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

The Role of Glutamate Receptors in Epilepsy

Glutamate is an essential excitatory neurotransmitter in the central nervous system, playing an indispensable role in neuronal development and memory formation. The dysregulation of glutamate receptors and the glutamatergic system is involved in numerous neurological and psychiatric disorders, especially epilepsy. There are two main classes of glutamate receptor, namely ionotropic and metabotropic (mGluRs) receptors. The former stimulate fast excitatory neurotransmission, are N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), and kainate; while the latter are G-protein-coupled receptors that mediate glutamatergic activity via intracellular messenger systems. Glutamate, glutamate receptors, and regulation of astrocytes are significantly involved in the pathogenesis of acute seizure and chronic epilepsy. Some glutamate receptor antagonists have been shown to be effective for the treatment of epilepsy, and research and clinical trials are ongoing.

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.

Carbamazepine enhances brain production of kynurenic acid in vitro

Disturbed formation of kynurenic acid, an endogenous antagonist of glutamate ionotropic receptors, might contribute to the pathogenesis of seizures. Here, the effect of anticonvulsant drug, carbamazepine on the production of kynurenic acid was studied. Carbamazepine (0.5-3 mM) enhanced kynurenic acid synthesis in rat cortical slices and also increased the activity of kynurenine aminotransferase (KAT) I at 0.1-3.0 mM concentration. Thus, anticonvulsant drugs, such as carbamazepine, might act partially via stimulation of kynurenic acid production.

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.

Pre- rather than co-application of vigabatrin increases the efficacy of tiagabine in hippocampal slices

PURPOSE

The antiepileptic drug vigabatrin (VGB) increases intracellular availability of the inhibitory transmitter gamma-aminobutyric acid (GABA) by inhibition of GABA-transaminase. A blockade of the GABA uptake is the main mechanism of action of tiagabine (TGB). Based on this, the two antiepileptic drugs (AEDs) can be speculated to act synergistically so that their combined antiepileptic efficacy is supraadditive.

METHODS

To test this, experiments were performed on hippocampal slices of guinea-pigs. As an epilepsy model, epileptiform field potentials (EFPs) were induced by omission of Mg2+ from the bath solution and recorded in stratum pyramidale of the CA3 region. VGB (7.5 microM) and TGB (0.75 microM) were added to the superfusate.

RESULTS

VGB, given alone, failed to decrease the repetition rate of EFPs. Similarly, TGB applied alone only transiently led to a nonsignificant reduction of the EFP frequency. Combining VGB and TGB, their suppressive efficacy increased, yielding a significant reduction of EFP frequency, which, however, again did not persist. Pretreatment of the preparations with VGB for 2 h, followed by additional application of TGB, or TGB alone, drastically and persistently potentiated the effects.

CONCLUSIONS

These results demonstrate that VGB and TGB show favorable pharmacodynamic interactions, provided VGB is allowed to block intracellular GABA degradation before GABA uptake block by TGB.

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.

Network and pharmacological mechanisms leading to epileptiform synchronization in the limbic system in vitro

Seizures in patients presenting with mesial temporal lobe epilepsy result from the interaction among neuronal networks in limbic structures such as the hippocampus, amygdala and entorhinal cortex. Mesial temporal lobe epilepsy, one of the most common forms of partial epilepsy in adulthood, is generally accompanied by a pattern of brain damage known as mesial temporal sclerosis. Limbic seizures can be mimicked in vitro using preparations of combined hippocampus-entorhinal cortex slices perfused with artificial cerebrospinal fluid containing convulsants or nominally zero Mg(2+), in order to produce epileptiform synchronization. Here, we summarize experimental evidence obtained in such slices from rodents. These data indicate that in control animals: (i) prolonged, NMDA receptor-dependent epileptiform discharges, resembling electrographic limbic seizures, originate in the entorhinal cortex from where they propagate to the hippocampus via the perforant path-dentate gyrus route; (ii) the initiation and maintenance of these ictal discharges is paradoxically contributed by GABA (mainly type A) receptor-mediated mechanisms; and (iii) CA3 outputs, which relay a continuous pattern of interictal discharge at approximately 1Hz, control rather than sustain ictal discharge generation in entorhinal cortex. Recent work indicates that such a control is weakened in the pilocarpine model of epilepsy (presumably as a result of CA3 cell damage). In addition, in these experiments electrographic seizure activity spreads directly to the CA1-subiculum regions through the temporoammonic pathway. Studies reviewed here indicate that these changes in network interactions, along with other mechanisms of synaptic plasticity (e.g. axonal sprouting, decreased activation of interneurons, upregulation of bursting neurons) can confer to the epileptic, damaged limbic system, the ability to produce recurrent limbic seizures as seen in patients with mesial temporal lobe epilepsy.

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.

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.

Electrical but not chemical kindling increases sensitivity to some phencyclidine-like behavioral effects induced by the competitive NMDA receptor antagonist D-CPPene in rats

We have previously reported that a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, DL-[E]-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849), produces stereotyped behaviors and hyperlocomotion in amygdala kindled rats at doses which do not induce such phencyclidine (PCP)-like behaviors in nonkindled rats, indicating that kindling predisposes rats to such adverse effects of competitive NMDA receptor antagonists. From these data we predicted that epileptic patients may exhibit a hypersensitivity to PCP-like adverse effects of competitive NMDA receptor antagonists, which was subsequently confirmed in a clinical trial with D-CPPene (SDZ EAA-494; 3-(2-carboxypiperazine-4-yl)propenyl-1-phosphonate). For further exploration of the functional alterations in NMDA receptor responsiveness produced by kindling, we studied whether the enhanced susceptibility of amygdala-kindled rats to PCP-like adverse effects of CGP 37849 is also observed with D-CPPene. Furthermore, we determined whether the enhanced susceptibility of kindled rats to such adverse effects occurs only after relatively short intervals following the last seizure, as used in our previous study, or is a more permanent phenomenon. For this purpose, we compared adverse effects in kindled rats not only with naive (non-implanted) controls, as done in our previous study, but used electrode-implanted nonkindled rats as an additional control to assess the possible bias of mere electrode-implantation. In addition, we studied whether the enhanced susceptibility to NMDA receptor antagonists of electrically kindled rats is also present in chemically kindled animals. In some experiments, the PCP-like uncompetitive NMDA receptor antagonist MK-801 (dizocilpine) was included for comparison. In amygdala kindled rats, D-CPPene produced significantly more stereotyped behaviors than in electrode-implanted or naive nonkindled controls. The enhanced sensitivity of electrically kindled rats to PCP-like stereotypies induced by D-CPPene was observed both 7 and 180 days after the last kindled seizure, indicating a long-lasting if not permanent hypersensitivity to these adverse effects. In addition, more intense circling was observed in amygdala kindled rats, whereas hyperlocomotion only tended to be more intense after D-CPPene in kindled rats. These alterations in D-CPPene-induced behaviors were not observed after chemical kindling with pentylenetetrazole, but D-CPPene induced significantly less hypothermia in chemically kindled rats both 7 and 70 days after the last seizure. The data demonstrate that kindling produces long-lasting alterations in some adverse effects of D-CPPene, substantiating that epileptogenesis as initiated by kindling renders the brain more susceptible to PCP-like behavioral side effects of competitive NMDA receptor antagonists.

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.

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.

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.

Influence of D-cycloserine on the anticonvulsant activity of phenytoin and carbamazepine against electroconvulsions in mice

PURPOSE

D-Cycloserine (DCS) is a high-efficacy partial agonist at the strychnine-insensitive glycine modulatory site within the N-methyl-D-aspartate (NMDA)-receptor/ionophore complex. Previous studies demonstrated that DCS exhibits anticonvulsant activity in a variety of experimental epilepsy models. In this study, we determined the influence of DCS in subprotective doses on the anticonvulsant action of phenytoin (PHT) and carbamazepine (CBZ) in mice.

METHODS

Two electroconvulsive tests were used, i.e., determination of seizure threshold and maximal electroshock seizures. Antiepileptic drug-induced motor and long-term memory deficits were quantified by using the chimney test and the passive-avoidance test, respectively. In addition, plasma levels of PHT and CBZ were measured by fluorescence polarization immunoassay to exclude any pharmacokinetic interactions.

RESULTS

DCS, when used alone in doses of 80 and 160 mg/kg, significantly increased the threshold for electroconvulsive seizures. DCS in a wide range of doses (1.25-40 mg/kg) was combined with either PHT or CBZ and tested in electroconvulsive tests. DCS, at doses of 2.5 and 10 mg/kg, was the most effective in potentiating the threshold-increasing action of PHT; higher doses of DCS (20 and 40 mg/kg) were required to achieve a similar effect of CBZ. In maximal electroshock-induced seizures, DCS (10 mg/kg) augmented the protective action of PHT, but was ineffective at a dose of 40 mg/kg with CBZ. DCS did not potentiate the neurotoxicity produced by PHT and CBZ in the chimney test. Both PHT and CBZ induced impairments of long-term memory; PHT-induced memory adverse effects were counteracted by DCS (10 mg/kg). There was no such effect on CBZ-induced memory impairment, and a worsening influence was observed. Any pharmacokinetic interactions were excluded by measuring total and free plasma levels of both antiepileptic drugs.

CONCLUSION

Our results suggest that combining DCS with PHT and CBZ may be beneficial in treating epileptic seizures.

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.

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.

Influence of combined treatment with NMDA and non-NMDA receptor antagonists on electroconvulsions in mice

alpha-Amino-3-hydroxy-5-methyl-isoxazole-4-propionate/kainate (AMPA/kainate) receptor antagonists (at subthreshold doses against electroconvulsions), 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466 at maximally 5 mg/kg) and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX at maximally 20 mg/kg) enhanced the protective effects of NMDA receptor antagonists, MK-801 (dizocilpine) or 2-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid (D-CPP-ene), against electroconvulsions. Similarly, MK-801 or D-CPP-ene reduced the ED50 values of both NBQX and GYKI 52466 against maximal electroshock. The adverse effects of D-CPP-ene, evaluated in the chimney and rotorod tests, were potentiated by both GYKI 52466 (2.5 mg/kg) and NBQX (10 mg/kg). Also, D-CPP-ene (0.1 mg/kg) worsened the motor performance of mice pretreated with GYKI 52466 in the rotorod test. Neither MK-801 (0.025 mg/kg) nor D-CPP-ene (0.1 mg/kg) affected the NBQX-induced impairment of motor coordination. Similarly, GYKI 52466 (2.5 mg/kg) or NBQX (10 mg/kg) did not influence the performance of mice treated with MK-801 (0.2 mg/kg). It may be concluded that the blockade of more than one subtype of glutamate receptors leads to a more pronounced anticonvulsive effect when compared with the effect of blockade of an individual receptor subtype. In some cases more efficient seizure protection was not associated with increased adverse effects.

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.