Hypothalamic levels of NPY, MCH, and prepro-orexin mRNA during pregnancy and lactation in the rat: role of prolactin

Transcriptome identification of differential mammary genes of Kazakh horses during early pregnancy

Kazakh mares have attracted widespread attention with their outstanding lactation traits. Lactation is a complex dynamic process regulated by multiple factors. The extensive application of transcriptome sequencing technology enables researchers to further explore this biological issue. This study selected three pregnant and three non-pregnant Kazakh mares as the research subject. Their mammary glands were taken for transcriptome sequencing. The results show that there are 9 lncRNAs and 122 mRNAs differentially expressed between the two groups. GO enrichment analysis shows that there are 175 molecular functions, 59 cellular components, and 555 biological processes, including cellular hormone metabolic process, hormone catabolic process, and I-kappaB kinase/NF-kappaB signaling. KEGG enrichment analysis exhibits that these differential genes are mainly enriched in the NF-kappa B signaling pathway, steroid hormone biosynthesis, breast cancer, ECM-receptor interaction, and MAPK signaling pathway. WNT4, DPP4, and NFKBIA are key nodes regulating breast activation. Conclusions: Through the comparative analysis of the transcriptome data of mammary tissues of pregnant and non-pregnant mares, relevant differentially expressed genes are screened and analyzed. This study provides valuable fundamental data for investigating candidate genes related to the lactation regulation and mammogenesis of Kazakh horses.

Hypocretins, sleep, and maternal behavior

The postpartum period is a demanding time during which mothers experience numerous physiological adaptations that enable them to care for their offspring while maintaining their wellbeing. Hypocretins, also known as orexins, are neuropeptides synthesized by hypothalamic neurons that play a fundamental role in several functions, including the promotion of wakefulness and motivated behaviors, such as maternal care. In this regard, several findings suggest that the activity of the hypocretinergic system increases in the early postpartum period and begins to decline as weaning approaches. In particular, hypocretins within the medial preoptic area, a crucial region during this period, modulate both maternal behavior and sleep. Although further studies are necessary to obtain a comprehensive understanding of the role of hypocretins in lactating females, current research suggests that this system participates in promoting active components of maternal behavior and regulating wakefulness and sleep adjustments during the postpartum period, potentially leading to increased wakefulness during this stage. These adaptive adjustments enable the mother to cope with the continuously changing demands of the pups.

Maternal adaptations to food intake across pregnancy: Central and peripheral mechanisms

A sufficient and balanced maternal diet is critical to meet the nutritional demands of the developing fetus and to facilitate deposition of fat reserves for lactation. Multiple adaptations occur to meet these energy requirements, including reductions in energy expenditure and increases in maternal food intake. The central nervous system plays a vital role in the regulation of food intake and energy homeostasis and responds to multiple metabolic and nutrient cues, including those arising from the gastrointestinal tract. This review describes the nutrient requirements of pregnancy and the impact of over- and undernutrition on the risk of pregnancy complications and adult disease in progeny. The central and peripheral regulation of food intake is then discussed, with particular emphasis on the adaptations that occur during pregnancy and the mechanisms that drive these changes, including the possible role of the pregnancy-associated hormones progesterone, estrogen, prolactin, and growth hormone. We identify the need for deeper mechanistic understanding of maternal adaptations, in particular, changes in gut-brain axis satiety signaling. Improved understanding of food intake regulation during pregnancy will provide a basis to inform strategies that prevent maternal under- or overnutrition, improve fetal health, and reduce the long-term health and economic burden for mothers and offspring.

Reduced insulin sensitivity and increased β/α cell mass is associated with reduced hepatic insulin-degrading enzyme activity in pregnant rats

AIMS

Pregnancy is associated with the development of a transitory insulin resistance that parallels with the upregulation of pancreatic β-cell function and mass. These metabolic adaptations guarantee the higher insulin demand, but there is no evidence of whether insulin clearance contributes to this process. Thus, we investigated some of the hepatic parameters related to insulin clearance during rat pregnancy. We also investigated some molecular parameters in the hypothalamus.

MAIN METHODS

We evaluated the body mass and food intake, insulin sensitivity, β- and α-cell masses, insulin clearance based on an exogenous insulin load, hepatic insulin-degrading enzyme (IDE) activity, and hepatic and hypothalamic protein content of IDE and carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM-1) in three periods of gestation in Wistar rats.

KEY FINDINGS

In the first week of pregnancy, both insulin sensitivity and clearance increased, a pattern that inverted in the third week of gestation (reduced insulin sensitivity and clearance). Diminished insulin clearance was associated with lower hepatic IDE activity and higher pancreatic β- and α-cell masses. No alteration in the hepatic IDE and CEACAM protein content was observed throughout pregnancy, but hypothalamic IDE protein content was significantly reduced in the late gestation period.

SIGNIFICANCE

In conclusion, elevated insulin demand in the late period of gestation occurs not only as a result of increased β-cell mass and function but also by a potential reduction in hepatic insulin clearance. Knowing this physiological process may be valuable when considering gestational diabetes mellitus results from a failure in insulin supply during pregnancy.

Central actions of insulin during pregnancy and lactation

Pregnancy and lactation are highly metabolically demanding states. Maternal glucose is a key fuel source for the growth and development of the fetus, as well as for the production of milk during lactation. Hence, the maternal body undergoes major adaptations in the systems regulating glucose homeostasis to cope with the increased demand for glucose. As part of these changes, insulin levels are elevated during pregnancy and lower in lactation. The increased insulin secretion during pregnancy plays a vital role in the periphery; however, the potential effects of increased insulin action in the brain have not been widely investigated. In this review, we consider the impact of pregnancy on brain access and brain levels of insulin. Moreover, we explore the hypothesis that pregnancy is associated with site-specific central insulin resistance that is adaptive, allowing for the increases in peripheral insulin secretion without the consequences of increased central and peripheral insulin functions, such as to stimulate glucose uptake into maternal tissues or to inhibit food intake. Conversely, the loss of central insulin actions may impair other functions, such as insulin control of the autonomic nervous system. The potential role of low insulin in facilitating adaptive responses to lactation, such as hyperphagia and suppression of reproductive function, are also discussed. We end the review with a list of key research questions requiring resolution.

Pregnancy-induced adaptation of central sensitivity to leptin and insulin

Pregnancy is a time of increased food intake and fat deposition in the mother, and adaptations of glucose homeostasis to meet the energy demands of the growing fetus. As part of these adaptations, leptin and insulin concentrations increase in the maternal circulation during pregnancy. Central effects of leptin and insulin, however, are counterproductive to pregnancy, as increased action of these hormones in the brain lead to suppression of food intake. To prevent this, it is well documented that pregnancy induces a state of leptin- and insulin-insensitivity in the brain, particularly the hypothalamus, in a range of species. While the mechanisms underlying leptin- or insulin-insensitivity during pregnancy vary between species, there is evidence of reduced transport into the brain, impaired activation of intracellular signalling pathways, including reduced leptin receptor expression, and attenuated activation of downstream neuronal pathways, especially for leptin insensitivity. Pregnancy-induced changes in prolactin, growth hormone and leptin are discussed in terms of their role in mediating this reduced response to leptin and insulin.

Folic acid content of diet during pregnancy determines post-birth re-set of metabolism in Wistar rat dams

Maternal metabolism begins to return to homeostasis (re-set) following birth and is accelerated by lactation. Delay in metabolic re-set may contribute to postpartum weight retention and later-life metabolic consequences. Folic acid (FA) is essential during pregnancy but inadequate intakes may alter 1-carbon metabolism, consequently affecting energy homeostatic systems. Our objectives were to examine the effects of FA content 1)below and 2)above requirements during pregnancy on the re-set of body weight, markers of hepatic 1-carbon metabolism and central and peripheral energy metabolic pathways in Wistar rat mothers early post-weaning (PW) compared to pregnant controls. Pregnant Wistar rats were fed an AIN-93G diet with FA at 0X, 1X (control, 2 mg FA/kg) or a range above requirements at 2.5X, 5X or 10X recommended levels then the control diet during lactation up to 1 week PW. Dams fed below (0X) or above (5X and 10X) FA requirements had delayed weight-loss from weaning up to 1 week PW, higher plasma insulin and HOMA-IR and changes in glucose and lipid metabolism-regulating genes in muscle, but not liver or adipose tissue compared to controls. Expression of folate-related genes in liver were lower in high FA fed dams. Central food intake neurons were not affected by FA diets. In conclusion, intakes of FA below (0X) or above (5X, 10X) requirements during pregnancy delayed weight-loss, dysregulated 1-carbon pathways in the liver and peripheral energy metabolic pathways in the Wistar rat mother up to 4 weeks after dietary exposure; potentially programming long-term negative metabolic effects and that of her future offspring.

Prolactin maintains transient melanin-concentrating hormone expression in the medial preoptic area during established lactation

A population of neurones in the medial part of the medial preoptic area (mPOA) transiently express melanin-concentrating hormone (MCH) in mid to late lactation in the rat, and this expression disappears on weaning. Prolactin is known to mediate many of the physiological adaptations that occur within the dam associated with lactation and the mPOA is well endowed with prolactin receptors (Prlr); hence, we hypothesised that these transiently MCH-expressing cells may be regulated by prolactin. By in situ hybridisation, we show that approximately 60% of the cells expressing prepro-MCH (Pmch) mRNA in the medial part of the mPOA on day 19 of lactation also express Prlr mRNA. To demonstrate that these transiently MCH-expressing cells can acutely respond to prolactin, dams were treated with bromocriptine on the morning of day 19 of lactation and then given vehicle or prolactin 4 hours later. In the prolactin-treated animals, over 80% of the MCH-immunopositive cells were also immunopositive for phosphorylated signal transducer and activator of transcription 5, an indicator of prolactin receptor activation: double immunopositive cells were rare in vehicle-treated animals. Finally, the effect of manipulating the circulating concentrations of prolactin on days 17, 18 and 19 on the number of MCH-immunopositive cells on day 19 was determined. Reducing circulating concentrations of prolactin over days 17, 18 and 19 of lactation with or without a suckling stimulus resulted in a reduction (P < 0.05) in the number of MCH-immunopositive cells in the medial part of the mPOA on day 19 of lactation. Further research is required to determine the functional role(s) of these prolactin-activated transiently MCH-expressing neurones; however, we suggest the most likely role involves adaptations in maternal metabolism to support the final week of lactation.

Neurophysiological and cognitive changes in pregnancy

The hormonal fluctuations in pregnancy drive a wide range of adaptive changes in the maternal brain. These range from specific neurophysiological changes in the patterns of activity of individual neuronal populations, through to complete modification of circuit characteristics leading to fundamental changes in behavior. From a neurologic perspective, the key hormone changes are those of the sex steroids, estradiol and progesterone, secreted first from the ovary and then from the placenta, the adrenal glucocorticoid cortisol, as well as the anterior pituitary peptide hormone prolactin and its pregnancy-specific homolog placental lactogen. All of these hormones are markedly elevated during pregnancy and cross the blood-brain barrier to exert actions on neuronal populations through receptors expressed in specific regions. Many of the hormone-induced changes are in autonomic or homeostatic systems. For example, patterns of oxytocin and prolactin secretion are dramatically altered to support novel physiological functions. Appetite is increased and feedback responses to metabolic hormones such as leptin and insulin are suppressed to promote a positive energy balance. Fundamental physiological systems such as glucose homeostasis and thermoregulation are modified to optimize conditions for fetal development. In addition to these largely autonomic changes, there are also changes in mood, behavior, and higher processes such as cognition. This chapter summarizes the hormonal changes associated with pregnancy and reviews how these changes impact on brain function, drawing on examples from animal research, as well as available information about human pregnancy.

Transcriptome sequencing to detect the potential role of long non-coding RNAs in bovine mammary gland during the dry and lactation period

BACKGROUND

It is known that long non-coding RNAs (lncRNAs) play an important role in various biological processes, including cell proliferation, differentiation and apoptosis. However, their functions and profiles in lactation cycle of dairy cows are largely unknown. In this study, lncRNA-seq technique was employed to compare the expression profiles of lncRNAs and mRNAs from Chinese Holstein mammary gland in dry and lactation period.

RESULT

Totally 3746 differentially expressed lncRNAs (DELs) and 2890 differentially expressed genes (DEGs) were identified from the dry and lactation mammary glands of Holstein cows. Functional enrichment analysis on target genes of lncRNAs indicated that these genes were involved in lactation-related signaling pathways, including cell cycle, JAK-STAT, cell adhesion, and PI3K-Akt signaling pathways. Additionally, the interaction between lncRNAs and their potential miRNAs was predicted and partly verified. The result indicated that the lactation-associated miR-221 might interact with lncRNAs TCONS_00040268, TCONS_00137654, TCONS_00071659 and TCONS_00000352, which revealed that these lncRNAs might be important regulators for lactation cycle.

CONCLUSION

This study provides a resource for lncRNA research on lactation cycle of bovine mammary gland. Besides, the interaction between lncRNAs and the specific miRNA is revealed. It expands our knowledge about lncRNA and miRNA biology as well as contributes to clarify the regulation of lactation cycle of bovine mammary gland.

Reduced representation bisulfite sequencing (RRBS) of dairy goat mammary glands reveals DNA methylation profiles of integrated genome-wide and critical milk-related genes

DNA methylation (DNAm), a major element of epigenetics, plays critical roles in individual development. Reduced representation bisulfite sequencing (RRBS) is an effective and economical method for analyzing the DNA methylation of a single base. The aims of this study were to determine the DNAm profiles of the methylation contexts (CGs and non-CGs) of lactation and dry periods of goat mammary glands using the RRBS, and to identify potential milk-related genes. The proportion of CG was the highest among all the sequence contexts. The highest CG levels (72.44% to 75.24%) occurred in the 3′ UTR region, followed by the gene body region (61.14% to 65.45%). The non-CG levels were low compared to the CG levels. Bioinformatic analysis demonstrated that the CGs were mainly enriched at high methylation levels (>90%), while non-CGs were enriched at low methylation levels. Methylation levels of 95 and 54 genes in the lactation period were up- or downregulated, respectively, relative to the dry period, such as PPARα, RXRα and NPY genes. The bisulfite sequencing PCR results showed that the methylation level of goat PPARα gene during the lactation period was significant lower than in the dry period, while the methylation level of the RXRα gene was lower in the dry period than in the lactation period. Meanwhile, the methylation levels of human PPARα and NPY genes were significantly higher in MCF-7 than in MCF-10A cells. These findings provide essential information for DNA methylation profiles of goat mammary gland and detect some potential milk-related genes in dairy goats.

Estradiol effects on hypothalamic AMPK and BAT thermogenesis: A gateway for obesity treatment?

In addition to their prominent roles in the control of reproduction, estrogens are important modulators of energy balance, as evident in conditions of deficiency of estrogens, which are characterized by increased feeding and decreased energy expenditure, leading to obesity. AMP-activated protein kinase (AMPK) is a ubiquitous cellular energy gauge that is activated under conditions of low energy, increasing energy production and reducing energy wasting. Centrally, the AMPK pathway is a canonical route regulating energy homeostasis, by integrating peripheral signals, such as hormones and metabolites, with neuronal networks. As a result of those actions, hypothalamic AMPK modulates feeding, as well as brown adipose tissue (BAT) thermogenesis and browning of white adipose tissue (WAT). Here, we will review the central actions of estrogens on energy balance, with particular focus on hypothalamic AMPK. The relevance of this interaction is noteworthy, because some agents with known actions on metabolic homeostasis, such as nicotine, metformin, liraglutide, olanzapine and also natural molecules, such as resveratrol and flavonoids, exert their actions by modulating AMPK. This evidence highlights the possibility that hypothalamic AMPK might be a potential target for the treatment of obesity.

Estradiol Regulation of Brown Adipose Tissue Thermogenesis

Physiologically, estrogens carry out a myriad of functions, the most essential being the regulation of the reproductive axis. Currently, it is also dogmatic that estrogens play an important role modulating energy balance and metabolism. In this sense, it is well known that low estrogens levels, occurring due to ovarian insufficiency, in conditions such as menopause or ovariectomy (OVX), are associated with increased food intake and decreased energy expenditure, leading to weight gain and obesity at long term. Concerning energy expenditure, the main effect of estradiol (E2) is on brown adipose tissue (BAT) thermogenesis. Thus, acting through a peripheral or a central action, E2 activates brown fat activity and increases body temperature, which is negatively associated with body weight. Centrally, the hypothalamic AMP-activated protein kinase (AMPK) mediates the E2 action on BAT thermogenesis. In this chapter, we will summarize E2 regulation of BAT thermogenesis and how this can influence energy balance and metabolism in general.

Brain STAT5 signaling and behavioral control

Several growth factors and cytokines recruit the signal transducer and activator of transcription 5 (STAT5) signaling pathway to control cell proliferation, differentiation and apoptosis. Nonetheless, the importance of this transcription factor for brain functions is still poorly understood. Because some STAT5-inducing hormones, such as prolactin and leptin, act in the brain to regulate the expression of motivated behaviors, this signaling pathway is likely involved in behavioral modulation. Therefore, the objective of the present review was to summarize and discuss the available data regarding the possible role of central STAT5 signaling in the regulation of brain functions, especially on behavioral control. We discussed studies that investigated the importance of STAT5 signaling in the regulation of maternal and feeding behaviors. Additionally, we highlighted other behaviors that could be potentially affected by STAT5 signaling. This knowledge may help to understand how motivated behaviors are regulated at the cellular level.

Inactivation of the melanin concentrating hormone system impairs maternal behavior

In order to prepare the mother for the demands of pregnancy and lactation, the maternal brain is subjected to a number of adaptations. Maternal behaviors are regulated by complex neuronal interactions. Here, we show that the melanin concentrating hormone (MCH) system is an important regulator of maternal behaviors. First, we report that melanin concentrating hormone receptor 1 knockout (MCHR1 KO) mice display a disruption of maternal behavior. Early postpartum MCHR1 KO females exhibit poor nesting, deficits in pup retrieval and maternal aggression. In addition, ablation of MCH receptors results in decreased milk production and prolactin mRNA levels. Then we show that these results are in line with those obtained in wild type mice (WT) treated with the specific MCHR1 antagonist GW803430. Furthermore, following pups retrieval, MCHR1 KO mice display a lower level of Fos expression than WT mice in the ventral tegmental area, and nucleus accumbens. With the progression of the lactation period, however, the MCHR1 KO mice improve maternal care towards their pups. This is manifested by an increase in the pups׳ survival rate and the decrease in pups׳ retrieval time beyond the second day after parturition. In conclusion, we show that the MCH system plays a significant role in the initiation of maternal behavior. In this context, MCH may play a role in integrating information from multiple sources, and connecting brain reward, homeostatic and regulatory systems.

Estradiol and brown fat

Ovarian steroids, such as estradiol (E2), control a vastness of physiological processes, such as puberty, reproduction, growth, development and metabolic rate. In fact, physiological, pathological, pharmacological or genetically-induced estrogen deficiency causes increased appetite and reduced energy expenditure, promoting weight gain and ultimately leading to obesity. Remarkably, estrogen replacement reverts those effects. Interestingly, although a wealth of evidence has shown that E2 can directly modulate peripheral tissues to exert their metabolic actions, novel data gathered in recent years have shown that those effects are mainly central and occur in the hypothalamus. Here, we will review what is known about the actions of E2 on energy homeostasis, with particular focus on brown adipose tissue (BAT) thermogenesis.

Neuroendocrine Regulation of Metabolism

Given the current environment in most developed countries, it is a challenge to maintain a good balance between calories consumed and calories burned, although maintenance of metabolic balance is key to good health. Therefore, understanding how metabolic regulation is achieved and how the dysregulation of metabolism affects health is an area of intense research. Most studies focus on the hypothalamus, which is a brain area that acts as a key regulator of metabolism. Among the nuclei that comprise the hypothalamus, the arcuate nucleus is one of the major mediators in the regulation of food intake. The regulation of energy balance is also a key factor ensuring the maintenance of any species as a result of the dependence of reproduction on energy stores. Adequate levels of energy reserves are necessary for the proper functioning of the hypothalamic-pituitary-gonadal axis. This review discusses valuable data presented in the 2015 edition of the International Workshop of Neuroendocrinology concerning the fundamental nature of the hormonal regulation of the hypothalamus and the impact on energy balance and reproduction.

Essential role of UCP1 modulating the central effects of thyroid hormones on energy balance

Objective

Classically, metabolic effects of thyroid hormones (THs) have been considered to be peripherally mediated, i.e. different tissues in the body respond directly to thyroid hormones with an increased metabolism. An alternative view is that the metabolic effects are centrally regulated. We have examined here the degree to which prolonged, centrally infused triiodothyronine (T3) could in itself induce total body metabolic effects and the degree to which brown adipose tissue (BAT) thermogenesis was essential for such effects, by examining uncoupling protein 1 (UCP1) KO mice.

Methods

Wildtype and UPC1 KO mice were centrally-treated with T3 by using minipumps. Metabolic measurements were analyzed by indirect calorimetry and expression analysis by RT-PCR or western blot. BAT morphology and histology were studied by immunohistochemistry.

Results

We found that central T3-treatment led to reduced levels of hypothalamic AMP-activated protein kinase (AMPK) and elevated body temperature (0.7 °C). UCP1 was essential for the T3-induced increased rate of energy expenditure, which was only observable at thermoneutrality and notably only during the active phase, for the increased body weight loss, for the increased hypothalamic levels of neuropeptide Y (NPY) and agouti-related peptide (AgRP) and for the increased food intake induced by central T3-treatment. Prolonged central T3-treatment also led to recruitment of BAT and britening/beiging (“browning”) of inguinal white adipose tissue (iWAT).

Conclusions

We conclude that UCP1 is essential for mediation of the central effects of thyroid hormones on energy balance, and we suggest that similar UCP1-dependent effects may underlie central energy balance effects of other agents.

•

Central thyroid hormone (TH) increases brown adipose tissue (BAT) thermogenesis.

•

Central TH increases oxygen consumption and energy expenditure.

•

Central TH increases food intake.

•

Central TH promotes britening/beiging of white adipose tissue (WAT).

•

All these metabolic effects of central TH are lost in UCP1 knockout mice.

Neuronal STAT5 signaling is required for maintaining lactation but not for postpartum maternal behaviors in mice

Prolactin and placental lactogens control mammary development and lactation as well as play an important role in maternal behaviors. However, the molecular mechanisms in the brain responsible for this regulation remain largely unknown. Therefore, the present study investigated whether Signal Transducer and Activator of Transcription 5 (STAT5) signaling in the brain, the key transcriptional factor recruited by prolactin receptor and other hormones, is required for postpartum maternal behavior, maintenance of lactation and offspring growth. Neuronal ablation of STAT5 impaired the control of prolactin secretion and reduced the hypothalamic expression of suppressors of cytokine signaling (i.e., SOCS3 and CISH). In addition, neuronal STAT5 deletion attenuated the hyperphagia commonly observed during lactation by decreasing the hypothalamic expression of orexigenic neurotransmitters such as the neuropeptide Y and agouti-related protein. The lower food intake of lactating neuron-specific STAT5 knockout females resulted in reduced milk production and offspring growth. Unexpectedly, postpartum maternal behavior expression was not impaired in neuron-specific STAT5 knockout females. On the contrary, the latency to retrieve and group the pups into the nest was reduced in mutant dams. Finally, we demonstrated that approximately 30% of recorded neurons in the medial preoptic area were acutely depolarized by prolactin suggesting that fast STAT5-independent signaling pathways may be involved in the regulation of maternal behaviors. Overall, our results revealed important information about the molecular mechanisms recruited by hormones to orchestrate the activation of neural circuitries engaged in the induction of maternal care.

Pregnancy induces resistance to the anorectic effect of hypothalamic malonyl-CoA and the thermogenic effect of hypothalamic AMPK inhibition in female rats

During gestation, hyperphagia is necessary to cope with the metabolic demands of embryonic development. There were three main aims of this study: Firstly, to investigate the effect of pregnancy on hypothalamic fatty acid metabolism, a key pathway for the regulation of energy balance; secondly, to study whether pregnancy induces resistance to the anorectic effect of fatty acid synthase (FAS) inhibition and accumulation of malonyl-coenzyme A (CoA) in the hypothalamus; and, thirdly, to study whether changes in hypothalamic AMPK signaling are associated with brown adipose tissue (BAT) thermogenesis during pregnancy. Our data suggest that in pregnant rats, the hypothalamic fatty acid pathway shows an overall state that should lead to anorexia and elevated BAT thermogenesis: decreased activities of AMP-activated protein kinase (AMPK), FAS, and carnitine palmitoyltransferase 1, coupled with increased acetyl-CoA carboxylase function with subsequent elevation of malonyl-CoA levels. This profile seems dependent of estradiol levels but not prolactin or progesterone. Despite the apparent anorexic and thermogenic signaling in the hypothalamus, pregnant rats remain hyperphagic and display reduced temperature and BAT function. Actually, pregnant rats develop resistance to the anorectic effects of central FAS inhibition, which is associated with a reduction of proopiomelanocortin (POMC) expression and its transcription factors phospho-signal transducer and activator of transcription 3, and phospho-forkhead box O1. This evidence demonstrates that pregnancy induces a state of resistance to the anorectic and thermogenic actions of hypothalamic cellular signals of energy surplus, which, in parallel to the already known refractoriness to leptin effects, likely contributes to gestational hyperphagia and adiposity.

Preoptic inputs and mechanisms that regulate maternal responsiveness

The preoptic area is a well-established centre for the control of maternal behaviour. An intact medial preoptic area (mPOA) is required for maternal responsiveness because lesion of the area abolishes maternal behaviours. Although hormonal changes in the peripartum period contribute to the initiation of maternal responsiveness, inputs from pups are required for its maintenance. Neurones are activated in different parts of the mPOA in response to pup exposure. In the present review, we summarise the potential inputs to the mPOA of rodent dams from the litter that can activate mPOA neurones. The roles of potential indirect effects through increased prolactin levels, as well as neuronal inputs to the preoptic area, are described. Recent results on the pathway mediating the effects of suckling to the mPOA suggest that neurones containing the neuropeptide tuberoinfundibular peptide of 39 residues in the posterior thalamus are candidates for conveying the suckling information to the mPOA. Although the molecular mechanism through which these inputs alter mPOA neurones to support the maintenance of maternal responding is not yet known, altered gene expression is a likely candidate. Here, we summarise gene expression changes in the mPOA that have been linked to maternal behaviour and explore the idea that chromatin remodelling during mother-infant interactions mediates the long-term alterations in gene expression that sustain maternal responding.

Estradiol regulates brown adipose tissue thermogenesis via hypothalamic AMPK

Estrogens play a major role in the modulation of energy balance through central and peripheral actions. Here, we demonstrate that central action of estradiol (E2) inhibits AMP-activated protein kinase (AMPK) through estrogen receptor alpha (ERα) selectively in the ventromedial nucleus of the hypothalamus (VMH), leading to activation of thermogenesis in brown adipose tissue (BAT) through the sympathetic nervous system (SNS) in a feeding-independent manner. Genetic activation of AMPK in the VMH prevented E2-induced increase in BAT-mediated thermogenesis and weight loss. Notably, fluctuations in E2 levels during estrous cycle also modulate this integrated physiological network. Together, these findings demonstrate that E2 regulation of the VMH AMPK-SNS-BAT axis is an important determinant of energy balance and suggest that dysregulation in this axis may account for the common changes in energy homeostasis and obesity linked to dysfunction of the female gonadal axis.

Effects of sex and reproductive experience on the number of orexin A-immunoreactive cells in the prairie vole brain

Large populations of cells synthesizing the neuropeptide orexin (OX) exist in the caudal hypothalamus of all species examined and are implicated in physiological and behavioral processes including arousal, stress, anxiety and depression, reproduction, and goal-directed behaviors. Hypothalamic OX expression is sexually dimorphic in different directions in laboratory rats (F>M) and mice (M>F), suggesting different roles in male and female physiology and behavior that are species-specific. We here examined if the number of hypothalamic cells immunoreactive for orexin A (OXA) differs between male and female prairie voles (Microtus ochrogaster), a socially monogamous species that pairbonds after mating and in which both sexes care for offspring, and if reproductive experience influences their number of OXA-immunoreactive (OXA-ir) cells. It was found that the total number of OXA-ir cells did not differ between the sexes, but females had more OXA-ir cells than males in anterior levels of the caudal hypothalamus, while males had more OXA-ir cells posteriorly. Sexually experienced females sacrificed 12 days after the birth of their first litter, or one day after birth of a second litter, had more OXA-ir cells in anterior levels but not posterior levels of the caudal hypothalamus compared to females housed with a brother (incest avoidance prevents sibling mating). Male prairie voles showed no effect of reproductive experience but showed an unexpected effect of cohabitation duration regardless of mating. The sex difference in the distribution of OXA-ir cells, and their increased number in anterior levels of the caudal hypothalamus of reproductively experienced female prairie voles, may reflect a sex-specific mechanism involved in pairbonding, parenting, or lactation in this species.

The orexigenic effect of orexin-A revisited: dependence of an intact growth hormone axis

Fifteen years ago orexins were identified as central regulators of energy homeostasis. Since then, that concept has evolved considerably and orexins are currently considered, besides orexigenic neuropeptides, key modulators of sleep-wake cycle and neuroendocrine function. Little is known, however, about the effect of the neuroendocrine milieu on orexins' effects on energy balance. We therefore investigated whether hypothalamic-pituitary axes have a role in the central orexigenic action of orexin A (OX-A) by centrally injecting hypophysectomized, adrenalectomized, gonadectomized (male and female), hypothyroid, and GH-deficient dwarf rats with OX-A. Our data showed that the orexigenic effect of OX-A is fully maintained in adrenalectomized and gonadectomized (females and males) rats, slightly reduced in hypothyroid rats, and totally abolished in hypophysectomized and dwarf rats when compared with their respective vehicle-treated controls. Of note, loss of the OX-A effect on feeding was associated with a blunted OX-A-induced increase in the expression of either neuropeptide Y or its putative regulator, the transcription factor cAMP response-element binding protein, as well as its phosphorylated form, in the arcuate nucleus of the hypothalamus of hypophysectomized and dwarf rats. Overall, this evidence suggests that the orexigenic action of OX-A depends on an intact GH axis and that this neuroendocrine feedback loop may be of interest in the understanding of orexins action on energy balance and GH deficiency.

The roles of melanin-concentrating hormone in energy balance and reproductive function: Are they connected?

Melanin-concentrating hormone (MCH) is an anabolic neuropeptide with multiple and diverse physiological functions including a key role in energy homoeostasis. Rodent studies have shown that the ablation of functional MCH results in a lean phenotype, increased energy expenditure and resistance to diet-induced obesity. These findings have generated interest among pharmaceutical companies vigilant for potential anti-obesity agents. Nutritional status affects reproductive physiology and behaviours, thereby optimising reproductive success and the ability to meet energetic demands. This complex control system entails the integration of direct or indirect peripheral stimuli with central effector systems and involves numerous mediators. A role for MCH in the reproductive axis has emerged, giving rise to the premise that MCH may serve as an integratory mediator between those discrete systems that regulate energy balance and reproductive function. Hence, this review focuses on published evidence concerning i) the role of MCH in energy homoeostasis and ii) the regulatory role of MCH in the reproductive axis. The question as to whether the MCH system mediates the integration of energy homoeostasis with the neuroendocrine reproductive axis and, if so, by what means has received limited coverage in the literature; evidence to date and current theories are summarised herein.

Interleukin 6 deficiency modulates the hypothalamic expression of energy balance regulating peptides during pregnancy in mice

Pregnancy is associated with hyperphagia, increased adiposity and multiple neuroendocrine adaptations. Maternal adipose tissue secretes rising amounts of interleukin 6 (IL6), which acts peripherally modulating metabolic function and centrally increasing energy expenditure and reducing body fat. To explore the role of IL6 in the central mechanisms governing dam's energy homeostasis, early, mid and late pregnant (gestational days 7, 13 and 18) wild-type (WT) and Il6 knockout mice (Il6-KO) were compared with virgin controls at diestrus. Food intake, body weight and composition as well as indirect calorimetry measurements were performed in vivo. Anabolic and orexigenic peptides: neuropeptide Y (Npy) and agouti-related peptide (Agrp); and catabolic and anorectic neuropeptides: proopiomelanocortin (Pomc), corticotrophin and thyrotropin-releasing hormone (Crh and Trh) mRNA levels were determined by in situ hybridization. Real time-PCR and western-blot were used for additional tissue gene expression and protein studies. Non-pregnant Il6-KO mice were leaner than WT mice due to a decrease in fat but not in lean body mass. Pregnant Il6-KO mice had higher fat accretion despite similar body weight gain than WT controls. A decreased fat utilization in absence of Il6 might explain this effect, as shown by increased respiratory exchange ratio (RER) in virgin Il6-KO mice. Il6 mRNA levels were markedly enhanced in adipose tissue but reduced in hypothalamus of mid and late pregnant WT mice. Trh expression was also stimulated at gestational day 13 and lack of Il6 blunted this effect. Conversely, in late pregnant mice lessened hypothalamic Il6 receptor alpha (Il6ra), Pomc and Crh mRNA were observed. Il6 deficiency during this stage up-regulated Npy and Agrp expression, while restoring Pomc mRNA levels to virgin values. Together these results demonstrate that IL6/IL6Ra system modulates Npy/Agrp, Pomc and Trh expression during mouse pregnancy, supporting a role of IL6 in the central regulation of body fat in this physiological state.

Pregnancy affects FOS rhythms in brain regions regulating sleep/wake state and body temperature in rats

Circadian rhythms in behavior and physiology change substantially as female mammals undergo the transition from a non-pregnant to a pregnant state. Here, we examined the possibility that site-specific changes in brain regions known to regulate the sleep/wake cycle and body temperature might reflect altered rhythms in these overt functions. Specifically, we compared daily patterns of immunoreactive FOS in early pregnant and diestrous rats in the medial septum (MS), vertical and horizontal diagonal bands of Broca (VDB and HDB), perifornical lateral hypothalamus (LH), and ventrolateral, medial, and median preoptic areas (VLPO, MPA, and MnPO, respectively). In the pregnant animals, FOS expression was reduced and the daily rhythms of expression were lost or attenuated in the MS, VDB, and LH, areas known to support wakefulness, and in the MPA, a brain region that may coordinate sleep/wake patterns with temperature changes. However, despite the well-documented differences in sleep patterns between diestrous and pregnant rats, reproductive state did not affect FOS expression in the VLPO or MnPO, two brain regions in which FOS expression usually correlates with sleep. These data indicate that plasticity in sleep/wake patterns during early pregnancy may be driven by a reduction in wakefulness-promotion by the brain, rather than by an increase in sleep drive.

Regulation of Peptide YY Levels by Age, Hormonal, and Nutritional Status

OBJECTIVE

Peptide YY (PYY) 3-36 has recently been recognized as an important gut hormone that influences food intake. Peripheral injections of PYY 3-36 in rats inhibit food intake in experimental animals as well as in lean and obese human subjects. This hormone has been suggested as an attractive therapeutic option for obesity. The aim of this study was to assess the influence of age, sex, thyroid status, growth hormone (GH), pregnancy, and food restriction on PYY levels in rat.

RESEARCH METHODS AND PROCEDURES

We determined plasma PYY levels in all experimental sets.

RESULTS

PYY levels were influenced by age, with the highest hormone levels achieved in early postnatal life (day 10) and decreasing thereafter. PYY levels were also dependent on thyroid hormone status being decreased in hyperthyroid rats. Exogenous GH administration led to a clear-cut decrease in PYY levels in both normal and GH-deficient rats. Acute food deprivation or chronic food restriction led to decreased PYY levels in virgin and pregnant rats. In pregnant rats with food available ad libitum, PYY levels were enhanced at late gestation.

DISCUSSION

Our observations indicate that PYY levels are influenced by age, thyroid hormones, and GH. These data indicate that PYY could be involved in the changes of food intake associated with these conditions. The PYY levels observed in acute and chronic food-restricted rats indicate that, in situations of decreased energy intake, the lower PYY levels could serve to disinhibit central pathways and facilitate food intake.

Food deprivation explains effects of mouthbrooding on ovaries and steroid hormones, but not brain neuropeptide and receptor mRNAs, in an African cichlid fish

Feeding behavior and reproduction are coordinately regulated by the brain via neurotransmitters, circulating hormones, and neuropeptides. Reduced feeding allows animals to engage in other behaviors important for fitness, including mating and parental care. Some fishes cease feeding for weeks at a time in order to provide care to their young by brooding them inside the male or female parent's mouth. Maternal mouthbrooding is known to impact circulating hormones and subsequent reproductive cycles, but neither the full effects of food deprivation nor the neural mechanisms are known. Here we ask what effects mouthbrooding has on several physiological processes including gonad and body mass, brain neuropeptide and receptor gene expression, and circulating steroid hormones in a mouthbrooding cichlid species, Astatotilapia burtoni. We ask whether any observed changes can be explained by food deprivation, and show that during mouthbrooding, ovary size and circulating levels of androgens and estrogens match those seen during food deprivation. Levels of gonadotropin-releasing hormone 1 (GnRH1) mRNA in the brain were low in food-deprived females compared to controls and in mouthbrooding females compared to gravid females. Levels of mRNA encoding two peptides involved in regulating feeding, hypocretin and cholecystokinin, were increased in the brains of food-deprived females. Brain mRNA levels of two receptors, GnRH receptor 2 and NPY receptor Y8c, were elevated in mouthbrooding females compared to the fed condition, but NPY receptor Y8b mRNA was differently regulated by mouthbrooding. These results suggest that many, but not all, of the characteristic physiological changes that occur during mouthbrooding are consequences of food deprivation.

Interleukin-6 regulates the expression of hypothalamic neuropeptides involved in body weight in a gender-dependent way

Interleukin (IL)-6 has been involved in the control of body weight and body fat. Nevertheless, the mechanisms underlying these effects are not completely understood because central and peripheral actions of IL-6 are plausible. To gain further insight into the central effects of IL-6, we used transgenic mice expressing the IL-6 gene under the control of the glial fibrillary acidic protein (GFAP) promoter (GFAP-IL-6 mice), therefore with central nervous system-restricted over-expression of IL-6, and we studied the expression of the main neuropeptides responsible for energy homeostasis in specific hypothalamic nuclei. Neuropeptide Y (NPY), agouti-related peptide (AgRP), melanin-concentrating hormone (MCH), prepro-orexin (preproOX) (orexigenic and anabolic neuropeptides), pro-opiomelanocortin (POMC) and corticotrophin-releasing hormone (CRH) (anorexigenic and catabolic peptides) mRNA levels were determined using in situ hybridisation in young (2-4 month-old) and old (10-12 month-old) female and male mice under different feeding conditions: normal diet (control) and high-fat diet (HFD), and 24 h-food deprivation. In GFAP-IL-6 females fed a control diet (GFAP-IL-6-control), we showed a significant decrease in NPY and AgRP mRNA levels at all ages, and a late increase in POMC expression (only significant in older animals). These differences were blunted in HFD mice. By contrast, GFAP-IL-6-control males showed a decrease in CRH mRNA content at early ages (2-4 months), and an increase in older mice (10-12 months). Interestingly, these differences were again blunted in HFD mice. Finally, central IL-6 was not able to counteract the effects of 24 h of fasting on body weight, plasma glucose levels and the mRNA content of the peptides evaluated in the present study. Our results demonstrate that IL-6 may regulate the expression of hypothalamic neuropeptides involved in the control of body weight and body fat acting at the central level in a gender- and age-dependent way.

Hyperphagia and central mechanisms for leptin resistance during pregnancy

The purpose of this work was to study the central mechanisms involved in food intake regulation and leptin resistance during gestation in the rat. Sprague Dawley rats of 7, 13, and 18 d of pregnancy [days of gestation (G) 7, G13, and G18] were used and compared with nonpregnant animals in diestrus-1. Food intake was already increased in G7, before hyperleptinemia and central leptin resistance was established in midpregnancy. Leptin resistance was due to a reduction in leptin transport through the blood-brain barrier (BBB) and to alterations in leptin signaling within the hypothalamus based on an increase in suppressor of cytokine signaling 3 levels and a blockade of signal transducer and activator of transcription-3 phosphorylation (G13), followed by a decrease in LepRb and of Akt phosphorylation (G18). In early gestation (G7), no change in hypothalamic neuropeptide Y (NPY), agouti-related peptide (AgRP), or proopiomelanocortin (POMC) expression was shown. Nevertheless, an increase in NPY and AgRP and a decrease in POMC mRNA were observed in G13 and G18 rats, probably reflecting the leptin resistance. To investigate the effect of maternal vs. placental hormones on these mechanisms, we used a model of pseudogestation. Rats of 9 d of pseudogestation were hyperphagic, showing an increase in body and adipose tissue weight, normoleptinemia, and normal responses to iv/intracerebroventricular leptin on hypothalamic leptin signaling, food intake, and body weight. Leptin transport through the BBB, and hypothalamic NPY, AgRP and POMC expression were unchanged. Finally, the transport of leptin through the BBB was assessed using a double-chamber culture system of choroid plexus epithelial cells or brain microvascular endothelial cells. We found that sustained high levels of prolactin significantly reduced leptin translocation through the barrier, whereas progesterone and β-estradiol did not show any effect. Our data demonstrate a dual mechanism of leptin resistance during mid/late-pregnancy, which is not due to maternal hormones and which allows the maintenance of hyperphagia in the presence of hyperleptinemia driven by an increase in NPY and AgRP and a decrease in POMC mRNA. By contrast, in early pregnancy maternal hormones induce hyperphagia without the regulation of hypothalamic NPY, AgRP, or POMC and in the absence of leptin resistance.

Limits to sustained energy intake. XIII. Recent progress and future perspectives

Several theories have been proposed to explain limits on the maximum rate at which animals can ingest and expend energy. These limits are likely to be intrinsic to the animal, and potentially include the capacity of the alimentary tract to assimilate energy – the ‘central limitation’ hypothesis. Experimental evidence from lactating mice exposed to different ambient temperatures allows us to reject this and similar ideas. Two alternative ideas have been proposed. The ‘peripheral limitation’ hypothesis suggests that the maximal sustained energy intake reflects the summed demands of individual tissues, which have their own intrinsic limitations on capacity. In contrast, the ‘heat dissipation limit’ (HDL) theory suggests that animals are constrained by the maximal capacity to dissipate body heat. Abundant evidence in domesticated livestock supports the HDL theory, but data from smaller mammals are less conclusive. Here, we develop a novel framework showing how the HDL and peripheral limitations are likely to be important in all animals, but to different extents. The HDL theory makes a number of predictions – in particular that there is no fixed limit on sustained energy expenditure as a multiple of basal metabolic rate, but rather that the maximum sustained scope is positively correlated with the capacity to dissipate heat.

Mechanism of orexin B-stimulated insulin and glucagon release from the pancreas of normal and diabetic rats

OBJECTIVES

To examine the pattern of distribution and effect of orexin B in the islets of normal and diabetic rats.

METHODS

Pancreatic tissue fragments collected from normal and diabetic (4 weeks after the onset of diabetes) rats were either processed for immunohistochemistry or treated with different concentrations (10 to 10 mol/L) of orexin B.

RESULTS

Orexin B-positive nerves were observed in the wall of blood vessels of both normal and diabetic rat pancreas. Orexin B is abundant in the islets of normal rats and colocalized with insulin in β cells. The number of orexin B-positive cells decreased after the onset of diabetes. Orexin B evoked significant (P<0.05) increases in insulin release from the pancreas of normal and diabetic rats. Propranolol, a β-adrenergic receptor antagonist, significantly (P<0.04) reduced the stimulatory effect of orexin B on insulin secretion. Orexin B also induced significant (P<0.05) increases in glucagon release from the pancreas of normal rats but failed to stimulate glucagon secretion from the pancreas of diabetic rats.

CONCLUSIONS

Orexin B stimulated insulin secretion in normal and diabetic rat pancreas through the β-adrenergic pathway. Orexin B may have an important role in the regulation of islet function.

Effects of leptin infusion during peak lactation on food intake, body composition, litter growth, and maternal neuroendocrine status in female Brandt's voles (Lasiopodomys brandtii)

During lactation, female small mammals frequently reduce their fat reserves to very low levels. The function of this reduction is unclear, as calculations suggest that the contribution of the withdrawn energy from fat to the total energy balance of lactation is trivial. An alternative hypothesis is that reducing fat leads to a reduction in circulating adipokines, such as leptin, that play a role in stimulating the hyperphagia of lactation. We investigated the role of circulating leptin in lactation by repleting leptin levels using miniosmotic pumps during the last 7 days of lactation in Brandt's voles (Lasiopodomys brandtii), a model small wild mammal we have extensively studied in the context of lactation energy demands. Repletion of leptin resulted in a dose-dependent reduction of body mass and food intake in lactating voles. Comparisons to nonreproducing individuals suggests that the reduced leptin in lactation, due to reduced fat stores, may account for ∼16% of the lactational hyperphagia. Reduced leptin in lactation may, in part, cause lactational hyperphagia via stimulatory effects on hypothalamic orexigenic neuropeptides (neuropeptide Y and agouti-related peptide) and inhibition of the anorexigenic neuropeptide (proopiomelanocortin). These effects were reversed by the experimental repletion of leptin. There was no significant effect of leptin treatment on daily energy expenditure, milk production or pup growth, but leptin repletion did result in a reversal of the suppression of uncoupling protein-1 levels in brown adipose tissue, indicating an additional role for reducing body fat and leptin during peak lacation.

Cross-talk between orexins (hypocretins) and the neuroendocrine axes (hypothalamic-pituitary axes)

Lesioning and electrical stimulation experiments carried out during the first half of the twentieth century showed that the lateral hypothalamic area (LHA) is involved in the neuroendocrine control of hormone secretion. However, the molecular basis of this phenomenon remained unclear until fifty years later when in 1998, two different laboratories discovered a new family of hypothalamic neuropeptides, the orexins or hypocretins (OX-A/Hcrt1 and OX-B/Hcrt2). Since then, remarkable evidence has revealed that orexins/hypocretins play a prominent role in regulating virtually all the neuroendocrine axes, acting as pivotal signals in the coordination of endocrine responses with regards to sleep, arousal and energy homeostasis. The clinical relevance of these actions is supported by human data showing impairment of virtually all the neuroendocrine axes in orexin/hypocretin-deficient narcoleptic patients. Here, we summarize more than ten years of knowledge about the orexins/hypocretins with particular focus on their role as neuroendocrine regulators. Understanding this aspect of orexin/hypocretin physiology could open new therapeutic possibilities in the treatment of sleep, energy homeostasis and endocrine pathologies.

Orexins (hypocretins) actions on the GHRH/somatostatin-GH axis

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system that includes two major hypothalamic regulators, namely GH-releasing hormone (GHRH) and somatostatin (SST) that stimulate and inhibit, respectively, GH release. Classical experiments involving damage and electrical stimulation suggested that the lateral hypothalamic area (LHA) modulated the somatotropic axis, but the responsible molecular mechanisms were unclear. Evidence obtained during the last decade has demonstrated that orexins/hypocretins, a family of peptides expressed in the LHA controlling feeding and sleep, play an important regulatory role on GH, by inhibiting its secretion modulating GHRH and SST neurones. Considering that GH release is closely linked to the sleep-wake cycle and feeding state, understanding orexin/hypocretin physiology could open new therapeutic possibilities in the treatment of sleep, energy homeostasis and GH-related pathologies, such as GH deficiency.

Role of orexins in the hypothalamic-pituitary-ovarian relationships

Appropriate nutritional and vigilance states are needed for reproduction. In previous works, we described the influence of the hormonal milieu of proestrus on the orexinergic system and we found that orexin receptor 1 expression in the hypothalamus, but not other neural areas, and the adenohypophysis was under the influence of oestradiol and the time of the day. Information from the sexual hormonal milieu of proestrous afternoon impacts on various components of the orexinergic system and alertness on this particular night of proestrus would be of importance for successful reproduction. In this review, we summarize the available experimental data supporting the participation of orexins in the hypothalamic-pituitary-ovarian relationships. All together, these results suggest a role of the orexinergic system as an integrative link among vital functions such as reproduction, food intake, alertness and the inner biological clock.

A Brief Review on How Pregnancy and Sex Hormones Interfere with Taste and Food Intake

Many physiological and behavioral changes take place during pregnancy, including changes in taste and an increase in food intake. These changes are necessary to ensure growth and development of a healthy fetus. Both hyperphagia and taste changes during pregnancy may be induced by sex hormones estrogen and progesterone that are increased during pregnancy. Indeed, it has been shown that estrogen decreases food intake, while progesterone increases food intake. This is for instance apparent from the fact that food intake changes during the menstrual cycle with variation in sex hormones. This review will give a short overview of the effects of pregnancy and sex hormones on food intake and taste.

Developmental and aging change of orexin-A and -B immunoreactive neurons in the male rat hypothalamus

Orexin/hypocretin is indicated to affect various physiological functions and behaviors, such as energy balance, feeding, wake-sleep cycle, stress response, and reproduction. This study investigated postnatal development and aging changes of the orexin neuron in the male rat hypothalamus. The brain tissue of rats from 1 week to 24 months old was analyzed by immunohistochemistry for two forms of orexin peptides, orexin-A and -B. The number of immunoreactive cells for each age group was counted and the immunoreactive intensity was also analyzed in order to reveal the changes in the number of expressing cells and the relative amount of the peptides. The number of orexin immunoreactive cells increased from postnatal 2 weeks to maturation, then slightly decreased and stabilized until the age of 8 months old, but it was significantly decreased by 24 months old. The intensity of the immunoreaction followed almost the same pattern. Our findings demonstrate that orexin neurons are increased during maturation and then are significantly decreased during the period from 8 to 24 months old, indicating an involvement of orexin in the physiological changes in rat aging such as energy balance, sleep, stress response, and reproduction.