l-2-(carboxypropyl)glycine and 2-(2-3-dicarboxycyclopropyl)glycine

1. The binding of the new selective group II metabotropic glutamate receptor radioligand, [3H]-(2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine ([3H]-DCG IV), was characterized in rat mGlu2 receptor-transfected CHO cell membranes. 2. [3H]-DCG IV binding was pH-dependent, but was not sensitive to temperature. Saturation analysis showed the presence of a single binding site, with a Kd value of 160 nM and a Bmax value of 10 pmol mg(-1) protein. Binding was not sensitive to Na+-dependent glutamate uptake blockers or Cl-dependent glutamate binding inhibitors. Furthermore, up to concentrations of 1 mM, the glutamate ionotropic receptor agonists, N-methyl-D-aspartic acid (NMDA), (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate, did not affect [3H]-DCG IV binding. 3. Of the compounds observed to inhibit [3H]-DCG IV binding, the most potent were the recently described selective group II agonist, (+)-2-aminobicyclo-[3.1.0]hexane-2,6-dicarboxylate (LY 354740; Ki value 16 nM) and antagonist, 2-amino-2-(2-carboxycyclopropan-1-yl)-3-(dibenzopyran-4-yl) propanoic acid (LY 341495; Ki value 19 nM). As expected, for a G-protein-coupled receptor, guanosine-5'-O-(3-thiotriphosphate) (GTPgammaS) inhibited [3H]-DCG IV binding in a concentration-dependent manner, with an IC50 value of 12 nNM. 4. A highly significant correlation was observed between the potencies of compounds able to inhibit [3H]-DCG IV binding and potencies obtained for agonist activity in a GTPgamma35S binding functional assay. In addition, these studies identified a number of compounds with previously unknown activity at mGlu2 receptors, including L(+)-2-amino-3-phosphonopropionic acid (L-AP3), L(+)-2-amino-5-phosphonopentanoic acid (L-AP5), 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (R-CPP), N-acetyl-L-aspartyl-L-glutamic acid (NAAG) and (RS)-alpha-methylserine-O-phosphate (MSOP).

Topics: Animals; Cell Membrane; CHO Cells; Colforsin; Cricetinae; Cyclic AMP; Cyclopropanes; Glycine; Guanosine 5'-O-(3-Thiotriphosphate); Protein Binding; Rats; Receptors, Metabotropic Glutamate; Recombinant Proteins; Sulfur Radioisotopes; Transfection; Tritium

In a previous study we reported that the addition of a carboxylic group to the mGlu receptor agonist aminocyclopentane-1,3-dicarboxylate (ACPD) changes its properties from agonist to antagonist at both mGlu1 and mGlu2 receptors, and resulted in an increase in affinity at mGlu4 receptors, with isomers being either agonists or antagonists. In the present study, the effect of gamma-carboxy-L-glutamic acid (Gla) and (2S,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), two carboxylic derivatives of non-selective agonists, were examined on all cloned mGlu receptors. We found that this additional carboxylic group on glutamate prevents its interaction with group-I mGlu receptors and generates a potent group-II antagonist (K(B) = 55 microM on mGlu2). At group-III mGlu receptors, Gla was found to be either an antagonist (mGlu7 and mGlu8 receptors) or a partial agonist (mGlu4 and mGlu6 receptors). We show here that L-CCG-I is a general mGlu receptor agonist activating all cloned receptors. We also confirm that DCG-IV, which corresponds to L-CCG-I with an additional carboxylic group, is a selective group-II agonist. However, this additional COOH group changes the properties of L-CCG-I from an agonist to an antagonist at all group-III receptors, making this compound one of the most potent group-III mGlu receptor antagonist known so far. These observations will be useful for the development of more potent and selective mGlu receptor agonists and antagonists.

Topics: 1-Carboxyglutamic Acid; Amino Acids, Dicarboxylic; Cell Line; Cloning, Molecular; Cyclopropanes; Excitatory Amino Acid Agonists; Glycine; Humans; Receptors, Metabotropic Glutamate; Structure-Activity Relationship