SELECTIVE RELEASE OF NITRIC-OXIDE FROM RETINAL AMACRINE AND BIPOLAR CELLS

PURPOSE. To investigate the cellular origin of nitric oxide released f rom the rabbit retina in response to physiological stimulation with li ght. METHODS. The release of nitric oxide from the retina was measured in rabbits anesthetized with urethane. An eyecup 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 1 0 minutes, and nitric oxide in the resultant samples was measured usin g nitrate reductase and a nitric oxide meter.RESULTS. in the unstimula ted dark-adapted retina there was a spontaneous resting release of nit ric oxide (1.20 nmol/min). When the retina was stimulated for 10 minut es with flickering light there was an increase in nitric oxide release to almost double the resting release. Stimulation of the retina for 1 0 minutes with Continuous light produced a similar increase in nitric oxide release. The exposure of the retina to L-amino-4-phosphonobutyra te (APB), which specifically blocks transmission between the photorece ptors and the depolarizing bipolar cells, abolished the evoked release of nitric oxide caused by flickering light and continuous light. In c ontrast, the nonselective excitatory amino acid antagonist cis-2,3-pip eridinedicarboxylic acid (PDA) had no effect on the flicker-evoked rel ease of nitric oxide, but it more than halved the release caused by co ntinuous light. A similar differential effect on release was found wit h glycine, which abolished the nitric oxide release evoked with contin uous light but did not affect the flicker-evoked release. The inhibito ry effect of glycine was blocked by strychnine. CONCLUSIONS. Nitric ox ide was released in the retina by flickering Light and by continuous l ight, but the two types of stimulation cause nitric oxide release from different cells. Because in the rabbit retina nitric oxide synthase o ccurs mainly in a subpopulation of amacrine cells and a few bipolar ce lls, our pharmacologic results suggest that continuous light causes ni tric oxide release from amacrine cells, whereas flickering light evoke s nitric oxide release from bipolar cells.