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

A Potential Neurotoxic Mechanism: Bisphenol S-Induced Inhibition of Glucose Transporter 1 Leads to ATP Excitotoxicity in the Zebrafish Brain

Many environmental pollutants have neurotoxic effects, but the initial molecular events involved in these effects are unclear. Here, zebrafish were exposed to the neurotoxicant bisphenol S (BPS, 1, 10, or 100 μg/L) from the embryonic stage to the larval stage to explore the ability of BPS to interfere with energy metabolism in the brain. BPS, which is similar to a glucose transporter 1 (GLUT1) inhibitor, inhibited GLUT1 function but increased mitochondrial activity in the brains of larval zebrafish. Interestingly, GLUT1 inhibitor treatment and BPS exposure did not reduce energy production in the brain; instead, they increased ATP production by inducing the preferential use of ketone bodies. Moreover, BPS promoted the protein expression of the purinergic 2X receptor but inhibited the purinergic 2Y-mediated phosphatidylinositol signaling pathway, indicating that excess ATP acts as a neurotransmitter to activate the purinergic 2X receptor under the BPS-induced restriction of GLUT1 function. BPS-induced inhibition of GLUT1 increased the number of neurons but promoted apoptosis by activating ATP-purinergic 2X receptors in the brain, causing ATP excitatory neurotoxicity. Our data reveal a potential neurotoxic mechanism induced by BPS that may represent a new adverse outcome pathway.

Unraveling brain palmitic acid: Origin, levels and metabolic fate

In the human brain, palmitic acid (16:0; PAM) comprises nearly half of total brain saturates and has been identified as the third most abundant fatty acid overall. Brain PAM supports the structure of membrane phospholipids, provides energy, and regulates protein stability. Sources underlying the origin of brain PAM are both diet and endogenous synthesis via de novo lipogenesis (DNL), primarily from glucose. However, studies investigating the origin of brain PAM are limited to tracer studies utilizing labelled (14C/11C/3H/2H) PAM, and results vary based on the model and tracer used. Nevertheless, there is evidence PAM is synthesized locally in the brain, in addition to obtained directly from the diet. Herein, we provide an overview of brain PAM origin, entry to the brain, metabolic fate, and factors influencing brain PAM kinetics and levels, the latter in the context of age, as well as neurological diseases and psychiatric disorders. Additionally, we briefly summarize the role of PAM in signaling at the level of the brain. We add to the literature a rudimentary summary on brain PAM metabolism.

β-Hydroxybutyrate and Medium-Chain Fatty Acids are Metabolized by Different Cell Types in Mouse Cerebral Cortex Slices

Ketogenic diets and medium-chain triglycerides are gaining attention as treatment of neurological disorders. Their major metabolites, β-hydroxybutyrate (βHB) and the medium-chain fatty acids (MCFAs) octanoic acid (C8) and decanoic acid (C10), are auxiliary brain fuels. To which extent these fuels compete for metabolism in different brain cell types is unknown. Here, we used acutely isolated mouse cerebral cortical slices to (1) compare metabolism of 200 µM [U-¹³C]C8, [U-¹³C]C10 and [U-¹³C]βHB and (2) assess potential competition between metabolism of βHB and MCFAs by quantifying metabolite ¹³C enrichment using gas chromatography-mass spectrometry (GC-MS) analysis. The ¹³C enrichment in most metabolites was similar with [U-¹³C]C8 and [U-¹³C]C10 as substrates, but several fold lower with [U-¹³C]βHB. The ¹³C enrichment in glutamate was in a similar range for all three substrates, whereas the ¹³C enrichments in citrate and glutamine were markedly higher with both [U-¹³C]C8 and [U-¹³C]C10 compared with [U-¹³C]βHB. As citrate and glutamine are indicators of astrocytic metabolism, the results indicate active MCFA metabolism in astrocytes, while βHB is metabolized in a different cellular compartment. In competition experiments, ¹²C-βHB altered ¹³C incorporation from [U-¹³C]C8 and [U-¹³C]C10 in only a few instances, while ¹²C-C8 and ¹²C-C10 only further decreased the low [U-¹³C]βHB-derived ¹³C incorporation into citrate and glutamine, signifying little competition for oxidative metabolism between βHB and the MCFAs. Overall, the data demonstrate that βHB and MCFAs are supplementary fuels in different cellular compartments in the brain without notable competition. Thus, the use of medium-chain triglycerides in ketogenic diets is likely to be beneficial in conditions with carbon and energy shortages in both astrocytes and neurons, such as GLUT1 deficiency.

Pathophysiology of Pediatric Traumatic Brain Injury

The national incidence of traumatic brain injury (TBI) exceeds that of any other disease in the pediatric population. In the United States the Centers for Disease Control and Prevention (CDC) reports 697,347 annual TBIs in children ages 0–19 that result in emergency room visits, hospitalization or deaths. There is a bimodal distribution within the pediatric TBI population, with peaks in both toddlers and adolescents. Preclinical TBI research provides evidence for age differences in acute pathophysiology that likely contribute to long-term outcome differences between age groups. This review will examine the timecourse of acute pathophysiological processes during cerebral maturation, including calcium accumulation, glucose metabolism and cerebral blood flow. Consequences of pediatric TBI are complicated by the ongoing maturational changes allowing for substantial plasticity and windows of vulnerabilities. This review will also examine the timecourse of later outcomes after mild, repeat mild and more severe TBI to establish developmental windows of susceptibility and altered maturational trajectories. Research progress for pediatric TBI is critically important to reveal age-associated mechanisms and to determine knowledge gaps for future studies.

Exogenous Ketone Bodies as Promising Neuroprotective Agents for Developmental Brain Injury

Ketone bodies are a promising area of neuroprotection research that may be ideally suited to the injured newborn. During normal development, the human infant is in significant ketosis for at least the first week of life. Ketone uptake and metabolism is upregulated in the both the fetus and neonate, with ketone bodies providing at least 10% of cerebral metabolic energy requirements, as well as being the preferred precursors for the synthesis of fatty acids and cholesterol. At the same time, ketone bodies have been shown to have multiple neuroprotective effects, including being anticonvulsant, decreasing oxidative stress and inflammation, and epigenetically upregulating the production of neurotrophic factors. While ketogenic diets and exogenous ketosis are largely being investigated in the setting of adult brain injury, the adaptation of the neonate to ketosis suggests that developmental brain injury may be the area most suited to the use of ketones for neuroprotection. Here, we describe the mechanisms by which ketone bodies exert their neuroprotective effects, and how these may translate to benefits within each of the phases of neonatal asphyxial brain injury.

The metabolic trinity, glucose-glycogen-lactate, links astrocytes and neurons in brain energetics, signaling, memory, and gene expression

Glucose, glycogen, and lactate are traditionally identified with brain energetics, ATP turnover, and pathophysiology. However, recent studies extend their roles to include involvement in astrocytic signaling, memory consolidation, and gene expression. Emerging roles for these brain fuels and a readily-diffusible by-product are linked to differential fluxes in glycolytic and oxidative pathways, astrocytic glycogen dynamics, redox shifts, neuron-astrocyte interactions, and regulation of astrocytic activities by noradrenaline released from the locus coeruleus. Disproportionate utilization of carbohydrate compared with oxygen during brain activation is influenced by catecholamines, but its physiological basis is not understood and its magnitude may be affected by technical aspects of metabolite assays. Memory consolidation and gene expression are impaired by glycogenolysis blockade, and prevention of these deficits by injection of abnormally-high concentrations of lactate was interpreted as a requirement for astrocyte-to-neuron lactate shuttling in memory and gene expression. However, lactate transport was not measured and evidence for presumed shuttling is not compelling. In fact, high levels of lactate used to preserve memory consolidation and induce gene expression are sufficient to shut down neuronal firing via the HCAR1 receptor. In contrast, low lactate levels activate a receptor in locus coeruleus that stimulates noradrenaline release that may activate astrocytes throughout brain. Physiological relevance of exogenous concentrations of lactate used to mimic and evaluate metabolic, molecular, and behavioral effects of lactate requires close correspondence with the normal lactate levels, the biochemical and cellular sources and sinks, and specificity of lactate delivery to target cells.

Cerebral glucose metabolism in an immature rat model of pediatric traumatic brain injury

Altered cerebral metabolism and mitochondrial function have been identified in experimental and clinical studies of pediatric traumatic brain injury (TBI). Metabolic changes detected using (1)H (proton) magnetic resonance spectroscopy correlate with long-term outcomes in children after severe TBI. We previously identified early (4-h) and sustained (24-h and 7-day) abnormalities in brain metabolites after controlled cortical impact (CCI) in immature rats. The current study aimed to identify specific alterations of cerebral glucose metabolism at 24 h after TBI in immature rats. Rats (postnatal days 16-18) underwent CCI to the left parietal cortex. Sham rats underwent craniotomy only. Twenty-four hours after CCI, rats were injected (intraperitoneally) with [1,6-(13)C]glucose. Brains were removed, separated into hemispheres, and frozen. Metabolites were extracted with perchloric acid and analyzed using (1)H and (13)C-nuclear magnetic resonance spectroscopy. TBI resulted in decreases in N-acetylaspartate in both hemispheres, compared to sham contralateral. At 24 h after TBI, there was significant decrease in the incorporation of (13)C label into [3-(13)C]glutamate and [2-(13)C]glutamate in the injured brain. There were no differences in percent enrichment of [3-(13)C]glutamate, [4-(13)C]glutamate, [3-(13)C]glutamine, or [4-(13)C]glutamine. There was significantly lower percent enrichment of [2-(13)C]glutamate in both TBI sides and the sham craniotomy side, compared to sham contralateral. No differences were detected in enrichment of (13)C glucose label in [2-(13)C]glutamine, [2-(13)C]GABA (gamma-aminobutyric acid), [3-(13)C]GABA, or [4-(13)C]GABA, [3-(13)C]lactate, or [3-(13)C]alanine between groups. Results suggest that overall oxidative glucose metabolism in the immature brain recovers at 24 h after TBI. Specific reductions in [2-(13)C]glutamate could be the result of impairments in either neuronal or astrocytic metabolism. Future studies should aim to identify pathways leading to decreased metabolism and develop cell-selective "metabolic rescue."

Monocarboxylate transporters in temporal lobe epilepsy: roles of lactate and ketogenic diet

Epilepsy is a serious neurological disorder that affects approximately 1 % of the general population, making it one of the most common disorders of the central nervous system. Furthermore, up to 40 % of all patients with epilepsy cannot control their seizures with current medications. More efficacious treatments for medication refractory epilepsy are therefore needed. A better understanding of the mechanisms that cause this disorder is likely to facilitate the discovery of such treatments. Impairment in cerebral energy metabolism has been proposed as a possible causative factor in the pathogenesis of temporal lobe epilepsy (TLE), which is one of the most common types of medication-refractory epilepsies in adults. In this review, we will discuss some of the current hypotheses regarding the possible causal relationship between brain energy metabolism and TLE. Emphasis will be placed on the role of energy substrates (lactate and ketone bodies) and their transporter molecules, particularly monocarboxylate transporters 1 and 2 (MCT1 and MCT2). We recently reported that the cellular distribution of MCT1 and MCT2 is perturbed in the hippocampus in patients with TLE. The changes may be an adaptive response aimed at keeping high levels of lactate in the epileptic tissue, which may serve to counteract epileptic activity by downregulating cAMP levels through the lactate receptor GPR81, newly discovered in hippocampus. We propose that the perturbation of MCTs may be further involved in the pathophysiology of TLE by influencing brain energy homeostasis, mitochondrial function, GABA-ergic and glutamatergic neurotransmission, and flux of lactate through the brain.

Cerebral Developmental Abnormalities in a Mouse with Systemic Pyruvate Dehydrogenase Deficiency

Pyruvate dehydrogenase (PDH) complex (PDC) deficiency is an inborn error of pyruvate metabolism causing a variety of neurologic manifestations. Systematic analyses of development of affected brain structures and the cellular processes responsible for their impairment have not been performed due to the lack of an animal model for PDC deficiency. METHODS: In the present study we investigated a murine model of systemic PDC deficiency by interrupting the X-linked Pdha1 gene encoding the α subunit of PDH to study its role on brain development and behavioral studies. RESULTS: Male embryos died prenatally but heterozygous females were born. PDC activity was reduced in the brain and other tissues in female progeny compared to age-matched control females. Immunohistochemical analysis of several brain regions showed that approximately 40% of cells were PDH⁻. The oxidation of glucose to CO₂ and incorporation of glucose-carbon into fatty acids were reduced in brain slices from 15 day-old PDC-deficient females. Histological analyses showed alterations in several structures in white and gray matters in 35 day-old PDC-deficient females. Reduction in total cell number and reduced dendritic arbors in Purkinje neurons were observed in PDC-deficient females. Furthermore, cell proliferation, migration and differentiation into neurons by newly generated cells were reduced in the affected females during pre- and postnatal periods. PDC-deficient mice had normal locomotor activity in a novel environment but displayed decreased startle responses to loud noises and there was evidence of abnormal pre-pulse inhibition of the startle reflex. CONCLUSIONS: The results show that a reduction in glucose metabolism resulting in deficit in energy production and fatty acid biosynthesis impairs cellular differentiation and brain development in PDC-deficient mice.

Fueling and imaging brain activation

Metabolic signals are used for imaging and spectroscopic studies of brain function and disease and to elucidate the cellular basis of neuroenergetics. The major fuel for activated neurons and the models for neuron–astrocyte interactions have been controversial because discordant results are obtained in different experimental systems, some of which do not correspond to adult brain. In rats, the infrastructure to support the high energetic demands of adult brain is acquired during postnatal development and matures after weaning. The brain's capacity to supply and metabolize glucose and oxygen exceeds demand over a wide range of rates, and the hyperaemic response to functional activation is rapid. Oxidative metabolism provides most ATP, but glycolysis is frequently preferentially up-regulated during activation. Underestimation of glucose utilization rates with labelled glucose arises from increased lactate production, lactate diffusion via transporters and astrocytic gap junctions, and lactate release to blood and perivascular drainage. Increased pentose shunt pathway flux also causes label loss from C1 of glucose. Glucose analogues are used to assay cellular activities, but interpretation of results is uncertain due to insufficient characterization of transport and phosphorylation kinetics. Brain activation in subjects with low blood-lactate levels causes a brain-to-blood lactate gradient, with rapid lactate release. In contrast, lactate flooding of brain during physical activity or infusion provides an opportunistic, supplemental fuel. Available evidence indicates that lactate shuttling coupled to its local oxidation during activation is a small fraction of glucose oxidation. Developmental, experimental, and physiological context is critical for interpretation of metabolic studies in terms of theoretical models.

Regulation of oligodendrocyte development and myelination by glucose and lactate

In the gray matter of the brain, astrocytes have been suggested to export lactate (derived from glucose or glycogen) to neurons to power their mitochondria. In the white matter, lactate can support axon function in conditions of energy deprivation, but it is not known whether lactate acts by preserving energy levels in axons or in oligodendrocytes, the myelinating processes of which are damaged rapidly in low energy conditions. Studies of cultured cells suggest that oligodendrocytes are the cell type in the brain that consumes lactate at the highest rate, in part to produce membrane lipids presumably for myelin. Here, we use pH imaging to show that oligodendrocytes in the white matter of the rat cerebellum and corpus callosum take up lactate via monocarboxylate transporters (MCTs), which we identify as MCT1 by confocal immunofluorescence and electron microscopy. Using cultured slices of developing cerebral cortex from mice in which oligodendrocyte lineage cells express GFP (green fluorescent protein) under the control of the Sox10 promoter, we show that a low glucose concentration reduces the number of oligodendrocyte lineage cells and myelination. Myelination is rescued when exogenous l-lactate is supplied. Thus, lactate can support oligodendrocyte development and myelination. In CNS diseases involving energy deprivation at times of myelination or remyelination, such as periventricular leukomalacia leading to cerebral palsy, stroke, and secondary ischemia after spinal cord injury, lactate transporters in oligodendrocytes may play an important role in minimizing the inhibition of myelination that occurs.

ß-Cell-specific pyruvate dehydrogenase deficiency impairs glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) by β-cells requires the generation of ATP from oxidation of pyruvate as well as generation of coupling factors involving three different pyruvate cycling shuttles. The roles of several key enzymes involved in pyruvate cycling in β-cells have been documented using isolated islets and β-cell clonal lines. To investigate the role of the pyruvate dehydrogenase (PDH) complex (PDC) in GSIS, a murine model of β-cell-specific PDH deficiency (β-PDHKO) was created. Pancreatic insulin content was decreased in 1-day-old β-PDHKO male pups and adult male mice. The plasma insulin levels were decreased and blood glucose levels increased in β-PDHKO male mice from neonatal life onward. GSIS was reduced in isolated islets from β-PDHKO male mice with about 50% reduction in PDC activity. Impairment in a glucose tolerance test and in vivo insulin secretion during hyperglycemic clamp was evident in β-PDHKO adults. No change in the number or size of islets was found in pancreata from 4-wk-old β-PDHKO male mice. However, an increase in the mean size of individual β-cells in islets of these mice was observed. These findings show a key role of PDC in GSIS by pyruvate oxidation. This β-PDHKO mouse model represents the first mouse model in which a mitochondrial oxidative enzyme deletion by gene knockout has been employed to demonstrate an altered GSIS by β-cells.

Effects of d-3-hydroxybutyrate treatment on hypoglycemic coma in rat pups

d-3-Hydroxybutyrate (3OHB) is an alternative energy substrate for the brain during hypoglycemia, especially in infancy. Knowledge of the capacity and limits of 3OHB to compensate for cerebral glucose depletion during hypoglycemia in developing brain is important for its potential clinical use, but is scarce. We studied the effect of 3OHB treatment during insulin-induced hypoglycemia in 13-day-old rat pups. 3OHB treatment resulted in increased 3OHB plasma levels in hypoglycemic animals (3-4mM vs. 0.5-1mM untreated), and delayed the onset of clinical coma by 70min and of burst-suppression coma by 90min. 3OHB treated animals did not survive after resuscitation with glucose, compared to 80% survival of untreated hypoglycemic pups. Cleaved-caspase-3 immunohistochemistry and double labeling studies demonstrated a 20-fold increase of apoptotic mature oligodendrocytes in white matter of 3OHB treated animals. 3OHB treatment delays the onset of clinical and burst-suppression coma during hypoglycemia, but the prolonged duration of hypoglycemia is associated with increased mortality after resuscitation and cellular white matter injury.

Exchange-mediated dilution of brain lactate specific activity: implications for the origin of glutamate dilution and the contributions of glutamine dilution and other pathways

The magnitude of metabolic activation is greatly underestimated in autoradiographic studies using [1- or 6-14C]glucose compared to parallel assays with [14C]deoxyglucose indicating that most of the label corresponding to the additional [14C]glucose consumed during activation compared to rest is quickly released from activated structures. Label could be lost by net release of [14C]lactate from brain or via lactate exchange between blood and brain. These possibilities were distinguished by comparison of glucose and lactate specific activities in arterial blood and brain before, during, and after generalized sensory stimulation and during spreading cortical depression. Over a wide range of brain lactate concentrations, lactate specific activity was close to the theoretical maximum, i.e. half that of [6-14C]glucose, indicating that exchange-mediated dilution of lactate is negligible and that efflux of [14C]lactate probably accounts for most of the label loss. Low lactate dilution also indicates that dilution of glutamate C4 fractional enrichment in [13C]glucose studies, currently ascribed predominantly to lactate exchange, arises from other unidentified pathways or factors. Alternative explanations for glutamate dilution (presented in Supporting Information) include poorly labeled amino acid pools and oxidative metabolism of minor substrates in astrocytes to first dilute the astrocytic glutamine pool, followed by dilution of glutamate via glutamate-glutamine cycling.

Imaging brain activation: simple pictures of complex biology

Elucidation of biochemical, physiological, and cellular contributions to metabolic images of brain is important for interpretation of images of brain activation and disease. Discordant brain images obtained with [(14)C]deoxyglucose and [1- or 6-(14)C]glucose were previously ascribed to increased glycolysis and rapid [(14)C]lactate release from tissue, but direct proof of [(14)C]lactate release from activated brain structures is lacking. Analysis of factors contributing to images of focal metabolic activity evoked by monotonic acoustic stimulation of conscious rats reveals that labeled metabolites of [1- or 6-(14)C]glucose are quickly released from activated cells as a result of decarboxylation reactions, spreading via gap junctions, and efflux via lactate transporters. Label release from activated tissue accounts for most of the additional [(14)C]glucose consumed during activation compared to rest. Metabolism of [3,4-(14)C]glucose generates about four times more [(14)C]lactate compared to (14)CO(2) in extracellular fluid, suggesting that most lactate is not locally oxidized. In brain slices, direct assays of lactate uptake from extracellular fluid demonstrate that astrocytes have faster influx and higher transport capacity than neurons. Also, lactate transfer from a single astrocyte to other gap junction-coupled astrocytes exceeds astrocyte-to-neuron lactate shuttling. Astrocytes and neurons have excess capacities for glycolysis, and oxidative metabolism in both cell types rises during sensory stimulation. The energetics of brain activation is quite complex, and the proportion of glucose consumed by astrocytes and neurons, lactate generation by either cell type, and the contributions of both cell types to brain images during brain activation are likely to vary with the stimulus paradigm and activated pathways.

Mathematical modeling of (13)C label incorporation of the TCA cycle: the concept of composite precursor function

A novel approach for the mathematical modeling of (13)C label incorporation into amino acids via the TCA cycle that eliminates the explicit calculation of the labeling of the TCA cycle intermediates is described, resulting in one differential equation per measurable time course of labeled amino acid. The equations demonstrate that both glutamate C4 and C3 labeling depend in a predictable manner on both transmitochondrial exchange rate, V(X), and TCA cycle rate, V(TCA). For example, glutamate C4 labeling alone does not provide any information on either V(X) or V(TCA) but rather a composite "flux". Interestingly, glutamate C3 simultaneously receives label not only from pyruvate C3 but also from glutamate C4, described by composite precursor functions that depend in a probabilistic way on the ratio of V(X) to V(TCA): An initial rate of labeling of glutamate C3 (or C2) being close to zero is indicative of a high V(X)/V(TCA). The derived analytical solution of these equations shows that, when the labeling of the precursor pool pyruvate reaches steady state quickly compared with the turnover rate of the measured amino acids, instantaneous labeling can be assumed for pyruvate. The derived analytical solution has acceptable errors compared with experimental uncertainty, thus obviating precise knowledge on the labeling kinetics of the precursor. In conclusion, a substantial reformulation of the modeling of label flow via the TCA cycle turnover into the amino acids is presented in the current study. This approach allows one to determine metabolic rates by fitting explicit mathematical functions to measured time courses.

Cerebral metabolic adaptation and ketone metabolism after brain injury

The developing central nervous system has the capacity to metabolize ketone bodies. It was once accepted that on weaning, the 'post-weaned/adult' brain was limited solely to glucose metabolism. However, increasing evidence from conditions of inadequate glucose availability or increased energy demands has shown that the adult brain is not static in its fuel options. The objective of this review is to summarize the body of literature specifically regarding cerebral ketone metabolism at different ages, under conditions of starvation and after various pathologic conditions. The evidence presented supports the following findings: (1) there is an inverse relationship between age and the brain's capacity for ketone metabolism that continues well after weaning; (2) neuroprotective potentials of ketone administration have been shown for neurodegenerative conditions, epilepsy, hypoxia/ischemia, and traumatic brain injury; and (3) there is an age-related therapeutic potential for ketone as an alternative substrate. The concept of cerebral metabolic adaptation under various physiologic and pathologic conditions is not new, but it has taken the contribution of numerous studies over many years to break the previously accepted dogma of cerebral metabolism. Our emerging understanding of cerebral metabolism is far more complex than could have been imagined. It is clear that in addition to glucose, other substrates must be considered along with fuel interactions, metabolic challenges, and cerebral maturation.

Biochemical and structural brain alterations in female mice with cerebral pyruvate dehydrogenase deficiency

Pyruvate dehydrogenase complex (PDC) deficiency is an inborn metabolic disorder associated with a variety of neurologic abnormalities. This report describes the development and initial characterization of a novel murine model system in which PDC deficiency has been introduced specifically into the developing nervous system. The absence of liveborn male and a roughly 50% reduction in female offspring following induction of the X-linked mutation indicate that extensive deficiency of PDC in the nervous system leads to pre-natal lethality. Brain tissue from surviving females at post-natal days 15 and 35 was shown to have approximately 75% of wild-type PDC activity, suggesting that a threshold of enzyme activity exists for post-natal survival. Detailed histological analyses of brain tissue revealed structural defects such as disordered neuronal cytoarchitecture and neuropil fibers in grey matter, and reduced size of bundles and disorganization of fibers in white matter. Many of the histologic abnormalities resemble those found in human female patients who carry mutations in the X-linked ortholog. These findings demonstrate a requirement for PDC activity within the nervous system for survival in utero and suggest that impaired pyruvate metabolism in the developing brain can affect neuronal migration, axonal growth and cell-cell interactions.

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

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

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Cerebral glucose metabolism in diabetes mellitus

The brain uses glucose as its primary fuel. Cerebral metabolism of glucose requires transport through the blood-brain barrier, glycolytic conversion to pyruvate, metabolism via the tricarboxylic acid cycle and ultimately oxidation to carbon dioxide and water for full provision of adenosine triphosphate (ATP) and its high-energy equivalents. When deprived of glucose, the brain becomes dysfunctional or can be even permanently damaged. Glucose is stored as glycogen within astrocytes with potential importance for tolerance of hypoglycemia. Glycogen may also be important for the metabolic response to somatosensory stimulation and coupling of blood flow and cellular metabolism. Uncontrolled diabetes has a variety of adverse effects upon brain metabolism and function. Many aspects of function that affect the brain may be indirectly linked to cerebral glucose metabolism. Neurotransmitter metabolism, cerebral blood flow, blood-brain barrier and microvascular function may all be affected to varying degrees by either hypoglycemia or uncontrolled diabetes mellitus.

Nutrition during brain activation: does cell-to-cell lactate shuttling contribute significantly to sweet and sour food for thought?

Functional activation of astrocytic metabolism is believed, according to one hypothesis, to be closely linked to excitatory neurotransmission and to provide lactate as fuel for oxidative metabolism in neighboring neurons. However, review of emerging evidence suggests that the energetic demands of activated astrocytes are higher and more complex than recognized and much of the lactate presumably produced by astrocytes is not locally oxidized during activation. In vivo activation studies in normal subjects reveal that the rise in consumption of blood-borne glucose usually exceeds that of oxygen, especially in retina compared to brain. When the contribution of glycogen, the brain's major energy reserve located in astrocytes, is taken into account the magnitude of the carbohydrate-oxygen utilization mismatch increases further because the magnitude of glycogenolysis greatly exceeds the incremental increase in utilization of blood-borne glucose. Failure of local oxygen consumption to equal that of glucose plus glycogen in vivo is strong evidence against stoichiometric transfer of lactate from astrocytes to neighboring neurons for oxidation. Thus, astrocytes, not nearby neurons, use the glycogen for energy during physiological activation in normal brain. These findings plus apparent compartmentation of metabolism of glycogen and blood-borne glucose during activation lead to our working hypothesis that activated astrocytes have high energy demands in their fine perisynaptic processes (filopodia) that might be met by glycogenolysis and glycolysis coupled to rapid lactate clearance. Tissue culture studies do not consistently support the lactate shuttle hypothesis because key elements of the model, glutamate-induced increases in glucose utilization and lactate release, are not observed in many astrocyte preparations, suggesting differences in their oxidative capacities that have not been included in the model. In vivo nutritional interactions between working neurons and astrocytes are not as simple as implied by "sweet (glucose-glycogen) and sour (lactate) food for thought."

Brain uptake and metabolism of ketone bodies in animal models

As a consequence of the high fat content of maternal milk, the brain metabolism of the suckling rat represents a model of naturally occurring ketosis. During the period of lactation, the rate of uptake and metabolism of the two ketone bodies, beta-hydroxybutyrate and acetoacetate is high. The ketone bodies enter the brain via monocarboxylate transporters whose expression and activity is much higher in the brain of the suckling than the mature rat. beta-Hydroxybutyrate and acetoacetate taken up by the brain are efficiently used as substrates for energy metabolism, and for amino acid and lipid biosynthesis, two pathways that are important for this period of active brain growth. Ketone bodies can represent about 30-70% of the total energy metabolism balance of the immature rat brain. The active metabolism of ketone bodies in the immature brain is related to the high activity of the enzymes of ketone body metabolism. Thus, the use of ketone bodies by the immature rodent brain serves to spare glucose for metabolic pathways that cannot be fulfilled by ketones such as the pentose phosphate pathway mainly. The latter pathway leads to the biosynthesis of ribose mandatory for DNA synthesis and NADPH which is not formed during ketone body metabolism and is a key cofactor in lipid biosynthesis. Finally, ketone bodies by serving mainly biosynthetic purposes spare glucose for the emergence of various functions such as audition, vision as well as more integrated and adapted behaviors whose appearance during brain maturation seems to critically relate upon active glucose supply and specific regional increased use.

Neuron-specific aminopeptidase and puromycin-sensitive aminopeptidase in rat brain development

Neuron-specific aminopeptidase (NAP) and the ubiquitous puromycin-sensitive aminopeptidase (PSA) were compared in the rat hippocampus during early development. Hippocampus contains the highest amount of NAP determined by a fast-protein liquid chromatography-aminopeptidase analyzer using Leu beta-naphthylamide as substrate. Both enzymes were found in the hippocampus in all ages. NAP was lower in immature rat; the 19th embryonic-day fetus contained the least. It increased steeply during the prenatal through the early postnatal period, 9-fold by the first month. The rate of increase diminished subsequently, increasing 20% in the second month and 13% in the third. The age-dependent increase in NAP activity was parallel to its protein expression as determined by Western blot. The specific molecular activity (hydrolytic activity/ NAP antigenicity) in newborn, 15-day-old, and 30-day-old rats were 1.00, 0.88, and 1.00, respectively. The PSA developmental profile without linear increase in activity was distinct from NAP. PSA activity was higher than NAP in decreasing order, 100-4 times, during the same development span. Similarly, different growth profiles for NAP and PSA were also found in the primary culture of developing cerebellar granule cells. Puromycin (1-5 microM) blocked neurite outgrowth and caused apoptosis by nonantibiotic effects. Our data suggest that the synaptosome-enriched NAP plays a role in neuron growth, differentiation, and information programming.

Effect of acute hyperketonemia on the cerebral uptake of ketone bodies in nondiabetic subjects and IDDM patients

Using R-beta-[1-(11)C]hydroxybutyrate and positron emission tomography, we studied the effect of acute hyperketonemia (range 0.7-1.7 micromol/ml) on cerebral ketone body utilization in six nondiabetic subjects and six insulin-dependent diabetes mellitus (IDDM) patients with average metabolic control (HbA(1c) = 8.1 +/- 1.7%). An infusion of unlabeled R-beta-hydroxybutyrate was started 1 h before the bolus injection of R-beta-[1-(11)C]hydroxybutyrate. The time course of the radioactivity in the brain was measured during 10 min. For both groups, the utilization rate of ketone bodies was found to increase nearly proportionally with the plasma concentration of ketone bodies (1.0 +/- 0.3 micromol/ml for nondiabetic subjects and 1.3 +/- 0.3 micromol/ml for IDDM patients). No transport of ketone bodies from the brain could be detected. This result, together with a recent study of the tissue concentration of R-beta-hydroxybutyrate in the brain by magnetic resonance spectroscopy, indicate that, also at acute hyperketonemia, the rate-limiting step for ketone body utilization is the transport into the brain. No significant difference in transport and utilization of ketone bodies could be detected between the nondiabetic subjects and the IDDM patients.

Alpha-cyano-4-hydroxycinnamate decreases both glucose and lactate metabolism in neurons and astrocytes: implications for lactate as an energy substrate for neurons

The rates of uptake and oxidation of [U-(14)C]lactate and [U-(14)C]glucose were determined in primary cultures of astrocytes and neurons from rat brain, in the presence and absence of the monocarboxylic acid transport inhibitor alpha-cyano-4-hydroxycinnamate (4-CIN). The rates of uptake for 1 mM lactate and glucose were 7.45 +/- 1.35 and 8.80 +/- 1.0 nmol/30 sec/mg protein in astrocytes and 2.36 +/- 0.19 and 1.93 +/- 0.16 nmol/30 sec/mg protein in neuron cultures, respectively. Lactate transport into both astrocytes and neurons was significantly decreased by 0.25-1.0 mM 4-CIN; however, glucose uptake was not affected. The rates of (14)CO(2) formation from 1 mM lactate and glucose were 12.49 +/- 0.77 and 3.42 +/- 0.67 nmol/hr/mg protein in astrocytes and 29.32 +/- 2.81 and 10.04 +/- 1.79 nmol/hr/mg protein in neurons, respectively. Incubation with 0.25 mM 4-CIN decreased the oxidation of lactate and glucose to 57.1% and 54.1% of control values in astrocytes and to 13.2% and 41.6% of the control rates in neurons, respectively. Preincubation with 4-CIN further decreased the oxidation of both glucose and lactate. Studies with glucose specifically labeled in the one and six positions demonstrated that 4-CIN decreased mitochondrial glucose oxidation but did not impair the metabolism of glucose via the pentose phosphate pathway in the cytosol. The lack of effect of 4-CIN on glutamate oxidation demonstrated that overall mitochondrial metabolism was not impaired. These findings suggest that the impaired neuronal function and tissue damage in the presence of 4-CIN observed in other studies may be due in part to decreased uptake of lactate; however, the effects of 4-CIN on mitochondrial transport would significantly decrease the oxidative metabolism of pyruvate derived from both glucose and lactate.

Glucose metabolism in the developing brain

As in adults, glucose is the predominant cerebral energy fuel for the fetus and newborn. Studies in experimental animals and humans indicate that cerebral glucose utilization initially is low and increases with maturation with increasing regional heterogeneity. The increases in cerebral glucose utilization with advancing age occurs as a consequence of increasing functional activity and cerebral energy demands. The levels of expression of the 2 primary facilitative glucose transporter proteins in brain, GLUT1 (blood-brain barrier and glia) and GLUT3 (neuronal), display a similar maturational pattern. Alternate cerebral energy fuels, specifically the ketone bodies and lactate, can substitute for glucose, especially during hypoglycemia, thereby protecting the immature brain from potential untoward effects of hypoglycemia. Unlike adults, glucose supplementation during hypoxia-ischemia is protective in the immature brain, whereas hypoglycemia is deleterious. Accordingly, glucose plays a critical role in the developing brain, not only as the primary substrate for energy production but also to allow for normal biosynthetic processes to proceed.

beta-Hydroxybutyrate fuels synaptic function during development. Histological and physiological evidence in rat hippocampal slices

To determine whether ketone bodies sustain neuronal function as energy substrates, we examined the effects of beta-hydroxybutyrate (betaHB) on synaptic transmission and morphological integrity during glucose deprivation in rat hippocampal slices. After the depression of excitatory postsynaptic potentials (EPSPs) by 60 min of glucose deprivation, administration of 0.5-10 mM D-betaHB restored EPSPs in slices from postnatal day (PND) 15 rats but not in slices from PND 30 or 120 rats. At PND 15, adding D-betaHB to the media allowed robust long-term potentiation of EPSPs triggered by high frequency stimulation, and prevented the EPSP-spike facilitation that suggests hyperexcitability of neurons. Even after PND 15,D-betaHB blocked morphological changes produced by either glucose deprivation or glycolytic inhibition. These results indicate that D-betaHB is not only able to substitute for glucose as an energy substrate but is also able to preserve neuronal integrity and stability, particularly during early development.

Progressive hippocampal loss of immunoreactive GLUT3, the neuron-specific glucose transporter, after global forebrain ischemia in the rat

Brain damage after global forebrain ischemia is worsened by prior hyperglycemia and ameliorated by antecedent hypoglycemia. To assess whether GLUT3, the neuron specific glucose transporter and its mRNA, are affected by cerebral ischemia, we investigated the hippocampal pattern of GLUT3 immunoreactivity and GLUT3 gene expression 1, 4 and 7 days after global forebrain ischemia in a rat 2-vessel occlusion model. We used a newly generated, specific, C-terminally directed polyclonal antiserum against GLUT3 to stain coronal frozen sections. Thionin staining and the microglial marker, OX42, indicated the extent of ischemic damage in hippocampus and correlated with GLUT3 loss. One day after ischemia, no significant change in hippocampal GLUT3 immunoreactivity was observed; by 4 days however, there was consistent and pronounced loss; and at 7 days the loss of GLUT3 staining was maximal. The greatest loss of GLUT3 staining was in the CA1 region, especially the strata oriens and radiatum of Ammon's horn. By contrast, GLUT3 staining was undiminished in the stratum lacunosum moleculare, in the mossy fibers of the lateral aspect of CA3 and in all but the inner-most portion of the molecular layer of the dentate gyrus, immediately adjacent to the granule cells. GLUT3 mRNA levels were not significantly altered at 24 hours and significantly declined at 4 and 7 days after ischemia in the CA1 pyramidal layer. These data are consistent with the pattern of neuronal loss and microglial activation in hippocampus. Loss of GLUT3 may affect the availability of glucose, and possibly the viability of ischemically damaged neurons.

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.

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.

Quantitative histochemical changes in enzymes involved in energy metabolism in the rat brain during postnatal development. II. Glucose-6-phosphate dehydrogenase and beta-hydroxybutyrate dehydrogenase

The postnatal maturation of glucose-6-phosphate and beta-hydroxybutyrate dehydrogenase activity was assessed by histochemistry in rats at eight postnatal stages, P0, P5, P10, P14, P17, P21, P35 and the adult stage. Enzyme activities were revealed on cryostat brain sections with nitroblue tetrazolium. Both enzyme activities were low and homogeneous at birth, and increased to reach a peak in all areas studied, at P17 for beta-hydroxybutyrate dehydrogenase and at P21 for glucose-6-phosphate dehydrogenase. Then, glucose-6-phosphate dehydrogenase activity decreased regularly by 20-49% from P21 to adult stage, except in cerebellar white matter where activity did not change after P21. beta-hydroxybutyrate dehydrogenase activity decreased regularly from P17 to adult stage in globus pallidus, hippocampus, thalamus, brainstem, genu of corpus callosum and cerebellar white matter. It sensorimotor cortex, medial geniculate body, caudate nucleus, hypothalamus and inferior colliculus, beta-hydroxybutyrate dehydrogenase activity stayed stable between P17 and P35 and decreased thereafter to adult levels. Finally, in parietal, auditory and cerebellar cortices, beta-hydroxybutyrate dehydrogenase activity either stayed stable or slightly increased after P17. The present study shows that there is a quite good correlation between postnatal changes in cerebral glucose-6-phosphate and beta-hydroxybutyrate dehydrogenase activities and the importance of pentose phosphate pathway and ketone body utilization in the developing brain. Our results also reflect the regional heterogeneity of beta-hydroxybutyrate utilization in the adult rat brain, translating into a remaining high activity of beta-hydroxybutyrate dehydrogenase in cerebral cortex.

Effect of methylmalonate on in vitro lactate release and carbon dioxide production by brain of suckling rats

Methylmalonate (MMA) accumulates in the tissues of patients with methylmalonic acidaemia, who present severe neurological signs soon after birth and later mental retardation. Attempting to understand the pathophysiology of the disorder, we investigated the effects of MMA on brain glucose uptake, lactate release and CO2 production. Glucose uptake and lactate release were studied by incubating 40 microns wide brain prisms from 15-day-old rats in Krebs-Ringer bicarbonate buffer, pH 7.0, containing 5.0 mmol/L glucose and one of three concentrations of MMA (1.0, 2.5 and 5.0 mmol/L). Controls did not contain MMA in the incubation medium. MMA induced a significant increase of lactate production in a dose-dependent pattern that was proportional to glucose uptake by the brain prisms. We also studied the influence of MMA on brain CO2 production from [2-14C]glucose and [U-14C]acetate by incubating brain prisms in the same buffer in the presence of the substrates with (experimental groups) or without (controls) 5.0 mmol/L MMA. MMA significantly reduced CO2 formation from both substrates.

Regional blood-brain barrier transport of ketone bodies in portacaval-shunted rats

The permeability of the blood-brain barrier to ketone bodies, substrates of the monocarboxylic acid carrier, was measured in individual brain structures of control and portacaval-shunted rats. The measurements were made 5-7 wk after the shunt or sham operation by quantitative autoradiography. Portacaval shunting caused the permeability to ketone bodies to decrease throughout the brain by approximately 70%. There was a striking change in the transport pattern in the cerebral cortex; deeper cortical layers were affected more than superficial layers. Ketone body consumption by brain is limited by the transport capacity of the monocarboxylic acid system. Therefore, in portacaval-shunted rats the very low activity of this system makes it unlikely that ketone bodies can make a substantial contribution during situations such as fasting. Likewise, other substrates of the monocarboxylic acid system, e.g., lactate and pyruvate, will have greatly restricted access to the brain after portacaval shunting. If the carrier is symmetrical, another consequence will be that exit of endogenously produced lactate will be retarded.

Effects of methylmalonate and propionate on uptake of glucose and ketone bodies in vitro by brain of developing rats

Methylmalonate (MMA) and propionate effects on glucose and ketone body uptake in vitro by brain of fed and 30-hour-fasted 15-day-old rats were studied. In some experiments cerebrum prisms were incubated in the presence of glucose and either MMA or propionate in Krebs-Ringer bicarbonate buffer, pH 7.0. In others, the incubation medium contained beta-hydroxybutyrate (HBA) or acetoacetate (AcAc) instead of glucose. We verified that MMA increased glucose uptake by brain of fasting animals, whereas propionate had no effect. In addition, MMA diminished HBA but not AcAc incorporation into brain prisms, whereas propionate provoked a diminished utilization of both ketone bodies by brain. The in vitro effect of MMA and propionate on brain and liver beta-hydroxybutyrate dehydrogenase activity was also investigated. It was shown that MMA but not propionate significantly inhibited this activity. Rats were also injected subcutaneously three times with a MMA buffered solution, and the in vivo effects of MMA on the above-mentioned parameters assessed. Results from these experiments confirmed the previously found in vitro MMA effects. Methylmalonic acidemic patients accumulate primarily methylmalonate and secondarily propionate and other metabolites in their tissues at levels comparable to those we used in our assays. Most patients who survive early stages of the disease show a variable degree of neuromotor delay. Since glucose and sometimes ketones are the vital substrates for brain metabolism, it is possible that our findings may contribute to a certain extent to an understanding of the biochemical basis of mental retardation in these patients.

Autoradiographic measurement of local cerebral beta-hydroxybutyrate uptake in the rat during postnatal development

An autoradiographic method has been developed for the regional assessment of cerebral tracer levels after the acute intravenous injection of [3-14C]beta-hydroxybutyrate in developing rats. The animals were studied at five postnatal stages, i.e. postnatal day 10 (P10), P14, P17, P21 and P35. Tracer levels were high from P10 to P17, reaching peak values at P14, which were two- to threefold higher than those at P10. At P17, tracer concentrations were about twice as low as at P14. Between P17 and P21, regional 14C concentrations were again reduced by about twofold in all areas studied and decreased further by about 50% after weaning reaching quite low levels by P35. The distribution of 14C inside sections appeared to be rather homogeneous throughout the brain at all stages studied, never exceeding a ratio higher than 2 at any stage studied. These results are in good agreement with previous data on the rate of uptake and utilization of beta-hydroxybutyrate by the immature rat brain.

Influence of early chronic phenobarbital treatment on cerebral arteriovenous differences of glucose and ketone bodies in the developing rat

The influence of an early chronic phenobarbital treatment on cerebral arteriovenous differences of glucose, lactate, pyruvate, beta-hydroxybutyrate and acetoacetate was studied in suckling rats. The animals were treated from day 2 to 21 after birth by a daily injection of 50 mg/kg phenobarbital or by saline and were studied at 10, 14 and 21 days. Phenobarbital treatment induced a decrease in cerebral arteriovenous difference of glucose at P14 and no change at P10 and P21. The barbiturate did not have any influence on cerebral arteriovenous difference of lactate and pyruvate at the three stages studied. Cerebral uptake of beta-hydroxybutyrate was unchanged at P10 and increased by two-fold at P14 and by threefold at P21 by phenobarbital. Cerebral arteriovenous difference of acetoacetate was low and did not change with the pharmacological treatment. At P14 and P21, the calculated amount of oxygen used by the brain for the oxidation of ketone bodies was twice as high in barbiturate- as in saline-treated rats and reached values of 47 and 16% respectively in phenobarbital-exposed animals. In addition, the barbiturate seemed to affect the carrier process of beta-hydroxybutyrate from blood to brain. The results of the present study are in good agreement with previous data from our laboratory showing that an early chronic phenobarbital treatment is able to induce a shift in the cerebral energy metabolism balance in favor of ketone bodies.

Inhibition of sterol but not fatty acid synthesis by valproate in developing rat brain in vivo

The effect of administration of valproate on lipogenesis in the developing rat brain in vivo was studied. Valproate inhibited by 21-38% the rate of 3H2O incorporation into brain sterols, without significantly affecting fatty acid synthesis. Similarly, R-[2-14C]mevalonate incorporation into sterols was inhibited by 33-54%; the low rate of fatty acid synthesis under these conditions was not affected by valproate. Plasma ketone bodies decreased after treatment with valproate. Valproate inhibited (about 50%) both sterol and fatty acid synthesis in livers of weanling rats. It is concluded that valproate can specifically inhibit sterol synthesis in the brain during development, in part at a stage after mevalonate formation, and also by decreased exogenous precursor supply.

Cortical microvessels during brain development: a morphometric study in the rat

In order to determine whether any structural changes occur in the blood-brain barrier during development which can be related to changes in barrier function over the same period, morphometric methods were used on pieces of visual cortex taken from rats between 16 days gestation and adult and processed for light and electron microscopy. Capillary volume fraction, numbers per unit area, and surface density increased in two phases, before birth and between 10 and 20 days after birth, with no subsequent change after 20 days. Brain parenchymal mitochondria, assumed to be a measure of brain oxidative activity, did not change before birth but increased gradually and continuously after birth from 1.7% to 5.7% in adults. The capillary endothelial mitochondrial volume fraction as a percentage of the endothelial cell cytoplasm, previously thought to be important for ion transport, did not change with age (mean = 5.4%), although there was evidence that mitochondria either divide or change in shape up to 30 days after birth. The endothelial cell thickness decreased with age from 0.6 to 0.2 microns, probably through cell elongation during vascular growth. By combining the data on endothelial mitochondria and vascularity, it was shown that the total endothelial mitochondrial volume per unit length of capillary decreased with age, whereas per unit volume of tissue it increased.

Lactate, 3-hydroxybutyrate, and glucose as substrates for the early postnatal rat brain

The dependence of cerebral energy metabolism upon glucose, 3-hydroxybutyrate, and lactate as fuel sources during the postnatal period was investigated. The brain of 6 day old suckling pups used very little glucose, but by the 15th postnatal day glucose was the major catabolite. Hydroxybutyrate was not a major brain fuel at either 6 or 15 days of age. Its utilization accounted for only 19% of the brain's total energy needs at 15 days of age, even through blood ketone concentrations are near maximal at this time. Seventy percent of the cerebral metabolic requirements were met by lactate in animals aged 6 days. The major role played by lactate as a substrate for brain metabolism in young pups was not a result of abnormally elevated blood lactate concentrations. The slow catabolism of glucose in young brain can not be explained by low rates of influx or inadequate enzymatic capacity.

Local cerebral glucose utilization during status epilepticus in newborn primates

The effect of bicuculline-induced status epilepticus (SE) on local cerebral metabolic rates for glucose (LCMRglc) was studied in 2-wk-old ketamine-anesthetized marmoset monkeys, using the 2-[14C]-deoxy-D-glucose autoradiographical technique. To estimate LCMRglc in cerebral cortex and thalamus during SE, the lumped constant (LC) for 2-deoxy-D-glucose (2-DG) and the rate constants for 2-DG and glucose were calculated for these regions. The control LC was 0.43 in frontoparietal cortex, 0.51 in temporal cortex, and 0.50 in thalamus; it increased to 1.07 in frontoparietal cortex, 1.13 in temporal cortex, and 1.25 in thalamus after 30 min of seizures. With control LC values, LCMRglc in frontoparietal cortex, temporal cortex, and dorsomedial thalamus appeared to increase four to sixfold. With seizure LC values, LCMRglc increased 1.5- to 2-fold and only in cortex. During 45-min seizures, LCMRglc in cortex and thalamus probably increases 4- to 6-fold initially and later falls to the 1.5- to 2-fold level as tissue glucose concentrations decrease. Together with our previous results demonstrating depletion of high-energy phosphates and glucose in these regions, the data suggest that energy demands exceed glucose supply. The long-term effects of these metabolic changes on the developing brain remain to be determined.

The uptake of [14C]deoxyglucose into brain of young rats with inherited hydrocephalus

The effect of hydrocephalus on cerebral glucose utilization as reflected by deoxyglucose uptake has been examined in rats with inherited hydrocephalus at 10, 20, and 28 days after birth using a semiquantitative method. Injection of [14C]deoxyglucose intraperitoneally was followed by freezing the brain, sectioning, and quantitative autoradiography of 10 brain regions. Brain [14C] concentration, cortical thickness, and plasma glucose concentrations were measured. Maximal thinning of the cerebral cortex had already occurred by 10 days after birth, although obvious symptoms such as gait disturbance developed after 20 days. In control rats, the cerebral isotope concentration was lower and more homogeneous at 10 days than at 20 or 28 days, which may be a reflection of the use of metabolic substrates other than glucose in younger animals. In order to make comparisons between control and hydrocephalic groups, tissue isotope concentrations were normalized to cerebellar cortex which was not affected by the hydrocephalus at any age. In hydrocephalic rats at 10 and 20 days, the concentration of [14C] was lower in all areas except the inferior colliculi and pons but the reduction was only significant in the sensory-motor cortex at 10 days and in the caudate nuclei at 20 days. By 28 days after birth, all areas except the cerebellum (six cortical regions, inferior colliculi, pons, and caudate) had significantly lower isotope concentrations in the hydrocephalic group. It is concluded that cerebral glucose metabolism is significantly reduced by 28 days after birth in H-Tx rats with congenital hydrocephalus and that less marked reductions occur prior to 28 days.

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.