Agonist analysis of 2-(carboxycyclopropyl)glycine isomers for cloned metabotropic glutamate receptor subtypes expressed in Chinese hamster ovary cells

Design, Synthesis, Pharmacology, and In Silico Studies of (1S,2S,3S)-2-((S)-Amino(carboxy)methyl)-3-(carboxymethyl)cyclopropane-1-carboxylic Acid (LBG30300): A Picomolar Potency Subtype-Selective mGlu2 Receptor Agonist

Metabotropic glutamate (Glu) receptors (mGlu receptors) play a key role in modulating excitatory neurotransmission in the central nervous system (CNS). In this study, we report the structure-based design and pharmacological evaluation of densely functionalized, conformationally restricted glutamate analogue (1S,2S,3S)-2-((S)-amino(carboxy)methyl)-3-(carboxymethyl)cyclopropane-1-carboxylic acid (LBG30300). LBG30300 was synthesized in a stereocontrolled fashion in nine steps from a commercially available optically active epoxide. Functional characterization of all eight mGlu receptor subtypes showed that LBG30300 is a picomolar agonist at mGlu2 with excellent selectivity over mGlu3 and the other six mGlu receptor subtypes. Bioavailability studies on mice (IV administration) confirm CNS exposure, and an in silico study predicts a binding mode of LBG30300 which induces a flipping of Tyr144 to allow for a salt bridge interaction of the acetate group with Arg271. The Tyr144 residue now prevents Arg271 from interacting with Asp146, which is a residue of differentiation between mGlu2 and mGlu3 and thus could explain the observed subtype selectivity.

Targeting mGluR group III for the treatment of neurodegenerative diseases

Glutamate, an excitatory neurotransmitter, is essential for neuronal function, and it acts on ionotropic or metabotropic glutamate receptors (mGluRs). A disturbance in glutamatergic signaling is a hallmark of many neurodegenerative diseases. Developing disease-modifying treatments for neurodegenerative diseases targeting glutamate receptors is a promising avenue. The understudied group III mGluR 4, 6-8 are commonly found in the presynaptic membrane, and their activation inhibits glutamate release. Thus, targeted mGluRs therapies could aid in treating neurodegenerative diseases. This review describes group III mGluRs and their pharmacological ligands in the context of amyotrophic lateral sclerosis, Parkinson's, Alzheimer's, and Huntington's diseases. Attempts to evaluate the efficacy of these drugs in clinical trials are also discussed. Despite a growing list of group III mGluR-specific pharmacological ligands, research on the use of these drugs in neurodegenerative diseases is limited, except for Parkinson's disease. Future efforts should focus on delineating the contribution of group III mGluR to neurodegeneration and developing novel ligands with superior efficacy and a favorable side effect profile for the treatment of neurodegenerative diseases.

Role of group II metabotropic glutamate receptors in ketamine's antidepressant actions

Major Depressive Disorder (MDD) is a serious neuropsychiatric disorder afflicting around 16-17 % of the global population and is accompanied by recurrent episodes of low mood, hopelessness and suicidal thoughts. Current pharmacological interventions take several weeks to even months for an improvement in depressive symptoms to emerge, with a significant percentage of individuals not responding to these medications at all, thus highlighting the need for rapid and effective next-generation treatments for MDD. Pre-clinical studies in animals have demonstrated that antagonists of the metabotropic glutamate receptor subtype 2/3 (mGlu_2/3 receptor) exert rapid antidepressant-like effects, comparable to the actions of ketamine. Therefore, it is possible that mGlu₂ or mGlu₃ receptors to have a regulatory role on the unique antidepressant properties of ketamine, or that convergent intracellular mechanisms exist between mGlu_2/3 receptor signaling and ketamine's effects. Here, we provide a comprehensive and critical evaluation of the literature on these convergent processes underlying the antidepressant action of mGlu_2/3 receptor inhibitors and ketamine. Importantly, combining sub-threshold doses of mGlu_2/3 receptor inhibitors with sub-antidepressant ketamine doses induce synergistic antidepressant-relevant behavioral effects. We review the evidence supporting these combinatorial effects since sub-effective dosages of mGlu_2/3 receptor antagonists and ketamine could reduce the risk for the emergence of significant adverse events compared with taking normal dosages. Overall, deconvolution of ketamine's pharmacological targets will give critical insights to influence the development of next-generation antidepressant treatments with rapid actions.

Activation of metabotropic glutamate receptor 4 regulates proliferation and neural differentiation in neural stem/progenitor cells of the rat subventricular zone and increases phosphatase and tensin homolog protein expression

Neural stem/progenitor cells (NSPCs) persist in the mammalian subventricular zone throughout life, where they can be activated in response to physiological and pathophysiological stimuli. A recent study indicates metabotropic glutamate receptor 4 (mGluR4) is involved in regulating NSPCs behaviors. Therefore, defining mGluR4 function in NSPCs is necessary for determining novel strategies to enhance the intrinsic potential for brain regeneration after injuries. In this study, mGluR4 was functionally expressed in SVZ-derived NSPCs from male Sprague-Dawley rats, in which the cyclic adenosine monophosphate concentration was reduced after treatment with the mGluR4-specific agonist VU0155041. Additionally, lateral ventricle injection of VU0155041 significantly decreased 5-bromo-2'-deoxyuridine (BrdU)⁺ and Ki67⁺ cells, while increased Doublecortin (DCX)/BrdU double-positive cells in SVZ. In cultured NSPCs, mGluR4 activation decreased the ratio of BrdU⁺ cells, G2/M-phase cells, and inhibited Cyclin D1 expression, whereas it increased neuron-specific class III β-tubulin (Tuj1) expression and the number of Tuj1, DCX, and PSA-NCAM-positive cells. However, pharmacological blocking mGluR4 with the antagonist MSOP or knockdown of mGluR4 abolished the effects of VU0155041 on NSPCs proliferation and neuronal differentiation. Further investigation demonstrated that VU0155041 treatment down-regulated AKT phosphorylation and up-regulated expression of the phosphatase and tensin homolog protein (PTEN) in NSPCs culture. Moreover VU0155041-induced proliferating inhibition and neuronal differentiating amplification in NSPCs were significantly hampered by VO-OHpic, a PTEN inhibitor. We conclude that activation of mGluR4 in SVZ-derived NSPCs suppresses proliferation and enhances their neuronal differentiation, and regulation of PTEN may be involved as a potential intracellular target of mGluR4 signal. Cover Image for this issue: https://doi.org/10.1111/jnc.15052.

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2

Metabotropic glutamate receptors (mGluRs) associated with the slow phase of the glutamatergic signaling pathway in neurons of the central nervous system have gained importance as drug targets for chronic neurodegenerative diseases. While extracellular Ca²⁺ was reported to exhibit direct activation and modulation via an allosteric site, the identification of those binding sites was challenged by weak binding. Herein, we review the discovery of extracellular Ca²⁺ in regulation of mGluRs, summarize the recent developments in probing Ca²⁺ binding and its co-regulation of the receptor based on structural and biochemical analysis, and discuss the molecular basis for Ca²⁺ to regulate various classes of drug action as well as its importance as an allosteric modulator in mGluRs.

The Role of mGlu Receptors in Hippocampal Plasticity Deficits in Neurological and Psychiatric Disorders: Implications for Allosteric Modulators as Novel Therapeutic Strategies

Long-term potentiation (LTP) and long-term depression (LTD) are two distinct forms of synaptic plasticity that have been extensively characterized at the Schaffer collateral-CA1 (SCCA1) synapse and the mossy fiber (MF)-CA3 synapse within the hippocampus, and are postulated to be the molecular underpinning for several cognitive functions. Deficits in LTP and LTD have been implicated in the pathophysiology of several neurological and psychiatric disorders. Therefore, there has been a large effort focused on developing an understanding of the mechanisms underlying these forms of plasticity and novel therapeutic strategies that improve or rescue these plasticity deficits. Among many other targets, the metabotropic glutamate (mGlu) receptors show promise as novel therapeutic candidates for the treatment of these disorders. Among the eight distinct mGlu receptor subtypes (mGlu1-8), the mGlu1,2,3,5,7 subtypes are expressed throughout the hippocampus and have been shown to play important roles in the regulation of synaptic plasticity in this brain area. However, development of therapeutic agents that target these mGlu receptors has been hampered by a lack of subtype-selective compounds. Recently, discovery of allosteric modulators of mGlu receptors has provided novel ligands that are highly selective for individual mGlu receptor subtypes. The mGlu receptors modulate the multiple forms of synaptic plasticity at both SC-CA1 and MF synapses and allosteric modulators of mGlu receptors have emerged as potential therapeutic agents that may rescue plasticity deficits and improve cognitive function in patients suffering from multiple neurological and psychiatric disorders.

Functional coupling between metabotropic glutamate receptors and G-proteins in rat cerebral cortex assessed by guanosine-5'-O-(3-[(35)S]thio)triphosphate binding assay

Stimulation of specific guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding by l-glutamate was pharmacologically characterized in rat cerebral cortical membranes. Optimization of the experimental conditions with respect to the concentrations of GDP, MgCl(2) and NaCl in assay buffer prompted us to adopt the incubation of rat cerebral cortical membranes with 0.2 nM [(35)S]GTPγS at 30°C for 60 min. in the presence of 20 μM GDP, 5 mM MgCl(2) and 100 mM NaCl as a standard condition. Specific [(35)S]GTPγS binding was stimulated by l-glutamate in a concentration-dependent manner but not by ionotropic glutamate receptor agonists. The stimulatory responses were also elicited by many agonists for metabotropic glutamate (mGlu) receptor, with (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268) being the most potent. l-glutamate-stimulated [(35)S]GTPγS binding was inhibited by several mGlu antagonists, with (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495) being the most potent. The pharmacological properties of a series of agonists and antagonists indicated the involvement of group II mGlu receptors, especially mGlu2. Supportive of this notion was the finding that l-glutamate-stimulated specific [(35)S]GTPγS binding was augmented by 2,2,2-trifluoro-N-[4-(2-methoxyphenoxy)phenyl]-N-(3-pyridinylmethyl)ethanesulphonamide hydrochloride (LY487379), a reportedly selective allosteric positive modulator for mGlu2, by means of upward and leftward shift of the concentration-response curve. In addition, LY487379 per se stimulated [(35)S]GTPγS binding, though, through a mechanism different from the stimulation by l-glutamate. Pre-treatment of the membranes with N-ethylmaleimide (NEM) cancelled l-glutamate-stimulated [(35)S]GTPγS binding in a concentration- and incubation time-dependent manner. Taken altogether, l-glutamate-stimulated [(35)S]GTPγS binding serves as a useful functional assay for the activation of NEM-sensitive G(i/o) -mediated group II mGlu receptors in rat cerebral cortical membranes.

4,4-Dimethyl- and diastereomeric 4-hydroxy-4-methyl- (2S)-glutamate analogues display distinct pharmacological profiles at ionotropic glutamate receptors and excitatory amino acid transporters

Subtype-selective ligands are of great interest to the scientific community, as they provide a tool for investigating the function of one receptor or transporter subtype when functioning in its native environment. Several 4-substituted (S)-glutamate (Glu) analogues were synthesized, and altogether this approach has provided important insight into the structure-activity relationships (SAR) for ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs), as well as the excitatory amino acid transporters (EAATs). In this work, three 4,4-disubstituted Glu analogues 1-3, which are hybrid structures of important 4-substituted Glu analogues 4-8, were investigated at iGluRs and EAATs. Collectively, their pharmacological profiles add new and valuable information to the SAR for the iGluRs and EAAT1-3.

A novel postsynaptic group II metabotropic glutamate receptor role in modulating baroreceptor signal transmission

The nucleus tractus solitarius (NTS) is essential for orchestrating baroreflex control of blood pressure. When a change in blood pressure occurs, the information is transmitted by baroreceptor afferent fibers to the central network by glutamate binding to ionotropic glutamate receptors on second-order baroreceptor neurons. Glutamate also activates presynaptic group II and III metabotropic glutamate receptors (mGluRs), depressing both glutamate and GABA release to modulate baroreceptor signal transmission. Here we present a novel role for postsynaptic group II mGluRs to further fine-tune baroreceptor signal transmission at the first central synapses. In a brainstem slice with ionotropic glutamate and GABA receptors blocked, whole-cell patch-clamp recordings of second-order baroreceptor neurons revealed that two group II mGluR agonists evoked concentration-dependent membrane hyperpolarizations. The hyperpolarization remained when a presynaptic contribution was prevented with Cd(2+), was blocked by a postsynaptic intervention of intracellular dialysis of the G-protein signaling inhibitor, was mimicked by endogenous release of glutamate by tractus solitarius stimulation, and was prevented by a group II mGluR antagonist. Postsynaptic localization of group II mGluRs was confirmed by fluorescent confocal immunohistochemistry and light microscopy. Group II mGluR induced-currents consisted of voltage-dependent outward and inward components, prevented by tetraethylammonium chloride and tetrodotoxin, respectively. In contrast to group II mGluR-induced hyperpolarization, there was no effect on intrinsic excitability as determined by action potential shape or firing in response to depolarizing current injections. The data suggest a novel mechanism for postsynaptic group II mGluRs to fine-tune baroreceptor signal transmission in the NTS.

Protein kinase Calpha mediates a novel form of plasticity in the accessory olfactory bulb

Modification of synapses in the accessory olfactory bulb (AOB) is believed to underlie pheromonal memory that enables mate recognition in mice. The memory, which is acquired with single-trial learning, forms only with coincident noradrenergic and glutamatergic inputs to the AOB. The mechanisms by which glutamate and norepinephrine (NE) alter the AOB synapses are not well understood. Here we present results that not only reconcile the earlier, seemingly contradictory, observations on the role of glutamate and NE in changing the AOB synapses, but also reveal novel mechanisms of plasticity. Our studies suggest that initially, glutamate acting at Group II metabotropic receptors and NE acting at alpha(2)-adrenergic receptors inhibit N-type and R-type Ca(2+) channels in mitral cells via a G-protein. The N-type and R-type Ca(2+) channel inhibition is reversed by activation of alpha(1)-adrenergic receptors and protein kinase Calpha (PKCalpha). Based on these results, we propose a hypothetical model for a new kind of synaptic plasticity in the AOB that accounts for the previous behavioral data on pheromonal memory. According to this model, initial inhibition of the Ca(2+) channels suppresses the GABAergic inhibitory feedback to mitral cells, causing disinhibition and Ca(2+) influx. NE also activates phospholipase C (PLC) through alpha(1)-adrenergic receptors generating inositol 1,4,5-trisphosphate and diacylglycerol (DAG). Calcium and DAG together activate PKCalpha which switches the disinhibition to increased inhibition of mitral cells. Thus, PKCalpha is likely to be a coincidence detector integrating glutamate and NE input in the AOB and bridging the short-term signaling to long-term structural changes resulting in enhanced inhibition of mitral cells that is thought to underlie memory formation.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Metabotropic glutamate receptor ligands as possible anxiolytic and antidepressant drugs

Depression and anxiety represent a major problem. However, the current treatment of both groups of diseases is not satisfactory. As the glutamatergic system may play an important role in pathophysiology of both depression and anxiety, we decided to discuss the recent data on possible anxiolytic and/or antidepressant effects of metabotropic glutamate (mGlu) receptor ligands. Preclinical data indicated that antagonists of group I mGlu receptors, particularly antagonists of mGlu5 receptors, produced both anxiolytic-like and antidepressant-like effects. Clinical data also demonstrated that mGlu5 receptor antagonist, fenobam, was an active anxiolytic drug. The anxiolytic effects exerted by mGlu5 receptor antagonists are profound, comparable with or stronger than those of benzodiazepines. However, the problem with the psychotomimetic activity of mGlu5 receptor antagonists and their possible influence on memory has to be further investigated. Among all mGlu receptor ligands, group II mGlu receptor agonists seem to be the drugs with the most promising therapeutic potential and a good safety profile. Animal studies showed anxiolytic-like effects of group II mGlu receptor agonists. Currently, group II mGlu receptor agonists are in phase III clinical trials for potential treatment of anxiety disorders. On the other hand, data has been accumulated, indicating that antagonists of group II mGlu receptors have an antidepressant potential. Group III mGlu receptor ligands represent the least investigated group of mGlu receptors. However, preclinical data also indicates that ligands of these receptors, both agonists and antagonists, may have an anxiolytic-like and antidepressant-like potential.

Role of peripheral group I and II metabotropic glutamate receptors in IL-1beta-induced mechanical allodynia in the orofacial area of conscious rats

The present study investigated the role of peripheral group I and II metabotropic glutamate receptors (mGluRs) in interleukin-1beta (IL-1beta)-induced mechanical allodynia in the orofacial area. Experiments were carried out on Sprague-Dawley rats weighing between 230 and 280 g. After subcutaneous administration of 0.01, 0.1, 1, or 10 pg of IL-1beta, we examined withdrawal behavioral responses produced by 10 successive trials of a ramp of air-puffs pressure applied ipsilaterally or contralaterally to the IL-1beta injection site. The thresholds of air puffs were measured 10, 30, 60, 120, or 180 min after 25 microl of IL-1beta was administered through an implanted tube. Subcutaneous injection of IL-1beta produced bilateral mechanical allodynia. While the IL-1beta-induced mechanical allodynia was blocked by pretreatment with an IL-1 receptor antagonist, the IL-1beta-induced mirror-image mechanical allodynia was not blocked by an IL-1 receptor antagonist injected into the contralateral side. Subcutaneous administration of CPCCOEt or LY367385, an mGluR1 antagonist, or MPEP or SIB1893, an mGluR5 antagonist, 10 min prior to injection of IL-1beta abolished IL-1beta-induced mechanical allodynia. Pretreatment with APDC or DCG4, a group II mGluR agonist, blocked the IL-1beta-induced mechanical allodynia. The anti-allodynic effect induced by APDC was inhibited by pretreatment with LY341495, a group II mGluR antagonist. These results suggest that peripheral group I and II mGluRs participate in IL-1beta-induced mechanical allodynia in the orofacial area. Peripheral group I mGluR antagonists blocked the IL-1beta-induced mechanical allodynia, while peripheral group II mGluR agonists produced anti-allodynic effects on IL-1beta-induced mechanical allodynia in the orofacial area of rats.

Rigid nonproteinogenic cyclic amino acids as ligands for glutamate receptors:trans-Tris(homoglutamic) acids

The second-generation asymmetric synthesis of the trans-tris(homoglutamic) acids reported herein proceeds via Strecker reaction of chiral ketimines, obtained from condensation of racemic 2-ethoxycarbonylmethylcyclopentanone and commercially available (S)- and (R)-1-phenylethylamine, respectively. In the key stereodifferentiating step, the cyanide addition leads to mixtures of diastereomeric alpha-amino nitrile-esters, the composition of which is independent of the reaction temperature and the type of the solvent, respectively. Hydrolysis of the alpha-amino nitrile-esters with concentrated H(2)SO(4) yielded diastereomeric mixtures of secondary alpha-amino amido-esters, which after separation were hydrogenolyzed and hydrolyzed each to the enantiomeric trans-1-amino-2-carboxymethylcyclopentanecarboxylic acids. Their configuration was completely established by NMR methods, CD spectra, and X-ray analysis of the trans-1S,2R-configured secondary alpha-amino amido-ester. In receptor binding assays and functional tests, trans-1S,2R-1-amino-2-carboxymethylcyclopentanecarboxylic acid hydrochloride was found to behave as a selective mGluR(2)-antagonist without relevant binding properties at iGluRs.

Metabotropic glutamate receptor type 4 is involved in autoinhibitory cascade for glucagon secretion by alpha-cells of islet of Langerhans

In islets of Langerhans, L-glutamate is stored in glucagon-containing secretory granules of alpha-cells and cosecreted with glucagon under low-glucose conditions. The L-glutamate triggers secretion of gamma-aminobutyric acid (GABA) from beta-cells, which in turn inhibits glucagon secretion from alpha-cells through the GABAA receptor. In the present study, we tested the working hypothesis that L-glutamate functions as an autocrine/paracrine modulator and inhibits glucagon secretion through a glutamate receptor(s) on alpha-cells. The addition of L-glutamate at 1 mmol/l; (R,S)-phosphonophenylglycine (PPG) and (S)-3,4-dicarboxyphenylglycine (DCPG), specific agonists for class III metabotropic glutamate receptor (mGluR), at 100 micromol/l; and (1S,3R,4S)-1-aminocyclopentane-1,3,4-tricarboxylic acid (ACPT-I) at 50 micromol/l inhibited the low-glucose-evoked glucagon secretion by 87, 81, 73, and 87%, respectively. This inhibition was dose dependent and was blocked by (R,S)-cyclopropyl-4-phosphonophenylglycine (CPPG), a specific antagonist of class III mGluR. Agonists of other glutamate receptors, including kainate and quisqualate, had little effectiveness. RT-PCR and immunological analyses indicated that mGluR4, a class III mGluR, was expressed and localized with alpha- and F cells, whereas no evidence for expression of other mGluRs, including mGluR8, was obtained. L-Glutamate, PPG, and ACPT-I decreased the cAMP content in isolated islets, which was blocked by CPPG. Dibutylyl-cAMP, a nonhydrolyzable cAMP analog, caused the recovery of secretion of glucagon. Pertussis toxin, which uncouples adenylate cyclase and inhibitory G-protein, caused the recovery of both the cAMP content and secretion of glucagon. These results indicate that alpha- and F cells express functional mGluR4, and its stimulation inhibits secretion of glucagon through an inhibitory cAMP cascade. Thus, L-glutamate may directly interact with alpha-cells and inhibit glucagon secretion.

Characterization of Novel L-threo-β-Benzyloxyaspartate Derivatives, Potent Blockers of the Glutamate Transporters

Nontransportable blockers of the glutamate transporters are important tools for investigating mechanisms of synaptic transmission. DL-threo-beta-Benzyloxyaspartate (DL-TBOA) is a potent blocker of all subtypes of the excitatory amino acid transporters (EAATs). We characterized novel L-TBOA analogs possessing a substituent on their respective benzene rings. The analogs significantly inhibited labeled glutamate uptake, the most potent of which was (2S,3S)-3-[3-[4-(trifluoromethyl)benzoylamino]benzyloxy]aspartate (TFB-TBOA). In an uptake assay using cells transiently expressing EAATs, the IC(50) values of TFB-TBOA for EAAT1, EAAT2, and EAAT3 were 22, 17, and 300 nM, respectively. TFB-TBOA was significantly more potent at inhibiting EAAT1 and EAAT2 compared with L-TBOA (IC(50) values for EAAT1-3 were 33, 6.2, and 15 microM, respectively). Electrophysiological analyses revealed that TBOA analogs block the transport-associated currents in all five EAAT subtypes and also block leak currents in EAAT5. The rank order of the analogs for potencies at inhibiting substrate-induced currents was identical to that observed in the uptake assay. However, the kinetics of TFBTBOA differed from the kinetics of L-TBOA, probably because of the strong binding affinity. Notably, TFB-TBOA did not affect other representative neurotransmitter transporters or receptors, including ionotropic and metabotropic glutamate receptors, indicating that it is highly selective for EAATs. Moreover, intracerebroventricular administration of the TBOA analogs induced severe convulsive behaviors in mice, probably because of the accumulation of glutamate. Taken together, these findings indicate that novel TBOA analogs, especially TFB-TBOA, should serve as useful tools for elucidating the physiological roles of the glutamate transporters.

(2S,1'S,2'R,3'R)-2-(2'-Carboxy-3'-hydroxymethylcyclopropyl) glycine is a highly potent group 2 and 3 metabotropic glutamate receptor agonist with oral activity

The asymmetric synthesis and biological activity of (2S,1'S,2'R,3'R)-2-(2'-carboxy-3'-hydroxymethylcyclopropyl) glycine ((+)-3) is described. This novel C-3' substituted carboxy cyclopropyl glycine is a highly potent group 2 and group 3 mGluR agonist that has proven to be orally active in both fear potentiated startle (animal model for anxiety) and PCP-induced motor activation (animal model for psychosis) assays in rats.

Ibotenic acid and thioibotenic acid: a remarkable difference in activity at group III metabotropic glutamate receptors

In this study, we have determined and compared the pharmacological profiles of ibotenic acid and its isothiazole analogue thioibotenic acid at native rat ionotropic glutamate (iGlu) receptors and at recombinant rat metabotropic glutamate (mGlu) receptors expressed in mammalian cell lines. Thioibotenic acid has a distinct pharmacological profile at group III mGlu receptors compared with the closely structurally related ibotenic acid; the former is a potent (low microm) agonist, whereas the latter is inactive. By comparing the conformational energy profiles of ibotenic and thioibotenic acid with the conformations preferred by the ligands upon docking to mGlu1 and models of the other mGlu subtypes, we propose that unlike other subtypes, group III mGlu receptor binding sites require a ligand conformation at an energy level which is prohibitively expensive for ibotenic acid, but not for thioibotenic acid. These studies demonstrate how subtle differences in chemical structures can result in profound differences in pharmacological activity.

Synapses between parallel fibres and stellate cells express long-term changes in synaptic efficacy in rat cerebellum

Various forms of synaptic plasticity underlying motor learning have already been well characterized at cerebellar parallel fibre (PF)-Purkinje cell (PC) synapses. Inhibitory interneurones play an important role in controlling the excitability and synchronization of PCs. We have therefore tested the possibility that excitatory synapses between PFs and stellate cells (SCs) are also able to exhibit long-term changes in synaptic efficacy. In the present study, we show that long-term potentiation (LTP) and long-term depression (LTD) were induced at these synapses by a low frequency stimulation protocol (2 Hz for 60 s) and that pairing this low frequency stimulation protocol with postsynaptic depolarization induced a marked shift of synaptic plasticity in favour of LTP. This LTP was cAMP independent, but required nitric oxide (NO) production from pre- and/or postsynaptic elements, depending on the stimulation or pairing protocol used, respectively. In contrast, LTD was not dependent on NO production but it required activation of postsynaptic group II and possibly of group I metabotropic glutamate receptors. Finally, stimulation of PFs at 8 Hz for 15 s also induced LTP at PF-SC synapses. But in this case, LTP was cAMP dependent, as was also observed at PF-PC synapses for presynaptic LTP induced in the same conditions. Thus, long-term changes in synaptic efficacy can be accomplished by PF-SCs synapses as well as by PF-PC synapses, suggesting that both types of plasticity might co-operate during cerebellar motor learning.

Synthesis and structure-activity relationship studies of novel 2-diarylethyl substituted (2-carboxycycloprop-1-yl)glycines as high-affinity group II metabotropic glutamate receptor ligands

The major excitatory neurotransmitter in the central nervous system, (S)-glutamic acid , activates both ionotropic and metabotropic excitatory amino acid receptors. Its importance in connection to neurological and psychiatric disorders has directed great attention to the development of compounds that modulate the effects of this endogenous ligand. Whereas L-carboxycyclopropylglycine (L-CCG-1) is a potent agonist at, primarily, group II metabotropic glutamate receptors, alkylation of at the alpha-carbon notoriously result in group II mGluR antagonists, of which the most potent compound described so far, LY341495, displays IC(50) values of 23 and 10 nM at the group II receptor subtypes mGlu2 and mGlu3, respectively. In this study we synthesized a series of structural analogues of in which the xanthyl moiety is replaced by two substituted-phenyl groups. The pharmacological characterization shows that these novel compounds have very high affinity for group II mGluRs when tested as their racemates. The most potent analogues demonstrate K(i) values in the range of 5-12 nM, being thus comparable to LY341495.

Behavioral evidence supporting a differential role for spinal group I and II metabotropic glutamate receptors in inflammatory hyperalgesia in sheep

A differential role for metabotropic glutamate receptors (mGluRs) in spinal nociception in normal animals has previously been identified. The present study examined the contribution of group I and group II mGluRs to the development and maintenance of inflammatory hyperalgesia produced by unilateral intradermal injection of carrageenan into the lower forelimb in sheep. Carrageenan (7.5 mg in 500 micro l) produced a significant bilateral reduction in forelimb mechanical withdrawal thresholds. Intrathecal administration of saline-vehicle or the group II mGluR antagonist (2S)-alpha-ethylglutamate (EGLU; 570 nmol) had no effect on either the development or maintenance of hyperalgesia. However, intrathecal administration of the group I mGluR antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 450 nmol) before carrageenan blocked the development of ipsilateral hyperalgesia, and when given 2 h after carrageenan, reversed both ipsilateral and contralateral hyperalgesia. Intrathecal administration of the group II mGluR agonist (2S,1S,2S)-2-(carboxycyclopropyl)glycine (L-CCG-I; 620 nmol) given either before or after carrageenan treatment produced analgesia and anti-hyperalgesia, an effect abolished by co-administration of EGLU (570 nmol). The magnitude of the analgesic response, assessed by the area under the response curve, was significantly greater than that produced by LCCG-I in normal animals. These data demonstrate that the development and maintenance of inflammatory hyperalgesia is dependent on activation of group I mGluRs in spinal cord. In addition, the analgesic and anti-hyperalgesic actions of group II mGluRs suggest that these receptors play a crucial role in modulating acute inflammatory hyperalgesia.

(2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-methylcyclopropyl) glycine is a potent and selective metabotropic group 2 receptor agonist with anxiolytic properties

The asymmetric synthesis and biological activity of (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-methylcyclopropyl) glycine 7 and its epimer at the C3' center 6 are described. Compound 7 is a highly potent and selective agonist for group 2 metabotropric glutamate receptors (mGluRs). It is also systemically 4 orders of magnitude more active in the fear-potentiated startle model of anxiety in rats than the rigid constrained bicyclic system LY354740. Therefore, we have shown that high molecular complexity of conformationally constrained bicyclic systems is not a requirement to achieve highly selective and potent group 2 mGluRs agonists.

2-Amino-3-(3-hydroxy-1,2,5-thiadiazol-4-yl)propionic acid: resolution, absolute stereochemistry and enantiopharmacology at glutamate receptors

In order to identify new subtype-selective (S)-glutamate (Glu) receptor ligands we have synthesized (RS)-2-amino-3-(3-hydroxy-1,2,5-thiadiazol-4-yl)propionic acid [(RS)-TDPA]. Resolution of (RS)-TDPA by chiral chromatography was performed using a Crownpac CR(+) column affording (R)- and (S)-TDPA of high enantiomeric purity (enantiomeric excess=99.9%). An X-ray crystallographic analysis revealed that the early eluting enantiomer has R-configuration. Both enantiomers showed high affinity as well as high agonist activity at (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptors, determined using a [(3)H]AMPA binding assay and an electrophysiological model, respectively. The affinities and agonist activities obtained for (R)-TDPA (IC(50)=0.265 microM and EC(50)=6.6 microM, respectively) and (S)-TDPA (IC(50)=0.065 microM and EC(50)=20 microM, respectively) revealed a remarkably low AMPA receptor stereoselectivity, (S)-TDPA showing the highest affinity and (R)-TDPA the most potent agonist activity. In addition, (S)-TDPA was shown to interact with synaptosomal Glu uptake sites displacing [(3)H](R)-aspartic acid (IC(50 ) approximately 390 microM). An enantiospecific and subtype-selective agonist activity was observed for (S)-TDPA at group I metabotropic Glu (mGlu) receptors (EC(50)=13 microM at mGlu(5) and EC(50)=95 microM at mGlu(1)).

Stereoselective synthesis of 2-amino-3-fluoro bicyclo[3.1.0]hexane-2,6-dicarboxylic acid

(+)-2-Aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740) is a conformationally restricted glutamate analogue that is a potent, selective and orally active group 2 metabotropic glutamate receptor agonist possessing anticonvulsant and anxiolytic properties. Herein, we describe a stereoselective and highly efficient synthesis of its 3-beta fluoro derivative using the Corey-Link methodology to create the amino acid stereogenic center.

Resolution, configurational assignment, and enantiopharmacology at glutamate receptors of 2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA) and demethyl-ACPA

We have previously described (RS)-2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA) as a potent agonist at the (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor subtype of (S)-glutamic acid (Glu) receptors. We now report the chromatographic resolution of ACPA and (RS)-2-amino-3-(3-carboxy-4-isoxazolyl)propionic acid (demethyl-ACPA) using a Sumichiral OA-5000 column. The configuration of the enantiomers of both compounds have been assigned based on X-ray crystallographic analyses, supported by circular dichroism spectra and elution orders on chiral HPLC columns. Furthermore, the enantiopharmacology of ACPA and demethyl-ACPA was investigated using radioligand binding and cortical wedge electrophysiological assay systems and cloned metabotropic Glu receptors. (S)-ACPA showed high affinity in AMPA binding (IC(50) = 0.025 microM), low affinity in kainic acid binding (IC(50) = 3.6 microM), and potent AMPA receptor agonist activity on cortical neurons (EC(50) = 0.25 microM), whereas (R)-ACPA was essentially inactive. Like (S)-ACPA, (S)-demethyl-ACPA displayed high AMPA receptor affinity (IC(50) = 0.039 microM), but was found to be a relatively weak AMPA receptor agonist (EC(50) = 12 microM). The stereoselectivity observed for demethyl-ACPA was high when based on AMPA receptor affinity (eudismic ratio = 250), but low when based on electrophysiological activity (eudismic ratio = 10). (R)-Demethyl-ACPA also possessed a weak NMDA receptor antagonist activity (IC(50) = 220 microM). Among the enantiomers tested, only (S)-demethyl-ACPA showed activity at metabotropic receptors, being a weak antagonist at the mGlu(2) receptor subtype (K(B) = 148 microM).

Design, synthesis and preliminary evaluation of novel 3'-substituted carboxycyclopropylglycines as antagonists at group 2 metabotropic glutamate receptors

Two novel 3'-substituted carboxycylopropylglycines, (2S,1'S,2'S,3'R)-2-(3'-xanthenylmethyl-2'-carboxycyclopropyl)glycine (8a) and (2S,1'S,2'S,3'R)-2-(3'-xanthenylethyl-2'-carboxycyclopropyl)glycine (8b), were synthesized and evaluated as mGluR ligands. Compound 8b showed to be a potent group II antagonist with submicromolar activity.

Synthesis, SARs, and pharmacological characterization of 2-amino-3 or 6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives as potent, selective, and orally active group II metabotropic glutamate receptor agonists

(+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (4, LY354740), a highly selective and orally active group II metabotropic glutamate receptor (mGluR) agonist, has increased interest in the study of group II mGluRs. Our interest focused on a conformationally constrained form of compound 4, because it appeared that the rigid form resulted in not only selectivity for group II mGluR but was orally active. Therefore, we introduced a fluorine atom to compound 4, based on the molecular size (close resemblance to hydrogen atom) and electronegativity (effects on the electron distribution in the molecule) of this atom and carbon-fluorine bond energy. Compound (+)-7 (MGS0008), the best compound among 3-fluoro derivatives 7-10, retained the agonist activity of compound 4 for mGluR2 and mGluR3 ((+)-7: EC(50) = 29.4 +/- 3.3 nM and 45.4 +/- 8.4 nM for mGluR2 and mGluR3, respectively; 4: EC(50) = 18.3 +/- 1.6 nM and 62.8 +/- 12 nM for mGluR2 and mGluR3, respectively) and increased the oral activity of compound 4 ((+)-7: ED(50) = 5.1 mg/kg and 0.26 mg/kg for phencyclidine (PCP)-induced hyperactivity and PCP-induced head-weaving behavior, respectively; 4: ED(50) = >100 mg/kg and 3.0 mg/kg for PCP-induced hyperactivity and PCP-induced head-weaving behavior, respectively). In addition, a compound [(3)H]-(+)-7 binding study using mGluR2 or 3 expressed in CHO cells was successful ((+)-7: K(i) = 47.7 +/- 17 nM and 65.9 +/- 7.1 nM for mGluR2 and mGluR3, respectively; 4: K(i) = 23.4 +/- 7.1 nM and 53.5 +/- 13 nM for mGluR2 and mGluR3, respectively). On the basis of a successful result of compound 7, we focused on the introduction of a fluorine atom on the C6 position of compound 4. (1R,2S,5R, 6R)-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid ((-)-11) exhibited a high degree of agonist activity for group II mGluRs equal to that of compound 4 or 7 ((-)-11: K(i) = 16.6 +/- 5.6 and 80.9 +/- 31 nM for mGluR2 and mGluR3, respectively). Our interest shifted to modification on CH(2) at C4 position of compound 11, since replacement of the CH(2) group with either an oxygen atom or sulfur atom yielded compound 5 or 6, resulting in increased agonist activity. We selected a carbonyl group instead of CH(2) at the C4 position of compound 11. The carbonyl group might slightly change the relative conformation of three functional groups, the amino group and two carboxylic acids, which have important roles in mediating the interaction between group II mGluRs and their ligand, compared with the CH(2) group of 4, oxygen atom of 5, and sulfur atom of 6. (1R,2S,5S,6S)-2-Amino-6-fluoro-4-oxobicyclo[3.1. 0]hexane-2,6-dicarboxylic acid monohydrate ((+)-14, MGS0028) exhibited a remarkably high degree of agonist activity for mGluR2 (K(i) = 0.570 +/- 0.10 nM) and mGluR3 (K(i) = 2.07 +/- 0.40 nM) expressed in CHO cells but not mGluR4, 6, 7, 1a, or 5 expressed in CHO cells (K(i) = >100 000 nM). Furthermore, compound (+)-14 strongly inhibited phencyclidine (PCP)-induced head-weaving behavior (ED(50) = 0.090 microg/kg) and hyperactivity (ED(50) = 0.30 mg/kg) in rats. Thus, (+)-7 and (+)-14 are potent, selective, and orally active group II mGluR agonists and might be useful not only for exploring the functions of mGluRs but in the treatment of schizophrenia.

The effects of DCG-IV and L-CCG-1 upon phencyclidine (PCP)-induced locomotion and behavioral changes in mice

The behavioral changes of mice induced by acute and repeated i.p. injection of phencyclidine (PCP) were observed by measuring locomotor activity and stereotyped behavior. Then, the effects of metabotropic glutamate receptor (mGluR) agonists, DCG-IV and L-CCG-1, on the above behavioral changes induced by PCP were found. The effects of DCG-IV were very strong and completely depressed the PCP-induced hyperlocomotion. The effects of L-CCG-1 were not so strong. Repeated injection of PCP for 20 days into mice induced lower locomotor activity than that in acutely injected mice. These behavioral changes may be related with the negative symptoms of schizophrenia. In order to examine some molecular mechanisms of PCP-induced behavioral changes, Northern blot analysis of total RNA from prefrontal cortical tissues of mice treated with PCP, DCG-IV, and L-CCG-1 was carried out.

Pharmacological characterization of metabotropic glutamate receptor-mediated high-affinity GTPase activity in rat cerebral cortical membranes

Activation of heterotrimeric guanine nucleotide-binding regulatory proteins (G-proteins) functionally coupled to metabotropic glutamate receptors (mGluRs) was assessed by agonist-induced high-affinity GTPase (EC3.6.1.-) activity in rat cerebral cortical membranes. L-Glutamate (1 mM) stimulated high-affinity GTPase activity to the same extent throughout the incubation period up to 20 min, in a Mg(2+)-dependent manner. The addition of 1 mM L-glutamate augmented V(max) of the enzyme activity (1670 to 3850 pmol mg(-1) protein 15 min(-1)) with slight increase in K(M) value (0.26 to 0.63 microM). The high-affinity GTPase activity was stimulated by the following compounds with a rank order of potency of (2S,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl) glycine (DCG-IV) > (2S,1'S, 2'S)-2-(carboxycyclopyropyl)glycine (L-CCG-I) > L-glutamate > or = 2R, 4R-4-aminopyrrolidine-2,4-dicarboxylate [(2R,4R)-APDC] > 1S, 3R-1-aminocyclopentane-1,3-dicarboxylate [(1S,3R)-ACPD] > (S)-4-carboxy-3-hydroxyphenylglycine [(S)-4C3HPG] > (S)-3-carboxy-4-hydroxyphenylglycine [(S)-3C4HPG] > ibotenate, but not by L-(+)-2-amino-4-phosphonobutyrate (L-AP4), (RS)-3, 5-dihydroxyphenylglycine [(RS)-3,5-DHPG], quisqualate, or L-serine-O-phosphate (L-SOP), indicative of involvement of group II mGluRs, in particular mGluR2. (2S)-alpha-Ethylglutamate (EGLU), a presumably selective antagonist against group II mGluRs, inhibited DCG-IV-stimulated high-affinity GTPase activity in a competitive manner with an apparent K(B) of 220 microM. L-Glutamate-stimulated activity was eliminated by pretreatment of the membranes with sulfhydryl alkylating agent N-ethylmaleimide (NEM) at 30-50 microM, indicating that G-proteins of the G(i) family are involved. These results indicate that mGluR agonist-induced high-affinity GTPase activity in rat cerebral cortical membranes may be used to detect the functional interaction between group II mGluRs, in particular mGluR2, and NEM-sensitive G(i) proteins.

Behavioural evidence supporting a differential role for group I and II metabotropic glutamate receptors in spinal nociceptive transmission

Metabotropic glutamate receptors (mGluRs) have been shown to contribute to nociceptive processing in spinal cord. This study examined the effects of intrathecal treatment with group I and II mGluR compounds on withdrawal thresholds to noxious mechanical stimuli, in the absence of tissue damage or inflammation, in adult female sheep. Both the group I/II mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 5.2-520 nmol) and the group II agonist (2S,1S, 2S)-2-(carboxycyclopropyl)glycine (L-CCG-I; 620 nmol) significantly increased mechanical withdrawal thresholds between 5-15 min post-injection. These anti-nociceptive effects were blocked by co-administration of the mGluR antagonist (2S)-alpha-ethylglutamate (EGLU; 570 nmol; group II), but not (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 450 nmol; group I). Intrathecal administration of the group I-specific agonist (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG; 50 nmol) produced a significant reduction in mechanical thresholds, which was blocked by co-administration of the group I antagonist AIDA. In contrast, the highest dose of (S)-3,5-DHPG tested, 5 micromol, significantly elevated response thresholds. These results demonstrate that both group I and II mGluRs play crucial, but contrasting roles in mediating acute mechanical nociceptive events in spinal cord.

Hydroxyindole-O-methyltransferase is another target for L-glutamate-evoked inhibition of melatonin synthesis in rat pinealocytes

Rat pinealocytes use L-glutamate as a modulator for melatonin synthesis. Upon binding of L-glutamate to the class II metabotropic glutamate receptor, norepinephrine (NE)-dependent formation of cAMP was inhibited, resulting in decreased serotonin-N-acetyltransferase (NAT) activity and melatonin output. Although L-glutamate at 1 mM caused 90% inhibition of melatonin synthesis, about 30% of the NAT activity remained, suggesting the presence of another target for L-glutamate. In this study, we found that L-glutamate also inhibits hydroxyindole-O-methyltransferase (HIOMT). The inhibition is reversible and dose-dependent: the maximal inhibition was obtained with more than 0.4 mM L-glutamate. Contrary to L-glutamate-evoked inhibition of NAT, agonists for class II metabotropic receptors such as (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) had no effect on HIOMT. Neither (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine (MCCG), an specific antagonist for class II mGluRs, nor dibutyryl cAMP restored the L-glutamate-evoked inhibition of HIOMT. Northern blot analyses revealed that L-glutamate significantly inhibits the expression of mRNA of NAT, but not that of HIOMT. These results indicated that HIOMT is an another target for L-glutamate due to its inhibition of melatonin synthesis, and the signaling pathway toward the inhibition is distinct from that of NAT.

Selective activation of group II mGluRs with LY354740 does not prevent neuronal excitotoxicity

Recent reports have suggested a role for group II metabotropic glutamate receptors (mGluRs) in the attenuation of excitotoxicity. Here we examined the effects of the recently available group II agonist (+)-2-Aminobicyclo[3.1.0]hexane-2-6-dicarboxylic acid (LY354740) on N-methyl-D-aspartate (NMDA)-induced excitotoxic neuronal death, as well as on hypoxic-ischemic neuronal death both in vitro and in vivo. At concentrations shown to be selective for group II mGluRs expressed in cell lines (0.1-100 nM), LY354740 did not attenuate NMDA-mediated neuronal death in vitro or in vivo. Furthermore, LY354740 did not attenuate oxygen-glucose deprivation-induced neuronal death in vitro or ischemic infarction after transient middle cerebral artery occlusion in rats. In addition, the neuroprotective effect of another group II agonist, (S)-4-carboxy-3-phenylglycine (4C3HPG), which has shown injury attenuating effects both in vitro and in vivo, was not blocked by the group II antagonists (2 S)-alpha-ethylglutamic acid (EGLU), (RS)-alpha-methyl-4-sulphonophenylglycine (MSPG), or the group III antagonist (S)-alpha-methyl-3-carboxyphenylalanine (MCPA), suggesting that this neuroprotection may be mediated by other effects such as upon group I mGluRs.

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.

[3H]-LY341495 as a novel antagonist radioligand for group II metabotropic glutamate (mGlu) receptors: characterization of binding to membranes of mGlu receptor subtype expressing cells

Metabotropic glutamate (mGlu) receptors are a family of eight known subtypes termed mGlu1-8. Currently, few ligands are available to study the pharmacology of mGlu receptor subtypes. In functional assays, we previously described LY341495 as a highly potent and selective mGlu2 and mGlu3 receptor antagonist. In this study, radiolabeled [3H]-LY341495 was used to investigate the characteristics of receptor binding to membranes from cells expressing human mGlu receptor subtypes. Using membranes from cells expressing human mGlu2 and mGlu3 receptors, [3H]-LY341495 (1 nM) specific binding was > 90% of total binding. At an approximate K(D) concentration for [3H]-LY341495 binding to human mGlu2 and mGlu3 receptors (1 nM), no appreciable specific binding of [3H-]LY341495 was found in membranes of cells expressing human mGlu1a, mGlu5a, mGlu4a, mGlu6, or mGlu7a receptors. However, modest (approximately 20% of mGlu2/3) specific [3H]-LY341495 (1 nM) binding was observed in human mGlu8 expressing cells. [3H]-LY341495 bound to membranes expressing human mGlu2 and mGlu3 receptors in a reversible and saturable manner with relatively high affinities (Bmax 20.5 +/- 5.4 and 32.0 +/- 7.0 pmol/mg protein; and K(D) = 1.67 +/- 0.20 and 0.75 +/- 0.43 nM, respectively). The pharmacology of [3H]-LY341495 binding in mGlu2 and mGlu3 expressing cells was consistent with that previously described for LY341495 in functional assays. [3H]-LY341495 binding provides a useful way to further investigate regulation of receptor expression and pharmacological properties of mGlu2 and mGlu3 receptor subtypes in recombinant systems.

Localization of metabotropic glutamate receptors mGluR1alpha and mGluR2/3 in the cat retina

The distribution of metabotropic glutamate receptors 1alpha (mGluR1alpha) and mGluR2/3 in the cat retina was studied through the use of preembedding immunocytochemistry for light and electron microscopy. Staining for mGluR1alpha in the outer plexiform layer was seen in numerous punctate structures that were identified as rod spherules. Cone pedicles remained unlabeled. A number of amacrine and ganglion cell somata also were stained with processes ramifying throughout the inner plexiform layer. These processes were postsynaptic to cone bipolar cells in both sublaminae, where they comprised one but not both of the postsynaptic elements at dyad contacts. Immunostaining for mGluR2/3 was observed in horizontal cells as well as in numerous amacrine and displaced amacrine cells. Labeled amacrine processes were postsynaptic to cone bipolar cells in both sublaminae but, similar to mGluR1alpha, comprised only one of the postsynaptic elements. Staining for mGluR2/3 also was seen in amacrine processes postsynaptic to rod bipolar terminals; these processes were identified as belonging to type A17 amacrine cells. The distribution patterns indicate that both mGluR1alpha and mGluR2/3 are positioned for postsynaptic function, whereas mGluR1alpha also may contribute to the presynaptic regulation of glutamate release from rod photoreceptors.

Agonist selectivity of mGluR1 and mGluR2 metabotropic receptors: a different environment but similar recognition of an extended glutamate conformation

To investigate the structural requirements for selective activation or blockade of metabotropic glutamate receptors, we developed a pharmacophore model for group I (mGluR1) and group II (mGluR2) agonists. The Apex-3D program was used with a training set of known active, inactive, and/or selective compounds with a wide structural diversity. The pharmacophore models were then validated by testing a set of additional known agonists. We also used competitive antagonist superpositions in order to define more precisely the topology of the mGluR1 and mGluR2 agonists' recognition site. Both models account for the activity of most potent compounds and show that the selectivity between mGluR1 and mGluR2 subtypes may be due to excluded volumes and additional binding sites, while the relative spatial position of functional groups (NH2, alpha- and gamma-CO2H) remains very similar. On both models glutamate lies in an extended form. An additional binding site is disclosed on mGluR1, while this region would be forbidden on mGluR2. This new site combines a closed and an open model for mGluR1 and accounts for the increased affinity of quisqualic acid. The models show another large hydrophobic region which is tolerated for mGluR2 and restricted for mGluR1.

Activation of metabotropic glutamate receptors modulates the voltage-gated sustained calcium current in a teleost horizontal cell

In the teleost retina, cone horizontal cells contain a voltage-activated sustained calcium current, which has been proposed to be involved in visual processing. Recently, several studies have demonstrated that modulation of voltage-gated channels can occur through activation of metabotropic glutamate receptors (mGluRs). Because glutamate is the excitatory neurotransmitter in the vertebrate retina, we have used whole cell electrophysiological techniques to examine the effect of mGluR activation on the sustained voltage-gated calcium current found in isolated cone horizontal cells in the catfish retina. In pharmacological conditions that blocked voltage-gated sodium and potassium channels, as well as N-methyl-D-aspartate (NMDA) and non-NMDA channels, application of L-glutamate or 1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) to voltage-clamped cone horizontal cells acted to increase the amplitude of the calcium current, expand the activation range of the calcium current by 10 mV into the cell's physiological operating range, and shift the peak calcium current by -5 mV. To identify and characterize the mGluR subtypes found on catfish cone horizontal cells, agonists of group I, group II, or group III mGluRs were applied via perfusion. Group I and group III mGluR agonists mimicked the effect of L-glutamate or 1S,3R-ACPD, whereas group II mGluR agonists had no effect on L-type calcium current activity. Inhibition studies demonstrated that group I mGluR antagonists significantly blocked the modulatory effect of the group I mGluR agonist, (S)-3,5-dihydroxyphenylglycine. Similar results were obtained when the group III mGluR agonist, L-2-amino-4-phosphonobutyric acid, was applied in the presence of a group III mGluR antagonist. These results provide evidence for two groups of mGluR subtypes on catfish cone horizontal cells. Activation of these mGluRs is linked to modulation of the voltage-gated sustained calcium current.