Role of the Y5 neuropeptide Y receptor in feeding and obesity

Deep brain stimulation in the treatment of obesity

Obesity is a growing global health problem frequently intractable to current treatment options. Recent evidence suggests that deep brain stimulation (DBS) may be effective and safe in the management of various, refractory neuropsychiatric disorders, including obesity. The authors review the literature implicating various neural regions in the pathophysiology of obesity, as well as the evidence supporting these regions as targets for DBS, in order to explore the therapeutic promise of DBS in obesity.

The lateral hypothalamus and ventromedial hypothalamus are the appetite and satiety centers in the brain, respectively. Substantial data support targeting these regions with DBS for the purpose of appetite suppression and weight loss. However, reward sensation associated with highly caloric food has been implicated in overconsumption as well as obesity, and may in part explain the failure rates of conservative management and bariatric surgery. Thus, regions of the brain's reward circuitry, such as the nucleus accumbens, are promising alternatives for DBS in obesity control.

The authors conclude that deep brain stimulation should be strongly considered as a promising therapeutic option for patients suffering from refractory obesity.

Ablation of neurons expressing agouti-related protein activates fos and gliosis in postsynaptic target regions

We have developed a mouse model in which a specific population of inhibitory neurons can be selectively ablated by the action of diphtheria toxin (DT). The model involves targeting the human DT receptor to the agouti-related protein (Agrp) locus so that systemic administration of DT kills all of the AgRP-expressing neurons, resulting in starvation of the mice. Ablation of AgRP neurons results in robust (5- to 10-fold) activation of Fos gene expression in many brain regions that are innervated by AgRP neurons, including the arcuate nucleus (ARC), the paraventricular nucleus, the medial preoptic area, the lateral septum, and nucleus of the solitary tract. As expected, there is robust increase in GFAP staining (astrocytes) as well as IBA1 and CD11b staining (microglia) in the ARC in response to AgRP neuron ablation. There is also a dramatic increase of these markers in most, but not all, postsynaptic targets of AgRP axons. We used a genetic approach to reduce melanocortin signaling, which attenuated Fos activation in some brain regions after ablation of AgRP neurons. We suggest that loss of inhibitory signaling onto target neurons results in unopposed excitation that is responsible for the activation of Fos and that dysregulation of these neuronal circuits is responsible for starvation. Furthermore, glial cell activation in target areas of AgRP neurons appears to be a result of excitotoxicity.

Relevance of animal models to human eating disorders and obesity

BACKGROUND AND RATIONALE

This review addresses the role animal models play in contributing to our knowledge about the eating disorders anorexia nervosa (AN) and bulimia nervosa (BN) and obesity.

OBJECTIVES

Explore the usefulness of animal models in complex biobehavioral familial conditions, such as AN, BN, and obesity, that involve interactions among genetic, physiologic, psychological, and cultural factors.

RESULTS AND CONCLUSIONS

The most promising animal model to mimic AN is the activity-based anorexia rodent model leading to pathological weight loss. The paradigm incorporates reward elements of the drive for activity in the presence of an appetite and allows the use of genetically modified animals. For BN, the sham-feeding preparation in rodents equipped with a gastric fistula appears to be best suited to reproduce the postprandial emesis and the defects in satiety. Animal models that incorporate genes linked to behavior and mood may clarify biobehavioral processes underlying AN and BN. By contrast, a relative abundance of animal models has contributed to our understanding of human obesity. Both environmental and genetic determinants of obesity have been modeled in rodents. Here, we consider single gene mutant obesity models, along with models of obesigenic environmental conditions. The contributions of animal models to obesity research are illustrated by their utility for identifying genes linked to human obesity, for elucidating the pathways that regulate body weight and for the identification of potential therapeutic targets. The utility of these models may be further improved by exploring the impact of experimental manipulations on the behavioral determinants of energy balance.

Y eat?

Feeding behavior is tightly regulated by peptidergic transmission within the hypothalamus. Neuropeptide Y (NPY) is one of the most potent known stimulators of food intake and has robust effects on the hypothalamic feeding neuronal networks. A vast body of literature has documented the substantial effects of NPY on feeding behavior. However, the cellular mechanisms underlying the actions of NPY have only recently begun to be explored. The NPYergic signal, including its expression in hypothalamic neurons, its release into the synaptic space, and its direct or indirect receptor-mediated actions, is highly responsive to decreases in the metabolic state. The orexigenic NPY signal can suppress the anorexigenic drive to restore energy balance homeostasis when energy levels are low, such as after food deprivation. The NPY signal interacts with glucose- and fat-sensitive signals arriving in the hypothalamus and effects changes in anorexigenic pathways, such as those mediated by the melanocortins. Recent applications of electrophysiological methods to examine the neuronal activity and pathways engaged by NPY-mediated signaling have advanced our understanding of this orexigenic system. Furthermore, crucial roles for NPY pathways in the development of hypothalamic feeding circuitry have been identified by these means. Orexigenic NPY signaling is critical during development and its absence is lethal in adults, thus reflecting the essential role of NPY for the regulation of energy homeostasis.

Synergistic attenuation of obesity by Y2- and Y4-receptor double knockout in ob/ob mice

OBJECTIVE

Neuropeptide Y regulates numerous processes including food intake, body composition, and reproduction by at least five different Y receptors. We previously demonstrated a synergistic interaction between Y2 and Y4 receptors in reducing adiposity in chow- or fat-fed Y2Y4-receptor double-knockout mice. In the present study, we investigated whether this synergy could reduce the massive obesity of leptin-deficient ob/ob mice.

METHODS

Mice with germline deletions of Y2 and Y4 receptors were crossed onto the ob/ob strain. Body weight was measured weekly until 15-18 wk of age before decapitation for collection of trunk blood and tissues.

RESULTS

Male and female Y24ob triple mutants showed highly significant reductions in body weight and white adipose tissue mass compared with ob/ob mice. This reduction in body weight was not evident in Y2ob or Y4ob double mutants, and the effect on adiposity was significantly greater than that seen in Y2ob or Y4ob mice. These changes were associated with significant attenuation of the increased brown adipose tissue mass and small intestinal hypertrophy seen in ob/ob mice and with normalization of the low circulating free thyroxine concentrations seen in female ob/ob mice and the high circulating corticosterone concentrations seen in male ob/ob mice.

CONCLUSION

These data reveal a synergistic interaction between Y2 and Y4 receptors in attenuating the massive obesity of ob/ob mice, possibly mediated by stimulation of thyroid function and inhibition of intestinal nutrient absorption. Dual pharmacologic antagonism of Y2 and Y4 receptors could help people to attain and maintain a healthy weight.

(9S)-9-(2-hydroxy-4,4-dimethyl-6-oxo-1-cyclohexen-1-yl)-3,3-dimethyl-2,3,4,9-tetrahydro-1H-xanthen-1-one, a selective and orally active neuropeptide Y Y5 receptor antagonist

(9S)-9-(2-Hydroxy-4,4-dimethyl-6-oxo-1-cyclohexen-1-yl)-3,3-dimethyl-2,3,4,9-tetrahydro-1H-xanthen-1-one ((S)-1) was identified as a selective and orally active neuropeptide Y Y5 receptor antagonist. The structure-activity relationship for this structural class was investigated and showed that limited substitution on the phenyl ring was tolerated and that modification of the 4,4-dimethyl group of the cyclohexenone and the 3,3-dimethyl group of the xanthenone parts slightly improved potency. The plasma concentration-time profile after oral administration of (S)-1 in Sprague-Dawley (SD) rats showed significant in vivo racemization of (S)-1 and that (S)-1 is cleared much more quickly than (R)-1. The duration of (S)-1 in SD rats after oral administration of (RS)-1 racemate was twice as long as that following oral administration of (S)-1. The C max values of (S)-1 after administration of (S)-1 and (RS)-1 were comparable, and the brain to plasma ratio for (S)-1 was 0.34 in SD rats. In our acute D-Trp (34)NPY-induced food intake model, both (S)-1 and (RS)-1 showed potent and dose-dependent efficacy. Therefore, the use of (RS)-1 is suitable for studies that require sustained plasma exposure of (S)-1.

Effects of a novel Y5 antagonist in obese mice: combination with food restriction or sibutramine

OBJECTIVE

To further address the function of the Y5 receptor in energy homeostasis, we investigated the effects of a novel spironolactone Y5 antagonist in diet-induced obese (DIO) mice.

METHODS AND PROCEDURES

Male C57BL/6 or Npy5r(-/-) mice were adapted to high-fat (HF) diet for 6-10 months and were submitted to three experimental treatments. First, the Y5 antagonist at a dose of 10 or 30 mg/kg was administered for 1 month to DIO C57BL/6 or Npy5r(-/-) mice. Second, the Y5 antagonist at 30 mg/kg was administered for 1.5 months to DIO C57BL/6 mice, and insulin sensitivity was evaluated using an insulin tolerance test. After a recovery period, nuclear magnetic resonance measurement was performed to evaluate body composition. Third, DIO mice were treated with the Y5 antagonist alone, or in combination with 10% food restriction, or with another anorectic agent, sibutramine at 10 mg/kg, for 1.5 months. Plasma glucose, insulin, and leptin levels, and adipose tissue weights were quantified.

RESULTS

The spironolactone Y5 antagonist significantly reduced body weight in C57BL DIO mice, but not in Npy5r(-/-) DIO mice. The Y5 antagonist produced a fat-selective loss of body weight, and ameliorated obesity-associated insulin resistance in DIO mice. In addition, the Y5 antagonist combined with either food restriction or sibutramine tended to produce greater body weight loss, as compared with single treatment.

DISCUSSION

These findings demonstrate that the Y5 receptor is an important mediator of energy homeostasis in rodents.

Peptide YY (PYY) levels and bone mineral density (BMD) in women with anorexia nervosa

INTRODUCTION

Anorexia nervosa (AN) is a psychiatric illness that results in significant bone loss. Studies examining the neuroendocrine dysregulation that occurs in AN may increase understanding of endocrine systems that regulate bone mass. Peptide YY (PYY) is an anorexigenic peptide derived primarily from the intestine, with actions mediated via activation of Y receptors. We have previously shown that PYY levels are elevated in adolescents with AN. Y2 receptor knockout mice have increased bone mineral density (BMD) and thus PYY may play a role in regulating bone mass. We hypothesized that PYY levels would be inversely associated with BMD in women with AN.

METHODS

This was a cross-sectional study performed in a General Clinical Research Center of 12 adult women with AN, (mean+/-SEM) mean age 30.9+/-1.8 years, BMI 17.1+/-0.4 kg/m2, and % ideal body weight 77.5+/-1.7%. PYY concentrations were measured hourly from 20:00 h to 08:00 h. BMD was measured using dual X-ray absorptiometry (DXA).

RESULTS

In women with AN, mean overnight PYY levels strongly inversely correlated with BMD at the PA spine (r=-0.77, p=0.003), lateral spine (r=-0.82, p=0.002), total hip (r=-0.75, p=0.005), femoral neck (r=-0.72, p=0.009), total radius (r=-0.72, p=0.009) and 1/3 distal radius (r=-0.81, p=0.002). Body mass index was inversely correlated with PYY level (r=-0.64, p=0.03). Multivariate stepwise regression analysis was performed to determine the contribution of age, duration of AN, BMI, fat-free mass, and PYY to BMD. For PA and lateral spine, PYY was the primary determinant of BMD, accounting for 59% and 67% of the variability, respectively. Fat-free mass and duration of anorexia nervosa were the primary determinants of BMD at other skeletal sites.

CONCLUSIONS

In women with anorexia nervosa, an elevated PYY level is strongly associated with diminished BMD, particularly at the spine. Therefore further investigation of the hypothesis that PYY may contribute to the prevalent bone pathology in this disorder is merited.

Pharmacological characterization and appetite suppressive properties of BMS-193885, a novel and selective neuropeptide Y(1) receptor antagonist

Treatment of obesity is still a large unmet medical need. Neuropeptide Y is the most potent orexigenic peptide in the animal kingdom. Its five cloned G-protein couple receptors are all implicated in the regulation of energy homeostasis evidenced by overexpression or deletion of neuropeptide Y or its receptors. Neuropeptide Y most likely exerts its orexigenic activity via the neuropeptide Y(1) and neuropeptide Y(5) receptors, although the involvement of the neuropeptide Y(2) and neuropeptide Y(4) receptors are also gaining importance. The lack of potent, selective, and brain penetrable pharmacologic agents at these receptors made our understanding of the modulation of food intake by neuropeptide Y-ergic agents elusive. BMS-193885 (1,4-dihydro-[3-[[[[3-[4-(3-methoxyphenyl)-1-piperidinyl]propyl]amino] carbonyl]amino]phenyl]-2,6-dimethyl-3,5-pyridinedicarboxylic acid, dimethyl ester) is a potent and selective neuropeptide Y(1) receptor antagonist. BMS-193885 has 3.3 nM affinity at the neuropeptide Y(1) receptor, acting competitively at the neuropeptide Y binding site. BMS-193885 increased the K(d) of [(125)I]PeptideYY from 0.35 nM to 0.65 nM without changing the B(max) (0.16 pmol/mg of protein) in SK-N-MC cells that endogenously express the neuropeptide Y(1) receptor. It is also found to be a full antagonist with an apparent K(b) of 4.5 nM measured by reversal of forskolin (FK)-stimulated inhibition of cAMP production by neuropeptide Y. Pharmacological profiling showed that BMS-193885 has no appreciable affinity at the other neuropeptide Y receptors, and is also 200-fold less potent at the alpha(2) adrenergic receptor. Testing the compound in a panel of 70 G-protein coupled receptors and ion channels resulted in at least 200-fold or greater selectivity, with the exception of the sigma(1) receptor, where the selectivity was 100-fold. When administered intracerebroventricularly or directly into the paraventricular nucleus of the hypothalamus, it blocked neuropeptide Y-induced food intake in rats. Intraperitoneal administration of BMS-193885 (10 mg/kg) also reduced one-hour neuropeptide Y-induced food intake in satiated rats, as well as spontaneous overnight food consumption. Chronic administration of BMS-193885 (10 mg/kg) i.p. for 44 days significantly reduced food intake and the rate of body weight gain compared to vehicle treated control without developing tolerance or affecting water intake. These results provide supporting evidence that BMS-193885 reduces food intake and body weight via inhibition of the central neuropeptide Y(1) receptor. BMS-193885 has no significant effect of locomotor activity up to 20 mg/kg dose after 1 h of treatment. It also showed no activity in the elevated plus maze when tested after i.p. and i.c.v. administration, indicating that reduction of food intake is unrelated to anxious behavior. BMS-193885 has good systemic bioavailability and brain penetration, but lacks oral bioavailability. The compound had no serious cardiovascular adverse effect in rats and dogs up to 30 and 10 mg/kg dose, respectively, when dosed intravenously. These data demonstrate that BMS-193885 is a potent, selective, brain penetrant Y(1) receptor antagonist that reduces food intake and body weight in animal models of obesity both after acute and chronic administration. Taken together the data suggest that a potent and selective neuropeptide Y(1) receptor antagonist might be an efficacious treatment for obesity in humans.

Pathogenic potential of adipose tissue and metabolic consequences of adipocyte hypertrophy and increased visceral adiposity

When caloric intake exceeds caloric expenditure, the positive caloric balance and storage of energy in adipose tissue often causes adipocyte hypertrophy and visceral adipose tissue accumulation. These pathogenic anatomic abnormalities may incite metabolic and immune responses that promote Type 2 diabetes mellitus, hypertension and dyslipidemia. These are the most common metabolic diseases managed by clinicians and are all major cardiovascular disease risk factors. 'Disease' is traditionally characterized as anatomic and physiologic abnormalities of an organ or organ system that contributes to adverse health consequences. Using this definition, pathogenic adipose tissue is no less a disease than diseases of other body organs. This review describes the consequences of pathogenic fat cell hypertrophy and visceral adiposity, emphasizing the mechanistic contributions of genetic and environmental predispositions, adipogenesis, fat storage, free fatty acid metabolism, adipocyte factors and inflammation. Appreciating the full pathogenic potential of adipose tissue requires an integrated perspective, recognizing the importance of 'cross-talk' and interactions between adipose tissue and other body systems. Thus, the adverse metabolic consequences that accompany fat cell hypertrophy and visceral adiposity are best viewed as a pathologic partnership between the pathogenic potential adipose tissue and the inherited or acquired limitations and/or impairments of other body organs. A better understanding of the physiological and pathological interplay of pathogenic adipose tissue with other organs and organ systems may assist in developing better strategies in treating metabolic disease and reducing cardiovascular disease risk.

Starvation after AgRP neuron ablation is independent of melanocortin signaling

Ablation of inhibitory agouti-related protein (AgRP)-expressing neurons in the arcuate nucleus that also synthesize gamma-amino-butyric acid (GABA) and neuropeptide Y in adult mice leads to starvation within 1 week. The removal of inhibition from the AgRP neurons onto neighboring proopiomelanocortin neurons and their common postsynaptic neurons is predicted to stimulate melanocortin signaling, which is known to inhibit appetite. To examine the importance of uncontrolled melanocortin signaling in mediating starvation in this model, we ablated AgRP neurons in A(y)/a mice that have chronic blockade of the melanocortin signaling. The blockade of melanocortin signaling did not ameliorate the rate of starvation. On both WT and A(y)/a genetic backgrounds, there was a progressive decrease in meal frequency after AgRP neuron ablation. Surprisingly, intraoral feeding also was dramatically reduced after the ablation of AgRP neurons. These results indicate that both the appetitive and consummatory aspects of feeding become impaired in a melanocortin-independent manner after AgRP neuron ablation.

Physiological roles of the Ca2+/CaM-dependent protein kinase cascade in health and disease

Numerous hormones, growth factors and physiological processes cause a rise in cytosolic Ca2+, which is translated into meaningful cellular responses by interacting with a large number of Ca2(+)-binding proteins. The Ca2(+)-binding protein that is most pervasive in mediating these responses is calmodulin (CaM), which acts as a primary receptor for Ca2+ in all eukaryotic cells. In turn, Ca2+/CaM functions as an allosteric activator of a host of enzymatic proteins including a considerable number of protein kinases. The topic of this review is to discuss the physiological roles of a sub-set of these protein kinases which can function in cells as a Ca2+/CaM-dependent kinase signaling cascade. The cascade was originally believed to consist of a CaM kinase kinase that phosphorylates and activates one of two CaM kinases, CaMKI or CaMKIV. The unusual aspect of this cascade is that both the kinase kinase and the kinase require the binding of Ca2+/CaM for activation. More recently, one of the CaM kinase kinases has been found to activate another important enzyme, the AMP-dependent protein kinase so the concept of the CaM kinase cascade must be expanded. A CaM kinase cascade is important for many normal physiological processes that when misregulated can lead to a variety of disease states. These processes include: cell proliferation and apoptosis that may conspire in the genesis of cancer; neuronal growth and function related to brain development, synaptic plasticity as well as memory formation and maintenance; proper function of the immune system including the inflammatory response, activation of T lymphocytes and hematopoietic stem cell maintenance; and the central control of energy balance that, when altered, can lead to obesity and diabetes. Although the study of the CaM-dependent kinase cascades is still in its infancy continued analysis of the pathways regulated by these Ca2(+)-initiated signaling cascades holds considerable promise for the future of disease-related research.

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.

NPY Y1 receptor antagonist prevents NPY-induced torporlike hypothermia in cold-acclimated Siberian hamsters

Siberian hamsters ( Phodopus sungorus) undergo bouts of daily torpor during which body temperature decreases by as much as 20°C and provides a significant savings in energy expenditure. Natural torpor in this species is normally triggered by winterlike photoperiods and low ambient temperatures. Intracerebroventricular injection of neuropeptide Y (NPY) reliably induces torporlike hypothermia that resembles natural torpor. NPY-induced torporlike hypothermia is also produced by intracerebroventricular injections of an NPY Y1 receptor agonist but not by injections of an NPY Y5 receptor agonist. In this research, groups of cold-acclimated Siberian hamsters were either coinjected with a Y1 receptor antagonist (1229U91) and NPY or were coinjected with a Y5 receptor antagonist ( CGP71683 ) and NPY in counterbalanced designs. Paired vehicle + NPY induced torporlike hypothermia in 92% of the hamsters, whereas coinjection of Y1 antagonist + NPY induced torporlike hypothermia in 4% of the hamsters. In contrast, paired injections of vehicle + NPY and Y5 antagonist + NPY induced torporlike hypothermia in 100% and 91% of the hamsters, respectively. Although Y5 antagonist treatment alone had no effect on body temperature, Y1 antagonist injections produced hyperthermia compared with controls. Both Y1 antagonist and Y5 antagonist injections significantly reduced food ingestion 24 h after treatment. We conclude that activation of NPY 1 receptors is both sufficient and necessary for NPY-induced torporlike hypothermia.

Neuropeptide y receptor selective ligands in the treatment of obesity

Obesity is a serious public health problem throughout the world, affecting both developed societies and developing countries. The central nervous system has developed a meticulously interconnected circuitry in order to keep us fed and in an adequate nutritional state. One of these consequences is that an energy-dense environment favors the development of obesity. Neuropeptide Y (NPY) is one of the most abundant and widely distributed peptides in the central nervous system of both rodents and humans and has been implicated in a variety of physiological actions. Within the hypothalamus, NPY plays an essential role in the control of food intake and body weight. Centrally administered NPY causes robust increases in food intake and body weight and, with chronic administration, can eventually produce obesity. NPY activates a population of at least six G protein-coupled Y receptors. NPY analogs exhibit varying degrees of affinity and specificity for these Y receptors. There has been renewed speculation that ligands for Y receptors may be of benefit for the treatment of obesity. This review highlights the therapeutic potential of Y(1), Y(2), Y(4), and Y(5) receptor agonists and antagonists as additional intervention to treat human obesity.

NPY5R antagonism does not augment the weight loss efficacy of orlistat or sibutramine

OBJECTIVE

Central counter-regulatory mechanisms, including those related to the orexigenic hormone neuropeptide Y (NPY), may limit the weight loss observed with conventional pharmacological monotherapy. This study evaluated whether blockade of the NPY Y5 receptor (NPY5R) with the selective antagonist MK-0557 potentiates sibutramine and orlistat weight loss effects.

RESEARCH METHODS AND PROCEDURES

Obese patients (497, BMI 30 to 43 kg/m2) were randomized to 1 of 5 treatment arms [placebo, n = 101; sibutramine 10 mg/d, n = 100; MK-0557 1 mg/d plus sibutramine 10 mg/d, n = 98; orlistat 120 mg TID, n = 99; MK-0557 1 mg/d plus orlistat 120 mg TID, n = 99] in conjunction with a hypocaloric diet for 24 weeks. The all-patients-treated population, imputing missing data using last observation carried forward, was used to assess weight loss from baseline.

RESULTS

The study was completed by 71% of patients in placebo, 76% in sibutramine alone, 79% in MK-0557 + sibutramine, 69% in orlistat alone, and 76% in MK-0557 + orlistat groups. Least squares (LS) mean difference [95% confidence interval (CI)] in weight change from baseline between MK-0557 + sibutramine and sibutramine alone was -0.1 (-1.6, 1.4) kg (p = 0.892) and between MK-0557 + orlistat and orlistat alone was -0.9 (-2.4, 0.6) kg (p = 0.250). Sibutramine alone induced a LS mean weight loss of -5.9 (-6.9, -4.9) kg vs. -4.6 (-5.7, -3.6) kg for orlistat (p = 0.097). There were no serious drug-related adverse events and MK-0557 was well tolerated.

DISCUSSION

Blockade of the NPY5R with the potent antagonist MK-0557 did not significantly increase the weight loss efficacy of either orlistat or sibutramine monotherapy.

Chronic intracerebroventricular administration of anti-neuropeptide Y antibody stimulates starvation-induced feeding via compensatory responses in the hypothalamus

To investigate how compensatory responses develop after the onset of inhibition of NPY signaling, we examined the effect of continuous intracerebroventricular (ICV) injection of neutralizing NPY antibodies (NPY-ab) on daily and fast-induced food intake in mice. A single ICV injection of NPY-ab reduced food intake in fasted mice. In contrast to a single injection, continuous ICV injection of NPY-ab for 13 days increased fast-induced food intake, although daily food intake was unaffected by continuous administration of NPY-ab. Immunohistochemistry indicated that the expression of NPY protein increases in the arcuate nucleus, lateral hypothalamic area, and paraventricular nucleus 7 days after onset of continuous NPY-ab infusion and remains at an elevated level, whereas the expression of the NPY Y1 receptor transiently increases in the same areas for 3 days and then gradually decreases. Similar results were obtained for the expression of NPY and NPY Y1 receptor mRNA. The mRNA level of agouti-related protein, another orexigenic neuropeptide, also increased in parallel with NPY, whereas that of pro-opiomelanocortin did not change over the 13 days of the NPY-ab administration. These results suggest that chronic central inhibition of NPY immediately activates orexigenic signaling in first-order hypothalamic neurons and that this compensatory mechanism normalizes the regulation of feeding and energy expenditure to maintain energy homeostasis. On the other hand, in mice that have acquired this compensation, fast-induced food intake further increases even after the energy deficit is corrected because of the dominant orexigenic signal.

Medical sequencing at the extremes of human body mass

Body weight is a quantitative trait with significant heritability in humans. To identify potential genetic contributors to this phenotype, we resequenced the coding exons and splice junctions of 58 genes in 379 obese and 378 lean individuals. Our 96-Mb survey included 21 genes associated with monogenic forms of obesity in humans or mice, as well as 37 genes that function in body weight-related pathways. We found that the monogenic obesity-associated gene group was enriched for rare nonsynonymous variants unique to the obese population compared with the lean population. In addition, computational analysis predicted a greater fraction of deleterious variants within the obese cohort. Together, these data suggest that multiple rare alleles contribute to obesity in the population and provide a medical sequencing-based approach to detect them.

Effect of NPY5R antagonist MK-0557 on weight regain after very-low-calorie diet-induced weight loss

OBJECTIVE

To evaluate whether MK-0557, a highly selective, orally administered neuropeptide Y Y5 receptor antagonist, could limit weight regain after very-low-calorie diet (VLCD)-induced weight loss.

RESEARCH METHODS AND PROCEDURES

We enrolled 502 patients 18 to 65 years of age with a BMI of 30 to 43 kg/m2. Patients were placed on a VLCD (800 kcal/d liquid diet) for 6 weeks. Patients who lost>or=6% of initial body weight (n=359) were randomized to 52 weeks of 1 mg/d MK-0557 or placebo and maintained on a hypocaloric diet (300 kcal below weight maintenance requirements).

RESULTS

In randomized patients, the VLCD was associated with an average weight loss of 9.1 kg. After 12 weeks of double-blind treatment, weight began to gradually increase for both placebo- and MK-0557-treated patients. The mean weight change (95% confidence interval) from baseline at the end of the VLCD to Week 52 was +3.1 (2.1, 4.0) and +1.5 (0.5, 2.4) kg for patients treated with placebo and MK-0557, respectively. The difference of 1.6 kg between the two groups was significant (p=0.014). Secondary endpoints, such as blood pressure, lipid profile, insulin, and leptin, as well as waist circumference and quality-of-life measurements, did not show significant differences between MK-0557 and placebo treatments.

DISCUSSION

Although the difference in weight regain between placebo- and MK-0557-treated patients was statistically significant, the magnitude of the effect was small and not clinically meaningful. Antagonism of the neuropeptide Y Y5 receptor is not an efficacious treatment strategy for reducing weight regain after VLCD.

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.

To subjugate NPY is to improve the quality of life and live longer

The interactive network of neuropeptide Y (NPY) and cohorts is necessary for integrating the hypothalamic regulation of appetite and energy expenditure with the endocrine and neuroendocrine systems on a daily basis. Genetic and environmental factors that produce an insufficiency of leptin restraint on NPY and cognate receptors deregulate the homeostasis to engender various life-threatening risk factors. Recent studies from our laboratory show that neurotherapy consisting of a single central administration of recombinant adeno-associated virus vector encoding the leptin gene can repress the hypothalamic NPY system for the lifetime of rodents. A major benefit of this stable tonic restraint is deceleration of pathophysiologic sequalae that shorten life span. These include suppression of weight gain, fat accumulation, circulating adipokines, amelioration of major symptoms of metabolic syndrome, improved reproduction and bone health. Thus, sustained repression of NPY signaling in the hypothalamus by leptin transgene expression can improve the quality of life and extend longevity.

Molecular signaling involved in regulating feeding and other mitivated behaviors

The metabolic and nutritional status of an organism influences multiple behaviors in addition to food intake. When an organism is hungry, it employs behaviors that help it locate and ingest food while suppressing behaviors that are not associated with this goal. Alternatively, when an organism is satiated, food-seeking behaviors are repressed so that the animal can direct itself to other goal-oriented tasks such as reproductive behaviors. Studies in both vertebrate and invertebrate model systems have revealed that food-deprived and -satiated behaviors are differentially executed and integrated via common molecular signaling mechanisms. This article discusses cellular and molecular mechanisms for how insulin, neuropeptide Y (NPY), and serotonin utilize common signaling pathways to integrate feeding and metabolic state with other motivated behaviors. Insulin, NPY, and serotonin are three of the most well-studied molecules implicated in regulating such behaviors. Overall, insulin signaling allows an organism to coordinate proper behavioral output with changes in metabolism, NPY activates behaviors required for locating and ingesting food, and serotonin modulates behaviors performed when an organism is satiated. These three molecules work to ensure that the proper behaviors are executed in response to the feeding state of an organism.

Emerging therapeutic strategies for obesity

The rising tide of obesity is one of the most pressing health issues of our time, yet existing medicines to combat the problem are disappointingly limited in number and effectiveness. Fortunately, a recent burgeoning of mechanistic insights into the neuroendocrine regulation of body weight provides an expanding list of molecular targets for novel, rationally designed antiobesity pharmaceuticals. In this review, we articulate a set of conceptual principles that we feel could help prioritize among these molecules in the development of obesity therapeutics, based on an understanding of energy homeostasis. We focus primarily on central targets, highlighting selected strategies to stimulate endogenous catabolic signals or inhibit anabolic signals. Examples of the former approach include methods to enhance central leptin signaling through intranasal leptin delivery, use of superpotent leptin-receptor agonists, and mechanisms to increase leptin sensitivity by manipulating SOCS-3, PTP-1B, ciliary neurotrophic factor, or simply by first losing weight with traditional interventions. Techniques to augment signaling by neurochemical mediators of leptin action that lie downstream of at least some levels of obesity-associated leptin resistance include activation of melanocortin receptors or 5-HT2C and 5-HT1B receptors. We also describe strategies to inhibit anabolic molecules, such as neuropeptide Y, melanin-concentrating hormone, ghrelin, and endocannabinoids. Modulation of gastrointestinal satiation and hunger signals is discussed as well. As scientists continue to provide fundamental insights into the mechanisms governing body weight, the future looks bright for development of new and better antiobesity medications to be used with diet and exercise to facilitate substantial weight loss.

Role of neuropeptides in appetite regulation and obesity--a review

Obesity represents the most prevalent nutritional problem worldwide which in the long run predisposes to development of diabetes mellitus, hypertension, endometrial carcinoma, osteoarthritis, gall stones and cardiovascular diseases. Despite significant reductions in dietary fat consumption, the prevalence of obesity is on a rise and is taking on pandemic proportions. Obesity develops when energy intake exceeds energy expenditure over time. Recently, a close evolutionary relationship between the peripheral and hypothalamic neuropeptides has become apparent. The hypothalamus being the central feeding organ mediates regulation of short-term and long-term dietary intake via synthesis of various orexigenic and anorectic neuropeptides. The structure and function of many hypothalamic peptides (neuropeptide Y (NPY), melanocortins, agouti-related peptide (AGRP), cocaine and amphetamine regulated transcript (CART), melanin concentrating hormone (MCH), orexins have been characterized in rodent models The peripheral neuropeptides such as cholecystokinin (CCK), ghrelin, peptide YY (PYY3-36), amylin, bombesin regulate important gastrointestinal functions such as motility, secretion, absorption, provide feedback to the central nervous system on availability of nutrients and may play a part in regulating food intake. The pharmacological potential of several endogenous peripheral peptides released prior to, during and/or after feeding are being explored. Long-term regulation is provided by the main circulating hormones leptin and insulin. These systems implicated in hypothalamic appetite regulation provide potential targets for treatment of obesity which could potentially pass into clinical development in the next 5 years. This review summarizes various effects and interrelationship of these central and peripheral neuropeptides in metabolism, obesity and their potential role as targets for treatment of obesity.

Behavioral and neurochemical sources of variability of circadian period and phase: studies of circadian rhythms of npy-/- mice

The cycle length or period of the free-running rhythm is a key characteristic of circadian rhythms. In this study we verify prior reports that locomotor activity patterns and running wheel access can alter the circadian period, and we report that these treatments also increase variability of the circadian period between animals. We demonstrate that the loss of a neurochemical, neuropeptide Y (NPY), abolishes these influences and reduces the interindividual variability in clock period. These behavioral and environmental influences, from daily distribution of peak locomotor activity and from access to a running wheel, both act to push the mean circadian period to a value < 24 h. Magnitude of light-induced resetting is altered as well. When photoperiod was abruptly changed from a 18:6-h light-dark cycle (LD18:6) to LD6:18, mice deficient in NPY were slower to respond to the change in photoperiod by redistribution of their activity within the prolonged dark and eventually adopted a delayed phase angle of entrainment compared with controls. These results support the hypothesis that nonphotic influences on circadian period serve a useful function when animals must respond to abruptly changing photoperiods and point to the NPYergic pathway from the intergeniculate leaflet innervating the suprachiasmatic nucleus as a circuit mediating these effects.

Neuropeptide Y deficiency attenuates responses to fasting and high-fat diet in obesity-prone mice

Neuropeptide Y (NPY) stimulates feeding and weight gain, but deletion of the NPY gene does not affect food intake and body weight in mice bred on a mixed genetic background. We reasoned that the orexigenic action of NPY would be evident in C57Bl/6J mice susceptible to obesity. NPY deficiency has no significant effect in mice fed a normal rodent diet. However, energy expenditure is elevated during fasting, and hyperphagia and weight gain are blunted during refeeding. Expression of agouti-related peptide (AGRP) in the hypothalamus is increased in NPY knockout (NPYko) than wild-type mice, but unlike wild type there is no further increase in AGRP when NPYko mice are fasted. Moreover, NPYko mice have higher oxygen consumption and uncoupling protein-1 expression in brown adipose tissue during fasting. The failure of an increase in orexigenic peptides and higher thermogenesis may contribute to attenuation of weight gain when NPYko mice are refed. C57Bl/6J mice lacking NPY are also less susceptible to diet-induced obesity (DIO) as a result of reduced feeding and increased energy expenditure. The resistance to DIO in NPYko mice is associated with a reduction in nocturnal feeding and increased expression of anorexigenic hypothalamic peptides. Insulin, leptin, and triglyceride levels increase with adiposity in both wild-type and NPYko mice.

Neuropeptide Y5 receptor antagonism does not induce clinically meaningful weight loss in overweight and obese adults

Neuropeptide Y (NPY) is a potent orexigenic neuropeptide, and antagonism of NPY Y1 and NPY Y5 receptors (NPYxR) is considered a potentially important anti-obesity drug target. We tested the hypothesis that blockade of the NPY5R will lead to weight loss in humans using MK-0557, a potent, highly selective, orally active NPY5R antagonist. The initial series of experiments reported herein, including a multiple-dose positron-emission tomography study and a 12 week proof-of concept/dose-ranging study, suggested an optimal MK-0557 dose of 1 mg/day. The hypothesis was then tested in a 52 week, multicenter, randomized, double-blind, placebo-controlled trial involving 1661 overweight and obese patients. Although statistically significant at 52 weeks, the magnitude of induced weight loss was not clinically meaningful. These observations provide the first clinical insight into the human NPY-energy homeostatic pathway and suggest that solely targeting the NPY5R in future drug development programs is unlikely to produce therapeutic efficacy.

Physiology and gene regulation of the brain NPY Y1 receptor

Neuropeptide Y (NPY) is one of the most prominent and abundant neuropeptides in the mammalian brain where it interacts with a family of G-protein coupled receptors, including the Y(1) receptor subtype (Y(1)R). NPY-Y(1)R signalling plays a prominent role in the regulation of several behavioural and physiological functions including feeding behaviour and energy balance, sexual hormone secretion, stress response, emotional behaviour, neuronal excitability and ethanol drinking. Y(1)R expression is regulated by neuronal activity and peripheral hormones. The Y(1)R gene has been isolated from rodents and humans and it contains multiple regulatory elements that may participate in the regulation of its expression. Y(1)R expression in the hypothalamus is modulated by changes in energetic balance induced by a wide variety of conditions (fasting, pregnancy, hyperglycaemic challenge, hypophagia, diet induced obesity). Estrogens up-regulate responsiveness to NPY to stimulate preovulatory GnRH and gonadotropin surges by increasing Y(1)R gene expression both in the hypothalamus and the pituitary. Y(1)R expression is modulated by different kinds of brain insults, such as stress and seizure activity, and alteration in its expression may contribute to antidepressant action. Chronic modulation of GABA(A) receptor function by benzodiazepines or neuroactive steroids also affects Y(1)R expression in the amygdala, suggesting that a functional interaction between the GABA(A) receptor and Y(1)R mediated signalling may contribute to the regulation of emotional behaviour. In this paper, we review the state of the art concerning Y(1)R function and gene expression, including our personal contribution to many of the subjects mentioned above.

[Trp3, Arg5]-ghrelin(1-5) stimulates growth hormone secretion and food intake via growth hormone secretagogue (GHS) receptor

Ghrelin, a 28 amino acid peptide identified as an endogenous ligand for growth hormone secretagogue (GHS) receptor, stimulates food intake and growth hormone (GH) secretion. We designed low molecular weight peptides with affinity for the GHS receptor based on the primary structure of ghrelin. We found that [Trp3, Arg5]-ghrelin(1-5) (GSWFR), a novel pentapeptide composed of all L-amino acids, had affinity for the GHS receptor (IC50 = 10 microM). GSWFR stimulated GH secretion after intravenous or oral administration. Centrally administered GSWFR increased food intake in non-fasted mice. The orexigenic action of GSWFR was inhibited by a GHS receptor antagonist, [D-Lys3]-GH-releasing peptide-6, suggesting that GSWFR stimulated food intake through the GHS receptor. The orexigenic action of GSWFR was also inhibited by a neuropeptide Y (NPY) Y1 receptor antagonist, BIBO3304. These results suggest that the GSWFR-induced feeding is mediated by the NPY Y1 receptor.

Neuropeptide Y (NPY) Y4 receptor selective agonists based on NPY(32-36): development of an anorectic Y4 receptor selective agonist with picomolar affinity

We have previously shown [Cys-Trp-Arg-Nva-Arg-Tyr-NH(2)](2), 1, to be a moderately selective neuropeptide Y (NPY) Y(4) receptor agonist. Toward improving the selectivity and potency for Y(4) receptors, we studied the effects of dimerizing H-Trp-Arg-Nva-Arg-Tyr-NH(2) using various diamino-dicarboxylic acids containing either di-, tri-, or tetramethylene spacers. These parallel dimers, 2A, 2B, 3, 4A, and 4B, and the corresponding linear tandem dimer and trimer analogues, 5 and 6, had enhanced selectivity and affinity for Y(4) receptors compared to 1 (Table 1). Substitution of Trp and Nva with Tyr and Leu, respectively, as in 2,7-d/l-diaminosuberic acid derivatized dimer, 7, resulted in a superior Y(4) selective agonist with picomolar affinity. Intraperitoneal (ip) injection of 7 potently inhibited food intake in fasted mice. Moreover, 7 (ip) inhibited the food intake in wild-type mice and not in Y(4)(-/-) knock-out mice, confirming that the actions of 7 on food intake are not due to global effects, but specifically mediated Y(4) receptors.

Pharmacological characterization and feeding-suppressive property of FMS586 [3-(5,6,7,8-tetrahydro-9-isopropyl-carbazol-3-yl)-1-methyl-1-(2-pyridin-4-yl-ethyl)-urea hydrochloride], a novel, selective, and orally active antagonist for neuropeptide Y Y5 receptor

We evaluated the pharmacological profiles of FMS586 [3-(5,6,7,8-tetrahydro-9-isopropyl-carbazol-3-yl)-1-methyl-1-(2-pyridin-4-yl-ethyl)-urea hydrochloride], a novel tetrahydrocarbazole derivative as a neuropeptide Y (NPY) Y5 receptor antagonist. This compound showed a highly selective in vitro affinity for Y5 (IC(50) = 4.3 +/- 0.4 nM) relative to other NPY receptor subtypes like Y1 or Y2. Its binding to Y5 was found to be fully antagonistic from cyclic AMP accumulation assays in human embryonic kidney 293 cells. Pharmacokinetic analysis revealed sufficient oral availability and brain permeability of this compound accompanied with clear dose relation. We attempted to assess the selectivity of FMS586 and, thereby, to infer the physiological role of Y5 in the following feeding experiments in normal rats. An intracerebroventricular injection of NPY and Y5-selective agonist peptide induced acute and robust feeding responses in satiated rats, and prior administration of FMS586 at the doses from 25 to 100 mg/kg clearly inhibited these responses by approximately 55 and 90%, respectively. This compound also showed dose-dependent but transient suppression in natural feeding models of both overnight fasting-induced hyperphagia and spontaneous daily intake. FMS586 did not modulate food intake induced by the topical injection of norepinephrine, galanin, or gamma-aminobutyric acid receptor agonist muscimol to the paraventricular nucleus. In addition, we confirmed the Y5-specific activity profile of FMS586 by immunohistochemical analysis. Taken together, we propose not only that our compound potentially expresses specific blockade of central Y5 signals but also that Y5 receptor would certainly contribute to physiological regulation of food intake in normal rats, as suggested from its origin.

A neuropeptide Y Y5 antagonist selectively ameliorates body weight gain and associated parameters in diet-induced obese mice

Neuropeptide Y (NPY) is thought to have a major role in the physiological control of energy homeostasis. Among five NPY receptors described, the NPY Y5 receptor (Y5R) is a prime candidate to mediate some of the effects of NPY on energy homeostasis, although its role in physiologically relevant rodent obesity models remains poorly defined. We examined the effect of a potent and highly selective Y5R antagonist in rodent obesity and dietary models. The Y5R antagonist selectively ameliorated diet-induced obesity (DIO) in rodents by suppressing body weight gain and adiposity while improving the DIO-associated hyperinsulinemia. The compound did not affect the body weight of lean mice fed a regular diet or genetically obese leptin receptor-deficient mice or rats, despite similarly high brain Y5R receptor occupancy. The Y5R antagonist acts in a mechanism-based manner, as the compound did not affect DIO of Y5R-deficient mice. These results indicate that Y5R is involved in the regulation and development of DIO and suggest utility for Y5R antagonists in the treatment of obesity.

The human obesity gene map: the 2005 update

This paper presents the 12th update of the human obesity gene map, which incorporates published results up to the end of October 2005. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTL) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2005, 176 human obesity cases due to single-gene mutations in 11 different genes have been reported, 50 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 244 genes that, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 408. The number of human obesity QTLs derived from genome scans continues to grow, and we now have 253 QTLs for obesity-related phenotypes from 61 genome-wide scans. A total of 52 genomic regions harbor QTLs supported by two or more studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably, with 426 findings of positive associations with 127 candidate genes. A promising observation is that 22 genes are each supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. The electronic version of the map with links to useful publications and relevant sites can be found at http://obesitygene.pbrc.edu.

Hypothalamic regulatory pathways and potential obesity treatment targets

With an ever-growing population of obese people as well as comorbidities associated with obesity, finding effective weight loss strategies is more imperative than ever. One of the challenges in curbing the obesity crisis is designing successful strategies for long-term weight loss and weight-loss maintenance. Currently, weight-loss strategies include promotion of therapeutic lifestyle changes (diet and exercise), pharmacological therapy, and bariatric surgery. This review focuses on several pharmacological targets that activate central nervous system pathways that normally limit food intake and body weight. Though it is likely that no single therapy will prove effective for everyone, this review considers several recent pre-clinical targets, and several compounds that have been in human clinical trials.

Modulation of neuropeptide Y expression in adult mice does not affect feeding

Despite numerous experiments showing that administration of neuropeptide Y (NPY) to rodents stimulates feeding and obesity, whereas acute interference with NPY signaling disrupts feeding and promotes weight loss, NPY-null mice have essentially normal body weight regulation. These conflicting observations suggest that chronic lack of NPY during development may lead to compensatory changes that normalize regulation of food intake and energy expenditure in the absence of NPY. To test this idea, we used gene targeting to introduce a doxycycline (Dox)-regulated cassette into the Npy locus, such that NPY would be expressed until the mice were given Dox, which blocks transcription. Compared with wild-type mice, adult mice bearing this construct expressed approximately 4-fold more Npy mRNA, which fell to approximately 20% of control values within 3 days after treatment with Dox. NPY protein also fell approximately 20-fold, but the half-life of approximately 5 days was surprisingly long. The biological effectiveness of these manipulations was demonstrated by showing that overexpression of NPY protected against kainate-induced seizures. Mice chronically overexpressing NPY had normal body weight, and administration of Dox to these mice did not suppress feeding. Furthermore, the refeeding response of these mice after a fast was normal. We conclude that, if there is compensation for changes in NPY levels, then it occurs within the time it takes for Dox treatment to deplete NPY levels. These observations suggest that pharmacological inhibition of NPY signaling is unlikely to have long-lasting effects on body weight.

PYY3-36 as an anti-obesity drug target

The neuropeptide Y (NPY)/peptide YY (PYY) system has been implicated in the physiology of obesity for several decades. More recently ignited enormous interest in PYY3-36, an endogenous Y2-receptor agonist, as a promising anti-obesity compound. Despite this interest, there have been remarkably few subsequent reports reproducing or extending the initial findings, while at the same time studies finding no anti-obesity effects have surfaced. Out of 41 different rodent studies conducted (in 16 independent labs worldwide), 33 (83%) were unable to reproduce the reported effects and obtained no change or sometimes increased food intake, despite use of the same experimental conditions (i.e. adaptation protocols, routes of drug administration and doses, rodent strains, diets, drug vendors, light cycles, room temperatures). Among studies by authors in the original study, procedural caveats are reported under which positive effects may be obtained. Currently, data speak against a sustained decrease in food intake, body fat, or body weight gain following PYY3-36 administration and make the previously suggested role of the hypothalamic melanocortin system unlikely as is the existence of PYY deficiency in human obesity. We review the studies that are in the public domain which support or challenge PYY3-36 as a potential anti-obesity target.

NPY/AgRP neurons are essential for feeding in adult mice but can be ablated in neonates

Hypothalamic neurons that express neuropeptide Y (NPY) and agouti-related protein (AgRP) are thought to be critical regulators of feeding behavior and body weight. To determine whether NPY/AgRP neurons are essential in mice, we targeted the human diphtheria toxin receptor to the Agrp locus, which allows temporally controlled ablation of NPY/AgRP neurons to occur after an injection of diphtheria toxin. Neonatal ablation of NPY/AgRP neurons had minimal effects on feeding, whereas their ablation in adults caused rapid starvation. These results suggest that network-based compensatory mechanisms can develop after the ablation of NPY/AgRP neurons in neonates but do not readily occur when these neurons become essential in adults.

Hormonal regulation of food intake

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

Energy metabolic profile of mice after chronic activation of central NPY Y1, Y2, or Y5 receptors

OBJECTIVE

Neuropeptide Y (NPY), a 36-amino acid peptide with orexigenic properties, is expressed abundantly in the central nervous system and binds to several NPY receptor subtypes. This study examines the roles of the NPY Y1, Y2, and Y5 receptor(s) in energy homeostasis.

RESEARCH METHODS AND PROCEDURES

We administered intracerebroventricular NPY (3 microg/d) or selective peptide agonists for the Y1, Y2, and Y5 receptor subtypes to C57Bl/6 mice for 6 days by mini-osmotic pumps to assess the role of each receptor subtype in NPY-induced obesity. Energy expenditure (EE) and respiratory quotient (RQ) were studied using indirect calorimetry. Adiposity was measured by DXA scanning and fat pad dissection. Insulin sensitivity was tested by whole-blood glucose measurement after an insulin challenge.

RESULTS

Central administration of the selective Y1 agonist, Y5 agonist, or NPY for 6 days in mice significantly increased body weight, adiposity, and RQ, with significant hyperphagia in the Y5 agonist- and NPY-treated groups but not in the Y1 agonist-treated group. The NPY, Y1, or Y5 agonist-treated mice had little change in total EE during ad libitum and pair-feeding conditions. Conversely, selective activation of the Y2 receptor reduced feeding and resulted in a significant, but transient, weight loss.

DISCUSSION

Central activation of both Y1 and Y5 receptors increases RQ and adiposity, whereas only Y5 receptor activation reduces energy expended per energy ingested. Selective activation of Y2 autoreceptors leads to hypophagia and transient weight loss, with little effect on total EE. Our study indicates that all three NPY receptor subtypes may play a role in regulating energy homeostasis in mice.

Compensatory changes in [125I]-PYY binding in Y receptor knockout mice suggest the potential existence of further Y receptor(s)

Gene knockout approaches have helped to better understand the functions of the different Y receptors. However, some results obtained from these knockout mice are unexpected and differ from the results of pharmacological intervention experiments. One possible explanation for this is that germ-line gene deletion of a particular Y receptor can influence expression and function of the remaining Y receptors. Here we show that such compensation in mRNA and protein expression does occur in Y receptor single, double and triple knockout models. Radio-ligand binding experiments using [(125)I]-PYY revealed significant up- and down-regulation of remaining Y receptor binding sites in various Y receptor knockout models compared to results from control mice employing Y receptor preferring agonist or antagonists for displacement of the radio-ligand. The most obvious change can be seen in the hippocampus of Y(1) knockout mice, where the level of the remaining Y receptors is strongly down-regulated. In Y(2) knockout mice no such trend can be seen, however, the expression pattern is significantly changed with a strong up-regulation of [(125)I]-PYY specific binding in the dentate gyrus. Interestingly, this pattern was also seen in Y(1)Y(2)Y(4) triple knockout mice. Y(5) receptor mRNA was approximately 20% higher in the hippocampus and dentate gyrus in the triple knockout mice compared to wild-type controls, while Y(6) mRNA expression could not be detected. However, competition binding experiments in Y(1)Y(2)Y(4) triple knockout mice with the Y(5) receptor preferring ligands [Leu(31), Pro(34)] NPY and [A(31), Aib(32)] NPY were able to replace only approximately 50% of [(125)I]-PYY binding in the dentate gyrus suggesting the existence of further yet unidentified Y receptor(s).