(S)-4-carboxy-3-hydroxyphenylglycine, an antagonist of metabotropic glutamate receptor (mGluR) 1a and an agonist of mGluR2, protects against audiogenic seizures in DBA/2 mice

Metabotropic glutamate receptors (mGluRs) in epileptogenesis: an update on abnormal mGluRs signaling and its therapeutic implications

Epilepsy is a neurological disorder characterized by high morbidity, high recurrence, and drug resistance. Enhanced signaling through the excitatory neurotransmitter glutamate is intricately associated with epilepsy. Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors activated by glutamate and are key regulators of neuronal and synaptic plasticity. Dysregulated mGluR signaling has been associated with various neurological disorders, and numerous studies have shown a close relationship between mGluRs expression/activity and the development of epilepsy. In this review, we first introduce the three groups of mGluRs and their associated signaling pathways. Then, we detail how these receptors influence epilepsy by describing the signaling cascades triggered by their activation and their neuroprotective or detrimental roles in epileptogenesis. In addition, strategies for pharmacological manipulation of these receptors during the treatment of epilepsy in experimental studies is also summarized. We hope that this review will provide a foundation for future studies on the development of mGluR-targeted antiepileptic drugs.

Dysregulation of Ambient Glutamate and Glutamate Receptors in Epilepsy: An Astrocytic Perspective

Given the important functions that glutamate serves in excitatory neurotransmission, understanding the regulation of glutamate in physiological and pathological states is critical to devising novel therapies to treat epilepsy. Exclusive expression of pyruvate carboxylase and glutamine synthetase in astrocytes positions astrocytes as essential regulators of glutamate in the central nervous system (CNS). Additionally, astrocytes can significantly alter the volume of the extracellular space (ECS) in the CNS due to their expression of the bi-directional water channel, aquaporin-4, which are enriched at perivascular endfeet. Rapid ECS shrinkage has been observed following epileptiform activity and can inherently concentrate ions and neurotransmitters including glutamate. This review highlights our emerging knowledge on the various potential contributions of astrocytes to epilepsy, particularly supporting the notion that astrocytes may be involved in seizure initiation via failure of homeostatic responses that lead to increased ambient glutamate. We also review the mechanisms whereby ambient glutamate can influence neuronal excitability, including via generation of the glutamate receptor subunit GluN2B-mediated slow inward currents, as well as indirectly affect neuronal excitability via actions on metabotropic glutamate receptors that can potentiate GluN2B currents and influence neuronal glutamate release probabilities. Additionally, we discuss evidence for upregulation of System x c - , a cystine/glutamate antiporter expressed on astrocytes, in epileptic tissue and changes in expression patterns of glutamate receptors.

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2

Metabotropic glutamate receptors (mGluRs) associated with the slow phase of the glutamatergic signaling pathway in neurons of the central nervous system have gained importance as drug targets for chronic neurodegenerative diseases. While extracellular Ca²⁺ was reported to exhibit direct activation and modulation via an allosteric site, the identification of those binding sites was challenged by weak binding. Herein, we review the discovery of extracellular Ca²⁺ in regulation of mGluRs, summarize the recent developments in probing Ca²⁺ binding and its co-regulation of the receptor based on structural and biochemical analysis, and discuss the molecular basis for Ca²⁺ to regulate various classes of drug action as well as its importance as an allosteric modulator in mGluRs.

Metabotropic Glutamate Receptors and Interacting Proteins in Epileptogenesis

Neurotransmitter and receptor systems are involved in different neurological and neuropsychological disorders such as Parkinson's disease, depression, Alzheimer’s disease and epilepsy. Recent advances in studies of signal transduction pathways or interacting proteins of neurotransmitter receptor systems suggest that different receptor systems may share the common signal transduction pathways or interacting proteins which may be better therapeutic targets for development of drugs to effectively control brain diseases. In this paper, we reviewed metabotropic glutamate receptors (mGluRs) and their related signal transduction pathways or interacting proteins in status epilepticus and temporal lobe epilepsy, and proposed some novel therapeutical drug targets for controlling epilepsy and epileptogenesis.

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.

Therapeutic potential of metabotropic glutamate receptor modulators

Glutamate is the main excitatory neurotransmitter in the central nervous system (CNS) and is a major player in complex brain functions. Glutamatergic transmission is primarily mediated by ionotropic glutamate receptors, which include NMDA, AMPA and kainate receptors. However, glutamate exerts modulatory actions through a family of metabotropic G-protein-coupled glutamate receptors (mGluRs). Dysfunctions of glutamatergic neurotransmission have been implicated in the etiology of several diseases. Therefore, pharmacological modulation of ionotropic glutamate receptors has been widely investigated as a potential therapeutic strategy for the treatment of several disorders associated with glutamatergic dysfunction. However, blockade of ionotropic glutamate receptors might be accompanied by severe side effects due to their vital role in many important physiological functions. A different strategy aimed at pharmacologically interfering with mGluR function has recently gained interest. Many subtype selective agonists and antagonists have been identified and widely used in preclinical studies as an attempt to elucidate the role of specific mGluRs subtypes in glutamatergic transmission. These studies have allowed linkage between specific subtypes and various physiological functions and more importantly to pathological states. This article reviews the currently available knowledge regarding the therapeutic potential of targeting mGluRs in the treatment of several CNS disorders, including schizophrenia, addiction, major depressive disorder and anxiety, Fragile X Syndrome, Parkinson’s disease, Alzheimer’s disease and pain.

Fragile x syndrome and autism: from disease model to therapeutic targets

Autism is an umbrella diagnosis with several different etiologies. Fragile X syndrome (FXS), one of the first identified and leading causes of autism, has been modeled in mice using molecular genetic manipulation. These Fmr1 knockout mice have recently been used to identify a new putative therapeutic target, the metabotropic glutamate receptor 5 (mGluR5), for the treatment of FXS. Moreover, mGluR5 signaling cascades interact with a number of synaptic proteins, many of which have been implicated in autism, raising the possibility that therapeutic targets identified for FXS may have efficacy in treating multiple other causes of autism.

Dissection of a QTL hotspot on mouse distal chromosome 1 that modulates neurobehavioral phenotypes and gene expression

A remarkably diverse set of traits maps to a region on mouse distal chromosome 1 (Chr 1) that corresponds to human Chr 1q21-q23. This region is highly enriched in quantitative trait loci (QTLs) that control neural and behavioral phenotypes, including motor behavior, escape latency, emotionality, seizure susceptibility (Szs1), and responses to ethanol, caffeine, pentobarbital, and haloperidol. This region also controls the expression of a remarkably large number of genes, including genes that are associated with some of the classical traits that map to distal Chr 1 (e.g., seizure susceptibility). Here, we ask whether this QTL-rich region on Chr 1 (Qrr1) consists of a single master locus or a mixture of linked, but functionally unrelated, QTLs. To answer this question and to evaluate candidate genes, we generated and analyzed several gene expression, haplotype, and sequence datasets. We exploited six complementary mouse crosses, and combed through 18 expression datasets to determine class membership of genes modulated by Qrr1. Qrr1 can be broadly divided into a proximal part (Qrr1p) and a distal part (Qrr1d), each associated with the expression of distinct subsets of genes. Qrr1d controls RNA metabolism and protein synthesis, including the expression of approximately 20 aminoacyl-tRNA synthetases. Qrr1d contains a tRNA cluster, and this is a functionally pertinent candidate for the tRNA synthetases. Rgs7 and Fmn2 are other strong candidates in Qrr1d. FMN2 protein has pronounced expression in neurons, including in the dendrites, and deletion of Fmn2 had a strong effect on the expression of few genes modulated by Qrr1d. Our analysis revealed a highly complex gene expression regulatory interval in Qrr1, composed of multiple loci modulating the expression of functionally cognate sets of genes.

Differential Expression of Cerebellar Metabotropic Glutamate Receptors mGLUR2/3 and mGLUR4a after the Administration of a Convulsant Drug and the Adenosine Analogue Cyclopentyladenosine

Metabotropic glutamate receptors (mGluR) play a role in synaptic transmission, neuronal modulation and plasticity but their action in epileptic activity is still controversial. On the other hand adenosine acts as a neuromodulator with endogenous anticonvulsive properties. Since cerebellum from epileptic patients has shown neuronal damage, sometimes associated with Purkinje cells loss, we have explored the effect of repetitive seizures on two types of mGluR in the cerebellum. Seizures were induced by the convulsant drug 3-mercaptopropionic acid (MP) and the effect of the adenosine analogue cyclopentyladenosine (CPA) alone or before MP administration (CPA+MP) were also evaluated. The expression of the receptors subtypes 2/3 (mGluR2/3) and 4a (mGluR4a) was assessed by immunocitochemistry. Granular cell layer was labeled with mGluR2/3 antibody and increased immunoreactivity was observed after MP (60%), CPA (53%) and CPA + MP (85%) treatments. Control cerebellum slices showed mGluR4a reactivity around Purkinje cells, while MP, CPA and CPA+MP treatment decreased this immunostaining. Repetitive administration of MP and CPA induces an increased cerebellar mGluR2/3 and a decreased mGluR4a immunostaining, suggesting a distinct participation of both receptors that may be related to the type of cell involved. A protective action and /or an apoptotic effect may not be discarded. CPA repetitive administration although increase seizure latency, cannot prevent seizure activity.

Metabotropic glutamate receptors and epilepsy

Metabotropic glutamate receptors (mGluRs) play an important role in the initiation of ictal discharges by participating in the interictal-ictal transition, and may play a crucial role in recruiting normal brain tissue into synchronized discharges, thereby facilitating propagation of seizure activity. In this article we present a review of mGluRs and epilepsy studies. Structural features of mGluRs offer multiple possibilities for synthetic compounds to modulate their activity, and for many reasons these compounds are good candidates for therapeutic applications. Group I mGluRs enhance excitatory transmission as much as groups II and III mGluRs can modulate those effects. Finally, main avenues to induce epileptogenesis are considered: activation of Ca2+ channels and Ca2+/CaMKII cascade, overexpression of AMPA and/or KA receptors, enhanced NMDARs function, activation of protooncogenes leading to a steady epileptogenic state, enhancement of INaP currents, blockade of A and/or M K(+) currents, calcium channelopathies, diminished number of GABARs or functions, and down-regulation of glutamate transporters. Deregulation of mGluR signaling functions including deficits in groups II and III mGluRs or hyperactivation of group I mGluRs may occur in some forms of epilepsy, therefore targeting these mechanisms with specific pharmacological tools could provide new developments for original therapeutic approaches.

Agonists and antagonists of metabotropic glutamate receptors: anticonvulsants and antiepileptogenic agents?

Anticonvulsant and neuroprotective effects of agonist and antagonist of metabotropic glutamate receptors (mGluRs) have been known for more than 10 years from multiple studies. However, it is not certain whether these candidate drugs are also antiepileptic and antiepileptogenic, as few studies included the chronic stages to determine whether spontaneous recurrent seizures could be prevented or stopped. Even in the acute stage, differences in experimental design such as timing and route of administration of candidate drugs, age, species and strain of experimental animal and experimental model make it difficult to determine the anticonvulsant and europrotective effects of each candidate drug. This paper, reviews in vivo neuropharmacological studies on agonsists and antagonists of mGluRs in different seizure and epilepsy models in last more than ten years. By combining with our neuropharmacological studies on the effect of mGluR agonists and antagonists in the mouse pilocarpine model of temporal lobe epilepsy, an ideal model for future development of mGluR agonists and antagonists as antiepileptogenic drugs will be proposed.

Anticonvulsant effects of LY456236, a selective mGlu1 receptor antagonist

Several lines of evidence suggest that mGlu1 metabotropic glutamate receptors may be involved in seizure disorders such as epilepsy. For example, the mGlu1 agonist DHPG produces limbic seizures and group I antagonists such as 4C3HPG and 4CPG are anticonvulsant when administered intracerebrally. The purpose of the present experiments was to characterize the anticonvulsant effects of the selective mGlu1 receptor antagonist LY456236 in mice and rats. In male and female DBA/2 mice, LY456236 produced a dose-related inhibition of sound-induced clonic-tonic seizures. In male CF1 mice, LY456236 produced a dose-related inhibition of tonic extensor seizures in the threshold electroshock model, and limbic seizures in the 6-Hz focal seizure model. However, this antagonist did not inhibit clonic seizures produced by pentylenetetrazol. In amygdala-kindled male Sprague-Dawley rats, LY456236 produced dose-related decreases in behavioral and electrographic seizures at threshold stimulus intensity. In addition, LY456236 produced a dose-related increase in the stimulus intensity required to produce generalized seizures. Taken together, the present results support the conclusion that mGlu1 receptor antagonists such as LY456236 may have clinical utility in the treatment of epilepsy and other seizure disorders.

Metabotropic glutamate receptor 2/3 in the hippocampus of patients with mesial temporal lobe epilepsy, and of rats and mice after pilocarpine-induced status epilepticus

A comparative study of the expression of metabotropic glutamate receptor 2/3 (mGluR2/3) was done in the hippocampus of rats and mice after pilocarpine-induced status epilepticus (APISE), and of patients with mesial temporal lobe epilepsy. At 1 day APISE, there was a marked increase in mGluR2/3 immunoreactivity in the stratum lacunosum moleculare (SLM) of CA1 area and in the middle one-third of the molecular layer (MM) of the dentate gyrus. Immuno-electron microscopic study showed degenerating mGluR2/3 positive axons in the SLM of CA1 area at 1 day APISE. From 7 days, mGluR2/3 immunopositive product decreased, and by 31 days APISE, it almost disappeared in two-thirds of the SLM near CA2. In the mouse model at 2 months APISE, mGluR2/3 immunopositive product in two-thirds of the SLM near the stratum radiatum disappeared, and so did in the whole SLM of CA1 area in patients with mesial temporal lobe epilepsy. Neuropharmacological study by intravenous injection of mGluR2/3 agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate [(2R,4R)-APDC] at different doses at 1h during pilocarpine induced status epilepticus showed that (2R,4R)-APDC could not stop seizures and neuronal death in the hilus of the dentate gyrus. The present study, therefore, suggests that the reduction of mGluR2/3 immunopositive product in the SLM of CA1 is a consequence of neuronal loss in either the entorhinal cortex or CA1 area of the hippocampus, and at the dosage range from 12.5 to 600 mg/kg, (2R,4R)-APDC may not be effective in the prevention of seizures or neuronal death in the hilus of the dentate gyrus.

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.

Attenuation of seizures induced by homocysteic acid in immature rats by metabotropic glutamate group II and group III receptor agonists

Previous studies demonstrated that selected agonists for metabotropic glutamate group II and group III receptors can provide protection against seizures in adult animals. The present study has examined the potential effect of some of these compounds on seizures induced in immature rats by intracerebroventricular infusion of DL-homocysteic acid (DL-HCA, 600 nmol/side). Rat pups were sacrificed during generalised clonic-tonic seizures, 50--60 min after infusion. Comparable time intervals were used for sacrificing the pups which had received the protective drugs. The anticonvulsant effect was evaluated according to the suppression of behavioural manifestations of seizures and the protection of energy metabolite changes which normally accompany these seizures (large decreases of glucose and glycogen, and approximately 7- to 10-fold accumulation of lactate). Partial protection was exhibited by group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV, 0.6 nmol) and this effect was abolished after pretreatment with an antagonist for group II mGluRs (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG, 100 nmol). In high doses (5--100 nmol), however, DCG IV evoked seizures which were prevented by AP7, suggesting that the convulsant effect was mediated by interaction with NMDA receptors. A pronounced anticonvulsant effect against DL-HCA-induced seizures was achieved with low doses of a highly selective group II mGluR agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC, 0.6 nmol), group II agonist and group I mGluR antagonist (S)-4-carboxy-3-hydroxyphenylglycine ((S)-4-C3HPG, 0.6 nmol) and group III mGluR agonist (RS)-1-amino-3-(phosphonomethylene) cyclobutane-carboxylic acid (32 nmol). Generalised clonic--tonic seizures were completely suppressed and the metabolic changes were markedly ameliorated, there being only a 1.5-, 2- and 2.5-fold rise of lactate, respectively. Higher doses of (S)-4-C3HPG (1--100 nmol) were, however, less anticonvulsant than low doses. The present results have confirmed that mGluRs may be considered a potential target for treatment of epilepsy.

Modulation of calcium channels by group I and group II metabotropic glutamate receptors in dentate gyrus neurons from patients with temporal lobe epilepsy

PURPOSE

Metabotropic glutamate receptors (mGluRs) might be promising new drug targets for the treatment of epilepsy because the expression of certain mGluRs is regulated in epilepsy and because activation of mGluRs results in distinctive anti- and proconvulsant effects. Therefore, we examined how mGluR activation modulates high-voltage-activated (HVA) Ca2+ channels.

METHODS

Whole-cell patch-clamp recordings were obtained from granule cells and interneuron-like cells acutely isolated from the dentate gyrus of patients with pharmacoresistent temporal lobe epilepsy.

RESULTS

Agonists selective for either group I or group II mGluRs rapidly and reversibly reduced HVA currents in most dentate gyrus cells. These modulatory effects were inhibited by the respective group I and group II mGluR antagonists. The specific Ca2+ channel antagonists nifedipine and omega-conotoxin GVIA potently occluded the effects of group I and II mGluR agonists, respectively, indicating that group I mGluRs acted on L-type channels and group II mGluRs affected N-type channels. About two thirds of the responsive neurons were sensitive either to group I or group II mGluRs, whereas a minority of cells showed effects to agonists of both groups, indicating a variable mGluR expression pattern.

CONCLUSIONS

Group I and group II mGluRs are expressed in human dentate gyrus neurons and modulate L- and N-type HVA channels, respectively. The data shed light on the possible cellular sequelae of the mGluR1 upregulation observed in human epileptic dentate gyrus as well as on possible mGluR-mediated anticonvulsant mechanisms.

Anticonvulsant activity of two metabotropic glutamate group I antagonists selective for the mGlu5 receptor: 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and (E)-6-methyl-2-styryl-pyridine (SIB 1893)

The selective mGlu5 antagonists, MPEP, 2-methyl-6-phenylethynyl-pyridine, and SIB1893, (E)-6-methyl-2-styryl-pyridine, have been evaluated as antiepileptic drugs in DBA/2 mice and lethargic mice. Clonic seizures induced by the selective mGlu5 agonist, (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG), 3 micromol intracerebroventricularly (i.c.v.), are potently suppressed by both compounds (MPEP, ED(50)=0.42 [0.28-0.62] mg/kg intraperitoneally (i.p.); SIB 1893 ED(50)=0.19 [0.11-0.33] mg/kg i.p. ). Clonic seizures induced by the mGlu1,5 agonist, 3, 5-dihydroxyphenylglycine (DHPG), 1.5 micromol i.c.v., are less potently suppressed by both compounds (MPEP, ED(50)=22 [13-38] mg/kg i.p., 110 [67-180] nmol i.c.v.; SIB1893, ED(50)=31 [18-54] mg/kg i.p. , 95 [82-110] nmol i.c.v.). Sound-induced seizures in DBA/2 mice are suppressed at 15 min by MPEP and SIB 1893 (MPEP ED(50) clonic seizures=18 [10-32] mg/kg i.p., 93 [69-125] nmol i.c.v.; tonic seizures=6.1 [4.5-8.3] mg/kg i.p., 46 [26-80] nmol i.c.v.; SIB 1893 ED(50) clonic seizures=27 [17-44] mg/kg i.p., 825 [615-1108] nmol i. c.v., tonic seizures=5.4 [3.4-8.6] mg/kg i.p., 194 [113-332] nmol i. c.v.). The ED(50) for MPEP for impaired rotarod performance is 128 [83-193] mg/kg i.p., at 15 min, i.e. a therapeutic index for sound-induced seizures of 5-20. In lethargic mice (lh/lh), a genetic absence model, MPEP, 50 mg/kg i.p., caused a marked reduction in the incidence of spontaneous spike-and-wave discharges. These selective antagonists of mGlu5 block seizures due to activation of mGlu5 at very low systemic doses. At rather higher doses they block convulsive and non-convulsive primary generalised seizures.

Glutamate and epilepsy

Epileptic syndromes have very diverse primary causes, which may be genetic, developmental or acquired. In rodent models, altering glutamate receptor or glutamate transporter expression by knockout or knockdown procedures can induce or suppress epileptic seizures. Regardless of the primary cause, synaptically released glutamate acting on ionotropic and metabotropic receptors appears to play a major role in the initiation and spread of seizure activity. In rodent models of acquired epilepsy and in human temporal lobe epilepsy, there is evidence for enhanced functional efficacy of ionotropic N-methyl-D-aspartate (NMDA) and metabotropic (Group I) receptors. In animal models of epilepsy, antagonists acting at NMDA receptors or at Group I metabotropic receptors have potent anticonvulsant actions.

Temporal lobe epilepsy associated up-regulation of metabotropic glutamate receptors: correlated changes in mGluR1 mRNA and protein expression in experimental animals and human patients

Aberrant axonal reorganization and altered distribution of neurotransmitter receptor subtypes have been proposed as major pathogenic mechanisms for hippocampal hyperexcitability in chronic temporal lobe epilepsies (TLE). Recent data point to excitatory class I metabotropic glutamate receptors (mGluR1 and mGluR5) as interesting candidates. Here, we have analyzed the hippocampal distribution and mRNA expression of mGluR1 and mGluR5 in two rat models of limbic seizures, i.e. electrical kindling and intraperitoneal kainate injections, as well as in human TLE. Quantitative RT-PCR analysis detected a significant increase of hippocampal mGluR1 gene transcript levels in kainate treated and kindled rats. In addition, microdissected hippocampal tissue samples localized this increase to the dentate gyrus. Using immunohistochemistry with mGluR1alpha subtype specific antibodies, increased labeling was observed within the dentate gyrus molecular layer (DG-ML). A similar pattern of increased mGluR1alpha neuropil staining was found within the DG-ML of epilepsy patients (n = 42) compared with peritumoral hippocampus specimens obtained from nonepileptic patients (biopsy controls, n = 3). This increase was detected in TLE patients with segmental hippocampal cell loss, as well as in TLE patients with focal lesions but no histopathological alterations of the hippocampus. In contrast, mGluR5 immunoreactivity and mRNA expression were not significantly altered in the DG-ML. Our data demonstrate a striking regional induction of mGluR1alpha in the hippocampal dentate gyrus of experimental animals with limbic seizures as well as in human patients with chronic, intractable TLE. This increase corresponds to functional alterations of class I mGluRs observed in seizure models and may significantly contribute to hippocampal hyperexcitability in focal human epilepsies.

Selective activation of group II mGluRs with LY354740 does not prevent neuronal excitotoxicity

Recent reports have suggested a role for group II metabotropic glutamate receptors (mGluRs) in the attenuation of excitotoxicity. Here we examined the effects of the recently available group II agonist (+)-2-Aminobicyclo[3.1.0]hexane-2-6-dicarboxylic acid (LY354740) on N-methyl-D-aspartate (NMDA)-induced excitotoxic neuronal death, as well as on hypoxic-ischemic neuronal death both in vitro and in vivo. At concentrations shown to be selective for group II mGluRs expressed in cell lines (0.1-100 nM), LY354740 did not attenuate NMDA-mediated neuronal death in vitro or in vivo. Furthermore, LY354740 did not attenuate oxygen-glucose deprivation-induced neuronal death in vitro or ischemic infarction after transient middle cerebral artery occlusion in rats. In addition, the neuroprotective effect of another group II agonist, (S)-4-carboxy-3-phenylglycine (4C3HPG), which has shown injury attenuating effects both in vitro and in vivo, was not blocked by the group II antagonists (2 S)-alpha-ethylglutamic acid (EGLU), (RS)-alpha-methyl-4-sulphonophenylglycine (MSPG), or the group III antagonist (S)-alpha-methyl-3-carboxyphenylalanine (MCPA), suggesting that this neuroprotection may be mediated by other effects such as upon group I mGluRs.

Molecular neuropathology of human mesial temporal lobe epilepsy

With the recent progress in surgical treatment modalities, human brain tissue from patients with intractable focal epilepsies will increasingly become available for studies on the molecular pathology, electrophysiological changes and pathogenesis of human focal epilepsies. An inherent problem for studies on human temporal lobe epilepsy (TLE) is the lack of suitable controls. Strategies to alleviate this obstacle include the use of human post mortem samples, hippocampus from experimental animals and, in particular, the comparative analysis of surgical specimens from patients with Ammon's horn sclerosis (AHS) and with focal temporal lesions but anatomically preserved hippocampal structures. In this review we focus on selected aspects of the molecular neuropathology of TLE: (1) the potential impact of persisting calretinin-immunoreactive neurons with Cajal-Retzius cell morphology, (2) astrocytic tenascin-C induction and redistribution as potential regulator of aberrant axonal sprouting and (3) alterations of Ca2+ -mediated hippocampal signalling pathways. The diverse and complex changes described so far in human TLE specimens require a systematic interdisciplinary approach to distinguish primary, epileptogenic alterations and secondary, compensatory mechanisms in the pathogenesis of human temporal lobe epilepsies.

Group I metabotropic glutamate receptors: implications for brain diseases

Glutamate is the major excitatory neurotransmitter in the brain and plays a unique role in a variety of central nervous system (CNS) functions. The discovery of the metabotropic receptors (mGluRs), a family of G-protein coupled receptors than can be activated by glutamate, has led to an impressive number of studies in recent years aimed at understanding their biochemical, physiological and pharmacological characteristics. The eight mGluRs now known are divided into three groups according to their sequence homology, signal transduction mechanisms, and agonist selectivity. Group I mGluRs include mGluR1 and mGluR5, which are linked to the activation of phospholipase C; Groups II and III include all others and are negatively coupled to adenylyl cyclases. The availability in recent years of agents selective for Group I mGluRs has made possible the study of the physiological roles of these receptors in the CNS. In addition to mediating glutamatergic neurotransmission, Group I mGluRs can modulate other neurotransmitter receptors, including GABA and the ionotropic glutamate receptors. Group I mGluRs are involved in many CNS functions and may participate in a variety of disorders such as pain, epilepsy, ischemia, and chronic neurodegenerative diseases. This class of receptor may provide important pharmacological therapeutic targets and elucidating its functions will be relevant to develop new treatments for neurological and psychiatric disorders in which glutamatergic neurotransmission is abnormally regulated. In this review anatomical, physiological and pharmacological results are presented with a special emphasis on the role of Group I mGluRs in functional and pathological processes.

Lysosphingomyelin prevents behavioral aberrations and hippocampal neuron loss induced by the metabotropic glutamate receptor agonist quisqualate

1. Excessive excitation of brain neurons by the excitatory neurotransmitter, glutamate, induces a cascade of events leading to increased intracellular Ca++, neuronal degeneration and death. 2. Recent in vitro research has demonstrated that a natural cationic amphiphile in the brain, lysosphingomyelin, may be able to prevent neuronal degeneration by repressing phosphosinositidase-C overactivation induced by excessive excitation of the metabotropic glutamate receptor. 3. This research tested the latter finding in vivo in a rat model of glutamate excitotoxicity. Intracerebroventricular (i.c.v.) administration of the Group 1 metabotropic glutamate receptor (mGluR) agonist, quisqualate, produced seizures, akinesia, destruction of hippocampal pyramidal cell dendritic microtubule-associated protein-2, and major loss of hippocampal CA sector neurons. 4. Prophylactic i.c.v. infusion of lysosphingomyelin powerfully attenuates these quisqualate-induced behaviors and prevents neuronal degeneration. 5. Lysosphingomyelin may be of clinical use in allaying progressive Group 1 mGluR-induced hippocampal cognitive and motor disorders including Alzheimer's disease, brain seizure, and stroke.

Differential effects of metabotropic glutamate receptor antagonists on bursting activity in the amygdala

Differential effects of metabotropic glutamate receptor antagonists on bursting activity in the amygdala. Metabotropic glutamate receptors (mGluRs) are implicated in both the activation and inhibition of epileptiform bursting activity in seizure models. We examined the role of mGluR agonists and antagonists on bursting in vitro with whole cell recordings from neurons in the basolateral amygdala (BLA) of amygdala-kindled rats. The broad-spectrum mGluR agonist 1S,3R-1-aminocyclopentane dicarboxylate (1S,3R-ACPD, 100 microM) and the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 20 microM) evoked bursting in BLA neurons from amygdala-kindled rats but not in control neurons. Neither the group II agonist (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (L-CCG-I, 10 microM) nor the group III agonist L-2-amino-4-phosphonobutyrate (L-AP4, 100 microM) evoked bursting. The agonist-induced bursting was inhibited by the mGluR1 antagonists (+)-alpha-methyl-4-carboxyphenylglycine [(+)-MCPG, 500 microM] and (S)-4-carboxy-3-hydroxyphenylglycine [(S)-4C3HPG, 300 microM]. Kindling enhanced synaptic strength from the lateral amygdala (LA) to the BLA, resulting in synaptically driven bursts at low stimulus intensity. Bursting was abolished by (S)-4C3HPG. Further increasing stimulus intensity in the presence of (S)-4C3HPG (300 microM) evoked action potential firing similar to control neurons but did not induce epileptiform bursting. In kindled rats, the same threshold stimulation that evoked epileptiform bursting in the absence of drugs elicited excitatory postsynaptic potentials in (S)-4C3HPG. In contrast (+)-MCPG had no effect on afferent-evoked bursting in kindled neurons. Because (+)-MCPG is a mGluR2 antagonist, whereas (S)-4C3HPG is a mGluR2 agonist, the different effects of these compounds suggest that mGluR2 activation decreases excitability. Together these data suggest that group I mGluRs may facilitate and group II mGluRs may attenuate epileptiform bursting observed in kindled rats. The mixed agonist-antagonist (S)-4C3HPG restored synaptic transmission to control levels at the LA-BLA synapse in kindled animals. The different actions of (S)-4C3HPG and (+)-MCPG on LA-evoked bursting suggests that the mGluR1 antagonist-mGluR2 agonist properties may be the distinctive pharmacology necessary for future anticonvulsant compounds.

Anticonvulsant actions of LY 367385 ((+)-2-methyl-4-carboxyphenylglycine) and AIDA ((RS)-1-aminoindan-1,5-dicarboxylic acid)

We have studied the effects in three rodent models of generalised convulsive or absence epilepsy of two antagonists of group I metabotropic glutamate receptors that are selective for the mGlu1 receptor. LY 367385 ((+)-2-methyl-4-carboxyphenylglycine) and AIDA ((RS)-1-aminoindan-1,5-dicarboxylic acid) have been administered intracerebroventricularly (i.c.v.) to DBA/2 mice and lethargic mice (lh/lh), and focally into the inferior colliculus of genetically epilepsy prone rats (GEPR). In DBA/2 mice both compounds produce a rapid, transient suppression of sound-induced clonic seizures (LY 367385: ED50 = 12 nmol, i.c.v., 5 min; AIDA: ED50 = 79 nmol, i.c.v., 15 min). In lethargic mice both compounds significantly reduce the incidence of spontaneous spike and wave discharges on the electroencephalogram, from <30 to >150 min after the administration of AIDA, 500 nmol, i.c.v., and from 30 to >150 min after the administration of LY 367385, 250 nmol, i.c.v. LY 367385, 50 nmol, suppresses spontaneous spike and wave discharges from 30 to 60 min. In genetically epilepsy prone rats both compounds reduce sound-induced clonic seizures. LY 367385, 160 nmol bilaterally, fully suppresses clonic seizures after 2-4 h. AIDA is fully effective 30 min after 100 nmol bilaterally. It is concluded that antagonists of mGlu1 receptors are potential anticonvulsant agents and that activation of mGlu1 receptors probably contributes to a variety of epileptic syndromes.

Glutamate receptors in epilepsy

Glutamatergic synapses play a critical role in all epileptic phenomena. Broadly enhanced activation of post-synaptic glutamate receptors (ionotropic and metabotropic) is proconvulsant. Antagonists of NMDA receptors and AMPA receptors are powerful anticonvulsants in many animal models of epilepsy. A clinical application of pure specific glutamate antagonists has not yet been established. Many different alterations in glutamate receptors or transporters can potentially contribute to epileptogenesis. Several genetic alterations have been shown to be epileptogenic in animal models but no specific mutation relating to glutamatergic function has yet been linked to a human epilepsy syndrome. There is clear evidence for altered NMDA receptor function in acquired epilepsy in animal models and in man. Changes in metabotropic receptor function may also play a key role in epileptogenesis.

The activation of metabotropic glutamate receptors protects nerve cells from oxidative stress

Metabotropic glutamate receptors (mGluRs) have been implicated in a variety of cellular responses to glutamic acid. The work described in this manuscript extends the role of mGluRs to include protection from oxidative stress-induced programmed cell death. Glutamate analogs regulate inositol-1,4,5 triphosphate mass accumulation in accordance with their ability to protect cells from oxidative glutamate toxicity, and protection appears to take place at the level of glutathione metabolism. Short-term exposure of cells to low concentrations of glutamate desensitizes cells to a subsequent challenge from glutamate. Glutamate exposure upregulates the expression of mGluR5 in hippocampal HT-22 cells and mGluR1 in cortical primary cultures. Finally, group I mGluR agonists also protect cells from death programs initiated by glucose starvation and cystine deprivation.

Identification and preclinical testing of novel antiepileptic compounds

Procedures for identifying novel antiepileptic drugs (AEDs) are changing and need to change more. Widespread reliance on two primary screens has led to the identification of novel compounds that resemble either phenytoin (suppressing high-frequency repetitive firing in cultured neurons and prolonging inactivation of voltage-dependent sodium channels identified by the maximal electroshock test) or benzodiazepines (potentiating the inhibitory effect of gamma-aminobutyric acid (GABA), identified by the threshold pentylenetetrazol test). Advances in molecular neurobiology have identified specific molecular targets (subunits of ion channels, neurotransmitter receptors, and transporters) and have made them available in a form permitting high-throughput screening. AEDs can be designed to interact with specific sites on the target molecules. Alternatively, the molecular screens can be used to identify active components in natural products, including folk remedies. Preclinical in vivo screens can be improved by using animals with genetic or acquired epilepsies that have similar modifications in the properties of the target molecules as do human epilepsy syndromes. Future work is likely to define molecular targets for AEDs that will block or reverse chronic epileptogenesis.

Enhanced type 1alpha metabotropic glutamate receptor-stimulated phosphoinositide signaling after pertussis toxin treatment

The regulation of phosphoinositide hydrolysis by the type 1alpha metabotropic glutamate receptor (mGluR1alpha) was investigated in stably transfected baby hamster kidney (BHK) cells. Incubation of the cells with L-glutamate, quisqualate, and 1-aminocyclopentane-1S, 3R-dicarboxylic acid resulted in a marked accumulation of [3H]inositol monophosphate (InsP1) and inositol-1,4,5-trisphosphate [Ins(1,4,5)P3] mass in a time- and concentration-dependent manner. Pretreatment of BHK-mGluR1alpha cells with pertussis toxin [ 100 ng/ml, 24 hr] led to a dramatic 12-16-fold increase in the accumulation of [3H]InsP1 and a 2-fold increase in Ins(1,4,5)P3 in the absence of added agonist. Although only very low levels (</=1 microM) of L-glutamate could be detected in medium taken from control and PTX-treated cell monolayers, the PTX-elicited effect on basal [3H]InsP1 was fully reversed by preincubation of cells in the presence of glutamic-pyruvic transaminase and pyruvate, suggesting that an increased sensitivity to endogenous glutamate was responsible for the apparent agonist-independent activation of phosphoinositidase C (PIC) after PTX treatment. Consistent with this hypothesis, in the presence of glutamic-pyruvic transaminase/pyruvate, the maximal [3H]InsP1 response to quisqualate was increased by >/=75%, and the EC50 shifted leftward by 65-fold [-log EC50 values (molar), 7.26 +/- 0.23 versus 5.45 +/- 0.07; n = 4) in PTX-treated compared with control cells. In contrast, antagonist effects on agonist-stimulated [3H]InsP1 responses were similar in control and PTX-treated BHK-mGluR1alpha cells. These changes in the concentration-effect curves for mGluR agonists are consistent with a model in which the receptor associates with PTX-sensitive inhibitory (Gi/o) and PTX-insensitive stimulatory (Gq/11) G proteins that can each influence PIC activity. The present observations are consistent with a dual regulation of mGluR1alpha-mediated PIC activity that could be fundamental in controlling the output of phosphoinositide-derived messengers.

The L-AP4 receptor

1. The L-2-amino-4-phosphonobutyric acid (L-AP4) receptor was originally discovered by the ability of L-AP4 to depress synaptic transmission in hippocampal glutamatergic pathways and in the retina. 2. The molecular identity of the L-AP4 receptor is not yet resolved; however, with the molecular cloning of subtypes of metabotropic glutamate receptors (mGluRs), high affinity targets for L-AP4 have been identified. 3. As the information on the pharmacology of the mGluRs and the electrophysiological and biochemical studies on L-AP4 receptor physiology becomes elaborated it seems evident that the L-AP4 receptor is not a single molecular target but may involve multiple receptor subtypes.

Prolonged anticonvulsant action of glutamate metabotropic receptor agonists in inferior colliculus of genetically epilepsy-prone rats

The anticonvulsant activity of (S)-4-carboxy-3-hydroxyphenylglycine ((S)-4C3HPG) (an antagonist of Group I and an agonist of Group II metabotropic glutamate (mGlu) receptors), of (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3S)-ACPD) (an agonist of Group II mGlu receptors), and of L-serine-O-phosphate (an agonist of Group III mGlu receptors) was studied against sound-induced seizures in genetically epilepsy-prone (GEP) rats following bilateral microinjection into the inferior colliculus. All 3 drugs produce dose-dependent suppression of all phases of sound-induced seizures (wild running, clonic and tonic). (S)-4C3HPG produces an immediate and short-lasting (< 2 h) protection against sound-induced seizures with an ED50 value of 4.3 (3.2-5.7) nmol, at 5 min. The preferential agonists of Group II and Group III mGlu receptors produce an immediate, transient (< 10 min) proconvulsant effect followed by a prolonged (> 1 day) anticonvulsant effect against sound-induced seizures. The anticonvulsant ED50 value for (1S,3S)-ACPD is 9 (5-18) nmol at 2 h, and for L-serine-O-phosphate is 36 (6.5-199) nmol at 2 days. It is concluded that mGlu receptor activation potently modifies seizure threshold.

Epileptogenesis in vivo enhances the sensitivity of inhibitory presynaptic metabotropic glutamate receptors in basolateral amygdala neurons in vitro

Modulation of excitatory synaptic transmission by presynaptic metabotropic glutamate receptors (mGluRs) was examined in brain slices from control rats and rats with amygdala-kindled seizures. Using whole-cell voltage-clamp and current-clamp recordings, this study shows for the first time that in control and kindled basolateral amygdala neurons, two pharmacologically distinct presynaptic mGluRs mediate depression of synaptic transmission. Moreover, in kindled neurons, agonists at either group II- or group III-like mGluRs exhibit a 28- to 30-fold increase in potency and suppress synaptically evoked bursting. The group II mGluR agonist (2S,3S,4S)-2-(carboxycyclopropyl)glycine (L-CCG) dose-dependently depressed monosynaptic EPSCs evoked by stimulation in the lateral amygdala with EC50 values of 36 nM (control) and 1.2 nM (kindled neurons). The group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) was less potent, with EC50 values of 297 nM (control) and 10.8 nM (kindled neurons). The effects of L-CCG and L-AP4 were fully reversible. Neither L-CCG (0.0001-10 microM) nor L-AP4 (0.001-50 microM) caused membrane currents or changes in the current-voltage relationship. The novel mGluR antagonists (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)-glycine (MCCG; 100 microM) and (S)-2-methyl-2-amino-4-phosphonobutyrate (MAP4; 100 microM) selectively reversed the inhibition by L-CCG and L-AP4 to 81.3 +/- 12% and 65.3 +/- 6.6% of predrug, respectively. MCCG and MAP4 (100-300 microM) themselves did not significantly affect synaptic transmission. The exquisite sensitivity of agonists in the kindling model of epilepsy and the lack of evidence for endogenous receptor activation suggest that presynaptic group II- and group III-like mGluRs might be useful targets for suppression of excessive synaptic activation in neurological disorders such as epilepsy.

Anticonvulsive and neuroprotective actions of a potent agonist (DCG-IV) for group II metabotropic glutamate receptors against intraventricular kainate in the rat

Anticonvulsive and neuroprotective effects of (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl) glycine (DCG-IV), a potent agonist for Group II metabotropic glutamate receptors, were examined in vivo against the excitotoxicity of kainic acid in the rat. Intraventricular injection of kainic acid (2 nmol) induced circling behavior and wet-dog shakes soon after injection, followed by episodes of limbic motor seizures at intervals of several minutes (sporadic limbic motor seizures). The frequency of sporadic limbic motor seizures gradually increased until seizures occurred incessantly (continuous limbic motor seizures). Intraventricular kainic acid also caused severe selective neuron damage in the hippocampal CA3 region, limbic lobe and medial geniculate body. Prolonged intraventricular infusion of DCG-IV (24-240 pmol/h) for 17 h before and 7 h after the application of kainic acid decreased the incidence of the continuous limbic motor seizures and the degree of neuronal damage in circumscribed brain areas. However, the behavioral changes observed immediately after the administration of kainic acid were unaffected by prolonged intraventricular infusion with DCG-IV (8-2400 pmol/h). Similarly, the occurrence of sporadic limbic motor seizures was only slightly reduced by the administration of DCG-IV (8-800 pmol/h). High doses of DCG-IV, greater than 800 pmol/h, afforded no protection against kainate-induced lesions; rather, the degradation of hippocampal CA1 pyramidal neurons was increased under such conditions. Single injections of DCG-IV (10-300 pmol/rat) in the lateral ventricle did not affect kainate neurotoxicity. Thus, prolonged infusion of DCG-IV showed a bell-shaped doso-response relationship with regard to protection against kainate-induced neurotoxicity.

Pharmacology and functions of metabotropic glutamate receptors

In the mid to late 1980s, studies were published that provided the first evidence for the existence of glutamate receptors that are not ligand-gated cation channels but are coupled to effector systems through GTP-binding proteins. Since those initial reports, tremendous progress has been made in characterizing these metabotropic glutamate receptors (mGluRs), including cloning and characterization of cDNA that encodes a family of eight mGluR subtypes, several of which have multiple splice variants. Also, tremendous progress has been made in developing new highly selective mGluR agonists and antagonists and toward determining the physiologic roles of the mGluRs in mammalian brain. These findings have exciting implications for drug development and suggest that the mGluRs provide a novel target for development of therepeutic agents that could have a significant impact on neuropharmacology.

Pharmacological characterization of metabotropic glutamate receptors potentiating NMDA responses in mouse cortical wedge preparations

1. Mouse cortical wedge preparations were used in order to study the effects of metabotropic glutamate receptor (mGluR) agonists and antagonists on the depolarization induced by N-methyl-D-aspartate (NMDA) or by (S)-alpha-amino-4-bromo-3-hydroxy-5-isoxazolepropionic acid (AMPA). 2. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (30-300 microM) significantly potentiated the depolarizations induced by NMDA, leaving unchanged those mediated by AMPA. This potentiation developed slowly and lasted for up to 60 min provided that the slices were continuously perfused with the mGluR agonist. 3. Concentration-response curves to NMDA in the absence and in the presence of 1S,3R-ACPD (100 microM) indicated that the potentiation was due to increased affinity of the NMDA receptor complex for its agonist. The maximal responses to NMDA were not potentiated. 4. Selective agonists of group 1 mGluR such as quisqualate (Quis) (30 microM) or (RS)-3,5-dihydroxyphenylglycine (DHPG) (300 microM) did not potentiate NMDA responses. Similarly, selective agonists of group 2 mGluRs, such as (2S,3S,4S)-alpha-carboxycyclopropyl-glycine (L-CCG-I) (3-30 microM), and of group 3, such as L-2-amino-4-phosphonobutyric acid (L-AP4) (100 microM) were inactive in our test. A number of other putative mGluR agents having partial agonist activity on mGluRs in brain slices and in expression systems, such as 1R,3S-ACPD (500 microM), DL-2-amino-3-phosphonopropionic acid (DL-AP3) (300 microM) and (S)-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG; 500 microM), when placed in the experimental protocol we used, did not change NMDA responses. 5. Available mGluR antagonists, such as DL-AP3 (1 mM), (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) (500 microM), S-4-carboxyphenylglycine (4CPG; 500 microM) and S-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG; 500 microM), did not reduce 1S,3R-ACPD potentiation of NMDA responses. 6. It is concluded that the potentiation of NMDA currents induced by the mGluR agonist 1S,3R-ACPD, in mouse cortical wedges, has a pharmacological profile which is different from that of the three mGluR groups so far described in expression systems.

Induction or protection of limbic seizures in mice by mGluR subtype selective agonists

The behavioral consequences of metabotropic glutamate receptor (mGluR) activation were investigated following intracerebral administration of the mGluR selective agonists (RS)3,5-dihydroxyphenyl-glycine (3,5-DHPG), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD), (1R,3S)-1-aminocyclopentane-1,3-dicarboxylate (1R,3S-ACPD), L-2-amino-4-phosphonobutyrate (L-AP4), L-serine-O-phosphate (L-SOP) and (2S,3S,4S)alpha-(carboxycyclopropyl)glycine (L-CCGI) into the thalamus in mice. Injections of 3,5-DHPG, 1S,3R-ACPD and L-CCGI produced dose-dependent increases in limbic seizures with a potency order of 3,5-DHPG = 1S,3R-ACPD > L-CCGI. This effect of 1S,3R-ACPD was stereoselective, since the inactive isomer (1R,3S-ACPD) did not elicit seizure activity. Limbic seizures induced by the phosphoinositide-coupled mGluR subtype selective agonist 3,5-DHPG were attenuated by the mGluR antagonist L-2-amino-3-phosphonopropanoic acid (L-AP3) and dantrolene, inhibitors of mGluR-mediated intracellular calcium mobilization. Interestingly, L-AP4, L-SOP and low doses of L-CCGI also protected against 3,5-DHPG seizures. These data indicate that mGluR agonist-induced limbic seizures in mice are mediated by activation of phosphoinositide-coupled mGluRs. Furthermore, these seizures can be protected against by activation of mGluRs that are negatively-linked to cAMP formation.

The actions of a range of excitatory amino acids at (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-depolarizing receptors on neonatal rat motoneurones

Depolarizations induced by a range of amino acids including some sulphur-containing excitatory transmitter candidates were evoked from motoneurones in the neonatal rat spinal cord under conditions that precluded activation of known ionotropic glutamate receptors. The responses could be partially and differentially depressed by continuous application of several metabotropic glutamate receptor (mGluR) antagonists or by receptor desensitization with the mGluR agonist, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD]. In most cases [the exceptions being (1S,3R)-ACPD and to a lesser extent, quisqualate], the major component of these depolarizations was resistant to antagonism by phenylglycine-derived mGluR antagonists or desensitization of (1S,3R)-ACPD-sensitive receptors. Of the excitatory responses observed with the tested agonists, those evoked by L-glutamate itself were generally the least affected by blockade of known glutamate receptors.