Glutamate release by the intact light-responsive photoreceptor layer of the Xenopus retina

In order to study glutamate release from light responsive photoreceptors, we used an eyecup preparation treated with detergent and distilled water, which permitted removal of the inner retina. The remaining 'reduced' retina consists mainly of photoreceptors attached to the pigment epithelium. The viability of the preparation was established by exclusion of trypan blue, light and electron microscopic examination of the photoreceptor layer and by intracellular recordings from rods. The 'reduced' retina was superfused at 1 ml/h and overflow samples were analyzed for their glutamate content by a fluorimetric enzyme assay. We tested the response to dark and light adaptation and to treatment with 100 mu M CdCl2. We found a baseline glutamate level in light-adapted preparation which was not affected by cadmium. Dark adaptation induced a 2-fold increase of glutamate release, which was completely blocked by cadmium.