Circadian and light-driven regulation of rod dark adaptation

The sensation of light is initiated in photoreceptor cells by the photoisomerization of a chromophore molecule from 11-cis to all-trans retinal. Continuous visual perception requires recycling of the spent chromophore back to the 11-cis form through the visual cycle, a series of reactions in the retinal pigmented epithelium (RPE). Light-driven chromophore consumption by photoreceptors is greater in daytime compared to night time, suggesting that correspondingly higher activity of the visual cycle may be required. On the other hand, as rod photoreceptors are saturated in bright light, the continuous turnover of their chromophore through the visual cycle during daytime would unnecessarily utilize precious energy and produce toxic byproducts. Here, we sought to determine whether the recycling of chromophore and the dark adaptation of rods is regulated by the circadian clock and light exposure. We demonstrate that in melatonin-proficient C3H/f+/+ mice, rod dark adaptation is slower during the day or after light exposure. This surprising daytime downregulation of the RPE visual cycle was further demonstrated by gene analysis, which revealed light-driven reduction in the expression of Rpe65, which encodes a key enzyme of the RPE visual cycle. In contrast, rods in melatonin-deficient strains (C57BL6/J and 129/Sv) were not affected by this daily visual cycle modulation. Our results demonstrate that the circadian clock and light exposure regulate the recycling of chromophore in the RPE visual cycle. This daily modulation of rod dark adaptation is mediated by melatonin and could potentially protect the retina from light-induced damage during the day. mRNA-seq of murine eyes (lens removed) in objective day (OD) vs. subjective day (SD) conditions (2 biological replicates per condition). Each biological replicate consisted of 4 eyes (from 1 female and 1 male).