NADPH-diaphorase distribution in the choroid after continuous illumination.

The role of nitric oxide (NO) in the regulation of the tone of the blood choroid vessels was investigated using the histochemical NADPH-diaphorase technique. Rats were continuously illuminated with white light, 12,000 lux, for 8 days and perfused after 1, 3, 10 and 20 days of total darkness. Cryostat sections were processed using the NADPH diaphorase histochemical method. An increase in reactive fibres was observed during days 1, 3 and 10, but after 20 days in darkness, reactivity was similar to that observed in control sections. These results indicate that NO plays a role in regulating the blood flow in the uveal vessels, and that there are changes in NO level dependent on the functional state of the retina.