Melatonin, Circadian Rhythms, and Nocturnal IOP Dynamics

This audio article is from VisualFieldTest.com.Read the full article here: https://visualfieldtest.com/en/melatonin-circadian-rhythms-and-nocturnal-iop-dynamicsTest your visual field online: https://visualfieldtest.comExcerpt:Melatonin and the Eye: Nighttime IOP and NeuroprotectionMelatonin is a neurohormone produced in a ~24-hour cycle (circadian rhythm) that plays key roles in sleep regulation and acts as a powerful antioxidant. In the eye, melatonin is synthesized locally (in the retina and ciliary body) and binds to MT1/MT2 melatonin receptors on ocular cells (). Its levels peak at night, coinciding with the normal drop in blood pressure and (in healthy individuals) the typical reduction in intraocular pressure (IOP) during sleep. These circadian patterns mean melatonin helps modulate aqueous humor (the watery fluid filling the front of the eye) dynamics. In turn, this affects nighttime IOP and retinal health, especially in aging. Recent studies suggest that impaired melatonin signaling may contribute to glaucoma risk, while melatonin analogs (drugs that mimic melatonin) show promise in lowering IOP and protecting retinal neurons () ().Ocular Melatonin and Circadian ControlMelatonin is not only made by the pineal gland but also produced in the eye itself. Photoreceptors in the retina generate melatonin at night, and the ciliary body (the gland that produces aqueous humor) also synthesizes melatonin and releases it into the aqueous () (). This means melatonin levels in the aqueous humor rise in darkness, peaking around midnight to 2–4 AM (). By contrast, light exposure (especially blue light) suppresses melatonin via melanopsin-containing retinal ganglion cells. Thus, melatonin is a bridge between circadian signals (day–night) and intraocular physiology. Receptors for melatonin (MT1, MT2 and possibly MT3) are found on cells of the eye, including the non-pigmented ciliary epithelial cells that secrete aqueous humor (). Activation of these receptors influences cellular pathways (via G-proteins) that control ion transport and fluid secretion. In simple terms, melatonin engagement tends to slow aqueous humor production, helping lower IOP. Conversely, loss of normal melatonin signaling (as may happen in glaucoma or with aging) can lead to higher nighttime IOP. For example, mice lacking the MT1 receptor have higher nocturnal IOP and suffer more retinal ganglion cell (RGC) loss () (). Similarly, human glaucoma patients often secrete abnormally timed melatonin due to damage of light-sensitive retinal cells, suggesting a chicken-and-egg problem: glaucoma may disturb circadian rhythms, and disrupted melatonin may worsen glaucoma () ().Melatonin in Aqueous Humor DynamicsThe generation and drainage of aqueous humor set eye pressure. Melatonin influences both sides of this balance. As noted, melatonin slows aqueous production by ciliary epithelial cells via MT1/MT2 receptor signaling (which lowers cAMP inside the cells) () (). Experiments in animals show melatonin analogs reduce IOP dramatically. For instance, the MT3 agonist 5-MCA-NAT produced a 43% IOP drop in rabbits (versus 24% by melatonin itself) (). In glaucoma-model monkeys, 5-MCA-NAT lowered IOP steadily over days, with effects lasting >18 hours (). Similarly, the MT2 agonist IIK7 and other analogs have shown significant pressure-lowering in animals. This suggests multiple melatonin receptors (especially MT3) mediate IOP control () (). In addition to reducing production, melatonin may help increase aqueous outflow. It modulates ion channels (e.g. chloride transport) and enzymes in the ciliary body. One study found melatonin boosted Cl⁻ transport in porcine ciliary cells, affecting fluid secretion (). Another showed a melatonin analog dSupport the show