Yellow filter effect on melatonin secretion in the eye: Role in IOP regulation.

PURPOSE: Melatonin is a neurohormone mainly synthesized in the pineal gland; however, it is also present in the aqueous humor. One of melatonins' functions in the eye is the regulation of intraocular pressure. Melatonin is known to be sensitive to light. Recently, the photopigment which controls melatonin synthesis, melanopsin, was found in the crystalline lens. Therefore, light conditions is an interesting possible way of regulating melatonin levels in the aqueous humor. The current study used yellow-filters, since melanopsin is activated by short-wavelength (blue light).

METHODS: New Zealand white rabbits were used, divided in two groups, one under controlled 12 h light/dark cycles, while the rest had their cages encased with a yellow filter (λ 465-480). IOP measurements were taken every week at the same time before they were anesthetized for aqueous humour extraction.

RESULTS: Keeping the rabbits under the yellow filter resulted in a decrease in IOP up to 43.8 ± 7.8% after 3 weeks. This effect was reversed after the topical application of selective and non-selective melatonin receptors antgonists, 4PPDOT and luzindole. Also, blocking melanopsin by its antagonist AA92593 under white light condition decreased IOP. Finally, melatonin levels were found significantly higher in the aqueous humor of rabbits developed under yellow filter compared to controls (37.4 ± 4.2 ng/ml and 15.3 ± 3.1 ng/ml, respectively).

CONCLUSION: Yellow filters modulate melatonin levels in the aqueous humour due to deactivating melanopsin activity. This effect leaded to a decrease in IOP mediated by melatonin receptors.