Nutrients for the brain: problems in supply

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Brain Glucose Metabolism: Integration of Energetics with Function

Glucose is the long-established, obligatory fuel for brain that fulfills many critical functions, including ATP production, oxidative stress management, and synthesis of neurotransmitters, neuromodulators, and structural components. Neuronal glucose oxidation exceeds that in astrocytes, but both rates increase in direct proportion to excitatory neurotransmission; signaling and metabolism are closely coupled at the local level. Exact details of neuron-astrocyte glutamate-glutamine cycling remain to be established, and the specific roles of glucose and lactate in the cellular energetics of these processes are debated. Glycolysis is preferentially upregulated during brain activation even though oxygen availability is sufficient (aerobic glycolysis). Three major pathways, glycolysis, pentose phosphate shunt, and glycogen turnover, contribute to utilization of glucose in excess of oxygen, and adrenergic regulation of aerobic glycolysis draws attention to astrocytic metabolism, particularly glycogen turnover, which has a high impact on the oxygen-carbohydrate mismatch. Aerobic glycolysis is proposed to be predominant in young children and specific brain regions, but re-evaluation of data is necessary. Shuttling of glucose- and glycogen-derived lactate from astrocytes to neurons during activation, neurotransmission, and memory consolidation are controversial topics for which alternative mechanisms are proposed. Nutritional therapy and vagus nerve stimulation are translational bridges from metabolism to clinical treatment of diverse brain disorders.

Brain barrier properties and cerebral blood flow in neonatal mice exposed to cerebral hypoxia-ischemia

Insults to the developing brain often result in irreparable damage resulting in long-term deficits in motor and cognitive functions. The only treatment today for hypoxic-ischemic encephalopathy (HIE) in newborns is hypothermia, which has limited clinical benefit. We have studied changes to the blood-brain barriers (BBB) as well as regional cerebral blood flow (rCBF) in a neonatal model of HIE to further understand the underlying pathologic mechanisms. Nine-day old mice pups, brain roughly equivalent to the near-term human fetus, were subjected to hypoxia-ischemia. Hypoxia-ischemia increased BBB permeability to small and large molecules within hours after the insult, which normalized in the following days. The opening of the BBB was associated with changes to BBB protein expression whereas gene transcript levels were increased showing direct molecular damage to the BBB but also suggesting compensatory mechanisms. Brain pathology was closely related to reductions in rCBF during the hypoxia as well as the areas with compromised BBB showing that these are intimately linked. The transient opening of the BBB after the insult is likely to contribute to the pathology but at the same time provides an opportunity for therapeutics to better reach the infarcted areas in the brain.

Therapeutic hypothermia achieves neuroprotection via a decrease in acetylcholine with a concurrent increase in carnitine in the neonatal hypoxia-ischemia

Although therapeutic hypothermia is known to improve neurologic outcomes after perinatal cerebral hypoxia-ischemia, etiology remains unknown. To decipher the mechanisms whereby hypothermia regulates metabolic dynamics in different brain regions, we used a two-step approach: a metabolomics to target metabolic pathways responding to cooling, and a quantitative imaging mass spectrometry to reveal spatial alterations in targeted metabolites in the brain. Seven-day postnatal rats underwent the permanent ligation of the left common carotid artery followed by exposure to 8% O2 for 2.5 hours. The pups were returned to normoxic conditions at either 38 °C or 30 °C for 3 hours. The brain metabolic states were rapidly fixed using in situ freezing. The profiling of 107 metabolites showed that hypothermia diminishes the carbon biomass related to acetyl moieties, such as pyruvate and acetyl-CoA; conversely, it increases deacetylated metabolites, such as carnitine and choline. Quantitative imaging mass spectrometry demarcated that hypothermia diminishes the acetylcholine contents specifically in hippocampus and amygdala. Such decreases were associated with an inverse increase in carnitine in the same anatomic regions. These findings imply that hypothermia achieves its neuroprotective effects by mediating the cellular acetylation status through a coordinated suppression of acetyl-CoA, which resides in metabolic junctions of glycolysis, amino-acid catabolism, and ketolysis.

Substrate competition studies demonstrate oxidative metabolism of glucose, glutamate, glutamine, lactate and 3-hydroxybutyrate in cortical astrocytes from rat brain

It is well established that astrocytes can utilize many substrates to support oxidative energy metabolism; however, use of energy substrates in the presence of other substrates, as would occur in vivo, has not been systematically evaluated. Substrate competition studies were used to determine changes in the rates of (14)CO(2) production since little is known about the interaction of energy substrates in astrocytes. The rates of (14)CO(2) production from 1 mM D-[6-(14)C]glucose, L-[U-(14)C]glutamate, L-[U-(14)C]glutamine, D-3-hydroxy[3-(14)C]butyrate, L-[U-(14)C]lactate and L-[U-(14)C]malate by primary cultures of astrocytes from rat brain were determined to be 1.17 ± 0.19, 85.30 ± 12.25, 28.04 ± 2.84, 13.55 ± 4.56, 14.84 ± 2.40 and 5.20 ± 1.20 nmol/h/mg protein (mean ± SEM), respectively. The rate of (14)CO(2) production from glutamate oxidation was higher than that of the other substrates Addition of unlabeled glutamate significantly decreased the rates of (14)CO(2) production from all other substrates studied; however, glutamate oxidation was not altered by the addition of any of the other substrates. The rate of (14)CO(2) production of glutamine was decreased by glutamate, but not altered by other substrates. The rate of (14)CO(2) production from glucose was significantly decreased by the addition of unlabeled glutamate, glutamine or lactate, but not by 3-hydroxybutyrate or malate. Addition of unlabeled glucose did not significantly alter the (14)CO(2) production from any other substrate. (14)CO(2) production from lactate was decreased by the addition of unlabeled glutamine or glutamate and increased by addition of malate. The (14)CO(2) production from malate was decreased by the addition of unlabeled glutamate or lactate, but was not altered by the other substrates. The substrate utilization for oxidative energy metabolism in astrocytes is very different than the profile previously reported for synaptic terminals. These studies demonstrate the potential use of multiple substrates including glucose, glutamate, glutamine, lactate and 3-hydroxybutyrate as energy substrates for astrocytes. The data also provide evidence of interactions of substrates and multiple compartments of TCA cycle activity in cultured astrocytes.

The ketogenic diet has no effect on the expression of spike-and-wave discharges and nutrient transporters in genetic absence epilepsy rats from Strasbourg

The genetic absence epilepsy rat from Strasbourg is considered an isomorphic, predictive, and homologous model of typical childhood absence epilepsy. It is characterized by the expression of spike-and-wave discharges (SWDs) in the thalamus and cortex. The ketogenic diet (KD) is successfully used in humans and animals with various types of seizures, but was not effective in children with intractable atypical absence epilepsy. Here, we studied its potential impact on the occurrence of SWDs in genetic absence epilepsy rat from Strasbourg. Rats were fed the KD for 3 weeks during which they were regularly subjected to the electroencephalographic recording of SWDs. The KD did not influence the number and duration of SWDs despite a 15-22% decrease in plasma glucose levels and a large increase in beta-hydroxybutyrate levels. Likewise, the KD did not affect the level of expression of the blood-brain barrier glucose transporter GLUT1 or of the monocarboxylate transporters, MCT1 and MCT2. This report extends the observation in humans that the KD does not appear to show effectiveness in intractable atypical absence epilepsy to this model of typical childhood absence epilepsy which responds to specific antiepileptic drugs.

Direct measurement of oxidative metabolism in the living brain by microdialysis: a review

This review summarizes microdialysis studies that address the question of which compounds serve as energy sources in the brain. Microdialysis was used to introduce 14C-labeled glucose, lactate, pyruvate, glutamate, glutamine, and acetate into the interstitial fluid of the brain to observe their metabolism to 14CO2. Although glucose uptake from the systemic system supplies the carbon source for these compounds, compounds synthesized from glucose by the brain are subject to recycling including complete metabolism to CO2. Therefore, the brain utilizes multiple compounds in its domain to provide the energy needed to fulfill its function. The physiological conditions controlling metabolism and the contribution of compartmentation into different brain regions, cell types, and subcellular spaces are still unresolved. The aconitase inhibitor fluorocitrate, with a lower inhibition threshold in glial cells, was used to identify the proportion of lactate and glucose that was oxidized in glial cells versus neurons. The fluorocitrate data suggest that glial and neuronal cells are capable of utilizing both lactate and glucose for energy metabolism.

Effect of fluorocitrate on cerebral oxidation of lactate and glucose in freely moving rats

Glucose is the primary carbon source to enter the adult brain for catabolic and anabolic reactions. Some studies suggest that astrocytes may metabolize glucose to lactate; the latter serving as a preferential substrate for neurons, especially during neuronal activation. The current study utilizes the aconitase inhibitor fluorocitrate to differentially inhibit oxidative metabolism in glial cells in vivo. Oxidative metabolism of 14C-lactate and 14C-glucose was monitored in vivo using microdialysis and quantitating 14CO2 in the microdialysis eluate following pulse labeling of the interstitial glucose or lactate pool. After establishing a baseline oxidation rate, fluorocitrate was added to the perfusate. Neither lactate nor glucose oxidation was affected by 5 micromol/L fluorocitrate. However, 20 and 100 micromol/L fluorocitrate reduced lactate oxidation by 55 +/- 20% and 68 +/- 12%, respectively (p < 0.05 for both). Twenty and 100 micromol/L fluorocitrate reduced 14C-glucose oxidation by 50 +/- 14% (p < 0.05) and 24 +/- 19% (ns), respectively. Addition of non-radioactive lactate to (14)C-glucose plus fluorocitrate decreased 14C-glucose oxidation by an additional 29% and 38%, respectively. These results indicate that astrocytes oxidize about 50% of the interstitial lactate and about 35% of the glucose. By subtraction, neurons metabolize a maximum of 50% of the interstitial lactate and 65% of the interstitial glucose.

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.

Age-dependent pathways of brain energy metabolism: the suckling rat, a natural model of the ketogenic diet

As a consequence of the high fat content of maternal milk, the suckling rat may be viewed as a 'natural model' of the ketogenic diet. Changes in energy metabolism during this period of development may give us some clues into the antiepileptic properties of the ketogenic diet. We have, therefore studied the postnatal evolution of local cerebral metabolic rates for glucose (LCMRglcs) and of regional rates of cerebral uptake of beta-hydroxybutyrate (betaHB) in the developing rat between postnatal day (PN) 10 and 35. LCMRglcs were low and homogeneous at PN10. They increased significantly in four auditory regions between PN10 and PN14, at the time of maturation of auditory function. Between PN14 and PN17, they increased further in two auditory regions, one visual area (the lateral geniculate nucleus), three limbic and three motor areas. These increases occurred simultaneously with the maturation of vision and the development of locomotion and general exploratory behavior. Between PN17 and PN21, LCMRglcs increased by 28-97% (depending on brain area) and by a mean value of 25% in all areas studied. In contrast to the function-related increases in LCMRglcs, regional rates of cerebral betaHB uptake underwent a generalized non-specific increase between PN1O and PN14, and stayed at a high level until PN17. Between PN17 and PN21, rates of cerebral betaHB uptake decreased significantly in all brain regions studied, and reached very low levels by PN35. Thus, even in the suckling rat, whose cerebral metabolic activity depends upon both glucose and ketone bodies, it is the postnatal increases in LCMRglcs that appear to be critical for the acquisition of new functions and neurological competence. Conversely, the homogeneous increase in cerebral betaHB uptake occurring between PN10 and PN17 at a period of active brain growth may rather reflect non-specific mechanisms of cell growth.

Effect of alpha-ketoisocaproate and leucine on the in vivo oxidation of glutamate and glutamine in the rat brain

Leucine and alpha-ketoisocaproate (alpha-KIC) were perfused at increasing concentrations into rat brain hippocampus by microdialysis to mimic the conditions of maple syrup urine disease. The effects of elevated leucine or alpha-KIC on the oxidation of L-[U-14C]glutamate and L-[U-14C]glutamine in the brain were determined in the non-anesthetized rat. 14CO2 generated by the metabolic oxidation of [14C]glutamate and [14C]glutamine in brain was measured following its diffusion into the eluant during the microdialysis. Leucine and alpha-KIC exhibited differential effects on 14CO2 generation from radioactive glutamate on glutamine. Infusion of 0.5 mM alpha-KIC increased [14C]glutamate oxidation approximately 2-fold; higher concentrations of alpha-KIC did not further stimulate [14C]glutamate oxidation. The enhanced oxidation of [14C]glutamate may be attributed to the function of alpha-KIC as a nitrogen acceptor from [14C]glutamate yielding [14C]alpha-ketoglutarate, an intermediate of the tricarboxylic acid cycle. [14C]glutamine oxidation was not stimulated as much as [14C]glutamate oxidation and only increased at 10 mM alpha-KIC reflecting the extra metabolic step required for its oxidative metabolism. In contrast, leucine had no effect on the oxidation of either [14C]glutamate or [14C]glutamine. In maple syrup urine disease elevated alpha-KIC may play a significant role in altered energy metabolism in brain while leucine may contribute to clinical manifestations of this disease in other ways.

Lactate spares glucose as a metabolic fuel in neurons and astrocytes from primary culture

The effect of lactate on glucose metabolism in neurons and astrocytes from primary culture has been studied. The rates of glucose metabolism through the pentose-phosphate shunt, the pyruvate dehydrogenase-catalyzed reaction, the tricarboxylic acid cycle, the total lipogenesis and the synthesis of glycerol-borne lipids in astrocytes were 2-3 fold higher than in neurons. However, the rate of glucose incorporation into sterols and esterified fatty acids was similar in both types of cells. Total glucose utilization was inhibited by lactate to the same extend in both neurons and astrocytes. Lactate strongly inhibited glucose oxidation through the pyruvate dehydrogenase-catalyzed reaction and the tricarboxylic acid cycle, in both neurons (60 and 44%, respectively) and astrocytes (64 and 62%, respectively). Glucose incorporation into sterols and fatty acids was also inhibited by lactate in both neurons and astrocytes (57 and 76%, respectively) while the oxidation of glucose in the pentose-phosphate shunt and the synthesis of glycerol-borne lipids was not significantly affected. These results suggest that in the presence of lactate both neurons and astrocytes can utilize lactate as the major metabolic substrate, sparing glucose for the synthesis of NADPH(H+), ribose-5-phosphate and/or glycerol-borne lipids. An interaction between glucose and lactate metabolism at the level of the pyruvate dehydrogenase-catalyzed reaction is suggested.

Transport of 3-hydroxybutyrate by cultured rat brain astrocytes

It is well established that 3-hydroxybutyrate can serve as an energy source for the brain. Since substrate utilization may be regulated in part by transport across the cellular membrane, we investigated the uptake of 3-hydroxybutyrate by primary cultures of rat brain astrocytes. Measurement of the net uptake indicated a saturable system and a Lineweaver-Burke type plot was consistent with a single carrier-mediated mechanism with a Km of 6.03 mM and a Vmax of 32.7 nmol/30 seconds/mg protein. The rate of uptake at pH 6.2 was more than ten times the rate at pH 8.2, with the rate at pH 7.4 being intermediate between these values, suggesting the possibility of cotransport with H+ or exchange with OH- (antiport). Mersalyl had only a slight effect on the transport of 3-hydroxybutyrate, suggesting that sulfhydryl groups are not involved in the transport of this monocarboxylic acid. Phenylpyruvate and alpha-ketoisocaproate also attenuated the transport, but lactate had only a marginal effect. These results suggest that the utilization of 3-hydroxybutyrate as an energy source by astrocytes is regulated in part by carrier-mediated transport and that the uptake system is different from the lactate transport system.

Regulation of malate dehydrogenases from neonatal, adolescent, and mature rat brain

Since the malate-aspartate shuttle in brain has been shown to be closely linked to brain energy metabolism and neurotransmitter synthesis, the activity of MDH, one of the enzymes of the malate-aspartate shuttle, was studied in cortical non-synaptic mitochondria (mMDH) and cytosol (cMDH) in 1-4 day, 18-20 day and 7-8 week old rats. The mean mMDH activity (nmol/min/mg protein) was 10,517 +/- 734 (mean +/- SEM), 8,882 +/- 241 and 10,323 +/- 561 and cMDH activity was 2,453 +/- 99, 4,673 +/- 152 and 6,821 +/- 205 in 1-4 day, 18-20 day and 7-8 week old rats, respectively. While cMDH activity increased with age (p < 0.0001), mMDH activity showed no change. This study also determined if endogenous compounds, previously shown to alter malate metabolism, affected MDH activities. Lactate inhibited only cMDH activity, by a competitive mechanism. Oxaloacetate inhibited mMDH by partial non-competitive inhibition and cMDH by competitive inhibition. Alpha-ketoglutarate competitively inhibited both enzymes; however, the inhibition of mMDH activity was more pronounced than that of cMDH activity. Citrate inhibited mMDH via an uncompetitive mechanism and cMDH via a noncompetitive mechanism. The mechanisms of inhibition of mMDH and cMDH by each of the effectors were the same over the three ages. The results suggest mMDH and cMDH activities show a dissimilar developmental pattern and may be regulated differently by endogenous effectors. The greater sensitivity of mMDH, compared to cMDH, to certain effectors may be related to the dual role of mMDH in the tricarboxylic acid cycle and the malate-aspartate shuttle.

Transport of L-lactate by cultured rat brain astrocytes

Several reports indicate that lactate can serve as an energy substrate for the brain. The rate of oxidation of this substrate by cultured rat brain astrocytes was 3-fold higher than the rate with glucose, suggesting that lactate can serve as an energy source for these cells. Since transport into the astrocytes may play an important role in regulating nutrient use by individuals types of brain cells, we investigated the uptake of L-[U-14C]lactate by primary cultures of rat brain astrocytes. Measurement of the net uptake suggested two carrier-mediated mechanisms and an Eadie-Hofstee type plot of the data supported this conclusion revealing 2 Km values of 0.49 and 11.38 mM and Vmax values of 16.55 and 173.84 nmol/min/mg protein, respectively. The rate of uptake was temperature dependent and was 3-fold higher at pH 6.2 than at 7.4, but was 50% less at pH 8.2. Although the lactate uptake carrier systems in astrocytes appeared to be labile when incubated in phosphate buffered saline for 20 minutes, the uptake process exhibited an accelerative exchange mechanism. In addition, lactate uptake was altered by several metabolic inhibitors and effectors. Potassium cyanide and alpha-cyano-4-hydroxycinnamate inhibited lactate uptake, but mersalyl had little or no effect. Phenylpyruvate, alpha-ketoisocaproate, and 3-hydroxybutyrate at 5 and 10 mM greatly attenuated the rate of lactate uptake. These results suggest that the availability of lactate as an energy source is regulated in part by a biphasic transport system in primary astrocytes.

Local cerebral glucose utilization in brains of lean and genetically obese (fa/fa) rats

The local cerebral glucose utilization (LCGU) of brains from Wistar, lean Zucker (FA/FA), and obese Zucker (fa/fa) rats was investigated using the method of Sokoloff et al. (L. Sokoloff, M. Reiwich, C. Kennedy, M.H. Des Rosiers, C.S. Patlak, K.D. Pettigrew, O. Sakurada, and M. Shinohara. J. Neurochem. 28: 897-916, 1977.). The LCGU of obese Zucker (fa/fa) rats was decreased in comparison to the relatively high values obtained for the lean Zucker (FA/FA) rats in all gray matter areas studied, on average to the extent of 50%. When compared with Wistar rats, several brain areas of lean Zucker (FA/FA) animals had a normal glucose uptake. When these normal areas were assessed for common efferent and afferent pathways, it was found that many of these common connections had normal glucose utilizations. In direct comparison to the obese fa/fa rat, the LCGU rates of these areas were decreased, hinting that this would also be the case for their functional activity. Because these areas (limbic, thalamic, hypothalamic, autonomic) have been reported to be potentially relevant for bringing about the behavioral and neuroendocrine alterations known to occur in obese fa/fa rats, it is proposed that they represent dysfunctions that are partly responsible for the obesity syndrome of the fa/fa strain.

Regional and temporal glycerol changes induced by forebrain ischemia in gerbils

Temporal ischemic changes in glycerol and energy metabolites were studied in the striatum, hippocampus and cortex of gerbils subjected to 15 min of bilateral carotid artery occlusion alone or with various periods of recirculation. The same tissue sample served for the determination of glycerol and energy reserve by a simple enzymatic fluoro- and spectrometric assay after perchloric acid extraction. Cerebral ischemia increased the levels of glycerol (8- to 10-fold) and depleted the energy stores. During the first hour of recirculation, the glycerol content decreased and thereafter (at 2 h), normalized in all structures. However, the glycerol content was still twice as high in the striatum and hippocampus as compared to their respective controls. At the same time, an incomplete restoration of energy reserves was observed in these structures. The findings indicate that glycerol is not a stable postischemic indicator of the ischemia-induced membrane damage.

Metabolism of lactate in the rat brain during the early neonatal period

The metabolism of lactate in isolated cells from early neonatal rat brain has been studied. In these circumstances, lactate was mainly oxidized to CO2, although a significant portion was incorporated into lipids (78% sterols, 4% phosphatidylcholine, 2% phosphatidylethanolamine, and 1% phosphatidylserine). The rate of lactate incorporation into CO2 and lipids was higher than those found for glucose and 3-hydroxybutyrate. Lactate strongly inhibited glucose oxidation through the pyruvate dehydrogenase-catalyzed reaction and the tricarboxylic acid cycle while scarcely affecting glucose utilization by the pentose phosphate pathway. Lipogenesis from glucose was strongly inhibited by lactate without relevant changes in the rate of glycerol phosphate synthesis. These results suggest that lactate inhibits glucose utilization at the level of the pyruvate dehydrogenase-catalyzed reaction, which may be a mechanism to spare glucose for glycerol and NADPH synthesis. The effect of 3-hydroxybutyrate inhibiting lactate utilization only at high concentrations of 3-hydroxybutyrate suggests that before ketogenesis becomes active, lactate may be the major fuel for the neonatal brain. (-)-Hydroxycitrate and aminooxyacetate markedly inhibited lipogenesis from lactate, suggesting that the transfer of lactate carbons through the mitochondrial membrane is accomplished by the translocation of both citrate and N-acetylaspartate.

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.

Lactate utilization by isolated cells from early neonatal rat brain

The utilization of lactate, glucose, 3-hydroxybutyrate, and glutamine has been studied in isolated brain cells from early newborn rats. Isolated brain cells actively utilized these substrates, showing saturation at concentrations near physiological levels during the perinatal period. The rate of lactate utilization was 2.5-fold greater than that observed for glucose, 3-hydroxybutyrate, or glutamine, suggesting that lactate is the main metabolic substrate for the brain immediately after birth. The apparent Km for glucose utilization suggested that this process is limited by the activity of hexokinase. However, lactate, 3-hydroxybutyrate, and glutamine utilization seems to be limited by their transport through the plasma membrane. The presence of fatty acid-free bovine serum albumin (BSA) in the incubation medium significantly increased the rate of lipogenesis from lactate or 3-hydroxybutyrate, although this was balanced by the decrease in their rates of oxidation in the same circumstances. BSA did not affect the rate of glucose utilization. The effect of BSA was due not to the removal of free fatty acid, but possibly to the binding of long-chain acyl-CoA, resulting in the disinhibition of acetyl-CoA carboxylase and citrate carrier.

Metabolic maturation of the brain: a study of local cerebral glucose utilization in the developing cat

Previously, using positron emission tomography (PET), we showed that local cerebral metabolic rates for glucose (lCMRglc) in children undergo dynamic maturational trends before reaching adult values. In order to develop an animal model that can be used to explore the biological significance of the different segments of the lCMRglc maturational curve, we measured lCMRglc in kittens at various stages of postnatal development and in adult cats using quantitative [14C]2-deoxyglucose autoradiography. In the kitten, very low lCMRglc levels (0.14 to 0.53 mumol min-1 g-1) were seen during the first 15 days of life, with phylogenetically older brain regions being generally more metabolically mature than newer structures. After 15 days of age, many brain regions (particularly telencephalic structures) underwent sharp increases of lCMRglc to reach, or exceed, adult rates by 60 days. This developmental period (15 to 60 days) corresponds to the time of rapid synaptic proliferation known to occur in the cat. At 90 and 120 days, a slight decline in lCMRglc was observed, but this was followed by a second, larger peak occurring at about 180 days, when sexual maturation occurs in the cat. Only after 180 days did lCMRglc decrease to reach final adult values (0.21 to 2.04 mumol min-1 g-1). In general, there was good correlation between the metabolic maturation of various neuroanatomical regions and the emergence of behaviors mediated by the specific region. At least in the kitten visual cortex, which has been extensively studied with respect to developmental plasticity, the "critical period" corresponded to that portion of the lCMRglc maturational curve surrounding the 60-day metabolic peak. These normal maturational lCMRglc data will serve as baseline values with which to compare anatomical and metabolic plasticity changes induced by age-related lesions in the cat.

The metabolism of malate by cultured rat brain astrocytes

Since malate is known to play an important role in a variety of functions in the brain including energy metabolism, the transfer of reducing equivalents and possibly metabolic trafficking between different cell types; a series of biochemical determinations were initiated to evaluate the rate of 14CO2 production from L-[U-14C]malate in primary cultures of rat brain astrocytes. The 14CO2 production from labeled malate was almost totally suppressed by the metabolic inhibitors rotenone and antimycin A suggesting that most of malate metabolism was coupled to the electron transport system. A double reciprocal plot of the 14CO2 production from the metabolism of labeled malate revealed biphasic kinetics with two apparent Km and Vmax values suggesting the presence of more than one mechanism of malate metabolism in these cells. Subsequent experiments were carried out using 0.01 mM and 0.5 mM malate to determine whether the addition of effectors would differentially alter the metabolism of high and low concentrations of malate. Effectors studied included compounds which could be endogenous regulators of malate metabolism and metabolic inhibitors which would provide information regarding the mechanisms regulating malate metabolism. Both lactate and aspartate decreased 14CO2 production from 0.01 mM and 0.5 mM malate equally. However, a number of effectors were identified which selectively altered the metabolism of 0.01 mM malate including aminooxyacetate, furosemide, N-acetylaspartate, oxaloacetate, pyruvate and glucose, but had little or no effect on the metabolism of 0.5 mM malate. In addition, alpha-ketoglutarate and succinate decreased 14CO2 production from 0.01 mM malate much more than from 0.5 mM malate. In contrast, a number of effectors altered the metabolism of 0.5 mM malate more than 0.01 mM. These included methionine sulfoximine, glutamate, malonate, alpha-cyano-4-hydroxycinnamate and ouabain. Both the biphasic kinetics and the differential action of many of the effectors on the 14CO2 production from 0.01 mM and 0.5 mM malate provide evidence for the presence of more than one pool of malate metabolism in cultured rat brain astrocytes.

Effect of nerve growth factor on the synthesis of amino acids in PC12 cells

Radioactive short-chain fatty acids preferentially label glutamine relative to glutamate in brain due to compartmentation of glutamine and glutamate. To determine whether this phenomenon occurs in a single cell culture model, we examined the effect of fatty acid chain length on the synthesis as well as pool size of selected amino acids in rat pheochromocytoma PC12 cells, a cell culture model of the large glutamate compartment in neurons. Intracellular 14C-amino acids were quantitated by HPLC, and the incorporation of [U-14C]-glucose, [1-14C]-butyrate, [1-14C]-octanoate, and [1-14C]-palmitate into five amino acids was measured in native and NGF-treated PC12 cells. NGF pretreatment decreased the intracellular concentration of amino acids as did addition of fatty acids but these effects were not additive. Specific activities of amino acids in native cells labelled by 14C-octanoate were 1,300 DPM/nmol, 490 DPM/nmol, 200 DPM/nmol, and 110 DPM/nmol for glutamate, aspartate, glutamine, and serine, respectively. No radioactivity was detected in alanine. Similar specific activities were noted when 14C-butyrate was the precursor; however, there was at least 5-fold less if 14C-palmitate was the precursor. Pretreatment of cells with NGF decreased the specific activity of amino acids by 25-65%. Specific activities of amino acids synthesized from 14C-glucose decreased in the following order: glutamate, 1,640 DPM/nmol; aspartate, 1,210 DPM/nmol; alanine, 580 DPM/nmol; glutamine, 275 DPM/nmol; and serine, 80 DPM/nmol for native cells. NGF pretreatment decreased the specific activities of glutamate and glutamine, but not of the other 3 amino acids. The preferred precursor for glutamate synthesis in native PC12 cells was glucose followed by octanoate, butyrate and palmitate (16:6:3:1).

Synthesis of glutamate and glutamine in dibutyryl cyclic AMP-treated astrocytes

The relative contributions of radioactively labeled fatty acids and glucose to synthesis of glutamate and glutamine were compared in native and dibutyryl cyclic AMP (diBcAMP)-treated primary rat astrocytes. The intracellular specific activities of glutamate and glutamine were 10-fold greater than the specific activities of aspartate or alanine. Butyrate, octanoate and palmitate were equally as effective as precursors for glutamate and glutamine while glucose was 50% as effective as the fatty acids. The specific activity of glutamate and glutamine were identical in the absence of diBcAMP. In diBcAMP treated cells the specific activity of glutamine was greater than that of glutamate when octanoate and palmitate were the labeled precursors. This suggests that cultured astrocytes preferentially utilize free fatty acids for glutamate/glutamine synthesis and that diBcAMP-treated astrocytes contain more than one glutamate compartment.

Glycerol oxidation in discrete areas of rat brain from young, adolescent, and adult rats

We have reported previously that the oxidation rate of [1,3-14C] glycerol to 14CO2 is lower in whole brain homogenates from neonatal rats and increases about 30% during the suckling period to adult levels. To determine whether there are developmental changes in glycerol oxidation in discrete regions of brain, we examined the oxidation of glycerol by homogenates of hypothalamus, cerebellum, brain stem, hippocampus, and cerebral cortex of young (4-6 days) and older (18-20 days) pups and adult (greater than 90 day) rats. The oxidation was measured at both low (0.2 mM) and high (2.0 mM) concentrations of glycerol, since the oxidation of glycerol by brain tissue has been shown to exhibit biphasic kinetics. At each age, and with both concentrations of glycerol, there were significant differences among the discrete brain regions. Although the rate of glycerol oxidation increased with age in most areas of brain, each brain region had a distinct developmental profile. In the hypothalamus, the oxidation of glycerol increased significantly between 4-6 days and adult levels at 18-20 days. The oxidation of glycerol was essentially the same in homogenates of cortex from young and older pups, but it was significantly increased in adults. In contrast with other brain regions, the oxidation of glycerol by brain stem was highest at 4-6 days and significantly decreased with age. The developmental profile of glycerol oxidation in hippocampus and cerebellum was particularly complex, since it increased with age at low, but not at high, concentrations of glycerol. This developmental pattern in hippocampus and cerebellum could be related to changes in the biphasic kinetics of glycerol oxidation and suggests that glycerol metabolism is different in these two areas of brain compared with other areas of brain.

Serum effects on substrate oxidation by dissociated brain cells: possible sites of action

This report is an extension of recent studies indicating the presence of a factor in serum that preferentially inhibits 14CO2 production from labeled glucose. Experiments with dissociated cells revealed that the inhibitory effects of serum were only slightly changed over more than a 50-fold range in initial glucose concentration. Serum had no effect on the rate of glucose transport (uptake of 1,3[3H]2-deoxyglucose). The inhibitory effect of serum was greater on 14CO2 production from [6-14C]glucose than [1-14C]glucose. Other studies revealed that 14CO2 production from [1-14C]pyruvate was more than 5 times the rate obtained using [3-14C]pyruvate; however, the inhibitory effect of serum was much greater on the latter (20% vs 60% inhibition respectively) at 2 mM pyruvate and in the presence of 1% fetal bovine serum. Attempts to characterize the factor using Amicon filtration showed the highest inhibitory activity in a 10,000 mol. wt. fraction, although some inhibitory activity was found in commercial preparations of bovine serum albumin. Delipidation of serum had no effect. Based on these results, we postulate that the observed decrease in labeled CO2 production reflects the regulation of substrate utilization at the pyruvate carboxylase step by one or more factors in serum (with a mol. wt. of approximately 10,000).

Competition of glycerol with other oxidizable substrates in rat brain

The rate of conversion of [1,3-14C]glycerol into 14CO2 was measured in the presence and absence of unlabelled alternative substrates in whole homogenates from the brains of young (4-6 and 18-20 days old) and adult rats. Unlabelled glucose decreased 14CO2 production from [1,3-14C]glycerol by about 40% at all ages studied. Unlabelled 3-hydroxybutyrate significantly decreased the 14CO2 production from both low (0.2 mM) and high (2.0 mM) concentrations of glycerol in 4-6- and 18-20-day-old rat pups. However, the addition of 3-hydroxybutyrate had no effect on the rate of 14CO2 production from 2.0 mM-glycerol in adult rats, suggesting that the interaction of 3-hydroxybutyrate with glycerol in adult rat brain is complex and may be related to the biphasic kinetics previously reported for glycerol oxidation. Unlabelled glutamine decreased the production of 14CO2 by brain homogenates from 18-20-day-old and adult rats, but not in 4-6-day-old rat pups. In the converse situation, the addition of unlabelled glycerol to whole brain homogenates had little effect on the rate of 14CO2 production from [6-14C]glucose, 3-hydroxy[3-14C]butyrate and [U-14C]glutamine, although some significant differences were noted. Collectively these results suggest that glycerol and these other substrates may be metabolized in separate subcellular compartments in brain such that the products of glucose, 3-hydroxybutyrate and glutamine metabolism can dilute the oxidation of glycerol, but the converse cannot occur. The data also demonstrate that there are complex age-related changes in the interaction of glycerol with 3-hydroxybutyrate and glutamine. The fact that glycerol oxidation was only partially suppressed by the addition of 1-5 mM-glucose, -3-hydroxybutyrate or -glutamine could also suggest that glycerol may be selectively utilized as an energy substrate in some discrete brain region.

Acetoacetate and glucose as lipid precursors and energy substrates in primary cultures of astrocytes and neurons from mouse cerebral cortex

Primary cultures of astrocytes and neurons derived from neonatal and embryonic mouse cerebral cortex, respectively, were incubated with [3-14C]acetoacetate or [2-14C]glucose. The utilization of glucose and acetoacetate, the production of lactate, D-3-hydroxybutyrate, and 14CO2, and the incorporation of 14C and of 3H from 3H2O into lipids and lipid fractions were measured. Both cell types used acetoacetate as an energy substrate and as a lipid precursor; lactate was the major product of glucose metabolism. About 60% of the acetoacetate that was utilized by neurons was oxidized to CO2, whereas this was only approximately 20% in the case of cultured astrocytes. This indicates that the rate at which 14C-labeled Krebs cycle intermediates exchange with pools of unlabeled intermediates is much higher in astrocytes than in neurons. Acetoacetate is a better precursor for the synthesis of fatty acids and cholesterol than glucose, presumably because it can be used directly in the cytosol for these processes; preferential incorporation into cholesterol was not observed in these in vitro systems. We conclude that ketone bodies can be metabolized both by the glial cells and by the neuronal cells of developing mouse brain.

Substrate oxidation by isolated rat brain mitochondria and synaptosomes

The rates of [6-14C]-glucose oxidation by reconstituted systems of cytosol and mitochondria or cytosol and synaptosomes were essentially the same as the rate of oxidation of [3-14C]-3-hydroxybutyrate. However, the rate of [U-14C]-glutamine oxidation by mitochondria was 2.5 times that by synaptosomes. The addition of glutamine (5 mM) caused a reduction in the rates of oxidation [6-14C]-glucose of 20% and 40% by mitochondria and synaptosomes, respectively. Conversely, the addition of glucose (5 mM) had little or no effect on the rate of [U-14C]-glutamine oxidation by either organelle. Amino-oxyacetate decreased [U-14C]-glutamine oxidation by mitochondria more than 35% but had little or no effect on the rate of glutamine oxidation by synaptosomes. When glucose (5 mM) was added to [3-14C]-3-hydroxybutyrate, the rates of oxidation by the mitochondria and synaptosomes were increased by 65% and 77%, respectively. However, in the reverse situation the addition of 3-hydroxybutyrate decreased [6-14C]-glucose oxidation by synaptosomes (35%) but did not decrease the rate by mitochondria. These results suggest that differences in the rates of substrate utilization by mitochondria and synaptosomes and differences in substrate interactions in these two subcellular organelles may contribute to metabolic compartmentation in the brain.

Competition among oxidizable substrates in brains of young and adult rats. Whole homogenates

The rates of conversion into 14CO2 of D-(-)-3-hydroxy[3-14C]butyrate, [3-14C]acetoacetate, [6-14C]glucose and [U-14C]glutamine were measured in the presence and absence of unlabelled alternative oxidizable substrates in whole homogenates from the brains of young and adult rats. The addition of unlabelled glutamine resulted in decreased 14CO2 production from [6-14C]glucose in brain homogenates from both young and adult rats. In contrast, glucose had no effect on [U-14C]glutamine oxidation. In suckling animals, both 3-hydroxybutyrate and acetoacetate decreased the rate of oxidation of [6-14C]glucose, but in adults only 3-hydroxybutyrate had an effect, and to a lesser degree. The addition of unlabelled glucose markedly enhanced the rates of oxidation of both ketone bodies in adult brain tissue and had little or no effect in the young. The rate of production of 14CO2 from [U-14C]glutamine was increased by the addition of unlabelled ketone bodies in brain homogenates from young, but not from adult rats. In the converse situation, unlabelled glutamine added to 14C-labelled ketone bodies diminished 14CO2 production in young rats, but had no effect in adult animals. These results revealed a complex age-dependent pattern of interaction in which certain substrates apparently competed with each other, whereas an enhanced rate of 14CO2 production was found with others.