Evidence that a positive modulator of AMPA-type glutamate receptors improves delayed recall in aged humans

Epigenome-wide association study on methamphetamine dependence

Repeated abuse of methamphetamine (METH) can cause dependence, repeated relapse of psychotic symptoms, compulsive drug-seeking behaviour, and various neurological symptoms. These long-term biological changes may be associated with epigenetic mechanisms; however, the association between METH use and epigenetic mechanisms has been poorly investigated. Thus, we performed an epigenome-wide association study of METH dependence using genomic DNA extracted from the blood samples of 24 patients with METH dependence and 24 normal controls. All participants were of Japanese descent. We tested the association between METH dependence and DNA methylation using linear regression analysis. We found epigenome-wide significant associations at four CpG sites, one of which occurred in the CNOT1 gene and another in the PUM1 gene. We especially noted the CNOT1 and PUM1 genes as well as several other genes that indicated some degree of association with METH dependence. Among the relatively enriched Gene Ontology terms, we were interested in terms of mRNA metabolism, respirasome, and excitatory extracellular ligand-gated ion channel activity. Among the relatively enriched Kyoto Encyclopedia of Genes and Genome pathways, we noted pathways of several neurological diseases. Our results indicate that genetic changes akin to those in other psychiatric or neurodegenerative disorders may also occur via epigenetic mechanisms in patients with METH dependence.

Distinct contributions of GluA1-containing AMPA receptors of different hippocampal subfields to salience processing, memory and impulse control

Schizophrenia is associated with a broad range of severe and currently pharmacoresistant cognitive deficits. Prior evidence suggests that hypofunction of AMPA-type glutamate receptors (AMPARs) containing the subunit GLUA1, encoded by GRIA1, might be causally related to impairments of selective attention and memory in this disorder, at least in some patients. In order to clarify the roles of GluA1 in distinct cell populations, we investigated behavioural consequences of selective Gria1-knockout in excitatory neurons of subdivisions of the prefrontal cortex and the hippocampus, assessing sustained attention, impulsivity, cognitive flexibility, anxiety, sociability, hyperactivity, and various forms of short-term memory in mice. We found that virally induced reduction of GluA1 across multiple hippocampal subfields impaired spatial working memory. Transgene-mediated ablation of GluA1 from excitatory cells of CA2 impaired short-term memory for conspecifics and objects. Gria1 knockout in CA3 pyramidal cells caused mild impairments of object-related and spatial short-term memory, but appeared to partially increase social interaction and sustained attention and to reduce motor impulsivity. Our data suggest that reduced hippocampal GluA1 expression-as seen in some patients with schizophrenia-may be a central cause particularly for several short-term memory deficits. However, as impulse control and sustained attention actually appeared to improve with GluA1 ablation in CA3, strategies of enhancement of AMPAR signalling likely require a fine balance to be therapeutically effective across the broad symptom spectrum of schizophrenia.

Furazans in Medicinal Chemistry

Incorporation of heterocycles into drug molecules can enhance physical properties and biological activity. A variety of heterocyclic groups is available to medicinal chemists, many of which have been reviewed in detail elsewhere. Oxadiazoles are a class of heterocycle containing one oxygen and two nitrogen atoms, available in three isomeric forms. While the 1,2,4- and 1,3,4-oxadiazoles have seen widespread application in medicinal chemistry, 1,2,5-oxadiazoles (furazans) are less common. This Review provides a summary of the application of furazan-containing molecules in medicinal chemistry and drug development programs from analysis of both patent and academic literature. Emphasis is placed on programs that reached clinical or preclinical stages of development. The examples provided herein describe the pharmacology and biological activity of furazan derivatives with comparative data provided where possible for other heterocyclic groups and pharmacophores commonly used in medicinal chemistry.

Cellular and Molecular Changes in Hippocampal Glutamate Signaling and Alterations in Learning, Attention, and Impulsivity Following Prenatal Nicotine Exposure

Over 70 million European pregnant women are smokers during their child-bearing years. Consumption of tobacco-containing products during pregnancy is associated with several negative behavioral outcomes for the offspring, including a higher susceptibility for the development of attention-deficit/hyperactive disorder (ADHD). In efforts to minimize fetal exposure to tobacco smoke, many women around the world switch to nicotine replacement therapies (NRTs) during the gestational period; however, prenatal nicotine exposure (PNE) in any form has been associated with alterations in cognitive processes, including learning, memory, and attention. These processes are controlled by glutamatergic signaling of hippocampal pyramidal neurons within the CA1 region, suggesting actions of nicotine on glutamatergic transmission in this region if present prenatally. Accordingly, we aimed to investigate hippocampal glutamatergic function following PNE treatment in NMRI mice employing molecular, cellular electrophysiology, and pharmacological approaches, as well as to evaluate cognition in the rodent continuous performance task (rCPT), a recently developed mouse task allowing assessment of learning, attention, and impulsivity. PNE induced increases in the expression levels of mRNA coding for different glutamate receptors and subunits within the hippocampus. Functional alterations in AMPA and NMDA receptors on CA1 pyramidal neurons of PNE mice were suggestive of higher GluA2-lacking and lower GluN2A-containing receptors, respectively. Finally, PNE was associated with reduced learning, attention, and enhanced impulsivity in the rCPT. Alterations in glutamatergic functioning in CA1 neurons parallel changes seen in the spontaneously hypertensive rat ADHD model and likely contribute to the lower cognitive performance in the rCPT.

Electrophysiological characterization of a novel AMPA receptor potentiator, TAK-137, in rat hippocampal neurons

We have recently discovered an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) potentiator TAK-137, 9-(4-phenoxyphenyl)-3,4-dihydropyrido[2,1-c][1,2,4] thiadiazine 2,2-dioxide with little agonistic effect. Under preclinical evaluation, TAK-137 demonstrated potent pro-cognitive effects with lower risks of seizure and bell-shaped dose response than LY451646, a potent AMPA-R potentiator, in rodents and monkeys. In this study, using rat primary cultured hippocampal neurons we explored the electrophysiological characterization of TAK-137 on native AMPA-Rs. TAK-137 dose-dependently enhanced AMPA-induced inward currents; its potency in the presence of AMPA was comparable to that of LY451646. The inward currents enhanced by TAK-137 were almost completely inhibited by GYKI53655, a selective AMPA-R blocker. Moreover, TAK-137 did not affect N-methyl-_D-aspartate (NMDA)-activated inward currents, which suggests the AMPA-R-selective activation by TAK-137. In the absence of AMPA-R agonist, LY451646 at 30 μM induced slowly developing large inward currents, whereas TAK-137 at 30 μM exhibited a slight impact on baseline holding currents, further supporting the lower agonistic properties of TAK-137 than LY451646. Similar to LY451646, TAK-137 also increased the potency and binding affinity of AMPA for AMPA-Rs. These results indicate that TAK-137 is a highly potent and selective potentiator with little agonistic effect against native AMPA-Rs. Much greater agonistic effects of LY451646 than of TAK-137 may contribute to the increased risks of seizure and bell-shaped dose response in vivo.

A 24-week double-blind placebo-controlled study of the efficacy and safety of the AMPA modulator S47445 in patients with mild to moderate Alzheimer's disease and depressive symptoms

Introduction

S47445 is a novel positive allosteric modulator of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors that may emerge as a favorable candidate for the symptomatic treatment of cognitive and depressive disorders in patients suffering from Alzheimer's disease (AD) of mild to moderate severity and with depressive symptoms.

Methods

For this double-blind, placebo-controlled 24-week phase II trial, 520 outpatients aged between 55 and 85 years, with probable AD at mild to moderate stages (a Mini-Mental State Examination score of 24-15 inclusive) and exhibiting depressive symptoms (Cornell Scale for Depression in Dementia [CSDD] ≥ 8) were recruited in twelve countries and randomized to 3 doses of S47445 (5-15-50 mg) or placebo. The primary end point was the change from baseline in the 11-item Alzheimer's Disease Assessment Scale-Cognitive subscale (ADAS-Cog) total score at week 24. Secondary measures included the Disability Assessment for Dementia, Mini-Mental State Examination, ADAS-Cog 13-item, CSDD, Clinical Global Impression of Change (Alzheimer's Disease Cooperative Study-CGIC), Neuropsychiatric Inventory (NPI), and safety criteria.

Results

Baseline characteristics were comparable between the 4 groups. After 24 weeks, no statistically significant treatment difference was demonstrated between S47445 (5, 15 or 50 mg/d) and placebo on cognition (ADAS-Cog), function (Disability Assessment for Dementia), or depressive symptoms (CSDD). An improvement on neuropsychiatric symptoms assessed by NPI was evidenced at the lower dose 5 mg/d (Δ -2.55, P = .023, post hoc analysis) compared to placebo. CSDD and total NPI scores improved in all groups including placebo. There were no specific and/or unexpected safety signals observed with any of the S47445 doses.

Discussion

S47445 administered for 24 weeks was safe and well tolerated by patients with mild to moderate AD; the compound did not show significant benefits over placebo on cognition, function, or depressive symptoms.

Aniracetam does not improve working memory in neurologically healthy pigeons

Understanding the effects of cognitive enhancing drugs is an important area of research. Much of the research, however, has focused on restoring memory following some sort of disruption to the brain, such as damage or injections of scopolamine. Aniracetam is a positive AMPA-receptor modulator that has shown promise for improving memory under conditions when the brain has been damaged, but its effectiveness in improving memory in neurologically healthy subjects is unclear. The aim of the present study was to examine the effects of aniracetam (100mg/kg and 200 mg/kg) on short-term memory in "neurologically healthy" pigeons. Pigeons were administered aniracetam via either intramuscular injection or orally, either 30 or 60 minutes prior to testing on a delayed matching-to-sample task. Aniracetam had no effect on the pigeons' memory performance, nor did it affect response latency. These findings add to the growing evidence that, while effective at improving memory function in models of impaired memory, aniracetam has no effect in improving memory in healthy organisms.

Tarps differentially affect the pharmacology of ampakines

Transmembrane AMPA receptor regulatory proteins (TARPs) govern AMPA receptor cell surface expression and distinct physiological properties including agonist affinity, desensitization and deactivation kinetics. The prototypical TARP, STG or γ2 and TARPs γ3, γ4, γ7 and γ8 are all expressed to varying degrees in the mammalian brain and differentially regulate AMPAR gating parameters. Positive allosteric AMPA receptor modulators or ampakines alter receptor rates of agonist binding/unbinding, channel opening and can offset receptor desensitization and deactivation. The effects of the two ampakines, CX614 and cyclothiazide (CTZ) were evaluated on homomeric GluR1-flip receptors and GluR2-flop receptors expressed on HEK293 cells by transient transfection with or without different TARPs γ2, γ3, γ4 or γ8 genes. γ4 was the most robust TARP in increasing the affinities of CX614 and CTZ on GluR1-flip receptors, but had no such effect on GluR2-flop receptors. However, γ8 gave the most significant increases in affinities of CX614 and CTZ on GluR2-flop. These data show that TARPs differentially affect the surface expression and kinetics of the AMPA receptor, as well as the pharmacology of ampakines for the AMPA receptor. The modulatory effects of TARPs on ampakine pharmacology are complex, being dependent on both the TARP subtype and the AMPA receptor subtypes/isoforms.

Stargazin differentially modulates ampakine gating kinetics and pharmacology

It was previously reported that Stargazin (STG) enhances the surface expression of AMPA receptors, controls receptor gating and slows channel desensitization as an auxiliary subunit of the receptors. Ampakines are a class of AMPA receptor positive allosteric modulators that modify rates of transmitter binding, channel activity and desensitization parameters. As such, they have shown efficacy in animal models of neurodegenerative diseases, where excitatory synaptic transmission is compromised. Given the functional similarities between STG and ampakines, the current study sought to probe interactions between STG and ampakine gating properties. The effects of the high impact ampakines, CX614 and cyclothiazide (CTZ), were compared with homomeric GluR1-flip (Glur1i) and GluR2-flop (Glur2o) receptors expressed in HEK293 cells by transient transfection with or without STG gene. STG dramatically enhanced the surface expression of AMPA receptors and increased glutamate-induced steady-state currents during desensitization. STG also increased ratios of 500 μM kainate and 500 μM glutamate activated steady-state currents. STG reduced association rates of ampakines and differentially affected the dissociation rates for both CX614 and CTZ on desensitized receptors. The estimated Kd value for CX614 was lowered from 340 μM to 70 μM, whereas that for CTZ was lowered from 170 μM to 6 μM by STG. The data suggest that Stargazin can dramatically alter the conformation of the receptor dimer interface where CX614 and CTZ are known to bind. This work also demonstrates the importance of considering STG interactions when developing ampakines to treat neurodegenerative diseases in which AMPAergic signaling is compromised.

Glutamatergic regulation of cognition and functional brain connectivity: insights from pharmacological, genetic and translational schizophrenia research

The pharmacological modulation of glutamatergic neurotransmission to improve cognitive function has been a focus of intensive research, particularly in relation to the cognitive deficits seen in schizophrenia. Despite this effort, there has been little success in the clinical use of glutamatergic compounds as procognitive drugs. Here, we review a selection of the drugs used to modulate glutamatergic signalling and how they impact on cognitive function in rodents and humans. We highlight how glutamatergic dysfunction, and NMDA receptor hypofunction in particular, is a key mechanism contributing to the cognitive deficits observed in schizophrenia and outline some of the glutamatergic targets that have been tested as putative procognitive targets for this disorder. Using translational research in this area as a leading exemplar, namely, models of NMDA receptor hypofunction, we discuss how the study of functional brain network connectivity can provide new insight into how the glutamatergic system impacts on cognitive function. Future studies characterizing functional brain network connectivity will increase our understanding of how glutamatergic compounds regulate cognition and could contribute to the future success of glutamatergic drug validation. Linked Articles This article is part of a themed section on Pharmacology of Cognition: a Panacea for Neuropsychiatric Disease? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.19/issuetoc.

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.

Immunolocalization of AMPA receptor subunits within the enteric nervous system of the mouse colon and the effect of their activation on spontaneous colonic contractions

BACKGROUND

The appropriate expression of specific neurotransmitter receptors within the cellular networks that compose the enteric nervous system (ENS) is central to the regulation of gastrointestinal (GI) functions. While the ENS expression patterns of the neurotransmitter glutamate have been well documented, the localization of its receptors on ENS neurons remains to be fully characterized. We investigated the expression patterns of glutamate receptor AMPA subunits within ENS neurons of the mouse colon and the consequences of their pharmacological activation on spontaneous colonic contractility.

METHODS

RT-PCR was used to detect individual AMPA receptor (GluR 1-4) subunit expression at the mRNA level in mouse colon tissue. Immunohistochemistry and confocal microscopy was used to localize the expression of the GluR1 and 4 subunits in colon tissue. Brain tissue was used as a positive control. Organ bath preparations were used to determine the effect of AMPA receptors activation on the force and frequency of colonic longitudinal smooth muscle spontaneous contractions.

KEY RESULTS

GluR1, 3, 4 mRNA was detected in the mouse colon. Immunoreactivity for GluR1 and 4 subunits was detected on the somatic and dendritic surfaces of subpopulations of neurochemically defined ENS neurons. The pharmacological activation of AMPA receptors increased the force but not frequency of spontaneous colonic contractions.

CONCLUSIONS & INFERENCES

Molecularly distinct AMPA receptor subtypes are differentially expressed within the neural networks of the mouse colon and have a direct role in motility. These data provide the rationale for the development of AMPA-selective ligands for the therapeutic delivery to the GIT in motility disorders.

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.

Innovative mechanisms of action for pharmaceutical cognitive enhancement: A systematic review

Pharmacological cognitive enhancement refers to improvement in cognitive functions after drug use in healthy individuals. This popular topic attracts attention both from the general public and the scientific community. The objective was to explore innovative mechanisms of psychostimulant's action, whose potential effectiveness was assessed in randomized placebo-controlled trials (RCTs). A systematic review was carried out, using the words "attention", "memory", "learning", "executive functions", and "vigilance/wakefulness" combined to "cognitive enhancer" or "smart drug". Methylphenidate, amphetamines, modafinil, nicotine, acetylcholine esterase inhibitors and antidepressants were extensively studied in previous meta-analyses and were not included in the present work. Drugs were classified according to their primary mode of action, namely catecholaminergic drugs (tolcapone, pramipexole, guanfacine), cholinergic drugs (anticholinergics), glutamatergic drugs (ampakines), histaminergic drugs, and non-specified (glucocorticoids). Overall, 50 RCTs were included in the present review. In conclusion, a number of new active drugs were found to improve some cognitive functions, in particular verbal episodic memory. However the number of RCTs was limited, and most of the studies found negative results. Future studies should assess both effectiveness and tolerance of repeated doses administration, and individual variability in dose response (including baseline characteristics and potential genetic polymorphisms). One explanation for the limited number of recent RCTs with new psychostimulants seems to be the ethical debate surrounding pharmaceutical cognitive enhancement in healthy subjects.

Signaling pathways relevant to cognition-enhancing drug targets

Aging is generally associated with a certain cognitive decline. However, individual differences exist. While age-related memory deficits can be observed in humans and rodents in the absence of pathological conditions, some individuals maintain intact cognitive functions up to an advanced age. The mechanisms underlying learning and memory processes involve the recruitment of multiple signaling pathways and gene expression, leading to adaptative neuronal plasticity and long-lasting changes in brain circuitry. This chapter summarizes the current understanding of how these signaling cascades could be modulated by cognition-enhancing agents favoring memory formation and successful aging. It focuses on data obtained in rodents, particularly in the rat as it is the most common animal model studied in this field. First, we will discuss the role of the excitatory neurotransmitter glutamate and its receptors, downstream signaling effectors [e.g., calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase C (PKC), extracellular signal-regulated kinases (ERK), mammalian target of rapamycin (mTOR), cAMP response element-binding protein (CREB)], associated immediate early gene (e.g., Homer 1a, Arc and Zif268), and growth factors [insulin-like growth factors (IGFs) and brain-derived neurotrophic factor (BDNF)] in synaptic plasticity and memory formation. Second, the impact of the cholinergic system and related modulators on memory will be briefly reviewed. Finally, since dynorphin neuropeptides have recently been associated with memory impairments in aging, it is proposed as an attractive target to develop novel cognition-enhancing agents.

Enhanced AMPA receptor function promotes cerebellar long-term depression rather than potentiation

Ampakines are allosteric modulators of AMPA receptors that facilitate hippocampal long-term potentiation (LTP) and learning, and have been considered for the treatment of cognition and memory deficits. Here, we show that the ampakine CX546 raises the amplitude and slows the decay time of excitatory postsynaptic currents (EPSCs) at cerebellar parallel fiber (PF) to Purkinje cell synapses, thus resembling CX546 effects described at hippocampal synapses. Using the fluorescent calcium indicator dye Oregon Green BAPTA-2 and an ultra-high-speed CCD camera, we also monitored calcium transients in Purkinje cell dendrites. In the presence of CX546 in the bath, PF-evoked calcium transients were enhanced and prolonged, suggesting that CX546 not only enhances synaptic transmission, but also boosts dendritic calcium signaling at cerebellar synapses. In contrast to previous observations in the hippocampus, however, CX546 applied during cerebellar recordings facilitates long-term depression (LTD) rather than LTP at PF synapses. These findings show that ampakines selectively modify the LTP–LTD balance depending on the brain area and type of synapse, and may provide tools for the targeted regulation of synaptic memories.

Neuroethical issues in pharmacological cognitive enhancement

Neuroethics is an emerging field that in general deals with the ethics of neuroscience and the neuroscience of ethics. In particular, it is concerned with the ethical issues in the translation of neuroscience to clinical practice and in the public domain. Numerous ethical issues arise when healthy individuals use pharmacological substances known as pharmacological cognitive enhancers (PCEs) for non-medical purposes in order to boost higher-order cognitive processes such as memory, attention, and executive functions. However, information regarding their actual use, benefits, and harms to healthy individuals is currently lacking. Neuroethical issues that arise from their use include the unknown side effects that are associated with these drugs, concerns about the modification of authenticity and personhood, and as a result of inequality of access to these drugs, the lack of distributive justice and competitive fairness that they may cause in society. Healthy individuals might be coerced by social institutions that force them to take these drugs to function better. These drugs might enable or hinder healthy individuals to gain better moral and self-understanding and autonomy. However, how these drugs might achieve this still remains speculative and unknown. Hence, before concrete policy decisions are made, the cognitive effects of these drugs should be determined. The initiation of accurate surveys to determine the actual usage of these drugs by healthy individuals from different sections of the society is proposed. In addition, robust empirical research need to be conducted to delineate not only whether or not these drugs modify complex higher-order cognitive processes but also how they might alter important human virtues such as empathy, moral reasoning, creativity, and motivation in healthy individuals. WIREs Cogn Sci 2014, 5:533-549. doi: 10.1002/wcs.1306 For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The author has declared no conflicts of interest for this article.

Structural analysis of the positive AMPA receptor modulators CX516 and Me-CX516 in complex with the GluA2 ligand-binding domain

Positive allosteric modulators of the ionotropic glutamate receptor A2 (GluA2) can serve as lead compounds for the development of cognitive enhancers. Several benzamide-type (S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor modulators such as aniracetam, CX516 and CX614 have been shown to inhibit the deactivation of AMPA receptors with a less pronounced effect on desensitization. Despite CX516 being an extensively investigated AMPA receptor modulator and one of the few clinically evaluated compounds, the binding mode of CX516 to AMPA receptors has not been reported. Here, the structures of a GluA2 ligand-binding domain mutant in complex with CX516 and the 3-methylpiperidine analogue of CX516 (Me-CX516) are reported. The structures show that the binding modes of CX516 and Me-CX516 are similar to those of aniracetam and CX614 and that there is limited space for substitution at the piperidine ring of CX516. The results therefore support that CX516, like aniracetam and CX614, modulates deactivation of AMPA receptors.

Glutamatergic Approaches for the Treatment of Schizophrenia

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system and plays a key role in most aspects of normal brain function including cognition, learning and memory. Dysfunction of glutamatergic neurotransmission has been implicated in a number of neurological and psychiatric disorders with a growing body of evidence suggesting that hypofunction of glutamatergic neurotransmission via the N-methyl-d-aspartate (NMDA) receptor plays an important role in the pathophysiology of schizophrenia. It thus follows that potentiation of NMDA receptor function via pharmacological manipulation may provide therapeutic utility for the treatment of schizophrenia and a number of different approaches are currently being pursued by the pharmaceutical industry with this aim in mind. These include strategies that target the glycine/d-serine site of the NMDA receptor (glycine transporter GlyT1, d-serine transporter ASC-1 and d-amino acid oxidase (DAAO) inhibitors) together with those aimed at enhancing glutamatergic neurotransmission via modulation of AMPA receptor and metabotropic glutamate receptor function. Such efforts are now beginning to bear fruit with compounds such as the GlyT1 inhibitor RG1678 and mGlu2 agonist LY2140023 proving to have clinical meaningful effects in phase II clinical trials. While more studies are required to confirm long-term efficacy, functional outcome and safety in schizophrenic agents, these agents hold real promise for addressing unmet medical needs, in particular refractory negative and cognitive symptoms, not currently addressed by existing antipsychotic agents.

Therapeutic approaches to age-associated neurocognitive disorders

The United Nations projects that the number of individuals with dementia in developed countries alone will be approximately 36,7 million by the year 2050. International recognition of the significant emotional and economic burden of Alzheimer's disease has been matched by a dramatic increase in the development of pharmacological and nonpharmacological approaches to this illness in the past decade. Changing demographics have underscored the necessity to develop similar approaches for the remediation of the cognitive impairment associated with more benign syndromes, such as mild cognitive impairment (MCI) and age-associated cognitive decline (AACD). The present article aims to provide an overview of the most current therapeutic approaches to age-associated neurocognitive disorders. Additionally, it discusses the conceptual and methodological issues that surround the design, implementation, and interpretation of such approaches.

Piracetam, an AMPAkine drug, facilitates memory consolidation in the day-old chick

Piracetam is an AMPAkine drug that may have a range of different mechanisms at the cellular level, and which has been shown to facilitate memory, amongst its other effects. This series of experiments demonstrated that a 10mg/kg dose of piracetam facilitated memory consolidation in the day-old chick when injected from immediately until 120min after weak training (i.e. using a 20% v/v concentration of methyl anthranilate) with the passive avoidance learning task. Administration of piracetam immediately after training led to memory facilitation which lasted for up to 24h following training. This dose of the AMPAkine was not shown to facilitate memory reconsolidation. These findings support the contention that application of the AMPAkine piracetam facilitates memory using a weak training task, and extend the range of actions previously noted with NMDA-related agents to those which also facilitate the AMPA receptor.

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'.

DRUG FOCUS: S 18986: A positive allosteric modulator of AMPA-type glutamate receptors pharmacological profile of a novel cognitive enhancer

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) type glutamate receptors are critical for synaptic plasticity and induction of long-term potentiation (LTP), considered as one of the synaptic mechanisms underlying learning and memory. Positive allosteric modulators of AMPA receptors could provide a therapeutic approach to the treatment of cognitive disorders resulting from aging and/or neurodegenerative diseases, such as Alzheimer disease (AD). Several AMPA potentiators have been described in the last decade, but for the moment their clinical efficacy has not been demonstrated due to the complexity of the target, AMPA receptors, and the difficulty in studying cognition in animals and humans. A better understanding of the mechanism of action of this type of drug remains an important issue, if knowledge of these compounds is to be increased and if this novel therapeutic approach is to be an interesting research area. Among the AMPA potentiators, S 18986 is emerging as a new selective positive allosteric modulator of AMPA-type glutamate receptors. S 18986, as with other positive AMPA receptor modulators, increased induction and maintenance of LTP in the hippocampus as well as the expression of brain-derived neurotrophic factor (BDNF) both in vitro and in vivo. Its cognitive-enhancing properties have been demonstrated in various behavioral models (procedural, spatial, "episodic," working, and relational/declarative memory) in young-adult and aged rodents. It is interesting to note that memory-enhancing effects appeared more robust in middle-aged animals compared with aged ones and in "episodic" and spatial memory tasks. From these results, S 18986 is expected to treat memory deficits associated with early cerebral aging and neurological diseases in elderly people.

The neuropharmacology of implicit learning

Two decades of pharmacologic research on the human capacity to implicitly acquire knowledge as well as cognitive skills and procedures have yielded surprisingly few conclusive insights. We review the empirical literature of the neuropharmacology of implicit learning. We evaluate the findings in the context of relevant computational models related to neurotransmittors such as dopamine, serotonin, acetylcholine and noradrenalin. These include models for reinforcement learning, sequence production, and categorization. We conclude, based on the reviewed literature, that one can predict improved implicit acquisition by moderately elevated dopamine levels and impaired implicit acquisition by moderately decreased dopamine levels. These effects are most prominent in the dorsal striatum. This is supported by a range of behavioral tasks in the empirical literature. Similar predictions can be made for serotonin, although there is yet a lack of support in the literature for serotonin involvement in classical implicit learning tasks. There is currently a lack of evidence for a role of the noradrenergic and cholinergic systems in implicit and related forms of learning. GABA modulators, including benzodiazepines, seem to affect implicit learning in a complex manner and further research is needed. Finally, we identify allosteric AMPA receptors modulators as a potentially interesting target for future investigation of the neuropharmacology of procedural and implicit learning.

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.

Regulation of ionotropic glutamate receptors by their auxiliary subunits

Glutamate receptors are major excitatory receptors in the brain. Recent findings have established auxiliary subunits of glutamate receptors as critical modulators of synaptic transmission, synaptic plasticity, and neurological disorder. The elucidation of the molecular rules governing glutamate receptors and subunits will improve our understanding of synapses and of neural-circuit regulation in the brain.

High-throughput screening in embryonic stem cell-derived neurons identifies potentiators of alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate-type glutamate receptors

Stem cell biology offers advantages to investigators seeking to identify new therapeutic molecules. Specifically, stem cells are genetically stable, scalable for molecular screening, and function in cellular assays for drug efficacy and safety. A key hurdle for drug discoverers of central nervous system disease is a lack of high quality neuronal cells. In the central nervous system, alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA) subtype glutamate receptors mediate the vast majority of excitatory neurotransmissions. Embryonic stem (ES) cell protocols were developed to differentiate into neuronal subtypes that express AMPA receptors and were pharmacologically responsive to standard compounds for AMPA potentiation. Therefore, we hypothesized that stem cell-derived neurons should be predictive in high-throughput screens (HTSs). Here, we describe a murine ES cell-based HTS of a 2.4 x 10(6) compound library, the identification of novel chemical "hits" for AMPA potentiation, structure function relationship of compounds and receptors, and validation of chemical leads in secondary assays using human ES cell-derived neurons. This reporting of murine ES cell derivatives being formatted to deliver HTS of greater than 10(6) compounds for a specific drug target conclusively demonstrates a new application for stem cells in drug discovery. In the future new molecular entities may be screened directly in human ES or induced pluripotent stem cell derivatives.

Reversal of cognitive deficits by an ampakine (CX516) and sertindole in two animal models of schizophrenia--sub-chronic and early postnatal PCP treatment in attentional set-shifting

RATIONALE

Therapies treating cognitive impairments in schizophrenia especially deficits in executive functioning are not available at present.

OBJECTIVE

The current study evaluated the effect of ampakine CX516 in reversing deficits in executive functioning as represented in two animal models of schizophrenia and assessed by a rodent analog of the intradimensional-extradimensional (ID-ED) attentional set-shifting task. The second generation antipsychotic, sertindole, provided further validation of the schizophrenia-like disease models.

METHODS

Animals were subjected to (a) sub-chronic or (b) early postnatal phencyclidine (PCP) treatment regimes: (a) Administration of either saline or PCP (5 mg/kg, intraperitonally b.i.d. for 7 days) followed by a 7-day washout period and testing on day 8. (b) On postnatal days (PNDs) 7, 9, and 11, rats were subjected to administration of either saline or PCP (20 mg/kg, subcutaneously (s.c.)) and tested on PNDs 56-95, after reaching adulthood. The single test session required rats to dig for food rewards in a series of discriminations following acute administration of either vehicle, or CX516 (5-40 mg/kg, s.c.), or sertindole (1.25 mg/kg, perorally).

RESULTS

The specific extradimensional deficits produced by sub-chronic or early postnatal PCP treatment were significantly attenuated by sertindole and dose-dependently by CX516.

CONCLUSION

Findings here further establish PCP treatment as model of executive functioning deficits related to schizophrenia and provide evidence that direct glutamatergic interventions could improve these, when assessed in the ID-ED attentional set-shifting task.

Modulation of agonist binding to AMPA receptors by 1-(1,4-benzodioxan-6-ylcarbonyl)piperidine (CX546): differential effects across brain regions and GluA1-4/transmembrane AMPA receptor regulatory protein combinations

Ampakines are cognitive enhancers that potentiate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor currents and synaptic responses by slowing receptor deactivation. Their efficacy varies greatly between classes of neurons and brain regions, but the factor responsible for this effect remains unclear. Ampakines also increase agonist affinity in binding tests in ways that are related to their physiological action. We therefore examined 1) whether ampakine effects on agonist binding vary across brain regions and 2) whether they differ across receptor subunits expressed alone and together with transmembrane AMPA receptor regulatory proteins (TARPs), which associate with AMPA receptors in the brain. We found that the maximal increase in agonist binding (E(max)) caused by the prototypical ampakine 1-(1,4-benzodioxan-6-ylcarbonyl)piperidine (CX546) differs significantly between brain regions, with effects in hippocampus and cerebellum being nearly three times larger than that in thalamus, brainstem, and striatum, and cortex being intermediate. These differences can be explained at least in part by regional variations in receptor subunit and TARP expression because combinations prevalent in hippocampus (GluA2 with TARPs gamma3 and gamma8) exhibited E(max) values nearly twice those of combinations abundant in thalamus (GluA4 with gamma2 or gamma4). TARPs seem to be critical because GluA2 and GluA4 alone had comparable E(max) and also because hippocampal and thalamic receptors had similar E(max) after solubilization with Triton X-100, which probably removes associated proteins. Taken together, our data suggest that variations in physiological drug efficacy, such as the 3-fold difference previously seen in recordings from hippocampus versus thalamus, may be explained by region-specific expression of GluA1-4 as well as TARPs.

Pharmaceutical treatment for cognitive deficits in Alzheimer's disease and other neurodegenerative conditions: exploring new territory using traditional tools and established maps

RATIONALE

Over 30 years ago, we began to develop a nonhuman primate model to study cognitive deficits of age-related neurodegenerative diseases and their neuroanatomical-neurochemical underpinnings for purposes of translating this work toward first pharmacotherapies. This effort produced several notable findings that eventually received consensus support, which we have been asked to review.

OBJECTIVES

A discussion of these findings, in the context of issues and obstacles confronted and principles applied, might facilitate the development of even more effective models and treatments, not only for Alzheimer's disease (AD) but for many other disorders involving cognitive deficits.

RESULTS

Collectively, our research provided first evidence of the following: aged primates can be used as 'models' for human age-related neurodegenerative diseases; key cognitive deficits in early AD share important conceptual similarities to deficits in both aged monkeys as well as non-demented humans (e.g., age-associated memory impairment and mild cognitive impairment); pharmacological intervention can reduce age-related cognitive impairments in animals that are conceptually similar to those seen in human diseases, including AD; cholinergics would likely be the first approved therapeutics for AD; and that many other classes of drugs would not likely succeed.

CONCLUSIONS

Despite the early promise shown by behavioral/functional approaches to develop treatment strategies, the dramatic shift in focus away from behavioral outcomes in animal neurodegenerative research that began 20 years ago has compromised further progress and continues to impede our ability to understand how these diseases impair human cognition and what pathways might lead to effective therapies. Principles applied successfully in the past should provide guidance for facilitating efforts in the future.

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

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

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.

The AMPA modulator S 18986 improves declarative and working memory performances in aged mice

The aim of this study was to further characterize the memory-enhancing profile of S 18986 a positive allosteric modulator of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors. S 18986 was studied in two mouse models of age-related memory deficits, using radial maze paradigms involving long-term/declarative memory and short-term/working memory. Aged mice exhibited severe deficits when compared with their younger counterparts in the two behavioural tests. S 18986 at the dose of 0.1 mg/kg selectively improved aged mouse performance in the test of long-term/declarative memory flexibility and exerted a beneficial effect on short-term retention of successive arm-visits in the short-term/working memory test. This study confirms the memory-enhancing properties of S 18986 and, in line with emerging data on multiple AMPA modulators, highlights the relevance of targeting AMPA receptors in the development of new memory enhancers.

Cholinergic and glutamatergic alterations beginning at the early stages of Alzheimer disease: participation of the phospholipase A2 enzyme

RATIONALE

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of AD, and thus, treatment strategies should address impairments in both systems. Evidence suggests the involvement of phospholipase A(2) (PLA(2)) enzyme in memory impairment and neurodegeneration in AD via actions on both cholinergic and glutamatergic systems.

OBJECTIVES

To review cholinergic and glutamatergic alterations underlying cognitive impairment and neuropathology in AD and attempt to link PLA(2) with such alterations.

METHODS

Medline databases were searched (no date restrictions) for published articles with links among the terms Alzheimer disease (mild, moderate, severe), mild cognitive impairment, choline acetyltransferase, acetylcholinesterase, NGF, NGF receptor, muscarinic receptor, nicotinic receptor, NMDA, AMPA, metabotropic glutamate receptor, atrophy, glucose metabolism, phospholipid metabolism, sphingolipid, membrane fluidity, phospholipase A(2), arachidonic acid, attention, memory, long-term potentiation, beta-amyloid, tau, inflammation, and reactive species. Reference lists of the identified articles were checked to identify additional studies of interest.

RESULTS

Overall, results suggest the hypothesis that persistent inhibition of cPLA(2) and iPLA(2) isoforms at early stages of AD may play a central role in memory deficits and beta-amyloid production through down-regulation of cholinergic and glutamate receptors. As the disease progresses, beta-amyloid induced up-regulation of cPLA(2) and sPLA(2) isoforms may play critical roles in inflammation and oxidative stress, thus participating in the neurodegenerative process.

CONCLUSION

Activation and inhibition of specific PLA(2) isoforms at different stages of AD could be of therapeutic importance and delay cognitive dysfunction and neurodegeneration.

Calcium-permeable AMPA receptors containing Q/R-unedited GluR2 direct human neural progenitor cell differentiation to neurons

We identify calcium-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors on human neural progenitor cells (NPCs) and present a physiological role in neurogenesis. RNA editing of the GluR2 subunit at the Q/R site is responsible for making most AMPA receptors impermeable to calcium. Because a single-point mutation could eliminate the need for editing at the Q/R site and Q/R-unedited GluR2 exists during embryogenesis, the Q/R-unedited GluR2 subunit presumably has some important actions early in development. Using calcium imaging, we found that NPCs contain calcium-permeable AMPA receptors, whereas NPCs differentiated to neurons and astrocytes express calcium-impermeable AMPA receptors. We utilized reverse-transcription polymerase chain reaction and BbvI digestion to demonstrate that NPCs contain Q/R-unedited GluR2, and differentiated cells contain Q/R-edited GluR2 subunits. This is consistent with the observation that the nuclear enzyme responsible for Q/R-editing, adenosine deaminase (ADAR2), is increased during differentiation. Activation of calcium-permeable AMPA receptors induces NPCs to differentiate to the neuronal lineage and increases dendritic arbor formation in NPCs differentiated to neurons. AMPA-induced differentiation of NPCs to neurons is abrogated by overexpression of ADAR2 in NPCs. This elucidates the role of AMPA receptors as inductors of neurogenesis and provides a possible explanation for why the Q/R editing process exists.

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.

AMPA receptors and stargazin-like transmembrane AMPA receptor-regulatory proteins mediate hippocampal kainate neurotoxicity

Naturally occurring glutamate analogs, such as kainate and domoate, which cause excitotoxic shellfish poisoning, induce nondesensitizing responses at neuronal alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. In addition to acting on AMPA receptors, kainate and domoate also activate high-affinity kainate-type glutamate receptors. The receptor type that mediates their neurotoxicity remains uncertain. Here, we show that the transmembrane AMPA receptor-associated protein (TARP) gamma-2 (or stargazin) and the related TARP gamma-8 augment responses to kainate and domoate by making these neurotoxins more potent and more efficacious AMPA receptor agonists. Genetic deletion of hippocampal enriched gamma-8 selectively abolishes sustained depolarizations in hippocampus mediated by kainate activation of AMPA receptors. gamma-8 knockout mice display typical kainate-induced seizures; however, the associated neuronal cell death in the hippocampus is attenuated in mice lacking gamma-8. This work decisively demonstrates that TARP-associated AMPA receptors mediate kainate neurotoxicity and identifies TARPs as targets for modulating neurotoxic properties of AMPA receptors.

Mutations in ionotropic AMPA receptor 3 alter channel properties and are associated with moderate cognitive impairment in humans

Ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (iGluRs) mediate the majority of excitatory synaptic transmission in the CNS and are essential for the induction and maintenance of long-term potentiation and long-term depression, two cellular models of learning and memory. We identified a genomic deletion (0.4 Mb) involving the entire GRIA3 (encoding iGluR3) by using an X-array comparative genomic hybridization (CGH) and four missense variants (G833R, M706T, R631S, and R450Q) in functional domains of iGluR3 by sequencing 400 males with X-linked mental retardation (XLMR). Three variants were found in males with moderate MR and were absent in 500 control males. Expression studies in HEK293 cells showed that G833R resulted in a 78% reduction of iGluR3 due to protein misfolding. Whole-cell recording studies of iGluR3 homomers in HEK293 cells revealed that neither iGluR3-M706T (S2 domain) nor iGluR3-R631S (near channel core) had substantial channel function, whereas R450Q (S1 domain) was associated with accelerated receptor desensitization. When forming heteromeric receptors with iGluR2 in HEK293 cells, all four iGluR3 variants had altered desensitization kinetics. Our study provides the genetic and functional evidence that mutant iGluR3 with altered kinetic properties is associated with moderate cognitive impairment in humans.

Comparison of single vs. multiple administrations of the AMPA receptors modulator S 18986 in the object recognition task in rats

The present study aimed at defining the best scheme of administration of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-positive modulator (S)-2,3-dihydro-[3,4]-cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide (S 18986) [once daily (o.d.) administration of 1 mg/kg for 3 days vs. three times daily (t.i.d.) administration of 0.3 mg/kg for 3 days] to get an optimal procognitive activity in the object recognition task in rats. Memory performance [Recognition Index (RI)] of rats was significantly improved 1 h (RI = 41%, P < 0.01) and 3 h (RI = 46%, P < 0.001) following oral administration of S 18986 (1 mg/kg, o.d.) when compared with animals receiving the vehicle (RI = 6%). When the interval between administration and testing was increased to 6 h and 9 h, no statistically significant improvement in memory performance was observed (RI = 42% for 6 h and RI = 18% for 9 h vs. 20% for the vehicle group). When S 18986 was administered at 0.3 mg/kg t.i.d., no statistically significant improvement in memory performance was observed (RI = 36%). These findings show a long-lasting efficacy of the AMPA receptor allosteric modulator in the object recognition task despite a short half-life in plasma and in brain (approximately 1 h). Accordingly, multiple administrations of S 18986 are not required to obtain a maximal efficacy in this paradigm, because a o.d. schedule of administration leads to a powerful procognitive activity.

Acute effects of the ampakine farampator on memory and information processing in healthy elderly volunteers

Ampakines act as positive allosteric modulators of AMPA-type glutamate receptors and facilitate hippocampal long-term potentiation (LTP), a mechanism associated with memory storage and consolidation. The present study investigated the acute effects of farampator, 1-(benzofurazan-5-ylcarbonyl) piperidine, on memory and information processes in healthy elderly volunteers. A double-blind, placebo-controlled, randomized, cross-over study was performed in 16 healthy, elderly volunteers (eight male, eight female; mean age 66.1, SD 4.5 years). All subjects received farampator (500 mg) and placebo. Testing took place 1 h after drug intake, which was around Tmax for farampator. Subjects performed tasks assessing episodic memory (wordlist learning and picture memory), working and short-term memory (N-back, symbol recall) and motor learning (maze task, pursuit rotor). Information processing was assessed with a tangled lines task, the symbol digit substitution test (SDST) and the continuous trail making test (CTMT). Farampator (500 mg) unequivocally improved short-term memory but appeared to impair episodic memory. Furthermore, it tended to decrease the number of switching errors in the CTMT. Drug-induced side effects (SEs) included headache, somnolence and nausea. Subjects with SEs had significantly higher plasma levels of farampator than subjects without SEs. Additional analyses revealed that in the farampator condition the group without SEs showed a significantly superior memory performance relative to the group with SEs. The positive results on short-term memory and the favorable trends in the trail making test (CTMT) are interesting in view of the development of ampakines in the treatment of Alzheimer's disease and schizophrenia.

AMPA potentiator treatment of cognitive deficits in Alzheimer disease

OBJECTIVE

To investigate the efficacy and safety of the positive alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid modulator LY451395 in patients with mild to moderate Alzheimer disease (AD) (Mini-Mental State Examination scores 14 to 26).

METHODS

One hundred eighty-one patients were randomized to treatment in an 11-week, double-blind, placebo-controlled trial. Patients received either LY451395 0.2 mg BID for 28 days and 1.0 mg BID thereafter (n = 90) or placebo (n = 91). The primary outcome measurement was the Alzheimer's Disease Assessment Scale-Cognitive Subscale (ADAS-Cog) with several secondary outcome measurements: Clinician's Interview-Based Impression of Change, Trail Making Part A, Stylus Tapping Test, Single Digit Modality Test, and Neuropsychiatric Inventory (NPI).

RESULTS

Baseline demographics were similar between the two groups. Patients did not show any mean change from baseline in the ADAS-Cog after treatment with LY451395 for 4 weeks (p = 0.60) or 8 weeks (p = 0.83). The only secondary outcome measurement that showed changes from baseline compared with placebo was the NPI Total Score: p = 0.06 (marginal significance) after 4 weeks of treatment and p = 0.03 after 8 weeks of treatment. Ninety-two percent of LY451395-treated patients and 95% of placebo-treated patients completed the trial. Adverse events were experienced by 83% of LY451395-treated patients and 86% of placebo-treated patients, the majority of which were rated mild in severity.

CONCLUSION

Patients treated with LY451395 did not show a statistically significant separation from patients taking placebo on the Alzheimer's Disease Assessment Scale-Cognitive Subscale, the primary outcome measure.

Stargazin interacts functionally with the AMPA receptor glutamate-binding module

Neuronal AMPA receptors comprise pore forming glutamate receptor (GluR) proteins and auxiliary transmembrane AMPA receptor regulatory (TARP) subunits. TARPs traffic AMPA receptors to synapses and regulate channel gating. Both intracellular and extracellular regions in TARPs regulate AMPA receptors; however, the details for these interactions remain unknown. Here, we employ site-directed mutagenesis to determine functional interactions between GluR1 and the prototypical TARP, stargazin. We find that a point mutation in the glutamate-binding region of GluR1 corresponding to the Lurcher allele of GluRdelta2, abolishes stargazin's effects on receptor trafficking and channel gating. A point mutation that prevents receptor desensitization modulates the effects of stargazin on channel gating but preserves receptor trafficking. These studies identify a functional interaction of stargazin with the extracellular glutamate-binding domain of AMPA receptors.