Synthesis of vesicular GABA from glutamine involves TCA cycle metabolism in neocortical neurons

A tribute to Arne Schousboe's contributions to neurochemistry and his innovative and enduring research in GABA, glutamate, and brain energy metabolism

This is a tribute to Arne Schousboe, Professor Emeritus at the University of Copenhagen, an eminent neurochemist and neuroscientist who was a leader in the fields of GABA, glutamate, and brain energy metabolism. Arne was known for his keen intellect, his wide-ranging expertise in neurochemistry and neuropharmacology of GABA and glutamate and brain energy metabolism. Arne was also known for his strong leadership, his warm and engaging personality and his enjoyment of fine wine and great food shared with friends, family, and colleagues. Sadly, Arne passed away on February 27, 2024, after a short illness. He is survived by his wife Inger Schousboe, his two children, and three wonderful grandchildren. His death is a tremendous loss to the neuroscience community. He will be greatly missed by his friends, family, and colleagues. Some of the highlights of Arne's career are described in this tribute.

Mechanistic stoichiometric relationship between the rates of neurotransmission and neuronal glucose oxidation: Reevaluation of and alternatives to the pseudo-malate-aspartate shuttle model

The ~1:1 stoichiometry between the rates of neuronal glucose oxidation (CMRglc-ox-N) and glutamate (Glu)/γ-aminobutyric acid (GABA)-glutamine (Gln) neurotransmitter (NT) cycling between neurons and astrocytes (VNTcycle) has been firmly established. However, the mechanistic basis for this relationship is not fully understood, and this knowledge is critical for the interpretation of metabolic and brain imaging studies in normal and diseased brain. The pseudo-malate-aspartate shuttle (pseudo-MAS) model established the requirement for glycolytic metabolism in cultured glutamatergic neurons to produce NADH that is shuttled into mitochondria to support conversion of extracellular Gln (i.e., astrocyte-derived Gln in vivo) into vesicular neurotransmitter Glu. The evaluation of this model revealed that it could explain half of the 1:1 stoichiometry and it has limitations. Modifications of the pseudo-MAS model were, therefore, devised to address major knowledge gaps, that is, submitochondrial glutaminase location, identities of mitochondrial carriers for Gln and other model components, alternative mechanisms to transaminate α-ketoglutarate to form Glu and shuttle glutamine-derived ammonia while maintaining mass balance. All modified models had a similar 0.5 to 1.0 predicted mechanistic stoichiometry between VNTcycle and the rate of glucose oxidation. Based on studies of brain β-hydroxybutyrate oxidation, about half of CMRglc-ox-N may be linked to glutamatergic neurotransmission and localized in pre-synaptic structures that use pseudo-MAS type mechanisms for Glu-Gln cycling. In contrast, neuronal compartments that do not participate in transmitter cycling may use the MAS to sustain glucose oxidation. The evaluation of subcellular compartmentation of neuronal glucose metabolism in vivo is a critically important topic for future studies to understand glutamatergic and GABAergic neurotransmission.

Mutant Tau protein-induced abnormalities in the Na+-dependent glutamine translocation and recycling and their impact on astrocyte-neuron integrity in vitro

Tau-dependent neurodegeneration is accompanied by astrocytosis in a mouse trans-genic model, which replicates the neuropathological characteristic of tauopathy and other human neurodegenerative disorders where astrocyte activation precedes neuronal loss and is associated with disease progression. This indicates an important role of astrocytes in the development of the disease. Astrocytes derived from a transgenic mouse model expressing human Tau, exhibit changes in cellular markers of astrocyte neuroprotective function related to the glutamate-glutamine cycle (GGC), representing a key part of astrocyte-neuron integrity. Here, we focused on investigating the functional properties of key GGC components involved in the astrocyte-neuron network associated with Tau pathology in vitro. Mutant recombinant Tau (rTau) carrying the P301L mutation was added to the neuronal cultures, with or without control astrocyte-conditioned medium (ACM), to study glutamine translocation through the GGC. We demonstrated that mutant Tau in vitro induces neuronal degeneration, while control astrocytes response in neuroprotective way by preventing neurodegeneration. In parallel with this observation, we noticed the Tau-dependent decline of neuronal microtubule associated protein 2 (MAP2), followed by changes in glutamine (Gln) transport. Exposure to rTau decreases sodium-dependent Gln uptake in neurons and that effect was reversed when cells were co-incubated with control ACM after induction of rTau dependent pathology. Further, we found that neuronal Na⁺-dependent system A is the most specific system that is affected under rTau exposure. In addition, in rTau-treated astrocytes total Na⁺-dependent uptake of Gln, which is mediated by the N system, increases. Altogether, our study suggest mechanisms operating in Tau pathology may be related to the alterations in glutamine transport and recycling that affect neuronal-astrocytic integrity.

Glial Glutamine Homeostasis in Health and Disease

Glutamine is an essential cerebral metabolite. Several critical brain processes are directly linked to glutamine, including ammonia homeostasis, energy metabolism and neurotransmitter recycling. Astrocytes synthesize and release large quantities of glutamine, which is taken up by neurons to replenish the glutamate and GABA neurotransmitter pools. Astrocyte glutamine hereby sustains the glutamate/GABA-glutamine cycle, synaptic transmission and general brain function. Cerebral glutamine homeostasis is linked to the metabolic coupling of neurons and astrocytes, and relies on multiple cellular processes, including TCA cycle function, synaptic transmission and neurotransmitter uptake. Dysregulations of processes related to glutamine homeostasis are associated with several neurological diseases and may mediate excitotoxicity and neurodegeneration. In particular, diminished astrocyte glutamine synthesis is a common neuropathological component, depriving neurons of an essential metabolic substrate and precursor for neurotransmitter synthesis, hereby leading to synaptic dysfunction. While astrocyte glutamine synthesis is quantitatively dominant in the brain, oligodendrocyte-derived glutamine may serve important functions in white matter structures. In this review, the crucial roles of glial glutamine homeostasis in the healthy and diseased brain are discussed. First, we provide an overview of cellular recycling, transport, synthesis and metabolism of glutamine in the brain. These cellular aspects are subsequently discussed in relation to pathological glutamine homeostasis of hepatic encephalopathy, epilepsy, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. Further studies on the multifaceted roles of cerebral glutamine will not only increase our understanding of the metabolic collaboration between brain cells, but may also aid to reveal much needed therapeutic targets of several neurological pathologies.

Global Metabolic Shifts in Age and Alzheimer's Disease Mouse Brains Pivot at NAD+/NADH Redox Sites

Age and Alzheimer’s disease (AD) share some common features such as cognitive impairments, memory loss, metabolic disturbances, bioenergetic deficits, and inflammation. Yet little is known on how systematic shifts in metabolic networks depend on age and AD. In this work, we investigated the global metabolomic alterations in non-transgenic (NTg) and triple-transgenic (3xTg-AD) mouse brain hippocampus as a function of age by using untargeted Ultrahigh Performance Liquid Chromatography-tandem Mass Spectroscopy (UPLC-MS/MS). We observed common metabolic patterns with aging in both NTg and 3xTg-AD brains involved in energy-generating pathways, fatty acids oxidation, glutamate, and sphingolipid metabolism. We found age-related downregulation of metabolites from reactions in glycolysis that consumed ATP and in the TCA cycle, especially at NAD⁺/NADH-dependent redox sites, where age- and AD-associated limitations in the free NADH may alter reactions. Conversely, metabolites increased in glycolytic reactions in which ATP is produced. With age, inputs to the TCA cycle were increased including fatty acid β-oxidation and glutamine. Overall age- and AD-related changes were > 2-fold when comparing the declines of upstream metabolites of NAD⁺/NADH-dependent reactions to the increases of downstream metabolites (p = 10^-5, n = 8 redox reactions). Inflammatory metabolites such as ceramides and sphingosine-1-phosphate also increased with age. Age-related decreases in glutamate, GABA, and sphingolipid were seen which worsened with AD genetic load in 3xTg-AD brains, possibly contributing to synaptic, learning- and memory-related deficits. The data support the novel hypothesis that age- and AD-associated metabolic shifts respond to NAD(P)⁺/NAD(P)H redox-dependent reactions, which may contribute to decreased energetic capacity.

NCX1 and EAAC1 transporters are involved in the protective action of glutamate in an in vitro Alzheimer's disease-like model

Increasing evidence suggests that metabolic dysfunctions are at the roots of neurodegenerative disorders such as Alzheimer's disease (AD). In particular, defects in cerebral glucose metabolism, which have been often noted even before the occurrence of clinical symptoms and histopathological lesions, are now regarded as critical contributors to the pathogenesis of AD. Hence, the stimulation of energy metabolism, by enhancing the availability of specific metabolites, might be an alternative way to improve ATP synthesis and to positively affect AD progression. For instance, glutamate may serve as an intermediary metabolite for ATP synthesis through the tricarboxylic acid (TCA) cycle and the oxidative phosphorylation. We have recently shown that two transporters are critical for the anaplerotic use of glutamate: the Na⁺-dependent Excitatory Amino Acids Carrier 1 (EAAC1) and the Na⁺-Ca²⁺ exchanger 1 (NCX1). Therefore, in the present study, we established an AD-like phenotype by perturbing glucose metabolism in both primary rat cortical neurons and retinoic acid (RA)-differentiated SH-SY5Y cells, and we explored the potential of glutamate to halt cell damage by monitoring neurotoxicity, AD markers, ATP synthesis, cytosolic Ca²⁺ levels and EAAC1/NCX1 functional activities. We found that glutamate significantly increased ATP production and cell survival, reduced the increase of AD biomarkers (amyloid β protein and the hyperphosphorylated form of tau protein), and recovered the increase of NCX reverse-mode activity. The RNA silencing of either EAAC1 or NCX1 caused the loss of the beneficial effects of glutamate, suggesting the requirement of a functional interplay between these transporters for glutamate-induced protection. Remarkably, our results indicate, as proof-of-principle, that facilitating the use of alternative fuels, like glutamate, may be an effective approach to overcome deficits in glucose utilization and significantly slow down neuronal degenerative process in AD.

Astroglial contribution to tau-dependent neurodegeneration

Astrocytes, by maintaining an optimal environment for neuronal function, play a critical role in proper function of mammalian nervous system. They regulate synaptic transmission and plasticity and protect neurons against toxic insults. Astrocytes and neurons interact actively via glutamine-glutamate cycle (GGC) that supports neuronal metabolic demands and neurotransmission. GGC deficiency may be involved in different diseases of the brain, where impaired astrocytic control of glutamate homeostasis contributes to neuronal dysfunction. This includes tau-dependent neurodegeneration, where astrocytes lose key molecules involved in regulation of glutamate/glutamine homeostasis, neuronal survival and synaptogenesis. Astrocytic dysfunction in tauopathy appears to precede neurodegeneration and overt tau neuropathology such as phosphorylation, aggregation and formation of neurofibrillary tangles. In this review, we summarize recent studies demonstrating that activation of astrocytes is strictly associated with neurodegenerative processes including those involved in tau related pathology. We propose that astrocytic dysfunction, by disrupting the proper neuron-glia signalling early in the disease, significantly contributes to tauopathy pathogenesis.

The impact of manganese on neurotransmitter systems

BACKGROUND

Manganese (Mn) is a metal ubiquitously present in nature and essential for many living organisms. As a trace element, it is required in small amounts for the proper functioning of several important enzymes, and reports of Mn deficiency are indeed rare.

METHODS

This mini-review will cover aspects of Mn toxicokinetics and its impact on brain neurotransmission, as well as its Janus-faced effects on humans and other animal's health.

RESULTS

The estimated safe upper limit of intracellular Mn for physiological function is in anarrow range of 20-53 μM.Therefore, intake of higher levels of Mn and the outcomes, especially to the nervous system, have been well documented.

CONCLUSION

The metal affects mostly the brain by accumulating in specific areas, altering cognitive functions and locomotion, thus severely impacting the health of the exposed organisms.

Disturbed neurotransmitter homeostasis in ether lipid deficiency

Plasmalogens, the most prominent ether (phospho)lipids in mammals, are structural components of most cellular membranes. Due to their physicochemical properties and abundance in the central nervous system, a role of plasmalogens in neurotransmission has been proposed, but conclusive data are lacking. Here, we targeted this issue in the glyceronephosphate O-acyltransferase (Gnpat) KO mouse, a model of complete deficiency in ether lipid biosynthesis. Throughout the study, focusing on adult male animals, we found reduced brain levels of various neurotransmitters. In the dopaminergic nigrostriatal tract, synaptic endings but not neuronal cell bodies were affected. Neurotransmitter turnover was altered in ether lipid-deficient murine as well as human post-mortem brain tissue. A generalized loss of synapses did not account for the neurotransmitter deficits, since the levels of several presynaptic proteins appeared unchanged. However, reduced amounts of vesicular monoamine transporter indicate a compromised vesicular uptake of neurotransmitters. As exemplified by norepinephrine, the release of neurotransmitters from Gnpat KO brain slices was diminished in response to strong electrical and chemical stimuli. Finally, addressing potential phenotypic correlates of the disturbed neurotransmitter homeostasis, we show that ether lipid deficiency manifests as hyperactivity and impaired social interaction. We propose that the lack of ether lipids alters the properties of synaptic vesicles leading to reduced amounts and release of neurotransmitters. These features likely contribute to the behavioral phenotype of Gnpat KO mice, potentially modeling some human neurodevelopmental disorders like autism or attention deficit hyperactivity disorder.

Metabolism of Amino Acids in the Brain and Their Roles in Regulating Food Intake

Amino acids (AAs) and their metabolites play an important role in neurological health and function. They are not only the building blocks of protein but are also neurotransmitters. In the brain, glutamate and aspartate are the major excitatory neurotransmitters, whereas γ-aminobutyrate (GABA, a metabolite of glutamate) and glycine are the major inhibitory neurotransmitters. Nitric oxide (NO, a metabolite of arginine), H₂S (a metabolite of cysteine), serotonin (a metabolite of tryptophan) and histamine (a metabolite of histidine), as well as dopamine and norepinephrine (metabolites of tyrosine) are neurotransmitters to modulate synaptic plasticity, neuronal activity, learning, motor control, motivational behavior, emotion, and executive function. Concentrations of glutamine (a precursor of glutamate and aspartate), branched-chain AAs (precursors of glutamate, glutamine and aspartate), L-serine (a precursor of glycine and D-serine), methionine and phenylalanine in plasma are capable of affecting neurotransmission through the syntheses of glutamate, aspartate, and glycine, as well as the competitive transport of tryptophan and tyrosine across from the blood-brain barrier. Adequate consumption of AAs is crucial to maintain their concentrations and the production of neurotransmitters in the central nervous system. Thus, the content and balance of AAs in diets have a profound impact on food intake by animals. Knowledge of AA transport and metabolism in the brain is beneficial for improving the health and well-being of humans and animals.

Role of Astrocytes in Manganese Neurotoxicity Revisited

Manganese (Mn) overexposure is a public health concern due to its widespread industrial usage and the risk for environmental contamination. The clinical symptoms of Mn neurotoxicity, or manganism, share several pathological features of Parkinson's disease (PD). Biologically, Mn is an essential trace element, and Mn in the brain is preferentially localized in astrocytes. This review summarizes the role of astrocytes in Mn-induced neurotoxicity, specifically on the role of neurotransmitter recycling, neuroinflammation, and genetics. Mn overexposure can dysregulate astrocytic cycling of glutamine (Gln) and glutamate (Glu), which is the basis for Mn-induced excitotoxic neuronal injury. In addition, reactive astrocytes are important mediators of Mn-induced neuronal damage by potentiating neuroinflammation. Genetic studies, including those with Caenorhabditis elegans (C. elegans) have uncovered several genes associated with Mn neurotoxicity. Though we have yet to fully understand the role of astrocytes in the pathologic changes characteristic of manganism, significant strides have been made over the last two decades in deciphering the role of astrocytes in Mn-induced neurotoxicity and neurodegeneration.

Glutamate metabolism in temporal lobe epilepsy as revealed by dynamic proton MRS following the infusion of [U13-C] glucose

Focal metabolic dysfunction commonly observed in temporal lobe epilepsy (TLE), and is associated with the development of medical intractability and neurocognitive deficits. It has not been established if this dysfunction is due to cell loss or biochemical dysfunction in metabolic pathways. To explore this question, dynamic ¹H MRS following an infusion of [U¹³- C] glucose was performed to measure glutamate (Glu) metabolism. Subjects (n=6) showed reduced Glu levels (p<0.01) in the ipsilateral mesial temporal lobe (MTL) compared with controls (n=4). However, the rate of ¹³C incorporation into Glu did not differ between those with epilepsy and controls (p=0.77). This suggests that reduced Glu concentrations in the region of the seizure focus are not due to disruptions in metabolic pathways, but may instead be due to neuronal loss or simplification.

The Eating-Disorder Associated HDAC4A778T Mutation Alters Feeding Behaviors in Female Mice

BACKGROUND

While eating disorders (EDs) are thought to result from a combination of environmental and psychological stressors superimposed on genetic vulnerability, the neurobiological basis of EDs remains incompletely understood. We recently reported that a rare missense mutation in the gene for the transcriptional repressor histone deacetylase 4 (HDAC4) is associated with the risk of developing an ED in humans.

METHODS

To understand the biological consequences of this missense mutation, we created transgenic mice carrying this mutation by introducing the alanine to threonine mutation at position 778 of mouse Hdac4 (corresponding to position 786 of the human protein). Bioinformatic analysis to identify Hdac4-regulated genes was performed using available databases.

RESULTS

Male mice heterozygous for HDAC4^A778T did not show any metabolic or behavioral differences. In contrast, female mice heterozygous for HDAC4^A778T display several ED-related feeding and behavioral deficits depending on housing condition. Individually housed HDAC4^A778T female mice exhibit reduced effortful responding for high-fat diet and compulsive grooming, whereas group-housed female mice display increased weight gain on high-fat diet, reduced behavioral despair, and increased anxiety-like behaviors. Bioinformatic analysis identifies mitochondrial biogenesis including synthesis of glutamate/gamma-aminobutyric acid as a potential transcriptional target of HDAC4^A778T activity relevant to the behavioral deficits identified in this new mouse model of disordered eating.

CONCLUSIONS

The HDAC4^A778T mouse line is a novel model of ED-related behaviors and identifies mitochondrial biogenesis as a potential molecular pathway contributing to behavioral deficits.

Comparison of Glutamate Turnover in Nerve Terminals and Brain Tissue During [1,6-13C2]Glucose Metabolism in Anesthetized Rats

The ¹³C turnover of neurotransmitter amino acids (glutamate, GABA and aspartate) were determined from extracts of forebrain nerve terminals and brain homogenate, and fronto-parietal cortex from anesthetized rats undergoing timed infusions of [1,6-¹³C₂]glucose or [2-¹³C]acetate. Nerve terminal ¹³C fractional labeling of glutamate and aspartate was lower than those in whole cortical tissue at all times measured (up to 120 min), suggesting either the presence of a constant dilution flux from an unlabeled substrate or an unlabeled (effectively non-communicating on the measurement timescale) glutamate pool in the nerve terminals. Half times of ¹³C labeling from [1,6-¹³C₂]glucose, as estimated by least squares exponential fitting to the time course data, were longer for nerve terminals (Glu_C4, 21.8 min; GABA_C2 21.0 min) compared to cortical tissue (Glu_C4, 12.4 min; GABA_C2, 14.5 min), except for Asp_C3, which was similar (26.5 vs. 27.0 min). The slower turnover of glutamate in the nerve terminals (but not GABA) compared to the cortex may reflect selective effects of anesthesia on activity-dependent glucose use, which might be more pronounced in the terminals. The ¹³C labeling ratio for glutamate-C4 from [2-¹³C]acetate over that of ¹³C-glucose was twice as large in nerve terminals compared to cortex, suggesting that astroglial glutamine under the ¹³C glucose infusion was the likely source of much of the nerve terminal dilution. The net replenishment of most of the nerve terminal amino acid pools occurs directly via trafficking of astroglial glutamine.

Glucose, Lactate, β-Hydroxybutyrate, Acetate, GABA, and Succinate as Substrates for Synthesis of Glutamate and GABA in the Glutamine-Glutamate/GABA Cycle

The glutamine-glutamate/GABA cycle is an astrocytic-neuronal pathway transferring precursors for transmitter glutamate and GABA from astrocytes to neurons. In addition, the cycle carries released transmitter back to astrocytes, where a minor fraction (~25 %) is degraded (requiring a similar amount of resynthesis) and the remainder returned to the neurons for reuse. The flux in the cycle is intense, amounting to the same value as neuronal glucose utilization rate or 75-80 % of total cortical glucose consumption. This glucose:glutamate ratio is reduced when high amounts of β-hydroxybutyrate are present, but β-hydroxybutyrate can at most replace 60 % of glucose during awake brain function. The cycle is initiated by α-ketoglutarate production in astrocytes and its conversion via glutamate to glutamine which is released. A crucial reaction in the cycle is metabolism of glutamine after its accumulation in neurons. In glutamatergic neurons all generated glutamate enters the mitochondria and its exit to the cytosol occurs in a process resembling the malate-aspartate shuttle and therefore requiring concomitant pyruvate metabolism. In GABAergic neurons one half enters the mitochondria, whereas the other one half is released directly from the cytosol. A revised concept is proposed for the synthesis and metabolism of vesicular and nonvesicular GABA. It includes the well-established neuronal GABA reuptake, its metabolism, and use for resynthesis of vesicular GABA. In contrast, mitochondrial glutamate is by transamination to α-ketoglutarate and subsequent retransamination to releasable glutamate essential for the transaminations occurring during metabolism of accumulated GABA and subsequent resynthesis of vesicular GABA.

Enzyme Complexes Important for the Glutamate-Glutamine Cycle

Transient multienzyme and/or multiprotein complexes (metabolons) direct substrates toward specific pathways and can significantly influence the metabolism of glutamate and glutamine in the brain. Glutamate is the primary excitatory neurotransmitter in brain. This neurotransmitter has essential roles in normal brain function including learning and memory. Metabolism of glutamate involves the coordinated activity of astrocytes and neurons and high affinity transporter proteins that are selectively distributed on these cells. This chapter describes known and possible metabolons that affect the metabolism of glutamate and related compounds in the brain, as well as some factors that can modulate the association and dissociation of such complexes, including protein modifications by acylation reactions (e.g., acetylation, palmitoylation, succinylation, SUMOylation, etc.) of specific residues. Development of strategies to modulate transient multienzyme and/or enzyme-protein interactions may represent a novel and promising therapeutic approach for treatment of diseases involving dysregulation of glutamate metabolism.

A new role for α-ketoglutarate dehydrogenase complex: regulating metabolism through post-translational modification of other enzymes

This Editorial highlights a study by Gibson et al. published in this issue of JNeurochem, in which the authors reveal a novel role for the α-ketoglutarate dehydrogenase complex (KGDHC) in post-translational modification of proteins. KGDHC may catalyze post-translational modification of itself as well as several other proteins by succinylation of lysine residues. The authors' report of an enzyme responsible for succinylation of key mitochondrial enzymes represents a major step toward our understanding of the complex functional metabolome. TCA, tricarboxylic acid; KG, α-ketoglutarate; KGDHC, α-ketoglutarate dehydrogenase complex; FUM, fumarase; MDH, malate dehydrogenase; ME, malic enzyme; GDH, glutamate dehydrogenase; AAT, aspartate aminotransferase; GS, glutamine synthetase; PAG, phosphate-activated glutaminase; SIRT3, silent information regulator 3; SIRT5, silent information regulator 5.

Expression of glutamine transporter isoforms in cerebral cortex of rats with chronic hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric disorder that occurs due to acute and chronic liver diseases, the hallmark of which is the increased levels of ammonia and subsequent alterations in glutamine synthesis, i.e. conditions associated with the pathophysiology of HE. Under physiological conditions, glutamine is fundamental for replenishment of the neurotransmitter pools of glutamate and GABA. The different isoforms of glutamine transporters play an important role in the transfer of this amino acid between astrocytes and neurons. A disturbance in the GABA biosynthetic pathways has been described in bile duct ligated (BDL) rats, a well characterized model of chronic HE. Considering that glutamine is important for GABA biosynthesis, altered glutamine transport and the subsequent glutamate/GABA-glutamine cycle efficacy might influence these pathways. Given this potential outcome, the aim of the present study was to investigate whether the expression of the glutamine transporters SAT1, SAT2, SN1 and SN2 would be affected in chronic HE. We verified that mRNA expression of the neuronal glutamine transporters SAT1 and SAT2 was found unaltered in the cerebral cortex of BDL rats. Similarly, no changes were found in the mRNA level for the astrocytic transporter SN1, whereas the gene expression of SN2 was increased by two-fold in animals with chronic HE. However, SN2 protein immuno-reactivity did not correspond with the increase in gene transcription since it remained unaltered. These data indicate that the expression of the glutamine transporter isoforms is unchanged during chronic HE, and thus likely not to participate in the pathological mechanisms related to the imbalance in the GABAergic neurotransmitter system observed in this neurologic condition.

Effects of γ-Aminobutyric acid transporter 1 inhibition by tiagabine on brain glutamate and γ-Aminobutyric acid metabolism in the anesthetized rat In vivo

γ-Aminobutyric acid (GABA) clearance from the extracellular space after release from neurons involves reuptake into terminals and astrocytes through GABA transporters (GATs). The relative flows through these two pathways for GABA released from neurons remains unclear. This study determines the effect of tiagabine, a selective inhibitor of neuronal GAT-1, on the rates of glutamate (Glu) and GABA metabolism and GABA resynthesis via the GABA-glutamine (Gln) cycle. Halothane-anesthetized rats were administered tiagabine (30 mg/kg, i.p.) and 45 min later received an intravenous infusion of either [1,6-(13)C2]glucose (in vivo) or [2-(13)C]acetate (ex vivo). Nontreated rats served as controls. Metabolites and (13)C enrichments were measured with (1)H-[(13)C]-nuclear magnetic resonance spectroscopy and referenced to their corresponding endpoint values measured in extracts from in situ frozen brain. Metabolic flux estimates of GABAergic and glutamatergic neurons were determined by fitting a metabolic model to the (13)C turnover data measured in vivo during [1,6-(13)C2]glucose infusion. Tiagabine-treated rats were indistinguishable (P > 0.05) from controls in tissue amino acid levels and in (13)C enrichments from [2-(13)C]acetate. Tiagabine reduced average rates of glucose oxidation and neurotransmitter cycling in both glutamatergic neurons (↓18%, CMR(glc(ox)Glu): control, 0.27 ± 0.05 vs. tiagabine, 0.22 ± 0.04 µmol/g/min; ↓11%, V(cyc(Glu-Gln)): control 0.23 ± 0.05 vs. tiagabine 0.21 ± 0.04 µmol/g/min and GABAergic neurons (↓18-25%, CMR(glc(ox)GABA): control 0.09 ± 0.02 vs. tiagabine 0.07 ± 0.03 µmol/g/min; V(cyc(GABA-Gln)): control 0.08 ± 0.02 vs. tiagabine 0.07 ± 0.03 µmol/g/min), but the changes in glutamatergic and GABAergic fluxes were not significant (P > 0.10). The results suggest that any reduction in GABA metabolism by tiagabine might be an indirect response to reduced glutamatergic drive rather than direct compensatory effects.

GABA deficit in the visual cortex of patients with neurofibromatosis type 1: genotype-phenotype correlations and functional impact

Alterations in the balance between excitatory and inhibitory neurotransmission have been implicated in several neurodevelopmental disorders. Neurofibromatosis type 1 is one of the most common monogenic disorders causing cognitive deficits for which studies on a mouse model (Nfl(+/-)) proposed increased γ-aminobutyric acid-mediated inhibitory neurotransmission as the neural mechanism underlying these deficits. To test whether a similar mechanism translates to the human disorder, we used magnetic resonance spectroscopy to measure γ-aminobutyric acid levels in the visual cortex of children and adolescents with neurofibromatosis type 1 (n = 20) and matched control subjects (n = 26). We found that patients with neurofibromatosis type 1 have significantly lower γ-aminobutyric acid levels than control subjects, and that neurofibromatosis type 1 mutation type significantly predicted cortical γ-aminobutyric acid. Moreover, functional imaging of the visual cortex indicated that blood oxygen level-dependent signal was correlated with γ-aminobutyric acid levels both in patients and control subjects. Our results provide in vivo evidence of γ-aminobutyric acidergic dysfunction in neurofibromatosis type 1 by showing a reduction in γ-aminobutyric acid levels in human patients. This finding is relevant to understand the physiological profile of the disorder and has implications for the identification of targets for therapeutic strategies.

Valine but not leucine or isoleucine supports neurotransmitter glutamate synthesis during synaptic activity in cultured cerebellar neurons

Synthesis of neuronal glutamate from α-ketoglutarate for neurotransmission necessitates an amino group nitrogen donor; however, it is not clear which amino acid(s) serves this role. Thus, the ability of the three branched-chain amino acids (BCAAs), leucine, isoleucine, and valine, to act as amino group nitrogen donors for synthesis of vesicular neurotransmitter glutamate was investigated in cultured mouse cerebellar (primarily glutamatergic) neurons. The cultures were superfused in the presence of (15) N-labeled BCAAs, and synaptic activity was induced by pulses of N-methyl-D-aspartate (300 μM), which results in release of vesicular glutamate. At the end of the superfusion experiment, the vesicular pool of glutamate was released by treatment with α-latrotoxin (3 nM, 5 min). This experimental paradigm allows a separate analysis of the cytoplasmic and vesicular pools of glutamate. Amount and extent of (15) N labeling of intracellular amino acids plus vesicular glutamate were analyzed employing HPLC and LC-MS analysis. Only when [(15) N]valine served as precursor did the labeling of both cytoplasmic and vesicular glutamate increase after synaptic activity. In addition, only [(15) N]valine was able to maintain the amount of vesicular glutamate during synaptic activity. This indicates that, among the BCAAs, only valine supports the increased need for synthesis of vesicular glutamate.

Ischemic cerebral damage: an appraisal of synaptic failure

In the human brain, ≈30% of the energy is spent on synaptic transmission. Disappearance of synaptic activity is the earliest consequence of cerebral ischemia. The changes of synaptic function are generally assumed to be reversible and persistent damage is associated with membrane failure and neuronal death. However, there is overwhelming experimental evidence of isolated, but persistent, synaptic failure resulting from mild or moderate cerebral ischemia. Early failure results from presynaptic damage with impaired transmitter release. Proposed mechanisms include dysfunction of adenosine triphosphate-dependent calcium channels and a disturbed docking of glutamate-containing vesicles resulting from impaired phosphorylation. We review energy distribution among neuronal functions, focusing on energy usage of synaptic transmission. We summarize the effect of ischemia on neurotransmission and the evidence of long-lasting synaptic failure as a cause of persistent symptoms in patients with cerebral ischemia. Finally, we discuss the implications of synaptic failure in the diagnosis of cerebral ischemia, including the limited sensitivity of diffusion-weighted MRI in those cases in which damage is presumably limited to the synapses.

Succinic semialdehyde dehydrogenase: biochemical-molecular-clinical disease mechanisms, redox regulation, and functional significance

Succinic semialdehyde dehydrogenase (SSADH; aldehyde dehydrogenase 5a1, ALDH5A1; E.C. 1.2.1.24; OMIM 610045, 271980) deficiency is a rare heritable disorder that disrupts the metabolism of the inhibitory neurotransmitter 4-aminobutyric acid (GABA). Identified in conjunction with increased urinary excretion of the GABA analog gamma-hydroxybutyric acid (GHB), numerous patients have been identified worldwide and the autosomal-recessive disorder has been modeled in mice. The phenotype is one of nonprogressive neurological dysfunction in which seizures may be prominently displayed. The murine model is a reasonable phenocopy of the human disorder, yet the severity of the seizure disorder in the mouse exceeds that observed in SSADH-deficient patients. Abnormalities in GABAergic and GHBergic neurotransmission, documented in patients and mice, form a component of disease pathophysiology, although numerous other disturbances (metabolite accumulations, myelin abnormalities, oxidant stress, neurosteroid depletion, altered bioenergetics, etc.) are also likely to be involved in developing the disease phenotype. Most recently, the demonstration of a redox control system in the SSADH protein active site has provided new insights into the regulation of SSADH by the cellular oxidation/reduction potential. The current review summarizes some 30 years of research on this protein and disease, addressing pathological mechanisms in human and mouse at the protein, metabolic, molecular, and whole-animal level.

Glutamate dehydrogenase in brain mitochondria: do lipid modifications and transient metabolon formation influence enzyme activity?

Metabolism of glutamate, the primary excitatory neurotransmitter in brain, is complex and of paramount importance to overall brain function. Thus, understanding the regulation of enzymes involved in formation and disposal of glutamate and related metabolites is crucial to understanding glutamate metabolism. Glutamate dehydrogenase (GDH) is a pivotal enzyme that links amino acid metabolism and TCA cycle activity in brain and other tissues. The allosteric regulation of GDH has been extensively studied and characterized. Less is known about the influence of lipid modifications on GDH activity, and the participation of GDH in transient heteroenzyme complexes (metabolons) that can greatly influence metabolism by altering kinetic parameters and lead to channeling of metabolites. This review summarizes evidence for palmitoylation and acylation of GDH, information on protein binding, and information regarding the participation of GDH in transient heteroenzyme complexes. Recent studies suggest that a number of other proteins can bind to GDH altering activity and overall metabolism. It is likely that these modifications and interactions contribute additional levels of regulation of GDH activity and glutamate metabolism.

Synthesis of neurotransmitter GABA via the neuronal tricarboxylic acid cycle is elevated in rats with liver cirrhosis consistent with a high GABAergic tone in chronic hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric complication to liver disease. It is known that ammonia plays a role in the pathogenesis of HE and disturbances in the GABAergic system have been related to HE. Synthesis of GABA occurs by decarboxylation of glutamate formed by deamidation of astrocyte-derived glutamine. It is known that a fraction of glutamate is decarboxylated directly to GABA (referred to as the direct pathway) and that a fraction undergoes transamination with formation of alpha-ketoglutarate. The latter fraction is cycled through the neuronal tricarboxylic acid cycle, an energy-generating pathway, prior to being employed for GABA synthesis (the indirect pathway). We have previously shown that ammonia induces an elevation of the neuronal tricarboxylic acid cycle activity. Thus, the aims of the present study were to determine if increased levels of ammonia increase GABA synthesis via the indirect pathway in a rat model of HE induced by bile-duct ligation and in co-cultures of neurons and astrocytes exposed to ammonia. Employing (13) C-labeled precursors and subsequent analysis by mass spectrometry, we demonstrated that more GABA was synthesized via the indirect pathway in bile duct-ligated rats and in co-cultures subjected to elevated ammonia levels. Since the indirect pathway is associated with synthesis of vesicular GABA, this might explain the increased GABAergic tone in HE.

Knockout of GAD65 has major impact on synaptic GABA synthesized from astrocyte-derived glutamine

γ-Aminobutyric acid (GABA) synthesis from glutamate is catalyzed by glutamate decarboxylase (GAD) of which two isoforms, GAD65 and GAD67, have been identified. The GAD65 has repeatedly been shown to be important during intensified synaptic activity. To specifically elucidate the significance of GAD65 for maintenance of the highly compartmentalized intracellular and intercellular GABA homeostasis, GAD65 knockout and corresponding wild-type mice were injected with [1-(13)C]glucose and the astrocyte-specific substrate [1,2-(13)C]acetate. Synthesis of GABA from glutamine in the GABAergic synapses was further investigated in GAD65 knockout and wild-type mice using [1,2-(13)C]acetate and in some cases γ-vinylGABA (GVG, Vigabatrin), an inhibitor of GABA degradation. A detailed metabolic mapping was obtained by nuclear magnetic resonance (NMR) spectroscopic analysis of tissue extracts of cerebral cortex and hippocampus. The GABA content in both brain regions was reduced by ∼20%. Moreover, it was revealed that GAD65 is crucial for maintenance of biosynthesis of synaptic GABA particularly by direct synthesis from astrocytic glutamine via glutamate. The GAD67 was found to be important for synthesis of GABA from glutamine both via direct synthesis and via a pathway involving mitochondrial metabolism. Furthermore, a severe neuronal hypometabolism, involving glycolysis and tricarboxylic acid (TCA) cycle activity, was observed in cerebral cortex of GAD65 knockout mice.

Metabolism of acetyl-L-carnitine for energy and neurotransmitter synthesis in the immature rat brain

Acetyl-L-carnitine (ALCAR) is an endogenous metabolic intermediate that facilitates the influx and efflux of acetyl groups across the mitochondrial inner membrane. Exogenously administered ALCAR has been used as a nutritional supplement and also as an experimental drug with reported neuroprotective properties and effects on brain metabolism. The aim of this study was to determine oxidative metabolism of ALCAR in the immature rat forebrain. Metabolism was studied in 21-22 day-old rat brain at 15, 60 and 120 min after an intraperitoneal injection of [2-(13)C]acetyl-L-carnitine. The amount, pattern, and fractional enrichment of (13)C-labeled metabolites were determined by ex vivo(13)C-NMR spectroscopy. Metabolism of the acetyl moiety from [2-(13)C]ALCAR via the tricarboxylic acid cycle led to incorporation of label into the C4, C3 and C2 positions of glutamate (GLU), glutamine (GLN) and GABA. Labeling patterns indicated that [2-(13)C]ALCAR was metabolized by both neurons and glia; however, the percent enrichment was higher in GLN and GABA than in GLU, demonstrating high metabolism in astrocytes and GABAergic neurons. Incorporation of label into the C3 position of alanine, both C3 and C2 positions of lactate, and the C1 and C5 positions of glutamate and glutamine demonstrated that [2-(13)C]ALCAR was actively metabolized via the pyruvate recycling pathway. The enrichment of metabolites with (13)C from metabolism of ALCAR was highest in alanine C3 (11%) and lactate C3 (10%), with considerable enrichment in GABA C4 (8%), GLN C3 (approximately 4%) and GLN C5 (5%). Overall, our (13)C-NMR studies reveal that the acetyl moiety of ALCAR is metabolized for energy in both astrocytes and neurons and the label incorporated into the neurotransmitters glutamate and GABA. Cycling ratios showed prolonged cycling of carbon from the acetyl moiety of ALCAR in the tricarboxylic acid cycle. Labeling of compounds formed from metabolism of [2-(13)C]ALCAR via the pyruvate recycling pathway was higher than values reported for other precursors and may reflect high activity of this pathway in the developing brain. This is, to our knowledge, the first study to determine the extent and pathways of ALCAR metabolism for energy and neurotransmitter biosynthesis in the brain.

The glutamate-glutamine cycle is not stoichiometric: fates of glutamate in brain

Although glutamate is usually thought of as the major excitatory neurotransmitter in brain, it is important to note that glutamate has many other fates in brain, including oxidation for energy, incorporation into proteins, and formation of glutamine, gamma-aminobutyric acid (GABA), and glutathione. The compartmentation of glutamate in brain cells is complex and modulated by the presence and concentration of glutamate per se as well as by other metabolites. Both astrocytes and neurons distinguish between exogenous glutamate and glutamate formed endogenously from glutamine via glutaminase. There is evidence of multiple subcellular compartments of glutamate within both neurons and astrocytes, and the carbon skeleton of glutamate can be derived from other amino acids and many energy substrates including glucose, lactate, and 3-hydroxybutyrate. Both astrocytes and neurons utilize glutamate, albeit for cell-specific metabolic fates. Glutamate is readily formed in neurons from glutamine synthesized in astrocytes, released into the extracellular space, and taken up by neurons. However, the glutamate-glutamine cycle is not a stoichiometric cycle but rather an open pathway that interfaces with many other metabolic pathways to varying extents depending on cellular requirements and priorities.

Stirred vessel cultures of rat brain cells aggregates: characterization of major metabolic pathways and cell population dynamics

We report a study on neural metabolism of long-term three-dimensional cultures of rat embryonic brain cells in stirred vessels. Our experimental setup was optimized to keep viable aggregate cultures with neuronal maintenance for up to 44 days. Results show that aggregate size and shape could be hydrodynamically controlled depending on the impeller design, avoiding necrotic centers or significant losses in cell viability. Aggregates were composed mainly of neurons until day 16, whereas an effective growth of the glial population was observed after day 21. Cell metabolic status was evaluated by quantification of several metabolites in the culture medium; amino acid metabolism was used as a marker of metabolic interrelationships between neural cell types. Furthermore, (13)C-NMR spectroscopy was used on day 31 to explore specific metabolic pathways: incubation with [1-(13)C]glucose for 45 hr produced an increase in label incorporation in extracellular alanine, lactate, and glutamine, reflecting mainly astrocytic metabolism. The contribution of anaplerotic vs. oxidative pathways for glutamine synthesis was determined: a 92% reduction in the pyruvate carboxylase flux during the first 41 hr of incubation suggested a decrease in the need for replacing tricarboxylic acid cycle intermediates. We believe that our data corroborate the aggregating cultures as an attractive system to analyze brain cell metabolism being a valuable tool to model metabolic fluxes for in vitro brain diseases.

Regulation of excitation by GABA neurotransmission: focus on metabolism and transport

The vast majority of excitatory synapses in the central nervous system (CNS) utilize glutamate as the neurotransmitter. The level of excitation appears to be under regulatory control by the major inhibitory neurotransmitter GABA, which is synthesized from glutamate by its decarboxylation catalysed by glutamate decarboxylase (GAD). The inactivation of GABA is brought about by high affinity GABA transporters located in the presynaptic GABAergic neurons as well as surrounding astrocytes and subsequently GABA may be metabolized by GABA-transaminase (GABA-T) ultimately allowing the carbon skeleton to enter the tricarboxylic acid (TCA) cycle for oxidative metabolism. In the presynaptic GABAergic neuron, GABA taken up seems, however, preferentially to enter the vesicular GABA pool and hence it is recycled as a transmitter. It has become clear that compounds acting as inhibitors at either the transporters or GABA-T are capable of regulating the inhibitory tonus thus controlling excitation. This has led to development of clinically efficatious antiepileptic drugs. This paper shall review recent progress in targeting these pharmacological entities.

Glutamine as a precursor for transmitter glutamate, aspartate and GABA in the cerebellum: a role for phosphate-activated glutaminase

Phosphate-activated glutaminase is present at high levels in the cerebellar mossy fiber terminals. The role of this enzyme for the production of glutamate from glutamine in the parallel-fiber terminals is unclear. In order to address this, we used light miroscopic immunoperoxidase and electron microscopic immunogold methods to study the localization of glutamate in rat cerbellar slices incubated with physiological K+ (3 mmol/L) and depolarizing K+ (40 mmol/L) concentrations, and during depolarizing conditions with the addition of glutamine and the glutaminase inhibitor 6-diazo-5-oxo-l-norleucine. During K+-induced depolarization glutamate labeling was redistributed from parallel-fiber terminals to glial cells. The nerve terminal content of glutamate was sustained when the slices were supplied with glutamine, which also reduced the accumulation of glutamate in glia. In spite of glutamine supplementation, the depolarized slices treated with 6-diazo-5-oxo-l-norleucine showed depletion of glutamate from parallel-fiber terminals and accumulation in glial cells. We conclude that cerebellar parallel-fiber terminals contain a glutaminase activity enabling them to synthesize glutamate from glutamine. Our results confirm that this is also true for the mossy fiber terminals. In addition, we show that, like for glutamate, the levels of aspartate in parallel-fiber terminals and GABA in Golgi fiber terminals can be maintained during depolarization if glutamine is present. This process is dependent on the activity of a glutaminase, as it can be inhibited by 6-diazo-5-oxo-l-norleucine, suggesting that the glutaminase reaction is important for glutamine to act as a precursor also for aspartate and GABA. The low levels of the kidney type of glutaminase that previously has been shown to be present in the parallel and Golgi fiber terminals could be sufficient to produce the transmitter amino acids. Alternatively, the amino acids could be produced from the liver type of glutaminase, which is not yet localized on the cellular level, or from an unknown glutminase.

Glutamine uptake by System A transporters maintains neurotransmitter GABA synthesis and inhibitory synaptic transmission

GABA synthesis is necessary to maintain synaptic vesicle filling, and key proteins in its biosynthetic pathways may play a role in regulating inhibitory synaptic stability and strength. GABAergic neurons require a source of precursor glutamate, possibly from glutamine, although it is controversial whether glutamine contributes to the synaptic pool of GABA. Here we report that inhibition of System A glutamine transporters with alpha-(methyl-amino) isobutyric acid rapidly reduced the amplitude of inhibitory post-synaptic currents and miniature inhibitory post-synaptic currents (mIPSCs) recorded in rat hippocampal area cornu ammonis 1 (CA1) pyramidal neurons, indicating that synaptic vesicle content of GABA was reduced. After inhibiting astrocytic glutamine synthesis by either blocking glutamate transporters or the glutamine synthetic enzyme, the effect of alpha-(methyl-amino) isobutyric acid on mIPSC amplitudes was abolished. Exogenous glutamine did not affect mIPSC amplitudes, suggesting that the neuronal transporters are normally saturated. Our findings demonstrate that a constitutive supply of glutamine is provided by astrocytes to inhibitory neurons to maintain vesicle filling. Therefore, glutamine transporters, like those for glutamate, are potential regulators of inhibitory synaptic strength. However, in contrast to glutamate, extracellular glutamine levels are normally high. Therefore, we propose a supportive role for glutamine, even under resting conditions, to maintain GABA vesicle filling.

GABAergic structures in the ventral part of the oral pontine reticular nucleus: An ultrastructural immunogold analysis

GABA mediates inhibitory effects in neurons of the ventral part of the oral pontine reticular nucleus (vRPO). Evidence increasingly suggests that GABA plays an important role in the modulation of rapid eye movement (REM) sleep generation in the cat vRPO. Here, we investigate the anatomical substrate of this modulation using GABA immunocytochemistry. Immunoperoxidase labeling revealed a few small GABA-immunoreactive cell bodies scattered throughout the vRPO. The numerical densities of all vRPO synapses and the GABA-immunoreactive synapses were estimated, at the electron microscopical level, by using a combination of the physical disector and the post-embedding immunogold techniques. We estimated that 30% of all vRPO synaptic terminals were immunoreactive to GABA. Our findings support the hypothesis that vRPO neuron activity is significantly controlled by inhibitory GABAergic terminals that directly target somata and the different parts of the dendritic tree, including distal regions. GABAergic input could inhibit vRPO REM sleep-inducing neurons during other states of the sleep-wakefulness cycle such as wakefulness or non-REM sleep.

Co-localization of glutamic acid decarboxylase and phosphate-activated glutaminase in neurons of lateral reticular nucleus in feline thalamus

Immunohistochemical methods were used to label singly and/or in combination glutamic acid decarboxylase (GAD, the sole synthesizing enzyme for the inhibitory neurotransmitter gamma-aminobutyric acid) and phosphate-activated glutaminase (GLN, a synthesizing enzyme for glutamate) in neurons of lateral reticular nucleus (LRN) of thalamus of adult cats. (1) GAD- and GLN-immunoreactivity (IR) exhibited matching regional patterns of organization within LRN. (2) GAD- and GLN-IR co-localized within most if not all LRN neuronal cell bodies as shown by light microscopy. (3) GAD- and GLN-IR had distinct subcellular localizations in LRN neurons as shown by correlative light/electron microscopy. LRN neurons are important conceptual models where strongly inhibitory cells receive predominant excitatory glutamatergic afferents (from neocortex). Consistent with known actions of intermediary astrocytes, LRN neurons demonstrate GLN enrichment synergistically coupled with glutamatergic innervation to supplement the glutamate pool for GABA synthesis (via GAD) and for metabolic utilization (via the GABA shunt/tricarboxylic acid cycle) but not, apparently, for excitatory neurotransmission.

Evidence that GAD65mediates increased GABA synthesis during intense neuronal activityin vivo

In this study we tested the hypothesis that the 65-kDa isoform of glutamate decarboxylase (GAD(65)) mediates activity-dependent GABA synthesis as invoked by seizures in anesthetized rats. GABA synthesis was measured following acute GABA-transaminase inhibition by gabaculine using spatially localized (1)H NMR spectroscopy before and after bicuculline-induced seizures. Experiments were conducted with animals pre-treated with vigabatrin 24 h earlier in order to reduce GAD(67) protein and also with non-treated controls. GAD isoform content was quantified by immunoblotting. GABA was higher in vigabatrin-treated rats compared to non-treated controls. In vigabatrin-treated animals, GABA synthesis was 28% lower compared to controls [p < 0.05; vigabatrin-treated, 0.043 +/- 0.011 micromol/(g min); non-treated, 0.060 +/- 0.014 micromol/(g min)] and GAD(67) was 60% lower. No difference between groups was observed for GAD(65). Seizures increased GABA synthesis in both control [174%; control, 0.060 +/- 0.014 micromol/(g min) vs. seizures, 0.105 +/- 0.043 micromol/(g min)] and vigabatrin-treated rats [214%; control, 0.043 +/- 0.011 micromol/(g min); seizures, 0.092 +/- 0.018 micromol/(g min)]. GAD(67) could account for at least half of basal GABA synthesis but only 20% of the two-fold increase observed in vigabatrin-treated rats during seizures. The seizure-induced activation of GAD(65) in control cortex occurs concomitantly with a 2.3-fold increase in inorganic phosphate, known to be a potent activator of apoGAD(65)in vitro. Our results are consistent with a major role for GAD(65) in activity-dependent GABA synthesis.

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.

Neurochemical and oedematous changes in 1,3-dinitrobenzene-induced astroglial injury in rat brain from a 1H-nuclear magnetic resonance perspective

1,3-Dinitrobenzene (1,3-DNB), an intermediate used in the chemical industry, has toxic effects in the brain and testes. It produces focal lesions with marked astroglial necrosis in the rat brain upon repeated administration. Astrocytic death occurs in parallel with elevated local blood flow and is followed by damage to the cerebral vasculature and neurones. (1)H-nuclear magnetic resonance spectroscopic analysis before the onset of astrocytic damage, showed a global elevation of lactate, whereas choline containing compounds increased in the non-vulnerable cerebral cortex, yet decreased in the vulnerable brainstem. Similarly, glutamate increased in the cerebral cortex, cerebellum and midbrain, but decreased in the susceptible brainstem. In vivo T2-weighted NMR imaging showed high signal intensities in brain nuclei shown to develop astroglial loss by conventional neuropathology at 24 hours after completion of dosing, but not at 6-10 hours. Hence the early neurochemical changes in susceptible areas contribute to the aetiology of degeneration, and those seen elsewhere may represent adaptive responses dependent on the particular phenotype of different cell groups and underlying metabolic relationships.

Role of glutamine and neuronal glutamate uptake in glutamate homeostasis and synthesis during vesicular release in cultured glutamatergic neurons

Glutamate exists in a vesicular as well as a cytoplasmic pool and is metabolically closely related to the tricarboxylic acid (TCA) cycle. Glutamate released during neuronal activity is most likely to a large extent accumulated by astrocytes surrounding the synapse. A compensatory flux from astrocytes to neurons of suitable precursors is obligatory as neurons are incapable of performing a net synthesis of glutamate from glucose. Glutamine appears to play a major role in this context. Employing cultured cerebellar granule cells, as a model system for glutamatergic neurons, details of the biosynthetic machinery have been investigated during depolarizing conditions inducing vesicular release. [U-13C]Glucose and [U-13C]glutamine were used as labeled precursors for monitoring metabolic pathways by nuclear magnetic resonance (NMR) spectroscopy and liquid chromatography-mass spectrometry (LC-MS) technologies. To characterize release mechanisms and influence of glutamate transporters on maintenance of homeostasis in the glutamatergic synapse, a quantification was performed by HPLC analysis of the amounts of glutamate and aspartate released in response to depolarization by potassium (55 mM) in the absence and presence of DL-threo-beta-benzyloxyaspartate (TBOA) and in response to L-trans-pyrrolidine-2,4-dicarboxylate (t-2,4-PDC), a substrate for the glutamate transporter. Based on labeling patterns of glutamate the biosynthesis of the intracellular pool of glutamate from glutamine was found to involve the TCA cycle to a considerable extent (approximately 50%). Due to the mitochondrial localization of PAG this is unlikely only to reflect amino acid exchange via the cytosolic aspartate aminotransferase reaction. The involvement of the TCA cycle was significantly lower in the synthesis of the released vesicular pool of glutamate. However, in the presence of TBOA, inhibiting glutamate uptake, the difference between the intracellular and the vesicular pool with regard to the extent of involvement of the TCA cycle in glutamate synthesis from glutamine was eliminated. Surprisingly, the intracellular pool of glutamate was decreased after repetitive release from the vesicular pool in the presence of TBOA indicating that neuronal reuptake of released glutamate is involved in the maintenance of the neurotransmitter pool and that 0.5 mM glutamine exogenously supplied is inadequate to sustain this pool.

Compartmentation of glutamine, glutamate, and GABA metabolism in neurons and astrocytes: functional implications

The classical concept regarding compartmentation of brain metabolism pertinent to the two neurotransmitter amino acids, glutamate and GABA (gamma-aminobutyrate), operates with different pools of glutamate and glutamine in different cell types, that is, pools that have different sizes and turnover rates. As a result of more sophisticated technology (e.g., nuclear magnetic resonance spectroscopy and mass spectrometry used in relation to cultured neurons and astrocytes), a more complex scenario is emerging. Hence, both neurons and astrocytes exhibit a compartmentalized metabolism that very likely relates to individual cells containing mitochondrial populations having different metabolic roles. Models for this in neurons and astrocytes, respectively, are presented. The functional implications of this for the homeostatic mechanisms regulating the levels of neurotransmitter glutamate and GABA are discussed in relation to development of therapeutic strategies for neurological disorders in which these transmitters are believed to play important roles.

Characterization of depolarization-coupled release of glutamate from cultured mouse cerebellar granule cells using DL-threo-beta-benzyloxyaspartate (DL-TBOA) to distinguish between the vesicular and cytoplasmic pools

Release of preloaded [3H]D-aspartate in response to depolarization induced by N-methyl-D-aspartate (NMDA) or the endogenous agonist glutamate was characterized using cultured glutamatergic cerebellar granule neurons. Release from the vesicular and the cytoplasmic glutamate pools, respectively, was distinguished employing the competitive, non-transportable glutamate transport inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA). NMDA (300 microM)-induced release was enhanced (50%) by a simultaneous elevation of the extracellular potassium concentration to 15 mM, which lifts the voltage-dependent magnesium block of the NMDA receptors. This NMDA/K(+)-induced release was not sensitive to DL-TBOA (100 microM) but was inhibited by 75% in the presence of the unspecific calcium channel antagonist La(3+) (100 microM). Glutamate (100 microM) induced a large fractional release of the preloaded [3H]D-aspartate and in the presence of DL-TBOA the release was reduced by approximately 50%. In contrast, release evoked by 25 microM glutamate was not inhibited by DL-TBOA. These results indicate that the release elicited by 100 microM glutamate is comprised of a significant glutamate transporter-mediated component in addition to the vesicular release while the NMDA/K(+)-induced release is vesicular in nature. It is likely that the high glutamate concentration (100 microM) may facilitate heteroexchange of the preloaded [3H]D-aspartate.

Age-related alterations in the diffusional transport of amino acids across the human Bruch's-choroid complex

Photoreceptor maintenance is dependent on effective delivery of nutrients from the choroidal circulation by way of the acellular Bruch's membrane and the retinal pigment epithelium. Aging of Bruch's membrane is associated with thickening, increased cross linking of fibers, and deposition of debris culminating in reduced porosity. The present study has investigated the effects of aging on the diffusional transport of eight amino acids across Bruch's membrane in 19 human donors. Diffusion studies were carried out in Ussing chambers, and the amount of time-dependent transfer of amino acids across the preparation was quantified by reverse-phase high-performance liquid chromatography. Diffusion rates for all amino acids showed a significant linear decline with aging of donor. The importance of this reduction in delivery of amino acids is discussed with reference to both normal physiology and age-related macular degeneration.

In vivo effects of adenosine A(2) receptor agonist and antagonist on neuronal and astrocytic intermediary metabolism studied with ex vivo (13)C MR spectroscopy

The effect of adenosine A(2) receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5'-ethylcarboxamidoadenosine (CGS 21680) and antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) on [1-(13)C]glucose and [1,2-(13)C]acetate metabolism was studied in rats by (13)C magnetic resonance (MR) spectroscopy and HPLC. In the cortex a significant reduction was observed in the amounts of [2-(13)C]GABA and [3-(13)C]aspartate from [1-(13)C]glucose in CGS 21680. In the subcortex the concentration of labelled [4-(13)C]glutamate was increased in both treatment groups. The amounts of [2 + 3-(13)C]succinate and [3-(13)C]lactate were increased in the CGS 21680 group compared to control, and the DMPX group showed an increase in the total amount of [6-(13)C]N-acetyl aspartate compared to control in the subcortex. Astrocyte metabolism was only affected in the cortex as shown by a decrease in the pyruvate carboxylase/pyruvate dehydrogenase ratio in glutamate and glutamine in the treatment groups. Labelling from [1,2-(13)C]acetate was not much affected by CGS 21680 or DMPX. However, the amount of [1,2-(13)C]acetate in cortex and subcortex was reduced in the DMPX group. In the cortex a reduction in the labelling of [3-(13)C]GABA in the DMPX group compared to control and an increase in the total amount of taurine in both treatment groups was detected. The present study shows that A(2) receptor agonist and antagonist have similar effects; however, in cortex GABAergic neurones and astrocytes were affected in contrast to subcortex, where glutamatergic neurones showed the greatest changes.