Inhibiting Glutamate Receptors Research Articles

Narrative Review of Topiramate: Clinical Uses and Pharmacological Considerations.

Due to the diverse mechanisms of action of antiseizure drugs, there has been a rise in prescriptions of these drugs for non-epileptic pathologies. One drug that is now being used for a variety of conditions is topiramate. This is a narrative review that used PubMed, Google Scholar, MEDLINE, and ScienceDirect to review literature on the clinical and pharmacologic properties of topiramate. Topiramate is a commonly prescribed second-generation antiseizure drug. The drug works through multiple pathways to prevent seizures. In this regard, topiramate blocks sodium and calcium voltage-gated channels, inhibits glutamate receptors, enhances gamma-aminobutyric acid (GABA) receptors, and inhibits carbonic anhydrase. Topiramate is approved by the Food and Drug Administration (FDA) for epilepsy treatment and migraine prophylaxis. Topiramate in combination with phentermine is also FDA-approved for weight loss in patients with a body mass index (BMI) > 30. The current target dosing for topiramate monotherapy is 400mg/day and 100mg/day to treat epilepsy and migraines, respectively. Commonly reported side effects include paresthesia, confusion, fatigue, dizziness, and change in taste. More uncommon and serious adverse effects can include acute glaucoma, metabolic acidosis, nephrolithiasis, hepatotoxicity, and teratogenicity. Related to a broad side effect profile, physicians prescribing this drug should routinely monitor for side effects and/or toxicity. The present investigation reviews various anti-seizure medications before summarizing indications of topiramate, off-label uses, pharmacodynamics, pharmacokinetics, adverse effects, and drug-drug interactions.

Cochlear hair cell innervation is dependent on a modulatory function of Semaphorin-3A.

Proper connectivity between type I spiral ganglion neurons (SGNs) and inner hair cells (IHCs) in the cochlea is necessary for conveying sound information to the brain in mammals. Previous studies have shown that type I SGNs are heterogeneous in form, function and synaptic location on IHCs, but factors controlling their patterns of connectivity are not well understood. During development, cochlear supporting cells and SGNs express Semaphorin-3A (SEMA3A), a known axon guidance factor. Mice homozygous for a point mutation that attenuates normal SEMA3A repulsive activity (Sema3aK108N ) show cochleae with grossly normal patterns of IHC innervation. However, genetic sparse labeling and three-dimensional reconstructions of individual SGNs show that cochleae from Sema3aK108N mice lacked the normal synaptic distribution of type I SGNs. Additionally, Sema3aK108N cochleae show a disrupted distribution of GLUA2 postsynaptic patches around the IHCs. The addition of SEMA3A-Fc to postnatal cochleae led to increases in SGN branching, similar to the effects of inhibiting glutamate receptors. Ca2+ imaging studies show that SEMA3A-Fc decreases SGN activity. Contrary to the canonical view of SEMA3A as a guidance ligand, our results suggest SEMA3A may regulate SGN excitability in the cochlea, which may influence the morphology and synaptic arrangement of type I SGNs.

Phaseic Acid, an Endogenous and Reversible Inhibitor of Glutamate Receptors in Mouse Brain

Phaseic acid (PA) is a phytohormone regulating important physiological functions in higher plants. Here, we show the presence of naturally occurring (-)-PA in mouse and rat brains. (-)-PA is exclusively present in the choroid plexus and the cerebral vascular endothelial cells. Purified (-)-PA has no toxicity and protects cultured cortical neurons against glutamate toxicity through reversible inhibition of glutamate receptors. Focal occlusion of the middle cerebral artery elicited a significant induction in (-)-PA expression in the cerebrospinal fluid but not in the peripheral blood. Importantly, (-)-PA induction only occurred in the penumbra area, indicting a protective role of PA in the brain. Indeed, elevating the (-)-PA level in the brain reduced ischemic brain injury, whereas reducing the (-)-PA level using a monoclonal antibody against (-)-PA increased ischemic injury. Collectively, these studies showed for the first time that (-)-PA is an endogenous neuroprotective molecule capable of reversibly inhibiting glutamate receptors during ischemic brain injury.

Dietary Polyphenols as Potential Remedy for Dementia.

With increasing life expectancy as a result of better quality of life and improved health care, the incidence of aging related diseases and disorders is heading toward epidemic proportions. Dementia, a spectrum of neurological diseases associated with aging, is an increasingly prevalent disease. No cure exists yet for dementia; however, there are many potential candidates for treatment of dementia that merit more exploration. Polyphenols, which constitute one such class of compounds, are dietary agents that are globally found in commonly consumed food. Many processes that are associated with the pathophysiology of dementia can be modulated by polyphenols. Polyphenolic compounds can alleviate oxidative stress by acting as direct scavengers of free radicals and clearing superoxide and hydroxyl radicals and by increasing the level of antioxidant enzymes such as glutathione peroxidase. They also chelate metal ions to prevent free radical formation. Polyphenols can also combat inflammation by affecting transcription factors such as NF-κB. Some polyphenols may have the potential to inhibit excitotoxicity by regulating intracellular calcium ion concentration, inhibiting glutamate receptors and increasing glutamate reuptake at the synapse. The cognitive decline in dementia due to decreased availability of acetylcholine can also be countered by polyphenols that inhibit acetyl-cholinesterase activity. Taken together, these findings suggest that increasing the consumption of polyphenol rich food may alleviate the effects of dementia. Moreover, their effects on controlling multiple mechanisms that are associated with dementia may also prevent or slow down the onset and progress of this devastating disease.

Chronic Glutamate Toxicity in Neurodegenerative Diseases-What is the Evidence?

Together with aspartate, glutamate is the major excitatory neurotransmitter in the brain. Glutamate binds and activates both ligand-gated ion channels (ionotropic glutamate receptors) and a class of G-protein coupled receptors (metabotropic glutamate receptors). Although the intracellular glutamate concentration in the brain is in the millimolar range, the extracellular glutamate concentration is kept in the low micromolar range by the action of excitatory amino acid transporters that import glutamate and aspartate into astrocytes and neurons. Excess extracellular glutamate may lead to excitotoxicity in vitro and in vivo in acute insults like ischemic stroke via the overactivation of ionotropic glutamate receptors. In addition, chronic excitotoxicity has been hypothesized to play a role in numerous neurodegenerative diseases including amyotrophic lateral sclerosis, Alzheimer's disease and Huntington's disease. Based on this hypothesis, a good deal of effort has been devoted to develop and test drugs that either inhibit glutamate receptors or decrease extracellular glutamate. In this review, we provide an overview of the different pathways that are thought to lead to an over-activation of the glutamatergic system and glutamate toxicity in neurodegeneration. In addition, we summarize the available experimental evidence for glutamate toxicity in animal models of neurodegenerative diseases.

Selective increases of AMPA, NMDA, and kainate receptor subunit mRNAs in the hippocampus and orbitofrontal cortex but not in prefrontal cortex of human alcoholics

Glutamate is the main excitatory transmitter in the human brain. Drugs that affect the glutamatergic signaling will alter neuronal excitability. Ethanol inhibits glutamate receptors. We examined the expression level of glutamate receptor subunit mRNAs in human post-mortem samples from alcoholics and compared the results to brain samples from control subjects. RNA from hippocampal dentate gyrus (HP-DG), orbitofrontal cortex (OFC), and dorso-lateral prefrontal cortex (DL-PFC) samples from 21 controls and 19 individuals with chronic alcohol dependence were included in the study. Total RNA was assayed using quantitative RT-PCR. Out of the 16 glutamate receptor subunits, mRNAs encoding two AMPA [2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid] receptor subunits GluA2 and GluA3; three kainate receptor subunits GluK2, GluK3 and GluK5 and five NMDA (N-methyl-D-aspartate) receptor subunits GluN1, GluN2A, GluN2C, GluN2D, and GluN3A were significantly increased in the HP-DG region in alcoholics. In the OFC, mRNA encoding the NMDA receptor subunit GluN3A was increased, whereas in the DL-PFC, no differences in mRNA levels were observed. Our laboratory has previously shown that the expression of genes encoding inhibitory GABA-A receptors is altered in the HP-DG and OFC of alcoholics (Jin et al., 2011). Whether the changes in one neurotransmitter system drives changes in the other or if they change independently is currently not known. The results demonstrate that excessive long-term alcohol consumption is associated with altered expression of genes encoding glutamate receptors in a brain region-specific manner. It is an intriguing possibility that genetic predisposition to alcoholism may contribute to these gene expression changes.

Neurobehavioral and genotoxic evaluation of (−)-linalool in mice

(-)-Linalool is a monoterpene compound commonly found as a major component of the essential oil of several aromatic species. It has been shown to exert several actions in the central nervous system (CNS) and is able to inhibit glutamate receptors. This study investigated the effect of (-)-linalool in depression and genotoxicity models. Mice were given (-)-linalool (10, 50, 100 or 200 mg/kg i.p.) and were evaluated using the tail suspension test (TST). Genotoxic and antigenotoxic effects in blood and brain were investigated using the alkaline comet assay. In the TST, the animals that received doses of 100 and 200 mg/kg presented a decrease in immobility times. No increase in DNA damage was observed in either tissue, and resistance to DNA oxidative damage induced by hydrogen peroxide did not increase. (-)-Linalool showed an antidepressant-like activity in the TST and was unable to cause damage/protection to DNA in brain tissue and peripheral blood. This investigation provides evidence of an important effect of (-)-linalool on the CNS; however, more studies are necessary to support its possible clinical uses.

Abstract WP259: CaMKII Inhibitor tatCN21 Decreases Neuronal Damage Following Cardiac Arrest and Cardiopulmonary Resuscitation in Mice

INTRODUCTION: Glutamate excitotoxicity is considered an important mechanism of ischemic neuronal damage. However, directly inhibiting glutamate receptors can cause severe side effects due to their key physiological functions in the central nervous system (CNS). Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a major downstream target of glutamate-induced Ca2+ signaling. The aim of this study is to examine the neuroprotective potential of the CaMKII inhibitor tatCN21 on experimental cardiac arrest and cardiopulmonary resuscitation (CA/CPR). Methods: Global cerebral ischemia was induced with 8 min of cardiac arrest followed by CPR in C57BL/6 male mice. The mice were randomly administered either tatCN21 (1 mg/kg, n=14) or vehicle (saline, n=13) 2 minutes after resuscitation (intravenous). Hippocampal neuronal damage was analyzed three days after resuscitation by H&E staining. Primary hippocampal neurons were insulted with 400 μm glutamate for 5 min and treated with tatCN21 (5 μm), the scrambled CN peptide (tatRev, 5 μm) or KN93 (10 μm) 1 hr after removal of glutamate, cell viability was measure by LDH release 24 hrs later. Statistical analyses were performed using ANOVA and t tests, with P<0.05 considered statistically significant. Results: Analysis of histological damage 72 hrs after resuscitation revealed that tatCN21 significantly reduces hippocampal CA1 neuronal injury compared to vehicle-treated mice, reducing damage from 72.9±4.5% (n=13) for vehicle to 54.1±7.6% (n=14, P<0.05) for tatCN21 treated mice. tatCN21 significantly reduced glutamate-induced neuronal cell death, but not the classical CaMKII inhibitor KN93 (P<0.05). No difference in body weight, cardiac arrest parameters and survival rates were observed between each groups. Conclusions: Our data showed that the novel CaMKII inhibitor tatCN21 decreases neuronal damage both in vivo and in vitro. Therefore, this study indicated that CaMKII may have an important role in glutamate excititoxicity and that tatCN21 may serve as a potential therapeutic strategy for minimizing brain injury following cardiac arrest.

The Structure of (−)-Kaitocephalin Bound to the Ligand Binding Domain of the (S)-α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA)/Glutamate Receptor, GluA2

Glutamate receptors mediate the majority of excitatory synaptic transmission in the central nervous system, and excessive stimulation of these receptors is involved in a variety of neurological disorders and neuronal damage from stroke. The development of new subtype-specific antagonists would be of considerable therapeutic interest. Natural products can provide important new lead compounds for drug discovery. The only natural product known to inhibit glutamate receptors competitively is (-)-kaitocephalin, which was isolated from the fungus Eupenicillium shearii and found to protect CNS neurons from excitotoxicity. Previous work has shown that it is a potent antagonist of some subtypes of glutamate receptors (AMPA and NMDA, but not kainate). The structure of kaitocephalin bound to the ligand binding domain of the AMPA receptor subtype, GluA2, is reported here. The structure suggests how kaitocephalin can be used as a scaffold to develop more selective and high affinity antagonists for glutamate receptors.

Topiramate and Alcohol Dependence

The anticonvulsant topiramate stimulates γ-aminobutyric acid receptors and inhibits glutamate receptors; because these neurotransmitter effects are

Endogenous factors regulating neuronal death induced by radical stress.

Neurons in the central nervous system (CNS) are vulnerable to radical stress caused by reactive oxygen species, including nitric oxide (NO). Those radicals play crucial roles in glutamate neurotoxicity associated with ischemic brain injury and a wide range of neurodegenerative disorders. In our previous studies, we have shown evidence suggesting that glutamate neurotoxicity is regulated by certain endogenous substances such as neurotrophins, nicotinic acetylcholine, prostanoids and vitamins. Based on those findings, we have used the term 'neuroprotective factor' for endogenous substances possessing protective activity against glutamate neurotoxicity, and have further searched for a candidate with unique structure. We isolated a novel neuroprotective substance named 'serofendic acid' derived from fetal calf serum. The compound exhibited potent protective action against neurotoxicity induced by glutamate and by an NO donor without inhibiting glutamate receptors. Electron spin resonance analysis demonstrated that serofendic acid had no direct scavenging activity on NO, but was capable of inhibiting the generation of a hydroxyl radical, a presumed 'executor' radical in the nitric oxide-mediated neurotoxic cascade. The chemical structure was determined by mass spectrometry and nuclear magnetic resonance spectroscopy, and was confirmed by synthesis. The structure was unique among known endogenous substances because the compound was a sulfur-containing atisane type diterpenoid. The discovery of serofendic acid may provide a new scope for the investigation of low-molecular weight bioactive factors promoting the survival of CNS neurons.

S-methyl-N,N-diethylthiocarbamate sulfoxide elicits neuroprotective effect against N-methyl-D-aspartate receptor-mediated neurotoxicity.

Glutamatergic neurotransmission, particularly of the NMDA receptor type, has been implicated in the excitotoxic response to several external and internal stimuli. In the present investigation, we report that S-methyl-N,N-diethylthiocarbamate sulfoxide (DETC-MeSO) selectively and specifically blocks the NMDA receptor subtype of the glutamate receptors, and attenuates glutamate-induced neurotoxicity in rat-cultured primary neurons. Other major ionotropic glutamate receptor subtypes, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate, were insensitive to DETC-MeSO both in vitro and in vivo. Disulfiram, the parent compound of DETC-MeSO, also inhibits glutamate receptors partially in vivo; however, it fails to inhibit glutamate receptors in mice pretreated with N-butyl imidazole, a cytochrome P450 enzyme inhibitor, implicating the need for bioactivation of disulfiram to be an effective antagonist. We showed that glutamate-induced increase in (45)Ca2+ was attenuated in rat-cultured primary neurons following pretreatment with DETC-MeSO. The Ca2+ influx into primary neurons, studied by confocal microscopy of the fluorescent Ca2+ dye fura-2, demonstrated a complete attenuation of NMDA-induced Ca2+ influx. Similarly, DETC-MeSO attenuated NMDA-induced (45)Ca2+ uptake. Glutamate-induced (45)Ca2+ uptake and Ca2+ influx, however, were partially blocked by DETC-MeSO, and this is consistent with both in vitro and in vivo studies in which DETC-MeSO partially blocked mouse brain glutamate receptors. In addition, DETC-MeSO pretreatment effectively prevented seizures in mice induced either by NMDA, ammonium acetate, or ethanol-induced kindling seizures, all of which are believed to be mediated by NMDA receptors. These data demonstrate that DETC-MeSO produces the neuroprotective effect through antagonism of NMDA receptors in vivo.

Effects of isoflurane and hypothermia on glutamate receptor-mediated calcium influx in brain slices.

To understand how volatile anesthetics protect neurons during cerebral ischemia, we studied the effects of isoflurane on cerebral glutamate receptor-mediated calcium influx. Calcium influx via these key excitatory receptors may mediate pain transmission, memory, and the pathophysiologic sequelae of cerebral anoxia or ischemia. Because cerebral protection by hypothermia may involve a decrease in glutamate receptor activity, we also examined the interaction of temperature and isoflurane on glutamate receptor inhibition. We measured glutamate receptor-mediated changes in cytosolic calcium in 300-microns-thick rat cortical brain slices. Temperature was varied to 28, 34, 37, or 39 degrees C and isoflurane partial pressure to 0.016-0.019 atm (equivalent to 1.16 minimum alveolar concentration [MAC], adjusted for temperature and age). Brain slices were loaded with fura-2 to permit measurement of cytosolic free calcium. Calcium changes due to the glutamate receptor agonist N-methyl-D-aspartate (NMDA) (50 microM), to ischemia levels of L-glutamate (1.0 mM) or to simulated ischemia (1.0 mM glutamate, 100 microM NaCN, and 3.5 mM iodoacetate) was then measured. Slice lactate dehydrogenase leakage and adenosine triphosphate were measured as indices of cellular integrity. Isoflurane reduced both L-glutamate and NMDA-mediated calcium fluxes by approximately 60%. Neither the activity of the NMDA receptor nor its inhibition by isoflurane was altered by temperature. The rate of calcium influx during ischemia was significantly reduced both by temperature and by isoflurane (P < 0.05). Adenosine triphosphate loss and lactate dehydrogenase leakage were reduced by isoflurane during simulated ischemia by 37% and 73% (P < 0.05), respectively. (1) At 1.16 MAC, isoflurane potently inhibits glutamate receptors and delays cellular injury induced by simulated ischemia, and (2) hypothermia does not reduce the intrinsic activity of cortical glutamate receptors but delays calcium accumulation during simulated ischemia. Isoflurane reduces the severity of key pathophysiologic events in an in vitro model of simulated cerebral ischemia.

Glutamate receptor inhibitors as potential insecticides

Philanthotoxin (PhTX) is a neurotoxic constituent of the paralytic venom of the digger wasp, Philanthus triangulum. PhTX inhibits glutamate receptors of insect muscles mostly as a channel blocker, thereby producing muscle paralysis. Since glutamate receptor blockers may be of value as selective insect control agents, numerous derivatives of PhTX were synthesized and tested for their potencies as inhibitors of insect skeletal muscle glutamate receptors. Structure-activity relationship studies revealed that shortening the polyamine chain length reduced potency, and quaternarization of the nitrogen destroyed it. The potency was increased by a bulky anchoring group with moderate hydrophobicity at the end of the polyamine chain. The conversion of the tryosyl moiety to 3,5-diiodo-tyrosyl also increased potency and so did lengthening the butyryl chain from 4 to 10 carbons. Not only did PhTXs inhibit different subtypes of glutamate receptors, including the mammalian N-methyl-D-aspartate receptor, but also nicotinic receptors of insects and vertebrates. Because of this low selectively, and the hydrophilicity of the derivatives tested, which interferes with their penetration to the target receptor, these compounds cannot be used as insecticides. Nevertheless, the insect skeletal muscle glutamate receptor is a viable target for selective insecticides and major changes in PhTX structure may possibly produce derivatives that can be potential insecticides.

Kynurenate and 2-amino-5-phosphonovalerate interact with multiple binding sites of the [formula omitted] glutamate receptor domain

By studying the binding of [ 3H]glycine and [ 3H]glutamate to rat synaptic membranes in the presence of 2-amino-5-phosphonovalerate (APV) and kynurenate (KYN) we have demonstrated that KYN is more potent than APV in displacing [ 3H]glycine, while an opposite order of potency was seen in displacing [ 3H]glutamate. Moreover, 2-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) inhibited only [ 3H]glutamate binding. The [ 3H]MK-801 specific binding was inhibited by all of the above antagonists; this action was abolished by glutamate, while glycine partially reversed only the action of KYN. Hence, KYN inhibits glutamate receptors by preferentially interfering with glycine recognition sites, while APV preferentially interacts with N- methyl- d-aspartate (NMDA) recognition sites.