Anti-epileptogenic and anticonvulsant activity of L-2-amino-4-phosphonobutyrate, a presynaptic glutamate receptor agonist

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.

Bypassing the Blood-Brain Barrier: Direct Intracranial Drug Delivery in Epilepsies

Epilepsies are common chronic neurological diseases characterized by recurrent unprovoked seizures of central origin. The mainstay of treatment involves symptomatic suppression of seizures with systemically applied antiseizure drugs (ASDs). Systemic pharmacotherapies for epilepsies are facing two main challenges. First, adverse effects from (often life-long) systemic drug treatment are common, and second, about one-third of patients with epilepsy have seizures refractory to systemic pharmacotherapy. Especially the drug resistance in epilepsies remains an unmet clinical need despite the recent introduction of new ASDs. Apart from other hypotheses, epilepsy-induced alterations of the blood-brain barrier (BBB) are thought to prevent ASDs from entering the brain parenchyma in necessary amounts, thereby being involved in causing drug-resistant epilepsy. Although an invasive procedure, bypassing the BBB by targeted intracranial drug delivery is an attractive approach to circumvent BBB-associated drug resistance mechanisms and to lower the risk of systemic and neurologic adverse effects. Additionally, it offers the possibility of reaching higher local drug concentrations in appropriate target regions while minimizing them in other brain or peripheral areas, as well as using otherwise toxic drugs not suitable for systemic administration. In our review, we give an overview of experimental and clinical studies conducted on direct intracranial drug delivery in epilepsies. We also discuss challenges associated with intracranial pharmacotherapy for epilepsies.

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 and neuroprotective effect of (S)-3,4-dicarboxyphenylglycine against seizures induced in immature rats by homocysteic acid

The present study has examined the anticonvulsant and neuroprotective effect of (S)-3,4-dicarboxyphenylglycine ((S)-3,4-DCPG), a highly selective agonist for subtype 8 of group III metabotropic glutamate receptors (mGluRs), against seizures induced in immature 12-day-old rats by bilateral icv infusion of DL-homocysteic acid (DL-HCA, 600 nmol/side). For biochemical analyses, rat pups were sacrificed during generalized clonic-tonic seizures, approximately 45-50 min after infusion. Comparable time intervals were used for sacrificing the animals which had received (S)-3,4-DCPG (0.25 nmol/each side, 15-20 min prior to infusion of DL-HCA or saline). This agonist provided a pronounced anticonvulsant effect, generalized clonic-tonic seizures were completely suppressed and cortical energy metabolite changes which normally accompany these seizures were either normalized (decrease of glucose and glycogen) or markedly reduced (an accumulation of lactate). Anticonvulsant effect of (S)-3,4-DCPG was also evident from the EEG recordings, nevertheless, it was not complete. In spite of the absence of obvious motor phenomena, sporadic ictal activity could be seen in some animals. Isolated spikes could also be observed in some animals after administration of (S)-3,4-DCPG alone. The neuroprotective effect of (S)-3,4-DCPG was evaluated after 24 h and 6 days of survival following DL-HCA-induced seizures. Massive neuronal degeneration was observed in a number of brain regions following infusion of DL-HCA alone (seizure group), whereas pretreatment with (S)-3,4-DCPG provided substantial neuroprotection. The present findings suggest that receptor subtype 8 of group III mGluRs may be considered a promising target for drug therapy in childhood epilepsies in the future.

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.

Group III metabotropic glutamate receptor-mediated, chemically induced long-term depression differentially affects cell viability in the hippocampus

In vivo, activation of group III metabotropic glutamate (mGlu) receptors leads to a reduction of basal synaptic transmission in the hippocampus, and depending on the experimental conditions in vitro, leads to neuroprotection or neurotoxicity. Here, the cellular response to cerebral application of L(+)-2-amino-4-phosphonobutanoic acid (AP4) was investigated in the CA1 region and dentate gyrus of freely moving rats. Drugs were applied via the lateral ventricle, and electrophysiological measurements were obtained via chronically implanted electrodes. AP4 produced a slowly developing depression of evoked responses in both hippocampal regions which lasted for over 4 h. Effects could be reversed by application of high frequency tetanus. Histological evaluation, 4 h or 7 d, following a single, acute AP4 injection into the lateral cerebral ventricle showed that AP4-mediated synaptic depression either amplified (CA1) or attenuated (dentate gyrus) excitotoxic neuronal death, strongly depending on the sub-region investigated. Effects were long-lasting, being still evident 7 days after AP4 application. In both hippocampal areas, the effects obtained were subtle, however, with the CA1 region being more potently affected. Interestingly, effects in the dentate gyrus comprised a slight enhancement of live cell number coupled with deterioration in cell area, suggesting that cell proliferation triggered by group III mGlu receptor activation may have masked neurotoxic effects mediated by activation of this receptor. These results show that although AP4 induces a slow-onset synaptic depression in both sub-regions, cell viability is differentially influenced by activation of group III mGlu receptors in the CA1 region and dentate gyrus.

In vivo modulatory action of extracellular glutamate on the anticonvulsant effects of hippocampal dopamine and serotonin

PURPOSE

Our recent work (Clinckers et al., J Neurochem 2004;89:834-43) demonstrated that intrahippocampal perfusion of 2 nM dopamine or serotonin via a microdialysis probe offered complete protection against focal pilocarpine-induced limbic seizures and did not influence basal extracellular hippocampal glutamate levels. Ten nanomolar dopamine or serotonin perfusion, however, worsened seizures and was accompanied by significant extracellular glutamate increases to approximately 200%. The significance of these glutamate elevations in seizure generation remains unclear. The present microdialysis study investigated the modulatory role of extracellular hippocampal glutamate levels in these monoaminergic protective and proconvulsant effects.

METHODS

A first group of male Wistar albino rats was perfused intrahippocampally for 240 min with 6.25 microM glutamate alone to increase extracellular levels by 200%. Other animals were perfused with anticonvulsant concentrations of monoamines throughout the experiments while receiving continuous coperfusions of 6.25 microM glutamate either before, during, and after (240 min) or only after (100 min) pilocarpine perfusion (40 min). Rats were scored for epileptic behavior, and the mean scores were compared with those of the control group. Microdialysates were analyzed for monoamine and glutamate content with microbore liquid chromatography.

RESULTS

No convulsions occurred during glutamate perfusion alone. When monoamines and glutamate were coperfused before pilocarpine administration, the anticonvulsant effect of the monoamines was lost. Glutamate addition after pilocarpine administration did not affect monoaminergic seizure protection.

CONCLUSIONS

These results indicate that extracellular glutamate increases per se do not necessarily induce seizures but that they can modulate the anticonvulsant effects exerted by hippocampal monoamines.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Medial perforant path inhibition mediated by mGluR7 is reduced after status epilepticus

Metabotropic glutamate receptor (mGluR)-mediated inhibition within the dentate gyrus is altered after epilepsy. Whether these changes occur during the developmental period of the disease (i.e., the latent period) has not yet been investigated. Field excitatory postsynaptic potentials (fEPSPs) were recorded in the lateral (LPP) and medial perforant path (MPP) simultaneously in adult mouse hippocampal slices 3-9 days after pilocarpine (PILO)-induced status epilepticus. Genetically manipulated mice (mGluR8 knockout and mGluR4/8 double knockout) and pharmacologically selective agonists were used to identify specific mGluR subtypes affected after PILO. Pharmacological activation of mGluR7 by L-AP4 in both wild-type and mGluR4/8 double knockout mice selectively reduced fEPSPs in the MPP, but not LPP, and this level of inhibition was significantly reduced 3-9 days after PILO-induced SE. Activation of mGluR2/3 reversibly depressed the fEPSP slopes in both the MPP and LPP, but no alterations were noted after PILO. mGluR8 activation selectively inhibited evoked responses in the LPP, but not in the MPP, and this level of inhibition did not change after PILO treatment. These data suggest that reduced presynaptic inhibition mediated by mGluR7, but not mGluR2/3 or mGluR8, may play a role during the latent period in generating hyperexcitability in the dentate and thereby contribute to epileptogenesis.

Inhibition of non-vesicular glutamate release by group III metabotropic glutamate receptors in the nucleus accumbens

Previous in vitro studies have shown that group III metabotropic glutamate receptors (mGluRs) regulate synaptic glutamate release. The present study used microdialysis to characterize this regulation in vivo in rat nucleus accumbens. Reverse dialysis of the group III mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (L-AP4) decreased, whereas the antagonist (R,S)-alpha-methylserine-O-phosphate (MSOP) increased the extracellular level of glutamate. The decrease by L-AP4 or the increase by MSOP was antagonized by co-administration of MSOP or L-AP4, respectively. Activation of mGluR4a by (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid or mGluR6 by 2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid had no effect on extracellular glutamate. (R,S)-4-Phosphonophenylglycine (PPG), another group III agonist with high affinity for mGluR4/6/8, reduced extracellular glutamate only at high concentrations capable of binding to mGluR7. The increase in extracellular glutamate by MSOP was tetrodotoxin-independent, and resistant to both the L-type and N-type Ca2+ channel blockers. L-AP4 failed to block 30 mm K+-induced vesicular glutamate release. Blockade of glutamate uptake by d,l-threo-beta-benzyloxyaspartate caused a Ca2+-independent elevation in extracellular glutamate that was reversed by L-AP4. Finally, (S)-4-carboxyphenylglycine, an inhibitor of cystine-glutamate antiporters, attenuated the L-AP4-induced reduction in extracellular glutamate. Together, these data indicate that group III mGluRs regulate in vivo extracellular glutamate in the nucleus accumbens by inhibiting non-vesicular glutamate release.

Characterisation of the actions of group I metabotropic glutamate receptor subtype selective ligands on excitatory amino acid release and sodium-dependent re-uptake in rat cerebrocortical minislices

In this study we have tested the effects of a wide range of metabotropic glutamate receptor ligands on (i) depolarisation-evoked efflux of pre-accumulated d-[3H]aspartic acid (d-[3H]asp) from rapidly superfused rat cerebrocortical minislices, and (ii) Na+-dependent uptake of d-[3H]asp into cerebrocortical tissue. Transient elevations in extracellular K+ produced concentration-dependent increases in d-[3H]asp efflux. A submaximally effective concentration (50 mm) was used in all subsequent experiments. The broad-spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; EC50 17.8 microm], the group I mGlu-selective agonist (S)-3,5-dihydroxyphenylglycine [(S)-3,5-DHPG; EC50 0.5 microm] and the mGlu5 receptor subtype-selective agonist (RS)-2-chloro-5-hydroxyphenylglycine [(RS)-CHPG; EC50 7.3 microm] all concentration-dependently potentiated high K+-evoked d-[3H]asp efflux in the absence of effects on basal outflow of radiolabel. At concentrations selective for mGlu1 receptors, the antagonists (RS)-1-aminoindan-1,5-dicarboxylic acid [(RS)-AIDA; 10-300 microm]; (+)-2-methyl-4-carboxyphenylglycine [LY367385; 1-100 microm] and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylate ethyl ester [CPCCOEt, 1-30 microm] all failed to inhibit responses to (S)-3,5-DHPG. However, the broad-spectrum mGlu receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine [(S)-MCPG; IC50 88.5 microm] together with the recently described mGlu5-selective antagonists, 2-methyl-6-(phenylethynyl)-pyridine (MPEP; IC50 0.6 microm), 6-methyl-2-(phenyl-azo)-3-pyridinol (SIB-1757; IC50 4.4 microm) and (E)-2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893; IC50 3.1 microm), at mGlu5-selective concentrations, all powerfully and concentration-dependently inhibited (S)-3,5-DHPG-evoked responses. Two selective excitatory amino acid (EAA) uptake inhibitors, l-trans-2,4-pyrrolidine dicarboxylate (l-trans-2,4-PDC; IC50 229 microm) and dl-threo-beta-benzyloxyaspartate (dl-TBOA; IC50 665 microm) both inhibited the Na+-dependent uptake of d-[3H]asp into cerebrocortical minislices. Importantly, none of the mGlu ligands utilized in the present study significantly inhibited d-[3H]asp uptake at concentrations shown to potentiate K+-evoked efflux. These data demonstrate for the first time that mGlu5 ligands modulate extracellular EAA concentrations by a direct effect on mGlu5-type autoreceptors on EAA nerve terminals as they evoke clear changes in EAA release in the absence of any effects on EAA uptake. Selective mGlu5 receptor antagonists that show high potency and good central bioavailability may provide novel classes of neuroprotective agents for the treatment of brain disorders associated with abnormal EAAergic neurotransmission.

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.

Neuroprotective activity of metabotropic glutamate receptor ligands

Metabotropic glutamate receptors form a family of currently eight subtypes (mGluR1-8), subdivided into three groups (I-III). Activation of group-II (mGluR2 and -3) or group-III metabotropic glutamate receptors (mGluR4, -6, -7 and -8) has been established to be neuroprotective in vitro and in vivo. In contrast, group-I mGluRs (mGluR1 and -5) need to be antagonized in order to evoke protection. Initially, all neuroprotective mGluR ligands were analogues of L-glutamate. Those compounds were valuable to demonstrate protection in vitro, but showed limited applicability in animal models, particularly in chronic tests, due to low blood-brain-barrier penetration. Recently, systemically active and more potent and selective ligands became available, e.g., the group-II mGluR agonists LY354740 and LY379268 or group-I antagonists like MPEP (mGluR5-selective) and BAY36-7620 (mGluR1-selective). This new generation of pharmacological agents allows a more stringent assessment of the role of individual mGluR-subtypes or groups of receptors in various nervous system disorders, including ischaemia-induced brain damage, traumatic brain injury, Huntington's and Parkinson's-like pathology or epilepsy. Moreover, the use of genetically modified animals (e.g., knock-out mice) is starting to shed light on specific functions of mGluR-subtypes in experimental neuropathologies.

Modulation of spinal nociceptive processing through the glutamate transporter GLT-1

GLT-1 is the predominant glutamate transporter in most brain regions and therefore plays a major role in terminating synaptic transmission and protecting neurons from glutamate neurotoxicity. In the present study we assessed (i) the regulation of GLT-1 expression in the spinal cord after peripheral nociceptive stimulation and (ii) the nociceptive behavior of rats following inhibition or transient knockdown of spinal GLT-1. Formalin injection into one hindpaw caused a rapid transient upregulation of GLT-1 protein expression in the spinal cord which did not occur when rats were pretreated with morphine (10 mg/kg, i.p.) suggesting that the nociceptive input specifically caused the increase of GLT-1 transcription. Inhibition of GLT-1 by the transportable inhibitor trans-pyrrolidine-2,4-dicarboxylic acid resulted in a significant reduction of nociceptive behavior in the rat formalin assay. Similar results were obtained with a transient reduction of GLT-1 protein expression by antisense oligonucleotides. These data suggest that inhibition of GLT-1 activity or expression reduces excitatory synaptic efficacy and thereby nociception. Mechanisms that might explain this phenomenon may include activation of inhibitory metabotropic glutamate receptors, postsynaptic desensitization or disturbance of glutamate recycling.

Down-regulation of mGluR8 in pilocarpine epileptic rats

Activation of presynaptic metabotropic glutamate receptors (mGluRs) leads to a powerful inhibition of glutamate release from many synaptic terminals throughout the CNS. mGluRs as autoreceptors are believed to provide a negative feedback system that prevents potentially toxic accumulation of glutamate in the extracellular space during synchronous synaptic activity such as epileptic seizures. In this study we analyzed the function of presynaptic mGluR8 on terminals of the lateral perforant pathway in the pilocarpine model of limbic epilepsy. Field excitatory postsynaptic potentials (fEPSPs) recorded in hippocampal slices of rats that developed spontaneous recurrent seizures after pilocarpine-induced status epilepticus (SRS group) showed a significantly reduced sensitivity to Group III mGluR agonists and severe mossy fiber sprouting. The Group III mGluR agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4, 10 microM) depressed fEPSPs in the SRS group only by 26 +/- 21% compared to 50 +/- 18% in untreated rats. Similarly, the mGluR8 preferring agonist (R,S)-4-phosphonophenylglycine (PPG, 5 microM) was significantly less effective in slices from SRS rats (43 +/- 4% vs. 83 +/- 5%). Concentration-response curves for L-AP4 revealed that the EC(50) values were not different between the control and SRS group (13 +/- 7 microM vs. 9 +/- 9 microM), while the maximal depressing effect was significantly reduced. The remaining depressing effect of L-AP4 in the SRS group could be blocked by the Group III specific antagonists (RS)-alpha-methyl-4-phosphonophenylglycine (MPPG) and alpha-methyl-L-AP4 (MAP4). Rats that did not develop SRS following pilocarpine-induced status epilepticus were indistinguishable from control rats: fEPSPs were highly sensitive to L-AP4 and there was no mossy fiber sprouting. The results show that pilocarpine-induced status epilepticus can lead to a downregulation of mGluR8 and suggest that the condition of SRS is associated with a deteriorated autoregulation of glutamate release.

Seizures induced by homocysteic acid in immature rats are prevented by group III metabotropic glutamate receptoragonist (R,S)-4-phosphonophenylglycine

The potential anticonvulsant effect of group III metabotropic glutamate receptor (mGluR) agonist (R,S)-4-phosphonophenylglycine ((R,S)-PPG) against seizures induced in immature 12-day-old rats by bilateral intracerebroventricular (icv) infusion of DL-homocysteic acid (DL-HCA, 600 nmol/side) was examined in the present study. Rat pups were sacrificed during generalized clonic-tonic seizures, approximately 45 to 50 min after infusion. Comparable time intervals were used for sacrificing the pups which had received (R,S)-PPG. Low doses of (R,S)-PPG (10 nmol, icv) provided a pronounced anticonvulsant effect which was abolished by pretreatment with a selective group III mGluR antagonist (R,S)-alpha-methylserine-O-phosphate. Generalized clonic-tonic seizures were completely suppressed and cortical energy metabolite changes which normally accompany these seizures were either normalized (glucose and glycogen decreases) or markedly ameliorated (an accumulation of lactate). Despite the absence of obvious motor phenomena, EEG recordings revealed sporadic ictal activity, mostly in the dorsal hippocampus. Spreading of this activity into the frontal cortex was rather exceptional. The latency of ictal EEG in pretreated rats was significantly prolonged. Our data suggest that the predominant effect of (R,S)-PPG might concern seizure spread. The administration of (R,S)-PPG alone did not cause any overt behavioral side effects; it did not change the EEG pattern and did not influence cortical metabolite levels, with the exception of increased concentrations of glucose. The present findings suggest that group III mGlu receptor agonists may be of therapeutic significance for treating childhood epilepsies.

Glutamatergic modulation of GABAergic signaling among hippocampal interneurons: novel mechanisms regulating hippocampal excitability

PURPOSE

Because interneurons play a central role in regulating the excitability of the hippocampal formation, it is important to understand the mechanisms that modulate gamma-aminobutyric acid (GABA)ergic signaling among them. This study addresses the modulation of GABA release from interneuron terminals by presynaptic glutamate receptors.

METHODS

Whole-cell recordings were obtained from CA1 stratum radiatum interneurons in guinea pig hippocampal slices. Selective agonists and blockers of glutamate receptors were used to study modulation of GABAergic transmission by group III metabotropic receptors or kainate receptors. Antidromic action-potential initiation also was analyzed by stimulating the axons of interneurons.

RESULTS

Agonists of group III metabotropic glutamate receptors attenuated monosynaptic GABAergic signals in interneurons, but not in pyramidal neurons, in agreement with anatomic evidence on the distribution of these receptors. Submicromolar kainate enhanced the frequency and amplitude of spontaneous GABAergic signals in interneurons. Kainate also depolarized the axons of hippocampal interneurons, and triggered spontaneous ectopic action potentials in axons. Synaptically released glutamate reproduced many of the effects of both agonists, implying that these receptors can sense the ambient glutamate concentration, and therefore indirectly respond to the excitatory traffic in the hippocampus. When the two classes of receptors were stimulated simultaneously, complex interactions were obtained.

CONCLUSIONS

Group III metabotropic receptors and kainate receptors profoundly affect GABAergic signaling among interneurons of the hippocampus. Glutamatergic modulation of GABAergic signaling among interneurons represents a novel class of mechanisms that potentially plays a major role in determining the initiation, propagation, and termination of seizures.

Molecular determinants of high affinity binding to group III metabotropic glutamate receptors

The amino-terminal domain containing the ligand binding site of the G protein-coupled metabotropic glutamate receptors (mGluRs) consists of two lobes that close upon agonist binding. In this study, we explored the ligand binding pocket of the Group III mGluR4 receptor subtype using site-directed mutagenesis and radioligand binding. The selection of 16 mutations was guided by a molecular model of mGluR4, which was based on the crystal structure of the mGluR1 receptor. Lysines 74 and 405 are present on lobe I of mGluR4. The mutation of lysine 405 to alanine virtually eliminated the binding of the agonist [(3)H]l-amino-4-phosphonobutyrate ([(3)H]l-AP4). Thus lysine 405, which is conserved in all eight mGluRs, likely represents a fundamental recognition residue for ligand binding to the mGluRs. Single point mutations of lysines 74 or 317, which are not conserved in the mGluRs, to alanine had no effect on agonist affinity, whereas mutation of both residues together caused a loss of ligand binding. Mutation of lysine 74 in mGluR4, or the analogous lysine in mGluR8, to tyrosine (mimicking mGluR1 at this position) produced a large decrease in binding. The reduction in binding is likely due to steric hindrance of the phenolic side chain of tyrosine. The mutation of glutamate 287 to alanine, which is present on lobe II and is not conserved in the mGluR family, caused a loss of [(3)H]l-AP4 binding. We conclude that the determinants of high affinity ligand binding are dispersed across lobes I and II. Our results define a microenvironment within the binding pocket that encompasses several positively charged amino acids that recognize the negatively charged phosphonate group of l-AP4 or the endogenous compound l-serine-O-phosphate.

Long-term potentiation and contextual fear conditioning increase neuronal glutamate uptake

Induction and expression of long-term potentiation (LTP) in area CA1 of the hippocampus require the coordinated regulation of several cellular processes. We found that LTP in area CA1 was associated with an N-methyl-D-aspartate (NMDA) receptor-dependent increase in glutamate uptake. The increase in glutamate uptake was inhibited by either removal of Na+ or addition of D,L-threo-beta-hydroxyaspartate. Dihydrokainate (DHK), a specific inhibitor of the glial glutamate transporter GLT-1, did not block the increase in glutamate uptake. LTP was also associated with a translocation of the EAAC1 glutamate transporter from the cytosol to the plasma membrane. Contextual fear conditioning increased the maximum rate (Vmax) of glutamate uptake and membrane expression of EAAC1 in area CA1. These results indicate that regulation of glutamate uptake may be important for maintaining the level of synaptic strength during long-term changes in synaptic efficacy.

Activation of presynaptic group III metabotropic glutamate receptors depresses spontaneous inhibition in layer V of the rat entorhinal cortex

Whole cell voltage clamp recording was used to investigate neurotransmitter release onto neurones in deep and superficial layers of rat entorhinal cortex in vitro. Activation of metabotropic glutamate receptors with the agonist (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid depressed spontaneous release of the inhibitory neurotransmitter GABA in layer V, but not in layer II. Depression of transmitter release did not persist in the presence of the sodium channel blocker tetrodotoxin. It seems likely that activation of presynaptic glutamate heteroreceptors inhibits action potential dependent release of neurotransmitter via a direct action at the presynaptic terminal. We confirmed that depression of inhibitory neurotransmission in layer V was mediated by group III metabotropic glutamate receptors using a specific group III antagonist, (RS)-cyclopropyl-4-phosphonophenylglycine. Application of the antagonist alone did not alter the frequency of spontaneous neurotransmitter release, suggesting that the metabotropic glutamate receptor is not tonically active. In layer V of the entorhinal cortex, activation of presynaptic metabotropic glutamate receptors enhances spontaneous glutamate release, and inhibits spontaneous release of GABA. These effects may combine to increase random action potential firing in this layer, thereby reducing its capacity for synchrony generation. Our results are consistent with an anticonvulsant action for group III metabotropic glutamate receptors in the entorhinal cortex.

Metabotropic glutamate receptor subtypes as targets for neuroprotective drugs

Metabotropic glutamate (mGlu) receptors have been considered as potential targets for neuroprotective drugs, but the lack of specific drugs has limited the development of neuroprotective strategies in experimental models of acute or chronic central nervous system (CNS) disorders. The advent of potent and centrally available subtype-selective ligands has overcome this limitation, leading to an extensive investigation of the role of mGlu receptor subtypes in neurodegeneration during the last 2 years. Examples of these drugs are the noncompetitive mGlu1 receptor antagonists, CPCCOEt and BAY-36-7620; the noncompetitive mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)pyridine, SIB-1893, and SIB-1757; and the potent mGlu2/3 receptor agonists, LY354740 and LY379268. Pharmacologic blockade of mGlu1 or mGlu5 receptors or pharmacologic activation of mGlu2/3 or mGlu4/7/8 receptors produces neuroprotection in a variety of in vitro or in vivo models. MGlu1 receptor antagonists are promising drugs for the treatment of brain ischemia or for the prophylaxis of neuronal damage induced by synaptic hyperactivity. MGlu5 receptor antagonists may limit neuronal damage induced by a hyperactivity of N-methyl-d-aspartate (NMDA) receptors, because mGlu5 and NMDA receptors are physically and functionally connected in neuronal membranes. A series of observations suggest a potential application of mGlu5 receptor antagonists in chronic neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer disease. MGlu2/3 receptor agonists inhibit glutamate release, but also promote the synthesis and release of neurotrophic factors in astrocytes. These drugs may therefore have a broad application as neuroprotective agents in a variety of CNS disorders. Finally, mGlu4/7/8 receptor agonists potently inhibit glutamate release and have a potential application in seizure disorders. The advantage of all these drugs with respect to NMDA or AMPA receptor agonists derives from the evidence that mGlu receptors do not "mediate," but rather "modulate" excitatory synaptic transmission. Therefore, it can be expected that mGlu receptor ligands are devoid of the undesirable effects resulting from the inhibition of excitatory synaptic transmission, such as sedation or an impairment of learning and memory.

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.

Anticonvulsant activity of a mGlu(4alpha) receptor selective agonist, (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid

The metabotropic Group III agonist, (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid (ACPT-1), selective for the mGlu(4alpha) receptor, suppresses sound-induced seizures in DBA/2 mice following its intracerebroventricular (i.c.v.) administration (ED(50) 5.6 [2.9-10.7], nmol i.c.v., 15 min, clonic phase) and in genetically epilepsy-prone (GEP) rats following focal administration into the inferior colliculus (ED(50) 0.08 [0.01-0.50], nmol, 60 min, clonic phase). ACPT-1 also protects against clonic seizures induced in DBA/2 mice by the Group I agonist, (RS)-3,5-dihydroxyphenylglycine (3,5-DHPG) (ED(50) 0.60 [0.29-1.2], nmol i.c.v.) and by the Group III antagonist, (RS)-alpha-methylserine-O-phosphate (MSOP) (ED(50) 49.3 [37.9-64.1], nmol i.c.v.). Another Group III agonist, (RS)-4-phosphonophenyl-glycine (PPG), preferentially activating the mGlu(8) receptor, previously shown to protect against sound-induced seizures in DBA/2 mice and GEP rats, also protects against seizures induced in DBA/2 by 3,5-DHPG (ED(50) 3.7 [2.4-5.7], nmol i.c.v.) and by the Group III antagonist, MSOP (ED(50) 40.2 [21.0-77.0], nmol i.c.v.). At very high doses (500 nmol i.c.v. and above), Group III antagonists have pro-convulsant and convulsant activity. The anticonvulsant protection against sound-induced seizures in DBA/2 mice provided by a fully protective dose (20 nmol, i.c.v.) of the mGlu(4) receptor agonist ACPT-1, is partially reversed by the co-administration of the Group III antagonists, MSOP, (RS)-alpha-methyl-4-phosphonophenylglycine (MPPG) or (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4), in the 20-50 nmol dose range. At doses of 50-200 nmol, MPPG and MAP4 cause further reversal of the ACPT-1 anticonvulsant protection, while the MSOP effect on ACPT-1 protection is abolished at higher doses. In contrast, the anticonvulsant protection against sound-induced seizures in DBA/2 mice provided by a fully protective dose (20 nmol, i.c.v.) of the mGlu(8) receptor agonist PPG, is not significantly affected by the co-administration of the same Group III antagonists, MSOP, MPPG or MAP4. We conclude that activation of either mGlu(4alpha) or mGlu(8) receptors confer anticonvulsant protection in DBA/2 mice. Furthermore, the metabotropic Group III receptor antagonists, MSOP, MPPG, and MAP4 appear to be functionally selective for the mGlu(4) receptor in this system.

Selective activation of group I metabotropic glutamate receptors upregulates preprodynorphin, substance P, and preproenkephalin mRNA expression in rat dorsal striatum

Group I metabotropic glutamate receptors (mGluRs) are positively coupled to phosphoinositide hydrolysis through G-proteins and are densely expressed in the medium-sized spiny neurons of striatum. Activation of this group of mGluRs in the striatum produces long-lasting stimulation of behavioral activity. In this study, the role of group I mGluRs in the modulation of neuropeptide mRNA expression in striatal neurons was investigated using a Group I-selective agonist, 3,5-dihydroxyphenylglycine (DHPG) in chronically cannulated rats. Unilateral injections of DHPG into the dorsal striatum (caudoputamen) at behaviorally active doses of 20, 40, and 80 nmol elevated basal levels of preprodynorphin (PPD), substance P (SP), and preproenkephalin (PPE) mRNAs in the injected dorsal striatum as revealed by quantitative in situ hybridization. The elevation of all three mRNAs was dose-dependent and the responsiveness of opioid peptide mRNAs (PPD and PPE) to acute injection of DHPG at each dose surveyed was greater than that of SP mRNA. Induction of the mRNAs was delayed and prolonged as increases in hybridization signal became evident at 2 (SP and PPE) or 3 (PPD) h, reached a peak between 3 and 6 h, and returned to normal levels 24 h after DHPG injection. Coadministration of a Group I-selective antagonist, n-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carbo xamide (PHCCC, 10 nmol), with DHPG markedly attenuated DHPG-stimulated PPD, PPE, and, to a lesser extent, SP expression. Administration of PHCCC alone had no significant effect on basal levels of three mRNA expression in the striatum. This study provides a detailed description of the dose- and time-related alterations in striatonigral PPD/SP and striatopallidal PPE mRNA expression in response to a single injection of the Group I agonist DHPG. Data obtained demonstrate a facilitatory, dynamic regulation of constitutive expression of PPD, SP, and PPE mRNAs by local enhancement of glutamatergic tone on DHPG- and PHCCC-sensitive Group I mGluRs.

Long-term regulation of neuronal high-affinity glutamate and glutamine uptake in Aplysia

An increase in transmitter release accompanying long-term sensitization and facilitation occurs at the glutamatergic sensorimotor synapse of Aplysia. We report that a long-term increase in neuronal Glu uptake also accompanies long-term sensitization. Synaptosomes from pleural-pedal ganglia exhibited sodium-dependent, high-affinity Glu transport. Different treatments that induce long-term enhancement of the siphon-withdrawal reflex, or long-term synaptic facilitation increased Glu uptake. Moreover, 5-hydroxytryptamine, a treatment that induces long-term facilitation, also produced a long-term increase in Glu uptake in cultures of sensory neurons. The mechanism for the increase in uptake is an increase in the V(max) of transport. The long-term increase in Glu uptake appeared to be dependent on mRNA and protein synthesis, and transport through the Golgi, because 5,6-dichlorobenzimidazole riboside, emetine, and brefeldin A inhibited the increase in Glu uptake. Also, injection of emetine and 5,6-dichlorobenzimidazole into Aplysia prevented long-term sensitization. Synthesis of Glu itself may be regulated during long-term sensitization because the same treatments that produced an increase in Glu uptake also produced a parallel increase in Gln uptake. These results suggest that coordinated regulation of a number of different processes may be required to establish or maintain long-term synaptic facilitation.

Plasticity of excitatory amino acid transporters in experimental epilepsy

PURPOSE

To examine the relationship between seizures and excitatory amino acid transporter (EAAT) activity and whether up-regulation of EAAT activity alters epileptogenicity.

METHODS

In this study, we exposed rat hippocampal slices to different convulsants before measuring EAAT activity. Rats were exposed to the EAAT inhibitor pyrrolidine-2,4-dicarboxylic acid (PDC) before entorhinal cortex/hippocampal slices were obtained. These slices were exposed to low-Mg2+ buffer while electrophysiological recordings were obtained from the entorhinal cortex. mGluR III acting agents were used to study whether activation of mGluR III could regulate EAAT activity and if this regulation could overcome the effects on EAAT activity induced by the convulsants.

RESULTS

Veratridine, kainic acid (KA), and pilocarpine reduced EAAT activity in rat hippocampal slices. L-2-Amino-4-phosphonobutyric acid (an mGluR III agonist) restored EAAT activity and reduced epileptiform activity to near control levels. The saturation curve for glutamate uptake in slices from KA-seized rats killed 2 hours after the first forelimb clonus was displaced to the left, suggesting a compensatory change for the enhanced excitation. On the other hand, rats injected with the EAAT inhibitor PDC (by intracerebroventricular injection) had more severe KA-induced seizures and N-methyl-D-aspartate epileptiform activity than control rats. Furthermore, hippocampal slices from KA- or KA+PDC-treated rats exposed to low Mg2+ reduced their firing rate to nearly zero once they returned to normal solution, whereas their control counterparts continued to fire, although at a lower rate.

CONCLUSIONS

These results suggest a significant contribution of EAATs in some experimental epilepsy models and point to their short-term regulation by mGluR III as a possible source of their plasticity.

Neuroprotection against hypoxic/hypoglycaemic injury after the insult by the group III metabotropic glutamate receptor agonist (R, S)-4-phosphonophenylglycine

The role of group III metabotropic glutamate receptors (mGluR) in ischaemic neurodegeneration is still unsettled. In order to examine a possible modulatory effect of these receptors on ischaemia-induced damage we tested the novel selective agonist (R, S)-4-phosphonophenylglycine [(R,S)-PPG] after an hypoxic/hypoglycaemic insult in rat hippocampal slices. The recovery of population spike amplitudes in the CA1-region was used as parameter for neuronal viability. (R,S)-PPG significantly improved the recovery of synaptic transmission in the CA1-region even when applied only during the recovery period. The results imply that presynaptic glutamate release after an insult contributes to neurodegeneration. Since agonists of group III mGluR reduce neurotransmitter release - probably via presynaptic autoreceptors - we interpret the results obtained in our in vitro model of hypoxia/hypoglycaemia as support of the hypothesis that group III mGluR agonists might be beneficial drugs against diseases where excitotoxicity is one of the dominant pathological mechanisms.

Activation of group III metabotropic glutamate receptors inhibits basal and amphetamine-stimulated dopamine release in rat dorsal striatum: an in vivo microdialysis study

Group III metabotropic glutamate (mGlu) receptors are negatively coupled to adenylate cyclase and are distributed pre-synaptically in the striatum. A behavioral study previously conducted in this laboratory shows that activation of this group of mGlu receptors attenuates acute amphetamine-stimulated motor activity. By administering a group III selective agonist or antagonist via the dialysis probe, the present study employed in vivo microdialysis to evaluate the capacity of the group III selective agents to alter extracellular levels of dopamine in the dorsal striatum of normal and amphetamine-treated rats. It was found that the group III agonist L-2-amino-4-phosphonobutyrate (L-AP4) dose-dependently (1, 10 and 100 microM) reduced basal levels of extracellular dopamine. In contrast, the group III antagonist alpha-methyl-4-phosphonophenylglycine (MPPG) dose-dependently (10, 50 and 250 microM) elevated the basal release of extracellular dopamine. This elevation was antagonized by co-perfusion of L-AP4. Perfusion of 5-microM amphetamine through the dialysis probe increased extracellular dopamine in the dorsal striatum. Co-perfusion of L-AP4 (100 microM) significantly reduced amphetamine-stimulated dopamine levels, whereas co-perfusion of L-AP4 (100 microM) and MPPG (100 microM) did not alter the capacity of amphetamine to elicit dopamine release. The data obtained from this study demonstrate the presence of a tonically active glutamatergic tone on group III mGlu receptors in the dorsal striatum to pre-synaptically regulate basal dopamine release in an inhibitory fashion. Moreover, activation of L-AP4-sensitive group III mGlu receptors can suppress the phasic release of dopamine induced by a dopamine stimulant amphetamine.

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.

Distinct inhibition of acute cocaine-stimulated motor activity following microinjection of a group III metabotropic glutamate receptor agonist into the dorsal striatum of rats

Group III metabotropic glutamate receptors (mGluRs) are negatively coupled to adenylate cyclase through G-proteins. Activation of this group of mGluRs shows an inhibition of dopaminergic transmission in the forebrain. To define the role of striatal group III mGluRs in the regulation of basal and dopamine-stimulated motor behavior, the recently developed agonist and antagonist relatively selective for group III mGluRs were utilized to pharmacologically enhance and reduce group III mGluR glutamatergic tone in the dorsal striatum of chronically cannulated rats. Bilateral injections of a group III agonist, L-2-amino-4-phosphonobutyrate (L-AP4), did not alter basal levels of motor activity at three doses surveyed (1, 10, and 100 nmol). Neither did intracaudate injection of a group III antagonist, alpha-methyl-4-phosphonophenylglycine (MPPG), at 10, 30, and 100 nmol. However, pretreatment with L-AP4 (10 and 100 nmol) dose dependently blocked hyperlocomotion induced by acute injection of cocaine (20 mg/kg, i.p.), amphetamine (2.5 mg/kg, i.p.), or apomorphine (1 mg/kg, s.c.). The behavioral activity induced by cocaine was much more sensitive to L-AP4 than that induced by amphetamine and apomorphine. At 100 nmol, L-AP4 completely blocked cocaine effect whereas amphetamine- and apomorphine-stimulated behaviors were blocked only by 28% and 31%, respectively. The blocking effect of L-AP4 on cocaine action was reversed by pretreatment with MPPG. MPPG itself did not modify behavioral responses to cocaine, amphetamine, or apomorphine. These data indicate that the glutamatergic tone on the group III mGluRs is not active in the regulation of basal and acute dopamine-stimulated motor activity. However, enhanced group III mGluR glutamatergic transmission by an exogenous ligand is capable of suppressing behavioral responses to acute exposure of dopamine stimulants.

Modulation of absence seizures by the GABA(A) receptor: a critical rolefor metabotropic glutamate receptor 4 (mGluR4)

Experimental absence seizures are associated with perturbations in the presynaptic release of GABA and glutamate within thalamocortical circuitry. The release of both glutamate and GABA is regulated by group III metabotropic glutamate receptors (mGluRs). Therefore, we examined the susceptibility of mice lacking the mGluR4 subtype of mGluR (mGluR4(-/-)) versus their wild-type controls (mGluR4(+/+)) to absence seizures induced either by gamma-hydroxybutyrate (GHB) or the GABA(B) agonist (-) baclofen or by low doses of the GABA(A) receptor (GABA(A)R) antagonists pentylenetetrazole, bicuculline, or picrotoxin. There was no difference between mGluR4(-/-) and mGluR4(+/+) mice in threshold to absence seizures induced by either GHB or (-) baclofen. In contrast, the mGluR4(-/-) mice were markedly resistant to absence seizures induced by low doses of GABA(A)R antagonists. No differences were observed between mGluR4(-/-) and mGluR4(+/+) mice in threshold to clonic or tonic seizures induced by higher doses of GABA(A)R antagonists, strychnine, or electroshock, indicating that seizure resistance in the mGluR4(-/-) mice was restricted solely to absence seizures. The resistance of mGluR4(-/-) mice to absence seizures induced by GABA(A)R antagonists was mimicked by bilateral administration of a mGluR4 antagonist into the nucleus reticularis thalami (nRT) of mGluR4(+/+) mice. Conversely, intra-nRT administration of a mGluR4 agonist in mGluR4(+/+) mice exacerbated GABA(A)R-induced absence seizures. These data indicate that the presence of mGluR4 within nRT is critical to GABAergic modulation of thalamocortical synchronization in normal and pathological states, such as generalized absence epilepsy.

Metabotropic glutamate autoreceptors of the mGlu(5) subtype positively modulate neuronal glutamate release in the rat forebrain in vitro

In the present study we have examined the role of presynaptic group I metabotropic glutamate (mGlu) receptors in the control of neuronal glutamate release using rat forebrain slices pre-loaded with [(3)H]D-aspartate. We have also addressed the question of which group I mGlu receptor subtype, mGlu(1) or mGlu(5), mediates the facilitatory response observed by the use of a range of established and some more novel agonists and antagonists showing selectivity for these receptors. The electrically-stimulated release of pre-loaded [(3)H]D-aspartate from rat forebrain slices was markedly potentiated by the potent group I mGlu receptor agonist, L-quisqualic acid (L-QUIS), in a concentration-dependent manner (EC(50) 17.31 microM). This response was inhibited by the mGlu receptor antagonists (S)-MCPG (100 microM) and (RS)-MTPG (100 microM) but not by the AMPA-type ionotropic glutamate receptor antagonist, NBQX (100 microM). The selective group I mGlu receptor agonist (S)-3, 5-dihydroxyphenylglycine ((S)-DHPG) also enhanced electrically-stimulated efflux of label, although responses diminished with high (10-100 microM) concentrations of the agonist. Maximum responses were fully restored when (S)-DHPG (10 microM) was applied in the presence of the proposed mGlu(5) receptor desensitization inhibitor, cyclothiazide (10 microM). The positive modulatory response to (S)-DHPG (1 microM) was powerfully inhibited by (S)-MCPG (IC(50) 0.08 microM) but was resistant to the mGlu(1) receptor antagonists, (RS)-AIDA (1-500 microM), CPCCOEt (0.1-100 microM) and (+)-2-methyl-4-carboxyphenylglycine (LY367385) (0.1-10 microM). The recently developed, selective mGlu(5) receptor agonist (RS)-2-chloro-5-hydroxyphenylglycine ((RS)-CHPG) enhanced electrically-stimulated [(3)H]D-aspartate efflux from rat forebrain slices with a similar concentration-response profile to that of (S)-DHPG. Responses to this receptor subtype-selective agonist were also blocked by (S)-MCPG (IC(50) 1.13 microM) but were unaffected by (RS)-AIDA (500 microM), CPCCOEt (100 microM) or LY367385 (10 microM). These results indicate that the positive modulation of neuronal glutamate release seen in the rat forebrain in the presence of group I mGlu receptor agonists is mediated by presynaptically located mGlu(5) glutamate autoreceptors. The pharmacological profile of these receptors appears to be distinct from that of postsynaptic mGlu receptors. Novel antagonists acting at these presynaptic receptors may provide new drugs for the experimental therapy of a range of acute or chronic neurodegenerative disorders.

Distinct influence of the group III metabotropic glutamate receptor agonist (R,S)-4-phosphonophenylglycine [(R,S)-PPG] on different forms of neuronal damage

With this study we evaluated the influence of (R, S)-4-phosphonophenylglycine [(R,S)-PPG], a selective group III metabotropic glutamate receptor agonist, on excitotoxic, hypoxic/hypoglycaemic and ischaemic cerebral damage in rodents. Consistent with previous data showing neuroprotective and anticonvulsive effects (Gasparini, F., Bruno, V., Battaglia, G., Lukic, S., Leonhardt, T., Inderbitzin, W., et al., 1999. (R, S)-4-Phosphonophenylglycine, a potent and selective group III metabotropic glutamate receptor agonist, is anticonvulsive and neuroprotective in vivo. Journal of Pharmacology and Experimental Therapeutics 290, 1678-1687), we found pronounced neuroprotective effects with (R,S)-PPG (300 nmol) in a model of excitotoxicity, i.e. quinolinic acid-induced striatal lesions in rats. However, neither in focal cerebral ischaemia in mice nor in global cerebral ischaemia in gerbils or rats did (R,S)-PPG have any significant influence on the extent of neuronal damage. In a model of hypoxia/hypoglycaemia in acutely isolated hippocampal slices, however, (R,S)-PPG led to an improved recovery of population spike amplitude. As acutely isolated hippocampal slices are only viable for a few hours, these electrophysiological recordings can only be performed in a limited time window after the challenge-when most probably excitotoxicity is still the predominant influence in hypoxic pathophysiology. From this we conclude that group III mGluR agonists might be promising drugs against damage mediated mainly by excitotoxicity, but less likely against development of neuronal death due to ischaemia.

Modulation of GABAergic signaling among interneurons by metabotropic glutamate receptors

Synapses between hippocampal interneurons are an important potential target for modulatory influences that could affect overall network behavior. We report that the selective group III metabotropic receptor agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depresses GABAergic transmission to interneurons more than to pyramidal neurons. The L-AP4-induced depression is accompanied by changes in trial-to-trial variability and paired-pulse depression that imply a presynaptic site of action. Brief trains of stimuli in Schaffer collaterals also depress GABAergic transmission to interneurons. This depression persists when GABA(B) receptors are blocked, is enhanced by blocking glutamate uptake, and is abolished by the group III metabotropic receptor antagonist (alpha-methylserine-O-phosphate (MSOP). The results imply that GABAergic transmission among interneurons is modulated by glutamate spillover from excitatory afferent terminals.

New anti-epileptic drugs

Epilepsy represents the most common serious neurological disorder, with a prevalence of 0.4 - 1%. Approximately 30% of patients are resistant to currently available drugs. New anti-epileptic drugs are needed to treat refractory epilepsy, improve upon current therapies, improve the prognosis of epilepsy and to prevent the epileptogenic process. Designing compounds with specific physiological targets would seem the most rational method of anti-epileptic drug development, but results from this approach have been disappointing; the widespread screening of compounds in animal models has been much more fruitful. Older methods of animal screening have used acute seizure models, which bear scant relationship to the human condition. More modern methods have included the development of animal models of chronic epilepsy; although more expensive, it is likely that these models will be more sensitive and more specific in determining anti-epileptic efficacy. In this review, we consider the possible physiological targets for anti-epileptic drugs, the animal models of epilepsy, problems with clinical trials and ten promising anti-epileptic drugs in development (AWD 131-138, DP16 (DP-VPA), ganaxolone, levetiracetam, losigamone, pregabalin, remacemide, retigabine, rufinamide and soretolide). Perhaps the most important advances will come about from the realisation that epilepsy is a symptom, not a disease. Preclinical testing should be used to determine the spectrum of epilepsies that a drug can treat, and to direct later clinical trials, which need to select patients based on carefully defined epilepsy syndromes and aetiologies. Not only will such an approach improve the sensitivity of clinical trials, but also will lead to a more rational basis on which to treat.

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.

Pentylenetetrazol-induced seizures increase [3H]L-2-amino-4-phosphonobutyrate binding in discrete regions of the rat brain

Convulsions were induced in rats by a single injection of pentylenetetrazol (PTZ) and [3H]L-2-amino-4-phosphonobutyrate ([3H]L-AP4) in vitro receptor autoradiography was used to evaluate the effects on the expression of group III metabotropic glutamate receptors. Significant increases by 60-80% in [3H]L-AP4 binding was observed in the frontal parts of cortex (sensory, motor and cingulate cortex) and by 28% in the molecular layer of the cerebellar cortex 24 h after PTZ-induced seizures. Since group III metabotropic glutamate receptors has an inhibitory effect on glutamatergic transmission, the observed increases in binding may indicate that these receptors serve as a mechanism for preventing further seizure activity.

Role of metabotropic glutamate receptors in the hypersusceptibility to pentylenetetrazole-induced seizure during diazepam withdrawal

To clarify the contribution of metabotropic glutamate (mGlu) receptors in brain to benzodiazepine withdrawal signs, we now examine the effects in mice of selective ligands for three subgroups of mGlu receptor on the hypersusceptibility to pentylenetetrazole-induced seizure during diazepam withdrawal. The seizure threshold for pentylenetetrazole was significantly decreased by the discontinuation of chronic diazepam treatment. The decrease in the seizure threshold for pentylenetetrazole during diazepam withdrawal was significantly suppressed by intracerebroventricular (i.c.v.) pretreatment with the group 1 mGlu receptor antagonist, (S)-4-carboxyphenylglycine ((S)-4CPG: 56 and 100 nmol). These doses of (S)-4CPG did not alter the seizure threshold in chronically vehicle-treated (control) mice. Pretreatment i.c.v. with a presynaptic mGlu receptor agonist (the group 2 mGlu receptor agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)-glycine (L-CCG-I: 3.0 and 5.6 nmol) and the group 3 mGlu receptor agonist, L-amino-4-phosphonobutyric acid (L-AP4: 3.0 and 5.6 nmol)) failed to suppress the decrease in seizure threshold in diazepam-withdrawn mice, but increased the seizure threshold in control mice. Pretreatment i.c.v. with the group 1 mGlu receptor antagonist/group 2 mGlu receptor agonist, (S)-4-carboxy-3-hydroxyphenylglycine ((S)-4C3HPG: 56 and 100 nmol), significantly increased the seizure threshold in control mice and suppressed the decrease in seizure threshold in diazepam-withdrawn mice. These findings suggest that enhancement of group 1 mGlu receptor function and a decline in both group 2 and group 3 mGlu receptor functions may play an important role in the hypersusceptibility to pentylenetetrazole-induced seizure during diazepam withdrawal.

Analysing metabotropic glutamate group III receptor mediated modulation of synaptic transmission in the amygdala-kindled dentate gyrus of the rat

Metabotropic glutamate receptors (mGluRs) provide a powerful control of synaptic transmission in the hippocampus and may serve as a target for drug development in human temporal lobe epilepsies. Agonists and antagonists at these receptors influence the development and propagation of seizures in some animal models of epilepsy. Experimental seizures can change the level of expression of mGluRs in the rat hippocampus. In the human dentate gyrus of patients suffering from temporal lobe epilepsy (TLE), group III mGluR mediated inhibition of synaptic transmission is almost lost in the sub-group with Ammon's horn sclerosis. We tested the modulation of synaptic transmission by the group III mGluR specific agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) in the dentate gyrus outer molecular layer in control and amygdala-kindled rats, a common model for TLE. Extracellular field potential recordings upon subthreshold stimulation of lateral perforant path fibers were measured simultaneously in the outer molecular layer and granule cell layer. Analysis of 'paired-pulse' characteristics in the absence and presence of L-AP4 and group III mGluR mediated inhibition of synaptic transmission in the lateral perforant path revealed no significant alterations in fully kindled rats. Since there is no evidence of altered L-AP4 responses, a loss of group III mGluR function, particularly that of subtype mGluR8, seems not necessary for the kindling epilepsy.

Selective agonists of metabotropic glutamate receptors elicit significant EEG effects when infused in the nucleus accumbens of rats

The effect of intra-accumbens infusion of selective group I ((S)-3,5-dihydroxyphenylglycine, DHPG), group II ((2S,3S,4S)-CCG/(2S,1'S,2'S)-2-(carboxycyclopropyl)glycine, L-CCG-I) and group III ((L-(+)-2-amino-4-phosphonobutyric acid, L-AP4) metabotropic glutamate (mGlu) receptor agonists was studied in male Wistar rats. A computerised electroencephalographic (EEG) power spectral analysis was performed. While DHPG (400 nmoles) induced EEG and behavioural limbic seizures, L-CCG-I (400 nmoles) and L-AP4 (800 nmoles) induced a 'depressant' EEG with an increase in relative power in the slow-frequency bands and a decrease in relative power in the high-frequency bands) and behavioural effects. These results show for the first time that the stimulation of groups I, II and III mGlu receptors located in the nucleus accumbens significantly influences the EEG tracing in rats.

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.

Roles of metabotropic glutamate receptor subtypes in modulation of pentylenetetrazole-induced seizure activity in mice

The anticonvulsant or proconvulsant properties of ligands at metabotropic glutamate receptors (mGluRs) were examined in a chemoconvulsant model using pentylenetetrazole (PTZ). Mice received mGluR ligands by intracerebroventricular (i.c.v.) infusion prior to a subcutaneous injection of PTZ and the latency to onset of tonic convulsions was recorded. The group I mGluR antagonist 1-aminoindan-1,5-dicarboxylic acid (AIDA) dose-dependently antagonised PTZ-induced seizures with a mean ED50 value of 465 nmol. In contrast, the selective group I mGluR agonist, (S)-3,5-dihydroxyphenylglycine [(S)-DHPG], was proconvulsive and decreased the PTZ-induced seizure latency (ED50=60 nmol i.c.v.). A selective agonist of group II mGluRs, (1S,3S)-1-aminocyclopentane dicarboxylic acid [(1S,3S)-ACPD], was proconvulsive but did not affect PTZ-induced seizure latency. Moreover, the proconvulsant effect of (IS,3S)-ACPD was not blocked by the mGluR2 antagonist, alpha-methylserine-O-phosphate monophenyl ester but was blocked by AIDA suggesting the involvement of group I mGluRs. (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCG-IV), which is a potent mGluR2 antagonist and a group III mGluR agonist at higher doses, increased the PTZ-induced seizure latency (ED50=51 nmol) and this effect was fully reversed by the group III mGluR antagonist, (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4). Similarly, the group III mGluR agonist 1-amino-3-(phosphonomethylene)cyclobutanecarboxylate (cyclobutylene-AP5) increased the PTZ-induced seizure latency (ED50=12 nmol) in a MAP4-sensitive manner. Collectively, these data suggest that mGluR ligands modulate PTZ-induced seizure activity in mice by either antagonizing group I mGluRs or activating group III mGluRs.

The effect of 2-amino-3-arsonopropionate and 2-amino-4-arsonobutyrate on the development and maintenance of amygdala kindled seizures

The effects of 2-a-3-arsonopropionate and 2-a-4-arsonobutyrate, the arsono analogues of aspartate and glutamate respectively, on the development of electrically-induced kindling in the amygdala, and on seizures induced in fully kindled rats, were compared to the effects of 3-amino-propylarsonate the arsono analogue of GABA. Intra-amygdaloid micro-injection of 2-a-3-arsonopropionate and 2-a-4-arsonobutyrate (10 nmol in 0.5 microl buffer phosphate) reduced the rate of epileptogenesis without preventing the development of generalized seizure responses, after 14 daily stimulations. In fully electrically kindled animals with stage 5 amygdala-kindled seizures, 3-aminopropy-larsonate (10 nmol/0.5 microl) increased after-discharge threshold (ADT) by 82% (P< or =0.005) without having any effect on mean seizure score or after-discharge duration. Chemical reduction of 3-aminopropylarsonate with glutathione diminished the anti-seizure activity of the drug. 2-a-3-arsonopropionate and 2-a-4-arsonobutyrate the arsono analogues of aspartate and glutamate were not effective when they were micro-injected into the amygdala of fully kindled animals at equivalent doses i.e. (10 nmol/0.5 microl). Higher doses (100 nmol/0.5 microl) of 2-a-3-arsonopropionate the analogue of aspartate increased the generalized seizure threshold by 40% (P < or = 0.025), while 2-a-4-arsonobutyrate was not effective even at high doses.