Glial contribution to seizure: carbonic anhydrase activity in epileptic mammalian brain.

The activity of carbonic anhydrase (CA), a glial enzyme, was measured in the epileptic cortex of audiogenic DBA/2 mice and of cats with a freeze lesion. In mice, the activity increased with age from birth to 24 days, but were always higher in audiogenic mice than in normal C57/BL mice, reflecting species differences. The difference between the two strains increased sharply from 25 to 40 days of age, after the period of maximal audiogenic susceptibility. Acetazolamide, a CA-specific inhibitor, greatly decreased the seizure severity score of DBA/2 mice after a single intraperitoneal (i.p.) administration (150 mg/kg). After 24 days of age, when CA activities were high, the effect of acetazolamide was less important, suggesting that the increased cortical CA activity might reflect a protective mechanism. In cats with a freeze lesion, no significant changes in CA activities were observed in the actively discharging primary and secondary foci as compared with the nonepileptogenic perifocal cortex and the control cortex of sham-operated animals. The results indicate that the cortex of genetically susceptible audiogenic mice has an increased CA activity. The hypothesis of an adaptive glial mechanism, relating to the age-dependent decrease of seizure susceptibility in DBA/2 mice, is postulated.

Effect of milacemide on audiogenic seizures and cortical (Na+, K+)-ATPase of DBA/2J mice.

Milacemide (MLM, CP 1552 S, 2-N-pentylaminoacetamide), a glycinamide derivative, is currently being evaluated clinically for antiepileptic activity. Anticonvulsant properties have been shown in various animal models, but the mechanism of action of MLM is unclear. We studied its activity in audiogenic seizures of DBA/2J mice. MLM was effective in inhibiting the convulsions induced by sound with a biphasic dose-effect relation. The ED50 was 109 mg/kg orally against tonic extension. Higher doses were necessary to abolish clonic convulsion and running response. Because impaired cerebral (Na+, K+)-ATPase activity is supposed to play a role in epileptogenesis, we tested MLM on in vitro cortical enzymatic activity of DBA/2J mice. Basal (Na+, K+)-ATPase activity was unchanged by several concentrations of MLM in normal C57BL/6J and audiogenic DBA/2J mice. K+ activation (from 3 to 18 mM) of (Na+, K+)-ATPase is abolished in DBA/2J mice as compared with C57BL/6J mice, suggesting impaired glial (Na+, K+)-ATPase. In the presence of MLM (from 30 to 1000 mg/L), cortical (Na+, K+)-ATPase of DBA/2J mice is activated by high concentrations of K+, as in C57BL/6J mice. Results suggest that the antiepileptic activity of MLM in audiogenic mice may be secondary to an activation of a deficient glial (Na+, K+)-ATPase.

Milacemide stimulates deficient glial Na+, K(+)-ATPase in freezing-induced epileptogenic cortex of cats.

We investigated the influence of milacemide, a glycinamide derivative with putative antiepileptic activity, on the K(+)-activation of Na+,K(+)-ATPase in bulk isolated glial cells and synaptosomes of control and epileptogenic cortex of cats with a chronic freeze lesion. In the primary and secondary epileptic foci of non-treated animals, glial Na+,K(+)-ATPase lost its physiological K(+)-activation, while the synaptosomal enzyme was unchanged. These data reproduced previous work done on the kinetic measurement of the enzymic activities. In treated animals (500 mg/kg milacemide given orally for 2 weeks after the freeze lesion), the glial enzyme showed a normal K(+)-activation in the epileptic foci. These results confirm the existence of an abnormal glial Na+,K(+)-ATPase in cold-induced focal epilepsy and suggest that the antiepileptic activity of milacemide might be secondary to an activation of glial Na+,K(+)-ATPase, contributing to antagonize ictal transformation and seizure spread.

Phenytoin dephosphorylates the alpha(-) catalytic subunit of (Na+, K+)-ATPase. A study in mouse, cat and human brain.

Phenytoin, a potent antiepileptic drug, has been thought to stimulate Na+, K+ transport across cell membranes, but its influence on (Na+, K+)-ATPase activity remains highly controversial. We have investigated the effects of the drug on the phosphorylation level of (Na+, K+)-ATPase partially purified from mouse, cat and human brain. (Na+, K+)-ATPase catalytic subunits [alpha(+) and alpha(-)] were resolved by sodium dodecylsulfate polyacrylamide gel electrophoresis. Previous experiments had shown that phenytoin dephosphorylates the (Na+, K+)-ATPase catalytic subunit by +/- 50% in C57/BL mice. In the present study, we showed that phenytoin (10(-4) M) decreases the phosphorylation level of (Na+, K+)-ATPase catalytic subunit by the same value in cat and human cortex. Moreover, that effect is predominant on the alpha(-) subunit, thought to be the predominant enzymatic form in non-neuronal or glial cells. The results are thus favoring the hypothesis that phenytoin stimulates the brain (Na+, K+)-ATPase. They further suggest that phenytoin mainly activates the glial enzymatic form, providing central nervous system with an enhanced ability to regulate extracellular K+.