Role of group I metabotropic glutamate receptors in the patterning of epileptiform activities in vitro

Audiogenic kindling activates glutamatergic system in the hippocampus of rats with genetic predisposition to audiogenic seizures

Temporal lobe epilepsy (TLE) development is associated with dysregulation of glutamatergic transmission in the hippocampus; however, detailed molecular mechanisms of pathological changes are still poorly understood. In the present study, we performed the complex analysis of glutamatergic system in the hippocampus of Krushinsky-Molodkina (KM) rats genetically prone to audiogenic seizures (AGS). Daily AGS stimulations (audiogenic kindling) were used to reproduce the dynamics of TLE development. Naïve KM rats were used as a control. After 14 AGS, at the stage of developing TLE, KM rats demonstrated significant upregulation of extracellular signal-regulated kinases (ERK) 1 and 2, cAMP response element-binding protein (CREB), and c-Fos in the hippocampus indicating activation of the hippocampal cells. These changes were accompanied with an increase in glutaminase and vesicular glutamate transporter (VGLUT) 2 suggesting the activation of glutamate production and loading into the synaptic vesicles. After 21 AGS, when TLE was fully-established, alterations were similar but more pronounced, with higher activation of glutaminase, increase in glutamate production, upregulation of VGLUT1 and 2, and Fos-related antigen 1 (Fra-1) along with c-Fos. Analysis of glutamate receptors showed variable changes. Thus, after 14 AGS, simultaneous increase in metabotropic glutamate receptor mGluR1 and decrease in ionotropic N-methyl-D-aspartate (NMDA) receptors could reflect compensatory anti-epileptic mechanism, while further kindling progression induced upregulation of ionotropic receptors, probably, contributing to the hippocampal epileptization. However, we revealed practically no alterations in the expression of synaptic proteins. Altogether, obtained results suggested that overactivation of glutamate production in the hippocampus strongly contributed to TLE development in KM rats.

Sex-dependence of synaptic depression induced by activation of metabotropic glutamate receptors in rat hippocampus

The modulation of synaptic efficacy by group I metabotropic glutamate receptors is dysregulated in several neurodevelopmental and neurodegenerative disorders impacting cognitive function. The progression and severity of these and other disorders are affected by biological sex, and differences in metabotropic glutamate receptor signalling have been implicated in this effect. In this study, we have examined whether there are any sex-dependent differences in a form of long-term depression of synaptic responses that is triggered by application of the group I metabotropic glutamate receptor agonist 3,5-dihydroxyphenylglycine (DHPG). We studied DHPG-induced long-term depression at the Schaffer collateral-commissural pathway in area CA1 of hippocampal slices prepared from three separate age groups of Sprague Dawley rats. In both juvenile (2-week-old) and young adult (3-month-old) rats, there were no differences between sexes in the magnitude of long-term depression. However, in older adult (>1-year-old) rats, DHPG-induced long-term depression was greater in males. In contrast, there were no differences between sexes with respect to basal synaptic transmission or paired-pulse facilitation in any age group. The specific enhancement of metabotropic glutamate receptor-dependent long-term depression in older adult males, but not females, reinforces the importance of considering sex as a factor in the study and treatment of brain disorders.

The Role of Glutamate Receptors in Epilepsy

Glutamate is an essential excitatory neurotransmitter in the central nervous system, playing an indispensable role in neuronal development and memory formation. The dysregulation of glutamate receptors and the glutamatergic system is involved in numerous neurological and psychiatric disorders, especially epilepsy. There are two main classes of glutamate receptor, namely ionotropic and metabotropic (mGluRs) receptors. The former stimulate fast excitatory neurotransmission, are N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), and kainate; while the latter are G-protein-coupled receptors that mediate glutamatergic activity via intracellular messenger systems. Glutamate, glutamate receptors, and regulation of astrocytes are significantly involved in the pathogenesis of acute seizure and chronic epilepsy. Some glutamate receptor antagonists have been shown to be effective for the treatment of epilepsy, and research and clinical trials are ongoing.

Enhanced burst discharges in the CA1 area of the immature versus adult hippocampus: patterns and cellular mechanisms

Burst discharges in the immature brain may contribute to its enhanced seizure susceptibility. The cellular mechanisms underlying burst discharges in the CA1 area of the immature versus adult hippocampus were investigated with simultaneous whole-cell and field-potential recordings. When GABAA receptors were blocked pharmacologically, bursts in CA1 were either graded or all-or-none (or mixed) as a function of electrical stimulation intensity. Most CA1 minislices from immature rats displayed all-or-none or mixed bursts, whereas the slices from adult rats predominantly elicited graded bursts. The frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) were greater in CA1 pyramidal cells from the immature than the adult slices. The developmental differences in CA1 bursting were also detected in slices adjusted for maturational changes in brain volume (i.e., 350 µm thick for immature vs. 450 µm thick for adult rats). Neither N-methyl-d-aspartate (NMDA) nor group I metabotropic glutamate (mGlu1) receptor antagonists blocked the network-driven bursts in immature CA1, but an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor blocker abolished them. Robust excitatory postsynaptic potentials (EPSPs) occurred after bursts in some immature CA1 slices (23%) but never in slices from the adult. The input-output (amount of current injected vs. number of action potentials generated) relationship was markedly greater in CA1 pyramidal cells in the immature compared with the adult hippocampus. These data suggest that the CA1 area of the immature brain is capable of generating network-driven bursts, which declines in adult rats. The increased propensity of burst generation in immature CA1 appears to involve a greater AMPA receptor-mediated synaptic network and an increased intrinsic spike-generating ability.NEW & NOTEWORTHY Burst discharges in the developing brain can provide valuable insights into epileptogenesis. We show that the immature hippocampal CA1 area is capable of generating all-or-none (i.e., network) bursts, which transitions to graded (i.e., nonnetwork) bursts in the mature brain via both synaptic and intrinsic mechanisms. Our results provide new clues to help understand possible mechanisms that may be shared in the immature and epileptic brain and how the normal brain becomes seizure prone (i.e., epileptogenesis).

Glutamate receptors and synaptic plasticity: The impact of Evans and Watkins

On the occasion of the 40 year anniversary of the hugely impactful review by Richard (Dick) Evans and Jeff Watkins, we describe how their work has impacted the field of synaptic plasticity. We describe their influence in each of the major glutamate receptor subtypes: AMPARs, NMDARs, KARs and mGluRs. Particular emphasis is placed on how their work impacted our own studies in the hippocampus. For example, we describe how the tools and regulators that they identified for studying NMDARs (e.g., NMDA, D-AP5 and Mg²⁺) led to the understanding of the molecular basis of the induction of LTP. We also describe how other tools that they introduced (e.g., (1S,3R)-ACPD and MCPG) helped lead to the concept of metaplasticity.

Sensitivity, specificity and limitation of in vitro hippocampal slice and neuron-based assays for assessment of drug-induced seizure liability

An effective in vitro screening assay to detect seizure liability in preclinical development can contribute to better lead molecule optimization prior to candidate selection, providing higher throughput and overcoming potential brain exposure limitations in animal studies. This study explored effects of 26 positive and 14 negative reference pharmacological agents acting through different mechanisms, including 18 reference agents acting on glutamate signaling pathways, in a brain slice assay (BSA) of adult rat to define the assay's sensitivity, specificity, and limitations. Evoked population spikes (PS) were recorded from CA1 pyramidal neurons of hippocampus (HPC) in the BSA. Endpoints for analysis were PS area and PS number. Most positive references (24/26) elicited a concentration-dependent increase in PS area and/or PS number. The negative references (14/14) had little effect on the PS. Moreover, we studied the effects of 15 reference agents testing positive in the BSA on spontaneous activity in E18 rat HPC neurons monitored with microelectrode arrays (MEA), and compared these effects to the BSA results. From these in vitro studies we conclude that the BSA provides 93% sensitivity and 100% specificity in prediction of drug-induced seizure liability, including detecting seizurogenicity by 3 groups of metabotropic glutamate receptor (mGluR) ligands. The MEA results seemed more variable, both quantitatively and directionally, particularly for endpoints capturing synchronized electrical activity. We discuss these results from the two models, comparing each with published results, and provide potential explanations for differences and future directions.

Group I mGluR-Mediated Activation of Martinotti Cells Inhibits Local Cortical Circuitry in Human Cortex

Group I metabotropic glutamate receptors (mGluRs) mediate a range of signaling and plasticity processes in the brain and are of growing importance as potential therapeutic targets in clinical trials for neuropsychiatric and neurodevelopmental disorders (NDDs). Fundamental knowledge regarding the functional effects of mGluRs upon pyramidal neurons and interneurons is derived largely from rodent brain, and their effects upon human neurons are predominantly untested. We therefore addressed how group I mGluRs affect microcircuits in human neocortex. We show that activation of group I mGluRs elicits action potential firing in Martinotti cells, which leads to increased synaptic inhibition onto neighboring neurons. Some other interneurons, including fast-spiking interneurons, are depolarized but do not fire action potentials in response to group I mGluR activation. Furthermore, we confirm the existence of group I mGluR-mediated depression of excitatory synapses in human pyramidal neurons. We propose that the strong increase in inhibition and depression of excitatory synapses onto layer 2/3 pyramidal neurons upon group I mGluR activation likely results in a shift in the balance between excitation and inhibition in the human cortical network.

Carbachol-Induced theta-like oscillations in the rodent brain limbic system: Underlying mechanisms and significance

Theta oscillations (4-12 Hz) represent one of the most prominent physiological oscillatory activity in the mammalian EEG. They are observed in several areas of the hippocampus and in parahippocampal structures. Theta oscillations play important roles in modulating synaptic plasticity during memory and learning; moreover, they are dependent on septal cholinergic inputs. Theta oscillations can be reproduced in vitro in several regions of the temporal lobe in the absence of the septum by employing the cholinergic agonist carbachol (CCh). Here, we review the mechanisms underlying CCh-induced theta oscillations. We address: (i) the ability of temporal lobe neuronal networks to oscillate independently at theta frequency during CCh treatment; (ii) the contribution of intrinsic ionic currents; (iii) the participation of principal cells and interneurons; and (iv) their pharmacological profiles. We also discuss the similarities between CCh-induced theta oscillations and physiological type II theta activity, as well as their roles in synaptic plasticity. Finally, we consider experimental evidence pointing to the contribution of spontaneous and CCh-induced theta activity to epileptiform synchronization.

Lovastatin suppresses hyperexcitability and seizure in Angelman syndrome model

Epilepsy is prevalent and often medically intractable in Angelman syndrome (AS). AS mouse model (Ube3a^m-/p+) shows reduced excitatory neurotransmission but lower seizure threshold. The neural mechanism linking the synaptic dysfunction to the seizure remains elusive. We show that the local circuits of Ube3a^m-/p+in vitro are hyperexcitable and display a unique epileptiform activity, a phenomenon that is reminiscent of the finding in fragile X syndrome (FXS) mouse model. Similar to the FXS model, lovastatin suppressed the epileptiform activity and audiogenic seizures in Ube3a^m-/p+. The in vitro model of Ube3a^m-/p+ is valuable for dissection of neural mechanism and epilepsy drug screening in vivo.

Early-Onset Network Hyperexcitability in Presymptomatic Alzheimer's Disease Transgenic Mice Is Suppressed by Passive Immunization with Anti-Human APP/Aβ Antibody and by mGluR5 Blockade

Cortical and hippocampal network hyperexcitability appears to be an early event in Alzheimer’s disease (AD) pathogenesis, and may contribute to memory impairment. It remains unclear if network hyperexcitability precedes memory impairment in mouse models of AD and what are the underlying cellular mechanisms. We thus evaluated seizure susceptibility and hippocampal network hyperexcitability at ~3 weeks of age [prior to amyloid beta (Aβ) plaque deposition, neurofibrillary pathology, and cognitive impairment] in a triple transgenic mouse model of familial AD (3xTg-AD mouse) that harbors mutated human Aβ precursor protein (APP), tau and presenilin 1 (PS1) genes. Audiogenic seizures were elicited in a higher proportion of 3xTg-AD mice compared with wild type (WT) controls. Seizure susceptibility in 3xTg-AD mice was attenuated either by passive immunization with anti-human APP/Aβ antibody (6E10) or by blockade of metabotropic glutamate receptor 5 (mGluR5) with the selective antagonist, 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP). In in vitro hippocampal slices, suppression of synaptic inhibition with the GABA_A receptor antagonist, bicuculline, induced prolonged epileptiform (>1.5 s in duration) ictal-like discharges in the CA3 neuronal network in the majority of the slices from 3xTg-AD mice. In contrast, only short epileptiform (<1.5 s in duration) interictal-like discharges were observed following bicuculline application in the CA3 region of WT slices. The ictal-like activity in CA3 region of the hippocampus was significantly reduced in the 6E10-immunized compared to the saline-treated 3xTg-AD mice. MPEP acutely suppressed the ictal-like discharges in 3xTg-AD slices. Remarkably, epileptiform discharge duration positively correlated with intraneuronal human (transgenic) APP/Aβ expression in the CA3 region of the hippocampus. Our data suggest that in a mouse model of familial AD, hypersynchronous network activity underlying seizure susceptibility precedes Aβ plaque pathology and memory impairment. This early-onset network hyperexcitability can be suppressed by passive immunization with an anti-human APP/Aβ antibody and by mGluR5 blockade in 3xTg-AD mice.

Theiler's murine encephalomyelitis virus infection of SJL/J and C57BL/6J mice: Models for multiple sclerosis and epilepsy

Mouse models are great tools to study the mechanisms of disease development. Theiler's murine encephalomyelitis virus is used in two distinct viral infection mouse models to study the human diseases multiple sclerosis (MS) and epilepsy. Intracerebral (i.c.) infection of the SJL/J mouse strain results in persistent viral infection of the central nervous system and a MS-like disease, while i.c. infection of the C57BL/6J mouse strain results in acute seizures and epilepsy. Our understanding of how the immune system contributes to the development of two disparate diseases caused by the same virus is presented.

Differential ability of the dorsal and ventral rat hippocampus to exhibit group I metabotropic glutamate receptor-dependent synaptic and intrinsic plasticity

Background:

The hippocampus is critically involved in learning and memory processes. Although once considered a relatively homogenous structure, it is now clear that the hippocampus can be divided along its longitudinal axis into functionally distinct domains, responsible for the encoding of different types of memory or behaviour. Although differences in extrinsic connectivity are likely to contribute to this functional differentiation, emerging evidence now suggests that cellular and molecular differences at the level of local hippocampal circuits may also play a role.

Methods:

In this study, we have used extracellular field potential recordings to compare basal input/output function and group I metabotropic glutamate receptor-dependent forms of synaptic and intrinsic plasticity in area CA1 of slices taken from the dorsal and ventral sectors of the adult rat hippocampus.

Results:

Using two extracellular electrodes to simultaneously record field EPSPs and population spikes, we show that dorsal and ventral hippocampal slices differ in their basal levels of excitatory synaptic transmission, paired-pulse facilitation, and EPSP-to-Spike coupling. Furthermore, we show that slices taken from the ventral hippocampus have a greater ability than their dorsal counterparts to exhibit long-term depression of synaptic transmission and EPSP-to-Spike potentiation induced by transient application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine.

Conclusions:

Together, our results provide further evidence that the information processing properties of local hippocampal circuits differ in the dorsal and ventral hippocampal sectors, and that these differences may in turn contribute to the functional differentiation that exists along the hippocampal longitudinal axis.

APP Causes Hyperexcitability in Fragile X Mice

Amyloid-beta protein precursor (APP) and metabolite levels are altered in fragile X syndrome (FXS) patients and in the mouse model of the disorder, Fmr1KO mice. Normalization of APP levels in Fmr1KO mice (Fmr1KO /APPHET mice) rescues many disease phenotypes. Thus, APP is a potential biomarker as well as therapeutic target for FXS. Hyperexcitability is a key phenotype of FXS. Herein, we determine the effects of APP levels on hyperexcitability in Fmr1KO brain slices. Fmr1KO /APPHET slices exhibit complete rescue of UP states in a neocortical hyperexcitability model and reduced duration of ictal discharges in a CA3 hippocampal model. These data demonstrate that APP plays a pivotal role in maintaining an appropriate balance of excitation and inhibition (E/I) in neural circuits. A model is proposed whereby APP acts as a rheostat in a molecular circuit that modulates hyperexcitability through mGluR5 and FMRP. Both over- and under-expression of APP in the context of the Fmr1KO increases seizure propensity suggesting that an APP rheostat maintains appropriate E/I levels but is overloaded by mGluR5-mediated excitation in the absence of FMRP. These findings are discussed in relation to novel treatment approaches to restore APP homeostasis in FXS.

Alteration of pentylenetetrazole-induced seizure threshold by chronic administration of ginger (Zingiber officinale) extract in male mice

CONTEXT

Zingiber officinale Roscoe (Zingiberaceae), or ginger, used in traditional Chinese medicine, has antioxidant activity and neuroprotective effects. The effects of this plant on clonic seizure have not yet been studied.

OBJECTIVE

The present study evaluated the anticonvulsant effect of ginger in a model of clonic seizures induced with pentylenetetrazole (PTZ) in male mice.

MATERIALS AND METHODS

The anticonvulsant effect of Z. officinale was investigated using i.v. PTZ-induced seizure models in mice. Different doses of the hydroethanolic extract of Z. officinale (25, 50, and 100 mg/kg) were administered intraperitonal (i.p.), daily for 1 week before induction of PTZ. Phenobarbital sodium (30 mg/kg), a reference standard, was also tested for comparison. The effect of ginger on to the appearance of three separate seizure endpoints, e.g., myoclonic, generalized clonic, and tonic extension phase, was recorded.

RESULTS

Hydroethanolic extract of Z. officinale significantly increased the onset time of myoclonic seizure at doses of 25-100 mg/kg (55.33 ± 1.91 versus 24.47 ± 1.33 mg/kg, p < 0.001) and significantly prevented generalized clonic (74.64 ± 3.52 versus 47.72 ± 2.31 mg/kg, p < 0.001) and increased the threshold for the forelimb tonic extension (102.6 ± 5.39 versus 71.82 ± 7.82 mg/kg, p < 0.01) seizure induced by PTZ compared with the control group.

DISCUSSION AND CONCLUSION

Based on the results, the hydroethanolic extract of ginger has anticonvulsant effects, possibly through an interaction with inhibitory and excitatory systems, antioxidant mechanisms, and oxidative stress inhibition.

Pannexin-1-mediated ATP release from area CA3 drives mGlu5-dependent neuronal oscillations

The activation of Group I metabotropic glutamate receptors (GI mGluRs) in the hippocampus results in the appearance of persistent bursts of synchronised neuronal activity. In response to other stimuli, such activity is known to cause the release of the purines ATP and its neuroactive metabolite, adenosine. We have thus investigated the potential release and role of the purines during GI mGluR-induced oscillations in rat hippocampal areas CA3 and CA1 using pharmacological techniques and microelectrode biosensors for ATP and adenosine. The GI mGluR agonist DHPG induced both persistent oscillations in neuronal activity and the release of adenosine in areas CA1 and CA3. In contrast, the DHPG-induced release of ATP was only observed in area CA3. Whilst adenosine acting at adenosine A1 receptors suppressed DHPG-induced burst activity, the activation of mGlu5 and P2Y1 ATP receptors were necessary for the induction of DHPG-induced oscillations. Selective inhibition of pannexin-1 hemichannels with a low concentration of carbenoxolone (10 μM) or probenecid (1 mM) did not affect adenosine release in area CA3, but prevented both ATP release in area CA3 and DHPG-induced bursting. These data reveal key aspects of GI mGluR-dependent neuronal activity that are subject to bidirectional regulation by ATP and adenosine in the initiation and pacing of burst firing, respectively, and which have implications for the role of GI mGluRs in seizure activity and neurodevelopmental disorders.

Enhanced excitatory synaptic network activity following transient group I metabotropic glutamate activation

Prolonged activation of group I metabotropic glutamate receptors (mGluRs) using the agonist (S)-3,5-dihydroxyphenylglycine (DHPG) produces long-lasting changes in the CA3 region of the hippocampal slice. Changes in CA3 pyramidal neuron excitability that follow DHPG exposure result in abnormal network activity manifest by epileptiform activity that consists of interictal and longer lasting ictal epileptiform discharges. In this study we evaluated changes in synaptic activity of CA3 neurons in rat hippocampal slices that occurred after exposure to DHPG. Whole-cell voltage-clamp recordings were made from visually identified CA3 neurons in control artificial cerebrospinal fluid at times greater than 1h after DHPG exposure. Compared to control slices, neurons from slices exposed to DHPG showed enhanced amplitude and frequency of spontaneously occurring excitatory postsynaptic currents (EPSCs) without a concurrent change in inhibitory postsynaptic current (IPSC) amplitude or frequency. Miniature EPSCs were not affected by DHPG exposure but mIPSCs occurred less frequently and were of reduced amplitude. IPSCs recorded in the presence of ionotropic glutamate receptor blockade occurred less frequently in neurons that had been exposed to DHPG. Monosynaptic-evoked IPSPs were also reduced in amplitude in neurons that had been exposed to DHPG. Taken together, these findings demonstrated an enhanced network excitability of the CA3 region and failure of compensatory synaptic inhibition. We propose that prolonged activation of group I mGluR that may occur under conditions of pathological glutamate release results in long-lasting changes in CA3 synaptic network activity and epileptiform activity driven by excessive synaptic excitation.

Acute administration of ginger (Zingiber officinale rhizomes) extract on timed intravenous pentylenetetrazol infusion seizure model in mice

PURPOSE

Zingiber officinale (Zingiberaceae) or ginger, which is used in traditional medicine has antioxidant activity and neuroprotective effects. The effects of this plant on clonic seizure have not yet been studied. The present study evaluated the anticonvulsant effect of ginger in a model of clonic seizures induced with pentylenetetrazole (PTZ) in male mice.

MATERIALS AND METHODS

The anticonvulsant effect of Z. officinale was investigated using i.v. PTZ-induced seizure models in mice. Different doses of the hydroethanolic extract of Z. officinale (25, 50, and 100mg/kg) were administered intraperitonal (i.p.), 2 and 24h before induction of PTZ. Phenobarbital sodium (30mg/kg), a reference standard, was also tested for comparison. The effect of ginger on to the appearance of three separate seizure endpoints (myoclonic, generalized clonus and forelimb tonic extension phase) was recorded.

RESULTS

The results showed that the ginger extract has anticonvulsant effects in all the experimental treatment groups of seizure tested as it significantly increased the seizure threshold. Hydroethanolic extract of Z. officinale significantly increased the onset time of myoclonic seizure at doses of 25-100mg/kg (p<0.001) and significantly prevented generalized clonic (p<0.001) and increased the threshold for the forelimb tonic extension (p<0.01) seizure 2 and 24h before induction of PTZ compared with control group.

CONCLUSION

Based on the results the hydroethanolic extract of ginger has anticonvulsant effects, possibly through an interaction with inhibitory and excitatory system, antioxidant mechanisms, oxidative stress and calcium channel inhibition.

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.

Reduced juvenile long-term depression in tuberous sclerosis complex is mitigated in adults by compensatory recruitment of mGluR5 and Erk signaling

A mouse model of the human genetic disorder tuberous sclerosis complex fails to undergo developmental down-regulation of mGluR5 expression and activation of Erk signaling, probably contributing to the aberrant plasticity and epilepsy in this disease.

Tuberous sclerosis complex (TSC) is a multisystem genetic disease that manifests with mental retardation, tumor formation, autism, and epilepsy. Heightened signaling through the mammalian target of rapamycin (mTOR) pathway is involved in TSC pathology, however it remains unclear how other signaling pathways are perturbed and contribute to disease symptoms. Reduced long-term depression (LTD) was recently reported in TSC mutant mice. We find that although reduced LTD is a feature of the juvenile mutant hippocampus, heightened expression of metabotropic glutamate receptor 5 and constitutively activated Erk signaling in the adult hippocampus drives wild-type levels of LTD. Increased mGluR5 and Erk results in a novel mTOR-independent LTD in CA1 hippocampus of adult mice, and contributes to the development of epileptiform bursting activity in the TSC2^+/− CA3 region of the hippocampus. Inhibition of mGluR5 or Erk signaling restores appropriate mTOR-dependence to LTD, and significantly reduces epileptiform bursting in TSC2^+/− hippocampal slices. We also report that adult TSC2^+/− mice exhibit a subtle perseverative behavioral phenotype that is eliminated by mGluR5 antagonism. These findings highlight the potential of modulating the mGluR5-Erk pathway in a developmental stage-specific manner to treat TSC.

Tuberous sclerosis complex (TSC) is a genetic disorder that afflicts around 1 in 6,000 people and results from a mutation in one of two genes, TSC1 or TSC2. TSC patients suffer a number of neuronal symptoms including various degrees of autism, mental retardation, and epilepsy, the latter found in more than 80% of cases within the first year of life. In the TSC mutant mice that are used to model the disease, a region of the brain called the hippocampus fails to undergo long-term depression (LTD), a neuronal process that is important for learning and memory. We find that while this is the case in juvenile mutant mice, adult mice appear to have fixed this deficit. The “fix” involves the ramping up of signaling pathways involving mGluR5 and Erk. Although increased mGluR5 and Erk signaling outwardly fixes the problem of diminished LTD in adulthood, it renders the brain insensitive to the cues and inputs that normally work to control LTD. Moreover, the hippocampus in adult TSC mice is prone to seizures and impaired in learning and memory tasks. We find that drugs that target mGluR5 or Erk signaling repair the problems with excitability and learning deficits.

Persistent receptor activity underlies group I mGluR-mediated cellular plasticity in CA3 neuron

Plastic changes in cortical activities induced by group I metabotropic glutamate receptor (mGluR) stimulation include epileptogenesis, expressed in vitro as the conversion of normal neuronal activity to persistent, prolonged synchronized (ictal) discharges. At present, the mechanism that maintains group I mGluR-induced plasticity is not known. We examined this issue using hippocampal slices from guinea pigs and mice. Agonist [(S)-3,5-dihydroxyphenylglycine (DHPG), 30-50 μm)] stimulation of group I mGluRs induces persistent prolonged synchronized (ictal-like) discharges in CA3 that are associated with three identified excitatory cellular responses-suppression of spike afterhyperpolarizations, activation of a voltage-dependent cationic current, and increase in neuronal input resistance. Persistent prolonged synchronized discharges and the underlying excitatory cellular responses maintained following induction were reversibly blocked by mGluR1 antagonists [(S)-+-α-amino-4-carboxy-2-methylbenzeneacetic acid (LY 367385), 50, 100 μm; CPCCOEt (hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester, 100 μm], and to a lesser extent by the mGluR5 antagonist MPEP [2-methyl-6-(phenylethynyl)pyridine hydrochloride, 50 μm]. Activation of persistent cellular responses to DHPG were unaffected by tetrodotoxin (0.5-1 μm) or perfusion with low Ca(2+)(0.2 mm)-Mn(2+)(0.5 mm) media-conditions that suppress endogenous glutamate release. The pharmacological profile of the blocking action of the group I mGluR antagonist MCPG [(RS)-α-methyl-4-carboxyphenylglycine, 50-500 μm] on persistent cellular responses was different from that on cellular responses directly activated by DHPG. These data indicate that transient stimulation of group I mGluRs alters receptor properties, rendering them persistently active in the absence of applied agonist or endogenous glutamate activation. Persistent receptor activities, primarily involving mGluR1, maintain excitatory cellular responses and emergent prolonged synchronized discharges.

Antagonists reversibly reverse chemical LTD induced by group I, group II and group III metabotropic glutamate receptors

Metabotropic glutamate (mGlu) receptors are implicated in many neurological and psychiatric diseases and are the targets of therapeutic agents currently in clinical development. Their activation has diverse effects in the central nervous system (CNS) that includes an involvement in synaptic plasticity. We previously reported that the brief exposure of hippocampal slices to dihydroxyphenylglycine (DHPG) can result in a long-term depression (LTD) of excitatory synaptic transmission. Surprisingly, this LTD could be fully reversed by mGlu receptor antagonists in a manner that was itself fully reversible upon washout of the antagonist. Here, 15 years after the discovery of DHPG-LTD and its reversible reversibility, we summarise these initial findings. We then present new data on DHPG-LTD, which demonstrates that evoked epileptiform activity triggered by activation of group I mGlu receptors can also be reversibly reversed by mGlu receptor antagonists. Furthermore, we show that the phenomenon of reversible reversibility is not specific to group I mGlu receptors. We report that activation of group II mGlu receptors in the temporo-ammonic pathway (TAP) and mossy fibre pathway within the hippocampus and in the cortical input to neurons of the lateral amygdala induces an LTD that is reversed by LY341495, a group II mGlu receptor antagonist. We also show that activation of group III mGlu8 receptors induces an LTD at lateral perforant path inputs to the dentate gyrus and that this LTD is reversed by MDCPG, an mGlu8 receptor antagonist. In conclusion, we have shown that activation of representative members of each of the three groups of mGlu receptors can induce forms of LTD than can be reversed by antagonists, and that in each case washout of the antagonist is associated with the re-establishment of the LTD. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Pathological plasticity in fragile X syndrome

Deficits in neuronal plasticity are common hallmarks of many neurodevelopmental disorders. In the case of fragile-X syndrome (FXS), disruption in the function of a single gene, FMR1, results in a variety of neurological consequences directly related to problems with the development, maintenance, and capacity of plastic neuronal networks. In this paper, we discuss current research illustrating the mechanisms underlying plasticity deficits in FXS. These processes include synaptic, cell intrinsic, and homeostatic mechanisms both dependent on and independent of abnormal metabotropic glutamate receptor transmission. We place particular emphasis on how identified deficits may play a role in developmental critical periods to produce neuronal networks with permanently decreased capacity to dynamically respond to changes in activity central to learning, memory, and cognition in patients with FXS. Characterizing early developmental deficits in plasticity is fundamental to develop therapies that not only treat symptoms but also minimize the developmental pathology of the disease.

Metabotropic glutamate receptors for new treatments in schizophrenia

Metabotropic glutamate receptors (mGluRs) represent exciting targets for the development of novel therapeutic agents for schizophrenia. Recent studies indicate that selective activation of specific mGluR subtypes may provide potential benefits for not only the positive symptoms, but also the negative symptoms and cognitive impairments observed in individuals with schizophrenia. Although optimization of traditional orthosteric agonists may still offer a feasible approach for the activation of mGluRs, important progress has been made in the discovery of novel subtype-selective allosteric ligands, including positive allosteric modulators (PAMs) of mGluR2 and mGluR5. These allosteric mGluR ligands have improved properties for clinical development and have served as key preclinical tools for a more in-depth understanding of the potential roles of these different mGluR subtypes for the treatment of schizophrenia.

Metabotropic glutamate receptors regulate hippocampal CA1 pyramidal neuron excitability via Ca²⁺ wave-dependent activation of SK and TRPC channels

Group I metabotropic glutamate receptors (mGluRs) play an essential role in cognitive function. Their activation results in a wide array of cellular and molecular responses that are mediated by multiple signalling cascades. In this study, we focused on Group I mGluR activation of IP3R-mediated intracellular Ca2+ waves and their role in activating Ca2+-dependent ion channels in CA1 pyramidal neurons. Using whole-cell patch-clamp recordings and high-speed Ca2+ fluorescence imaging in acute hippocampal brain slices, we show that synaptic and pharmacological stimulation of mGluRs triggers intracellular Ca2+ waves and a biphasic electrical response composed of a transient Ca2+-dependent SK channel-mediated hyperpolarization and a TRPC-mediated sustained depolarization. The generation and magnitude of the SK channel-mediated hyperpolarization depended solely on the rise in intracellular Ca2+ concentration ([Ca2+]i), whereas the TRPC channel-mediated depolarization required both a small rise in [Ca2+]i and mGluR activation. Furthermore, the TRPC-mediated current was suppressed by forskolin-induced rises in cAMP. We also show that SK- and TRPC-mediated currents robustly modulate pyramidal neuron excitability by decreasing and increasing their firing frequency, respectively. These findings provide additional evidence that mGluR-mediated synaptic transmission makes an important contribution to regulating the output of hippocampal neurons through intracellular Ca2+ wave activation of SK and TRPC channels. cAMP provides an additional level of regulation by modulating TRPC-mediated sustained depolarization that we propose to be important for stabilizing periods of sustained firing.

Persistent ictal-like activity in rat entorhinal/perirhinal cortex following washout of 4-aminopyridine

Application of 4-aminopyridine (4-AP, 100μM) in a solution containing 0.6mM Mg(2+) and 1.2mM Ca(2+) to hippocampal-entorhinal-perirhinal slices of adult rat brain induced ictal-like epileptiform activity in entorhinal and perirhinal cortices as revealed by electrophysiological field potential recordings. The ictal-like activity persisted after washing out the 4-AP. This persistence indicated that a change had occurred in the slice so that it was now "epileptic" in the absence of the convulsant 4-AP. Induction of persistent ictal-like activity was dependent upon the concentration of divalent cations during 4-AP exposure; that is, although 4-AP caused ictal-like activity in approximately half the slices in solution containing 1.6mM Mg(2+) and 2.0mM Ca(2+), this ictal-like activity did not persist upon washout of the 4-AP. Expression of the persistent ictal-like epileptiform activity required ionotropic glutamate-mediated synaptic transmission: application of the AMPA/kainate receptor antagonist NBQX after 4-AP washout reduced persistent ictal-like activity, and the combined application of NBQX and the NMDA receptor antagonist d-AP5 completely blocked it. In order to investigate the mechanism of induction of persistent ictal-like activity, several agents were applied before the introduction of 4-AP. Application of d-AP5 did not block the onset of ictal-like activity upon introduction of 4-AP but did prevent the persistence of the ictal-like activity upon washout of the 4-AP. In contrast, induction of persistent ictal-like activity was not prevented by simultaneous application of the group I metabotropic glutamate receptor (mGluR) antagonists LY 367385 and MPEP or by application of the protein synthesis inhibitor cycloheximide. In conclusion, we have characterized a new in vitro model of epileptogenesis in which induction of ictal-like activity is dependent upon NMDA receptor activation but not upon group I mGluR activation or protein synthesis.

Dual regulation of fragile X mental retardation protein by group I metabotropic glutamate receptors controls translation-dependent epileptogenesis in the hippocampus

Group I metabotropic glutamate receptors (mGluRs) stimulation activates translation-dependent epileptogenesis in the hippocampus. This translation is regulated by repressors, including BC1 RNA and fragile X mental retardation protein (FMRP). Recent data indicate that group I mGluR stimulation exerts bidirectional control over FMRP level by activating translation and ubiquitin-proteasome system (UPS)-dependent proteolysis for the up- and downregulation of the protein, respectively. At present, the temporal relationship of translation and proteolysis on FMRP and their interplay for group I mGluR-mediated translation and epileptogenesis are unknown. We addressed these issues by using mouse hippocampal slices. Agonist [(S)-3,5-dihydroxyphenylglycine (DHPG)] stimulation of group I mGluRs caused a biphasic change in FMRP level. An initial decrease (within 10 min) was followed by an increase at 30 min. When slices were pretreated with translation inhibitor (anisomycin or cycloheximide), group I mGluRs elicited a sustained decrease in FMRP. This decrease was prevented by a proteasome inhibitor [Z-Leu-Leu-Leu-CHO (MG-132)]. When slices were pretreated with MG-132 alone, DHPG no longer elicited any change in FMRP. MG-132 also suppressed increase in other proteins, including postsynaptic density-95 and α-calcium/calmodulin-dependent protein kinase II, normally elicited by group I mGluR stimulation. Physiological experiments showed that proteasome inhibitor suppressed group I mGluR-induced prolonged synchronized discharges. However, proteasome inhibitor did not affect group I mGluR-induced prolonged synchronized discharges in Fmr1(-/-) preparations, where functional FMRP is absent. The results suggest that constitutive FMRP in hippocampal cells acts as a brake on group I mGluR-mediated translation and epileptogenesis. FMRP downregulation via UPS removes this brake enabling group I mGluR-mediated translation and epileptogenesis.

Interneuron subtype specific activation of mGluR1/5 during epileptiform activity in hippocampus

PURPOSE

Specific inhibitory interneurons in area CA1 of the hippocampus, notably those located in stratum oriens-alveus (O/A-INs), are selectively vulnerable in patients and animal models of temporal lobe epilepsy (TLE). The excitotoxic mechanisms underlying the selective vulnerability of interneurons have not been identified but could involve group I metabotropic glutamate receptor subtypes (mGluR1/5), which have generally proconvulsive actions and activate prominent cationic currents and calcium responses specifically in O/A-INs.

METHODS

In this study, we examine the role of mGluR1/5 in interneurons during epileptiform activity using whole-cell recordings from CA1 O/A-INs and selective antagonists of mGluR1α (LY367385) and mGluR5 (MPEP) in a disinhibited rat hippocampal slice model of epileptiform activity.

RESULTS

Our data indicate more prominent epileptiform burst discharges and paroxysmal depolarizations (PDs) in O/A-INs than in interneurons located at the border of strata radiatum and lacunosum/moleculare (R/LM-INs). In addition, mGluR1 and mGluR5 significantly contributed to epileptiform responses in O/A-INs but not in R/LM-INs. Epileptiform burst discharges in O/A-INs were partly dependent on mGluR5. PDs and associated postsynaptic currents were dependent on both mGluR1α and mGluR5. These receptors contributed differently to postsynaptic currents underlying PDs, with mGluR5 contributing to the fast and slow components and mGluR1α to the slow component.

DISCUSSION

These findings support interneuron subtype-specific activation and differential contributions of mGluR1α and mGluR5 to epileptiform activity in O/A-INs, which could be important for their selective vulnerability in TLE.

Distinctions between persistent and reversible group I mGluR-induced epileptiform burst prolongation

We have previously shown that selective activation of group I metabotropic glutamate receptors (mGluRs) results in long-lasting enhancement of synchronized network activity in the hippocampal slice. Data herein suggest that activation of group I mGluRs need not result in this potentially epileptogenic effect. (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (ACPD), a nonselective mGluR agonist, elicits ictaform bursts identical in appearance to those induced by selective agonists, but ACPD-induced bursts do not persist following removal of the agent. Like the bursts induced by selective agonist, the ACPD bursts are blocked with group I mGluR antagonists and are not dependent on activation of either N-methyl-D-aspartate (NMDA) receptors or protein kinase C. However, they differ from the persistent bursts in that they do not require active protein synthesis and they are not suppressed with L-cysteine sulfinic acid, an agonist at a phospholipase D-coupled metabotropic receptor. These novel findings provide evidence that group I mGluR-induced epileptogenesis may be preventable.

Regulatory RNAs in brain function and disorders

Regulatory RNAs are being increasingly investigated in neurons, and important roles in brain function have been revealed. Regulatory RNAs are non-protein-coding RNAs (npcRNAs) that comprise a heterogeneous group of molecules, varying in size and mechanism of action. Regulatory RNAs often exert post-transcriptional control of gene expression, resulting in gene silencing or gene expression stimulation. Here, we review evidence that regulatory RNAs are implicated in neuronal development, differentiation, and plasticity. We will also discuss npcRNA dysregulation that may be involved in pathological states of the brain such as neurodevelopmental disorders, neurodegeneration, and epilepsy.

Both synaptic and intrinsic mechanisms underlie the different properties of population bursts in the hippocampal CA3 area of immature versus adult rats

Pharmacological blockade of GABA(A) receptors on CA3 pyramidal cells in hippocampal slices from immature rats (i.e. second to third postnatal weeks), compared to CA3 slices from adult rats, is known to cause prolonged burst discharges (i.e. several seconds vs. tens of milliseconds). Synaptic and intrinsic mechanisms responsible for this developmental difference in burst duration were analysed in isolated minislices of the CA3 area. The frequency and amplitude of spontaneous EPSCs in CA3 pyramidal cells were greater in slices from immature than mature rats. In the presence of GABA(A)- and GABA(B)-receptor antagonists, the burst discharges of immature CA3 pyramidal cells were still prolonged in thinner slices (350 microm, vs. 450 microm in adults, to compensate for developmental differences in neuronal density) and in NMDA- and mGlu1-receptor antagonists. The AMPA receptor antagonist DNQX blocked the remaining burst discharges, suggesting that differences in recurrent excitatory circuits contributed to the prolonged bursts of immature CA3 pyramidal cells. In slices from immature versus adult rats, the CA3 recurrent synaptic responses showed potentiation to repetitive stimulation, suggestive of a lower transmitter release probability. The intrinsic firing ability was greater in CA3 pyramidal neurons from immature than adult rats, and the medium-duration afterhyperpolarization was smaller. These data suggest that, compared to adults, the CA3 area of immature rats contains a more robust recurrent excitatory synaptic network, greater intrinsic membrane excitability, and an increased capacity for sustained transmitter release, which together may account for the more prolonged network bursts in immature versus adult CA3.

BC1 regulation of metabotropic glutamate receptor-mediated neuronal excitability

Regulatory RNAs have been suggested to contribute to the control of gene expression in eukaryotes. Brain cytoplasmic (BC) RNAs are regulatory RNAs that control translation initiation. We now report that neuronal BC1 RNA plays an instrumental role in the protein-synthesis-dependent implementation of neuronal excitation-repression equilibria. BC1 repression counter-regulates translational stimulation resulting from synaptic activation of group I metabotropic glutamate receptors (mGluRs). Absence of BC1 RNA precipitates plasticity dysregulation in the form of neuronal hyperexcitability, elicited by group I mGluR-stimulated translation and signaled through the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway. Dysregulation of group I mGluR function in the absence of BC1 RNA gives rise to abnormal brain function. Cortical EEG recordings from freely moving BC1(-/-) animals show that group I mGluR-mediated oscillations in the gamma frequency range are significantly elevated. When subjected to sensory stimulation, these animals display an acute group I mGluR-dependent propensity for convulsive seizures. Inadequate RNA control in neurons is thus causally linked to heightened group I mGluR-stimulated translation, neuronal hyperexcitability, heightened gamma band oscillations, and epileptogenesis. These data highlight the significance of small RNA control in neuronal plasticity.

The fragile X mental retardation protein: a valuable partner in the battle against epileptogenesis

Correction of Fragile X Syndrome in Mice. Dölen G, Osterweil E, Rao BSS, Smith GB, Auerbach BD, Chattarji S, Bear MF. Neuron 2007;56:955–962. Fragile X syndrome (FXS) is the most common form of heritable mental retardation and the leading identified cause of autism. FXS is caused by transcriptional silencing of the FMR1 gene that encodes the fragile X mental retardation protein (FMRP), but the pathogenesis of the disease is unknown. According to one proposal, many psychiatric and neurological symptoms of FXS result from unchecked activation of mGluR5, a metabotropic glutamate receptor. To test this idea we generated Fmr1 mutant mice with a 50% reduction in mGluR5 expression and studied a range of phenotypes with relevance to the human disorder. Our results demonstrate that mGluR5 contributes significantly to the pathogenesis of the disease, a finding that has significant therapeutic implications for fragile X and related developmental disorders.

Limbic Epileptogenesis in a Mouse Model of Fragile X Syndrome. Qiu LF, Lu TJ, Hu XL, Yi YH, Liao WP, Xiong ZQ. Cereb Cortex 2009 in press. (doi:10.1093/cercor/bhn163) Fragile X syndrome (FXS), caused by silencing of the Fmr1 gene, is the most common form of inherited mental retardation. Epilepsy is reported to occur in 20–25% of individuals with FXS. However, no overall increased excitability has been reported in Fmr1 knockout (KO) mice, except for increased sensitivity to auditory stimulation. Here, we report that kindling increased the expressions of Fmr1 mRNA and protein in the forebrain of wild-type (WT) mice. Kindling development was dramatically accelerated in Fmr1 KO mice, and Fmr1 KO mice also displayed prolonged electrographic seizures during kindling and more severe mossy fiber sprouting after kindling. The accelerated rate of kindling was partially repressed by inhibiting N-methyl-D-aspartic acid receptor (NMDAR) with MK-801 or mGluR5 receptor with 2-methyl-6-(phenylethynyl)-pyridine (MPEP). The rate of kindling development in WT was not effected by MPEP, however, suggesting that FMRP normally suppresses epileptogenic signaling downstream of metabotropic glutamate receptors. Our findings reveal that FMRP plays a critical role in suppressing limbic epileptogenesis and predict that the enhanced susceptibility of patients with FXS to epilepsy is a direct consequence of the loss of an important homeostatic factor that mitigates vulnerability to excessive neuronal excitation.

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.

Cellular plasticity for group I mGluR-mediated epileptogenesis

Stimulation of group I metabotropic glutamate receptors (mGluRs) by the agonist (S)-dihydroxyphenylglycine in the hippocampus transforms normal neuronal activity into prolonged epileptiform discharges. The conversion is long lasting in that epileptiform discharges persist after washout of the inducing agonist and serves as a model of epileptogenesis. The group I mGluR model of epileptogenesis took on special significance because epilepsy associated with fragile X syndrome (FXS) may be caused by excessive group I mGluR signaling. At present, the plasticity mechanism underlying the group I mGluR-mediated epileptogenesis is unknown. I(mGluR(V)), a voltage-gated cationic current activated by group I mGluR agonists in CA3 pyramidal cells in the hippocampus, is a possible candidate. I(mGluR(V)) activation is associated with group I mGluR agonist-elicited epileptiform discharges. For I(mGluR(V)) to play a role in epileptogenesis, long-term activation of the current must occur after group I mGluR agonist exposure or synaptic stimulation. We observed that I(mGluR(V)), once induced by group I mGluR agonist stimulation in CA3 pyramidal cells, remained undiminished for hours after agonist washout. In slices prepared from FXS model mice, repeated stimulation of recurrent CA3 pyramidal cell synapses, effective in eliciting mGluR-mediated epileptiform discharges, also induced long-lasting I(mGluR(V)) in CA3 pyramidal cells. Similar to group I mGluR-mediated prolonged epileptiform discharges, persistent I(mGluR(V)) was no longer observed in preparations pretreated with inhibitors of tyrosine kinase, of extracellular signal-regulated kinase 1/2, or of mRNA protein synthesis. The results indicate that I(mGluR(V)) is an intrinsic plasticity mechanism associated with group I mGluR-mediated epileptogenesis.

Metabotropic glutamate 1 receptor: current concepts and perspectives

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

Physiology and pathophysiology of canonical transient receptor potential channels

The existence of a mammalian family of TRPC ion channels, direct homologues of TRP, the visual transduction channel of flies, was discovered during 1995-1996 as a consequence of research into the mechanism by which the stimulation of the receptor-Gq-phospholipase Cbeta signaling pathway leads to sustained increases in intracellular calcium. Mammalian TRPs, TRPCs, turned out to be nonselective, calcium-permeable cation channels, which cause both a collapse of the cell's membrane potential and entry of calcium. The family comprises 7 members and is widely expressed. Many cells and tissues express between 3 and 4 of the 7 TRPCs. Despite their recent discovery, a wealth of information has accumulated, showing that TRPCs have widespread roles in almost all cells studied, including cells from excitable and nonexcitable tissues, such as the nervous and cardiovascular systems, the kidney and the liver, and cells from endothelia, epithelia, and the bone marrow compartment. Disruption of TRPC function is at the root of some familial diseases. More often, TRPCs are contributing risk factors in complex diseases. The present article reviews what has been uncovered about physiological roles of mammalian TRPC channels since the time of their discovery. This analysis reveals TRPCs as major and unsuspected gates of Ca(2+) entry that contribute, depending on context, to activation of transcription factors, apoptosis, vascular contractility, platelet activation, and cardiac hypertrophy, as well as to normal and abnormal cell proliferation. TRPCs emerge as targets for a thus far nonexistent field of pharmacological intervention that may ameliorate complex diseases.

Impact of protein kinase C activation on epileptiform activity in the hippocampal slice

There is evidence suggesting that protein kinase C (PKC) activation can prevent the enhanced network excitability associated with status epilepticus and group I metabotropic glutamate receptor (mGluR)-induced epileptogenesis. However, we observed no suppression of mGluR-induced burst prolongation in the guinea pig hippocampal slice when applied in the presence of the PKC activator phorbol-12,13-dibutyrate (PDBu). Furthermore, PDBu alone converted picrotoxin-induced interictal bursts into ictal-length discharges ranging from 2 to 6s in length. This effect could not be elicited by the inactive analog 4-alpha-PDBu and was suppressed with the PKC inhibitor chelerythrine, indicating PKC dependence. PKC activation can enhance neurotransmitter release, and both glutamate and acetylcholine are capable of eliciting similar prolonged synchronized discharges. However, neither mGluR1 nor NMDA receptor antagonist suppressed PDBu-driven burst prolongation, suggesting that increased glutamate release alone is unlikely to account for the PKC-induced expression of ictaform discharges. Similarly, atropine, a broad-spectrum muscarinic receptor antagonist, had no effect on PKC-induced burst prolongation. By contrast, AMPA/kainate receptor antagonist abolished PKC-induced burst prolongation, and mGluR5 antagonist significantly blunted the maximum burst length induced by PKC. These data suggest that PKC-induced prolongation of epileptiform bursts is dependent on changes specific to mGluR5 and AMPA/kainate receptors and not mediated simply by a generalized increase in transmitter release.

The induction of long-term plasticity of non-synaptic, synchronized activity by the activation of group I mGluRs

It is well established that activation of group I metabotropic glutamate receptors (mGluRs) produces long-lasting alterations in synaptic efficacy. We now demonstrate that activation of mGluRs can also induce long-term alterations in synchronised network activity that are both induced and expressed in the absence of chemical synaptic transmission. Specifically, in hippocampal slices in which synaptic transmission was eliminated by perfusing with a Ca2+-free medium, the selective group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) induced a persistent (>3h) enhancement (>2-fold) of the frequency of synchronised bursting activity. The underlying biochemical mechanism responsible for the induction of this form of plasticity was similar to that for DHPG-induced long-term depression (LTD) in that it required the activation of tyrosine phosphatases. Also, like DHPG-induced LTD, this form of neuronal plasticity could be reversed by application of the mGluR antagonist alpha-methyl-4-carboxyphenylglycine (MCPG). This unusual form of plasticity, which presumably also occurs when synaptic transmission is intact, could contribute to long-term alterations in synchronised activity in hippocampal neuronal networks.

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 glutamate receptor-dependent TRPC channel trafficking in hippocampal neurons

The group I metabotropic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine (DHPG) elicited two phases of synchronized neuronal (epileptiform) discharges in hippocampal slices: an initial phase of short duration discharges followed by a phase of prolonged discharges. We assessed the involvement of transient receptor potential canonical (TRPC) channels in these responses. Pre-treatment of hippocampal slices with TRPC channel blockers, 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF96365) or 2-aminoethoxydiphenyl borate, did not affect the short epileptiform discharges but blocked the prolonged epileptiform discharges. SKF96365 suppressed ongoing DHPG-induced prolonged epileptiform discharges. Western blot analysis showed that the total TRPC4 or TRPC5 proteins in hippocampal slices were unchanged following DHPG. DHPG increased TRPC4 and TRPC5 in the cytoplasmic compartment and decreased these proteins in the plasma membrane. Translocation of TRPC4 and TRPC5 was suppressed when the epileptiform discharges were blocked by ionotropic glutamate receptor blockers. Translocation of TRPC4 and TRPC5 was also prevented in slices from phospholipase C (PLC) beta1 knockout mice, even when synchronized discharges were elicited by the convulsant 4-aminopyridine. The results suggest that TRPC channels are involved in generating DHPG-induced prolonged epileptiform discharges. This function of TRPC channels is associated with a neuronal activity- and PLCbeta1-dependent translocation of TRPC4 and TRPC5 proteins from the plasmalemma to the cytoplasmic compartment.

A large-scale mutagenesis screen to identify seizure-resistant zebrafish

PURPOSE

Zebrafish are a vertebrate organism ideally suited to mutagenesis screening strategies. Although a genetic basis for seizure susceptibility and epilepsy is well established, no efforts have been made to study seizure resistance. Here we describe a novel strategy to isolate seizure-resistant zebrafish mutants from a large-scale mutagenesis screen.

METHODS

Seizures were induced with pentylenetetrazole (PTZ). Zebrafish were analyzed between 3 and 7 days postfertilization (dpf). Genome mutations were induced in founders by using N-ethyl-nitrosourea (ENU). Seizure behavior was monitored by using a high-speed camera and quantified by locomotion-tracking software. Electrographic activity was monitored by using a field-recording electrode placed in the optic tectum of agar-immobilized zebrafish.

RESULTS

Short-term PTZ exposure elicited a burst-suppression seizure pattern in 3-dpf zebrafish and more complex activity consisting of interictal- and ictal-like discharges at 7 dpf. Prolonged exposure to PTZ induced status epilepticus-like seizure activity and fatality in wild-type zebrafish larvae. With a PTZ survival assay at 6-7 dpf, we identified six zebrafish mutants in a forward-genetic screen covering nearly 2,000 F(2) families. One mutant (s334) also was shown to exhibit reduced behavioral activity on short-term PTZ exposure and an inability to generate long-duration ictal-like discharge.

CONCLUSIONS

Zebrafish offers a powerful tool for the identification and study of a genetic basis for seizure resistance.

Molecular signaling mechanisms underlying epileptogenesis

Epilepsy, a disorder of recurrent seizures, is a common and frequently devastating neurological condition. Available therapy is only symptomatic and often ineffective. Understanding epileptogenesis, the process by which a normal brain becomes epileptic, may help identify molecular targets for drugs that could prevent epilepsy. A number of acquired and genetic causes of this disorder have been identified, and various in vivo and in vitro models of epileptogenesis have been established. Here, we review current insights into the molecular signaling mechanisms underlying epileptogenesis, focusing on limbic epileptogenesis. Study of different models reveals that activation of various receptors on the surface of neurons can promote epileptogenesis; these receptors include ionotropic and metabotropic glutamate receptors as well as the TrkB neurotrophin receptor. These receptors are all found in the membrane of a discrete signaling domain within a particular type of cortical neuron--the dendritic spine of principal neurons. Activation of any of these receptors results in an increase Ca2+ concentration within the spine. Various Ca2+-regulated enzymes found in spines have been implicated in epileptogenesis; these include the nonreceptor protein tyrosine kinases Src and Fyn and a serine-threonine kinase [Ca2+-calmodulin-dependent protein kinase II (CaMKII)] and phosphatase (calcineurin). Cross-talk between astrocytes and neurons promotes increased dendritic Ca2+ and synchronous firing of neurons, a hallmark of epileptiform activity. The hypothesis is proposed that limbic epilepsy is a maladaptive consequence of homeostatic responses to increases of Ca2+ concentration within dendritic spines induced by abnormal neuronal activity.

Metabotropic glutamate receptor subtype 1 regulates sodium currents in rat neocortical pyramidal neurons

Brain sodium channels (NaChs) are regulated by various neurotransmitters such as acetylcholine, serotonin and dopamine. However, it is not known whether NaCh activity is regulated by glutamate, the principal brain neurotransmitter. We show here that activation of metabotropic glutamate receptor (mGluR) subtype 1 regulates fast transient (I(NaT)) and persistent Na(+) currents (I(NaP)) in cortical pyramidal neurons. A selective agonist of group I mGluR, (S)-3,5-dihydroxyphenylglycine (DHPG), reduced action potential amplitude and decreased I(NaT). This reduction was blocked when DHPG was applied in the presence of selective mGluR1 antagonists. The DHPG-induced reduction of the current was accompanied by a shift of both the inactivation curve of I(NaT) and the activation curve of I(NaP). These effects were dependent on the activation of PKC. The respective role of these two regulatory processes on neuronal excitability was determined by simulating transient and persistent Na(+) conductances (G(NaT) and G(NaP)) with fast dynamic-clamp techniques. The facilitated activation of G(NaP) increased excitability near the threshold, but, when combined with the down-regulation of G(NaT), repetitive firing was strongly decreased. Consistent with this finding, the mGluR1 antagonist LY367385 increased neuronal excitability when glutamatergic synaptic activity was stimulated with high external K(+). We conclude that mGluR1-dependent regulation of Na(+) current depresses neuronal excitability, which thus might constitute a novel mechanism of homeostatic regulation acting during intense glutamatergic synaptic activity.

Metabotropic glutamate receptors and epilepsy

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

Prolonged epileptiform discharges induced by altered group I metabotropic glutamate receptor-mediated synaptic responses in hippocampal slices of a fragile X mouse model

Mutations in FMR1, which encodes the fragile X mental retardation protein (FMRP), are the cause of fragile X syndrome (FXS), an X-linked mental retardation disorder. Inactivation of the mouse gene Fmr1 confers a number of FXS-like phenotypes including an enhanced susceptibility to epileptogenesis during development. We find that in a FXS mouse model, in which the function of FMRP is suppressed, synaptically released glutamate induced prolonged epileptiform discharges resulting from enhanced group I metabotropic glutamate receptor (mGluR)-mediated responses in hippocampal slices. The induction of the group I mGluR-mediated, prolonged epileptiform discharges was inhibited in preparations that were pretreated with inhibitors of ERK1/2 (extracellular signal-regulated kinase 1/2) phosphorylation or of mRNA translation, and their maintenance was suppressed by group I mGluR antagonists. The results suggest that FMRP plays a key role in the control of signaling at the recurrent glutamatergic synapses in the hippocampus. The absence of this control causes the synaptically activated group I mGluRs to elicit translation-dependent epileptogenic activities.

Repeated administration of group I mGluR antagonists prevents seizure-induced long-term aberrations in hippocampal synaptic plasticity

Kindling induced by repeated application of the convulsant pentylenetetrazole (PTZ) is a validated model of epilepsy and epilepsy-related neuromorphological, neurophysiological and behavioural alterations. In this study, we examined whether kindling-induced long-term aberrations in hippocampal synaptic plasticity can be prevented by application of group I mGluR antagonists. Kindling resulted in a higher magnitude of long-term potentiation (LTP) induced by a strong high-frequency stimulation in the hippocampal CA1 region in vitro. When the specific mGluR1 antagonist LY 367385 (0.40 microMol) or the specific mGluR5 inhibitor MPEP (0.06 microMol) were given 30 min prior to PTZ, this kindling-induced enhancement of LTP was almost completely prevented. In addition, application of MPEP led to an impaired maintenance of population spike LTP in kindled animals. LY 367385 applied to unkindled control animals caused a reduction of the initial magnitude of population spike LTP. MPEP, in contrast, left the initial magnitude untouched but resulted in a faster decay of potentiation. A single administration of LY 367385 (200 microM) and MPEP (50 microM), respectively, directly into the bath had almost no effect. Our data suggest that the long-lasting aberrations of hippocampal synaptic plasticity induced by the repeated occurrence of generalized epileptic seizures ultimately require a concurrent operation of mGluR1 and mGluR5.