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. These authors contributed equally to this work.     

A Physiological Increase of Insulin in the Olfactory Bulb Decreases Detection of a Learned Aversive Odor and Abolishes Food Odor-Induced Sniffing Behavior in Rats

Insulin is involved in multiple regulatory mechanisms, including body weight and food intake, and plays a critical role in metabolic disorders such as obesity and diabetes. An increasing body of evidence indicates that insulin is also involved in the modulation of olfactory function. The olfactory bulb (OB) contains the highest level of insulin and insulin receptors (IRs) in the brain. However, a role for insulin in odor detection and sniffing behavior remains to be elucidated. Using a behavioral paradigm based on conditioned olfactory aversion (COA) to isoamyl-acetate odor, we demonstrated that an intracerebroventricular (ICV) injection of 14 mU insulin acutely decreased olfactory detection of fasted rats to the level observed in satiated animals. In addition, whereas fasted animals demonstrated an increase in respiratory frequency upon food odor detection, this effect was absent in fasted animals receiving a 14 mU insulin ICV injection as well as in satiated animals. In parallel, we showed that the OB and plasma insulin levels were increased in satiated rats compared to fasted rats, and that a 14 mU insulin ICV injection elevated the OB insulin level of fasted rats to that of satiated rats. We further quantified insulin receptors (IRs) distribution and showed that IRs are preferentially expressed in the caudal and lateral parts of the main OB, with the highest labeling found in the mitral cells, the main OB projection neurons. Together, these data suggest that insulin acts on the OB network to modulate olfactory processing and demonstrate that olfactory function is under the control of signals involved in energy homeostasis regulation and feeding behaviors.     

Lipid sensing and insulin resistance in the brain

Lipid sensing and insulin signaling in the brain independently triggers a negative feedback system to lower glucose production and food intake. Here, we discuss the underlying molecular and neuronal mechanisms of lipid sensing and insulin signaling in the hypothalamus and how these mechanisms are affected in response to high-fat feeding. We propose that high-fat feeding concurrently disrupts hypothalamic insulin-signaling and lipid-sensing mechanisms and that experiments aimed to restore both insulin action and lipid sensing in the brain could effectively lower glucose production and food intake to restore metabolic homeostasis in type 2 diabetes and obesity.     

Neuropeptide Y: stimulation of feeding and drinking by injection into the paraventricular nucleus

Neuropeptide Y (NPY), a peptide contained within numerous presynaptic terminals in the hypothalamic paraventricular nucleus (PVN), was injected directly into the PVN of satiated, brain-cannulated rats, and food and water intake were measured 0.5, 1, 2 and 4 hrs postinjection. Neuropeptide Y (24 and 78 pmoles/0.3 microliter isotonic saline) caused a dose-dependent increase in food intake, as well as a small, dose-dependent increase in water intake. This effect on feeding occurred even when food was not presented until 4 hrs postinjection. To determine the behavioral specificity of this effect, the impact of PVN injection of NPY (78 pmoles) on various behaviors was observed. With food available, only feeding and drinking behavior were affected. No change in other behaviors, including grooming, rearing, sleeping, resting or different levels of activity, was observed. With food absent, NPY still elicited drinking, suggesting that this is a primary effect, rather than secondary to the feeding. In addition to drinking, NPY reliably increased activity while decreasing sleep and grooming. These results suggest an important role for hypothalamic NPY, or a structurally-related peptide, in the regulation of feeding and drinking behavior.     

The role of cholecystokinin octapeptide in the central control of food intake in the dog

Cholecystokinin octapeptide (CCK-8, 1, 190 pmol/5 min) decreased food intake and water consumption in two models of ingestive behavior, i.e., food deprivation-induced feeding and insulin-induced feeding, when administered into the third (3V) and lateral (LV) cerebral ventricles. In fasted dogs, the suppression of food intake was more prominent after 3V CCK-8, whereas intravenously administered CCK-8 was without effect. Neuropeptide Y (NPY, 1, 190 pmol) had no significant stimulatory effect on food intake and water consumption in fasted as well as satiated dogs, and actually reduced both food and water intake in insulin-treated dogs. There was a slight but significant decrease in food and water intake after 275 nmol naloxone administration in both feeding models, and some of the dogs vomited. In insulin-treated animals, CCK-8 reversed, but NPY potentiated the hypothermic phase of temperature response observed after saline administration, whereas naloxone failed to alter rectal temperature. These results suggest that the effect of CCK-8 on feeding seems to involve central mechanisms in the dog, and that the mechanisms by which CCK-8, NPY and naloxone affect feeding behavior are different.     

Selective activation of central NPY Y1 vs. Y5 receptor elicits hyperinsulinemia via distinct mechanisms

Central administration of neuropeptide Y (NPY) stimulates hyperphagia and hyperinsulinemia. Recent evidence has suggested that the Y1 and Y5 receptor subtypes may both mediate NPY-stimulated feeding. The present study attempts to further characterize the role of central NPY receptor subtypes involved in hyperinsulinemia. NPY and peptide analogs of NPY that selectively activated the NPY Y1 or Y5 receptor subtype induced feeding and hyperinsulinemia in satiated Long Evans rats, whereas NPY analogs that selectively activated the NPY Y2 or Y4 receptor subtype did not. To determine whether NPY-induced hyperinsulinemia is secondary to its hyperphagic effect, we compared the plasma insulin levels in the presence and absence of food after a 1-min central infusion of NPY and its analogs at 15, 60, and 120 min postinfusion. Our data suggest that selective activation of central NPY Y1 receptor subtype induced hyperinsulinemia independent of food ingestion, whereas the NPY Y5 receptor-induced hyperinsulinemia was dependent on food ingestion. Central administration of the selective Y1 receptor agonist D-Arg25 NPY eventually decreased plasma glucose levels 2 h postinfusion in Long Evans rats.     

Thee effect of food intake on the central monoaminergic system in the snail, Lymnaea stagnalis

We investigated the effect of food intake on the serotonin and dopamine levels of the CNS as well as on the spontaneous firing activity of the CGC in isolated preparations from starved, feeding and satiated animals. Furthermore we investigated the effects of 1 microM serotonin and/or dopamine and their mixture on the firing activity of the CGC. The HPLC assay of serotonin and dopamine showed that during food intake both the serotonin and dopamine levels of the CNS increased whereas in satiated animals their levels were not significantly more than the control levels. Recording from the CGC in isolated CNS preparation from starved, feeding or satiated animals showed that feeding increased the firing frequency of the CGC compared to the starved control. The application of 1 microM dopamine decreased the firing frequency whereas the application of 1 microM serotonin increased the firing frequency of the CGC. We conclude that during food intake the external and internal food stimuli increase the activity of the central monoaminergic system and also increase the levels of monoamines in the CNS. Furthermore, we also suggest that the increased dopamine and serotonin levels both affect the activity of the serotonergic neurons during the different phases of feeding.     

Minibrain/Dyrk1a Regulates Food Intake through the Sir2-FOXO-sNPF/NPY Pathway in Drosophila and Mammals

Feeding behavior is one of the most essential activities in animals, which is tightly regulated by neuroendocrine factors. Drosophila melanogaster short neuropeptide F (sNPF) and the mammalian functional homolog neuropeptide Y (NPY) regulate food intake. Understanding the molecular mechanism of sNPF and NPY signaling is critical to elucidate feeding regulation. Here, we found that minibrain (mnb) and the mammalian ortholog Dyrk1a target genes of sNPF and NPY signaling and regulate food intake in Drosophila melanogaster and mice. In Drosophila melanogaster neuronal cells and mouse hypothalamic cells, sNPF and NPY modulated the mnb and Dyrk1a expression through the PKA-CREB pathway. Increased Dyrk1a activated Sirt1 to regulate the deacetylation of FOXO, which potentiated FOXO-induced sNPF/NPY expression and in turn promoted food intake. Conversely, AKT-mediated insulin signaling suppressed FOXO-mediated sNPF/NPY expression, which resulted in decreasing food intake. Furthermore, human Dyrk1a transgenic mice exhibited decreased FOXO acetylation and increased NPY expression in the hypothalamus, as well as increased food intake. Our findings demonstrate that Mnb/Dyrk1a regulates food intake through the evolutionary conserved Sir2-FOXO-sNPF/NPY pathway in Drosophila melanogaster and mammals.     

Central mechanisms involved in the orexigenic actions of ghrelin

Ghrelin is a stomach hormone, secreted into the bloodstream, that initiates food intake by activating NPY/AgRP neurons in the hypothalamic acruate nucleus. This review focuses on recent evidence that details the mechanisms through which ghrelin activate receptors on NPY neurons and downstream signaling within NPY neurons. The downstream signaling involves a novel CaMKK-AMPK-CPT1-UCP2 pathway that enhances mitochondrial efficiency and buffers reactive oxygen species in order to maintain an appropriate firing response in NPY. Recent evidence that shows metabolic status affects ghrelin signaling in NPY is also described. In particular, ghrelin does not activate NPY neurons in diet-induced obese mice and ghrelin does not increase food intake. The potential mechanisms and implications of ghrelin resistance are discussed.     

Expression changes of hypothalamic Ahi1 in mice brain: implication in sensing insulin signaling

Growing evidence suggests that the brain, in particular the hypothalamus, directly senses hormones and nutrients to initiate feeding behavior and metabolic responses in the control of energy homeostasis. However, the molecular mechanisms underlying this important process have remained largely unknown. Our study provides the evidence for the role of Abelson helper integration site 1 (Ahi1) protein as a sensor of insulin signaling in the hypothalamus. We found that fasting increased the expression of hypothalamic Ahi1 which was accompanied by lower levels of circulating insulin compared with satiated mice, while re-feeding decreased the expression of hypothalamic Ahi1 which was accompanied by higher levels of circulating insulin. We also found the up-regulated expression of hypothalamic Ahi1 in high-fat induced obese mice, db/db mice, and streptozotocin induced diabetic mice. In addition, we demonstrated that insulin could decrease the expression of Ahi1 in neuroblastoma cell line N18TG2. Taken together, our results indicate that hypothalamic Ahi1 functions as a sensor of insulin signaling.     

Brain insulin: regulation,mechanisms of action and functions

1. While many questions remain unanswered, it is now well documented that, contrary to earlier views, insulin is an important neuromodulator, contributing to neurobiological processes, in particular energy homeostasis and cognition. A specific role on cognitive functions related to feeding is proposed, and it is suggested that brain insulin from different sources might be involved in the above vital functions in health and disease.2. A molecule identical to pancreatic insulin, and specific insulin receptors, are found widely distributed in the central nervous system networks related to feeding, reproduction, or cognition.3. The actions of insulin in the central nervous system may be under both multilevel and multifactorial controls. The amount of blood insulin reaching the brain, brain insulin stores and secretion, potential local biosynthesis and degradation of the peptide, and insulin receptors and signal transduction can be affected by metabolic factors induced by nutrients, hormones, neurotransmitters, and regulatory peptides, peripherally or in the central nervous system.4. Glucose and serotonin regulate insulin directly in the hypothalamus and may be of importance for its biological effects. Central mechanisms regulating glucose-induced insulin secretion show some analogy with the mechanisms operating in the pancreas.5. A cross-talk between insulin and leptin receptors has been observed in the brain, and a regulation of central insulin actions, potentially via serotonin modulation, by leptin, galanin, melanocortins, and neuropeptide Y (NPY) is suggested.6. A more complete knowledge of the biological role of insulin in brain function and dysfunction, and of the regulatory mechanisms involved in these processes, constitutes a real advancement in the understanding of the pathophysiology of metabolic and mental diseases and could lead to important medical benefits.     

Mapping of neuropeptide Y expression in Octopus brains

Neuropeptide Y (NPY) is an evolutionarily conserved neurosecretory molecule implicated in a diverse complement of functions across taxa and in regulating feeding behavior and reproductive maturation in Octopus. However, little is known about the precise molecular circuitry of NPY-mediated behaviors and physiological processes, which likely involve a complex interaction of multiple signal molecules in specific brain regions. Here, we examined the expression of NPY throughout the Octopus central nervous system. The sequence analysis of Octopus NPY precursor confirmed the presence of both, signal peptide and putative active peptides, which are highly conserved across bilaterians. In situ hybridization revealed distinct expression of NPY in specialized compartments, including potential “integration centers,” where visual, tactile, and other behavioral circuitries converge. These centers integrating separate circuits may maintain and modulate learning and memory or other behaviors not yet attributed to NPY-dependent modulation in Octopus. Extrasomatic localization of NPY mRNA in the neurites of specific neuron populations in the brain suggests a potential demand for immediate translation at synapses and a crucial temporal role for NPY in these cell populations. We also documented the presence of NPY mRNA in a small cell population in the olfactory lobe, which is a component of the Octopus feeding and reproductive control centers. However, the molecular mapping of NPY expression only partially overlapped with that produced by immunohistochemistry in previous studies. Our study provides a precise molecular map of NPY mRNA expression that can be used to design and test future hypotheses about molecular signaling in various Octopus behaviors.     

Reduced serotonin reuptake transporter (SERT) function causes insulin resistance and hepatic steatosis independent of food intake

Serotonin reuptake transporter (SERT) is a key regulator of serotonin neurotransmission and a major target of antidepressants. Antidepressants, such as selectively serotonin reuptake inhibitors (SSRIs), that block SERT function are known to affect food intake and body weight. Here, we provide genetic evidence that food intake and metabolism are regulated by separable mechanisms of SERT function. SERT-deficient mice ate less during both normal diet and high fat diet feeding. The reduced food intake was accompanied with markedly elevated plasma leptin levels. Despite reduced food intake, SERT-deficient mice exhibited glucose intolerance and insulin resistance, and progressively developed obesity and hepatic steatosis. Several lines of evidence indicate that the metabolic deficits of SERT-deficient mice are attributable to reduced insulin-sensitivity in peripheral tissues. First, SERT-deficient mice exhibited beta-cell hyperplasia and islet-mass expansion. Second, biochemical analyses revealed constitutively elevated JNK activity and diminished insulin-induced AKT activation in the liver of SERT-deficient mice. SERT-deficient mice exhibited hyper-JNK activity and hyperinsulinemia prior to the development of obesity. Third, enhancing AKT signaling by PTEN deficiency corrected glucose tolerance in SERT-deficient mice. These findings have potential implications for designing selective SERT drugs for weight control and the treatment of metabolic syndromes.     

Continuous intraventricular infusion of neuropeptide Y evokes episodic food intake in satiated female rats: effects of adrenalectomy and cholecystokinin

In these studies the pattern of feeding behavior during continuous intraventricular (IVT) infusion of NPY for 4 hr in the satiated female rat was monitored. Whereas saline infusion was ineffective, each of the three doses of NPY (117, 470 or 1175 pmol/hr) increased feeding during the entire 4 hr infusion and 2 hr postinfusion period. The cumulative food intake at the end of 4 hr of NPY infusion was enhanced in a dose-related fashion between 0, 117 and 470 pmol/hr; at 1175 pmol/hr food intake plateaued. In addition, the latency to initiate feeding response decreased in a dose-related fashion and feeding occurred in discrete (35-45) episodes during the 4 hr infusion period. Further, the total time feeding and local eating rate (g/min) increased significantly in response to the higher rates of NPY infusion. Concurrent infusion of cholecystokinin (CCK) at either equimolar or 2.5 x NPY dose, affected neither the NPY-induced cumulative food intake nor any other parameter of feeding behavior. On the other hand, cumulative food intake was significantly decreased in adrenalectomized rats in response to NPY infusion (470 pmol/hr); a response due primarily to a marked suppression in some, and almost complete cessation of food consumption in other rats during the second 2 hr period of NPY infusion. These studies show that continuous central infusion of NPY can produce sustained, intermittent feeding behavior and adrenalectomy significantly curtailed the duration of NPY effectiveness.(ABSTRACT TRUNCATED AT 250 WORDS)     

Paraventricular nucleus injections of peptide YY and neuropeptide Y preferentially enhance carbohydrate ingestion

Neuropeptide Y (NPY) injected into the paraventricular nucleus (PVN) is known to elicit a powerful feeding response in satiated, brain-cannulated rats [41, 42, 43]. The present experiment investigates the effect of peptide YY (PYY), a structurally-related peptide, on feeding behavior and, in addition, the effects of both PYY and NPY on the pattern of macronutrient selection. Injection of PYY directly into the PVN, in doses ranging from 7.8 to 235 pmol/0.3 μl, caused a strong, dose-dependent stimulation of feeding behavior, as well as a small stimulation of drinking behavior, in satiated rats. The mean latency to eat was 9.3 min, with substantial feeding occurring within 30 min of the injection. At low doses, the increase in feeding was seen predominantly during the first hr. At the highest dose, in contrast, food intake continued to increase progressively over the next few hr, such that by 4 hr postinjection food intake was more than 20 g over vehicle baseline. In 1 hr tests with 3 pure macronutrient (protein, fat and carbohydrate) diets simulataneously available, PYY and NPY (78 pmol/0.3 μl) both elicited a strong and selective increase in carbohydrate consumption, with little or no effect on protein or fat consumption. These results suggest that hypothalamic receptors sensitive to PYY and NPY may participate in the control of carbohydrate consumption.     

The influence of dehydroepiandrosterone and 8-OH-DPAT on the caloric intake and hypothalamic neurotransmitters of lean and obese Zucker rats

The 5 HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetraline (8-OH-DPAT) increases the food intake of satiated Zucker rats, both lean and obese. Associated with this increased intake are changes in the hypothalamic content of serotonin and its metabolite, 5-HIAA (5-hydroxyindole-3-acetic acid); serotonin is increased while the level of 5-HIAA is decreased. Analysis of individual 5-HIAA/5-hydroxytryptamine (5-HT) ratios, a measure of serotonin turnover indicate that 8-OH DPAT affected serotonin turnover equally and dramatically in both phenotypes. This would be an expected physiological action of an autofeedback mechanism by a 5-HT(1A) receptor agonist. Dehydroepiandrosterone (DHEA) at doses as low as 10 mg/kg blocks the 8-OH-DPAT-induced increase in food intake but does not alter food intake of control satiated Zucker rats. The mechanism of DHEA's action was investigated by monitoring the steroid's effect on hypothalamic neurotransmitters in this satiated model. DHEA by itself induced some change in 5-HIAA in the obese satiated model but not the lean. 8-OH-DPAT, by itself, dramatically decreased serotonin turnover in either lean or obese rats, and DHEA combined with 8-OH-DPAT did not further change serotonin turnover, suggesting DHEA may work through mechanisms other than monoamines to cause its inhibition of 8-OH-DPAT-induced behavioral effects at such low doses.     

Agouti-related protein in the hypothalamic paraventricular nucleus: effect on feeding

Agouti-related protein (Agrp) is an endogenous melanocortin-4 receptor antagonist implicated in the regulation of food intake. Effects of Agrp on feeding under varying conditions were investigated. Agrp (10 to 100 pmol) was injected into the hypothalamic paraventricular nucleus of satiated (a.m. and p.m. injections) and food-deprived rats, or was co-administered with 117 pmol Neuropeptide Y (NPY). Agrp significantly stimulated light-phase feeding by 24 h post-injection. However, Agrp stimulated dark-phase and deprivation-induced feeding by 4 and 2 h, respectively. Animals receiving NPY and Agrp consumed more than animals receiving either peptide alone, the effect remaining by 24 h.     

High sugar-induced insulin resistance in Drosophila relies on the lipocalin Neural Lazarillo

In multicellular organisms, insulin/IGF signaling (IIS) plays a central role in matching energy needs with uptake and storage, participating in functions as diverse as metabolic homeostasis, growth, reproduction and ageing. In mammals, this pleiotropy of action relies in part on a dichotomy of action of insulin, IGF-I and their respective membrane-bound receptors. In organisms with simpler IIS, this functional separation is questionable. In Drosophila IIS consists of several insulin-like peptides called Dilps, activating a unique membrane receptor and its downstream signaling cascade. During larval development, IIS is involved in metabolic homeostasis and growth. We have used feeding conditions (high sugar diet, HSD) that induce an important change in metabolic homeostasis to monitor possible effects on growth. Unexpectedly we observed that HSD-fed animals exhibited severe growth inhibition as a consequence of peripheral Dilp resistance. Dilp-resistant animals present several metabolic disorders similar to those observed in type II diabetes (T2D) patients. By exploring the molecular mechanisms involved in Drosophila Dilp resistance, we found a major role for the lipocalin Neural Lazarillo (NLaz), a target of JNK signaling. NLaz expression is strongly increased upon HSD and animals heterozygous for an NLaz null mutation are fully protected from HSD-induced Dilp resistance. NLaz is a secreted protein homologous to the Retinol-Binding Protein 4 involved in the onset of T2D in human and mice. These results indicate that insulin resistance shares common molecular mechanisms in flies and human and that Drosophila could emerge as a powerful genetic system to study some aspects of this complex syndrome.     

Molecular docking analysis of beta-caryophyllene with IRS-1, cSrc and Akt

Diabetes mellitus (DM) is a common metabolic illness defined by hyperglycemia caused by insufficient production or absent of pancreatic insulin, with or without concomitant insulin action impairment. Hence, novel problem-solving approaches for assessing early metabolic diseases, notably insulin resistance, are urgently needed. Screening of natural compounds for drug discovery to combat diabetes is common in modern medical research and development. Therefore, it is of interest to document the molecular docking analysis data of beta-Caryophyllene, a naturally occurring sequiterpene with the downstream insulin signaling molecules such as IRS-1, cSrc and Akt for the management of type-2 diabetes. The molecular docking analysis data of beta-caryophyllene with the insulin downstream signaling molecules such as IRS-1, cSrc and Akt reveals its ability and further studies are needed to elucidate its complete mechanism of action against type-2 diabetes.