Melatonin stimulates the secretion of progesterone along with the expression of cholesterol side-chain cleavage enzyme (P450scc) and steroidogenic acute regulatory protein (StAR) in corpus luteum of pregnant sows.
The direct effect of melatonin on porcine luteal function during the pregnancy remains unknown. The objective of the study was to analyse the molecular mechanism(s) by which melatonin directly affects progesterone (P4) production in the corpus luteum (CL) of pregnant sows. We evaluated the localization of melatonin membrane receptors (MT1 and MT2) in CL, and investigated the effect of melatonin on P4 secretion along with the expression of P4 synthesis intermediates in luteal cells. Immunohistochemistry analysis showed that MT1 and MT2 were predominantly localized in luteal cells in pregnant luteal tissues. The results of our in vitro experiments showed that melatonin from 5 to 625 pg/mL was able to significantly increase P4 release (P < 0.05) in a dose-dependent manner. And at the dose of 125 pg/mL treatment, the time-dependent effect on P4 secretion was observed. Furthermore, melatonin from 5 to 625 pg/mL up-regulated both P450scc and StAR expression (P < 0.05) in a dose-dependent manner, and the effect was also time-dependent. No difference of 3β hydroxysteroid dehydrogenase (3β-HSD) expression was observed between control and treatment groups. In addition, melatonin induced a dose- and time-dependent promotion on cell viability. Additionally, the stimulatory effects of melatonin were blocked by luzindole, a non-selective MT1 and MT2 receptor antagonist, or partially blocked by a selective MT2 ligand, 4-phenyl-2-propionamidotetralin (4P-PDOT). The data support the presence of MT1 and MT2 in porcine CL and a regulatory role for melatonin in luteal function through MT1 and MT2-mediated signal transduction pathways.