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670nm Protects against Retinopathy of Prematurity Figure 6. Number of retinal haemorrhages in the rat oxygen induced retinopathy animals. The number of haemorrhages was reduced from 2.3 per retina in the OIR animals to 0.4 in the 670nm+OIR animals. The 670nm+OIR was not statistically significant from controls, but was from OIR animals (* indicates a p-value < 0.05). OIR – Oxygen Induced Retinopathy. Graph bar fills indicate OIR (light grey) and 670nm+OIR (white). doi: 10.1371/journal.pone.0072135.g006 alterations in normal development. When administered in conjunction with an OIR paradigm, the present study shows that in both the mouse OIR paradigm, and the rat OIR paradigm, 670nm red light treatment (i) reduces vaso- obliteration in the retinal vasculature; (ii) reduces neovascularization and retinal haemorrhage; (iii) preserves retinal vascular branching architecture, and (iv) reduces the incidence of neural cell death. Furthermore, we find that treatment of OIR rats with 670nm red light reduces the incidence of lung pathology to levels that are comparable with controls. The animal models used in this study are established models for mimicking the sort of pathologies seen in ROP (reviewed in 23), with strengths and weaknesses to both models. The differences between the mouse and rat OIR models include differences in the patterns of vaso-obliteration, such that in the mouse OIR model vaso-obliterations occur centrally, around the optic disc [27], while in rats the vaso-obliteration occurs in the periphery [28], as they do in human infants. The mouse model, however, is better suited to quantifying changes in levels of neovascularisation [27]. The retina of albino neonates (reviewed in 29 also respond differently to fluctuations of oxygen [30] and have developmental mutations, such as the inability to regulate retinal cell cycle [31]. Despite these differences we observed a similar and significant reduction in the severity of vaso-obliteration in both rat and mouse OIR animals treated with 670nm red light. Figure 7. Animal weight (A), length (B), mortality rate (C) and organ weight (D) taken daily in animals exposed to 670nm compared to controls in normal atmospheric oxygen. (A) From P12 there was a statistically significant (* indicated p<0.05) deviation from normal weight gain with animals gaining weight at an increased rate. (B) There were no noticeable differences in the length of the animals over the 21 day period. (C) There was no change in the weight of the organs investigated except for the lungs which showed a significant increase in weight (* indicates p < 0.05). (D) 670nm red light appears to reduce the mortality rate from 27% in controls (n=27) to 15% in the 670nm (n=21) exposed animals (* indicates p<0.05). All animals were raised in normal atmospheric oxygen. Littermates were used to reduce variability across litters. Graph bar fills indicate controls (grey bars), 670nm treated (white bars). doi: 10.1371/journal.pone.0072135.g007 ROP involves vaso-obliteration and neovascularisation, and in the worst cases results in neural damage, retinal detachment and blindness [6,32]. The hyperoxic phase of OIR results from inspired oxygen being administered to neonates in order to maintain normal oxygen saturations in infants affected by lung disease of prematurity. Although, inspired oxygen therapy does not account for all occurrences of ROP, it is widely accepted as one of the key factors in the pathogenesis of the disease [2,33]. This makes the OIR models useful for studying changes in retinal pathology due to exposure to increased (mouse) or fluctuating (rat) oxygen levels. Our results suggest that the severity of the hypoxic phase of ROP can be reduced, by maintaining a normal vessel development during the hyperoxic phase, thus reducing the impact of hypoxia on return to room air. It is unclear how normal development of the retinal vasculature is maintained by 670nm red light in these models of OIR. This is largely because the precise mechanism underling the effect of LLLT is largely unknown. The most widely accepted hypothesis is that cytochrome c oxidase is the photoacceptor for 670nm red light [20,22,34,35], and promotes an increase in oxidative metabolism (reviewed in 36). Thus it is suggested that 670nm red light acts to promote mitochondrial function, increasing oxygen usage, reducing oxidative damage, PLOS ONE | www.plosone.org 6 August 2013 | Volume 8 | Issue 8 | e72135PDF Image | 670nm Photobiomodulation as a Novel Protection to Retinopathy
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