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C. Altimus, A. D. Guler, S. Hattar; Individual Contribution of Rods and Cones to Circadian Photoentrainment. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3239.
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Three photoreceptors in the mammalian retina account for light detection for both image formation and light dependent physiological functions. Rods and cones, located in the outer segment of the retina, detect light information for image formation and send this information on to the brain via retinal ganglion cells. A subtype of retinal ganglion cells not only receive rod/cone input but are themselves intrinsically photosensitive because they express a photopigment, melanopsin. These melanopsin containing intrinsically photosensitive retinal ganglion cells (ipRGCs) are necessary for photoentrainment and normal pupil constriction. Recent studies have conclusively demonstrated that rods/cones contribute to circadian photoentrainment through the ipRGCs. However, the relative contribution of rods and cones to circadian photoentrainment is at best controversial. Here we sought to determine the role of each photoreceptor for circadian photoentrainment.
We used animals that are modified to contain only functional rods or functional cones with minimum retinal degeneration. We utilized wheel running activity system to determine the ability of these animals to photoentrain to light.
We have shown that mice lacking functional rods and melanopsin protein (cone only animals), weakly photoentrain to high light intensity and are unable to photoentrain at low light intensities (below 1 lux). These results are in disagreement with photoentrainment models in the circadian field that predicted a major contribution for cones in photoentrainment. Surprisingly, we found that mice lacking functional cones and melanopsin protein (rod only animals) also have difficulty photoentraining at most light intensities. However, at low light intensities the rod only animals were able to photoentrain. We then used short pulses of light followed by 25 minutes of darkness during the light portion of a 12:12 light dark cycle to allow rods to dark adapt between pulses of light and not to be constantly bleached. With this paradigm, animals that have only functional rod were also able to photoentrain to light.
These studies show that both rods and cones contribute to photoentrainment and that their involvement is dependent on the light environment.
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