The pharmacology of neuropeptide Y (NPY) receptor-mediated feeding in rats characterizes better Y5 than Y1, but not Y2 or Y4 subtypes

Chemical modification of neuropeptide Y for human Y1 receptor targeting in health and disease

As a very abundant neuropeptide in the brain and widely distributed peptide hormone in the periphery, neuropeptide Y (NPY) appears to be a multisignaling key peptide. Together with peptide YY, pancreatic polypeptide and the four human G protein-coupled receptor subtypes hY1R, hY2R, hY4R and hY5R it forms the NPY/hYR multiligand/multireceptor system, which is involved in essential physiological processes as well as in human diseases. In particular, NPY-induced hY1R signaling plays a central role in the regulation of food intake and stress response as well as in obesity, mood disorders and cancer. Thus, several hY1R-preferring NPY analogs have been developed as versatile tools to unravel the complex NPY/hY1R signaling in health and disease. Further, these peptides provide basic lead structures for the development of innovative drugs. Here, the current research is summarized focusing on the development of differently sized hY1R-preferring NPY analogs as well as their advances with respect to hY1R profiling, potential therapeutic applications and targeted cancer imaging and therapy. Finally, major limitations and innovative strategies for next generation hY1R-preferring NPY analogs are addressed.

An insurmountable NPY Y5 receptor antagonist exhibits superior anti-obesity effects in high-fat diet-induced obese mice

Neuropeptide Y (NPY) Y5 receptor plays a key role in the effects of NPY, an important neurotransmitter in the control of energy homeostasis including stimulation of food intake and inhibition of energy expenditure. The NPY-Y5 receptor system has been an attractive drug target for potential use in treating obesity. Here we report the discovery and characterization of two novel Y5 receptor antagonists, S-2367 and S-234462. Both compounds displayed high affinity for the Y5 receptor in the radio-ligand binding assay, while in the cell-based functional assay, S-2367 and S-234462 showed, respectively, surmountable and insurmountable antagonism. In cell-based washout experiments, S-234462 dissociated from the Y5 receptor more slowly than S-2367. In vivo study showed that S-234462 effectively suppressed food intake induced by acute central injection of a selective Y5 receptor agonist. Furthermore, high-fat diet-induced obese (DIO) mice treated with S-234462 for 5 weeks showed a significant decrease in body weight gain and food intake compared to those treated with S-2367. In conclusion, S-234462 exhibits insurmountable antagonism of NPY Y5 receptor in vitro and superior anti-obesity effects to the surmountable NPY Y5 antagonist S-2367 in DIO mice.

The stimulatory effect of neuropeptide Y on growth hormone expression, food intake, and growth in olive flounder (Paralichthys olivaceus)

Neuropeptide Y (NPY) is a 36-amino acid peptide known to be a strong orexigenic (appetite-stimulating) factor in many species. In this study, we investigated the effect of NPY on food intake and growth in the olive flounder (Paralichthys olivaceus). Recombinant full-length NPY was injected intraperitoneally into olive flounder at the dose of 1 μg/g body weight; phosphate buffered saline was used as the negative control. In a long-term experiment, NPY and control groups were injected every fifth day over a period of 30 days. In a short-term experiment, NPY and control groups were given intraperitoneal injections and maintained for 24 h. Food intake and growth rates were significantly higher in fish injected with recombinant NPY than in the control fish (P < 0.05). Higher growth hormone (GH) and NPY mRNA transcript levels were observed in both experiments, indicating a stimulatory effect of NPY on GH release. These findings demonstrate that NPY is an effective appetite-stimulating factor in olive flounder with the potential to improve the growth of domestic fish species and enhance efficiency in aquaculture.

Cross-talk between reproduction and energy homeostasis: central impact of estrogens, leptin and kisspeptin signaling

The central nervous system receives hormonal cues (e.g., estrogens and leptin, among others) that influence reproduction and energy homeostasis. 17β-estradiol (E2) is known to regulate gonadotropin-releasing hormone (GnRH) secretion via classical steroid signaling and rapid non-classical membrane-initiated signaling. Because GnRH neurons are void of leptin receptors, the actions of leptin on these neurons must be indirect. Although it is clear that the arcuate nucleus of the hypothalamus is the primary site of overlap between these two systems, it is still unclear which neural network(s) participate in the cross-talk of E2 and leptin, two hormones essential for reproductive function and metabolism. Herein we review the progress made in understanding the interactions between reproduction and energy homeostasis by focusing on the advances made to understand the cellular signaling of E2 and leptin on three neural networks: kisspeptin, pro-opiomelanocortin (POMC) and neuropeptide Y (NPY). Although critical in mediating the actions of E2 and leptin, considerable work still remains to uncover how these neural networks interact in vivo.

Up-regulation of Neuropeptide Y Receptors in the Hypothalamus of Monosodium Glutamate-lesioned Sprague-Dawley Rats

Monosodium-glutamate (MSG) is neurotoxic for brain regions devoid of blood-brain barrier when it is injected at high doses during the neonatal period. Neuropeptide Y (NPY) neurons in the arcuate nucleus are particularly sensitive to MSG treatment. But, despite of the large decrease of this potent orexigenic peptide, feeding behavior is only slightly affected. We hypothesized that the hypothalamic NPY receptor system might be modified in these rats. The present study characterizes hypothalamic NPY and NPY receptors in normal and MSG-treated rats. MSG-treated rats were lighter (p < 0.01) and ate 17% less than the control rats (p < 0.01). NPY levels in the mediobasal and mediodorsal hypothalamus were reduced in MSG-treated rats compared to normal rats (-26% and -43%, p < 0.05 and p < 0.01, respectively). Combined hypothalamic Y1 and Y5 NPY receptor density was increased in MSG-treated rats compared to normal rats (+25%, p < 0.04), but affinity remained unaltered. Blockade with a selective Y1 antagonist showed that the Y1 receptor subtype represented more than 90% of the combined Y1 and Y5 receptor populations. The up-regulation of the NPY receptors is an element necessary to maintain food intake at a sufficient level to allow survival and growth of the lesioned rats.

Inhibiting neuropeptide Y Y1 receptor modulates melanocortin receptor- and NF-κB-mediated feeding behavior in phenylpropanolamine-treated rats

Neuropeptide Y (NPY) and nuclear factor-kappa B (NF-κB) are involved in regulating anorexia elicited by phenylpropanolamine (PPA), a sympathomimetic drug. This study explored whether NPY Y1 receptor (Y1R) is involved in this process, and a potential role for the proopiomelanocortin system was identified. Rats were given PPA once a day for 4days. Changes in the hypothalamic expression of the NPY, Y1R, NF-κB, and melanocortin receptor 4 (MC4R) levels were assessed and compared. The results indicated that food intake and NPY expression decreased, with the largest reductions observed on Day 2 (approximately 50% and 45%, respectively), whereas NF-κB, MC4R, and Y1R increased, achieving maximums on Day 2 (160%, 200%, and 280%, respectively). To determine the role of Y1R, rats were pretreated with Y1R antisense or a Y1R antagonist via intracerebroventricular injection 1h before the daily PPA dose. Y1R knockdown and inhibition reduced PPA anorexia and partially restored the normal expression of NPY, MC4R, and NF-κB. The data suggest that hypothalamic Y1R participates in the appetite-suppression from PPA by regulating MC4R and NF-κB. The results of this study increase our understanding of the molecular mechanisms in PPA-induced anorexia.

Role of GnRH Neurons and Their Neuronal Afferents as Key Integrators between Food Intake Regulatory Signals and the Control of Reproduction

Reproductive function is regulated by a plethora of signals that integrate physiological and environmental information. Among others, metabolic factors are key components of this circuit since they inform about the propitious timing for reproduction depending on energy availability. This information is processed mainly at the hypothalamus that, in turn, modulates gonadotropin release from the pituitary and, thereby, gonadal activity. Metabolic hormones, such as leptin, insulin, and ghrelin, act as indicators of the energy status and convey this information to the reproductive axis regulating its activity. In this review, we will analyse the central mechanisms involved in the integration of this metabolic information and their contribution to the control of the reproductive function. Particular attention will be paid to summarize the participation of GnRH, Kiss1, NPY, and POMC neurons in this process and their possible interactions to contribute to the metabolic control of reproduction.

Central nervous system neuropeptide Y signaling via the Y1 receptor partially dissociates feeding behavior from lipoprotein metabolism in lean rats

Elevated plasma triglyceride (TG) levels contribute to an atherogenic dyslipidemia that is associated with obesity, diabetes, and metabolic syndrome. Numerous models of obesity are characterized by increased central nervous system (CNS) neuropeptide Y (NPY) tone that contributes to excess food intake and obesity. Previously, we demonstrated that intracerebroventricular (icv) administration of NPY in lean fasted rats also elevates hepatic production of very low-density lipoprotein (VLDL)-TG. Thus, we hypothesize that elevated CNS NPY action contributes to not only the pathogenesis of obesity but also dyslipidemia. Here, we sought to determine whether the effects of NPY on feeding and/or obesity are dissociable from effects on hepatic VLDL-TG secretion. Pair-fed, icv NPY-treated, chow-fed Long-Evans rats develop hypertriglyceridemia in the absence of increased food intake and body fat accumulation compared with vehicle-treated controls. We then modulated CNS NPY signaling by icv injection of selective NPY receptor agonists and found that Y1, Y2, Y4, and Y5 receptor agonists all induced hyperphagia in lean, ad libitum chow-fed Long-Evans rats, with the Y2 receptor agonist having the most pronounced effect. Next, we found that at equipotent doses for food intake NPY Y1 receptor agonist had the most robust effect on VLDL-TG secretion, a Y2 receptor agonist had a modest effect, and no effect was observed for Y4 and Y5 receptor agonists. These findings, using selective agonists, suggest the possibility that the effect of CNS NPY signaling on hepatic VLDL-TG secretion may be relatively dissociable from effects on feeding behavior via the Y1 receptor.

Direct regulation of GnRH neuron excitability by arcuate nucleus POMC and NPY neuron neuropeptides in female mice

Hypothalamic neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons act to sense and coordinate the brain's responses to metabolic cues. One neuronal network that is very sensitive to metabolic status is that controlling fertility. In this study, we investigated the impact of neuropeptides released by NPY and POMC neurons on the cellular excitability of GnRH neurons, the final output cells of the brain controlling fertility. The majority (∼70%) of GnRH neurons were activated by α-melanocyte-stimulating hormone, and this resulted from the direct postsynaptic activation of melanocortin receptor 3 and melanocortin receptor 4. A small population of GnRH neurons (∼15%) was excited by cocaine and amphetamine-regulated transcript or inhibited by β-endorphin. Agouti-related peptide, released by NPY neurons, was found to have variable inhibitory (∼10%) and stimulatory (∼25%) effects upon subpopulations of GnRH neurons. A variety of NPY and pancreatic polypeptide analogs was used to examine potential NPY interactions with GnRH neurons. Although porcine NPY (Y1/Y2/Y5 agonist) directly inhibited the firing of approximately 45% of GnRH neurons, [Leu(31),Pro(34)]-NPY (Y1/Y4/Y5 agonist) could excite (56%) or inhibit (19%). Experiments with further agonists indicated that Y1 receptors were responsible for suppressing GnRH neuron activity, whereas postsynaptic Y4 receptors were stimulatory. These results show that the activity of GnRH neurons is regulated in a complex manner by neuropeptides released by POMC and NPY neurons. This provides a direct route through which different metabolic cues can regulate fertility.

Involvement of neuropeptide Y Y1 receptor in the regulation of amphetamine-mediated appetite suppression

Recently, we reported that an initial decrease followed by recovery of food intake was observed during four days of amphetamine (AMPH) treatment and suggested that these changes in response were mediated by changes in neuropeptide Y (NPY) and proopiomelanocortin (POMC). Here we investigated if Y1 receptor (Y1R) and/or Y5 receptor (Y5R) might be involved in this regulation. Rats were treated daily with AMPH for four days. Changes in the expression levels of Y1R, Y5R, melanocortin receptor 3 (MC3R), and NPY were assessed and compared. Results showed that Y1R and MC3R increased, with a maximal increase of about 210% on Day 2 but with a restoration to the normal level on Day 4. In contrast, NPY decreased with a biggest reduction of about 45% on Day 2 and the pattern of expression during AMPH treatment was opposite to those of Y1R and MC3R, while the expression of Y5R was not changed. Central inhibitions of NPY formation or Y1R activity modulated the anorectic response of AMPH and the reciprocal regulation of NPY and MC3R, revealing a crucial role of Y1R in this action. It is suggested that Y1R participates in the reciprocal regulation of NPY- and MC3R-containing neurons in the hypothalamus during the anorectic effect of AMPH. These results may further the understanding of Y1R in the control of eating.

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.

The role of NPY in hypothalamic mediated food intake

Neuropeptide Y (NPY) is a highly conserved neuropeptide with orexigenic actions in discrete hypothalamic nuclei that plays a role in regulating energy homeostasis. NPY signals via a family of high affinity receptors that mediate the widespread actions of NPY in all hypothalamic nuclei. These actions are also subject to tight, intricate regulation by numerous peripheral and central energy balance signals. The NPY system is embedded within a densely-redundant network designed to ensure stable energy homeostasis. This redundancy may underlie compensation for the loss of NPY or its receptors in germline knockouts, explaining why conventional knockouts of NPY or its receptors rarely yield a marked phenotypic change. We discuss insights into the hypothalamic role of NPY from studies of its physiological actions, responses to genetic manipulations and interactions with other energy balance signals. We conclude that numerous approaches must be employed to effectively study different aspects of NPY action.

GPC receptors and not ligands decide the binding mode in neuropeptide Y multireceptor/multiligand system

Many G protein-coupled receptors belong to families of different receptor subtypes, which are recognized by a variety of distinct ligands. To study such a multireceptor/multiligand system, we investigated the Y-receptor family. This family consists of four G protein-coupled Y receptors in humans (hY 1R, hY 2R, hY 4R, and hY 5R) and is activated by the so-called NPY hormone family, which itself consists of three native peptide ligands named neuropeptide Y (NPY), pancreatic polypeptide (PP), and peptide YY (PYY). The hY 5R shows high affinity for all ligands, although for PP binding, the affinity is slightly decreased. As a rational explanation, we suggest that Tyr (27) is lost as a contact point between PP and the hY 5R in contrast to NPY or PYY. Furthermore, several important residues for ligand binding were identified by the first extensive mutagenesis study of the hY 5R. Using a complementary mutagenesis approach, we were able to discover a novel interaction point between hY 5R and NPY. The interaction between NPY(Arg (25)) and hY 5R(Asp (2.68)) as well as between NPY(Arg (33)) and hY 5R(Asp (6.59)) is maintained in the binding of PYY and PP to hY 5R but different to the PP-hY 4R and NPY-hY 1R contact points. Therefore, we provide evidence that the receptor subtype and not the pre-orientated conformation of the ligand at the membrane decides the binding mode. Furthermore, the first hY 5R model was set up on the basis of the crystal structure of bovine rhodopsin. We can show that most of the residues identified to be critical for ligand binding are located within the now postulated binding pocket.

Responsiveness of obese Zucker rats to [D-Trp34]-NPY supports the targeting of Y5 receptor for obesity treatment

The increased synthesis and release of neuropeptide Y (NPY) in the hypothalamus participate in the development of overeating and obesity in the Zucker fa/fa rat. The orexigenic effects of NPY are mediated through the Y1 and Y5 receptors. The substitution of [D-Trp34] in the NPY amino-acid sequence increases selectivity without lowering potency at the Y5 receptor. In the present study, to address the role of the NPY Y5 receptor in obesity, we investigated the acute effect of [D-Trp 34]-NPY in lean and obese Zucker rats. Obese rats were markedly hyperphagic (27.1 +/- 0.6 vs. 18.7 +/- 0.4 (lean) g/day; p < 0.01). Injection of [D-Trp34]-NPY in the lateral brain ventricle at a dose of 16 microg stimulated food intake to the same extent in both lean (p < 0.01) and obese (p < 0.01) rats 1 h after injection. This effect was still observed after 6 h (p < 0.01). These results indicate, therefore, that the obese rats are responsive to [D-Trp34]-NPY. They support the role of the neuropeptide Y5 receptor in the regulation of food intake and suggest that NPY Y5 antagonism might be useful for treating 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.

NPY Y1 receptor is involved in ghrelin- and fasting-induced increases in foraging, food hoarding, and food intake

Fasting triggers a constellation of physiological and behavioral changes, including increases in peripherally produced ghrelin and centrally produced hypothalamic neuropeptide Y (NPY). Refeeding stimulates food intake in most species; however, hamsters primarily increase foraging and food hoarding with smaller increases in food intake. Fasting-induced increases in foraging and food hoarding in Siberian hamsters are mimicked by peripheral ghrelin, central NPY, and NPY Y1 receptor agonist injections. Because fasting stimulates ghrelin and subsequently NPY synthesis/release, it may be that fasting-induced increased hoarding is mediated by NPY Y1 receptor activation. Therefore, we asked: Can an Y1 receptor antagonist block fasting- or ghrelin-induced increases in foraging, food hoarding, and food intake? This was accomplished by injecting the NPY Y1 receptor antagonist 1229U91 intracerebroventricularly in hamsters fasted, fed, or given peripheral ghrelin injections and housed in a running wheel-based food delivery foraging system coupled with simulated-burrow housing. Three foraging conditions were used: 1) no running wheel access, free food, 2) running wheel access, free food, or 3) foraging requirement (10 revolutions/pellet) for food. Fasting was a more potent stimulator of foraging and food hoarding than ghrelin. Concurrent injections of 1229U91 completely blocked fasting- and ghrelin-induced increased foraging and food intake and attenuated, but did not always completely block, fasting- and ghrelin-induced increases in food hoarding. Collectively, these data suggest that the NPY Y1 receptor is important for the effects of ghrelin- and fasting-induced increases in foraging and food intake, but other NPY receptors and/or other neurochemical systems are involved in increases in food hoarding.

Paradoxical effects of NPY in the suprachiasmatic nucleus

The circadian clock in the suprachiasmatic nucleus (SCN) is synchronized by the 24 h, light : dark cycle, and is reset by photic and non-photic cues. The acute effects of light in the SCN include the increase of mRNA levels of the circadian clock gene Per1 and a dramatic reduction of pineal melatonin. Neuropeptide Y (NPY), which appears to mediate the phase-resetting effects of non-photic stimuli, prevents the ability of light, and stimuli that mimic light, to phase shift the circadian clock when injected into the SCN. The purpose of the present study was to determine if NPY inhibits the ability of light to suppress pineal melatonin. Surprisingly, NPY injected into the SCN of hamsters mimicked the effects of light by suppressing pineal melatonin levels. To confirm that NPY inhibited the effects of light on the induction of Per1 mRNA levels, Per1 mRNA levels in the SCN were measured in these same animals. NPY significantly reduced Per1 mRNA levels induced by the light pulse. The suppression of melatonin by NPY appears to be mediated by the same subtype of NPY receptors in the SCN that mediate the modulation of phase shifts. Injection of Y5 receptor agonists mimicked the effects of NPY on pineal melatonin, while injection of a Y2 agonist did not. Thus, these data are the first to demonstrate the paradoxical effects of NPY within the SCN. NPY mimics the effects of light on pineal melatonin and inhibits the effects of light on the induction of Per1 mRNA.

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.

The neuropeptide-Y Y5 receptor antagonist L-152,804 decreases alcohol self-administration in inbred alcohol-preferring (iP) rats

Neuropeptide-Y (NPY) is the most abundant and widely distributed peptide in the mammalian central nervous system and increases feeding behavior through actions at the Y5 receptor subtype. Recent pharmacological evidence indicates that NPY activity at this receptor subtype can modulate ethanol reinforcement. The purpose of this study was to determine if NPY Y5 receptor antagonism reduces ethanol self-administration and reinforcement in a rodent genetic animal model of alcoholism. Selectively inbred alcohol-preferring (iP) rats were trained to voluntarily consume ethanol (10% vol/vol) versus H2O in a 24-h two-bottle choice test. An additional group of iP rats was trained in operant ethanol self-administration to lever press on a fixed-ratio 1 schedule for ethanol (10% vol/vol) reinforcement. Following establishment of baseline intake or ethanol-reinforced responding, iP rats were injected with L-152,804 (0-20 mg/kg) prior to two-bottle or operant ethanol self-administration sessions. In the two-bottle choice test, L-152,804 (3 and 10 mg/kg, ip) significantly reduced ethanol intake (g/kg) at 4- and 6-h postinjection and had no effect on food intake. In the operant procedure, L-152,804 (10 and 20 mg/kg, ip) significantly reduced both the dosage of self-administered ethanol (g/kg/1-h) and the total number of ethanol-reinforced responses. No effect was observed on latency to the first response or the number of inactive lever presses. These results indicate that blockade of NPY Y5 receptor activity decreases both voluntary ethanol drinking and ethanol reinforcement in a rodent genetic animal model of alcoholism. For this reason, NPY Y5 receptor antagonists may be useful in medical management of alcohol abuse and alcoholism in the human population.

NPY mediates the feeding elicited by muscimol injections into the nucleus accumbens shell

Injections of muscimol into the nucleus accumbens shell (AcbSh) induce large increases in food intake in satiated rats and also activate neurons in a number of feeding-related brain regions, including NPY-containing neurons in the arcuate hypothalamic nucleus and cells in the paraventricular hypothalamic nucleus. This suggests that the NPY system may participate in the expression of AcbSh-mediated feeding behavior. Therefore, we examined the effects of intraventricular administration of the Y1 receptor antagonist 1229U91 or the Y5 receptor antagonist L-152,804 on AcbSh-mediated food intake. Intra-AcbSh muscimol elicited a large increase in food intake which was potently suppressed by blocking either central Y1 or Y5 receptors. Our results suggest that the AcbSh influences food intake, in part, through the release of NPY.

Circadian control during the day and night: Role of neuropeptide Y Y5 receptors in the suprachiasmatic nucleus

Circadian rhythms are reset by light during the night or by nonphotic stimuli during the day. Neuropeptide Y (NPY), which appears to mediate at least some nonphotic phase shifts by its actions in the suprachiasmatic nucleus (SCN), induces phase advances during the day and inhibits light-induced phase advances during the night. In this study, we used a highly selective Y5-like agonist to test whether activation of NPY Y5 receptors is sufficient to mimic NPY during the day and late night in Syrian hamsters. We also tested whether NPY in the early night reduces light-induced phase delays in a dose-dependent manner. Microinjection of a selective Y5 receptor agonist, (Ala(31), Aib(32))-NPY, into the SCN significantly inhibited light-induced phase advances during the late night, but did not induce phase advances during the day. In addition, concentrations of NPY ranging from 0.23 to 23 mM did not attenuate light-induced phase delays in the early night. These results suggest that activation of Y5-like receptors is sufficient to inhibit light-induced phase advances during the late night but is not sufficient to induce phase advances during the day. Furthermore, this study provided no evidence that NPY can inhibit light-induced phase shifts early in the night.

Enhancement of feeding suppression by PYY(3-36) in rats with area postrema ablations

We investigated suppression of food intake by intraperitoneal (IP) injections of peptide YY(3-36) (PYY(3-36)) (24, 60, or 150 microg/kg) in rats with ablations of the area postrema (APX) and in controls with sham ablations. In controls, PYY(3-36)-induced suppression was modest and short-lived, averaging 20% at most and persisting less than 6h. The highest dose tested (150 microg/kg) was even less effective than were the two lesser doses after 3h. APX did not diminish the potency of these effects of PYY(3-36). In fact, the magnitude of suppression produced by the greatest dose of PYY(3-36) in APX rats was significantly greater than in controls and PYY(3-36)-induced suppression was still present at 24h.

Synthesis and structure activity relationship of guanidines as NPY Y5 antagonists

A series of bis-aryl substituted guanidines have been discovered as potent NPY Y5 antagonists. The SAR and in vitro metabolic stability of these compounds are discussed.

Neuropeptide Y, ubiquitous and elusive

This paper reviews aspects of NPY research that were emerging in 1985, shortly after the isolation and characterization of the peptide. NPY had become known for its widespread distribution especially in the central and peripheral nervous systems, but also in the gastro-intestinal and respiratory tracts and in fibers innervating smooth muscle around blood vessels. Consistent with its distribution, it was determined that NPY is a potent vasoconstrictor, affects neuroendocrine systems and is involved in appetite regulation--areas of research still relevant today. Through advances in technology knowledge about NPY's role in these and newly discovered physiological functions has deepened considerably. Successful cloning of a series of NPY receptors has opened up new and complex research vistas. Lately, the creation of mice genetically modified for NPY as well as for several receptor subtypes has brought many puzzling observations--followed by questions yet to be answered.

The Y1 receptor subtype mediates the cardiovascular changes evoked by NPY administered into the posterior hypothalamic nucleus of conscious rat

An earlier study showed that the neuropeptide Y (NPY) receptor antagonist PYX-2 blocks the enhancement of a carbachol (CCh)-evoked pressor response produced by prior NPY administration into the posterior hypothalamic nucleus (PHN). The Y receptor subtype that mediates this response, and an increase in mean arterial pressure (MAP) and heart rate, remained unknown due to the lack of selectivity of PYX-2 for the Y receptor subtypes. Thus, the present study was undertaken to elucidate the Y receptor subtype responsible for mediating the NPY-evoked cardiovascular responses from the PHN by determining the rank order of potency of several NPY-related peptides for increasing MAP, and by correlating the pressor response evoked by these peptides to reported K(i)'s and IC(50)'s for the Y(1), Y(2), Y(4) and Y(5) receptor subtypes. The pharmacological profile (PYY>or=NPY>or=[Leu(31),Pro(34)]NPY>or=NPY(13-36)>or=hPP) and correlations suggest that the Y(1) and/or Y(5) receptor subtypes mediate these cardiovascular changes. Administration of the relatively non-selective Y receptor antagonist PYX-2 or the selective Y(1) receptor antagonist BIBP 3226 into the PHN prior to NPY completely blocked the cardiovascular responses. BIBP 3226 also blocked the cardiovascular changes evoked by [Leu(31),Pro(34)]NPY, NPY(13-36) and human pancreatic polypeptide (hPP). In contrast, neither BIBP 3226 nor PYX-2 inhibited the cardiovascular changes induced by peptide YY (PYY) or CCh microinjection into the PHN. These results show that NPY and PYY act on different receptors to mediate their respective cardiovascular changes from the PHN with NPY stimulating the Y(1) receptor.

A potent and selective NPY Y5 antagonist reduces food intake but not through blockade of the NPY Y5 receptor

AIM

These studies were performed to test the hypothesis that endogenous neuropeptide Y (NPY) acting on the NPY Y(5) receptor subtype contributes to the control of food intake. The hypothesis was tested using S 25585-a newly synthesized NPY Y(5) receptor antagonist.

METHODS AND RESULTS

S 25585 was shown to be a high-affinity antagonist of the NPY Y(5) receptor subtype (IC(50) 5 nM) with no significant affinity toward other NPY receptor subtypes and over 40 other receptors, channels or uptake systems. S 25585 (7.5 mg/kg, i.p.) did not induce a conditioned taste aversion, significantly alter need-induced sodium appetite or induce pica, suggesting that at this dose the compound did not induce illness or malaise. In satiated rats, S 25585 (5.0 and 7.5 mg/kg, i.p.) significantly decreased the overfeeding induced by i.c.v. injection of NPY (1 microg) and the highly selective NPY Y(5) receptor agonist [hPP(1-17), Ala(31), Aib(32)]NPY (0.7 microg). In rats fasted for 4 h immediately before the dark phase, analysis of the microstructure of feeding behavior revealed that S 25585 significantly increased latency to eat and significantly decreased the duration and size of the meals without altering the meal number or eating rate. Analysis of the behavioral satiety sequence at this time revealed that the animals passed through the normal pattern of feeding, grooming and resting. Although S 25585 appeared to be influencing a physiological system controlling appetite, this does not involve the NPY Y(5) receptor since the antagonist also markedly reduced food intake in the NPY Y(5) knockout mouse.

CONCLUSIONS

The results presented do not support a role for the NPY Y(5) receptor in the control of food intake. The results further illustrate that it is imperative that the activity of any new NPY Y(5) antagonist be assessed in the NPY Y(5) knockout mouse before assuming that its effect on food intake is due to blockade of this receptor.

Neuropeptide-Y Y5 Receptors Modulate the Onset and Maintenance of Operant Ethanol Self-Administration

BACKGROUND

Neuropeptide Y (NPY) is the most abundant and widely distributed peptide in the mammalian central nervous system and increases feeding behavior at NPY Y1 or Y5 receptor subtypes. Recent pharmacological and mutant mouse data indicate that NPY activity at its receptors can influence ethanol self-administration, although the direction and strength of this influence are not clear.

METHODS

Effects of the novel NPY Y5 receptor antagonist L-152,804 on the onset and maintenance of operant self-administration were examined in male C57BL/6J mice, which were trained to self-administer ethanol (10% v/v) versus water via the sucrose substitution method during 16 hr overnight sessions. After 4 months of baseline responding, mice were injected with L-152,804 (0, 10, 30, or 60 mg/kg, intraperitoneally) before operant sessions. Potential locomotor effects of L-152,804 and possible interaction with the sedative properties of ethanol also were examined.

RESULTS

All three doses of L-152,804 significantly delayed the onset of ethanol-reinforced responding relative to vehicle injection. L-152,804 produced no effect on the total number of ethanol- or water-reinforced responses per 16 hr session. However, L-152,804 selectively modulated the temporal distribution of ethanol-reinforced responding depending on the dose (10 and 60 mg/kg) and time point measured in a manner consistent with blockade of ethanol reinforcement. Additional experiments determined that L-152,804 (10 or 60 mg/kg) did not alter spontaneous locomotor activity or influence the sedative effects of ethanol (4 g/kg).

CONCLUSIONS

These results indicate that blockade NPY Y5 receptor activity modulates the onset and maintenance of ethanol self-administration. For this reason, NPY-Y5 receptor antagonists may be useful in medical management of alcohol abuse and alcoholism.

Structure–activity relationship studies on 2-heteroaryl-4-arylimidazoles NPY5 receptor antagonists

A series of 2-heteroaryl-4-arylimidazoles with potent in vitro activity at the NPY5 receptor was developed. Introduction of electron-withdrawing groups on the 4-aryl ring led to a significant improvement of in vitro potency. Several analogues from this series had anorectic activity in rodent feeding models, but were also found to have undesired behavioral effects in spontaneous locomotor activity.

Agonists for neuropeptide Y receptors Y1 and Y5 stimulate different phases of feeding in guinea pigs

1. The stimulatory effect of neuropeptide Y (NPY) on food intake is well established but the roles of the receptor subtypes Y(1) and Y(5) have been difficult to define. We have studied the effects of two novel Y(1)-preferring and two Y(5)-preferring agonists on feeding in guinea pigs. 2. The Y(1)-preferring receptor agonists [Arg(6),Pro(34)]pNPY and [Phe(7),Pro(34)]pNPY had high affinity for the Y(1) receptor (K(i) values 0.07 and 0.04 nM, respectively) and nanomolar affinity for the Y(5) receptor. Administration of either compound into the third brain ventricle increased food intake equally to NPY. 3. The Y(5) agonist [Ala(31),Aib(32)]pNPY displayed a moderate affinity for the Y(5) receptor (K(i) 7.42 nM) and a low affinity for Y(1) (K(i) 1.7 micro M). This compound had only a modest effect on feeding. 4. The other Y(5)-preferring peptide [cPP(1-7),NPY(19-23),Ala(31),Aib(32),Gln(34)]hPP had a higher affinity at the Y(5) receptor (K(i) 1.32 nM) and also at the Y(1) receptor (K(i) 85 nM). It potently stimulated feeding: the food consumption after administration of this peptide was two-fold compared to NPY. 5. Our results support the view that both the receptor subtypes Y(1) and Y(5) are involved in the stimulation of feeding. As the action profiles of the Y(1) and Y(5) agonists on feeding parameters were different, it seems that they influence different phases of eating.

Structure-activity relationships in a series of NPY Y5 antagonists: 3-amido-9-ethylcarbazoles, core-modified analogues and amide isosteres

Beginning with carbazole 1a, the amide and alkyl substituents were optimized to maintain potency while adding solubilizing groups. Efforts to replace the 3-amino-9-ethylcarbazole core, a known carcinogen, used the SAR generated in the carbazole series for guidance and led to the synthesis of a number of core-modified analogues. In addition, an isosteric series, in which the amide was replaced with an imidazole, was prepared. Two potent new series lacking the putative toxicophore were identified from these endeavors.

NPY ablation in C57BL/6 mice leads to mild obesity and to an impaired refeeding response to fasting

Neuropeptide Y (NPY) is an orexigenic (appetite-stimulating) peptide that plays an important role in regulating energy balance. When administered directly into the central nervous system, animals exhibit an immediate increase in feeding behavior, and repetitive injections or chronic infusions lead to obesity. Surprisingly, initial studies of Npy(-/-) mice on a mixed genetic background did not reveal deficits in energy balance, with the exception of an attenuation in obesity seen in ob/ob mice in which the NPY gene was also deleted. Here, we show that, on a C57BL/6 background, NPY ablation is associated with an increase in body weight and adiposity and a significant defect in refeeding after a fast. This impaired refeeding response in Npy(-/-) mice resulted in a deficit in weight gain in these animals after 24 h of refeeding. These data indicate that genetic background must be taken into account when the biological role of NPY is evaluated. When examined on a C57BL/6 background, NPY is important for the normal refeeding response after starvation, and its absence promotes mild obesity.

Recent developments in our understanding of the physiological role of PP-fold peptide receptor subtypes

The three peptides pancreatic polypeptide (PP), peptide YY (PYY), and neuropeptide Y (NPY) share a similar structure known as the PP-fold. There are four known human G-protein coupled receptors for the PP-fold peptides, namely Y1, Y2, Y4, and Y5, each of them being able to bind at least two of the three endogenous ligands. All three peptides are found in the circulation acting as hormones. Although NPY is only released from neurons, PYY and PP are primarily found in endocrine cells in the gut, where they exert such effects as inhibition of gall bladder secretion, gut motility, and pancreatic secretion. However, when PYY is administered in an experimental setting to animals, cloned receptors, or tissue preparations, it can mimic the effects of NPY in essentially all studies, making it difficult to study the effects of PP-fold peptides and to delineate what receptor and peptide accounts for a particular effect. Initial studies with transgenic animals confirmed the well-established action of NPY on metabolism, food-intake, vascular systems, memory, mood, neuronal excitability, and reproduction. More recently, using transgenic techniques and novel antagonists for the Y1, Y2, and Y5 receptors, NPY has been found to be a key player in the regulation of ethanol consumption and neuronal development.

Effects of a high-fat diet and strain on hypothalamic gene expression in rats

OBJECTIVE

This study was designed to investigate whether dietary fat and genetic background might differentially alter the expression of hypothalamic genes involved in food intake.

RESEARCH METHODS AND PROCEDURES

Three-month-old Osborne-Mendel (OM) and S5B/Pl rats were fed either a high-fat or a low-fat diet for 14 days. mRNA for neuropeptide Y (NPY), corticotrophin-releasing hormone, NPY Y-1 receptor and Y-5 receptor, and serotonin 2c (5-HT2c) receptors were measured using Northern blotting or ribonuclease protection assays.

RESULTS

OM rats showed an increased expression of NPY and corticotrophin-releasing hormone compared with S5B/Pl rats. The expression of NPY-Y1 and -Y5 receptor mRNA was significantly higher in the hypothalamus of OM rats compared with S5B/Pl rats. The expression of 5HT-2c receptor mRNA was significantly reduced in both strains of rats eating a high-fat diet when compared with the animals eating the low-fat diet.

DISCUSSION

These data suggest that over activity of the NPY system may contribute to the development of obesity in OM rats and that expression of the 5HT-2c receptor gene may be modulated by dietary fat.

Differential response to NPY of PVH and dopamine-responsive VMH neurons in overweight rats

Neuronal responses to neuropeptide Y and dopamine were recorded in brain slices of hypothalamic paraventricular (PVH) and ventromedial (VMH) nuclei in normal and hyperphagic overweight rats reared in small litters of three pups. NPY significantly activated PVH neurons of normal rats, but inhibited neurons of overweight rats. In the VMH, a significantly higher coincidence of inhibition induced by NPY and dopamine was found in overweight rats. Similar neuronal responses were evoked by a NPY Y5 receptor agonist. Effects of NPY could be blocked by a Y1 receptor antagonist. The altered response of PVH neurons to the feeding-inducing NPY and the increased inhibition by NPY and dopamine in the VMH might contribute to the persisting hyperphagia and overweight of postnatally overnourished rats.

Selective antagonism of the NPY Y5 receptor does not have a major effect on feeding in rats

Neuropeptide Y (NPY) is thought to play a key role in stimulating feeding, thus making NPY receptors attractive appetite suppressant drug targets for treating obesity. Because the orexigenic effects of NPY have been ascribed to actions at the NPY Y5 receptor, we have determined the role of this receptor in feeding in rats, using a small molecule antagonist of this receptor. NPY5RA-972 is a selective and potent (<10 nmol/l) NPY Y5 receptor antagonist. This compound is central nervous system (CNS) penetrant, and an oral dose of 10 mg/kg NPY5RA-972 to rats produced concentrations in cerebrospinal fluid that greatly exceeded the in vitro IC(50) (inhibitory concentration 50%). Indeed, at doses to rats as low as 1 mg/kg, NPY5RA-972 inhibited feeding induced by intracerebroventricular (ICV) administration of a selective NPY Y5 agonist ([cPP(1-7),NPY(19-23),Ala(31),Aib(32),Gln(34)]-hPP). However, in the dose range 1-10 mg/kg, NPY5RA-972 had no significant effect on food intake in Wistar rats induced to feed by either ICV NPY or 24 h fasting or in free-feeding Wistar or obese Zucker rats. Chronic administration of NPY5RA-972 (10 mg/kg twice daily) had no effect on food intake or body weight in either free-feeding Wistar rats or dietary obese rats. These data indicate that NPY5RA-972 is a potent, selective, orally active, and CNS-penetrant antagonist of the NPY Y5 receptor that prevents feeding driven by activation of this receptor. The data obtained with this antagonist indicate that the NPY Y5 receptor is not a major regulator of feeding in the rat.

Food intake inhibition and reduction in body weight gain in lean and obese rodents treated with GW438014A, a potent and selective NPY-Y5 receptor antagonist

Numerous reports have implicated theY5 receptor as the 'feeding' receptor mediating the orexigenic action of neuropeptide Y (NPY). This notion is supported by the correlation between the in vitro functional and binding activities of different peptide agonists and their potent stimulation of food intake in rodents. We have discovered a series of small molecule heterocycles with high affinity, selectivity, and functional antagonism for Y5 receptors. Intraperitoneal (i.p.) administration of GW438014A into rodents, resulted in a potent reduction of NPY-induced and normal overnight food intake. Brain levels of GW438014A were detected well in excess of its binding IC(50) for up to 3 h post-dosing. Daily (i.p., BID, 10 mg/kg) administration of this compound to Zucker Fatty rats for a period of 4 days resulted in a marked decrease in the rate of weight gain and a reduction in fat mass. No effect on food intake was observed following oral administration of GW438014A (25-100 mg/kg), consistent with the poor oral bioavailability (<3%) and low brain levels observed.

A convenient 3-step synthesis of 3-acetamido-6-arylpyridazines directed to novel Y5 receptor antagonist

A 3-step synthesis of 3-acetamido-6-arylpyridazines as potential NPY5 antagonists.

Appetite suppression based on selective inhibition of NPY receptors

AIM

The aim of this review is to critically assess available evidence that blockade of the actions of NPY at one of the five NPY receptor subtypes represents an attractive new drug discovery target for the development of an appetite suppressant drug.

RESULTS

Blockade of the central actions of NPY using anti-NPY antibodies, antisense oligodeoxynucleotides against NPY and NPY receptor antagonists results in a decrease in food intake in energy-deprived animals. These results appear to show that endogenous NPY plays a role in the control of appetite. The fact that NPY receptors exist as at least five different subtypes raises the possibility that the actions of endogenous NPY on food intake can be adequately dissociated from other effects of the peptide. Current drug discovery has produced a number of highly selective NPY receptor antagonists which have been used to establish the NPY Y(1) receptor subtype as the most critical in regulating short-term food intake. However, additional studies are now needed to more clearly define the relative contribution of NPY acting through the NPY Y2 and NPY Y5 receptors in the complex sequence of physiological and behavioral events that underlie the long-term control of appetite.

CONCLUSIONS

Blockade of the NPY receptor may produce appetite-suppressing drugs. However, it is too early to state with certainty whether a single subtype selective drug used alone or a combination of NPY receptor selective antagonists used in combination will be necessary to adequately influence appetite regulation.

Multiple NPY receptors Inhibit GABA(A) synaptic responses of rat medial parvocellular effector neurons in the hypothalamic paraventricular nucleus

We have recently shown that NPY and alpha-melanocyte-stimulating hormone, which potently induce or inhibit feeding, respectively, have opposing modulatory actions on GABAergic synapses in the medial parvocellular region of the paraventricular hypothalamic nucleus (mpPVN). Because this action might underlie the effects of NPY on feeding, we have examined the pharmacology of NPY responses using electrophysiological recordings. Focal electrical stimulation within the PVN elicited a GABA(A) synaptic response in some mpPVN neurons, which was reversibly inhibited by NPY in a concentration-dependent manner (EC(50) = 28 nM). NPY did not alter the response to the GABA(A) agonist, muscimol. Agonist responses to NPY analogs were not consistent with a single NPY receptor subtype; the most subtype selective agonists were less effective than the more broadly selective ones. Antagonist blockade of individual receptor subtypes partly inhibited NPY action, while fully blocking effects of selective agonists. Combining Y1 and Y5 antagonists blocked actions of NPY entirely, but the Y2 antagonist also completely blocked actions of NPY in some neurons. NPY inhibits GABA(A) synaptic transmission onto mpPVN neurons, but this can be mediated by three different NPY receptors. Controversy regarding the receptor or receptor subtypes involved in NPY-mediated feeding may arise from the multiple NPY receptors present.

Mouse feeding behavior: ethology, regulatory mechanisms and utility for mutant phenotyping

Ingestive behaviors, feeding and drinking, constitute unconditioned, obligatory functions that are tightly regulated in the rodent according to demands of the external and internal milieu. Dependent measures of food intake have been used extensively in rats to infer the identity and function of neurochemical pathways, which mediate energy balance. A recent interest in application of appetitive measures in mice can be attributed jointly to the discovery of novel markers of energy balance in genetically obese mice as well as systematic targeting of known feeding regulatory pathways in bioengineered mutant mice. Accordingly, this review will attempt to provide the reader interested in behavioral phenotyping of knockout or transgenic mice with information regarding the ethology of mouse eating behavior, known mechanisms of appetitive regulation and examples of successes and pitfalls encountered when studying food intake in mutant mice.

Central and peripheral dysregulation of melanin-concentrating hormone in obese Zucker rats

Melanin concentrating hormone (MCH) is a peptide synthesized in the lateral hypothalamus which stimulates food ingestion and leptin secretion in rodents. In this experiment, we measured the expressions of MCH as well as of its receptor (SLC-1) in the hypothalamus of obese hyperphagic and lean Zucker rats by quantitative real time RT-PCR. MCH mRNA expression in the obese rats was significantly increased by a factor of five (P<0.01) whereas expression of SLC-1 was decreased by more than 50% (P<0.05). Circulating levels of leptin and MCH were increased in the plasma of obese Zucker rats when compared to lean rats (38-fold and 1.7-fold, respectively, P<0.001 and P<0.01). However, individual MCH levels were not directly correlated to leptin levels in the lean (functional leptin receptor) or in the obese (non-functional leptin receptor) Zucker rats. These results indicate that the absence of leptin signaling in rats is associated with an increased hypothalamic expression and circulating release of MCH, contributing to their obesity syndrome.

Novel analogues of neuropeptide Y with a preference for the Y1-receptor

Neuropeptide Y (NPY) is one of the most abundant neuropeptides in the mammalian brain and acts in humans via at least three receptor subtypes: Y1, Y2, and Y5. Whereas selective agonists and antagonists are known for the Y2- and Y5-receptors, the Y1-receptor still lacks a highly selective agonist. This work presents the first NPY-based analogues with Y1-receptor preference and agonistic properties. Furthermore, the importance of specific amino acids of NPY for binding to the Y-receptor subtypes is presented. Amongst the analogues tested, [Phe7,Pro34]pNPY (where pNPY is porcine neuropeptide Y) showed the most significant Y1-receptor preference (> 1 : 3000-fold), with subnanomolar affinity to the Y1-receptor, and Ki values of approximately 30 nM for the Y2- and Y5-subtype, respectively. Variations of position 6, especially [Arg6,Pro34]pNPY and variations within positions 20-23 of NPY were found to result in further analogues with significant Y1-receptor preference (1 : 400-1 : 2000). In contrast, cyclo S-S [Cys20,Cys24]pNPY was found to be a highly selective ligand at the Y2-receptor, binding only threefold less efficiently than NPY. Analogues containing variations of positions 31 and 32 showed highly reduced affinity to the Y1-receptor, while binding to the Y5-receptor was affected less. Inhibition of cAMP-accumulation of selected peptides with replacements within position 20-23 of NPY showed preserved agonistic properties. The NPY analogues tested give insights into ligand-receptor interaction of NPY at the Y1-, Y2- and Y5-receptor and contribute to our understanding of subtype selectivity. Furthermore, the Y1-receptor-preferring peptides are novel tools that will provide insight into the physiological role of the Y1-receptor.