The agonist selectivity of a class III metabotropic glutamate receptor, human mGluR4a, is determined by the N-terminal extracellular domain

To test the hypothesis that the determinants for agonist selectivity of class III metabotropic glutamate receptors (mGluRs) are localized in the N-terminal extracellular domain, a chimaeric cDNA was constructed where 519 amino acids of the N-terminal extracellular domain of human mGluR1b were exchanged with the corresponding region of human mGluR4. The pharmacological profile of the chimaera, designated hmGluR41-519/1b, was analysed by recordings of intracellular calcium concentration ([Ca2+]i) in transiently transfected HEK 293 cells and compared with that of human mGluR1b and human mGluR4a stably expressed in Chinese hamster ovary cells. Application of 100 µM l-2-amino-4-phosphonobutyrate (l-AP4), a class III mGluR-specific agonist, induced a rise in [Ca2+]i in hmGluR41-519/1b but not in hmGluR1b expressing cells. In contrast, application of quisqualate (100 µM) induced a rise in [Ca2+]i at hmGluR1b but not at hmGluR41-519/1b. Dose–response analysis with l-AP4 and l-glutamate at hmGluR41-519/1b revealed a half-maximal effect (EC50) of 16.0 µM and 196 µM, respectively. The EC50 values for quisqualate, glutamate and (1S,3R)-ACPD at hmGluR1b were 10.25 µM, 225 µM and 3060 µM, respectively. The rank order of agonist potency of hmGluR41-519/1b corresponds to that of hmGluR4 (l-AP4 > l-glutamate > (1S,3R)-ACPD > quisqualate) but is different from that of hmGluR1b (quisqualate > glutamate [U00BB] (1S,3R)-ACPD).