Effects of varying the expression level of recombinant human mGlu1 alpha 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 c ells using an isopropyl beta-D-thiogalactopyranoside (IPTG) inducible syste m. Expression of mGlu1 alpha receptors could not be detected using immunobl otting or immunocytochemical approaches in non-induced cells, however, cont rolled expression could be induced following IPTG addition in a time- and c oncentration-dependent manner. 2 In induced cells (100 mu M IPTG, 20 h) the agonists L-quisqualate or 1-am inocyclopentane-1S,3R-dicarboxylic acid stimulated large increases in [H-3] -inositol (poly)phosphate (in the presence of Lif) and inositol, 1,4,5-tris phosphate levels. 3 Induction with 1-100 mu M IPTG allowed the receptor density to be increas ed incrementally and this not only resulted in an increase in the maximum r esponse 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-d icarboxylic acid dramatically increased with increasing receptor expression . 5 The activities of the competitive mGlu1 alpha receptor antagonists (S)-al pha-methyl-4-carboxyphenylglycine and (S)-4-carboxy-3-hydroxyphenylglycine for inhibition of the effects of L-quisqualate or (S)3,5-dihydroxyphenylgly cine were found to be independent of the receptor expression level. 6 When the mGlu1 alpha receptor was expressed at very high levers, no evide nce for receptor constitutive activity could be detected, and none of the a ntagonists tested revealed either any intrinsic activity or negative effica cy. 7 These data demonstrate that both the potency and efficacy of mGlu1 alpha: receptor agonists are influenced by expression level, whilst mGlu1 alpha r eceptor antagonist activities are independent of expression level.