A [ 35S]GTP γS binding assessment of metabotropic glutamate receptor standards in Chinese hamster ovary cell lines expressing the human metabotropic receptor subtypes 2 and 4

Abstract

The activities of metabotropic glutamate receptor (mGluR) standards were evaluated in the [ 35S]GTP γS binding assay and in the forskolin (FSK)-enhanced cyclic AMP assay using Chinese hamster ovary (CHO) cells or homogenates which expressed the human mGluR (hmGluR) subtypes 2 and 4. Though distinct rank orders of activities were determined for the agonists between the cell lines expressing individual hmGluRs, similar rank orders of agonist activities were determined for the standards between assays. O-phospho- l-serine ( l-SOP) and ( S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4) antagonized agonist EC 90 responses in the cell lines expressing the hmGluR 2 and 4 subtypes, respectively. In addition to its antagonist effect, l-SOP increased the baseline level of cAMP when tested in the absence of agonist. In spite of this anomalous effect, l-SOP was found to be a competitive antagonist in the cAMP assay as well as in the [ 35S]GTP γS binding assay with a pA 2 value of 5.2 in both assays. MAP4 was a competitive antagonist of l(+)-2-amino-4-phosphonobutyric acid ( l-AP4)-induced responses in the CHO cell line expressing hmGluR4 with pA 2 values of 4.4 and 4.5 determined in the [ 35S]GTP γS binding and cAMP assays, respectively.