Influences of exogenous melatonin on the oocyte growth and oxidative status of ovary during different reproductive phases of an annual cycle in carp Catla catla.

The present study aimed to evaluate antioxidant role of melatonin in determining seasonality of ovarian growth in adult carp Catla catla. Accordingly, an identical regimen of exogenous melatonin administration (100 μg/100 g body weight per day for 15 days) was followed during the preparatory, prespawning, and spawning phases of an annual reproductive cycle. The study did not include postspawning phase, when the ovaries were completely regressed and devoid of any healthy growing follicles. The ovarian response was evaluated by determining relative number of developing oocytes as well as measuring the levels of melatonin, oxidative stress (using malondialdehyde [MDA] as the marker), both enzymatic (superoxide dismutase [SOD], catalase [CAT], glutathione peroxidase [GPx], and glutathione S-transferase [GST]) and nonenzymatic (reduced glutathione [GSH]) antioxidants in the ovarian homogenates. Due to melatonin treatment, oocyte growth was accelerated in the preparatory phase but retarded in the prespawning and spawning phases of annual cycle. Conversely, melatonin administration in each reproductive phase led to a significant reduction of MDA and elevations of SOD, CAT, GPx, GST, GSH, as well as melatonin levels in the ovary. As a result, melatonin titers in the ovary always reported a negative correlation with MDA and a positive correlation with SOD, CAT, GST, GPx, as well as GSH levels. However, melatonin content of ovary and the values of gonosomatic index in melatonin-treated carp displayed a positive correlation in the preparatory phase and a negative correlation in the remaining parts of reproductive cycle. Thus, it seems likely that melatonin by acting as an antioxidant reduces intraovarian oxidative stress throughout the seasons of follicular growth, whereas exogenous melatonin administration exerts progonadal influences during the preparatory phase, but antigonadal effects during the prespawning and spawning phases of reproductive cycle.