Anticonvulsant action of a non-competitive antagonist of NMDA receptors (MK-801) in the kindling model of epilepsy

Transcranial Focal Electric Stimulation Avoids P-Glycoprotein Over-Expression during Electrical Amygdala Kindling and Delays Epileptogenesis in Rats

Recent evidence suggests that P-glycoprotein (P-gp) overexpression mediates hyperexcitability and is associated with epileptogenesis. Transcranial focal electrical stimulation (TFS) delays epileptogenesis and inhibits P-gp overexpression after a generalized seizure. Here, first we measured P-gp expression during epileptogenesis and second, we assessed if TFS antiepileptogenic effect was related with P-gp overexpression avoidance. Male Wistar rats were implanted in right basolateral amygdala and stimulated daily for electrical amygdala kindling (EAK), P-gp expression was assessed during epileptogenesis in relevant brain areas. Stage I group showed 85% increase in P-gp in ipsilateral hippocampus (p < 0.001). Stage III group presented 58% and 57% increase in P-gp in both hippocampi (p < 0.05). Kindled group had 92% and 90% increase in P-gp in both hippocampi (p < 0.01), and 93% and 143% increase in both neocortices (p < 0.01). For the second experiment, TFS was administrated daily after each EAK stimulation for 20 days and P-gp concentration was assessed. No changes were found in the TFS group (p > 0.05). Kindled group showed 132% and 138% increase in P-gp in both hippocampi (p < 0.001) and 51% and 92% increase in both cortices (p < 0.001). Kindled + TFS group presented no changes (p > 0.05). Our experiments revealed that progression of EAK is associated with increased P-gp expression. These changes are structure-specific and dependent on seizure severity. EAK-induced P-gp overexpression would be associated with neuronal hyperexcitability and thus, epileptogenesis. P-gp could be a novel therapeutical target to avoid epileptogenesis. In accordance with this, TFS inhibited P-gp overexpression and interfered with EAK. An important limitation of the present study is that P-gp neuronal expression was not evaluated under the different experimental conditions. Future studies should be carried out to determine P-gp neuronal overexpression in hyperexcitable networks during epileptogenesis. The TFS-induced lessening of P-gp overexpression could be a novel therapeutical strategy to avoid epileptogenesis in high-risk patients.

Targeting NMDA Receptor Complex in Management of Epilepsy

N-methyl-D-aspartate receptors (NMDARs) are widely distributed in the central nervous system (CNS) and play critical roles in neuronal excitability in the CNS. Both clinical and preclinical studies have revealed that the abnormal expression or function of these receptors can underlie the pathophysiology of seizure disorders and epilepsy. Accordingly, NMDAR modulators have been shown to exert anticonvulsive effects in various preclinical models of seizures, as well as in patients with epilepsy. In this review, we provide an update on the pathologic role of NMDARs in epilepsy and an overview of the NMDAR antagonists that have been evaluated as anticonvulsive agents in clinical studies, as well as in preclinical seizure models.

Methadone's effects on pentylenetetrazole-induced seizure threshold in mice: NMDA/opioid receptors and nitric oxide signaling

Methadone is a synthetic opioid used to treat opiate withdrawal and addiction. Studies have demonstrated the impact of methadone on seizure susceptibility. This study investigated the modulatory impacts of acute and subchronic (three times daily for 5 days) intraperitoneal methadone treatment on pentylenetetrazole-induced clonic seizure threshold (CST) in mice, as well as the involvement of the nitric oxide, N-methyl-d-aspartate (NMDA), and µ-opioid pathways. Acute administration of different doses of methadone (0.1, 0.3, 1, and 3 mg/kg) 45 min before CST significantly decreased the seizure threshold. Additionally, pretreatment with noneffective doses of an opioid receptor antagonist (naltrexone) and NMDA receptor antagonists (ketamine and MK-801) inhibited methadone's proconvulsive activity in the acute phase, while l-NAME (a nonspecific nitric oxide synthase (NOS) inhibitor) did not affect that activity. In the subchronic phase, methadone (3 mg/kg) demonstrated an anticonvulsive effect. Although subchronic pretreatment with noneffective doses of l-NAME and 7-nitroindazole (a specific neuronal NOS inhibitor) reversed methadone's anticonvulsive activity, aminoguanidine (a specific inducible NOS inhibitor), naltrexone, MK-801, and ketamine did not change methadone's anticonvulsive characteristic. Our results suggest that NMDA and µ-opioid receptors may be involved in methadone's proconvulsive activity in the acute phase, while methadone's anticonvulsive activity may be modulated by neuronal NOS in the subchronic phase.

Undernourishment and recurrent seizures early in life impair Long-Term Potentiation and alter NMDAR and AMPAR expression in rat hippocampus

Undernourishment is a global issue, especially in developing countries, affecting newborns and children in a vulnerable period of brain development. Previous studies of undernourishment models suggested a relationship between undernourishment and epilepsy. The exposure to both undernourishment and recurrent seizures early in life appears to have detrimental effects on the developing brain. This study aims to investigate the neurobiological consequences of undernourishment and recurrent seizures exposure early in life, investigating Long-Term Potentiation (LTP) induction and gene expression of NMDA receptor subunits in the hippocampus during adulthood (P60). Animals were exposed to maternal deprivation protocol from P2 to P15 to control food intake in rat pups and Flurothyl-induced seizures from P7 to P10. Electrophysiological records of hippocampal slices were recorded and gene expression of NR1A, NR2A, NR2B, NR2C, NR2D and BDNF were investigated. Animals exposed to undernourishment or recurrent seizures failed to promote LTP after stimulation. Furthermore, seizure exposure early in life led to increased expression of hippocampal NR1A, NR2A, NR2B, NR2C and NR2D when compared to controls. Interestingly, when animals were exposed to undernourishment paradigm early in life, this upregulation of NDMA subunits was absent. In conclusion, our study showed impaired LTP after undernourishment and recurrent seizures early in life, together with differential expression of NDMA expression in the hippocampus during adulthood.

The Effects of Ginsenoside Compound K Against Epilepsy by Enhancing the γ-Aminobutyric Acid Signaling Pathway

The imbalance between the GABA-mediated inhibition and the glutamate-mediated excitation is the primary pathological mechanism of epilepsy. GABAergic and glutamatergic neurotransmission have become the most important targets for controlling epilepsy. Ginsenoside compound K (GCK) is a main metabolic production of the ginsenoside Rb1, Rb2, and Rc in the intestinal microbiota. Previous studies show that GCK promoted the release of GABA from the hippocampal neurons and enhanced the activity of GABA_A receptors. GCK is shown to reduce the expression of NMDAR and to attenuate the function of the NMDA receptors in the brain. The anti-seizure effects of GCK have not been reported so far. Therefore, this study aimed to investigate the effects of GCK on epilepsy and its potential mechanism. The rat model of seizure or status epilepticus (SE) was established with either Pentylenetetrazole or Lithium chloride-pilocarpine. The Racine's scale was used to evaluate seizure activity. The levels of the amino acid neurotransmitters were detected in the pilocarpine-induced epileptic rats. The expression levels of GABA_ARα1, NMDAR1, KCC2, and NKCC1 protein in the hippocampus were determined via western blot or immunohistochemistry after SE. We found that GCK had deceased seizure intensity and prolonged the latency of seizures. GCK increased the contents of GABA, while the contents of glutamate remained unchanged. GCK enhanced the expression of GABA_ARα1 in the brain and exhibited a tendency to decrease the expression of NMDAR1 protein in the hippocampus. The expression of KCC2 protein was elevated by the treatment of GCK after SE, while the expression of NKCC1 protein was reversely down-regulated. These findings suggested that GCK exerted anti-epileptic effects by promoting the hippocampal GABA release and enhancing the GABA_AR-mediated inhibitory synaptic transmission.

Anticonvulsant effect of minocycline on pentylenetetrazole-induced seizure in mice: involvement of nitric oxide and N-methyl-d-aspartate receptor

Anticonvulsant effects of minocycline have been explored recently. This study was designed to examine the anticonvulsant effect of acute administration of minocycline on pentylenetetrazole-induced seizures in mouse considering the possible role of the nitric oxide/N-methyl-d-aspartate (NMDA) pathway. We induced seizure using intravenous administration of pentylenetetrazole. Our results showed that acute administration of minocycline increased the seizure threshold. Furthermore, co-administration of subeffective doses of the nonselective nitric oxide synthase (NOS) inhibitor NG-l-arginine methyl ester (10 mg/kg) and the neuronal NOS inhibitor 7-nitroindazole (40 mg/kg) enhanced the anticonvulsant effect of subeffective doses of minocycline (40 mg/kg). We found that inducible NOS inhibitor aminoguanidine (100 mg/kg) had no effect on the antiseizure effect of minocycline. Moreover, l-arginine (60 mg/kg), as a NOS substrate, reduced the anticonvulsant effect of minocycline. We also demonstrated that pretreatment with the NMDA receptor antagonists ketamine (0.5 mg/kg) and MK-801 (0.05 mg/kg) increased the anticonvulsant effect of subeffective doses of minocycline. Results showed that minocycline significantly decreased the hippocampal nitrite level. Furthermore, co-administration of a neuronal NOS inhibitor like NMDA receptor antagonists augmented the effect of minocycline on the hippocampal nitrite level. In conclusion, we revealed that anticonvulsant effect of minocycline might be, at least in part, due to a decline in constitutive hippocampal nitric oxide activity as well as inhibition of NMDA receptors.

MK-801, but not naloxone, attenuates high-dose dextromethorphan-induced convulsive behavior: Possible involvement of the GluN2B receptor

Dextromethorphan (DM) is a dextrorotatory isomer of levorphanol, a typical morphine-like opioid. When administered at supra-antitussive doses, DM produces psychotoxic and neurotoxic effects in humans. Although DM abuse has been well-documented, few studies have examined the effects of high-dose DM. The present study aimed to explore the effects of a single high dose of DM on mortality and seizure occurrence. After intraperitoneal administration with a high dose of DM (80mg/kg), Sprague-Dawley rats showed increased seizure occurrence and intensity. Hippocampal expression levels of N-methyl-d-aspartate (NMDA) receptor subunits (GluN1<GluN2A<GluN2B), c-Fos and pro-apoptotic factors (Bax and cleaved caspase-3) were upregulated by DM treatment; while levels of anti-apoptotic factors (Bcl-2 and Bcl-xL) were downregulated. Consistently, DM also induced ultrastructural degeneration in the hippocampus. A non-competitive NMDA receptor antagonist, MK-801, attenuated these effects of high-dose DM, whereas an opioid antagonist, naloxone, did not affect DM-induced neurotoxicity. Moreover, pretreatment with a highly specific GluN2B subunit inhibitor, traxoprodil, was selectively effective in preventing DM-induced c-Fos expression and apoptotic changes. These results suggest that high-dose DM produces convulsive behaviors by activating GluN2B/NMDA signaling that leads to pro-apoptotic changes.

Anticonvulsant effect of dextrometrophan on pentylenetetrazole-induced seizures in mice: Involvement of nitric oxide and N-methyl-d-aspartate receptors

Dextrometrophan (DM), widely used as an antitussive, has recently generated interest as an anticonvulsant drug. Some effects of dextrometrophan are associated with alterations in several pathways, such as inhibition of nitric oxide synthase (NOS) enzyme and N-methyl d-aspartate (NMDA) receptors. In this study, we aimed to investigate the anticonvulsant effect of acute administration of dextrometrophan on pentylenetetrazole (PTZ)-induced seizures and the probable involvement of the nitric oxide (NO) pathway and NMDA receptors in this effect. For this purpose, seizures were induced by intravenous PTZ infusion. All drugs were administrated by intraperitoneal (i.p.) route before PTZ injection. Our results demonstrate that acute DM treatment (10-100mg/kg) increased the seizure threshold. In addition, the nonselective NOS inhibitor L-NAME (10mg/kg) and the neural NOS inhibitor, 7-nitroindazole (40mg/kg), at doses that had no effect on seizure threshold, augmented the anticonvulsant effect of DM (3mg/kg), while the inducible NOS inhibitor, aminoguanidine (100mg/kg), did not affect the anticonvulsant effect of DM. Moreover, the NOS substrate l-arginine (60mg/kg) blunted the anticonvulsant effect of DM (100mg/kg). Also, NMDA antagonists, ketamine (0.5mg/kg) and MK-801 (0.05mg/kg), augmented the anticonvulsant effect of DM (3mg/kg). In conclusion, we demonstrated that the anticonvulsant effect of DM is mediated by a decline in neural nitric oxide activity and inhibition of NMDA receptors.

What are the arguments for and against rational therapy for epilepsy?

Although more than a dozen new anti-seizure drugs (ASDs) have entered the market since 1993, a substantial proportion of patients (~30 %) remain refractory to current treatments. Thus, a concerted effort to identify and develop new therapies that will help these patients continues. Until this effort succeeds, it is reasonable to re-assess the use of currently available therapies and to consider how these therapies might be utilized in a more efficacious manner. This applies to the selection of monotherapies in newly-diagnosed epilepsy, but perhaps, more importantly, to the choice of combination treatments in otherwise drug-refractory epilepsy. Rational polytherapy is a concept that is predicated on the combination of drugs with complementary mechanisms of action (MoAs) that work synergistically to maximize efficacy and minimize the potential for adverse events. Furthermore, rational polytherapy requires a detailed understanding of the MoA subclasses amongst available ASDs and an appreciation of the empirical evidence that supports the use of specific combinations. The majority of ASDs can be loosely categorized into those that target neurotransmission and network hyperexcitability, modulate intrinsic neuronal properties through ion channels, or possess broad-spectrum efficacy as a result of multiple mechanisms. Within each of these categories, there are discrete pharmacological profiles that differentiate individual ASDs. This chapter will consider how knowledge of MoA can help guide therapy in a rational manner, both in the selection of monotherapies for specific seizure types and syndromes, but also in the choice of drug combinations for patients whose epilepsy is not optimally controlled with a single ASD.

Upregulation of calcium binding protein, S100A6, in activated astrocytes is linked to glutamate toxicity

S100A6 (calcyclin), an EF-hand calcium binding protein, is considered to exert various functions, e.g., cell proliferation and differentiation, calcium homeostasis, and neuronal degeneration. In this study, we aimed to investigate whether S100A6 might be linked to glutamate toxicity using three animal models and pharmacological interventions. We first examined the age-related changes in S100A6 immunoreactivity in the mouse hippocampus, considering that an important negative aspect of brain aging is linked to increased extracellular glutamate. The surface area of S100A6-positive (+) astrocytes was significantly larger in aged mice than in young mice, while the numbers of S100β+ astrocytes did not change with age. In the second experiment, we examined the alterations in S100A6 immunoreactivity in the injured hypoglossal nucleus, because glutamate toxicity is considered to contribute to neuronal death after axotomy. There was no apparent S100A6 immunoreactivity in the hypoglossal nucleus of sham control animals. However, intense labeling for S100A6 in activated astrocytes was observed in the axotomized hypoglossal nucleus of mice. Administration of ceftriaxone, an astrocyte glutamate transporter enhancer, to axotomized mice significantly decreased the immunoreactivity for S100A6. In the third experiment, we tested an animal model of epilepsy using kainic acid (KA), a glutamate analog. In the mouse hippocampus after KA injection, S100A6 immunoreactivity was significantly increased in astrocytes, and pyknotic changes were observed in CA3 pyramidal neurons. Treatment of MK-801, an N-methyl-d-aspartate receptor antagonist, counteracted the KA-induced increase in S100A6 immunoreactivity, and reduced the numbers of pyknotic neurons. Our results indicate that upregulation of astrocytic S100A6 in response to extracellular glutamate may be involved in neuronal damage under pathophysiological conditions.

Neuroimaging of frontal-limbic dysfunction in schizophrenia and epilepsy-related psychosis: toward a convergent neurobiology

Psychosis is a devastating, prevalent condition considered to involve dysfunction of frontal and medial temporal limbic brain regions as key nodes in distributed brain networks involved in emotional regulation. The psychoses of epilepsy represent an important, though understudied, model relevant to understanding the pathophysiology of psychosis in general. In this review, we (1) discuss the classification of epilepsy-related psychoses and relevant neuroimaging and other studies; (2) review structural and functional neuroimaging studies of schizophrenia focusing on evidence of frontal-limbic dysfunction; (3) report our laboratory's PET, fMRI, and electrophysiological findings; (4) describe a theoretical framework in which frontal hypoactivity and intermittent medial temporal hyperactivity play a critical role in the etiopathology of psychosis both associated and unassociated with epilepsy; and (5) suggest avenues for future research.

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

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

MK-801 increases the baseline level of anxiety in mice introduced to a spatial memory task without prior habituation

C57BL/6J mice were introduced to a nine arm radial maze without prior habituation and trained in the acquisition of a working memory task in 16 sessions, one session per day. In this maze mice need to climb onto an upward inclined bridge in order to reach and cross onto an arm. They received in each session an i.p. injection of MK-801 (0.1 mg/kg) 30 min before training or immediately after training. MK-801 pre-treated mice made significantly more entries onto the bridges, fewer entries onto the arms and took significantly longer time to make a first arm visit compared to saline and MK-801 post-treated mice during the first 3 session blocks (4 sessions per block). These results indicate that MK-801 induced anxiety which was extended throughout the first 3 session blocks. MK-801 pre-treated mice made also significantly more errors and required more sessions to reach the criterion compared to saline and MK-801 post-treated mice. Administration of MK-801 after training did not affect the acquisition of the task. The present results indicate that MK-801 pre-treatment impaired the acquisition of a spatial task and this can be accounted for by its effect on the baseline level of anxiety which was elevated. The introduction of mice to the acquisition of the task without prior habituation demonstrates that a drug treatment can affect learning and memory by increasing and/or prolonging anxiety. Such effect may be confounded with learning and memory performance and not detected with pre-habituation training procedures, particularly when the number of sessions is determined a-priori.

Is epilepsy a preventable disorder? New evidence from animal models

Epilepsy accounts for 0.5% of the global burden of disease, and primary prevention of epilepsy represents one of the three 2007 NINDS Epilepsy Research Benchmarks. In the past decade, efforts to understand and intervene in the process of epileptogenesis have yielded fruitful preventative strategies in animal models.This article reviews the current understanding of epileptogenesis, introduces the concept of a "critical period" for epileptogenesis, and examines strategies for epilepsy prevention in animal models of both acquired and genetic epilepsies. We discuss specific animal models, which may yield important insights into epilepsy prevention including kindling, poststatus epilepticus, prolonged febrile seizures, traumatic brain injury, hypoxia, the tuberous sclerosis mouse model, and the WAG/Rij rat model of primary generalized epilepsy. Hopefully, further investigation of antiepileptogenesis in animal models will soon enable human therapeutic trials to be initiated, leading to long-term epilepsy prevention and improved patient quality of life.

Role of glutamate and GABA transporters in development of pentylenetetrazol-kindling

Kindling is a form of epileptogenesis that can be induced with pentylenetetrazol (PTZ). We undertook this study to evaluate the contribution of glutamate and GABA transporters to the process of PTZ kindling. Rats were injected i.p. three times per week with PTZ (40 mg/kg) until they were fully kindled. In rats who achieved full kindling, measurement of hippocampal glutamate and GABA transporters within 24 h by western blot showed that GLAST, GLT-1, and EAAC1 were elevated significantly. However, fully kindled rats at 30 days after their last seizure had no change in either glutamate or GABA transporters proteins. These sequential observations suggest that glutamate transporters may contribute to the occurrence of seizures, but were not associated with maintenance of epileptogenesis. During this experiment, we collected data from animals that had kindled easily and animals who were resistant to kindling. Easily-kindled rats reached full kindling with less than five injections of PTZ. Kindling resistant animals failed to achieve full kindling even after administration of 12 consecutive injections of PTZ. Levels of EAAC1 and GAT-1 in easily-kindled rats were decreased by 30% when compared to kindling resistant animals at 30 days after the last PTZ injection. Since decreased EAAC1 and GAT-1 would diminish GABA function, less quantity of these proteins would appear to be associated with the convulsive threshold at the beginning of kindling development. We wonder if glutamate and GABA transporters might be operant in a convulsion threshold set factor or as a pace factor for kindling.

Limbic epileptogenesis in a mouse model of fragile X syndrome

Fragile X syndrome (FXS), caused by silencing of the Fmr1 gene, is the most common form of inherited mental retardation. Epilepsy is reported to occur in 20-25% of individuals with FXS. However, no overall increased excitability has been reported in Fmr1 knockout (KO) mice, except for increased sensitivity to auditory stimulation. Here, we report that kindling increased the expressions of Fmr1 mRNA and protein in the forebrain of wild-type (WT) mice. Kindling development was dramatically accelerated in Fmr1 KO mice, and Fmr1 KO mice also displayed prolonged electrographic seizures during kindling and more severe mossy fiber sprouting after kindling. The accelerated rate of kindling was partially repressed by inhibiting N-methyl-D-aspartic acid receptor (NMDAR) with MK-801 or mGluR5 receptor with 2-methyl-6-(phenylethynyl)-pyridine (MPEP). The rate of kindling development in WT was not effected by MPEP, however, suggesting that FMRP normally suppresses epileptogenic signaling downstream of metabolic glutamate receptors. Our findings reveal that FMRP plays a critical role in suppressing limbic epileptogenesis and predict that the enhanced susceptibility of patients with FXS to epilepsy is a direct consequence of the loss of an important homeostatic factor that mitigates vulnerability to excessive neuronal excitation.

Role of hippocampal sodium channel Nav1.6 in kindling epileptogenesis

PURPOSE

Central nervous system plasticity is essential for normal function, but can also reinforce abnormal network behavior, leading to epilepsy and other disorders. The role of altered ion channel expression in abnormal plasticity has not been thoroughly investigated. Nav1.6 is the most abundantly expressed sodium channel in the nervous system. Because of its distribution in the cell body and axon initial segment, Nav1.6 is crucial for action potential generation. The goal of the present study was to investigate the possible role of changes in Nav1.6 expression in abnormal, activity-dependent plasticity of hippocampal circuits.

METHODS

We studied kindling, a form of abnormal activity-dependent facilitation. We investigated: (1) sodium channel protein expression by immunocytochemistry and sodium channel messenger RNA (mRNA) by in situ hybridization, (2) sodium current by patch clamp recordings, and (3) rate of kindling by analysis of seizure behavior. The initiation, development, and expression of kindling in wild-type mice were compared to Nav1.6 +/-med(tg) mice, which have reduced expression of Nav1.6.

RESULTS

We found that kindling was associated with increased expression of Nav1.6 protein and mRNA, which occurred selectively in hippocampal CA3 neurons. Hippocampal CA3 neurons also showed increased persistent sodium current in kindled animals compared to sham-kindled controls. Conversely, Nav1.6 +/-med(tg) mice resisted the initiation and development of kindling.

DISCUSSION

These findings suggest an important mechanism for enhanced excitability, in which Nav1.6 may participate in a self-reinforcing cycle of activity-dependent facilitation in the hippocampus. This mechanism could contribute to both normal hippocampal function and to epilepsy and other common nervous system disorders.

Pathology and pathophysiology of the amygdala in epileptogenesis and epilepsy

Acute brain insults, such as traumatic brain injury, status epilepticus, or stroke are common etiologies for the development of epilepsy, including temporal lobe epilepsy (TLE), which is often refractory to drug therapy. The mechanisms by which a brain injury can lead to epilepsy are poorly understood. It is well recognized that excessive glutamatergic activity plays a major role in the initial pathological and pathophysiological damage. This initial damage is followed by a latent period, during which there is no seizure activity, yet a number of pathophysiological and structural alterations are taking place in key brain regions, that culminate in the expression of epilepsy. The process by which affected/injured neurons that have survived the acute insult, along with well-preserved neurons are progressively forming hyperexcitable, epileptic neuronal networks has been termed epileptogenesis. Understanding the mechanisms of epileptogenesis is crucial for the development of therapeutic interventions that will prevent the manifestation of epilepsy after a brain injury, or reduce its severity. The amygdala, a temporal lobe structure that is most well known for its central role in emotional behavior, also plays a key role in epileptogenesis and epilepsy. In this article, we review the current knowledge on the pathology of the amygdala associated with epileptogenesis and/or epilepsy in TLE patients, and in animal models of TLE. In addition, because a derangement in the balance between glutamatergic and GABAergic synaptic transmission is a salient feature of hyperexcitable, epileptic neuronal circuits, we also review the information available on the role of the glutamatergic and GABAergic systems in epileptogenesis and epilepsy in the amygdala.

Effect of YM872, a selective and highly water-soluble AMPA receptor antagonist, in the rat kindling and rekindling model of epilepsy

We examined antiepileptogenic and anticonvulsant effects of [2,3-dioxo-7-(1H-imidazol-1-yl)-6-nitro-1,2,3,4-tetrahydro-1-quinoxalinyl]-acetic acid monohydrate (YM872), a potent and highly water-soluble alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptor antagonist, in the rat amygdala kindling model of epilepsy. Administration of YM872 significantly suppressed fully kindled seizures. Daily pretreatment with YM872 markedly retarded development of kindling during drug sessions. We also used the rekindling method to investigate the antiepileptogenic effect of YM872 in an attempt to differentiate between true and false effects in the conventional method of daily administration. The results using the rekindling method suggested that the effect of YM872 was truly antiepileptogenic, indicating its possible clinical usefulness as an antiepileptogenic drug. We also affirmed the importance of AMPA receptors in the seizure expression mechanism and development of kindling-induced epileptogenesis.

Pentylenetetrazol-induced seizures affect the levels of prolyl oligopeptidase, thimet oligopeptidase and glial proteins in rat brain regions, and attenuation by MK-801 pretreatment

The regulatory mechanisms of neuropeptide-metabolizing enzymes often play a critical role in the pathogenesis of neuronal damage. A systemic administration of pentylenetetrazol (PTZ), an antagonist of GABA(A) receptor ion channel binding site, causes generalized epilepsy in an animal model. In the present study, we examined the involvement of prolyl oligopeptidase (POP), thimet oligopeptidase/neurolysin (EP 24.15/16) and glial proteins in PTZ-treated rat brain regions, and the suppressive effect of MK-801, a non-competitive NMDA receptor antagonist, pretreatment for their proteins. The activity of POP significantly decreased in the hippocampus at 30min and 3h, and in the frontal cortex at 3h after PTZ treatment, and pretreatment with MK-801 recovered the activity in the cortex at 3h. The activity of EP 24.15/16 significantly decreased in the hippocampus at 3h and 1 day, and in the cortex at 3h after the PTZ administration, whereas pretreatment with MK-801 recovered the change of the activity. The Western blot analysis of EP 24.15 showed significant decrease of the protein level in the hippocampus 3h after the PTZ treatment, whereas pretreatment with MK-801 recovered. The expression of GFAP and CD11b immunohistochemically increased in the hippocampus of the PTZ-treated rat as compared with controls. Pretreatment with MK-801 also recovered the GFAP and CD11b expression. These data suggest that PTZ-induced seizures of the rats cause indirect activation of glutamate NMDA receptors, then decrease POP and EP 24.15/16 enzyme activities and EP 24.15 immunoreactivity in the neuronal cells of the hippocampal formation. We speculate that changes of those peptidases in the brain may be related to the levels of the neuropeptides regulating PTZ-induced seizures.

Glutamate Receptor Antagonists and Benzodiazepine Inhibit the Progression of Granule Cell Dispersion in a Mouse Model of Mesial Temporal Lobe Epilepsy

PURPOSE

Unilateral intrahippocampal injection of kainic acid (KA) in adult mice induces the progressive dispersion of dentate granule cells, one of the characteristic pathologic changes of mesial temporal lobe epilepsy. However, little is known about the mechanisms that trigger this dispersion. In this study, the possible involvement of glutamatergic and gamma-aminobutyric acid (GABA)ergic neurotransmissions in the development of granule cell dispersion (GCD) was examined in this model.

METHODS

Antagonists of N-methyl-d-aspartate (NMDA) receptor (MK-801) and non-NMDA receptor (GYKI52466), and an agonist of benzodiazepine-GABA(A) receptor (midazolam) were injected before and after KA in various ways, and the morphologic changes of the hippocampus, especially GCD, were examined.

RESULTS

MK-801 (5 mg/kg, i.p.) did not reduce GCD when injected 2 h before KA injection but inhibited GCD almost completely for <or=14 days, when injected 4 h after KA. However, mild to moderate dispersion was observed at 28 days, indicating that MK-801 may delay the progression of GCD. Similarly, daily treatment with MK-801 (2 x 1 mg/kg i.p./day) for the first 26 days after KA significantly reduced GCD. In contrast, GYKI52466 (30 mg/kg, s.c.) was effective only when it was injected before KA. A significant reduction of GCD was also observed after continuous administration of midazolam (10 mg/kg/h) after KA.

CONCLUSIONS

These data show that GCD in this mouse model is triggered by either the stimulation of the NMDA receptor or reduction of GABA(A)-mediated inhibition after intrahippocampal injection of KA. It is suggested that the increased excitation or the reduced inhibition or both could be one of the factors triggering or maintaining or both the process of GCD.

Aging alters electroencephalographic and clinical manifestations of kainate-induced status epilepticus

PURPOSE

The elderly exhibit an increased risk for developing status epilepticus and status-related morbidity and mortality. However, it is unclear how aging alters the progression of electroencephalographic (EEG) activity and behavioral manifestations during status epilepticus.

METHODS

A repetitive low-dose kainate treatment protocol (2.5 mg/kg/h; i.p.) was used in this study in conjunction with EEG and behavioral monitoring from freely behaving adult (7-8 months) and aged (22-25 months) Fischer 344 rats to assess the effects of aging on status epilepticus.

RESULTS

During kainate treatment, both groups exhibited an increase in EEG power that corresponded with the time course of kainate treatment. However, visual inspection and spectral analysis revealed a reduction of the faster frequencies (12.5-35 Hz) in the EEGs of aged rodents. A similar progression of behavioral manifestations was observed in adult and aged rodents during kainate treatment, although the frequency of preseizure manifestations (e.g., wet-dog shakes; aged rats, 110 events/h vs. adults, 25 events/h; median values) was greater, and latency to onset for any given behavioral manifestation (e.g., class V seizures; aged median, 60 min, vs. adult median, 145 min) was consistently shorter within the aged group.

CONCLUSIONS

These data reveal that aged Fischer 344 rats exhibit altered EEG activity (reduction of higher frequencies) and clinical manifestations during kainate-induced status epilepticus. Taken together, these data indicate an age-related change in seizure onset and spread after exposure to glutamate analogues.

Convulsive seizures induced by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid microinjection into the mesencephalic reticular formation in rats

Effects of microinjections of a single 2 or 10 nmol dose of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) into the unilateral mesencephalic reticular formation (MRF) on behavior and on the electroencephalogram were examined in rats (n=30) over a 15-min period (Exp. 1); subsequent effects of sound stimulation with key jingling applied at 15, 30, and 45 min after the injection were observed (Exp. 2). The microinjections of a 2 nmol dose of AMPA (n=15) induced hyperactivity (15 of 15 rats) and running/circling (10 of 15 rats) in Exp. 1, and hyperactivity (5 of 15 rats) in Exp. 2. Moreover, the microinjections of a 10 nmol dose of AMPA (n=15) induced hyperactivity (15 of 15 rats), running/circling (13 of 15 rats), generalized tonic-clonic seizures (GTCS) (4 of 15 rats), and amygdala kindling-like seizures (AMKS) (8 of 15 rats) in Exp. 1; electroencephalographic seizure discharges were predominantly observed in the MRF during hyperactivity, running/circling and GTCS, while those predominantly observed in the amygdala were during AMKS. In Exp. 2, hyperactivity (15 of 15 rats), running/circling (14 of 15 rats) and GTCS (6 of 15 rats) were elicited by sound stimulation, although AMKS were not. The control group of rats (n=15) which received a single dose of saline microinjection into the unilateral MRF showed no behavioral or electroencephalographic changes in both Exp. 1 and 2. These findings suggest that potentiation of excitatory amino acid neurotransmission induced by AMPA injection into the MRF plays an important role not only in the development of hyperactivity, running/circling, GTCS and AMKS, but also in the development of audiogenic seizures.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Epileptiform activity in rat hippocampus strengthens excitatory synapses

Although epileptic seizures are characterized by excessive excitation, the role of excitatory synaptic transmission in the induction and expression of epilepsy remains unclear. Here, we show that epileptiform activity strengthens excitatory hippocampal synapses by increasing the number of functional (RS)-alpha-amino-3hydroxy-5methyl-4-isoxadepropionate (AMPA)-type glutamate receptors in CA3-CA1 synapses. This form of synaptic strengthening occludes long-term potentiation (LTP) and enhances long-term depression (LTD), processes involved in learning and memory. These changes in synaptic transmission and plasticity, which are fully blocked with N-methyl-D-aspartate (NMDA) receptor antagonists, may underlie epilepsy induction and seizure-associated memory deficits.

Dose-dependent effect of MK-801 on the levels of neuropeptides processing enzymes in rat brain regions

The appropriate levels of neuropeptides and their processing enzyme activities are required to continue a normal cell life, and the dysfunction of these peptides and enzymes are responsible for many neuronal abnormalities. Systemic administration of (+) MK-801 (dizocilpine maleate), a noncompetitive N-methyl-[D]-aspartate (NMDA) receptor antagonist, causes both neuroprotective and neurotoxic activities depending on doses and conditions. In the present study, we investigated the dose dependent effect of (+) MK-801 on prolyl endopeptidase (PEP), endopeptidase EC 24.15 (EP 24.15) and beta-D-glucuronidase activities as well as the protein levels of EP 24.15 and neuron specific enolase (NSE) in the posterior cingulate/retrosplenial cortices (PC/RSC), hippocampus, frontal cortex and striatum of female rats 3 days after the treatment. The activity of PEP was significantly increased compared with controls (saline) in the PC/RSC at 1.0 and 5.0 mg/kg doses, and in the frontal cortex at 5.0 mg/kg dose. beta-D-Glucuronidase activity was dose-dependently increased in all brain regions examined. The activity of EP 24.15 was unchanged in all regions after the treatment, whereas the Western blot analysis for EP 24.15 showed the increased protein level in the PC/RSC. These results suggest that a low dose treatment with MK-801 causes neurotoxicity in the PC/RSC and hippocampus, and the high dose treatment causes neurotoxicity in all the brain regions examined.

Neuroadaptive processes in GABAergic and glutamatergic systems in benzodiazepine dependence

Knowledge of the neural mechanisms underlying the development of benzodiazepine (BZ) dependence remains incomplete. The gamma-aminobutyric acid (GABA(A)) receptor, being the main locus of BZ action, has been the main focus to date in studies performed to elucidate the neuroadaptive processes underlying BZ tolerance and withdrawal in preclinical studies. Despite this intensive effort, however, no clear consensus has been reached on the exact contribution of neuroadaptive processes at the level of the GABA(A) receptor to the development of BZ tolerance and withdrawal. It is likely that changes at the level of this receptor are inadequate in themselves as an explanation of these neuroadaptive processes and that neuroadaptations in other receptor systems are important in the development of BZ dependence. In particular, it has been hypothesised that as part of compensatory mechanisms to diazepam-induced chronic enhancement of GABAergic inhibition, excitatory mechanisms (including the glutamatergic system) become more sensitive [Behav. Pharmacol. 6 (1995) 425], conceivably contributing to BZ tolerance development and/or expression of withdrawal symptoms on cessation of treatment, including increased anxiety and seizure activity. Glutamate is a key candidate for changes in excitatory transmission mechanisms and BZ dependence, (1) since there are defined neuroanatomical relationships between glutamatergic and GABAergic neurons in the CNS and (2) because of the pivotal role of glutamatergic neurotransmission in mediating many forms of synaptic plasticity in the CNS, such as long-term potentiation and kindling events. Thus, it is highly possible that glutamatergic processes are also involved in the neuroadaptive processes in drug dependence, which can conceivably be considered as a form of synaptic plasticity. This review provides an overview of studies investigating changes in the GABAergic and glutamatergic systems in the brain associated with BZ dependence, with particular attention to the possible differential involvement of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in these processes.

Cessation of repeated administration of MK-801 changes the anticonvulsant effect against flurothyl-induced seizure in mice

The effects of acute and repeated administration of MK-801 on flurothyl (FE)-induced seizure were investigated in mice. In the acute effect of MK-801 (0.01-0.1 mg/kg ip) in naive and FE-kindled mice, there were no changes on the latencies of clonic seizures. However, MK-801 dose-dependently inhibited both latencies and incidence of tonic seizures in mice and suppressed the grade of seizure severity in FE-kindled mice. Repeated administration of MK-801 at doses of 0.01 and 0.1 mg/kg 2 h prior to each exposure to FE for 8 days did not show any effects on the latencies of clonic seizure. However, seizure severity was significantly exacerbated in the 0.1 mg/kg treated group when mice were re-exposed to FE without MK-801 1 week after the last administration. A week after the repeated administration of MK-801 at a dose of 0.1 mg/kg for 8 days without exposure to FE, mice were exposed to FE 2 h after readministration of MK-801 until tonic seizure occurred. The latencies of clonic seizures were almost the same in the acute experiment in naive controls. The latency of tonic seizure was significantly delayed compared to the acute experiment with MK-801 at a dose of 0.1 mg/kg. These findings suggested that MK-801 possessed an anticonvulsant action against FE-induced tonic seizure. However, the efficacy of this acute effect of MK-801 was impaired at 1 week of withdrawal after repeated administrations. This may be related in part to the changes in sensitivity to NMDA receptors.

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.

Functional and morphological changes in the hippocampal neuronal circuits associated with epileptic seizures

PURPOSE

We review what is currently known about functional and morphologic plasticity of hippocampal neuronal circuits in animal epilepsy models.

RESULTS

Perforant-path kindling has been shown to enhance excitatory synaptic transmission in the dentate gyrus significantly, and this enhancement has lasted for > or =1 month after kindling. However, in our rapid kindling experiment with an interstimulus interval of 5 min, perforant-path kindling resulted in behavioral convulsions and prolonged the afterdischarge duration, but dentate synaptic transmission remained depressed during kindling.

CONCLUSIONS

These results indicate that kindling-induced synaptic potentiation is not necessary for kindling development. Conversely, epileptic seizures have led to the anomalous sprouting of mossy fibers, a process thought to induce hyperexcitation of granule cells. We conclude that the sprouting of mossy fibers did not play a critical role in kindling, because we observed no significant correlation between the mossy fiber sprouting and kindling development. Epileptic seizures also have been shown to increase gamma-aminobutyric acid (GABA)A receptor-mediated inhibition during interictal periods. Although this strengthening of GABAergic inhibition is likely to be a defensive measure against seizure initiation, some studies have suggested that another type of GABAergic response facilitates seizure activity. The possibility of GABAA receptor-mediated excitation during ictal periods is theoretically examined.

Secretory phospholipase A2-mediated neuronal cell death involves glutamate ionotropic receptors

To define the significance of glutamate ionotropic receptors in sPLA -mediated neuronal cell death we used the NMDA receptor antagonist MK-801 and the AMPA receptor antagonist PNQX. In primary neuronal cell cultures both MK-801 and PNQX inhibited sPLA - and glutamate-induced neuronal death. [ H]Arachidonic acid release induced by both sPLA and glutamate was partially blocked by MK-801, indicating that the glutamate-NMDA-cPLA pathway contributes to sPLA -induced arachidonic acid release. Systemic administration of MK-801 to rats that had sPLA injected into the right striatum significantly decreased neuronal cell death. We conclude that glutamatergic synaptic activity modulates sPLA -induced neuronal cell death.

Kindling enhances kainate receptor-mediated depression of GABAergic inhibition in rat granule cells

Several lines of evidence indicate a substantial contribution of kainate receptors to temporal lobe seizures. The activation of kainate receptors located on hippocampal inhibitory interneurons was shown to reduce GABA release. A reduced GABA release secondary to kainate receptor activation could contribute to an enhanced seizure susceptibility. As the dentate gyrus serves a pivotal gating function in the spread of limbic seizures, we tested the role of kainate receptors in the regulation of GABA release in the dentate gyrus of control and kindled animals. Application of glutamate (100 micro m) in the presence of the NMDA receptor antagonist d-APV and the AMPA receptor antagonist, SYM 2206 caused a slight depression of evoked monosynaptic inhibitory postsynaptic currents (IPSCs) in control, but a substantial decrease in kindled dentate granule cells. The observation that kainate receptor activation altered paired-pulse depression and reduced the frequency of TTX-insensitive miniature IPSCs without affecting their amplitude is consistent with a presynaptic action on the inhibitory terminal to reduce GABA release. In kindled preparations, neither glutamate (100 micro m) nor kainate (10 micro m) applied in a concentration known to depolarize hippocampal interneurons led to an increase of the TTX-sensitive spontaneous IPSC frequency nor to changes of the postsynaptic membrane properties. Consistently, the inhibitory effect on evoked IPSCs was not affected by the presence of the GABAB receptor antagonist, CGP55845A, thus excluding a depression by an enhanced release of GABA acting on presynaptic GABAB receptors. The enhanced inhibition of GABA release following presynaptic kainate receptor activation favours a use-dependent hyperexcitability in the epileptic dentate gyrus.

Consequences of amygdala kindling and repeated withdrawal from ethanol on amphetamine-induced behaviours

It has been shown previously that chronic ethanol treatment in mice leads to accelerated behavioural sensitization to psychomotor stimulants [Manley & Little (1997) J. Pharmacol. Exp. Ther., 281, 1330-1339], whilst repeated experience of ethanol withdrawal sensitizes pathways underlying seizure activity (Becker & Hale (1993) Alcohol Clin. Exp. Res., 17, 94-98]. The aim of the current experiment was to investigate the consequences of repeated withdrawal from ethanol on amphetamine-induced behaviours in the rat and compare this with animals with electrical kindling of the amygdala, a procedure that has been shown to enhance alcohol withdrawal seizures [Pinel et al. (1975) Can. J. Neurol. Sci., 2, 467-475]. For the kindling experiments, electrodes were surgically implanted in the left basolateral amygdala and were stimulated daily at the afterdischarge threshold until a criterion of three consecutive stage 5 seizures was reached. Fully kindled rats showed a marginally significant reduction in sensitivity to the locomotor stimulant effects of acute amphetamine compared with sham and partially kindled rats which had experienced subthreshold stimulation of the amygdala. Sham and partially kindled rats sensitized readily to the locomotor activating effects of amphetamine (0.125 mg/kg) following repeated treatments, but the fully kindled rats did not. Fully kindled rats also failed to show place preference conditioning to amphetamine (0.5 mg/kg). Rats, withdrawn three times from chronic ethanol (liquid-diet), kindled more quickly to PTZ (30 mg/kg, i.p.) than rats with the same overall exposure to ethanol (24 days) followed by a single withdrawal or control animals. However, there was no difference in the locomotor stimulating effects of acute amphetamine (0.25-1 mg/kg, i.p.), the rate of sensitization to amphetamine (0.125 mg/kg, i.p.) or amphetamine induced conditioned place preference (1 mg/kg, i.p.). These observations suggest that, in rats, repeated withdrawal from a relatively mild chronic ethanol treatment modulates neuronal systems that may also be involved in PTZ-induced kindling but not those involved in either the acute stimulant effects of amphetamine or behavioural sensitization or appetitive conditioning following repeated amphetamine administration. Behavioural changes following amygdala kindling differed from those following repeated ethanol withdrawal, suggesting that withdrawal kindling from a mild ethanol treatment differs in its effects from amygdala kindling.

Expression of glutamate transporters and ionotropic glutamate receptors in GLAST knockout mice

In order to investigate the molecular mechanism underlying high seizure susceptibility of GLAST knockout mice, we carried out Western blotting for the expression of GLT-1, EAAC-1, and several kinds of glutamate receptors in the hippocampus and the cortex. Although no significant difference was observed between GLAST (+/+) and (-/-) mice in terms of expression of GLT-1 and EAAC-1 in the hippocampus, these proteins were over-expressed in the frontal cortex in GLAST (-/-) mice (GLT-1, about 210% increase; EAAC-1, about 180% increase). Expression of hippocampal Glu-R1 and Glu-R2 in GLAST (-/-) mice was remarkably increased (Glu-R1, about 140% increase; Glu-R2, about 160% increase), while Glu-R3 and NMDA receptors levels (NMDA-R1, 2A and 2B) were equal to those in control. Cortical levels of Glu-R1, -R2 and -R3 receptors in GLAST (-/-) mice were remarkably decreased (Glu-R1, about 60% decrease; Glu-R2, about 60% decrease; Glu-R3, about 70% decrease), while NMDA receptors were remarkably increased in comparison to those in GLAST (+/+) mice (N-R1, about 150% increase; N-R2A, about 150% increase; N-R2B, about 140% increase). These data suggest that the increased susceptibility to seizures in GLAST (-/-) mice might be derived from increased expression of Glu-R1 in the hippocampus coupled with decreased cortical expression of Glu-R2 and increased NMDA-R1 and -2A, -2B expression.

Effects of MK-801 and electroconvulsive shock on c-Fos expression in the rat hippocampus and frontal cortex

Both the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 and electroconvulsive shock (ECS) have been reported to induce c-Fos in rat brain. However, the former has anticonvulsant and psychotomimetic effects and the latter has proconvulsant and antipsychotic effects. To understand the mode of action of these treatments, the authors examined the effect of MK-801 and the interaction between MK-801 and ECS on the induction of c-Fos in the rat hippocampus and frontal cortex. MK-801 induced c-Fos in these brain regions in a nonlinear dose-response relationship. Maximum effect was achieved with 1-2 mg/kg of MK-801. The level of c-Fos paralleled animal hyperkinetic behavior, suggesting the role of c-Fos in the induced psychotomimetic behaviors. Pretreatment with MK-801 dose-dependently attenuated both the seizures and c-Fos expression by ECS. However, at an MK-801 pretreatment dose of 8 mg/kg, which completely blocked ECS-induced seizure, the induction of c-Fos was not completely blocked, suggesting non-NMDA mediated pathways of the induction of c-Fos by ECS.

Epileptic-like activity induces multiple forms of plasticity in hippocampal area CA1

Mesial temporal lobe epilepsy is a relatively common form of epilepsy that afflicts many thousands of people. It has been suggested that the development of primary and secondary foci may involve mechanisms similar to long-term potentiation (LTP). In vitro seizure models typically involve an increase in spontaneous asynchronous bursting activity (epileptiform activity) induced either by increasing excitation or decreasing inhibition. Previous experiments have indicated that these models often generate bursting activity that closely resembles epileptic activity. LTP is often observed following epileptiform activity. In area CA1 of the hippocampus two forms of LTP that are dependent on the activation of either the L-type voltage dependent calcium channel (vdccLTP) or the N-methyl-D-aspartate receptor/channel (nmdaLTP) have been described. It is unclear from previous experiments which type of LTP results from epileptiform activity. Recent evidence indicates that nmdaLTP is most likely a short-term type of plasticity while vdccLTP may be a long-lasting form of synaptic plasticity. Given the characteristics of vdccLTP it is a likely candidate mechanism to underlie the development and formation of secondary seizure foci. We have therefore tested the ability of epileptiform activity induced by elevated potassium chloride to induce multiple forms of LTP in area CA1 of the rat hippocampus. Elevation of extracellular potassium chloride resulted in spontaneous asynchronous bursting. The net result of the spontaneous asynchronous bursting was to induce a compoundLTP consisting of nmdaLTP and vdccLTP components.

Low doses of AMPA exert anticonvulsant effects on pentylenetetrazol-kindled seizures

Excitatory amino acids (EAAs) are critically involved in the initiation and propagation of seizures. N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors appear to be of special interest in this regard. Besides receptor binding by antagonists, the function of glutamatergic synapses can be altered via autoreceptor-mediated mechanisms or by receptor desensitisation. Therefore, the effect of AMPA (1, 10 or 100 pmol per animal, intracerebroventricular injection) was tested on acutely induced pentylenetetrazol (PTZ) seizures. The lowest dose exerted clear anticonvulsant effects. Furthermore, 1 and 10 pmol AMPA were tested for their efficacy to suppress PTZ kindling. The lower dose reduced seizure severity significantly but 10 pmol AMPA was ineffective. In reaction to a test dose of PTZ, the kindled groups pretreated with AMPA reached seizure scores similar to saline-pretreated kindled rats, suggesting that the kindled state was reached. In a further experiment, we tested the effect of cyclothiazide (CYC, which blocks AMPA receptor desensitisation) on the 1 pmol AMPA-mediated anticonvulsant effect. The AMPA response was not altered. These results suggest that autoreceptor-mediated mechanisms rather than desensitisation might contribute to the anticonvulsant effect found.

Sequential changes in AMPA and NMDA protein levels during Fe(3+)-induced epileptogenesis

Seizure susceptibility is related to enhanced glutamatergic excitatory synaptic transmission with alterations in the expressions of ionotropic glutamate receptors. We wondered if levels of AMPA and NMDA receptor subunits changed following epileptogenesis induced by amygdalar FeCl(3) injection. We used Western blots to measure levels of subunits in the ipsilateral and contralateral hippocampus at various times after FeCl(3) injection into the amygdaloid body. With acute seizures, at +5 days after the injection, levels of GluR1, NMDAR1, and NMDAR2 were markedly increased in both hippocampi, with quantities at least 2-4 times baseline. By +15 and +30 days after injection, when chronic spontaneous seizures were occurring, the levels of GluR2 were increased, while GluR1 and NMDAR1&2A/B were decreased. Increased NMDAR1&2A/B levels at +5 days are consistent with the occurrence of upregulation of NMDA receptor production in the early stages of epileptogenesis. Since GluR2 suppresses glutamate receptor-mediated Ca(2+)-influx, increased expression of GluR2 with development of chronic, recurrent seizures may be a compensatory effect during epileptogenesis from neural responses to propagated seizures.

Effect of riluzole on the acquisition and expression of amygdala kindling

PURPOSE

Riluzole possesses various synaptic effects including an inhibitory action on glutamate release. The drug has been shown to inhibit kindled seizures, while its effect on the acquisition of kindling has not been reported. We investigated effects of riluzole on the kindling development in addition to effects on kindled seizures.

METHODS

A bipolar electrode was implanted in the right amygdala of rats. Riluzole was injected intraperitoneally 30 min before kindling stimulation. To investigate effects of riluzole on the kindling development, rats were stimulated once daily for the drug session of 14 days at a current of 200 microA, 60 Hz, 1 ms for 2 s and thereafter stimulated without drugs (drug-free session) until completion of kindling. Seizure ranks and after discharge duration were observed every day. To investigate effects of riluzole on kindled seizures, fully-kindled rats were stimulated at the current of generalized seizure threshold (GST) before and after the administration of riluzole. Seizure ranks and after discharge duration were measured. GST after the treatment was also determined.

RESULTS

The number of stimuli required for the first appearance of stage five seizure was significantly larger in rats treated with 8 mg/kg of riluzole than in vehicle controls. Riluzole at a dose of 8 mg/kg significantly retarded the development of seizure stages in the drug session. By comparison, effects on the duration of after discharge was relatively mild, though significantly different from the vehicle control. Riluzole at a dose of either 4 or 8 mg/kg markedly inhibited behavioral seizures and reduced the duration of after discharge in kindled seizures provoked by GST. The drug also significantly increased GST at both doses, suggesting that the anticonvulsant effects were attributed to the increase in GST.

CONCLUSION

It was demonstrated that inhibitory effects of riluzole on both kindled seizures and the development of behavioral seizures in kindling acquisition with relatively mild correlation to afterdischarge duration. These effects might be attributed to inhibitory actions of riluzole on glutamate release and NMDA-receptor mediated events.

Preclinical Evaluation of CHF3381 as a Novel Antiepileptic Agent

CHF3381 [n-(2-indanyl)-glycinamide hydrochloride] has been selected on the basis of a screening program as the compound displaying the highest anticonvulsant activity in the maximal electroshock seizure (MES) test and the best therapeutic index with reference to the rotarod test in mice and rats. In this study, the antiepileptic activity and the behavioural toxicity of CHF3381 were characterised in multiple model systems. CHF3381 effectively prevented MES-induced convulsions when administered i.p. (ED50, 24 mg/kg and 7.5 mg/kg) or p.o. (ED50, 21 mg/kg and 21 mg/kg) in both mice and rats, respectively. The time course of oral anti-MES activity in the rat was related to the brain concentration profile of unchanged CHF3381. Interestingly, the brain drug levels were about 4-5 times higher than in plasma. CHF3381 was very effective in mice against picrotoxin-, and i.c.v. N-methyl-D-aspartate (NMDA)-induced hind limb tonic extension (ED50 Approximately/=10 mg/kg), but was a weaker antagonist of 4-amynopyridine- and bicuculline-induced tonic seizures (ED50 approximately/=100 mg/kg), and ineffective against pentylentetrazole- and picrotoxin-induced clonic seizures. CHF3381 antagonised the behavioural effects and lethality of i.p. administered NMDA (ED50 = 57 mg/kg p.o.), indicating that the compound may act as a functional NMDA antagonist. In keeping with this idea, CHF3381 weakly displaced [(3)H]-TCP from binding to NMDA receptor channels (Ki, 8.8 microM). In the rat amygdala kindling model, CHF3381 was more efficient against kindling development than against kindled seizures (minimally active dose = 80 vs. 120 mg/kg i.p). Furthermore, it significantly increased the seizure threshold in kindled rats at relatively low doses (40 mg/kg i.p.). In contrast with MK-801-induced hyperactivity, CHF3381 moderately reduced the spontaneous locomotor activity in mice at anticonvulsant doses. Toxic effects on motor performance (rotarod test) were found at high doses only (TD50 approximately/= 300 mg/kg p.o., congruent with 100 mg/kg i.p. in both mice and rats). Furthermore, CHF3381 did not impair passive avoidance and Morris water maze responding in the therapeutic range of doses. Finally, the development of tolerance after repeated doses was negligible. These data indicate that CHF3381 exerts anticonvulsant and antiepileptogenic effects in various seizure models and possesses good therapeutic window, with scarce propensity to cause neurological side-effects.

Kindling Induces Transient NMDA Receptor–Mediated Facilitation of High-Frequency Input in the Rat Dentate Gyrus

To elucidate the gating mechanism of the epileptic dentate gyrus on seizure-like input, we investigated dentate gyrus field potentials and granule cell excitatory postsynaptic potentials (EPSPs) following high-frequency stimulation (10–100 Hz) of the lateral perforant path in an experimental model of temporal lobe epilepsy (i.e., kindled rats). Although control slices showed steady EPSP depression at frequencies greater than 20 Hz, slices taken from animals 48 h after the last seizure presented pronounced EPSP facilitation at 50 and 100 Hz, followed by steady depression. However, 28 days after kindling, the EPSP facilitation was no longer detectable. Using the specific N-methyl-d-aspartate (NMDA) and RS-α-amino-3-hydroxy-5-methyl-4-isoxazoleproponic acid (AMPA) receptor antagonists 2-amino-5-phosphonovaleric acid and SYM 2206, we examined the time course of alterations in glutamate receptor–dependent synaptic currents that parallel transient EPSP facilitation. Forty-eight hours after kindling, the fractional AMPA and NMDA receptor–mediated excitatory postsynaptic current (EPSC) components shifted dramatically in favor of the NMDA receptor–mediated response. Four weeks after kindling, however, AMPA and NMDA receptor–mediated EPSCs reverted to control-like values. Although the granule cells of the dentate gyrus contain mRNA-encoding kainate receptors, neither single nor repetitive perforant path stimuli evoked kainate receptor–mediated EPSCs in control or in kindled rats. The enhanced excitability of the kindled dentate gyrus 48 h after the last seizure, as well as the breakdown of its gating function, appear to result from transiently enhanced NMDA receptor activation that provides significantly slower EPSC kinetics than those observed in control slices and in slices from kindled animals with a 28-day seizure-free interval. Therefore, NMDA receptors seem to play a critical role in the acute throughput of seizure activity and in the induction of the kindled state but not in the persistence of enhanced seizure susceptibility.

Role of AMPA and GluR5 kainate receptors in the development and expression of amygdala kindling in the mouse

The role of AMPA and GluR5-containing kainate receptors in the development and expression of amygdala kindling was examined using the selective 2,3-benzodiazepine AMPA receptor antagonist GYKI 52466 [(1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2, 3-benzodiazepine] and the decahydroisoquinoline mixed AMPA receptor and GluR5 kainate receptor antagonist LY293558 {(3S,4aR,6R, 8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]decahydroisoquinoline- 3-carboxy lic acid)}. Administration of GYKI 52466 (5-40 mg/kg, intraperitoneally) and LY293558 (10-40 mg/kg, intraperitoneally) prior to daily kindling stimulation in mice produced a dose-dependent suppression of the rate of development of behavioral kindled seizure activity and reduced the duration of the stimulation-induced electrographic afterdischarge. In drug-free stimulation sessions after the initial drug-treatment sessions, there was an acceleration in the rate of kindling development compared with the rate during the preceding drug-administration period; the "rebound" rate was also greater than the kindling rate in saline-treated control animals. In fully kindled animals, both GYKI 52466 and LY293558 produced a dose-dependent suppression of evoked seizures (ED(50), 19.3 and 16.7 mg/kg, respectively). Although AMPA receptors appear to be critical to the expression of kindled seizures, since kindling development progressed despite the suppression of behavioral seizure activity, AMPA receptors are less important to the kindling process. LY293558 was modestly less effective at suppressing behavioral seizures during kindling and was not superior to GYKI 52466 in retarding the overall extent of kindling development, indicating that GluR5 kainate receptors do not contribute to epileptogenesis in this model.

Effects of MK-801, dantrolene, and FK506 on convulsive seizures and brain nitric oxide production in seizure-susceptible EL mice

To clarify the role of nitric oxide (NO) in the pathogenesis of seizures in susceptible EL mice, we investigated effects of three drugs potentially related to NO production, MK-801, dantrolene, and FK506, on convulsive seizures and brain NO metabolites (NOx). MK-801 or dantrolene, but not FK506, suppressed convulsive seizures in EL mice; only MK-801 reduced NOx in the brain. Our results suggested involvement of the N-methyl-D-aspartate receptor-channel complex and intracellular calcium mobilization, but not calcineurin, in the convulsions of EL mice.

Antiepileptogenic properties of phenobarbital: behavior and neurochemical analysis

Chronic in vivo models of epilepsy provide a suitable strategy for quantifying epileptogenesis, as well as investigating neurochemical changes associated with neuronal plasticity that leads to seizuring conditions. The aim of this paper was to investigate antiepileptogenic properties of phenobarbital, focusing on the neurochemical changes associated with repeated seizures induced by low convulsive dose of pentylenetetrazol (PTZ) (60 mg/kg, sc) in mice. Phenobarbital (10 and 30 mg/kg, ip) significantly diminished the severity of seizures induced by PTZ. Repeated PTZ administration was associated with an increase in [3H]glutamate binding (B(max) 196.6+/-10.2 pmol/mgxcontrol B(max) 137.7+/-17.0 pmol/mg). Regarding NMDA receptors, repeated PTZ administration was likewise associated with an increase in [3H]MK-801 binding (0.55+/-0.02 pmol/mgxcontrol 0.32+/-0.01 pmol/mg). In addition, phenobarbital (10 mg/kg) prevented the increase in [3H]glutamate binding (B(max) 133.7+/-11.4 pmol/mg), as well as in [3H]MK-801 binding (phenobarbital 10 and 30 mg/kg, 0.33+/-0.01 and 0.34+/-0.01 pmol/mg, respectively). This study reveals an interesting capability of phenobarbital in interfering with the establishment of both the behavioral expression and associated neurochemical changes induced by the repeated administration of low convulsive dose of PTZ, which may be important in the context of preventing epileptogenesis.

Convulsive seizures induced by N-methyl-D-aspartate microinjection into the mesencephalic reticular formation in rats

Effects of microinjections of a single 2 or 10 nmol dose of N-methyl-D-aspartate (NMDA) into the unilateral mesencephalic reticular formation (MRF) on behavior and electroencephalogram were examined in rats (n=18) during a 15 min period (Exp. 1), and subsequent effects of sound stimulation with key jingling applied at 15, 30, and 45 min after the injections were observed (Exp. 2). The microinjections of 2 nmol dose of NMDA (n=10) induced hyperactivity (9 of 10 rats) and running/circling (8 of 10 rats) in Exp. 1, and hyperactivity (3 of 10 rats) in Exp. 2. Moreover, the microinjections of 10 nmol dose of NMDA (n=8) induced not only hyperactivity (8 of 8 rats) and running/circling (7 of 8 rats) but also generalized tonic-clonic seizures (GTCS) (5 of 8 rats) in Exp. 1; these seizure patterns were also elicited by sound stimulation in Exp. 2. The seizure patterns were accompanied by electroencephalographic seizure discharges in the MRF and the motor cortex. In contrast, the control group rats (n=10) which received a single dose of saline microinjection into the unilateral MRF showed no behavioral or electroencephalographic changes in both Exp. 1 and 2. These findings suggest that the MRF has an important role in the development of GTCS, which follows hyperactivity and running/circling, and that potentiation of excitatory neurotransmission in the MRF participates in the development of audiogenic seizures as well as GTCS.

Lasting changes in NMDAR1 mRNA level in various regions of cerebral cortex in epileptogenesis of amygdaloid-kindled rat

The involvement of NMDA receptor subunit, NR1, with kindling phenomenon has been reported, but the role of NR1 in epileptogenesis is still unknown. We have examined the expression levels of NR1 mRNA in the cerebral cortices of amygdaloid-kindled rats. Northern blot analysis showed a significant increase in NR1 mRNA expression level in the ipsilateral frontal and temporal cortices at 4 weeks after the last generalized seizure. At the same time, NR1 mRNA decreased in the bilateral piriform cortices. These data suggest that NR1-mediated transmission may have an impact in the neurobiological basis of enduring epileptogenesis.

Dorsal hippocampal kindling produces long-lasting changes in the origin of spontaneous discharges in the piriform versus perirhinal cortex in vitro

In an in vitro slice preparation of the amygdala-piriform-perirhinal cortex (A-P area), it was shown previously (McIntyre, D.C., Plant, J. R., 1993. Long-lasting changes in the origin of spontaneous discharges from amygdala-kindled rats: piriform vs. perirhinal cortex in vitro, Brain Res. 624, 268-276) that the infrequent spontaneous field potentials that initially originated in or near the perirhinal (PRh) cortex of slices from control rats began instead in the piriform (Pir) cortex of amygdala-kindled rats. This change in onset was only observed in the A-P area ipsilateral to the kindled amygdala. In the present experiment, we determined whether similar changes in activity were evident following kindling from a different limbic site, the dorsal hippocampus (DH). Kindling of the DH resulted in changes in the origin of the spontaneous discharges in the A-P area similar to amygdala kindling but, importantly, the changes involved both hemispheres. In addition, the origin of spontaneous discharges in slices from partial kindled rats (those that received as many hippocampal afterdischarges as the fully kindled rats but had not developed generalized convulsive responses) initially were similar to control tissue, but, during 0 Mg(2+) perfusion, changed more quickly than control tissue to mimic the profile of generalized kindled rats. The enduring changes in A-P area excitability caused by previous generalized kindling highlights the importance of the A-P area in convulsive generalization of limbic-kindled seizures.

Kindling induced by pentylenetetrazole in rats is not directly associated with changes in the expression of NMDA or benzodiazepine receptors

Repeated injections of a subconvulsant dose of pentylenetetrazole (PTZ, 30 mg/kg IP three times weekly for 13 injections) in Wistar and hooded Lister rats resulted in kindled seizures, the extent of which varied between strains. Wistar rats achieved stage 4 of clonic-tonic seizures, whereas hooded Lister rats only reached stage 2 of convulsive waves axially through the body. Rats were killed 10 days after their final injection, and radioligand binding was used to measure the expression of NMDA receptors in cortex and hippocampus using [3H]MK-801 and [3H]L-689,560, the latter binding specifically to the NR1 subunit. [3H]Ro 15-1788 measured expression of GABA(A)-benzodiazepine binding sites containing alpha1, alpha2, alpha3, or alpha5 subunits. Specific analysis of GABA(A) receptors containing the alpha5 subunit, which are preferentially localized in the hippocampus, was assessed with [3H]L-655,708. In the cortex, there was no effect of strain or treatment on the K(D) or B(max) of any of the ligands. Similarly, there was no effect of strain or treatment on hippocampal [3H]L-689,560 or [3H]Ro 15-1788 binding. However, in the hippocampus there was a significant, albeit modest, effect of treatment on the B(max) of [3H]MK-801 binding and the B(max) and K(D) of [3H]L-655,708 binding, i.e., PTZ-treated rats had fewer [3H]MK-801 and [3H]L-655,708 binding sites (NMDA and alpha5-containing GABA(A) receptors, respectively), but, these reductions were significant only in the relatively seizure-insensitive hooded Lister strain. This suggests that the increased susceptibility to kindling in Wistar rats is not directly related to alterations in the expression of NMDA or GABA(A) receptors.