Dissociation of two distinct luteotropic effects of prolactin: regulation of luteinizing hormone-receptor content and progesterone secretion during pregnancy

Seasonality of prolactin in birds and mammals

In most animals, annual rhythms in environmental cues and internal programs regulate seasonal physiology and behavior. Prolactin, an evolutionarily ancient hormone, serves as a molecular correlate of seasonal timing in most species. Prolactin is highly pleiotropic with a wide variety of well-documented physiological effects; in a seasonal context prolactin is known to regulate annual changes in pelage and molt. While short-term homeostatic variation of prolactin secretion is under the control of the hypothalamus, long-term seasonal rhythms of prolactin are programmed by endogenous timers that reside in the pituitary gland. The molecular basis of these rhythms is generally understood to be melatonin dependent in mammals. Prolactin rhythmicity persists for several years in many species, in the absence of hypothalamic signaling. Such evidence in mammals has supported the hypothesis that seasonal rhythms in prolactin derive from an endogenous timer within the pituitary gland that is entrained by external photoperiod. In this review, we describe the conserved nature of prolactin signaling in birds and mammals and highlight its role in regulating multiple diverse physiological systems. The review will cover the current understanding of the molecular control of prolactin seasonality and propose a mechanism by which long-term rhythms may be generated in amniotes.

Screening and Identification of Differential Ovarian Proteins before and after Induced Ovulation via Seminal Plasma in Bactrian Camels

Simple Summary

Camelidae are induced ovulators whose ovulation is tightly regulated by multiple factors. Understanding the biological mechanisms underlying follicular development, hormone secretion, and ovulation requires investigating the potential molecular pathways involved in these mechanisms. However, little is known about these molecular pathways in Bactrian camels. To screen and identify candidate biomarkers after seminal plasma (SP)-induced ovulation in the ovaries, we performed comprehensive proteomic and molecular biological analyses of the ovaries from camels that were intramuscularly injected with either seminal plasma or phosphate-buffered saline. Identification of these candidate biomarkers will enable a better understanding of reproduction in Bactrian camels. Our findings suggest candidate proteins for further studies on the molecular mechanisms of induced ovulation.

Abstract

Camelidae are induced ovulators whose ovulation is tightly regulated by multiple factors. Understanding the biological mechanisms underlying follicular development, hormone secretion, and ovulation requires investigating the potential molecular pathways involved. However, little is known about these pathways in Bactrian camels. To screen and identify candidate biomarkers after inducing ovulation, this study performed comprehensive proteomic and molecular biological analyses of the ovaries from two camel groups (n = 6). We identified 5075 expressed ovarian proteins, of which 404 were differentially expressed (264 upregulated, 140 downregulated) (p < 0.05 or p < 0.01), in samples from plasma-induced versus control camels. Gene ontology annotation identified the potential functions of the differentially expressed proteins (DEPs). These results validated the differential expression for a subset of these proteins using Western blot (p < 0.05) and immunofluorescence staining. Three DEPs (FST, NR5A1, and PRL) were involved in neurochemical signal transduction, as well as endocrine and reproductive hormone regulatory processes. The Kyoto Encyclopedia of Genes and Genomes analysis indicated the involvement of several pathways, including the calcium, cAMP, gonadotropin-releasing hormone, MAPK, and neuroactive ligand–receptor signaling pathways, suggesting that induced ovulation depends on the hypothalamic–pituitary–ovarian axis. Identifying these candidate biomarkers enables a better understanding of Bactrian camel reproduction. Ovarian proteomic profiling and the measurement of selected proteins using more targeted methods is a promising approach for studying induced-ovulation mechanisms.

Prolactin decreases LPS-induced inflammatory cytokines by inhibiting TLR-4/NFκB signaling in the human placenta

Prolactin (PRL) plays an important role in trophoblast growth, placental angiogenesis and immunomodulation within the feto-maternal interface, where different cell types secrete PRL and express its receptor. During pregnancy, inflammatory signalling is a deleterious event that has been associated with poor fetal outcomes. The placenta is highly responsive to the inflammatory stimulus; however, the actions of PRL in placental immunity and inflammation remain largely unknown. The aim of this study was to evaluate PRL effects on the TLR4/NFkB signalling cascade and associated inflammatory targets in cultured explants from healthy term human placentas. An in utero inflammatory scenario was mimicked using lipopolysaccharides (LPS) from Escherichia coli. PRL significantly reduced LPS-dependent TNF-α, IL-1β and IL-6 secretion and intracellular levels. Mechanistically, PRL prevented LPS-mediated upregulation of TLR-4 expression and NFκB phosphorylation. In conclusion, PRL limited inflammatory responses to LPS in the human placenta, suggesting that this hormone could be critical in inhibiting exacerbated immune responses to infections that could threaten pregnancy outcome. This is the first evidence of a mechanism for anti-inflammatory activity of PRL in the human placenta, acting as a negative regulator of TLR-4/NFkB signaling.

60 YEARS OF NEUROENDOCRINOLOGY: The hypothalamo-prolactin axis

The hypothalamic control of prolactin secretion is different from other anterior pituitary hormones, in that it is predominantly inhibitory, by means of dopamine from the tuberoinfundibular dopamine neurons. In addition, prolactin does not have an endocrine target tissue, and therefore lacks the classical feedback pathway to regulate its secretion. Instead, it is regulated by short loop feedback, whereby prolactin itself acts in the brain to stimulate production of dopamine and thereby inhibit its own secretion. Finally, despite its relatively simple name, prolactin has a broad range of functions in the body, in addition to its defining role in promoting lactation. As such, the hypothalamo-prolactin axis has many characteristics that are quite distinct from other hypothalamo-pituitary systems. This review will provide a brief overview of our current understanding of the neuroendocrine control of prolactin secretion, in particular focusing on the plasticity evident in this system, which keeps prolactin secretion at low levels most of the time, but enables extended periods of hyperprolactinemia when necessary for lactation. Key prolactin functions beyond milk production will be discussed, particularly focusing on the role of prolactin in inducing adaptive responses in multiple different systems to facilitate lactation, and the consequences if prolactin action is impaired. A feature of this pleiotropic activity is that functions that may be adaptive in the lactating state might be maladaptive if prolactin levels are elevated inappropriately. Overall, my goal is to give a flavour of both the history and current state of the field of prolactin neuroendocrinology, and identify some exciting new areas of research development.

Short-term pharmacological suppression of the hyperprolactinemia of infertile hCG-overproducing female mice persistently restores their fertility

Female infertility is often associated with deregulation of hormonal networks, and hyperprolactinemia is one of the most common endocrine disorders of the hypothalamic-pituitary axis affecting the reproductive functions. We have shown previously that transgenic female mice overexpressing human chorionic gonadotropin β-subunit (hCGβ+ mice), and producing elevated levels of bioactive LH/hCG, exhibit increased production of testosterone and progesterone, are overweight and infertile, and develop hyperprolactinemia associated with pituitary lactotrope adenomas in adult age. In the present study, we analyzed the influence of the hyperprolactinemia of hCGβ+ females on their reproductive phenotype by treating them with the dopamine agonists, bromocriptine and cabergoline. Long-term bromocriptine treatment of adult mice was effective in the control of obesity, pituitary growth, and disturbances in the hormone profile, demonstrating that hyperprolactinemia was the main cause of the hCGβ+ female phenotype. Interestingly, short-term treatment (1 wk) with cabergoline applied on 5-wk-old mice corrected hyperprolactinemia, hyperandrogenism, and hyperprogesteronemia, prevented pituitary overgrowth, normalized gonadal function, and recovered fertility of adult hCGβ+ females after hormone-induced and natural ovulation. The same cabergoline treatment in the short term applied on 3-month-old hCGβ+ females failed to recover their reproductive function. Hence, we demonstrated that the short-term cabergoline treatment applied at a critical early stage of the phenotype progression effectively prevented the hyperprolactinemia-associated reproductive dysfunction of hCG-overproducing females.

Prolactin signaling mechanisms in ovary

Prolactin is a hormone that is essential for normal reproduction and signals through two types of receptors. Not only is the classical long form of the prolactin receptor identified, but so are many short form receptors in rodents and human tissues. Mouse mutagenesis studies have offered insight into the biology of prolactin family, providing compelling evidence that the different isoforms have independent biological activity. The possibility that short forms mediate cell proliferation is important for a variety of tissues including mammary gland and ovarian follicles. This review summarizes our current knowledge about prolactin signaling and its role in reproduction through either long or short isoform receptors.

Impact of prolactin receptor isoforms on reproduction

Prolactin is a hormone involved in growth, development, reproduction, metabolism, water and electrolyte balance, brain and behavior, and immunoregulation. Its actions on reproductive processes represent the largest group of functions identified for this hormone. Besides the classic long form of the prolactin receptor, many short form receptors have been identified in rodents and human tissues. Mouse mutagenesis studies have offered insight into the biology of the prolactin family, providing compelling evidence that different isoforms have independent biological activity. The possibility that short forms mediate cell proliferation is important for a variety of tissues including mammary glands and ovarian follicles. This review summarizes the current knowledge about prolactin signaling and its role in reproduction through either long or short isoform receptors.

Prolactin independent rescue of mouse corpus luteum life span: identification of prolactin and luteinizing hormone target genes

The corpus luteum (CL) plays a central role in the maintenance of pregnancy in rodents, mainly by secreting progesterone. Female mice lacking prolactin (PRL) receptor (R) are sterile due to a failure of embryo implantation, which is a consequence of decreased luteinizing hormone (LH) receptor expression in the CL and inadequate levels of progesterone. We attempted to treat PRLR(-/-) females with human chorionic gonadotropin (hCG) and showed a de novo expression of LHR mRNA in the corpora lutea. Binding analysis confirmed that the LHR in hCG-treated PRLR(-/-) animals was functional. This was accompanied with increased expression of steroidogenic enzymes involved in progesterone synthesis. Despite these effects, no embryo implantation was observed because of high expression of 20alpha-hydroxysteroid dehydrogenase. To better appreciate the molecular mechanisms underlying maintenance of the CL, a series of mRNA expression-profiling experiments was performed on isolated corpora lutea of PRLR(-/-) and hCG-treated PRLR(-/-) mice. This approach revealed several novel candidate genes with potentially pivotal roles in ovarian function, among them, p27, VE-cadherin, Pten, and sFRP-4, a member of the Wnt/frizzled family. This study showed the differential role of PRL and LH in CL function and identified new targets of these hormones in luteal cells.

PRL-induced ERalpha gene expression is mediated by Janus kinase 2 (Jak2) while signal transducer and activator of transcription 5b (Stat5b) phosphorylation involves Jak2 and a second tyrosine kinase

In the rat corpus luteum of pregnancy, PRL stimulation of ER expression is a prerequisite for E2 to have any luteotropic effect. Previous work from our laboratory has established that PRL stimulates ERalpha expression at the level of transcription and that the transcription factor Stat5 (signal transducer and activator of transcription 5) mediates this stimulation. Since it is well established that PRL activates Stat5 through the tyrosine kinase, Janus kinase 2 (Jak2), the role of Jak2 in PRL regulation of ERalpha expression was investigated. In primary luteinized granulosa cells, the general tyrosine kinase inhibitors, genistein and AG18, and the Jak2 inhibitor, AG490, prevented PRL stimulation of ERalpha mRNA levels, suggesting that PRL signaling to the ERalpha gene requires Jak2 activity. However, using an antibody that recognizes the tyrosine-phosphorylated forms of both Stat5a and Stat5b (Y694/Y699), it was found that AG490 could inhibit PRL-induced Stat5a phosphorylation only and had little or no effect on Stat5b phosphorylation. These effects of AG490 were confirmed in COS cells overexpressing Stat5b. Also in COS cells, a kinase-negative Jak2 prevented PRL stimulation of ERalpha promoter activity and Stat5b phosphorylation while a constitutively active Jak2 could stimulate both in the absence of PRL. Furthermore, kinase-negative-Jak2, but not AG490, could inhibit Stat5b nuclear translocation and DNA binding. Therefore, it seems that in the presence of AG490, Stat5b remains phosphorylated, is located in the nucleus and capable of binding DNA, but is apparently transcriptionally inactive. These findings suggest that PRL may activate a second tyrosine kinase, other than Jak2, that is capable of phosphorylating Stat5b without inducing transcriptional activity. To investigate whether another signaling pathway is involved, the src kinase inhibitor PP2 and the phosphoinositol-3 kinase inhibitor (PI3K), LY294002, were used. Neither inhibitor alone had any major effect on PRL regulation of ERalpha promoter activity or on PRL-induced Stat5b phosphorylation. However, the combination of AG490 and LY294002 largely prevented PRL-induced Stat5b phosphorylation. These findings indicate that PRL stimulation of ERalpha expression requires Jak2 and also that PRL can induce Stat5b phosphorylation through two tyrosine kinases, Jak2 and one downstream of PI3K. Furthermore, these results suggest that the role of Jak2 in activating Stat5b may be through a mechanism other than simply inducing Stat5b phosphorylation.

Luteal expression of cytochrome P450 side-chain cleavage, steroidogenic acute regulatory protein, 3beta-hydroxysteroid dehydrogenase, and 20alpha-hydroxysteroid dehydrogenase genes in late pregnant rats: effect of luteinizing hormone and RU486

A decrease in serum progesterone at the end of pregnancy is essential for the induction of parturition in rats. We have previously demonstrated that LH participates in this process through: 1) inhibiting 3beta-hydroxysteroid dehydrogenase (3beta-HSD) activity and 2) stimulating progesterone catabolism by inducing 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) activity. The objective of this investigation was to determine the effect of LH and progesterone on the luteal expression of the steroidogenic acute regulatory protein (StAR), cytochrome P450 side-chain cleavage (P450(scc)), 3beta-HSD, and 20alpha-HSD genes. Gene expression was analyzed by Northern blot analysis 24 and 48 h after administration of LH or vehicle on Day 19 of pregnancy. StAR and 3beta-HSD mRNA levels were lower in LH-treated rats than in rats administered with vehicle at both time points studied. P450(scc) mRNA levels were unaffected by LH. The 20alpha-HSD mRNA levels were not different between LH and control rats 24 h after treatment; however, greater expression of 20alpha-HSD, with respect to controls, was observed in LH-treated rats 48 h after treatment. Luteal progesterone content dropped in LH-treated rats at both time points studied, whereas serum progesterone decreased after 48 h only. In a second set of experiments, the anti-progesterone RU486 was injected intrabursally on Day 20 of pregnancy. RU486 had no effect on 3beta-HSD or P450(scc) expression but increased 20alpha-HSD mRNA levels after 8 h treatment. In conclusion, the luteolytic effect of LH is mediated by a drop in StAR and 3beta-HSD expression without effect on P450(scc) expression. We also provide the first in vivo evidence indicating that a decrease in luteal progesterone content may be an essential step toward the induction of 20alpha-HSD expression at the end of pregnancy in rats.

Strain comparisons of atrazine-induced pregnancy loss in the rat

Atrazine was administered by gavage, in 1% methylcellulose, to F344 Sprague-Dawley (SD), and Long Evans (LE) rats at 0, 25, 50, 100, or 200 mg/kg/day on gestation days 6 through 10. The dams were allowed to deliver and litters were examined postnatally. The F344 strain was the most sensitive to atrazine's effects on pregnancy, showing full-litter resorption (FLR) at >/=50 mg/kg. In surviving F344 litters, prenatal loss was increased at 200 mg/kg. In SD and LE rats, FLR occurred only at 200 mg/kg. Delayed parturition was seen at >/=100 mg/kg in F344 and SD rats. Regarding maternal toxicity, the SD dams were the most sensitive, with weight loss at >/=25 mg/kg. When 200 mg/kg was administered to F344 rats on days 11 through 15 (after the LH-dependent period of pregnancy), no FLR was seen. These findings suggest that atrazine-induced FLR is maternally mediated, and consistent with loss of LH support of the corpora lutea.

Luteotropic hormone receptors in the ovary of the mink (Mustela vison) during delayed implantation and early-postimplantation gestation

The reproductive cycle of the mink displays rigid seasonality and obligate embryonic diapause. After ovulation, the corpus luteum (CL) involutes, and it secretes basal progesterone until activated prior to implantation. To study changes in the relative abundance of luteal prolactin and LH receptor mRNA through gestation, ovaries and serum were collected from pregnant female mink at 2-day intervals (n = 3 per date) through embryonic diapause and CL activation (March 19-31) and at 5-day intervals through implantation and early-postimplantation gestation (March 31-April 15). To determine the effect of endogenous prolactin, mink received Alzet osmotic minipumps releasing 2 mg/day 2-bromo-alpha-ergocryptine (bromocriptine) or saline on March 19. Ovaries and serum were taken from 3 animals every 2 days until March 31. Prolactin receptor mRNA in ovaries was low during CL activation but increased 3-fold through embryo implantation. Its abundance correlated with prolactin binding to ovarian membranes and with circulating prolactin. Bromocriptine suppressed endogenous prolactin levels and prevented the increase in prolactin receptor mRNA. There was a transient peak in LH receptor mRNA in the ovaries at March 19-23, which declined to basal levels by March 25 and remained constant through midgestation. Bromocriptine prevented the preimplantation peak in LH receptor mRNA and reduced its abundance below pretreatment levels. The results suggest that prolactin up-regulates its receptor and maintains the LH receptor in the mink CL. The pattern of LH receptor mRNA argues for a role for LH in CL reactivation and termination of embryonic diapause in mink.

Critical period of carbon tetrachloride-induced pregnancy loss in Fischer-344 rats, with insights into the detection of resorption sites by ammonium sulfide staining

Several low-molecular weight halocarbons have been shown to cause full-litter resorption (FLR), i.e., pregnancy loss, in Fischer-344 rats treated during organogenesis. To determine periods of gestation sensitive to acute exposure, a single dose of 150 mg carbon tetrachloride (CCl4)/kg was administered on gestation day (GD) 6, 7, 8, 10, or 12. Fetuses were delivered by cesarean section on GD 20. Non-gravid uteri were examined for resorption sites, placed in 10% ammonium sulfide, and re-examined for stained resorption sites approximately 1 and 4.5 hr later. FLR was seen in 4% (1/27) of control dams and 36% (4/11), 54% (7/13), 72% (18/25), 54% (7/13), and 0% (0/12) of dams treated on GD 6, 7, 8, 10, and 12, respectively. Ammonium sulfide staining clearly yielded a more accurate account of the incidence of FLR. The technique was most effective when the staining period was extended to 4.5 hr, as two cases of FLR were revealed that had been undetected after 1 hr of staining. For dams with FLR, staining was required to detect resorption sites in all dams treated on GD 6 or 7, most dams treated on GD 8, and one dam treated on GD 10. Fewer implantation sites were detected in the dams treated on GD 6, and the size of the stained resorption sites increased as the day of treatment was delayed. These findings demonstrate a relationship between the time of toxicant exposure and the size and detectability of resorption sites near term, suggesting that the size of the resorption site may reliably reflect the time of embryonic death. Treatment on GD 8 caused the highest incidence of FLR and will be used in subsequent mechanistic research.

In vivo investigation of luteal function in dogs: effects of cabergoline, a dopamine agonist, and prolactin on progesterone secretion during mid-pregnancy and -diestrus

The role of prolactin on luteal function in dogs was investigated in vivo. The function of prolactin in mid-luteal phase was compared in pregnant and nonpregnant dogs. A dopamine agonist, cabergoline, known for its prolactin secretion inhibitory effects, was injected subcutaneously at a dose of 5 micrograms/kg body weight in five pregnant and five nonpregnant Beagle bitches. Mean plasma prolactin and progesterone were dramatically suppressed for 4 to 5 days after injection in both groups when compared with control pregnant and non-pregnant animals, whereas no effect on luteinizing hormone (LH) secretion was observed. The decline in plasma progesterone occurred after that in prolactin, suggesting plasma progesterone was impaired by inhibition of prolactin secretion. These results confirm the luteotropic importance of prolactin in pregnant bitches, and also demonstrate its importance in luteal phase of the nonpregnant dog. Second, to demonstrate that the effects of cabergoline were mediated by prolactin inhibition and not by a direct action on the corpus luteum, concomitant administration on Day 30 of cabergoline and prolactin (375 micrograms i.v. twice daily on Days 30 and 31) or cabergoline and LH (750 micrograms i.v. twice daily on Days 30 and 31) was affected in two groups of five pregnant animals each. Results showed that only prolactin was able to reverse the negative effects of cabergoline on circulating progesterone. This confirms the indirect mode of action of the dopamine agonist, cabergoline on corpus luteum function. Third, further investigation on the precise luteotropic role of prolactin was made by IV injection of 375 micrograms pure canine prolactin twice daily in five pregnant bitches on Days 30 and 31, and in five pregnant bitches on Days 40 and 41. No direct stimulatory effect of prolactin on plasma progesterone secretion occurred. Nor was there a noticeable effect on plasma LH secretion. These results suggest that prolactin is unable to directly stimulate progesterone secretion by the corpus luteum of pregnancy. The results of this study suggest that prolactin is an essential luteotropin in the dog from mid-luteal phase in both pregnant and nonpregnant animals. However, it appears to act by sustaining corpus luteum lifespan and function rather than by direct stimulatory effects on progesterone secretion.

Cloning and characterization of an ovarian-specific protein that associates with the short form of the prolactin receptor

Prolactin (PRL) is essential for progesterone biosynthesis and luteal cell hypertrophy of the rat corpus luteum during pregnancy. Both the long and short form of the PRL receptor have been identified in the corpus luteum of pregnant rat. The long form has been shown to transduce PRL signal in other cells, whereas no information is available on the role of the short form, especially in the corpus luteum. In the present study, we have cloned a rat ovarian-specific phosphoprotein, PRAP (PRL Receptor Associated Protein), which has no significant homology to other known proteins. We have demonstrated that this protein is immunoprecipitated by anti-PRL receptor and anti-phosphotyrosine antibodies. To determine whether PRAP associates with either the long or the short form of the PRL receptor, fusion proteins with glutathione S-transferase containing the cytoplasmic domain of the long or short form of the PRL receptor were produced, purified, and incubated with luteal proteins. Our results indicate that PRAP preferentially binds to the short form of the PRL receptor. Thus, the long form and short forms of the PRL receptor may signal through distinct pathways. These data provide evidence for the involvement of a novel protein in PRL signal transduction and suggest that PRAP may contribute to the luteotropic effects of PRL on the corpus luteum during pregnancy.

Prolactin stimulates transcription of growth-related genes in Nb2 T lymphoma cells

The pituitary peptide hormone prolactin exerts a profound effect on various physiological processes involving both cellular proliferation and differentiation. The rat Nb2 T lymphoma cell line has been used as a model system for studying prolactin regulation of cell proliferation. Several genes associated with cell growth (c-myc, ornithine decarboxylase (ODC), heat shock protein 70 (hsp 70)-homologue, and beta-actin) are induced rapidly within 4 h after prolactin addition. Nuclear run-on transcription assays indicate that prolactin induction of these growth-related genes occurs primarily at the transcriptional level. According to the different kinetics of transcriptional response to prolactin, these growth-related genes can be divided into immediate-early (actin, c-myc), early (ODC) and mid-G1 (hsp 70-homologue) genes. Thus, prolactin may regulate Nb2 T cell-proliferative responses by modulating the transcriptional induction of various growth-related genes. These studies also represent a first report of a transcriptional cascade set off in rapid response to prolactin in cultured T cells.

Effects of prolactin on steroidogenesis and cAMP accumulation in rat luteal cell cultures

The effects of prolactin (PRL), alone and as a modulator of human chorionic gonadotropin (hCG) action, on steroidogenesis and cAMP accumulation in rat luteal cell cultures were examined. Cultured rat luteal cells were prepared from immature rats primed with pregnant mare serum gonadotropin and hCG. In vitro treatment was performed with 0.1 and 0.2 IU/ml hCG and 1-100 ng/ml PRL. Cultures were incubated for 48 h for evaluation of progesterone (P4) secretion and for 1 h for measurement of cAMP accumulation. The same doses of hormones and incubation periods were also used in preovulatory rat granulosa cell cultures and found to cause a significant, dose-dependent inhibition in estradiol, P4 and cAMP accumulation. In luteal cell cultures, on the other hand, P4 secretion was significantly elevated, in a dose-dependent manner, by PRL. Moreover, identical doses of PRL caused a significant, dose-dependent stimulation of cAMP accumulation. Basal levels of P4 were also significantly elevated by PRL alone, but no such stimulation by PRL was detected in basal levels of cAMP. Addition of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, increased the stimulation of P4 and cAMP by hCG + PRL in a manner dependent on PRL concentrations. The overall data therefore demonstrate divergent effects of PRL on cAMP accumulation and steroidogenesis in the ovary: inhibitory in the preovulatory and stimulatory in the postovulatory state. Moreover, these findings suggest a possible common mechanism linking the effects of PRL before and after ovulation: inhibition of cAMP accumulation via enhanced breakdown of the nucleotide.

An analysis of factors responsible for resorption of embryos in cisplatin-treated rats

Pregnant rats were injected ip with 4 or 7 mg cisplatin/kg on gestation day (gd) 6 to study its effect on embryonal resorption. Serum concentrations of prolactin, luteinizing hormone (LH), and progesterone were determined by radioimmunoassay in pregnant rats, and related to the effects of cisplatin on the maintenance of pregnancy. The nocturnal prolactin surge on gd 9 was abolished in cisplatin-treated rats. Within 3 days after drug injection, LH concentrations decreased 39%, while serum progesterone decreased 63% by Day 10. A histochemical study of 20 alpha-hydroxysteroid dehydrogenase activity revealed no enzyme activity by gd 10. It is proposed that the cause of cisplatin-related embryonal resorption in rats may be due to decreases in hormone concentrations observed after drug treatment.

Identification and partial characterization of a prolactin-like hormone produced by rat decidual tissue

Previous studies have strongly, but indirectly, suggested that rat decidual tissue produces a prolactin-like hormone, decidual luteotropin, which markedly affects luteal cell function. However, it was also found that extracts of decidual tissue do not cross-react with antisera to either rat or ovine prolactin (PRL). The purpose of this study was to determine whether the decidual tissue contains a substance that binds to PRL receptors in rat luteal membranes and, if so, to identify, quantitate, and characterize this molecule with the use of an ovarian radioreceptor assay. Decidual tissue was induced in day 5 pseudopregnant rats by scratching the antimesometrial wall of the uterus; it was collected on day 9 and homogenized and extracted. Decidual tissue extracts bound specifically to ovarian PRL receptors. Graded dilutions of the extracts yielded curves that were parallel to the ovine PRL standard, indicating that decidual luteotropin competes for the same receptor sites on rat luteal membranes. To determine the levels of decidual luteotropin throughout pseudopregnancy, decidual tissue was obtained on each day between days 6-12. The PRL-like activity was detectable in decidual tissue as early as day 6, reached a maximum on day 9, and declined thereafter. The elution profile obtained from gel filtration of a day 9 decidual tissue extract displayed a major component of decidual luteotropin eluting at a Ve/Vo ratio of approximately equal to 2.0. Column chromatography indicated that decidual luteotropin corresponds to a protein with a molecular weight of 23,500. The hormone was heat labile, digestible by trypsin, and appears to contain disulfide linkages. In summary, this study reports the identification, quantitation, and partial characterization of a PRL-like hormone produced by the decidual tissue of the rat.

Gonadotropin and lipoprotein receptor levels in rat luteal cells of early pregnancy

Dispersed cells from rat corpora lutea isolated on pregnancy days, 5, 7, 10 and 13, yielded three distinct sub-populations of luteal cells when centrifuged in a continuous, 0-40%, Metrizamide density gradient. Bands I and II (refract. ind. 1.340-1.346) constituted highly enriched, functional, luteal cell fractions. The more dense fraction (Band III, refract. ind. 1.348-1.355), while contaminated with non-luteal cells, also contained cells capable of progesterone production. Progesterone synthesis by all three sub-populations was stimulated by LH, and each sub-population bound [125I]-labeled hCG and [125I]-labeled HDL with high affinity. However, the concentrations of LH/hCG and lipoprotein binding sites were greater in Band I + II than in Band III. Progesterone secretion during the selected days of pregnancy, being high on days 5 and 13 with a nadir on day 10, was shown to correlate with the concentration of LH/hCG-binding sites present on Band I + II cells. This correlation was not seen when comparing Band III cells. The concentration of HDL-binding sites did not vary significantly with the sub-populations of luteal cells isolated on the selected days of pregnancy.

Comparative progesterone concentrations in two Peromyscus species

1. Plasma progesterone concentrations were measured by radioimmunoassay in the oldfield mouse (Peromyscus polionotus) and the deermouse (P. maniculatus) at estrus and at 5-day intervals during pregnancy. 2. P. polionotus had significantly higher progesterone levels, both at estrus and during gestation. 3. The level was markedly less in female P. polionotus bearing interspecific (P. polionotus x P. maniculatus) hybrid conceptuses than in those with conspecific conceptuses. 4. It is postulated that the reduced progesterone level in females bearing hybrid fetuses may be responsible for reproductive failure which regularly occurs when this cross is attempted. 5. An endocrine difference of this magnitude could represent an incipient reproductive isolating mechanism.

Antiserum to LH reverses the abortifacient effect of Bromergocryptine treatment in early rat pregnancy

In vivo experiments showed that anti-bovine luteinizing hormone rabbit serum (a-LH) reversed the abortifacient effect of Bromergocryptine (BEC) on day 6 of pregnancy in 13 of 19 rats when injected 5-15 h after the initial BEC administration. Radioreceptor assay studies revealed that BEC treatment entailed an 85% drop of ovarian LH-receptor (LH-rec) concentration and a drastic decrease in progesterone serum levels. a-LH restored both the ovarian LH-rec content and progesterone serum levels in those rats that were found to have macroscopically intact implantations. A time course study of LH serum levels after BEC treatment showed a trend for BEC to stimulate LH secretion with a maximum 12-18 h after the initial BEC injection. These experiments indicate that (i) BEC terminates early rat pregnancy primarily by inducing a process of desensitization of ovarian LH-rec by blocking prolactin (PRL) secretion and possibly also by additionally stimulating LH secretion, which in the absence of endogenous PRL causes HL-rec and serum progesterone levels to drop; (ii) a-LH administration without simultaneous PRL substitution may rescue LH-rec of BEC treated rats from being desensitized as well as prevent fetuses from being resorbed. It is suggested that the contribution of PRL in the luteotropic complex is permissive for the luteotropic action of LH and/or for keeping the corpus luteum in a functional state rather than being luteotropic by itself.

Oestrogens regulate divergent effects of prolactin in the ovary

Prolactin exerts well established stimulatory effects both in vitro and in vivo in the rat ovary. However, in the ovary of higher mammals, prolactin action is not well characterised, and may differ from that observed in rodents. For example, in human clinical conditions of physiological or pathological prolactin excess, ovarian function is depressed, and in isolated, human granulosa cells in vitro, prolactin seems to inhibit progesterone production. To examine direct prolactin effects in the higher mammalian ovary, we have used an in vitro porcine granulosa-cell model. In this system, prolactin action is bipotential, depending critically on the degree of granulosa-cell differentiation attained in vivo. Prolactin suppresses steroid production by cultured granulosa cells isolated from small (1-2 mm), immature follicles, but stimulates progesterone secretion by granulosa cells collected from large (greater than 6 mm) mature follicles. The present studies show that oestrogens may play an important part in regulating these divergent actions of prolactin in the ovary.

The effect of pinealectomy on serum progesterone and oestradiol levels during the last stage of pregnancy in rats

Pinealectomy leads to significantly higher levels of progesterone (on days 19 and 20 of gestation) and oestradiol (on days 21 and 22) in the serum of pregnant rats. These results indicate that during the last phase of pregnancy the pineal gland may be modifying the levels of gonadal hormones, although not affecting the sequence of the physiological events.