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

Glucocorticoids act upon the hypothalamic paraventricular nucleus (PVN ) and anterior pituitary in a classic negative feedback loop to regula te ACTH biosynthesis and secretion. Evidence exists to indicate that g lucocorticoid feedback may be attenuated during late gestation in the sheep fetus to allow the preterm rise in fetal plasma cortisol necessa ry for parturition in theis species. The present studies were undertak en to determine the effect of glucocorticoids placed adjacent to the f etal PVN on messenger RNA (mRNA) for CRH in the PVN and mRNA for POMC in the anterior pituitary during late gestation. We performed our stud ies at two critical stages during late gestation to determine if gesta tional 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 we re 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 separat ed from neurointermediate and posterior lobes after necropsy. Total RN A was subjected to Northern analysis using specific complementary DNA probes to CRH and POMC, and specific message was normalized to actin m RNA content in each individual sample. Anterior pituitary POMC mRNA wa s 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 CO NT; 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 f or CRH was greater in group I compared to group II (P less-than-or-equ al-to 0.05). We conclude that 1) CRH expression in the PVN of fetal sh eep is suppressible by glucocorticoids; 2) suppression can occur direc tly at the level of the PVN and 3) that the efficacy of negative feedb ack decreases with increasing gestational age. Furthermore, the lack o f effect of hypothalamic administration of DEX on anterior pituitary P OMC mRNA indicates that basal expression of POMC in fetal sheep may be independent from support from the PVN at this stage of gestation.