MOLECULAR, FUNCTIONAL, AND PHARMACOLOGICAL CHARACTERIZATION OF THE METABOTROPIC GLUTAMATE-RECEPTOR TYPE-5 SPLICE VARIANTS - COMPARISON WITHMGLUR1

The main excitatory neurotransmitter in the brain, glutamate (Glu), ac tivates not only receptor-channels, but also receptors coupled to G-pr otein called metabotropic Glu receptors (mGluRs), Eight genes coding f or mGluRs have been characterized to date giving rise to even more pro teins due to alternative splicing phenomena, Here we characterized a s plice variant of mGluR5, called mGluR5b which contains a 32 amino acid fragment inserted in the cytoplasmic tail, 50 residues after the 7th transmembrane domain. mGluR5b mRNAs are present in different regions o f the adult rat brain and are expressed at a higher level than mGluR5a mRNA, functional analysis of mGluR5a and mGluR5b revealed that they s hare all the properties of mGluR1a, but not those of mGluR1b or 1c. Li ke mGluR1a, both mGluR5a and mGluR5b activate a rapid and transient cu rrent in Xenopus oocytes. When expressed in LLC-PK1 cells, they show t he same subcellular distribution as mGluR1a, and stimulate both inosit ol phosphate (IP) and cAMP production. Moreover, cells expressing mGlu R5a or mGluR5b, like those expressing mGluR1a have a higher basal PLC activity that is not inhibited by glutamate-pyruvate transaminase (GPT ), suggesting that these receptors have an intrinsic activity. Interes tingly, the pharmacological profiles of mGluR5a and b are identical, b ut different from that of mGluR1a, Most agonists, except glutamate, ar e more potent on mGluR5a/b than on mGluR1a, Interestingly, the mGluR1a antagonists MCPG and 4CPG have no effect on mGluR5a/b; 4C3HPG which i s a full antagonist at mGluR1a is a partial agonist at mGluR5a/b. Thes e results indicate that the long C-terminal intracellular domain prese nt only in mGluR1a and mGluR5a/b, although not well conserved, is like ly to be involved in the specific functional properties of these recep tors. Although the ligand recognition sites of mGluR5a/b and mGluR1a a re highly conserved, these receptors have different pharmacology.