The effect of non-NMDA antagonists and phorbol esters on excitatory amino acid stimulated inositol phosphate formation in rat cerebral cortex.

The effects of various modulators of excitatory amino acid stimulated inositol phosphate (IP) formation were investigated in slices of rat cerebral cortex. Both quisqualic acid (Quis) and ibotenic acid (Ibo) stimulated IP formation in a dose-dependent manner. Quis (0.3mM) stimulated the rapid formation of inositol bis, tris and tetrakis-phosphates, with a slower linear rise in the monophosphate which plateaued after 20 min. The responses to both Ibo (0.3 mM) and Quis (0.3 mM) were dose-dependently inhibited by phorbol dibutyrate (PDBu); the Ibo response was particularly sensitive to PDBu, with a 90% inhibition of the response at 1 ?M and an IC(50) of 200 nM; Quis stimulated IP formation was less sensitive with a 50% inhibition observed at 10 ?M PDBu. PMA (1 ?M) and dioctanoylglycerol (30 ?M) also inhibited Ibo-stimulated IP formation though 4-?-phorbol (10 ?M) was ineffective. The inhibition by PDBu was reversed with staurosporine (10 ?M) and polymyxin (10 ?M), and both protein kinase C inhibitors potentiated the Ibo response in the absence of PDBu. The putative Quis-kainate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and 6-cyano-7-nitroquinoxaline-2,3-dione did not inhibit the responses to either Quis or Ibo at a concentration of 100 ?M. Both responses were, however, inhibited by dl-2-amino-4-phosphonobutyric acid and O- phospho- l -serine (IC(50)s were 1-2 mM), and also by the putative Quis receptor agonist dl-?-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA; IC(50) 10?M). Our data suggest that excitatory amino acid stimulated IP formation in rat cerebral cortex is via a site distinct from previously defined amino acid receptors.