Dynamic changes in glucose and lactate in the cortex of the freely moving rat monitored using microdialysis

Metabolomics-based study of the potential interventional effects of Xiao-Xu-Ming Decoction on cerebral ischemia/reperfusion rats

ETHNOPHARMACOLOGICAL RELEVANCE

Xiao-Xu-Ming Decoction (XXMD) is a classical Chinese medicinal compound for the treatment of ischemic stroke, which has good efficacy in clinical studies and also plays a neuroprotective role in pharmacological studies.

AIM OF THE STUDY

The purpose of this study is to investigate the potential and integral interventional effects of XXMD on cerebral ischemia/reperfusion rat model.

MATERIALS AND METHODS

In this study, ¹H NMR metabolomics was used, combined with neurological functional assessments, cerebral infarct area measurements, and pathological staining including Nissl staining, immunofluorescence staining of NeuN and TUNEL, and immunohistochemical staining of MCT2, to analyze the metabolic effects of XXMD in the treatment of an ischemia/reperfusion rat model.

RESULTS

It's observed that XXMD treatment could improve the neurological deficit scores and reduce the cerebral infarct areas on cerebral ischemia/reperfusion rat model. The pathological staining results performed that XXMD treatment could improve the decrease of Nissl bodies and the expression of NeuN and MCT2, reduce the high expression of TUNEL. In ¹H NMR study, it revealed that the metabolic patterns among three experimental groups were different, the level of lactate, acetate, NAA, glutamate, and GABA were improved to varying degrees in different brain area.

CONCLUSION

Our findings indicated that XXMD has positive effect on neuroprotection and improvement of metabolism targeting cerebral ischemic injury in rats, which showed great potential for ischemic stroke.

Massive efflux of adenosine triphosphate into the extracellular space immediately after experimental traumatic brain injury

The aim of the current study was to determine effects of mild traumatic brain injury (TBI), with or without blockade of purinergic ATP Y1 (P2Y1) receptors or store-operated calcium channels, on extracellular levels of ATP, glutamate, glucose and lactate. Concentrations of ATP, glutamate, glucose and lactate were measured in cerebral microdialysis samples obtained from the ipsilateral cortex and underlying hippocampus of rats with mild unilateral controlled cortical impact (CCI) or sham injury. Immediately after CCI, a large release of ATP was observed in the cortex (3.53-fold increase of pre-injury value) and hippocampus (2.97-fold increase of pre-injury value), with ATP returning to the baseline levels within 20 min post-injury and remaining stable for during the 3-h sampling period. In agreement with the results of previous studies, there was a significant increase in glutamate 20 min after CCI, which was concomitant with a decrease in extracellular glucose (20 min) and an increase in lactate (40-60 min) in both brain regions after CCI. Addition of a selective P2Y1 receptor blocker (MRS2179 ammonium salt hydrate) to the microdialysis perfusate significantly lowered pre-injury ATP and glutamate levels, and eliminated the post-CCI peaks. Addition of a blocker of store-operated calcium channels [2-aminoethoxy diphenylborinate (2-APB)] to the microdialysis perfusate significantly lowered pre-injury ATP in the hippocampus, and attenuated the post-CCI peak in both the cortex and hippocampus. 2-APB treatment significantly increased baseline glutamate levels, but the values post-injury did not differ from those in the sham group. Pre-injury glucose levels, but not lactate levels, were increased by MRS2179 and decreased by 2-APB. However, none of these treatments substantially altered the CCI-induced reduction in glucose and increase in lactate in the cortex. In conclusion, the results of the present study demonstrated that a short although extensive release of ATP immediately after experimental TBI can be significantly attenuated by blockade of P2Y1 receptors or store-operated calcium channels.

Minimally Invasive Microelectrode Biosensors Based on Platinized Carbon Fibers for in Vivo Brain Monitoring

The ability to monitor the chemical composition of brain interstitial fluid remains an important challenge in the field of bioanalytical chemistry. In particular, microelectrode biosensors are a promising resource for the detection of neurochemicals in interstitial fluid in both animals and humans. These biosensors can provide second-by-second temporal resolution and enzymatic recognition of virtually any redox or nonredox molecule. However, despite miniaturization of these sensors to 50-250 μm in diameter to avoid vascular and cellular injury, inflammation and foreign-body reactions still occur following their implantation. Here, we fabricated microelectrodes with platinized carbon fibers to create biosensors that have an external diameter that is less than 15 μm. Platinization was achieved with physical vapor deposition, and increased sensitivity to hydrogen peroxide and improved enzymatic detection were observed for these carbon fiber microelectrodes. When these devices were implanted in the brains of rats, no injuries to the parenchyma or brain blood vessels were detected. In addition, these microelectrodes provided different estimates of basal glucose, lactate, and oxygen concentrations compared to conventional biosensors. Induction of spreading depolarization in the cerebral cortex further demonstrated the greater sensitivity of our microelectrodes to dynamic neurochemical changes. Thus, these minimally invasive devices represent a major advance in our ability to analyze brain interstitial fluid.

Microdialysis for Monitoring the Process of Functional Tissue Culture

Continuous monitoring is important during tissue culture. However, there are still technical difficulties in monitoring the internal status of cells or tissues. In this paper, microdialysis is adopted to monitor functional tissue growth in a bioreactor. Explanted bovine caudal intervertebral disc (IVD) was used as the test tissue. A microdialysis membrane probe of 100 kDa molecular weight cut-off was employed and in situ calibration methods with phenol red and fluorescent 40 kDa dextran were developed to measure the relative recovery of the solute of interest, and membrane fouling, respectively. Tissue metabolism was monitored successfully. At the same time soluble macromolecules were picked up by the probe and were detected and quantified by Fast Protein Liquid Chromatography (FPLC) and/or Sodium Dodecyl Sulphate Polyacrylamide Gel Electrophoresis (SDS-PAGE). These proteins were believed to be associated with biofunction of engineered tissue. Monitoring of phenol red content in the dialysate indicated that there was no significant fouling of the membrane probe during a 7-day culture period and the Relative Recovery of macromolecules of interests remained roughly 9%. We concluded that microdialysis could be used to sample a wide range of molecular species released during cell metabolism and extracellular matrix turnover, which were direct or indirect indications of cell and tissue functions. The application of the developed system could be extended to monitor tissue repair in vivo, and the development of the engineered tissue.

Neurometabolic coupling between neural activity, glucose, and lactate in activated visual cortex

Neural activity is closely coupled with energy metabolism but details of the association remain to be identified. One basic area involves the relationships between neural activity and the main supportive substrates of glucose and lactate. This is of fundamental significance for the interpretation of non-invasive neural imaging. Here, we use microelectrodes with high spatial and temporal resolution to determine simultaneous co-localized changes in glucose, lactate, and neural activity during visual activation of the cerebral cortex in the cat. Tissue glucose and lactate concentration levels are measured with electrochemical microelectrodes while neural spiking activity and local field potentials are sampled by a microelectrode. These measurements are performed simultaneously while neurons are activated by visual stimuli of different contrast levels, orientations, and sizes. We find immediate decreases in tissue glucose concentration and simultaneous increases in lactate during neural activation. Both glucose and lactate signals return to their baseline levels instantly as neurons cease firing. No sustained changes or initial dips in glucose or lactate signals are elicited by visual stimulation. However, co-localized measurements of cerebral blood flow and neural activity demonstrate a clear delay in the cerebral blood flow signal such that it does not correlate temporally with the neural response. These results provide direct real-time evidence regarding the coupling between co-localized energy metabolism and neural activity during physiological stimulation. They are also relevant to a current question regarding the role of lactate in energy metabolism in the brain during neural activation. Dynamic changes in energy metabolites can be measured directly with high spatial and temporal resolution by use of enzyme-based microelectrodes. Here, to examine neuro-metabolic coupling during brain activation, we use combined microelectrodes to simultaneously measure extracellular glucose, lactate, and neural responses in the primary visual cortex to visual stimulation. We demonstrate rapid decreases in glucose and increases in lactate during neural activation. Changes in glucose and lactate signals are transient and closely coupled with neuronal firing.

Microdialysis in the neurocritical care unit

Effective monitoring is critical for neurologically compromised patients, and several techniques are available. One of these tools, cerebral microdialysis (MD), was designed to detect derangements in cerebral metabolism. Although this monitoring device began as a research instrument, favorable results and utility have broadened its clinical applications. Combined with other brain monitoring techniques, MD can be used to estimate cerebral vulnerability, to assess tissue outcome, and possibly to prevent secondary ischemic injury by guiding therapy. This article reviews the literature regarding the past, present, and future uses of MD along with its advantages and disadvantages in the intensive care unit setting.

Recent trends in microdialysis sampling integrated with conventional and microanalytical systems for monitoring biological events: a review

Microdialysis (MD) is a sampling technique that can be employed to monitor biological events both in vivo and in vitro. When it is coupled to an analytical system, microdialysis can provide near real-time information on the time-dependent concentration changes of analytes in the extracellular space or other aqueous environments. Online systems for the analysis of microdialysis samples enable fast, selective and sensitive analysis while preserving the temporal information. Analytical methods employed for online analysis include liquid chromatography (LC), capillary (CE) and microchip electrophoresis and flow-through biosensor devices. This review article provides an overview of microdialysis sampling and online analysis systems with emphasis on in vivo analysis. Factors that affect the frequency of analysis and, hence, the temporal resolution of these systems are also discussed.

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.

Glutamate receptor-dependent increments in lactate, glucose and oxygen metabolism evoked in rat cerebellum in vivo

Neuronal activity is tightly coupled with brain energy metabolism. Numerous studies have suggested that lactate is equally important as an energy substrate for neurons as glucose. Lactate production is reportedly triggered by glutamate uptake, and independent of glutamate receptor activation. Here we show that climbing fibre stimulation of cerebellar Purkinje cells increased extracellular lactate by 30% within 30 s of stimulation, but not for briefer stimulation periods. To explore whether lactate production was controlled by pre- or postsynaptic events we silenced AMPA receptors with CNQX. This blocked all evoked rises in postsynaptic activity, blood flow, and glucose and oxygen consumption. CNQX also abolished rises in lactate concomitantly with marked reduction in postsynaptic currents. Rises in lactate were unaffected by inhibition of glycogen phosphorylase, suggesting that lactate production was independent of glycogen breakdown. Stimulated lactate production in cerebellum is derived directly from glucose uptake, and coupled to neuronal activity via AMPA receptor activation.

Time-resolved microdialysis for in vivo neurochemical measurements and other applications

Monitoring changes in chemical concentrations over time in complex environments is typically performed using sensors and spectroscopic techniques. Another approach is to couple sampling methods, such as microdialysis, with chromatographic, electrophoretic, or enzymatic assays. Recent advances of such coupling have enabled improvements in temporal resolution, multianalyte capability, and automation. In a sampling and analysis method, the temporal resolution is set by the mass sensitivity of the analytical method, analysis time, and zone dispersion during sampling. Coupling methods with high speed and mass sensitivity to microdialysis sampling help to reduce some of these contributions to yield methods with temporal resolution of seconds. These advances have been primarily used in monitoring neurotransmitters in vivo. This review covers the problems associated with chemical monitoring in the brain, recent advances in using microdialysis for time-resolved in vivo measurements, sample applications, and other potential applications of the technology such as determining reaction kinetics and process monitoring.

Supply and demand in cerebral energy metabolism: the role of nutrient transporters

Glucose is the obligate energetic fuel for the mammalian brain, and most studies of cerebral energy metabolism assume that the majority of cerebral glucose utilization fuels neuronal activity via oxidative metabolism, both in the basal and activated state. Glucose transporter (GLUT) proteins deliver glucose from the circulation to the brain: GLUT1 in the microvascular endothelial cells of the blood-brain barrier (BBB) and glia; GLUT3 in neurons. Lactate, the glycolytic product of glucose metabolism, is transported into and out of neural cells by the monocarboxylate transporters (MCT): MCT1 in the BBB and astrocytes and MCT2 in neurons. The proposal of the astrocyte-neuron lactate shuttle hypothesis suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energetics, especially during activation. Since the identification of the GLUTs and MCTs in brain, much has been learned about their transport properties, that is capacity and affinity for substrate, which must be considered in any model of cerebral glucose uptake and utilization. Using concentrations and kinetic parameters of GLUT1 and -3 in BBB endothelial cells, astrocytes, and neurons, along with the corresponding kinetic properties of the MCTs, we have successfully modeled brain glucose and lactate levels as well as lactate transients in response to neuronal stimulation. Simulations based on these parameters suggest that glucose readily diffuses through the basal lamina and interstitium to neurons, which are primarily responsible for glucose uptake, metabolism, and the generation of the lactate transients observed on neuronal activation.

Cortical spreading depression: an adverse but treatable factor in intensive care?

PURPOSE OF REVIEW

The aetiology and management of secondary deterioration in patients with acute traumatic or ischaemic brain injury remain serious challenges for clinicians and also for basic neuroscientists. The occurrence of spreading depolarization events and some of their features in the cerebral cortex in patients with traumatic brain injury and aneurysmal subarachnoid haemorrhage, as documented in recent papers, represent a novel pathophysiological mechanism in this setting.

RECENT FINDINGS

The history and definitions of two critically different patterns of depolarization are reviewed on the basis of their physiology and pathophysiology, particularly the responses of the cerebral microcirculation to depolarization as seen in the laboratory. It is now becoming possible to conduct similar assessments in the brain-injured patient. Currently the recorded incidence of depolarization events in patients undergoing craniotomy for traumatic contusions is in the region of 50-60%, rising to 72% following major subarachnoid haemorrhage.

SUMMARY

Realization of the therapeutic potential of the new findings will depend on clear knowledge of the impact of the different patterns of depolarization on outcome. Meantime, current results call for even stricter attention during clinical management of acute brain injury to secondary factors such as body temperature and plasma glucose.

Monitoring of metabolite gradients in tissue-engineered constructs

At present, the assessment of developing tissue-engineered constructs is almost always carried out destructively using biochemical or histological methods to determine cell number, viability and tissue growth throughout the construct. Since many of these experiments are long, taking weeks or even months to complete, simple and readily applicable non-destructive methods of monitoring changes in cell metabolism, viability and tissue deposition within the construct would be invaluable; such methods could point out adverse responses during the early stages of culture. Here, we describe the use of microdialysis for detecting local changes in cellular metabolism within a tissue-engineered construct. Three-dimensional constructs consisting of bovine articular chondrocytes entrapped in an alginate gel were cultured in a bioreactor for two weeks. Glucose and lactate were monitored by microdialysis, as the major nutrient and metabolite, respectively. Concentration gradients within the construct were evident, with the highest lactate concentrations in the construct centre. The local lactate concentration was a measure of cellular metabolic activity, decreasing as cellular activity fell and increasing as cellular activity was stimulated. Nutrient starvation and cell death in the construct centre could be readily detected in constructs deliberately cultured under adverse conditions. The results show that probe measurements can give an early warning of inappropriate local metabolic changes. Such information during the growth of tissue-engineered constructs would allow either corrective action or else an early end to an unsuccessful test.

Changes in cerebral energy metabolites induced by impact-acceleration brain trauma and hypoxic-hypotensive injury in rats

The aim of this study was to describe, in rats, brain energy metabolites changes after different levels of head trauma (T) complicated by hypoxia-hypotension (HH). Male Sprague Dawley rats (n = 7 per groups) were subjected to T by impact-acceleration with 450-g weight drop from 1.50 or 1.80 m (T 1.50 or T 1.80), or to a 15-min period of HH (controlled hemorrhage to mean arterial pressure [MAP] of 40 mm Hg, and mechanical ventilation with N(2) 90%/O(2) 10%), or to their association (T followed by HH). Invasive MAP, intraparenchymental intracranial pressure (ICP), and cerebral blood flow (CBF using Laser Doppler flowmetry) were recorded during the 5 post-traumatic hours. Cerebral microdialysis was used to measure each hour interstitial brain glucose, lactate, pyruvate, and glutamate. For the entire period, the levels of cerebral glucose, pyruvate, and glutamate were not statistically different between groups. In addition, there were no differences associated with the lactate-glucose ratio. Lactate was significantly higher overtime only in T 1.80 + HH group (p < 0.001 vs. every other groups). The lactate-pyruvate ratio increased with trauma level, and was significantly different vs. sham for the entire study period in T 1.50 + HH, in T 1.80, and in T 1.80 + HH. There was no correlation between CBF variations and the lactate-pyruvate ratio (r(2) = 0.00001). The cerebral perfusion pressure was greater than 70 mm Hg in all groups. The prolonged post-traumatic impairment in brain energy metabolism may be related to traumatic brain injury (TBI) severity. It became worse when T was complicated by HH, but was not related to changes in CBF.

Brain metabolism and extracellular space diffusion parameters during and after transient global hypoxia in the rat cortex

Hypoxia results in both reversible and irreversible changes in the brain extracellular space (ECS). This study utilized microdialysis to monitor changes in the energy-related metabolites lactate, pyruvate, glucose and glutamate in the rat cortex before, during and after 30-min transient global hypoxia, induced in anesthetized rats by reducing inspired oxygen to 6% O(2) in nitrogen. Changes in metabolite levels were compared with ECS diffusion parameters calculated from diffusion curves of tetramethylammonium applied by iontophoresis. Significant increases in lactate concentration and the lactate/pyruvate ratio, as well as decreased glucose levels, were found in the cortex immediately after the induction of hypoxia. Following recovery to ventilation with air, extracellular lactate and glucose levels and the lactate/pyruvate ratio returned to control levels within 40, 20 and 30 min, respectively. Glutamate levels started to increase 20-30 min after the onset of hypoxia and returned to prehypoxic values within 30-40 min of reoxygenation. The ECS volume fraction alpha decreased by about 5% from 0.18+/-0.01 during the first 20-25 min of hypoxia; after 25 min alpha dropped a further 22% to 0.14+/-0.01. Within 10 min of reoxygenation, alpha returned to control values, then increased to 0.20+/-0.01 and remained at this level until the end of the experiment. The observed 22% decrease in alpha markedly influences dialysate levels measured during hypoxia. In our study, the complete posthypoxic recovery of cortical metabolite levels and ECS diffusion properties suggests that metabolic enzymes and related cellular components (e.g., mitochondria) may tolerate prolonged hypoxic periods and recover to prehypoxic values.

Application of rapid-sampling, online microdialysis to the monitoring of brain metabolism during aneurysm surgery

OBJECTIVE

To introduce rapid-sampling microdialysis for the early detection of adverse metabolic changes in tissue at risk during aneurysm surgery.

METHODS

A microdialysis catheter was inserted under direct vision into at-risk cortex at the start of surgery. This monitoring was sustained throughout the course of the operation, during which intraoperative events, for example, temporary arterial occlusion or lobe retraction, were precisely documented. A continuous online flow of dialysate was fed into a mobile bedside glucose and lactate analyser. This comprises flow-injection dual-assay enzyme-based biosensors capable of determining values of metabolites every 30 seconds.

RESULTS

Eight patients underwent clipping or wrapping of intracranial aneurysms and were monitored. Time between events and detection: 9 minutes. Mean change in metabolite value +/- standard deviation: temporal lobe retraction lactate, +656 +/- 562 micromol/L (n = 7, P < 0.05); glucose, -123 +/- 138 micromol/L (n = 6, P = 0.08). Glucose intravenous bolus infusion glucose, +512 +/- 244 micromol/L (n = 5, P < 0.01); peak at mean time after bolus, 16 minutes. Temporary proximal clip lactate, +731 +/- 346 micromol/L (n = 6, P < 0.01); glucose, -139 +/- 96 micromol/L (n = 5, P < 0.05); mean clip time, 8.6 minutes.

CONCLUSION

The technique detects changes 9 minutes after intraoperative events occur (limited only by probe-to-sensor tubing length and dialysate flow rate). This provides reliable information to the surgeon and anesthetist promptly. It is a useful method for monitoring glucose and lactate in dialysate, particularly when rapid, transient changes in brain analyte levels need to be determined and the alternative offline methodology would be inadequate.

Transient changes in cortical glucose and lactate levels associated with peri-infarct depolarisations, studied with rapid-sampling microdialysis

Peri-infarct depolarisations (PIDs) contribute to infarct expansion in experimental focal ischaemia; furthermore, depolarisations propagate in the injured human brain. Glucose utilisation is increased under both conditions, and depletion of brain glucose carries a poor prognosis. We studied dynamics of cerebral glucose and lactate in relation to PID patterns in experimental stroke. The middle cerebral artery was occluded for 3 h in 23 cats under terminal chloralose anaesthesia. We used fluorescence imaging to detect occurrence of PIDs, and rapid-sampling online microdialysis (rsMD), coupled to a flow-injection assay, to examine changes in cerebral cortical extracellular glucose and lactate at intervals of 30 sec each. After 30 min' ischaemia, lactate had increased by 43.6+/-s.d. 45.9 micromol/L, and stabilised in that range for 3 h. In contrast, glucose fell only slightly initially (11.9+/-9.7 micromol/L), but progressively decreased to a reduction of 56.7+/-47.2 micromol/L at 3 h, with no evidence of stabilisation. There was a highly significant inverse relationship of frequency of PIDs with plasma glucose (P<0.001). The results also characterise a metabolic signature for PIDs for possible application in clinical work, and emphasise potential risks in the use of insulin to control plasma glucose in patients with brain injury.

On-line monitoring of striatum glucose and lactate in the endothelin-1 rat model of transient focal cerebral ischemia using microdialysis and flow-injection analysis with biosensors

In vivo studies on cerebral glucose and lactate metabolism following a brain insult require fast and sensitive monitoring techniques. Here we report on-line monitoring of ischemic events and metabolic changes following reperfusion in striatum of freely moving rats subjected to endothelin-1 (60-240 pmol) induced, transient focal cerebral ischemia using slow microdialysis (0.5 microl/min), fast sampling (every minute) and flow-injection analysis with biosensors for glucose and lactate. The high-time resolution provides detailed information on lactate rise times and duration of low glucose. In rats, developing large striatal lesions, lactate increased from 1.0 +/- 0.1 to 4.2 +/- 0.7 mM within 37 +/- 1 min, whereas glucose dropped from 0.3 +/- 0.1 mM to below detection levels (<0.05 mM) for a period of 80 +/- 18 min. The lactate increase measured over a 2-h period after endothelin-1 infusion was highly correlated with striatal infarct size. In some rats oscillatory changes are observed which cannot be detected in traditional assays. The here-described monitoring technique applied in a clinically relevant rat model is a sensitive tool to study post-ischemic energy metabolism, effects of therapeutic interventions and its relationship with histological outcome.

Modeling cerebral arteriovenous lactate kinetics after intravenous lactate infusion in the rat

Venous-arterial lactate differences across the brain during lactate infusion in rats were studied, and the fate of lactate was described with a mathematical model that includes both cerebral and extracerebral kinetics. Ultrafiltration was used to sample continuously and simultaneously arterial and venous blood. Subsequent application of flow injection analysis and biosensors allowed the measurement of glucose and lactate concentrations every minute. Because of the high temporal resolution, arteriovenous lactate kinetics could be modeled in individual experiments. The existence of both a cerebral lactate sink and a lactate exchangeable compartment, representing approximately 24% of brain volume, was thus modeled.

Lactate as a pivotal element in neuron-glia metabolic cooperation

Lactate has been considered for a long time as a metabolic waste and/or a sign of hypoxia in the central nervous system. Nevertheless, clear evidence that lactate can constitute an adequate energy substrate for brain tissue has been provided as early as in the 1950s with the pioneering work of McIlwain in brain slices. Over the years, several studies using different approaches have confirmed that lactate is efficiently oxidized by brain cells in vitro. Moreover, lactate has been shown under certain circumstances to have a neuroprotective effect and support neuronal activity. Similar confirmation of lactate utilization in vivo as well as putative neuroprotection in various excitotoxic models has been provided. Lactate was even shown to restore cognitive performance upon an hypoglycemic episode in humans. More recently, it was proposed that lactate could be produced by astrocytes and released in the extracellular space to form a pool readily available for neurons in case of high energy demands. Several elements support the concept of a lactate shuttle between astrocytes and neurons in the central nervous system. Among them, the description of specific monocarboxylate transporters found on both astrocytes and neurons is an important observation consistent with this concept. Interestingly, lactate shuttles between different cell types within the same organ have been described outside the central nervous system, notably in muscle and testis. Thus, lactate is emerging as a valuable intercellular exchange molecule in different systems including the brain where it might be an essential element of neuron-glia metabolic interactions.

Persistently low extracellular glucose correlates with poor outcome 6 months after human traumatic brain injury despite a lack of increased lactate: a microdialysis study

Disturbed glucose brain metabolism after brain trauma is reflected by changes in extracellular glucose levels. The authors hypothesized that posttraumatic reductions in extracellular glucose levels are not due to ischemia and are associated with poor outcome. Intracerebral microdialysis, electroencephalography, and measurements of brain tissue oxygen levels and jugular venous oxygen saturation were performed in 30 patients with traumatic brain injury. Levels of glucose, lactate, pyruvate, glutamate, and urea were analyzed hourly. The 6-month Glasgow Outcome Scale extended (GOSe6) score was assessed for each patient. In regions of increased glucose utilization defined by positron emission tomography, the extracellular glucose concentration was less than 0.2 mmol/l. Extracellular glucose values were less than 0.2 mmol during postinjury days 0 to 7 in 19% to 30% of hourly samples on each day. Transient decreases in glucose levels occurred with electrographic seizures and nonischemic reductions in cerebral perfusion pressure and jugular venous oxygen saturation. Glutamate levels were elevated in the majority of low-glucose samples, but the lactate/pyruvate ratio did not indicate focal ischemia. Terminal herniation resulted in reductions in glucose with increases in the lactate/pyruvate ratio but not in lactate concentration alone. GOSe6 scores correlated with persistently low glucose levels, combined early low glucose levels and low lactate/glucose ratio, and with the overall lactate/glucose ratio. These results suggest that the level of extracellular glucose is typically reduced after traumatic brain injury and associated with poor outcome, but is not associated with ischemia.

Analysis of glucose and lactate in hippocampal dialysates of rats during the operant conditioned reflex using microdialysis

Changes of extracellular glucose and lactate in hippocampus for freely moving rats during the operant conditioned reflex were examined simultaneously. Samples of the dialysate were assayed for both glucose and lactate using in vivo microdialysis and a microbore flow injection analysis-immobilized enzyme reactor-electrochemical detection (FIA-IMER-ECD) system. Microdialysis samplings were conducted in a Skinner box where lights were delivered as conditioned stimuli (CS) paired with foot shocks as unconditioned stimuli (US). In the treatment group the concentration of glucose and lactate showed no fluctuations during the whole process. However, in the control group in which the rats were exposed to many foot shocks, lactate levels decreased by 19% below baseline during the behavioral session and glucose showed a delayed decrease (by 18%). Compared with glucose, lactate can immediately indicate the dynamic changes in brain.

Perfusate oxygen and carbon dioxide concentration influence basal microdialysate levels of striatal glucose and lactate in conscious rats

O(2) concentration ([O(2)]) in air equilibrated solutions at room temperature is three fold higher than that in brain extracellular fluid (ECF), and CO(2) concentration ([CO(2)]) is 100 times lower. Using microdialysis the ECF is routinely dialyzed against glucose free isotonic perfusates containing 200 microM O(2) and 10 microM CO(2). In conscious rats, 2 days after probe implantation, decreasing perfusate [O(2)] from 200 to 68 microM (physiologic level) for 60 min, while maintaining a low [CO(2)] (10 microM), increased striatal dialysate glucose and lactate by 12% and 33%, respectively. The same protocol on the third day essentially had no effect on monoamine metabolites. Decreasing [O(2)] concurrent with increasing [CO(2)] to 1.3 mM (physiologic level) for 60 min increased glucose and lactate by 17% and 37%, respectively. This study demonstrates for dialysis studies of glucose and lactate, perfusates that mimic physiologic ECF [O(2)] and [CO(2)] are more appropriate.

Quantitative on-line monitoring of hippocampus glucose and lactate metabolism in organotypic cultures using biosensor technology

Quantitative glucose and lactate metabolism was assessed in continuously perfused organotypic hippocampal slices under control conditions and during exposure to glutamate and drugs that interfere with aerobic and anaerobic metabolism. On-line detection was possible with a system based on slow perfusion rates, a half-open (medium/air interface) tissue chamber and a flow injection analytic system equipped with biosensors for glucose and lactate. Under basal conditions about 50% of consumed glucose was converted to lactate in hippocampal slice cultures. Using medium containing lactate (5 mm) instead of glucose (5 mm) significant lactate uptake was observed, but this uptake was less than the net uptake of lactate equivalents in glucose-containing medium. Glucose deprivation experiments suggested lactate efflux from glycogen stores. The effects of drugs compromising or stimulating energy metabolism, i.e. 2-deoxyglucose, 3-nitropropionic acid, alpha-cyano-4-hydroxycinnamate, l-glutamate, d-asparate, ouabain and monensin, were tested in this flow system. The data show that maintaining Na+ and K+ gradients consumed much of the energy but do not support the hypothesis that l-glutamate stimulates glycolysis in hippocampal slice cultures.

Analysis of glucose and lactate in dialysate from hypothalamus of rats after exhausting swimming using microdialysis

A microbore flow injection analysis-immobilized enzyme reactor-electrochemical detection (FIA-IMER-ECD) system for glucose and lactate detection was built up. The assays were precise, sensitive and practicable for determination of glucose and lactate levels in hypothalamic dialysate. The method had been used to detect the dynamic changes of glucose and lactate levels during rat exhausting swimming and recovery. The data showed that after exhausting swimming, the concentration of glucose in hypothalamic dialysate that reflected the concentration in the hypothalamic extracellular fluid decreased. The level fell to its nadir at day 1 after the exercise and then went back to the basal level at day 3 after the swimming. However, lactate levels increased to a maximum at day 3 and went back to the basal level at day 5 after the swimming.

Glutamate infusion coupled with hypoxia has a neuroprotective effect in the rat

Traumatic brain injury leads to a rise in glutamate, interference with oxygen supply and secondary neuronal death in the region surrounding the primary lesion. In the present experiments we have examined the effect of combining glutamate infusion with hypoxia on both brain metabolism and neuronal death. We have used microdialysis in unanaesthetised rats with a novel dual assay for glucose and lactate to monitor the temporal relation of changes in these metabolites resulting from infusion of 100 mM glutamate alone or combined with a reduction of inspired oxygen to 8%. In a parallel series of experiments we have compared the size of neuronal lesions under the same experimental conditions. We have used MAP2 antibody staining to measure the size of the neuronal lesion. Our results demonstrate that a 30 min glutamate infusion causes an immediate increase in neuronal glucose utilisation and a rise in lactate production. When hypoxia is added during the last 15 min of glutamate infusion there is a small rise in glucose and a large additional increase in lactate. The size of the neuronal lesions produced by infusion of 100 mM glutamate is reduced by the addition of hypoxia.

Feeding active neurons: (re)emergence of a nursing role for astrocytes

Despite unquestionable evidence that glucose is the major energy substrate for the brain, data collected over several decades with different approaches suggest that lactate may represent a supplementary metabolic substrate for neurons. Starting with the pioneering work of McIlwain in the early 1950s which showed that lactate can sustain the respiratory rate of small brain tissue pieces, this idea receives confirmation with more recent studies using nuclear magnetic resonance spectroscopy undoubtedly demonstrating that lactate is efficiently oxidized by neurons, both in vitro and in vivo. Not only is lactate able to maintain ATP levels and promote neuronal survival but it was also found to support neuronal activity, at least if low levels of glucose are present. Despite the early suggestion for a role of astrocytes in metabolic supply to neurons, it is only recently however that they have been considered as a potential source of lactate for neurons. Moreover, it has been proposed that astrocytes might provide lactate to neurons in response to enhanced synaptic activity by a well-characterized mechanism involving glutamate uptake. The description of specific transporters for lactate on both astrocytes and neurons further suggest that there exist a coordinated mechanism of lactate exchange between the two cell types. Thus it is proposed that astrocytes play a nursing role toward neurons by providing lactate as an additional energy substrate especially during periods of enhanced synaptic activity. The importance of this metabolic cooperation within the central nervous system, although not unique if compared to other organs, still remains to be explored.

Intracellular compartmentation of pyruvate in primary cultures of cortical neurons as detected by (13)C NMR spectroscopy with multiple (13)C labels

The intracellular compartmentation of pyruvate in primary cultures of cortical neurons was investigated by high resolution (13)C NMR using mixtures of different pyruvate precursors conveniently labeled with (13)C or unlabeled. Cells were incubated with 1-5 mM (1-(13)C, 1,2-(13)C(2) or U-(13)C(6)) glucose only or with mixtures containing 1.5 mM (1-(13)C or U-(13)C(6)) glucose, 0.25-2.5 mM (2-(13)C or 3-(13)C) pyruvate and 1 mM malate. Extracts from cells and incubation media were analyzed by (13)C NMR to determine the relative contributions of the different precursors to the intracellular pyruvate pool. When ((13)C) glucose was used as the sole substrate fractional (13)C enrichments and (13)C isotopomer populations in lactate and glutamate carbons were compatible with a unique intracellular pool of pyruvate. When mixtures of ((13)C) glucose, ((13)C) pyruvate and malate were used, however, the fractional (13)C enrichments of the C2 and C3 carbons of lactate were higher than those of the C2 and C3 carbons of alanine and depicted a different (13)C isotopomer distribution. Moreover, neurons incubated with 1 mM (1,2-(13)C(2)) glucose and 0.25-5 mM (3-(13)C) pyruvate produced exclusively (3-(13)C) lactate, revealing that extracellular pyruvate is the unique precursor of lactate under these conditions. These results reveal the presence of two different pools of intracellular pyruvate; one derived from extracellular pyruvate, used mainly for lactate and alanine production and one derived from glucose used primarily for oxidation. A red-ox switch using the cytosolic NAD(+)/NADH ratio is proposed to modulate glycolytic flux, controlling which one of the two pyruvate pools is metabolized in the tricarboxylic acid cycle when substrates more oxidized or reduced than glucose are used.