Suckling-induced prolactin release potentiates mifepristone-induced lactogenesis in pregnant rats

Circadian clocks and their role in lactation competence

Circadian rhythms are 24 h cycles of behavior, physiology and gene expression that function to synchronize processes across the body and coordinate physiology with the external environment. Circadian clocks are central to maintaining homeostasis and regulating coordinated changes in physiology in response to internal and external cues. Orchestrated changes occur in maternal physiology during the periparturient period to support the growth of the fetus and the energetic and nutritional demands of lactation. Discoveries in our lab made over a decade ago led us to hypothesize that the circadian timing system functions to regulate metabolic and mammary specific changes that occur to support a successful lactation. Findings of studies that ensued are summarized, and point to the importance of circadian clocks in the regulation of lactation competence. Disruption of the circadian timing system can negatively affect mammary gland development and differentiation, alter maternal metabolism and impair milk production.

Mifepristone Treatment in Pregnant Murine Model Induced Mammary Gland Dysplasia and Postpartum Hypogalactia

Mammary gland dysplasia and postpartum hypogalactia often occur in humans and in the livestock breeding industry. However, their underlying mechanisms are not clear yet. Mifepristone, which has a high affinity for progesterone (P4) and glucocorticoid receptors, was exploited here to induce the disorders of mammary gland development and lactation. Four strategies were devised for treating pregnant mice with mifepristone. In the first strategy, mice were administered 1.20 mg mifepristone/kg body weight (BW) on pregnancy day 4 (Pd4). In the second strategy, mifepristone was administered to mice twice, with 1.20 mg/kg BW on Pd4 and 0.40 mg/kg BW on Pd8. In the third strategy, mice were treated with a single dose of 0.40 mg mifepristone/kg BW on Pd8. In the fourth strategy, mice were administered 0.40 mg mifepristone/kg BW on Pd8 and 0.20 mg mifepristone/kg BW on Pd12. The results suggested that mifepristone administration at the dose of 1.20 mg/kg BW on Pd4 caused significant reduction in milk production on lactation day 1 (Ld1), Ld2, and Ld3, as assessed using a weigh-suckle-weigh assay. Mammary β-casein expression, milk yields, litter growth rates, gland structure, and serum concentrations of 17-β estrogen (E2), P4, prolactin (PRL), growth hormone (GH), corticosterone (CORT) and oxytocin (OT) as well as the receptors of these hormones were determined during pregnancy or lactation after performing the first (Pd4) strategy. The results demonstrated that mifepristone administration during early pregnancy decreased β-casein expression, milk yields and litter growth rates, induced fewer alveoli, enlarged alveolar lumina, and altered the levels of E2, P4, PRL, GH, CORT, and OT as well as the mRNA expression of these hormonal receptors during pregnancy or early lactation. The present study on pregnant mice treated with mifepristone offers an innovative murine model to study the mechanism underlying mammary gland dysplasia and postpartum hypogalactia.

Effects of parity and serum prolactin levels on the incidence and regression of DMBA-induced tumors in OFA hr/hr rats

Prolactin (PRL) is a key player in the development of mammary cancer. We studied the effects of parity or hyperprolactinemia on mammary carcinogenesis in OFA hr/hr treated with 7,12-dimethylbenzanthracene. They were divided into three groups: nulliparous (Null), primiparous (PL, after pregnancy and lactation), and hyperprolactinemic rats (I, implanted in the arcuate nucleus with 17β-estradiol). The tumor incidence was similar in the three groups. However, a higher percentage of regressing tumors was evident in the PL group. Serum PRL, mammary development, and mammary β-casein content were higher in I rats compared to Null. The expression of hormone receptors was similar in the different groups. However, mammary tissue from PL rats bearing tumors had increased expression of PRL and estrogen alpha receptors compared to rats free of tumors. Our results suggest that serum PRL levels do not have relevance on the incidence of tumors, probably because the low levels of PRL in OFA rats are not further decreased by PL like in other strains. However, supraphysiological levels of PRL affect carcinogenesis. PL induces regression of the tumors due to the differentiation produced on the mammary cells. Alterations in the expression of hormonal receptors may be involved in progression and regression of tumors.

The inhibitory effect of progesterone on lactogenesis during pregnancy is already evident by mid- to late gestation in rodents

Lactogenesis is a very complex process highly dependent on hormonal regulation. In the present study the time-course of the inhibitory actions of progesterone on prolactin secretion, mammary gland morphology and lactogenesis from mid- to late gestation in rodents was investigated. Groups of pregnant rats were luteectomised or administered with mifepristone on Day 10, 13, 15 or 17 of gestation and decapitated 28 or 48 h later. Whole-blood samples and the inguinal mammary glands were taken for determinations of hormone levels and for measurement of mammary content of casein and lactose and for tissue morphology analyses, respectively. Luteectomy or mifepristone evoked prolactin increases only after Day 17 of gestation. Mammary content of casein was increased by both treatments regardless of timing or duration. Mifepristone was less effective than luteectomy in inducing lactose production and the effect was only observed after Day 15 of gestation. Analysis of mammary gland morphology confirmed the observed effect of progesterone on lactogenesis. Both treatments triggered remarkable secretory activity in the mammary gland, even without a parallel epithelial proliferation, demonstrating that the mammary epithelium is able to synthesise milk compounds long before its full lobulo–alveolar development is achieved, provided that progesterone action is abolished. Thus, the present study demonstrates that progesterone is a potent hormonal switch for the prolactin and prolactin-like effects on mammary gland development and its milk-synthesising capacity during pregnancy, and that its inhibitory action is already evident by mid-pregnancy in rodents.

Effect of progesterone withdrawal on hypothalamic mechanisms related to prolactin release in late pregnant rats

BACKGROUND/AIMS

Progesterone (P(4)) fall provoked by spontaneous or prostaglandin F2α (PGF2α)-induced luteolysis in late pregnant rats triggers a prolactin (PRL) surge 12-24 h later.

METHODS

To investigate the hypothalamic mechanism mediating this response, we determined expression of tyrosine hydroxylase (TH), PRL receptors (long form, PRLR(long)), estrogen-α (ERα) and ERβ, P(4) (PR) A and B receptors, and STAT5a, STAT5b, suppressors of cytokine signaling 1 (SOCS1), SOCS3 and CIS at mRNA (by semiquantitative and real-time RT-PCR) and protein (by Western blot only for TH, ERα and PRs) levels, and dopamine and DOPAC (by high-performance liquid chromatography) contents in the mediobasal hypothalamus (MBH) 24 h after luteolysis induced by a PGF2α analogue (cloprostenol, 25 μg/rat s.c. at 8 and 12 h on day 19 of pregnancy).

RESULTS

PGF2α treatment decreased circulating P(4) and estradiol and increased PRL and the estradiol/P(4) ratio. MBH DOPAC and DOPAC/dopamine ratio fell, indicating decreased dopaminergic transmission. PRLR(long), PRB and ERα mRNA increased. ERα and PR proteins were not modified. However, TH protein and mRNA did not change. PRA, the small PR isoform, was much more abundant than PRB, the isoform considered to mediate P(4) genomic actions. STAT5a, SOCS1 and SOCS3 mRNA were also increased.

CONCLUSION

The P(4) fall induced by PGF2α treatment induces PRL release through diminution in MBH dopaminergic transmission without change in TH expression. The increased PRLR along with elevated circulating PRL may be responsible for maintaining high TH expression through activation of short-loop feedback mechanisms, counteracting the effect of the fall in circulating P(4). In parallel, SOCS expression contributes to limit PRL signaling.

Hormonal profile and reproductive performance in lactation deficient (OFA hr/hr) and normal (Sprague–Dawley) female rats

Lactation deficiency may have important consequences on infant health, particularly in populations of low socioeconomic status. The OFAhr/hr(OFA) strain of rats, derived from Sprague–Dawley (SD) rats, has deficient lactation and is a good model of lactation failure. We examined the reproductive performance and hormonal profiles in OFA and SD strains to determine the cause(s) of the lactation failure of the OFA strain. We measured hormonal (PRL, GH, gonadotropins, oxytocin, and progesterone) levels by RIA in cycling, pregnant, and lactating rats and in response to suckling. Dopaminergic metabolism was assessed by determination of mediobasal hypothalamic dopamine and dihydroxyphenylacetic acid (DOPAC) concentrations by HPLC and tyrosine hydroxylase expression by immunocytochemistry and western blot. OFA rats have normal fertility but 50% of the litters die of malnutrition on early lactation; only 6% of the mothers show normal lactation. The OFA rats showed lower circulating PRL during lactation, increased hypothalamic dopamine and DOPAC, and impaired milk ejection with decreased PRL and oxytocin response to suckling. Before parturition, PRL release and lactogenesis were normal, but dopaminergic metabolism was altered, suggesting activation of the dopaminergic system in OFA but not in SD rats. The number of arcuate and periventricular neurons expressing tyrosine hydroxylase was higher in SD rats, but hypothalamic expression of TH was higher in OFA rats at the end of pregnancy and early lactation. These results suggest that the OFA rats have impaired PRL release linked with an augmented dopaminergic tone which could be partially responsible for the lactational failure.

Effect of intrauterine injection of casein on fetal survival in rat: a new pharmacological approach for contraception

BACKGROUND

The incidental finding of casein as a possible new local pharmacological contraceptive prompted us to assess its validity in an experimental rat model.

METHODS

The intrauterine injection of 150 microg of bovine alpha-casein dissolved in 150 microL phosphate-buffered saline (PBS) was performed on one uterine rat horn on days L(5)-L(7), whereas the contralateral horn was used for injection of 150 microL PBS as a control. Intraperitoneal injection of alpha-casein (5 mg/mL, 2 mL/rat) was performed on day L(5). The rats were killed by cervical dislocation on the day L(14).

RESULTS

The effect of an alpha-casein on fetal resorption rate was assessed following the unilateral injection of 150 microL of alpha-casein (1 mg/mL in PBS) and compared with the effect obtained following the contralateral injection of 150 microL PBS. The unilateral injection of alpha-casein on day L(5) caused a significant increase in fetal resorption rate as compared with the contralateral uterine horn injected with PBS (p<.00001). The decrease in alpha-casein concentration from 1 to 0.3 mg/mL caused a reduced, but still significant, effect on fetal resorption rate (p<.0001). The injection on days L(6)-L(7) caused a local effect of resorption near the injection site. There was no effect on fetal resorption rate following the injection of alpha-casein intraperitoneally.

CONCLUSION

Our data suggest a new pharmacological approach for contraception, based on local intrauterine effect of alpha-casein in an experimental rat model. The exact mechanism of action related to casein should be further studied.

In vivo hormonal environment leads to differential susceptibility of the corpus luteum to apoptosis in vitro

We evaluated the involvement of the in vivo hormonal environment on the ability of the rat corpus luteum (CL) to undergo apoptosis. Gel electrophoretic DNA fragmentation analysis revealed no apoptosis in CL isolated either the 2 last days of pregnancy (Days 21 and 22) or throughout the 4 days following parturition, suggesting that the number of cells undergoing apoptosis at the same time is not sufficient to allow for visualization of DNA breakdown. In contrast, CL incubated in serum-free medium underwent significant apoptosis, as evaluated by chromatin condensation and DNA fragmentation, regardless of their developmental stage in pregnancy. However, CL obtained on Day 7 of pregnancy and on Day 4 postpartum demonstrated higher sensitivity to apoptosis in vitro, but lactation reduced significantly the capacity of the CL to undergo apoptosis when maintained in culture. These data suggest that the exposure of the CL to different hormonal environments throughout pregnancy and after parturition is responsible for the differential susceptibility to apoptosis observed in vitro. We have previously shown that progesterone is a direct factor for survival of the CL. Prolactin stimulates luteal progesterone production; therefore, we examined whether prolactin prevents apoptosis in luteal cells independently of its stimulatory action on progesterone production. We used a luteal cell line (GG-CL) that expresses the prolactin receptor but does not produce progesterone. These cells undergo apoptosis under conditions of serum starvation, and addition of prolactin to the culture medium significantly reduced DNA fragmentation. These results indicate that the extent of luteal cell death induced by incubation of CL under serum-free conditions depends on the hormonal environment to which this endocrine gland is exposed in vivo. These results also indicate an important role for lactation in preventing apoptosis, which is further supported by the antiapoptotic activity of prolactin observed in luteal cells.

Estradiol implants in the arcuate nucleus induce lactogenesis in virgin rats. Role of progesterone

The aim of this study was to determine the effect of the centrally administered estradiol, and the effects of the consequent hypersecretion of prolactin (PRL) and progesterone, on lactogenesis as evaluated by mammary accumulation of casein and lactose. Bilateral cannulae containing 17beta-estradiol or cholesterol were implanted in the arcuate nucleus of virgin rats on the day of estrus (Day 0). In the first experiment different groups of rats were killed on Days 6, 9, 15, 17, or 19. Trunk blood was collected and abdominal mammary glands were taken. In the second experiment, estradiol-implanted rats received the progesterone antagonist mifepristone or vehicle at 14.00 h on Day 8 or 16 post-implant, and were killed 28 or 48 h later. Serum PRL and progesterone and mammary casein were measured by RIA and lactose was determined by an enzymatic assay. Estradiol-implanted rats showed a significant increase in both milk components at all time points after implant compared to controls. On Day 9 after estradiol implant, mifepristone had no effect on mammary content of casein or lactose. By contrast, on Day 16, mifepristone markedly increased both casein and lactose contents without modifying serum PRL and progesterone concentrations. In conclusion, 17beta-estradiol implants in the arcuate nucleus of virgin rats results in hyperprolactinaemia and stimulates mammary accumulation of casein and lactose in the absence of feto-placental units. Despite the prolonged luteal activation, the sustained high levels of circulating progesterone become inhibitory to lactogenesis after a relatively long period after implant.