Targeting Ionotropic Glutamate Receptors in the Treatment of Epilepsy

BackgroundA dysfunction in glutamate neurotransmission is critical for seizure. Glutamate is the major excitatory drive in the cerebral cortex, where seizures occur. Glutamate acts via (i) ionotropic (iGlu) receptors, which are ligand-gated ion channels mediating fast excitatory synaptic transmission; and (ii) G proteins coupled metabotropic (mGlu) receptors.ObjectiveTo overview the evidence on the role of iGlu receptors in the onset, duration, and severity of convulsive and non-convulsive seizures to lay the groundwork for novel strategies for drug-resistant epilepsy.MethodsWe used PubMed crossed-search for “glutamate receptor and epilepsy” (sorting 3,170 reports), searched for “ionotropic glutamate receptors”, “AMPA receptors”, “NMDA receptors”, “kainate receptors”, “convulsive seizures”, “absence epilepsy”, and selected those papers focusing this Review’s scope.ResultsiGlu receptor antagonists inhibit, whereas agonists worsen experimental seizures in various animal species. Clinical development of iGlu receptor antagonists has been limited by the occurrence of adverse effects caused by inhibition of fast excitatory synaptic transmission. To date, only one drug (perampanel) selectively targeting iGlu receptors is marketed for the treatment of focal epilepsy. However, other drugs, such as topiramate and felbamate, inhibit iGlu receptors in addition to other mechanisms.ConclusionThis review is expected to help dissect those steps induced by iGlu receptors activation, which may be altered to provide antiepileptic efficacy without altering key physiological brain functions, thus improving the safety and tolerability of iGlu-receptor directed antiepileptic agents. This effort mostly applies to drug resistant seizures, which impact the quality of life and often lead to status epilepticus, which is a medical urgency.     

Capsaicin-Induced Glutamate Release Is Implicated in Nociceptive Processing Through Activation of Ionotropic Glutamate Receptors and Group I Metabotropic Glutamate Receptor in Primary Afferent Fibers

Glutamate (Glu) is the major excitatory neurotransmitter in the central nervous system. The role of peripheral Glu and Glu receptors (GluRs) in nociceptive transmission is, however, still unclear. In the present study, we examined Glu levels released in the subcutaneous perfusate of the rat hind instep using a microdialysis catheter and the thermal withdrawal latency using the Plantar Test following injection of drugs associated with GluRs with/without capsaicin into the hindpaw. The injection of capsaicin into the rat hind instep caused an increase of Glu level in the s.c. perfusate. Capsaicin also significantly decreased withdrawal latency to irradiation. These effects of capsaicin were inhibited by pretreatment with capsazepine, a transient receptor potential vanilloid receptor 1 (TRPV1) competitive antagonist. Capsaicin-induced Glu release was also suppressed by combination with each antagonist of ionotropic GluRs (iGluRs: NMDA/AMPA receptors) and group I metabotropic GluR (mGluR), but not group II and group III mGluRs. Furthermore, these GluRs antagonists showed remarkable inhibition against capsaicin-induced thermal hyperalgesia. These results suggest that Glu is released from the peripheral endings of small-diameter afferent fibers by noxious stimulation and then activates peripheral iGluRs and group I mGluR in development and/or maintenance of nociception. Furthermore, the activation of peripheral NMDA/AMPA receptors and group I mGluR may be important in mechanisms whereby capsaicin evokes nociceptive responses.     

The Role of Ionotropic Glutamate Receptors in Childhood Neurodevelopmental Disorders: Autism Spectrum Disorders and Fragile X Syndrome

Autism spectrum disorder (ASD) and Fragile X syndrome (FXS) are relatively common childhood neurodevelopmental disorders with increasing incidence in recent years. They are currently accepted as disorders of the synapse with alterations in different forms of synaptic communication and neuronal network connectivity. The major excitatory neurotransmitter system in brain, the glutamatergic system, is implicated in learning and memory, synaptic plasticity, neuronal development. While much attention is attributed to the role of metabotropic glutamate receptors in ASD and FXS, studies indicate that the ionotropic glutamate receptors (iGluRs) and their regulatory proteins are also altered in several brain regions. Role of iGluRs in the neurobiology of ASD and FXS is supported by a weight of evidence that ranges from human genetics to in vitro cultured neurons. In this review we will discuss clinical, molecular, cellular and functional changes in NMDA, AMPA and kainate receptors and the synaptic proteins that regulate them in the context of ASD and FXS. We will also discuss the significance for the development of translational biomarkers and treatments for the core symptoms of ASD and FXS.     

Effect of Chronic Administration of Magnesium Sulfate on 1–Methyl–4–phenyl–1,2,3,6–tetrahydropyridine–Induced Neurotoxicity in Mice

Abstract: This investigation was aimed to study the effect of magnesium on 1–methyl–4–phenyl–1,2,3,6–tetrahydropyridine (MPTP)–induced neurotoxicity in mice. Four groups of mice were given magnesium sulfate (MgSO₄.7H₂O) in drinking water at four different concentrations of 0.0 g/l (control), 2.5 g/l (low), 5.0 g/l (medium) and 10.0 g/l (high) respectively for a period of 16 weeks; these animals also received MPTP (30 mg/kg, intraperitoneally daily) during the last five days of Mg treatment. Other four groups of mice were given similar dose regimen of MgSO₄ but received injections of saline instead of MPTP. Seventy–two hr after the last dose of MPTP, neurobehavioural studies including locomotor activity, pole climbing test and heat nociception test were performed and striata were collected for the analysis of dopamine. The results of this study show that treatment of mice with MgSO₄ or MPTP individually has no effect on their behaviour. Concomitant administration of low dose of MgSO₄ (2.5 g/l) along with MPTP produced increase in motor activity and latency to heat stimuli; whereas medium and high doses of MgSO₄ in combination with MPTP produced opposite (as compared to low dose) effects resulting in a decrease in motor activity and latency to heal stimuli and increase in pole climbing time. However, MgSO₄ dose–dependently exacerbated MPTP–induced depletion of striatal dopamine. The mortality was drastically increased (30–55%) in the animals receiving combined treatments of MPTP and MgSO₄ as compared to the mice treated with MPTP alone (12%). This study clearly points towards the ability of MgSO₄ to modify MPTP–induced neurotoxicity.     

Low Non-NMDA Receptor Current Density as Possible Protection Mechanism from Neurotoxicity of Circulating Glutamate on Subfornical Organ Neurons in Rats

The subfornical organ (SFO) is one of circumventricular organs characterized by the lack of a normal blood brain barrier. The SFO neurons are exposed to circulating glutamate (60~100 µM), which may cause excitotoxicity in the central nervous system. However, it remains unclear how SFO neurons are protected from excitotoxicity caused by circulating glutamate. In this study, we compared the glutamate-induced whole cell currents in SFO neurons to those in hippocampal CA1 neurons using the patch clamp technique in brain slice. Glutamate (100 µM) induced an inward current in both SFO and hippocampal CA1 neurons. The density of glutamate-induced current in SFO neurons was significantly smaller than that in hippocampal CA1 neurons (0.55 vs. 2.07 pA/pF, p<0.05). To further identify the subtype of the glutamate receptors involved, the whole cell currents induced by selective agonists were then compared. The current densities induced by AMPA (0.45 pA/pF) and kainate (0.83 pA/pF), non-NMDA glutamate receptor agonists in SFO neurons were also smaller than those in hippocampal CA1 neurons (2.44 pA/pF for AMPA, p<0.05; 2.34 pA/pF for kainate, p< 0.05). However, the current density by NMDA in SFO neurons was not significantly different from that of hippocampal CA1 neurons (1.58 vs. 1.47 pA/pF, p>0.05). These results demonstrate that glutamate-mediated action through non-NMDA glutamate receptors in SFO neurons is smaller than that of hippocampal CA1 neurons, suggesting a possible protection mechanism from excitotoxicity by circulating glutamate in SFO neurons.     

Glutamate Receptors in Extinction and Extinction-Based Therapies for Psychiatric Illness

Some psychiatric illnesses involve a learned component. For example, in posttraumatic stress disorder, memories triggered by trauma-associated cues trigger fear and anxiety, and in addiction, drug-associated cues elicit drug craving and withdrawal. Clinical interventions to reduce the impact of conditioned cues in eliciting these maladaptive conditioned responses are likely to be beneficial. Extinction is a method of lessening conditioned responses and involves repeated exposures to a cue in the absence of the event it once predicted. We believe that an improved understanding of the behavioral and neurobiological mechanisms of extinction will allow extinction-like procedures in the clinic to become more effective. Research on the role of glutamate—the major excitatory neurotransmitter in the mammalian brain—in extinction has led to the development of pharmacotherapeutics to enhance the efficacy of extinction-based protocols in clinical populations. In this review, we describe what has been learned about glutamate actions at its three major receptor types (N-methyl--aspartate (NMDA) receptors, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and metabotropic glutamate receptors) in the extinction of conditioned fear, drug craving, and withdrawal. We then discuss how these findings have been applied in clinical research.     

Spider Neurotoxins as Tools for the Investigation of Glutamate Receptors

Abstract

Progress in the investigation of glutamate receptors structure and function requires the use of specific chemical tools affecting the responses to glutamate and other excitatory amino acids. Several low molecular weight spider and wasp toxins may be used as valuable tools for identification of certain subtypes of glutamate receptors and the study of their function. Some results obtained on different preparations with the help of argiotoxin 636 (argiopine), neurotoxin isolated from the venom Argiope lobata are reviewed in the present paper.     

Harmine–, LON–954– and 5–Hydroxytryptophan–induced Tremors in Rats Withdrawn from Ethanol

Abstract: The tremors induced by harmine, LON–954 (N–carbamoyl–2–(2,6–dichlorophenyl)acctamidine hydrochloride) and 5–hydroxytryptophan (5–HTP) were studied in control rats and in rats withdrawn for 16–48 hrs from 6 to 9 days' ethanol administration. The frequencies and the intensities of the tremors were determined electronically. In control rats the frequency spectra of the tremors induced by harmine (20 mg/ kg) and LON–954 (10 mg/kg) showed a narrow peak frequency at about 10 Hz. Atropine (1.2 mg/kg) altered neither the frequency nor the intensity of these tremors. 5–HTP (50 mg/kg) when given 3.5 hrs after iproniazid (100 mg/kg) induced tremor with peak frequencies at 6–7 Hz and 12 Hz. In ethanol–withdrawn rats treated with harmine or LON–954 the frequency analysis of tremor revealed a narrow peak frequency at about 12 Hz, which was neither the characteristic frequency of ethanol withdrawal tremor (6 Hz) nor that of harmine or LON–954 (10 Hz). The intensity of both harmine– and LON–954–induced tremor was significantly increased in ethanol–withdrawn rats. The ethanol–withdrawn rats were markedly sensitized to the effect of iproniazid + 5–HTP, shown by deaths. The peak frequencies of this tremor were the same as those in control rats. The results suggest that harmine–induced tremor involves a dopaminergic–5–HT'ergic imbalance and the tremor induced by LON–954 a dopaminergic–cholinergic imbalance in the brain. The tremor in ethanol–withdrawn rats seems to be mediated by alterations in the activity of the cerebral 5–HT'ergic system     

NMR structural characterization of cecropin A(1–8) – magainin 2(1–12) and cecropin A(1–8) – melittin(1–12) hybrid peptides

Abstract: In order to elucidate the structure–antibiotic activity relationships of the peptides, the three-dimensional structures of two hybrid peptides, CA(1–8) – MA(1–12) and CA(1–8) – ME(1–12) in trifluoroethanol-containing aqueous solution were investigated by NMR spectroscopy. Both CA(1–8) – MA(1–12) and CA(1–8) – ME(1–12) have strong antibacterial activity but only CA(1–8) – ME(1–12) has hemolytic activity against human erythrocytes. CA(1–8) – MA(1–12) has a hydrophobic 3₁₀-helix of only two turns combined with one short helix in the N-terminus with a flexible hinge section in between. CA(1–8) – MA(1–12) has a severely bent structure in the middle of the peptide. These structural features as well as the low hydrophobicity of CA(1–8) – MA(1–12) seem to be crucial for the selective lysis against the membrane of prokaryotic cells. CA(1–8) – ME(1–12) has an α-helical structure of about three turns in the melittin domain and a flexible structure with one turn in the cecropin domain connected with a flexible hinge section in between, and these might be the structural features required for membrane distruption against prokaryotic and eukaryotic cells. The central hinge region (Gly9-Ile10-Gly11) in an amphipathic antibacterial peptide is considered to play an important role in providing the conformational flexibility required for ion channel formation of the C-terminal hydrophobic α-helix on cell membrane.     

GABA and Glutamate Receptors as Therapeutic Targets in Neurodegenerative Disorders*

Abstract: The amiloride derivatives, 2′,3′-benzobenzamil (BB), 3′,4′-dichlorobenzamil (DCB), and 5-(N-4-chlorobenzyl)-2′,4′-dimethylbenzamil (CBDB) are known as inhibitors of the Na⁺/Ca²⁺ exchange. This kind of drug action was recently suggested to be a new inotropic mechanism. In guinea-pig myocardium, we have studied the inotropic and the accompanying electrophysiological effects of the three compounds in order to assess their selectivity of action. In left atria and in papillary muscle, force of contraction increased with DCB and CBDB (atria only) at a high concentration (5 × 10^-5 ? 10^-4 mol/l) and after long exposure time, whereas BB produced a negative inotropic effect. In the isolated perfused Langendorff heart, the amiloride derivatives tested decreased spontaneous heart rate and force of contraction and prolonged the duration of contraction. In isolated cardiac myocytes, sodium current, calcium current and the delayed rectifier were reduced by concentrations of BB, DCB and CBDB similar to the IC₅₀ values reported for the inhibition of the Na⁺/Ca²⁺ exchange. Our results demonstrate that the amiloride derivatives have multiple sites of action. It is concluded that more specific modulators of the Na⁺/Ca²⁺ exchange are required in order to define their contribution to the regulation of contractile activation of the heart.     

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The hottest developments in the fields of cancer research, neuroscience, genomics and proteomics, anti-virals and more, with a pick of the key research papers in these areas.     

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Synthesis of cystine peptides 21–25/70–73 and 35–39/56–59 of the β-subunit of human choriogonadotropin

Syntheses of two asymmetrical cystine peptides with the amino acid residues 21–25/70–73 and 35–39/56–59, based on the linear amino acid sequence and the disulfide bond assignment in the β-subunit of human choriogonadotropin (hCG-β), are described. S-trityl and S-acetamidomethyl peptide fragments of each cystine peptide were prepared in solution phase and were subjected to oxidation with I₂/MeOH to form the disulfide bridge. The cystine peptides were characterized by their amino acid analyses and fast atom bombardment mass spectrometry. Immunological characterization by several homologous radioimmunoas-say systems showed that peptide 21–25/70–73 had significant hCG, hCG-β, and hLH activities while peptide 35–39/56–59 failed to reveal any immunoreactivity.     

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An insight into the latest developments in drug discovery through brief synopses of recent publications, summarizing the chemistry, pharmacological significance and biological relevance of new molecules.