CLONING AND STABLE EXPRESSION OF THE MGLUR1B SUBTYPE OF HUMAN METABOTROPIC RECEPTORS AND PHARMACOLOGICAL COMPARISON WITH THE MGLUR5A SUBTYPE

We isolated and characterized a cDNA encoding the human metabotropic g lutamate receptor subtype 1b (hmGluR1b). In situ hybridization studies in human brain regions revealed a higher distribution of mGluR1 mRNA in the dentate gyrus of the hippocampus, the substantia nigra pars com pacta and the Purkinje cell layer of the cerebellum compared to other regions studied. We established stable expression of recombinant hmGlu R1b in L(tk(-)) mouse fibroblast and Chinese hamster ovary (CHO-dhfr(- )) cells. In both expression systems, agonist activation of hmGluR1b s timulated inositol phosphate (InsP) formation and elevation of the cyt osolic free calcium ([Ca2+](i)), and both responses were blocked by (S )-MCPG. The rank order of potency for agonists was quisqualate > gluta mate, (IS,3R)-ACPD in both expression systems. Comparison of the agoni st profiles of hmGluR1b and hmGluR5a, both stably expressed in L(tk(-) ) cells, indicated the same rank order of potency (quisqualate > gluta mate greater than or equal to (RS)-3,5-DHPG greater than or equal to ( 1S,3R)-ACPD), but each of the four agonists were more potent on hmGluR 5a than on hmGluR1b. In antagonist studies, (S)-MCPG inhibited the ago nist-induced InsP formation and elevation of [Ca2+](i) in both hmGluR1 b- and hmGluR5a-expressing cells. (S)-4CPG and (S)-4C3HPG both inhibit ed agonist responses only in hmGluR1b-expressing cells. However, in hm GluR5a-expressing cells the antagonist activity of(S)-4CPG and (S)-4C3 HPG was dependent on the agonist used in the study, since they inhibit ed responses to glutamate but not to quisqualate. Stable cell lines ex pressing specific subtypes of human mGluRs represent valuable tools fo r the study of the mechanism of action of mGluRs at the molecular and cellular level and as screening targets for identification of subtype- selective agonists or antagonists. (C) 1997 Elsevier Science Ltd.