Potent NMDA-like actions and potentiation of glutamate responses by conformational variants of a glutamate analogue in the rat spinal cord

The 1980s: D-AP5, LTP and a Decade of NMDA Receptor Discoveries

In the 1960s and 70s, biochemical and pharmacological evidence was pointing toward glutamate as a synaptic transmitter at a number of distinct receptor classes, known as NMDA and non-NMDA receptors. The field, however, lacked a potent and highly selective antagonist to block these putative postsynaptic receptors. So, the discoveries in the early 1980s of d-AP5 as a selective NMDA receptor antagonist and of its ability to block synaptic events and plasticity were a major breakthrough leading to an explosion of knowledge about this receptor subtype. During the next 10 years, the role of NMDA receptors was established in synaptic transmission, long-term potentiation, learning and memory, epilepsy, pain, among others. Hints at pharmacological heterogeneity among NMDA receptors were followed by the cloning of separate subunits. The purpose of this review is to recognize the important contributions made in the 1980s by Graham L. Collingridge and other key scientists to the advances in our understanding of the functions of NMDA receptors throughout the central nervous system.

Synthesis, Molecular Modeling Studies, and Preliminary Pharmacological Characterization of All Possible 2-(2‘-Sulfonocyclopropyl)glycine Stereoisomers as Conformationally Constrained L-Homocysteic Acid Analogs

Bioisosteric replacements of the distal acidic group of L-glutamic acid (L-Glu, 1) and conformational constraining of its carbon skeleton, have been widely exploited to discover competitive modulators of glutamate receptors. Noteworthy, L-homocysteic acid (L-HCA, 18), a neurotransmitter belonging to the class of excitatory sulfur-containing amino acids, may be considered an endogenous occurring bioisoster of L-Glu (1). L-HCA (18) has been reported to mediate signaling between glial cells and postsynaptic neurons through the activation of glutamate receptors and others hitherto not well-characterized receptors. As a continuation of our work in the preparation of conformationally constrained glutamate analogs, we report the synthesis and the preliminary pharmacological characterization at iGluRs and mGluRs of all eight stereoisomers of 2-(2'-sulfonocyclopropyl)glycine (SCGs, 8-15). Among the reported compounds, S-SCG-4 (15) showed to be a potent and relatively selective AMPA ligand. Docking experiments coupled to molecular electrostatic potential calculations allowed insight into the molecular basis of the activity of this compound to be gained. The library of SCGs (8-15), while providing a novel source of modulators of the glutamate receptors, represents a valuable chemical tool to better characterize L-HCA pathways in the CNS.

Effects of amino acid antagonists on spontaneous dorsal root activity and evoked dorsal horn field potentials in an isolated preparation of rat spinal cord

Fast and slow dorsal horn field potentials and spontaneous dorsal root activity were recorded from 19-23-day-old rat isolated spinal cord preparations. The effects of GABA, glycine, and glutamate antagonists were tested on these recordings. CNQX, an AMPA/kainate antagonist, reduced all 3 components of the dorsal horn field potential whereas MK801, an NMDA ion channel antagonist, reduced the fast S2 component and the slow wave. Both reduced spontaneous dorsal root activity. NMDA antagonists, D-AP5, 7-chlorokynurenic acid and arcaine, and the metabotropic glutamate antagonists L-AP3 and ethylglutamic acid, while having little effect on the fast components of the field potential, all reduced the slow component. The GABA antagonist, bicuculline, and the glycine antagonist, strychnine, while having no effect on the fast S1 and slow components of the field potential, reduced both the fast S2 component of the field potential and spontaneous dorsal root activity. These results suggest that non-NMDA glutamate receptors are involved in low and high threshold transmission to dorsal horn neurones while NMDA and metabotropic glutamate receptors are primarily involved in high threshold transmission and both GABA and glycine have roles in the transmission or modulation of sensory information within the dorsal horn of the cord.

Molecular pharmacology of glutamate transporters, EAATs and VGLUTs

L-Glutamate serves as a major excitatory neurotransmitter in the mammalian central nervous system (CNS) and is stored in synaptic vesicles by an uptake system that is dependent on the proton electrochemical gradient (VGLUTs). Following its exocytotic release, glutamate activates fast-acting, excitatory ionotropic receptors and slower-acting metabotropic receptors to mediate neurotransmission. Na+-dependent glutamate transporters (EAATs) located on the plasma membrane of neurons and glial cells rapidly terminate the action of glutamate and maintain its extracellular concentration below excitotoxic levels. Thus far, five Na+-dependent glutamate transporters (EAATs 1-5) and three vesicular glutamate transporters (VGLUTs 1-3) have been identified. Examination of EAATs and VGLUTs in brain preparations and by heterologous expression of the various cloned subtypes shows these two transporter families differ in many of their functional properties including substrate specificity and ion requirements. Alterations in the function and/or expression of these carriers have been implicated in a range of psychiatric and neurological disorders. EAATs have been implicated in cerebral stroke, epilepsy, Alzheimer's disease, HIV-associated dementia, Huntington's disease, amyotrophic lateral sclerosis (ALS) and malignant glioma, while VGLUTs have been implicated in schizophrenia. To examine the physiological role of glutamate transporters in more detail, several classes of transportable and non-transportable inhibitors have been developed, many of which are derivatives of the natural amino acids, aspartate and glutamate. This review summarizes the development of these indispensable pharmacological tools, which have been critical to our understanding of normal and abnormal synaptic transmission.

Novel Betaines from a Micronesian Sponge Dysidea herbacea

Three new betaines, dysibetaine PP (1), dysibetaine CPa (2), and dysibetaine CPb (3), were isolated from an aqueous extract of the marine sponge Dysidea herbacea collected in Yap state, Micronesia. The structure of 1 was determined by spectral methods as well as chemical degradation to be a novel dipeptide betaine, and those for 2 and 3 were determined to be unprecedented cyclopropane betaines. Compounds 2 and 3 showed weak affinity toward the N-methyl-D-aspartic acid-type and the kainic acid-type glutamate receptors, respectively, in a radioligand binding assay.

(1R,3S)-1-Aminocyclopentane-1,3-dicarboxylic acid (RS-ACPD) reduces intracellular glutamate levels in astrocytes

(+/-)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) is an equimolar mixture of two enantiomers: (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (SR-ACPD) and 1R,3S-1-Aminocyclopentane-1,3-dicarboxylic acid (RS-ACPD). t-ACPD and SR-ACPD have been commonly used as agonists for metabotropic glutamate receptors (mGluR). Here we demonstrated that RS-ACPD, but not SR-ACPD, is transported into astrocytes with a K(m) of 6.51 +/- 2.38 mM and V(max) of 22.8 +/- 3.4 nmol/mg/min. This low-affinity transport is Na(+)-dependent and is competitively blocked by glutamate or other substrates for the glutamate transporter. RS-ACPD therefore is probably taken up by the glutamate transporter. Prolonged incubation with high levels of RS-ACPD (> 500 microM) induced significant swelling of astrocytes. At lower concentrations (100 microM), RS-ACPD reduced intracellular glutamate content ([Glu](i)) by > 50% without obvious morphological changes. The reduction in [Glu](i) was accompanied by an increase in [glutamine](i). The RS-ACPD's effect on [Glu](i) required glutamine and high levels of phosphate, suggesting that RS-ACPD inhibited phosphate-activated glutaminase (PAG). These data suggest that astrocytic PAG is actively involved in determining the equilibrium between intracellular glutamate and glutamine. By reducing [Glu](i), RS-ACPD reduces the amount of glutamate available for release.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Neurochemistry of L-Glutamate Transport in the CNS: A Review of Thirty Years of Progress

The review highlights the landmark studies leading from the discovery and initial characterization of the Na+-dependent "high affinity" uptake in the mammalian brain to the cloning of individual transporters and the subsequent expansion of the field into the realm of molecular biology. When the data and hypotheses from 1970's are confronted with the recent developments in the field, we can conclude that the suggestions made nearly thirty years ago were essentially correct: the uptake, mediated by an active transport into neurons and glial cells, serves to control the extracellular concentrations of L-glutamate and prevents the neurotoxicity. The modern techniques of molecular biology may have provided additional data on the nature and location of the transporters but the classical neurochemical approach, using structural analogues of glutamate designed as specific inhibitors or substrates for glutamate transport, has been crucial for the investigations of particular roles that glutamate transport might play in health and disease. Analysis of recent structure/activity data presented in this review has yielded a novel insight into the pharmacological characteristics of L-glutamate transport, suggesting existence of additional heterogeneity in the system, beyond that so far discovered by molecular genetics. More compounds that specifically interact with individual glutamate transporters are urgently needed for more detailed investigations of neurochemical characteristics of glutamatergic transport and its integration into the glutamatergic synapses in the central nervous system. A review with 162 references.

Isolation of pure (2S,1'S,2'S)-2-(2'-carboxycyclopropyl)glycine from Blighia sapida (Akee)

The isolation of (2S,1'S,2'S)-2-(2'-carboxycyclopropyl)glycine (CCG I, 2) from Blighia sapida (Akee) was achieved through column chromatography on deactivated silica gel followed by ion-exchange chromatography. A HPLC method has also been devised in order to assess the purity of the isolated product.

Inhibition of uptake and release of a novel mGluR agonist (L-F2CCG-I) by anion transport blockers in the rat spinal cord

A new metabotropic glutamate receptor (mGluR) agonist, (2S,1'S,2'S)-2-(2-carboxy-3,3-difluorocyclopropyl)glycine (L-F2CCG-I), induces a priming effect on (RS)-alpha-aminopimelate in the isolated spinal cord of newborn rats. Similar to (RS)-alpha-aminopimelate, L-glutamate (30-100 microM) neither affected spinal reflexes nor the resting membrane potentials of motoneurones, but preferentially potentiated the depression of monosynaptic excitation caused by L-F2CCG-I (0.4 microM). Following L-F2CCG-I treatment (1-2 microM), L-glutamate decreased the monosynaptic spinal reflexes in a concentration dependent manner, indicating a priming' effect of L-F2CCG-I. Thus L-glutamate is completely compatible with (RS)-alpha-aminopimelate in revealing the priming effect. An anion transport blocker, 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS) (100 microM), markedly inhibited both the response to (RS)-alpha-aminopimelate and the induction of the L-F2CCG-I priming effect. The data suggest that L-F2CCG-I is Cl- -dependently incorporated into certain stores, and that (RS)-alpha-aminopimelate or L-glutamate must stimulate the release of L-F2CCG-I from the storage site. There were pharmacological similarities between the quisqualate and L-F2CCG-I priming effect. The physiological significance of the quisqualate or L-F2CCG-I priming is not yet established. L-F2CCG-I would be expected to be a useful pharmacological probe for elucidating the mechanism of the priming.

Roles of metabotropic glutamate receptor subtypes in modulation of pentylenetetrazole-induced seizure activity in mice

The anticonvulsant or proconvulsant properties of ligands at metabotropic glutamate receptors (mGluRs) were examined in a chemoconvulsant model using pentylenetetrazole (PTZ). Mice received mGluR ligands by intracerebroventricular (i.c.v.) infusion prior to a subcutaneous injection of PTZ and the latency to onset of tonic convulsions was recorded. The group I mGluR antagonist 1-aminoindan-1,5-dicarboxylic acid (AIDA) dose-dependently antagonised PTZ-induced seizures with a mean ED50 value of 465 nmol. In contrast, the selective group I mGluR agonist, (S)-3,5-dihydroxyphenylglycine [(S)-DHPG], was proconvulsive and decreased the PTZ-induced seizure latency (ED50=60 nmol i.c.v.). A selective agonist of group II mGluRs, (1S,3S)-1-aminocyclopentane dicarboxylic acid [(1S,3S)-ACPD], was proconvulsive but did not affect PTZ-induced seizure latency. Moreover, the proconvulsant effect of (IS,3S)-ACPD was not blocked by the mGluR2 antagonist, alpha-methylserine-O-phosphate monophenyl ester but was blocked by AIDA suggesting the involvement of group I mGluRs. (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCG-IV), which is a potent mGluR2 antagonist and a group III mGluR agonist at higher doses, increased the PTZ-induced seizure latency (ED50=51 nmol) and this effect was fully reversed by the group III mGluR antagonist, (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4). Similarly, the group III mGluR agonist 1-amino-3-(phosphonomethylene)cyclobutanecarboxylate (cyclobutylene-AP5) increased the PTZ-induced seizure latency (ED50=12 nmol) in a MAP4-sensitive manner. Collectively, these data suggest that mGluR ligands modulate PTZ-induced seizure activity in mice by either antagonizing group I mGluRs or activating group III mGluRs.

Preparation of difluoro analogs of CCGs and their pharmacological evaluations

All the stereoisomers of 2-(2-carboxy-3,3-difluorocyclopropyl)glycines (F2CCGs) were synthesized in enantiomerically pure forms using (R)-2,3-O-isopropyl-ideneglyceraldehyde as a chiral precursor. L-F2CCG-I, one of the stereoisomers corresponding to an extended form of L-glutamate was found to be a potent agonist for metabotropic glutamate receptors (mGluRs).

Modulation of glutamate receptor pathways in the search for new neuroprotective agents

Excessive stimulation of excitatory amino acid (EAA) receptors is responsible for a wide variety of acute and chronic neurological impairments. A separate line of investigation has focused on oxidative stress as one of the main reasons for several of these degenerative disorders. Current evidence has confirmed that activation of both ionotropic and metabotropic glutamate receptors can also result in either neuroprotection or neurodegeneration according to the role played by oxidative stress mechanisms. An outline of this research, together with our recent results aimed at the discovery of new subtype selective modulators of the central nervous system pathways as well as new classes of free radical scavengers, is presented.

Potentiation by DL-alpha-aminopimelate of the inhibitory action of a novel mGluR agonist (L-F2CCG-I) on monosynaptic excitation in the rat spinal cord

1. Neuropharmacological actions of all the possible stereoisomers of 3',3'-difluoro-2-(carboxycyclopropyl)glycine (3',3'-difluoro-CCG) were compared with those of the corresponding 2-(carboxycyclopropyl)glycine (CCG) isomers in the isolated spinal cord of newborn rats. (2S,1'S,2'S)- and (2S,1'R,2'S)-2-(2-carboxy-3,3-difluorocyclopropyl)glycine (L-F2CCG-I and L-F2CCG-IV) were the most potent in causing depolarization, their threshold concentrations being approximately 1 microM. 2. The depolarization evoked by L-F2CCG-I (30 microM) was depressed by (+)-alpha-methyl-4-carboxyphenylglycine (MCPG, 1 mM (n=4)) to 17+/-3% of the control: this depolarizing action was not decreased by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 100 microM), and only slightly decreased by high concentrations of D-2-amino-5-phosphonopentanoic acid (D-AP5, 100 microM), suggesting that L-F2CCG-I activates mainly metabotropic glutamate receptors. 3. L-F2CCG-I preferentially depressed the monosynaptic component of the spinal reflex approximately 3 times more effectively than (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I). The depressant action of L-F2CCG-I (0.2 microM-0.7 microM) on monosynaptic excitation was antagonized by (2S,1'S,2'S)-2-methyl-2-(carboxycyclopropyl)glycine (MCCG, 0.3 mM-1 mM) and (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4, 0.3 mM). 4. DL-alpha-aminopimelate (10 and 100 microM) selectively potentiated the depression of monosynaptic excitation caused by L-CCG-I (0.2 microM) and L-F2CCG-I (0.1 microM). The actions of (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) (50 nM-0.2 microM), L-2-amino-4-phosphonobutanoic acid (L-AP4) (0.3-1 microM), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) (1-7 microM) and baclofen (0.1-0.7 microM) were unaffected by DL-alpha-aminopimelate. The threshold concentration for the potentiating actions of DL-alpha-aminopimelate was 3 microM. 5. The depolarization induced by quisqualate (3 microM, 10 s application) was increased to 115+/-2% and 137+/-5% of the control values during combined application of quisqualate with either 30 microM or 100 microM DL-alpha-aminopimelate, respectively. 6. Following the application and subsequent washout of L-F2CCG-I, DL-alpha-aminopimelate (3-100 microM) decreased the amplitude of the monosynaptic component of spinal reflexes in a concentration-dependent manner, indicating a 'priming' effect of L-F2CCG-I.

Selective metabotropic receptor agonists distinguish non-ionotropic glutamate binding sites

Metabotropic glutamate receptors (mGluRs) are thought to mediate diverse processes in brain including synaptic plasticity and excitotoxicity. These receptors are often divided into three groups by their pharmacological profiles. [3H]Glutamate binding in the presence of compounds selective for ionotropic glutamate receptors can be used as a general assay for these receptors; subtypes of this non-ionotropic [3H]glutamate binding differ in both pharmacology and anatomical distribution, and are differentially sensitive to quisqualate. The characteristics of these binding sites are consistent with those of group 1 (high-affinity quisqualate) and group 2 (low-affinity quisqualate) mGluRs. Under our assay conditions, no [3H]glutamate binding to group 3-like (L-AP4 sensitive) sites could be demonstrated. We have attempted to characterize particular agents which may selectively measure [3H]glutamate binding to mGluR subtypes. We used two isomers of 2-(carboxycyclopropyl)glycine, L-CCG-I and L-CCG-II, and the (2S,1'R,2'R,3'R) isomer of 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) as competitors of non-ionotropic [3H]glutamate binding sites. DCG-IV clearly distinguishes two binding sites. Quantitative levels of DCG-IV binding by anatomic region correlate with quisqualate-defined binding subtypes: high-affinity DCG-IV binding correlates with low-affinity quisqualate binding, whereas low-affinity DCG-IV binding correlates with high-affinity quisqualate binding. L-CCG-II displaces only one type of non-ionotropic [3H]glutamate binding, corresponding to high-affinity quisqualate binding. Therefore DCG-IV and L-CCG-II at appropriate concentrations appear to distinguish binding to putative group 2 vs. group 1 mGluRs. L-CCG-I displaces both high- and low-affinity quisqualate binding sites, but unlike the other two compounds, does not clearly distinguish between them.

Synthesis and pharmacological characterization of all sixteen stereoisomers of 2-(2'-carboxy-3'-phenylcyclopropyl)glycine. Focus on (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine, a novel and selective group II metabotropic glutamate receptors antagonist

All 16 2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCGs) stereoisomers 32-47 have been prepared from the corresponding racemic aldehydes 12-15 following an enantiodivergent synthetic protocol. Compounds 32-47 were evaluated by a number of binding and functional experiments as potential ligands for several classes of excitatory amino acid receptors, including metabotropic glutamate receptors (mGluR1a, mGluR2, mGluR4) and ionotropic glutamate receptors (NMDA, KA, AMPA) as well as sodium-dependent and calcium/ chloride-dependent glutamate transport systems. The stereolibrary of compounds 32-47 appears to be endowed with a peculiar pharmacological profile. PCCG-2 (33) and PCCG-3 (34) displaced labeled kainate at low micromolar concentration; PCCG-9 (40) and PCCG-11 (42) weakly interacted with the NMDA site; PCCG-5 (36), PCCG-10 (41), and PCCG-12 (43) showed to be potent inhibitors of Ca2+/Cl(-)-dependent glutamate transport system. Most interestingly, PCCG-4 (35) has been shown to be able to antagonize (IC50 = 8 microM) the effects of glutamate on forskolin-stimulated cAMP formation in BHK cells expressing mGluR2. Uneffective at mGluR1, 35 is a weak mGluR4 agonist (EC50 = 156 microM) and has no effect on either ionotropic receptors or glutamate transport systems, thus demonstrating to be a novel selective mGluR2 antagonist with a 6-fold increase in potency over previously reported antagonists.

Syntheses and conformational analyses of glutamate analogs: 2-(2-carboxy-3-substituted-cyclopropyl)glycines as useful probes for excitatory amino acid receptors

An hypothesis that each subtype of glutamate receptors requires a specific conformation of L-glutamate for its selective activation was examined using the conformationally constrained analogs of L-glutamate, L-2-(2-carboxycyclopropyl)glycines (CCGs), and L-2-[2-carboxy-3-(methoxymethyl)cyclopropyl)glycines (MCGs). All MCG isomers were newly synthesized in a stereoselective manner via the common synthetic intermediate 5a starting with the oxazolidine aldehyde 1. The synthesis of the four MCG isomers was characterized by a stereoselective inversion of alpha-cyclopropyl acyl anion (e.g., from 10 to 11). The spectroscopic studies, in particular, pH vs J correlation experiments of CCGs and MCGs using 1H NMR and their molecular mechanics calculations, revealed that these analogs possessed an antiperiplanar conformation regarding the H-C2-C1'-H bond as a majority among the other possible rotamers in aqueous solution. The fact that each CCG and MCG exhibited potent and selective activities to the distinct types of glutamate receptors allowed us to extract an active conformation of L-glutamate. Thus, the conformational requirement of metabotropic glutamate receptors was speculated to be the anti-anti conformation (aa-A) because the conformations of CCG-1 and cis and trans-MCG-I, selective agonists of the receptors, closely mimicked the rotamer A of L-glutamate. On the other hand, N-methyl-D-aspartate and kainate receptors, representative ionotropic glutamate receptors, would require glutamate g+g+ rotamer E which was deduced from the conformation-activity relationship studies of the selective agonists CCG-IV, cis-MCG-IV, and trans-MCG-IV and the related analogs.

Novel agonists for metabotropic glutamate receptors: trans- and cis-2-(2-carboxy-3-methoxymethylcyclopropyl)glycine (trans- and cis-MCG-I)

New derivatives of 2-(carboxycyclopropyl)glycine (CCG), (2S,1'S,2'R,3'S)- and (2S,1'S,2'R,3'R)-2-(2-carboxy-3-methoxymethylcyclopropyl) glycine (trans- and cis-MCG-I), effectively inhibited forskolin-stimulated cyclic AMP formation in a concentration dependent manner in cultured spinal neurones of rats. They effectively depressed monosynaptic excitation in the spinal reflex of newborn rats with IC50 values of 0.3 and 3 microM, respectively, which was sensitive to (+)-MCPG. They did not cause any depolarization even when the concentration was increased up to 0.3 mM. However, after treatment with quisqualate, cis-MCG-I caused a depolarization of motoneurones in the newborn rat spinal cord in a concentration dependent manner with a threshold concentration of 1 microM (quisqualate effect). The depolarizing activity developed after quisqualate treatment gradually decreased but lasted for more than 2 hr. The depolarization induced by cis-MCG-I seemed pharmacologically similar to that of phosphonate-containing analogues of glutamate such as L-AP4 or L-AP6 under the "quisqualate effect". These novel CCG derivatives would be expected to provide useful probes for elucidating the physiological function of mGluRs.

Inhibition by folded isomers of L-2-(carboxycyclopropyl)glycine of glutamate uptake via the human glutamate transporter hGluT-1

The effects of isomers of 2-(carboxycyclopropyl)glycine (CCG) on uptake of L-glutamate were investigated in COS-7 cells that expressed a cloned human glutamate transporter (hGluT-1). The (2S, 3S, 4R)-isomer (L-CCG-III) and the (2S, 3R, 4S)-isomer (L-CCG-IV) markedly inhibited glutamate uptake with a 50% inhibitory concentration of 290 nM and 1.1 microM, respectively. The (2S, 3S, 4S)-isomer (L-CCG-I) and the (2S, 3R, 4R)-isomer (L-CCG-II) did not inhibit glutamate uptake at concentrations of < or = 10 microM. Thus, hGluT-1 showed a markedly higher affinity for L-CCG-III and L-CCG-IV with a folded conformation of the glutamate skeleton, than for L-CCG-I or L-CCG-II with an extended conformation.

NMDA receptor heterogeneity in mammalian tissues: focus on two agonists, (2S,3R,4S) cyclopropylglutamate and the sulfate ester of 4-hydroxy-(S)-pipecolic acid

Several potent and selective agonists of the glutamate (L-GLU) receptors of N-methyl-D-aspartate (NMDA) type have been tested on the L-[3H]GLU binding to rat cortical membranes, on the depolarization of mouse cortical wedges and on the contraction of guinea pig longitudinal muscle myenteric plexus preparations with the aim of comparing the NMDA receptors present in the cortex and those present in the gut. When the depolarization of the cortical wedges was evaluated, the EC50 values of the agonists were (microM): (R,S)-(tetrazol-5-yl)-glycine (TG) 0.3; trans-4-hydroxy-(S)-pipecolic acid-4-sulfate (t-HPIS) 0.7; 1-aminocyclobutane-cis-1,3-dicarboxylic acid (ACBD) 0.8; NMDA 8; (2S,3R,4S) cyclopropylglutamate (L-CGA C) 12; quinolinic acid (QUIN) 400. When the contraction of the longitudinal muscle myenteric plexus was evaluated, the EC50 values were (microM): L-CGA C 1; TG 8; ACBD 50; t-HPIS 100; QUIN 500 and NMDA 680. When the displacement of NMDA specific L-[3H]GLU binding from rat cortical membranes was evaluated, the IC50 values were (microM): L-CGA C 0.003; TG 0.005; ACBD 0.044; t-HPIS 0.062; NMDA 0.31 and QUIN 15. No significant correlation was found when the EC50 values obtained in the ileum were plotted against the EC50 values obtained in the cortex (r = 0.47). In particular it was noted that L-CGA C was approximately three orders of magnitude more potent than NMDA when tested in the ileum but had a potency not significantly different from that of NMDA when tested in the cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and expression of a bovine glutamate transporter

We have isolated a 3845 base-pair cDNA (BNGLUAS) encoding a bovine glutamate transporter (bovine GLAST) by screening a bovine retina cDNA library with an oligonucleotide probe corresponding to conserved regions of known glutamate transporters. The cDNA sequence predicted a protein of 542 amino acids and displayed 96% and 97% amino acid identity with the rat GLAST/GluT-1 and human GLAST, respectively. Expression of the bovine GLAST in Xenopus oocytes revealed Na(+)-dependent [14C]L-glutamate uptake and electrogenic glutamate uptake.

A novel antagonist, phenylbenzene omega-phosphono-alpha-amino acid, for strychnine-sensitive glycine receptors in the rat spinal cord

1. 3-[2'-Phosphonomethyl[1,1'-biphenyl]-3-yl]alanine (PMBA) is a novel glycine antagonist at strychnine-sensitive receptors. The chemical structure of PMBA, possessing both a glycine moiety and a phosphono group, is quite different from that of strychnine. 2. In the spinal motoneurone of newborn rats, glycine (100 microM-1 mM) induced depolarizing responses in a concentration-dependent manner. PMBA effectively inhibited depolarizing responses to glycine and other agonists, such as taurine and beta-alanine. The dose-response curves for glycine were shifted to the right in an almost parallel manner (pA2 value: 5.30 +/- 0.23, n = 5) by PMBA which was about 60 times less potent than strychnine (pA2 value: 7.08 +/- 0.21, n = 5) as a glycine antagonist. 3. PMBA (1-100 microM) did not interact with modulatory glycine sites on N-methyl-D-aspartate (NMDA) receptors, which suggests a high selectivity of PMBA for strychnine-sensitive glycine receptors. At considerably high concentrations (0.1 mM-1 mM), PMBA depressed responses to GABA (pA2 value: 3.57 +/- 0.24, n = 3). 4. PMBA inhibited the binding of [3H]-strychnine to synaptosomes from adult rat spinal cords; the IC50 values of PMBA, glycine and strychnine were 8 +/- 2, 9 +/- 3 and 0.08 +/- 0.04 microM, respectively (n = 5) for [3H]-strychnine (4.8 nM). 5. PMBA is a central excitant drug with relatively high potency and selectivity and should be useful as a pharmacological probe for analysing the mechanisms underlying physiological functions of glycine receptors.

Agonists for metabotropic glutamate receptors in the rat delay recovery from halothane anesthesia

Intraventricular injection of (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), a potent agonist for metabotropic glutamate receptors, to rats retarded dose-dependently the recovery from halothane anesthesia at a dose range from 30 to 300 pmol/rat. At a dose of 300 pmol/rat, recovery was prolonged up to about 630% of the control. (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) also delayed recovery, but doses much higher than those of DCG-IV were required (about 300 and 1000 times, respectively). These results suggest that some agonists for a certain kind of metabotropic glutamate receptors have a central depressant action in the rat.

A novel metabotropic glutamate receptor agonist: (2S,1'S,2'R,3'R)-2-(2-carboxy-3-methoxymethylcyclopropyl)glycine (cis-MCG-I)

A novel agonist for metabotropic glutamate receptors, (2S,1'S,2'R,3'R)-2-(2-carboxy-3-methoxymethylcyclopropyl)glycine (cis-MCG-I), effectively inhibited monosynaptic excitation in newborn rat spinal cords with EC50 of 3 microM without causing any depolarization. Furthermore, cis-MCG-I inhibited the forskolin-stimulated cyclic AMP formation in rat cultured cortical cells with IC50 of 1.5 microM. cis-MCG-I neither stimulated the phosphoinositide hydrolysis nor activated ionotropic glutamate receptors even in high concentrations. However, after a brief exposure of spinal cords to quisqualate, it caused depolarization in a dose-dependent manner. This compound would provide useful information for elucidating physiological functions of metabotropic glutamate receptors.

A conformationally constrained competitive inhibitor of the sodium-dependent glutamate transporter in forebrain synaptosomes: L-anti-endo-3,4-methanopyrrolidine dicarboxylate

A series of L-3,4-methanopyrrolidine dicarboxylate isomers were investigated as potential inhibitors of the high affinity, sodium-dependent glutamate transporter in rat forebrain synaptosomes. Of the isomers tested, only L-anti-endo-3,4-methanopyrrolidine dicarboxylate (L-anti-endo-MPDC) blocked the uptake of [3H]D-aspartate, a non-metabolized substrate. Kinetic analysis demonstrated that L-anti-endo-MPDC is a potent competitive inhibitor (Ki = 5 microM) comparable to that of L-glutamate and L-trans-2,4-pyrrolidine dicarboxylate (L-trans-2,4-PDC). Conformational analysis of L-glutamate, L-trans-2,4-PDC and L-anti-endo-MPDC are used to refine the pharmacophore model of the transporter binding site.

The depolarization-induced outflow of D-[3H]aspartate from rat brain slices is modulated by metabotropic glutamate receptors

Rat brain slices were used to study the effects of different metabotropic glutamate receptor ligands on (i) the depolarization (30 mM KCl)-induced outflow of previously taken up D-[3H]aspartate; (ii) the inhibition of forskolin (30 microM)-induced cyclic AMP accumulation; and (iii) the hydrolysis of phosphoinositides. In addition, the localization of mRNAs coding for different metabotropic glutamate receptor subtypes was detected using in situ hybridization. (1S-3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (30-300 microM), a non selective metabotropic glutamate receptor agonist, significantly increased the KCl-induced output of radioactivity from cortical slices, whereas it inhibited the output from striatal slices. Conversely, (1S,3S,4S)-carboxycyclopropylglycine (0.1-1 microM), a relatively selective agonist of the mGluR2 metabotropic glutamate receptor subtype, had an inhibitory effect on the output of D-[3H]aspartate from both cortical and striatal slices and proved to be the most potent metabotropic glutamate receptor agonist in inhibiting cyclic AMP accumulation, but not in stimulating phosphoinositide hydrolysis. Since 2-amino-4-phosphonobutyrate (a mGluR4, mGluR6 and mGluR7 agonist) was not active in any of the assays tested, we hypothesized that the mGluR2 subtype could be involved in these events. Accordingly, mGluR2 mRNA expression was abundant in cortical neurons projecting to the striatum. Our experiments suggest that the stimulation of metabotropic glutamate receptors may either decrease or increase transmitter release depending on the subtype that prevails in the region under study.

Pharmacological characterization of the metabotropic glutamate receptor inhibiting D-[3H]-aspartate output in rat striatum

1. The effects of several agonists of the metabotropic glutamate receptor (mGluR) were studied in adult rat striatal slices by measuring (i) KCl (30 mM)-induced output of previously taken up D-[3H]-aspartate (Asp), (ii) forskolin (30 microM)-induced adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation and (iii) phophoinositide (PI) hydrolysis. 2. K(+)-induced efflux of D-[3H]-Asp was inhibited by the following mGluR agonists: (1S,3S,4S)-(carboxycyclopropyl)glycine (L-CCG-I), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and quisqualic acid (Quis). 2-Amino-4-phosphonobutyrate (L-AP4) was inactive up to 300 microM. The maximal inhibition of D-[3H]-Asp output was 60 +/- 8%. The EC50s of mGluR agonists were: 0.5 microM for L-CCG-I, 100 microM for 1S,3R-ACPD and 100 microM for Quis. 3. Forskolin-induced cyclic AMP accumulation was also inhibited by mGluR agonists. The maximal inhibition was 50 +/- 4% and was obtained at a concentration of 10 microM for L-CCG-I and 100 microM for 1S,3R-ACPD. The EC50s for this inhibition were: 0.9 microM for L-CCG-I and 20 microM for 1S,3R-ACPD. Quis (300 microM) inhibited cyclic AMP accumulation by approximately 20%. L-AP4 slightly potentiated cyclic AMP accumulation. 4. PI hydrolysis was stimulated by mGluR agonists. The most potent compound was Quis (100 microM), which increased inositol phosphate formation up to 2.2 fold over control values. Its EC50 was 15 microM. L-CCG-I and 1S,3R-ACPD increased inositol phosphate formation by approximately 1.8 fold and their EC50 values were 30 and 25 microM, respectively. L-AP4 did not affect PI hydrolysis. 5. In conclusion, mGluR agonists that reduce D-[3H]-Asp output have a pharmacological profile similar to that of mGluR agonists inhibiting cyclic AMP accumulation. L-CCG-I appears to be a relatively selective agonist for the mGluR receptor which inhibits D-[3H]-Asp efflux and cyclic AMP accumulation,while Quis appears to act preferentially on the mGluR receptor linked to the metabolism of PIs.

Metabotropic glutamate receptors: an original family of G protein-coupled receptors

In 1985, we discovered a new glutamate receptor which was coupled to phospholipase C via a G protein and which was later termed metabotropic glutamate receptor (mGluR). In this review, both the diversity of mGluRs and the cellular events they control are discussed, as well as their roles in physiological regulation and brain function.

(2S,3S,4R)-2-(carboxycyclopropyl)glycine, a potent and competitive inhibitor of both glial and neuronal uptake of glutamate

The effects of several diastereoisomers of L-2-(carboxycyclopropyl)glycine (CCG) on L-glutamate uptake were compared among three different preparations, glial plasmalemmal vesicles (GPV), synaptosomes and cultured astrocytes from rat hippocampus. The (2S,3S,4R)-isomer (L-CCG-III) inhibited a Na(+)-dependent high-affinity L-glutamate uptake in GPV and synaptosomes in a dose dependent manner at a micromolar range. The potency was quite similar to that of L-threo-beta-hydroxyaspartate in both subcellular fractions and much higher than L-aspartate-beta-hydroxamate, which were known as potent inhibitors of glutamate uptake. The (2S,3R,4S)-isomer (L-CCG-IV) also inhibited the glutamate uptake in GPV and synaptosomes, but it was about 100 times less active than L-CCG-III. The (2S,3S,4S)- and (2S,3R,4R)-isomers (L-CCG-I and L-CCG-II, respectively) hardly showed any inhibitory action on the glutamate uptake. Dixon plot analysis of the initial uptake rate revealed that the inhibition was in a competitive manner and the value of the inhibition constant (Ki) was about 1 microM in both GPV and synaptosomes. L-CCG-III effectively inhibited the glutamate uptake by cultured hippocampal astrocytes as well. These results suggested that L-CCG-III inhibited the glutamate uptake in both neurones and glial cells of the mammalian central nervous system in a similar manner.

A novel metabotropic glutamate receptor agonist: marked depression of monosynaptic excitation in the newborn rat isolated spinal cord

1. Neuropharmacological actions of a novel metabotropic glutamate receptor agonist, (2S,1'R,2'R,3'R)-2(2,3-dicarboxycyclopropyl)glycine (DCG-IV), were examined in the isolated spinal cord of the newborn rat, and compared with those of the established agonists of (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD). 2. At concentrations higher than 10 microM, DCG-IV caused a depolarization which was completely blocked by selective N-methyl-D-aspartate (NMDA) antagonists. The depolarization was pharmacologically quite different from that caused by L-CCG-I and (1S,3R)-ACPD. 3. DCG-IV reduced the monosynaptic excitation of motoneurones rather than polysynaptic discharges in the nanomolar range without causing postsynaptic depolarization of motoneurones. DCG-IV was more effective than L-CCG-I, (1S,3R)-ACPD or L-2-amino-4-phosphonobutanoic acid (L-AP4) in reducing the monosynaptic excitation of motoneurones. 4. DCG-IV (30 nM-1 microM) did not depress the depolarization induced by known excitatory amino acids in the newborn rat motoneurones, but depressed the baseline fluctuation of the potential derived from ventral roots. Therefore, DCG-IV seems to reduce preferentially transmitter release from primary afferent nerve terminals. 5. Depression of monosynaptic excitation caused by DCG-IV was not affected by any known pharmacological agents, including 2-amino-3-phosphonopropanoic acid (AP3), diazepam, 2-hydroxysaclofen, picrotoxin and strychnine. 6. DCG-IV has the potential of providing further useful information on the physiological function of metabotropic glutamate receptors.

Subtypes of sodium-dependent high-affinity L-[3H]glutamate transport activity: pharmacologic specificity and regulation by sodium and potassium

Some data suggest that the sodium-dependent, high-affinity L-glutamate (Glu) transport sites in forebrain are different from those in cerebellum. In the present study, sodium-dependent transport of L-[3H]Glu was characterized in cerebellum and cortex. In both cerebellar and cortical tissue, activity was enriched in synaptosomes. Approximately 100 excitatory amino acid analogues were tested as potential inhibitors of transport activity. Many of the compounds tested inhibited transport activity by < 65% at 1 mM and were not studied further. One group of compounds exhibited inhibition conforming to theoretical curves with Hill coefficients of 1 and were < 10-fold selective as inhibitors of transport activity. These included three of the putative endogenous substrates for transport: L-Glu, L-aspartate, and L-cysteate. Four of the compounds exhibited inhibition conforming to theoretical curves with Hill coefficients of 1 and were > 10-fold selective as inhibitors. These included beta-N-oxalyl-L-alpha,beta-diaminopropionate, alpha-methyl-DL-glutamate, (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine, and (2S,1'S,2'S,3'S)-2-(2-carboxy-3-methoxymethylcyclopropyl)glycine. Data obtained with a few of the inhibitors were consistent with two sites in one or both of the brain regions. (2S,1'R,2'R)-2-(Carboxycyclopropyl)glycine (L-CCG-II) was identified as the most potent (IC50 = 5.5 microM) and selective (60-100-fold) inhibitor of transport activity in cerebellum. One of the potential endogenous substrates, L-homocysteate, was also a selective inhibitor of cerebellar transport activity. The data for inhibition of transport activity in cortex by both L-CCG-II and L-homocysteate were best fit to two sites. Kainate was equipotent as an inhibitor of transport activity, and in both brain regions the data for inhibition were best fit to two sites. The possibility that there are four subtypes of excitatory amino acid transport is discussed. Altering sodium and potassium levels affects cerebellar and cortical transport activity differently, suggesting that the differences extend to other recognition sites on these transporters.

Molecular diversity of glutamate receptors and implications for brain function

The glutamate receptors mediate excitatory neurotransmission in the brain and are important in memory acquisition, learning, and some neurodegenerative disorders. This receptor family is classified in three groups: the N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate, and metabotropic receptors. Recent molecular studies have shown that many receptor subtypes exist in all three groups of the receptors and exhibit heterogeneity in function and expression patterns. This article reviews the molecular and functional diversity of the glutamate receptors and discusses their implications for integrative brain function.

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

1. 2-(Carboxycyclopropyl)glycines (CCGs) are conformationally restricted glutamate analogues and consist of eight isomers including L- and D-forms. The agonist potencies and selectivities of these compounds for metabotropic glutamate receptors (mGluRs) were studied by examining their effects on the signal transduction of representative mGluR1, mGluR2 and mGluR4 subtypes in Chinese hamster ovary cells expressing the individual cloned receptors. 2. Two extended isomers of L-CCG, L-CCG-I and L-CCG-II, effectively stimulated phosphatidylinositol hydrolysis in mGluR1-expressing cells. The rank order of potencies of these compounds was L-glutamate > L-CCG-I > L-CCG-II. 3. L-CCG-I and L-CCG-II were effective in inhibiting the forskolin-stimulated adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation in mGluR2-expressing cells. Particularly, L-CCG-I was a potent agonist for mGluR2 with an EC50 value of 3 x 10(-7) M, which was more than an order of potency greater than that of L-glutamate. 4. L-CCG-I evoked an inhibition of the forskolin-stimulated cyclic AMP production characteristic of mGluR4 with a potency comparable to L-glutamate. 5. In contrast to the above compounds, the other CCG isomers showed no appreciable effects on the signal transduction involved in the three mGluR subtypes. 6. This investigation demonstrates not only the importance of a particular isomeric structure of CCGs in the interaction with the mGluRs but also a clear receptor subtype specificity for the CCG-receptor interaction, and indicates that the CCG isomers would serve as useful agonists for investigation of functions of the mGluR family.

A metabotropic l-Glutamate receptor agonist: Pharmacological difference between rat central neurones and crayfish neuromuscular junctions

1. 2S,3S,4S-2-(carboxycyclopropyl)glycine (L-CCG-I), a conformationally restricted glutamate analogue, is a potent metabotropic L-glutamate receptor agonist in the mammalian central nervous system. 2. Depolarizing actions of L-CCG-I and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) in the newborn rat spinal motoneurone are temperature-sensitive, and are not depressed by 3-[(+/-)-2-carboxypiperazin-4-yl] propyl-1-phosphonic acid (CPP) and/or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 3. L-CCG-I and trans-ACPD induced oscillatory responses in Xenopus oocytes injected with rat brain mRNA. Oocytes with oscillatory responses to L-CCG-I and trans-ACPD showed reversal potential of about -20 mV, which was very close to the equilibrium potential of chloride ions. 4. In rat hippocampal synaptoneurosomes, L-CCG-I stimulated phosphoinositide hydrolysis in a concentration dependent manner. L-CCG-I was less potent than quisqualate but more potent than trans-ACPD. 5. At low concentrations, L-CCG-I did not cause any depolarization of newborn rat spinal motoneurones, but reduced substantially amplitudes of monosynaptic reflexes. 6. At the crayfish neuromuscular junction L-CCG-I, acting presynaptically, reduced the amplitude of excitatory junctional potentials. This action was prevented by application of picrotoxin but not pertussis toxin. The actions of trans-ACPD differ from those of either L-CCG-I or ibotenate at the crayfish neuromuscular junction. 7. L-CCG-I has a potential to provide further useful information on metabotropic L-glutamate receptor function.