Anticonvulsant effects of LY456236, a selective mGlu1 receptor antagonist

Astrocyte-induced mGluR1 activates human lung cancer brain metastasis via glutamate-dependent stabilization of EGFR

There are limited methods to stably analyze the interactions between cancer cells and glial cells in vitro, which hinders our molecular understanding. Here, we develop a simple and stable culture method of mouse glial cells, termed mixed-glial culture on/in soft substrate (MGS), which serves well as a platform to study cancer-glia interactions. Using this method, we find that human lung cancer cells become overly dependent on metabotropic glutamate receptor 1 (mGluR1) signaling in the brain microenvironment. Mechanistically, interactions with astrocytes induce mGluR1 in cancer cells through the Wnt-5a/prickle planar cell polarity protein 1 (PRICKLE1)/RE1 silencing transcription factor (REST) axis. Induced mGluR1 directly interacts with and stabilizes the epidermal growth factor receptor (EGFR) in a glutamate-dependent manner, and these cells then become responsive to mGluR1 inhibition. Our results highlight increased dependence on mGluR1 signaling as an adaptive strategy and vulnerability of human lung cancer brain metastasis.

MTEP, a Selective mGluR5 Antagonist, Had a Neuroprotective Effect but Did Not Prevent the Development of Spontaneous Recurrent Seizures and Behavioral Comorbidities in the Rat Lithium-Pilocarpine Model of Epilepsy

Metabotropic glutamate receptors (mGluRs) are expressed predominantly on neurons and glial cells and are involved in the modulation of a wide range of signal transduction cascades. Therefore, different subtypes of mGluRs are considered a promising target for the treatment of various brain diseases. Previous studies have demonstrated the seizure-induced upregulation of mGluR5; however, its functional significance is still unclear. In the present study, we aimed to clarify the effect of treatment with the selective mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP) on epileptogenesis and behavioral impairments in rats using the lithium–pilocarpine model. We found that the administration of MTEP during the latent phase of the model did not improve survival, prevent the development of epilepsy, or attenuate its manifestations in rats. However, MTEP treatment completely prevented neuronal loss and partially attenuated astrogliosis in the hippocampus. An increase in excitatory amino acid transporter 2 expression, which has been detected in treated rats, may prevent excitotoxicity and be a potential mechanism of neuroprotection. We also found that MTEP administration did not prevent the behavioral comorbidities such as depressive-like behavior, motor hyperactivity, reduction of exploratory behavior, and cognitive impairments typical in the lithium–pilocarpine model. Thus, despite the distinct neuroprotective effect, the MTEP treatment was ineffective in preventing epilepsy.

Group 1 Metabotropic Glutamate Receptors in Neurological and Psychiatric Diseases: Mechanisms and Prospective

Metabotropic glutamate receptors (mGluRs) are G-protein coupled receptors that are activated by glutamate in the central nervous system (CNS). Basically, mGluRs contribute to fine-tuning of synaptic efficacy and control the accuracy and sharpness of neurotransmission. Among eight subtypes, mGluR1 and mGluR5 belong to group 1 (Gp1) family, and are implicated in multiple CNS disorders, such as Alzheimer’s disease, autism, Parkinson’s disease, and so on. In the present review, we systematically discussed underlying mechanisms and prospective of Gp1 mGluRs in a group of neurological and psychiatric diseases, including Alzheimer’s disease, Parkinson’s disease, autism spectrum disorder, epilepsy, Huntington’s disease, intellectual disability, Down’s syndrome, Rett syndrome, attention-deficit hyperactivity disorder, addiction, anxiety, nociception, schizophrenia, and depression, in order to provide more insights into the therapeutic potential of Gp1 mGluRs.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Advances in CNS PET: the state-of-the-art for new imaging targets for pathophysiology and drug development

PURPOSE

A limit on developing new treatments for a number of central nervous system (CNS) disorders has been the inadequate understanding of the in vivo pathophysiology underlying neurological and psychiatric disorders and the lack of in vivo tools to determine brain penetrance, target engagement, and relevant molecular activity of novel drugs. Molecular neuroimaging provides the tools to address this. This article aims to provide a state-of-the-art review of new PET tracers for CNS targets, focusing on developments in the last 5 years for targets recently available for in-human imaging.

METHODS

We provide an overview of the criteria used to evaluate PET tracers. We then used the National Institute of Mental Health Research Priorities list to identify the key CNS targets. We conducted a PubMed search (search period 1st of January 2013 to 31st of December 2018), which yielded 40 new PET tracers across 16 CNS targets which met our selectivity criteria. For each tracer, we summarised the evidence of its properties and potential for use in studies of CNS pathophysiology and drug evaluation, including its target selectivity and affinity, inter and intra-subject variability, and pharmacokinetic parameters. We also consider its potential limitations and missing characterisation data, but not specific applications in drug development. Where multiple tracers were present for a target, we provide a comparison of their properties.

RESULTS AND CONCLUSIONS

Our review shows that multiple new tracers have been developed for proteinopathy targets, particularly tau, as well as the purinoceptor P2X7, phosphodiesterase enzyme PDE10A, and synaptic vesicle glycoprotein 2A (SV2A), amongst others. Some of the most promising of these include 18F-MK-6240 for tau imaging, 11C-UCB-J for imaging SV2A, 11C-CURB and 11C-MK-3168 for characterisation of fatty acid amide hydrolase, 18F-FIMX for metabotropic glutamate receptor 1, and 18F-MNI-444 for imaging adenosine 2A. Our review also identifies recurrent issues within the field. Many of the tracers discussed lack in vivo blocking data, reducing confidence in selectivity. Additionally, late-stage identification of substantial off-target sites for multiple tracers highlights incomplete pre-clinical characterisation prior to translation, as well as human disease state studies carried out without confirmation of test-retest reproducibility.

Fragment-Based Approaches for Allosteric Metabotropic Glutamate Receptor (mGluR) Modulators

Metabotropic glutamate receptors (mGluR) are members of the class C G-Protein Coupled Receptors (GPCR-s) and have eight subtypes. These receptors are responsible for a variety of functions in the central and peripheral nervous systems and their modulation has therapeutic utility in neurological and psychiatric disorders. It was previously established that selective orthosteric modulation of these receptors is challenging, and this stimulated the search for allosteric modulators. Fragment-Based Drug Discovery (FBDD) is a viable approach to find ligands binding at allosteric sites owing to their limited size and interactions. However, it was also observed that the structure-activity relationship of allosteric modulators is often sharp and inconsistent. This can be attributed to the characteristics of the allosteric binding site of mGluRs that is a water channel where ligand binding is accompanied with induced fit and interference with the water network, both playing a role in receptor activation. In this review, we summarize fragment-based drug discovery programs on mGluR allosteric modulators and their contribution identifying of new mGluR ligands with better activity and selectivity.

Regulation of the Hippocampal Network by VGLUT3-Positive CCK- GABAergic Basket Cells

Hippocampal interneurons release the inhibitory transmitter GABA to regulate excitation, rhythm generation and synaptic plasticity. A subpopulation of GABAergic basket cells co-expresses the GABA/glycine vesicular transporters (VIAAT) and the atypical type III vesicular glutamate transporter (VGLUT3); therefore, these cells have the ability to signal with both GABA and glutamate. GABAergic transmission by basket cells has been extensively characterized but nothing is known about the functional implications of VGLUT3-dependent glutamate released by these cells. Here, using VGLUT3-null mice we observed that the loss of VGLUT3 results in a metaplastic shift in synaptic plasticity at Shaeffer's collaterals - CA1 synapses and an altered theta oscillation. These changes were paralleled by the loss of a VGLUT3-dependent inhibition of GABAergic current in CA1 pyramidal layer. Therefore presynaptic type III metabotropic could be activated by glutamate released from VGLUT3-positive interneurons. This putative presynaptic heterologous feedback mechanism inhibits local GABAergic tone and regulates the hippocampal neuronal network.

Influence of MPEP (a selective mGluR5 antagonist) on the anticonvulsant action of novel antiepileptic drugs against maximal electroshock-induced seizures in mice

The aim of this study was to determine the effects of 2-methyl-6-(phenylethynyl)pyridine (MPEP - a selective antagonist for the glutamate metabotropic receptor subtype mGluR5) on the protective action of some novel antiepileptic drugs (lamotrigine, oxcarbazepine, pregabalin and topiramate) against maximal electroshock-induced seizures in mice. Brain concentrations of antiepileptic drugs were measured to determine whether MPEP altered pharmacokinetics of antiepileptic drugs. Intraperitoneal injection of 1.5 and 2mg/kg of MPEP significantly elevated the threshold for electroconvulsions in mice, whereas MPEP at a dose of 1mg/kg considerably enhanced the anticonvulsant activity of pregabalin and topiramate, but not that of lamotrigine or oxcarbazepine in the maximal electroshock-induced seizures in mice. Pharmacokinetic results revealed that MPEP (1mg/kg) did not alter total brain concentrations of pregabalin and topiramate, and the observed effect in the mouse maximal electroshock seizure model was pharmacodynamic in nature. Collectively, our preclinical data suggest that MPEP may be a safe and beneficial adjunct to the therapeutic effects of antiepileptic drugs in human patients.

Anticonvulsant effects of structurally diverse GABA(B) positive allosteric modulators in the DBA/2J audiogenic seizure test: Comparison to baclofen and utility as a pharmacodynamic screening model

The GABA(B) receptor has been indicated as a promising target for multiple CNS-related disorders. Baclofen, a prototypical orthosteric agonist, is used clinically for the treatment of spastic movement disorders, but is associated with unwanted side-effects, such as sedation and motor impairment. Positive allosteric modulators (PAM), which bind to a topographically-distinct site apart from the orthosteric binding pocket, may provide an improved side-effect profile while maintaining baclofen-like efficacy. GABA, the major inhibitory neurotransmitter in the CNS, plays an important role in the etiology and treatment of seizure disorders. Baclofen is known to produce anticonvulsant effects in the DBA/2J mouse audiogenic seizure test (AGS), suggesting it may be a suitable assay for assessing pharmacodynamic effects. Little is known about the effects of GABA(B) PAMs, however. The studies presented here sought to investigate the AGS test as a pharmacodynamic (PD) screening model for GABA(B) PAMs by comparing the profile of structurally diverse PAMs to baclofen. GS39783, rac-BHFF, CMPPE, A-1295120 (N-(3-(4-(4-chloro-3-fluorobenzyl)-6-methoxy-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide), and A-1474713 (N-(3-(4-(4-chlorobenzyl)-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide) all produced robust, dose-dependent anticonvulsant effects; a similar profile was observed with baclofen. Pre-treatment with the GABA(B) antagonist SCH50911 completely blocked the anticonvulsant effects of baclofen and CMPPE in the AGS test, indicating such effects are likely mediated by the GABA(B) receptor. In addition to the standard anticonvulsant endpoint of the AGS test, video tracking software was employed to assess potential drug-induced motor side-effects during the acclimation period of the test. This analysis was sensitive to detecting drug-induced changes in total distance traveled, which was used to establish a therapeutic index (TI = hypoactivity/anticonvulsant effects). Calculated TIs for A-1295120, CMPPE, rac-BHFF, GS39783, and A-1474713 were 5.31x, 5.00x, 4.74x, 3.41x, and 1.83x, respectively, whereas baclofen was <1. The results presented here suggest the DBA/2J mouse AGS test is a potentially useful screening model for detecting PD effects of GABA(B) PAMs and can provide an initial read-out on target-related motor side-effects. Furthermore, an improved TI was observed for PAMs compared to baclofen, indicating the PAM approach may be a viable therapeutic alternative to baclofen.

Why does a high-fat diet induce preeclampsia-like symptoms in pregnant rats

Changes in neurotransmitter levels in the brain play an important role in epilepsy-like attacks after pregnancy-induced preeclampsia-eclampsia. Metabotropic glutamate receptor 1 participates in the onset of lipid metabolism disorder-induced preeclampsia. Pregnant rats were fed with a high-fat diet for 20 days. Thus, these pregnant rats experienced preeclampsia-like syndromes such as tension and proteinuria. Simultaneously, metabotropic glutamate receptor 1 mRNA and protein expressions were upregulated in the rat hippocampus. These findings indicate that increased sion of metabotropic glutamate receptor 1 promotes the occurrence of high-fat diet-induced preeclampsia in pregnant rats.

A novel class of succinimide-derived negative allosteric modulators of metabotropic glutamate receptor subtype 1 provides insight into a disconnect in activity between the rat and human receptors

Recent progress in the discovery of mGlu₁ allosteric modulators has suggested the modulation of mGlu₁ could offer possible treatment for a number of central nervous system disorders; however, the available chemotypes are inadequate to fully investigate the therapeutic potential of mGlu₁ modulation. To address this issue, we used a fluorescence-based high-throughput screening assay to screen an allosteric modulator-biased library of compounds to generate structurally diverse mGlu₁ negative allosteric modulator hits for chemical optimization. Herein, we describe the discovery and characterization of a novel mGlu₁ chemotype. This series of succinimide negative allosteric modulators, exemplified by VU0410425, exhibited potent inhibitory activity at rat mGlu₁ but was, surprisingly, inactive at human mGlu₁. VU0410425 and a set of chemically diverse mGlu₁ negative allosteric modulators previously reported in the literature were utilized to examine this species disconnect between rat and human mGlu₁ activity. Mutation of the key transmembrane domain residue 757 and functional screening of VU0410425 and the literature compounds suggests that amino acid 757 plays a role in the activity of these compounds, but the contribution of the residue is scaffold specific, ranging from critical to minor. The operational model of allosterism was used to estimate the binding affinities of each compound to compare to functional data. This novel series of mGlu₁ negative allosteric modulators provides valuable insight into the pharmacology underlying the disconnect between rat and human mGlu₁ activity, an issue that must be understood to progress the therapeutic potential of allosteric modulators of mGlu₁.

Octahydropyrrolo[3,4-c]pyrrole negative allosteric modulators of mGlu1

Development of SAR in an octahydropyrrolo[3,4-c]pyrrole series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. The octahydropyrrolo[3,4-c]pyrrole scaffold was chosen as an isosteric replacement for the piperazine ring found in the initial hit compound. Characterization of selected compounds in protein binding assays was used to identify the most promising analogs, which were then profiled in P450 inhibition assays in order to further assess the potential for drug-likeness within this series of compounds.

N-Acyl-N'-arylpiperazines as negative allosteric modulators of mGlu1: identification of VU0469650, a potent and selective tool compound with CNS exposure in rats

Development of SAR in an N-acyl-N'-arylpiperazine series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. Characterization of selected compounds in protein binding assays was used to aid in selecting VU0469650 for further profiling in ancillary pharmacology assays and pharmacokinetic studies. VU0469650 demonstrated an excellent selectivity profile and good exposure in both plasma and brain samples following intraperitoneal dosing in rats.

Synthesis and SAR development of novel mGluR1 antagonists for the treatment of chronic pain

High throughput screening identified the pyridothienopyrimidinone 1 as a ligand for the metabotropic glutamate receptor 1 (mGluR1=10 nM). Compound 1 has an excellent in vivo profile; however, it displays unfavorable pharmacokinetic issues and metabolic stability. Therefore, using 1 as a template, novel analogues (10i) were prepared. These analogues displayed improved oral exposure and activity in the Spinal Nerve Ligation (SNL) pain model.

Allosteric ligands for G protein-coupled receptors: a novel strategy with attractive therapeutic opportunities

Allosteric receptor ligands bind to a recognition site that is distinct from the binding site of the endogenous messenger molecule. As a consequence, allosteric agents may attach to receptors that are already transmitter-bound. Ternary complex formation opens an avenue to qualitatively new drug actions at G protein-coupled receptors (GPCRs), in particular receptor subtype selective potentiation of endogenous transmitter action. Consequently, suitable exploitation of allosteric recognition sites as alternative molecular targets could pave the way to a drug discovery paradigm different from those aimed at mimicking or blocking the effects of endogenous (orthosteric) receptor activators. The number of allosteric ligands reported to modulate GPCR function is steadily increasing and some have already reached routine clinical use. This review aims at introducing into this fascinating field of drug discovery and at providing an overview about the achievements that have already been made. Various case examples will be discussed in the framework of GPCR classification (family A, B, and C receptors). In addition, the behavior at muscarinic receptors of hybrid derivatives incorporating both an allosteric and an orthosteric fragment in a common molecular skeleton will be illustrated.

Discovery and SAR of 6-substituted-4-anilinoquinazolines as non-competitive antagonists of mGlu5

A high-throughput cell-based screen identified a series of 6-substituted-4-anilinoquinazolines as non-competitive antagonists of metabotropic glutamate receptor 5 (mGlu(5)). This Letter describes the SAR of this series and the profile of selected compounds in selectivity and radioligand binding assays.

Correlation of receptor occupancy of metabotropic glutamate receptor subtype 1 (mGluR1) in mouse brain with in vivo activity of allosteric mGluR1 antagonists

The aim of this study was to clarify the relationship between receptor occupancy and in vivo pharmacological activity of mGluR1 antagonists. The tritiated mGluR1-selective allosteric antagonist [(3)H]FTIDC (4-[1-(2-fluoropyridin-3-yl)-5-methyl-1H-1,2,3-triazol-4-yl]-N-isopropyl-N-methyl-3,6-dihydropyridine-1(2H)-carboxamide) was identified as a radioligand having high affinity for mGluR1-expressing CHO cells (K(D) = 2.1 nM) and mouse cerebellum (K(D) = 3.7 nM). [(3)H]FTIDC bound to mGluR1 was displaced by structurally unrelated allosteric antagonists, suggesting there is a mutual binding pocket shared with different allosteric antagonists. The binding specificity of [(3)H]FTIDC for mGluR1 in brain sections was demonstrated by the lack of significant binding to brain sections prepared from mGluR1-knockout mice. Ex vivo receptor occupancy with [(3)H]FTIDC revealed that the receptor occupancy level by FTIDC correlated well with FTIDC dosage and plasma concentration. Intracerebroventricular administration of (S)-3,5-dihydroxyphenylglycine is known to elicit face washing behavior that is mainly mediated by mGluR1. Inhibition of this behavioral change by FTIDC correlated with the receptor occupancy level of mGluR1 in the brain. A linear relationship between the receptor occupancy and in vivo activity was also demonstrated using structurally diverse mGluR1 antagonists. The receptor occupancy assays could help provide guidelines for selecting appropriate doses of allosteric mGluR1 antagonist for examining the function of mGluR1 in vivo.

Effects of subtype-selective group I mGluR antagonists on synchronous activity induced by 4-aminopyridine/CGP 55845 in adult guinea pig hippocampal slices

Co-application of the convulsant 4-aminopyridine (4-AP) and the GABA(B) receptor antagonist CGP 55845 to adult guinea pig hippocampal slices elicits giant GABA-mediated postsynaptic potentials (GPSPs) and epileptiform discharges. Here we tested the effects of the group I metabotropic glutamate receptor (mGluR) subtype-selective antagonists LY 367385 (mGlu1, 100 microM), MPEP (mGlu5, 10 microM), and MTEP (mGlu5, 500 nM) on this synchronous activity. Electrophysiological field recordings were performed in the CA3 region of hippocampal slices from adult guinea pigs. The mGlu5 receptor antagonists increased GPSP rate, but the mGlu1 receptor antagonist did not. This ability of mGlu5 receptor antagonists to increase the rate of GPSPs indicates that enough endogenous glutamate is released under these conditions to activate group I mGluR; nevertheless, co-application of a mGlu1 receptor antagonist (LY 367385 or JNJ 16259685) and MPEP did not decrease pre-existing epileptiform activity. Furthermore, co-application of LY 367,385 and MPEP did not prevent the emergence of epileptiform activity. When ionotropic glutamate receptor (iGluR) antagonists were present, neither MPEP nor the group I mGluR agonist DHPG changed GPSP rate, suggesting that pyramidal cell-to-interneuron iGluR-mediated synaptic connections are involved in the rate change mechanism. In contrast to the lack of effect of group I mGluR antagonists on epileptiform activity in the 4-AP/CGP 55845 model, group I mGluR antagonists blocked the emergence of longer epileptiform events and decreased the overall amount of synchronous activity in the GABA(A) antagonist/4-AP model. In conclusion, in the 4-AP/CGP 55845 model, enough glutamate was released to activate group I mGluRs and affect GPSP rate via mGlu5 receptors; however, this group I mGluR activation was not required for the generation of the epileptiform activity.

The mouse mutants recoil wobbler and nmf373 represent a series of Grm1 mutations

The identification of novel mutant alleles is important for understanding critical functional domains of a protein and establishing genotype:phenotype correlations. The recoil wobbler (rcw) allelic series of spontaneous ataxic mutants and the ENU-induced mutant nmf373 genetically mapped to a shared region of chromosome 10. Their mutant phenotypes are strikingly similar; all have an ataxic phenotype that is recessive, early-onset, and is not associated with neurodegeneration. In this study we used complementation tests to show that these series of mutants are allelic to a knockout mutant of Grm1. Subsequently, a duplication of exon 4 and three missense mutations were identified in Grm1: I160T, E292D, and G337E. All mutations occurred within the ligand-binding region and changed conserved amino acids. In the rcw mutant, the Grm1 gene is expressed and the protein product is properly localized to the molecular layer of the cerebellar cortex. Grm1 is responsible for the generation of inositol 1,4,5-trisphosphate (IP(3)). The inositol second messenger system is the central mechanism for calcium release from intracellular stores in cerebellar Purkinje cells. Several of the genes involved in this pathway are mutated in mouse ataxic disorders. The novel rcw mutants represent a resource that will have utility for further studies of inositol second-messenger-system defects in neurogenetic disorders.

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.

Metabotropic glutamate receptors as a strategic target for the treatment of epilepsy

Epilepsy is a chronic neurological disorder that has many known types, including generalized epilepsies that involve cortical and subcortical structures. A proportion of patients have seizures that are resistant to traditional anti-epilepsy drugs, which mainly target ion channels or postsynaptic receptors. This resistance to conventional therapies makes it important to identify novel targets for the treatment of epilepsy. Given the involvement of the neurotransmitter glutamate in the etiology of epilepsy, targets that control glutamatergic neurotransmission are of special interest. The metabotropic glutamate receptors (mGluRs) are of a family of eight G-protein-coupled receptors that serve unique regulatory functions at synapses that use the neurotransmitter glutamate. Their distribution within the central nervous system provides a platform for both presynaptic control of glutamate release, as well as postsynaptic control of neuronal responses to glutamate. In recent years, substantial efforts have been made towards developing selective agonists and antagonists which may be useful for targeting specific receptor subtypes in an attempt to harness the therapeutic potential of these receptors. We examine the possibility of intervening at these receptors by considering the specific example of absence seizures, a form of generalized, non-convulsive seizure that involves the thalamus. Views of the etiology of absence seizures have evolved over time from the "centrencephalic" concept of a diffuse subcortical pacemaker toward the "cortical focus" theory in which cortical hyperexcitability leads the thalamus into the 3-4 Hz rhythms that are characteristic of absence seizures. Since the cortex communicates with the thalamus via a massive glutamatergic projection, ionotropic glutamate receptor (iGluR) blockade has held promise, but the global nature of iGluR intervention has precluded the clinical effectiveness of drugs that block iGluRs. In contrast, mGluRs, because they modulate iGluRs at glutamatergic synapses only under certain conditions, may quell seizure activity by selectively reducing hyperactive glutamatergic synaptic communication within the cortex and thalamus without significantly affecting normal response rates. In this article, we review the circuitry and events leading to absence seizure generation within the corticothalamic network, we present a comprehensive review of the synaptic location and function of mGluRs within the thalamus and cerebral cortex, and review the current knowledge of mGluR modulation and seizure generation. We conclude by reviewing the potential advantages of Group II mGluRs, specifically mGluR2, in the treatment of both convulsive and non-convulsive seizures.

Seeking a mechanism of action for the novel anticonvulsant lacosamide

Lacosamide (LCM) is anticonvulsant in animal models and is in phase 3 assessment for epilepsy and neuropathic pain. Here we seek to identify cellular actions for the new drug and effects on recognised target sites for anticonvulsant drugs. Radioligand binding and electrophysiology were used to study the effects of LCM at well-established mammalian targets for clinical anticonvulsants. 10 microM LCM did not bind with high affinity to a plethora of rodent, guinea pig or human receptor sites including: AMPA; Kainate; NMDA (glycine/PCP/MK801); GABA(A) (muscimol/benzodiazepine); GABA(B); adenosine A1,2,3; alpha1, alpha2; beta1, beta2; M1,2,3,4,5; H1,2,3; CB1,2; D1,2,3,4,5; 5HT1A,1B,2A,2C,3,5A,6,7 and KATP. Weak displacement (25%) was evident at batrachotoxin site 2 on voltage gated Na+ channels. LCM did not inhibit neurotransmitter transport mechanisms for norepinephrine, dopamine, 5-HT or GABA, nor did it inhibit GABA transaminase. LCM at 100 microM produced a significant reduction in the incidence of excitatory postsynaptic currents (EPSC's) and inhibitory postsynaptic currents (IPSC's) in cultured cortical cells and blocked spontaneous action potentials (EC50 61 microM). LCM did not alter resting membrane potential or passive membrane properties following application of voltage ramps between -70 to +20 mV. The voltage-gated sodium channel (VGSC) blocker phenytoin potently blocked sustained repetitive firing (SRF) but, in contrast, 100 microM LCM failed to block SRF. No effect was observed on voltage-clamped Ca2+ channels (T-, L-, N- or P-type). Delayed-rectifier or A-type potassium currents were not modulated by LCM (100 microM). LCM did not mimic the effects of diazepam as an allosteric modulator of GABA(A) receptor currents, nor did it significantly modulate evoked excitatory neurotransmission mediated by NMDA or AMPA receptors (n > or = 5). Evidently LCM perturbs excitability in primary cortical cultures but does not appear to do so via a high-affinity interaction with an acknowledged recognition site on a target for existing antiepileptic drugs.

mGlu1 and mGlu5 receptor antagonists lack anticonvulsant efficacy in rodent models of difficult-to-treat partial epilepsy

Modulation of metabotropic glutamate (mGlu) receptors represents an interesting new approach for the treatment of a range of neurological and psychiatric disorders. Several lines of evidence suggest that functional blockade of group I (mGlu1 and mGlu5) receptors may be beneficial for treatment of epileptic seizures. This study was conducted to investigate whether mGlu1 or mGlu5 receptor antagonists have the potential to block partial or secondarily generalized seizures as occurring in partial epilepsy, the most common and difficult-to-treat type of epilepsy in patients. For this purpose, we systemically administered novel highly selective and brain penetrable group I mGlu receptor antagonists, i.e., the mGlu1 receptor antagonist EMQMCM [3-ethyl-2-methyl-quinolin-6-yl-(4-methoxy-cyclohexyl)-methanone methanesulfonate] and the mGlu5 receptor antagonist MTEP ([(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine), at doses appropriate for mGlu1 or mGlu5 receptor-mediated effects in rodent models of partial seizures. Two models were used: the 6-Hz electroshock model of partial seizures in mice and the amygdala-kindling model in rats. Clinically established antiepileptic drugs were included in the experiments for comparison. Antiepileptic drugs exerted significant anticonvulsant effects in both models, while EMQMCM and MTEP were ineffective in this regard, although both compounds were administered up to doses associated with essentially full receptor occupancy and with typical mGlu receptor-mediated effects in rodent models of anxiety or pain. Brain microdialysis for determining extracellular levels of MTEP following i.p. administration in rats substantiated that effective brain concentrations were reached at times of our experiments in seizure models. The present results do not support a significant anticonvulsant potential of group I mGlu receptor antagonists in rodent models of difficult-to-treat partial epilepsy.