Effects of leptin on gonadotropin-releasing hormone release from hypothalamic–infundibular explants and gonadotropin release from adenohypophyseal primary cell cultures: further evidence that fully nourished cattle are resistant to leptin

Interaction of pre- and postnatal nutrition on expression of leptin receptor variants and transporter molecules, leptin transport, and functional response to leptin in heifers†

Perinatal nutrition modulates the hypothalamic neurocircuitries controlling GnRH release, thus programming pubertal maturation in female mammals. Objectives of experiments reported here were to test the hypotheses that prenatal nutrition during mid- to late gestation interacts with postnatal nutrition during the juvenile period in heifer offspring to alter expression of leptin receptor (LepR) variants (ObRa, ObRb, ObRc, ObRt), and lipoprotein transporter molecules (LRP1 and 2) in the choroid plexus, leptin transport across the blood-brain barrier, and hypothalamic-hypophyseal responsiveness to exogenous ovine leptin (oleptin) during fasting. Nutritional programming of heifers employed a 3 × 2 factorial design of maternal (high, H; low, L; and moderate, M) × postnatal (H and L) dietary treatments. Results (Expt. 1) demonstrated that prepubertal heifers born to L dams, regardless of postnatal diet, had reduced expression of the short isoform of ObRc compared to H and M dams, with sporadic effects of undernutrition (L or LL) on ObRb, ObRt, and LRP1. Intravenous administration of oleptin to a selected postpubertal group (HH, MH, LL) of ovariectomized, estradiol-implanted heifers fasted for 56 h (Expt. 2) did not create detectable increases in third ventricle cerebrospinal fluid but increased gonadotropin secretion in all nutritional groups tested. Previous work has shown that leptin enhances gonadotropin secretion during fasting via effects at both hypothalamic and anterior pituitary levels in cattle. Given the apparent lack of robust transfer of leptin across the blood-brain barrier in the current study, effects of leptin at the adenohypophyseal level may predominate in this experimental model.

Review: The effect of nutrition on timing of pubertal onset and subsequent fertility in the bull

The advent of genomic selection has led to increased interest within the cattle breeding industry to market semen from young bulls as early as possible. However, both the quantity and quality of such semen is dictated by the age at which these animals reach puberty. Enhancing early life plane of nutrition of the bull stimulates a complex biochemical interplay involving metabolic and neuroendocrine signalling and culminating in enhanced testicular growth and development and earlier onset of sexual maturation. Recent evidence suggests that an enhanced plane of nutrition leads to an advancement of testicular development in bulls at 18 weeks of age. However, as of yet, much of the neuronal mechanisms regulating these developmental processes remain to be elucidated in the bull. While early life nutrition clearly affects the sexual maturation process in bulls, there is little evidence for latent effects on semen traits post-puberty. Equally the influence of prevailing nutritional status on the fertility of mature bulls is unclear though management practices that result in clinical or even subclinical metabolic disease can undoubtedly impact upon normal sexual function. Dietary supplements enriched with various polyunsaturated fatty acids or fortified with trace elements do not consistently affect reproductive function in the bull, certainly where animals are already adequately nourished. Further insight on how nutrition mediates the biochemical interaction between neuroendocrine and testicular processes will facilitate optimisation of nutritional regimens to optimise sexual maturation and subsequent semen production in bulls.

Hot topic: Ceramide inhibits insulin sensitivity in primary bovine adipocytes

In nonruminants, the sphingolipid ceramide inhibits insulin sensitivity by inactivating protein kinase B (AKT) within the insulin-signaling pathway. We have established that ceramide accrual develops with impaired systemic insulin action in ruminants during the transition from gestation to lactation, dietary palmitic acid supplementation, or controlled nutrient restriction. We hypothesized that ceramide promotes AKT inactivation and antagonizes insulin sensitivity in primary bovine adipocytes. Stromal-vascular cells were grown from bovine adipose tissue explants and cultured in differentiation media. To modify ceramide supply, we treated differentiated adipocytes with (1) myriocin, an inhibitor of de novo ceramide synthesis, or (2) cell-permeable C2:0-ceramide. Insulin-stimulated AKT activation (i.e., phosphorylation) and 2-deoxy-D-[³H]-glucose (2DOG) uptake were measured. Treatment of adipocytes with myriocin consistently decreased concentrations of ceramide, monohexosylceramide, and lactosylceramide. The insulin-stimulated ratio of phosphorylated AKT to total AKT was increased with myriocin but decreased with C2:0-ceramide. Moreover, adipocyte insulin-stimulated 2DOG uptake was decreased with C2:0-ceramide and increased with myriocin. We conclude that ceramide inhibits insulin-stimulated glucose uptake by downregulating AKT activation in primary bovine adipocytes.

Effect of nutrition during calfhood and peripubertal period on serum metabolic hormones, gonadotropins and testosterone concentrations, and on sexual development in bulls

The objective of the present study was to characterize the effects of nutrition on circulating concentrations of metabolic hormones, gonadotropins, and testosterone during sexual development in bulls. Nutrition regulated the hypothalamus-pituitary-testes axis through effects on the GnRH pulse generator in the hypothalamus and through direct effects on the testes. Pituitary function (gonadotropin secretion after GnRH challenge) was not affected by nutrition. However, nutrition affected LH pulse frequency and basal LH concentration during the early gonadotropin rise (10-26 weeks of age). There were close temporal associations between changes in insulin-like growth factor-I (IGF-I) concentrations and changes in LH pulse frequency, suggesting a role for IGF-I in regulating the early gonadotropin rise in bulls. The peripubertal increase in testosterone concentration was delayed in bulls with lesser serum IGF-I concentrations (low nutrition), suggesting a role for IGF-I in regulating Leydig cell function. Serum IGF-I concentrations accounted for 72 and 67% of the variation in scrotal circumference and paired-testes volume, respectively (at any given age), indicating that IGF-I may regulate testicular growth. Bulls with a more sustained elevated LH pulse frequency during the early gonadotropin rise (high nutrition) had greater testicular mass at 70 weeks of age relative to the control group (medium nutrition), despite no differences in metabolic hormone concentrations after 26 weeks of age. Therefore, gonadotropin-independent mechanism regulating testicular growth might be dependent on previous gonadotropin milieu.

Aetiology and pathogenesis of cystic ovarian follicles in dairy cattle: a review

Cystic ovarian follicles (COF) are an important ovarian dysfunction and a major cause of reproductive failure in dairy cattle. Due to the complexity of the disorder and the heterogeneity of the clinical signs, a clear definition is lacking. A follicle becomes cystic when it fails to ovulate and persists on the ovary. Despite an abundance of literature on the subject, the exact pathogenesis of COF is unclear. It is generally accepted that disruption of the hypothalamo-pituitary-gonadal axis, by endogenous and/or exogenous factors, causes cyst formation. Secretion of GnRH/LH from the hypothalamus-pituitary is aberrant, which is attributed to insensitivity of the hypothalamus-pituitary to the positive feedback effect of oestrogens. In addition, several factors can influence GnRH/LH release at the hypothalamo-pituitary level. At the ovarian level, cellular and molecular changes in the growing follicle may contribute to anovulation and cyst formation, but studying follicular changes prior to cyst formation remains extremely difficult. Differences in receptor expression between COF and dominant follicles may be an indication of the pathways involved in cyst formation. The genotypic and phenotypic link of COF with milk yield may be attributed to negative energy balance and the associated metabolic and hormonal adaptations. Altered metabolite and hormone concentrations may influence follicle growth and cyst development, both at the level of the hypothalamus-pituitary and the ovarian level.

Adipokines: implications for female fertility and obesity

Obesity is associated with a diverse set of metabolic disorders, and has reproductive consequences that are complex and not well understood. The adipose tissue-produced leptin has dominated the literature with regards to female fertility complications, but it is pertinent to explore the likely role of other adipokines--adiponectin and resistin--as our understanding of their biological functions emerge. Leptin influences the developing embryo, the functioning of the ovary and the endometrium, interacts with the release and activity of gonadotrophins and the hormones that control their synthesis. In this review such biological actions and potential roles of the adipokines leptin, adiponectin and resistin are explored in relation to female fertility and the complexity of the obese metabolic state.

Obesity is associated with altered metabolic and reproductive activity in the mare: effects of metformin on insulin sensitivity and reproductive cyclicity

In mares, obesity is associated with continuous reproductive activity during the non-breeding season. To investigate the effect of obesity and associated alterations in metabolic parameters on the oestrous cycle, two related studies were conducted. In Experiment 1, obese (body condition score > 7) mares were fed ad libitum or were moderately feed restricted during the late summer and autumn months. Feed restriction did not alter the proportion of mares entering seasonal anoestrus. However, obese mares exhibited a significantly longer duration of the oestrous cycle, significant increases in circulating concentrations of leptin and insulin, and decreased insulin sensitivity and concentrations of thyroxine compared with feed-restricted mares throughout the experiment. Experiment 2 was designed to investigate the effects of administration of the insulin-sensitising drug metformin hydrochloride on insulin sensitivity and the characteristics of the oestrous cycle in obese mares. In a dose–response trial, metformin increased insulin sensitivity after 30 days following administration of 3 g day–1, but not 6 or 9 g day–1, compared with controls receiving vehicle only. However, there were no differences in insulin sensitivity or oestrous cycle characteristics between control and metformin-treated groups when the 3 g day–1 dose was tested for a longer period of 2 months. These results demonstrate that obesity is associated with aberrations in the oestrous cycle and perturbations in several markers of metabolic status. The results also indicate that metformin is not an effective long-term monotherapy for increasing insulin sensitivity in horses at the doses tested. Additional studies are needed to examine possible effects of increasing insulin sensitivity on reproductive activity in obese mares.

Regulatory roles of leptin in reproduction and metabolism: a comparative review

Leptin plays an important role in signaling nutritional status to the central reproductive axis of mammals and appears to be at least a permissive factor in the initiation of puberty. The expression and secretion of leptin are correlated with body fat mass and are acutely affected by changes in feed intake. Moreover, circulating leptin increases during pubertal development in rodents, human females and heifers. Effects of leptin are mediated mainly via receptor activation of the JAK-STAT pathway; however, activation of alternative pathways, such as MAP kinase, has also been reported. Although the leptin receptor (LR) has not been found on GnRH neurons, leptin stimulates the release of GnRH from rat and porcine hypothalamic explants. Moreover, leptin increases the release of LH in rats and from adenohypophyseal explants and/or cells from full-fed rats and pigs. In contrast, stimulation of the hypothalamic-gonadotropic axis by leptin in cattle and sheep is observed predominantly in animals and tissues pre-exposed to profound negative energy balance. For example, leptin prevents fasting-mediated reductions in the frequency of LH pulses in peripubertal heifers, augments the magnitude of LH and GnRH pulses in fasted cows, and enhances basal secretion of LH in vivo and from adenohypophyseal explants of fasted cows. However, leptin is incapable of accelerating the frequency of LH pulses in prepubertal heifers, regardless of nutrient status, and has no effect on the secretion of GnRH and LH in full-fed cattle or hypothalamic/hypophyseal explants derived thereof. Similar to results obtained with LH, basal secretion of GH from anterior pituitary explants of fasted, but not normal-fed cows, was potentiated acutely by low, but not high, doses of leptin. Mechanisms through which undernutrition hypersensitize the hypothalamic-gonadotropic axis to leptin may involve up-regulation of the LR. However, an increase in LR mRNA expression is not a requisite feature of heightened adenohypophyseal responses in fasted cattle. To date, leptin has not been successful for inducing puberty in ruminants. Future therapeutic uses for recombinant leptin that exploit states of nutritional hypersensitization, and identification of genetic markers for genotypic variation in leptin resistance, are currently under investigation.