Placental-derived corticotropin-releasing hormone (CRH) seems to play a major role in the mechanisms controlling human pregnancy and parturition. It has been suggested that CRH directly modulates the endocrine function of placental trophoblasts, including the production of estrogen, ACTH, and prostaglandin. In this study, we sought to investigate the effect of CRH, locally produced by placenta, on progesterone production. Percoll-purified placental trophoblasts were obtained from uncomplicated term pregnancies and cultured for 72 h. Progesterone concentration in culture media was measured by RIA. The mRNA transcripts encoding CYP11A1 and HSD3B1, the enzymes for progesterone synthesis, were determined by quantitative real-time reverse transcription (RT)-PCR. Results showed that CRH (10(-8)-10(-6) mol/l) caused a significant decrease in progesterone levels in a dose-dependent manner. The CRH antagonist, alpha-helical CRH 9-41, at 10(-7)-10(-5) mol/l stimulated progesterone secretion. Consistent with this thesis, CRH decreased, whereas alpha-helical CRH increased, the mRNA levels of CYP11A1 and HSD3B1. Since CRH has been shown to activate the phospholipase C-protein kinase C (PKC) signal pathway in placenta, we examined whether the effect of CRH on progesterone synthesis was dependent on PKC signal pathway. Treatment of cells with PKC inhibitor, Gö6976, resulted in a significant increase in progesterone production, and exogenous CRH restored progesterone production. In conclusion, placental CRH exhibits a tonic inhibitory effect on progesterone production in a PKC-dependent fashion.