Sprague-Dawley rats 10-12 weeks of age were entrained to a standard light-dark cycle with lights turned on at 6 am and off at 6 pm. Variations of 24-hour aqueous humor protein concentration were determined. Samples were taken every 4 hours (N = 10-14) under the standard light-dark condition at 8 pm, midnight, 4 am, 8 am, noon, and 4 pm. Under an acute constant dark condition, when lights were not turned on at 6 am, samples were collected at 8 am, noon, 4 pm, and 8 pm. Aqueous humor protein concentrations under the standard light-dark condition were found in the range of 0.305 ± 0.115 mg/ml (mean ± SD, N = 10) at midnight to 1.505 ± 0.342 mg/ml (N = 14) at noon. The 3 lightphase protein concentrations were each higher than the 3 dark-phase concentrations. Aqueous humor protein concentrations at 8 am, noon, and 4 pm under the acute constant dark condition were each higher than the concentrations at 8 pm (after both 2 hours and 26 hours in the dark), midnight, and 4 am, demonstrating an endogenously driven 24-hour pattern. At 8 am, noon, and 4 pm, protein concentrations were 56-147% higher when exposed to light. Intraocular pressure (IOP) was monitored using telemetry in separate groups of light-dark entrained rats under the standard lightdark condition and the acute constant dark condition. The 24-hour IOP pattern was inverse to the 24-hour pattern of aqueous humor protein concentration under the standard light-dark condition, and this IOP pattern was not altered by the acute constant dark condition. In conclusion, an endogenously driven 24-hour variation of aqueous humor protein concentration occurred in Sprague-Dawley rats with higher concentrations during the light-phase than the dark-phase. This endogenous pattern of protein concentration was accentuated by a direct effect of light, which was unrelated to the 24-hour pattern of IOP.

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