Neurochemical applications of liquid chromatography with electrochemical detection

Pattern recognition of neurotransmitters using multimode sensing

BACKGROUND

Pattern recognition is essential in chemical analysis of biological fluids. Reliable and sensitive methods for neurotransmitters analysis are needed.

NEW METHOD

Therefore, we developed for pattern recognition of neurotransmitters: dopamine, epinephrine, norepinephrine a method based on multimode sensing. Multimode sensing was performed using microsensors based on diamond paste modified with 5,10,15,20-tetraphenyl-21H,23H-porphyrine, hemin and protoporphyrin IX in stochastic and differential pulse voltammetry modes.

RESULTS

Optimized working conditions: phosphate buffer solution of pH 3.01 and KCl 0.1mol/L (as electrolyte support), were determined using cyclic voltammetry and used in all measurements. The lowest limits of quantification were: 10(-10)mol/L for dopamine and epinephrine, and 10(-11)mol/L for norepinephrine. The multimode microsensors were selective over ascorbic and uric acids and the method facilitated reliable assay of neurotransmitters in urine samples, and therefore, the pattern recognition showed high reliability (RSD<1% for more than 6 months) for the simultaneous determination of dopamine, epinephrine and norepinephrine from urine and whole blood samples.

COMPARISON WITH EXISTING METHOD(S)

The proposed method can perform pattern recognition of the three neurotransmitters on biological fluids at a lower determination level than chromatographic methods. The sampling of the biological fluids referees only to the buffering (1:1, v/v) with a phosphate buffer pH 3.01, while for chromatographic methods the sampling is laborious.

CONCLUSIONS

Accordingly with the statistic evaluation of the results at 99.00% confidence level, both modes can be used for pattern recognition and quantification of neurotransmitters with high reliability. The best multimode microsensor was the one based on diamond paste modified with protoporphyrin IX.

Diethylation labeling combined with UPLC/MS/MS for simultaneous determination of a panel of monoamine neurotransmitters in rat prefrontal cortex microdialysates

The primary challenge associated with the development of an LC/MS/MS-based assay for simultaneous determination of biogenic monoamine neurotransmitters such as norepinephrine (NE), dopamine (DA), serotonin (5-HT), and normetanephrine (NM) in rat brain microdialysates is to improve detection sensitivity. In this work, a UPLC/ MS/MS-based method combined with a diethyl labeling technique was developed for simultaneous determination of a panel of monoamines in rat prefrontal cortex microdialysates. The chromatographic run time is 3.5 min/ sample. The limits of detection of the UPLC/MS/MS-based method for NE, DA, 5-HT/ and NM, with/without diethyl labeling of monoamines, are 0.005/0.4 (30/2367 pM), 0.005/0.1 (33/653 pM), 0.005/0.2 (28/1136 pM), and 0.002/0.2 ng/mL (11/1092 pM), respectively. Diethyl labeling of amino groups of monoamines affords 20-100 times increased detection sensitivity of corresponding native monoamines during the UPLC/MS/MS analysis. This could result from the following: (1) improved fragmentation patterns; (2) increased hydrophobicity and concomitantly increased ionization efficiency in ESI MS and MS/MS analysis; (3) reduced matrix interference. This labeling reaction employs a commercially available reagent, acetaldehyde-d4, to label the amine groups on the monoamines via reductive amination. It is also simple, fast (approximately 25-min reaction time), specific, and quantitative under mild reaction conditions. Data are also presented from the application of this assay to monitor the drug-induced changes of monoamine concentrations in rat prefrontal cortex microdialysate samples followed by administration of SKF 81297, a selective D1 dopamine receptor agonist known to elevate the extracellular level of the neurotransmitters DA and NE in the central nervous system.

Online extraction of 5-hydroxyindole acetic acid from urine for analysis by liquid chromatography–tandem mass spectrometry

Background

Urinary 5-hydroxyindole acetic acid (5-HIAA) is a useful marker for the turnover of tryptophan metabolites in the diagnosis and monitoring of carcinoid tumours and the carcinoid syndrome. We have developed a simple and cost-effective assay for urinary 5-HIAA using liquid chromatography–tandem mass spectrometry (LC–MS/MS) incorporating an online sample clean-up process to replace a liquid chromatography electrochemical (LC-EC) technique.

Methods

Acidified urine was serially diluted in ammonium acetate buffer followed by ammonium acetate buffer enriched with 5-hydroxyindole-3-acetic-2,2-D2 acid internal standard. A 2.1 × 10 mm C18 column was used for primary online clean-up and eluted with 100% methanolic mobile phase onto a second dC18 Atlantis 2.1 × 20 mm column. Analytes were detected by mass spectrometry using transitions 192.1 > 146.3 and 194.1 > 148.0 for 5-HIAA and deuterated analyte, respectively.

Results

Run time was 3 min with 5-HIAA eluting at 1.37 min. The inter and intra-assay imprecision and accuracies of the three levels of inhouse quality control (QC) (30, 300 and 600 μmol/L) were acceptable with coefficient of variations (CVs) and deviation from target values <12% ( n = 15). The average recovery of 5-HIAA spiked into urine was 93.7% with no ion suppression observed. The limit of detection was 2.8 and lower limit of quantification 4.0 μmol/L. Passing–Bablok regression of LC-EC with LC–MS/MS results showed good agreement between the methods, the relationship described as LC–MS/MS = 1.01(LC–EC)–1.22. No systematic or proportional biases were observed over the working range of the method. The assay was linear to at least 2000 μmol/L.

Conclusions

We have developed a robust method offering a more than six-fold improvement in linearity compared to the existing LC–EC method.

Recent advances in methods for the analysis of catecholamines and their metabolites

Catecholamines, for example epinephrine, norepinephrine, and dopamine, are widely distributed and are important neurotransmitters and hormones in mammalian species. Several methods have been developed for analysis of catecholamines and related compounds. Determination of catecholamines in biological fluids has enabled us to clarify the physiological role played by these amines. Catecholamine levels in plasma and/or urine are also useful for diagnosis of several diseases, for example hypertension, pheochromocytoma, and neuroblastoma. This review covers reports from 2000 to the present of methods for the analysis of catecholamines and their metabolites.

Simultaneous determination of biogenic monoamines in rat brain dialysates using capillary high-performance liquid chromatography with photoluminescence following electron transfer

Simultaneous determination of biogenic monoamines such as dopamine, serotonin, and 3-methoxytyramine in brain is important in understanding neurotransmitter activity. This study presents a sensitive determination of biogenic monoamines in rat brain striatum microdialysates using capillary high-performance liquid chromatography with the photoluminescence following electron-transfer detection technique. Separation conditions were optimized by changing the concentration of an ion-interaction agent and the percentage of an organic modifier. The high concentration of ion-interaction agent enabled the amines as a class to be separated from interfering acids, but also made the separation very long. To shorten the separation time, 10% (v/v) acetonitrile was used as the organic modifier. Eight chromatographic runs during a 3-h period were analyzed in terms of retention times, peak heights, and peak widths. Chromatograms are very reproducible, with less than 1% changes in peak height over 3 h. Typical concentration detection limits at the optimum separation conditions were less than 100 pM for metabolic acids and approximately 200 pM for monoamines. The injection volume of the sample was 500 nL. Thus, the mass detection limits were less than 50 amol for metabolic acids and approximately 100 amol for monoamines. Typical separation time was less than 10 min. To validate the technique, the separation method was applied to the observation of drug-induced changes of monoamine concentrations in rat brain microdialysis samples. Local perfusion of tetrodotoxin, a sodium channel blocker, into the striatum of an anesthetized rat decreased dopamine, 3-methoxytyramine, and serotonin concentrations in dialysates. Successive monitoring of striatal dialysates at a temporal resolution of 7.7 min showed that the injection of nomifensine transiently increased dopamine and 3-methoxytyramine concentrations in rat brain dialysate.

Quantitation of tryptophan, kynurenine and kynurenic acid in human plasma by capillary liquid chromatography-electrospray ionization tandem mass spectrometry

Concentrations of tryptophan and its metabolites in plasma are of great interest in determining proper diagnosis and medication of several neurological diseases like, for example, Alzheimer's disease. A method of standard addition was developed to determine total level of tryptophan and two of its metabolites, kynurenine and kynurenic acid, in human plasma by capillary liquid chromatography-electrospray ionization tandem mass spectrometry. Plasma samples were simply deproteinized by addition of diluted perchloric acid. Samples were then mixed with trichloroacetic acid and injected onto a capillary column. Analytes were separated by a fast gradient elution of the injected samples. Detection was performed by sheathless electrospray tandem mass spectrometry in the multiple reaction monitoring mode. Linear calibration curves were obtained for spiked plasma sample with up to 100% of the expected analytes concentrations. The determined concentrations were well within ranges previously reported (i.e., 6 nM-95 microM) and limit of detections were around 3 nM for each analyte.

Psychoactive tryptamines from basidiomycetes

The review lists natural sources, i.e. strains and species of fungi producing predominantly psychoactive tryptamines (indolealkylamines), their chemical structure and properties, toxic effects on the man and psychic symptoms of intoxication. It describes the biosynthesis and production of some tryptamines by the mycelial culture of Psilocybe bohemica Sebek, a survey of methods for their analysis and isolation. It evaluates the worldwide use and abuse of psychoactive fungi as sources of drugs in general and in the Czechia in particular during the last two and a half decades.

Clinical chemistry of serotonin and metabolites

Analyses of serotonin and other 5-hydroxyindoles, such as its precursor 5-hydroxytryptophan and major metabolite 5-hydroxyindoleacetic acid (5-HIAA), are indispensable for the elucidation of their (patho)physiological roles. In clinical chemistry attention is mainly focused on the diagnosis and follow-up of carcinoid tumours. For this most laboratories routinely measure urinary 5-HIAA. More recently, measurements of serotonin in platelets and urine have been advocated. Platelet serotonin may be the most sensitive indole marker for the detection of carcinoid tumours that secrete only small amounts of serotonin and/or its precursor 5-hydroxytryptophan. Although several chromatographic techniques have emerged for the analysis of tryptophan-related indoles, HPLC with either electrochemical or fluorometric detection have become the methods of choice for their quantification. HPLC-based methods combine selectivity, sensitivity and high precision, and enable the simultaneous investigation of several metabolically related indoles. This review aims to place the analysis of indoles in biological matrices in a biochemical, physiological and clinical perspective and highlights several important steps in their chromatographic analysis and quantification.

An automatic analyzer for catecholamines and their 3-O-methyl metabolites using a micro coulometric flow cell as a postcolumn reactor for fluorogenic reaction

A coulometric flow cell for a miniaturized LC system was developed. The cell was examined, as 3-O-methyl catecholamines were converted to their relative omicron-quinones for subsequent fluorometric and chemiluminescence detection. Its performance was evaluated in comparison with commercially available amperometric and coulometric detectors in terms of specification of the low dead volume and high conversion efficiency. The fully automated small-bore LC analyzer for simultaneous determination of catecholamines and their 3-O-methyl metabolites included precolumn pretreatment, column switching, column separation, postcolumn oxidative conversion, fluorometric derivatization, and chemiluminescence detection. The detection limits were 0.3-2.0 fmol for catecholamines and their 3-O-methyl metabolites. Because of the high sensitivity, the required volume of rat plasma sample was only 15 microL.

Determination of norepinephrine in microdialysis samples by microbore column liquid chromatography with fluorescence detection following derivatization with benzylamine

A microbore column liquid chromatographic method is described for the determination of norepinephrine (NE) in microdialysis samples from rat brain. The method is based on precolumn derivatization of NE with benzylamine in the presence of potassium hexacyanoferrate(III) resulting in a highly fluorescent and stable benzoxazole derivative. Typically, a 10-microl sample was mixed with 10 microl derivatization reagent containing 0.45 M Caps buffer (pH 12.0), 0.2 M benzylamine, 10 mM potassium hexacyanoferrate(III), and N,N-dimethylformamide (1:1:1:15, v/v). The derivatization was carried out at 50 degrees C for 20 min. Under these conditions only NE and epinephrine produced high fluorescence yields at excitation and emission wavelengths of 345 and 480 nm, respectively, while fluorescence of other catechols and 5-hydroxyindoles was quenched by 10-100 times. The NE derivative was separated on a reversed-phase column (100 x 1.0 mm i.d., packed with C18 silica, 5 microm) within 10 min with no late eluting peaks. The mobile phase consisted of 40 mM Britton-Robinson buffer (pH 7.5) containing 1 mM didodecyldimethylammonium bromide and acetonitrile (34%, v/v), the flow rate was 40 microl/min. The limit of detection (signal-to-noise ratio of 3) for NE was 90 amol in 10 microl sample injected. Microdialysis samples were collected in 5-min intervals from the probes implanted in the hippocampus, frontal cortex, or hypothalamus of awake rats. The basal extracellular NE levels in the respective areas were 4.7 +/- 0.9, 1.8 +/- 0.3, and 0.8 +/- 0.2 fmol/10 microl (mean +/- SE, n = 7). Perfusion with a Ringer solution containing 100 mM K+ increased hippocampal NE levels by 700%, while NE uptake inhibitors maprotiline and amitriptyline administered orally or subcutaneously increased extracellular NE in the frontal cortex by about 300%. On the other hand, reserpine (5 mg/kg) reduced cortical NE levels by 40% 3 h after the administration. This new fluorescence derivatization method provides better selectivity, sensitivity, and speed for NE determination than the electrochemical detection since no late-eluting compounds such as dopamine, serotonin, and their metabolites are detectable in the chromatograms of the microdialysis samples.

Procedure for the sample preparation and handling for the determination of amino acids, monoamines and metabolites from microdissected brain regions of the rat

A method is described for the analysis of amino acids, monoamines and metabolites by high-performance liquid chromatography with electrochemical detection (HPLC-ED) from individual brain areas. The chromatographic separations were achieved using microbore columns. For amino acids we used a 100x1 mm I.D. C8, 5 microm column. A binary mobile phases was used: mobile phase A consisted of 0.1 M sodium acetate buffer (pH 6.8)-methanol-dimethylacetamide (69:24:7, v/v) and mobile phase B consisted of sodium acetate buffer (pH 6.8)-methanol-dimethylacetamide (15:45:40, v/v). The flow-rate was maintained at 150 microl/min. For monoamines and metabolites we used a 150X1 mm I.D. C18 5 microm reversed-phase column. The mobile phase consisted of 25 mM monobasic sodium phosphate, 50 mM sodium citrate, 27 microM disodium EDTA, 10 mM diethylamine, 2.2 mM octane sulfonic acid and 10 mM sodium chloride with 3% methanol and 2.2% dimethylacetamide. The potential was +700 mV versus Ag/AgCl reference electrode for both the amino acids and the biogenic amines and metabolites. Ten rat brain regions, including various cortical areas, the cerebellum, hippocampus, substantia nigra, red nucleus and locus coeruleus were microdissected or micropunched from frozen 300-microm tissue slices. Tissue samples were homogenized in 50 or 100 microl of 0.05 M perchloric acid. The precise handling and processing of the tissue samples and tissue homogenates are described in detail, since care must be exercised in processing such small volumes while preventing sample degradation. An aliquot of the sample was derivatized to form the tert.-butylthiol derivatives of the amino acids and gamma-aminobutyric acid. A second aliquot of the same sample was used for monamine and metabolite analyses. The results indicate that the procedure is ideal for processing and analyzing small tissue samples.

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.

High-performance liquid chromatographic analysis with electrochemical detection of biogenic amines using microbore columns

High-performance liquid chromatography with electrochemical detection (HPLC-ED) is a popular method for measuring biogenic amines, owing to its simplicity, versatility, sensitivity, and specificity. Recent developments in microbore column HPLC-ED have been facilitated by miniaturization of solvent delivery, column packing, sample injection and micro-flow cell construction. The aim of this paper is to present an overview of recent developments in microbore column HPLC-ED, in terms of advantages and limitations. This paper covers the recent advancements and important factors of HPLC-ED analysis of biogenic amines using microbore columns. Particular emphasis is placed on applying this technique to microdialysis, for which great sensitivity is required. Its potential in future biomedical applications is also discussed.

Overview of chemical sampling techniques

Although many of the ideas for sampling the chemical microenvironment of the brain were present, at least in nascent form, three decades ago or more, the last 10 years have witnessed a particularly spectacular surge of development, refinement, and use. We are now able to measure virtually any endogenous brain chemical in vivo at commendable levels of sensitivity, selectivity, and speed. The long-dreamt-of goal of being able to correlate neurochemical events with ongoing behavior and/or presentation of salient environmental cues and stimuli has already been largely achieved. Further refinements of existing techniques may well lead to levels of analysis inconceivable even a few years ago. The implications for theory-building and hypothesis-testing are enormous, particularly within such essentially virgin domains as behavioral neuroscience and biological psychiatry. These are truly exciting times.

Plasma catecholamines--analytical challenges and physiological limitations

Catecholamines in plasma may be measured to assess sympathoadrenal activity. Numerous assay methodologies have been published, illustrating the fact that there are many analytical problems. Different methodologies are discussed briefly. A plea for better validation, especially with regard to specificity (which should not be confused with sensitivity or reproducibility), is made. Plasma NA is a frequently used marker for sympathetic nerve activity in humans, but the data obtained are often misinterpreted due to lack of appreciation of the physiological determinants of the NA concentration measured. NA overflow from an organ gives a good reflection of nerve activity in that organ. However, sympathetic nerve activity is highly differentiated, particularly during stress, and conventional plasma NA levels (usually forearm venous samples) cannot be taken as an indication of 'sympathetic tone' in the whole individual. NA is rapidly removed from plasma, resulting in meaningless net veno-arterial concentration differences over organs unless its removal from arterial plasma is taken into account. In the forearm, for example, 40-50% of catecholamines are removed during one passage; about half of the NA in a venous sample is derived from the arm and half from the rest of the body. Therefore, conventional venous sampling overemphasizes local (mainly skeletal muscle) nerve activity. Whole-body sympathetic nerve activity may be monitored in arterial or mixed venous (i.e. pulmonary arterial) samples, which reflect NA overflow from all organs in the body. NA levels are determined both by overflow to plasma and clearance from plasma. NA turnover studies with 3H-NA infusions may be needed to assess clearance, but the simpler concentration measurements usually yield adequate information if the sampling site is relevant. NA overflow from an organ can be assessed (using 3H-NA or ADR as a marker for NA extraction in the organ) and provides valuable information on local sympathetic activity. Mental stress elicits marked circulatory responses, with mainly cardiorenal sympathetic activation and minor elevations of conventional venous plasma NA levels, thus illustrating the differentiated firing pattern of the sympathetic nerves. Circulating ADR is less important than neurogenic mechanisms in the responses to stress. Concentration-effect studies for infused catecholamines may be used for receptor sensitivity studies in vivo, but reflexogenic contributions to responses need to be determined. However, prejunctional mechanisms cannot be assessed without knowledge of the nerve activity present; for example, ADR infusion leads to increased nerve activity. When correctly sampled, measured and interpreted, plasma catecholamines can yield very valuable information on sympathoadrenal activity.

Analysis of the biogenic amines in the central nervous system of the tobacco hornworm by high-performance liquid chromatography with 16-sensor electrochemical detection

A method was developed to analyze biogenic amines in extracts of the central nervous system of the tobacco hornworm, Manduca sexta by high-performance liquid chromatography with 16-sensor electrochemical detection (n-EC-HPLC). The amines, precursors, and metabolites were separated in two dimensions. The first dimension involves separation based upon retention time by reversed-phase HPLC, while the second dimension involves separation based upon the characteristic oxidation potentials achieved by n-EC. Biogenic amine identification was based upon maximum oxidation potential and peak height ratios in addition to retention time. The improved resolving power of this method allows for a simplified sample preparation procedure and simultaneous determination of a wide range of compounds, including phenylethylamine, catecholamines, indoleamines, and some of their precursors and metabolites.

Simultaneous solid-phase extraction and chromatographic analysis of morphine and hydromorphone in plasma by high-performance liquid chromatography with electrochemical detection

High-performance liquid chromatography has become an important analytical tool for the quantitation of opioid drugs. Using solid-phase extraction and coulometric electrochemical detection, we have developed a chromatographic method for the simultaneous measurement of morphine and hydromorphone which is both sensitive and specific. Using 1 ml of plasma, intra-assay and inter-assay data show that the detection limit for accurate quantitation of these compounds is about 1.2 ng/ml (coefficient of variation 11.6%) for morphine and 2.5 ng/ml (coefficient of variation 10.5%) for hydromorphone. The method is simple and readily adaptable to most pharmacokinetic studies and toxic screens involving these drugs.

Analysis of terbutaline in human plasma by high-performance liquid chromatography with electrochemical detection using a micro-electrochemical flow cell

A high-performance liquid chromatographic method is described for the determination of terbutaline in human plasma in the range 1-35 ng/ml. Detection was achieved using a carbon fibre micro-electrochemical detector and a column-switching system. The microelectrode cell has advantages over conventional glassy carbon electrode-based detection systems in that it is easy to prepare, flexible in its operation and suffers less trouble from problems such as air bubbles and leaks. Furthermore, it has a better detection limit for terbutaline (0.8 ng/ml) to that obtained using a conventional glassy carbon electrode flow detector (2 ng/ml). Sample clean-up was by on-line solid-phase extraction with column switching, providing a method which was sensitive and reproducible, where the mean overall coefficient of variation was 5.60% and drug recovery in excess of 86% at the concentration levels studied.

Preparation of an optimum mobile phase for the simultaneous determination of neurochemicals in mouse brain tissues by high-performance liquid chromatography with electrochemical detection

A systematic method is described for the optimization of a mobile phase for the simultaneous determination of 24 neurochemicals consisting of catecholamine, serotonin, their precursors and metabolites and related materials. This mobile phase contained sodium acetate (0.04 M), citric acid (0.01 M), sodium chloride (0.0126 M), sodium octyl sulfate (91 mg/l), tetrasodium EDTA (50 mg/l) and 10% (v/v) methanol. When this optimum mobile phase was applied to the analysis of brain tissues of the Swiss male mouse, twelve neurochemicals were quantified in the free state: tyrosine, L-beta-3,4-dihydroxyphenylanine, dopamine, 3,4-dihydroxyphenylacetic acid, 4-hydroxy-3-methoxyphenylacetic acid, norepinephrine, 3-methoxy-4-hydroxyphenylglycol, DL-3,4-dihydroxymandelic acid, DL-4-hydroxy-3-methoxymandelic acid, serotonin, L-tryptophan, 5-hydroxyindole-3-acetic acid and DL-synephrine and normetanephrine, appearing as a fused peak. This fused peak was present on the chromatogram tracings of all the mouse brain tissues. The separable neurochemicals not found by this procedure in the Swiss male mouse tissues were DL-3,4-dihydroxyphenylglycol,5-hydroxytryptophan, epinephrine, DL-octopamine, metanephrine, deoxyepinephrine, homovanillyl alcohol, N-acetylserotonin, tyramine and 3-methyltyramine.

Two-parameter mobile phase optimization for the simultaneous high-performance liquid chromatographic determination of dopamine, serotonin and related compounds in microdissected rat brain nuclei

A new high-pressure liquid chromatography method with electrochemical detection is described that allows the simultaneous determination of dopamine, serotonin, 3,4-dihydroxyphenylacetic acid, 3-methoxy-4-hydroxyphenylacetic acid, 5-hydroxytryptophan and 5-hydroxyindoleacetic acid in microdissected nuclei from individual rat brains. No sample pre-treatment steps are required. Resolution and analysis time were optimized by a simple limited optimization procedure, involving two-parameter factorial design.

Determination of heat-exposure effects on the concentration of catecholamines in bovine plasma and milk

A reversed-phase high-performance liquid chromatographic method with electrochemical detection has been adapted for the determination of picogram concentrations of norepinephrine and epinephrine in bovine plasma and milk. This method has been used to monitor the levels of these catecholamines when lactating cows are exposed to heat stress under controlled conditions. In response to heat stress, epinephrine concentrations in milk and plasma were similar. However, norepinephrine concentrations in milk were one tenth of that in plasma.

Evaluation of a simple plasma catecholamine extraction procedure prior to high-performance liquid chromatography and electrochemical detection

The modified extraction method for catecholamines described in this study is reproducible, simple, rapid, economical and relatively hazard-free. This method is based on the principle that plasma catecholamines are selectively adsorbed on acid-washed alumina at pH 8.6 and then eluted at a pH between 1.0 and 2.0. No statistically significant differences were obtained by using either 0.5 or 1.0 ml of plasma with 0.5 or 1.0 ml of Tris buffer. A 15-min mixing time during the adsorption and desorption steps was found to be practical, but any standardized time up to 1 h can be used. If the washing step was omitted, the catecholamines could not be eluted from the acid-washed alumina. To prevent dilution, the alumina had to be centrifuged and not aspirated to dryness after the washing step. An amount of 50 mg of WA-4 alumina was found to be the most practical in this study. Extracted or unextracted plasma as well as catecholamine standards were stable for four months at -20 degrees C.

A sensitive method for simultaneous determination of histamine and noradrenaline with high-performance liquid chromatography/electrochemistry

A simple and reliable high-performance liquid chromatographic method is described for the simultaneous determination of histamine (His), which cannot be directly oxidized, and noradrenaline (NA), which can be directly oxidized within the useful working potential range. The isoindole products formed by precolumn derivatization of His and NA with o-phthalaldehyde (OPA) and 2-mercaptoethanol (2-ME) yielded a linear relationship of detection between the electrochemical signal and the compound content to a minimum detectable limit of 50 pg (signal-to-noise ratio = 3:1) for both compounds at 0.5 nA of detector range. Without 2-ME, OPA derivatives of both His and NA were not detectable electrochemically at the oxidation potential range from 0 to + 1 V. Although the peak potential was +0.85 V for both His and NA, we used +0.7 V for both compounds to keep background noise minimal. The capacity factors of some electrochemically interfering compounds were also determined. The significance of OPA/2-ME derivative of NA is discussed relative to the direct oxidation of catecholamines. An example of a practical application of the method to the determination of His and NA in rat cardiac tissue is presented.

Separation and quantification of histamine and N tau-methylhistamine in brain extracts

Histamine and its N tau-methyl derivative can be separated from perchloric acid extracts of rat brain by high performance liquid chromatography on a C18 column under isocratic conditions eluting with 0.1 M sodium phosphate buffer containing 0.19 mM sodium dodecyl sulphate and 25% methanol. Using electrochemical detection, histamine and N tau-methylhistamine can be detected at levels of less than 40 pg/microL tissue extract (less than 1 pmol). The retention times for histamine and N tau-methylhistamine were 15 min and 23 min, respectively, at a flow rate of 1.2 mL/min, and both compounds eluted as acceptably sharp peaks. The concentrations of histamine and N tau-methylhistamine in brain from seven-day-old rats were found to be very similar to those obtained by other analytical procedures.

High-pressure liquid chromatographic analysis of hemolymph plasma catecholamines in immune-reactive Aedes aegypti

Tyrosine and catecholamines have been implicated as substrates for the encapsulation reactions involved in the immune response of mosquitoes to microfilariae (mff). Identification and quantitation of tyrosine and catecholamines present in Aedes aegypti hemolymph plasma were accomplished by ion-pair high-pressure liquid chromatography with electrochemical detection at either +650 or +850 mV vs Ag/AgCl. Tyrosine, dopamine, and N-beta-alanyldopamine were detected in the hemolymph plasma of naive A. aegypti. Although no differences in these compounds were observed in hemolymph plasma from A. aegypti inoculated with Dirofilaria immitis mff, the chromatogram showed a single major peak (PI) (65 microM, expressed as dopamine equivalents) that was not present in naive hemolymph plasma. Saline-inoculated controls contained only 5% of the PI in immune reactive hemolymph plasma. A high concentration of PI (127 +/- 39 microM) was also detected after treatment of hemolymph plasma with mild alkaline conditions (pH 9.0), indicating that it is normally present as an electrochemically inert form in naive mosquitoes. High concentrations of PI were also detected in the naive hemolymph plasma from three other mosquito species, but no PI was found in A. trivittatus under any conditions. PI did not cochromatograph with any of the catecholamines commonly thought to be involved in immune responses of dipterans against metazoan parasites, suggesting that it may be a unique substrate for these reactions. The biological relevance of PI was evidenced by its appearance in the hemolymph plasma of two strains of D. immitis-inoculated A. aegypti.

Simultaneous measurement of tetrahydrobiopterin (THBP) and biogenic amines by liquid chromatography with electrochemical detection

This report describes a rapid and sensitive method for measuring tetrahydrobiopterin (THBP) and biogenic amines simultaneously by liquid chromatography with electrochemical detection (LC-ECD). The coefficient of variation for THBP was 4.87% and the minimum detectable amount of THBP was approximately 20 pg. These results indicate that this simple reverse-phase ion-pair chromatography system can be used for the simultaneous analysis of endogenous THBP and biogenic amines without long sample preparation time.