Pyruvate incubation enhances glycogen stores and sustains neuronal function during subsequent glucose deprivation

Glycolytic System in Axons Supplement Decreased ATP Levels after Axotomy of the Peripheral Nerve

Wallerian degeneration (WD) occurs in the early stages of numerous neurologic disorders, and clarifying WD pathology is crucial for the advancement of neurologic therapies. ATP is acknowledged as one of the key pathologic substances in WD. The ATP-related pathologic pathways that regulate WD have been defined. The elevation of ATP levels in axon contributes to delay WD and protects axons. However, ATP is necessary for the active processes to proceed WD, given that WD is stringently managed by auto-destruction programs. But little is known about the bioenergetics during WD. In this study, we made sciatic nerve transection models for GO-ATeam2 knock-in rats and mice. We presented the spatiotemporal ATP distribution in the injured axons with in vivo ATP imaging systems, and investigated the metabolic source of ATP in the distal nerve stump. A gradual decrease in ATP levels was observed before the progression of WD. In addition, the glycolytic system and monocarboxylate transporters (MCTs) were activated in Schwann cells following axotomy. Interestingly, in axons, we found the activation of glycolytic system and the inactivation of the tricarboxylic acid (TCA) cycle. Glycolytic inhibitors, 2-deoxyglucose (2-DG) and MCT inhibitors, a-cyano-4-hydroxycinnamic acid (4-CIN) decreased ATP and enhanced WD progression, whereas mitochondrial pyruvate carrier (MPC) inhibitors (MSDC-0160) did not change. Finally, ethyl pyruvate (EP) increased ATP levels and delayed WD. Together, our findings suggest that glycolytic system, both in Schwann cells and axons, is the main source of maintaining ATP levels in the distal nerve stump.

Effects of Rumen-Protected Creatine Pyruvate on Meat Quality, Hepatic Gluconeogenesis, and Muscle Energy Metabolism of Long-Distance Transported Beef Cattle

Pre-slaughter long-distance transport resulted in a rapid depletion of muscle glycogen and led to a higher rate of dark, firm and dry (DFD) meat. Therefore, enhancing muscle glycogen reserves is critical for beef cattle prior to transportation. Creatine pyruvate (CrPyr) can provide simultaneous pyruvate and creatine and both are proven to promote the glycogen reserves. This study aimed to investigate the effects of transport treatment and dietary supplementation of rumen-protected (RP)-CrPyr on the meat quality, muscle energy metabolism, and hepatic gluconeogenesis of beef cattle. Twenty 18 month-old male Simmental crossbred cattle (659 ± 16 kg) were allotted 4 treatments based on a 2 × 2 factorial arrangement with two RP-CrPyr levels (140 g/d or 0 g/d) and two transport treatments (12 h or 5 min): ST_CrPyr0, ST_CrPyr140, LT_CrPyr0 and LT_CrPyr140. Three cattle per group were slaughtered after 30 days of feeding. The interaction of transport and RP-CrPyr had a significant effect on the muscle pH45 min, redness, glycogen content, GP, and AMP level (P < 0.05). Compared with short-distance transport, long-distance transport increased the muscle pH45 min value, redness, yellowness, drip loss, creatine level (P < 0.05), decreased muscle glycogen content, glycolytic potential (GP), and liver glucose amount (P < 0.05). Supplementation of RP-CrPyr decreased the activities of creatine kinase and lactate dehydrogenase in serum, muscle pH24 h value, redness, yellowness, lactate content, AMP level, and AMP/ATP (P < 0.05), increased the muscle glycogen content, GP, hexokinase activity, ATP and ADP levels, and ATP/ADP, liver pyruvate and glucose contents, activity of pyruvate carboxylase in the liver of cattle than those in the nonsupplemented treatments (P < 0.05). These results indicated that dietary RP-CrPyr supplementation might be favorable to improve meat quality and regulatory capacity of energy metabolism of beef cattle suffering long-distance transport followed with recovery treatment by increasing muscle glycogen storage, energy supply, and hepatic gluconeogenesis.

Protection from systemic pyruvate at resuscitation in newborn lambs with asphyxial cardiac arrest

BACKGROUND

Infants with hypoxic-ischemic injury often require cardiopulmonary resuscitation. Mitochondrial failure to generate adenosine triphosphate (ATP) during hypoxic-ischemic reperfusion injury contributes to cellular damage. Current postnatal strategies to improve outcome in hypoxic-ischemic injury need sophisticated equipment to perform servo-controlled cooling. Administration of intravenous pyruvate, an antioxidant with favorable effects on mitochondrial bioenergetics, is a simple intervention that can have a global impact. We hypothesize that the administration of pyruvate following the return of spontaneous circulation (ROSC) would improve cardiac function, systemic hemodynamics, and oxygen utilization in the brain in newborn lambs with cardiac arrest (CA).

METHODS

Term lambs were instrumented, delivered by C-section and asphyxia induced by umbilical cord occlusion along with clamping of the endotracheal tube until asystole; Lambs resuscitated following 5 min of CA; upon ROSC, lambs were randomized to receive pyruvate or saline infusion over 90 min and ventilated for 150 min postinfusion. Pulmonary and systemic hemodynamics and arterial gases monitored. We measured plasma pyruvate, tissue lactate, and ATP levels (heart and brain) in both groups.

RESULTS

Time to ROSC was not different between the two groups. Systolic and diastolic blood pressures, stroke volume, arterial oxygen content, and cerebral oxygen delivery were similar between the two groups. The cerebral metabolic rate of oxygen was higher following pyruvate infusion; higher oxygen consumption in the brain was associated with lower plasma levels but higher brain ATP levels compared to the saline group.

CONCLUSIONS

Pyruvate promotes energy generation accompanied by efficient oxygen utilization in the brain and may facilitate additional neuroprotection in the presence of hypoxic-ischemic injury.

Metabolic switching is impaired by aging and facilitated by ketosis independent of glycogen

The ability to switch between glycolysis and ketosis promotes survival by enabling metabolism through fat oxidation during periods of fasting. Carbohydrate restriction or stress can also elicit metabolic switching. Keto-adapting from glycolysis is delayed in aged rats, but factors mediating this age-related impairment have not been identified. We measured metabolic switching between glycolysis and ketosis, as well as glycogen dynamics, in young and aged rats undergoing time-restricted feeding (TRF) with a standard diet or a low carbohydrate ketogenic diet (KD). TRF alone reversed markers of insulin-related metabolic deficits and accelerated metabolic switching in aged animals. A KD+TRF, however, provided additive benefits on these variables. Remarkably, the ability to keto-adapt was not related to glycogen levels and KD-fed rats showed an enhanced elevation in glucose following epinephrine administration. This study provides new insights into the mechanisms of keto-adaptation demonstrating the utility of dietary interventions to treat metabolic impairments across the lifespan.

Brain Glucose Metabolism: Integration of Energetics with Function

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

Impact of Hypoglycemia on Brain Metabolism During Diabetes

Diabetes is a metabolic disease afflicting millions of people worldwide. A substantial fraction of world's total healthcare expenditure is spent on treating diabetes. Hypoglycemia is a serious consequence of anti-diabetic drug therapy, because it induces metabolic alterations in the brain. Metabolic alterations are one of the central mechanisms mediating hypoglycemia-related functional changes in the brain. Acute, chronic, and/or recurrent hypoglycemia modulate multiple metabolic pathways, and exposure to hypoglycemia increases consumption of alternate respiratory substrates such as ketone bodies, glycogen, and monocarboxylates in the brain. The aim of this review is to discuss hypoglycemia-induced metabolic alterations in the brain in glucose counterregulation, uptake, utilization and metabolism, cellular respiration, amino acid and lipid metabolism, and the significance of other sources of energy. The present review summarizes information on hypoglycemia-induced metabolic changes in the brain of diabetic and non-diabetic subjects and the manner in which they may affect brain function.

Glucose and lactate as metabolic constraints on presynaptic transmission at an excitatory synapse

KEY POINTS

Synapses have high energy demands which increase during intense activity. We show that presynaptic terminals can utilise extracellular glucose or lactate to generate energy to maintain synaptic transmission. Reducing energy substrates induces a metabolic stress: presynaptic ATP depletion impaired synaptic transmission through a reduction in the number of functional synaptic vesicle release sites and a slowing of vesicle pool replenishment, without a consistent change in release probability. Metabolic function is compromised in many pathological conditions (e.g. stroke, traumatic brain injury and neurodegeneration). Knowledge of how synaptic transmission is constrained by metabolic stress, especially during intense brain activity, will provide insights to improve cognition following pathological insults.

ABSTRACT

The synapse has high energy demands, which increase during intense activity. Presynaptic ATP production depends on substrate availability and usage will increase during activity, which in turn could influence transmitter release and information transmission. We investigated transmitter release at the mouse calyx of Held synapse using glucose or lactate (10, 1 or 0 mm) as the extracellular substrates while inducing metabolic stress. High-frequency stimulation (HFS) and recovery paradigms evoked trains of EPSCs monitored under voltage-clamp. Whilst postsynaptic intracellular ATP was stabilised by diffusion from the patch pipette, depletion of glucose increased EPSC depression during HFS and impaired subsequent recovery. Computational modelling of these data demonstrated a reduction in the number of functional release sites and slowed vesicle pool replenishment during metabolic stress, with little change in release probability. Directly depleting presynaptic terminal ATP impaired transmitter release in an analogous manner to glucose depletion. In the absence of glucose, presynaptic terminal metabolism could utilise lactate from the aCSF and this was blocked by inhibition of monocarboxylate transporters (MCTs). MCT inhibitors significantly suppressed transmission in low glucose, implying that lactate is a presynaptic substrate. Additionally, block of glycogenolysis accelerated synaptic transmission failure in the absence of extracellular glucose, consistent with supplemental supply of lactate by local astrocytes. We conclude that both glucose and lactate support presynaptic metabolism and that limited availability, exacerbated by high-intensity firing, constrains presynaptic ATP, impeding transmission through a reduction in functional presynaptic release sites as vesicle recycling slows when ATP levels are low.

Metabolic correction by pyruvate halts acquired epilepsy in multiple rodent models

Metabolic intervention strategy of epilepsy treatment has been gaining broader attention due to accumulated evidence that hypometabolism, manifested in humans as reduced brain glucose consumption, is a principal factor in acquired epilepsy. Therefore, targeting deficient energy metabolism may be an effective approach for treating epilepsy. To confront this pathology we utilized pyruvate, which besides being an anaplerotic mitochondrial fuel possesses a unique set of neuroprotective properties as it: (i) is a potent reactive oxygen species scavenger; (ii) abates overactivation of Poly [ADP-ribose] polymerase 1 (PARP-1); (iii) facilitates glutamate efflux from the brain; (iv) augments brain glycogen stores; (v) is anti-inflammatory; (vi) prevents neuronal hyperexcitability; and (vii) normalizes the cytosolic redox state. In vivo, chronic oral pyruvate administration completely abolished established epileptic phenotypes in three accepted and fundamentally different rodent acquired epilepsy models. Our study reports metabolic correction by pyruvate as a potentially highly effective treatment of acquired epilepsies.

The vicious circle of hypometabolism in neurodegenerative diseases: Ways and mechanisms of metabolic correction

Hypometabolism, characterized by decreased brain glucose consumption, is a common feature of many neurodegenerative diseases. Initial hypometabolic brain state, created by characteristic risk factors, may predispose the brain to acquired epilepsy and sporadic Alzheimer's and Parkinson's diseases, which are the focus of this review. Analysis of available data suggests that deficient glucose metabolism is likely a primary initiating factor for these diseases, and that resulting neuronal dysfunction further promotes the metabolic imbalance, establishing an effective positive feedback loop and a downward spiral of disease progression. Therefore, metabolic correction leading to the normalization of abnormalities in glucose metabolism may be an efficient tool to treat the neurological disorders by counteracting their primary pathological mechanisms. Published and preliminary experimental results on this approach for treating Alzheimer's disease and epilepsy models support the efficacy of metabolic correction, confirming the highly promising nature of the strategy. © 2017 Wiley Periodicals, Inc.

Chronic Pyruvate Supplementation Increases Exploratory Activity and Brain Energy Reserves in Young and Middle-Aged Mice

Numerous studies have reported neuroprotective effects of pyruvate when given in systemic injections. Impaired glucose uptake and metabolism are found in Alzheimer's disease (AD) and in AD mouse models. We tested whether dietary pyruvate supplementation is able to provide added energy supply to brain and thereby attenuate aging- or AD-related cognitive impairment. Mice received ~800 mg/kg/day Na-pyruvate in their chow for 2-6 months. In middle-aged wild-type mice and in 6.5-month-old APP/PS1 mice, pyruvate facilitated spatial learning and increased exploration of a novel odor. However, in passive avoidance task for fear memory, the treatment group was clearly impaired. Independent of age, long-term pyruvate increased explorative behavior, which likely explains the paradoxical impairment in passive avoidance. We also assessed pyruvate effects on body weight, muscle force, and endurance, and found no effects. Metabolic postmortem assays revealed increased energy compounds in nuclear magnetic resonance spectroscopy as well as increased brain glycogen storages in the pyruvate group. Pyruvate supplementation may counteract aging-related behavioral impairment, but its beneficial effect seems related to increased explorative activity rather than direct memory enhancement.

Contributions of glycogen to astrocytic energetics during brain activation

Glycogen is the major store of glucose in brain and is mainly in astrocytes. Brain glycogen levels in unstimulated, carefully-handled rats are 10-12 μmol/g, and assuming that astrocytes account for half the brain mass, astrocytic glycogen content is twice as high. Glycogen turnover is slow under basal conditions, but it is mobilized during activation. There is no net increase in incorporation of label from glucose during activation, whereas label release from pre-labeled glycogen exceeds net glycogen consumption, which increases during stronger stimuli. Because glycogen level is restored by non-oxidative metabolism, astrocytes can influence the global ratio of oxygen to glucose utilization. Compensatory increases in utilization of blood glucose during inhibition of glycogen phosphorylase are large and approximate glycogenolysis rates during sensory stimulation. In contrast, glycogenolysis rates during hypoglycemia are low due to continued glucose delivery and oxidation of endogenous substrates; rates that preserve neuronal function in the absence of glucose are also low, probably due to metabolite oxidation. Modeling studies predict that glycogenolysis maintains a high level of glucose-6-phosphate in astrocytes to maintain feedback inhibition of hexokinase, thereby diverting glucose for use by neurons. The fate of glycogen carbon in vivo is not known, but lactate efflux from brain best accounts for the major metabolic characteristics during activation of living brain. Substantial shuttling coupled with oxidation of glycogen-derived lactate is inconsistent with available evidence. Glycogen has important roles in astrocytic energetics, including glucose sparing, control of extracellular K(+) level, oxidative stress management, and memory consolidation; it is a multi-functional compound.

Age-related metabolic fatigue during low glucose conditions in rat hippocampus

Previous reports have indicated that with aging, intrinsic brain tissue changes in cellular bioenergetics may hamper the brain's ability to cope with metabolic stress. Therefore, we analyzed the effects of age on neuronal sensitivity to glucose deprivation by monitoring changes in field excitatory postsynaptic potentials (fEPSPs), tissue Po2, and NADH fluorescence imaging in the CA1 region of hippocampal slices obtained from F344 rats (1-2, 3-6, 12-20, and >22 months). Forty minutes of moderate low glucose (2.5 mM) led to approximately 80% decrease of fEPSP amplitudes and NADH decline in all 4 ages that reversed after reintroduction of 10 mM glucose. However, tissue slices from 12 to 20 months and >22-month-old rats were more vulnerable to low glucose: fEPSPs decreased by 50% on average 8 minutes faster compared with younger slices. Tissue oxygen utilization increased after onset of 2.5 mM glucose in all ages of tissue slices, which persisted for 40 minutes in younger tissue slices. But, in older tissue slices the increased oxygen utilization slowly faded and tissue Po2 levels increased toward baseline values after approximately 25 minutes of glucose deprivation. In addition, with age the ability to regenerate NADH after oxidation was diminished. The NAD(+)/NADH ratio remained relatively oxidized after low glucose, even during recovery. In young slices, glycogen levels were stable throughout the exposure to low glucose. In contrast, with aging utilization of glycogen stores was increased during low glucose, particularly in hippocampal slices from >22 months old rats, indicating both inefficient metabolism and increased demand for glucose. Lactate addition (20 mM) improved oxidative metabolism by directly supplementing the mitochondrial NADH pool and maintained fEPSPs in young as well as aged tissue slices, indicating that inefficient metabolism in the aging tissue can be improved by directly enhancing NADH regeneration.

Energy substrates protect hippocampus against endogenous glutamate-mediated neurodegeneration in awake rats

Excitotoxicity due to excessive glutamatergic neurotransmission is a well-studied phenomenon that has been related to the mechanisms of neuronal death occurring in some disorders of the CNS. We have previously shown that the intrahippocampal perfusion by microdialysis of 4-aminopyridine (4-AP) in rats stimulates endogenous glutamate release from nerve endings and this results in excitotoxic effects such as immediate seizures and delayed neuronal death, due to the overactivation of N-methyl-D-aspartate (NMDA) receptors. To study whether mitochondrial energy dysfunction and oxidative stress could be involved in this 4-AP-induced excitotoxicity, we evaluated in awake rats the protective effect of several energy substrates and antioxidant compounds, using microdialysis, electroencephalographic (EEG) recording and histological analysis. The 4-AP-induced behavioral and EEG seizures, which progressed to status epilepticus in about 30 min, were prevented by the NMDA receptor antagonist MK-801, whereas acetoacetate, DL- and L-β-hydroxybutyrate did not protect against seizures but increased the latency to the onset of status epilepticus; pyruvate, α-ketoglutarate and glutathione ethyl ester did not show any protective effect. 4-AP also produced nearly complete loss of pyramidal neurons in CA1 and CA3 regions of the ipsilateral hippocampus 24 h after the experiment. MK-801 totally prevented this neuronal death and the energy substrates tested protected by about 50%, whereas the antioxidants showed only a weak protection. We conclude that ketone bodies possess weak anticonvulsant effects and that energy metabolism impairment plays a more important role than oxidative stress in the delayed hippocampal neurodegeneration resulting from the excitotoxic action of 4-AP mediated by endogenous glutamate.

Direct evidence for activity-dependent glucose phosphorylation in neurons with implications for the astrocyte-to-neuron lactate shuttle

Previous (13)C magnetic resonance spectroscopy experiments have shown that over a wide range of neuronal activity, approximately one molecule of glucose is oxidized for every molecule of glutamate released by neurons and recycled through astrocytic glutamine. The measured kinetics were shown to agree with the stoichiometry of a hypothetical astrocyte-to-neuron lactate shuttle model, which predicted negligible functional neuronal uptake of glucose. To test this model, we measured the uptake and phosphorylation of glucose in nerve terminals isolated from rats infused with the glucose analog, 2-fluoro-2-deoxy-D-glucose (FDG) in vivo. The concentrations of phosphorylated FDG (FDG6P), normalized with respect to known neuronal metabolites, were compared in nerve terminals, homogenate, and cortex of anesthetized rats with and without bicuculline-induced seizures. The increase in FDG6P in nerve terminals agreed well with the increase in cortical neuronal glucose oxidation measured previously under the same conditions in vivo, indicating that direct uptake and oxidation of glucose in nerve terminals is substantial under resting and activated conditions. These results suggest that neuronal glucose-derived pyruvate is the major oxidative fuel for activated neurons, not lactate-derived from astrocytes, contradicting predictions of the original astrocyte-to-neuron lactate shuttle model under the range of study conditions.

Nicotinamide pre-treatment ameliorates NAD(H) hyperoxidation and improves neuronal function after severe hypoxia

Prolonged hypoxia leads to irreversible loss of neuronal function and metabolic impairment of nicotinamide adenine dinucleotide recycling (between NAD(+) and NADH) immediately after reoxygenation, resulting in NADH hyperoxidation. We test whether the addition of nicotinamide (to enhance NAD(+) levels) or PARP-1 inhibition (to prevent consumption of NAD(+)) can be effective in improving either loss of neuronal function or hyperoxidation following severe hypoxic injury in hippocampal slices. After severe, prolonged hypoxia (maintained for 3min after spreading depression) there was hyperoxidation of NADH following reoxygenation, an increased soluble NAD(+)/NADH ratio, loss of neuronal field excitatory post-synaptic potential (fEPSP) and decreased ATP content. Nicotinamide incubation (5mM) 2h prior to hypoxia significantly increased total NAD(H) content, improved neuronal recovery, enhanced ATP content, and prevented NADH hyperoxidation. The nicotinamide-induced increase in total soluble NAD(H) was more significant in the cytosolic compartment than within mitochondria. Prolonged incubation with PJ-34 (>1h) led to enhanced baseline NADH fluorescence prior to hypoxia, as well as improved neuronal recovery, NADH hyperoxidation and ATP content on recovery from severe hypoxia and reoxygenation. In this acute model of severe neuronal dysfunction prolonged incubation with either nicotinamide or PJ-34 prior to hypoxia improved recovery of neuronal function, enhanced NADH reduction and ATP content, but neither treatment restored function when administered during or after prolonged hypoxia and reoxygenation.

Neuronal damage and cognitive impairment associated with hypoglycemia: An integrated view

The aim of the present review is to offer a current perspective about the consequences of hypoglycemia and its impact on the diabetic disorder due to the increasing incidence of diabetes around the world. The main consequence of insulin treatment in type 1 diabetic patients is the occurrence of repetitive periods of hypoglycemia and even episodes of severe hypoglycemia leading to coma. In the latter, selective neuronal death is observed in brain vulnerable regions both in humans and animal models, such as the cortex and the hippocampus. Cognitive damage subsequent to hypoglycemic coma has been associated with neuronal death in the hippocampus. The mechanisms implicated in selective damage are not completely understood but many factors have been identified including excitotoxicity, oxidative stress, zinc release, PARP-1 activation and mitochondrial dysfunction. Importantly, the diabetic condition aggravates neuronal damage and cognitive failure induced by hypoglycemia. In the absence of coma prolonged and severe hypoglycemia leads to increased oxidative stress and discrete neuronal death mainly in the cerebral cortex. The mechanisms responsible for cell damage in this condition are still unknown. Recurrent moderate hypoglycemia is far more common in diabetic patients than severe hypoglycemia and currently important efforts are being done in order to elucidate the relationship between cognitive deficits and recurrent hypoglycemia in diabetics. Human studies suggest impaired performance mainly in memory and attention tasks in healthy and diabetic individuals under the hypoglycemic condition. Only scarce neuronal death has been observed under moderate repetitive hypoglycemia but studies suggest that impaired hippocampal synaptic function might be one of the causes of cognitive failure. Recent studies have also implicated altered mitochondrial function and mitochondrial oxidative stress.

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.

Effects of hypoglycaemia on neuronal metabolism in the adult brain: role of alternative substrates to glucose

Hypoglycaemia is characterized by decreased blood glucose levels and is associated with different pathologies (e.g. diabetes, inborn errors of metabolism). Depending on its severity, it might affect cognitive functions, including impaired judgment and decreased memory capacity, which have been linked to alterations of brain energy metabolism. Glucose is the major cerebral energy substrate in the adult brain and supports the complex metabolic interactions between neurons and astrocytes, which are essential for synaptic activity. Therefore, hypoglycaemia disturbs cerebral metabolism and, consequently, neuronal function. Despite the high vulnerability of neurons to hypoglycaemia, important neurochemical changes enabling these cells to prolong their resistance to hypoglycaemia have been described. This review aims at providing an overview over the main metabolic effects of hypoglycaemia on neurons, covering in vitro and in vivo findings. Recent studies provided evidence that non-glucose substrates including pyruvate, glycogen, ketone bodies, glutamate, glutamine, and aspartate, are metabolized by neurons in the absence of glucose and contribute to prolong neuronal function and delay ATP depletion during hypoglycaemia. One of the pathways likely implicated in the process is the pyruvate recycling pathway, which allows for the full oxidation of glutamate and glutamine. The operation of this pathway in neurons, particularly after hypoglycaemia, has been re-confirmed recently using metabolic modelling tools (i.e. Metabolic Flux Analysis), which allow for a detailed investigation of cellular metabolism in cultured cells. Overall, the knowledge summarized herein might be used for the development of potential therapies targeting neuronal protection in patients vulnerable to hypoglycaemic episodes.

Dietary energy substrates reverse early neuronal hyperactivity in a mouse model of Alzheimer's disease

Deficient energy metabolism and network hyperactivity are the early symptoms of Alzheimer's disease (AD). In this study, we show that administration of exogenous oxidative energy substrates (OES) corrects neuronal energy supply deficiency that reduces the amyloid-beta-induced abnormal neuronal activity in vitro and the epileptic phenotype in AD model in vivo. In vitro, acute application of protofibrillar amyloid-β1-42 (Aβ1-42) induced aberrant network activity in wild-type hippocampal slices that was underlain by depolarization of both the neuronal resting membrane potential and GABA-mediated current reversal potential. Aβ1-42 also impaired synaptic function and long-term potentiation. These changes were paralleled by clear indications of impaired energy metabolism, as indicated by abnormal NAD(P)H signaling induced by network activity. However, when glucose was supplemented with OES pyruvate and 3-beta-hydroxybutyrate, Aβ1-42 failed to induce detrimental changes in any of the above parameters. We administered the same OES as chronic supplementation to a standard diet to APPswe/PS1dE9 transgenic mice displaying AD-related epilepsy phenotype. In the ex-vivo slices, we found neuronal subpopulations with significantly depolarized resting and GABA-mediated current reversal potentials, mirroring abnormalities we observed under acute Aβ1-42 application. Ex-vivo cortex of transgenic mice fed with standard diet displayed signs of impaired energy metabolism, such as abnormal NAD(P)H signaling and strongly reduced tolerance to hypoglycemia. Transgenic mice also possessed brain glycogen levels twofold lower than those of wild-type mice. However, none of the above neuronal and metabolic dysfunctions were observed in transgenic mice fed with the OES-enriched diet. In vivo, dietary OES supplementation abated neuronal hyperexcitability, as the frequency of both epileptiform discharges and spikes was strongly decreased in the APPswe/PS1dE9 mice placed on the diet. Altogether, our results suggest that early AD-related neuronal malfunctions underlying hyperexcitability and energy metabolism deficiency can be prevented by dietary supplementation with native energy substrates.

Cellular Links between Neuronal Activity and Energy Homeostasis

Neuronal activity, astrocytic responses to this activity, and energy homeostasis are linked together during baseline, conscious conditions, and short-term rapid activation (as occurs with sensory or motor function). Nervous system energy homeostasis also varies during long-term physiological conditions (i.e., development and aging) and with adaptation to pathological conditions, such as ischemia or low glucose. Neuronal activation requires increased metabolism (i.e., ATP generation) which leads initially to substrate depletion, induction of a variety of signals for enhanced astrocytic function, and increased local blood flow and substrate delivery. Energy generation (particularly in mitochondria) and use during ATP hydrolysis also lead to considerable heat generation. The local increases in blood flow noted following neuronal activation can both enhance local substrate delivery but also provides a heat sink to help cool the brain and removal of waste by-products. In this review we highlight the interactions between short-term neuronal activity and energy metabolism with an emphasis on signals and factors regulating astrocyte function and substrate supply.