Altered neuropeptide Y concentrations in specific hypothalamic regions of obese (fa/fa) Zucker rats. Possible relationship to obesity and neuroendocrine disturbances

Sensitivity of the Neuroendocrine Stress Axis in Metabolic Diseases

Metabolic diseases are prevalent in modern society and have reached pandemic proportions. Metabolic diseases have systemic effects on the body and can lead to changes in the neuroendocrine stress axis, the critical regulator of the body's stress response. These changes may be attributed to rising insulin levels and the release of adipokines and inflammatory cytokines by adipose tissue, which affect hormone production by the neuroendocrine stress axis. Chronic stress due to inflammation may exacerbate these effects. The increased sensitivity of the neuroendocrine stress axis may be responsible for the development of metabolic syndrome, providing a possible explanation for the high prevalence of severe comorbidities such as heart disease and stroke associated with metabolic disease. In this review, we address current knowledge of the neuroendocrine stress axis in response to metabolic disease and discuss its role in developing metabolic syndrome.

Links between HPA axis and adipokines: clinical implications in paradigms of stress-related disorders

INTRODUCTION

In the human organism, a constant interplay exists between the stress system [which includes the activity of the hypothalamic-pituitary-adrenal (HPA) axis] and the adipose tissue. This interplay is mediated by hormones of the HPA axis such as corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH) and glucocorticoids (GCs) and adipokines secreted by the adipose tissue.

AREAS COVERED

In this critical review, the bi-directional interactions between HPA axis and the most studied adipokines such as leptin and adiponectin, as well as the pro-inflammatory adipocytokines tumor necrosis factor (TNF) and interleukin (IL) 6 are presented. Furthermore, these interactions are described in normalcy as well as in specific clinical paradigms of stress-related disorders such as eating disorders, hypothalamic amenorrhea, and stress-related endogenous hypercortisolism states. Wherever new therapeutic strategies emerge, they are presented accordingly.

EXPERT COMMENTARY

Additional research is needed to clarify the mechanisms involved in the interplay between the HPA axis and the adipose tissue. Research should be focused, in particular, on the development of new therapeutic means targeting dysfunctional adipose tissue in stress-related situations.

Neuropeptide Y: An Anti-Aging Player?

Accumulating evidence suggests that neuropeptide Y (NPY) has a role in aging and lifespan determination. In this review, we critically discuss age-related changes in NPY levels in the brain, together with recent findings concerning the contribution of NPY to, and impact on, six hallmarks of aging, specifically: loss of proteostasis, stem cell exhaustion, altered intercellular communication, deregulated nutrient sensing, cellular senescence, and mitochondrial dysfunction. Understanding how NPY contributes to, and counteracts, these hallmarks of aging will open new avenues of research on limiting damage related to aging.

Hypothalamic control of hepatic lipid metabolism via the autonomic nervous system

Our body is well designed to store energy in times of nutrient excess, and release energy in times of food deprivation. This adaptation to the external environment is achieved by humoral factors and the autonomic nervous system. Claude Bernard, in the 19th century, showed the importance of the autonomic nervous system in the control of glucose metabolism. In the 20th century, the discovery of insulin and the development of techniques to measure hormone concentrations shifted the focus from the neural control of metabolism to the secretion of hormones, thus functionally "decapitating" the body. Just before the end of the 20th century, starting with the discovery of leptin in 1994, the control of energy metabolism went back to our heads. Since the start of 21st century, numerous studies have reported the involvement of hypothalamic pathways in the control of hepatic insulin sensitivity and glucose production. The autonomic nervous system is, therefore, acknowledged to be one of the important determinants of liver metabolism and a possible treatment target. In this chapter, we review research to date on the hypothalamic control of hepatic lipid metabolism.

Expression Levels of Genes Likely Involved in Glucose-sensing in the Obese Zucker Rat Brain

It has been suggested that certain cells in the brain, like pancreatic beta-cells, use glucose transporter-2 (GLUT-2), glucokinase and glucagon-like peptide-1 receptor (GLP-1R) to sense glucose in the service of multiple aspects of energy balance. The obese Zucker rat displays numerous disturbances in energy homeostasis and may provide a model of dysfunctional expression of genes related to nutrient control systems. Using real-time RT-PCR we measured gene expression for three of the pancreatic glucose-sensing markers and neuropeptide Y (NPY) in the medial, lateral hypothalamus and hindbrain of lean and obese Zucker rats of both genders. Additionally, we measured circulating levels of glucose, leptin, insulin, corticosterone and glucagon. The results indicate that GLUT-2 mRNA expression is decreased, whereas glucokinase is increased in the hindbrain of obese rats. NPY mRNA level is significantly higher, whereas GLP-1R is significantly lower in the medial hypothalamus in obese individuals. Gender-related differences were found in the hindbrain and medial hypothalamus for GLUT-2 and in the lateral hypothalamus for GLP-1R and they may be related to the fact that the female Zucker rats do not develop diabetes as readily as males. Furthermore, the hindbrain may be an important site for glucose-sensing where major phenotypic changes occur for glucose-sensing genes expression.

Anthocyanins attenuate body weight gain via modulating neuropeptide Y and GABAB1 receptor in rats hypothalamus

Anthocyanins in a variety of plant species have been identified and are known for its hypolipidemic and anti-obesity effects. The effect of anthocyanins extracted from black soybean on body weight and daily food intake in adult rats raised on normal diet were studied. Male Sprague-Dawley rats were daily intra-gastric administered water or anthocyanins 6 mg/kg and 24 mg/kg for 40 days. During this period daily food intake and body weight were measured prior to anthocyanins treatment. These findings showed that anthocyanins treatment resulted in significantly lowered body weight and food intake compared with water treated rats. In addition, anthocyanins dose dependently reduced the adipose tissue size compared with control group. Western blot analysis showed that high dose of anthocyanins treatment significantly reduced the expression of neuropeptide Y (NPY) and increased γ-amino butyric acid receptor (GABAB1R) in hypothalamus. Furthermore, these events were followed by a decreased in expression of GABAB1R downstream signaling molecules protein kinase A-α (PKA) and phosphorylated cAMP-response element binding protein (p-CREB) in hypothalamus. These data support the concept that anthocyanins even in normal circumstances have the capability to reduce body weight and food intake through its modulatory effect on NPY and GABAB1R in hypothalamus. These results suggest that anthocyanins from black soybean seed coat might have a novel role in preventing obesity in rats on normal diet.

Dehydroepiandrosterone (DHEA) blocks the increase in food intake caused by neuropeptide Y (NPY) in the Zucker rat

Recent studies have demonstrated that neuropeptide Y (NPY) reduced the neural production of dehydroepiandrosterone (DHEA) in frog hypothalamic explants. The objective of this study was to assess if DHEA can block the NPY induced increase in food intake in lean and obese Zucker rats. Rats were given one of the following four treatments: sterile water/dimethylsulfoxide (DMSO), NPY/DMSO, water/DHEA, and NPY/DHEA. Immediately after administration of their respective treatment, rats were exposed to macronutrients for 4 h and food intake was monitored. NPY caused a significant increase in total calories consumed compared to control. Co-administration of DHEA along with NPY blocked this NPY dependent effect. These results suggest that DHEA blocks the over-eating in satiated rats induced by NPY. Measurement of changes in regional hypothalamic and raphe monoamine neurotransmitters known to affect food intake suggested a possible role of serotonin fluctuations in the ventromedial hypothalamus (VMH) guiding this behaviour.

Effects of Fluoxetine Administration on Regional Galanin Expression in Obese Zucker Rat Hypothalamus

The aim of the present work was to study the potential involvement of hypothalamic galanin system in the anorectic mechanism of fluoxetine in obese Zucker rats. Male obese Zucker (fa/fa) rats were administered fluoxetine (10 mg/kg; i.p.) daily for two weeks. The control group was given 0.9% NaCl solution. Significant decreases in food intake, final body weight and total body fat were observed after fluoxetine treatment. Although fluoxetine-treated rats showed a decrease in urine elimination, this effect was not enough to compensate decreased water intake, leading to dehydration, as showed by decreased body water content. Chronic fluoxetine administration increased the numbers of galanin positively immunostained neural cells in medial and lateral preoptic areas, lateral hypothalamic area and paraventricular nucleus (rostral and magnocellular regions), without changes in dorsomedial, ventromedial, supraoptic, suprachiasmatic and arcuate nuclei. Taken into account that galanin stimulates appetite, these results could represent rather a compensatory response against reduced food intake than a direct anorectic mechanism. Changes in the magnocellular region of the hypothalamic paraventricular nucleus suggest a role for galanin neural circuits at this level in fluoxetine-induced hydro-osmotic impairment.

Biological mechanisms that promote weight regain following weight loss in obese humans

Weight loss dieting remains the treatment of choice for the vast majority of obese individuals, despite the limited long-term success of behavioral weight loss interventions. The reasons for the near universal unsustainability of behavioral weight loss in [formerly] obese individuals have not been fully elucidated, relegating researchers to making educated guesses about how to improve obesity treatment, as opposed to developing interventions targeting the causes of weight regain. This article discusses research on several factors that may contribute to weight regain following weight loss achieved through behavioral interventions, including adipose cellularity, endocrine function, energy metabolism, neural responsivity, and addiction-like neural mechanisms. All of these mechanisms are engaged prior to weight loss, suggesting that these so called "anti-starvation" mechanisms are activated via reductions in energy intake, rather than depletion of energy stores. Evidence suggests that these mechanisms are not necessarily part of a homeostatic feedback system designed to regulate body weight, or even anti-starvation mechanisms per se. Although they may have evolved to prevent starvation, they appear to be more accurately described as anti-weight loss mechanisms, engaged with caloric restriction irrespective of the adequacy of energy stores. It is hypothesized that these factors may combine to create a biological disposition that fosters the maintenance of an elevated body weight and works to restore the highest sustained body weight, thus precluding the long-term success of behavioral weight loss. It may be necessary to develop interventions that attenuate these biological mechanisms in order to achieve long-term weight reduction in obese individuals.

Hypothalamic neuropeptide Y (NPY) controls hepatic VLDL-triglyceride secretion in rats via the sympathetic nervous system

Excessive secretion of triglyceride-rich very low-density lipoproteins (VLDL-TG) contributes to diabetic dyslipidemia. Earlier studies have indicated a possible role for the hypothalamus and autonomic nervous system in the regulation of VLDL-TG. In the current study, we investigated whether the autonomic nervous system and hypothalamic neuropeptide Y (NPY) release during fasting regulates hepatic VLDL-TG secretion. We report that, in fasted rats, an intact hypothalamic arcuate nucleus and hepatic sympathetic innervation are necessary to maintain VLDL-TG secretion. Furthermore, the hepatic sympathetic innervation is necessary to mediate the stimulatory effect of intracerebroventricular administration of NPY on VLDL-TG secretion. Since the intracerebroventricular administration of NPY increases VLDL-TG secretion by the liver without affecting lipolysis, its effect on lipid metabolism appears to be selective to the liver. Together, our findings indicate that the increased release of NPY during fasting stimulates the sympathetic nervous system to maintain VLDL-TG secretion at a postprandial level.

Central and peripheral mechanisms of the NPY system in the regulation of bone and adipose tissue

Skeletal research is currently undergoing a period of marked expansion. The boundaries of "bone" research are being re-evaluated and with this, a growing recognition of a more complex and interconnected biology than previously considered. One aspect that has become the focus of particular attention is the relationship between bone and fat homeostasis. Evidence from a number of avenues indicates that bone and adipose regulation are both related and interdependent. This review examines the neuropeptide Y (NPY) system, known to exert powerful control over both bone and fat tissue. The actions of this system are characterized by signaling both within specific nuclei of the hypothalamus and also the target tissues, mediated predominantly through two G-protein coupled receptors (Y1 and Y2). In bone tissue, elevated NPY levels act consistently to repress osteoblast activity. Moreover, both central Y2 receptor and osteoblastic Y1 receptor signaling act similarly to repress bone formation. Conversely, loss of NPY expression or receptor signaling induces increased osteoblast activity and bone mass in both cortical and cancellous envelopes. In fat tissue, NPY action is more complex. Energy homeostasis is powerfully altered by elevations in hypothalamic NPY, resulting in increases in fat accretion and body-wide energy conservation, through the action of locally expressed Y1 receptors, while local Y2 receptors act to inhibit NPY-ergic tone. Loss of central NPY expression has a markedly reduced effect, consistent with a physiological drive to promote fat accretion. In fat tissue, NPY and Y1 receptors act to promote lipogenesis, consistent with their roles in the brain. Y2 receptors expressed in adipocytes also act in this manner, showing an opposing action to their role in the hypothalamus. While direct investigation of these processes has yet to be completed, these responses appear to be interrelated to some degree. The starvation-based signal of elevated central NPY inducing marked inhibition of osteoblast activity, whilst promoting fat accretion, indicating skeletal tissue is a component of the energy conservation system. Moreover, when NPY expression is reduced, consistent with high calorie intake and weight gain, bone formation is stimulated, strengthening the skeleton. In conclusion, NPY acts to regulate both bone and fat tissue in a coordinated manner, and remains a strong candidate for mediating interactions between these two tissues.

The role of neuropeptide Y in energy homeostasis

When administered into the brain, NPY acts at Y1 and Y5 receptors to increase food intake. The response occurs with a short latency and is quite robust, such that exogenous NPY is generally considered to be the most potent of a growing list of orexigenic compounds that act in the brain. The role of endogenous NPY is not so straightforward, however. Evidence from diverse types of experiments suggests that rather than initiating behavioral eating per se, endogenous NPY elicits autonomic responses that prepare the individual to better cope with consuming a calorically large meal.

Biology's response to dieting: the impetus for weight regain

Dieting is the most common approach to losing weight for the majority of obese and overweight individuals. Restricting intake leads to weight loss in the short term, but, by itself, dieting has a relatively poor success rate for long-term weight reduction. Most obese people eventually regain the weight they have worked so hard to lose. Weight regain has emerged as one of the most significant obstacles for obesity therapeutics, undoubtedly perpetuating the epidemic of excess weight that now affects more than 60% of U.S. adults. In this review, we summarize the evidence of biology's role in the problem of weight regain. Biology's impact is first placed in context with other pressures known to affect body weight. Then, the biological adaptations to an energy-restricted, low-fat diet that are known to occur in the overweight and obese are reviewed, and an integrative picture of energy homeostasis after long-term weight reduction and during weight regain is presented. Finally, a novel model is proposed to explain the persistence of the "energy depletion" signal during the dynamic metabolic state of weight regain, when traditional adiposity signals no longer reflect stored energy in the periphery. The preponderance of evidence would suggest that the biological response to weight loss involves comprehensive, persistent, and redundant adaptations in energy homeostasis and that these adaptations underlie the high recidivism rate in obesity therapeutics. To be successful in the long term, our strategies for preventing weight regain may need to be just as comprehensive, persistent, and redundant, as the biological adaptations they are attempting to counter.

NPY neuron-specific Y2 receptors regulate adipose tissue and trabecular bone but not cortical bone homeostasis in mice

BACKGROUND

Y2 receptor signalling is known to be important in neuropeptide Y (NPY)-mediated effects on energy homeostasis and bone physiology. Y2 receptors are located post-synaptically as well as acting as auto receptors on NPY-expressing neurons, and the different roles of these two populations of Y2 receptors in the regulation of energy homeostasis and body composition are unclear.

METHODOLOGY/PRINCIPAL FINDINGS

We thus generated two conditional knockout mouse models, Y2(lox/lox) and NPYCre/+;Y2(lox/lox), in which Y2 receptors can be selectively ablated either in the hypothalamus or specifically in hypothalamic NPY-producing neurons of adult mice. Specific deletion of hypothalamic Y2 receptors increases food intake and body weight compared to controls. Importantly, specific ablation of hypothalamic Y2 receptors on NPY-containing neurons results in a significantly greater adiposity in female but not male mice, accompanied by increased hepatic triglyceride levels, decreased expression of liver carnitine palmitoyltransferase (CPT1) and increased expression of muscle phosphorylated acetyl-CoA carboxylase (ACC). While food intake, body weight, femur length, bone mineral content, density and cortical bone volume and thickness are not significantly altered, trabecular bone volume and number were significantly increased by hypothalamic Y2 deletion on NPY-expressing neurons. Interestingly, in situ hybridisation reveals increased NPY and decreased proopiomelanocortin (POMC) mRNA expression in the arcuate nucleus of mice with hypothalamus-specific deletion of Y2 receptors in NPY neurons, consistent with a negative feedback mechanism between NPY expression and Y2 receptors on NPY-ergic neurons.

CONCLUSIONS/SIGNIFICANCE

Taken together these data demonstrate the anti-obesogenic role of Y2 receptors in the brain, notably on NPY-ergic neurons, possibly via inhibition of NPY neurons and concomitant stimulation of POMC-expressing neurons in the arcuate nucleus of the hypothalamus, reducing lipogenic pathways in liver and/or skeletal muscle in females. These data also reveal as an anti-osteogenic effect of Y2 receptors on hypothalamic NPY-expressing neurons on trabecular but not on cortical bone.

Reduced expression of the KATP channel subunit, Kir6.2, is associated with decreased expression of neuropeptide Y and agouti-related protein in the hypothalami of Zucker diabetic fatty rats

The link between obesity and diabetes is not fully understood but there is evidence to suggest that hypothalamic signalling pathways may be involved. The hypothalamic neuropeptides, pro-opiomelanocortin (POMC), neuropeptide Y (NPY) and agouti-related protein (AGRP) are central to the regulation of food intake and have been implicated in glucose homeostasis. Therefore, the expression of these genes was quantified in hypothalami from diabetic Zucker fatty (ZDF) rats and nondiabetic Zucker fatty (ZF) rats at 6, 8, 10 and 14 weeks of age. Although both strains are obese, only ZDF rats develop pancreatic degeneration and diabetes over this time period. In both ZF and ZDF rats, POMC gene expression was decreased in obese versus lean rats at all ages. By contrast, although there was the expected increase in both NPY and AGRP expression in obese 14-week-old ZF rats, the expression of NPY and AGRP was decreased in 6-week-old obese ZDF rats with hyperinsulinaemia and in 14-week-old rats with the additional hyperglycaemia. Therefore, candidate genes involved in glucose, and insulin signalling pathways were examined in obese ZDF rats over this age range. We found that expression of the ATP-sensitive potassium (K(ATP)) channel component, Kir6.2, was decreased in obese ZDF rats and was lower compared to ZF rats in each age group tested. Furthermore, immunofluorescence analysis showed that Kir6.2 protein expression was reduced in the dorsomedial and ventromedial hypothalamic nuclei of 6-week-old prediabetic ZDF rats compared to ZF rats. The Kir6.2 immunofluorescence colocalised with NPY throughout the hypothalamus. The differences in Kir6.2 expression in ZF and ZDF rats mimic those of NPY and AGRP, which could infer that the changes occur in the same neurones. Overall, these data suggest that chronic changes in hypothalamic Kir6.2 expression may be associated with the development of hyperinsulinaemia and hyperglycaemia in ZDF rats.

Y2, Y4 receptors and obesity

The neuropeptide Y system - comprising neuropeptide Y, peptide YY, pancreatic polypeptide and the Y receptors through which they act (Y1, Y2, Y4, Y5 and y6) - has been at the center of attention with regards to regulation of feeding behavior and its possible involvement in obesity. In the past, research has focused mainly on the orexigenic and obesogenic action of this system, with Y1 and Y5 receptors being prime candidates as mediators of neuropeptide Y-induced hyperphagia and obesity. However, in recent years, the role of other members of the neuropeptide Y family, peptide YY, pancreatic polypeptide and the Y2 and Y4 receptors through which they predominantly act, have commanded increasing attention on account of their effects to mediate satiety and promote weight loss via actions in key brain structures, such as the arcuate nucleus of the hypothalamus and the brain stem. This review focuses on the role of peptide YY- and pancreatic polypeptide-like compounds as possible antiobesity drugs, taking into account their effects, not only on energy balance, but also in the regulation of bone formation, and highlights potential benefits of using Y2 and/or Y4 antagonists (as opposed to agonists such as peptide YY or pancreatic polypeptide) in the treatment of obesity.

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.

Combined deletion of Y1, Y2, and Y4 receptors prevents hypothalamic neuropeptide Y overexpression-induced hyperinsulinemia despite persistence of hyperphagia and obesity

Neuropeptide Y (NPY) is a key regulator of energy homeostasis and is implicated in the development of obesity and type 2 diabetes. Whereas it is known that hypothalamic administration of exogenous NPY peptides leads to increased body weight gain, hyperphagia, and many hormonal and metabolic changes characteristic of an obesity syndrome, the Y receptor(s) mediating these effects is disputed and unclear. To investigate the role of different Y receptors in the NPY-induced obesity syndrome, we used recombinant adeno-associated viral vector to overexpress NPY in mice deficient of selective single or multiple Y receptors (including Y1, Y2, and Y4). Results from this study demonstrated that long-term hypothalamic overexpression of NPY lead to marked hyperphagia, hypogonadism, body weight gain, enhanced adipose tissue accumulation, hyperinsulinemia, and other hormonal changes characteristic of an obesity syndrome. NPY-induced hyperphagia, hypogonadism, and obesity syndrome persisted in all genotypes studied (Y1(-/-), Y2(-/-), Y2Y4(-/-), and Y1Y2Y4(-/-) mice). However, triple deletion of Y1, Y2, and Y4 receptors prevented NPY-induced hyperinsulinemia. These findings suggest that Y1, Y2, and Y4 receptors under this condition are not crucially involved in NPY's hyperphagic, hypogonadal, and obesogenic effects, but they are responsible for the central regulation of circulating insulin levels by NPY.

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.

NPY presynaptic actions are reduced in the hypothalamic mpPVN of obese (fa/fa), but not lean, Zucker rats in vitro

1. Neuropeptide Y (NPY) profoundly enhances feeding when injected intracerebroventricularly, or directly into hypothalamic nuclei, such as the paraventricular nucleus (PVN). Paradoxically, NPY has a reduced action on feeding in obese Zucker rats relative to lean Zucker rats, although the obese rats have much higher levels of hypothalamic NPY expression. GABAergic inputs to a subpopulation of medial parvocellular PVN (mpPVN) neurons are sensitive to NPY. Here, we tested the hypothesis that the blunted eating response to NPY observed in obese Zucker rats will be reflected in a reduced NPY action at mpPVN GABAergic synapses. 2. 'Blind' whole-cell patch-clamp recordings made from mpPVN neurons in acute brain slices of lean and obese Zucker rats revealed GABAergic inhibitory postsynaptic currents (IPSC) responses which were inhibited by NPY. While the maximum response in the obese Zucker rats was significantly less than in lean Zucker or Sprague-Dawley rats, there was no difference in the EC(50). 3. Experiments using blocking concentrations of Y(1)- or Y(5)-receptor antagonists revealed no differences between lean and obese Zucker rats in the contributions of either of these receptors to the total NPY response in mpPVN. 4. NPY is less effective at the mpPVN GABA synapse in obese than in lean Zucker rats. This is not associated with a change in the proportion of Y(1) or Y(5) receptors mediating the NPY response, and is consistent with the downregulation of NPY receptors or a reduction in receptor-effector coupling, and with the reduced sensitivity of obese rats to NPY.

Galanin-like peptide mRNA alterations in arcuate nucleus and neural lobe of streptozotocin-diabetic and obese zucker rats. Further evidence for leptin-dependent and independent regulation

Galanin-like peptide (GALP) is a 60-amino-acid peptide with structural similarities to galanin and a high affinity for galanin receptors. GALP is expressed by a discrete population of neurons in the arcuate nucleus (ARC) and median eminence of the hypothalamus of several species, including the rat. GALP neurons express leptin receptors and GALP mRNA levels are decreased slightly in fasted rats and stimulated significantly by acute leptin treatment in combination with fasting. In studies to further explore the leptin dependence of GALP expression, we examined GALP mRNA levels in the hypothalamus of obese Zucker and streptozotocin-induced diabetic (STZ-DM) rats. In leptin receptor-deficient obese Zucker rats, with 75% higher body weight than lean littermates, GALP mRNA levels in the ARC were decreased by 75%, while neuropeptide Y (NPY) mRNA levels were increased 7-fold (n = 5, p < 0.001), consistent with earlier reports. In hypoleptinemic diabetic rats with 4.5-fold higher blood glucose and 15% lower body weight than controls, GALP mRNA levels in the ARC were decreased by 90%, while NPY mRNA levels were increased 9-fold (n = 5, p < 0.001). GALP is also expressed by pituicytes in the neural lobe of the rat pituitary gland and GALP expression is increased by osmotic stimulation such as dehydration and salt loading. Thus, in STZ-DM rats that are in a hyperosmotic state with elevated plasma vasopressin levels, GALP mRNA levels were increased by approximately 20-fold in the neural lobe relative to control (n = 4, p < 0.001). The current findings are consistent with a strong tonic influence of leptin receptor signalling on hypothalamic GALP expression under normal conditions, and possible abnormalities in GALP neuronal signalling and their putative targets, thyrotropin-releasing hormone and gonadotropin hormone-releasing hormone neurons, under pathophysiological conditions such as diabetes and obesity. Our data in STZ-DM rats also clearly demonstrate that GALP gene expression is differentially regulated in neurons and pituicytes.

Sibutramine decreases body weight gain and increases energy expenditure in obese Zucker rats without changes in NPY and orexins

The aim of the present work was to describe the effects of sibutramine on body weight and adiposity and to establish the potential involvement of neuropeptide Y (NPY) and orexins in the anorectic action of this drug. Male obese Zucker rats were daily administered with sibutramine (10 mg/kg, intraperitoneal) for two weeks. Carcass composition was assessed using the official methods of the Association of Official Analytical Chemists. Total body oxygen consumption was measured daily for 60 min before sibutramine or saline injection and for 30 min (from 60 to 90 min) after drug or saline injection. Hypothalamic arcuate and paraventricular nuclei, and the lateral hypothalamic area were immunostained for NPY, orexin A and orexin B. Commercial kits were used for serum determinations. Reductions in body weight and adipose tissue weights were observed after sibutramine treatment in obese Zucker rats. No changes in NPY immunostaining in the arcuate and paraventricular nuclei were found. Orexin A and orexin B immunostaining was not modified in the lateral hypothalamic area in treated rats. The reduction in body weight and adiposity induced by sibutramine was achieved by both a reduction in food intake and an increase in energy expenditure. NPY and orexins do not seem to be involved in the anorectic effect of sibutramine.

Neuropeptides and appetite control

Obesity is important in the aetiology of type 2 diabetes, and presents a major barrier to its successful prevention and management. Obesity develops when energy intake exceeds energy expenditure over time. A complex system has evolved to maintain energy homeostasis, but this is biased towards weight gain. Meal size is controlled by a series of short-term hormonal and neural signals that derive from the gastrointestinal tract, such as cholecystokinin whereas others may initiate meals, such as the recently discovered hormone, ghrelin. Other hormones such as insulin and leptin, together with circulating nutrients, indicate long-term energy stores. All these signals act at several central nervous system (CNS) sites but the pathways converge on the hypothalamus, which contains a large number of peptide and other neurotransmitters that influence food intake. As energy deficit is most likely to compromise survival, it is not surprising that the most powerful of these pathways are those that increase food intake and decrease energy expenditure when stores are depleted. When energy stores are low, production of leptin from adipose tissue, and thus circulating leptin concentrations fall, leading to increased production of hypothalamic neurotransmitters that strongly increase food intake, such as neuropeptide Y (NPY), galanin and agouti-related protein (AGRP) and decreased levels of alpha-melanocyte-stimulating hormone (alpha-MSH), cocaine and amphetamine-regulated transcript (CART) and neurotensin that reduce food intake and increase energy expenditure. The finding that mutations in leptin and POMC lead to severe early onset obesity in humans has highlighted the importance of these peptides in humans. This new understanding may eventually lead to new treatments for obesity that will be of particular benefit in the prevention and treatment of type 2 diabetes.

Effects of fluoxetine administration on neuropeptide y and orexins in obese zucker rat hypothalamus

OBJECTIVE

The aim of this work was to study the potential involvement of neuropeptide Y (NPY) and orexins in the anorexigenic mechanism of fluoxetine in obese Zucker rats, assessing the effects of chronic fluoxetine treatment on NPY and orexin immunostaining in several hypothalamic regions.

RESEARCH METHODS AND PROCEDURES

Male obese Zucker (fa/fa) rats were administered fluoxetine (10 mg/kg intraperitoneally) daily for 2 weeks. The control group was administered 0.9% NaCl solution. Carcass composition was assessed using the official methods of the Association of Official Analytical Chemists. To test the potential thermogenic effect of fluoxetine administration, total body oxygen consumption was measured daily for 60 minutes before fluoxetine or saline injection and for 30 minutes after drug or saline injection. Hypothalamic arcuate and paraventricular nuclei, and the lateral hypothalamic area were immunostained for NPY, orexin A, and orexin B. Commercial kits were used for serum determinations.

RESULTS

Chronic fluoxetine administration in obese Zucker rats generated a reduction in body weight gain, food intake, adipocyte size, fat mass, and body protein. A decrease in NPY immunostaining in the paraventricular nucleus, without changes in the arcuate, was observed. However, no changes were observed in the number of neural cells immunostained for orexin A or orexin B in the lateral hypothalamic area.

DISCUSSION

Due to the hyperphagic effect of NPY in the paraventricular nucleus, these results suggest that NPY, but not orexins, could be involved in the anorexigenic effect of fluoxetine in obese Zucker rats.

Blockade of body weight gain and plasma corticosterone levels in Zucker fatty rats using an orally active neuropeptide Y Y1 antagonist

1. An experiment was conducted to examine whether a potent, orally active and highly selective neuropeptide Y Y1 receptor antagonist attenuates hyperphagia and obesity in genetically obese Zucker fatty rats. 2. Oral administration of the Y1 antagonist (30 and 100 mg x kg(-1), once daily for 2 weeks) significantly suppressed the daily food intake and body weight gain in Zucker fatty rats accompanied with a reduction of fat cell size and plasma corticosterone levels. 3. Despite the fact that food intake was gradually returned to near the control level, the body weight of the treated animals remained significantly less when compared to that of the controls for the duration of the treatment. 4. These results suggest that the Y1 receptor, at least in part, participate in pathophysiological feeding and/or fat accumulation observed in Zucker fatty rats. Y1 antagonists might be useful for the treatment of obesity.

Injection of alpha-MSH, but not beta-endorphin, into the PVN decreases POMC gene expression in the ARC

beta-Endorphin (beta-END) and alpha-melanocyte stimulating hormone (alpha-MSH), neuropeptides derived from proopiomelanocortin (POMC), have opposite effects on eating behavior. We injected rats with alpha-MSH (0.6 nmol) or beta-END (1 nmol) into the PVN (three times in a 26 h period). These doses of alpha-MSH and beta-END decreased and increased feeding respectively. Following alpha-MSH administration into the PVN, mRNA levels of POMC decreased by 17%, whereas there was no significant change in gene expression of either proDynorphin or proEnkephalin. PVN injection of beta-END failed to alter gene expression of POMC, proDynorphin or proEnkephalin. These data suggest that a feedback pathway exists between the PVN and ARC for alpha-MSH and POMC, but not for beta-END and POMC.

Disruption of arcuate/paraventricular nucleus connections changes body energy balance and response to acute stress

The mediobasal hypothalamus regulates functions necessary for survival, including body energy balance and adaptation to stress. The purpose of this experiment was to determine the contribution of the arcuate nucleus (ARC) in controlling these two functions by the paraventricular nucleus (PVN). Circular, horizontal cuts (1.0 mm radius) were placed immediately above the anterior ARC to sever afferents to the PVN. In shams the knife was lowered to the same coordinates but was not rotated. Food intake and body weight were monitored twice daily, at the beginning and end of the light cycle, for 1 week. On the final day the animals were restrained for 30 min. Lesioned animals had increased food intake in light and dark periods, higher weight gain per day, and more body fat as compared with shams. There was no difference in caloric efficiency. Unlike shams, lesioned rats had no predictable relationship between plasma insulin and leptin. Plasma ACTH was increased at 0 min in lesioned rats but was decreased 15 and 30 min after restraint as compared with shams. There was no difference in plasma corticosterone. Immunostaining revealed that alpha-melanocortin (alphaMSH) and neuropeptide Y (NPY) accumulated below the cuts, and both were decreased in PVN. Food intake and body weight were correlated negatively to alphaMSH, but not NPY in PVN. There was no difference in proopiomelanocortin (POMC) mRNA, but NPY mRNA was reduced in the ARC of lesioned animals. We conclude that ARC controls body energy balance in unstressed rats, possibly by alphaMSH input to PVN, and that ARC also is necessary for PVN regulation of ACTH.

The hypothalamus and the regulation of energy homeostasis: lifting the lid on a black box

The hypothalamus is the focus of many peripheral signals and neural pathways that control energy homeostasis and body weight. Emphasis has moved away from anatomical concepts of 'feeding' and 'satiety' centres to the specific neurotransmitters that modulate feeding behaviour and energy expenditure. We have chosen three examples to illustrate the physiological roles of hypothalamic neurotransmitters and their potential as targets for the development of new drugs to treat obesity and other nutritional disorders. Neuropeptide Y (NPY) is expressed by neurones of the hypothalamic arcuate nucleus (ARC) that project to important appetite-regulating nuclei, including the paraventricular nucleus (PVN). NPY injected into the PVN is the most potent central appetite stimulant known, and also inhibits thermogenesis; repeated administration rapidly induces obesity. The ARC NPY neurones are stimulated by starvation, probably mediated by falls in circulating leptin and insulin (which both inhibit these neurones), and contribute to the increased hunger in this and other conditions of energy deficit. They therefore act homeostatically to correct negative energy balance. ARC NPY neurones also mediate hyperphagia and obesity in the ob/ob and db/db mice and fa/fa rat, in which leptin inhibition is lost through mutations affecting leptin or its receptor. Antagonists of the Y5 receptor (currently thought to be the NPY 'feeding' receptor) have anti-obesity effects. Melanocortin-4 receptors (MC4-R) are expressed in various hypothalamic regions, including the ventromedial nucleus and ARC. Activation of MC4-R by agonists such as alpha-melanocyte-stimulating hormone (a cleavage product of pro-opiomelanocortin which is expressed in ARC neurones) inhibits feeding and causes weight loss. Conversely, MC4-R antagonists such as 'agouti' protein and agouti gene-related peptide (AGRP) stimulate feeding and cause obesity. Ectopic expression of agouti in the hypothalamus leads to obesity in the AVY mouse, while AGRP is co-expressed by NPY neurones in the ARC. Synthetic MC4-R agonists may ultimately find use as anti-obesity drugs in human subjects Orexins-A and -B, derived from prepro-orexin, are expressed in specific neurones of the lateral hypothalamic area (LHA). Orexin-A injected centrally stimulates eating and prepro-orexin mRNA is up regulated by fasting and hypoglycaemia. The LHA is important in receiving sensory signals from the gut and liver, and in sensing glucose, and orexin neurones may be involved in stimulating feeding in response to falls in plasma glucose.

ARC POMC mRNA and PVN alpha-MSH are lower in obese relative to lean zucker rats

Effects of obesity on gene expression for opioid peptides and neuropeptide-Y (NPY) in the arcuate nucleus (ARC), and on opioid peptides and alpha-melanocyte stimulating hormone (alpha-MSH) in the paraventricular nucleus (PVN) were examined in obese Zucker rats (18 weeks old). Obese Zucker rats are insulin-resistant, diabetic and hyperleptinemic as indicated by high serum glucose, insulin and leptin levels. ARC proOpiomelanocortin (POMC) mRNA levels were significantly lower in the obese relative to lean Zucker rats and ARC proNeuropeptide Y (proNPY) mRNA levels were higher (P<0.05). There were no differences in proDynorphin and proEnkephalin mRNA levels in the ARC (0.05). Obese Zucker rats had lower alpha-MSH and dynorphin A(1-17) peptide levels in the paraventricular nucleus (PVN) (P<0.05), but did not have lower PVN beta-endorphin peptide levels (0.05). The decrease in POMC in the ARC and decrease in alpha-MSH in the PVN seen in the obese Zucker rat in the present study suggest that reduced activity of the melanocortin system in the ARC to PVN pathway may contribute to the related hyperphagia. Reduced activity of the melanocortin system in the ARC to PVN pathway may be due to a disturbance of leptin signaling coupling to POMC.

The effect of moxonidine on feeding and body fat in obese Zucker rats: role of hypothalamic NPY neurones

The antihypertensive agent moxonidine, an imidazoline Ii-receptor agonist, also induces hypophagia and lowers body weight in the obese spontaneously hypertensive rat, but the central mediation of this action and the neuronal pathways that moxonidine may interact with are not known. We studied whether moxonidine has anti-obesity effects in the genetically-obese and insulin-resistant fa/fa Zucker rat, and whether these are mediated through inhibition of the hypothalamic neuropeptide Y (NPY) neurones. Lean and obese Zucker rats were given moxonidine (3 mg kg(-1) day(-1)) or saline by gavage for 21 days. Moxonidine decreased food intake throughout by 20% in obese rats (P<0.001) and by 8% in lean rats (P<0.001), and reduced weight gain that final body weight was 15% lower in obese (P<0.001) and 7% lower in lean (P<0.01) rats than their untreated controls. Plasma insulin and leptin levels were decreased in moxonidine-treated obese rats (P<0.01 and P<0.05), but unchanged in treated lean rats. Uncoupling protein-1 gene expression in brown adipose tissue was stimulated by 40-50% (P< or =0.05) in both obese and lean animals given moxonidine. Obese animals given moxonidine showed a 37% reduction in hypothalamic NPY mRNA levels (P = 0.01), together with significantly increased NPY concentrations in the paraventricular nucleus (P<0.05), but no changes in the arcuate nucleus or other nuclei; this is consistent with reduced NPY synthesis in the arcuate nucleus and blocked release of NPY in the paraventricular nucleus. In lean animals, moxonidine did not affect NPY levels or NPY mRNA. The hypophagic, thermogenic and anti-obesity effects of moxonidine in obese Zucker rats may be partly due to inhibition of the NPY neurones, whose inappropriate overactivity may underlie obesity in this model.

Neuropeptide Y and corticotropin-releasing hormone concentrations within specific hypothalamic regions of lean but not ob/ob mice respond to food-deprivation and refeeding

Leptin is proposed to control food intake at least in part by regulating hypothalamic neuropeptide Y (NPY), a stimulator of food intake, and corticotropin-releasing hormone (CRH), an inhibitor of food intake. Ob/ob mice are leptin-deficient and would thus be expected to exhibit alterations in hypothalamic NPY and CRH. We therefore measured concentrations of NPY and CRH in discrete regions of the hypothalamus (i.e., ARC, arcuate nucleus; PVN, paraventricular nucleus; VMH, ventromedial nucleus; DMH, dorsomedial nucleus; and SCN, suprachiasmatic nucleus) of 6.5-7-wk-old ob/ob and lean mice with free access to stock diet, 24 h after food deprivation, and 1 h after refeeding. Fed ob/ob mice had 55-75% higher concentrations of NPY in the ARC, VMH and SCN than lean mice. Food deprivation increased NPY concentrations approximately 70% in the ARC, PVN and VMH of lean mice, and refeeding lowered NPY concentrations approximately 70% in the PVN of these mice. NPY in these hypothalamic regions of ob/ob mice was unresponsive to food deprivation or refeeding. The most pronounced change in CRH concentrations within the regions examined (i.e., ARC, PVN and VMH) occurred in the ARC of lean mice where refeeding lowered CRH concentrations by 75% without influencing ARC CRH concentrations in ob/ob mice. The hypothalamic concentrations of two neuropeptides involved in body weight regulation (i.e., NPY and CRH) in leptin-deficient ob/ob mice respond abnormally to abrupt changes in nutritional status.

Regulation of expression of neuropeptide Y Y1 and Y2 receptors in the arcuate nucleus of fasted rats

Neuropeptide Y is expressed in neurons of the hypothalamic arcuate nucleus and has been ascribed a role as a stimulant of food intake. Neuropeptide Y Y1 and Y2 receptors are also localised in the arcuate nucleus, and it has been suggested that the Y1 receptor mediates part of the effect of neuropeptide Y on feeding behaviour. In the present study, immunohistochemistry and in situ hybridization were used to investigate the effect of food deprivation on the expression of Y1 and Y2 receptors in the arcuate nucleus of the rat. Fasting for 48 h induced a decrease in the number and area of Y1 receptor immunoreactive neurons in the arcuate nucleus. Furthermore, arcuate Y1 receptor mRNA levels also decreased after food deprivation. The decrease in the number of the Y1 receptor immunoreactive neurons was partially attenuated by supplementing the drinking water with 10% glucose. In contrast, fasting did not significantly change Y2 receptor mRNA levels in the arcuate nucleus. These results support the view that Y1 receptors in the arcuate nucleus play a role in the feeding pattern induced by neuropeptide Y.

A potent neuropeptide Y antagonist, 1229U91, suppressed spontaneous food intake in Zucker fatty rats

Neuropeptide Y (NPY) is one of the most potent orexigenic substances known. 1229U91 was found to be a potent and selective NPY antagonist. To elucidate a physiological role of NPY in hyperphagia in obese animals, we studied the effect of 1229U91 on spontaneous food intake in obese and lean Zucker rats. The food intake of Zucker rats was suppressed by intracerebroventricular administration of 1229U91 more potently in obese than in lean animals without abnormal behavior (31.7 and 67.3% inhibition at doses of 10 and 30 micrograms, respectively, in Zucker fatty rats and 22.2% inhibition at 30 micrograms in lean rats). This compound markedly suppressed NPY-induced food intake at 30 micrograms but did not affect galanin-induced food intake, suggesting that the feeding suppression seen in Zucker fatty and lean rats is pharmacologically and behaviorally specific. These results suggest that NPY is involved in feeding behavior in Zucker fatty rats and that NPY contributes to feeding to a greater degree in Zucker fatty than in lean rats. The hyperphagia in Zucker fatty rats may be due to the abnormal overactivation of the NPYergic system.

Hypothalamic and hypophyseal regulation of growth hormone secretion

1. Regulation of pulsatile secretion of growth hormone (GH) relies on hypothalamic neuronal loops, major transmitters involved in their operation are growth hormone releasing hormone (GHRH) synthetized mostly in arcuate nucleus (ARC) neurons, and somatostatin (SRIH), synthetized both in hypothalamus periventricular (PVe) and ARC neurons. 2. Neurons synthetizing both peptides can inhibit each other in a reciprocal manner. Other neuropeptides synthetized in ARC neurons, such as galanin, or in ARC interneurons, such as neuropeptide Y (NPY), are able to modulate synthesis and release of GHRH and SRIH into the hypothalamohypophyseal portal system. 3. In addition, the hitherto uncharacterized endogenous ligand of the recently cloned growth hormone releasing peptide receptor, expressed mostly in the ARC, triggers GH release, presumably by actions on ARC interneurons. 4. Thyroid, gonadal, and adrenal steroid hormones also affect the GHRH-SRIH balance; a differential distribution of sex steroid receptors in the ARC and the PVe is likely to account for the different pattern of GH secretion in male and female animals. 5. Growth hormone itself is able to inhibit the amplitude of GH secretory episodes and to increase their frequency, by entering the brain (presumably by receptor-mediated internalization at the level of the choroid plexus) and acting subsequently on ARC neurons. 6. At the pituitary level, major neurotransmitters regulating GH cells act on receptors of the VIP/PACAP/GHRH family and of the somatostatin family, in particular, sst2 and sst3. Those are coupled to accumulation of cAMP as a second messenger. 7. In addition, patch-clamp experiments and measurement of intracellular Ca2+ indicate that GH cells present characteristic, GHRH-dependent, but self-maintained Ca2+ spikes and [Ca2+]i transients, which reflect adaptive mechanisms to constraints of episodic release. 8. Recent data on transcription factors affecting GH gene expression and somatotrope differentiation are also summarized. 9. Regulation and differentiation of somatotropes also depend upon paracrine processes within the pituitary itself and involve growth factors and several neuropeptides, for instance, vasoactive intestinal peptide, angiotensin 2, endothelin, and activin. 10. Finally, characteristic changes occur in the GH secretory pattern under discrete, pathological conditions, such as abnormal growth and dwarfism, diabetes, and acromegaly, as well as during inflammatory processes.

Phosphorylation modulates the activity of the ATP-sensitive K+ channel in the ventromedial hypothalamic nucleus

Regulation of the ATP-sensitive K+ (K-ATP) channel was examined in cell-attached and inside-out membrane patches of freshly isolated neurons from the ventromedial hypothalamic nucleus (VMN) of 7-14 day old male Sprague-Dawley rats. When inside-out patches were exposed to symmetrical K+, the reversal potential was -2.85 +/- 1.65 mV, the single channel conductance 46 pS, and the total conductance varied as a multiple of this value. Glucose (10 mM) reversibly inhibited channel activity in cell-attached preparations by 81%. In the presence of 0.1 mM ADP, 10, 5, and 1 mM ATP reversibly inhibited VMN K-ATP channels in inside-out patches by 88, 83, and 60%, respectively. This inhibition was not dependent on phosphorylation since 5 mM AMPPNP, the non-hydrolyzable analog of ATP, reversibly inhibited channel activity by 67%. Relatively high concentrations of glibenclamide (100 microM) also reversibly inhibited VMN K-ATP channel activity in cell attached and inside-out patches by 67 and 79%, respectively. Finally, the non-specific kinase inhibitor H7 (200 microM) decreased channel activity by 53% while the non-specific phosphatase inhibitor microcystin (250 nM) increased channel activity by 218%. These data suggest that while the inhibitory effect of ATP is not phosphorylation dependent, phosphorylation state is an important regulator of the VMN K-ATP channel.

Increased feeding in fatty Zucker rats by the thiazolidinedione BRL 49653 (rosiglitazone) and the possible involvement of leptin and hypothalamic neuropeptide Y

1. The thiazolidinedione BRL 49653 (rosiglitazone) induces hyperphagia and weight gain in obese, insulin-resistant fatty Zucker rats but not in lean insulin-sensitive rats. We investigated whether these responses might involve neuropeptide Y (NPY), leptin and insulin. 2. BRL 49653 (1 mg kg(-1) day(-1), orally) was given for 7 or 20 days to fatty and lean Zucker and Wistar rats. 3. In lean rats of either strain, BRL 49653 had no effect on food intake, body weight, plasma insulin and corticosterone, NPY or NPY mRNA levels. 4. Fatty rats given BRL 49653 showed a 30% increase in food intake and accelerated body weight gain (both P<0.01) after 7 and 20 days, but without significant changes in regional hypothalamic NPY or NPY mRNA levels. 5. Plasma leptin levels were twice as high in untreated fatty Zucker rats as in lean rats (P<0.01), but were unaffected by BRL 49653 given for 20 days. However, BRL 49653 reduced insulin levels by 42% and increased corticosterone levels by 124% in fatty rats (both P<0.01). 6. Hyperphagia induced in fatty Zucker rats by BRL 49653 does not appear to be mediated by either a fall in circulating leptin levels or increased activity of hypothalamic NPYergic neurones. The fall in plasma insulin and/or rise in corticosterone levels during BRL 49653 treatment may be involved, consistent with the postulated role of these hormones in the control of food intake.

Reciprocal regional changes in brain NPY receptor density during dietary restriction and dietary-induced obesity in the rat

Neuropeptide Y (NPY) potently induces feeding, reduces thermogenesis and induces obesity in rats when injected into the cerebral ventricles. Groups of male Wistar rats were either restricted to 60% of their normal daily food intake over 10 days or made obese by presenting them with a high-calorie diet rich in sugars and fat over 6 weeks. Food restricted rats lost up to 20% of their body weight, compared with control rats and had large reductions in their body fat mass. By contrast, rats with dietary-induced obesity weighed 26% more than controls due mainly to increased body fat mass. Quantitative receptor autoradiography demonstrated reduced [(125)I]PYY binding in the hypothalamic lateral (perifornical) and dorsal areas, hypothalamic ventromedial, arcuate and dorsomedial nuclei, hippocampal CA3 region, centromedial amygdaloid nucleus and thalamic paraventricular and reuniens nuclei in dietary restricted rats compared with controls. By contrast, regional [(125)I]PYY binding was significantly increased in hypothalamic lateral and dorsal areas, hypothalamic arcuate and dorsomedial nuclei, amygdaloid medial and centromedial nuclei, thalamic centromedial and paraventricular nuclei of dietary obese rats versus controls. Masking NPY Y1 receptors with 1 microM BIBP3226, a selective Y1 receptor antagonist, revealed that the changes in [(125)I]PYY binding in brains of food-restricted and dietary-obese rats were due to changes in BIBP3226-insensitive binding sites, presumably Y2 or Y5 NPY receptors. These data suggest that dietary-restriction stimulates NPY release resulting in down-regulation of NPY Y5 'feeding' and/or Y2 receptors and reduced BAT thermogenesis thereby providing an increased drive to eat to restore normal caloric intake whilst reducing thermogenesis in order to conserve fat reserves. By contrast, the up-regulation of NPY Y5 and/or Y2 receptors in dietary-induced obesity is consistent with inhibition of NPY release in the hypothalamus, amygdala and thalamus. Overall, we suggest that there is a regional increase in NPY release during negative energy balance, such as during food-restriction and a reduced regional release of NPY in positive energy balance, such as during hyperphagia associated with the development of obesity.

Increased feeding and neuropeptide Y (NPY) but not NPY mRNA levels in the hypothalamus of the rat following central administration of the serotonin synthesis inhibitor p-chlorophenylalanine

Neurons containing serotonin (5-HT), a potent anorexic agent, come into contact with neuropeptide Y-ergic neurons, that project from the arcuate nucleus (ARC) to the paraventricular nucleus (PVN). NPY powerfully stimulates feeding and induces obesity when injected repeatedly into PVN. We hypothesize that 5-HT tonically inhibits the ARC-PVN neurons and that balance between the two systems determines feeding and energy homeostasis. This study aimed to determine whether central injection of the 5-HT synthesis inhibitor p-chlorophenylalanine (pCPA), which increases feeding, increased hypothalamic NPY and NPY mRNA levels. pCPA (10 mg/kg in 3 microliters) was administered into the third ventricle either as a single injection (n = 8) or daily for 7 days (n = 8). Control rats received a similar injection of saline. pCPA significantly increased food intake compared with controls after both single and repeated injections (P < 0.05). NPY levels were measured by radioimmunoassay in microdissected hypothalamic extracts. NPY levels in the acutely treated group were significantly increased in the paraventricular nucleus (PVN; by 41%, P = 0.01), anterior hypothalamic area (AHA; by 34%, P < 0.01) and lateral hypothalamic area (LHA; by 41%, P < 0.02). In the 7-day-treated group, NPY levels were also increased in the same areas, i.e. PVN (by 24%, P < 0.01), AHA (by 30%, P < 0.01) and LHA (by 38%, P = 0.01). There were no significant changes in the ARC or any other region or in hypothalamic NPY mRNA levels. pCPA administration increased NPY levels in several regions notably the PVN. This is a major site of NPY release, where NPY injection induces feeding. We suggest that the hyperphagia induced by pCPA is mediated by increased NPY levels and secretion in the PVN. This is further evidence for interactions between NPY and 5-HT in the control of energy homeostasis.

The serotonin (5-HT) antagonist methysergide increases neuropeptide Y (NPY) synthesis and secretion in the hypothalamus of the rat

NPY is synthesized in neurons of the hypothalamic arcuate nucleus (ARC) which project to the paraventricular nucleus (PVN), an important site of NPY release. Serotonin (5-HT) has been suggested to induce satiety and 5-HT fibers contact NPY neurons in the ARC and PVN, suggesting that 5-HT could inhibit the ARC-PVN projection. Methysergide is a 5-HT antagonist which stimulates feeding in rats and increases NPY levels in the hypothalamus. To clarify the effects of methysergide on NPY, we examined its effects on NPY synthesis and on NPY secretion in the PVN using push-pull sampling. Hypothalamic NPY mRNA levels were measured in rats (n = 8/group) given either saline or methysergide (10 mg/kg) and killed after 4 h or after 7 days. Food intake was increased by 33% in the acute study and by 9% in the 7-day study (both P < 0.01). NPY mRNA levels were 80% higher in the 7-day study (P < 0.05) and unchanged in the acute study. NPY secretion was measured over a 3-h period after an i.p. injection of methysergide or saline (10 mg/kg, n = 12) with a flow rate of 15 microliters/min. Mean NPY secretion in the methysergide-injected rats was increased by 34% (P < 0.01). We conclude that methysergide induced feeding is associated with increased activity of the NPY neurons in the ARC-PVN projection. This is consistent with our previous findings suggesting that the NPYergic ARC-PVN projection may mediate, at least in part the effects of 5-HT on feeding and energy balance.

Brain peptides and obesity: pharmacologic treatment

Obesity results from an imbalance between nutrient ingestion and metabolism, with more calories being ingested than utilized. The brain plays an important role in coordinating these complex behavioral and physiological functions, operating through multiple neurochemical systems with distinct properties. This review focuses on two hypothalamic peptide systems, neuropeptide Y (NPY) and galanin (GAL), that illustrate how the brain operates through different mechanisms to control the body's nutrient stores, in different states or conditions. These peptides have different behavioral and physiological effects and are, themselves, differentially responsive to feedback signals from circulating steroids, peptides, and nutrients. They can be distinguished by their relation to natural feeding patterns and endogenous hormones and by their specificity of action in relation to natural biological rhythms. The neuroanatomical substrates involved in these actions of NPY and GAL are also distinct. The neurocircuit mediating NPY's actions originates in the arcuate nucleus and terminates in the medial portion of the paraventricular nucleus; the GAL-containing neurons, in contrast, are concentrated in the lateral portion of the paraventricular nucleus, in addition to the medial preoptic area, which contribute to local GAL innervation as well as projections to the median eminence. Regarding their distinct functions, the evidence suggests that the NPY system is more closely related to patterns of carbohydrate ingestion and carbohydrate utilization, channeling nutrients towards the synthesis of fat. It is most strongly activated at the start of the active feeding cycle or after weaning, in close association with the adrenal steroid, corticosterone. The GAL system, in contrast, is more closely associated with patterns of fat consumption and signals related to fat oxidation. This peptide system is most active during the middle of the feeding cycle or immediately after puberty, in close association with the gonadal steroids. The gene expression and synthesis of these peptides in their respective neuronal cell groups is inhibited by circulating insulin and altered by dietary nutrients. Disturbances in sensitivity to insulin and steroid feedback regulation in the brain are believed to be involved in producing abnormal patterns of peptide function that result in overeating and body weight gain.

Interleukin-1 beta-induced anorexia and pyrexia in rat: relationship to hypothalamic neuropeptide Y

We investigated the effect of recombinant human interleukin-1 beta (rhIL-1 beta)-induced anorexia and pyrexia on the hypothalamic neuropeptide Y (NPY)-ergic system, which stimulates feeding and reduces thermogenesis. In meal-fed rats, food intake decreased by 83%, 90 min after IL-1 beta treatment (1.3 micrograms/100 g ip; n - 8) vs. controls. NPY concentrations were significantly higher in the medial preoptic area (MPO), paraventricular (PVN), ventromedial (VMN), and dorsomedial (DMN) nuclei but unchanged in the arcuate nucleus (ARC) in both IL-1 beta-treated and pair-fed groups. Indomethacin (0.25 mg/100 g ip) reduced IL-1 beta-induced anorexia and tended to normalize NPY concentrations. In study 2, IL-1 beta increased core temperature by 1.1 degrees C above preinjection values (P < 0.001) and significantly raised NPY concentrations in the MPO, PVN, VMN, and DMN compared with controls, 60 min postinjection. Indomethacin prevented the pyrexia and normalized hypothalamic NPY levels. As NPY concentrations were not increased in the ARC (the hypothalamic site of synthesis), we suggest that the increased NPY levels may result from blocked release, which would be in accord with the known experimental effects of NPY.

Increased neuropeptide Y secretion in the hypothalamic paraventricular nucleus of obese (fa/fa) Zucker rats

NPY is synthesized in the hypothalamic arcuate nucleus (ARC), and NPY injected into the paraventricular nucleus (PVN), the main site of NPY release, induces hyperphagia and reduces energy expenditure. Hypothalamic NPY mRNA and NPY levels are increased in fatty Zucker rats, consistent with increased NPY release. This could explain the hyperphagia and reduced energy expenditure, which lead to obesity in the fatty Zucker rat. We have therefore compared NPY secretion in the PVN of conscious fatty and lean Zucker rats using push-pull sampling. The NPY secretory profile was consistently higher in fatty Zucker rats than in lean rats throughout the 3-h study period (P < 0.01), and mean NPY secretion over the whole 3 h was increased 2-fold in the fatty rats (P < 0.001). We conclude that fatty Zucker rats have increased NPY release in the PVN. This observation further supports the hypothesis that increased activity of the NPYergic ARC-PVN pathway may contribute to obesity in the fatty Zucker syndrome.

Effects of opioid antagonists naloxone and naltrexone on neuropeptide Y-induced feeding and brown fat thermogenesis in the rat. Neural site of action

Neuropeptide Y administered intracerebroventricularly and into the paraventricular nucleus of the hypothalamus stimulates feeding and decreases brown adipose tissue thermogenesis. Although specific neuropeptide Y antagonists are not yet available, previous studies had shown that the opioid antagonist naloxone blocked neuropeptide Y-induced feeding when both drugs were injected intracerebroventricularly. We wanted to find out if naloxone injected into specific brain sites would block neuropeptide Y effects on feeding and brown fat thermogenesis. Rats were double injected in specific brain sites with neuropeptide Y and either naloxone or naltrexone (a congener of naloxone). Food intake and brown fat measures were assessed. Naloxone or naltrexone in the paraventricular nucleus weakly decreased paraventricular nucleus neuropeptide Y-induced feeding and did not affect neuropeptide Y-induced reductions in brown fat activity. Peripheral naloxone blocked intracerebroventricular neuropeptide Y-induced feeding and brown fat alterations. Fourth ventricular naloxone decreased paraventricular nucleus neuropeptide Y-induced feeding, and naltrexone given into the nucleus of the solitary tract blocked paraventricular nucleus neuropeptide Y-induced alterations in feeding and brown fat. These data indicate that neuropeptide Y in the paraventricular nucleus may act on feeding and brown fat thermogenesis through opioidergic pathways in the nucleus of the solitary tract.