AMPA Receptor Modulators Research Articles

Transcranial magnetic stimulation as a translational biomarker for AMPA receptor modulation

TAK-653 is a novel α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-positive allosteric modulator being developed as a potential therapeutic for major depressive disorder (MDD). Currently, there are no translational biomarkers that evaluate physiological responses to the activation of glutamatergic brain circuits available. Here, we tested whether noninvasive neurostimulation, specifically single-pulse or paired-pulse motor cortex transcranial magnetic stimulation (spTMS and ppTMS, respectively), coupled with measures of evoked motor response captures the pharmacodynamic effects of TAK-653 in rats and healthy humans. In the rat study, five escalating TAK-653 doses (0.1–50 mg/kg) or vehicle were administered to 31 adult male rats, while measures of cortical excitability were obtained by spTMS coupled with mechanomyography. Twenty additional rats were used to measure brain and plasma TAK-653 concentrations. The human study was conducted in 24 healthy volunteers (23 males, 1 female) to assess the impact on cortical excitability of 0.5 and 6 mg TAK-653 compared with placebo, measured by spTMS and ppTMS coupled with electromyography in a double-blind crossover design. Plasma TAK-653 levels were also measured. TAK-653 increased both the mechanomyographic response to spTMS in rats and the amplitude of motor-evoked potentials in humans at doses yielding similar plasma concentrations. TAK-653 did not affect resting motor threshold or paired-pulse responses in humans. This is the first report of a translational functional biomarker for AMPA receptor potentiation and indicates that TMS may be a useful translational platform to assess the pharmacodynamic profile of glutamate receptor modulators.

Structural basis for positive allosteric modulation of AMPA and kainate receptors.

This paper summarizes the present knowledge on how positive allosteric modulators (PAMs) interact with the ligand-binding domain (LBD) of AMPA and kainate receptors, based on structure determinations. AMPA and kainate receptors belong to the family of ionotropic glutamate receptors that are responsible for mediating the majority of fast excitatory neurotransmission. These receptors have been related to brain disorders, e.g. Alzheimer's disease and attention deficit hyperactivity disorder. PAMs are small molecules that potentiate AMPA and kainate receptor currents by interfering with receptor desensitization. Therefore, PAMs are considered to be of interest for the development of pharmacological tools. Whereas PAMs for AMPA receptors have been known for several years, only recently have PAMs for kainate receptors been reported. Today, >80 structures are available for AMPA receptors with PAMs. These PAMs bind at the interface between two LBD subunits in the vicinity of residue 775, which is important for functional differences between flip and flop isoforms of AMPA receptors. PAMs can be divided into five classes based on their binding mode. The most potent PAM reported to date belongs to class 3, which comprises dimerized PAMs. Three structures of the kainate receptor GluK1 were determined with PAMs belonging to class 2. One PAM enhances kainate receptor currents 5- to 59-fold but shows 100-fold lower potency compared to AMPA receptors. Selective PAMs for kainate receptors will be of great use as pharmacological tools for functional investigations in vivo and might potentially prove useful as drugs in controlling the activity of neuronal networks.

Ampakines enhance phrenic nerve output in a ZFN rat model of Pompe Disease

Pompe disease (PD) is caused by mutations in the gene encoding acid-α-glucosidase (GAA), an enzyme that degrades lysosomal glycogen. The resultant glycogen accumulation is associated with neurodegeneration, cardiac deficits, and impaired phrenic motoneuron and diaphragm activation. Cardiorespiratory failure is the leading cause of death in early-onset PD patients. We previously reported that a drug which acts as a positive allosteric modulator of AMPA receptors, ampakine CX717, could stimulate phrenic motor output and increase minute ventilation in a mouse model of Pompe disease (Gaa-/-). We recently created a new rat model of PD that better recapitulates the clinical phenotype of early onset PD, called the Zinc Finger Nuclease (ZFN) KO model. Phrenic nerve recordings were performed on urethane anesthetized ZFN KO rats of both sexes, and age-matched wild type (WT) controls during a baseline normoxic period, followed by a brief exposure to hypoxia (PaO2 ~ 30mmHg) and a maximal respiratory challenge with combined hypoxia-hypercapnia. Recordings were done before and after treatment with ampakine CX717 (15mg/kg) or its vehicle control (2-Hydroxypropyl)-β-cyclodextrin. Our goal was to build upon the prior data from the Gaa-/-mouse, by using the ZFN KO model to test the hypothesis that in addition to stimulating basal or “eupneic” phrenic output, ampakine CX717 could also increase the ability to respond to acute respiratory challenge. We also aimed to determine if the extent of phrenic motor deficits were different between male and female ZFN KO rats. The data confirm that both male and female ZFN KO rats exhibit cardiac irregularities and phrenic motor deficits when compared to WT controls. However, a sex difference was apparent as male ZFN KO rats exhibit greater phrenic motor deficits and blunted responses to gas challenges compared to female ZFN KO rats. Ampakine treatment enhanced phrenic output and improved the ability to respond to respiratory challenge in both sexes of the ZNF rat model of PD. We conclude that 1) sex differences are present in the phrenic motor phenotype of ZFN KO rats, 2) the ability of low dose CX717to stimulate phrenic motor output in PD is reproducible across two species, and 3) low dose CX717 can enhance the ability to respond to respiratory challenge in PD rats These data suggest that ampakines may serve as a viable adjunctive therapy for respiratory insufficiency in PD patients.

Pharmacological modulation of AMPA receptors rescues specific impairments in social behavior associated with the A350V Iqsec2 mutation

In this study we tested the hypothesis that pharmacological modulation of glutamatergic neurotransmission could rescue behavioral deficits exhibited by mice carrying a specific mutation in the Iqsec2 gene. The IQSEC2 protein plays a key role in glutamatergic synapses and mutations in the IQSEC2 gene are a frequent cause of neurodevelopmental disorders. We have recently reported on the molecular pathophysiology of one such mutation A350V and demonstrated that this mutation downregulates AMPA type glutamatergic receptors (AMPAR) in A350V mice. Here we sought to identify behavioral deficits in A350V mice and hypothesized that we could rescue these deficits by PF-4778574, a positive AMPAR modulator. Using a battery of social behavioral tasks, we found that A350V Iqsec2 mice exhibit specific deficits in sex preference and emotional state preference behaviors as well as in vocalizations when encountering a female mouse. The social discrimination deficits, but not the impaired vocalization, were rescued with a single dose of PF-4778574. We conclude that social behavior deficits associated with the A350V Iqsec2 mutation may be rescued by enhancing AMPAR mediated synaptic transmission.

New nanomolar negative modulators of AMPA receptors

Bisamides of 1,4-bis(aminomethyl)benzene with cyclopropane-, cyclobutane- and cyclopentanecarboxylic acids reveal the negative modulation of kainate-induced currents in rat Purkinje neurons observed using patch clamp technique. The most pronounced effect has been found for the bisamide of cyclobutanecarboxylic acid, which blocks the currents by 41 and 49% at concentrations of 1 × 10−10 and 1 × 10−9 mol dm−3, respectively.

Novel bivalent positive allosteric AMPA receptor modulator of bis-amide series

A series of bis-amides of bis(aminoalkyl)benzenes and diaminoalkanes were synthesized. Positive modulation of kainate-induced transmembrane currents was registered on Purkinje neurons for isobutyric acid derivative with 1,4-bis(aminomethyl)benzene linker (40% potentiation at 1 nm). This compound also demonstrated a neuroprotective effect at 0.1 nm in an oxidative stress model in HT-22 cells when applied 24 h before H2O2.

Ligand-independent activity of the ghrelin receptor modulates AMPA receptor trafficking and supports memory formation.

The biological signals of hunger, satiety, and memory are interconnected. The role of the hormone ghrelin in regulating feeding and memory makes ghrelin receptors attractive targets for associated disorders. We investigated the effects of the high ligand-independent activity of the ghrelin receptor GHS-R1a on the physiology of excitatory synapses in the hippocampus. Blocking this activity produced a decrease in the synaptic content of AMPA receptors in hippocampal neurons and a reduction in GluA1 phosphorylation at Ser845 Reducing the ligand-independent activity of GHS-R1a increased the surface diffusion of AMPA receptors and impaired AMPA receptor-dependent synaptic delivery induced by chemical long-term potentiation. Accordingly, we found that blocking this GHS-R1a activity impaired spatial and recognition memory in mice. These observations support a role for the ligand-independent activity of GHS-R1a in regulating AMPA receptor trafficking under basal conditions and in the context of synaptic plasticity that underlies learning.

The novel scaffold 1,2,4-benzothiadiazine-1,1-dioxide: a review

The aim of this review is an attempt to highlight the progress in the chemistry and to present an outline of various pharmacological activities of 1,2,4-benzothiadiazine-1,1-dioxide ring. The biological activities of 1,2,4-benzothiadiazine-1,1-dioxide, for example, antimicrobial, antiviral, antihypertensive, antidiabetic, anticancer, KATP channel activators, and AMPA receptor modulators etc., are reported. Many functional groups attached to the ring are responsible for the activity of 1,2,4-benzothiadiazine-1,1-dioxide like a halo, the group at 7 and 8 positions of the ring gave active compounds. Meanwhile, other functional groups like alkyl, aryl, alkylamino, benzyl, keto, etc. at different position of 1,2,4-benzothiadiazine-1,1-dioxide ring are responsible for the activity. The review aims are to collect literature work stated by scholars on 1,2,4-benzothiadiazine-1,1-dioxide for their different therapeutic activities and report the current efforts made on this ring.

Characterizing the binding and function of TARP γ8-selective AMPA receptor modulators

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid(AMPA)-type glutamate receptors (AMPARs) are the predominant excitatory neurotransmitter receptors in the brain, where they mediate synaptic transmission and plasticity. Excessive AMPAR activation leads to diseases such as epilepsy. AMPAR properties are modulated by auxiliary proteins and foremost by the transmembrane AMPAR regulatory proteins (TARPs). These distribute in unique expression patterns across the brain, rendering AMPAR/TARP complexes promising targets for region-specific therapeutic intervention. TARP γ8 is predominantly expressed in the forebrain and is enriched in the hippocampus, a region associated with temporal lobe epilepsy. Recent high-throughput medicinal chemistry screens have identified multiple promising compounds that selectively target AMPARs associated with γ8 and hold promise for epilepsy treatment. However, how these modulators target the receptor complex is currently unknown. Here, we use a combination of ligand docking, molecular dynamics simulations, and electrophysiology to address this question. We identify a conserved oxindole isostere, shared between three structurally diverse modulators (LY-3130481, JNJ-55511118, and JNJ-61432059) as the major module engaging γ8 by an H-bond to Asn-172 (γ8). The remaining variable region of each molecule likely targets the receptor complex in ligand-selective modes. Functional data reveal parallels in the underlying modulatory action of two prominent compounds. This work will aid development of refined AMPAR epilepsy therapeutics and facilitate to uncover the mechanisms by which TARPs modulate the receptor.

Fxr1 regulates sleep and synaptic homeostasis

The fragile X autosomal homolog 1 (Fxr1) is regulated by lithium and has been GWAS‐associated with schizophrenia and insomnia. Homeostatic regulation of synaptic strength is essential for the maintenance of brain functions and involves both cell‐autonomous and system‐level processes such as sleep. We examined the contribution of Fxr1 to cell‐autonomous homeostatic synaptic scaling and neuronal responses to sleep loss, using a combination of gene overexpression and Crispr/Cas9‐mediated somatic knockouts to modulate gene expression. Our findings indicate that Fxr1 is downregulated during both scaling and sleep deprivation via a glycogen synthase kinase 3 beta (GSK3β)‐dependent mechanism. In both conditions, downregulation of Fxr1 is essential for the homeostatic modulation of surface AMPA receptors and synaptic strength. Preventing the downregulation of Fxr1 during sleep deprivation results in altered EEG signatures. Furthermore, sequencing of neuronal translatomes revealed the contribution of Fxr1 to changes induced by sleep deprivation. These findings uncover a role of Fxr1 as a shared signaling hub between cell‐autonomous homeostatic plasticity and system‐level responses to sleep loss, with potential implications for neuropsychiatric illnesses and treatments.

Cannabidiol inhibits febrile seizure by modulating AMPA receptor kinetics through its interaction with the N-terminal domain of GluA1/GluA2

Cannabidiol (CBD) is a major phytocannabinoid in Cannabis sativa. CBD is being increasingly reported as a clinical treatment for neurological diseases. Febrile seizure is one of the most common diseases in children with limited therapeutic options. We investigated possible therapeutic effects of CBD on febrile seizures and the underlying mechanism. Use of a hyperthermia-induced seizures model revealed that CBD significantly prolonged seizure latency and reduced the severity of thermally-induced seizures. Hippocampal neuronal excitability was significantly decreased by CBD. Further, CBD significantly reduced the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) mediated evoked excitatory postsynaptic currents (eEPSCs) and the amplitude and frequency of miniature EPSCs (mEPSCs). Furthermore, CBD significantly accelerated deactivation in GluA1 and GluA2 subunits. Interestingly, CBD slowed receptor recovery from desensitization of GluA1, but not GluA2. These effects on kinetics were even more prominent when AMPAR was co-expressed with γ-8, the high expression isoform 8 of transmembrane AMPAR regulated protein (TARPγ8) in the hippocampus. The inhibitory effects of CBD on AMPAR depended on its interaction with the distal N-terminal domain of GluA1/GluA2. CBD inhibited AMPAR activity and reduced hippocampal neuronal excitability, thereby improving the symptoms of febrile seizure in mice. The putative binding site of CBD in the N-terminal domain of GluA1/GluA2 may be a drug target for allosteric gating modulation of AMPAR.

Characterization of a New Positive Allosteric Modulator of AMPA Receptors - PAM-43: Specific Binding of the Ligand and its Ability to Potentiate AMPAR Currents.

Currently, the most dynamic areas in the glutamate receptor system neurobiology are the identification and development of positive allosteric modulators (PAMs) of glutamate ionotropic receptors. PAM-based drugs are of great interest as promising candidates for the treatment of neurological diseases, such as epilepsy, Alzheimer's disease, schizophrenia, etc. Understanding the molecular mechanisms underlying the biological action of natural and synthetic PAMs is a key point for modifying the original chemical compounds as well as for new drug design. We are trying to elaborate a system of molecular functional screening of ionotropic glutamate receptor probable PAMs. The system will be based on the radioligand - receptor method of analysis and will allow rapid quantification of new AMPAR probable PAMs molecular activity. We plan to use a tritiumlabeled analogue of recently elaborated ionotropic GluR probable PAM ([3H]PAM-43) as the main radioligand. Here, we characterized the specific binding of the ligand and its ability to potentiate ionotropic GluR currents. The existence of at least two different sites of [3H]PAM-43 specific binding has been shown. One of the above sites is glutamate-dependent and is characterized by higher affinity. "Patchclamp" technique showed the ability of PAM-43 to potentiate ionotropic GluR currents in rat cerebellar Purkinje neurons in a concentration-dependent manner. The possibility of using PAM-43 as a model compound to study different allosteric effects of potential regulatory drugs (AMPAR allosteric regulators) was shown. [3H]PAM-43 based screening system will allow rapid selection of new AMPAR probable PAM structures and quantification of their molecular activity.

The Anxiolytic Effect of the Neuropeptide Cycloprolylglycine Is Mediated by AMPA and TrkB Receptors.

Previously we have shown that the neuropeptide cycloprolylglycine is an endogenous positive modulator of AMPA receptors and assumed that the pharmacological effects of CPG are associated with the brain neurotrophic factor. In this paper, we have first demonstrated that DNQX, an inhibitor of AMPA receptors, and K252A, an ihibitor of Trk receptors, prevented the anxiolytic effect of CPG, which confirms the formulated hypothesis.

Allosteric modulation of AMPA receptors counteracts Tau-related excitotoxic synaptic signaling and memory deficits in stress- and Aβ-evoked hippocampal pathology.

Despite considerable progress in the understanding of its neuropathology, Alzheimer's disease (AD) remains a complex disorder with no effective treatment that counteracts the memory deficits and the underlying synaptic malfunction triggered by the accumulation of amyloid beta (Aβ) and Tau protein. Mounting evidence supports a precipitating role for chronic environmental stress and glutamatergic excitotoxicity in AD, suggesting that targeting of glutamate receptor signaling may be a promising approach against both stress and AD pathologies. In light of the limited cognitive benefit of the direct antagonism of NMDA receptors in AD, we here focus on an alternative way to modify glutamatergic signaling through positive allosteric modulation of AMPA receptors, by the use of a PAM-AMPA compound. Using non-transgenic animal model of Aβ oligomer injection as well as the combined stress and Aβ i.c.v. infusion, we demonstrate that positive allosteric modulation of AMPA receptors by PAM-AMPA treatment reverted memory, but not mood, deficits. Furthermore, PAM-AMPA treatment reverted stress/Aβ-driven synaptic missorting of Tau and associated Fyn/GluN2B-driven excitotoxic synaptic signaling accompanied by recovery of neurotransmitter levels in the hippocampus. Our findings suggest that positive allosteric modulation of AMPA receptors restores synaptic integrity and cognitive performance in stress- and Aβ-evoked hippocampal pathology. As the prevalence of AD is increasing at an alarming rate, novel therapeutic targeting of glutamatergic signaling should be further explored against the early stages of AD synaptic malfunction with the goal of attenuating further synaptic damage before it becomes irreversible.

Spinal delivery of ampakine CX717 impacts phrenic motor output in adult rats

Throughout the brainstem and spinal cord, α‐amino‐3‐hydroxy‐5‐methy‐4‐isoxazole propionate receptors (AMPARs) play an essential role in respiratory motor control. Ampakines are positive allosteric modulators of AMPARs which can enhance glutamatergic currents. Our laboratory recently reported that intravenous delivery of low‐dose ampakine CX717 can evoke increases in phrenic nerve inspiratory burst amplitude. However, this increased phrenic bursting could reflect an impact of the ampakine on pre‐motor brainstem circuits or a direct action on spinal neurons. In ongoing experiments, we are testing the hypothesis that delivery of ampakines to the cervical spinal cord is sufficient to evoke a sustained increase in phrenic nerve inspiratory burst amplitude. Phrenic motor output was measured in urethane‐anesthetized, mechanically ventilated, vagotomized adult male Sprague‐Dawley rats. Along with amplitude of inspiratory phrenic motor output, arterial blood pressure and end‐tidal CO2 were recorded. For intrathecal delivery of ampakine CX717 (15μl, 500μM, dissolved in 10% HPCD), the spinal cord was exposed via a dorsal approach followed by laminectomy and durotomy to enable drug administration over the mid‐cervical spinal cord. At least 15 min of stable phrenic burst amplitude was recorded as baseline after which intrathecal CX717 was applied. The initial results (n=3 sham, n=3 CX717) indicate that spinally directed CX717 causes a gradual and sustained increase in phrenic inspiratory burst amplitude lasting up to 60 min. Conversely, intrathecal delivery of 10% HPCD (sham) had no consistent impact on phrenic burst amplitude. These preliminary results are consistent with the hypothesis that allosteric modulation of spinal AMPA receptors can increase phrenic motor output. Phrenic motor neurons express AMPA receptors and are a likely candidate for the site of CX717 action.Support or Funding Information5 R01 HL139708 02 (DDF)

Impact of Ampakine CX‐717 on Diaphragm EMG Activity and Breathing in Non‐anesthetized Freely Behaving Rats

Glutamatergic neurotransmission is partly mediated by ionotropic AMPA receptors, and these receptors play an important role in the neural control of breathing. Ampakines are positive allosteric modulators of AMPA receptors. These drugs can stimulate breathing during hypo‐ventilated states, such as following an opioid overdose. Our laboratory has recently shown that acute exposure to an ampakine, CX717 (15 mg/kg), can acutely stimulate neural drive to the diaphragm in anesthetized but otherwise healthy rats. The goal of the current work is to evaluate the impact of ampakine on diaphragm muscle activation in freely moving non‐anesthetized rats. Ventilation and metabolic rate (VCO2) were measured using whole body plethysmography and diaphragm EMG was assessed using intramuscular electrodes. Spinally intact rats (n=2) were given a single intravenous bolus of CX717 at four months after implantation of diaphragm EMG electrodes. Delivering CX717 at 15 mg/kg had no detectable impact on VCO2, inspiratory tidal volume or diaphragm EMG burst amplitude. In addition, at 30 minutes following IV ampakine, both animals showed a robust increase in diaphragm EMG activity, tidal volume, and the ratio of ventilation VCO2 (VE/VCO2) during an acute hypoxic challenge (5 min, 10.5% inspired O2). Qualitatively, we observed a possible sedative effect as shown of lack of movement and exploratory behavior following ampakine exposure. Our preliminary conclusion is that ampakine CX717 at a dose of 15 mg/kg has little impact on breathing in the otherwise healthy rat. Ongoing studies are evaluating impact of the vehicle (2‐hydroxypropyl‐beta‐cyclodextrin) versus ampakine using a cross‐over study design in spinally intact rats and in rats with spinal cord injury.Support or Funding Information5 R01 HL139708 02 (DDF)F31 HL145831‐01 (MDS)

Auxiliary Proteins are the Predominant Determinants of Differential Efficacy of Clinical Candidates Acting as AMPA Receptor Positive Allosteric Modulators.

Positive allosteric modulators (PAMs) of AMPA receptors boost cognitive performance in preclinical and clinical studies. Their therapeutic window is narrow, however, and clinical application will likely only occur if greater discrimination in activity is achieved. Toward that end, we compared the modulatory activity of two PAMs recently considered as clinical candidates, LY451395 (mibampator) and PF-04958242/BIIB104, on recombinant and native AMPA receptors (AMPARs). We found that the principle molecular determinant that shaped modulatory activity of both PAMs on deactivation (recombinant) and decay (synaptic) of AMPARs was the auxiliary protein incorporated into the receptor complexes. AMPARs containing the stargazin/γ2 transmembrane AMPAR regulatory protein (TARP) were slowed to a >10-fold degree by both PAMs as compared with those incorporating γ8 TARP. Neither subunit composition nor flip/flop splice variation had substantive effect. Similarly, stargazin/γ2-containing mossy fiber EPSCs in cerebellar granule neurons were slowed to a ∼5-fold greater degree than EPSCs in hippocampal CA1 pyramidal cell neurons, which express the γ8 TARP. LY451395 exhibited greater efficacy than BIIB104 at both synapses. These studies provide insight into the receptor constituents that determine efficacy of sulfonamide PAMs. We conclude that compounds that discriminate between AMPARs complexed with distinct TARPs, and particularly those with lower stargazin/γ2 efficacy such as BIIB104, could act as viable procognitive therapeutics. SIGNIFICANCE STATEMENT: Positive allosteric modulators (PAMs) of AMPA receptors enhance cognitive function in a variety of preclinical models. A clearer understanding of the critical determinants of PAM activity could yield critical insight into pathways to maximize their therapeutic index. Here we show that auxiliary proteins for AMPARs play a major, but thus far underappreciated, role in shaping recombinant and neuronal AMPAR modulation by two clinical candidate PAMs. These data will inform both clinical outcomes as well as future rational development of new modulators.

Mechanism of Modulation of AMPA Receptors by TARPγ8

Fast excitatory synaptic transmission in the mammalian central nervous system is mediated by glutamate activated AMPA receptors (AMPARs). The core GluA subunits of AMPARs in neurons co-assemble with auxiliary subunits, including transmembrane AMPAR regulatory proteins (TARPs), several of which modifiy AMPA receptor-channel gating kinetics. In the case of AMPARs containing four copies of TARP γ8, achieved by expression of the tandem GluA2Q∗TARPγ8 subunit developed in the lab, AMPARs exhibit resensitization, or partial recovery from desensitization in continued presence of glutamate. We have recorded single channel currents in cell-attached patch recordings and show that under resensitizing conditions, these AMPARs show transitions primarily from closed to higher conductance levels, similar to activation of homomeric GluA receptors in the presence of cyclothiazide. To study the conformation associated with these states we used single molecule FRET and show that this high conductance state produced by GluA2Q∗TARPγ8 channels exhibits tighter coupling between subunits in the extracellular parts of the receptor.

Synthesis and evaluation of 6-(11C-methyl(4-(pyridin-2-yl)thiazol-2-yl)amino)benzo[d]thiazol-2(3H)-one for imaging γ-8 dependent transmembrane AMPA receptor regulatory protein by PET

Transmembrane AMPA receptor regulatory proteins (TARPs) are a recently discovered family of proteins that modulate AMPA receptors activity. Based on a potent and selective TARP subtype γ-8 antagonist, 6-(methyl(4-(pyridin-2-yl)thiazol-2-yl)amino)benzo[d]thiazol-2(3H)-one (compound 9), we perform the radiosynthesis of its 11C-isotopologue 1 and conduct preliminary PET evaluation to test the feasibility of imaging TARP γ-8 dependent receptors in vivo.

Mechanism of modulation of AMPA receptors by TARP-γ8.

Fast excitatory synaptic transmission in the mammalian central nervous system is mediated by glutamate-activated α-amino-5-methyl-3-hydroxy-4-isoxazole propionate (AMPA) receptors. In neurons, AMPA receptors coassemble with transmembrane AMPA receptor regulatory proteins (TARPs). Assembly with TARP γ8 alters the biophysical properties of the receptor, producing resensitization currents in the continued presence of glutamate. Using single-channel recordings, we show that under resensitizing conditions, GluA2 AMPA receptors primarily transition to higher conductance levels, similar to activation of the receptors in the presence of cyclothiazide, which stabilizes the open state. To study the conformation associated with these states, we have used single-molecule FRET and show that this high-conductance state exhibits tighter coupling between subunits in the extracellular parts of the receptor. Furthermore, the dwell times for the transition from the tightly coupled state to the decoupled states correlate to longer open durations of the channels, thus correlating conformation and function at the single-molecule level.