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Celeste R. Quinones, Andrew T. Hartwick; Action Spectrum for Light-Induced Suppression of Nocturnal Intraocular Pressure in Rats. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2432.
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Intraocular pressure (IOP) exhibits a predictable circadian rhythm in mammals, with peak levels occurring during the night. Previous studies on rabbits have shown that this nocturnal elevation in IOP can be acutely suppressed by exposure to light. The aims of this study are to construct an action spectrum for light-induced IOP suppression in rats, and to use the resulting spectral sensitivity curve to establish the retinal photoreceptor that mediates this effect.
Adult male Brown Norway rats were maintained in a controlled 12 h light: 12 h dark cycle. First, IOP was measured at 6 time-points spaced throughout both the light and dark cycle using a TonoLab rebound tonometer (n=16). Second, rats were exposed to white light (250 and 2500 lux) or maintained in the dark for 30 minutes (n=10 each group) during the animals’ night-time (1 to 3 hours after lights out). IOP was measured before and after the light or dark exposure. Third, rats were exposed to monochromatic light (420, 450, 480, 510, 540, 570 and 600 nm) at different irradiances (1011 to 10 15 photons/s/cm2) for 30 minutes (n=6-8 each group) and the light-induced change in IOP was determined.
The intraocular pressure in the rats exhibited a circadian rhythm, with the peak level occurring 1 hour after lights off (19.6 ± 1.0 mmHg). After 30 min exposure to white light (250 and 2500 lux), there was an acute decrease in IOP (by 4.7 ± 1.3 mmHg and 6.7 ± 1.0 mmHg, respectively) that was greater than the animals restrained in the dark for 30 minutes (2.3 ± 1.2 mmHg decrease). Short-wavelength light was shown to be more effective than long-wavelength light at suppressing the nocturnal rise in IOP. For example, exposure to bright 480 nm light (1014 phots/s/cm2) induced a 6.7 ± 1.5 mmHg decrease, while exposure to bright 600 nm light (1014 phots/s/cm2) induced a 3.0 ± 1.3 mmHg decrease in IOP.
There is a circadian rhythm for IOP in rats, and the nocturnal elevation in IOP can be acutely suppressed by light exposure. Our results suggest that short-wavelength light is a potent stimulus for this effect. The effectiveness of blue light is consistent with the potential involvement of melanopsin-expressing retinal ganglion cells that are intrinsically photosensitive.
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