Differences in some properties of newborn and adult brain glutamate decarboxylase

Memantine Improves the Disturbed Glutamine and γ-Amino Butyric Acid Homeostasis in the Brain of Rats Subjected to Experimental Autoimmune Encephalomyelitis

Glutamine (Gln), glutamate (Glu), and γ-amino butyric acid (GABA) are essential amino acids for brain metabolism and function. Astrocyte-derived Gln is the precursor for the two most important neurotransmitters in the central nervous system (CNS), which are the excitatory neurotransmitter Glu and the inhibitory neurotransmitter GABA. In addition to their roles in neurotransmission, these amino acids can be used as alternative substrates in brain metabolism that enable metabolic coupling between astrocytes and neurons in the glutamate-glutamine cycle (GGC). The disturbed homeostasis of these amino acids within the tripartite synapse may be involved in the pathogenesis of various neurological diseases. Interactions between astrocytes and neurons in terms of Gln, Glu, and GABA homeostasis were studied in different phases of experimental allergic encephalomyelitis (EAE) in Lewis rats. The results of the study showed a decrease in the transport (uptake and release) of Gln and GABA in both neuronal and astrocyte-derived fractions. These effects were fully or partially reversed when the EAE rats were treated with memantine, a NMDA receptor antagonist. Changes in the expression and activity of selected glutamine/glutamate metabolizing enzymes, such as glutamine synthase (GS) and phosphate-activated glutaminase (PAG), which were affected by memantine, were observed in different phases of EAE. The results suggested perturbed homeostasis of Gln, Glu, and GABA during EAE, which may indicate alterations in neuron-astrocyte coupling and dysfunction of the tripartite synapse. Memantine appears to partially regulate the disturbed relationships between Gln, Glu, and GABA.

Nicotine Exposure Along with Oral Contraceptive Treatment in Female Rats Exacerbates Post-cerebral Ischemic Hypoperfusion Potentially via Altered Histamine Metabolism

Smoking-derived nicotine (N) and oral contraceptives (OCs) synergistically exacerbate ischemic brain damage in the female, and the underlying mechanisms remain elusive. Our published study showed that N toxicity is exacerbated by OC via altered mitochondrial electron transport chain function. Because mitochondria play an important role in cellular metabolism, we investigated the global metabolomic profile of brains of adolescent and adult female Sprague-Dawley rats exposed to N with or without OC (N+/-OC). Rats were randomly exposed to saline or N+/-OC for 16-21 days followed by random allocation into two cohorts. The first cohort was used to characterize the cortical metabolome. Pathway enrichment analysis showed a significant increase in several histamine metabolites including 1-methylhistamine, 1-methyl-4-imidazoleacetate, and 1-ribosyl-imidazleacetate, along with carnosine and homocarnosine in adolescent and adult animals treated with N and N+OC in relation to respective saline controls, which may be reflective of altered histamine metabolism with nicotine treatment. We also observed reduced levels of the neurotransmitters N-acetyl-aspartyl-glutamate (NAAG), gamma-aminobutyrate (GABA), and N-methyl-GABA in N+OC treatment in adolescent animals. The second cohort underwent bilateral carotid artery occlusion and hypotension followed by cerebral blood flow (CBF) assessment a day later. Autoradiographic images of the brain 24 h after ischemic episodes showed severe reduction in cortical and hippocampal local CBF in N+/-OC-exposed rats compared with saline treated. Because GABA and histamine are critical for CBF maintenance, altered metabolism of these neurotransmitters may be responsible for observed severe post-ischemic hypoperfusion, which in turn exacerbates ischemic brain damage.

Relationships between glutamine, glutamate, and GABA in nerve endings under Pb-toxicity conditions

Glutamine (Gln), glutamate (Glu) and gamma-amino butyric acid (GABA) are essential amino acids for brain metabolism and function. Astrocytic-derived glutamine is the precursor of the two most important neurotransmitters: glutamate, an excitatory neurotransmitter, and GABA, an inhibitory neurotransmitter. In addition to their roles in neurotransmission these neurotransmitters act as alternative metabolic substrates that enable metabolic coupling between astrocytes and neurons. The relationships between Gln, Glu and GABA were studied under lead (Pb) toxicity conditions using synaptosomal fractions obtained from adult rat brains to investigate the cause of Pb neurotoxicity-induced seizures. We have found that diminished transport of [(14)C]GABA occurs after Pb treatment. Both uptake and depolarization-evoked release decrease by 40% and 30%, respectively, relative to controls. Lower expression of glutamate decarboxylase (GAD), the GABA synthesizing enzyme, is also observed. In contrast to impaired synaptosomal GABA function, the GABA transporter GAT-1 protein is overexpressed (possibly as a compensative mechanism). Furthermore, similar decreases in synaptosomal uptake of radioactive glutamine and glutamate are observed. However, the K(+)-evoked release of Glu increases by 20% over control values and the quantity of neuronal EAAC1 transporter for glutamate reaches remarkably higher levels after Pb treatment. In addition, Pb induces decreased activity of phosphate-activated glutaminase (PAG), which plays a role in glutamate metabolism. Most noteworthy is that the overexpression and reversed action of the EAAC1 transporter may be the cause of the elevated extracellular glutamate levels. In addition to the impairment of synaptosomal processes of glutamatergic and GABAergic transport, the results indicate perturbed relationships between Gln, Glu and GABA that may be the cause of altered neuronal-astrocytic interactions under conditions of Pb neurotoxicity.

Inhibition of brain glutamate decarboxylase activity is related to febrile seizures in rat pups

Because previous work showed that in the newborn brain, but not in the adult brain, glutamate decarboxylase (GAD) is notably susceptible to heat, we have studied the possible involvement of GAD inhibition in febrile convulsions and the related changes in gamma-aminobutyric acid (GABA) content. Rats of different ages were subjected to hyperthermia, and GAD activity was determined in brain homogenates by measuring the release of 14CO2 from labeled glutamate and by measuring the formation of GABA. The latter method gave considerably lower values than the former in the youngest rats, and was considered more reliable. With this method, we found a 37-48% inhibition of GAD activity in rat pups 2-5 days old, which showed febrile seizures at progressively higher body temperatures, whereas in 10- and 15-day-old animals, which did not show convulsions, GAD activity was not affected by hyperthermia. Whole-brain GABA levels, however, did not change at any age. In contrast to GAD, choline acetyltransferase and lactic dehydrogenase activities were not altered by hyperthermia at any of the ages studied. These results suggest that a decreased efficiency of the inhibitory neurotransmission mediated by GABA, consequent to the inhibition of GAD activity, may be a factor related to febrile convulsions.

GAD and GABA in an enriched population of cultured GABAergic neurons from rat cerebral cortex

To study various aspects of GABAergic metabolism in an easily accessible system, dissociated cells from postnatal rat cerebral cortex were cultured in a serum-based medium and characterized morphologically and biochemically. The majority (70-96%) of the neurons were GABAergic as determined by three double-labeling procedures. The specific activity of glutamine synthetase in the cultures was 4-5% of the levels in rat astrocyte cultures and intact rat brain, indicating that glia were a minor component. The developmental increase of GABA levels preceded the increase of GAD activity in both immunocytochemical and biochemical experiments. GABA turnover rates also increased with culture age and were 20-30% of GAD activity. Four anti-GAD antibodies, which recognize GAD subunits with differing molecular masses to varying degrees, were used to stain cultured neurons and make immunoblots. Immunoblots showed that the neurons contained two major subunits of GAD which differed in mass by 2 kDa. All four antibodies immunostained both neuronal perikarya and neurites but one antibody, which on the immunoblots predominantly labeled the GAD protein with the lower molecular weight, showed a somewhat more pronounced punctate staining, possibly indicating a principal localization to neurites.

Regulatory properties of brain glutamate decarboxylase

1. Glutamate decarboxylase is a focal point for controlling gamma-aminobutyric acid (GABA) synthesis in brain. Several factors that appear to be important in the regulation of GABA synthesis have been identified by relating studies of purified glutamate decarboxylase to conditions in vivo. 2. The interaction of glutamate decarboxylase with its cofactor, pyridoxal 5'-phosphate, is a regulated process and appears to be one of the major means of controlling enzyme activity. The enzyme is present in brain predominantly as apoenzyme (inactive enzyme without bound cofactor). Studies with purified enzyme indicate that the relative amounts of apo- and holoenzyme are determined by the balance in a cycle that continuously interconverts the two. 3. The cycle that interconverts apo- and holoenzyme is part of the normal catalytic mechanism of the enzyme and is strongly affected by several probable regulatory compounds including pyridoxal 5'-phosphate, ATP, inorganic phosphate, and the amino acids glutamate, GABA, and aspartate. ATP and the amino acids promote apoenzyme formation and pyridoxal 5'-phosphate and inorganic phosphate promote holoenzyme formation. 4. Numerous studies indicate that brain contains multiple molecular forms of glutamate decarboxylase. Multiple forms that differ markedly in kinetic properties including their interactions with the cofactor have been isolated and characterized. The kinetic differences among the forms suggest that they play a significant role in the regulation of GABA synthesis.

Brain L-glutamate decarboxylase: purification and subunit structure

Glutamate decarboxylase (GDCase; L-glutamate-1-carboxy-lyase, EC 4.1.1.15) was purified from whole rat brain approximately equal to 1300-fold to apparent homogeneity with a specific activity of 2.4 units per mg of protein by a combination of column chromatographies on DEAE-cellulose, hydroxylapatite, and gel filtration, and preparative nondenaturing polyacrylamide gel electrophoresis. The purified preparation contained a single protein band that comigrated with GDCase activity in three diverse analyses: nondenaturing regular (5%) and gradient (3.6-25%) polyacrylamide gel electrophoresis and isoelectric focusing at pH 4-7. The native molecular mass was calculated to be 120 +/- 10 kDa from gradient polyacrylamide gel electrophoresis and 110 +/- 10 kDa from gel filtration. Under the treatment with NaDodSO4 and 2-mercaptoethanol, GDCase dissociated into two subunits of 40 +/- 2 and 80 +/- 4 kDa, as estimated from NaDodSO4 gel electrophoresis. However, only a 40-kDa subunit was detected when GDCase was treated with 4 M urea plus NaDodSO4 and 2-mercaptoethanol, suggesting that the 80-kDa subunit is the dimer of the 40-kDa subunit. In immunoblotting, polyclonal antibodies against GDCase reacted with both 40- and 80-kDa subunits, while monoclonal antibody reacted with only 80-kDa subunits. The isoelectric point of the native enzyme was 5.4. The Km for glutamate was 1.59 X 10(-3) M. In addition to L-glutamate, cysteine sulfinic acid was also decarboxylated at approximately equal to 10% of the rate of glutamate. The pH optimum was fairly broad, with a maximum at approximately equal to 7.3. The enzyme was strongly inhibited by carbonyl-trapping agents, sulfhydryl reagents, thiol compounds, and beta-methylene-DL-aspartate.

Some characteristics of glutamic acid decarboxylase of chick ampullary cristae

In a previous study, it was demonstrated that enzyme-mediated gamma-aminobutyric acid (GABA) synthesis occurs in the vestibule of the chick inner ear. As deeper knowledge of the properties of its synthesizing enzyme might contribute to the understanding of the role of GABA in inner ear function, some characteristics of glutamate decarboxylase (GAD) were studied in chick isolated ampullary cristae under conditions in which 14CO2 release from [1-14C]glutamate and [14C]GABA formation from [U-14C]glutamate for estimating GAD activity were equal. It was found that Km for glutamate is 5 mM and that the enzyme pH optimum is 7.3. These values fall within the range described for the corresponding enzyme in nervous tissue of other species. Pyridoxal phosphate (PLP) activates the enzyme and aminooxyacetic acid inhibits it, the same as these agents activate or inhibit GAD from several nervous tissue sources. 2-Mercaptoethanol shows some protection from inactivation of the PLP-dependent enzyme and Triton X-100 exerts some inhibition of vestibular GAD activity, as previously shown in other nervous tissue preparations. Although its cellular localization is at present uncertain, these results indicate that GAD of chick vestibular tissue possesses properties resembling those of the brain enzyme and might be controlled in a manner similar to that of GAD in brain, thus possibly participating in the regulation of inner ear function.

GABA-mediated control of glutamate decarboxylase (GAD) in cell aggregate culture of chick embryo retina

Glutamate decarboxylase (L-glutamate 1-carboxylase, (EC 4.1.1.15) activity increased 7-fold during the course of differentiation of retina cell aggregate cultures prepared from 8-day-old embryos. The addition of 5 mM GABA in the culture medium almost completely prevented the appearance of GAD activity normally observed during the differentiation of the aggregates. This effect was readily reversible after shifting the aggregates to a GABA-free medium. Differentiated cultures, characterized by high GAD specific activity were also sensitive to GABA treatment, which promoted 50% decrease in the enzyme activity after 7 h incubation of the aggregates in the presence of 0.01 mM GABA. Maximal inhibition was obtained at 0.1 mM GABA. However, GABA up to 10 mM concentration had no effect on GAD activity when added directly to homogenates of retinal tissue. The GABA-mediated inhibition of GAD was antagonized by picrotoxin. The ED50 for picrotoxin to revert GAD inhibition by 0.01 mM GABA was approximately 2 microM. The time course for the decay in GAD activity of cultures exposed to 5 mM GABA, revealed two first-order kinetic components. For an 8-day-old culture the first component had a T1/2 of approximately 60 min and the second decayed with a T1/2 of approximately 315 min. In 13-day-old cultures the first component of GAD decay was identical to the one observed in 8-day-old cultures. The second component, however, was insensitive to GABA inhibition.

Assay and properties of glutamic acid decarboxylase in homogenates of crayfish nervous tissue

The activity of glutamic acid decarboxylase (GAD) was measured in homogenates of crayfish nervous tissue. Radioactive GABA and CO2 were formed from radioactive glutamic acid in approximately equimolar amounts. Product formation was linear for 9.5 hr at 11-32 degrees C with about 1-30 micrograms homogenate protein. Enzyme activity remained high at pH 7-10 but declined steeply above pH 10.5 and below pH 7. Enzyme activity was stimulated by pyridoxal phosphate, 2-mercaptoethanol, and potassium phosphate; at higher than optimal concentrations of each the activity was reduced. Sodium phosphate altered the stimulatory effect of potassium phosphate. Crayfish GAD behaves like a typical neural GAD but is distinguishable biochemically from GAD of other species.

GABA synthesis in isolated vestibulary tissue of chick inner ear

This study was undertaken to assess the possible participation of GABA in neurotransmission in the inner ear of higher vertebrates utilizing isolated chick vestibulary cristae as a model. GABA synthesis was investigated as a suitable criterion for postulating this amino acid as a putative neurotransmitter. Enzyme-mediated GABA synthesis was demonstrated implying a possible neurotransmitter role of this amino acid in the chick vestibule.

Glutamic acid decarboxylase in sea lamprey (Petromyzon marinus): characterization, localization, and developmental changes

We have carried out assays for glutamic acid decarboxylase (GAD) in homogenates of brain and spinal cord from larval and adult sea lamprey (Petromyzon marinus). The enzyme had similar characteristics in both stages. Optimal pH was 6.8; optimal temperature was 27-30 degrees C; Km at 27 degrees C was 5 mM. GAD activity was distributed uniformly along the length of the spinal cord. Specific activities for the larval cord and brain were 26 and 63 nm CO2/mg protein/h, respectively. The specific activities for the adult cord and brain were 29 and 236 nm CO2/mg protein/h, respectively. Thus, the activity of cord homogenates did not change significantly between larval and adult stages, but that of the brain increased about fourfold.

Glutamate decarboxylase activity in chick brain and retina. Inhibition of the immature enzyme by Triton-X-100

We have studied the effect of Triton-X-100 on glutamate decarboxylase (GAD) activity in brain and retina from chick embryos of 12 and 16 days' incubation and from chicks 4--6 weeks old. GAD activity was measured in five different homogenization media. Triton-X-100 inhibited the enzyme by about 60% in both brain and retina of 12-day embryos and by about 50% in 16-day embryos, independently of the homogenization medium. In chicks only about 20% inhibition by the detergent was observed in brain whereas no effect was found in retina. These results indicate that the evaluation of the experimental conditions of enzyme assays at different ages is essential for developmental studies of GAD activity in nervous tissue.

Seizure susceptibility in the developing mouse and its relationship to glutamate decarboxylase and pyridoxal phosphate in brain

The relationship between the susceptibility to convulsions, the content of pyridoxal 5'-phosphate and the activity of pyridoxal kinase (EC 2.7.1.35) and glutamate decarboxylase (EC 4.1.1.15) in brain, was studied in the developing mouse. Seizures were induced by pyridoxal phosphate-gamma-glutamyl hydrazone (PLPGH), a drug previously reported to reduce the levels of pyridoxal 5'-phosphate and as a consequence to inhibit the activity of glutamate decarboxylase in brain of adult mice. It was found that the seizure pattern, as well as the time of appearance of convulsions, differed between 2- and 5-day old mice and 10-day old or older mice, indicating a progressive increase in seizure susceptibility during development. In brain, pyridoxal kinase activity and pyridoxal 5'-phosphate levels were decreased by the administration of PLPGH at all ages studied, whereas glutamate decarboxylase activity was inhibited less than 25% in 2- and 5-day old mice, and about 50% thereafter. Parallelly, the activation of glutamate decarboxylase by pyridoxal 5'-phosphate added in vitro to control homogenates was less in 2- and 5-day old mice than in older animals. It is concluded that the increase in the susceptibility to seizures induced by PLPGH during development is probably related to the increase observed in the sensitivity of glutamate decarboxylase in vivo to a decrease of pyridoxal 5'-phosphate levels. The correlation between pyridoxal 5'-phosphate, glutamate decarboxylase, and seizure susceptibility seems to be established at about 10 days of age.