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C. Bobu, M.-P. Felder-Schmittbuhl, M. Masson-Pévet, D. Hicks; Rhythmicity of Visual Pigment Synthesis and Photoreceptor Phagocytosis in the Diurnal Rodent Arvicanthis ansorgei. Invest. Ophthalmol. Vis. Sci. 2008;49(13):157.
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The daily renewal of photoreceptors (PR) constitutes a essential process for the structure, function and survival of retina. This renewal consists of synthetic (transcription of specific mRNA’s, protein synthesis), transport [insertion of new membranes in the outer segment (OS)], and catabolic stages [the phagocytosis of OS debris by retinal pigment epithelium (RPE)]. Each process is cyclic and coordinated, and errors in different aspects (alterations in expression levels of visual pigment, perturbation of phagocytosis) cause retinal degenerations in humans and animals. Molecular mechanisms implicated in renewal are known principally for rods, whereas the cones constitute the most important population of PR for high acuity, chromatic vision. Conventional animal models (rats, mouse), being nocturnal, are very poor in cones. We used Arvicanthis ansorgei possessing a cone-rich retina (33 %) to establish the pattern of two important retinal processes: the phagocytosis of cone PR and the synthesis of visual pigments in different lighting conditions.
Arvicanthis Rhodopsin, MW Opsin, SW Opsin, Cone Arrestin and Beta Actin cDNAs were cloned by RT-PCR using selected primers. Retinas were dissected from adult animals (n=6) every 4 hrs in standard light/dark (12h/12h) cycles, and total RNA isolated. We then quantified the respective transcript levels by qRT-PCR-SYBR GREEN.For the quantification of phagocytosis, we employed a published immunohistochemical double immunolabeling technique (Bobu et al., 2006, IOVS 47: 3109-18) on retinal cryosections. Primary antibodies were: anti-rhodopsin monoclonal (rho-4D2); anti-mouse red/green (MW) cone opsin polyclonal.
We observed rythmicity of synthesis of all visual pigments and cone arrestin in adult Arvicanthis ansorgei retinas under a 24h light/dark cycle. Amplitude of cycles varied from 0.2 to 2. For all RNAs, a maximum peak of synthesis was seen at the light /dark transition. Quantification of phagocytosis showed a circadian rythmicity under dark/dark conditions for both cone and rod shedding, with maximal values 1-2h after onset of subjective day for both cell types.
Cyclic variations in visual pigment synthesis show similar profiles for nocturnal (mouse) and diurnal (Arvicanthis) rodents. Rod and cone phagocytosis occur with similar profiles (overlapping maxima) in the diurnal rodent. These data demonstrate that Arvicanthis represents an interesting animal model for investigating cone cellular physiology.
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