Effect of nociceptin/orphanin FQ on food intake in rats that differ in diet preference

A 40-year analysis of central neuroanatomical and neurochemical circuits mediating homeostatic intake and hedonic intake and preferences in rodents

This perspective review was written in response to the celebration of the 60th anniversary of the journal, Brain Research, and covers the evolving focus of my laboratory's work over 40 years in the neurobiological substrates of ingestive behavior in rodents. Following our initial work examining the effects of systemic and ventricular administration of general and selective opioid receptor agonists and antagonists on food intake under spontaneous, deprivation, glucoprivic and hedonic conditions, my laboratory in close collaboration with Drs. Gavril Pasternak and Ying-Xian Pan utilized an antisense oligodoxynucleotide knock-down technique affecting MOR-1, DOR-1, KOR-1 and ORL-1 genes as well as against G-protein subunits to study receptor mediation of opioid receptor agonist-induced feeding as well as feeding following regulatory challenges. Our laboratory employed intracerebral microinjection techniques to map limbic nucleus accumbens and ventral tegmental area central brain circuits mediating homeostatic and hedonic feeding responses through the use of selective mu, delta1, delta2 and kappa opioid receptor subtype agonists in combination with general and selective opioid, dopamineric, glutamatergic and GABAergic antagonists administered into the same site or the reciprocal site, allowing for the identification of a distributed brain network mediating these ingestive effects. Our laboratory in close collaboration with Dr. Anthony Sclafani then focused on the pharmacological, neuroanatomical and learning mechanisms related to the development of sugar- (sucrose, glucose and fructose) and fat- (corn oil) conditioned flavor preferences (CFP) in rats, and on murine genetic variance in food intake, preferences and the process of appetition.

Role of Nociceptin/Orphanin FQ-NOP Receptor System in the Regulation of Stress-Related Disorders

Nociceptin/orphanin FQ (N/OFQ) is a 17-residue neuropeptide that binds the nociceptin opioid-like receptor (NOP). N/OFQ exhibits nucleotidic and aminoacidics sequence homology with the precursors of other opioid neuropeptides but it does not activate either MOP, KOP or DOP receptors. Furthermore, opioid neuropeptides do not activate the NOP receptor. Generally, activation of N/OFQ system exerts anti-opioids effects, for instance toward opioid-induced reward and analgesia. The NOP receptor is widely expressed throughout the brain, whereas N/OFQ localization is confined to brain nuclei that are involved in stress response such as amygdala, BNST and hypothalamus. Decades of studies have delineated the biological role of this system demonstrating its involvement in significant physiological processes such as pain, learning and memory, anxiety, depression, feeding, drug and alcohol dependence. This review discusses the role of this peptidergic system in the modulation of stress and stress-associated psychiatric disorders in particular drug addiction, mood, anxiety and food-related associated-disorders. Emerging preclinical evidence suggests that both NOP agonists and antagonists may represent a effective therapeutic approaches for substances use disorder. Moreover, the current literature suggests that NOP antagonists can be useful to treat depression and feeding-related diseases, such as obesity and binge eating behavior, whereas the activation of NOP receptor by agonists could be a promising tool for anxiety.

PNOCARC Neurons Promote Hyperphagia and Obesity upon High-Fat-Diet Feeding

Calorie-rich diets induce hyperphagia and promote obesity, although the underlying mechanisms remain poorly defined. We find that short-term high-fat-diet (HFD) feeding of mice activates prepronociceptin (PNOC)-expressing neurons in the arcuate nucleus of the hypothalamus (ARC). PNOC^ARC neurons represent a previously unrecognized GABAergic population of ARC neurons distinct from well-defined feeding regulatory AgRP or POMC neurons. PNOC^ARC neurons arborize densely in the ARC and provide inhibitory synaptic input to nearby anorexigenic POMC neurons. Optogenetic activation of PNOC^ARC neurons in the ARC and their projections to the bed nucleus of the stria terminalis promotes feeding. Selective ablation of these cells promotes the activation of POMC neurons upon HFD exposure, reduces feeding, and protects from obesity, but it does not affect food intake or body weight under normal chow consumption. We characterize PNOC^ARC neurons as a novel ARC neuron population activated upon palatable food consumption to promote hyperphagia.

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Acute high-fat-diet feeding activates PNOC neurons in the arcuate nucleus (ARC)

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GABAergic PNOC^ARC neurons inhibit anorexigenic POMC neurons

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Optogenetic activation of PNOC^ARC neurons promotes feeding

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Ablation of PNOC^ARC neurons protects from obesity

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

N/OFQ-NOP System in Food Intake

While lifestyle modifications should be the first-line actions in preventing and treating obesity and eating disorders, pharmacotherapy also provides a necessary tool for the management of these diseases.However, given the limitations of current anti-obesity drugs, innovative treatments that improve efficacy and safety are needed.Since the discovery that the activation of the Nociceptin/Orphanin (N/OFQ) FQ peptide (NOP) receptor by N/OFQ induces an increase of food intake in laboratory animals, and the finding that this effect can be blocked by NOP antagonists, many NOP agonists and antagonists have been synthesized and tested in vitro and in vivo for their potential regulation of feeding behavior. Promising results seem to suggest that the N/OFQergic system may be a potential therapeutic target for the neural control of feeding behavior and related pathologies, especially in binge-like eating behavior.

Parsing the hedonic and motivational influences of nociceptin on feeding using licking microstructure analysis in mice

Opioid peptides are implicated in processes related to reward and aversion; however, how specific opioid peptides are involved remains unclear. We investigated the role of nociceptin (NOC) in voluntary licking for palatable and aversive tastants by studying the effect of intracerebroventricularly administered NOC on licking microstructure in wild-type and NOC receptor knockout (NOP KO) mice. Compared with the wild-type mice, NOP KO mice emitted fewer bouts of licking when training to lick for a 20% sucrose solution. Correspondingly, intracerebroventricular administration of NOC increased the number of licking bouts for sucrose and sucralose in wild-type, but not in NOP KO mice. The ability of NOC to initiate new bouts of licking for sweet solutions suggests that NOC may drive motivational aspects of feeding behavior. Conversely, adulterating a sucrose solution with the aversive tastant quinine reduced licking bout lengths in wild-type and NOP KOs, suggesting that NOC signaling is not involved in driving voluntary consumption of semiaversive tastants. Interestingly, when consuming sucrose following 20 h of food deprivation, NOP KO mice emitted longer bouts of licking than wild types, suggesting that under hungry conditions, NOC may also contribute toward hedonic aspects of feeding. Together, these results suggest differential roles for NOC in the motivational and hedonic aspects of feeding.

Nociceptin receptor antagonist SB 612111 decreases high fat diet binge eating

Binge eating is a dysregulated form of feeding behavior that occurs in multiple eating disorders including binge-eating disorder, the most common eating disorder. Feeding is a complex behavioral program supported through the function of multiple brain regions and influenced by a diverse array of receptor signaling pathways. Previous studies have shown the overexpression of the opioid neuropeptide nociceptin (orphanin FQ, N/OFQ) can induce hyperphagia, but the role of endogenous nociceptin receptor (NOP) in naturally occurring palatability-induced hyperphagia is unknown. In this study we adapted a simple, replicable form of binge eating of high fat food (HFD). We found that male and female C57BL/6J mice provided with daily one-hour access sessions to HFD eat significantly more during this period than those provided with continuous 24h access. This form of feeding is rapid and entrained. Chronic intermittent HFD binge eating produced hyperactivity and increased light zone exploration in the open field and light-dark assays respectively. Treatment with the potent and selective NOP antagonist SB 612111 resulted in a significant dose-dependent reduction in binge intake in both male and female mice, and, unlike treatment with the serotonin selective reuptake inhibitor fluoxetine, produced no change in total 24-h food intake. SB 612111 treatment also significantly decreased non-binge-like acute HFD consumption in male mice. These data are consistent with the hypothesis that high fat binge eating is modulated by NOP signaling and that the NOP system may represent a promising novel receptor to explore for the treatment of binge eating.

Central histaminergic system interplay with suppressive effects of immune challenge on food intake in chicken

The aim of the current study was to investigate the interaction of the lipopolysaccharide (LPS) and histaminergic systems on appetite regulation in broilers. Effects of intracerebroventricular (ICV) injection of α-fluoromethylhistidine (α-FMH, histidine decarboxylase inhibitor), chlorpheniramine (histamine H1 receptor antagonist), famotidine (histamine H2 receptor antagonist) and thioperamide (histamine H3 receptor antagonist) on LPS-induced hypophagia in broilers were studied. A total of 128 broilers were randomly allocated into 4 experiments (4 groups and 8 replications in each experiment). A cannula was surgically implanted into the lateral ventricle. In Experiment 1, broilers were ICV injected with LPS (20 ng) prior to α-FMH (250 nmol). In Experiment 2, chickens were ICV injected with LPS followed by chlorpheniramine (300 nmol). In Experiment 3, broilers were ICV injected with famotidine (82 nmol) after LPS (20 ng). In Experiment 4, ICV injection of LPS was followed by thioperamide (300 nmol). Then, cumulative food intake was recorded until 4 h post-injection. According to the results, LPS significantly decreased food intake. Chlorpheniramine significantly amplified food intake, and LPS-induced hypophagia was lessened by injection of chlorpheniramine. α-FMH, famotidine and thioperamide had no effect on LPS-induced hypophagia. These results suggest that there is an interaction between central LPS and the histaminergic system where LPS-induced hypophagia is mediated by H1 histamine receptors in 3 h food-deprived broilers.

A Novel Nociceptin Receptor Antagonist LY2940094 Inhibits Excessive Feeding Behavior in Rodents: A Possible Mechanism for the Treatment of Binge Eating Disorder

Nociceptin/orphanin FQ (N/OFQ), a 17 amino acid peptide, is the endogenous ligand of the ORL1/nociceptin-opioid-peptide (NOP) receptor. N/OFQ appears to regulate a variety of physiologic functions including stimulating feeding behavior. Recently, a new class of thienospiro-piperidine-based NOP antagonists was described. One of these molecules, LY2940094 has been identified as a potent and selective NOP antagonist that exhibited activity in the central nervous system. Herein, we examined the effects of LY2940094 on feeding in a variety of behavioral models. Fasting-induced feeding was inhibited by LY2940094 in mice, an effect that was absent in NOP receptor knockout mice. Moreover, NOP receptor knockout mice exhibited a baseline phenotype of reduced fasting-induced feeding, relative to wild-type littermate controls. In lean rats, LY2940094 inhibited the overconsumption of a palatable high-energy diet, reducing caloric intake to control chow levels. In dietary-induced obese rats, LY2940094 inhibited feeding and body weight regain induced by a 30% daily caloric restriction. Last, in dietary-induced obese mice, LY2940094 decreased 24-hour intake of a high-energy diet made freely available. These are the first data demonstrating that a systemically administered NOP receptor antagonist can reduce feeding behavior and body weight in rodents. Moreover, the hypophagic effect of LY2940094 is NOP receptor dependent and not due to off-target or aversive effects. Thus, LY2940094 may be useful in treating disorders of appetitive behavior such as binge eating disorder, food choice, and overeating, which lead to obesity and its associated medical complications and morbidity.

Role of nociceptin/orphanin FQ in thermoregulation

Nociceptin/Orphanin FQ (N/OFQ) is a 17-amino acid peptide that binds to the nociceptin receptor (NOP). N/OFQ and NOP receptors are expressed in numerous brain areas. The generation of specific agonists, antagonists and receptor-deficient mice or rats has enabled progress in elucidating the biological functions of N/OFQ. These tools have been employed to identify the biological significance of the N/OFQ system and how it interacts with other endogenous systems to regulate several body functions. The present review focuses on the role of N/OFQ in the regulation of body temperature and its relationship with energy balance. Critical evaluation of the literature data suggests that N/OFQ, acting through the NOP receptor, may cause hypothermia by influencing the complex thermoregulatory system that operates as a federation of independent thermoeffector loops to control body temperature at the hypothalamic level. Furthermore, N/OFQ counteracts hyperthermia elicited by cannabinoids or µ-opioid agonists. N/OFQ-induced hypothermia is prevented by ω-conotoxin GVIA, an N-type calcium channel blocker. Hypothermia induced by N/OFQ is considered within the framework of the complex action that this neuropeptide exerts on energy balance. Energy stores are regulated through the complex neural controls exerted on both food intake and energy expenditure. In laboratory rodents, N/OFQ stimulates consummatory behavior and decreases energy expenditure. Taken together, these studies support the idea that N/OFQ contributes to the regulation of energy balance by acting as an "anabolic" neuropeptide as it elicits effects similar to those produced in the hypothalamus by other neuropeptides such as orexins and neuropeptide Y.

The biology of Nociceptin/Orphanin FQ (N/OFQ) related to obesity, stress, anxiety, mood, and drug dependence

Nociceptin/Orphanin FQ (N/OFQ) is a 17 amino acid peptide that was deorphanized in 1995. The generation of specific agonists, antagonists and receptor deficient mice and rats has enabled progress in elucidating the biological functions of N/OFQ. Additionally, radio-imaging technologies have been advanced for investigation of this system in animals and humans. Together with traditional neurobehavioral techniques, these tools have been utilized to identify the biological significance of the N/OFQ system and its interacting partners. The present review focuses on the role of N/OFQ in the regulation of feeding, body weight homeostasis, stress, the stress-related psychiatric disorders of depression and anxiety, and in drug and alcohol dependence. Critical evaluation of the current scientific preclinical literature suggests that small molecule modulators of nociceptin opioid peptide receptors (NOP) might be useful in the treatment of diseases related to these biological functions. In particular, the literature data suggest that antagonism of NOP receptors will produce anti-obesity and antidepressant activities in humans. However, there are also contradictory data discussed. The current literature on the role of N/OFQ in anxiety and addiction, on the other hand points primarily to a role of agonist modulation being potentially therapeutic. Some drug-like molecules that function either as agonists or antagonists of NOP receptors have been optimized for human clinical study to test some of these hypotheses. The discovery of PET ligands for NOP receptors, combined with the pharmacological tools and burgeoning preclinical data set discussed here bodes well for a rapid advancement of clinical understanding and potential therapeutic benefit.

Evidence that orphanin FQ mediates progesterone negative feedback in the ewe

Orphanin FQ (OFQ), a member of the opioid family, is found in many areas of the hypothalamus and, when given centrally OFQ inhibits episodic LH secretion in rodents and sheep. Because GnRH neurons are devoid of the appropriate receptors to mediate steroid negative feedback directly, neurons that release OFQ may be involved. Using immunocytochemistry, we first determined that most OFQ neurons in the arcuate nucleus (ARC) and other hypothalamic regions of luteal phase ewes contained both estrogen receptor α and progesterone (P) receptor. Given a similar high degree of steroid receptor colocalization in other ARC subpopulations, we examined whether OFQ neurons of the ARC contained those other neuropeptides and neurotransmitters. OFQ did not colocalize with kisspeptin, tyrosine hydroxylase, or agouti-related peptide, but all ARC OFQ neurons coexpressed proopiomelanocortin. To test for a role for endogenous OFQ, we examined the effects of an OFQ receptor antagonist, [Nphe1,Arg14,Lys15]Nociceptin-NH2 (UFP-101) (30 nmol intracerebroventricular/h), on LH secretion in steroid-treated ewes in the breeding season and ovary-intact ewes in anestrus. Ovariectomized ewes with luteal phase concentrations of P and estradiol showed a significant increase in LH pulse frequency during infusion of UFP-101 (4.5 ± 0.5 pulses/6 h) compared with saline infusion (2.6 ± 0.4 pulses/6 h), whereas ewes implanted with only estradiol did not. Ovary-intact anestrous ewes displayed no significant differences in LH pulse amplitude or frequency during infusion of UFP-101. Therefore, we conclude that OFQ mediates, at least in part, the negative feedback action of P on GnRH/LH pulse frequency in sheep.

Deciphering intracellular localization and physiological role of nociceptin and nocistatin

Nociceptin and nocistatin are endogenous ligands of G protein coupled receptor family. Numerous techniques have been used to study the diverse parameters including, localization, distribution and ultrastructure of these peptides. The majority of the study parameters are based on their physiological roles in different organ systems. The present study presents an overview of the different methods used for the study of nociceptin, nocistatin and their receptors. Nociceptin has been implicated in many physiological functions including, nociception, locomotion, stressed-induced analgesia, learning and memory, neurotransmitter and hormone release, renal function, neuronal differentiation, sexual and reproductive behavior, uterine contraction, feeding, anxiety, gastrointestinal motility, cardiovascular function, micturition, cough, hypoxic-ischemic brain injury, diuresis and sodium balance, temperature regulation, vestibular function, and mucosal transport. It has been noted that the use of light and electron microscopy was less frequent, though it may be one of the most promising tools to study the intracellular localization of these neuropeptides. In addition, more studies on the level of circulating nociceptin and nocistatin are also necessary for investigating their clinical roles in health and disease. A variety of modern tools including physiological, light and electron microscopy (EM) are needed to decipher the extent of intracellular localization, tissue distribution and function of these peptides. The intracellular localization of nociceptin and nocistatin will require a high resolution transmission EM capable of identifying these peptides and other supporting molecules that co-localize with them. A tracing technique could also elucidate a possible migratory ability of nociceptin and nocistatin from one cellular compartment to the other.

Changes in feeding behavior after peripheral loperamide administration in rats

Changes in the parameters of operant feeding behavior and body weight were studied in rats after intragastric administration of μ-opioid receptor agonist loperamide. Loperamide administration significantly suppressed foraging behavior in rats and reduced their body weight. Our findings suggest that peripheral loperamide administration, according to the hypothesis of reciprocal interactions between the central and peripheral parts of the endogenous opioid system, suppresses activity of central opioid mechanisms of feeding behavior organization. Changes in feeding behavior can appear due to disturbances in the mechanisms of assessment of food reward. We hypothesized that natural activation of μ-opioid receptors of the stomach with food-derived peptides can be associated with "sensory satiation" mechanism limiting excessive food intake.

Effect of mosapride citrate on gastric emptying in interferon-induced gastroparesis

BACKGROUND AND OBJECTIVES

Gastroparesis, a gastrointestinal autonomic neuropathy, is a common adverse reaction in chronic hepatitis C (CHC) patients receiving interferon therapy. Current therapeutic options are limited. We evaluated the efficacy of mosapride for IFN-induced gastroparesis.

METHODS

Twenty-four consecutive CHC patients were randomly assigned to either the control group, which received pegylated interferon α-2b at 1.5 μg/kg/week and ribavirin at 600-1,000 mg/day, depending on body weight (PegIFN/RBV), or the mosapride group, which received PegIFN/RBV plus mosapride at 15 mg/person/day. The solid-phase gastric emptying half-times (T1/2) of the total, proximal, and distal stomach (scintigraphy) and digestive symptoms (questionnaire) were measured within one week before and four weeks after initiation of the assigned therapy. The test meal comprised a 200-g pancake containing Tc-99m diethylenetriamine pentaacetic acid.

RESULTS

In the control group, after PegIFN/RBV initiation, a significant increase was observed in the total T1/2 (before: 84.0 ± 22.1 min versus after: 100.8 ± 28.9 min, P = 0.03), the distal T1/2 (before: 95.3 ± 32.2 min versus after: 115.3 ± 41.4 min, P = 0.03), and digestive symptom score (before: 3.2 ± 1.4 versus after: 8.1 ± 4.8, P = 0.02); proximal T1/2 change was not significant. In the mosapride group, no significant delays were observed in the total, proximal, and distal T1/2 values; the change in symptom scores was not significant.

CONCLUSIONS

Mosapride improved total and distal gastric motility in IFN-induced gastroparesis, and consequently relieved symptoms.

Molecular, immunohistochemical, and pharmacological evidence of oxytocin's role as inhibitor of carbohydrate but not fat intake

Oxytocin (OT) facilitates feeding termination stemming from high osmolality, stomach distention, and malaise. Recent knockout (KO) studies suggested a crucial function for OT in carbohydrate intake: OT-/- mice had increased preference for carbohydrates, including sucrose, but not fat (Intralipid). In striking contrast, sugar appetite was unaffected in the OT receptor KO mouse; data from wild-type animals have been insufficient. Therefore, we examined the involvement of OT in the regulation of sucrose vs. fat intake in C57BL/6 mice that served as a background KO strain. We exposed mice to a meal of sucrose or Intralipid and determined that the percentage of c-Fos-immunoreactive paraventricular hypothalamic OT neurons was elevated at termination of intake of either of the tastants, but this increase was 2-fold higher in sucrose-fed mice. A 48-h exposure to sucrose compared with Intralipid caused up-regulation of OT mRNA, whereas inherent individual preferences for sucrose vs. fat were not associated with differences in baseline OT expression as established with quantitative PCR. We found that L-368,899, an OT receptor antagonist, increased sugar intake when sucrose was presented alone or concurrently with Intralipid; it had no effect on Intralipid or total calorie consumption. L-368,899 affected Fos immunoreactivity in the paraventricular hypothalamus, arcuate nucleus, amygdala, and nucleus of the solitary tract, areas involved in aversion, satiety, and reward. This pattern serves as neuroanatomical basis of OT's complex role in food intake, including sucrose intake. The current findings expand our knowledge on OT and suggest that it acts as a carbohydrate-specific inhibitor of feeding.

Central nociceptin/orphanin FQ system elevates food consumption by both increasing energy intake and reducing aversive responsiveness

Nociceptin/orphanin FQ (N/OFQ), the nociceptin opioid peptide (NOP) receptor ligand, increases feeding when injected centrally. Initial data suggest that N/OFQ blocks the development of a conditioned taste aversion (CTA). The current project further characterized the involvement of N/OFQ in the regulation of hunger vs. aversive responses in rats by employing behavioral, immunohistochemical, and real-time PCR methodology. We determined that the same low dose of the NOP antagonist [Nphe(1)]N/OFQ(1-13)NH(2) delivered via the lateral ventricle diminishes both N/OFQ- and deprivation-induced feeding. This anorexigenic effect did not stem from aversive consequences, as the antagonist did not cause the development of a CTA. When [Nphe(1)]N/OFQ(1-13)NH(2) was administered with LiCl, it moderately delayed extinction of the LiCl-induced CTA. Injection of LiCl + antagonist compared with LiCl alone generated an increase in c-Fos immunoreactivity in the central nucleus of the amygdala. The antagonist alone elevated Fos immunoreactivity in the paraventricular nucleus of the hypothalamus, nucleus of the solitary tract, and central nucleus of the amygdala. Hypothalamic NOP mRNA levels were decreased during energy intake restriction induced by aversion, as well as in non-CTA rats food-restricted to match CTA-reduced consumption. Brain stem NOP was upregulated only in aversion. Prepro-N/OFQ mRNA showed a trend toward upregulation in restricted rats (P = 0.068). We conclude that the N/OFQ system promotes feeding by affecting the need to replenish lacking calories and by reducing aversive responsiveness. It may belong to mechanisms that shift a balance between the drive to ingest energy and avoidance of potentially tainted food.

Hypothalamic FTO is associated with the regulation of energy intake not feeding reward

Background

Polymorphism in the FTO gene is strongly associated with obesity, but little is known about the molecular bases of this relationship. We investigated whether hypothalamic FTO is involved in energy-dependent overconsumption of food. We determined FTO mRNA levels in rodent models of short- and long-term intake of palatable fat or sugar, deprivation, diet-induced increase in body weight, baseline preference for fat versus sugar as well as in same-weight animals differing in the inherent propensity to eat calories especially upon availability of diverse diets, using quantitative PCR. FTO gene expression was also studied in organotypic hypothalamic cultures treated with anorexigenic amino acid, leucine. In situ hybridization (ISH) was utilized to study FTO signal in reward- and hunger-related sites, colocalization with anorexigenic oxytocin, and c-Fos immunoreactivity in FTO cells at initiation and termination of a meal.

Results

Deprivation upregulated FTO mRNA, while leucine downregulated it. Consumption of palatable diets or macronutrient preference did not affect FTO expression. However, the propensity to ingest more energy without an effect on body weight was associated with lower FTO mRNA levels. We found that 4-fold higher number of FTO cells displayed c-Fos at meal termination as compared to initiation in the paraventricular and arcuate nuclei of re-fed mice. Moreover, ISH showed that FTO is present mainly in hunger-related sites and it shows a high degree of colocalization with anorexigenic oxytocin.

Conclusion

We conclude that FTO mRNA is present mainly in sites related to hunger/satiation control; changes in hypothalamic FTO expression are associated with cues related to energy intake rather than feeding reward. In line with that, neurons involved in feeding termination express FTO. Interestingly, baseline FTO expression appears linked not only with energy intake but also energy metabolism.

Chronic intracerebroventricular infusion of nociceptin/orphanin FQ produces body weight gain by affecting both feeding and energy metabolism in mice

Nociceptin/orphanin FQ (N/OFQ), an endogenous ligand for opioid receptor-like 1 (ORL1), is involved in various central functions, such as pain, psychological stress, locomotor activity, learning and memory, and feeding regulation. Of these functions, the role of N/OFQ in the regulation of feeding has been suggested by the fact that the central administration of N/OFQ leads to feeding behavior. However, the manner in which N/OFQ influences body weight control and subsequent obesity is unclear. To clarify the involvement of N/OFQ in the development of obesity, we evaluated the effects of intracerebroventricular infusion of N/OFQ on food intake and body weight in C57BL/6J mice that were fed a regular chow diet or moderately high-fat (MHF) diet (32.6% kcal fat). N/OFQ significantly increased food intake and body weight both in the regular diet- and MHF diet-fed mice, and these changes were more apparent in the MHF diet-fed mice. When we performed a pair-feeding study in N/OFQ intracerebroventricularly infused mice, N/OFQ did not cause body weight gain but increased white adipose tissue weight and plasma leptin, insulin, and cholesterol levels. N/OFQ reduced rectal temperature in pair-fed mice, in keeping with decreased UCP1 mRNA expression in brown adipose tissue. These results suggest that N/OFQ contributes to the development of obesity not only by inducing hyperphagia but also by decreasing energy expenditure.

Endogenous nociceptin modulates diet preference independent of motivation and reward

Previous studies show that the opioid peptide nociceptin stimulates food intake. Here, we studied nociceptin receptor knockout (NOP KO) mice in various behavioral paradigms designed to differentiate psychological and physiological loci at which endogenous nociceptin might control feeding. When presented a choice under food restriction, NOP KO mice displayed reduced preference for high sucrose diet, but lower intake of high fat diet under no-choice conditions. These responses were absent under ad libitum feeding conditions. Conditioned place preference to high fat diet under food-deprived conditions was unaltered in NOP KO mice, suggesting no difference in reward responses. Furthermore, operant food self-administration under a variety of conditions showed no genotype-dependent differences, suggesting no differences in the motivational properties of food. Taste reactivity to sucrose was unchanged in NOP KO mice, though NOP KO mice had altered aversive reactions to quinine solutions under ad libitum feeding, suggesting minor differences in the affective impact of palatable and unpalatable tastants. Although NOP KO mice re-fed following food-deprivation showed normal increases in plasma glucose and insulin, multidimensional scaling analysis showed that the relationship between these measures, body weight and plasma leptin was substantially disrupted in NOP KO, particularly in fasted mice. Additionally, the typical positive relationship between body weight and plasma leptin was considerably weaker in NOP KO mice. Together, these findings suggest that endogenous nociceptin differentially modulates diet preference depending on macronutrient content and homeostatic state, independently of the motivating, rewarding or orosensory properties of food, but may involve metabolic or postingestive processes.

Inverse association of high-fat diet preference and anxiety-like behavior: a putative role for urocortin 2

The aim of this study was to investigate whether the preference for a palatable high-fat diet (HFD) is associated with response to novelty and with anxiety-like behavior in rats and whether such fat preference correlates with gene expression of hypothalamic neuropeptides related to feeding. We subjected male rats to two tests of exploration of novel environments: the multivariate concentric square field (MCSF) and the elevated plus maze (EPM). The rats were then exposed to a 5-day test of preference for a palatable HFD versus reference diets. Messenger RNA (mRNA) levels of 21 neuropeptides were investigated by quantitative polymerase chain reaction. We found a strong positive correlation of HFD preference and open-arm activity in the EPM (% open-arm time, r(s) = 0.629, df = 26, P < 0.001). Thus, HFD preference was inversely associated with anxiety-like behavior. The same association was found for HFD preference and behavior in the MCSF (bridge entries, r(s) = 0.399, df = 23, P = 0.048). In addition, the HFD preference was positively correlated (r(s) = 0.433, df = 25, P = 0.021) with hypothalamic mRNA levels of urocortin 2 (Ucn 2). Moreover, behavior in the EPM was significantly correlated with expression levels of the receptor for Ucn 2, the corticotropin-releasing factor receptor 2, in the hypothalamus (r(s) = 0.382, df = 33, P = 0.022, pituitary (r(s) = 0.494, df = 31, P = 0.004) and amygdala (r(s) = 0.381, df = 30, P = 0.032). We conclude that preference for palatable HFD is inversely associated with anxiety and propose that Ucn 2 signaling may play a role in this association.

Ghrelin in the CNS: from hunger to a rewarding and memorable meal?

Ghrelin, the endogenous agonist of the growth hormone secretagogue receptor, has been shown to induce robust feeding responses in numerous experimental models. Although ghrelin comes from both peripheral and central sources, its hyperphagic properties, to a large extent, arise from activity at the brain level. The current review focuses on describing central mechanisms through which this peptide affects consumption. We address the issue of whether ghrelin serves just as a signal of energy needs of the organism or - as suggested by the most recent findings - also affects food intake via other feeding-related mechanisms, including reward and memory. Complexity of ghrelin's role in the regulation of ingestive behavior is discussed by characterizing its influence on consumption, reward and memory as well as by defining its function within the brain circuitry and interplay with other neuropeptides.

Paraventricular opioids alter intake of high-fat but not high-sucrose diet depending on diet preference in a binge model of feeding

Previous work from our laboratory indicates that when rats are given a choice between a high-fat and a high-sucrose diet, opioid blockade with naltrexone (NTX) in a reward-related site (central amygdala) inhibits intake of the preferred diet only, whereas NTX injected into a homeostasis-related site, such as the hypothalamic paraventricular nucleus (PVN), inhibits intake of both diets. However, other work suggests that opioids increase intake of fat specifically. The present study further investigates the role of PVN opioids in food choices made by calorically-replete animals. We used a binge model with chow-maintained rats given 3-h access to a choice of a high-fat or high-sucrose diet 3 days a week. We hypothesized that intra-PVN injection of the mu-opioid agonist, DAMGO (0, 0.025, 0.25, and 2.5 nmol) would enhance, and NTX (0, 10, 30, and 100 nmol) would inhibit intake of both diets to an equal extent. We found that when animals were divided into groups according to sucrose or fat preference, DAMGO increased fat intake in fat-consuming animals, while having no effect on intake of either diet in sucrose-consuming animals. NTX, however, inhibited fat intake in both groups. Intra-PVN NTX did not inhibit intake of sucrose when presented in the absence of a fat choice, but did so when injected peripherally. Furthermore, intra-PVN and systemic NTX inhibited intake of chow by 24-h-food-deprived animals. These results indicate a complex role for PVN opioids in food intake with preference, nutrient type, and energy state affecting the ability of these compounds to change behavior.

Central opioids and consumption of sweet tastants: when reward outweighs homeostasis

Numerous reports have described opioids as peptides involved in the regulation of food intake. The role of these endogenous substances appears to be linked with reward-dependent feeding, since injection of opioid receptor ligands alters consumption of palatable foods and solutions more readily than of non-palatable ones, and intake of such tastants affects the activity of the opioid system within the brain. Among a variety of available foods, those rich in sucrose and other sweet tastants, are extremely appealing to humans and laboratory animals. In the current review, we focus on the rewarding aspects of consummator behavior driven by opioids. We attempt to delineate opioid-dependent central mechanisms responsible for overconsumption of "rewarding" palatable diets, especially foods high in sugar that can potentially jeopardize homeostasis.

Functional interaction between nociceptin/orphanin FQ and alpha-melanocyte-stimulating hormone in the regulation of feeding

Nociceptin/orphanin FQ (N/OFQ), an endogenous agonist of the opioid N/OFQ (NOP) receptor, increases food intake when administered centrally. As N/OFQ is part of a larger neural network that governs consummatory behavior, presumably its orexigenic properties stem from interplay with other neuropeptidergic components of the feeding-related circuitry. One such peptide may be the ligand of the melanocortin-3 and -4 receptors, alpha-melanocyte-stimulating hormone (alpha-MSH), which is known to inhibit food intake. The aim of the present study was to establish whether there is a functional "interaction" between N/OFQ and alpha-MSH in the regulation of feeding. By using double immunostaining for c-Fos and alpha-MSH, we found that intracerebroventricular (i.c.v.) injection of N/OFQ at a 10nmol dose that moderately prolongs deprivation-induced food intake in rats, decreases activation of alpha-MSH neurons involved in feeding termination. However, i.c.v. injections of alpha-MSH at doses previously established to reduce deprivation-induced feeding, do not decrease hyperphagia generated by N/OFQ in ad libitum-fed animals. Our results suggest that while alpha-MSH does not appear to modify the orexigenic response to N/OFQ in sated rats, the NOP receptor ligand promotes a decrease in activation of neurons synthesizing the anorexigenic peptide, alpha-MSH, at the time of re-feeding. Thus, to some degree, the stimulatory effect of N/OFQ on consumption may arise from this peptide's inhibitory influence on activity of anorexigenic pathways containing alpha-MSH.

Nociceptin/orphanin FQ-induced delay in gastric emptying: role of central corticotropin-releasing factor and glucocorticoid receptors

When injected intracerebroventricularly (i.c.v.) in rats, nociceptin/orphanin FQ (N/OFQ) delays gastric emptying and increases plasma corticosterone levels. Our aim in this study was to investigate changes in gastric emptying of a phenol red meal, and the plasma corticosterone response to N/OFQ in adrenalectomized (ADX) rats, in ADX rats injected with corticosterone at 1, 24 and 72 h before the gastric emptying assay, and in intact rats i.c.v. pretreated with a glucocorticoid antagonist (RU486) and with a corticotropin-releasing factor receptor antagonist (alpha-helical CRF9-41). In adrenal intact rats, i.c.v. injection of N/OFQ (2.5 nmol rat-1) significantly delayed gastric emptying (by 70%) and increased plasma corticosterone concentrations. Conversely, in ADX rats, N/OFQ left gastric emptying unchanged. In ADX rats, corticosterone injected at 1, 24 and 72 h before the gastric emptying assay almost restored the N/OFQ-induced delay in gastric emptying. Finally, pretreatment with RU486- and alpha-helical CRF9-41 abolished the N/OFQ-induced inhibition of gastric emptying. These findings suggest that central N/OFQ inhibits gastric emptying through an integrated orphaninergic system-CRF interaction in which corticosterone plays a permissive role.

Minireview: Characterization of influence of central nociceptin/orphanin FQ on consummatory behavior

Nociceptin/orphanin FQ (N/OFQ), a peptide closely related to dynorphin A, is the endogenous agonist of the NOP receptor that moderately increases food intake under various conditions. Its orexigenic properties are mediated by the brain circuitry. In the present review, we focus on discussing the nature of hyperphagic effects of N/OFQ with special emphasis on its function within feeding-related neural networks. Although some of N/OFQ's orexigenic effects resemble those induced by opioids, reward-dependent feeding appears to be affected in a different manner by agonists of the NOP and classical opioid receptors. Also, data suggest that N/OFQ may not only promote feeding initiation, but rather its role may be to inhibit signaling responsible for inhibition of consummatory behavior. Central systems involved in termination of feeding that seem to be influenced by N/OFQ encompass oxytocin, alpha-MSH, and CRH.

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

Dexamethasone exposure during the neonatal period alters ORL1 mRNA expression in the hypothalamic paraventricular nucleus and hippocampus of the adult rat

Dexamethasone is commonly used to limit the severity of chronic lung disease in premature infants with severe respiratory distress syndrome. Recent literature has demonstrated an association between dexamethasone exposure in critically ill premature neonates and later development of cerebral palsy. However, the majority of children exposed to dexamethasone in the neonatal period do not develop cerebral palsy or global developmental delay, and other more subtle effects of early life glucocorticoid exposure may go unnoticed. Presently, little is known regarding possible effects of early dexamethasone exposure on development of neuropeptide systems that are sensitive to glucocorticoid modulation. One such system is the pain-related opioid system that interacts with the stress-related limbic-hypothalamic pituitary adrenal (LHPA) axis. In the present study, a neonatal rat model was used to expose newborn rats to dexamethasone. Using a within-litter design, on postnatal days P3 through P6, pups were either handled, or they received a daily intramuscular injection of saline or dexamethasone. Adult animals were sacrificed on day of life P120, their brains were removed and quick-frozen. Using in situ hybridization histochemistry, mRNA expression of the opioid receptor-like (ORL1) receptor was measured in the paraventricular nucleus of the hypothalamus (PVN) and the hippocampal formation. In dexamethasone-treated adult male rats, ORL1 mRNA expression was increased in the PVN and dentate gyrus, but decreased in area CA1, when compared to handled and vehicle controls. These results suggest that prolonged glucocorticoid receptor (GR) occupation in the neonatal period leads to permanent alterations in ORL1 expression in the LHPA stress axis of the adult rat.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).