Circulating estradiol suppresses luteinizing hormone pulse frequency during dietary restriction

Effect of feed restriction on hepatic estradiol metabolism and liver function in cows

In this study, the effects of restriction feeding on the liver function, hepatic uridine diphosphate glucuronosyltransferase (UGT) activity, hepatic insulin-like growth factor (IGF)-1 mRNA expression and response to high-dose estradiol-17β (E₂) administration were investigated in non-lactating cows. Cows were assigned to either restricted feeding (30% of total digestible nutrient requirement) or ad libitum feeding of a dent corn-based concentrate and roughage for a 2-week feeding trial (Day 1=day of beginning the feeding trial). On day 14, a high-dose E₂ administration study was carried out to examine plasma E₂ levels as an indicator of hepatic E₂ metabolism. Plasma E₂ concentration in the restricted feeding group was consistently higher after high-dose E₂ administration than in the control group. In addition, indocyanine green half-life value was prolonged by restricted feeding for 13 days, and increased liver triglyceride concentration and decreased liver UGT activity were caused by this restriction over 14 days. Restricted feeding did not affect plasma IGF-1 concentration or hepatic IGF-1 mRNA expression. These results suggest that two weeks of restriction feeding led to accumulation of triglyceride, decreased liver blood flow, and slightly impaired liver function, which in turn slowed down the hepatic metabolism of E₂ without significantly impacting hepatic IGF-1 production.

New concepts of the central control of reproduction, integrating influence of stress, metabolic state, and season

Gonadotropin releasing hormone is the primary driver of reproductive function and pulsatile GnRH secretion from the brain causes the synthesis and secretion of LH and FSH from the pituitary gland. Recent work has revealed that the secretion of GnRH is controlled at the level of the GnRH secretory terminals in the median eminence. At this level, projections of kisspeptin cells from the arcuate nucleus of the hypothalamus are seen to be closely associated with fibers and terminals of GnRH cells. Direct application of kisspeptin into the median eminence causes release of GnRH. The kisspeptin cells are activated at the time of a natural "pulse" secretion of GnRH, as reflected in the secretion of LH. This appears to be due to input to the kisspeptin cells from glutamatergic cells in the basal hypothalamus, indicating that more than 1 neural element is involved in the secretion of GnRH. Because the GnRH secretory terminals are outside the blood-brain barrier, factors such as kisspeptin may be administered systemically to cause GnRH secretion; this offers opportunities for manipulation of the reproductive axis using factors that do not cross the blood-brain barrier. In particular, kisspeptin or analogs of the same may be used to activate reproduction in the nonbreeding season of domestic animals. Another brain peptide that influences reproductive function is gonadotropin inhibitory hormone (GnIH). Work in sheep shows that this peptide acts on GnRH neuronal perikarya, but projections to the median eminence also allow secretion into the hypophysial portal blood and action of GnIH on pituitary gonadotropes. GnIH cells are upregulated in anestrus, and infusion of GnIH can block the ovulatory surge in GnRH and/or LH secretion. Metabolic status may also affect the secretion of reproduction, and this could involve action of gut peptides and leptin. Neuropeptide Y and Y-receptor ligands have a negative impact on reproduction, and Neuropeptide Y production is markedly increased in negative energy balance; this may be the cause of lowered GnRH and gonadotropin secretion in this state. There is a complex interaction between appetite-regulating peptide neurons and kisspeptin neurons that enables the former to regulate the latter both positively and negatively. In terms of how GnRH secretion is reduced during stress, recent data indicate that GnIH cells are integrally involved, with increased input to the GnRH cells. The secretion of GnIH into the portal blood is not increased during stress, so the negative effect is most likely effected at the level of GnRH neuronal cell bodies.

Interface between metabolic balance and reproduction in ruminants: focus on the hypothalamus and pituitary

This article is part of a Special Issue "Energy Balance". The interface between metabolic regulators and the reproductive system is reviewed with special reference to the sheep. Even though sheep are ruminants with particular metabolic characteristics, there is a broad consensus across species in the way that the reproductive system is influenced by metabolic state. An update on the neuroendocrinology of reproduction indicates the need to account for the way that kisspeptin provides major drive to gonadotropin releasing hormone (GnRH) neurons and also mediates the feedback effects of gonadal steroids. The way that kisspeptin function is influenced by appetite regulating peptides (ARP) is considered. Another newly recognised factor is gonadotropin inhibitory hormone (GnIH), which has a dual function in that it suppresses reproductive function whilst also acting as an orexigen. Our understanding of the regulation of food intake and energy expenditure has expanded exponentially in the last 3 decades and historical perspective is provided. The function of the regulatory factors and the hypothalamic cellular systems involved is reviewed with special reference to the sheep. Less is known of these systems in the cow, especially the dairy cow, in which a major fertility issue has emerged in parallel with selection for increased milk production. Other endocrine systems--the hypothalamo-pituitary-adrenal axis, the growth hormone (GH) axis and the thyroid hormones--are influenced by metabolic state and are relevant to the interface between metabolic function and reproduction. Special consideration is given to issues such as season and lactation, where the relationship between metabolic hormones and reproductive function is altered.

Effects of photoperiod and food restriction on the reproductive physiology of female California mice

Many temperate-zone animals use changes in photoperiod to time breeding. Shorter term cues, like food availability, are integrated with photoperiod to adjust reproductive timing under unexpected conditions. Many mice of the genus Peromyscus breed in the summer. California mice (Peromyscus californicus), however, can breed year round, but tend to begin breeding in the winter. Glial cells may be involved in transduction of environmental signals that regulate gonadotrophin releasing hormone I (GnRH) activity. We examined the effects of diet and photoperiod on reproduction in female California mice. Mice placed on either short days (8L:16D) or long days (16L:8D) were food restricted (80% of normal intake) or fed ad libitum. Short day-food restricted mice showed significant regression of the reproductive system. GnRH-immunoreactivity was increased in the tuberal hypothalamus of long day-food restricted mice. This may be associated with the sparing effect long days have when mice are food restricted. The number of GFAP-immunoreactive fibers in proximity to GnRH nerve terminals correlated negatively with uterine size in ad libitum but not food restricted mice, suggesting diet may alter glial regulation of the reproductive axis. There was a trend towards food restriction increasing uterine expression of c-fos mRNA, an estrogen dependent gene. Similar to other seasonally breeding rodents, short days render the reproductive system of female California mice more susceptible to effects of food restriction. This may be vestigial, or it may have evolved to mitigate consequences of unexpectedly poor winter food supplies.