Association of NMDA receptor sites and seizures of El mice

Variations of dopamine, serotonin, and amino acid concentrations in Noda epileptic rat (NER) retina

Noda epileptic rats (NER) exhibit frequent spontaneous tonic-clonic convulsions which represent a valuable model of human epilepsy. If implication of brain neurotransmitters was largely reported, little is known about retina. However, it has been reported that human epilepsy syndrome varies not only with the location of seizure foci but also according to rhythmic patterns, for which retina has a major role in the transmission of external light-dark cycle information. The purpose of this work was to evaluate dopamine (DA), DA metabolites, serotonin (5-HT), and amino acid [glutamate, aspartate, glycine, gamma aminobutyric acid (GABA), and taurine] level variations in retina from NER, at two different nycthemeral periods (11 a.m. and 11 p.m.) and at different ages (2, 6, and 12 months). In NER, retinal dopaminergic function was decreased as soon as 2 months, whereas GABA levels were increased, even if no differences among the different ages could be distinguished. These variations were associated to a slight increase in 5-HT. Other amino acids tested were not affected by epilepsy, whereas taurine decreased with aging in NER as well as in control rats. Retinal 5-HT occurs principally as a precursor of melatonin (MEL). A triangular interaction may be hypothesized: MEL could decrease DA synthesis or release by enhancing GABA activity. Taken together, these results suggest that the retinal physiology is affected by the epileptic status and that information transmitted from retina to the brain should be affected by epilepsy in NER.

Systemic overexpression of the alpha 1B-adrenergic receptor in mice: an animal model of epilepsy

PURPOSE

A lack of selective alpha1-adrenergic receptor (alpha1-ARs) agonists and antagonists has made it difficult to clarify the precise function of these receptors in the CNS. We recently generated transgenic mice that overexpress either wild-type or a constitutively active mutant alpha 1B-AR in tissues that normally express the receptor. Both wild-type and mutant mice showed an age-progressive neurodegeneration with locomotor impairment and probable stress-induced motor events, which can be partially reversed by alpha 1-AR antagonists. We hypothesized that the wild-type and mutant mice may exhibit spontaneous epileptogenicity as compared with normal (nontransgenic) mice.

METHODS

Normal, wild-type, and mutant mice were studied. Twenty mice (1 year old) underwent prolonged video-EEG monitoring over a 4-week period. Raw EEG data were blindly analyzed by visual inspection for the presence of interictal and ictal epileptic activities.

RESULTS

During the acute postoperative period (< or = 3 days), both wild-type (26.1 +/- 8.07 spikes/day) and mutant mice (116.87 +/- 55.13) exhibited more frequent interictal spikes than did normal mice (2.17 +/- 0.75; p value, <0.05), but all three groups showed EEG and clinical seizures. During the later monitoring periods (>3 days), wild-type and mutant mice showed more frequent interictal spikes (15.44 +/- 4.07; p < 0.01; and 6.05 +/- 2.46; p < 0.05, respectively) as compared with normal mice (0.41 +/- 0.41), but only mutant mice had spontaneous clinical seizures (means +/- SEM).

CONCLUSIONS

The selective overexpression of the alpha 1B-AR is associated with increased in vivo spontaneous interictal epileptogenicity and EEG/behavioral seizures. These results suggest a possible role (direct or indirect) for the alpha 1B-ARs in the development and expression of epileptogenicity.

Regional excitatory and inhibitory amino acid concentrations in Noda epileptic rat (NER) brain

We measured regional concentrations of excitatory and inhibitory amino acids in Noda epileptic rat (NER) brains to investigate the mechanisms responsible for spontaneous generalized seizures. Compared with Fisher 344 (F344) rats and F1 hybrid (female NER x male F344 rat) rats, NERs had significantly higher concentrations of glycine in the cerebellum. NERs and F1 hybrid rats had significantly lower concentrations of taurine in the cerebellum than did F344 rats. Although our findings do not explain sufficiently the mechanisms directly responsible for spontaneous seizures, they suggest that the cerebellum of NER may be in an excited state to dampen seizures, while the cerebellum of F1 hybrid rats may also be slightly excited to inhibit milder forms of seizures. Further studies, including microdialysis and receptor binding assays, will be required to elucidate these mechanisms.

Increased expression of 3-hydroxyanthranilate 3,4-dioxygenase gene in brain of epilepsy-prone El mice

The El mouse is an established animal model for human epilepsy. We previously reported that the level of quinolinic acid (QUIN), an excitotoxin, was high in the brain of epilepsy-prone El mice and that the increased production of QUIN was secondary to an increased activity of 3-hydroxyanthranilate 3,4-dioxygenase (3-HAO, EC 1.13.11. 6) in the brains of these mice. In this study, we cloned and sequenced the cDNA for 3-HAO and showed that its expression in the brain of El mice was higher than that of control ddY mice. These results suggest that a genetic defect leading to derepression of the 3-HAO gene expression in the brain may be involved in the pathogenesis for the epileptic diseases of El mice.

Paroxysmal discharges in the EL mouse, a genetic model of epilepsy

The EL/Suz (EL) mouse is a strain that is highly susceptible to convulsive seizures after repeated sensory stimulation. Its control strain, DDY/Jc1 (DDY), is less susceptible under similar conditions. The seizure prone phenotype is the result of differences at several genetic loci. In vivo electrical recordings from the seizure prone EL mouse brain have shown that the appearance of abnormal discharges in the hippocampus are critical to the onset of generalized seizures, indicating that the hippocampus plays an important role in EL mouse seizure activity. In the present study, electrophysiological differences between EL and DDY mice (9-15 weeks of age) were examined by comparing field potentials recorded from the dentate granule cell layer of hippocampal brain slices from mice that had not been stimulated to induce seizures. In control physiological solution, no significant differences were observed in characteristics of perforant path evoked field potentials or in paired pulse depression of evoked field potentials using 20 to 300 ms interstimulus intervals. After 60 min of disinhibition following bicuculline (10 microM) exposure, however, prolonged large amplitude potentials, paroxysmal discharges, were evoked by perforant path stimulation in the dentate gyrus of EL mice but were absent in the DDY strain. Paroxysmal discharges were curtailed by APV and were similar to responses recorded from the dentate gyrus in hippocampal brain slices from temporal lobe epileptic patients. The field response to hilar stimulation was identical in both strains and was composed of a single population spike before and after bicuculline exposure. Mossy fiber terminals were not present in the molecular layer of either strain. We propose that the mechanisms leading to a greater likelihood of paroxysmal discharge generation in EL mouse may be important in the development and/or generation of epileptic seizures in this mouse strain and may be a significant phenotypic difference between the EL mouse and its parent strain.

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.

Changes of NMDA receptor binding in spontaneously epileptic rat and parent strains

We measured the binding of [3H]3-[(+/-)2-carboxypiperazin-4-yl] propyl-1-phosphonic acid ([3H]CPP), a competitive ligand for N-methyl-D-aspartate (NMDA) receptors, in double mutant spontaneously epileptic rats (SER: zi/zi, tm/tm) and their parent strains, zitter rats and tremor rats, and WTC rats (control rats from tremor rats derived from Kyoto:Wistar rats) before and after the onset of seizures in tremor rats and SER. Significantly lower [3H]CPP binding receptor density (Bmax) was found in the cortex of SER and zitter rats at 12-15 weeks of age than in that of WTC rats and tremor rats, and at 4 weeks of age the Bmax in zitter rats was lower than that in the other strains. The reduction of Bmax in SER at 12-15 weeks of age may reflect a down regulation of NMDA receptors due to repetitive tonic seizures in SER.

Age- and seizure-related changes in noradrenaline and dopamine in several brain regions of epileptic El mice

Noradrenaline (NA) and dopamine (DA) levels in six brain regions of stimulated and nonstimulated El (El[s] and El[ns]) mice and their maternal ddY mice were determined at various ages and various times after a convulsion. The NA levels in the striatum and hippocampus of 12-week-old El[s] and El[ns] mice were lower than in ddY mice, and remained lower in 23-week-old El[s] mice, but not in El[ns] mice. DA levels were lower in the striatum of El[s] mice than in El[ns] and ddY mice at 16 and 23 weeks of age. NA levels decreased during seizure in the striatum and hippocampus of El[s] mice, and returned to preconvulsive levels 1 hr after convulsion in the striatum and 30 min in the hippocampus. DA levels in the striatum of El[s] mice decreased during convulsion and increased from 1 to 10 min after convulsion. These changes suggest that the NAergic systems in the striatum and hippocampus and the DAergic system in the striatum have important roles in relation to seizure susceptibility in El mice.