Group I mGluR-induced epileptogenesis: distinct and overlapping roles of mGluR1 and mGluR5 and implications for antiepileptic drug design

Epilepsy Characteristics in Neurodevelopmental Disorders: Research from Patient Cohorts and Animal Models Focusing on Autism Spectrum Disorder

Epilepsy, a heterogeneous group of brain-related diseases, has continued to significantly burden society and families. Epilepsy comorbid with neurodevelopmental disorders (NDDs) is believed to occur due to multifaceted pathophysiological mechanisms involving disruptions in the excitation and inhibition (E/I) balance impeding widespread functional neuronal circuitry. Although the field has received much attention from the scientific community recently, the research has not yet translated into actionable therapeutics to completely cure epilepsy, particularly those comorbid with NDDs. In this review, we sought to elucidate the basic causes underlying epilepsy as well as those contributing to the association of epilepsy with NDDs. Comprehensive emphasis is put on some key neurodevelopmental genes implicated in epilepsy, such as MeCP2, SYNGAP1, FMR1, SHANK1-3 and TSC1, along with a few others, and the main electrophysiological and behavioral deficits are highlighted. For these genes, the progress made in developing appropriate and valid rodent models to accelerate basic research is also detailed. Further, we discuss the recent development in the therapeutic management of epilepsy and provide a briefing on the challenges and caveats in identifying and testing species-specific epilepsy models.

G protein-coupled receptors in acquired epilepsy: Druggability and translatability

As the largest family of membrane proteins in the human genome, G protein-coupled receptors (GPCRs) constitute the targets of more than one-third of all modern medicinal drugs. In the central nervous system (CNS), widely distributed GPCRs in neuronal and nonneuronal cells mediate numerous essential physiological functions via regulating neurotransmission at the synapses. Whereas their abnormalities in expression and activity are involved in various neuropathological processes. CNS conditions thus remain highly represented among the indications of GPCR-targeted agents. Mounting evidence from a large number of animal studies suggests that GPCRs play important roles in the regulation of neuronal excitability associated with epilepsy, a common CNS disease afflicting approximately 1-2% of the population. Surprisingly, none of the US Food and Drug Administration (FDA)-approved (>30) antiepileptic drugs (AEDs) suppresses seizures through acting on GPCRs. This disparity raises concerns about the translatability of these preclinical findings and the druggability of GPCRs for seizure disorders. The currently available AEDs intervene seizures predominantly through targeting ion channels and have considerable limitations, as they often cause unbearable adverse effects, fail to control seizures in over 30% of patients, and merely provide symptomatic relief. Thus, identifying novel molecular targets for epilepsy is highly desired. Herein, we focus on recent progresses in understanding the comprehensive roles of several GPCR families in seizure generation and development of acquired epilepsy. We also dissect current hurdles hindering translational efforts in developing GPCRs as antiepileptic and/or antiepileptogenic targets and discuss the counteracting strategies that might lead to a potential cure for this debilitating CNS condition.

Expression of mGlu Receptor Genes in the Hippocampus After Intoxication with Trimethyltin

A variety of localization and signaling properties of eight subtypes of metabotropic glutamate receptors (mGluRs) in the brain provide glutamate an important regulatory role in many processes, including neurodegeneration and repair of neuronal damage. To identify specific subtypes of mGluRs, which are involved in neurodegeneration process, we assessed expression levels of their genes under pathophysiological conditions. Using quantitative real-time RT-PCR analysis, we studied transcription levels of mGlu2-5 and mGlu7 genes in the hippocampus after its damage by neurotoxicant trimethyltin chloride (TMT) in Wistar rats. This organotin compound is known to cause neurodegeneration in the brain, especially in the hippocampus. Morphological studies confirmed neuronal damage in CA3-CA4 subfields of the hippocampus 6 weeks after the treatment with TMT. Step-through passive avoidance test revealed memory deterioration in rat-treated TMT. Interestingly, 3 and 6 weeks after the treatment with TMT, expression levels of the mGlu2 and mGlu7 genes were not changed in comparison to the control values while expression level of mGlu4 genes was upregulated throughout the whole studied period of TMT action. The dynamics of mGlu3 gene expression revealed the existence of neuroinflammation 3 weeks after the treatment with TMT, which was further confirmed by the upregulation of cyclooxygenase-2 gene expression. The expression level of mGlu5 receptors was downregulated 6 weeks after the treatment with TMT. Our results revealed a significant role of mGlu4, mGlu5, and mGlu3 receptors in the neurodegenerative/reparative processes in the hippocampus after the treatment with TMT. Ligands of these receptor subtypes can be, therefore, considered potential therapeutic targets for prevention or reduction of neurodegeneration.

Neurotoxic Agent-Induced Injury in Neurodegenerative Disease Model: Focus on Involvement of Glutamate Receptors

Glutamate receptors play a crucial role in the central nervous system and are implicated in different brain disorders. They play a significant role in the pathogenesis of neurodegenerative diseases (NDDs) such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Although many studies on NDDs have been conducted, their exact pathophysiological characteristics are still not fully understood. In in vivo and in vitro models of neurotoxic-induced NDDs, neurotoxic agents are used to induce several neuronal injuries for the purpose of correlating them with the pathological characteristics of NDDs. Moreover, therapeutic drugs might be discovered based on the studies employing these models. In NDD models, different neurotoxic agents, namely, kainic acid, domoic acid, glutamate, β-N-Methylamino-L-alanine, amyloid beta, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1-methyl-4-phenylpyridinium, rotenone, 3-Nitropropionic acid and methamphetamine can potently impair both ionotropic and metabotropic glutamate receptors, leading to the progression of toxicity. Many other neurotoxic agents mainly affect the functions of ionotropic glutamate receptors. We discuss particular neurotoxic agents that can act upon glutamate receptors so as to effectively mimic NDDs. The correlation of neurotoxic agent-induced disease characteristics with glutamate receptors would aid the discovery and development of therapeutic drugs for NDDs.

Tau and amyloid-related pathologies in the entorhinal cortex have divergent effects in the hippocampal circuit

The entorhinal cortex (EC) is affected early in Alzheimer's disease, an illness defined by a co-occurrence of tau and amyloid-related pathologies. How the co-occurrence of these pathologies in the EC affects the hippocampal circuit remains unknown. Here we address this question by performing electrophysiological analyses of the EC circuit in mice that express mutant human amyloid precursor protein (hAPP) or tau (hTau), or both in the EC. We show that the alterations in the hippocampal circuit are divergent, with hAPP increasing but hTau decreasing neuronal/circuit excitability. Most importantly, mice co-expressing hAPP and hTau show that hTau has a dominant effect, dampening the excitatory effects of hAPP. Additionally, compensatory synaptic downscaling, in response to increased excitability in EC was observed in subicular neurons of hAPP mice. Based on simulations, we propose that EC interneuron pruning can account for both EC hyperexcitability and subicular synaptic downscaling found in mice expressing hAPP.

Metabotropic glutamate receptor, mGlu5, regulates hippocampal synaptic plasticity and is required for tetanisation-triggered changes in theta and gamma oscillations

Hippocampal synaptic plasticity and learning are regulated by metabotropic glutamate receptors (mGlu) and particularly by mGlu5. In the hippocampus, synaptic plasticity is tightly linked to neuronal network oscillations in theta (5-10 Hz) and gamma (∼30-100 Hz) frequency ranges, and specific changes in theta and gamma spectral power can predict for the success of patterned afferent stimulation in inducing robust long-term potentiation (LTP). In this study, we hypothesized that activation of mGlu5 mediates tetanisation-driven changes in network oscillations and thereby determines the longevity of LTP. To explore this, we applied high-frequency stimulation (HFS) to the perforant path input to the dentate gyrus (DG), in the presence of the negative allosteric modulator, 2-methyl-6-(phenylethynyl)pyridine (MPEP), or the positive allosteric modulator (S)-(4-fluorophenyl)-[3-(3-(3-(4-fluorophenyl)-[1,2,4]oxadiazol-5-yl)-piperidin-1-yl)]methanone (ADX47273). In freely behaving rats, administration of MPEP resulted in a significant impairment, whereas treatment with ADX47273 led to a significant enhancement, of LTP (>24 h) compared to vehicle-treated controls. Allosteric potentiation of mGlu5 also resulted in a significantly greater increase of the spectral power of theta and gamma oscillations within the period of 300 s after HFS, as compared to MPEP-treated animals or controls. Our findings show that the regulation of hippocampal LTP by mGlu5 is associated with modulation of network oscillatory activity in the period shortly after LTP induction. Taken together, these data demonstrate that changes in the spectral contents of local field activity that occur in response to patterned afferent stimulation require activation of mGlu5 and may be instrumental for the successful expression of persistent LTP. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Mechanisms of tau and Aβ-induced excitotoxicity

Excitotoxicity was originally postulated to be a late stage side effect of Alzheimer׳s disease (AD)-related neurodegeneration, however more recent studies indicate that it may occur early in AD and contribute to the neurodegenerative process. Tau and amyloid beta (Aβ), the main components of neurofibrillary tangles (NFTs) and amyloid plaques, have been implicated in cooperatively and independently facilitating excitotoxicity. Our study investigated the roles of tau and Aβ in AD-related excitotoxicity. In vivo studies showed that tau knockout (tau(-/-)) mice were significantly protected from seizures and hippocampal superoxide production induced with the glutamate analog, kainic acid (KA). We hypothesized that tau accomplished this by facilitating KA-induced Ca(2+) influx into neurons, however lentiviral tau knockdown failed to ameliorate KA-induced Ca(2+) influx into primary rat cortical neurons. We further investigated if tau cooperated with Aβ to facilitate KA-induced Ca(2+) influx. While Aβ biphasically modulated the KA-induced Cacyt(2+) responses, tau knockdown continued to have no effect. Therefore, tau facilitates KA-induced seizures and superoxide production in a manner that does not involve facilitation of Ca(2+) influx through KA receptors (KAR). On the other hand, acute pretreatment with Aβ (10 min) enhanced KA-induced Ca(2+) influx, while chronic Aβ (24 h) significantly reduced it, regardless of tau knockdown. Given previously published connections between Aβ, group 1 metabotropic glutamate receptors (mGluRs), and KAR regulation, we hypothesized that Aβ modulates KAR via a G-protein coupled receptor pathway mediated by group 1 mGluRs. We found that Aβ did not activate group 1 mGluRs and inhibition of these receptors did not reverse Aβ modulation of KA-induced Ca(2+) influx. Therefore, Aβ biphasically regulates KAR via a mechanism that does not involve group 1mGluR activation.

Development of Novel, CNS Penetrant Positive Allosteric Modulators for the Metabotropic Glutamate Receptor Subtype 1 (mGlu1), Based on an N-(3-Chloro-4-(1,3-dioxoisoindolin-2-yl)phenyl)-3-methylfuran-2-carboxamide Scaffold, That Potentiate Wild Type and Mutant mGlu1 Receptors Found in Schizophrenics

The therapeutic potential of selective mGlu1 activation is vastly unexplored relative to the other group I mGlu receptor, mGlu5; therefore, our lab has focused considerable effort toward developing mGlu1 positive allosteric modulators (PAMs) suitable as in vivo proof of concept tool compounds. Optimization of a series of mGlu1 PAMs based on an N-(3-chloro-4-(1,3-dioxoisoindolin-2-yl)phenyl)-3-methylfuran-2-carboxamide scaffold provided 17e, a potent (mGlu1 EC50 = 31.8 nM) and highly CNS penetrant (brain to plasma ratio (Kp) of 1.02) mGlu1 PAM tool compound, that potentiated not only wild-type human mGlu1 but also mutant mGlu1 receptors derived from deleterious GRM1 mutations found in schizophrenic patients. Moreover, both electrophysiological and in vivo studies indicate the mGlu1 ago-PAMs/PAMs do not possess the same epileptiform adverse effect liability as mGlu5 ago-PAMs/PAMs and maintain temporal activity suggesting a broader therapeutic window.

Group I and group II metabotropic glutamate receptor allosteric modulators as novel potential antipsychotics

Recently, there has been a shift in the schizophrenia field focusing on restoring glutamate signaling. Extensive preclinical data suggests that mGlu5 PAMs could have efficacy in all three symptom domains but there is concern of potential adverse effects. New insights into mechanisms underlying this toxicity may provide a path for discovery of safe mGlu5 PAMs. Genetic mutations in mGlu1 have been described in schizophrenics creating interest in this receptor as a therapeutic target. Preclinical data demonstrated the antipsychotic potential of mGlu2/3 agonists but clinical trials were not successful. However, studies have suggested that mGlu2 is the subtype mediating antipsychotic effects and selective mGlu2 PAMs are now in clinical development. Finally, recent genetic studies suggest mGlu3 modulators may be pro-cognitive.

Opportunities and challenges in the discovery of allosteric modulators of GPCRs for treating CNS disorders

Novel allosteric modulators of G protein-coupled receptors (GPCRs) are providing fundamental advances in the development of GPCR ligands with high subtype selectivity and novel modes of efficacy that have not been possible with traditional approaches. As new allosteric modulators are advancing as drug candidates, we are developing an increased understanding of the major advantages and broad range of activities that can be achieved with these agents through selective modulation of specific signalling pathways, differential effects on GPCR homodimers versus heterodimers, and other properties. This understanding creates exciting opportunities, as well as unique challenges, in the optimization of novel therapeutic agents for disorders of the central nervous system.

Exploration of allosteric agonism structure-activity relationships within an acetylene series of metabotropic glutamate receptor 5 (mGlu5) positive allosteric modulators (PAMs): discovery of 5-((3-fluorophenyl)ethynyl)-N-(3-methyloxetan-3-yl)picolinamide (ML254)

Positive allosteric modulators (PAMs) of metabotropic glutamate receptor 5 (mGlu5) represent a promising therapeutic strategy for the treatment of schizophrenia. Both allosteric agonism and high glutamate fold-shift have been implicated in the neurotoxic profile of some mGlu5 PAMs; however, these hypotheses remain to be adequately addressed. To develop tool compounds to probe these hypotheses, the structure-activity relationship of allosteric agonism was examined within an acetylenic series of mGlu5 PAMs exhibiting allosteric agonism in addition to positive allosteric modulation (ago-PAMs). PAM 38t, a low glutamate fold-shift allosteric ligand (maximum fold-shift ~ 3.0), was selected as a potent PAM with no agonism in the in vitro system used for compound characterization and in two native electrophysiological systems using rat hippocampal slices. PAM 38t (ML254) will be useful to probe the relative contribution of cooperativity and allosteric agonism to the adverse effect liability and neurotoxicity associated with this class of mGlu5 PAMs.

Biotransformation of a novel positive allosteric modulator of metabotropic glutamate receptor subtype 5 contributes to seizure-like adverse events in rats involving a receptor agonism-dependent mechanism

Activation of metabotropic glutamate receptor subtype 5 (mGlu5) represents a novel strategy for therapeutic intervention into multiple central nervous system disorders, including schizophrenia. Recently, a number of positive allosteric modulators (PAMs) of mGlu5 were discovered to exhibit in vivo efficacy in rodent models of psychosis, including PAMs possessing varying degrees of agonist activity (ago-PAMs), as well as PAMs devoid of agonist activity. However, previous studies revealed that ago-PAMs can induce seizure activity and behavioral convulsions, whereas pure mGlu5 PAMs do not induce these adverse effects. We recently identified a potent and selective mGlu5 PAM, VU0403602, that was efficacious in reversing amphetamine-induced hyperlocomotion in rats. The compound also induced time-dependent seizure activity that was blocked by coadministration of the mGlu5 antagonist, 2-methyl-6-(phenylethynyl) pyridine. Consistent with potential adverse effects induced by ago-PAMs, we found that VU0403602 had significant allosteric agonist activity. Interestingly, inhibition of VU0403602 metabolism in vivo by a pan cytochrome P450 (P450) inactivator completely protected rats from induction of seizures. P450-mediated biotransformation of VU0403602 was discovered to produce another potent ago-PAM metabolite-ligand (M1) of mGlu5. Electrophysiological studies in rat hippocampal slices confirmed agonist activity of both M1 and VU0403602 and revealed that M1 can induce epileptiform activity in a manner consistent with its proconvulsant behavioral effects. Furthermore, unbound brain exposure of M1 was similar to that of the parent compound, VU0403602. These findings indicate that biotransformation of mGlu5 PAMs to active metabolite-ligands may contribute to the epileptogenesis observed after in vivo administration of this class of allosteric receptor modulators.

Unique signaling profiles of positive allosteric modulators of metabotropic glutamate receptor subtype 5 determine differences in in vivo activity

BACKGROUND

Metabotropic glutamate receptor subtype 5 (mGlu5) activators have emerged as a novel approach to the treatment of schizophrenia. Positive allosteric modulators (PAMs) of mGlu5 have generated tremendous excitement and fueled major drug discovery efforts. Although mGlu5 PAMs have robust efficacy in preclinical models of schizophrenia, preliminary reports suggest that these compounds may induce seizure activity. Prototypical mGlu5 PAMs do not activate mGlu5 directly but selectively potentiate activation of mGlu5 by glutamate. This mechanism may be critical to maintaining normal activity-dependence of mGlu5 activation and achieving optimal in vivo effects.

METHODS

Using specially engineered mGlu5 cell lines incorporating point mutations within the allosteric and orthosteric binding sites, as well as brain slice electrophysiology and in vivo electroencephalography and behavioral pharmacology, we found that some mGlu5 PAMs have intrinsic allosteric agonist activity in the absence of glutamate.

RESULTS

Both in vitro mutagenesis and in vivo pharmacology studies demonstrate that VU0422465 is an agonist PAM that induces epileptiform activity and behavioral convulsions in rodents. In contrast, VU0361747, an mGlu5 PAMs optimized to eliminate allosteric agonist activity, has robust in vivo efficacy and does not induce adverse effects at doses that yield high brain concentrations.

CONCLUSIONS

Loss of the absolute dependence of mGlu5 PAMs on glutamate release for their activity can lead to severe adverse effects. The finding that closely related mGlu5 PAMs can differ in their intrinsic agonist activity provides critical new insights that is essential for advancing these molecules through clinical development for treatment of schizophrenia.

Beyond dopamine: glutamate as a target for future antipsychotics

The dopamine hypothesis of schizophrenia remains the primary theoretical framework for the pharmacological treatment of the disorder. Despite various lines of evidence of dopaminergic abnormalities and reasonable efficacy of current antipsychotic medication, a significant proportion of patients show suboptimal treatment responses, poor tolerability, and a subsequent lack of treatment concordance. In recent decades, intriguing evidence for the critical involvement of other neurotransmitter systems in the pathophysiology of schizophrenia has emerged, most notably of dysfunctions within the glutamate pathways. Consequently, the glutamate synapse has arisen as a promising target for urgently needed novel antipsychotic compounds—particularly in regards to debilitating negative and cognitive symptoms poorly controlled by currently available drugs. In this paper, recent findings integrating glutamatergic and dopaminergic abnormalities in schizophrenia and their implications for novel pharmacological targets are discussed. An overview of compounds in various stages of development is given: drugs enhancing NMDA receptor function as well as metabotropic glutamate receptor (mGluR) agonist and positive allosteric modulators (PAMs) are emphasised. Together with other agents more indirectly affecting glutamatergic neurotransmission, their potential future role in the pharmacotherapy of schizophrenia is critically evaluated.

Metabotropic glutamate receptors as therapeutic targets for schizophrenia

Treatment options for schizophrenia that address all symptom categories (positive, negative, and cognitive) are lacking in current therapies for this disorder. Compounds targeting the metabotropic glutamate (mGlu) receptors hold promise as a more comprehensive therapeutic alternative to typical and atypical antipsychotics and may avoid the occurrence of extrapyramidal side effects that accompany these treatments. Activation of the group II mGlu receptors (mGlu(2) and mGlu(3)) and the group I mGlu(5) are hypothesized to normalize the disruption of thalamocortical glutamatergic circuitry that results in abnormal glutamaterigic signaling in the prefrontal cortex (PFC). Agonists of mGlu(2) and mGlu(3) have demonstrated efficacy for the positive symptom group in both animal models and clinical trials with mGlu(2) being the subtype most likely responsible for the therapeutic effect. Limitations in the chemical space tolerated by the orthosteric site of the mGlu receptors has led to the pursuit of compounds that potentiate the receptor's response to glutamate by acting at less highly conserved allosteric sites. Several series of selective positive allosteric modulators (PAMs) for mGlu(2) and mGlu(5) have demonstrated efficacy in animal models used for the evaluation of antipsychotic agents. In addition, evidence from animal studies indicates that mGlu(5) PAMs hold promise for the treatment of cognitive deficits that occur in schizophrenia. Hopefully, further optimization of allosteric modulators of mGlu receptors will yield clinical candidates that will allow full evaluation of the potential efficacy of these compounds in the treatment of multiple symptom domains in schizophrenia patients in the near future.

Targeting glutamate synapses in schizophrenia

Although early clinical observations implicated dopamine dysfunction in the neuropathology of schizophrenia, accumulating evidence suggests that multiple neurotransmitter pathways are dysregulated. The psychotomimetic actions of NMDA receptor antagonists point to an imbalance of glutamatergic signaling. Encouragingly, numerous preclinical and clinical studies have elucidated several potential targets for increasing NMDA receptor function and equilibrating glutamatergic tone, including the metabotropic glutamate receptors 2, 3 and 5, the muscarinic acetylcholine receptors M(1) and M(4), and the glycine transporter GlyT1. Highly specific allosteric and orthosteric ligands have been developed that modify the activity of these novel target proteins, and in this review we summarize both the glutamatergic mechanisms and the novel compounds that are increasing the promise for a multifaceted pharmacological approach to treat schizophrenia.

Therapeutic promise and principles: metabotropic glutamate receptors

For a number of disease entities, oxidative stress becomes a significant factor in the etiology and progression of cell dysfunction and injury. Therapeutic strategies that can identify novel signal transduction pathways to ameliorate the toxic effects of oxidative stress may lead to new avenues of treatment for a spectrum of disorders that include diabetes, Alzheimer's disease, Parkinson's disease and immune system dysfunction. In this respect, metabotropic glutamate receptors (mGluRs) may offer exciting prospects for several disorders since these receptors can limit or prevent apoptotic cell injury as well as impact upon cellular development and function. Yet the role of mGluRs is complex in nature and may require specific mGluR modulation for a particular disease entity to maximize clinical efficacy and limit potential disability. Here we discuss the potential clinical translation of mGluRs and highlight the role of novel signal transduction pathways in the metabotropic glutamate system that may be vital for the clinical utility of mGluRs.

Interictal spikes and epileptogenesis

Interictal spikes are widely accepted diagnostically as a sign of epilepsy, but reasons for the presence of interictal activity in the epileptic brain are unknown. Interictal spikes are easily generated in normal brain by pharmacologically reducing inhibition, and experimental studies of acquired epilepsy indicate that spikes precede seizures. These data lead to the hypothesis that interictal spikes are correlated with epilepsy because they play a fundamental role in epileptogenesis following brain injury. Spikes may guide sprouting axons back to their network of origin, increase and sustain the strength of the synapses formed by sprouted axons, and alter the balance of ion channels in the epileptic focus, such that seizures become possible. This hypothesis has implications that are testable: altering spiking or the calcium signals generated by spikes should alter epileptogenesis and spikes should precede seizures in brain-injured human patients.

High-frequency network activity, global increase in neuronal activity, and synchrony expansion precede epileptic seizures in vitro

How seizures start is a major question in epilepsy research. Preictal EEG changes occur in both human patients and animal models, but their underlying mechanisms and relationship with seizure initiation remain unknown. Here we demonstrate the existence, in the hippocampal CA1 region, of a preictal state characterized by the progressive and global increase in neuronal activity associated with a widespread buildup of low-amplitude high-frequency activity (HFA) (>100 Hz) and reduction in system complexity. HFA is generated by the firing of neurons, mainly pyramidal cells, at much lower frequencies. Individual cycles of HFA are generated by the near-synchronous (within approximately 5 ms) firing of small numbers of pyramidal cells. The presence of HFA in the low-calcium model implicates nonsynaptic synchronization; the presence of very similar HFA in the high-potassium model shows that it does not depend on an absence of synaptic transmission. Immediately before seizure onset, CA1 is in a state of high sensitivity in which weak depolarizing or synchronizing perturbations can trigger seizures. Transition to seizure is characterized by a rapid expansion and fusion of the neuronal populations responsible for HFA, associated with a progressive slowing of HFA, leading to a single, massive, hypersynchronous cluster generating the high-amplitude low-frequency activity of the seizure.

mGluR5 positive allosteric modulators facilitate both hippocampal LTP and LTD and enhance spatial learning

Highly selective positive allosteric modulators (PAMs) of metabotropic glutamate receptor subtype 5 (mGluR5) have emerged as a potential approach to treat positive symptoms associated with schizophrenia. mGluR5 plays an important role in both long-term potentiation (LTP) and long-term depression (LTD), suggesting that mGluR5 PAMs may also have utility in improving impaired cognitive function. However, if mGluR5 PAMs shift the balance of LTP and LTD or induce a state in which afferent activity induces lasting changes in synaptic function that are not appropriate for a given pattern of activity, this could disrupt rather than enhance cognitive function. We determined the effect of selective mGluR5 PAMs on the induction of LTP and LTD at the Schaffer collateral-CA1 synapse in the hippocampus. mGluR5-selective PAMs significantly enhanced threshold theta-burst stimulation (TBS)-induced LTP. In addition, mGluR5 PAMs enhanced both DHPG-induced LTD and LTD induced by the delivery of paired-pulse low-frequency stimulation. Selective potentiation of mGluR5 had no effect on LTP induced by suprathreshold TBS or saturated LTP. The finding that potentiation of mGluR5-mediated responses to stimulation of glutamatergic afferents enhances both LTP and LTD and supports the hypothesis that the activation of mGluR5 by endogenous glutamate contributes to both forms of plasticity. Furthermore, two systemically active mGluR5 PAMs enhanced performance in the Morris water maze, a measure of hippocampus-dependent spatial learning. Discovery of small molecules that enhance both LTP and LTD in an activity-appropriate manner shows a unique action on synaptic plasticity that may provide a novel approach for the treatment of impaired cognitive function.

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

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

Group I metabotropic receptor neuroprotection requires Akt and its substrates that govern FOXO3a, Bim, and beta-catenin during oxidative stress

Metabotropic glutamate receptors are expressed throughout the nervous system, but their function as well as their ability to promote neuronal survival rests heavily upon the intracellular mechanisms governed by this family of G-proteins. In this regard, we examined one of the primary pathways that can oversee cell survival, namely protein kinase B (Akt1), and its functional integration with some of its substrates that may work in concert with group I metabotropic glutamate receptor (mGluRI) activation to protect primary hippocampal neurons during oxidative stress. We demonstrate that neuroprotection against free radical injury through mGluRI activation with DHPG requires the activation of Akt1, since loss of Akt1 activity assessed through its GSK-3alpha/beta substrate by pharmacological blockade of the phosphatidylinositide-3-kinase pathway or the gene silencing of Akt1 expression prevents neuronal protection during mGluRI activation. Closely coupled to the robust neuroprotection by mGluRI activation are the inhibitory phosphorylation and prevention of caspase 3 cleavage of the Forkhead transcription factor FOXO3a, the down-regulation of Bim expression, and the protection of beta-catenin by Akt1 against phosphorylation and degradation to promote its translocation from the cytoplasm to the nucleus and allow it to assist with a "pro-survival" cellular program. Further insight into the cellular mechanisms that determine neuronal protection by the metabotropic glutamate system will foster the successful therapeutic development of mGluRs for neurodegenerative disorders.

Driving cellular plasticity and survival through the signal transduction pathways of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) share a common molecular morphology with other G protein-linked receptors, but there expression throughout the mammalian nervous system places these receptors as essential mediators not only for the initial development of an organism, but also for the vital determination of a cell's fate during many disorders in the nervous system that include amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, Multiple Sclerosis, epilepsy, trauma, and stroke. Given the ubiquitous distribution of these receptors, the mGluR system impacts upon neuronal, vascular, and glial cell function and is activated by a wide variety of stimuli that includes neurotransmitters, peptides, hormones, growth factors, ions, lipids, and light. Employing signal transduction pathways that can modulate both excitatory and inhibitory responses, the mGluR system drives a spectrum of cellular pathways that involve protein kinases, endonucleases, cellular acidity, energy metabolism, mitochondrial membrane potential, caspases, and specific mitogen-activated protein kinases. Ultimately these pathways can converge to regulate genomic DNA degradation, membrane phosphatidylserine (PS) residue exposure, and inflammatory microglial activation. As we continue to push the envelope for our understanding of this complex and critical family of metabotropic receptors, we should be able to reap enormous benefits for both clinical disease as well as our understanding of basic biology in the nervous system.

Contrasting Roles of Protein Kinase C in Induction Versus Suppression of Group I mGluR-Mediated Epileptogenesis In Vitro

Activation of group I metabotropic glutamate receptors (mGluRs) elicits persistent ictaform discharges in guinea pig hippocampal slices, providing an in vitro model of epileptogenesis. The induction of these persistent ictaform bursts is prevented by l-cysteine sulfinic acid (CSA), an agonist at phospholipase D (PLD)–coupled mGluRs. Studies described herein examined the role of protein kinase C (PKC) in both the group I mGluR–mediated induction and CSA-mediated suppression of this form of epileptogenesis. Intracellular recordings were performed from CA3 stratum pyramidale and synchronized burst length was monitored. In the presence of 50 μM picrotoxin, a γ-aminobutyric acid type A antagonist, 250- to 500-ms synchronized bursts were elicited. ( S)-3,5-Dihydroxyphenylglycine (DHPG, 50 μM), an agonist at group I mGluRs, increased the burst length to 1–3 s in duration, a change that persisted after agonist washout. This persistent change in burst length was elicited in the presence of 10 μM chelerythrine, a PKC inhibitor, indicating that DHPG-induced epileptogenesis is PKC independent. However, although PLD activation with CSA (100 μM) was highly effective at suppressing group I mGluR–mediated induction of burst prolongation, CSA application in the presence of chelerythrine was no longer effective and resulted in the expression of persistent ictaform bursts. These data suggest that CSA-mediated suppression of group I mGluR–induced epileptogenesis is PKC dependent. We propose that CSA mediates its effect by PLD-driven activation of PKC, which may desensitize the phospholipase C–linked group I mGluRs and thereby prevent group I mGluR–induced epileptogenesis.