Changes of brain glycogen in the spreading EEG-depression of Leao

Spreading depolarizations pose critical energy challenges in acute brain injury

Spreading depolarization (SD) is an electrochemical wave of neuronal depolarization mediated by extracellular K+ and glutamate, interacting with voltage-gated and ligand-gated ion channels. SD is increasingly recognized as a major cause of injury progression in stroke and brain trauma, where the mechanisms of SD-induced neuronal injury are intimately linked to energetic status and metabolic impairment. Here, I review the established working model of SD initiation and propagation. Then, I summarize the historical and recent evidence for the metabolic impact of SD, transitioning from a descriptive to a mechanistic working model of metabolic signaling and its potential to promote neuronal survival and resilience. I quantify the energetic cost of restoring ionic gradients eroded during SD, and the extent to which ion pumping impacts high-energy phosphate pools and the energy charge of affected tissue. I link energy deficits to adaptive increases in the utilization of glucose and O2, and the resulting accumulation of lactic acid and CO2 downstream of catabolic metabolic activity. Finally, I discuss the neuromodulatory and vasoactive paracrine signaling mediated by adenosine and acidosis, highlighting these metabolites' potential to protect vulnerable tissue in the context of high-frequency SD clusters.

The Neuroprotective Effects of Long-Term Repetitive Transcranial Magnetic Stimulation on the Cortical Spreading Depression-induced Damages in Rat's Brain

INTRODUCTION

Cortical Spreading Depression (CSD) is a propagating wave of neural and glial cell depolarization with important role in several clinical disorders. Repetitive Transcranial Magnetic Stimulation (rTMS) is a potential tool with preventive treatment effects in psychiatric and neuronal disorders. In this paper, we study the effects of rTMS on CSD by using behavioral and histological approaches in hippocampus and cortical regions.

METHODS

Twenty-four rats were divided into four groups. A group of control rats were kept in their home cage during the experiment. The CSD group received four CSD inductions during 4 weeks with 1 week intervals. The CSD-rTMS group were treated with rTMS stimulation (figure-eight coils, 20 Hz, 10 min/d) for 4 weeks. The fourth group, i.e. rTMS group received rTMS stimulation similar to the CSD-rTMS group without CSD induction.

RESULTS

Long-term rTMS application in treated groups significantly reduced production of dark neurons, increased the mean volume of normal neurons, and decreased the number of apoptotic neurons in cortical regions compared to the control group. The protective effects of long-term treatment by rTMS in the hippocampal regions were also studied. It was effective in some regions; however, rTMS effects on hippocampal regions were lower than cortical ones.

CONCLUSION

Based on the study results, rTMS has significant preventive and protective effects in CSD-induced damages in cortical and hippocampal regions of the rat's brain.

Regulation of cerebral metabolism during cortical spreading depression

We analyzed the metabolic response to cortical spreading depression that drastically increases local energy demand to restore ion homeostasis. During single and multiple cortical spreading depressions in the rat cortex, we simultaneously monitored extracellular levels of glucose and lactate using rapid sampling microdialysis and glucose influx using 18 F-fluorodeoxyglucose positron emission tomography while tracking cortical spreading depression using laser speckle imaging. Combining the acquired data with steady-state requirements we developed a mass-conserving compartment model including neurons and glia that was consistent with the observed data. In summary, our findings are: (1) Early breakdown of glial glycogen provides a major source of energy during increased energy demand and leaves 80% of blood-borne glucose to neurons. (2) Lactate is used solely by neurons and only if extracellular lactate levels are >80% above normal. (3) Although the ratio of oxygen and glucose consumption transiently reaches levels <3, the major part (>90%) of the overall energy supply is from oxidative metabolism. (4) During cortical spreading depression, brain release of lactate exceeds its consumption suggesting that lactate is only a circumstantial energy substrate. Our findings provide a general scenario for the metabolic response to increased cerebral energy demand.

Spreading Depression, Spreading Depolarizations, and the Cerebral Vasculature

Spreading depression (SD) is a transient wave of near-complete neuronal and glial depolarization associated with massive transmembrane ionic and water shifts. It is evolutionarily conserved in the central nervous systems of a wide variety of species from locust to human. The depolarization spreads slowly at a rate of only millimeters per minute by way of grey matter contiguity, irrespective of functional or vascular divisions, and lasts up to a minute in otherwise normal tissue. As such, SD is a radically different breed of electrophysiological activity compared with everyday neural activity, such as action potentials and synaptic transmission. Seventy years after its discovery by Leão, the mechanisms of SD and its profound metabolic and hemodynamic effects are still debated. What we did learn of consequence, however, is that SD plays a central role in the pathophysiology of a number of diseases including migraine, ischemic stroke, intracranial hemorrhage, and traumatic brain injury. An intriguing overlap among them is that they are all neurovascular disorders. Therefore, the interplay between neurons and vascular elements is critical for our understanding of the impact of this homeostatic breakdown in patients. The challenges of translating experimental data into human pathophysiology notwithstanding, this review provides a detailed account of bidirectional interactions between brain parenchyma and the cerebral vasculature during SD and puts this in the context of neurovascular diseases.

Multifaceted roles for astrocytes in spreading depolarization: A target for limiting spreading depolarization in acute brain injury?

Spreading depolarizations (SDs) are coordinated waves of synchronous depolarization, involving large numbers of neurons and astrocytes as they spread slowly through brain tissue. The recent identification of SDs as likely contributors to pathophysiology in human subjects has led to a significant increase in interest in SD mechanisms, and possible approaches to limit the numbers of SDs or their deleterious consequences in injured brain. Astrocytes regulate many events associated with SD. SD initiation and propagation is dependent on extracellular accumulation of K(+) and glutamate, both of which involve astrocytic clearance. SDs are extremely metabolically demanding events, and signaling through astrocyte networks is likely central to the dramatic increase in regional blood flow that accompanies SD in otherwise healthy tissues. Astrocytes may provide metabolic support to neurons following SD, and may provide a source of adenosine that inhibits neuronal activity following SD. It is also possible that astrocytes contribute to the pathophysiology of SD, as a consequence of excessive glutamate release, facilitation of NMDA receptor activation, brain edema due to astrocyte swelling, or disrupted coupling to appropriate vascular responses after SD. Direct or indirect evidence has accumulated implicating astrocytes in many of these responses, but much remains unknown about their specific contributions, especially in the context of injury. Conversion of astrocytes to a reactive phenotype is a prominent feature of injured brain, and recent work suggests that the different functional properties of reactive astrocytes could be targeted to limit SDs in pathophysiological conditions.

Glycogenolysis and purinergic signaling

Both ATP and glutamate are on one hand essential metabolites in brain and on the other serve a signaling function as transmitters. However, there is the major difference that the flux in the pathway producing transmitter glutamate is comparable to the rate of glucose metabolism in brain, whereas that producing transmitter ATP is orders of magnitude smaller than the metabolic turnover between ATP and ADP. Moreover, de novo glutamate production occurs exclusively in astrocytes, whereas transmitter ATP is produced both in neurons and astrocytes. This chapter deals only with ATP and exclusively with its formation and release in astrocytes, and it focuses on potential associations with glycogenolysis, which is known to be indispensable for the synthesis of glutamate. Glycogenolysis is dependent upon an increase in free intracellular Ca(2+) concentration (Ca(2+)]i). It can be further stimulated by cAMP, but in contrast to widespread beliefs, cAMP can on its own not induce glycogenolysis. Astrocytes generate ATP from accumulated adenosine, and this process does not seem to require glycogenolysis. A minor amount of the generated ATP is utilized as a transmitter, and its synthesis requires the presence of the mainly intracellular nucleoside transporter ENT3. Many transmitters as well as extracellular K(+) concentrations high enough to open the voltage-sensitive L-channels for Ca(2+) cause a release of transmitter ATP from astrocytes. Adenosine and ATP induce release of ATP by action at several different purinergic receptors. The release evoked by transmitters or elevated K(+) concentrations is abolished by DAB, an inhibitor of glycogenolysis.

Brain lactate metabolism: the discoveries and the controversies

Potential roles for lactate in the energetics of brain activation have changed radically during the past three decades, shifting from waste product to supplemental fuel and signaling molecule. Current models for lactate transport and metabolism involving cellular responses to excitatory neurotransmission are highly debated, owing, in part, to discordant results obtained in different experimental systems and conditions. Major conclusions drawn from tabular data summarizing results obtained in many laboratories are as follows: Glutamate-stimulated glycolysis is not an inherent property of all astrocyte cultures. Synaptosomes from the adult brain and many preparations of cultured neurons have high capacities to increase glucose transport, glycolysis, and glucose-supported respiration, and pathway rates are stimulated by glutamate and compounds that enhance metabolic demand. Lactate accumulation in activated tissue is a minor fraction of glucose metabolized and does not reflect pathway fluxes. Brain activation in subjects with low plasma lactate causes outward, brain-to-blood lactate gradients, and lactate is quickly released in substantial amounts. Lactate utilization by the adult brain increases during lactate infusions and strenuous exercise that markedly increase blood lactate levels. Lactate can be an 'opportunistic', glucose-sparing substrate when present in high amounts, but most evidence supports glucose as the major fuel for normal, activated brain.

Persistent increase in oxygen consumption and impaired neurovascular coupling after spreading depression in rat neocortex

Cortical spreading depression (CSD) is associated with a dramatic failure of brain ion homeostasis and increased energy metabolism. There is strong clinical and experimental evidence to suggest that CSD is the mechanism of migraine, and involved in progressive neuronal injury in stroke and head trauma. Here we tested the hypothesis that single episodes of CSD induced acute hypoxia, and prolonged impairment of neurovascular and neurometabolic coupling. Cortical spreading depression was induced in rat frontal cortex, whereas cortical electrical activity and local field potentials (LFPs) were recorded by glass microelectrodes, cerebral blood flow (CBF) by laser-Doppler flowmetry, and tissue oxygen tension (tpO(2)) with polarographic microelectrodes. Cortical spreading depression increased cerebral metabolic rate of oxygen (CMRO(2)) by 71%+/-6.7% and CBF by 238%+/-48.1% for 1 to 2 mins. For the following 2 h, basal tpO(2) and CBF were reduced whereas basal CMRO(2) was persistently elevated by 8.1%+/-2.9%. In addition, within first hour after CSD we found impaired neurovascular coupling (LFP versus CBF), whereas neurometabolic coupling (LFP versus CMRO(2)) remained unaffected. Impaired neurovascular coupling was explained by both reduced vascular reactivity and suppressed function of cortical inhibitory interneurons. The protracted effects of CSD on basal CMRO(2) and neurovascular coupling may contribute to cellular dysfunction in patients with migraine and acutely injured cerebral cortex.

Persisting depletion of brain glucose following cortical spreading depression, despite apparent hyperaemia: evidence for risk of an adverse effect of Leão's spreading depression

Rapid sampling microdialysis (rsMD) directed towards the cerebral cortex has allowed identification of a combined time-series signature for glucose and lactate that characterizes peri-infarct depolarization in experimental focal ischaemia, but no comparable data exist for 'classical' cortical spreading depression (CSD) associated with hyperaemia in the normally perfused brain. Here, we examined the rsMD responses of dialysate glucose and lactate to five hyperaemic spreading depressions induced with intracortical microinjections, typically of 1 mol/L KCl, in open-skull preparations in five cats under chloralose anaesthesia. Depolarization was verified with microelectrodes, and laser speckle flowmetry was used to examine propagation of the events and perfusion responses near the MD probe. Ten minutes after depolarization, dialysate glucose fell and lactate rose by 28% and 58% respectively. There was no recovery of dialysate glucose 30 mins after depolarization. Mean baseline indicative cerebral blood flow was 25.5+/-4.1 mL/100 g/min and mean maximum hyperaemic increase was by 29.6+/-6 mL/100 g/min; hyperaemia remained present 30 mins after CSD. As CSD events are repetitive, frequent, and often clustered temporally in human acute brain injury, these results indicate a high risk of depletion of extracellular glucose in association with depolarization events of a pattern previously thought to be largely benign.

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.

Labeling of metabolic pools by [6-14C]glucose during K(+)-induced stimulation of glucose utilization in rat brain

[6-14C]Glucose is the tracer sometimes recommended to assay cerebral glucose utilization (CMRglc) during transient or brief functional activations, but when used to study visual stimulation and seizures in other laboratories, it underestimated CMRglc. The metabolic fate of [6-14C]glucose during functional activation of cerebral metabolism is not known, and increased labeling of diffusible metabolites might explain underestimation of CMRglc and also reveal trafficking of metabolites. In the current studies cerebral cortex in conscious rats was unilaterally activated metabolically by KCl application, and CMRglc was determined in activated and contralateral control cortex with [6-14C]glucose or 2-[14C]deoxy-glucose ([14C]DG) over a 5- to 7-min interval. Local 14C concentrations were determined by quantitative autoradiography. Labeled precursor and products were measured bilaterally in paired cortical samples from funnel-frozen brains. Left-right differences in 14C contents were small with [6-14C]glucose but strikingly obvious in [14C]DG autoradiographs. CMRglc determined with [6-14C]glucose was slightly increased in activated cortex but 40-80% below values obtained with [14C]DG. [14C]Lactate was a major metabolite of [6-14C]glucose in activated but not control cortex and increased proportionately with unlabeled lactate. These results demonstrate significant loss of labeled products of [6-14C]glucose from metabolically activated brain tissue and indicate that [14C]DG is the preferred tracer even during brief functional activations of brain.

Cortical slow potential changes during convulsions induced by maximal electroshock or penicillin focus

We have described the occurrence in freely moving gerbils of slow potential changes (SPC) in two different models of experimental epilepsy: 1) maximal electroshock and 2) bilateral epileptic foci induced by penicillin. SPC is considered a by-product of epileptiform activity in both models and correlates to the SPC which occurs during spreading depression. In the first model there develops a cortical SPC simultaneous with a depression of EEG activity, although there is no propagation of the wave. We suggest that a non-propagated multifocal depression (MD) occurs in the MES model. In the model of focal epilepsy, all requirements are fulfilled, and the SPC is characterized as the one which occurs during spreading depression propagating with an average velocity of 8 mm/min.

Transient changes in energy metabolites and intracellular pH during spreading depression in the chick retina

The in vitro preparation of the chick retina can be used to show the occurrence of transient changes in the intracellular pH and of energy metabolites which occurs during spreading depression (SD). There is an initial increase in intracellular pH associated with elevated values for ADP, P-Creatine, lactate and pyruvate, an intermediary acid shift with increases in ATP values and decreases in ADP, and a late alkaline rebound where P-Creatine levels are reduced and the content of ADP and lactate are elevated. These transient changes in intracellular pH observed during SD, when correlated to the levels of energy metabolites, supports the hypothesis that the intracellular pH can be used by the tissue as a mechanism to rapidly modify the metabolic activities of neurons and glial cells. We suggest that the first alkaline shift is caused by glial cells and the intermediary acid shift by neurons. However, a specific cell could not be pointed out as responsible for the late alkaline shift but it could explain the refractoriness of the neurons during the phenomenon.

Blood-brain glucose transfer in spreading depression

Spreading depression in rat brain cortex is associated with a twofold increase of cerebral blood flow. It is not known whether this increase is coupled to increases of cerebral metabolic rate and glucose transport from blood to brain. During the passage of a single spreading depression, we measured blood-brain glucose transport and glucose metabolism in rat cerebral cortex by single intravenous injection of tracer glucose. Blood flow and tissue content of glucose were measure as well. Reduction of tissue glucose and the consequent increase of net transfer of glucose from blood to brain were consistent with a threefold increase of the consumption of glucose before the increase of blood flow. There was no increase of unidirectional blood-brain transfer.

Cerebral glucose utilization: local changes during and after recovery from spreading cortical depression

Cerebral glucose utilization is markedly increased in most areas of the cerebral cortex and reduced in many subcortical structures during spreading cortical depression. During recovery, cortical glucose utilization is still elevated, but the increased metabolic activity is distributed in columns running perpendicularly through the cortex.

Brain cyclic adenosine 3',5'-monophosphate during depolarization of the cerebral cortical cells in vivo

Cyclic AMP (cAMP) was determined in that part of the rat cerebral cortex which was invaded by slow potential change (SPC) accompanying cortical spreading depression (CSD). cAMP was determined at various phases of SPC development and at several time intervals after SPC recovery. At maximum SPC, cAMP was increased by 100% above control and by 116% at the time of SPC recovery. Five, 10 and 15 min after SPC recovery, +54%, +34% and 0% increase in cAMP was found, respectively. The activity of phosphorylase a increased by 112% at SPC recovery (maximal cAMP increase), and by 13% 15 min later (complete cAMP recovery). Some depolarizing agents which differ in their ability to stimulate cAMP formation in vitro (veratridine, cyanide and glutamate) when applied topically onto the cerebral cortex in concentrations evoking CSD (2 mM veratridine, 10 mM cyanide and 100 mM glutamate), induced uniform increase of cAMP by about 100%. The same cAMP augmentation was found in the cortical region under maximum SPC induced by these agents.