Actions of phenylglycine analogs at subtypes of the metabotropic glutamate receptor family

Metabotropic Glutamate Receptor Subtype 5 Positron-Emission-Tomography Radioligands as a Tool for Central Nervous System Drug Development: Between Progress and Setbacks

The metabotropic glutamate receptor subtype 5 (mGluR5) is a class C G-protein-coupled receptor (GPCR) that has been implicated in various neuronal processes and, consequently, in several neuropsychiatric or neurodevelopmental disorders. Over the past few decades, mGluR5 has become a major focus for pharmaceutical companies, as an attractive target for drug development, particularly through the therapeutic potential of its modulators. In particular, allosteric binding sites have been targeted for better specificity and efficacy. In this context, Positron Emission Tomography (PET) appears as a useful tool for making decisions along a drug candidate's development process, saving time and money. Thus, PET provides quantitative information about a potential drug candidate and its target at the molecular level. However, in this area, particular attention has to be given to the interpretation of the PET signal and its conclusions. Indeed, the complex pharmacology of both mGluR5 and radioligands, allosterism, the influence of endogenous glutamate and the choice of pharmacokinetic model are all factors that may influence the PET signal. This review focuses on mGluR5 PET radioligands used at several stages of central nervous system drug development, highlighting advances and setbacks related to the complex pharmacology of these radiotracers.

Metabotropic glutamate receptor subtype 5: molecular pharmacology, allosteric modulation and stimulus bias

The metabotropic glutamate receptor subtype 5 (mGlu5 ) is a family C GPCR that has been implicated in various neuronal processes and, consequently, in several CNS disorders. Over the past few decades, GPCR-based drug discovery, including that for mGlu5 receptors, has turned considerable attention to targeting allosteric binding sites. Modulation of endogenous agonists by allosteric ligands offers the advantages of spatial and temporal fine-tuning of receptor activity, increased selectivity and reduced adverse effects with the potential to elicit improved clinical outcomes. Further, with greater appreciation of the multifaceted nature of the transduction of mGlu5 receptor signalling, it is increasingly apparent that drug discovery must take into consideration unique receptor conformations and the potential for stimulus-bias. This novel paradigm proposes that different ligands may differentially modulate distinct signalling pathways arising from the same receptor. We review our current understanding of the complexities of mGlu5 receptor signalling and regulation, and how these relate to allosteric ligands. Ultimately, a deeper appreciation of these relationships will provide the foundation for targeted drug design of compounds with increased selectivity, not only for the desired receptor but also for the desired signalling outcome from the receptor. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Behavioral effects of alpha-alkylated amino acid analogs in the C57BL/6J mouse

Although a series of amino acid analogs have been shown to modulate brain function, information on the pharmacology of alpha-alkylated amino acids (AAAA) is limited. In particular there is no information on the effect of these amino acid analogs (AAA) on the elevated plus maze, the tail suspension test and the forced swim test. It was therefore the aim of the study to test a series of AAAA in these paradigms in order to explore behavioral activities of this compound class. 10 male mice per group aged between 10 and 14 weeks were used. Vehicle-treated controls were used in addition to intraperitoneal injections of 1, 10 and 100mg/kg body weight of each, alpha-amino-isobutyic acid (AIB), isovaline (IVA), alpha-propyl-alanine (APA), alpha-butyl-alanine (ABA), alpha-pentyl-alanine (APnA), alpha-ethylphenylglycine (AEPG) and alpha-methyl-valine (AMV). The elevated plus maze (EPM), the tail suspension test (TST) and forced swim test (FST) were used for behavioral testing. There were dose-dependent results: all compounds increased time and pathlength in the open arm of the EPM at least at one dose administered. In the TST and in the FST only the 100mg dose was showing an effect. The results show pharmacological activity modifying the EPM in low doses suggesting the use in treatment of behavioral traits and symptoms represented by or linked to the EPM including anxiety-related behavior including depression. Compounds acting at higher doses may be used to induce behavioral changes and thus serve as neurobiological-neuropharmacological tools.

D-Phenylglycinium bromide

In the crystal of the title salt, C₈H₁₀NO₂⁺·Br⁻, the bromide anions and the phenylglycinium cations are ­linked through N—H⋯Br, O—H⋯Br and C—H⋯O hydrogen bonds, generating sheets lying parallel to (001).

Interhemispheric regulation of the medial prefrontal cortical glutamate stress response in rats

While stressors are known to increase medial prefrontal cortex (PFC) glutamate (GLU) levels, the mechanism(s) subserving this response remain to be elucidated. We used microdialysis and local drug applications to investigate, in male Long-Evans rats, whether the PFC GLU stress response might reflect increased interhemispheric communication by callosal projection neurons. We report here that tail-pinch stress (20 min) elicited comparable increases in GLU in the left and right PFC that were sodium and calcium dependent and insensitive to local glial cystine-GLU exchanger blockade. Unilateral ibotenate-induced PFC lesions abolished the GLU stress response in the opposite hemisphere, as did contralateral mGlu(2/3) receptor activation. Local dopamine (DA) D(1) receptor blockade in the left PFC potently enhanced the right PFC GLU stress response, whereas the same treatment applied to the right PFC had a much weaker effect on the left PFC GLU response. Finally, the PFC GLU stress response was attenuated and potentiated, respectively, following alpha(1)-adrenoreceptor blockade and GABA(B) receptor activation in the opposite hemisphere. These findings indicate that the PFC GLU stress response reflects, at least in part, activation of callosal neurons located in the opposite hemisphere and that stress-induced activation of these neurons is regulated by GLU-, DA-, norepinephrine-, and GABA-sensitive mechanisms. In the case of DA, this control is asymmetrical, with a marked regulatory bias of the left PFC DA input over the right PFC GLU stress response. Together, these findings suggest that callosal neurons and their afferentation play an important role in the hemispheric specialization of PFC-mediated responses to stressors.

Involvement of Group II mGluRs in mossy fiber LTD

Mossy fiber long-term depression (LTD) has been shown to be triggered by either pharmacological or synaptic activation of Group II metabotropic glutamate receptors (mGluRs) whereas other studies indicate that synaptic activation of mGluRs is very limited. Therefore, we reexamined the role of Group II mGluRs for the induction of mossy fiber LTD. The complete depression of field potentials (fEPSPs) by 1 microM (2S,2'R,3'R)-2-(2',3'-Dicarboxycyclopropyl)glycine (DCG-IV) only partially reversed upon removal of the drug but fEPSPs were completely restored by the Group II antagonist 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid (LY341495) (3 microM). In contrast, fEPSPs returned back to baseline within 30 min after a brief application of 0.2 microM DCG-IV suggesting that the incomplete reversal of higher concentrations may be due to a residual receptor occupancy rather than to an induction of LTD. LY341495 itself did not increase fEPSPs and also blocked the inhibition of (2S,1'S,2'S)-2-(2-carboxycyclopropyl)glycine (L-CCG-I) (20 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) (10 microM) and its effect was mimicked by CPPG (50 microM). Furthermore, stimulation at 1 Hz for 15 min induced an LTD of 81% +/- 3% and 80% +/- 4% in the absence and presence of LY341495, respectively (n = 7, 5). Finally, we found that synaptic activation of Group II mGluRs during 15 min of 1-Hz stimulation only produces an inhibition of release by 8% +/- 1% (30 degrees C, n = 3). Our data suggests that pharmacological activation of Group II mGluRs is fully reversible per se and does not produce a long lasting depression and that activation of Group II mGluRs is neither necessary nor sufficient for the induction of mossy fiber LTD.

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.

Blunted cystine-glutamate antiporter function in the nucleus accumbens promotes cocaine-induced drug seeking

Repeated cocaine alters glutamate neurotransmission, in part, by reducing cystine-glutamate exchange via system xc-, which maintains glutamate levels and receptor stimulation in the extrasynaptic compartment. In the present study, we undertook two approaches to determine the significance of plasticity involving system xc-. First, we examined whether the cysteine prodrug N-acetylcysteine attenuates cocaine-primed reinstatement by targeting system xc-. Rats were trained to self-administer cocaine (1 mg/kg/200 microl, i.v.) under extended access conditions (6 h/day). After extinction training, cocaine (10 mg/kg, i.p.) primed reinstatement was assessed in rats pretreated with N-acetylcysteine (0-60 mg/kg, i.p.) in the presence or absence of the system xc- inhibitor (S)-4-carboxyphenylglycine (CPG; 0.5 microM; infused into the nucleus accumbens). N-acetylcysteine attenuated cocaine-primed reinstatement, and this effect was reversed by co-administration of CPG. Secondly, we examined whether reduced system xc- activity is necessary for cocaine-primed reinstatement. To do this, we administered N-acetylcysteine (0 or 90 mg/kg, i.p.) prior to 12 daily self-administration sessions (1 mg/kg/200 microl, i.v.; 6 h/day) since this procedure has previously been shown to prevent reduced activity of system xc-. On the reinstatement test day, we then acutely impaired system xc- in some of the rats by infusing CPG (0.5 microM) into the nucleus accumbens. Rats that had received N-acetylcysteine prior to daily self-administration sessions exhibited diminished cocaine-primed reinstatement; this effect was reversed by infusing the cystine-glutamate exchange inhibitor CPG into the nucleus accumbens. Collectively these data establish system xc- in the nucleus accumbens as a key mechanism contributing to cocaine-primed reinstatement.

Extracts of retina and brain that excite afferent fibers innervating hair cells contain a compound related to hydroxyphenylglycine-N-carbamoyl

To identify new neurotransmitter and modulator candidates that might be important in transmission from sensory hair cells to afferent nerves, we examined extracts of neural tissue for compounds that excite afferent fibers innervating hair cells. Here, we describe the extraction and purification from retina and brain of a potent, unstable, excitatory compound with pharmacological activity similar to glutamate on afferent fibers innervating hair cells. This compound, however, was clearly distinguished from glutamate, other common amino acids, and known endogenous glutamate-receptor agonists. After derivatization and analysis by gas chromatography-mass spectrometry, the major compound found in highly purified neuroactive chromatographic fractions had the same gas chromatographic elution time and mass spectrum as the compound formed by derivatization of L-p-hydroxyphenylglycine-N-carbamoyl. Hydroxyphenylglycine-N-carbamoyl, however, did not copurify with the neuroactive compound and was not neuroactive. We thus hypothesize that the detected compound was produced from a precursor, structurally related to L-p-hydroxyphenylglycine-N-carbamoyl, that was a major component of the neuroactive chromatographic fractions. Because several compounds related to hydroxyphenylglycine are known to act on glutamate receptors, such a compound is an interesting candidate to be an endogenous glutamate-receptor ligand in the mammalian nervous system.

Ionotropic and metabotropic glutamate receptor structure and pharmacology

RATIONALE

L: -Glutamate is the major excitatory neurotransmitter in the central nervous system (CNS) and mediates its actions via activation of both ionotropic and metabotropic receptor families. The development of selective ligands, including competitive agonists and antagonists and positive and negative allosteric modulators, has enabled investigation of the functional roles of glutamate receptor family members.

OBJECTIVE

In this review we describe the subunit structure and composition of the ionotropic and metabotropic glutamate receptors and discuss their pharmacology, particularly with respect to selective tools useful for investigation of their function in the CNS.

RESULTS

A large number of ligands are now available that are selective either for glutamate receptor subfamilies or for particular receptor subtypes. Such ligands have enabled considerable advances in the elucidation of the physiological and pathophysiological roles of receptor family members. Furthermore, efficacy in animal models of neurological and psychiatric disorders has supported the progression of several glutamatergic ligands into clinical studies. These include ionotropic glutamate receptor antagonists, which have entered clinical trials for disorders including epilepsy and ischaemic stroke, alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptor positive allosteric modulators which are under evaluation as cognitive enhancers, and metabotropic glutamate receptor 2 (mGluR2) agonists which are undergoing clinical evaluation as anxiolytics. Furthermore, preclinical studies have illustrated therapeutic potential for ligands selective for other receptor subtypes in various disorders. These include mGluR1 antagonists in pain, mGluR5 antagonists in anxiety, pain and drug abuse and mGluR5 positive allosteric modulators in schizophrenia.

CONCLUSIONS

Selective pharmacological tools have enabled the study of glutamate receptors. However, pharmacological coverage of the family is incomplete and considerable scope remains for the development of novel ligands, particularly those with in vivo utility, and for the their use together with existing tools for the further investigation of the roles of receptor family members in CNS function and as potentially novel therapeutics.

Metabotropic glutamate receptors modulate ischemia-induced GABA release in mouse hippocampal slices

The involvement of glutamate receptors in GABA release in ischemia was investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice. For in vitro ischemia, the slices were superfused in glucose-free media under nitrogen. Ionotropic glutamate receptor agonists failed to affect the ischemia-induced basal GABA release at either age. The K(+)-stimulated release in the immature hippocampus was potentiated by N-methyl-D-aspartate receptors, whereas in adults this release was reduced by both kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate receptor activation. The group I metabotropic receptor agonist (1+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate enhanced the basal ischemic GABA release in a receptor-mediated manner in adults, this being concordant with the positive modulation of GABAergic neurotransmission by group I metabotropic glutamate receptors. (1 +/-)-1-Aminocyclopentane-trans-1,3-dicarboxylate and (S)-3,5-dihydroxyphenylglycine also enhanced the K(+)-stimulated release in the developing hippocampus in a receptor-mediated manner. Because group I receptors generally increase neuronal excitability, the enhanced GABA release may attenuate hyperexcitation or strengthen inhibition, being thus neuroprotective, particularly under ischemic conditions. Group III metabotropic glutamate receptors were not at all involved in ischemic GABA release in the immature mice, but in adults their activation by O-phospho-L-serine potentiated the basal release and reduced the K(+)-stimulated release. These opposite effects were abolished by the antagonist (RS)-2-cyclopropyl-4-phosphonophenylglycine. Metabotropic glutamate receptors, namely group I and III receptors, are able to modify the release of GABA from hippocampal slices under ischemic conditions, both positive and negative effects being discernible, depending on the age and type of receptor activated.

Analogues of homoibotenic acid show potent and selective activity following sensitisation by quisqualic acid

Quisqualic acid induces sensitisation of neurones to depolarisation by analogues of 2-amino-4-phosphonobutyric acid (AP4), phenylglycine, and homoibotenic acid (HIBO). Thus, after administration of quisqualate these analogues become active at concentrations at which they are otherwise inactive. The mechanisms behind quisqualate-induced sensitisation are poorly understood and have not previously been quantified properly. In this study, we have tested the activity of a number of 4-alkyl- and 4-aryl-substituted analogues of HIBO as regards quisqualate-sensitisation, and present a method for quantifying the sensitisation induced by quisqualate at cortical neurones. These analogues are generally more potent and selective than (S)-AP4 or its homologue (S)-AP5 following quisqualate-sensitisation. Furthermore, we found a statistically significant correlation between the ligands' ability to inhibit CaCl(2)-dependent (S)-[(3)H]glutamate uptake into rat cortical synaptosomes, and their potency following quisqualate-induced depolarisation. This demonstrates the involvement of a transport system in the mechanism underlying the quisqualate-effect.

The novel and systemically active metabotropic glutamate 1 (mGlu1) receptor antagonist 3-MATIDA reduces post-ischemic neuronal death

We examined the pharmacological properties of 3-methyl-aminothiophene dicarboxylic acid (3-MATIDA) by measuring second messenger responses in baby hamster kidney cells stably transfected with mGlu1a, mGlu2, mGlu4a or mGlu5a receptors and ionotropic glutamate receptor agonist-induced depolarizations in mouse cortical wedges. 3-MATIDA was a potent (IC(50)=6.3 microM, 95% confidence limits 3-15) and relatively selective mGlu1 receptor antagonist. When tested on mGlu2, mGlu4 or mGlu5 receptors its IC(50) was >300 microM. When tested in cortical wedges, however, 3-MATIDA was also able to antagonize AMPA or NMDA responses with an IC(50) of 250 microM. When present in the incubation medium of cultured murine cortical cells, 3-MATIDA (1-100 microM) significantly reduced the death of neurons induced by 60 min of oxygen and glucose deprivation (OGD), even when added up to 60 min after OGD. A similar neuroprotective activity was observed when 3-MATIDA was present at 10-100 microM in the medium of rat organotypic hippocampal slice cultures exposed to 30 min OGD. Systemic administration of 3-MATIDA (3-10 mg/kg, immediately and 1 h after the onset of ischemia) reduced the volume of brain infarcts following permanent middle cerebral artery occlusion in rats. Our results show that 3-MATIDA is a potent and possibly selective mGlu 1 receptor antagonist that may be considered as a novel prototype neuroprotective agent.

Supersensitivity of human metabotropic glutamate 1a receptor signaling in L929sA cells

The effect of antagonist pretreatment on the signaling properties of the human metabotropic glutamate 1a (hmGlu1a) receptor was examined in stably transfected L929sA cells. Pre-exposure of hmGlu1a receptor-expressing cells to the mGlu1 receptor antagonists (S)-4-carboxy-3-hydroxyphenylglycine and 7-(hydroxyimino)cyclo-propa[b]chromen-1a-carboxylate ethyl ester dramatically enhanced subsequent glutamate-induced phosphoinositide hydrolysis and intracellular [Ca(2+)] rise. We found clear indications that the antagonist-mediated enhancement of glutamate-evoked mGlu1a receptor signaling is caused by the development of mGlu1a receptor supersensitivity: the potency of glutamate was increased by 3-fold after 24 h antagonist pretreatment and the potency of the antagonists was significantly decreased in antagonist-pretreated cells. The kinetic profile of the antagonist-mediated enhancement showed that the maximal increase in intracellular [Ca(2+)] was already reached after 30-min pretreatment, suggesting that de novo receptor synthesis is not involved in the process of mGlu1a receptor supersensitization. Glutamate-mediated phosphoinositide hydrolysis increased up to 24 h after antagonist treatment. Although it seemed likely that the hmGlu1a receptor could desensitize after activation by endogenously present glutamate, removal of glutamate from the extracellular medium with GPT resulted in a much smaller enhancement of glutamate responsiveness. Moreover, the magnitude of antagonist-mediated receptor supersensitivity was much larger than the magnitude of agonist-induced receptor desensitization. These results suggest that antagonist-evoked mGlu1 receptor supersensitivity is not merely the result of a blockade of agonist-induced desensitization. Finally, we found that antagonist pretreatment doubled the amount of receptors at the cell surface. Our findings are the first lines of evidence that prolonged antagonist treatment can supersensitize the hmGlu1a receptor. In view of the potential therapeutic application of mGlu1 receptor antagonists, it will be important to know whether these phenomena occur in vivo.

Activation of group I mGluRs is necessary for induction of long-term depression at striatal synapses

Activation of metabotropic glutamate receptors (mGluRs), which are coupled to G proteins, has important roles in certain forms of synaptic plasticity including corticostriatal long-term depression (LTD). In the present study, extracellular field potential and whole cell voltage-clamp recording techniques were used to investigate the effect of mGluR antagonists with different subtype specificity on high-frequency stimulation (HFS)-induced LTD of synaptic transmission in the striatum of brain slices obtained from 15-to 25-day-old rats. Induction of LTD was prevented during exposure to the nonselective mGluR antagonist (RS)-alpha-methyl-4-carboxyphenylglycine (500 microM). The group I mGluR-selective antagonists (S)-4-carboxy-phenylglycine (50 microM) and (RS)-1-aminoindan-1,5-dicarboxylic acid (100 microM) prevented induction of LTD when applied before and during HFS. The mGluR1-selective antagonist 7-(Hydroxyimino) cyclopropa[b]chromen-1a-carboxylate ethyl ester (80 microM) also blocked LTD induction. Unexpectedly, the mGluR5-selective antagonist 2-methyl-6-(phenylethyl)-pyridine (10 microM) also prevented LTD induction. The group II mGluR antagonist LY307452 (10 microM) did not block LTD induction at corticostriatal synapses, but LY307452 was able to block transient synaptic depression induced by the group II agonist LY314593. None of the antagonists had any effect on basal synaptic transmission at the concentrations used, and mGluR antagonists did not reverse LTD when applied beginning 20 min after HFS. These results suggest that both group I mGluR subtypes contribute to the induction of LTD at corticostriatal synapses.

mGlu1 receptors mediate a post-tetanic depression at parallel fibre-Purkinje cell synapses in rat cerebellum

Metabotropic glutamate (mGlu) receptors are located pre- and postsynaptically at central synapses. Activation of the receptors by exogenous agonists usually results in a reversible depression of fast glutamatergic neurotransmission. Evidence that synaptically released glutamate has such an action, however, is scarce. Sharp microelectrode recordings were used to investigate the modulatory role of mGlu receptors at a well-studied glutamatergic synapse, the one between parallel fibres and Purkinje cells in rat cerebellar slices. Brief, tetanic stimulation of the parallel fibres caused a depression of subsequent fast EPSPs. This post-tetanic depression (PTD) reached its maximum 4.5 s after the tetanus. Measured at this point, PTD was frequency-dependent; 10 stimuli at 20 Hz produced no significant depression, whereas, at 100 Hz the same number of stimuli was maximally effective (approximately 50% depression). The nonselective mGlu antagonist, (S)-alpha-methyl-4-carboxyphenylglycine 1 mm or the GABAB antagonist, CGP35348 (1 mm), both decreased the magnitude of the PTD. In the presence of CGP35348 the mGlu1 antagonist, 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester (300 microm), inhibited PTD further. A group II/III mGlu antagonist had no effect. These observations indicate that synaptically activated mGlu1 receptors not only generate a slow EPSP and induce Ca2+ mobilization in Purkinje cells, as reported previously, but also produce a transient depression of fast synaptic transmission. This short-term plasticity may be important for shaping the output of cerebellar circuits and/or it could provide a substrate for long-term depression when additional mechanisms are superimposed.

Metabotropic glutamate receptor subtypes modulating neurotransmission at parallel fibre-Purkinje cell synapses in rat cerebellum

The actions of reportedly group-selective metabotropic glutamate (mGlu) receptor agonists and antagonists on neurotransmission at parallel fibre-Purkinje cell synapses in the rat cerebellum have been characterised using sharp microelectrode recording and an in vitro slice preparation. Application of the group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) or the group III selective agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed synaptic transmission in a reversible and concentration-dependent manner (EC(50)=18 and 5 microM, respectively). The depression produced by DHPG was unrelated to the depolarisation observed in some Purkinje cells. The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV, 1 microM) had no effect. The effects of DHPG were inhibited by the group I-selective antagonist 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester (CPCCOEt), but not by the group II/III antagonist alpha-methyl-4-phosphonophenylglycine (MPPG). The effect of L-AP4 was inhibited by MPPG, but not by the group I/II antagonist (S)-alpha-methyl-4-carboxyphenylglycine (MCPG). By themselves, the antagonists did not affect the EPSPs, suggesting that neither receptor is activated during low frequency neurotransmission. It is concluded that, in addition to the excitatory role for group I receptors described previously, both group I and III (but not group II) mGlu receptors operate at this synapse to inhibit synaptic transmission. The specific receptor subtypes involved are likely to be mGlu1 and mGlu4.

(S)-4C3HPG, a mixed group I mGlu receptor antagonist and a group II agonist, administered intrastriatally, counteracts parkinsonian-like muscle rigidity in rats

The aim of the present study was to determine whether S-4-carboxy-3-hydroxyphenylglycine (S)-4C3HPG, a mixed group I glutamate metabotropic receptor antagonist and a group II agonist, attenuated parkinsonian-like muscle rigidity in rats. Muscle tone was examined using a combined mechano and electromyographic method, which measured simultaneously the muscle resistance (MMG) of the rat's hind foot to passive extension and flexion in the ankle joint and the electromyographic activity (EMG) of the antagonistic muscles of that joint: gastrocnemius and tibialis anterior. Muscle rigidity was induced by pretreatment with haloperidol (1 mg/kg i.p.). (S)-4C3HPG injected in doses of 5 and 15 microg/0.5 microl bilaterally, into the rostral region of the striatum, decreased both the haloperidol-induced muscle rigidity (MMG) and the enhanced electromyographic activity (EMG). The present results suggest that blockade of mGluR1 receptors and/or activation of mGluR2 ones, localized in the rostral part of the striatum, may be responsible for the anti-parkinsonian effect of (S)-4C3HPG.

(S)-3,4-DCPG, a potent and selective mGlu8a receptor agonist, activates metabotropic glutamate receptors on primary afferent terminals in the neonatal rat spinal cord

(S)-3,4-Dicarboxyphenylglycine (DCPG) has been tested on cloned human mGlu1-8 receptors individually expressed in AV12-664 cells co-expressing a rat glutamate/aspartate transporter and shown to be a potent and selective mGlu8a receptor agonist (EC(50) value 31+/-2 nM, n=3) with weaker effects on the other cloned mGlu receptors (EC(50) or IC(50) values >3.5 microM on mGlu1-7). Electrophysiological characterisation on the neonatal rat spinal cord preparation revealed that (S)-3,4-DCPG depressed the fast component of the dorsal root-evoked ventral root potential (fDR-VRP) giving a biphasic concentration-response curve showing EC(50) values of 1.3+/-0.2 microM (n=17) and 391+/-81 microM (n=17) for the higher and lower affinity components, respectively. The receptor mediating the high-affinity component was antagonised by 200 microM (S)-alpha-methyl-2-amino-4-phosphonobutyrate (MAP4, K(D) value 5.4+/-1.5 microM (n=3)), a group III metabotropic glutamate (mGlu) receptor antagonist. The alpha-methyl substituted analogue of (S)-3,4-DCPG, (RS)-3,4-MDCPG (100 microM), antagonised the effects of (S)-3,4-DCPG (K(D) value 5.0+/-0.4 microM, n=3) in a similar manner to MAP4. (S)-3,4-DCPG-induced depressions of the fDR-VRP in the low-affinity range of the concentration-response curve were potentiated by 200 microM (S)-alpha-ethylglutamate (EGLU), a group II mGlu receptor antagonist, and were relatively unaffected by MAP4 (200 microM). However, depressions of the fDR-VRP mediated by the AMPA selective antagonist (R)-3,4-DCPG were not potentiated by EGLU, suggesting that the low-affinity component of the concentration-response curve for (S)-3,4-DCPG is not due to antagonism of postsynaptic AMPA receptors. It is suggested that the receptor responsible for mediating the high-affinity component is mGlu8. The receptor responsible for mediating the low-affinity effect of (S)-3,4-DCPG has yet to be identified but it is unlikely to be one of the known mGlu receptors present on primary afferent terminals or an ionotropic glutamate receptor of the AMPA or NMDA subtype.

Metabotropic glutamate receptors modulate GABA release from mouse hippocampal slices

The effects of metabotropic glutamate receptor agonists on the basal and potassium (50 mM K+)-stimulated release of [3H]GABA from mouse hippocampal slices were investigated using a superfusion system. The group I agonist (1+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate enhanced the basal GABA release and reduced the K+-evoked release by a mechanism antagonized by (RS)-1-aminoindan-1,5-dicarboxylate in both cases. The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine failed to have any effect on the basal release, but inhibited the stimulated release. This inhibition was not affected by the antagonist (2S)-2-ethylglutamate. The group III agonists L(+)-amino-4-phosphonobutyrate and O-phospho-L-serine inhibited the basal GABA release, which effects were blocked by the antagonist (RS)-2-cyclopropyl-4-phosphonophenylglycine. Moreover, the suppression of the K+-evoked release by L(+)2-amino-4-phosphonobutyrate was apparently receptor-mediated, being blocked by (RS)-2-cyclopropyl-4-phosphonophenylglycine. The results show that activation of metabotropic glutamate receptors of group I is able to potentiate the basal release of GABA, whereas activation of groups I and III receptors reduce K+-stimulated release in mouse hippocampal slices.

Developmental regulation of hippocampal excitatory synaptic transmission by metabotropic glutamate receptors

The aims of this study were, to use agonists selective for the 3 mGlu receptor groups to identify developmental changes in their effects, and to assess the usefulness of proposed selective antagonists as pharmacological tools. Hippocampal slices (400 microm) were prepared from neonate (9 - 14 days) and young adult (5 - 7 weeks) Sprague-Dawley rats. Field excitatory postsynaptic potentials (fEPSP) were recorded from CA1. DHPG (100 microM), a group I agonist, produced a slowly developing enhancement of fEPSP slope in slices from adults. In slices from neonates, DHPG (75 microM) depressed fEPSP slope. DCG-IV (500 nM), a group II agonist, did not affect the fEPSP recorded from slices from adults whereas perfusion in neonate slices produced a sustained depression. The group III agonist L-AP4 (50 microM) was ineffective in adult slices but depressed fEPSP slope in slices prepared from neonates. DHPG-induced depression of fEPSP slope was inhibited by 4-CPG (400 microM), a group I antagonist, but was unaffected by MCCG (500 microM) and MAP4 (500 microM), group II and III receptor antagonists respectively. MCCG but not MAP4 antagonized the effects of DCG-IV with 4-CPG producing variable effects. The effect of L-AP4 was unaffected by MCCG, blocked by MAP4, and enhanced by 4-CPG. The results show that the effects of the agonists for all groups of mGlu receptors are developmentally regulated. Furthermore, MCCG and MAP4 behave as effective and selective antagonists for group II and group III mGlu receptors respectively, whereas the usefulness of 4-CPG as a group I antagonist may be limited.

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.

Involvement of metabotropic glutamate receptors in ischemia-induced taurine release in the developing and adult hippocampus

Metabotropic glutamate receptors have recently been envisaged as involved in both potentiation and prevention of ischemic and excitotoxic neuronal damage. The release of the inhibitory amino acid taurine is markedly enhanced in ischemia in both the immature and mature mouse hippocampus. The modulation of [3H]taurine release by metabotropic receptor agonists and antagonists was studied in hippocampal slices from developing (7-day-old) and adult (3-month-old) mice using a superfusion system. Agonists of group I, II and III metabotropic glutamate receptors generally reduced the ischemia-induced release in adult animals. In the immature hippocampus the group I agonists (S)-3,5-dihydroxyphenylglycine and (1+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate, which mainly enhance neuronal excitation, potentiated initial taurine release in ischemia. Ionotropic glutamate receptor agonists also enhance the ischemia-induced taurine release in developing mice. This glutamate-activated taurine release may thus constitute an important protective mechanism against excitotoxicity in the immature hippocampus.

Evidence that DHPG-induced nociception depends on glutamate release from primary afferent C-fibres

We examined whether enhanced glutamate release contributes to the expression of persistent spontaneous nociceptive behaviours (SNBs) in rats induced by intrathecal (i.t.) administration of the selective group I mGluR agonist, (RS)-3,5-dihydroxyphenylglycine ((RS)-DHPG). Pretreatment with drugs that have been shown to inhibit glutamate release, including a group II metabotropic glutamate receptor (mGluR) agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate ((2R,4R)-APDC), a group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4), or the use-dependent sodium channel blockers 3,5-diamino-6-(2,3-diclorophenyl)-1,2,4-triazine (lamotrigine) and 2-amino-6-trifluoromethoxybenzothiazole (riluzole), produced dose-dependent reductions in (RS)-DHPG-induced SNBs. We have also shown that incubation of rat lumbar spinal cord slices with (RS)-DHPG potentiates 4-aminopyridine-evoked (4-AP) release of glutamate. Furthermore, we found that destruction of unmyelinated primary afferent C-fibres by neonatal capsaicin treatment significantly reduced (RS)-DHPG-induced SNBs in adult rats. Together, these results suggest that (RS)-DHPG-induced nociception is dependent on spinal glutamate release, probably from primary afferent C-fibres.

Selective inhibition of NAALADase, which converts NAAG to glutamate, reduces ischemic brain injury

We describe here a new strategy for the treatment of stroke, through the inhibition of NAALADase (N-acetylated-alpha-linked-acidic dipeptidase), an enzyme responsible for the hydrolysis of the neuropeptide NAAG (N-acetyl-aspartyl-glutamate) to N-acetyl-aspartate and glutamate. We demonstrate that the newly described NAALADase inhibitor 2-PMPA (2-(phosphonomethyl)pentanedioic acid) robustly protects against ischemic injury in a neuronal culture model of stroke and in rats after transient middle cerebral artery occlusion. Consistent with inhibition of NAALADase, we show that 2-PMPA increases NAAG and attenuates the ischemia-induced rise in glutamate. Both effects could contribute to neuroprotection. These data indicate that NAALADase inhibition may have use in neurological disorders in which excessive excitatory amino acid transmission is pathogenic.

Platelet-activating factor and group I metabotropic glutamate receptors interact for full development and maintenance of long-term potentiation in the rat medial vestibular nuclei

In rat brainstem slices, we investigated the interaction between platelet-activating factor and group I metabotropic glutamate receptors in mediating long-term potentiation within the medial vestibular nuclei. We analysed the N1 field potential wave evoked in the ventral portion of the medial vestibular nuclei by primary vestibular afferent stimulation. The group I metabotropic glutamate receptor antagonist, (R,S)-1-aminoindan-1,5-dicarboxylic acid, prevented long-term potentiation induced by a platelet-activating factor analogue [1-O-hexadecyl-2-O-(methylcarbamyl)-sn-glycero-3-phosphocholine], as well as the full development of potentiation, induced by high-frequency stimulation under the blocking agent for synaptosomal platelet-activating factor receptors (ginkolide B), at drug washout. However, potentiation directly induced by the group I glutamate metabotropic receptor agonist, (R,S)-3,5-dihydroxyphenylglycine, was reduced by ginkolide B. These findings suggest that platelet-activating factor, whether exogenous or released following potentiation induction, exerts its effect through presynaptic group I metabotropic glutamate receptors, mediating the increase of glutamate release. In addition, we found that this mechanism, which led to full potentiation through presynaptic group I metabotropic glutamate receptor activation, was inactivated soon after application of potentiation-inducing stimulus. In fact, the long-lasting block of the platelet-activating factor and metabotropic glutamate receptors prevented the full potentiation development and the induced potentiation progressively declined to null. Moreover, ginkolide B, given when high-frequency-dependent potentiation was established, only reduced it within 5 min after potentiation induction. We conclude that to fully develop vestibular long-term potentiation requires presynaptic events. Platelet-activating factor, released after the activation of postsynaptic mechanisms which induce potentiation, is necessary for coupling postsynaptic and presynaptic phenomena, through the activation of group I metabotropic glutamate receptors, and its action lasts only for a short period. If this coupling does not occur, a full and long-lasting potentiation cannot develop.

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.

Type I and II metabotropic glutamate receptors mediate depressor and bradycardic actions in the nucleus of the solitary tract of anaesthetized rats

The potential role of metabotropic glutamate (mGlu) receptors in cardiovascular function in the nucleus of the solitary tract was examined following the microinjection of a number of selective mGlu receptor compounds into this site of anaesthetized rats. The prototypic mGlu receptor selective agonist 1S,3R-1-amino-cyclopentane dicarboxylate elicited depressor and bradycardic actions following microinjection into the nucleus tractus solitarius, which were similar to those produced by L-glutamate. Similarly, decreases in blood pressure and heart rate were observed upon administration of the type I and II selective mGlu receptor agonists, (R,S)-3,5-dihydroxyphenylglycine (DHPG) and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC), respectively. These actions of DHPG were selectively attenuated by (+/-)-1-aminoindane-1,5-dicarboxylate, a type I mGlu receptor antagonist, whilst cardiovascular responses to APDC were unaffected by this compound. Interestingly, the proposed type II antagonist, (2S,4S)-2-amino-4-(4,4-diphenylbut-1-yl)-pentane-1,5-doic acid, reduced the cardiovascular responses to intra-nucleus tractus solitarius administration of both APDC and DHPG. The type III mGlu receptor agonist, L-2-amino-4-phosphonobutyrate, however, failed to elicit any cardiovascular actions when microinjected into the nucleus tractus solitarius. These studies provide new evidence for functional type I and II mGlu receptors in modulating cardiovascular responses in the nucleus tractus solitarius.

Biochemical and electrophysiological studies on (S)-(+)-2-(3'-carboxybicyclo(1.1.1)pentyl)-glycine (CBPG), a novel mGlu5 receptor agonist endowed with mGlu1 receptor antagonist activity

The pharmacological profile of (S)-(+)-2-(3'-carboxybicyclo[1.1.1]pentyl)-glycine (CBPG) and of other group 1 metabotropic glutamate (mGlu) receptor agents were studied in BHK cells transfected with mGlu receptor subtypes or in native receptors in brain slices by measuring second messenger responses. The mGlu receptor-mediated changes in the electrophysiological properties of CA1 pyramidal cells of the hippocampus were also evaluated. In mGlu5a receptor transfected cells, CBPG behaved as a partial agonist, while in mGlu1alpha receptor transfected cells, it behaved as a glutamate antagonist. No effect was found on cAMP formation in cells transfected with mGlu2 receptors or mGlu4 receptors. In brain slices, CBPG neither affected phospholipase D-coupled glutamate receptors nor did it modify the responses to ionotropic receptor stimulation (at concentrations up to 1 mM). When tested in CA1 pyramidal cells of the hippocampus, CBPG (50-100 microM) caused depolarization, increased cell input resistance, and decreased action potential frequency adaptation and afterhyperpolarization. DHPG (3-100 microM), an agonist of both mGlu1 and mGlu5 receptors, and CHPG (1000 microM), a low affinity mGlu5 agonist, produced qualitatively similar effects. The actions of CBPG or CHPG were not modified by AIDA (300 microM), a selective mGlu1 receptor antagonist. Our results suggest that CBPG could be a useful tool for discriminating between mGlu1 receptor and mGlu5 receptor effects and that mGlu5 receptors are the receptors which are mainly responsible for the direct excitatory effects of mGlu receptor agonists on CA1 pyramidal cells.

Regulation of synaptic transmission in the mossy fibre-granule cell pathway of rat cerebellum by metabotropic glutamate receptors

The role of metabotropic glutamate receptors (mGluRs) in the mossy fibre-granule cell pathway in rat cerebellum was studied using slice preparations and electrophysiological techniques. Application of the group I selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) evoked, in a concentration-dependent manner (EC50 = 33 microM), a depolarising/hyperpolarising complex response from granule cells which was preferentially inhibited by the group I selective antagonist (S)-4-carboxyphenylglycine (4CPG). The group III selective agonist L-amino-4-phosphonobutyrate (AP4) evoked a hyperpolarising response (EC50 = 10 microM) which was inhibited by the group II/III selective antagonist (S)-alpha-methyl-4-phosphonophenylglycine (MPPG). The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxylcyclopropyl)glycine (DCG-IV) elicited no measurable voltage change. The amplitude of the synaptically-mediated mossy fibre response in granule cells was unaffected during application of AP4, was reduced by DHPG and was enhanced by DCG-IV (EC50 = 80 nM). These effects were inhibited by the group selective antagonists 4CPG and (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCG-4), respectively. Further investigation using patch-clamp recording revealed that DCG-IV potently inhibited spontaneous GABAergic currents. We conclude that group I and III (but not group II) mGluRs are functionally expressed by granule cells, whereas unexpectedly group II or III mGluRs do not appear to be present presynaptically on mossy fibre terminals. Group II mGluRs are located on Golgi cell terminals; when activated these receptors cause disinhibition, a function which may be important for gating information transfer from the mossy fibres to the granule cells.

Involvement of metabotropic glutamate receptors in taurine release in the adult and developing mouse hippocampus

The inhibitory amino acid taurine has been held to function as an osmoregulator and modulator of neural activity, being particularly important in the immature brain. Ionotropic glutamate receptor agonists are known markedly to potentiate taurine release. The effects of different metabotropic glutamate receptor (mGluR) agonists and antagonists on the basal and K(+)-stimulated release of [3H]taurine from hippocampal slices from 3-month-old (adult) and 7-day-old mice were now investigated using a superfusion system. Of group I metabotropic glutamate receptor agonists, quisqualate potentiated basal taurine release in both age groups, more markedly in the immature hippocampus. This action was not antagonized by the specific antagonists of group I but by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX), which would suggest an involvement of ionotropic glutamate receptors. (S)-3,5-dihydroxyphenylglycine (DHPG) potentiated the basal release by a receptor-mediated mechanism in the immature hippocampus. The group II agonist (2S, 2'R, 3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) markedly potentiated basal taurine release at both ages. These effects were antagonized by dizocilpine, indicating again the participation of ionotropic receptors. Group III agonists slightly potentiated basal taurine release, as did several antagonists of the three metabotropic receptor groups. Potassium-stimulated (50 mM K+) taurine release was generally significantly reduced by mGluR agents, mainly by group I and II compounds. This may be harmful to neurons in hyperexcitatory states. On the other hand, the potentiation by mGluRs of basal taurine release, particularly in the immature hippocampus, together with the earlier demonstrated pronounced enhancement by activation of ionotropic glutamate receptors, may protect neurons against excitotoxicity.

Cyclobutane quisqualic acid analogues as selective mGluR5a metabotropic glutamic acid receptor ligands

The conformationally constrained cyclobutane analogues of quisqualic acid (Z)- and (E)-1-amino-3-[2'-(3',5'-dioxo-1',2', 4'-oxadiazolidinyl)]cyclobutane-1-carboxylic acid, compounds 2 and 3, respectively, were synthesized. Both 2 and 3 stimulated phosphoinositide (PI) hydrolysis in the hippocampus with EC50 values of 18 +/- 6 and 53 +/- 19 microM, respectively. Neither analogue stimulated PI hydrolysis in the cerebellum. The effects of 2 and 3 were also examined in BHK cells which expressed either mGluR1a or mGluR5a receptors. Compounds 2 and 3 stimulated PI hydrolysis in cells expressing mGluR5a but not in those cells expressing mGluR1a. The EC50 value for 2 was 11 +/- 4 microM, while that for 3 was 49 +/- 25 microM. Both 2 and 3 did not show any significant effect on cells expressing the mGluR2 and mGluR4a receptors. In addition, neither compound blocked [3H]glutamic acid uptake into synaptosomal membranes, and neither compound was able to produce the QUIS effect as does quisqualic acid. This pharmacological profile indicates that 2 and 3 are selective ligands for the mGluR5a metabotropic glutamic acid receptor.

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.

Induction of long-lasting depression by (+)-alpha-methyl-4-carboxyphenylglycine and other group II mGlu receptor ligands in the dentate gyrus of the hippocampus in vitro

Application of several well characterized group II mGlu receptor ligands was found to induce a long-lasting depression of synaptic transmission in the medial perforant path of the dentate gyrus. These ligands were N-acetylaspartylglutamate (NAAG), which is a dipeptide located in the brain and possibly functioning as a neurotransmitter, two agents widely used previously as mGluR antagonists, (+)-alpha-methyl-4-carboxyphenylglycine (MCPG), and (S)-alpha-ethylglutamate (EGLU), and the well characterized group II mGluR agonist (2S,1R,2R,3R)-2-(2S,1'R,2'R,3'R)-2(2'3'-dicarboxycyclopropyl)glyci ne (DCG-IV). It is postulated that all these ligands induced the long-lasting depression by an agonist/partial agonist action at group II mGlu receptor. The long-lasting depression induced by the ligands showed mutual occlusion with low frequency stimulation-induced long-term depression, demonstrating common induction or maintenance mechanisms. The induction of the long-lasting depression by the mGlu receptor ligands are suggested to occur postsynaptically as the induction was not associated with a change in paired pulse depression of excitatory postsynaptic potentials (EPSPs).

Effects of varying the expression level of recombinant human mGlu1alpha receptors on the pharmacological properties of agonists and antagonists

1. Different expression levels of the human type 1alpha metabotropic glutamate (mGlu1alpha) receptor were obtained in transfected Chinese hamster ovary cells using an isopropyl beta-D-thiogalactopyranoside (IPTG) inducible system. Expression of mGlu1alpha receptors could not be detected using immunoblotting or immunocytochemical approaches in non-induced cells, however, controlled expression could be induced following IPTG addition in a time- and concentration-dependent manner. 2. In induced cells (100 microM IPTG, 20 h) the agonists L-quisqualate or 1-aminocyclopentane-1S,3R-dicarboxylic acid stimulated large increases in [3H]-inositol (poly)phosphate (in the presence of Li+) and inositol, 1,4,5-trisphosphate levels. 3. Induction with 1-100 microM IPTG allowed the receptor density to be increased incrementally and this not only resulted in an increase in the maximum response to L-quisqualate, 1-aminocyclopentane-1S,3R-dicarboxylic acid and (S)-3,5-dihydroxy-phenylglycine, but also in an increase in the respective potencies of each agent to activate phosphoinositide hydrolysis. 4. The intrinsic activity of the partial agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid dramatically increased with increasing receptor expression. 5. The activities of the competitive mGlu1alpha receptor antagonists (S)-alpha-methyl-4-carboxyphenylglycine and (S)-4-carboxy-3-hydroxyphenylglycine for inhibition of the effects of L-quisqualate or (S)-3,5-dihydroxyphenylglycine were found to be independent of the receptor expression level. 6. When the mGlu1alpha receptor was expressed at very high levels, no evidence for receptor constitutive activity could be detected, and none of the antagonists tested revealed either any intrinsic activity or negative efficacy. 7. These data demonstrate that both the potency and efficacy of mGlu1alpha receptor agonists are influenced by expression level, whilst mGlu1alpha receptor antagonist activities are independent of expression level.

Antagonist activity of alpha-substituted 4-carboxyphenylglycine analogues at group I metabotropic glutamate receptors expressed in CHO cells

1. We have investigated the antagonist properties of 6 alpha-substituted phenylglycine analogues based on the structure of 4-carboxyphenylglycine (4-CPG) for group I metabotropic glutamate receptors (mGlu1alpha and mGlu5a) permanently expressed in CHO cells. 2. (S)-4-CPG and (S)-MCPG were the most selective mGlu1alpha receptor antagonists. Longer chain alpha-carbon substitutions resulted in a progressive loss of antagonist affinity at mGlu1alpha receptors but not at mGlu5a receptors. Thus mGlu1alpha receptor antagonists require small aliphatic groups at the alpha-position. Alpha-cyclopropyl-4-CPG showed a tendency towards mGlu5a selectivity, suggesting that bulky groups at this position may favour mGlu5a receptor antagonism. 3. We demonstrate that the mGlu5a receptor displays agonist-dependent antagonism. L-glutamate-induced Ca2+ release in mGlu5a receptor expressing cells was more susceptible to antagonism by cyclic alpha-carbon derivatives than (S)-3,5-dihydroxyphenylglycine (DHPG)-induced Ca2+ release in the same cell line. 4. The data presented suggests that mGlu1alpha and mGlu5a receptors have different steric and/or conformational requirements for the binding of antagonists and different amino acids which could interact with agonists. 5. These phenylglycine analogues could provide leads for the development of subtype selective antagonists.

Effects of metabotropic glutamate receptor block on the synaptic transmission and plasticity in the rat medial vestibular nuclei

In rat brainstem slices, we investigated the possible role of metabotropic glutamate receptors in modulating the synaptic transmission within the medial vestibular nuclei, under basal and plasticity inducing conditions. We analysed the effect of the metabotropic glutamate receptor antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine on the amplitude of the field potentials and latency of unitary potentials evoked in the ventral portion of the medial vestibular nuclei by primary vestibular afferent stimulation, and on the induction and maintenance of long-term potentiation, after high-frequency stimulation. Two effects were observed, consisting of a slight increase of the field potentials and reduction of unit latency during the drug infusion, and a further long-lasting development of these modifications after the drug wash-out. The long-term effect depended on N-methyl-D-aspartate receptor activation, as D,L-2-amino-5-phosphonopentanoic acid prevented its development. We suggest that (R,S)-alpha-methyl-4carboxyphenylglycine enhances the vestibular responses and induces N-methyl-D-aspartate-dependent long-term potentiation by increasing glutamate release, through the block of presynaptic metabotropic glutamate receptors which actively inhibit it. The block of these receptors was indirectly supported by the fact that the agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid reduced the vestibular responses and blocked the induction of long-term potentiation by high-frequency stimulation. The simultaneous block of metabotropic glutamate receptors facilitating synaptic plasticity, impedes the full expression of the long-term effect throughout the (R,S)-alpha-methyl-4-carboxyphenylglycine infusion. The involvement of such a facilitatory mechanism in the potentiation is supported by its reversible reduction following a second (R,S)-alpha-methyl-4-carboxyphenylglycine infusion. The drug also reduced the expression of potentiation induced by high-frequency stimulation. Conversely the electrical long-term potentiation was still induced, but it was occluded by the previous drug potentiation. We conclude that metabotropic glutamate receptors play a dual functional role in the medial vestibular nuclei, consisting in the inhibition of glutamate release under basal conditions, and the facilitation of N-methyl-D-aspartate-dependent plasticity phenomena.

Coexpression of multiple metabotropic glutamate receptors in axon terminals of single suprachiasmatic nucleus neurons

Glutamate is the primary excitatory transmitter in axons innervating the hypothalamic suprachiasmatic nucleus (SCN) and is responsible for light-induced phase shifts of circadian rhythms generated by the SCN. By using self-innervating single neuron cultures and patch-clamp electrophysiology, we studied metabotropic glutamate receptors (mGluRs) expressed by SCN neurons. The selective agonists for group I (3,5-dihydroxy-phenylglycine), group II ((S)-4-carboxy-3-hydroxyphenylglycine), and group III ((+)-2-amino-4-phosphonobutyric acid) mGluRs all depressed the evoked IPSC in a subset (33%) of single autaptic neurons, suggesting a coexpression of all three groups of mGluRs in the same axon terminals of a single neuron. Other neurons showed a variety of combinations of mGluRs, including an expression of only one group of mGluR (18%) or coexpression of two groups of mGluRs (27%). Some neurons had no response to any of the three agonists (22%). The three mGluR agonists had no effect on postsynaptic gamma-aminobutyric acid (GABA) receptor responses, indicating a presynaptic modulation of GABA release by mGluRs. We conclude that multiple mGluRs that act through different second messenger pathways are coexpressed in single axon terminals of SCN neurons where they modulate the release of GABA presynaptically, usually inhibiting release.

Antagonists for group I mGluRs attenuate excitotoxic neuronal death in cortical cultures

Activation of ion channel-linked glutamate receptors, especially N-methyl-D-aspartate (NMDA) receptors, mediates the excitotoxic effects of glutamate upon central neurons. We examined the hypothesis that activation of group I metabotropic glutamate receptors (mGluRs) would increase NMDA receptor-mediated cortical neuronal death. Addition of the selective group I mGluR agonists, dihydroxyphenylglycine (DHPG) or trans-azetidine-2,4-dicarboxylic acid (t-ADA) potentiated NMDA-induced neuronal death, and application of the group I mGluR-selective antagonist, aminoindan-1,5-dicarboxylic acid (AIDA), as well as the non-selective antagonists methyl-4-carboxyphenylglycine (MCPG) or 4-carboxyphenylglycine (4CPG) reduced NMDA- and kainate-induced neuronal death in murine cortical cultures. The pro-excitotoxic effect of group I mGluR activation may be mediated largely by enhancement of glutamate release, as DHPG potentiated high potassium-stimulated glutamate release, and the protective effects of both AIDA and MCPG were abolished when NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptors were blocked immediately after toxic NMDA receptor overstimulation. The present data support the possibility that antagonizing group I mGluRs may be a useful strategy for attenuating excitotoxic neuronal death in certain disease states.

LY341495 is a nanomolar potent and selective antagonist of group II metabotropic glutamate receptors

The in vitro pharmacology of a structurally novel compound, LY341495, was investigated at human recombinant metabotropic glutamate (mGlu) receptor subtypes expressed in non-neuronal (RGT, rat glutamate transporter) cells. LY341495 was a nanomolar potent antagonist of 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD)-induced inhibition of forskolin-stimulated cAMP formation at mGlu2 and mGlu3 receptors (respective IC50S of 0.021 and 0.014 microM). At group I mGlu receptor expressing cells, LY341495 was micromolar potent in antagonizing quisqualate-induced phosphoinositide (PI) hydrolysis, with IC50 values of 7.8 and 8.2 microM for mGlu1a and mGlu5a receptors, respectively. Among the human group III mGlu receptors, the most potent inhibition of L-2-amino-4-phosphonobutyric acid (L-AP4) responses was seen for LY341495 at mGlu8, with an IC50 of 0.17 microM. LY341495 was less potent at mGlu7 (IC50 = 0.99 microM) and least potent at mGlu4 (IC50 = 22 microM). Binding studies in rat brain membranes also demonstrated nanomolar potent group II mGlu receptor affinity for LY341495, with no appreciable displacement of ionotropic glutamate receptor ligand binding. Thus, LY341495 has a unique range of selectivity across the mGlu receptor subtypes with a potency order of mGlu3 > or = mGlu2 > mGlu8 > mGlu7 >> mGlu1a = mGlu5a > mGlu4. In particular, LY341495 is the most potent antagonist yet reported at mGlu2, 3 and 8 receptors. Thus, it represents a novel pharmacological agent for elucidating the function of mGlu receptors in experimental systems.

Type I and II metabotropic glutamate receptors regulate the outflow of [3H]D-aspartate and [14C]gamma-aminobutyric acid in rat solitary nucleus

Metabotropic glutamate (mGlu) receptors modulating amino acid outflow were examined in a model system in order to further characterize the pharmacological nature of the mGlu receptors involved in viscerosensory processing in the nucleus tractus solitarii. The actions of a number of subtype-selective mGlu receptor agonists and antagonists were monitored on the K+-evoked outflow of [3H]D-aspartate and [14C]gamma-aminobutyric acid (GABA) from superfused slices of rat nucleus tractus solitarii. (+/-)1S,3R-1-Amino-cyclopentane-1,3-dicarboxylate (10-300 microM), produced a concentration-dependent increase in outflow, which was attenuated by a number of phenylglycine antagonists. (2S,3S,4S)-alpha-(Carboxycyclopropyl)-glycine (30-300 microM) had mixed effects on outflow. The type I-selective agonist (RS)-3,5-dihydroxyphenylglycine (300 microM) also increased outflow and these effects were reversed by the type I antagonist (RS)-1-aminoindan-1,5-dicarboxylate (100 microM). Activation of type II mGlu receptors with (2R,4R)-aminopyrrolidine-2,4-dicarboxylate (300 microM), however, decreased outflow, and this effect was antagonized by the type II antagonist LY307452 (200 microM). Interestingly, LY307452 (200 microM) alone, enhanced outflow of [3H]D-aspartate, but not [14C]GABA. Type III mGlu receptors may not be involved in outflow of [3H]D-aspartate and [14C]GABA in the nucleus tractus solitarii, as L-2-amino-4-phosphonobutyrate (30-300 microM) had no effect under the present experimental conditions. These in vitro studies provide new evidence for roles for Type I and II mGlu receptors in viscerosensory processing in nucleus tractus solitarii.