Hypothalamo-pituitary disconnection in the fetal sheep

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.

Cortisol infusion in late-gestation hypothalamo-pituitary disconnected sheep fetus restores pituitary cell responsiveness to arginine vasopressin

Corticotrophs in the fetal sheep become increasingly responsive to arginine vasopressin (AVP) in late gestation. We previously reported that this may be due in part to corresponding increases in signal transduction (inositol 1,4,5-trisphosphate, IP(3)). These ontogenic changes are prevented by hypothalamo-pituitary disconnection (HPD), which also prevents fetal plasma cortisol concentrations from increasing in late gestation. This led us to hypothesize that cortisol is involved in mediating the changes in pituitary responsiveness. HPD was performed on fetal sheep at 120 days gestational age (dGA). Half of the HPD fetuses were infused with cortisol for 3 days beginning at 135-137 dGA (HPD+C). The remaining HPD fetuses and a group of sham-operated control fetuses were infused with saline. Pituitary cells were isolated and cultured. After 48 h, a subset of cells was stimulated with 100 nM AVP for 2 h, and the medium was collected for ACTH analysis. Another subset of cells was stimulated with 100 nM AVP for 30 min, and the formation of IP(3) was determined. Plasma cortisol concentrations increased rapidly within the first 6 h after infusion (5.2 +/- 1.9 to 29.7 +/- 4.9 ng/ml) but did not increase thereafter. Cells from HPD+C and sham-operated fetuses secreted significantly more ACTH than those from HPD fetuses (% increase from control: 33.0 +/- 8.8%, 47.9 +/- 10.6%, and 11.9 +/- 2.4%, respectively). IP(3) formation was significantly increased in cells from HPD+C and sham-operated compared with HPD fetuses (% increase from control: 17.7 +/- 4.4%, 18.9 +/- 4.3%, and 4.6 +/- 1.5%, respectively). These findings support the idea that cortisol plays a role in mediating the increase in pituitary responsiveness to AVP in the late-gestation fetal sheep.

The impact of ACTH receptor knockdown on fetal and adult ovine adrenocortical cell function

Preparing the mammalian fetus for birth requires an increase in fetal plasma glucocorticoid levels. The mechanisms facilitating this increase are not fully known. It has been shown in sheep that the prepartum elevation in fetal plasma cortisol is accompanied by increases in adrenocorticotropin receptor (ACTH-R) expression in the fetal adrenal and in the adrenal responsiveness to stimulation. To determine the significance of the upregulation in ACTH-R expression on fetal adrenal function, the authors used small interfering RNA targeted to the ovine ACTH-R to reduce receptor expression and studied responses to stimulation in ovine adrenal cells. They studied fetal cells from late gestation after responsiveness had increased. They also studied adult cells to determine if maturation would influence the impact of receptor expression suppression on responsiveness. Fetal and adult cells were obtained, dispersed, transfected with receptor-targeted small interfering RNA or scrambled small interfering RNA, and subsequently stimulated with ACTH. Cells and media were harvested for measurements of gene and protein expression and cyclic adenosine monophosphate (cAMP) and cortisol levels. The ability of ACTH to upregulate its receptor or steroid acute regulatory protein was attenuated in fetal (P < .01) and adult cells (P < .01) by small interfering RNA treatment; the blockade was more pronounced in the adult cells (P < .01). The small interfering RNA treatment also blocked the cAMP response to ACTH in fetal (P < .001) and adult (P < .05) cells. This was accompanied by marked reductions in cortisol responses in both (P < .001 and P < .01, respectively). These data suggest that upregulation of the ACTH-R expression in late gestation is essential for the increase in adrenal steroidogenic capacity occurring then. The data also indicate that a reduction in the ACTH-R expression blocks the ability of the peptide to stimulate early steps in the steroidogenic pathway event after maturation is complete.

Hypothalamic input is required for development of normal numbers of thyrotrophs and gonadotrophs, but not other anterior pituitary cells in late gestation sheep

To evaluate the hypothalamic contribution to the development of anterior pituitary (AP) cells we surgically disconnected the hypothalamus from the pituitary (hypothalamo-pituitary disconnection, HPD) in fetal sheep and collected pituitaries 31 days later. Pituitaries (n = 6 per group) were obtained from fetal sheep (term = 147 +/- 3 days) at 110 days (unoperated group) of gestation and at 141 days from animals that had undergone HPD or sham surgery at 110 days. Cells were identified by labelling pituitary sections with antisera against the six AP hormones. Additionally, we investigated the colocalization of glycoprotein hormones. The proportions of somatotrophs and corticotrophs were unchanged by age or HPD. Lactotrophs increased 80% over time, but the proportion was unaffected by HPD. Thyrotrophs, which were unaffected by age, increased 70% following HPD. Gonadotrophs increased with gestational age (LH+ cells 55%; FSH+ cells 19-fold), but this was severely attenuated by HPD. We investigated the possible existence of a reciprocal effect of HPD on multipotential glycoprotein-expressing cells. Co-expression of LH and TSH was extremely rare (< 1%) and unchanged over the last month of gestation or HPD. The increase of gonadotrophs expressing FSH only or LH and FSH was attenuated by HPD. Therefore, the proportions of somatotrophs, lactotrophs and corticotrophs are regulated independently of hypothalamic input in the late gestation fetal pituitary. In marked contrast, the determination of the thyrotroph and gonadotroph lineages over the same time period is subject to complex mechanisms involving hypothalamic factors, which inhibit differentiation and/or proliferation of thyrotrophs, but stimulate gonadotrophs down the FSH lineage. Development of a distinct population of gonadotrophs, expressing only LH, appears to be subject to alternative mechanisms.

Intrafetal insulin-like growth factor-I infusion stimulates adrenal growth but not steroidogenesis in the sheep fetus during late gestation

We investigated the effects of an intrafetal infusion of IGF-I on adrenal growth and expression of the adrenal steroidogenic and catecholamine-synthetic enzyme mRNAs in the sheep fetus during late gestation. Fetal sheep were infused for 10 d with either IGF-I (26 microg/kg.h; n = 14) or saline (n = 10) between 120 and 130 d gestation, and adrenal glands were collected for morphological analysis and determination of the mRNA expression of steroidogenic and catecholamine-synthetic enzymes. Fetal body weight was not altered by IGF-I infusion; however, adrenal weight was significantly increased by 145% after IGF-I infusion. The density of cell nuclei within the fetal adrenal cortex (the zona glomerulosa and zona fasciculata), and within the adrenaline synthesizing zone of the adrenal medulla, was significantly less in the IGF-I-infused fetuses compared with the saline-infused group. Thus, based on cell-density measurements, there was a significant increase in cell size in the zona glomerulosa and zona fasciculata of the adrenal cortex and in the adrenaline-synthesizing zone of the adrenal medulla. There was no effect of IGF-I infusion on the adrenal mRNA expression of the steroidogenic or catecholamine-synthetic enzymes or on fetal plasma cortisol concentrations. In summary, infusion of IGF-I in late gestation resulted in a marked hypertrophy of the steroidogenic and adrenaline-containing cells of the fetal adrenal in the absence of changes in the mRNA levels of adrenal steroidogenic or catecholamine-synthetic enzymes or in fetal plasma cortisol concentrations. Thus, IGF-I infusion results in a dissociation of adrenal growth and function during late gestation.

Ontogeny and effects of hypothalamic pituitary disconnection on formation of inositol trisphosphate in fetal sheep pituitary cells

In late gestation fetal sheep, the pituitary becomes increasingly responsive to stimulation by arginine vasopressin (AVP). This change appears to be one important factor mediating the plasma cortisol surge, a critical developmental event. It is not known precisely why pituitary corticotropes become more responsive at this time. In this study we examined the possibility that changes in second messenger generation [inositol trisphosphate (IP(3))] are responsible. Two studies were undertaken. The first was an ontogeny study, where pituitaries were isolated from 100-, 120-, and 140-d gestational age (dGA) fetal sheep. Cells were cultured, stimulated with AVP, and the formation of IP(3) assessed. The amount of IP(3) generated increased with gestational age (percent increases from unstimulated controls were 4.6, 11.5, and 21.5 for 100, 120, and 140 dGA, respectively), with significant differences between the 140-dGA group and both earlier groups apparent. The second study examined the impact of 120-dGA hypothalamo-pituitary disconnection (HPD), which prevents corticotrope maturation, on responsiveness of pituitary cells isolated from 140-dGA fetuses. Cells were stimulated with AVP, and the formation of IP(3) and secretion of ACTH were assessed. Significantly less IP(3) was formed, and ACTH secreted in cells from HPD compared with control fetuses (IP(3) and ACTH levels were 50% and 35% lower, respectively). Results from the HPD study demonstrate that the ontogenic changes in IP(3) after AVP require an intact hypothalamic-pituitary-adrenal axis. These findings suggest that heightened second messenger generation may be a key reason for increased ACTH secretory responsiveness to AVP in the late gestation sheep fetus.

Infusion of ACTH stimulates expression of adrenal ACTH receptor and steroidogenic acute regulatory protein mRNA in fetal sheep

The late-gestation plasma cortisol surge in the sheep fetus is critical for stimulating organ development and parturition. Increased adrenal responsiveness is one of the key reasons for the surge; however, the underlying mechanisms are not fully understood. Our recent studies suggest that ACTH-mediated increased expression of ACTH receptor (ACTH-R) and steroid acute regulatory protein (StAR) may play a role in enhancing responsiveness. Hence, we examined effects of ACTH infusion in fetal sheep on mRNA expression of these two mediators of adrenal responsiveness and assessed the functional consequences of this treatment in vitro. Fetuses of approximately 118 and 138 days of gestational age (dGA) were infused with ACTH-(1-24) for 24 h. Controls received saline infusion. Arterial blood was sampled throughout the infusion. Adrenals were isolated and analyzed for ACTH-R and StAR mRNA, or cells were cultured for 48 h. Cells were stimulated with ACTH, and medium was collected for cortisol measurement. Fetal plasma ACTH and cortisol concentrations increased over the infusion period in both groups. ACTH-R mRNA levels were significantly higher in ACTH-infused fetuses in both the 118 and 138 dGA groups. StAR mRNA increased significantly in both the 118 and 138 dGA groups. Adrenal cells from ACTH-infused fetuses were significantly more responsive to ACTH stimulation in terms of cortisol secretion than those from saline-infused controls. These findings demonstrate that increases in circulating ACTH levels promote increased expression of ACTH-R and StAR mRNA and are coupled to heightened adrenal responsiveness.

Hypothalamic-pituitary disconnection in fetal sheep blocks the peripartum increases in adrenal responsiveness and adrenal ACTH receptor expression

Although it has been recognized for over a decade that hypothalamic-pituitary disconnection (HPD) in fetal sheep prevents the late gestation rise in plasma cortisol concentrations, the underlying mechanisms remain unclear. We hypothesized that reductions in adrenal responsiveness and ACTH receptor (ACTH-R) expression may be mediating factors. HPD or sham surgery was performed at 120 days of gestation, and catheters were placed for blood sampling. At approximately 138 days of gestation, fetuses were killed, and adrenals were removed for cell culture and analyses of ACTH-R mRNA and protein. After 48 h, adrenocortical cells were stimulated with ACTH for 2 h, and the medium was collected for cortisol measurement. The same cells were incubated overnight with medium or medium containing ACTH or forskolin (FSK), followed by ACTH stimulation (as above) and cortisol and cellular ACTH-R mRNA analyses. HPD prevented the late gestation increase in plasma cortisol and bioactive ACTH and reduced adrenal ACTH-R mRNA and protein levels by over 35%. HPD cells secreted significantly less cortisol than sham cells (3.2 +/- 1.2 vs. 47.3 +/- 11.1 ng.ml(-1).2 h(-1)) after the initial ACTH stimulation. Overnight incubation of HPD cells with ACTH or FSK restored cortisol responses to acute stimulation to levels seen in sham cells initially. ACTH-R mRNA levels in cells isolated from HPD fetuses were decreased by over 60%, whereas overnight incubation with ACTH or FSK increased levels by approximately twofold. Our findings indicate that the absence of the cortisol surge in HPD fetuses is a consequence, at least in part, of decreased ACTH-R expression and adrenal responsiveness.

The effect of hypothalamo-pituitary disconnection on the renin-angiotensin system in the late-gestation fetal sheep

The activity of the renin-angiotensin system (RAS) increases significantly in the late-gestation fetal sheep. Fetal cortisol is also increased during this time, and it is thought that the increase in cortisol may modulate the RAS changes. Previous studies have examined the effects of cortisol infusion on RAS activity, but the effects of blocking the peripartum increase in cortisol concentrations on the developmental changes in the RAS are not known. Therefore, we utilized the technique of hypothalamic-pituitary disconnection (HPD), which prevents the cortisol surge from occurring, to investigate the importance of the late-gestation increase in cortisol on the ontogenic changes in RAS activity. HPD of fetal sheep was performed at 120 days of gestational age (dGA), and fetuses were delivered between 135 and 139 dGA. Control fetuses were sham operated. HPD blocked the late-gestation cortisol increase but did not alter renal renin mRNA, renal renin or prorenin protein content, nor plasma renin levels compared with sham operated. However, HPD fetuses had increased ANG II receptor subtype 1 (AT1) mRNA and protein expression in the kidney and lungs. ANG II receptor subtype 2 (AT2) expression was not altered in these tissues at either mRNA or protein level. HPD did not change AT1 or AT2 mRNA in the left ventricle but did result in decreased protein levels for both receptors. These studies demonstrate that blockade of the naturally occurring increase in fetal cortisol concentration in late gestation is associated with tissue-specific alterations in expression of AT1 and AT2 receptors. These changes may impact on fetal tissue maturation and hence have consequences in postnatal life.

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.

Site and mechanism of action of dynorphin A-(1-13) and N-methyl-D-aspartate on ACTH release in fetal sheep

Dynorphin A (Dyn A) stimulates the release of ACTH in fetal sheep, a response that involves N-methyl-D-aspartate (NMDA) receptors but not the secretogogues corticotropin-releasing hormone or arginine vasopressin. We now find that neither Dyn A-(1-13) (0.5 mg/kg, i.v.) nor NMDA (4 mg/kg, i.v.) elicits ACTH release in postnatal lambs. This led us to hypothesize that Dyn A-(1-13) and NMDA might act to release placental ACTH. However, the ability of Dyn A-(1-13), NMDA, and the kappa-opioid receptor agonist U-50488H (1 mg/kg, i.v.) to release ACTH was lost after either fetal hypophysectomy (n = 4) or hypothalamo-pituitary disconnection (n = 4). These results indicate that neither the placenta nor the fetal pituitary is the site of action for these agonists and suggest a hypothalamic or suprahypothalamic site of action. Furthermore, the release of ACTH by Dyn A-(1-13) and NMDA was abolished after pretreatment with indomethacin, suggesting that they might cause the release of a prostanoid, possibly from the placenta, that subsequently acts at the hypothalamus or serves as a permissive factor in the action of Dyn A-(1-13) and NMDA at the hypothalamus.

Fetal hypothalamus-pituitary-adrenal axis on the road to parturition

1. Activity of the fetal hypothalamus-pituitary-adrenal (HPA) axis waxes and wanes as a function of gestational age. 2. In a number of species, including sheep, at the end of gestation there is an increase in HPA activity, as characterized by an increase in fetal plasma glucocorticoids. 3. To a certain degree, the hypothalamus, pituitary and adrenal all act autonomously and, therefore, may be thought of as contributing to the initiation of the signal that results in the increase in steroidogenesis before birth. 4. Because it integrates sensory information from beyond as well as within the HPA axis and likely triggers developmental changes within the pituitary, the hypothalamus may be a 'first among equals' in being the ultimate source of triggering information for the HPA axis.

The regulation of prolactin receptor messenger ribonucleic acid levels in the sheep liver before birth: relative roles of the fetal hypothalamus, cortisol, and the external photoperiod

We have investigated the separate actions of hypothalamo-pituitary disconnection (HPD), with or without cortisol administration, and changes in the external photoperiod on the regulation of the levels of messenger RNA (mRNA) encoding long (PRLR1) and short (PRLR2) forms of PRL receptor in the liver of the fetal lamb. In pregnant Merino ewes (n = 20), the hypothalamus and pituitary were surgically disconnected in 13 fetuses (HPD group), and fetal vascular catheters were implanted in the HPD group and in an additional 7 fetuses (intact + saline group) between 104-120 days gestation (d). Fetal sheep in the HPD group were infused with either cortisol (3.5 mg/4.8 ml saline/24 h; HPD + F; n = 5) or saline for 5 days between 134-141 d, and saline was also infused in the intact group within the same gestational age range. A second group of pregnant ewes (n = 12) was kept in a 12-h light, 12-h dark cycle from 70 d until implantation of fetal vascular catheters between 106-120 d, after which ewes were allocated to either a long photoperiod (16 h of light, 8 h of darkness; LL group; n = 6) or a short photoperiod (8 h of light, 16 h of darkness; SL group; n = 6) regimen. Circulating cortisol concentrations were higher (P < 0.05) in the intact fetal sheep (18.7 +/- 3.8 nmol/liter) than in the HPD + saline group (1.5 +/- 0.6 nmol/liter), and were further increased (P < 0.05) in the HPD + cortisol group (97.4 +/- 23.7 nmol/liter). Fetal PRL concentrations were lower (P < 0.05) in the HPD + saline (10.6 +/- 4.3 ng/ml) and HPD + cortisol (5.6 +/- 2 ng/ml) groups compared with those in the intact group (38.9 +/- 6.8 ng/ml). The levels of hepatic PRLR mRNA were higher (P < 0.05) in the intact (PRLR1, 27.4 +/- 6.1; PRLR2, 17.7 +/- 2.5) and HPD + cortisol (PRLR1, 23.4 +/- 0.4; PRLR2, 15.3 +/- 3.0) groups than in the HPD + saline group (PRLR1, 10.6 +/- 1.8; PRLR2, 8.9 +/- 1.8) at 140/141 d. The mean plasma PRL concentration in the LL group (70 +/- 9 ng/ml) was higher (P < 0.05) than that in the SL group (34 +/- 15 ng/ml), whereas the levels of hepatic PRLR1 mRNA (LL group, 4.6 +/- 0.9; SL group, 4.3 +/- 0.8) and PRLR2 mRNA (LL group, 3.4 +/- 0.4; SL group, 3.0 +/- 0.5) at 140-141 d were not different. These data indicate that cortisol acts directly or indirectly to maintain hepatic PRLR mRNA levels in the sheep fetus during late pregnancy. In contrast, changes in the external photoperiod and circulating PRL concentrations in the sheep fetus do not directly alter PRLR expression in the fetal liver. These studies provide further insight into the role that the PRL axis may play in the transduction of signals about the external environment to the fetus as it prepares for the transition to extrauterine life.

Adrenal responsiveness and the timing of parturition in hypothalamo-pituitary disconnected ovine foetuses with and without constant adrenocorticotrophin infusion

Ovine parturition results from an increase in foetal cortisol secretion in late gestation which is dependent on an intact hypothalamo-pituitary connection. The cortisol surge and parturition fails in hypothalamo-pituitary disconnected (HPD) foetuses but, paradoxically, immunoreactive (ir)-ACTH concentrations and secretory dynamics appear normal. This study compares the occurrence and timing of labour, basal ir-ACTH and cortisol concentrations and adrenal responsiveness in HPD foetuses (HPD/ACTH) receiving constant ACTH(1-24) infusion (43 ng/h/kg) from surgery (114+/-1 days gestational age (GA)) with those of saline-infused HPD or intact foetuses (HPD/SAL and INT/SAL). HPD/ACTH foetuses initiated labour at 147+/-2 days GA, which was not significantly different from INT/SAL foetuses (149+/-1 day GA). HPD/SAL foetuses were killed electively at 146+/-3 days GA with no signs of labour. Foetal ir-ACTH concentrations in all groups were indistinguishable, but only HPD/ACTH and INT/SAL foetuses had a significant cortisol surge. Adrenal responsiveness to ACTH(1-24)(1 microg/kg) was greater in HPD/ACTH foetuses than in HPD/SAL or INT/SAL foetuses at all GAs studied. Adrenal responsiveness in HPD/SAL foetuses exceeded that in INT/SAL foetuses at 120 and 130 days GA but did not change with GA. In summary, the basal cortisol and parturition defect in HPD foetuses was reversed by low-dose ACTH(1-24) infusion. Basal cortisol concentrations were unrelated to adrenal responsiveness. HPD/SAL foetuses had hyper-responsive adrenals compared to those of INT/SAL foetuses until 130 days GA, suggesting that the foetal hypothalamus exerts a negative influence on adrenal cortisol responses before 130 days GA, after which time stimulatory influences predominate.

Hypothalamo-pituitary disconnection of the late-gestation ovine fetus results in profound changes in cortisol secretion that are not reflected in commensurate changes in adrenocorticotropin secretion

A prepartum increase in fetal glucocorticoid concentrations is essential for the perinatal transition to extrauterine life for many mammalian species. In the case of the sheep, this increase in cortisol is also the trigger for parturition, and depends upon an intact hypothalamo-pituitary unit. Fetal sheep that have undergone hypothalamo-pituitary disconnection (HPD) fail to have a prepartum cortisol surge or initiate labor, despite apparently normal fetal ACTH concentrations in late gestation. We have investigated whether a defect exists in the regulation of pulsatile neurohormone secretion in the pituitaryadrenal axis of the HPD sheep fetus, by comparing immunoreactive (ir) ACTH and cortisol secretory dynamics in intact and HPD fetuses at 126 and 145 days of gestation (normal gestation length, 147 days). The fetal surgery was conducted at 115 days of gestation. Blood samples were collected at 5-min intervals for 2 h on each experimental day, and the resulting irACTH and cortisol concentrations were analyzed by multiple-parameter deconvolution and cross-correlation analysis. Basal irACTH secretion was less (P < 0.01) in HPD fetuses than intact fetuses at 126 days, but it had recovered by 145 days. There were no differences in irACTH half-life or the number or duration of irACTH secretory bursts between the two groups of fetuses or the two gestational ages (GAs). The size of the irACTH secretory bursts was not affected by the operation, but it increased with GA to a similar extent in both groups of fetuses (P < 0.01). In keeping with the observations for irACTH secretion, there was no effect of age or the operation on cortisol half-life or on the number or duration of cortisol secretory bursts. In contrast, there were dramatic age-related increases (P < 0.01) in the basal cortisol secretion rate and the size of the cortisol secretory bursts in the intact, but not the HPD, fetuses. Cross-correlation analysis revealed a significant (P < 0.01) concordance between irACTH and cortisol secretion in only the intact fetuses at 126 days; this was not apparent in the intact fetuses at 145 days, or in the young or old HPD fetuses. These findings confirm a major defect in cortisol secretion in the late-gestation HPD fetus but suggest that this is not caused by defects in irACTH secretion. Together with other observations, these data suggest that ACTH may not be the sole, or primary, regulator of adrenal cortisol secretion in the late-gestation ovine fetus.

Photoperiodic history and hypothalamic control of prolactin secretion before birth

We investigated whether the fetal lamb can construct a photoperiodic history in utero. We measured the fetal PRL response to a 12-h photoperiod in intact fetal sheep and in fetal sheep after hypothalamo-pituitary disconnection (HPD), following exposure of the ewe to either a long (16 h L) or short (8 h L) photoperiod for 50 days in early pregnancy. Ewes were maintained on either a long light (LL, n = 20) or a short light (SL, n = 19) regimen from 57 days gestation until fetal HPD (pre-LL, n = 7; pre-SL, n = 7) or sham surgery (pre-LL, n = 13; pre-SL, n = 12) was performed at 99-113 days gestation. All ewes were housed in a 12-h photoperiod from surgery until 140 days gestation. In HPD fetal sheep previously exposed to SL, fetal PRL concentrations were significantly higher (P < 0.05) after 20 days in the 12-h L regimen than previously (0-5 days, 3.2 +/- 0.6 ng/ml; 21-25 days, 5.6 +/- 1.4 ng/ml). In the HPD fetal sheep previously exposed to LL, however, fetal PRL concentrations significantly decreased (P < 0.05) after 5 days exposure to the 12-h L regimen (6.7 +/- 2.9 ng/ml) and remained low throughout the remaining study period (31-35 days, 1.7 +/- 0.5 ng/ml). In contrast, in the sham group there was no effect of photoperiodic history on the gestational age profile of fetal PRL, and PRL concentrations increased significantly (F = 22.4, P < 0.001) in fetal sheep previously exposed to either SL or LL. Fetal PRL concentrations were significantly higher (P < 0.05) after 121 days gestation in the 12-h L regimen in all sham fetal sheep (<110 days, pre-SL 6.4 +/- 0.3 ng/ml, pre-LL 12.0 +/- 3.3 ng/ml; 121-125 days, pre-SL 20.0 +/- 3.9 ng/ml, pre-LL 25.9 +/- 4.4 ng/ml). TRH (50 microg) was administered i.v. to all fetal sheep at 130-134 days gestation. There was a significant fetal PRL response to TRH in both the HPD (F = 20.9, P < 0.001) and sham (F = 31.3, P < 0.001) groups. There was no difference, however, in the PRL response to TRH in fetal sheep previously exposed to SL or LL in either the HPD or sham groups. The maximum percentage changes in PRL occurred at +10 min after TRH administration in the HPD (pre-SL, 421 +/- 75%; pre-LL, 555 +/- 76%) and sham groups (pre-SL, 394 +/- 68%; pre-LL, 369 +/- 59%). In summary, therefore, we have demonstrated that there is an effect of photoperiodic history on the PRL response to an intermediate photoperiod in utero in HPD fetal sheep. It appears, however, that the effect of photoperiodic history on PRL secretion in intact fetal sheep is either masked or suppressed by the stimulatory effect of factors associated with an increase in gestational age acting at the fetal hypothalamus.

The relative roles of the hypothalamus and cortisol in the control of prolactin gene expression in the anterior pituitary of the sheep fetus

The neuroendocrine control of prolactin synthesis and secretion before birth is not well understood. We have measured the changes in the level of prolactin mRNA in the anterior pituitary of the fetal sheep throughout the last 15 days of pregnancy (term = 147 +/- 3 days gestation). We have also investigated the effects of surgical disconnection of the fetal hypothalamus and pituitary (HPD) with or without long term cortisol infusion on pituitary prolactin mRNA levels and plasma prolactin concentrations in the late gestation sheep fetus. Prolactin mRNA levels were measured in anterior pituitaries collected from a series of fetal sheep (130-134 days, n = 6; 135-140 days, n = 6; 141-145 days, n = 6) in late gestation. HPD was carried out in ten fetal sheep at 105-115 days gestation and five intact fetal sheep were used as controls. In the HPD group, either saline (HPD + saline group, n = 5) or cortisol was infused (3.5 mg/24 h) for 5 days from 134-136 days gestation (HPD + cortisol group, n = 5). There was an increase in the ratio of prolactin mRNA: 18S rRNA in the fetal pituitary between 130-134 days (0.46 +/- 0.08, n = 6) and 135-140 days (1.27 +/- 0.17 n = 6) which was maintained after 141 days gestation, (1.27 +/- 0.11, n = 6). The mean prolactin mRNA: 18 S rRNA ratio was significantly higher (P < 0.05) in intact fetal sheep (1.41 +/- 0.16, n = 4) than in the HPD fetal sheep after either saline (0.54 +/- 0.14, n = 4) or cortisol (0.74 +/- 0.24, n = 5) administration. The mean plasma concentration of prolactin was also higher in the intact group (28.3 +/- 3.9 ng/ml) when compared with the HPD + saline group (8.0 +/- 3.3 ng/ml) or the HPD + cortisol group (5.6 +/- 1.9 ng/ml). We have demonstrated that there is a strong hypothalamic drive to prolactin synthesis and secretion in the fetus and that cortisol does not act directly at the fetal pituitary to stimulate prolactin synthesis and secretion in late gestation.

The peptide ACTH(1-39), adrenal growth and steroidogenesis in the sheep fetus after disconnection of the hypothalamus and pituitary

1. We have investigated the role of the fetal hypothalamo-pituitary axis in the control of adrenocortical growth and steroidogenesis in the sheep fetus during late gestation. Plasma concentrations of ACTH(1-39) increased between 120-125 and 136-142 days (P < 0.05), but did not change after surgical disconnection of the fetal hypothalamus and pituitary (HPD) at 106-120 days gestation. There was no effect of either gestational age or HPD on the circulating concentrations of the ACTH-containing precursors pro-opiomelanocortin (POMC) and pro-ACTH (the 22 kDa N-terminal portion of POMC). 2. In the fetal sheep adrenal, the relative abundance of the mRNAs of the steroidogenic enzymes CYPIIA1 and CYP21A1 increased between 130-135 and 136-140 days gestation (P < 0.05) and remained high after 141 days, whereas that of CYP17 mRNA increased after 141 days gestation (P < 0.05). The abundance of adrenal 3 beta-HSD mRNA did not change between 130 and 145 days. 3. Hypothalamo-pituitary disconnection significantly reduced the abundance of of CYPIIA1 mRNA, 3 beta-HSD mRNA and CYP17 mRNA by 3.4, 3.1 and 3.7 times, respectively, at 140-142 days gestation (P < 0.05). 4. In the intact group of fetal sheep, adrenal weight increased between 130-135 and 141-145 days (P < 0.05), but there was no change in the abundance of adrenal insulin-like growth factor II (IGF-II) mRNA across this gestational age range. Hypothalamo-pituitary disconnection significantly reduced fetal adrenal weight to 66% that of intact sheep (P < 0.01), but did not alter the abundance of IGF-II mRNA in the fetal adrenal at 140-142 days. 5. Our results suggest that the prepartum changes in adrenal growth and steroidogenesis are under the control of an intact hypothalamo-pituitary axis in late gestation and are dependent on an increase in circulating ACTH(1-39), rather than on ACTH precursors. We have found no evidence, however, for a direct-relationship between fetal adrenal growth or steroidogenesis and adrenal IGF-II mRNA between 130 and 145 days gestation.

Effect of late hypothalamo-pituitary disconnection on the development of the HPA axis in the ovine fetus and the initiation of parturition

The studies of Liggins et al. (1) in which fetuses stalk-sectioned from day 116 onwards delivered at or near term, suggested that a connection between the fetal hypothalamus and pituitary is not essential for parturition to occur. The objective of this study was to repeat these experiments on the effects of pituitary stalk sections at different gestational ages and include information on the plasma concentrations of key fetal hormones. We have used the more sophisticated technique of hypothalamo-pituitary disconnection (HPD) at either of two gestational age ranges (123-127 days or 133-135 days). Completeness of the procedure was assessed by demonstrating an attenuated prolactin response to chlorpromazine challenge. Following HPD, gestation was prolonged for at least eight days beyond term (146.2 +/- 1.5 days) in 9 of the 10 fetuses operated. Fetal plasma ACTH1-39 concentrations were not different between the HPD and control fetuses, increasing in all groups with increasing gestation. Fetal plasma cortisol concentrations increased (P < 0.01) in control fetuses over gestation. Cortisol concentrations did not change significantly in the day 125 HPD group following HPD but increased in the day 135 HPD group (P < 0.05) with advancing gestation. These latter concentrations, however were markedly less (P < 0.001) than those for control fetuses prior to parturition. Fetal and maternal plasma PGE2 concentrations increased (P < 0.01) in the control group over gestation but did not change following HPD. Maternal plasma progesterone concentrations decreased (P < 0.05) after day 143 in the control group but did not change in the HPD group.(ABSTRACT TRUNCATED AT 250 WORDS)