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O. Dkhissi–Benyahya, W. De Vanssay, F. Flamant, H.M. Cooper; Role of M–Cones and Melanopsin in Circadian Photoreception . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2981.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In order to dissect out the role of different photoreceptors in clock mechanisms, we used a knockout mouse Trß1–/ß2– which is characterized by an absence of MW cones coupled with an over–expression of SW opsin. Methods: Quantification of Opsin content (MW, SW opsins, rhodopsin and melanopsin) was performed using real–time PCR in wild–type and KO mouse. Entrainment to light and light–induced phase shifts was assessed by monitoring locomotor activity in both groups of mice. Photic entrainment was assayed using 3 levels of irradiances (1, 10 and 100 lux) combined with a 6 hour shift of the light–dark cycle. To measure light–induced phase shift, mice were exposed to two set of experiments 1) 15 min of monochromatic light at three wavelengths (360, 480 and 530 nm) and 4 irradiance levels (1010 to 1014photons/cm2/s) and 2) stimulation with 480 nm– light at one irradiance level (1014photons /cm2/s) and 3 durations (1s, 5 and 15 min). Results: Immunohistochemical labelling using specific antibodies against SW and MW opsins demonstrates the absence of MW cones and the over–expression of SW cones in KO mouse. Real time PCR confirms this result and also shows over–expression of melanopsin in the KO mouse. The rhodopsin content is similar in both strains.Knockout mice entrain to a light/dark cycle with reduced sensitivity compared to wild type mice. The KO mice show a robust phase–shift after a light pulse at 360 and 480nm, however, the magnitude of the response at 530 nm is significantly reduced compared to wild type mice. Light–stimulation (480 nm) at different durations shows that KO mouse present a reduced sensitivity of the system compared to wild–type mouse. Conclusions: These results demonstrate for the first time that M cones and melanopsin contribute significantly whereas rods play a minor role in the photic signal that is transmitted to the circadian clock in the SCN.
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