Electroconvulsive Threshold Research Articles

Influence of sexual hormone antagonists on the anticonvulsant action of conventional antiepileptic drugs against electrically- and pentylenetetrazol-induced seizures in mice

The present results refer to the action of three gonadal steroid antihormones, tamoxifen (TXF, an estrogen antagonist), cyproterone acetate (CYP, an antiandrogen) and mifepristone (MIF, a progesterone antagonist) on seizure phenomena in mice. TXF and CYP at their lowest protective dose in the electroconvulsive threshold test, enhanced the antiseizure efficacy of some antiepileptic drugs. TXF (20 mg/kg) potentiated the protective activity of valproate, diphenylhydantoin and clonazepam, but not that of carbamazepine or phenobarbital, against maximal electroshock-induced convulsions in female mice. CYP (40 mg/kg) enhanced the anticonvulsant action of valproate, carbamazepine, diphenylhydantoin and clonazepam, but not that of phenobarbital, against maximal electroshock in male animals. MIF failed to affect the electroconvulsive threshold or the efficacy of antiepileptic drugs in maximal electroshock. The effect of TXF or CYP upon the electroconvulsive threshold and on the action of antiepileptics was not reversed by sex steroid hormones (estradiol, testosterone, progesterone). However, the TXF-induced elevation of the electroconvulsive threshold was abolished by bicuculline, N-methyl- d-aspartic acid and kainic acid, and partially reversed by aminophylline, strychnine being ineffective in this respect. The action of CYP on the threshold for electroconvulsions was partially reversed by bicuculline and aminophylline. Both glutamatergic agonists and strychnine remained ineffective in this respect. Moreover, the action of TXF or CYP on the activity of antiepileptics was not influenced by strychnine, and reversed to various extents by the remaining convulsants. In contrast to maximal electroshock, none of the three antihormones affected the protective action of antiepileptic drugs against pentylenetetrazol-induced seizures in mice. Neither TXF nor CYP altered the free plasma levels of antiepileptic drugs, so a pharmacokinetic interaction is not probable. The combined treatment of the two antihormones with antiepileptic drugs, providing 50% protection against maximal electroshock, did not affect motor performance in mice, and did not result in significant long-term memory deficits. Our data indicate that steroid receptor-mediated events may be indirectly associated with seizure phenomena in the central nervous system and can modulate the protective activity of some conventional antiepileptic drugs.

Interaction of astemizole, an H 1 receptor antagonist, with conventional antiepileptic drugs in mice

Histamine is one of the aminergic neurotransmitters, playing an important role in the regulation of a number of physiological processes. There are several subtypes of histamine receptors—H 1, H 2, H 3 and the recently discovered H 4. H 1 receptors exist on mast cells, basophils, enterochromaffin cells and in the central nervous system, being located postsynaptically. H 1 receptor antagonists, including classical antiallergy drugs, occasionally have been expected to induce convulsions in children and epileptics. The aim of this study was to evaluate the effects of astemizole-given intraperitoneally, singly or for 7 days on the anticonvulsant activity of antiepileptic drugs (AEDs) against maximal electroshock (MES)-induced convulsions in mice. The following AEDs were administered intraperitoneally: valproate magnesium, carbamazepine, diphenylhydantoin and phenobarbital. Adverse effects were evaluated in the chimney test (motor performance) and passive avoidance task (long-term memory). Brain and plasma levels of AEDs were measured by immunofluorescence. Astemizole (a single dose and following a 7-day treatment at 2–6 mg/kg) reduced the threshold for electroconvulsions, being without effect upon this parameter at lower doses. Astemizole (1 mg/kg) did not significantly alter the protective effect of AEDs against MES (after acute and 7-day administration). Also, acute astemizole (2 mg/kg) remained ineffective in this respect. Astemizole (2 mg/kg), following chronic administration, significantly reduced the protective efficacy of phenobarbital and diphenylhydantoin, reflected by an increase in their ED 50 values (50% effective dose necessary to protect 50% of animals tested against MES) from 21.1 to 34.0 mg/kg and from 10.4 to 19.2 mg/kg, respectively. Astemizole (2 mg/kg) did not alter the protective activity of the remaining AEDs. Moreover, astemizole (2 mg/kg) did not influence the free plasma levels and brain concentration of the studied AEDs. Also, this H 1 receptor antagonist did not impair long-term memory or motor coordination when given acutely. However, 7-day treatment with astemizole (2 mg/kg) significantly decreased TD 50 (50% toxic dose required to induce motor impairment in 50% of animals) value of phenobarbital, being without effect on carbamazepine, valproate and diphenylhydantoin in this respect. Similarly, phenobarbital and diphenylhydantoin, administered alone at their ED 50s against MES, or combined with astemizole, disturbed long-term memory in mice. The results of this study indicate that astemizole may need to be used with caution in epileptic patients.

L-histidine is a beneficial adjuvant for antiepileptic drugs against maximal electroshock-induced seizures in mice.

Endogenous histamine has been reported to be involved in regulation of seizure susceptibility. Enhancement of histamine neurotransmission engendered by L-histidine treatment produces anticonvulsant effects in experimental animals. The present study investigated the influence of L-histidine on the protective effects of carbamazepine and phenytoin against maximal electroshock-induced seizures in mice.L-Histidine, administered at the doses that did not influence the threshold for electroconvulsions (250-500 mg/kg), enhanced by nearly 30% the protective effects of carbamazepine against maximal electroshock-induced seizures. D-Histidine (1000 mg/kg), an inactive isomer of histidine, was without any effect in this regard. L-Histidine (500 mg/kg) also augmented the protective effects of phenytoin. Importantly, the enhancement of the anticonvulsant effects of these antiepileptic drugs produced by L-histidine co-administration was not associated with augmentation of their unwanted effects on memory and motor performance. A pharmacokinetic interaction was also excluded since the free plasma levels of these antiepileptics remained unchanged in the presence of L-histidine. It may be suggested that L-histidine could serve as a beneficial adjuvant for selected antiepileptic drugs.

Spontaneous deletion of epilepsy gene orthologs in a mutant mouse with a low electroconvulsive threshold.

The electroconvulsive threshold (ECT) test has been used extensively to determine the protection conferred by antiepileptic drug candidates against induced seizures in rodents. Despite its clinical relevance, the potential of ECT to identify mouse epilepsy models in genetic studies has not been thoroughly assessed. We adopted the ECT test to screen the progeny of ethylnitrosourea treated male C57BL/6J mice. In a small-scale screen, several mutant lines conferring a low threshold to ECT minimal clonic seizures were mapped to the telomeric region of mouse chromosome 2 in independent founder families. This high incidence was suggestive of a single spontaneous event that pre-existed in the founders of mutagenized stock. Genetic and physical mapping led to the discovery that several lines shared a single mutation, Szt1 (seizure threshold-1), consisting of a 300 kb deletion of genomic DNA involving three known genes. Two of these genes, Kcnq2 and Chrna4, are known to be mutated in human epilepsy families. Szt1 homozygotes and heterozygotes display similar phenotypes to those found in the respective Kcnq2 knockout mutant mice, suggesting that Kcnq2 haploinsufficiency is at the root of the Szt1 seizure sensitivity. Our results provide a novel genetic model for epilepsy research and demonstrate that the approach of using ECT to study seizures in mice has the potential to lead to the identification of human epilepsy susceptibility genes.

Interactions of tiagabine with some antiepileptics in the maximal electroshock in mice

Tiagabine (TGB), a new potent γ-aminobutyric acid (GABA) uptake inhibitor, is widely applied in adjunctive treatment of partial seizures in humans. Although, polytherapy is not an initial method of epilepsy treatment, clinicians often combine TGB with other antiepileptics as add-on therapy for assuring the anticonvulsant protection in patients with refractory seizures. To evaluate the character of pharmacological interactions between TGB and some antiepileptics, the isobolographic analysis was used as a suitable method for determining the exact types of interactions. Determination of an influence of TGB on the protective effects of diphenylhydantoin (DPH), carbamazepine (CBZ), valproate (VPA), phenobarbital (PB), lamotrigine (LTG), topiramate (TPM), and felbamate (FBM) in maximal electroshock-induced seizures was essential for this study. To exclude or confirm a pharmacokinetic character of observed interactions, the free plasma and brain concentrations of antiepileptic drugs (AEDs) studied were evaluated by using the immunofluorescence or high-pressure liquid chromatography (HPLC). TGB (up to 2.5 mg/kg) remained ineffective upon the electroconvulsive threshold, whilst the drug in doses of 5 and 10 mg/kg significantly raised the electroconvulsive threshold in mice. According to the isobolography, TGB appears to act synergistically with VPA. The remaining combinations tested exerted additive interactions. A pharmacokinetic character of interaction between TGB and VPA was evidently corroborated either in plasma or brains. Moreover, TGB significantly reduced the plasma and brain concentrations of DPH; however, pharmacokinetic events were not accompanied by any changes in anticonvulsant activity of the latter. Finally, the isobolographic analysis revealed that combinations of TGB with VPA exerted synergistic (supra-additive) interaction resulting from a pharmacokinetic interaction.

Low-affinity kainate receptor-mediated events reduce the protective activity of phenobarbital and diphenylhydantoin against maximal electroshock in mice

(2S,2R)-4-Methylglutamic acid (SYM 2081), a potent selective agonist of GluR5 and GluR6 kainate receptor subtypes, applied at the dose of 15.5 mg/kg, equal to its CD 16 value (i.e., a dose required to induce convulsions in 16% of mice), significantly decreased the electroconvulsive threshold from 7.0 to 5.8 mA. When administered at the dose of 11.5 mg/kg, equal to 75% of its CD 16, it markedly attenuated the protective activity of phenobarbital and diphenylhydantoin, but not that of valproate, carbamazepine, or diazepam against maximal electroshock-induced seizures in mice. The respective ED 50 values were increased from 18.5 to 23.8 mg/kg for phenobarbital, and from 11.7 to 14.7 mg/kg for diphenylhydantoin. Since the free plasma levels of both antiepileptic drugs were not influenced by SYM 2081, the pharmacokinetic interaction does not seem to be involved in the observed results. In conclusion, low-affinity kainate receptor-mediated events might be a factor reducing the protective efficacy of some antiepileptic drugs. Furthermore, the activation of GluR5 and GluR6 kainate receptor subtypes by endogenous glutamate during seizures may be associated with the drug-resistance phenomenon.

Interaction of the neurosteroid alphaxalone with conventional antiepileptic drugs in different types of experimental seizures

A number of neurosteroids exert antiseizure and/or neuroprotective properties. The aim of this study was to evaluate the effect of the neurosteroid alphaxalone on the protective action of conventional antiepileptics in four seizure tests. Alphaxalone (up to 5 mg/kg) did not exert a significant action against amygdala-kindled seizures in rats, or against pentetrazole- or aminophylline-induced convulsions in mice. The neuroactive steroid at the dose of 2.5 mg/kg significantly raised the threshold for electroconvulsions in mice. At 2.5 mg/kg, alphaxalone diminished the protective activity of valproate against maximal electroshock and at 2.5–5 mg/kg against pentetrazole-induced seizures in mice. However, alphaxalone (2.5 mg/kg) did not affect the protective activity of carbamazepine, diphenylhydantoin, phenobarbital or clonazepam against maximal electroshock and at 5 mg/kg did not affect that of phenobarbital, clonazepam and ethosuximide against pentetrazole-induced convulsions. Insignificant results were also obtained in the case of co-administration of alphaxalone with phenobarbital, valproate, clonazepam and carbamazepine against aminophylline-evoked seizures in mice. Also, in the kindling model of epilepsy, combinations of the neuroactive steroid (2.5 mg/kg) with valproate, carbamazepine, phenobarbital, diphenylhydantoin or clonazepam at their subprotective doses did not result in pro- or anticonvulsant activity. Valproate (284 mg/kg; the dose used in combination with alphaxalone) produced significant memory deficits in mice. Alphaxalone (2.5 mg/kg), valproate (at its ED50 value of 226 mg/kg) and the combination of valproate (284 mg/kg) with alphaxalone (2.5 mg/kg) did not affect long-term memory, evaluated in the passive avoidance task with mice. Alphaxalone administered alone or in combination with valproate caused no motor impairment in experimental animals. Finally, alphaxalone (2.5 and 5 mg/kg) significantly increased the free plasma levels of valproate, strongly indicating that the neuroactive steroid-induced reduction of the protective activity of valproate is not related to pharmacokinetic phenomena. Summing up, alphaxalone does not seem to be a promising candidate for adjunctive treatment of epilepsy.

Influence of agents affecting voltage-dependent calcium channels and dantrolene on the anticonvulsant action of the AMPA/kainate receptor antagonist LY 300164 in mice

It was previously documented that calcium (Ca 2+) channel inhibitors intensified the protective effects of conventional antiepileptics against electroconvulsions in mice. The aim of this study was to evaluate the effects of Ca 2+ channel inhibitors (nifedipine, nicardipine and flunarizine) on the anticonvulsant action of the new AMPA/kainate receptor antagonist, 7-acetyl-3-(4-aminophenyl)-8,9-dihydro-8-methyl-7H-1,3-dioxazolo[4,5-h][2,3]-benzodiazepine (LY 300164), against maximal electroshock (MES)-induced seizures in mice. Dantrolene (an inhibitor of Ca 2+release from intracellular stores) was also included. Nifedipine (30 mg/kg) and flunarizine (15 mg/kg) raised the threshold for electroconvulsions, being ineffective at lower doses. Nicardipine (up to 30 mg/kg) and dantrolene (up to 20 mg/kg) did not affect this parameter. Flunarizine (10 mg/kg), nicardipine (20 mg/kg) and dantrolene (20 mg/kg) potentiated the efficacy of LY 300164 against MES. However, nicardipine (at 20 mg/kg) raised the free plasma concentration of LY 300164. Nifedipine (30 mg/kg), given even in a dose raising the electroconvulsive threshold, did not significantly alter the protective effect of LY 300164 against MES. Furthermore, the Ca 2+ channel agonist—BAY k-8644 (at 5 mg/kg) did not influence the protection offered by LY 300164 against MES. Finally, this Ca 2+ channel activator did not affect the enhanced efficacy of LY 300164 by Ca 2+ channel modulators. The only exception was the combination of LY 300164 with flunarizine. Combined treatment with LY 300164 and dantrolene (20 mg/kg), compared to LY 300164 alone, resulted in an impairment of motor performance in mice. Ca 2+ channel inhibitors were without effect upon this parameter evaluated in the chimney test. As shown in the passive avoidance task, LY 300164 alone (at its ED 50) or combined with agents affecting neuronal Ca 2+ concentration did not disturb long-term memory. The present results suggest that agents preventing influx of Ca 2+ ions into neurons may enhance the protective action of LY 300164.

Do structural properties explain the anticonvulsant activity of valproate metabolites? A QSAR analysis.

Differences in potency among valproate (VPA) metabolites could be explained by structural properties. Therefore, a quantitative structure-activity relation (QSAR) analysis was performed to study the relation between structural parameters and the effect of the following VPA metabolites: 4-en-VPA, 2-en-VPA, 3-en-VPA, 2,4'-dien-VPA, 4,4'-dien-VPA, 4-hydroxy-VPA, 3-ceto-VPA, 3-hydroxy-VPA, 5-hydroxy-VPA, and propylglutaric acid. By using the CAChe program package for biomolecules (Oxford Molecular, Ltd), we performed molecular modeling. The anticonvulsant activity determined on the threshold for maximal electroconvulsions in mice was obtained from a study of Löscher and Nau. Structural parameters were compared between metabolites with a double bond and metabolites with oxygen at either side chain (unpaired Student's t test). A single linear regression analysis between each structural parameter and the relative anticonvulsant potency was also performed. Similar parameters were found between the cis and trans and R and S isomers. Biologic activity and most of the structural parameters were significantly different between metabolites with a double bond and metabolites with oxygen at either side chain. Activity was directly related to log P(oct) (r(2) = 0.77) and to reactivity parameters and was inversely related to stability parameters and to molecular weight and surface. The most potent metabolites had a log P(oct) value of higher than 2 units. Similar data were identified between cis and trans, and R and S isomers of VPA metabolites. Anticonvulsant activity was mainly related to log P(oct), probably reflecting the ability of VPA metabolites to cross the blood-brain barrier.

Nociceptin, OP4 receptor ligand in different models of experimental epilepsy

The anticonvulsive activity of nociceptin, endogenous OP4 receptors agonist was investigated in pentylenetetrazole (PTZ), N-methyl D-aspartic acid (NMDA), bicucculine (BCC) and electrically evoked seizure models of experimental epilepsy. Nociceptin, at the dose of 10 nmol, suppressed the clonic seizures induced by PTZ, NMDA and BCC. [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1–13)-NH2 which has been proposed to be selective antagonist OP4 receptors, did not prevent the action of nociceptin. The effect of [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1–13)-NH2 on seizures induced by PTZ, NMDA and BCC was very similar to that of nociceptin. These data support the hypothesis that it possesses agonistic properties. Naloxone did not reverse the anticonvulsive action of nociceptin as well as [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1–13)-NH2 which excludes the participation of opioid receptor in this action. On the other hand in the electroconvulsive model of generalized seizures, nociceptin as well as [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1–13)-NH2 influenced neither the electroconvulsive threshold nor the maximal electroshock test. The data suggest that nociceptin and [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1–13)-NH2 can exert anticonvulsive action. These properties depend on OP4 but not opioid receptors activation.

2-Chloroadenosine, a preferential agonist of adenosine A 1 receptors, enhances the anticonvulsant activity of carbamazepine and clonazepam in mice

2-Chloroadenosine (0.25–1 mg/kg) significantly raised the threshold for electroconvulsions in mice. This preferential adenosine A 1 receptor agonist (at 0.125 mg/kg) significantly potentiated the protective activity of carbamazepine against maximal electroshock-induced seizures in mice. 2-Chloroadenosine (1 mg/kg) showed also anticonvulsive efficacy against pentylenetetrazol-evoked seizures, raising the CD 50 value for pentylenetetrazol from 77.2 to 93.7 mg/kg. The drug (at 0.5 mg/kg) significantly enhanced the protective action of clonazepam in this test, decreasing its ED 50 value from 0.033 to 0.011 mg/kg. Moreover, aminophylline, a non-selective adenosine receptor antagonist (5 mg/kg), and 8-cyclopentyl-1,3-dimethylxanthine (8-CPX), a selective A 1 adenosine receptor antagonist (5 mg/kg) reversed the 2-chloroadenosine (0.125 mg/kg)-induced enhancement of the protective activity of carbamazepine and clonazepam. 2-Chloroadenosine administered alone or combined with antiepileptic drugs, caused neither motor nor long-term memory impairment. Finally, the adenosine A 1 agonist did not change the free plasma concentration of antiepileptics, so a pharmacokinetic factor is not probable. Summing up, 2-chloroadenosine potentiated the protective activity of both carbamazepine and clonazepam, which seems to be associated with the enhancement of purinergic transmission mediated through adenosine A 1 receptors.

Propranolol and metoprolol enhance the anticonvulsant action of valproate and diazepam against maximal electroshock

The anticonvulsive potential of classical antiepileptics co-administered with β-adrenergic receptor antagonists against generalized tonic–clonic seizures was evaluated in the model of maximal electroshock (MES)-induced convulsions. Propranolol, acebutolol, metoprolol and atenolol were tested in the doses not affecting the electroconvulsive threshold. Propranolol and metoprolol lowered the ED 50 of valproate and diazepam. Acebutolol reduced valproate's but not diazepam's ED 50 value. In contrast, hydrophilic atenolol, not penetrating via blood–brain barrier, affected neither the action of valproate nor diazepam. None of the studied drugs changed the protective activity of carbamazepine and phenytoin against MES. β-blockers per se did not alter the motor performance of mice. Moreover, propranolol and metoprolol did not influence diazepam-evoked impairment of locomotor activity. The free plasma and brain levels of antiepileptic drugs were not affected by β-blockers. In conclusion, the use of certain β-adrenoceptor antagonists, such as propranolol and metoprolol, might improve the antiepileptic potential of valproate and diazepam.

β-Adrenoceptor blockade enhances the anticonvulsant effect of glutamate receptor antagonists against maximal electroshock

In this study, we evaluated whether β-adrenoceptor antagonists may modify the protective efficacy of dizocilpine (MK-801), a NMDA receptor antagonist, and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), a non-NMDA (AMPA/kainate) receptor antagonist, against maximal electroshock-induced seizures in mice. Propranolol, acebutolol, metoprolol and atenolol were used in doses that did not alter the electroconvulsive threshold. Propranolol potentiated the anticonvulsant activity of MK-801 and GYKI 52466, significantly lowering their ED 50 values from 0.38 and 15.0 to 0.15 ( P<0.001) and 8.4 mg/kg ( P<0.001), respectively. Similarly, metoprolol lowered the ED 50 of MK-801 and GYKI 52466 from 0.38 and 15.0 to 0.17 ( P<0.05) and 11.2 mg/kg ( P<0.05). Acebutolol enhanced the protective action of GYKI 52466, lowering its ED 50 value from 15.0 to 12.2 mg/kg ( P<0.05), but not that of MK-801. Atenolol, not penetrating the blood–brain barrier, did not affect the anticonvulsive efficacy of MK-801 and GYKI 52466. In conclusion, β-adrenoceptor antagonists may act synergistically with excitatory amino acid receptor antagonists to inhibit generalised tonic-clonic seizures.

Electroconvulsive Thresholds of Inbred Mouse Strains

The electroconvulsive threshold (ECT) test is used commonly in the screening of anti-epileptic drugs in rodent models, but little is known about its genetic or mechanistic basis. Thresholds for minimal clonic, maximal tonic, or psychomotor (partial) seizures were determined in 16 different inbred mouse strains in two different laboratories. A wide range of thresholds was observed, suggesting that a variety of neuroexcitability alleles exist in inbred strains. Although there was generally good cross-strain correlation between the three seizure types, several outlier strains were detected, showing that genetically encoded differences can affect the ability of a particular seizure type to spread through the brain. Furthermore, the relative seizure susceptibility of a strain was comparable between the two laboratories, suggesting that despite different test sites, instrumentation, and personnel, the ECT assay is portable and that common inbred strains can often be relied upon as calibration standards. Last, the ECT paradigm was also sensitive enough to detect single locus differences, laying the groundwork for mutation screens for new neuroexcitability models.

Anticonvulsant and sodium channel blocking activity of higher doses of clenbuterol.

Clenbuterol, a lipophilic beta2-adrenoceptor agonist, was investigated in various seizure models of experimental epilepsy. In the maximal electroshock seizure threshold test, clenbuterol (> or =4 mg/kg i.p.) increased the electroconvulsive threshold for tonic seizures in mice. In the traditional maximal electroshock seizure (MES) test in mice, ED50 values of 11 mg/kg i.p. or s.c. were determined. In both models, the beta2-receptor antagonist ICI 118.551 did not antagonize the anticonvulsant activity of clenbuterol. Combinations of clenbuterol with standard antiepileptics revealed additive anticonvulsant effects. Repeated administration of clenbuterol (5 mg/kg s.c., twice daily for 14 days) to mice did not significantly influence its anticonvulsant potency or the effectiveness of phenobarbital in the MES test. In various chemically-induced seizure tests with tonic convulsions, clenbuterol inhibited or tended to suppress the tonic phase. However, this drug was not effective in preventing clonic seizures in the pentylenetetrazol (85 mg/kg s.c.) seizure threshold test. In the rotarod ataxia test (mice), a minimal "neurotoxic" dose (TD50) of 41 mg/kg i.p. was determined. In unrestrained rats with chronically implanted electrodes in the dorsal hippocampus, clenbuterol (2 mg/kg and 4 mg/kg i.p.) significantly reduced the duration of electrically evoked hippocampal afterdischarges. In amygdala-kindled rats, clenbuterol (5 mg/kg and 10 mg/kg i.p.) reduced the seizure severity to stage 3. Additional studies indicated that clenbuterol (6 mg/kg i.p.) increased the heart rate and decreased the blood pressure, but this drug did not alter the plasma level of the two tested antiepileptics phenobarbital and carbamazepine. Furthermore, in whole-cell voltage-clamp experiments on cultured neonatal rat cardiomyocytes, clenbuterol (1-100 microM) depressed the fast sodium current in a concentration- and frequency-dependent manner. In conclusion, the anticonvulsant effects of higher doses of clenbuterol against generalized tonic-clonic and complex partial seizures seem to be related to the inhibition of voltage-dependent sodium channels and not to the modulation of beta-adrenoceptors.

Low propensity of conventional antiepileptic drugs for interaction with felbamate against maximal electroshock-induced seizures in mice

The objective of this study was to evaluate an interaction of the novel antiepileptic drug felbamate (2-phenyl-1,3-propanediol dicarbamate) with conventional antiepileptic drugs against maximal electroshock-induced convulsions in mice. Electroconvulsions were produced by means of an alternating current (ear-clip electrodes, 0.2-s stimulus duration, tonic hindlimb extension taken as the endpoint). Adverse effects were evaluated in the chimney test (motor performance) and passive avoidance task (long-term memory). Brain and plasma levels of antiepileptic drugs were measured by immunofluorescence. Felbamate (7.5-30 mg/kg) significantly increased the electroconvulsive threshold. Felbamate at the subprotective dose of 5 mg/kg did not affect the anticonvulsive action of antiepileptics studied. On the other hand, this drug used at the lowest protective dose of 7.5mg/kg remained without effect upon the activity of valproate, carbamazepine or phenobarbital, but significantly potentiated the protective potential of diphenylhydantoin. No adverse effects were observed with combinations of felbamate with these antiepileptics. Neither brain nor free plasma levels of antiepileptic drugs were changed by felbamate. The results indicate that the combination of felbamate with conventional antiepileptic drugs is not promising from the experimental point of view.

Interaction of topiramate with conventional antiepileptic drugs in mice

Topiramate [2,3:4,5-bis- O-(1-methyl-ethylidene-)-β- d-fructopyranose sulfamate], administered intraperitoneally (i.p.) up to 5 mg/kg, did not influence the threshold for electroconvulsions. In doses of 10–30 mg/kg, topiramate significantly raised the threshold. This novel antiepileptic drug, in subprotective doses, enhanced the protective activity of i.p. given valproate, carbamazepine, dihenylhydantoin and phenobarbital against maximal electroshock-induced convulsions in mice. The potentiation induced by topiramate (2.5–5 mg/kg) was most profound for carbamazepine and phenobarbital. The anticonvulsive activity of valproate and diphenylhydantoin was potentiated by topiramate only at 5 mg/kg. Topiramate (5 mg/kg) combined with valproate, phenobarbital and diphenylhydantoin did not alter their free plasma levels but its combination with carbamazepine resulted in an increased free plasma level of this antiepileptic drug. Treatment with topiramate (5 mg/kg) alone or in combination with the studied antiepileptics (providing 50% protection against maximal electroshock) resulted in no adverse effects, as measured in the chimney test (motor coordination) or passive avoidance task (long-term memory). In contrast, valproate administered alone at its ED 50 against maximal electroshock impaired motor coordination. It is noteworthy that valproate and carbamazepine at their respective ED 50 values of 248 and 11.2 mg/kg disturbed long-term memory. The results provide an experimental basis for rational polytherapy.

The Chinese Herbal Medicine Chai-Hu-Long-Ku-Mu-Li-Tan (TW-001) Exerts Anticonvulsant Effects Against Different Experimental Models of Seizure in Rats

We evaluated the anticonvulsant effect of Chai-Hu-Long-Ku-Mu-Li-Tan (TW-001), a Chinese herbal medicine, and its mechanisms in several standard rodent models of generalized seizure. TW-001 (4 g/kg, p.o.) significantly increased the threshold for tonic electroconvulsions and the threshold for tonic seizures in response to i.v. infusion of pentylenetetrazole (PTZ). In the s.c. PTZ seizure test, both the incidence and severity of seizures were decreased by TW-001. TW-001 (1-10 mg/ml) did not alter resting membrane potential or input resistance of the hippocampal CA1 neurons, but elicited a reversible suppression of stimulus-triggered epileptiform activity in area CA1 and spontaneously occuring epileptiform burst discharges in area CA3 elicited by picrotoxin. Both field excitatory postsynaptic potentials and population spikes were reversibly depressed by TW-001 (0.5- 15 mg/ml) in a concentration-dependent manner. The sensitivity of postsynaptic neurons to a glutamate-receptor agonist, α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid or N-methyl-D-aspartate, was not altered by TW-001 (10 mg/ml). However, TW-001 (5 mg/ml) clearly increased the magnitude of paired-pulse facilitation. TW-001 (5-10 mg/ml) reversibly limited the repetitive firing and reduced the maximal rate of rise of action potentials elicited by injection of depolarizing current pulses (0.4 nA, 200 ms) into the pyramidal cells. TW-001 (1-10 mg/ml) exerted a concentration-dependent reduction of the tetrodotoxin-sensitive sodium currents and high voltage-activated calcium currents. These results suggest that TW-001 is an interesting new anticonvulsant agent that exerts its anticonvulsant activity through inhibition of sodium and calcium channels, stabilizing neuronal membrane excitability and inhibiting glutamate release.

INFLUENCE OF 3-PPP, A SIGMA RECEPTOR LIGAND, ON THE ANTICONVULSIVE ACTION OF CONVENTIONAL ANTIEPILEPTIC DRUGS

(+)-3-(3-Hydroxyphenyl)- N -(1-propyl)-piperidine (3-PPP; a sigma receptor ligand), administered at 30 mg kg−1, 30 min before the test, significantly decreased the electroconvulsive threshold in mice, being ineffective in lower doses. 3-PPP (20 mg kg−1) diminished the protective activity of diphenylhydantoin, phenobarbital and valproate, but not that of carbamazepine against maximal electroshock. The effect of 3-PPP upon the electroconvulsive threshold and the 3-PPP-induced inhibition of the protective action of antiepileptics was reversed by haloperidol (0.5 mg kg−1). Moreover, 3-PPP did not alter the total and free plasma levels of antiepileptic drugs, so a pharmacokinetic interaction is not probable. The combined treatment of 3-PPP with antiepileptic drugs, providing a 50% protection against maximal electroshock, did not affect motor performance in mice, although resulted in significant long-term memory deficits. Our data indicate that sigma receptor-mediated events may play some role in seizure processes in the central nervous system and can modulate the protective activity of some conventional antiepileptic drugs.pc 1999 Academic Press@p$hr

NMDA- but not kainate-mediated events reduce efficacy of some antiepileptic drugs against generalized tonic-clonic seizures in mice.

The aim of this study was to evaluate the efficacy of conventional antiepileptic drugs (AEDs) against the generalized tonic-clonic seizures in mice subjected to the subconvulsive doses of N-methyl-D-aspartate (NMDA) or kainate. Mice were given NMDA and kainate in the doses of 50.0 and 9.0 mg/kg i.p., respectively [i.e., equal to 75% of their CD16 values (convulsive dose in 16% of the animals studied)]. Subsequently the anticonvulsive potential of conventional AEDs against the maximal electroshock-induced seizures was estimated. Where necessary, the plasma levels of AEDs were assessed. NMDA or kainate application did not affect the electroconvulsive threshold. NMDA, but not kainate, diminished the antiepileptic activity of diazepam (DZP) and carbamazepine (CBZ), increasing their 50% effective doses (ED50s) from 14.1 and 8.6 to 19.0 and 12.1 mg/kg i.p., respectively. Neither NMDA nor kainate affected the ED50 for valproate (VPA), phenobarbital (PB), or diphenylhydantoin (DPH) against electroconvulsions. NMDA-evoked effects were reversed with the use of the NMDA antagonist, D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116) and were not accompanied by the alterations in the free plasma levels of AEDs. The NMDA-mediated events, but not kainate-related ones, seem to be involved in the protective action of DZP and CBZ against maximal electroshock-induced seizures. Moreover, it might be concluded that when subthreshold activation of NMDA receptors adds to other epileptogenic factors, DZP and CBZ are less efficacious. Presented data indicate that in such situations, adding the NMDA receptor antagonist (at very low doses) to the AED may yield beneficial therapeutic effects.