Attenuation of corticotropin-releasing hormone and arginine vasopressin responsiveness during late-gestation pregnancy in sheep

Parity and housing effects on the behavioural and hypothalamic-pituitary-adrenal axis responses of pregnant ewes

It is common in many countries for sheep to be housed during winter from mid-gestation until lambing to protect ewes and lambs from adverse conditions and improve late gestation nutritional management. Keeping ewes indoors, however, has its own challenges as the animals may be mixed with unfamiliar conspecifics, have limited floor and feeding space, experience changes to their diet and increased handling by humans. Therefore, the objective of this study was to investigate the effect of variation in housing management (space allowance and social stability) on the behaviour and hypothalamic-pituitary-adrenal (HPA) axis responses of pregnant ewes from mid-to-late gestation (weeks 11-18 of pregnancy). Seventy-seven ewes (41 primiparous, 36 multiparous) were divided into two groups: 'Control' and 'Restricted space and mixed' (RS-Mix), where RS-Mix ewes were allocated half the amount of space (1.27 vs 2.5 m2 for RS-Mix and Control, respectively) and feedface (concentrate feeder space) allowance (36 vs 71 cm per ewe) given to the Control group and were also subjected to two social mixing events. Aggressive behaviour at the feedface and time spent standing, lying, walking, feeding and ruminating were recorded and faecal samples were collected for assessment of faecal glucocorticoid metabolite (FGM) concentrations. Higher aggression was observed in RS-Mix ewes during the first week of observation (P = 0.044), which gradually declined to the same level as Control ewes by the end of the study (P = 0.045). RS-Mix ewes were significantly less likely to be able to freely join the feedface compared to Controls (P = 0.022). No other significant treatment effects on aggressive behaviour or FGM during gestation were found. RS-Mix ewes displayed significantly higher ruminating behaviour at week 18 of gestation compared to Control ewes (P < 0.001), but no other effects were seen on general pen behaviour. However, the effect of indoor housing had a significant impact on primiparous ewes, who had lower weight gain (P = 0.015) and higher FGM concentrations (P = 0.014) compared to multiparous ewes regardless of treatment group. The data suggest that, although no sustained effects on behaviour or HPA axis responses were seen with the differences in space and feeder allowance or social stability at the levels used in this study, inexperienced (primiparous) ewes may find indoor housing more stressful; and are less able to adapt compared to multiparous ewes. These effects may influence the behaviour of the ewe at lambing time, and her offspring.

Fetal and Neonatal HPA Axis

Stress is an integral part of life. Activation of the hypothalamus-pituitary-adrenal (HPA) axis in the adult can be viewed as mostly adaptive to restore homeostasis in the short term. When stress occurs during development, and specifically during periods of vulnerability in maturing systems, it can significantly reprogram function, leading to pathologies in the adult. Thus, it is critical to understand how the HPA axis is regulated during developmental periods and what are the factors contributing to shape its activity and reactivity to environmental stressors. The HPA axis is not a passive system. It can actively participate in critical physiological regulation, inducing parturition in the sheep for instance or being a center stage actor in the preparation of the fetus to aerobic life (lung maturation). It is also a major player in orchestrating mental function, metabolic, and cardiovascular function often reprogrammed by stressors even prior to conception through epigenetic modifications of gametes. In this review, we review the ontogeny of the HPA axis with an emphasis on two species that have been widely studied-sheep and rodents-because they each share many similar regulatory mechanism applicable to our understanding of the human HPA axis. The studies discussed in this review should ultimately inform us about windows of susceptibility in the developing brain and the crucial importance of early preconception, prenatal, and postnatal interventions designed to improve parental competence and offspring outcome. Only through informed studies will our public health system be able to curb the expansion of many stress-related or stress-induced pathologies and forge a better future for upcoming generations.

Development and programming of the hypothalamus-pituitary-adrenal axis

The fetal hypothalamus-pituitary-adrenal (HPA) axis plays a critical role in fetal development and physiology in utero: appropriate function of the fetal HPA axis is critical for preparation of the fetus for birth and survival in postnatal life. Because of the critical importance of appropriate physiological regulation of HPA activity in postnatal life, there has been intense interest in the possibility that fetal or neonatal stressors can permanently "program" the axis to hyperrespond or hyporespond to stimuli. This is a review of the literature relevant to normal development and "programming" of the HPA axis.

On the function of placental corticotropin-releasing hormone: a role in maternal-fetal conflicts over blood glucose concentrations

Throughout the second and third trimesters, the human placenta (and the placenta in other anthropoid primates) produces substantial quantities of corticotropin-releasing hormone (placental CRH), most of which is secreted into the maternal bloodstream. During pregnancy, CRH concentrations rise over 1000-fold. The advantages that led selection to favour placental CRH production and secretion are not yet fully understood. Placental CRH stimulates the production of maternal adrenocorticotropin hormone (ACTH) and cortisol, leading to substantial increases in maternal serum cortisol levels during the third trimester. These effects are puzzling in light of widespread theory that cortisol has harmful effects on the fetus. The maternal hypothalamic-pituitary-adrenal (HPA) axis becomes less sensitive to cortisol during pregnancy, purportedly to protect the fetus from cortisol exposure. Researchers, then, have often looked for beneficial effects of placental CRH that involve receptors outside the HPA system, such as the uterine myometrium (e.g. the placental clock hypothesis). An alternative view is proposed here: the beneficial effect of placental CRH to the fetus lies in the fact that it does stimulate the production of cortisol, which, in turn, leads to greater concentrations of glucose in the maternal bloodstream available for fetal consumption. In this view, maternal HPA insensitivity to placental CRH likely reflects counter-adaptation, as the optimal rate of cortisol production for the fetus exceeds that for the mother. Evidence pertaining to this proposal is reviewed.

Changes in ovine maternal temperature, and serum cortisol and interleukin-6 concentrations after challenge with Escherichia coli lipopolysaccharide during pregnancy and early lactation

Major changes in maternal physiology during pregnancy and lactation can have a large impact on the immune and neuroendocrine systems. One of the most significant changes, observed in rats and mice, is hyporesponsiveness of the hypothalamic pituitary adrenal axis (HPAA) in response to inflammation, restraint, and other psychological stressors during late pregnancy and lactation. This attenuation, however, has not been well characterized in ruminant animals and may be relevant to their susceptibility to inflammatory diseases during these periods. Thus, the intent of this study was to characterize responsiveness of the ovine HPAA to inflammatory challenge during pregnancy and lactation. Ewes from early (33 d), middle (55 d), and late (138 d) pregnancy, as well as early lactation (10 d), were challenged i.v. with a bolus dose of 400 ng of Escherichia coli lipopolysaccharide (LPS)/kg of BW or saline. A corresponding group of nonpregnant ewes was also challenged with LPS to serve as positive control animals for each pregnancy and lactation study. Responsiveness of the HPAA was assessed by measuring the 4-h change in serum cortisol concentration after LPS challenge. The cortisol increase after LPS challenge was elevated (P < 0.01) in pregnant ewes during late pregnancy over that of nonpregnant animals. In contrast, the characteristic temperature response associated with systemic LPS challenge was decreased (P < 0.01) during early pregnancy and lactation compared with nonpregnant or nonlactating animals. Serum IL-6 concentrations were measured to assess whether changes in HPAA responsiveness during pregnancy or lactation were attributed to changes in proinflammatory signaling to the HPAA. Interestingly, enhanced cortisol responsiveness during late pregnancy was correlated with increased (P < 0.01) serum IL-6 concentrations, indicating that IL-6 may contribute to enhanced HPAA responsiveness during this period. Serum IL-6 concentrations during early and midpregnancy did not increase in response to LPS challenge, indicating that HPAA activation during periods of pregnancy may be independent of IL-6 production.

Subpopulations of corticotrophs in the sheep pituitary during late gestation: effects of development and placental restriction

The prepartum surge in fetal plasma cortisol is essential for the normal timing of parturition in sheep and may result from an increase in the ratio of ACTH to proopiomelanocortin (POMC) in the fetal circulation. In fetuses subjected to experimental induction of placental restriction, the prepartum surge in fetal cortisol is exaggerated, whereas pituitary POMC mRNA levels are decreased, and in vitro, unstimulated ACTH secretion is elevated in corticotrophs nonresponsive to CRH. We therefore investigated the changes in the relative proportions of cells expressing POMC, ACTH, and the CRH type 1 receptor (CRHR(1)) shortly before birth and during chronic placental insufficiency. Placental restriction (PR) was induced by removal of the majority of placental attachment sites in five ewes before mating. Pituitaries were collected from control and PR fetal sheep at 140 d (control, n = 4; PR, n = 4) and 144 d (control, n = 6; PR, n = 4). Pituitary sections were labeled with specific antisera raised against POMC, ACTH, and CRHR(1). Three major subpopulations of corticotrophs were identified that expressed POMC + ACTH + CRHR(1), ACTH + CRHR(1), or POMC only. The proportion of pituitary corticotrophs expressing POMC + ACTH + CRHR(1) decreased (P < 0.05) between 140 (control, 60 +/- 1%; PR, 66 +/- 4%) and 144 (control, 45 +/- 2%; PR, 56 +/- 6%) d. A significantly higher (P < 0.05) proportion of corticotrophs expressed POMC + ACTH + CRHR(1) in the pituitary of the PR group compared with controls. This study is the first to demonstrate subpopulations of corticotrophs in the fetal sheep pituitary that differentially express POMC, ACTH, and CRHR(1) and the separate effects of gestational age and placental restriction on these subpopulations of corticotrophs.

The role of hypothalamic input on corticotroph maturation in fetal sheep

Corticotropin-releasing hormone receptor type 1 (CRH-R1) expression and vasopressin type 1b (V1b) receptor protein decrease in late-gestation fetal sheep. Because hypothalamo-pituitary disconnection (HPD) has been demonstrated to prevent the morphological maturation of corticotrophs, we hypothesized that hypothalamic input is necessary for the maturational changes in CRH-R1 and V1b receptor levels. We measured CRH-R1 and V1b receptor expression in the anterior pituitaries of fetuses at 140 days gestational age (dGA) that underwent HPD or sham surgery at 120 dGA. CRH-R1 mRNA decreased similarly in HPD and sham-operated fetuses compared with 120 dGA naive fetuses. However, CRH-R1 protein levels were elevated in HPD fetuses compared with sham and were not different from 120 dGA values. V1b protein levels decreased similarly in HPD and sham-operated fetuses compared with 120 dGA naive fetuses. We conclude that hypothalamic input to the pituitary is necessary for the decrease in CRH-R1 receptor protein levels in late-gestation fetal sheep. However, hypothalamic input is not necessary for the decrease in V1b receptor expression seen in late gestation.

Ontogeny and effect of cortisol on vasopressin-1b receptor expression in anterior pituitaries of fetal sheep

Corticotroph responsiveness to arginine vasopressin (AVP) increases during late gestation in fetal sheep. The mechanism of this increase in AVP responsiveness is currently unknown but could be related to an increase in vasopressin type 1b (V1b) receptor expression in the pituitary during development. To determine if there are ontogenic changes in V1b receptor expression that may help explain the changes in ACTH responses to AVP, we studied pituitaries from three groups of fetal sheep [100, 120, or 140 days gestational age (dGA)]. V1b receptor mRNA and protein significantly decreased by 140 dGA. Peak V1b mRNA levels were detected at 100 dGA, while peak V1b protein levels were detected at 120 dGA. The reduction in V1b receptor expression in late gestation may be due to the naturally occurring peripartum increase in fetal plasma cortisol because cortisol infusion at 122-130 dGA decreased V1b receptor mRNA. Thus there is a marked decrease in the expression of the V1b receptor in the pituitary during fetal development, leaving the role of the V1b receptor in increasing AVP responsiveness uncertain.