Expression of metabotropic glutamate receptor 1alpha in the hippocampus of rat pilocarpine model of status epilepticus

Proposed mechanisms of tau: relationships to traumatic brain injury, Alzheimer's disease, and epilepsy

Traumatic brain injury (TBI), Alzheimer's disease (AD), and epilepsy share proposed mechanisms of injury, including neuronal excitotoxicity, cascade signaling, and activation of protein biomarkers such as tau. Although tau is typically present intracellularly, in tauopathies, phosphorylated (p-) and hyper-phosphorylated (hp-) tau are released extracellularly, the latter leading to decreased neuronal stability and neurofibrillary tangles (NFTs). Tau cleavage at particular sites increases susceptibility to hyper-phosphorylation, NFT formation, and eventual cell death. The relationship between tau and inflammation, however, is unknown. In this review, we present evidence for an imbalanced endoplasmic reticulum (ER) stress response and inflammatory signaling pathways resulting in atypical p-tau, hp-tau and NFT formation. Further, we propose tau as a biomarker for neuronal injury severity in TBI, AD, and epilepsy. We present a hypothesis of tau phosphorylation as an initial acute neuroprotective response to seizures/TBI. However, if the underlying seizure pathology or TBI recurrence is not effectively treated, and the pathway becomes chronically activated, we propose a "tipping point" hypothesis that identifies a transition of tau phosphorylation from neuroprotective to injurious. We outline the role of amyloid beta (Aβ) as a "last ditch effort" to revert the cell to programmed death signaling, that, when fails, transitions the mechanism from injurious to neurodegenerative. Lastly, we discuss targets along these pathways for therapeutic intervention in AD, TBI, and epilepsy.

Transient Switching of NMDA-Dependent Long-Term Synaptic Potentiation in CA3-CA1 Hippocampal Synapses to mGluR1-Dependent Potentiation After Pentylenetetrazole-Induced Acute Seizures in Young Rats

The mechanisms of impairment in long-term potentiation after status epilepticus (SE) remain unclear. We investigated the properties of LTP induced by theta-burst stimulation in hippocampal slices of rats 3 h and 1, 3, and 7 days after SE. Seizures were induced in 3-week old rats by a single injection of pentylenetetrazole (PTZ). Only animals with generalized seizures lasting more than 30 min were included in the experiments. The results revealed that LTP was strongly attenuated in the CA1 hippocampal area after PTZ-induced SE as compared with that in control animals. Saturation of synaptic responses following epileptic activity does not explain weakening of LTP because neither the quantal size of the excitatory responses nor the slopes of the input-output curves for field excitatory postsynaptic potentials changed in the post-SE rats. After PTZ-induced SE, NMDA-dependent LTP was suppressed, and LTP transiently switched to the mGluR₁-dependent form. This finding does not appear to have been reported previously in the literature. An antagonist of NMDA receptors, D-2-amino-5-phosphonovalerate, did not block LTP induction in 3-h and 1-day post-SE slices. An antagonist of mGluR₁, FTIDS, completely prevented LTP in 1-day post-SE slices; whereas it did not affect LTP induction in control and post-SE slices at the other studied times. mGluR₁-dependent LTP was postsynaptically expressed and did not require NMDA receptor activation. Recovery of NMDA-dependent LTP occurred 7 day after SE. Transient switching between NMDA-dependent LTP and mGluR1-dependent LTP could play a role in the pathogenesis of acquired epilepsy.

In Vivo Monitoring for Regional Changes of Metabotropic Glutamate Receptor Subtype 1 (mGluR1) in Pilocarpine-Induced Epileptic Rat Brain by Small-Animal PET

Metabotropic glutamate receptor subtype 1 (mGluR1) is a crucial pharmacological target for several central nervous system disorders. In this study, we aimed to monitor in vivo regional changes of mGluR1 related to neuroinflammation in the brains of rats after pilocarpine-induced status epilepticus (PISE) using longitudinal positron emission tomography (PET). PISE was induced in rats by administering lithium chloride, followed by repeated pilocarpine hydrochloride treatments. PET assessments were conducted using N-[4-[6-(isopropylamino)-pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methyl-4-[¹¹C]methylbenzamide ([¹¹C]ITDM), a selective radioligand for mGluR1, and N-benzyl-N-[¹¹C]methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide ([¹¹C]DAC), a selective translocator protein PET ligand for neuroinflammation monitoring. PET scans were conducted on PISE rats at 1 day (acute), 1 week (subacute) and 3 weeks (chronic) after repeated seizures. PET with [¹¹C]ITDM showed significant decreases of mGluR1 availability (BP_ND) in the thalamus and hippocampus after PISE over the chronic period. Conversely, PET with [¹¹C]DAC exhibited a significant increase of radioactive uptake in the forebrain after the acute period, especially in the thalamus. These conflicting changes in the thalamus indicated negative correlation. In conclusion, PET with [¹¹C]ITDM could successfully visualize hippocampal and thalamic declines of mGluR1 related to neuroinflammation, which would help further understanding for mGluR1 functions in neuroexcitotoxicity.

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.

Astrocytic Cx 43 and Cx 40 in the mouse hippocampus during and after pilocarpine-induced status epilepticus

Astrocytes have now been well accepted to play important roles in epileptogenesis by controlling gliotransmitter release and neuronal excitability, contributing to blood-brain barrier dysfunction and involving in brain inflammation. Recent studies indicate that abnormal expression of gap junction protein connexin (Cx) may also be a contributing factor for seizure generation. To further address this issue, we investigated the progressive changes of Cx 43 and Cx 40 in the mouse hippocampus at 4 h, 1 day, 1 week and 2 months during and after pilocarpine-induced status epilepticus (PISE). The co-localization of Cx 43 and Cx 40 with glial fibrillary acidic protein (GFAP) was also examined. We observed that Cx 43 and Cx 40 protein expression remained unaltered at 4 h during and at 1 day (acute stage) after PISE. However, their expression was significantly increased in CA1 and CA3 areas and in the dentate gyrus at 1 week (latent stage) and 2 months (chronic stage) after PISE. Double immunofluorescence labeling indicated the localization of Cx 43 and Cx 40 in astrocytes. Combined with progressive neuronal loss in the mouse hippocampus, our results suggest that the increase in gap junctions in the neuronoglial syncytium of reactive astrocytes may be implicated in synchronization of hippocampal hyperactivity leading to neuronal loss and epileptogenesis.

Morpho-physiologic characteristics of dorsal subicular network in mice after pilocarpine-induced status epilepticus

The goal of this study was to examine the morpho-physiologic changes in the dorsal subiculum network in the mouse model of temporal lobe epilepsy using extracellular recording, juxtacellular and immunofluorescence double labeling, and anterograde tracing methods. A significant loss of total dorsal subicular neurons, particularly calbindin, parvalbumin (PV) and immunopositive interneurons, was found at 2 months after pilocarpine-induced status epilepticus (SE). However, the sprouting of axons from lateral entorhinal cortex (LEnt) was observed to contact with surviving subicular neurons. These neurons had two predominant discharge patterns: bursting and fast irregular discharges. The bursting neurons were mainly pyramidal cells, and their dendritic spine density and bursting discharge rates were increased significantly in SE mice compared with the control group. Fast irregular discharge neurons were PV-immunopositive interneurons and had less dendritic spines in SE mice when compared with the control mice. When LEnt was stimulated, bursting and fast irregular discharge neurons had much shorter latency and stronger excitatory response in SE mice compared with the control group. Our results illustrate that morpho-physiologic changes in the dorsal subiculum could be part of a multilevel pathologic network that occurs simultaneously in many brain areas to contribute to the generation of epileptiform activity.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Two-methyl-6-phenylethynyl-pyridine (MPEP), a metabotropic glutamate receptor 5 antagonist, with low doses of MK801 and diazepam: A novel approach for controlling status epilepticus

By intravenous administration of group I metabotropic glutamate receptor antagonists at 1 or 2h during pilocarpine induced status epilepticus (PISE), we showed that mGluR1 antagonists AIDA or LY367385 (at dosages ranging from 25 to 200mg/kg), mGluR5 antagonists SIB1757 (at dosages ranging from 25 to 200mg/kg), SIB1893 (from 25 to 100mg/kg), MPEP (from 25 to 100mg/kg) injected at 1 or 2h during PISE were ineffective in controlling status epilepticus (SE). However, when administered at 1h during PISE, MPEP at 200mg/kg, combination of MPEP (200mg/kg) with MK801 (0.1mg/kg) or with MK801 (0.1mg/kg) and diazepam (0.5mg/kg), combination of SIB1893 (200mg/kg) with MK801 (0.1mg/kg) could effectively control behavioral SE, and were neuroprotective. In particular, the combination of MPEP with MK801 and diazepam could stop both behavioral SE and electrical SE (under EEG monitoring) within a few minutes after the administration. HPLC study showed that a high level of MPEP was maintained in the blood and its metabolism rate was slow in experimental mice with PISE. We therefore concluded that the combination of MPEP (200mg/kg) with MK801 (0.1mg/kg) and diazepam (0.5mg/kg) could effectively stop SE and its subsequent neuronal loss in the hippocampus when administered 1h during PISE. It may provide a new approach to effectively control intractable SE.

Loss of metabotropic glutamate receptor-dependent long-term depression via downregulation of mGluR5 after status epilepticus

Synaptic plasticity is thought to be a key mechanism of information storage in the CNS. Different forms of synaptic long-term potentiation have been shown to be impaired in neurological disorders. Here, we show that metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), but not NMDA receptor-dependent LTD at Schaffer collateral-CA1 synapses, is profoundly impaired after status epilepticus. Brief application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (100 microM; 5 min) induced mGluR LTD in control, but not in pilocarpine-treated rats. Experiments in the presence of selective inhibitors of either mGluR5 [2-methyl-6-(phenylethynyl)-pyridine] or mGluR1 [7-(hydroxyimino)cyclopropachromen-carboxylate ethyl ester and (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid] demonstrate that loss of mGluR LTD is most likely attributable to a loss of mGluR5 function. Quantitative real-time reverse transcription PCR revealed a specific downregulation of mGluR5 mRNA, but not of mGluR1 mRNA in the CA1 region. Furthermore, we detected a strong reduction in mGluR5 protein expression by immunofluorescence and quantitative immunoblotting. Additionally, the scaffolding protein Homer that mediates coupling of mGluR5 to downstream signaling cascades was downregulated. Thus, we conclude that the reduction of mGluR LTD after pilocarpine-induced status epilepticus is the result of the subtype-specific downregulation of mGluR5 and associated downstream signaling components.

Cav1.2, Cav1.3, and Cav2.1 in the mouse hippocampus during and after pilocarpine-induced status epilepticus

Calcium binding proteins are well known to be expressed by different groups of hippocampal interneurons; however, whether voltage-dependent calcium channels (Ca(v)) are also localized in these neurons, changed during and after status epilepticus (SE), and involved in epileptic activity have not been reported. In the present study, we showed the colocalization of three subtypes of voltage-gated calcium channels (Ca(v)1.2, Ca(v)1.3, or Ca(v)2.1) with different calcium binding proteins such as calbindin (CB), calretinin (CR), and parvalbumin (PV). At early stages during and after pilocarpine-induced status epilepticus (PISE), significant changes of expression of Ca(v)1.2, Ca(v)1.3 (L-type), and Ca(v)2.1 (P/Q-type) were found in different groups of hippocampal neurons. Induced expression of Ca(v)1.3 or Ca(v)2.1 in reactive astrocytes was shown at 1 week and 2 months after PISE. At the latter time point, higher percentages of colocalization of PV and Ca(v)1.2, CB, or PV and Ca(v)1.3 or Ca(v)2.1, lower percentages of CR and Ca(v)1.3 or Ca(v)2.1 immunoposivie neurons were observed in gliotic CA1 area. We therefore conclude that voltage-gated calcium channels are expressed by different groups of hippocampal interneurons in the mouse. At acute stages during and after PISE, up- or down-regulation of Ca(v)1.2, Ca(v)1.3, or Ca(v)2.1 in functionally different groups of interneurons in CA1 area may be related to the changes of their plasticity. Up-regulation of Ca(v)1.2, Ca(v)1.3, or Ca(v)2.1 in granule cells may be directly related to the occurrence of SE. The induced expression of Ca(v)1.3 or Ca(v)2.1 in reactive astrocytes at 1 week and 2 months after PISE suggests that Ca(v)1.3 or Ca(v)2.1-related calcium signaling in reactive astrocytes may be involved in initiation, maintenance or spread of seizure activity. In gliotic CA1 area at chronic stage (i.e., 2 months after PISE), the occurrence of higher percentages of colocalization of PV and Ca(v)1.2, CB, or PV and Ca(v)1.3 or Ca(v)2.1, lower percentages of CR and Ca(v)1.3 or Ca(v)2.1 immunopositive neurons may suggest that such colocalizations may be linked to the survival or loss of particular group of hippocampal neurons.

Reorganization of CA3 area of the mouse hippocampus after pilocarpine induced temporal lobe epilepsy with special reference to the CA3-septum pathway

We showed that when CA3 pyramidal neurons in the caudal 80% of the dorsal hippocampus had almost disappeared completely, the efferent pathway of CA3 was rarely detectable. We used the mouse pilocarpine model of temporal lobe epilepsy (TLE), and injected iontophoretically the anterograde tracer phaseolus vulgaris leucoagglutinin (PHA-L) into gliotic CA3, medial septum and the nucleus of diagonal band of Broca, median raphe, and lateral supramammillary nuclei, or the retrograde tracer cholera toxin B subunit (CTB) into gliotic CA3 area of hippocampus. In the afferent pathway, the number of neurons projecting to CA3 from medial septum and the nucleus of diagonal band of Broca, median raphe, and lateral supramammillary nuclei increased significantly. In the hippocampus, where CA3 pyramidal neurons were partially lost, calbindin, calretinin, parvalbumin immunopositive back-projection neurons from CA1-CA3 area were observed. Sprouting of Schaffer collaterals with increased number of large boutons in both sides of CA1 area, particularly in the stratum pyramidale, was found. When CA3 pyramidal neurons in caudal 80% of the dorsal hippocampus have almost disappeared completely, surviving CA3 neurons in the rostral 20% of the dorsal hippocampus may play an important role in transmitting hyperactivity of granule cells to surviving CA1 neurons or to dorsal part of the lateral septum. We concluded that reorganization of CA3 area with its downstream or upstream nuclei may be involved in the occurrence of epilepsy.

Calcium binding protein containing neurons in the gliotic mouse hippocampus with special reference to their afferents from the medial septum and the entorhinal cortex

In CA1 area and the hilus of the dentate gyrus of the mouse hippocampus, drastic reduction of NeuN, calbindin, calretinin, or parvalbumin immunopositive neurons was shown at 3, 7 and 60 days after pilocarpine-induced status epilepticus. In gliotic CA1 area at 60 days, few dendritic branches of calcium binding protein immunopositive neurons could be found suggesting reorganization of the afferents of surviving calcium binding protein immunopositive neurons. Calbindin, calretinin, or parvalbumin and 5-bromo-2'-deoxyuridine (BrdU) double labeling showed that calcium binding protein immunopositive neurons in gliotic CA1 area at 60 days were surviving instead of newly generated neurons. Iontophoretic injection of Phaseolus vulgaris leucoagglutinin into the medial septum and the nucleus of the diagonal band of Broca or the lateral entorhinal cortex showed contacts between Phaseolus vulgaris leucoagglutinin immunopositive en passant and terminal boutons and surviving calcium binding protein immunopositive neurons in the hippocampus. The presence in the gliotic hippocampus of enlarged and/or aggregated bouton-like structures 60 days after pilocarpine-induced status epilepticus is indicative for the reorganization of connections between the hippocampal afferents and surviving hippocampal neurons. This reconstruction could be a factor in the ongoing epileptic activity in this model of mesial temporal lobe epilepsy.

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.

Glutamate receptor 1-immunopositive neurons in the gliotic CA1 area of the mouse hippocampus after pilocarpine-induced status epilepticus

Significant reduction in glutamate receptor 1 (GluR1)- and GluR2/3-immunopositive neurons was demonstrated in the hilus of the dentate gyrus in mice killed on days 1, 7 and 60 after pilocarpine-induced status epilepticus (PISE). In addition, GluR1 and GluR2/3 immunostaining in the strata oriens, radiatum and lacunosum moleculare of areas CA1-3 decreased drastically on days 7 and 60 after PISE. Neuronal loss observed in the above regions may account, at least in part, for a decrease in GluR immunoreactivity. By contrast, many GluR1-immunopositive neurons were observed in the gliotic area of CA1. Of these, about 42.8% were immunopositive for markers for hippocampal interneurons, namely calretinin (7.6%), calbindin (12.8%) and parvalbumin (22.4%). GluR1 or GluR2/3 and BrdU double-labelling showed that the GluR1- and GluR2/3-immunopositive neurons at 60 days after PISE were neurons that had survived rather than newly generated neurons. Furthermore, anterograde tracer and double-labelling studies performed on animals at 60 days after PISE indicated a projection from the hilus of the dentate gyrus to gliotic areas in both CA3 and CA1, where the projecting fibres apparently established connections with GluR1-immunopositive neurons. The projection to CA1 was unexpected. These novel findings suggest that the intrinsic hippocampal neuronal network is altered after PISE. We speculate that GluR1-immunopositive neurons in gliotic CA1 act as a bridge between dentate gyrus and subiculum contributing towards epileptogenesis.

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.

Expression of different isoforms of protein kinase C in the rat hippocampus after pilocarpine-induced status epilepticus with special reference to CA1 area and the dentate gyrus

At 4 h during pilocarpine-induced status epilepticus (DPISE) in rat, protein kinase C (PKC)beta1, PKCbeta2, and PKCgamma were induced at the border between the stratum oriens and alveus (O/A border) of CA1 in the hippocampus. Induced PKCgamma was colocalized with metabotropic glutamate receptor alpha (mGluR alpha). By intracerebroventricular injection of mGluR1alpha antagonists, (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), PKCbeta1, PKCbeta2, and PKCgamma immunoreactive products decreased dramatically; however, intracerebroventricular injection of saline did not change the expression of PKCbeta1, PKCbeta2, and PKCgamma, suggesting that these three PKC isoforms might be involved in mGluR1alpha-related excitoneurotoxicity. One day after pilocarpine-induced status epilepticus (APISE), PKCdelta was induced in microglial cells. At this time point, both PKCgamma and PKCepsilon immunopositive products decreased in the inner molecular layer of upper blade of the stratum granulosum. At 7-31 days APISE, induced PKCbeta1, PKCdelta, PKCeta, and PKCzeta positive astrocytes were demonstrated in all parts of hippocampus, suggesting that they may be involved in gliosis. By this time, both PKCgamma and PKCepsilon immunopositive products in the inner molecular layer had almost disappeared, suggesting that they may be involved in the inhibition of granule cells by controlling neurotransmitter release presynaptically in the dentate gyrus of normal rats.

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.