Antiepileptogenic and anticonvulsant effects of NBQX, a selective AMPA receptor antagonist, in the rat kindling model of epilepsy

Impact of Perampanel for First-Episode Seizures versus Usual Care on Clinical Outcome and Safety Profile Aspects of the Thai Experience

Background and Purpose

Epilepsy increases poor outcomes in patients with post-traumatic brain injury and brain tumor-related epilepsy, for whom early seizure control is essential. Perampanel (PER) was a known third-generation antiepileptic drug for treatment all types of seizures. The objective of the study is to compare clinical outcomes and safety of PER administration as monotherapy.

Methods

A prospective study of all 84 patients assigned to PER monotherapy (PER group, n=36) and other first-line antiepileptic drugs (n=48). Clinical outcomes parameters were measured by the prevalence of patients with a diminish in seizure frequency at 50% in 28 days. From November 1, 2020 to April 30, 2024, comparing the PER group with usual care. Clinical outcomes included adherence rate and seizure-free proportion at 28 days and 6 months. Adverse drug reactions were recorded in both groups.

Results

There was no difference in demographic data and incidence of adverse drug reactions between two groups. Median PER dosage was 4 mg (range, 2-12 mg). Compared to other antiepileptic drugs, the PER group had a prevalence of 50% responder rate at 28 days and 6 months significantly were 75%, 81%, 65%, and 51% respectively. Common adverse drug reactions were somnolence and dizziness.

Conclusions

PER administration as monotherapy demonstrated good efficacy and less adverse drug reactions. Low dosages helped to decrease adverse drug reactions and improved retention rate.

Efficacy and safety of perampanel as early add-on therapy in Chinese patients with focal-onset seizures: a multicenter, open-label, single-arm study

Background

No interventional study has been conducted in China to assess efficacy and safety of perampanel in treating Chinese patients with epilepsy, nor has there been any study on perampanel early add-on therapy in China. This interventional study aimed to assess efficacy and safety of perampanel as an early add-on treatment of focal-onset seizures (FOS) with or without focal-to-bilateral tonic-clonic seizures (FBTCS) in Chinese patients.

Methods

In this multicenter, open-label, single-arm, phase 4 interventional study, Chinese patients ≥ 12 years old with FOS with or without FBTCS who failed anti-seizure medication (ASM) monotherapy from 15 hospitals in China were enrolled and treated with perampanel add-on therapy (8-week titration followed by 24-week maintenance). The primary endpoint was 50% responder rate. Secondary endpoints included seizure-freedom rate and changes in seizure frequency from baseline. Treatment-emergent adverse events (TEAEs) and drug-related TEAEs were recorded.

Results

The full analysis set included 150 patients. The mean maintenance perampanel dose was 5.9 ± 1.5 mg/day and the 8-month retention rate was 72%. The 50% responder rate and seizure-freedom rate for all patients during maintenance were 67.9 and 30.5%, respectively. Patients with FBTCS had higher 50% responder rate (96.0%) and seizure-freedom rate (76.0%) during maintenance. Patients on concomitant sodium valproate had a significantly higher seizure-freedom rate than those on concomitant oxcarbazepine. Eight-six (55.1%) patients experienced treatment-related TEAEs, and the most common TEAEs were dizziness (36.5%), hypersomnia (11.5%), headache (3.9%), somnolence (3.2%), and irritability (3.2%). Withdrawal due to TEAEs occurred to 14.7% of the patients.

Conclusion

Perampanel early add-on was effective and safe in treating Chinese patients≥12 years old with FOS with or without FBTCS.Clinical trial registrationwww.chictr.org.cn, Identifier ChiCTR2000039510.

A retrospective, real-world experience of perampanel monotherapy in patient with first new onset focal seizure: A Thailand experience

Objective

Real‐world data on efficacy and tolerability of perampanel (PER) monotherapy in treatment‐naïve patients with focal onset seizures (FOS) and/or focal‐to‐bilateral tonic‐clonic seizures (FBTCS) to assess efficacy effectiveness and tolerability.

Methods

This is a retrospective review of study patients with new FOS with or without FBTCS, aged ≥15 years, who had been prescribed PER as monotherapy. Treatment outcome included retention rate, responder, and seizure‐free rate at observational point 3, 6, and 12 months (OP3, OP6, and OP12). Treatment‐emergent adverse events (TEAEs) and adverse drug reactions were recorded.

Results

A total of 41 patients enrolled in the study (male:female; 17:22, mean age =46.1 ± 21.8 years), with new FOS and/or FBTCS. The proportions of individuals remaining on PER monotherapy at 3, 6, and 12 months were evaluated. The median PER dosage was 4 mg (range 2‐8 mg). The retention rates at OP3, OP6, and OP12 were 88%, 73%, and 61%, respectively. The seizure freedom rates at OP3, OP6, and OP12 were 78%, 80%, and 76%, respectively. About 14% had discontinued the PER monotherapy because of lack of efficacy. Sixteen individuals (41%) had TEAEs; common AEs were dizziness, somnolence, and ataxia; and only one case had depression. The AEs with somnolence and ataxia were found higher in elderly (15% and 30%) than adult patients (7% and 3%), respectively. Only 14% had intolerant adverse events, and it was found higher in elderly (23%).

Significance

Real‐world data of PER monotherapy in treatment‐naïve patients with focal onset seizures demonstrated good effectiveness and a good safety profile at relatively low doses. By starting with low dosage and slow titration of PER help to minimize the impact of adverse effects, maximize adherence, and increase patient retention. PER has a once‐daily dosing schedule that supports patient adherence contributes to achieving seizure freedom.

The Effect of Anti-seizure Medications on the Propagation of Epileptic Activity: A Review

The propagation of epileptiform events is a highly interesting phenomenon from the pathophysiological point of view, as it involves several mechanisms of recruitment of neural networks. Extensive in vivo and in vitro research has been performed, suggesting that multiple networks as well as cellular candidate mechanisms govern this process, including the co-existence of wave propagation, coupled oscillator dynamics, and more. The clinical importance of seizure propagation stems mainly from the fact that the epileptic manifestations cannot be attributed solely to the activity in the seizure focus itself, but rather to the propagation of epileptic activity to other brain structures. Propagation, especially when causing secondary generalizations, poses a risk to patients due to recurrent falls, traumatic injuries, and poor neurological outcome. Anti-seizure medications (ASMs) affect propagation in diverse ways and with different potencies. Importantly, for drug-resistant patients, targeting seizure propagation may improve the quality of life even without a major reduction in simple focal events. Motivated by the extensive impact of this phenomenon, we sought to review the literature regarding the propagation of epileptic activity and specifically the effect of commonly used ASMs on it. Based on this body of knowledge, we propose a novel classification of ASMs into three main categories: major, minor, and intermediate efficacy in reducing the propagation of epileptiform activity.

Ionotropic Glutamate Receptors in Epilepsy: A Review Focusing on AMPA and NMDA Receptors

It is widely accepted that glutamate-mediated neuronal hyperexcitation plays a causative role in eliciting seizures. Among glutamate receptors, the roles of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors in physiological and pathological conditions represent major clinical research targets. It is well known that agonists of NMDA or AMPA receptors can elicit seizures in animal or human subjects, while antagonists have been shown to inhibit seizures in animal models, suggesting a potential role for NMDA and AMPA receptor antagonists in anti-seizure drug development. Several such drugs have been evaluated in clinical studies; however, the majority, mainly NMDA-receptor antagonists, failed to demonstrate adequate efficacy and safety for therapeutic use, and only an AMPA-receptor antagonist, perampanel, has been approved for the treatment of some forms of epilepsy. These results suggest that a misunderstanding of the role of each glutamate receptor in the ictogenic process may underlie the failure of these drugs to demonstrate clinical efficacy and safety. Accumulating knowledge of both NMDA and AMPA receptors, including pathological gene mutations, roles in autoimmune epilepsy, and evidence from drug-discovery research and pharmacological studies, may provide valuable information enabling the roles of both receptors in ictogenesis to be reconsidered. This review aimed to integrate information from several studies in order to further elucidate the specific roles of NMDA and AMPA receptors in epilepsy.

Decreased epileptogenesis in mice lacking the System xc - transporter occurs in association with a reduction in AMPA receptor subunit GluA1

OBJECTIVE

Although the cystine/glutamate antiporter System xc - (Sxc -) plays a permissive role in glioma-associated seizures, its contribution to other acquired epilepsies has not been determined. As such, the present study investigates whether and how Sxc - contributes to the pentylenetetrazole (PTZ) chemical kindling model of epileptogenesis.

METHODS

Male Sxc - null (sut/sut) mice and their wild-type littermates were administered PTZ (i.p.) daily for up to 21 days (kindling paradigm). Seizure severity was scored on a 5-point behavioral scale. Mossy fiber sprouting, cellular degeneration, and Sxc - light chain (xCT) messenger RNA (mRNA) were explored using Timm staining, thionin staining, and real-time quantitative polymerase chain reaction (qPCR), respectively. Levels of reduced and oxidized glutathione and cysteine were determined via high-performance liquid chromatography (HPLC). Plasma membrane protein levels of glutamate and γ-aminobutyric acid (GABA) receptor subunits as well as the K+/Cl- co-transporter KCC2 were quantified via western blot analysis.

RESULTS

Repeated administration of PTZ produced chemical kindling in only 50% of Sxc - null mice as compared to 82% of wild-type littermate control mice. Kindling did not result in any changes in xCT mRNA levels assessed in wild-type mice. No cellular degeneration or mossy fiber sprouting was discernible in either genotype. Except for a small, but significant, decrease in oxidized cysteine in the hippocampus, no other change in measured redox couples was determined in Sxc - null mice. Cortical levels of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluA1 were decreased in Sxc - null mice as compared to wild-type littermates, whereas all other proteins tested showed no difference between genotypes.

SIGNIFICANCE

This study provides the first evidence that Sxc - signaling contributes to epileptogenesis in the PTZ kindling model of acquired epilepsy. Further data indicate that a reduction in AMPA receptor signaling could underlie the resistance to PTZ kindling uncovered in Sxc - null mice.

Impaired neural stem cell expansion and hypersensitivity to epileptic seizures in mice lacking the EGFR in the brain

Mice lacking the epidermal growth factor receptor (EGFR) develop an early postnatal degeneration of the frontal cortex and olfactory bulbs and show increased cortical astrocyte apoptosis. The poor health and early lethality of EGFR^−/− mice prevented the analysis of mechanisms responsible for the neurodegeneration and function of the EGFR in the adult brain. Here, we show that postnatal EGFR‐deficient neural stem cells are impaired in their self‐renewal potential and lack clonal expansion capacity in vitro. Mice lacking the EGFR in the brain (EGFR^Δbrain) show low penetrance of cortical degeneration compared to EGFR^−/− mice despite genetic recombination of the conditional allele. Adult EGFR^Δ mice establish a proper blood–brain barrier and perform reactive astrogliosis in response to mechanical and infectious brain injury, but are more sensitive to Kainic acid‐induced epileptic seizures. EGFR‐deficient cortical astrocytes, but not midbrain astrocytes, have reduced expression of glutamate transporters Glt1 and Glast, and show reduced glutamate uptake in vitro, illustrating an excitotoxic mechanism to explain the hypersensitivity to Kainic acid and region‐specific neurodegeneration observed in EGFR‐deficient brains.

A combination of NMDA and AMPA receptor antagonists retards granule cell dispersion and epileptogenesis in a model of acquired epilepsy

Epilepsy may arise following acute brain insults, but no treatments exist that prevent epilepsy in patients at risk. Here we examined whether a combination of two glutamate receptor antagonists, NBQX and ifenprodil, acting at different receptor subtypes, exerts antiepileptogenic effects in the intrahippocampal kainate mouse model of epilepsy. These drugs were administered over 5 days following kainate. Spontaneous seizures were recorded by video/EEG at different intervals up to 3 months. Initial trials showed that drug treatment during the latent period led to higher mortality than treatment after onset of epilepsy, and further, that combined therapy with both drugs caused higher mortality at doses that appear safe when used singly. We therefore refined the combined-drug protocol, using lower doses. Two weeks after kainate, significantly less mice of the NBQX/ifenprodil group exhibited electroclinical seizures compared to vehicle controls, but this effect was lost at subsequent weeks. The disease modifying effect of the treatment was associated with a transient prevention of granule cell dispersion and less neuronal degeneration in the dentate hilus. These data substantiate the involvement of altered glutamatergic transmission in the early phase of epileptogenesis. Longer treatment with NBQX and ifenprodil may shed further light on the apparent temporal relationship between dentate gyrus reorganization and development of spontaneous seizures.

EGAR, A Food Protein-Derived Tetrapeptide, Reduces Seizure Activity in Pentylenetetrazole-Induced Epilepsy Models Through α-Amino-3-Hydroxy-5-Methyl-4-Isoxazole Propionate Receptors

A primary pathogeny of epilepsy is excessive activation of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs). To find potential molecules to inhibit AMPARs, high-throughput screening was performed in a library of tetrapeptides in silico. Computational results suggest that some tetrapeptides bind stably to the AMPAR. We aligned these sequences of tetrapeptide candidates with those from in vitro digestion of the trout skin protein. Among salmon-derived products, Glu-Gly-Ala-Arg (EGAR) showed a high biological affinity toward AMPAR when tested in silico. Accordingly, natural EGAR was hypothesized to have anticonvulsant activity, and in vitro experiments showed that EGAR selectively inhibited AMPAR-mediated synaptic transmission without affecting the electrophysiological properties of hippocampal pyramidal neurons. In addition, EGAR reduced neuronal spiking in an in vitro seizure model. Moreover, the ability of EGAR to reduce seizures was evaluated in a rodent epilepsy model. Briefer and less severe seizures versus controls were shown after mice were treated with EGAR. In conclusion, the promising experimental results suggest that EGAR inhibitor against AMPARs may be a target for antiepilepsy pharmaceuticals. Epilepsy is a common brain disorder characterized by the occurrence of recurring, unprovoked seizures. Twenty to 30 % of persons with epilepsy do not achieve adequate seizure control with any drug. Here we provide a possibility in which a natural and edible tetrapeptide, EGAR, can act as an antiepileptic agent. We have combined computation with in vitro experiments to show how EGAR modulates epilepsy. We also used an animal model of epilepsy to prove that EGAR can inhibit seizures in vivo. This study suggests EGAR as a potential pharmaceutical for the treatment of epilepsy.

Learning as a Functional State of the Brain: Studies in Wild-Type and Transgenic Animals

Contemporary neuroscientists are paying increasing attention to subcellular, molecular, and electrophysiological mechanisms underlying learning and memory processes. Recent studies have examined the development of transgenic mice affected at different stages of the learning process, or have emulated in animals various human pathological conditions involving cognition and motor learning. However, a parallel effort is needed to develop stimulating and recording techniques suitable for use in behaving mice in order to understand activity-dependent synaptic changes taking place during the very moment of the learning process. The in vivo models should incorporate information collected from different molecular and in vitro approaches. Long-term potentiation (LTP) has been proposed as the neural mechanism underlying synaptic plasticity, and NMDA receptors have been proposed as the molecular substrate of LTP. It now seems necessary to study the relationship of both LTP and NMDA receptors to functional changes in synaptic efficiency taking place during actual learning in selected cerebral cortical structures. Here, we review data collected in our laboratory during the past 10 years on the involvement of different hippocampal synapses in the acquisition of the classically conditioned eyelid responses in behaving mice. Overall the results indicate a specific contribution of each cortical synapse to the acquisition and storage of new motor and cognitive abilities. Available data show that LTP, evoked by high-frequency stimulation of Schaffer collaterals, disturbs both the acquisition of conditioned eyelid responses and the physiological changes that occur at hippocampal synapses during learning. Moreover, the administration of NMDA-receptor antagonists is able not only to prevent LTP induction in vivo, but also to hinder both the formation of conditioned eyelid responses and functional changes in the strength of the CA3-CA1 synapse. Nevertheless, many other neurotransmitter receptors, intracellular mediators, and transcription factors are also involved in learning and memory processes. In summary, it can be proposed that learning and memory in behaving mammals are the result of the activation of complex and distributed functional states involving many different cerebral cortical synapses, with the participation also of various neurotransmitter systems.

AMPA Receptor Antagonist NBQX Decreased Seizures by Normalization of Perineuronal Nets

Epilepsy is a serious brain disorder with diverse seizure types and epileptic syndromes. AMPA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzoquinoxaline-2,3-dione (NBQX) attenuates spontaneous recurrent seizures in rats. However, the anti-epileptic effect of NBQX in chronic epilepsy model is poorly understood. Perineuronal nets (PNNs), specialized extracellular matrix structures, surround parvalbumin-positive inhibitory interneurons, and play a critical role in neuronal cell development and synaptic plasticity. Here, we focused on the potential involvement of PNNs in the treatment of epilepsy by NBQX. Rats were intraperitoneally (i.p.) injected with pentylenetetrazole (PTZ, 50 mg/kg) for 28 consecutive days to establish chronic epilepsy models. Subsequently, NBQX (20 mg/kg, i.p.) was injected for 3 days for the observation of behavioral measurements of epilepsy. The Wisteria floribundi agglutinin (WFA)-labeled PNNs were measured by immunohistochemical staining to evaluate the PNNs. The levels of three components of PNNs such as tenascin-R, aggrecan and neurocan were assayed by Western blot assay. The results showed that there are reduction of PNNs and decrease of tenascin-R, aggrecan and neurocan in the medial prefrontal cortex (mPFC) in the rats injected with PTZ. However, NBQX treatment normalized PNNs, tenascin-R, aggrecan and neurocan levels. NBQX was sufficient to decrease seizures through increasing the latency to seizures, decrease the duration of seizure onset, and reduce the scores for the severity of seizures. Furthermore, the degradation of mPFC PNNs by chondroitinase ABC (ChABC) exacerbated seizures in PTZ-treated rats. Finally, the anti-epileptic effect of NBQX was reversed by pretreatment with ChABC into mPFC. These findings revealed that PNNs degradation in mPFC is involved in the pathophysiology of epilepsy and enhancement of PNNs may be effective for the treatment of epilepsy.

Activation of the sigma receptor 1 modulates AMPA receptor-mediated light-evoked excitatory postsynaptic currents in rat retinal ganglion cells

Sigma receptor (σR), a unique receptor family, is classified into three subtypes: σR1, σR2 and σR3. It was previously shown that σR1 activation induced by 1μM SKF10047 (SKF) suppressed N-methyl-d-aspartate (NMDA) receptor-mediated responses of rat retinal ganglion cells (GCs) and the suppression was mediated by a distinct Ca(2+)-dependent phospholipase C (PLC)-protein kinase C (PKC) pathway. In the present work, using whole-cell patch-clamp techniques in rat retinal slice preparations, we further demonstrate that SKF of higher dosage (50μM) significantly suppressed AMPA receptor (AMPAR)-mediated light-evoked excitatory postsynaptic currents (L-EPSCs) of retinal ON-type GCs (ON GCs), and the effect was reversed by the σR1 antagonist BD1047, suggesting the involvement of σR1. The SKF (50μM) effect was unlikely due to a change in glutamate release from bipolar cells, as suggested by the unaltered paired-pulse ratio (PPR) of AMPAR-mediated EPSCs of ON GCs. SKF (50μM) did not change L-EPSCs of ON GCs when the G protein inhibitor GDP-β-S or the protein kinase G (PKG) inhibitor KT5823 was intracellularly infused. Calcium imaging further revealed that SKF (50μM) did not change intracellular calcium concentration in GCs and persisted to suppress L-EPSCs when intracellular calcium was chelated by BAPTA. The SKF (50μM) effect was intact when protein kinase A (PKA) and phosphatidylinostiol (PI)-PLC signaling pathways were both blocked. We conclude that the SKF (50μM) effect is Ca(2+)-independent, PKG-dependent, but not involving PKA, PI-PLC pathways.

BET protein Brd4 activates transcription in neurons and BET inhibitor Jq1 blocks memory in mice

Precise regulation of transcription is crucial for the cellular mechanisms underlying memory formation. However, the link between neuronal stimulation and the proteins that directly interact with histone modifications to activate transcription in neurons remains unclear. Brd4 is a member of the bromodomain and extra-terminal domain (BET) protein family, which binds acetylated histones and is a critical regulator of transcription in many cell types, including transcription in response to external cues. Small molecule BET inhibitors are in clinical trials, yet almost nothing is known about Brd4 function in the brain. Here we show that Brd4 mediates the transcriptional regulation underlying learning and memory. The loss of Brd4 function affects critical synaptic proteins, which results in memory deficits in mice but also decreases seizure susceptibility. Thus Brd4 provides a critical link between neuronal activation and the transcriptional responses that occur during memory formation.

The AMPA receptor antagonist NBQX exerts anti-seizure but not antiepileptogenic effects in the intrahippocampal kainate mouse model of mesial temporal lobe epilepsy

The AMPA receptor subtype of glutamate receptors, which mediates fast synaptic excitation, is of primary importance in initiating epileptiform discharges, so that AMPA receptor antagonists exert anti-seizure activity in diverse animal models of partial and generalized seizures. Recently, the first AMPA receptor antagonist, perampanel, was approved for use as adjunctive therapy for the treatment of resistant partial seizures in patients. Interestingly, the competitive AMPA receptor antagonist NBQX has recently been reported to prevent development of spontaneous recurrent seizures (SRS) in a neonatal seizure model in rats, indicating the AMPA antagonists may exert also antiepileptogenic effects. This prompted us to evaluate competitive (NBQX) and noncompetitive (perampanel) AMPA receptor antagonists in an adult mouse model of mesial temporal lobe epilepsy. In this model, SRS develop after status epilepticus (SE) induced by intrahippocampal injection of kainate. Focal electrographic seizures in this model are resistant to several major antiepileptic drugs. In line with previous studies, phenytoin was not capable of blocking such seizures in the present experiments, while they were markedly suppressed by NBQX and perampanel. However, perampanel was less tolerable than NBQX in epileptic mice, so that only NBQX was subsequently tested for antiepileptogenic potential. When mice were treated over three days after kainate-induced SE with NBQX (20 mg/kg t.i.d.), no effect on development or frequency of seizures was found in comparison to vehicle controls. These results suggest that AMPA receptor antagonists, while being effective in suppressing resistant focal seizures, are not exerting antiepileptogenic effects in an adult mouse model of partial epilepsy.

Alterations in microRNA-124 and AMPA receptors contribute to social behavioral deficits in frontotemporal dementia

Neurodegenerative diseases, such as frontotemporal dementia (FTD), are often associated with behavioral deficits, but the underlying anatomical and molecular causes remain poorly understood. Here we show that forebrain-specific expression of FTD-associated mutant CHMP2B in mice causes several age-dependent neurodegenerative phenotypes, including social behavioral impairments. The social deficits were accompanied by a change in AMPA receptor (AMPAR) composition, leading to an imbalance between Ca(2+)-permeable and Ca(2+)-impermeable AMPARs. Expression of most AMPAR subunits was regulated by the brain-enriched microRNA miR-124, whose abundance was markedly decreased in the superficial layers of the cerebral cortex of mice expressing the mutant CHMP2B. We found similar changes in miR-124 and AMPAR levels in the frontal cortex and induced pluripotent stem cell-derived neurons from subjects with behavioral variant FTD. Moreover, ectopic miR-124 expression in the medial prefrontal cortex of mutant mice decreased AMPAR levels and partially rescued behavioral deficits. Knockdown of the AMPAR subunit Gria2 also alleviated social impairments. Our results identify a previously undescribed mechanism involving miR-124 and AMPARs in regulating social behavior in FTD and suggest a potential therapeutic avenue.

Targeting α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors in epilepsy

INTRODUCTION

Despite epilepsies being between the oldest and most studied neurological diseases, new treatment remains an unmet need of scientific research due to the high percentage of refractory patients. Several studies have identified new suitable anti-seizure targets. Glutamate activation of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) have long ago been identified as suitable targets for the development of anti seizure drugs.

AREAS COVERED

Here, we describe: i) AMPARs' structure and their involvement and role during seizures and in epilepsy and ii) the efficacy of AMPAR antagonists in preclinical models of seizures and epilepsy.

EXPERT OPINION

The physiological and pathological role of AMPAR in the CNS and the development of AMPAR antagonists have recently gained attention considering their recent involvement in status epilepticus and the marketing of perampanel. The need for new anti-seizure drugs represents a major challenge in both preclinical and clinical epilepsy. The introduction into the market of perampanel for the treatment of epilepsy will shed new light on the real potential of AMPAR antagonism in clinical settings outside the limited world of clinical trials. While research will go on in this area, fundamental will be the post-marketing evaluation of perampanel efficacy and tolerability and a better definition of the role of this receptor in the epileptic brain.

AMPA receptors as a molecular target in epilepsy therapy

Epileptic seizures occur as a result of episodic abnormal synchronous discharges in cerebral neuronal networks. Although a variety of non-conventional mechanisms may play a role in epileptic synchronization, cascading excitation within networks of synaptically connected excitatory glutamatergic neurons is a classical mechanism. As is the case throughout the central nervous system, fast synaptic excitation within and between brain regions relevant to epilepsy is mediated predominantly by AMPA receptors. By inhibiting glutamate-mediated excitation, AMPA receptor antagonists markedly reduce or abolish epileptiform activity in in vitro preparations and confer seizure protection in a broad range of animal seizure models. NMDA receptors may also contribute to epileptiform activity, but NMDA receptor blockade is not sufficient to eliminate epileptiform discharges. AMPA receptors move into and out of the synapse in a dynamic fashion in forms of synaptic plasticity, underlying learning and memory. Often, the trigger for these dynamic movements is the activation of NMDA receptors. While NMDA receptor antagonists inhibit these forms of synaptic plasticity, AMPA receptor antagonists do not impair synaptic plasticity and do not inhibit memory formation or retrieval. The demonstrated clinical efficacy of perampanel, a high-potency, orally active non-competitive AMPA receptor antagonist, supports the concept that AMPA receptors are critical to epileptic synchronization and the generation and spread of epileptic discharges in human epilepsy.

Mechanisms of cardiovascular actions of urocortins in the hypothalamic arcuate nucleus of the rat

The presence of urocortins (UCNs) and corticotropin-releasing factor (CRF) receptors has been reported in the hypothalamic arcuate nucleus (ARCN). We have previously reported that UCNs are involved in central cardiovascular regulation. Based on this information, we hypothesized that the ARCN may be one of the sites where UCNs exert their central cardiovascular actions. Experiments were done in artificially ventilated, adult male Wistar rats anesthetized with urethane. Unilateral microinjections (30 nl) of UCN1 (0.12-2 mM) elicited decreases in mean arterial pressure (MAP) and heart rate (HR). Maximum cardiovascular responses were elicited by a 1 mM concentration of UCN1. Microinjections of UCN2 and UCN3 (1 mM each) into the ARCN elicited similar decreases in MAP and HR. UCN1 was used as a prototype for the other experiments described below. HR responses elicited by UCN1 were significantly attenuated by bilateral vagotomy. Prior microinjections of NBI-27914 (CRF-1 receptor antagonist) and astressin (CRF-1 receptor and CRF-2 receptor antagonist) (1 mM each) into the ARCN significantly attenuated the cardiovascular responses elicited by UCN1 microinjections at the same site. Microinjections of UCN1 into the ARCN decreased efferent renal sympathetic nerve activity. It was concluded that microinjections of UCN1, UCN2, and UCN3 into the ARCN elicited decreases in MAP and HR. Decreases in MAP, HR, and renal sympathetic nerve activity elicited by UCN1 microinjections into the ARCN were mediated via CRF receptors. Bradycardic responses to UCN1 were mediated via the activation of vagus nerves, and decreases in MAP may be mediated via decreases in sympathetic nerve activity.

A series of structurally novel heterotricyclic alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor-selective antagonists

BACKGROUND AND PURPOSE

A new class of heterotricyclic glutamate analogues recently was generated by incorporating structural elements of two excitotoxic marine compounds, kainic acid and neodysiherbaine A. Rather than acting as convulsants, several of these 'IKM' compounds markedly depressed CNS activity in mice. Here, we characterize the pharmacological profile of the series with a focus on the most potent of these molecules, IKM-159.

EXPERIMENTAL APPROACH

The pharmacological activity and specificity of IKM compounds were characterized using whole-cell patch clamp recording from neurons and heterologous receptor expression systems, in combination with radioligand binding techniques.

KEY RESULTS

The majority of the IKM compounds tested reduced excitatory synaptic transmission in neuronal cultures, and IKM-159 inhibited synaptic currents from CA1 pyramidal neurons in hippocampal slices. IKM-159 inhibited glutamate-evoked whole-cell currents from recombinant GluA2- and GluA4-containing alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptors most potently, whereas kainate and NMDA receptor currents were not reduced by IKM-159. Antagonism of steady-state currents was agonist concentration dependent, suggesting that its mechanism of action was competitive, although it paradoxically did not displace [(3)H]-AMPA from receptor binding sites. IKM-159 reduced spontaneous action potential firing in both cultured hippocampal neurons in control conditions and during hyperactive states in an in vitro model of status epilepticus.

CONCLUSIONS AND IMPLICATIONS

IKM-159 is an AMPA receptor-selective antagonist. IKM-159 and related nitrogen heterocycles represent structurally novel AMPA receptor antagonists with accessible synthetic pathways and potentially unique pharmacology, which could be of use in exploring the role of specific populations of receptors in neurophysiological and neuropathological processes.

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.

Choosing the correct antiepileptic drugs: from animal studies to the clinic

Epilepsy is a chronic condition caused by an imbalance of normal excitatory and inhibitory forces in the brain. Antiepileptic drug therapy is directed primarily toward reducing excitability through blockage of voltage-gated Na(+) or Ca(2+) channels, or increasing inhibition through enhancement of gamma-aminobutyric acid currents. Prior to clinical studies, putative antiepileptic drugs are screened in animals (usually rodents). Maximal electrical shock, pentylenetetrazol, and kindling are typically used as nonmechanistic screens for antiseizure properties, and the rotorod test assesses acute toxicity. Whereas antiseizure drug screening has been successful in bringing drugs to the market and improving our understanding of the pathophysiology of seizures, it merits emphasis that the vast majority of drug screening occurs in mature male rodents and involves models of seizures, not epilepsy. Effective drugs in acute seizures may not be effective in chronic models of epilepsy. Seizure type, clinical and electroencephalographic phenotype, syndrome, and etiology are often quite different in children with epilepsy than in adults. Despite these age-related unique features, drugs used in children are generally the same as those in adults. As awareness of the unique features of seizures during development increases, more drug screening in the immature animal will likely occur.

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.

Activity-dependent changes of the hippocampal CA3-CA1 synapse during the acquisition of associative learning in conscious mice

Contemporary neuroscientists are paying increasing attention to subcellular, molecular and electrophysiological mechanisms underlying learning and memory processes. Recent efforts have addressed the development of transgenic mice affected at different stages of the learning process, or emulating pathological conditions involving cognition and motor-learning capabilities. However, a parallel effort is needed to develop stimulating and recording techniques suitable for use in behaving mice, in order to grasp activity-dependent neural changes taking place during the very moment of the process. These in vivo models should integrate the fragmentary information collected by different molecular and in vitro approaches. In this regard, long-term potentiation (LTP) has been proposed as the neural mechanism underlying synaptic plasticity. Moreover, N-methyl-d-aspartate (NMDA) receptors are accepted as the molecular substrate of LTP. It now seems necessary to study the relationship of both LTP and NMDA receptors with the plastic changes taking place, in selected neural structures, during actual learning. Here, we review data on the involvement of the hippocampal CA3-CA1 synapse in the acquisition of classically conditioned eyelid conditioned responses (CRs) in behaving mice. Available data show that LTP, evoked by high-frequency stimulation of Schaffer collaterals, disturbs both the acquisition of CRs and the physiological changes that occur at the CA3-CA1 synapse during learning. Moreover, the administration of NMDA-receptor antagonists is able not only to prevent LTP induction in vivo, but also to hinder the formation of both CRs and functional changes in strength of the CA3-CA1 synapse. Thus, there is experimental evidence relating activity-dependent synaptic changes taking place during actual learning with LTP mechanisms and with the role of NMDA receptors in both processes.

Effects of ionotropic glutamate receptor channel blockers on the development of pentylenetetrazol kindling in mice

Experiments on mice were performed to study the ability of monocationic and dicationic adamantane and phenylcyclohexyl derivatives to prevent the development of kindling induced by i.p. administration of pentylenetetrazol (Corasol, 35 mg/kg). The monocationic phenylcyclohexyl derivative IEM-1921 effectively slowed the development of kindling, this being seen over a wide range of doses (0.0001-0.1 micromol/kg). A monocationic adamantane derivative (memantine), also a selective non-competitive blocker of NMDA receptors, produced a similar effect at doses 100 times higher. The anticonvulsive activity of the dicationic phenylcyclohexyl derivative IEM-1925, which could block both types of glutamate receptors, differed from the activity of the monocationic derivative by having a more complex dose-response relationship. Thus, the development of kindling was suppressed by essentially the same doses as needed for the monocation IEM-1921 (0.001 micromol/kg). However, on reducing the dose by a factor of 10 (0.0001 micromol/kg), IEM-1925 facilitated the development of kindling. This difference in the prophylactic activities of selective NMDA receptor blockers and substances able to block both NMDA and AMPA receptors provides evidence that the mechanism of kindling involves both types of ionotropic glutamate receptor and the effects of compounds depend not only on the ratio of the contributions of these receptors, but also on the kinetic characteristics of the blocking action.

Competitive AMPA receptor antagonists

Glutamic acid (Glu) is the major excitatory neurotransmitter in the mammalian central nervous system (CNS) where it is involved in the physiological regulation of different processes. It has been well established that excessive endogenous Glu is associated with many acute and chronic neurodegenerative disorders such as cerebral ischaemia, epilepsy, amiotrophic lateral sclerosis, Parkinson's, and Alzheimer's disease. These data have consequently added great impetus to the research in this field. In fact, many Glu receptor antagonists acting at the N-methyl-D-aspartic acid (NMDA), 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA), and/or kainic acid (KA) receptors have been developed as research tools and potential therapeutic agents. Ligands showing competitive antagonistic action at the AMPA type of Glu receptors were first reported in 1988, and the systemically active 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo[f]quinoxaline (NBQX) was first shown to have useful therapeutic effects in animal models of neurological disease in 1990. Since then, the quinoxaline template has represented the backbone of various competitive AMPA receptor antagonists belonging to different classes which had been developed in order to increase potency, selectivity and water solubility, but also to prolong the "in vivo" action. Compounds that present better pharmacokinetic properties and less serious adverse effects with respect to the others previously developed are undergoing clinical evaluation. In the near future, the most important clinical application for the AMPA receptor antagonists will probably be as neuroprotectant in neurodegenerative diseases, such as epilepsy, for the treatment of patients not responding to current therapies. The present review reports the history of competitive AMPA receptor antagonists from 1988 up to today, providing a systematic coverage of both the open and patent literature.

Different effects of nonNMDA and NMDA receptor antagonists (NBQX and dizocilpine) on cortical epileptic afterdischarges in rats

Excitatory amino acids play an important role in generation of epileptic seizures. To study the participation of different types of their receptors in cortical epileptic afterdischarges, a noncompetitive NMDA receptor antagonist dizocilpine and a competitive AMPA receptor antagonist NBQX were used. Adult rats with implanted epidural stimulation and registration electrodes were pretreated either with NBQX (30 or 60 mg/kg i.p.) or with dizocilpine (0.1 or 0.5 mg/kg i.p.) and low-frequency stimulation of sensorimotor cortical area was repeatedly applied with stepwise increased current intensities. Lower dose of NBQX unexpectedly decreased thresholds for elicitation of spike-and-wave afterdischarges (ADs), clonic seizures accompanying this type of ADs and for transition into the second, limbic type of ADs. Lower dose of dizocilpine increased these three thresholds. Higher doses of either drug did not significantly change threshold intensities. Duration of ADs was also influenced by the two antagonists in opposite directions: higher dose of NBQX resulted in prolongation of ADs mainly due to an increased duration of the spike-and-wave part of ADs whereas dizocilpine shortened ADs in a dose-dependent manner affecting both types of ADs. In addition, NBQX did not influence interhemispheric responses meanwhile dizocilpine moderately suppressed these evoked potentials. According to our results, NMDA receptors are important for generation of cortical epileptic afterdischarges meanwhile the role of AMPA receptors is not clear and has to be analyzed.

Alteration in sensitivity of ionotropic glutamate receptors and tachykinin receptors in spinal cord contribute to development and maintenance of nerve injury-evoked neuropathic pain

Allodynia or hyperalgesia induced by peripheral nerve injury may be involved in changes in the sensitivity of neurotransmitters at the spinal cord level. In order to clarify the functional role of neurotransmitters in peripheral nerve injury, we used rats with nerve injury induced by chronic constriction of the sciatic nerve (CCI rat model) and estimated the effects of the intrathecal injection of drugs known to affect glutamate and tachykinin receptors. In sham-operated rats, the NMDA receptor agonist NMDA and AMPA-kinate receptor agonist RS-(5)-bromowillardin reduced withdrawal latency. The non-competitive NMDA receptor antagonist MK-801, competitive NMDA receptor antagonist AP-5 and AMPA-kinate receptor antagonist NBQX increased withdrawal latency. Substance P (SP) increased the withdrawal latency but only transitorily. The NK1 receptor antagonist RP67580 increased withdrawal latency, but the NK2 receptor antagonist SR48968 did not show an effect. In CCI rats, RS-(5)-bromowillardin reduced withdrawal latency, but NMDA did not show an effect. NBQX increased withdrawal latency, while MK-801 and AP-5 showed little or no effect. SP reduced withdrawal latency, and both RP67580 and SR48968 increased it. These results indicate that the alteration in sensitivity of ionotropic glutamate receptors and tachykinin receptors in the spinal cord contribute to development and maintenance of nerve injury-evoked neuropathic pain.

Role of the medial prefrontal cortex in cardiovascular responses to acute restraint in rats

The medial prefrontal cortex (mPFC) modulates neurovegetative and behavioral responses, being involved in memory, attention, motivational and executive processes. There is evidence indicating that mPFC modulates cardiovascular correlates of behavioral responses to stressful stimuli. Acute restraint is an unavoidable stress situation that evokes marked and sustained cardiovascular changes, characterized by elevated blood pressure (BP) and intense heart rate (HR) increase. We presently report effects of mPFC pharmacological manipulations on BP and HR responses evoked by acute restraint in rats. Bilateral microinjection of 200 nl of the unspecific synaptic blocker CoCl2 (1 mM) in the mPFC prelimbic area (PL) increased HR response to acute restraint, without significant effect on the BP response. This result indicates that PL synaptic mechanisms have an inhibitory influence on restraint-evoked HR changes. Injections of the non-selective glutamatergic receptor antagonist kynurenic acid (0.02 M) or the selective N-methyl-d-aspartic acid (NMDA) receptor glutamatergic antagonist (LY235959) (0.02 M) caused effects similar to cobalt, suggesting that local glutamatergic neurotransmission and NMDA receptors mediate the PL inhibitory influence on restraint-related HR responses. Pretreatment with the non-non-N-methyl-D-aspartic acid glutamatergic antagonist glutamatergic antagonist glutamatergic receptor antagonist NBQX (0.02 M) did not affect restraint-related cardiovascular responses, reinforcing the idea that NMDA receptors mediate PL-related inhibitory influence. Pretreatment with the glutamatergic-receptor antagonists did not affect baseline BP or HR values. I.v. pretreatment with the quaternary ammonium anticholinergic drug homatropine methyl bromide (0.2 mg/kg) also increased the restraint-related HR response to values similar to those observed after treatment with kynurenic acid or LY235959, thus, suggesting that PL inhibitory influence on restraint-evoked heart rate increase could be related to increased parasympathetic activity. This dose of homatropine had no significant effects on baseline BP or HR values. Results suggest a PL inhibitory influence on restraint-evoked HR increase. They also indicate that local NMDA receptors involved in parasympathetic activation mediate PL inhibitory influence on restraint-evoked HR increase.

Effects of uridine in models of epileptogenesis and seizures

Due to the limited efficacy and side effects of current antiepileptic drugs (AEDs), the search for new therapeutic agents is critical. Uridine, a possible endogenous antiepileptic modulator, has been demonstrated to have anticonvulsant effects in some models of epilepsy, but not others. In this study, we examined possible neuroprotective effects of uridine by administering the agent following lithium-pilocarpine induced status epilepticus. The effects of uridine were assessed on EEG patterns, visual-spatial memory in the water maze and histopathology. There was a trend for reduced EEG spike frequency, improved visual spatial memory and better histology score in rats receiving uridine. The antiepileptogenic and anticonvulsant effects of uridine were studied by administering uridine to rats undergoing rapid kindling or following full kindling. In the rapid kindling models, uridine had a moderate antiepileptogenic and anticonvulsant effect. These results suggest uridine may have potential to aid in the prevention and treatment of epilepsy.

Involvement of the CA3-CA1 synapse in the acquisition of associative learning in behaving mice

One of the brain sites more directly related with learning and memory processes is the hippocampus. We recorded, in conscious mice, the activity-dependent changes taking place at the hippocampal CA3-CA1 synapse during the acquisition, extinction, recall, and reconditioning of an associative task. Mice were classically conditioned to evoke eyelid responses using a trace [conditioned stimuli (CS), tone; unconditioned stimuli (US), shock] paradigm. A single electrical pulse presented to the Schaffer collateral-commissural pathway during the CS-US interval evoked a monosynaptic field EPSP (fEPSP) at ipsilateral CA1 pyramidal cells. The slope of evoked fEPSPs increased across conditioning sessions and decreased during extinction, being linearly related to learning evolution. In contrast, fEPSPs were not modified when evoked in control mice in the absence of a conditioning protocol. Long-term potentiation (LTP) evoked by high-frequency stimulation of Schaffer collaterals prevented acquisition, extinction, recall, or reconditioning, depending on the moment when it was triggered. Learning and memory impairments evoked by LTP induction resulted probably from the functional saturation of the CA3-CA1 synapse, although an additional disturbance of the subsequent information transfer toward postsynaptic circuits cannot be discarded. CGP 39551 [(E)-(+/-)-2-amino-4-methyl-5-phosphono-3-pentenoic acid ethyl ester] (an NMDA antagonist) prevented LTP induction in behaving mice, as well as the acquisition of an eyelid learned response, and the synaptic changes taking place at the CA3-CA1 synapse across conditioning. In conclusion, the responsivity of the CA3-CA1 synapse seems to be modulated during associative learning, and both processes are prevented by experimental LTP or NMDA-receptor inactivation. Our results provide evidence of a relationship between activity-dependent synaptic plasticity and associative learning in behaving mice.

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.

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate receptor autoradiography in mouse brain after single and repeated withdrawal from diazepam

Withdrawal from chronic treatment with benzodiazepines is associated with increased neuronal excitability leading to anxiety, aversive effects and increased seizure sensitivity. After repeated withdrawal experiences, seizure sensitivity increases while withdrawal-induced anxiety and aversion decrease. We used autoradiographical methods employing [(3)H]Ro48 8587, a selective ligand for glutamatergic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, to study withdrawal-induced changes in AMPA receptor binding in areas of the mouse brain postulated to be involved in these responses. Mice were given 21 days treatment with diazepam (15 mg/kg, s.c. in sesame oil) followed by withdrawal (single withdrawal) or three blocks of 7 days treatment interspersed with 3-day periods to allow washout of drug (repeated withdrawal). In keeping with heightened excitability in withdrawal from chronic diazepam treatment, the single withdrawal group showed, 72 h after their final dose of diazepam, increased [(3)H]Ro48 8587 binding in several brain areas associated with emotional responses or seizure activity, including hippocampal subfields, amygdalar and thalamic nuclei and motor cortex. In contrast, the repeated withdrawal group showed no changes in [(3)H]Ro48 8587 binding in any brain area studied. These observations are consistent with up-regulation of AMPA receptor-mediated transmission being important in withdrawal-induced anxiety and aversion but not in increased seizure sensitivity associated with repeated withdrawal. As changes in AMPA receptor subunit expression alter the functionality of the receptor, future studies will address this possibility.

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.

Suppression of fully kindled seizure and retardation of kindling acquisition by YM928 in the rat kindling model of epilepsy

We investigated the effects of 2-[N-(4-chlorophenyl)-N-methylamino]-4H-pyrido[3.2-e]-1,3-thiazin-4-one (YM928), a selective alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist, in the rat kindling model of complex partial seizures. YM928 (10 and 30 mg/kg p.o.) markedly suppressed the motor seizures and afterdischarge induced by electrical stimulation of the amygdala at generalized seizure-triggering threshold intensity. YM928 (10 mg/kg p.o.) did not induce apparent abnormal behavior, but did induce sedation at a dose of 30 mg/kg p.o. YM928 (30 mg/kg p.o.) showed a similar anticonvulsant effect at twice the threshold intensity as it did at threshold intensity. Diazepam (10 mg/kg p.o.) and phenobarbital (60 mg/kg p.o.) also exerted anticonvulsant activities. Diazepam (10 mg/kg) showed a similar effect at twice the threshold as at threshold, but the anticonvulsant effect of phenobarbital (60 mg/kg p.o.) was reversed when the stimulus was doubled. When YM928 (10 mg/kg p.o.) was administered 60 min before daily stimulation of the amygdala, the development of kindling seizure was significantly retarded. These results indicate that YM928 has anticonvulsant effects and suppresses kindling acquisition without sedative effects, and may be suitable as an antiepileptic drug for the treatment of complex partial seizures in humans.

GluR2(B) knockdown accelerates CA3 injury after kainate seizures

Ca2+ currents are thought to enhance glutamate excitotoxicity. To investigate whether reduced expression of the Ca2+ limiting GluR2(B) subunit enhances seizure-induced vulnerability to either CA1 or CA3 neurons, we delivered GluR2(B) oligodeoxynucleotides (AS-ODNs) to the dorsal hippocampus of adult rats before inducing kainate (KA) seizures. After knockdown, no changes in behavior, electrographic activity, or histology were observed. In contrast, GluR2(B) knockdown and KA-induced status epilepticus produced accelerated histological injury to the ipsilateral CA3a-b and hilar subregions. At 8 to 12 h, the CA3a was preferentially labeled by both silver and TUNEL methods. TUNEL staining revealed 2 types of nuclei. They were round with uniform label, features of necrosis, or had DNA clumping or speckled chromatin deposits within surrounding cytosol, features of apoptosis. At 16 to 24 h, many CA3a-c neurons were shrunken, eosinophilic, argyrophilic, or completely absent. Immunohistochemistry revealed marked decreases in GluR2(B) subunits throughout the hippocampus, NR1 immunoreactivity was also reduced but to a lesser extent. In contrast, GluR1 and NR2A/B immunohistochemistry was relatively uniform except in regions of cell loss or within close proximity to the CA1 infusion site. At 144 h, the CA3 was still preferentially injured although bilateral CA1 injury was also observed in some AS-ODN-, S-ODN-, and KA-only-treated animals. Glutamate receptor antibodies revealed generalized decreases in the CA3 with all probes tested at this delayed time. In contrast, GluR2(B) expression was increased within CA1 irregularly shaped, injured neurons. Therefore, hippocampal deprivation of GluR2(B) subunits is insufficient to induce cell death in mature animals but may accelerate the already known CA3/hilar lesion, possibly by triggering apoptosis within CA3 neurons. CA1 and DG survive the first week despite their loss of GluR2(B) subunits, suggesting that other intrinsic properties such as increased Na+ conductance and reduced ability of the GluR2(B) subunit to interact with certain cytoplasmic proteins may be responsible for the augmented cell death rather than changes in AMPA receptor-mediated Ca2+ permeability. Alternatively, changes in allosteric interactions that affect other receptor classes of high density at the mossy fiber synapse (e.g. KA receptors) may augment KA neurotoxicity. Latent GluR2(B) increases in CA1 injured neurons support a role for AMPA receptor subunit alterations in seizure-induced tolerance.

Effects of clozapine, haloperidol and iloperidone on neurotransmission and synaptic plasticity in prefrontal cortex and their accumulation in brain tissue: an in vitro study

The mode of action of the antipsychotic drugs clozapine, haloperidol and iloperidone was investigated in layer V of prefrontal cortex slices using extracellular field potential, intracellular sharp-electrode as well as whole-cell voltage clamp recording techniques. Intracellular investigations on a broad range of concentrations revealed that the typical neuroleptic haloperidol at higher concentrations significantly depressed the excitatory postsynaptic component induced by electrical stimulation of layer II. This was not seen with the atypical neuroleptics clozapine and iloperidone. None of the three compounds had any effect on the resting membrane potential, spike amplitude or input resistance at relevant concentrations. Synaptic plasticity was assessed by means of extracellular field potential recordings. Clozapine significantly facilitated the potentiation of synaptic transmission, whereas haloperidol and iloperidone showed no effects. In line with its facilitating effect on synaptic plasticity, it could be demonstrated by whole-cell voltage clamp recordings that clozapine increased N-methyl-D-aspartic acid receptor-mediated excitatory postsynaptic currents in the majority of prefrontal cortical neurones. These investigations were made with neuroleptic drugs applied to the bath in the micromolar concentration range in order to approach clinical brain concentrations that are reached after administration of therapeutic doses. The drug concentrations reached in the slices after the experiments were assessed by means of high-pressure liquid chromatography coupled with mass-spectrometric detection. Surprisingly, drug accumulation in the in vitro preparation was of similar degree as reported in vivo. In conclusion, the typical neuroleptic haloperidol significantly depressed excitatory synaptic transmission in layer V neurones of the prefrontal cortex. In contrast, the two atypical neuroleptics iloperidone and clozapine revealed no depressing effects. This feature of the atypical neuroleptics might be beneficial since a hypofunctionality of this brain area is thought to be linked with the pathophysiology of schizophrenia. Additionally, clozapine facilitated long-term potentiation, which might be linked with the clinically observed beneficial effects on certain cognitive parameters. The clozapine-induced increase of N-methyl-D-aspartic acid receptor-mediated currents suggests that clozapine facilitates the induction of long-term potentiation. Furthermore, the present study points to the importance of considering the significant accumulation of neuroleptic drugs in in vitro studies.

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.

CP-465,022, a selective noncompetitive AMPA receptor antagonist, blocks AMPA receptors but is not neuroprotective in vivo

BACKGROUND AND PURPOSE

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor inhibition has been hypothesized to provide neuroprotective efficacy after cerebral ischemia on the basis of the activity in experimental ischemia models of a variety of compounds with varying selectivity for AMPA over other glutamate receptor subtypes. CP-465,022 is a new, potent, and selective noncompetitive AMPA receptor antagonist. The present study investigated the ability of this compound to reduce neuronal loss after experimental cerebral ischemia to probe the neuroprotective potential of AMPA receptor inhibition.

METHODS

To demonstrate that CP-465,022 gains access to the brain, the effects of systemic administration of CP-465,022 were investigated on AMPA receptor-mediated electrophysiological responses in hippocampus and on chemically induced seizures in rats. The compound was then investigated for neuroprotective efficacy in rat global and focal ischemia models at doses demonstrated to be maximally effective in the electrophysiology and seizure models.

RESULTS

CP-465,022 potently and efficaciously inhibited AMPA receptor-mediated hippocampal synaptic transmission and the induction of seizures. However, at comparable doses, CP-465,022 failed to prevent CA1 neuron loss after brief global ischemia or to reduce infarct volume after temporary middle cerebral artery occlusion.

CONCLUSIONS

Given the high selectivity of CP-465,022 for AMPA over kainate and N-methyl-D-aspartate subtypes of glutamate receptors, the lack of neuroprotective efficacy of the compound calls into question the neuroprotective efficacy of AMPA receptor inhibition after ischemia.