Coexpression of leptin receptor and preproneuropeptide Y mRNA in arcuate nucleus of mouse hypothalamus

Intraventricular (i3vt) ghrelin increases food intake in fatty Zucker rats

Ghrelin is an orexigenic peptide secreted from the stomach and also made in the brain. Ghrelin receptors are expressed on hypothalamic cells important in appetite and energy balance. We determined that intra-3rd-ventricular (i3vt) ghrelin dose-dependently increases acute (1 and 2 h) food intake in lean and fatty Zucker rats (0, 0.01, 0.1 and 1.0 nmol ghrelin). The percentage increase of food intake in fatty Zucker rats was significantly greater than that in lean rats. Fatty Zucker rats had 4.5 times more ghrelin receptor mRNA in the hypothalamus than lean Zucker rats, suggesting a possible mechanism for the increased sensitivity.

Neuropeptide Y in normal eating and in genetic and dietary-induced obesity

Neuropeptide Y (NPY) is one the most potent orexigenic peptides found in the brain. It stimulates food intake with a preferential effect on carbohydrate intake. It decreases latency to eat, increases motivation to eat and delays satiety by augmenting meal size. The effects on feeding are mediated through at least two receptors, the Y1 and Y5 receptors. The NPY system for feeding regulation is mostly located in the hypothalamus. It is formed of the arcuate nucleus (ARC), where the peptide is synthesized, and the paraventricular (PVN), dorsomedial (DMN) and ventromedial (VMN) nuclei and perifornical area where it is active. This activity is modulated by the hindbrain and limbic structures. It is dependent on energy availability, e.g. upregulation with food deprivation or restriction, and return to baseline with refeeding. It is also sensitive to diet composition with variable effects of carbohydrates and fats. Leptin signalling and glucose sensing which are directly linked to diet type are the most important factors involved in its regulation. Absence of leptin signalling in obesity models due to gene mutation either at the receptor level, as in the Zucker rat, the Koletsky rat or the db/db mouse, or at the peptide level, as in ob/ob mouse, is associated with increased mRNA abundance, peptide content and/or release in the ARC or PVN. Other genetic obesity models, such as the Otsuka-Long-Evans-Tokushima Fatty rat, the agouti mouse or the tubby mouse, are characterized by a diminution in NPY expression in the ARC nucleus and by a significant increase in the DMN. Further studies are necessary to determine the exact role of NPY in these latter models. Long-term exposure to high-fat or high-energy palatable diets leads to the development of adiposity and is associated with a decrease in hypothalamic NPY content or expression, consistent with the existence of a counter-regulatory mechanism to diminish energy intake and limit obesity development. On the other hand, an overactive NPY system (increased mRNA expression in the ARC associated with an upregulation of the receptors) is characteristic of rats or rodent strains sensitive to dietary-induced obesity. Finally, NPY appears to play an important role in body weight and feeding regulation, and while it does not constitute the only target for drug treatment of obesity, it may nevertheless provide a useful target in conjunction with others.

Appetite and energy balance signals from adipocytes

Interest in the biology of white adipose tissue has risen markedly with the recent surge in obesity and its associated disorders. The tissue is no longer viewed simply as a vehicle for lipid storage; instead, it is recognized as a major endocrine and secretory organ. White adipocytes release a multiplicity of protein hormones, signals and factors, termed adipokines, with an extensive range of physiological actions. Foremost among these various adipokines is the cytokine-like hormone, leptin, which is synthesized predominantly in white fat. Leptin plays a critical role in the control of appetite and energy balance, with mutations in the genes encoding the hormone or its receptor leading to profound obesity in both rodents and man. Leptin regulates appetite primarily through an interaction with hypothalamic neuroendocrine pathways, inhibiting orexigenic peptides such as neuropeptide Y and orexin A, and stimulating anorexigenic peptides such as proopiomelanocortin. White fat also secretes several putative appetite-related adipokines, which include interleukin-6 and adiponectin, but whether these are indeed significant signals in the regulation of food intake has not been established. Through leptin and the other adipokines it is evident that adipose tissue communicates extensively with other organs and plays a pervasive role in metabolic homeostasis.

Altered expression of SOCS3 in the hypothalamic arcuate nucleus during seasonal body mass changes in the field vole, Microtus agrestis

We have previously shown that cold-acclimated (8 degrees C) male field voles (Microtus agrestis) transferred from short day (SD, 8 h light) to long day (LD, 16 h light) photoperiod exhibit an increase in body mass lasting 4 weeks, after which they stabilise at a new plateau approximately 7.5 g (24.8%) higher than animals maintained in SD. By infusing voles with exogenous leptin, we have also demonstrated that SD voles respond to the hormone by reducing body mass and food intake, whereas LD animals increasing body mass are resistant to leptin treatment. In the present study, we investigated whether seasonal changes in body mass could be linked to modulation of the leptin signal by suppressor of cytokine signalling-3 (SOCS3). We used in situ hybridisation to examine hypothalamic arcuate nucleus (ARC) expression of SOCS3, neuropeptide Y (NPY), agouti-related peptide (AgRP), pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) genes in 90 voles exposed to either SD or LD for up to 11 weeks. LD voles increasing body mass had significantly higher levels of SOCS3 mRNA than SD or LD voles with a stable body mass. There were no associated changes in expression of NPY, AgRP, POMC and CART genes. These results suggest that voles that regulate body mass at either the lower (SD) or upper (LD) plateau remain sensitive to leptin action, whereas SOCS3-mediated leptin resistance is a short-term mechanism that enables animals to move between the stable body mass plateaus. Our data provide evidence that expression of SOCS3 in the ARC is involved in the modulation of the strength of the leptin signal to facilitate seasonal cycles in body mass and adiposity.

Role of dorsomedial hypothalamic neuropeptide Y in energy homeostasis

Hypothalamic neuropeptide Y (NPY) is primarily expressed in the arcuate nucleus (Arc) and the dorsomedial hypothalamus (DMH). Although Arc NPY gene expression is responsive to circulating leptin, the regulation of DMH NPY expression is leptin-independent. DMH NPY expression is increased in response to chronic food restriction, but not acute food deprivation. DMH NPY expression is elevated in pair-fed OLETF rats lacking cholecystokinin (CCK)-1 receptors. A role for CCK in controlling DMH NPY expression is demonstrated by the down-regulation of DMH NPY by parenchymal DMH CCK administration in intact rats. Moreover, access to running wheels normalizes body weight and prevents altered DMH NPY expression of OLETF rats. Together, these data suggest that DMH NPY plays an important role in feeding and body weight control.

Neuropeptide signaling in the integration of metabolism and reproduction

Fertility is gated by nutrition and the availability of stored energy reserves, but the cellular and molecular mechanisms that link energy stores and reproduction are not well understood. Neuropeptides including galanin-like peptide (GALP), neuropeptide Y (NPY), products of the proopiomelanocortin (POMC; e.g., alpha-MSH and beta-endorphin), and kisspeptin are thought to be involved in this process for several reasons. First, the neurons that express these neuropeptides all reside in the hypothalamic arcuate nucleus, a critical site for the regulation of both metabolism and reproduction. Second, these neuropeptides are all targets for regulation by metabolic hormones, such as leptin and insulin. And third, these neuropeptides have either direct or indirect effects on feeding and metabolism, as well as on the secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). As the target for the action of metabolic hormones and sex steroids, these neuropeptides serve as molecular motifs integrating the control of metabolism and reproduction.

G protein-coupled receptor 101 mRNA expression in the mouse brain: altered expression in the posterior hypothalamus and amygdala by energetic challenges

GPCR101 is a recently identified orphan G protein-coupled receptor (GPCR) expressed abundantly in the human and mouse hypothalamus. In the absence of a ligand, a direct approach to determine the function(s) of this receptor is not possible. However, clues to the possible functions of GPCR101 may yield from information on the distribution of the receptor and the effect of in vivo manipulation upon the expression level of the receptor. In situ hybridisation on mouse brain sections revealed GPCR101 expression in a number of nuclei, including the amygdala, lateral parabrachial nucleus and nucleus of the solitary tract, as well as in the arcuate nucleus, posterior hypothalamus and paraventricular nucleus of the hypothalamus. Food-deprivation was found to increase GPCR101 mRNA level in the posterior hypothalamus and amygdala. In obese mice bearing the ob gene mutation, GPCR101 mRNA level decreased in the posterior hypothalamus and remained unaltered in the amygdala. By contrast, in both nuclei, GPCR101 mRNA level did not change significantly in obese ob/ob mice after intraperitoneal injection of leptin or in mice fed with a high fat diet. These data suggest that GPCR101 mRNA expression in the posterior hypothalamus and amygdala is regulated by a factor(s) other than leptin. Dual in situ hybridisation was used to establish the relationship between GPCR101 and neuropeptides expressed in the hypothalamus. In the arcuate nucleus, GPCR101 mRNA was expressed in approximately half of the population of neurones expressing the mRNA for the anorexigenic neuropeptide, pro-opiomelanocortin, which suggests a potential functional relationship.

Leptin levels in infertile male patients are correlated with inhibin B, testosterone and SHBG but not with sperm characteristics

In the present study, differences in leptin levels between different groups of male patients presenting with infertility problems and possible correlations between leptin levels and clinical, spermiological, histological and hormonal characteristics were examined. Two hundred and ten male partners from infertile couples were included in the study. Based on clinical examination, spermiogram and testicular histology results, patients were divided into four groups: 42 men with non-obstructive azoospermia, 15 men with obstructive azoospermia, 68 men with oligoasthenoteratozoospermia and 85 men with normozoospermia. Serum levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), inhibin B, testosterone, sex hormone binding globulin (SHBG) and leptin were measured. After adjustment for body mass index, there was a negative correlation between serum levels of leptin and inhibin B, total testosterone and SHBG (r = -0.189, p = 0.009, r = -0.250, p = 0.001 and r =-0.221, p = 0.003 respectively) but there was no correlation between leptin and classical sperm characteristics. Our results therefore demonstrate a link between leptin and testicular function, independently of FSH and LH, possibly involving testosterone and SHBG through a regulation of Leydig cell function.

Effects of pinealectomy and exogenous melatonin on immunohistochemical ghrelin staining of arcuate nucleus and serum ghrelin leves in the rat

Although the main source of circulating ghrelin is the stomach, it is also present in physiologically relevant amounts in the hypothalamus. It is reported that pharmacological doses of melatonin decrease blood levels of ghrelin. Thus, melatonin (MT) may be a candidate for the regulation of ghrelin synthesis in the brain. This study was therefore undertaken to investigate possible effects of pinealectomy and exogenous melatonin on hypothalamic ghrelin amount. Serum ghrelin levels following pinealectomy and administration of melatonin were also sought. Adult male Sprague-Dawley rats were divided into four groups as sham-operated (SHAM), sham-operated with melatonin treatment (SHAM-MT), pinealectomised (PNX) and melatonin-treated PNX (PNX-MT) groups. Ghrelin staining in the hypothalamus was determined by immunohistochemistry. Hypothalamic ghrelin was not observed in PNX rats. Much higher staining was detected in SHAM-MT rats compared to SHAM group. Lack of effect of melatonin on hypothalamic ghrelin in PNX rats implicates that exogenous melatonin requires an intact pineal to exert its effects. Although there were remarkable changes in the immunohistochemical activity of ghrelin in the hypothalamic arcuate nucleus, neither pinealectomy nor exogenous melatonin significantly changed serum levels of ghrelin. We have demonstrated for the first time that the pineal gland may play a role in ghrelin amount in the hypothalamus.

Metabolic signals modulate hypothalamic-pituitary-adrenal axis activation during maternal separation of the neonatal mouse

The postnatal development of the mouse is characterised by a period of hypo-responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis to moderate stressors. Maternal separation disinhibits this blockade of the HPA axis, but the mechanism responsible is not clear. The present study examined the influence of metabolic signals on the central and peripheral components of the HPA axis in neonatal mice aged 8 days in absence or presence of the mother. Reductions in plasma glucose and leptin as well as rapid increases in plasma ghrelin were apparent in the neonate 4 h following maternal deprivation and maximal at 8 h. In addition, maternal separation induced an increase of neuropeptide Y (NPY) mRNA expression in the arcuate nucleus, a decrease of corticotrophin-releasing hormone (CRH) mRNA expression in the paraventricular nucleus and a rise in serum corticosterone. Pharmacological manipulation of the metabolic signals attenuated the HPA response to maternal separation. Thus, the rise in plasma corticosterone induced by maternal separation was ameliorated by prevention of reduction in blood glucose or blockade of the ghrelin signalling pathway, as were the hypothalamic changes in NPY and CRH mRNAs. By contrast, leptin treatment did not affect the HPA axis response to maternal separation. Together these results suggest that metabolic signals play an important role in triggering the HPA response of the neonate to maternal separation.

The hypothalamic paraventricular nucleus is not essential for orexigenic NPY or anorexigenic melanocortin action

Bilateral electrolytic lesions of the paraventricular nucleus of the hypothalamus (PVN) produce hyperphagia with excess weight gain. The orexigenic neuropeptide Y (NPY) system and the anorexigenic melanocortin system act in the PVN to regulate food intake, and participate in mediating the anorexic effects of leptin. We hypothesized that changes in the responsiveness of these systems may contribute to the hyperphagia observed in PVN-lesioned rats. Adult female Sprague-Dawley rats received either sham or electrolytic lesions in the PVN immediately followed by implantation of a guide cannula into the third cerebroventricle. Twenty-five days following surgery groups of sham and hyperphagic PVN-lesioned rats were injected intracerebroventricularly (i.c.v.) with either 118 pmole or 470 pmole of NPY and food intake was measured for 3 h. Food intake in response to NPY was nearly three-fold higher in PVN-lesioned rats as compared to sham rats. However, the response to 5 microg leptin i.c.v. was not different in lesioned versus sham rats. The effect of the melanocortin agonist MTII on food intake was tested in additional rats beginning either 7-14 days or 30-40 days following surgery. Doses of 0.1 nmole or 1.0 nmole of MTII were injected immediately before lights-off and food intake was measured at 2 h, 24 h and 48 h post-injection. Suppression of food intake in PVN-lesioned rats was not different from that in sham-lesioned rats. These data suggest that hyper-responsiveness to NPY may account in part for the hyperphagia observed in PVN-lesioned rats. Furthermore, based on the similarities of responses of PVN-lesioned and sham control rats to the anorexigenic agents MTII and leptin and the hypersensitivity of lesioned rats to NPY, we conclude that the PVN is not essential for NPY stimulation of food intake or for melanocortin suppression of food intake and that NPY and melanocortin receptors outside of the PVN are sufficient to produce these effects.

Leptin: a potential biomarker for childhood obesity?

Leptin, a hormone made by adipocytes, is an important circulating signal for the regulation of body weight. A review of the scientific literature (PubMed Search 1994 to 2005) for studies examining the relationship among leptin, pediatric obesity and the impact of exercise intervention programmes on leptin concentrations are summarized. The potential utility of leptin as a biomarker for identifying children at risk of obesity is discussed. This literature review demonstrated that (1) leptin directly interacts with the hypothalamus for energy balance regulation; (2) the measurement of free, bound and total leptin as well as soluble leptin receptor concentration are critical for our understanding of obesity in children; and (3) leptin concentration may be an important factor for determining intervention programme responsiveness in pediatric obesity.

Synaptic plasticity in energy balance regulation

Leptin regulates energy balance, in part, by modulating the activity of neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons in the hypothalamic arcuate nucleus. Leptin-deficient (ob/ob) mice differ from wild-type mice in the number of excitatory and inhibitory post-synaptic densities and currents onto NPY and POMC neurons. When leptin was delivered to ob/ob mice, the synaptic density rapidly normalized, an effect detectable within 6 hours, several hours before leptin's effect on food intake. Synaptic currents were also shifted toward wild-type values in leptin-replaced ob/ob mice. These data suggest that leptin-mediated plasticity in the ob/ob hypothalamus may underlie some of the hormone's behavioral effects. In an effort to determine whether the observed synaptic plasticity is leptin specific, we analyzed the effects of an orexigenic hormone, ghrelin, and anorexigenic hormone, estradiol. Ghrelin rearranged synapses in wild type animals to support suppressed POMC tone, whereas the estradiol triggered a robust increase in the number of excitatory, glutamate inputs of POMC neurons. The rearrangement of synapses by estradiol was leptin independent, because it was also evident in leptin- (ob/ob) and leptin receptor-deficient (db/db) mice and was paralleled with decreased food intake and increased energy expenditure in these mutant, obese animals. Such plasticity was also observed in other hypothalamic regions and extrahypothalamic sites. These observations raise the notion that synaptic plasticity is a major way through which peripheral metabolic hormones influence brain functions.

The effect of intracerebroventricular infusions of leptin on the immunoreactivity of neuropeptide Y and gonadotrophin releasing hormone neurons in the hypothalamus of prepubertal sheep in conditions of short fasting

In the study we evaluated the effects of infusion of exogenous leptin to the third ventricle of the brain on the expression of immunoreactive (ir) neuropeptide Y (NPY) neurons in the hypothalamus and ir gonadotrophin releasing hormone (GnRH) nerve terminals in the median eminence of prepubertal lambs in the conditions of short fasting. Merino female sheep (n=16) were randomly divided into four groups, two fed with standard feeds and two fasted for 72 h. One standard and one fasted groups were infused with Ringer saline (controls), remaining standard and fasted groups with leptin (25 microg/120 microl/h), for 4 h during three consecutive days, and then slaughtered. Ir NPY and ir GnRH were localized by immunohistochemistry using specific polyclonal antibodies. Detection of both hormones was followed by the image analysis and expressed as the percent area stained and integral density of immunostaining. In the hypothalami from all groups the ir NPY perikarya and varicose nerve fibers were localized in three distinct sub-areas, in the arcuate (ARC), paraventricular and periventricular nuclei. In fasted sheep the percent area and integral density for immunoreactivity of NPY increased significantly (P<0.001) in three sub-areas compared to the standard-fed animals. Leptin infusion lowered the both parameters (P<0.001) but solely in the ARC NPY population of fasted sheep. The percent area and integral density of immunostaining for ir GnRH in fasted sheep revealed the augmentation (P<0.001) compared to standard-fed sheep. Leptin infusions diminished (P<0.001) both parameters in fasted, without effects in standard-fed lambs. In conclusion, the enhanced by fasting immunoreactivity of the ARC NPY perikarya and varicose nerve fibers and restrained immunoreaction of GnRH terminals in the median eminence were reversed by exogenous leptin. It is suggested that leptin can affect GnRH release via ARC NPY neurons in conditions of deficit of nutrients in prepubertal, female lambs.

Role of leptin in regulating appetite, neuroendocrine function, and bone remodeling

Leptin, a hormone secreted by adipocytes, acts on the hypothalamus to regulate appetite and neuroendocrine function. In the hypothalamus, both the arcuate nucleus and the ventromedial nucleus express leptin receptors. Specific neurons in the arcuate nucleus regulate appetite and reproduction. In contrast, neurons in the ventromedial nucleus regulate bone mass. The melanocortin system is the downstream pathway for regulating appetite and neuroendocrine function. In contrast, the sympathetic nervous system is the downstream pathway for regulating bone mass. Leptin, in regulating food intake and body weight, acts, in part, by inhibiting the synthesis of neuropeptide Y and its release from the hypothalamus. The leptin and insulin pathways may interact and may be important in the pathogenesis of the metabolic syndrome.

KiSS-1 neurones are direct targets for leptin in the ob/ob mouse

Leptin is an adipocyte-derived hormone that acts on the hypothalamus to influence feeding, metabolism and reproduction, but the cellular and molecular targets for the action of leptin in the brain have yet to be fully elucidated. Kisspeptins are encoded by the Kiss1 gene, which is expressed in the hypothalamus and has been implicated in the neuroendocrine regulation of gonadotrophin-releasing hormone secretion. We tested the hypothesis that kisspeptin-expressing neurones are targets for leptin. First, we examined whether leptin regulates the expression of Kiss1 by comparing levels of KiSS-1 mRNA in the arcuate nucleus among groups of mice having different circulating levels of leptin: (i) wild-type (WT); (ii) leptin-deficient ob/ob; and (iii) ob/ob mice treated with leptin. All mice were castrated to control for endogenous concentrations of gonadal steroids. KiSS-1 mRNA was significantly reduced in ob/ob compared to WT mice and levels of KiSS-1 mRNA in ob/ob mice treated with leptin were increased, but not fully restored to that found in WT animals. Second, we performed double-label in situ hybridisation for KiSS-1 mRNA and the leptin receptor (Ob-Rb) mRNA and found that almost one-half (approximately 40%) of KiSS-1 mRNA-expressing cells in the arcuate nucleus expressed Ob-Rb mRNA. These results demonstrate that KiSS-1 neurones are direct targets for regulation by leptin and suggest that the reproductive deficits associated with leptin-deficient states may be attributable, in part, to diminished expression of Kiss1.

Adipokines: implications for female fertility and obesity

Obesity is associated with a diverse set of metabolic disorders, and has reproductive consequences that are complex and not well understood. The adipose tissue-produced leptin has dominated the literature with regards to female fertility complications, but it is pertinent to explore the likely role of other adipokines--adiponectin and resistin--as our understanding of their biological functions emerge. Leptin influences the developing embryo, the functioning of the ovary and the endometrium, interacts with the release and activity of gonadotrophins and the hormones that control their synthesis. In this review such biological actions and potential roles of the adipokines leptin, adiponectin and resistin are explored in relation to female fertility and the complexity of the obese metabolic state.

Astrocytes in the arcuate nucleus and median eminence that take up a fluorescent dye from the circulation express leptin receptors and neuropeptide Y Y1 receptors

Following systemic injection, several different dyes and markers are found to accumulate rapidly in cells in the arcuate nucleus and median eminence, and the capillaries in this region appear specialised for exchange of molecules. The present study used hydroxystilbamidine (FluoroGold equivalent) to identify cells that take up molecules from the circulation in these regions; 2-6 h following injection, uptake was seen in the external and intermediate zones of the median eminence and the adjacent ventral part of the arcuate nucleus, but not in other regions of the hypothalamus. The labelled cells were small; double-labelling experiments revealed that they expressed glial fibrillary acid protein (GFAP), but not NeuN, Agouti-related protein (AgRP) or beta-endorphin. They had the morphology of astrocytes and were readily distinguished from tanycytes by staining for vimentin. Many of these labelled astrocytes also expressed leptin receptors and neuropeptide Y Y1 receptors. The surrounding neurons that expressed these receptors did not take up this dye. This demonstrates that astrocytes take up molecules from the circulation in the median eminence and adjacent arcuate nucleus, and may have a significant signalling role in regulation of food intake.

Leptin inhibits hypothalamic Npy and Agrp gene expression via a mechanism that requires phosphatidylinositol 3-OH-kinase signaling

Phosphatidylinositol 3-OH-kinase (PI3K) and STAT3 are signal transduction molecules activated by leptin in brain areas controlling food intake. To investigate their role in leptin-mediated inhibition of hypothalamic neuropeptide Y (Npy) and agouti-related peptide (Agrp) gene expression, male Sprague-Dawley rats (n = 5/group) were either fed ad libitum or subjected to a 52-h fast. At 12-h intervals, the PI3K inhibitor LY-294002 (LY, 1 nmol) or vehicle was injected intracerebroventricularly (ICV) as a pretreatment, followed 1 h later by leptin (3 microg icv) or vehicle. Fasting increased hypothalamic Npy and Agrp mRNA levels (P < 0.05), and ICV leptin administration prevented this increase. As predicted, LY pretreatment blocked this inhibitory effect of leptin, such that Npy and Agrp levels in LY-leptin-treated animals were similar to fasted controls. By comparison, leptin-mediated activation of hypothalamic STAT3 signaling, as measured by induction of both phospho-STAT3 immunohistochemistry and suppressor of cytokine signaling-3 (Socs3) mRNA, was not significantly attenuated by ICV LY pretreatment. Because NPY/AgRP neurons project to the hypothalamic paraventricular nucleus (PVN), we next investigated whether leptin activation of PVN neurons is similarly PI3K dependent. Compared with vehicle, leptin increased the number of c-Fos positive cells within the parvocellular PVN (P = 0.001), and LY pretreatment attenuated this effect by 35% (P = 0.043). We conclude that leptin requires intact PI3K signaling both to inhibit hypothalamic Npy and Agrp gene expression and activate neurons within the PVN. In addition, these data suggest that leptin activation of STAT3 is insufficient to inhibit expression of Npy or Agrp in the absence of PI3K signaling.

Immunohistochemical evidence of functional leptin receptor expression in neuronal and endothelial cells of the rat brain

Leptin binding to its functional receptor leads to activation of the JAK-STAT-signaling pathway and especially to the activation of the signal transducer and activator of transcription factor 3 (STAT3). The immunohistochemical detection of nuclear STAT3 translocation is used as a neuroanatomical mapping tool to determine leptin-responsive cells in the rat brain. This study neuroanatomically identifies those brain cell phenotypes showing STAT3 activation after intraperitoneal leptin treatment (5 mg/kg) using immunohistochemical colocalization with neuronal and endothelial cell marker proteins. Leptin treatment induced nuclear STAT3 signals with the strongest response observed 90min after the treatment. The caudobasal hypothalamus showed a particularly pronounced STAT3 response. Leptin-induced nuclear STAT3 signals were additionally determined in the solitary tract nucleus, the choroid plexus and in the brain endothelium. The vast majority of STAT3-responsive cells proved to be neurons located in the caudobasal hypothalamus, however, a marked number of brain endothelial cells distributed throughout the entire brain got activated as well. In conclusion, neurons and non-neuronal brain cells, e.g., endothelial or choroid plexus cells, seem to express functional leptin receptors and might thereby mediate leptin-dependent functions in the rat brain.

Photoperiod regulates leptin sensitivity in field voles, Microtus agrestis

We have previously shown that cold-acclimated (8 degrees C) male field voles (Microtus agrestis) transferred from short (SD, 8:16 h L:D) to long photoperiod (LD, 16:8 h L:D) exhibit increases in body mass, adiposity and food intake. To assess whether these increases were associated with decreased leptin sensitivity, we infused LD and SD voles with physiological doses of murine leptin (or saline) delivered peripherally for 7 days via mini-osmotic pumps. Measurements were made of body mass (weight-reducing effect of leptin), food intake (anorectic effect of leptin) and gene expression of uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) (thermogenic effect of leptin). The SD animals were sensitive to the weight-reducing effects of leptin (mean body mass decrease of 1.2 g over 7 days) and appetite-reducing effect of leptin (mean food intake decrease of 2.5 g over 7 days), whereas LD voles were resistant to the hormone treatment. The switch from a leptin-sensitive to leptin-resistant state appears to act as a desensitisation mechanism that allows voles transferred from SD to LD to ignore elevated leptin levels generated by increased body fat and accumulate adipose tissue without stimulating compensatory changes opposing the weight gain. Neither SD nor LD voles responded to infusion of leptin by changes in BAT UCP1 gene expression, suggesting dissociation of anorectic and thermogenic effects of leptin, possibly related to chronic cold exposure. Our results indicate that cold-acclimated voles show photoperiod-regulated changes in leptin sensitivity and may provide an attractive model for elucidating molecular mechanisms of leptin resistance.

Hormonal regulation of food intake

Our knowledge of the physiological systems controlling energy homeostasis has increased dramatically over the last decade. The roles of peripheral signals from adipose tissue, pancreas, and the gastrointestinal tract reflecting short- and long-term nutritional status are now being described. Such signals influence central circuits in the hypothalamus, brain stem, and limbic system to modulate neuropeptide release and hence food intake and energy expenditure. This review discusses the peripheral hormones and central neuronal pathways that contribute to control of appetite.

Endocrine and exocrine secretion of leptin by the gastric mucosa

Leptin is a hormone that plays important roles in nutritional status and in obesity. By means of immunocytochemistry, two populations of leptin-secreting cells were found in the lower half of the gastric mucosa. One consists of numerous large cells located around the gastric pits, the Chief epithelial cells, whereas the second refers to much smaller cells, strongly stained, few in number, and scattered between the gastric pits, the endocrine cells. By double immunostaining, leptin and pepsinogen were colocalized in the Chief cells, whereas the endocrine cells were positive only for leptin. Immunoelectron microscopy showed that leptin is present along the rough endoplasmic reticulum-Golgi-granules secretory pathways of the Chief and endocrine cells. On the other hand, leptin-receptor (long and short forms) immunolabelings, although absent in the gastric epithelial cell plasma membranes, were present in enterocytes at the level of their apical and basolateral membranes. Duodenal, jejunal, and ileal enterocytes displayed similar labelings for the leptin receptor. Thus, exocrine and endocrine secretions of leptin together with the presence of leptin receptors on enterocyte plasma membranes constitute a gastroenteric axis that coordinates the role played by leptin in the digestive tract.

Fetal and perinatal programming of appetite

There is increasing concern about the rapidly rising incidence of obesity worldwide and its impact both on mortality, morbidity and the cost of healthcare. In the last 15 years, a large volume of research has linked low birth weight to many adult diseases in humans, such as Type II diabetes, cardiovascular disease, hypertension and the metabolic syndrome. Obesity is a causal factor in all these conditions. There are epidemiological studies linking low birth weight to increased adiposity, but the timing of the insult during gestation seems crucial, as reducing maternal nutrition in late gestation and during lactation causes a reduction in later obesity. Recent studies in animal models have provided clues towards mechanisms of altered appetite regulation following alterations in fetal and neonatal growth. The outcome of these and future studies could prove clinically crucial, particularly in the debate over the benefits of breast feeding, which provides a lower plane of nutrition compared with formula feeding.

Limits to sustained energy intake IX: a review of hypotheses

Several lines of evidence indicate that animals in the wild may be limited in their maximal rates of energy intake by their intrinsic physiology rather than food availability. Understanding the limits to sustained energy intake is important because this defines an envelope within which animals must trade-off competing activities. In the first part of this review, we consider the initial ideas that propelled this area and experimental evidence connected with them. An early conceptual advance in this field was the idea that energy intake could be centrally limited by aspects of the digestive process, or peripherally limited at the sites of energy utilisation. A model system that has been widely employed to explore these ideas is lactation in small rodents. Initial studies in the late 1980s indicated that energy intake might be centrally limited, but work by Hammond and colleagues in the 1990s suggested that it was more likely that the limits were imposed by capacity of the mammary glands, and other works tended to support this view. This consensus, however, was undermined by studies that showed milk production was higher in mice at low temperatures, suggesting that the capacity of the mammary gland is not a limiting factor. In the second part of the review we consider some additional hypotheses that might explain these conflicting data. These include the heat dissipation limits hypothesis, the seasonal investment hypothesis and the saturated neural control hypothesis. Current evidence with respect to these hypotheses is also reviewed. The limited evidence presently available does not unambiguously support any one of them.

Transcriptional regulation of neuronal genes and its effect on neural functions: transcriptional regulation of neuropeptide Y gene by leptin and its effect on feeding

Leptin is an adipose tissue-derived secretory hormone that suppresses appetite by inhibition of neuropepeptide Y (NPY) gene expression in arcuate nucleus (ARC) in the hypothalamus. To investigate the transcriptional regulation of NPY gene by leptin, we carried out a luciferase assay using NPY gene promotor plasmid (NPY-luc) in NPY expressing cells such as N18TG2, NG108-15, and PC12 cells. In these cells, the NPY gene was transactivated by leptin through activation of leptin receptor. Leptin-induced transactivation was mediated through the 221-bp region of the NPY gene promotor, which possesses two putative STAT3 binding sites. To investigate the mechanism of in vivo suppression of NPY gene transcription in ARC by leptin, the effect of SOCS members on the leptin-induced transactivation of NPY gene was studied. In vivo SOCS2 and SOCS3 mRNAs were induced in mouse hypothalamus by leptin. Although leptin (125 ng/ml) induced significant increase in NPY gene transcriptional activity in mock-transfected cells, the leptin-induced NPY gene transcriptional activity was completely abolished in SOCS3-transfected cells. SOCS3 also suppressed the basal NPY gene transcription. These finding suggested that leptin inhibits NPY gene transcription in the hypothalamus in vivo and SOCS3 is a negative regulator of the NPY gene.

Regulatory roles of leptin in reproduction and metabolism: a comparative review

Leptin plays an important role in signaling nutritional status to the central reproductive axis of mammals and appears to be at least a permissive factor in the initiation of puberty. The expression and secretion of leptin are correlated with body fat mass and are acutely affected by changes in feed intake. Moreover, circulating leptin increases during pubertal development in rodents, human females and heifers. Effects of leptin are mediated mainly via receptor activation of the JAK-STAT pathway; however, activation of alternative pathways, such as MAP kinase, has also been reported. Although the leptin receptor (LR) has not been found on GnRH neurons, leptin stimulates the release of GnRH from rat and porcine hypothalamic explants. Moreover, leptin increases the release of LH in rats and from adenohypophyseal explants and/or cells from full-fed rats and pigs. In contrast, stimulation of the hypothalamic-gonadotropic axis by leptin in cattle and sheep is observed predominantly in animals and tissues pre-exposed to profound negative energy balance. For example, leptin prevents fasting-mediated reductions in the frequency of LH pulses in peripubertal heifers, augments the magnitude of LH and GnRH pulses in fasted cows, and enhances basal secretion of LH in vivo and from adenohypophyseal explants of fasted cows. However, leptin is incapable of accelerating the frequency of LH pulses in prepubertal heifers, regardless of nutrient status, and has no effect on the secretion of GnRH and LH in full-fed cattle or hypothalamic/hypophyseal explants derived thereof. Similar to results obtained with LH, basal secretion of GH from anterior pituitary explants of fasted, but not normal-fed cows, was potentiated acutely by low, but not high, doses of leptin. Mechanisms through which undernutrition hypersensitize the hypothalamic-gonadotropic axis to leptin may involve up-regulation of the LR. However, an increase in LR mRNA expression is not a requisite feature of heightened adenohypophyseal responses in fasted cattle. To date, leptin has not been successful for inducing puberty in ruminants. Future therapeutic uses for recombinant leptin that exploit states of nutritional hypersensitization, and identification of genetic markers for genotypic variation in leptin resistance, are currently under investigation.

Neuropeptide Y Y1 receptor mRNA in rodent brain: distribution and colocalization with melanocortin-4 receptor

The central neuropeptide Y (NPY) Y1 receptor (Y1-R) system has been implicated in feeding, endocrine, and autonomic regulation. In the present study, we systematically examined the brain distribution of Y1-R mRNA in rodents by using radioisotopic in situ hybridization histochemistry (ISHH) with a novel sensitive cRNA probe. Within the rat hypothalamus, Y1-R-specific hybridization was observed in the anteroventral periventricular, ventromedial preoptic, suprachiasmatic, paraventricular (PVH), dorsomedial, ventromedial, arcuate, and mamillary nuclei. In the rat, Y1-R mRNA expression was also seen in the subfornical organ, anterior hypothalamic area, dorsal hypothalamic area, and in the lateral hypothalamic area. In addition, Y1-R hybridization was evident in several extrahypothalamic forebrain and hindbrain sites involved in feeding and/or autonomic regulation in the rat. A similar distribution pattern of Y1-R mRNA was observed in the mouse brain. Moreover, by using a transgenic mouse line expressing green fluorescent protein under the control of the melanocortin-4 receptor (MC4-R) promoter, we observed Y1-R mRNA expression in MC4-R-positive cells in several brain sites such as the PVH and central nucleus of the amygdala. Additionally, dual-label ISHH demonstrated that hypophysiotropic PVH cells coexpress Y1-R and pro-thyrotropin-releasing hormone mRNAs in the rat. These observations are consistent with the proposed roles of the central NPY/Y1-R system in energy homeostasis.

A role for the central histaminergic system in the leptin-mediated increase in cardiovascular dynamics

The central nervous system (CNS) histaminergic neurons have been shown to regulate feeding behavior and are a target of leptin in the brain. The present study aimed to examine the involvement of the histaminergic system in the leptin-mediated regulation of cardiovascular dynamics. We investigated the cardiovascular responses to the CNS administration of histamine, leptin and alpha-melanocyte stimulating hormone (alpha-MSH) both in the presence and absence of the histamine H1 antagonist, chlorpheniramine. The intracerebroventricular (i.c.v.) administration of histamine resulted in an immediate increase in both mean arterial pressure (MAP) and heart rate (HR) and vasoconstricted the iliac, renal and superior mesenteric vessels. The i.c.v. pretreatment with chlorpheniramine attenuated the histamine-induced increase in MAP, HR and decreased vascular conductance. The i.c.v. administration of leptin increased MAP and HR and decreased vascular conductance. The i.c.v. pretreatment with chlorpheniramine decreased the leptin-induced increase in MAP and the leptin-mediated iliac vasoconstriction. The i.c.v. administration of alpha-MSH also increased MAP, HR and decreased vascular conductance. However, pretreatment with chlorpheniramine did not influence the central alpha-MSH-mediated increase in MAP, HR and decreased vascular conductance. These results indicate that the central histaminergic system mediated by H1 receptors have a role in the central signaling pathway and is involved in leptin's regulation of cardiovascular dynamics. It appears that leptin directly or indirectly stimulates histaminergic neurons that lead to increased cardiovascular activity.

PI3K integrates the action of insulin and leptin on hypothalamic neurons

Central control of energy balance depends on the ability of proopiomelanocortin (POMC) or agouti-related protein (Agrp) hypothalamic neurons to sense and respond to changes in peripheral energy stores. Leptin and insulin have been implicated as circulating indicators of adiposity, but it is not clear how changes in their levels are perceived or integrated by individual neuronal subtypes. We developed mice in which a fluorescent reporter for PI3K activity is targeted to either Agrp or POMC neurons and used 2-photon microscopy to measure dynamic regulation of PI3K by insulin and leptin in brain slices. We show that leptin and insulin act in parallel to stimulate PI3K in POMC neurons but in opposite ways on Agrp neurons. These results suggest a new view of hypothalamic circuitry, in which the effects of leptin and insulin are integrated by anorexigenic but not by orexigenic neurons.

Emerging antiobesity drugs

The healthcare burden that the obesity epidemic now poses in highly significant, in part due to increased risk of secondary chronic diseases such as hypertension. A lack of physical activity and high fat diets are major factors contributing to this condition. However, increasingly apparent is the genetic predisposition of individuals and ethnic groups to obesity. Present treatment strategies are currently inadequate and unlikely to have a major effect on the future prevalence of obesity. To slow the obesity epidemic, the source needs to be tackled now through fundamental research into the mechanisms by which obesity is manifest, and education on the risks and how to prevent it. This article will describe current and emerging treatments for obesity and review the recent advances in research that may provide the antiobesity treatments of the future. Research into obesity has escalated at considerable pace, catalysed by the discovery of the obese gene product leptin. Leptin is secreted by adipose tissue and acts via specific receptors in the brain to engage central neural pathways involved in regulating energy homeostasis. Since this discovery, numerous significant advances have been made in our understanding of how the brain integrates and responds to central and peripheral signals involved in maintaining energy homeostasis, and how disruption of these signalling mechanisms can manifest as obesity. As a consequence of these findings, numerous potential sites for therapeutic intervention into this condition have and are materializing. The aim of this review is to highlight current treatment strategies for obesity, recent advances in our understanding of the central neural control of energy balance, and what the authors consider to be the most promising targets for the development of novel antiobesity drugs in the future. Thus, the review focuses on leptin, neuropeptide Y, melanocortin and ghrelin signalling at the level of the CNS, and strategies targeting the sympathetic innervation of fat cells at the periphery.

A possible role of neuropeptide Y, agouti-related protein and leptin receptor isoforms in hypothalamic programming by perinatal feeding in the rat

AIM/HYPOTHESIS

Perinatal overfeeding predisposes humans and rats to obesity and diabetes in later life. One classical model for studying the effect of early feeding is manipulation of the size of rat litters. Rats growing up in small litters gain more weight than rats growing up in normal-sized litters. Interestingly, these obese rats maintain this phenotype in adulthood. Conversely, rats raised in large litters show a delay in growth and a decrease in body weight. The aim of this work was to assess the hypothalamic control mechanisms of food intake regulated by perinatal feeding.

METHODS

Leptin levels were analysed using RIA. Leptin receptor mRNA levels were analysed using RT-PCR. Neuropeptide mRNA levels were analysed using in situ hybridisation.

RESULTS

Perinatally overfed neonatal male rats exhibited hyperleptinaemia and a decrease in hypothalamic mRNA levels of the long isoform of the leptin receptor (OB-Rb), explaining their leptin resistance. Moreover, this obese model showed an increase in the mRNA expression of cocaine- and amphetamine-regulated transcript, neuropeptide Y and agouti-related protein in the hypothalamic arcuate nucleus (ARC). In contrast, perinatally underfed neonatal male rats with hypoleptinaemia showed an increase in hypothalamic mRNA of the short isoforms of the leptin receptor. Furthermore, they exhibited an increase in expression of neuropeptide Y and agouti-related protein in the ARC.

CONCLUSIONS/INTERPRETATION

Rats overfed during early postnatal life show a leptin-resistant state mediated by down-regulation of the hypothalamic OB-Rb. These data, together with the increased expression of neuropeptide Y and agouti-related protein in specific neurons in the ARC, might indicate the existence of regulated programming in this nucleus and may provide a new aetiopathogenic concept in susceptibility to obesity.

Localization and role of leptin in the thyroid gland of the lizardPodarcis sicula (reptilia, lacertidae)

Leptin, the product of the ob gene, is a hormone secreted by adipocytes that regulates food intake and energy expenditure. The hypothalamus-pituitary-thyroid axis is markedly influenced by the metabolism status, being suppressed during food deprivation. The present study was designed to ascertain whether (1) lizard thyroid gland expresses the long form of leptin receptor (Ob-Rb) and (2) the leptin administration affects the thyroid gland activity in this species (and to verify whether leptin plays a similar role in reptiles as observed in the other vertebrates). The presence of leptin receptor in the thyroid gland of Podarcis sicula was demonstrated by immunohistochemical technique (avidin-biotin-peroxidase complex--ABC method). The role of leptin in the control of thyroid gland activity was studied in vivo using light microscopy (LM) technique coupled to a specific radioimmunoassay for thyroid-stimulating hormone (TSH) and thyroid hormones (T4 and T3). Leptin (0.1 mg/100 g body wt)/day increased T4 and T3 release for 3 days but decreased the plasma concentration of TSH; using LM clear signs of stimulation in the thyroid gland were observed. These findings suggest that systemic administration of leptin stimulates the morphophysiology of the thyroid gland in the lizard through a direct mechanism involving Ob-Rb.

NPY neurons express somatostatin receptor subtype 1 in the arcuate nucleus

The regulation of growth hormone (GH) secretion involves hypothalamic somatostatin and its specific receptors (sst1-sst5). sst1 is highly expressed in the arcuate nucleus (AN), and several data suggest that sst1 receptors are preferentially involved in the somatotropic hypothalamic network. Neuropeptide Y (NPY)-containing neurons function as direct transducers for GH feedback. Interestingly, there is an overlap in the distribution of NPY and sst1 containing cells in the AN. To determine whether these NPY cells are target for somatostatin we used a double label in situ hybridization histochemistry. Image analysis revealed that approximately 7% of NPY-hybridizing neurons coexpressed sst1 mRNA. These results further support the evidence for the direct interactions between the somatotropic axis and the neuroendocrine regulatory loops of energy homeostasis.

Physiologic estradiol levels enhance hypothalamic expression of the long form of the leptin receptor in intact rats

Estradiol is a potent hypophagic agent that reduces food intake and body weight without a concomitant fall in plasma leptin levels. We investigated whether the hypophagic effect of estradiol is mediated by stimulating POMC and/or inhibiting NPY neuronal pathways in the hypothalamus, which respectively inhibit and stimulate feeding. We examined hypothalamic gene expression of Ob-Rb, NPY, POMC, MC4-R, and AgRP in intact Wistar rats treated with estradiol for 48 hours. Food intake and body weight were reduced in estradiol-treated rats but fat mass was unchanged; plasma leptin and insulin levels were not significantly different from untreated, freely fed controls. In untreated rats that were pair-fed to match the estradiol-treated group, body weight was also reduced without changes in fat mass, although leptin and insulin levels decreased significantly. Ob-Rb expression was increased in both hypophagic groups despite serum leptin were only decreased in pair-fed animals, suggesting an estradiol-stimulating effect on Ob-Rb expression. No significant differences were found in POMC, AgRP, or MC4-R expression among any of the experimental groups. A significant but small decrease in NPY expression was also found in both hypophagic groups; this was explained by the combined effect of both surgery and reduced food intake. These results indicate that estradiol mediated hypophagia in intact rats could be brought about by an enhanced hypothalamic leptin sensitivity but is unlikely to be driven by changes in NPY or melanocortin system.

The electrophysiology of feeding circuits

Obesity is quickly becoming one of the most common and debilitating disorders of the developed world. More than 60% of American adults are now overweight or obese, predisposing them to a host of chronic diseases. To understand the etiology of obesity, and to discover new therapies for obesity, we must understand the components of energy balance. In simple terms, energy intake (feeding) must equal energy expenditure (physical activity, basal metabolism and adaptive thermogenesis) for body weight homeostasis. To maintain homeostasis, neurocircuitry must sense both immediate nutritional status and the amount of energy stored in adipose tissue, and must be able to provide appropriate output to balance energy intake and energy expenditure. The brain receives various signals that carry information about nutritional and metabolic status including neuropeptide PYY(3-36), ghrelin, cholecystokinin, leptin, glucose and insulin. Circulating satiety signals access the brain either by "leakage" across circumventricular organs or transport across the blood-brain barrier. Signals can also activate sensory vagal terminals that innervate the whole gastrointestinal tract.

Pre-obese and obese agouti mice are sensitive to the anorectic effects of peptide YY(3-36) but resistant to ghrelin

OBJECTIVE

The role of the melanocortin system in the feeding effects of peripheral peptide YY(3-36) (PYY(3-36)) and ghrelin was investigated using the agouti (A(y)/a) mouse as a model of abnormal melanocortin signalling. Furthermore, we examined whether the ectopic expression of agouti protein in A(y)/a mice results in complete MC4-R inhibition, by studying the effects of peripheral alpha-melanocyte-stimulating hormone (alpha-MSH) and leptin on food intake.

DESIGN

Adult A(y)/a mice were studied in the pre-obese state (7-8 weeks) and obese state (14-15 weeks). Animals received PYY(3-36) (0.02 micromol/kg), NDP-alpha-MSH (0.2 micromol/kg), leptin (2 micromol/kg) (all 24 h fasted state) and ghrelin (0.2 micromol/kg) (fed state) by intraperitoneal (i.p.) injection. Age-matched A(y)/a controls received i.p. saline. A separate cohort of wild-type (WT), age-matched controls received the same peptide dose or saline. Food intake was measured at 1, 2, 4, 8 and 24 h post-injection and compared in all four groups. Plasma leptin-, ghrelin- and PYY-like immunoreactivity (IR) were measured using radioimmunoassay (RIA).

RESULTS

At 2 h post-injection, PYY(3-36) reduced food intake in pre-obese and obese A(y)/a mice, whereas ghrelin had no effect. Plasma ghrelin levels were significantly reduced in pre-obese and obese A(y)/a mice compared to WT controls. Peripheral administration of NDP-alpha-MSH and leptin acutely suppressed feeding (0-2 h) in pre-obese and obese A(y)/a mice.

CONCLUSIONS

Responsiveness of pre-obese and obese A(y)/a mice to PYY(3-36) suggests that the melanocortin system may not be essential for the anorectic effects of this peptide. Melanocortinergic antagonism by agouti protein in A(y)/a mice may be sufficient to block the effects of endogenous, but not exogenous PYY(3-36), alpha-MSH and leptin. The mechanism underlying ghrelin resistance in A(y)/a mice may result from antagonism of hypothalamic melanocortin receptors-4 by agouti protein, supporting a role for the melanocortin system in mediating ghrelin's actions.

Regulation of growth hormone secretagogue receptor gene expression in the arcuate nuclei of the rat by leptin and ghrelin

The anorexigenic and orexigenic hormones leptin and ghrelin act in opposition to one another. When leptin signaling is reduced, as in the Zucker fatty rat, or when circulating ghrelin is increased during fasting, the effect of ghrelin becomes more dominant, indicating an influence of both hormones on ghrelin action. This effect could be mediated via the level of expression of ghrelin receptor (growth hormone secretagogue receptor [GHS-R]). For testing this, GHS-R expression was measured using in situ hybridization in Zucker fatty versus lean rats; in fed versus fasted (48 h) rats, treated with either ghrelin or leptin; and in GH-deficient, dwarf versus control rats. In the arcuate nuclei of the Zucker fatty rat and in fasted rats, GHS-R expression is significantly increased. A single leptin intracerebroventricular injection attenuated the fasting-induced increase in GHS-R but had no effect in fed rats 2 h after injection, whereas leptin infusion for 24 h or longer significantly decreased GHS-R expression in fed rats. Ghrelin significantly increased GHS-R expression but not in dwarf rats. These results show that the level of GHS-R expression in the ARC is reduced by leptin and increased by ghrelin and that the effect of ghrelin may be GH dependent.

Obesity: the integrated roles of environment and genetics

Obesity represents one of the most serious global health issues with approximately 310 million people presently affected. It develops because of a mismatch between energy intake and expenditure that results from behavior (feeding behavior and time spent active) and physiology (resting metabolism and expenditure when active). Both of these traits are affected by environmental and genetic factors. The dramatic increase in the numbers of obese people in Western societies reflects mostly changing environmental factors and is linked to reduced activity and perhaps also increased food intake. However, in all societies and subpopulations, there are both obese and nonobese subjects. These differences are primarily a consequence of genetic factors as is revealed by the high heritability for body mass index. Most researchers agree that energy balance and, hence, body weight are regulated phenomena. There is some disagreement about exactly how this regulation occurs. However, a common model is the "lipostatic" regulation system, whereby our energy stores generate signals that are compared with targets encoded in the brain, and differences between these drive our food intake levels, activity patterns, and resting and active metabolisms. Considerable advances were made in the last decade in understanding the molecular basis of this lipostatic system. Some obese people have high body weight because they have broken lipostats, but these are a rare minority. This suggests that for the majority of obese people, the lipostat is set at an inappropriately high level. When combined with exposure to an environment where there is ready availability of food at low energy costs to obtain it, obesity develops. The evolutionary background to how such a system might have evolved involves the evolution of social behavior, the harnessing of fire, and the development of weapons that effectively freed humans from the risks of predation. The lipostatic model not only explains why some people become obese whereas others do not, but also allows us to understand why energy-controlled diets do not work. Drug-based solutions to the obesity problem that work with the lipostat, rather than against it, are presently under development and will probably be in regular use within 5-10 y. However, several lines of evidence including genetic mapping studies of quantitative trait loci associated with obesity suggest that our present understanding of the regulatory system is still rudimentary. In particular, we know nothing about how the target body weight in the brain is encoded. As our understanding in this field advances, new drug targets are likely to emerge and allow us to treat this crippling disorder.

Leptin-Receptor Gene Transfer into the Arcuate Nucleus of Female Fatty Zucker Rats Using Recombinant Adeno-Associated Viral Vectors Stimulates the Hypothalamo-Pituitary-Gonadal Axis1

Fatty fa/fa Zucker rats with a missense mutation in the leptin receptor (OB-R) are obese and infertile with prolonged estrous cycles. To determine whether their reproductive deficits could be corrected by OB-R installation, we employed viral vectors to introduce the OB-R gene into either the arcuate nucleus (ARC) or the paraventricular nucleus (PVN) of the hypothalamus, sites of OB-R expression in wild-type rats. Recombinant adeno-associated viral (rAAV) vectors encoding the human leptin-receptor gene (rAAV-OB-Rb) were microinjected intraparenchymally to produce doxycycline-regulatable OB-R gene expression. Expression of the OB-R gene in the ARC and PVN was verified using reverse transcription-polymerase chain reaction. Expression of OB-R in the ARC, but not in the PVN, resulted in normalization of estrous cycle length, increased ovarian follicular development, and decreased serum progesterone levels. Compared to saline-injected rats, hypothalamic expression of neuropeptide Y (NPY) and pro-opiomelanocortin were decreased in ARC rAAV-OB-Rb-injected rats. Parallel decreases were noted in NPY and beta-endorphin (beta-END) concentrations in the hypothalamus, whereas luteinizing hormone-releasing hormone (LHRH) levels increased. These studies showed that rAAV vectors can be successfully used to install functional OB-R in the hypothalamus for extended periods. The resultant stimulation of the hypothalamo-pituitary-gonadal (HPG) axis in ARC-injected rats was probably brought about by the observed decreases in NPY and beta-END, which inhibit hypothalamic LHRH. Because these changes were seen in ARC-injected, but not in PVN-injected, rats, the results suggest that the ARC may be the primary site where leptin acts to regulate the HPG axis.

Adaptation of ghrelin levels to limit body weight gain in the obese Zucker rat

In this study, we measured the ghrelin, leptin, and insulin variations in lean and obese Zucker fa/fa rats during the acute phase of body weight gain. At 2 months of age, plasma insulin and leptin concentrations in fa/fa rats were, respectively, 470% and 3700% higher than in lean rats (p <0.0001). Plasma ghrelin was significantly lower (-24.6%; p <0.02) than in lean rats. At 6 months of age, ghrelin increased in both genotypes but the difference was no more significant. The inverse correlations existing between ghrelin and either body weight (BW), insulin or leptin at 2 months of age were no more observable in 6-month-old rats. At 6 months of age, the lean rats had the same body weight as the 2-month-old obese rats. In these body weight-matched rats, ghrelin was not correlated with BW but it remained negatively correlated with insulin and leptin. At the same body weight, obese rats had a much lower plasma ghrelin than lean rats (717+/-42 vs. 1754+/-83 pg/ml; p <0.0001). These data indicate that body composition rather than body weight is the primary factor for the down-regulation of the ghrelin system. This down-regulation constitutes a mechanism of defense of the organism against the development of obesity at least during the first part of life.

Rapid rewiring of arcuate nucleus feeding circuits by leptin

The fat-derived hormone leptin regulates energy balance in part by modulating the activity of neuropeptide Y and proopiomelanocortin neurons in the hypothalamic arcuate nucleus. To study the intrinsic activity of these neurons and their responses to leptin, we generated mice that express distinct green fluorescent proteins in these two neuronal types. Leptin-deficient (ob/ob) mice differed from wild-type mice in the numbers of excitatory and inhibitory synapses and postsynaptic currents onto neuropeptide Y and proopiomelanocortin neurons. When leptin was delivered systemically to ob/ob mice, the synaptic density rapidly normalized, an effect detectable within 6 hours, several hours before leptin's effect on food intake. These data suggest that leptin-mediated plasticity in the ob/ob hypothalamus may underlie some of the hormone's behavioral effects.

Neuropeptide Y-Mediated Inhibition of Proopiomelanocortin Neurons in the Arcuate Nucleus Shows Enhanced Desensitization in ob/ob Mice

NPY and alphaMSH are expressed in distinct neurons in the arcuate nucleus of the hypothalamus, where alphaMSH decreases and NPY increases food intake and body weight. Here we use patch-clamp electrophysiology from GFP-labeled POMC and NPY neurons to demonstrate that NPY strongly hyperpolarized POMC neurons through the Y1R-mediated activation of GIRK channels, while the alphaMSH analog, MTII, had no effect on activity of NPY neurons. While initially NPY had similar effects on POMC neurons derived from ob/ob mice, further studies revealed a significant increase in desensitization of the NPY-induced currents in POMC neurons from ob/ob mice. This increase in desensitization was specific to NPY, as GABA(B) and microOR agonists showed unaltered desensitization in POMC neurons from ob/ob mice. These data reveal an intricate and asymmetric interplay between NPY and POMC neurons in the hypothalamus and have important implications for the delineation of the neural circuits that regulate feeding behavior.

Leptin resistance and enhancement of feeding facilitation by melanin-concentrating hormone in mice lacking bombesin receptor subtype-3

Mice lacking either bombesin receptor subtype (BRS)-3 or gastrin-releasing peptide receptor (GRP-R) exhibit feeding abnormalities. However, it is unclear how these receptors are associated with feeding regulation. In BRS-3-deficient mice, we found hyperphagia, subsequent hyperleptinemia, and brain leptin resistance that occurred after the onset of obesity. To explore the cause of this phenomenon, we examined changes in feeding responses to appetite-related neuropeptides in BRS-3-deficient, GRP-R-deficient, and wild-type littermate mice. Among orexigenic neuropeptides, the hyperphagic response to melanin-concentrating hormone (MCH) was significantly enhanced in BRS-3-deficient mice but not in GRP-R-deficient mice. In addition, the levels of MCH-R and prepro-MCH mRNAs in the hypothalamus of BRS-3-deficient mice were significantly more elevated than those of wild-type littermates. There was no significant difference in feeding between BRS-3-deficient and wild-type littermate mice after treatment with bombesin (BN), although the hypophagic response to low-dose BN was significantly suppressed in the GRP-R-deficient mice. These results suggest that upregulation of MCH-R and MCH triggers hyperphagia in BRS-3-deficient mice. From these results, we assume that the BRS-3 gene deletion upsets the mechanism by which leptin decreases the expression of MCH-R and that this effect may be mediated through neural networks independent of BN-related peptides such as GRP-R.

Adaptive responses in hypothalamic neuropeptide Y in the face of prolonged high-fat feeding in the rat

While a dysregulation in neuropeptide Y (NPY) signaling has been described in rodent models of obesity, few studies have investigated the time-course of changes in NPY content and responsiveness during development of diet-induced obesity. Therefore we investigated the effect of differing lengths (2-17 weeks) of high-fat diet on hypothalamic NPY peptide content, release and NPY-induced hyperphagia. Male Sprague-Dawley rats (211 +/- 3 g) were fed either a high-fat diet (30% fat) or laboratory chow (5% fat). Animals were implanted with intracerebroventricular cannulae to investigate feeding responses to NPY (0.5 nmol, 1 nmol) after 4 or 12 weeks of diet. At the earlier stage of obesity, NPY-induced hyperphagia was not altered; however, animals maintained on the high-fat diet for the longer duration were hyper-responsive to NPY, compared to chow-fed control rats (p < 0.05). Overall, hypothalamic NPY peptide content tended to be decreased from 9 to 17 weeks of diet (p < 0.05). Total hypothalamic NPY content was negatively correlated with plasma leptin concentration (p < 0.05), suggesting the hypothalamic NPY system remains responsive to leptin's inhibitory signal. In addition, hypothalamic NPY overflow was significantly reduced in high-fat fed animals (p < 0.05). Together these results suggest a reduction in hypothalamic NPY activity in high-fat fed animals, perhaps in an attempt to restore energy balance.

Leptin signaling in the hypothalamus: emphasis on energy homeostasis and leptin resistance

Leptin, the long-sought satiety factor of adipocytes origin, has emerged as one of the major signals that relay the status of fat stores to the hypothalamus and plays a significant role in energy homeostasis. Understanding the mechanisms of leptin signaling in the hypothalamus during normal and pathological conditions, such as obesity, has been the subject of intensive research during the last decade. It is now established that leptin action in the hypothalamus in regulation of food intake and body weight is mediated by a neural circuitry comprising of orexigenic and anorectic signals, including NPY, MCH, galanin, orexin, GALP, alpha-MSH, NT, and CRH. In addition to the conventional JAK2-STAT3 pathway, it has become evident that PI3K-PDE3B-cAMP pathway plays a critical role in leptin signaling in the hypothalamus. It is now established that central leptin resistance contributes to the development of diet-induced obesity and ageing associated obesity. Central leptin resistance also occurs due to hyperleptinimia produced by exogenous leptin infusion. A defective nutritional regulation of leptin receptor gene expression and reduced STAT3 signaling may be involved in the development of leptin resistance in DIO. However, leptin resistance in the hypothalamic neurons may occur despite an intact JAK2-STAT3 pathway of leptin signaling. Thus, in addition to defective JAK2-STAT3 pathway, defects in other leptin signaling pathways may be involved in leptin resistance. We hypothesize that defective regulation of PI3K-PDE3B-cAMP pathway may be one of the mechanisms behind the development of central leptin resistance seen in obesity.

Acute food deprivation and chronic food restriction differentially affect hypothalamic NPY mRNA expression

Although acute food deprivation and chronic food restriction both result in body weight loss, they produce different metabolic states. To evaluate how these two treatments affect hypothalamic peptide systems involved in energy homeostasis, we compared patterns of hypothalamic neuropeptide Y (NPY), agouti-related protein (AgRP), proopiomelanocotin (POMC), and leptin receptor gene expression in acutely food-deprived and chronically food-restricted rats. Both acute food deprivation and chronic food restriction reduced body weight and circulating leptin levels and resulted in increased arcuate NPY and decreased arcuate POMC gene expression. Arcuate AgRP mRNA levels were only elevated in acutely deprived rats. NPY gene expression was increased in the compact subregion of the dorsomedial hypothalamus (DMH) in response to chronic food restriction, but not in response to acute food deprivation. Leptin receptor expression was not affected by either treatment. Double in situ hybridization histochemistry revealed that, in contrast to the situation in the arcuate nucleus, NPY and leptin receptor mRNA-expressing neurons were not colocalized in the DMH. Together, these data suggest that arcuate and DMH NPY gene expression are differentially regulated. DMH NPY-expressing neurons do not appear to be under the direct control of leptin signaling.

Distribution, function, and properties of leptin receptors in the brain

Leptin, a peptide hormone, is implicated in the modulation of food intake and maintenance of energy balance in many vertebrates including humans. It is considered to act via its receptor mainly through several hypothalamic nuclei that play critical roles in the regulation of appetite. This article looks mainly at the functional significance of leptin in rat brain by drawing on published reports of morphological and physiological analyses. Our immunohistochemical observations indicate that the leptin receptor is distributed throughout the brain, including the hypothalamus, and interestingly, is found in the hippocampus and neocortex. Physiological experiments with single living cells isolated from fresh rat hypothalamus clearly demonstrate that leptin has a significant effect on feeding-regulating neurons in the hypothalamus. Studies to date support a role for leptin not only in modulating food intake and appetite in rats and humans, but also in relation to learning and memory processes.