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BOR - Papers in Press, published online ahead of print February 11, 2004.
Biol Reprod 2004, 10.1095/biolreprod.103.025254
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BIOLOGY OF REPRODUCTION 70, 1650–1657 (2004)
DOI: 10.1095/biolreprod.103.025254
© 2004 by the Society for the Study of Reproduction, Inc.

Pregnancy

Effects of Leptin on Fetal Plasma Adrenocorticotropic Hormone and Cortisol Concentrations and the Timing of Parturition in the Sheep1

B.S.J. Yuen3, P.C. Owens4, M.E. Symonds5, D.H. Keisler6, J.R. McFarlane7, K.G. Kauter7, and I.C. McMillen2,3

Departments of Physiology3 Obstetrics and Gynaecology,4 University of Adelaide, Adelaide,South Australia 5005, Australia Academic Division of Child Health,5 University of Nottingham, Nottingham, NG7 2UH, United Kingdom Department of Animal Sciences,6 University of Missouri, Columbia, Missouri 65211 Department of Physiology,7 University of New England, Armidale, New South Wales 2350, Australia


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

adrenal, cortisol, leptin, pregnancy


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Leptin is a 16-kDa polypeptide hormone, which is principally synthesized and secreted by adipose tissue and acts to regulate energy homeostasis and a range of neuroendocrine and reproductive functions [1–3]. In the human infant, there is a positive relationship between cord blood concentrations of leptin at delivery and either birth weight or neonatal adiposity [4–6]. In animal species such as the sheep and pig, in which fat is deposited before birth, leptin is synthesized in fetal adipose tissue and is present in the fetal circulation throughout late gestation [7–13]. In the sheep fetus, the expression of leptin mRNA in fetal adipose tissue is positively correlated with circulating leptin concentrations and there is also a positive relationship between fetal plasma leptin concentrations and the relative mass of lipid locules present within fetal adipose tissue [11, 13]. We have recently shown that intravascular infusion of leptin in the sheep fetus during late gestation altered the lipid storage characteristics and suppressed leptin mRNA expression within fetal adipose tissue [14]. Thus, leptin may act as a circulating signal of fetal adiposity and have a "lipostatic" role before birth.

It is well established in the sheep that the prepartum increase in circulating cortisol is required for the differentiation and maturation of key fetal organs such as the fetal lung, liver, kidney, and brain and for the normal timing of parturition and the successful transition to extrauterine life [15]. Forhead et al. [10] reported that in the sheep fetus, plasma cortisol and leptin concentrations increased in parallel and were positively related between 130 and 140 days gestation and that fetal adrenalectomy resulted in lower plasma leptin concentrations after 136 days. These findings are consistent with studies that have demonstrated that glucocorticoids stimulate both leptin gene expression and secretion from adult adipocytes in vivo and in vitro [16–19] and suggest that there is a positive relationship between the level of activation of the fetal hypothalamus-pituitary-adrenal (HPA) axis and leptin synthesis and/or secretion in late gestation. A separate study, however, investigated the effects of intracerebroventricular (icv) infusion of leptin between 135 and 140 days gestation on the characteristics of plasma adrenocorticotropic hormone (ACTH) and cortisol pulses occurring during a 4-h sampling period on the first and last day of the infusion period [20]. These authors found that icv leptin administration blunted the size of the increase that occurred in the amplitude and mean value of plasma ACTH and cortisol pulses between 135 and 140 days gestation [20]. These data are consistent with studies in the adult rat, which have shown that administration of leptin can attenuate fasting or restraint-induced stimulation of the HPA axis [21–22].

Given the conflicting nature of the previous studies, we directly determined whether leptin can act to suppress the normal prepartum activation of the fetal HPA axis and delay the timing of parturition. First, we measured the effects of a 4-day intrafetal infusion of leptin on fetal plasma ACTH and cortisol concentrations starting from 136 days gestation (i.e., at the onset of the prepartum activation of the fetal HPA axis). Second, we infused leptin into fetal sheep from 144 days gestation until delivery and measured the effects of an increase in circulating leptin on the prepartum changes in fetal plasma ACTH and cortisol concentrations and on the timing of parturition.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals and Surgery

These studies were approved by the University of Adelaide Animal Ethics Committee. Dated pregnant Merino ewes (n = 25) were housed in individual pens in rooms with a 12L:12D cycle and fed once daily (~10– 12 MJ/kg metabolizable energy) with a diet consisting of lucerne chaff (85% dry matter) and concentrated pellets containing straw, cereal, hay, clover, barley, oats, lupins, almond shells, oat husks, and limestone (90% dry matter; Johnsons and Sons, Kapunda, Australia) at 1100 h with water provided ad libitum. Surgery was performed between 110 and 126 days gestation, as previously described [13, 23]. Briefly, general anesthesia was induced in ewes by an i.v. injection of sodium thiopentone (1.25 g, Pentothal; Rhone Merieux, Pinkenba, Australia) and maintained by 2.5% to 4% halothane (Fluothane; ICI, Melbourne, VIC, Australia). Under aseptic conditions catheters were inserted into a maternal jugular vein, a fetal carotid artery and jugular vein, and the amniotic cavity. Catheters were filled with heparinized saline and the fetal vascular and amniotic catheters exteriorized through an incision made in the ewe's flank. Ewes and their fetuses received a 2-ml i.m. injection of antibiotics (procaine penicillin 250 mg/ml; dihydrostreptomycin 250 mg/ml; procaine hydrochloride 20 mg/ml Penstrep Illium, Troy Laboratories, Smithfield, Australia). Animals were allowed to recover for at least 4 days after surgery before routine fetal arterial blood samples (3 ml) were collected every 2–3 days before the infusion studies commenced.

Experimental Protocols

Leptin Infusion from 136 or 137 Days Gestation In 13 pregnant ewes at 136–137 days gestation, fetal arterial blood samples (3 ml) were collected at –3 h, –2 h, –1 h and –30 min relative to the start of the infusion period at 1300 h. A bolus of either sterile saline (0.5 ml, n = 7) or recombinant ovine leptin (0.25 mg in 0.5 ml sterile saline, n = 6; provided by Professor Duane Keisler, Department of Animal Sciences, University of Missouri, Columbia, MO) was infused into the fetal jugular vein, immediately followed by a continuous infusion (0.16 ml/h) of either sterile saline or leptin (0.48 mg/kg/day) [24], respectively. Fetal arterial blood samples were collected at +2 min, +30 min, +1 h, +2 h, +4 h, and +8 h on the first day of the infusion and at 0900, 1300, and 1700 h on the second and third days and at 0900 and 1300 h on the fourth day of the infusion. Blood samples were centrifuged at 1500 x g for 10 min, and plasma aliquots were separated and stored at –20°C. Fetal arterial blood samples (0.5 ml) were also collected daily to monitor fetal blood gases and pH (ABL 520 blood gas analyzer; Radiometer, Copenhagen, Denmark). After 96 h (at 140 or 141 days gestation), ewes were killed with an overdose of sodium pentobarbitone (Virbac Pty. Ltd., Peakhurst, Australia) and fetuses (saline infused group: five singletons and two twins; leptin-infused group: three singletons and three twins) were delivered by hysterotomy, weighed, and decapitated.

Leptin Infusion from 144 Days Gestation In 12 pregnant ewes at 144 days gestation, a bolus of saline (n = 5) or recombinant ovine leptin (n = 7, 0.5 mg/0.5 ml sterile saline) was infused into the fetal jugular vein immediately followed by a continuous infusion of saline or recombinant ovine leptin (1.0 mg kg day). On the first day of infusion, fetal arterial blood samples (3.5 ml) were collected at–3 h, –2 h, –1 h, and –30 min and at +2 min, +30 min, +1 h, +2 h, +4 h, and +8 h relative to the start of the infusion at 1300 h. On the second, third, and fourth days of the infusion, fetal blood samples were collected at 0900, 1300, and 1700 h, and on subsequent days, fetal blood samples were collected at 0900 and 1700 h until either the ewe was in late labor (n = 2) or the fetus delivered (n = 10). There were four singletons and one twin lamb (three male, two female) in the saline-infused group and seven singletons (four male, two female, one unknown) in the leptin-infused group. Ewes were defined as being in late labor when the pressure of repeated intrauterine contractions was greater than 20 mm Hg in amplitude. Intrauterine pressure was measured using a MacLab 1050 displacement transducer (ADInstruments, Castle Hill, NSW, Australia) connected to the saline-filled amniotic catheter [24]. A MacLab data acquisition system (ADInstruments) was attached to the transducer and MacLab Chart software was used to analysis the intrauterine pressure recordings.

Leptin ELISA

A competitive ELISA specific for ovine leptin was used to measure plasma leptin concentrations in fetal sheep, as previously described [13, 25]. Briefly, 6 ng recombinant bovine leptin was coated onto an ELISA plate by overnight incubation at 37°C. The plate was blocked with 200 µl of 5% skim milk in ELISA buffer for 1 h at 37°C. Fetal plasma samples (100 µl) were added to wells containing a biotinylated chicken anti-recombinant bovine leptin in 100% Triton X-100, 0.5% SDS, and 5% sodium deoxycholate (50 µl), and the plate was incubated overnight at 37°C. A biotinylated second antibody was added to the plate and incubated overnight at 37°C. The plate was then washed and incubated for 1 h with streptavidin conjugated to alkaline phosphatase (Amrad Biotech, Boronia, Australia) and then developed with p-nitrophenylphosphate disodium salt hexahydrate. The sensitivity of the assay was 0.5 ng/ml, and the interassay and intra-assay coefficients of variation were 11% and 9%, respectively.

ACTH Radioimmunoassay

ACTH concentrations in fetal sheep plasma were measured by radioimmunoassay (DiaSorin, Stillwater, MN), previously validated for fetal sheep plasma [26]. The cross-reactivity of the rabbit anti-ACTH antisera was <0.01% with {alpha}-melanocyte-stimulating hormone, ß-endorphin, ß-lipotropin, parathyroid hormone, vasopressin, and growth hormone. Briefly, rabbit anti-ACTH serum (50 µl) was added to each sample (50 µl) and incubated overnight at 4°C. Radiolabeled [125I]-ACTH (50 µl) was added to each tube and incubated overnight at 4°C. Rabbit serum (200 µl), preprecipitated with goat anti-rabbit serum and polyethylene glycol, was added to samples that were then centrifuged. The interassay coefficient of variation was 11.5%, and the intra-assay coefficient of variation was 5.2%.

Cortisol Radioimmunoassay

Cortisol was extracted from fetal plasma using dichloromethane as previously described [27]. The efficiency of recovery of radiolabeled [125I]- cortisol from fetal plasma using this extraction procedure was >90%. The cross-reactivity of the rabbit anticortisol antisera was <1% with pregnenolone, aldosterone, progesterone, and estradiol. Fetal cortisol concentrations were then measured using a Amersham radioimmunoassay kit (Amersham Pharmacia Biotech Inc., Piscataway, NJ). Briefly, 100 nM of hydrocortisone (Sigma Chemical Co., St. Louis, MO) was serially diluted in buffer (0.1 mol/L Tris-HCl [pH 7.4] 0.5% BSA, 0.1% sodium azide) to generate a standard curve. Plasma extracts (100 µl) were incubated with rabbit anticortisol antisera (100 µl) overnight at 4°C. Radiolabeled [125I]- cortisol (100 µl) was then added to the samples that were then incubated overnight at 4°C. The inter- and intra-assay coefficients of variation were 10% and 5%, respectively.

Statistical Analyses

Data are presented as the mean ± SEM. Fetal hormone data were logarithmically transformed where required to reduce heterogeneity of variance. ANOVA with repeated measures was performed using the Statistical Package for Social Sciences (SPSSX, Chicago, IL) on a VAX mainframe computer. The Duncan new multiple range test was used post hoc to identify significant differences between mean values.

Leptin Infusion from 136 to 137 Days Gestation Mean values for fetal arterial pO2, pCO2, and pH between 0 and +96 h were calculated and a Student unpaired t-test was performed to determine whether fetal blood gases and pH values were different between the saline- and leptin-infused groups.

A mean value for the basal plasma concentration of leptin, ACTH, or cortisol was calculated for each fetus as the average of the five values during the preinfusion period. The change in fetal hormones during the infusion period was then calculated by subtraction of the mean preinfusion hormonal value at each time point, and the effects of leptin infusion on fetal hormone concentrations were analyzed using a two-way ANOVA with treatment and the length of time relative to the start of the infusion as the specified factors.

Leptin Infusion from 144 Days Gestation The effects of leptin on fetal hormone concentrations during the first 20 h of the infusion period were determined by a two-way ANOVA with treatment and time relative to the start of the infusion as the specified factors. Because the length of gestation varied among animals (147–153 days), the hormonal data from each animal were expressed relative to the known or estimated time of delivery. Intrauterine pressure traces from animals that delivered were analyzed to establish the relationship between frequency of contractions with an amplitude of >20 mm Hg and time before birth. For the two fetuses that were killed during late labor, the intrauterine pressure traces were analyzed to determine the frequency of contractions (>20 mm Hg) during labor, and the time of delivery was then estimated. Hormone data were then grouped into 12-h time blocks in relation to the actual or estimated time of delivery. In the leptin-infused group, 94 plasma samples were assayed for ACTH, 93 samples were assayed for cortisol, and 92 samples were assayed for leptin. In the saline-infused group, 67 plasma samples were assayed for ACTH, 68 samples were assayed for cortisol, and 38 samples were assayed for leptin. The effects of leptin infusion on fetal leptin, ACTH, and cortisol concentrations were determined by a two-way ANOVA using treatment and time before birth as the specified factors.

Linear regression analysis was used to assess relationships between the plasma leptin and either ACTH or cortisol concentrations in samples collected between 125 and 137 days (i.e., prior to the onset of the prepartum increase in circulating cortisol) and between 138 and 146 days gestation (i.e., after the onset of the prepartum increase in cortisol in fetuses that were infused with saline from 144 days gestation).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Leptin Infusion from 136 to 137 Days Gestation

Plasma Leptin Concentrations and Fetal Blood Gas Status Plasma leptin concentrations increased (P < 0.001) during the leptin infusion period (+24 h: 22.9 ± 3.9 ng/ ml; +92–96 h: 20.1 ± 1.5 ng/ml) but not during the saline infusion period (+24h: 1.9 ± 0.7 ng/ml; +92–96h: 4.1 ± 1.6 ng/ml). There was no significant difference in mean fetal arterial blood gas and pH values between the saline- and leptin-infused groups during the infusion period (pO2- saline infused: 21.7 ± 0.6 mm Hg; leptin infused: 22.3 ± 0.5 mm Hg; pCO2-saline infused: 50.4 ± 1.2 mm Hg; leptin infused: 49.1 ± 0.8 mm Hg; pH-saline infused: 7.404 ± 0.006; leptin infused: 7.401 ± 0.005).

Fetal Plasma ACTH and Cortisol Concentrations During the infusion period, there was a significant interaction between the effects of treatment and time on ACTH concentrations expressed relative to those in the preinfusion period (P < 0.05; Fig. 1, A and B). In the saline-infused group, the change in plasma ACTH concentrations relative to baseline values was greater at 96 h (13.7 ± 7.8 pg/ml, P < 0.01) than at between 2 h and 44 h after the start of the infusion (–6.8 ± 3.4 pg/ml). In contrast, in fetuses infused with leptin, there was no change in plasma ACTH concentrations during the 96-h infusion period (+96 h: –4.1 ± 2.0 pg/ml). There was a significant interaction between the effects of treatment and time on cortisol concentrations relative to basal levels (P < 0.02; Fig. 1, C and D). In the saline-infused group, the change in plasma cortisol concentrations was greater at 96 h (54.1 ± 7.5 nmol/L; P < 0.001) when compared with between 3 h before and 24 h after the start of the infusion (14.9 ± 2.9 nmol/L). In fetuses infused with leptin, there was no change, however, in plasma cortisol concentrations throughout the infusion period (+96 h: 7.0 ± 3.9 nmol/L). The ratios of plasma cortisol:ACTH concentrations were significantly higher (P < 0.001) in both treatment groups at +96 h (saline infused: 1.66 ± 0.23 nmol/ng; leptin infused: 1.12 ± 0.25 nmol/ng) when compared with the period from 3 h before until 52 h after the start of the saline (0.82 ± 0.26 nmol/ng) or leptin (0.63 ± 0.22 nmol/ng) infusion (Fig. 1, E and F).



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  		FIG. 1. Plasma ACTH, cortisol, and the ratio of plasma cortisol:ACTH concentrations in fetuses infused with saline (open circles; A, C, E) or leptin (closed circles; B, D, F) for 96 h from 136 to 137 days gestation. Different alphabetic superscripts denote mean values that are significantly different (P < 0.05) from each other within a treatment group

Fetal Plasma Leptin, Cortisol, and ACTH Concentrations in Saline-Infused Fetuses Between 125 and 150 Days Gestation

There was a significant increase in plasma cortisol concentrations between 125 and 150 days gestation in saline- infused fetuses. There was no change, however, in plasma leptin concentrations in these fetuses during this period (Fig. 2). Although there was no relationship between fetal plasma cortisol (y) and leptin (x) concentrations at 125– 137 days gestation, there was a significant negative relationship between the plasma concentrations of these two hormones at 138–146 days gestation (y = 81.4 – 7.7x, n = 84, r = 0.38, P < 0.005; Fig. 3).



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  		FIG. 2. Plasma cortisol and leptin concentrations between 125 and 150 days gestation in fetal sheep that were infused with saline from 144 days gestation. Different alphabetic superscripts denote mean hormone values that are significantly different (P < 0.05) from each other during late gestation



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  		FIG. 3. The relationship between plasma cortisol and leptin concentrations between 138 and 146 days gestation in those fetal sheep that were infused with saline from 144 days gestation. The relationship is described by the equation, y = 81.4 – 7.7x (r = 0.38, P < 0.05).

Leptin Infusion from 144 Days Gestation

Plasma Leptin, ACTH, and Cortisol Concentrations During the First 20 h of the Infusion Period During the first 20 h of infusion, plasma leptin concentrations increased significantly (P < 0.001) in the leptin-infused (+20 h: 18.3 ± 2.1 ng/ml) and not in the saline-infused fetuses (+20 h: 2.0 ± 0.7 ng/ml). At 20 h after the start of the infusion, there was no difference in either plasma cortisol (saline infused: 61.4 ± 5.1 nmol/L; leptin infused: 81.4 ± 22.2 nmol/L) or ACTH (saline infused: 41.0 ± 4.9 pg/ml; leptin infused: 34.1 ± 5.0 pg/ml) concentrations between the saline and leptin infused groups.

Effects of Leptin on the Timing of Delivery and Plasma ACTH and Cortisol Concentrations Preceding Delivery There was no difference in the length of gestation (saline infused: 150.2 ± 0.5 d; leptin infused: 149.8 ± 1.0 days) or birth weight (saline infused: 4.9 ± 0.3 kg; leptin infused: 5.2 ± 0.1 kg) between the saline- and leptin-infused groups.

Circulating leptin concentrations were higher in leptin- infused than saline-infused fetuses from 114 h before and up to delivery (leptin infused: 16.3 ± 2.9 ng/ml; saline infused: 2.5 ± 0.7 ng/ml; P < 0.001; Fig. 4).



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  		FIG. 4. Plasma leptin concentrations in saline-infused (open bars) and leptin-infused (closed bars) fetuses during the period from 114 h until 6 h before birth

There was no significant difference in mean fetal arterial blood gas and pH values between the saline- and leptin- infused groups during the infusion period (pO2-saline infused, 20.4 ± 1.3 mm Hg; leptin infused, 20.1 ± 1.2 mm Hg, O2 saturation–saline infused, 52.8 ± 2.8 mm Hg; leptin infused, 52.3 ± 5.4 mm Hg, pH-saline infused, 7.396 ± 0.008; leptin infused, 7.372 ± 0.032).

There was no difference in plasma ACTH concentrations between the saline-infused (70.8 ± 46.8 pg/ml) and leptin- infused (69.7 ± 22.6 pg/ml) fetuses during the period 114–6 h before delivery. In both the saline- and leptin- infused groups, plasma ACTH concentrations were significantly higher (P < 0.001) during the period from 18 to 6 h before delivery when compared with either 90–78 h or 114–102 h before delivery (Fig. 5A).



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  		FIG. 5. Plasma ACTH (A), cortisol (B), and the ratio of plasma cortisol: ACTH (C) concentrations in saline-infused (open bars) and leptin-infused (closed bars) fetuses from 114 h before delivery. Asterisks denote significant differences (P < 0.05) between mean values in the saline- and leptin- infused groups

There was a significant interaction (P < 0.05) between the effects of leptin infusion and time before delivery on fetal plasma cortisol concentrations. Between 90 and 42 h before delivery, circulating cortisol concentrations were significantly higher in the saline-infused fetuses, compared with the leptin-infused fetuses (90–42 h, saline infused: 142.5 ± 27.6 nmol/L; leptin infused: 84.3 ± 22.7 nmol/L; P < 0.05; Fig. 5B). During the period 42–6 h before delivery, however, there was no difference in plasma cortisol concentrations between the saline-infused (188.0 ± 42.4 nmol/L) and leptin-infused groups (244.5 69.3 nmol/L). Plasma cortisol concentrations were highest (P < 0.001) in both leptin- and saline-infused groups from between 30 and 6 h before delivery (Fig. 5B).

There was a significant interaction between the effects of treatment and the time relative to delivery on the ratio of plasma cortisol:ACTH concentrations (P < 0.005; Fig. 5C). There was no significant difference between the saline- and leptin-infused fetuses in the ratio of plasma cortisol: ACTH concentrations between 114 and 90 h before delivery (saline infused: 1.89 ± 0.45; leptin infused: 2.04 ± 0.42). The plasma cortisol:ACTH ratios were lower, however, in the leptin-infused fetuses between 90 and 30 h before delivery (saline infused, 2.78 ± 0.53; leptin infused, 1.62 ± 0.34; Fig. 5C).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
We have demonstrated that infusion of leptin into fetal sheep, resulting in a 4- to 5-fold increase in circulating leptin concentrations, suppressed the normal increase in fetal cortisol concentrations at the onset of the prepartum activation of the fetal HPA between 136 and 140 days gestation. Furthermore, intrafetal infusion of leptin from 144 days gestation until delivery also suppressed fetal plasma cortisol concentrations for an extended period from 90 to 42 h before delivery. Although plasma cortisol concentrations were reduced by around 40% during this period in the leptin-infused group, there was no difference in the timing of parturition between leptin- and saline-infused fetuses.

In the present study, there was an increase in fetal plasma ACTH and cortisol concentrations when saline was infused for a 96-h period between 136 and 141 days gestation, as expected. There was no increase, however, in either plasma ACTH or cortisol concentrations when leptin was infused during this gestational age range. In a previous study, Howe and colleagues [20] infused leptin via the lateral cerebral ventricle in fetal sheep between 135 and 140 days gestation and measured plasma ACTH and cortisol concentrations during a 4-h sampling period at 135 and 140 days gestation. They found that the increases in the mean value and amplitude of the pulses in plasma ACTH and cortisol concentrations between 135 and 140 days gestation were less in the leptin-infused compared with the vehicle- infused fetuses.

In the present study, when leptin was infused continuously from 144 days gestation, there was no effect on fetal ACTH concentrations during the week before delivery. In marked contrast, leptin infusion from 144 days gestation suppressed fetal plasma cortisol concentrations and the ratio of fetal plasma cortisol:ACTH concentrations for an extended period from 90 h until around 42 to 30 h before delivery. The suppression of fetal plasma cortisol concentrations and the decrease in the ratio of plasma cortisol: ACTH concentrations was not maintained, however, during the last 30 h before delivery, despite continued infusion of leptin. Plasma cortisol concentrations were similar in both the leptin- and saline-infused fetuses on the day before delivery, and there was no difference between these two groups in the timing of delivery. In summary, evidence from the current study suggests that an increase in circulating leptin concentrations in the fetus during late gestation can blunt the prepartum activation of the HPA axis but not block or delay the timing of delivery. Although there may be a transient impact of leptin on fetal plasma ACTH concentrations during the early phase of activation of the fetal pituitary-adrenal axis in late gestation, the predominant action of leptin appears to suppress the normal prepartum increase in circulating cortisol and adrenal responsiveness to ACTH.

In adult sheep it has been demonstrated that icv infusion of leptin suppressed food intake and resulted in a decrease in the expression of the mRNA for the orexigenic peptide, neuropeptide Y (NPY), in the hypothalamic arcuate nucleus [28]. This is consistent with the localization of the long form of the leptin receptor in around 60% of NPY-containing cells in the sheep hypothalamus [29]. There is also evidence in the sheep that hypothalamic NPY can regulate the synthesis and secretion of the ACTH secretagogues, corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) [30]. NPY is present within the arcuate nucleus of the fetal sheep hypothalamus during late gestation [31], and it is possible that leptin acts centrally via leptin receptors located within the fetal hypothalamus to suppress NPY, CRH, and AVP secretion and hence result in a decrease in fetal plasma ACTH concentrations in late gestation. It appears from the present study, however, that any inhibitory effect of an increase in circulating leptin concentrations on fetal ACTH secretion is not maintained during the week before delivery. The sustained increase in circulating leptin concentrations may induce resistance to the central actions of leptin because this has been proposed to underlie reduced sensitivity to peripherally administered leptin in genetically wild-type mice, primates, and lambs [32–34]. There is evidence that high circulating leptin concentrations may induce a decrease in the transport or access of leptin to the brain [35–37]. Although this is possible, it should be noted that we found no evidence that leptin infusion at 144 days resulted in an initial decrease in fetal ACTH concentrations during the first day of the infusion period. An alternative explanation is that the hypothalamic mechanisms that stimulate fetal pituitary ACTH synthesis and secretion during the prepartum period are not suppressed by an increase in peripheral leptin concentrations.

A range of studies have reported that the long form of the leptin receptor is expressed in human, rat, and mouse adrenal and that leptin acts directly to inhibit ACTH-stimulated glucocorticoid secretion by the bovine [16], human, and rat adrenal gland [38]. Leptin acts to decrease the expression of the steroidogenic enzymes, cytochrome P450 C21-hydroxylase, side chain cleavage, and C17 {alpha}-hydroxylase in the bovine adrenal, and it has recently been reported that leptin reduces the ACTH stimulation of steroidogenic acute regulatory protein expression in the rat adrenal [39, 40]. It has been proposed that in the adult, a leptin-mediated feedback loop exists between adipose tissue and the HPA axis because glucocorticoids can stimulate leptin expression and secretion from the adipocyte [17, 19] whereas rising circulating leptin concentrations can directly downregulate adrenal cortisol synthesis and secretion. Thus, it is possible that leptin acts directly at the fetal adrenal and that there is a similar endocrine feedback loop between fetal adipose tissue and the HPA axis in late gestation.

In the present study, intrafetal leptin infusion resulted in circulating leptin concentrations of around 15–20 ng/ml. Although these concentrations are similar to those measured in well-fed pregnant ewes in which rapid maternal weight gain has occurred [41], they are significantly higher than those measured by us or others in the fetal sheep of well-nourished ewes in late gestation [9, 11, 13, 42]. It has been shown, however, that fetal plasma leptin concentrations are increased up to 9-fold higher in human pregnancies that are complicated by maternal glucose intolerance and fetal hyperglycemia when compared with fetuses in normal pregnancies [43–45], and in these pregnancies it is possible that such an increase in fetal leptin concentrations may regulate adrenal responsiveness to ACTH and other stimulatory hormones. What is currently unclear is the extent of the endocrine interaction between fetal adipose tissue and the HPA axis in normal pregnancy. In the sheep fetus, circulating leptin concentrations are positively correlated with the relative mass of lipid stored in dominant cellular lipid locules within the fetal perirenal adipose tissue [11], and leptin is therefore an endocrine signal of the lipid storage capacity of this tissue. Forhead and colleagues [10] have reported that plasma cortisol and leptin concentrations increase in parallel during late gestation and are positively related between 130 and 140 days in the sheep fetus. Furthermore, they reported that fetal adrenalectomy resulted in lower plasma leptin concentrations in fetal sheep after 136 days [10]. Cortisol infusion or fetal adrenalectomy, however, did not alter leptin mRNA levels in perirenal adipose tissue in the late-gestation sheep fetus [46]. In the present study, we found that in saline-infused fetuses, there was no change in fetal plasma leptin concentrations during the last 3 weeks of gestation, and there was also no relationship between plasma cortisol and leptin concentrations between 125 and 137 days gestation. The difference between studies in the extent to which plasma cortisol and leptin are related between 130 and 140 days gestation may be related to the differences in circulating fetal leptin concentrations between the sheep breeds used in the studies. The fetus of the Welsh mountain ewe appears relatively hypoleptinemic [10] when compared with the fetus of the Merino ewe used in the current and previous studies [11, 13].

One further potential source of circulating leptin in the fetus is the placenta. Although the placenta has been proposed as a possible source of fetal leptin in the human, baboon, and rat [47–50], the levels of leptin mRNA present in the sheep placenta are negligible [9, 41]. It should be noted, however, that the leptin receptor is expressed in the sheep placenta [41], that there is evidence for transplacental transfer of leptin in the rat [51], and that maternal and fetal plasma leptin concentrations are correlated during late gestation in the sheep [13]. Whether there is a major contribution of maternal leptin to circulating leptin in the fetus and the extent to which this may vary across different breeds of sheep has yet to be determined.

In the present study, there was a negative relationship between circulating cortisol and leptin in the fetus in the week before delivery such that around 14% of the variation in plasma cortisol in the saline-infused group was explained by the variation in fetal leptin concentrations. Thus, although the initiation of the prepartum increase in fetal plasma cortisol does not appear to be related to any concomitant fall in circulating leptin, leptin may act to inhibit the output of cortisol from the fetal adrenal during the week before delivery.

In summary, we have demonstrated that an increase in circulating leptin concentrations in fetal sheep suppressed the normal increase in fetal cortisol concentrations at the onset of the prepartum activation of the fetal HPA between 136 and 140 days gestation. Furthermore, intrafetal infusion of leptin from 144 days gestation until delivery also suppressed fetal plasma cortisol concentrations for an extended period from between 90 and 42 h before delivery, although there was no difference in the timing of parturition between the leptin- and saline-infused groups. This study provides evidence, therefore, that fetal hyperleptinemia, which is present in pregnancies complicated by gestational diabetes, may act to limit the fetal adrenal responsiveness to ACTH and other trophic factors during the transition from intrauterine to extrauterine life.


   ACKNOWLEDGMENTS
 
We are also grateful to Anne Jurisevic, Laura O'Carroll, and Frank Carbone for their expert assistance with the surgical procedures.


   FOOTNOTES
 
1 This work was supported by the National Health and Medical Research Council of Australia. Back

2 Correspondence: I.C. McMillen, Physiology, School of Molecular and Biomedical Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia. FAX: 61 8 8 303 3356;caroline.mcmillen{at}adelaide.edu.au Back

Received: 6 November 2003.

First decision: 20 November 2003.

Accepted: 9 January 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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