Constant infusion of [13C6]glucose: simultaneous measurement of turnover of GABA and glutamate in defined regions of the brain of individual animals

Mass spectrometry strategies for clinical metabolomics and lipidomics in psychiatry, neurology, and neuro-oncology

Metabolomics research has the potential to provide biomarkers for the detection of disease, for subtyping complex disease populations, for monitoring disease progression and therapy, and for defining new molecular targets for therapeutic intervention. These potentials are far from being realized because of a number of technical, conceptual, financial, and bioinformatics issues. Mass spectrometry provides analytical platforms that address the technical barriers to success in metabolomics research; however, the limited commercial availability of analytical and stable isotope standards has created a bottleneck for the absolute quantitation of a number of metabolites. Conceptual and financial factors contribute to the generation of statistically under-powered clinical studies, whereas bioinformatics issues result in the publication of a large number of unidentified metabolites. The path forward in this field involves targeted metabolomics analyses of large control and patient populations to define both the normal range of a defined metabolite and the potential heterogeneity (eg, bimodal) in complex patient populations. This approach requires that metabolomics research groups, in addition to developing a number of analytical platforms, build sufficient chemistry resources to supply the analytical standards required for absolute metabolite quantitation. Examples of metabolomics evaluations of sulfur amino-acid metabolism in psychiatry, neurology, and neuro-oncology and of lipidomics in neurology will be reviewed.

Neurochemical analysis of amino acids, polyamines and carboxylic acids: GC-MS quantitation of tBDMS derivatives using ammonia positive chemical ionization

The GC-MS quantitation of a large number of neurochemicals utilizing a single derivatization step is not common but is provided by the reagent N-(tert-butyldimethylsilyl)-N-methyltrifluro-acetamide (MTBSTFA). Previous workers have utilized this derivative for GC-MS analyses of amino acids, carboxylic acids and urea with electron impact (EI) and with positive chemical ionization (PCI; methane as reagent gas). However, these conditions yield significant fragmentation, decreasing sensitivity and in some cases reducing specificity for quantitation with selected ion monitoring (SIM). Additionally, the majority of studies have used a single internal standard to quantitate many compounds. In this study we demonstrate that using isotopic dilution combined with ammonia as the reagent gas for PCI analyses, results in high precision and sensitivity in analyzing complex neurochemical mixes. We also demonstrate for the first time the utility of this derivative for the analysis of brain polyamines and the dipeptide cysteinyl glycine. In the case of ammonia as the reagent gas, all amino acids, polyamines and urea yielded strong [MH](+) ions with little or no fragmentation. In the case of carboxylic acids, [M+18](+) ions predominated but [MH](+) ions were also noted. This approach was used to analyze superfusates from hippocampal brain slices and brain tissue extracts from brain lesion studies. The advantages of this methodology include: (i) simple sample preparation; (ii) a single derivatization step; (iii) direct GC-MS analysis of the reaction mix; (iv) high precision as a result of isotopic dilution analyses; (v) high sensitivity and specificity as a result of strong [MH](+) ions with ammonia reagent gas; (vi) no hydrolysis of glutamine to glutamate or asparagine to aspartate; and (vii) applicability to a wide range of neurochemicals.

Cocaine-induced increases in medial prefrontal cortical GABA transmission involves glutamatergic receptors

A recent study showed that cocaine-induced sensitization is associated with an increase in GABA (gamma-aminobutyric acid) transmission in the medial prefrontal cortex. Since previous studies have demonstrated that sensitization is associated with enhanced medial prefrontal cortex glutamatergic transmission, the present study examined the role of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate (AMPA/KA) receptors in cocaine-induced increases in medial prefrontal cortex GABA levels. Male Sprague-Dawley rats received four daily injections of saline (1 ml/kg, i.p.) or cocaine (15 mg/kg). One day later, animals were infused with NMDA or AMPA/KA antagonists 3-[(R)-2 carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) and 6,7-dinitroquinoxaline-2,3-dione (DNQX), respectively, into medial prefrontal cortex via microdialysis probe for 60 min before receiving systemic challenge injections of saline or cocaine. Cocaine-sensitized animals showed an increase in extracellular medial prefrontal cortex GABA levels that was blocked by prior medial prefrontal cortex infusion of DNQX, but not CPP. These data indicate that enhanced medial prefrontal cortex GABA transmission seen in cocaine-sensitized animals involves glutamatergic stimulation of AMPA receptors.

In vivo detection of cortical GABA turnover from intravenously infused [1-13C]D-glucose

In this study [2-(13)C] gamma-aminobutyric acid (GABA) was spectrally resolved in vivo and detected simultaneously with [4-(13)C]glutamate (Glu) and [4-(13)C]glutamine (Gln) in the proton spectra obtained from a localized 40 microL voxel in rat neocortex with the use of an adiabatic (1)H-observed, (13)C-edited (POCE) spectroscopy method and an 89-mm-bore vertical 11.7 Tesla microimager. The time-resolved kinetics of (13)C label incorporation from intravenously infused [1-(13)C]glucose into [4-(13)C]Glu, [4-(13)C]Gln, and [2-(13)C]GABA were measured after acute administration of gabaculine, a potent and specific inhibitor of GABA-transaminase. In contrast to previous observations of a rapid turnover of [2-(13)C]GABA from [1-(13)C]glucose in intact rat brain, the rate of (13)C incorporation from [1-(13)C]glucose into [2-(13)C]GABA in the gabaculine-treated rats was found to be significantly reduced as a result of the blockade of the GABA shunt.

Effects of cocaine and the cocaine analog CFT on glutamatergic neurons

The effects of cocaine and the cocaine analog methyl-3-beta-(p-fluorophenyl)-1 alpha H, 5 alpha H-tropane-2b-carboxylate (CFT) on glutamate turnover rate were studied in the nucleus accumbens, striatum, frontal cortex, and parietal-cingulate cortex of the rat, using neurotransmitter turnover rate as an estimate of the activity of the glutamatergic neurons. Both cocaine [15 or 30 mg/kg, intraperitoneally (IP)] and CFT (2.2 mg/kg, IP) increased glutamate turnover in the nucleus accumbens, although the time course of their actions differed. These effects on glutamate turnover appeared at times after maximal motor activation of the animals had occurred. On the other hand, neither cocaine nor CFT affected glutamate turnover in the frontal cortex, parietal-cingulate cortex, or striatum. Neither cocaine nor CFT affected the content of glutamate or glucose in any brain region studied. Thus, although cocaine and CFT affect glutamatergic neurons in the CNS, these actions are not generalized across the CNS, but are restricted to a specific brain region.

The compartmentation of phosphorylated thiamine derivatives in cultured neuroblastoma cells

Thiamine transport in cultured neuroblastoma cells is mediated by a high-affinity carrier (KM = 40 nM). In contrast, the uptake of the more hydrophobic sulbutiamine (isobutyrylthiamine disulfide) is unsaturable and its initial transport rate is 20-times faster than for thiamine. In the cytoplasm, sulbutiamine is rapidly hydrolyzed and reduced to free thiamine, the overall process resulting in a rapid and concentrative thiamine accumulation. Incorporation of radioactivity from [14C]thiamine or [14C]sulbutiamine into intracellular thiamine diphosphate is slow in both cases. Despite the fact that the diphosphate is probably the direct precursor for both thiamine monophosphate and triphosphate, the specific radioactivity increased much faster for the latter two compounds than for thiamine diphosphate. This suggests the existence of two pools of thiamine diphosphate, the larger one having a very slow turnover (about 17 h); a much smaller, rapidly turning over pool would be the precursor of thiamine mono- and triphosphate. The turnover time for thiamine triphosphate could be estimated to be 1-2 h. When preloading the cells with [14C]sulbutiamine was followed by a chase with the same concentration of the unlabeled compound, the specific radioactivities of thiamine and thiamine monophosphate decreased exponentially as expected, but labeling of the diphosphate continued to increase slowly. Specific radioactivity of thiamine triphosphate increased first, but after 30 min it began to slowly decrease. These results show for the first time the existence of distinct thiamine diphosphate pools in the same homogeneous cell population. They also suggest a complex compartmentation of thiamine metabolism.

Stimulation of GABAB receptors in the basal forebrain selectively impairs working memory of rats in the double Y-maze

The present experiments were conducted to evaluate the possible contribution of GABAergic inputs to the basal forebrain in the region of the nucleus basalis magnocellularis (nbm) to memory. In two experiments, rats implanted with bilateral intra-nbm guide cannulae were trained in the double Y-maze task to perform working- and reference-memory components. Animals were placed in one of two start arms of the first "Y" and the reference-memory component required travelling to its central stem for food. Access to the second "Y" then was given and the working-memory component for Expt. 1 required travelling to the goal arm diagonally opposite the start arm in the first "Y" of that trial. In Expt. 2, the working-memory component required travelling to the goal arm opposite to the goal arm entered in the second "Y" on the preceding trial, with 0- and 15-s delays between trials. In Expt. 1, pretrained rats (n = 8) received the GABAA agonist, muscimol (0.1 microgram in 0.5 microliter), the GABAB agonist, R(+)-baclofen (0.01, 0.05 and 0.1 microgram), and its less active enantiomer, S(-)-baclofen (0.1 microgram), in a counterbalanced order with retraining to criterion between injections. In Expt. 2, pretrained rats (n = 9) received saline (0.5 microliter), R(+)-baclofen (0.1 microgram), the GABAB antagonist, phaclofen (1 microgram), and R(+)-baclofen+phaclofen. Results of Expt. 1 revealed that intra-nbm muscimol and, in a dose-dependent manner, R(+)-baclofen differentially affected working but not reference memory. In Expt. 2, the differential mnemonic impairment produced by R(+)-baclofen was replicated and co-injection with phaclofen reversed this effect. A 15-s delay between trials significantly impaired working but not reference memory. Results suggest that both GABAA and GABAB receptors may be involved in modulating the possible mnemonic functions of nbm cholinergic neurons.

Time-related loss of glutamine from hippocampal slices and concomitant changes in neurotransmitter amino acids

A dramatic, time-dependent loss of L-glutamine was observed in mouse and rat hippocampal slices equilibrated in normal artificial CSF under static (no-flow) and superfused (constant-flow) conditions. Concomitant with the decline in L-glutamine, there was a significant, but less pronounced, decrease in levels of the neurotransmitter amino acids, gamma-aminobutyric acid, L-aspartate, and L-glutamate. The disappearance of L-glutamine was a result of diffusion from the tissue to the artificial CSF rather than chemical or biochemical transformation. The loss of amino acids from the hippocampal slices was prevented to different degrees by the addition of 0.5 mM exogenous L-glutamine to the artificial CSF. The levels of newly synthesized amino acids were also determined, because they may be more indicative of the neuronal activity than the total tissue levels of amino acids. The effects of perturbations in glutamine (length of the equilibration time and addition of exogenous glutamine) on newly synthesized glutamate were more pronounced under 4-aminopyridine-stimulated than control (unstimulated) conditions. Therefore, a loss of L-glutamine from the hippocampal slices may have neurophysiological effects and warrants further investigation.

Regional variation in gamma-aminobutyric acid turnover: effect of castration on gamma-aminobutyric acid turnover in microdissected brain regions of the male rat

This study compared the turnover of GABA neurons in different brain areas of the male rat and examined the effect of castration on GABA turnover in regions of the brain associated with the control of gonadotropin secretion. To estimate GABA turnover, GABA was quantified by HPLC in microdissected brain regions 0, 30, 60, 90, and 120 min after inhibition of GABA degradation by aminooxyacetic acid (100 mg/kg, i.p.). GABA accumulation was linear in all areas for 90 min (p < 0.01), and GABA turnover was estimated as the slope of the line formed by increased GABA concentration versus time, determined by linear regression. There was considerable regional variation both in the initial steady-state concentrations of GABA and in the rates of GABA turnover. Of 10 discrete brain structures, GABA turnover was highest in the medial preoptic nucleus and lowest in the caudate nucleus. Turnover times in the terminal fields of known GABAergic projection neurons ranged sevenfold, from 2.6 h in the substantia nigra to 0.4 h in the lateral vestibular nucleus. The effect of castration on GABA turnover in 13 microdissected brain regions was investigated by measuring regional GABA concentrations before and 30 min after injection of aminooxyacetic acid in intact rats or 2 or 6 days postcastration. Following castration, steady-state GABA concentrations were increased, and GABA turnover decreased in the diagonal band of Broca, the medial preoptic area, and the median eminence. GABA turnover increased in the medial septal nucleus and was unaffected in the cortex, striatum, and hindbrain. These results are consistent with the hypothesis that testosterone negative-feedback control of luteinizing hormone-releasing hormone involves steroid-sensitive GABAergic neurons in the rostral and medial basal hypothalamus.

Abnormalities in Amino Acid Neurotransmitters in Discrete Brain Regions of Genetically Dystonic Hamsters

The concentrations of 11 amino acids, including the neurotransmitters gamma-aminobutyric acid, glutamate, aspartate, glycine, and taurine, were determined by HPLC in 12 brain regions of genetically dystonic (dtSZ) hamsters and age-matched nondystonic controls. Since dystonia in mutant dtSZ hamsters is transient and disappears after about 70 days of age, amino acids were determined at the age of maximum severity of dystonia (30-40 days) and after disappearance of the disease, to examine which neurochemical changes were related to dystonia. In dtSZ hamsters with the maximum severity of dystonia, significant changes in concentrations of the neurotransmitters gamma-aminobutyric acid, glutamate, aspartate, and taurine were found in several regions involved in motor functions, e.g., cerebellum, thalamus, and corpus striatum. Most of these changes were not permanent but disappeared in parallel with dystonia, implicating a causal relationship between altered aminoacidergic neurotransmission and dystonia in mutant dtSZ hamsters.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional alterations in brain amino acids after administration of the N-methyl-D-aspartate receptor antagonists MK-801 and CGP 39551 in rats

The effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 and the novel competitive NMDA receptor antagonist CGP 39551 on levels of 11 amino acids, including several excitatory and inhibitory neurotransmitters, were studied in 12 brain regions of rats. Both drugs were administered at doses which produced comparable behavioural effects (ataxia, hyperactivity). Amino acids were determined in brain tissue by high-performance liquid chromatography after o-phthaldialdehyde precolumn derivatization. MK-801 (0.1 mg/kg, i.p.) moderately increased the concentration of glutamate and GABA in several brain regions. Other amino acids (glutamine, taurine, asparagine, alanine, serine) were only altered in single brain regions, or were not altered at all (aspartate, glycine, threonine, arginine). In contrast to MK-801, CGP 39551 (10 mg/kg, i.p.) increased glutamate levels only in the cerebellum, and produced no significant alterations in levels of GABA. The data demonstrate differences in alterations of amino acid levels in response to competitive and non-competitive NMDA receptor antagonists and support the assumption that competitive NMDA antagonists may be more selective than non-competitive antagonists.

Glycine, glycinamide and D-serine act as positive modulators of signal transduction at the N-methyl-D-aspartate (NMDA) receptor in vivo: differential effects on mouse cerebellar cyclic guanosine monophosphate levels

Direct intracerebellar (icb) administration of glycine, glycinamide and D-serine produced time- and dose-dependent changes in mouse cerebellar cGMP levels, indicating a modulation of ongoing neuronal activity through the NMDA receptor complex. Intracerebroventricular administration of glycinamide also produced a time-dependent change in cGMP levels, indicating a central mechanism of action. The icb dose-response data indicated a unimolecular interaction for these compounds. D-serine-, glycine-, and glycinamide-mediated increases in cGMP levels were reversed by the competitive NMDA antagonist, CPP and the NMDA-associated glycine receptor antagonist, HA-966, indicating mediation via the NMDA receptor complex. Glycine and D-serine were less effective than glycinamide at increasing cerebellar cGMP levels. In contrast, L- and D-serinamide did not affect cGMP levels. These results indicate that glycine receptor is not saturated under physiological conditions and also suggest possible existence of multiple glycine pools.

Use of stable isotopes and gas chromatography-mass spectrometry in the study of different pools of neurotransmitter amino acids in brain slices

A method was developed for simultaneous determination of endogenous and newly synthesized neurotransmitter amino acids (4-aminobutyric acid, glutamate and aspartate) and glutamine in brain in vitro. Brain slices were incubated in artificial cerebrospinal fluid in the presence of 13C-labeled precursors (glucose, pyruvate or acetate). After the incubation, the slices were homogenized in cold 80% ethanol and the supernatants were evaporated to dryness. The resultant residues were derivatized with N-methyl-N-(tert.-butyldimethylsilyl)trifluoroacetamide and analyzed by capillary gas chromatography-mass spectrometry in the electron-impact mode. N(O)-tert.-Butyldimethylsilyl derivatives of the naturally occurring amino acids, their 13C-enriched counterparts and deuterated internal standards were detected as their [M-57]+ fragments using selected-ion monitoring. The method was shown applicable to studying compartmentation of neurotransmitter amino acids.

Use of inhibitors of gamma-aminobutyric acid (GABA) transaminase for the estimation of GABA turnover in various brain regions of rats: a reevaluation of aminooxyacetic acid

The technique of estimating gamma-aminobutyric acid (GABA) turnover by inhibiting its major degrading enzyme GABA-T (4-aminobutyrate:2-oxoglutarate aminotransferase; EC 2.6.1.19) and measuring GABA accumulation has been used repeatedly, but, at least in rats, its usefulness has been limited by several difficulties, including marked differences in the degree of GABA-T inhibition in different brain regions after systemic injection of GABA-T inhibitors. In an attempt to improve this type of approach for measuring GABA turnover, the time course of GABA-T inhibition and accumulation of GABA in 12 regions of rat brain has been studied after systemic administration of aminooxyacetic acid (AOAA), injected at various doses and with different routes of administration. A total and rapidly occurring inhibition of GABA-T in all regions was obtained with intraperitoneal injection of 100 mg/kg AOAA, whereas after lower doses, marked regional differences in the degree of GABA-T inhibition were found, thus leading to underestimation of GABA synthesis rates, e.g., in substantia nigra. The activity of the GABA-synthesizing enzyme GAD (L-glutamate-1-decarboxylase; EC 4.1.1.15) was not reduced significantly at any time after intraperitoneal injection of AOAA, except for a small decrease in olfactory bulbs. Even the highest dose of AOAA tested (100 mg/kg) was not associated with toxicity in rats, but induced motor impairment, which was obviously related to the marked GABA accumulation found with this dose. The increase in GABA concentrations induced with intraperitoneal injection of 100 mg/kg AOAA was rapid in onset, allowing one to estimate GABA turnover rates from the initial rate of GABA accumulation, i.e., during the first 30 min after AOAA injection. GABA turnover rates thus determined were correlated in a highly significant fashion with the GAD activities determined in brain regions, with highest turnover rates measured in substantia nigra, hypothalamus, olfactory bulb, and tectum. Pretreatment of rats with diazepam, 5 mg/kg i.p., 5-30 min prior to AOAA, reduced the AOAA-induced GABA accumulation in all 12 regions examined, most probably as a result of potentiation of postsynaptic GABA function. The data indicate that AOAA is a valuable tool for regional GABA turnover studies in rats, provided the GABA-T inhibitor is administered in sufficiently high doses to obtain complete inhibition of GABA degradation.

Changes with aging in the levels of amino acids in rat CNS structural elements. I. Glutamate and related amino acids

Glutamate and related amino acids were determined in 53 discrete brain areas of 3- and 29-month-old male Fischer 344 rats microdissected with the punch technique. The levels of amino acids showed high regional variation - the ratio of the highest to lowest level was 9 for aspartate, 5 for glutamate, 6 for glutamine, and 21 for GABA. Several areas were found to have all four amino acids at very high or at very low level, but also some areas had some amino acids at high, others at low level. With age, in more than half of the areas, significant changes could be observed; decrease occurred 5 times more frequently than increase. Changes occurred more often in levels of aspartate and GABA than in those of glutamate or glutamine. The regional levels of glutamate and its related amino acids show severalfold variations, with the levels tending to decrease in the aged brain.