ABSTRACT

In sheep, birth is preceded by an increase in fetal plasma concentrations of ACTH and cortisol. Activation of the fetal pituitary-adrenal axis is pivotal to the onset of parturition in this species and may be regulated, at least in part, by corticotrophin-releasing factor (CRF). Pulsed administration of CRF has been shown to activate the fetal pituitary-adrenal axis in immature fetal sheep. However, pituitary ACTH responsiveness declined after continued administration of CRF, as a result of increasing negative feedback effects of increased concentrations of endogenous cortisol. To test the hypothesis that arginine vasopressin (AVP) is required, in addition to CRF, to produce the necessary trophic stimulus to the pituitary-adrenal axis, we administered saline, CRF (1 μg), AVP (200 ng) or CRF plus AVP as pulses every 4 h for 7 days to fetal sheep beginning at days 117–120 of pregnancy (term =145 days). Pituitary-adrenal responses were assessed by measuring plasma concentrations of immunoreactive (ir) ACTH and cortisol in response to one of the pulses on each of the 7 days of treatment.

On day 1, CRF and AVP significantly increased plasma concentrations of ir-ACTH and there was a synergistic interaction when the two peptides were given together (P<0·05). However, as pulsed treatment continued there was a decline in the pituitary ir-ACTH response to all treatments (P<0·05). This decline in pituitary response occurred over a much longer period of time when CRF and AVP were given together when compared with the two peptides given separately. In contrast, the cortisol response to endogenously released ir-ACTH after administration of CRF, AVP or CRF plus AVP was small on day 1 but gradually increased as treatment progressed. This was particularly apparent when the two peptides were given together. A significant inverse correlation (r = 0·781, P<0·01) between basal cortisol concentrations and the ir-ACTH response to CRF plus AVP was observed over the 7 days of treatment. Premature delivery was not induced by any of the treatments despite significant increases in fetal adrenal weight. Furthermore, there were no changes in the circulating maternal plasma concentrations of progesterone or oestrone during the 7 days of the experiment.

We conclude that combination of CRF and AVP administered as pulses to immature fetal sheep results in a greater degree of pituitary-adrenal activation when compared with the two peptides given independently. However, even after this combined treatment regimen pituitary responsiveness eventually declines, an effect which may be due to increased negative feedback effects of increased endogenous cortisol.

Journal of Endocrinology (1990) 124, 27–35

Abstract

The fetal hypothalamo–pituitary–gonadal axis reaches a peak in activity at mid-gestation and this is followed by a period of suppression which persists until the onset of puberty. The decline in gonadotrophic activity during late gestation is thought to reflect the maturation of central and peripheral feedback signals. In order to establish if sustained pituitary responsiveness is rate limiting to the reinstatement of reproductive function, we have examined the endocrine consequences of repeated pulsatile GnRH administration to male and fetal sheep during late gestation. Beginning on day 121 of gestation (term=145 days) chronically catheterized fetal sheep were given i.v. pulses of either 500 ng GnRH or saline every 2 h for 14 days. Pituitary and gonadal responses were assessed by measuring changes in plasma concentrations of LH, FSH, inhibin and testosterone (in male fetuses) in response to the first pulse of GnRH on day 1 and to the corresponding pulse on days 4, 7, 10 and 14. In response to the first pulse of GnRH there was an immediate release of LH, with the peak response being significantly (P<0·01) greater than on subsequent days. In male fetuses each pulse of LH was followed by a rise in plasma testosterone concentrations within 40–60 min. The amplitude of these testosterone responses increased significantly (P<0·01) after 9 days of treatment despite a decline in the plasma LH response. Basal FSH concentrations increased progressively (P<0·05) during pituitary stimulation with GnRH in both male and female fetuses. Immunoreactive inhibin concentrations were significantly (P<0·05) higher in males than in females, and there was a gradual increase throughout the experimental period irrespective of treatment. We observed no inverse correlation between inhibin and FSH concentrations. These data show that pulsatile administration of GnRH to fetal sheep during late gestation results in sustained re-activation of pituitary–gonadal function. The decline in fetal gonadotrophins, which is a characteristic feature of late gestation, is therefore likely to result from inadequate GnRH secretion from the fetal hypothalamus rather than an inhibition of pituitary function by peripheral feedback signals.

Journal of Endocrinology (1997) 153, 385–391

ABSTRACT

This study examined the ability of prostaglandin E₂ (PGE₂) to regulate ACTH secretion from cultured anterior pituitary cells of fetal sheep between days 130 and 140 of gestation (term = 145 days). Corticotrophin-releasing factor (CRF) and arginine vasopressin (AVP) induced dose-dependent (0·1–1000 nmol/l) increases in ACTH secretion from fetal sheep pituitary cells maintained in culture for 6 days, with AVP being significantly (P<0·01) more potent than CRF. PGE₂ (1000 nmol/l) significantly (P<0·05) enhanced the ability of AVP, but not CRF, to stimulate ACTH secretion. However, PGE₂ given alone (0·1–1000 nmol/l) had no effect on ACTH secretion. Concomitant administration of CRF and AVP induced a greater release of ACTH than after treatment with either peptide alone, a synergistic interaction which was unaffected by simultaneous administration of PGE₂.

These results provide evidence for a direct action of PGE₂ on ACTH secretion from the fetal sheep pituitary gland via a specific interaction with AVP. This interaction may allow increased fetal plasma concentrations of PGE₂, seen during late gestation, to stimulate fetal pituitary–adrenal maturation.

Journal of Endocrinology (1992) 132, 33–38

Abstract

During ovine fetal development there is a progressive maturation of the hypothalamo–pituitary–adrenal axis which culminates in the onset of birth. The role and regulation of the intermediate lobe of the fetal pituitary gland in relation to this maturational process remains controversial. To test the hypothesis that the pars intermedia of the ovine fetal pituitary is under tonic inhibitory dopaminergic control we treated fetal sheep at day 131 of gestation with a 3-day intravenous infusion of one of the following: the dopamine antagonist sulpiride (0·3 mg/0·5 ml/h; n=12), the dopamine agonist bromocriptine (0·03 mg/0·5 ml/h; n=7) or vehicle (0·1 m tartaric acid in saline; n=8) alone. Fetal plasma concentrations of α-MSH were significantly (P<0·01) increased by treatment with sulpiride and decreased (P<0·05) by bromocriptine. The increase in α-MSH after sulpiride was characterised by an increase in the amplitude of α-MSH pulses whereas bromocriptine virtually abolished all pulses of α-MSH. Immunoreactive ACTH (IR-ACTH) concentrations were significantly (P<0·05) elevated after sulpiride but were unaffected by bromocriptine. There were no changes in the pulsatile characteristics of IR-ACTH secretion. Cortisol concentrations were unchanged by either treatment. We conclude that fetal α-MSH and IR-ACTH are secreted in a pulsatile fashion and are tonically inhibited by dopaminergic pathways. The lack of an effect of endogenously raised α-MSH on plasma cortisol concentrations provides evidence that this POMC-derived peptide is not responsible for the regulation of cortisol biosynthesis and/or secretion.

Journal of Endocrinology (1996) 151, 439–447

ABSTRACT

To test the hypothesis that endogenous opioids participate in the regulation of the ontogenic development of the hypothalamic-pituitary-adrenal axis in fetal sheep, we measured changes in immunoreactive (ir) ACTH and cortisol concentrations in response to bolus injections of either the [Met]-enkephalin analogue, [d-Ala²,N-Phe⁴,Met(0)ol⁵]-enkephalin (FK 33-824; 25 μg), the opioid antagonist naloxone (1 mg), a combination of both, or saline vehicle, administered to chronically catheterized fetal sheep through late gestation.

There were no effects of either FK 33-824, naloxone or saline on the release of ir-ACTH and cortisol at the earliest stage of gestation studied (days 110–115). By days 125–130, FK 33-824 caused a rapid but short-lived (30 min) increase in plasma ir-ACTH (P <0·05) which was accompanied by a smaller but nonsignificant increase in cortisol. Naloxone given concurrently with FK 33-824 abolished this response, thus providing evidence for a specific effect through opioid receptors. Naloxone given alone was without effect. At days 135–140, FK 33-824 caused a significant increase in ir-ACTH which was of similar duration and magnitude to that which occurred at days 125–130. There was a larger basal variation in plasma concentrations of cortisol than at days, 125–130, and a greater increase in cortisol after FK 33-824, although this did not reach statistical significance. Naloxone again reversed the effects of FK 33-824 but was without effect when given alone.

We conclude that opioid receptors capable of mediating the effects of exogenously administered opioid peptides on the pituitary-adrenal axis are present in fetal sheep by days 123–130. However, we have been unable to demonstrate tonic control of this axis by endogenous opioids, since naloxone is ineffective when given alone.

J. Endocr. (1988) 119, 389–395

Abstract

In order to investigate the regulation of the hypothalamo-pituitary-gonadal axis during fetal development, sheep fetuses at day 70 of gestation were implanted subcutaneously with a biodegradable implant containing the longacting gonadotrophin-releasing hormone (GnRH) agonist, buserelin. The treatment of fetuses with a GnRH agonist throughout the last half of gestation (term=145 days) abolished the increase in plasma LH concentrations that was seen in 2-day-old control lambs in response to an injection of GnRH. This attenuated response was associated with corresponding reductions in the pituitary content of LH and FSH. Immunolocalization studies revealed that pituitary glands from newborn lambs implanted with a GnRH agonist during fetal development were devoid of immunopositive LH- and FSH-containing cells. At birth the testicular weights of GnRH agonist-treated ram lambs were significantly decreased by 40% when compared with controls. This was associated with a 45% reduction in the total number of Sertoli cells per testis. In newborn ewe lambs GnRH agonist treatment had no effect on ovarian weight or on the morphological appearance of the ovaries. GnRH agonist treatment had no effect on the plasma concentrations of progesterone and oestrone in the maternal circulation or on the length of gestation. These results show (1) that GnRH positively regulates the synthesis and secretion of gonadotrophins in the fetus, (2) that reduced fetal gonadotrophic support during the last half of gestation results in a reduction in testicular growth, and (3) that fetal gonadotrophins do not affect maternal steroid secretion.

Journal of Endocrinology (1994) 141, 317–324

Abstract

The changes in gonadotrophin secretion in childhood and puberty involve changes in the pulsatile pattern as well as mean concentration. We have examined pulsatile secretion using 24-h LH profiles in 78 children aged 4·2–15·6 years and six adult men. The profiles were analysed by a method which gives an estimate of peak and baseline levels and by spectral analysis. Prepubertal children were divided into groups by age and pubertal children by pubertal stage.

Baseline LH levels in children aged 4·2–6·9 years were higher than in those aged 7·1–9·8 years. Pulse frequency in both groups was slow (periodicities 140–200 min). In the oldest prepubertal group there was an increase in peak levels with increased spectral power at periodicities of 100–120 min. Pulse frequency did not change in puberty (periodicity 120–160 min). Girls demonstrated an increase in both peak and baseline LH concentrations in early puberty. Boys had an increase in only peak levels in early puberty; there was no increase in baseline concentrations until a testicular volume of 10 ml was attained.

In conclusion, these data show that LH levels do not reach a nadir until 8 years of age. We have detected a sex difference in the pattern of LH secretion seen in early puberty, and this mirrors clinical findings. The mature pattern attained by the end of puberty in both sexes is probably important for fertility.

Journal of Endocrinology (1994) 141, 169–176

Abstract

Anti-Mullerian hormone (AMH) and androgenic steroids are key factors regulating the masculinisation of the internal and external genitalia during fetal development. AMH is produced in Sertoli cells and causes regression of the Mullerian ducts in the male. 3β-Hydroxysteroid dehydrogenase (3β-HSD) is one of the key steroidogenic enzymes regulating testosterone production in Leydig cells. The objective of this experiment was to elucidate the development of the ovine fetal testes by identifying the spatio-temporal expression of AMH, 3β-HSD and androgen receptor expression within them. Fetuses from days 30 and 40 of gestation were fixed intact, while the gonads were dissected from the fetuses on days 70, 100 and 130 of gestation. Tissue was fixed in Bouin's fixative for 6 h, processed into paraffin wax and sections immunostained using rabbit anti-human AMH, 3β-HSD or androgen receptor antibodies.

While seminiferous cords were absent on day 30 of gestation, pre-cord organisation was apparent and the gonad could be clearly distinguished from surrounding tissue by the presence of AMH and 3β-HSD immunopositive cells. Androgen receptor expression was not apparent at this stage. By day 40 of gestation the testis was organised into distinct seminiferous cords and intense immunostaining for AMH and 3β-HSD was present in Sertoli cells within the cords and Leydig cells in the interstitium respectively. Androgen receptor immunopositive cells were present in the interstitium but cells destined to develop into rete testis were immunonegative. By day 70 of gestation, the rete testis was organised in the centre of the testis and was strongly androgen receptor immunopositive. AMH and 3β-HSD expression was present in Sertoli and Leydig cells respectively. The expression of AMH, 3β-HSD and androgen receptor in the 100 and 130 day gestation fetuses was similar to that identified in the 70 day fetuses. In conclusion, Sertoli and Leydig precursor cells are present in the gonad prior to seminiferous cord formation and contain AMH and 3β-HSD at all stages of gestation examined. While androgen receptor immunoexpression was present in nuclei of interstitial cells from day 40 of gestation and in the rete testis from day 70 of gestation, Sertoli cells were immunonegative for androgen receptor at all of the stages examined.

Journal of Endocrinology (1997) 153, 27–32

ABSTRACT

Corticotrophin-releasing factor (CRF) is thought to be an important physiological regulator of the pituitary-adrenal axis in fetal sheep and, as such, plays a fundamental role in the initiation of parturition in this species. However, little is known of the controls of CRF secretion from the fetal hypothalamus.

We looked for the presence of CRF in fetal hypothalami, and examined whether the hypothalamic CRF concentration or molecular species changed in relation to gestational age. We established an in-vitro perifusion system to examine the release of CRF from perifused hypothalami taken from fetuses at day 100 and day 140 of pregnancy, under basal conditions and in response to potassium depolarization and/or dexamethasone administration.

Immunoreactive CRF was present in fetal hypothalami as early as day 100 (2·42 ± 0·99 (s.e.m.) μg/g protein, n = 9) and in similar concentrations at day 140 (2·31 ± 0·69 μg/g protein, n = 9). There was a significant (P < 0·05) increase in hypothalamic CRF content to 14·79 ± 4·09 μg/g protein (n = 16) between day 122 and day 135 of gestation. Using Sephadex G-75 chromatography, hypothalamic extracts at day 100, days 122–135 and day 140 eluted with a single peak of immunoreactivity which corresponded to synthetic ovine CRF(1–41).

The basal release of CRF from perifused hypothalami at day 140 (76·6 ± 10·4 pg/fraction, n = 8) was significantly (P < 0·05) greater than at day 100 (50·1 ± 10·2 pg/fraction, n = 11). Dexamethasone significantly inhibited basal CRF release at day 140 of gestation but not at day 100. Potassium depolarization caused a rapid release of CRF in all cases, a response which was independent of gestational age or treatment with dexamethasone.

We conclude that the fetal hypothalamus contains immunoreactive CRF as early as day 100 of gestation and that this material may be released when perifused in vitro under basal conditions and in response to a depolarizing agent. The basal release of CRF from perifused hypothalami of day-140 fetuses was greater than at day 100 and was inhibited by dexamethasone, suggesting maturation of negative feedback control of CRF output between days 100 and 140. Since dexamethasone had no effect on potassium-stimulated release of CRF, we suggest that its effects are at sites other than the hypothalamic CRF nerve terminals.

Journal of Endocrinology (1989) 122, 15–22

Abstract

In order to investigate the ontogeny of gonadal inhibin production in the male fetal sheep, testes were collected from male fetuses at days 70, 100, 130 and 140 of gestation (term=145 days). The expression and localization of inhibin α- and inhibin β_A-subunit mRNA and protein were evaluated using in situ hybridization and immunocytochemistry. The expression of inhibin α-subunit mRNA was localized within the seminiferous cords of the developing fetal testis and progressively increased with gestational age. Immunostaining corresponding to immunoreactive inhibin α-subunit was detected in Sertoli cells within the seminiferous cords at days 100, 130 and 140 of gestation. In addition, immunostaining was detectable in a small proportion of Leydig cells. No expression of inhibin β_A-subunit mRNA or immunoreactivity was detected in any testicular tissue at any stage of gestation. These data show that the Sertoli cells of the developing fetal sheep testis have the capacity to produce inhibin α-subunit by day 100 of gestation and that production increases during late gestation.

Journal of Endocrinology (1995) 145, 35–42