NADPH DIAPHORASE ACTIVITY IN MAMMALIAN RETINAS IS MODULATED BY THE STATE OF VISUAL-ADAPTATION

NADPH diaphorase histochemistry is commonly used to identify cells con taining nitric oxide synthase (NOS), the enzyme catalyzing the product ion of nitric oxide from L-arginine. NADPH diaphorase activity and NOS immunostaining was demonstrated in different cells of the vertebrate retina; photoreceptors, horizontal cells, amacrine cells, ganglion cel ls, and Muller cells. However, the physiological role of nitric oxide (NO) in the retina has yet to be elucidated. In this study, we tested the assumption that NADPH diaphorase activity in the retinas of rabbit s and rats depended on the state of visual adaptation. In the rabbit, light adaptation enhanced NADPH diaphorase activity in amacrine cells and practically eliminated it in horizontal cells. Dark adaptation ind uced the opposite effects; the NADPH diaphorase activity was reduced i n amacrine cells and enhanced in horizontal cells. Retinas from eyes t hat were injected intravitreally with L-glutamate exhibited a pattern of NADPH diaphorase activity that was similar to that seen in dark-ada pted retinas. In rats, the NADPH diaphorase activity of amacrine and h orizontal cells exhibited adaptation dependency similar to that of the rabbit retina. But, the most pronounced effect of dark adaptation in the rat's retina was an enhancement of NADPH diaphorase activity in Mu ller cells, especially of the endfoot region. Assuming that NADPH diap horase activity is a marker for NOS, these findings suggest that NO pr oduction in the mammalian retina is modulated by the level of ambient illumination and support the notion that NO plays a physiological role in the retina.