Myers D A, McDonald T J, Dunn T G, Moss G E, Nathanielsz P W
Laboratory for Pregnancy and Newborn Research, New York State College of Veterinary Medicine, Cornell University, Ithaca 14853-6401.
Endocrinology. 1992 Apr;130(4):2167-72. doi: 10.1210/endo.130.4.1547733.
Glucocorticoids act upon the hypothalamic paraventricular nucleus (PVN) and anterior pituitary in a classic negative feedback loop to regulate ACTH biosynthesis and secretion. Evidence exists to indicate that glucocorticoid feedback may be attenuated during late gestation in the sheep fetus to allow the preterm rise in fetal plasma cortisol necessary for parturition in this species. The present studies were undertaken to determine the effect of glucocorticoids placed adjacent to the fetal PVN on messenger RNA (mRNA) for CRH in the PVN and mRNA for POMC in the anterior pituitary during late gestation. We performed our studies at two critical stages during late gestation to determine if gestational age related changes occur in the efficacy of negative feedback regulation of expression of CRH and subsequently POMC. Dexamethasone (DEX) implants were placed bilaterally 2 mm lateral to the fetal PVN at 105 to 107 days gestational age (dGA; group I, n = 4) and 121-123 dGA (group II; n = 4). Gestational-age matched, sham implanted fetuses were used as controls (CONT) for both groups (n = 4 per group). Fetuses were recovered at 126-128 (group I) and 136 dGA (group II). Fetal PVN were isolated by micropunching, and the anterior pituitary was separated from neurointermediate and posterior lobes after necropsy. Total RNA was subjected to Northern analysis using specific complementary DNA probes to CRH and POMC, and specific message was normalized to actin mRNA content in each individual sample. Anterior pituitary POMC mRNA was not different in DEX fetuses compared to CONT for either group I (78 +/- 26% of CONT; mean +/- SEM) or group II (84 +/- 17% of CONT). PVN CRH mRNA content was lower in DEX fetuses in group I (28 +/- 14% of CONT; P less than or equal to 0.01) and group II (65 +/- 12% of CONT; P less than or equal to 0.01). The degree to which DEX suppressed mRNA for CRH was greater in group I compared to group II (P less than or equal to 0.05). We conclude that 1) CRH expression in the PVN of fetal sheep is suppressible by glucocorticoids; 2) suppression can occur directly at the level of the PVN and 3) that the efficacy of negative feedback decreases with increasing gestational age. Furthermore, the lack of effect of hypothalamic administration of DEX on anterior pituitary POMC mRNA indicates that basal expression of POMC in fetal sheep may be independent from support from the PVN at this stage of gestation.
糖皮质激素通过经典的负反馈回路作用于下丘脑室旁核（PVN）和垂体前叶，以调节促肾上腺皮质激素（ACTH）的生物合成和分泌。有证据表明，绵羊胎儿在妊娠后期糖皮质激素反馈可能减弱，以允许该物种分娩所需的胎儿血浆皮质醇在早产时升高。本研究旨在确定在妊娠后期将糖皮质激素置于胎儿PVN附近对PVN中促肾上腺皮质激素释放激素（CRH）的信使核糖核酸（mRNA）以及垂体前叶中阿黑皮素原（POMC）的mRNA的影响。我们在妊娠后期的两个关键阶段进行了研究，以确定与胎龄相关的变化是否发生在CRH以及随后的POMC表达的负反馈调节效能中。在妊娠105至107天胎龄（dGA；I组，n = 4）和121 - 123 dGA（II组；n = 4）时，将地塞米松（DEX）植入物双侧放置在胎儿PVN外侧2 mm处。两组均使用胎龄匹配的假植入胎儿作为对照（CONT，每组n = 4）。在126 - 128天（I组）和136 dGA（II组）时取出胎儿。通过微量打孔分离胎儿PVN，尸检后将垂体前叶与神经中间叶和后叶分离。使用针对CRH和POMC的特异性互补DNA探针进行Northern分析，将每个样本中特异的信使RNA标准化为肌动蛋白mRNA含量。对于I组（为CONT的78 +/- 26%；平均值 +/- 标准误）或II组（为CONT的84 +/- 17%），DEX处理的胎儿垂体前叶POMC mRNA与CONT相比无差异。I组（为CONT的28 +/- 14%；P≤0.01）和II组（为CONT的65 +/- 12%；P≤0.01）中，DEX处理的胎儿PVN中CRH mRNA含量较低。与II组相比，I组中DEX抑制CRH mRNA程度更大（P≤0.05）。我们得出结论：1）糖皮质激素可抑制绵羊胎儿PVN中CRH的表达；2）抑制可直接发生在PVN水平；3）负反馈效能随胎龄增加而降低。此外，下丘脑给予DEX对垂体前叶POMC mRNA无影响，表明在妊娠这个阶段，绵羊胎儿垂体前叶POMC的基础表达可能独立于PVN的支持。
