The non-competitive AMPA/kainate receptor antagonist, GYKI 52466, potentiates the anticonvulsant activity of conventional antiepileptics

In vivo modulation of the behavioral effects of nicotine by the coumarins xanthotoxin, bergapten, and umbelliferone

RATIONALE

Nicotine, a dominant alkaloid found in tobacco, is responsible for physical dependence, as well as addiction to cigarette smoking; consequently, smoking cessation is a very difficult process. Hepatic cytochrome P-450 2A6 (CYP2A6) is involved in the 70-80 % of the initial metabolism of nicotine and its co-metabolites. As this metabolism is slowed by inhibitors of CYP2A6, this kind of enzymatic inhibition has been proposed as a novel target for smoking cessation.

OBJECTIVES

Nicotine administered alone improved memory acquisition and consolidation as well as exerted antidepressive activity in animal models. These effects persist for 24 h. However, they are completely extinguished 48 h after administration.

METHODS

To investigate if the coumarins prolong the behavioral effects of nicotine, the forced swimming test (FST)-animal models of depression, and passive avoidance (PA) test-memory and learning paradigm were used.

RESULTS

This study revealed that three CYP2A6 inhibitors: two furanocoumarins, xanthotoxin (15 mg/kg) and bergapten (25 mg/kg), and the simple coumarin umbelliferone (25 mg/kg), prolonged the antidepressive and procognitive effects of nicotine.

CONCLUSIONS

These natural products may offer a new approach to the treatment of nicotinism as antidepressant and memory improvement actions are one of the main factors of nicotine dependence.

Correlations between the Memory-Related Behavior and the Level of Oxidative Stress Biomarkers in the Mice Brain, Provoked by an Acute Administration of CB Receptor Ligands

The endocannabinoid system, through cannabinoid (CB) receptors, is involved in memory-related responses, as well as in processes that may affect cognition, like oxidative stress processes. The purpose of the experiments was to investigate the impact of CB1 and CB2 receptor ligands on the long-term memory stages in male Swiss mice, using the passive avoidance (PA) test, as well as the influence of these compounds on the level of oxidative stress biomarkers in the mice brain. A single injection of a selective CB1 receptor antagonist, AM 251, improved long-term memory acquisition and consolidation in the PA test in mice, while a mixed CB1/CB2 receptor agonist WIN 55,212-2 impaired both stages of cognition. Additionally, JWH 133, a selective CB2 receptor agonist, and AM 630, a competitive CB2 receptor antagonist, significantly improved memory. Additionally, an acute administration of the highest used doses of JWH 133, WIN 55,212-2, and AM 630, but not AM 251, increased total antioxidant capacity (TAC) in the brain. In turn, the processes of lipids peroxidation, expressed as the concentration of malondialdehyde (MDA), were more advanced in case of AM 251. Thus, some changes in the PA performance may be connected with the level of oxidative stress in the brain.

Targeting α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors in epilepsy

INTRODUCTION

Despite epilepsies being between the oldest and most studied neurological diseases, new treatment remains an unmet need of scientific research due to the high percentage of refractory patients. Several studies have identified new suitable anti-seizure targets. Glutamate activation of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) have long ago been identified as suitable targets for the development of anti seizure drugs.

AREAS COVERED

Here, we describe: i) AMPARs' structure and their involvement and role during seizures and in epilepsy and ii) the efficacy of AMPAR antagonists in preclinical models of seizures and epilepsy.

EXPERT OPINION

The physiological and pathological role of AMPAR in the CNS and the development of AMPAR antagonists have recently gained attention considering their recent involvement in status epilepticus and the marketing of perampanel. The need for new anti-seizure drugs represents a major challenge in both preclinical and clinical epilepsy. The introduction into the market of perampanel for the treatment of epilepsy will shed new light on the real potential of AMPAR antagonism in clinical settings outside the limited world of clinical trials. While research will go on in this area, fundamental will be the post-marketing evaluation of perampanel efficacy and tolerability and a better definition of the role of this receptor in the epileptic brain.

Effects of imperatorin on nicotine-induced anxiety- and memory-related responses and oxidative stress in mice

The purpose of the reported experiments was to examine the effects of imperatorin [9-[(3-methylbut-2-en-1-yl)oxy]-7H-furo[3,2-g]chromen-7-one] on anxiety and memory-related responses induced by nicotine in mice and their relation to the level of nicotine-induced oxidative stress in brain as well as in the hippocampus and the prefrontal cortex. Male Swiss mice were tested for anxiety in the elevated plus maze test (EPM), and for cognition using passive avoidance (PA) procedures. Imperatorin, purified by high-speed counter-current chromatography from methanol extract of fruits of Angelica officinalis, acutely administered at the doses of 10 and 20mg/kg impaired the anxiogenic effect of nicotine (0.1mg/kg, s.c.). Furthermore, acute injections of subthreshold dose of imperatorin (1mg/kg, i.p.) improved processes of memory acquisition when co-administered with nicotine used at non-active dose of 0.05 mg/kg, s.c. Additionally, repeated administration of imperatorin (1mg/kg, i.p., twice daily, for 6 days) improved different stages of memory processes (both acquisition and consolidation) when injected in combination with non-active dose of nicotine (0.05 mg/kg, s.c.) in the PA task. Oxidative stress was assessed by determination of antioxidant enzymes (glutathione peroxidases (GPx), superoxide dismutase (SOD), glutathione reductase (GR)) activities as well as of malondialdehyde (MDA) concentration in the whole brain, the hippocampus and the prefrontal cortex after repeated administration of imperatorin (1mg/kg, 6 days) and single nicotine injection (0.05 mg/kgs.c.) on the seventh day. The results of our research suggest strong behavioural interaction between imperatorin and nicotine at the level of anxiety- and cognitive-like processes. Furthermore, imperatorin inhibited nicotine-induced changes in examined indicators of oxidative stress, especially in the hippocampus and the cortex.

Pharmacological Preconditioning with GYKI 52466: A Prophylactic Approach to Neuroprotection

Some toxins and drugs can trigger lasting neuroprotective mechanisms that enable neurons to resist a subsequent severe insult. This “pharmacological preconditioning” has far-reaching implications for conditions in which blood flow to the brain is interrupted. We have previously shown that in vitro preconditioning with the AMPA receptor antagonist GYKI 52466 induces tolerance to kainic acid (KA) toxicity in hippocampus. This effect persists well after washout of the drug and may be mediated via inverse agonism of G-protein coupled receptors (GPCRs). Given the amplifying nature of metabotropic modulation, we hypothesized that GYKI 52466 may be effective in reducing seizure severity at doses well below those normally associated with adverse side effects. Here we report that pharmacological preconditioning with low-dose GYKI imparts a significant protection against KA-induced seizures in vivo. GYKI (3 mg/kg, s.c.), 90–180 min prior to high-dose KA, markedly reduced seizure scores, virtually abolished all level 3 and level 4 seizures, and completely suppressed KA-induced hippocampal c-FOS expression. In addition, preconditioned animals exhibited significant reductions in high frequency/high amplitude spiking and ECoG power in the delta, theta, alpha, and beta bands during KA. Adverse behaviors often associated with higher doses of GYKI were not evident during preconditioning. The fact that GYKI is effective at doses well-below, and at pre-administration intervals well-beyond previous studies, suggests that a classical blockade of ionotropic AMPA receptors does not underlie anticonvulsant effects. Low-dose GYKI preconditioning may represent a novel, prophylactic strategy for neuroprotection in a field almost completely devoid of effective pharmaceuticals.

Antiepileptic drug-induced neuronal cell death in the immature brain: effects of carbamazepine, topiramate, and levetiracetam as monotherapy versus polytherapy

The aim of this study was to test the potential neurotoxicity of three antiepileptic drugs (AEDs), carbamazepine (5H-dibenzepine-5-carboxamide), topiramate [2,3:4,5-bis-O-(1-methylethylidene)-beta-d-fructopyranose sulfamate], and levetiracetam [2-(2-oxopyrrolidin-1-yl)butanamide], in the developing rat brain, when given alone or in combinations. The extent of cell death induced by AEDs was measured in several brain regions of rat pups (postnatal day 8) by terminal deoxynucleotidyl transferase dUTP nick-end labeling assay 24 h after drug treatment. Carbamazepine alone did not increase neurodegeneration when given in doses up to 50 mg/kg, but it induced significant cell death at 100 mg/kg. When combined with phenytoin, carbamazepine, 50 but not 25 mg/kg, significantly exacerbated phenytoin-induced cell death. Although topiramate (20-80 mg/kg) alone caused no neurodegeneration, all doses exacerbated phenytoin-induced neurodegeneration. Levetiracetam (250-1000 mg/kg) alone did not induce cell death, nor did it exacerbate phenytoin-induced neurodegeneration. Of the combinations examined, only that of levetiracetam (250 mg/kg) with carbamazepine (50 mg/kg) did not induce neurodegeneration. Our data underscore the importance of evaluating the safety of combinations of AEDs given during development and not merely extrapolating from the effects of exposure to single drugs. Although carbamazepine and topiramate alone did not induce neuronal death, both drugs exacerbated phenytoin-induced cell death. In contrast, because cotreatment with levetiracetam and carbamazepine did not enhance cell death in the developing brain, it may be possible to avoid proapoptotic effects, even in polytherapy, by choosing appropriate drugs. The latter drugs, as monotherapy or in combination, may be promising candidates for the treatment of women during pregnancy and for preterm and neonatal infants.

Synergistic combinations of anticonvulsant agents: what is the evidence from animal experiments?

PURPOSE

Combination therapy is often used in the treatment of seizures refractory to monotherapy. At the same time, the pharmacodynamic mechanisms that determine the combined efficacy of antiepileptic drugs (AEDs) are unknown, and this prevents a rational use of these drug combinations. We critically evaluate the existing evidence for pharmacodynamic synergism between AEDs from preclinical studies in animal models of epilepsy to identify useful combinations of mechanisms and to determine whether study outcome depends on the various research methods that are in use.

METHODS

Published articles were included if the studies were placebo-controlled, in vivo, or ex vivo animal studies investigating marketed or experimental AEDs. The animal models that were used in these studies, the primary molecular targets of the tested drugs, and the methods of interpretation were recorded. The potential association of these factors with the study outcome (synergism: yes or no) was assessed through logistic regression analysis.

RESULTS

In total, 107 studies were identified, in which 536 interaction experiments were conducted. In 54% of these experiments, the possibility of a pharmacokinetic interaction was not investigated. The majority of studies were conducted in the maximal electroshock model, and other established models were the pentylenetetrazole model, amygdala kindling, and the DBA/2 model. By far the most widely used method for interpretation of the results was evaluation of the effect of a threshold dose of one agent on the median effective dose (ED50) of another agent. Experiments relying on this method found synergism significantly more often compared with experiments relying on other methods (p<0.001). Furthermore, experiments including antagonists of the AMPA receptor were more likely to find synergism in comparison with all other experiments (p<0.001).

CONCLUSIONS

Intensive preclinical research into the effects of AED combinations has not led to an understanding of the pharmacodynamic properties of AED combinations. Specifically, the majority of the preclinical studies are not adequately designed to distinguish between additive, synergistic, and antagonistic interactions. Quantitative pharmacokinetic-pharmacodynamic studies of selectively acting AEDs in a battery of animal models are necessary for the development of truly synergistic drug combinations.

Talampanel

Talampanel is a novel anticonvulsant that acts as an allosteric inhibitor of the AMPA receptor. Talampanel has a broad spectrum of action in animal models of epilepsy and neuroprotection. Clinical experience to date has been able to show effectiveness in reduction of seizures in patients with refractory partial seizures.

Effect of topiramate on the anticonvulsant activity of conventional antiepileptic drugs in two models of experimental epilepsy

PURPOSE

The objective of this study was to evaluate the interaction of the novel antiepileptic drug (AED), topiramate (TPM), with conventional AEDs against amygdala-kindled seizures in rats and pentylenetetrazol-induced convulsions in mice.

METHODS

Experiments were performed on mice and fully kindled rats. In pentylenetetrazol test, the chemoconvulsant was used at its CD97 dose of 105 mg/kg, producing clonic seizures in 97% of mice. Adverse effects were evaluated with the chimney test and passive avoidance task. Plasma levels of AEDs were measured with immunofluorescence.

RESULTS

TPM at 20 mg/kg exerted a significant anticonvulsant effect as regards seizure and afterdischarge durations in amygdala-kindled seizures in rats, being ineffective at lower doses. Coadministration of TPM (10 mg/kg) with valproate (VPA; at a subtherapeutic dose of 50 mg/kg) resulted in essential reductions of seizure and afterdischarge durations. TPM (10 mg/kg) combined with carbamazepine (CBZ; at a subtherapeutic dose of 15 mg/kg) significantly increased afterdischarge threshold, simultaneously decreasing the remaining seizure parameters (duration or severity of seizures and afterdischarge duration). TPM (10 mg/kg) given with phenobarbital (PB; 15 mg/kg) markedly shortened seizure severity and seizure and afterdischarge durations. Combinations of TPM with diphenylhydantoin (PHT) were ineffective against kindled seizures in rats. TPM combined with VPA and PB did not alter their plasma levels, but its combination with CBZ resulted in an increased free plasma CBZ concentration. TPM (10 and 20 mg/kg) alone and its combinations with conventional AEDs affected neither motor coordination nor long-term memory, evaluated in the chimney and passive avoidance tests, respectively, in rats. In pentylenetetrazol-evoked convulsions in mice, TPM (175 and 200 mg/kg) showed anticonvulsant effects per se. Moreover, TPM (at its subtherapeutic dose of 150 mg/kg), significantly potentiated the anticonvulsant action of ethosuximide (ESM), but not that of VPA, PB, or clonazepam (CZP) against pentylenetetrazol-induced seizures. Either TPM alone (150 mg/kg) or its combination with ESM did not result in significant undesired effects.

CONCLUSIONS

The experimental data indicate that except for PHT, the combinations of TPM with conventional AEDs are beneficial against amygdala-kindled seizures in rats. In the pentylenetetrazol test, this novel AED potentiated only the protection offered by ESM.

Polytherapy in epilepsy: the experimental evidence

Monotherapy is recommended preferentially among newly diagnosed epileptic patients. In monotherapy-resistant patients polytherapy may be necessary. Two antiepileptic drugs may produce antagonistic, additive, and supra-additive (synergistic) anticonvulsant effects. The drug combination providing the supra-additive effect seems of clinical significance. However, when the supra-additive anticonvulsant efficacy is also associated by a distinct increase in toxicity, the protective index may be not affected or even lowered. Synergistic interactions have been shown for the combinations of valproate-phenytoin/ethosuximide, topiramate-carbamazepine/phenobarbital and felbamate-all major conventional antiepileptics. In contrast, the protective action of conventional antiepileptics has not been affected by felbamate at subprotective doses against maximal electroshock in mice. This is indicative that synergism is evident at only some drug ratios. Potential antiepileptic drugs, excitatory amino acid antagonists and calcium channel inhibitors, generally enhanced the protection offered by antiepileptic drugs. The experimental data may be helpful for predicting which drug combinations may prove effective in epileptic patients.

Protective effect of the antiepileptic drug candidate talampanel against AMPA-induced striatal neurotoxicity in neonatal rats

2,3-Benzodiazepines represent a family of specific, noncompetitive AMPA receptor antagonists with anticonvulsant and neuroprotective properties. In this study, the antiexcitotoxic potency of the clinical antiepileptic drug candidate, talampanel (4 x 2 mg/kg), and that of two related 2,3-benzodiazepines, 5-(4-aminophenyl)-8-methyl-9H-1,3-dioxolo[4,5-h][2,3]-benzodiazepine (GYKI 52466) (4 x 10 mg/kg) and GYKI 53784 (4 x 2 mg/kg), was investigated in 7-day-old rats. The AMPA antagonists were applied in four consecutive i.p. injections at 1-h intervals, the first dosage was given shortly after the intrastriatal injection of (S)-alpha-amino-3-hydroxy-5,7-methylisoxazole-4-propionic acid (AMPA) (2.5 nmol). All tested compounds protected animals from brain damage induced by AMPA as assessed 5 days later by using a tissue volume determination method based on computer-aided serial section reconstruction. GYKI 53784 (56.1 +/- 5.0% protection) and talampanel (42.5 +/- 5.3% protection) were more potent neuroprotective agents than GYKI 52466 (21.8 +/- 2.8% protection). Furthermore, the three compounds attenuated the unilateral AMPA injection-induced turning behavior and seizure-like events.Our present findings are in agreement with those of other investigators who found talampanel neuroprotective in various in vivo experimental models. These data indicate that besides being a promising antiepileptic drug candidate talampanel may have a value in the pharmacotherapy of acute and chronic neurodegenerative diseases, including perinatal ischemia/hypoxia-induced brain injuries, as well.

Interaction of topiramate with conventional antiepileptic drugs in mice

Topiramate [2,3:4, 5-bis-O-(1-methyl-ethylidene-)-beta-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.

Protection by conventional and new antiepileptic drugs against lindane-induced seizures and lethal effects in mice

Toxic effects caused by the organochlorine xenobiotic lindane may result from too excessive antiscabicidal treatment and in cases of accidental or intentional poisoning. Predominant symptoms of lindane intoxication concern the central nervous system, e.g. different manifestation of hyperexcitability and epileptiform activity. The inhibition of GABA-ergic neurotransmission seems to be responsible for the convulsant activity of lindane. This study was intended to compare the protective activity of conventional and new antiepileptic drugs against convulsions and lethal effects elicited by lindane administration in mice. Diazepam, clonazepam and phenobarbital protected against full seizure pattern and lethal effects evoked by lindane. Carbamazepine, phenytoin, gabapentin, felbamate and lamotrigine inhibited only lindane-induced tonic convulsions and mortality. It may be concluded that apart from benzodiazepines, phenobarbital and, to a lesser extent, carbamazepine, phenytoin, gabapentin, felbamate and lamotrigine could be used in lindane poisoning. Vigabatrin proved completely ineffective against seizures or lethal effects elicited by lindane.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Influence of LY 300164, an antagonist of AMPA/kainate receptors, on the anticonvulsant activity of clonazepam

LY 300164 [7-acetyl-5-(4-aminophenyl)-8,9-dihydro-8-methyl-7H-1,3-dioxolo(4, 5H)-2,3-benzodiazepine], an antagonist of AMPA/kainate receptors, at 5 mg/kg exerted a significant anticonvulsant effect, as regards seizure and afterdischarge durations in amygdala-kindled seizures in rats. At lower doses, LY 300164 did not exert anticonvulsant activity. Clonazepam alone (0.003-0.1 mg/kg) significantly diminished seizure severity, seizure and afterdischarge durations. Coadministration of LY 300164 (2 mg/kg) with clonazepam (0.001 mg/kg) resulted in the significant anticonvulsant activity. Seizure severity score, seizure and afterdischarge durations were reduced from 5 to 4, from 32.6 s to 12.3 s, and 42.7 s to 23.2 s. LY 300164 (2 mg/kg), clonazepam (0.001-0.1 mg/kg) and the combination of clonazepam (0.001 mg/kg) with LY 300164 (2 mg/kg) did not affect long-term memory evaluated in the passive avoidance task in rats. LY 300164 (at the subprotective dose of 2 mg/kg) significantly potentiated the anticonvulsant action of clonazepam against maximal electroshock but not against pentylenetetrazol-induced convulsions in mice. The results indicate that blockade of glutamate-mediated events at AMPA/kainate receptors may differently affect the protection offered by clonazepam, which seems dependent upon the model of experimental seizures.

Effect of a glutamate receptor antagonist (GYKI 52466) on 4-aminopyridine-induced seizure activity developed in rat cortical slices

In the present experiments we have tested the effect of the noncompetitive AMPA antagonist GYKI 52466 (20-80 microM) on spontaneous epileptic discharges developed as the consequence of 4-aminopyridine application in neocortex slices of adult rats. Parallel to the changes of spontaneous activity, the field potentials, evoked by electrical stimulation of the corpus callosum, were also analyzed. Glass microcapillary extracellular recording electrode was positioned in the third layer of the somatosensory cortex slice, while the stimulating electrode was placed at the border of the white and gray matter. 4-aminopyridine and GYKI 52466 were bath-applied. The application of 40 microM GYKI 52466 caused about 40% decrease in the frequency and the amplitude of spontaneous seizures as well as the duration of each discharges developed in 4-amino-pyridine. Pre-incubation with the AMPA antagonist effectively inhibited both the development of seizure activity and the maintenance of the discharges. GYKI 52466 also decreased the duration and amplitude of field responses evoked by stimulation of the corpus callosum. This inhibitory effect was dose-dependent. Our data in the in vitro cortex slice epilepsy model suggest that the non-competitive AMPA antagonist GYKI 52466 is a potent anticonvulsant and neuroprotective compound because it reduced the fully developed epileptic discharges or prevented their development.

LY 300164, a novel antagonist of AMPA/kainate receptors, potentiates the anticonvulsive activity of antiepileptic drugs

LY 300164 [7-acetyl-5-(4-aminophenyl)-8,9-dihydro-8-methyl-7H-1,3-dioxolo(4, 5H)-2,3-benzodiazepine], administered intraperitoneally up to 2 mg/kg, did not influence the threshold for electroconvulsions. In doses of 2.5-4 mg/kg, LY 300164 significantly raised the threshold. In subprotective doses against electroconvulsions, this excitatory amino acid receptor antagonist enhanced the protective activity of intraperitoneally given valproate, carbamazepine and diphenylhydantoin against maximal electroshock-induced convulsions in mice. The anticonvulsive action of phenobarbital was potentiated by LY 300164 only at 2 mg/kg. The non-N-methyl-D-aspartate receptor antagonist did not affect the plasma levels of the antiepileptic drugs, so a pharmacokinetic interaction is not probable. Combined treatment with LY 300164 (2 mg/kg) and the antiepileptics studied (providing 50% protection against maximal electroshock) did not impair the motor performance of mice, evaluated in the chimney test. Valproate, at its ED50 of 280 mg/kg against maximal electroshock, produced motor impairment. As shown in the passive avoidance task, combination of LY 300164 (2 mg/kg) with valproate or diphenylhydantoin resulted in impairment of long-term memory. Alone among the antiepileptics, valproate (280 mg/kg) and phenobarbital (28.5 mg/kg) disturbed long-term memory. The results suggest that blockade of glutamate-mediated events via non-NMDA receptors leads to enhancement of the anticonvulsive activity of conventional antiepileptics. Some combinations of LY 300164 with antiepileptic drugs were superior to these antiepileptics alone in terms of their lack of adverse effects.

A potential anti-asthmatic drug, CR 2039, enhances the anticonvulsive activity of some antiepileptic drugs against pentetrazol in mice

CR 2039 (4-(1H-tetrazol-5-yl)-N-[4-(1H-tetrazol-5-yl]phenylbenzam ide), in doses of 10, 20, and 100 mg/kg i.p., did not modify the seizure pattern observed after subcutaneous pentetrazol, administered at its CD97 of 90 mg/kg for the clonic phase. However, when combined with antiepileptic drugs, this phenylbenzamide derivative (20 mg/kg) converted the subprotective doses of ethosuximide (100 mg/kg) or valproate (100 mg/kg) against the clonic phase into anticonvulsive ones. The protection observed was comparable to that noted after doubling the doses of these antiepileptics. Also, a combination of valproate (100 mg/kg) with CR 2039 (10 mg/kg) resulted in a clear-cut protection against clonic seizures induced by pentetrazol. The protective efficacy of clonazepam was not affected by the phenylbenzamide derivative up to 40 mg/kg. The potentiation of the anticonvulsive activity of ethosuximide or valproate was not accompanied by increased adverse effects, evaluated in the chimney test (motor coordination) and passive avoidance task (long-term memory). Finally, CR 2039 (20 mg/kg) did not alter the plasma levels of the antiepileptic drugs studied, which speaks against a pharmacokinetic mechanism in the observed results. In conclusion, CR 2039 seems devoid of a hazardous influence of the anti-asthmatic drug, aminophylline, on the anticonvulsive effects of conventional antiepileptics.

[31P]/[1H] nuclear magnetic resonance study of mitigating effects of GYKI 52466 on kainate-induced metabolic impairment in perfused rat cerebrocortical slices

PURPOSE

Kainic acid (KA) has long been used in experimental animals to induce status epilepticus (SE). A mechanistic implication of this is the association between excitotoxicity and brain damage during or after SE. We evaluated KA-induced metabolic impairment and the potential mitigating effects of GYKI 52466 [1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine] in superfused rat cerebral cortical slices.

METHODS

Interleaved [31P]/[1H] magnetic resonance spectroscopy (MRS) was used to assess energy metabolism, intracellular pH (pHi), N-acetyl-L-aspartate (NAA) level, and lactate (Lac) formation before, during, and after a 56-min exposure to 4 mM KA in freshly oxygenated artificial cerebrospinal fluid (oxy-ACSF).

RESULTS

In the absence of GYKI 52466 and during the KA exposure, NAA, PCr, and ATP levels were decreased to 91.1 +/- 0.8, 62.4 +/- 3.9, and 59.1 +/- 4.3% of the control, respectively; Lac was increased to 118.2 +/- 2.1 %, and pH, was reduced from 7.27 +/- 0.02 to 7.13 +/- 0.02. During 4-h recovery with KA-free ACSF, pHi rapidly and Lac gradually recovered, NAA decreased further to 85.5 +/- 0.3%, and PCr and ATP showed little recovery. Removal of Mg2+ from ACSF during KA exposure caused a more profound Lac increase (to 147.1 +/- 4.0%) during KA exposure and a further NAA decrease (to 80.4 +/- 0.5%) during reperfusion, but did not exacerbate PCr, ATP, and pHi changes. Inclusion of 100 microM GYKI 52466 during KA exposure significantly improved energy metabolism: the PCr and ATP levels were above 76.6 +/- 2.1 and 82.0 +/- 2.9% of the control, respectively, during KA exposure and recovered to 101.4 +/- 2.4 and 95.0 +/- 2.4%, respectively, during reperfusion. NAA level remained at 99.8 +/- 0.6% during exposure and decreased only slightly at a later stage of reperfusion.

CONCLUSIONS

Our finding supports the notion that KA-induced SE causes metabolic disturbance and neuronal injury mainly by overexcitation through non-N-methyl-D-aspartate (NMDA) receptor functions.

GYKI 52466 [1-(4-aminophenyl)-4-methoxy-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride] and the anticonvulsive activity of conventional antiepileptics against pentetrazol in mice

Excitatory amino acids participate in the generation of seizure activity. Consequently, the effects of GYKI 52466 [1-(4-aminophenyl)-4-methoxy-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride], an antagonist of glutamate-mediated events, on the protective activity of conventional antiepileptic drugs against pentetrazol were studied. GYKI 52466 (up to 10 mg/kg, i.p.) did not affect the clonic phase of pentetrazol (injected s.c. at its CD97 of 90 mg/kg) convulsions. Only the antipentetrazol activity of valproate (100 mg/kg) was enhanced by GYKI 52466 (10 mg/kg)--the percentage of mice protected was significantly increased from 20 to 90%. The anticonvulsive activity of clonazepam (at 0.01), ethosuximide (at 50), and phenobarbital (at 2.5 mg/kg) was not modified by GYKI 52466 (up to 10 mg/kg). The combination of valproate (100 mg/kg) with GYKI 52466 (10 mg/kg) did not affect the performance of mice evaluated in the chimney test. However, this combination resulted in significant memory deficits, measured in the passive avoidance task. In no case did GYKI 52466 (10 mg/kg) affect either total or free plasma levels of antiepileptic drugs (as measured by immunofluorescence), so a pharmacokinetic interaction is not probable. Finally, the interaction of the non-NMDA receptor antagonist with antiepileptic drugs does not seem promising in the pentetrazol test, recognized as a model of human myoclonic epilepsy.

Anticonvulsant and adverse effects of MK-801, LY 235959, and GYKI 52466 in combination with Ca2+ channel inhibitors in mice

This study was designed to investigate the influence of the calcium (Ca2+) channel inhibitors nicardipine, nifedipine, and flunarizine on the protective action of MK-801, LY 235959 [N-methyl-D-aspartate (NMDA) receptor antagonists], and GYKI 52466 (a non-NMDA receptor antagonist) against electroconvulsions in mice. Unlike nicardipine (15 mg/kg) or flunarizine (10 mg/kg) nifedipine (7.5 and 15 mg/kg) potentiated the protective potency of MK-801 (0.05 mg/kg), as reflected by significant elevation of the convulsive threshold (a CS50 value of the current strength in mA producing tonic hind limb extension in 50% of the animals). The protective activity of LY 235959 and GYKI 52466 was reflected by their ED50 values in mg/kg, at which the drugs were expected to protect 50% of mice against maximal electroshock-induced tonic extension of the hind limbs. Nicardipine (3.75 15 mg/kg), nifedipine (0.94-15 mg/kg), and flunarizine (2.5-10 mg/kg) in a dose-dependent manner markedly potentiated the antiseizure efficacy of LY 235959. Flunarizine (5 and 10 mg/kg) was the only Ca2+ channel inhibitor to enhance the protective action of GYKI 52466 against electroconvulsions. Except with MK-801 + flunarizine (motor performance) or GYKI 52466 + flunarizine (long-term memory), combination of NMDA or non-NMDA receptor antagonists with Ca2+ channel inhibitors produced an impairment of motor performance (evaluated in the chimney test) and long-term memory acquisition (measured in the passive avoidance task) as compared with vehicle treatment.

Ca2+ channel blockade and the antielectroshock activity of NMDA receptor antagonists, CGP 40116 and CGP 43487, in mice

Nicardipine, nifedipine and flunarizine showed anticonvulsive activity (reflected by significant elevations of the seizure threshold for tonic hindlimb extension) in doses of 20, 20 and 15 mg/kg, respectively. In combination studies, CGP 40116 [D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid] or its methyl ester derivative (CGP 43487) was administered in a constant dose of 0.25 and 3.5 mg/kg, respectively. At these doses both competitive NMDA receptor antagonists were able to elevate significantly the convulsive threshold. Nicardipine, nifedipine, and flunarizine were administered at maximal doses (or lower) not affecting the convulsive threshold (15, 15 and 10 mg/kg, respectively). The protective activity of CGP 40116 and CGP 43487 was dose dependently potentiated by all three Ca2+ channel inhibitors. The combined treatment caused motor impairments (evaluated in the chimney test) and long-term memory deficits (measured in the passive avoidance task) similar to these produced by CGP 40116 or CGP 43487 alone. Our results indicate that nicardipine, nifedipine and flunarizine significantly potentiate the protective activity, but not the adverse effects, of CGP 40116 and CGP 43487 in mice.

Competitive NMDA-receptor antagonists, LY 235959 and LY 233053, enhance the protective efficacy of various antiepileptic drugs against maximal electroshock-induced seizures in mice

PURPOSE

The objective of this study was to evaluate an interaction of two competitive N-methyl-D-aspartate (NMDA)-receptor antagonists, LY 235959 l(-)-3R,4aS,6R,8aR-6-(phosphonomethyl)-decahydroiso-qu inoline-3-carboxylic acid; < or = 0.5 mg/kg] or LY 233053 cis-(+/-)-4-[(2H-tetrazol-5-yl) methyl]piperidine-2-carboxylic acid; < or = 5 mg/kg] with carbamazepine, diphenylhydantoin, phenobarbital, or valproate magnesium against maximal electroshock-induced convulsions in mice.

METHODS

Electroconvulsions were produced by means of an alternating current (ear-clip electrodes, 0.2-s stimulus duration, tonic hindlimb extension taken as the end point) delivered by a Hugo-Sachs stimulator (Type 221, reiburg, FRG). Adverse effects were evaluated in the chimney test (motor performance) and passive-avoidance ask (long-term memory). Plasma levels of antiepileptic rugs were measured by immunofluorescence.

RESULTS

Both LY 235959 and LY 233053 ( < or = 0.5 and 5 mg/kg, respectively) did not influence the electroconvulsive threshold but potentiated the anticonvulsant action of all antiepileptics studied. The combined treatment of LY 233053 (5 mg/kg) with carbamazepine, diphenylhydantoin, or phenobarbital (providing a 50% protection against maximal electroshock) resulted in the impairment of long-term memory. No adverse effects were observed with combinations of LY 235959 with these antiepileptics. The combined treatment of valproate with either LY 235959 or LY 233053 was superior to valproate alone, as regards motor impairment, but not the impairment of long-term memory. Neither NMDA-receptor antagonist elevated the total plasma levels of antiepileptic drugs studied.

CONCLUSIONS

It may be concluded that NMDA-receptor blockade leads to the enhanced anticonvulsive action of conventional antiepileptics against maximal electroshock-induced seizures. A pharmacokinetic interaction does not seem probable.

Interactions of excitatory amino acid antagonists with conventional antiepileptic drugs

Excitatory amino acid antagonists possess anticonvulsant properties in many experimental models of epilepsy and were shown to potentiate the protective activity of conventional antiepileptics against maximal electroshock-induced seizures in mice. Combined treatments of valproate with either D,L-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid or dizocilpine (NMDA antagonists), which provided a 50% protection against maximal electroshock, produced no side-effects, as measured in the chimney test (motor coordination) or passive avoidance task (long-term memory). Valproate alone at its ED50 against maximal electroshock, induced severe adverse effects. The NMDA antagonists, D-3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid, memantine, procyclidine, and trihexyphenidyl also potentiated the protective activity of conventional antiepileptics but these treatments were associated with considerable side-effects. The non-NMDA receptor antagonists, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline and 1-(amino-phenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine, also enhanced the anticonvulsive action of antiepileptic drugs against maximal electroshock, and these combinations generally resulted in no adverse effects. The potential clinical importance of some combinations of common antiepileptics with excitatory amino acid antagonists is postulated.

Influence of aminophylline and strychnine on the protective activity of excitatory amino acid antagonists against maximal electroshock-induced convulsions in mice

Aminophylline reversed the protective action of both, D-3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid (D-CPP-ene-a competitive NMDA antagonist) and valproate (used as a conventional antiepileptic drug for comparative purposes) against maximal electroshock-induced seizures. The respective ED50 values of aminophylline were 55.7 and 98.4 mg/kg i.p. However, aminophylline (up to 100 mg/kg i.p.) did not influence the protective efficacy of 1-(4-aminophenyl)-4-methyl-7,8-methyl- enedioxy-5H-2,3-benzodiazepine (GYKI 52466-a non-NMDA antagonist). Strychnine affected the protection provided by D-CPP-ene, GYKI 52466, and valproate against maximal electroshock-the ED50 values of strychnine for the reversal of the anticonvulsive effects of D-CPP-ene, GYKI 52466 or valproate were 0.082, 0.35 and 0.28 mg/kg s.c., respectively. An involvement of strychnine sensitive glycinergic receptor-mediated events in the mechanism of the anticonvulsive activity of excitatory amino acid antagonists and valproate may be postulated. The ineffectiveness of aminophylline to reduce the anticonvulsive effects of GYKI 52466 may distinguish a new class of antiepileptic drugs offering an advantage over conventional antiepileptics in patients with epilepsy, requiring aminophylline for pulmonary reasons.

Influence of a potential anti-asthmatic drug, CR 2039, upon the anticonvulsive activity of conventional antiepileptics against maximal electroshock-induced seizures in mice

CR 2039 [[4-(1H-tetrazol-5-yl)-N-(4-(1H-tetrazol-5-yl]phenylbenza m ide], in doses of 10, 50, and 100 mg/kg i.p., significantly elevated the threshold for electroconvulsions, increasing the CS50 (current strength 50% in mA) values from 6.3 to 7.2, 7.5, and 7.6 mA, respectively. When combined with carbamazepine, diphenylhydantoin, or valproate, CR 2039 (5 and 10 mg/kg) potentiated the anticonvulsive action of these antiepileptics against maximal electroshock-induced convulsions which was reflected by significant decreases in the respective ED50s (in mg/kg). The protective efficacy of phenobarbital was not affected by the phenylbenzamide derivative. The potentiation of the anticonvulsive activity of three antiepileptics was not accompanied by increased adverse effects, evaluated in the chimney test (motor coordination) and passive avoidance task (long-term memory). Finally, CR 2039 (10 mg/kg) did not alter the plasma levels of the antiepileptic drugs studied which speaks against a pharmacokinetic mechanism in the observed results. It is concluded that CR 2039 may prove a safer anti-asthmatic drug for the use in epileptic patients than aminophylline which, either acutely or chronically, considerably impaired the anticonvulsive activity of conventional antiepileptics.