Genetically epilepsy-prone rats (GEPRs) and DBA/2 mice: Two animal models of audiogenic reflex epilepsy for the evaluation of new generation AEDs.

This review summarizes the current knowledge about DBA/2 mice and genetically epilepsy-prone rats (GEPRs) and discusses the contribution of such animal models on the investigation of possible new therapeutic targets and new anticonvulsant compounds for the treatment of epilepsy. Also, possible chemical or physical agents acting as proconvulsant agents are described. Abnormal activities of enzymes involved in catecholamine and serotonin synthesis and metabolism were reported in these models, and as a result of all these abnormalities, seizure susceptibility in both animals is greatly affected by pharmacological manipulations of the brain levels of monoamines and, prevalently, serotonin. In addition, both genetic epileptic models permit the evaluation of pharmacodynamic and pharmacokinetic interactions among several drugs measuring plasma and/or brain level of each compound. Audiogenic models of epilepsy have been used not only for reflex epilepsy studies, but also as animal models of epileptogenesis. The seizure predisposition (epileptiform response to sound stimulation) and substantial characterization of behavioral, cellular, and molecular alterations in both acute and chronic (kindling) protocols potentiate the usefulness of these models in elucidating ictogenesis, epileptogenesis, and their mechanisms. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Concept of activity-induced cell death in epilepsy: historical and contemporary perspectives.

Selective neuronal loss following status epilepticus was first described just under 100 years ago. The acute pathology following status epilepticus was shown to be 'ischemic cell change' and was assumed to arise through hypoxia/ischemia. Less than 30 years ago it was proposed, from experiments in primates, that the selective neuronal loss in hippocampus and cortex resulted from the abnormal electrical discharges. Selectively vulnerable neurons show swollen, calcium-loaded mitochondria in the soma and focally in dendrites. Burst firing with a massive Ca2+ entry needs to be sustained for 30-120 min to produce necrotic cell death. Lesser stress may produce apoptosis or immediate early gene expression with enhanced expression of many enzymes and receptor subunits. Changes in enzyme, transporter, ion-channel or receptor function or in network properties may lead to altered vulnerability to the effects of seizures. This type of modification and the cumulative effect of oxidative damage to proteins and lipids may explain the long-term consequences of repetitive brief seizures.

Implications for neuroprotective treatments.

Pharmacological neuroprotection against the consequences of seizures can be considered as primary neuroprotection where the object is to diminish the initial insult by suppressing the seizure activity or diminishing the associated ionic fluxes (of which the entry of Na+ and Ca2+ are the most significant), and secondary neuroprotection where the target is some later event in the chain linking ionic changes to altered brain morphology or function. Thus primary neuroprotection is provided by antiepileptic drugs and compounds acting on voltage-sensitive Na+ and Ca2+ channels or on glutamate receptors (NMDA, AMPA/KA or Group I metabotropic). Secondary neuroprotection may be a result of acting on the cascade leading to necrosis (e.g. free radical scavengers, NitricOxide synthase inhibitors, CycloOxygenase-2 inhibitors) or the cascades leading to apoptosis (e.g. MAP-kinase inhibitors, caspase-3 inhibitors). Other approaches may diminish the long-term morphological and functional effects of seizures (e.g. neurotrophin-related therapies). We need improved preclinical tests for identifying novel compounds with potential for providing secondary neuroprotection and antiepileptogenesis. Clinical trials of neuroprotective agents in chronic epilepsy in adults pose major practical difficulties but the severe childhood epilepsies provide opportunities for aggressive testing of novel compounds.

Group III mGlu receptor agonists potentiate the anticonvulsant effect of AMPA and NMDA receptor block.

We report the anticonvulsant action in DBA/2 mice of two mGlu Group III receptor agonists: (R,S)-4-phosphonophenylglycine, (R,S)-PPG, a compound with moderate mGlu8 selectivity, and of (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid, ACPT-1, a selective agonist for mGlu4alpha receptors. Both compounds, given intracerebroventricularly at doses which did not show marked anticonvulsant activity, produced a consistent shift to the left of the dose-response curves (i.e. enhanced the anticonvulsant properties) of 1-(4'-aminophenyl)-3,5-dihydro-7,8-dimethoxy-4H-2,3-benzodiazepin-4-one hydrochloride, CFM-2, a noncompetitive AMPA receptor antagonist, and 3-((+/-)-2-carboxypiperazin-4-yl)-1-phosphonic acid, CPPene, a competitive NMDA receptor antagonist, in DBA/2 mice. In addition, (R,S)-PPG and ACPT-1 administered intracerebroventricularly prolonged the time course of the anticonvulsant properties of CFM-2 (33 micromol/kg, i.p.) and CPPene (3.3 micromol/kg, i.p.) administered intraperitoneally. We conclude that modest reduction of synaptic glutamate release by activation of Group III metabotropic receptors potentiates the anticonvulsant effect of AMPA and NMDA receptor blockade.

Glutamate metabotropic receptors as targets for drug therapy in epilepsy.

Metabotropic glutamate (mGlu) receptors have multiple actions on neuronal excitability through G-protein-linked modifications of enzymes and ion channels. They act presynaptically to modify glutamatergic and gamma-aminobutyric acid (GABA)-ergic transmission and can contribute to long-term changes in synaptic function. The recent identification of subtype-selective agonists and antagonists has permitted evaluation of mGlu receptors as potential targets in the treatment of epilepsy. Agonists acting on group I mGlu receptors (mGlu1 and mGlu5) are convulsant. Antagonists acting on mGlu1 or mGlu5 receptors are anticonvulsant against 3,5-dihydroxyphenylglycine (DHPG)-induced seizures and in mouse models of generalized motor seizures and absence seizures. The competitive, phenylglycine mGlu1/5 receptor antagonists generally require intracerebroventricular administration for potent anticonvulsant efficacy but noncompetitive antagonists, e.g., (3aS,6aS)-6a-naphthalen-2-ylmethyl-5-methyliden-hexahydrocyclopenta[c]furan-1-on (BAY36-7620), 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), and 2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893) block generalized seizures with systemic administration. Agonists acting on group II mGlu receptors (mGlu2, mGlu3) to reduce glutamate release are anticonvulsant, e.g., 2R,4R-aminopyrrolidine-2,4-dicarboxylate [(2R,4R)-APDC], (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740), and (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268). The classical agonists acting on group III mGlu receptors such as L-(+)-2-amino-4-phosphonobutyric acid, and L-serine-O-phosphate are acutely proconvulsant with some anticonvulsant activity. The more recently identified agonists (R,S)-4-phosphonophenylglycine [(R,S)-PPG] and (S)-3,4-dicarboxyphenylglycine [(S)-3,4-DCPG] and (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid [ACPT-1] are all anticonvulsant without proconvulsant effects. Studies in animal models of kindling reveal some efficacy of mGlu receptor ligands against fully kindled limbic seizures. In genetic mouse models, mGlu1/5 antagonists and mGlu2/3 agonists are effective against absence seizures. Thus, antagonists at group I mGlu receptors and agonists at groups II and III mGlu receptors are potential antiepileptic agents, but their clinical usefulness will depend on their acute and chronic side effects. Potential also exists for combining mGlu receptor ligands with other glutamatergic and non-glutamatergic agents to produce an enhanced anticonvulsant effect. This review also discusses what is known about mGlu receptor expression and function in rodent epilepsy models and human epileptic conditions.

Novel quinolinone-phosphonic acid AMPA antagonists devoid of nephrotoxicity.

We reported previously the synthesis and structure-activity relationships (SAR) in a series of 2-(1H)-oxoquinolines bearing different acidic functions in the 3-position. Exploiting these SAR, we were able to identify 6,7-dichloro-2-(1H)-oxoquinoline-3-phosphonic acid compound 3 (S 17625) as a potent, in vivo active AMPA antagonist. Unfortunately, during the course of the development, nephrotoxicity was manifest at therapeutically effective doses. Considering that some similitude exists between S 17625 and probenecid, a compound known to protect against the nephrotoxicity and/or slow the clearance of different drugs, we decided to synthesise some new analogues of S 17625 incorporating some of the salient features of probenecid. Replacement of the chlorine in position 6 by a sulfonylamine led to very potent AMPA antagonists endowed with good in vivo activity and lacking nephrotoxicity potential. Amongst the compounds evaluated, derivatives 7a and 7s appear to be the most promising and are currently evaluated in therapeutically relevant stroke models.

Molecular targets for antiepileptic drug development.

This review considers how recent advances in the physiology of ion channels and other potential molecular targets, in conjunction with new information on the genetics of idiopathic epilepsies, can be applied to the search for improved antiepileptic drugs (AEDs). Marketed AEDs predominantly target voltage-gated cation channels (the alpha subunits of voltage-gated Na+ channels and also T-type voltage-gated Ca2+ channels) or influence GABA-mediated inhibition. Recently, alpha2-delta voltage-gated Ca2+ channel subunits and the SV2A synaptic vesicle protein have been recognized as likely targets. Genetic studies of familial idiopathic epilepsies have identified numerous genes associated with diverse epilepsy syndromes, including genes encoding Na+ channels and GABA(A) receptors, which are known AED targets. A strategy based on genes associated with epilepsy in animal models and humans suggests other potential AED targets, including various voltage-gated Ca2+ channel subunits and auxiliary proteins, A- or M-type voltage-gated K+ channels, and ionotropic glutamate receptors. Recent progress in ion channel research brought about by molecular cloning of the channel subunit proteins and studies in epilepsy models suggest additional targets, including G-protein-coupled receptors, such as GABA(B) and metabotropic glutamate receptors; hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel subunits, responsible for hyperpolarization-activated current Ih; connexins, which make up gap junctions; and neurotransmitter transporters, particularly plasma membrane and vesicular transporters for GABA and glutamate. New information from the structural characterization of ion channels, along with better understanding of ion channel function, may allow for more selective targeting. For example, Na+ channels underlying persistent Na+ currents or GABA(A) receptor isoforms responsible for tonic (extrasynaptic) currents represent attractive targets. The growing understanding of the pathophysiology of epilepsy and the structural and functional characterization of the molecular targets provide many opportunities to create improved epilepsy therapies.

Regional expression of multidrug resistance genes in genetically epilepsy-prone rat brain after a single audiogenic seizure.

PURPOSE: The multidrug resistance (mdr) gene family encodes the drug transport macromolecule P-glycoprotein (P-gp), which contributes to the functionality of the blood-brain barrier. Recent evidence suggests that P-gp-mediated drug extrusion may play a facilitatory role in refractory epilepsy. We investigated the regional expression of mdr genes in genetically epilepsy-prone rat (GEPR) brain after a single audiogenic seizure. METHODS: Three groups of adult male GEPRs (n = 5/group) were exposed to a seizure-inducing audiogenic stimulus and killed at 4 h, 24 h, and 7 days thereafter. A further group (n = 5) served as a stimulus-naïve control. Expression of mdr1a and mdr1b in distinct anatomic brain regions (cortex, midbrain, pons/medulla, hippocampus) was determined by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in the presence of competitive internal standards. RESULTS: When compared with control, mdr1a expression in cortex and midbrain was significantly (p < 0.05) increased at 24 h after a single audiogenic seizure. Cortical mdr1a expression remained elevated at 7 days after stimulus. In contrast, mdr1a expression in pons/medulla and hippocampus was unchanged. The mdr1b isoform was quantifiable in hippocampus alone and not influenced by seizure activity. CONCLUSIONS: These findings suggest that acute seizures in the GEPR can induce the expression of mdr genes. The pattern of increased expression appears to follow the anatomic pathway of audiogenic seizures in these animals, with initiation in the midbrain and propagation to the cortex. Further studies are required to investigate the effects of recurrent seizure activity and to characterise mdr expression in other experimental seizure models.

The 'glial' glutamate transporter, EAAT2 (Glt-1) accounts for high affinity glutamate uptake into adult rodent nerve endings.

The excitatory amino acid transporters (EAAT) removes neurotransmitters glutamate and aspartate from the synaptic cleft. Most CNS glutamate uptake is mediated by EAAT2 into glia, though nerve terminals show evidence for uptake, through an unknown transporter. Reverse-transcriptase PCR identified the expression of EAAT1, EAAT2, EAAT3 and EAAT4 mRNAs in primary cultures of mouse cortical or striatal neurones. We have used synaptosomes and glial plasmalemmal vesicles (GPV) from adult mouse and rat CNS to identify the nerve terminal transporter. Western blotting showed detectable levels of the transporters EAAT1 (GLAST) and EAAT2 (Glt-1) in both synaptosomes and GPVs. Uptake of [3H]D-aspartate or [3H]L-glutamate into these preparations revealed sodium-dependent uptake in GPV and synaptosomes which was inhibited by a range of EAAT blockers: dihydrokainate, serine-o-sulfate, l-trans-2,4-pyrrolidine dicarboxylate (PDC) (+/-)-threo-3-methylglutamate and (2S,4R )-4-methylglutamate. The IC50 values found for these compounds suggested functional expression of the 'glial, transporter, EAAT2 in nerve terminals. Additionally blockade of the majority EAAT2 uptake sites with 100 micro m dihydrokainate, failed to unmask any functional non-EAAT2 uptake sites. The data presented in this study indicate that EAAT2 is the predominant nerve terminal glutamate transporter in the adult rodent CNS.