Glutamate metabotropic receptors as targets for drug therapy in epilepsy

Audiogenic epileptic DBA/2 mice strain as a model of genetic reflex seizures and SUDEP

Epilepsy is a chronic neurological disease characterized by abnormal brain activity, which results in repeated spontaneous seizures. Sudden unexpected death in epilepsy (SUDEP) is the leading cause of seizure-related premature death, particularly in drug-resistant epilepsy patients. The etiology of SUDEP is a structural injury to the brain that is not fully understood, but it is frequently associated with poorly controlled and repeated generalized tonic-clonic seizures (GTCSs) that cause cardiorespiratory and autonomic dysfunctions, indicating the involvement of the brainstem. Both respiratory and cardiac abnormalities have been observed in SUDEP, but not much progress has been made in their prevention. Owing to the complexity of SUDEP, experimental animal models have been used to investigate cardiac and/or respiratory dysregulation due to or associated with epileptic seizures that may contribute to death in humans. Numerous rodent models, especially mouse models, have been developed to better understand epilepsy and SUDEP physiopathology. This review synthesizes the current knowledge about dilute brown agouti coat color (DBA/2) mice as a possible SUDEP model because respiratory arrest (RA) and sudden death induced by audiogenic generalized seizures (AGSs) have been observed in these animals. Respiratory/cardiac dysfunction, brainstem arousal system dysfunction, and alteration of the neurotransmitter systems, which are observed in human SUDEP, have also been observed in these mice. In particular, serotonin (5-HT) alteration and adenosine neurotransmission appear to contribute to not only the pathophysiological mechanisms of medication but also seizure-related respiratory dysfunctions in this animal model. These neurotransmitter systems could be the relevant targets for medication development for chronic epilepsy and SUDEP prevention. We reviewed data on AGSs in DBA/2 mice and the relevance of this model of generalized tonic-clonic epilepsy to human SUDEP. Furthermore, the advantages of using this strain prone to AGSs for the identification of possible new therapeutic targets and treatment options have also been assessed.

Recent Scaffold Hopping Applications in Central Nervous System Drug Discovery

The concept of bioisosterism and the implementation of bioisosteric replacement is fundamental to medicinal chemistry. The exploration of bioisosteres is often used to probe key structural features of candidate pharmacophores and enhance pharmacokinetic properties. As the understanding of bioisosterism has evolved, capabilities to undertake more ambitious bioisosteric replacements have emerged. Scaffold hopping is a broadly used term in the literature referring to a variety of different bioisosteric replacement strategies, ranging from simple heterocyclic replacements to topological structural overhauls. In this work, we have highlighted recent applications of scaffold hopping in the central nervous system drug discovery space. While we have highlighted the benefits of using scaffold hopping approaches in central nervous system drug discovery, these are also widely applicable to other medicinal chemistry fields. We also recommend a shift toward the use of more refined and meaningful terminology within the realm of scaffold hopping.

Changes in Metabotropic Glutamate Receptor Gene Expression in Rat Brain in a Lithium-Pilocarpine Model of Temporal Lobe Epilepsy

Preventing epileptogenesis in people at risk is an unmet medical need. Metabotropic glutamate receptors (mGluRs) are promising targets for such therapy. However, drugs acting on mGluRs are not used in the clinic due to limited knowledge of the involvement of mGluRs in epileptogenesis. This study aimed to analyze the changes in gene expression of mGluR subtypes (1-5, 7, 8) in various rat brain regions in the latent and chronic phases of a lithium-pilocarpine model of epilepsy. For this study, multiplex test systems were selected and optimized to analyze mGluR gene expression using RT-qPCR. Region- and phase-specific changes in expression were revealed. During the latent phase, mGluR5 mRNA levels were increased in the dorsal and ventral hippocampus, and expression of group III genes was decreased in the hippocampus and temporal cortex, which could contribute to epileptogenesis. Most of the changes in expression detected in the latent stage were absent in the chronic stage, but mGluR8 mRNA production remained reduced in the hippocampus. Moreover, we found that gene expression of group II mGluRs was altered only in the chronic phase. The study deepened our understanding of the mechanisms of epileptogenesis and suggested that agonists of group III mGluRs are the most promising targets for preventing epilepsy.

MTEP, a Selective mGluR5 Antagonist, Had a Neuroprotective Effect but Did Not Prevent the Development of Spontaneous Recurrent Seizures and Behavioral Comorbidities in the Rat Lithium-Pilocarpine Model of Epilepsy

Metabotropic glutamate receptors (mGluRs) are expressed predominantly on neurons and glial cells and are involved in the modulation of a wide range of signal transduction cascades. Therefore, different subtypes of mGluRs are considered a promising target for the treatment of various brain diseases. Previous studies have demonstrated the seizure-induced upregulation of mGluR5; however, its functional significance is still unclear. In the present study, we aimed to clarify the effect of treatment with the selective mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP) on epileptogenesis and behavioral impairments in rats using the lithium–pilocarpine model. We found that the administration of MTEP during the latent phase of the model did not improve survival, prevent the development of epilepsy, or attenuate its manifestations in rats. However, MTEP treatment completely prevented neuronal loss and partially attenuated astrogliosis in the hippocampus. An increase in excitatory amino acid transporter 2 expression, which has been detected in treated rats, may prevent excitotoxicity and be a potential mechanism of neuroprotection. We also found that MTEP administration did not prevent the behavioral comorbidities such as depressive-like behavior, motor hyperactivity, reduction of exploratory behavior, and cognitive impairments typical in the lithium–pilocarpine model. Thus, despite the distinct neuroprotective effect, the MTEP treatment was ineffective in preventing epilepsy.

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 II metabotropic glutamate receptor (mGlu2 and mGlu3 ) roles in thalamic processing

BACKGROUND AND PURPOSE

As the thalamus underpins almost all aspects of behaviour, it is important to understand how the thalamus operates. Group II metabotropic glutamate (mGlu₂ /mGlu₃ ) receptor activation reduces inhibition in thalamic nuclei originating from the surrounding thalamic reticular nucleus (TRN). Whilst an mGlu₂ component to this effect has been reported, in this study, we demonstrate that it is likely, largely mediated via mGlu₃ .

EXPERIMENTAL APPROACH

The somatosensory ventrobasal thalamus (VB) is an established model for probing fundamental principles of thalamic function. In vitro slices conserving VB-TRN circuitry from wild-type and mGlu₃ knockout mouse brains were used to record IPSPs and mIPSCs. In vivo extracellular recordings were made from VB neurons in anaesthetised rats. A range of selective pharmacological agents were used to probe Group II mGlu receptor function (agonist, LY354740; antagonist, LY341495; mGlu₂ positive allosteric modulator, LY487379 and mixed mGlu₂ agonist/mGlu₃ antagonist LY395756).

KEY RESULTS

The in vitro and in vivo data are complementary and suggest that mGlu₃ receptor activation is largely responsible for potentiating responses to somatosensory stimulation by reducing inhibition from the TRN.

CONCLUSIONS AND IMPLICATIONS

mGlu₃ receptor activation in the VB likely enables important somatosensory information to be discerned from background activity. These mGlu₃ receptors are likely to be endogenously activated via 'glutamate spillover'. In cognitive thalamic nuclei, this mechanism may be of importance in governing attentional processes. Positive allosteric modulation of endogenous mGlu₃ receptor activation may therefore enhance cognitive function in pathophysiological disease states, such as schizophrenia, thus representing a highly specific therapeutic target.

Group 1 Metabotropic Glutamate Receptors in Neurological and Psychiatric Diseases: Mechanisms and Prospective

Metabotropic glutamate receptors (mGluRs) are G-protein coupled receptors that are activated by glutamate in the central nervous system (CNS). Basically, mGluRs contribute to fine-tuning of synaptic efficacy and control the accuracy and sharpness of neurotransmission. Among eight subtypes, mGluR1 and mGluR5 belong to group 1 (Gp1) family, and are implicated in multiple CNS disorders, such as Alzheimer’s disease, autism, Parkinson’s disease, and so on. In the present review, we systematically discussed underlying mechanisms and prospective of Gp1 mGluRs in a group of neurological and psychiatric diseases, including Alzheimer’s disease, Parkinson’s disease, autism spectrum disorder, epilepsy, Huntington’s disease, intellectual disability, Down’s syndrome, Rett syndrome, attention-deficit hyperactivity disorder, addiction, anxiety, nociception, schizophrenia, and depression, in order to provide more insights into the therapeutic potential of Gp1 mGluRs.

Presynaptic Release-regulating Metabotropic Glutamate Receptors: An Update

Metabotropic glutamate (mGlu) receptors represent the largest family of glutamate receptors in mammals and act as fine tuners of the chemical transmission in central nervous system (CNS). In the last decade, results concerning the expression and the subcellular localization of mGlu receptors further clarified their role in physio-pathological conditions. Concomitantly, their pharmacological characterization largely improved thanks to the identification of new compounds (chemical ligands and antibodies recognizing epitopic sequences of the receptor proteins) that allowed to decipher the protein compositions of the naive receptors. mGlu receptors are expressed at the presynaptic site of chemical synapses. Here, they modulate intraterminal enzymatic pathways controlling the migration and the fusion of vesicles to synaptic membranes as well as the phosphorylation of colocalized receptors. Both the control of transmitter exocytosis and the phosphorylation of colocalized receptors elicited by mGlu receptors are relevant events that dictate the plasticity of nerve terminals, and account for the main role of presynaptic mGlu receptors as modulators of neuronal signalling. The role of the presynaptic mGlu receptors in the CNS has been the matter of several studies and this review aims at briefly summarizing the recent observations obtained with isolated nerve endings (we refer to as synaptosomes). We focus on the pharmacological characterization of these receptors and on their receptor-receptor interaction / oligo-dimerization in nerve endings that could be relevant to the development of new therapeutic approaches for the cure of central pathologies.

Preconditioning by ultra-low dose of tramadol reduces the severity of tramadol-induced seizure: Contribution of glutamate receptors

Tramadol, a weak agonist of mu-opioid receptors, causes seizure via several mechanisms. Preconditioning has been purposed to reduce the epileptic seizures in animal models of epilepsy. The preconditioning effect of tramadol on seizure is not studied yet. This study was designed to evaluate the preconditioning effect of ultra-low dose of tramadol on the seizures induced by tramadol at high dose. Furthermore, regarding the critical role of glutamate signaling in the pathogenesis of epilepsy, the effect of preconditioning on some glutamate signaling elements was also examined. Male Wistar rats received tramadol (2 mg/kg, i.p) or normal saline (1 mL/kg, i.p) in preconditioning and control groups, respectively. After 4 days, the challenging tramadol dose (150 mg/kg) was injected to all rats. Epileptic behaviors were recorded during 50 min. The expression of Norbin (as a regulator of metabotropic glutamate receptor 5), Calponin3 (as a regulator of excitatory synaptic markers), NR1 (NMDA receptor subunit 1) and GluR1 (AMPA receptor subunit 1) was measured in hippocampus, prefrontal cortex (PFC) and amygdala. Preconditioning decreased the number and duration of tremors and tonic-clonic seizures. Norbin, Calponin3, NR1 and GluR1 expression were decreased in hippocampus, and preconditioning had no effect on them. In contrast, it increased Norbin expression in PFC and amygdala, and attenuated NR1 and GluR1 upregulation following tramadol at high dose. These findings indicated that preconditioning by ultra-low dose of tramadol protected the animals against seizures following high dose of tramadol mediated, at least in part, by Norbin up regulation, and NR1 and GluR1 down regulation.

Neuropharmacological potentials of β-carboline alkaloids for neuropsychiatric disorders

Neuropsychiatric disorders are diseases of the central nervous system (CNS) which are characterised by complex pathomechanisms that including homeostatic failure, malfunction, atrophy, pathology remodelling and reactivity anomaly of the neuronal system where treatment options remain challenging. β-Carboline (βC) alkaloids are scaffolds of structurally diverse tricyclic pyrido[3,4-b]indole alkaloid with vast occurrence in nature. Their unique structural features which favour interactions with enzymes and protein receptor targets account for their potent neuropharmacological properties. However, our current understanding of their biological mechanisms for these beneficial effects, especially for neuropsychiatric disorders is sparse. Therefore, we present a comprehensive review of the scientific progress in the last two decades on the prospective pharmacology and physiology of the βC alkaloids in the treatment of some neuropsychiatric conditions such as depression, anxiety, Alzheimer's disease, Parkinson's disease, brain tumour, essential tremor, epilepsy and seizure, licking behaviour, dystonia, agnosia, spasm, positive ingestive response as demonstrated in non-clinical models. The current evidence supports that βC alkaloids offer potential therapeutic agents against most of these disorders and amenable for further drug design.

Neuropolybin: A new antiseizure peptide obtained from wasp venom

Epilepsy accounts for one of the most serious neurological disorders, and its treatment remains a challenge, due to high cost and harmful side effects. Bioactive molecules extracted from arthropod venoms are considered a promising therapy since these compounds are known for their highly selective and potent profiles. The purpose of this study was to identify and characterize the potential antiseizure effect of the peptide Ppnp7, extracted from the venom of the social wasp Polybia paulista, and also the effect of the bioinspired peptide, named Neuropolybin, in the same parameters. Additionally, we also evaluated the electroencephalographic (EEG) profile in the PTZ-induced acute seizures in animals treated with Neuropolybin, and potential adverse effects of both peptides in general spontaneous activity (Open Field analysis). Interestingly, Ppnp7 and Neuropolybin showed a noteworthy antiseizure effect in rats and mice, respectively. Curves of protection against the maximum seizure were obtained for both peptides, and EEG records demonstrated that Neuropolybin protected 80% of animals from tonic-clonic seizures when applied with a dose of 3 nmol (an approximate Ppnp7 ED₅₀ found in rats). Neuropolybin and Ppnp7 did not cause changes in the general spontaneous activity of the animals in any of the doses evaluated. Therefore, this study demonstrated how compounds isolated from wasps' venom may be essential resources in the search for new drugs, and can also be considered valuable therapeutic and biotechnological tools for the study and future treatment of epileptic disorders.

Radiosynthesis of the diastereomerically pure (E)-[11 C]ABP688

We report an efficient protocol for the radiosynthesis of diastereomerically pure (E)-[¹¹ C]ABP688, a positron emission tomography (PET) tracer for metabotropic glutamate type 5 (mGlu5) receptor imaging. The protocol reliably provides sterile and pyrogen-free formulation of (E)-[¹¹ C]ABP688 suitable for preclinical and clinical PET imaging with >99% diastereomeric excess (d.e.), >99% overall radiochemical purity (RCP), 14.9 ± 4.3% decay-corrected radiochemical yield (RCY), and 148.86 ± 79.8 GBq/μmol molar activity in 40 minutes from the end of bombardment.

Neurochemical investigation of multiple locally induced seizures using microdialysis sampling: Epilepsy effects on glutamate release

The objective of this study was to develop an in vivo model for locally induced epilepsy. Epilepsy is a prominent neurological disorder that affects millions of people worldwide. Patients may experience either global seizures, affecting the entire brain, or focal seizures, affecting only one brain region. The majority of epileptic patients experience focal seizures but they go undiagnosed because such seizures can be difficult to detect. To better understand the effects of focal epilepsy on the neurochemistry of a brain region with high seizure diathesis, an animal model for locally induced seizures in the hippocampus was developed. In this model, two seizure events were chemically induced by administering the epileptogenic agent, 3-mercaptopropionic acid (3-MPA), to the hippocampus to disturb the balance between excitatory and inhibitory neurotransmitters in the brain. Microdialysis was used for local delivery of 3-MPA as well as for collection of dialysate for neurochemical analyses. Two periods of seizures separated by varying inter-seizure recovery times were employed, and changes in the release of the excitatory transmitter, glutamate, were measured. Significant differences in glutamate release were observed between the first and second seizure episodes. Diminished glutamate biosynthesis, enhanced glutamate re-uptake, and/or neuronal death were considered possible causes of the attenuated glutamate release during the second seizure episode. Biochemical measurements were indicative that a combination of these factors led to the attenuation in glutamate release.

Effects of GRM4, SCN2A and SCN3B polymorphisms on antiepileptic drugs responsiveness and epilepsy susceptibility

BACKGROUND

Pharmacotherapy of epilepsy including antiepileptic drugs (AEDs) is one of the main treatment approaches. As a biological target, sodium channels (Nav channels) and glutamate receptor genes are playing a major role in the etiology and treatment of epilepsy.

OBJECTIVE

This study aims to investigate the genetic associations of certain genetic polymorphisms with increased risk of epilepsy susceptibility and variability in response to AEDs treatment in a Jordanian Arab population.

METHOD

A pharmacogenetics and case-control study on 296 unrelated epileptic Jordanian patients recruited from the pediatric neurology clinic at the Queen Rania Al-Abdullah Hospital (QRAH) in Amman, Jordan and 299 healthy individuals was conducted. Children up to 15 years old which receiving AEDs for at least three months were scanned for genetic association of 7 single nucleotide polymorphisms (SNPs) within three candidate genes (SCN2A, SCN3B and GRM4) with epilepsy susceptibility.

RESULTS

SCN2A rs2304016 (P = 0.04) and GRM4 rs2499697 (P = 0.031) were statistically significant with generalized epilepsy. Haplotype of CAACG GRM4 was genetically associated with epilepsy and partial epilepsy (P = 0.036; P = 0.024, respectively). This study also found that TGTAA genetic haplotype formed within GRM4 gene was associated with generalized epilepsy susceptibility (P = 0.006). While, no significant linkage of SCN3B rs3851100 to either disease susceptibility or drug responsiveness was found.

CONCLUSION

This study identified no significant associations of allelic or genotypic SNPs with the susceptibility of epilepsy and medication response with an exception of rs2304016 and rs2499697 SNPs that were associated with the generalized type of epilepsy among Jordanian population. Further studies are required in different populations to confirm our results and identify genetic factors that involved in susceptibility and treatment response.

Synaptic adhesion protein ELFN1 is a selective allosteric modulator of group III metabotropic glutamate receptors in trans

Functional characterization of the GPCR interactome has been focused predominantly on intracellular interactions, yet GPCRs are increasingly found in complex with extracellular proteins. Extracellular leucine-rich repeat fibronectin type III domain containing 1 (ELFN1) was recently reported to physically anchor mGluR6 and mGluR7 across retinal and hippocampal synapses, respectively; however, the consequence of transsynaptic interactions on properties and pharmacology of these receptors are unknown. In the current study, we explore the effects of ELFN1 on mGluR signaling and pharmacology. First, we established the binding specificity of ELFN1 and found it to be recruited selectively to all group III mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8), but not other mGluR species. Using site-directed mutagenesis we mapped binding determinants of this interaction to two distinct sites on the ELFN1 ectodomain. To evaluate functional aspects of the interaction, we developed a transcellular signaling assay in reconstituted HEK293 cells which monitors changes in mGluR activity in one cell following its exposure to separate ELFN1-containing cells. Using this platform, we found that ELFN1 acts as an allosteric modulator of class III mGluR activity in suppressing cAMP accumulation: altering both agonist-induced and constitutive receptor activity. Using bioluminescence resonance energy transfer-based real-time kinetic assays, we established that ELFN1 alters the ability of mGluRs to activate G proteins. Our findings demonstrate that core properties of class III mGluRs can be altered via extracellular interactions with ELFN1 which serves as a transsynaptic allosteric modulator for these receptors. Furthermore, our unique assay platform opens avenues for exploring transcellular/transsynaptic pharmacology of other GPCR transcomplexes.

Hippocampal protein kinase D1 is necessary for DHPG-induced learning and memory impairments in rats

Background

Understanding molecular mechanisms underlying the induction of learning and memory impairments remains a challenge. Recent investigations have shown that the activation of group I mGluRs (mGluR1 and mGluR5) in cultured hippocampal neurons by application of (S)-3,5-Dihydroxyphenylglycine (DHPG) causes the regulated internalization of N-methyl-D-aspartate receptors (NMDARs), which subsequently activates protein kinase D1 (PKD1). Through phosphorylating the C-terminals of the NMDAR GluN2 subunits, PKD1 down-regulates the activity of remaining (non-internalized) surface NMDARs. The knockdown of PKD1 does not affect the DHPG-induced inhibition of AMPA receptor-mediated miniature excitatory post-synaptic currents (mEPSCs) but prevents the DHPG-induced inhibition of NMDAR-mediated mEPSCs in vitro. Thus, we investigated the in vivo effects of bilateral infusions of DHPG into the hippocampal CA1 area of rats in the Morris water maze (MWM) and the novel object discrimination (NOD) tests.

Methods

A total of 300 adult male Sprague Dawley rats (250–280 g) were used for behavioral tests. One hundred ninety four were used in MWM test and the other 106 rats in the NOD test. Following one week of habituation to the vivarium, rats were bilaterally implanted under deep anesthesia with cannulas aimed at the CA1 area of the hippocampus (CA1 coordinates in mm from Bregma: AP -3.14; lateral +/-2; DV -3.0). Through implanted cannulas artificial cerebrospinal fluid (ACSF), the group1 mGluR antagonist 6-Methyl-2-(phenylethynyl)pyridine (MPEP), the dynamin-dependent internalization inhibitor Dynasore, or the PKD1 inhibitor CID755673 were infused into the bilateral hippocampal CA1 areas (2 μL per side, over 5 min). The effects of these infusions and the effects of PKD1 knockdown were examined in MWM or NOD test.

Results

DHPG infusion increased the latency to reach the platform in the MWM test and reduced the preference for the novel object in the NOD task. We found that the DHPG effects were dose-dependent and could be maintained for up to 2 days. Notably, these effects could be prevented by pre-infusion of the group1 mGluR antagonist MPEP, the dynamin-dependent internalization inhibitor Dynasore, the PKD1 inhibitor CID755673, or by PKD1 knockdown in the hippocampal CA1 area.

Conclusion

Altogether, these findings provide direct evidence that PKD1-mediated signaling may play a critical role in the induction of learning and memory impairments by DHPG infusion into the hippocampal CA1 area.

Potent and selective pharmacodynamic synergy between the metabotropic glutamate receptor subtype 2-positive allosteric modulator JNJ-46356479 and levetiracetam in the mouse 6-Hz (44-mA) model

OBJECTIVE

We previously demonstrated that positive allosteric modulators (PAMs) of metabotropic glutamate subtype 2 (mGlu₂ ) receptors have potential synergistic interactions with the antiseizure drug levetiracetam (LEV). The present study utilizes isobolographic analysis to evaluate the combined administration of JNJ-46356479, a selective and potent mGlu₂ PAM, with LEV as well as sodium valproate (VPA) and lamotrigine (LTG).

METHODS

The anticonvulsant efficacy of JNJ-46356479 was evaluated in the 6-Hz model of psychomotor seizures in mice. JNJ-46356479 was administered in combination with LEV using 3 fixed dose-ratio treatment groups in the mouse 6-Hz (44-mA) seizure test. The combination of JNJ-46356479 with LEV was also evaluated in the mouse corneal kindling model. The potential interactions of JNJ-46356479 with the antiseizure drugs VPA and LTG were also evaluated using fixed dose-ratio combinations. Plasma levels were obtained for analysis of potential pharmacokinetic interactions for each combination studied in the mouse 6-Hz model.

RESULTS

JNJ-46356479 was active in the 6-Hz model at both 32-mA and 44-mA stimulus intensities (median effective dose = 2.8 and 10.2 mg/kg, respectively). Using 1:1, 1:3, and 3:1 fixed dose-ratio combinations (LEV:JNJ-46356479), coadministration was significantly more potent than predicted for additive effects, and plasma levels suggest this synergism was not due to pharmacokinetic interactions. Studies in kindled mice further demonstrate the positive pharmacodynamic interaction of LEV with JNJ-46356479. Using 1:1 dose-ratio combinations of JNJ-46356479 with either VPA or LTG, there were no significant differences observed for coadministration.

SIGNIFICANCE

These studies demonstrate a synergistic interaction of JNJ-46356479 with LEV, whereas no such effect occurred for JNJ-46356479 with either VPA or LTG. The synergy seems therefore to be specific to LEV, and the combination LEV/mGlu₂ PAM has the potential to result in a rational polypharmacy approach to treat patients with refractory epilepsy, once it has been confirmed in clinical studies.

Novel Targets for Developing Antiseizure and, Potentially, Antiepileptogenic Drugs

Epilepsy is a chronic neurological disorder caused by abnormal changes in the functions of neuronal circuits and manifested by seizures. It affects patients of all age, substantially worsens the quality of life for the patients as well as their families, and imposes a huge economic burden on the healthcare system. Historically, efforts for discovering and developing antiseizure therapies have been focused on modulating the functions of receptors, transporters, and enzymes expressed by neurons. These drug development efforts have paid off, as we have over 25 antiseizure drugs available in the clinic. However, these drugs mainly provide symptomatic relief from seizures and often cause serious adverse effects. Importantly, almost one-third of patients with epilepsy do not have their seizures adequately controlled by available drugs. To address this problem, researchers are investigating cellular and molecular mechanisms fundamental to the optimal function of neuronal circuits. Evidence shows that disruptions in these mechanisms cause impairment in neuroglial interactions, uncontrolled inflammation, aberrant synaptogenesis, and neurodegeneration in genetic and acquired epilepsies. Many novel therapeutic targets have been identified to target these mechanisms for developing new antiseizure drugs. In addition, the field is exploring new drug targets which may impede the development of epilepsy. We have summarized some of these novel targets in this brief review.

Genetically epilepsy-prone rats (GEPRs) and DBA/2 mice: Two animal models of audiogenic reflex epilepsy for the evaluation of new generation AEDs

This review summarizes the current knowledge about DBA/2 mice and genetically epilepsy-prone rats (GEPRs) and discusses the contribution of such animal models on the investigation of possible new therapeutic targets and new anticonvulsant compounds for the treatment of epilepsy. Also, possible chemical or physical agents acting as proconvulsant agents are described. Abnormal activities of enzymes involved in catecholamine and serotonin synthesis and metabolism were reported in these models, and as a result of all these abnormalities, seizure susceptibility in both animals is greatly affected by pharmacological manipulations of the brain levels of monoamines and, prevalently, serotonin. In addition, both genetic epileptic models permit the evaluation of pharmacodynamic and pharmacokinetic interactions among several drugs measuring plasma and/or brain level of each compound. Audiogenic models of epilepsy have been used not only for reflex epilepsy studies, but also as animal models of epileptogenesis. The seizure predisposition (epileptiform response to sound stimulation) and substantial characterization of behavioral, cellular, and molecular alterations in both acute and chronic (kindling) protocols potentiate the usefulness of these models in elucidating ictogenesis, epileptogenesis, and their mechanisms. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Epilepsy, Antiepileptic Drugs, and Aggression: An Evidence-Based Review

Antiepileptic drugs (AEDs) have many benefits but also many side effects, including aggression, agitation, and irritability, in some patients with epilepsy. This article offers a comprehensive summary of current understanding of aggressive behaviors in patients with epilepsy, including an evidence-based review of aggression during AED treatment. Aggression is seen in a minority of people with epilepsy. It is rarely seizure related but is interictal, sometimes occurring as part of complex psychiatric and behavioral comorbidities, and it is sometimes associated with AED treatment. We review the common neurotransmitter systems and brain regions implicated in both epilepsy and aggression, including the GABA, glutamate, serotonin, dopamine, and noradrenaline systems and the hippocampus, amygdala, prefrontal cortex, anterior cingulate cortex, and temporal lobes. Few controlled clinical studies have used behavioral measures to specifically examine aggression with AEDs, and most evidence comes from adverse event reporting from clinical and observational studies. A systematic approach was used to identify relevant publications, and we present a comprehensive, evidence-based summary of available data surrounding aggression-related behaviors with each of the currently available AEDs in both adults and in children/adolescents with epilepsy. A psychiatric history and history of a propensity toward aggression/anger should routinely be sought from patients, family members, and carers; its presence does not preclude the use of any specific AEDs, but those most likely to be implicated in these behaviors should be used with caution in such cases.

Astrocytes modulate thalamic sensory processing via mGlu2 receptor activation

Astrocytes possess many of the same signalling molecules as neurons. However, the role of astrocytes in information processing, if any, is unknown. Using electrophysiological and imaging methods, we report the first evidence that astrocytes modulate neuronal sensory inhibition in the rodent thalamus.

We found that mGlu2 receptor activity reduces inhibitory transmission from the thalamic reticular nucleus to the somatosensory ventrobasal thalamus (VB): mIPSC frequencies in VB slices were reduced by the Group II mGlu receptor agonist LY354740, an effect potentiated by mGlu2 positive allosteric modulator (PAM) LY487379 co-application (30 nM LY354740: 10.0 ± 1.6% reduction; 30 nM LY354740 & 30 μM LY487379: 34.6 ± 5.2% reduction).

We then showed activation of mGlu2 receptors on astrocytes: astrocytic intracellular calcium levels were elevated by the Group II agonist, which were further potentiated upon mGlu2 PAM co-application (300 nM LY354740: ratio amplitude 0.016 ± 0.002; 300 nM LY354740 & 30 μM LY487379: ratio amplitude 0.035 ± 0.003).

We then demonstrated mGlu2-dependent astrocytic disinhibition of VB neurons in vivo: VB neuronal responses to vibrissae stimulation trains were disinhibited by the Group II agonist and the mGlu2 PAM (LY354740: 156 ± 12% of control; LY487379: 144 ± 10% of control). Presence of the glial inhibitor fluorocitrate abolished the mGlu2 PAM effect (91 ± 5% of control), suggesting the mGlu2 component to the Group II effect can be attributed to activation of mGlu2 receptors localised on astrocytic processes within the VB.

Gating of thalamocortical function via astrocyte activation represents a novel sensory processing mechanism. As this thalamocortical circuitry is important in discriminative processes, this demonstrates the importance of astrocytes in synaptic processes underlying attention and cognition.

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Thalamic inhibition is mediated by both neuronal and astrocytic mechanisms.

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Group II mGlu receptor (mGlu2/3) activation can modulate this thalamic inhibition.

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Thalamic astrocytes can be activated upon mGlu2 receptor stimulation.

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This process may enable relevant activity to be discerned from background noise.

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Targeting astrocytic mGlu2 receptors may therefore affect attention and cognition.

Efficacy of mGlu2 -positive allosteric modulators alone and in combination with levetiracetam in the mouse 6 Hz model of psychomotor seizures

OBJECTIVE

The metabotropic glutamate receptor subtype 2 (mGlu₂ ) possesses both orthosteric and allosteric modulatory sites, are expressed in the frontal cortex and limbic structures, and can affect excitatory synaptic transmission. Therefore, mGlu₂ is a potential therapeutic target in the treatment of epilepsy. The present study seeks to evaluate the anticonvulsant potential of mGlu₂ -acting compounds.

METHODS

The anticonvulsant efficacy of two selective mGlu₂ -positive allosteric modulators (PAMs) (JNJ-42153605 and JNJ-40411813/ADX71149) and one mGlu_2/3 receptor agonist (LY404039) were evaluated alone and in combination with the antiseizure drug levetiracetam (LEV) in the mouse 6 Hz model.

RESULTS

In the 6 Hz (32 mA stimulus intensity) model, median effective dose (ED₅₀ ) values were determined for JNJ-42153605 (3.8 mg/kg), JNJ-40411813 (12.2 mg/kg), and LY404039 (10.9 mg/kg). At the 44 mA stimulus intensity, ED₅₀ values were determined for JNJ-42153605 (5.9 mg/kg), JNJ-40411813 (21.0 mg/kg), LY404039 (14.1 mg/kg), and LEV (345 mg/kg). In addition, subprotective doses of each mGlu₂ -acting compound, administered in combination with various doses of LEV, were able to shift the 6 Hz 44 mA ED₅₀ for LEV by >25-fold. When JNJ-42153605 was administered at varying doses in combination with a single dose of LEV (10 mg/kg), the potency of JNJ-42153605 was increased 3.7-fold. Similarly, when a moderately effective dose of LEV (350 mg/kg) was administered in combination with varying doses of JNJ-40411813, the potency of JNJ-40411813 was increased approximately 14-fold. Plasma levels of JNJ-40411813 and LEV were not different when administered alone or in combination, suggesting that increases in potency are not due to pharmacokinetic effects.

SIGNIFICANCE

These studies suggest a potential positive pharmacodynamic effect of mGlu₂ -acting compounds in combination with LEV. If this effect is translated in a clinical setting, it can support a rational polypharmacy concept in treatment of epilepsy patients.

Autoimmune Aspects of Neurodegenerative and Psychiatric Diseases: A Template for Innovative Therapy

Neurodegenerative and psychiatric diseases (NPDs) are today's most important group of diseases, surpassing both atherosclerotic cardiovascular disease and cancer in morbidity incidence. Although NPDs have a dramatic impact on our society because of their high incidence, mortality, and severe debilitating character, remarkably few effective interventions have become available. The current treatments, if available, comprise the lifelong intake of general immunosuppressants to delay disease progression or neurotransmitter antagonists/agonists to dampen undesired behaviors. The long-term usage of such medication, however, coincides with often severe adverse side effects. There is, therefore, an urgent need for safe and effective treatments for these diseases. Here, we discuss that many NPDs coincide with subtle chronic or flaring brain inflammation sometimes escalating with infiltrations of lymphocytes in the inflamed brain parts causing mild to severe or even lethal brain damage. Thus, NPDs show all features of autoimmune diseases. In this review, we postulate that NPDs resemble autoimmune-driven inflammatory diseases in many aspects and may belong to the same disease spectrum. Just like in autoimmune diseases, NPD symptoms basically are manifestations of a chronic self-sustaining inflammatory process with detrimental consequences for the patient. Specific inhibition of the destructive immune responses in the brain, leaving the patient's immune system intact, would be the ultimate solution to cure patients from the disease. To reach this goal, the primary targets, e.g., the primary self-antigens (pSAgs) of the patient's chronic (auto)immune response, need to be identified. For a few major NPDs, immunological studies led to the identification of the pSAgs involved in the autoimmune damage of specific brain parts. However, further research is needed to complete the list of pSAgs for all NPDs. Such immunological studies will not only provide crucial insights into NPD pathogenesis but also ultimately enable the development of a new generation of safe and effective immunotherapies for NPDs. Interventions that will dramatically improve the life expectancy and quality of life of individual patients and, moreover, will significantly reduce the health-care costs of the society in general.

2-Methyl-6-(phenylethynyl)pyridine Hydrochloride Modulates Metabotropic Glutamate 5 Receptors Endogenously Expressed in Zebrafish Brain

Due to phylogenetic proximity to the human, zebrafish has been recognized as a reliable model to study Alzheimer's disease (AD) and other central nervous system disorders. Furthermore, metabotropic glutamate receptors have been previously reported to be impaired in brain from AD patients. Metabotropic glutamate 5 (mGlu₅) receptors are G-protein coupled receptors proposed as potential targets for therapy of different neurodegenerative disorders. Thus, MPEP (2-methyl-6-(phenylethynyl)pyridine hydrochloride), a selective noncompetitive mGlu₅ receptor antagonist, has been suggested for pharmacological treatment of AD. The aim of the present work was to quantify mGlu₅ receptors in brain from zebrafish and to study the possible modulation of these receptors by MPEP treatment. To this end, radioligand binding assay and open field test were used. Results showed a slightly higher presence of mGlu₅ receptors in brain from male than in that from female zebrafish. However, a significant increase of mGlu₅ receptor in male without variation in female was observed after MPEP treatment. This gender specific response was also observed in locomotor behavior, being significantly decreased only in male zebrafish. These results confirm the presence of mGlu₅ receptors in brain from zebrafish and their gender specific modulation by selective antagonist treatment and suggest a role of these receptors on locomotor activity, which is affected in many disorders. In addition, our data point to zebrafish as a useful model to study mGlu receptor function in both healthy and pathological conditions.

5-HT2A and mGlu2/3 receptor interactions: on their relevance to cognitive function and psychosis

Serotonin [5-hydroxytryptamine (5-HT)] and glutamate have both been implicated in the pathophysiology of neuropsychiatric disorders but also in the mechanism of antipsychotic and hallucinogenic drug actions. Furthermore, close antagonistic interactions between 5-HT2A and metabotropic glutamate (mGlu)2/3 receptors have been established over the last decades on the basis of numerous electrophysiological, biochemical, and behavioral studies. Besides synaptic mechanisms, more recent findings suggested that heterodimeric 5-HT2A-mGlu2 receptor complexes in the prefrontal cortex may account for the functional crosstalk between these two receptor subtypes. In this review, we focus on in-vitro and in-vivo studies documenting the important relationship between 5-HT2A and mGlu2/3 receptors, with relevance to both normal behavioral function and psychosis.

Influence of MPEP (a selective mGluR5 antagonist) on the anticonvulsant action of novel antiepileptic drugs against maximal electroshock-induced seizures in mice

The aim of this study was to determine the effects of 2-methyl-6-(phenylethynyl)pyridine (MPEP - a selective antagonist for the glutamate metabotropic receptor subtype mGluR5) on the protective action of some novel antiepileptic drugs (lamotrigine, oxcarbazepine, pregabalin and topiramate) against maximal electroshock-induced seizures in mice. Brain concentrations of antiepileptic drugs were measured to determine whether MPEP altered pharmacokinetics of antiepileptic drugs. Intraperitoneal injection of 1.5 and 2mg/kg of MPEP significantly elevated the threshold for electroconvulsions in mice, whereas MPEP at a dose of 1mg/kg considerably enhanced the anticonvulsant activity of pregabalin and topiramate, but not that of lamotrigine or oxcarbazepine in the maximal electroshock-induced seizures in mice. Pharmacokinetic results revealed that MPEP (1mg/kg) did not alter total brain concentrations of pregabalin and topiramate, and the observed effect in the mouse maximal electroshock seizure model was pharmacodynamic in nature. Collectively, our preclinical data suggest that MPEP may be a safe and beneficial adjunct to the therapeutic effects of antiepileptic drugs in human patients.

In the grey zone between epilepsy and schizophrenia: alterations in group II metabotropic glutamate receptors

Glutamate is the major excitatory neurotransmitter in the brain. The glutamate system plays an important role in the formation of synapses during brain development and synaptic plasticity. Dysfunctions in glutamate regulation may lead to hyperexcitatory neuronal networks and neurotoxicity. Glutamate excess is possibly of great importance in the pathophysiology of several neurological and psychiatric disorders such as epilepsy and schizophrenia. Interestingly, cross talk between these disorders has been well documented: psychiatric comorbidities are frequent in epilepsy and temporal lobe epilepsy is one of the highest risk factors for developing psychosis. Therefore, dysfunctions in glutamatergic neurotransmission might constitute a common pathological mechanism. A major negative feedback system is regulated by the presynaptic group II metabotropic glutamate (mGlu) receptors including mGlu2/3 receptors. These receptors are predominantly localised extrasynaptically in basal ganglia and limbic structures. Hence, mGlu2/3 receptors are an interesting target for the treatment of disorders like epilepsy and schizophrenia. A dysfunction in the glutamate system may be associated with alterations in mGlu2/3 receptor expression. In this review, we describe the localization of mGlu2/3 receptors in the healthy brain of mice, rats and humans. Secondly, changes in mGlu2/3 receptor density of the brain regions affected in epilepsy and schizophrenia are summarised. Increased mGlu2/3 receptor density might represent a compensatory mechanism of the brain to regulate elevated glutamate levels, while reduced mGlu2/3 receptor density in some brain regions may further contribute to the aberrant hyperexcitability. Further research considering the mGlu2/3 receptor can contribute significantly to the understanding of the etiological and therapeutic role of group II mGlu receptor in epilepsy, epilepsy with psychosis and schizophrenia.

Recurrent Nonconvulsive Status Epilepticus in a Patient with Coffin-Lowry Syndrome

Coffin-Lowry syndrome (CLS) is a rare neurodevelopmental condition caused by heterogeneous mutations in the RPS6KA3 gene on the X chromosome, leading to severe intellectual disability and dysmorphism in men, while women are carriers and only weakly affected. CLS is well known for stimulus-induced drop episodes; however, epilepsy is not commonly reported in this condition. We report on a CLS patient presenting with recurrent episodes of nonconvulsive status epilepticus (NCSE) with generalized epileptic activity, for which investigations did not find any other cause than the patient's genetic condition. This case underlines that the possibility of nonconvulsive epileptic seizures and status epilepticus should, therefore, be considered in those patients. The treatable diagnosis of NCSE may easily be overlooked, as symptoms can be unspecific.

Evaluation of the Effects of Charged Amino Acids on Uncontrolled Seizures

Introduction. Epilepsy is one of the most common diseases of the central nervous system. The prevalence of epilepsy throughout the world is 0.5 to 1%, and the same rate is 7.8 per 1000 in Kerman. Almost 20 to 30% of epileptic patients do not respond properly to common medications. The present study investigated patients who did not respond to common and, even in some cases, adjuvant therapies, with two seizures or more per week, regardless of the type of the inflicted epilepsy. Methodology. The participants of the present double-blind study were randomly selected into three 10-member groups of uncontrolled epileptic patients (arginine, glutamic acid, and lysine). The patients used amino acid powder dissolved in water (three times the daily need) every day for two weeks before breakfast. The number of seizures was recorded one week prior to commencing amino acid use, as well as the first and the second weeks subsequent to use. Results. A total of 32 patients were studied in three groups. The decline rates of seizures were 53%, 41%, and 13%, and the P value was 0.013, 0.027, and 0.720, respectively. Conclusion. Administration of the charged amino acids, arginine, and glutamic acid can decrease the seizures of patients suffering from uncontrolled epilepsy.

Cross-species pharmacological characterization of the allylglycine seizure model in mice and larval zebrafish

Treatment-resistant seizures affect about a third of patients suffering from epilepsy. To fulfill the need for new medications targeting treatment-resistant seizures, a number of rodent models offer the opportunity to assess a variety of potential treatment approaches. The use of such models, however, has proven to be time-consuming and labor-intensive. In this study, we performed pharmacological characterization of the allylglycine (AG) seizure model, a simple in vivo model for which we demonstrated a high level of treatment resistance. (d,l)-Allylglycine inhibits glutamic acid decarboxylase (GAD) - the key enzyme in γ-aminobutyric acid (GABA) biosynthesis - leading to GABA depletion, seizures, and neuronal damage. We performed a side-by-side comparison of mouse and zebrafish acute AG treatments including biochemical, electrographic, and behavioral assessments. Interestingly, seizure progression rate and GABA depletion kinetics were comparable in both species. Five mechanistically diverse antiepileptic drugs (AEDs) were used. Three out of the five AEDs (levetiracetam, phenytoin, and topiramate) showed only a limited protective effect (mainly mortality delay) at doses close to the TD50 (dose inducing motor impairment in 50% of animals) in mice. The two remaining AEDs (diazepam and sodium valproate) displayed protective activity against AG-induced seizures. Experiments performed in zebrafish larvae revealed behavioral AED activity profiles highly analogous to those obtained in mice. Having demonstrated cross-species similarities and limited efficacy of tested AEDs, we propose the use of AG in zebrafish as a convenient and high-throughput model of treatment-resistant seizures.

AMN082, a metabotropic glutamate receptor 7 allosteric agonist, attenuates locomotor sensitization and cross-sensitization induced by cocaine and morphine in mice

Previous studies have indicated that metabotropic glutamate receptors 7 (mGluR7s) are involved in drug addiction. However, the role of these receptors in drug-induced behavioral sensitization is unknown. The aim of the present study was to determine whether systemic injection of AMN082, a selective mGluR7 allosteric agonist, reduces the cocaine- and morphine-induced hyperactivity and the development and expression of locomotor sensitization, and also affects the reciprocal cross-sensitization to the stimulant effect of cocaine and morphine in mice. AMN082 (1.25-10.0 mg/kg, i.p.) did not have an impact on locomotion of naive mice and did not affect the acute cocaine- or morphine-induced hyperactivity, except the dose of 10 mg/kg that suppressed the locomotor effect of both drugs. Repeated exposure to cocaine or morphine (10 mg/kg, 5× every 3 days) gradually increased locomotion during induction of sensitization and after 4 (cocaine) or 7 day (morphine) withdrawal phase when challenged with cocaine (10 mg/kg, i.p.) or morphine (10 mg/kg, i.p.) on day 17 or 20, respectively. Pretreatment of animals with the lower doses of AMN082 (1.25-5.0 mg/kg, i.p.), 30 min before every cocaine or morphine injection during repeated drug administration or before cocaine or morphine challenge, dose-dependently attenuated the development, as well as the expression of cocaine or morphine locomotor sensitization. AMN082 also inhibited the reciprocal cross-sensitization between these drugs. Prior to administration of MMPIP (10 mg/kg, i.p.), a selective mGluR7 antagonist reversed the inhibitory effect of AMN082 on the development or expression of cocaine or morphine sensitization. These data indicate that AMN082 attenuated the development and expression of cocaine and morphine sensitization, and the reciprocal cross-sensitization via a mechanism that involves mGluR7s. Thus, AMN082 might have therapeutic implications not only in the treatment of cocaine or opioid addiction but also in the treatment of cocaine/opioid polydrug-abusers.

Citrus aurantium increases seizure latency to PTZ induced seizures in zebrafish thru NMDA and mGluR's I and II

Epilepsy is a serious neurological condition and pharmacotherapy is not effective for all patients and causes serious adverse effects and pharmacokinetic and pharmacodynamic interactions. Natural products and ethnobotanical resources can help develop new therapeutic options for conditions like epilepsy. In Puerto Rico, ethnobotanical resources highlight the anxiolytic properties of a tea like preparation made from the leaves of the Citrus aurantium tree or bitter orange. Studies performed with essential oils from the peel of the fruit have shown to increase seizure latency to pentylenetetrazole (PTZ) and maximal electroshock seizure in mice. We characterized the extract composition, and used a model of PTZ induces seizures in the zebrafish and a receptor-ligand binding assay to determine if this preparation has anticonvulsant properties and its mechanism of action. We determined that the aqueous extract made from the leaves of the C. aurantium tree contains hesperidin, neohesperidin, and neohesperidin dihydrochalcone. Using our zebrafish model, we determined that exposure to the C. aurantium 28 mg/mL extract in aquarium water increases seizure latency by 119% compared to controls. We ruled out a mechanism involving GABA_A receptors using the selective antagonist gabazine. We used two approaches to study the role of glutamate in the mechanism of the C. aurantium extract. The ligand binding assay revealed C. aurantium extracts at concentrations of 0.42 to 5.6 mg/mL significantly reduced [³H]Glu binding indicating an interaction with glutamate receptors, in particular with NMDA receptors and mGluR II. This interaction was confirmed with our animal model using selective receptor antagonists and we identified an interaction with mGluR I, not observed in the ligand binding experiment. These study provide evidence of the anticonvulsant properties of the aqueous extract made from the leaves of the C. aurantium tree and a mechanism involving NMDA and mGluR's I and II.

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.

Neuronal activity patterns in the mediodorsal thalamus and related cognitive circuits are modulated by metabotropic glutamate receptors

The mediodorsal thalamus (MD) likely plays an important role in cognition as it receives abundant afferent connections from the amygdala and prefrontal cortex (PFC). Indeed, disturbed activity within the MD is thought to precipitate cognitive deficits associated with schizophrenia. As compounds acting at the Group II metabotropic glutamate (mGlu) receptors (subtypes mGlu2/mGlu3) have efficacy in animal models of schizophrenia, we investigated whether a Group II agonist and an mGlu2 positive allosteric modulator (PAM) could modulate MD activity. Extracellular single-unit recordings were made in vivo from MD neurones in anaesthetised rats. Responses were elicited by electrical stimulation of the PFC and/or amygdala, with Group II compounds locally applied as required. The Group II agonist reduced inhibition evoked in the MD: an effect manifested as an increase in short-latency responses, and a decrease in long-latency burst-firing. This disinhibitory action of the Group II receptors in the MD represents a mechanism of potential therapeutic importance as increased inhibition in the MD has been associated with cognitive deficit-onset. Furthermore, as co-application of the mGlu2 PAM did not potentiate the Group II agonist effects in the MD, we suggest that the Group II disinhibitory effect is majority-mediated via mGlu3. This heterogeneity in Group II receptor thalamic physiology bears consequence, as compounds active exclusively at the mGlu2 subtype are unlikely to perturb maladapted MD firing patterns associated with cognitive deficits, with activity at mGlu3 receptors possibly more appropriate. Indeed, polymorphisms in the mGlu3, but not the mGlu2, gene have been detected in patients with schizophrenia.

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There is heterogeneity in Group II receptor physiology across thalamic nuclei.

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This differential distribution may facilitate multimodal thalamic nuclei functions.

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Group II receptor activation reduced burst firing via reducing thalamic inhibition.

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Increased thalamic inhibition precipitates impairments in cognitive function.

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Activating the Group II receptors may therefore enhance cognitive function.

Group III metabotropic glutamate receptors: pharmacology, physiology and therapeutic potential

Glutamate, the primary excitatory neurotransmitter in the central nervous system (CNS), exerts neuromodulatory actions via the activation of metabotropic glutamate (mGlu) receptors. There are eight known mGlu receptor subtypes (mGlu1-8), which are widely expressed throughout the brain, and are divided into three groups (I-III), based on signalling pathways and pharmacological profiles. Group III mGlu receptors (mGlu4/6/7/8) are primarily, although not exclusively, localised on presynaptic terminals, where they act as both auto- and hetero-receptors, inhibiting the release of neurotransmitter. Until recently, our understanding of the role of individual group III mGlu receptor subtypes was hindered by a lack of subtype-selective pharmacological tools. Recent advances in the development of both orthosteric and allosteric group III-targeting compounds, however, have prompted detailed investigations into the possible functional role of these receptors within the CNS, and revealed their involvement in a number of pathological conditions, such as epilepsy, anxiety and Parkinson's disease. The heterogeneous expression of group III mGlu receptor subtypes throughout the brain, as well as their distinct distribution at glutamatergic and GABAergic synapses, makes them ideal targets for therapeutic intervention. This review summarises the advances in subtype-selective pharmacology, and discusses the individual roles of group III mGlu receptors in physiology, and their potential involvement in disease.

Group III mGlu receptor agonist, ACPT-I, exerts potential neuroprotective effects in vitro and in vivo

Many evidence suggest that metabotropic glutamate receptors (mGluRs) may modulate glutamatergic transmission, hence, these receptors are regarded as potential targets for neuroprotective drugs. Since group III mGlu receptor agonists are known to reduce glutamatergic transmission by inhibiting glutamate release, we decided to investigate the neuroprotective potential of the group III mGlu receptor agonist, (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid (ACPT-I) against kainate (KA)-induced excitotoxicity in vitro and in vivo. In primary neuronal cell cultures ACPT-I (1-200 μM), applied 30 min-3 h after starting the exposure to KA (150 μM), significantly attenuated the KA-induced LDH release, increased cell viability, and inhibited caspase-3 activity both in cortical and hippocampal cell cultures. The effects were dose-, time- and structure-dependent. The neuroprotective effects of ACPT-I were reversed by (RS)-alpha-cyclopropyl-4-phosphonophenyl glycine, a group III mGluR antagonist. In the in vivo studies, KA (2.5 nmol/1 μl) was unilaterally injected into the rat dorsal CA1 hippocampal region and the size of degeneration was examined by stereological counting of surviving neurons in the CA pyramidal layer. It was found that ACPT-I (7.5 or 15 nmol/1 μl), injected into the dorsal hippocampus 30 min, 1 or 3 h after KA in dose-dependent manner prevented the KA-induced neuronal damage. Moreover, in vivo microdialysis studies in the rat hippocampus showed that ACPT-I (200 μM) given simultaneously with KA (50 μM) significantly diminished the KA-induced glutamate release in the hippocampus. This mechanism seems to play a role in mediating the neuroprotective effect of ACPT-I.

Inhibitory actions of mGlu4 receptor ligands on cocaine-, but not nicotine-, induced sensitizing and conditioning locomotor responses in rats

BACKGROUND

Male Wistar rats were used to verify the hypothesis that metabotropic glutamate 4 (mGlu4) receptor ligands may modulate the locomotor effects evoked by cocaine or nicotine.

METHODS

The preferential mGlu4 receptor orthosteric agonist (2S)-2-amino-4-[hydroxy[hydroxy(4-hydroxy-3-methoxy-5-nitrophenyl)methyl]phosphoryl]butanoic acid (LSP1-2111) and the mGlu4 receptor positive allosteric modulator (+)-cis-N(1)-(3,4-dichlorophenyl)cyclohexane-1,2-dicarboxamide (Lu AF21934) were used in the study. Rats were given repeated pairings of a test environment with cocaine (10mg/kg), nicotine (0.4 mg/kg) or the respective vehicles for 5 days. On day 10, animals were challenged with cocaine (10mg/kg, cocaine sensitization), nicotine (0.4 mg/kg, nicotine sensitization) or vehicle (conditioned hyperlocomotion) in experimental cages.

RESULTS

Given on day 10, LSP1-2111 (3mg/kg) as well as Lu AF21934 (2.5-5mg/kg) decreased the expression of cocaine sensitization. In another set of experiments, LSP1-2111 (3mg/kg) and Lu AF21934 (5mg/kg) administered on day 10 attenuated the conditioned hyperlocomotion in rats treated repeatedly with cocaine. Neither LSP1-2111 (1-3mg/kg) nor Lu AF21934 (2.5-5mg/kg) changed the expression of nicotine sensitization and conditioned hyperlocomotion in rats treated repeatedly with nicotine. None of the mGlu4 receptor agonist/modulator altered the basal locomotor activity or acute hyperactivity to cocaine or nicotine.

CONCLUSIONS

The present data indicate that pharmacological stimulation of mGlu4 receptors reduces the cocaine-induced expression of sensitization as well as conditioned hyperactivity. In contrast, mGlu4 receptor activation seems to be devoid of any effect on the locomotor effects of nicotine.

Translational enhancers of EAAT2: therapeutic implications for neurodegenerative disease

Glutamate excitotoxicity contributes to the neuronal injury and death associated with many neurodegenerative diseases. The glutamate transporter EAAT2, which is primarily localized on astrocytic processes, facilitates glutamate clearance from synapses, thus preventing neuronal damage. In this issue of the JCI, Kong et al. characterize a compound that upregulates EAAT2 translation, thereby increasing glutamate uptake by glial cells. Furthermore, this strategy for alleviating excitotoxicity was found to be beneficial in mouse models of both amyotrophic lateral sclerosis (ALS) and epilepsy, suggesting that future development in this chemical series may lead to much-needed treatments for these disorders.

Activation of group 2 metabotropic glutamate receptors reduces behavioral and electrographic correlates of pilocarpine induced status epilepticus

Novel treatments for epilepsy are necessary because many epilepsy patients are resistant to medication. Metabotropic glutamate receptors (mGluRs), specifically mGluR 2 and 3, may serve as antiepileptic drug targets because of their role in controlling synaptic release. In this study, we administered a Group 2 mGluR agonist, LY379268, one of two mGluR2-specific positive allosteric modulators, BINA or CBiPES, or a cocktail of both BINA and LY379268 in a series of experiments using the pilocarpine model of SE. In one study, groups received treatments 15 min prior to pilocarpine, while in a second study groups received treatments after SE had been initiated to determine whether the drugs could reduce development and progression of SE. We measured bouts of stage 5 seizures, latency to the first seizure, and the maximum Racine score to characterize the seizure severity. We analyzed mouse EEG with implanted electrodes using a power analysis. We found that pretreatment and posttreatment with LY379268 was effective at reducing both behavioral correlates and power in EEG bandwidths associated with seizure, while CBiPES was less effective and BINA was ineffective. These data generally support continued development of mGluR2 pharmacology for novel antiepileptic drugs, though further study with additional drugs and concentrations will be necessary.

What's hAPPening at synapses? The role of amyloid β-protein precursor and β-amyloid in neurological disorders

Accumulating evidence suggests that dysregulated levels of amyloid β-protein precursor (APP) and its catabolites contribute to the impaired synaptic plasticity and seizure incidence observed in several neurological disorders, including Alzheimer's disease, fragile X syndrome, Down's syndrome, autism, epilepsy and Parkinson's disease as well as in brain injury. This review article summarizes what is known regarding the synaptic synthesis, processing and function of APP and amyloid-beta (Aβ), as well as discusses how these proteins could contribute to the altered synaptic plasticity and pathology of the aforementioned disorders. In addition, APP and its proteolytic fragments are emerging as biomarkers for neurological health, and pharmacological interventions that modulate their levels, such as secretase inhibitors, passive immunotherapy against Aβ and mGluR5 antagonists, are reviewed.

Piperine decreases pilocarpine-induced convulsions by GABAergic mechanisms

Piperine, an alkaloid present in the Piper genus, was shown to have an anticonvulsant activity, evaluated by the pilocarpine-induced model, in mice. Pilocarpine (350mg/kg, i.p.) was administered 30min after piperine (2.5, 5, 10 and 20mg/kg, i.p.) which significantly increased latencies to 1st convulsion and to death, and percentage of survivals. These parameters were also increased in the pilocarpine groups pretreated with atropine plus piperine (10 and 2.5mg/kg, respectively), as related to the pilocarpine group. However, they were not altered in the pilocarpine groups pretreated with memantine (a NMDA-type glutamate receptors blocker, 2mg/kg, p.o.) or nimodipine (a calcium channel blocker, 10mg/kg, p.o.), both associated with piperine (1 or 2.5mg/kg), as compared to the piperine plus pilocarpine group. Moreover, the pilocarpine group pretreated with diazepam (which binds to the GABAA receptor, 0.2 and 0.5mg/kg, i.p.) plus piperine (1 and 2.5mg/kg) significantly increased latency to the 1st convulsion, as related to the pilocarpine group, suggesting that the GABAergic system is involved with the piperine action. Furthermore, the piperine effect was blocked by flumazenil (2mg/kg, i.p.), a benzodiazepine antagonist. Untreated P350 animals showed decreased striatal DA and increased DOPAC and HVA levels that were not affected in the piperine plus pilocarpine groups. Piperine increased striatal levels of GABA, glycine and taurine, and reversed pilocarpine-induced increases in nitrite contents in sera and brain. Hippocampi from the untreated pilocarpine group showed an increased number of TNF-α immunostained cells in all areas, as opposed to the pilocarpine group pretreated with piperine. Taken together, piperine anticonvulsant effects are the result of its anti-inflammatory and antioxidant actions, as well as TNF-α reduction. In addition, piperine effects on inhibitory amino acids and on the GABAergic system may certainly contribute to the drug anticonvulsant activity.

GABA and Glutamate Transporters in Brain

The mammalian genome contains four genes encoding GABA transporters (GAT1, slc6a1; GAT2, slc6a13; GAT3, slc6a11; BGT1, slc6a12) and five glutamate transporter genes (EAAT1, slc1a3; EAAT2, slc1a2; EAAT3, slc1a1; EAAT4, slc1a6; EAAT5, slc1a7). These transporters keep the extracellular levels of GABA and excitatory amino acids low and provide amino acids for metabolic purposes. The various transporters have different properties both with respect to their transport functions and with respect to their ability to act as ion channels. Further, they are differentially regulated. To understand the physiological roles of the individual transporter subtypes, it is necessary to obtain information on their distributions and expression levels. Quantitative data are important as the functional capacity is limited by the number of transporter molecules. The most important and most abundant transporters for removal of transmitter glutamate in the brain are EAAT2 (GLT-1) and EAAT1 (GLAST), while GAT1 and GAT3 are the major GABA transporters in the brain. EAAT3 (EAAC1) does not appear to play a role in signal transduction, but plays other roles. Due to their high uncoupled anion conductance, EAAT4 and EAAT5 seem to be acting more like inhibitory glutamate receptors than as glutamate transporters. GAT2 and BGT1 are primarily expressed in the liver and kidney, but are also found in the leptomeninges, while the levels in brain tissue proper are too low to have any impact on GABA removal, at least in normal young adult mice. The present review will provide summary of what is currently known and will also discuss some methodological pitfalls.

Pharmacodynamic potentiation of antiepileptic drugs' effects by some HMG-CoA reductase inhibitors against audiogenic seizures in DBA/2 mice

It is known that the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are effective in both the primary and the secondary prevention of ischemic heart disease. Increasing evidence indicates that statins have protective effects in several neurological diseases including stroke, cerebral ischemia, Parkinson disease, multiple sclerosis, traumatic brain injury and epilepsy. The aim of the present research was to evaluate the effects of some HMG-CoA reductase inhibitors (i.e. lovastatin, simvastatin, atorvastatin, fluvastatin and pravastatin) commonly used for the treatment of hypercholesterolemia in the DBA/2 mice, an animal model of generalized tonic-clonic seizures. Furthermore, the co-administration of these compounds with some antiepileptic drugs (AEDs; i.e. carbamazepine, diazepam, felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, phenobarbital, phenytoin, topiramate and valproate) was studied in order to identify possible positive pharmacological interactions. Simvastatin only was active against both the tonic and clonic phase of audiogenic seizures, whereas the other statins tested were only partially effective against the tonic phase with the following order of potency: lovastatin>fluvastatin>atorvastatin; pravastatin was completely ineffective up to the dose of 150mg/kg. The co-administration of ineffective doses of all statins with AEDs generally increased the potency of the latter reducing their ED50 values. In particular, simvastatin was the most active in potentiating the activity of AEDs and the combinations of statins with carbamazepine, diazepam, felbamate, lamotrigine, topiramate and valproate were the most favorable, whereas, the co-administrations with the other AEDs studied was in most cases neutral. The increase in potency was generally associated with an enhancement of motor impairment (TD50); however, the therapeutic index (TD50/ED50) of combined treatment of AEDs with statins was predominantly more favorable than control. Statins administration did not significantly affect the total plasma but, in some cases, it increased the free plasma levels and the brain concentrations of some of the AEDs studied (i.e. carbamazepine, diazepam, phenytoin and valproate); however, these alterations where not statistically significant. Therefore, with the exception of the latter compounds, we might exclude pharmacokinetic interactions and conclude that for the most of AEDs, potentiation was of pharmacodynamic nature. In conclusion, simvastatin, fluvastatin, lovastatin and atorvastatin showed an additive anticonvulsant effect when co-administered with some AEDs, most notably carbamazepine, diazepam, felbamate, lamotrigine, topiramate and valproate, implicating a possible therapeutic relevance of such drug combinations. The present results suggest that statins, besides the beneficial cardiovascular effects, might be able to affect brain areas, which might participate in the regulation of seizure susceptibility.