Neuronal-glial contributions to transmitter amino acid metabolism: studies with kainic acid-induced lesions of rat striatum

Effects of hyperammonaemia on brain function

Neuropsychiatric symptoms of hyperammonaemia include alterations of mood and personality, cognitive impairment, ataxia, convulsions and coma. The nature and severity of CNS dysfunction depend upon the aetiology and degree of hyperammonaemia, its acuteness of onset and the age of the patient. Neuropathological studies reveal Alzheimer type II astrocytosis in the adult hyperammonaemic patient, whereas hyperammonaemia in the infant resulting from congenital urea cycle disorders or Reye syndrome is accompanied by cerebral atrophy, neuronal loss and cerebral oedema. Several electrophysiological and biochemical mechanisms have been proposed to explain the deleterious effects of ammonia on CNS function. Such mechanisms include direct effects of the ammonium ion on excitatory and inhibitory neurotransmission and a deficit in cerebral energy metabolism due to ammonia-induced inhibition of alpha-ketoglutarate dehydrogenase. In addition, ammonia has been shown to interfere with normal processes of uptake, storage and release of various neurotransmitters. Ammonia disrupts monoamine storage, inhibits the high-affinity uptake of glutamate by both astrocytic and neuronal elements and activates 'peripheral-type' benzodiazepine receptors leading to the potential synthesis of neuroactive steroids in brain. On the basis of these actions, it has been proposed that ammonia disrupts neuron-astrocyte trafficking of amino acids and monoamines in brain. The increased formation of brain glutamine in hyperammonaemic syndromes could be responsible for the phenomenon of brain oedema in these disorders. Therapies aimed at either decreasing ammonia production in the gastrointestinal tract or increasing ammonia removal by liver or skeletal muscle are the mainstay in the prevention and treatment of the CNS consequences of hyperammonaemia. New therapeutic approaches aimed at correction of the neurotransmitter and cerebral energy deficits in these syndromes could hold promise for the future.

Reduction of striatal N-methyl-D-aspartate toxicity by inhibition of nitric oxide synthase

Coronal slices of rat brain were incubated for 40 min in 300 microM kainate (KA) or 500 microM N-methyl-D-aspartate (NMDA). Histological examination showed neuronal degeneration accompanied by significant losses in the activity of neuron-specific enolase (NSE; EC 4.2.1.11) (-23% KA; -26% NMDA). The activity of the glial enzyme glutamine synthetase (GS; EC 6.3.1.2) was also reduced (-32% KA; -27% NMDA). Pre-incubation with 100 microM L-NG-nitroarginine (L-N-ARG), an inhibitor of nitric oxide (NO) synthase (EC 1.14.23.-), for 20 min attenuated the toxicity of toxicity of NMDA, but not KA. NSE levels after successive incubation in L-N-ARG and NMDA were 95% of controls incubated in Krebs bicarbonate medium only (GS activity 89% of controls). In contrast, pre-incubation with L-N-ARG prior to the addition of KA resulted in neuronal degeneration and significant reductions in NSE levels and GS activities. These observations suggest that the unrestricted function of NO synthase is significant in mediating NMDA neurotoxicity whereas KA toxicity is associated with alternative mechanisms not linked to NO production.

Increase in nucleoside diphosphatase in rat brain striatum lesioned with kainic acid

The activity of ammoniagenesis from guanine nucleotides was found to increase significantly in rat brain after infusion of kainic acid into the striatum. Among the enzymes involved in degrading guanine nucleotides, nucleoside diphosphatase was markedly increased in the lesioned striatum. The enzyme activity began to increase 2 days after the infusion, and reached the maximum on the 13th day, the level being 4 times as high as that of the intact contralateral region. The increased activity was due to Type L enzyme, judging from its substrate specificity. Puromycin and cycloheximide inhibited this increase, indicating that the increased activity resulted from an increase in the net synthesis of the enzyme. These findings suggest that Type L NDPase might play some important roles in gliosis after neuronal lesion.

Glutamine synthetase in oligodendrocytes and astrocytes: new biochemical and immunocytochemical evidence

The results of recent immunocytochemical experiments suggest that glutamine synthetase (GS) in the rat CNS may not be confined to astrocytes. In the present study, GS activity was assayed in oligodendrocytes isolated from bovine brain and in oligodendrocytes, astrocytes, and neurons isolated from rat forebrain, and the results were compared with new immunochemical data. Among the cells isolated from rat brain, astrocytes had the highest specific activities of GS, followed by oligodendrocytes. Oligodendrocytes isolated from white matter of bovine brain had GS specific activities almost fivefold higher than those in white matter homogenates. Immunocytochemical staining also showed the presence of GS in both oligodendrocytes and astrocytes in bovine forebrain, in three white-matter regions of rat brain, and in Vibratome sections as well as paraffin sections.

Glutamate dysfunction and selective motor neuron degeneration in amyotrophic lateral sclerosis: a hypothesis

Recent studies provided evidence for a generalized defect in glutamate metabolism in patients with amyotrophic lateral sclerosis, associated with widespread alterations in the central nervous system levels of this excitatory amino acid putative transmitter. Present data support the hypothesis that altered presynaptic glutamatergic mechanisms may be responsible for a neuroexcitotoxic cell loss in this disorder. High local concentrations of glycine, released from glycinergic terminals, may disrupt adaptive processes contributing to abnormal potentiation of excitatory transmission mediated by glutamate receptors and resultant selective degeneration of motor neurons. These considerations offer new therapeutic strategies for amyotrophic lateral sclerosis.

Glutamate dehydrogenase: some properties of the rat brain enzyme from different cellular compartments

1. Differences in the GDH activity of neuronal, glial cells and synaptosomes were detected. 2. The enzyme was measured in both directions: synthesis and degradation of glutamate. 3. Synaptosomes were the region with the highest GDH activity. 4. ADP plays an important role in the regulation of the reaction sense. 5. This effector produced higher activation on the enzyme measured in the direction of glutamate synthesis than in the sense of its degradation. 6. The enhancement produced by ADP was dependent on the enzyme localization. The ADP effect is discussed.

Intrahippocampal kainic acid reduces glutamine synthetase

Kainic acid was injected into the hippocampus of rats and glutamine synthetase was measured to determine whether astrocytes are involved in the early effects of this neurotoxic agent. Glutamine synthetase was reduced by 38%, 24 h after the stereotaxic application of 4 nmol of kainic acid to this region. The reduction in glutamine synthetase by kainic acid was not due to direct inhibition of the brain enzyme. This effect also was not due to seizure activity since rats peripherally injected with a convulsant dose of kainic acid were found to have normal hippocampal glutamine-synthetase activity. Exposure of astrocyte cultures to kainic acid for 24 h produced no evidence of gliotoxicity and no change in glutamine synthetase activity. The effect of intrahippocampal kainic acid on glutamine synthetase appears to be indirect, most likely produced secondarily to its neuronal effects. Several studies have shown that endogenous glutamate is involved in kainate neurotoxicity. A reduction in glutamine synthetase by kainic acid may impair the capacity for astrocytes to metabolize glutamate. Such an impairment could contribute to the glutamate-mediated cell death following kainic acid exposure.

Cellular localization of soluble and membrane-bound forms of arylsulfatase in rat brain

The cellular localization of the soluble and membrane-bound forms of the enzyme, arylsulfatase (ArS), in rat brain was investigated by measuring their activities in rat striatum after unilateral lesioning with the neurotoxin, kainic acid. Membrane-bound ArS (C form of ArS) activity was found to increase after lesioning and the increase paralleled that of the astroglial marker enzyme, glutamine synthetase. Total soluble ArS (A and B forms of ArS) was shown to decrease on day 2 after the kainic acid injection but rapidly increase thereafter. When the two soluble forms of arylsulfatase were measured separately, the activity associated with the A form was found to initially decrease followed by a rapid increase in activity, whereas the activity of the B form of the enzyme increased over the entire duration of the experiment. These data suggest that the ArS-C and B form of arylsulfatase predominate in proliferating astroglial cells, whereas the A form of arylsulfatase is present both in neuronal cell bodies and astroglia associated with the rat striatum.

Detoxification enzymes following intrastriatal kainic acid

A complete explanation of the neurotoxicity that follows kainic acid (KA) injection into the rat striatum is lacking. An assessment of the chronological course after intrastriatal KA injection of the activities of enzymes preferentially concentrated in glia or involved in the detoxification of oxygen metabolites is accomplished. An enhancement of the specific activities of glutathione peroxidase (GP) and catalase is found without an alteration in the specific activity of superoxide dismutase (SOD). There is no increase in the in vivo striatal levels of malondialdehyde, a putative indicator of lipid peroxidation, the expected result of cell membrane damage from oxygen metabolites. Understanding the mechanism and importance of the preferential induction of the activities of the detoxification enzymes will require further study.

In vivo studies on the extracellular, and veratrine-releasable, pools of endogenous amino acids in the rat striatum: effects of corticostriatal deafferentation and kainic acid lesion

The effects of corticostriatal deafferentation (decortication) and destruction of intrinsic neurons (intrastriatal kainate injection) on the extracellular concentration, and veratrine-releasable pools, of endogenous amino acids in the rat striatum were examined using the in vivo brain dialysis technique. Intracellular amino acid content was also determined. Decortication reduced selectively intra- and extracellular levels of glutamate (Glu) and aspartate (Asp). Extracellular changes were more pronounced than those in tissue content. gamma-Aminobutyric acid (GABA), taurine (Tau), and phosphoethanolamine (PEA) levels were not affected, whereas nonneuroactive amino acids were increased at 1 week but not at 1 month post-lesion. The intracellular pool of Glu and Asp was also reduced in kainate-lesioned striata. However, extracellular levels of these compounds were not affected significantly by this treatment. The tissue content of all other amino acids was decreased, the most prominent change being in the concentration of GABA. Extracellular GABA concentration was also reduced dramatically, whereas the concentrations of noneuroactive amino acids were increased to varying degrees. These data suggest that transmitter pools of neuroactive amino acids are an important supply for their extracellular pools. Lesion-induced alterations in nonneuroactive amino acids are discussed with regard to the loss of metabolic pools, glial reactivity, and changes in blood-brain barrier transport. Veratrine induced a massive release of neuroactive amino acids such as Glu, Asp, GABA, and Tau into the extracellular fluid, and a delayed increase in PEA. Extracellular levels of neuroactive amino acids were raised slightly. Decortication reduced, selectively, the amounts of Glu and Asp released by veratrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral ammonia metabolism in normal and hyperammonemic rats

Brain ammonia is generated from many enzymatic reactions, including glutaminase, glutamate dehydrogenase, and the purine nucleotide cycle. In contrast, the brain possesses only one major enzyme for the removal of exogenous ammonia, i.e., glutamine synthetase. Thus, following administration of [13N]ammonia to rats [via either the carotid artery or cerebrospinal fluid (csf)], most metabolized label was in glutamine (amide) and little was in glutamate (plus aspartate). Since blood-and csf-borne ammonia are converted to glutamine largely, if not entirely, in the astrocytes, it is not possible from these types of experiments to predict with certainty the metabolic fate of the bulk of endogenously produced ammonia. By comparing the specific activity of L-[13N]glutamate to that of L-[amine-13N]glutamine following intracarotid [13N]ammonia administration it was concluded that metabolic compartmentation is no longer intact in the brains of rats treated with the glutamine synthetase inhibitor L-methionine-SR-sulfoximine (MSO) and that blood and brain ammonia pools mix in such animals. In MSO-treated animals, recovery of label in brain was low (approximately 20% of controls), and of the label remaining, a prominent portion was in glutamine (amide) (despite an 87% decrease in brain glutamine synthetase activity). These data are consistent with the hypothesis that glutamine synthetase is the major enzyme for metabolism of endogenously--as well as exogenously--produced ammonia. The rate of turnover of blood-derived ammonia to glutamine in normal rat brain is extremely rapid (t1/2 less than or equal to 3 s), but is slowed in the brains of chronically (12-14-wk portacaval-shunted) or acutely (urease-treated) hyperammonemic rats (t1/2 less than or equal to 10 s). The slowed turnover rate may be caused by an increased astrocytic ammonia, decreased glutamine synthetase activity, or both. In the hyperammonemic rat brain, glutamine synthetase is still the only important enzyme for the removal of blood-borne ammonia. Hyperammonemia causes an increase in brain lactate/pyruvate ratios and decreases in brain glutamate and brainstem ATP, consistent with an interference with the malate-aspartate shuttle. In vitro, pathological levels of ammonia also inhibit brain alpha-ketoglutarate dehydrogenase complex and, less strongly, pyruvate dehydrogenase complex. The rat brain does not adapt to prolonged hyperammonemia by increasing its glutamine synthetase activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphologic evidence of a primary response of gila to kainic acid administration into the rat neostriatum; studied in vivo and in vitro

Glial changes that follow kainic acid administration were studied in the rat neostriatum at many different time intervals after the lesion, both in the animal model of Huntington's chorea and in an organotypic culture of striatum. The glial reaction showed striking similarities between in vivo and in vitro conditions and resulted in extensive production and accumulation of gliofilaments leading to transformation of the protoplasmic type of astroglia into the fibrous type. The earliest ultrastructural study in vivo revealed severe swelling of the astrocytic cytoplasm and additional morphologic changes of cytoplasmic organelles, i.e., enlargement of mitochondria, dilation of rough endoplasmic reticulum, and presence of numerous vacuoles. The glial pathology progressed parallel to neuronal degeneration. The same reaction was observed in culture both in the explanted tissue in which neurons remained intact and in the distant outgrowth zone containing a pure population of glial cells. This study proved that kainic acid might act directly on astroglia cells and that glial changes were independent of neuronal damage. Because kainic acid is a structural analog of glutamate, the presented results may be interpreted to reflect changes in the metabolism of this amino acid occurring in astroglia independently of neuronal changes. This interpretation is consistent with the existence of two independent metabolic compartments of glutamate.

The subcellular localization of glutamate dehydrogenase (GDH): is GDH a marker for mitochondria in brain?

Glutamate dehydrogenase (GDH, EC 1.4.1.2) has long been used as a marker for mitochondria in brain and other tissues, despite reports indicating that GDH is also present in nuclei of liver and dorsal root ganglia. To examine whether GDH can be used as a marker to differentiate between mitochondria and nuclei in the brain, we have measured GDH by enzymatic activity and on immunoblots in rat brain mitochondria and nuclei which were highly enriched by density-gradient centrifugation methods. The activity of GDH was enriched in the nuclear fraction as well as in the mitochondrial fraction, while the activities of other "mitochondrial" enzymes (fumarase, NAD-isocitrate dehydrogenase and pyruvate dehydrogenase complex) were enriched only in the mitochondrial fraction. Immunoblots using polyclonal antibodies against bovine liver GDH confirmed the presence of GDH in the rat brain nuclear and mitochondrial fractions. The GDH in these two subcellular fractions had a very similar molecular weight of 56,000 daltons. The mitochondrial and nuclear GDH differed, however, in their susceptibility to solubilization by detergents and salts. The mitochondrial GDH could be solubilized by extraction with low concentrations of detergents (0.1% Triton X-100 and 0.1% Lubrol PX), while the nuclear GDH could be solubilized only by elevated concentrations of detergents (0.3% each) plus KCl (greater than 150 mM). Our results indicate that GDH is present in both nuclei and mitochondria in rat brain. The notion that GDH may serve as a marker for mitochondria needs to be re-evaluated.

Abnormal platelet glutamate dehydrogenase activity and activation in dominant and nondominant olivopontocerebellar atrophy

Glutamate dehydrogenase (GDH) activity and its allosteric modulation by purine nucleotides were studied in platelet preparations from 4 patients with a nondominant form of adult-onset olivopontocerebellar atrophy (OPCA) and in affected and nonaffected members of two families with a dominant form of OPCA. A partial deficiency of GDH activity (40 to 50% of control values) was present in 3 patients with nondominant OPCA and in 2 patients, father and son, with a dominant form of OPCA. Platelet GDH from these patients and controls was regularly inactivated by 2 mM guanosine-5'-triphosphate (GTP) and simulated one- to twofold by 2 mM adenosine-5'-diphosphate (ADP). In the presence of 0.2% Triton X-100, the activating effect of ADP was enhanced four- to sixfold. The partial deficiency in maximum catalytic activity observed in these patients persisted under all conditions used for enzyme assay. In affected members, but not in one unaffected member of another family with a dominant type of OPCA, GDH activity was in the control range but was not activated by ADP in either the presence or absence of Triton. These results suggest that there may be at least two possible alterations of GDH in patients with OPCA: one which decreases the maximum catalytic activity and one which impairs the regulatory properties of the enzyme. Furthermore, this study suggests that platelet GDH determination in patients with OPCA may provide a simple and useful tool to classify these disorders and to understand the basic pathophysiological mechanisms involved.

Heterogeneity of sodium-dependent excitatory amino acid uptake mechanisms in rat brain

The pharmacologic and kinetic characteristics of sodium-dependent uptake of [3H]L-glutamate, [3H]D-aspartate, and [3H]L-aspartate into crude synaptosomal preparations of rat corpus striatum and cerebellum have been examined in vitro. In cerebellum the apparent Kts and Vmax for the three excitatory amino acids were identical whereas in striatal synaptosomes, the Vmax for [3H]L-glutamate was 30% greater (P less than or equal to .001) than for [3H]D-aspartate and 50% greater (P less than or equal to .001) than for [3H]L-aspartate. L-Amino adipic acid inhibited the uptake of the three amino acids in both regions of brain was 15- to 20-fold more potent in cerebellum than in striatum. In contrast, dihydrokainic acid inhibited transport processes in the corpus striatum but was without activity in cerebellar preparations. The neurotoxin kainic acid blocked only a portion (60%) of [3H]L-glutamate and [3H]D-aspartate uptake in cerebellum while completely inhibiting amino acid transport in corpus striatum. Three days post kainic acid lesion, [3H]D-aspartate uptake was attenuated more than [3H]L-glutamate uptake in the corpus striatum; destruction of corticostriatal afferents reduced [3H]L-glutamate to a greater extent than [3H]D-aspartate. Various lesions of the cerebellum affected excitatory amino acid transport processes to a similar extent. These results suggest that excitatory amino acid transport systems are pharmacologically distinct in different brain regions and may be heterogeneous within a single region.

Effects of intrahypothalamic kainic acid injection on taurine levels, binding and uptake

The neurochemical effects of unilateral intrahypothalamic injection of kainic acid on taurine levels and synaptosomal uptake and binding of taurine were investigated. Seven days after the kainic acid injections, there were no changes in either taurine uptake or binding. However, taurine levels were significantly increased by 54% over the control contralateral side. These data are consistent with the hypothesis that taurine is localized in glial cells; the increased levels being a result of gliosis after kainic acid injections.

Cellular localization of MAO A and B in brain: evidence from kainic acid lesions in striatum

The cellular localization of the two forms of monoamine oxidase (MAO A and MAO B) was studied by measuring their activities in rat striatum following unilateral stereotaxic injection of kainic acid to produce selective degeneration of striatal neurons and subsequent proliferation of astrocytes. The results demonstrated a persistent loss of 15-20% in MAO A activity, whereas MAO B activity decreased initially by 25% and then increased to more than twice the control value by 54 days after lesions. The changes in activity were compared to parallel estimates of the postsynaptic neuronal enzyme markers glutamic acid decarboxylase (GAD) and neuron-specific enolase (NSE), astroglial enzyme markers glutamine synthetase (GS) and non-neuronal enolase (NNE), and the presynaptic enzyme marker DOPA decarboxylase (DDC). The results suggest that a small amount of striatal MAO A is present in kainic acid-sensitive postsynaptic striatal neurons and that MAO B is probably localized in both neurons and astrocytes.

Changes in regional neurotransmitter amino acid levels in rat brain during seizures induced by L-allylglycine, bicuculline, and kainic acid

Changes in amino acid concentrations were studied in the cortex, cerebellum, and hippocampus of the rat brain, after 20 min of seizure activity induced by kainic acid, 47 mumol/kg i.v.; L-allylglycine, 2.4 mmol/kg i.v.; or bicuculline, 3.27 mumol/kg i.v. in paralysed, mechanically ventilated animals. Metabolic changes associated with kainic acid seizures predominate in the hippocampus, where there are decreases in aspartate (-26%), glutamate (-45%), taurine (-20%), and glutamine (-32%) concentrations and an increase in gamma-aminobutyric acid (GABA) concentration (+ 26%). L-Allylglycine seizures are associated with generalized decreases in GABA concentrations (-32 to -54%), increases in glutamine concentrations (+10 to +53%), and a decrease in cortical aspartate concentration (-14%). Bicuculline seizures, in fasted rats, are associated with marked increases in the levels of hippocampal GABA (+106%) and taurine (+40%). In the cerebellum, there are increases in glutamine (+50%) and taurine concentrations (+36%). These changes can be explained partially in terms of known biochemical and neurophysiological mechanisms, but uncertainties remain, particularly concerning the cerebellar changes and the effects of kainic acid on dicarboxylic amino acid metabolism.

Glutamine oxidation by dissociated cells and homogenates of rat brain: kinetics and inhibitor studies

The rates of [U-14C]glutamine oxidation to 14CO2 were determined under a variety of experimental conditions using whole homogenates and dissociated cells from rat brain. The pattern of glutamine oxidation by homogenates differed from that by dissociated brain cells in several respects. The rates of glutamine oxidation by dissociated brain cells showed saturation kinetics with an apparent Km of 0.30 mM. Lineweaver-Burk plots of glutamine oxidation by homogenates revealed two linear segments with two apparent Km values (0.58 mM and 3.0 mM). In the presence of aminooxyacetate, however, the Lineweaver-Burk plots for homogenates were linear with a single Km of 0.47 mM. The oxidation of glutamine by homogenates was inhibited by both rotenone and antimycin A (80-85%), as were glutamate and glucose oxidation, suggesting that a significant amount of glutamine is oxidized via the tricarboxylic acid cycle. In the presence of aminooxyacetate, glutamine oxidation was inhibited less than 40%, whereas the oxidation of glutamate was inhibited 75%; in contrast, glucose oxidation was enhanced 50%. The rates of glutamine oxidation by homogenates were highest in the presence of high levels of potassium (50 mM) and low levels of sodium (2.5 mM). Varying ionic composition, however, had little or no effect on the rates of glutamine oxidation by dissociated brain cells. Measurements of glutamine oxidation by homogenates prepared from 2-, 10-, 15-, 25-, and 90-day-old rats revealed little or no age-dependent difference. In contrast, the oxidation by dissociated brain cells from 2-day-old animals was significantly less than that obtained for animals 10 days or older (7.76 vs. 15.6 nmol/h/mg).(ABSTRACT TRUNCATED AT 250 WORDS)

Competition among oxidizable substrates in brains of young and adult rats. Whole homogenates

The rates of conversion into 14CO2 of D-(-)-3-hydroxy[3-14C]butyrate, [3-14C]acetoacetate, [6-14C]glucose and [U-14C]glutamine were measured in the presence and absence of unlabelled alternative oxidizable substrates in whole homogenates from the brains of young and adult rats. The addition of unlabelled glutamine resulted in decreased 14CO2 production from [6-14C]glucose in brain homogenates from both young and adult rats. In contrast, glucose had no effect on [U-14C]glutamine oxidation. In suckling animals, both 3-hydroxybutyrate and acetoacetate decreased the rate of oxidation of [6-14C]glucose, but in adults only 3-hydroxybutyrate had an effect, and to a lesser degree. The addition of unlabelled glucose markedly enhanced the rates of oxidation of both ketone bodies in adult brain tissue and had little or no effect in the young. The rate of production of 14CO2 from [U-14C]glutamine was increased by the addition of unlabelled ketone bodies in brain homogenates from young, but not from adult rats. In the converse situation, unlabelled glutamine added to 14C-labelled ketone bodies diminished 14CO2 production in young rats, but had no effect in adult animals. These results revealed a complex age-dependent pattern of interaction in which certain substrates apparently competed with each other, whereas an enhanced rate of 14CO2 production was found with others.

Competition among oxidizable substrates in brains of young and adult rats. Dissociated cells

The rates of conversion of D-(-)-3-hydroxy[3-14C]butyrate, [3-14C]acetoacetate, [6-14C]glucose and [U-14C]glutamine into 14CO2 were measured in the presence and absence of alternative oxidizable substrates in intact dissociated cells from the brains of young and adult rats. When unlabelled glutamine was added to [6-14C]glucose or unlabelled glucose was added to [U-14C]glutamine, the rate of 14CO2 production was decreased in both young and adult rats. The rate of oxidation of 3-hydroxy[3-14C]butyrate was also decreased by the addition of unlabelled glutamine in both age groups, but in the reverse situation, i.e. unlabelled 3-hydroxybutyrate added to [U-14C]glutamine, only the brain cells from young rats were affected. No significant effects were seen when glutamine and acetoacetate were combined. The addition of either of the two ketone bodies to [6-14C]glucose markedly lowered the rate of 14CO2 production in young rats, but in the adult only 3-hydroxybutyrate was effective and the magnitude of decrease in the rate of [6-14C]glucose oxidation was much lower than in young animals. Unlabelled glucose decreased the rate of [3-14C]acetoacetate oxidation to a minor extent in brain cells from both age groups; when added to 3-hydroxy[3-14C]butyrate, glucose had no effect in young rats and greatly enhanced 14CO2 production in adult brain cells. Many of these patterns of substrate interaction in dissociated brain cells differ from those in whole homogenates; they may be a function of the plasma membranes and the role of a carrier-mediated transport system or a reflection of a difference in the population of cell types or subcellular organelles in these two preparations.

Glial contribution to amino acid content and metabolism of the deafferented dentate gyrus

The time course of tissue content and evoked release of endogenous amino acids was analyzed in the partially deafferented dentate gyrus of the rat hippocampus 2-24 days following unilateral lesion of the perforant path. Amino acids in tissue extracts and perfusates were determined after precolumn derivatization and hplc separation. The astrocytic glial cell reaction was monitored with immunohistochemistry of S-100. The tissue content of glutamate decreased significantly on the lesioned side, whereas only a moderate reduction in taurine, aspartate, and alanine occurred. Glutamine was significantly elevated at 7 days. The evoked efflux of glutamate was reduced at 2 and 7 days, whereas no change was seen at longer survival periods. The evoked release of GABA and aspartate increased on the denervated side after 12 and 24 days. The rate of carbon utilization into amino acid pools was followed with 14C-glucose and 14C-acetate. The incorporation of acetate showed a peak 2-9 days following lesion, which paralleled in time the hypertrophic glial cells. The incorporation of glucose decreased during this period. The metabolic events are discussed in relation to the morphological changes in synapses and glial cells.

Feline generalized penicillin epilepsy: changes of glutamic acid and taurine parallel the progressive increase in excitability of the cortex

A coinciding temporal sequence of electrophysiological and biochemical correlates of developing generalized penicillin epilepsy in cats may indicate a "cause and effect" relationship between the two phenomena. After intramuscular injection of penicillin, in the pre-epileptic state prior to the onset of spike-and-wave discharge, the cortical content of glutamic acid decreases. This change occurs when an increased amplitude of visual evoked potentials in association cortex heralds the approach of spike-and-wave activity. The decrease of glutamic acid and that of aspartic acid occur in parallel with an almost stoichiometric increase of glutamine, gamma-aminobutyric acid (GABA), or both, while taurine levels in the pre-epileptic state remain near normal. As the pre-epileptic progresses to the epileptic state, characterized by generalized 4-5 cycles/s spike-and-wave discharges, a failure of the glial capture mechanisms for taurine and glutamate appears to occur, since both amino acids are lost from the tissue and glutamine levels fall while GABA levels are maintained or become elevated but increasingly at the expense of aspartic acid. A presumed increase in interstitial glutamic acid concentration possibly in combination with subsequent failure of GABA inhibition appears the most plausible explanation for the increasing hyperexcitability during the development of feline generalized penicillin epilepsy.

Activity of neuron-specific enolase in normal and lesioned rat brain

Rat brain contains 3 forms of enolase, a neuron-specific form (NSE), a hybrid form, and a non-neuronal form (NNE) which were separated by DEAE-cellulose column chromatography. The enolase activity corresponding to each form of the enzyme eluted from the columns was determined spectrophotometrically. Using this assay procedure, an activity ratio (%NNE/%NSE) was calculated for cerebellum, brainstem, sciatic nerve, adrenals and liver. The results indicated excellent agreement between this enzymatically determined ratio and published values of a similar ratio (NNE/NSE) determined by radioimmunoassay for enzyme protein. Following in vivo destruction of neurons by intracerebral injection of the selective neurotoxin, kainic acid, there was a significant decrease in the activity of NSE and hybrid enolase (neuronal forms) and no change in the activity of NNE (glial form). These data indicate that separation and measurement of enolase species is useful to determine levels of these species in normal tissue and to estimate neuronal damage biochemically in brain lesions.

Distinct cellular localization of membrane-bound and soluble forms of catechol-O-methyltransferase in brain

The cellular localization of the two forms of catechol-O-methyltransferase (COMT) was investigated by measuring their activities in rat striatum following unilateral stereotaxic injection of kainic acid, which causes degeneration of striatal neurons followed by proliferation of astroglial cells. Membrane-bound COMT activity was decreased in the lesioned striatum, while soluble COMT activity was increased. There was a statistically significant correlation between the ratio of lesioned to control activity for membrane-bond COMT and the neuronal marker enzyme glutamate decarboxylase. Similarly the increase in soluble COMT activity paralleled that of the astroglial marker enzyme, glutamine synthetase. These results indicate that the low-Km membrane-bound catechol-O-methyltransferase may be localized predominantly in neurons, whereas the high-Km soluble enzyme is found in glial cells.

GABA-transaminase in the basal ganglia: a pharmacohistochemical study

A pharmacohistochemical procedure to demonstrate histochemically those cells with the ability to synthesize the GABA-metabolizing enzyme GABA-transaminase has been applied to determine the localization of this enzyme in the basal ganglia. In normal, pharmacologically unmanipulated animals, strong GABA-transaminase activity is present throughout the neuropil in many nuclei of the basal ganglia. The striatum, globus pallidus, entopeduncular nucleus, subthalamic nucleus and substantia nigra are all heavily stained. White matter such as the corpus callosum and internal capsule are unstained. Pretreatment of rats with the irreversible GABA-transaminase inhibitor ethanolamine-O-sulfate resulted in a marked reduction in the general neuropil staining. At a suitable post-injection survival time, cells which had synthesized new GABa-transaminase molecules could be detected. Small, positive neurons were present in the striatum and entopeduncular nucleus, while the globus pallidus, substantia nigra zona reticulata, and the ventral pallidal region contained many large, intensely GABA-transaminase-positive neurons. The results indicate that much of the GABA-transaminase in the basal ganglia is neuronal in origin. The relationship of GABA-transaminase to GABA neurons is discussed.

Glutamine synthetase activity in Huntington's disease

Glutamine synthetase activity was measured in seven brain areas post-mortem from control patients, and those with Huntington's disease. The activity of the enzyme was reduced in the frontal and temporal cortex, putamen and cerebellum, but not in the hippocampus, thalamus or olivary nucleus. The results do not suggest a generalised deficiency of glutamine synthetase in Huntington's disease. However, as this enzyme is localised to astrocytic cells, the reduction in activity in areas of neuronal devastation, where the ration of astrocytes to neurones is increased, may reflect a greater functional deficit. The enzyme plays a crucial role in cerebral ammonia assimilation and its inhibition in laboratory animals is known to produce neuronal toxicity. A reduction in its activity in Huntington's disease may well contribute to the neuronal pathology in certain areas.

Induction of glutamine synthetase by dibutyryl cyclic AMP in C-6 glioma cells

Glutamine synthetase was found to be increased in C-6 glioma cells as a result of increasing culture passage and N-6,2'-O-dibutyryl cyclic AMP (dbcAMP) treatment. At low passage dbcAMP produced a 2.5-fold increase in glutamine synthetase activity per unit of cellular protein. At high passage control glutamine synthetase was approximately double that seen at low passage, but dbcAMP produced an additional 65% increase. Lactate dehydrogenase activity was also increased by dbcAMP treatment at both low and high passage, but culture passage produced no change in the lactate dehydrogenase. With increasing culture passage, the ratio of cellular protein to DNA doubled. Therefore, expression of data per unit of protein tended to minimize the apparent changes in activity. The maximum increase in glutamine synthetase activity produced by both dbcAMP and increasing culture passage and expressed on a DNA basis was 5.6-fold. The increase in glutamine synthetase activity was generally linear during the first 20 h of drug treatment, after which enzyme activity remained nearly constant up to 72 h. Ninety percent or more of the dbcAMP remained in the medium at the end of 48-h exposure of cells to dbcAMP. 8-br-Cyclic AMP also increased glutamine synthetase activity of C-6-cels, but n-butyrate did not. Isoproterenol, which increases cyclic AMP in C-6-cells, increased glutamine synthetase activity. The effect of isoproterenol on glutamine synthetase was inhibited by the beta-adrenergic blocking agent sotalol. Cycloheximide (10 micrograms/ml) inhibited the dbcAMP effect on glutamine synthetase activity and also decreased the control enzyme activity by 60%.

Glutamine and glutamate transport in cultured neuronal and glial cells

The uptake of L-glutamine in neuronal and glial cultures derived from rat cerebral hemispheres was found to be mediated by a low affinity-high capacity mechanism which was concentrative in both cell types; the calculated Km and Vmax were twice as high in glial than in neuronal cultures. In contrast L-glutamate was taken up by a high affinity system which was particularly efficient and concentrative in the glial cells. Different transport mechanisms for L-glutamine appeared to operate in the two cell types: L-glutamine uptake in neurons was sodium-dependent, specifically inhibited by L-glutamine but not affected by high potassium concentrations in the external medium; on the other hand, glial glutamine transport was decreased when potassium concentration increased, was sodium-independent and significantly inhibited by 3 structurally related amino acids. No significant contribution of homoexchange could be detected in either cell type. After [14C]glutamine preincubation, the radioactivity released into the superfusion medium by neuronal cells was increased in the presence of a high potassium concentration; no such effect could be seen in the case of glial cultures. A regulatory mechanism is suggested where astrocyte depolarization and repolarization would channel a flux of glutamine toward the neurons, subsequent to a glutamate flux in the opposite direction.

Metabolism and brain uptake of gamma-aminobutyric acid in galactosamine-induced hepatic encephalopathy in rats

Kinetic studies of [3H]gamma-aminobutyric acid ([3H]GABA) after an intravenous injection were performed in normal rats and in rats with severe degree of hepatic encephalopathy due to fulminant hepatic failure induced by galactosamine. Moreover, plasma and brain GABA levels, and GABA and glutamic acid decarboxylase activity were studied in some brain areas. After intravenous injection, [3H]GABA disappeared very rapidly in the blood of normal rats, with a prompt increase of 3H metabolites. In comatose rats, a delayed disappearance of [3H]GABA was parallelled by a lower amount of metabolites, indirectly indicating a peripheral decrease of GABA-transaminase activity. The amount of [3H]GABA in brain was lightly but constantly lower in comatose rats than in controls, indicating that the change in permeability of the blood-brain barrier in hepatic encephalopathy does not affect the [3H]GABA uptake of the brain. Furthermore, the assay of endogenous GABA in blood, whole brain, and brain areas did not show any significant difference in any of the two groups. The finding that glutamic acid decarboxylase activity in brain was reduced, together with the indirect evidence of a reduction in GABA-transaminase, may account for the steady state of GABA in hepatic encephalopathy. However, the reduction in glutamic acid decarboxylase activity is in favor of a functional derangement at the GABA-ergic nerve terminals in this pathological condition.

Regulation of glutamate biosynthesis and release in vitro by low levels of ammonium ions

The content and release of endogenous amino acids from isolated rat hippocampal slices were measured. The tissue was perfused with control media and pulsed with high potassium media in order to induce synaptic release. Pathophysiological concentrations of ammonium ions (3--5 mM) were added to the control medium for 60 min prior to the induced release. Amino acids belonging to the putative transmitter group were released extensively during potassium perfusion and, except for glutamate, even after ammonium ion perfusion. The spontaneous secretion of glutamate increased, however, slowly after the addition of ammonia. The incorporation of 14C from radiolabelled glucose and acetate into the amino acid fraction was studied in the presence of ammonia-containing media. Glucose was utilized to a moderately increasing extent, but acetate-derived radioactivity was strikingly decreased in the amino acid fraction during ammonia perfusion. The decreased acetate incorporation into amino acids was mainly due to an inhibition by ammonia of the accumulation of acetate by the CNS tissue.

Distribution and uptake of glycine, glutamate and gamma-aminobutyric acid in the vagal nuclei and eight other regions of the rat medulla oblongata

In order to study the central neurochemical control of the vagus nerve, the contents of glycine, GABA, glutamate and five other amino acids have been measured in ten anatomically distinct regions of the rat medulla oblongata. Additionally, the high affinity uptake of glycine, GABA, glutamate, and leucine were measured in the same ten medullary regions. The data support published evidence for glutamatergic and GABAergic transmission in the nucleus of the tractus solitarius (NTS), and glycinergic inhibition in the hypoglossal nucleus. The data also lead to the suggestion that GABA and glutamate may be taken up into glial cells which exist along fiber tracts.

Alteration in neuronal-glial metabolism of glutamate by the neurotoxin kainic acid

The effect of the excitotoxin kainic acid on glutamate and glutamine metabolism was studied in cerebellar slices incubated with D-[2-14C]glucose, [U-14C]gamma-aminobutyric acid, [3H]acetate, [U-14C]glutamate, and [U-14C]glutamine as precursors. Kainic acid (1 mM) strongly inhibited the labeling of glutamine relative to that of glutamate from all precursors except [2-14C]glucose and [U-14C]glutamine. Kainic acid did not inhibit glutamine synthetase directly. The data indicate that in the cerebellum kainic acid inhibits the synthesis of glutamine from the small pool of glutamate that is thought to be associated with glial cells. Kainic acid also markedly stimulated the efflux of glutamate from cerebellar slices and this release was not sensitive to tetrodotoxin. Kainic acid stimulated efflux of both glucose- and acetate-labeled glutamate. In contrast, veratridine released glucose-labeled glutamate preferentially via a tetrodotoxin-sensitive mechanism. Kainic acid did not release [U-14C]glutamate from synaptosomal fractions. These results suggest that the bulk of the glutamate released from cerebellar slices by kainic acid comes from nonsynaptic pools.

Effects of kainic acid injection and cortical lesion on ornithine and aspartate aminotransferases in rat striatum

The effects of cortical lesions and intrastriatal kainic acid injections on various striatal enzyme activities were investigated. Ornithine aminotransferase decreased concomitantly with glutamate uptake in decorticated and chronic kainic acid-treated rats. It was also decreased in acute kainic acid-lesioned striatum where glutamate uptake was unaffected. Aspartate aminotransferase, however, decreased only after acute kainic acid treatment. Results for glutamate uptake, glutamate decarboxylase, and choline acetyltransferase were in agreement with previous findings. These results suggest that ornithine may act as a precursor for glutamate in nerve terminals, although the nonspecific localization does not allow ornithine aminotransferase to be a convenient biochemical marker. The decrease in aspartate aminotransferase is thought to be due to the widespread cell degeneration after acute kainic acid. Aspartate aminotransferase activities were also found to be reduced in the frontal cortex, caudate nucleus and putamen of Huntington's disease brains.

Alterations in the compartmentalized metabolism of glutamic acid with changed cerebral conditions

Data obtained from combined determinations of the nervous tissue content of glutamic acid, taurine and glutamine were examined in terms of the well established concept of a compartmentalized metabolism for glutamic acid. Three different situations associated with altered cortical conditions were studied: cortical hyperexcitability induced by cobalt epilepsy (mouse); chronic stimulation of the optic tectum by light adaptation (fish); and anatomic alteration of the optic tectum following unilateral enucleation (fish). All 3 situations appear to cause a reduction in the ability of glial elements to capture free glutamic acid released from neuronal structures. However, the underlying causes for such an insufficiency seem to differ in each instance. In epilepsy the release of glutamic acid and taurine exceeds the glial capture rate; during chronic stimulation of a normal cortex a diminished glial uptake rate for both amino acids seems apparent; anatomical degenerative changes seem to diminish especially the glutamine retention capacity of the cortex, possibly in combination with a reduced glial taurine uptake.

Control of aerobic glycolysis in the brain in vitro

Protoveratrine-(5 microM) stimulated aerobic glycolysis of incubated rat brain cortex slices that accompanies the enhanced neuronal influx of Na+ is blocked by tetrodotoxin (3 microM) and the local anesthetics, cocaine (0.1 mM) and lidocaine (0.5 mM). On the other hand, high [K+]-stimulated aerobic glycolysis that accompanies the acetylcholine-sensitive enhanced glial uptakes of Na+ and water is unaffected by acetylcholine (2 mM). Experiments done under a variety of metabolic conditions show that there exists a better correlation between diminished ATP content of the tissue and enhanced aerobic glycolysis than between tissue ATP and the ATP-dependent synthesis of glutamine. Whereas malonate (2 mM) and amino oxyacetate (5 mM) suppress ATP content and O2 uptake, stimulate lactate formation, but have little effect on glutamine levels, fluoroacetate (3 mM) suppresses glutamine synthesis in glia, presumably by suppressing the operation of the citric acid cycle, with little effect on ATP content, O2 uptake, and lactate formation. Exogenous citrate (5 mM), which may be transported and metabolized in glia but not in neurons, inhibits lactate formation by cell free acetone-dried powder extracts of brain cortex but not by brain cortex slices. These results suggest that the neuron is the major site of stimulated aerobic glycolysis in the brain, and that under our experimental conditions glycolysis in glia is under lesser stringent metabolic control than that in the neuron. Stimulation of aerobic glycolysis by protoveratrine occurs due to diminution of the energy charge of the neuron as a result of stimulation of the sodium pump following tetrodotoxin-sensitive influx of Na+; stimulation by high [K+], NH4+, or Ca2+ deprivation occurs partly by direct stimulation of key enzymes of glycolysis and partly by a fall in the tissue ATP concentration.

Glutamate dehydrogenase deficiency in three patients with spinocerebellar syndrome

Four nicotinamide-adenine dinucleotide phosphate-requiring enzymes were measured in disrupted cultured skin fibroblasts from a 19-year-old patient with juvenile onset of a spinocerebellar and extrapyramidal syndrome. There was marked reduction in the activity of glutamate dehydrogenase (GDH) (22% of mean control activity); GDH activity was also decreased in homogenates of leukocytes from this patient (38% of mean control activity). GDH activity was measured in the leukocytes of two siblings afflicted with adult-onset spinocerebellar syndrome and found to be decreased in both (29% and 31% of mean control activity); an unaffected sibling had normal GDH activity. Mixing experiments with control fibroblast and leukocyte homogenates did not show the presence of a GDH inhibitor in cells from these patients. This allosterically regulated enzyme was stimulated by adenosine 5'-diphosphate (10(-3) M) and inhibited by guanosine 5'-triphosphate (10(-3) M) in both fibroblast and leukocyte homogenates; these changes occurred in equal proportions in the patients and controls. The decreased fibroblast and leukocyte GDH activity persisted at different concentrations of the enzyme's substrates and with successive passages of cultured fibroblasts. GDH may have an important role in the metabolism of glutamate, a putative neurotransmitter in cerebellum, brainstem, and spinal cord. A genetic deficiency of GDH may underlie some forms of spinocerebellar ataxias.

Effect of kainate on ATP levels and glutamate metabolism in cerebellar slices

The levels of ATP and amino acids were measured in rat cerebellar slices incubated in the presence of the neurotoxin, kainic acid (KA). 0.1--1 mM KA caused a significant decrease in tissue content of ATP, glutamate, aspartate and glutamine. The levels of glutamate and aspartate, but not glutamine, rose concomitantly in the incubation medium. The results are consistent with a multiaction mechanism for the neurotoxicity of KA.