MOLECULAR AND FUNCTIONAL-CHARACTERIZATION OF RECOMBINANT HUMAN METABOTROPIC GLUTAMATE-RECEPTOR SUBTYPE-5

We have isolated and characterized overlapping cDNAs that encode two i soforms of the human metabotropic glutamate receptor subtype 5 (hmGluR 5). The deduced amino acid sequences of human and rat mGluR5a are 94.5 % identical. However, a region in the putative cytoplasmic domain (SER 926-ALA1121) displays significant sequence divergence. Genomic analysi s of this region showed that the sequence divergence results from spec ies-specific differences in the genomic sequences, not from alternativ e splicing. The distribution of mGluR5 mRNA in human brain was most st rongly detected throughout the hippocampus, with moderate levels in th e caudate-putamen, cerebral cortex, thalamus, and deep cerebellar nucl ei, and at low levels in the cerebellar cortex. Activation of both hmG luR5a and hmGluR5b transiently expressed in Xenopus oocytes and HEK293 cells was coupled to inositol phosphate (InsP) formation and elevatio n of the intracellular free calcium ([Ca2+](i)). The agonist rank orde r of potency for activating recombinant hmGluR5a receptors in either s ystem was quisqualate > L-glutamate > IS,3R-ACPD. Both the quisqualate stimulated InsP and [Ca2+](i) were inhibited by (+)-MCPG. Recombinant human mGluR5a was also stably expressed in mouse fibroblast Ltk(-) ce lls, in which the efficacy and potency of quisqualate were unchanged f or more than 30 cell passages.