Influence of MK-801 on the anticonvulsant activity of antiepileptics

Assessment of drug-drug interactions between moxonidine and antiepileptic drugs in the maximal electroshock seizure test in mice

Hypertension is a common comorbid condition with epilepsy, and drug interactions between antihypertensive and antiepileptic drugs (AEDs) are likely in patients. Experimental studies showed that centrally active imidazoline compounds belonging to antihypertensive drugs can affect seizure susceptibility. The purpose of this study was to assess the effect of moxonidine, an I₁ -imidazoline receptor agonist, on the anticonvulsant efficacy of numerous AEDs (carbamazepine, phenobarbital, valproate, phenytoin, oxcarbazepine, topiramate and lamotrigine) in the mouse model of maximal electroshock. Besides, the combinations of moxonidine and AEDs were investigated for adverse effects in the passive avoidance task and the chimney test. Drugs were administered intraperitoneally (ip). Moxonidine at doses of 1 and 2 mg/kg ip did not affect the convulsive threshold. Among tested AEDs, moxonidine (2 mg/kg) potentiated the protective effect of valproate against maximal electroshock. This interaction could be pharmacodynamic because the brain concentration of valproate was not significantly changed by moxonidine. The antihypertensive drug did not cause adverse effects when combined with AEDs. This study shows that moxonidine may have a neutral or positive effect on the anticonvulsant activity of AEDs in patients with epilepsy. The enhancement of the anticonvulsant action of valproate by moxonidine needs further investigations to elucidate potential mechanisms involved.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

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.

Interactions between non-barbiturate injectable anesthetics and conventional antiepileptic drugs in the maximal electroshock test in mice--an isobolographic analysis

The aim of this study was the isobolographic evaluation of interactions between three non-barbiturate intravenous anesthetics and conventional antiepileptic drugs in the maximal electroshock-induced seizures 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). Plasma levels of antiepileptic drugs were measured by immunofluorescence. Obtained results indicate that ketamine acts synergistically with valproate and carbamazepine. Also the combinations of propofol and valproate or phenobarbital led to synergistic interactions. An antagonism was found between etomidate and carbamazepine or phenobarbital. On the other hand, interactions between diphenylhydantoin and injectable anesthetics proved to be additive. The only exception was the combination of diphenylhydantoin and propofol (1:3). Pharmacokinetic phenomena do not seem to interfere with the observed interactions, since none of anesthetics influenced the free plasma concentrations of antiepileptic drugs. Referring to undesired effects, only propofol impaired long-term memory. Although propofol did not disturbed motor coordination, it enhanced motor impairment caused by carbamazepine and diphenylhydantoin. Results of the present study suggest that etomidate needs to be avoided in epileptic patients due to a possibility of negative interactions with some antiepileptic drugs and seizure precipitation.

Antiepileptic drugs and agents that inhibit voltage-gated sodium channels prevent NMDA antagonist neurotoxicity

N-methyl-D-aspartate (NMDA) glutamate receptor antagonists are used in clinical anesthesia and are being developed as therapeutic agents for preventing neurodegeneration in stroke, epilepsy, and brain trauma. However, the ability of these agents to produce neurotoxicity in adult rats and psychosis in adult humans compromises their clinical usefulness. In addition, an NMDA receptor hypofunction (NRHypo) state might play a role in neurodegenerative and psychotic disorders, like Alzheimer's disease, bipolar disorder and schizophrenia. Thus, developing pharmacological means of preventing these NRHypo-induced effects could have significant clinically relevant benefits. NRHypo neurotoxicity appears to be mediated by a complex disinhibition mechanism that results in the excessive stimulation of certain vulnerable neurons. Here we report our findings that five agents (phenytoin, carbamazepine, valproic acid, lamotrigine, and riluzole), thought to possess anticonvulsant activity because they inhibit voltage-gated sodium channels, prevent NRHypo neurotoxicity. The ability of tetrodotoxin, a highly selective inhibitor of voltage-gated sodium channels, to prevent the same neurotoxicity suggests that inhibition of this ion channel is the likely mechanism of action of these five agents. We also found that three other anticonvulsants (felbamate, gabapentin and ethosuximide), whose mechanism is less clear, also prevent NRHypo neurotoxicity, suggesting that inhibition of voltage-gated sodium channels is not the only mechanism via which anticonvulsants can act to prevent NRHypo neurotoxicity. Several of these agents have been found to be of clinical use in bipolar disorder. It would be of interest to determine whether these agents might have therapeutic benefits for conditions in which a NRHypo state may exist.

Coadministration of gabapentin or MK-801 with lamotrigine slows tolerance to its anticonvulsant effects on kindled seizures

The development of tolerance to therapeutic effects of antiepileptic drugs can be a problem in the treatment of epilepsy, bipolar disorder, and pain syndromes. In the present study, acute treatment with the new antiepileptic drug lamotrigine (LTG, 15 mg/kg) markedly suppressed seizure stage and seizure duration in amygdala-kindled rats; but this antiseizure effect was rapidly lost following 4-8 days of repeated treatment. When gabapentin (GBP, 20 mg/kg) was coadministered with LTG, the ability of LTG to suppress seizure stage, seizure duration, and after-discharge (AD) duration was markedly extended. In addition, GBP coadministration with LTG decreased the number of animals that developed LTG-related running fits (Stage 6 seizures) and lengthened the number of days required to develop running fits or complete tolerance. Neither acute nor repeated treatment with MK-801 (0.3 mg/kg), a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, had effects on kindled seizures. However, cotreatment with MK-801 markedly extended the anticonvulsant effects of LTG on the three seizure indices and reduced running fits. These data indicate that cotreatment with either GBP or MK-801 slows tolerance development to the anticonvulsant effects of LTG on kindled seizures. Therapeutic implications of the present study remain to be explored.

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.

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

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

AMPA/kainate-related mechanisms contribute to convulsant and proconvulsant effects of 3-nitropropionic acid

The role of the glutamatergic system in the convulsant and proconvulsant action of a mitochondrial toxin, 3-nitropropionic acid, was studied in mice. The occurrence of 3-nitropropionic acid-induced seizures was inhibited by the alpha-amino-2,3-dihydro-5-methyl-3-oxo-isoxazole-propionate (AMPA)/kainate receptor antagonists, 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione disodium (NBQX) and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine HCI (GYKI 52466), with ED50 of 14.1 (7.9-25.2) and 7.2 (5.3-9.6) mg/kg, respectively. The N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine (MK-801) and 3-(2-carboxypiperazine-4-yl)propenyl-1-phosphonic acid (CPPene), were ineffective. Moreover, 3-nitropropionic acid given in a subthreshold dose potently enhanced seizures generated by intracerebroventricular administration of AMPA and kainate, lowering their CD50 from 0.98 (0.83-1.17) and 0.73 (0.64-0.83) to 0.55 (0.45-0.66) (P<0.001) and 0.58 (0.51-0.65) (P<0.05) nmol, respectively. In contrast, NMDA action was not changed by 3-nitropropionic acid application. We conclude that AMPA/kainate-mediated events are involved in proconvulsive and convulsive effects of 3-nitropropionic acid.

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.

Pharmacology of glutamate receptor antagonists in the kindling model of epilepsy

It is widely accepted that excitatory amino acid transmitters such as glutamate are involved in the initiation of seizures and their propagation. Most attention has been directed to synapses using NMDA receptors, but more recent evidence indicates potential roles for ionotropic non-NMDA (AMPA/kainate) and metabotropic glutamate receptors as well. Based on the role of glutamate in the development and expression of seizures, antagonism of glutamate receptors has long been thought to provide a rational strategy in the search for new, effective anticonvulsant drugs. Furthermore, because glutamate receptor antagonists, particularly those acting on NMDA receptors, protect effectively in the induction of kindling, it was suggested that they may have utility in epilepsy prophylaxis, for example, after head trauma. However, first clinical trials with competitive and uncompetitive NMDA receptor antagonists in patients with partial (focal) seizures, showed that these drugs lack convincing anticonvulsant activity but induce severe neurotoxic adverse effects in doses which were well tolerated in healthy volunteers. Interestingly, the only animal model which predicted the unfavorable clinical activity of competitive NMDA antagonists in patients with chronic epilepsy was the kindling model of temporal lobe epilepsy, indicating that this model should be used in the search for more effective and less toxic glutamate receptor antagonists. In this review, results from a large series of experiments on different categories of glutamate receptor antagonists in fully kindled rats are summarized and discussed. NMDA antagonists, irrespective whether they are competitive, high- or low-affinity uncompetitive, glycine site or polyamine site antagonists, do not counteract focal seizure activity and only weakly, if at all, attenuate propagation to secondarily generalized seizures in this model, indicating that once kindling is established, NMDA receptors are not critical for the expression of fully kindled seizures. In contrast, ionotropic non-NMDA receptor antagonists exert potent anticonvulsant effects on both initiation and propagation of kindled seizures. This effect can be markedly potentiated by combination with low doses of NMDA antagonists, suggesting that an optimal treatment of focal and secondarily generalized seizures may require combined use of both non-NMDA and NMDA antagonists. Given the promising results obtained with novel AMPA/kainate antagonists and glycine/NMDA partial agonists in the kindling model, the hope for soon having potentially useful glutamate antagonists for use in epileptic patients is increasing.

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.

Chlormethiazole anticonvulsive efficacy diminished by N-methyl-D-aspartate but not kainate in mice

The aim of this study was to evaluate the effect of N-methyl-D-aspartate (NMDA) and kainate used at nonconvulsive doses upon protective efficacy of chlormethiazole against maximal electroshock-induced seizures. NMDA (50 mg/kg, i.p.) reduced the anticonvulsant potency of chlormethiazole increasing its ED50 value from 126.9 to 155.0 mg/kg. The effect of NMDA was completely reversed by the competitive NMDA receptor antagonist D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116) (0.06 mg/kg i.p.). Kainic acid (9 mg/kg i.p.) did not affect the anticonvulsive properties of chlormethiazole. Our results suggest that NMDA but not kainate receptor-mediated events participate in the anticonvulsant action of chlormethiazole.

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.

The effects of anticonvulsant drugs on NMDA-EPSP, AMPA-EPSP, and GABA-IPSP in the rat hippocampus

The effects of phenobarbital, phenytoin, and valproic acid on pharmacologically isolated NMDA-EPSP, AMPA-EPSP, and GABA-IPSPs were examined in rat hippocampal slices. Phenobarbital (0.05 mg/ml) had no effect on the NMDA-EPSP, but decreased the slope of the AMPA-EPSP by 13.4% and facilitated the GABA-IPSP slope by 77.12%. Phenytoin (0.02 mg/ml) had no effects on the NMDA-EPSP, AMPA-EPSP, or GABA-IPSP. Valproic acid (0.1 mg/ml) decreased the NMDA-EPSP slope by 14.3%, increased the GABA-IPSP slope by 54.34%, and had no effect on the AMPA-EPSP. These data suggest that the mechanisms of action of these anticonvulsant drugs may be via their actions on different neurotransmitter systems or ion channels.

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.

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

1-(4-Aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466), up to 5 mg/kg, did not influence the electroconvulsive threshold but potentiated the anticonvulsant activity of valproate, carbamazepine and diphenylhydantoin against maximal electroshock-induced convulsions in mice. No potentiation was observed in the case of phenobarbital. Moreover, this non-NMDA receptor antagonist did not influence the plasma levels of the antiepileptic drugs studied, so a pharmacokinetic interaction, in terms of total and free plasma levels, is not probable. The combined treatment of GYKI 52466 with either carbamazepine or diphenylhydantoin (providing a 50% protection against maximal electroshock) was devoid of significant side effects (motor and long-term memory impairment). Valproate applied at a dose equal to its ED50 caused serious worsening of motor coordination and long-term memory. It is noteworthy that the combined treatment of GYKI 52466 with valproate was superior to valproate alone, as regards adverse effects. The results suggest that concomitant administration of GYKI 52466 with some conventional antiepileptic drugs may offer a novel approach in the treatment of epilepsy.

Effect of the glycine/NMDA receptor partial agonist, D-cycloserine, on seizure threshold and some pharmacodynamic effects of MK-801 in mice

Acute treatment of mice with D-cycloserine (a high efficacy, partial agonist at strychnine-insensitive glycine receptors) resulted in dose- and time-dependent increases in the threshold for electrically induced tonic seizures. This anticonvulsant effect was observed at doses which did not induce motor impairment, as determined by the rotarod test. Despite the relatively high intrinsic efficacy of D-cycloserine at glycine receptors, this drug did not produce proconvulsant effects in mice at any of the doses (5-320 mg/kg) or time points examined. Prolonged treatment with D-cycloserine led to a reduction of its anticonvulsant effect. Similar to D-cycloserine, the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801 (dizocilpine), dose dependently increased the electroconvulsive threshold. Combined treatment with MK-801 and D-cycloserine led to significant anticonvulsant effects, but these effects were simply additive and not synergistic. In contrast to anticonvulsant activity, the motor impairing effect of MK-801 was markedly potentiated by D-cycloserine. The data substantiate that high efficacy glycine/NMDA receptor partial agonists such as D-cycloserine exert anticonvulsant activity at non-toxic doses. The finding that motor impairing but not anticonvulsant effects of MK-801 were potentiated by D-cycloserine suggests that different pharmacodynamic actions of NMDA receptor antagonists are differentially modulated by the glycine receptor, which could be related to the regional heterogeneity of the NMDA receptor complex in the brain.

The competitive NMDA antagonist, D-CPP-ene, potentiates the anticonvulsant activity of conventional antiepileptics against maximal electroshock-induced seizures in mice

D-CPP-ene[3-(2-carboxy-piperazine-4-yl)-1-propenyl-1-phosphonic acid; a competitive antagonist of N-methyl-D-aspartic acid] in a dose of 2 mg/kg (i.p.) significantly increased the threshold for electroconvulsions. When given in a dose half that affecting the electroconvulsive threshold, D-CPP-ene potentiated the anticonvulsant activity of carbamazepine, diazepam, diphenylhydantoin, phenobarbital and valproate against maximal electroshock (50 mA)-induced seizures in mice. However, this NMDA antagonist did not influence the plasma levels of the antiepileptic drugs studied, so a pharmacokinetic interaction, in terms of total plasma levels at least, is not probable. The chimney test and retention test in mice revealed that the combined treatment of D-CPP-ene at 1.0 mg/kg (i.p.) with either diazepam, diphenylhydantoin, phenobarbital or valproate (providing a 50% protection against maximal electroshock convulsions) resulted in motor impairment and caused impairment of long-term memory. On the other hand, a combination of D-CPP-ene and carbamazepine was devoid of adverse effects. It can be concluded that the potential utility of D-CPP-ene in combination with conventional antiepileptic drugs does not seem promising, except for carbamazepine.

Antiepileptics inhibit cortical N-methyl-D-aspartate-evoked [3H]norepinephrine efflux

The antiepileptic drugs phenytoin, valproic acid and phenobarbital were examined for their ability to inhibit N-methyl-D-aspartate (NMDA)-stimulated [3H]norepinephrine efflux from rat brain cortical slices. All three drugs inhibited efflux at varying concentrations. Valproic acid was the most potent and inhibited efflux at 0.01 mg/ml. Phenytoin and phenobarbital inhibited efflux at 0.1 mg/ml. These results indicate that some antiepileptic drugs are capable of inhibiting NMDA receptor function in the therapeutic range.

The NMDA antagonist procyclidine, but not ifenprodil, enhances the protective efficacy of common antiepileptics against maximal electroshock-induced seizures in mice

Procyclidine (up to 20 mg/kg i.p.) did not influence the electroconvulsive threshold per se, but when given in a dose of 10 mg/kg, it potentiated the protective activity of carbamazepine, diphenylhydantoin, phenobarbital and valproate, and in a dose of 20 mg/kg, that of diazepam against maximal electroshock-induced convulsions in mice. Ifenprodil increased the threshold for electroconvulsions when applied at 20 and 40 mg/kg (i.p.), but surprisingly, when combined with all antiepileptics tested, it did not influence their anticonvulsant actions. The chimney test in mice revealed, that application of procyclidine at 10 mg/kg together with phenobarbital and valproate, and procyclidine at 20 mg/kg with diazepam resulted in motor impairment. However, when procyclidine was applied at 10 mg/kg together with carbamazepine or diphenylhydantoin, no motor impairment was noted. The combined treatment of procyclidine (10 mg/kg) with carbamazepine, diphenylhydantoin, phenobarbital or valproate, as well as procyclidine (20 mg/kg) with diazepam caused significant worsening of long-term memory. Finally, procyclidine did not alter the total plasma levels of carbamazepine, diazepam, diphenylhydantoin, phenobarbital and valproate. It may be concluded that not all agents interfering with NMDA receptor complex-mediated events lead to the potentiation of the anticonvulsant activity of antiepileptic drugs.

Competitive NMDA receptor antagonists enhance the antielectroshock activity of various antiepileptics

CGP 37849 (1 mg/kg i.p.) enhanced the protective action of carbamazepine, diphenylhydantoin and phenobarbital against maximal electroshock-induced convulsions in mice. At 0.25 mg/kg CGP 37849 was inactive and at 0.5 mg/kg it potentiated the anticonvulsive activity of phenobarbital. CGP 39551 (5 mg/kg i.p.) reduced the ED50 values of diphenylhydantoin and phenobarbital, being without influence on carbamazepine. In the dose of 1.25 mg/kg, CGP 39551 potentiated the antielectroshock action of diphenylhydantoin and at 2.5 mg/kg that of phenobarbital. Neither NMDA receptor antagonist elevated the total plasma levels of antiepileptic drugs. Consequently, a pharmacokinetic interaction (in terms of total plasma levels at least) seems unlikely to be responsible for the observed potentiation of the antiepileptic drugs' activity. Combinations of CGP 37849 with either carbamazepine or phenobarbital resulted in a motor and memory impairment quantified by the chimney test and passive avoidance task, respectively. Moreover, combined treatment with phenobarbital and CGP 39551 caused a memory deficit. In contrast, diphenylhydantoin combined with either CGP 37849 or 39551 was devoid of adverse effects. It may be concluded that NMDA receptor blockade results in enhanced anticonvulsive action of common antiepileptics against maximal electroshock-induced seizures.

2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline enhances the protective activity of common antiepileptic drugs against maximal electroshock-induced seizures in mice

NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline), a novel and selective AMPA antagonist, was tested to evaluate its influence upon anticonvulsant activity of common antiepileptic drugs in mice. NBQX (10, 20, 40 mg/kg, i.p.) had no influence upon the threshold for electroconvulsions. NBQX (10 mg/kg) enhanced the activity of anticonvulsant drugs decreasing their ED50S against maximal electroshock from 321 to 190 mg/kg for valproate, from 19.5 to 14.5 mg/kg for carbamazepine, from 31.0 to 21.4 mg/kg for phenobarbital, from 17.8 to 9.5 mg/kg for diphenylhydantoin and from 19.5 to 10.5 mg/kg for diazepam. In addition, NBQX (10 mg/kg) failed to impair motor performance and long-term memory determined in the chimney test and passive avoidance task. The combinations of NBQX (10 mg/kg) and carbamazepine, diphenylhydantoin or phenobarbital resulted in no adverse effects. Diazepam (10.5 mg/kg) alone impaired the motor performance and long-term memory and so it did when combined with NBQX. Also retention of the passive avoidance task and motor performance were impaired by valproate alone or given together with NBQX. Finally, NBQX (10 mg/kg) did not affect the plasma level of any antiepileptic drug. It is concluded that non-NMDA glutamate receptor blockade results in the considerable enhancement of the efficacy of common antiepileptic drugs.

Effects of the competitive NMDA receptor antagonist, CGP 37849, on anticonvulsant activity and adverse effects of valproate in amygdala-kindled rats

The effects of combined treatment with low doses (1-5 mg/kg i.p.) of the competitive NMDA receptor antagonist, CGP 37849 (DL-[E]-2-amino-4-methyl-5-phosphono-3-pentenoic acid), and the antiepileptic drug, valproate, were studied in amygdala-kindled and non-kindled rats. CGP 37849, 5 mg/kg, did not exert anticonvulsant effects in fully kindled rats but increased the anticonvulsant potency of valproate, 80 mg/kg i.p. However, the increase in anticonvulsant activity was parallelled by a marked increase in motor impairment, resulting in a considerable reduction of the therapeutic index of the combined treatment compared to valproate alone. Furthermore, at doses of 2.5-5 mg/kg, CGP 37849 potentiated the adverse effects but not the anticonvulsant activity of 50 mg/kg valproate. In non-kindled rats, combined treatment with CGP 37849 and valproate induced significantly less marked adverse effects than in kindled rats. The data on combined treatment with CGP 37849 and valproate substantiate that kindling alters the susceptibility to manipulations of NMDA receptor-mediated events. Since kindling is thought to be a predictive model of complex partial seizures, these results suggest that competitive NMDA receptor antagonists such as CGP 37849 may be of limited usefulness against this seizure type in humans.

Competitive antagonists of NMDA receptors, CGP 37849 and CGP 39551, enhance the anticonvulsant activity of valproate against electroconvulsions in mice

Two novel N-methyl-D-aspartic acid (NMDA) competitive antagonists, CGP 37849 (1.25 and 2.5 mg/kg) and CGP 39551 (10 mg/kg), significantly raised the threshold for electroconvulsions in mice. CGP 37849 in doses of 0.125-1.0 mg/kg and CGP 39551 in doses of 0.625-5 mg/kg i.p. considerably potentiated the protective activity of magnesium valproate against maximal electroshock-induced convulsions. The anticonvulsant activity of sodium valproate was potentiated by CGP 37849 (1 mg/kg) to a similar degree, which suggests that magnesium is not involved in the observed interaction. Neither CGP agent influenced the plasma level of valproate, so a pharmacokinetic interaction, in terms of total plasma levels, is not probable. Furthermore, the performance of mice injected with magnesium valproate (91 mg/kg) and CGP 37849 (0.25 mg/kg), which provided 50% protection against maximal electroshock-induced convulsions, in the long-term memory test and chimney test did not differ significantly from that of the control animals. The combination of magnesium valproate and CGP 39551 had a neurotoxic potential comparable to that of valproate alone. The results suggest that a combined treatment of valproate and some competitive NMDA antagonists may be important from a clinical point of view.

Kindled rats are more sensitive than non-kindled rats to the behavioural effects of combined treatment with MK-801 and valproate

The effects of combined treatment with low doses (0.025-0.05 mg/kg i.p.) of the non-competitive NMDA receptor antagonist, MK-801 (dizocilpine), and the antiepileptic drug, valproate, were studied in amygdala-kindled and non-kindled rats. MK-801, 0.05 mg/kg, did not exert anticonvulsant effects in fully kindled rats but increased the anticonvulsant potency of valproate, 100 mg/kg i.p. However, the increase in anticonvulsant activity was paralleled by a marked increase in adverse effects such as motor impairment and hyperactivity, resulting in a considerable reduction of the therapeutic index of the combined treatment compared to valproate alone. Furthermore, MK-801 potentiated the adverse effects but not the anticonvulsant activity of 50 mg/kg valproate. Combined treatment with MK-801 and valproate induced much less marked adverse effects in non-kindled rats than in kindled rats. The competitive NMDA receptor antagonist, CGP 37849 1 mg/kg i.p., did not alter the effects of valproate in kindled rats. The data on combined treatment with MK-801 and valproate substantiate the conclusion that kindling alters the susceptibility to manipulations of NMDA receptor-mediated events.

Influence of antidepressant drugs on seizure susceptibility and the anticonvulsant activity of valproate in mice

The tricyclic antidepressants, amitriptyline (20-30 mg/kg, i.p.) and imipramine (30-40 mg/kg), provided a significant protection against electro-convulsions (12 mA, 0.2 s stimulus duration) but desipramine (up to 40 mg/kg) remained ineffective. On the other hand, all drugs, amitriptyline (10 mg/kg), desipramine (20 mg/kg), and imipramine (20 mg/kg) distinctly potentiated the protective efficacy of valproate against maximal electroshock, reducing its ED 50 values from 255 mg/kg to 150, 135, and 128 mg/kg, respectively. In one case the plasma valproate level was measured and it was evident that desipramine (20 mg/kg) did not affect the plasma level of this antiepileptic.