Characterization of [ 3 H]‐(2S,2′R,3′R)‐2‐(2′,3′‐dicarboxy‐ cyclopropyl)glycine ([ 3 H]‐DCG IV) binding to metabotropic mGlu 2 receptor‐transfected cell membranes

The binding of the new selective group II metabotropic glutamate receptor radioligand, [ 3 H]‐(2 S ,2′ R ,3′ R )‐2‐(2′,3′‐dicarboxycyclopropyl)glycine ([ 3 H]‐DCG IV), was characterized in rat mGlu 2 receptor‐transfected CHO cell membranes. [ 3 H]‐DCG IV binding was pH‐dependent, but was not sensitive to temperature. Saturation analysis showed the presence of a single binding site, with a K d value of 160 n M and a B max 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 m M , the glutamate ionotropic receptor agonists, N‐methyl‐ D ‐aspartic acid (NMDA), ( S )‐α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) and kainate, did not affect [ 3 H]‐DCG IV binding. Of the compounds observed to inhibit [ 3 H]‐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; K i value 16 n M ) and antagonist, 2‐amino‐2‐(2‐carboxycyclopropan‐1‐yl)‐3‐(dibenzopyran‐4‐yl) propanoic acid (LY 341495; K i value 19 n M ). As expected, for a G‐protein‐coupled receptor, guanosine‐5′‐O‐(3‐thiotriphosphate) (GTPγS) inhibited [ 3 H]‐DCG IV binding in a concentration‐dependent manner, with an IC 50 value of 12 n M . A highly significant correlation was observed between the potencies of compounds able to inhibit [ 3 H]‐DCG IV binding and potencies obtained for agonist activity in a GTPγ 35 S binding functional assay. In addition, these studies identified a number of compounds with previously unknown activity at mGlu 2 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 )‐α‐methylserine‐O‐phosphate (MSOP).British Journal of Pharmacology (1998) 123 , 497–504; doi: 10.1038/sj.bjp.0701647