Astrocyte influences on ischemic neuronal death

Glutamate excitotoxicity, oxidative stress, and acidosis are primary mediators of neuronal death during ischemia and reperfusion. Astrocytes influence these processes in several ways. Glutamate uptake by astrocytes normally prevents excitotoxic glutamate elevations in brain extracellular space, and this process appears to be a critical determinant of neuronal survival in the ischemic penumbra. Conversely, glutamate efflux from astrocytes by reversal of glutamate uptake, volume sensitive organic ion channels, and other routes may contribute to extracellular glutamate elevations. Glutamate activation of neuronal N-methyl-D-aspartate (NMDA) receptors is modulated by glycine and D-serine: both of these neuromodulators are transported by astrocytes, and D-serine production is localized exclusively to astrocytes. Astrocytes influence neuronal antioxidant status through release of ascorbate and uptake of its oxidized form, dehydroascorbate, and by indirectly supporting neuronal glutathione metabolism. In addition, glutathione in astrocytes can serve as a sink for nitric oxide and thereby reduce neuronal oxidant stress during ischemia. Astrocytes probably also influence neuronal survival in the post-ischemic period. Reactive astrocytes secrete nitric oxide, TNFalpha, matrix metalloproteinases, and other factors that can contribute to delayed neuronal death, and facilitate brain edema via aquaporin-4 channels localized to the astrocyte endfoot-endothelial interface. On the other hand erythropoietin, a paracrine messenger in brain, is produced by astrocytes and upregulated after ischemia. Erythropoietin stimulates the Janus kinase-2 (JAK-2) and nuclear factor-kappaB (NF-kB) signaling pathways in neurons to prevent programmed cell death after ischemic or excitotoxic stress. Astrocytes also secrete several angiogenic and neurotrophic factors that are important for vascular and neuronal regeneration after stroke.     

Cellular mechanisms of brain energy metabolism and their relevance to functional brain imaging.

Despite striking advances in functional brain imaging, the cellular and molecular mechanisms that underlie the signals detected by these techniques are still largely unknown. The basic physiological principle of functional imaging is represented by the tight coupling existing between neuronal activity and the associated local increase in both blood flow and energy metabolism. Positron emission tomography (PET) signals detect blood flow, oxygen consumption and glucose use associated with neuronal activity; the degree of blood oxygenation is currently thought to contribute to the signal detected with functional magnetic resonance imaging, while magnetic resonance spectroscopy (MRS) identifies the spatio-temporal pattern of the activity-dependent appearance of certain metabolic intermediates such as glucose or lactate. Recent studies, including those of neurotransmitter-regulated metabolic fluxes in purified preparations and analyses of the cellular localization of enzymes and transporters involved in energy metabolism, as well as in vivo microdialysis and MRS approaches have identified the neurotransmitter glutamate and astrocytes, a specific type of glial cell, as pivotal elements in the coupling of synaptic activity with energy metabolism. Astrocytes are ideally positioned to sense increases in synaptic activity and to couple them with energy metabolism. Indeed they possess specialized processes that cover the surface of intraparenchymal capillaries, suggesting that astrocytes may be a likely site of prevalent glucose uptake. Other astrocyte processes are wrapped around synaptic contacts which possess receptors and reuptake sites for neurotransmitters. Glutamate stimulates glucose uptake into astrocytes. This effect is mediated by specific glutamate transporters present on these cells. The activity of these transporters, which is tightly coupled to the synaptic release of glutamate and operates the clearance of glutamate from the extracellular space, is driven by the electrochemical gradient of Na+. This Na(+)-dependent uptake of glutamate into astrocytes triggers a cascade of molecular events involving the Na+/K(+)-ATPase leading to the glycolytic processing of glucose and the release of lactate by astrocytes. The stoichiometry of this process is such that for one glutamate molecule taken up with three Na+ ions, one glucose molecule enters an astrocyte, two ATP molecules are produced through aerobic glycolysis and two lactate molecules are released. Within the astrocyte, one ATP molecule fuels one 'turn of the pump' while the other provides the energy needed to convert glutamate to glutamine by glutamine synthase. Evidence has been accumulated from structural as well as functional studies indicating that, under aerobic conditions, lactate may be the preferred energy substrate of activated neurons. Indeed, in the presence of oxygen, lactate is converted to pyruvate, which can be processed through the tricarboxylic acid cycle and the associated oxidative phosphorylation, to yield 17 ATP molecules per lactate molecule. These data suggest that during activation the brain may transiently resort to aerobic glycolysis occurring in astrocytes, followed by the oxidation of lactate by neurons. The proposed model provides a direct mechanism to couple synaptic activity with glucose use and is consistent with the notion that the signals detected during physiological activation with 18F-deoxyglucose (DG)-PET may reflect predominantly uptake of the tracer into astrocytes. This conclusion does not question the validity of the 2-DG-based techniques, rather it provides a cellular and molecular basis for these functional brain imaging techniques.     

A metabolic switch in brain: glucose and lactate metabolism modulation by ascorbic acid

In this review, we discuss a novel function of ascorbic acid in brain energetics. It has been proposed that during glutamatergic synaptic activity neurons preferably consume lactate released from glia. The key to this energetic coupling is the metabolic activation that occurs in astrocytes by glutamate and an increase in extracellular [K⁺]. Neurons are cells well equipped to consume glucose because they express glucose transporters and glycolytic and tricarboxylic acid cycle enzymes. Moreover, neuronal cells express monocarboxylate transporters and lactate dehydrogenase isoenzyme 1, which is inhibited by pyruvate. As glycolysis produces an increase in pyruvate concentration and a decrease in NAD⁺/NADH, lactate and glucose consumption are not viable at the same time. In this context, we discuss ascorbic acid participation as a metabolic switch modulating neuronal metabolism between rest and activation periods. Ascorbic acid is highly concentrated in CNS. Glutamate stimulates ascorbic acid release from astrocytes. Ascorbic acid entry into neurons and within the cell can inhibit glucose consumption and stimulate lactate transport. For this switch to occur, an ascorbic acid flow is necessary between astrocytes and neurons, which is driven by neural activity and is part of vitamin C recycling. Here, we review the role of glucose and lactate as metabolic substrates and the modulation of neuronal metabolism by ascorbic acid.     

Lactate involvement in neuron-glia metabolic interaction: (13)C-NMR spectroscopy contribution

Glucose is commonly admitted to be the main substrate for brain energy requirement. However, it has been recently proposed that lactate, generated from glucose via glycolysis, would be the oxidative substrate for neurons, particularly during neuronal activation, according to a mechanism called the astrocyte-neuron lactate shuttle hypothesis (ANLSH). In that mechanism, glutamate released in the synaptic cleft during brain activation is taken up by astrocytes. This uptake, via the glutamate/Na(+) transporter, induces the entry of sodium, which is then excluded from the astrocytes via the Na(+)/K(+) ATPase. This exclusion consumes ATP, which stimulates glycolysis and thus lactate formation in astrocytes. This lactate is then transferred to neurons where it is utilized as oxidative substrate. This review tries to gather the recent evidences that support this hypothesis and presents the contribution of NMR to this matter.     

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.     

Glial perspectives of metabolic states during cerebral hypoxia--calcium regulation and metabolic energy

Cooperation between astrocytes and neurons is a unique interaction between two highly specialized cell types of the brain. Therefore, lack of nutrient supply during ischemia requires tight coordination of metabolism between astrocytes and neurons to keep the brain functions intact. To understand the impact of energy limitation on astrocytes, the functions of astrocytes have to be considered: (i) supplementation of neuronal cells, (ii) modulation of the extracellular milieu, mainly of the glutamate level, and (iii) elimination of reactive oxygen species (ROS). In cultured astrocytes and neurons inhibition of oxidative phosphorylation, using rotenone, was tested. Interestingly, this had only a negligible effect on Ca2+ homeostasis in astrocytes, even in combination with a severe glutamate stress. In contrast, in neurons glutamate in the presence of rotenone induced Ca2+ deregulation. Ca2+ homeostasis is very critical for cell survival. A massive and prolonged Ca2+ rise will lead to deregulation of many processes in such a way that the cells affected can hardly survive. Ca2+ homeostasis depends on the energy-consuming processes, which maintain the steep gradient between intracellular and extracellular Ca2+ concentration. Deprivation of oxygen and glucose during ischemia leads to a depletion of ATP in the brain, due to inhibited glycolytic and mitochondrial activity, whereas energy-consuming processes like ion pumps drain the ATP pools. On the other hand, specific mechanisms can protect brain structures against the massive insult of ischemia. Glycogen, stored in astrocytes, can maintain both neurons and astrocytes alive during short limitation of oxygen and glucose. Moreover, astrocytes can fuel ATP generation by providing lactate for neurons.     

Lactate produced by glycogenolysis in astrocytes regulates memory processing

When administered either systemically or centrally, glucose is a potent enhancer of memory processes. Measures of glucose levels in extracellular fluid in the rat hippocampus during memory tests reveal that these levels are dynamic, decreasing in response to memory tasks and loads; exogenous glucose blocks these decreases and enhances memory. The present experiments test the hypothesis that glucose enhancement of memory is mediated by glycogen storage and then metabolism to lactate in astrocytes, which provide lactate to neurons as an energy substrate. Sensitive bioprobes were used to measure brain glucose and lactate levels in 1-sec samples. Extracellular glucose decreased and lactate increased while rats performed a spatial working memory task. Intrahippocampal infusions of lactate enhanced memory in this task. In addition, pharmacological inhibition of astrocytic glycogenolysis impaired memory and this impairment was reversed by administration of lactate or glucose, both of which can provide lactate to neurons in the absence of glycogenolysis. Pharmacological block of the monocarboxylate transporter responsible for lactate uptake into neurons also impaired memory and this impairment was not reversed by either glucose or lactate. These findings support the view that astrocytes regulate memory formation by controlling the provision of lactate to support neuronal functions.     

Neighborly interactions of metabolically-activated astrocytes in vivo

Metabolic responses of brain cells to a stimulus are governed, in part, by their enzymatic specialization and interrelationships with neighboring cells, and local shifts in functional metabolism during brain activation are likely to be influenced by the neurotransmitter system, subcellular compartmentation, and anatomical structure. Selected examples of functional activation illustrate the complexity of metabolic interactions in working brain and of interpretation of changes in brain lactate levels. The major focus of this article is the disproportionately higher metabolism of glucose compared to oxygen in normoxic brain, a phenomenon that occurs during activation in humans and animals. The glucose utilized in excess of oxygen is not fully explained by accumulation of glucose, lactate, or glycogen in brain or by lactate efflux from brain to blood. Thus, any lactate derived from the excess glucose could not have been stoichiometrically exported to and metabolized by neighboring neurons because oxygen consumption would have otherwise increased and matched that of glucose. Metabolic labeling of tricarboxylic acid cycle-derived amino acids increased during brief sensory stimulation, reflecting a rise in oxidative metabolism. Brain glycogen is mainly in astrocytes, and its level falls throughout the stimulus and early post-activation interval. Glycogenolysis cannot be accounted for by lactate accumulation or oxidation; there must be rapid product clearance. Glycogen restoration is slow and diversion of glucose from oxidative pathways for its re-synthesis could reduce the global O(2)/glucose uptake ratio; astrocytes could downshift this ratio for up to an hour after 5 min stimulus. Morphological studies of astrocytes reveal a paucity of cytoplasm and organelles in the fine processes that surround synapses and form gap junction connections with neighboring astrocytes. Specialized regions of astrocytes, e.g. their endfeet and thin peripheral lamellae, are likely to have compartmentalized metabolic activities. Anatomical constraints imposed upon the fine processes might require preferential utilization of glycolysis to satisfy their energy demands, but rapid lactate clearance would then be essential, since its accumulation would inhibit glycolysis. Gap junctional connections between neighboring astrocytes provide a mechanism for rapid metabolite spreading via the astrocytic syncytium and elimination of by-products. Local structure-function relationships need to be incorporated into experimental models of neuron-astrocyte and astrocyte-astrocyte interactions in working brain.     

Zinc inhibits astrocyte glutamate uptake by activation of poly(ADP-ribose) polymerase-1

Several processes by which astrocytes protect neurons during ischemia are now well established. However, less is known about how neurons themselves may influence these processes. Neurons release zinc (Zn2+) from presynaptic terminals during ischemia, seizure, head trauma, and hypoglycemia, and modulate postsynaptic neuronal function. Peak extracellular zinc may reach concentrations as high as 400 microM. Excessive levels of free, ionic zinc can initiate DNA damage and the subsequent activation of poly(ADP-ribose) polymerase 1 (PARP-1), which in turn lead to NAD+ and ATP depletion when DNA damage is extensive. In this study, cultured cortical astrocytes were used to explore the effects of zinc on astrocyte glutamate uptake, an energy-dependent process that is critical for neuron survival. Astrocytes incubated with 100 or 400 microM of zinc for 30 min showed significant decreases in ATP levels and glutamate uptake capacity. These changes were prevented by the PARP inhibitors benzamide or DPQ (3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-isoquinolinone) or PARP-1 gene deletion (PARP-1 KO). These findings suggest that release of Zn2+ from neurons during brain insults could induce PARP-1 activation in astrocytes, leading to impaired glutamate uptake and exacerbation of neuronal injury.     

ATP and glutamate released via astroglial connexin 43 hemichannels mediate neuronal death through activation of pannexin 1 hemichannels

Inflammation contributes to neurodegeneration in post-ischemic brain, diabetes, and Alzheimer's disease. Participants in this inflammatory response include activation of microglia and astrocytes. We studied the role of microglia treated with amyloid-β peptide (Aβ) on hemichannel activity of astrocytes subjected to hypoxia in high glucose. Reoxygenation after 3?h hypoxia in high glucose induced transient astroglial permeabilization via Cx43 hemichannels and reduction in intercellular communication via Cx43 cell-cell channels. Both responses were greater and longer lasting in astrocytes previously exposed for 24 h to conditioned medium from Aβ-treated microglia (CM-Aβ). The effects of CM-Aβ were mimicked by TNF-α and IL-1β and were abrogated by neutralizing TNF-α with soluble receptor and IL-1β with a receptor antagonist. Astrocytes under basal conditions protected neurons against hypoxia, but exposure to CM-Aβ made them toxic to neurons subjected to a sub-lethal hypoxia/reoxygenation episode, revealing the additive nature of the insults. Astrocytes exposed to CM-Aβ induced permeabilization of cortical neurons through activation of neuronal pannexin 1 (Panx1) hemichannels by ATP and glutamate released through astroglial Cx43 hemichannels. In agreement, inhibition of NMDA or P2X receptors only partially reduced the activation of neuronal Panx1 hemichannels and neuronal mortality, but simultaneous inhibition of both receptors completely prevented the neurotoxic response. Therefore, we suggest that responses to ATP and glutamate converge in activation of neuronal Panx1 hemichannels. Thus, we propose that blocking hemichannels expressed by astrocytes and/or neurons in the inflamed nervous system could represent a novel and alternative strategy to reduce neuronal loss in various pathological states including Alzheimer's disease, diabetes and ischemia.     

Astrocytic energetics during excitatory neurotransmission: What are contributions of glutamate oxidation and glycolysis?

Astrocytic energetics of excitatory neurotransmission is controversial due to discrepant findings in different experimental systems in vitro and in vivo. The energy requirements of glutamate uptake are believed by some researchers to be satisfied by glycolysis coupled with shuttling of lactate to neurons for oxidation. However, astrocytes increase glycogenolysis and oxidative metabolism during sensory stimulation in vivo, indicating that other sources of energy are used by astrocytes during brain activation. Furthermore, glutamate uptake into cultured astrocytes stimulates glutamate oxidation and oxygen consumption, and glutamate maintains respiration as well as glucose. The neurotransmitter pool of glutamate is associated with the faster component of total glutamate turnover in vivo, and use of neurotransmitter glutamate to fuel its own uptake by oxidation-competent perisynaptic processes has two advantages, substrate is supplied concomitant with demand, and glutamate spares glucose for use by neurons and astrocytes. Some, but not all, perisynaptic processes of astrocytes in adult rodent brain contain mitochondria, and oxidation of only a small fraction of the neurotransmitter glutamate taken up into these structures would be sufficient to supply the ATP required for sodium extrusion and conversion of glutamate to glutamine. Glycolysis would, however, be required in perisynaptic processes lacking oxidative capacity. Three lines of evidence indicate that critical cornerstones of the astrocyte-to-neuron lactate shuttle model are not established and normal brain does not need lactate as supplemental fuel: (i) rapid onset of hemodynamic responses to activation delivers oxygen and glucose in excess of demand, (ii) total glucose utilization greatly exceeds glucose oxidation in awake rodents during activation, indicating that the lactate generated is released, not locally oxidized, and (iii) glutamate-induced glycolysis is not a robust phenotype of all astrocyte cultures. Various metabolic pathways, including glutamate oxidation and glycolysis with lactate release, contribute to cellular energy demands of excitatory neurotransmission.     

Astrocytes and energy metabolism

Astrocytes are glial cells, which play a significant role in a number of processes, including the brain energy metabolism. Their anatomical position between blood vessels and neurons make them an interface for effective glucose uptake from blood. After entering astrocytes, glucose can be involved in different metabolic pathways, e.g. in glycogen production. Glycogen in the brain is localized mainly in astrocytes and is an important energy source in hypoxic conditions and normal brain functioning. The portion of glucose metabolized into glycogen molecules in astrocytes is as high as 40%. It is thought that the release of gliotransmitters (such as glutamate, neuroactive peptides and ATP) into the extracellular space by regulated exocytosis supports a significant part of communication between astrocytes and neurons. On the other hand, neurotransmitter action on astrocytes has a significant role in brain energy metabolism. Therefore, understanding the astrocytes energy metabolism may help understanding neuron-astrocyte interactions.     

Glycolysis-mediated control of blood-brain barrier development and function

The blood-brain barrier (BBB) consists of differentiated cells integrating in one ensemble to control transport processes between the central nervous system (CNS) and peripheral blood. Molecular organization of BBB affects the extracellular content and cell metabolism in the CNS. Developmental aspects of BBB attract much attention in recent years, and barriergenesis is currently recognized as a very important and complex mechanism of CNS development and maturation. Metabolic control of angiogenesis/barriergenesis may be provided by glucose utilization within the neurovascular unit (NVU). The role of glycolysis in the brain has been reconsidered recently, and it is recognized now not only as a process active in hypoxic conditions, but also as a mechanism affecting signal transduction, synaptic activity, and brain development. There is growing evidence that glycolysis-derived metabolites, particularly, lactate, affect barriergenesis and functioning of BBB. In the brain, lactate produced in astrocytes or endothelial cells can be transported to the extracellular space via monocarboxylate transporters (MCTs), and may act on the adjoining cells via specific lactate receptors. Astrocytes are one of the major sources of lactate production in the brain and significantly contribute to the regulation of BBB development and functioning. Active glycolysis in astrocytes is required for effective support of neuronal activity and angiogenesis, while endothelial cells regulate bioavailability of lactate for brain cells adjusting its bidirectional transport through the BBB. In this article, we review the current knowledge with regard to energy production in endothelial and astroglial cells within the NVU. In addition, we describe lactate-driven mechanisms and action of alternative products of glucose metabolism affecting BBB structural and functional integrity in developing and mature brain.     

A Temporary Local Energy Pool Coupled to Neuronal Activity: Fluctuations of Extracellular Lactate Levels in Rat Brain Monitored with Rapid-Response Enzyme-Based Sensor

Abstract: A successfully developed enzyme-based lactate microsensor with rapid response time allows the direct and continuous in vivo measurement of lactic acid concentration with high temporal resolution in brain extracellular fluid. The fluctuations coupled to neuronal activity in extracellular lactate concentration were explored in the dentate gyrus of the hippocampus of the rat brain after electrical stimulation of the perforant pathway. Extracellular glucose and oxygen levels were also detected simultaneously by coimplantation of a fast-response glucose sensor and an oxygen electrode, to provide novel information of trafficking of energy substances in real time related to local neuronal activity. The results first give a comprehensive picture of complementary energy supply and use of lactate and glucose in the intact brain tissue. In response to acute neuronal activation, the brain tissue shifts immediately to significant energy supply by lactate. A local temporary fuel "reservoir" is established behind the blood-brain barrier, evidenced by increased extracellular lactate concentration. The pool can be depleted rapidly, up to 28% in 10–12 s, by massive, acute neuronal use after stimulation and can be replenished in ∼20 s. Glutamate-stimulated astrocytic glycolysis and the increase of regional blood flow may regulate the lactate concentration of the pool in different time scales to maintain local energy homeostasis.     

The redox switch/redox coupling hypothesis

We provide an integrative interpretation of neuroglial metabolic coupling including the presence of subcellular compartmentation of pyruvate and monocarboxylate recycling through the plasma membrane of both neurons and glial cells. The subcellular compartmentation of pyruvate allows neurons and astrocytes to select between glucose and lactate as alternative substrates, depending on their relative extracellular concentration and the operation of a redox switch. This mechanism is based on the inhibition of glycolysis at the level of glyceraldehyde 3-phosphate dehydrogenase by NAD(+) limitation, under sufficiently reduced cytosolic NAD(+)/NADH redox conditions. Lactate and pyruvate recycling through the plasma membrane allows the return to the extracellular medium of cytosolic monocarboxylates enabling their transcellular, reversible, exchange between neurons and astrocytes. Together, intracellular pyruvate compartmentation and monocarboxylate recycling result in an effective transcellular coupling between the cytosolic NAD(+)/NADH redox states of both neurons and glial cells. Following glutamatergic neurotransmission, increased glutamate uptake by the astrocytes is proposed to augment glycolysis and tricarboxylic acid cycle activity, balancing to a reduced cytosolic NAD(+)/NADH in the glia. Reducing equivalents are transferred then to the neuron resulting in a reduced neuronal NAD(+)/NADH redox state. This may eventually switch off neuronal glycolysis, favoring the oxidation of extracellular lactate in the lactate dehydrogenase (LDH) equilibrium and in the neuronal tricarboxylic acid cycles. Finally, pyruvate derived from neuronal lactate oxidation, may return to the extracellular space and to the astrocyte, restoring the basal redox state and beginning a new loop of the lactate/pyruvate transcellular coupling cycle. Transcellular redox coupling operates through the plasma membrane transporters of monocarboxylates, similarly to the intracellular redox shuttles coupling the cytosolic and mitochondrial redox states through the transporters of the inner mitochondrial membrane. Finally, transcellular redox coupling mechanisms may couple glycolytic and oxidative zones in other heterogeneous tissues including muscle and tumors.     

Ex vivo analysis of lactate and glucose metabolism in the rat brain under different states of depressed activity

Brain metabolism of glucose and lactate was analyzed by ex vivo NMR spectroscopy in rats presenting different cerebral activities induced after the administration of pentobarbital, alpha-chloralose, or morphine. The animals were infused with a solution of either [1-(13)C]glucose plus lactate or glucose plus [3-(13)C]lactate for 20 min. Brain metabolite contents and enrichments were determined from analyses of brain tissue perchloric acid extracts according to their post-mortem evolution kinetics. When amino acid enrichments were compared, both the brain metabolic activity and the contribution of blood glucose relative to that of blood lactate to brain metabolism were linked with cerebral activity. The data also indicated the production in the brain of lactate from glycolysis in a compartment other than the neurons, presumably the astrocytes, and its subsequent oxidative metabolism in neurons. Therefore, a brain electrical activity-dependent increase in the relative contribution of blood glucose to brain metabolism occurred via the increase in the metabolism of lactate generated from brain glycolysis at the expense of that of blood lactate. This result strengthens the hypothesis that brain lactate is involved in the coupling between neuronal activation and metabolism.     

The Role of Astrocytic Glycogen in Supporting the Energetics of Neuronal Activity

Energy homeostasis in the brain is maintained by oxidative metabolism of glucose, primarily to fulfil the energy demand associated with ionic movements in neurons and astrocytes. In this contribution we review the experimental evidence that grounds a specific role of glycogen metabolism in supporting the functional energetic needs of astrocytes during the removal of extracellular potassium. Based on theoretical considerations, we further discuss the hypothesis that the mobilization of glycogen in astrocytes serves the purpose to enhance the availability of glucose for neuronal glycolytic and oxidative metabolism at the onset of stimulation. Finally, we provide an evolutionary perspective for explaining the selection of glycogen as carbohydrate reserve in the energy-sensing machinery of cell metabolism.     

Intercellular metabolic compartmentation in the brain: past, present and future

The first indication of 'metabolic compartmentation' in brain was the demonstration that glutamine after intracisternal [14C]glutamate administration is formed from a compartment of the glutamate pool that comprises at most one-fifth of the total glutamate content in the brain. This pool, which was designated 'the small compartment,' is now known to be made up predominantly or exclusively of astrocytes, which accumulate glutamate avidly and express glutamine synthetase activity, whereas this enzyme is absent from neurons, which eventually were established to constitute 'the large compartment.' During the following decades, the metabolic compartment concept was refined, aided by emerging studies of energy metabolism and glutamate uptake in cellularly homogenous preparations and by the histochemical observations that the two key enzymes glutamine synthetase and pyruvate carboxylase are active in astrocytes but absent in neurons. It is, however, only during the last few years that nuclear magnetic resonance (NMR) spectroscopy, assisted by previously obtained knowledge of metabolic pathways, has allowed accurate determination in the human brain in situ of actual metabolic fluxes through the neuronal tricarboxylic acid (TCA) cycle, the glial, presumably mainly astrocytic, TCA cycle, pyruvate carboxylation, and the 'glutamate-glutamine cycle,' connecting neuronal and astrocytic metabolism. Astrocytes account for 20% of oxidative metabolism of glucose in the human brain cortex and accumulate the bulk of neuronally released transmitter glutamate, part of which is rapidly converted to glutamine and returned to neurons in the glutamate-glutamine cycle. However, one-third of released transmitter glutamate is replaced by de novo synthesis of glutamate from glucose in astrocytes, suggesting that at steady state a corresponding amount of glutamate is oxidatively degraded. Net degradation of glutamate may not always equal its net production from glucose and enhanced glutamatergic activity, occurring during different types of cerebral stimulation, including the establishment of memory, may be associated with increased de novo synthesis of glutamate. This process may contribute to a larger increase in glucose utilization rate than in rate of oxygen consumption during brain activation. The energy yield in astrocytes from glutamate formation is strongly dependent upon the fate of the generated glutamate.