An ibotenate-selective metabotropic glutamate receptor mediates protein phosphorylation in cultured hippocampal...

Previous results showed that within 30 s after glutamate stimulation of cultured rat hippocampal pyramidal neurons there occurred an elevation of Ca2+ and diacylglycerol, and the phosphorylation of three acidic protein kinase C substrates, i.e., an 87-kDa protein known as myristoylated alanine-rich C kinase substrate and a 120- and a 48-kDa protein. In addition, it was suggested that a metabotropic-type glutamate receptor might be responsible for the phosphorylation observed. This work examines the ability of metabotropic and inotropic glutamate receptor agonists to quickly activate phospholipases in 1.26 mM versus 50 nM extracellular Ca2+ by measuring the generation of inositol phosphates. NMDA, quisqualate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid did not stimulate the generation of inositol phosphates in the presence of normal or low extracellular Ca2+ in pyramidal neurons. Kainate stimulated the production of inositol phosphates in the presence of 1.26 mM extracellular Ca2+ but not in 50 nM extracellular Ca2+. Other than glutamate, only ibotenate was able to stimulate the generation of inositol phosphatases in both normal and low extracellular Ca2+. The maximal response to ibotenate was approximately equal to that of glutamate, when pyramidal neurons were stimulated in 50 nM extracellular Ca2+. The generation of inositol phosphates by glutamate and ibotenate could be partially blocked (50-60% reduction) by pretreatment of neurons with pertussis toxin (250 ng/ml), suggesting that a GTP-binding protein might be involved. In addition, ibotenate stimulated the immediate phosphorylation of the same three protein kinase C substrates as glutamate. The NMDA receptor blocker MK-801 had no effect on this phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)