Amino acid metabolism in the brain with convulsive disorders. Part 2: The effects of anticonvulsants on convulsions and free amino acid patterns in the brain of El mouse

Intracerebroventricular administration of histidine reduces kainic acid-induced convulsive seizures in mice

Kainic acid (KA)-induced seizures and other experimental models of epilepsy have been proven to be instrumental in identifying novel targets that could be responsible for human icto- and epileptogenesis. We have previously shown that the ablation of pharmacoresistant voltage-gated Ca²⁺ channels with Ca_v2.3 as central ion-conducting pore (R-type Ca²⁺ channel) reduces the sensitivity towards KA-induced epilepsy in mice. In vivo, Ca_v2.3 channels are thought to be under tight allosteric control by endogenous loosely bound trace metal cations (Zn²⁺ and Cu²⁺) that suppress channel gating via a high-affinity trace metal-binding site. Metal dyshomeostasis in the brain, which is a common feature of (KA-induced) seizures, could therefore alter the normal function of Ca_v2.3 channels and may shift hippocampal and neocortical signaling towards hyperexcitation. To investigate the role of loosely bound metal ions for KA-induced hyperexcitation in vivo, we examined the effects of manipulating brain trace metal homeostasis in mice. To this end, we developed a murine system for intracerebroventricular administration of trace metal ions and/or histidine (His), which can bind Zn²⁺ and Cu²⁺ and is involved in their transendothelial transport at the blood-brain barrier. Unexpectedly, our preliminary findings indicate that application of His alone but not in the presence of Zn²⁺ has substantial beneficial effects on the outcome of KA-induced epilepsy in mice. As such, our results emphasize previous findings on the complex, two-sided role of loosely bound metal ions with regard to neuronal excitation and degeneration under pathophysiological conditions.

Thioethers as markers of hydrogen sulfide production in homocystinurias

Two enzymes in the transsulfuration pathway of homocysteine -cystathionine beta-synthase (CBS) and gamma-cystathionase (CTH)-use cysteine and/or homocysteine to produce the important signaling molecule hydrogen sulfide (H2S) and simultaneously the thioethers lanthionine, cystathionine or homolanthionine. In this study we explored whether impaired flux of substrates for H2S synthesis and/or deficient enzyme activities alter production of hydrogen sulfide in patients with homocystinurias. As an indirect measure of H2S synthesis we determined by LC-MS/MS concentrations of thioethers in plasma samples from 33 patients with different types of homocystinurias, in 8 patient derived fibroblast cell lines, and as reaction products of seven purified mutant CBS enzymes. Since chaperoned recombinant mutant CBS enzymes retained capacity of H2S synthesis in vitro it can be stipulated that deficient CBS activity in vivo may impair H2S production. Indeed, in patients with classical homocystinuria we observed significantly decreased cystathionine and lanthionine concentrations in plasma (46% and 74% of median control levels, respectively) and significantly lower cystathionine in fibroblasts (8% of median control concentrations) indicating that H2S production from cysteine and homocysteine may be also impaired. In contrast, the grossly elevated plasma levels of homolanthionine in CBS deficient patients (32-times elevation compared to median of controls) clearly demonstrates a simultaneous overproduction of H2S from homocysteine by CTH. In the remethylation defects the accumulation of homocysteine and the increased flux of metabolites through the transsulfuration pathway resulted in elevation of cystathionine and homolanthionine (857% and 400% of median control values, respectively) indicating a possibility of an increased biosynthesis of H2S by both CBS and CTH. This study shows clearly disturbed thioether concentrations in homocystinurias, and modeling using these data indicates that H2S synthesis may be increased in these conditions. Further studies are needed to confirm our findings and to explore the possible implications for pathophysiology of these disorders.

The EL mouse: a natural model of autism and epilepsy

PURPOSE

Autism is a multifactorial disorder that involves impairments in social interactions and communication, as well as restricted and repetitive behaviors. About 30% of individuals with autism develop epilepsy by adulthood. The EL mouse has long been studied as a natural model of multifactorial idiopathic generalized epilepsy with complex partial seizures. Because epilepsy is a comorbid trait of autism, we evaluated the EL mouse for behaviors associated with autism.

METHODS

We compared the behavior of EL mice to age-matched control DDY mice, a genetically related nonepileptic strain. The mice were compared in the open field and in the light-dark compartment tests to measure activity, exploratory behavior, and restricted and repetitive behaviors. The social transmission of food preference test was employed to evaluate social communication. Home-cage behavior was also evaluated in EL and DDY mice as a measure of repetitive activity.

KEY FINDINGS

We found that EL mice displayed several behavioral abnormalities characteristic of autism. Impairments in social interaction and restricted patterns of interest were evident in EL mice. Activity, exploratory behavior, and restricted behavior were significantly greater in EL mice than in DDY mice. EL mice exhibited impairment in the social transmission of food preference assay. In addition, a stereotypic myoclonic jumping behavior was observed in EL mice, but was not seen in DDY mice. It is of interest to note that seizure activity within 24 h of testing exacerbated the autistic behavioral abnormalities found in EL mice.

SIGNIFICANCE

These findings suggest that the EL mouse expresses behavioral abnormalities similar to those seen in persons with autism. We propose that the EL mouse can be utilized as a natural model of autism and epilepsy.

Antiepileptic action induced by a combination of vigabatrin and tiagabine

Vigabatrin, an inhibitor of GABA breakdown by GABA transaminase and of GABA transporter isoform 1 (GAT1), and tiagabine, a highly specific inhibitor of GAT1, have successfully been applied in the treatment of epilepsy. We investigated the effects of individual and combined application of these drugs on GAT1 expressed in Xenopus oocytes, and examined the effects on epileptiform discharges in the CA3 area of brain slices of genetically epileptic El and control ddY mice, and on the occurrence of seizures in El mice. Simultaneous application of vigabatrin and tiagabine inhibited epileptiform discharges induced by high-K+ solution in the brain slices in an antagonistic fashion. The degree of inhibition by tiagabine after pre-treatment with vigabatrin was additive in ddY mice and synergistic in El mice. In Mg2+-free solution, co-treatment by the two drugs produced additive inhibition in slices from both mouse strains, but pre-treatment with vigabatrin produced synergistic inhibition in slices only from ddY mice. In the slices from El mice, a combination of drugs resulted in additive effects in both co- and pre-treatment by the drugs. Although these drugs are also effective in vivo at suppressing seizure occurrence in El mice, the combined application does not show synergistic effects, but rather is antagonistic under the experimental conditions in this particular variant of epilepsy. The synergistic inhibition of epileptiform discharges in brain slices may, in part, have originated from the complex interaction with GAT1. In experiments on the GAT1 expressed in oocytes it could be demonstrated that synergistic inhibition occurs only at low concentration (0.1 nM) of vigabatrin. This illustrates that the oocytes may form a powerful test system for drug screening and investigation of complex drug interactions. These results present a novel interpretation of synergistic inhibition of certain epileptic discharges using vigabatrin and another drug, and that for successful synergistic treatment of epilepsies carefully designed timed dosage regimens are essential.

Sexual dysfunction and sudden death in epileptic male EL mice: inheritance and prevention with the ketogenic diet

PURPOSE

The EL mouse is an animal model for multifactorial idiopathic epilepsy. Although EL mice have been studied extensively for >45 years, the etiology of male sudden death and its relation to seizures have not been defined. Here we investigated the cause of EL male sudden death and its relation to epilepsy.

METHODS

For histopathologic analysis, the terminally ill EL mice (n = 15) were killed, and the tissues were fixed. Blood chemical composition was compared between the terminally ill EL (n = 9) and the healthy age-matched EL (n = 17) and DDY (n = 11) males. To determine the effect of the ketogenic diet (KD) on sudden male death, young male EL mice (P30) were randomly separated into two groups that were fed ad libitum with either Agway lab chow (control n = 38) or with the KD (treated, n = 39) for 5 months. The genetic predisposition to sudden death was analyzed in the backcross generation (n = 106) of a cross between EL and the nonepileptic ABP strains.

RESULTS

Sudden death coincided with the onset of seizures (70-80 days) and affected 94% of male EL mice by age 300 days. Urethral plugs were observed histologically in 13 of 15 longitudinally sectioned penises. Concentrations of blood urea nitrogen, creatinine, phosphorus, and calcium in the terminally ill mice were significantly elevated when compared with those of healthy animals. None of the mice treated with the KD experienced sudden death, whereas 15 (39%) of the untreated control mice died by age 5 months. The sudden death in male EL mice was inherited as an autosomal recessive sex-limited lethal trait.

CONCLUSIONS

The cause of sudden death in male EL mice arises from abnormal ejaculation, which produces a urethral plug with consequent urinary retention and acute severe uremia. The coincident onset of seizures and sudden death in EL males suggests that a sexual dysfunction is associated with epilepsy in this model.

Infantile spasm: the effect of corticotropin (ACTH) on the free amino acid profile in cerebrospinal fluid

Increased excitatory amino acid neurotransmission has been implicated in the pathogenesis of infantile spasm. In this study we studied the profile of free amino acids in cerebrospinal fluid (CSF) from 16 patients with infantile spasm before corticotropin (ACTH) treatment. After 10 weeks ACTH therapy the profile of amino acids in CSF was studied once more in eight of the patients. Eleven patients were in the symptomatic group, and five in the cryptogenic group. Increased aspartate levels were measured in CSF following ACTH therapy (P<0.05). It was concluded that aspartate might have an important role in hypothalamic-hypophyseal axis in patients with infantile spasm.

Relationships between convulsive seizures and serum and brain concentrations of phenobarbital and zonisamide in mutant inbred strain EL mouse

We evaluated the anticonvulsive effects of phenobarbital (PB) and zonisamide (ZNS) in the EL mouse, a strain that is highly susceptible to seizures. The concentration of each agent was analyzed in the serum and brain. PB suppressed the seizures dose-dependently, whereas even the higher dose of ZNS was ineffective in achieving a complete suppression. Serum and brain concentrations of these two drugs increased in proportion to the higher dose injected intraperitoneally. Brain concentration of PB was lower than the serum concentration, while the brain concentration of ZNS exceeded that in serum. Although serum concentration of ZNS was essential unchanged after the combined administration of PB and ZNS, the concentration of ZNS in brain tended to rise in proportion to the highly dose of PB. Combined administration was more effective than other treatment alone. Results indicated that brain concentration of ZNS especially after concomitant PB administration, were higher than that would be expected from the concentration of ZNS in serum.

Peripheral-type benzodiazepine receptors in association with epileptic seizures in EL mice

Peripheral-type benzodiazepine receptors (PBR) in the brain were studied in association with epileptic seizures using EL mice, an animal model of epilepsy, and DDY mice as controls. Ro 5-4864 (i.p.), a specific agonist for PBR, elicited tonic-clonic convulsions in EL mice 2.6-times more potently than in DDY mice with CD50s of 11.9 and 31.2 mg/kg for EL and DDY mice, respectively. In contrast, pentylenetetrazole (i.p.) exerted convulsant actions on EL and DDY mice in a less differential way with CD50s of 29.2 and 48.1 mg/kg for EL and DDY mice, respectively. PK 11195 (i.v.), a specific antagonist for PBR, raised seizure thresholds of EL mice at a dose of 2 mg/kg. Binding assay revealed a 50% higher density of [3H]Ro 5-4864 binding sites in the mitochondrial fraction isolated from the cerebrum of EL mice in comparison with DDY mice. Similarly, a 40% higher density of [3H]flunitrazepam binding was observed in the mitochondrial fraction of EL mice. The results support the hypothesis that PBR, particularly those associated with mitochondria, are involved in the pathogenesis of epileptic seizures in EL mice.

New seizure frequency QTL and the complex genetics of epilepsy in EL mice

EL/Suz (EL) mice experience recurrent seizures that are similar to common partial complex epilepsy in humans. In the mice, seizures occur naturally at 90-100 days of age, but can be induced in younger mice and analyzed as a semi-quantitative trait after gentle rhythmic stimulation. A previous genetic mapping study of EL backcrosses to the strains ABP/LeJ or DBA/2J showed two quantitative trait loci (QTL) with large effects on seizure frequency (El1, Chr 9; El2, Chr 2) and implied the existence of other QTL with lesser effects. To further the understanding of EL-derived seizure alleles, we examined intercross progeny of EL and the strains ABP/LeJ and DDY/Jcl, and also a backcross of (EL x DDY)F1 hybrids to DDY. A new large-effect seizure frequency QTL was found (El5, Chr 14), a more minor QTL confirmed (El3, Chr 10), and two additional QTL proposed (El4, Chr 9; El6, Chr 11). The serotonin receptor gene, Htr2a, maps near and is a candidate for El5, and linkages of other serotonin receptor genes to seizure frequency QTL are noted. In addition, a strong gender effect was revealed, and epistasis was found between Chr 9 and Chr 14 markers. Despite this progress, however, our results revealed a more complex determinism of epilepsy in EL mice than previously described. In particular, no single El locus or pair was essential for frequent seizures, as QTL with large effects, such as El5, El2, and El1, were highly dependent on genetic context. Our studies highlight the importance of gene interaction in some complex mammalian traits defined by natural variation.

Biochemical correlates of epilepsy in the E1 mouse: analysis of glial fibrillary acidic protein and gangliosides

The E1 (epileptic) mouse is considered a model for complex partial seizures in humans. Seizures in E1 mice begin around 7-8 weeks of age and persist throughout life. To determine if astrocytic gliosis was present in adult seizing E1 mice, the distribution of glial fibrillary acidic protein (GFAP) was studied in the hippocampus using an antibody to GFAP. The mean number of GFAP-positive cells per square millimeter of hippocampus was approximately 15- to 40-fold higher in adult E1 mice than in nonseizing control C57BL/6J (B6) mice or in young nonseizing E1 mice. Relative GFAP concentration (expressed per milligram of total tissue protein) in hippocampus and cerebellum was estimated by densitometric scanning of peroxidase-stained western blots. GFAP concentration was 2.7-fold greater in hippocampus of adult seizing E1 mice than in the control B6 mice. No differences in GFAP content were detected between the strains in the cerebellum. Because gangliosides can serve as cell surface markers for changes in neuronal cytoarchitecture, they were analyzed to determine if the gliotic response in E1 mice was associated with changes in neural composition. Although the total ganglioside concentration of hippocampus, cerebral cortex, and cerebellum was similar in adult E1 and control B6 mice, a synaptic membrane enriched ganglioside, GD1a, was elevated in the adult E1 cerebral cortex and hippocampus. The findings indicate that E1 mice express a type of gliosis that is not accompanied by obvious neuronal loss.

Genes for epilepsy mapped in the mouse

The neurological mutant mouse strain E1 is a model for complex partial seizures in humans. The inheritance of epileptic seizures with seven conventional chromosomal markers and over 60 endogenous proviral markers was studied by means of back-crosses of E1 with two seizure-resistant strains, DBA/2J and ABP/LeJ. The major gene responsible for this epileptic phenotype (El-1) was localized to a region distal with respect to the centromere on chromosome 9. At least one other gene, El-2, linked to proviral markers on chromosome 2, also influences the seizure phenotype. In addition, a potential modifier of seizures was detected in the DBA/2J background. The location of El-1 on distal chromosome 9 may allow identification of an epilepsy candidate gene in humans on the basis of conserved synteny with human chromosome 3.

Enhanced aspartate release from hippocampal slices of epileptic (El) mice

The release of putative neurotransmitters [aspartate, glutamate, and gamma-aminobutyric acid (GABA)] was studied in hippocampal slices from adult normal C57BL/6J (B6) and El (epileptic) mice. The El mice, a genetic model of temporal lobe epilepsy, had an average of 86 seizures. Sets of B6 and El hippocampal slices (400 microns thick) were incubated in a series of normal and high potassium (60 mM) buffers in the presence or absence of calcium. The calcium-dependent and calcium-independent potassium-induced release of amino acids was compared in each mouse strain. Release of endogenous amino acids was measured using liquid chromatography with electrochemical detection and was expressed as picomoles of amino acid released per milliliter of incubation buffer per minute of incubation per slice +/- SEM. No significant differences were found between the El and B6 mice for the calcium-dependent potassium-evoked release of glutamate (18.20 +/- 2.62 and 15.41 +/- 3.56), or GABA (17.28 +/- 2.90 and 12.73 +/- 1.37), respectively. Aspartate release, however, was significantly higher in the El mice (6.62 +/- 0.69) than in the B6 mice (3.31 +/- 0.72). These findings suggest that enhanced aspartate release may be related to seizure expression in El mice.

El mouse as a model of focal epilepsy: a review

The El mouse is a model of hereditary sensory precipitated temporal lobe epilepsy. All adult El mice given rhythmic vestibular stimulation (e.g. tossing, rocking) during development will experience tonic-clonic convulsions when given similar stimulation as adults. The seizures have prodromal, convulsive, and postictal stages. EEG and 2-deoxyglucose studies have localized the seizures to the temporal lobe, with onset in the hippocampus. El mice have a decreased threshold for convulsion by electrical or pharmacologic stimulation. A variety of anticonvulsant medications eliminate El mouse seizures, including phenytoin (PHT), phenobarbital (PB), valproate (VPA), and ethosuximide (ESM). Anatomic studies have shown subtle differences in the thalamus and hippocampus of El mice. Immunohistochemistry of the El mouse hippocampus has revealed changes in peptidergic and gabaergic cell populations. Numerous biochemical differences have been found between El and nonconvulsive mice, including increased acetylcholine (ACh), dopamine (DA), GABA, serotonin (5-HT), and decreased norepinephrine (NE).

VIP-, SS-, and GABA-like immunoreactivity in the mid-hippocampal region of El (epileptic) and C57BL/6 mice

The El (epileptic) mouse is a model of hereditary sensory precipitated temporal lobe epilepsy. We compared vasoactive intestinal polypeptide-like immunoreactivity (VIP-LI), somatostatin-like immunoreactivity (SS-LI), and gamma-aminobutyric acid-like immunoreactivity (GABA-LI) in the mid-hippocampal region of El and C57BL/6 mice. Specific interneuron populations with VIP-LI and GABA-LI were elevated in the El mice, whereas SS-LI populations were unchanged. These neurochemical alterations may be contributing to the epileptic predisposition of El mice.

Depressed convulsions by diazepam and its effects on brain monoamines and amino acids in E1 mice

The effect of diazepam on inbred mutant E1 mice, which develop convulsive seizures after repeated sessions of being tossed up, was examined. Acute administration of diazepam (32 mg/kg, i.p.) completely inhibited the convulsions. At that time, the dopamine level was increased in the cortex and hippocampus, and the norepinephrine level in the cerebellum was decreased. 5-Hydroxytryptamine levels were not changed. As for amino acids, the glutamine level increased and the levels of GABA, glutamic acid, aspartic acid, alanine and other amino acids were not changed.