Selective suppression of hippocampal region hyperexcitability related to seizure susceptibility in epileptic El mice by the GABA-transporter inhibitor tiagabine

Changes in the content of fatty acids in CA1 and CA3 areas of the hippocampus of Krushinsky-Molodkina rats after single and fivefold audiogenic seizures

Audiogenic seizures (AS) are generalized seizures evoked by high frequency sounds. Since the hippocampus is involved in the generation and maintenance of seizures, the effect of AS on the composition and content of fatty acids in the CA1 and CA3 hippocampal areas of AS-susceptible Krushinsky-Molodkina (KM) rats on days 1, 3, and 14 after single and fivefold seizures were examined. The total content of all fatty acids in field СА1 was found to be lower compared with the control at all times of observation after both a single seizure or fivefold seizures. The total content of fatty acids in field СА3 decreased at all times of examination after a single seizure, whereas it remained unchanged on days 3 and 14 following five AS. The content of omega-3 fatty acids in both fields at all times of observation after a single seizure and fivefold AS did not significantly differ from that in intact animals. The absence of significant changes in the content of stearic and α-linolenic acids and a considerable decrease in the levels of palmitic, oleic, and eicosapentaenoic acids were common to both fields at all times after both a single seizure or fivefold AS. The changes in the content of fatty acids in the СА3 and СА1 fields of the brain of AS-susceptible rats indicate that fatty acids are involved in both the development of seizure activity and neuroprotective anticonvulsive processes.

Inhibition of different GABA transporter systems is required to attenuate epileptiform activity in the CA3 region of the immature rat hippocampus

GABA transporters (GATs) are an essential element of the GABAergic system, which regulate excitability in the central nervous system and are thus used as targets for anticonvulsive therapy. However, in the immature nervous system the functions of the GABAergic system and the expression profile of GATs are distinct from the adult situation, obscuring to predict how different GAT isoforms influence epileptiform activity. Therefore we analyzed the effects of subtype specific GAT inhibitors on repetitive epileptiform discharges using field potential and whole-cell patch-clamp recordings in the CA3 region of hippocampal slices of immature (postnatal days 4-7) rats. These experiments revealed that inhibition of GAT-1 with either tiagabine (30 μM) or NO-711 (10 μM) exhibited only a minor anticonvulsive effect on repetitive epileptiform discharges. Blockade of GAT-2/3 with SNAP-5114 (40 μM) had no anticonvulsive effect, but significantly prolonged the decay of spontaneous GABAergic postsynaptic currents. In contrast, the combined application of 10 μM NO-711 and 40 μM SNAP-5114 blocked epileptiform activity in 33% of all slices and reduced the occurrence of epileptiform discharges by 54% in the remaining slices. In addition, the input resistance decreased by 10.5 ± 1.0% under this condition. These results indicate that both GAT-1 and GAT-2/3 are functional in the immature hippocampus and that only the combined inhibition of GAT 1-3 is sufficient to promote a considerable anticonvulsive effect. We conclude from these results that both GAT-1 and GAT-2/3 act synergistically to regulate the excitability in the immature hippocampus.

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.

Phenotyping the untouchables: environmental enhancement of behavioral and physiological activation in seizure-prone El mice

The onset and frequency of spontaneous and tail suspension-induced seizures in El mice appear to be influenced strongly by developmental and experiential factors over the first 3 months of life. To assess the impact of social factors on behavioral characteristics of El mice prior to the age of seizure susceptibility, locomotor activity and exploratory measures of arousal were recorded in 40-day-old El and control DDY mice exposed to group and isolation housing conditions. Once mice reached maturity, physiological reactivity to a tail suspension stressor was evaluated. The locomotor activity measure revealed circadian entrainment in both strains, nocturnal hyperactivity in El mice, and a locomotor activity-attenuating effect of group housing in El mice. In the two-compartment model of exploration, latency to enter, transitions to and from, and rearing in a brightly lit compartment were 50% higher in El relative to DDY mice, again suggesting a hyperactive phenotype. Finally, an acute 2-minute tail suspension stressor applied to 75-day-old mice implanted with radiotelemetry transmitters revealed a reactive tachycardia in El, but not DDY, mice. No seizures were observed during any of the experimental manipulations. Taken together, these results suggest that spontaneously occurring deviations in behavioral and cardiovascular measures of arousal characterize preseizure El mice and that motor features of hyperarousal can be exaggerated by the environmental manipulation of isolation housing.

Down-regulation of GABA-transporter function by hippocampal translation products: its possible role in epilepsy

The genetically epileptic mouse strain (El) is used as a model for human temporal lobe epilepsy. To address the question of whether altered function of the neuronal GABA transporter GAT1 is involved in the pathology of epilepsy of El mice, we expressed in Xenopus oocytes cloned GAT1 of mouse brain by injection of complementary ribonucleic acid (cRNA) and co-injected messenger ribonucleic acid (mRNA) isolated from the hippocampus of non-epileptic control mother strain (ddY) mice and from El mice. GABA transporter activity was investigated by measurements of [(3)H]-GABA uptake as well as by steady-state and transient current measurements under voltage clamp.Co-injection of hippocampal mRNA into oocytes reduced GAT1-mediated transport. This effect was more pronounced for mRNA from ddY mice than for that from El mice that never experienced seizures, El(-), and being absent for mRNA from El mice that have had high seizure experience, El(+). The pronounced inhibition of GABA transport after injection of mRNA from the ddY strain results from reduced expression of functional GAT1, but to about one third also from a reduced GABA translocation rate. The reduced translocation can be attributed to a reduced forward rate of a step associated with extracellular Na(+) binding. If the results can be applied to the mouse brain, we may hypothesise that in ddY mice some GAT down-regulating factor translated from hippocampal mRNA may be involved to keep GAT1 activity low, and hence GABA concentration in synaptic cleft high. In El(-) mice such regulatory mechanism may be reduced or counteracted by another unknown factor present in El(-) brain. The repeated seizure experience in El(+) mice enhances this compensatory effect.

Hyperexcitability and changes in activities of Ca2+/calmodulin-dependent kinase II and mitogen-activated protein kinase in the hippocampus of rats exposed to 1-bromopropane

Chronic inhalation of 1-bromopropane (1-BP), a substitute of ozone-depleting chlorofluorocarbons, has been suspected of having central neurotoxicity (Clinical Neurology and Neurosurgery 101 (1999) 199; Journal of Occupational Health 44 (2002) 1) for humans. In animal experiments, 1-BP inhalation (1500 ppm) caused hyperexcitability in the CA1 and the dentate gyrus (DG) [Journal of Occupational Health 42 (2000) 149, Journal of Occupational Health 44 (2002) 156]. We studied whether the hyperexcitability is associated with changes of Ca2+/calmodulin-dependent kinase II (CaMKII), mitogen-activated protein kinase (MAPK), and protein kinase C (PKC). Male Wistar rats were exposed to 1-BP for 6 hours in a day in an exposure chamber with a concentration of 700 ppm for 8 weeks. After the inhalation, paired-pulse ratios of field excitatory postsynaptic potentials and population spikes (PSs) were analyzed in the CA1 and DG of hippocampal slices. Control rats were then given fresh air in the inhalation chamber. Semiquantitative immunoblotting analyses of protein kinases using antibodies against active and conventional protein kinases were done using the whole hippocampus. A paired-pulse ratio of PS was increased at the 5 ms interpulse interval in the CA1 and at the 10-20 ms interpulse intervals in the DG. The amount of active MAPK and total amount of CaMKIIalpha and beta were significantly increased by 28, 29, and 46% compared to control, respectively, without any change in PKC activity. In contrast, the amount of active CaMKIIbeta was decreased to 78%. These results suggest that modifications of intracellular signaling cascades are associated with hyperexcitability that occurred in the hippocampal formation of rats exposed to the chronic inhalation of 1-BP.