Glutamate receptors of the kainate type and synaptic transmission

Signal Peptides and Their Fragments in Post-Translation: Novel Insights of Signal Peptides

Signal peptides (SPs), peptide sequences located at the N-terminus of newly synthesized proteins, are primarily known for their role in targeting proteins to the endoplasmic reticulum (ER). It has traditionally been assumed that cleaved SPs are rapidly degraded and digested near the ER. However, recent evidence has demonstrated that cleaved SP fragments can be detected in extracellular fluids such as blood flow, where they exhibit bioactivity. In addition, SP fragments are delivered to extracellular fluids via extracellular vesicles such as exosomes and microvesicles, which are important mediators of intercellular communication. These findings suggest that SPs and their fragments may have physiological roles beyond their classical function. This review aims to provide a comprehensive overview of these novel roles and offer new insights into the potential functions of SPs and their fragments in post-translational regulation and intercellular communication.

A glutamatergic pathway between the medial habenula and the rostral ventrolateral medulla may regulate cardiovascular function in a rat model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a serious psychiatric disorder, and there is an association between it and the development of cardiovascular disease. The aim of this study was to explore whether there is a glutamatergic pathway connecting the medial habenula (MHb) with the rostral ventrolateral medulla (RVLM) that is involved in the regulation of cardiovascular function in a rat model of PTSD. Vesicular glutamate transporter 2 (VGLUT2)-positive neurons in the MHb region were retrogradely labeled with FluoroGold (FG) by the double-labeling technique of VGLUT2 immunofluorescence and FG retrograde tracing. Rats belonging to the PTSD model group were microinjected with artificial cerebrospinal fluid (ACSF) or kynurenic acid (KYN; a nonselective glutamate receptor blocker) into their RVLM. Subsequently, with electrical stimulation of MHb, the discharge frequency of the RVLM neurons, heart rate, and blood pressure were found to be significantly increased after microinjection of ACSF using an in vivo multichannel synchronous recording technology; however, this effect was inhibited by injection of KYN. The expression of N-methyl-D-aspartic acid (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits was significantly increased in RVLM of PTSD model rats analyzed by the Western blotting technique. These findings suggest that there may be a glutamatergic pathway connection between MHb and RVLM and that this pathway may be involved in the regulation of cardiovascular function in the PTSD model rats, by acting on NMDA and AMPA receptors in the RVLM.

Synthesis and pharmacological characterisation of arctigenin analogues as antagonists of AMPA and kainate receptors

(-)-Arctigenin and a series of new analogues have been synthesised and then tested for their potential as AMPA and kainate receptor antagonists of human homomeric GluA1 and GluK2 receptors expressed in HEK293 cells using a Ca2+ influx assay. In general, these compounds showed antagonist activity at both receptors with greater activity evident at AMPARs. Schild analysis indicates that a spirocyclic analogue 6c acts as a non-competitive antagonist. Molecular docking studies in which 6c was docked into the X-ray crystal structure of the GluA2 tetramer suggest that (-)-arctigenin and its analogues bind in the transmembrane domain in a similar manner to the known AMPA receptor non-competitive antagonists GYKI53655 and the antiepileptic drug perampanel. The arctigenin derivatives described herein may serve as novel leads for the development of drugs for the treatment of epilepsy.

Magnetic Resonance Spectroscopy in Schizophrenia: Evidence for Glutamatergic Dysfunction and Impaired Energy Metabolism

In the past couple of decades, major efforts were made to increase reliability of metabolic assessments by magnetic resonance methods. Magnetic resonance spectroscopy (MRS) has been valuable for providing in vivo evidence and investigating biomarkers in neuropsychiatric disorders, namely schizophrenia. Alterations of glutamate and glutamine levels in brains of schizophrenia patients relative to healthy subjects are generally interpreted as markers of glutamatergic dysfunction. However, only a small fraction of MRS-detectable glutamate is involved in neurotransmission. Here we review and discuss brain metabolic processes that involve glutamate and that are likely to be implicated in neuropsychiatric disorders.

Protease activated receptor 1 (PAR1) enhances Src-mediated tyrosine phosphorylation of NMDA receptor in intracerebral hemorrhage (ICH)

It has been demonstrated that Src could modulate NMDA receptor, and PAR1 could also affect NMDAR signaling. However, whether PAR1 could regulate NMDAR through Src under ICH has not yet been investigated. In this study, we demonstrated the role of Src-PSD95-GluN2A signaling cascades in rat ICH model and in vitro thrombin challenged model. Using the PAR1 agonist SFLLR, antagonist RLLFS and Src inhibitor PP2, electrophysiological analysis showed that PAR1 regulated NMDA-induced whole-cell currents (INMDA) though Src in primary cultured neurons. Both in vivo and in vitro results showed the elevated phosphorylation of tyrosine in Src and GluN2A and enhanced interaction of the Src-PSD95-GluN2A under model conditions. Treatment with the PAR1 antagonist RLLFS, AS-PSD95 (Antisense oligonucleotide against PSD95) and Src inhibitor PP2 inhibited the interaction among Src-PSD95-GluN2A, and p-Src, p-GluN2A. Co-application of SFLLR and AS-PSD95, PP2, or MK801 (NMDAR inhibitor) abolished the effect of SF. In conclusion, our results demonstrated that activated thrombin receptor PAR1 induced Src activation, enhanced the interaction among Src-PSD95-GluN2A signaling modules, and up-regulated GluN2A phosphorylation after ICH injury. Elucidation of such signaling cascades would possibly provide novel targets for ICH treatment.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Synaptic Targeting of Kainate Receptors

When native and recombinant kainate receptors (KARs) are compared, there is a mismatch in several of their functional properties. While both generate currents, synaptic responses mediated by KARs have rarely observed in cultured hippocampal neurons. The recent discovery of auxiliary proteins for KARs, such as Netos, offers an explanation for these discrepancies. We found that the GluK5 KAR subunit and the ancillary proteins, Neto1 and Neto2, are not expressed by hippocampal neurons in culture. Therefore, we used this model to directly test whether these proteins are required for the synaptic localization of KARs. Transfection of GluK4, GluK5, Neto1, or Neto2 into hippocampal neurons was associated with the appearance of synaptic KAR-mediated EPSCs. However, GluK4 or GluK5 alone produced synaptic activity in a significant proportion of cells and with reliable event frequency. While neurons expressing GluK4 or GluK5 subunits displayed synaptic responses with rapid kinetics, the expression of Neto proteins conferred these synaptic responses with their characteristic slow onset and decay rates. These data reveal some requirements for KAR targeting to the synapse, indicating a fundamental role of high affinity KAR subunits in this process.

Kainate receptors in health and disease

Our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology has progressed significantly over the last 30 years. A plethora of studies indicate that kainate receptors are important mediators of the pre- and postsynaptic actions of glutamate, although the mechanisms underlying such effects are still often a topic for discussion. Three clear fields related to their behavior have emerged: there are a number of interacting proteins that pace the properties of kainate receptors; their activity is unconventional since they can also signal through G proteins, behaving like metabotropic receptors; they seem to be linked to some devastating brain diseases. Despite the significant progress in their importance in brain function, kainate receptors remain somewhat puzzling. Here we examine discoveries linking these receptors to physiology and their probable implications in disease, in particular mood disorders, and propose some ideas to obtain a deeper understanding of these intriguing proteins.

Contribution of different classes of glutamate receptors in the corticostriatal polysynaptic responses from striatal direct and indirect projection neurons

Background

Previous work showed differences in the polysynaptic activation of GABAergic synapses during corticostriatal suprathreshold responses in direct and indirect striatal projection neurons (dSPNs and iSPNs). Here, we now show differences and similarities in the polysynaptic activation of cortical glutamatergic synapses on the same responses. Corticostriatal contacts have been extensively studied. However, several questions remain unanswered, e.g.: what are the differences and similarities in the responses to glutamate in dSPNs and iSPNs? Does glutamatergic synaptic activation exhibits a distribution of latencies over time in vitro? That would be a strong suggestion of polysynaptic cortical convergence. What is the role of kainate receptors in corticostriatal transmission? Current-clamp recordings were used to answer these questions. One hypothesis was: if prolonged synaptic activation distributed along time was present, then it would be mainly generated from the cortex, and not from the striatum.

Results

By isolating responses from AMPA-receptors out of the complex suprathreshold response of SPNs, it is shown that a single cortical stimulus induces early and late synaptic activation lasting hundreds of milliseconds. Prolonged responses depended on cortical stimulation because they could not be elicited using intrastriatal stimulation, even if GABAergic transmission was blocked. Thus, the results are not explained by differences in evoked inhibition. Moreover, inhibitory participation was larger after cortical than after intrastriatal stimulation. A strong activation of interneurons was obtained from the cortex, demonstrating that polysynaptic activation includes the striatum. Prolonged kainate (KA) receptor responses were also elicited from the cortex. Responses of dSPNs and iSPNs did not depend on the cortical area stimulated. In contrast to AMPA-receptors, responses from NMDA- and KA-receptors do not exhibit early and late responses, but generate slow responses that contribute to plateau depolarizations.

Conclusions

As it has been established in previous physiological studies in vivo, synaptic invasion over different latencies, spanning hundreds of milliseconds after a single stimulus strongly indicates convergent polysynaptic activation. Interconnected cortical neurons converging on the same SPNs may explain prolonged corticostriatal responses. Glutamate receptors participation in these responses is described as well as differences and similarities between dSPNs and iSPNs.

Neuroprotection of co-activation of GABA receptors by preventing caspase-3 denitrosylation in KA-induced seizures

Previous studies have demonstrated that kainic acid (KA)-induced seizures can cause the enhancement of excitation and lead to neuronal death in rat hippocampus. Co-activation of the inhibitory GABA receptors can attenuate the excitatory JNK3 apoptotic signaling pathway via inhibiting the increased assembly of the GluR6-PSD-95-MLK3 signaling module induced by KA in epileptic rat hippocampal CA1 and CA3 regions. Caspase-3 is a cysteine protease located in both the cytoplasm and mitochondrial intermembrane space that is a central effector of many apoptotic pathways. We designed experiments to elucidate the underlying molecular mechanisms of procaspase-3 activation and neuroprotection of co-activation of GABA receptors against neuronal death induced by KA. In this study, we show that co-activation of GABA receptors can attenuate the Fas/FasL apoptotic signaling pathway and inhibit the increased of thioredoxin reductase activity induced by KA, subsequently inhibit the activation of procaspase-3 by diminishing the denitrosylation of its active-site thiol and decreasing the cleavage of the caspase-3 zymogen to its active subunits. These results indicate that co-activation of GABA receptors results in neuroprotection by preventing caspase-3 denitrosylation in KA-induced seizure of rats.

Synaptic targeting and functional modulation of GluK1 kainate receptors by the auxiliary neuropilin and tolloid-like (NETO) proteins

Auxiliary proteins modify the biophysical function and pharmacological properties of ionotropic glutamate receptors and likely are important components of receptor signaling complexes in vivo. The neuropilin and tolloid-like proteins (NETO) 1 and NETO2, two closely related CUB domain-containing integral membrane proteins, were identified recently as auxiliary proteins that slowed GluK2a kainate receptor current kinetics without impacting receptor membrane localization. Here we demonstrate that NETO2 profoundly slows the desensitization rate of GluK1 kainate receptors, promotes plasma membrane localization of transfected receptors in heterologous cells and rat hippocampal neurons, and targets GluK1-containing receptors to synapses. Conversely, the closely related protein NETO1 increases the rate of GluK1 receptor desensitization. Incorporation of NETO proteins into kainate receptor-signaling complexes therefore extends the temporal range of receptor gating by over an order of magnitude. The presence of these auxiliary proteins could underlie some of the unusual aspects of kainate receptor function in the mammalian CNS.

Deep brain stimulation changes basal ganglia output nuclei firing pattern in the dystonic hamster

Dystonia is a heterogeneous syndrome of movement disorders characterized by involuntary muscle contractions leading to abnormal movements and postures. While medical treatment is often ineffective, deep brain stimulation (DBS) of the internal pallidum improves dystonia. Here, we studied the impact of DBS in the entopeduncular nucleus (EP), the rodent equivalent of the human globus pallidus internus, on basal ganglia output in the dt(sz)-hamster, a well-characterized model of dystonia by extracellular recordings. Previous work has shown that EP-DBS improves dystonic symptoms in dt(sz)-hamsters. We report that EP-DBS changes firing pattern in the EP, most neurons switching to a less regular firing pattern during DBS. In contrast, EP-DBS did not change the average firing rate of EP neurons. EP neurons display multiphasic responses to each stimulation impulse, likely underlying the disruption of their firing rhythm. Finally, neurons in the substantia nigra pars reticulata display similar responses to EP-DBS, supporting the idea that EP-DBS affects basal ganglia output activity through the activation of common afferent fibers.

Metabotropic glutamate receptor ligands as potential therapeutics for addiction

There is now compelling evidence that the excitatory amino acid neurotransmitter glutamate plays a pivotal role in drug addiction and alcoholism. As a result, there has been increasing interest in developing glutamate-based therapies for the treatment of addictive disorders. Receptors for glutamate are primarily divided into two classes: ionotropic glutamate receptors (iGluRs) that mediate fast excitatory glutamate transmission, and metabotropic glutamate receptors (mGluRs), which are G-protein coupled receptors that mediate slower, modulatory glutamate transmission. Most iGluR antagonists, while showing some efficacy in animal models of addiction, exhibit serious side effects when tested in humans. mGluR ligands, on the other hand, which have been advanced to testing in clinical trials for various medical conditions, have demonstrated the ability to reduce drug reward, reinforcement, and relapse-like behaviors in animal studies. mGluR ligands that have been shown to be primarily effective are Group I (mGluR1 and mGluR5) negative allosteric modulators and Group II (mGluR2 and mGluR3) orthosteric presynaptic autoreceptor agonists. In this review, we will summarize findings from animal studies suggesting that these mGluR ligands may be of potential benefit in reducing on-going drug self-administration and may aid in the prevention of relapse. The neuroanatomical distribution of mGluR1, mGluR2/3, and mGluR5 receptors and the pharmacological properties of Group I negative allosteric modulators and Group II agonists will also be overviewed. Finally, we will discuss the current status of mGluR ligands in human clinical trials.

Selectivity and cooperativity of modulatory ions in a neurotransmitter receptor

Ions play a modulatory role in many proteins. Kainate receptors, members of the ionotropic glutamate receptor family, require both monovalent anions and cations in the extracellular milieu for normal channel activity. Molecular dynamics simulations and extensive relative binding free energy calculations using thermodynamic integration were performed to elucidate the rank order of binding of monovalent cations, using x-ray crystal structures of the GluR5 kainate receptor dimers with bound cations from the alkali metal family. The simulations show good agreement with experiments and reveal that the underlying backbone structure of the binding site is one of the most rigid regions of the protein. A simplified model where the partial charge of coordinating oxygens was varied suggests that selectivity arises from the presence of two carboxylate groups. Furthermore, using a potential of mean force derived from umbrella sampling, we show that the presence of cations lower the energy barrier for anion approach and binding in the buried anion binding cavity.

Role of glutamate in schizophrenia: integrating excitatory avenues of research

Schizophrenia is a debilitating lifelong disorder affecting up to 1% of the population worldwide, producing significant financial and emotional hardship for patients and their families. As yet, the causes of schizophrenia and the mechanism of action of antipsychotic drugs are unknown, and many patients do not respond well to currently available medications. Attempts to find risk factors for the disorder using epidemiological methods have shown that schizophrenia is highly heritable, and path analyses predict that the disorder is caused by several genes in combination with nongenetic factors. Therefore, intensive research efforts have been made to identify genes creating vulnerability to schizophrenia and also genes predicting response to treatment. Interactions of the glutamatergic system with dopaminergic and serotonergic circuitry are crucial for normal brain function, and their disruption may be a mechanism by which the pathophysiology of schizophrenia is manifest. Genes within the glutamatergic system are therefore strong candidates for investigation, and these include the glutamate receptor genes in addition to genes encoding neuregulin, dysbindin, D-amino acid oxidase and G72/G30. These genetic studies could eventually reveal new targets for antipsychotic drug treatment, which currently focuses on inhibition of the dopaminergic system. However, a recent breakthrough indicates clinical efficacy of a drug stimulating the metabotropic glutamate receptor II, LY2140023, which has improved efficacy for negative and cognitive symptoms of schizophrenia. Studies of larger patient samples are required to consolidate these data. Further investigation of glutamatergic targets is likely to reinvigorate antipsychotic drug development.

The history of the pharmacology and cloning of ionotropic glutamate receptors and the development of idiosyncratic nomenclature

In this article, the beginnings of glutamate pharmacology are traced from the early doubts about 'non-specific' excitatory effects, through glutamate- and aspartate-preferring receptors, to NMDA, quisqualate/AMPA and kainate subtypes, and finally to the cloning of genes for these receptor subunits. The development of selective antagonists, crucial to the subtype classification, allowed the fundamental importance of glutamate receptors to synaptic activity throughout the CNS to be realised. The ability to be able to express and manipulate cloned receptor subunits is leading to huge advances in our understanding of these receptors. Similarly the tortuous path of the nomenclature is followed from naming with reference to exogenous agonists, through abortive early attempts at generic schemes, and back to the NC-IUPHAR system based on the natural agonist, the defining exogenous agonist and the gene names.

Epilepsies and neuronal plasticity: for better or for worse?

Extensive experimental investigations have confirmed that "seizures beget seizures." Thus, in adults, limbic seizures lead to cell loss, followed by the formation of novel excitatory synapses that contribute to generating further seizures. The triggering signal is an enhancement of synaptic efficacy, followed by a molecular cascade that triggers axonal sprouting. New synapses are aberrant, since they are formed in regions in which they are not present in controls. They also involve receptors that are not present in controls, and this facilitates the generation of seizures. Therefore, an aberrant form of reactive neuronal plasticity provides a substrate for the long-lasting sequelae of seizures. Since these events take place in brain structures involved in integrative and mnemonic functions, they will have an important impact. Reactive plasticity is documented for other insults and disorders, and may be the basis for the long-term progression of neurodegenerative disorders.

Protective effect of IL-18 on kainate- and IL-1 beta-induced cerebellar ataxia in mice

The pathogenesis of sporadic cerebellar ataxia remains unknown. In this study, we demonstrate that proinflammatory cytokines, IL-18 and IL-1beta, reciprocally regulate kainate-induced cerebellar ataxia in mice. We show that systemic administration of kainate activated IL-1beta and IL-18 predominantly in the cerebellum of mice, which was accompanied with ataxia. Mice deficient in caspase-1, IL-1R type I, or MyD88 were resistant to kainate-induced ataxia, while IL-18- or IL-18R alpha-deficient mice displayed significant delay of recovery from ataxia. A direct intracerebellar injection of IL-1beta-induced ataxia and intracerebellar coinjection of IL-18 counteracted the effect of IL-1beta. Our data firstly show that IL-18 and IL-1beta display differential direct regulation in kainate-induced ataxia in mice. Our results might contribute toward the development of a new therapeutic strategy for cerebellar ataxia in humans.

Neuroprotective effects of lactation against kainic acid treatment in the dorsal hippocampus of the rat

Marked hippocampal changes in response to excitatory amino acid agonists occur during pregnancy (e.g. decreased frequency in spontaneous recurrent seizures in rats with KA lesions of the hippocampus) and lactation (e.g. reduced c-Fos expression in response to N-methyl-d,l-aspartic acid but not to kainic acid). In this study, the possibility that lactation protects against the excitotoxic damage induced by KA in hippocampal areas was explored. We compared cell damage induced 24 h after a single systemic administration of KA (5 or 7.5 mg/kg bw) in regions CA1, CA3, and CA4 of the dorsal hippocampus of rats in the final week of lactation to that in diestrus phase. To determine cellular damage in a rostro-caudal segment of the dorsal hippocampus, we used NISSL and Fluorojade staining, immunohistochemistry for active caspase-3 and TUNEL, and we observed that the KA treatment provoked a significant loss of neurons in diestrus rats, principally in the pyramidal cells of CA1 region. In contrast, in lactating rats, pyramidal neurons from CA1, CA3, and CA4 in the dorsal hippocampus were significantly protected against KA-induced neuronal damage, indicating that lactation may be a natural model of neuroprotection.

Mechanisms involved in the cerebrovascular dilator effects of N-methyl-d-aspartate in cerebral cortex

Glutamate and its synthetic analogues N-methyl-d-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) are potent dilator agents in the cerebral circulation. The close linkage between neural activity-based release and actions of glutamate on neurons and the related decrease in cerebral vascular resistance is a classic example in support of the concept of tight coupling between increased neural activity and cerebral blood flow. However, mechanisms involved in promoting cerebral vasodilator responses to glutamatergic agents are controversial. Here we review the development and current status of this important field of research especially in respect to cerebrovascular responses to NMDA receptor activation.

Glutamate receptors in neuroinflammatory demyelinating disease

Multiple sclerosis (MS) is a chronic demyelinating disease of the human central nervous system (CNS). The condition predominantly affects young adults and is characterised by immunological and inflammatory changes in the periphery and CNS that contribute to neurovascular disruption, haemopoietic cell invasion of target tissues, and demyelination of nerve fibres which culminate in neurological deficits that relapse and remit or are progressive. The main features of MS can be reproduced in the inducible animal counterpart, experimental autoimmune encephalomyelitis (EAE). The search for new MS treatments invariably employs EAE to determine drug activity and provide a rationale for exploring clinical efficacy. The preclinical development of compounds for MS has generally followed a conventional, immunotherapeutic route. However, over the past decade, a group of compounds that suppress EAE but have no apparent immunomodulatory activity have emerged. These drugs interact with the N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-isoxazolepropionic acid (AMPA)/kainate family of glutamate receptors reported to control neurovascular permeability, inflammatory mediator synthesis, and resident glial cell functions including CNS myelination. The review considers the importance of the glutamate receptors in EAE and MS pathogenesis. The use of receptor antagonists to control EAE is also discussed together with the possibility of therapeutic application in demyelinating disease.

Variable kainate receptor distributions of oriens interneurons

Interneuron kainate receptor (KAR) activation regulates normal network activity and modulates cell excitability. As a result, determining the subcellular distribution of KARs in a cell-specific manner is a necessary step toward understanding their role in network function. We have functionally mapped synaptic and extrasynaptic dendritic KARs on hippocampal oriens interneurons using local photolysis of caged glutamate. We find that the majority of trilaminar and oriens lacunosum-moleculare (O-LM) cells have uniform and continuous current densities along the lengths of their dendrites. However, there is a subpopulation of interneurons that have no KAR currents or currents exclusively at "hot spots" on the soma and dendrites. Finally, bistratified cells have KAR currents on all dendrites except those extending into the stratum radiatum. Thus KARs are functionally distributed in a cell-specific and cell-independent manner that may reflect the physiologically distinct roles they play in the hippocampal network.

Binding site flexibility: molecular simulation of partial and full agonists within a glutamate receptor

Ionotropic glutamate receptors mediate fast synaptic transmission in the mammalian central nervous system and play an important role in many different functions, including memory and learning. They have also been implicated in a variety of neuropathologies and as such have generated widespread interest in their structure and function. Molecular Dynamics simulations (5 x 20 ns) of the ligand-binding core of the GluR2 glutamate receptor were performed. Through simulations of both wild type and the L650T mutant, we show that the degree of protein flexibility can be correlated with the extent to which the binding cleft is open. In agreement with recent experiments, the simulations of kainate with the wild-type construct show a slight increase in beta-sheet content that we are able to localize to two specific regions. During one simulation, the protein made a transition from an open-cleft conformation to a closed-cleft conformation. This closed cleft conformation closely resembles the closed-cleft crystal structure, thus indicating a potential pathway for conformational change associated with receptor activation. Analysis of the binding pocket suggests that partial agonists possess a greater degree of flexibility within the pocket that may help to explain why they are less efficient at opening the channel than full agonists. Examination of water molecules surrounding the ligands reveals that mobility in distinct subsites can be a discriminator between full and partial agonism and will be an important consideration in the design of drugs against these receptors.

Excitatory amino acid neurotransmission

In recent years great progress has been made in understanding the function of ionotropic and metabotropic glutamate receptors; their pharmacology and potential therapeutic applications. It should be stressed that there are already N-methyl-D-aspartate (NMDA) antagonists in clinical use, such as memantine, which proves the feasibility of their therapeutic potential. It seems unlikely that competitive NMDA receptor antagonists and high-affinity channel blockers will find therapeutic use due to limiting side-effects, whereas agents acting at the glycineB site, NMDA receptor subtype-selective agents and moderate-affinity channel blockers are far more promising. This is supported by the fact that there are several glycineB antagonists, NMDA moderate-affinity channel blockers and NR2B-selective agents under development. Positive and negative modulators of AMPA receptors such as the AMPAkines and 2,3-benzodiazepines also show more promise than e.g. competitive antagonists. Great progress has also been made in the field of metabotropic glutamate receptors since the discovery of novel, allosteric modulatory sites for these receptors. Selective agents acting at these transmembrane sites have been developed that are more drug-like and have a much better access to the central nervous system than their competitive counterparts. The chapter will critically review preclinical and scarce clinical experience in the development of new ionotropic and metabotropic glutamate receptor modulators according to the following scheme: rational, preclinical findings in animal models and finally clinical experience, where available.

Localization of kainate receptor subunit GluR5-immunoreactive cells in the rat hypothalamus

Glutamate is the major excitatory neurotransmitter in the hypothalamus, which exerts its effects by activating ion channel-forming (ionotropic) or G-protein-coupled (metabotropic) receptors. Kainate-preferring glutamate receptor subunits (GluR5, GluR6, GluR7, KA1, and KA2) form one of the three ionotropic receptor families. In the present study, we analyzed the distribution of GluR5 subunit protein in the rat hypothalamus with immunohistochemistry. GluR5 immunoreactivity was observed in perikarya and processes of many hypothalamic cells some of which, based upon their morphological differentiation by size and structure, appeared to be neurons and others glial cells. Analyses revealed that higher number of glial cells were GluR5 positive when compared to the moderate number of GluR5-labeled neurons in the anteroventral periventricular nucleus. Numerous GluR5-expressing neurons and similar number of glia were detected in the suprachiasmatic nucleus. In the arcuate nucleus more glial cells were identified with GluR5 immunoreactivity than the number of labeled neurons. Scattered GluR5-positive cells were present in the periventricular nucleus. Specific immunostaining was not seen in the ventromedial nucleus or dorsomedial nucleus. In conclusion, it is suggested that the GluR5 subunits participate in the glutamatergic regulation of several neuroendocrine systems, such as the tubero-infundibular systems as well as in the control of circadian output through neuron-to-neuron and/or neuron-to-glia interactions.

Provocation of kainic acid receptor mRNA changes in the rat paraventricular nucleus by insulin-induced hypoglycaemia

Hypoglycaemia induced by insulin injection is a powerful stimulus to the hypothalamic-pituitary-adrenal (HPA) axis and drives the secretion of corticotropin-releasing hormone and vasopressin from the neurones in the paraventricular nucleus (PVN), as well as the downstream hormones, adrenocorticotropic hormone and corticosterone. In some brain regions, hypoglycaemia also provokes increases in extracellular fluid concentrations of glutamate. Regulation of glutamatergic mechanisms could be involved in the control of the HPA axis during hypoglycaemic stress and one potential site of regulation might be at the receptors for glutamate, which are expressed in the PVN. Insulin (2.0 IU/kg, i.p.) or saline was administered to adult male Sprague-Dawley rats and the animals were sacrificed 30 min, 180 min and 24 h after injection. The amount of several kainic acid-preferring glutamate receptor mRNAs (i.e. KA2, GluR5 and GluR6) were assessed in the PVN by in situ hybridisation histochemistry. Injection of insulin induced a rapid fall in plasma glucose concentrations, which was mirrored by an increase in plasma corticosterone concentrations. KA2 and GluR5 mRNAs are highly expressed within the rat PVN, and responded to hypoglycaemia with robust increases in expression that endured beyond the period of hypoglycaemia itself. However, GluR6 mRNA is expressed in the areas adjacent to the PVN and hypoglycaemic stress failed to alter expression of this mRNA. These experiments suggest that kainic acid-preferring glutamate receptors are responsive to changes in plasma glucose concentrations and may participate in the activation of the PVN neurones during hypoglycaemic stress.

Activity-dependent endocytic sorting of kainate receptors to recycling or degradation pathways

Kainate receptors (KARs) play important roles in the modulation of neurotransmission and plasticity, but the mechanisms that regulate their surface expression and endocytic sorting remain largely unknown. Here, we show that in cultured hippocampal neurons the surface expression of GluR6-containing KARs is dynamically regulated. Furthermore, internalized KARs are sorted into recycling or degradative pathways depending on the endocytotic stimulus. Kainate activation causes a Ca2+- and PKA-independent but PKC-dependent internalization of KARs that are targeted to lysosomes for degradation. In contrast, NMDAR activation evokes a Ca2+-, PKA- and PKC-dependent endocytosis of KARs to early endosomes with subsequent reinsertion back into the plasma membrane. These results demonstrate that GluR6-containing KARs are subject to activity-dependent endocytic sorting, a process that provides a mechanism for both rapid and chronic changes in the number of functional receptors.

Comparative anticonvulsant activity of N-acetyl-1-aryl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline derivatives in rodents

The anticonvulsant activity of competitive 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo (F)-quinoxaline (NBQX) and noncompetitive 2,3-benzodiazepines and tetrahydroisoquinolines (THIQs) AMPA/kainate receptor antagonists, was tested in different experimental seizure models and compared with diazepam, a conventional antiepileptic drug acting on GABAergic neurotransmission. In particular, the compounds were evaluated against audiogenic and maximal electroshock seizures (MES) test and pentetrazol (PTZ) seizures model, and all of them showed protective action. In addition, NBQX, 2,3-benzodiazepines and THIQs, but not diazepam, were also protective against clonic and tonic seizures and lethality induced by kainate, AMPA and ATPA, but were ineffective against NMDA-induced seizures. Only 2,3-benzodiazepines and some THIQs were able to affect 4-aminopyridine- and mercaptopropionic-acid-induced seizures. The duration of anticonvulsant action of 33 micromol/kg of some 2,3-benzodiazepines and THIQs was also investigated in DBA/2 mice, a strain genetically susceptible to audiogenic seizures, and it was observed that the derivative THIQ-10c, possessing an acetyl group at the N-2 and a chlorine atom on the C-1 phenyl ring, showed higher anticonvulsant activity and longer-lasting protective effects.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

Astrocyte-mediated activation of neuronal kainate receptors

Exogenous kainate receptor agonists have been shown to modulate inhibitory synaptic transmission in the hippocampus, but the pathways involved in physiological activation of the receptors remain largely unknown. Accumulating evidence indicates that astrocytes can release glutamate in a Ca(2+)-dependent manner and signal to neighboring neurons. We tested the hypothesis that astrocyte-derived glutamate activates kainate receptors on hippocampal interneurons. We report here that elevation of intracellular Ca(2+) in astrocytes, induced by uncaging Ca(2+), o-nitrophenyl-EGTA, increased action potential-driven spontaneous inhibitory postsynaptic currents in nearby interneurons in rat hippocampal slices. This effect was blocked by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate glutamate receptor antagonists, but not by selective AMPA receptor or N-methyl-d-aspartate receptor antagonists. This pharmacological profile indicates that kainate receptors were activated during Ca(2+) elevation in astrocytes. Kainate receptors containing the GluR5 subunit seemed to mediate the observed effect because a selective GluR5-containing kainate receptor antagonist blocked the changes in sIPSCs induced by Ca(2+) uncaging, and bath application of a selective GluR5-containing receptor agonist robustly potentiated sIPSCs. When tetrodotoxin was included to block action potentials, Ca(2+) uncaging induced a small decrease in the frequency of miniature inhibitory postsynaptic currents, which was not affected by AMPA/kainate receptor antagonists. Our data suggest that an astrocyte-derived, nonsynaptic source of glutamate represents a signaling pathway that can activate neuronal kainate receptors. By modulating the activity of interneurons, astrocytes may play a critical role in circuit function of hippocampus.

Cloning and characterization of an ionotropic glutamate receptor subunit expressed in the squid nervous system

In this paper we describe the cloning of a putative ionotropic glutamate receptor subunit, SqGluR, and its distribution in the nervous system of the squid. A full-length cDNA was assembled from a cDNA library of the stellate ganglion/giant fibre lobe complex of Loligo opalescens. The deduced amino acid sequence of the mature SqGluR displayed 44-46% amino acid identity with mammalian GluR1-GluR4 and 53% with Lym-eGluR1 from Lymnaea stagnalis. In situ hybridizations in adult squid confirmed that the SqGluR mRNA is abundant in giant fibre lobe neurons, in large, presumptive motor neurons of the stellate ganglion proper and in the supraoesophageal and optic lobes of the central nervous system. In newborn squid, SqGluR mRNA expression was detected throughout the nervous system but not elsewhere. A synthetic peptide corresponding to the last 15 amino acids of the SqGluR C-terminus was used to generate polyclonal antibodies, which were used for immunoblot analysis to demonstrate widespread expression in the squid central and peripheral nervous systems. Injection of the synthetic peptide into the postsynaptic side of the giant synapse inhibited synaptic transmission.

Comparative anticonvulsant activity of some 2,3-benzodiazepine derivatives in rodents

The anticonvulsant activities of some 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA)/kainate receptor antagonists, noncompetitive (2,3-benzodiazepines) and a competitive 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline (NBQX), were compared in different experimental seizure models. In particular, compounds were evaluated against audiogenic seizure in DBA/2 mice, maximal electroshock seizure (MES) test and various chemoconvulsant models; both groups showed a protective action against audiogenic seizure, MES- and pentylenetetrazole (PTZ)-induced seizures. All 2,3-benzodiazepines were also protective against clonic and tonic seizures and lethality induced by 4-aminopyridine, kainate, AMPA and 3-mercaptopropionic acid but were ineffective against NMDA-induced seizures. NBQX was unable to affect 4-aminopyridine-, mercaptopropionic acid- and NMDA-induced seizures. The duration of anticonvulsant action of 33 micromol/kg of some 2,3-benzodiazepine in DBA/2 mice, genetically susceptible to audiogenic seizures, was also investigated. The derivatives possessing a thiocarbonyl group at the C-4 position of heptatomic ring showed higher anticonvulsant activities and longer lasting protective effects. We conclude that all 2,3-benzodiazepines studied are effective against various models of experimental epilepsy and the presence of thiocarbonyl groups at the C-4 position of heptatomic ring is able to increase the anticonvulsant effect of these compounds.

Pre-synaptic kainate receptors in GABAergic and glutamatergic axon terminals in the monkey globus pallidus

Although the localization and role of kainate receptors in the CNS remain poorly known, complex, and rather unusual, pre-synaptic auto- and heteroreceptor functions have been disclosed in various brain regions. Basal ganglia nuclei, including the globus pallidus, are enriched in GluR6/7 immunoreactivity. Using electron microscopic immunocytochemistry for GluR6/7 combined with post-embedding immunogold labeling for GABA, we demonstrate that GluR6/7 immunoreactivity is enriched in a large subpopulation of small unmyelinated, presumably pre-terminal, axons as well as GABAergic and putative glutamatergic axon terminals in the internal and external segments of the globus pallidus in monkey. Our findings suggest that kainate receptors are located to subserve pre-synaptic modulation of inhibitory and excitatory transmission in the primate globus pallidus.

Kindling enhances kainate receptor-mediated depression of GABAergic inhibition in rat granule cells

Several lines of evidence indicate a substantial contribution of kainate receptors to temporal lobe seizures. The activation of kainate receptors located on hippocampal inhibitory interneurons was shown to reduce GABA release. A reduced GABA release secondary to kainate receptor activation could contribute to an enhanced seizure susceptibility. As the dentate gyrus serves a pivotal gating function in the spread of limbic seizures, we tested the role of kainate receptors in the regulation of GABA release in the dentate gyrus of control and kindled animals. Application of glutamate (100 micro m) in the presence of the NMDA receptor antagonist d-APV and the AMPA receptor antagonist, SYM 2206 caused a slight depression of evoked monosynaptic inhibitory postsynaptic currents (IPSCs) in control, but a substantial decrease in kindled dentate granule cells. The observation that kainate receptor activation altered paired-pulse depression and reduced the frequency of TTX-insensitive miniature IPSCs without affecting their amplitude is consistent with a presynaptic action on the inhibitory terminal to reduce GABA release. In kindled preparations, neither glutamate (100 micro m) nor kainate (10 micro m) applied in a concentration known to depolarize hippocampal interneurons led to an increase of the TTX-sensitive spontaneous IPSC frequency nor to changes of the postsynaptic membrane properties. Consistently, the inhibitory effect on evoked IPSCs was not affected by the presence of the GABAB receptor antagonist, CGP55845A, thus excluding a depression by an enhanced release of GABA acting on presynaptic GABAB receptors. The enhanced inhibition of GABA release following presynaptic kainate receptor activation favours a use-dependent hyperexcitability in the epileptic dentate gyrus.

Contribution of AMPA and NMDA receptors to excitatory responses in the inferior colliculus

Brain slice studies of neurons in the central nucleus of the inferior colliculus (ICC) indicate that excitatory responses evoked by electrical stimulation of the lateral lemniscus consist of two components, an early, rapid response mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and a later, a slower one mediated by N-methyl-D-aspartate (NMDA) receptors. The early response can be selectively blocked by AMPA receptor antagonists (1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium [NBQX]; or 6-cyano-7-nitroquinoxaline-2,3-dione) [CNQX], and the later one by NMDA receptor antagonists ((+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CPP]; or (+/-)-2-amino-5-phosphonovaleric acid) [APV]. Both AMPA and NMDA receptor-mediated responses can be elicited at resting potential, although the NMDA response is voltage dependent and makes a greater contribution when the cell membrane is depolarized. In vivo studies indicate that both AMPA and NMDA receptors contribute to sound-evoked responses. Both AMPA and NMDA receptor antagonists reduce the firing rate of single neurons in the ICC to contralaterally presented tones. Both classes of antagonist lower evoked activity over a wide range of sound intensities from threshold to maximum sound pressure levels. Thus, both NMDA and AMPA receptors contribute to responses over the full dynamic range of auditory sensitivity. The AMPA receptor antagonist, NBQX, is more effective than the NMDA receptor antagonist, CPP, in blocking responses of onset cells. Furthermore, NBQX and CPP have preferential effects in blocking the early or late responses of neurons that exhibited sustain activity to a 100 ms tone. Excitatory responses to sinusoidally amplitude-modulated stimuli are also reduced by application of either AMPA or NMDA antagonists. However, the synchrony of firing of action potentials to the modulation period (vector strength) is largely unaffected. The data suggest that the synchrony of firing of neurons in the inferior colliculus is determined primarily by the pattern of activity at lower levels of the auditory pathway and/or the local intrinsic properties of the cells.

Molecular dynamics simulations of the ligand-binding domain of the ionotropic glutamate receptor GluR2

Ionotropic glutamate receptors are essential for fast synaptic nerve transmission. Recent x-ray structures for the ligand-binding (S1S2) region of the GluR2 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-sensitive receptor have suggested how differences in protein/ligand interactions may determine whether a ligand will behave as a full agonist. We have used multiple molecular dynamics simulations of 2-5 ns duration to explore the structural dynamics of GluR2 S1S2 in the presence and absence of glutamate and in a complex with kainate. Our studies indicate that not only is the degree of domain closure dependent upon interactions with the ligand, but also that protein/ligand interactions influence the motion of the S2 domain with respect to S1. Differences in domain mobility between the three states (apo-S1S2, glutamate-bound, and kainate-bound) are surprisingly clear-cut. We discuss how these changes in dynamics may provide an explanation relating the mechanism of transmission of the agonist-binding event to channel opening. We also show here how the glutamate may adopt an alternative mode of binding not seen in the x-ray structure, which involves a key threonine (T480) side chain flipping into a new conformation. This new conformation results in an altered pattern of hydrogen bonding at the agonist-binding site.

AMPA-evoked acetylcholine release from cultured spinal cord motoneurons and its inhibition by GABA and glycine

The release of [(3)H]acetylcholine evoked by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and its inhibition mediated by GABA(A) and glycine receptors were studied in superfused cultured rat embryo spinal cord motoneurons prelabeled with [(3)H]choline. AMPA elicited tritium release, possibly representing [(3)H]acetylcholine release in a concentration-dependent manner. The release was external Ca(2+)-dependent and was sensitive to Cd(2+) ions, omega-conotoxin GVIA and omega-conotoxin MVIIC, but not to nifedipine, suggesting the involvement of N-, P/Q-, but not L-type Ca(2+) channels. The AMPA effect was insensitive to tetrodotoxin. The glutamate receptors involved are AMPA type since the AMPA-evoked [(3)H]acetylcholine release was blocked by LY303070 and was potentiated by the antidesensitizing agent cyclothiazide. Muscimol inhibited completely the AMPA effect on [(3)H]acetylcholine release; muscimol was potentiated by diazepam and antagonized by SR95531, indicating the involvement of benzodiazepine-sensitive GABA(A) receptors. Glycine, acting at strychnine-sensitive receptors, also inhibited the effect of AMPA, but only in part. The inhibitory effects of muscimol and glycine are additive. We conclude that glutamate can act at AMPA receptors sited on spinal motoneurons to evoke release of acetylcholine. GABA and glycine, possibly released as cotransmitters from spinal interneurons, inhibit glutamate-evoked acetylcholine release by activating GABA(A) and glycine receptors on motoneurons.

Subcellular and subsynaptic localization of presynaptic and postsynaptic kainate receptor subunits in the monkey striatum

The localization and functions of kainate receptors (KARs) in the CNS are still poorly known. In the striatum, GluR6/7 and KA2 immunoreactivity is expressed presynaptically in a subpopulation of glutamatergic terminals and postsynaptically in dendrites and spines. The goal of this study was to further characterize the subcellular and subsynaptic localization of kainate receptor subunits in the monkey striatum. Immunoperoxidase data reveal that the relative abundance of GluR6/7- and KA2-immunoreactive terminals is homogeneous throughout the striatum irrespective of the differential degree of striatal degeneration in Huntington's disease. Pre-embedding and post-embedding immunogold data indicate that >70% of the presynaptic or postsynaptic GluR6/7 and KA2 labeling is expressed intracellularly. In material stained with the post-embedding immunogold method, approximately one-third of plasma membrane-bound gold particles labeling in axon terminals and spines is associated with asymmetric synapses, thereby representing synaptic kainate receptor subunits. On the other hand, >60% of the plasma-membrane bound labeling is extrasynaptic. Both GluR6/7 and KA2 labeling in glutamatergic terminals often occurs in clusters of gold particles along the membrane of large vesicular organelles located at various distances from the presynaptic grid. Anterograde labeling from the primary motor cortex or the centromedian thalamic nucleus indicate that both corticostriatal and thalamostriatal terminals express presynaptic GluR6/7 and KA2 immunoreactivity in the postcommissural putamen. In conclusion, these data demonstrate that kainate receptors in the striatum display a pattern of subcellular distribution different from other ionotropic glutamate receptor subtypes, but consistent with their metabotropic-like functions recently shown in the hippocampus.

Molecular physiology of kainate receptors

A decade ago, our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology was essentially null. A plethora of recent studies has altered this situation profoundly such that kainate receptors are now regarded as key players in the modulation of transmitter release, as important mediators of the postsynaptic actions of glutamate, and as possible targets for the development of antiepileptic and analgesic drugs. In this review, we summarize our current knowledge of the properties of kainate receptors focusing on four key issues: 1) their structural and biophysical features, 2) the important progress in their pharmacological characterization, 3) their pre- and postsynaptic mechanisms of action, and 4) their involvement in a series of physiological and pathological processes. Finally, although significant progress has been made toward the elucidation of their importance for brain function, kainate receptors remain largely an enigma and, therefore, we propose some new roads that should be explored to obtain a deeper understanding of this young, but intriguing, class of proteins.

Insulin-like growth factor I potentiates kainate receptors through a phosphatidylinositol 3-kinase dependent pathway

Neurotrophic factors modulate synaptic plasticity through mechanisms that include regulation of membrane ion channels and neurotransmitter receptors. Recently, it was shown that insulin-like growth factor I (IGF-I) induces depression of AMPA-mediated currents without affecting NMDA-receptor function in neurons. We now report that IGF-I markedly potentiates the kainate-preferring ionotropic glutamate receptor in young cerebellar granule neurons expressing functional kainate-, but not AMPA-mediated currents. Potentiation of kainate responses by IGF-I is blocked by wortmannin, a phosphatidylinositol 3-kinase (P13K) inhibitor, indicating a role for this kinase in the effect of IGF-I. These results reinforce the notion that modulation of ionotropic glutamate receptors are involved in the regulatory actions of IGF-I on neuronal plasticity.

Kindling Induces Transient NMDA Receptor–Mediated Facilitation of High-Frequency Input in the Rat Dentate Gyrus

To elucidate the gating mechanism of the epileptic dentate gyrus on seizure-like input, we investigated dentate gyrus field potentials and granule cell excitatory postsynaptic potentials (EPSPs) following high-frequency stimulation (10–100 Hz) of the lateral perforant path in an experimental model of temporal lobe epilepsy (i.e., kindled rats). Although control slices showed steady EPSP depression at frequencies greater than 20 Hz, slices taken from animals 48 h after the last seizure presented pronounced EPSP facilitation at 50 and 100 Hz, followed by steady depression. However, 28 days after kindling, the EPSP facilitation was no longer detectable. Using the specific N-methyl-d-aspartate (NMDA) and RS-α-amino-3-hydroxy-5-methyl-4-isoxazoleproponic acid (AMPA) receptor antagonists 2-amino-5-phosphonovaleric acid and SYM 2206, we examined the time course of alterations in glutamate receptor–dependent synaptic currents that parallel transient EPSP facilitation. Forty-eight hours after kindling, the fractional AMPA and NMDA receptor–mediated excitatory postsynaptic current (EPSC) components shifted dramatically in favor of the NMDA receptor–mediated response. Four weeks after kindling, however, AMPA and NMDA receptor–mediated EPSCs reverted to control-like values. Although the granule cells of the dentate gyrus contain mRNA-encoding kainate receptors, neither single nor repetitive perforant path stimuli evoked kainate receptor–mediated EPSCs in control or in kindled rats. The enhanced excitability of the kindled dentate gyrus 48 h after the last seizure, as well as the breakdown of its gating function, appear to result from transiently enhanced NMDA receptor activation that provides significantly slower EPSC kinetics than those observed in control slices and in slices from kindled animals with a 28-day seizure-free interval. Therefore, NMDA receptors seem to play a critical role in the acute throughput of seizure activity and in the induction of the kindled state but not in the persistence of enhanced seizure susceptibility.

Consolidation of transient ionotropic glutamate signals through nuclear transcription factors in the brain

Long-lasting alterations of neuronal functions could involve mechanisms associated with consolidation of transient extracellular signals through modulation of de novo synthesis of particular functional proteins in the brain. In eukaryotes, protein de novo synthesis is mainly under the control at the level of gene transcription by transcription factors in the cell nucleus. Transcription factors are nuclear proteins with an ability to recognize particular core nucleotides at the upstream and/or downstream of target genes, and thereby to modulate the activity of RNA polymerase II that is responsible for the formation of mRNA from double stranded DNA. Gel retardation electrophoresis is widely employed for conventional detection of DNA binding activities of a variety of transcription factors with different protein motifs. Extracellular ionotropic glutamate (Glu) signals lead to rapid and selective potentiation of DNA binding of the nuclear transcription factor activator protein-1 (AP1) that is a homo- and heterodimeric complex between Jun and Fos family members, in addition to inducing expression of the corresponding proteins, in a manner unique to each Glu signal in murine hippocampus. Therefore, extracellular Glu signals may be differentially transduced into the nucleus to express AP1 with different assemblies between Jun and Fos family members, and thereby to modulate de novo synthesis of the individual target proteins at the level of gene transcription in the hippocampus. Such mechanisms may be operative on synaptic plasticity as well as delayed neuronal death through consolidation of alterations of a variety of cellular functions induced by transient extracellular signals in the brain.

Water maze and radial maze learning and the density of binding sites of glutamate, GABA, and serotonin receptors in the hippocampus of inbred mouse strains

Correlations between the densities of ionotropic glutamate, GABA(A), and serotonin binding sites in the hippocampus of seven inbred mouse strains and strain-specific learning capacities in two types of maze were studied. Binding site densities were measured with quantitative receptor autoradiography. Learning capacities were determined in a water maze task as well as in spatial and nonspatial versions of an eight-arm radial maze. The densities of most binding sites differed significantly between the strains in the subfields of Ammon's horn (CA1 and CA3) and the dentate gyrus, except for serotonin binding sites in CA1. By comparing the different strains, significant receptor-behavioral correlations between the densities of the GABA(A) receptors and the activity-dependent behavior in the water maze as well as the spatial learning in the radial maze were found. The densities of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionate (AMPA) and kainate receptors correlated positively with learning capacity in the spatial eight-arm radial maze. We conclude that hereditary variations mainly in AMPA, kainate, and GABA(A) receptor densities are involved in behavioral variations in spatial and nonspatial learning tasks.

In situ Ca2+ imaging reveals neurotransmitter receptors for glutamate in taste receptor cells

The neurotransmitters at synapses in taste buds are not yet known with confidence. Here we report a new calcium-imaging technique for taste buds that allowed us to test for the presence of glutamate receptors (GluRs) in living isolated tissue preparations. Taste cells of rat foliate papillae were loaded with calcium green dextran (CaGD). Lingual slices containing CaGD-labeled taste cells were imaged with a scanning confocal microscope and superfused with glutamate (30 micromter to 1 mm), kainate (30 and 100 micrometer), AMPA (30 and 100 micrometer), or NMDA (100 micrometer). Responses were observed in 26% of the cells that were tested with 300 micrometer glutamate. Responses to glutamate were localized to the basal processes and cell bodies, which are synaptic regions of taste cells. Glutamate responses were dose-dependent and were induced by concentrations as low as 30 microm. The non-NMDA receptor antagonists CNQX and GYKI 52466 reversibly blocked responses to glutamate. Kainate, but not AMPA, also elicited Ca(2+) responses. NMDA stimulated increases in [Ca(2+)](i) when the bath medium was modified to optimize for NMDA receptor activation. The subset of cells that responded to glutamate was either NMDA-unresponsive (54%) or NMDA-responsive (46%), suggesting that there are presumably two populations of glutamate-sensitive taste cells-one with NMDA receptors and the other without NMDA receptors. The function of GluRs in taste buds is not yet known, but the data suggest that glutamate is a neurotransmitter there. GluRs in taste cells might be presynaptic autoreceptors or postsynaptic receptors at afferent or efferent synapses.

In vivo, the direct and seizure-induced neuronal cytotoxicity of kainate and AMPA is modified by the non-competitive antagonist, GYKI 52466

The 2,3-benzodiazepine GYKI 52466, administered intracerebrally or systemically, was assessed for its ability to protect against the neuronal death in the brain caused by intra-hippocampal injections of the non-N-methyl-D-aspartate (NMDA) receptor agonists, kainate and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). In contrast to a previous report, a low intra-hippocampal dose of GYKI 52466 (25 nmol) did not protect against kainate toxicity. In order to achieve higher doses of GYKI 52466, solubilization in 2-hydroxypropyl-beta-cyclodextrin was used, and limited protection against AMPA, but not kainate toxicity was found. There was a commensurate reduction in seizure-related neuronal loss in the limbic regions of the brain. When diazepam was used to prevent seizures, GYKI 52466 had no effect on hippocampal neuronal loss caused by the direct toxicity of AMPA and kainate on hippocampal neurons. Systemic administration of GYKI 52466 had only a minimal effect on preventing neuronal death caused by AMPA. In vivo, GYKI 52466 is only weakly effective as a neuroprotective agent.

The role of RNA editing of kainate receptors in synaptic plasticity and seizures

The ionotropic glutamate receptor subunit GluR6 undergoes developmentally and regionally regulated Q/R site RNA editing that reduces the calcium permeability of GluR6-containing kainate receptors. To investigate the functional significance of this editing in vivo, we engineered mice deficient in GluR6 Q/R site editing. In these mutant mice but not in wild types, NMDA receptor-independent long-term potentiation (LTP) could be induced at the medial perforant path-dentate gyrus synapse. This indicates that kainate receptors with unedited GluR6 subunits can mediate LTP. Behavioral analyses revealed no differences from wild types, but mutant mice were more vulnerable to kainate-induced seizures. Together, these results suggest that GluR6 Q/R site RNA editing may modulate synaptic plasticity and seizure vulnerability.