Fetal Hypothalamic-pituitary-adrenal Axis Research Articles

A Blood Test to Predict Preterm Birth: Don’t Mess with Maternal-Fetal Stress

The timely onset of labor and birth is a critical determinant of perinatal outcome. The mean duration of human pregnancy is 40 wk (280 d) from the first day of the last normal menstrual period. Both preterm birth (PTB; defined as delivery before 37 wk gestation) and postterm pregnancy (delivery after 42 wk) are associated with adverse perinatal outcome. In developed countries, PTB complicates 7–12% of all deliveries but accounts for over 70% of perinatal morbidity and mortality (1). Although the incidence of PTB has continued to rise every year for the past three decades, perinatal survival and intact survival have increased steadily during this same time period due in large part to the appropriate and timely use of antenatal corticosteroids and to improvements in neonatal resuscitation and infant care. For these reasons, early and accurate identification of women at risk of PTB is critical to optimize perinatal outcome. Early identification of women at risk will become even more important if recent reports on the use of progesterone supplementation to prevent PTB can be replicated (2–4). Obstetric care providers have traditionally relied on clinical symptoms (regular uterine contractions, pelvic pressure) to identify women at risk of spontaneous PTB. Numerous studies have now shown that reliance on clinical symptoms alone is inaccurate and unreliable. Such an approach will fail to identify the majority of womenwhodeliverpreterm(5), and, contrary topopularbelief, the presence of symptoms is not associated with an increased risk of spontaneous PTB (6, 7). For these reasons, extensive efforts have been made to develop objective tests to identify women at risk of PTB. Two such tests are currently recommended by the American College of Obstetricians and Gynecologists (ACOG) for the prediction of PTB, namely cervicovaginal fetal fibronectin (fFN) and measurement of cervical length by transvaginal ultrasound (8). A number of other predictive tests are under investigation, the most promising of which are measurements of steroid and stress hormones in the maternal circulation. The biological plausibility behind the use of maternal serum stress and sex steroid hormone levels to predict PTB is well established. Considerable evidence suggests that the hormonal milieu of the fetoplacental unit is in control of the timing of labor, although maternal and genetic factors are also involved (9–11). It is likely that a “parturition cascade” exists at term that is responsible for the removal of mechanisms maintaining uterine quiescence and the recruitment of factors acting to promote uterine activity. Although a detailed review of the proposed cascade is beyond the scope of this editorial, it is clear that endocrine factors are involved. As in other mammalian viviparous species, the final common pathway toward parturition in the human appears to be maturation and activation of the fetal hypothalamic-pituitary-adrenal (HPA) axis, with an increase in the major adrenal glucocorticoid product in the fetal circulation in late gestation (cortisol in the sheep and human; corticosterone in the rat and mouse). The cellular and molecular factors responsible for the maturation of the fetal HPA axis are incompletely understood, but are associated with the gestational age-dependent up-regulation of a number of critical genes, most importantly fetoplacental CRH. Levels of CRH in the maternal circulation increase from 10–100 pg/ml in nonpregnant women to 500–3,000 pg/ml in the third trimester of pregnancy (12). The source of this excess CRH is the placenta. Shortly before the onset of labor, both at term and preterm, CRH levels increase and hepatic-derived CRH-binding protein levels decrease, resulting in a marked increase in free (biologically active) CRH (13). CRH, in turn, acts via specific nuclear receptors in the myometrium, placenta, and fetal membranes to differentially regulate target genes. Among its effects on term uterine tissues, CRH promotes myometrial contractility by stimulating prostaglandin production from the decidua and fetal membranes (14) and potentiates the contractile effects of oxytocin and prostaglandins on the myometrium (15). At term, CRH also directly stimulates the production of the C-19 steroid, dehydroepiandrostenedione sulfate, from the intermediate (fetal) zone of the fetal adrenal glands (16, 17). Dehydroepiandrostenedione sulfate is then 16-hydroxylated in the fetal liver and converted to estriol

Fetal stress. Focus on “Effects of acute acidemia on the fetal cardiovascular defense to acute hypoxemia” by Thakor and Giussani

what constitutes fetal stress, and what are the implications of fetal stress in the course of fetal development? Thakor and Giussani ([17][1]) address a fundamental component of the fetal stress response: acidemia and it role as a modulator of the hypothalamus-pituitary-adrenal (HPA) axis

The Endocrinology of Equine Parturition

Delivery of viable young requires co-ordination of fetal maturation with the onset of labour at term. In turn, this depends on a cascade of fetal and maternal endocrine events. The sequence of these events is broadly similar in most mammals but there are differences in placental hormone synthesis and in the timing and magnitude of key prepartum endocrine changes between species. In most farm animals, maternal progesterone (P4) levels decline and oestrogen levels increase in the last 5 - 10 days before delivery in response to activation of the fetal hypothalamicpituitary-adrenal (HPA) axis and increased fetal cortisol concentrations. This cortisol surge is also responsible for fetal maturation and increasing uteroplacental prostaglandin (PG) synthesis. In the mare, there is little, if any, P4 in the maternal plasma during late gestation and both progestagens and oestrogens are produced by a feto-placental unit which uses precursors supplied by the fetus to synthesise a range of C21 and C18 steroids, many of which are unique to the horse. Regulation of uterine quiescence and activation is, therefore, complex in the mare near term. Indeed, total progestagen concentrations rise and total oestrogen levels fall in the mare during the last 20 - 30 days of gestation and only show the changes typical of impending parturition in other species in the last 24 - 48 h before delivery. Fetal cortisol concentrations also rise late in gestation in the horse compared to other species. In common with other species, the prepartum endocrine cascade appears to begin in the fetal horse with activation of the fetal HPA axis but, initially, the primary product of the fetal equine adrenal appears to be pregnenolone (P5) and not cortisol. This leads to increased progestagen production by the uteroplacental tissues, which maintains uterine quiescence in the face of increasing uterine stretch caused by the rapidly growing fetus. Very close to term in association with increasing fetal ACTH levels, the fetal equine adrenals appear to switch to producing cortisol. This late cortisol surge induces a period of rapid fetal maturation and may also contribute to increased uteroplacental oestradiol-17 beta and PG production. The fall in P5 availability may reduce uteroplacental progestagen production and lift the block on myometrial contractility. Finally, increased PG secretion activates myometrial contractions, which stimulate oxytocin release via a neuroendocrine reflex. The endocrine regulation of equine parturition, therefore, involves progestagens, oestrogens, PGs and oxytocin as in other species. However, further studies are required to establish the causes and consequences of the rise and fall in maternal progestagens and the extent to which initiation of equine labour depends on the fetal HPA axis.

Effects of Twinning and Periconceptional Undernutrition on Late-Gestation Hypothalamic-Pituitary-Adrenal Axis Function in Ovine Pregnancy

The relationships between reduced size at birth, increased activity of the hypothalamic-pituitary-adrenal (HPA) axis, and increased risk of disease in adulthood are well described in singletons but are much less clear in twins. This may be because the physiological processes underlying reduced size at birth are different in singletons and twins. Periconceptional undernutrition can cause altered activity of the fetal and postnatal HPA axis without altering size at birth. However, the independent effects of periconceptional undernutrition and twinning on activity of the maternal and fetal HPA axes are not well described. We therefore studied maternal and fetal HPA axis function during late gestation in twin and singleton sheep pregnancies, either undernourished around conception or fed ad libitum. We found that twinning led to suppressed baseline HPA axis function and decreased adrenal sensitivity to ACTH stimulation but increased fetal pituitary ACTH response both to direct stimulation by CRH (ACTH area under the curve response: 29.7 +/- 2.2 vs. 17.1 +/- 1.6 ng/min x ml, P < 0.01) and to decreased cortisol negative feedback. In contrast, periconceptional undernutrition resulted in a decreased pituitary response (ACTH area under the curve response: 19.4 +/- 1.6 vs. 26.1 +/- 2.2 ng/min x ml, P = 0.02) but no difference in adrenal response. Thus, the HPA axis function of twin sheep fetuses in late gestation is very different from that of control and undernourished singletons. If the HPA axis is an important mediator between fetal adaptations and adult disease, these data may help explain why the relationship between fetal growth and postnatal physiology and disease risk is inconsistent in twins.

Blockade of Estrogen Receptors Decreases CNS and Pituitary Prostaglandin Synthase Expression in Fetal Sheep

Background/Aims: Both prostaglandin E₂ (PGE₂) and estradiol stimulate fetal ACTH secretion and augment fetal ACTH responses to stress. We have reported that estradiol increases prostaglandin endoperoxide synthase-2 (PGHS-2), and we have proposed that there is a positive feedback relationship between estrogen and fetal hypothalamus-pituitary-adrenal (HPA) axis activity that is dependent upon PGHS activity in the fetal brain. The present study was designed to test the hypothesis that blockade of estrogen receptors in the fetal brain decrease PGHS-2 expression and reduces fetal HPA axis activity. Methods: In study 1, six time-dated pregnant ewes with chronically-catheterized twin fetuses were used. In each pregnancy, one twin was treated intracerebroventricularly (icv) with the estrogen receptor antagonist ICI 182,780 (25 µg/day; n = 6) while the other twin served as an age-matched control. In study 2, plasma samples were drawn from 10 singleton chronically-catheterized fetuses on alternating days until the time of spontaneous parturition. Results: ICI infusion caused significantly decreased PGHS-2 mRNA abundance in fetal central nervous system and pituitary, with the greatest decreases occurring in hippocampus and pituitary. There were no statistically significant changes in PGHS-1 mRNA. ICI infusion did not significantly change fetal plasma concentrations of pro-opiomelanocortin (POMC), ACTH, or cortisol in fetuses 130–134 days ges- tation (study 1) but did decrease the preparturient rise in plasma pro-opiomelanocortin concentrations in study 2. Conclusion: We conclude that PGHS-2 expression in the late-gestation fetal brain is in part stimulated by circulating estrogens in fetal plasma. Blockade of CNS estrogen receptors reduces preparturient plasma concentrations of POMC, but does not reduce fetal HPA axis activity in 130–134 day fetal sheep.

Intra-amniotic inflammation activates the fetal hypothalamic-pituitary adrenal axis in utero as identified by 3D ultrasound assessment of fetal adrenal gland volume

Intra-amniotic inflammation activates the fetal hypothalamic-pituitary adrenal axis in utero as identified by 3D ultrasound assessment of fetal adrenal gland volume

The effect of tobacco exposure on the fetal hypothalamic–pituitary–adrenal axis

Our objective was to determine if maternal smoking is associated with programming of the fetal hypothalamic-pituitary-adrenal (HPA) axis. Cigarette smoking, which induces a state of hypoxia in the fetus, may promote in utero'programming' of the HPA axis. In utero, adaptations to the HPA axis, which become maladaptive later in life, have been hypothesised to contribute to the development of adult cardiovascular disease and metabolic disorders. This was a prospective cohort study of term infants. The study involved 104 infants born by elective caesarean section, 21 of whom were exposed to in utero tobacco and 83 were nonexposed. Healthy women with healthy pregnancies were recruited if they were undergoing elective caesarean section. Maternal blood was drawn for cortisol and cotinine in the morning, and the umbilical blood was drawn immediately after delivery of the baby. Umbilical arterial cortisol and adrenocorticotropin hormone (ACTH) levels. ACTH levels were significantly elevated in smoke-exposed infants [17 (4-22) pmol/l versus 4 (2-11) pmol/l, respectively, P= 0.005], while cortisol levels were similar [182 (130-240) nmol/l versus 192 (127-265) nmol/l, respectively, P= 0.541]. For the first time, it was shown that infants exposed to in utero tobacco smoke have significantly elevated ACTH levels compared with nonexposed infants. The results of this study warrant further exploration of the effect of smoking on the neonatal HPA axis as a potential set up for 'programming'.

Cyclooxygenase‐1 and ‐2 inhibition alter hypothalamic‐pituitary‐adrenal axis (HPA) activity and COX expression in fetal brain.

In fetal sheep, reflex responses to hypotension are in part mediated by generation of prostaglandins (PG) in the CNS. We have previously demonstratd that inhibition of COX1 reduces the responsiveness of the fetal HPA to hypotension and increases expression of COX2 in specific regions of the fetal CNS. We hypothesized that generation of PG alters the transcription of COX2. The present study was designed to test this hypothesis. Chronically catheterized twin fetal sheep received cerebral ventricular infusions of inhibitor or vehicle. We used either the COX2 inhibitor nimesulide (NIM) at 1mg (HD) or 10 ug/day (LD), or the COX1 inhibitor resveratrol (RES) at 1 mg/day. Blood samples were collected to measure fetal blood gases, proopiomelanocortin (POMC), ACTH, and cortisol. After 5 days, fetal hypothalami and pituitaries were harvested, and COX1 and 2 mRNA were analyzed by real-time PCR. Arterial blood gases were unchanged by HD nimesulide or resveratrol. LD nimesulide significantly decreased pO2 of vehicle treated fetuses. HD NIM significantly reduced hypothalamic COX1 and 2 mRNA, but did not alter pituitary levels. COX1 and 2 were unchanged in the RES and LD NIM groups. Hypothalamic AVP, CRH, and POMC mRNA levels were unchanged in all groups. Plasma ACTH, POMC, and cortisol concentrations were unchanged in the HD NIM group, yet RES and LD NIM treatment significantly reduced plasma cortisol without changing POMC or ACTH. We conclude that blockade of COX2 with HD NIM alters the expression of COX1 and 2 mRNA; and 2) unstimulated fetal ACTH secretion is not altered by blockade of CNS COX isoforms, although adrenal sensitivity to ACTH appears to be influenced by both COX1 and 2 activity in the CNS. Supported by HD42135 and a Predoctoral Fellowship from the American Heart Association.

Does Reduction of Circulating Prostaglandin E Reduce Fetal Hypothalamic-Pituitary-Adrenal Axis Activity?

Placental production of prostaglandin E2 (PGE2) increases in fetal sheep as term approaches. It has been suggested that placental PGE2 may act as a hormone to activate the fetal hypothalamic-pituitary-adrenal (HPA) axis. Alternatively, we have proposed that local generation of prostaglandins in the fetal brain and/or pituitary might play a more prominent role in the stimulation of fetal adrenocorticotropin (ACTH) secretion. We performed the present experiments to test the hypothesis that the elevated concentrations of PGE2 in fetal plasma do not tonically stimulate fetal ACTH secretion. We studied chronically catheterized late-gestation fetal sheep. Selective inhibitors of prostaglandin synthase-1 and -2 (PGHS-1 and PGHS-2) were injected intravenously. Fetal blood pressure and heart rate were monitored continuously, and circulating concentrations of PGE2, ACTH, and cortisol were measured by specific immunoassay. Injection of vehicle did not have an effect on circulating levels of PGE2 or ACTH, but it did have a mild stimulatory effect on cortisol. The selective PGHS-2 inhibitor, Nimesulide (Cayman Chemical, Ann Arbor, MI), significantly decreased plasma PGE2 concentrations. The selective PGHS-1 inhibitor, Resveratrol (Cayman Chemical), produced smaller decreases in plasma PGE2 concentrations and significantly increased mean arterial blood pressure. Neither inhibitor significantly altered plasma ACTH or cortisol concentrations. These results demonstrate that reduction of circulating PGE2 concentrations in response to intravenous injection of PGHS-1 and PGHS-2 inhibitors does not reduce fetal HPA axis activity. We conclude that PGE2 in late-gestation ovine fetal plasma does not tonically stimulate fetal ACTH secretion.

Estrogen/Hypothalamus-Pituitary-Adrenal Axis Interactions in the Fetus: the Interplay Between Placenta and Fetal Brain

The hormonal interactions between the placenta and the fetal hypothalamus-pituitary-adrenal (HPA) axis are reviewed. This review addresses data obtained from the chronically catheterized fetal sheep, drawing relevant comparisons to human fetuses. In the sheep, and perhaps in primate species, parturition is initiated by an increase in the activity of the HPA axis. The endogenous mechanisms underlying the increase in activity of the fetal HPA axis are incompletely understood but might involve an interplay between placenta and fetal hypothalamus and pituitary. Various hypotheses have been proposed, involving placental secretion of prostaglandins and various components of the fetal HPA axis. In the sheep, the influence of estradiol appears to be potent, and various experiments have suggested the possibility that, in late gestation, there exists a positive feedback relationship between placental estrogen secretion and pituitary adrenocorticotropin (ACTH) secretion. Estradiol circulates in concentrations known to stimulate fetal ACTH secretion. Additionally, estradiol circulates in the form of estradiol-3-sulfate, a molecular form that is taken up by the fetal brain and deconjugated by steroid sulfatase, which is expressed in the fetal brain. Recent evidence suggests that the interaction between estradiol and ACTH might involve production of paracrine or autocrine substances in the fetal brain. One candidate mediator is prostaglandin E2 (PGE2), highlighted by the action of estradiol on the expression of prostaglandin endoperoxide synthase-2 (PGHS-2 or COX-2) in brain regions known to be important for controlling HPA activity. Estradiol, secreted by the placenta in increasing amounts in late gestation, is a potent stimulator of fetal ACTH secretion. The interactions between estradiol and the fetal HPA axis might function as a positive feedback loop that increases the concentrations of both hormones before birth.

Serum biomarkers of spontaneous preterm birth

The rationale for the prediction of spontaneous preterm birth (sPTB) is three-fold. First, delineating those factors predictive of PTB, we may obtain a better understanding of the mechanisms and biologic pathways that lead to spontaneous preterm parturition. Second, the use of predictors of sPTB permits the identification of a group of women at the highest risk for whom an intervention may be tested and for whom intervention is most critically needed. The third motivation for the prediction of sPTB is a corollary of the second; by identifying women at low risk of PTB, unnecessary, costly, and sometime hazardous interventions might be avoided. With respect to the source of biomarkers, there are two fundamental compartments of interest: the maternal and the fetal. The maternal compartment may be subdivided into serum, saliva, urine, cervix, and vagina. The fetal compartment may be subdivided into cord blood and amniotic fluid. This brief review will focus solely on the maternal serum compartment. The maternal serum compartment offers several distinct theoretical advantages. Obtaining maternal serum is less invasive than obtaining amniotic fluid or fetal blood. Maternal blood is drawn routinely at several points in time during prenatal care, and thus, a serum biomarker could be incorporated into the usual provision of care. Using maternal serum for biomarkers of PTB also facilitates central processing and analysis. After minimal processing at the point of care, blood specimens may be transported to a central facility for processing and analysis. This model of central processing and analysis has already been successfully incorporated into the aspects of routine prenatal care, such as serum screening for open neural tube defects and serum screening for aneuploidy (1, 2). Serum biomarkers of sPTB may be categorized by using the notion of the syndrome of PTB (3) and the four distinct pathways to PTB forwarded by Lockwood and Kuczynski (4): activation of maternal or fetal hypothalamic–pituitary–adrenal axis (e.g. corticotropin-releasing hormone), upper genital tract infection (e.g. defensins and tumor necrosis factor-α), decidual hemorrhage (e.g. thrombin–anti-thrombin III complex), and pathologic uterine overdistension. To date, no serum biomarkers of pathologic uterine overdistension have been described. Given the known pathophysiologic heterogeneity of sPTB, the clinical utility of any individual biomarker for predicting PTB is limited. To that end, Goldenberg and colleagues sought to evaluate a ‘multiple marker test’ for PTB in The Preterm Prediction Study of the Maternal–Fetal Medicine Units Network of the National Institutes of Child Health and Human Development (5). This was a nested case-control study performed within a cohort of 2929 women with a singleton gestation recruited from the general obstetric population of the 10 participating centers. Women, in this study, underwent serial assessment of serum, cervical, vaginal, ultrasound, and historical markers or risk factors. The serum biomarkers considered in this study have been listed below. The markers associated with sPTB at <32 weeks were α-fetoprotein, alkaline phosphatase, granulocyte-macrophage colony-stimulating factor, and defensins. Only α-fetoprotein and alkaline phosphatase were related to sPTB at <35 weeks. Importantly, the overlap between the biomarkers was small, supporting the notion that there are several heterogeneous pathways to sPTB. In fact, the utilization of the serum markers in concert improved the predictive ability. Any one ‘positive’ serum marker had 81% sensitivity and 78% specificity for sPTB at <32 weeks and 60% sensitivity and 73% specificity for sPTB at <35 weeks. Because this was a nested case-control study, the true positive and negative predictive values are uncertain. No individual serum biomarker of sPTB is useful when used alone in an symptomatic low-risk obstetric population. The notion of a ‘multiple marker’ approach may improve the predictive ability, but no prediction model, to date, provides adequate utility to justify routine clinical use. The future development of an effective prediction model may permit the evaluation of targeted therapies. Future directions for research in this arena will involve the optimization and validation of multiple markers in diverse prospective cohorts. Novel marker development will be facilitated by the use of high-throughput proteomic and metabolomic technologies and the consideration of more complex modeling techniques, such as neural networks and artificial intelligence. Corticotropin-releasing hormone, α-fetoprotein, alkaline phosphatase, β2-macroglobulin, ferritin, ICAM-1, interleukin-6, C-reactive protein, cortisol, lactoferrin, defensins, relaxin, interleukin-10, granulocyte-macrophage colony-stimulating factor, and activin.

Nutritional paradigms of ovine fetal growth restriction: Implications for human pregnancy

Low birth weight and prematurity are associated with short inter-pregnancy intervals, low pre-pregnancy weights, insufficient maternal weight gains during pregnancy, multifetal pregnancies and a young maternal age. Improvements in maternal nutritional status are arguably imperative for ensuring an appropriate pregnancy outcome in these vulnerable groups, but ethical boundaries limit these investigations. Experimental paradigms using the pregnant sheep have been widely used to identify the nutritionally sensitive periods of conceptus development. In adult sheep, severe undernutrition during the periconceptual period accelerates maturation of the fetal hypothalamic-pituitary adrenal axis and results in pre-term delivery. Low pre-pregnancy weight, followed by undernutrition during mid-pregnancy, results in reduced placental growth and lower birth weights at term. Studies that have restricted nutrients during mid-gestation only reveal variable effects on the placental and fetal growth trajectory, however if undernutrition is prolonged during late-pregnancy, fetal growth is compromised, particularly in twin pregnancies. In contrast, overnourishing the adolescent sheep to promote rapid maternal growth, results in the premature delivery of low birth weight lambs. These effects are mediated by impaired placental growth, uteroplacental blood flows and fetal nutrient uptakes. At the other end of the nutritional spectrum, undernourishing the adolescent sheep to gradually deplete nutrient reserves, results in fetal growth restriction which is independent of alterations in placental mass.

Induction of Uterine Prostaglandin H Synthase 2 by Estradiol Following Fetal Adrenalectomy

In sheep, fetal cortisol stimulates the conversion of progesterone to estradiol in late gestation initiating labor. It is unclear whether an intact fetal hypothalamic-pituitary-adrenal (HPA) axis is required to induce the estradiol-triggered subsequent endocrine changes including enhanced intrauterine prostaglandin (PG) synthesis associated with the onset of labor. We have shown that maternal estradiol administration stimulates PG H synthase (PGHS)-2 expressions in pregnant ovine intra-uterine tissues. The current study was undertaken to determine whether the fetal adrenal mediates estradiol's stimulation of the intrauterine PGHS-2 in pregnant sheep. Placenta, myometrium, and endometrium were collected from two groups of ewes at 123-127 d of gestational age (dGA) after fetal adrenalectomy and vehicle treatment (ADX; n = 5); or fetal ADX and maternal estradiol administration (5 mg twice a day for 2 d, ADX+E2, n = 5). PGHS-2 mRNA and protein were analyzed by Northern and Western Blot analyses in both groups and presented as the ratios to beta actin mRNA for Northern and G protein beta subunit for Western blot analysis. Fetal plasma cortisol was measured by radioimmunoassay. Data were analyzed by Student's t test. Fetal plasma cortisol levels were low in ADX and ADX+ E2 groups (<6 ng/mL). The cervix of all ADX+E2 treated ewes was dilated at necropsy. Three out of five ADX+ E2-treated ewes delivered within 48 h. The cervix was closed in all fetal ADX ewes at necropsy. PGHS-2 mRNA and protein increased (p < 0.05) in myometrium and endometrium, but not placenta in ADX+E2-treated ewes compared with ADX group. These data provide the first in vivo evidence for estradiol upregulation of intrauterine PGHS-2 in late gestation in the absence of an intact fetal HPA axis. Thus, the fetal adrenal is not required to mediate estradiol's stimulation of uterine PGHS-2 expression associated with the onset of labor.

Effects of periconceptional undernutrition on the initiation of parturition in sheep.

In sheep, parturition is initiated by increased fetal hypothalamic-pituitary-adrenal axis (HPAA) activity leading to PGE(2) and PGF(2alpha) production and a rise in the 17beta-estradiol-progesterone (E(2)/P(4)) ratio. Uteroplacental PG production can also increase fetal HPAA activity. Periconceptional maternal undernutrition accelerates fetal HPAA maturation resulting in preterm labor. We determined whether preterm labor was preceded by an increase in PG concentrations and E(2)/P(4) ratio and whether these increases preceded or followed the corresponding rise in cortisol concentrations. Singleton-bearing ewes were nourished ad libitum (N, n = 9) or undernourished (UN, n = 10) to reduce maternal weight by 15% from -61 days (d) to +30 d after mating with ad libitum intake thereafter. Paired maternal and fetal blood samples were collected from 126 d until delivery. Half the UN group delivered prematurely (>2 SD below mean gestation for the flock). PG and cortisol concentrations and E(2)/P(4) ratio increased before delivery in the same way in both groups. However, the increases occurred 7-10 d earlier in UN than in N animals. In both UN and N fetuses cortisol concentrations rose before fetal and maternal PG concentrations and maternal E(2)/P(4) ratio. Periconceptional maternal undernutrition induces preterm delivery in sheep by advancing the expected prepartum rise in cortisol and PG concentrations and E(2)/P(4) ratio. The rise in fetal cortisol concentration precedes the rise in fetal and maternal PG concentrations and maternal E(2)/P(4) ratio, suggesting that the underlying mechanism is likely to be acceleration of fetal HPAA maturation, resulting in initiation of the normal process of parturition.

Effects of leptin on fetal plasma adrenocorticotropic hormone and cortisol concentrations and the timing of parturition in the sheep.

We investigated whether leptin can suppress the prepartum activation of the fetal hypothalamus-pituitary-adrenal (HPA) axis and delay the timing of parturition in the sheep. First, we investigated the effects of a 4-day intravascular infusion of recombinant ovine leptin (n = 7) or saline (n = 6) on fetal plasma adrenocorticotropic hormone (ACTH) and cortisol concentrations, starting from 136 days gestation (i.e., at the onset of the prepartum activation of the fetal HPA axis. The effects of a continuous intrafetal infusion of leptin (n = 7) or saline (n = 5) from 144 days gestation on fetal plasma ACTH and cortisol concentrations and the timing of delivery were also determined in a separate study. There was an increase in fetal plasma ACTH (P < 0.01) and cortisol (P < 0.001) concentrations when saline was infused between 136-137 and 140-141 days gestation. Plasma ACTH and cortisol concentrations did not rise, however, when leptin was infused during this period of gestation. When leptin was infused after 144 days gestation, there was no effect of a 4- to 5-fold increase in circulating leptin on fetal ACTH concentrations. In contrast, leptin infusion from 144 days gestation suppressed (P < 0.05) fetal plasma cortisol concentrations by around 40% between 90 and 42 h before delivery. There was no difference, however, in the length of gestation between the saline- and leptin-infused groups (saline infused, 150.2 +/- 0.5 days; leptin infused, 149.8 +/- 1.0 days). In saline-infused fetuses, there was a significant negative relationship between the plasma concentrations of cortisol (y) and leptin (x) between 138 and 146 days gestation (y = 81.4 - 7.7x, r = 0.38, P < 0.005). This study provides evidence for an endocrine negative feedback loop between leptin and the HPA axis in fetal life.

Central nervous system prostaglandin endoperoxide synthase-1 and -2 responses to oestradiol and cerebral hypoperfusion in late-gestation fetal sheep.

Previous work in this laboratory has demonstrated that cerebral hypoperfusion increases the expression of prostaglandin endoperoxide synthase-2 (PGHS-2) in ovine fetal brain regions. Endogenously produced prostaglandins, in turn, partially mediate the fetal hypothalamus- pituitary-adrenal (HPA) axis response to arterial hypotension. In separate experiments, we have found that oestradiol stimulates fetal HPA axis activity. The present experiments were designed to test the hypothesis that oestradiol increases the expression of PGHS isoforms, and that oestradiol augments the PGHS response to cerebral hypoperfusion. Sixteen fetal sheep of known gestational ages (124-128 days' gestation at the time of study) were subjected to chronic catheterization and implantation of extravascular occluder around the brachiocephalic artery. Eight fetuses were subjected to subcutaneous implantation of a pellet containing 17beta-oestradiol (release rate 5 mg (21 days)-1). Brachiocephalic occlusion (BCO) stimulated adrenocorticotropin (ACTH), cortisol and arginine vasopressin (AVP) secretion, responses that were augmented by oestradiol. One hour after the beginning of a 10 min period of BCO, PGHS-1 mRNA was increased in fetal brainstem and hypothalamus, and PGHS-2 mRNA was increased in fetal brainstem. Oestradiol, by itself, increased the abundance of PGHS-2 mRNA in brainstem and cerebellum, and augmented the PGHS-2 mRNA response to BCO in brainstem. In contrast, oestradiol had no significant effect on the abundance of PGHS-1 mRNA in any brain region. PGHS-1 and PGHS-2 protein levels did not reflect the changes in the respective mRNAs. The abundance of both proteins was increased in cerebral cortex in response to BCO, and the abundance of PGHS-2 protein was increased by both oestradiol and BCO in the hippocampus. The results of this study confirm and extend the results of our previous studies, demonstrating an effect of cerebral hypoperfusion on the expression of both isoforms of PGHS. We conclude that oestradiol increases the expression of PGHS-2 in specific fetal brain regions, and that there is an interaction between oestradiol and BCO in the control of PGHS-2 expression in the fetal brainstem. We expect that at later time-points, the changes in mRNA would be followed by similar changes in enzyme abundance at the protein level. We speculate that at least a part of the effect of oestradiol on fetal HPA axis function is mediated by an interaction between oestradiol and prostaglandin biosynthesis in the fetal brain.

Biological activity of 17beta-estradiol-3-sulfate in ovine fetal plasma and uptake in fetal brain.

In sheep, the fetal hypothalamus-pituitary-adrenal axis plays a central role in the initiation of parturition. We have reported that estradiol dramatically increases the activity of the fetal hypothalamus-pituitary-adrenal (HPA) axis. Sulfoconjugated estrogens are known to circulate in high concentrations in fetal plasma. We have reported the expression and abundant activity of steroid sulfatase within the fetal brain regions important for HPA axis control, and we have proposed that sulfoconjugated estrogens in fetal plasma are deconjugated (and therefore converted to a biologically active form) in fetal brain. The present study was designed to test the hypothesis that exogenous estradiol-3-sulfate stimulates HPA axis activity in late gestation fetal sheep and that it is concentrated by fetal brain tissue. We infused estradiol-3-sulfate iv into fetal sheep (125-135 d gestation; term = 147 d) at rates of 0, 0.25, and 1.0 mg/d for 5 d and performed serial sampling of fetal blood before and at the end of the infusion periods. Infusions increased fetal plasma estradiol-3-sulfate concentrations and produced dose-related increases in HPA axis activity. The action of the steroid on the fetal brain was also demonstrated as dose-related increases in the abundance of Fos in fetal cerebellum. In a second study we measured the uptake of sulfoconjugated and unconjugated estrogen (estrone-3sulfate and estrone, respectively) into the fetal brain (124-128 d gestation) in vivo. Both forms of estrogen were concentrated in fetal brain, with the uptake of estrone greater than that of estrone-3-sulfate. We conclude that sulfoconjugated estrogens augment fetal HPA axis activity and that they can cross the fetal blood-brain barrier. We propose that in late gestation the large circulating pool of sulfoconjugated estrogen is a biologically important source of active hormone that might play a role in the timing of parturition in sheep.

Pituitary-Adrenal Physiology during Pregnancy

In many species, including human beings, pregnancy results in an activation of the hypothalamus-pituitary-adrenal (HPA) axis, resulting in increased maternal cortisol secretion. This increase is important for maintaining an environment to support fetal growth and development, because adrenal excess or insufficiency results in disturbances in maternal and fetal homeostasis. The mechanism of the HPA axis activation is not completely understood, and in the human, may involve placental secretion of corticotropin releasing hormone. However, a common mechanism is likely to be the interaction of progesterone with mineralocorticoid receptors in the brain, resulting in an increase in set-point for cortisol. In the human, maternal cortisol can cross the placenta and affect fetal physiology before the fetal adrenal maturation. As the fetus matures, transplacental passage of cortisol decreases, and the fetal adrenal cortisol and androgen secretion increases. The maturing fetal HPA axis ultimately controls the timing of parturition by determining the rate of estrogen secretion from the placenta, thereby stimulating increased myometrial activity during labor. At term, interactions between the HPA and placental hormones may result in positive feedback cycles, which amplify and coordinate the increases in HPA secretion, myometrial activity, and fetal organ maturation.

Effects of Cortisol and Estradiol on Pituitary Expression of Proopiomelanocortin, Prohormone Convertase-1, Prohormone Convertase-2, and Glucocorticoid Receptor mRNA in Fetal Sheep

We hypothesized that in the late-gestation sheep fetus there is an interaction between the prepartum rise in cortisol and the increase in placental estradiol production that allows expression of key components of the fetal hypothalamic-pituitary-adrenal (HPA) axis. Therefore, the goal of this study was to investigate the effects of cortisol on the fetal HPA axis in the presence and absence of increased placental estradiol production. We obtained fetal plasma samples and pituitary tissue from animals that had received an infusion of either cortisol, cortisol and 4-hydroxyandrostenedione (40HA, an aromatase inhibitor), saline, or saline + 40HA controls. Cortisol significantly decreased plasma adrenocorticotropic hormone concentrations, and in the presence of 40HA reduced pituitary proopiomelanocortin (POMC) mRNA levels in the pars distalis. There was no effect of any treatment on the expression of the key POMC processing enzymes, prohormone convertase-1 or -2 in the fetal pituitary. Conversely, levels of glucocorticoid receptor (GR) mRNA in the pituitary were increased with cortisol treatment in the absence of increased estradiol. We suggest that in the late-gestation sheep fetus, cortisol and estradiol have opposite effects on pituitary POMC and GR mRNA expression, and interact to regulate these key components of the fetal HPA axis.

Altered fetal pituitary-adrenal function in the ovine fetus treated with RU486 and meloxicam, an inhibitor of prostaglandin synthase-II.

Term and preterm labor are associated with increased fetal hypothalamic-pituitary-adrenal (HPA) activation and synthesis of prostaglandins (PGs) generated through the increased expression of prostaglandin H synthase-II (PGHS-II) in the placenta. Inhibition of PGHS-II has been advocated as a means of producing uterine tocolysis, but the effects of such treatment on fetal endocrine functions have not been thoroughly examined. Because PGE(2) is known to activate the fetal HPA axis, we hypothesized that administration of meloxicam, a PGHS-II inhibitor, to sheep in induced labor would suppress fetal HPA function. Chronically catheterized pregnant ewes were treated with RU486, a progesterone receptor antagonist, to produce active labor, and then treated with either high-maintenance-dose meloxicam, graded-maintenance-dose meloxicam, or a saline infusion. Maternal uterine contraction frequency increased 24 h after the RU486 injection and the animals were in active labor by 48 +/- 4 h. RU486 injection led to increased concentrations of PGE(2), ACTH, and cortisol in the fetal circulation, and increased concentrations of 13,14 dihydro 15-ketoprostaglandin F(2 alpha) (PGFM) in the maternal circulation. Uterine activity was inhibited within 12 h of beginning meloxicam infusion at both infusion regimes. During meloxicam infusion there were significant decreases in fetal plasma PGE(2), ACTH, and cortisol concentrations, and PGFM concentrations in maternal plasma. In control animals, frequency of uterine contractions, maternal plasma PGFM, fetal plasma PGE(2), ACTH, and cortisol concentrations increased after RU486 administration, and continued to rise during saline infusion until delivery occurred. We conclude that RU486-provoked labor in sheep is associated with activation of fetal HPA function, and that this is attenuated during meloxicam treatment to a level considered compatible with pregnancy maintenance.