Direct in vivo effects of leptin on ovarian steroidogenesis in sheep

Spontaneous ovulation in the cat: incidence among queens presented at a veterinary teaching facility

OBJECTIVES

The queen is recognised as an induced ovulator. Ovulation without male contact is generally regarded as spontaneous. The aim of this study was to provide an estimate of the incidence of spontaneous ovulation in a population of intact queens presented to a veterinary care facility for both reproductive and non-reproductive reasons. The secondary objective was to determine the roles of age, breed, body weight, presence of tom cats or other cycling queens, and physical contact with humans on triggering spontaneous ovulation, along with its implications.

METHODS

Serum samples from post-pubertal intact queens presented between January 2020 and June 2023 to the Veterinary Teaching Hospital of the University of Padova, Italy, were retrieved and assayed for progesterone (P4) levels. Serum P4 above 2.0 ng/ml without a history of male contact was considered as proof of spontaneous ovulation.

RESULTS

In total, 31 serum samples from 29 intact post-pubertal queens were obtained. Of the 31 samples, 14 had a P4 concentration above 2.0 ng/ml and 9/29 (31.0%) queens ovulated spontaneously. The mean age and weight of the nine spontaneously ovulating queens were 4.3 ± 5.7 years and 3.7 ± 0.8 kg, respectively. One queen ovulated spontaneously at her first heat at 6 months of age, which makes it the earliest spontaneous ovulation reported.

CONCLUSIONS AND RELEVANCE

As both our findings and previous publications indicate that the incidence of spontaneous ovulation in queens is consistently ⩾30%, cats should not be considered strictly induced ovulators, but as a species in which ovulation can be either spontaneous or induced. Since the risk of progesterone-dependent conditions (cystic endometrial hyperplasia - pyometra complex, feline mammary hypertrophy) is increased in these queens, veterinarians should be aware and advise breeders and clients accordingly.

Role of adipokines in the ovarian function: Oogenesis and steroidogenesis

Adipokines are mainly produced by adipose tissue; however, their expression has been reported in other organs including female reproductive tissues. Therefore, adipokines have opened new avenues of research in female fertility. In this regard, studies reported different roles for certain adipokines in ovarian function, although the role of other recently identified adipokines is still controversial. It seems that adipokines are essential for normal ovarian function and their abnormal levels could be associated with ovarian-related disorders. The objective of this study is to review the available information regarding the role of adipokines in ovarian functions including follicular development, oogenesis and steroidogenesis and also their involvement in ovary-related disorders.

Adipocyte and steroidogenic cell cross-talk in polycystic ovary syndrome

BACKGROUND

Metabolic and endocrine alterations in women with polycystic ovary syndrome (PCOS) affect adipose tissue mass and distribution. PCOS is characterised by hyperandrogenism, obesity and adipocyte dysfunction. Hyperandrogenism in PCOS drives dysfunctional adipocyte secretion of potentially harmful adipocytokines. Glucocorticoids and sex-steroids modulate adipocyte development and function. For their part, adipocyte products interact with adrenal and ovarian steroidogenic cells. Currently, the relationship between adipocyte and steroidogenic cells is not clear, and for these reasons, it is important to elucidate the interrelationship between these cells in women with and without PCOS.

OBJECTIVE AND RATIONALE

This comprehensive review aims to assess current knowledge regarding the interrelationship between adipocytes and adrenal and ovarian steroidogenic cells in animal models and humans with or without PCOS.

SEARCH METHODS

We searched for articles published in English and Portuguese in PubMed. Keywords were as follows: polycystic ovary syndrome, steroidogenesis, adrenal glands, theca cells, granulosa cells, adipocytes, adipocytokines, obesity, enzyme activation, and cytochrome P450 enzymes. We expanded the search into the references from the retrieved articles.

OUTCOMES

Glucocorticoids and sex-steroids modulate adipocyte differentiation and function. Dysfunctional adipocyte products play important roles in the metabolic and endocrine pathways in animals and women with PCOS. Most adipokines participate in the regulation of the hypothalamic-pituitary-adrenal and ovarian axes. In animal models of PCOS, hyperinsulinemia and poor fertility are common; various adipokines modulate ovarian steroidogenesis, depending on the species. Women with PCOS secrete unbalanced levels of adipocyte products, characterised by higher levels of leptin and lower levels of adiponectin. Leptin expression positively correlates with body mass index, waist/hip ratio and levels of total cholesterol, triglyceride, luteinising hormone, oestradiol and androgens. Leptin inhibits the production of oestradiol and, in granulosa cells, may modulate 17-hydroxylase and aromatase enzyme activities. Adiponectin levels negatively correlate with fat mass, body mass index, waist-hip ratio, glucose, insulin and triglycerides, and decrease androgen production by altering expression of luteinising hormone receptor, steroidogenic acute regulatory protein, cholesterol-side-chain cleavage enzyme and 17-hydroxylase. Resistin expression positively correlates with body mass index and testosterone, and promotes the expression of 17-hydroxylase enzyme in theca cells. The potential benefits of adipokines in the treatment of women with PCOS require more investigation.

WIDER IMPLICATIONS

The current data regarding the relationship between adipocyte products and steroidogenic cells are conflicting in animals and humans. Polycystic ovary syndrome is an excellent model to investigate the interrelationship among adipocyte and steroidogenic cells. Women with PCOS manifest some pathological conditions associated with hyperandrogenism and adipocyte products. In animals, cross-talk between cells may vary according to species, and the current review suggests opportunities to test new medications to prevent or even reverse several harmful sequelae of PCOS in humans. Further studies are required to investigate the possible therapeutic application of adipokines in women with obese and non-obese PCOS. Meanwhile, when appropriate, metformin use alone, or associated with flutamide, may be considered for therapeutic purposes.

Adipokines change the balance of proliferation/apoptosis in the ovarian cells of human and domestic animals: A comparative review

Adipose tissue secretes multiple hormones termed adipokines, which are important regulators of many processes. There are four types of evidence supporting an association between adipokines and female fertility which are effects that occur: centrally at the pituitary; peripherally and locally at the ovary and reproductive tract; directly on the oocyte/embryo and during pregnancy. In this review, there was a focus on the description of adipokines (leptin, apelin, resistin, chemerin, adiponectin, vaspin and visfatin) on ovarian cell proliferation, cell cycle progression and apoptosis in comparison to effects on human and domestic animal ovaries including pigs, cattle and chickens. Knowledge about molecules which regulate the balance between proliferation and apoptosis so that these processes are optimal for ovarian function is essential for understanding the physiology and reducing the incidence of infertility. Furthermore, oogenesis, folliculogenesis, oocyte loss/selection and atresia are important processes for optimal ovarian physiological functions. There, however, is ovulation from only a few follicles, while the majority undergo atresia that is induced by apoptosis.

Association of genetic polymorphism in leptin gene with growth, reproduction and production traits in Sahiwal cows

The present study explored the association between single nucleotide polymorphisms (SNPs) in leptin gene with various economic traits (body weight at different age, first lactation reproduction and production traits) in Sahiwal cows. PCR-RFLP of 522 bp fragment comprising partial intron 2 and exon 3 of leptin gene, carried out using BsaAI restriction endonuclease, revealed three genotypes (GG, AG and AA) at position G93263736A in chromosome 4 with genotypic frequencies of 0.39, 0.52 and 0.09 respectively. The allele frequencies were 0.65 for the G allele and 0.35 for the A allele. General Linear Model (GLM) analysis revealed significant association of SNP G93263736A with growth and first lactation production traits. The present study supports that SNP in leptin gene could be used as an aid to selection for improving different economic traits in Sahiwal cows.

Relationship of Serum Leptin and Reproductive Hormones in Unexplained Infertile and Fertile Females

OBJECTIVE

To investigate the relationship between serum leptin and reproductive hormones in females with unexplained infertility (UI).

METHODOLOGY

It was a case-control study conducted in the Gynecology and Obstetrics Department and Infertility Clinic, Jinnah Postgraduate Medical Center (JPMC), Karachi, Pakistan. A total of 235 primary infertile females with an unidentified cause of infertility were selected from the Infertility Clinics. The patients were excluded if they were found to have polycystic ovary syndrome, endometriosis, tubal blockage, irregular menstrual cycles, hyperthyroidism, hypothyroidism, hyperprolactinemia, hyperandrogenemia, fasting blood sugar >110 mg/dl, and male factor infertility. A total of 205 healthy, fertile females were selected from the general population. The blood samples of both groups were collected on the 12th and 21st day of their menstrual cycle. Serum leptin, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and E2 levels were measured. Statistical analysis was executed using the SPSS version 16 (SPSS Inc., Chicago, IL).

RESULTS

No significant difference was observed in leptin values of fertile and UI females, 37.110±1.19 vs. 35.321±0.901. In the preovulatory phase (12th day) of the cycle, infertile subjects with body mass index (BMI) <20 and 20-24.9 had significantly higher values for leptin (p<0.05), whereas, with an increase in BMI, leptin levels were reduced in these females. Leptin was reduced further in the luteal phase of infertile females with BMI 25-30, with a significantly lower value for FSH (p<0.005), LH (p<0.005), and estradiol (p<0.005. In infertile subjects, it correlated with estradiol (r=0.501, p<0.005), BMI (r=0.903, p<0.001), and progesterone (r=0.146, p<0.05).

CONCLUSION

Low levels of leptin observed to have an increase in the BMI of UI females were associated with a reduced estradiol and progesterone production in the luteal phase of the cycle.

Spontaneous ovulation in cats-Uterine findings and correlations with animal weight and age

Cats are considered induced ovulating animals but ovulations occur in the absence of mating (i.e., spontaneous ovulations). Factors that stimulate such ovulations remain largely unknown. In this study, ovaries and uterine horn segments from 89 post-pubertal queens presented for ovariectomy were evaluated morphologically and histologically. It was hypothesized that corpora lutea (CL) are present in non-pregnant cats and can be associated with cystic endometrial hyperplasia (CEH). Cats were assigned to three age groups (7-12 months, n = 32; 13-24 months, n = 26 and ≥ 24 months, n = 31) and three weight groups (2.1-2.8 kg, n = 28; > 2.8-3.3 kg, n = 32 and > 3.3 kg, n = 29). Uterine horn diameter and thickness of the endometrium and myometrium were determined. Corpora lutea were detected in 39.3% of the cats and presence did not differ between age groups. The percentage of queens with CL increased with bodyweight (2.1-2.8 kg: 14.3%; > 2.8-3.3 kg: 37.5%, > 3.3 kg: 65.5%; P < 0.01). In cats with CL, the thickness of all layers of the uterus were greater than in cats without CL (P < 0.05). Of the cats, 22.5% had CEH but there was no difference between cats without and with CL. The percentage of CEH increased with age (P < 0.001) but did not differ between weight groups. In conclusion, ovulations occurred in the absence of mating in approximately one third of all queens.

Fetal programming by androgen excess in rats affects ovarian fuel sensors and steroidogenesis

Fetal programming by androgen excess is hypothesized as one of the main factors contributing to the development of polycystic ovary syndrome (PCOS). PCOS is more than a reproductive disorder, as women with PCOS also show metabolic and other endocrine alterations. Since both ovarian and reproductive functions depend on energy balance, the alterations in metabolism may be related to reproductive alterations. The present study aimed to evaluate the effect of androgen excess during prenatal life on ovarian fuel sensors and its consequences on steroidogenesis. To this end, pregnant rats were hyperandrogenized with testosterone and the following parameters were evaluated in their female offspring: follicular development, PPARG levels, adipokines (including leptin, adiponectin, and chemerin as ovarian fuel sensors), serum gonadotropins (LH and FSH), the mRNA of their ovarian receptors, and the expression of steroidogenic mediators. At 60 days of age, the prenatally hyperandrogenized (PH) female offspring displayed both an irregular ovulatory phenotype and an anovulatory phenotype with altered follicular development and the presence of cysts. Both PH groups showed altered levels of both proteins and mRNA of PPARG and a different expression pattern of the adipokines studied. Although serum gonadotropins were not impaired, there were alterations in the mRNA levels of their ovarian receptors. The steroidogenic mediators Star, Cyp11a1, Cyp17a1, and Cyp19a1 were altered differently in each of the PH groups. We concluded that androgen excess during prenatal life leads to developmental programming effects that affect ovarian fuel sensors and steroidogenesis in a phenotype-specific way.

The ovarian estrogen synthesis function was impaired in Y123F mouse and partly restored by exogenous FSH supplement

BACKGROUND

LepR tyrosine site mutation mice (Y123F) exhibit decreased serum E2 levels, immature reproductive organs, infertility as well as metabolic abnormalities. Although the actions of leptin and lepR in the control of reproductive function are thought to be exerted mainly via the hypothalamic-pituitary-gonadal axis, relatively less is known regarding their local effects on the peripheral ovary, especially on steroid hormone synthesis. Meanwhile, whether the decreased fertility of Y123F mouse could be restored by gonadotropin has not been clear yet.

METHODS

The serum levels of E2, P4, FSH, LH, T and leptin of Y123F and WT mice at the age of 12 weeks were measured by enzyme-linked immunosorbent assay. Immunohistochemistry was used to compare the distribution of hormone synthases (STAR, CYP11A1, CYP19A1, HSD17B7) and FSHR in adult mouse ovaries of two genotypes. Western blot and real-time PCR were used to detect the expression levels of four ovarian hormone synthases and JAK2-STAT3 / STAT5 signaling pathway in 4 and 12 weeks old mice, as well as the effects of exogenous hFSH stimulation on hormone synthases and FSHR.

RESULTS

Compared with WT mice, the serum levels of FSH, LH and E2 in 12-week-old Y123F mice were significantly decreased; T and leptin levels were significantly increased; but there was no significant difference of serum P4 levels. STAR, CYP11A1, HSD17B7 expression levels and the phosphorylation levels of JAK2 and STAT3 were significantly decreased in adult Y123F mice, while the expression of CYP19A1 and phospho-STAT5 were significantly increased. No significant differences were found between 4-week-old Y123F and WT mice. After exogenous hFSH stimulation, E2 levels and expression of CYP19A1 and HSD17B7 were significantly higher than that in the non-stimulated state, but significant differences still existed between Y123F and WT genotype mice under the same condition.

CONCLUSIONS

Abnormal sex hormone levels of Y123F mice were due to not only decreased gonadotropin levels in the central nervous system, but also ovarian hormone synthase abnormalities in the peripheral gonads. Both FSH signaling pathway and JAK2-STAT3/STAT5 signaling pathway were involved in regulation of ovarian hormone synthases expression. Exogenous FSH just partly improved the blood E2 levels and ovarian hormone synthase expression.

Effects of leptin administration on development, vascularization and function of Corpus luteum in alpacas submitted to pre-ovulatory fasting

The objective of this study was to determine the effect of leptin administration on the development, vascularization and function of Corpus luteum (CL) in alpacas submitted to pre-ovulatory fasting. Fourteen alpacas were kept in fasting conditions for 72h and received five doses of o-leptin (2μg/kg e.v.; Leptin group) or saline (Control group) every 12h. Ovulation was induced with a GnRH dose (Day 0). The ovaries were examined every other day by trans-rectal ultrasonography (7.5MHz; mode B and power Doppler) from Day 0 to 13 to determine the pre-ovulatory follicle diameter and ovulation, and then to monitor CL diameter and vascularization until the regression phase. Serial blood samples were taken after GnRH treatment to determine plasma LH concentration; and every other day from Days 1 to 13 to determine plasma progesterone and leptin concentrations. The pre-ovulatory follicle and CL diameter, LH, progesterone and leptin plasma concentrations were not affected by treatment (P>0.05). The vascularization area of the CL was, nevertheless, affected by the treatment (P<0.01) with significant differences between groups at Days 3, 7 and 9 (P<0.05). The Leptin group had a larger maximum vascularization area (0.67±0.1 compared with 0.35±0.1cm²; P<0.05). In addition, there was a positive correlation between CL vascularization, CL diameter and plasma progesterone. The exogenous administration of leptin during pre-ovulatory fasting increased the vascularization of the CL in alpacas in vivo.

Increased body condition score through increased lean muscle, but not fat deposition, is associated with reduced reproductive response to oestrus induction in beef cows

Energy reserve, estimated as body condition score (BCS), is the major determinant of the re-initiation of ovarian activity in postpartum cows. Leptin, IGF-I and insulin are positively related to BCS and are putative mediators between BCS and reproductive function. However, when BCS and body composition dissociates, concentrations of these metabolic hormones are altered. We hypothesized that increasing lean muscle tissue, but not fat tissue, would diminish the reproductive response to oestrus induction treatments. Thirty lactating beef cows with BCS of 3.10±1.21 and 75.94±12 days postpartum were divided in two groups. Control cows (n=15) were supplemented with 10.20 kg of concentrate daily for 60 days. Treated cows (n=15) were supplemented equally, and received a β-adrenergic receptor agonist (β-AA; 0.15 mg/kg BW) to achieve accretion of lean tissue mass and not fat tissue mass. Twelve days after ending concentrate supplementation/β-AA treatment, cows received a progestin implant to induce oestrus. Cows displaying oestrus were inseminated during the following 60 days, and maintained with a fertile bull for a further 21 days. Cows in both groups gained weight during the supplementation period (Daily weight gain: Control=0.75 kg v. β-AA=0.89 kg). Cows treated with β-AA had a larger increase in BCS (i.e. change in BCS: control=1 point (score 4.13) v. β-AA=2 points (score 5.06; P0.05) did not differ between groups. However, the number of cows displaying oestrus (control 13/15 v. β-AA 8/15; P<0.05) and the percentage cycling (control 6/8 v. β-AA 3/10; P=0.07) after progestin treatment and the pregnancy percentage at the end of the breeding period (control 13/15 v. β-AA 8/15; P<0.05) were lower in β-AA than control cows. In summary, the increase BCS through muscle tissue accretion, but not through fat tissue accretion, resulted in a lower response to oestrus induction, lower percentage of cycling animals and lower pregnancy percentage after progestin treatment; which was associated with a decrease in serum concentrations of leptin and IGF-I.

Leptin-Induced CART (Cocaine- and Amphetamine-Regulated Transcript) Is a Novel Intraovarian Mediator of Obesity-Related Infertility in Females

Obesity is considered detrimental to women's reproductive health. Although most of the attention has been focused on the effects of obesity on hypothalamic function, studies suggest a multifactorial impact. In fact, obesity is associated with reduced fecundity even in women with regular cycles, indicating that there may be local ovarian effects modulating fertility. Here we describe a novel mechanism for leptin actions directly in the ovary that may account for some of the negative effects of obesity on ovarian function. We find that normal cycling, obese, hyperleptinemic mice fed with a high-fat diet are subfertile and ovulate fewer oocytes compared with animals fed with a normal diet. Importantly, we show that leptin induces expression of the neuropeptide cocaine- and amphetamine-regulated transcript (CART) in the granulosa cells (GCs) of ovarian follicles both in vitro and in vivo. CART then negatively affects intracellular cAMP levels, MAPK signaling, and aromatase mRNA expression, which leads to lower estradiol synthesis in GCs and altered ovarian folliculogenesis. Finally, in human samples from patients undergoing in vitro fertilization, we show a significant positive correlation between patient body mass index, CART mRNA expression in GCs, and CART peptide levels in follicular fluid. These observations suggest that, under obese conditions, CART acts as a local mediator of leptin in the ovary to cause ovarian dysfunction and reduced fertility.

The Deep Correlation between Energy Metabolism and Reproduction: A View on the Effects of Nutrition for Women Fertility

In female mammals, mechanisms have been developed, throughout evolution, to integrate environmental, nutritional and hormonal cues in order to guarantee reproduction in favorable energetic conditions and to inhibit it in case of food scarcity. This metabolic strategy could be an advantage in nutritionally poor environments, but nowadays is affecting women's health. The unlimited availability of nutrients, in association with reduced energy expenditure, leads to alterations in many metabolic pathways and to impairments in the finely tuned inter-relation between energy metabolism and reproduction, thereby affecting female fertility. Many energetic states could influence female reproductive health being under- and over-weight, obesity and strenuous physical activity are all conditions that alter the profiles of specific hormones, such as insulin and adipokines, thus impairing women fertility. Furthermore, specific classes of nutrients might affect female fertility by acting on particular signaling pathways. Dietary fatty acids, carbohydrates, proteins and food-associated components (such as endocrine disruptors) have per se physiological activities and their unbalanced intake, both in quantitative and qualitative terms, might impair metabolic homeostasis and fertility in premenopausal women. Even though we are far from identifying a "fertility diet", lifestyle and dietary interventions might represent a promising and invaluable strategy to manage infertility in premenopausal women.

The Influence of LepR Tyrosine Site Mutations on Mouse Ovary Development and Related Gene Expression Changes

Leptin exerts many biological functions, such as in metabolism and reproduction, through binding to and activating the leptin receptor, LepRb, which is expressed in many regions of the brain. To better understand the roles of LepR downstream signaling pathways, Y123F mice, which expressed mutant leptin receptors with phenylalanine (F) substituted for three tyrosines (Y) (Tyr985, Tyr1077 and Tyr1138), were generated. The body weight and abdominal fat deposits of Y123F homozygous mice (HOM) were higher than those of wild-type mice (WT). HOM ovaries were atrophic and the follicles developed abnormally; however, the HOM ovaries did not exhibit polycystic phenotypes. Moreover, Y123F HOM adults had no estrous cycle and the blood estrogen concentration remained stable at a low level below detection limit of 5 pg/ml. LepR expression in HOM ovaries was higher than in WT ovaries. Using cDNA Microarrays, the mRNA expressions of 41 genes were increased, and 100 were decreased in HOM vs. WT ovaries, and many signaling pathways were evaluated to be involved significantly. The expressions of 19 genes were validated by real-time quantitative PCR, most of which were consistent with the microarray results. Thus, Y123F HOM mice were suggested as a new animal model of PCOS for research that mainly emphasizes metabolic disorders and anovulation, but not the polycystic phenotype. Meanwhile, using the model, we found that JAK-STAT and hormone biosynthesis pathways were involved in the follicular development and ovulation disorders caused by LepR deficiency in ovaries, although we could not exclude indirect actions from the brain.

Duplicated leptin receptors in two species of eel bring new insights into the evolution of the leptin system in vertebrates

Since its discovery in mammals as a key-hormone in reproduction and metabolism, leptin has been identified in an increasing number of tetrapods and teleosts. Tetrapods possess only one leptin gene, while most teleosts possess two leptin genes, as a result of the teleost third whole genome duplication event (3R). Leptin acts through a specific receptor (LEPR). In the European and Japanese eels, we identified two leptin genes, and for the first time in vertebrates, two LEPR genes. Synteny analyses indicated that eel LEPRa and LEPRb result from teleost 3R. LEPRb seems to have been lost in the teleost lineage shortly after the elopomorph divergence. Quantitative PCRs revealed a wide distribution of leptins and LEPRs in the European eel, including tissues involved in metabolism and reproduction. Noticeably, leptin1 was expressed in fat tissue, while leptin2 in the liver, reflecting subfunctionalization. Four-month fasting had no impact on the expression of leptins and LEPRs in control European eels. This might be related to the remarkable adaptation of silver eel metabolism to long-term fasting throughout the reproductive oceanic migration. In contrast, sexual maturation induced differential increases in the expression of leptins and LEPRs in the BPG-liver axis. Leptin2 was strikingly upregulated in the liver, the central organ of the reproductive metabolic challenge in teleosts. LEPRs were differentially regulated during sexual maturation, which may have contributed to the conservation of the duplicated LEPRs in this species. This suggests an ancient and positive role of the leptin system in the vertebrate reproductive function. This study brings new insights on the evolutionary history of the leptin system in vertebrates. Among extant vertebrates, the eel represents a unique case of duplicated leptins and leptin receptors as a result of 3R.

The effects of intravenous, glucose versus saline on ovarian follicles and their levels of some mediators of insulin signalling

BACKGROUND

A short-term increase in food intake and specifically dietary energy can stimulate folliculogenesis and increase ovulation rate in ewes. The mechanism appears to involve the insulin-glucose metabolic system and its interaction with FSH signalling pathways in the granulosa cells of ovarian follicles. This experiment was designed to investigate the interaction between these two systems in the granulosa cells of ovarian follicles.

METHODS

Thirty six Ile-de-France ewes were used in this controlled experiment to study the effects of intravenous glucose on folliculogenesis. Eighteen ewes were infused with glucose (10 mM/h for 72 h) from day 8 of the oestrous cycle, while the others (controls) received saline. Ovaries were collected when the infusions ended (luteal phase) or 30 h later and after a luteolytic dose of a PGF2α analogue (follicular phase). Follicles were dissected and granulosa cells and follicular fluid harvested. The blood concentrations of glucose, insulin, oestradiol and FSH were monitored over the experiment. The levels of Aromatase P450 and of the phosphorylated and non-phosphorylated forms of Akt, AMPK and ERK in granulosa cells and the concentration of oestradiol in follicular fluid, were determined.

RESULTS

Glucose increased the circulating concentration of glucose (P < 0.05) and insulin (P < 0.05). It also increased the total number of follicles >1.0 mm in diameter (P < 0.05) and small (P < 0.05) follicles (>1.0 to 2.0 mm in diameter) but not medium (>2.0 to 3.5 mm in diameter) or large (>3.5 mm in diameter) follicles. Glucose decreased circulating oestradiol (P < 0.05) but not that of FSH or progesterone. Glucose reduced aromatase P450 (P < 0.05) and decreased the phosphorylation of Akt (P < 0.05), ERK (P < 0.05) and AMPK (P < 0.05) in granulosa cells from oestrogenic follicles. The level of Aromatase P450 was greatest in large oestrogenic follicles and the phosphorylation of Akt (P < 0.05), ERK (P < 0.05) and AMPK (P < 0.05) was lower in small follicles compared to medium and large follicles.

CONCLUSIONS

These data suggest that the effect of glucose in small follicles is a direct action of glucose that increases the number of small follicles while the effect of glucose in oestrogenic follicles is an indirect insulin-mediated action.

Roles of leptin in reproduction, pregnancy and polycystic ovary syndrome: consensus knowledge and recent developments

As an essential function for perpetuation of species, reproduction, including puberty onset, is sensitive to the size of body energy stores and the metabolic state of the organism. Accordingly, impaired energy homeostasis, ranging from extreme leanness, such as in anorexia or cachexia, to morbid obesity has an impact on the timing of puberty and is often associated to fertility problems. The neuroendocrine basis for such phenomenon is the close connection between numerous metabolic hormones and nutritional cues with the various elements of the so-called hypothalamic-pituitary-gonadal (HPG) axis. Yet, despite previous fragmentary knowledge, it was only the discovery of the adipose-hormone, leptin, in 1994 what revolutionized our understanding on how metabolic and reproductive systems closely interplay and allowed the definition of the neurohormonal causes of perturbations of puberty and fertility in conditions of impaired body energy homeostasis. In this article, we aim to provide a synoptic view of the mechanisms whereby leptin engages in the regulation of different elements of the HPG axis, with special attention to its effects and mechanisms of action on the different elements of the reproductive brain and its proven direct effects in the gonads. In addition, we will summarize the state-of-the-art regarding the putative roles of leptin during gestation, including its potential function as placental hormone. Finally, comments will be made on the eventual leptin alterations in reproductive disorders, with special attention to the polycystic ovary syndrome (PCOS), a disease in which reproductive, metabolic and neuroendocrine alterations are commonly observed. All in all, we intend to provide an updated account of our knowledge on the physiological roles of leptin in the metabolic regulation of the reproductive axis and its eventual pathophysiological implications in prevalent reproductive disorders, such as PCOS.

Expressão gênica de adipocinas em ovelhas alimentadas com resíduos da indústria do biodiesel da mamona

A expressão de RNAm para leptina, receptor de leptina (obRb), adiponectina, receptor de adiponectina (AdipoR1) e resistina foi avaliada por meio da técnica de PCR em tempo real, em tecidos ovariano, hipofisário, adiposo do omento e da região perirrenal, em ovelhas alimentadas sem farelo de mamona ou com farelo de mamona detoxificada durante 14 meses. O tipo de dieta não afetou os níveis de RNAm para leptina, obRb, adiponectina, AdipoR1 e resistina nos diferentes tecidos avaliados (P>0,05). Nos tecidos ovariano e hipofisário, não foi verificada a expressão da adiponecina e da resistina, respectivamente. Como consequência, pode-se concluir que o farelo de mamona detoxificada pode ser utilizado como fonte proteica na dieta de ovelhas, sem afetar a expressão do gene resistina e dos genes leptina e adiponectina, bem como de seus receptores.

An updated view of leptin on implantation and pregnancy: a review

The hormone leptin, which is thought to be primarily produced by adipose tissue, is a polypeptide that was initially characterized by its ability to regulate food intake and energy metabolism. Leptin appears to signal the status of body energy stores to the brain, resulting in the regulation of food intake and whole-body energy expenditure. Subsequently, it was recognized as a cytokine with a wide range of peripheral actions and is involved in the regulation of a number of physiological systems including reproduction. In the fed state, leptin circulates in the plasma in proportion to body adiposity in all species studied to date. However other factors such as sex, age, body mass index (BMI), sex steroids and pregnancy may also affect leptin levels in plasma. In pregnant mice and humans, the placenta is also a major site of leptin expression. Leptin circulates in biological fluids both as free protein and in a form that is bound to the soluble isoform of its receptor or other binding proteins such as one of the immunoglobulin superfamily members Siglec-6 (OB-BP1). Although the actions of leptin in the control of reproductive function are thought to be exerted mainly via the hypothalamic-pituitary-gonadal axis, there have also been reports of local direct effects of leptin at the peripheral level, however, these data appear contradictory. Therefore, there is a need to summarize the current status of research outcomes and analyze the possible reasons for differing results and thus provide researchers with new insight in designing experiments to investigate leptin effect on reproduction. Most importantly, our recent experimental data suggesting that reproductive performance is improved by decreasing concentrations of peripheral leptin was unexpected and cannot be explained by hypotheses drawn from the experiments of excessive exogenous leptin administration to normal animals or ob/ob mice.

The effect of leptin on luteal angiogenic factors during the luteal phase of the estrous cycle in goats

Fibroblast growth factor 2 (FGF2), angiopoietin 1 (Ang1), and vascular endothelial growth factor (VEGF) are angiogenic factors implicated in the vascular development of the corpus luteum (CL). Each factor is regulated or influenced by leptin in non-ovarian tissues. Moreover, leptin and its receptor, ObRb, have been identified in luteal tissue throughout the luteal phase. Therefore, leptin is hypothesized to influence luteal vasculature through the regulation of FGF2, Ang1, and VEGF. Multiparous, cycling crossbred female goats (does) were allocated to early (n=12), mid (n=8), and late (n=11) stages of the luteal phase for CL collection. Luteal tissue was harvested and either snap frozen in liquid N2, paraffin embedded, or cultured with leptin (0, 10(-12), 10(-11), 10(-10), 10(-9), 10(-8)M). Tissue was analyzed for FGF2, Ang1, VEGF, ObRb, and leptin expression. Angiopoietin 1, FGF2, VEGF expression was higher (P≤0.001) in the mid-luteal stage than the early stage. Expression decreased (P≤0.001) during the late luteal stage with the exception of VEGF, which remained elevated. In contrast, leptin and ObRb were lowest (P≤0.003) during the mid-luteal stage compared to the early and late stages. All factors were detected in and/or around vessels in early stage tissue compared to mid and late stages. Leptin stimulated (P≤0.02) Ang1, FGF2, and VEGF expression only in early stage luteal cultures. Collectively, these data provide evidence that leptin may be involved in the luteal angiogenic process during the early stage of CL formation.

The effect of nutrition and metabolic status on the development of follicles, oocytes and embryos in ruminants

The impact of nutrition and energy reserves on the fertility of ruminants has been extensively described. However, the metabolic factors and the molecular mechanisms involved in the interactions between nutrition and ovarian function are still poorly understood. These factors could be hormonal (either reproductive and/or metabolic) and/or dietary and metabolic (glucose, amino acids and fatty acids). In this review, we briefly summarize the impact of those nutrients (fatty acids, glucose and amino acids) and metabolic hormones (insulin/IGF-I, growth hormone, T3/4, ghrelin, apelin and the adipokines (leptin, adiponectin and resistin)) implicated in the development of ovarian follicles, oocytes and embryos in ruminants. We then discuss the current hypotheses on the mechanisms of action of these factors on ovarian function. We particularly describe the role of some energy sensors including adenosine monophosphate-activated kinase and peroxisome proliferator-activated receptors in the ovarian cells.

Different levels of leptin regulate different target enzymes involved in progesterone synthesis

OBJECTIVE

To study the effects of different doses of leptin on the expression of proteins involved in P synthesis, such as steroidogenic acute regulatory protein (StAR), cytochrome P450 side chain cleavage (P450scc), and 3β-hydroxysteroid dehydrogenase (3βHSD).

DESIGN

Experimental studies.

SETTING

Research laboratory.

ANIMAL(S)

Immature rats primed with gonadotropins to induce ovulation.

INTERVENTION(S)

In vivo studies: rats received either an acute or daily treatment with leptin. In vitro studies: ovarian explants were cultured in the absence or presence of leptin (0.3-500 ng/mL).

MAIN OUTCOME MEASURE(S)

The expression of both messenger RNA and protein of StAR, P450scc, and 3βHSD were measured by reverse transcription-polymerase chain reaction (PCR) and Western blot, respectively.

RESULT(S)

The acute treatment with leptin, which inhibits the ovulatory process, caused a significant reduction in the ovarian expression of P450scc without changes in StAR or 3βHSD. In contrast, the daily treatment, which induces the ovulatory process, showed an increased expression of the ovarian 3βHSD protein, without differences in the other proteins measured. We also found that leptin increased the protein of both P450scc and 3βHSD at physiological levels and inhibited both messenger RNA and protein of 3βHSD at higher concentrations.

CONCLUSION(S)

The results indicate that 1) leptin is able to regulate the expression of the 3βHSD protein in a dose-dependent manner; and 2) leptin seems to exert its dual effects on P synthesis on different targets in a dose-dependent manner.

Integrative control of energy balance and reproduction in females

There is a strong association between nutrition and reproduction. Chronic dietary energy deficits as well as energy surpluses can impair reproductive capacity. Metabolic status impacts reproductive function at systemic level, modulating the hypothalamic GnRH neuronal network and/or the pituitary gonadotropin secretion through several hormones and neuropeptides, and at the ovarian level, acting through the regulation of follicle growth and steroidogenesis by means of the growth hormone-IGF-insulin system and local ovarian mediators. In the past years, several hormones and neuropeptides have been emerging as important mediators between energy balance and reproduction. The present review goes over the main sites implicated in the control of energy balance linked to reproductive success and summarizes the most important metabolic and neuroendocrine signals that participate in reproductive events with special emphasis on the role of recently discovered neuroendocrine peptides. Also, a little overview about the effects of maternal nutrition, affecting offspring reproduction, has been presented.

The role of anti-Müllerian hormone (AMH) during follicle development in a monovulatory species (sheep)

Knockout studies in mice have suggested that anti-Müllerian hormone (AMH) modulates primordial follicle recruitment and the response of growing follicles to FSH. Little is known of the physiology of AMH in monovular species, despite intense clinical interest in this factor. Using sheep as a model, we sought to investigate the functional role of AMH in modulating follicle development in monovular species. In contrast to the rodent, the results indicate that AMH does not affect the rate of primordial follicle recruitment but appears to regulate the rate at which follicles progress through the gonadotropin-responsive phase, during which it is maximally expressed. Thus, knockdown of AMH bioactivity by active immunization lead to a decline in the population of gonadotropin-responsive preantral and small antral follicles (P < 0.01) and increases in both the number of gonadotropin-dependent antral follicles (P < 0.01) and ovulation rate (P < 0.05). These in vivo findings were consistent with the results of other studies examining the pattern of expression of AMH, which was negatively correlated with aromatase (P < 0.001), and in vitro supplementation experiments, which supported an inhibitory role for AMH in modulating the response of both theca and granulosa cells to LH and FSH, respectively. The elucidation of a functional relationship between AMH and LH-stimulated thecal androgen production may be significant in terms of the etiology of common forms of anovulatory infertility in women. Furthermore, the observed increase in both the number of recruitable antral follicles and ovulatory quota in response to AMH knockdown may have therapeutic value in women who respond poorly to ovarian stimulation.

Leptin alters adrenal responsiveness by decreasing expression of ACTH-R, StAR, and P450c21 in hypoxemic fetal sheep

The late gestation increase in adrenal responsiveness to adrenocorticotropin (ACTH) is dependent upon the upregulation of the ACTH receptor (ACTH-R), steroidogenic acute regulatory protein (StAR), and steroidogenic enzymes in the fetal adrenal. Long-term hypoxia decreases the expression of these and adrenal responsiveness to ACTH in vivo. Leptin, an adipocyte-derived hormone which attenuates the peripartum increase in fetal plasma cortisol is elevated in hypoxic fetuses. Therefore, we hypothesized that increases in plasma leptin will inhibit the expression of the ACTH-R, StAR, and steroidogenic enzymes and attenuate adrenal responsiveness in hypoxic fetuses. Spontaneously hypoxemic fetal sheep (132 days of gestation, PO(2) ≈ 15 mm Hg) were infused with recombinant human leptin (n = 8) or saline (n = 7) for 96 hours. An ACTH challenge was performed at 72 hours of infusion to assess adrenal responsiveness. Plasma cortisol and ACTH were measured daily and adrenals were collected after 96 hours infusion for messenger RNA (mRNA) and protein expression measurement. Plasma cortisol concentrations were lower in leptin- compared with saline-infused fetuses (14.8 ± 3.2 vs 42.3 ± 9.6 ng/mL, P < .05), as was the cortisol:ACTH ratio (0.9 ± 0.074 vs 46 ± 1.49, P < .05). Increases in cortisol concentrations were blunted in the leptin-treated group after ACTH(1-24) challenge (F = 12.2, P < .0001). Adrenal ACTH-R, StAR, and P450c21 expression levels were reduced in leptin-treated fetuses (P < .05), whereas the expression of Ob-Ra and Ob-Rb leptin receptor isoforms remained unchanged. Our results indicate that leptin blunts adrenal responsiveness in the late gestation hypoxemic fetus, and this effect appears mediated by decreased adrenal ACTH-R, StAR, and P450c21 expression.

Leptin and reproductive function

Adipose tissue plays a dynamic role in whole-body energy homeostasis by acting as an endocrine organ. Collective evidence indicates a strong link between neural influences and adipocyte expression and secretion of leptin. Developmental changes in these relationships are considered important for pubertal transition in reproductive function. Leptin augments secretion of gonadotropin hormones, which are essential for initiation and maintenance of normal reproductive function, by acting centrally at the hypothalamus to regulate gonadotropin-releasing hormone (GnRH) neuronal activity and secretion. The effects of leptin on GnRH are mediated through interneuronal pathways involving neuropeptide-Y, proopiomelanocortin and kisspeptin. Increased infertility associated with diet induced obesity or central leptin resistance are likely mediated through the kisspeptin-GnRH pathway. Furthermore, Leptin regulates reproductive function by altering the sensitivity of the pituitary gland to GnRH and acting at the ovary to regulate follicular and luteal steroidogenesis. Thus leptin serves as a putative signal that links metabolic status with the reproductive axis. The intent of this review is to examine the biological role of leptin with energy metabolism, and reproduction.

The effect of leptin on maturing porcine oocytes is dependent on glucose concentration

Increased body weight is often accompanied by increased circulating levels of leptin and glucose, which alters glucose metabolism in various tissues, including perhaps the oocyte. Alteration of glucose metabolism impacts oocyte function and may contribute to the subfertility often associated with obese individuals. The objective of this study was to determine the effect of leptin (0, 10, and 100 ng/ml) on the oocyte and cumulus cells during in vitro maturation under differing glucose concentrations. We examined the effects of leptin on oocyte maturation, blastocyst development, and/or gene expression in oocytes and cumulus cells (IRS1, IGF1, PPARγ, IL6, GLUT1) in a physiological glucose (2 mM) and high glucose (50 mM) environment. We also evaluated the effect of leptin on glucose metabolism via glycolysis and the pentose phosphate pathway. In a physiological glucose environment, leptin did not have an influence on oocyte maturation, blastocyst development, or oocyte gene expression. Expression of GLUT1 in cumulus cells was downregulated with 100 ng/ml leptin treatment, but did not affect oocyte glucose metabolism. In a high glucose environment, oocyte maturation and glycolysis were decreased, but in the presence of 100 ng/ml leptin, these parameters were improved to levels similar to control. This effect is potentially mediated by an upregulation of oocyte IRS1 and a correction of cumulus cell IGF1 expression. The present study demonstrates that in a physiological glucose concentration, leptin plays a negligible role in oocyte function. However, leptin appears to modulate the deleterious impact of a high glucose environment on oocyte function.

Regulation of folliculogenesis and the determination of ovulation rate in ruminants

The paper presents an update of our 1993 model of ovarian follicular development in ruminants, based on knowledge gained from the past 15 years of research. The model addresses the sequence of events from follicular formation in fetal life, through the successive waves of follicular growth and atresia, culminating with the emergence of ovulatory follicles during reproductive cycles. The original concept of five developmental classes of follicles, defined primarily by their responses to gonadotrophins, is retained: primordial, committed, gonadotrophin-responsive, gonadotrophin-dependent and ovulatory follicles. The updated model has more extensive integration of the morphological, molecular and cellular events during folliculogenesis with systemic events in the whole animal. It also incorporates knowledge on factors that influence oocyte quality and the critical roles of the oocyte in regulating follicular development and ovulation rate. The original hypothetical mechanisms determining ovulation rate are retained but with some refinements; the enhanced viability of gonadotrophin-dependent follicles and increases in the number of gonadotrophin-responsive follicles by increases in the throughput of follicles to this stage of growth. Finally, we reexamine how these two mechanisms, which are thought not to be mutually exclusive, appear to account for most of the known genetic and environmental effects on ovulation rate.

The use of a ‘first-wave’ model to study the effect of nutrition on ovarian follicular dynamics and ovulation rate in the sheep

We have developed an experimental model in which groups of ewes are simultaneously experiencing the first ovarian follicular wave of their oestrous cycle. We used this ‘first-wave model’ in a 2×2 factorial experiment (ten ewes per group) to study the effect of body condition (BC) and a short-term supplement on follicular dynamics and ovulation rate. The ‘first-wave’ was established by giving ewes three injections of prostaglandin (PG), 7 days apart. The 6-day supplement (lupin grain) began 2 days after the second PG injection and continued until the third. Follicles were studied by ultrasound, and blood was sampled to measure glucose and hormones. The supplement increased (P<0.01) the concentrations of glucose, insulin and leptin, decreased FSH concentrations (P<0.01) and tended to increase oestradiol concentrations (P=0.06). The supplement tended to increase the number of 3 mm follicles (P=0.06). Compared with low-BC ewes, high-BC ewes had more follicular waves (P<0.05), higher concentrations of insulin, leptin and IGF1 (P<0.05) and tended to have higher FSH concentrations (P=0.09). Leptin and insulin concentrations remained high until the end of supplementation in high-BC ewes, whereas they decreased after the third day of supplementation in low-BC ewes. In conclusion, high concentrations of metabolic hormones in fat ewes are associated with the development of more follicular waves. When a supplement is superimposed on this situation, changes in glucose and metabolic hormones allow more follicles to be selected to ovulate.

The effect of systemic and ovarian infusion of glucose, galactose and fructose on ovarian function in sheep

Glucose is a critical metabolic fuel in most mammals although many foodstuffs also contain high levels of the monosaccharides, galactose and fructose. The aims of this work were to determine the insulin response to challenges of these sugars (experiment 1) and to examine the effect of systemic (experiment 2) and direct ovarian (experiment 3) infusion of these monosaccharides on ovarian function in ewes with autotransplanted ovaries. In experiment 1, both fructose (fourfold increase peaking in 2 h) and galactose (twofold increase; 30 min) elicited markedly different (P<0.001) insulin responses than glucose (sevenfold increase; 20 min) although the total amount released following fructose and glucose challenge was similar. In experiment 2, low-dose systemic fructose infusion had no acute effect on insulin but did depress FSH (P<0.05), and following the end of fructose infusion, a transient increase in FSH and insulin was observed (P<0.05), which was associated with an increase (P<0.05) in ovarian oestradiol and androstenedione secretion. Systemic infusion of neither glucose nor galactose had a significant effect on ovarian steroidogenesis although glucose acutely suppressed insulin levels. In contrast, ovarian arterial infusion of fructose and glucose had no effect on ovarian function whereas galactose suppressed ovarian follicle number and steroid secretion (P<0.05). In conclusion, this work indicates that fructose and galactose can influence ovarian functionin vivoin sheep and that different mechanisms are involved. Thus, fructose exerts stimulatory effects through indirect modulation of peripheral insulin and/or gonadotrophin levels whereas galactose exerts primarily suppressive effects by direct actions on the ovary.

The effect of monosaccharide sugars and pyruvate on the differentiation and metabolism of sheep granulosa cells in vitro

The objective of this study was to investigate the effect of three monosaccharides or pyruvate on the ability of gonadotrophins to induce cellular proliferation and differentiation of cultured sheep granulosa cells. Lactate production and levels of mRNA expression for the glucose transporters SLC2A1, SLC2A4, SLC2A5 and SLC2A8 were also determined. No energy source in the culture media reduced cell number (50%) and oestradiol (E2) production. Dose and type of monosaccharide had a highly significant (P<0.001) effect on FSH-induced differentiation of the granulosa cells, and there was a highly significant interaction (P<0.001). Glucose supported higher levels of E2production than fructose, which was in turn higher than galactose (P<0.001). In contrast, pyruvate at low doses supported similar levels of E2production as glucose, but higher doses were markedly inhibitory to E2production (P<0.001). Cells responded positively to insulin (P<0.001) in the presence of all three monosaccharides. Glucose and the high doses of fructose resulted in the accumulation of lactate (P<0.001), but pyruvate, galactose and the low dose of fructose resulted in low lactate production. SLC2A5 expression was not detected and SLC2A8 expression was not affected, but SLC2A1 and SLC2A4 expression was depressed (P<0.05) by culture in the presence of fructose and glucose. These data show that glucose, metabolised under anoxic conditions to lactate, is the preferred energy substrate to support the gonadotrophin-induced differentiation of ovine granulosa cellsin vitro, and that fructose and pyruvate, but not galactose, are alternative energy substrates despite marked differences in the way these substrates are metabolised.

Invited review: New perspectives on the roles of nutrition and metabolic priorities in the subfertility of high-producing dairy cows

Management, nutrition, production, and genetics are the main reasons for the decline in fertility in the modern dairy cow. Selection for the single trait of milk production with little consideration for traits associated with reproduction in the modern dairy cow has produced an antagonistic relationship between milk yield and reproductive performance. The outcome is a multi-factorial syndrome of subfertility during lactation; thus, to achieve a better understanding and derive a solution, it is necessary to integrate a range of disciplines, including genetics, nutrition, immunology, molecular biology, endocrinology, metabolic and reproductive physiology, and animal welfare. The common theme underlying the process is a link between nutritional and metabolic inputs that support complex interactions between the gonadotropic and somatotropic axes. Multiple hormonal and metabolic signals from the liver, pancreas, muscle, and adipose tissues act on brain centers regulating feed intake, energy balance, and metabolism. Among these signals, glucose, fatty acids, insulin-like growth factor-I, insulin, growth hormone, ghrelin, leptin, and perhaps myostatin appear to play key roles. Many of these factors are affected by changes in the somatotropic axis that are a consequence of, or are needed to support, high milk production. Ovarian tissues also respond directly to metabolic inputs, with consequences for folliculogenesis, steroidogenesis, and the development of the oocyte and embryo. Little doubt exists that appropriate nutritional management before and after calving is essential for successful reproduction. Changes in body composition are related to the processes that lead to ovulation, estrus, and conception. However, better indicators of body composition and measures of critical metabolites are required to form precise nutritional management guidelines to optimize reproductive outcomes. The eventual solution to the reduction in fertility will be a new strategic direction for genetic selection that includes fertility-related traits. However, this will take time to be effective, so, in the short term, we need to gain a greater understanding of the interactions between nutrition and fertility to better manage the issue. A greater understanding of the phenomenon will also provide markers for more targeted genetic selection. This review highlights many fruitful directions for research, aimed at the development of strategies for nutritional management of reproduction in the high-producing subfertile dairy cow.

Leptin infusion during the early luteal phase in ewes does not affect progesterone production

Infusion of leptin during the ovine follicular phase has been shown to increase progesterone secretion during the subsequent luteal phase. In this study, we have assessed the effects of infusing leptin during the early luteal phase. Infusion of leptin (2.5 microg/h) into the ovarian artery of ewes with ovarian autotransplants (n=5) on day 3 of the luteal phase for 12h did not affect progesterone estradiol or LH concentrations compared to control ewes (n=5). These results suggest no direct effect of leptin on ovarian function at this stage of the estrous cycle.

Role of leptin and ghrelin in the regulation of gonadal function

Gonadal development and function is sustained by the complex interaction of an array of regulatory signals that operate directly on the gonads and/or indirectly via modulation of gonadotropin secretion. During the last decade, different factors primarily involved in the control of food intake and energy balance have been demonstrated as putative modulators of different elements of the reproductive axis, including the gonads, thus helping to define the neuroendocrine basis for the link between body energy stores and fertility. These factors include not only the adipocyte-derived hormone leptin, which is indispensable for proper energy balance and reproduction, but also a number of neuropeptides and hormones of central and peripheral origin. In the latter, growing evidence strongly suggests the involvement of the stomach-secreted peptide ghrelin in the control of several aspects of gonadal function. Interestingly, leptin and ghrelin have been proposed as reciprocally related regulators of energy homeostasis; however, their potential interplay in the control of reproduction remains ill defined. This work will summarize the most salient findings concerning the potential roles of leptin and ghrelin in the functional control of the gonads. In addition, open issues regarding the reproductive facets of these metabolic signals will be highlighted. Overall, the authors propose that through complementary or antagonistic actions, leptin and ghrelin may jointly cooperate to modulate a wide set of reproductive functions, thereby contributing to the physiologic integration of energy balance and reproduction.

Roles of ghrelin and leptin in the control of reproductive function

Reproductive function in mammals, defined as the capacity to generate viable male and female gametes, and to support pregnancy and lactation selectively in the female, is sensitive to the metabolic state of the organism. This contention, long assumed on the basis of intuitive knowledge, became formulated on a scientific basis only recently, with the identification of a number of neuroendocrine signals which crucially participate in the joint control of energy balance and reproduction. A paradigmatic example in this context is the adipocyte-derived hormone, leptin; a satiety factor which signals the amount of body energy (fat) stores not only to the circuits controlling food intake but also to a number of neuroendocrine axes, including the reproductive system. More recently, the reproductive dimension of another metabolic hormone, namely the orexigenic stomach-secreted peptide, ghrelin, has been disclosed by observations on its putative roles in the control of gonadal function and gonadotropin secretion. Of note, leptin and ghrelin have been proposed to act as reciprocal regulators of energy homeostasis. However, their potential interplay in the control of reproduction remains largely unexplored. Based on the comparison of the biological actions of leptin and ghrelin at different levels of the hypothalamic-pituitary-gonadal axis, reviewed in detail herein, we propose that, through concurrent or antagonistic actions, the leptin-ghrelin pair is likely to operate also as modulator of different reproductive functions, thereby contributing to the physiological integration of reproduction and energy balance.

Reproduction in high body condition mares with high versus low leptin concentrations

Previous results from our laboratory indicated that a majority of mares with high body condition scores (BCS) displayed estrous cycles or had considerable follicular activity during the winter. Among these high BCS mares, about 35% of them exhibited a persistent hyperleptinemia and hyperinsulinemia. The current experiment was designed to compare the reproductive characteristics of high BCS mares with hyperleptinemia to those with normal (low) plasma concentrations of leptin during the winter and the first estrous cycle (or the first full cycle encountered for those already cycling). Light horse mares with high BCS (6-8.5) were assigned to groups based on leptin concentrations (8/group): low (<5 ng/mL) and high (>10 ng/mL). Beginning 7 January, mares were assessed every 3d for follicular activity and then daily once a follicle >25 mm was detected. Mares were subsequently monitored through their first and second ovulations. Leptin concentrations remained higher (P<0.001) in mares in the high leptin group over the duration of the experiment. Also, high leptin mares had greater (P<0.0001) insulin response to glucose infusion and a faster (P<0.05) rate of glucose clearance. One mare with high leptin and three mares with low leptin had progesterone concentrations indicative of the presence of a corpus luteum at the onset of the experiment. Plasma concentrations of LH, FSH, and progesterone did not differ between groups (P>0.1) during the first estrous cycle occurring after 7 January. Date of first ovulation after 7 January and interovulatory interval were similar (P>0.1) for the two groups, as were estimates of follicular numbers on the ovaries (small, medium, and large; P>0.1). It is concluded that the perturbations in leptin and insulin secretion observed in some high BCS mares are not associated with alterations in ovarian activity or the estrous cycle during winter and into the period of vernal transition.

A review of the effects of supplementary nutrition in the ewe on the concentrations of reproductive and metabolic hormones and the mechanisms that regulate folliculogenesis and ovulation rate

This paper discusses the phenomenon of nutritional flushing in ewes whereby increased nutrition stimulates folliculogenesis and ovulation rate. In addition the paper reviews recent findings on the effects of increased levels of nutrition on the blood concentrations of reproductive and metabolic hormones in the ewe and some of the intraovarian changes that take place in response to nutritional stimulation. Finally, in the paper, we propose a model of the physiological mechanism for the nutritional stimulation of folliculogenesis and we review how closely the model fits recent published and unpublished evidence examining the mechanism of flushing. Nutritional stimulation alters the blood concentrations of some metabolic hormones. By using short-term models of nutritional flushing, we have shown that as the blood concentrations of insulin and leptin increase that of growth hormone decreases while that of IGF-I appears unaffected by the nutritional flushing. Nutritional flushing also alters the blood concentrations of some reproductive hormones. Again, using the same model, we have shown that there is a transient increase in FSH and a decrease in oestradiol concentrations in the blood. The changes in oestradiol are particularly evident in the follicular phase of the oestrous cycle. In the ovary, the effect of nutrition is to stimulate folliculogenesis. These changes are associated with intra-follicular alterations in the insulin-glucose, IGF and leptin metabolic systems. The stimulation of these intra-follicular systems leads to a suppression in follicular oestradiol production. The consequence of these direct actions on the follicle is a reduced negative feedback to the hypothalamic-pituitary system and increased FSH secretion that leads to a stimulation of folliculogenesis.

The effect of nutrition on the seasonality of reproduction in ewes

The beneficial effects of nutrition on reproduction in sheep have been described, particularly on ovulation rate. However, the relationships between nutrition and reproductive seasonality are not well known. This review will deal with the effects of body fat or food intake on sexual and hypothalamic/pituitary activity in sheep, mainly focused on Mediterranean genotypes. Although only severe malnutrition can significantly extend the length of the seasonal anestrous period, the level of fat reserves can play a significant role on reproductive seasonality delaying the onset of seasonal anoestrus, particularly on the Mediterranean environment. The effect of overfeeding on LH secretion has also been reported, specially at short term. Several experimental approaches have elucidated that both high body fat and food intake are able to modify the sensitivity of the hypothalamus to oestradiol negative feedback during seasonal anoestrus, with those effects being associated to a reduced amount of NPY mRNA and to an increase of plasma insulin, glucose and leptin concentrations, particularly in the late scenario. However, the highest receptivity to nutritional stimulation in terms of increasing LH occurs when ewes are subjected to a photoperiodic state of early anoestrus or late breeding season rather than under a photoperiod characteristic of the end of anoestrus or the beginning of the breeding season.