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H Levy, G Twig, I Perlman; Nitric Oxide as a Light Adapting Signal in Cone Photoreceptors of the Turtle Retina . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3751.
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Purpose: Nitric Oxide (NO) has been identified to play an important role in inter-cellular communication in different tissues. In the retina, numerous cells were found to contain Nitric Oxide Synthase (NOS), the enzyme that synthesizes NO from L-arginine. In this study, we examined the modulatory effects of NO upon the distal retina of the turtle under different adaptation conditions, in order to test the suggestion that NO acts as a light adapting signal. Methods: The photoresponses of cone photoreceptors were recorded in the everted eyecup preparation of the turtle Mauremys caspica. Light stimuli of different intensities were applied in the dark-adapted state and during background illumination with different intensities. The intensity-response curve was fitted to a hyperbolic function in order to derive the maximal amplitude (Vmax) and semi-saturation constant (s). Retinal level of NO was modified by adding L-arginine, the substrate for NO synthesis or L-NAME, an inhibitor of NO synthesis. Sodium nitroprusside (SNP) was uesd as an exogenous donor of NO. Results: When a background light was turned on the cone photoreceptors initially hyperpolarized to reach a peak level and then gradually recovered to on intermediate potential. The degree of recovery was increased as the retinal level of NO was raised either by endogenous synthesis or by exogenous application. In a given state of adaptation, raising NO level in the retina caused augmentation of the cone photoresponses, while of the cones while inhibition of NO synthesis induced a decrease of the photoresponses. The effects of L-arginine and SNP were decreased as the level of background illumination was raised, while the effects of L-NAME did not change or even increased. Conclusion: Our findings are consistent with the notion that the rate of NO synthesis in the outer retina of the turtle is directly related to the intensity of ambient. The effects of NO upon the response of the cones to turning on a background light indicate that NO speeds up the rate at which turtle cones adapt to bright backgrounds.
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