Regulation of proopiomelanocortin messenger ribonucleic acid levels in theovine fetal anterior pituitary in vitro

Parturition in sheep is dependent upon maturation of the fetal hypothalamo- pituitary-adrenocortical (HPA) axis. Anterior pituitary expression of the A CTH precursor, proopiomelanocortin (POMC), increases during the final days of gestation in spite of exponentially increasing fetal plasma cortisol lev els. Lesion of the hypothalamic paraventricular nucleus prevents the late g estation increase in POMC mRNA. The purpose of this study was to examine gl ucocorticoid, corticotropin releasing factor (CRF) and arginine vasopressin (AVP) regulation of POMC mRNA levels in fetal anterior pituitary corticotr opes in vitro and to address potential interactions between glucocorticoids acid neuropeptides in regulating POMC. Anterior pituitaries from fetal she ep at two gestational ages (dGA; 118-125 dGA, n = 9; 140-144 dGA, n = 7) we re enzymatically dispersed. POMC mRNA levels were determined at 24, 48 and 72 h post-dispersion. CRF. AVP and dexamethasone (DEX) regulation of POMC m RNA were determined at 24 and 72 h post-dispersion. The capacity of CRF and AVP to modulate DEX suppression of POMC mRNA levels was also examined. POM C mRNA was elevated at 24 h (P < 0.01) and 48 h (P < 0.05) post-dispersion compared to 0 h (immediately post-dispersion) in 140-144 dGA but not 118-12 5 dGA corticotropes. DEX suppressed POMC mRNA in a dose-dependent manner (w hen administered at 24 h post-dispersion) in the 140-144 dGA anterior pitui tary cells but not 118-125 dGA anterior pituitary cells. Administration of DEX (10 nM) at 0 h prevented the increase in POMC mRNA levels observed at 2 4 h post dispersion in the 140-144 dGA group. Neither CRF nor AVP (administ ered at either 24 or 72 h post-dispersion) altered POMC mRNA levels in eith er 118-125 or 140-144 dGA anterior pituitary cells. Continuous exposure of anterior pituitary cells with either CRF or AVP (50 pM) through 96 h increa sed (P < 0.05) POMC mRNA. No synergistic or additive effects were observed with CRF and AVP. Four hour pretreatment with CRF but not AVP (100 nM at 24 h post-dispersion) attenuated (P < 0.05) DEX suppression of POMC mRNA leve ls in 140-144 dGA corticotropes. In conclusion, our results indicate that d irect glucocorticoid suppression of POMC expression in fetal sheep initiate s between similar to 120 and similar to 140 dGA, coincident with the period of gestation when fetal plasma cortisol is exponentially rising. Further, while short duration exposure of fetal corticotropes to either CRF or AVP h ad no effect on POMC mRNA, CRF appears capable of interfering with glucocor ticoid suppression of POMC mRNA. The latter observation provides a potentia l mechanism via which the fetal PVN may counter rising fetal plasma cortiso l concentrations resulting in the previously observed late gestation increa se in anterior pituitary POMC mRNA. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.