Regulation of retinal photoreceptor phagocytosis in a diurnal mammal by circadian clocks and ambient lighting.

Objective: To characterize light and circadian control of photoreceptor phagocytosis in a diurnal cone-rich rodent.

Methods: Diurnal Arvicanthis ansorgei were maintained under standard cyclic light (12 hours 300 lux white light [L]/12 hours dark [D]) and were divided into five groups: 1, maintained in LD; 2, transferred to constant darkness (DD); 3, transferred to constant light (LL, 300 lux); 4, subjected to 6-hour advance in light onset; and 5, subjected to 6-hour delay in light onset. Animals were killed every 3 hours during 24 hours, and their eyes were rapidly enucleated and fixed. Cryosections were stained using specific rod (rhodopsin) and cone (MW cone opsin) antibodies. Immunopositive inclusions within the retinal pigment epithelium layer were quantified for each time point.

Results: In LD, both rod and cone phagocytosis showed coincident synchronized profiles with sharp peaks 1 to 2 hours after light onset. In groups 2 and 3, phagocytic activity shifted partially to the new light schedules. In DD, the temporal phagocytic profile resembled largely that of LD. On the contrary, LL animals exhibited large alterations in both rod and cone phagocytosis. There was no longer any peak, with both showing relatively uniform profiles. In addition, the number of cone phagosomes was much higher ( approximately 250% increase) compared with LD or DD.

Conclusions: These data are the first to measure photoreceptor phagocytosis in a diurnal mammal under different lighting conditions and to highlight the disruptive effects of constant light, especially on cone photoreceptor function.