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Yunlu Xue, Vladimir Kefalov; Seeking For Circadian Rhythmicity in the Mammalian Photoreceptor Visual Cycle. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3425.
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© ARVO (1962-2015); The Authors (2016-present)
The circadian clock regulates many functions of the retina, such as photoreceptor disc shedding, melatonin synthesis and the rod-cone coupling. However, it is unclear whether the circadian clock controls the photoreceptor visual cycle in mammals. We examined this question by measuring rod dark-adaptation using scotopic Electroretinogram (ERG) recordings from C57BL/6 mice.
The animals we used for the study were 6-12 weeks old wild type (C57BL/6) mice. We used ERG to assess their photoreceptor responses. The animals were dark-adapted over night, anesthetized with Ketamine/Xylazine cocktail, and stabilized for 10 minutes in darkness before the experiment. We recorded the dark-adapted scotopic sensitivity and maximal response before exposing the animals’ eyes to brief bright light, which bleached 90% of the photopigment. We then monitored the dark-adaption of ERG responses, including a-wave sensitivity, and maximal scotopic a- and b-wave amplitudes. Four Zeitgeber time (ZT) points were taken to perform the dark-adaptation test: ZT0, ZT6, ZT 12 and ZT18. At least six retinas for each tested time were used for analysis.
We did not observe any significant difference in the rate of dark-adaptation among the four tested time points (i.e. ZT0, ZT6, ZT 12 and ZT18). The maximal a-wave amplitude recovered to 70% of its pre-bleach level at steady-state with a half-recovery time of about 30 minutes. The maximal b-wave amplitude recovered to 60% at steady-state with a half-recovery time of about 20 minutes. The a-wave sensitivity recovered to 30% at steady state with a half-recovery time of about 30 minutes.
We did not observe any diurnal variation of rod photoreceptor dark-adaptation in the melatonin-deficient C57BL/6 mice. Further studies of rod and cone dark-adaptation in melatonin-sufficient mice are under way.
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