Antioxidant and antiapoptotic effect of melatonin on cat vitrified oocytes

The development of cryopreservation protocols for feline gametes is a crucial part of conservation efforts. Preservation of oocytes is particularly challenging, since they often degenerate after warming or develop into embryos poorly. Two of the main causes are thought to be oxidative stress and apoptotic cell death. Thus, melatonin was employed in this study for its well-known antioxidant and antiapoptotic properties. After determining suitable concentrations of melatonin (Experiment I), the oxidative status (Experiment II), apoptotic status (Experiment III), and developmental competence (Experiment IV) of cat immature oocytes following vitrification and/or in vitro maturation (IVM) with the supplementation of melatonin were assessed. In Experiment I, 10-9 M was the lowest melatonin concentration with favorable maturation of fresh oocytes (58.06%, p=0.6 vs. control, 51.48%). In Experiment II, oocytes vitrified with melatonin showed decreased oxidative stress after IVM, close to negative control (p=0.849). In Experiment III, the addition of melatonin to vitrification-warming and IVM medium hindered apoptotic signaling by reducing caspase activity (p=0.9 vs. fresh control oocytes) and enhancing maturation rates (48.39% vs. 12.12% in control vitrified oocytes; p=0.002). In Experiment IV, melatonin brought maturation rates of treated vitrified oocytes close to those of fresh oocytes (46.38±10.68% and 68.42±10.88%, respectively, p=0.145), but there were no differences in embryo development or morphological quality (cleavage rates: melatonin-treated oocytes: 33.29±17.86%; control vitrified oocytes: 31.59±21.63%; p=0.992). In summary, the use of the antioxidant and antiapoptotic melatonin during vitrification-warming and IVM mitigated oxidative and apoptotic stress in vitrified oocytes, though its impact on developmental competence remained limited.