Pharmacological characterization of glutamatergic agonists and antagonists at recombinant human homomeric and heteromeric kainate receptors in vitro

The positive allosteric modulator BPAM344 and L-glutamate introduce an active-like structure of the ligand-binding domain of GluK2

Kainate receptors belong to the family of ionotropic glutamate receptors and contribute to the majority of fast excitatory neurotransmission. Consequently, they also play a role in brain diseases. Therefore, understanding how these receptors can be modulated is of importance. Our study provides a crystal structure of the dimeric ligand-binding domain of the kainate receptor GluK2 in complex with L-glutamate and the small-molecule positive allosteric modulator, BPAM344, in an active-like conformation. The role of Thr535 and Gln786 in modulating GluK2 by BPAM344 was investigated using a calcium-sensitive fluorescence-based assay on transiently transfected cells expressing GluK2 and mutants hereof. This study may aid in the design of compounds targeting kainate receptors, expanding their potential as targets for the treatment of brain diseases.

Small-molecule positive allosteric modulation of homomeric kainate receptors GluK1-3: development of screening assays and insight into GluK3 structure

The kainate receptors GluK1-3 (glutamate receptor ionotropic, kainate receptors 1-3) belong to the family of ionotropic glutamate receptors and are essential for fast excitatory neurotransmission in the brain, and are associated with neurological and psychiatric diseases. How these receptors can be modulated by small-molecule agents is not well understood, especially for GluK3. We show that the positive allosteric modulator BPAM344 can be used to establish robust calcium-sensitive fluorescence-based assays to test agonists, antagonists, and positive allosteric modulators of GluK1-3. The half-maximal effective concentration (EC50) of BPAM344 for potentiating the response of 100 μm kainate was determined to be 26.3 μm for GluK1, 75.4 μm for GluK2, and 639 μm for GluK3. Domoate was found to be a potent agonist for GluK1 and GluK2, with an EC50 of 0.77 and 1.33 μm, respectively, upon co-application of 150 μm BPAM344. At GluK3, domoate acts as a very weak agonist or antagonist with a half-maximal inhibitory concentration (IC50) of 14.5 μm, in presence of 500 μm BPAM344 and 100 μm kainate for competition binding. Using H523A-mutated GluK3, we determined the first dimeric structure of the ligand-binding domain by X-ray crystallography, allowing location of BPAM344, as well as zinc-, sodium-, and chloride-ion binding sites at the dimer interface. Molecular dynamics simulations support the stability of the ion sites as well as the involvement of Asp761, Asp790, and Glu797 in the binding of zinc ions. Using electron microscopy, we show that, in presence of glutamate and BPAM344, full-length GluK3 adopts a dimer-of-dimers arrangement.

Enhanced Membrane Incorporation of H289Y Mutant GluK1 Receptors from the Audiogenic Seizure-Prone GASH/Sal Model: Functional and Morphological Impacts on Xenopus Oocytes

Epilepsy is a neurological disorder characterized by abnormal neuronal excitability, with glutamate playing a key role as the predominant excitatory neurotransmitter involved in seizures. Animal models of epilepsy are crucial in advancing epilepsy research by faithfully replicating the diverse symptoms of this disorder. In particular, the GASH/Sal (genetically audiogenic seizure-prone hamster from Salamanca) model exhibits seizures resembling human generalized tonic-clonic convulsions. A single nucleotide polymorphism (SNP; C9586732T, p.His289Tyr) in the Grik1 gene (which encodes the kainate receptor GluK1) has been previously identified in this strain. The H289Y mutation affects the amino-terminal domain of GluK1, which is related to the subunit assembly and trafficking. We used confocal microscopy in Xenopus oocytes to investigate how the H289Y mutation, compared to the wild type (WT), affects the expression and cell-surface trafficking of GluK1 receptors. Additionally, we employed the two-electrode voltage-clamp technique to examine the functional effects of the H289Y mutation. Our results indicate that this mutation increases the expression and incorporation of GluK1 receptors into an oocyte's membrane, enhancing kainate-evoked currents, without affecting their functional properties. Although further research is needed to fully understand the molecular mechanisms responsible for this epilepsy, the H289Y mutation in GluK1 may be part of the molecular basis underlying the seizure-prone circuitry in the GASH/Sal model.

Aryl Hydrocarbon Receptor in Glia Cells: A Plausible Glutamatergic Neurotransmission Orchestrator

Glutamate is the major excitatory amino acid in the vertebrate brain. Glutamatergic signaling is involved in most of the central nervous system functions. Its main components, namely receptors, ion channels, and transporters, are tightly regulated at the transcriptional, translational, and post-translational levels through a diverse array of extracellular signals, such as food, light, and neuroactive molecules. An exquisite and well-coordinated glial/neuronal bidirectional communication is required for proper excitatory amino acid signal transactions. Biochemical shuttles such as the glutamate/glutamine and the astrocyte-neuronal lactate represent the fundamental involvement of glial cells in glutamatergic transmission. In fact, the disruption of any of these coordinated biochemical intercellular cascades leads to an excitotoxic insult that underlies some aspects of most of the neurodegenerative diseases characterized thus far. In this contribution, we provide a comprehensive summary of the involvement of the Aryl hydrocarbon receptor, a ligand-dependent transcription factor in the gene expression regulation of glial glutamate transporters. These receptors might serve as potential targets for the development of novel strategies for the treatment of neurodegenerative diseases.

Kainate Receptor Antagonists: Recent Advances and Therapeutic Perspective

Since the 1990s, ionotropic glutamate receptors have served as an outstanding target for drug discovery research aimed at the discovery of new neurotherapeutic agents. With the recent approval of perampanel, the first marketed non-competitive antagonist of AMPA receptors, particular interest has been directed toward 'non-NMDA' (AMPA and kainate) receptor inhibitors. Although the role of AMPA receptors in the development of neurological or psychiatric disorders has been well recognized and characterized, progress in understanding the function of kainate receptors (KARs) has been hampered, mainly due to the lack of specific and selective pharmacological tools. The latest findings in the biology of KA receptors indicate that they are involved in neurophysiological activity and play an important role in both health and disease, including conditions such as anxiety, schizophrenia, epilepsy, neuropathic pain, and migraine. Therefore, we reviewed recent advances in the field of competitive and non-competitive kainate receptor antagonists and their potential therapeutic applications. Due to the high level of structural divergence among the compounds described here, we decided to divide them into seven groups according to their overall structure, presenting a total of 72 active compounds.

Opioid-Induced Reductions in Amygdala Lateral Paracapsular GABA Neuron Circuit Activity

Opioid use and withdrawal evokes behavioral adaptations such as drug seeking and anxiety, though the underlying neurocircuitry changes are unknown. The basolateral amygdala (BLA) regulates these behaviors through principal neuron activation. Excitatory BLA pyramidal neuron activity is controlled by feedforward inhibition provided, in part, by lateral paracapsular (LPC) GABAergic inhibitory neurons, residing along the BLA/external capsule border. LPC neurons express µ-opioid receptors (MORs) and are potential targets of opioids in the etiology of opioid-use disorders and anxiety-like behaviors. Here, we investigated the effects of opioid exposure on LPC neuron activity using immunohistochemical and electrophysiological approaches. We show that LPC neurons, and other nearby BLA GABA and non-GABA neurons, express MORs and δ-opioid receptors. Additionally, DAMGO, a selective MOR agonist, reduced GABA but not glutamate-mediated spontaneous postsynaptic currents in LPC neurons. Furthermore, in LPC neurons, abstinence from repeated morphine-exposure in vivo (10 mg/kg/day, 5 days, 2 days off) decrease the intrinsic membrane excitability, with a ~75% increase in afterhyperpolarization and ~40-50% enhanced adenylyl cyclase-dependent activity in LPC neurons. These data show that MORs in the BLA are a highly sensitive targets for opioid-induced inhibition and that repeated opioid exposure results in impaired LPC neuron excitability.

Glutamate Signaling and Filopodiagenesis of Astrocytoma Cells in Brain Cancers: Survey and Questions

Astrocytes are non-excitable cells in the CNS that can cause life-threatening astrocytoma tumors when they transform to cancerous cells. Perturbed homeostasis of the neurotransmitter glutamate is associated with astrocytoma tumor onset and progression, but the factors that govern this phenomenon are less known. Herein, we review possible mechanisms by which glutamate may act in facilitating the growth of projections in astrocytic cells. This review discusses the similarities and differences between the morphology of astrocytes and astrocytoma cells, and the role that dysregulation in glutamate and calcium signaling plays in the aberrant morphology of astrocytoma cells. Converging reports suggest that ionotropic glutamate receptors and voltage-gated calcium channels expressed in astrocytes may be responsible for the abnormal filopodiagenesis or process extension leading to astrocytoma cells’ infiltration throughout the brain.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Architecture and structural dynamics of the heteromeric GluK2/K5 kainate receptor

Kainate receptors (KARs) are L-glutamate-gated ion channels that regulate synaptic transmission and modulate neuronal circuits. KARs have strict assembly rules and primarily function as heteromeric receptors in the brain. A longstanding question is how KAR heteromer subunits organize and coordinate together to fulfill their signature physiological roles. Here we report structures of the GluK2/GluK5 heteromer in apo, antagonist-bound, and desensitized states. The receptor assembles with two copies of each subunit, ligand binding domains arranged as two heterodimers and GluK5 subunits proximal to the channel. Strikingly, during desensitization, GluK2, but not GluK5, subunits undergo major structural rearrangements to facilitate channel closure. We show how the large conformational differences between antagonist-bound and desensitized states are mediated by the linkers connecting the pore helices to the ligand binding domains. This work presents the first KAR heteromer structure, reveals how its subunits are organized, and resolves how the heteromer can accommodate functionally distinct closed channel structures.

Subunit-selective iGluR antagonists can potentiate heteromeric receptor responses by blocking desensitization

Ionotropic glutamate receptors (iGluRs) are key molecules for synaptic signaling in the central nervous system, which makes them promising drug targets. Intensive efforts are being devoted to the development of subunit-selective ligands, which should enable more precise pharmacologic interventions while limiting the effects on overall neuronal circuit function. However, many AMPA and kainate receptor complexes in vivo are heteromers composed of different subunits. Despite their importance, little is known about how subunit-selective ligands affect the gating of heteromeric iGluRs, namely their activation and desensitization properties. Using fast ligand application experiments, we studied the effects of competitive antagonists that block glutamate from binding at part of the four subunits. We found that UBP-310, a kainate receptor antagonist with high selectivity for GluK1 subunits, reduces the desensitization of GluK1/GluK2 heteromers and fully abolishes the desensitization of GluK1/GluK5 heteromers. This effect is mirrored by subunit-selective agonists and heteromeric receptors that contain binding-impaired subunits, as we show for both kainate and GluA2 AMPA receptors. These findings are consistent with a model in which incomplete agonist occupancy at the four receptor subunits can provide activation without inducing desensitization. However, we did not detect significant steady-state currents during UBP-310 dissociation from GluK1 homotetramers, indicating that antagonist dissociation proceeds in a nonuniform and cooperativity-driven manner, which disfavors nondesensitizing occupancy states. Besides providing mechanistic insights, these results have direct implications for the use of subunit-selective antagonists in neuroscience research and envisioned therapeutic interventions.

Immunohistochemical Evidence for Glutamatergic Regulation of Nesfatin-1 Neurons in the Rat Hypothalamus

Nesfatin-1, identified as an anorexigenic peptide, regulates the energy metabolism by suppressing food intake. The majority of nesfatin-1-synthesizing neurons are concentrated in various hypothalamic nuclei, especially in the supraoptic (SON), arcuate (ARC) and paraventricular nuclei (PVN). We tested the hypothesis that the glutamatergic system regulates nesfatin-1 neurons through glutamate receptors. Therefore, the first aim of the proposed studies was to examine effects of different glutamate agonists in the activation of nesfatin-1 neurons using c-Fos double immunohistochemical labeling. Experimental groups were formed containing male and female rats which received intraperitoneal injections of glutamate agonists kainic acid, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) while the control rats received vehicle. The significant increase in the number of c-Fos-expressing nesfatin-1 neurons after agonist injections were observed both in female and male subjects and some of these effects were found to be sexually dimorphic. In addition, treatment with specific glutamate antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or dizocilpine (MK-801) before each of the three agonist injections caused a statistically significant reduction in the number of activated nesfatin-1 neurons in the hypothalamic nuclei including supraoptic, paraventricular and arcuate nuclei. The second aim of the study was to determine the expression of glutamate receptor subunit proteins in the nesfatin-1 neurons by using a double immunofluorescence technique. The results showed that the glutamate receptor subunits, which may form homomeric or heteromeric functional receptor channels, were expressed in the nesfatin-1 neurons. In conclusion, the results of this study suggest that nesfatin-1 neurons respond to glutamatergic signals in the form of neuronal activation and that the glutamate receptors that are synthesized by nesfatin-1 neurons may participate in the glutamatergic regulation of these neurons.

Algal neurotoxin biosynthesis repurposes the terpene cyclase structural fold into an N-prenyltransferase

Prenylation is a common biological reaction in all domains of life wherein prenyl diphosphate donors transfer prenyl groups onto small molecules as well as large proteins. The enzymes that catalyze these reactions are structurally distinct from ubiquitous terpene cyclases that, instead, assemble terpenes via intramolecular rearrangements of a single substrate. Herein, we report the structure and molecular details of a new family of prenyltransferases from marine algae that repurposes the terpene cyclase structural fold for the N-prenylation of glutamic acid during the biosynthesis of the potent neurochemicals domoic acid and kainic acid. We solved the X-ray crystal structure of the prenyltransferase found in domoic acid biosynthesis, DabA, and show distinct active site binding modifications that remodel the canonical magnesium (Mg²⁺)-binding motif found in terpene cyclases. We then applied our structural knowledge of DabA and a homologous enzyme from the kainic acid biosynthetic pathway, KabA, to reengineer their isoprene donor specificities (geranyl diphosphate [GPP] versus dimethylallyl diphosphate [DMAPP]) with a single amino acid change. While diatom DabA and seaweed KabA enzymes share a common evolutionary lineage, they are distinct from all other terpene cyclases, suggesting a very distant ancestor to the larger terpene synthase family.

A kainate receptor-selective RNA aptamer

Kainate and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are two major, closely related receptor subtypes in the glutamate ion channel family. Excessive activities of these receptors have been implicated in a number of central nervous system diseases. Designing potent and selective antagonists of these receptors, especially of kainate receptors, is useful for developing potential treatment strategies for these neurological diseases. Here, we report on two RNA aptamers designed to individually inhibit kainate and AMPA receptors. To improve the biostability of these aptamers, we also chemically modified these aptamers by substituting their 2'-OH group with 2'-fluorine. These 2'-fluoro aptamers, FB9s-b and FB9s-r, were markedly resistant to RNase-catalyzed degradation, with a half-life of ∼5 days in rat cerebrospinal fluid or serum-containing medium. Furthermore, FB9s-r blocked AMPA receptor activity. Aptamer FB9s-b selectively inhibited GluK1 and GluK2 kainate receptor subunits, and also GluK1/GluK5 and GluK2/GluK5 heteromeric kainate receptors with equal potency. This inhibitory profile makes FB9s-b a powerful template for developing tool molecules and drug candidates for treatment of neurological diseases involving excessive activities of the GluK1 and GluK2 subunits.

Zinc Modulates Olfactory Bulb Kainate Receptors

Kainate receptors (KARs) are glutamate receptors with ionotropic and metabotropic activity composed of the GluK1-GluK5 subunits. We previously reported that KARs modulate excitatory and inhibitory transmission in the olfactory bulb (OB). Zinc, which is highly concentrated in the OB, also appears to modulate OB synaptic transmission via actions at other ionotropic glutamate receptors (i.e., AMPA, NMDA). However, few reports of effects of zinc on recombinant and/or native KARs exist and none have involved the OB. In the present study, we investigated the effects of exogenously applied zinc on OB KARs expressed by mitral/tufted (M/T) cells. We found that 100 µM zinc inhibits currents evoked by various combinations of KAR agonists (kainate or SYM 2081) and the AMPA receptor antagonist SYM 2206. The greatest degree of zinc-mediated inhibition was observed with coapplication of zinc with the GluK1- and GluK2-preferring agonist SYM 2081 plus SYM 2206. This finding is consistent with prior reports of zinc's inhibitory effects on some recombinant (homomeric GluK1 and GluK2 and heteromeric GluK2/GluK4 and GluK2/GluK5) KARs, although potentiation of other (GluK3, GluK2/3) KARs has also been described. It is also of potential importance given our previously reported molecular data suggesting that OB neurons express relatively high levels of GluK1 and GluK2. Our present findings suggest that a physiologically relevant concentration of zinc modulates KARs expressed by M/T cells. As M/T cells are targets of zinc-containing olfactory sensory neurons, synaptically released zinc may influence odor information-encoding synaptic circuits in the OB via actions at KARs.

A Targeted Mutation Disrupting Mitochondrial Complex IV Function in Primary Afferent Neurons Leads to Pain Hypersensitivity Through P2Y1 Receptor Activation

As mitochondrial dysfunction is evident in neurodegenerative disorders that are accompanied by pain, we generated inducible mutant mice with disruption of mitochondrial respiratory chain complex IV, by COX10 deletion limited to sensory afferent neurons through the use of an Advillin Cre-reporter. COX10 deletion results in a selective energy-deficiency phenotype with minimal production of reactive oxygen species. Mutant mice showed reduced activity of mitochondrial respiratory chain complex IV in many sensory neurons, increased ADP/ATP ratios in dorsal root ganglia and dorsal spinal cord synaptoneurosomes, as well as impaired mitochondrial membrane potential, in these synaptoneurosome preparations. These changes were accompanied by marked pain hypersensitivity in mechanical and thermal (hot and cold) tests without altered motor function. To address the underlying basis, we measured Ca²⁺ fluorescence responses of dorsal spinal cord synaptoneurosomes to activation of the GluK1 (kainate) receptor, which we showed to be widely expressed in small but not large nociceptive afferents, and is minimally expressed elsewhere in the spinal cord. Synaptoneurosomes from mutant mice showed greatly increased responses to GluK1 agonist. To explore whether altered nucleotide levels may play a part in this hypersensitivity, we pharmacologically interrogated potential roles of AMP-kinase and ADP-sensitive purinergic receptors. The ADP-sensitive P2Y₁ receptor was clearly implicated. Its expression in small nociceptive afferents was increased in mutants, whose in vivo pain hypersensitivity, in mechanical, thermal and cold tests, was reversed by a selective P2Y₁ antagonist. Energy depletion and ADP elevation in sensory afferents, due to mitochondrial respiratory chain complex IV deficiency, appear sufficient to induce pain hypersensitivity, by ADP activation of P2Y₁ receptors.

Putative Receptors Underpinning l-Lactate Signalling in Locus Coeruleus

The importance of astrocytic l-lactate (LL) for normal functioning of neural circuits such as those regulating learning/memory, sleep/wake state, autonomic homeostasis, or emotional behaviour is being increasingly recognised. l-Lactate can act on neurones as a metabolic or redox substrate, but transmembrane receptor targets are also emerging. A comparative review of the hydroxy-carboxylic acid receptor (HCA1, formerly known as GPR81), Olfactory Receptor Family 51 Subfamily E Member 2 (OR51E2), and orphan receptor GPR4 highlights differences in their LL sensitivity, pharmacology, intracellular coupling, and localisation in the brain. In addition, a putative Gs-coupled receptor on noradrenergic neurones, LLRx, which we previously postulated, remains to be identified. Next-generation sequencing revealed several orphan receptors expressed in locus coeruleus neurones. Screening of a selection of these suggests additional LL-sensitive receptors: GPR180 which inhibits and GPR137 which activates intracellular cyclic AMP signalling in response to LL in a heterologous expression system. To further characterise binding of LL at LLRx, we carried out a structure–activity relationship study which demonstrates that carboxyl and 2-hydroxyl moieties of LL are essential for triggering d-lactate-sensitive noradrenaline release in locus coeruleus, and that the size of the LL binding pocket is limited towards the methyl group position. The evidence accumulating to date suggests that LL acts via multiple receptor targets to modulate distinct brain functions.

Kainate Receptors Play a Role in Modulating Synaptic Transmission in the Olfactory Bulb

Glutamate is the neurotransmitter used at most excitatory synapses in the mammalian brain, including those in the olfactory bulb (OB). There, ionotropic glutamate receptors including N-methyl-d-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) play a role in processes such as reciprocal inhibition and glomerular synchronization. Kainate receptors (KARs) represent another type of ionotropic glutamate receptor, which are composed of five (GluK1-GluK5) subunits. Whereas KARs appear to be heterogeneously expressed in the OB, evidence as to whether these KARs are functional, found at synapses, or modify synaptic transmission is limited. In the present study, coapplication of KAR agonists (kainate, SYM 2081) and AMPAR antagonists (GYKI 52466, SYM 2206) demonstrated that functional KARs are expressed by OB neurons, with a subset of receptors located at synapses. Application of kainate and the GluK1-selective agonist ATPA had modulatory effects on excitatory postsynaptic currents (EPSCs) evoked by stimulation of the olfactory nerve layer. Application of kainate and ATPA also had modulatory effects on reciprocal inhibitory postsynaptic currents (IPSCs) evoked using a protocol that evokes dendrodendritic inhibition. The latter finding suggests that KARs, with relatively slow kinetics, may play a role in circuits in which the relatively brief duration of AMPAR-mediated currents limits the role of AMPARs in synaptic transmission (e.g., reciprocal inhibition at dendrodendritic synapses). Collectively, our findings suggest that KARs, including those containing the GluK1 subunit, modulate excitatory and inhibitory transmission in the OB. These data further suggest that KARs participate in the regulation of synaptic circuits that encode odor information.

A pharmacological profile of the high-affinity GluK5 kainate receptor

Mouse GluK5 was expressed in Sf9 insect cells and radiolabelled with [(3)H]-kainate in receptor binding assays (Kd=6.9nM). Western immunoblotting indicated an Sf9 GluK5 band doublet corresponding to the glycosylated (128kDa) and deglycosylated (111kDa) protein, which was identical to the band pattern of native rat brain GluK5. A pharmacological profile of the high-affinity kainate receptor GluK5 is described which is distinct from the profiles of other kainate receptors (GluK1-3). The 27 tested ligands generally show a preferential affinity to GluK1 over GluK5, the exceptions being: dihydrokainate, (S)-5-fluorowillardiine, (S)-glutamate and quisqualate, where the affinity is similar at GluK1 and GluK5. In contrast, quisqualate shows 40-fold higher affinity at GluK5 over GluK3 whereas (S)-1-(2'-amino-2'-caboxyethyl)thienol[3,4-d]pyrimidin-2,4-dione (NF1231), (RS)-2-amino-3-(5-tert-butyl-3-hydroxyisoxazol-4-yl)propionate (ATPA), dihydrokainate and (2S,4R)-4-methyl-glutamate (SYM2081) have higher affinity at GluK3 compared to GluK5. Since some studies have indicated that GluK5 is associated with various diseases in the central nervous system (e.g. schizophrenia, temporal lobe epilepsy, bipolar disorder), selective GluK5 ligands could have therapeutic potential. The distinct pharmacological profile of GluK5 suggests that it would be possible to design ligands with selectivity towards GluK5.

Lessons from crystal structures of kainate receptors

Kainate receptors belong to the family of ionotropic glutamate receptors. These receptors assemble from five subunits (GluK1-5) into tetrameric ion channels. Kainate receptors are located at both pre- and postsynaptic membranes in the central nervous system where they contribute to excitatory synaptic transmission and modulate network excitability by regulating neurotransmitter release. Dysfunction of kainate receptors has been implicated in several neurological disorders such as epilepsy, schizophrenia and depression. Here we provide a review on the current understanding of kainate receptor structure and how they bind agonists, antagonists and ions. The first structure of the ligand-binding domain of the GluK1 subunit was reported in 2005, seven years after publication of the crystal structure of a soluble construct of the ligand-binding domain of the AMPA-type subunit GluA2. Today, a full-length structure has been determined of GluK2 by cryo electron microscopy to 7.6 Å resolution as well as 84 high-resolution crystal structures of N-terminal domains and ligand-binding domains, including agonist and antagonist bound structures, modulatory ions and mutations. However, there are still many unanswered questions and challenges in front of us. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Pharmacological Modulation of GluK1 and GluK2 by NETO1, NETO2, and PSD95

The association between the kainate receptors (KARs) GluK1 and GluK2 and the modifying proteins neuropilin- and tolloid-like 1 (NETO1), neuropilin- and tolloid-like 2 (NETO2), and postsynaptic density protein 95 (PSD95) is likely to produce distinct GluK1 and GluK2 pharmacology in postsynaptic neurons. However, little is known about their corresponding modulatory effects on GluK1 and GluK2 activity in high-throughput assays for cell-based drug discovery. Using heterologous cells that potentially mimic the response in native cells in a fluorescence imaging plate reader (FLIPR) assay, we have investigated assays that incorporate (1) coexpression of GluK1 or GluK2 with their modulatory proteins (NETO1, NETO2, PSD95) and/or (2) enablement of assays with physiological concentration of native GluK1 and GluK2 agonist (glutamate) in the absence of an artificial potentiator (e.g., concanavalin A [Con A]). We found that in the absence of Con A, both NETO1 and NETO2 accessory proteins are able to potentiate kainate- and glutamate-evoked GluK1-mediated Ca(2+) influx. We also noted the striking ability of PSD95 to enhance glutamate-stimulated potentiation effects of NETO2 on GluK1 without the need for Con A and with a robust signal that could be utilized for high-throughput FLIPR assays. These experiments demonstrate the utility of heterologous cells coexpressing PSD95/NETO2 with GluK1 or GluK2 in native cell-mimicking heterologous cell systems for high-throughput assays and represent new avenues into the discovery of KAR modulating therapies.

High-Throughput Screen of GluK1 Receptor Identifies Selective Inhibitors with a Variety of Kinetic Profiles Using Fluorescence and Electrophysiology Assays

GluK1, a kainate subtype of ionotropic glutamate receptors, exhibits an expression pattern in the CNS consistent with involvement in pain processing and migraine. Antagonists of GluK1 have been shown to reduce pain signaling in the spinal cord and trigeminal nerve, and are predicted to provide pain and migraine relief. We developed an ultra-high-throughput small-molecule screen to identify antagonists of GluK1. Using the calcium indicator dye fluo-4, a multimillion-member small-molecule library was screened in 1536-well plate format on the FLIPR (Fluorescent Imaging Plate Reader) Tetra against cells expressing a calcium-permeable GluK1. Following confirmation in the primary assay and subsequent counter-screen against the endogenous Par-1 receptor, 6100 compounds were selected for dose titration to assess potency and selectivity. Final triage of 1000 compounds demonstrating dose-dependent inhibition with IC50 values of less than 12 µM was performed in an automated whole-cell patch clamp electrophysiology assay. Although a weak correlation between electrophysiologically active and calcium-imaging active compounds was observed, the identification of electrophysiologically active compounds with a range of kinetic profiles revealed a broad spectrum of mechanisms of action.

AMPA receptor desensitization is the determinant of AMPA receptor mediated excitotoxicity in purified retinal ganglion cells

The ionotropic glutamate receptors (iGLuR) have been hypothesized to play a role in neuronal pathogenesis by mediating excitotoxic death. Previous studies on iGluR in the retina have focused on two broad classes of receptors: NMDA and non-NMDA receptors including the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) and kainate receptor. In this study, we examined the role of receptor desensitization on the specific excitotoxic effects of AMPAR activation on primary retinal ganglion cells (RGCs). Purified rat RGCs were isolated from postnatal day 4-7 Sprague-Dawley rats. Calcium imaging was used to identify the functionality of the AMPARs and selectivity of the s-AMPA agonist. Phosphorylated CREB and ERK1/2 expression were performed following s-AMPA treatment. s-AMPA excitotoxicity was determined by JC-1 mitochondrial membrane depolarization assay, caspase 3/7 luciferase activity assay, immunoblot analysis for α-fodrin, and Live (calcein AM)/Dead (ethidium homodimer-1) assay. RGC cultures of 98% purity, lacking Iba1 and GFAP expression were used for the present studies. Isolated prenatal RGCs expressed calcium permeable AMPAR and s-AMPA (100 μM) treatment of cultured RGCs significantly increased phosphorylation of CREB but not that of ERK1/2. A prolonged (6 h) AMPAR activation in purified RGCs using s-AMPA (100 μM) did not depolarize the RGC mitochondrial membrane potential. In addition, treatment of cultured RGCs with s-AMPA, both in the presence and absence of trophic factors (BDNF and CNTF), did not increase caspase 3/7 activities or the cleavage of α-fodrin (neuronal apoptosis marker), as compared to untreated controls. Lastly, a significant increase in cell survival of RGCs was observed after s-AMPA treatment as compared to control untreated RGCs. However, preventing the desensitization of AMPAR with the treatment with either kainic acid (100 μM) or the combination of s-AMPA and cyclothiazide (50 μM) significantly reduced cell survivability. Activation of the AMPAR in RGCs does not appear to activate a signaling cascade to apoptosis, suggesting that RGCs in vitro are not susceptible to AMPA excitotoxicity as previously hypothesized. Conversely, preventing AMPAR desensitization through differential agonist activation caused AMPAR mediated excitotoxicity. Activation of the AMPAR in increasing CREB phosphorylation was dependent on the presence of calcium, which may help explain this action in increasing RGC survival.

Kynurenic Acid Content in Selected Culinary Herbs and Spices

Previous studies demonstrated that kynurenic acid (KYNA) is present in various types of food in varying concentrations. Therefore, the aim of the study was to check whether KYNA is present in culinary herbs and spices. Achieved results indicate that KYNA is present in all 19 selected culinary herbs and spices. The highest concentration of KYNA was found in basil and thyme, 14.08 and 8.87 μg/g, respectively, while the lowest content of KYNA was found in cumin and black pepper, 0.64 and 0.10 μg/g, respectively. This is the first report on the concentration of KYNA in culinary herbs and spices. The need for more detailed investigation of dietary supplementation with culinary herbs and spices containing KYNA is suggested.

Contributions of different kainate receptor subunits to the properties of recombinant homomeric and heteromeric receptors

The tetrameric kainate receptors can be assembled from a combination of five different subunit subtypes. While GluK1-3 subunits can form homomeric receptors, GluK4 and GluK5 require a heteromeric partner to assemble, traffic to the membrane surface, and produce a functional channel. Previous studies have shown that incorporation of a GluK4 or GluK5 subunit changes both receptor pharmacology and channel kinetics. We directly compared the functional characteristics of recombinant receptors containing either GluK4 or GluK5 in combination with the GluK1 or GluK2 subunit. In addition, we took advantage of mutations within the agonist binding sites of GluK1, GluK2, or GluK5 to isolate the response of the wild-type partner within the heteromeric receptor. Our results suggest that GluK1 and GluK2 differ primarily in their pharmacological properties, but that GluK4 and GluK5 have distinct functional characteristics. In particular, while binding of agonist to only the GluK5 subunit appears to activate the channel to a non-desensitizing state, binding to GluK4 does produce some desensitization. This suggests that GluK4 and GluK5 differ fundamentally in their contribution to receptor desensitization. In addition, mutation of the agonist binding site of GluK5 results in a heteromeric receptor with a glutamate sensitivity similar to homomeric GluK1 or GluK2 receptors, but which requires higher agonist concentrations to produce desensitization. This suggests that onset of desensitization in heteromeric receptors is determined more by the number of subunits bound to agonist than by the identity of those subunits. The distinct, concentration-dependent properties observed with heteromeric receptors in response to glutamate or kainate are consistent with a model in which either subunit can activate the channel, but in which occupancy of both subunits within a dimer is needed to allow desensitization of GluK2/K5 receptors.

The neurotoxin domoate causes long-lasting inhibition of the kainate receptor GluK5 subunit

Ionotropic glutamate receptors (iGluRs) are responsible for fast excitatory neurotransmission in the mammalian brain, and are critical regulators of neuronal activity and synaptic plasticity. The three main types of iGluRs (AMPA, NMDA, and kainate receptors) are composed of distinct subunit populations. The tetrameric kainate receptors can be assembled from a combination of five different types of subunits (GluK1-GluK5). GluK1-3 subunits are able to produce functional homomeric receptors, while GluK4-5 are obligate heteromers, and must assemble with a GluK1-3 subunit. The neurotoxin domoate is widely used as an agonist at kainate-type receptors because it produces a less desensitizing response compared to glutamate. We have identified an additional, subunit-dependent action of domoate at recombinant kainate receptors. When applied to heteromeric GluK2/K5 receptors, domoate generates a small, long-lasting, tonic current. In addition, brief exposure to domoate inhibits the GluK5 subunit, preventing its activation by other agonists for several minutes. These characteristics are not associated with the GluK1, K2, or K4 subunits and can be prevented by a mutation in GluK5 that reduces agonist binding affinity. The results also show that the domoate-bound, GluK2/K5 heteromeric receptors can be fully activated by agonists acting through the GluK2 subunit, suggesting that the subunits within the tetramer can function independently to open the ion channel, and that the domoate-bound state is not a desensitized or blocked conformation. This study describes new properties associated with domoate action at kainate receptors, and further characterizes the distinct roles played by different subunits in heteromeric receptors.

Role of GluK1 kainate receptors in seizures, epileptic discharges, and epileptogenesis

Kainate receptors containing the GluK1 subunit have an impact on excitatory and inhibitory neurotransmission in brain regions, such as the amygdala and hippocampus, which are relevant to seizures and epilepsy. Here we used 2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA), a potent and selective agonist of kainate receptors that include the GluK1 subunit, in conjunction with mice deficient in GluK1 and GluK2 kainate receptor subunits to assess the role of GluK1 kainate receptors in provoking seizures and in kindling epileptogenesis. We found that systemic ATPA, acting specifically via GluK1 kainate receptors, causes locomotor arrest and forelimb extension (a unique behavioral characteristic of GluK1 activation) and induces myoclonic behavioral seizures and electrographic seizure discharges in the BLA and hippocampus. In contrast, the proconvulsant activity of systemic AMPA, kainate, and pentylenetetrazol is not mediated by GluK1 kainate receptors, and deletion of these receptors does not elevate the threshold for seizures in the 6 Hz model. ATPA also specifically activates epileptiform discharges in BLA slices in vitro via GluK1 kainate receptors. Olfactory bulb kindling developed similarly in wild-type, GluK1, and GluK2 knock-out mice, demonstrating that GluK1 kainate receptors are not required for epileptogenesis or seizure expression in this model. We conclude that selective activation of kainate receptors containing the GluK1 subunit can trigger seizures, but these receptors are not necessary for seizure generation in models commonly used to identify therapeutic agents for the treatment of epilepsy.

Kainate receptor subunit diversity underlying response diversity in retinal off bipolar cells

Postsynaptic kainate receptors mediate excitatory synaptic transmission over a broad range of temporal frequencies. In heterologous systems, the temporal responses of kainate receptors vary when different channel-forming and auxiliary subunits are co-expressed but how this variability relates to the temporal differences at central synapses is incompletely understood. The mammalian cone photoreceptor synapse provides advantages for comparing the different temporal signalling roles of kainate receptors, as cones release glutamate over a range of temporal frequencies, and three functionally distinct Off bipolar cell types receive cone signals at synapses that contain either AMPA or kainate receptors, all with different temporal properties. A disadvantage is that the different receptor subunits are not identified. We used in situ hybridization, immunocytochemistry, and pharmacology to identify the kainate receptor and auxiliary subunits in ground squirrel (Ictidomys tridecimlineatus) cb1a/b, cb2, and cb3a/b Off bipolar cell types. As expected, the types showed distinct subunit expression patterns. Kainate receptors mediated ∼80% of the synaptic response in cb3a/b cells and were heteromers of GluK1 and GluK5. Cb3a/b cells contained message for GluK1 and GluK5, and also GluK3 and the auxiliary subunit Neto1. The synaptic responses in cb1a/b cells were mediated by GluK1-containing kainate receptors that behaved differently from the receptors expressed by cb3a/b cells. AMPA receptors mediated the entire synaptic response in cb2 cells and the remaining synaptic response in cb3a/b cells. We conclude that GluK1 is the predominant kainate receptor subunit in cb1 and cb3 Off bipolar cells. Different temporal response properties may result from selective association with GluK3, GluK5, or Neto1.

Kynurenic Acid in the digestive system-new facts, new challenges

This review provides information on the most recent findings concerning presence, origin, and role of kynurenic acid (KYNA), a tryptophan metabolite, in the digestive system. KYNA is an antagonist of both the ionotropic glutamate receptors and the alpha7 nicotinic acetylcholine receptor, as well as an agonist of G-protein coupled GPR35 receptor. Since the GPR35 receptor is mainly present in the gastrointestinal tract, researchers have concentrated on the digestive system in recent years. They have found that KYNA content increases gradually and significantly along the gastrointestinal tract. Interestingly, the concentration of KYNA in the lumen is much higher than in the wall of intestine. It has been documented that KYNA may have a positive influence on the number of pathologies in the gastrointestinal tract, in particular ulcers, colon obstruction, or colitis. Future studies might determine whether it is advisable to supplement KYNA to a human organism.

Modal gating of GluN1/GluN2D NMDA receptors

GluN2D-containing NMDA receptors are characterized by an unusually low open probability (0.023), even in the presence of saturating glutamate and glycine. Here, we show that recombinant GluN1/GluN2D NMDA receptors can enter brief periods with exceptionally high open probability (0.65) in excised outside-out and cell-attached single channel recordings. GluN1/GluN2D channels during the enhanced gating mode have similar open durations as occurs outside of the high open probability burst of activity. However, the periods in the high gating mode only exhibit 4 brief closed duration exponential components similar to the briefest observed for openings outside the burst. GluN1/GluN2D receptors also open to a more prominent subconductance level compared to activity outside the high open probability burst. Evaluation of a five-state NMDA receptor gating model suggests that both the opening and closing rate constants differ for the periods of higher open probability compared to the high open probability arm of a gating model previously published for GluN1/GluN2D fit to a representative full length single channel recording. These data demonstrate that GluN2D-containing NMDA receptors can enter a conformation or mode that allows the pore to gate with high probability.

Activation of group I metabotropic glutamate receptors potentiates heteromeric kainate receptors

Kainate receptors (KARs), a family of ionotropic glutamate receptors, are widely expressed in the central nervous system and are critically involved in synaptic transmission. KAR activation is influenced by metabotropic glutamate receptor (mGlu) signaling, but the underlying mechanisms are not understood. We undertook studies to examine how mGlu modulation affects activation of KARs. Confocal immunohistochemistry of rat hippocampus and cultured rat cortex revealed colocalization of the high-affinity KAR subunits with group I mGlu receptors. In hippocampal and cortical cultures, the calcium signal caused by activation of native KARs was potentiated by activation of group I mGlu receptors. In Xenopus laevis oocytes, activation of group I mGlu receptors potentiated heteromeric but not homomeric KAR-mediated currents, with no change in agonist potency. The potentiation of heteromeric KARs by mGlu1 activation was attenuated by GDPβS, blocked by an inhibitor of phospholipase C or the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), prolonged by the phosphatase inhibitor okadaic acid, but unaffected by the tyrosine kinase inhibitor lavendustin A. Protein kinase C (PKC) inhibition reduced the potentiation by mGlu1 of GluK2/GluK5, and conversely, direct activation of PKC by phorbol 12-myristate,13-acetate potentiated GluK2/GluK5. Using site-directed mutagenesis, we identified three serines (Ser833, Ser836, and Ser840) within the membrane proximal region of the GluK5 C-terminal domain that, in combination, are required for mGlu1-mediated potentiation of KARs. Together, these data suggest that phosphorylation of key residues in the C-terminal domain changes the overall charge of this domain, resulting in potentiated agonist responses.

Molecular and pharmacological evidence for a facilitatory functional role of pre-synaptic GLUK2/3 kainate receptors on GABA release in rat trigeminal caudal nucleus

BACKGROUND

Gamma-aminobutyric acid (GABA) and glutamate (GLU) are involved in nociceptive signals processing in the trigeminal system. In this study, we investigated the influence of excitatory transmission on GABA release in nerve terminals isolated from the rat trigeminal caudal nucleus (TCN).

METHODS

We utilize biochemical (superfused synaptosomes loaded with [(3) H]GABA) and morphological (immunofluorescence experiments with specific antibody) techniques.

RESULTS

Our results show that GLU potentiates the release of [(3) H]GABA evoked by 9, 15 and 30 mM [K(+)](e); 15 mM [K(+)](e)-evoked [(3) H]GABA release was also reinforced by domoate and kainate (KA), two naturally occurring GLU-receptor agonists. The enhancement of 15 mM [K(+)](e)-evoked [(3) H]GABA release produced by 100 μM KA was abolished by NBQX, a mixed AMPA/KA receptor antagonist, but was not affected by GYKI52466, a selective AMPA receptor antagonist. ATPA, a selective agonist for KA receptors containing the GLUK1 subunit, had no effect on depolarization-induced [(3) H]GABA release, and UBP310, which selectively antagonizes these same receptors, failed to reverse the KA-induced potentiation of 15 mM [K(+)](e)-evoked [(3) H]GABA release. The KA-induced potentiation was also unaffected by concanavalin A (10 μM), a positive allosteric modulator of GLUK1- and GLUK2-containing KA receptors. Immunofluorescence experiments revealed that GABAergic nerve terminals in the TCN differentially expressed GLUK subunits, with GLUK2/3-positive terminals being twice more abundant than GLUK1-containing synaptosomes.

CONCLUSIONS

These findings indicate that pre-synaptic KA receptors facilitating GABA release from TCN nerve terminals mainly express GLUK2/GLUK3 subunits, supporting the notion that different types of KA receptors are involved in the various stages of pain transmission.

Synaptic pathways that shape the excitatory drive in an OFF retinal ganglion cell

Different types of retinal ganglion cells represent distinct spatiotemporal filters that respond selectively to specific features in the visual input. Much about the circuitry and synaptic mechanisms that underlie such specificity remains to be determined. This study examines how N-methyl-d-aspartate (NMDA) receptor signaling combines with other excitatory and inhibitory mechanisms to shape the output of small-field OFF brisk-sustained ganglion cells (OFF-BSGCs) in the rabbit retina. We used voltage clamp to separately resolve NMDA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and inhibitory inputs elicited by stimulation of the receptive field center. Three converging circuits were identified. First is a direct glutamatergic input, arising from OFF cone bipolar cells (CBCs), which is mediated by synaptic NMDA and AMPA receptors. The NMDA input was saturated at 10% contrast, whereas the AMPA input increased monotonically up to 60% contrast. We propose that NMDA inputs selectively enhance sensitivity to low contrasts. The OFF bipolar cells, mediating this direct excitatory input, express dendritic kainate (KA) receptors, which are resistant to the nonselective AMPA/KA receptor antagonist, 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide disodium salt (NBQX), but are suppressed by a GluK1- and GluK3-selective antagonist, (S)-1-(2-amino-2-carboxyethyl)-3-(2-carboxy-thiophene-3-yl-methyl)-5-methylpyrimidine-2,4-dione (UBP-310). The second circuit entails glycinergic crossover inhibition, arising from ON-CBCs and mediated by AII amacrine cells, which modulate glutamate release from the OFF-CBC terminals. The third circuit also comprises glycinergic crossover inhibition, which is driven by the ON pathway; however, this inhibition impinges directly on the OFF-BSGCs and is mediated by an unknown glycinergic amacrine cell that expresses AMPA but not KA receptors.

Binding site and interlobe interactions of the ionotropic glutamate receptor GluK3 ligand binding domain revealed by high resolution crystal structure in complex with (S)-glutamate

Ionotropic glutamate receptors (iGluRs) are involved in excitatory signal transmission throughout the central nervous system and their malfunction is associated with various health disorders. GluK3 is a subunit of iGluRs, belonging to the subfamily of kainate receptors (GluK1-5). Several crystal structures of GluK1 and GluK2 ligand binding domains have been determined in complex with agonists and antagonists. However, little is known about the molecular mechanisms underlying GluK3 ligand binding properties and no compounds displaying reasonable selectivity towards GluK3 are available today. Here, we present the first X-ray crystal structure of the ligand binding domain of GluK3 in complex with glutamate, determined to 1.6Å resolution. The structure reveals a conserved glutamate binding mode, characteristic for iGluRs, and a water molecule network in the glutamate binding site similar to that seen in GluK1. In GluK3, a slightly lower degree of domain closure around glutamate is observed compared to most other kainate receptor structures with glutamate. The volume of the GluK3 glutamate binding cavity was found to be of intermediate size between those of GluK1 and GluK2. The residues in GluK3 contributing to the subfamily differences in the binding sites are primarily: Thr520, Ala691, Asn722, Leu736 and Thr742. The GluK3 ligand binding domain seems to be less stabilized through interlobe interactions than GluK1 and this may contribute to the faster desensitization kinetics of GluK3.

Distinct functional roles of subunits within the heteromeric kainate receptor

Kainate receptors (KARs) have been implicated in a number of neurological disorders, including epilepsy. KARs are tetrameric, composed of a combination of GluK1-GluK5 subunits. We examined the contribution of GluK2 and GluK5 subunits to activation and desensitization of the heteromeric receptor. Heteromeric GluK2/K5 receptors expressed in HEK-293T cells showed markedly higher glutamate sensitivity than GluK2 homomers and did not desensitize at low glutamate concentrations. Mutation of residue E738 in GluK2 substantially lowered its glutamate sensitivity. However, heteromeric KARs containing this mutant GluK2 [GluK2(E738D)] assembled with wild-type GluK5 showed no change in glutamate EC(50) compared with wild-type heteromeric KARs. Instead, higher concentrations of glutamate were required to produce desensitization. This suggested that, within the heteromeric receptor, glutamate binding to the high-affinity GluK5 subunit alone was sufficient for channel activation but not desensitization, whereas agonist binding to the low-affinity GluK2 subunit was not necessary to open the channel but instead caused the channel to enter a closed, desensitized state. To test this hypothesis in wild-type receptors, we used the competitive antagonist kynurenate, which has higher affinity for the GluK2 than the GluK5 subunit. Coapplication of kynurenate with glutamate to heteromeric receptors reduced the onset of desensitization without affecting the peak current response, consistent with our hypothesis. Our results suggest that GluK2 and GluK5 subunits can be individually activated within the heteromeric receptor and that these subunits serve dramatically different functional roles.

Overview of glutamatergic neurotransmission in the nervous system

This introductory article to the special edition on glutamate neurotransmission in neuropsychiatric disorders provides an overview of glutamate neurotransmitter system physiology and pharmacology. Glutamate was only relatively recently recognized as the major excitatory neurotransmitter in the mammalian brain, in part due to its ubiquitous nature and diverse metabolic roles within the CNS. The extremely high concentration of glutamate in brain tissue paired with its excitotoxic potential requires tight physiological regulation of extracellular glutamate levels and receptor signaling in order to assure optimal excitatory neurotransmission but limits excitotoxic damage. In order to achieve this high level of control, the system has developed a complex physiology with multiple regulatory processes modulating glutamate metabolism, release, receptor signaling, and uptake. The basic physiology of the various regulatory components of the system including the rich receptor pharmacology is briefly reviewed. Potential contributions from each of the system's components to the pathophysiology of neuropsychiatric illnesses are briefly discussed, as are the many new pharmacological targets for drug development provided by the system, especially as they pertain to the proceeding preclinical and clinical articles in this issue.

Kainate receptors with a metabotropic signature enhance hippocampal excitability by regulating the slow after-hyperpolarization in CA3 pyramidal neurons

Most of our knowledge of the synaptic function of kainate receptors stems from a detailed analysis of synaptic transmission between dentate granule cells and CA3 pyramidal neurons, where kainate receptors mediate a slow excitatory current with integrative properties ideally suited for repetitive neuronal firing. Besides this well characterized ionotropic effect of kainate receptors, they can also enhance neuronal excitability by inhibiting the slow Ca(2+) activated K(+) current I(sAHP) via a G-protein coupled mechanism. This phenomenon is associated with Ca(2+) mobilization and protein-kinase activation and ultimately leads to modulation of ion channels responsible for intrinsic electrical properties such as firing adaptation. The significance for CNS function of these newly emerging metabotropic kainate receptors is poorly understood and as yet proteomic analysis of kainate receptors has yielded little information on signaling molecules associated with the kainate receptor ionophore. This chapter covers the key findings that have led to the proposal that high-affinity postsynaptic kainate receptors trigger a form of metabotropic signaling regulating I(sAH P) and neuronal firing in CA3 hippocampal neurons.

Medicinal chemistry of competitive kainate receptor antagonists

Kainic acid (KA) receptors belong to the group of ionotropic glutamate receptors and are expressed throughout in the central nervous system (CNS). The KA receptors have been shown to be involved in neurophysiological functions such as mossy fiber long-term potentiation (LTP) and synaptic plasticity and are thus potential therapeutic targets in CNS diseases such as schizophrenia, major depression, neuropathic pain and epilepsy. Extensive effort has been made to develop subtype-selective KA receptor antagonists in order to elucidate the physiological function of each of the five subunits known (GluK1-5). However, to date only selective antagonists for the GluK1 subunit have been discovered, which underlines the strong need for continued research in this area. The present review describes the structure-activity relationship and pharmacological profile for 10 chemically distinct classes of KA receptor antagonists comprising, in all, 45 compounds. To the medicinal chemist this information will serve as reference guidance as well as an inspiration for future effort in this field.

Kainate receptors coming of age: milestones of two decades of research

Two decades have passed since the first report of the cloning of a kainate-type glutamate receptor (KAR) subunit. The intervening years have seen a rapid growth in our understanding of the biophysical properties and function of KARs in the brain. This research has led to an appreciation that KARs play very distinct roles at synapses relative to other members of the glutamate-gated ion channel receptor family, despite structural and functional commonalities. The surprisingly diverse and complex nature of KAR signaling underlies their unique impact upon neuronal networks through their direct and indirect effects on synaptic transmission, and their prominent role in regulating cell excitability. This review pieces together highlights from the two decades of research subsequent to the cloning of the first subunit, and provides an overview of our current understanding of the role of KARs in the CNS and their potential importance to neurological and neuropsychiatric disorders.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Ion channel blockers for the treatment of neuropathic pain

Neuropathic pain, a severe chronic pain condition characterized by a complex pathophysiology, is a largely unmet medical need. Ion channels, which underlie cell excitability, are heavily implicated in the biological mechanisms that generate and sustain neuropathic pain. This review highlights the biological evidence supporting the involvement of voltage-, proton- and ligand-gated ion channels in the neuropathic pain setting. Ion channel modulators at different research or development stages are reviewed and referenced. Ion channel modulation is one of the main avenues to achieve novel, improved neuropathic pain treatments. Voltage-gated sodium and calcium channel and glutamate receptor modulators are likely to produce new, improved agents in the future. Rationally targeting subtypes of known ion channels, tackling recently discovered ion channel targets or combining drugs with different mechanism of action will be primary sources of new drugs in the longer term.

Subunit-specific desensitization of heteromeric kainate receptors

Kainate receptor subunits can form functional channels as homomers of GluK1, GluK2 or GluK3, or as heteromeric combinations with each other or incorporating GluK4 or GluK5 subunits. However, GluK4 and GluK5 cannot form functional channels by themselves. Incorporation of GluK4 or GluK5 into a heteromeric complex increases glutamate apparent affinity and also enables receptor activation by the agonist AMPA. Utilizing two-electrode voltage clamp of Xenopus oocytes injected with cRNA encoding kainate receptor subunits, we have observed that heteromeric channels composed of GluK2/GluK4 and GluK2/GluK5 have steady state concentration-response curves that were bell-shaped in response to either glutamate or AMPA. By contrast, homomeric GluK2 channels exhibited a monophasic steady state concentration-response curve that simply plateaued at high glutamate concentrations. By fitting several specific Markov models to GluK2/GluK4 heteromeric and GluK2 homomeric concentration-response data, we have determined that: (a) two strikingly different agonist binding affinities exist; (b) the high-affinity binding site leads to channel opening; and (c) the low-affinity agonist binding site leads to strong desensitization after agonist binding. Model parameters also approximate the onset and recovery kinetics of desensitization observed for macroscopic currents measured from HEK-293 cells expressing GluK2 and GluK4 subunits. The GluK2(E738D) mutation lowers the steady state apparent affinity for glutamate by 9000-fold in comparison to GluK2 homomeric wildtype receptors. When this mutant subunit was expressed with GluK4, the rising phase of the glutamate steady state concentration-response curve overlapped with the wildtype curve, whereas the declining phase was right-shifted toward lower affinity. Taken together, these data are consistent with a scheme whereby high-affinity agonist binding to a non-desensitizing GluK4 subunit opens the heteromeric channel, whereas low-affinity agonist binding to GluK2 desensitizes the whole channel complex.