min-1 x gm-1 in control to 0.80 ml.min-1 x gm-1 in fasted animals (p > 0.05). Brain Uptake Indices were significantly (p < 0.05) higher in fasted (plasma glucose = 4.7 mM) than control (plasma glucose = 6.5 mM) mice, while plasma glucose was significantly lower. The maximal velocity (Vmax) for glucose transport was 1562 +/- 303 nmoles.min-1 x g-1, and the half-saturation constant (Km =) 6.67 +/- 1.46 mM in normally fed mice. In fasted mice the Vmax was 2053 +/- 393 nmoles.min-1 x g-1 (p > 0.05), and the half-saturation constant (Km =) 7.40 +/- 1.60 mM (not significant, P > 0.05). A rabbit polyclonal antiserum to a synthetic peptide encoding the 13 C-terminal amino acids of the human erythrocyte glucose transporter (GLUT-1) immunocytochemically confirmed that the mouse brain capillary endothelial glucose transporter is a GLUT-1 transporter, and immunoreactivity was similar in brain endothelia from fed and fasted animals. In conclusion, after a 2-day fast in the mouse, we saw significant reductions in forebrain weight (7%), and plasma glucose levels (27%). Increased brain glucose extraction (25%, p < 0.05), and a 22% increase in the unsaturated permeability-surface area product (p < 0.05) was also observed.

Effect of starvation on glycogen and glucose metabolism in different areas of the rat brain

We have studied the changes in concentration of glycogen, glucose and the bisphosphorylated sugars, glucose 1,6-P2 and fructose 2,6-P2, in several rat brain regions during 72 h of starvation. The animals were killed by focused microwave irradiation. The activities of glycogen metabolizing enzymes in the different areas were measured. A large decrease in glycogen and glucose concentration was observed in all areas. The concentrations of bisphosphorylated sugars changed, suggesting that an increase in glycolysis could take place at the beginning of starvation, with blood glucose as a major energy source. Differences in metabolite concentration before starvation disappeared after 72 h. The activities of glycogen synthase, glycogen phosphorylase and glycogen phosphorylase kinase were similar in all areas, and they did not change during starvation.

Anterograde and retrograde enhancement of 24-h memory by glucose in elderly humans

The present experiment examined anterograde and retrograde enhancement of memory storage by glucose in elderly humans. Glucose (50 g) or saccharin was administered shortly before or immediately after acquisition of a narrative prose passage. Recall was tested 24 h later. Glucose administration before or after presentation of the material to be learned significantly improved recall 24 h later compared to performance in the saccharin condition. These findings suggest that glucose retroactively enhances memory storage processing in elderly humans and that the enhancement of memory outlasts the transient elevations in blood glucose levels after glucose ingestion.

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.

Amino acids and glucose in human cerebrospinal fluid after acute ischaemic brain damage

In the search for potential biochemical markers of value for prognosis after acute hypoxic brain damage, amino acids and glucose were assessed in the cerebrospinal fluid (CSF) and glucose in blood. Samples were taken by lumbar puncture 4, 28, 76 and 172 h after resuscitation from 20 patients and once from 10 control patients. Eight of the resuscitated patients recovered neurologically but 12 remained comatose. The concentrations of alanine (P less than 0.001) and phenylalanine (P less than 0.035) differed most in 4-h samples between the groups. The concentration of alanine was higher in all patient groups with hypoxic brain damage as compared to the controls, the concentrations in patients dying within 76 h (disabled-s group) being higher than in the recovered patients. Phenylalanine in the disabled-s group was significantly higher than the control value. Furthermore, there were significant differences between various patient groups in the concentrations of glutamine, isoleucine, leucine, lysine, serine, tyrosine and valine. When taking into account the permeability of the BBB to these amino acids, alanine, valine and isoleucine most clearly represent brain amino acid metabolism. CSF glucose in the control group and in the recovered patients was lower than in patients dying within 76 h.

The influence of biological and technical factors on the variability of global and regional brain metabolism of 2-[18F]fluoro-2-deoxy-D-glucose

This study investigated the influence of biological and technical factors on variations of global and regional cerebral metabolic rate of glucose (CMRglc) measured with 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG). Twelve male volunteers (22-40 years) were investigated on three or four occasions for a total of 42 studies. We calculated the variance/covariance of the following parameters: CMRglc, six parameters of the blood clearance of [18F]FDG, hour of injection, peak time of blood radioactivity, and six components of the operational equation (nonradioactive blood glucose concentration, brain radioactivity, two integrals, numerator, and denominator). There was correlation among these six components, except for nonradioactive blood glucose. However, the correlation between the CMRglc and the individual components of the operational equation was poor. The inter- and intrapersonal CMRglc coefficients of variations were 13.8 and 7.1%, respectively. In contrast, coefficients of variations of the numerator and denominator of the operational equation were 34.6 and 32.6%, respectively, and were always in the same direction. No correlation was found between CMRglc and the technical factors in the numerator and denominator of the operational equation. Factor analysis disclosed that a single factor was responsible for 70% of the variance. This factor included caudate, putamen, thalamus, frontal cortex, temporal cortex, and cingulate gyrus. These structures are involved with multiple complex functions, from autonomic motor control to behavior and emotions. The intrinsic metabolic variability of these structures, along with the basal metabolic processes that are continuously going on in the brain, may be the best explanation for the variance encountered in our investigation.

Alterations of local cerebral blood flow and glucose uptake by electroconvulsive shock in rats

The effects of single and repeated electroconvulsive shock (ECS) treatment on regional cerebral blood flow (rCBF) and on rates of glucose flow from blood to local brain areas (rCGF), were investigated in pentobarbital-anesthetized rats, using quantitative autoradiographic techniques. Effects of single ECS on rCBF were assessed at two average time points of 15 and 55 sec after the application of the electric current, whereas the effects on rCGF were assessed at 70 and 110 sec. Effects of repeated ECS were assessed 24 hr after the last ECS in a series of eight daily treatments. Single ECS caused marked increases in rCGF in different brain structures, but no significant effects were observed after repeated ECS. Similarly, substantial increases in rCBF were seen during and immediately after the ECS-induced seizure, but not 24 hr after the last treatment of repeated ECS. Single ECS appeared to have differential effects on rCBF in hind-brain structures as compared to more anterior regions. A linear relationship between rCBF and rCGF values was established in control animals, indicating coupling of these two variables with a constant rCBF/rCGF ratio. ECS caused an apparent increase in the CBF/CGF ratio, which might be attributed to the different temporal resolution of the two methods used here to estimate rCGF and rCBF. Analysis of the increments of rCGF and rCBF extrapolated to the same point in time after a single ECS (10 sec), revealed that in many of the examined structures the CBF/CGF ratio was similar to that observed in control animals, indicating that the coupling of CBF and CGF is maintained during the seizure. But in some brain stem structures such as the dorsal raphe, inferior colliculi, superior olivary nucleus, and the vestibular nucleus there were large increases in CGF associated with a marked drop in the CBF/CGF ratio. This observation suggests that high metabolic demands can be met by increased local blood flow up to a given "ceiling" keeping the glucose clearance from blood to brain tissue constant. However, when the metabolic demands exceed this upper limit, the additional demands could be met by an increased clearance of glucose without a change in CBF.

Autoradiographic measurement of cerebral lactate transport rate constants in normal and activated conditions

We used quantitative autoradiography to measure the regional rate constants of blood-to-brain transport of lactate in normal rats and rats treated with kainic acid. Mean cerebral values of lactate transport rate constants were not significantly different between the normal and treated rats, being 0.13 and 0.14 min-1 (ml/g), respectively. Regional values were also generally similar between the groups, but structures that are known to be activated by kainic acid showed increased values in the treated rats compared with rates in the controls. Our measured values of lactate transport rate constants are approximately 50% as great as those published for glucose, indicating that blood-brain transfer of lactate can be significant. This observation supports the hypothesis that radiolabel derived from glucose can leave the brain as radiolabeled lactate in conditions in which intracerebral lactate concentration rises, a hypothesis that has previously been presented to explain differences between rates of accumulation of radiolabel derived from deoxyglucose and glucose in such conditions.

Blood-brain barrier glucose transporter is asymmetrically distributed on brain capillary endothelial lumenal and ablumenal membranes: an electron microscopic immunogold study

It is generally assumed that there is symmetric distribution of the glucose transporter on the lumenal and ablumenal membranes of the brain capillary endothelial cell that makes up the blood-brain barrier (BBB) in vivo. However, the presence of brain endothelial tight junctions allows for asymmetric distribution of BBB plasma membrane proteins. Glucose transporter isoform 1 (GLUT-1), the principal glucose transporter at the BBB, was assessed in rat brain in the present studies using immunogold electron microscopy. The distribution of the immunoreactive GLUT-1 protein on the endothelial lumenal membrane, the ablumenal membrane, and the cytoplasmic compartment was 12%, 48%, and 40%, respectively, and no significant immunolabeling of the neuropil was measurable. These studies suggest (i) that GLUT-1 is asymmetrically distributed on the BBB plasma membrane with an approximately 4-fold greater abundance on the ablumenal membrane as compared to the lumenal membrane; (ii) that approximately 40% of the endothelial glucose transporter protein is contained within the cytoplasmic space, which provides a mechanism for rapid up-regulation of the transporter by altered distribution of transporter between cytoplasmic and plasma membrane compartments; and (iii) that no significant labeling of neuropil is found with antisera directed against the GLUT-1 protein. These studies also suggest mechanisms of regulation of glucose transport from blood to brain that involve differential distribution of the BBB glucose transporter in subcellular compartments of brain capillary endothelial cells.