Effect of placement of dexamethasone adjacent to the ovine fetal paraventricular nucleus on adrenocortical steroid hydroxylase messenger ribonucleic acid

The preterm rise in the concentration of ovine fetal plasma cortisol that initiates the events of parturition in sheep commences at approximately 125 days of gestational age (dGA; term = -147 dGA). Concurrent with the rise in fetal plasma cortisol, adrenocortical steroid hydroxylase enzyme activity increases. The purpose of this study was 1) to quantitate changes in levels of mRNA for the steroid hydroxylases, 17-hydroxylase cytochrome P450 (P450_17α), side-chain cleavage cytochrome P450 (P450_scc), 11β-hydxoxylase cytochrome P450 (P450_11β), and C21-hydroxylase cytochrome P450 (P450_C21); and 2) examine the role of the fetal paraventricular nucleus (PVN) in the onset of adrenocortical steroid hydroxylase mRNA expression. Unperturbed fetuses were collected by cesarian section under halo- thane anesthesia at 105 (n = 4), 120 (n = 4), 126-128 (n = 4), and 136 dGA (n = 3); neonatal animals were collected within 2 h after birth. To examine the role of the fetal PVN in regulation of adrenocortical steroid hydroxylase mRNA expression, ovine fetuses were stereotaxi- cally implanted bilaterally 2 mm lateral to the fetal PVN at 105-107 dGA with either cholesterol (n = 4) or dexamethasone (DEX; n = 3). Implanted fetuses were collected by cesarian section under halothane anesthesia at 126-128 dGA. mRNA for both cytochrome P450_17α and P450_scc declined 3-fold from 105 to 120 dGA (P < 0.05), and then increased by 126-128 dGA compared to that on 120 dGA (P < 0.05) and continued to increase through term. Cytochrome P450_C21 increased at 126-128 dGA compared to that on 105 and 120 dGA (P < 0.05) and remained elevated through term. Three distinct transcripts [-6.2,4.2, and 2.5 kilobases (kb)] were observed for cytochrome P450_11β; the 4.2-kb transcript was predominant. While total message for P450_11β declined over increasing gestational age, no differences were noted for the 4.2-kb transcript until after birth, when levels significantly declined (P < 0.025). Placement of DEX adjacent to the fetal PVN prevented reemergence of expression of mRNA for P450_17α and P450_scc at 126-128 dGA, but had no effect on mRNA for P450C21 or P45011β. We conclude that mRNA for P450_17α and P450_scc undergoes a decline in expression concurrent with the previously described period of adrenal hyporesponsiveness from 105-126 dGA, followed by an increase in mRNA that accompanies the preterm rise in fetal plasma cortisol. Since DEX placed adjacent to the PVN prevents the 126-128 dGA reemergence of expression of cytochrome P450 steroid hydroxylase mRNA, we hypothesize that normal function of the fetal PVN is necessary for the increase in mRNA for fetal adrenocortical steroid hydroxylase at this period of gestation.