Melatonin Protects Bovine Spermatozoa by Reinforcing Their Antioxidant Defenses

Cryopreserved semen is widely used in assisted reproductive techniques. Post-thawing spermatozoa endure oxidative stress due to the high levels of reactive oxygen and nitrogen species, which are produced during the freezing/thawing process, and the depletion of antioxidants. To counteract this depletion, supplementation of sperm preparation medium with antioxidants has been widely applied. Melatonin is a hormone with diverse biological roles and a potent antioxidant, with an ameliorative effect on spermatozoa. In the present study, we assessed the effect of melatonin on thawed bovine spermatozoa during their handling. Cryopreserved bovine spermatozoa were thawed and incubated for 60 min in the presence or absence of 100 μΜ melatonin. Also, the effect of melatonin was assessed on spermatozoa further challenged by the addition of 100 μΜ hydrogen peroxide. Spermatozoa were evaluated in terms of kinematic parameters (CASA), viability (trypan blue staining) and antioxidant capacity (glutathione and NBT assay, determination of iNOS levels by Western blot analysis). In the presence of melatonin, spermatozoa presented better kinematic parameters, as the percentage of motile and rapid spermatozoa was higher in the melatonin group. They also presented higher viability and antioxidant status, as determined by the increased cellular glutathione levels and the decreased iNOS protein levels.