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Abstract
PURPOSE: To investigate the cellular origin of nitric oxide released from the rabbit retina in response to physiological stimulation with light. METHODS: The release of nitric oxide from the retina was measured in rabbits anesthetized with urethane. An eye-cup was prepared and was filled with Krebs-Ringer bicarbonate. After washing for 45 minutes, 0.5 ml medium was placed in the eyecup. The medium was replaced every 10 minutes, and nitric oxide in the resultant samples was measured using nitrate reductase and a nitric oxide meter. RESULTS: In the unstimulated dark-adapted retina there was a spontaneous resting release of nitric oxide (1.20 nmol/min). When the retina was stimulated for 10 minutes with flickering light there was an increase in nitric oxide release to almost double the resting release. Stimulation of the retina for 10 minutes with continuous light produced a similar increase in nitric oxide release. The exposure of the retina to L-amino-4-phosphonobutyrate (APB), which specifically blocks transmission between the photoreceptors and the depolarizing bipolar cells, abolished the evoked release of nitric oxide caused by flickering light and continuous light. In contrast, the nonselective excitatory amino acid antagonist cis-2,3-piperidinedicarboxylic acid (PDA) had no effect on the flicker-evoked release of nitric oxide, but it more than halved the release caused by continuous light. A similar differential effect on release was found with glycine, which abolished the nitric oxide release evoked with continuous light but did not affect the flicker-evoked release. The inhibitory effect of glycine was blocked by strychnine. CONCLUSIONS: Nitric oxide was released in the retina by flickering light and by continuous light, but the two types of stimulation cause nitric oxide release from different cells. Because in the rabbit retina nitric oxide synthase occurs mainly in a subpopulation of amacrine cells and a few bipolar cells, our pharmacologic results suggest that continuous light causes nitric oxide release from amacrine cells, whereas flickering light evokes nitric oxide release from bipolar cells.