Functional characterization of CP-465,022, a selective, noncompetitive AMPA receptor antagonist

Thiazole Derivatives as Modulators of GluA2 AMPA Receptors: Potent Allosteric Effects and Neuroprotective Potential

Thiazole carboxamide derivatives were synthesized in this investigation, with a subsequent examination of their impact on GluA2 AMPA receptors. The synthesized compounds, namely MMH-1-5, were subjected to characterization using high-resolution mass spectrometry (HRMS), proton nuclear magnetic resonance (1H-NMR), and carbon-13 nuclear magnetic resonance (13C-NMR). The present work thoroughly investigates the impact of five thiazole derivatives on GluA2 AMPA receptors. This investigation examined their effects on both whole-cell currents and receptor kinetics. In addition, the cytotoxicity of the samples was assessed using the MTS test. The compound MMH-5 had the highest effect level, resulting in a notable drop in current amplitude by a factor of six. Similarly, MMH-4 and MMH-3 also caused major reductions in the current amplitude. The compounds mentioned above also influenced the rates of deactivation and desensitization. MMH-5 and MMH-4 exhibited an increase in deactivation, while MMH-5 showed reduced desensitization. Our research findings highlight the efficacy of MMH-5 as a negative allosteric modulator of GluA2 AMPA receptors, exerting substantial effects on both the magnitude and time course of receptor activity. Significantly, the compound MMH-2 demonstrated noteworthy cytotoxic effects, as evidenced by cell viability rates dropping below 6.79% for all cancer cell lines and 17.52% for the normal cell line (LX-2). Of particular interest is the pronounced cytotoxicity observed in MMH-5, suggesting its potential as a safe neuroprotective agent targeting the AMPA receptor, as indicated by cell viability percentages exceeding 85.44% across all cancer and normal cell lines. Docking simulations were performed to determine possible modes of interaction between MMH5 and the GluA2-AMPA receptor (PDB:7RZ5). The abovementioned facts and the well-documented effects of further thiazole derivatives provide a strong foundation for future research endeavors to enhance tailored treatments for neurological disorders that rely heavily on GluA2 signaling. The present study elucidates the intricate association between thiazole derivatives and GluA2 receptors, providing valuable perspectives on the prospects of enhanced and specific therapeutic interventions for diverse neurological conditions.

Acetylcholine Promotes Directionally Biased Glutamatergic Retinal Waves

Spontaneous retinal waves are a critical driving force for the self-organization of the mouse visual system prior to eye-opening. Classically characterized as taking place in three distinct stages defined by their primary excitatory drive, Stage II waves during the first postnatal week are propagated through the volume transmission of acetylcholine while Stage III retinal waves during the second postnatal week depend on glutamatergic transmission from bipolar cells. However, both late Stage II and early Stage III retinal waves share a defining propagation bias toward the temporal-to-nasal direction despite developmental changes in the underlying cholinergic and glutamatergic retinal networks. Here, we leverage genetic and pharmacological manipulations to investigate the relationship between cholinergic and glutamatergic neurotransmission during the transition between Stage II and Stage III waves in vivo. We find that the cholinergic network continues to play a vital role in the propagation of waves during Stage III after the primary mode of neurotransmission changes to glutamate. In the absence of glutamatergic waves, compensatory cholinergic activity persists but lacks the propagation bias typically observed in Stage III waves. In the absence of cholinergic waves, gap junction-mediated activity typically associated with Stage I waves persists throughout the developmental window in which Stage III waves usually emerge and lacks the spatiotemporal profile of normal Stage III waves, including a temporal-to-nasal propagation bias. Finally, we show that cholinergic signaling through β2 subunit-containing nicotinic acetylcholine receptors, essential for Stage II wave propagation, is also critical for Stage III wave directionality.

Positive and negative allosteric modulation of GluK2 kainate receptors by BPAM344 and antiepileptic perampanel

Kainate receptors (KARs) are a subtype of ionotropic glutamate receptors that control synaptic transmission in the central nervous system and are implicated in neurological, psychiatric, and neurodevelopmental disorders. Understanding the regulation of KAR function by small molecules is essential for exploring these receptors as drug targets. Here, we present cryoelectron microscopy (cryo-EM) structures of KAR GluK2 in complex with the positive allosteric modulator BPAM344, competitive antagonist DNQX, and negative allosteric modulator, antiepileptic drug perampanel. Our structures show that two BPAM344 molecules bind per ligand-binding domain dimer interface. In the absence of an agonist or in the presence of DNQX, BPAM344 stabilizes GluK2 in the closed state. The closed state is also stabilized by perampanel, which binds to the ion channel extracellular collar sites located in two out of four GluK2 subunits. The molecular mechanisms of positive and negative allosteric modulation of KAR provide a guide for developing new therapeutic strategies.

Affecting AMPA Receptor Biophysical Gating Properties with Negative Allosteric Modulators

Glutamatergic chemical synapses mediate excitatory neurotransmission by the ion flow through α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors in the central nervous system (CNS). AMPA receptor-mediated synaptic transmission abnormalities may play a role in neurologic and neurodegenerative diseases, and compounds that can modulate AMPA receptor (AMPAR) signaling have been studied for decades as possible therapies for Alzheimer's disease, Parkinson's disease, depression, and epilepsy. Here, we aimed to determine the modulating effect of allosteric regulators on AMPA receptors by comparing their actions on AMPA-evoked currents, desensitization, and deactivation rate in human embryonic kidney cells (HEK293T) recombinant AMPAR subunits. In this study, patch-clamp electrophysiology was performed to examine how the AMPA subunit responded to benzodioxole (BDZ) derivatives. Our results showed that the BDZ derivatives affected AMPARs as negative modulators, particularly BDZs (8, 9, and 15), where they increased the desensitization rate and delayed the deactivation process. The BDZ compounds were utilized in this study as AMPA modulators to investigate fundamental and clinical AMPA receptor processes. We test BDZs as negative allosteric AMPAR modulators to reestablish glutamatergic synaptic transmission. These efforts have resulted in important molecules with neuroprotective properties on AMPA receptors.

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.

AMPA Receptor Noncompetitive Inhibitors Occupy a Promiscuous Binding Site

Noncompetitive inhibitors of AMPA receptors have attracted interest in recent years as antiepileptic drugs. However, their development is hindered by a lack of detailed understanding of the protein-inhibitor interaction mechanisms. Recently, structures of AMPA receptor complexes with the structurally dissimilar, noncompetitive, small-molecule inhibitors pyridone perampanel (PMP), GYKI 53655 (GYKI), and CP 465022 (CP) were resolved, revealing that all three share a common binding site. However, due to the low resolution of the ligands, their exact binding modes and protein-ligand interactions remain ambiguous and insufficiently detailed. We carried out molecular dynamics (MD) simulations on X-ray-resolved and docked AMPA receptor complexes, including thermodynamic integration (TI) to compute ligand binding constants, in order to investigate the inhibitor binding modes in detail and identify key protein-ligand interaction mechanisms. Our analysis and simulations show that the ligand binding pocket at the interface of the receptor's transmembrane domain exhibits features also found in the binding pockets of the multidrug-resistance proteins. The inhibitors bind to such promiscuous pockets by forming multiple weak contacts, while the large, flexible pocket undergoes adjustments to accommodate structurally different ligands in different orientations. TI was able to identify a specific more favorable binding mode for GYKI, while PMP, which has a symmetric ring structure, produced several comparable poses indicating that it may bind in several orientations.

Mutational Analysis and Modeling of Negative Allosteric Modulator Binding Sites in AMPA Receptors

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) constitute a subclass of the ionotropic glutamate receptor superfamily, which functions as glutamate-gated cation channels to mediate the majority of excitatory neurotransmission in the central nervous system. AMPARs are therapeutic targets in a range of brain disorders associated with abnormal glutamate hyperactivity. Multiple classes of AMPAR inhibitors have been developed during the past decades, including competitive antagonists, ion channel blockers, and negative allosteric modulators (NAMs). At present, the NAM is the only class of AMPAR ligands that have been developed into safe and useful drugs in humans in the form of perampanel (Fycompa), which was recently approved for treatment of epilepsy. Compared with the detailed understanding of other AMPAR ligand classes, surprisingly little information has been available regarding the molecular mechanism of perampanel and other classes of NAMs at AMPARs; including the location and structure of NAM binding pockets in the receptor complex. However, structures of the AMPAR GluA2 in complex with NAMs were recently reported that unambiguously identified the NAM binding sites. In parallel with this work, our aim with the present study was to identify specific residues involved in the formation of the NAM binding site for three prototypical AMPAR NAMs. Hence, we have performed a mutational analysis of the AMPAR region that links the four extracellular ligand-binding domains to the central ion channel in the transmembrane domain region. Furthermore, we perform computational ligand docking of the NAMs into structural models of the homomeric GluA2 receptor and optimize side chain conformations around the NAMs to model how NAMs bind in this specific site. The new insights provide potentially valuable input for structure-based drug design of new NAMs. SIGNIFICANCE STATEMENT: The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are glutamate-gated ion channels that mediate the majority of excitatory neurotransmission in the brain. Negative allosteric modulators of AMPA receptors are considered to have significant therapeutic potential in diseases linked to glutamate hyperactivity. The present work employs mutational analysis and molecular modeling of the binding site for prototypical NAMs to provide new molecular insight into how NAMs interact with the AMPA receptor, which is of potential use for future design of new types of NAMs.

High-throughput-compatible assays using a genetically-encoded calcium indicator

Measurement of intracellular calcium in live cells is a key component of a wide range of basic life science research, and crucial for many high-throughput assays used in modern drug discovery. Synthetic calcium indicators have become the industry standard, due their ease of use, high reliability, wide dynamic range, and availability of a large variety of spectral and chemical properties. Genetically-encoded calcium indicators (GECIs) have been optimized to the point where their performance rivals that of synthetic calcium indicators in many applications. Stable expression of a GECI has distinct advantages over synthetic calcium indicators in terms of reagent cost and simplification of the assay process. We generated a clonal cell line constitutively expressing GCaMP6s; high expression of the GECI was driven by coupling to a blasticidin resistance gene with a self-cleaving cis-acting hydrolase element (CHYSEL) 2A peptide. Here, we compared the performance of the GECI GCaMP6s to the synthetic calcium indicator fluo-4 in a variety of assay formats. We demonstrate that the pharmacology of ion channel and GPCR ligands as determined using the two indicators is highly similar, and that GCaMP6s is viable as a direct replacement for a synthetic calcium indicator.

Catalyst- and Solvent-Free Coupling of 2-Methyl Quinazolinones and Isatins: An Environmentally Benign Access of Diastereoselective Schizocommunin Analogues

An environmentally benign highly atom-economic protocol for the construction of the C-C bond has been developed under catalyst- and solvent-free conditions. This protocol involves the efficient coupling of 2-methyl quinazolinones with isatin for the highly diastereoselective access of schizocommunin derivatives in excellent yields (up to 97%). Furthermore, the preliminary cytotoxicity screening of selected schizocommunin analogues displayed promising anticancer activity against human cancer cell lines, and the cytotoxic potential of active compound 12ac was also validated by in silico molecular docking simulation studies.

Structure based virtual screening of novel noncompetitive antagonist of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype ionotropic glutamate receptors are attractive antiepileptic targets responsible for mediating the majority of excitatory neurotransmission and plasticity. The noncompetitive antagonists obtain more and more attention as drug candidates for treatment of the neurological diseases involving excessive activity of AMPARs, due to they regulate AMPA receptors (AMPARs) activity independently of endogenous glutamate levels unlike the competitive antagonists. Development of novel AMPAR noncompetitive antagonists, which are safer and more efficacious than competitive antagonists, is highly under demand. Here, we present the discovery of novel antagonists against AMPAR through Structure-Based Virtual Screening (SBVS). Three compounds were successfully distinguished by several different filtering strategies, namely STOCK6S-10902, STOCK1N-49134 and STOCK5S-68665. The interaction mode of these compounds was further explored through molecular dynamics simulation, binding free energy calculation and the binding free energy decomposition. It is demonstrated that some residues within the binding pocket, which have been proved their importance in antagonist binding and gating, form strong hydrogen bond interactions with these three molecules. In particular, H-bond interactions with high occupancies between Ser516, Ser788 and STOCK6S-10902 and Ser516, Asn791 and STOCK1N-49134 were observed. The three hit compounds with new scaffolds and the detailed binding modes could potentially serve as a starting point for further optimization and development.

Allosteric Modulation of Ionotropic Glutamate Receptors: An Outlook on New Therapeutic Approaches To Treat Central Nervous System Disorders

The allosteric targeting of ionotropic glutamate receptors (iGluRs) is a valuable approach for treating various central nervous system (CNS) disorders. In this frame, this Innovations provides a summary of the state-of-the art in the development of allosteric modulators for iGluRs and offers an outlook regarding innovative strategies for treating neurological diseases.

Pharmacological modulation of AMPA receptor rescues social impairments in animal models of autism

Autism spectrum disorder (ASD) is a neurodevelopmental disorder, featuring social communication deficit and repetitive/restricted behaviors as common symptoms. Its prevalence has continuously increased, but, till now, there are no therapeutic approaches to relieve the core symptoms, particularly social deficit. In previous studies, abnormal function of the glutamatergic neural system has been proposed as a critical mediator and therapeutic target of ASD-associated symptoms. Here, we investigated the possible roles of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) in autism symptoms using two well-known autistic animal models, Cntnap2 knockout (KO) mice and in utero valproic acid-exposed ICR (VPA) mice. We found that Cntnap2 KO mice displayed decreased glutamate receptor expression and transmission. Contrarily, VPA mice exhibited increased glutamate receptor expression and transmission. Next, we investigated whether AMPAR modulators (positive-allosteric-modulator for Cntnap2 KO mice and antagonist for VPA mice) can improve autistic symptoms by normalizing the aberrant excitatory transmission in the respective animal models. Interestingly, the AMPAR modulation specifically ameliorated social deficits in both animal models. These results indicated that AMPAR-derived excitatory neural transmission changes can affect normal social behavior. To validate this, we injected an AMPAR agonist or antagonist in control ICR mice and, interestingly, these treatments impaired only the social behavior, without affecting the repetitive and hyperactive behaviors. Collectively, these results provide insight into the role of AMPARs in the underlying pathophysiological mechanisms of ASD, and demonstrate that modulation of AMPAR can be a potential target for the treatment of social behavior deficits associated with ASD.

Role of the Ion Channel Extracellular Collar in AMPA Receptor Gating

AMPA subtype ionotropic glutamate receptors mediate fast excitatory neurotransmission and are implicated in numerous neurological diseases. Ionic currents through AMPA receptor channels can be allosterically regulated via different sites on the receptor protein. We used site-directed mutagenesis and patch-clamp recordings to probe the ion channel extracellular collar, the binding region for noncompetitive allosteric inhibitors. We found position and substitution-dependent effects for introduced mutations at this region on AMPA receptor gating. The results of mutagenesis suggested that the transmembrane domains M1, M3 and M4, which contribute to the ion channel extracellular collar, undergo significant relative displacement during gating. We used molecular dynamics simulations to predict an AMPA receptor open state structure and rationalize the results of mutagenesis. We conclude that the ion channel extracellular collar plays a distinct role in gating and represents a hub for powerful allosteric modulation of AMPA receptor function that can be used for developing novel therapeutics.

Structural Bases of Noncompetitive Inhibition of AMPA-Subtype Ionotropic Glutamate Receptors by Antiepileptic Drugs

Excitatory neurotransmission plays a key role in epileptogenesis. Correspondingly, AMPA-subtype ionotropic glutamate receptors, which mediate the majority of excitatory neurotransmission and contribute to seizure generation and spread, have emerged as promising targets for epilepsy therapy. The most potent and well-tolerated AMPA receptor inhibitors act via a noncompetitive mechanism, but many of them produce adverse side effects. The design of better drugs is hampered by the lack of a structural understanding of noncompetitive inhibition. Here, we report crystal structures of the rat AMPA-subtype GluA2 receptor in complex with three noncompetitive inhibitors. The inhibitors bind to a novel binding site, completely conserved between rat and human, at the interface between the ion channel and linkers connecting it to the ligand-binding domains. We propose that the inhibitors stabilize the AMPA receptor closed state by acting as wedges between the transmembrane segments, thereby preventing gating rearrangements that are necessary for ion channel opening.

GluA2-Containing AMPA Receptors Distinguish Ribbon-Associated from Ribbonless Afferent Contacts on Rat Cochlear Hair Cells

Mechanosensory hair cells release glutamate at ribbon synapses to excite postsynaptic afferent neurons, via AMPA-type ionotropic glutamate receptors (AMPARs). However, type II afferent neurons contacting outer hair cells in the mammalian cochlea were thought to differ in this respect, failing to show GluA immunolabeling and with many “ribbonless” afferent contacts. Here it is shown that antibodies to the AMPAR subunit GluA2 labeled afferent contacts below inner and outer hair cells in the rat cochlea, and that synaptic currents in type II afferents had AMPAR-specific pharmacology. Only half the postsynaptic densities of type II afferents that labeled for PSD-95, Shank, or Homer were associated with GluA2 immunopuncta or presynaptic ribbons, the “empty slots” corresponding to ribbonless contacts described previously. These results extend the universality of AMPAergic transmission by hair cells, and support the existence of silent afferent contacts.

Functional analysis of a de novo GRIN2A missense mutation associated with early-onset epileptic encephalopathy

NMDA receptors (NMDARs), ligand-gated ion channels, play important roles in various neurological disorders, including epilepsy. Here we show the functional analysis of a de novo missense mutation (L812M) in a gene encoding NMDAR subunit GluN2A (GRIN2A). The mutation, identified in a patient with early-onset epileptic encephalopathy and profound developmental delay, is located in the linker region between the ligand-binding and transmembrane domains. Electrophysiological recordings revealed that the mutation enhances agonist potency, decreases sensitivity to negative modulators including magnesium, protons and zinc, prolongs the synaptic response time course and increases single-channel open probability. The functional changes of this amino acid apply to all other NMDAR subunits, suggesting an important role of this residue on the function of NMDARs. Taken together, these data suggest that the L812M mutation causes overactivation of NMDARs and drives neuronal hyperexcitability. We hypothesize that this mechanism underlies the patient's epileptic phenotype as well as cerebral atrophy.

Enantioselective synthesis of 3-arylquinazolin-4(3H)-ones via peptide-catalyzed atroposelective bromination

We report the development of a tertiary amine-containing β-turn peptide that catalyzes the atroposelective bromination of pharmaceutically relevant 3-arylquinazolin-4(3H)-ones (quinazolinones) with high levels of enantioinduction over a broad substrate scope. The structure of the free catalyst and the peptide-substrate complex were explored using X-ray crystallography and 2D-NOESY experiments. Quinazolinone rotational barriers about the chiral anilide axis were also studied using density functional theory calculations and are discussed in light of the high enantioselectivities observed. Mechanistic studies also suggest that the initial bromination event is stereodetermining, and the major monobromide intermediate is an atropisomerically stable, mono-ortho-substituted isomer. The observation of stereoisomerically stable monobromides stimulated the conversion of the tribromide products to other atropisomerically defined products of interest. For example, (1) a dehalogenation Suzuki-Miyaura cross-coupling sequence delivers ortho-arylated derivatives, and (2) a regioselective Buchwald-Hartwig amination procedure installs para-amine functionality. Stereochemical information was retained during these subsequent transformations.

Perampanel inhibition of AMPA receptor currents in cultured hippocampal neurons

Perampanel is an aryl substituted 2-pyridone AMPA receptor antagonist that was recently approved as a treatment for epilepsy. The drug potently inhibits AMPA receptor responses but the mode of block has not been characterized. Here the action of perampanel on AMPA receptors was investigated by whole-cell voltage-clamp recording in cultured rat hippocampal neurons. Perampanel caused a slow (τ∼1 s at 3 µM), concentration-dependent inhibition of AMPA receptor currents evoked by AMPA and kainate. The rates of block and unblock of AMPA receptor currents were 1.5×105 M-1 s-1 and 0.58 s-1, respectively. Perampanel did not affect NMDA receptor currents. The extent of block of non-desensitizing kainate-evoked currents (IC50, 0.56 µM) was similar at all kainate concentrations (3-100 µM), demonstrating a noncompetitive blocking action. Parampanel did not alter the trajectory of AMPA evoked currents indicating that it does not influence AMPA receptor desensitization. Perampanel is a selective negative allosteric AMPA receptor antagonist of high-affinity and slow blocking kinetics.

In vivo synaptic scaling is mediated by GluA2-lacking AMPA receptors in the embryonic spinal cord

When spiking activity within a network is perturbed for hours to days, compensatory changes in synaptic strength are triggered that are thought to be important for the homeostatic maintenance of network or cellular spiking activity. In one form of this homeostatic plasticity, called synaptic scaling, all of a cell's AMPAergic miniature postsynaptic currents (mEPSCs) are increased or decreased by some scaling factor. Although synaptic scaling has been observed in a variety of systems, the mechanisms that underlie AMPAergic scaling have been controversial. Certain studies find that synaptic scaling is mediated by GluA2-lacking calcium receptors (CP-AMPARs), whereas others have found that scaling is mediated by GluA2-containing calcium-impermeable receptors (CI-AMPARs). Spontaneous network activity is observed in most developing circuits, and in the spinal cord this activity drives embryonic movements. Blocking spontaneous network activity in the chick embryo by infusing lidocaine in vivo triggers synaptic scaling in spinal motoneurons; here we show that AMPAergic scaling occurs through increases in mEPSC conductance that appear to be mediated by the insertion of GluA2-lacking AMPA receptors at the expense of GluA2-containing receptors. We have previously reported that in vivo blockade of GABAA transmission, at a developmental stage when GABA is excitatory, also triggered AMPAergic synaptic scaling. Here, we show that this form of AMPAergic scaling is also mediated by CP-AMPARs. These findings suggest that AMPAergic scaling triggered by blocking spiking activity or GABAA receptor transmission represents similar phenomena, supporting the idea that activity blockade triggers scaling by reducing GABAA transmission.

Measurement of atropisomer racemization kinetics using segmented flow technology

When stable atropisomers are encountered by drug discovery teams, they can have important implications due to potential differences in their biological activity, pharmacokinetics, and toxicity. Knowledge of an atropisomer's activation parameters for interconversion is required to facilitate informed decisions on how to proceed. Herein, we communicate the development of a new method for the rapid measurement of atropisomer racemization kinetics utilizing segmented flow technology. This method leverages the speed, accuracy, low sample requirement, safety, and semiautomated nature of flow instrumentation to facilitate the acquisition of kinetics data required for experimentally probing atropisomer activation parameters. Measured kinetics data obtained for the atropo isomerization of AMPA antagonist CP-465021 using segmented flow and traditional thermal methods were compared to validate the method.

Revisiting AMPA receptors as an antiepileptic drug target

In the 1990s there was intense interest in ionotropic glutamate receptors as therapeutic targets for diverse neurological disorders, including epilepsy. NMDA receptors were thought to play a key role in the generation of seizures, leading to clinical studies of NMDA receptor blocking drugs in epilepsy. Disappointing results dampened enthusiasm for ionotropic glutamate receptors as a therapeutic target. Eventually it became appreciated that another type of ionotropic glutamate receptor, the AMPA receptor, is actually the predominant mediator of excitatory neurotransmission in the central nervous system and moreover that AMPA receptors are critical to the generation and spread of epileptic activity. As drugs became available that selectively target AMPA receptors, it was possible to demonstrate that AMPA receptor antagonists have powerful antiseizure activity in in vitro and in vivo models. A decade later, promising clinical studies with AMPA receptor antagonists, including the potent noncompetitive antagonist perampanel, are once again focusing attention on AMPA receptors as a drug target for epilepsy therapy.

Structural and mechanistic determinants of a novel site for noncompetitive inhibition of GluN2D-containing NMDA receptors

NMDA receptors are ionotropic glutamate receptors that mediate excitatory synaptic transmission and have been implicated in several neurological diseases. We have evaluated the mechanism of action of a class of novel subunit-selective NMDA receptor antagonists, typified by (E)-4-(6-methoxy-2-(3-nitrostyryl)-4-oxoquinazolin-3(4H)-yl)-benzoic acid (QNZ46). We found that QNZ46 inhibits NMDA receptor function in a noncompetitive and voltage-independent manner by an unconventional mechanism that requires binding of glutamate to the GluN2 subunit, but not glycine binding to the GluN1 subunit. This dependency of antagonist association on glutamate binding to GluN2 renders these compounds nominally use-dependent, since inhibition will rely on synaptic release of glutamate. Evaluation of the structural determinants responsible for the subunit-selectivity of QNZ46 revealed that these compounds act at a new site that has not previously been described. Residues residing in the part of the agonist binding domain immediately adjacent to the transmembrane helices appear to control selectivity of QNZ46 for GluN2C- and GluN2D-containing receptors. These residues are well-positioned to sense glutamate binding to GluN2 and thus to mediate glutamate-dependent actions. This new class of noncompetitive antagonists could provide an opportunity for the development of pharmacological tools and therapeutic agents that target NMDA receptors at a new site and modulate function by a novel mechanism.

Quinazolin-4-one derivatives: A novel class of noncompetitive NR2C/D subunit-selective N-methyl-D-aspartate receptor antagonists

We describe a new class of subunit-selective antagonists of N-methyl D-aspartate (NMDA)-selective ionotropic glutamate receptors that contain the (E)-3-phenyl-2-styrylquinazolin-4(3H)-one backbone. The inhibition of recombinant NMDA receptor function induced by these quinazolin-4-one derivatives is noncompetitive and voltage-independent, suggesting that this family of compounds does not exert action on the agonist binding site of the receptor or block the channel pore. The compounds described here resemble CP-465,022 ((S)-3-(2-chlorophenyl)-2-[2-(6-diethylaminomethyl-pyridin-2-yl)-vinyl]-6-fluoro-3H-quinazolin-4-one), a noncompetitive antagonist of AMPA-selective glutamate receptors. However, modification of ring substituents resulted in analogues with greater than 100-fold selectivity for recombinant NMDA receptors over AMPA and kainate receptors. Furthermore, within this series of compounds, analogues were identified with 50-fold selectivity for recombinant NR2C/D-containing receptors over NR2A/B containing receptors. These compounds represent a new class of noncompetitive subunit-selective NMDA receptor antagonists.

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.

Subunit-dependent modulation of kainate receptors by muscarinic acetylcholine receptors

Interactions between cholinergic and glutamatergic neurotransmitter systems influence synaptic transmission and plasticity. While previous studies have examined cross-talk between acetylcholine (ACh) and NMDA or AMPA receptors, little is known about the effect of ACh on kainate receptors (KARs). We show that stimulation of m1 or m3 muscarinic ACh receptors (mAChRs) for 2min potentiates recombinant KAR currents in a long lasting fashion. Muscarinic AChR activation potentiates heteromeric GluK2/GluK4 and GluK2/GluK5 receptors, but not homomeric GluK2 receptors. In hippocampal slices kainate potentiates mossy fiber axon excitability. Transient mAChR activation enhances this action of kainate, suggesting a novel mechanism through which acetylcholine could modulate synaptic transmission in the hippocampus. KAR over-activation has been implicated in excitotoxic cell death. To establish the functional significance of the interaction between mAChRs and KARs we examined the effect of mAChR activation on KAR-mediated excitotoxicity. We find that during pharmacological blockade of NMDA and AMPA receptors, KAR activation with AMPA produces significant cell death in primary cortical culture. Concanavalin A (Con A), which selectively blocks KAR desensitization, markedly increases this KAR-mediated neurotoxicity. Brief activation of mAChRs with pilocarpine significantly enhances KAR-mediated excitotoxicity both in the presence and absence of Con A. We conclude that KARs are modulated in a subunit dependent manner by mAChRs. We suggest that ACh may induce long lasting alterations in neuronal excitability and enhance excitotoxicity in part by potentiating KAR function.

The AMPA receptor as a therapeutic target: current perspectives and emerging possibilities

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is a subtype of the ionotropic glutamate receptors that plays a prominent role in neurotransmission and is widespread throughout the CNS. Because of this, its malfunction can result in a multitude of nervous system diseases. This review looks at compounds that are able to modulate AMPAR function by binding to one of several sites on the receptor that either downregulate its function (competitive, noncompetitive and uncompetitive antagonists) or upregulate its function (positive modulators). It will also give an account of the various diseases that have implicated AMPAR dysfunction and how specific types of AMPAR modulator may be beneficial in their treatment. The AMPAR remains an unexploited but important therapeutic target.

2,3-Benzodiazepine-type AMPA receptor antagonists and their neuroprotective effects

AMPA receptors are fast ligand-gated members of glutamate receptors in neuronal and many types of non-neuronal cells. The heterotetramer complexes are assembled from four subunits (GluR1-4) in region-, development- and function-selective patterns. Each subunit contains three extracellular domains (a large amino terminal domain, an agonist-binding domain and a transducer domain), and three transmembrane segments with a loop (pore forming domain), as well as the intracellular carboxy terminal tail (traffic and conductance regulatory domain). The binding of the agonist (excitatory amino acids and their derivatives) initiates conformational realignments, which transmit to the transducer domain and membrane spanning segments to gate the channel permeable to Na+, K+ and more or less to Ca2+. Several 2,3-benzodiazepines act as non-competitive antagonists of the AMPA receptor (termed also negative allosteric modulators), which are thought to bind to the transducer domains and inhibit channel gating. Analysing their effects in vitro, it has been possible to recognize a structure-activity relationship, and to describe the critical parts of the molecules involved in their action at AMPA receptors. Blockade of AMPA receptors can protect the brain from apoptotic and necrotic cell death by preventing neuronal excitotoxicity during pathophysiological activation of glutamatergic neurons. Animal experiments provided evidence for the potential usefulness of non-competitive AMPA antagonists in the treatment of human ischemic and neurodegenerative disorders including stroke, multiple sclerosis, Parkinson's disease, periventricular leukomalacia and motoneuron disease. 2,3-benzodiazepine AMPA antagonists can protect against seizures, decrease levodopa-induced dyskinesia in animal models of Parkinson's disease demonstrating their utility for the treatment of a variety of CNS disorders.

The new 2,3-benzodiazepine derivative EGIS-8332 inhibits AMPA/kainate ion channels and cell death

We observed in vitro neuroprotective and AMPA/kainate receptor antagonist effects of the new 2,3-benzodiazepine derivative EGIS-8332 (R,S-1-(4-aminophenyl)-7,8-methylenedioxy-4-cyano-4-methyl-3-N-acetyl-5H-3,4-dihydro-2,3-benzodiazepine) using the lactate dehydrogenase (LDH) release assay and patch clamp recordings on primary cultures of rat embryonic telencephalon neurons exposed to AMPA/kainate receptor agonists. EGIS-8332 potently decreased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and quisqualate induced LDH release (IC(50)=5.2+/-0.4 and 7.4+/-1.3 microM, respectively) from the cells. Whole-cell patch clamp studies carried out on the ionotropic glutamate receptors N-methyl D-aspartate (NMDA), as well as AMPA (and kainate) in cultured telencephalon neurons verified that EGIS-8332 blocked steady state responses to AMPA and kainate (IC(50)=1.7+/-0.4 and 6.2+/-1.6 microM, respectively), but hardly influenced currents evoked by NMDA. EGIS-8332 also inhibited kainate-evoked response in CHO cells expressing the flop variant of GluR1 receptor and, in cerebellar Purkinje cells at similar efficiency. The stereoselectivity of the inhibitory site is established by the clearly dissimilar inhibitory potency of the enantiomer components of EGIS-8332 differing in the configuration of methyl and cyano substituents on carbon C(4): the R(-) enantiomer was found to be the efficient species. This finding suggests that the inhibitory interaction between the channel protein and drug is promoted by presence of the C(4) methyl group. The inhibition of the AMPA/kainate ion channels by EGIS-8332 is non-competitive, not use dependent, and depends neither on the closed/open state of the channel, nor the membrane potential. These findings suggest an allosteric mechanism for the inhibition. These in vitro observations suggest that the compound might be useful in the treatments of certain acute and chronic neurological syndromes initiated by derangements of ionotropic glutamate receptor function.

Molecular Mechanism of AMPA Receptor Noncompetitive Antagonism

AMPA-type glutamate receptors are specifically inhibited by the noncompetitive antagonists GYKI-53655 and CP-465,022, which act through sites and mechanisms that are not understood. Using receptor mutagenesis, we found that these antagonists bind at the interface between the S1 and S2 glutamate binding core and channel transmembrane domains, specifically interacting with S1-M1 and S2-M4 linkers, thereby disrupting the transduction of agonist binding into channel opening. We also found that the antagonists' affinity is higher for agonist-unbound receptors than for activated nondesensitized receptors, further depending on the level of S1 and S2 domain closure. These results provide evidence for substantial conformational changes in the S1-M1 and S2-M4 linkers following agonist binding and channel opening, offering a conceptual frame to account for noncompetitive antagonism of AMPA receptors.

Ionotropic and metabotropic glutamate receptor structure and pharmacology

RATIONALE

L: -Glutamate is the major excitatory neurotransmitter in the central nervous system (CNS) and mediates its actions via activation of both ionotropic and metabotropic receptor families. The development of selective ligands, including competitive agonists and antagonists and positive and negative allosteric modulators, has enabled investigation of the functional roles of glutamate receptor family members.

OBJECTIVE

In this review we describe the subunit structure and composition of the ionotropic and metabotropic glutamate receptors and discuss their pharmacology, particularly with respect to selective tools useful for investigation of their function in the CNS.

RESULTS

A large number of ligands are now available that are selective either for glutamate receptor subfamilies or for particular receptor subtypes. Such ligands have enabled considerable advances in the elucidation of the physiological and pathophysiological roles of receptor family members. Furthermore, efficacy in animal models of neurological and psychiatric disorders has supported the progression of several glutamatergic ligands into clinical studies. These include ionotropic glutamate receptor antagonists, which have entered clinical trials for disorders including epilepsy and ischaemic stroke, alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptor positive allosteric modulators which are under evaluation as cognitive enhancers, and metabotropic glutamate receptor 2 (mGluR2) agonists which are undergoing clinical evaluation as anxiolytics. Furthermore, preclinical studies have illustrated therapeutic potential for ligands selective for other receptor subtypes in various disorders. These include mGluR1 antagonists in pain, mGluR5 antagonists in anxiety, pain and drug abuse and mGluR5 positive allosteric modulators in schizophrenia.

CONCLUSIONS

Selective pharmacological tools have enabled the study of glutamate receptors. However, pharmacological coverage of the family is incomplete and considerable scope remains for the development of novel ligands, particularly those with in vivo utility, and for the their use together with existing tools for the further investigation of the roles of receptor family members in CNS function and as potentially novel therapeutics.

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.

AMPA receptor ligands: Synthetic and pharmacological studies of polyamines and polyamine toxins

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPAR), subtype of the ionotropic glutamate receptors (IGRs), mediate fast synaptic transmission in the central nervous system (CNS), and are involved in many neurological disorders, as well as being a key player in the formation of memory. Hence, ligands affecting AMPARs are highly important for the study of the structure and function of this receptor, and in this regard polyamine-based ligands, particularly polyamine toxins, are unique as they selectively block Ca2+ -permeable AMPARs. Indeed, endogenous intracellular polyamines are known to modulate the function of these receptors in vivo. In this study, recent developments in the medicinal chemistry of polyamine-based ligands are given, particularly focusing on the use of solid-phase synthesis (SPS) as a tool for the facile generation of libraries of polyamine toxin analogues. Moreover, the recent development of highly potent and very selective AMPAR ligands is described. Additionally, we provide a detailed account on the mechanism and site of action of AMPAR blockade by polyamine-based ligands, including examples of how these ligands are used as tools to study AMPAR, and a comparison with their action on other ionotropic receptors.

Activation of Ionotropic Glutamate Receptors on Peripheral Axons of Primary Motoneurons Mediates Transmitter Release at the Zebrafish NMJ

The development and function of the vertebrate neuromuscular junction (NMJ) is continually being redefined. Previous studies have indicated that glutamate may play a role in the development or function of the NMJ by associating with presynaptic receptors. We have used larval zebrafish ( Danio rerio) to investigate the presence of presynaptic ionotropic glutamate receptors (iGluRs) at the NMJ in vivo. In whole-mount zebrafish larvae, antibody staining directed to NR2A subunits colocalized with specific staining of motoneuron axon tracts. Whole cell voltage-clamp recordings of miniature endplate currents (mEPCs) from axial white muscle were performed during application of iGluR agonists and antagonists. Local perfusion of the NMJ with iGluR agonists resulted in significant increases in the frequency of spontaneous acetylcholine (ACh) release. These increases were blocked by the N-methyl-d-aspartate (NMDA) receptor antagonist d-(-)-2-amino-5-phosphonopentanoic acid (50 μM) and by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione (50 μM). Further pharmacological investigation revealed no effect of the kainate receptor-specific antagonist (2S,4R)-4-methylglutamate (10 μM) on kainate-induced rises in the frequency of spontaneous ACh release. However, these were blocked with the AMPA receptor-specific antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (50 μM). Application of glutamate (1 mM) in the presence of the glutamate uptake inhibitor d-threo-β-benzyloxyaspartate(200 μM) resulted in a significant increase in the frequency of mEPCs. These results suggest the presence of AMPA and NMDA receptors in association with motoneuron axons of larval zebrafish.

Functional characterization of YM928, a novel moncompetitive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor is thought to play an important role in the pathogenesis of several neurological disorders as well as normal brain function. The search for AMPA receptor antagonists as potential therapeutics is ongoing. Here, we describe the functional characterization of a novel noncompetitive AMPA receptor antagonist, 2-[N-(4-chlorophenyl)-N-methylamino]-4H-pyrido[3,2-e]-1,3-thiazin-4-one (YM928). This compound inhibited AMPA receptor-mediated toxicity in primary rat hippocampal cultures with an IC50 of 2 micro M. Its manner of inhibition was noncompetitive as the agonist concentration was increased. YM928 blocked AMPA-induced intracellular calcium influx with an IC50 of 3 micro M and antagonized AMPA-induced inward currents with an IC50 of 1 micro M in cultured cells. YM928 displaced neither [3H]AMPA binding nor other existing glutamate receptor-related ligand binding in rat brain membranes. In terms of in vivo activity, YM928 had an anticonvulsant effect in sound-induced seizures in DBA/2 mice 45 min after oral administration at 3 mg/kg. Thus, YM928 has potential as an oral therapeutic drug for various types of neurological disorders.

Pharmacophore modeling as an efficient tool in the discovery of novel noncompetitive AMPA receptor antagonists

A three-dimensional pharmacophore model for the binding of noncompetitive AMPA receptor antagonists was developed in order to map common structural features of highly active compounds. This hypothesis, which consists of two hydrophobic regions, one hydrogen bond acceptor and one aromatic region, was successfully used as framework for the design of a new class of allosteric modulators containing a tetrahydroisoquinoline skeleton and for in silico screening. The promising biological results suggested that the identified molecules might be useful "lead compounds" for future drug development.

Binding modes of noncompetitive AMPA antagonists: a computational approach

The activity of functional AMPA receptors (AMPARs) is modulated by noncompetitive antagonists. So far, no information about the molecular mechanism of action and the localization of the binding pocket(s) is available. We speculated that the leucine/isoleucine/valine binding protein (LIVBP)-like domain of AMPAR, localized at the extracellular N-terminus of the receptor, might be involved in the binding of noncompetitive antagonists and we tested this hypothesis through a computational approach involving the comparison with NMDA and metabotropic glutamate receptors and the generation of a 3D homology model of the LIVBP-like domain of AMPAR. The results suggest that the interdomain cleft of the LIVBP-like domain of AMPAR may contain the noncompetitive antagonist binding pocket.

Discovery of a novel and highly potent noncompetitive AMPA receptor antagonist

N-Acetyl-1-aryl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline derivatives were designed and synthesized as potential noncompetitive AMPA receptor antagonists on the basis of molecular modeling studies. Sound-induced seizure testing showed that this class of compounds possessed anticonvulsant properties. In particular, 10c was more potent than talampanel (2), a noncompetitive AMPA receptor antagonist currently being investigated in phase III trials as an antiepileptic agent. Furthermore, electrophysiological studies indicated that 10c was a highly effective noncompetitive-type modulator of the AMPA receptor.