Effects And Mechanisms Of Different Monochromatic Lights On Eye Growth And Defocus-induced Myopia In Guinea Pigs

Part I Effects of430nm and530nm monochromatic light on refractive development and retinal cones in guinea pigsPURPOSE To investigate into the responses of guinea pigs’ eyes to the alteration of longitudinal chromatic aberration and the change of the overcompensation induced by monochromatic light when the lighting is recovered to normal, and to investigate the effects of different monochromatic light on retinal-cones distribution and ultrastructure in guinea pigs.METHODS Fifty pigmented guinea pigs (two weeks old) were randomly distributed to five groups:short-wavelength (SL), short-wavelength-recuperation (SLR), middle-wavelength (ML), middle-wavelength-recuperation (MLR) and broad-band group (BL)(10in each group). The SL, ML and BL groups were raised for20weeks under LED lighting of430nm,530nm and broad-band light respectively. The SLR and MLR groups were first reared under monochromatic light for10weeks, and then were shifted into broad-band light for another10weeks. The lighting conditions were set at equal quantum number. All animals underwent biometric and refractive measurements every2weeks. Another eighteen guinea pigs were randomly distributed to blue-light group (BL), green-light group (GL) and white-light group (WL)(n=6for each group). The illuminative parameters of all groups were identical and the light quantum number was3×10μmol·cm-2·s-1. At the end of10-weeks observation, the right eyes of each group were used for retinal-cones immunocytochemistry, and the left eyes of each group were checked by transmission electron microscope.RESULTS The eyes in the ML group developed toward myopia (P<0.001) accompanied by a higher speed of vitreous extension (P<0.001), but the eyes in the SL group appeared hyperopia (P<0.001) with a slower vitreous elongation (P<0.001) when compared with the BL group. From week0to10, the refraction and eye growth in the MLR group were similar for every time-point with the ML group (p>0.05). And it was same for the SLR and SL groups. But after recuperation the refraction of the MLR group increased suddenly (about0.6D, p=0.001) and the vitreous length decreased significantly (about0.07mm,p=0.0315) from week10to12, but the refraction of the MLR group was still more myopic (p=0.008) than the BL group in the end. However, in the SLR group the refraction decreased sharply (about1.3D, p<0.0001) and the vitreous length increased significantly (about0.08, p=0.0076) in two weeks, but at the end of the experiment the eyes of the SLR group were still more hyperopic (p<0.001) than the BL group. The result of retinal-cones immunocytochemistry indicated that, the density of M-cone in ventral retina of GL group increased (P<0.01), while the M-cone density in dorsal retina of BL group decreased (P<0.05). The S-cone density of BL retina increased significantly (P<0.05) in all the three zones, dorsal, central, and ventral. But, the density of S-cone in GL group decreased with significant difference (P<0.01) in all over the three zones.CONCLUSIONS The disparities of refraction and vitreous length forming under different monochromatic light couldn’t be totally eliminated under broad-band light because of the reservation of overcompensation. It was implied that the process of emmetropization was blocked when eyes were under the condition of short wave-length light and a promotional effect of the middle wave-length light on refraction occurred, which may result in overcompensation in guinea pigs. The effect of longitudinal chromatic aberration caused by illuminative change occurred quickly, which was probably mediated by choroid thickness modification.430nm and530nm monochromatic light could influence the density of retinal-cones distribution and the ultrastructures of cones. It was indicated that all of the different cones may play a role in the development of this ametropia induced by monochromatic light. Part II Effects of the chromatic defocus caused by interchange of two monochromatic lights on refraction and ocular dimension in guinea pigsPURPOSE To investigate refractive and axial responses to the shift of focal plane resulting from the interchange of two monochromatic lights separately corresponding to the peak wavelengths of the cones absorption spectrum in retina.METHODS Fifty2-week-old pigmented guinea pigs were randomly assigned to five groups based on the mode of illumination:short-wavelength light (SL), middle-wavelength light (ML) and broad-band white light (BL) for20weeks, SL for10weeks followed by ML for10weeks (STM), as well as ML for10weeks followed by SL for10weeks (MTS). Biometric and refractive measurements were then performed every2weeks. The illuminative parameters of all groups were identical and the light quantum number was3×10-4μmol·cm-2·s-1.RESULTS The SL and the STM groups became more hyperopic (p<0.001) and had less vitreous elongation (p<0.001) than the BL group after10weeks. However, the ML and the MTS groups became more myopic (p<0.001) and had more vitreous elongation (p<0.01) relative to the BL group after a10-week illumination. After the monochromatic light interchange, the refraction decreased rapidly by about1.93D (p<0.001) and the vitreous length increased by0.14mm (p<0.001) in the STM group from week10to12. After then, there were no significant differences (p>0.05) between the STM and the BL groups in refraction and vitreous length. The interchange from ML to SL quickly increased the refraction by about1.53D (p<0.001) and decreased the vitreous length by about0.13mm (p<0.001) in the MTS group after two weeks. At this time, there were also no significant differences (p>0.05) between the MTS and the BL groups in the two measurements. Because of the following increase in refraction, the MTS group was more hyperopic (about1.0D, p=0.001) but not shorter in vitreous length (p>0.05) than the BL group after10weeks in SL CONCLUSIONS The guinea pig eye can accurately detect the shift in focal plane caused by interchange of two monochromatic lights and rapidly generate refractive and axial responses. However, an excessive compensation was induced. Some properties of photoreceptors or retina may be changed by the monochromatic light to influence the following refractive development. Part III Effects of430nm monochromatic light on defocus-induced myopia in guinea pigsPURPOSE To investigate the effects of430nm monochromatic light on defocus-induced myopia in guinea pigs.METHODS Eighteen2-week-old pigmented guinea pigs were randomly assigned to two groups based on the mode of illumination:short-wavelength light (SL) for8weeks and broad-band white light (BL) for8weeks. All animals of the two groups were worn-5D lenses on right eye. Biometric and refractive measurements were then performed every2weeks. The illuminative parameters of all groups were identical and the light quantum number was3×10-4μmol·cm-2·s-1.RESULTS After the beginning of the experiment, the right eyes of the two groups decreased in refraction. At the end of the experiment, relative myopia of the right eye was about2.72D in the SL and about3.03D in the BL when compared with the fellow eye. But a relative hyperopia, about1.2D, was induced in the SL compared with the BL group in the end. From4to8week, there was significant difference in radius of corneal curvature between the two eyes of the SL group. But, there was no significant difference in corneal curvature between the two eyes of the BL group. At the end of the experiment, there was significant difference in radius of corneal curvature between the right eyes of the two groups. The difference was not significant in vitreous length of right eye between the two groups from beginning to the end of experiment. There was no significant difference in vitreous length between the two eyes of the SL group in the end. But finally, significant difference existed in vitreous length between the two eyes of the BL group. There were no significant inter-group or intra-group differences in length of anterior segment and Lens thickness.CONCLUSIONS430nm monochromatic light could interfere with the development of defocus-induced myopia in guinea pigs. The effect of the monochromatic light may be achieved by influencing the developments of vitreous chamber and corneal curvature. The recognition of defocus under monochromatic light may be achieved by the function from only one type of cone.