Extracellular glutamate: on-line monitoring using microdialysis coupled to enzyme-amperometric analysis

Microdialysis techniques and microdialysis-based patient-near diagnostics

This article will debate the usefulness of POCT measurements and the contribution microdialysis can make to generating valuable information. A particular theme will be the rarely considered difference between ex vivo sampling, which typically generates only a static measure of concentration, and in vivo measurements that are subject to dynamic changes due to mass transfer. Those dynamic changes provide information about the patients' physiological state.

Deuterated Glutamate-Mediated Neuronal Activity on Micro-Electrode Arrays

The excitatory synaptic transmission is mediated by glutamate in neuronal networks of the mammalian brain. In addition to the synaptic glutamate, extra-synaptic glutamate is known to modulate the neuronal activity. In neuronal networks, glutamate uptake is an important role of neurons and glial cells for lowering the concentration of extracellular glutamate and to avoid the excitotoxicity by glutamate. Monitoring the spatial distribution of intracellular glutamate is important to study the uptake of glutamate, but the approach has been hampered by the absence of appropriate glutamate analogs that report the localization of glutamate. Deuterium-labeled glutamate (GLU-D) is a promising tracer for monitoring the intracellular concentration of glutamate, but physiological properties of GLU-D have not been studied. Here we study the effects of extracellular GLU-D for the neuronal activity by using primary cultured rat hippocampal neurons that form neuronal networks on microelectrodes array. The frequency of firing in the spontaneous activity of neurons increased with the increasing concentration of extracellular GLU-D. The frequency of synchronized burst activity in neurons increased similarly as we observed in the spontaneous activity. These changes of the neuronal activity with extracellular GLU-D were suppressed by antagonists of glutamate receptors. These results suggest that GLU-D can be used as an analog of glutamate with equivalent effects for facilitating the neuronal activity. We anticipate GLU-D developing as a promising analog of glutamate for studying the dynamics of glutamate during neuronal activity.

Glutamate detection at the cellular level by means of polymer/enzyme multilayer modified carbon nanoelectrodes

Carbon nanoelectrodes in the sub-micron range were modified with an enzyme cascade immobilized in a spatially separated polymer double layer system for the detection of glutamate at the cellular level.

Flow biosensing and sampling in indirect electrochemical detection

Miniaturization in biological analyses has several advantages, such as sample volume reduction and fast response time. The integration of miniaturized biosensors within lab-on-a-chip setups under flow conditions is highly desirable, not only because it simplifies process handling but also because measurements become more robust and operator-independent. In this work, we study the integration of flow amperometric biosensors within a microfluidic platform when analyte concentration is indirectly measured. As a case study, we used a platinum miniaturized glucose biosensor, where glucose is enzymatically converted to [Formula: see text] that is oxidized at the electrode. The experimental results produced are strongly coupled to a theoretical analysis of fluid dynamic conditions affecting the electrochemical response of the sensor. We verified that the choice of the inlet flow rate is a critical parameter in flow biosensors, because it affects both glucose and [Formula: see text] transport, to and from the electrode. We identify optimal flow rate conditions for accurate sensing at high time resolution. A dimensionless theoretical analysis allows the extension of the results to other sensing systems according to fluid dynamic similarity principles. Furthermore, we developed a microfluidic design that connects a sampling unit to the biosensor, in order to decouple the sampling flow rate from that of the actual measurement.

Potential neuroprotective effects of acupuncture stimulation on diabetes mellitus in a global ischemic rat model

Acupuncture (ACU) is known to be effective in ischemia treatment, and glutamate (GLU) excitotoxicity is an important factor in neuronal cell death. We observed the effect of ACU on cerebral blood flow (%CBF) and DeltaGLU (the changes in GLU release) in the ischemic stroke rat model of diabetic mellitus (DM). A global ischemia was induced using the eleven-vessel occlusion (11-VO) method in 14 Sprague-Dawley rats (DM), which were randomly divided into two groups: the control group and the ACU-treatment group. Extracellular DeltaGLU was assessed using an intra-cerebral biosensor system measuring 256 samples per second, simultaneously with %CBF and electroencephalogram. ACU stimulation was applied to ACU points GB34 and GB39 during the ischemic period. Twenty-three diagnostic parameters were proposed first for a detailed analysis of changes in %CBF and GLU release during ischemia/reperfusion. ACU rats showed a significant decrease in ischemic (p < 0.05) and reperfusion %CBF (p < 0.0001) than control rats, and a significantly larger decrease in ischemic DeltaGLU (p < 0.05) and peak level of reperfusion DeltaGLU (p < 0.005) than control rats. From these results, we suggest that ACU stimulation is responsible for the potential protection of neurons through suppression of %CBF response in the increased plasma osmolality and extracellular DeltaGLU in diabetic rats under ischemic conditions.

Structural characterization of L-glutamate oxidase from Streptomyces sp. X-119-6

L-Glutamate oxidase (LGOX) from Streptomyces sp. X-119-6, which catalyzes the oxidative deamination of L-glutamate, has attracted increasing attention as a component of amperometric L-glutamate sensors used in the food industry and clinical biochemistry. The precursor of LGOX, which has a homodimeric structure, is less active than the mature enzyme with an alpha(2)beta(2)V(2) structure; enzymatic proteolysis of the precursor forms the stable mature enzyme. We solved the crystal structure of mature LGOX using molecular replacement with a structurally homologous model of L-amino acid oxidase (LAAO) from snake venom: LGOX has a deeply buried active site and two entrances from the surface of the protein into the active site. Comparison of the LGOX structure with that of LAAO revealed that LGOX has three regions that are absent from the LAAO structure, one of which is involved in the formation of the entrance. Furthermore, the arrangement of the residues composing the active site differs between LGOX and LAAO, and the active site of LGOX is narrower than that of LAAO. Results of the comparative analyses described herein raise the possibility that such a unique structure of LGOX is associated with its substrate specificity.

Microdialysis applications in neuroscience

BACKGROUND

Microdialysis is a technique to monitor extracellular changes in living tissue. Substances present in the extracellular space, such as neurotransmitters and metabolites transported between cells and capillaries in the extracellular fluid (ECF), are major object.

RESULTS

Since its introduction to the research of the nervous system, microdialysis has become a popular method for the measurements of brain chemistry and greatly affected in the fields of neuropharmacology, neuroanatomy and neurophysiology. Most of published papers using microdialysis have focused on the area of neuroscience, recently more biomedical application.

CONCLUSION

In this review, we focused on cerebral microdialysis as a monitoring tool for physiologic and pathophysiologic changes in chemical processes in the brain. Then we presented the principle and various applications of cerebral microdialysis.

Pyramidal, rodlike, spherical gold nanostructures for direct electron transfer of copper, zinc-superoxide dismutase: application to superoxide anion biosensors

It is the first time that direct electron transfer of copper, zinc-superoxide dismutase (Cu, Zn-SOD) is realized at nanospherical, nanorodlike, and nanopyramidal gold nanostructures, without any mediators or promoters. Thermodynamic and kinetic parameters of the electron transfer vary with the morphology of the electrodeposited gold nanostructures, suggesting the morphology-dependent electrochemistry of SOD. Experimental results reveal that SOD is strongly confined onto the nanostructured gold surfaces and processes its inherent enzymatic activity after being adsorbed on all three kinds of gold nanostructures, which also enable the direct electron transfer of SOD itself. A combination of the facilitated direct electron transfer and the bifunctional enzymatic catalytic activities of the SOD substantially offers a dual electrochemical approach to determination of O2(*-), in which O2(*-) could be detected both anodically and cathodically. In both the oxidation and reduction regions, the present O2(*-) biosensors display excellent analytical performance, such as wide linear range, low detection limit, quick response time, and good stability and reproducibility, while not being limited by interferences, for instance, uric acid, ascorbic acid, and hydrogen peroxide.

Electrochemical Determination of Minoxidil in Different Pharmaceutical Formulations by Flow Injection Analysis

The electrochemical behavior and amperometric-FIA quantification of minoxidil at a glassy carbon electrode is described. The procedure is based on electrochemical oxidation at 0.800 V (vs. Ag/AgCl/NaCl(3 M) in a phosphate buffer solution. Minoxidil was determined over the range 1 x 10(-7) - 1 x 10(-4) M. Different analytical parameters and electrode stability were analyzed to obtain the best electrode performance. The optimal conditions were: working potentials, 0.800 V; flow rate, 0.74 mL min(-1); and solution pH 7.0. This system allowed a sampling rate of 120 samples per hour without any pretreatment. The proposed method was used for minoxidil quantification in pharmaceutical preparations with satisfactory results. The accuracy of FIA-amperometric method was established by a comparison with the conventional UV determination technique using a paired t-test indicating the absence of systematic errors.

Real-time monitoring of L-glutamate release from mouse brain slices under ischemia with a glass capillary-based enzyme electrode

Real-time monitoring of L-glutamate release from various neuronal regions of mouse hippocampal slices under ischemia (a glucose-free hypoxia condition) is described. A glass capillary microelectrode with a tip size of approximately 10 microm containing a very small volume ( approximately 2 microL) of a solution of glutamate oxidase (GluOx) and ascorbate oxidase was used. First, the amperometric response behavior of the electrode at 0 V versus Ag/AgCl was characterized with a standard glutamate solution in terms of continuous measurements, effect of oxygen, viscosity of solution and concentration dependence. The electrode was applied to the real-time monitoring of L-glutamate released from different neuronal regions of acute hippocampal slices submerged in a hypoxia solution. The time-resolved amounts of L-glutamate released at various neuronal regions (CA1, CA3 and DG) of mouse hippocampal slices were quantified and compared with the reported L-glutamate fluxes using difference-image analysis during ischemia.

Effect of ifenprodil, a polyamine site NMDA receptor antagonist, on brain edema formation following asphyxial cardiac arrest in rats

OBJECTIVE

Brain edema occurs in experimental and clinical cardiac arrest (CA) and is predictive of a poor neurological outcome. N-Methyl--aspartate (NMDA) receptors contribute to brain edema elicited by focal cerebral ischemia/reperfusion (I/R). Ifenprodil, a NMDA receptor antagonist, attenuates brain edema and injury size in rats after focal cerebral I/R. We assessed the hypothesis that ifenprodil reduces CA-elicited brain edema.

METHODS

Eighteen male Sprague-Dawley rats were assigned to group 1 (normal control, n=6), group 2 (placebo-treated CA, n=6), or group 3 (ifenprodil-treated CA, n=6). CA was induced by 8 min of asphyxiation and the animals were resuscitated with cardiopulmonary resuscitation (CPR), ventilation, epinephrine (adrenaline), and sodium bicarbonate (NaHCO3). Ifenprodil of 10 mg/kg or a placebo vehicle was given intraperitoneally 5 min before CA. Brain edema was determined by brain wet-to-dry weight ratio at 1 h after resuscitation.

RESULTS

There were no differences between groups 2 and 3 in all physiological variables at baseline. Time from asphyxiation to CA was 201.5 +/- 7.5 s in group 2 and 160.7 +/- 10.4 s in group 3 (P<0.001). Resuscitation time was 68.2 +/- 13.3 s in group 2 and 92.8 +/- 18.2 s in group 3 (P<0.05). Ifenprodil decreased mean arterial pressure (MAP) before asphyxiation, from 128 +/- 7 in group 2 to 82 +/- 15 mmHg in group 3 (P<0.001), and negated immediate post-resuscitation hypertension. Brain wet-to-dry weight ratio was 5.64 +/- 0.44 in group 1, 7.34 +/- 0.95 in group 2 (P<0.01 versus group 1), and 5.93 +/- 0.40 in group 3 (P<0.05 versus group 2).

CONCLUSIONS

Ifenprodil reduces CA-elicited brain edema. In addition, we observed significant hemodynamic changes caused by ifenprodil.

Electrochemistry and Electrocatalytic Activities of Superoxide Dismutases at Gold Electrodes Modified with a Self-Assembled Monolayer

In this article, the electrochemical properties and electrocatalytic activity of three kinds of superoxide dismutases (SODs), that is, bovine erythrocyte copper-zinc superoxide dismutase (Cu/Zn-SOD), iron superoxide dismutase from Escherichia coli (Fe-SOD), and manganese superoxide dismutase from E. coli (Mn-SOD), in the SOD family were studied. It was revealed that the direct electron transfer of the three kinds of SODs could be efficiently promoted by a self-assembled monolayer (SAM) of 3-mercaptopropionic acid (MPA) confined on a gold electrode. The electrochemical properties of the SODs at the MPA-SAM electrode vary with the sort of SOD with respect to the formal potential, reversibility of electrode reactions, kinetic parameters, and pH dependence, suggesting different mechanisms for the electrode reactions of the individual SODs. A combination of the facilitated direct electron transfer and the bifunctional enzymatic catalytic activities of the SODs via a redox cycle of their active metals substantially offered a flexible electrochemical route to determination of O(2)(*)(-) where O(2)(*)(-) can be sensed with the SOD-based biosensors in both anodic and cathodic polarizations. Such an intrinsic feature of the SOD-based biosensors successfully enabled a sensitive determination scheme for O(2)(*)(-) free from the interference from some coexisting electroactive species, such as ascorbic acid (AA) and uric acid (UA). Further potential applications for in vivo determination of O(2)(*)(-) is also suggested.

Mechanisms of the anti-ischemic effect of vagus nerve stimulation in the gerbil hippocampus

The neuroprotective mechanisms of cervical vagus nerve stimulation (VNS) in transient ischemia were investigated. Left VNS (0.4 mA, 40 Hz) was performed during 5 min ischemia in gerbils. About 50% of the hippocampal neurons were rescued from ischemic insult by VNS, and this effect was prevented by transection of the vagus nerve centrally to the site of cervical stimulation. VNS significantly attenuated both ischemia-induced glutamate release and transient increase of hippocampal blood flow during reperfusion. Hyperemia as well as excessive glutamate release after ischemia is regarded as an important factor in ischemic brain damage as it leads to generate considerable reactive oxygen species. Thus, VNS might protect neurons from ischemia-induced glutamate excitotoxicity and reperfusion injury via the afferent path-way of the vagus.

Selective detection of L-glutamate using a microfluidic device integrated with an enzyme-modified pre-reactor and an electrochemical detector

A microfluidic device integrated with a nanoliter volume enzyme pre-reactor and an enzyme-modified electrode was developed for the highly selective continuous measurement of glutamate (Glu). The device consists mainly of two glass plates. One plate incorporates an electrochemical cell that consists of working electrode (WE), reference electrode (RE) and counter electrode (CE). The WE is modified with a bilayer film of Os-polyvinylpyrridine-based mediator containing horseradish peroxidase (Os-gel-HRP). The WE was operated at -50 mV versus Ag. The other plate has a thin layer flow channel integrated with a pre-reactor. The reactor has a number of micropillars (20 microm in diameter, 20 microm high and separated from each other by a 20 microm gap) modified with ascorbate oxidase (AAOx) to eliminate L-ascorbic acid (AA). The enzymatic oxidation of AA is superior to that obtained with our previously reported pre-electrolysis type micro-reactor since electrochemically reversible transmitters such as catecholamines do not provide a cathodic current at the WE. In addition, the high operation potential of the pre-reactor causes unknown electroactive species, which also cause interference at the detection electrode. As a result, we were able to detect 1 microM Glu continuously at a low flow rate even when AA concentration was 100 microM.

[Real-time detection of neurotransmitter release and its spatial distribution]

Neurotransmitters have been well known as information carriers for a long time. Recently, some of the research indicated their neurotoxicity, while some indicated their neurotrophic actions. It is very important to understand the role of neurotransmitters. Glutamate is one of the most important excitatory neurotransmitter in the brain. We developed a novel measurement method for glutamate. The method we describe here is based on the enzyme-mediated electrochemical detection. Glutamate oxidase and horseradish peroxidase were deposited together with polymer-mediator on the electrode. We applied this idea on ITO multi-array electrode and developed a 64 channel multi-array sensor. The sensor permits us to detect glutamate release from multiple regions simultaneously in real time. As it is possible to illustrate the distribution of glutamate release, the sensor could be used not only in the pharmacological field, but also in medical treatment in the near future.

A glass capillary microelectrode based on capillarity and its application to the detection of L-glutamate release from mouse brain slices

A new glass capillary microelectrode for L-glutamate is described using pulled glass capillaries (tip size, approximately 12.5 microm) with a very small volume (approximately 2 microl) of inner solution containing glutamate oxidase (GluOx) and ascorbate oxidase. The operation of the electrode is based on capillary action that samples L-glutamate into the inner solution. The enzyme reaction by GluOx generates hydrogen peroxide that is detected at an Os-gel-HRP polymer modified Pt electrode in a three-electrode configuration. The amperometric response behavior of the electrode was characterized in terms of the capillarity, response time, sensitivity and selectivity for measurements of L-glutamate. The currents at 0 V vs. Ag/AgCl increased linearly with the L-glutamate concentration from 10 to 150 microM for in vitro and in situ calibrations. The response was highly selective to L-glutamate over ascorbate, dopamine, serotonin and other amino acids. The detection of L-glutamate in the extracellular fluids of different regions of mouse hippocampal slices under stimulation of KCl was demonstrated.

In vivo temporal sequence of rat striatal glutamate, aspartate and dopamine efflux during apomorphine, nomifensine, NMDA and PDC in situ administration

In vivo microdialysis was used to investigate the interactions between dopamine (DA), glutamate (Glu) and aspartate (Asp) in anaesthetised-rat striatum. The combination of brain microdialysis and capillary electrophoresis with laser-induced fluorescence detection (CE-LIFD) allows the simultaneous monitoring of the efflux of these neurotransmitters up to every 10 s. DA and Glu reuptake inhibitors, nomifensine and L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) and, dopaminergic and glutamatergic receptor agonists, apomorphine and NMDA respectively, were administered by reverse dialysis. Reverse dialysis of 20 micro M nomifensine induced a rapid and marked increase (+3200% at 5 min) in extracellular DA, while a decrease in Glu and Asp (-11 and -25%, respectively) was observed simultaneously. Reverse dialysis of 10 micro M apomorphine led to progressive changes: -63% decrease in DA and +25% Glu increase at 36 min. Reverse dialysis of 1 mM NMDA induced a simultaneous increase in DA, Glu and Asp which peaked at +2 min (+840%, +40% and +150%, respectively). Surprisingly, a second increase in Glu was observed 5 min after the end of NMDA perfusion. Reverse dialysis of PDC (1 mM and 10 mM) induced a rapid increase in Glu and Asp levels, while DA increased with a 26-s delay. These findings indicate that, in the striatum, endogenous DA and Glu may act in opposition to regulate each other's efflux. These results have been obtained due to unique features offered by microdialysis coupled with CE-LIFD.

Continuous on-line measurement of cerebral hydrogen peroxide using enzyme-modified ring-disk plastic carbon film electrode

An amperometric method suitable for the continuous on-line measurement of cerebral hydrogen peroxide from a microdialysate has been successfully performed for the first time by using an enzyme-modified ring-disk plastic carbon film electrode (PCFE) in a thin-layer radial flow cell. PCFE consists of a ring electrode modified with horseradish peroxidase to detect H2O2 at 0.0 V (vs Ag/ AgCl) and a disk electrode coated with ascorbate oxidase (AOx) to preoxidize ascorbic acid (AA) and thus suppress interference via direct oxidation. Analytes in solution (brain dialysates or standards) are mixed on-line with a phosphate-buffered solution containing dissolved oxygen and chelating agent, EDTA. The buffered solution is used to provide the O2 necessary for the AOx catalytic reaction, stabilize the changes in dialysate pH that are associated with the in vivo formation of H2O2, and remove heavy metal ion impurities and thus suppress reactions between AA and H2O2. This procedure enables trace levels of H2O2 to be readily monitored, virtually interference-free from physiological levels of AA, uric acid, electroactive neurotransmitters and their principle metabolites, in a continuous-flow system.

Glucocorticoid feedback increases the sensitivity of the limbic system to stress

In the hypothalamus, corticotropin-releasing hormone (CRH) has a well-described role initiating the hypothalamic-pituitary-adrenal (HPA) axis response to stress. Cortisol, released from the adrenal gland, exerts negative feedback on this axis. The role of extrahypothalamic CRH in stress responses is less well known. The purpose of this study was to measure the response of CRH in the amygdala to acute and repeated stress and to examine if cortisol had any effect on this response. Immunosensor-based microdialysis probes were used to measure CRH and cortisol in the amygdala and cortisol systemically in sheep exposed to a predator stress (a dog). Upon presentation of a dog, CRH increased in the amygdala of the sheep and then fell off. Cortisol levels rose both systemically and in the amygdala, and as they peaked, a second CRH response was observed. Repeated stress changed this response, with the magnitude of the first CRH peak decreasing while the second peak increased. Repeated stress also produced an exaggeration in both of the CRH peaks to presentation of a subsequent novel stress (a forelimb electric shock). Animals that had an escape route from the repeated dog stress did not show this exaggeration when faced subsequently with the novel stress. Administration of mifepristone, a glucocorticoid receptor antagonist, prior to the delivery of the repeat stress prevented subsequent changes in the CRH response. The data suggest that the amygdala shows a CRH response to presentation of a stressor acutely and repeatedly and that repeated stress can alter subsequent amygdala responsiveness to the same or a different stressor. This alteration appears dependent on circulatory glucocorticoids.

Measuring of extracellular cortisol and corticotropin-releasing hormone in the amygdala using immunosensor coupled microdialysis

A method is described for the measurement of cortisol or corticotropin-releasing hormone (CRH) from the brain, within, or in the dialysate of, a microdialysis probe using an antibody-linked assay. Polyclonal antibodies for either cortisol or CRH provide a specificity of measurement. These antibodies are affixed on a platinum electrode within the probe. Determination of bound cortisol or CRH is performed via an indirect assessment of competitive ligand also bound, and conjugated to HRP. This is activated and measured as current change. These probes were used for extracellular measurements in the amygdala, a limbic brain region, of sheep. Direct measurement in vivo compared favorably to the use of either the immunosensor or standard RIA techniques on dialysate emerging from a classical microdialysis probe, suggesting that the method could be used directly in-vivo. This is the first report in literature of rapid on-line measurement of CRH and cortisol from a discrete brain area. The probes have fast response times (>90% of maximum response within 30 s of start of analysis), high sensitivity (<0.1 pg/ml for CRH) and can acquire data every 2 min. They are stable in-vivo (>72 h) and regenerable, offering on-line measurement, with rapid time resolution, of neurohumoral substances.

A flow-through amperometric sensor based on dialysis tubing and free enzyme reactors

A generic flow-through amperometric microenzyme sensor is described, which is based on semi-permeable dialysis tubing carrying the sample to be analyzed. This tubing (300 microm OD) is led through a small cavity, containing the working and reference electrode. By filling this cavity with a few microl of an appropriate enzyme solution, an amperometric enzyme sensor results. As the dialysis tubing is impermeable for large molecular species such as enzymes, this approach does not require any immobilization chemistry, and as a consequence the enzyme is present in its natural free form. Based on this principle, amperometric sensors for lactate, glucose, and glutamate were formed by filling cavities, precision machined in Perspex, with buffered solutions containing respectively, lactate-, glucose-, and glutamate-oxidase. All sensors showed a large linear range (0-35 mM for glucose, 0-3 mM for lactate, and 0-5 mM for glutamate) covering the complete physiological range. The lower detection limit was in the order of 15-50 microM. Applicability in flow injection analysis systems is demonstrated.

Extracellular glutamate changes in rat striatum during ischemia determined by a novel dialysis electrode and conventional microdialysis

Our newly developed method using a dialysis electrode has made it possible to perform real time monitoring of extracellular glutamate concentration ([Glu]e) utilizing the oxygen-independent reaction with glutamate oxidase and ferrocene. In this study, we therefore, investigated [Glu]e changes during brain ischemia using both the conventional microdialysis method and the dialysis electrode method. A comparison between our newly developed dialysis electrode and conventional microdialysis methods provided the following results. When the conventional microdialysis method was employed: (1) the elevation of [Glu]e during complete global ischemia was delayed; and (2) the elevation of concentration and reuptake of glutamate were delayed during 10-min transient ischemia, and the elevation of [Glu]e reached a maximum later using conventional microdialysis than using our dialysis electrode. (3) The biphasic [Glu]e elevation of glutamate concentration detected using the dialysis electrode method was not observed using the conventional microdialysis method. It was additionally investigated why the conventional microdialysis method provides inferior time resolution. In this study, we also demonstrated with the chromatographic SMART procedure coupled to UV detection that biogenic substances, i.e. low molecular weight proteins and peptides, are released during ischemic injury, and they may cause a delay in the time resolution in the microdialysis method.

Quinolinic acid accumulation during neuroinflammation. Does it imply excitotoxicity?

It is often proposed that quinolinic acid (QUIN) contributes to the pathophysiology of neuroinflammation because this kynurenine pathway metabolite is a selective agonist of N-methyl-D-aspartate (NMDA) receptors, and both its brain tissue and cerebrospinal fluid concentrations increase markedly with inflammation. However, whether or not the extracellular levels of QUIN reached during neuroinflammation are high enough to promote excitotoxicity, remains unclear because QUIN is a weak NMDA receptor agonist. We have addressed this issue by evaluating the extracellular concentrations of QUIN that must be reached to initiate potentially excitotoxic changes in the cerebral cortex of rats, under normal conditions, and when superimposed on another insult. We have also examined the increase in extracellular lactate associated with the perfusion of increasing concentrations of QUIN through a microdialysis probe. The extracellular EC50 for induction of local depolarisation was 228 microM with QUIN alone; that is, about 30 times the levels of QUIN measured previously in immune activated brain. Furthermore, at least 20 microM extracellular QUIN needed to be reached to reduce K+ induced spreading depression, an unexpected effect since spreading depression is inhibited by NMDA receptor antagonists. Our data suggest that, although synthesis of QUIN from activated microglia and invading macrophages can increase its extracellular concentration 10-100-fold, the levels that are reached in these conditions remain far below the concentrations of QUIN that are necessary for excessive NMDA receptor activation. However, the possibility that QUIN accumulation may be a deleterious feature of neuroinflammation cannot be ruled out at this stage.

Glutamatergic neurotransmission in aging: a critical perspective

The effects of aging on glutamate neurotransmission in the brain is reviewed and evaluated. Glutamate is the neurotransmitter in most of the excitatory synapses and appears to be involved in functions such as motor behaviour, cognition and emotion, which alter with age. However, relatively few studies have been conducted to study the relationship between glutamate and aging of the brain. The studies presented here indicate the existence of a number of changes in the glutamatergic system during the normal process of aging. First, an age-related decrease of glutamate content in tissue from cerebral cortex and hippocampus has been reported, although it may be mainly a consequence of changes in metabolic activity rather than glutamatergic neurotransmission. On the other hand, studies in vitro and in vivo have shown no changes in glutamate release during aging. Since glutamate sampled in most of these studies is the result of a balance between release and uptake processes, the lack of changes in glutamate release may be due to compensatory changes in glutamate uptake. In fact, a reduced glutamate uptake capacity, as well as a loss in the number of high affinity glutamate transporters in glutamatergic terminals of aged rats, have been described. However, the most significant and consistent finding is the decrease in the density of glutamatergic NMDA receptors with age. A new perspective, in which glutamate interacts with other neurotransmitters to conform the substrates of specific circuits of the brain and its relevance to aging, is included in this review. In particular, studies from our laboratory suggest the existence of age-related changes in the interaction between glutamate and other neurotransmitters, e.g. dopamine and GABA, which are regionally specific.

Microdialysis coupled to online enzymatic assays

In vivo sampling of interstitial fluid by using microdialysis fibers has become a standard and accepted procedure. This sampling method is generally coupled to offline analysis of consecutive dialysate samples by high-performance liquid chromatography or capillary electrophoresis, but this combination is not the best approach for some applications, especially those which require high temporal resolution and rapid data collection. The purpose of this review is to provide information on enzyme-based online assays, i.e., continuous analysis of the dialysate as it emerges from the outlet of the sampling device. We have focused on methods developed specifically for the analysis of solutions perfused at a very slow flow rate, i.e., a feature of microdialysis and ultrafiltration techniques. These methods include flow enzyme-fluorescence assays, flow enzyme-amperometric assays, and sequential enzyme-amperometric detection. Each type of assay is discussed in terms of principle, applications, advantages, and limitations. We also comment on implantable biosensors, an obvious next step forward for in vivo monitoring of molecules in neuroscience.

Analytical considerations for microdialysis sampling

Adaptations in microdialysis probe designs have made it possible to obtain samples from the extracellular fluid of a variety of tissues with high temporal resolution. The resulting small volume samples, often with low concentration of the analyte(s) of interest, present a particular challenge to the analytical system. Rapid separations can be coupled on-line with microdialysis to provide near real-time data. By combining microdialysis sampling with a liquid chromatographic or capillary electrophoretic separation and a highly sensitive detection method, a separation-based sensor can be developed. Such sensors have been applied to the investigation of drug entities as well as to study endogenous analytes.

Oxygen-independent real-time monitoring of distinct biphasic glutamate release using dialysis electrode in rat striatum during anoxia: in vivo evaluation of glutamate release and reversed uptake

Using a dialysis electrode, previous studies showed a clear biphasic release of glutamate during anoxia and ischemia. In this study, we examined two hypotheses: (1) glutamate is of vesicular origin and its release is thus Ca2+- and ATP-dependent in the first phase, while in the second phase glutamate is derived primarily from the metabolic pool, and (2) reversed glutamate uptake, due to electrogenic stoichiometry, produces the second phase during anoxic insult in the rat brain. A dialysis electrode continuously perfused with glutamate oxidase and ferrocene-conjugated bovine serum albumin (BSA) optimized the time resolution of monitoring, allowing quantitative oxygen-independent, real-time measurement of the extracellular glutamate concentration ([Glu]e) during anoxia. [Glu]e dynamics were analyzed during anoxia by combining the dialysis electrode with focal microinjection of substances inducing glutamate release. Following anoxia in the rat brain, a sharp and rapid [Glu]e elevation took place (first phase). The [Glu]e elevation then shifted, continuing a gently sloping rise throughout the anoxic period (second phase). This first phase disappeared with intracranial administration of either Co2+ or omega-conotoxin. The second phase rise increased with focal microinjection of KCl (300 mM, 1 microL) and decreased with NaCl (300 mM, 1 microL), ultimately reaching a plateau in both cases. Preloading with a novel glutamate transporter inhibitor (tPDC) decreased both the first and second phases of [Glu]e elevation. This dialysis electrode system provides data supporting in vivo evidence that the peak of the first phase of [Glu]e elevation is derived from the "neurotransmitter pool," while the second phase is derived from the neuronal and glial "metabolic pool," which is, at least, partly related to a "reversed uptake" mechanism in the anoxic rat brain.

Quantitative evaluation of extracellular glutamate concentration in postischemic glutamate re-uptake, dependent on brain temperature, in the rat following severe global brain ischemia

Changes in brain temperature are known to modulate the marked neuronal damage caused by an approximately 10-min intra-ischemic period. Numerous studies have suggested that the extracellular glutamate concentration ([Glu](e)) in the intra-ischemic period and the initial postischemia period is strongly implicated in such damage. In this study, the effects of intra-ischemic brain temperature (32, 37, 39 degrees C) on [Glu](e) were investigated utilizing a dialysis electrode combined with ferrocene bovine serum albumin (BSA), which allows oxygen-independent real-time measurement of [Glu](e). This system allowed separate quantitative evaluation of intra-ischemic biphasic glutamate release from the neurotransmitter and metabolic pools, and of postischemic glutamate re-uptake in ischemia-reperfusion models. The biphasic [Glu](e) elevation in the intra-ischemic period did not differ markedly among intra-ischemic brain temperatures ranging from 32 to 39 degrees C. Intra-ischemic normothermia (37 degrees C) and mild hyperthermia (39 degrees C) markedly inhibited [Glu](e) re-uptake during the postischemic period, although the intra-ischemic [Glu](e) elevation did not differ from that during intra-ischemic hypothermia (32 degrees C). It was assumed that normothermia or mild hyperthermia in the intra-ischemic period influences intracellular functional abnormalities other than the intra-ischemic [Glu](e) elevation, thereby inhibiting glutamate re-uptake after reperfusion rather than directly modulating intra-ischemic [Glu](e) dynamics.

Excitotoxicity in neurological disorders--the glutamate paradox

Beneficial effects of glutamate-receptor antagonists in models of neurological disorders are often used to support the notion that endogenous excitotoxicity (i.e. resulting from extracellular accumulation of endogenous glutamate) is a major contributor to neuronal death associated with these conditions. However, this interpretation conflicts with a number of robust and important experimental evidence. Here, emphasis is placed on two key elements: (i) very high extracellular levels of glutamate must be reached to initiate neuronal death, far above those measured in models of neurological disorders; and (ii) changes in extracellular glutamate as measured by microdialysis are not related to changes in the synaptic cleft, i.e. the compartment where neurotransmitter glutamate interacts with its receptors. It has become clear that the diversity and complexity of glutamate-mediated processes allow for a wide range of potential abnormalities (e.g. loss of selectivity of glutamate-operated ion channels, abnormal modulation of glutamate receptors). In addition, as neuronal death subsequent to ischemia and other insults is likely to result from multifactorial processes that may be inter-related, inhibition of glutamate-mediated synaptic transmission may be neuroprotective by increasing the resistance of neurons to other deleterious mechanisms (e.g. inadequate energy supply) that are not directly related to glutamatergic transmission.

Subnanoliter volume wall-jet cells combined with interdigitated microarray electrode and enzyme modified planar microelectrode

Miniaturized wall-jet type flow cells with an active volume of 0.042-15 nL were fabricated for use as highly sensitive electrochemical detectors for capillary electrophoresis/electrochemical detection and small on-line enzyme sensors. The cells consisted of three glass plates and a fused-silica capillary. Two of the plates had microfabricated flow channels and guide trenches for the capillary and working, reference, and counter electrodes. The other plate had a film electrode. When an interdigitated microarray electrode (total area, 66 microm x 64 microm; bandwidth and gap, 2 microm) was installed in the flow cell, the redox cycling enhanced the current at flow rates of less than 100 nL/min even though there were only eight pairs of microbands. A sharp dopamine peak enhanced by the redox cycling was observed when the cell was used for capillary electrophoresis. A square film electrode modified with glutamate oxidase and Os-poly(vinylpyridine) containing HRP was also installed in the flow cell and used to measure neurotransmitter release from cultured nerve cells. When the flow rate was relatively high, the response time of the modified electrode was comparable to that of a cylindrical carbon fiber electrode (33 microm o.d.) modified with the same enzyme and mediator. We observed a transient cathodic current response assigned to the glutamate release with the electrode in the flow cell in a suction mode measurement when we stimulated cultured nerve cells electrically with a dual microelectrode.

Intraoperative enzyme-amperometric monitoring of extracellular glutamate concentration with a dialysis electrode in ischemic human brain

Changes in the extracellular concentration of glutamate in the brain ([Glu]e) were monitored continuously by an enzyme-amperometric technique employing a dialysis electrode during ischemia caused by isolation of the brain tissue in rats and human patients. In the rat (n = 10), the dialysis electrode was placed in the frontal cortex and the frontal lobe was transected. A transient sharp increase in [Glu]e was frequently observed during the transection. A biphasic elevation (a rapid increase followed by a slowly continuing increase) subsequently occurred with a latent period of 1-3 min after the transection of the rat frontal lobe. In patients (n = 7), the dialysis electrode was placed in tumor-free cortical areas which were planned to be resected together with gliomas. Progressive increases in [Glu]e were observed in all of the patients as the isolation of the brain tissue progressed. A biphasic increase, similar to that seen in the rat, was identified in 2 patients in whom the cortical area surrounding the dialysis electrode was rapidly isolated. The present enzyme-amperometric technique employing a dialysis electrode appears to be useful for detecting the occurrence of potentially harmful ischemia and for securing minimal metabolic stress caused during various surgical manipulations.

The relationship between the apparent diffusion coefficient measured by magnetic resonance imaging, anoxic depolarization, and glutamate efflux during experimental cerebral ischemia

A reduction in the apparent diffusion coefficient (ADC) of water measured by magnetic resonance imaging (MRI) has been shown to occur early after cerebrovascular occlusion. This change may be a useful indicator of brain tissue adversely affected by inadequate blood supply. The objective of this study was to test the hypothesis that loss of membrane ion homeostasis and depolarization can occur simultaneously with the drop in ADC. Also investigated was whether elevation of extracellular glutamate ([GLU]e) would occur before ADC changes. High-speed MRI of the trace of the diffusion tensor (15-second time resolution) was combined with simultaneous recording of the extracellular direct current (DC) potential and on-line [GLU]e from the striatum of the anesthetized rat. After a control period, data were acquired during remote middle cerebral artery occlusion for 60 minutes, followed by 30 minutes of reperfusion, and cardiac arrest-induced global ischemia. After either focal or global ischemia, the ADC was reduced by 10 to 25% before anoxic depolarization occurred. After either insult, the time for half the maximum change in ADC was significantly shorter than the corresponding DC potential parameter (P < 0.05). The [GLU]e remained at low levels during the entire period of varying ADC and DC potential and did not peak until much later after either ischemic insult. This study demonstrates that ADC changes can occur before membrane depolarization and that high [GLU]e has no involvement in the early rapid ADC decrease.

Monitoring glutamate and ascorbate in the extracellular space of brain tissue with electrochemical microsensors

This paper describes electrochemical microsensors for the in vivo measurement of glutamate and ascorbate in the extracellular space of brain tissue. To prepare glutamate microsensors, carbon fiber microelectrodes (10 microns in diameter and 300-400 microns long) were modified with a cross-linked redox polymer film containing enzymes. The microsensors were coated with a thin Nafion film before use. The glutamate microsensors were both selective and sensitive toward glutamate, with detection limits in the low micromolar range. Physiologically relevant concentrations of several electroactive compounds found in brain tissue produced no response at the glutamate microsensors and also did not affect their glutamate response, the only exception being glutamine, for which a small response was observed in the absence, but not in the presence, of glutamate. The ascorbate microsensors were used in conjunction with cyclic voltammetry. They were sensitive and selective toward ascorbate, but did exhibit a small sensitivity toward the dopamine metabolite, dihydroxyphenylacetic acid. The in vivo measurements performed establish the ability of the glutamate microsensors to monitor the component of the basal extracellular glutamate level that is derived from the neuronal activity of brain tissue.