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J. J. Hejkal, M. Zhao, L. Stapp, T. Rudebush, C. B. Camras, C. B. Toris; Circadian Rhythms of Aqueous Humor Dynamics in the Rabbit. Invest. Ophthalmol. Vis. Sci. 2009;50(13):810.
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Humans exhibit a higher intraocular pressure (IOP) and aqueous flow during the day versus night, as well as decreased corneal thickness and increased outflow facility. Rabbits, as nocturnal animals, also experience a circadian rhythm of IOP and aqueous flow which is about 12 hours out of phase with humans. This study examines the circadian variations in corneal thickness and outflow facility in the rabbit.
Ten male New Zealand white (NZW) rabbits (each 3.4-4.6 kg) were entrained to a 12-hour light-dark cycle (lights on 0600-1800). IOP, central corneal thickness, and outflow facility were measured in both eyes of each awake rabbit in the daytime (1145-1500 hrs) and at night (2300-0200 hrs) by pneumatonometry, ultrasound pachymetry, and tonography, respectively. Results were analyzed using the Student’s two-tailed, paired t-test.
Compared with daytime, IOP significantly (p<0.001) increased at night from 14.9±1.8 mm Hg (mean ± standard deviation) to 21.0±1.8 mm Hg. Central corneal thickness significantly (p<0.001) decreased at night from 384±23 to 348±18 microns. Outflow facility increased from 0.19±0.06 in the daytime to 0.34±0.25 ul/min/mm Hg at night.
Like humans, NZW rabbits exhibit circadian variations in IOP, corneal thickness, and outflow facility. In contrast to the rhythm in humans, these variations are approximately 12 hours out of phase in the rabbit. The rabbit represents a useful model to study diurnal and nocturnal fluctuations in aqueous humor dynamics.
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