Selective blockade of mGlu5 metabotropic glutamate receptors protects rat hepatocytes against hypoxic damage

Western blot analysis of protein extracts from rat liver revealed the prese nce of the mGlu5 receptor, one of the G-protein-coupled receptors activated by glutamate (named "metabotropic glutamate receptors" or mGlu receptors). mGlu5 expression was particularly high in extracts from isolated hepatocyt es, where levels were comparable with those seen in the rat cerebral cortex . The presence of mGlu5 receptors in hepatocytes was confirmed by reverse-t ranscription polymerase chain reaction (RT-PCR) analysis, immunohistochemis try in neonate or adult rat liver, as well as by, immunocytochemical analys is in HepG2 hepatoma cells, where the receptor appeared to be preferentiall y distributed in cell membranes. Interestingly, mGlu1 receptors (which are structurally and functionally homologous to mGlu5 receptors) were never fou nd in rat liver or hepatocytes, In hepatocytes exposed to anoxic conditions for 90 minutes, glutamate, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic ac id (1S,3R-ACPD) and quisqualate, which all activate mGlu5 receptors, accele rated the onset and increased the extent of cell damage, while 4-carboxy-3- hydroxyphenylglycine (4C3HPG), an agonist of mGlu2/3 receptors, was inactiv e. 2-Methyl-6-(2-phenyl-1-ethynyl)-pyridine (MPEP), a novel, noncompetitive , highly selective mGlu5 receptor antagonist, not only abolished the toxic effect of 1S,3R-ACPD, but, unexpectedly, was protective by itself against a noxic damage. This suggests that hepatocytes express mGlu5 receptors and th at activation of these receptors by endogenous glutamate facilitates the de velopment of anoxic damage in hepatocytes.