Light, dark, and melatonin: emerging evidence for the importance of melatonin in ocular physiology

Abstract

Melatonin is a hormone, which is mainly produced by the pineal gland, a vestigial eye. Rather than the rods and cones, it is a newly discovered subgroup of photosensitive retinal ganglion cells, which is responsible for mediating the light-dark cycles, thus regulating melatonin's secretion. One of the correlates of the circadian rhythm of melatonin release is the habitual sleep pattern. Patients with circadian rhythm sleep disorders, including some blind patients with no light-induced suppression of melatonin, benefit from melatonin treatment. Melatonin is synthesized in the retina, lens, ciliary body as well as other parts of the body. In this review, we discuss the physiological roles of melatonin in the eye, as well as the potential therapeutic avenues currently under study.

Relative effectiveness of a blue light–filtering intraocular lens for photoentrainment of the circadian rhythm

“…These circadian rhythms are synchronized by the endogenous master circadian rhythm created by a pacemaker located in the suprachiasmatic nucleus of the hypothalamic region of the brain. 1,2 This master circadian rhythm, referred to in this paper as the circadian rhythm, is free running and is synchronized by external cues called Zeitgebers (from the German for ''time givers''). Any compromise in this synchronization may have a significant effect on various health aspects of patients including sleep patterns, mood, seasonal affective disorder, and other more serious diseases that are still under investigation.…”

Reactions of the melatonin metaboliteN1-acetyl-5-methoxykynuramine (AMK) with the tyrosine side-chain fragment, 4-ethylphenol

“…Nevertheless, an eventual modification of crystallins by AMK cannot be entirely ruled out. Melatonin was reported to be formed in lens, ciliary body and retina, 38,39 findings which are, however, uncertain for primates because of the poor expression of hydroxyindole O-methyltransferase in their eyes. 40,41 Nevertheless, melatonin could be taken up from the circulation, especially after pharmacological administration.…”

Improving Melatonin Delivery Within the Eye

“…It exists in almost every organism starting from the primitive ones as prokaryotes to the very complex organisms as the humans. Melatonin is known as a circadian rhythm regulator [2]. This indoleamide is a neurohormone considered classically to be secreted by the pineal gland, nonetheless, it is currently known to be synthesized in other organs and tissues such as the retina and cerebellum [3,4], iris, ciliary body [5], crystalline lens [6,7], Harderian gland [8] and the lacrimal gland [9], spleen, heart, skeletal muscle, liver, stomach, gut, placenta, testes, ovaries, cerebral cortex and striatum [10].…”