Analysis of Circadian Rhythm Gene Expression With Reference to Diurnal Pattern of Intraocular Pressure in Mice.

Elevated intraocular pressure (IOP) is a strong risk factor for primary open-angle glaucoma (POAG), and current standard of care focuses on the reduction in IOP as the primary modality to reduce optic nerve damage and prevent vision loss.1 Characterization of molecular regulators of IOP would help identify novel molecules that can be used as therapeutic targets for the treatment of POAG. Intraocular pressure is the result of a balance between secretion and outflow of aqueous humor, a colorless fluid in the anterior segment of the eye.2 Three variables are of interest in this balance: the rate of aqueous humor formation, the resistance to outflow, and the episcleral venous pressure.3 The rate of formation and resistance to outflow are the primary targets for current drug therapy aimed at lowering IOP. The ciliary body is responsible for aqueous humor production, primarily via active secretion from its nonpigmented epithelium.4 Aqueous humor outflow is mediated through the trabecular meshwork (conventional outflow) and the uveoscleral region (uveoscleral outflow).5 The trabecular meshwork is responsible for approximately 75% of outflow resistance in humans and mice.6,7 Topical medications currently in use effectively lower IOP by decreasing ciliary body production of aqueous humor and/or increasing removal of aqueous humor from the anterior segment of the eye through the trabecular meshwork and uveoscleral pathway. Aqueous humor secretion is known to vary in a circadian manner, decreasing by up to 50% during sleep.8 Epinephrine may play a role in this change, but the mechanism behind decreased secretion remains poorly understood.9 Outflow facility also decreases slightly at night.10,11 However, the decrease in outflow does not appear to be significant enough to compensate for the decrease in aqueous secretion, and the cause of this decrease in outflow remains unknown.10 While the decrease in aqueous humor secretion and the slight decrease in aqueous humor outflow suggest a nocturnal normality or slight decrease in IOP, multiple studies have repeatedly demonstrated a sinusoidal pattern of IOP that peaks at night when animals are maintained on a 12-hour light–dark cycle.12–,16 Interestingly, IOP is higher at night irrespective of diurnal or nocturnal habits of the animals, which suggests that IOP is regulated by light intensity and not by the level of activity.17 This circadian pattern of IOP is abolished in the absence of a distinct light–dark cycle.17,18 Previous studies to determine the cause of this pattern have demonstrated that impairments in the sympathetic nervous system abolish the nocturnal rise in IOP, implicating a role for adrenergic stimulation in the circadian pattern.19,20 Additionally, exposure to short-wavelength light during the circadian dark phase eliminates the nocturnal IOP rise.21 Melatonin, which is synthesized by the ciliary epithelium in a circadian pattern with peak levels at night, has also been suggested to have an involvement in aqueous humor secretion from the nonpigmented epithelium.22–,24 While the molecular mechanism for the diurnal variation in IOP is not yet known, these results suggest that the presence or absence of light may be a trigger for the molecular regulators of IOP. To examine this, we measured the diurnal variation in IOP of C57BL/6 wild-type mice and correlated this with clock gene expression in the iris–ciliary body complex.