Sensory stimulation induces local cerebral glycogenolysis: demonstration by autoradiography

Non-invasive quantification of brain glycogen absolute concentration

The only currently available method to measure brain glycogen in vivo is 13C NMR spectroscopy. Incorporation of 13C-labeled glucose (Glc) is necessary to allow glycogen measurement, but might be affected by turnover changes. Our aim was to measure glycogen absolute concentration in the rat brain by eliminating label turnover as variable. The approach is based on establishing an increased, constant 13C isotopic enrichment (IE). 13C-Glc infusion is then performed at the IE of brain glycogen. As glycogen IE cannot be assessed in vivo, we validated that it can be inferred from that of N-acetyl-aspartate IE in vivo: After [1-13C]-Glc ingestion, glycogen IE was 2.2 +/- 0.1 fold that of N-acetyl-aspartate (n = 11, R(2) = 0.77). After subsequent Glc infusion, glycogen IE equaled brain Glc IE (n = 6, paired t-test, p = 0.37), implying isotopic steady-state achievement and complete turnover of the glycogen molecule. Glycogen concentration measured in vivo by 13C NMR (mean +/- SD: 5.8 +/- 0.7 micromol/g) was in excellent agreement with that in vitro (6.4 +/- 0.6 micromol/g, n = 5). When insulin was administered, the stability of glycogen concentration was analogous to previous biochemical measurements implying that glycogen turnover is activated by insulin. We conclude that the entire glycogen molecule is turned over and that insulin activates glycogen turnover.

Cerebral non-oxidative carbohydrate consumption in humans driven by adrenaline

During brain activation, the decrease in the ratio between cerebral oxygen and carbohydrate uptake (6 O(2)/(glucose + (1)/(2) lactate); the oxygen-carbohydrate index, OCI) is attenuated by the non-selective beta-adrenergic receptor antagonist propranolol, whereas OCI remains unaffected by the beta(1)-adrenergic receptor antagonist metroprolol. These observations suggest involvement of a beta(2)-adrenergic mechanism in non-oxidative metabolism for the brain. Therefore, we evaluated the effect of adrenaline (0.08 microg kg(-1) min(-1) i.v. for 15 min) and noradrenaline (0.5, 0.1 and 0.15 microg kg(-1) min(-1) i.v. for 20 min) on the arterial to internal jugular venous concentration differences (a-v diff) of O(2), glucose and lactate in healthy humans. Adrenaline (n = 10) increased the arterial concentrations of O(2), glucose and lactate (P < 0.05) and also increased the a-v diff for glucose from 0.6 +/- 0.1 to 0.8 +/- 0.2 mM (mean +/- s.d.; P < 0.05). The a-v diff for lactate shifted from a net cerebral release to an uptake and OCI was lowered from 5.1 +/- 1.5 to 3.6 +/- 0.4 (P < 0.05) indicating an 8-fold increase in the rate of non-oxidative carbohydrate uptake during adrenaline infusion (P < 0.01). Conversely, noradrenaline (n = 8) did not affect the OCI despite an increase in the a-v diff for glucose (P < 0.05). These results support that non-oxidative carbohydrate consumption for the brain is driven by a beta(2)-adrenergic mechanism, giving neurons an abundant provision of energy when plasma adrenaline increases.

The energy hypothesis of sleep revisited

One of the proposed functions of sleep is to replenish energy stores in the brain that have been depleted during wakefulness. Benington and Heller formulated a version of the energy hypothesis of sleep in terms of the metabolites adenosine and glycogen. They postulated that during wakefulness, adenosine increases and astrocytic glycogen decreases reflecting the increased energetic demand of wakefulness. We review recent studies on adenosine and glycogen stimulated by this hypothesis. We also discuss other evidence that wakefulness is an energetic challenge to the brain including the unfolded protein response, the electron transport chain, NPAS2, AMP-activated protein kinase, the astrocyte-neuron lactate shuttle, production of reactive oxygen species and uncoupling proteins. We believe the available evidence supports the notion that wakefulness is an energetic challenge to the brain, and that sleep restores energy balance in the brain, although the mechanisms by which this is accomplished are considerably more complex than envisaged by Benington and Heller.

Unlocking the Energy Dynamics of Executive Functioning: Linking Executive Functioning to Brain Glycogen

Past work suggests that executive functioning relies on glucose as a depletable energy, such that executive functioning uses a relatively large amount of glucose and is impaired when glucose is low. Glucose from the bloodstream is one energy source for the brain, and glucose stored in the brain as glycogen is another. A review of the literature on glycogen suggests that executive functioning uses it in much the same way as glucose, such that executive functioning uses glycogen and is impaired when glycogen is low. Findings on stress, physical persistence, glucose tolerance, diabetes, sleep, heat, and other topics provide general support for this view.

Effect of acute and recurrent hypoglycemia on changes in brain glycogen concentration

Our objective was to evaluate whether excessive brain glycogen deposition might follow episodes of acute hypoglycemia (AH) and thus play a role in the hypoglycemia-associated autonomic failure seen in diabetic patients receiving intensive insulin treatment. We determined brain glucose and glycogen recovery kinetics after AH and recurrent hypoglycemia (RH), an established animal model of counterregulatory failure. A single bout of insulin-induced AH or RH for 3 consecutive days was used to deplete brain glucose and glycogen stores in rats. After microwave fixation and glycogen extraction, regional recovery kinetics in the brain was determined using a biochemical assay. Both AH and RH treatments reduced glycogen levels in the cerebellum, cortex, and hypothalamus from control levels of 7.78 +/- 0.55, 5.4 +/- 0.38, and 4.45 +/- 0.37 micromol/g, respectively, to approximately 50% corresponding to a net glycogen utilization rate between 0.6 and 1.2 micromol/g.h. After hypoglycemia, glycogen levels returned to baseline within 6 h in both the AH and the RH group. However, recovery of brain glycogen tended to be faster in rats exposed to RH. This effect followed more rapid recovery of brain glucose levels in the RH group, despite similar blood glucose levels in both groups. There was no statistically significant increase above baseline glycogen levels in either group. In particular, brain glycogen was not increased 24 h after the last of recurrent episodes of hypoglycemia, when a significant counterregulatory defect could be documented during a hyperinsulinemic hypoglycemic clamp study. We conclude that glycogen supercompensation is not a major contributory factor to the pathogenesis of hypoglycemia-associated autonomic failure.

Energy metabolism and memory processing: role of glucose transport and glycogen in responses to adrenoceptor activation in the chicken

From experiments using a discriminated bead task in young chicks, we have defined when and where adrenoceptors (ARs) are involved in memory modulation. All three ARs subtypes (alpha(1)-, alpha(2)- and beta-ARs) are found in the chick brain and in regions associated with memory. Glucose and glycogen are important in the role of memory consolidation in the chick since increasing glucose levels improves memory consolidation while inhibiting glucose transporters (GLUTs) or glycogen breakdown inhibits memory consolidation. The selective beta(3)-AR agonist CL316243 enhances memory consolidation by a glucose-dependent mechanism and the administration of the non-metabolized glucose analogue 2-deoxyglucose reduces the ability of CL316243 to enhance memory. Agents that reduce glucose uptake by GLUTs and its incorporation into the glycolytic pathway also reduce the effectiveness of CL316243, but do not alter the dose-response relationship to the beta(2)-AR agonist zinterol. However, beta(2)-ARs do have a role in memory related to glycogen breakdown and inhibition of glycogenolysis reduces the ability of zinterol to enhance memory. Both beta(2)- and beta(3)-ARs are found on astrocytes from chick forebrain, and the actions of beta(3)-ARs on glucose uptake, and beta(2)-ARs on the breakdown of glycogen is consistent with an effect on astrocytic metabolism at the time of memory consolidation 30 min after training. We have shown that both beta(2)- and beta(3)-ARs can increase glucose uptake in chick astrocytes but do so by different mechanisms. This review will focus on the role of ARs on memory consolidation and specifically the role of energy metabolism on AR modulation of memory.

Energy metabolism in brain cells: effects of elevated ammonia concentrations

Both neurons and astrocytes have high rates of glucose utilization and oxidative metabolism. Fully 20% of glucose consumption is used for astrocytic production of glutamate and glutamine, which during intense glutamatergic activity leads to an increase in glutamate content, but at steady state is compensated for by an equally intense oxidation of glutamate. The amounts of ammonia used for glutamine synthesis and liberated during glutamine hydrolysis are large, compared to the additional demand for glutamine synthesis in hyperammonemic animals and patients with hepatic encephalopathy. Nevertheless, elevated ammonia concentrations lead to an increased astrocytic glutamine production and an elevated content of glutamine combined with a decrease in glutamate content, probably mainly in a cytosolic pool needed for normal activity of the malate-asparate shuttle (MAS); another compartment generated by glutamine hydrolysis is increased. As a result of reduced MAS activity the pyruvate/lactate ratio is decreased in astrocytes but not in neurons and decarboxylation of pyruvate to form acetyl coenzyme A is reduced. Elevated ammonia concentrations also inhibit decarboxylation of alpha-ketoglutarate in the TCA cycle. This effect occurs in both neurons and astrocytes, is unrelated to MAS activity and seen after chronic treatment with ammonia even in the absence of elevated ammonia concentrations.

Glycogen is a preferred glutamate precursor during learning in 1-day-old chick: Biochemical and behavioral evidence

Bead discrimination training in chicks sets in motion a tightly timed series of biochemical events, including glutamate release, increase in forebrain level of glutamate and utilization of glycogen and glucose. Inhibition of glycogen breakdown by the glycogen phosphorylase inhibitor 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) around the time of training abolishes the increase in glutamate 5 min posttraining in the left hemisphere, in spite of uninhibited glucose metabolism. It also reduces the contents of glutamate, glutamine, and aspartate in the right hemisphere. Behavioral evidence supports the conclusion that glucose breakdown serves to provide energy, whereas glycogen acts as a substrate for glutamate, glutamine, and aspartate formation, requiring both pyruvate dehydrogenation to acetyl coenzyme A and pyruvate carboxylation in astrocytes. Inhibition of memory consolidation caused by DAB or 2-deoxyglucose (2-DG), an inhibitor of glucose phosphorylation without effect on glycogen metabolism, was challenged by intracerebral administration of acetate, aspartate, glutamine, lactate or glucose. DAB-mediated memory inhibition was successfully challenged by administration at 0 or 20 min posttraining of acetate (an astrocyte-specific acetyl CoA precursor) together with aspartate, substituting for pyruvate carboxylation, or of glutamine at 0-2.5 or 30 min posttraining. 2-DG-mediated memory impairment was not challenged by acetate with or without aspartate at 0 time but was challenged by acetate without aspartate at 20 min. Lactate, a substrate for both dehydrogenation and pyruvate carboxylation challenged both DAB and 2-DG. Doses of DAB and 2-DG which, on their own were subeffective, were not additive, further supporting the existence of one pathway using glucose and another using glycogen.

The brain at work: A cerebral metabolic manifestation of central fatigue?

Central fatigue refers to circumstances in which strength appears to be limited by the ability of the central nervous system to recruit motoneurons. Central fatigue manifests when the effort to contract skeletal muscles is intense and, thus, is aggravated when exercise is performed under stress, whereas it becomes attenuated following training. Central fatigue has not been explained, but the cerebral metabolic response to intense exercise, as to other modalities of cerebral activation, is a reduction in its "metabolic ratio" (MR), i.e., the brain's uptake of oxygen relative to that of carbohydrate. At rest the MR is close to 6 but during intense whole-body exercise it decreases to less than 3, with the uptake of lactate becoming as important as that of glucose. It remains debated what underlies this apparent inability of the brain to oxidize the carbohydrate taken up, but it may approach approximately 10 mmol glucose equivalents. In the case of exercise, a concomitant uptake of ammonium for formation of amino acids may account for only approximately 10% of this "extra" carbohydrate taken up. Also, accumulation of intermediates in metabolic pathways and compartmentalization of metabolism between astrocytes and neurons are avenues that have to be explored. Depletion of glycogen stores and subsequent supercompensation during periods of low neuronal activity may not only play a role but also link brain metabolism to its function.

A glycogen phosphorylase inhibitor selectively enhances local rates of glucose utilization in brain during sensory stimulation of conscious rats: implications for glycogen turnover

Glycogen is degraded during brain activation but its role and contribution to functional energetics in normal activated brain have not been established. In the present study, glycogen utilization in brain of normal conscious rats during sensory stimulation was assessed by three approaches, change in concentration, release of (14)C from pre-labeled glycogen and compensatory increase in utilization of blood glucose (CMR(glc)) evoked by treatment with a glycogen phosphorylase inhibitor. Glycogen level fell in cortex, (14)C release increased in three structures and inhibitor treatment caused regionally selective compensatory increases in CMR(glc) over and above the activation-induced rise in vehicle-treated rats. The compensatory rise in CMR(glc) was highest in sensory-parietal cortex where it corresponded to about half of the stimulus-induced rise in CMR(glcf) in vehicle-treated rats; this response did not correlate with metabolic rate, stimulus-induced rise in CMR(glc) or sequential station in sensory pathway. Thus, glycogen is an active fuel for specific structures in normal activated brain, not simply an emergency fuel depot and flux-generated pyruvate greatly exceeded net accumulation of lactate or net consumption of glycogen during activation. The metabolic fate of glycogen is unknown, but adding glycogen to the fuel consumed during activation would contribute to a fall in CMR(O2)/CMR(glc) ratio.

Direct validation of in vivo localized 13C MRS measurements of brain glycogen

With the use of localized 13C MRS in conjunction with [1-(13)C]-D-glucose infusion, it is possible to study brain glycogen metabolism in vivo. The purpose of this study was to validate in vivo 13C MRS measurements by comparing them with results from a standard biochemical assay. To increase the [1-(13)C] glycogen concentration, 11 rats were subjected to an episode of acute hypoglycemia followed by a mild hyperglycemic recovery period during which [1-(13)C]-D-glucose was infused. The total brain [1-(13)C] glycogen of the same animal was determined from the enzymatically determined total brain glycogen content, which was fixed by focused microwave irradiation (4 kW in 1.4 s) immediately after the end of the in vivo NMR measurements. The corresponding isotopic enrichment (IE) of glycogen was measured by in vitro 1H MRS of protons bound to glucose C1-alpha. The in vivo [1-(13)C] glycogen concentration was strongly correlated to the in vitro [1-(13)C] glycogen content determined by biochemical measurement in a linear manner (R=0.79). The results are consistent with the notion that localized 13C MRS measurements closely reflect 13C glycogen content in the brain.

Human brain glycogen content and metabolism: implications on its role in brain energy metabolism

The adult brain relies on glucose for its energy needs and stores it in the form of glycogen, primarily in astrocytes. Animal and culture studies indicate that brain glycogen may support neuronal function when the glucose supply from the blood is inadequate and/or during neuronal activation. However, the concentration of glycogen and rates of its metabolism in the human brain are unknown. We used in vivo localized 13C-NMR spectroscopy to measure glycogen content and turnover in the human brain. Nine healthy volunteers received intravenous infusions of [1-(13)C]glucose for durations ranging from 6 to 50 h, and brain glycogen labeling and washout were measured in the occipital lobe for up to 84 h. The labeling kinetics suggest that turnover is the main mechanism of label incorporation into brain glycogen. Upon fitting a model of glycogen metabolism to the time courses of newly synthesized glycogen, human brain glycogen content was estimated at approximately 3.5 micromol/g, i.e., three- to fourfold higher than free glucose at euglycemia. Turnover of bulk brain glycogen occurred at a rate of 0.16 micromol.g-1.h-1, implying that complete turnover requires 3-5 days. Twenty minutes of visual stimulation (n=5) did not result in detectable glycogen utilization in the visual cortex, as judged from similar [13C]glycogen levels before and after stimulation. We conclude that the brain stores a substantial amount of glycogen relative to free glucose and metabolizes this store very slowly under normal physiology.

Energy metabolism in astrocytes: high rate of oxidative metabolism and spatiotemporal dependence on glycolysis/glycogenolysis

Astrocytic energy demand is stimulated by K(+) and glutamate uptake, signaling processes, responses to neurotransmitters, Ca(2+) fluxes, and filopodial motility. Astrocytes derive energy from glycolytic and oxidative pathways, but respiration, with its high-energy yield, provides most adenosine 5' triphosphate (ATP). The proportion of cortical oxidative metabolism attributed to astrocytes ( approximately 30%) in in vivo nuclear magnetic resonance (NMR) spectroscopic and autoradiographic studies corresponds to their volume fraction, indicating similar oxidation rates in astrocytes and neurons. Astrocyte-selective expression of pyruvate carboxylase (PC) enables synthesis of glutamate from glucose, accounting for two-thirds of astrocytic glucose degradation via combined pyruvate carboxylation and dehydrogenation. Together, glutamate synthesis and oxidation, including neurotransmitter turnover, generate almost as much energy as direct glucose oxidation. Glycolysis and glycogenolysis are essential for astrocytic responses to increasing energy demand because astrocytic filopodial and lamellipodial extensions, which account for 80% of their surface area, are too narrow to accommodate mitochondria; these processes depend on glycolysis, glycogenolysis, and probably diffusion of ATP and phosphocreatine formed via mitochondrial metabolism to satisfy their energy demands. High glycogen turnover in astrocytic processes may stimulate glucose demand and lactate production because less ATP is generated when glucose is metabolized via glycogen, thereby contributing to the decreased oxygen to glucose utilization ratio during brain activation. Generated lactate can spread from activated astrocytes via low-affinity monocarboxylate transporters and gap junctions, but its subsequent fate is unknown. Astrocytic metabolic compartmentation arises from their complex ultrastructure; astrocytes have high oxidative rates plus dependence on glycolysis and glycogenolysis, and their energetics is underestimated if based solely on glutamate cycling.

Activity-dependent regulation of energy metabolism by astrocytes: An update

Astrocytes play a critical role in the regulation of brain metabolic responses to activity. One detailed mechanism proposed to describe the role of astrocytes in some of these responses has come to be known as the astrocyte-neuron lactate shuttle hypothesis (ANLSH). Although controversial, the original concept of a coupling mechanism between neuronal activity and glucose utilization that involves an activation of aerobic glycolysis in astrocytes and lactate consumption by neurons provides a heuristically valid framework for experimental studies. In this context, it is necessary to provide a survey of recent developments and data pertaining to this model. Thus, here, we review very recent experimental evidence as well as theoretical arguments strongly supporting the original model and in some cases extending it. Aspects revisited include the existence of glutamate-induced glycolysis in astrocytes in vitro, ex vivo, and in vivo, lactate as a preferential oxidative substrate for neurons, and the notion of net lactate transfer between astrocytes and neurons in vivo. Inclusion of a role for glycogen in the ANLSH is discussed in the light of a possible extension of the astrocyte-neuron lactate shuttle (ANLS) concept rather than as a competing hypothesis. New perspectives offered by the application of this concept include a better understanding of the basis of signals used in functional brain imaging, a role for neuron-glia metabolic interactions in glucose sensing and diabetes, as well as novel strategies to develop therapies against neurodegenerative diseases based upon improving astrocyte-neuron coupled energetics.

Exercise and heat stress: cerebral challenges and consequences

This review deals with new aspects of exercise in the heat as a challenge that not only influences the locomotive and cardiovascular systems, but also affects the brain. Activation of the brain during such exercise is manifested in the lowering of the cerebral glucose to oxygen uptake ratio, the elevated ratings of perceived exertion and increased release of hypothalamic hormones. While the slowing of the electroencephalographic (EEG), the decreased endurance and hampered ability to activate the skeletal muscles maximally during sustained isometric and repeated isokinetic contractions appear to relate to central fatigue arising as the core/brain increases, the central fatigue during exercise with hyperthermia thus can be considered as the ultimate safety break against catastrophic hyperthermia. This would force the subject to stop exercising or decrease the internal heat production. It appears that the dopaminergic system is important, but several other factors may interact and feedback from the skeletal muscles and internal temperature sensors are probably also involved. The complexity of brain fatigue response is discussed based on our own investigations and in the light of recent literature.

Inhibition of glycogenolysis in astrocytes interrupts memory consolidation in young chickens

Glycolysis and glycogenolysis are involved in memory processing in day-old chickens and, aside from the provision of energy for neuronal and astrocytic energy metabolism these pathways enable astrocytes to supply neurones with precursor for transmitter glutamate by glucose-based de novo synthesis. We have previously shown that memory processing for bead discrimination learning is dependent on glycolysis; however, the metabolic inhibitor used, iodoacetate, inhibits pyruvate formation from both glucose and glycogen. At specific time points after training transient reductions in brain glycogen content occur, mirrored by increases in glutamate/glutamine content. In the present study, we used intracerebral injection of a glycogen phosphorylase inhibitor, 1,4-dideoxy-1,4-imino-D-arabinitol (DAB), which does not affect glucose breakdown, to evaluate the role of glycogen metabolism in memory consolidation. Dose-dependent inhibition of learning occurred when DAB was administered at specific time periods in relation to training: (i) 5 min before training, (ii) around 30 min posttraining, and (iii) 55 min posttraining. After injection at either of the two earlier periods, memory disappeared after consolidation 30 min postlearning, and after injection 55 min after learning memory was absent at 70 min. The memory loss caused by early administration could be prevented after training by central injection of the glutamate precursor glutamine or the astrocyte-specific substrate acetate together with aspartate, substituting for pyruvate carboxylation. Thus, glycogenolysis is essential for learning in this paradigm and, aside from energy supply considerations, we suggest that an important role for glycogenolysis is to provide neurones with glutamine as the precursor for neuronal glutamate and GABA.

Brain work and brain imaging

Functional brain imaging with positron emission tomography and magnetic resonance imaging has been used extensively to map regional changes in brain activity. The signal used by both techniques is based on changes in local circulation and metabolism (brain work). Our understanding of the cell biology of these changes has progressed greatly in the past decade. New insights have emerged on the role of astrocytes in signal transduction as has an appreciation of the unique contribution of aerobic glycolysis to brain energy metabolism. Likewise our understanding of the neurophysiologic processes responsible for imaging signals has progressed from an assumption that spiking activity (output) of neurons is most relevant to one focused on their input. Finally, neuroimaging, with its unique metabolic perspective, has alerted us to the ongoing and costly intrinsic activity within brain systems that most likely represents the largest fraction of the brain's functional activity.

Fuelling cerebral activity in exercising man

The metabolic response to brain activation in exercise might be expressed as the cerebral metabolic ratio (MR; uptake O2/glucose + 1/2 lactate). At rest, brain energy is provided by a balanced oxidation of glucose as MR is close to 6, but activation provokes a 'surplus' uptake of glucose relative to that of O2. Whereas MR remains stable during light exercise, it is reduced by 30% to 40% when exercise becomes demanding. The MR integrates metabolism in brain areas stimulated by sensory input from skeletal muscle, the mental effort to exercise and control of exercising limbs. The MR decreases during prolonged exhaustive exercise where blood lactate remains low, but when vigorous exercise raises blood lactate, the brain takes up lactate in an amount similar to that of glucose. This lactate taken up by the brain is oxidised as it does not accumulate within the brain and such pronounced brain uptake of substrate occurs independently of plasma hormones. The 'surplus' of glucose equivalents taken up by the activated brain may reach approximately 10 mmol, that is, an amount compatible with the global glycogen level. It is suggested that a low MR predicts shortage of energy that ultimately limits motor activation and reflects a biologic background for 'central fatigue'.

Glycogen content in the cerebral cortex increases with sleep loss in C57BL/6J mice

We hypothesized that a function of sleep is to replenish brain glycogen stores that become depleted while awake. We have previously tested this hypothesis in three inbred strains of mice by measuring brain glycogen after a 6h sleep deprivation (SD). Unexpectedly, glycogen content in the cerebral cortex did not decrease with SD in two of the strains and was even found to increase in mice of the C57BL/6J (B6) strain. Manipulations that initially induce glycogenolysis can also induce subsequent glycogen synthesis thereby elevating glycogen content beyond baseline. It is thus possible that in B6 mice, cortical glycogen content decreased early during SD and became elevated later in SD. In the present study, we therefore measured changes in brain glycogen over the course of a 6 h SD and during recovery sleep in B6 mice. We found no evidence of a decrease at any time during the SD, instead, cortical glycogen content monotonically increased with time-spent-awake and, when sleep was allowed, started to revert to control levels. Such a time-course is opposite to the one predicted by our initial hypothesis. These results demonstrate that glycogen synthesis can be achieved during prolonged wakefulness to the extent that it outweighs glycogenolysis. Maintaining this energy store seems thus not to be functionally related to sleep in this strain.

Astrocyte activation in working brain: energy supplied by minor substrates

Glucose delivered to brain by the cerebral circulation is the major and obligatory fuel for all brain cells, and assays of functional activity in working brain routinely focus on glucose utilization. However, these assays do not take into account the contributions of minor substrates or endogenous fuel consumed by astrocytes during brain activation, and emerging evidence suggests that glycogen, acetate, and, perhaps, glutamate, are metabolized by working astrocytes in vivo to provide physiologically significant amounts of energy in addition to that derived from glucose. Rates of glycogenolysis during sensory stimulation of normal, conscious rats are high enough to support the notion that glycogen can contribute substantially to astrocytic glucose utilization during activation. Oxidative metabolism of glucose provides most of the ATP for cultured astrocytes, and a substantial contribution of respiration to astrocyte energetics is supported by recent in vivo studies. Astrocytes preferentially oxidize acetate taken up into brain from blood, and calculated local rates of acetate utilization in vivo are within the range of calculated rates of glucose oxidation in astrocytes. Glutamate may also serve as an energy source for activated astrocytes in vivo because astrocytes in tissue culture and in adult brain tissue readily oxidize glutamate. Taken together, contributions of minor metabolites derived from endogenous and exogenous sources add substantially to the energy obtained by astrocytes from blood-borne glucose. Because energy-generating reactions from minor substrates are not taken into account by routine assays of functional metabolism, they reflect a "hidden cost" of astrocyte work in vivo.

Glycogen phosphorylase reactivity in the entorhinal complex in familiar and novel environments: Evidence for labile glycogenolytic modules in the rat

Active and total glycogen phosphorylase were measured histochemically in the entorhinal complex of male Sprague-Dawley rats. Rats were sacrificed from their home cage, or after 5 min in a novel holeboard. Hemispheres from each group were paired, sectioned and processed together. Glycogen phosphorylase reactivity highlighted entorhinal cortex in contrast to less densely stained perirhinal cortex or neocortex. The presubiculum, but not parasubiculum, was strongly reactive for glycogen phosphorylase. Within medial and lateral entorhinal cortex, modularity of active glycogen phosphorylase reactivity was apparent. In inner Layer I there were small ( approximately 50 microm) intense patches of active glycogen phosphorylase. In Layer III there were both small and larger ( approximately 200 microm), patches of active glycogen phosphorylase. Lamina dessicans was reactive. Layers V and VI were relatively unreactive. Exposure to a holeboard intensified the small patches of active glycogen phosphorylase in inner Layer I, while attenuating active glycogen phosphorylase reactivity in Layer III. Total glycogen phosphorylase was unaffected by exposure to the novel environment and exhibited a pattern of continuous dense reactivity suggesting enzyme reserves, particularly in superficial layers of entorhinal cortex. These patterns confirm earlier evidence that glycogenolytic demand in Layers I and III of rat entorhinal cortex is organized in a modular fashion and show that such demand can be modified by brief exposure to a novel holeboard.

Compartmentation of Lactate Originating from Glycogen and Glucose in Cultured Astrocytes

Brain glycogen metabolism was investigated by employing isofagomine, an inhibitor of glycogen phosphorylase. Cultured cerebellar and neocortical astrocytes were incubated in medium containing [U-(13C)]glucose in the absence or presence of isofagomine and the amounts and percent labeling of intra- and extracellular metabolites were determined by mass spectrometry (MS). The percent labeling in glycogen was markedly decreased in the presence of isofagomine. Surprisingly, the percent labeling of intracellular lactate was also decreased demonstrating the importance of glycogen turnover. The decrease was limited to the percent labeling in the intracellular pool of lactate, which was considerably lower compared to that observed in the medium in which it was close to 100%. These findings indicate compartmentation of lactate derived from glycogenolysis and that derived from glycolysis. Inhibiting glycogen degradation had no effect on the percent labeling in citrate. However, the percent labeling of extracellular glutamine was slightly decreased in neocortical astrocytes exposed to isofagomine, indicating an importance of glycogen turnover in the synthesis of releasable glutamine. In conclusion, the results demonstrate that glycogen in cultured astrocytes is continuously synthesized and degraded. Moreover, it was found that lactate originating from glycogen is compartmentalized from that derived from glucose, which lends further support to a compartmentalized metabolism in astrocytes.

Idazoxan activates rat forebrain glycogen phosphorylase in vivo: A histochemical study

In vitro experiments show norepinephrine activates glycogen phosphorylase and glycogenolysis in forebrain glia. The present study used idazoxan (5 mg/kg) to elevate NE in vivo and examined patterns of active (aGP) and total (tGP) glycogen phosphorylase reactivity in selected neocortical, hippocampal, diencephalic, and striatal sites using a histochemical method. In somatosensory neocortex, aGP reactivity was highest in Layer 4 with consistent reactivity in the barrel fields in vehicle-treated brains. In the hippocampus, the stratum lacunosum moleculare was highly reactive, while cell layers were least reactive. The dentate gyrus and CA3 were more reactive for aGP than CA1. In the diencephalon, the medial habenula was most reactive followed by the reticular nucleus of the thalamus. In the striatum, globus pallidus was most reactive. Reactivity patterns for tGP were similar to those for aGP, but more intense. The neocortex had the highest overall reactivity for tGP. An estimate of the percentage of aGP relative to tGP suggested the regions sampled had similar levels of median basal activation (approximately 65%). Idazoxan increased aGP reactivity in all regions of the neocortex assessed (layers 3-6 of primary and secondary somatosensory cortex and the barrel fields). The neuropil layers, but not the cell layers, of hippocampus were more reactive following idazoxan treatment. Idazoxan also increased aGP reactivity in the laterodorsal, paraventricular, and reticular nuclei of the thalamus. The largest idazoxan-induced changes, as an estimated percentage of tGP, occurred in the hippocampus (approximately 16% for stratum lacunosum moleculare and for CA1 stratum oriens). Increases ranged from approximately 3 to 6% in neocortex and were less than 3% in the diencephalic and striatal areas. These effects of idazoxan are consistent with a role for norepinephrine in activating forebrain glycogenolyis in vivo and supporting increased brain metabolism. They contrast with earlier evidence showing that idazoxan reduces 2-deoxyglucose uptake in these brain areas. Idazoxan, and norepinephrine, may preferentially recruit glycolytic over oxidative metabolism in the rat forebrain.

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

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

Glutamatergic neurotransmission and neuronal glucose oxidation are coupled during intense neuronal activation

13C nuclear magnetic resonance (NMR) experiments have previously shown that glutamatergic neurotransmitter flux (Vcycle(Glu/Gln)) changes proportionately with neuronal glucose oxidation (CMRglc(ox)N) in the nonactivated cortex of anesthetized rats. Positron Emission Tomography measurements of glucose and oxygen uptake during sensory stimulation had shown that the incremental glucose utilization is greater than oxygen leading to the suggestion that the energy required for stimulated neuronal activity arises from nonoxidative glucose metabolism. In this study, the authors used spatially localized 1H-observed, 13C-edited NMR spectroscopy during an infusion of [1,6-13C2]glucose to assess the relationship between changes in Vcycle(Glu/Gln) and glucose utilization (CMRglc(ox)N and CMRglc(nonox)) during the intense cortical activity associated with bicuculline-induced seizures. Metabolic fluxes were determined by model-based analysis of the 13C-enrichment time courses of glutamate-C4 and glutamine-C4 (CMRglc(ox)N, Vcycle(Glu/Gln)) and lactate-C3 (CMRglc(nonox)). The exchange rate between alpha-ketoglutarate and glutamate was found to be significantly faster than TCA cycle flux both for control (41 micromol.g(-1).min(-1); 95% CI, 5 to 109 micromol.g(-1).min(-1)) and during seizures (21 micromol.g(-1).min(-1); 95% CI, 4.4 to 51.8 micromol.g(-1).min(-1)). During seizures, total glucose utilization (CMRglc(ox+nonox)) increased substantially (466% between 0 and 6 minutes; 277% between 6 and 55 minutes). Glucose oxidation (CMRglc(ox)N) also increased (214%; from 0.26 +/- 0.02 to 0.57 +/- 0.07 micromol.g(-1).min(-1)) but to a lesser degree, resulting in a large increase in cortical lactate concentration. Vcycle(Glu/Gln) increased 233% (from 0.22 +/- 0.04 to 0.52 +/- 0.07 micromol.g(-1).min(-1)), which was similar to the increase in glucose oxidation. The value of Vcycle(Glu/Gln) and CMRglc(ox)N obtained here lie on the line predicted in a previous study. These results indicate that neuronal glucose oxidation and not total glucose utilization is coupled to the glutamate/glutamine cycle during intense cortical activation.

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

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

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.

Cerebral metabolism during upper and lower body exercise

When continuation of exercise calls for a "will," the cerebral metabolic ratio of O2 to (glucose + lactate) decreases, with the largest reduction (30-50%) at exhaustion. Because a larger effort is required to exercise with the arms than with the legs, we tested the hypothesis that the reduction in the cerebral metabolic ratio would become more pronounced during arm cranking than during leg exercise. The cerebral arterial-venous differences for blood-gas variables, glucose, and lactate were evaluated in two groups of eight subjects during exhaustive arm cranking and leg exercise. During leg exercise, exhaustion was elicited after 25 +/- 6 (SE) min, and the cerebral metabolic ratio was reduced from 5.6 +/- 0.2 to 3.5 +/- 0.2 after 10 min and to 3.3 +/- 0.3 at exhaustion (P < 0.05). Arm cranking lasted for 35 +/- 4 min and likewise decreased the cerebral metabolic ratio after 10 min (from 6.7 +/- 0.4 to 5.0 +/- 0.3), but the nadir at exhaustion was only 4.7 +/- 0.4, i.e., higher than during leg exercise (P < 0.05). The results demonstrate that exercise decreases the cerebral metabolic ratio when a conscious effort is required, irrespective of the muscle groups engaged. However, the comparatively small reduction in the cerebral metabolic ratio during arm cranking suggests that it is influenced by the exercise paradigm.

Astrocytic beta2-adrenergic receptors and multiple sclerosis

Despite intensive research, the cause and a cure of multiple sclerosis (MS) have remained elusive and many aspects of the pathogenesis are not understood. Immunohistochemical experiments have shown that astrocytic beta(2)-adrenergic receptors are lost in MS. Because norepinephrine mediates important supportive and protective actions of astrocytes via activation of these beta(2)-adrenergic receptors, we postulate that this abnormality may play a prominent role in the pathogenesis of MS. First, it may allow astrocytes to act as facultative antigen-presenting cells, thereby initiating T-cell mediated inflammatory responses that lead to the characteristic demyelinated lesions. Second, it may contribute to inflammatory injury by stimulating the production of nitric oxide and proinflammatory cytokines, and reducing glutamate uptake. Third, it may lead to apoptosis of oligodendrocytes by reducing the astrocytic production of trophic factors, including neuregulin, nerve growth factor and brain-derived neurotrophic factor. Fourth, it may impair astrocytic glycogenolysis, which supplies energy to axons, and this may represent a mechanism underlying axonal degeneration that is hold responsible for the progressive chronic disability.

Glucocorticoids influence brain glycogen levels during sleep deprivation

We investigated whether glucocorticoids [i.e., corticosterone (Cort) in rats] released during sleep deprivation (SD) affect regional brain glycogen stores in 34-day-old Long-Evans rats. Adrenalectomized (with Cort replacement; Adx+) and intact animals were sleep deprived for 6 h beginning at lights on and then immediately killed by microwave irradiation. Brain and liver glycogen and glucose and plasma glucose levels were measured. After SD in intact animals, glycogen levels decreased in the cerebellum and hippocampus but not in the cortex or brain stem. By contrast, glycogen levels in the cortex of Adx+ rats increased by 43% ( P < 0.001) after SD, while other regions were unaffected. Also in Adx+ animals, glucose levels were decreased by an average of 28% throughout the brain after SD. Intact sleep-deprived rats had elevations of circulating Cort, blood, and liver glucose that were absent in intact control and Adx+ animals. Different responses between brain structures after SD may be due to regional variability in metabolic rate or glycogen metabolism. Our findings suggest that the elevated glucocorticoid secretion during SD causes brain glycogenolysis in response to energy demands.

Cerebral carbohydrate cost of physical exertion in humans

Above a certain level of cerebral activation the brain increases its uptake of glucose more than that of O(2), i.e., the cerebral metabolic ratio of O(2)/(glucose + 12 lactate) decreases. This study quantified such surplus brain uptake of carbohydrate relative to O(2) in eight healthy males who performed exhaustive exercise. The arterial-venous differences over the brain for O(2), glucose, and lactate were integrated to calculate the surplus cerebral uptake of glucose equivalents. To evaluate whether the amount of glucose equivalents depends on the time to exhaustion, exercise was also performed with beta(1)-adrenergic blockade by metoprolol. Exhaustive exercise (24.8 +/- 6.1 min; mean +/- SE) decreased the cerebral metabolic ratio from a resting value of 5.6 +/- 0.2 to 3.0 +/- 0.4 (P < 0.05) and led to a surplus uptake of glucose equivalents of 9 +/- 2 mmol. beta(1)-blockade reduced the time to exhaustion (15.8 +/- 1.7 min; P < 0.05), whereas the cerebral metabolic ratio decreased to an equally low level (3.2 +/- 0.3) and the surplus uptake of glucose equivalents was not significantly different (7 +/- 1 mmol; P = 0.08). A time-dependent cerebral surplus uptake of carbohydrate was not substantiated and, consequently, exhaustive exercise involves a brain surplus carbohydrate uptake of a magnitude comparable with its glycogen content.

Brain glycogen re-awakened

The mammalian brain contains glycogen, which is located predominantly in astrocytes, but its function is unclear. A principal role for brain glycogen as an energy reserve, analogous to its role in the periphery, had been universally dismissed based on its relatively low concentration, an assumption apparently reinforced by the limited duration that the brain can function in the absence of glucose. However, during insulin-induced hypoglycaemia, where brain glucose availability is limited, glycogen content falls first in areas with the highest metabolic rate, suggesting that glycogen provides fuel to support brain function during pathological hypoglycaemia. General anaesthesia results in elevated brain glycogen suggesting quiescent neurones allow glycogen accumulation, and as long ago as the 1950s it was shown that brain glycogen accumulates during sleep, is mobilized upon waking, and that sleep deprivation results in region-specific decreases in brain glycogen, implying a supportive functional role for brain glycogen in the conscious, awake brain. Interest in brain glycogen has recently been re-awakened by the first continuous in vivo measurements using NMR spectroscopy, by the general acceptance of metabolic coupling between glia and neurones involving intercellular transfer of energy substrate, and by studies supporting a prominent physiological role for brain glycogen as a provider of supplemental energy substrate during periods of increased tissue energy demand, when ambient normoglycaemic glucose is unable to meet immediate energy requirements.

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.

Glycogen metabolism in the rat retina

It has been reported that glycogen levels in retina vary with retinal vascularization. However, the electrical activity of isolated retina depends on glucose supply, suggesting that it does not contain energetic reserves. We determined glycogen levels and pyruvate and lactate production under various conditions in isolated retina. Ex vivo retinas from light- and dark-adapted rats showed values of 44 +/- 0.3 and 19.5 +/- 0.4 nmol glucosyl residues/mg protein, respectively. The glycogen content of retinas from light-adapted animals was reduced by 50% when they were transferred to darkness. Glycogen levels were low in retinas incubated in glucose-free media and increased in the presence of glucose. The highest glycogen values were found in media containing 20 mm of glucose. A rapid increase in lactate production was observed in the presence of glucose. Surprisingly, glycogen levels were the lowest and lactate production was also very low in the presence of 30 mm glucose. Our results suggest that glycogen can be used as an immediate accessible energy reserve in retina. We speculate on the possibility that gluconeogenesis may play a protective role by removal of lactic acid.

Glucocorticoids modulate neurotransmitter-induced glycogen metabolism in cultured cortical astrocytes

Glucocorticoids (GC) are considered as key modulators of glycogen homeostasis in peripheral tissues, but their role in the central nervous system has only partially been characterized. Exposure of primary cultures of cortical astrocytes to dexamethasone (DEX), a synthetic glucocorticoid, results in the reduction of noradrenaline (NA)-induced glycogen synthesis in a concentration-dependent manner with a IC50 of 4.88 nm and a maximum inhibition of 51%. Such an effect is mediated via glucocorticoid receptors (GRs), since it is mimicked by the glucocorticoid analogue RU28362 (100 nm) and prevented by the GR antagonist RU38486 (1 micro m). DEX does not act through alteration of signal transduction mechanisms, as cAMP formation induced by noradrenergic stimulation was unchanged. Moreover, glycogen synthesis was inhibited to the same extent when DEX was applied either together or only after a brief NA application. Neither [3H]2-deoxyglucose uptake nor lactate release was altered by DEX in the presence of NA, demonstrating that inhibition of glycogen synthesis is not a consequence of reduced glucose utilization or availability. Interestingly, enhancement of glycogen synthase activity induced by NA was reduced in the presence of DEX (-27%). These results suggest that GC could have a significant influence on neuroenergetics as they could modulate activity-related changes in brain glycogen metabolism.

Cerebral perturbations provoked by prolonged exercise

This review addresses cerebral metabolic and neurohumoral alterations during prolonged exercise in humans with special focus on associations with fatigue. Global energy turnover in the brain is unaltered by the transition from rest to moderately intense exercise, apparently because exercise-induced activation of some brain regions including cortical motor areas is compensated for by reduced activity in other regions of the brain. However, strenuous exercise is associated with cerebral metabolic and neurohumoral alterations that may relate to central fatigue. Fatigue should be acknowledged as a complex phenomenon influenced by both peripheral and central factors. However, failure to drive the motorneurons adequately as a consequence of neurophysiological alterations seems to play a dominant role under some circumstances. During exercise with hyperthermia excessive accumulation of heat in the brain due to impeded heat removal by the cerebral circulation may elevate the brain temperature to >40 degrees C and impair the ability to sustain maximal motor activation. Also, when prolonged exercise results in hypoglycaemia, perceived exertion increases at the same time as the cerebral glucose uptake becomes low, and centrally mediated fatigue appears to arise as the cerebral energy turnover becomes restricted by the availability of substrates for the brain. Changes in serotonergic activity, inhibitory feed-back from the exercising muscles, elevated ammonia levels, and alterations in regional dopaminergic activity may also contribute to the impaired voluntary activation of the motorneurons after prolonged and strenuous exercise. Furthermore, central fatigue may involve depletion of cerebral glycogen stores, as signified by the observation that following exhaustive exercise the cerebral glucose uptake increases out of proportion to that of oxygen. In summary, prolonged exercise may induce homeostatic disturbances within the central nervous system (CNS) that subsequently attenuates motor activation. Therefore, strenuous exercise is a challenge not only to the cardiorespiratory and locomotive systems but also to the brain.

Astrocyte glycogen metabolism is required for neural activity during aglycemia or intense stimulation in mouse white matter

We tested the hypothesis that inhibiting glycogen degradation accelerates compound action potential (CAP) failure in mouse optic nerve (MON) during aglycemia or high-intensity stimulation. Axon function was assessed as the evoked CAP, and glycogen content was measured biochemically. Isofagomine, a novel inhibitor of central nervous system (CNS) glycogen phosphorylase, significantly increased glycogen content under normoglycemic conditions. When MONs were bathed in artificial cerebrospinal fluid (aCSF) containing 10 mM glucose, the CAP failed 16 min after exposure to glucose-free aCSF. MONs bathed in aCSF plus isofagomine displayed accelerated CAP failure on glucose removal. Similar results were obtained in MONs bathed in 30 mM glucose, which increased baseline glycogen concentration. The ability of isofagomine to increase glycogen content thus was not translated into delayed CAP failure. This is likely due to the inability of the tissue to metabolize glycogen in the presence of isofagomine, highlighting the importance of glycogen in sustaining neural function during aglycemia. The hypothesis that glycogen breakdown supports intense neural activity was tested by blocking glycogen breakdown during periods of high-frequency stimulation. The CAP area declined more rapidly when glycogen metabolism was inhibited by isofagomine, explicitly showing an important physiological role for glycogen metabolism during neural activity.

Glycogen in the brain of Drosophila melanogaster: diurnal rhythm and the effect of rest deprivation

One function of sleep is thought to be the restoration of energy stores in the brain depleted during wakefulness. One such energy store found in mammalian brains is glycogen. Many of the genes involved in glycogen regulation in mammals have also been found in Drosophila melanogaster and rest behavior in Drosophila has recently been shown to have the characteristics of sleep. We therefore examined, in the fly, variation in the glycogen contents of the brain, the whole head and the body throughout the rest/activity cycle and after rest deprivation. Glycogen in the brain varies significantly throughout the day (p=0.001) and is highest during rest and lowest while flies are active. Glycogen levels in the whole head and body do not show diurnal variation. Brain glycogen drops significantly when flies are rest deprived for 3 h (p=0.034) but no significant differences are observed after 6 h of rest deprivation. In contrast, glycogen is significantly depleted in the body after both 3 and 6 h of rest deprivation (p<0.0001 and p<0.0001, respectively). Glycogen in the fly brain changes in relationship to rest and activity and demonstrates a biphasic response to rest deprivation similar to that observed in mammalian astrocytes in culture.

Neurohumoral responses during prolonged exercise in humans

This study examined neurohumoral alterations during prolonged exercise with and without hyperthermia. The cerebral oxygen-to-carbohydrate uptake ratio (O2/CHO = arteriovenous oxygen difference divided by arteriovenous glucose difference plus one-half lactate), the cerebral balances of dopamine, and the metabolic precursor of serotonin, tryptophan, were evaluated in eight endurance-trained subjects during exercise randomized to be with or without hyperthermia. The core temperature stabilized at 37.9 +/- 0.1 degrees C (mean +/- SE) in the control trial, whereas it increased to 39.7 +/- 0.2 degrees C in the hyperthermic trial, with a concomitant increase in perceived exertion (P < 0.05). At rest, the brain had a small release of tryptophan (arteriovenous difference of -1.2 +/- 0.3 micromol/l), whereas a net balance was obtained during the two exercise trials. Both the arterial and jugular venous dopamine levels became elevated during the hyperthermic trial, but the net release from the brain was unchanged. During exercise, the O2/CHO was similar across trials, but, during recovery from the hyperthermic trial, the ratio decreased to 3.8 +/- 0.3 (P < 0.05), whereas it returned to the baseline level of approximately 6 within 5 min after the control trial. The lowering of O2/CHO was established by an increased arteriovenous glucose difference (1.1 +/- 0.1 mmol/l during recovery from hyperthermia vs. 0.7 +/- 0.1 mmol/l in control; P < 0.05). The present findings indicate that the brain has an increased need for carbohydrates during recovery from strenuous exercise, whereas enhanced perception of effort as observed during exercise with hyperthermia was not related to alterations in the cerebral balances of dopamine or tryptophan.

In vivo 13C NMR assessment of brain glycogen concentration and turnover in the awake rat

Brain glycogen metabolism was recently observed in vivo and found to be very slow in the lightly alpha-chloralose anesthetized rat [J. Neurochem. 73 (1999) 1300]. Based on that slow turnover, the total glycogen content in the awake rat brain and its turnover time were assessed after administering 13C-labeled glucose for 48 h. Label incorporation into glycogen, glucose, amino acid, and N-acetyl-aspartate (NAA) resonances was observed. The amount of 13C label incorporated into glycogen was variable and did not correlate with that in glutamate (r=-0.1, P>0.86). However, the amount of 13C label incorporated into glycogen was very similar to that in NAA (r=0.93), implying similar turnover times between brain glycogen and NAA (approximately 10 h). Absolute quantification of the total concentration of brain glycogen in the awake, normoglycemic rat yielded 3.3+/-0.8 micromol/g (n=6, mean+/-S.D.).

Direct, noninvasive measurement of brain glycogen metabolism in humans

The concentration and metabolism of the primary carbohydrate store in the brain, glycogen, is unknown in the conscious human brain. This study reports the first direct detection and measurement of glycogen metabolism in the human brain, which was achieved using localized 13C NMR spectroscopy. To enhance the NMR signal, the isotopic enrichment of the glucosyl moieties was increased by administration of 80 g of 99% enriched [1-13C]glucose in four subjects. 3 h after the start of the label administration, the 13C NMR signal of brain glycogen C1 was detected (0.36+/-0.07 micromol/g, mean+/-S.D., n=4). Based on the rate of 13C label incorporation into glycogen and the isotopic enrichment of plasma glucose, the flux through glycogen synthase was estimated at 0.17+/-0.05 micromol/(gh). This study establishes that brain glycogen can be measured in humans and indicates that its metabolism is very slow in the conscious human. The noninvasive detection of human brain glycogen opens the prospect of understanding the role and function of this important energy reserve under various physiological and pathophysiological conditions.

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.

Changes in brain glycogen after sleep deprivation vary with genotype

Sleep has been functionally implicated in brain energy homeostasis in that it could serve to replenish brain energy stores that become depleted while awake. Sleep deprivation (SD) should therefore lower brain glycogen content. We tested this hypothesis by sleep depriving mice of three inbred strains, i.e., AKR/J (AK), DBA/2J (D2), and C57BL/6J (B6), that differ greatly in their sleep regulation. After a 6-h SD, these mice and their controls were killed by microwave irradiation, and glycogen and glucose were quantified in the cerebral cortex, brain stem, and cerebellum. After SD, both measures significantly increased by approximately 40% in the cortex of B6 mice, while glycogen significantly decreased by 20-38% in brain stem and cerebellum of AK and D2 mice. In contrast, after SD, glucose content increased in all three structures in AK mice and did not change in D2 mice. The increase in glycogen after SD in B6 mice persisted under conditions of food deprivation that, by itself, lowered cortical glycogen. Furthermore, the strains that differ most in their compensatory response to sleep loss, i.e., AK and D2, did not differ in their glycogen response. Thus glycogen content per se is an unlikely end point of sleep's functional role in brain energy homeostasis.

In vivo NMR studies of the glutamate neurotransmitter flux and neuroenergetics: implications for brain function

Until very recently, non-invasive measurement of the glutamate-glutamine cycle in the intact mammalian brain had not been possible. In this review, we describe some studies that have led to quantitative assessment of the glutamate-glutamine cycle (Vcyc), as well as other important metabolic fluxes (e.g., glucose oxidation, CMRglc(ox)), with (13)C magnetic resonance spectroscopy (MRS) in vivo. These (13)C MRS studies clearly demonstrate that glutamate released from presynaptic neurons is taken up by the astrocyte for subsequent glutamine synthesis. Contrary to the earlier concept of a small, metabolically inactive neurotransmitter pool, in vivo (13)C MRS studies demonstrate that glutamate release and recycling is a major metabolic pathway that cannot be distinguished from its actions of neurotransmission. Furthermore, the in vivo (13)C MRS studies demonstrate in the rat cerebral cortex that increases in Vcyc and neuronal CMRglc(ox) are linearly related with a close to 1:1 slope. Measurements in human cerebral cortex are in agreement with this result. This relationship is consistent with more than two thirds of the energy yielded by glucose oxidation being used to support events associated with glutamate neurotransmission, and it supports a molecular model of a stoichiometric coupling between glutamate neurotransmission and functional glucose oxidation. (13)C MRS measurements of resting human cerebral cortex have found a high level of glutamate-glutamine cycling. This high resting neuronal activity, which is subtracted away in brain mapping studies by positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), has significant implications for the interpretations of functional imaging data. Here we review and discuss the importance of neurotransmission and neuroenergetics as measured by (13)C MRS for understanding brain function and interpreting fMRI.

Glycogen regulation and functional role in mouse white matter

CNS glycogen, contained predominantly in astrocytes, can be converted to a monocarboxylate and transported to axons as an energy source during aglycaemia. We analysed glycogen regulation and the role of glycogen in supporting neural activity in adult mouse optic nerve, a favourable white matter preparation. Axon function was quantified by measuring the compound action potential (CAP) area. During aglycaemia, axon function persisted for 20 min, then declined in conjunction with glycogen content. Lactate fully supported CAPs in the absence of glucose, but was unable to sustain glycogen content; thus, axon failure occurred rapidly when lactate was withdrawn. Glycogen content in the steady state was directly proportional to bath glucose concentration. Increasing [K+]o to 10 mM caused a rapid decrease in glycogen content. Latency to onset of CAP failure during aglycaemia was directly proportional to glycogen content and varied from about 2 to 30 min. Intense neural activity reduced glycogen content in the presence of 10 mM bath glucose and CAP area gradually declined. CAP area declined more rapidly during high frequency stimulation if monocarboxylate transport was inhibited. This suggested that astrocytic glycogen was broken down to a monocarboxylate(s) that was used by rapidly discharging axons. Likewise, depleting glycogen by brief periods of high frequency axon stimulation accelerated onset of CAP decline during aglycaemia. In summary, these experiments indicated that glycogen content was under dynamic control and that glycogen was used to support the energy needs of CNS axons during both physiological as well as pathological processes.

A2B receptor activation promotes glycogen synthesis in astrocytes through modulation of gene expression

Adenosine has been proposed as a key factor regulating the metabolic balance between energy supply and demand in the central nervous system. Because astrocytes represent an important cellular element in the control of brain energy metabolism, we investigated whether adenosine could induce long-term changes of glycogen levels in primary cultures of mouse cortical astrocytes. We observed that adenosine increased glycogen content, up to 300%, in a time- (maximum at 8 h) and concentration-dependent manner with an EC(50) of 9.69 microM. Pharmacological experiments using the broad-spectrum agonist 5'-(N-ethylcarboxamido)adenosine (NECA) and specific agonists for the A(1), A(2A), and A(3) receptors [N(6)-cyclopentyladenosine (CPA), CGS-21680, and IB-MECA, respectively] suggest that the effect of adenosine is mediated through activation of the low-affinity A(2B) adenosine receptor subtype. Interestingly, adenosine induces in parallel the expression of the protein targeting to glycogen (PTG), one of the protein phosphatase-1 glycogen-targeting subunits that has been implicated in the control of glycogen levels in various tissues. These results indicate that adenosine can exert long-term control over glycogen levels in astrocytes and might therefore play a significant role in physiological and/or pathological processes involving long-term modulation of brain energy metabolism.