Characterization of metabotropic glutamate receptors in rat C6 glioma cells.

Metabotropic glutamate receptors in rat C6 glioma cells have been characterized by pharmacological and kinetic binding experiments, using both L-[3H]glutamate and [3H(+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (3H-trans-ACPD) radioligands. Saturation experiments revealed a single binding site with a Kd = 1250 +/- 101 nM and Bmax = 12.1 +/- 1.8 pmol/mg protein when the assays were performed with L-[3H]glutamate as radioligand in the presence of AMPA, kainate, NMDA and DL-threo-beta-hydroxyaspartic acid. When 3H-trans-ACPD was used as radioligand, the kinetic parameters obtained were Kd = 2605 +/- 1042 nM and Bmax = 13.66 +/- 5.01 pmol/mg protein. Pharmacological characterization indicated that specific binding of L-[3H]glutamate was sensitive to different agonists of mGlu receptors, showing a rank order of affinity L-glutamate > L-quisqualic acid > (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) > ibotenic acid >>> (2S, 'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I). Specific binding of L-[3H]glutamate to mGlu receptors is regulated by guanine nucleotides. Guanylyl imidodiphosphate (Gpp(NH)p) causes an affinity shift on the L-glutamate dose-response curve, increasing the IC50 value. These results support the evidence that metabotropic glutamate receptors are present in rat C6 glioma cells and they are coupled to a G-protein.