Effects of age and long-term ovariectomy on the estrogen-receptor containing subpopulations of beta-endorphin-immunoreactive neurons in the arcuate nucleus of female C57BL/6J mice

The hypothalamus for whole-body physiology: from metabolism to aging

Obesity and aging are two important epidemic factors for metabolic syndrome and many other health issues, which contribute to devastating diseases such as cardiovascular diseases, stroke and cancers. The brain plays a central role in controlling metabolic physiology in that it integrates information from other metabolic organs, sends regulatory projections and orchestrates the whole-body function. Emerging studies suggest that brain dysfunction in sensing various internal cues or processing external cues may have profound effects on metabolic and other physiological functions. This review highlights brain dysfunction linked to genetic mutations, sex, brain inflammation, microbiota, stress as causes for whole-body pathophysiology, arguing brain dysfunction as a root cause for the epidemic of aging and obesity-related disorders. We also speculate key issues that need to be addressed on how to reveal relevant brain dysfunction that underlines the development of these disorders and diseases in order to develop new treatment strategies against these health problems.

Carving the senescent phenotype by the chemical reactivity of catecholamines: An integrative review

Macromolecules damaged by covalent modifications produced by chemically reactive metabolites accumulate in the slowly renewable components of living bodies and compromise their functions. Among such metabolites, catecholamines (CA) are unique, compared with the ubiquitous oxygen, ROS, glucose and methylglyoxal, in that their high chemical reactivity is confined to a limited set of cell types, including the dopaminergic and noradrenergic neurons and their direct targets, which suffer from CA propensities for autoxidation yielding toxic quinones, and for Pictet-Spengler reactions with carbonyl-containing compounds, which yield mitochondrial toxins. The functions progressively compromised because of that include motor performance, cognition, reward-driven behaviors, emotional tuning, and the neuroendocrine control of reproduction. The phenotypic manifestations of the resulting disorders culminate in such conditions as Parkinson's and Alzheimer's diseases, hypertension, sarcopenia, and menopause. The reasons to suspect that CA play some special role in aging accumulated since early 1970-ies. Published reviews address the role of CA hazardousness in the development of specific aging-associated diseases. The present integrative review explores how the bizarre discrepancy between CA hazardousness and biological importance could have emerged in evolution, how much does the chemical reactivity of CA contribute to the senescent phenotype in mammals, and what can be done with it.

Hypothalamic Perineuronal Nets Are Regulated by Sex and Dietary Interventions

Perineuronal nets (PNNs) are widely present in the hypothalamus, and are thought to provide physical protection and ion buffering for neurons and regulate their synaptic plasticity and intracellular signaling. Recent evidence indicates that PNNs in the mediobasal hypothalamus play an important role in the regulation of glucose homeostasis. However, whether and how hypothalamic PNNs are regulated are not fully understood. In the present study, we examined whether PNNs in various hypothalamic regions in mice can be regulated by sex, gonadal hormones, dietary interventions, or their interactions. We demonstrated that gonadal hormones are required to maintain normal PNNs in the arcuate nucleus of hypothalamus in both male and female mice. In addition, PNNs in the terete hypothalamic nucleus display a sexual dimorphism with females higher than males, and high-fat diet feeding increases terete PNNs only in female mice but not in male mice. On the other hand, PNNs in other hypothalamic regions are not influenced by sex, gonadal hormones or dietary interventions. In summary, we demonstrated that hypothalamic PNNs are regulated in a region-specific manner and these results provide a framework to further investigate the potential functions of PNNs in regulating energy/glucose homeostasis at the interplay of sex, gonadal hormones and diets.

Mechanisms for Sex Differences in Energy Homeostasis

Sex differences exist in the regulation of energy homeostasis. Better understanding of the underlying mechanisms for sexual dimorphism in energy balance may facilitate development of gender-specific therapies for human diseases, e.g. obesity. Multiple organs, including the brain, liver, fat and muscle, play important roles in the regulations of feeding behavior, energy expenditure and physical activity, which therefore contribute to the maintenance of energy balance. It has been increasingly appreciated that this multi-organ system is under different regulations in male vs. female animals. Much of effort has been focused on roles of sex hormones (including androgens, estrogens and progesterone) and sex chromosomes in this sex-specific regulation of energy balance. Emerging evidence also indicates that other factors (not sex hormones/receptors and not encoded by the sex chromosomes) exist to regulate energy homeostasis differentially in males vs. females. In this review, we summarize factors and signals that have been shown to regulate energy homeostasis in a sexually dimorphic fashion and propose a framework where these factors and signals may be integrated to mediate sex differences in energy homeostasis.

Central regulation of energy metabolism by estrogens

BACKGROUND

Estrogenic actions in the brain prevent obesity. Better understanding of the underlying mechanisms may facilitate development of new obesity therapies.

SCOPE OF REVIEW

This review focuses on the critical brain regions that mediate effects of estrogens on food intake and/or energy expenditure, the molecular signals that are involved, and the functional interactions between brain estrogens and other signals modulating metabolism. Body weight regulation by estrogens in male brains will also be discussed.

MAJOR CONCLUSIONS

17β-estradiol acts in the brain to regulate energy homeostasis in both sexes. It can inhibit feeding and stimulate brown adipose tissue thermogenesis. A better understanding of the central actions of 17β-estradiol on energy balance would provide new insight for the development of therapies against obesity in both sexes.

Progress in the molecular understanding of central regulation of body weight by estrogens

OBJECTIVE

Estrogens can act in the brain to prevent body weight gain. Tremendous research efforts have been focused on estrogen physiology in the brain in the context of body weight control; estrogen receptors and the related signals have been attractive targets for development of new obesity therapies. The objective is to review recent findings on these aspects.

METHODS

Recent studies that used conventional and conditional knockout mouse strains to delineate the cellular and molecular mechanisms for the beneficial effects of estrogens on body weight balance are reviewed. Emerging genetic tools that could further benefit the field of estrogen research and a newly developed estrogen-based regimen that produces body weight-lowering benefits also are discussed.

RESULTS

The body weight-lowering effects of estrogens are mediated by multiple forms of estrogen receptors in different brain regions through distinct but coordinated mechanisms. Both rapid signals and "classic" nuclear receptor actions of estrogen receptors appear to contribute to estrogenic regulation of body weight.

CONCLUSIONS

Estrogen receptors and associated signal networks are potential targets for obesity treatment, and further investigations are warranted.

G protein-coupled estrogen receptor in energy homeostasis and obesity pathogenesis

Obesity and its related metabolic diseases have reached a pandemic level worldwide. There are sex differences in the prevalence of obesity and its related metabolic diseases, with men being more vulnerable than women; however, the prevalence of these disorders increases dramatically in women after menopause, suggesting that sex steroid hormone estrogens play key protective roles against development of obesity and metabolic diseases. Estrogens are important regulators of several aspects of metabolism, including body weight and body fat, caloric intake and energy expenditure, and glucose and lipid metabolism in both males and females. Estrogens act in complex ways on their nuclear estrogen receptors (ERs) ERα and ERβ and transmembrane ERs such as G protein-coupled estrogen receptor. Genetic tools, such as different lines of knockout mouse models, and pharmacological agents, such as selective agonists and antagonists, are available to study function and signaling mechanisms of ERs. We provide an overview of the evidence for the physiological and cellular actions of ERs in estrogen-dependent processes in the context of energy homeostasis and body fat regulation and discuss its pathology that leads to obesity and related metabolic states.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Central nervous control of energy and glucose balance: focus on the central melanocortin system

Studies have suggested that manipulations of the central melanocortin circuitry by pharmacological agents produce robust effects on the regulation of body weight and glucose homeostasis. In this review, we discuss recent findings from genetic mouse models that have further established the physiological relevance of this circuitry in the context of glucose and energy balance. In addition, we will discuss distinct neuronal populations that respond to central melanocortins to regulate food intake, energy expenditure, insulin sensitivity, and insulin secretion, respectively. Finally, multiple hormonal and neural cues (e.g., leptin, estrogen, and serotonin) that use the melanocortin systems to regulate energy and glucose homeostasis will be reviewed. These findings suggest that targeting the specific branches of melanocortin circuits may be potential avenues to combat the current obesity and diabetes epidemics.

Molecular fingerprint of neuropeptide S-producing neurons in the mouse brain

Neuropeptide S (NPS) has been associated with a number of complex brain functions, including anxiety-like behaviors, arousal, sleep-wakefulness regulation, drug-seeking behaviors, and learning and memory. In order to better understand how NPS influences these functions in a neuronal network context, it is critical to identify transmitter systems that control NPS release and transmitters that are co-released with NPS. For this purpose, we generated several lines of transgenic mice that express enhanced green-fluorescent protein (EGFP) under control of the endogenous NPS precursor promoter. NPS/EGFP-transgenic mice show anatomically correct and overlapping expression of both NPS and EGFP. A total number of ∼500 NPS/EGFP-positive neurons are present in the mouse brain, located in the pericoerulear region and the Kölliker-Fuse nucleus. NPS and transgene expression is first detectable around E14, indicating a potential role for NPS in brain development. EGFP-positive cells were harvested by laser-capture microdissection, and mRNA was extracted for expression profiling by using microarray analysis. NPS was found co-localized with galanin in the Kölliker-Fuse nucleus of the lateral parabrachial area. A dense network of orexin/hypocretin neuronal projections contacting pericoerulear NPS-producing neurons was observed by immunostaining. Expression of a distinct repertoire of metabotropic and ionotropic receptor genes was identified in both NPS neuronal clusters that will allow for detailed investigations of incoming neurotransmission, controlling neuronal activity of NPS-producing neurons. Stress-induced functional activation of NPS-producing neurons was detected by staining for the immediate-early gene c-fos, thus supporting earlier findings that NPS might be part of the brain stress response network.

Cross-Talk between Metabolism and Reproduction: The Role of POMC and SF1 Neurons

Energy homeostasis and reproduction require tight coordination, but the mechanisms underlying their interaction are not fully understood. Two sets of hypothalamic neurons, namely pro-opiomelanocortin (POMC) neurons in the arcuate nucleus and steroidogenic factor-1 (SF1) neurons in the ventromedial hypothalamic nucleus, are emerging as critical nodes where metabolic and reproductive signals communicate. This view is supported by recent genetic studies showing that disruption of metabolic signals (e.g., leptin and insulin) or reproductive signals (e.g., estradiol) in these neurons leads to impaired regulation of both energy homeostasis and fertility. In this review, we will examine the potential mechanisms of neuronal communication between POMC, SF1, and gonadotropin-releasing hormone neurons in the regulation of metabolism and reproduction.

Reproductive experience may positively adjust the trajectory of senescence

Although aging is inexorable, aging well is not. From the perspective of research in rats and complementary models, reproductive experience has significant effects; indeed, benefits, which include better-than-average cognitive skills, a slowing of the slope of decline, and a healthier brain and/or nervous system well later into life. Work from our lab and others has suggested that the events of pregnancy and parturition, collectively referred to as reproductive experience-an amalgam of hormone exposure, sensory stimulation, and offspring behavioral experience and interaction-may summate to flatten the degree of decline normally associated with aging. Mimicking the effects of an enriched environment, reproductive experience has been shown to: enhance/protect cognition and decrease anxiety well out to two-plus years; result in fewer hippocampal deposits of the Alzheimer's disease herald, amyloid precursor protein (APP); and, in general, lead to a healthier biology. Based on a suite of recent work in organisms as diverse as nematodes, flies, and mammals, the ubiquitous hormone insulin and its large family of related substances and receptors may play a major role in mediating some of the effects of RE on the parameters of aging studied thus far. We will discuss the current set of data that suggest mechanisms for successful biological and neurobiological aging, and the implications for understanding aging and senescence in their broadest terms.

Reproduction-induced neuroplasticity: natural behavioural and neuronal alterations associated with the production and care of offspring

As a female transitions into motherhood, many neurobiological adaptations are required to meet the demands presented by her offspring. In addition to the traditional maternal responses (e.g. crouching, nursing, retrieving, grooming), our laboratories have observed several behavioural modifications accompanying parity, especially in the areas of foraging and emotional resilience. Additionally, brain modifications have been observed in the hippocampus and amygdala, providing support for neural plasticity extending beyond the expected hypothalamic alterations. Interestingly, we have observed parenting-induced neuroplasticity to persist into late adulthood, even providing protection against age-related brain and memory deficits. Although the majority of work on the parental brain has been conducted on females, preliminary research suggests similar changes in the biparental male California deer mouse. Taken together, research suggests that the parental brain is dynamic and changeable as it undergoes diverse and, in some cases, long-lasting, modifications to facilitate the production and care of offspring.

Evidence for cell-specific changes with age in expression of oestrogen receptor (ER) alpha and beta in bone fractures from men and women

Oestrogen is recognized as important for maintaining bone mass in men and women. Oestrogen receptor (ER) alpha and the recently described ER-beta are both expressed in bone cells, but have different affinities for oestrogen agonists and plant oestrogens, which could be important in developing treatments for bone loss in both men and women. It is unclear, however, which isoform predominates in bone; cell type and age may influence their relative expression. The present study has compared ER-alpha and ER-beta expression in serial sections of human fracture callus from males (n = 19, age range 5-72 years) and females (n = 15, age range 3-86 years) by indirect immunoperoxidase. Fracture callus was used as it can be readily obtained from individuals over a wide age range and contains a variety of bone cells. Antibody specificity was confirmed by western blotting and comparison of immunoreactivity in sections of breast tumour and benign prostate hyperplasia. No gender difference in ER expression was found in bone from individuals less than 40 years old. Proliferative chondrocytes were positive for both isoforms, but few larger hypertrophic cells were immunoreactive. ER-alpha and ER-beta were co-expressed in osteoclasts, suggesting that oestrogen may act directly on these cells. Osteoblasts, osteocytes, and mesenchymal cells also expressed both isoforms. In women over 40 years of age, however, relatively fewer biopsies contained osteocytes positive for ER-alpha and ER-beta. Likewise, the proportions of osteoblasts and mesenchymal cells expressing ER-beta were reduced but ER-alpha remained unaffected. In contrast, in men over 40 years, only the proportion of biopsies containing ER-beta-positive mesenchymal cells was lower. In these older men and women, ER-alpha and ER-beta expression was retained by the small proliferative chondrocytes. These results demonstrate that gender, age, and cell type are important determinants of ER isoform expression in skeletal cells.

Hypothalamic and vagal neuropeptide circuitries regulating food intake

It has been recognized for some time that a number of different neuropeptides exert powerful effects on food intake. During the last few years, the neurocircuitry within which these peptides operate has also begun to be elucidated. Peptidergic feeding-regulatory neurones are found both in the hypothalamus and the brainstem, where they act as input stations for hormonal and gastrointestinal information, respectively. These cell populations both project to several other brain regions and interconnect extensively. The present review summarizes the neuroanatomy and connectivity of some prominent peptides involved in food intake control, including neuropeptide Y, melanocortin peptides, agouti gene-related protein, cocaine- and amphetamine-regulated transcript, orexin/hypocretin, melanin-concentrating hormone and cholecystokinin. Disturbances in the hypothalamic neuropeptide systems have been implicated in the phenotype of a genetic model of fatal hypophagia, the mouse anorexia (anx) mutation, which is also discussed.

Estrogen modulates spontaneous alternation and the cholinergic phenotype in the basal forebrain

We report that a small population of neurons expresses both choline acetyltransferase and classical estrogen receptor immunoreactivity and they are found primarily in the bed nucleus of the stria terminalis. In short-term ovariectomized ageing mice (24 months, n = 5) there were 41.0 +/- 4.1% fewer of these double-labeled cells than in young (five months, n = 5) short-term ovariectomized C57BL/6J mice. To study cholinergic neuron estrogen responsiveness, young mice (n = 8) were ovariectomized at puberty (five weeks). After three months half of the mice (n = 4) were given physiological levels of 17beta estradiol for 10 days. Bed nucleus double-labeled neurons increased by 32.9% (P < or = 0.003) in the young mice given estrogen. In a gel shift assay, double-stranded oligonucleotides with putative estrogen response elements from the choline acetyltransferase gene were used as competitors against estrogen receptor binding to consensus estrogen response elements. A sequence with 60% homology to the vitellogenin estrogen response element was found to compete at 500- and 1000-fold excess. Young mice (five months) with ovaries demonstrated significantly (P < or = 0.04) better performance in the spontaneous alternation T-maze test than did old (19 month) mice with ovaries (young = 66.3 +/- 3.3% correct choices; vs old = 55.0 +/- 4.0% in old mice with ovaries). Young mice (five months old), ovariectomized for one month and treated with estrogen, showed significantly more spontaneous alternation than ovariectomized controls (69.1 +/- 2.8% vs 58.3 +/- 3.9%; P < or = 0.04). Estrogen also increased spontaneous alternation in old, short-term ovariectomized mice (61.5 +/- 2.7% vs 48 +/- 3.3%; P < or = 0.005). In either young or old ovariectomized mice, estrogen increased spontaneous alternation to levels seen in young animals with ovaries. Estrogen increases the number of choline acetyltransferase-immunoreactive and choline acetyltransferase/estrogen receptor-immunoreactive cells in old or young mice lacking estrogen, and enhances working memory in old or young mice lacking estrogen. Our data suggest that estrogen may act at the level of the choline acetyltransferase gene, but in view of the limited distribution of cholinergic cells expressing the classical estrogen receptor, it is unlikely that these cells can account for a memory enhancing effect of estrogen replacement.

Proopiomelanocortin gene expression is decreased in the infundibular nucleus of postmenopausal women

Previous studies have shown that estrogen withdrawal decreases the secretion of beta-endorphin from the monkey hypothalamus. In addition, there are consistent age-associated changes in beta-endorphin neurons in the rodent. Based on these findings, we hypothesized that the activity of hypothalamic beta-endorphin neurons would be decreased in the hypothalamus of postmenopausal women. In the present study, we examined the expression of proopiomelanocortin (POMC) mRNA, the precursor mRNA for beta-endorphin, in the medial basal hypothalamus of premenopausal and postmenopausal women. Every 20th sagittal section through the hypothalamus was hybridized with a synthetic [35S]labeled, 48-base oligonucleotide probe complementary to POMC mRNA. Labeled neurons were counted and their somatic profile areas were measured with an image-combining computer microscope system. The number of POMC mRNA-containing neurons/section in the infundibular nucleus was reduced by 65% in postmenopausal women. In contrast, there was no significant difference in the number of neurons expressing POMC gene transcripts in the retrochiasmatic region. The POMC neurons in the retrochiasmatic area were also distinct morphologically from those in the infundibular nucleus. The differences between the infundibular and retrochiasmatic regions suggest that functional subgroups of POMC neurons exist in the human hypothalamus. Our findings provide evidence that the activity of hypothalamic POMC neurons is decreased in the infundibular nucleus of postmenopausal women. Both aging and gonadal steroid withdrawal may contribute to the decline in POMC gene expression in postmenopausal women.

Brain opioid receptor measurements by positron emission tomography in normal cycling women: relationship to luteinizing hormone pulsatility and gonadal steroid hormones

The regulation of central mu-opioid receptors in women during the menstrual cycle was explored with positron emission tomography and the selective radiotracer [11C]carfentanil. Ten healthy women were studied twice, during their follicular and luteal phases. Plasma concentrations of estradiol, progesterone, testosterone, and beta-endorphin were determined immediately before scanning. LH pulsatility was measured over the 9 h preceding each of the two positron emission tomography scans. No significant differences in the binding potential of mu-opioid receptors (binding capacity/Kd) were observed between phases of the menstrual cycle. However, significant negative correlations were observed between circulating levels of estradiol during the follicular phase and mu-receptor binding measures in the amygdala and hypothalamus, two regions thought to be involved in the regulation of GnRH pulsatility. LH pulse amplitude was positively correlated with mu binding in the amygdala, whereas LH pulse number was negatively correlated with binding in this same region. No significant associations were noted between LH pulse measures and the hypothalamus for this sample. These results suggest that amygdalar mu-opioid receptors exert a modulatory effect on GnRH pulsatility, and that circulating levels of estradiol also regulate central mu-opioid function.

Estrogen, the ovary, and neutotransmitters: factors associated with aging

Our studies in the C57BL/6J mouse have been designed to examine the interactions of aging and the ovary, and their mutual effects on neuroendocrine function. In the pituitary, ovarian status and not age determines responsiveness to gonadotropin hormone releasing hormone (GnRH), but estrogen (E2) is an important mediator in CNS changes, and removal of the ovary (OVX) is deleterious to the neuroendocrine hypothalamus. OVX for just six days in young animals results in synaptic loss between noradrenergic terminals and gonadotropin hormone releasing hormone (GnRH) neurons. Long-term OVX, hypothesized to protect against neuroendocrine aging, fails to guard against any studied age-related changes. Some age-related changes occur as early as midlife. Although neuron number remains constant at middle age, opiatergic neurons undergo significant functional changes by producing opiate antagonist peptides. This change appears to be caused by alterations in the prohormone convertases, which cleave propeptide to peptide. Altered peptides may trigger the loss of reproductive capacity. The midlife shift in opiate peptide production is a component of natural developmental processes that begin in the neonate and continue through old age. In the cholinergic system, E2 mediates numbers of cholinergic receptors, cholinergic neurons, and cholinergic-modulated memory systems in both young and old animals. Regardless of age, ovarian steroids, if present at physiologic levels, are beneficial to the neuroendocrine CNS, and long-term deprivation from ovarian-produced factors is deleterious in the systems we have examined. Our studies have shown that deprivation from ovarian steroid hormones in the female appears to be a major factor in the health of the CNS and in events associated with aging.