A Novel Binding Assay For Metabotropic Glutamate Receptors Using [3H] ʟ-Quisqualic Acid And Recombinant Receptors

Abstract

We established a methodology to analyze radioligand binding to the recombinant type 1a metabotropic glutamate receptor (mGluR1a). A full-length ᴄDNA encoding mGluR1a, which was isolated from a λ gt 11 ᴄDNA library of human cerebellar origin, was expressed in a baculovirus/Sf9 insect cell system. Membrane fractions with recombinant receptor expression were analyzed for the binding of [3H]L-quisqualic acid (ʟ-QA), which is known to be a potent agonist of mGluR1a. Efficient binding of the radioligand to the human receptor was observed in a saturable manner, giving an apparent Kd= 0.091 μᴍ. [3H]ʟ-QA bound to the human mGluR1a was displaced by known ligands such as ʟ-QA, ʟ-Glu, t-ACPD(( ±)-1- aminocyclopentane-trans-1,3-dicarboxylic acid) with IC50s = 0.056, 0.97 and 4.0 μᴍ, respectively. MCPG (α-methyl-4-carboxyphenylglycine) displaced the radioligand binding with lower potency. Using this binding protocol, we then evaluated the ligand ability of synthetic dipeptides. Among peptides tested, only Glu-containing dipeptides inhibited the radioligand binding, e.g. IC50 of ʟ-Met-ʟ-Glu was 4.3 μᴍ. When phosphatidyl inositol turnover was assayed in mGluR1a-expressing CHO cells, ʟ-Met-ʟ-Glu was partially agonistic. We further expanded this [3H]ʟ-QA binding protocol to type 5a mGluR, another member of group I mGluRs, as well as to AMPA receptor, a member of ionotropic glutamate receptors, since ʟ-QA is also known to be a potent ligand for these receptors. Data shown here will provide a novel system not only to search for ligands for the glutamate receptors, but also to biochemically analyze the interaction modes between glutamate receptors and their ligands