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A. Polosa, W. Liu, S. Goyer, P. Lachapelle; Melanin Does Protect From Light-Induced Retinopathy, But Only to a Certain Extent. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5591.
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© ARVO (1962-2015); The Authors (2016-present)
Albino Sprague Dawley (SD) rats exposed to a bright light develop a severe retinopathy, the hallmark of which being a region of near complete destruction located in the supero-temporal quadrant at a position where the macula would be if rats had one (Polosa et al., ARVO 2009). We investigated if a similar region exists in pigmented Long-Evans (LE) rats known to be more resistant to light damage.
Juvenile (J; n=12) and adult (A; n=12) LE rats were exposed to a bright cyclic light (10 000lux) from postnatal day 14-28 (JLE) or for 6 consecutive days (ALE). Scotopic (-6.3 to 0.6 log cd.sec.m-2) and photopic ( 0.9 log cd.sec.m-2; background: 30 cd.m-2) ERGs and retinal histology were performed at days 1, 3, 10 and 31 post light exposure.
ERGs were significantly attenuated (p<.05) in ALE rats, with only partial recovery [scotopic b-wave: (D1: 11%, D3: 21%, D10: 21% and D31: 46% of control)]. Full recovery was noticed in the JLE rats [scotopic b-wave: (D1: 56%, D3: 108%, D10: 129% and D31: 97% of control)]. Photopic ERGs yielded similarly results. In ALE, the outer retina was severely affected (RPE, OS and IS destroyed and only 1 row of ONL cells) in the same region (supero-temporal quadrant) as that previously reported in SD rats. The extent of this region increased (p.<05) from 736±66µm at D1 to 1602±208µm at D31 and was accompanied by a similar (p<05) growth in the melanin deprived area adjacent to it from 995±260 µm at D1 to 1643±151µm at D31 (normal LE rats: 775±107µm). In the JLE retinas, despite a non-significant loss of melanin [D1: 798±209 and D31: 987±207µm] no significant alteration of the retinal structure was seen.
Our results confirm that the structural and functional consequences of LIR are significantly less severe in LE compared to SD rats and are significantly less severe in JLE compared to ALE rats. As in SD rats, the LIR of LE rats targets the same supero-temporal quadrant of the retina, a region normally devoid of melanin pigment in LE. Given that at D1 the ALE show an area of ONL destruction (736 µm) smaller than the corresponding melanin devoid area (995 µm) but nearly the same size as that of normal rats (775 µm) and by D31, both are of nearly equal size (1643 vs 1602µm) strongly suggests that light initially destroy the pigmented RPE and as a result of the loss in melanin protection, the adjacent ONL is also destroyed. Notwithstanding the latter claim, one must also postulate an increase in sensitivity to light damage for the photoreceptors found in the supero-temporal retina as this is also where the most severe consequences of LIR are found in the albino SD rats.
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