| Summary: | Melatonin enhances the quality and in vitro maturation (IVM) of oocytes under heat stress (HS), but the mechanism of melatonin in reducing HS injury on oocytes is not fully understood. In this study, porcine cumulus-oocyte complexes (COCs) were randomly divided into three groups. The COCs of the control group were cultured at 38.5 °C for 42 h, and the COCs of the HS group were cultured at 41.5 °C for 4 h, and then transferred into 38.5 °C for 38 h. The COCs of the HS + melatonin group were cultured with 10<sup>−9</sup> M melatonin under the same conditions as the HS group. The survival rate, maturation rate, distribution of α-tubulin and F-actin of the oocytes were assessed. In addition, the expression profiles for genes related to the oocyte maturation, including heat shock protein 70 (<i>HSP70</i>), nuclear factor erythroid 2-related factor 2 (<i>N</i><i>RF2</i>), cyclin-dependent kinase 1 (<i>CDK1</i>), growth differentiation factor 9 (<i>GDF9</i>) were analyzed by real-time quantitative PCR. The results showed that HS decreased the survival rate and maturation rate, distribution of α-tubulin and F-actin, but melatonin treatment could partly counteract these adverse effects. In addition, HS increased expression of <i>HSP70</i> and <i>NRF2</i> mRNA, and melatonin treatment had a similar effect on <i>HSP70</i> expression, but had a contrary effect on <i>NRF2</i> expression. Furthermore, HS inhibited expression of <i>CDK1</i> and <i>GDF9</i> mRNA, but melatonin treatment could weaken the effect on <i>GDF9</i> expression induced by HS. In summary, melatonin treatment could attenuate the unfavorable effects induced by HS to enhance developmental competence of porcine oocytes during IVM.
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