Paraquat (PQ), a broad-spectrum agricultural pesticide, causes cellular toxicity by increasing oxidative stress levels in various biological systems, including the reproductive system. PQ exposure causes embryotoxicity and reduces the developmental abilities of embryos. However, there is little information regarding the toxic effects of PQ on oocyte maturation. In this study, we studied the toxic effects of PQ exposure and the effects of melatonin on PQ-induced damage in bovine oocytes. PQ exposure disrupted nuclear and cytoplasmic maturation, which was manifested as decreased cumulus cell expansion, reduced first polar body extrusion, and abnormal distribution patterns of cortical granules and mitochondria. In addition, PQ treatment severely disrupted the ability of the resulted in vitro-produced embryos to develop to the blastocyst stage. Moreover, PQ exposure significantly increased the intracellular reactive oxygen species (ROS) level and early apoptotic rate, and decreased the glutathione (GSH) level, antioxidative CAT and GPx4 mRNA, and apoptotic-related Bcl-2/Bax mRNA ratio. These results indicated that PQ causes reproductive toxicity in bovine oocytes. Melatonin application resulted in significant protection against the toxic effects of PQ in PQ-exposed oocytes. The mechanisms underlying the role of melatonin included the inhibition of PQ-induced p38 mitogen-activated protein kinase (MAPK) activation, and restoration of abnormal trimethyl-histone H3 lysine 4 (H3K4me3) and trimethyl-histone H3 lysine 9 (H3K9me3) levels. These results reveal that melatonin serves as a powerful agent against experimental PQ-induced toxicity during bovine oocyte maturation and could form a basis for further studies to develop therapeutic strategies against PQ poisoning. K E Y W O R D Sbovine, melatonin, oocyte maturation, oxidative damage, paraquat 2 of 15 | PANG et Al.

To establish a foundation for further researches on the improvement of polymorphonuclear neutrophil leukocytes (PMN) functions in dairy cow during perinatal period, the counting of PMN, as well as the mRNA and protein expression of toll-like receptor-4 (TLR-4) on PMN was studied during this critical period. Blood samples were taken 21, 14 and 7 days, and at calving (0) day before expected calving time, and 7, 14 and 21 days after calving. The PMN changes were measured by automatic blood cell analyzer, and mRNA and protein expression of TLR-4 were analyzed by quantity real-time PCR (RT-PCR) and western blot. The results show that the quantity of neutrophil leukocytes reached the peak (3.12 ± 0.26 × 10 9 , p<0.05) at 0 day. The mRNA expression of TLR-4 was down-regulated from the -21days before calving to the 14 day after calving (P<0.01). The protein of expression TLR-4 was lower from 7 to 14 days. The down-regulation of TLR-4 expression may be the major factor of PMN dysfunction of cows from 7 to 14 days after calving.