The role of melatonin as an antioxidant in the follicle

Reactive oxygen species (ROS) are formed continuously in cells as a consequence of both oxidative biochemical reactions and external factors. While physiological levels of oxygen are necessary for cells to live, ROS such as superoxide radical (O2-), hydroxyl radical (·OH), and hydrogen peroxide (H2O2) are generated from oxygen. ROS can regulate cell function by controlling the production and activation of substances that have biological activities and by activating key downstream cell-signaling pathways [1-3]. However, surplus generation of ROS interact with lipid, protein and nucleic acid, resulting in a loss of membrane integrity, structural or functional changes in proteins, and damage in nucleic acids. Therefore, an increase in the production of ROS have detrimental effects on cell function and contributes significantly to several diseases, including those that may compromise reproduction and fertility [4].ROS are produced within the follicle, especially during the ovulatory process. ROS play a physiological role in the process of ovulation, e.g. follicle rupture. However, an excessive amount of ROS cause oxidative stress and may damage oocyte and granulosa cells. Accumulating data have shown that ROS accelarate oocyte aging and deteriorate oocyte quality [5,6]. On the other hand, antioxidant defense systems, such as superoxide dismutase (SOD) or glutathione (GSH), are present in follicles. The balance between ROS and antioxidants within the follicle seems to be critical to the function of oocyte and granulosa cells.Melatonin, a hormone mainly synthesized in the pineal gland, has multiple effects on a number of different physiological processes. Melatonin plays a key role in a variety of important physiological functions, including circadian rhythms [7], reproductive [8], neuroendocrine [9], cardiovascular [10], neuroimmunological [11], oncostatic actions [12]. We already reported that melatonin plays a role in lipid metabolism [13], pregnancy and parturition ti