Effects of a centrally active benzoylpyrrolidine drug on AMPA receptor kinetics

Pharmacological characterization of N-[(2S)-5-(6-fluoro-3-pyridinyl)-2, 3-dihydro-1H-inden-2-yl]-2-propanesulfonamide: a novel, clinical AMPA receptor positive allosteric modulator

BACKGROUND AND PURPOSE

AMPA receptor positive allosteric modulators represent a potential therapeutic strategy to improve cognition in people with schizophrenia. These studies collectively constitute the preclinical pharmacology data package used to build confidence in the pharmacology of this molecule and enable a clinical trial application.

EXPERIMENTAL APPROACH

[N-[(2S)-5-(6-fluoro-3-pyridinyl)-2,3-dihydro 1H-inden-2-yl]-2-propanesulfonamide] (UoS12258) was profiled in a number of in vitro and in vivo studies to highlight its suitability as a novel therapeutic agent.

KEY RESULTS

We demonstrated that UoS12258 is a selective, positive allosteric modulator of the AMPA receptor. At rat native hetero-oligomeric AMPA receptors, UoS12258 displayed a minimum effective concentration of approximately 10 nM in vitro and enhanced AMPA receptor-mediated synaptic transmission at an estimated free brain concentration of approximately 15 nM in vivo. UoS12258 reversed a delay-induced deficit in novel object recognition in rats after both acute and sub-chronic dosing. Sub-chronic dosing reduced the minimum effective dose from 0.3 to 0.03 mg·kg^-1 . UoS12258 was also effective at improving performance in two other cognition models, passive avoidance in scopolamine-impaired rats and water maze learning and retention in aged rats. In side-effect profiling studies, UoS12258 did not produce significant changes in the maximal electroshock threshold test at doses below 10 mg·kg^-1 .

CONCLUSION AND IMPLICATIONS

We conclude that UoS12258 is a potent and selective AMPA receptor modulator exhibiting cognition enhancing properties in several rat behavioural models superior to other molecules that have previously entered clinical evaluation.

Copper-catalyzed tandem aryl–halogen hydroxylation and CH2Cl2-based N,O-acetalization toward the synthesis of 2,3-dihydrobenzoxazinones

Copper-catalyzed tandem reactions consisting of Ar–halogen hydroxylation andN,O-acetalization are designed for the concise synthesis of 2,3-dihydrobenzoxazinones.

AMPA receptor-positive allosteric modulators for the treatment of schizophrenia: an overview of recent patent applications

The role of glutamate and its receptors in central nervous system biology and disease has long been of interest to scientists involved in both fundamental research and drug discovery, however the complex pharmacology and lack of highly selective compounds has severely hampered drug discovery efforts in this area. Recent advances in the identification and profiling of positive allosteric modulators of the AMPA receptor offer a potential way forward and the hope of a new treatment for schizophrenia. This article will review recent patent applications published in this area.

Pharmacological enhancement of memory or cognition in normal subjects

The possibility of expanding memory or cognitive capabilities above the levels in high functioning individuals is a topic of intense discussion among scientists and in society at large. The majority of animal studies use behavioral endpoint measures; this has produced valuable information but limited predictability for human outcomes. Accordingly, several groups are pursuing a complementary strategy with treatments targeting synaptic events associated with memory encoding or forebrain network operations. Transcription and translation figure prominently in substrate work directed at enhancement. Notably, the question of why new proteins would be needed for a now-forming memory given that learning-driven synthesis presumably occurred throughout the immediate past has been largely ignored. Despite this conceptual problem, and some controversy, recent studies have reinvigorated the idea that selective gene manipulation is a plausible route to enhancement. Efforts to improve memory by facilitating synaptic encoding of information have also progressed, in part due of breakthroughs on mechanisms that stabilize learning-related, long-term potentiation (LTP). These advances point to a reductionistic hypothesis for a diversity of experimental results on enhancement, and identify under-explored possibilities. Cognitive enhancement remains an elusive goal, in part due to the difficulty of defining the target. The popular view of cognition as a collection of definable computations seems to miss the fluid, integrative process experienced by high functioning individuals. The neurobiological approach obviates these psychological issues to directly test the consequences of improving throughput in networks underlying higher order behaviors. The few relevant studies testing drugs that selectively promote excitatory transmission indicate that it is possible to expand cortical networks engaged by complex tasks and that this is accompanied by capabilities not found in normal animals.

Functional analysis of a novel positive allosteric modulator of AMPA receptors derived from a structure-based drug design strategy

Positive allosteric modulators of α-amino-3-hydroxy-5-methyl-isoxazole-propionic acid (AMPA) receptors facilitate synaptic plasticity and can improve various forms of learning and memory. These modulators show promise as therapeutic agents for the treatment of neurological disorders such as schizophrenia, ADHD, and mental depression. Three classes of positive modulator, the benzamides, the thiadiazides, and the biarylsulfonamides differentially occupy a solvent accessible binding pocket at the interface between the two subunits that form the AMPA receptor ligand-binding pocket. Here, we describe the electrophysiological properties of a new chemotype derived from a structure-based drug design strategy (SBDD), which makes similar receptor interactions compared to previously reported classes of modulator. This pyrazole amide derivative, JAMI1001A, with a promising developability profile, efficaciously modulates AMPA receptor deactivation and desensitization of both flip and flop receptor isoforms. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

The likelihood of cognitive enhancement

Whether drugs that enhance cognition in healthy individuals will appear in the near future has become a topic of considerable interest. We address this possibility using a three variable system (psychological effect, neurobiological mechanism, and efficiency vs. capabilities) for classifying candidates. Ritalin and modafinil, two currently available compounds, operate on primary psychological states that in turn affect cognitive operations (attention and memory), but there is little evidence that these effects translate into improvements in complex cognitive processing. A second category of potential enhancers includes agents that improve memory encoding, generally without large changes in primary psychological states. Unfortunately, there is little information on how these compounds affect cognitive performance in standard psychological tests. Recent experiments have identified a number of sites at which memory drugs could, in principle, manipulate the cell biological systems underlying the learning-related long-term potentiation (LTP) effect; this may explain the remarkable diversity of memory promoting compounds. Indeed, many of these agents are known to have positive effects on LTP. A possible third category of enhancement drugs directed specifically at integrated cognitive operations is nearly empty. From a neurobiological perspective, two plausible candidate classes have emerged that both target the fast excitatory transmission responsible for communication within cortical networks. One acts on nicotinic receptors (alpha7 and alpha4) that regulate release of the neurotransmitter glutamate while the other ('ampakines') allosterically modulates the glutamate receptors mediating the post-synaptic response (EPSCs). Brain imaging in primates has shown that ampakines expand cortical networks engaged by a complex task; coupled with behavioral data, these findings provide evidence for the possibility of generating new cognitive capabilities. Finally, we suggest that continuing advances in behavioral sciences provide new opportunities for translational work, and that discussions of the social impact of cognitive enhancers have failed to consider the distinction between effects on efficiency vs. new capabilities.

Structural and functional analysis of two new positive allosteric modulators of GluA2 desensitization and deactivation

At the dimer interface of the extracellular ligand-binding domain of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors a hydrophilic pocket is formed that is known to interact with two classes of positive allosteric modulators, represented by cyclothiazide and the ampakine 2H,3H,6aH-pyrrolidino(2,1-3',2')1,3-oxazino(6',5'-5,4)benzo(e)1,4-dioxan-10-one (CX614). Here, we present structural and functional data on two new positive allosteric modulators of AMPA receptors, phenyl-1,4-bis-alkylsulfonamide (CMPDA) and phenyl-1,4-bis-carboxythiophene (CMPDB). Crystallographic data show that these compounds bind within the modulator-binding pocket and that substituents of each compound overlap with distinct moieties of cyclothiazide and CX614. The goals of the present study were to determine 1) the degree of modulation by CMPDA and CMPDB of AMPA receptor deactivation and desensitization; 2) whether these compounds are splice isoform-selective; and 3) whether predictions of mechanism of action could be inferred by comparing molecular interactions between the ligand-binding domain and each compound with those of cyclothiazide and CX614. CMPDB was found to be more isoform-selective than would be predicted from initial binding assays. It is noteworthy that these new compounds are both more potent and more effective and may be more clinically relevant than the AMPA receptor modulators described previously.

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.

Challenges for and current status of research into positive modulators of AMPA receptors

AMPA receptors consist of a family of hetero-oligomeric (tetrameric) receptors arising from four genes, each of which encodes a distinct receptor subunit (GluA1-4). Recombinant homo-tetrameric AMPA receptors, comprising four identical subunits, are functionally active and have been used in in vitro assays. However, the many different subunit permutations make possible the functional and anatomical diversity of AMPA receptors throughout the CNS. Furthermore, AMPA receptor subunit stoichiometry influences the biophysical and functional properties of the receptor. A number of chemically diverse positive modulators of AMPA receptor have been identified which potentiate AMPA receptor-mediated activity in vitro as well as improving cognitive performance in rodents and non-human primates with several being taken further in the clinic. This review article summarizes the current status in the research on positive allosteric modulation of AMPA receptors and outlines the challenges involved in identifying a chemically distinct series of AMPA receptor positive modulators, addressing the challenges created by the heterogeneity of the AMPA receptor populations and the development of structure-activity relationships driven by homomeric, recombinant systems on high-throughput platforms. We also review the role of X-ray crystallography in the selection and prioritization of targets for lead optimization for AMPA receptor positive modulators.

Glutamate receptor-mediated restoration of experience-dependent place field expansion plasticity in aged rats

Place fields of hippocampal pyramidal cells expand asymmetrically when adult rats repeatedly follow the same route. This behaviorally induced expression of neuronal plasticity uses an NMDAR-dependent, LTP-like mechanism and could be used by hippocampal networks to store information. Aged spatial memory-impaired rats exhibit defective experience-dependent place field expansion plasticity. One possible explanation for this aged-associated deficit is alterations in glutamatergic function. In fact, both NMDAR- and AMPAR-mediated field excitatory postsynaptic potentials in CA1 decrease with aging. The current study investigated whether modulation of either AMPA or NDMA receptor activity could restore this experience-dependent plasticity by prolonging AMPAR activity with the ampakine CX516 and modulating the NMDAR with the noncompetitive antagonist memantine. The spatial firing characteristics of multiple CA1 pyramidal cells were monitored under both treatment conditions as aged rats repeatedly traversed a circular track. Compared to the saline baseline condition, acute administration of memantine, but not CX516, reinstated experience-dependent place field expansion. Taken together, these data suggest that pharmacological manipulation of the NMDAR can improve the function of hippocampal networks critical to optimal cognition in aging.

Physiological significance of high- and low-affinity agonist binding to neuronal and recombinant AMPA receptors

Radioligand binding studies have shown that AMPA receptors exist in two variants that differ about twenty-fold in their binding affinities, with brain receptors being mainly of the low-affinity type and recombinantly expressed receptors having almost exclusively high affinity. However, the physiological correlate of high- and low-affinity binding is not yet known. In this study we examined if physiological experiments similarly reveal evidence for two distinct receptor variants. We therefore measured equilibrium desensitization by glutamate and determined IC(50) values for neuronal receptors and for the homomeric receptors GluR1-4 expressed in HEK293 cells. Contrary to the prediction that these IC(50) values exhibit large differences commensurate with those of high- and low-affinity binding, values for homomeric receptors (1-18 microM) were on an average not different from those of neuronal receptors (3-10 microM). Moreover, simulations with kinetic receptor models suggest that the IC(50) values for neuronal and recombinant receptors correspond to the binding affinity of the low-affinity receptor variant. These findings indicate that the high-affinity binding measured in heterologous expression systems represents an immature receptor variant that does not contribute to the currents recorded from these cells, and that the functional low-affinity receptors are present in such small number that they are effectively masked in binding assays by the high-affinity receptors. Thus, in order to compare experimentally determined saturation binding profiles with those predicted by kinetic receptor models and with dose-response curves from physiological studies, it will be imperative to develop methods for isolating first the low-affinity receptors.

The fast kinetics of AMPA GluR3 receptors is selectively modulated by the TARPs gamma 4 and gamma 8

Transmembrane AMPA receptor-associated regulatory proteins (TARPs) modulate the kinetics of AMPA receptors and increase their surface expression. This study compared the effects of the TARPs gamma2, gamma3, gamma4 and gamma8 on the AMPA receptor subunits GluR1-3. We found that the deactivation and desensitization time constants of GluR3 receptors were greatly increased by gamma4 and gamma8 (approximately 3 fold) whereas gamma2 and gamma3 caused minimal changes. This stands in contrast to the effects on GluR1 and GluR2 receptors which were similar across all four TARPs. Other response parameters like the current density, the EC(50) for glutamate-induced peak currents, and kainate-induced currents were in general more effectively modulated by gamma2 and gamma3 in all subunits, including GluR3. All TARPs were effective in increasing the unitary conductance. The differential TARP effects on deactivation time constants of the GluR3 subunit are likely to have a significant impact on synaptic responses across different neurons in the brain.

A placebo-controlled add-on trial of the Ampakine, CX516, for cognitive deficits in schizophrenia

AMPA-receptor-positive modulators (Ampakines) facilitate learning and memory in animal models and in preliminary trials in human subjects. CX516 is the first Ampakine to be studied for cognitive enhancement in schizophrenia. Stable schizophrenia patients treated with clozapine (n=52), olanzapine (n=40), or risperidone (n=13) were randomly assigned to add-on treatment with CX516 900 mg three times daily or placebo for 4 weeks. Subjects were assessed with a cognitive battery at baseline, week 4, and at 4-week follow-up. Clinical scales and safety monitoring were also performed. The primary endpoint was the change from baseline in a composite cognitive score at week 4 for the intent-to-treat sample. Additional analyses examined change in symptom rating scores and examined drug effects on patients treated with clozapine separately from patients treated with either olanzapine or risperidone. A total of 105 patients were randomized and 95 (90%) completed the 4-week trial. Patients treated with CX516 did not differ from placebo in change from baseline on the composite cognitive score, or on any cognitive test at weeks 4 or 8. The between groups effect size at week 4 for the cognitive composite score was -0.19 for clozapine-treated patients and 0.24 for patients treated with olanzapine or risperidone. The placebo group improved more on the PANSS total score than the CX516 group; no other clinical rating differed between treatment groups. CX516 was associated with fatigue, insomnia and epigastric discomfort compared to placebo, but was generally well tolerated. CX516 was not effective for cognition or for symptoms of schizophrenia when added to clozapine, olanzapine, or risperidone.

Factors affecting guanine nucleotide binding to rat AMPA receptors

Glutamate receptors are competitively inhibited by guanine nucleotides. Insight into the physiological function of this inhibition would be greatly advanced if nucleotide binding could be eliminated through mutations without altering other aspects of receptor function, or if compounds were discovered that selectively prevent nucleotide binding. It was previously reported that a lysine in the chick kainate binding protein (cKBP) is specifically involved in guanine nucleotide binding. In the present study we mutated the equivalent lysine in the rat AMPA receptor subunit GluR1 flip to alanine (K445A) and assessed changes in nucleotide affinity from the displacement of [(3)H]fluorowillardiine. As in the cKBP, the affinity for nucleotides was greatly reduced while the binding affinity for agonists remained unchanged. The reduction in affinity was largest for GTP (factor of 5.8) and GDP (4.4) and minor for GMP and guanosine. This suggests that K445 is involved in stabilizing the second phosphate of the nucleotide. Given that bulkier analogs like GDP-fucose are also accommodated at this site, it seems likely that nucleotides bind in such a way that their phosphates project out of the cleft. In excised-patch recordings using short pulses of glutamate, the K445A mutation increased the EC(50) for the peak response 1.8-fold and accelerated desensitization and deactivation. This indicates that the effects of this mutation are not as specific as previously suggested. Efforts to selectively eliminate inhibition by nucleotides may therefore depend on mapping out further the docking site. In a first attempt using point mutations we ruled out several amino acids around the cleft as being involved in nucleotide binding. Also, the AMPA receptor modulator PPNDS which competitively inhibits nucleotide binding to purinergic receptors did not affect nucleotide inhibition, suggesting that there are major differences in the topography between purinergic and glutamate receptors. Thus new approaches, including crystallography, may be called for to identify residues uniquely involved in nucleotide binding.

Ampakines and the threefold path to cognitive enhancement

Ampakines are the first peripherally administered drugs that increase excitatory monosynaptic responses in the brain. Because of this effect, the compounds improve communication in complex networks, potently facilitate long-term potentiation (LTP) and induce the expression of neurotrophic factors. Ampakines are effective in animal models of neuropsychiatric disorders and have produced positive results in a small number of human studies. Neurobiological considerations and modeling studies suggest that the drugs, in addition to their effects on disturbed behavior, will alter the encoding and organization of information in normal brains. Results from physiological and behavioral studies accord with this prediction. Building on these findings, this article considers how the threefold effects of ampakines will modify, and enhance, cognition.

Mechanism of positive allosteric modulators acting on AMPA receptors

Ligand-gated ion channels involved in the modulation of synaptic strength are the AMPA, kainate, and NMDA glutamate receptors. Small molecules that potentiate AMPA receptor currents relieve cognitive deficits caused by neurodegenerative diseases such as Alzheimer's disease and show promise in the treatment of depression. Previously, there has been limited understanding of the molecular mechanism of action for AMPA receptor potentiators. Here we present cocrystal structures of the glutamate receptor GluR2 S1S2 ligand-binding domain in complex with aniracetam [1-(4-methoxybenzoyl)-2-pyrrolidinone] or CX614 (pyrrolidino-1,3-oxazino benzo-1,4-dioxan-10-one), two AMPA receptor potentiators that preferentially slow AMPA receptor deactivation. Both potentiators bind within the dimer interface of the nondesensitized receptor at a common site located on the twofold axis of molecular symmetry. Importantly, the potentiator binding site is adjacent to the "hinge" in the ligand-binding core "clamshell" that undergoes conformational rearrangement after glutamate binding. Using rapid solution exchange, patch-clamp electrophysiology experiments, we show that point mutations of residues that interact with potentiators in the cocrystal disrupt potentiator function. We suggest that the potentiators slow deactivation by stabilizing the clamshell in its closed-cleft, glutamate-bound conformation.

AMPA receptor modulators have different impact on hippocampal pyramidal cells and interneurons

Positive modulators of AMPA receptors enhance synaptic plasticity and memory encoding. Facilitation of AMPA receptor currents not only results in enhanced activation of excitatory neurons but also increases the activity of inhibitory interneurons by up-modulating their excitatory input. However, little is known about the effects of these modulators on cells other than pyramidal neurons and about their impact on local microcircuits. This study examined the effects of members from three subfamilies of modulators (mainly CX516, CX546 and cyclothiazide) on excitatory synaptic responses in four classes of hippocampal CA1 neurons and on excitatory and disynaptically induced inhibitory field potentials in hippocampal slices. Effects on excitatory postsynaptic currents (EPSCs) were examined in pyramidal cells, in two types of inhibitory interneurons located in stratum radiatum and oriens, and in stratum radiatum giant cells, a novel type of excitatory neuron. With CX516, increases in EPSC amplitude in pyramidal cells were two to three times larger than in interneurons and six times larger than in radiatum giant cells. The effects of CX546 on response duration similarly were largest in pyramidal cells. However, this drug also strongly differentiated between stratum oriens and radiatum interneurons with increases being four times larger in the latter. In contrast, cyclothiazide had similar effects on response duration in all cell types. In field recordings, CX516 was several times more potent in enhancing excitatory postsynaptic potentials (EPSPs) than feedback or feedforward circuits, as expected from its larger influence on pyramidal cells. In contrast, BDP-20, a CX546 analog, was more potent in enhancing feedforward inhibition than either EPSPs or feedback inhibition. This preference for feedforward over feedback circuits is probably related to its higher potency in stratum radiatum versus oriens interneurons. Taken together, AMPA receptor modulators differ substantially in their potency and/or efficacy across major classes of neurons which is likely to have consequences with regard to their impact on circuits and behavior.

Use of [3H]fluorowillardiine to study properties of AMPA receptor allosteric modulators

Compounds which modulate AMPA receptor function through allosteric mechanisms were examined for their effect on the binding of the agonist [3H]fluorowillardiine (FW). Benzamide-type positive modulators (ampakinestrade mark) under all experimental circumstances increased [3H]FW binding to native receptors in rat brain membranes. Benzothiadiazide drugs had more variable effects ranging from large reductions with cyclothiazide and JM-13 to increases produced by more recent compounds like PEPA, D1 and LY392098. These effects on binding were moderately influenced by the assay conditions, including temperature and the presence or absence of thiocyanate. Significant changes in agonist binding were also produced by other modulatory agents such as noncompetitive blockers (GYKI 53655, SYM 2206), polycationic compounds (spermine, Naspm, philanthotoxin) and polyanionic compounds (Evans Blue, suramin, PPNDS). EC50 values usually were similar to those from physiological studies, which validates using binding tests to assess drug potencies. Moreover, direction and magnitude of the binding change (Emax) provide information about which kinetic aspects are affected by a drug. For example, the magnitude of the binding increase produced by positive modulators was strongly correlated with their ability to slow response deactivation in excised patch recordings. Binding also provides a reliable method to examine whether interactions between agents are competitive. Thus, thiocyanate did not significantly influence the EC50 of cyclothiazide, suggesting distinct sites of action. Taken together, [3H]FW binding can yield important information about drug-receptor and drug-drug interactions for a wide range of modulatory agents. One potential limitation of [3H]FW is a large preference for subunits GluR1 and GluR2 (KD 4-10 nM) over GluR3 and GluR4 (160-600 nM) which implies that tests with brain membranes preferentially reveal drug effects produced at the former two subunits. Lastly, data are shown which highlight the importance of optimizing experimental conditions in filtration assays, for instance by always including thiocyanate in wash buffers.

C-terminal truncation affects kinetic properties of GluR1 receptors

GluR1flop receptors in which the C-terminal 52 amino acids had been recombinantly removed were characterized with whole-cell recording and binding assays. Compared to wildtype GluR1, truncated receptors showed faster desensitization and deactivation and they recovered more slowly from desensitization. The EC50 for glutamate was increased 2-fold. In binding tests, K(D)s for [3H]fluorowillardiine were 1.5 times larger for truncated receptors. According to receptor simulations, most differences can be explained if the C-terminal domain is assumed to stabilize the ligand-bound closed and open states. The effects on response waveforms are different from those caused by phosphorylation, suggesting that the C-terminus influences receptor function in multiple ways. Truncated forms of GluR1 identical or similar to the one examined here may also be generated by calcium-activated proteases during intense synaptic activity. The lowered affinity and faster inactivation of these receptors suggests that their presence does not represent a risk for neuronal viability.

Divergent effects of the purinoceptor antagonists suramin and pyridoxal-5'-phosphate-6-(2'-naphthylazo-6'-nitro-4',8'-disulfonate) (PPNDS) on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors

Suramin is a large naphthyl-polysulfonate compound that inhibits an array of receptors and enzymes, and it has also been reported to block currents mediated by glutamate receptors. This study shows that suramin and several structurally related compounds [8,8'-[carbonylbis(imino-3,1-phenylenecarbonylamino)]bis-(1,3,5-naphthalenetrisulfonic acid), 6Na (NF023), 8,8'-(carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino))bis-1,3,5-naphthalenetrisulfonic acid, Na (NF279), and 4,4',4'',4'''-[carbonyl-bis[imino-5,1,3-benzenetriyl-bis-(carbonylimino)]]tetrakis-benzene-1,3-disulfonic acid, 8Na (NF449)] reduce binding of [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and [3H]fluorowillardiine to rat brain membranes and homomeric GluR1-4 receptors, with IC50 values in the range of 5 to 180 microM. Inhibition often was less than complete at saturating drug concentrations and thus seems to be noncompetitive in nature. Pyridoxal-5'-phosphate-6-(2'-naphthylazo-6'-nitro-4',8'-disulfonate) (PPNDS) is a potent antagonist of purinoceptors that shares some structural elements with suramin yet is smaller than the latter. PPNDS also had potent effects on AMPA receptors (EC50 value of 4 microM) but of a kind not seen with the other compounds in that it increased binding affinity for radioagonists severalfold. In addition, PPNDS slowed association and dissociation rates more than 10 times. In physiological experiments with GluR2 receptors, PPNDS at 50 microM reduced the peak current by 30 to 50% but had only small effects on other waveform aspects such desensitization and steady-state currents. This pattern of effects differentiates PPNDS from other compounds such as thiocyanate and up-modulators, which increase agonist binding by enhancing desensitization or slowing deactivation, respectively. Receptor model simulations indicate that most effects can be accounted for by assuming that PPNDS slows agonist binding/unbinding and stabilizes the bound-closed state of the receptor. By extension, suramin is proposed to stabilize the unbound state and thereby to reduce affinity for agonists. These drugs thus act through a novel type of noncompetitive antagonism.

Ampakines reduce methamphetamine-driven rotation and activate neocortex in a regionally selective fashion

It has been proposed that glutamatergic and dopaminergic systems are functionally opposed in their regulation of striatal output. The present study tested the effects of drugs that enhance AMPA-receptor-mediated glutamatergic transmission (ampakines) for their effects on dopamine-related alterations in cortical activity and locomotor behavior. Rats with unilateral 6-hydroxydopamine lesions of the ascending nigro-striatal dopamine system were sensitized to methamphetamine and then tested for methamphetamine-induced circling behavior in the presence and absence of ampakines CX546 and CX614. Both ampakines produced rapid, dose-dependent reductions in circling that were evident within 15 min and sustained through 1 h of behavioral testing. In situ hybridization maps of c-fos mRNA expression showed that in the intact hemisphere, ampakine cotreatment markedly increased c-fos expression in parietal, sensori-motor neocortex above that found in rats treated with methamphetamine alone. Ampakine cotreatment did not augment c-fos expression in frontal, sensori-motor cortex or striatum. Still larger ampakine-elicited effects were obtained in parietal cortex of the dopamine-depleted hemisphere where labeling densities were increased by approximately 60% above values found in methamphetamine-alone rats. With these effects, the hemispheric asymmetry of cortical activation was less pronounced in the ampakine-cotreatment group as compared with the methamphetamine-alone group. These results indicate that positive modulation of AMPA-type glutamate receptors 1) can offset behavioral disturbances arising from sensitized dopamine receptors and 2) increases aggregate neuronal activity in a regionally selective manner that is probably dependent upon behavioral demands.

The mechanism of action of aniracetam at synaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors: indirect and direct effects on desensitization

The mechanism of action of aniracetam on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors was examined in outside-out patches and at glutamatergic synapses in neurons of the chick cochlear nucleus. A combination of rapid-flow analysis, using glutamate as an agonist, and kinetic modeling indicated that aniracetam slows both the rate of channel closing, and the microscopic rates of desensitization, even for partially liganded receptors. Little effect was observed on the rate of recovery from desensitization or on the response to the weakly desensitizing agonist kainate. Aniracetam's effects on receptor deactivation saturated at lower concentrations than its effects on desensitization, suggesting that cooperativity between homologous binding sites was required to regulate desensitization. Analysis of responses to paired pulses of agonist also indicated that AMPA receptors must desensitize partially even after agonist exposures too brief to permit rebinding. In the presence of aniracetam, evoked excitatory synaptic currents (EPSCs) and miniature EPSCs in low quantal-content conditions had decay times similar to the time course of receptor deactivation. Under these conditions, the time course of both transmitter release and clearance must be <1 to 2 ms. However, in high quantal-content conditions, the evoked EPSC in aniracetam decayed with a time course intermediate between deactivation and desensitization, suggesting that the time course of transmitter clearance is prolonged because of pooling of transmitter in the synaptic cleft. Moreover, by comparing the amounts of paired-pulse synaptic depression and patch desensitization prevented by aniracetam, we conclude that significant desensitization occurs in response to rebinding of transmitter to the AMPA receptors.

Identification of a site in GluR1 and GluR2 that is important for modulation of deactivation and desensitization

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype of ionotropic glutamate receptors consists of rapidly gating ion channels. Positive modulation of channel gating may slow gating kinetics through at least two distinct mechanisms, evidenced by the predominant slowing of either the rate of receptor desensitization or the rate of offset after agonist withdrawal (deactivation). This study compares the actions of two positive allosteric modulators [cyclothiazide, which modulates desensitization, and 1-(1,4-benzodioxan-6-ylcarbonyl)piperidine (CX546), which modulates deactivation] in a mutant shown previously to impede modulation by cyclothiazide. These experiments test the hypothesis that the point mutation, GluR1(S493T), would also cause a loss of modulation by CX546. Wild-type GluR1 through -4 receptors were modulated by CX546, as assayed by the potentiation of steady-state currents in the Xenopus laevis oocyte expression system. CX546 potentiated steady-state currents of both splice isoforms of GluR1. Modulation by CX546 was completely abolished in GluR1(S493T) and its homolog, GluR2(S497T), although this mutation did not affect apparent agonist affinity in the absence of CX546. Thus, the GluR1(S493T) mutation has a similar impairment of modulation by either cyclothiazide or CX546, indicating that some residues at the subunit interface of glutamate receptors play an important role in channel deactivation and desensitization.

Benzamide-type AMPA receptor modulators form two subfamilies with distinct modes of action

CX516 (BDP-12) and CX546, two first-generation benzamide-type AMPA receptor modulators, were compared with regard to their influence on AMPA receptor-mediated currents, autaptic responses in cultured hippocampal neurons, hippocampal excitatory postsynaptic currents, synaptic field potentials, and agonist binding. The two drugs exhibited comparable potencies in most tests but differed in their efficacy and in their relative impact on various response parameters. CX546 greatly prolonged the duration of synaptic responses, and it slowed 10-fold the deactivation of excised-patch currents following 1-ms pulses of glutamate. The effects of CX516 on those measures were, by comparison, small; however, the drug was equally or more efficacious than CX546 in increasing the amplitude of synaptic responses. This double dissociation suggests that amplitude and duration of synaptic responses are governed by different aspects of receptor kinetics, which are differentially modified by the two drugs. These effects can be reproduced in receptor simulations if one assumes that CX516 preferentially accelerates channel opening while CX546 slows channel closing. In binding tests, CX546 caused an approximately 2-fold increase in the affinity for radiolabeled agonists, whereas CX516 was ineffective. More importantly, even millimolar concentrations of CX516 did not influence the dose-response relation for CX546, suggesting the possibility that they bind to different sites. Taken together, the evidence suggests that benzamide modulators from the Ampakine family form two subgroups with different modes and sites of action. Of these, CX516-type drugs may have the greater therapeutic utility because of their limited efficacy in prolonging synaptic responses and in attenuating receptor desensitization.

Interactions between recording technique and AMPA receptor modulators

Whole cell recording (EPSCs) and extracellular recording (field EPSPs) were compared in hippocampal field CA1 with regard to the effects of experimental treatments that increase AMPA receptor gated currents. Cyclothiazide, which maintains AMPA receptors in the sensitized state, caused a rapid and pronounced increase in EPSCs but only minor changes in field EPSPs. This difference was evident in recordings carried out at 22 and 32 degrees C and with different solutions in the clamp pipette. The larger effect of cyclothiazide on EPSCs was unaffected by blockade of GABA and NMDA receptors. Two-dimensional current source density analyses derived from 64 recording sites were used to provide extracellular estimates of AMPA receptor mediated synaptic currents. With this method, cyclothiazide again had much smaller effects than were obtained with whole cell clamp. Differences between whole cell and extracellular recordings were present, although not as pronounced, for the ampakines, a class of drugs that slow both deactivation and desensitization of AMPA receptors. Additionally, increases in synaptic responses produced by frequency facilitation, a manipulation that enhances the number of bound receptors, were not qualitatively different between recording techniques. These results support the conclusion that the whole cell clamp technique may alter AMPA receptors in such a way as to increase the relative importance of desensitization.

Effects of 5'-alkyl-benzothiadiazides on (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor biophysics and synaptic responses

Alkyl-substituted benzothiadiazides (BTDs) were tested for their effects on (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors. In excised patches, the 5'-ethyl derivative "D1" blocked the desensitization of AMPA receptor currents during prolonged application of glutamate (EC(50), 36 microM), and it slowed deactivation of responses elicited by 1-ms glutamate pulses greater than 10-fold. [(3)H]Fluorowillardiine binding to rat synaptic membranes was increased by D1 by a factor of 3.6 (EC(50), 17 microM) with a Hill coefficient near 2. In hippocampal slices, the compound reversibly increased excitatory postsynaptic currents and field excitatory postsynaptic potentials (EPSPs) with thresholds around 10 microM. The size of the alkyl substituent influenced both the potency and nature of the drug effect on synaptic currents: 5'-methyl compounds had a 2-fold greater effect on response amplitude than on response duration, whereas 5'-ethyl compounds like D1 caused greater increases in duration than amplitude. In tests with recombinantly expressed AMPA receptor subunits, D1 preferred the glutamate receptor (GluR) subunit GluR4 flip (0.64 microM) over GluR4 flop (5.3 microM); similar affinities but with smaller flip-flop differences were obtained for GluR1 through 3. These results show that D1 and congeners are significantly more potent than the parent compound IDRA-21 and that they differ in two fundamental aspects from cyclothiazide, the most widely studied BTD: 1) D1 markedly increases the agonist affinity of AMPA receptors and 2) it has immediate and large effects on field EPSPs. The large gain in potency conferred by alkyl substitution suggests that the 5' substituent is in intimate contact with the receptor, with the size of the substituent determining the way in which receptor kinetics is changed.

A desensitization-selective potentiator of AMPA-type glutamate receptors

1: We examined the effects of PEPA, an allosteric potentiator of AMPA receptors, on AMPA receptor kinetics. 2: PEPA did not affect the deactivation of glutamate responses but potently attenuated the extent of receptor desensitization without slowing the onset of desensitization in most of the recombinant AMPA receptors (GluR1-flip, GluR1-flop, GluR3-flip, GluR3-flip+GluR2-flip, and GluR3-flop+GluR2-flop) expressed in Xenopus oocytes. For the GluR3-flop subunit, PEPA attenuated the extent of desensitization and only weakly prolonged deactivation (1.3 fold). 3: PEPA did not significantly affect recovery from desensitization in oocytes expressing GluR3-flip, GluR1-flop, and GluR1-flop, but weakly accelerated (2.6 fold) recovery from desensitization in oocytes expressing GluR3-flop. 4: PEPA's effect on desensitization of GluR3-flop-containing receptors is unique in that onset is very slow. 5: Simulation studies using simplified kinetic models for AMPA receptors are utilized to explore the differential effects of PEPA on GluR3-flip and -flop. It is possible to simulate the action on GluR3-flip by modulating two rate constants in a 12-state kinetic model. For simulation of the action on GluR3-flop, the 12-state kinetic model is not enough, and it is necessary to invoke a 13th state, a PEPA-bound receptor to which glutamate cannot bind. 6: These results suggest that attenuation of extent of desensitization represents the principal mechanism underlying the potentiation of AMPA receptors by PEPA, and that PEPA exhibits different mechanisms with respect to GluR3-flip and GluR3-flop.

AMPA receptor-mediated miniature synaptic calcium transients in GluR2 null mice

AMPA-type glutamate receptors are normally Ca(2+) impermeable due to the expression of the GluR2 receptor subunit. By using GluR2 null mice we were able to detect miniature synaptic Ca(2+) transients (MSCTs) associated with AMPA-type receptor-mediated miniature synaptic currents at single synapses in primary cortical cultures. MSCTs and associated Ca(2+) transients were monitored under conditions that isolated responses mediated by AMPA or N-methyl-D-aspartate (NMDA) receptors. As expected, addition of the antagonist 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX, 3 microM) blocked the AMPA receptor-mediated MSCTs. Voltage-gated Ca(2+) channels did not contribute to AMPA MSCTs because CdCl(2) (0.1-0.2 mM) did not significantly alter the frequency or the amplitude of the MSCTs. The amplitude of AMPA MSCTs appeared to be regulated independently from event frequency since the two measures were not correlated (R = 0.023). Synapses were identified that only expressed MSCTs attributed to either NMDA or AMPA receptors. At synapses with only NMDA responses, MSCT amplitude was significantly lower (by 40%) than synapses expressing both NMDA and AMPA responses. At synapses that showed MSCTs mediated by both AMPA and NMDA receptors, the amplitude of the transients in each condition was positively correlated (R = 0.94). Our results suggest that when AMPA and NMDA receptors are co-expressed at synapses, mechanisms exist to ensure proportional scaling of each receptor type that are distinct from the presynaptic factors controlling the frequency of miniature release.

Long-term potentiation alters the modulator pharmacology of AMPA-type glutamate receptors

Changes in the biophysical properties of AMPA-type glutamate receptors have been proposed to mediate the expression of long-term potentiation (LTP). The present study tested if, as predicted from this hypothesis, AMPA receptor modulators differentially affect potentiated versus control synaptic currents. Whole cell recordings were collected from CA1 pyramidal neurons in hippocampal slices from adult rats. Within-neuron comparisons were made of the excitatory postsynaptic currents (EPSCs) elicited by two separate groups of Schaffer-collateral/commissural synapses. LTP was induced by theta burst stimulation in one set of inputs; cyclothiazide (CTZ), a drug that acts on the desensitization kinetics of AMPA receptors, was infused 30 min later. The decay time constants of the potentiated EPSCs prior to drug infusion were slightly, but significantly, shorter than those of control EPSCs. CTZ slowed the decay of the EPSCs, as reported in prior studies, and did so to a significantly greater degree in the potentiated synapses. Additionally, infusion of CTZ resulted in significantly greater effects on amplitude in potentiated pathways as compared with control pathways. The interaction between LTP and CTZ was also obtained in a separate set of experiments in which GABA receptor antagonists were used to block inhibitory postsynaptic currents. Additionally, there was no significant change in paired-pulse facilitation in the presence of CTZ, indicating that presynaptic effects of the drug were negligible. These findings provide new evidence that LTP modifies AMPA receptor kinetics. Candidates for the changes responsible for the observed effects of LTP were evaluated using a model of AMPA receptor kinetics; a simple increase in the channel opening rate provided the most satisfactory match with the LTP data.

5'-alkyl-benzothiadiazides: a new subgroup of AMPA receptor modulators with improved affinity

AMPA receptors form a major subdivision of the glutamate receptor family that mediates excitatory synaptic transmission in the brain. Currents through AMPA receptors can be up- or down-regulated by compounds that allosterically modulate receptor kinetics through binding sites distinct from that for glutamate. One of those modulators is the benzothiadiazide IDRA-21 which has been reported to enhance synaptic transmission and be effective in behavioral tests, but typically requires threshold concentrations of at least 100 microM to be active in vitro. In this study, new benzothiadiazides were developed with IDRA-21 as lead compound and examined for their potency in modulating AMPA receptor kinetics. A significant increase in drug affinity was obtained by alkyl substitution at the 5'-position of IDRA-21; substitutions at other positions of the benzothiadiazide core generally did not yield a further gain in affinity and in some cases abolished drug binding. The 5'-ethyl derivative exhibited an EC(50) value in the order of 22 microM which represents about a 30-fold gain in affinity over that of IDRA-21. The EC(50) value is comparable to that of cyclothiazide, the most potent benzothiadiazide drug, but the effects on AMPA receptors differed substantially between these two compounds in that the 5'-ethyl derivative of IDRA-21 greatly increased the binding affinity for receptor agonists whereas cyclothiazide is known to reduce agonist binding. The structure--activity relationships reported here thus offer to provide new insights how receptor kinetics is linked to particular aspects of receptor--drug interactions.

Antidepressant activity of memory-enhancing drugs in the reduction of submissive behavior model

The present study tests the activity of nootropic drugs in a behavioral test linked to depression. This test measures the reduction of submissive behavior in a competition test as the relative success of two food-restricted rats to gain access to a feeder. Nootropic drugs tested include piracetam (2-oxo-1-pyrrolidineacetamide), aniracetam (1-(4-methoxybenzoyl)-2-pyrrolidinone), the Ampakine, Ampalex, 1-(quinoxalin-6-ylcarbonyl)piperidine, and analogs were compared to the antidepressants, fluoxetine ((+/-)-N-methyl-gamma-(4-[trifluoromethyl]phenoxy)-benzenepropanamine) and desimpramine (5H-dibenz[b,f]azepine-5-propanamine, 10,11-dihydro-N-methyl-, monohydrochloride), while the anxiolytic diazepam (7-chloro-1-methyl-5-phenyl-3H-1,4-benzodiazepin-2(1H)-one) served as a control. Drugs were given intraperitoneally for 3 weeks. The antidepressant and nootropic drugs reduced submissive behavior over time. The effect was dose dependent as measured for fluoxetine and Ampakines. The reduction of submissive behavior by Ampakines gradually faded after cessation of treatment and had a more rapid onset of activity (during the 1st week of treatment) than fluoxetine (after 2 weeks). The results suggest that Ampakines may have antidepressant activity. The potential of depression treatment with memory-enhancing drugs is hypothesized and the link between cognition and depression is discussed.

Mechanism and impact of allosteric AMPA receptor modulation by the ampakine CX546

Glutamate release at central synapses is transduced into a characteristic fast postsynaptic response by AMPA receptor gating and agonist affinity. The effect of two classes of modulators of AMPA receptor desensitization, the benzothiadiazides (cyclothiazide and IDRA 21) and the benzoylpiperidines (CX516 and CX546), were studied on gating kinetics of recombinant, native AMPA receptors and on synaptic currents. CX546 reduced the degree of desensitization more potently than CX516 or IDRA 21, but not as efficiently as cyclothiazide. In presence of CX516/CX546, desensitization of GluR2(flip) receptors was inhibited more than of GluR1(flip), whereas they had no effect upon response shape or conductance. CX546 increased agonist affinity threefold on nondesensitizing AMPA receptors by slowing agonist unbinding. Analysis of modulatory action suggests that, in contrast to cyclothiazide or IDRA 21, the Ampakine CX546 binds specifically to the agonist bound nondesensitized receptor, most likely acting by destabilizing the desensitized receptor conformation. All modulators tested showed higher efficiency on native receptors as compared to homomeric receptors. At the glutamatergic synapse, evoked synaptic amplitudes were weakly potentiated, while EPSC decay was slowed by nearly a factor of three in the presence of CX546 or cyclothiazide. In the presence of CX546, the current induced by short pulses of glutamate from recombinant GluR2 receptors decayed with a time course that was approximately twentyfold faster than EPSCs. The unique properties of CX546 may be beneficial for therapeutical use.

The AMPA receptor allosteric potentiator PEPA ameliorates post-ischemic memory impairment

PEPA (4-[2-(Phenylsulphonylamino)ethylthio]-2,6-difluorophenoxyacetamide) is a recently developed allosteric potentiator of AMPA receptors that preferentially affects flop splice variants. We tested the effects of PEPA on ischemia-induced memory deficit in rats. Permanent unilateral occlusion of the middle cerebral artery induced severe impairment of performance of rats in the Morris water maze test. Repeated intravenous administration of PEPA (1, 3, 10 mg/kg/day for 10 days) improved test performance. In contrast, a corresponding dose of aniracetam, a representative potentiator of AMPA receptor, did not significantly improve test performance. Thus, PEPA is more effective than aniracetam in reversing impaired memory function as assessed by the Morris water maze test; and PEPA may be an effective compound for the treatment of impaired memory.

Binding of an AMPA receptor potentiator ([3H]LY395153) to native and recombinant AMPA receptors

LY395153 is a member of a newly described class of arylpropylsulfonamide AMPA receptor potentiators. Here, we characterize and compare [(3)H]LY395153 binding to native AMPA receptors from rat cerebral cortex and recombinant human GluR4(flip) receptors expressed in HEK293 cells. L-Glutamate and AMPA increased [(3)H]LY395153 binding to both native and recombinant AMPA receptors in a concentration dependent and stereoselective manner; this effect of AMPA receptor agonists reflects an apparent increase in ligand affinity. In the presence of L-glutamate (500 microM), [(3)H]LY395153 binding is saturable; the affinity of this radioligand is slightly, albeit statistically significantly higher at human GluR4(flip) (K(d)=55.6+/-5.3nM) than rat cortical receptors (K(d)=110+/-15.1nM). NBQX competitively inhibited L-glutamate-induced increases in [(3)H]LY395153 binding in both native and recombinant receptors, whilst LY303070 (the active isomer of GYKI53655) noncompetitively inhibited this effect in native, but not recombinant receptors. The prototypic AMPA receptor potentiator cyclothiazide competitively inhibited [(3)H]LY395153 binding with a potency (K(i) approximately 7 microM) comparable to EC(50) values reported in electrophysiological studies. In contrast, the structurally unrelated AMPA receptor potentiator CX 516 did not inhibit [(3)H]LY395153 binding at concentrations of up to 600 microM. Further, at concentrations reported to facilitate AMPA receptor desensitization, thiocyanate acts as a competitive inhibitor of [(3)H]LY395153 binding. [(3)H]LY395153 binding was unaffected by a variety of structurally (and mechanistically) diverse compounds tested at a concentration of 10 microM. These data indicate [(3)H]LY395153 is a useful probe for labeling a unique modulatory site on both native and recombinant AMPA receptors.

Effects of developmental lead exposure on inhibitory avoidance learning and glutamate receptors in rats

Chronic lead (Pb) exposure during development is known to produce learning deficits. AMPA and NMDA receptors have been shown to participate in the synaptic mechanisms involved in certain forms of learning and memory. We investigated whether the effects of Pb on AMPA and NMDA receptors are associated with Pb-induced impairment in learning and memory. Rats were exposed to 0.2% lead acetate at different developmental stages including a maternally exposed group (including gestation and lactation period), a postweaning exposed group, and a continuously exposed group. Lead treatment impaired learning acquisition, but not memory retention in step-down avoidance learning task in all treatment groups. In parallel with the behavioral data, autoradiographic analyses of brain sections indicated that the [3H]AMPA binding was decreased in the CA1 and dentate gyrus of the hippocampus and entorhinal cortex in all three Pb-exposed groups. However, an increase in [3H]MK801 binding was only observed in CA1 of the hippocampus in the continuously Pb-exposed rats. The findings suggest that alterations in AMPA receptor may contribute to the Pb-induced deficits in learning acquisition of inhibitory avoidance.

GYKI 52466 has positive modulatory effects on AMPA receptors

The 2,3-benzodiazepine derivative GYKI 52466 has been well characterized as a negative modulator of AMPA-type glutamate receptors. The present study re-examined the effects of GYKI 52466 on AMPA receptor-mediated currents in patches excised from pyramidal neurons in the hippocampal CA1 field and found that this drug has positive modulatory effects in addition to its receptor blocking action. A low concentration of GYKI 52466 (10 microM) reliably increased the steady-state current by about three-fold, while the peak current was reduced by 30% only. Higher drug concentrations produced parallel reductions in both the steady-state and peak currents. The increase in the steady-state current was not accompanied by a change in the deactivation time constant and thus, is more likely to result from a change in desensitization than a slowing of channel closing. The results indicate that GYKI 52466 modulates AMPA receptor-mediated currents in a complex manner, perhaps by acting through more than one binding site.

Effects of the potent ampakine CX614 on hippocampal and recombinant AMPA receptors: interactions with cyclothiazide and GYKI 52466

R,S-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor up-modulators of the benzamide type ("ampakines") have previously been shown to enhance excitatory synaptic transmission in vivo and in vitro and AMPA receptor currents in excised patches. The present study analyzed the effects of an ampakine (CX614; 2H,3H, 6aH-pyrrolidino[2",1"-3',2']1,3-oxazino[6',5'-5,4]benz o[e]1, 4-dioxan-10-one) that belongs to a benzoxazine subgroup characterized by greater structural rigidity and higher potency. CX614 enhanced the size (amplitude and duration) of field excitatory postsynaptic potentials in hippocampal slices and autaptically evoked excitatory postsynaptic currents in neuronal cultures with EC(50) values of 20 to 40 microM. The compound blocked desensitization (EC(50) = 44 microM) and slowed deactivation of responses to glutamate by a factor of 8.4 in excised patches. Currents through homomeric, recombinant AMPA receptors were enhanced with EC(50) values that did not differ greatly across GluR1-3 flop subunits (19-37 microM) but revealed slightly lower potency at corresponding flip variants. Competition experiments using modulation of [(3)H]fluorowillardiine binding suggested that CX614 and cyclothiazide share a common binding site but cyclothiazide seems to bind to an additional site not recognized by the ampakine. CX614 did not reverse the effect of GYKI 52466 on responses to brief glutamate pulses, which indicates that they act through separate sites, a conclusion that was confirmed in binding experiments. In sum, these results extend prior evidence that ampakines are effective in enhancing synaptic responses, most likely by slowing deactivation, and that their effects are exerted through sites that are only in part shared with other modulators.

Synaptic plasticity and memory: an evaluation of the hypothesis

Changing the strength of connections between neurons is widely assumed to be the mechanism by which memory traces are encoded and stored in the central nervous system. In its most general form, the synaptic plasticity and memory hypothesis states that "activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the information storage underlying the type of memory mediated by the brain area in which that plasticity is observed." We outline a set of criteria by which this hypothesis can be judged and describe a range of experimental strategies used to investigate it. We review both classical and newly discovered properties of synaptic plasticity and stress the importance of the neural architecture and synaptic learning rules of the network in which it is embedded. The greater part of the article focuses on types of memory mediated by the hippocampus, amygdala, and cortex. We conclude that a wealth of data supports the notion that synaptic plasticity is necessary for learning and memory, but that little data currently supports the notion of sufficiency.

NMDA-induced changes in a cortical network in vivo are prevented by AMPA

Analogues of glutamic acid including N-methyl-D-aspartic acid (NMDA) depolarise neurones of the cerebral cortex in vivo and thus change the size of the somatosensory evoked potentials (SEPs). The potentials recover rapidly despite maintained superfusion with NMDA, suggesting a form of neuronal desensitisation or network adaptation. In this study potentials were evoked at the cortical surface by electrical stimulation of the contralateral forepaw and compounds applied topically to the cortical surface by a cortical cup. NMDA at 50-250 microM caused a concentration-dependent decrease in the amplitude of the SEPs, with the highest concentration always abolishing them. AMPA at 50 microM did not affect evoked potentials when applied alone, but prevented the NMDA. Such AMPA-NMDA interactions were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and enhanced by cyclothiazide (which prevents AMPA desensitisation). Superfusion with potassium did not change sensitivity to NMDA. These results suggest that, in the rat cerebral cortex in vivo, activation of AMPA receptors can induce a loss of the network response to activation of NMDA receptors. Such a phenomenon may have physiological and therapeutic implications.

Interactions of glutamate receptor agonists with long-term potentiation in the rat hippocampal slice

Previous work has described the apparent desensitisation of neuronal networks in the rat neocortex to amino acid agonists, following prior exposure several minutes earlier. Since long-term potentiation is believed to involve activation of amino acid receptors, we have now sought to determine whether long-term potentiation can modify the sensitivity of neurones to glutamate receptor agonists in rat hippocampal slices. Responses were measured as the change in population spike or postsynaptic potential (e.p.s.p.) size. Two applications of N-methyl-D-aspartate (NMDA), quinolinic acid, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or kainate, 45 min apart, did not exhibit any apparent desensitisation. However, the induction of long-term potentiation produced a marked loss of sensitivity to quinolinic acid, with smaller effects on NMDA, AMPA and kainate responses. No marked changes were obtained of e.p. s.p. size. In order to localise the cellular sites of these changes, agonists were also applied by microiontophoresis to the cell bodies or dendritic regions of CA1 neurones. Responses to quinolinic acid showed apparent desensitisation at both sites, whereas no decrease was observed in responses to NMDA or AMPA application. The induction of long-term potentiation again produced a decrease in the size of responses to NMDA and AMPA. Inhibition of nitric oxide (NO) synthase prevented the long-term potentiation-induced loss of responsiveness to NMDA, but not AMPA, implying a role for NO in the loss of NMDA sensitivity. Recordings of single cell activity during the iontophoretic application of agonists and induction of long-term potentiation showed that responses to NMDA were often suppressed to a greater extent than to quinolinic acid. The results indicate that long-term potentiation can modify the sensitivity of hippocampal neurones to glutamate receptor agonists, and that differences exist in the pharmacology of NMDA and quinolinic acid.

Differential modulation of the GYKI 53784-induced inhibition of AMPA currents by various AMPA-positive modulators in cerebellar Purkinje cells

The effects of various (S)-alpha-amino-3-hydroxy-5-methyl-4-izoxazole-propionate (AMPA) receptor modulators on AMPA-induced whole-cell currents were compared in isolated rat cerebellar Purkinje cells. The positive modulators, aniracetam, cyclothiazide, 1-(1, 3-benzodioxol-5-ylcarbonyl)-piperidine (1-BCP), and 1-(quinoxaline-6-ylcarbonyl)-piperidine (BDP-12), dose-dependently potentiated the steady-state component of AMPA currents. The negative modulator, (-)1-(4-aminophenyl)-4-methyl-7, 8-methylenedioxy-4,5-dihydro-3-methylcarbamoyl-2,3-benzodiazepine (GYKI 53784), dose-dependently suppressed AMPA responses. Its concentration-response curve was shifted to the right in a parallel fashion by all positive modulators, indicating a competitive type of interaction. However, the relative potencies of the positive modulators were different with regard to the enhancement of AMPA responses and the reversal of GYKI 53784-induced inhibition, respectively. It is supposed that positive modulators act at multiple allosteric sites and that they interact with GYKI 53784 at only one of these sites.

Therapeutic potential of positive AMPA receptor modulators in the treatment of neurological disease

Excitatory neurotransmission in the CNS depends heavily upon alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-type glutamate receptors. Derangements in AMPA receptor mediated synaptic transmission may be a contributing factor in neurological and neurodegenerative diseases and could be a target for therapeutic intervention. Recently, drugs that positively modulate AMPA receptors have been identified, having differential effects upon certain AMPA receptor subunits and different effects upon physiological properties of AMPA receptors. These drugs facilitate AMPA receptor mediated processes and may have beneficial therapeutic effects. For example, certain AMPA modulators facilitate long-term potentiation, which is considered a cellular mechanism that may be important for memory storage and they also facilitate memory encoding in behavioural experiments. Thus, AMPA modulators might ameliorate memory deficits that occur in dementia, such as Alzheimer's disease (AD). However, AMPA receptor mediated excitotoxicity may occur with excessive AMPA receptor activation which occurs in seizures or ischaemia and positive AMPA modulators could promote neuronal injury in those conditions. Ultimately, the clinical utility of positive AMPA modulators will be dependent upon understanding the role of AMPA receptors in certain neurological disorders, identifying receptor subtypes involved in specific neurological disorders and developing drugs with selective actions upon specific AMPA receptor properties that also possess receptor subtype specificity. Currently available drugs have provided significant insight into the physiology and structural determinants of important AMPA receptor properties and some insight into potential clinical uses as well as potential dangers of such drugs.

The contributions of GluR2 to allosteric modulation of AMPA receptors

Native AMPA receptor complexes in the CNS are composed of hetero-oligomers of the GluR1-4 subunits, and generally contain the GluR2 subunit. To determine the contributions of GluR2 to pharmacological properties of receptor complexes, the effect of hetero-oligomerization with GluR2 on allosteric modulation of recombinant AMPA receptors was studied. The study of homo-oligomeric GluR2 was facilitated with a site-directed mutant of the pore, GluR2(R607Q), which allowed robust currents from this normally low-conducting subunit. The efficacy of the allosteric modulators was tested on homo-oligomeric GluR1-4, and then compared with hetero-oligomeric GluR1/GluR2, GluR3/GluR2 and GluR4/GluR2. Two selective allosteric modulators were tested, a positive modulator, cyclothiazide, and a negative modulator, LY300164. The results show that the pharmacological properties of homo-oligomeric GluR2 are not significantly different from those of GluR1, GluR3 or GluR4. The apparent affinity of cyclothiazide is not significantly changed upon hetero-oligomerization. However, the extent of potentiation of kainate responses by cyclothiazide is significantly decreased upon hetero-oligomerization. Hetero-oligomerization increases the apparent affinity of LY300164, a (-) isomer of the 2,3-benzodiazepine LY293606. These data indicate that although GluR2 has a dominant effect on the permeation properties, this subunit does not have a similarly dominant effect on pharmacological properties of native receptors. However, the state of hetero-oligomerization can alter the pharmacological properties of AMPA receptors.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Interactions of GYKI 52466 and NBQX with cyclothiazide at AMPA receptors: experiments with outside-out patches and EPSCs in hippocampal neurones

In outside-out patches from cultured hippocampal neurones, glutamate (1 mM) applied for 1 ms evoked currents which rose rapidly (tau(on) 451 +/- 31 micros) to a peak and then deactivated with slower kinetics (1.95 +/- 0.13 ms). Offset time constants were significantly slower with longer application durations (tau(off) 3.10 +/- 0.19, 3.82 +/- 0.25, 4.80 +/- 0.65 and 7.56 +/- 0.65 ms with 10, 20, 100 and 500 ms applications respectively). Desensitization was complete within 100 ms with a similar rate for all application durations (4.74 +/- 0.34 ms with 100 ms applications). GYKI 52466 reduced inward peak currents with an IC50 of 11.7 +/- 0.6 microM and had similar potency on steady-state currents to longer glutamate applications. GYKI 52466 had no significant effect on desensitization or deactivation time constants but caused a modest and significant prolongation of onset kinetics at higher concentrations. Cyclothiazide (100 microM) potentiated steady-state currents 25-fold at 100 ms and caused a modest but significant slowing in onset kinetics (601 +/- 49 micros with 1 ms applications) but a more pronounced prolongation of deactivation time constants (5.55 +/- 0.66 ms with 1 ms applications). In 50% of neuronal patches cyclothiazide completely eliminated desensitization. In those patches with residual desensitization, the rate was not significantly different to control (5.36 +/- 0.43 ms with 100 ms applications). Following 100 ms applications of glutamate, GYKI 52466 had IC50s of 11.7 +/- 1.1 microM and 75.1 +/- 7.0 microM in the absence and presence of cyclothiazide (100 microM) respectively. Onset kinetics were slowed from 400 +/- 20 micros to 490 +/- 30 micros by cyclothiazide (100 microM) and then further prolonged by GYKI 52466 (100 microM) to a double exponential function (tau(on1) 1.12 +/- 0.13 ms and tau(on2) 171.5 +/- 36.5 ms). GYKI 52466 did not re-introduce desensitization but concentration-dependently weakened cyclothiazide's prolongation of deactivation time constants (1 ms applications: 5.01 +/- 0.71, 4.47 +/- 0.80 and 2.28 +/- 0.64 ms with GYKI 52466 30, 100 and 300 microM respectively). NBQX reduced peak current responses with an IC50 of 28.2 +/- 1.3 nM. Paradoxically, steady-state currents with 500 ms applications of glutamate were potentiated from 3.3 +/- 1.2 pA to 29.4 +/- 6.4 pA by NBQX (1 nM). Higher concentrations of NBQX then antagonized this potentiated response. The potency of NBQX in antagonizing steady-state currents to 500 ms applications of glutamate (IC50 120.9 +/- 30.2 nM) was 2-fold less than following 100 ms applications (IC50 67.7 +/- 2.6 nM). NBQX had no effect on rapid onset, desensitization or deactivation time constants. However, a slow relaxation of inhibition was seen with longer applications. NBQX was 2-5-fold less potent against inward currents in the presence of cyclothiazide (100 microM) depending on the application duration but had no effect on the rapid onset, desensitization or deactivation time constants. The same relaxation of inhibition was seen as with NBQX alone. NBQX (1 microM) reduced AMPA receptor-mediated EPSC amplitude to 7 +/- 1% of control with no effect on kinetics. Cyclothiazide (330 microM) caused a 2.8-fold prolongation of the decay time constant (control 26.6 +/- 2.2 ms, cyclothiazide 74.2 +/- 7.6 ms, n = 9). Additional application of NBQX (1 microM) partly reversed this prolongation to 1.9 fold (47.7 +/- 2.5 ms, n = 5). These results support previous findings that cyclothiazide also allosterically influences AMPA receptor agonist/antagonist recognition sites. There were no interactions between NBQX and cyclothiazide on desensitization or deactivation time constants of glutamate-induced currents but clear interactions on EPSC deactivation kinetics. This raises the possibility that the interactions of NBQX, GYKI 52466 and cyclothiazide on AMPA-receptor-mediated EPSC kinetics observed are due to modulation of glutamate-release at presynaptic AMPA receptors.

The diazoxide derivative 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine-S,S-dioxide augments AMPA- and GABA-mediated synaptic responses in cultured hippocampal neurons

The diazoxide derivative 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine-S,S-dioxide (IDRA21) enhances memory and learning in rodents, most likely by potentiating AMPAergic synaptic activity. We examined IDRA21's effect upon AMPAergic synaptic currents and whole-cell glutamate currents in cultured rat hippocampal neurons to determine whether IDRA21 was a partial modulator of AMPA receptor desensitization and deactivation. Comparable to cyclothiazide, IDRA21 prolonged AMPAergic autaptic currents (5.6 times control, EC50 150 microM) and slowed the rate of AMPA deactivation (3 times control) following 1-ms applications of 1 mM glutamate to excised, outside-out membrane patches. IDRA21 also augmented autaptic GABA currents by 27 +/- 8.1%, although it had two opposing effects, reducing the peak amplitude versus prolonging autaptic GABA currents. IDRA21 (200 microM) inhibited whole-cell GABA currents elicited by exogenously applied 1 mM GABA by 41 +/- 11%. At sufficient concentrations, IDRA21 reduced AMPA receptor desensitization and slowed the rate of deactivation, most consistent with full agonist activity with lower potency compared to cyclothiazide. IDRA21 slightly augments GABAergic synaptic currents.

Modulating excitatory synaptic neurotransmission: potential treatment for neurological disease?

Excitatory neurotransmission at many CNS synapses depends upon AMPA-type glutamate receptors. Derangements in AMPA receptor-mediated synaptic transmission may be a contributing factor in neurological and neurodegenerative diseases and could be a target for therapeutic intervention. Drugs that positively modulate AMPA receptors by reducing AMPA receptor desensitization and/or slowing AMPA receptor deactivation, such as thiazide derivative (cyclothiazide, diazoxide, IDRA 21) and benzoylpiperidine derivatives (1-BCP, CX516, aniracetam), facilitate AMPA receptor-mediated processes and may have beneficial therapeutic effects. For example, AMPA modulators facilitate long-term potentiation, which may be important for memory storage, and facilitate memory encoding in behavioral experiments. Thus, AMPA modulators might ameliorate memory deficits that occur in dementia, such as Alzheimer's disease. However, AMPA receptor-mediated excitotoxicity may occur with excessive AMPA receptor activation such as in seizures or ischemia, and positive AMPA modulators would promote neuronal injury under those conditions. Regardless of the ultimate clinical utility of positive AMPA modulators, their discovery and study have already provided significant insight into the physiology and structural determinants of important AMPA receptor properties. This review attempts to synthesize a variety of studies that have utilized these AMPA modulators to gain insight into fundamental as well as clinically relevant AMPA receptor-mediated processes.