Effects of varying the expression level of recombinant human mGlu1α receptors on the pharmacological properties of agonists and antagonists

1 Different expression levels of the human type Ice metabotropic glutamate (mGlu1 alpha) receptor were obtained in transfected Chinese hamster ovary cells using an isopropyl beta-D-thiogalactopyranoside (IPTG) inducible system. Expression of mGlu1 alpha receptors could not be detected using immunoblotting or immunocytochemical approaches in non-induced cells, however, controlled expression could be induced following IPTG addition in a time- and concentration-dependent manner. 2 In induced cells (100 mu M IPTG, 20 h) the agonists L-quisqualate or 1-aminocyclopentane-1S,3R-dicarboxylic acid stimulated large increases in [H-3]-inositol (poly)phosphate (in the presence of Lif) and inositol, 1,4,5-trisphosphate levels. 3 Induction with 1-100 mu M IPTG allowed the receptor density to be increased incrementally and this not only resulted in an increase in the maximum response to. L-quisqualate, 1-aminocyclopentane-1S,3R-dicarboxylic acid and (S)-3,5-dihydroxy-phenylglycine, but also in an increase in the respective potencies of each agent to activate phosphoinositide hydrolysis. 4 The intrinsic activity of the partial agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid dramatically increased with increasing receptor expression. 5 The activities of the competitive mGlu1 alpha receptor antagonists (S)-alpha-methyl-4-carboxyphenylglycine and (S)-4-carboxy-3-hydroxyphenylglycine for inhibition of the effects of L-quisqualate or (S)3,5-dihydroxyphenylglycine were found to be independent of the receptor expression level. 6 When the mGlu1 alpha receptor was expressed at very high levers, no evidence for receptor constitutive activity could be detected, and none of the antagonists tested revealed either any intrinsic activity or negative efficacy. 7 These data demonstrate that both the potency and efficacy of mGlu1 alpha: receptor agonists are influenced by expression level, whilst mGlu1 alpha receptor antagonist activities are independent of expression level.