Molecular characterization of a novel metabotropic glutamate receptor mGluR5 coupled to inositol phosphate/Ca²⁺ signal transduction

A cDNA clone for a new metabotropic glutamate receptor, mGluR5, was isolated through polymerase chain reaction-mediated DNA amplification by using primer sequences conserved among the metabotropic glutamate receptor (mGluR) family and by the subsequent screening of a rat brain cDNA library. The cloned receptor consists of 1171 amino acid residues and exhibits a structural architecture common to the mGluR family, possessing a large extracellular domain preceding the seven putative membrane-spanning segments. mGluR5 shows the highest sequence similarity to mGluR1 among the mGluR members and is coupled to the stimulation of phosphatidylinositol hydrolysis/Ca²⁺ signal transduction in Chinese hamster ovary cells transfected with the cloned cDNA. This receptor also resembles mGluR1 in its agonist selectivity and antagonist responses; the potency rank order of agonists for mGluR5 was determined to be quisqualate > L-glutamate ≥ ibotenate > trans-1-aminocyclopentane-1,3-dicarboxylate. Blot and in situ hybridization analyses indicated that mGluR5 mRNA is widely distributed in neuronal cells of the central nervous system and is expressed differently from mGluR1 mRNA in many brain regions. This investigation thus demonstrates that there is an additional mGluR subtype which closely resembles mGluR1 in its signal transduction and pharmacological properties and is expressed in specialized neuronal cells in the central nervous system.