Trapping of glutamate and glycine during open channel block of rat hippocampal neuron NMDA receptors by 9-aminoacridine

Structural insights into gating mechanism and allosteric regulation of NMDA receptors

N-methyl-d-aspartate receptors (NMDARs) belong to the ionotropic glutamate receptors (iGluRs) superfamily and act as coincidence detectors that are crucial to neuronal development and synaptic plasticity. They typically assemble as heterotetramers of two obligatory GluN1 subunits and two alternative GluN2 (from 2A to 2D) and/or GluN3 (3A and 3B) subunits. These alternative subunits mainly determine the diverse biophysical and pharmacological properties of different NMDAR subtypes. Over the past decade, the unprecedented advances in structure elucidation of these tetrameric NMDARs have provided atomic insights into channel gating, allosteric modulation and the action of therapeutic drugs. A wealth of structural and functional information would accelerate the artificial intelligence-based drug design to exploit more NMDAR subtype-specific molecules for the treatment of neurological and psychiatric disorders.

Light-Sensitive Open Channel Block of Ionotropic Glutamate Receptors by Quaternary Ammonium Azobenzene Derivatives

Glutamate ionotropic receptors mediate fast excitation processes in the central nervous system of vertebrates and play an important role in synaptic plasticity, learning, and memory. Here, we describe the action of two azobenene-containing compounds, AAQ (acrylamide-azobenzene-quaternary ammonium) and QAQ (quaternary ammonium-azobenzene-quaternary ammonium), which produced rapid and fully reversible light-dependent inhibition of glutamate ionotropic receptors. The compounds demonstrated voltage-dependent inhibition with only minor voltage-independent allosteric action. Calcium-impermeable AMPA receptors had weaker sensitivity compared to NMDA and calcium-permeable AMPA receptors. We further revealed that the compounds bound to NMDA and calcium-permeable AMPA receptors in different modes. They were able to enter the wide selectivity filter of AMPA receptors, and strong negative voltages caused permeation into the cytoplasm. The narrow selectivity filter of the NMDA receptors did not allow the molecules to bypass them; therefore, QAQ and AAQ bound to the shallow channel site and prevented channel closure by a foot-in-the-door mechanism. Computer simulations employing available AMPA and NMDA receptor structures readily reproduced the experimental findings, allowing for the structure-based design of more potent and selective drugs in the future. Thus, our work creates a framework for the development of light-sensitive blockers of calcium-permeable AMPA receptors, which are desirable tools for neuroscience.

Tricyclic Antidepressant Structure-Related Alterations in Calcium-Dependent Inhibition and Open-Channel Block of NMDA Receptors

N-methyl-D-aspartate receptors (NMDARs) are an essential target for the analgetic action of tricyclic antidepressants (TCAs). Their therapeutic blood concentrations achieve 0.5–1.5 μM, which, however, are insufficient to cause in vitro the open-channel block known as the only effect of TCAs on NMDARs. Whereas structures of amitriptyline (ATL), desipramine (DES), and clomipramine (CLO) are rather similar these compounds manifest different therapeutic profiles and side effects. To study structure-activity relationships of DES and CLO on NMDARs, we measured IC₅₀s as a function of extracellular calcium ([Ca²⁺]) and membrane voltage (V_m) of NMDAR currents recorded in cortical neurons. Here two components of TCA action on NMDARs are described, which could be characterized as the Ca²⁺-dependent inhibition and the open-channel block. DES demonstrated a profound Ca²⁺-dependent inhibition of NMDARs, while the CLO effect was weak. DES IC₅₀ exhibited an e-fold change with a [Ca²⁺] shift of 0.59 mM, which is consistent with ATL. The Ca²⁺ dependence of NMDAR inhibition by DES disappeared in BAPTA loaded neurons, suggesting that Ca²⁺ acts from the inside. Since CLO differs from DES and ATL by the presence of Cl-atom in the structure, most likely, this is the atom which is responsible for the loss of pronounced [Ca²⁺] dependence. As for the NMDAR open-channel block, both DES and CLO were about 5-folds more potent than ATL due to their slow rates of dissociation either from open and closed states. DES demonstrated stronger V_m-dependence than CLO, suggesting a deeper location of the DES binding site within the ion pore. Because DES and CLO differ from ATL by the nitrogen-containing tricycle, presumably this moiety of the molecules determines their high-affinity binding with the NMDAR channel, while the aliphatic chain mono-methyl amino-group of DES allows a deep permeation in the channel. Thus, different structure-activity relationships of the Ca²⁺-dependent inhibition and V_m-dependent open-channel block of NMDARs by DES and CLO suggest that these processes are independent and most likely may represent an action on different molecular targets. The proposed model of TCA action on NMDARs predicts well the experimental values of IC₅₀s at physiological [Ca²⁺] and within a wide range of V_ms.

Mechanisms of NMDA receptor inhibition by nafamostat, gabexate and furamidine

N-methyl-D-aspartate (NMDA) receptors are affected by many pharmaceuticals. In this work, we studied the action of the serine protease inhibitors nafamostat, gabexate and camostat, and an antiprotozoal compound, furamidine, on native NMDA receptors in rat hippocampal pyramidal neurons. Nafamostat, furamidine and gabexate inhibited these receptors with IC₅₀ values of 0.20 ± 0.04, 0.64 ± 0.13 and 16 ± 3 μM, respectively, whereas camostat was ineffective. Nafamostat and furamidine showed voltage-dependent inhibition, while gabexate showed practically voltage-independent inhibition. Nafamostat and furamidine demonstrated tail currents, implying a 'foot-in-the-door' mechanism of action; gabexate did not demonstrate any signs of 'foot-in-the-door' or trapping channel block. Gabexate action was also not competitive, suggesting allosteric inhibition of NMDA receptors. Furamidine and nafamostat are structurally similar to the previously studied diminazene and all three demonstrated a 'foot-in-the-door' mechanism. They have a rather rigid, elongated structures and cannot fold into more compact forms. By contrast, the gabexate molecule can fold, but its folded structure differs drastically from that of typical NMDA receptor blockers, in agreement with its voltage-independent inhibition. These findings provide a better understanding of the structural determinants of NMDA receptor antagonism, while also supporting the potential clinical repurposing of these drugs as neuroprotectors for glaucoma and other neurodegenerative diseases.

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.

Gating mechanism and a modulatory niche of human GluN1-GluN2A NMDA receptors

N-methyl-D-aspartate (NMDA) receptors are glutamate-gated calcium-permeable ion channels that are widely implicated in synaptic transmission and plasticity. Here, we report a gallery of cryo-electron microscopy (cryo-EM) structures of the human GluN1-GluN2A NMDA receptor at an overall resolution of 4 Å in complex with distinct ligands or modulators. In the full-length context of GluN1-GluN2A receptors, we visualize the competitive antagonists bound to the ligand-binding domains (LBDs) of GluN1 and GluN2A subunits, respectively. We reveal that the binding of positive allosteric modulator shortens the distance between LBDs and the transmembrane domain (TMD), which further stretches the opening of the gate. In addition, we unexpectedly visualize the binding cavity of the "foot-in-the-door" blocker 9-aminoacridine within the LBD-TMD linker region, differing from the conventional "trapping" blocker binding site at the vestibule within the TMD. Our study provides molecular insights into the crosstalk between LBDs and TMD during channel activation, inhibition, and allosteric transition.

Interplay between Gating and Block of Ligand-Gated Ion Channels

Drugs that inhibit ion channel function by binding in the channel and preventing current flow, known as channel blockers, can be used as powerful tools for analysis of channel properties. Channel blockers are used to probe both the sophisticated structure and basic biophysical properties of ion channels. Gating, the mechanism that controls the opening and closing of ion channels, can be profoundly influenced by channel blocking drugs. Channel block and gating are reciprocally connected; gating controls access of channel blockers to their binding sites, and channel-blocking drugs can have profound and diverse effects on the rates of gating transitions and on the stability of channel open and closed states. This review synthesizes knowledge of the inherent intertwining of block and gating of excitatory ligand-gated ion channels, with a focus on the utility of channel blockers as analytic probes of ionotropic glutamate receptor channel function.

Dual action of amitriptyline on NMDA receptors: enhancement of Ca-dependent desensitization and trapping channel block

Although the tricyclic antidepressant amitriptyline (ATL) is widely used in the clinic, the mechanism underlying its high therapeutic efficacy against neuropathic pain remains unclear. NMDA receptors (NMDARs) represent a target for ATL and are involved in sensitization of neuropathic pain. Here we describe two actions of ATL on NMDARs: 1) enhancement of Ca²⁺-dependent desensitization and 2) trapping channel block. Inhibition of NMDARs by ATL was found to be dependent upon external Ca²⁺ concentration ([Ca²⁺]) in a voltage-independent manner, with an IC₅₀ of 0.72 μM in 4 mM [Ca²⁺]. The ATL IC₅₀ value increased exponentially with decreasing [Ca²⁺], with an e-fold change observed per 0.69 mM decrease in [Ca²⁺]. Loading neurons with BAPTA abolished Ca²⁺-dependent inhibition, suggesting that Ca²⁺ affects NMDARs from the cytosol. Since there is one known Ca²⁺-dependent process in gating of NMDARs, we conclude that ATL most likely promotes Ca²⁺-dependent desensitization. We also found ATL to be a trapping open-channel blocker of NMDARs with an IC₅₀ of 220 µM at 0 mV. An e-fold change in ATL IC₅₀ was observed to occur with a voltage shift of 50 mV in 0.25 mM [Ca²⁺]. Thus, we disclose here a robust dependence of ATL potency on extracellular [Ca²⁺], and demonstrate that ATL bound in the NMDAR pore can be trapped by closure of the channel.

The mechanisms of potentiation and inhibition of GABAA receptors by non-steroidal anti-inflammatory drugs, mefenamic and niflumic acids

Fenamates mefanamic and niflumic acids (MFA and NFA) induced dual potentiating and inhibitory effects on GABA currents recorded in isolated cerebellar Purkinje cells using the whole-cell patch-clamp and fast-application techniques. Regardless of the concentration, both drugs induced a pronounced prolongation of the current response. We demonstrated that the same concentration of drugs can produce both potentiating and inhibitory effects, depending on the GABA concentration, which indicates that both processes take place simultaneously and the net effect depends on the concentrations of both the agonist and fenamate. We found that the NFA-induced block is strongly voltage-dependent. The Woodhull analysis of the block suggests that NFA has two binding sites in the pore - shallow and deep. We built a homology model of the open GABA_AR based on the cryo-EM structure of the open α1 GlyR and applied Monte-Carlo energy minimization to optimize the ligand-receptor complexes. A systematic search for MFA/NFA binding sites in the GABA_AR pore revealed the existence of two sites, the location of which coincides well with predictions of the Woodhull model. In silico docking suggests that two fenamate molecules are necessary to occlude the pore. We showed that MFA, acting as a PAM, competes with an intravenous anesthetic etomidate for a common binding site. We built structural models of MFA and NFA binding at the transmembrane β(+)/α(-) intersubunit interface. We suggested a hypothesis on the molecular mechanism underlying the prolongation of the receptor lifetime in open state after MFA/NFA binding and β subunit specificity of the fenamate potentiation.

Hydrophobic Amines and Their Guanidine Analogues Modulate Activation and Desensitization of ASIC3

Acid-sensing ion channel 3 (ASIC3) is an important member of the acid-sensing ion channels family, which is widely expressed in the peripheral nervous system and contributes to pain sensation. ASICs are targeted by various drugs and toxins. However, mechanisms and structural determinants of ligands' action on ASIC3 are not completely understood. In the present work we studied ASIC3 modulation by a series of "hydrophobic monoamines" and their guanidine analogs, which were previously characterized to affect other ASIC channels via multiple mechanisms. Electrophysiological analysis of action via whole-cell patch clamp method was performed using rat ASIC3 expressed in Chinese hamster ovary (CHO) cells. We found that the compounds studied inhibited ASIC3 activation by inducing acidic shift of proton sensitivity and slowed channel desensitization, which was accompanied by a decrease of the equilibrium desensitization level. The total effect of the drugs on the sustained ASIC3-mediated currents was the sum of these opposite effects. It is demonstrated that drugs' action on activation and desensitization differed in their structural requirements, kinetics of action, and concentration and state dependencies. Taken together, these findings suggest that effects on activation and desensitization are independent and are likely mediated by drugs binding to distinct sites in ASIC3.

Structure, function, and allosteric modulation of NMDA receptors

Hansen et al. review recent structural data that have provided insight into the function and allosteric modulation of NMDA receptors.

NMDA-type glutamate receptors are ligand-gated ion channels that mediate a Ca²⁺-permeable component of excitatory neurotransmission in the central nervous system (CNS). They are expressed throughout the CNS and play key physiological roles in synaptic function, such as synaptic plasticity, learning, and memory. NMDA receptors are also implicated in the pathophysiology of several CNS disorders and more recently have been identified as a locus for disease-associated genomic variation. NMDA receptors exist as a diverse array of subtypes formed by variation in assembly of seven subunits (GluN1, GluN2A-D, and GluN3A-B) into tetrameric receptor complexes. These NMDA receptor subtypes show unique structural features that account for their distinct functional and pharmacological properties allowing precise tuning of their physiological roles. Here, we review the relationship between NMDA receptor structure and function with an emphasis on emerging atomic resolution structures, which begin to explain unique features of this receptor.

Effects of Mg2+ on recovery of NMDA receptors from inhibition by memantine and ketamine reveal properties of a second site

Memantine and ketamine are NMDA receptor (NMDAR) open channel blockers that are thought to act via similar mechanisms at NMDARs, but exhibit divergent clinical effects. Both drugs act by entering open NMDARs and binding at a site deep within the ion channel (the deep site) at which the endogenous NMDAR channel blocker Mg2+ also binds. Under physiological conditions, Mg2+ increases the IC50s of memantine and ketamine through competition for binding at the deep site. Memantine also can inhibit NMDARs after associating with a second site accessible in the absence of agonist, a process termed second site inhibition (SSI) that is not observed with ketamine. Here we investigated the effects of 1 mM Mg2+ on recovery from inhibition by memantine and ketamine, and on memantine SSI, of the four main diheteromeric NMDAR subtypes. We found that: recovery from memantine inhibition depended strongly on the concentration of memantine used to inhibit the NMDAR response; Mg2+ accelerated recovery from memantine and ketamine inhibition through distinct mechanisms and in an NMDAR subtype-dependent manner; and Mg2+ occupation of the deep site disrupted memantine SSI in a subtype-dependent manner. Our results support the hypothesis that memantine associates with, but does not inhibit at the second site. After associating with the second site, memantine can either slowly dissociate directly to the extracellular solution, or transit to the deep site, resulting in typical channel block. Memantine's relatively slow dissociation from the second site underlies the dependence of NMDAR recovery from inhibition on both memantine concentration and on Mg2+.

Potentiation and Block of ASIC1a by Memantine

Acid-sensing ion channels (ASICs) are modulated by various classes of ligands, including the recently described hydrophobic monoamines, which inhibit and potentiate ASICs in a subunit-specific manner. In particular, memantine inhibits ASIC1a and potentiates ASIC2a homomers. The aim of the present work was to characterize action mechanism of memantine on recombinant ASIC1a expressed in CHO (Chinese hamster ovary) cells. We have demonstrated that effect of memantine on ASIC1a strongly depends on membrane voltage, conditioning pH value and application protocol. When applied simultaneously with activating acidification at hyperpolarized voltages, memantine caused the strongest inhibition. Surprisingly, application of memantine between ASIC1a activations at zero voltage caused significant potentiation. Analysis of the data suggests that memantine produces two separate effects, voltage-dependent open-channel block and shift of steady-state desensitization curve to more acidic values. Putative binding sites are discussed based on the computer docking of memantine to the acidic pocket and the pore region.

Semiclosed Conformations of the Ligand-Binding Domains of NMDA Receptors during Stationary Gating

NMDA receptors are tetrameric ligand-gated ion channels. In the continuous presence of saturating agonists, NMDA receptors undergo stationary gating, in which the channel stochastically switches between an open state that permits ion conductance and a closed state that prevents permeation. The ligand-binding domains (LBDs) of the four subunits are expected to have closed clefts in the channel-open state. On the other hand, there is little knowledge about the conformational status of the LBDs in the channel-closed state during stationary gating. To probe the latter conformational status, Kussius and Popescu engineered interlobe disulfide cross-links in NMDA receptors and found that the cross-linking produced stationary gating kinetics that differed only subtly from that produced by agonist binding. These authors assumed that the cross-linking immobilized the LBDs in cleft-closed conformations, and consequently concluded that throughout stationary gating, agonist-bound LBDs also stayed predominantly in cleft-closed conformations and made only infrequent excursions to cleft-open conformations. Here, by calculating the conformational free energies of cross-linked and agonist-bound LBDs, we assess whether cross-linking actually traps the LBDs in cleft-closed conformations and delineate semiclosed conformations of agonist-bound LBDs that may potentially be thermodynamically and kinetically important during stationary gating. Our free-energy results show that the cross-linked LBDs are not locked in the fully closed form; rather, they sample semiclosed conformations almost as readily as the agonist-bound LBDs. Several lines of reasoning suggest that LBDs are semiclosed in the channel-closed state during stationary gating. Our free-energy simulations suggest possible structural details of such semiclosed LBD conformations, including intra- and intermolecular interactions that serve as alternatives to those in the cleft-closed conformations.

NMDA Receptors in the Central Nervous System

NMDA-type glutamate receptors are ligand-gated ion channels that mediate a major component of excitatory neurotransmission in the central nervous system (CNS). They are widely distributed at all stages of development and are critically involved in normal brain functions, including neuronal development and synaptic plasticity. NMDA receptors are also implicated in the pathophysiology of numerous neurological and psychiatric disorders, such as ischemic stroke, traumatic brain injury, Alzheimer's disease, epilepsy, mood disorders, and schizophrenia. For these reasons, NMDA receptors have been intensively studied in the past several decades to elucidate their physiological roles and to advance them as therapeutic targets. Seven NMDA receptor subunits exist that assemble into a diverse array of tetrameric receptor complexes, which are differently regulated, have distinct regional and developmental expression, and possess a wide range of functional and pharmacological properties. The diversity in subunit composition creates NMDA receptor subtypes with distinct physiological roles across neuronal cell types and brain regions, and enables precise tuning of synaptic transmission. Here, we will review the relationship between NMDA receptor structure and function, the diversity and significance of NMDA receptor subtypes in the CNS, as well as principles and rules by which NMDA receptors operate in the CNS under normal and pathological conditions.

Modulating synaptic NMDA receptors

Recent structural information on ligand-gated glutamate receptors and newly-discovered clinical uses for NMDA receptor antagonists has renewed interest in understanding the mechanisms of drug action at these receptors. Although the voltage-dependence and calcium permeability of NMDA receptors are well-studied, the mechanisms affecting the time course of synaptic NMDA receptor activation may be of more therapeutic value by serving as a rheostat for the total synaptic response. The NMDA receptor-mediated EPSC time course has been thought of as a fixed parameter based simply on receptor subunit composition as variably constrained by anatomical and developmental expression patterns, albeit subject to modification by kinetic behaviors such as modal gating. However, the EPSC time course also can be manipulated by endogenous and exogenous ligands. In this commentary we discuss insights into the in situ composition and kinetic behavior of synaptic NMDA receptors and propose new opportunities to target modulatory sites on NMDA receptors and to develop useful therapeutics. The emerging data on the atomic structure of NMDA receptors and knowledge of the kinetics of native receptors in neurons provide a roadmap in this regard. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Block of NMDA receptor channels by endogenous neurosteroids: implications for the agonist induced conformational states of the channel vestibule

N-methyl-D-aspartate receptors (NMDARs) mediate synaptic plasticity, and their dysfunction is implicated in multiple brain disorders. NMDARs can be allosterically modulated by numerous compounds, including endogenous neurosteroid pregnanolone sulfate. Here, we identify the molecular basis of the use-dependent and voltage-independent inhibitory effect of neurosteroids on NMDAR responses. The site of action is located at the extracellular vestibule of the receptor's ion channel pore and is accessible after receptor activation. Mutations in the extracellular vestibule in the SYTANLAAF motif disrupt the inhibitory effect of negatively charged steroids. In contrast, positively charged steroids inhibit mutated NMDAR responses in a voltage-dependent manner. These results, in combination with molecular modeling, characterize structure details of the open configuration of the NMDAR channel. Our results provide a unique opportunity for the development of new therapeutic neurosteroid-based ligands to treat diseases associated with dysfunction of the glutamate system.

The Effect of Hydrophobic Monoamines on Acid-Sensing Ion Channels ASIC1B

Acid-sensing ion channels (ASICs) are widely distributed in both the central and peripheral nervous systems of vertebrates. The pharmacology of these receptors remains poorly investigated, while the search for new ASIC modulators is very important. Recently, we found that some monoamines, which are blockers of NMDA receptors, inhibit and/or potentiate acid-sensing ion channels, depending on the subunit composition of the channels. The effect of 9-aminoacridine, IEM-1921, IEM-2117, and memantine both on native receptors and on recombinant ASIC1a, ASIC2a, and ASIC3 homomers was studied. In the present study, we have investigated the effect of these four compounds on homomeric ASIC1b channels. Experiments were performed on recombinant receptors expressed in CHO cells using the whole-cell patch clamp technique. Only two compounds, 9-aminoacridine and memantine, inhibited ASIC1b channels. IEM-1921 and IEM-2117 were inactive even at a 1000 μM concentration. In most aspects, the effect of the compounds on ASIC1b was similar to their effect on ASIC1a. The distinguishing feature of homomeric ASIC1b channels is a steep activation-dependence, indicating cooperative activation by protons. In our experiments, the curve of the concentration dependence of ASIC1b inhibition by 9-aminoacridine also had a slope (Hill coefficient) of 3.8, unlike ASIC1a homomers, for which the Hill coefficient was close to 1. This finding indicates that the inhibitory effect of 9-aminoacridine is associated with changes in the activation properties of acid-sensing ion channels.

Recent insights into the mode of action of memantine and ketamine

The clinical benefits of the glutamate receptor antagonists memantine and ketamine have helped sustain optimism that glutamate receptors represent viable targets for development of therapeutic drugs. Both memantine and ketamine antagonize N-methyl-D-aspartate receptors (NMDARs), a glutamate receptor subfamily, by blocking the receptor-associated ion channel. Although many of the basic characteristics of NMDAR inhibition by memantine and ketamine appear similar, their effects on humans and to a lesser extent on rodents are strongly divergent. Some recent research suggests that preferential inhibition by memantine and ketamine of distinct NMDAR subpopulations may contribute to the drugs' differential clinical effects. Here we review studies that shed light on possible explanations for differences between the effects of memantine and ketamine.

Block of GABA(A) receptor ion channel by penicillin: electrophysiological and modeling insights toward the mechanism

GABA(A) receptors (GABA(A)R) mainly mediate fast inhibitory neurotransmission in the central nervous system. Different classes of modulators target GABA(A)R properties. Penicillin G (PNG) belongs to the class of noncompetitive antagonists blocking the open GABA(A)R and is a prototype of β-lactam antibiotics. In this study, we combined electrophysiological and modeling approaches to investigate the peculiarities of PNG blockade of GABA-activated currents recorded from isolated rat Purkinje cells and to predict the PNG binding site. Whole-cell patch-сlamp recording and fast application system was used in the electrophysiological experiments. PNG block developed after channel activation and increased with membrane depolarization suggesting that the ligand binds within the open channel pore. PNG blocked stationary component of GABA-activated currents in a concentration-dependent manner with IC50 value of 1.12mM at -70mV. The termination of GABA and PNG co-application was followed by a transient tail current. Protection of the tail current from bicuculline block and dependence of its kinetic parameters on agonist affinity suggest that PNG acts as a sequential open channel blocker that prevents agonist dissociation while the channel remains blocked. We built the GABA(A)R models based on nAChR and GLIC structures and performed an unbiased systematic search of the PNG binding site. Monte-Carlo energy minimization was used to find the lowest energy binding modes. We have shown that PNG binds close to the intracellular vestibule. In both models the maximum contribution to the energy of ligand-receptor interactions revealed residues located on the level of 2', 6' and 9' rings formed by a bundle of M2 transmembrane segments, indicating that these residues most likely participate in PNG binding. The predicted structural models support the described mechanism of PNG block.

Monoamine NMDA receptor channel blockers inhibit and potentiate native and recombinant proton-gated ion channels

Acid-sensing ion channels (ASICs) are widely distributed in the peripheral and central nervous system. Although they are involved in many physiological functions, the actual processes that activate ASICs remain unclear. This is particularly true for brain ASICs, which produce only a transient response to a fast drop in pH and cannot mediate sustained current. Therefore, the search for ASIC inhibitors and, especially, potentiators/activators is important. We report that NMDA receptor channel blockers with a comparatively simple structure (9-aminoacridine, memantine, IEM-2117 and IEM-1921) potentiate and/or inhibit ASICs in submillimolar concentrations. The experiments were performed using the patch clamp technique on native ASICs from rat hippocampal interneurons and recombinant ASICs of different subunit compositions expressed in CHO cells. Native ASICs were potentiated by IEM-1921 and IEM-2117, and inhibited by memantine and 9-aminoacridine. Homomeric ASIC1a were inhibited by memantine, IEM-2117 and 9-aminoacridine while IEM-1921 was ineffective. In contrast, homomeric ASIC2a were potentiated by IEM-2117, memantine and IEM-1921, whereas 9-aminoacridine was inactive. The compounds caused a complex effect on ASIC3. 9-aminoacridine and IEM-1921 potentiated the steady-state response of ASIC3 and inhibited the peak component. IEM-2117 not only potentiated ASIC3-mediated currents caused by acidification but also evoked steady-state currents at neutral pH. Our results demonstrate that, depending on the subunit composition, ASICs can be activated or inhibited by simple compounds that possess only amino group and aromatic/hydrophobic moieties. This opens up the possibility to search for new ASIC modulators among a number of endogenous ligands.

Celecoxib and ion channels: a story of unexpected discoveries

Celecoxib (Celebrex), a highly popular selective inhibitor of cyclooxygenase-2, can modulate ion channels and alter functioning of neurons and myocytes at clinically relevant concentrations independently of cyclooxygenase inhibition. In experimental systems varying from Drosophila to primary mammalian and human cell lines, celecoxib inhibits many voltage-activated Na(+), Ca(2+), and K(+) channels, including NaV1.5, L- and T-type Ca(2+) channels, KV1.5, KV2.1, KV4.3, KV7.1, KV11.1 (hERG), while stimulating other K(+) channels-KV7.2-5 and, possibly, KV11.1 (hERG) channels under certain conditions. In this review, we summarize the information currently available on the effects of celecoxib on ion channels, examine mechanistic aspects of drug action and the concomitant changes at the cellular and organ levels, and discuss these findings in the therapeutic context.

The neuroactive steroid pregnenolone sulfate stimulates trafficking of functional N-methyl D-aspartate receptors to the cell surface via a noncanonical, G protein, and Ca2+-dependent mechanism

N-methyl D-aspartate (NMDA) receptors (NMDARs) mediate fast excitatory synaptic transmission and play a critical role in synaptic plasticity associated with learning and memory. NMDAR hypoactivity has been implicated in the pathophysiology of schizophrenia, and clinical studies have revealed reduced negative symptoms of schizophrenia with a dose of pregnenolone that elevates serum levels of the neuroactive steroid pregnenolone sulfate (PregS). This report describes a novel process of delayed-onset potentiation whereby PregS approximately doubles the cell's response to NMDA via a mechanism that is pharmacologically and kinetically distinct from rapid positive allosteric modulation by PregS. The number of functional cell-surface NMDARs in cortical neurons increases 60-100% within 10 minutes of exposure to PregS, as shown by surface biotinylation and affinity purification. Delayed-onset potentiation is reversible and selective for expressed receptors containing the NMDAR subunit subtype 2A (NR2A) or NR2B, but not the NR2C or NR2D, subunits. Moreover, substitution of NR2B J/K helices and M4 domain with the corresponding region of NR2D ablates rapid allosteric potentiation of the NMDA response by PregS but not delayed-onset potentiation. This demonstrates that the initial phase of rapid positive allosteric modulation is not a first step in NMDAR upregulation. Delayed-onset potentiation by PregS occurs via a noncanonical, pertussis toxin-sensitive, G protein-coupled, and Ca(2+)-dependent mechanism that is independent of NMDAR ion channel activation. Further investigation into the sequelae for PregS-stimulated trafficking of NMDARs to the neuronal cell surface may uncover a new target for the pharmacological treatment of disorders in which NMDAR hypofunction has been implicated.

Mechanistic and structural determinants of NMDA receptor voltage-dependent gating and slow Mg2+ unblock

NMDA receptor (NMDAR)-mediated currents depend on membrane depolarization to relieve powerful voltage-dependent NMDAR channel block by external magnesium (Mg(o)(2+)). Mg(o)(2+) unblock from native NMDARs exhibits a fast component that is consistent with rapid Mg(o)(2+) -unbinding kinetics and also a slower, millisecond time scale component (slow Mg(o)(2+) unblock). In recombinant NMDARs, slow Mg(o)(2+) unblock is prominent in GluN1/2A (an NMDAR subtype composed of GluN1 and GluN2A subunits) and GluN1/2B receptors, with slower kinetics observed for GluN1/2B receptors, but absent from GluN1/2C and GluN1/2D receptors. Slow Mg(o)(2+) unblock from GluN1/2B receptors results from inherent voltage-dependent gating, which increases channel open probability with depolarization. Here we examine the mechanisms responsible for NMDAR subtype dependence of slow Mg(o)(2+) unblock. We demonstrate that slow Mg(o)(2+) unblock from GluN1/2A receptors, like GluN1/2B receptors, results from inherent voltage-dependent gating. Surprisingly, GluN1/2A and GluN1/2B receptors exhibited equal inherent voltage dependence; faster Mg(o)(2+) unblock from GluN1/2A receptors can be explained by voltage-independent differences in gating kinetics. To investigate the absence of slow Mg(o)(2+) unblock in GluN1/2C and GluN1/2D receptors, we examined the GluN2 S/L site, a site responsible for several NMDAR subtype-dependent channel properties. Mutating the GluN2 S/L site of GluN2A subunits from serine (found in GluN2A and GluN2B subunits) to leucine (found in GluN2C and GluN2D) greatly diminished both voltage-dependent gating and slow Mg(o)(2+) unblock. Therefore, the residue at the GluN2 S/L site governs the expression of both slow Mg(o)(2+) unblock and inherent voltage dependence.

A fast model of voltage-dependent NMDA receptors

NMDA receptors are among the crucial elements of central nervous system models. Recent studies show that both conductance and kinetics of these receptors are changing voltage-dependently in some parts of the brain. Therefore, several models have been introduced to simulate their current. However, on the one hand, kinetic models-which are able to simulate these voltage-dependent phenomena-are computationally expensive for modeling of large neural networks. On the other hand, classic exponential models, which are computationally less expensive, are not able to simulate the voltage-dependency of these receptors, accurately. In this study, we have modified these classic models to endow them with the voltage-dependent conductance and time constants. Temperature sensitivity and desensitization of these receptors are also taken into account. We show that, it is possible to simulate the most important physiological aspects of NMDA receptor's behavior using only three to four differential equations, which is significantly smaller than the previous kinetic models. Consequently, it seems that our model is both fast and physiologically plausible and therefore is a suitable candidate for the modeling of large neural networks.

Influence of external magnesium ions on the NMDA receptor channel block by different types of organic cations

The NMDA type of ionotropic glutamate receptors plays a unique role in synaptic functions because of high permeability for calcium and because of a voltage-dependent block by endogenous Mg(2+). Activity and voltage dependence of the NMDA receptor channel block by organic cations are strongly affected by competition with magnesium ions for the binding site in the channel pore. It complicates prediction of action of NMDA receptor channel blockers in vivo. In the present work we studied the NMDA receptor channel block in the presence of Mg(2+) by several organic blockers with different characteristics of voltage dependence and mechanism of action. The action of NMDA receptor channel antagonists was studied in native NMDA receptors of hippocampus CA1 pyramidal neurons isolated from rat brain slices. It was demonstrated that the IC(50) values of NMDA receptor channel blockers at -30 mV are increased 1.5-5 times compared with magnesium-free conditions. The voltage dependence of the channel block is decreased, abolished or even inversed in the presence of magnesium. Although simple competition between magnesium ion and organic channel blockers provides a general explanation of the observed effects, certain disagreements were revealed. Diversity in Mg(2+) effects on the NMDAR channel block by different organic cations reported herein likely reflects interaction of NMDAR channel blockers with additional binding site(s) and suggests that individual analysis in the presence of Mg(2+) is required for newly developed NMDAR channel blocking drugs.

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.

Memantine binding to a superficial site on NMDA receptors contributes to partial trapping

Although many nervous system disorders are associated with N-methyl-D-aspartate (NMDA) receptor overactivation, pharmacological inhibition of NMDA receptors has typically demonstrated limited clinical value due to debilitating psychotomimetic side-effects. Memantine, however, induces far fewer behavioural side-effects than other NMDA receptor channel blockers such as ketamine, and slows the progressive cognitive decline associated with Alzheimer's disease. Memantine and ketamine inhibit NMDA receptors with similar affinity and kinetics. A prominent mechanistic difference between memantine and ketamine is the degree to which they are 'trapped' within the closed channel of NMDA receptors following removal of agonist: ketamine becomes trapped in nearly all NMDA receptors to which it was bound before agonist removal, whereas some bound memantine molecules dissociate after agonist removal, a phenomenon called partial trapping. Here we investigated the mechanism underlying partial trapping of memantine by recombinant NR1/2A NMDA receptors. We found that memantine dissociation from NR1/2A receptors after agonist removal (the process that results in partial trapping) followed an exponential time course with tau = 0.79 +/- 0.32 s. Neither membrane voltage depolarization nor maintained presence of memantine after agonist removal affected partial trapping, suggesting that partial trapping does not result from memantine escape through open channels. We tested the hypothesis that partial trapping results from binding of memantine to two sites, a superficial 'non-trapping' site and a deep 'trapping' site, which cannot be occupied simultaneously. This hypothesis was supported by the lack of ketamine binding to the superficial site, the voltage dependence of partial trapping, and the effect on partial trapping of a mutation near the deep site. The superficial binding site for memantine may, by causing partial trapping, contribute to memantine's unique therapeutic utility.

Mechanism of bis(7)-tacrine inhibition of GABA-activated current in cultured rat hippocampal neurons

Bis(7)-tacrine is a novel dimeric acetylcholinesterase inhibitor derived from tacrine that shows promise for the treatment of Alzheimer's disease. We have previously reported that bis(7)-tacrine inhibits GABA(A) receptors. In the present study we investigated the mechanism of bis(7)-tacrine inhibition of GABA(A) receptor function using whole-cell patch-clamp recording in cultured rat hippocampal neurons. Bis(7)-tacrine produced a gradual decline of GABA-activated current to a steady-state, but this was not an indication of use-dependence, as the gradually declining component could be eliminated by exposure to bis(7)-tacrine prior to GABA application. In addition, bis(7)-tacrine inhibition did not require the presence of agonist, and GABA-activated current recovered completely from inhibition by bis(7)-tacrine in the absence of agonist. The slow onset of inhibition by bis(7)-tacrine was not apparently due to an action at an intracellular site, as inclusion of 25 microM bis(7)-tacrine in the recording pipette did not alter inhibition by bis(7)-tacrine applied externally. Bis(7)-tacrine shifted the GABA concentration-response curve to the right in a parallel manner and the pA(2) value estimated from a Schild plot was 5.7. Bis(7)-tacrine increased the time constant of activation of GABA-gated ion channels without affecting the time constants of deactivation or desensitization. These results suggest that bis(7)-tacrine is a competitive GABA(A) receptor antagonist with slow onset and offset kinetics. The competitive inhibition of GABA receptors by bis(7)-tacrine could contribute to its ability to enhance memory.

Blockade of NMDA receptor channels by 9-aminoacridine and its derivatives

9-Aminoacridine is known as "foot-in-the-door" NMDA receptor channel blocker because its binding prevents channel closure. Structural determinants of this mechanism of block were studied using a series of 9-aminoacridine derivatives. Experiments were performed on native NMDA receptors of hippocampal pyramidal neurons, isolated from rat brain slices. The use-dependence of block and kinetics of recovery from block were used to characterize mechanism of block produced by the compounds. Modifications, which preserve the flat structure of the tricyclic 9-aminoacridine moiety, affect blocking activity and kinetics but not the foot-in-the-door mechanism. On the contrary, disruption of the flat structure changes the mechanism of block to trapping. It is concluded that flat aromatic structure is one of the critical determinants of the action mechanism of 9-aminoacridine.

Voltage-dependent gating of NR1/2B NMDA receptors

Ligand-gated ion channels are activated by agonist binding, but may also be modulated by membrane voltage. N-Methyl-d-aspartate receptors (NMDARs) exhibit especially strong voltage dependence due to channel block by external Mg(2+) (Mg(o)(2+)). Here we demonstrate that activity of NMDARs composed of NR1 and NR2B subunits (NR1/2B receptors) is enhanced by depolarization even in 0 Mg(o)(2+), causing slow current relaxations in response to rapid voltage changes. We present a kinetic model of receptor activation that incorporates voltage-dependent gating-associated NR2B subunit conformational changes. The model accurately reproduces current relaxations during depolarizations and subsequent repolarizations in 0 Mg(o)(2+). Model simulations in physiological Mg(o)(2+) concentrations show that voltage-dependent receptor gating also underlies the slow component of Mg(o)(2+) unblock, a phenomenon that previously was shown to influence Mg(o)(2+) unblock kinetics during dendritic spikes. We propose that voltage-dependent gating of NR1/2B receptors confers enhanced voltage and time dependence on NMDAR-mediated signalling.

The NR1 M3 domain mediates allosteric coupling in the N-methyl-D-aspartate receptor

N-Methyl-D-aspartate (NMDA) receptors play a critical role in both development of the central nervous system and adult neuroplasticity. However, although the NMDA receptor presents a valuable therapeutic target, the relationship between its structure and functional properties has yet to be fully elucidated. To further explore the mechanism of receptor activation, we characterized two gain-of-function mutations within the NR1 M3 segment, a transmembrane domain proposed to couple ligand binding and channel opening. Both mutants (A7Q and A7Y) displayed significant glycine-independent currents, indicating that their M3 domains may preferentially adopt a more activated conformation. Substituted cysteine modification experiments revealed that the glycine binding clefts of both A7Q and A7Y are inaccessible to modifying reagents and resistant to competitive antagonism. These data suggest that perturbation of M3 can stabilize the ligand binding domain in a closed cleft conformation, even in the absence of agonist. Both mutants also displayed significant glutamate-independent current and insensitivity to glutamate-site antagonism, indicating partial activation by either glycine or glutamate alone. Furthermore, A7Q and A7Y increased accessibility of the NR2 M3 domain, providing evidence for intersubunit coupling at the transmembrane level and suggesting that these NR1 mutations dominate the properties of the intact heteromeric receptor. The equivalent mutations in NR2 did not exhibit comparable phenotypes, indicating that the NR1 and NR2 M3 domains may play different functional roles. In summary, our data demonstrate that the NR1 M3 segment is functionally coupled to key structural domains in both the NR1 and NR2 subunits.

Ion channels of glutamate receptors: structural modeling

Ionotropic glutamate receptors belong to the superfamily of P-loop channels as well as K(+), Na(+), and Ca(2+) channels. However, the structural similarity between ion channels of the glutamate receptors and K(+) channels is a matter of discussion. The aim of this study was to analyze differences between the structures of K(+) channels and glutamate receptor channels. For this purpose, homology models of NMDA and AMPA receptor channels (M2 and M3 segments) were built using X-ray structures of K(+) channels as templates. The models were optimized and used to reproduce specific data on the structure of glutamate receptor channels. Particular attention was paid to the data of the binding of channel blockers and to the results of scanning mutagenesis. The modeling demonstrates that properties of glutamate receptor channel can be reproduced assuming only local structural deformations of the K(+) channel templates. The most valuable differences were found in the selectivity-filter region, whereas helical parts of M2 and M3 segments could have similar spatial organization with homologous segments in K(+) channels. It is concluded that the current experimental data on glutamate receptor channels does not reveal global structural differences with K(+) channels.

NMDA receptor NR2 subunit dependence of the slow component of magnesium unblock

NMDA receptor activity is important for many physiological functions, including synapse formation and alterations in synaptic strength. NMDA receptors are composed most commonly of NR1 and NR2 subunits. There are four NR2 subunits (NR2A-NR2D). NR2 subunit expression varies across both brain regions and developmental stages. The identity of the NR2 subunit within a functional NMDA receptor helps to determine many pharmacological and biophysical receptor properties, including strength of block by external Mg2+ (Mg(o)2+). Mg(o)2+ block confers strong voltage dependence to NMDA receptor-mediated responses and is critically important for many of the functions that the NMDA receptor plays within the CNS. Here we describe the NR2 subunit dependence of the kinetics of Mg(o)2+ unblock after rapid depolarizations. We find that Mg(o)2+ unblocks from NR1/2A and NR1/2B receptors with a prominent slow component similar to that previously described in native hippocampal and cortical NMDA receptors. Strikingly, this slow component of Mg(o)2+ unblock is completely absent from NR1/2C and NR1/2D receptors. Thus currents from NR1/2C and NR1/2D receptors respond more rapidly to fast depolarizations than currents from NR1/2A and NR1/2B receptors. In addition, the slow component of Mg(o)2+ unblock from NR1/2B receptors is consistently slower than from NR1/2A receptors. This makes rapid depolarizations, such as action potential waveforms, more efficacious at stimulating Mg(o)2+ unblock from NR1/2A than from NR1/2B receptors. These NR2 subunit differences in the kinetics of Mg(o)2+ unblock are likely to help determine the contribution of each NMDA receptor subtype to current flow during synaptic activity.

Concanavalin-A reports agonist-induced conformational changes in the intact GluR6 kainate receptor

The agonist-binding domain of ionotropic glutamate receptors (iGluRs) has recently been crystallized as two polypeptide chains with a linker region. Although work on the structure of this isolated ligand-binding core has been invaluable, there is debate over how it relates to conformations adopted by intact receptors. iGluR crystals are proposed to represent the activated state as their degree of domain closure correlates well with agonist efficacy. However, iGluR crystals exhibit high agonist affinity that more closely matches that of desensitized receptors. Consequently, conformations adopted by iGluR crystals may represent this state. To test this, we have employed the plant lectin, concanavalin-A (Con-A) to report conformational changes elicited by kainate (KA) iGluR agonists during desensitization. When GluR6 KA receptors (KARs) were pre-incubated with Con-A, equilibrium responses evoked by the full agonist, l-glutamate (l-Glu), increased almost 30-fold. However, in the continued presence of l-Glu, Con-A exerted no effect suggesting that it has restricted access to its binding sites when the agonist is bound. However, Con-A does not discriminate well between agonist-bound or -unbound states with the weak partial agonist, domoate. Accessibility experiments with KA were intermediate in nature consistent with its equilibrium efficacy at GluR6 KARs. Our results suggest that full and partial agonists elicit distinct conformational changes in KARs during desensitization. This finding can be reconciled with crystallographic data if the agonist-binding domain adopts the same conformation in the activated and desensitized states. However, other interpretations are possible suggesting future work is required if this issue is to be resolved.

Amantadine inhibits NMDA receptors by accelerating channel closure during channel block

The channel of NMDA receptors is blocked by a wide variety of drugs. NMDA receptor channel blockers include drugs of abuse that induce psychotic behavior, such as phencyclidine, and drugs with wide therapeutic utility, such as amantadine and memantine. We describe here the molecular mechanism of amantadine inhibition. In contrast to most other described channel-blocking molecules, amantadine causes the channel gate of NMDA receptors to close more quickly. Our results confirm that amantadine binding inhibits current flow through NMDA receptor channels but show that its main inhibitory action at pharmaceutically relevant concentrations results from stabilization of closed states of the channel. The surprising variation in the clinical utility of NMDA channel blockers may in part derive from their diverse effects on channel gating.

Subtype-dependence of N-methyl-d-aspartate receptor modulation by pregnenolone sulfate

N-methyl-D-aspartate receptors play a critical role in synaptogenesis, synaptic plasticity, and excitotoxicity. They are heteromeric complexes of NR1 combined with NR2A-D and/or NR3A-B subunits. The subunit composition determines the biophysical and pharmacological properties of the N-methyl-D-aspartate receptor channel complex. In this study, we report that responses mediated by recombinant rat N-methyl-D-aspartate receptors expressed in human embryonic kidney HEK293 cells are differentially affected by naturally occurring neurosteroid pregnenolone sulfate. We show that responses induced by 1mM glutamate in NR1-1a/NR2A and NR1-1a/NR2B receptors are potentiated five- to eight-fold more by pregnenolone sulfate than responses of NR1-1a/NR2C and NR1-1a/NR2D receptors with no differences in the concentration of pregnenolone sulfate that produced 50% potentiation. In addition to potentiation, pregnenolone sulfate also has an inhibitory effect at recombinant N-methyl-D-aspartate receptors, with values of the concentration of pregnenolone sulfate that produces 50% inhibition of NR1/NR2D=NR1/NR2C<NR1/NR2B<NR1/NR2A. In addition, we show that the structure of the extracellular loop between the third and fourth transmembrane domains of the NR2 subunit is critical for both the potentiating and inhibitory effects of pregnenolone sulfate. The modulatory effects of pregnenolone sulfate are consistent with a model in which this neurosteroid acts at two distinct binding sites on the N-methyl-D-aspartate receptor. These data provide insight into the mechanisms by which pregnenolone sulfate and related sulfated neurosteroids modulate activity of N-methyl-D-aspartate receptor channels.

ProbingN-Methyl-d-aspartate Receptor Desensitization with the Substituted-Cysteine Accessibility Method

Several forms of macroscopic N-methyl-D-aspartate (NMDA) receptor desensitization affect the amplitude and duration of postsynaptic responses. In addition to its functional significance, desensitization provides one means to examine the conformational coupling of ligand binding to channel gating. Segments flanking the ligand binding domain in the extracellular N terminus of the NMDA receptor NR2 subunit influence the glycine-independent form of desensitization. The NR2A pre-M1 region, the linker between the glutamate binding domain and the channel pore, plays a critical role in desensitization. Thus, we used the substituted-cysteine accessibility method to scan the accessibility of residues in the pre-M1 region and the first transmembrane domain (M1) of NR2A. Cysteine mutants were expressed with NR1 in human embryonic kidney 293 cells and were assayed by whole-cell recording. With activation of the receptor by glutamate and glycine, only a single mutant, V557C, which is located at the beginning of M1, led to irreversible inhibition by the methanethiosulfonate derivative methanethiosulfonate ethyltrimethylammonium (MTSET). The NR2 ligand glutamate was insufficient on its own to induce modification of V557C by MTSET, suggesting that the change in accessibility required channel gating. The rate of MTSET modification of the homologous residue on NR1 (NR1-1a(L562C)/NR2A) was much slower than V557C. We also substituted cysteine in the V557 site of mutant subunits that exhibit either enhanced or reduced desensitization. Modification by MTSET correlated with the degree of desensitization for these subunits, suggesting that V557C is a sensitive detector of desensitization gating.

20-oxo-5beta-pregnan-3alpha-yl sulfate is a use-dependent NMDA receptor inhibitor

NMDA receptors are ligand-gated ion channels permeable to calcium and play a critical role in excitatory synaptic transmission, synaptic plasticity, and excitotoxicity. They are heteromeric complexes of NR1 combined with NR2A-D and/or NR3A-B subunits that are activated by glutamate and glycine and whose activity is modulated by allosteric modulators. In this study, patch-clamp recordings from human embryonic kidney 293 cells expressing NR1/NR2 receptors were used to study the molecular mechanism of the endogenous neurosteroid 20-oxo-5beta-pregnan-3alpha-yl sulfate (3alpha5betaS) action at NMDA receptors. 3alpha5betaS was a twofold more potent inhibitor of responses mediated by NR1/NR2C-D receptors than those mediated by NR1/NR2A-B receptors. The structure of the extracellular loop between the third and fourth transmembrane domains of the NR2 subunit was found to be critical for the neurosteroid inhibitory effect. The degree of 3alpha5betaS-induced inhibition of responses to glutamate was voltage independent, with recovery lasting several seconds. In contrast, application of 3alpha5betaS in the absence of agonist had no effect on the subsequent response to glutamate made in the absence of the neurosteroid. A kinetic model was developed to explain the use-dependent action of 3alpha5betaS at NMDA receptors. In accordance with the model, 3alpha5betaS was a less potent inhibitor of NMDA receptor-mediated EPSCs and responses induced by a short application of 1 mm glutamate than of those induced by a long application of glutamate. These results suggest that 3alpha5betaS is a use-dependent but voltage-independent inhibitor of NMDA receptors, with more potent action at tonically than at phasically activated receptors. This may be important in the treatment of excitotoxicity-induced neurodegeneration.

Polyamines Modulate AMPA Receptor–Dependent Synaptic Responses in Immature Layer V Pyramidal Neurons

α-Amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs) mediate the majority of fast excitation in the CNS. Receptors lacking GluR2 exhibit inward rectification and paired-pulse facilitation (PPF) due to polyamine (PA)-dependent block and unblock, respectively. In this study, we tested whether rectification and PPF in immature, but not mature, pyramidal neurons depend not only on the absence of functional GluR2 but also on the level of endogenous PAs. Whole cell recordings were obtained from layer V pyramidal neurons of P12–P14 or P16–P20 rats in the presence or absence of spermine in the pipette (50 μM). Isolated minimal excitatory synaptic responses were obtained, and paired (20 Hz) stimuli were used to investigate the rectification index (RI) and paired-pulse ratio (PPR). Spermine and its synthetic enzyme, ornithine decarboxylase (ODC), expression was examined using immunostaining and Western blot, respectively. At the immature stage (<P15) inclusion of intracellular spermine increased rectification and PPF for evoked excitatory postsynaptic currents (EPSCs) but had little or no effect on either measure in more mature (P16–P20) pyramidal neurons. Depletion of PAs reduced rectification suggesting that endogenous PAs play a critical role in functional regulation of AMPARs. Spermine immunoreactivity and ODC expression in immature rat neocortex (<P15) were greater than more mature tissue by ∼20 and 60%, respectively. These results provide further support for the idea that excitatory synapses on immature neocortical pyramidal neurons ubiquitously contain AMPA receptors lacking the GluR2 subunit and that the level of endogenous PAs plays an important role in modulating AMPAR-dependent neurotransmission.

Fast and slow voltage-dependent dynamics of magnesium block in the NMDA receptor: the asymmetric trapping block model

The NMDA receptor (NMDAR) produces a long-lasting component of the glutamatergic EPSC in mammalian central neurons. The current through NMDARs is voltage dependent as a result of block by extracellular magnesium, which has recently been shown to give rise to a complex time dependence, with fast and slow components of responses to changes in membrane potential. Here, we studied the dynamics of block and unblock by measuring voltage step responses in conjunction with fast perfusion of agonist in nucleated patches isolated from rat cortical pyramidal neurons. We found that slow unblock shows a progressive onset during synaptic-like responses to brief pulses of agonist. Repolarizing briefly from +40 to -70 mV revealed that slow unblock is reestablished with a time constant of approximately 5 msec at room temperature. Also, the time course of deactivation, in response to a pulse of agonist, slows twofold over the potential range -30 to +40 mV. An asymmetric "trapping block" model in which the voltage-independent closing rate constant of the blocked channel is approximately three times that of the unblocked channel accounts quantitatively for all of these phenomena and for responses to action potential waveform clamp. This model allows much more accurate prediction of NMDAR current in physiological conditions of magnesium concentration and changing membrane potential than previously possible. It suggests a positive allosteric link between occupation of the NMDAR pore by magnesium and closure of the permeation gate.

Potassium, sodium, calcium and glutamate-gated channels: pore architecture and ligand action

In the last decade, the idea of common organization of certain ion channel families exhibiting diverse physiological and pharmacological properties has received strong experimental support. Transmembrane topologies and patterns of the pore-facing residues are conserved in P-loop channels that include high-selective cation channels and certain ligand-gated channels. X-ray structures of bacterial K+ channels, KcsA, MthK and KvAP, help to understand structure-function relationships of other P-loop channels. Data on binding sites and mechanisms of action of ligands of K+, Na+, Ca2+ and glutamate gated ion channels are considered in view of their possible structural similarity to the bacterial K+ channels. Emphasized are structural determinants of ligand-receptor interactions within the channels and mechanisms of state-dependent action of the ligands.

The neuropharmacological basis for the use of memantine in the treatment of Alzheimer's disease

Memantine has been demonstrated to be safe and effective in the symptomatic treatment of Alzheimer's disease (AD). While the neurobiological basis for the therapeutic activity of memantine is not fully understood, the drug is not a cholinesterase inhibitor and, therefore, acts differently from current AD therapies. Memantine can interact with a variety of ligand-gated ion channels. However, NMDA receptors appear to be a key target of memantine at therapeutic concentrations. Memantine is an uncompetitive (channel blocking) NMDA receptor antagonist. Like other NMDA receptor antagonists, memantine at high concentrations can inhibit mechanisms of synaptic plasticity that are believed to underlie learning and memory. However, at lower, clinically relevant concentrations memantine can under some circumstances promote synaptic plasticity and preserve or enhance memory in animal models of AD. In addition, memantine can protect against the excitotoxic destruction of cholinergic neurons. Blockade of NMDA receptors by memantine could theoretically confer disease-modifying activity in AD by inhibiting the "weak" NMDA receptor-dependent excitotoxicity that has been hypothesized to play a role in the progressive neuronal loss that underlies the evolving dementia. Moreover, recent in vitro studies suggest that memantine abrogates beta-amyloid (Abeta) toxicity and possibly inhibits Abeta production. Considerable attention has focused on the investigation of theories to explain the better tolerability of memantine over other NMDA receptor antagonists, particularly those that act by a similar channel blocking mechanism such as dissociative anesthetic-like agents (phencyclidine, ketamine, MK-801). A variety of channel-level factors could be relevant, including fast channel-blocking kinetics and strong voltage-dependence (allowing rapid relief of block during synaptic activity), as well as reduced trapping (permitting egress from closed channels). These factors may allow memantine to block channel activity induced by low, tonic levels of glutamate--an action that might contribute to symptomatic improvement and could theoretically protect against weak excitotoxicity--while sparing synaptic responses required for normal behavioral functioning, cognition and memory.