The contribution of changes in milk delivery to the prolongation of lactational infertility induced by food restriction or increased litter size

Energetic demands of lactation produce an increase in the expression of growth hormone secretagogue receptor in the hypothalamus and ventral tegmental area of the rat despite a reduction in circulating ghrelin

Lactating rats show changes in the secretion of hormones and brain signals that promote hyperphagia and facilitate the production of milk. Little is known, however, about the role of ghrelin in the mechanisms sustaining lactational hyperphagia. Here, we used Wistar female rats that underwent surgery to sever the galactophores to prevent milk delivery (GC rats) and decrease the energetic drain of milk delivery. We compared plasma acyl-ghrelin concentrations and growth hormone secretagogue receptor (GHSR) mRNA expression in different brain regions of GC rats with those of sham operated lactating and nonlactating rats. Additional lactating and nonlactating rats were implanted with cannulae aimed at the lateral ventricles and were used to compare feeding responses to central ghrelin or GHSR antagonist infusions to those of nonlactating rats receiving similar infusions on day 14-16 postpartum (pp). Results show lower plasma acyl-ghrelin concentrations on day 15 pp sham operated lactating rats compared to GC or nonlactating rats. These changes occur in association with increased GHSR mRNA expression in the hypothalamic arcuate nucleus (ARC) and ventral tegmental area (VTA) of sham operated lactating rats. Despite lactational hyperphagia, infusions of ghrelin (0.25 or 1 μg) resulted in similar increases in food intake in lactating and nonlactating rats. In addition, infusions of the GHSR antagonist JMV3002 (4 μg in 1 μl of vehicle) produced greater suppression of food intake in lactating rats than in nonlactating rats. These data suggest that, despite lower plasma ghrelin, the energetic drain of lactation increases sensitivity to the orexigenic effects of ghrelin in brain regions important for food intake and energy balance, and these events are associated with lactational hyperphagia.

The role of prolactin in co-ordinating fertility and metabolic adaptations during reproduction

Mammalian pregnancy and lactation is accompanied by a period of infertility that takes place in the midst of a sustained increase in food intake. Indeed, successful reproduction in females is dependent on co-ordination of the distinct systems that regulate reproduction and metabolism. Rather than arising from different mechanisms during pregnancy and lactation, we propose that elevations in lactogenic hormones (predominant among these being prolactin and the placental lactogens), are ideally placed to influence both of these systems at the appropriate time. We review the literature examining the impacts of lactogens on fertility and energy homeostasis in the virgin state, during pregnancy and lactation and potential long-term impacts of reproductive experience. Taken together, the literature indicates that duration and pattern of lactogen exposure is a vital factor in the ability of these hormones to alter reproduction and food intake. Transient increases in prolactin, as typically seen in healthy virgin females and males, are unable to exert lasting impacts. Importantly, both suppression of fertility and increased food intake are only observed following exposure to chronically-elevated levels of lactogens. Physiologically, the only time this pattern of lactogenic secretion is maintained in the healthy female is during pregnancy and lactation, when co-ordination between these regulatory systems emerges. This article is part of the special issue on 'Neuropeptides'.

Mood, Food, and Fertility: Adaptations of the Maternal Brain

Successfully rearing young places multiple demands on the mammalian female. These are met by a wide array of alterations in maternal physiology and behavior that are coordinated with the needs of the developing young, and include adaptations in neuroendocrine systems not directly involved in maternal behavior or lactation. In this article, attenuations in the behavioral and neuroendocrine responses to stressors, the alterations in metabolic pathways facilitating both increased food intake and conservation of energy, and the changes in fertility that occur postpartum are described. The mechanisms underlying these processes as well as the factors that contribute to them and the relative contributions of these stimuli at different times postpartum are also reviewed. The induction and maintenance of the adaptations observed in the postpartum maternal brain are dependent on mother-young interaction and, in most cases, on suckling stimulation and its consequences for the hormonal profile of the mother. The peptide hormone prolactin acting on receptors within the brain makes a major contribution to changes in metabolic pathways, suppression of fertility and the attenuation of the neuroendocrine response to stress during lactation. Oxytocin is also released, both into the circulation and in some hypothalamic nuclei, in response to suckling stimulation and this hormone has been implicated in the decrease in anxiety behavior seen in the early postpartum period. The relative importance of these hormones changes across lactation and it is becoming increasingly clear that many of the adaptations to motherhood reviewed here reflect the outcome of multiple influences. © 2016 American Physiological Society. Compr Physiol 6:1493-1518, 2016.

Food restriction during lactation suppresses Kiss1 mRNA expression and kisspeptin-stimulated LH release in rats

Among the numerous physiological changes that accompany lactation is the suppression of the reproductive axis. The aim of this study was to investigate a possible role for the kisspeptin system in the restoration of the hypothalamic-pituitary-gonadal axis during late lactation in rats using a food restriction model that allows manipulation of the duration of lactational anovulation. Kiss1 mRNA expression and kisspeptin-immunoreactive cell counts were examined in both food-restricted dams and ad libitum (AL)-fed dams across late lactation when LH concentrations begin to increase. In the arcuate nucleus, Kiss1 mRNA expression and kisspeptin-positive cell counts were suppressed during late lactation. In the anteroventral periventricular (AVPV), day 15 food-restricted dams had significantly lower AVPV Kiss1 mRNA expression and a decreased LH response to exogenous kisspeptin compared with the AL-fed dams. Following 5 days of ad libitum food intake, these values were restored to levels similar to those in dams that had been fed ad libitum throughout lactation. In conclusion, this study shows that delayed restoration of the reproductive axis due to food restriction is associated with a decrease in kisspeptin sensitivity and low AVPV Kiss1 mRNA in late lactation.

Many mouths to feed: the control of food intake during lactation

Providing nutrients to their developing young is perhaps the most energetically demanding task facing female mammals. In this paper we focus primarily on studies carried out in rats to describe the changes in the maternal brain that enable the dam to meet the energetic demands of her offspring. In rats, providing milk for their litter is associated with a dramatic increase in caloric intake, a reduction in energy expenditure and changes in the pattern of energy utilization as well as storage. These behavioral and physiological adaptations result, in part, from alterations in the central pathways controlling energy balance. Differences in circulating levels of metabolic hormones such as leptin, ghrelin and insulin as well as in responsiveness to these signals between lactating and nonlactating animals, contribute to the modifications in energy balance pathways seen postpartum. Suckling stimulation from the pups both directly, and through the hormonal state that it induces in the mother, plays a key role in facilitating these adaptations.

The neuroendocrine basis of lactation-induced suppression of GnRH: role of kisspeptin and leptin

Lactation is an important physiological model of the integration of energy balance and reproduction, as it involves activation of potent appetitive neuropeptide systems coupled to a profound inhibition of pulsatile GnRH/LH secretion. There are multiple systems that contribute to the chronic hyperphagia of lactation: 1) suppression of the metabolic hormones, leptin and insulin, 2) activation of hypothalamic orexigenic neuropeptide systems NPY, AGRP, orexin (OX) and melanin concentrating hormone (MCH), 3) special induction of NPY expression in the dorsomedial hypothalamus, and 4) suppression of anorexigenic systems POMC and CART. These changes ensure adequate energy intake to meet the metabolic needs of milk production. There is significant overlap in all of the systems that regulate food intake with the regulation of GnRH, suggesting there could be several redundant factors acting to suppress GnRH/LH during lactation. In addition to an overall increase in inhibitory tone acting directly on GnRH cell bodies that is brought about by increases in orexigenic systems, there are also effects at the ARH to disrupt Kiss1/neurokinin B/dynorphin neuronal function through inhibition of Kiss1 and NKB. These changes could lead to an increase in inhibitory auto-regulation of the Kiss1 neurons and a possible disruption of pulsatile GnRH release. While the low levels of leptin and insulin contribute to the changes in ARH appetitive systems, they do not appear to contribute to the suppression of ARH Kiss1 or NKB. The inhibition of Kiss1 may be the key factor in the suppression of GnRH during lactation, although the mechanisms responsible for its inhibition are unknown.

Sexual behavior in lactating rats: role of estrogen-induced progesterone receptors

Lactation is associated with suppression of reproductive function, the duration of which depends on the number of young suckled and food availability. Although previous studies have documented increasing responsivity to the positive feedback effects of estrogen on luteinizing hormone (LH) secretion with time postpartum, changes in the ability of estrogen to stimulate sexual behavior across these time points and the influence of food restriction on response to estrogen have not been investigated. Thus, we compared the ability of exogenous estrogen administration to stimulate proceptive and receptive behavior in ad libitum fed and food restricted rats on Days 15 and 20 postpartum. Because the ability of estrogen to induce sexual behavior depends on activation of both estrogen receptors and estrogen-induced progesterone receptors, a second study compared estrogen and progesterone-ir within the VMH and MPOA in similar groups. Finally, we investigated the role of the high levels of progesterone typical of lactation in the suppression of estrogen-induced sexual behavior by transient blockade of the progesterone receptor using RU486. As expected there was an increase across time in the ability of estrogen to stimulate sexual behavior that correlated with an increased ability of estrogen to induce progesterone receptors in the MPOA that was most evident in ad libitum fed rats. RU486 administration concomitant with estrogen administration increased solicitation behavior and was most effective in ad libitum fed rats suggesting an inhibitory role of progesterone on estrogen-induced sexual proceptivity in lactating rats.

Hypothalamic and pituitary expression of ghrelin receptor message is increased during lactation

In the lactating rat there is a dramatic increase in food intake that peaks at around day 15 postpartum, a time when pups are near weaning age, yet still fully dependant on maternal nourishment. We examined whether the orexigenic hormone ghrelin plays a role in increasing food intake during lactation. To do this, we compared plasma levels ghrelin, as well as brain and pituitary expression of the growth hormone secretagogue receptor (GHS-R 1a) rats in one of three groups: (1) dams whose litters were removed the day after giving birth (non-lactating); (2) dams whose litters were removed on day 13 postpartum (litter removed), and dams allowed keeping their litters (lactating). On day 15 postpartum, all dams were decapitated and trunk blood collected for plasma analysis of active ghrelin levels. Also, brain and pituitaries were collected and snap frozen using liquid nitrogen and stored at -80 degrees C before mRNA extraction and RT-PCR analysis. Results show no differences in ghrelin concentrations between lactating and non-lactating rats. Hypothalamic and pituitary expression of GHS-R 1a, however, was significantly increased in lactating animals compared to non-lactating animals. Interestingly, litter removed dams had higher levels of plasma ghrelin concentrations than either lactating or non-lactating females. Furthermore, GHS-R mRNA expression in these animals remained elevated in the pituitary but not the hypothalamus. These data suggest that the hypothalamus and pituitary of lactating rats are more sensitive to the effects of ghrelin, and that hypothalamic sensitivity to ghrelin depends on the presence of a suckling litter.

Ligation of the Uterine Artery and Early Postnatal Food Restriction – Animal Models for Growth Retardation

Intrauterine growth restriction (IUGR) is one of the major causes of short stature in child- and adulthood. The cause of IUGR is unknown, however, an impaired uteroplacental function during the second half of human pregnancy might be an important factor, by affecting the programming of somatotropic axis and leading to postnatal growth failure into adulthood. Two rat models with perinatally induced growth retardation were used to examine the long-term effects of perinatal insults on growth. IUGR rats were prepared from pregnant dams, with a bilateral uterine artery ligation at day 17 of their pregnancy. Since the rat is relatively immature at birth, an early postnatal food restriction model was included as another model to broaden the time window of sensitive period of organogenesis. An individual growth curve was calculated of each animal (n = 813). From these individual growth curves the predicted growth curve for each experimental group was calculated by multilevel analysis. The proposed mathematical model allows us to estimate the growth potentials of these rat models with precision and could provide basic information to investigate the relationships among a number of other variables in future studies. Furthermore, we concluded that both pre- and early postnatal malnutrition leads to irreversible slowing down of postnatal growth.

Food restriction during lactation results in prolonged hyposensitivity to the positive-feedback effects of oestradiol

Food restriction prolongs lactational infertility in rats. Here, we investigated whether an attenuated response to the positive-feedback effects of oestrogen on luteinizing hormone release contributed to this effect. The ability of oestrogen to induce surges in luteinizing hormone in ad libitum fed and food-restricted dams at different times of lactation was compared. The results showed that, on day 20 postpartum, ad libitum fed dams showed luteinizing hormone surges after oestrogen treatment, but food-restricted dams did not. Ovariectomy or RU486 treatment restored the ability of oestrogen to induce luteinizing hormone surges in food-restricted dams, and chronic progesterone exposure reduced oestrogen-induced surges of luteinizing hormone in ad libitum fed ovariectomized dams. Food restriction also resulted in a reduced ability of oestrogen to induce progesterone receptor immunoreactivity, but did not reduce the number of oestrogen receptors (ERalpha) in the anteroventralperiventricular area. As with the surge in luteinizing hormone, the effects of food restriction on oestrogen induction of progesterone receptors were mediated by progesterone. Together, these results suggest that the ability of food restriction to extend the length of lactational diestrus is mediated, in part, by a decrease in sensitivity to the positive-feedback effects of oestrogen. This results from high circulating concentrations of progesterone which apparently reduce the ability of oestrogen to induce progesterone receptor expression.

Integration of the regulation of reproductive function and energy balance: lactation as a model

Lactation is a physiological model for studying how the hypothalamus integrates peripheral signals, such as sensory signals (suckling stimulus) and those denoting energy balance (leptin), to alter hypothalamic function regulating food intake/energy balance and reproduction. The characteristics of food intake/energy balance during lactation are extreme hyperphagia, coupled with negative energy balance. The arcuate nucleus Neuropeptide Y (ARH-NPY) system is activated by: (1) brainstem projections specifically activated by the suckling stimulus, and (2) the decrease in leptin in response to the metabolic drain of milk production. NPY neurons from the ARH make direct contact with GnRH neurons and with CRH neurons in the PVH. NPY neurons also make contact with orexin and MCH neurons in the LHA, which, in turn, make contacts with GnRH neurons. Thus, the ARH-NPY system provides a neuroanatomical framework by which to integrate changes in food intake/energy with the regulation of cyclic reproductive function.

Neuroanatomical specificity of prolactin-induced hyperphagia in virgin female rats

Intracerebroventricular (i.c.v.) administration of PRL increases food intake in virgin female rats but the brain site(s) at which PRL acts to promote feeding behavior is not known. The present studies investigated the role of the paraventricular nucleus (PVN), ventromedial nucleus (VMH), and medial preoptic nucleus (MPOA) in the hyperphagic actions of PRL. Ad-libitum-fed virgin female rats received twice daily site-specific injections of PRL (800 ng) over a period of 10 days. Only subjects demonstrating regular vaginal cyclicity were included in the study. Food intake, body weight, and vaginal cyclicity were measured daily. Results showed that PRL significantly increased food intake when injected into the PVN. A nonsignificant trend towards a hyperphagic response in the last 5 days of testing was observed in rats receiving intra-VMH injections of PRL, and the MPOA was not responsive to the feeding-stimulating properties of PRL. None of the manipulations affected body weight or vaginal cyclicity as demonstrated by vaginal smears. In sum, the present results reveal that one brain site at which PRL acts to increase food intake is the PVN, but these studies do not rule out the possibility that the effects of PRL on food intake may also involve other brain areas.

Changes in leptin levels during lactation: implications for lactational hyperphagia and anovulation

In these studies we investigated the time course of changes in circulating leptin levels in lactating rats and the dependence of these changes on the energetic cost of lactation and evaluated the contribution of changes in leptin levels to lactational hyperphagia and infertility. In the first experiment, plasma leptin levels were measured on Days 5, 10, 15, 20, and 25 postpartum in freefeeding lactating rats and age-matched virgin females. Retroperitoneal and parametrial fat pads weights were obtained from the same females. In the second experiment the same measures, together with plasma insulin and prolactin levels, were taken on Days 15 and 20 postpartum from galactophore-cut and sham-operated females. In Experiments 3 and 4, the effects of exogenous leptin administration, either subcutaneously (sc) or intracerebroventricularly (icv), on lactational anovulation, maternal food intake, and dam and litter weights were examined. Circulating leptin levels decreased in lactating rats. Leptin levels were highly positively correlated with fat pad weight. Eliminating the energetic costs of lactation by preventing milk delivery induced dramatic increases in plasma leptin and insulin levels and also increased adiposity. Exogenous leptin administration did not affect length of lactational anovulation but reduced food intake, maternal body weight, and litter weight gain when given centrally and maternal body weight when given systemically. Together, these data show that the energetic costs of lactation are associated with a fall in circulating leptin levels but that these do not make a major contribution to the suppression of reproduction in lactating rats; however, they may be permissive to the hyperphagia of lactation.