Neuroprotective effect of felbamate after kainic acid-induced status epilepticus

Protective effect of hispidulin on kainic acid-induced seizures and neurotoxicity in rats

Hispidulin is a flavonoid compound which is an active ingredient in a number of traditional Chinese medicinal herbs, and it has been reported to inhibit glutamate release. The purpose of this study was to investigate whether hispidulin protects against seizures induced by kainic acid, a glutamate analog with excitotoxic properties. The results indicated that intraperitoneally administering hispidulin (10 or 50mg/kg) to rats 30 min before intraperitoneally injecting kainic acid (15 mg/kg) increased seizure latency and decreased seizure score. In addition, hispidulin substantially attenuated kainic acid-induced hippocampal neuronal cell death, and this protective effect was accompanied by the suppression of microglial activation and the production of proinflammatory cytokines such as interleukin-1β, interleukin-6, and tumor necrosis factor-α in the hippocampus. Moreover, hispidulin reduced kainic acid-induced c-Fos expression and the activation of mitogen-activated protein kinases in the hippocampus. These data suggest that hispidulin has considerable antiepileptic, neuroprotective, and antiinflammatory effects on kainic acid-induced seizures in rats.

First-in-class thyrotropin-releasing hormone (TRH)-based compound binds to a pharmacologically distinct TRH receptor subtype in human brain and is effective in neurodegenerative models

JAK4D, a first-in-class thyrotropin-releasing hormone (TRH)-based compound, is a prospective therapeutic candidate offering a multifaceted approach to treating neurodegeneration and other CNS conditions. The purpose of these studies was to determine the ability of JAK4D to bind to TRH receptors in human brain and to evaluate its neuropharmacological effects in neurodegenerative animal models. Additionally, JAK4D brain permeation was examined in mouse, and initial toxicology was assessed in vivo and in vitro. We report that JAK4D bound selectively with nanomolar affinity to native TRH receptors in human hippocampal tissue and showed for the first time that these receptors are pharmacologically distinct from TRH receptors in human pituitary, thus revealing a new TRH receptor subtype which represents a promising neurotherapeutic target in human brain. Systemic administration of JAK4D elicited statistically significant and clinically-relevant neuroprotective effects in three established neurodegenerative animal models: JAK4D reduced cognitive deficits when administered post-insult in a kainate (KA)-induced rat model of neurodegeneration; it protected against free radical release and neuronal damage evoked by intrastriatal microdialysis of KA in rat; and it reduced motor decline, weight loss, and lumbar spinal cord neuronal loss in G93A-SOD1 transgenic Amyotrophic Lateral Sclerosis mice. Ability to cross the blood-brain barrier and a clean initial toxicology profile were also shown. In light of these findings, JAK4D is an important tool for investigating the hitherto-unidentified central TRH receptor subtype reported herein and an attractive therapeutic candidate for neurodegenerative disorders.

Characteristics of selected second-generation antiepileptic drugs used in dogs

A significant number of cases of clinical canine epilepsy remain difficult to control in spite of the applied treatment. At the same time, the range of antiepileptic drugs is increasingly wide, which allows efficient treatment. In the present paper we describe the pharmacodynamics and pharmacokinetics of the newer antiepileptic drugs which were licensed after 1990 but are still not widely used in veterinary medicine. The pharmacokinetic profiles of six of these drugs were tested on dogs. The results of experimental studies suggest that second generation antiepileptic drugs may be applied in mono- as well as in poli- treatment of canine epilepsy because of the larger safety margin and more advantageous pharmacokinetic parameters. Knowledge of the drugs' pharmacokinetics allows its proper clinical appliance, which, in turn, gives the chance to improve the efficiency of pharmacotherapy of canine epilepsy.

New insight on the mechanisms of epileptogenesis in the developing brain

The incidence of epilepsy is at its highest in childhood and seizures can persist for a lifetime. As brain tissue from pediatric patients with epilepsy is rarely available, the analysis of molecular and cellular changes during epileptogenesis, which could serve as targets for treatment approaches, has to rely largely on the analysis of tissue from animal models. However, these data have to be analyzed in the context of the developmental stage when the insult occurs. Here we review the current status of the available animal models, the molecular analysis done in these models, as well as treatment attempts to prevent epileptogenesis in the immature brain. Considering that epilepsy is one of the major childhood neurological diseases, it is remarkable how little is known on epileptogenesis in the immature brain at a molecular level. It is a true challenge for the future to expand the armamentarium of clinically relevant animal models, and systematic analysis of molecular and cellular data to enhance the probability of developing syndrome specific antiepileptogenic treatments and biomarkers for acquired pediatric epileptogenesis.

Effect of TTX suppression of hippocampal activity following status epilepticus

Status epilepticus (SE) is a severe neurological condition that can result in brain damage. In animals, SE is associated with cell loss and aberrant synaptogenesis. These pathological processes appear to be activity-dependent and may continue after the SE has ended. We postulated that suppression of electrical activity following SE at the site of the epileptic focus will reduce seizure-induced damage. To achieve this goal, tetrodotoxin (TTX) was used to suppress electrical activity in the hippocampi bilaterally following SE. Adult rats experienced lithium-pilocarpine-induced SE for 2h while controls underwent sham-SE with saline injections. Starting 12h after the SE or sham-SE rats received either continuous TTX (1 microM) or saline infusions through cannulas implanted in the bilateral hippocampi for 5h daily for 4 days. TTX resulted in significant EEG suppression and reduction in spikes and sharp waves. Rats were sacrificed 2 weeks after SE and the brains examined for cell loss and sprouting. Rats receiving TTX following SE had significantly more cell loss as well as a trend toward more mossy fiber sprouting than saline-treated rats following SE. TTX injection in sham-SE rats caused no cell loss or mossy fiber sprouting. These results suggest that suppression of electrical activity following SE is detrimental.

Levetiracetam and felbamate interact both pharmacodynamically and pharmacokinetically: an isobolographic analysis in the mouse maximal electroshock model

PURPOSE

Polytherapy with two or more antiepileptic drugs (AEDs) is generally required for approximately 30% of patients with epilepsy, who do not respond satisfactorily to monotherapy. The potential usefulness of AED combinations, producing synergistic anticonvulsant efficacy and minimal adverse effects, is therefore of significant importance. The present study sought to ascertain the potential usefulness of levetiracetam (LEV) and felbamate (FBM) in combination in the mouse maximal electroshock (MES)-induced seizure model.

METHODS

The anticonvulsant interaction profile between LEV and FBM in the mouse MES-induced seizure model was determined using type II isobolographic analysis. Acute adverse effects (motor performance) were ascertained by use of the chimney test. LEV and FBM brain concentrations were measured by HPLC in order to determine any pharmacokinetic contribution to the observed antiseizure effect.

RESULTS

LEV in combination with FBM, at the fixed ratios of 1:2, 1:1, 2:1, and 4:1, were supraadditive, whereas at the fixed ratio of 1:4, additivity was observed in the mouse MES model. Furthermore, none of the investigated combinations altered motor performance in the chimney test. Brain FBM concentrations were unaffected by concomitant LEV administration. In contrast, FBM significantly increased LEV brain concentrations.

CONCLUSIONS

LEV in combination with FBM was associated with pharmacodynamic supraadditivity in the MES test. However, this anticonvulsant supraadditivity was associated with a concurrent increase in brain LEV concentrations indicating a pharmacokinetic contribution to the observed pharmacodynamic interaction between LEV and FBM.

Interaction between lamotrigine and felbamate in the maximal electroshock-induced seizures in mice: an isobolographic analysis

Isobolographic profile of interactions between lamotrigine (LTG) and felbamate (FBM), two second-generation antiepileptic drugs, against maximal electroshock (MES)-induced seizures, and neurotoxic adverse effects in the chimney test in mice were determined. LTG combined with FBM at the fixed ratios of 1:3, 1:1, and 3:1 exerted merely additive interactions against MES-induced seizures. In the chimney test, isobolography revealed that LTG coadministered with FBM at the fixed ratio of 1:1 displayed subadditivity (antagonism), whereas the remaining combinations tested (1:3 and 3:1) exerted additivity in terms of their neurotoxic side effects. LTG (at the dose of 2.3 mg/kg) coadministered with FBM (25.7 mg/kg) at the fixed ratio of 1:1 from the MES test did not impair long-term memory of mice challenged with the passive avoidance task. Furthermore, FBM (25.7 mg/kg) altered neither the free plasma nor brain concentration of LTG, hence pharmacokinetic events, which might affect the observed interactions in the MES test, are unlikely. Considering benefit indices for the respective fixed ratio combinations, it may be concluded that the combination of LTG with FBM at the fixed ratio of 1:1 is advantageous from a preclinical point of view, offering the highest benefit index reaching the value of 1.46. Likewise, the two-drug combination of 1:3 was also beneficial and is worth recommendation with benefit index amounting to 1.36. Only the combination of 3:1 was neutral with a benefit index of 1.08. Protection offered by LTG in combination with FBM against maximal electroconvulsions and its favorable neurotoxic side effect profile might provide the patients with intractable seizures with an efficacious treatment, as the rational polytherapy however, it requires to be clinically confirmed and verified.

Antiepileptic drugs in neuroprotection

Antiepileptic drugs (AEDs) are designed to prevent and suppress seizure activity. Their effects on calcium influx and molecular cascades contributing to necrotic and apoptotic neuronal death, however, suggests that they have functions other than just suppression of excitability. The neuroprotective effects of 20 AEDs currently in use or being investigated in Phase II - III clinical trials for treatment of epilepsy are reviewed. Data analyses is complicated by several factors. Firstly, the available data on the neuroprotective effects of different AEDs varies largely. Secondly, most of the evidence demonstrating neuroprotective effects comes from stroke models and it is uncertain whether these data can be extrapolated to other conditions, such as status epilepticus (SE) or traumatic brain injury. Thirdly, data obtained in adult animals cannot be extrapolated to young animals without caution. For example, AEDs protecting adult brain from stroke or SE-induced injury can cause apoptosis in immature brain. Finally, data comparison is complicated by the variability in study designs and methodologies between studies. With these caveats in mind, an analysis of the available data suggests that AEDs with different mechanisms of action can have mild-to-moderate neuroprotective effects. It is difficult, however, to associate the neuroprotective effects with a favourable functional outcome. For example, it is difficult to conclude that administration of AEDs during the latency phase would have an effect on the molecular cascades underlying epileptogenesis. The few favourable data demonstrating a decrease in the incidence of epilepsy after SE are probably related to the administration of AEDs during SE, which resulted in modification/alleviation of the insult itself and consequently, reduced its epileptogenecity. These experimental data, however, are clinically important because they show that early intervention of SE has an effect on long-term functional outcome. These observations emphasise the need to use additional outcome measures, such as markers of normal development or cognitive performance, when the benefits of neuroprotection achieved by the use of neuroprotective AEDs are assessed.

Temporal Lobe Epileptogenesis and Epilepsy in the Developing Brain: Bridging the Gap Between the Laboratory and the Clinic. Progression, But in What Direction?

The origins of human mesial temporal lobe epilepsy and hippocampal sclerosis are still not well understood. Hippocampal sclerosis and temporal lobe epileptogenesis involve a series of pathologies including hippocampal neuronal loss and gliosis, axonal reorganization, and maybe hippocampal neoneurogenesis. However, the causality of these events is unclear as well as their relation to the factors that may precipitate epileptogenesis. Significant differences between temporal lobe epileptogenesis in the adult and immature brain may require differential approaches. Hereditary factors also may participate in some cases of hippocampal sclerosis. The key point is to identify the significance of these age-dependent changes and to design preventive treatments. Novel strategies for the prevention and treatment of mesial temporal lobe epilepsy and hippocampal sclerosis may include rational use of neuroprotective agents, hormonotherapy, immunizations, and immunotherapy.

An anticonvulsant profile of the ketogenic diet in the rat

The present study was designed to evaluate the anticonvulsant effects of a high-fat ketogenic diet (KD) in rats. Animals were maintained on one of four experimental diets: (1) calorie-restricted ketogenic (KCR); (2) calorie-restricted normal (NCR); (3) ad libitum ketogenic (KAL); or (4) ad libitum normal (NAL). The calorie-restricted diets were fed in quantities such that they were calorically equivalent. All animals began diet treatment at age P37 and each was subjected to one of five chemically-induced seizure tests: bicuculline (BIC; s.c.), picrotoxin (PIC; s.c.), kainate (KA, i.p. or s.c.) and gamma-butyrolactone (GBL, i.p.), strychnine (s.c.). Bipolar epidural electrodes were implanted under ketamine/xylazine anesthesia to permit recording the spike and wave discharges (SWD) characteristic of electroencephalograms during absence seizures. Ketonemia was assayed by measuring blood levels of beta-hydroxybutyrate (BHB) spectrophotometrically prior to induction of seizures in each experiment. Animals fed ketogenic diets (i.e. either calorie restricted or ad libitum) exhibited greater blood levels of BHB compared to control groups. Seizure results show that treatment with a KD: (1) reduced the incidence of bicuculline-induced convulsions; (2) diminished the number of picrotoxin-induced seizures (KCR group only); (3) increased latency to GBL-induced SWD and reduced both the number and duration of SWD; but (4) conferred no protection from strychnine-induced seizures; and (5) made KA-induced seizures more severe. Together these results indicate a spectrum of anticonvulsant action for the KD in rats that includes threshold seizures induced via GABA receptors (BIC, PIC, GBL) but not those induced at glycine (strychnine) or the KA-subclass of glutamate receptors. Uniquely, the KD is the only treatment described that protects against both convulsive and non-convulsive (absence) seizures in rats.

New insights from the use of pilocarpine and kainate models

Local or systemic administration of pilocarpine and kainate in rodents leads to a pattern of repetitive limbic seizures and status epilepticus, which can last for several hours. A latent period follows status epilepticus and precedes a chronic phase, which is characterized by the occurrence of spontaneous limbic seizures. These distinct features, in a single animal preparation, of an acute damage induced by status epilepticus, a silent interval between injury and the onset of spontaneous seizures, and a chronic epileptic state have allowed antiepileptic drug (AED) studies with different purposes, (a) in the acute phase, identification of compounds with efficacy against refractory status epilepticus and/or neuroprotection against damage induced by sustained seizures; (b) in the latent period, identification of agents with a potential for preventing epileptogenesis and/or against seizure-induced long-term behavioral deficits and (c) in the chronic phase, testing drugs effective against partial and secondarily generalized seizures. Studies on pilocarpine and kainate models have pointed out that some AEDs or other compounds exert an antiepileptogenic effect. The analogy of the latent phase of pilocarpine and kainate models with the acquisition of amygdala kindling should encourage testing of drugs that have proved to suppress the evolution of amygdala kindling. Drug testing in the chronic phase should not address only the suppression of secondarily generalized motor seizures. Most of current tools used to quantify spontaneous seizure events need to be coupled to electrophysiology and more sophisticated systems for recording and analyzing behavior.

Nonconvulsive Kainic Acid-Induced Seizures Elicit Age-Dependent Impairment of Memory for the Elevated Plus-Maze

The goal of this study was to evaluate changes in spatial learning in adult and immature rats during and after nonconvulsive seizures. An elevated plus-maze was used in 18- and 25-day-old and adult rats. Kainic acid (KA 6 mg/kg) was administered 60 minutes before the first exposure (Experiment 1) or after a 3-day pretraining (Experiment 2, only adult rats). Animals were retested three times with 24-hour intervals. EEG activity was monitored in 18-day-old rats. KA prolonged the transfer latency (TL) in all age groups. In the youngest group the TL was prolonged 24 hours after KA when epileptic EEG graphoelements were still registered. In both older groups, prolonged TL was measured only 60 minutes after KA. In the pretrained adults, significantly prolonged TLs persisted for 24 hours after KA. KA changed the performance of adult and immature rats in the elevated plus maze not only during nonconvulsive seizures but also 24 hours later.

Neurochemical studies with the anticonvulsant felbamate in mouse brain

Felbamate (FBM) is a relatively novel anticonvulsant agent which has been reported to exert its antiepileptic effects by blockade of the glycine recognition site on the N-methyl-D-aspartate subtype of glutamate receptor and potentiation at the gamma-aminobutyric acid (GABA) type A receptor. An increasing number of antiepileptic drugs have, however, additional, neurochemical actions on the GABA and glutamate systems which may contribute to their anticonvulsant activity. As a result, we have investigated the effects of FBM on several GABA- and glutamate-related neurochemical parameters in mouse brain. Adult male ICR mice were randomised into two groups and administered FBM (0-100 mg kg(-1)) intraperitoneally either as a single dose or twice daily for 5 days. Four hours after the final dose, animals were killed and their brains removed for analysis of GABA, glutamate and glutamine concentrations and activities of GABA-transaminase and glutamic acid decarboxylase. Single and repeated doses of FBM were without effect on all of the parameters investigated. These results appear to exclude the possibility that FBM, in addition to its known effects on GABA and glutamate receptors, exerts its antiepileptic effects via an action on the GABA- and glutamate-related neurochemical parameters chosen for investigation.

Behavioural effects of a subconvulsive dose of kainic acid in rats

Kainic acid can induce a continuum of non-convulsive seizures characterised by epileptic automatisms and convulsive motor seizures depending on the dose. There are scarce data on the behavioural effects of low doses of kainate inducing only non-convulsive seizures. Therefore, we studied spontaneous behaviour of adult male rats using a method of positive habituation based on a detailed analysis of patterns and attention of animals to a stimulus object. Twenty-three animals were individually tested in the experimental arena on two consecutive days. Comparing the data from the first two exposures, a conspicuous habituation in all animals was observed. On experimental day 3, 12 rats received kainate (6 mg/kg intraperitoneally) and the remaining 11 animals received a physiological saline. After 1 h, animals were put into the arena with an object localised in the centre. It was found that both kainate and saline treated animals exhibited a significant increase in the total number of central area visits, and both the total and mean time spent in the vicinity of the object. However, the mean time spent was significantly shorter in kainate treated rats. Furthermore. kainate rats exhibited a significant decrease in rearing as compared with the controls. In addition, an epileptic automatism (wet dog shakes) was observed in seven out of 12 animals given kainate. The comparison of transition matrices between consecutive behavioural categories showed significant differences between the kainate and control groups. Our results demonstrate that a non-convulsive dose of kainate induced changes in the structure of spontaneous behaviour and impaired the processes related to maintenance of attention.

Status epilepticus causes long-term NMDA receptor-dependent behavioral changes and cognitive deficits

PURPOSE

The role of N-methyl-D-aspartate (NMDA)-receptor activation on behavioral and cognitive changes after status epilepticus (SE) is unknown. In this study, behavioral and cognitive changes after SE were evaluated in the short and long term and in rats in which the NMDA receptor was inactivated during SE.

METHODS

Pilocarpine (350 mg/kg) was injected to induce SE. Inhibition of the NMDA receptor during SE was achieved with MK-801 (4 mg/kg). Seizure intensity during SE was monitored by electroencephalography (EEG). After SE, behavioral studies were performed to identify abnormal behavior by using behavioral tests adapted from Moser's functional observational battery. Cognitive changes were assessed by using the Morris Water Maze (MWM).

RESULTS

Pilocarpine-treated animals scored significantly higher on two of the behavioral tests: the Touch test and the Pick-Up test. These behavioral changes occurred very soon after SE, with the earliest changes observed 2 days after SE and persisting for the life of the animal. Inhibition of the NMDA receptor with MK-801 completely inhibited these behavioral changes under conditions that did not alter the duration of SE. In addition, pilocarpine-treated animals exhibited cognitive deficits as determined by using the MWM. Six weeks after SE, the animals displayed significantly longer latencies to locate the hidden platform on this test. The impaired performance on the MWM also occurred as early as 5 days after SE. These cognitive deficits were prevented in animals treated with MK-801 during SE.

CONCLUSIONS

The results indicate that behavioral and cognitive changes occur soon after SE, are permanent, and are dependent on NMDA-receptor activation during SE. NMDA-receptor activation may play an important role in causing cognitive and behavioral morbidity after recovery from SE.

Qualitative changes of anticonvulsant action of felbamate during development in rats

The anticonvulsant action of felbamate (25, 50, 100 and 150 mg/kg i.p.) was tested against motor seizures induced by pentylenetetrazol in five age-groups of rats (7, 12, 18, 25 and 90 days old). In adult rats, felbamate suppressed generalized tonic-clonic seizures leaving intact minimal clonic seizures. In all groups of rat pups felbamate exhibited a specific action against the tonic phase of generalized tonic-clonic seizures. In addition, the highest dose of felbamate was found to suppress minimal seizures in 18-day-old rats. The changes of felbamate action during ontogeny might be due to multiple mechanisms of anticonvulsant action with an uneven developmental profile.

Advances in the medical treatment of epilepsy

Treatment options for epilepsy, especially using antiepileptic drugs, have increased substantially in the past five years. Since 1993, four novel antiepileptic drugs have been approved and marketed in the United States: felbamate, gabapentin, lamotrigine, and topiramate. Two others, tiagabine and vigabatrin, are likely to be approved in the near future. For many patients, these agents offer the realistic promise of improved seizure control, often with fewer adverse effects and less significant drug interactions compared with older agents. In addition, fosphenytoin, a water-soluble phenytoin prodrug with a number of advantages over intravenous phenytoin, has been released. There are new administration options for carbamazepine, diazepam, and valproic acid. For drug-resistant or -intolerant patients, there has been renewed interest in alternative therapies, especially the ketogenic diet. Taken together, these represent significant therapeutic advances that are benefiting patients with epilepsy. At the same time, improved understanding of the basic mechanisms of epileptogenesis, and of the cellular and molecular actions of available antiepileptic drugs, creates a framework for designing unique therapeutic strategies that are targeted at key sites of vulnerability involved in the development and maintenance of the epileptic state.

Anticonvulsant activity of felbamate in amygdala kindling model of temporal lobe epilepsy in rats

PURPOSE

Previous studies have demonstrated that felbamate (FBM, 2-phenyl-1,3-propanediol dicarbamate) at nontoxic doses exerts potent anticonvulsant activity in a variety of animal epilepsy or seizure models. We further characterized the anticonvulsant activity of FBM by using the kindling model of temporal lobe epilepsy (TLE).

METHODS

The experiments were performed in fully kindled rats. The anticonvulsant effect of FBM was assessed by determining seizure severity, afterdischarge (AD) duration and seizure duration either at the focal seizure threshold, or after suprathreshold stimulation. In addition, the neurological performance of kindled rats after FBM administration was evaluated in the open field and by the rotorod test.

RESULTS

FBM at doses of 12.5-50 mg/kg, given intraperitoneally (i.p.) 60 min before testing, dose-dependently increased the AD threshold (ADT). The maximal effect was achieved after the highest dose tested and reached almost 600% of the control ADT. This dose of FBM significantly diminished other seizure parameters, e.g., seizure severity, seizure duration, and AD duration. When the rats were stimulated with suprathreshold current (500 microA) seizure severity was moderately but significantly reduced. No behavioral abnormalities were noted in kindled rats after administration of either of the doses.

CONCLUSIONS

FBM potently increases the threshold for focal seizures and reduces seizure severity, seizure duration, and AD duration at doses that produce no adverse behavioral effects in amygdala-kindled rats. These data are thus compatible with clinical experience with FBM in TLE and substantiate that kindling is a good predictor of anticonvulsant activity against TLE.

Voltage-activated calcium channels: targets of antiepileptic drug therapy?

Voltage-gated calcium currents play important roles in controlling neuronal excitability. They also contribute to the epileptogenic discharge, including seizure maintenance and propagation. In the past decade, selective calcium channel blockers have been synthesized, aiding in the analysis of calcium channel subtypes by patch-clamp recordings. It is still a matter of debate whether whether any of the currently available antiepileptic drugs (AEDs) inhibit these conductances as part of their mechanism of action. We tested oxcarbazepine, lamotrigine, and felbamate and found that they consistently inhibited voltage-activated calcium currents in cortical and striatal neurons at clinically relevant concentrations. Low micromolar concentrations of GP 47779 (the active metabolite of oxcarbazepine) and lamotrigine reduced calcium conductances involved in the regulation of transmitter release. In contrast, felbamate blocked nifedipine-sensitive conductances at concentrations significantly lower than those required to modify N-methyl-D-aspartate (NMDA) responses or sodium currents. Aside from contributing to AED efficacy, this mechanism of action may have profound implications for preventing fast-developing cellular damage related to ischemic and traumatic brain injuries. Moreover, the effects of AEDs on voltage-gated calcium signals may lead to new therapeutic strategies for the treatment of neurodegenerative disorders.

Basic fibroblast growth factor is highly neuroprotective against seizure-induced long-term behavioural deficits

Basic fibroblast growth factor has been reported to protect neurons of various structures from excitotoxic damage. To study the effects of basic fibroblast growth factor on seizure-induced brain damage we infused the growth factor into the lateral ventricles of 35-day-old rats receiving convulsant dosages of kainic acid. Artificial cerebrospinal fluid or basic fibroblast growth factor at dosages of 0.5 ng/h or 2.5 ng/h was infused into the lateral ventricle continuously for seven days starting two days before and continuing for five days after the animals had kainic acid-induced status epilepticus. At age 80 days the animals underwent behavioural testing using the water maze, open field, and handling tests and at age 95 days were tested for seizure threshold using flurothyl inhalation. Neither artificial cerebrospinal fluid or basic fibroblast growth factor modified the latency or duration of the acute seizures following kainic acid. However, rats infused with 2.5 ng/h, but not 0.5 ng/h of basic fibroblast growth factor, had fewer spontaneous recurrent seizures, a higher seizure threshold, better performance in the handling, open field and water maze test, and less cell loss in the hippocampus when compared to rats receiving artificial cerebrospinal fluid or 0.5 ng/h of basic fibroblast growth factor. These results show that basic fibroblast growth factor has a dose-related neuroprotective effect against seizure-induced long-term behavioural deficits when administered by osmotic pump prior to seizure onset. This neuroprotective effect is not related to an anticonvulsant effect.

Anticonvulsant activity of 5,7DCKA, NBQX, and felbamate against some chemoconvulsants in DBA/2 mice

The anticonvulsant effects of felbamate (10-300 mg/kg, intraperitoneally, IP), and those of two representative antagonists of the excitatory amino acid receptors, 5-7 dichlorokynurenic acid (5-7DCKA; 0.6-30 nmol/mouse, intracerebroventricularly, ICV), and 2, 3-dihydroxy-6 nitro-7-sulfamoylbenzo (F) quinoxoline (NBQX; 1.1-33.6 mg/kg, IP) were studied in the DBA/2 mice. All drugs protected the animals from sound-induced seizures. The drugs were also effective against seizures induced by stimulation of the excitatory amino acid receptor complex using the agonists N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5 methyl-4-isoxazolepropionic acid (AMPA). In separate studies, felbamate protected mice from seizures induced by ICV administration of the activator of dihydropyridine-sensitive calcium channels, methyl-1, 4-dihydro-2, 6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl) pyridine-5-carboxylate (Bay k 8644), with ED50 values of 26 and 46.9 mg/kg for tonus and clonus, respectively. Using Bay k 8644, NBQX (1-40 mg/kg IP) was uneffective, while 5,7DCKA (5-90 nmol/mouse, ICV) protected mice against tonus. Moreover, felbamate prevented seizures induced by blocking voltage-dependent K+ channels using alpha-dendrotoxin, with ED50 values of 22.6 mg/kg for tonus and of 34.8 mg/kg for clonus. Conversely, 5,7DCKA or NBQX did not significantly antagonize seizures induced by alpha-dendrotoxin. The present data indicate that felbamate is an effective anticonvulsant drug in DBA/2 mice with a broader anticonvulsant spectrum than 5,7DCKA and NBQX.

Long-term effects of excitatory amino acid antagonists NBQX and MK-801 on the developing brain

Because of the critical role of excitatory amino acids (EAAs) in epileptogenesis and seizure-induced brain damage, EAA antagonists are now being considered as a possible therapy for seizures. However, during development EAAs play a pivotal role in learning, memory, and brain plasticity. To evaluate the long-term effects of a short course of EAA antagonists on the developing brain, a non-NMDA antagonist, NBQX, or a NMDA antagonist, MK-801, were administered over 7 days by osmotic pumps stereotaxically implanted into the lateral ventricles of normal 10 day old rats. Alternatively, 10 and 20 day old rats received a 7 day course of intraperitoneal (i.p.) NBQX. One month later, the NBQX-, MK-801-treated rats, and controls underwent a series of behavioral studies: handling test, open field, and Morris water maze. Flurothyl inhalation was used to test seizure susceptibility in all groups. Although all of the rats treated with NBQX via osmotic pumps has spontaneous seizure, rats surviving infusion of EAAs had no deficits in learning, memory, or behavior and did not differ from controls in seizure susceptibility with flurothyl. In the developing animal, a short-term course of EAA antagonists leads to no long-term adverse effects on behavior or seizure susceptibility.

Recurrent seizures in immature rats: effect on auditory and visual discrimination

The effect of recurrent seizures in developing rats on subsequent long-term behavior was studied. Fifteen day old rats received a convulsant dosage of flurothyl three times daily for five consecutive days. When the rats were fully mature, they underwent behavioral testing using the water maze and auditory quality or location discrimination. With serial flurothyl administration seizure duration increased progressively but latency to seizure onset did not change. Compared to controls, flurothyl-treated rats had impaired performance in the water maze and on auditory location, but not on quality discrimination. Histological examination showed no gross cell loss in the hippocampus. This study demonstrates that serial seizures in the developing brain cause detrimental effects on visual and auditory spatial learning.

7-chlorokynurenic acid prevents in vitro epileptiform and neurotoxic effects due to kainic acid

1. The effects of 7-chlorokynurenic acid were studied against the epileptiform and neurotoxic effects due to the non-NMDA excitatory amino acid, kainic acid, in rat hippocampal slices. 2. Slice perfusion with 7-chlorokynurenic acid (100 microM), significantly (p < 0.05) decreased the duration of the CA1 epileptiform bursting due to 1 microM kainic acid. 3. Slice perfusion with 7-chlorokynurenic acid (100 microM) significantly (p < 0.05) increased the probability of recovery of the CA1 population spike after a neurotoxic concentration (12 microM) of kainic acid. 4. The results indicate that 7-chlorokynurenic acid affects, with a similar potency, epileptiform and neurotoxic effects due to kainic acid.

Excitatory amino acids in health and disease

PURPOSE

To review the role of excitatory neurotransmitters in normal mammalian brain function, the concept of excitotoxic neuronal death as an important final common path in a variety of diseases, and modification of excitatory synaptic transmission as an important new pharmacological principle. These principles are discussed, with special emphasis on diseases of importance to older adults.

DATA SOURCES

A MEDLINE search from 1966 to May 1995 was undertaken, as well as a manual search of current issues of clinical and basic neuroscience journals, for articles that addressed glutamate N-methyl-D-aspartate and/or excitotoxicity.

STUDY SELECTION

A total of 5398 original and 68 review articles were identified that addressed animal and human experimentation relevant to excitotoxic neuronal death. There were 364 articles with potential significance for clinical application identified; 132 of the most recent references are provided.

DATA EXTRACTION

All articles were classified into three categories: general receptor, biology pathogenesis of disease, and pharmacotherapy.

RESULTS

Glutamic and aspartic acids are the physiological mediators of most excitatory synaptic transmission. This is critical to several normal nervous system functions, including memory and long-term modification of synaptic transmission and nociception. Activation of the inotropic NMDA and non-NMDA receptors increases transmembrane calcium and sodium fluxes, and the metabotropic glutamate receptor activation results in generation of inositol triphosphate and inhibition of adenylate cyclase. Numerous modulatory sites exist, especially on the NMDA receptor. Nitric oxide, arachidonic acid, superoxide, and intracellular calcium overload are the ultimate mediators of neuronal death. Glutamate re-uptake transporters belong to a unique family of amino acid transport systems, the malfunction of which is intricately involved in disease pathogenesis. Ischemic stroke, hypoglycemia, Parkinson's disease, alcohol intoxication and withdrawal, Alzheimer's disease, epilepsy, and chronic pain syndromes are only some of the important clinical neurological disorders with a major pathogenic role for the excitatory amino acids.

CONCLUSIONS

Pharmacological manipulation of the excitatory amino acid receptors is likely to be of benefit in important and common diseases of the nervous system. Only a few of the currently available drugs that modify excitatory neurotransmission, such as remacemide, lamotrigine, and tizanidine, have an acceptable therapeutic index. The identification of numerous receptor subtypes, topographic variabilities of distribution, and multiple modulatory sites will provide a true challenge to the neuropharmacologist.

Electrophysiological actions of felbamate on rat striatal neurones

1. We have investigated the effects of the anticonvulsant drug, felbamate (FBM), on striatal neurones, recorded in vitro by using both intracellular and extracellular conventional recordings in slices and whole-cell recordings in acutely isolated neurones. 2. FBM, at therapeutically relevant concentrations (30-300 microM) showed multiple mechanisms of action. Like other antiepileptic drugs, FBM (30-300 microM) showed a direct inhibitory action on current-evoked firing discharge of striatal neurones. A patch-clamp analysis of this effect revealed a dose-related reduction of voltage-dependent sodium (Na+) currents (10-100 microM), with a half inhibiton dose (IC50) value of 28 microM. 3. We also tested whether FBM affected corticostriatal glutamate transmission. In control medium (1.2 mM external magnesium), both extracellularly recorded field potentials and intracellularly recorded excitatory postsynaptic potentials (e.p.s.ps) evoked by cortical stimulation were no affected by bath application of 30-300 microM FBM. 4. When magnesium was removed from the perfusing solution, a procedure which reveals a N-methyl-D-aspartate (NMDA)-mediated component in the corticostriatal synaptic potential, FBM (30-300 microM) produced a dose-dependent reduction of the amplitude of both the field potential and the e.p.s.p. 5. FBM reduced the inward currents produced either by bath or by focal applications of 30 microM NMDA, finding consistent with the hypothesis that the observed reduction of the NMDA-mediated component of the synaptic potentials may be caused at postsynaptic level. 6. The reduction of the NMDA-mediated component of the synaptic transmission by FBM and its depressant effect on the voltage-dependent Na+ channels, may account for the antiepileptic action of this drug. Moreover, the pharmacological properties of FBM might render this drug interesting as a neuroprotectant agent.

NMDA antagonists: antiepileptic-neuroprotective drugs with diversified neuropharmacological profiles

In conclusion, NMDA antagonists as anticonvulsants are especially active in preventing the generalization of the behavioural and electrical seizures and display a typical spectrum of in vitro antiepileptiform activities. In addition, based on in vitro and in vivo limbic kindled studies, the drugs should be regarded more as an antiepileptiform than as an anticonvulsant drugs. As neuroprotective drugs, NMDA antagonists are effective against many types of neuronal injury and show a window of activity which does not exceed 1-2 h, thus suggesting an influence of NMDA receptors in the 'early' or 'acute' mechanisms of brain damage. Among NMDA antagonists, glycine antagonists or the morphinans dextromethorphan and dextrorphan showed a spectrum of antiepileptiform and neuroprotective activities broader than other NMDA antagonists. The primary pharmacological activities of NMDA antagonists are accompanied by some effects including perturbation of many sensory, psychological or motor processes. Typical behavioural and EEG changes were also induced by the drugs. In spite of the side-effects elicited by the drugs, differential effects detected among the various classes of NMDA antagonists (i.e. lack of induction of typical EEG-behavioural effects and of typical cortical neurotoxicity) might render some of these suitable for full clinical application as anticonvulsant-neuroprotective drugs.

Felbamate selectively blocks in vitro hippocampal kainate-induced irreversible electrical changes

The influence of the anticonvulsant felbamate has been tested on in vitro excitotoxicity induced by treatment of hippocampal slices with elevated concentrations of NMDA, AMPA and kainic acid. For comparison, the effects of the glutamate antagonist 7-chlorokynurenic acid and of the anticonvulsants pentobarbitone and lamotrigine, were also studied. Slice perfusion with 50 microM NMDA or 25 microM AMPA or 12 microM kainic acid produced within 30 min a disappearance or a pronounced irreversible amplitude reduction of the CA1 electrical synaptic responses. Slice perfusion with 1.2-1.6 mM felbamate or 100 microM lamotrigine significantly decreased the incidence of the irreversible disappearance of the CA1 electrical response induced by kainic acid. On the contrary, slice perfusion with the same concentrations of the drugs did not affect the irreversible disappearance of the CA1 electrical response induced by NMDA or AMPA. By contrast, slice perfusion with 100 microM of 7-chlorokynurenic acid significantly prevented the neurotoxic effects induced by both NMDA and kainic acid, while 100 microM of pentobarbitone failed to affect kainic acid-induced neurotoxicity. The different profile of neuroprotection elicited by felbamate with respect to reference drugs indicates that a different mechanism of action than antagonism of NMDA response or potentiation of GABA response underlies the neuroprotectant effects of felbamate.

Felbamate

After the first year of clinical experience, felbamate (FBM) appears to be a valuable antiepileptic drug (AED) for the treatment of intractable epilepsy. However, many patients experience side effects that may discourage continued usage. These may be decreased by using a slower dose-escalation schedule and/or by being more aggressive in decreasing co-medication. The most common troublesome side effects are nausea and insomnia. With the recent observation of aplastic anemia, FBM should be considered only for persons with intractable epilepsy under the care of a physician familiar with FBM. Nevertheless, many patients have benefited significantly from FBM and have made a decision to continue receiving FBM at the presently known risk profile. A few more years of experience may be needed to more accurately determine the final place of FBM in the treatment of epilepsy.

Long-term behavioral deficits following pilocarpine seizures in immature rats

The effect of seizures on subsequent long-term behavior was studied in immature rats. A similar severity of seizures were induced in 20-day old rats (P20) and 45-day old rats (P45) by intraperitoneal injections of pilocarpine at doses of 200 mg/kg and 380 mg/kg, respectively. Immediately after injection of pilocarpine, prolonged seizures with electroencephalographic ictal discharges were observed in both groups of rats. These seizures were followed by seemingly complete neurological recovery. In rats that received pilocarpine at P45 spontaneous recurrent seizures appeared after 4-10 days and persisted until completion of the study at P100. Behavioral tests performed when the rats were fully mature demonstrated that they were more aggressive when handled, more active in open field, and had deficits in learning platform position in the water maze as compared to controls. Furthermore, flurothyl seizure latency was significantly lower in pilocarpine-treated P45 rats than controls. Histology examination showed gross cell loss in the CA3 subfield of the hippocampus in four out of six pilocarpine-treated rats while no cell loss was found in control rats. Rats that received pilocarpine at P20, despite having more severe seizures than the P45 rats, had no histological lesions, did not develop spontaneous recurrent seizures, and had no significant difference in the flurothyl seizure latency test when compared to their controls. While there was no difference between the control and pilocarpine-treated rats in the handling and open field test, P20 rats receiving pilocarpine were slower in learning platform position in the water maze than the controls. Rats receiving pilocarpine at P45 performed significantly more poorly than rats treated at P20 in the water maze. These results suggest that prolonged seizures in immature rats can cause long-term behavioral deficits. However, the severity and nature of these deficits are highly age dependent.

Excitatory amino acid neurotransmission through both NMDA and non-NMDA receptors is involved in the anticonvulsant activity of felbamate in DBA/2 mice

The anticonvulsant activity of felbamate against sound-induced seizures was studied in the DBA/2 mouse model. Felbamate (10-300 mg/kg, i.p.) produced dose-dependent effects with ED50 values for the suppression of tonic, clonic and wild running phases of 23.1, 48.8 and 114.6 mg/kg, respectively. Felbamate also protected DBA/2 mice from N-methyl-D-aspartate (NMDA)-induced seizures with ED50 values of 12.1 and 29 mg/kg for tonus and clonus, respectively. Pretreatment with glycine, an agonist to the glycine/NMDA receptors, shifted the dose-response effect of felbamate to the right (ED50 = 56.8 against tonus and 94.8 mg/kg versus clonus). Similarly, D-serine, an agonist at the glycine site, shifted the ED50 of felbamate against the tonic component of audiogenic seizures from 23.1 to 78.1, and that against clonus from 48.8 to 90.3 mg/kg. Felbamate was also potent to prevent seizures induced by administration of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), an AMPA/kainate receptor agonist (ED50 = 11.8 and 20.9 mg/kg, against tonus and clonus, respectively). The data indicate that felbamate is an effective anticonvulsant drug in the genetic model of seizure-prone DBA/2 mice. Our findings suggest that the anticonvulsant properties of felbamate depend upon its interaction with neurotransmission mediated by both the glycine/NMDA and the AMPA/kainate receptor complex.

Newer antiepileptic drugs. Towards an improved risk-benefit ratio

Epilepsy is one of the most common neurological disorders. Even though existing antiepileptic drugs can render 80% of newly diagnosed patients seizure free, a significant number of patients have chronic intractable epilepsy causing disability with considerable socioeconomic implications. There is, therefore, a need for more potent and effective antiepileptic drugs and drugs with fewer adverse effects, particularly CNS effects. Drugs for the treatment of partial seizures are particularly needed. With major advances in our understanding of the basic neuropathology, neuropharmacology and neurophysiology of epilepsy, numerous candidate novel antiepileptic drugs have been developed in recent years. This review comparatively evaluates the pharmacokinetics, efficacy and adverse effects of 12 new antiepileptic drugs namely vigabatrin, lamotrigine, gabapentin, oxcarbazepine, felbamate, tiagabine, eterobarb, zonisamide, remacemide, stiripentol, topiramate and levetiracetam (ucb-L059). Of the 12 drugs, vigabatrin, lamotrigine and gabapentin have recently been marketed in the UK. Five of these new drugs have known mechanisms of action (vigabatrin, lamotrigine, tiagabine, oxcarbazepine and eterobarb), which may provide for a more rational approach to the treatment of epilepsy. Oxcarbazepine, remacemide and eterobarb are prodrugs. Vigabatrin, gabapentin and topiramate are more promising on the basis of their pharmacokinetic characteristics in that they are excreted mainly unchanged in urine and not susceptible to significant pharmacokinetic interactions. In contrast, lamotrigine, felbamate and stiripentol exhibit significant drug interactions. Essentially, all the drugs are effective in partial or secondarily generalised seizures and are effective to varying degrees in other seizure types. Particularly welcome is the possible effectiveness of zonisamide in myoclonus and felbamate in Lennox-Gastaut syndrome. In relation to adverse effects, CNS effects are observed with all drugs, however, gabapentin, remacemide and levetiracetam appear to exhibit least. There is also the possibility of rational duotherapy, using drugs with known mechanisms of action, as an additional therapeutic approach. The efficacy of these 12 antiepileptic drug occurs despite the fact that candidate antiepileptic drugs are evaluated under highly unfavourable conditions, namely as add-on therapy in patients refractory to drug management and with high seizure frequency. Thus, whilst candidate drugs which do become licensed are an advance in that they are effective and/or are associated with less adverse effects than currently available antiepileptic drugs in these patients, it is possible that these drugs may exhibit even more improved risk-benefit ratios when used in normal clinical practice.