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

Inflammation as a target for the treatment of fever-associated epilepsy in zebrafish larvae

Inflammation is considered to be involved in epileptogenesis. However, the relationship between fever and inflammation as well as the mechanisms of fever in the occurrence and development of childhood epilepsy need further investigation. Here, we described an in vivo model of hyperthermia-induced seizures in zebrafish larvae with pentylenetetrazole (PTZ) exposure. Hyperthermia increased the susceptibility to seizure and the production of pro-inflammatory factors in PTZ-induced zebrafish larvae. As mutations in GABRG2 have been associated with fever-associated epilepsy, we used a Tg(hGABRG2^F343L) zebrafish model expressing mutant human GABRG2(F343L) to further investigate the involvement of inflammation in fever-induced seizures. Our data indicated that hyperthermia also increased the locomotor activity in Tg(hGABRG2^F343L) zebrafish larvae. Although the production of pro-inflammatory factors was upregulated by GABRG2 mutation, hyperthermia did not change the production of pro-inflammatory factors significantly. Lipopolysaccharide (LPS) stimulation was sufficient to increase the locomotor activity in zebrafish larvae, suggesting that inflammation contributed to fever-associated epilepsy. The expression of GABRG2 was increased with PTZ induction, especially at a higher temperature. Moreover, inhibition of inflammation by dexamethasone (DEX) reduced the excitability of zebrafish larvae, especially at a higher temperature. Finally, in vitro experiments proved that LPS stimulation increased the production of IL-1β and IL-6 in GABRG2(F343L) transfected cells. Collectively, our study demonstrated that neuroinflammation was induced in febrile seizures, and the increased expression of IL-1β and IL-6 might be responsible for epileptogenesis. The vicious cycle between fever and inflammation might induce seizure onset, and anti-inflammatory strategies might be a potential treatment for fever-associated epilepsy.

Chronic lead exposure disrupts neurometabolic activity in mouse brain: An ex vivo1H-[13C]-NMR study

Lead poisoning has been identified as a problem in adults as well as in children. Chronic exposure to lead has been implicated in neurological disorders such as amyotrophic lateral sclerosis, Parkinson's disease, and Alzheimer's disease. In the present study, we evaluated the impact of chronic lead exposure on cerebral glutamatergic and GABAergic metabolic activity in mice. C57BL6 mice were provided lead acetate in drinking water for two months. The regional cerebral metabolic activity was measured using ¹H-[¹³C]-NMR spectroscopy in conjunction with infusion of [1,6-¹³C₂]glucose. The blood Pb²⁺ increased significantly in lead acetate treated mice. Concomitantly, there was a significant reduction in the forelimb strength. The level of myo-inositol was elevated in the cerebral cortex of mice chronically exposed to lead. The glutamatergic neurometabolic activity was found to be reduced following chronic lead exposure in the cerebral cortex, hippocampus, and striatum. In contrast, the GABAergic fluxes were impaired in the hippocampus and thalamus only. The metabolic fluxes in the cerebellum were unperturbed to Pb²⁺ toxicity. In conclusion, we report that chronic lead exposure in mice leads to an impairment in forelimb strength, and a perturbation in neurometabolism in brain regions involving cognition and movement.

Phosphorylation of pyridoxal 5'-phosphate enzymes: an intriguing and neglected topic

Pyridoxal 5'-phosphate (PLP)-dependent enzymes catalyze a wide range of reactions of amino acids and amines, with the exception of glycogen phosphorylase which exhibits peculiar both substrate preference and chemical mechanism. They represent about 4% of the gene products in eukaryotic cells. Although structure-function investigations regarding these enzymes are copious, their regulation by post-translational modifications is largely unknown. Protein phosphorylation is the most common post-translational modification fundamental in mediating diverse cellular functions. This review aims at summarizing the current knowledge on regulation of PLP enzymes by phosphorylation. Starting from the paradigmatic PLP-dependent glycogen phosphorylase, the first phosphoprotein discovered, we collect data in literature regarding functional phosphorylation events of eleven PLP enzymes belonging to different fold types and discuss the impact of the modification in affecting their activity and localization as well as the implications on the pathogenesis of diseases in which many of these enzymes are involved. The pivotal question is to correlate the structural consequences of phosphorylation among PLP enzymes of different folds with the functional modifications exerted in terms of activity or conformational changes or others. Although the literature shows that the phosphorylation of PLP enzymes plays important roles in mediating diverse cellular functions, our recapitulation of clue findings in the field makes clear that there is still much to be learnt. Besides mass spectrometry-based proteomic analyses, further biochemical and structural studies on purified native proteins are imperative to fully understand and predict how phosphorylation regulates PLP enzymes and to find the relationship between addition of a phosphate moiety and physiological response.

The influence of temperature on adult zebrafish sensitivity to pentylenetetrazole

Pentylenetetrazole (PTZ) is one of the most valuable drugs used to induce seizure-like state in zebrafish especially considering the pharmacological screening for anticonvulsants and the study of basic mechanisms of epilepsy. Here, the effect of gender, weight and changes in temperature on latency to adult zebrafish reach classical seizure states induced by PTZ (10mM) was evaluated. Gender and weight (200-250mg versus 400-500mg) did not affect the profile of response to PTZ. When water temperature was changed from 22 to 30°C the lower temperature increased the latency time to reach seizure states and the higher temperature significantly decreased it, in comparison to the control group maintained at 26°C. The blockage of kainate receptors by DNQX (10μM) were unable to prevent the increased susceptibility of adult zebrafish exposed to hyperthermia and PTZ-induced seizures. The NMDA block by MK-801 (2.5μM) prevented the additive effect of hyperthermia on PTZ effects in adult zebrafish. This report emphasize that PTZ model in adult zebrafish exhibits no confounder factors from gender and weight, but water temperature is able to directly affect the response to PTZ, especially through a mechanism related to NMDA receptors.

Reduced glucose utilization underlies seizure protection with dietary therapy in epileptic EL mice

Dietary therapy has been used to treat many individuals with epilepsy whose seizures are refractory to antiepileptic drugs. The mechanisms for how dietary therapy confers seizure protection are currently not well understood. We evaluated the acute effects of glucose and β-hydroxybutyrate (the major circulating ketone body) in conferring seizure protection to the EL mouse, a model of multifactorial idiopathic generalized epilepsy. EL mice were fed either an unrestricted standard diet or a calorie-restricted standard diet to achieve a body weight reduction of 20-23%. D-Glucose, 2-deoxy-D-glucose, and β-hydroxybutyrate were supplemented in the drinking water of calorie-restricted mice for 2.5 h prior to seizure testing to simulate the effect of increased glucose availability, decreased glucose utilization, and increased ketone availability, respectively. Seizure susceptibility, body weight, plasma glucose, and β-hydroxybutyrate were measured over a nine-week treatment period. Additionally, excitatory and inhibitory amino acids were measured in the brains of mice using (1)H NMR. Glutamate decarboxylase activity was also measured to evaluate the connection between dietary therapy and brain metabolism. We found that lowering of glucose utilization is necessary to confer seizure protection with long-term (>4 weeks) calorie restriction, whereas increased ketone availability did not affect seizure susceptibility. In the absence of long-term calorie restriction, however, reduced glucose utilization and increased ketone availability did not affect seizure susceptibility. Brain excitatory and inhibitory amino acid content did not change with treatment, and glutamate decarboxylase activity was not associated with seizure susceptibility. We demonstrated that reduced glucose utilization is necessary to confer seizure protection under long-term calorie restriction in EL mice, while acute ketone supplementation did not confer seizure protection. Further studies are needed to uncover the mechanisms by which glucose utilization influences seizure susceptibility.

Convulsions induced by methylmalonic acid are associated with glutamic acid decarboxylase inhibition in rats: A role for GABA in the seizures presented by methylmalonic acidemic patients?

Methylmalonic acid (MMA) is an endogenous convulsing compound that accumulates in methylmalonic acidemia, an inborn error of the metabolism characterized by severe neurological dysfunction, including seizures. The mechanisms by which MMA causes seizures involves the activation of the N-methyl-D-aspartate (NMDA) receptors, but whether GABAergic mechanisms are involved in the convulsions induced by MMA is not known. Therefore, in the current study we investigated the involvement of GABAergic mechanisms in the convulsions induced by MMA. Adult rats were injected (i.c.v.) with muscimol (46 pmol/1 microl), baclofen (0.03, 0.1 and 0.3 micromol/1 microl), MK-801 (6 nmol/1 microl), pyridoxine (2 micromol/4 microl) or physiological saline (0.15 micromol/1 microl). After 30 min, MMA (0.3, 0.1 and 3 micromol/1 microl) or NaCl (6 micromol/1 microl, i.c.v.) was injected. The animals were immediately transferred to an open field and observed for the appearance of convulsions. After behavioral evaluation, glutamic acid decarboxylase (GAD) activity was determined in cerebral cortex homogenates by measuring the 14CO2 released from l-[14C]-glutamic acid. Convulsions were confirmed by electroencephalographic recording in a subset of animals. MMA caused the appearance of clonic convulsions in a dose-dependent manner and decreased GAD activity in the cerebral cortex ex vivo. GAD activity negatively correlated with duration of MMA-induced convulsions (r=-0.873, P<0.01), in an individual basis. Muscimol, baclofen, MK-801 and pyridoxine prevented MMA-induced convulsions, but only MK-801 and pyridoxine prevented MMA-induced GAD inhibition. These data suggest GABAergic mechanisms are involved in the convulsive action of MMA, and that GAD inhibition by MMA depends on the activation of NMDA receptors. While in this study we present novel data about the role of the GABAergic system in MMA-induced convulsions, the central role of NMDA receptors in the neurochemical actions of MMA is further reinforced since they seem to trigger GABAergic failure.

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.

Association analysis of gamma 2 subunit of gamma- aminobutyric acid type A receptor polymorphisms with febrile seizures

An alternation of gamma-aminobutyric acid (GABA)-ergic neurotransmission has been implicated as an etiologic factor in epileptogenesis. Missense mutations in the GABRG2 gene, which encodes the gamma2 subunit of central nervous GABAA receptors, have recently been described in one family with childhood absence epilepsy and febrile seizures (FSs). FSs represent the majority of childhood seizures and have a genetic predisposition. It is not known, however, whether polymorphisms in those genes involved in familial epilepsies also contribute to the pathogenesis of FSs. By performing an association study, we used single-nucleotide polymorphisms to investigate the distribution of genotypes of GABRG2 in patients with FSs. A total of 104 children with FSs and 83 normal control subjects were included in the study. PCR was used to identify the C/T and A/G polymorphisms of the GABRG2 gene on chromosome 5q33. Genotypes and allelic frequencies for the GABRG2 gene polymorphisms in both groups were compared. The GABRG2 (nucleotide position 3145 in intron G-->A) gene in both groups was not significantly different. In contrast, the number of individuals with the GABRG2 (SNP211037)-C/C genotype in patients with FSs was significantly greater compared with that in healthy control subjects (p = 0.017), and the GABRG2 (SNP211037)-C allele frequency in patients with FSs was significantly higher than that in healthy control subjects (p = 0.009). The odds ratio for developing FSs in individuals with the GABRG2 (SNP211037)-C/C genotype was 2.56 compared with individuals with the GABRG2 (SNP211037)-T/T genotype. These data suggest that the GABRG2 gene might be one of the susceptibility factors for FSs.

Intrinsic thermal resistance of the canine brain

Hyperthermia above a critical threshold results in multisystemic changes that include neurological manifestations of heat stroke. It is unknown if the latter represents an intrinsic thermal sensitivity of the CNS or whether injury is secondary to physiological responses of non-CNS origin. To address this issue, the present work examined functional, structural, and biochemical changes in the CNS of dogs subjected to a thermal dosage immediately below that which induces disseminated intravascular coagulation with secondary multiple organ injury. The experimental approach is previously reported, inducing a 42.5 degrees C, 90 min, whole body hyperthermia while preventing other physiological responses to treatment, including respiratory alkalosis and significant reductions in mean arterial pressure. Functional analyses included neurologic examinations and brainstem auditory evoked potential recordings in the post-treatment interval in both hyperthermic and euthermic control populations. Biochemical and structural analyses examined the expression of 70-kDa heat shock proteins, cytokines, markers of astroglial and microglial injury/activation, evidence of vascular endothelial damage, and evidence of neuronal and axonal injury in brain between 0.5 h and 8 days from the end of the treatment. The only significant change associated with treatment was induction of the major inducible 70-kDa heat shock protein, this being most prominent in the cerebellum with maximal expression at 6 h and a return to baseline by 8 days.Collectively, from these results we suggest that the canine brain is intrinsically resistant to sublethal hyperthermia such that when CNS lesions occur, they do so in the presence of other physiological derangements.

PTZ-induced seizures in rats: effects of age and strain

The susceptibility to pentylenetetrazol (PTZ)-induced seizures during postnatal ontogeny [postnatal day (PN) 10-220] was investigated in two rat strains. The WAG/Rij strain, genetically prone for developing generalized absence epilepsy, and Wistar rats were tested and compared at PN 10, 26, 30, 70, 90, 125, and 220 on the PTZ-convulsive threshold. A subconvulsive dose of 25-mg/kg PTZ was administered every 15 min, and the occurrence of clonic and tonic-clonic seizures was scored. The 10-day-old pups were quite sensitive to PTZ and showed mainly clonic seizures. The highest threshold and latency of PTZ-induced clonic and tonic-clonic convulsions were observed at PN 26 in both strains. From that age onwards, the seizure threshold significantly decreased and reached a minimum at PN 220. Between strain comparisons showed that WAG/Rij rats have a lower tonic-clonic seizure threshold than Wistar rats. The data indicate that changes in susceptibility first quickly decreases until PN 26-30 and then tend to monotonically increase with age, and that genetically prone nonconvulsive WAG/Rij rats are more vulnerable to convulsive seizures induced by PTZ than Wistar rats.

Heat-stress-induced hyperthermia alters CSF osmolality and composition in conscious rabbits

Amino acids have received increased attention with regard to their thermoregulatory effects and possible role as neurotransmitters within the thermoregulatory system. The purpose of the present work was to evaluate in conscious rabbits the changes in cerebrospinal fluid (CSF) concentration of taurine, GABA, aspartate, and glutamate during exposure to high ambient temperature (50 min, 40 degrees C) to investigate their involvement in heat stress (HS). CSF and plasma osmolality and CSF concentrations of some cations and proteins were also determined. HS animals underwent transient hyperthermia and thereafter fully recovered. This was accompanied by a significant rise in CSF and plasma osmolality, CSF protein, calcium, taurine, and GABA. Artificial CSF osmolality measurements after addition of CaCl(2) or taurine demonstrated that the increased CSF osmolality after HS is accounted for, only in part, by the increased concentrations of either calcium and taurine. It is suggested that, during HS, taurine and GABA are released in the extracellular space of brain tissues in higher amounts, possibly to counteract the resulting hyperthermia.

The effect of GABAergic system activity on hyperthermia-induced seizures in rats

Developing rats were given GABA antagonists and agonist before electrical seizure discharges were induced by heating the brain of a rat with infra-red rays. The thresholds for the GABA antagonist groups were significantly lower than that for the control, and the threshold for the GABA agonist group was significantly higher than that for the control. These results support the hypothesis that reduced GABAergic system activity underlies febrile seizures.

Hyperthermia depletes adenosine triphosphate and decreases glutamate uptake in rat hippocampal slices

The central nervous system is especially vulnerable to hyperthermia-induced dysfunction, yet the mechanism for this susceptibility is poorly understood. High levels of adenosine triphosphate are necessary to maintain normal re-uptake of glutamate and aspartate, the major excitatory amino acids, by excitatory amino acid co-transporters. We hypothesized that excitotoxic neurotransmitters accumulate extracellularly when hyperthermia depletes adenosine triphosphate, leading to decreased uptake or release of excitatory amino acids by these co-transporters. Incubation of hippocampal slices at 42 degrees C, a temperature that results in coma in vivo, reduced adenosine triphosphate to 70% of control values and decreased uptake of the transportable excitatory amino acid analogue, D,L threo-beta-hydroxyaspartate, to 50% of control values. The degree of adenosine triphosphate depletion induced by hyperthermia was highly correlated with decreases in excitatory amino acid uptake. Severe adenosine triphosphate depletion (< or = 20% of control) induced by hyperthermia in combination with metabolic insults was highly correlated with the release of endogenous glutamate and aspartate. Preloading slices with excitatory amino acid analogues potentiated hyperthermia-induced alterations of excitatory amino acid transport, strongly suggesting that the hyperthermia-induced changes were largely due to altered excitatory amino acid co-transporter activity. Immunocytochemical studies suggested glutamate-like immunoreactivity was lost from axonal terminals during hyperthermia in a similar manner to losses induced by metabolic toxins. Hyperthermia due to infectious diseases or heat stroke my induce disorientation and coma. These dysfunctions may be due, in part, to altered excitatory amino acid transport induced by adenosine triphosphate depletion.

Brain L-glutamate decarboxylase. Inhibition by phosphorylation and activation by dephosphorylation

Previously we showed that the activity of the gamma-aminobutyric acid-synthesizing enzyme L-glutamate decarboxylase (GAD) in crude brain extract is inhibited by ATP and protein phosphatase inhibitors. We suggested that GAD activity is regulated by protein phosphorylation. In this paper we further present evidence to support our hypothesis that protein kinase A and calcineurin may be involved in regulation of GAD activity through phosphorylation and dephosphorylation fo GAD, respectively. In addition, the effect of neuronal stimulation on GAD activity in cultured neurons is also included. A model to link neuronal excitation and activation of GAD by Ca(2+)-dependent phosphatase is proposed.

Histamine in cerebrospinal fluid of children with febrile convulsions

Febrile convulsions (FC) are frequent acute neurologic disturbances of childhood. The cellular and neurochemical mechanisms causing FC are unclear. Among other mechanisms, the CNS histamine (HA) has been suggested to participate in seizure control and thermoregulation. We evaluated the possible role of HA in regulation of FC by measuring HA and tele-methylhistamine (t-MH) concentrations in the cerebrospinal fluid (CSF) of children with FC. The study group consisted of 35 children treated for acute FC in the hospital. The control groups consisted of (a) feverish children without seizures (n = 23), (b) convulsive children without fever (n = 7), and (c) children with neither fever nor convulsions (n = 21). HA was assayed by high-performance liquid chromatography (HPLC) with fluorescence detection, and t-MH was measured by gas chromatography-mass spectrometry. CSF HA concentration in the group of febrile children without seizures was significantly higher (0.69 +/- 0.16 pmol/ml, mean +/- SE) than in children with FC (0.36 +/- 0.07 pmol/ml, p < 0.05, analysis of variance, ANOVA). HA concentration was 0.37 +/- 0.18 pmol/ml in the group of nonfebrile convulsive children and 0.36 +/- 0.08 pmol/ml in the nonfebrile nonconvulsive group. No statistical differences in t-MH were detected between groups. The increased susceptibility to seizures during fever may be connected to the lack of increase in CSF HA in the FC group. The data support the hypothesis that the central histaminergic neuron system may be involved in inhibition of seizures associated with febrile illnesses in childhood.

Role of Protein Phosphorylation in Regulation of Brain L-Glutamate Decarboxylase Activity

In the brain, the gamma-aminobutyric acid (GABA) level is primarily controlled by the activity of its synthesizing enzyme, L-glutamate decarboxylase (GAD). At present, mechanisms responsible for regulation of GAD activity remain largely unknown. Here we report that GAD activity is inhibited by conditions favoring protein phosphorylation, and this inhibition can be reversed by phosphatase treatment. Furthermore, this inhibition appears to result from the suppression of a Ca(2+)-dependent phosphatase. Phosphorylation of GAD is demonstrated by direct incorporation of (32)P into the GAD protein. These results suggest that GAD activity in the brain is inhibited by phosphorylation and activated by dephosphorylation. A model for regulation of GABA synthesis related to neuronal excitation is discussed. Copyright 1994 S. Karger AG, Basel

Convulsions and inhibition of glutamate decarboxylase by pyridoxal phosphate-gamma-glutamyl hydrazone in the developing rat

We have previously shown that in the adult rat the inhibition of brain glutamate decarboxylase (GAD) activity by pyridoxal phosphate-gamma-glutamyl hydrazone (PLPGH) administration does not result in convulsions, whereas in the adult mouse intense convulsions invariably occur. In the present study we report that, surprisingly, immature rats from 2 to 20 days of age treated with PLPGH (80 mg/kg) showed generalized tonic-clonic convulsions, whereas no convulsions at all were present in 30 days-old or older rats. GAD activity, measured by enzymic determination of GABA formed in forebrain homogenates, was inhibited by about 60% at the time of convulsions in 15 days-old and younger rats, whereas the inhibition was between 40 and 50% in older animals. The addition of the coenzyme pyridoxal 5'-phosphate to the incubation medium completely reversed this inhibition. In all treated animals GABA levels were lower compared to controls. The results indicate that the susceptibility of GAD in vivo to a diminished cofactor concentration decreases with age. It seems possible that changes in the expression of enzyme forms are reflected in developmental variations in the susceptibility to seizures induced by vitamin B6 depletion, but alterations of other B6-dependent biochemical pathways cannot be discarded.

A membrane form of brain L-glutamate decarboxylase: identification, isolation, and its relation to insulin-dependent mellitus

A membrane form of L-glutamate decarboxylase (GAD) was identified and purified to apparent homogeneity from hog brain. The purified GAD was established as an integral membrane protein by phase-partitioning assay, charge-shift electrophoresis, and chromatography on a hydrophobic interaction column. This membrane GAD has a native molecular mass of 96 +/- 5 kDa and is a homodimer of 48 +/- 3-kDa subunits. Immunoprecipitation and immunoblotting tests revealed the presence of antibodies against this membrane GAD in sera from patients with insulin-dependent diabetes mellitus. Since this form of GAD appears to be an integral membrane protein and is presumed to have extracellular domains exposed, it seems reasonable to suggest that membrane GAD is more likely than soluble GAD to be involved in the pathogenesis of insulin-dependent diabetes and related autoimmune disorders such as stiff-man syndrome.