A new highly selective metabotropic excitatory amino acid agonist: 2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid

The influence of glutamate receptors on insulin release and diabetic neuropathy

Diabetes causes macrovascular and microvascular complications such as peripheral neuropathy. Glutamate regulates insulin secretion in pancreatic β-cells, and its increased activity in the central nervous system is associated with peripheral neuropathy in animal models of diabetes. One strategy to modulate glutamatergic activity consists in the pharmacological manipulation of metabotropic glutamate receptors (mGluRs), which, compared to the ionotropic receptors, allow for a fine-tuning of neurotransmission that is compatible with therapeutic interventions. mGluRs are a family of eight G-protein coupled receptors classified into three groups (I-III) based on sequence homology, transduction mechanisms, and pharmacology. Activation of group II and III or inhibition of group I represents a strategy to counteract the glutamatergic hyperactivity associated with diabetic neuropathy. In this review article, we will discuss the role of glutamate receptors in the release of insulin and the development/treatment of diabetic neuropathy, with particular emphasis on their manipulation to prevent the glutamatergic hyperactivity associated with diabetic neuropathy.

Glutamate acts on acid-sensing ion channels to worsen ischaemic brain injury

Glutamate is traditionally viewed as the first messenger to activate NMDAR (N-methyl-D-aspartate receptor)-dependent cell death pathways in stroke1,2, but unsuccessful clinical trials with NMDAR antagonists implicate the engagement of other mechanisms3-7. Here we show that glutamate and its structural analogues, including NMDAR antagonist L-AP5 (also known as APV), robustly potentiate currents mediated by acid-sensing ion channels (ASICs) associated with acidosis-induced neurotoxicity in stroke4. Glutamate increases the affinity of ASICs for protons and their open probability, aggravating ischaemic neurotoxicity in both in vitro and in vivo models. Site-directed mutagenesis, structure-based modelling and functional assays reveal a bona fide glutamate-binding cavity in the extracellular domain of ASIC1a. Computational drug screening identified a small molecule, LK-2, that binds to this cavity and abolishes glutamate-dependent potentiation of ASIC currents but spares NMDARs. LK-2 reduces the infarct volume and improves sensorimotor recovery in a mouse model of ischaemic stroke, reminiscent of that seen in mice with Asic1a knockout or knockout of other cation channels4-7. We conclude that glutamate functions as a positive allosteric modulator for ASICs to exacerbate neurotoxicity, and preferential targeting of the glutamate-binding site on ASICs over that on NMDARs may be strategized for developing stroke therapeutics lacking the psychotic side effects of NMDAR antagonists.

Tandem mass spectrometry of homologous 3-hydroxyfurazan and nitrile amino acids: Analysis of cooperative interactions and fragmentation processes

The assignment of structure by tandem mass spectrometry (MS/MS) relies on the interpretation of the fragmentation behavior of gas-phase ions. Mass spectra were acquired for a series of heterocyclic mimetics of acidic amino acids and a related series of nitrile amino acids. All amino acids were readily protonated or deprotonated by electrospray ionization (ESI), and distinctive fragmentation processes were observed when the ions were subjected to collision-induced dissociation (CID). The deprotonated heterocycles showed bond cleavages of the 3-hydroxyfurazan ring with formation of oxoisocyanate and the complementary deprotonated nitrile amino acid. Further fragmentation of the deprotonated nitrile amino acids was greatly dependent on the length of the alkyl nitrile side chain. Competing losses of CO2 versus HCN occurred from α-cyanoglycinate (shortest chain), whereas water was lost from 2-amino-5-cyanopentanoate (longest chain). Interestingly, loss of acrylonitrile by a McLafferty-type fragmentation process was detected for 2-amino-4-cyanobutanoate, and several competing processes were observed for β-cyanoalanate. In one process, cyanide ion was formed either by consecutive losses of ammonia, carbon dioxide, and acetylene or by a one-step decarboxylative elimination. In another, complementary ions were obtained from β-cyanoalanate by loss of acetonitrile or HN=CHCO2H. Fragmentation of the protonated 3-hydroxyfurazan and nitrile amino acids resulted in the cumulative loss (H2O + CO), a loss that is commonly observed for protonated aliphatic α-amino acids. Overall, the distinct fragmentation behavior of the multifunctional 3-hydroxyfurazan amino acids correlated with the charged site, whereas fragmentations of the deprotonated nitrile amino acids showed cooperative interactions between the nitrile and the carboxylate groups.

Clavulanic Acid Improves Memory Dysfunction and Anxiety Behaviors through Upregulating Glutamatergic Transporters in the Nucleus Accumbens of Mice Repeatedly Exposed to Khat Extract

Khat (Catha edulis) is an evergreen shrub whose buds and leaves give a state of delight and euphoria when chewed. Cathinone, an amphetamine-like stimulant that is among the active ingredients in khat, is able to downregulate glutamate transporter subtype I (GLT-1). Neurobehavioral dysfunctions such as altered locomotor activity, anorexia, and nociception have been observed in animals exposed to cathinone. Interestingly, treatment with a β-lactam antibiotic such as ceftriaxone, which upregulates GLT-1, normalizes cathinone-induced conditioned place preference, and alters repetitive movements in rats. However, little is known about the role of the glutamatergic system in memory dysfunction and anxiety-like behaviors in mice exposed to khat. We found here that clavulanic acid, a β-lactam-containing compound and GLT-1 upregulator, would modulate the neurobehavioral changes, including memory impairment and anxiety-like behaviors, associated with repeated exposure of mice to khat. Our data supported that clavulanic acid could improve memory impairment and anxiety-like behaviors through upregulating GLT-1 in the nucleus accumbens (NAc), an effect abolished with a selective GLT-1 blocker. This upregulation was associated with restored glutamate/cystine antiporter expression in the NAc using a Western blotting assay. Cathine and cathinone were identified in khat extract using the gas chromatography technique. Our work provides preclinical insight into the efficacy of β-lactam-containing compounds for the attenuation of neurobehavioral changes induced by khat exposure.

The Cold Case of Metabotropic Glutamate Receptor 6: Unjust Detention in the Retina?

It is a common opinion that metabotropic glutamate receptor subtype 6 (mGluR6) is expressed exclusively in the retina, and in particular in the dendrites of ON-bipolar cells. Glutamate released in darkness from photoreceptors activates mGluR6, which is negatively associated with a membrane non-selective cation channel, the transient receptor potential melanoma-related 1, TRPM1, resulting in cell hyperpolarization. The evidence that mGluR6 is expressed not only in the retina but also in other tissues and cell populations has accumulated over time. The expression of mGluR6 has been identified in microglia, bone marrow stromal and prostate cancer cells, B lymphocytes, melanocytes and keratinocytes and non-neural tissues such as testis, kidney, cornea, conjunctiva, and eyelid. The receptor also appears to be expressed in brain areas, such as the hypothalamus, cortex, hippocampus, nucleus of tractus solitarius, superior colliculus, axons of the corpus callosum and accessory olfactory bulb. The pharmacological activation of mGluR6 in the hippocampus produced an anxiolytic-like effect and in the periaqueductal gray analgesic potential. This review aims to collect all the evidence on the expression and functioning of mGluR6 outside the retina that has been accumulated over the years for a broader view of the potential of the receptor whose retinal confinement appears understimated.

Unusual Amino Acids in Medicinal Chemistry

Unusual amino acids are fundamental building blocks of modern medicinal chemistry. The combination of readily functionalized amine and carboxyl groups attached to a chiral central core along with one or two potentially diverse side chains provides a unique three-dimensional structure with a high degree of functionality. This makes them invaluable as starting materials for syntheses of complex molecules, highly diverse elements for SAR campaigns, integral components of peptidomimetic drugs, and potential drugs on their own. This Perspective highlights the diversity of unnatural amino acid structures found in hit-to-lead and lead optimization campaigns and clinical stage and approved drugs, reflecting their increasingly important role in medicinal chemistry.

Functional imaging of oxidative stress with a novel PET imaging agent, 18F-5-fluoro-L-aminosuberic acid

Glutathione is the predominant endogenous cellular antioxidant, playing a critical role in the cellular defensive response to oxidative stress by neutralizing free radicals and reactive oxygen species. With cysteine as the rate-limiting substrate in glutathione biosynthesis, the cystine/glutamate transporter (system xc(-)) represents a potentially attractive PET biomarker to enable in vivo quantification of xc(-) activity in response to oxidative stress associated with disease. We have developed a system xc(-) substrate that incorporates characteristics of both natural substrates, L-cystine and L-glutamate (L-Glu). L-aminosuberic acid (L-ASu) has been identified as a more efficient system xc(-) substrate than L-Glu, leading to an assessment of a series of anionic amino acids as prospective PET tracers. Herein, we report the synthesis and in vitro and in vivo validation of a lead candidate, (18)F-5-fluoro-aminosuberic acid ((18)F-FASu), as a PET tracer for functional imaging of a cellular response to oxidative stress with remarkable tumor uptake and retention.

METHODS

(18)F-FASu was identified as a potential PET tracer based on an in vitro screening of compounds similar to L-cystine and L-Glu. Affinity toward system xc(-) was determined via in vitro uptake and inhibition studies using oxidative stress-induced EL4 and SKOV-3 cells. In vivo biodistribution and PET imaging studies were performed in mice bearing xenograft tumors (EL4 and SKOV-3).

RESULTS

In vitro assay results determined that L-ASu inhibited system xc(-) as well as or better than L-Glu. The direct comparison of uptake of tritiated compounds demonstrated more efficient system xc(-) uptake of L-ASu than L-Glu. Radiosynthesis of (18)F-FASu allowed the validation of uptake for the fluorine-bearing derivative in vitro. Evaluation in vivo demonstrated primarily renal clearance and uptake of approximately 8 percentage injected dose per gram in SKOV-3 tumors, with tumor-to-blood and tumor-to-muscle ratios of approximately 12 and approximately 28, respectively. (18)F-FASu uptake was approximately 5 times greater than (18)F-FDG uptake in SKOV-3 tumors. Dynamic PET imaging demonstrated uptake in EL4 tumor xenografts of approximately 6 percentage injected dose per gram and good tumor retention for at least 2 h after injection.

CONCLUSION

(18)F-FASu is a potentially useful metabolic tracer for PET imaging of a functional cellular response to oxidative stress. (18)F-FASu may provide more sensitive detection than (18)F-FDG in certain tumors.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Synthesis of C5-tetrazole derivatives of 2-amino-adipic acid displaying NMDA glutamate receptor antagonism

Five derivatives of 2-amino-adipic acid bearing a tetrazole-substituted in C5 position were synthesized. These compounds displayed selective antagonism towards N-methyl-D: -aspartate (NMDA) receptors compared with AMPA receptors, and they were devoid of any neurotoxicity. Among these five analogues, one exhibited a higher affinity for synaptic NMDA responses than the other four. Therefore, C5 tetrazole-substituted of 2-amino-adipic acid represent an interesting series of new NMDA receptor antagonists. This approach may be considered as a new strategy to develop ligands specifically targeted to synaptic or extra-synaptic NMDA receptors.

Metabotropic glutamate receptors: their therapeutic potential in anxiety

Psychiatric and neurological disorders are linked to changes in synaptic excitatory processes with a key role for glutamate, that is, the most abundant excitatory amino-acid. Molecular cloning of the metabotropic glutamate (mGlu) receptors has led to the identification of eight mGlu receptors, which, in contrast to ligand-gated ion channels (responsible for fast excitatory transmission), modulate and fine-tune the efficacy of synaptic transmission. mGlu receptors are G protein-coupled and constitute a new group of "drugable" targets for the treatment of various CNS disorders. The recent discovery of small molecules that selectively bind to receptors of Groups I (mGlu1 and mGlu5) and II (mGlu2 and mGlu3) allowed significant advances in our understanding of the roles of these receptors in brain function and dysfunction including anxiety. Although investigation of the role of the Group III (mGlu4, 6, 7, and 8) receptors is less advanced, the generation of genetically manipulated animals and recent advances in the identification of subtype-selective compounds have revealed some first insights into the therapeutic potential of this group of receptors.

Group III metabotropic glutamate receptors (mGluRs) modulate transmission of gustatory inputs in the brain stem

Glutamate is the principal neurotransmitter at the primary sensory afferent synapse in the medulla for the taste system. At this synapse, glutamate activates N-methyl-D-aspartate (NMDA) and non-NMDA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and kainate) ionotropic receptors to effect a response in the second-order neurons. The current experiment is the first to examine the role of metabotropic glutamate receptors (mGluRs) in the transmission of taste information. In an in vitro slice preparation of the primary vagal gustatory nucleus in goldfish, primary gustatory afferent fibers were stimulated electrically, whereas evoked dendritic field potentials were recorded in the sensory layers. Recordings were made before, during, and after bath application of mGluR agonists for various mGluR groups and subtypes. Whereas L-AP4, a group III agonist, reduced the field potential, group I and group II agonists had no effect. Furthermore, the selective mGluR4 agonist ACPT-III and mGluR8 agonist PPG were effective at reducing the field potential, whereas agonists selective for mGluR6 and 7 were not. MAP4, a group III mGluR antagonist, attenuated frequency-dependent depression, indicating that endogenous glutamate binds to presynaptic mGluRs under normal conditions. Furthermore, polymerase chain reaction showed that mRNA for mGluR4 and 8 is expressed in the vagal ganglia, a prerequisite if those receptors are expressed presynaptically in the vagal lobe. Collectively, these experiments indicate that mGluR4 and 8 are presynaptic at the primary gustatory afferent synapse and that their activation inhibits glutamatergic release.

Glutamate stimulates oligodendrocyte progenitor migration mediated via an alphav integrin/myelin proteolipid protein complex

In the mammalian CNS, oligodendrocyte precursor cells (OPCs) express most neurotransmitter receptors, but their function remains unclear. The current studies suggest a physiological role for glutamate (AMPA and/or kainate) receptors in OPC migration. AMPA stimulated alphav integrin-mediated OPC migration by increasing both the rate of cell movement and the frequency of Ca2+ transients. A protein complex containing the myelin proteolipid protein (PLP) and alphav integrin modulated the AMPA-stimulated migration, and stimulation of OPC AMPA receptors resulted in increased association of the AMPA receptor subunits themselves with the alphav integrin/PLP complex. Thus, after AMPA receptor stimulation, an alphav integrin/PLP/neurotransmitter receptor protein complex forms that reduces binding to the extracellular matrix and enhances OPC migration. To assess the extent to which PLP was involved in the AMPA-stimulated migration, OPCs from the myelin-deficient (MD) rat, which has a PLP gene mutation, were analyzed. OPCs from the MD rat had a normal basal migration rate, but AMPA did not stimulate the migration of these cells, suggesting that the PLP/alphav integrin complex was important for the AMPA-mediated induction. AMPA-induced modulation of OPC migration was abolished by pertussis toxin, although baseline migration was normal. Thus, G-protein-dependent signaling is crucial for AMPA-stimulated migration of OPCs but not for basal OPC migration. Other signaling pathways involved in this AMPA-stimulated OPC migration were also determined. These studies highlight novel signaling determinants of OPC migration and suggest that glutamate could play a pivotal role in regulating integrin-mediated OPC migration.

The respective N-hydroxypyrazole analogues of the classical glutamate receptor ligands ibotenic acid and (RS)-2-amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid

We have determined the pharmacological activity of N-hydroxypyrazole analogues (3a and 4a) of the classical glutamate receptor ligands ibotenic acid and (RS)-2-amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid (AMAA), as well as substituted derivatives of these two compounds. The pharmacological profile of 3a is closer to that of thioibotenic acid rather than ibotenic acid, while 4a is a selective N-methyl-D-aspartic acid (NMDA) receptor agonist. Ring substitution of 3a and 4a leads to NMDA receptor antagonists. Whereas efficacy of 3a derivatives at mglu2 receptor decreases from agonism via partial agonism to antagonism with increasing substituent size, substitution abolishes affinity for mglu1 and mglu4 receptors. Ligand- and receptor-based modelling approaches assist in explaining these pharmacological trends among the metabotropic receptors and suggest a mechanism of partial agonism at mglu2 receptor similar to that proposed for the GluR2 glutamate receptor.

Group III mGlu receptor agonists produce anxiolytic- and antidepressant-like effects after central administration in rats

It was well established that compounds which decrease glutamatergic transmission via blockade of NMDA or group I mGlu receptors produce anxiolytic- and antidepressant-like action in animal tests and models. Since group III metabotropic glutamate receptor (mGluR) agonists are known to reduce glutamatergic neurotransmission by the inhibition of glutamate release, we decided to investigate potential anxiolytic- and/or antidepressant-like effects of group III mGluR agonists, after central administration in rats. It was found that group III mGluR agonists, (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid (ACPT-I) and 2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid (HomoAMPA), given intrahippocampally, produced a dose-dependent anxiolytic-like effect in the conflict drinking test. The effects of ACPT-I and HomoAMPA were reversed by (RS)-alpha-cyclopropyl-4-phosphonophenyl glycine (CPPG), group III mGluR antagonist. Moreover, a dose-dependent antidepressant-like action of group III mGluR agonists, ACPT-I and (RS)-4-phosphonophenylglycine (RS-PPG), but not HomoAMPA, was found in behavioral despair test, after intracerebroventricular injections, and the effect of ACPT-I was reversed by CPPG. The results obtained indicate that group III mGluR agonists produce anxiolytic- as well as antidepressant-like effects in behavioral tests, after central administration in rats. The reduction of glutamate release by group III mGluR activation may be a possible mechanism underlying anxiolytic- and antidepressant-like properties of the tested compounds. In conclusion, the results of our studies indicate that group III mGlu receptor agonists may play a role in the therapy of both anxiety and depression.

Synthesis and anticonvulsant activity of novel bicyclic acidic amino acids

Bicyclic acidic amino acids (+/-)-6 and (+/-)-7, which are conformationally constrained homologues of glutamic acid, were prepared via a strategy based on a 1,3-dipolar cycloaddition. The new amino acids were tested toward ionotropic and metabotropic glutamate receptor subtypes; both of them behaved as antagonists at mGluR1,5 and as agonists at mGluR2. Furthermore, whereas (+/-)-6 was inactive at all ionotropic glutamate receptors, (+/-)-7 displayed a quite potent antagonism at the NMDA receptors. In the in vivo tests on DBA/2 mice, the compounds displayed an anticonvulsant activity. The interesting pharmacological profile of (+/-)-7 qualifies it as a lead of novel neuroprotective agents.

(S)-2-Amino-3-(3-hydroxy-7,8-dihydro-6H-cyclohepta[d]isoxazol-4-yl)propionic acid, a potent and selective agonist at the GluR5 subtype of ionotropic glutamate receptors. Synthesis, modeling, and molecular pharmacology

We have previously described (RS)-2-amino-3-(3-hydroxy-7,8-dihydro-6H-cyclohepta[d]isoxazol-4-yl)propionic acid (4-AHCP) as a highly effective agonist at non-N-methyl-d-aspartate (non-NMDA) glutamate (Glu) receptors in vivo, which is more potent than (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) but inactive at NMDA receptors. However, 4-AHCP was found to be much weaker than AMPA as an inhibitor of [(3)H]AMPA binding and to have limited effect in a [(3)H]kainic acid binding assay using rat cortical membranes. To shed light on the mechanism(s) underlying this quite enigmatic pharmacological profile of 4-AHCP, we have now developed a synthesis of (S)-4-AHCP (6) and (R)-4-AHCP (7). At cloned metabotropic Glu receptors mGluR1alpha (group I), mGluR2 (group II), and mGluR4a (group III), neither 6 nor 7 showed significant agonist or antagonist effects. The stereoisomer 6, but not 7, activated cloned AMPA receptor subunits GluR1o, GluR3o, and GluR4o with EC(50) values in the range 4.5-15 microM and the coexpressed kainate-preferring subunits GluR6 + KA2 (EC(50) = 6.4 microM). Compound 6, but not 7, proved to be a very potent agonist (EC(50) = 0.13 microM) at the kainate-preferring GluR5 subunit, equipotent with (S)-2-amino-3-(5-tert-butyl-3-hydroxyisothiazol-4-yl)propionic acid [(S)-Thio-ATPA, 4] and almost 4 times more potent than (S)-2-amino-3-(5-tert-butyl-3-hydroxyisoxazol-4-yl)propionic acid [(S)-ATPA, 3]. Compound 6 thus represents a new structural class of GluR5 agonists. Molecular modeling and docking to a crystal structure of the extracellular binding domain of the AMPA subunit GluR2 has enabled identification of the probable active conformation and binding mode of 6. We are able to rationalize the observed selectivities by comparing the docking of 4 and 6 to subtype constructs, i.e., a crystal structure of the extracellular binding domain of GluR2 and a homology model of GluR5.

Rational design, synthesis, and pharmacological evaluation of 2-azanorbornane-3-exo,5-endo-dicarboxylic acid: a novel conformationally restricted glutamic acid analogue

The design and synthesis of conformationally restricted analogues of alpha-amino acids is an often used strategy in medicinal chemistry research. Here we present the rational design, synthesis, and pharmacological evaluation of 2-azanorbornane-3-exo,5-endo-dicarboxylic acid (1), a novel conformationally restricted (S)-glutamic acid (Glu) analogue intended as a mimic of the folded Glu conformation. The synthesis of 1 was completed in its racemic form in eight steps from commercially available starting materials. As a key step, the first facially selective hydroboration of a 5-methylidene[2.2.1]bicyclic intermediate was investigated. In this transformation, the catalytic methodology of Wilkinson's/catechol borane proved superior to stoichiometric borane or dialkyl borane reagents, in terms of higher diastereomeric excess and chemical yield. To our surprise (+/-)-1 did not show affinity in binding studies on native 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) (IC(50) > 300 microM, [(3)H]AMPA) or kainic acid (IC(50) > 160 microM, [(3)H]kainic acid) receptors nor in binding studies on the cloned iGluR5,6 subtypes (IC(50) > 300 microM, [(3)H]kainic acid).

Synthesis and pharmacology of 3-hydroxy-delta2-isoxazoline-cyclopentane analogues of glutamic acid

The synthesis and pharmacology of two potential glutamic acid receptor ligands are described. Preparation of the bicyclic 3-hydroxy-delta2-isoxazoline-cyclopentane derivatives (+/-)-7 and (+/-)-8 was accomplished via 1,3-dipolar cycloaddition of bromonitrile oxide to suitably protected 1-amino-cyclopent-3-enecarboxylic acids. Their structure was established using a combination of 1H NMR spectroscopy and molecular mechanics calculations carried out on the intermediate cycloadducts (+/-)-11 and (+/-)-12. Amino acid derivatives (+/-)-7 and (+/-)-8 were assayed at ionotropic and metabotropic glutamic acid receptor subtypes and their activity compared with that of trans-ACPD and cis-ACPD. The results show that the replacement of the omega-carboxylic group of the model compounds with the 3-hydroxy-delta2-isoxazoline moiety abolishes or reduces drastically the activity at the metabotropic glutamate receptors. Conversely, on passing from cis-ACPD to derivative (+/-)-8, the agonist activity at NMDA receptors is almost unaffected.

Selective agonists at group II metabotropic glutamate receptors: synthesis, stereochemistry, and molecular pharmacology of (S)- and (R)-2-amino-4-(4-hydroxy[1,2,5]thiadiazol-3-yl)butyric acid

Homologation of analogues of the central excitatory neurotransmitter glutamic acid (Glu), in which the distal carboxy group has been bioisosterically replaced by acidic heterocyclic units, has previously provided subtype selective ligands for metabotropic Glu receptors (mGluRs). The (S)-form of the 1,2,5-thiadiazol-3-ol Glu analogue, 2-amino-3-(4-hydroxy[1,2,5]thiadiazol-3-yl)propionic acid (TDPA, 6), is an 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor agonist, which in addition stereospecifically activates group I mGluRs. We have now synthesized the (S)- and (R)-forms of 2-amino-4-(4-hydroxy[1,2,5]thiadiazol-3-yl)butyric acid (homo-TDPA, 7) and shown that whereas neither enantiomer interacts with AMPA receptors, (S)- and (R)-7 appear to be selective and equipotent agonists at group II mGluRs as represented by the mGluR2 subtype. The activities of (S)- and (R)-7 are rationalized by conformational analysis, comparison with the potent and specific group II mGluR agonist (-)-LY379268 [(-)-12], and docking to a homology model of mGluR2.

Glutamate receptor ligands: synthesis, stereochemistry, and enantiopharmacology of methylated 2-aminoadipic acid analogs

Homologation and substitution on the carbon backbone of (S)-glutamic acid [(S)-Glu, 1], as well as absolute stereochemistry, are structural parameters of key importance for the pharmacological profile of (S)-Glu receptor ligands. We describe a series of methyl-substituted 2-aminoadipic acid (AA) analogs, and the synthesis, stereochemistry, and enantiopharmacology of 3-methyl-AA (4a-d), 4-methyl-AA (5a-d), 5-methyl-AA (6a-d), and (E)-Delta(4)-5-methyl-AA (7a and 7b) are reported. The compounds were resolved using chiral HPLC and the configurational assignments of the enantiomers were based on X-ray crystallographic analyses, chemical correlation, and CD spectral analyses. The effects of the individual stereoisomers at ionotropic and metabotropic (S)-Glu receptors (iGluRs and mGluRs) were characterized. Compounds with S-configuration at the alpha-carbon generally showed mGluR2 agonist activity of similar or slightly lower potencies than (S)-AA [e.g., EC(50) = 76 microM for (2S,4S)-4-methyl-AA (5a) as compared to EC(50) = 35 microM for (S)-AA]. The position of the methyl substituent had a profound effect on the observed pharmacology, whereas the absolute stereochemistry at the methylated carbon atom had a very limited effect on pharmacology. Structure-activity relationships at iGluRs in the rat cortical wedge preparation showed a complex pattern, some compounds being NMDA receptor agonists [e.g., EC(50) =110 microM for (2S,5RS)-5-methyl-AA (6a,b)] and some compounds showing NMDA receptor antagonist effects [e.g., IC(50) = 300 microM for (2R,4S)-4-methyl-AA (5d)]. The two unsaturated analogs (S)- (7a) and (R)-(E)-Delta(4)-5-methyl-AA (7b) turned out to be a weak AMPA receptor agonist and a weak mixed NMDA/AMPA receptor antagonist, respectively.

Selective antagonists at group I metabotropic glutamate receptors: synthesis and molecular pharmacology of 4-aryl-3-isoxazolol amino acids

Homologation of (S)-glutamic acid (Glu, 1) and Glu analogues has previously provided ligands with activity at metabotropic Glu receptors (mGluRs). The homologue of ibotenic acid (7), 2-amino-3-(3-hydroxy-5-isoxazolyl)propionic acid (HIBO, 8), and the 4-phenyl derivative of 8, compound 9a, are both antagonists at group I mGluRs. Here we report the synthesis and molecular pharmacology of HIBO analogues 9b-h containing different 4-aryl substituents. All of these compounds possess antagonist activity at group I mGluRs but are inactive at group II and III mGluRs.

Signaling pathways activated by chemokine receptor CXCR2 and AMPA-type glutamate receptors and involvement in granule cells survival

We show that treatment of cerebellar granules with interleukin-8 (IL-8), growth-related gene product beta (GRObeta) or AMPA induced activation of PI3-K/Akt and of ERK pathways, the latter being independent of PI3-K and dependent on PTX-sensitive G proteins. We also show that AMPA-mediated neuron survival was abolished both by ERK kinase inhibitor PD98059 and AMPA-Rs blocker CNQX, and that chemokine-mediated survival was blocked by the PI3-K inhibitors LY294002 and wortmannin. We conclude that the neurotrophic effects of AMPA need the contemporary activation of ERKs and stimulation of AMPA-Rs, and that PI3-K/Akt activation is a determinant pathway for the IL-8/GRObeta anti-apoptotic activity.

Novel 1-hydroxyazole bioisosteres of glutamic acid. Synthesis, protolytic properties, and pharmacology

A number of 1-hydroxyazole derivatives were synthesized as bioisosteres of (S)-glutamic acid (Glu) and as analogues of the AMPA receptor agonist (R,S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA, 3b). All compounds were subjected to in vitro pharmacological studies, including a series of Glu receptor binding assays, uptake studies on native as well as cloned Glu uptake systems, and the electrophysiological rat cortical slice model. Compounds 7a,b, analogues of AMPA bearing a 1-hydroxy-5-pyrazolyl moiety as the distal carboxylic functionality, showed only moderate affinity for [3H]AMPA receptor binding sites (IC(50) = 2.7 +/- 0.4 microM and IC(50) = 2.6 +/- 0.6 microM, respectively), correlating with electrophysiological data from the rat cortical wedge model (EC(50) = 280 +/- 48 microM and EC(50) = 586 +/- 41 microM, respectively). 1-Hydroxy-1,2,3-triazol-5-yl analogues of AMPA, compounds 8a,b, showed high affinity for [3H]AMPA receptor binding sites (IC(50) = 0.15 +/- 0.03 microM and IC(50) = 0.13 +/- 0.02 microM, respectively). Electrophysiological data showed that compound 8a was devoid of activity in the rat cortical wedge model (EC(50) > 1000 microM), whereas the corresponding 4-methyl analogue 8b was a potent AMPA receptor agonist (EC(50) = 15 +/- 2 microM). In accordance with this disparity, compound 8a was found to inhibit synaptosomal [3H]D-aspartic acid uptake (IC(50) = 93 +/- 25 microM), as well as excitatory amino acid transporters (EAATs) EAAT1 (IC(50) = 100 +/- 30 microM) and EAAT2 (IC(50) = 300 +/- 80 microM). By contrast, compound 8b showed no appreciable affinity for Glu uptake sites, neither synaptosomal nor cloned. Compounds 9a-c and 10a,b, possessing 1-hydroxyimidazole as the terminal acidic function, were devoid of activity in all of the systems tested. Protolytic properties of compounds 7a,b, 8b, and 9b were determined by titration, and a correlation between the pK(a) values and the activity at AMPA receptors was apparent. Optimized structures of all the synthesized ligands were fitted to the known crystal structure of an AMPA-GluR2 construct. Where substantial reduction or abolition of affinity at AMPA receptors was observed, this could be rationalized on the basis of the ability of the ligand to fit the construct. The results presented in this article point to the utility of 1-hydroxypyrazole and 1,2,3-hydroxytriazole as bioisosteres of carboxylic acids at Glu receptors and transporters. None of the compounds showed significant activity at metabotropic Glu receptors.

Synthesis and receptor binding affinity of new selective GluR5 ligands

Two hybrid analogues of the kainic acid receptor agonists, 2-amino-3-(5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid (ATPA) and (2S,4R)-4-methylglutamic acid ((2S,4R)-4-Me-Glu), were designed, synthesized, and characterized in radioligand binding assays using cloned ionotropic and metabotropic glutamic acid receptors. The (S)-enantiomers of E-4-(2,2-dimethylpropylidene)glutamic acid ((S)-1) and E-4-(3,3-dimethylbutylidene)glutamic acid ((S)-2) were shown to be selective and high affinity GluR5 ligands, with Ki values of 0.024 and 0.39 microM, respectively, compared to Ki values at GluR2 of 3.0 and 2.0 microM. respectively. Their affinities in the [3H]AMPA binding assay on native cortical receptors were shown to correlate with their GluR2 affinity rather than their GluR5 affinity. No affinity for GluR6 was detected (IC50 > 100 microM).

3-Pyrazolone analogues of the 3-isoxazolol metabotropic excitatory amino acid receptor agonist homo-AMPA. Synthesis and pharmacological testing

We have previously shown that the higher homologue of (S)-glutamic acid [(S)-Glu], (S)-alpha-aminoadipic acid [(S)-alpha-AA] is selectively recognized by the mGlu(2) and mGlu(6) subtypes of the family of metabotropic glutamic acid (mGlu) receptors. Furthermore, a number of analogues of (S)-alpha-AA, in which the terminal carboxyl group has been replaced by various bioisosteric groups, such as phosphonic acid or 3-isoxazolol groups, have been shown to interact selectively with different subtypes of mGlu receptors. In this paper we report the synthesis of the 3-pyrazolone bioisosteres of alpha-AA, compounds (RS)-2-amino-4-(1,2-dihydro-5-methyl-3-oxo-3H-pyrazol-4-yl)butyric acid (1) and (RS)-2-amino-4-(1,2-dihydro-1,5-dimethyl-3-oxo-3H-pyrazol-4-yl)butyric acid (2). At a number of steps in the reaction sequences used, the reactions took unexpected courses and provided products which could not be transformed into the target compounds, and attempts to synthesize the 2,5-dimethyl isomer of 2, compound 3, failed. An X-ray crystallographic analysis of the intermediate 1,2-dihydro-4-(2-hydroxyethyl)-2,5-dimethyl-3H-pyrazol-3-one (5b) confirmed the expected regioselectivity of the reaction between methylhydrazine and alpha-acetylbutyrolactone (4). Neither 1 nor 2 showed significant effects at the different types of ionotropic glutamic acid receptors or at mGlu(1a) (group I), mGlu(2) (group II), and mGlu(4a) and mGlu(6) (group III) receptors, representing the three indicated groups of mGlu receptors.

Synthesis and enantiopharmacology of new AMPA-kainate receptor agonists

Regioisomeric 3-carboxyisoxazolinyl prolines [CIP-A (+/-)-6 and CIP-B (+/-)-7] and 3-hydroxyisoxazolinyl prolines [(+/-)-8 and (+/-)-9] were synthesized and assayed for glutamate receptor activity. The tests were carried out in vitro by means of receptor binding techniques, second messenger assays, and the rat cortical wedge preparation. CIP-A showed a good affinity for both 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) and kainic acid (KAIN) receptors. These results were confirmed in the cortical slice model where CIP-A displayed an EC(50) value very close to that of AMPA. The convulsant properties of all the compounds were evaluated in vivo on DBA/2 mice after icv injection. CIP-A showed a convulsant activity, measured as tonus and clonus seizures, 18-65 times higher than that produced by AMPA. It was also quite active after ip administration, since it induced seizures in mice at doses as low as 3.2 nmol/mouse. On the basis of the above-reported results we prepared and tested the enantiomers of CIP-A and CIP-B, obtained by reacting (S)-3,4-didehydroproline and (R)-3,4-didehydroproline, respectively, with ethoxycarbonylformonitrile oxide. In all the tests the S-form, CIP-AS [(-)-6], emerged as the eutomer evidencing common stereochemical requirements with the reference compounds AMPA and KAIN. Through modeling studies, carried out on CIP-A, AMPA, and KAIN, active conformations for CIP-AS and AMPA at AMPA receptors as well as for CIP-AS and KAIN at KAIN receptors are suggested.

Pharmacological agents acting at subtypes of metabotropic glutamate receptors

Metabotropic (G-protein-coupled) glutamate (mGlu) receptors have now emerged as a recognized, but still relatively new area of excitatory amino acid research. Current understanding of the roles and involvement of mGlu receptor subtypes in physiological/pathophysiological functions of the central nervous system has been recently propelled by the emergence of various structurally novel, potent, and mGlu receptor selective pharmacological agents. This article reviews the evolution of pharmacological agents that have been reported to target mGlu receptors, with a focus on the known receptor subtype selectivities of current agents.

Synthesis and pharmacology of N-alkylated derivatives of the excitotoxin ibotenic acid

Three amino-alkylated derivatives of the naturally occurring excitatory amino acid (EAA) receptor agonist ibotenic acid (Ibo) have been synthesized and tested pharmacologically. N-Methyl-Ibo (1a) and N-ethyl-Ibo (1b) were shown to be agonists at NMDA receptors (EC50 = 140 and 320 microM, respectively), though with activities considerably lower than Ibo (EC50 = 9.6 microM). N-Benzyl-Ibo (1c) was inactive at ionotropic EAA receptors and all three compounds were, in contrast to Ibo, inactive at metabotropic EAA receptors. Molecular mechanics calculations have been performed on Ibo, 1a-c and the potent NMDA agonist 2-amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid (AMAA) in order to elucidate the observed structure-activity data.

Molecular pharmacology of homologues of ibotenic acid at cloned metabotropic glutamic acid receptors

We have studied the effects of the enantiomers of 2-amino-3-(3-hydroxyisoxazol-5-yl)propionic acid (homoibotenic acid, HIBO) and analogues substituted with a methyl, bromo or butyl group in the four position of the ring at cloned metabotropic glutamate (mGlu) receptors expressed in Chinese hamster ovary (CHO) cells. In contrast to the parent compound ibotenic acid, which is a potent group I and II agonist, the (S)-forms of homoibotenic acid and its analogues are selective and potent group I antagonists whereas the (R)-forms are inactive both as agonists and antagonists at group I, II, and III mGlu receptors. Interestingly, (S)-homoibotenic acid and the analogues display equal potency at both mGlu1alpha and mGlu5a with Ki values in the range of 97 to 490 microM, (S)-homoibotenic acid and (S)-2-amino-3-(4-butyl-3-hydroxyisoxazol-5-yl)propionic acid [(S)-4-butylhomoibotenic acid] displaying the lowest and highest potency, respectively. The homoibotenic acid analogues thereby differ from mGlu receptor antagonists derived from phenylglycine such as (S)-4-carboxyphenylglycine which only antagonizes mGlu1alpha (Ki = 18 microM) showing no effect at mGlu5a (Ki > 300 microM).

Expression and purification of the extracellular ligand binding region of metabotropic glutamate receptor subtype 1

Each metabotropic glutamate receptor possesses a large extracellular domain that consists of a sequence homologous to the bacterial periplasmic binding proteins and a cysteine-rich region. Previous experiments have proposed that the extracellular domain is responsible for ligand binding. However, it is currently unknown whether the extracellular ligand binding site can bind ligands without other domains of the receptor. We began by obtaining a sufficient amount of receptor protein on a baculovirus expression system. In addition to the transfer vector that encodes the entire coding region, transfer vectors that encode portions of the extracellular domain were designed. Here, we report a soluble metabotropic glutamate receptor that encodes only the extracellular domain and retains a ligand binding characteristic similar to that of the full-length receptor. The soluble receptor secreted into culture medium showed a dimerized form. Furthermore, we have succeeded in purifying it to homogeneity. Dose-response curves of agonists for the purified soluble receptor were examined. The effective concentration for half-maximal inhibition (IC50) of quisqualate for the soluble receptor was 3.8 x 10(-8) M, which was comparable to that for the full-length receptor. The rank order of inhibition of the agonists was quisqualate >> ibotenate >/= L-glutamate approximately (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid. These data demonstrate that a ligand binding event in metabotropic glutamate receptors can be dissociated from the membrane domain.

Pharmacology of (S)-homoquisqualic acid and (S)-2-amino-5-phosphonopentanoic acid [(S)-AP5] at cloned metabotropic glutamate receptors

1 In this study we have determined the pharmacological profile of (S)-quisqualic acid, (S)-2-amino-4-phosphonobutyric acid ((S)-AP4) and their higher homologues (S)-homoquisqualic acid, (S)-2-amino-5-phosphonopentanoic acid ((S)-AP5), respectively, and (R)-AP5 at subtypes of metabotropic (S)-glutamic acid (mGlu) receptors expressed in Chinese hamster ovary cells. 2 (S)-Quisqualic acid was a potent mGlu1/mGlu5 agonist (EC50 values of 1.1 microM and 0.055 microM, respectively) showing no activity at mGlu2 and weak agonism at mGlu4 (EC50 approximately 1000 microM). 3 (S)-Homoquisqualic acid displayed competitive antagonism at mGlu1 (KB = 184 microM) and full agonism at mGlu5 (EC50 = 36 microM) and mGlu2 (EC50 = 23 microM), but was inactive at mGlu4. 4 (S)-AP4 was a potent and selective mGlu4 agonist (EC50 = 0.91 microM) being inactive at mGlu1, mGlu2 and mGlu5 both as agonist and antagonist. 5 (S)-AP5 displayed very weak agonist activity at mGlu4. At the mGlu2 receptor subtype (S)-AP5 acted as a competitive antagonist (KB = 205 microM), whereas the compound was inactive at mGlu, and mGlu5. (R)-AP5 was inactive at all mGlu receptor subtypes tested both as agonist and antagonist. 6 These studies demonstrate that incorporation of an additional carbon atom into the backbone of (S)-glutamic acid and its analogues, to give the corresponding homologues, and replacement of the terminal carboxyl groups by isosteric acidic groups have profound effects on the pharmacological profiles at mGlu receptor subtypes. Furthermore, (S)-homoquisqualic acid has been shown to be a potentially useful tool for differentiating mGlu1 and mGlu5.

(S)-homo-AMPA, a specific agonist at the mGlu6 subtype of metabotropic glutamic acid receptors

Our previous publication (J. Med. Chem. 1996, 39, 3188-3194) described (RS)-2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid (Homo-AMPA) as a highly selective agonist at the mGlu6 subtype of metabotropic excitatory amino acid (EAA) receptors. Homo-AMPA has already become a standard agonist for the pharmacological characterization of mGlu6 (Trends Pharmacol. Sci. Suppl. 1997, 37-39), and we here report the resolution, configurational assignment, and pharmacology of (S)- (6) and (R)- (7) Homo-AMPA. Using the "Ugi four-component condensation", 3-(3-ethoxy-5-methylisoxazol-4-yl)propanal (10) was converted into the separable diastereomeric derivatives of 6 and 7, compounds 12 and 11, respectively. Deprotection of 12, in one or two steps, gave extensively racemized 6, which was converted in low yield into 6 (99.0% ee) through several crystallizations. 6 (99.7% ee) and 7 (99.9% ee) were finally obtained by preparative chiral HPLC. The configurational assignments of 6 and 7 were based on 1H NMR spectroscopic studies on 12 and 11, respectively, and circular dichroism studies on 6 and 7. Values of optical rotations using different solvents and the chiral HPLC elution order of 6 and 7 supported the results of the spectroscopic configurational assignments. The activities of 6 and 7 at ionotropic EAA (iGlu) receptors and at mGlu1-7 were studied. (S)-Homo-AMPA (6) was shown to be a specific agonist at mGlu6 (EC50 = 58 +/- 11 microM) comparable in potency with the endogenous mGlu agonist (S)-glutamic acid (EC50 = 20 +/- 3 microM). Although Homo-AMPA did not show significant effects at iGlu receptors, (R)-Homo-AMPA (7), which was inactive at mGlu1-7, turned out to be a weak N-methyl-D-aspartic acid (NMDA) receptor antagonist (IC50 = 131 +/- 18 microM).

AMPA receptor-mediated regulation of a Gi-protein in cortical neurons

Excitatory synaptic transmission in the central nervous system is mediated primarily by the release of glutamate from presynaptic terminals onto postsynaptic channels gated by N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors. The myriad intracellular responses arising from the activation of the NMDA and AMPA receptors have previously been attributed to the flow of Ca2+ and/or Na+ through these ion channels. Here we report that the binding of the agonist AMPA to its receptor can generate intracellular signals that are independent of Ca2+ and Na+ in rat cortical neurons. In the absence of intracellular Ca2+ and Na+, AMPA, but not NMDA, brought about changes in a guanine-nucleotide-binding protein (Galpha[il]) that inhibited pertussis toxin-mediated ADP-ribosylation of the protein in an in vitro assay. This effect was observed in intact neurons treated with AMPA as well as in isolated membranes exposed to AMPA, and was also found in MIN6 cells, which express functional AMPA receptors but have no metabotropic glutamate receptors. AMPA also inhibited forskolin-stimulated activity of adenylate cyclase in neurons, demonstrating that Gi proteins were activated. Moreover, both Gbetagamma blockage and co-precipitation experiments demonstrated that the modulation of the Gi protein arose from the association of Galpha(il) with the glutamate receptor-1 (GluR1) subunit. These results suggest that, as well as acting as an ion channel, the AMPA receptor can exhibit metabotropic activity.

Molecular pharmacology of 4-substituted glutamic acid analogues at ionotropic and metabotropic excitatory amino acid receptors

The pharmacology of (2S,4R)-4-methylglutamic acid, (2S,4S)-4-methylglutamic acid and (S)- and (R)-4-methyleneglutamic acids (obtained in high chemical and enantiomeric purity from racemic 4-methyleneglutamic acid by chiral HPLC using a Crownpak CR(+) column), was examined in binding experiments using rat brain ionotropic glutamate receptors, and in functional assays using cloned metabotropic glutamate (mGlu) receptors. As a notable result of these studies, (2S,4R)-4-methylglutamic acid and (2S,4S)-4-methylglutamic acid were shown to be selective for kainic acid receptors and mGlu receptors (subtypes 1alpha and 2), respectively, whereas (S)-4-methyleneglutamic acid showed high but rather non-selective affinity for the (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA), kainic acid, NMDA and mGlu receptors (subtypes 1alpha and 2). Although none of the compounds were specific for any of the receptor subtypes, the results demonstrate that each of these structurally related compounds has a distinct pharmacological profile.

New analogues of ACPD with selective activity for group II metabotropic glutamate receptors

In this study we have determined the pharmacology of a series of 1-aminocyclopentane-1,3-dicarboxylic acid (1,3-ACPD) analogues at cloned metabotropic glutamic acid (mGlu) receptors. The new analogues comprise the four possible stereoisomers of 1-amino-1-carboxycyclopentane-3-acetic acid (1,3-homo-ACPD) and the racemic mixture of (1RS,2RS)-1-amino-1-carboxycyclopentane-2-acetic acid (1RS,2RS-homo-ACPD), (1RS,2RS)-Homo-ACPD was shown to be a competitive mGlu2 receptor antagonist with a KB of 391 microM. (1S,3R)-Homo-ACPD and (1R,3R)-homo-ACPD were both shown to be mGlu2 receptor agonists with EC50 values of 122 and 105 microM, respectively. Compared to (S)-Glu both compounds displayed partial agonism with intrinsic activities of 79% and 47%, respectively. (1S,3S)-Homo-ACPD was also found to be a partial mGlu2 receptor agonist with an intrinsic activity of 27% compared to (S)-Glu. None of the compounds tested showed any activity at mGlu1a or mGlu4a receptors. These homo-ACPD's show a higher degree of subtype selectivity than the parent compound (1SR,3RS)-ACPD. In addition none of the compounds demonstrated any activity at ionotropic Glu receptors.