A specific protein abnormality associated with cobalt-induced epilepsy in mice

Purification and characterization of a novel 70-kDa brain protein associated with seizure activities

Using ion exchange HPLC and ammonium sulfate precipitation, we have purified a 70-kDa protein (P70) specific to the cobalt-induced epileptogenic cortex of rat cerebrum and determined certain of its biochemical properties. P70 has a similar isoelectric point (pI; 4.6-4.8), amino acid composition and N-terminal amino acid sequence to rat serum albumin (RSA). Intracortical application of purified P70 to the motor area of normal rat cerebrum induces both ECoG seizure discharges and behavioral seizures. The data suggest that P70 is a novel albumin-like protein linked to the generation of seizure activities. However, it can be clearly distinguished from RSA, since it is able to produce seizure, is a glycoprotein and can be readily separated from RSA by 2-dimensional electrophoresis.

A specific 70K protein found in epileptic rat cortex: induction of bursting activity and negative resistance by its intracellular application in Euhadra neurons

The effect of 70-kD protein (P70, a specific protein found in cobalt-induced epileptogenic focus of rat cerebral cortex) on membrane properties was examined in identified neurons of the snail, Euhadra peliomphala, using the pressure injection method combined with the voltage-clamp technique. In neurons that normally exhibited spontaneous regular firing, intracellular injection of P70 elicited bursting activity and a negative slope resistance (NSR) region in their current-voltage (I-V) curve in a manner corresponding to the duration of its injection. These responses were suppressed by prior injection of an antibody to P70 into the neurons, and were markedly inhibited by a reduction of extracellular Na+ ions and the anticonvulsant agent phenytoin, but not by Co(2+)-substituted Ca(2+)-free saline. In addition, intracellularly applied P70 potentiated both bursting activity and the NSR induced by a Na channel activator, veratridine. However, prior application of a saturating dose of this activator occluded the effect of P70. These results suggest that P70 elicits a Na(+)-dependent negative resistance, which may contribute to the generation of bursting activity.

Protein changes associated with cobalt-induced epileptic activity in rat cerebral cortex

In cobalt-induced epileptogenic cortex in rats, a marked increase in two proteins of about 84 kDa and 70 kDa, a slight increase in 12-kDa and 10-kDa proteins and a decrease in a protein of about 57 kDa were noted, as determined by SDS-polyacrylamide gel electrophoresis. The initiation of these protein changes was ahead of the generation of epileptogenic activities. The anticonvulsant drugs phenytoin (PHT) and phenobarbital (PB) attenuated the cobalt-induced epileptogenic activities, but failed to suppress the protein changes. Among these proteins, a 70-kDa protein, when injected intracortically into the motor region of the normal rat cerebrum, evoked epileptic discharges on the electrocorticograph and behavioral seizure, which were abolished by prior treatment with PHT or PB. These findings suggest that the above protein changes are not an indirect offshoot of secondary stimulation of neurons by neurotransmitters or neuromodulators, and that P70 may contribute to the generation of epileptic seizure activities.

A novel protein induced in cortical epileptic focus of rat cerebrum: a possible linkage to epileptogenesis

To precisely evaluate a protein-related mechanism of epileptogenesis, we quantitatively analyzed the 70-kDa protein (namely, P70), a specific protein found in the cobalt-induced epileptic focus, and examined its effect on the electrocorticogram (ECoG) and cortical neurons in cerebral slices and its immunocytochemical localization in rats. Cobalt-induced cortical epileptogenic cortex exhibited a marked induction of P70. Its initiation time was ahead of the generation of epileptogenic activities. The anticonvulsant phenytoin (PHT) attenuated the cobalt-induced epileptogenic activities, but failed to suppress protein induction. Injection of this protein into the motor region of normal rat cerebral cortex elicited an epileptic ECoG and behavioral seizures. It also caused epileptiform activity with paroxysmal depolarization shifts in cortical neurons. These epileptogenic phenomena elicited by P70 were abolished by prior treatment with PHT or phenobarbital. Immunocytochemical analysis with an antiserum against P70 revealed that the reactivity was confined to pyramidal cells only in the region of the focus and was mainly localized on somatic, dendritic, and nuclear membranes and microtubles. These findings suggest that P70 may be linked to epileptogenesis.

The 70-kDa epileptic cortical protein elicits bursting activity accompanied by a reduction of outward current in Euhadra neurons which is inhibited by an antibody against this protein

The effect on membrane properties of 70-kDa protein (namely P70), a specific protein found in cobalt-induced epileptic cortex of rats, was examined electrophysiologically in identified neurons of the snail, Euhadra peliomphala. Injection of P70 into the neurons elicited a bursting activity resembling the paroxysmal depolarization shift seen in mammalian epileptic neurons and a reduction of the outward current, which were suppressed by Ca2(+)-free saline or tetraethylammonium. The injection of P70-antibody into neurons, preceding the P70 injection, markedly inhibited the bursting activity and the reduced outward current elicited by P70. These findings suggest that P70 causes a reduction of Ca2(+)-dependent potassium conductance which may be one of the mechanisms generating epileptic bursting activity.

A specific 70 kDa protein induced in the epileptogenic cortex of rats elicits bursting activity and inactivation of potassium current in snail neurons

The capability of a specific 70 kDa (P70) protein produced in cobalt-induced epileptogenic cortex of rat cerebrum in promoting epileptic bursting activity was examined in snail neurons, using the voltage clamp method in combination with the pressure injection technique. Injection of P70 into the neurons elicited bursting activity with paroxysmal depolarization shift together with a reduction in the delayed outward current which was suppressed by tetraethylammonium. These changes of membrane properties induced by P70 were dose-dependently inhibited by prior treatment with the anticonvulsant phenytoin. These findings indicate that P70 causes an inactivation of potassium conductance which may generate epileptic bursting activity.

Induction of epileptic seizure activity by a specific protein from cobalt-induced epileptogenic cortex of rats

To evaluate the protein-related mechanism for epileptogenesis, we examined protein behavior during cobalt-induced epileptic seizure activity and the effect of injection of cobalt-induced proteins into cerebral cortex on electrocorticographic discharge in rats. Cobalt-induced epileptic seizure activity caused an increase in two proteins that migrated at about 84 kDa (P84) and 70-71 kDa (P70) and a decrease in a protein of about 57 kDa (P57) as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Among these proteins, P70 evoked epileptic discharges on the electroencorticograph and behavioral seizure, when it was intracortically injected into the motor region of the normal rat cerebrum. The data suggest that P70 may be a factor for epileptogenesis.

In vivo inhibition of incorporation of [U-14C]glucose into proteins in experimental focal epilepsy

The in vivo incorporation of [14C] from [U-14C]-glucose into rat brain proteins from different cortical areas was examined in three different experimental focal epilepsies: cobalt, freeze-lesions, and tityustoxin. When [U-14C]-glucose was injected intraperitoneally into awake and unrestrained animals with marked signs of epileptic hyperactivity, the inhibition of incorporation of [14C]-amino acids into trichloracetic acid (TCA)-insoluble proteins was highest in the focal (sensorimotor) area when compared with distant regions (approx. 60%), but less when compared with the contralateral (sensorimotor) region (approx. 23%). Greatly decreased incorporation caused by both cobalt and freeze-lesion-induced epilepsies was also observed in the contralateral area when a comparison was made with distant regions (approx. 50%), but there were no significant differences in protein-specific radioactivity between the different distant areas.

Alterations in the compartmentalized metabolism of glutamic acid with changed cerebral conditions

Data obtained from combined determinations of the nervous tissue content of glutamic acid, taurine and glutamine were examined in terms of the well established concept of a compartmentalized metabolism for glutamic acid. Three different situations associated with altered cortical conditions were studied: cortical hyperexcitability induced by cobalt epilepsy (mouse); chronic stimulation of the optic tectum by light adaptation (fish); and anatomic alteration of the optic tectum following unilateral enucleation (fish). All 3 situations appear to cause a reduction in the ability of glial elements to capture free glutamic acid released from neuronal structures. However, the underlying causes for such an insufficiency seem to differ in each instance. In epilepsy the release of glutamic acid and taurine exceeds the glial capture rate; during chronic stimulation of a normal cortex a diminished glial uptake rate for both amino acids seems apparent; anatomical degenerative changes seem to diminish especially the glutamine retention capacity of the cortex, possibly in combination with a reduced glial taurine uptake.

Sodium outflux and influx within nerve terminals from chronic epileptogenic foci

The Na+-pump (oMpNa) or maximal sodium outflux inhibited by ouabain was studied in nerve terminals isolated from the primary and mirror epileptogenic foci of freezing lesions. In addition, the Na+ for Na+ exchange diffusion that is sensitive to ethacrynic acid and furosemide was also analyzed. oMpNa in control and epileptogenic states had similar and indistinguishable activation curves for K0 (extracellular K) in a sodium medium except for a two-fold difference in magnitude. However, the apparent affinity of the Na+-pump for K0 as measured by K1/2 was shifted to the right or decreased in epileptogenic foci (2 mM K0 compared to 0.5 mM in controls) when measured in a magnesium medium. A decrease in the apparent affinity for internal sodium was also observed. Of the total sodium outflux (67.7 nmol/mg/min), Na+ movements which are insensitive to ouabain and external K+ but stimulated by external Na+ and inhibited by furosemide and ethacrynic acid (20 nmol/mg/min or 32% of sodium outflux) represented the Na+ for Na+ exchange diffusion in nerve terminals. Na+ influx rising with increasing internal sodium in the presence of ouabain and blocked by furosemide represent the corresponding inward Na+ movement. No differences were observed between controls and epileptogenic states.

Influence of taurine dosage on cobalt epilepsy in mice

The present work addresses itself to answering several questions in relations to the antiepileptic action of taurine (see "Introduction"). For this purpose, cobalt epileptic mice, treated with isoosmolar saline or not treated, was compared with groups of mice that had received doses of taurine varying between 1.0 and 10.0 mg/kg. Whereas all the taurine doses employed effectively reduced seizure incidence, with 10.0 mg/kg giving the best results, none of these doses had ameliorated the amino acid abnormalities in the cortex after two days of treatment in comparison with the group that had received isoosmolar saline. On the contrary, the largest amino acid abnormalities occurred in the group of mice (10.0 mg/kg) that had improved the most clinically. We conclude that the acute antiseizure action of taurine, and its effect on restoring normal amino acid patterns in the cortex, represent two separate properties of taurine.