Lactate and pH change in close correlation in the extracellular space of the rat brain during cortical spreading depression

Alterations in metabolic flux in migraine and the translational relevance

BACKGROUND

Migraine is a highly prevalent disorder with significant economical and personal burden. Despite the development of effective therapeutics, the causes which precipitate migraine attacks remain elusive. Clinical studies have highlighted altered metabolic flux and mitochondrial function in patients. In vivo animal experiments can allude to the metabolic mechanisms which may underlie migraine susceptibility. Understanding the translational relevance of these studies are important to identifying triggers, biomarkers and therapeutic targets in migraine.

MAIN BODY

Functional imaging studies have suggested that migraineurs feature metabolic syndrome, exhibiting hallmark features including upregulated oxidative phosphorylation yet depleted available free energy. Glucose hypometabolism is also evident in migraine patients and can lead to altered neuronal hyperexcitability such as the incidence of cortical spreading depression (CSD). The association between obesity and increased risk, frequency and worse prognosis of migraine also highlights lipid dysregulation in migraine pathology. Calcitonin gene related peptide (CGRP) has demonstrated an important role in sensitisation and nociception in headache, however its role in metabolic regulation in connection with migraine has not been thoroughly explored. Whether impaired metabolic function leads to increased release of peptides such as CGRP or excessive nociception leads to altered flux is yet unknown.

CONCLUSION

Migraine susceptibility may be underpinned by impaired metabolism resulting in depleted energy stores and altered neuronal function. This review discusses both clinical and in vivo studies which provide evidence of altered metabolic flux which contribute toward pathophysiology. It also reviews the translational relevance of animal studies in identifying targets of biomarker or therapeutic development.

Extracellular Alterations in pH and K+ Modify the Murine Brain Endothelial Cell Total and Phospho-Proteome

Pathologies of the blood-brain barrier (BBB) have been linked to a multitude of central nervous system (CNS) disorders whose pathology is poorly understood. Cortical spreading depression (CSD) has long been postulated to be involved in the underlying mechanisms of these disease states, yet a complete understanding remains elusive. This study seeks to utilize an in vitro model of the blood-brain barrier (BBB) with brain endothelial cell (b.End3) murine endothelioma cells to investigate the role of CSD in BBB pathology by characterizing effects of the release of major pronociceptive substances into the extracellular space of the CNS. The application of trans-endothelial electrical resistance (TEER) screening, transcellular uptake, and immunoreactive methods were used in concert with global proteome and phospho-proteomic approaches to assess the effect of modeled CSD events on the modeled BBB in vitro. The findings demonstrate relocalization and functional alteration to proteins associated with the actin cytoskeleton and endothelial tight junctions. Additionally, unique pathologic mechanisms induced by individual substances released during CSD were found to have unique phosphorylation signatures in phospho-proteome analysis, identifying Zona Occludins 1 (ZO-1) as a possible pathologic "checkpoint" of the BBB. By utilizing these phosphorylation signatures, possible novel diagnostic methods may be developed for CSD and warrants further investigation.

Monitoring Neurochemistry in Traumatic Brain Injury Patients Using Microdialysis Integrated with Biosensors: A Review

In a traumatically injured brain, the cerebral microdialysis technique allows continuous sampling of fluid from the brain’s extracellular space. The retrieved brain fluid contains useful metabolites that indicate the brain’s energy state. Assessment of these metabolites along with other parameters, such as intracranial pressure, brain tissue oxygenation, and cerebral perfusion pressure, may help inform clinical decision making, guide medical treatments, and aid in the prognostication of patient outcomes. Currently, brain metabolites are assayed on bedside analysers and results can only be achieved hourly. This is a major drawback because critical information within each hour is lost. To address this, recent advances have focussed on developing biosensing techniques for integration with microdialysis to achieve continuous online monitoring. In this review, we discuss progress in this field, focusing on various types of sensing devices and their ability to quantify specific cerebral metabolites at clinically relevant concentrations. Important points that require further investigation are highlighted, and comments on future perspectives are provided.

Role of Carbonic Anhydrase in Cerebral Ischemia and Carbonic Anhydrase Inhibitors as Putative Protective Agents

Ischemic stroke is a leading cause of death and disability worldwide. The only pharmacological treatment available to date for cerebral ischemia is tissue plasminogen activator (t-PA) and the search for successful therapeutic strategies still remains a major challenge. The loss of cerebral blood flow leads to reduced oxygen and glucose supply and a subsequent switch to the glycolytic pathway, which leads to tissue acidification. Carbonic anhydrase (CA, EC 4.2.1.1) is the enzyme responsible for converting carbon dioxide into a protons and bicarbonate, thus contributing to pH regulation and metabolism, with many CA isoforms present in the brain. Recently, numerous studies have shed light on several classes of carbonic anhydrase inhibitor (CAI) as possible new pharmacological agents for the management of brain ischemia. In the present review we summarized pharmacological, preclinical and clinical findings regarding the role of CAIs in strokes and we discuss their potential protective mechanisms.

The Role of Metabolism in Migraine Pathophysiology and Susceptibility

Migraine is a highly prevalent and disabling primary headache disorder, however its pathophysiology remains unclear, hindering successful treatment. A number of key secondary headache disorders have headaches that mimic migraine. Evidence has suggested a role of mitochondrial dysfunction and an imbalance between energetic supply and demand that may contribute towards migraine susceptibility. Targeting these deficits with nutraceutical supplementation may provide an additional adjunctive therapy. Neuroimaging techniques have demonstrated a metabolic phenotype in migraine similar to mitochondrial cytopathies, featuring reduced free energy availability and increased metabolic rate. This is reciprocated in vivo when modelling a fundamental mechanism of migraine aura, cortical spreading depression. Trials assessing nutraceuticals successful in the treatment of mitochondrial cytopathies including magnesium, coenzyme q10 and riboflavin have also been conducted in migraine. Although promising results have emerged from nutraceutical trials in patients with levels of minerals or vitamins below a critical threshold, they are confounded by lacking control groups or cohorts that are not large enough to be representative. Energetic imbalance in migraine may be relevant in driving the tissue towards maximum metabolic capacity, leaving the brain lacking in free energy. Personalised medicine considering an individual's deficiencies may provide an approach to ameliorate migraine.

Tissue Acidosis Associated with Ischemic Stroke to Guide Neuroprotective Drug Delivery

Simple Summary

Ischemic stroke is caused by the blockade of a blood vessel in the brain. Consequently, the brain region supplied by the blocked vessel suffers brain damage and becomes acidic. Here we provide a summary of the causes and consequences of acid accumulation in the brain tissue. Ischemic stroke requires immediate medical attention to minimize the damage of brain tissue, and to save function. It would be desirable for the medical treatment to target the site of injury selectively, to enrich the site of ongoing injury with the protective agent, and to avoid undesirable side effects at the same time. We propose that acid accumulation at the sight of brain tissue injury can be used to delineate the region that would benefit most from medical treatment. Tiny drug carriers known as nanoparticles may be loaded with drugs that protect the brain tissue. These nanoparticles may be designed to release their drug cargo in response to an acidic environment. This would ensure that the therapeutic agent is directed selectively to the site where it is needed. Ultimately, this approach may offer a new way to treat stroke patients with the hope of more effective therapy, and better stroke outcome.

Abstract

Ischemic stroke is a leading cause of death and disability worldwide. Yet, the effective therapy of focal cerebral ischemia has been an unresolved challenge. We propose here that ischemic tissue acidosis, a sensitive metabolic indicator of injury progression in cerebral ischemia, can be harnessed for the targeted delivery of neuroprotective agents. Ischemic tissue acidosis, which represents the accumulation of lactic acid in malperfused brain tissue is significantly exacerbated by the recurrence of spreading depolarizations. Deepening acidosis itself activates specific ion channels to cause neurotoxic cellular Ca²⁺ accumulation and cytotoxic edema. These processes are thought to contribute to the loss of the ischemic penumbra. The unique metabolic status of the ischemic penumbra has been exploited to identify the penumbra zone with imaging tools. Importantly, acidosis in the ischemic penumbra may also be used to guide therapeutic intervention. Agents with neuroprotective promise are suggested here to be delivered selectively to the ischemic penumbra with pH-responsive smart nanosystems. The administered nanoparticels release their cargo in acidic tissue environment, which reliably delineates sites at risk of injury. Therefore, tissue pH-targeted drug delivery is expected to enrich sites of ongoing injury with the therapeutical agent, without the risk of unfavorable off-target effects.

Spreading depression as a preclinical model of migraine

Spreading depression (SD) is a slowly propagating wave of near-complete depolarization of neurons and glial cells across the cortex. SD is thought to contribute to the underlying pathophysiology of migraine aura, and possibly also an intrinsic brain activity causing migraine headache. Experimental models of SD have recapitulated multiple migraine-related phenomena and are considered highly translational. In this review, we summarize conventional and novel methods to trigger SD, with specific focus on optogenetic methods. We outline physiological triggers that might affect SD susceptibility, review a multitude of physiological, biochemical, and behavioral consequences of SD, and elaborate their relevance to migraine pathophysiology. The possibility of constructing a recurrent episodic or chronic migraine model using SD is also discussed.

Age or ischemia uncouples the blood flow response, tissue acidosis, and direct current potential signature of spreading depolarization in the rat brain

Spreading depolarization (SD) events contribute to lesion maturation in the acutely injured human brain. Neurodegeneration related to SD is thought to be caused by the insufficiency of the cerebral blood flow (CBF) response; yet the mediators of the CBF response, or their deficiency in the aged or ischemic cerebral cortex, remain the target of intensive research. Here, we postulated that tissue pH effectively modulates the magnitude of hyperemia in response to SD, the coupling of which is prone to be dysfunctional in the aged or ischemic cerebral cortex. To test this hypothesis, we conducted systematic correlation analysis between the direct current (DC) potential signature of SD, SD-associated tissue acidosis, and hyperemic element of the CBF response in the isoflurane-anesthetized, young or old, and intact or ischemic rat cerebral cortex. The data demonstrate that the amplitude of the SD-related DC potential shift, tissue acidosis, and hyperemia are tightly coupled in the young intact cortex; ischemia and old age uncouples the amplitude of hyperemia from the amplitude of the DC potential shift and acidosis; the duration of the DC potential shift, hyperemia and acidosis positively correlate under ischemia alone; and old age disproportionally elongates the duration of acidosis with respect to the DC potential shift and hyperemia under ischemia. The coincidence of the variables supports the view that local CBF regulation with SD must have an effective metabolic component, which becomes dysfunctional with age or under ischemia. Finally, the known age-related acceleration of ischemic neurodegeneration may be promoted by exaggerated tissue acidosis.

NEW & NOTEWORTHY The hyperemic element of the cerebral blood flow response to spreading depolarization is effectively modulated by tissue pH in the young intact rat cerebral cortex. This coupling becomes dysfunctional with age or under ischemia, and tissue acidosis lasts disproportionally longer in the aged cortex, making the tissue increasingly more vulnerable.

Neurometabolic and electrophysiological changes during cortical spreading depolarization: multimodal approach based on a lactate-glucose dual microbiosensor arrays

Spreading depolarization (SD) is a slow propagating wave of strong depolarization of neural cells, implicated in several neuropathological conditions. The breakdown of brain homeostasis promotes significant hemodynamic and metabolic alterations, which impacts on neuronal function. In this work we aimed to develop an innovative multimodal approach, encompassing metabolic, electric and hemodynamic measurements, tailored but not limited to study SD. This was based on a novel dual-biosensor based on microelectrode arrays designed to simultaneously monitor lactate and glucose fluctuations and ongoing neuronal activity with high spatial and temporal resolution. In vitro evaluation of dual lactate-glucose microbiosensor revealed an extended linear range, high sensitivity and selectivity, fast response time and low oxygen-, temperature- and pH- dependencies. In anesthetized rats, we measured with the same array a significant drop in glucose concentration matched to a rise in lactate and concurrently with pronounced changes in the spectral profile of LFP-related currents during episodes of mechanically-evoked SD. This occurred along with the stereotypical hemodynamic response of the SD wave. Overall, this multimodal approach successfully demonstrates the capability to monitor metabolic alterations and ongoing electrical activity, thus contributing to a better understanding of the metabolic changes occurring in the brain following SD.

Glymphatic clearance controls state-dependent changes in brain lactate concentration

Brain lactate concentration is higher during wakefulness than in sleep. However, it is unknown why arousal is linked to an increase in brain lactate and why lactate declines within minutes of sleep. Here, we show that the glymphatic system is responsible for state-dependent changes in brain lactate concentration. Suppression of glymphatic function via acetazolamide treatment, cisterna magna puncture, aquaporin 4 deletion, or changes in body position reduced the decline in brain lactate normally observed when awake mice transition into sleep or anesthesia. Concurrently, the same manipulations diminished accumulation of lactate in cervical, but not in inguinal lymph nodes when mice were anesthetized. Thus, our study suggests that brain lactate is an excellent biomarker of the sleep-wake cycle and increases further during sleep deprivation, because brain lactate is inversely correlated with glymphatic-lymphatic clearance. This analysis provides fundamental new insight into brain energy metabolism by demonstrating that glucose that is not fully oxidized can be exported as lactate via glymphatic-lymphatic fluid transport.

Spreading depolarization remarkably exacerbates ischemia-induced tissue acidosis in the young and aged rat brain

Spreading depolarizations (SDs) occur spontaneously in the cerebral cortex of subarachnoid hemorrhage, stroke or traumatic brain injury patients. Accumulating evidence prove that SDs exacerbate focal ischemic injury by converting zones of the viable but non-functional ischemic penumbra to the core region beyond rescue. Yet the SD-related mechanisms to mediate neurodegeneration remain poorly understood. Here we show in the cerebral cortex of isoflurane-anesthetized, young and old laboratory rats, that SDs propagating under ischemic penumbra-like conditions decrease intra and- extracellular tissue pH transiently to levels, which have been recognized to cause tissue damage. Further, tissue pH after the passage of each spontaneous SD event remains acidic for over 10 minutes. Finally, the recovery from SD-related tissue acidosis is hampered further by age. We propose that accumulating acid load is an effective mechanism for SD to cause delayed cell death in the ischemic nervous tissue, particularly in the aged brain.

Traumatic brain injury-induced autoregulatory dysfunction and spreading depression-related neurovascular uncoupling: Pathomechanisms, perspectives, and therapeutic implications

Traumatic brain injury (TBI) is a major health problem worldwide. In addition to its high mortality (35-40%), survivors are left with cognitive, behavioral, and communicative disabilities. While little can be done to reverse initial primary brain damage caused by trauma, the secondary injury of cerebral tissue due to cerebromicrovascular alterations and dysregulation of cerebral blood flow (CBF) is potentially preventable. This review focuses on functional, cellular, and molecular changes of autoregulatory function of CBF (with special focus on cerebrovascular myogenic response) that occur in cerebral circulation after TBI and explores the links between autoregulatory dysfunction, impaired myogenic response, microvascular impairment, and the development of secondary brain damage. We further provide a synthesized translational view of molecular and cellular mechanisms involved in cortical spreading depolarization-related neurovascular dysfunction, which could be targeted for the prevention or amelioration of TBI-induced secondary brain damage.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

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.

Functional t1ρ imaging in panic disorder

BACKGROUND

Abnormal brain pH has been suggested to play a critical role in panic disorder. To investigate this possibility, we employed a pH-sensitive magnetic resonance (MR) imaging strategy (T1 relaxation in the rotating frame [T1ρ]) and conventional blood oxygen level-dependent (BOLD) imaging.

METHODS

Thirteen panic disorder participants and 13 matched control subjects were enrolled in the study. T1ρ and BOLD were used to study the functional response to a visual flashing checkerboard and their relationship to panic symptoms assessed using the Beck Anxiety Inventory.

RESULTS

In response to visual stimulation, T1ρ imaging revealed a significantly greater increase in the visual cortex of panic disorder participants. T1ρ also detected a stimulus-evoked decrease in the anterior cingulate cortex. Blood oxygen level-dependent imaging detected no functional differences between groups. The correspondence between panic symptoms and functional T1ρ response identified significant relationships within the left inferior parietal lobe, left middle temporal gyrus, and right insula. No relationships were found between panic symptoms and the BOLD signal.

CONCLUSIONS

The data suggest greater activity-evoked T1ρ changes in the visual cortex and anterior cingulate cortex of panic disorder participants. These observations are consistent with a pH dysregulation in panic disorder. In addition, our data suggest that T1ρ imaging may provide information about panic disorder that is distinct from conventional BOLD imaging and may reflect abnormalities in pH and/or brain metabolism.

Spreading depolarization, a pathophysiological mechanism of stroke and migraine aura

Spreading depolarization is a mechanism of abrupt, massive ion translocation between intraneuronal and extracellular space that entails cytotoxic edema in the brain’s gray matter. It is observed in patients as a large change of the slow electrical potential. Dependent on the energy status of the tissue, spreading depolarization is either preceded by nonspreading silencing due to neuronal hyperpolarization or accompanied by spreading silencing of electrical brain activity due to a depolarization block. Nonspreading silencing seems to translate into the initial clinical symptoms of ischemic stroke and spreading silencing translates into migraine aura. Direct electrophysiological evidence exists that spreading depolarization occurs in abundance in aneurysmal subarachnoid hemorrhage, delayed ischemic stroke after subarachnoid hemorrhage, malignant hemispheric stroke, spontaneous intracerebral hemorrhage and traumatic brain injury. Indirect evidence suggests its occurrence during migraine aura. In animals, spreading depolarizations facilitate neuronal death when they invade metabolically compromised tissue, whereas they are relatively innocuous in healthy tissue. Therapies targeting spreading depolarization may potentially treat these neurological conditions.

Thresholds for thermal damage to normal tissues: an update

The purpose of this review is to summarise a literature survey on thermal thresholds for tissue damage. This review covers published literature for the consecutive years from 2002-2009. The first review on this subject was published in 2003. It included an extensive discussion of how to use thermal dosimetric principles to normalise all time-temperature data histories to a common format. This review utilises those same principles to address sensitivity of a variety of tissues, but with particular emphasis on brain and testis. The review includes new data on tissues that were not included in the original review. Several important observations have come from this review. First, a large proportion of the papers examined for this review were discarded because time-temperature history at the site of thermal damage assessment was not recorded. It is strongly recommended that future research on this subject include such data. Second, very little data is available examining chronic consequences of thermal exposure. On a related point, the time of assessment of damage after exposure is critically important for assessing whether damage is transient or permanent. Additionally, virtually no data are available for repeated thermal exposures which may occur in certain recreational or occupational activities. For purposes of regulatory guidelines, both acute and lasting effects of thermal damage should be considered.

Sustained visual cortex hyperexcitability in migraine with persistent visual aura

Persistent aura without infarction, a rare migraine disorder, is defined by aura symptoms that persist for >1 week without radiological evidence of cerebral infarction. To unveil its pathophysiological mechanisms, this study used magnetoencephalography to characterize the visual cortex excitability in persistent aura by comparison with episodic and chronic migraine. We recruited six patients with persistent visual aura, 39 patients with episodic migraine [12 in ictal phase; 27 in interictal phase (with aura, n = 9; without aura, n = 18)], 18 patients with chronic migraine and 24 healthy controls. Five sequential blocks of 50 neuromagnetic prominent 100 ms responses were obtained, and the dynamic change in visual cortex excitability was evaluated by the percentage changes of individual mean prominent 100 ms amplitudes at blocks 2-5 compared with block 1, with a significant increase indicating potentiation. We found that in patients with persistent aura, there was significant potentiation during ictal periods (P = 0.009 and 0.006 at blocks 2 and 5, respectively), and the excitability change was inversely correlated with the duration of aura persistence (correlation coefficient -0.812, P = 0.050, block 2). The interictal recordings (n = 3) also showed potentiation. In terms of the other migraine spectrum disorders, persistent aura differed from episodic migraine in the presence of ictal potentiation. Persistent aura further differed from chronic migraine in the absence of interictal potentiation in chronic migraine. There was a higher percentage change of response amplitude at the end of stimulation (block 5) in persistent aura (43.3 ± 11.7) than in chronic migraine (-7.6 ± 5.5, P = 0.006) and ictal recordings of episodic migraine (-4.9 ± 9.6, P = 0.020). Normal control subjects had no significant response changes. This magnetoencephalographic study showed that the visual cortex in patients with persistent visual aura maintains a steady-state hyperexcitability without significant dynamic modulation. The excitability characteristic supports persistent visual aura as a nosological entity in migraine spectrum disorders and suggests a pathophysiological link to sustained excitatory effects possibly related to reverberating cortical spreading depression.

Simultaneous monitoring of intracellular pH changes and hemodynamic response during cortical spreading depression by fluorescence-corrected multimodal optical imaging

Cortical spreading depression (CSD) plays an important role in trauma, migraine and ischemia. CSD could induce pronounced hemodynamic changes and the disturbance of pH homeostasis which has been postulated to contribute to cell death following ischemia. In this study, we described a fluorescence-corrected multimodal optical imaging system to simultaneously monitor CSD associated intracellular pH (pH(i)) changes and hemodynamic response including hemoglobin concentrations and cerebral blood flow (CBF). CSD was elicited by application of KCl on rat cortex and direct current (DC) potential was recorded as a typical characteristic of CSD. The pH(i) shift was mapped by neutral red (NR) fluorescence which was excited at 516-556 nm and emitted at 625 nm. The changes in hemoglobin concentrations were determined by dual-wavelength optical intrinsic signal imaging (OISI) at 550 nm and 625 nm. Integration of fluorescence imaging and dual-wavelength OISI was achieved by a time-sharing camera equipped with a liquid crystal tunable filter (LCTF). CBF was visualized by laser speckle contrast imaging (LSCI) through a separate camera. Besides, based on the dual-wavelength optical intrinsic signals (OISs) obtained from our system, NR fluorescence was corrected according to our method of fluorescence correction. We found that a transient intracellular acidification followed by a small alkalization occurred during CSD. After CSD, there was a prolonged intracellular acidification and the recovery of pH(i) from CSD took much longer time than those of hemodynamic response. Our results suggested that the new multimodal optical imaging system had the potential to advance our knowledge of CSD and might work as a useful tool to exploit neurovascular coupling under physiological and pathological conditions.

Acids in the brain: a factor in panic?

Several methods to experimentally induce panic cause profound acid-base disturbances. Evidence suggests that CO(2) inhalations, lactate infusions and, to a certain extent, voluntary hyperventilation can conceivably lead to a common scenario of brain acidosis in the face of disparate intravascular pH alterations. The importance of this event is reflected in data that support a model in which experimental panic attacks, as proxy to those occurring spontaneously, constitute a response to acute brain acidosis. Given that central CO(2)/H(+) chemoreception is an important drive for ventilation, and many chemosensitive neurons are related to respiration and arousal, this model can explain much of the connection between panic and respiration. We propose that the shared characteristics of CO(2)/H(+) sensing neurons overlap to a point where threatening disturbances in brain pH homeostasis, such as those produced by CO(2) inhalations, elicit a primal emotion that can range from breathlessness to panic.

Elevated brain lactate responses to neural activation in panic disorder: a dynamic 1H-MRS study

Converging evidence suggests that patients with panic disorder have a metabolic disturbance that may influence the regulation of arousal systems and confer vulnerability to 'spontaneous' panic attacks. The consistent finding of elevated brain lactate responses to various metabolic challenges in panic disorder appears to support this model, although the mechanism of this effect is not understood. Several mechanisms have been proposed to account for elevated brain lactate responses in panic disorder, including (1) brain hypoxia due to excessive cerebral vasoconstriction, and (2) a metabolic disturbance affecting lactate metabolism. Recent studies have shown that neural activation (for example, sensory stimulation) causes local lactate accumulation in the presence of increased oxygen availability. The current study used proton magnetic resonance spectroscopic measures of visual cortex lactate changes during visual stimulation in 15 untreated patients with panic disorder and 15 matched volunteers to critically test these two proposed mechanisms of elevated brain lactate responses in panic disorder. Visual cortex lactate/N-acetylaspartate increased during visual stimulation in both groups. The increase was significantly greater in the panic patients than in the comparison group. There were no group differences in end-tidal pCO(2). The finding that visual stimulation leads to significantly greater visual cortex lactate responses in panic patients is not predicted by the hypoxia model. These results suggest that a metabolic disturbance affecting the production or clearance of lactate is the cause of the elevated brain lactate responses consistently observed in panic disorder and provide further support for metabolic models of vulnerability to this illness.

Spatiotemporal pH dynamics following insertion of neural microelectrode arrays

Insertion trauma is a critical issue when assessing intracortical electrophysiological and neurochemical recordings. Previous reports document a wide variety of insertion techniques with speeds ranging from 10 microm/s to 10 m/s. We hypothesize that insertion speed has an effect on tissue trauma induced by implantation of a neural probe. In order to monitor the neural interface during and after probe insertion, we have developed a silicon-substrate array with hydrous iridium oxide microelectrodes for potentiometric recording of extracellular pH (pH(e)), a measure of brain homeostasis. Microelectrode sites were sensitive to pH in the super-Nernstian range (-85.9 mV/pH unit) and selective over other analytes including ascorbic acid, Na(+), K(+), Ca(2+), and Mg(2+). Following insertion, arrays recorded either triphasic or biphasic pH(e) responses, with a greater degree of prolonged acidosis for insertions at 50 microm/s than at 0.5 mm/s or 1.0 mm/s (p<0.05). Spatiotemporal analysis of the recordings also revealed micro-scale variability in the pH(e) response along the array, even when using the same insertion technique. Implants with more intense acidosis were often associated histologically with blood along the probe tract. The potentiometric microsensor array has implications not only as a useful tool to measure extracellular pH, but also as a feedback tool for delivery of pharmacological agents to treat surgical brain trauma.

Glucose metabolites in the striatum of freely behaving rats following infusion of elevated potassium

The outcome of patients with traumatic brain injury (TBI) can be predicted by the extracellular potassium concentration and the change in energy homeostasis. In this study, the authors investigated the effects of high potassium concentrations on extracellular levels of glucose, pyruvate and lactate in the rat striatum. Applying artificial cerebrospinal fluid (ACSF) enriched with 120 mM potassium by reverse microdialysis leads to an increase in lactate and reduction in glucose and pyruvate. Consequently, the lactate to pyruvate ratio was also increased. These data are discussed in the context of recent studies on lactate/pyruvate conversion and the potential mechanisms whereby high potassium could affect this equilibrium. We conclude that ischemic-like events are unlikely to explain these K(+)-induced changes.

Cortical spreading depression and peri-infarct depolarization in acutely injured human cerebral cortex

Electrocorticographic (ECoG) activity was recorded for up to 129 h from 12 acutely brain-injured human patients using six platinum electrodes placed near foci of damaged cortical tissue. The method probes ECoG activity in the immediate vicinity of the injured cortex and in adjacent supposedly healthy tissue. Six out of twelve patients displayed a total of 73 spontaneous episodes of spreading depression of the ECoG. Of the remaining 6 patients 1 displayed an episode of synchronous depression of ECoG during surgery. Using the same electrodes we also measured the slow potential changes (SPC) (0.005-0.05 Hz) to test the hypothesis that the ECoG depressions were identical to Leao's cortical spreading depression (CSD), and to be able to record peri-infarct depolarisations (PIDs) in electrically 'silent' cortical tissue. Changes in the SPC indicate depolarization of brain tissue. For the analysis, the SPCs were enhanced by calculating the time integral of the ECoG signal. Spreading ECoG depressions were accompanied at every single recording site by stereotyped SPCs, which spread across the cortical mantle at 3.3 (0.41-10) mm/min (median, range), i.e. at the same speed of spread as the depression of the ECoG activity. The amplitude of the SPCs was 0.06-3 mV. In 4 out of 6 patients the ECoG recovered spontaneously. In 2 patients we subsequently recorded recurrent SPCs, but without recovery of the initial ECoG background activity until 2-5 h later. This represents the first direct recording of PIDs in acutely injured human brain. Evidence from this and our previous study of 14 brain-injured patients suggests that CSDs in acute brain disorders occur at higher incidence in patients <30 years (83%) than above (33%). CSD was recorded in 4 out of 5 traumatic brain injury patients, and in 2 out of 7 patients with spontaneous haemorrhages. We conclude that the spreading ECoG depressions recorded in patients are identical to CSDs recorded in animal experiments. We furthermore provide direct electrophysiological evidence for the existence of PIDs and hence a penumbra in the human brain. We hypothesize that the depolarization events might contribute to tissue damage in acute disorders in the human brain.

Intrinsic optical signal of retinal spreading depression: Second phase depends on energy metabolism and nitric oxide

Spreading depression (SD) is a wave-like phenomenon that spreads through the gray matter of central nervous tissue. The aim of this work is to investigate how cellular energy supply and nitric oxide (NO) influence the recovery period after SD wave propagation. We have examined the SD wave in chicken retina by registration of the intrinsic optical signal (IOS). The changes of the IOS were observed via a microscope, transferred to a photomultiplier and amplified. The IOS of the SD wave consists of two phases. The first phase of IOS coexists with cellular swelling induced by ion distribution; the second phase is thought to reflect metabolic changes and reflects the refractory (recovery) period. To analyze the IOS, the amplitude, the duration and the front and the back maximal slopes of the both phases were analyzed. To reduce the cellular level of ATP the blocker of glucose transport-dexamethasone (glucocorticoid hormone) and the blocker of the respiratory chain-potassium cyanide were used. Sodium nitroprusside and trinitroglycerine were chosen as NO-donors. Our results show that during and after SD wave propagation (i) increased NO concentration changes the first and the second phases of IOS (duration of both phases is NO independent), (ii) reduced glucose uptake leads to an increased second phase duration and (iii) block of the respiratory chain prolongs the first phase. According to the results here presented, we propose that glycogen synthesis is one of the mechanisms reflected by the second phase of the IOS.

Regulation and modulation of pH in the brain

The regulation of pH is a vital homeostatic function shared by all tissues. Mechanisms that govern H+ in the intracellular and extracellular fluid are especially important in the brain, because electrical activity can elicit rapid pH changes in both compartments. These acid-base transients may in turn influence neural activity by affecting a variety of ion channels. The mechanisms responsible for the regulation of intracellular pH in brain are similar to those of other tissues and are comprised principally of forms of Na+/H+ exchange, Na+-driven Cl-/HCO3- exchange, Na+-HCO3- cotransport, and passive Cl-/HCO3- exchange. Differences in the expression or efficacy of these mechanisms have been noted among the functionally and morphologically diverse neurons and glial cells that have been studied. Molecular identification of transporter isoforms has revealed heterogeneity among brain regions and cell types. Neural activity gives rise to an assortment of extracellular and intracellular pH shifts that originate from a variety of mechanisms. Intracellular pH shifts in neurons and glia have been linked to Ca2+ transport, activation of acid extrusion systems, and the accumulation of metabolic products. Extracellular pH shifts can occur within milliseconds of neural activity, arise from an assortment of mechanisms, and are governed by the activity of extracellular carbonic anhydrase. The functional significance of these compartmental, activity-dependent pH shifts is discussed.

Extracellular potassium alters frequency and profile of retinal spreading depression waves

The phenomenon of spreading depression (SD) was observed in chicken retina by means of optical registration via a microscope and a CCD camera applying modern methods of image processing for optimized evaluation of the wave profiles. The propagation dynamics of SD waves was investigated as a function of extracellular potassium. Two main findings were obtained. Firstly, the frequency of spontaneous wave generation increased with the increase of K+ concentration. Secondly, there was an effect of potassium on the wave profile. In particular, the recovery zone of SD waves was shortened at increased K+. This effect was not only due to the dispersion relation of waves in excitable media as shown by the result of the mechanically induced wave trains. Applying the basic principles of chemical excitability for the interpretation of the data led us to the conclusion that these potassium effects are due to perturbations of an autocatalytic reaction to be further explored.

Mechanisms of spreading depression and hypoxic spreading depression-like depolarization

Spreading depression (SD) and the related hypoxic SD-like depolarization (HSD) are characterized by rapid and nearly complete depolarization of a sizable population of brain cells with massive redistribution of ions between intracellular and extracellular compartments, that evolves as a regenerative, "all-or-none" type process, and propagates slowly as a wave in brain tissue. This article reviews the characteristics of SD and HSD and the main hypotheses that have been proposed to explain them. Both SD and HSD are composites of concurrent processes. Antagonists of N-methyl-D-aspartate (NMDA) channels or voltage-gated Na(+) or certain types of Ca(2+) channels can postpone or mitigate SD or HSD, but it takes a combination of drugs blocking all known major inward currents to effectively prevent HSD. Recent computer simulation confirmed that SD can be produced by positive feedback achieved by increase of extracellular K(+) concentration that activates persistent inward currents which then activate K(+) channels and release more K(+). Any slowly inactivating voltage and/or K(+)-dependent inward current could generate SD-like depolarization, but ordinarily, it is brought about by the cooperative action of the persistent Na(+) current I(Na,P) plus NMDA receptor-controlled current. SD is ignited when the sum of persistent inward currents exceeds persistent outward currents so that total membrane current turns inward. The degree of depolarization is not determined by the number of channels available, but by the feedback that governs the SD process. Short bouts of SD and HSD are well tolerated, but prolonged depolarization results in lasting loss of neuron function. Irreversible damage can, however, be avoided if Ca(2+) influx into neurons is prevented.

Transporter reversal as a mechanism of glutamate release from the ischemic rat cerebral cortex: studies with DL-threo-beta-benzyloxyaspartate

Elevated levels of the excitotoxic amino acids, glutamate and aspartate, have been implicated in the pathogenesis of neuronal injury and death induced by cerebral ischemia. This study evaluated the contribution of reversed high-affinity, Na(+)-dependent, glutamate transport to the ischemia-evoked release of glutamate and aspartate using DL-threo-beta-benzyloxyaspartate (DL-TBOA), a newly developed competitive, non-transported blocker of the EAAT 1-3 transporters. Changes in the extracellular levels of these and other amino acids, and of glucose and lactate in cerebral cortical superfusates during four-vessel occlusion-elicited global cerebral ischemia were examined using a cortical window technique. Basal and ischemia-evoked amino acid, glucose and lactate efflux were compared in control versus DL-TBOA (100 microM; applied topically for 35 min prior to ischemia) animals. Twenty minutes of ischemia caused large increases in aspartate, glutamate, GABA and taurine effluxes into cortical superfusates, with non-significant effects on the efflux of glycine, glutamine, alanine and serine. Application of DL-TBOA caused a 2-fold increase in basal, preischemic, extracellular glutamate levels, but did not affect those of the other compounds. In the presence of DL-TBOA, ischemia-evoked release of aspartate, glutamate, taurine and glutamine was significantly reduced; that of the other amino acids was not affected. The ischemia-evoked declines in glucose were significantly attenuated, and lactate release was enhanced above that in control animals. The amino acid data are interpreted as indicating that aspartate and glutamate releases were reduced as a consequence of DL-TBOA inhibition of reversed transport by high-affinity, Na-dependent carriers, predominantly involving the glial EAAT 2 transporter. The reduction in ischemia-evoked taurine release is interpreted as being due to a decrease in cell swelling prior to and during the initial phase of ischemia due to reduced entry of the Na(+), and other ions, associated with a decreased glutamate uptake. Glucose-sparing and availability for lactate formation would also result from a reduced glutamate/Na(+) uptake. These results indicate that reversed transport, primarily from glial cells by the EAAT 2 carrier, is responsible for a substantial (42 and 56%) portion of the ischemia-evoked increase in extracellular glutamate and aspartate levels, respectively. As a potent, competitive, non-transported blocker of high-affinity, Na(+)-dependent, glutamate transporters, DL-TBOA promises to be a valuable new compound for the study of glutamatergic mechanisms.

Extracellular acidification decreases the basal motility of cultured mouse microglia via the rearrangement of the actin cytoskeleton

The present study was undertaken to examine the effect of extracellular pH (pH(0)) on the locomotor function of murine microglial cells in vitro. We have found that basal motility of microglia, as measured by a computer-assisted video assay, decreased in an acidic, but not in an alkaline environment. Extracellular acidification affected the architecture of F-actin cytoskeleton, inducing bundling of actin and the formation of stress fibers. The change in intracellular pH (pH(i)) resulting from the change in pH(0) seems to be a prerequisite for the motility decrease since other means to decrease pH(i), namely Na(+)-free solution (in the absence of HCO(-)(3)) and nigericin-containing solution, mimicked the extracellular acidification. In contrast to its pronounced effect on basal motility of microglial cells, the motility increase, as induced by the chemoattractant complement 5a (C5a), was not affected by the acidic environment. The relationship of pH(0) to the locomotor function was also studied in a long-term microchemotaxis assay where microglia migrated within a pH gradient. Intracellular acidification induced by lowering pH(0) to 6.0 or removal of Na(+) from the assay medium decreased basal microglial cell migration. The C5a-induced chemotactic migration was moderately decreased by the acidic environment. In conclusion, our results suggest that acidification of the microglial extracellular milieu leads to a decrease in pH(i) and thereby reduces the basal microglial motility and C5a-induced chemotaxis via a rearrangement of the cytoskeleton. We would therefore like to speculate that changes in pH(i) constitute an important control mechanism in regulating the locomotor function of microglia in culture and probably also in the intact tissue.

Response properties of trigeminal brain stem neurons with input from dura mater encephali in the rat

The responsiveness of trigeminal brain stem neurons to selective local mechanical and chemical stimulation of the cranial dura mater was examined in a preparation in the rat. The dura mater encephali was exposed and its surface stimulated with electrical pulses through bipolar electrodes. Extracellular recordings were made from neurons in the subnucleus caudalis of the spinal trigeminal nucleus. Single neurons driven by meningeal input were identified by their responses to electrical stimulation and to probing their receptive fields on the dura. Facial receptive fields were defined mechanically. Chemical stimuli (a combination of inflammatory mediators, bradykinin, prostaglandin E2, serotonin, capsaicin and acidic Tyrode's solution) were applied topically to the dura and by injection through a catheter into the superior sagittal sinus. All neurons with input from the parietal dura mater had convergent input from the facial skin, with preponderance of the periorbital region. Proportions of units were activated by the combination of inflammatory mediators (55%), bradykinin (64.5%), acidic Tyrode's solution (64.1%) and capsaicin (78.6%). We conclude that, among the chemical mediators of inflammation, bradykinin and low pH are the most effective chemical stimuli in activating meningeal nociceptors. These stimuli may be important during meningeal inflammatory processes that lead to the generation of headaches.

Rapid efflux of lactate from cerebral cortex during K+ -induced spreading cortical depression

Rapid transport of lactate from activated brain regions to blood, perhaps reflecting enhanced metabolite trafficking, would prevent local trapping of labeled metabolites of [6-14C]glucose and cause underestimation of calculated CMRglc. Because the identities of glucose metabolites lost from activated structures and major routes of their removal are not known, arteriovenous differences across brains of conscious normoxic rats for derivatives of [6-14C]glucose were determined under steady-state conditions in blood during K+ -induced spreading cortical depression. Lactate was identified as the major labeled product lost from brain. Its entry to blood was detected within 2 minutes after a pulse of [6-14C]glucose, and it accounted for 96% of the 14C lost from brain within approximately 8 minutes. Lactate efflux corresponded to 20% of glucose influx, but accounted for only half the magnitude of underestimation of CMRglc when [14C]glucose is the tracer, suggesting extensive [14C]lactate trafficking within brain. [14C]Lactate spreading within brain is consistent with (1) relatively uniform pattern labeling of K+ -treated cerebral cortex by [6-14C]glucose contrasting heterogeneous labeling by [14C]deoxyglucose, and (2) transport of 14C-labeled lactate and inulin up to 1.5 and 2.4 mm, respectively, within 10 minutes. Thus, newly synthesized lactate exported from activated cells rapidly flows to blood and probably other brain structures.

pH regulation and proton signalling by glial cells

The regulation of H+ in nervous systems is a function of several processes, including H+ buffering, intracellular H+ sequestering, CO2 diffusion, carbonic anhydrase activity and membrane transport of acid/base equivalents across the cell membrane. Glial cells participate in all these processes and therefore play a prominent role in shaping acid/base shifts in nervous systems. Apart from a homeostatic function of H(+)-regulating mechanisms, pH transients occur in all three compartments of nervous tissue, neurones, glial cells and extracellular spaces (ECS), in response to neuronal stimulation, to neurotransmitters and hormones as well as secondary to metabolic activity and ionic membrane transport. A pivotal role for H+ regulation and shaping these pH transients must be assigned to the electrogenic and reversible Na(+)-HCO3-membrane cotransport, which appears to be unique to glial cells in nervous systems. Activation of this cotransporter results in the release and uptake of base equivalents by glial cells, processes which are dependent on the glial membrane potential. Na+/H+ and Cl-/HCO3-exchange, and possibly other membrane carriers, accomplish the set of tools in both glial cells and neurones to regulate their intracellular pH. Due to the pH dependence of a great variety of processes, including ion channel gating and conductances, synaptic transmission, intercellular communication via gap junctions, metabolite exchange and neuronal excitability, rapid and local pH transients may have signalling character for the information processing in nervous tissue. The impact of H+ signalling under both physiological and pathophysiological conditions will be discussed for a variety of nervous system functions.

The measurement of extracellular inorganic phosphate gives a more reliable indication for severe impairment of cerebral cell function and cell death than the measurement of extracellular lactate

The measurement of cerebral extracellular lactate levels has been suggested to be used to monitor cerebral function in intensive care However, although an increase of extracellular lactate levels is a sensitive parameter for increased cellular activity in general, it will be shown that its prognostic value is limited in regard to the severity of the impairment of cellular function. As an alternative the measurement of the extracellular levels of inorganic phosphate (IP) or adenosine is proposed here: Whereas extracellular lactate levels increased rapidly to about the same extents during ischemia (IS) and spreading depression (SD), IP rose during IS only. Adenosine, on the other hand, increased during both events to a different degree. If, therefore, lactate was the only parameter to be monitored after a cerebral insult, the results would not allow to discriminate between a transient, spontaneously recovering event as a SD and a long lasting or an irreversible loss of cell function as in persisting ischemia/hypoxia. The measurement of IP, therefore, seems to be more suitable than that of lactate or adenosine since IP will appear within the extracellular space only after a sustained failure of membrane function. Thus, the measurement of IP changes turned out to be the more useful parameter for intensive care supervision.

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.

Time course of changes in extracellular lactate evoked by transient K(+)-induced depolarisation in the rat striatum

The purpose of this study was to establish whether excessive lactate production associated with local application of K+ is reflected at the extracellular level during or after the K+ challenge. Changes in extracellular lactate were continuously monitored by microdialysis coupled to on-line fluorimetric analysis. K(+)-induced changes in dialysate lactate were closely related to those of the direct current potential. High K+ evoked a large and sustained negative shift of direct current potential onto which were superimposed a variable number of transient peaks of further depolarisation. The initial negative shift in direct current potential was associated with a decrease in dialysate lactate, but after each transient depolarisation, the positive shift in direct current potential indicating cell repolarisation was associated with a marked increase in extracellular lactate. When repetitive transient depolarisations occurred during a stimulus, only a small increase after each depolarisation was observed. However, recordings consistently revealed a marked and rapid increase in extracellular lactate after the K+ stimulus. These data indicate that extracellular lactate mostly increased during periods of repolarisation. This suggests strongly that lactic acid transport out of brain cells may be impaired when their transmembrane ionic gradients are disrupted.

A previous potassium stimulation enhances the increases of striatal extracellular dopamine and 5-hydroxytryptamine during global ischaemia under simulated penumbral conditions

The effect of a previous K+ stimulation on striatal extracellular monoamine levels during global ischaemia, under simulated penumbral conditions, was investigated. Rats were implanted with microdialysis probes in both striata, monoamine release was stimulated unilaterally by adding K+ (100 mM, 20 min) to the artificial CSF perfused through one probe, and bilateral partial ischaemia was imposed after monoamine levels had returned to basal values or below. Resultant increases in dialysate levels of dopamine and 5-hydroxytryptamine were markedly and significantly greater on the side previously exposed to K+, even though electrophysiological measurements indicated similarly severe ischaemia on both sides. Associated monoamine metabolite changes did not differ significantly between the two sides. There was no evidence of greater neuronal loss in the K(+)-stimulated striata 7 days after ischaemia. However, striatal tissue probably exposed to the highest concentrations of K+ could not be examined because of extensive gliosis around the probe.

Microdialysis of interstitial amino acids during spreading depression and anoxic depolarization in rat neocortex

We have examined the effect of cortical spreading depression (SD) and anoxic depolarization (AD) on the interstitial concentration changes of amino acids (AA) in the neocortex of anesthetized rats using microdialysis and HPLC. Accompanying SD alanine increased to 126 +/- 11%, arginine to 116 +/- 3%, aspartate to 160 +/- 17%, glutamate to 163 +/- 9%, glycine to 158 +/- 21%, serine to 125 +/- 9%, and taurine to 172 +/- 15% (mean +/- 1 S.E.M.). The increases lasted for about 1 min. Histidine decreased to 74% +/- 4% at 1 min following SD, and returned to normal 4 min later. Cardiac arrest triggered AD after approximately 2 min, immediately followed by changes of interstitial AAs. At 5 min after AD alanine had increased to 183 +/- 13%, aspartate to 3,458 +/- 656%, GABA to 338 +/- 35%, glutamate to 1,696 +/- 546%, glycine to 297 +/- 37%, serine to 153 +/- 12%, and taurine to 1721 +/- 98% as compared to control values (mean +/- 1 S.E.M.). Histidine decreased to 78 +/- 2% at 3 min following AD while arginine exhibited insignificant variations around the baseline. The increase of glutamate during SD is consistent with activation of NMDA-receptors as an essential requirement for this reaction. The increase of AAs may also contribute to the sequence of events leading to AD, though the exact mechanism remains unknown. SD is an important pathophysiological mechanism of the ischemic penumbra associated with focal cerebral ischemia, while AD reflects the electrophysiological status of the infarct core.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological stimulation increases nonoxidative glucose metabolism in the brain of the freely moving rat

The effects of mild stress on nonoxidative glucose metabolism were studied in the brain of the freely moving rat. Extracellular lactate levels in the hippocampus and striatum were monitored at 2.5-min intervals with microdialysis coupled with an enzyme-based flow injection analysis system. Ten minutes of restraint stress led to a 235% increase in extracellular lactate levels in the striatum. A 5-min tail pinch caused an increase of 193% in the striatum and 170% in the hippocampus. Local application of tetrodotoxin in the striatum blocked the rise in lactate following tail pinch and inhibited the subsequent clearance of lactate from the extracellular fluid. Local application of the noncompetitive N-methyl-D-aspartate receptor antagonist MK-801 had no effect on the tail pinch-stimulated increase in lactate in the striatum. These results show that mild physiological stimulation can lead to a rapid increase in nonoxidative glucose metabolism in the brain.

Lactate-proton co-transport and its contribution to interstitial acidification during hypoxia in isolated rat spinal roots

Exposure of nervous tissue to hypoxia results in interstitial acidification. There is evidence for concomitant decrease in extracellular pH to the increase in tissue lactate. In the present study, we used double-barrelled pH-sensitive microelectrodes to investigate the link between lactate transport and acid-base homeostasis in isolated rat spinal roots. Addition of different organic anions to the bathing solution at constant bath pH caused transient alkaline shifts in extracellular pH; withdrawal of these compounds resulted in transient acid shifts in extracellular pH. With high anion concentrations (30 mM), the largest changes in extracellular pH were observed with propionate > L-lactate approximately pyruvate > 2-hydroxy-2-methylpropionate. Changes in extracellular pH induced by 10 mM L- and D-lactate were of similar size. Lactate transport inhibitors alpha-cyano-4-hydroxycinnamic acid and 4,4'-dibenzamidostilbene-2,2'-disulphonic acid significantly reduced L-lactate-induced extracellular pH shifts without affecting propionate-induced changes in extracellular pH. Hypoxia produced an extracellular acidification that was strongly reduced in the presence of alpha-cyano-4-hydroxycinnamic acid and 4,4'-dibenzamidostilbene-2,2'-disulphonic acid. In contrast, amiloride and 4,4'-di-isothiocyanostilbene-2,2'-disulphonate were without effect on hypoxia-induced acid shifts. The results indicate the presence of a lactate-proton co-transporter in rat peripheral nerves. This transport system and not Na+/H+ or Cl-/HCO3- exchange seems to be the dominant mechanism responsible for interstitial acidification during nerve hypoxia.

Extracellular changes of inorganic phosphate are different during spreading depression and global cerebral ischemia of rats

Tissue levels of inorganic phosphate (iP-) and lactate (lac) increase during cerebral ischemia and cortical spreading depression (SD). Since cell membranes become leaky during these insults, iP- and lac were expected to leak into the extracellular space (ECS). In order to find out whether this occurs or does not, a microdialysis (MD) fiber was implanted into the cortex of anesthetized rats and extracellular lactate (lac(e)) and extracellular iP- (iPe-) were determined during various insults. Extracellular lactate increased to about the same extent during ischemia and SD. In contrast, iPe- increased during ischemia but not during SD. Instead, iPe- started to rise after SD and reached its maximum about 45 min later. The distinct pattern of iPe- in comparison to lac(e) during the above mentioned insults points to a qualitative difference of the underlying mechanisms: whereas lac appears within the ECS at any stressful situation, elevation of iP- within the ECS indicates depletion of energy stores in parallel to the lack of control of ion homeostasis.