Nutritional influences on reproductive neuroendocrine output: insulin, leptin, and orexigenic neuropeptide signaling in the ovine hypothalamus

Dual-energy X-ray absorptiometry scans accurately predict differing body fat content in live sheep

Background

There is considerable interest in implementing mobile scanning technology for on-farm body composition analysis on live animals. These experiments evaluated the use of dual energy X-ray absorptiometry (DXA) as an accurate method of total body fat measurement in live sheep.

Results

In Exp. 1, visceral and whole body fat analysis was undertaken in sheep with body condition scores (BCS) in the range 2 to 3.25 (scale 1: thin to 5: fat). The relationship of BCS was moderately correlated with visceral fat depot mass (r = 0.59, P < 0.01, n = 24) and whole body fat (r = 0.70, P < 0.001, n = 24). In Exp. 2, sheep with BCS in the range 2.25 to 3.75 were blood sampled to analyse circulating leptin concentrations, and were DXA scanned immediately post mortem for total body fat. Plasma leptin concentrations had low correlations with BCS (r = 0.50, P < 0.05, n = 17) and DXA body fat (r = 0.42, P < 0.05, n = 17), and no correlation with chemical body fat (r = 0.17, P > 0.05, n = 9). There was a moderate correlation between DXA body fat and BCS (r = 0.70, P < 0.01, n = 17), and DXA body fat was highly correlated with chemical body fat (r = 0.81, P < 0.001, n = 9). In Exp. 3, a series of five DXA scans, at 8-week intervals, was performed on growing sheep over a 32-week period. The average BCS ranged from 2.39 ± 0.07 (S.E.M.) to 3.05 ± 0.11 and the DXA body fat (%) ranged from 16.8 ± 0.8 to 24.2 ± 1.2. There was a moderate correlation between DXA body fat and BCS over the 32 weeks (r = 0.61, P < 0.001, n = 24).

Conclusions

Overall, these experiments indicated that there was good agreement between BCS, DXA and chemical analysis for measuring total body fat in sheep, and that DXA scanning is a valid method for longitudinal measurement of total body fat in live sheep.

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.

Interactions between nutrition and reproduction in the management of the mature male ruminant

In mature male sheep and goats, changes in feed intake seem to have little effect on gonadal endocrine function but induce profound changes on sperm production. These outcomes are due to changes in size of the seminiferous tubules and in spermatogenic efficiency. Except with severe underfeeding, there are only minor changes in the endocrine function of the testis (testosterone production) unless season-long treatments are imposed. For cattle, nutrition clearly affects testicular development and the production of spermatozoa in young bulls, as it does in other species but, after the period of rapid growth has ended, there appears to be little or no response to nutrition. We are developing a clear picture of the metabolic signals, neuroendocrine processes and hormonal control systems that are involved, particularly for the mature male sheep. The energetic components of the diet, rather than protein, seem to be responsible, so we have envisaged a model of the relationship between energy balance and reproduction that has 4 'dimensions': genotype, structure (organs), communication (chemical and neural signals, nutrient sensing) and time (dynamics, metabolic memory, programming). We have linked these perspectives to 'resource allocation theory' and incorporated them into strategies for 'clean, green and ethical animal production'. In contrast to the clear outcomes with respect to spermatogenesis, the effects of nutrition on sexual behaviour are more difficult to define, perhaps because the behaviour is affected by a complex mix of physiological factors and because of flawed methods for quantifying male behaviour. For example, sexual behaviour is compromised by severe feed restriction, but male sexual behaviour requires intensive motor activity so a decline in libido could be caused by general weakness rather than specific nutritional limitations. The interaction between sexual activity and feeding behaviour also complicates the issue under field conditions. At the other end of the scale, overweight males can show reduced sexual success because they have difficulty courting and mounting. For this reason, exercise can enhance the fertilising capacity of rams. This will be important in extensive mating systems where males need to assemble and guard a harem and then mate many times for several weeks. For artificial insemination centres, there seems to be very few data on the nutritional management of males, but problems with overfed animals appear to be a risk. Future research should concentrate on the intra-testicular systems mediating the effects of nutrition on the production of spermatozoa.

Selected hormonal and neurotransmitter mechanisms regulating feed intake in sheep

Appetite control is a major issue in normal growth and in suboptimal growth performance settings. A number of hormones, in particular leptin, activate or inhibit orexigenic or anorexigenic neurotransmitters within the arcuate nucleus of the hypothalamus, where feed intake regulation is integrated. Examples of appetite regulatory neurotransmitters are the stimulatory neurotransmitters neuropeptide Y (NPY), agouti-related protein (AgRP), orexin and melanin-concentrating hormone and the inhibitory neurotransmitter, melanocyte-stimulating hormone (MSH). Examination of messenger RNA (using in situ hybridization and real-time PCR) and proteins (using immunohistochemistry) for these neurotransmitters in ruminants has indicated that physiological regulation occurs in response to fasting for several of these critical genes and proteins, especially AgRP and NPY. Moreover, intracerebroventricular injection of each of the four stimulatory neurotransmitters can increase feed intake in sheep and may also regulate either growth hormone, luteinizing hormone, cortisol or other hormones. In contrast, both leptin and MSH are inhibitory to feed intake in ruminants. Interestingly, the natural melanocortin-4 receptor (MC4R) antagonist, AgRP, as well as NPY can prevent the inhibition of feed intake after injection of endotoxin (to model disease suppression of appetite). Thus, knowledge of the mechanisms regulating feed intake in the hypothalamus may lead to mechanisms to increase feed intake in normal growing animals and prevent the wasting effects of severe disease in animals.

In vivo changes in central and peripheral insulin sensitivity in a large animal model of obesity

Obesity disrupts homeostatic energy balance circuits leading to insulin resistance. Here we examined in vivo peripheral and central insulin sensitivity, and whether central insensitivity in terms of the voluntary food intake (VFI) response occurs within the hypothalamus or at blood-brain transfer level, during obesity and after subsequent weight loss. Sheep with intracerebroventricular (i.c.v.) cannulae were fed complete diet for 40 wk ad libitum (obese group) or at control level (controls). Thereafter, obese sheep were food restricted (slimmers) and controls fed ad libitum (fatteners) for 16 wk. Dual-energy x-ray absorptiometry (DEXA) measured total body fat, insulin analyses in blood and cerebrospinal fluid (CSF) assessed blood-brain transfer, i.v. glucose tolerance test (GTT) and insulin tolerance test (ITT) measured peripheral insulin sensitivity, and VFI responses to icv insulin assessed intrahypothalamic sensitivity. Insulinemia was higher in obese than controls; plasma insulin correlated with DEXA body fat and CSF insulin. Insulinemia was higher in fatteners than slimmers but ratio of CSF to plasma insulin correlated only in fatteners. Plasma glucose baseline and area under the curve were higher during GTT and ITT in obese than controls and during ITT in fatteners than slimmers. GTT and ITT glucose area under the curve correlated with DEXA body fat. VFI decreased after i.c.v. insulin, with response magnitude correlating negatively with DEXA body fat. Overall, insulin resistance developed first in the periphery and then within the brain, thereafter correlating with adiposity; central resistance in terms of VFI response resulted from intrahypothalamic insensitivity rather than impaired blood-brain transfer; modest weight loss improved peripheral but not central insulin sensitivity and induced central hypoinsulinemia.

The interaction between photoperiod and nutrition and its effects on seasonal rhythms of reproduction in the ewe

In sheep, the seasonal patterns of reproductive activity are driven primarily by the annual photoperiodic cycle, but can also respond to other environmental factors, such as nutrition, yet little is known about the mechanisms underlying this interaction. This study was designed to define the interaction between photoperiodic and nutritional cues on seasonal patterns of ovarian activity, and to determine if there is a central interaction between these cues. Groups of Ile-de-France ewes were maintained in two nutritional states (restricted and well fed) under a simulated annual photoperiod of 8-16 h of light per day over two breeding seasons. At the end of the first breeding season, half of the animals of each group were ovariectomized (OVX) and fitted subcutaneously with estradiol implants. Low nutritional status shortened the season of ovarian activity, determined from the pattern of progesterone concentrations, by modifying the timing of seasonal transitions between periods of ovarian activity and anestrus. The same results were observed for the seasonal rhythm of neuroendocrine activity, assessed in the OVX ewes, from the pattern of luteinizing hormone concentrations. These results were then confirmed for neuroendocrine activity induced by a photoperiodic treatment. We conclude that nutrition centrally modulates the interpretation of photoperiod to affect seasonal reproductive transitions. The mechanisms of this interaction are discussed in the paper.

Effects of nutritional cues on the duration of the winter anovulatory phase and on associated hormone levels in adult female Welsh pony horses (Equus caballus)

BACKGROUND

Mares have an annual reproductive rhythm, with a phase of inactivity in midwinter. The aim of this study was to determine the impact of food restriction on physiological and metabolic hallmarks of this rhythm.

METHODS

Over three successive years, 3 groups of 10 mares were kept under natural photoperiod. A 'well-fed' group was fed to maintain the mares in good body condition; a 'restricted' group received a diet calculated to keep the mares thin and a 'variable' group was fed during some periods like the 'restricted' group and during some other periods like the 'well-fed' group, with the aim of mimicking the natural seasonal variation of pasture availability, but a few months in advance of this natural rhythm.

RESULTS

Winter ovarian inactivity always occurred and was long in the restricted group. In contrast, in the 'well-fed' group, 40% of mares showed this inactivity, which was shorter than in the other groups. Re-feeding the 'variable' group in autumn and winter did not advance the first ovulation in spring, compared with the 'restricted' group. Measurements of glucose and insulin concentrations in mares from the 'restricted' group during two 24 h periods of blood sampling, revealed no post-prandial peaks. For GH (Growth hormone), IGF-1 and leptin levels, large differences were found between the 'well-fed' group and the other groups. The glucose, insulin, GH and leptin levels but not melatonin level are highly correlated with the duration of ovulatory activity.

CONCLUSIONS

The annual rhythm driven by melatonin secretion is only responsible for the timing of the breeding season. The occurrence and length of winter ovarian inactivity is defined by metabolic hormones.

Adiposity and plane of nutrition influence reproductive neuroendocrine and appetite responses to intracerebroventricular insulin and neuropeptide-Y in sheep

Long-term nutritional background is thought to influence hypothalamic appetite and reproductive neuroendocrine responses to short-term nutritional feedback. In order to investigate this phenomenon, the effects of intracerebroventricular administration of insulin or neuropeptide-Y (NPY) on LH secretion and voluntary food intake (VFI) were examined in sheep that were initially thin and kept on an increasing nutritional plane (INP), or initially fat and kept on a decreasing nutritional plane (DNP), for 10 weeks. Intracerebroventricular insulin stimulated LH secretion and suppressed VFI in INP sheep when initially thin, but not when they became fat, and had no effect on LH in DNP sheep when initially fat, and stimulated LH secretion when they became thin. Intracerebroventricular NPY had no effect on LH or VFI in INP sheep when initially thin, decreased LH secretion and increased VFI when they became fat, and decreased LH secretion in DNP sheep when initially fat but had no effect when they became thin. Therefore, sensitivity to insulin increases with low or decreasing nutritional status and decreases with high or increasing nutritional status, whereas sensitivity to NPY increases with high or increasing nutritional status and decreases with low or decreasing nutritional status. In conclusion, reproductive neuroendocrine and appetite responses to acute changes in nutritional feedback signals depend on the individual's longer-term nutritional background.

The effect of gestational undernutrition on maternal weight change and fetal weight in lines of mice selected for different growth characteristics

The present study investigated whether the genetic growth characteristics (fast or slow growing, lean or fat) of a mother influences her ability to partition nutrients to developing offspring. A total of sixty-one pregnant mice of three selected lines were used: fast-growing, relatively fat (FF, n 19); fast-growing, relatively lean (FL, n 23); and normal growth, relatively lean (NL, n 19). On day 1 of pregnancy, mice were given either ad libitum access to food (control (C): n 32) or pair-fed at 80 % of C intake (restricted (R): n 29). Feed intake and dam weight were measured daily. The weight of the mouse, organs, mammary tissue and the weight of fetuses and placentas were determined at day 18 of gestation. Overall, R dams gained less than half the weight of C dams during gestation. NL dams gained the most weight, and FF dams gained the least weight (P < 0·001). R dams in the fast-growing lines mobilised significantly more body fat during gestation than the NL line (P < 0·001) and had a greater reduction in mammary tissue growth. The relative weight of the litter increased in R dams of the FF line but was reduced in both the lean lines. Undernutrition reduced fetal and placental weight, and reduced placental efficiency in all the lines. The reduction was least in the FF line and greatest in the FL line. The data suggest that selection of animals for different growth characteristics alters their response to undernutrition during pregnancy, the relatively fat line was better able to buffer its offspring from the effects of undernutrition than the lean lines, regardless of their underlying rate of growth.

Decreased blood-brain leptin transfer in an ovine model of obesity and weight loss: resolving the cause of leptin resistance

OBJECTIVE

Hypothalamic resistance to the anorexigenic actions of the peripheral adipostat hormone leptin is characteristic of obesity. Here, we use an obese animal model of similar body weight to that of the human to test in vivo whether leptin resistance is due to decreased blood-brain leptin transport or intra-hypothalamic insensitivity, and whether sensitivity to leptin is restored by weight loss. For 40 weeks, adult sheep surgically prepared with intra-cerebroventricular (ICV) cannulae were given a complete natural diet ad libitum ('Obese' group) or in restricted quantities ('Lean' group), and then the dietary amounts were reversed for 16 weeks until mean group body weights converged ('Slimmers' and 'Fatteners', respectively).

RESULTS

ICV leptin injection (0.5 mg) at 8-week intervals acutely decreased voluntary food intake by approximately 35% in the 'Obese' group on each occasion and in 'Slimmers' and 'Fatteners' at the end, providing no evidence of intra-hypothalamic insensitivity. The ratio between endogenous leptin concentrations in ventricular cerebrospinal fluid (CSF) and peripheral blood decreased with increasing leptinaemia in 'Obese' sheep, indicating decreased efficiency of blood-brain leptin transport, whereas leptin concentrations remained low and the CSF:blood ratio remained high in 'Lean' sheep. Compared with 'Fatteners' of similar body weight, 'Slimmers' were hypoleptinaemic, but their CSF:blood leptin concentration ratio remained low. Thus, the obesity-induced impairment of leptin blood-brain transport was sustained despite an approximately 15% weight loss.

CONCLUSION

These results support the hypothesis that central resistance to leptin in obesity with associated peripheral hyperleptinaemia is attributable to decreased efficiency of leptin transport into the brain and not to intra-hypothalamic leptin insensitivity. However, leptin transport efficiency is not restored after weight loss by caloric restriction despite the prevailing hypoleptinaemia.

A test of the lipostat theory in a seasonal (ovine) model under natural conditions reveals a close relationship between adiposity and melanin concentrating hormone expression

A study was undertaken in Corriedale ewes to test the lipostat theory using data obtained from a model of seasonal change in food intake and body composition. The theory predicts adipose-derived factors signal to the brain and vice versa, to maintain homeostasis. It is held that leptin acts on cells in the brain to regulate food intake and energy expenditure, through "first order" neurons in the arcuate nucleus (ARC). These cells are thought to receive information that is relayed to "second order" neurons, to regulate food intake and other functions. In this study, groups (n=4-5) of ovariectomized ewes were maintained under natural conditions and sampled at various points across the year. Food intake, body composition and indices of metabolic function were measured prior to collection of brains for in situ hybridization analysis. Expression of genes encoding for neuropeptide Y (NPY), pro-opiomelanocortin (POMC), orexin (ORX), melanin concentrating hormone (MCH) and leptin receptor (ObRb) was quantified. NPY gene expression was high when food intake was also high but, across the year, changes in NPY and POMC gene expression did not correspond predictably to plasma leptin levels or leptin receptor gene expression. Negative correlation was found between adiposity (omental and whole body fat) and gene expression of MCH and ORX, suggesting that changes in expression of genes for "second order" orexigenic peptides are closely linked to changes of metabolic state, even when similar relationships cannot be shown for expression of genes in "first order" neurons. These data provide support for the lipostat theory.