Melatonin in the physiology and pathology of transparent structures and media of the eye (cornea, lens, and intraocular fluid)

The organ of vision is the most important means of understanding the external world. Up to 90% of all afferent information enters the brain through this analyzer. The eye, along with other sensory organs, functions as a device for perceiving signals that are subsequently transmitted to the brain. The visual system consists of a peripheral component — the eyeball with its light-perceiving apparatus, the optic nerve, and central components — the lateral geniculate body and the visual cortex. Naturally, the normal functioning of the components of this system ensures a number of biologically important effects, adequate perception of the environment, and a high quality of life. The condition of the transparent structures and media of the eye (cornea, lens, and intraocular fluid) is of great importance for the functioning of the visual apparatus, since the light stream is conducted through them to the retina. These structures are primarily exposed to adverse environmental factors (toxic agents, ultraviolet radiation, etc.), which results in the development of pathological conditions that cause disturbances in light conduction. Both surgical and conservative treatment methods are used to correct them, but these methods are not always successful. The indole compound melatonin has recently been considered one of the alternative and effective treatments for ophthalmic pathology and has been attracting the attention of specialists in various fields of medicine for more than 60 years since its discovery. This is because melatonin has a wide range of valuable pharmacological properties, including anti-inflammatory, antioxidant, and immunotropic effects, as well as the ability to influence the functioning of the cardiovascular system, the gastrointestinal tract, and exert a number of other effects. Melatonin is produced in the pineal gland and enters peripheral tissues via the bloodstream, where it exerts its properties. However, indole synthesis also occurs in peripheral tissues, where this extraepiphyseal melatonin exhibits its effects. Melatonin is intensively synthesized in various structures of the eye, especially in the retina, where a full set of enzymes is present for the synthesis of indole and for specific melatonin receptors. Such information has led to the use of melatonin in the treatment of various ophthalmic pathologies.