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

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.

The adverse event profile of perampanel: meta-analysis of randomized controlled trials

BACKGROUND AND PURPOSE

To identify adverse events (AEs) significantly associated with perampanel treatment in double-blind clinical studies (RCTs). Serious AEs, study withdrawals due to AEs and dose-effect responses of individual AEs were also investigated.

METHODS

All placebo controlled, double-blind RCTs investigating therapeutic effects of oral perampanel were searched. AEs were assessed for their association with perampanel after exclusion of synonyms, rare AEs and non-assessable AEs. Risk difference (RD) was used to evaluate the association of any AE (99% confidence intervals) and withdrawals or serious AEs (95% confidence intervals) with perampanel.

RESULTS

Nine RCTs (five in pharmacoresistant epilepsy and four in Parkinson's disease) were included in our study. Almost 4000 patients had been recruited, 2627 of whom were randomized to perampanel and treated with drug doses of 0.5 mg/day (n = 68), 1 mg/day (n = 65), 2 mg/day (n = 753), 4 mg/day (n = 1017), 8 mg/day (n = 431) or 12 mg/day (n = 293). Serious AEs were not significantly associated with perampanel treatment. The experimental drug was significantly associated with an increased risk of AE-related study withdrawals at 4 mg/day [RD (95% confidence interval) 0.03 (0.00, 0.06)] and 12 mg/day [RD (95% confidence interval) 0.13 (0.07, 0.18)]. Of 15 identified AEs, five (dizziness, ataxia, somnolence, irritability and weight increase) were found to be significantly associated with perampanel and one (seizure worsening) was significantly associated with placebo.

CONCLUSIONS

Vestibulocerebellar AEs (dizziness, ataxia), sedative effects (somnolence), irritability and weight increase were significantly associated with perampanel treatment.

Revisiting AMPA receptors as an antiepileptic drug target

In the 1990s there was intense interest in ionotropic glutamate receptors as therapeutic targets for diverse neurological disorders, including epilepsy. NMDA receptors were thought to play a key role in the generation of seizures, leading to clinical studies of NMDA receptor blocking drugs in epilepsy. Disappointing results dampened enthusiasm for ionotropic glutamate receptors as a therapeutic target. Eventually it became appreciated that another type of ionotropic glutamate receptor, the AMPA receptor, is actually the predominant mediator of excitatory neurotransmission in the central nervous system and moreover that AMPA receptors are critical to the generation and spread of epileptic activity. As drugs became available that selectively target AMPA receptors, it was possible to demonstrate that AMPA receptor antagonists have powerful antiseizure activity in in vitro and in vivo models. A decade later, promising clinical studies with AMPA receptor antagonists, including the potent noncompetitive antagonist perampanel, are once again focusing attention on AMPA receptors as a drug target for epilepsy therapy.

Neuropsychopharmacological understanding for therapeutic application of morphinans

Morphinans are a class of compounds containing the basic structure of morphine. It is well-known that morphinans possess diverse pharmacological effects on the central nervous system. This review will demonstrate novel neuroprotective effects of several morphinans such as, dextromethorphan, its analogs and naloxone on the models of multiple neurodegenerative disease by modulating glial activation associated with the production of a host of proinflammatory and neurotoxic factors, although dextromethorphan possesses neuropsychotoxic potentials. The neuroprotective effects and the therapeutic potential for the treatment of excitotoxic and inflammatory neurodegenerative diseases, and underlying mechanism of morphinans are discussed.

ANTICONVULSANT, BUT NOT ANTIEPILEPTIC, ACTION OF VALPROATE ON AUDIOGENIC SEIZURES IN METAPHIT?TREATED RATS

1. The blocking effects of valproate (2-propylpentanoic acid), a standard anti-epileptic drug, on metaphit (1-[1-(3-isothiocyanatophenyl)-cyclohexyl]-piperidine)-induced audiogenic seizures as a model of generalized, reflex audiogenic epilepsy in adult Wistar male rats were studied. 2. Rats were stimulated using an electric bell (100 +/- 3 dB, 5-8 kHz, 60 s) 60 min after i.p. metaphit (10 mg/kg) injection and afterwards at hourly intervals. For power spectra and electroencephalograph (EEG) recordings, three gold-plated screws were implanted into the skull. Different doses of valproate (50, 75 and 100 mg/kg) were injected i.p. into rats with fully developed metaphit seizures after the eighth audiogenic testing. 3. In metaphit-treated animals, the EEG appeared as polyspikes, spike-wave complexes and sleep-like patterns, whereas the power spectra were increased compared with the corresponding controls. 4. Valproate reduced the incidence and intensity of convulsions and prolonged the duration of the latency period in a dose-dependent manner 4 h after administration. 5. The ED(50) of valproate in the first hour after injection was 63.19 mg/kg (95% confidence interval 51.37-77.71 mg/kg). 6. None of the doses of valproate applied eliminated the EEG signs of metaphit-provoked epileptiform activity. 7. Taken together, these results suggest that all doses of valproate examined acted to suppresse behavioural but not epileptic EEG spiking activity in metaphit-induced seizures.

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.

Conventional anticonvulsant drugs in the guinea-pig kindling model of partial seizures: effects of repeated administration

This study addressed the anticonvulsant effects of repeated administration of phenytoin, carbamazepine, phenobarbital, valproate, and ethosuximide in kindled guinea-pigs in order to further substantiate this novel model of partial seizures for the screening of future anticonvulsant drugs. Behavioral toxic effects were assessed at 30 min following drug administration using scores on a sedation/muscle relaxation rating index. In response to suprathreshold stimulation, the anticonvulsant efficacy of the drugs were evaluated from measurements of afterdischarge duration (ADD) and behavioral seizure severity (SS) during a repeated drug treatment schedule in kindled guinea-pigs. All drugs exerted slight to moderate sedative effects in guinea-pigs on our rating index. We found that phenytoin, carbamazepine, and phenobarbital exhibited effective anticonvulsant properties in kindled guinea-pigs by reducing both ADD and SS. We found that valproate consistently reduced ADD throughout the treatment schedule but failed to significantly reduce SS. Lastly, ethosuximide failed to exhibit effective anticonvulsant properties. Our results indicate that the guinea-pig kindling model correctly predicted the actions of these common anticonvulsant drugs in the treatment of partial seizures. Guinea-pig amygdala kindling appears to serve as a useful and valid model for partial epilepsy.

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.

Valproate prevents the induction and the expression of MK-801 sensitization

Repeated administration of psychostimulants such as amphetamine, cocaine, and methylphenidate has been shown to induce behavioral sensitization. Sodium valproate, an anticonvulsant agent that enhances GABA activity, and dizocilpine (MK-801), a non-competitive NMDA receptor antagonist, can block the sensitization elicited by psychostimulants. MK-801 also has been demonstrated to sensitize to itself. The objective of the present study was to determine whether valproate disrupts the behavioral sensitization elicited by MK-801. Male Sprague-Dawley rats were given a regimen of repeated MK-801 injections (0.3 mg/kg, i.p.) that produced behavioral sensitization. They were also given valproate, at a dosage (50 mg/kg, i.p.) that prevented behavioral sensitization to stimulants, either during or after multiple MK-801 injections. After the washout period, animals were then re-challenged with MK-801 to determine whether valproate disrupted the behavioral sensitization elicited by MK-801. An activity monitoring system recorded horizontal activity, total distance, and vertical activity of the animals following drug treatment. Results of their locomotor responses demonstrated that valproate disrupted the development/induction and the expression of sensitization to MK-801, as it did to methylphenidate.

Anti-convulsant and adverse effects of the glycineB receptor ligands, D-cycloserine and L-701,324: comparison with competitive and non-competitive N-methyl-D-aspartate receptor antagonists

In this study, the anticonvulsant and adverse effects of compounds that belong to four different categories of systemically available N-methyl-D-aspartate (NMDA) receptor ligands were compared, namely the competitive antagonist CGP 40116, the noncompetitive antagonist MK-801 (dizocilpine), the glycineB receptor antagonist L-701,324, and the glycineB receptor high-efficacy partial agonist D-cycloserine. The maximal electroshock seizures (MES), which are widely used to detect drug efficacy against generalized tonic-clonic seizures in humans, were produced by transcorneal electrical stimulation. Abolition of tonic hind-limb extension was taken as the end-point. The drug-induced motor and long-term memory deficits were quantified by using the inverted screen test and the step-through passive-avoidance test, respectively. All tested compounds exhibited significant anticonvulsant effect. The rank order of potency for the respective compounds was: MK-801 = CGP 40116 > L-701,324 >> D-cycloserine. All of these compounds induced motor impairment at doses close to those found to be anticonvulsant, however, hyperlocomotion and stereotyped behavior occurred only with MK-801. The highest protective indices [PI = TD50 (inverted screen)/ED50 (MES)] were calculated for CGP 40116 and D-cycloserine (2.4 and 2.2, respectively). When tested for memory impairment at one-half the MES ED50, again only MK-801 induced significant memory disruption in the passive avoidance test. In conclusion, these results suggest that glycineB receptor high-efficacy partial agonists and competitive NMDA receptor antagonists may be advantageous to noncompetitive NMDA antagonists and glycineB receptor antagonists as potential antiepileptic drugs.

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.

Pharmacokinetics of dextromethorphan and dextrorphan in epileptic patients

The present report describes the pharmacokinetic characteristics of dextromethorphan (DM) and its main active metabolite dextrorphan (DX) in a group of epileptic patients receiving comedication. Patients were sequentially dosed with DM 40 mg/6 h (8 weeks) and 50 mg/6 h (8 weeks) while concurrent antiepileptic drugs were kept stable. During baseline period, patients were phenotyped with regard to their drug metabolizing capacity. At the end of each treatment period, timed plasma DM and DX levels were determined post-dose by HPLC. Urine and cerebrospinal fluid f1p4) samples were also collected. The pharmacokinetic parameters of DM showed a wide intersubject variation. The genetic polymorphism of DM metabolism was identified as the possible cause of the observed variability. For both DM and DX mean values for Cmax and AUC increased in a linear fashion with dose, while the mean values of tmax and t 1/2 were not dependent on dose. The mean values of CL/F and Vss/F for DM were also dose-dependent. 3-Methoxymorphinan, an N-demethylated metabolite of DM was detected in plasma and CSF of some patients and warrants further investigation as to its possible CNS effects. In conclusion, DM given in doses up to 50 mg/6 h can produce plasma and brain concentrations similar to the in vitro antiepileptic levels, without causing significant adverse effects.

Kindling increases the sensitivity of rats to adverse effects of certain antiepileptic drugs

Development of novel antiepileptic drugs (AEDs) requires determining the margin between the desired anticonvulsant effect and undesired adverse effects (AE) (therapeutic index). For this purpose, drug-induced "minimal neurological deficits" (e.g., motor dysfunctions) are commonly quantified by simple tests, such as the rotarod test, in normal, i.e., nonepileptic animals. However, increasing evidence shows that chronic brain dysfunction associated with epilepsy may increase susceptibility to the AE of certain AEDs, e.g., N-methyl-D-aspartate (NMDA) receptor antagonists. The increased AE potential of such investigational drugs can be predicted by using kindled rats instead of normal rodents in preclinical drug evaluation studies. In the present experiments, we wished to determine whether kindled rats also exhibit an altered susceptibility to neurological adverse effects of standard AEDs, i.e., carbamazepine (CBZ), phenobarbital (PB), valproate (VPA), and diazepam (DZP). Abecarnil, a novel benzodiazepine (BZD) receptor agonist, was included in the study for comparison. All drugs were administered in diverse doses in kindled and nonkindled rats, and all behavioral alterations were scored in the cage and open field. Furthermore, the rotarod test was used to detect and quantify motor impairment induced by drug treatments. Kindled rats were more susceptible than nonkindled rats to motor impairment (ataxia and/or rotarod failures) induced by high doses of AEDs, although differences were noted between the drugs tested. VPA was the only drug that induced stereotyped behavior; it was much more potent in this respect in kindled than nonkindled rats. Abecarnil did not differ substantially in its AE in either subgroup of animals. Our data indicate that epileptogenesis induced by kindling renders the brain more susceptible to certain AE of AEDs.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in anticonvulsant potency and adverse effects between dextromethorphan and dextrorphan in amygdala-kindled and non-kindled rats

The anticonvulsant and adverse effects of dextromethorphan, a non-opioid antitussive, and its metabolite dextrorphan were examined in amygdala-kindled rats. Both drugs have repeatedly been proposed to be functional non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, but they also exert effects distinct from antagonism at NMDA receptors, such as blockade of voltage-gated calcium channels and sigma-site mediated actions. Since recent data have demonstrated that kindled rats are more susceptible to the adverse effects of NMDA receptor antagonists than non-kindled rats, the time course, characteristics and severity of adverse effects of dextromethorphan and dextrorphan were also determined in non-kindled animals. Dextromethorphan dose dependently increased the focal seizure threshold (i.e. the threshold for induction of afterdischarges recorded from the amygdala) in fully kindled rats. This anticonvulsant effect was found at relatively low doses (7.5-15 mg/kg i.p.) which were almost free of any adverse effects. At higher doses, dextromethorphan induced motor impairment and seizures, but no phenyclidine (PCP)-like adverse effects, such as hyperlocomotion or stereotypies. In contrast, such adverse effects were seen after dextrorphan, although only infrequently. Dextrorphan was less potent in inducing anticonvulsant but more potent in inducing motor impairing effects than dextromethorphan in kindled rats. In non-kindled rats, the motor impairment induced by dextrorphan was significantly less severe than in kindled rats, whereas no marked differences between kindled and non-kindled rats were found for dextromethorphan. The data indicate that dextromethorphan and dextrorphan differ in their mechanisms of action. Only dextrorphan exerts effects which are characteristic for NMDA receptor antagonism, whereas the potent anticonvulsant effect of dextromethorphan in presumably unrelated to the NMDA receptor complex.

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.