In vitro ADME characterization of a very potent 3-acylamino-2-aminopropionic acid-derived GluN2C-NMDA receptor agonist and its ester prodrugs.

The GluN2C subunit exists predominantly, but not exclusively in NMDA receptors within the cerebellum. Antagonists such as UBP1700 and positive allosteric modulators including PYD-106 and 3-acylamino-2-aminopropionic acid derivatives such as UA3-10 ((R)-2-amino-3-{[5-(2-bromophenyl)thiophen-2-yl]carboxamido}propionic acid) represent promising tool compounds to investigate the role of GluN2C-containing NMDA receptors in the signal transduction in the brain. However, due to its high polarity the bioavailability and CNS penetration of the amino acid UA3-10 are expected to be rather low. Herein, three ester prodrugs 12a-c of the NMDA receptor glycine site agonist UA3-10 were prepared and pharmacokinetically characterized. The esters 12a-c showed higher lipophilicity (higher logD 7.4 values) than the acid UA3-10 but almost the same binding at human serum albumin. The acid UA3-10 was rather stable upon incubation with mouse liver microsomes and NADPH, but the esters 12a-c were fast hydrolyzed to afford the acid UA3-10. Incubation with pig liver esterase and mouse serum led to rapid hydrolysis of the esters 12a-c. The isopropyl ester 12c showed a promising logD 7.4 value of 3.57 and the highest stability in the presence of pig liver esterase and mouse serum. These results demonstrate that ester prodrugs of UA3-10 can potentially afford improved bioavailability and CNS penetration.

Discovery of (R)-2-amino-3-triazolpropanoic acid derivatives as NMDA receptor glycine site agonists with GluN2 subunit-specific activity.

N-Methyl-d-aspartate (NMDA) receptors play critical roles in central nervous system function and are involved in variety of brain disorders. We previously developed a series of (R)-3-(5-furanyl)carboxamido-2-aminopropanoic acid glycine site agonists with pronounced variation in activity among NMDA receptor GluN1/2A-D subtypes. Here, a series of (R)-2-amino-3-triazolpropanoic acid analogues with a novel chemical scaffold is designed and their pharmacological properties are evaluated at NMDA receptor subtypes. We found that the triazole can function as a bioisostere for amide to produce glycine site agonists with variation in activity among NMDA receptor subtypes. Compounds 13g and 13i are full and partial agonists, respectively, at GluN1/2C and GluN1/2D with 3- to 7-fold preference in agonist potency for GluN1/2C-D over GluN1/2A-B subtypes. The agonist binding mode of these triazole analogues and the mechanisms by which the triazole ring can serve as a bioisostere for amide were further explored using molecular dynamics simulations. Thus, the novel (R)-2-amino-3-triazolpropanoic acid derivatives reveal insights to agonist binding at the GluN1 subunit of NMDA receptors and provide new opportunities for the design of glycine site agonists.

Derivatives of (R)-3-(5-Furanyl)carboxamido-2-aminopropanoic Acid as Potent NMDA Receptor Glycine Site Agonists with GluN2 Subunit-Specific Activity.

NMDA receptors mediate glutamatergic neurotransmission and are therapeutic targets due to their involvement in a variety of psychiatric and neurological disorders. Here, we describe the design and synthesis of a series of (R)-3-(5-furanyl)carboxamido-2-aminopropanoic acid analogues 8a-s as agonists at the glycine (Gly) binding site in the GluN1 subunit, but not GluN3 subunits, of NMDA receptors. These novel analogues display highly variable potencies and agonist efficacies among the NMDA receptor subtypes (GluN1/2A-D) in a manner dependent on the GluN2 subunit. Notably, compound 8p is identified as a potent partial agonist at GluN1/2C (EC50 = 0.074 µM) with an agonist efficacy of 28% relative to activation by Gly and virtually no agonist activity at GluN1/2A, GluN1/2B, and GluN1/2D. Thus, these novel agonists can modulate the activity of specific NMDA receptor subtypes by replacing the full endogenous agonists Gly or d-serine (d-Ser), thereby providing new opportunities in the development of novel therapeutic agents.

GHB analogs confer neuroprotection through specific interaction with the CaMKIIα hub domain.

Ca2+/calmodulin-dependent protein kinase II alpha subunit (CaMKIIα) is a key neuronal signaling protein and an emerging drug target. The central hub domain regulates the activity of CaMKIIα by organizing the holoenzyme complex into functional oligomers, yet pharmacological modulation of the hub domain has never been demonstrated. Here, using a combination of photoaffinity labeling and chemical proteomics, we show that compounds related to the natural substance γ-hydroxybutyrate (GHB) bind selectively to CaMKIIα. By means of a 2.2-Å x-ray crystal structure of ligand-bound CaMKIIα hub, we reveal the molecular details of the binding site deep within the hub. Furthermore, we show that binding of GHB and related analogs to this site promotes concentration-dependent increases in hub thermal stability believed to alter holoenzyme functionality. Selectively under states of pathological CaMKIIα activation, hub ligands provide a significant and sustained neuroprotection, which is both time and dose dependent. This is demonstrated in neurons exposed to excitotoxicity and in a mouse model of cerebral ischemia with the selective GHB analog, HOCPCA (3-hydroxycyclopent-1-enecarboxylic acid). Together, our results indicate a hitherto unknown mechanism for neuroprotection by a highly specific and unforeseen interaction between the CaMKIIα hub domain and small molecule brain-penetrant GHB analogs. This establishes GHB analogs as powerful tools for investigating CaMKII neuropharmacology in general and as potential therapeutic compounds for cerebral ischemia in particular.

Erythrina Alkaloid Analogues as nAChR Antagonists-A Flexible Platform for Leads in Drug Discovery.

Erythrina alkaloids and their central nervous system effects have been studied for over a century, mainly due to their potent antagonistic actions at ß2-containing nicotinic acetylcholine receptors (nAChRs). In the present work, we report a synthetic approach giving access to a diverse set of Erythrina natural product analogues and present the enantioselective total synthesis of (+)-Cocculine and (+)-Cocculidine, both found to be potent antagonists of the ß2-containing nAChRs.

Stereoselective synthesis of novel 2'-(S)-CCG-IV analogues as potent NMDA receptor agonists.

We developed a versatile stereoselective route for the synthesis of new 2'-(S)-CCG-IV analogues. The route allows for late stage diversification and thereby provides access to a great variety of conformationally restricted cyclopropyl glutamate analogues. A selection of the 2'-(S)-CCG-IV analogues were evaluated using two-electrode voltage-clamp electrophysiology at recombinant GluN1/GluN2A-D receptors, demonstrating that agonists can be developed with GluN2 subunit-dependent potency and agonist efficacy. We also describe a crystal structure of the GluN2A agonist binding domain in complex with 2'-butyl-(S)-CCG-IV that determines the position of 2'-substituents in (S)-CCG-IV agonists in the glutamate binding site and provides further insight to the structural determinants of their agonist efficacy. The stereoselective synthesis described here enables versatile and straight-forward modifications to diverse analogues of interest for the development of potent subtype-specific NMDA receptor agonists and other applications.

Pharmacological Characterization of a Betaine/GABA Transporter 1 (BGT1) Inhibitor Displaying an Unusual Biphasic Inhibition Profile and Anti-seizure Effects.

Focal epileptic seizures can in some patients be managed by inhibiting γ-aminobutyric acid (GABA) uptake via the GABA transporter 1 (GAT1) using tiagabine (Gabitril®). Synergistic anti-seizure effects achieved by inhibition of both GAT1 and the betaine/GABA transporter (BGT1) by tiagabine and EF1502, compared to tiagabine alone, suggest BGT1 as a target in epilepsy. Yet, selective BGT1 inhibitors are needed for validation of this hypothesis. In that search, a series of BGT1 inhibitors typified by (1R,2S)-2-((4,4-bis(3-methylthiophen-2-yl)but-3-en-yl)(methyl)amino)cyclohexanecarboxylic acid (SBV2-114) was developed. A thorough pharmacological characterization of SBV2-114 using a cell-based [3H]GABA uptake assay at heterologously expressed BGT1, revealed an elusive biphasic inhibition profile with two IC50 values (4.7 and 556 µM). The biphasic profile was common for this structural class of compounds, including EF1502, and was confirmed in the MDCK II cell line endogenously expressing BGT1. The possibility of two binding sites for SBV2-114 at BGT1 was assessed by computational docking studies and examined by mutational studies. These investigations confirmed that the conserved residue Q299 in BGT1 is involved in, but not solely responsible for the biphasic inhibition profile of SBV2-114. Animal studies revealed anti-seizure effects of SBV2-114 in two mouse models, supporting a function of BGT1 in epilepsy. However, as SBV2-114 is apparent to be rather non-selective for BGT1, the translational relevance of this observation is unknown. Nevertheless, SBV2-114 constitutes a valuable tool compound to study the molecular mechanism of an emerging biphasic profile of BGT1-mediated GABA transport and the putative involvement of two binding sites for this class of compounds.

Use of the 4-Hydroxytriazole Moiety as a Bioisosteric Tool in the Development of Ionotropic Glutamate Receptor Ligands.

We report a series of glutamate and aspartate analogues designed using the hydroxy-1,2,3-triazole moiety as a bioisostere for the distal carboxylic acid. Compound 6b showed unprecedented selectivity among ( S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor subtypes, confirmed also by an unusual binding mode observed for the crystal structures in complex with the AMPA receptor GluA2 agonist-binding domain. Here, a methionine (Met729) was highly disordered compared to previous agonist-bound structures. This observation provides a possible explanation for the pharmacological profile. In the structure with 7a, an unusual organization of water molecules around the bioisostere arises compared to previous structures of ligands with other bioisosteres. Aspartate analogue 8 with the hydroxy-1,2,3-triazole moiety directly attached to glycine was unexpectedly able to activate both the glutamate and glycine agonist-binding sites of the N-methyl-d-aspartic acid receptor. These observations demonstrate novel features that arise when employing a hydroxytriazole moiety as a bioisostere for the distal carboxylic acid in glutamate receptor agonists.

Elucidation of fluorine's impact on pKa and in vitro Pgp-mediated efflux for a series of PDE9 inhibitors.

P-Glycoprotein (Pgp)-mediated cellular efflux is recognized as a common challenge in CNS drug discovery. In this study, the influence of replacing a hydrogen atom with fluorine on the pKa and Pgp-mediated efflux is elucidated for a series of PDE9 inhibitors. The PDE9 inhibitors with and without fluorine were synthesized using a novel condensation-oxidation approach, providing access to several analogues, all from the same stereoenriched aldehyde building block. The incorporation of fluorine was found to influence two acid-base functionalities concomitantly, both of which were involved in Pgp-recognition. By methylating the acidic functionality, it was possible to isolate the effect responsible for lowering the Pgp-mediated efflux.

Inhibition of GABA transporters fails to afford significant protection following focal cerebral ischemia.

Brain ischemia triggers excitotoxicity and cell death, yet no neuroprotective drugs have made it to the clinic. While enhancing GABAergic signaling to counterbalance excitotoxicity has shown promise in animal models, clinical studies have failed. Blockade of GABA transporters (GATs) offers an indirect approach to increase GABA inhibition to lower the excitation threshold of neurons. Among the GATs, GAT1 is known to promote neuroprotection, while the protective role of the extrasynaptic transporters GAT3 and BGT1 is elusive. A focal lesion was induced in the motor cortex in two to four-month-old C57BL/6 J male mice by photothrombosis. The GAT1 inhibitor, tiagabine (1 and 10 mg/kg), the GAT2/3 inhibitor, ( S)-SNAP-5114 (5 and 30 mg/kg) and the GAT1/BGT1 inhibitor, EF-1502 (1 and 10 mg/kg) were given i.p. 1 and 6 h post-stroke to assess their impact on infarct volume and motor performance seven days post-stroke. One mg/kg tiagabine improved motor performance, while 10 mg/kg tiagabine, ( S)-SNAP-5114 and EF-1502 had no effect. None of the compounds affected infarct volume. Interestingly, treatment with tiagabine induced seizures and ( S)-SNAP-5114 led to increased mortality. Although we show that tiagabine can promote protection, our findings indicate that caution should be had when using GAT1 and GAT3 inhibitors for conditions of brain ischemia.

Structure-Activity Relationship, Pharmacological Characterization, and Molecular Modeling of Noncompetitive Inhibitors of the Betaine/γ-Aminobutyric Acid Transporter 1 (BGT1).

N-(1-Benzyl-4-piperidinyl)-2,4-dichlorobenzamide 5 (BPDBA) is a noncompetitive inhibitor of the betaine/GABA transporter 1 (BGT1). We here report the synthesis and structure-activity relationship of 71 analogues. We identify 26m as a more soluble 2,4-Cl substituted 3-pyridine analogue with retained BGT1 activity and an improved off-target profile compared to 5. We performed radioligand-based uptake studies at chimeric constructs between BGT1 and GAT3, experiments with site-directed mutated transporters, and computational docking in a BGT1 homology model based on the newly determined X-ray crystal structure of the human serotonin transporter (hSERT). On the basis of these experiments, we propose a binding mode involving residues within TM10 in an allosteric site in BGT1 that corresponds to the allosteric binding pocket revealed by the hSERT crystal structure. Our study provides first insights into a proposed allosteric binding pocket in BGT1, which accommodates the binding site for a series of novel noncompetitive inhibitors.

Astrocytic GABA Transporters: Pharmacological Properties and Targets for Antiepileptic Drugs.

Inactivation of GABA-mediated neurotransmission is achieved by high-affinity transporters located at both GABAergic neurons and the surrounding astrocytes. Early studies of the pharmacological properties of neuronal and glial GABA transporters suggested that different types of transporters might be expressed in the two cell types, and such a scenario was confirmed by the cloning of four distinctly different GABA transporters from a number of different species. These GABA-transport entities have been extensively characterized using a large number of GABA analogues of restricted conformation, and several of these compounds have been shown to exhibit pronounced anticonvulsant activity in a variety of animal seizure models. As proof of concept of the validity of this drug development approach, one GABA-transport inhibitor, tiagabine, has been developed as a clinically active antiepileptic drug. This review provides a detailed account of efforts to design new subtype-selective GABA-transport inhibitors aiming at identifying novel antiepileptic drug candidates.

Identification of AICP as a GluN2C-Selective N-Methyl-d-Aspartate Receptor Superagonist at the GluN1 Glycine Site.

N-methyl-d-aspartate (NMDA)-type ionotropic glutamate receptors mediate excitatory neurotransmission in the central nervous system and are critically involved in brain function. NMDA receptors are also implicated in psychiatric and neurological disorders and have received considerable attention as therapeutic targets. In this regard, administration of d-cycloserine (DCS), which is a glycine site NMDA receptor agonist, can enhance extinction of conditioned fear responses. The intriguing behavioral effects of DCS have been linked to its unique pharmacological profile among NMDA receptor subtypes (GluN1/2A-D), in which DCS is a superagonist at GluN2C-containing receptors compared with glycine and a partial agonist at GluN2B-containing receptors. Here, we identify (R)-2-amino-3-(4-(2-ethylphenyl)-1H-indole-2-carboxamido)propanoic acid (AICP) as a glycine site agonist with unique GluN2-dependent differences in agonist efficacy at recombinant NMDA receptor subtypes. AICP is a full agonist at GluN1/2A (100% response compared with glycine), a partial agonist at GluN1/2B and GluN1/2D (10% and 27%, respectively), and a highly efficacious superagonist at GluN1/2C receptors (353%). Furthermore, AICP potencies are enhanced compared with DCS with EC50 values in the low nanomolar range (1.7 nM at GluN1/2C). We show that GluN1/2C superagonism of AICP and DCS is mediated by overlapping but distinct mechanisms and that AICP selectively enhances responses from recombinant GluN1/2C receptors in the presence of physiological glycine concentrations. This functional selectivity of AICP for GluN2C-containing NMDA receptors is more pronounced compared with DCS, suggesting that AICP can be a useful tool compound for uncovering the roles of GluN2C subunits in neuronal circuit function and in the development of new therapeutic strategies.

Subtype-Specific Agonists for NMDA Receptor Glycine Binding Sites.

A series of analogues based on serine as lead structure were designed, and their agonist activities were evaluated at recombinant NMDA receptor subtypes (GluN1/2A-D) using two-electrode voltage-clamp (TEVC) electrophysiology. Pronounced variation in subunit-selectivity, potency, and agonist efficacy was observed in a manner that was dependent on the GluN2 subunit in the NMDA receptor. In particular, compounds 15a and 16a are potent GluN2C-specific superagonists at the GluN1 subunit with agonist efficacies of 398% and 308% compared to glycine. This study demonstrates that subunit-selectivity among glycine site NMDA receptor agonists can be achieved and suggests that glycine-site agonists can be developed as pharmacological tool compounds to study GluN2C-specific effects in NMDA receptor-mediated neurotransmission.

Radiosynthesis and Evaluation of [11C]3-Hydroxycyclopent-1-enecarboxylic Acid as Potential PET Ligand for the High-Affinity γ-Hydroxybutyric Acid Binding Sites.

γ-Hydroxybutyric acid (GHB) is an endogenous neuroactive substance and proposed neurotransmitter with affinity for both low- and high-affinity binding sites. A radioligand with high and specific affinity toward the high-affinity GHB binding site would be a unique tool toward a more complete understanding of this population of binding sites. With its high specific affinity and monocarboxylate transporter (MCT1) mediated transport across the blood-brain barrier in pharmacological doses, 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) seems like a suitable PET radiotracer candidate. Here, we report the 11C-labeling and subsequent evaluation of [11C]HOCPCA in a domestic pig, as a PET-radioligand for visualization of the high-affinity GHB binding sites in the live pig brain. To investigate the regional binding of HOCPCA in pig brain prior to in vivo PET studies, in vitro quantitative autoradiography on sections of pig brain was performed using [3H]HOCPCA. In vivo evaluation of [11C]HOCPCA showed no brain uptake, possibly due to a limited uptake of HOCPCA by the MCT1 transporter at tracer doses of [11C]HOCPCA.

The labeling of unsaturated γ-hydroxybutyric acid by heavy isotopes of hydrogen: iridium complex-mediated H/D exchange by C─H bond activation vs reduction by boro-deuterides/tritides.

3-Hydroxycyclopent-1-ene-1-carboxylic acid (HOCPCA (1)) is a potent ligand for high-affinity γ-hydroxybutyric acid binding sites in the central nervous system. Various approaches to the introduction of a hydrogen label onto the HOCPCA skeleton are reported. The outcomes of the feasible C─H activation of olefin carbon (C-2) by iridium catalyst are compared with the reduction of the carbonyl group (C-3) by freshly prepared borodeuterides. The most efficient iridium catalysts proved to be Kerr bulky phosphine N-heterocyclic species providing outstanding deuterium enrichment (up to 91%) in a short period of time. The highest deuterium enrichment (>99%) was achieved through the reduction of ketone precursor 2 by lithium trimethoxyborodeuteride. Hence, analogical conditions were used for the tritiation experiment. [3 H]-HOCPCA selectively labeled on the position C-3 was synthetized with radiochemical purity >99%, an isolated yield of 637 mCi and specific activity = 28.9 Ci/mmol.

A parallel panning scheme used for selection of a GluA4-specific Fab targeting the ligand-binding domain.

A method for development of murine Fab fragments towards extracellular domains of a surface receptor is presented. The GluA4 ionotropic glutamate receptor is used as a model system. Recombinant GluA4 ectodomain comprising both the N-terminal domain (NTD) and the ligand-binding domain (LBD) in one molecule was used for immunization. A Fab-phage library was constructed and a parallel panning approach enabled selection of murine Fab fragments towards either intact ectodomain or the isolated LBD of the GluA4 receptor. One LBD-Fab (FabL9) showed exclusive selectivity for the GluA4 LBD, over a panel of LBDs from GluA2, GluK1, GluK2 and GluD2. Soluble FabL9 was produced in amounts suitable for characterization. Competitive ELISA and rat-brain immunoprecipitation experiments confirmed that the FabL9 epitope is conserved in the LBD and in the intact native receptor. By an alignment of GluA2 and GluA4, the likely binding epitope for FabL9 was predicted. This study demonstrates a simple approach for development of antibody fragments towards specific sub-domains of a large ligand-gated ion channel, and this method could be utilized for all multi-domain surface receptors where antibody domain-selectivity may be desirable. Furthermore, we present for the first time a GluA4 subtype-specific murine Fab fragment targeting the LBD of the receptor.

Positive allosteric modulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptors differentially modulates the behavioural effects of citalopram in mouse models of antidepressant and anxiolytic action.

Drugs that increase monoamine neurotransmission are effective in both anxiety and depression. The therapeutic effects of monoamine-based antidepressant drugs may involve indirect effects on neurotransmission through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptors (AMPAR). Thus, chronic antidepressant treatment increases AMPAR-mediated neurotransmission and AMPAR-positive allosteric modulators have shown antidepressant-like efficacy in rodents. Here, the effect of enhanced AMPAR neurotransmission on the antidepressant-like and anxiolytic-like actions of the selective serotonin reuptake inhibitor citalopram (0-10 mg/kg) was investigated in mice using the AMPAR-positive allosteric modulator LY451646 (0-3 mg/kg). Antidepressant-like effects were assessed using the forced-swim test (FST), whereas anxiolytic-like effects were tested using the elevated zero maze (EZM) and the marble burying test. LY451646 (3 mg/kg) increased swim distance in the FST and a subactive dose of LY451646 (1 mg/kg) enhanced the effect of citalopram in the FST. In the EZM, LY451646 (3 mg/kg) did not show anxiogenic effects alone, but blocked the anxiolytic-like action of citalopram in the EZM, as reflected by an increase in the latency to enter the open areas and a decrease in the number of entries and time spent in the open areas in citalopram-treated mice. In the marble burying test, LY451646 (3 mg/kg) showed no effect alone, but significantly attenuated the anxiolytic-like effect of citalopram (1.25-2.5 mg/kg) by increasing the number of marbles buried in citalopram-treated mice. These results suggest that AMPAR neurotransmission plays opposite roles in anxiety and depression as AMPAR potentiation facilitated the antidepressant-like effects of citalopram while attenuating its anxiolytic-like effect. These findings have ramifications in the search for AMPAR-based novel anxiolytic and antidepressant treatments.

Synthesis and Characterisation of Substrate-Based Peptides as Inhibitors of Histone Demethylase KDM4C.

The design and synthesis of modified pentapeptides based on a truncated version of the substrate for KDM4C, a histone lysine demethylase (KDM), and investigation of their inhibitory activity at KDM4C is reported. By modifying the lysine residue corresponding to lysine 9 at histone 3 (H3K9), three different series of peptides were designed and synthesized. One series contained N-acylated H3K9 and two series introduced triazoles in this position via click chemistry to enable facile variation of headgroups. The click reaction is compatible with free amino acids and this was performed on an azido containing deprotected pentapeptide demonstrating a highly facile and convergent synthetic strategy for making substrate-based inhibitors. One of the 14 peptides showed inhibitory activity at KDM4C demonstrating the need for an iron chelator in the pentapeptide series.

Pharmacology and Structural Analysis of Ligand Binding to the Orthosteric Site of Glutamate-Like GluD2 Receptors.

The GluD2 receptor is a fundamental component of postsynaptic sites in Purkinje neurons, and is required for normal cerebellar function. GluD2 and the closely related GluD1 are classified as members of the ionotropic glutamate receptor (iGluR) superfamily on the basis of sequence similarity, but do not bind l-glutamate. The amino acid neurotransmitter D-Ser is a GluD2 receptor ligand, and endogenous D-Ser signaling through GluD2 has recently been shown to regulate endocytosis of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type iGluRs during synaptic plasticity in the cerebellum, such as long-term depression. Here, we investigate the pharmacology of the orthosteric binding site in GluD2 by examining the activity of analogs of D-Ser and GluN1 glycine site competitive antagonists at GluD2 receptors containing the lurcher mutation (GluD2(LC)), which promotes spontaneous channel activation. We identify several compounds that modulate GluD2(LC), including a halogenated alanine analog as well as the kynurenic acid analog 7-chloro-4-oxo-1H-quinoline-2-carboxylic acid (7-chlorokynurenic acid; 7-CKA). By correlating thermodynamic and structural data for 7-CKA binding to the isolated GluD2 ligand binding domain (GluD2-LBD), we find that binding 7-CKA to GluD2-LBD differs from D-Ser by inducing an intermediate cleft closure of the clamshell-shaped LBD. The GluD2 ligands identified here can potentially serve as a starting point for development of GluD2-selective ligands useful as tools in studies of the signaling role of the GluD2 receptor in the brain.