In Vivo Monitoring of Brain Glutamate by Microdialysis Coupled to Capillary Electrophoresis and Laser Induced Fluorescence Detection

Advancements in Brain Research: The In Vivo/In Vitro Electrochemical Detection of Neurochemicals

Neurochemicals, crucial for nervous system function, influence vital bodily processes and their fluctuations are linked to neurodegenerative diseases and mental health conditions. Monitoring these compounds is pivotal, yet the intricate nature of the central nervous system poses challenges. Researchers have devised methods, notably electrochemical sensing with micro-nanoscale electrodes, offering high-resolution monitoring despite low concentrations and rapid changes. Implantable sensors enable precise detection in brain tissues with minimal damage, while microdialysis-coupled platforms allow in vivo sampling and subsequent in vitro analysis, addressing the selectivity issues seen in other methods. While lacking temporal resolution, techniques like HPLC and CE complement electrochemical sensing's selectivity, particularly for structurally similar neurochemicals. This review covers essential neurochemicals and explores miniaturized electrochemical sensors for brain analysis, emphasizing microdialysis integration. It discusses the pros and cons of these techniques, forecasting electrochemical sensing's future in neuroscience research. Overall, this comprehensive review outlines the evolution, strengths, and potential applications of electrochemical sensing in the study of neurochemicals, offering insights into future advancements in the field.

Glutathione sulfinamide serves as a selective, endogenous biomarker for nitroxyl after exposure to therapeutic levels of donors

Nitroxyl (HNO) donors exhibit promising pharmacological characteristics for treatment of cardiovascular disorders, cancer, and alcoholism. However, whether HNO also serves as an endogenous signaling agent is currently unknown, largely because of the inability to selectively and sensitively detect HNO in a cellular environment. Although a number of methods to detect HNO have been developed recently, sensitivity and selectivity against other nitrogen oxides or biological reductants remain problematic. To improve selectivity, the electrophilic nature of HNO has been harnessed to generate modifications of thiols and phosphines that are unique to HNO, especially compared to nitric oxide (NO). Given high bioavailability, glutathione (GSH) is expected to be a major target of HNO. As a result, the putative selective product glutathione sulfinamide (GS(O)NH2) may serve as a high-yield biomarker of HNO production. In this work, the formation of GS(O)NH2 after exposure to HNO donors was investigated. Fluorescent labeling followed by separation and detection using capillary zone electrophoresis with laser-induced fluorescence allowed quantitation of GS(O)NH2 with nanomolar sensitivity, even in the presence of GSH and derivatives. Formation of GS(O)NH2 was found to occur exclusively upon exposure of GSH to HNO donors, thus confirming selectivity. GS(O)NH2 was detected in the lysate of cells treated with low-micromolar concentrations of HNO donors, verifying that this species has sufficient stability to server as a biomarker of HNO. Additionally, the concentration-dependent formation of GS(O)NH2 in cells treated with an HNO donor suggests that the concentration of GS(O)NH2 can be correlated to intracellular levels of HNO.

Contributions of capillary electrophoresis to neuroscience

Capillary electrophoresis (CE) is a small-volume separation approach amenable to the analysis of complex samples for their small molecule, peptide and protein content. A number of the features of CE make it a method of choice for addressing questions related to neurochemistry. The figures of merit inherent to CE that make it well suited for studying cell-to-cell and intracellular signaling include small sample volumes, high separation efficiency, the ability for online analyte concentration, and compatibility with sensitive and high-information content detection methods. A variety of instrumental aspects are detailed, including detection methods and sampling techniques that are particularly useful for the analysis of signaling molecules. Studies that have used these techniques to increase our understanding of neurobiology are emphasized throughout. One notable application is single neuron chemical analysis, a research area that has been greatly advanced by CE.

Glutamate release in the nucleus accumbens is involved in behavioral depression during the PORSOLT swim test

An abnormality in glutamate function has been implicated in the neural substrate of depressive disorders. To investigate this in rats, the Porsolt swim test was used to assess the role of glutamate in the nucleus accumbens. Glutamate injected into the nucleus accumbens dose-dependently decreased swimming time on the test day (day 2), whereas N-methyl-D-aspartate antagonists dizocilpine and 2-amino-5-phosphonovalerate increased swimming, like an antidepressant. Dizocilpine injected before the conditioning trial (day 1) did not modify the swimming times during the first day but abolished behavioral depression on day 2. Microdialysis coupled to capillary-zone electrophoresis was then used to determine in vivo changes in glutamate release in 1-min samples during the swim test. On day 1, glutamate increased significantly and reached a maximum of 222% after 3 min of swimming. On day 2, baseline glutamate levels were back to normal, but when the animal was placed in the water, glutamate increased to 419% during the first minute, and the animals swam significantly less. For comparison, tail pinch on consecutive days was used as a nonspecific, repeated stressor while accumbens glutamate levels were measured. Tail pinch on the first day increased glutamate similar to the effect obtained during the first day of swimming; however, a second day of tail pinch decreased glutamate levels, instead of the potentiated response observed during the second day of swimming. These results show that accumbens glutamate plays a role in causing the behavioral aspects of depressed behavior as modeled in the swim test. The accumbens may be a potential site of action for drugs that alter behavioral depression.

Extracellular glutamate increases in the lateral hypothalamus during meal initiation, and GABA peaks during satiation: microdialysis measurements every 30 s

Glutamate injected into the lateral hypothalamus can initiate eating, and gamma-aminobutyric acid (GABA) can stop it. This leads to the hypothesis that glutamate inputs are active at the beginning of a meal, and GABA is released at the end. To test this theory, the authors used microdialysis to sample glutamate and GABA simultaneously before, during, and after a meal. Food-deprived rats ate a meal of chow. Glutamate increased during the first third of the meal, then decreased to below baseline while the rats were still eating. GABA also increased at the start of the meal but continued rising and peaked during the last third of the meal. Glutamate may drive a hypothalamic system for eating, and GABA may oppose it.

Biomedical applications of capillary electrophoresis with laser-induced fluorescence detection

Capillary electrophoresis (CE) is a high-efficiency analytical technique that has had a great impact as a tool in biomedical research, clinical and forensic practice in the last ten years. Only in one of the applications, the DNA analysis, it has had an explosive exponential growth in the last few years. This impact is expressed in an enormous amount of CE articles and many reviews. The CE advantages with respect to other analytical techniques: the required very small sample volume, rapid analysis, great resolution power and low costs, have made this technique ideal for the analysis of a numerous endogenous and exogenous substances present in biological fluids. The different modes of CE have been coupled to different detection techniques such as UV-absorbance, electrochemical, mass spectrometry and laser-induced fluorescence detection (LIFD) to detect different nature and molecular size separated analytes. This review focuses mostly on the applications of CE-LIFD, to measure drugs and endogenous neuroactive substances such as amino acids and monoamines, especially in microdialysis samples from experimental animals and humans. CE-LIFD trends are discussed: automated faster analysis with capillary array systems, resolution power improvement, higher detection sensitivity, and CE systems miniaturization for extremely small sample volume, in order to make CE easier and affordable to the lab bench or the clinical bed.

Coupling continuous separation techniques to capillary electrophoresis

One of the weak points of capillary electrophoresis is the need to implement rigorously sample pretreatment because its great impact on the quality of the qualitative and quantitative results provided. One of the approaches to solve this problem is through the symbiosis of automatic continuous flow systems (CFSs) and capillary electrophoresis (CE). In this review a systematic approach to CFS-CE coupling is presented and discussed. The design of the corresponding interface depends on three factors, namely: (a) the characteristics of the CFS involved which can be non-chromatographic and chromatographic; (b) the type of CE equipment: laboratory-made or commercially available; and (c) the type of connection which can be in-line (on-capillary), on-line or mixed off/on-line. These are the basic criteria to qualify the hyphenation of CFS (solid-phase extraction, dialysis, gas diffusion, evaporation, direct leaching) with CE described so far and applied to determine a variety of analytes in many different types of samples. A critical discussion allows one to demonstrate that this symbiosis is an important topic in research and development, besides separation and detection, to consolidate CE as a routine analytical tool.

Application of capillary electrophoresis with laser-induced fluorescence detection to the determination of biogenic amines and amino acids in brain microdialysate and homogenate samples

A procedure is described to derivatize 16 primary-amine-containing biogenic amines and amino acids in brain mixtures with the fluorogenic reagent 5-furoylquinoline-3-carboxaldehyde (FQ). These FQ-tagged compounds in the brain sample were resolved in less than 16 min based on micellar electrokinetic chromatography and laser-induced fluorescence. There was a linear relationship between the concentration of analyte and the fluorescence intensity, with correlation coefficients in the range of 0.96-1.00. The utility of this method for the quantification of the important inhibitory neurotransmitter gamma-aminobutyric acid in microdialysates and brain homogenates from rats is illustrated.

Medial prefrontal transection enhances social interaction. II: neurochemical studies

Medial prefrontal cortex (MPFC) transection enhances social interaction in an open arena test. Social interaction enhances dopaminergic activity in the nucleus accumbens (NAC). In the present set of experiments, microdialysis probes were implanted in the NAC, and glutamate, gamma-aminobutyric acid (GABA) and dopamine (DA) were measured during electrical stimulation of the MPFC, after coronal transection caudal to the MPFC and after a systemic injection of amphetamine in transected rats. Electrical stimulation of the MPFC caused a transient enhancement of glutamate release in the NAC, no change in GABA levels and a long lasting increase in DA levels. Medial prefrontal transection did not change basal glutamate or GABA levels in the NAC, but increased basal DA levels. Amphetamine administration decreased GABA levels in medial prefrontal transected rats, had no effect on glutamate and increased DA levels more than in controls. The experiments suggest that glutamatergic activity in the accumbens decreases dopamine release. Medial prefrontal transection reduces glutamatergic tone and enhances dopamine release, which probably decreases GABAergic activity in the NAC. Presumably, GABA inhibition in the NAC enhances social interaction.

Effects of feeding on extracellular levels of glutamate in the medial and lateral portion of the globus pallidus of freely moving rats

The globus pallidus (GP) is considered to be part of the basal ganglia and previous research has determined that it might be involved in feeding behavior. For example, it has been shown that the GP becomes active during feeding and that disinhibition of this nucleus, by locally injecting a GABA antagonist, is sufficient to induce feeding in a satiated rat. However, few studies have measured extracellular levels of glutamate during free feeding in the GP of rats. For this reason brain microdialysis coupled to capillary electrophoresis with laser-induced fluorescence was used to determine FTC-glutamate levels, either in the medial or lateral portion of the GP, of freely moving rats. Retrograde labeling of the neurons projecting to the two areas was also examined in an attempt to gain some insight on the identity of the neurons that released glutamate in the GP during feeding. Extracellular levels of glutamate-FTC differentially increased in both portions of the GP during a 2-min interval of free feeding. Retrograde labeling also showed that both areas received projections from different brain areas suggesting that each of the GP portions could be involved in separate aspects of the feeding behavior.

Neutral amino acids monitoring in phenylketonuric plasma microdialysates using micellar electrokinetic chromatography and laser-induced fluorescence detection

Neutral and non-polar amino acids such as phenylalanine (Phe), valine (Val), tyrosine (Tyr), threonine (Thre) and GABA are hard to resolve by capillary zone electrophoresis (CZE). Their separation is possible by adding a surfactant to the mobile phase. This method is called micellar electrokinetic chromatography (MEKC). We used MEKC with laser-induced fluorescence detection (LIFD) to separate and quantitate these amino acids in plasma microdialysates of patients with phenylketonuria (PKU). This disease is an inborn enzymatic defect with decreased conversion of Phe to Tyr that causes severe neurological damage and mental deterioration, which is diagnosed by measuring plasma Phe and Phe/Tyr ratio. The amino acids tested had linear concentration-signal relation. PKU patients had significantly higher Phe, lower Tyr, 21 times higher Phe/Tyr ratio and decreased values of Val and Thre than controls. These results show that microdialysis of biological fluids coupled with MEKC-LIFD is a convenient technique to measure neutral amino acids in clinical disorders such as PKU.

Monitoring gamma-aminobutyric acid in human brain and plasma microdialysates using micellar electrokinetic chromatography and laser-induced fluorescence detection

Due to its low electrophoretic mobility, few studies have been able to measure gamma aminobutyric acid (GABA) in biological samples by means of capillary zone electrophoresis. Nevertheless, in micellar electrokinetic chromatography (MEKC) by adding a surfactant to the mobile phase separation can be carried out on the basis of the partition coefficient of the molecules rather than their electrophoretic mobility. In the present study microdialysis coupled to MEKC with laser induced fluorescence detection was used to successfully monitor GABA from cerebrospinal fluid and plasma dialysates. Moreover, we monitored changes in extracellular GABA from a human brain. Microdialysis samples were collected from a Parkinson's disease patient undergoing a thallamotomy as part of her treatment. Significant decreases in extracellular GABA were detected during high frequency electrical stimulation and following a thermolesion of the thalamus. These results demonstrate the feasibility of MEKC coupled to laser-induced fluorescence detection in resolving neutral amino acids, specifically GABA, from different human body fluids.

Coupling of biological sample handling and capillary electrophoresis

The analysis of biological samples (e.g., blood, urine, saliva, tissue homogenates) by capillary electrophoresis (CE) requires efficient sample preparation (i.e., concentration and clean-up) procedures to remove interfering solutes (endogenous/exogenous and/or low-/high-molecular-mass), (in)organic salts and particulate matter. The sample preparation modules can be coupled with CE either off-line (manual), at-line (robotic interface), on-line (coupling via a transfer line) or in-line (complete integration between sample preparation and separation system). Sample preparation systems reported in the literature are based on chromatographic, electrophoretic or membrane-based procedures. The combination of automated sample preparation and CE is especially useful if complex samples have to be analyzed and helps to improve both selectivity and sensitivity. In this review, the different modes of solid-phase (micro-) extraction will be discussed and an overview of the potential of chromatographic, electrophoretic (e.g., isotachophoresis, sample stacking) and membrane-based procedures will be given.

Determination of submicromolar concentrations of neurotransmitter amino acids by fluorescence detection using a modification of the 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate method for amino acid analysis

A sensitive method for quantitatively determining submicromolar levels of neurotransmitter amino acids (e.g. Asp, Glu and gamma-aminobutyric acid) in microdialysates from brain and cerebrospinal fluids is reported. 6-Aminoquinolyl-N-hydroxy-succinimidyl carbamate (AQC) was employed as the derivatization reagent, followed by HPLC separation and fluorescence detection of the derivatives. The derivatization was conducted simply by mixing the AQC directly with the microdialysis samples. The reaction was complete within seconds after mixing at room temperature. Separation development optimizing the gradient profile, eluent pH and column temperature resulted in an excellent separation of the required amino acids in less than 30 min. Other resolved amino acids in the same profile include Gly, taurine, and Pro. Recoveries for the amino acids of interest spiked into high salt containing perfusion buffers were greater than 97%. The sensitivity of the method was increased by employing a 16-microliter flow cell in the detector and analyzing 20-microliter aliquots of the derivatization mixtures. With the optimized conditions, the detection limits were 3-7 nM (fmol/microliter). Typical reproducibility (%R.S.D.) for quantitation of these amino acids at submicromolar levels was approximately 2%. Excellent linearity (r2 > 0.999) was achieved over the range 0.2-20 microM. The low detection limits permitted the analysis of a number of different microdialysate samples including those from cerebrospinal fluid, as well as substantia nigra and hypothalamus from brain samples, even at basal levels where gamma-aminobutyric acid concentration may be < 50 nM. The excellent sensitivity made it easy to distinguish basal from stimulated levels of neurotransmitter amino acids, even from sample sizes as small as 10 microliters.

Role of glutamate in the amygdala and lateral hypothalamus in conditioned taste aversion

The role of glutamate in conditioned taste aversion was investigated. Both, in the amygdala (AMYG) and in the lateral hypothalamus (LH) extracellular levels of glutamate were assessed by microdialysis and capillary electrophoresis with laser induced fluorescence detection. Rats were conditioned by pairing a novel flavor (strawberry flavor) with an intraperitoneal injection of lithium chloride. When the conditioned stimulus (strawberry flavored solution) was injected into the mouth of conditioned rats, there was an increase of glutamate release in the AMYG, and a decrease in glutamate release in the LH. These results predicted that glutamate release in the AMYG and the LH was involved in CTA. This possibility was tested by MK-801 (glutamate antagonist) and glutamate microinjections. MK-801 injections in AMYG attenuated the rejection of the novel flavor, and in the LH did not cause any effect on CTA. Glutamate microinjections in the AMYG caused CTA. These results suggest that glutamatergic activity in the AMYG might be a relevant neurochemical correlate and cause of conditioned taste aversion.

Determination of the amino acid neurotransmitters in the dorsal root ganglion of the rat by capillary electrophoresis with a laser-induced fluorescence-charge coupled device

A sensitive, rapid and accurate analysis of the amino acid neurotransmitters in dorsal root ganglion (DRG) of rat was accomplished by capillary electrophoresis with a laser-induced fluorescence-charged coupled device (LIF-CCD) and fluorescein isothiocyanate (FITC) derivatization. Since the CCD is an image detector with a matrix array of photoelements, the photogenerated charge generated from single photoelements was summed up by on-chip charge binning, and used to improve the detection sensitivity. A self-compiled data processing program was used to calculate digitally the summation of the multiwavelength data and transform three-dimensional electropherograms into two-dimensional electropherograms. Use of this program improved the detection sensitivity, the extraction of useful statistical data and the performance of the quantitative analysis. Owing to its excellent detectability and the unique characteristic of its two-dimensional format, which allows to perform analog and digital summation, high detection sensitivity with detection limits ranging from 1.2 to 17.2 nM was obtained for the amino acid neurotransmitters by LIF-CCD. This method was demonstrated to be a powerful tool for analyzing the complex biological samples through quantitative determination of the six amino acid neurotransmitters in the DRG of rat.

High temporal resolution monitoring of glutamate and aspartate in vivo using microdialysis on-line with capillary electrophoresis with laser-induced fluorescence detection

An automated method for high temporal resolution monitoring of the neurotransmitters glutamate and aspartate in vivo using capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection was developed. Microdialysis probes placed in the striatum of anesthetized rats were coupled on-line with the CE system by an automated flow-gated interface. Analytes were derivatized on-line with o-phthaldialdehyde/beta-mercaptoethanol and detected by LIF using the 354 nm line (7 mW) of a He-Cd laser for excitation. With dialysis flow rates of 1.2 microL/ min, the detection limit at the dialysis probe was 200 nM for glutamate and aspartate. Glutamate and aspartate could be resolved in less than 5 s with over 200,000 theoretical plates. The sampling time was limited by the separation time while the temporal resolution was limited to approximately 12 s because of band broadening that occurs within the probe and its associated tubing. The high temporal resolution allowed the first simultaneous monitoring of glutamate and aspartate overflow during acute electrical stimulation in the rat brain.

Critical review of recent developments in fluorescence detection for capillary electrophoresis

Recent developments in capillary electrophoresis with fluorescence detection are reviewed. Instrumental advances have led to increased sensitivity and, therefore, a growing number of applications. Capillary electrophoresis has been coupled with various techniques to achieve multi-dimensional separations. Other advances have focused on temporal resolution when sampling from biological environments, increased sample throughput especially for DNA analysis, and fast separation times. New technologies including chip and channel electrophoretic separations with fluorescence detection are also discussed.

Capillary electrophoresis and microdialysis: current technology and applications

Microdialysis sampling has become an important method for the continuous monitoring from an in vivo environment. This technique has been used to monitor many endogenous molecules, such as neurotransmitters, as well as exogenous species such as drug substances. Microdialysis samples have traditionally been analyzed by liquid chromatographic (LC) methods to gain resolution and quantification of the molecules of interest. However, LC separations have a relatively large injection volume requirement which, as a consequence, increases microdialysis sampling times. Capillary electrophoresis (CE), with its very small sample volume requirements and high resolving power, has therefore gained popularity as an alternative to LC. Reviewed here are many of the technologies currently available for CE and examples of how this technique has been effectively applied to the analysis of microdialysis samples.

Extracellular glutamate increases in the lateral hypothalamus and decreases in the nucleus accumbens during feeding

Glutamate release was monitored in the lateral hypothalamus and the nucleus accumbens during a meal using 30 s resolution microdialysis and capillary zone electrophoresis with laser-induced fluorescence. A significant increase in hypothalamic glutamate and a decrease in accumbens glutamate were observed. These results, added to previous pharmacological studies, suggest that glutamatergic synapses in the lateral hypothalamus and the nucleus accumbens might be involved in the control of feeding behavior.

New approaches in clinical chemistry: on-line analyte concentration and microreaction capillary electrophoresis for the determination of drugs, metabolic intermediates, and biopolymers in biological fluids

The use of capillary electrophoresis (CE) for clinically relevant assays is attractive since it often presents many advantages over contemporary methods. The small-diameter tubing that holds the separation medium has led to the development of multicapillary instruments, and simultaneous sample analysis. Furthermore, CE is compatible with a wide range of detectors, including UV-Vis, fluorescence, laser-induced fluorescence, electrochemistry, mass spectrometry, radiometric, and more recently nuclear magnetic resonance, and laser-induced circular dichroism systems. Selection of an appropriate detector can yield highly specific analyte detection with good mass sensitivity. Another attractive feature of CE is the low consumption of sample and reagents. However, it is paradoxical that this advantage also leads to severe limitation, namely poor concentration sensitivity. Often high analyte concentrations are required in order to have injection of sufficient material for detection. In this regard, a series of devices that are broadly termed 'analyte concentrators' have been developed for analyte preconcentration on-line with the CE capillary. These devices have been used primarily for non-specific analyte preconcentration using packing material of the C18 type. Alternatively, the use of very specific antibody-containing cartridges and enzyme-immobilized microreactors have been demonstrated. In the current report, we review the likely impact of the technology of capillary electrophoresis and the role of the CE analyte concentrator-microreactor on the analysis of biomolecules, present on complex matrices, in a clinical laboratory. Specific examples of the direct analysis of physiologically-derived fluids and microdialysates are presented, and a personal view of the future of CE in the clinical environment is given.

Improved method for the measurement of glutamate and aspartate using capillary electrophoresis with laser induced fluorescence detection and its application to brain microdialysis

We have previously published data on the analysis of glutamate in microdialysis samples using a commercially available CE apparatus. Here we demonstrate further improvements in the analysis of both glutamate and aspartate from very small volume microdialysates. The limit of detection of our system has been increased to 10(-9) M for both glutamate and aspartate. This permits microdialysis sampling time to be reduced to 2 min, thus improving the temporal resolution of microdialysis sampling. Concurrently, migration time has also been reduced such that resolution of both amino acids can be achieved inside 2 min. This new analytical method has been applied to the measurement of the EAA from microdialysis samples from the dentate gyrus of the hippocampus. Extracellular concentrations of both glutamate and aspartate increased to a maximum of 5- and 4.5-fold of preinfusion values, respectively, during infusion of 100 mM K+ through the microdialysis probe. This is consistent with the depolarization-evoked release of both amino acids from this brain region.

Glutamate measured by 6-s resolution brain microdialysis: capillary electrophoretic and laser-induced fluorescence detection application

In the present experiment the combination of brain microdialysis and CZE-LIFD permitted the measurement of glutamate in 100 nl microdialysis samples collected every 5 or 6 s. Samples were collected every 6 s, in rats anesthetized with two different anesthetic agents (ketamine and sodium thiopental). A microdialysis probe was inserted in the cortex of an anesthetized rat in the territory irrigated by the middle cerebral artery. The artery was clamped for 30 s and then released. The samples were derivatized with fluorescein isothiocyanate I (FITC) by means of a continuous-flow reactor, collected and injected into a home-made CZE-LIFD instrument. Glutamate decreased immediately after clamping the artery in ketamine anesthetized rats and increased 1 min after the onset of the ischemia in sodium thiopental anesthetized rats. In another experiment a 60 mM KCl solution was injected through a microdialysis probe inserted in the hippocampus of an anesthetized rat. In the first 5 s after the KCl solution reached the tissue, glutamate increased but gamma-aminobutytic acid and glutamine did not. The experiments show that time resolution of brain microdialysis can be reduced to a few seconds if the analytical technique is the proper one.

Microdialysis monitoring of extracellular glutamate combined with the simultaneous recording of evoked field potentials in hippocampal organotypic slice cultures

These experiments combined extracellular electrophysiological multirecordings from hippocampal organotypic slice cultures with application of drugs to and sampling of extracellular fluid from a restricted region of the slice using a microdialysis probe. Glutamate (Glu) concentrations were monitored in 0.5 or 2 min microdialysis samples, while evoked field potentials responses (EvFPR) in the CA1 region of the hippocampus (stimulation in the CA3 area) were simultaneously recorded using a multi-electrodes array (Physiocard). Glu was assayed by capillary electrophoresis with laser-induced fluorescence detection combined with a continuous flow derivatization of dialysates. The performance of this combined approach was demonstrated by monitoring extracellular Glu concentrations and EvFPR after K+ induced depolarisation, Glu uptake blockade by trans-pyrrolidine-2,4-dicarboxylic acid (PDC), and electrical stimulation. Such an approach allows a global monitoring of the neuronal functioning with a fine time resolution (up to 30 s) on a simple in vitro brain slice model, to be used as a complement to conventional in vivo microdialysis studies.

In vivo monitoring of extracellular noradrenaline and glutamate from rat brain cortex with 2-min microdialysis sampling using capillary electrophoresis with laser-induced fluorescence detection

The measurement of neurotransmitters by capillary electrophoresis (CE) has emerged as a reliable and sensitive method for microdialysis sample analysis. This paper describes a method which employs laser-induced fluorescence detection (LIFD) of catecholamines and excitatory amino acid derivatives formed after reaction with naphthalene-2,3-dicarboxaldehyde. On-line derivatization of very small volumes of microdialysis samples (500 nl) is developed before two off-line analyses (total run time of less than 10 min) are performed to detect derivatives of catecholamines and excitatory amino acids formed in each sample. High microdialysis temporal resolution is reached (2-min fractions) for the simultaneous monitoring of noradrenaline (NA) and glutamate concentrations from rat brain cortex microdialysates. The system performance is evaluated and pharmacological characterization of the determination of NA in cortical dialysates by CE-LIFD is reported.

Enhanced temporal resolution for the microdialysis monitoring of catecholamines and excitatory amino acids using capillary electrophoresis with laser-induced fluorescence detection. Analytical developments and in vitro validations

This paper reports the development of a method based on capillary electrophoresis with laser-induced fluorescence detection for the simultaneous determination of catecholamines and excitatory amino acids on submicroliter microdialysis samples, with short analysis times (3 min or less), high sensitivity (nanomolar range, i.e., attomoles detected) and high separation efficiency (up to 1.10(6) theoretical plates). A continuous flow derivatization of small volumes of microdialysate (500 nl) using naphthalene-2,3-dicarboxaldehyde as derivatizing reagent is described. Thereafter, two subsequent off-line analyses are performed on each of the 30-s dialysates to determine catecholamines and amino acids. The performances of the present method are demonstrated in vitro by monitoring rapid fluctuations in the concentration of catecholamines and amino acids in the external microdialysis medium.

Microdialysis monitoring of variations in extracellular levels of serotonin, GABA and excitatory amino acids in the frontal cortex of awake rats in response to a single peripheral or central administration of dexfenfluramine

The effects of a single dexfenfluramine (D-fen) administration on the release of endogenous serotonin (5-hydroxytryptamine, 5-HT), excitatory (glutamate, Glu, aspartate, Asp) and inhibitory (gamma-aminobutyric acid, GABA) amino acids from the frontal cortex were studied by using in vivo microdialysis in freely-moving rats. Extracellular levels of these neurotransmitters were measured with HPLC coupled to electrochemical detection or with capillary electrophoresis coupled to laser-induced fluoresence detection (CE-LIFD). In a first study, single intraperitoneal administration of D-fen (0.5, 1.3, 5 and 10 mg/kg) increased extracellular 5-HT levels in a dose-dependent manner (maximal increase by 982% over baseline for the highest dose) while changes in Glu, Asp or GABA never reached statistical significance. In a second study, 73 nM of D-fen applied locally through the frontocortical dialysis probe, at a flow rate of 1.5 microliters/min in 30 microliters of perfusion fluid for 20 min, increased extracellular 5-HT and Asp levels [the maximal increases were to 1804% and 280% of the respective basal values (100%)] without altering extracellular levels of Glu and GABA. Thus, the order of magnitude of the changes induced by systemic administration or local infusion of D-fen on frontocortical extracellular levels of several neurotransmitters (5-HT > > Asp > GABA = Glu) demonstrate that D-fen, an indirect serotoninergic agonist, mainly increases 5-HT release while producing slight (Asp) or no (Glu, GABA) short-term in vivo variations in amino acid extracellular levels in the rat frontal cortex.

Quantitative in vivo monitoring of primary amines in rat caudate nucleus using microdialysis coupled by a flow-gated interface to capillary electrophoresis with laser-induced fluorescence detection

A method for monitoring primary amines in vivo using microdialysis coupled on-line with capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC) with laser-induced fluorescence detection was explored. Dialysates were derivatized on-line with o-phthaldialdehyde/beta-mercaptoethanol and automatically transferred to a separation capillary by a flow-gated interface. Analytes were detected on-column using the 2 mW, 354 nm line of a He-Cd laser for excitation. Dialysis probes were perfused at 79 nL/min, resulting in relative recoveries of nearly 100%, which allowed quantitative monitoring. On-line detection limits were in the 20-50 nM range, and the response was linear up to 50 microM. Temporal resolution was between 45 s and 3 min and was limited by separation time or broadening of sample zones during transfer to the separation capillary, depending on the operational parameters. The system was applied to measurement of primary amines in the caudate nucleus of anesthetized rats. Using CZE for separation, it was possible to resolve and monitor several compounds, including aspartate and glutamate. The measured basal concentrations of aspartate and glutamate were 1.2 +/- 0.1 and 5.0 +/- 0.4 microM, respectively, which agrees well with literature values. Increases in in vivo aspartate and glutamate were monitored with 90 s temporal resolution during K+ depolarization using dialysis flow rates of 79 nL/min; however, temporal resolution of 45 s was possible at the expense of lower relative recovery if the dialysis flow rate was increased to 155 nL/min. The use of MEKC as the separation mode significantly increased the number of compounds that could be resolved and detected. Using MEKC to separate the dialysate samples allowed aspartate, glutamate, isoleucine, leucine, lysine, methionine, phenylalanine, taurine, tyrosine, and valine to be resolved and detected. The basal concentrations for these compounds using MEKC were 1.9 +/- 0.2, 4.1 +/- 0.2, 4.6 +/- 0.7, 2.6 +/- 0.3, 5.4 +/- 0.4, 1.8 +/- 0.2, 2.0 +/- 0.2, 11.3 +/- 1.3, 3.3 +/- 0.9, and 5.3 +/- 0.3 microM, respectively. The concentrations of these primary amines in the striatum were monitored after K+ depolarization with 3 min temporal resolution. This is the first microdialysis system to generate high relative recoveries and good temporal resolution simultaneously for multiple neurotransmitters.

Capillary electrophoresis with laser-induced fluorescence detection: a sensitive method for monitoring extracellular concentrations of amino acids in the periaqueductal grey matter

The use of capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) for the analysis of microdialysate samples from the periaqueductal grey matter (PAG) of freely moving rats is described. By employing 3-(4-carboxybenzoyl)-2-quinoline-carboxaldehyde (CBQCA) as a derivatization agent, we simultaneously monitored the concentrations of 8 amino acids (arginine, glutamine, valine, gamma-amino-n-butyric acid (GABA), alanine, glycine, glutamate, and aspartate), with nanomolar and subnanomolar detection limits. Two of the amino acids (GABA and glutamate) were analysed in parallel by conventional high-performance liquid chromatography (HPLC) in order to directly compare the two analytical methods. Other CE methods for analysis of microdialysate have been previously described, and this improved method offers greater sensitivity, ease of use, and the possibility to monitor several amino acids simultaneously. By using this technique together with an optimised form of microdialysis technique, the tiny sample consumption and the improved detection limits permit the detection of fast and transient transmitter changes.

High-speed separation of subnanomolar concentrations of noradrenaline and dopamine using capillary zone electrophoresis with laser-induced fluorescence detection

Capillary zone electrophoresis with laser-induced fluorescence detection has been shown to give rapid separations with high resolution and sensitivity. These advantages are documented for catecholamines analysis in the present work, which shows that separation of subnanomolar concentrations of dopamine and noradrenaline (detection limits of 8.6 x 10(-11) M, corresponding to a detected amount of 143 zeptomoles of each catecholamine) can be performed in 82 s with an efficiency of several million theoretical plates.

Physiological release of excitatory amino acids

The contribution of in vivo monitoring to the study of glutamate release is reviewed. Physiological stimulation increases both glutamate and aspartate in the extracellular compartment of the brain and both amino acids show Ca(2+)-dependent K(+)-evoked release. However, the finding that only glutamate is stored in synaptic vesicles implies that glutamate is the excitatory transmitter. Released glutamate is taken up into both neurones and glia by glutamate transporters. Uptake of glutamate, in addition to clearing the synapse, has a number of additional functions. Uptake into glia leads to the release of glutamine, which is involved in the recycling of transmitter glutamate; uptake into both neurones and glia leads to the release of ascorbate; uptake into glia leads to an increase glycolysis and export of lactate, an energy substrate for neuronal metabolism. Reversal of the glutamate transporter accounts for the parallel release of glutamate and aspartate from the cytoplasmic compartment. The basal concentration of extracellular glutamate is in the micromolar range. Such levels could lead to desensitisation of both NMDA and non-NMDA receptors. The functional implications of the level of basal glutamate are difficult to assess at present in view of the existence of multiple glutamate receptor subunits with different functional properties and distributions.

In vivo monitoring of glutamate in the brain by microdialysis and capillary electrophoresis with laser-induced fluorescence detection

Glutamic acid, an excitatory neurotransmitter, was monitored in vivo in the corpus striatum of freely moving rats by brain microdialysis and capillary electrophoresis with laser-induced fluorescence detection. A procedure to derivatize glutamate in complex matrices was developed. Capillary electrophoresis in 12 microns I.D. capillaries was performed to determine glutamate with a migration time of 195 s. Laser-induced fluorescence detection with 488-nm radiation from an argon ion laser and with colinear geometry was used. An injection of haloperidol decreased the concentration of glutamic acid in the dialysates. These experiments support the hypothesis that dopamine receptor blockade decreases glutamate release. The potential of these techniques for the study of chemicals in biomedical experiments is discussed.