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

GluR2 can Drive Neuroinflammation and Cognitive Impairments Following Peripherally Repeated Lipopolysaccharide Exposures

Neuroinflammation is being increasingly recognized as a vital factor in the development of various neurological and neuropsychiatric diseases. Lipopolysaccharides (LPS), an outer membrane component of gram-negative bacteria, can trigger innate immune responses, resulting in neuroinflammation and subsequent cognitive deficits. The expression of glutamate receptors (GluRs) on glial cells can induce glial activation. Therefore, we hypothesized that repeated LPS exposure can increase GluR levels, promoting microglial activation and ultimately affecting synaptic plasticity and cognitive function. In this study, C57/BL6 mice were repeatedly exposed to LPS to construct a neuroinflammation animal model. The levels of GluRs, inflammatory cytokines, ionized calcium-binding adaptor molecule 1, postsynaptic density protein 95, synaptophysin 38, NMDA receptor 2 A, and NMDA receptor 2B (GluN2B) were measured in the hippocampi. Furthermore, dendritic spine density in the CA1 hippocampal region was determined. Repeated LPS exposure induced cognitive impairments and microglial activation and increased GluR1 and GluR2 levels. This was accompanied by a significant decrease in GluN2B expression and dendritic spine density in the hippocampi. However, CFM-2, an α-amino-3- hydroxy-5-methyl-4-isoxazolepropionate receptor antagonist, reversed these anomalies. Furthermore, minocycline, a microglial inhibitor, reversed these anomalies and downregulated GluR2 but not GluR1 expression. In summary, we demonstrated that GluR2 plays an essential role in microglia-induced neuroinflammation, resulting in synaptic plasticity and cognitive impairment induced by repeated exposure to LPS.

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".

The Anticonvulsant Activity of a Flavonoid-Rich Extract from Orange Juice Involves both NMDA and GABA-Benzodiazepine Receptor Complexes

The usage of dietary supplements and other natural products to treat neurological diseases has been growing over time, and accumulating evidence suggests that flavonoids possess anticonvulsant properties. The aim of this study was to examine the effects of a flavonoid-rich extract from orange juice (OJe) in some rodent models of epilepsy and to explore its possible mechanism of action. The genetically audiogenic seizures (AGS)-susceptible DBA/2 mouse, the pentylenetetrazole (PTZ)-induced seizures in ICR-CD1 mice and the WAG/Rij rat as a genetic model of absence epilepsy with comorbidity of depression were used. Our results demonstrate that OJe was able to exert anticonvulsant effects on AGS-sensible DBA/2 mice and to inhibit PTZ-induced tonic seizures, increasing their latency. Conversely, it did not have anti-absence effects on WAG/Rij rats. Our experimental findings suggest that the anti-convulsant effects of OJe are likely mediated by both an inhibition of NMDA receptors at the glycine-binding site and an agonistic activity on benzodiazepine-binding site at GABA_A receptors. This study provides evidences for the antiepileptic activity of OJe, and its results could be used as scientific basis for further researches aimed to develop novel complementary therapy for the treatment of epilepsy in a context of a multitarget pharmacological strategy.

Group III mGlu receptor agonist, ACPT-I, exerts potential neuroprotective effects in vitro and in vivo

Many evidence suggest that metabotropic glutamate receptors (mGluRs) may modulate glutamatergic transmission, hence, these receptors are regarded as potential targets for neuroprotective drugs. Since group III mGlu receptor agonists are known to reduce glutamatergic transmission by inhibiting glutamate release, we decided to investigate the neuroprotective potential of the group III mGlu receptor agonist, (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid (ACPT-I) against kainate (KA)-induced excitotoxicity in vitro and in vivo. In primary neuronal cell cultures ACPT-I (1-200 μM), applied 30 min-3 h after starting the exposure to KA (150 μM), significantly attenuated the KA-induced LDH release, increased cell viability, and inhibited caspase-3 activity both in cortical and hippocampal cell cultures. The effects were dose-, time- and structure-dependent. The neuroprotective effects of ACPT-I were reversed by (RS)-alpha-cyclopropyl-4-phosphonophenyl glycine, a group III mGluR antagonist. In the in vivo studies, KA (2.5 nmol/1 μl) was unilaterally injected into the rat dorsal CA1 hippocampal region and the size of degeneration was examined by stereological counting of surviving neurons in the CA pyramidal layer. It was found that ACPT-I (7.5 or 15 nmol/1 μl), injected into the dorsal hippocampus 30 min, 1 or 3 h after KA in dose-dependent manner prevented the KA-induced neuronal damage. Moreover, in vivo microdialysis studies in the rat hippocampus showed that ACPT-I (200 μM) given simultaneously with KA (50 μM) significantly diminished the KA-induced glutamate release in the hippocampus. This mechanism seems to play a role in mediating the neuroprotective effect of ACPT-I.

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.

Synergistic combinations of anticonvulsant agents: what is the evidence from animal experiments?

PURPOSE

Combination therapy is often used in the treatment of seizures refractory to monotherapy. At the same time, the pharmacodynamic mechanisms that determine the combined efficacy of antiepileptic drugs (AEDs) are unknown, and this prevents a rational use of these drug combinations. We critically evaluate the existing evidence for pharmacodynamic synergism between AEDs from preclinical studies in animal models of epilepsy to identify useful combinations of mechanisms and to determine whether study outcome depends on the various research methods that are in use.

METHODS

Published articles were included if the studies were placebo-controlled, in vivo, or ex vivo animal studies investigating marketed or experimental AEDs. The animal models that were used in these studies, the primary molecular targets of the tested drugs, and the methods of interpretation were recorded. The potential association of these factors with the study outcome (synergism: yes or no) was assessed through logistic regression analysis.

RESULTS

In total, 107 studies were identified, in which 536 interaction experiments were conducted. In 54% of these experiments, the possibility of a pharmacokinetic interaction was not investigated. The majority of studies were conducted in the maximal electroshock model, and other established models were the pentylenetetrazole model, amygdala kindling, and the DBA/2 model. By far the most widely used method for interpretation of the results was evaluation of the effect of a threshold dose of one agent on the median effective dose (ED50) of another agent. Experiments relying on this method found synergism significantly more often compared with experiments relying on other methods (p<0.001). Furthermore, experiments including antagonists of the AMPA receptor were more likely to find synergism in comparison with all other experiments (p<0.001).

CONCLUSIONS

Intensive preclinical research into the effects of AED combinations has not led to an understanding of the pharmacodynamic properties of AED combinations. Specifically, the majority of the preclinical studies are not adequately designed to distinguish between additive, synergistic, and antagonistic interactions. Quantitative pharmacokinetic-pharmacodynamic studies of selectively acting AEDs in a battery of animal models are necessary for the development of truly synergistic drug combinations.

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.

The preferential mGlu2/3 receptor antagonist, LY341495, reduces the frequency of spike-wave discharges in the WAG/Rij rat model of absence epilepsy

We examined the expression and function of group-II metabotropic glutamate (mGlu) receptors in an animal model of absence seizures using genetically epileptic WAG/Rij rats, which develop spontaneous non-convulsive seizures after 2-3 months of age. Six-month-old WAG/Rij rats showed an increased expression of mGlu2/3 receptors in the ventrolateral regions of the somatosensory cortex, ventrobasal thalamic nuclei, and hippocampus, but not in the reticular thalamic nucleus and in the corpus striatum, as assessed by immunohistochemistry and Western blotting. In contrast, mGlu2/3 receptor signalling was reduced in slices prepared from the somatosensory cortex of 6-month-old WAG/Rij rats, as assessed by the ability of the agonist, LY379268, to inhibit forskolin-stimulated cAMP formation. None of these changes was found in "pre-symptomatic" 2-month-old WAG/Rij rats. To examine whether pharmacological activation or inhibition of mGlu2/3 receptors affects absence seizures, we recorded spontaneous spike-wave discharges (SWDs) in 6-month-old WAG/Rij rats systemically injected with saline, the mGlu2/3 receptor agonist LY379268 (0.33 or 1 mg/kg, i.p.), or with the preferential mGlu2/3 receptor antagonist, LY341495 (0.33, 1 or 5 mg/kg, i.p.). Injection of 1mg/kg of LY379268 (1 mg/kg, i.p.) increased the number of SWDs during 3-7 h post-treatment, whereas injection with LY341495 reduced the number of seizures in a dose-dependent manner. It can be concluded that mGlu2/3 receptors are involved in the generation of SWDs and that an upregulation of these receptors in the somatosensory cortex might be involved in the pathogenesis of absence epilepsy.

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.