The Yin and Yang of the oxytocin and stress systems: opposites, yet interdependent and intertwined determinants of lifelong health trajectories

During parturition and the immediate post-partum period there are two opposite, yet interdependent and intertwined systems that are highly active and play a role in determining lifelong health and behaviour in both the mother and her infant: the stress and the anti-stress (oxytocin) system. Before attempting to understand how the environment around birth determines long-term health trajectories, it is essential to understand how these two systems operate and how they interact. Here, we discuss together the hormonal and neuronal arms of both the hypothalamic-pituitary-adrenal (HPA) axis and the oxytocinergic systems and how they interact. Although the HPA axis and glucocorticoid stress axis are well studied, the role of oxytocin as an extremely powerful anti-stress hormone deserves more attention. It is clear that these anti-stress effects depend on oxytocinergic nerves emanating from the supraoptic nucleus (SON) and paraventricular nucleus (PVN), and project to multiple sites at which the stress system is regulated. These, include projections to corticotropin releasing hormone (CRH) neurons within the PVN, to the anterior pituitary, to areas involved in sympathetic and parasympathetic nervous control, to NA neurons in the locus coeruleus (LC), and to CRH neurons in the amygdala. In the context of the interaction between the HPA axis and the oxytocin system birth is a particularly interesting period as, for both the mother and the infant, both systems are very strongly activated within the same narrow time window. Data suggest that the HPA axis and the oxytocin system appear to interact in this early-life period, with effects lasting many years. If mother-child skin-to-skin contact occurs almost immediately postpartum, the effects of the anti-stress (oxytocin) system become more prominent, moderating lifelong health trajectories. There is clear evidence that HPA axis activity during this time is dependent on the balance between the HPA axis and the oxytocin system, the latter being reinforced by specific somatosensory inputs, and this has long-term consequences for stress reactivity.

Chronic maternal hypercortisolemia in late gestation alters fetal cardiac function at birth

Studies in our laboratory have shown that modest chronic increases in maternal cortisol concentrations over the last 0.20 of gestation impair maternal glucose metabolism and increase the incidence of perinatal stillbirth. Previous studies had found that an increase in maternal cortisol concentrations from 115 to 130 days of gestation in sheep increased both proliferation in fetal cardiomyocytes and apoptosis in the fetal cardiac Purkinje fibers. We hypothesized that the adverse effects of excess cortisol may result in defects in cardiac conduction during labor and delivery. In the present study, we infused cortisol (1 mg·kg^-1·day^-1) into late gestation pregnant ewes and continuously monitored fetal aortic pressure and ECG through labor and delivery. We found that, although the fetuses of cortisol infused ewes had normal late gestation patterns of arterial pressure and heart rate, there was a significant decrease in fetal aortic pressure and heart rate on the day of birth, specifically in the final hour before delivery. Significant changes in the fetal ECG were also apparent on the day of birth, including prolongation of the P wave and P-R interval. We speculate that chronic exposure to glucocorticoids alters cardiac metabolism or ion homeostasis, contributing to cardiac dysfunction, precipitated by active labor and delivery.

Cyclic decidualization of the human endometrium in reproductive health and failure

Decidualization denotes the transformation of endometrial stromal fibroblasts into specialized secretory decidual cells that provide a nutritive and immunoprivileged matrix essential for embryo implantation and placental development. In contrast to most mammals, decidualization of the human endometrium does not require embryo implantation. Instead, this process is driven by the postovulatory rise in progesterone levels and increasing local cAMP production. In response to falling progesterone levels, spontaneous decidualization causes menstrual shedding and cyclic regeneration of the endometrium. A growing body of evidence indicates that the shift from embryonic to maternal control of the decidual process represents a pivotal evolutionary adaptation to the challenge posed by invasive and chromosomally diverse human embryos. This concept is predicated on the ability of decidualizing stromal cells to respond to individual embryos in a manner that either promotes implantation and further development or facilitates early rejection. Furthermore, menstruation and cyclic regeneration involves stem cell recruitment and renders the endometrium intrinsically capable of adapting its decidual response to maximize reproductive success. Here we review the endocrine, paracrine, and autocrine cues that tightly govern this differentiation process. In response to activation of various signaling pathways and genome-wide chromatin remodeling, evolutionarily conserved transcriptional factors gain access to the decidua-specific regulatory circuitry. Once initiated, the decidual process is poised to transit through distinct phenotypic phases that underpin endometrial receptivity, embryo selection, and, ultimately, resolution of pregnancy. We discuss how disorders that subvert the programming, initiation, or progression of decidualization compromise reproductive health and predispose for pregnancy failure.

Elevated maternal cortisol leads to relative maternal hyperglycemia and increased stillbirth in ovine pregnancy

In normal pregnancy, cortisol increases; however, further pathological increases in cortisol are associated with maternal and fetal morbidities. These experiments were designed to test the hypothesis that increased maternal cortisol would increase maternal glucose concentrations, suppress fetal growth, and impair neonatal glucose homeostasis. Ewes were infused with cortisol (1 mg·kg(-1)·day(-1)) from day 115 of gestation to term; maternal glucose, insulin, ovine placental lactogen, estrone, progesterone, nonesterified free fatty acids (NEFA), β-hydroxybutyrate (BHB), and electrolytes were measured. Infusion of cortisol increased maternal glucose concentration and slowed the glucose disappearance after injection of glucose; maternal infusion of cortisol also increased the incidence of fetal death at or near parturition. The design of the study was altered to terminate the study prior to delivery, and post hoc analysis of the data was performed to test the hypothesis that maternal metabolic factors predict the fetal outcome. In cortisol-infused ewes that had stillborn lambs, plasma insulin was increased relative to control ewes or cortisol-infused ewes with live lambs. Maternal cortisol infusion did not alter maternal food intake or plasma NEFA, BHB, estrone, progesterone or placental lactogen concentrations, and it did not alter fetal body weight, ponderal index, or fetal organ weights. Our study suggests that the adverse effect of elevated maternal cortisol on pregnancy outcome may be related to the effects of cortisol on maternal glucose homeostasis, and that chronic maternal stress or adrenal hypersecretion of cortisol may create fetal pathophysiology paralleling some aspects of maternal gestational diabetes.

Differential effects of mineralocorticoid blockade on the hypothalamo-pituitary-adrenal axis in pregnant and nonpregnant ewes

During pregnancy, plasma ACTH and cortisol are chronically increased; this appears to occur through a reset of hypothalamo-pituitary-adrenal (HPA) activity. We have hypothesized that differences in mineralocorticoid receptor activity in pregnancy may alter feedback inhibition of the HPA axis. We tested the effect of MR antagonism in pregnant and nonpregnant ewes infused for 4 h with saline or the MR antagonist canrenoate. Pregnancy significantly increased plasma ACTH, cortisol, angiotensin II, and aldosterone. Infusion of canrenoate increased plasma ACTH, cortisol, and aldosterone in both pregnant and nonpregnant ewes; however, the temporal pattern of these responses differed between these two reproductive states. In nonpregnant ewes, plasma ACTH and cortisol transiently increased at 1 h of infusion, whereas in pregnant ewes the levels gradually increased and were significantly elevated from 2 to 4 h of infusion. MR blockade increased plasma aldosterone from 2 to 4 h in the pregnant ewes but only at 4 h in the nonpregnant ewes. In both pregnant and nonpregnant ewes, the increase in plasma aldosterone was significantly related to the timing and magnitude of the increase in plasma potassium. The results indicate a differential effect of MR activity in pregnant and nonpregnant ewes and suggest that the slow changes in ACTH, cortisol, and aldosterone are likely to be related to blockade of MR effects in the kidney rather than to effects of MR blockade in hippocampus or hypothalamus.

Regulation of maternal ACTH in ovine pregnancy: does progesterone play a role?

Pregnancy is characterized by increased plasma adrenocorticotropic hormone (ACTH) and cortisol. Studies suggest that progesterone acts as an antagonist at mineralocorticoid receptors. Therefore, we tested the hypothesis that chronic progesterone, produced by treatment of nonpregnant ewes or during pregnancy, will result in increased plasma ACTH relative to the plasma cortisol concentrations. We studied three groups of ewes: ovariectomized nonpregnant, nonpregnant treated with progesterone, and pregnant ewes. In two series of studies, ewes were adrenalectomized and replaced with 0.35 mg x kg(-1) x day(-1) or 0.5 mg x kg(-1) x day(-1) cortisol. In both studies, aldosterone was infused at 3 microg x kg(-1) x day(-1). In the first study, additional infusions of cortisol over 24 h were used to increase daily replacement doses to 0.5, 1, or 1.5 mg x kg(-1) x day(-1), and intact pregnant and nonpregnant ewes were studied with infusions of cortisol at 0, 0.5, and 1 mg x kg(-1) x day(-1). In adrenalectomized ewes chronically replaced to 0.35 mg x kg(-1) x day(-1) cortisol, plasma ACTH concentrations were decreased significantly in the nonpregnant progesterone-treated ewes compared with the ovariectomized nonpregnant ewes. With 0.5 mg x kg(-1) x day(-1) cortisol, plasma ACTH levels were greater in pregnant ewes than in nonpregnant ewes with or without progesterone. Overall plasma ACTH levels at 0.35 mg x kg(-1) x day(-1) were significantly related to the plasma protein concentration, suggesting that the ACTH levels in the hypocorticoid ewes are most closely related to plasma volume. Across all steroid doses, ACTH was positively related to plasma proteins and progesterone, and negatively related to cortisol. We conclude that increased progesterone does not alter the feedback relation of cortisol to ACTH, but may modulate ACTH indirectly through plasma volume.

Review article: steroid hormones and uterine vascular adaptation to pregnancy

Pregnancy is a physiological state that involves a significant decrease in uterine vascular tone and an increase in uterine blood flow, which is mediated in part by steroid hormones, including estrogen, progesterone, and cortisol. Previous studies have demonstrated the involvement of these hormones in the regulation of uterine artery contractility through signaling pathways specific to the endothelium and the vascular smooth muscle. Alterations in endothelial nitric oxide synthase expression and activity, nitric oxide production, and expression of enzymes involved in PGI(2) production contribute to the uterine artery endothelium-specific responses. Steroid hormones also have an effect on calcium-activated potassium channel activity, PKC signaling pathway and myogenic tone, and alterations in pharmacomechanical coupling in the uterine artery smooth muscle. This review addresses current understanding of the molecular mechanisms by which steroid hormones including estrogen, progesterone, and cortisol modulate uterine artery contractility to alter uterine blood flow during pregnancy with an emphasis on the pregnant ewe model.

Adrenalectomy alters regulation of blood pressure and endothelial nitric oxide synthase in sheep: modulation by estradiol

Hypoadrenocorticism produces more severe hypotension during the peripartal period in pregnant ewes and women. We hypothesized that estradiol increases the severity of hypotension after withdrawal of corticosteroids and that this results from combined effects of adrenalectomy and estradiol to increase endothelial nitric oxide synthase (eNOS). In study I, blood pressure and eNOS mRNA and protein in aorta, uterine, renal, and mesenteric arteries were measured in intact ewes or adrenalectomized ewes 18–20 h after cessation of infusion of cortisol and aldosterone; half of each group ewes were treated with estradiol. In study II, adrenalectomized ewes were similarly studied 22–28 h after withdrawal of corticosteroids. Estradiol treatment in both studies significantly increased eNOS mRNA and protein in uterine artery, whereas corticosteroid withdrawal decreased expression of eNOS mRNA and protein in uterine artery. In both studies, adrenalectomy and steroid withdrawal decreased mean arterial pressure. In study II, four of six adrenalectomized ewes not treated with estradiol showed dramatic phasic variations in blood pressure and heart rate with a period of ∼20 s, developing within 22–28 h after corticosteroid withdrawal. Although there was no effect of estradiol on blood pressure in study I, in study II, ewes treated with estradiol did not develop this pattern. Estradiol also slowed both the decline in plasma sodium and the rise in plasma potassium after corticosteroid withdrawal. These results disprove the hypothesis that estradiol increases the severity of hypotension during hypoadrenocorticism. However, the study reveals an important effect of corticosteroid withdrawal on blood pressure, consistent with corticosteroid modulation of baroreflex responsiveness.

Increased maternal cortisol in late-gestation ewes decreases fetal cardiac expression of 11beta-HSD2 mRNA and the ratio of AT1 to AT2 receptor mRNA

Moderately elevated maternal cortisol levels late in gestation cause enlargement of the fetal sheep heart. We have used quantitative real-time PCR to examine expression of candidate genes in fetal hearts from mothers in whom cortisol levels were increased (by infusion of 1 mg cortisol.kg(-1).day(-1)) or decreased (by adrenalectomy and replacement to 0.5 mg cortisol.kg(-1).day(-1)) from 115 to 130 days gestation. Control ewes were not treated with steroid. Expression of mineralocorticoid receptor (MR), glucocorticoid receptor (GR), 11beta-hydroxysteroid dehydrogenases 1 and 2 (11beta-HSD1 and -2), IGF I and II, IGF receptors 1 and 2 (IGF-1R and IGF-2R), endothelial nitric oxide synthase, VEGF, myotrophin, angiotensinogen, the angiotensin receptors 1 and 2 (AT1R and AT2R), and the angiotensin converting enzymes 1 and 2 were measured. MR mRNA abundance in fetal hearts was found to be similar to that in adult kidney and hippocampus. Although there were no significant changes in most genes, 11beta-HSD2 and IGF-1R expression were significantly decreased in the high cortisol group and 11beta-HSD2 expression negatively correlated to left ventricular wall thickness. There was also a significant change in the ratio of AT receptor expression, with increased AT2R and decreased AT1R in the high cortisol group. MR, GR, and 11beta-HSD1 immunoreactivity was found in cardiomyocytes and cardiac blood vessels in 126-128 day fetal sheep; in contrast 11beta-HSD2 staining was predominantly in blood vessels. These results indicate that cortisol could indeed act in the fetal heart to induce enlargement and suggest that the renin-angiotensin system may play a role.

Plasma protein and blood volume restitution after hemorrhage in conscious pregnant and ovarian steroid-replaced rats

We have previously shown that both plasma protein restitution and plasma volume restitution are significantly enhanced in female rats hemorrhaged during the proestrus phase of the estrous cycle. Estradiol and progesterone levels are markedly elevated during proestrus and also increase during pregnancy. The present studies were therefore designed to determine whether the ability to restore plasma protein and blood volume after hemorrhage is augmented during pregnancy and by chronically elevated estradiol levels. The response to moderate hemorrhage (22–23% blood loss) was evaluated in conscious pregnant rats during early and midgestation and compared with that of virgin female rats studied during metestrus. At 22 h posthemorrhage, plasma volume had increased to greater than basal levels, and blood volume was restored to 93 ± 1% (metestrus), 91 ± 2% (early pregnancy), and 98 ± 2% (midgestation) of control ( P > 0.05). Animals hemorrhaged during metestrus or early pregnancy restored the same amount of protein to the plasma as had been removed, whereas those hemorrhaged during midgestation restored nearly 50% more plasma protein than had been removed ( P < 0.01). In ovariectomized animals with chronic steroid replacement that maintained plasma progesterone at metestrus levels (15 ± 2 ng/ml) but raised plasma estradiol to twofold that of midgestation (22 ± 3 pg/ml), the blood volume and plasma protein restitution responses to hemorrhage did not differ from those of ovariectomized animals with no steroid replacement. In summary, posthemorrhage restoration of plasma protein content is significantly augmented during midgestation, but not during early pregnancy. This augmented response cannot be attributed to chronic elevation of plasma estradiol levels alone.

Genomic analysis of neuroendocrine development of fetal brain-pituitary-adrenal axis in late gestation

The present study was performed to identify the changes in genomic expression of critical components of the hypothalamus-pituitary-adrenal (HPA) axis in the second half of gestation in fetal sheep. We isolated mRNA from pituitary, hypothalamus, hippocampus, and brain stem in fetal sheep at 80, 100, 120, 130, and 145 days of gestation and 1 and 7 days after delivery (n = 4-5/group). Using real-time RT-PCR, we measured mRNA expression levels of glucocorticoid receptor (GR), mineralocorticoid receptor (MR), serum- and glucocorticoid-induced kinase-1 (sgk1), proopiomelanocortin (POMC), CRF, and arginine vasopressin (AVP). Both MR and GR were highly expressed in pituitary and hippocampus; in all tissues GR was more highly expressed than MR. AVP was more highly expressed than CRF in hypothalamus. MR, GR, and sgk1 expression were increased postnatally in brain stem, and sgk1 expression was increased postnatally in hypothalamus. GR expression was reduced in pituitary in term fetuses compared with younger ages. Hypothalamic CRF expression was increased at the end of gestation compared with younger ages, and AVP expression was increased in newborn lambs. Pituitary POMC was increased at 100 days of gestation compared with 80 days; hypothalamic POMC was increased at 120 days. Overall, the results demonstrate the expression of both MR and GR in brain regions important for control of the HPA axis. Decreases in expression of GR in pituitary at the end of gestation might contribute to the decreased corticosteroid negative feedback sensitivity at term in this species.

Chronic alterations in ovine maternal corticosteroid levels influence uterine blood flow and placental and fetal growth

Previous work from this laboratory demonstrated that the elevation of maternal plasma corticosteroid concentrations during pregnancy is important for the support of fetal development. Reducing ovine maternal plasma cortisol concentrations to nonpregnant levels stimulates homeostatic responses that defend fetal blood volume. The present study was designed to test the hypothesis that chronic decreases or increases in maternal plasma cortisol concentration alter uterine and placental blood flow and morphology. Three groups of pregnant ewes and their fetuses were chronically catheterized and studied: ewes infused with cortisol (1 mg·kg−1·day−1; high cortisol), ewes adrenalectomized and underreplaced with cortisol (0.5 mg·kg−1·day−1; low cortisol), and control ewes. The normal increment in uterine blood flow between 120 and 130 days was eliminated in the low-cortisol ewes; conversely, uterine blood flow was increased in the high-cortisol group compared with the control group. Fetal arterial blood pressure was increased in the high-cortisol group compared with controls, but there was no increase in fetal arterial pressure from 120 to 130 days of gestation in the low-cortisol group. The fetuses of both low-cortisol and high-cortisol groups had altered placental morphology, with increased proportions of type B placentomes, and overall reduced fetal placental blood flow. The rate of fetal somatic growth was impaired in both low-cortisol and high-cortisol groups compared with the fetuses in the intact group. The results of this study demonstrate that maternal plasma cortisol during pregnancy is an important contributor to the maternal environment supporting optimal conditions for fetal homeostasis and somatic growth.

Baseline and stress-induced glucocorticoids during reproduction in the variable flying fox,Pteropus hypomelanus (Chiroptera: Pteropodidae)

Baseline and stress-responsive glucocorticoid (GC) levels were assessed during early pregnancy, late pregnancy, and lactation in female variable flying foxes (Pteropus hypomelanus) and in males over the same time period. Animals were maintained in a breeding colony in captivity. High levels of both cortisol and corticosterone were detected, with total plasma GC levels being among the highest documented in vertebrates (up to 3000 ng/ml in individual animals, with cortisol being the primary GC, accounting for approximately 78% of total GCs), and significantly greater in males than in females. Plasma levels of cortisol and corticosterone showed nearly identical profiles within each sex, with the exception of females in late pregnancy, in which corticosterone, but not cortisol, increased significantly. Baseline levels of plasma cortisol were highest in September (when pups were between 1 and 2 months of age) in both sexes, which may be related to the approaching onset of the mating period. There was a continuum in the magnitude of the response to stress (handling and sampling) over time in females, with the greatest stress response in early pregnancy, a dampened response during late pregnancy, and no significant stress response during lactation. Surprisingly, males failed to exhibit elevated GCs after this stress, but did have significant stress-induced hyperglycemia and suppression of plasma testosterone levels. This may be due to their high (perhaps maximal) baseline levels, which suggests that being in a breeding group was chronically stressful for males.