Role of nitric oxide in control of light adaptive cone photomechanical movements in retinas of lower vertebrates: a comparative species study.

The possible role of nitric oxide (NO) as a novel light adaptive neuromodulator of cone plasticity (photomechanical movements) in retinae of two contrasting species of fish (freshwater carp and marine bream) and an example of an amphibian (Xenopus laevis) was studied pharmacologically by cytomorphometric measurements. Application of a NO donor [S-nitroso-N-acetyl-d, l-penicillamine] (500-700 microM) to dark-adapted retinae induced contraction of cones with an efficiency (CE) relative to full light adaptation of around 54% in all three species. Pretreatment with a NO scavenger [2-(4-Carboxyphenyl)-4,4,5,5-tetrametylimidazoline-1-oxil-3-oxide] (30-35 microM) produced a consistent significant inhibition of the light adaptation-induced cone contraction (CE = 15-20%). These results strongly suggest the involvement of endogenous NO in the cone contractions that occur in freshwater and marine fish and amphibian retinae as a part of the light adaptation process.