Neurotransmitter amino acids in the CNS. I. Regional changes in amino acid levels in rat brain during ischemia and reperfusion

The hypoxic respiratory response of the pre-Bötzinger complex

Since the discovery of the pre-Bötzinger Complex (preBötC) as a crucial region for generating the main respiratory rhythm, our understanding of its cellular and molecular aspects has rapidly increased within the last few decades. It is now apparent that preBötC is a highly flexible neuronal network that reconfigures state-dependently to produce the most appropriate respiratory output in response to various metabolic challenges, such as hypoxia. However, the responses of the preBötC to hypoxic conditions can be varied based on the intensity, pattern, and duration of the hypoxic challenge. This review discusses the preBötC response to hypoxic challenges at the cellular and network level. Particularly, the involvement of preBötC in the classical biphasic response of the respiratory network to acute hypoxia is illuminated. Furthermore, the article discusses the functional and structural changes of preBötC neurons following intermittent and sustained hypoxic challenges. Accumulating evidence shows that the preBötC neural circuits undergo substantial changes following hypoxia and contribute to several types of the respiratory system's hypoxic ventilatory responses.

Neurovascular Unit as a Source of Ischemic Stroke Biomarkers-Limitations of Experimental Studies and Perspectives for Clinical Application

Cerebral stroke, which is one of the most frequent causes of mortality and leading cause of disability in developed countries, often leads to devastating and irreversible brain damage. Neurological and neuroradiological diagnosis of stroke, especially in its acute phase, is frequently uncertain or inconclusive. This results in difficulties in identification of patients with poor prognosis or being at high risk for complications. It also makes difficult identification of these stroke patients who could benefit from more aggressive therapies. In contrary to the cardiovascular disease, no single biomarker is available for the ischemic stroke, addressing the abovementioned issues. This justifies the need for identifying of effective diagnostic measures characterized by high specificity and sensitivity. One of the promising avenues in this area is studies on the panels of biomarkers characteristic for processes which occur in different types and phases of ischemic stroke and represent all morphological constituents of the brains' neurovascular unit (NVU). In this review, we present the current state of knowledge concerning already-used or potentially applicable biomarkers of the ischemic stroke. We also discuss the perspectives for identification of biomarkers representative for different types and phases of the ischemic stroke, as well as for different constituents of NVU, which concentration levels correlate with extent of brain damage and patients' neurological status. Finally, a critical analysis of perspectives on further improvement of the ischemic stroke diagnosis is presented.

Rate dependent influence of arterial desaturation on self-selected exercise intensity during cycling

The purpose of this study was to clarify if Ratings of Perceived Exertion (RPE) and self-selected exercise intensity are sensitive not only to alterations in the absolute level of arterial saturation (SPO2) but also the rate of change in SPO2. Twelve healthy participants (31.6 ± 3.9 y, 175.5 ± 7.7 cm, 73.3 ± 10.3 kg, 51 ± 7 mL·kg-1·min-1 [Formula: see text]) exercised four times on a cycle ergometer, freely adjusting power output (PO) to maintain RPE at 5 on Borg's 10-point scale with no external feedback to indicate their exercise intensity. The fraction of inspired oxygen (FIO2) was reduced during three of those trials such that SPO2 decreased during exercise from starting values (>98%) to 70%. These trials were differentiated by the time over which the desaturation occurred: 3.9 ± 1.4 min, -8.7 ± 4.2%•min-1 (FAST), 11.0 ± 3.7 min, -2.8 ± 1.3%•min-1 (MED), and 19.5 ± 5.8 min, -1.5 ± 0.8%•min-1 (SLOW) (P < 0.001). Compared to stable PO throughout the control condition (no SPO2 manipulation), PO significantly decreased across the experimental conditions (FAST = 2.8 ± 2.1 W•% SPO2-1; MED = 2.5 ± 1.8 W•% SPO2-1; SLOW = 1.8 ± 1.6 W•% SPO2-1; P < 0.001). The rates of decline in PO during FAST and MED were similar, with both greater than SLOW. Our results confirm that decreases in absolute SPO2 impair exercise performance and that a faster rate of oxygen desaturation magnifies that impairment.

Interactions in the Metabolism of Glutamate and the Branched-Chain Amino Acids and Ketoacids in the CNS

Glutamatergic neurotransmission entails a tonic loss of glutamate from nerve endings into the synapse. Replacement of neuronal glutamate is essential in order to avoid depletion of the internal pool. In brain this occurs primarily via the glutamate-glutamine cycle, which invokes astrocytic synthesis of glutamine and hydrolysis of this amino acid via neuronal phosphate-dependent glutaminase. This cycle maintains constancy of internal pools, but it does not provide a mechanism for inevitable losses of glutamate N from brain. Import of glutamine or glutamate from blood does not occur to any appreciable extent. However, the branched-chain amino acids (BCAA) cross the blood-brain barrier swiftly. The brain possesses abundant branched-chain amino acid transaminase activity which replenishes brain glutamate and also generates branched-chain ketoacids. It seems probable that the branched-chain amino acids and ketoacids participate in a "glutamate-BCAA cycle" which involves shuttling of branched-chain amino acids and ketoacids between astrocytes and neurons. This mechanism not only supports the synthesis of glutamate, it also may constitute a mechanism by which high (and potentially toxic) concentrations of glutamate can be avoided by the re-amination of branched-chain ketoacids.

Severity of experimental traumatic brain injury modulates changes in concentrations of cerebral free amino acids

In this study, concentrations of free amino acids (FAA) and amino group containing compounds (AGCC) following graded diffuse traumatic brain injury (mild TBI, mTBI; severe TBI, sTBI) were evaluated. After 6, 12, 24, 48 and 120 hr aspartate (Asp), glutamate (Glu), asparagine (Asn), serine (Ser), glutamine (Gln), histidine (His), glycine (Gly), threonine (Thr), citrulline (Cit), arginine (Arg), alanine (Ala), taurine (Tau), γ‐aminobutyrate (GABA), tyrosine (Tyr), S‐adenosylhomocysteine (SAH), l‐cystathionine (l‐Cystat), valine (Val), methionine (Met), tryptophane (Trp), phenylalanine (Phe), isoleucine (Ile), leucine (Leu), ornithine (Orn), lysine (Lys), plus N‐acetylaspartate (NAA) were determined in whole brain extracts (n = 6 rats at each time for both TBI levels). Sham‐operated animals (n = 6) were used as controls. Results demonstrated that mTBI caused modest, transient changes in NAA, Asp, GABA, Gly, Arg. Following sTBI, animals showed profound, long‐lasting modifications of Glu, Gln, NAA, Asp, GABA, Ser, Gly, Ala, Arg, Citr, Tau, Met, SAH, l‐Cystat, Tyr and Phe. Increase in Glu and Gln, depletion of NAA and Asp increase, suggested a link between NAA hydrolysis and excitotoxicity after sTBI. Additionally, sTBI rats showed net imbalances of the Glu‐Gln/GABA cycle between neurons and astrocytes, and of the methyl‐cycle (demonstrated by decrease in Met, and increase in SAH and l‐Cystat), throughout the post‐injury period. Besides evidencing new potential targets for novel pharmacological treatments, these results suggest that the force acting on the brain tissue at the time of the impact is the main determinant of the reactions ignited and involving amino acid metabolism.

Mitochondria: a multimodal hub of hypoxia tolerance

Decreased oxygen availability impairs cellular energy production and, without a coordinated and matched decrease in energy consumption, cellular and whole organism death rapidly ensues. Of particular interest are mechanisms that protect brain from low oxygen injury, as this organ is not only the most sensitive to hypoxia, but must also remain active and functional during low oxygen stress. As a result of natural selective pressures, some species have evolved molecular and physiological mechanisms to tolerate prolonged hypoxia with no apparent detriment. Among these mechanisms are a handful of responses that are essential for hypoxia tolerance, including (i) sensors that detect changes in oxygen availability and initiate protective responses; (ii) mechanisms of energy conservation; (iii) maintenance of basic brain function; and (iv) avoidance of catastrophic cell death cascades. As the study of hypoxia-tolerant brain progresses, it is becoming increasingly apparent that mitochondria play a central role in regulating all of these critical mechanisms. Furthermore, modulation of mitochondrial function to mimic endogenous neuroprotective mechanisms found in hypoxia-tolerant species confers protection against otherwise lethal hypoxic stresses in hypoxia-intolerant organs and organisms. Therefore, lessons gleaned from the investigation of endogenous mechanisms of hypoxia tolerance in hypoxia-tolerant organisms may provide insight into clinical pathologies related to low oxygen stress.

Sensitivity and source of amine-proton exchange and amide-proton transfer magnetic resonance imaging in cerebral ischemia

PURPOSE

Amide-proton transfer (APT) and amine-water proton exchange (APEX) MRI can be viable to map pH-decreasing ischemic regions. However, their exact contributions are unclear.

METHODS

We measured APEX- and APT-weighted magnetization transfer ratio asymmetry (denoted as APEXw and APTw), apparent diffusion coefficient, T2 , and T1 images and localized proton spectra in rats with permanent middle cerebral artery occlusion at 9.4 T. Phantoms and theoretical studies were also performed.

RESULTS

Within 1-h postocclusion, APEXw and APTw maps showed hyperintensity (3.1% of M0 ) and hypointensity (-1.8%), respectively, in regions with decreased apparent diffusion coefficient. Ischemia increased lactate and gamma aminobutyric acid concentrations, but decreased glutamate and taurine concentrations. Over time, the APEXw contrast decreased with glutamate, taurine, and creatine, whereas the APTw contrast and lactate level were similar. Phantom and theoretical studies suggest that the source of APEXw signal is mainly from proteins at normal pH, whereas at decreased pH, gamma aminobutyric acid and glutamate contributions increase, inducing the positive APEXw contrast in ischemic regions. The APTw contrast is sensitive to lactate concentration and pH, but contaminated from contributions of the faster APEX processes.

CONCLUSION

Positive APEXw contrast is more sensitive to ischemia than negative APTw contrast. They may provide complementary tissue metabolic information.

Endogenous GABA(A) and GABA(B) receptor-mediated electrical suppression is critical to neuronal anoxia tolerance

Anoxic insults cause hyperexcitability and cell death in mammalian neurons. Conversely, in anoxia-tolerant turtle brain, spontaneous electrical activity is suppressed by anoxia (i.e., spike arrest; SA) and cell death does not occur. The mechanism(s) of SA is unknown but likely involves GABAergic synaptic transmission, because GABA concentration increases dramatically in anoxic turtle brain. We investigated this possibility in turtle cortical neurons exposed to anoxia and/or GABA(A/B) receptor (GABAR) modulators. Anoxia increased endogenous slow phasic GABAergic activity, and both anoxia and GABA reversibly induced SA by increasing GABA(A)R-mediated postsynaptic activity and Cl(-) conductance, which eliminated the Cl(-) driving force by depolarizing membrane potential (∼8 mV) to GABA receptor reversal potential (∼-81 mV), and dampened excitatory potentials via shunting inhibition. In addition, both anoxia and GABA decreased excitatory postsynaptic activity, likely via GABA(B)R-mediated inhibition of presynaptic glutamate release. In combination, these mechanisms increased the stimulation required to elicit an action potential >20-fold, and excitatory activity decreased >70% despite membrane potential depolarization. In contrast, anoxic neurons cotreated with GABA(A+B)R antagonists underwent seizure-like events, deleterious Ca(2+) influx, and cell death, a phenotype consistent with excitotoxic cell death in anoxic mammalian brain. We conclude that increased endogenous GABA release during anoxia mediates SA by activating an inhibitory postsynaptic shunt and inhibiting presynaptic glutamate release. This represents a natural adaptive mechanism in which to explore strategies to protect mammalian brain from low-oxygen insults.

Chronic electrographic seizure reduces glutamine and elevates glutamate in the extracellular fluid of rat brain

Effects of spontaneous seizures on extracellular glutamate and glutamine were studied in the kainate-induced rat model of epilepsy in the chronic phase. Extracellular fluid from the CA1-CA3 regions of the hippocampus was collected with a 2-mm microdialysis probe every 2 min for 5h. EEG seizures with no or mild behavioral components caused 2- to 6-fold elevation of extracellular glutamate. Concomitantly, extracellular glutamine decreased at t=5h to 48% of the initial value (n=6). The changes in extracellular glutamate and glutamine correlated with the frequency and magnitude of seizure activity. In contrast, no change in either metabolite was observed in kainate-injected rats that did not undergo seizure during microdialysis (n=6). In hippocampal tissue (9.4 ± 1.1mg) that contained the region sampled by microdialysis and the site of kainate injection, intracellular glutamine concentration was significantly reduced in the seizure group, compared to that in no-seizure group. The observed elevation of extracellular glutamate strongly suggests that neurotransmitter glutamate was released at a rate faster than the rate of its uptake into glia, possibly due to down-regulation of the transporter. This reduces the availability of substrate glutamate for glutamine synthesis, as corroborated by the observed reduction of intracellular glutamine. This is likely to reduce the rate of glutamine efflux from glia and result in the observed decrease of extracellular glutamine. There remains an intriguing possibility that seizure-induced decrease of extracellular glutamine also reflects its increased uptake into neurons to replenish neurotransmitter glutamate during enhanced epileptiform activity.

Extracellular amino acids and lipid peroxidation products in periventricular white matter during and after cerebral ischemia in preterm fetal sheep

It is widely hypothesized that accumulation of excitatory amino acids, and oxygen free radicals during or after exposure to hypoxia-ischemia play a pivotal role in preterm periventricular white matter injury; however, there is limited evidence in the intact brain. In preterm fetal sheep (0.65 gestation; term 147 days) we found no significant increase in extracellular levels of excitatory amino acids measured by microdialysis in the periventricular white matter during cerebral ischemia induced by bilateral carotid occlusion. There was no significant change in 8-isoprostane or malondialdehyde levels in the early phase of recovery after occlusion. In contrast, there was a significant delayed increase in most amino acids and in malondialdehyde (but not 8-isoprostane) that was maximal approximately 2-3 days after occlusion. The increase in glutamate was significantly correlated with a secondary increase in cortical impedance, an index of cytotoxic edema, and with white matter damage 3 days post-insult. In conclusion, no significant accumulation of cytotoxins was found within immature white matter during cerebral ischemia. Although a minority of fetuses showed a delayed increase in some cytotoxins, this occurred many days after ischemia, in association with secondary cytotoxic edema, strongly suggesting that these changes are mainly a consequence of evolving cell death.

The human GLUD2 glutamate dehydrogenase: localization and functional aspects

In all mammals, glutamate dehydrogenase (GDH), an enzyme central to the metabolism of glutamate, is encoded by a single gene (GLUD1 in humans) which is expressed widely (housekeeping). Humans and other primates also possess a second gene, GLUD2, which encodes a highly homologous GDH isoenzyme (hGDH2) expressed predominantly in retina, brain and testis. There is evidence that GLUD1 was retro-posed <23 million years ago to the X chromosome, where it gave rise to GLUD2 through random mutations and natural selection. These mutations provided the novel enzyme with unique properties thought to facilitate its function in the particular milieu of the nervous system. hGDH2, having been dissociated from GTP control (through the Gly456Ala change), is mainly regulated by rising levels of ADP/l-leucine. To achieve full-range regulation by these activators, hGDH2 needs to set its basal activity at low levels (<10% of full capacity), a property largely conferred by the evolutionary Arg443Ser change. Studies of structure/function relationships have identified residues in the regulatory domain of hGDH2 that modify basal catalytic activity and regulation. In addition, enzyme concentration and buffer ionic strength can influence basal enzyme activity. While mature hGDH1 and hGDH2 isoproteins are highly homologous, their predicted leader peptide sequences show a greater degree of divergence. Study of the subcellular sites targeted by hGDH2 in three different cultured cell lines using a GLUD2/EGFP construct revealed that hGDH2 localizes mainly to mitochondria and to a lesser extent to the endoplasmic reticulum of these cells. The implications of these findings for the potential role of this enzyme in the biology of the nervous system in health and disease are discussed.

Glutamate and GABA metabolism in transient and permanent middle cerebral artery occlusion in rat: Importance of astrocytes for neuronal survival

The aim of the present study was to identify the distinguishing metabolic characteristics of brain tissue salvaged by reperfusion following focal cerebral ischemia. Rats were subjected to 120 min of middle cerebral artery occlusion followed by 120 min of reperfusion. The rats received an intravenous bolus injection of [1-(13)C]glucose plus [1,2-(13)C]acetate. Subsequently two brain regions considered to represent penumbra and ischemic core, i.e. the frontoparietal cortex and the lateral caudoputamen plus lower parietal cortex, respectively, were analyzed with (13)C NMRS and HPLC. The results demonstrated four metabolic events that distinguished the reperfused penumbra from the ischemic core. (1) Improved astrocytic metabolism demonstrated by increased amounts of [4,5-(13)C]glutamine and improved acetate oxidation. (2) Neuronal mitochondrial activity was better preserved although the flux of glucose via pyruvate dehydrogenase into the tricarboxylic acid (TCA) cycle in glutamatergic and GABAergic neurons was halved. However, NAA content was at control level. (3) Glutamatergic and GABAergic neurons used relatively more astrocytic metabolites derived from the pyruvate carboxylase pathway. (4) Lactate synthesis was not increased despite decreased glucose metabolism in the TCA cycle via pyruvate dehydrogenase. In the ischemic core both neuronal and astrocytic TCA cycle activity declined significantly despite reperfusion. The utilization of astrocytic precursors originating from the pyruvate carboxylase pathway was markedly reduced compared the pyruvate dehydrogenase pathway in glutamate, and completely stopped in GABA. The NAA level fell significantly and lactate accumulated. The results demonstrate that preservation of astrocytic metabolism is essential for neuronal survival and a predictor for recovery.

Estradiol alters only GAD67 mRNA levels in ischemic rat brain with no consequent effects on GABA

The present study tested the hypothesis that estradiol reduces tissue infarction after middle cerebral artery occlusion (MCAO) in estradiol-deficient females by augmenting glutamic acid decarboxylase (GAD) expression and thus activity, leading to increases in gamma-amino-butyric acid (GABA) tissue levels. Glutamic acid decarboxylase is the principal enzyme for GABA synthesis and has two isoforms, GAD65 and GAD67, which differ in size and cellular distribution. Rats were ovariectomized 7 to 8 days before receiving no hormone, placebo, or 25 microg estradiol via subcutaneous implant 7 to 10 days before harvesting tissue in either ischemic cohorts after 2 h of MCAO (end-ischemia) or in nonischemic cohorts. Selected cortical and striatal regions were microdissected from harvested brains. GAD65/67 mRNA levels were determined by microlysate ribonuclease protection assay. End-ischemic GABA concentrations were determined by HPLC. Steroid treatment selectively decreased ischemic cortical GAD67 mRNA levels. In most brain regions evaluated, regional GABA concentrations increased with ischemia regardless of treatment. Estradiol blocked MCAO-induced increases in GABA concentration only in dorsomedial cortex. These data suggest that estradiol repletion in ischemic rat brain selectively decreases GAD67 mRNA levels but does not alter steady-state GABA concentrations. It may be that estradiol under ischemic conditions is attenuating GABA metabolism rather than enhancing synthesis or is augmenting other aspects of GABAergic transmission such as GABA transporters and receptors.

Brain amino acid requirements and toxicity: the example of leucine

Glutamic acid is an important excitatory neurotransmitter of the brain. Two key goals of brain amino acid handling are to maintain a very low intrasynaptic concentration of glutamic acid and also to provide the system with precursors from which to synthesize glutamate. The intrasynaptic glutamate level must be kept low to maximize the signal-to-noise ratio upon the release of glutamate from nerve terminals and to minimize the risk of excitotoxicity consequent to excessive glutamatergic stimulation of susceptible neurons. The brain must also provide neurons with a constant supply of glutamate, which both neurons and glia robustly oxidize. The branched-chain amino acids (BCAAs), particularly leucine, play an important role in this regard. Leucine enters the brain from the blood more rapidly than any other amino acid. Astrocytes, which are in close approximation to brain capillaries, probably are the initial site of metabolism of leucine. A mitochondrial branched-chain aminotransferase is very active in these cells. Indeed, from 30 to 50% of all alpha-amino groups of brain glutamate and glutamine are derived from leucine alone. Astrocytes release the cognate ketoacid [alpha-ketoisocaproate (KIC)] to neurons, which have a cytosolic branched-chain aminotransferase that reaminates the KIC to leucine, in the process consuming glutamate and providing a mechanism for the "buffering" of glutamate if concentrations become excessive. In maple syrup urine disease, or a congenital deficiency of branched-chain ketoacid dehydrogenase, the brain concentration of KIC and other branched-chain ketoacids can increase 10- to 20-fold. This leads to a depletion of glutamate and a consequent reduction in the concentration of brain glutamine, aspartate, alanine, and other amino acids. The result is a compromise of energy metabolism because of a failure of the malate-aspartate shuttle and a diminished rate of protein synthesis.

Neuro-overprotection? A functional evaluation of clomethiazole-induced neuroprotection following hypoxic-ischemic injury

Hypoxic-ischemic (H-I) injury produces extensive damage to the hippocampus of young rats. We have recently shown that administration of 125 mg kg-1 clomethiazole (CMZ), a GABA(A)-agonist, provides complete histological protection against H-I injury if administered 3 h post-H-I (Brain Res 1035 (2005) 194). However, whether that histological protection translates into lasting functional preservation is unclear. To determine whether hippocampal-based circuits remain functionally intact in CMZ-protected H-I rats, we administered 125 mg kg-1 (high dose [CMZ-HD]) or 65 mg kg-1 (low dose [CMZ-LD]) CMZ, 3 h post-H-I, and examined numerous kindling parameters in the dorsal hippocampus 60 days following H-I. Kindling parameters included afterdischarge (AD) thresholds (ADTs), AD durations and kindling rates. Additional groups assessed included vehicle-injected H-I (VIH), hypoxic, ligated and naive rats. VIH, CMZ-HD, CMZ-LD and hypoxic rats all exhibited significantly faster kindling rates than naive rats. Thus, a previous traumatic event, even hypoxia alone, facilitated subsequent seizure propagation. Still, a significantly slower kindling rate was evident in CMZ-HD rats than in hypoxic, VIH or CMZ-LD rats. Moreover, while longer pre-kindling AD durations were observed in the damaged hippocampus of VIH compared with naive rats, this was not true for either CMZ-treated groups, hypoxic or ligated rats. Collectively, these findings suggest CMZ can suppress the epileptogenic effects of H-I. Surprisingly, however, both groups of CMZ-treated rats exhibited a four to nine times greater ADT than any other group and this effect was most profound in the CMZ-protected hippocampus. Thus, CMZ administration protected local neurons against terminal insult and left network excitability relatively normal with respect to seizure offset mechanisms but also caused profound elevation of local ADTs, which suggests a local hypoexcitability/increased inhibition. Finally, this study demonstrates, for the first time, that the kindling model can serve as a sensitive measure of function-related neuroprotective efficacy in animal models of ischemia.

Glutamic acid decarboxylase and GABA immunoreactivities in the cerebellar cortex of adult rat after prenatal exposure to a low concentration of carbon monoxide

Glutamic acid decarboxylase and GABA immunoreactivities were qualitatively and quantitatively evaluated in the cerebellar cortex of adult rats prenatally exposed to a low concentration of carbon monoxide (75 parts per million). Carbon monoxide-exposed and control rats were perfused with modified Bouin's fluid and their cerebella were embedded in paraffin. Sections from the vermis of each cerebellum were stained with Toluidine Blue or assayed with anti-glutamic acid decarboxylase 65/67 or with anti-GABA antisera. In the Toluidine Blue-stained sections, no differences were observed in the microscopic structure of the cerebellar cortex between carbon monoxide-exposed rats and controls. The distribution patterns of glutamic acid decarboxylase and GABA immunoreactivities in the cerebellar cortex of the treated animals were qualitatively comparable to those of the controls, and in accordance with previous descriptions of glutamic acid decarboxylase and GABA immunoreactivities in the rat cerebellar cortex. However, quantitative analyses demonstrated a significant reduction of immunoreactivities to both substances in the exposed rats in comparison with the controls. The reduction regarded: in the molecular layer, the number of glutamic acid decarboxylase/GABA-immunoreactive neuronal bodies and of axon terminals and the area they covered; in the Purkinje neuron layer, the number and the area covered by glutamic acid decarboxylase/GABA immunoreactive axon terminals. The differences detected in the prenatally exposed adult rats could be due to carbon monoxide-induced impairment of the differentiation of cerebellar GABA synthesizing neurons. A consequently diminished synthesis of GABA might account for some behavioral disorders detected in adult rats submitted to the same experimental procedure.

Reversal or reduction of glutamate and GABA transport in CNS pathology and therapy

A dysfunction of amino acid neurotransmitter transporters occurs in a number of central nervous system disorders, including stroke, epilepsy, cerebral palsy and amyotrophic lateral sclerosis. This dysfunction can comprise a reversal of transport direction, leading to the release of neurotransmitter into the extracellular space, or an alteration in transporter expression level. This review analyses the role of glutamate and GABA transporters in the pathogenesis and therapy of a number of acute and chronic neurological disorders.

Sequential release of GABA by exocytosis and reversed uptake leads to neuronal swelling in simulated ischemia of hippocampal slices

GABA release during cerebral energy deprivation (produced by anoxia or ischemia) has been suggested either to be neuroprotective, because GABA will hyperpolarize neurons and reduce release of excitotoxic glutamate, or to be neurotoxic, because activation of GABA(A) receptors facilitates Cl- entry into neurons and consequent cell swelling. We have used the GABA(A) receptors of hippocampal area CA1 pyramidal cells to sense the rise of [GABA](o) occurring in simulated ischemia. Ischemia evoked, after several minutes, a large depolarization to approximately -20 mV. Before this "anoxic depolarization," there was an increase in GABA release by exocytosis (spontaneous IPSCs). After the anoxic depolarization, there was a much larger, sustained release of GABA that was not affected by blocking action potentials, vesicular release, or the glial GABA transporter GAT-3 but was inhibited by blocking the neuronal GABA transporter GAT-1. Blocking GABA(A) receptors resulted in a more positive anoxic depolarization but decreased cell swelling at the time of the anoxic depolarization. The influence of GABA(A) receptors diminished in prolonged ischemia because glutamate release evoked by the anoxic depolarization inhibited GABA(A) receptor function by causing calcium entry through NMDA receptors. These data show that ischemia releases GABA initially by exocytosis and then by reversal of GAT-1 transporters and that the resulting Cl- influx through GABA(A) receptor channels causes potentially neurotoxic cell swelling.

High level of orexin A observed in the phenylketonuria mouse brain is due to the abnormal expression of prepro-orexin

Orexins/hypocretins are recently discovered neuropeptides, synthesized mainly in the lateral hypothalamus of the brain. Orexins regulate various functions including sleep and apetite. We recently reported increased amount of orexin A in the phenylketonuria (PKU) mouse brain. Whether this is caused by overexpression of the precursor for orexins, prepro-orexin was studied in the PKU mouse brain. Microarray expression analysis revealed overexpression of orexin gene in the brain of PKU mouse. Quantitative real-time RT-PCR showed increased level of prepro-orexin mRNA in the PKU mouse brain. In addition, expression of genes associated with cell signal and growth regulation was also affected in the PKU mouse brain, as observed by microarray analysis. These data suggest that up-regulation of orexin mRNA expression is the possible factor for inducing high orexin A in the brain of PKU mouse. The metabolic environment in the brain of PKU mouse affects normal expression of other genes possibly to result in pathophysiology seen in the PKU mouse, if documented also in patients with PKU.

Implication of ionotropic glutamate receptors in the release of noradrenaline in hippocampal CA1 and CA3 subregions under oxygen and glucose deprivation

A strong linkage between adrenergic and glutamatergic systems exists in the CNS but it is still unclear whether the excessive release of noradrenaline under ischemic conditions is modulated by excitatory amino acids. We studied the effect of selective glutamate receptor antagonists on the release of [3H]noradrenaline evoked by glucose and oxygen deprivation in hippocampal CA1, CA3 and dentate gyrus subregions. The release of glutamate, aspartate and GABA was measured by HPLC. Omission of oxygen and glucose increased the release of [3H]noradrenaline as well as the release of amino acids. Maximum effect on noradrenaline release was observed in CA1 region. The relative increase of the release after 30 min energy deprivation (R(2)) versus the basal release under normal conditions (R(1)), i.e. the R(2)/R(1) ratio was 7.1+/-1.0, 3.87+/-0.4 and 3.26+/-0.27 for CA1, CA3 and dentate gyrus, respectively. The [3H]noradrenaline outflow in response to glucose and oxygen deprivation was abolished at low temperature, but not by Ca(2+) removal, suggesting a cytoplasmic release process. In CA1 and CA3 [3H]noradrenaline release was significantly attenuated by MK-801, an NMDA receptor antagonist. The AMPA receptor antagonist GYKI-53784 had no effect in CA3, but partly reduced noradrenaline release in CA1. Our results suggest that ionotropic glutamate receptors seem to be implicated in the massive cytoplasmic release of noradrenaline in CA1 what may contribute to its selective vulnerability.

Expression of glutamate transporter, GABRA6, serine proteinase inhibitor 2 and low levels of glutamate and GABA in the brain of knock-out mouse for Canavan disease

Canavan disease (CD) is an autosomal recessive leukodystrophy characterized by spongy degeneration of the brain. The clinical features of CD are hypotonia, megalencephaly, and mental retardation leading to early death. While aspartoacylase (ASPA) activity increases with age in the wild type mouse brain, there is no ASPA activity in the CD mouse brain. So far ASPA deficiency and elevated NAA have been ascribed with the CD. Other factors affecting the brain that result from ASPA deficiency may lead pathophysiology of CD. The NMR spectra and amino acid analysis showed lower levels of glutamate and gamma-aminobutyric acid in the CD mouse brain compared to the wild type. Microarray gene expression on CD mouse brain showed glutamate transporter-EAAT4 and gamma-aminobutyric acid-A receptor, subunit alpha6 (GABRA6) were lower 9.7- and 119.1-fold, respectively. Serine proteinase inhibitor 2 (Spi2) was 29.9-fold higher in the CD mouse brain compared to the wild type. The decrease of GABRA6 and high expression of Spi2 in CD mouse brain were also confirmed by real-time RT-PCR. This first report showing abnormal expression of EAAT4, GABRA6, Spi2 combined with lower levels of glutamate and GABA are likely to be associated with the pathophysiology of CD.

Differences in neurotransmitter synthesis and intermediary metabolism between glutamatergic and GABAergic neurons during 4 hours of middle cerebral artery occlusion in the rat: the role of astrocytes in neuronal survival

Astrocytes are intimately involved in both glutamate and gamma-aminobutyric acid (GABA) synthesis, and ischemia-induced disruption of normal neuroastrocytic interactions may have important implications for neuronal survival. The effects of middle cerebral artery occlusion (MCAO) on neuronal and astrocytic intermediary metabolism were studied in rats 30, 60, 120, and 240 minutes after MCAO using in vivo injection of [1-13C]glucose and [1,2- 13C]acetate combined with ex vivo 13C magnetic resonance spectroscopy and high-performance liquid chromatography analysis of the ischemic core (lateral caudoputamen and lower parietal cortex) and penumbra (upper frontoparietal cortex). In the ischemic core, both neuronal and astrocytic metabolism were impaired from 30 minutes MCAO. There was a continuous loss of glutamate from glutamatergic neurons that was not replaced as neuronal glucose metabolism and use of astrocytic precursors gradually declined. In GABAergic neurons astrocytic precursors were not used in GABA synthesis at any time after MCAO, and neuronal glucose metabolism and GABA-shunt activity declined with time. No flux through the tricarboxylic acid cycle was found in GABAergic neurons at 240 minutes MCAO, indicating neuronal death. In the penumbra, the neurotransmitter pool of glutamate coming from astrocytic glutamine was preserved while neuronal metabolism progressively declined, implying that glutamine contributed significantly to glutamate excitotoxicity. In GABAergic neurons, astrocytic precursors were used to a limited extent during the initial 120 minutes, and tricarboxylic acid cycle activity was continued for 240 minutes. The present study showed the paradoxical role that astrocytes play in neuronal survival in ischemia, and changes in the use of astrocytic precursors appeared to contribute significantly to neuronal death, albeit through different mechanisms in glutamatergic and GABAergic neurons.

Cerebral hypoxemia-ischemia and reoxygenation with 21% or 100% oxygen in newborn piglets: effects on extracellular levels of excitatory amino acids and microcirculation

OBJECTIVE

To determine whether reoxygenation with 21% oxygen is preferable to 100% oxygen in normalizing extracellular levels of excitatory amino acids in the brains of hypoxic-ischemic newborn piglets and to compare this model of combined hypoxemia-ischemia to a previously used model of global hypoxemia.

DESIGN

Prospective, randomized animal study.

SETTING

Surgical research laboratory.

SUBJECTS

Twenty-four anesthetized piglets, 1-3 days old.

INTERVENTIONS

Hypoxemia-ischemia was achieved by normoventilation with 8% oxygen and temporary occlusion of the common carotid arteries. After 20 mins, reoxygenation-reperfusion was started with 21% oxygen (HI 21% group, n = 12) or 100% oxygen (HI 100% group, n = 12) for 30 mins followed by 21% oxygen. All piglets were observed for 2 hrs.

MEASUREMENTS AND MAIN RESULTS

We measured extracellular concentrations of amino acids in striatum and hypoxanthine in cerebral cortex (microdialysis), microcirculation in cerebral cortex (laser Doppler), plasma hypoxanthine, and mean arterial pressure. During the 2-hr reoxygenation-reperfusion period, levels of amino acids were significantly higher in the HI 21% group compared with the HI 100% group (glutamate, p = 0.02; aspartate, p = 0.03). Mean arterial pressure was significantly lower in the HI 21% group (p = 0.04). Microcirculation decreased to <10% of baseline during hypoxemia-ischemia and normalized during reoxygenation-reperfusion in the HI 100% group, but it remained at a significantly lower level in the HI 21% group (p = 0.03).

CONCLUSIONS

Significantly higher levels of excitatory amino acids in striatum, significantly lower mean arterial pressure, and a significantly greater degree of hypoperfusion in cerebral cortex were found after reoxygenation with 21% oxygen compared with 100% oxygen in normocapnic, hypoxemic-ischemic newborn piglets. This suggests a less favorable outcome in the group receiving room air.

Is ischemia involved in the pathogenesis of murine cerebral malaria?

Sequestration of parasitized erythrocytes in the central nervous system microcirculation and increased cerebrospinal fluid lactate are prominent features of cerebral malaria (CM), suggesting that sequestration causes mechanical obstruction and ischemia. To examine the potential role of ischemia in the pathogenesis of CM, Plasmodium berghei ANKA (PbA) infection in CBA mice was compared to infection with P. berghei K173 (PbK) which does not cause CM (the non-CM model, NCM). Cerebral metabolite pools were measured by (1)H nuclear magnetic resonance spectroscopy during PbA and PbK infections. Lactate and alanine concentrations increased significantly at the terminal stage of CM, but not in NCM mice at any stage. These changes did not correlate with parasitemia. Brain NAD/NADH ratio was unchanged in CM and NCM mice at any time studied, but the total NAD pool size decreased significantly in the CM mice on day 7 after inoculation. Brain levels of glutamine and several essential amino acids were increased significantly in CM mice. There was a significant linear correlation between the time elapsed after infection and small, progressive decreases in the cell density/cell viability markers glycerophosphocholine and N-acetylaspartate in CM, indicative of gradual loss of cell viability. The metabolite changes followed a different pattern, with a sudden significant alteration in the levels of lactate, alanine, and glutamine at the time of terminal CM. In NCM, there were significant decreases with time of glutamate, the osmolyte myo-inositol, and glycerophosphocholine. These results are consistent with an ischemic change in the metabolic pattern of the brain in CM mice, whereas in NCM mice the changes were more consistent with hypoxia without vascular obstruction. Mild obstructive ischemia is a likely cause of the metabolic changes during CM, but a role for immune cell effector molecules cannot be ruled out.

Modulation of absence seizures by branched-chain amino acids: correlation with brain amino acid concentrations

The occurrence of absence seizures might be due to a disturbance of the balance between excitatory and inhibitory neurotransmissions in the thalamo-cortical loop. In this study, we explored the consequences of buffering the glutamate content of brain cells on the occurrence and duration of seizures in Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a genetic model of generalized non-convulsive epilepsy. Branched-chain amino acids (BCAAs) and alpha-ketoisocaproate (alpha-KIC), the ketoacid of leucine were repeatedly shown to have a critical role in brain glutamate metabolism. Thus, GAERS were injected by intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) route with these compounds, then the effects on seizures were evaluated on the electroencephalographic recording. We also measured the concentration of amino acids in thalamus and cortex after an i.p. injection of leucine or alpha-KIC. Intracerebroventricular injections of leucine or alpha-KIC did not influence the occurrence of seizures, possibly because the substances reached only the cortex. BCAAs and alpha-KIC, injected intraperitoneally, increased the number of seizures whereas they had only a slight effect on their duration. Leucine and alpha-KIC decreased the concentration of glutamate in thalamus and cortex without affecting GABA concentrations. Thus, BCAAs and alpha-KIC, by decreasing the effects of glutamatergic neurotransmission could facilitate those of GABAergic neurotransmission, which is known to increase the occurrence of seizures in GAERS.

Nerve tissue-specific (GLUD2) and housekeeping (GLUD1) human glutamate dehydrogenases are regulated by distinct allosteric mechanisms: implications for biologic function

Human glutamate dehydrogenase (GDH), an enzyme central to the metabolism of glutamate, is known to exist in housekeeping and nerve tissue-specific isoforms encoded by the GLUD1 and GLUD2 genes, respectively. As there is evidence that GDH function in vivo is regulated, and that regulatory mutations of human GDH are associated with metabolic abnormalities, we sought here to characterize further the functional properties of the two human isoenzymes. Each was obtained in recombinant form by expressing the corresponding cDNAs in Sf9 cells and studied with respect to its regulation by endogenous allosteric effectors, such as purine nucleotides and branched chain amino acids. Results showed that L-leucine, at 1.0 mM:, enhanced the activity of the nerve tissue-specific (GLUD2-derived) enzyme by approximately 1,600% and that of the GLUD1-derived GDH by approximately 75%. Concentrations of L-leucine similar to those present in human tissues ( approximately 0.1 mM:) had little effect on either isoenzyme. However, the presence of ADP (10-50 microM:) sensitized the two isoenzymes to L-leucine, permitting substantial enzyme activation at physiologically relevant concentrations of this amino acid. Nonactivated GLUD1 GDH was markedly inhibited by GTP (IC(50) = 0.20 microM:), whereas nonactivated GLUD2 GDH was totally insensitive to this compound (IC(50) > 5,000 microM:). In contrast, GLUD2 GDH activated by ADP and/or L-leucine was amenable to this inhibition, although at substantially higher GTP concentrations than the GLUD1 enzyme. ADP and L-leucine, acting synergistically, modified the cooperativity curves of the two isoenzymes. Kinetic studies revealed significant differences in the K:(m) values obtained for alpha-ketoglutarate and glutamate for the GLUD1- and the GLUD2-derived GDH, with the allosteric activators differentially altering these values. Hence, the activity of the two human GDH is regulated by distinct allosteric mechanisms, and these findings may have implications for the biologic functions of these isoenzymes.

Levels of neurotransmitter amino acids in the cerebrospinal fluid of patients with acute ischemic insult

The dynamics of excitatory (glutamate, aspartate) and inhibitory (GABA, glycine) neurotransmitter amino acid contents in the cerebrospinal fluid were studied in 110 patients with hemispheric ischemic insult. These studies revealed significant increases in the levels of glutamate and aspartate in the first six hours of illness, and the level and duration of these changes correlated with the severity of the insult. Peak GABA and glycine levels were seen at the end of the first day after strokes, reflecting the delayed activation of the mechanisms of protective inhibition. The insufficiency of GABAergic mediation in strokes located in the hemispheres to a significant extent mirrored the severity of clinical features and the potential of restorative processes. Early significant biochemical criteria were identified for objective assessment of the severity of brain ischemia, and these had prognostic value for the course and outcome of strokes. The most unfavorable prognostic signs were the presence of low (or undetectable) GABA levels in the first days after insult and progressive increases in aspartate levels to the third day on the background of sharp reductions in glutamate levels (after initial elevation on the first day).

Excitation of phrenic and sympathetic output during acute hypoxia: contribution of medullary oxygen detectors

Severe brain hypoxia results in respiratory excitation and an increase in sympathetic nerve activity. Respiratory excitation takes the form of gasping which is characterized by an abrupt onset, high amplitude, short duration burst of inspiratory activity. Recent evidence suggests that centrally-mediated hypoxic respiratory and sympathetic excitation may result from direct hypoxic stimulation of discrete hypoxia chemosensitive sites in the medulla. Thus, medullary regions involved in the generation and modulation of respiratory and sympathetic vasomotor output may contain neurons which function as central oxygen detectors, acting as medullary analogs to the peripheral (arterial) chemoreceptors. This review focuses on the medullary sites and mechanisms proposed to mediate hypoxic respiratory and sympathetic excitation in anesthetized, chemodeafferented animals, and provides the evidence suggesting a role for central oxygen detectors in the control of breathing and sympathetic vasomotor output.

Glutamate dependence of GABA levels in neurons of hypoxic and hypoglycemic rat hippocampal slices

Hypoxia may increase GABA levels in neurons by ATP depletion-induced activation of glutamate decarboxylase and by inhibiting GABA transaminase. Hypoglycemia, which also depletes ATP, reduces neuronal levels of GABA and its precursor glutamate. We examined whether differences in glutamate levels may contribute to these altered GABA levels in hippocampal slices. GABA levels were highly correlated with endogenous glutamate levels during both hypoxia and hypoglycemia (R=0.93 for combined data). Hypoxia maximally increased GABA levels (146+/-6.3% of control, S.E.M.) when glutamate remained above 90% of control levels and ATP was at 30% of control levels. Hypoglycemia with similar ATP levels and glutamate levels at 40% of control decreased GABA levels to 55% of control. Effects of inhibitors of glutamate decarboxylase and GABA transaminase suggested that increased synthesis and decreased catabolism may both contribute to increased hypoxic GABA levels. Immunocytochemical studies suggested that hypoxia increased GABA concentrations primarily in neurons and their processes, but not in glial cells. Severe hypoxic ATP depletion increased the release of both GABA and glutamate. Hypoxia increased GABA levels in neurons, while hypoglycemia with a similar severity of ATP depletion decreased GABA levels. Much of the difference may be related to lower levels of precursor glutamate during hypoglycemia. The twofold higher levels of neuroprotective GABA available for release during hypoxia may contribute to differences in the pathophysiology of these metabolic insults.

Striatal outflow of adenosine, excitatory amino acids, gamma-aminobutyric acid, and taurine in awake freely moving rats after middle cerebral artery occlusion: correlations with neurological deficit and histopathological damage

BACKGROUND AND PURPOSE

While a number of studies have investigated transmitter outflow in anesthetized animals after middle cerebral artery occlusion (MCAO) performed by craniectomy, studies have never been performed after MCAO induced by intraluminal filament. In addition, it has been reported that after MCAO, infarct volume correlates with functional outcome and with transmitter outflow, although there are no studies that demonstrate a direct correlation between transmitter outflow and functional outcome. The purpose of the present study was to assess excitatory amino acids, gamma-aminobutyric acid, taurine, and adenosine outflow in awake rats after intraluminal MCAO and to determine whether, in the same animal, outflow was correlated with neurological outcome and histological damage.

METHODS

Vertical microdialysis probes were placed in the striatum of male Wistar rats. After 24 hours, permanent MCAO was induced by the intraluminal suture technique. The transmitter concentrations in the dialysate were determined by high-performance liquid chromatography. Twenty-four hours after MCAO, neurological deficit and histological outcome were evaluated.

RESULTS

All transmitters significantly increased after MCAO. Twenty-four hours after MCAO, the rats showed a severe sensorimotor deficit and massive ischemic damage in the striatum and in the cortex (9+/-2% and 25+/-6% of hemispheric volume, respectively). Significant correlations were found between the efflux of all transmitters, neurological score, and striatal infarct volume.

CONCLUSIONS

In this study, for the first time, amino acid and adenosine extracellular concentrations during MCAO by the intraluminal suture technique were determined in awake and freely moving rats, and a significant correlation was found between transmitter outflow and neurological deficit. The evaluation of neurological deficit, histological damage, and transmitter outflow in the same animal may represent a useful approach for studying neuroprotective properties of new drugs/agents against focal ischemia.

Forebrain ischemia: effect on pharmacologically induced seizure thresholds in the rat

Seizures are common after severe cerebral ischemia. To examine the mechanisms underlying these seizures, we determined the impact of prior forebrain ischemia on the seizure thresholds of four convulsants with differing modes of action: lidocaine, pentylenetetrazol (PTZ), N-methyl-D-aspartate (NMDA), and picrotoxin. Anesthetized Sprague-Dawley rats were chronically instrumented with screw electrodes and vascular catheters, and then subjected to 10 min of forebrain ischemia, produced by carotid occlusion and hypotension (mean arterial pressure to 30 mmHg). Animals were then awakened. 6, 24 or 48 h later, groups of awake animals received intravenous infusions of the four drugs. The total dose of drug infused prior to either electrical seizures (lidocaine, PTZ, and picrotoxin) or tonic-clonic convulsions (all drugs) were noted. For each drug, a group of Sham animals (no ischemia) served as controls. There were markedly different patterns of changes in the convulsant thresholds for the drugs. For example, at 6 h post-ischemia, rats treated with lidocaine died before convulsing, while the threshold for PTZ increased by 86%. There was no change in the picrotoxin threshold at 6 h, but the dose of NMDA needed to induce tonic-clonic seizure activity was reduced by 70%. By 48 h, lidocaine and PTZ thresholds had returned to values similar to those in Shams, but the NMDA threshold had now increased to a value 62% greater than Sham. Ten minutes of cerebral ischemia is followed by a complex and changing pattern of susceptibility to chemical convulsants. Finding suggests that early post-ischemic seizures may be related to increased NMDA receptor sensitivity.

In vivo injection of [1-13C]glucose and [1,2-13C]acetate combined with ex vivo 13C nuclear magnetic resonance spectroscopy: a novel approach to the study of middle cerebral artery occlusion in the rat

Astrocytes play a pivotal role in cerebral glutamate homeostasis. After 90 minutes of middle cerebral artery occlusion in the rat, the changes induced in neuronal and astrocytic metabolism and in the neuronal-astrocytic interactions were studied by combining in vivo injection of [1-13C]glucose and [1,2-13C]acetate with ex vivo 13C nuclear magnetic resonance spectroscopy and HPLC analysis of amino acids of the lateral caudoputamen and lower parietal cortex, representing the putative ischemic core, and the upper frontoparietal cortex, corresponding to the putative penumbra. In the putative ischemic core, evidence of compromised de novo glutamate synthesis located specifically in the glutamatergic neurons was detected, and a larger proportion of glutamate was derived from astrocytic glutamine. In the same region, pyruvate carboxylase activity, representing the anaplerotic pathway in the brain and exclusively located in astrocytes, was abolished. However, astrocytic glutamate uptake and conversion to glutamine took place, and cycling of intermediates in the astrocytic tricarboxylic acid cycle was elevated. In the putative penumbra, glutamate synthesis was improved compared with the ischemic core, the difference appeared to be brought on by better neuronal de novo glutamate synthesis, combined with normal levels of glutamate formed from astrocytic glutamine. In both ischemic regions, gamma-aminobutyric acid synthesis directly from glucose was reduced to about half, indicating impaired pyruvate dehydrogenase activity; still, gamma-aminobutyric acid reuptake and cycling was increased. The results obtained in the current study demonstrate that by combining in vivo injection of [1-13C]glucose and [1,2-13C]acetate with ex vivo 13C nuclear magnetic resonance spectroscopy, specific metabolic alterations in small regions within the rat brain suffering a focal ischemic lesion can be studied.

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

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

Orphan neurones and amine excess: the functional neuropathology of Parkinsonism and neuropsychiatric disease

The aetiology and treatment of Parkinsonism is currently conceptualised within a dopamine (DA) deficiency-repletion framework. Loss of striatal DA is thought to cause motor impairment of which tremor, bradykinaesia and rigidity are prominent features. Repletion of deficient DA should at least minimise parkinsonian signs and symptoms. In Section 2, based on extensive pre-clinical and clinical findings, the instability of this approach to Parkinsonism is scrutinised as the existing negative findings challenging the DA deficiency hypothesis are reviewed and reinterpreted. In Section 3 it is suggested that Parkinsonism is due to a DA excess far from the striatum in the area of the posterior lateral hypothalamus (PLH) and the substantia nigra (SN). This unique area, around the diencephalon/mesencephalon border (DCMCB), is packed with many ascending and descending fibres which undergo functional transformation during degeneration, collectively labelled 'orphan neurones'. These malformed cells remain functional resulting in pathological release of transmitter and perpetual neurotoxicity. Orphan neurone formation is commonly observed in the PLH of animals and in man exhibiting Parkinsonism. The mechanism by which orphan neurones impair motor function is analogous to that seen in the diseased human heart. From this perspective, to conceptualise orphan neurones at the DCMCB as 'Time bombs in the brain' is neither fanciful nor unrealistic [E.M. Stricker, M.J. Zigmond, Comments on effects of nigro-striatal dopamine lesions, Appetite 5 (1984) 266-267] as the DA excess phenomenon demands a different therapeutic approach for the management of Parkinsonism. In Section 4 the focus is on this novel concept of treatment strategies by concentrating on non-invasive, pharmacological and surgical modification of functional orphan neurones as they affect adjacent systems. The Orphan neurone/DA excess hypothesis permits a more comprehensive and defendable interpretation of the interrelationship between Parkinsonism and schizophrenia and other related disorders.

Activation of two types of brain glutamate dehydrogenase isoproteins by gabapentin

The stimulatory effects of gabapentin on the activities of two types of glutamate dehydrogenase (GDH) isoproteins homogeneously purified from bovine brain have been studied at various conditions. When the effects of different gabapentin concentrations on GDH activities were studied in the direction of reductive amination of 2-oxoglutarate with NADPH as a coenzyme, a marked activation was observed for both isoproteins, whereas both isoproteins showed activation to a lesser extent with NADH as a coenzyme. Stimulatory effects of gabapentin on GDH activities in the direction of the oxidative deamination of glutamate were also observed, but to a much lesser extent than reductive amination. There were big differences between the two GDH isoproteins in their sensitivity to the action of gabapentin. The largest activation was observed with GDH II when NADPH was used as a coenzyme. Half-maximal stimulation was reached at around 1.5 mM. Gabapentin relieved the inhibition of GDH isoproteins by GTP and this resulted in an increase in the apparent activation by gabapentin in the presence of GTP. 2-Oxoglutarate was found to give rise to high substrate inhibition and gabapentin reduced the substrate inhibition in the presence of 0.2 mM NADH. Since there are neurodegenerative disorders in which GDH activity is decreased, the therapeutic modulation of the activity of this enzyme may be clinically useful.

Changes in excitatory amino acid levels and tissue energy metabolites of neonate rat brain after hypoxia and hypoxia-ischemia

Lactate accumulation, amino acid aspartate and glutamate levels, and hypoxanthine, xanthine and malondialdehyde (MDA) concentrations were compared in neonate rat brain after transient global hypoxia induced alone or in association with unilateral ligation of a carotid artery. Lactate production in both hemispheres was higher in cerebral hypoxia-ischemia (CHI) than in cerebral hypoxia (CH), and was lower in CHI after 2 h than at 15 min of recovery. Aspartate and glutamate levels were reduced 15 min after CHI in both hemispheres, but aspartate alone was decreased 2 h after CHI in the ipsilateral (left) hemisphere and 15 min after CH in both hemispheres. Hypoxanthine was increased 15 min after CHI in the ipsilateral hemisphere but decreased at 2 h, whereas xanthine was increased. MDA production was not modified after CH or CHI. These data, compared to those obtained in adult animals suggest that glutamate release and the capacity to generate oxygen-derived radicals are lower in neonates after ischemia. These differences might explain why the brain of the mammalian neonate is much more resistant to CH and CHI than that of the adult.

Delayed decrease of excitatory amino acid neurotransmitters in parietotemporal cortex and hippocampus after complete cerebral ischemia in aged rats

A 15-min period of complete cerebral ischemia (CCI) was used in aged rats to investigate changes in tissue contents of the amino acid neurotransmitters (AANT) glutamate (glu), aspartate (asp), gamma aminobutyric acid (gaba) and glycine (gly) in parietotemporal cortex and hippocampus during ischemia and up to 96 h of postischemic recirculation. The AANT were determined by HPLC. The excitatory AANT glu and asp showed a first decrease at the end of CCI in hippocampus and after 1 h of postischemic recirculation in parietotemporal cortex, reflecting a release of these AANT into the extracellular space with a further loss into the blood. A second decrease in glu and asp was seen after 24 h of postischemic recirculation in hippocampus and after 48 h in parietotemporal cortex. This coincides with previously described disturbances in energy metabolism from 24 h to 96 h in hippocampus and from 48 h to 72 h in parietotemporal cortex in the same experimental model in aged rats. This might be a causal factor in delayed postischemic neuronal damage. A comparison with investigations in young animals reveals an enhanced decrease of glutamate and aspartate tissue contents during the postischemic recirculation period in old rats indicating an enhanced release of these amino acid neurotransmitters. A significant decrease of gaba tissue content seen in hippocampus at 48 h and 72 h of postischemic recirculation with subsequent disproportion of inhibitory AANT (lower) to excitatory AANT (higher) might reflect a greater vulnerability of hippocampus than of parietotemporal cortex to ischemia. A significant decrease in the NMDA-receptor coactivator gly in parietotemporal cortex at the end of CCI and at 48 h, 72 h, and 96 h of postischemic recirculation, which was not seen in hippocampus, might be another reason for higher vulnerability of hippocampus to ischemia.

Methylmalonyl-CoA mutase induction by cerebral ischemia and neurotoxicity of the mitochondrial toxin methylmalonic acid

Differential screening of gerbil brain hippocampal cDNA libraries was used to search for genes expressed in ischemic, but not normal, brain. The methylmalonyl-CoA mutase (MCM) cDNA was highly expressed after ischemia and showed a 95% similarity to mouse and 91% similarity to the human MCM cDNAs. Transient global ischemia induced a fourfold increase in MCM mRNA on Northern blots from both hippocampus and whole forebrain. MCM protein exhibited a similar induction on Western blots of gerbil cerebral cortex 8 and 24 hr after ischemia. Treatment of primary brain astrocytes with either the branched-chain amino acid (BCAA) isoleucine or the BCAA metabolite, propionate, induced MCM mRNA fourfold. Increased concentrations of BCAAs and odd-chain fatty acids, both of which are metabolized to propionate, may contribute to inducing the MCM gene during ischemia. Methylmalonic acid, which is formed from the MCM substrate methylmalonyl-CoA and which inhibits succinate dehydrogenase (SDH), produced dose-related cell death when injected into the basal ganglia of adult rat brain. This neurotoxicity is similar to that of structurally related mitochondrial SDH inhibitors, malonate and 3-nitropropionic acid. Methylmalonic acid may contribute to neuronal injury in human conditions in which it accumulates, including MCM mutations and B12 deficiency. This study shows that methylmalonyl-CoA mutase is induced by several stresses, including ischemia, and would serve to decrease the accumulation of an endogenous cellular mitochondrial inhibitor and neurotoxin, methylmalonic acid.

Ischemic disruption of glutamate homeostasis in brain: quantitative immunocytochemical analyses

More than 10 years ago, it was shown by microdialysis that the excitatory transmitter glutamate accumulates in the interstitial space of brain subjected to ischemic insult. This was one of the key observations leading to the formulation of the "glutamate hypothesis' of ischemic cell death. It is now assumed that even a transient glutamate overflow may set in motion a number of events that ultimately cause cell loss in vulnerable neuronal populations. The aim of the present review is to discuss the intracellular changes that underlie the dysregulation of extracellular glutamate during and after ischemia, with emphasis on data obtained by postembedding, electron microscopic immunogold cytochemistry. While the time resolution of this approach is necessarily limited, it can reveal, quantitatively and at a high level of spatial resolution, how the intracellular pools of glutamate and metabolically related amino acids are perturbed during and after an ischemic insult. Moreover, this can be done in animals whose extracellular amino acid levels are monitored by microdialysis, allowing a direct correlation of extra- and intracellular changes. Immunogold analyses of brains subjected to ischemia have identified dendrites and neuronal somata as likely sources of glutamate efflux, probably mediated by reversal of glutamate uptake. The vesicular glutamate pool has been found to be largely unchanged after 20 min of ischemia. Ischemia causes an increased glutamate content and an increased glutamate/glutamine ratio in glial cells, as revealed by double immunogold labelling. This argues against the idea that glial cells contribute to the extracellular overflow of glutamate in the ischemic brain.

Development of GABA and calcium binding proteins immunoreactivity in the rat hippocampus following neonatal anoxia

The consequences of neonatal anoxia (N2 100% for 25 min at 30 h after birth) on the rat hippocampus were studied 7-60 days postnatally with immunocytochemistry for gamma-aminobutyric acid (GABA), parvalbumin (PV) and calbindin-D28k (CB). In both sham-treated and anoxic rats, GABA immunoreactivity presented a mature expression since early stages, while PV and CB immunoreactivity showed a major postnatal development. In anoxic animals, a significant reduction in the number of hippocampal GABA-immunoreactive neurons was observed at all time-points analysed, a transitory effect on PV immunoreactivity was seen at P7 and P21, while no modifications in the number of CB-immunoreactive neurons could be found. Thus, selective vulnerability of GABA-containing neurons and relative resistance of neurons in which PV or CB immunoreactivity is present or is expressed later, occur in the hippocampus after neonatal anoxia. The role of calcium binding proteins (CBP) in nerve cell protection is discussed.

Effect of hypoxia and glucose deprivation on ATP level, adenylate energy charge and [Ca2+]o-dependent and independent release of [3H]dopamine in rat striatal slices

Release of [3H]dopamine ([3H]DA) from rat striatal slices kept under hypoxic or/and glucose-free conditions was measured using a microvolume perfusion method. The corresponding changes in nucleotide content were determined by reverse-phase high-performance liquid chromatography (RPHPLC). The resting release of [3H]DA was not affected by hypoxia, but under glucose-free conditions massive [Ca2+]o-independent release of [3H]DA was observed. Hypoxia reduced the energy charge (E.C.) and the total purine content from 19.36 +/- 4.15 to 6.98 +/- 1.83 nmol/mg protein. Glucose deprivation by itself, or in combination with hypoxia, markedly reduced the levels of adenosine 5'-triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP). The E.C under glucose-free conditions was significantly reduced from 0.73 +/- 0.04 to 0.44 +/- 0.20. When the tissue was exposed to hypoxic and glucose-free conditions for 18 min the level of ATP was reduced to 3.15 +/- 0.11 nmol/mg protein. However, when the exposure time was 30 min the ATP level was further reduced to 1.11 +/- 0.37 nmol/mg protein. The resting release was enhanced in a [Ca2+]o-independent manner, but there was no release in response to stimulation, and tetrodotoxin did not affect the enhanced resting release, indicating that the release was not associated with axonal activity. Similarly, 50 microM ouabain, inhibitor of Na+/K(+)-activated ATPase, enhanced the release of [3H]DA at rest in a [Ca2+]o-independent manner. It seems very likely that the reduced ATP level under glucose-free conditions leads to an inhibition of the activity of Na+/K(+)-ATPase that results in reversal of the uptake processes and in [Ca2+]o-independent [3H]DA release from the axon terminals.

Relationships between ATP depletion, membrane potential, and the release of neurotransmitters in rat nerve terminals. An in vitro study under conditions that mimic anoxia, hypoglycemia, and ischemia

BACKGROUND AND PURPOSE

It is known that the extracellular accumulation of glutamate during anoxia/ischemia is responsible for initiating neuronal injury. However, little information is available on the release of monoamines and whether the mechanism of its release resembles that of glutamate, which may itself influence the release of monoamines by activating presynaptic receptors. This study was designed to characterize the release of both amino acids and monoamines under chemical conditions that mimic anoxia, hypoglycemia, and ischemia.

METHODS

The contents of synaptosomes in adenine nucleotides (ATP, ADP, and AMP), amino acids (aspartate, glutamate, taurine, and gamma-aminobutyric acid), and monoamines (dopamine, noradrenaline, and 5-hydroxytryptamine) were measured by high-performance liquid chromatography, after the synaptosomes were subjected to anoxia (KCN + oligomycin), hypoglycemia (2 mmol/L 2-deoxyglucose in glucose-free medium), and ischemia (anoxia plus hypoglycemia).

RESULTS

The anoxia- and ischemia-induced release or noradrenaline, dopamine, 5-hydroxytryptamine, and glutamate correlated well with ATP depletion. The correlation observed between glutamate levels and the release of dopamine and 5-hydroxytryptamine in ischemic conditions suggests a functional linkage between the two transmitter systems. However, the antagonists of presynaptic glutamate receptors failed to alter the amount of monoamines released. The inhibition of Na+,K+-ATPase by ouabain had an effect similar to that produced by ischemia.

CONCLUSIONS

The decrease in Na+ and K+ gradients resulting from the energy depletion of the synaptosomes under ischemic conditions or resulting from the inhibition of Na+, K+-ATPase by ouabain promotes the reversal of the neurotransmitter transporters. The decrease in uptake of neurotransmitters may also contribute to the rise in the extracellular concentration of different transmitters observed during brain ischemia.

Secondary elevation of extracellular neurotransmitter amino acids in the reperfusion phase following focal cerebral ischemia

The purpose of this study was to evaluate amino acid neurotransmitter dynamics in the reperfusion phase after transient cerebral ischemia. In vivo microdialysis was used to measure extracellular amino acid levels in a rabbit model of focal ischemia. During 30 min of transient ischemia (n = 5), small but significant (p < 0.05) increases in glutamate, aspartate, gamma-aminobutyric acid (GABA), and taurine were noted. These elevations rapidly returned to baseline levels upon recirculation and remained constant for up to 5.5 h of reperfusion. In rabbits subjected to 2 h of transient ischemia (n = 5), two phases of amino acid release were seen. During ischemia, large (5- to 50-fold) elevations in glutamate, aspartate, GABA, and taurine occurred, as expected. These elevations rapidly normalized upon unocclusion. However, significant (p < 0.05) secondary elevations in glutamate, aspartate, and GABA occurred after 2-4 h of reperfusion. Regression analysis demonstrated significant correlations between primary (ischemic) and secondary (reperfusion) efflux. In permanent ischemia (n = 5), amino acid levels remained elevated throughout the entire experiment. Secondary elevations in excitatory amino acids may further contribute to the excitotoxic cascade during reperfusion.

Effects of vitamin B6 and its related compounds on survival of cultured brain neurons

The effects of pyridoxine and its derived cofacter, pyridoxal phosphate (PLP) on the survival of primary cultured neurons from fetal rat brain were investigated. Pyridoxine and PLP significantly promoted the neuronal survival of various brain regions in high cell density culture (10(5) cells/cm2), but showed no positive effects on hippocampal neurons in low cell density culture (5 x 10(3) cells/cm2). This neurotrophic effect of PLP was remarkably suppressed by picrotoxin and ifenprodil. Aminooxyacetic acid (AOAA), an inhibitor of PLP dependent enzymes, caused significant neuronal loss by itself, and largely counteracted the neurotrophic effect of PLP. Taken together, we presume that vitamin B6 afforded the survival-promoting activities of cultured neurons by virtue of its crucial coenzymatic actions in the biosynthesis of putative neurotransmitters.

Effects of anticonvulsant drug gabapentin on the enzymes in metabolic pathways of glutamate and GABA

Gabapentin is a novel anticonvulsant drug. The anticonvulsant mechanism of gabapentin is not known. Based on the amino acid structure of gabapentin we explored its possible effects on glutamate and gamma-aminobutyric acid (GABA) metabolism in brain as they may relate to its anticonvulsant mechanisms of action. Gabapentin was tested for its effects on seven enzymes in the metabolic pathways of these two neurotransmitters: alanine aminotransferase (AL-T), aspartate aminotransferase (AS-T), GABA aminotransferase (GABA-T), branched-chain amino acid aminotransferase (BCAA-T), glutamine synthetase (Gln-S), glutaminase (GLNase), and glutamate dehydrogenase (GDH). In the presence of 10 mM gabapentin, only GABA-T, BCAA-T, and GDH activities were affected by this drug. Inhibition of GABA-T by gabapentin was weak (33%). The Ki values for inhibition of cytosolic and mitochondrial forms of GABA-T (17-20 mM) were much higher than the Km values for GABA (1.5-1.9 mM). It is, therefore, unlikely that inhibition of GABA-T by gabapentin is clinically relevant. As with leucine, gabapentin stimulated GDH activity. The GDH activity in rat brain synaptosomes was activated 6-fold and 3.4-fold, respectively, at saturating concentrations (10 mM) of leucine and gabapentin. The half-maximal stimulation by gabapentin was observed at approximately 1.5 mM. Gabapentin is not a substrate of BCAA-T, but it exhibited a potent competitive inhibition of both cytosolic and mitochondrial forms of brain BCAA-T. Inhibition of BCAA-T by this drug was reversible. The Ki values (0.8-1.4 mM) for inhibition of transamination by gabapentin were close to the apparent Km values for the branched-chain amino acids (BCAA) L-leucine, L-isoleucine, and L-valine (0.6-1.2 mM), suggesting that gabapentin may significantly reduce synthesis of glutamate from BCAA in brain by acting on BCAA-T.