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

Activation of AMPA receptor in the infralimbic cortex facilitates extinction and attenuates the heroin-seeking behavior in rats

Infralimbic cortex (IL) is proposed to suppress cocaine seeking after extinction, but whether the IL regulates the extinction and reinstatement of heroin-seeking behavior is unknown. To address this issue, the male SD rats were trained to self-administer heroin under a FR1 schedule for consecutive 14 days, then the rats underwent 7 daily 2h extinction session in the operant chamber. The activation of IL by microinjection PEPA, an allosteric AMPA receptor potentiator into IL before each of extinction session facilitated the extinction responding after heroin self-administration, but did not alter the locomotor activity in an open field testing environment. Other rats were first trained under a FR1 schedule for heroin self-administration for 14 days, followed by 14 days of extinction training, and reinstatement of heroin-seeking induced by cues was measured for 2h. Intra-IL microinjecting of PEPA at 15min prior to test inhibited the reinstatement of heroin-seeking induced by cues. Moreover, the expression of GluR1 in the IL and NAc remarkably increased after treatment with PEPA during the reinstatement. These finding suggested that activation of glutamatergic projection from IL to NAc shell may be involved in the extinction and reinstatement of heroin-seeking.

Cognitive enhancers for facilitating drug cue extinction: insights from animal models

Given the success of cue exposure (extinction) therapy combined with a cognitive enhancer for reducing anxiety, it is anticipated that this approach will prove more efficacious than exposure therapy alone in preventing relapse in individuals with substance use disorders. Several factors may undermine the efficacy of exposure therapy for substance use disorders, but we suspect that neurocognitive impairments associated with chronic drug use are an important contributing factor. Numerous insights on these issues are gained from research using animal models of addiction. In this review, the relationship between brain sites whose learning, memory and executive functions are impaired by chronic drug use and brain sites that are important for effective drug cue extinction learning is explored first. This is followed by an overview of animal research showing improved treatment outcome for drug addiction (e.g. alcohol, amphetamine, cocaine, heroin) when explicit extinction training is conducted in combination with acute dosing of a cognitive-enhancing drug. The mechanism by which cognitive enhancers are thought to exert their benefits is by facilitating consolidation of drug cue extinction memory after activation of glutamatergic receptors. Based on the encouraging work in animals, factors that may be important for the treatment of drug addiction are considered.

Molecular mechanism of flop selectivity and subsite recognition for an AMPA receptor allosteric modulator: structures of GluA2 and GluA3 in complexes with PEPA

Glutamate receptors are important potential drug targets for cognitive enhancement and the treatment of schizophrenia in part because they are the most prevalent excitatory neurotransmitter receptors in the vertebrate central nervous system. One approach to the application of therapeutic agents to the AMPA subtype of glutamate receptors is the use of allosteric modulators, which promote dimerization by binding to a dimer interface thereby reducing the degree of desensitization and deactivation. AMPA receptors exist in two alternatively spliced variants (flip and flop) that differ in desensitization and receptor activation profiles. Most of the structural information about modulators of the AMPA receptor targets the flip subtype. We report here the crystal structure of the flop-selective allosteric modulator, PEPA, bound to the binding domains of the GluA2 and GluA3 flop isoforms of AMPA receptors. Specific hydrogen bonding patterns can explain the preference for the flop isoform. This includes a bidentate hydrogen bonding pattern between PEPA and N754 of the flop isoforms of GluA2 and GluA3 (the corresponding position in the flip isoform is S754). Comparison with other allosteric modulators provides a framework for the development of new allosteric modulators with preferences for either the flip or flop isoforms. In addition to interactions with N/S754, specific interactions of the sulfonamide with conserved residues in the binding site are characteristics of a number of allosteric modulators. These, in combination with variable interactions with five subsites on the binding surface, lead to different stoichiometries, orientations within the binding pockets, and functional outcomes.

Positive modulation of AMPA receptors prevents downregulation of GluR2 expression and activates the Lyn-ERK1/2-CREB signaling in rat brain ischemia

alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are responsible for excitotoxicity induced by ischemic injury in hippocampal CA1 neurons, whereas the molecular mechanisms responsible for their neurotrophic activities are much less studied. Here, we examined the neuroprotective effect of positive modeulation of AMPARs by coapplication of AMPA with PEPA, an allosteric potentiator of AMPARs. We showed that coapplication of AMPA with PEPA protected hippocampal CA1 neurons from brain ischemia-induced death. Coapplication of AMPA with PEPA could prevent downregulated expression of GluR2 subunit caused by ischemia and increase BDNF expression via Lyn-ERK1/2-CREB signaling. Furthermore, TrkB receptor-mediated PI3K/Akt signal pathway was activated after coapplication of AMPA with PEPA, which was related to MAPK pathway and protected CA1 neurons against ischemic insults through depression of JNK3 activity, release of cytochrome c to cytosol and depression of capase-3 activity. Our results revealed that positive modulation of AMPARs could exert neuroprotective effects and the possible signaling pathways underlied.

Methods for evaluation of positive allosteric modulators of glutamate AMPA receptors

Hypofunctioning of glutamate synaptic transmission in the central nervous system (CNS) has been proposed as a factor that may contribute to cognitive deficits associated with various neurological and psychiatric disorders. Positive allosteric modulation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) subtype of glutamate receptors has been proposed as a novel therapeutic approach, because these receptors mediate the majority of rapid excitatory neurotransmission and are intimately involved in long-term changes in synaptic plasticity thought to underlie mnemonic processing. By definition, positive allosteric modulators do not affect AMPA receptor activity alone but can markedly enhance ion flux through the ion channel pore in the presence of bound agonist. Despite this commonality, positive allosteric modulators can be segregated on the basis of the preferential effects on AMPA receptor subunits, their alternatively spliced variants and/or their biophysical mechanism of action. This chapter provides a detailed description of the methodologies used to evaluate the potency/efficacy and biophysical mechanism of action of positive allosteric modulators of AMPA receptors.

Stargazin controls the pharmacology of AMPA receptor potentiators

Glutamate is the major excitatory neurotransmitter in brain, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) mediate the majority of postsynaptic depolarization. AMPAR ion channels display rapid gating, and their deactivation and desensitization determine the timing of synaptic transmission. AMPAR potentiators slow channel deactivation and desensitization, and these compounds represent exciting therapies for mental and neurodegenerative diseases. Previous studies showed that the AMPAR potentiators cyclothiazide and 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluorophenoxyacetamide display a preference for flip and flop alternatively spliced versions of glutamate receptor subunits, respectively. Here, we find that the AMPAR auxiliary subunit stargazin changes this pharmacology and makes both spliced forms of glutamate receptor subunit 1 sensitive to both classes of potentiator. Stargazin also enhances the effect of AMPAR potentiators on channel deactivation. This work demonstrates that stargazin controls AMPAR potentiator pharmacology, which has important implications for development of AMPAR potentiators as therapeutic agents.

Therapeutic potential of positive AMPA modulators and their relationship to AMPA receptor subunits. A review of preclinical data

BACKGROUND

Positive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) modulators enhance glutamate transmission via the AMPA receptor by altering the rate of desensitization; alone they have no intrinsic activity. They are the only class of compounds known that may pharmacologically separate AMPA subtypes.

OBJECTIVE

This manuscript will review preclinical work on positive AMPA modulators, with clinical examples where relevant.

RESULTS

The activity of these compounds appears to be determined by the AMPA receptor subunit composition. Studies have shown that splice variant and/or subunit combinations change the desensitization rate of this receptor. Also, these subunits are heterogeneously expressed across the central nervous system. Therefore, the functional outcome of different positive AMPA modulators could indeed be different. The origins of this pharmacological class come from hippocampal long-term potentiation studies, so quite naturally they were first studied in models of short- and long-term memory (e.g., delayed match to sample, maze performance). In general, these agents were procognitive. However, more recent work with different chemical classes has suggested additional therapeutic effects in models of schizophrenia (e.g., amphetamine locomotor activity), depression (e.g., forced swim test), neuroprotection (e.g., NMDA agonist lesions) and Parkinson's disease (e.g., 6-hydroxydopamine lesion).

CONCLUSIONS

In conclusion, positive modulation of AMPA may offer numerous therapeutic avenues for central nervous system drug discovery.

Protective Effect of Buckwheat Polyphenols Against Long-Lasting Impairment of Spatial Memory Associated With Hippocampal Neuronal Damage in Rats Subjected to Repeated Cerebral Ischemia

In the present experiment, we studied the action of buckwheat polyphenol (BWP, from Fagopyrum esculentum MOENCH) in a repeated cerebral ischemia model, which induced a strong and long-lasting impairment of spatial memory in 8-arm radial maze with hippocampal CA1 cell death in rats. BWP (600 mg/kg, continuous 21-day p.o.) significantly ameliorated not only the impairment of spatial memory in the 8-arm radial maze, but also necrosis and TUNEL-positive cells in the hippocampal CA1 area subjected to repeated cerebral ischemia (10 min x 2 times occlusion, 1-h interval) in rats. In order to investigate the mechanism of BWP protective action, we measured the release of glutamate and NO(x)(-) (NO(2)(-) + NO(3)(-)) production induced by repeated cerebral ischemia in the rat dorsal hippocampus using microdialysis. A 14-day BWP treatment significantly inhibited the excess release of glutamate after the second occlusion. In addition, the BWP remarkably suppressed a delayed increase in NO(x)(-) (NO(2)(-) + NO(3)(-)) induced by repeated cerebral ischemia in the dorsal hippocampus as determined in vivo by microdialysis. However, the 14-day treatment did not affect hippocampal blood flow in either intact rats or rats subjected to repeated ischemia measured by lasser Doppler flowmeter. These results suggested that BWP might ameliorate spatial memory impairment by inhibiting glutamate release and the delayed generation of NO(x)(-) in rats subjected to repeated cerebral ischemia.

Heterogeneity and potentiation of AMPA type of glutamate receptors in rat cultured microglia

alpha-amino-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor in rat cultured microglia were analyzed precisely using flop- and flip-preferring allosteric modulators of AMPA receptors, 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyacetamide (PEPA) and cyclothiazide (CTZ), respectively. Glutamate (Glu)- or kainite (KA)-induced currents were completely inhibited by a specific blocker of AMPA receptor, LY300164, indicating that functional Glu-receptors in cultured microglia are mostly AMPA receptor but not KA receptor in many cells. Glu- and KA-induced currents were potentiated by PEPA and CTZ in a concentration-dependent manner. The ratio of the potentiation by PEPA to the potentiation by cyclothiazide varied with cells between 0.1 and 0.9, suggesting cell-to-cell heterogeneity of AMPA receptor subunits expressed in microglia. Quantitative RT-PCR revealed that GluR1-3 mainly occurred in the flip forms, which agreed with the stronger potentiation of receptor currents by CTZ vs. PEPA. Finally, the potentiation of microglial AMPA receptors by PEPA and CTZ inhibited the Glu-induced release of tumor necrosis factor-alpha (TNF-alpha) unpredictably. The increase in TNF-alpha release by Glu or KA required extracellular Na+ and Ca2+ ions but not mitogen-activated protein kinase (MAPK), suggesting the effects of PEPA and CTZ were not due to the inhibition of MAPK. These results suggest that potentiation of microglial AMPA receptors serves as a negative feedback mechanism for the regulation of TNF-alpha release and may contribute to the ameliorating effects of allosteric modulators of AMPA receptors.

Multiple molecular determinants for allosteric modulation of alternatively spliced AMPA receptors

Positive allosteric regulation of glutamate AMPA receptors involves conformational changes that can attenuate receptor desensitization and enhance ion flux through the channel pore. Many allosteric modulators (e.g., cyclothiazide and aniracetam) preferentially affect the flip (i) or flop (o) alternatively spliced isoform of AMPA receptors, implicating residues in the flip-flop domain as critical determinants of splice variant sensitivity. Indeed, previous mutational analyses have demonstrated that the differential sensitivity to cyclothiazide and aniracetam depends on a single amino acid, Ser (flip) and Asn (flop), suggesting that this residue may be solely responsible for differences in modulation of AMPA receptor isoforms. The present studies tested this hypothesis by investigating the molecular determinants of modulation of AMPA receptor splice variants by a structurally distinct compound, LY404187, which displays strikingly different and opposing kinetics of allosteric regulation characterized by a time-dependent enhancement in potentiation of homomeric GluR1-GluR4i and a time-dependent reduction in potentiation of GluR1-GluR4o. Site-directed mutagenesis of residues in the flip-flop domain of GluR2 revealed that, although exchange of Asn775 for Ser in GluR2o was sufficient to confer the GluR2i phenotype of potentiation, the corresponding mutation, Ser775Asn, in GluR2i did not impart the GluR2o response. In fact, the GluR2o kinetics of modulation depended on a novel set of substitutions in GluR2i, including Thr765Asn, Pro766Ala, and Val779Leu in combination with Ser775Asn. Collectively, these results show that, unlike cyclothiazide and aniracetam, the residues that confer splice variant differences in modulation by LY404187 are not identical and indicate that allosteric regulation of AMPA receptors can arise from multiple molecular determinants.

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.