ACTIVATION OF METABOTROPIC GLUTAMATE RECEPTORS COUPLED TO INOSITOL PHOSPHOLIPID HYDROLYSIS AMPLIFIES NMDA-INDUCED NEURONAL DEGENERATION IN CULTURED CORTICAL-CELLS

We have studied the influence of class I metabotropic glutamate recept ors (mGluRs) on excitotoxic neuronal degeneration in cultured murine c ortical neurons grown on a monolayer of astrocytes. These cultures exp ressed high levels of mGluRS mRNA, which were comparable to those foun d in RNA extracts from cerebral cortex. Cortical neurons in mixed cult ures were heavily stained with antibodies raised against mGluRS and we re also stained-albeit to a much lower extent-with mGluR1a but not wit h mGluR1b or c antibodies. Preferential agonists of class I mGluRs, su ch as quisqualate, 3,5-dihydroxyphenylglycine (DHPG), and trans-azetid ine-2,4-dicarboxylic acid (t-ADA), as well as the mixed mGluR agonist, 1(S),3(R)-1-aminocyclopentane-1 ,3-dicarboxylic acid (1(S),3(R)-ACPD) all stimulated PPI hydrolysis in cultured cortical cells. The potency of N-methyl-D-aspartate (NMDA) in inducing neuronal degeneration was substantially enhanced when the drug was coincubated with quisqualate, DHPG or t-ADA during a 10-min pulse (paradigm of ''fast'' toxicity). None of the mGluR agonists influenced neuronal viability by itself. Th e amplification of NMDA toxicity by quisqualate or DHPG was attenuated by a series of protein kinase C (PKC) inhibitors, suggesting that cla ss I mGluRs operate, at least in part, through activation of PKC. Quis qualate and, in particular, DHPG enhanced excitotoxic neuronal degener ation even when applied after the toxic pulse with NMDA. This action i s likely to occur early in the maturation of excitotoxic damage, becau se the functional activity of class I mGluRs was substantially reduced at 2 or 3 hr after the NMDA pulse. These results suggest that activat ion of class I mGluRs enhances NMDA-receptor mediated neuronal toxicit y and encourage the search for selective antagonists for the experimen tal therapy of acute or chronic neurodegenerative diseases.