Neurochemical applications of liquid chromatography with electrochemical detection

Metabolic and drug profiling

Over recent years, advances in analytical technology have greatly improved our ability to study the metabolism of compounds from either endogenous or erogenous sources. The application of gas-liquid chromatography, mass spectrometry, high-performance liquid chromatography and immunological approaches are discussed in relation to the analysis of steroids, bile acids, organic acids, prostaglandins, porphyrins and bile pigments, amino acids, proteins, nucleotides, catecholamines, vitamins and drugs.

Hypothalamic catecholamine biosynthesis and neuropeptides

In conclusion, both steady state and non-steady state methods for investigating the turnover of central catecholamines can provide valuable information regarding the central control of neuroendocrine function. The system described here, utilizing HPLC with electrochemical detection, offers a relatively easy and reliable method for determining steady state catecholamine biosynthesis in the hypothalamus and other brain areas.

Determination of norepinephrine in brain perfusates using high-performance liquid chromatography with electrochemical detection

A procedure for determining catecholamine levels in brain perfusates is described. A single-step extraction on alumina followed by reversed-phase ion-pair chromatography and electrochemical detection was employed. Levels of norepinephrine are reported in push-pull perfusates of the rat hypothalamus under baseline and various ion substitution conditions. The resulting estimated norepinephrine release values are correlated with the behavior of the animal. The data are discussed in terms of the validity of the measures and the noradrenergic mediation of behavior.

Determination of dopamine and its acidic metabolites in brain tissue by HPLC with electrochemical detection in a single run after minimal sample pretreatment

A method, based on reverse-phase liquid-liquid chromatography, has been developed for the determination, in a single run, of dopamine (DA) and its acidic metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), combined with electrochemical detection (ECD). If applied to brain tissue, sample pretreatment can be reduced to centrifugation, filtration and adjustment of pH and perchlorate concentration prior to introduction into the liquid chromatograph. The relation between the perchlorate (counterion) concentration of the mobile phase and the retention (k') of the amines is linear, as is the relation between the H+ concentration of the mobile phase and the retention of the acidic metabolites. This flexible phase system, combined with a simple and therefore reproducible sample pretreatment, warrants a high throughput of samples. The procedure offers good possibilities for routine analysis of catecholamines and their acidic metabolites in the picogram range. Some typical examples of the behaviour of this phase system and the electrochemical detector are presented and discussed.

Dopamine turnover estimated by simultaneous LCEC assay of dopamine and dopamine metabolites

The estimation of changes in tissue levels of dopamine (DA) and its metabolites has been employed to approximate the turnover rate or release of DA. We have developed an assay using high pressure liquid chromatography with electrochemical detection to quantify DA, dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3-MT), and homovanillic acid (HVA) simultaneously in brain homogenates. The assay is simple, inexpensive, rapid and accurate; the sensitivity of the assay for DA and DOPAC was 10 pg, for HVA 50 pg, and 200 pg for 3-MT. Apomorphine treatment resulted in a reduction in tissue levels of DOPAC and HVA, while haloperidol administration increased tissue levels of DOPAC and HVA. Neither drug treatment caused changes in DA levels. Administration of d-amphetamine 2.5 mg/kg S.C. resulted 1 hour later in increases in the striatal concentration of DA (57%) and 3-MT (44%) and decreases in DOPAC (40%) and HVA (39%). These data are consistent with the agonistic effect of apomorphine and antagonistic effect of haloperidol on DA autoreceptors and post-synaptic receptors. In the case of amphetamine, the increases in the levels of DA and 3-MT probably reflect the ability of d-amphetamine to induce DA release and block re-uptake. The reduction of DOPAC and HVA levels are consistent with the ability of d-amphetamine to competitively inhibit monoamine oxidase activity as well as the neuronal reuptake of DA. The simultaneous estimation of DA, DOPAC, 3-MT, and HVA should facilitate the understanding of dopaminergic drugs with complicated mechanisms of action.

Routine determination of plasma catecholamines using reversed-phase, ion-pair high-performance liquid chromatography with electrochemical detection

A procedure is described for the determination of plasma catecholamines using reversed-phase, ion-pair high-performance liquid chromatography coupled with electrochemical detection. Optimisation of chromatographic conditions with respect to detector performance and adherence to procedures and precautions described, render the method applicable to both neurochemical research and routine clinical analysis. The limit of quantitative detection of the method was found to be approximately 30 pg per injection for individual catecholamines. A single chromatographic run, providing adequate resolution of each component, could be completed in approximately 12 min.

Determination of mianserin and metabolites in plasma by liquid chromatography with electrochemical detection

A procedure for the determination of mianserin, desmethylmianserin, and 8-hydroxymianserin in plasma at therapeutic concentrations by liquid chromatography with electrochemical detection is described. Following a multiple-step extraction from alkaline plasma into methyl-tert-butyl ether, the reconstituted extract was injected onto a reversed-phase trimethylsilyl-packed column and eluted with an acetate-acetonitrile mobile phase containing an ion-paired reagent. The method provides an absolute recovery of 71-76% and a day-to-day precision of 5.4-9.1% for each compound at 25 ng/ml. The minimum quantifiable level for all three compounds was 5 ng/ml (RSD greater than 11%), and the detector response was linear up to 500 ng/ml. Fixed-dose steady-state plasma level data for 34 patients are reported.

Reversed-phase liquid chromatography with amperometric detection of lipophilic dopamine analogues and determination of brain and serum concentrations after sample clean-up on small sephadex G-10 columns

The liquid chromatographic determination of N-alkylated analogues of dopamine is described. The retention of these compounds, ranging from dopamine to N,N-dibutyl-dopamine, was studied on four bonded-phase columns, of which Nucleosil 5 C18 was chosen for routine use. The compounds were detected by a rotating disc amperometric detector. Samples of rat brain and serum were taken through a clean-up step on small Sephadex G-10 columns from which the dopamine analogues eluted in the same fraction as dopamine. The overall recovery was 70--90% from brain tissue and 60--70% from serum or plasma. The limit of detection for the catechol-containing compounds in tissue was 40--100 pg, for O-methylated ones 100--200 pg. The method is applied to the determination of dopamine analogues in rat brain after peripheral administration.

Rapid liquid chromatographic determination of 5-hydroxyindoles and dihydroxyphenylacetic acid in cerebrospinal fluid of the rat

A stainless steel guide was implanted in the anterior third ventricle of the anesthetized rat and an internal needle shorter than the guide was used to continuously collect cerebrospinal fluid (CSF) at a constant outflow of 1 microliter/min. Five microliter samples were injected directly into a liquid chromatographic column. The mobile phase was adjusted for selective separation of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindolacetic acid (5-HIAA). Electrochemical detection with a limit of 0.05 pmol was used, 5-HTP and 5-HT concentrations were in the 10(-8) M range in controls while DOPAC and 5-HIAA were in the 10(-7) and 10(-6) M range. Brain aromatic amino acid decarboxylase inhibition with high doses of benserazide corresponded to an increased CSF level of 5-HTP. Monoamine oxidase inhibition with tranylcypromine resulted in a diminution of DOPAC and 5-HIAA. L-Tryptophan loading associated with monoamine oxidase inhibition induced an increase in CSF level of serotonin. These pharmacologically induced changes in serotonin and dopamine metabolite levels exemplify the usefulness of these CSF determinations as indices of brain function.

Investigations of catecholamine metabolism using high-performance liquid chromatography: analytical methodology and clinical applications

High-performance liquid chromatography, particularly in its reversed-phase mode, coupled with electrochemical or fluorometric detection, is becoming increasingly popular as an analytical tool for metabolic profiling of substances of neurochemical interest, such as catecholamines and their metabolites. During the last decade, a continued effort has been made to improve and simplify the analytical methodology for routine use in clinical laboratories where this technique is tremendously needed. New developments in column technology, reliable detectors, simplified sample cleanup procedures, and particularly better understanding of the complex physicochemical phenomena underlying the operation of electrochemical detection, have resulted in a steady and encouraging progress. The purpose of this review was to describe the current analytical methodology and recent applications of HPLC in the field of catecholamine metabolism. Although this discussion is by no means detailed and complete, it, at least, hints at the impact of this technique on biochemical investigations and its future potential in clinical laboratories.

Determination of catecholamines in rat heart tissue and plasma samples by liquid chromatography with electrochemical detection

Liquid chromatography with electrochemical detection is used for the determination of adrenaline, noradrenaline and dopamine in rat heart tissue, and the method has also been applied to the determination of basic levels of these compounds in blood plasma. The catecholamines are isolated from the biological sample by adsorption onto alumina and are then desorbed by elution with perchloric acid. The stability of the compounds during the different stages in the work-up process has been studied. A greatly simplified procedure for the preparation of alumina is presented. Both ion-pair reversed-phase and ion-exchange liquid chromatography have been used for the separation of the catecholamines. For plasma samples the method has been validated against radioenzymatic assay and the choice of method is discussed.

Determination of gamma-aminobutyric acid by liquid chromatography with electrochemical detection

gamma-Aminobutyric acid (GABA) has been determined in rat brain by derivatization with 2,4,6-trinitrobenzenesulfonic acid. The derivative and an internal standard, 2,4,6-trinitrophenyl-delta-aminovaleric acid, are extracted into toluene and separated by reversed-phase chromatography. Electrochemical reduction of these derivatives permits picomole measurements of GABA in microgram amounts of brain tissue.

Catecholamines in honey bee (Apis mellifera L.) and various vespid (Hymenoptera) venoms

Liquid chromatography with electrochemical detection was used to separate, identify and quantitate the levels of dopamine (DA) and noradrenaline (NA) present in the venoms of honey bees of known ages and of DA and 5-hydroxytryptamine (5-HT) in various species of vespid wasps. Fluorescence histochemistry corroborated the presence of amines in honey bee venom glands and reservoirs and supported the quantitative results. The results show: (i) There is an age dependent variation in DA and NA levels in honey bee venom and there may be a similar variation in DA and 5-HT in the venom of queen Dolichovespula arenaria. (ii) There appears to be a seasonal variation in the quantities of DA and NA present in honey bee venom. Such a seasonal variation may be one of the factors involved in the variation in the levels of amine components present in the various vespid wasp venom samples. (iii) 5-HT is present in much greater quantities than DA in vespid venoms. Low levels of NA and DOPA are present in vespid venoms.

Identification and localization of catecholamines in the nervous system of Limulus polyphemus

The concentrations of various catecholamines in the nervous system of the horseshoe crab Limulus polyphemus have been determined by high-performance liquid chromatography with electrochemical detection. Dopamine, norepinephrine, epinephrine, and their precursor L-Dopa were present in appreciable quantities in discrete regions of the central nervous system and cardiac ganglion. The catecholamines were localized more precisely by use of the glyoxylic-acid-histofluorescence technique of de la Torre and Surgeon (1976). Catecholamine fluorescence appeared in protocerebral and tritocerebral neuropile, including regions of the central body and optic medulla. Posterior to these brain areas, tracts extended through the circumesophageal ganglionic ring and laterally out each of the pedal ganglia. Small clusters of large fluorescent somata were present in the protocerebrum. No fluorescence was observed in the corpora pedunculata.

A rapid and simple method to determine the specific activities of serotonin, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophan in brain by HPLC with electrochemical detection

The specific activities of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) have been determined in the brain of rats by HPLC using electrochemical detection. The method allows, from a single sample, the simultaneous measurement of all three compounds and collection of each peak for radioactivity determinations. Five male Wistar rats were injected i.v. with 2.0 mCi/kg of DL-5-hydroxy-[G-3H]tryptophan (2.6 Ci/mmol) and 30 min later the animals were killed by near freezing. Whole brains were removed and homogenized in an acid medium. The content of 5-HTP, 5-HT, and 5-HIAA were determined by HPLC. Each peak of interest was immediately collected after detection in scintillation vials by use of a small dead space detector (TL-9A, Bioanalytical Systems, Inc.). The amounts of radioactivity were determined and specific activities calculated from the results. A second chromatography system (TLC) was used to check the authenticity and purity of compounds separated by the HPLC.

Plasma catecholamines in hypertension and pheochromocytoma determined using ion-pair reversed-phase chromatography with amperometric detection: investigation of the separation mechanism and clinical methodology

The retention behavior of catecholamines (CAs) in ion-pair reversed-phase chromatography is examined. From the effects of pH, ionic strength and a secondary ion-pairing reagent (citric acid), under our chromatographic conditions, the retention behavior can be explained by assuming a mixed ion-exchange mechanism with octyl sulfate and citrate, on the column and in the mobile phase, respectively. The developed separation method was applied to the analysis of CAs in plasma samples purified by alumina adsorption and detected amperometrically. This method provides the basis for the determination of the short-term magnitude of CA response to physical and physiological interventions, as well as the baseline CA levels in essential hypertension and pheochromocytoma. The results seen for norepinephrine and epinephrine are consistent with eh funcitonal roles of these CAs as hormones or peripheral transmitters.