Effect of implantation of dexamethasone adjacent to the paraventricular nucleus on messenger ribonucleic acid for corticotropin-releasing hormone and proopiomelanocortin during late gestation in fetal sheep

Abstract

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.