| Summary: | Melatonin can be found in all types of organisms, from multicellular to unicellular. It is widely known for its role in circadian system synchrony, including reproductive cycle control. This molecule is multifunctional, contributing to various cellular mechanisms involving cell homeostasis and oxidative damage protection. Thus, melatonin potential in sperm biology has gain attention as it could improve spermatozoa performance. Although in other vertebrates, melatonin supplementation has exhibited great improvements, in seawater fish species, there are no reports concerning its effects. Therefore, in a first experiment, the effects of melatonin in sperm motility using different concentrations and exposure times were evaluated in two cultured fish species with reproductive dysfunctions, the European eel, and the Senegalese sole. This enabled to better understand the cellular mechanisms by which melatonin can take part in spermatozoa motility. Regarding the European eel, there were no differences in sperm motility among different melatonin concentrations or exposure times, which may be related to MeOH tolerance in this species sperm. In the Senegalese sole, the sperm motility was significantly lower throughout melatonin exposure time suggesting that sperm quality decrease could be allied with DMSO toxicity. However, in 0.1 and 10 mM concentrations, there was a slight increase in spermatozoa motility. Although the motility descriptors were not significantly different, this improvement could be related with melatonin ability to freely cross the spermatozoa membrane cell. A second experiment was conducted to explore melatonin protective effect during sperm cryopreservation in Senegalese sole, following a described protocol for this species as a control versus two melatonin supplementations (0.1 and 10 mM). The post-thawed sperm was evaluated for motility, viability, DNA fragmentation, lipid peroxidation, ROS, and apoptosis detection. The 10 mM melatonin supplement exhibited significantly lower spermatozoa viability ...
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