Excitatory amino acid-induced inositol phosphate formation in cultured retinal pigment epithelium

Excitatory amino acid (EAA)-induced production of inositolphosphates (IPs) was studied in primary cultures of chick retinal pigment epithelium (RPE) following in vitro incorporation of [(3)H] myo-inositol. Glutamic acid (L-glu) significantly increased [(3)H]-IPs accumulation (215%). L-glu agonists stimulated [(3)H]IPs accumulation in the following order of efficiency: N-methyl-D-aspartate (NMDA) greater than or equal to L-glu > quisqualate greater than or equal to kainate > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD). Stimulation was dependent on external Ca(2+). The NMDA-induced response was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo-cyclohepten-5,10-imine maleate (MK-801) and 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and was decreased by the L-Ca(2+)-channel blockers verapamil and nifedipine as well as by dantrolene. The metabotropic glutamate receptor (mGluR) antagonist (+)-alpha-methyl-4-carboxyphenylglycine (+)MCPG inhibited 3,5-dihydroxyphenylglycine (DHPG) and ACPD-induced stimulation, which demonstrates the presence in RPE of mGluRs 1 and/or 5, as well as NMDA receptors coupled directly, or through the influx of external Ca(2+), to phospholipase C activation. L-glu agonists showed no effect either on basal level of intracellular cyclic adenosine monophosphate, nor on forskolin- or carbachol-induced stimulation of adenylyl cyclase. Since L-glu is released from the retina upon illumination, and receptors for this compound are present in RPE, the activation of the inositide pathway could be involved in the regulation of retina-RPE interaction, which is essential for the visual process.