Effect of fasting and leptin deficiency on hypothalamic neuropeptide Y gene transcription in vivo revealed by expression of a lacZ reporter gene

Metabolic sensing in AgRP neurons integrates homeostatic state with dopamine signalling in the striatum

Agouti-related peptide (AgRP) neurons increase motivation for food, however, whether metabolic sensing of homeostatic state in AgRP neurons potentiates motivation by interacting with dopamine reward systems is unexplored. As a model of impaired metabolic-sensing, we used the AgRP-specific deletion of carnitine acetyltransferase (Crat) in mice. We hypothesised that metabolic sensing in AgRP neurons is required to increase motivation for food reward by modulating accumbal or striatal dopamine release. Studies confirmed that Crat deletion in AgRP neurons (KO) impaired ex vivo glucose-sensing, as well as in vivo responses to peripheral glucose injection or repeated palatable food presentation and consumption. Impaired metabolic-sensing in AgRP neurons reduced acute dopamine release (seconds) to palatable food consumption and during operant responding, as assessed by GRAB-DA photometry in the nucleus accumbens, but not the dorsal striatum. Impaired metabolic-sensing in AgRP neurons suppressed radiolabelled 18F-fDOPA accumulation after ~30 min in the dorsal striatum but not the nucleus accumbens. Impaired metabolic sensing in AgRP neurons suppressed motivated operant responding for sucrose rewards during fasting. Thus, metabolic-sensing in AgRP neurons is required for the appropriate temporal integration and transmission of homeostatic hunger-sensing to dopamine signalling in the striatum.

Neurobiology: How to ask a mouse if it's hungry

Neural circuits influence food intake by responding to interoceptive hunger cues and/or hedonic cues. A new study utilizes a hunger discrimination behavioural task combined with opto- and chemo-genetic manipulation to identify hunger and non-hunger sensing neural circuits driving food intake.

Hypothalamic and Cell-Specific Transcriptomes Unravel a Dynamic Neuropil Remodeling in Leptin-Induced and Typical Pubertal Transition in Female Mice

Epidemiological and genome-wide association studies (GWAS) have shown high correlation between childhood obesity and advance in puberty. Early age at menarche is associated with a series of morbidities, including breast cancer, cardiovascular diseases, type 2 diabetes, and obesity. The adipocyte hormone leptin signals the amount of fat stores to the neuroendocrine reproductive axis via direct actions in the brain. Using mouse genetics, we and others have identified the hypothalamic ventral premammillary nucleus (PMv) and the agouti-related protein (AgRP) neurons in the arcuate nucleus (Arc) as primary targets of leptin action in pubertal maturation. However, the molecular mechanisms underlying leptin's effects remain unknown. Here we assessed changes in the PMv and Arc transcriptional program during leptin-stimulated and typical pubertal development using overlapping analysis of bulk RNA sequecing, TRAP sequencing, and the published database. Our findings demonstrate that dynamic somatodendritic remodeling and extracellular space organization underlie leptin-induced and typical pubertal maturation in female mice.

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MBH DEGs between lean and Lep^ob mice are highly represented in development

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Short-term leptin to Lep^ob mice alters MBH DEGs associated with reproduction

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PMv/Arc LepRb DEGs between lean and Lep^ob mice are abundant in extracellular space

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DEGs in developing PMv/Arc are conspicuous in extracellular and neuropil remodeling

MitoPark transgenic mouse model recapitulates the gastrointestinal dysfunction and gut-microbiome changes of Parkinson's disease

Gastrointestinal (GI) disturbances are one of the earliest symptoms affecting most patients with Parkinson's disease (PD). In many cases, these symptoms are observed years before motor impairments become apparent. Hence, the molecular and cellular underpinnings that contribute to this early GI dysfunction in PD have actively been explored using a relevant animal model. The MitoPark model is a chronic, progressive mouse model recapitulating several key pathophysiological aspects of PD. However, GI dysfunction and gut microbiome changes have not been categorized in this model. Herein, we show that decreased GI motility was one of the first non-motor symptoms to develop, evident as early as 8 weeks with significantly different transit times from 12 weeks onwards. These symptoms were observed well before motor symptoms developed, thereby paralleling PD progression in humans. At age 24 weeks, we observed increased colon transit time and reduced fecal water content, indicative of constipation. Intestinal inflammation was evidenced with increased expression of iNOS and TNFα in the small and large intestine. Specifically, iNOS was observed mainly in the enteric plexi, indicating enteric glial cell activation. A pronounced loss of tyrosine hydroxylase-positive neurons occurred at 24 weeks both in the mid-brain region as well as the gut, leading to a corresponding decrease in dopamine (DA) production. We also observed decreased DARPP-32 expression in the colon, validating the loss of DAergic neurons in the gut. However, the total number of enteric neurons did not significantly differ between the two groups. Metabolomic gas chromatography-mass spectrometry analysis of fecal samples showed increased sterol, glycerol, and tocopherol production in MitoPark mice compared to age-matched littermate controls at 20 weeks of age while 16 s microbiome sequencing showed a transient temporal increase in the genus Prevotella. Altogether, the data shed more light on the role of the gut dopaminergic system in maintaining intestinal health. Importantly, this model recapitulates the chronology and development of GI dysfunction along with other non-motor symptoms and can become an attractive translational animal model for pre-clinical assessment of the efficacy of new anti-Parkinsonian drugs that can alleviate GI dysfunction in PD.

AgRP Neurons Require Carnitine Acetyltransferase to Regulate Metabolic Flexibility and Peripheral Nutrient Partitioning

AgRP neurons control peripheral substrate utilization and nutrient partitioning during conditions of energy deficit and nutrient replenishment, although the molecular mechanism is unknown. We examined whether carnitine acetyltransferase (Crat) in AgRP neurons affects peripheral nutrient partitioning. Crat deletion in AgRP neurons reduced food intake and feeding behavior and increased glycerol supply to the liver during fasting, as a gluconeogenic substrate, which was mediated by changes to sympathetic output and peripheral fatty acid metabolism in the liver. Crat deletion in AgRP neurons increased peripheral fatty acid substrate utilization and attenuated the switch to glucose utilization after refeeding, indicating altered nutrient partitioning. Proteomic analysis in AgRP neurons shows that Crat regulates protein acetylation and metabolic processing. Collectively, our studies highlight that AgRP neurons require Crat to provide the metabolic flexibility to optimize nutrient partitioning and regulate peripheral substrate utilization, particularly during fasting and refeeding.

Anti-leptin receptor antibodies strengthen leptin biofunction in growing chickens

Antibodies against the extracellular domains of the chicken leptin receptor were used to study the biological function of leptin in growing chickens. Both polyclonal and monoclonal anti-LEPR antibodies were administered intramuscularly to 30-d-old Chinese indigenous Gushi pullets. Both antibody preparations increased feed intake for 6 h after injection and reduced plasma concentrations of glucose, triglycerides, and both high- and low-density lipoproteins. The antibody treatments also upregulated agouti-related peptide and neuropeptide Y in the hypothalamus and downregulated proopiomelanocortin, melanocortin 4 receptor, and leptin receptor. The treatments also upregulated leptin receptor, acetyl CoA carboxylase beta, and acyl-CoA oxidase in the liver, abdominal fat, and breast muscle and downregulated sterol regulatory element-binding protein-1 and fatty acid synthase. Furthermore, even though the anti-leptin receptor antibodies failed to affect leptin receptor signaling transduction when administered alone, they did augment the induction of leptin receptor signaling transduction by leptin. These results demonstrate that antibodies against the extracellular domains of leptin-specific receptor enhance, but do not mimic, the ability of leptin to activate receptors. Furthermore, the enhanced leptin bioactivity observed after the intramuscular injection of anti-LEPR antibodies confirmed the occurrence of de novo leptin in the peripheral tissues and blood of treated chickens.

Control of appetite, blood glucose, and blood pressure during melanocortin-4 receptor activation in normoglycemic and diabetic NPY-deficient mice

Although central melanocortin 4 receptor (MC4R) blockade abolishes the central nervous system (CNS)-mediated anorexogenic, antidiabetic, and cardiovascular actions of leptin, chronic MC4R stimulation fails to completely mimic the effects of leptin. Because neuropeptide Y (NPY) and MC4R exert opposite effects on cardiovascular and metabolic functions, we tested the role of NPY in offsetting the long-term actions of MC4R activation. Wild-type (WT) and NPY-deficient (NPY^-/-) mice were implanted with telemetry probes for measuring mean arterial pressure (MAP) and heart rate (HR) 24 h/day. After the mice recovered from surgery and stable baseline measurements, the MC3/4R agonist melanotan II (MTII, 120 μg·kg^-1·day^-1 iv) was infused for 7 days followed by a recovery period. No major differences between groups were observed at baseline except for slightly higher food intake and HR in NPY^-/- mice (4.3 ± 0.2 vs. 3.4 ± 0.2 g/day and 567 ± 14 vs. 522 ± 13 beats/min). Chronic MTII infusion reduced food intake in both groups while causing transient increases in MAP and HR only in WT mice (peaks of 11 ± 3 mmHg and 126 ± 13 beats/min). To examine whether NPY deficiency would amplify the antidiabetic effects of MC4R activation, diabetes was induced with streptozotocin (STZ) 1 wk before baseline measurements were taken, and the same experimental protocol was followed. In WT and NPY^-/- mice, STZ-induced diabetes led to similar hyperphagia, hyperglycemia, and weight loss, which were not reversed by chronic MTII treatment. Our results demonstrate that chronic MC4R activation, even in NPY-deficient mice, does not mimic chronic antidiabetic, cardiovascular, or metabolic actions of leptin, and that NPY is not essential for hyperphagia or cardiovascular changes associated with diabetes.

Effects of Leptin on Hypothalamic Arcuate Neurons in Wistar and Zucker Rats: An In Vitro Study

Leptin, a product of the ob gene, decreases food intake and body weight in both Wistar and Zucker obese rats when administered centrally or peripherally. To examine whether these leptin effects might be mediated through a neuropeptide Y (NPY) signaling pathway in the medial part of the arcuate nucleus of the hypothalamus (vmARC), the effects of leptin on vmARC neurons in Wistar and Zucker obese rats were examined electrophysiologically using brain slice preparations. Bath application of leptin inhibited about 60% of the vmARC neurons recorded in slices from Wistar rats. Similar inhibitory effects of leptin on vmARC neurons were also observed under low-Ca2+, high-Mg2+ Ringer's solution. However, inhibitory effects were almost absent under Ringer's solution containing a protein kinase C inhibitor, chelerythrine chloride. In slices from Zucker obese rats, leptin inhibited only about 25% of the vmARC neurons recorded, and the proportion of neurons inhibited was significantly smaller for these rats than for Wistar rats. These results suggest that reductions in food intake and body weight induced by leptin in both Wistar and Zucker obese rats are partly mediated via inhibition of an NPY signaling pathway in the vmARC.

No Change in Serum Incretins Levels but Rise of Leptin Levels After Smoking Cessation: a Pilot Study

The mechanisms behind the changes of body weight after smoking cessation are only partially understood. To this end, we explored the possible effects of smoking cessation on incretin hormones, leptin and selected anthropometric, biochemical and other hormonal parameters. Twenty-two non-obese male adult smokers attending an ambulatory smoking cessation program in Prague, Czech Republic, were examined at the baseline. Thirteen patients (mean age 37.92±2.66 years, mean body mass index 25.56±0.69 kg/m2) successfully quit smoking and were examined three months after smoking cessation; relapsed smokers were not followed up. The patients underwent 2-h liquid meal test with Fresubin and repeated blood sampling for measurements of blood glucose, gastric inhibitory polypeptide (GIP), glucagon-like peptide 1 (GLP-1), amylin, insulin, leptin, peptide-YY (PYY) and pancreatic polypeptide (PP). Three months after smoking cessation, body weight increased (4.35±3.32 kg, p<0.001). Leptin levels increased significantly in all repeated samples, while levels of GIP, GLP-1, amylin, insulin, PYY and PP remained unchanged. In conclusions, smoking cessation increased leptin levels probably owing to weight gain while it did not influence incretin levels.

60 YEARS OF NEUROENDOCRINOLOGY: The structure of the neuroendocrine hypothalamus: the neuroanatomical legacy of Geoffrey Harris

In November 1955, Geoffrey Harris published a paper based on the Christian A Herter Lecture he had given earlier that year at Johns Hopkins University in Baltimore, MD, USA. The paper reviewed the contemporary research that was starting to explain how the hypothalamus controlled the pituitary gland. In the process of doing so, Harris introduced a set of properties that helped define the neuroendocrine hypothalamus. They included: i) three criteria that putative releasing factors for adenohypophysial hormones would have to fulfill; ii) an analogy between the representation of body parts in the sensory and motor cortices and the spatial localization of neuroendocrine function in the hypothalamus; and iii) the idea that neuroendocrine neurons are motor neurons and the pituitary stalk functions as a Sherringtonian final common pathway through which the impact of sensory and emotional events on neuroendocrine neurons must pass in order to control pituitary hormone release. Were these properties a sign that the major neuroscientific discoveries that were being made in the early 1950s were beginning to influence neuroendocrinology? This Thematic Review discusses two main points: the context and significance of Harris's Herter Lecture for how our understanding of neuroendocrine anatomy (particularly as it relates to the control of the adenohypophysis) has developed since 1955; and, within this framework, how novel and powerful techniques are currently taking our understanding of the structure of the neuroendocrine hypothalamus to new levels.

The interaction between nutrition and the brain and its consequences for body weight gain and metabolism; studies in rodents and men

Aberrant feeding behavior can lead to obesity and obesity-related medical consequences, such as insulin resistance and diabetes. Although alterations in glucose metabolism (i.e. insulin resistance), in the presence of excessive fat tissue are often explained by the consequences of dysfunctional adipose tissue, evidence is emerging that also altered brain functions might be an important determinant of insulin resistance. In this review, we provide an overview of how feeding behavior and obesity interact with brain circuitry and how these interactions affect glucose metabolism. Because brain circuitries involved in food intake have been shown to partly control glucose metabolism as well, targeting these circuitries in obese subjects might not only affect food intake and body weight but also glucose metabolism.

Leptin and smoking cessation: secondary analyses of a randomized controlled trial assessing physical activity as an aid for smoking cessation

Background

Smokers have a lower body weight compared to non-smokers. Smoking cessation is associated with weight gain in most cases. A hormonal mechanism of action might be implicated in weight variations related to smoking, and leptin might be implicated. We made secondary analyses of an RCT, with a hypothesis-free exploratory approach to study the dynamic of leptin following smoking cessation.

Methods

We measured serum leptin levels among 271 sedentary smokers willing to quit who participated in a randomized controlled trial assessing a 9-week moderate-intensity physical activity intervention as an aid for smoking cessation. We adjusted leptin for body fat levels. We performed linear regressions to test for an association between leptin levels and the study group over time.

Results

One year after smoking cessation, the mean serum leptin change was +3.23 mg/l (SD 4.89) in the control group and +1.25 mg/l (SD 4.86) in the intervention group (p of the difference < 0.05). When adjusted for body fat levels, leptin was higher in the control group than in the intervention group (p of the difference < 0.01). The mean weight gain was +2.91 (SD 6.66) Kg in the intervention and +3.33 (SD 4.47) Kg in the control groups, respectively (p not significant).

Conclusions

Serum leptin levels significantly increased after smoking cessation, in spite of substantial weight gain. The leptin dynamic might be different in chronic tobacco users who quit smoking, and physical activity might impact the dynamic of leptin in such a situation.

Clinical trial registration number

NCT00521391

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2458-14-911) contains supplementary material, which is available to authorized users.

Evidence that diet-induced hyperleptinemia, but not hypothalamic gliosis, causes ghrelin resistance in NPY/AgRP neurons of male mice

High-fat diet (HFD) feeding causes ghrelin resistance in arcuate neuropeptide Y (NPY)/Agouti-related peptide neurons. In the current study, we investigated the time course over which this occurs and the mechanisms responsible for ghrelin resistance. After 3 weeks of HFD feeding, neither peripheral nor central ghrelin increased food intake and or activated NPY neurons as demonstrated by a lack of Fos immunoreactivity or whole-cell patch-clamp electrophysiology. Pair-feeding studies that matched HFD calorie intake with chow calorie intake show that HFD exposure does not cause ghrelin resistance independent of body weight gain. We observed increased plasma leptin in mice fed a HFD for 3 weeks and show that leptin-deficient obese ob/ob mice are still ghrelin sensitive but become ghrelin resistant when central leptin is coadministered. Moreover, ob/ob mice fed a HFD for 3 weeks remain ghrelin sensitive, and the ability of ghrelin to induce action potential firing in NPY neurons was blocked by leptin. We also examined hypothalamic gliosis in mice fed a chow diet or HFD, as well as in ob/ob mice fed a chow diet or HFD and lean controls. HFD-fed mice exhibited increased glial fibrillary acidic protein-positive cells compared with chow-fed mice, suggesting that hypothalamic gliosis may underlie ghrelin resistance. However, we also observed an increase in hypothalamic gliosis in ob/ob mice fed a HFD compared with chow-fed ob/ob and lean control mice. Because ob/ob mice fed a HFD remain ghrelin sensitive, our results suggest that hypothalamic gliosis does not underlie ghrelin resistance. Further, pair-feeding a HFD to match the calorie intake of chow-fed controls did not increase body weight gain or cause central ghrelin resistance; thus, our evidence suggests that diet-induced hyperleptinemia, rather than diet-induced hypothalamic gliosis or HFD exposure, causes ghrelin resistance.

JAK-STAT and feeding

The regulation of energy balance requires a complex system to homeostatically maintain the adult body at a precise set point. The central nervous system, particularly the hypothalamus, plays a key role in integrating a variety of signals that can relay information about the body's energy stores. As part of this system, numerous cytokines and hormones contribute to the regulation of food intake and energy homeostasis. Cytokines, and some hormones, are known to act through JAK-STAT intracellular signaling pathways. The hormone leptin, which plays a vital role in appetite regulation, signals through the JAK-STAT pathway, and it is through this involvement that the JAK-STAT pathway has become an established component in the mechanisms regulating food intake within the body. Emerging research, however, is now showing that this involvement of JAK-STAT is not limited to its activation by leptin. Furthermore, while the JAK-STAT pathway may simply act to transmit the anorectic signal of circulating factors, this intracellular signaling pathway may also become impaired when normal regulation of energy balance is disrupted. Thus, altered JAK-STAT signaling may contribute to the breakdown of the normal homeostatic mechanisms maintaining body weight in obesity.

An updated view of leptin on implantation and pregnancy: a review

The hormone leptin, which is thought to be primarily produced by adipose tissue, is a polypeptide that was initially characterized by its ability to regulate food intake and energy metabolism. Leptin appears to signal the status of body energy stores to the brain, resulting in the regulation of food intake and whole-body energy expenditure. Subsequently, it was recognized as a cytokine with a wide range of peripheral actions and is involved in the regulation of a number of physiological systems including reproduction. In the fed state, leptin circulates in the plasma in proportion to body adiposity in all species studied to date. However other factors such as sex, age, body mass index (BMI), sex steroids and pregnancy may also affect leptin levels in plasma. In pregnant mice and humans, the placenta is also a major site of leptin expression. Leptin circulates in biological fluids both as free protein and in a form that is bound to the soluble isoform of its receptor or other binding proteins such as one of the immunoglobulin superfamily members Siglec-6 (OB-BP1). Although the actions of leptin in the control of reproductive function are thought to be exerted mainly via the hypothalamic-pituitary-gonadal axis, there have also been reports of local direct effects of leptin at the peripheral level, however, these data appear contradictory. Therefore, there is a need to summarize the current status of research outcomes and analyze the possible reasons for differing results and thus provide researchers with new insight in designing experiments to investigate leptin effect on reproduction. Most importantly, our recent experimental data suggesting that reproductive performance is improved by decreasing concentrations of peripheral leptin was unexpected and cannot be explained by hypotheses drawn from the experiments of excessive exogenous leptin administration to normal animals or ob/ob mice.

Rictor/mTORC2 facilitates central regulation of energy and glucose homeostasis

Insulin signaling in the central nervous system (CNS) regulates energy balance and peripheral glucose homeostasis. Rictor is a key regulatory/structural subunit of the mTORC2 complex and is required for hydrophobic motif site phosphorylation of Akt at serine 473. To examine the contribution of neuronal Rictor/mTORC2 signaling to CNS regulation of energy and glucose homeostasis, we utilized Cre-LoxP technology to generate mice lacking Rictor in all neurons, or in either POMC or AgRP expressing neurons. Rictor deletion in all neurons led to increased fat mass and adiposity, glucose intolerance and behavioral leptin resistance. Disrupting Rictor in POMC neurons also caused obesity and hyperphagia, fasting hyperglycemia and pronounced glucose intolerance. AgRP neuron specific deletion did not impact energy balance but led to mild glucose intolerance. Collectively, we show that Rictor/mTORC2 signaling, especially in POMC-expressing neurons, is important for central regulation of energy and glucose homeostasis.

Exercise training does not enhance hypothalamic responsiveness to leptin or ghrelin in male mice

The detection of hormone and nutrient signals by the hypothalamus is blunted in obesity and contributes to dysregulated energy homeostasis. We investigated whether aerobic exercise training would improve long-term hypothalamic sensitivity to both leptin and ghrelin, independent of acute exercise-induced signalling. Male C57Bl/6J mice were fed either a chow or high-fat diet for 6 weeks, then remained sedentary on their respective diet, or completed 6 weeks of treadmill exercise training with a progressive increase in exercise volume and intensity. Food intake and hypothalamic signalling were assessed in mice injected with leptin or ghrelin at least 24 h after the last exercise bout. Exercise training reduced body mass, increased daily food intake and improved glucose tolerance. Intraperitoneal leptin administration reduced food intake in lean and obese mice, and this was not enhanced after exercise training. Leptin-mediated activation of phosphorylated signal transducer and activator of transcription 3 in the arcuate nucleus and ventromedial nucleus of the hypothalamus was not enhanced with exercise training. Ghrelin increased food intake and c-Fos positive neurones in the hypothalamus in lean and obese mice, and these physiological and molecular responses were not enhanced with exercise training. This suggests that the previously reported exercise effects on sensitising hypothalamic signalling and food intake responses may be limited to the period immediately after an exercise bout, and are not a result of stable structural or molecular changes that occur with exercise training.

Calorie-restricted weight loss reverses high-fat diet-induced ghrelin resistance, which contributes to rebound weight gain in a ghrelin-dependent manner

Twelve weeks of high-fat diet feeding causes ghrelin resistance in arcuate neuropeptide Y (NPY)/agouti-related protein (AgRP) neurons. In the current study, we investigated whether diet-induced weight loss could restore NPY/AgRP neuronal responsiveness to ghrelin and whether ghrelin mediates rebound weight gain after calorie-restricted (CR) weight loss. Diet-induced obese (DIO) mice were allocated to one of two dietary interventions until they reached the weight of age-matched lean controls. DIO mice received chow diet ad libitum or chow diet with 40% CR. Chow-fed and high-fat-fed mice served as controls. Both dietary interventions normalized body weight, glucose tolerance, and plasma insulin. We show that diet-induced weight loss with CR increases total plasma ghrelin, restores ghrelin sensitivity, and increases hypothalamic NPY and AgRP mRNA expression. We propose that long-term DIO creates a higher body weight set-point and that weight loss induced by CR, as seen in the high-fat CR group, provokes the brain to protect the new higher set-point. This adaptation to weight loss likely contributes to rebound weight gain by increasing peripheral ghrelin concentrations and restoring the function of ghrelin-responsive neuronal populations in the hypothalamic arcuate nucleus. Indeed, we also show that DIO ghrelin-knockout mice exhibit reduced body weight regain after CR weight loss compared with ghrelin wild-type mice, suggesting ghrelin mediates rebound weight gain after CR weight loss.

Minireview: The neural regulation of the hypothalamic-pituitary-thyroid axis

Thyroid hormone (TH) signaling plays an important role in development and adult life. Many organisms may have evolved under selective pressure of exogenous TH, suggesting that thyroid hormone signaling is phylogenetically older than the systems that regulate their synthesis. Therefore, the negative feedback system by TH itself was probably the first mechanism of regulation of circulating TH levels. In humans and other vertebrates, it is well known that TH negatively regulates its own production through central actions that modulate the hypothalamic-pituitary-thyroid (HPT) axis. Indeed, primary hypothyroidism leads to the up-regulation of the genes encoding many key players in the HPT axis, such as TRH, type 2 deiodinase (dio2), pyroglutamyl peptidase II (PPII), TRH receptor 1 (TRHR1), and the TSH α- and β-subunits. However, in many physiological circumstances, the activity of the HPT axis is not always a function of circulating TH concentrations. Indeed, circadian changes in the HPT axis activity are not a consequence of oscillation in circulating TH levels. Similarly, during reduced food availability, several components of the HPT axis are down-regulated even in the presence of lower circulating TH levels, suggesting the presence of a regulatory pathway hierarchically higher than the feedback system. This minireview discusses the neural regulation of the HPT axis, focusing on both TH-dependent and -independent pathways and their potential integration.

Neuron-specific deletion of peroxisome proliferator-activated receptor delta (PPARδ) in mice leads to increased susceptibility to diet-induced obesity

Central nervous system (CNS) lipid accumulation, inflammation and resistance to adipo-regulatory hormones, such as insulin and leptin, are implicated in the pathogenesis of diet-induced obesity (DIO). Peroxisome proliferator-activated receptors (PPAR α, δ, γ) are nuclear transcription factors that act as environmental fatty acid sensors and regulate genes involved in lipid metabolism and inflammation in response to dietary and endogenous fatty acid ligands. All three PPAR isoforms are expressed in the CNS at different levels. Recent evidence suggests that activation of CNS PPARα and/or PPARγ may contribute to weight gain and obesity. PPARδ is the most abundant isoform in the CNS and is enriched in the hypothalamus, a region of the brain involved in energy homeostasis regulation. Because in peripheral tissues, expression of PPARδ increases lipid oxidative genes and opposes inflammation, we hypothesized that CNS PPARδ protects against the development of DIO. Indeed, genetic neuronal deletion using Nes-Cre loxP technology led to elevated fat mass and decreased lean mass on low-fat diet (LFD), accompanied by leptin resistance and hypothalamic inflammation. Impaired regulation of neuropeptide expression, as well as uncoupling protein 2, and abnormal responses to a metabolic challenge, such as fasting, also occur in the absence of neuronal PPARδ. Consistent with our hypothesis, KO mice gain significantly more fat mass on a high-fat diet (HFD), yet are surprisingly resistant to diet-induced elevations in CNS inflammation and lipid accumulation. We detected evidence of upregulation of PPARγ and target genes of both PPARα and PPARγ, as well as genes of fatty acid oxidation. Thus, our data reveal a previously underappreciated role for neuronal PPARδ in the regulation of body composition, feeding responses, and in the regulation of hypothalamic gene expression.

Increased feeding and food hoarding following food deprivation are associated with activation of dopamine and orexin neurons in male Brandt's voles

Small mammals usually face energetic challenges, such as food shortage, in the field. They have thus evolved species-specific adaptive strategies for survival and reproductive success. In the present study, we examined male Brandt's voles (Lasiopodomys brandtii) for their physiological, behavioral, and neuronal responses to food deprivation (FD) and subsequent re-feeding. Although 48 hr FD induced a decrease in body weight and the resting metabolic rate (RMR), such decreases did not reach statistical significance when compared to the control males that did not experience FD. During the first 2 hr of re-feeding following 48 hr FD, voles showed higher levels of feeding than controls. However, when permitted to hoard food, FD voles showed an increase in food hoarding, rather than feeding, compared to the controls. Further, both feeding and food hoarding induced an increase in neuronal activation, measured by Fos-ir, in a large number of brain areas examined. Interestingly, feeding and food hoarding also induced an increase in the percentage of tyrosine hydroxylase immunoreactive (TH-ir) cells that co-expressed Fos-ir in the ventral tegmental area (VTA), whereas both FD and feeding induced an increase in the percentage of orexin-ir cells that co-expressed Fos-ir in the lateral hypothalamus (LH). Food hoarding also increased orexin-ir/Fos-ir labeling in the LH. Together, our data indicate that food-deprived male Brandt's voles display enhanced feeding or food hoarding dependent upon an environmental setting. In addition, changes in central dopamine and orexin activities in selected brain areas are associated with feeding and hoarding behaviors following FD and subsequent re-feeding.

PACAP neurons in the hypothalamic ventromedial nucleus are targets of central leptin signaling

The adipose-derived hormone, leptin, was discovered over 10 years ago, but only now are we unmasking its downstream pathways which lead to reduced energy intake (feeding) and increased energy expenditure (thermogenesis). Recent transgenic models have challenged the long-standing supposition that the hypothalamic arcuate nucleus (Arc) is omnipotent in the central response to leptin, and research focus is beginning to shift to examine roles of extra-arcuate sites. Dhillon et al. (2006) demonstrated that targeted knock out of the signaling form of the leptin receptor (lepr-B) in steroidogenic factor 1 (SF-1) cells of the hypothalamic ventromedial nucleus (VMN) produces obesity of a similar magnitude to the pro-opiomelanocortin (POMC)-driven lepr-B deleted mouse, via a functionally distinct mechanism. These findings reveal that SF-1 cells of the VMN could be equally as important as POMC cells in mediating leptin's anti-obesity effects. However, the identification of molecular and cellular correlates of this relationship remains tantalizingly unknown. Here, we have shown that mRNA expression of the VMN-expressed neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is regulated according to energy status and that it exerts catabolic effects when administered centrally to mice. Furthermore, we have shown that SF-1 and PACAP mRNAs are colocalized in the VMN, and that leptin signaling via lepr-B is required for normal PACAP expression in these cells. Finally, blocking endogenous central PACAP signaling with the antagonist PACAP(6-38) markedly attenuates leptin-induced hypophagia and hyperthermia in vivo. Thus, it appears that PACAP is an important mediator of central leptin effects on energy balance.

Corticotropin-releasing factor-overexpressing mice exhibit reduced neuronal activation in the arcuate nucleus and food intake in response to fasting

Corticotropin-releasing factor (CRF) overexpressing (OE) mice are a genetic model that exhibits features of chronic stress. We investigated whether the adaptive feeding response to a hypocaloric challenge induced by food deprivation is impaired under conditions of chronic CRF overproduction. Food intake response to a 16-h overnight fast and ip injection of gut hormones regulating food intake were compared in CRF-OE and wild type (WT) littermate mice along with brain Fos expression, circulating ghrelin levels, and gastric emptying of a nonnutrient meal. CRF-OE mice injected ip with saline showed a 47 and 44% reduction of 30-min and 4-h cumulative food intake response to an overnight fast, respectively, compared with WT. However, the 30-min food intake decrease induced by ip cholecystokinin (3 microg/kg) and increase by ghrelin (300 microg/kg) were similar in CRF-OE and WT mice. Overnight fasting increased the plasma total ghrelin to similar levels in CRF-OE and WT mice, although CRF-OE mice had a 2-fold reduction of nonfasting ghrelin levels. The number of Fos-immunoreactive cells induced by fasting in the arcuate nucleus was reduced by 5.9-fold in CRF-OE compared with WT mice whereas no significant changes were observed in other hypothalamic nuclei. In contrast, fasted CRF-OE mice displayed a 5.6-fold increase in Fos-immunoreactive cell number in the dorsal motor nucleus of the vagus nerve and a 34% increase in 20-min gastric emptying. These findings indicate that sustained overproduction of hypothalamic CRF in mice interferes with fasting-induced activation of arcuate nucleus neurons and the related hyperphagic response.

Effects of age and strain on yolk sac utilization and leptin levels in newly hatched broilers

The dynamics of yolk sac utilization and changes of leptin levels in serum, hypothalamus, and yolk sac with age were investigated in Beijing-You (BY) and Arbor Acres (AA) male broilers during 11 d after hatch. The growth rate and feed intake of BY broilers were lower (P < 0.0001) than those of AA broilers, but the dynamics of the weights and total energy contents of yolk sacs were similar between both strains and decreased exponentially with age. Leptin levels in yolk sacs of both broiler strains increased with age during 3 d posthatching. Compared with those of AA broilers, leptin levels in yolk sacs of BY broilers were greater (P <or= 0.0413) on d 0 and 3. There was no change in serum leptin levels in BY broilers, whereas in AA broilers, serum leptin levels on d 1 and 3 were greater (P <or= 0.0306) than that on d 0 and then decreased with age. Compared with AA broilers, BY broilers showed lower (P <or= 0.0254) levels of serum leptin on d 1 and 3. Hypothalamic leptin levels of both strains decreased with age except AA broilers on d 0. Hypothalamic neuropeptide Y (NPY) levels of BY and AA broilers increased with age until d 7 and then decreased. There were no differences in hypothalamic leptin and NPY levels between both strains during 11 d after hatch. Correlation analysis showed that average daily feed intake had a negative correlation with serum and hypothalamic leptin and positive correlation with hypothalamic NPY. Our results indicated that the dynamics of yolk sac utilization were similar between BY and AA broilers and decreased exponentially with age. The developmental changes of leptin and NPY in serum and hypothalamus with age varied in parameter and strain, and both signal molecules might be involved in the early programming of feed intake in chickens, but the mechanisms need further studies.

Regulation of agouti-related protein messenger ribonucleic acid transcription and peptide secretion by acute and chronic inflammation

Agouti-related protein (AgRP) is an orexigenic neuropeptide produced by neurons in the hypothalamic arcuate nucleus (ARC) that is a key component of central neural circuits that control food intake and energy expenditure. Disorders in energy homeostasis, characterized by hypophagia and increased metabolic rate, frequently develop in animals with either acute or chronic diseases. Recently, studies have demonstrated that proopiomelanocortin-expressing neurons in the ARC are activated by the proinflammatory cytokine IL-1beta. In the current study, we sought to determine whether inflammatory processes regulate the expression of AgRP mRNA and to characterize the response of AgRP neurons to IL-1beta. Here, we show by real-time RT-PCR and in situ hybridization analysis that AgRP mRNA expression in rodents is increased in models of acute and chronic inflammation. AgRP neurons were found to express the type I IL-1 receptor, and the percentage of expression was significantly increased after peripheral administration of lipopolysaccharide. Furthermore, we demonstrate that IL-1beta inhibits the release of AgRP from hypothalamic explants. Collectively, these data indicate that proinflammatory signals decrease the secretion of AgRP while increasing the transcription of the AgRP gene. These observations suggest that AgRP neurons may participate with ARC proopiomelanocortin neurons in mediating the anorexic and metabolic responses to acute and chronic disease processes.

Food deprivation increases the mRNA expression of micro-opioid receptors in the ventral medial hypothalamus and arcuate nucleus

Activation of micro-opioid receptors makes animals hyperphagic and increases their preference for a high-fat diet. Previous studies have suggested that this receptor population plays a role in mediating the hyperphagia that is associated with food deprivation. In this paper, we tested the hypothesis that food deprivation will increase the expression of micro-opioid receptors in the ventral medial hypothalamus and arcuate nucleus (VMH/ARC). Food deprivation resulted in a significant increase in the mRNA expression of micro-opioid receptors in the VMH/ARC and the lateral hypothalamus (LH) after 48 h of fasting but not after 24 or 12 h of fasting in either the light or dark. We did not observe a change in the mRNA expression of kappa- or delta-opioid receptors after food deprivation. When food-deprived animals were given a choice between a low-fat diet and a high-fat diet, they were hyperphagic and consumed significantly more of the high-fat diet. When the micro-opioid receptors were blocked with beta-funaltrexamine (selective mu-opioid receptor antagonist), prior to giving food-deprived animals access to both a low-fat and high-fat diet, it significantly decreased the percentage of high-fat diet consumed. These data demonstrate that hypothalamic micro-opioid receptors may contribute to the hyperphagia and increased preference for a high-fat diet that is associated with food deprivation.

Maternal nutrition and the programming of obesity: The brain

The increasing incidence of obesity in the developed and developing world in the last decade has led to a need to define our understanding of the physiological mechanisms which can predispose individuals to weight gain in infancy, childhood and adulthood. There is now a considerable body of evidence which has shown that the pathway to obesity may begin very early in life, and that exposure to an inappropriate level of nutrition during prenatal and/or early postnatal development can predispose individuals to obesity in later life The brain is at the heart of the regulation of appetite and food preferences, and it is increasingly being recognized that the development of central appetitive structures is acutely sensitive to the nutritional environment both before and immediately after birth. This review will summarize the body of work which has highlighted the critical role of the brain in the early origins of obesity and presents some perspectives as to the potential application of these research findings in the clinical setting.

Role of agouti-related protein-expressing neurons in lactation

Hypothalamic neurons that express agouti-related protein (AgRP) and neuropeptide Y (NPY) are thought to be important for regulation of feeding, especially under conditions of negative energy balance. The expression of NPY and AgRP increases during lactation and may promote the hyperphagia that ensues. We explored the role of AgRP neurons in reproduction and lactation, using a mouse model in which AgRP-expressing neurons were selectively ablated by the action of diphtheria toxin. We show that ablation of AgRP neurons in neonatal mice does not interfere with pregnancy, parturition, or lactation, suggesting that early ablation allows compensatory mechanisms to become established. However, ablation of AgRP neurons after lactation commences results in rapid starvation, indicating that both basal feeding and lactation-induced hyperphagia become dependent on AgRP neurons in adulthood. We also show that constitutive inactivation of Npy and Agrp genes does not prevent pregnancy or lactation, nor does it protect lactating dams from diphtheria toxin-induced starvation.

Chronic fasting-induced changes of neuropeptide Y immunoreactivity in the lateral septum of intact and ovariectomized female rats

The effect of 40% food deprivation for 1 week on the immunohistochemically detectable amount of neuropeptide Y (NPY) was studied in the lateral septum (LS) of intact and ovariectomized (OVX) female rats. Animals were either fed ad libitum or 40% food-deprived. Densitometric analysis of immunostained material showed a significant decrease in NPY-immunoreactivity (NPY-IR) in OVX rats compared to the control group. Food deprivation increased the density of punctate NPY-IR profiles in both intact and OVX animals, however, the density in food-deprived OVX animals was increased compared to baseline but remained reduced compared to intact rats. Our study indicates that the lack of gonadal hormones - most likely estrogen - results in a decrease in the density of NPY-IR axonal fibers within the LS, while food deprivation induced considerable elevation in NPY density. Food restriction-induced changes in the density of NPY-containing neural elements suggest that the LS may play a crucial role in the regulation of food intake and energy balance, in concert with the relevant hypothalamic areas.

gp130 receptor ligands as potential therapeutic targets for obesity

Obesity and its related cluster of pathophysiologic conditions including insulin resistance, glucose intolerance, dyslipidemia, and hypertension are recognized as growing threats to world health. It is now estimated that 10% of the world's population is overweight or obese. As a result, new therapeutic options for the treatment of obesity are clearly warranted. Recent research has focused on the role that gp130 receptor ligands may play as potential therapeutic targets in obesity. One cytokine in particular, ciliary neurotrophic factor (CNTF), acts both centrally and peripherally and mimics the biologic actions of the appetite control hormone leptin, but unlike leptin, CNTF appears to be effective in obesity and as such may have therapeutic potential. In addition, CNTF suppresses inflammatory signaling cascades associated with lipid accumulation in liver and skeletal muscle. This review examines the potential role of gp130 receptor ligands as part of a therapeutic strategy to treat obesity.

FAS inhibitor cerulenin reduces food intake and melanocortin receptor gene expression without modulating the other (an)orexigenic neuropeptides in chickens

Cerulenin, a natural fatty acid synthase (FAS) inhibitor, and its synthetic analog C75 are hypothesized to alter the metabolism of neurons in the hypothalamus that regulate ingestive behavior to cause a profound decrease of food intake and an increase in metabolic rate, leading to body weight loss. The bulk of data exclusively originates from mammals (rodents); however, such effects are currently lacking in nonmammalian species. We have, therefore, addressed this issue in broiler chickens because this species is selected for high growth rate and high food intake and is prone to obesity. First, we demonstrate that FAS messenger and protein are expressed in the hypothalamus of chickens. FAS immunoreactivity was detected in a number of brain regions, including the nucleus paraventricularis magnocellularis and the nucleus infundibuli hypothalami, the avian equivalent of the mammalian arcuate nucleus, suggesting that FAS may be involved in the regulation of food intake. Second, we show that hypothalamic FAS gene expression was significantly (P < 0.05) decreased by overnight fasting similar to that in liver, indicating that hypothalamic FAS gene is regulated by energy status in chickens. Finally, to investigate the physiological consequences of in vivo inhibition of fatty acid synthesis on food intake, we administered cerulenin by intravenous injections (15 mg/kg) to 2-wk-old broiler chickens. Cerulenin administration significantly reduced food intake by 23 to 34% (P < 0.05 to P < 0.0001) and downregulated FAS and melanocortin receptors 1, 4, and 5 gene expression (P < 0.05). However, the known orexigenic (neuropeptide Y, agouti gene-related peptide, orexin, and orexin receptor) and anorexigenic (pro-opiomelanocortin and corticotropin-releasing hormone) neuropeptide mRNA levels remained unchanged after cerulenin treatment. These results suggest that the catabolic effect of cerulenin in chickens may be mediated through the melanocortin system rather than the other neuropeptides known to be involved in food intake regulation.

Helicobacter pylori and overweight status in the United States: data from the Third National Health and Nutrition Examination Survey

Obesity is an important public health problem in the United States. Because of its potential effects on gastric leptin homeostasis, Helicobacter pylori may play a role in regulating body weight. The authors' aim in this study was to examine the association between H. pylori colonization and overweight status. Nonpregnant participants in the Third National Health and Nutrition Examination Survey (1988-1994) aged > or = 20 years who had had H. pylori testing performed and body mass index (weight (kg)/height (m2)) measured were studied. Overweight was defined as a body mass index greater than or equal to 25. On the basis of serologic results, the participants were categorized into three H. pylori status groups: H. pylori-positive and cytotoxin-associated gene A (cagA)-positive (H. pylori+ cagA+), H. pylori-positive and cagA-negative (H. pylori+ cagA-), and H. pylori-negative (H. pylori-). Of the 7,003 subjects with complete body mass index and H. pylori data, 2,634 (weighted percentage, 22.9%) were H. pylori+ cagA+, 1,385 (15.1%) were H. pylori+ cagA-, and 2,984 (62.0%) were H. pylori-. The adjusted odds of being overweight were 1.17 (95% confidence interval: 0.98, 1.39; p = 0.075) for the H. pylori+ cagA+ group and 0.99 (95% confidence interval: 0.80, 1.22; p = 0.92) for the H. pylori+ cagA- group in comparison with H. pylori- subjects. Serum leptin levels did not differ significantly between the three H. pylori groups. In this US population-based study, there was no significant association between H. pylori colonization, cagA+ strains of H. pylori, and being overweight.

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.

The role of insulin and the adipocytokines in regulation of vascular endothelial function

Vascular integrity in the healthy endothelium is maintained through the release of a variety of paracrine factors such as NO (nitric oxide). Endothelial dysfunction, characterized by reduced NO bioavailability, is associated with obesity, insulin resistance and Type II diabetes. Insulin has been demonstrated to have direct effects on the endothelium to increase NO bioavailability. Therefore altered insulin signalling in the endothelium represents a candidate mechanism underlying the association between insulin resistance and endothelial dysfunction. In recent years, it has become apparent that insulin sensitivity is regulated by the adipocytokines, a group of bioactive proteins secreted by adipose tissue. Secretion of adipocytokines is altered in obese individuals and there is increasing evidence that the adipocytokines have direct effects on the vascular endothelium. A number of current antidiabetic strategies have been demonstrated to have beneficial effects on endothelial function and to alter adipocytokine concentrations in addition to their effects on glucose homoeostasis. In this review we will explore the notion that the association between insulin resistance and endothelial dysfunction is accounted for by adipocytokine action on the endothelium. In addition, we examine the effects of weight loss, exercise and antidiabetic drugs on adipocytokine availability and endothelial function.

Interactions between leptin, neuropeptide-Y and insulin with chronic diurnal fasting during Ramadan

BACKGROUND

Fasting during the month of Ramadan for Muslims is a unique metabolic model that includes abstinence from food and fluid intake during the period from dawn to sunset as well as a reduction in meal frequency and alterations in the sleep-wakefulness cycle. Leptin, neuropeptide-Y and insulin are thought to play an important role in long-term regulation of caloric intake and energy expenditure. However, the long-term changes and interactions between these factors during this pattern of fasting are not known.

SUBJECTS AND METHODS

The study was conducted on 46 healthy female volunteers (age, 22+/-2 years; BMI, 25.3+/-0.7 kg/m2). Anthropometrical measurements, estimation of body fat and fasting serum levels of neuropeptide Y, leptin, insulin and glucose were estimated at baseline (day 1), days 14 and 28 of the month of Ramadan and 2 weeks after Ramadan.

RESULTS

Baseline serum levels of leptin correlated positively with body fat (r=0.87, P=0.0002). Serum leptin levels exhibited a significant increase by approximately 41% and neuropeptide-Y levels were decreased by 30.4% throughout the month of Ramadan. In addition, a significant correlation (r=0.63, P=0.0001) was found between changes in serum leptin and serum insulin. However, changes in serum neuropeptide-Y levels did not correlate with those of leptin or insulin

CONCLUSIONS

Long-term fasting with interrupted nocturnal eating is associated with significant elevations in serum leptin and insulin and reduction in serum neuropeptide-Y. The changes in serum leptin are likely mediated through insulin. However, changes in neuropeptide-Y appears to be mediated independently of leptin or insulin during this type of fasting

Metabolic fuels, neuropeptide Y, and estrous behavior in Syrian hamsters

Of the various environmental factors influencing reproduction, food availability plays a particularly significant role, and an insufficient supply of oxidizable metabolic fuels inhibits reproduction in female mammals. When ovariectomized, steroid-primed hamsters are food deprived for 48 h, estrous behavior is suppressed. However, the specific neuroendocrine alterations that mediate the suppression of estrous behavior are unknown. Several conditions that inhibit female sexual behavior are thought to be associated with altered neuropeptide Y (NPY) activity in the brain. Intracerebroventricular (ICV) infusion of NPY inhibits estrous behavior in ovariectomized steroid-primed rats and hamsters. Furthermore, food-deprived rats have an increase in NPY mRNA in the arcuate nucleus (ARC) of the hypothalamus. Unlike rats, studies in Syrian hamsters have failed to detect any alterations in ARC NPY mRNA following food deprivation. Here we show that ARC NPY immunoreactivity and mRNA is increased in food-deprived hamsters but not in hamsters given other metabolic challenges that inhibit estrous behavior. These findings support the hypothesis that NPY contribute to, but not be critical for, the nutritional inhibition of sexual receptivity.

Links between the appetite regulating systems and the neuroendocrine hypothalamus: lessons from the sheep

The hypothalamus is integral to the regulation of energy homeostasis and the secretion of hormones from the pituitary gland. Consequently, hypothalamic systems may have a dual purpose in regulating both neuroendocrine function and appetite. To date, most studies investigating the interface between appetite and hormone secretion have been performed in rats or mice that have been acutely fasted or baring a genetic abnormality causing either obesity or aphagia. By contrast, various physiological models, including chronic food-restriction or photoperiodically driven changes in voluntary food intake, add further perspective to the issue. In this regard, sheep provide an innovative model whereby long-term changes in body weight or extended feeding rhythms can be investigated. This review compares and contrasts data obtained in different species with regard to the neuroendocrinology of appetite, and discusses the benefits and knowledge gained from using various nonrodent models with a particular emphasis on a ruminant species.

Abnormal response of the neuropeptide Y-deficient mouse reproductive axis to food deprivation but not lactation

Neuropeptide Y (NPY) plays a key role in both food intake and GnRH secretion. Food deprivation elevates hypothalamic NPY activity and suppresses LH and gonadal steroid secretion. Similarly, lactation up-regulates NPY expression as food consumption increases and estrous cycles cease. These observations suggest that NPY coordinates reproductive suppression in response to energy deficiency; if so, the reproductive axis of NPY knockout (KO) mice should be impervious to lactation and food deprivation. We monitored food consumption, body weight, and estrous cyclicity during lactation in NPY KO mice with large and small litters. NPY KO mice with either litter size resembled wild types (WTs) in weight regulation and food consumption. Large-litter mothers had longer anestrous periods and smaller pups at weaning, but NPY KOs and WTs did not differ in either respect. We also examined the LH response of NPY KO mice to 48 h without food. Basal levels of LH in ovariectomized NPY KO animals decreased in response to fasting, but LH levels in intact and estrogen-treated ovariectomized NPY KO animals did not. In contrast, WTs consistently showed fasting-induced suppression of LH. Our findings suggest that other systems can sustain the hyperphagia of lactation and NPY alone is not responsible for suppressing cyclicity during lactation. Nevertheless, the suppression of basal LH release that accompanies food deprivation in normal female mice appears to require the steroid-dependent actions of NPY.

Leptin directly acts within the hypothalamus to stimulate gonadotropin-releasing hormone secretion in vivo in rats

It is still not known whether leptin, an adipocyte-derived hormone, acts directly within the hypothalamus to stimulate the gonadotropin-releasing hormone (GnRH)-luteinizing hormone (LH) system. In order to address this question, the present study examined the effects of direct intrahypothalamic perfusions with leptin on the in vivo release of GnRH in ovarian steroid-primed ovariectomized rats utilizing the push-pull perfusion technique. Both alpha-melanocyte-stimulating hormone (alpha-MSH) and neuropeptide Y were also measured in the hypothalamic perfusates. In normally fed animals, the leptin infusion was without effect on the release of these three hypothalamic peptides and also without effect on plasma LH and prolactin (PRL), whether leptin was infused into the medial preoptic area (where the majority of GnRH neuronal cell bodies exist) or the median eminence-arcuate nucleus complex (where axon terminals of GnRH neurons are located). In contrast, in 3-day fasted rats leptin was effective in stimulating the secretion of GnRH, alpha-MSH, and LH, regardless of the site of perfusion. These three hormones were increased in a temporal order of alpha-MSH, GnRH and LH. Irrespective of the site of perfusion, leptin was without effect on the release of neuropeptide Y. Only when leptin was infused into the median eminence-arcuate nucleus complex was PRL secretion also stimulated, although its onset was 1 h behind that of LH. The leptin-induced elevations of GnRH, alpha-MSH, LH and PRL were all dose-dependently stimulated by subnormal (1.0 ng ml(-1)) and normal (3.0 ng ml(-1)) concentrations of leptin, but at higher concentrations (10 ng ml(-1)) it did not produce additional effects. Leptin infusion into the anterior hypothalamic area, a control site equidistant from both the medial preoptic area and the median eminence-arcuate nucleus complex, did not produce a significant change in any of the hormones in either the fed or fasted rats. These results demonstrate for the first time that leptin can act at both the cell bodies and axon terminals of GnRH neurons to stimulate the release of the neurohormone in vivo, and they also suggest that alpha-MSH may play a significant intermediary role in linking leptin and GnRH secretion.

Leptin regulates growth hormone-releasing factor, somatostatin, and alpha-melanocyte-stimulating hormone but not neuropeptide Y release in rat hypothalamus in vivo: relation with growth hormone secretion

It is known that leptin, an adipocyte-derived hormone, exerts a stimulatory effect on growth hormone (GH) secretion in various animal species. However, no previous study examined in vivo whether leptin affects the secretion of GH-releasing factor (GRF), somatostatin (SRIH), and some other closely relevant neurohormones in the hypothalamus. Therefore, in this study we investigated the effects of direct leptin infusion into the hypothalamus on the in vivo release of GRF, SRIH, alpha-melanocyte-stimulating hormone (alpha-MSH), and neuropeptide Y (NPY) in freely moving adult male rats using the push-pull perfusion. Leptin was infused into the median eminence-arcuate nucleus complex at three different concentrations, i.e., 1.0 (normal feeding level), 3.0, and 10 ng/ml (mild obesity level). In normally fed rats, only 10 ng/ml leptin was able to stimulate GH secretion, whereas in 3 d fasted rats, GH release was dose-dependently stimulated by 1.0 and 3.0 ng/ml leptin, although its 10 ng/ml dose did not produce additional effects. The facilitation of GH secretion occurred as increased pulse amplitudes without significant changes in the pulse frequency. During the leptin infusion, the hypothalamic GRF increased and SRIH decreased in magnitudes that approximately paralleled those of GH changes. Leptin stimulated the release of alpha-MSH in the fasted but not fed rats. It is likely that the fasting-induced increase in the hypothalamic alpha-MSH sensitivity to leptin is relevant to ingestive behavior involving leptin. Leptin was without effect on NPY release in either the fed or fasted group. Although it is certain that NPY mediates at least part of the metabolic actions of leptin, NPY is unlikely to be involved in the acute effects of leptin on GH, GRF, and SRIH secretion. These results demonstrate for the first time that leptin can alter the in vivo release of both GRF and SRIH in rat hypothalamus concurrently with the stimulation of GH secretion.

Peptide signals regulating food intake and energy homeostasis

The adiposity hormone leptin has been shown to decrease food intake and body weight by acting on neuropeptide circuits in the hypothalamus. However, it is not clear how this primary hypothalamic action of leptin is translated into a change in food intake. We hypothesize that the behavioral effect of leptin ultimately involves the integration of neuronal responses in the forebrain with those in the nucleus tractus solitarius in the caudal brainstem, where ingestive behavior signals are received from the gastrointestinal system and the blood. One example is the peptide cholecystokinin, which is released from the gut following ingestion of a meal and acts via vagal afferent nerve fibers to activate medial nucleus tractus solitarius neurons and thereby decrease meal size. While it is established that leptin acts in the arcuate nucleus in the hypothalamus to stimulate anorexigenic neurons that inhibit food intake while simulataneously inhibiting orexigenic neurons that increase food intake, the mechanisms linking these effects with regions of the caudal brainstem that integrate cues related to meal termination are unclear. Based on an increasing body of supportive data, we hypothesize that this integration involves a pathway comprising descending projections from neurons from the paraventricular nucleus to neurons within the nucleus tractus solitarius that are activated by meal-related satiety factors. Leptin's anorexic effect comprises primarily decreased meal size, and at subthreshold doses for eliciting an effect on food intake, leptin intensifies the satiety response to circulating cholecystokinin. The location of neurons subserving the effects of intracerebroventricular administration of leptin and intraperitoneal injection of cholecystokinin on food intake has been identified by analysis of Fos expression. These studies reveal a distribution that includes the paraventricular nucleus and regions within the caudal brainstem, with the medial nucleus tractus solitarius having the most pronounced Fos expression in response to leptin and cholecystokinin, and support the hypothesis that the long-term adiposity signal leptin and the short-term satiety signal cholecystokinin act in concert to maintain body weight homeostasis.

Continuous fatty acid oxidation and reduced fat storage in mice lacking acetyl-CoA carboxylase 2

Malonyl-coenzyme A (malonyl-CoA), generated by acetyl-CoA carboxylases ACC1 and ACC2, is a key metabolite in the regulation of energy homeostasis. Here, we show that Acc2-/- mutant mice have a normal life span, a higher fatty acid oxidation rate, and lower amounts of fat. In comparison to the wild type, Acc2-deficient mice had 10- and 30-fold lower levels of malonyl-CoA in heart and muscle, respectively. The fatty acid oxidation rate in the soleus muscle of the Acc2-/- mice was 30% higher than that of wild-type mice and was not affected by addition of insulin; however, addition of insulin to the wild-type muscle reduced fatty acid oxidation by 45%. The mutant mice accumulated 50% less fat in their adipose tissue than did wild-type mice. These results raise the possibility that pharmacological manipulation of ACC2 may lead to loss of body fat in the context of normal caloric intake.

The regulation of body weight: lessons from the seasonal animal

The hypothalamus is a major regulatory centre involved in the control of many important physiological axes. One of these axes is the regulation of ingestive behaviour. Recent work using a combination of genetic-mutant mouse models together with targeted gene deletions has contributed much to our understanding of how neural pathways of the hypothalamus are involved in the regulation of energy balance in animals. These pathways are also relevant to human energy homeostasis. as mutations in key genes are correlated with obesity. Many of the genes identified mediate the effects of leptin, and are therefore primarily involved in sensing and responding to peripheral signals. In seasonal animals, such as the Siberian hamster (Phodopus sungorus), there is evidence for a higher level of regulation. The systems involved regulate body weight around an apparent 'set-point' through the action of photoperiod via the neurohormone, melatonin. The ability to manipulate energy balance through photoperiod (and melatonin) in the seasonal-animal model offers novel opportunities to identify further fundamental aspects of the control mechanisms involved in the central control of energy homeostasis and body weight.

Effect of food deprivation and leptin repletion on the plasma levels of estrogen (E2) and NADPH-d reactivity in the ventromedial and arcuate nuclei of the hypothalamus in the female rats

The exact role of leptin in fasting has not been completely elucidated. To determine whether leptin can act in fasting to influence plasma estrogen levels and nitric oxide synthase reactivity in food regulating centers of the brain, we fasted female rats for 4 days and treated them i.p. with vehicle or 100 microg of recombinant mouse leptin as 1 ml on the 3rd and 4th day twice daily (10.00 and 17.00 h). Proestrus blood was collected at 10.00, 14.00, 18.00 and at 22.00 h, plasma obtained and assayed for estrogen (E2) and leptin levels. Verification of ovulation occurrence was by examining the oviduct for extruded ovum. The rat brains were removed and processed for nitric oxide synthase reactivity in the ventromedial hypothalamus (VMH) and arcuate nucleus (ARC) using NADPH-diaphorase histochemistry, a marker for neurons expressing NOS enzyme. Leptin effect on dependable variables such as food intake, water intake and body weight gain was also investigated. Four days fasting significantly decreased body weight, estrogen and postfast leptin levels, nitric oxide reactivity in the VMH and ARC nucleus and stopped ovulation in many (4 out of 5) rats fasted and given vehicle. Leptin treatment significantly increased plasma estrogen and postfast leptin levels, restored ovulation in many (4 out of 5) rats and increased nitric oxide reactivity in the VMH and ARC. Leptin significantly inhibited food intake, water intake and gain in body weight during recommenced feeding. These observations suggest that leptin could act in the pituitary-ovarian axis during fasting to improve reproductive function by partly stimulating estrogen secretion.

Fluorescence in situ hybridization of scarce leptin receptor mRNA using the enzyme-labeled fluorescent substrate method and tyramide signal amplification

To increase the sensitivity of fluorescence in situ hybridization (FISH) for detection of low-abundance mRNAs, we performed FISH on cryostat sections of rat hypothalamus with biotin-labeled riboprobes to leptin receptor (ObRb) and amplified the signal by combining tyramide signal amplification (TSA) and Enzyme-Labeled Fluorescent alkaline phosphatase substrate (ELF) methods. First, TSA amplification was done with biotinylated tyramide. Second, streptavidin-alkaline phosphatase was followed by the ELF substrate, producing a bright green fluorescent reaction product. FISH signal for ObRb was undetectable when TSA or ELF methods were used alone, but intense ELF FISH signal was visible in hypothalamic neurons when the ELF protocol was preceded by TSA. The TSA-ELF was combined with FISH for pro-opiomelanocortin (POMC) and neuropeptide Y (NPY) mRNAs by hybridizing brain sections in a cocktail containing digoxigenin-labeled riboprobes to NPY or POMC mRNA and biotin-labeled riboprobes to ObRb mRNA. Dioxigenin-labeled NPY or POMC mRNA hybrids were subsequently detected first with IgG-Cy3. Then biotin-labeled leptin receptor hybrids were detected with the TSA-ELF method. Combining the ELF and TSA amplification techniques enabled FISH detection of scarce leptin receptor mRNAs and permitted the identification of leptin receptor mRNA in cells that also express NPY and POMC gene products.

Plasma leptin concentrations, basal metabolic rates and respiratory quotients in young and older adults

OBJECTIVE

To-examine the interrelationship of circulating leptin concentrations, basal metabolic rates (BMR) and respiratory quotients (RQ) in young and older adults.

DESIGN

Cross sectional study.

SUBJECTS

Seventy-six Australian men and women, 48 young (< 35 y) and 28 older ( > or = 50 y).

MEASUREMENTS

Fasting plasma leptin concentrations by RIA, BMR and RQ by indirect calorimetry, percentage body fat (BF%), fat mass (FM) and fat-free mass (FFM) from total body water (TBW) based on deuterium dilution, waist and hip circumferences from anthropometry.

RESULTS

Older subjects had significantly higher BF%, FM and waist-to-hip ratio (WHR), but significantly lower FFM and absolute BMR as compared to younger subjects. Absolute leptin concentrations were 60% higher in older subjects but did not achieve statistical significance. There was, however, a significant gender x age group interaction in leptin concentrations. This was reflected in a significant inverse relationship between age group and leptin in women when data was controlled for waist circumference (r = -0.38, P = 0.028), or FM (r = -0.36, P = 0.042). A similar relationship was not observed in men on controlling for BF% or FM. Log transformed plasma leptin was best explained by a model that included BF%, gender, age-group, gender x age-group and WHR r = 0.75, adjusted r2 = 0.56, standard error of estimate (SEE) = 0.73 ng/ml). BMR was best explained by FFM, FM and age group r = 0.94, adjusted r2 = 0.87; SEE = 429 kJ/day). On controlling for BF%, WHR and FFM, leptin was negatively related to RQ only in older men (r = -0.67, P = 0.033). There was no relationship of leptin to BMR in the groups studied.

CONCLUSION

The study demonstrates an age-related modification of the gender bias in leptin, and a gender-specific inverse relationship between leptin and RQ in older people. The decline in leptin and the lack of a relationship between RQ and leptin in older women may indicate an increased risk of weight gain relative to older men.

Reduced food intake and body weight in mice treated with fatty acid synthase inhibitors

With the escalation of obesity-related disease, there is great interest in defining the mechanisms that control appetite and body weight. We have identified a link between anabolic energy metabolism and appetite control. Both systemic and intracerebroventricular treatment of mice with fatty acid synthase (FAS) inhibitors (cerulenin and a synthetic compound C75) led to inhibition of feeding and dramatic weight loss. C75 inhibited expression of the prophagic signal neuropeptide Y in the hypothalamus and acted in a leptin-independent manner that appears to be mediated by malonyl-coenzyme A. Thus, FAS may represent an important link in feeding regulation and may be a potential therapeutic target.

How tight are your genes? Transcriptional and posttranscriptional regulation of the leptin receptor, NPY, and POMC genes

In the past few years, there has been exponential growth in our knowledge of genes that control food intake and metabolism. Most of this research has demonstrated either an increased or decreased expression of these "obesity genes" in response to changes in nutritional status. Ultimately, these changes reflect modifications in the rate of gene transcription, mRNA stability, translation initiation, or posttranslational processing. Few laboratories have examined specifically which of these molecular mechanisms are responsible for obesity gene regulation, and thus, the field is wide open for exploration. In addition, it is possible that some forms of human obesity may be caused by inherited mutations in transcription factors or other regulatory molecules rather than base pair mutations in the obesity genes themselves. This article focuses on the regulation of the leptin receptor, NPY, and POMC genes, and explores what is known about the regulation of these obesity genes in response to food intake or changes in body fat stores. Connections between regulation of these genes and some inherited forms of human obesity are made.