Pretreatment of cerebellar granule cells with concanavalin A potentiates quisqualate-stimulated phosphoinositide hydrolysis

Cyclothiazide selectively inhibits mGluR1 receptors interacting with a common allosteric site for non-competitive antagonists

Metabotropic glutamate receptors mGluR1 and mGluR5 stimulate phospholipase C, leading to an increased inositol trisphosphate level and to Ca(2+) release from intracellular stores. Cyclothiazide (CTZ), known as a blocker of AMPA receptor desensitization, produced a non-competitive inhibition of [Ca(2+)](i) increases induced by mGluR agonists in HEK 293 cells transfected with rat mGluR1a but had no effect on the [Ca(2+)](i) signals in cells expressing rat mGluR5a. In cells expressing mGluR1, CTZ also inhibited phosphoinositide hydrolysis, as well as cAMP accumulation and arachidonic acid release induced by mGluR1 agonists, indicating a direct inhibition of the receptor and not of a particular signal transduction system. However, CTZ failed to antagonize cAMP inhibition stimulated by rat mGluR2, -3, -4, -6, -7 and -8 receptors confirming its selectivity for mGluR1. The use of chimeric receptors with substituted N-terminal domains showed that CTZ did not interact with the N-terminal mGluR1a domain. Instead, mutation analysis revealed that CTZ interacts with the Thr-815 and Ala-818 residues, located at the 7th transmembrane domain, similarly as the mGluR1-selective antagonist CPCCOEt. In primary cultures of cerebellar granule neurons, expressing native metabotropic and ionotropic glutamate receptors, the final outcome of CTZ effects depended on its combined ability to potentiate AMPA receptors and inhibit mGluR1 receptors.

Synthesis and Structure−Activity Relationships of 3-[(2-Methyl-1,3-thiazol-4-yl)ethynyl]pyridine Analogues as Potent, Noncompetitive Metabotropic Glutamate Receptor Subtype 5 Antagonists; Search for Cocaine Medications

Recent genetic and pharmacological studies have suggested that the metabotropic glutamate receptor subtype 5 (mGluR5) may represent a druggable target in identifying new therapeutics for the treatment of various central nervous system disorders including drug abuse. In particular, considerable attention in the mGluR5 field has been devoted to identifying ligands that bind to the allosteric modulatory site, distinct from the site for the primary agonist glutamate. Both 2-methyl-6-(phenylethynyl)pyridine (MPEP) and its analogue 3-[(2-methyl-4-thiazolyl)ethynyl]pyridine (MTEP) have been shown to be selective and potent noncompetitive antagonists of mGluR5. Because of results presented in this study showing that MTEP prevents the reinstatement of cocaine self-administration caused by the presentation of environmental cues previously associated with cocaine availability, we have prepared a series of analogues of MTEP with the aim of gaining a better understanding of the structural features relevant to its antagonist potency and with the ultimate aim of investigating the effects of such compounds in blunting the self-administration of cocaine. These efforts have led to the identification of compounds showing higher potency as mGluR5 antagonists than either MPEP or MTEP. Two compounds 19 and 59 exhibited functional activity as mGluR5 antagonists that are 490 and 230 times, respectively, better than that of MTEP.

Synthesis and receptor assay of aromatic-ethynyl-aromatic derivatives with potent mGluR5 antagonist activity

Noncompetitive antagonists of the human metabotropic glutamate receptor subtype 5 (mGluR5) have been implicated as potential therapeutics for the treatment of a variety of nervous system disorders, including pain, anxiety, and drug addiction. To discover novel noncompetitive antagonists to the mGluR5, we initiated an SAR study around the known lead compounds MPEP and M-MPEP. Our results pointed out the critical role of the para position of the two aromatic rings, which leads to inactive products and permitted the discovery of potent mGluR5 antagonists (e.g., 16, 25, 28, 34 IC(50) = 13.5, 11.9, 21, 15 nM, respectively).

Functionalization at position 3 of the phenyl ring of the potent mGluR5 noncompetitive antagonists MPEP

We described the synthesis and biological evaluation of MPEP analogs functionalized at the position 3 of the phenyl ring. The results point out the limitation in the choice of a functional group at this position; the only substituents leading to retention of activity are NO(2) (IC(50)=13 nM) and CN (IC(50)=8 nM).

L-homocysteine sulfinic acid and other acidic homocysteine derivatives are potent and selective metabotropic glutamate receptor agonists

Moderate hyperhomocysteinemia is associated with several diseases, including coronary artery disease, stroke, Alzheimer's disease, schizophrenia, and spina bifida. However, the mechanisms for their pathogenesis are unknown but could involve the interaction of homocysteine or its metabolites with molecular targets such as neurotransmitter receptors, channels, or transporters. We discovered that L-homocysteine sulfinic acid (L-HCSA), L-homocysteic acid, L-cysteine sulfinic acid, and L-cysteic acid are potent and effective agonists at several rat metabotropic glutamate receptors (mGluRs). These acidic homocysteine derivatives 1) stimulated phosphoinositide hydrolysis in the cells stably expressing the mGluR1, mGluR5, or mGluR8 (plus Galpha(qi9)) and 2) inhibited the forskolin-induced cAMP accumulation in the cells stably expressing mGluR2, mGluR4, or mGluR6, with different potencies and efficacies depending on receptor subtypes. Of the four compounds, L-HCSA is the most potent agonist at mGluR1, mGluR2, mGluR4, mGluR5, mGluR6, and mGluR8. The effects of the four agonists were selective for mGluRs because activity was not discovered when L-HCSA and several other homocysteine derivatives were screened against a large panel of cloned neurotransmitter receptors, channels, and transporters. These findings imply that mGluRs are candidate G-protein-coupled receptors for mediating the intracellular signaling events induced by acidic homocysteine derivatives. The relevance of these findings for the role of mGluRs in the pathogenesis of homocysteine-mediated phenomena is discussed.

1-amino-APDC, a partial agonist of group II metabotropic glutamate receptors with neuroprotective properties

The synthesis of the 1-amino derivative of (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylic acid (1-amino-APDC), a selective metabotropic glutamate ligand, is disclosed. This compound acts as a partial agonist of the group II mGluRs and shows pronounced neuroprotective properties in the NMDA model of cell toxicity.

Synthesis and biology of the conformationally restricted ACPD analogue, 2-aminobicyclo[2.1.1]hexane-2,5-dicarboxylic acid-I, a potent mGluR agonist

To better characterize the roles of metabotropic glutamate receptors (mGluRs) in physiological and pathophysiological processes, there is an important need to learn more about the structural features relevant to the design of novel, high-affinity ligands that are family and subtype specific. To date, many of the biological studies that have been conducted in the area of mGluR research have made use of the agonist (1S,3R)-ACPD. This compound has been shown to act as an agonist at both the group I and group II receptors while showing little selectivity among the four subtypes belonging to these two groups. Moreover, (1S,3S)-ACPD, the cis isomer, shows negligible activity at group I receptors and is a good agonist of mGluR2. Since ACPD is itself somewhat flexible, with four distinctive conformations being identified from molecular modeling studies for the trans isomer and five conformations for the cis isomer, we believed that it would be of interest to examine the activity of an ACPD analogue that has been constrained through the introduction of a single carbon atom bridge. Accordingly, we have prepared an aminobicyclo[2.1.1]hexanedicarboxylic acid (ABHxD-I) analogue of ACPD. The synthesis of this compound was accomplished by use of an intramolecular [2 + 2] photocycloaddition reaction, in which four distinct isomers were isolated. Of these four compounds, only a single isomer, ABHxD-I (6a), was found to be a potent agonist of the mGluRs. This compound, which expresses the fully extended glutamate conformation, was found to be more potent than ACPD at all six of the eight mGluR subtypes that were investigated and to be comparable to or more potent than the endogenous ligand, glutamate, for these receptors. Interestingly, despite its fixed conformation, ABHxD-I, like glutamate, shows little subtype selectivity. Through modeling studies of ABHxD-I (6a), ABHD-VI, LY354740, (1S,3R)-ACPD, (1S, 3S)-ACPD, and l-glutamate, we conclude that the aa conformation of l-glutamate is the active conformation for both group I and group II mGluRs. Moreover, the modeling-based comparisons of these ligands suggest that the selectivity exhibited by LY354740 between the group I and group II mGluRs is not a consequence of different conformations of L-glutamate being required for recognition at these mGluRs but rather is related to certain structural elements within certain regions having a very different impact on the group I and group II mGluR activity. The enhanced potency of ABHxD-I relative to trans-ACPD commends it as a useful starting point in the design of subtype selective mGluR ligands.

Effects of L-trans-pyrrolidine-2,4-dicarboxylate, a glutamate uptake inhibitor, on NMDA receptor-mediated calcium influx and extracellular glutamate accumulation in cultured cerebellar granule neurons

Glutamate uptake inhibitor, L-trans-pyrrolidine-2,4-dicarboxylate (PDC, 20 muM) elevated basal and N-methyl-D-aspartate (NMDA, 100 muM)-induced extracellular glutamate accumulation, while it did not augment kainate (100 muM)-induced glutamate accumulation in cultured cerebellar granule neurons. However, pretreatment with PDC for 1 h significantly reduced NMDA-induced glutamate accumulation, but did not affect kainate-induced response. Pretreatment with glutamate (5 muM) for 1 h also reduced NMDA-induced glutamate accumulation, but did not kainate-induced response. Upon a brief application (3-10 min), PDC did neither induce elevation of intracellular calcium concentration ([Ca(2+)](i)) nor modulate NMDA-induced [Ca(2+)](i) elevation. Pretreatment with PDC for 1 h reduced NMDA-induced [Ca(2+)](i) elevation, but it did not reduce kainate-induced [Ca(2+)](i) elevation. These results suggest that glutamate concentration in synaptic clefts of neuronal cells is increased by prolonged exposure (1 h) of the cells to PDC, and the accumulated glutamate subsequently induces selective desensitization of NMDA receptor.

Lectins implicate specific carbohydrate domains in electric field stimulated nerve growth and guidance

Both endogenous lectins and DC electric fields may control aspects of early nerve growth and nerve guidance. To test whether such endogenous cues interact, lectins of varying sugar affinity and valency were studied for effects on electric field induced growth and reorientation of cultured Xenopus neurites. Concanavalin A (Con A), succinylated concanavalin A (S-Con A), and wheat germ agglutinin all completely inhibited field-induced cathodal reorientation. Lentil and pea lectins, which share the same sugar affinity as Con A/S-Con A, were only partially effective in inhibiting reorientation. Because S-Con A does not alter lateral mobility of membrane receptors, the previously accepted notion that Con A inhibited field-induced reorientation by preventing receptors from translocating and becoming redistributed asymmetrically in the membrane may be oversimplified. There are likely to be additional steric interactions that Con A and S-Con A share that inactivate asymmetrically redistributed receptors and prevent reorientation. Additionally, nerves growing in an applied field branch more commonly toward the cathode. Con A and S-Con A alone prevented this development of asymmetric branching. All the lectins tested prevented the normal field-induced increase in nerve growth rate, while all, except peanut agglutinin, prevented the usual faster growth cathodally than anodally. We suggest that lectin interactions with electric field effects in vitro may involve modulation of neuronal nicotinic acetylcholine receptors, neurotrophin receptors, or voltage-dependent calcium channels. Similar interactions between endogenous lectins and endogenous electric fields are to be expected.

Intrauterine hypoxia-ischemia reduces phosphoinositide hydrolysis stimulated by metabotropic glutamate receptor agonists in cultured rat cerebellar granule cells

Effects of intrauterine hypoxia-ischemia (HI) on receptor-stimulated phosphoinositide (PPI) hydrolysis were studied in rat cerebellar granule cell cultures prepared from an in utero HI model. On gestation day 17, HI conditions were achieved by complete clamping of the uterine vasculature for 30 min followed by removal of the clamps to permit reperfusion. Sham operation (SH, surgery without vasculature ligation) was performed as the control. Intrauterine HI did not affect the basal level of PPI hydrolysis (in the absence of stimulants) in cells prepared from either the SH or the HI group. PPI hydrolysis stimulated by quisqualate (QA) or trans-(1S,3R)-1-amino-1,3-cyclo-pentanedicarboxylic acid (trans-ACPD) was significantly reduced in cells prepared from the HI group, whereas intrauterine HI did not affect the PPI hydrolysis induced by ionotropic glutamate receptor agonists or by norepinephrine or serotonin. At a dose range of 100-300 microM, QA-stimulated PPI hydrolysis in cells prepared from the SH group increased by 3-to 4.5-fold, while this increase was only 2- to 2.5-fold in cells prepared from the HI group. Presence of L-NG-monomethyl-arginine (L-NMMA), a nitric oxide (NO) synthase inhibitor, did not increase QA-stimulated PPI hydrolysis in cells prepared from either the SH or the HI group, indicating that stimulation of NO formation is unlikely involved in the suppressive effects of intrauterine HI on QA-induced PPI hydrolysis. The QA-stimulated PPI hydrolysis in cells prepared from the HI group, but not from the SH group, was further inhibited by L-(+)-2-amino-3-phosphono-propionic acid (L-AP3). The overall results suggest that intrauterine HI has long-lasting suppressive effects on metabotropic glutamate receptor agonist-stimulated PPI hydrolysis and these effects might be associated with alterations in expression of metabotropic glutamate receptor subtypes.

Prenatal ethanol exposure reduces phosphoinositide hydrolysis stimulated by quisqualate in rat cerebellar granule cell cultures

Prenatal ethanol exposure-induced alteration in poly-phosphoinositide (PPI) hydrolysis stimulated by excitatory amino acids (EAA) was studied in rat cerebellar granule cells previously labeled with [3H]myoinositol. The prenatal exposure to ethanol was achieved via maternal consumption of a Sustacal (chocolate flavored) liquid diet containing either 5% ethanol (w/v, 35% of calories) or isocaloric sucrose (pair-fed) substituted for ethanol from gestation d 11 until the day of parturition. The ionotropic glutamate receptor agonists, N-methyl-D-aspartate, kainate or (+/-)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) (100 microM each) induced a two- to four-fold increase in PPI hydrolysis over the basal level, regardless of the liquid dietary treatment. Stimulation with quisqualate (QA), an agonist activating both metabotropic and ionotropic glutamate receptors, resulted in a much stronger and dose-dependent response in PPI hydrolysis and exposure in utero to ethanol significantly reduced this response. Tetrodotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), or (+/-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP) had no effect on QA-stimulated PPI hydrolysis nor on the suppression of this hydrolysis by ethanol. Exposure in utero to ethanol did not affect PPI hydrolysis stimulated by a selective metabotropic glutamate receptor agonist, trans-(+/-)-l-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD). Although the PPI hydrolysis stimulated by t-ACPD could be blocked by (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG), an antagonist of the metabotropic glutamate receptor, MCPG was incapable of affecting QA-induced PPI hydrolysis and the suppressive effects of prenatal ethanol exposure on this hydrolysis. Taken together, the data suggest that the long-lasting suppressive effects of prenatal ethanol exposure on QA-stimulated PPI hydrolysis in cerebellar granule cell cultures is through a metabotropic QA receptor pathway that may be different from the one activated by t-ACPD.