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M.L. Woodruff, J. Fan, M.C. Cilluffo, R.K. Crouch, G.L. Fain; Opsin–dependent Activation of Transduction in Mouse Rods . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4630.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To estimate quantitatively the activation of transduction by the apo–protein opsin in mouse rods.Methods: RPE65 knockout mice unable to synthesize 11–cis retinal were reared in complete darkness up to 36 weeks, which causes accumulation of small amounts of 9–cis retinal and its corresponding visual pigment isorhodopsin. Rod responses were recorded conventionally with suction electrodes, and Ca2+ concentrations were measured as previously described. Light sensitivity was compared with pigment concentration in the same mouse by measuring light responses from several rods of one eye and pigment concentration from the retina of the other eye, calculated from the difference spectrum in 20 mM hydroxylamine.Results: Rods from dark–reared Rpe65–/– mice had 5–17% of the WT pigment level, and their light responses had the expected spectral sensitivity of isorhodopsin; however, they were about 70 times less sensitive than could be accounted for by the decrease in pigment concentration, that is the by the loss in quantum catch. This extra desensitization could only have been caused by adaptation produced by opsin activation of the visual cascade. By comparing this desensitization to that produced in normal rods by background light, we estimated that this opsin is of the order of 1–2 x 10–4 as efficient in activating transduction as Rh*. Dark–reared Rpe65–/– rods were less desensitized than light cycle–reared Rpe65–/– rods, had a ∼50% larger photocurrent, and degenerated at a somewhat slower rate. Retinas sectioned after 36 weeks in darkness showed a larger number of photoreceptor nuclei than in light cycle–reared animals, though both had fewer nuclei than light cycle–reared WT retinas. Both also had a smaller outer segment length and Ca2+ concentration, though we detected no difference in Ca2+ between dark–reared and light cycle–reared mutant rods. Conclusions: These experiments provide the first quantitative measurement of opsin–dependent activation in physiologically responding mouse rods. Comparison of opsin activity in this mouse model with that previously determined in salamander indicates that mammalian rod opsin is about 100 times noiser. Because the volume of a mouse rod is smaller by about 100–fold, the same percentage bleach produces about the same desensitization in the two species. This result indicates that bleaching desensitization is not just molecular noise but serves some useful role in the physiology of the photoreceptor.
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