Glutamate release evoked by glutamate receptor agonists in cultured chick retina cells: Modulation by arachidonic acid

We studied the effect of ionotropic glutamate receptor agonists on the release of endogenous glutamate or of ³H]D -aspartate from reaggregate cultures (retinospheroids) or from monolayer cultures of chick retinal cells, respectively. Kainate increased the fluorescence ratio of the Na⁺ indicator SBFI and stimulated a dose-dependent release of glutamate in low (0.1 mM) Ca²⁺ medium, as measured using a fluorometric assay. Under the same experimental conditions, the release evoked by N-methyl-D -aspartate (NMDA; 400 μM) was about half of that evoked by the same kainate concentration; α-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA; 400 μM) did not trigger a significant response. In the presence of 1 mM CaCl₂, all of the agonists increased the Ca²⁺]_i, as determined with the fluorescence dye Indo-1, but the glutamate release evoked by NMDA and kainate was significantly lower than that measured in 0.1 mM CaCl₂ medium. Inhibition by Ca²⁺ of the kainate-stimulated release of glutamate was partially reversed by the phospholipase A₂ inhibitor oleiloxyethyl phosphorylcholine (OPC), suggesting that the effect was mediated by the release of arachidonic acid, which inhibits the glutamate carrier. Accordingly, kainate, NMDA, and AMPA stimulated a Ca²⁺-dependent release of ³H]arachidonic acid, and the direct addition of the exogenous fatty acid to the medium decreased the release of glutamate evoked by kainate in low (0.1 mM) CaCl₂ medium. In monolayer cultures, we showed that NMDA, kainate, and AMPA also stimulated the release of ³H]D -aspartate, but in this case release in the presence of 1 mM CaCl₂ was significantly higher than that evoked in media with no added Ca²⁺. The ranking order of efficacy for stimulation of Ca²⁺-dependent release of ³H]D -aspartate was NMDA ? kainate < AMPA. © 1996 Wiley-Liss, Inc.