Lactate utilization by isolated cells from early neonatal rat brain

Metabolite transport across the mammalian and insect brain diffusion barriers

The nervous system in higher vertebrates is separated from the circulation by a layer of specialized endothelial cells. It protects the sensitive neurons from harmful blood-derived substances, high and fluctuating ion concentrations, xenobiotics or even pathogens. To this end, the brain endothelial cells and their interlinking tight junctions build an efficient diffusion barrier. A structurally analogous diffusion barrier exists in insects, where glial cell layers separate the hemolymph from the neural cells. Both types of diffusion barriers, of course, also prevent influx of metabolites from the circulation. Because neuronal function consumes vast amounts of energy and necessitates influx of diverse substrates and metabolites, tightly regulated transport systems must ensure a constant metabolite supply. Here, we review the current knowledge about transport systems that carry key metabolites, amino acids, lipids and carbohydrates into the vertebrate and Drosophila brain and how this transport is regulated. Blood-brain and hemolymph-brain transport functions are conserved and we can thus use a simple, genetically accessible model system to learn more about features and dynamics of metabolite transport into the brain.

Pyruvate to Lactate Metabolic Changes during Neurodevelopment Measured Dynamically Using Hyperpolarized 13C Imaging in Juvenile Murine Brain

Hyperpolarized 13C magnetic resonance imaging has recently been used to dynamically image metabolism in vivo. This technique provides the capability to investigate metabolic changes in mouse brain development over multiple time points. In this study, we used 13C magnetic resonance spectroscopic imaging and hyperpolarized 13C-1-labeled pyruvate to analyze its conversion into lactate. We also applied T2-weighted anatomical imaging to examine brain volume changes starting from postnatal day 18 (P18). We combined these results with body weight measurements for a comprehensive interpretation of mouse brain maturation. Both the produced lactate level and pyruvate to lactate conversion rate decreased with increasing age in a linear manner. Total brain volume remained the same after P18, even though body weight continued to grow exponentially. Our results have shown that the rate of metabolism of 13C-1 pyruvate to lactate in brain is high in the young mouse and decreases with age. The brain at P18 is still relatively immature and continues to develop even as the total brain volume remains the same.

Spatial expression and functional flexibility of monocarboxylate transporter isoforms in the zebrafish brain

The present study provides in vivo evidence to prove the functional plasticity of monocarboxylate transporters (MCTs) in brains of vertebrates using zebrafish (Danio rerio) as a model. In the mammalian central nervous system (CNS), energy demands are largely met by oxidation of glucose. In recent studies, in addition to glucose, lactate is also considered an energy substrate for the CNS. Astrocytes were demonstrated to play an important role in transporting lactate as metabolic substrate from capillaries to neurons through monocarboxylate transporters (MCTs). The present study was to use zebrafish as an in vivo model to test the hypothesis of whether the various MCT homologs play differential roles in the development and functioning of the CNS. Using RT-PCR and double in situ hybridization coupling with immunocytochemical staining experiments, zebrafish MCTs1-4 were all found to be expressed in brains of embryos, and were further elucidated to be localized in both neurons and astrocytes. Loss-of-functions by morpholino knockdown further provided in vivo evidences to infer that zMCTs1, -2, and -4 may be involved in metabolite transport and functioning in the developing brain. Subsequent rescue experiments with capped mRNAs of specific isoforms further indicated that zMCT2 is an indispensable monocarboxylate-transporting route for CNS development and function in zebrafish. This information is essential for identifying proper candidates of MCT isoforms that are involved in the development and functioning of the CNS.

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.

Effect of 2-deoxy-D-glucose on aminoacids metabolism in rats' cerebral cortex slices

We studied the effect of different concentrations of 2-deoxy-D-glucose on the L-[U-14C]leucine, L-[1-14C]leucine and [1-14C]glycine metabolism in slices of cerebral cortex of 10-day-old rats. 2-deoxy-D-glucose since 0.5 mM concentration has inhibited significantly the protein synthesis from L-[U-14C]leucine and from [1-14C]glycine in relation to the medium containing only Krebs Ringer bicarbonate. Potassium 8.0 mM in incubation medium did not stimulate the protein synthesis compared to the medium containing 2.7 mM, and at 50 mM diminishes more than 2.5 times the protein synthesis compared to the other concentration. Only at the concentration of 5.0 mM, 2-deoxy-D-glucose inhibited the CO2 production and lipid synthesis from L-[U-14C] leucine. This compound did not inhibit either CO2 production, or lipid synthesis from [1-14C]glycine. Lactate at 10 mM and glucose 5.0 mM did not revert the inhibitory effect of 2-deoxy-D-glucose on the protein synthesis from L-[U-14C]leucine. 2-deoxy-D-glucose at 2.0 mM did not show any effect either on CO2 production, or on lipid synthesis from L-[U-14C]lactate 10 mM and glucose 5.0 mM.

Glucose Metabolism in Rat Retinal Pigment Epithelium

The retinal pigment epithelium (RPE) is the major transport pathway for exchange of metabolites and ions between choroidal blood supply and the neural retina. To gain insight into the mechanisms controlling glucose metabolism in RPE and its possible relationship to retinopathy, we studied the influence of different glucose concentrations on glycogen and lactate levels and CO(2) production in RPE from normal and streptozotocin-treated diabetic rats. Incubation of normal RPE in the absence of glucose caused a decrease in lactate production and glycogen content. In normal RPE, increasing glucose concentrations from 5.6 mM to 30 mM caused a four-fold increase in glucose accumulation and CO(2) yield, as well as reduction in lactate and glycogen production. In RPE from diabetic rats glucose accumulation did not increase in the presence of high glucose substrate, but it showed a four- and a seven-fold increase in CO(2) production through the mitochondrial and pentose phosphate pathways, respectively. We found high glycogen levels in RPE which can be used as an energy reserve for RPE itself and/or neural retina. Findings further show that the RPE possesses a high oxidative capacity. The large increase in glucose shunting to the pentose phosphate pathway in diabetic retina exposed to high glucose suggests a need for reducing capacity, consistent with increased oxidative stress.

Essential role of aralar in the transduction of small Ca2+ signals to neuronal mitochondria

Aralar, the neuronal Ca(2+)-binding mitochondrial aspartate-glutamate carrier, has Ca(2+) binding domains facing the extramitochondrial space and functions in the malate-aspartate NADH shuttle (MAS). Here we showed that MAS activity in brain mitochondria is stimulated by extramitochondrial Ca(2+) with an S(0.5) of 324 nM. By employing primary neuronal cultures from control and aralar-deficient mice and NAD(P)H imaging with two-photon excitation microscopy, we showed that lactate utilization involves a substantial transfer of NAD(P)H to mitochondria in control but not aralar-deficient neurons, in agreement with the lack of MAS activity associated with aralar deficiency. The increase in mitochondrial NAD(P)H was greatly potentiated by large [Ca(2+)](i) signals both in control and aralar-deficient neurons, showing that these large signals activate the Ca(2+) uniporter and mitochondrial dehydrogenases but not MAS activity. On the other hand, small [Ca(2+)](i) signals potentiate the increase in mitochondrial NAD(P)H only in control but not in aralar-deficient neurons. We concluded that neuronal MAS activity is selectively activated by small Ca(2+) signals that fall below the activation range of the Ca(2+) uniporter and plays an essential role in mitochondrial Ca(2+) signaling.

Brain lactate kinetics: Modeling evidence for neuronal lactate uptake upon activation

A critical issue in brain energy metabolism is whether lactate produced within the brain by astrocytes is taken up and metabolized by neurons upon activation. Although there is ample evidence that neurons can efficiently use lactate as an energy substrate, at least in vitro, few experimental data exist to indicate that it is indeed the case in vivo. To address this question, we used a modeling approach to determine which mechanisms are necessary to explain typical brain lactate kinetics observed upon activation. On the basis of a previously validated model that takes into account the compartmentalization of energy metabolism, we developed a mathematical model of brain lactate kinetics, which was applied to published data describing the changes in extracellular lactate levels upon activation. Results show that the initial dip in the extracellular lactate concentration observed at the onset of stimulation can only be satisfactorily explained by a rapid uptake within an intraparenchymal cellular compartment. In contrast, neither blood flow increase, nor extracellular pH variation can be major causes of the lactate initial dip, whereas tissue lactate diffusion only tends to reduce its amplitude. The kinetic properties of monocarboxylate transporter isoforms strongly suggest that neurons represent the most likely compartment for activation-induced lactate uptake and that neuronal lactate utilization occurring early after activation onset is responsible for the initial dip in brain lactate levels observed in both animals and humans.

How astrocytes feed hungry neurons

For years glucose was thought to constitute the sole energy substrate for neurons; it was believed to be directly provided to neurons via the extracellular space by the cerebral circulation. It was recently proposed that in addition to glucose, neurons might rely on lactate to sustain their activity. Therefore, it was demonstrated that lactate is a preferred oxidative substrate for neurons not only in vitro but also in vivo. Moreover, the presence of specific monocarboxylate transporters on neurons as well as on astrocytes is consistent with the hypothesis of a transfer of lactate from astrocytes to neurons. Evidence has been provided for a mechanism whereby astrocytes respond to glutamatergic activity by enhancing their glycolytic activity, resulting in increased lactate release. This is accomplished via the uptake of glutamate by glial glutamate transporters, leading to activation of the Na+/K+ ATPase and a stimulation of astrocytic glycolysis. Several recent observations obtained both in vitro and in vivo with different approaches have reinforced this view of brain energetics. Such an understanding might be critically important, not only because it forms the basis of some classical functional brain imaging techniques but also because several neurodegenerative diseases exhibit diverse alterations in energy metabolism.

Monocarboxylate transporters in the central nervous system: distribution, regulation and function

Monocarboxylate transporters (MCTs) are proton-linked membrane carriers involved in the transport of monocarboxylates such as lactate, pyruvate, as well as ketone bodies. They belong to a larger family of transporters composed of 14 members in mammals based on sequence homologies. MCTs are found in various tissues including the brain where three isoforms, MCT1, MCT2 and MCT4, have been described. Each of these isoforms exhibits a distinct regional and cellular distribution in rodent brain. At the cellular level, MCT1 is expressed by endothelial cells of microvessels, by ependymocytes as well as by astrocytes. MCT4 expression appears to be specific for astrocytes. By contrast, the predominant neuronal monocarboxylate transporter is MCT2. Interestingly, part of MCT2 immunoreactivity is located at postsynaptic sites, suggesting a particular role of monocarboxylates and their transporters in synaptic transmission. In addition to variation in expression during development and upon nutritional modifications, new data indicate that MCT expression is regulated at the translational level by neurotransmitters. Understanding how transport of monocarboxylates is regulated could be of particular importance not only for neuroenergetics but also for areas such as functional brain imaging, regulation of food intake and glucose homeostasis, or for central nervous system disorders such as ischaemia and neurodegenerative diseases.

Lactate utilization by brain cells and its role in CNS development

We studied the role played by lactate as an important substrate for the brain during the perinatal period. Under these circumstances, lactate is the main substrate for brain development and is used as a source of energy and carbon skeletons. In fact, lactate is used actively by brain cells in culture. Neurons, astrocytes, and oligodendrocytes use lactate as a preferential substrate for both energy purposes and as precursor of lipids. Astrocytes use lactate and other metabolic substrates for the synthesis of oleic acid, a new neurotrophic factor. Oligodendrocytes mainly use lactate as precursor of lipids, presumably those used to synthesize myelin. Neurons use lactate as a source of energy and as precursor of lipids. During the perinatal period, neurons may use blood lactate directly to meet the need for the energy and carbon skeletons required for proliferation and differentiation. During adult life, however, the lactate used by neurons may come from astrocytes, in which lactate is the final product of glycogen breakdown. It may be concluded that lactate plays an important role in brain development.

Monocarboxylate transport inhibition alters retinal function and cellular amino acid levels

We assessed the effect of the in vivo application of monocarboxylate transport inhibitors on retinal function and amino acid immunocytochemistry. We wanted to determine the impact that altered aerobic metabolite availability has on retinal function and the characteristics of amino acid shunting into metabolic pools. Electroretinograms were collected from anaesthetized rats at various times after intravitreal injection of the monocarboxylate transport inhibitors alpha-cyano-4-hydroxycinnamate (4-CIN; 2 micro L, 0.1-10 mm) or p-(dipropylsulphamoyl)benzoic acid (probenecid; 1-10 mm). Changes in retinal function were compared with quantitative amino acid immunocytochemical changes in retinas harvested 20 and 40 min after either 4-CIN or vehicle treatment. The injection of 4-CIN resulted in a dose-dependent reduction of the ON-bipolar cell P2 wave amplitude (20-80%) and delay in its implicit time. The phototransduction sensitivity was mildly reduced whereas the ON-bipolar cell P2 sensitivity was unaffected. Probenecid induced functional changes similar to those observed with 4-CIN. We also mapped the amino acid alterations within specific cell classes induced by 4-CIN application. All neurones displayed a reduced glutamate content averaging 48%; reduced GABA (31%) and glycine (28%) were found within amacrine cells and glutamine was reduced in all cell classes except photoreceptor and Müller cells. All cell classes in the retina demonstrated increases in aspartate (57%), whereas leucine (24%) and ornithine (21%) were only significantly increased in photoreceptor and bipolar cells. The reduction in glutamate immunolabelling in specific retinal cell classes was mirrored by an increase in aspartate levels at these locations. In addition, attenuated glutamine immunolabelling also closely matched the spatial pattern observed for glutamate. Our immunocytochemical analysis provides evidence that monocarboxylate transport inhibition induces a shift in the equilibrium of glutamate transamination reactions involving aspartate throughout the retina whereas photoreceptor and bipolar cells also use glutamate transamination reactions involving ornithine and leucine. The distribution pattern of glutamine secondary to monocarboxylate inhibition suggests that this amino acid is a major precursor for glutamate throughout the retina.

Neuroenergetics: calling upon astrocytes to satisfy hungry neurons

Classical neuroenergetics states that glucose is the exclusive energy substrate of brain cells and its full oxidation provides all the necessary energy to support brain function. Recent data have revealed a more intricate picture in which astrocytes play a key role in supplying lactate as an additional energy substrate in register with glutamatergic activity.

Lactate is a preferential oxidative energy substrate over glucose for neurons in culture

The authors investigated concomitant lactate and glucose metabolism in primary neuronal cultures using 13C- and 1H-NMR spectroscopy. Neurons were incubated in a medium containing either [1-13C]glucose and different unlabeled lactate concentrations, or unlabeled glucose and different [3-13C]lactate concentrations. Overall, 13C-NMR spectra of cellular extracts showed that more 13C was incorporated into glutamate when lactate was the enriched substrate. Glutamate 13C-enrichment was also found to be much higher in lactate-labeled than in glucose-labeled conditions. When glucose and lactate concentrations were identical (5.5 mmol/L), relative contributions of glucose and lactate to neuronal oxidative metabolism amounted to 21% and 79%, respectively. Results clearly indicate that when neurons are in the presence of both glucose and lactate, they preferentially use lactate as their main oxidative substrate.

Lactate as a pivotal element in neuron-glia metabolic cooperation

Lactate has been considered for a long time as a metabolic waste and/or a sign of hypoxia in the central nervous system. Nevertheless, clear evidence that lactate can constitute an adequate energy substrate for brain tissue has been provided as early as in the 1950s with the pioneering work of McIlwain in brain slices. Over the years, several studies using different approaches have confirmed that lactate is efficiently oxidized by brain cells in vitro. Moreover, lactate has been shown under certain circumstances to have a neuroprotective effect and support neuronal activity. Similar confirmation of lactate utilization in vivo as well as putative neuroprotection in various excitotoxic models has been provided. Lactate was even shown to restore cognitive performance upon an hypoglycemic episode in humans. More recently, it was proposed that lactate could be produced by astrocytes and released in the extracellular space to form a pool readily available for neurons in case of high energy demands. Several elements support the concept of a lactate shuttle between astrocytes and neurons in the central nervous system. Among them, the description of specific monocarboxylate transporters found on both astrocytes and neurons is an important observation consistent with this concept. Interestingly, lactate shuttles between different cell types within the same organ have been described outside the central nervous system, notably in muscle and testis. Thus, lactate is emerging as a valuable intercellular exchange molecule in different systems including the brain where it might be an essential element of neuron-glia metabolic interactions.

Tolerance of neonatal rat brain to acute hyperammonemia

The aim of the present work was to study the effects of hyperammonemia on brain energy metabolism in neonatal rats. Rats were rendered hyperammonemic by ammonium acetate administration. This decreased brain ATP concentrations but enhanced brain ammonia and lactate levels in both adult and neonatal rats. In adult rats, the decrease in brain ATP concentrations was accompanied by a plunge in the respiratory control rate (RCR) of brain mitochondria. However, the ammonia-induced effect on RCR was not observed in neonatal rats, suggesting that the fall in ATP levels observed in neonatal rats would not be due to an impairment of mitochondrial respiratory efficiency. However, in neonatal rats the increase in blood and brain ammonia concentrations did not change brain glutamate concentrations but decreased glutamine contents. These results may be of relevance for the understanding of the resistance of neonatal rats observed in this work to acute ammonia toxicity

Highly differential expression of the monocarboxylate transporters MCT2 and MCT4 in the developing rat brain

Monocarboxylate transporters (MCTs) play an important role in the metabolism of all cells. They mediate the transport of lactate and pyruvate but also some other substrates such as ketone bodies. It has been proposed that glial cells release monocarboxylates to fuel neighbouring neurons. A key element in this hypothesis is the existence of neuronal MCTs. Amongst the three MCTs known to be expressed in the brain (MCT1, 2 and 4) only MCT2 has been found in neurons. Here we have studied the expression pattern of MCT2 during postnatal development. By use of immunoperoxidase and double immunofluorescence microscopy we report that neuronal MCT2 occurs in most brain areas, including the hippocampus and cerebellum, from birth to adult. MCT2 is also expressed in specific subpopulations of astrocytes. Neuronal MCT2 is most abundant in the first 3 postnatal weeks and thereafter decreases toward adulthood. In contrast to MCT2, MCT4 is exclusively present in astroglia during all stages of development. Furthermore, MCT4 expression is very low at birth and reaches adult level by P14. Our results are consistent with previous data suggesting that in the immature brain much of the energy demand is met by monocarboxylates and ketone bodies.

Long-term neuroprotective effects of hypothermia on neonatal hypoxic-ischemic brain injury in rats, assessed by auditory brainstem response

A method to assess long-term neurofunctional outcome of hypothermia on immature brains has not yet been clearly established. To investigate the effects of hypothermia on long-term neurofunctional outcome, we studied brainstem function using auditory brainstem response in adult rats after neonatal hypoxic-ischemic brain injury. Seven-day-old rats underwent a combination of left common carotid artery ligation and subsequent exposure to 8% O(2) for 1 h (n = 17). The rats were divided into three groups: hypothermia group (n = 6), normothermia group (n = 6), and sham control group (n = 5). During recovery from the hypoxic-ischemic insult, body temperature was reduced to 30 degrees C for 24 h in the hypothermia group, but was kept at 37 degrees C in the normothermia and sham control group. Three months later the rats were assessed by auditory brainstem response, then killed. The normothermia group showed increased III-V latencies and wave V abnormalities. Hypothermia significantly ameliorated wave V abnormalities. Injury to the ipsilateral inferior colliculus was also reduced in the hypothermia group compared with that in the normothermia group, and the degree of damage assessed histologically correlated well with auditory brainstem response findings. The current study demonstrates that postischemic hypothermia may provide effective and long-lasting neurofunctional as well as histopathologic protection to the immature brain.

Lactate utilization as an energy substrate in ischemic preconditioned rat brain slices

We examined the utilization of lactate as an energy substrate in ischemic preconditioned slices obtained from the rat brain left hemisphere, of which the contralateral middle cerebral artery was occluded 48 h before the slice preparation. The levels of high-energy phosphates in the brain slices were measured using 31P NMR with a time resolution of 4 min at 25 degrees C. When iodoacetic acid-pretreated brain slices were further treated with fluorocitrate, a glial toxin, for 2 h (neuron-rich slices), the recovery of the phosphocreatine (PCr) level in artificial cerebrospinal fluid (ACSF) containing lactate after high-K+ stimulation was completely abolished in intact slices, whereas the PCr level in ischemic preconditioned slices well recovered in otherwise similar conditions. These results indicated that neurons, when preconditioned with ischemia, acquire the ability to utilize lactate as an energy substrate. In parallel experiments, we recorded population excitatory postsynaptic potentials and spikes from granule cells in hippocampal slices. Population spikes of intact slices in ACSF containing lactate were completely abolished in 30 min, but those of the ischemic preconditioned slices were maintained well over 50%. These results show that ischemic preconditioning may induce certain systematic changes in neurons, such as the expression of lactate transporters and/or the activation of lactate dehydrogenase.

Albumin promotes neuronal survival by increasing the synthesis and release of glutamate

It is well known that the presence of albumin within the brain and the CSF is developmentally regulated. However, the physiological relevance of this phenomenon is not well established. We have previously shown that albumin specifically increases the flux of glucose and lactate through the pyruvate dehydrogenase reaction in astrocytes. Here we show that, in neurones, albumin also increases the oxidation of glucose and lactate through the pyruvate dehydrogenase-catalysed reaction, the final purpose of this being the synthesis of glutamate. Thus, in neurones, the presence of albumin strongly increased the synthesis and release of glutamate to the extracellular medium. Our results also suggest that glutamate release caused by albumin is designed to promote neuronal survival. Thus, under culture conditions in which neurones die by apoptosis, the presence of albumin promoted neuronal survival and maintained the differentiation programme of these cells, as judged by the expression of the axonal protein, GAP-43. The effect of albumin on neuronal survival was counteracted by the presence of DNQX, an antagonist of non-NMDA-glutamate receptors, suggesting that the glutamate synthesized and released due to the presence of albumin is responsible for neuronal survival. In addition, the effect of albumin seemed to depend on the activity of the NGF receptor, TrkA, suggesting that the glutamate synthesized and released due to the presence of albumin promotes neuronal survival through the activity of TrkA.

Feeding active neurons: (re)emergence of a nursing role for astrocytes

Despite unquestionable evidence that glucose is the major energy substrate for the brain, data collected over several decades with different approaches suggest that lactate may represent a supplementary metabolic substrate for neurons. Starting with the pioneering work of McIlwain in the early 1950s which showed that lactate can sustain the respiratory rate of small brain tissue pieces, this idea receives confirmation with more recent studies using nuclear magnetic resonance spectroscopy undoubtedly demonstrating that lactate is efficiently oxidized by neurons, both in vitro and in vivo. Not only is lactate able to maintain ATP levels and promote neuronal survival but it was also found to support neuronal activity, at least if low levels of glucose are present. Despite the early suggestion for a role of astrocytes in metabolic supply to neurons, it is only recently however that they have been considered as a potential source of lactate for neurons. Moreover, it has been proposed that astrocytes might provide lactate to neurons in response to enhanced synaptic activity by a well-characterized mechanism involving glutamate uptake. The description of specific transporters for lactate on both astrocytes and neurons further suggest that there exist a coordinated mechanism of lactate exchange between the two cell types. Thus it is proposed that astrocytes play a nursing role toward neurons by providing lactate as an additional energy substrate especially during periods of enhanced synaptic activity. The importance of this metabolic cooperation within the central nervous system, although not unique if compared to other organs, still remains to be explored.

Astrocyte-synthesized oleic acid behaves as a neurotrophic factor for neurons

Unlike in the adult brain, the newborn brain specifically takes up serum albumin during the postnatal period, coinciding with the stage of maximal brain development. Here we shall summarize our knowledge about the role played by albumin in brain development. The role of this protein in brain development is intimately related to its ability to carry fatty acids. Thus, albumin stimulates oleic acid synthesis by astrocytes from the main metabolic substrates available during brain development. Astrocytes internalize albumin in vesicle-like structures by receptor-mediated endocytosis, which is followed by transcytosis, including passage through the endoplasmic reticulum (ER). The presence of albumin in the ER activates the sterol regulatory element-binding protein-1 (SREBP-1) and increases stearoyl-CoA 9-desaturase (SCD) mRNA, the key enzyme in oleic acid synthesis. Oleic acid released by astrocytes is used by neurons for the synthesis of phospholipids and is specifically incorporated into growth cones. In addition, oleic acid promotes axonal growth, neuronal clustering, and the expression of the axonal growth associated protein, GAP-43. All of these observations indicate neuronal differentiation. The effect of oleic acid on GAP-43 synthesis is brought about by the activation of protein kinase C. The expression of GAP-43 is significantly increased by the presence of albumin in neurons co-cultured with astrocytes, indicating that neuronal differentiation takes place by the presence of oleic acid synthesized and released by astrocytes in situ. In conclusion, during brain development the presence of albumin could play an important role by triggering the synthesis and release of oleic acid by astrocytes, thereby inducing neuronal differentiation.

Utilization of energy nutrients by cerebellar slices

We performed an ontogenetic study about the utilization of glycine, glutamine, beta-hydroxybutyrate and glycerol as energy nutrients by rat cerebellum slices. Production of CO2 from glycerol and glutamine increased with the animals' age and glutamine was the most used nutrient for CO2 production. In adult age, glutamine oxidation to CO2 was 15 to 35 times higher than all other nutrients studied. CO2 production from glycine decreased markedly with age and 10 day-old rats showed an oxidation 7.5 times higher than that of adult rats. At fetal age and at 10 postnatal days, glycine oxidation to CO2 was only 2 times lower than glutamine oxidation to CO2. Lipid synthesis from beta-hydroxybutyrate was highest in adult rats. We did not observe any difference in the utilization of beta-hydroxybutyrate between slices of cerebral cortex and cerebellum at the ages of 10 days and adult. The main nutrients used for lipid synthesis were glycerol and beta-hydroxybutyrate.

Energy metabolism of fish brain

This review focuses on recent research on the metabolic function of fish brain. Fish brain is isolated from the systemic circulation by a blood-brain barrier that allows the transport of glucose, monocarboxylates and amino acids. The limited information available in fishes suggests that oxidation of exogenous glucose and oxidative phosphorylation provide most of the ATP required for brain function in teleosts, whereas oxidation of ketones and amino acids occurs preferentially in elasmobranchs. In several agnathans and benthic teleosts brain glycogen levels rather than exogenous glucose may be the proximate glucose source for oxidation. In situations when glucose is in limited supply, teleost brains utilize other fuels such as lactate or ketones. Information on use of lipids and amino acids as fuels in fish brain is scarce. The main pathways of brain energy metabolism are changed by several effectors. Thus, several parameters of brain energy metabolism have been demonstrated to change post-prandially in teleostean fishes. The absence of food in teleosts elicits profound changes in brain energy metabolism (increased glycogenolysis and use of ketones) in a way similar to that demonstrated in mammals though delayed in time. Environmental factors induce changes in brain energy parameters in teleosts such as the enhancement of glycogenolysis elicited by pollutants, increased capacity for anaerobic glycolysis under hypoxia/anoxia or changes in substrate utilization elicited by adaptation to cold. Furthermore, several studies demonstrate effects of melatonin, insulin, glucagon, GLP-1, cortisol or catecholamines on energy parameters of teleost brain, although in most cases the results are quite preliminary being difficult to relate the effects of those hormones to physiological situations. The few studies performed with the different cell types available in the nervous system of fish allow us to hypothesize few functional relationships among those cells. Future research perspectives are also outlined.

Neuronal differentiation is triggered by oleic acid synthesized and released by astrocytes

Unlike in the adult brain, the newborn brain specifically takes up serum albumin during the postnatal period, coinciding with the stage of maximal brain development. Here we report that albumin stimulates oleic acid synthesis by astrocytes from the main metabolic substrates available during brain development. Oleic acid released by astrocytes is used by neurons for the synthesis of phospholipids and is specifically incorporated into growth cones. Oleic acid promotes axonal growth, neuronal clustering, and expression of the axonal growth-associated protein-43, GAP-43; all these observations indicating neuronal differentiation. The effect of oleic acid on GAP-43 synthesis is brought about by the activation of protein kinase C, since it was prevented by inhibitors of this kinase, such as H-7, polymyxin or sphingosine. The expression of GAP-43 was significantly increased in neurons co-cultured with astrocytes by the presence of albumin indicating that neuronal differentiation takes place in the presence of oleic acid synthesized and released by astrocytes in situ. In conclusion, during brain development the presence of albumin could play an important role by triggering the synthesis and release of oleic acid by astrocytes, which induces neuronal differentiation.

Neuroprotective role of monocarboxylate transport during glucose deprivation in slice cultures of rat hippocampus

The effects of energy substrate removal and metabolic pathway block have been examined on neuronal and glial survival in organotypic slice cultures of rat hippocampus. Slice cultures resisted 24 h of exogenous energy substrate deprivation. Application of 0.5 mM alpha-cyano-4-hydroxycinnamate (4-CIN) for 24 h resulted in specific damage to neuronal cell layers, which could be reversed by co-application of 5 mM lactate. Addition of 10 mM 2-deoxyglucose in the absence of exogenous energy supply produced widespread cell death throughout the slice. This was partly reversed by co-application of 5 mM lactate. These effects of metabolic blockade on cell survival were qualitatively similar to the effects on population spikes recorded in the CA1 cell layer following 60 min application of these agents. The data suggest that monocarboxylate trafficking from glia to neurons is an essential route for supply of energy substrates to neurons particularly when exogenous energy supply is restricted.

Study of developmental changes on hexoses metabolism in rat cerebral cortex

We have studied the developmental changes of glucose, mannose, fructose and galactose metabolism in rat cerebral cortex. As the animals aged, glucose, mannose and fructose oxidation to CO2 increased, whereas galactose oxidation decreased. Lipid synthesis from glucose and fructose also increased with age, that from mannose decreased and galactose did not change. Cytochalasin B, a potent non-competitive inhibitor of sodium-independent glucose transport, significantly impaired glucose, mannose and galactose metabolism, but had no effect on fructose metabolism. Both galactose or fructose did not change, whereas mannose declined the glucose metabolism. Glucose decreased fructose, galactose and mannose metabolism. Our results show that besides glucose, the metabolism of mannose, galactose and fructose present developmental changes from fetal to adult age, and reinforce the literature data indicating that mannose and galactose are transported by glucose carriers, while fructose is not.

Brain regional development of the activity of alpha-ketoglutarate dehydrogenase complex in the rat

This study was initiated to test the hypothesis that the development of alpha-ketoglutarate dehydrogenase complex (KGDHC) activity, like that of pyruvate dehydrogenase complex, is one of the late developers of tricarboxylic acid (TCA) cycle enzymes. The postnatal development of KGDHC in rat brain exhibits four distinct region-specific patterns. The age-dependent increases in olfactory bulb (OB) and hypothalamus (HYP) form one pattern: low in postnatal days (P) 2 and 4, KGDHC activity rose linearly to attain adult level at P30. The increases in mid-brain (MB) and striatum (ST) constitute a second pattern: being <40% of adult level at P2 and P4, KGDHC activity rose steeply between P10 and P17 and attained adult level by P30. The increases in cerebellum (CB), cerebral cortex (CC), and hippocampus (HIP) form a third pattern: being 25-30% of adult level at P2 and P4, KGDHC activity doubled between P10 and P17 and rose to adult level by P30. KGDHC activity development is unique in pons and medulla (PM): being >60% of the adult level at P2, it rose rapidly to adult level by P10. Thus, KGDHC activity develops earlier in phylogenetically older regions (PM) than in phylogenetically younger regions (CB, CC, HIP). Being lowest in activity among all TCA cycle enzymes, KGDHC activity in any region at any age will exert a limit on the maximum TCA cycle flux therein. The results may have functional and pathophysiological implications in control of brain glucose oxidative metabolism, energy metabolism, and neurotransmitter syntheses.

Characterization of cerebral white matter damage in preterm infants using 1H and 31P magnetic resonance spectroscopy

The biochemical characteristics of white matter damage (WMD) in preterm infants were assessed using magnetic resonance spectroscopy (MRS). The authors hypothesized that preterm infants with WMD at term had a persisting cerebral lactic alkalosis and reduced N-acetyl aspartate (NAA)/ creatine plus phosphocreatine (Cr), similar to that previously documented in term infants weeks after perinatal hypoxiaischemia (HI). Thirty infants (gestational age 27.9 +/- 3.1 weeks, birth weight 1,122 +/- 445 g) were studied at postnatal age of 9.8 +/- 4.1 weeks (corrected age 40.3 +/- 3.9 weeks). Infants were grouped according to the presence or absence of WMD on magnetic resonance (MR) images. The peak area ratios of lactate/Cr, NAA/Cr, myo-inositol/Cr, and choline (Cho)/Cr were measured from an 8-cm3 voxel in the posterior periventricular white matter (WM) using proton MRS. Intracellular pH (pHi) was calculated using phosphorus MRS. Eighteen infants had normal WM on MR imaging; 12 had WMD. For infants with WMD, lactate/Cr and myo-inositol/Cr were related (P < 0.01); lactate/Cr and pHi were not (P = 0.8). In the WMD group, mean lactate/Cr and myo-inositol/Cr were higher (P < 0.001, P < 0.05, respectively) than the normal WM group. There was no difference in the NAA/Cr, Cho/Cr, or pHi between the two groups, although pHi was not measured in all infants. These findings suggest that WMD in the preterm infant at term has a different biochemical profile compared with the term infant after perinatal HI.

Lactate prevents the alterations in tissue amino acids, decline in ATP, and cell damage due to aglycemia in retina

Under conditions of energy impairment, CNS tissue can utilize substrates other than glucose to maintain energy metabolism. Retinas produce large amounts of lactate, although it has not been shown that lactate can be utilized by retina to prevent the cell damage associated with hypoglycemia. To investigate this, intact, isolated retinas were subjected to aglycemic conditions in the presence or absence of 20 mM lactate. Retinas incubated in the absence of glucose for 60 min showed a threefold elevation in tissue aspartate and 60% decreases in tissue glutamate and glutamine, demonstrating a mobilization of carbon from glutamine and glutamate to the tricarboxylic acid cycle. Lactate prevented these changes in tissue amino acids, indicating metabolism of lactate with sparing of tissue glutamate and glutamine. Tissue ATP was 20 and 66% of control values with zero glucose or zero glucose plus lactate, respectively. Consistent with previous findings, incubation of retinas in the absence of glucose caused acute swelling of retinal neurons and release of GABA into the medium at 60 min. These acute toxic affects caused by the absence of glucose were completely prevented by the presence of lactate. At 24 h of recovery following 60 min of zero glucose, many pyknotic profiles were observed and lactate dehydrogenase (LDH) release into the medium was elevated sevenfold, indicating the extent of cell death. In contrast, no elevation in LDH was found and histology appeared normal in retinas exposed to zero glucose in the presence of lactate. alpha-Cyano-4-hydroxy cinnamate (4-CIN; 0.5 mM), an inhibitor of the monocarboxylic acid transporter and mitochondrial pyruvate carrier, blocked the ability of lactate to maintain ATP and protect retinas from aglycemia but had no effect on ATP or toxicity per se. Derangements in tissue aspartate, glutamate, and glutamine, which were prevented by lactate during zero glucose incubation, were again observed with lactate plus zero glucose in the presence of 4-CIN. However, 0.5 mM 4-CIN alone in the presence of glucose produced similar increases in aspartate and decreases in glutamate and glutamine as observed with zero glucose while having only modest inhibitory effects on [U-(14)C]lactate uptake, suggesting the mitochondrial pyruvate carrier as the main site of action. The above findings show that lactate is readily utilized by the chick retina during glucose deprivation to prevent derangements in tissue amino acids and ATP and retinal neuronal cell death.

Monocarboxylic acid transporters, MCT1 and MCT2, in cortical astrocytes in vitro and in vivo

We used sequence-specific antibodies to characterize two monocarboxylic acid transporters, MCT1 and MCT2, in astrocytes. Both proteins are expressed in primary cultures of cortical astrocytes, as indicated by immunoblotting and immunofluorescence. Both MCT1 and MCT2 are present in small, punctate structures in the cytoplasm and at the cell membrane. Cells showing very low levels of labeling for glial fibrillary acidic protein (GFAP) also label more dimly for MCT2, but not for MCT1. In vivo, double-label immunofluorescence studies coupled with confocal microscopy indicate that MCT1 and MCT2 are rare in astrocytes in the cortex. However, they are specifically labeled in astrocytes of the glial limiting membrane and in white matter tracts. Both transporters are also present in the microvasculature. Comparison of labeling for MCT1 and MCT2 with markers of the blood-brain barrier shows that the transporters are not always limited to the astrocytic endfeet in vivo. Our results suggest that the level of expression of monocarboxylic acid transporters MCT1 and MCT2 by cortical astrocytes in vivo is significantly lower than in vitro but that astrocytes in some other regions of the brain can express one or both proteins in significant amounts.

Association between cell swelling and glycogen content in cultured astrocytes

Treatment of cultured rat astrocytes with hypotonic media or with 1 mM glutamate for 90 min caused cell swelling and a significant increase in glycogen content. Conversely, treatment with hypertonic media caused cell shrinkage with a corresponding decrease in astrocyte glycogen, which was proportional to the increasing osmolality of the hypertonic media. The glutamate receptor antagonist, MK-801, lowered both the glutamate-induced swelling and glycogen increase. These findings demonstrate a correlation between changes in cell volume and astrocyte glycogen content. This may explain the increased astrocytic glycogen observed in many neuropathological conditions where astrocyte swelling occurs. Because glycogen represents the largest energy reserve in the central nervous system, a swelling-induced disturbance in glycogen metabolism may lead to abnormal glial-neuronal interactions resulting in impaired brain bioenergetics.

Mitochondria and neuronal survival

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca(2+), generate reactive oxygen species, and undergo Ca(2+)-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

Effects of vaginal birth versus caesarean section birth with general anesthesia on blood gases and brain energy metabolism in neonatal rats

Using a rat model, several laboratories have demonstrated long-term effects of Caesarean section (C-section) birth or of global hypoxia during C-section birth on a variety of central nervous system (CNS) parameters. These studies used C-section delivery from rapidly decapitated dams, to avoid confounding anesthetic effects, or from dams anesthetized with halothane or ether under unspecified conditions. Systemic oxygenation or cerebral energy metabolites in the pups at birth have not been systematically measured in this model. To develop and characterize a C-section model with relevance to the human situation, the present study measured arterial/venous blood gases and pH and brain ATP and lactate, a widely accepted measure of CNS hypoxia, in pups born either vaginally, by C-section from decapitated dams, or by C-section from dams anesthetized with nitrous oxide (N2O) and increasing concentrations of isoflurane under well-defined conditions. Immediately after birth, pups born vaginally, by C-section with maternal decapitation, or by C-section with 2.5% isoflurane showed no group differences in systemic pO2 or pH or brain ATP levels, but pCO2 was elevated in the C-section/2.5% isoflurane group. Pups born by C-section with 3.0, 3.5, or 4.0% isoflurane, showed progressive reductions in blood pO2 and increases in pCO2 and blood pH was reduced with 3.5% isoflurane. Relative to vaginal birth, brain lactate levels were unchanged in pups born by C-section with any concentration (2.5-4.0%) of isoflurane, but reduced in pups born by C-section from decapitated dams. At 1 h (and 4 h) after birth, in both vaginally born controls and the 2.5% isoflurane group, brain lactate fell while blood pO2 and brain ATP remained stable. In the 3.0, 3.5, or 4.0% isoflurane groups, blood gases and pH and brain lactate also normalized to control values. In conclusion, rat neonates show minimal signs of systemic or CNS hypoxia following C-section birth under 2.5% isoflurane with N2O. However, there is a rather narrow window of isoflurane concentrations which produces effective maternal anesthesia without producing respiratory compromise in the neonate. Thus the results indicate that the level of maternal anesthesia employed is an important factor influencing neonatal systemic and CNS oxygenation during C-section birth.

Hyperthermic preconditioning prevents blood-brain barrier disruption produced by hypoxia-ischemia in newborn rat

Effect of hyperthermic preconditioning on disruption of blood-brain barrier induced by hypoxic-ischemic insult was examined. Seven-day-old Wistar rats were separated into (1) pre-heated (15 min of hyperthermia at 41.5-42.0 degrees C) or (2) non-heated group (33 degrees C). Twenty-four hours after conditioning, all rats were subjected to 2 h hypoxia-ischemia (8% oxygen/92% nitrogen, at 33 degrees C), then 24 h later all brains were examined for immunohistological staining of endogenous macromolecule, IgG extravasation and staining of microtubule-associated protein 2 (MAP2) with MAP2 loss being an early marker of neuronal damage. Significant reduction in the area of IgG staining and loss of MAP2 staining was observed in the pre-heated group as compared to that in non-heated group. There was a close relationship between IgG staining and MAP2 staining loss, with the former area always found in the latter, suggesting that extravasation associates neuronal injury. Serum cortisol concentration in pre-heated group was significantly higher than that in non-heated group 24 h after hyperthermic treatment (14.8+/-0.6 vs. 12.5+/-0.3 ng/ml, p<0.01). These data indicate that hyperthermic preconditioning prevents disruption of blood-brain barrier, resulting in amelioration of hypoxic-ischemic neuronal damage in newborn rat.

Monocarboxylate transporter (MCT1) abundance in brains of suckling and adult rats: a quantitative electron microscopic immunogold study

Transcellular transport of energy substrates across the vascular endothelial cells of the brain is accomplished by integral membrane carrier proteins, such as the glucose transporter GLUT1 and the monocarboxylic acid transporter MCT1. The abundance of these proteins may vary depending on age and nutritional status. In this study we compared the expression of MCT1 in cerebral cortex of suckling and adult rats to determine whether the former, which use considerably more monocarboxylates such as lactate and ketone bodies as fuel than do older rats, correspondingly express more MCT1 than adults. Using electron microscopic immunogold methods, we found that 17-day old suckling rat pups had 25 times more MCT1 labeling in the membranes of capillary endothelial cells than adults. This transporter was nearly equally distributed in luminal and abluminal membranes with less than 10% of the immunogold particles in the endothelial cytoplasmic compartment. The suckling rats also had 15 times more immunogold particles associated with pericyte membranes and 19 times heavier labeling of membranes associated with astrocytic end feet adjacent to microvessels. Neuropil and choroid plexus were lightly labeled. Some MCT1-positive astrocyte and neuron cell bodies were observed, suggesting active synthesis of MCT1 by these cells. The potential for regulation of expression of MCTs by dietary or other factors may have important consequences for the progression and treatment of cerebrovascular disorders and other diseases.

Monocarboxylate transporter expression in mouse brain

Although glucose is the major metabolic fuel needed for normal brain function, monocarboxylic acids, i.e., lactate, pyruvate, and ketone bodies, can also be utilized by the brain as alternative energy substrates. In most mammalian cells, these substrates are transported either into or out of the cell by a family of monocarboxylate transporters (MCTs), first cloned and sequenced in the hamster. We have recently cloned two MCT isoforms (MCT1 and MCT2) from a mouse kidney cDNA library. Northern blot analysis revealed that MCT1 mRNA is ubiquitous and can be detected in most tissues at a relatively constant level. MCT2 expression is more limited, with high levels of expression confined to testes, kidney, stomach, and liver and lower levels in lung, brain, and epididymal fat. Both MCT1 mRNA and MCT2 mRNA are detected in mouse brain using antisense riboprobes and in situ hybridization. MCT1 mRNA is found throughout the cortex, with higher levels of hybridization in hippocampus and cerebellum. MCT2 mRNA was detected in the same areas, but the pattern of expression was more specific. In addition, MCT1 mRNA, but not MCT2, is localized to the choroid plexus, ependyma, microvessels, and white matter structures such as the corpus callosum. These results suggest a differential expression of the two MCTs at the cellular level.

Mitochondria and neuronal glutamate excitotoxicity

The role of mitochondria in the control of glutamate excitotoxicity is investigated. The response of cultured cerebellar granule cells to continuous glutamate exposure is characterised by a transient elevation in cytoplasmic free calcium concentration followed by decay to a plateau as NMDA receptors partially inactivate. After a variable latent period, a secondary, irreversible increase in calcium occurs (delayed calcium deregulation, DCD) which precedes and predicts subsequent cell death. DCD is not controlled by mitochondrial ATP synthesis since it is unchanged in the presence of the ATP synthase inhibitor oligomycin in cells with active glycolysis. However, mitochondrial depolarisation (and hence inhibition of mitochondrial calcium accumulation) without parallel ATP depletion (oligomycin plus either rotenone or antimycin A) strongly protects the cells against DCD. Glutamate exposure is associated with an increase in the generation of superoxide anion by the cells, but superoxide generation in the absence of mitochondrial calcium accumulation is not neurotoxic. While it is concluded that mitochondrial calcium accumulation plays a critical role in the induction of DCD we can find no evidence for the involvement of the mitochondrial permeability transition.

Expression of monocarboxylate transporter mRNAs in mouse brain: support for a distinct role of lactate as an energy substrate for the neonatal vs. adult brain

Under particular circumstances like lactation and fasting, the blood-borne monocarboxylates acetoacetate, beta-hydroxybutyrate, and lactate represent significant energy substrates for the brain. Their utilization is dependent on a transport system present on both endothelial cells forming the blood-brain barrier and on intraparenchymal brain cells. Recently, two monocarboxylate transporters, MCT1 and MCT2, have been cloned. We report here the characterization by Northern blot analysis and by in situ hybridization of the expression of MCT1 and MCT2 mRNAs in the mouse brain. In adults, both transporter mRNAs are highly expressed in the cortex, the hippocampus and the cerebellum. During development, a peak in the expression of both transporters occurs around postnatal day 15, declining rapidly by 30 days at levels observed in adults. Double-labeling experiments reveal that the expression of MCT1 mRNA in endothelial cells is highest at postnatal day 15 and is not detectable at adult stages. These results support the notion that monocarboxylates are important energy substrates for the brain at early postnatal stages and are consistent with the sharp decrease in blood-borne monocarboxylate utilization after weaning. In addition, the observation of a sustained intraparenchymal expression of monocarboxylate transporter mRNAs in adults, in face of the seemingly complete disappearance of their expression on endothelial cells, reinforces the view that an intercellular exchange of lactate occurs within the adult brain.

Endogenous monocarboxylates sustain hippocampal synaptic function and morphological integrity during energy deprivation

The ability to fuel neurons via glycogenolysis is believed to be an important function of glia. Indeed, the slow, rather than immediate, depression of synaptic transmission in hippocampal slices during exogenous glucose deprivation suggests that intrinsic energy reservoirs help to sustain neurotransmission. It is believed that glia fuel neighboring neurons via diffusible monocarboxylates such as pyruvate and lactate, although a role for glucose has been proposed also. Using alpha-cyano-4-hydroxycinnamate (4-CIN) to inhibit monocarboxylate transport and cytochalasin B (CCB) to inhibit glucose transport, we examined the role of glucose and monocarboxylates in supporting the functional and morphological integrity of hippocampal neurons during glucose deprivation. Although 200 microM 4-CIN failed to depress EPSPs supported by 10 mM glucose, pretreatment with 4-CIN accelerated the depression of EPSPs during glucose deprivation. 4-CIN also accelerated the decline in glucose-supported EPSPs after administration of 50 microM CCB, whereas CCB failed to alter the slow decay of pyruvate-supported EPSPs during pyruvate deprivation. 4-CIN did not alter the morphology of pyramidal neurons in the presence of 10 mM glucose but produced significant damage during glucose deprivation or CCB administration. These results suggest that endogenous monocarboxylates rather than glucose maintain neuronal integrity during energy deprivation. Furthermore, EPSPs supported by 2-3.3 mM glucose were sensitive to 4-CIN, suggesting that endogenous monocarboxylates are involved in maintaining neuronal function even under conditions of mild glucose deprivation.

Effect of tactile stimulation on serum lactate in the newborn rat

Maturation of the CNS in neonatal animals is dependent upon both sensory input and the constant availability of metabolic fuel. Previous reports indicate that the preferred metabolic substrate for the developing rat brain is lactate. In this study, we used the neonatal Sprague-Dawley rat to investigate a possible interactive role between touch and the regulation of serum lactate. Two hundred and fifty rats (postnatal d 0-7) were exposed to a standard tactile stimulation (TS) regimen to mimic nonspecific maternal stimulation. This regimen consisted of stroking the dorsum with a soft camel hair brush for 30 s every minute for 10 min. Serum lactate and glucose levels were measured after TS. In newborn (d 0) rats, lactate levels were increased by 207% in stroked pups versus controls. This elevation of serum lactate persisted for 30 min after cessation of TS. On d 7, TS increased lactate only 11%. Glucose levels were unaffected at all ages. In neonatal pups, pretreatment with pentobarbital blocked the effect of TS, whereas epidermal growth factor evoked a synergistic response. Capsaicin pretreatment had no effect. Mixed arteriovenous blood gases revealed a mild increase in pH and a decrease in Pco2 after TS. We conclude that TS in newborn rats is a regulator of circulating lactate. This response is maximal in the immediate postnatal period and wanes over the 1st wk of life. We speculate that the transduction of sensory signals by the skin is a mechanism regulating the availability of cerebral energy substrates in the newborn mammal.

Amino acid metabolism in rat hippocampus during the period of brain growth spurt

We studied protein synthesis, lipid synthesis and CO2 production by oxidation of glycine, alanine and leucine by slices of rat hippocampus during the period of brain growth spurt. The metabolism of the three amino acids decreased with the age of the animals. A major reduction was observed in protein synthesis, which was 4 times higher at 7 days of age than at 21 days of age for all amino acids studied. Glycine oxidation to CO2 was twice as high as alanine oxidation and ten times higher than leucine oxidation. The major pathway of leucine utilization was incorporation into proteins. Glycine was the amino acid that had the highest metabolic rate.

Effect of valproate on the metabolism of the central nervous system

The effects of valproate on brain energy and lipid metabolism is reviewed. Increasing evidence suggests that valproate uses the monocarboxylic acid carrier in order to cross the blood brain barrier (BBB) and the neural cell plasma membranes. The uptake of valproate into the brain through this mechanism would compete with the uptake of energy precursors, such as the monocarboxylic acids 3-hydroxybutyrate, lactate or pyruvate and with some amino acids, but not with glucose. This could impair brain fuel utilization, specially during the neonatal period or childhood, when lactate or 3-hydroxybutyrate furnishes alternative substrates to glucose for the brain. It is concluded that valproate interference with energy metabolism may have implications for the therapeutic action of the drug, stressing the possibility that valproate-mediated alterations in brain lipid synthesis may contribute to the pharmacological action of the drug.

Reduced glucose metabolism enhances the glutamate-evoked release of arachidonic acid from striatal neurons

Glucose deprivation potentiates the glutamate receptor-evoked release of arachidonic acid from cultured mouse striatal neurons. In this study we investigated whether this potentiation would be modified by the end-products of glycolysis. These enhanced responses were completely reversed by the addition of increasing concentrations of either lactate or pyruvate. This reversal was not due to increased osmolarity as substituting sucrose for lactate or pyruvate did not mimic their effects. In contrast, in the presence of glucose, neither lactate nor pyruvate was effective. Furthermore, these monocarboxylic acids rescued neuronal respiration in the absence of glucose. Inhibiting glycolysis with iodoacetate in the presence of glucose reproduced the potentiated glutamate-evoked release of arachidonic acid observed following glucose deprivation and reduced neuronal respiration to the same extent as that observed in the absence of glucose. All of these effects were overcome by the addition of either lactate or pyruvate. The reversal of the potentiated glutamate-evoked release of arachidonic acid by lactate or pyruvate was inhibited by a specific inhibitor of monocarboxylic acid transport, alpha-cyano-4-hydroxycinnamic acid, suggesting that lactate and pyruvate act intracellularly. Therefore, we propose that the enhanced release of arachidonic acid evoked by glutamate during glucose deprivation results from reduced glycolysis and hence from a depletion of lactate or pyruvate.

Fuel utilization by early newborn brain is preserved under congenital hypothyroidism in the rat

Mental retardation associated with hypothyroidism may be caused by impairment of brain ketone body-metabolizing enzymes during the suckling period. However, much evidence suggests that, immediately after delivery, lactate, instead of ketone bodies or glucose, may be the best substrate for the brain. In this work, we have studied the effect of experimentally induced congenital hypothyroidism on the rate of lactate, glucose, and 3-hydroxybutyrate utilization in early neonatal brain slices. Methimazole (MMI) administration to the mothers caused a 5.4- and 1.7-fold decrease in neonatal plasma concentrations of L-thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3), respectively. Propylthiouracil (PTU) administration to the mothers caused a 7.3- and > 2-fold decrease in plasma T4 and T3 concentrations, respectively. MMI-induced hypothyroidism did not significantly modify the rate of lactate, glucose, or 3-hydroxybutyrate oxidation to CO2 and their incorporation into lipids by the neonatal brain. However, PTU-induced hypothyroidism decreased the rate of lactate and glucose oxidation to CO2 and their incorporation into lipids by 17% (p < 0.05). 3-Hydroxybutyrate utilization was not modified by this treatment. Separation by HPLC of the lipids revealed that PTU-mediated inhibition of lipid synthesis from lactate and glucose may be accounted for by specific inhibition of the rate of sterol synthesis (15%, p < 0.05), whereas the rate of phospholipid synthesis was unaffected. These results suggest that the early newborn may develop mechanisms aimed at avoiding the possible brain damage caused by the inhibition of lipid synthesis brought about by mild neonatal hypothyroidism.

Metabolic fuel utilization and pyruvate oxidation during the postnatal period

The transplacental supply of nutrients is interrupted at birth, which diverts maternal metabolism to lactation. After birth, energy homeostasis is rapidly regained through milk nutrients which supply the newborn with the fatty acids and ketone bodies required for neonatal development. However, immediately after birth and before the onset of suckling there is a time lapse in which the newborn undergoes a unique kind of starvation. During this period glucose is scarce and ketone bodies are not available owing to the delay in ketogenesis. Under these circumstances, the newborn is supplied with another metabolic fuel, lactate, which is utilized as a source of energy and carbon skeletons. Neonatal rat lung, heart, liver and brain utilize lactate for energy production and lipogenesis. Lactate is also utilized by the brain of human babies with type I glycogenosis. Both rat neurons and astrocytes in primary culture actively use lactate as an oxidizable substrate and as a precursor of phospholipids and sterols. Lactate oxidation is enhanced by dichloroacetate, an inhibitor of the pyruvate dehydrogenase kinase in neurons but not in astrocytes, suggesting that the pyruvate dehydrogenase is regulated differently in each type of cell. Despite the low activity of this enzyme in newborn brain, pyruvate decarboxylation is the main fate of glucose in both neurons and astrocytes. The occurrence of a yeast-like pyruvate decarboxylase activity in neonatal brain may explain these results.