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

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.

Adaptive Changes in the Central Control of Energy Homeostasis Occur in Response to Variations in Energy Status

Energy homeostasis is regulated in coordinate fashion by the brain-gut axis, the homeostatic energy balance circuitry in the hypothalamus and the hedonic energy balance circuitry comprising the mesolimbcortical A10 dopamine pathway. Collectively, these systems convey and integrate information regarding nutrient status and the rewarding properties of ingested food, and formulate it into a behavioral response that attempts to balance fluctuations in consumption and food-seeking behavior. In this review we start with a functional overview of the homeostatic and hedonic energy balance circuitries; identifying the salient neural, hormonal and humoral components involved. We then delve into how the function of these circuits differs in males and females. Finally, we turn our attention to the ever-emerging roles of nociceptin/orphanin FQ (N/OFQ) and pituitary adenylate cyclase-activating polypeptide (PACAP)-two neuropeptides that have garnered increased recognition for their regulatory impact in energy homeostasis-to further probe how the imposed regulation of energy balance circuitry by these peptides is affected by sex and altered under positive (e.g., obesity) and negative (e.g., fasting) energy balance states. It is hoped that this work will impart a newfound appreciation for the intricate regulatory processes that govern energy homeostasis, as well as how recent insights into the N/OFQ and PACAP systems can be leveraged in the treatment of conditions ranging from obesity to anorexia.

The Melanocortin System behind the Dysfunctional Eating Behaviors

The dysfunction of melanocortin signaling has been associated with obesity, given the important role in the regulation of energy homeostasis, food intake, satiety and body weight. In the hypothalamus, the melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) contribute to the stability of these processes, but MC3R and MC4R are also localized in the mesolimbic dopamine system, the region that responds to the reinforcing properties of highly palatable food (HPF) and where these two receptors seem to affect food reward and motivation. Loss of function of the MC4R, resulting from genetic mutations, leads to overeating in humans, but to date, a clear understanding of the underlying mechanisms and behaviors that promote overconsumption of caloric foods remains unknown. Moreover, the MC4R demonstrated to be a crucial modulator of the stress response, factor that is known to be strictly related to binge eating behavior. In this review, we will explore the preclinical and clinical studies, and the controversies regarding the involvement of melanocortin system in altered eating patterns, especially binge eating behavior, food reward and motivation.

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.

Modulatory influences of estradiol and other anorexigenic hormones on metabotropic, Gi/o-coupled receptor function in the hypothalamic control of energy homeostasis

The appetite suppressant actions of estradiol are due to its ability to attenuate orexigenic signals and potentiate anorexigenic signals. The work from my laboratory has shown that male guinea pigs are more sensitive to the hyperphagic and hypothermic effects of cannabinoids than their female counterparts. Cannabinoid sensitivity is further dampened by the activational effects of estradiol. This occurs via the hypothalamic feeding circuitry, where estradiol rapidly attenuates the cannabinoid CB1 receptor-mediated presynaptic inhibition of glutamatergic input onto anorexigenic proopiomelanocortin (POMC) neurons in the arcuate nucleus. This disruption is blocked by the estrogen receptor antagonist ICI 182,780, and associated with increased expression of phosphatidylinositol-3-kinase (PI3K). Moreover, the ability of estradiol to reduce both the cannabinoid-induced hyperphagia and glutamate release onto POMC neurons is abrogated by the PI3K inhibitor PI 828. The peptide orphanin FQ/nociceptin (OFQ/N) activates opioid receptor-like (ORL)1 receptors to hyperpolarize and inhibit POMC neurons via the activation of postsynaptic G protein-gated, inwardly-rectifying (GIRK) channels. We have demonstrated that the fasting-induced hyperphagia observed in ORL1-null mice is blunted compared to wild type controls. In addition, the ORL1 receptor-mediated activation of GIRK channels in POMC neurons from ovariectomized female rats is markedly impaired by estradiol. The estrogenic attenuation of presynaptic CB1 and postsynaptic ORL1 receptor function may be part of a more generalized mechanism through which anorexigenic hormones suppress orexigenic signaling. Indeed, we have found that leptin robustly suppresses the OFQ/N-induced activation of GIRK channels in POMC neurons. Furthermore, its ability to augment excitatory input onto POMC neurons is blocked by PI 828. Thus, estradiol and other hormones like leptin reduce energy intake at least partly by activating PI3K to disrupt the pleiotropic functions of Gi/o-coupled receptors that inhibit anorexigenic POMC neurons.

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.

Endogenous opioids and feeding behavior: A decade of further progress (2004-2014). A Festschrift to Dr. Abba Kastin

Functional elucidation of the endogenous opioid system temporally paralleled the creation and growth of the journal, Peptides, under the leadership of its founding editor, Dr. Abba Kastin. He was prescient in publishing annual and uninterrupted reviews on Endogenous Opiates and Behavior that served as a microcosm for the journal under his stewardship. This author published a 2004 review, "Endogenous opioids and feeding behavior: a thirty-year historical perspective", summarizing research in this field between 1974 and 2003. The present review "closes the circle" by reviewing the last 10 years (2004-2014) of research examining the role of endogenous opioids and feeding behavior. The review summarizes effects upon ingestive behavior following administration of opioid receptor agonists, in opioid receptor knockout animals, following administration of general opioid receptor antagonists, following administration of selective mu, delta, kappa and ORL-1 receptor antagonists, and evaluating opioid peptide and opioid receptor changes in different food intake models.

Estradiol upregulates progesterone receptor and orphanin FQ colocalization in arcuate nucleus neurons and opioid receptor-like receptor-1 expression in proopiomelanocortin neurons that project to the medial preoptic nucleus in the female rat

BACKGROUND

Ovarian steroids regulate sexual receptivity in the female rat by acting on neurons that converge on proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH) that project to the medial preoptic nucleus (MPN). Estradiol rapidly activates these neurons to release β-endorphin that activates MPN μ-opioid receptors (MOP) to inhibit lordosis. Lordosis is facilitated by the subsequent action of progesterone that deactivates the estradiol-induced MPN MOP activation. Orphanin FQ (OFQ/N; also known as nociceptin) infusions into the ARH, like progesterone, deactivate MPN MOP and facilitate lordosis in estradiol-primed rats. OFQ/N reduces the activity of ARH β-endorphin neurons through post- and presynaptic mechanisms via its cognate receptor, ORL-1.

METHODS

We tested the hypotheses that progesterone receptors (PR) are expressed in ARH OFQ/N neurons by immunohistochemistry and ORL-1 is expressed in POMC neurons that project to the MPN by combining Fluoro-Gold injection into the MPN and double-label fluorescent in situ hybridization (FISH). We also hypothesized that estradiol increases coexpression of PR-OFQ/N and ORL-1-POMC in ARH neurons of ovariectomized rats.

RESULTS

The number of PR- and OFQ/N-immunopositive ARH neurons was increased as was their colocalization by estradiol treatment. FISH for ORL-1 and POMC mRNA revealed a subpopulation of ARH neurons that was triple labeled, indicating these neurons project to the MPN and coexpress ORL-1 and POMC mRNA. Estradiol was shown to upregulate ORL-1 and POMC expression in MPN-projecting ARH neurons.

CONCLUSION

Estradiol upregulates the ARH OFQ/N-ORL-1 system projecting to the MPN that regulates lordosis.

Gonadal steroids differentially modulate the actions of orphanin FQ/nociceptin at a physiologically relevant circuit controlling female sexual receptivity

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of pro-opiomelanocortin (POMC) neurones located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behaviour and energy balance. We tested the hypothesis that gonadal steroids differentially modulate the ability of OFQ/N to inhibit these cells via presynaptic inhibition of transmitter release and postsynaptic activation of G protein-gated, inwardly-rectifying K(+) (GIRK)-1 channels. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomised rats. OFQ/N (1 μm) decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs), and also caused a robust outward current in the presence of tetrodotoxin, in ARH neurones from vehicle-treated animals. A priming dose of oestradiol benzoate (EB; 2 μg) increased basal mEPSC frequency, markedly diminished both the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents, and potentiated the OFQ/N-induced decrease in mIPSC frequency. Steroid treatment regimens that facilitate sexual receptivity reinstate the basal mEPSC frequency, the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents to levels observed in vehicle-treated controls, and largely abolish the ability of OFQ/N to decrease mIPSC frequency. These effects were observed in an appreciable population of identified POMC neurones, almost one-half of which projected to the medial preoptic nucleus. Taken together, these data reveal that gonadal steroids influence the pleiotropic actions of OFQ/N on ARH neurones, including POMC neurones, in a disparate manner. These temporal changes in OFQ/N responsiveness further implicate this neuropeptide system as a critical mediator of the gonadal steroid regulation of reproductive behaviour.

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.

Estradiol negatively modulates the pleiotropic actions of orphanin FQ/nociceptin at proopiomelanocortin synapses

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of proopiomelanocortin (POMC) neurons located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behavior and energy balance. We tested the hypothesis that estradiol modulates the ability of OFQ/N to pre- and postsynaptically decrease the excitability of these cells. To this end, whole-cell patch-clamp recordings were performed in hypothalamic slices prepared from ovariectomized rats, including some that were injected with the retrograde tracer Fluorogold in the medial preoptic nucleus (MPN) to label the POMC neurons regulating sexual receptivity. OFQ/N (1 µM) evoked a robust outward current in ARH neurons from vehicle-treated animals that was blocked by the opioid receptor-like (ORL)1 receptor antagonist UFP-101 (100 nM) and the G protein-gated, inwardly rectifying K⁺ (GIRK-1) channel blocker tertiapin (10 nM). OFQ/N also produced a decrease in the frequency of glutamatergic, miniature excitatory postsynaptic currents (mEPSCs), which was also antagonized by UFP-101. Estradiol benzoate (2 µg) increased basal mEPSC frequency and markedly diminished both the OFQ/N-induced activation of postsynaptic GIRK-1 channel currents and the presynaptic inhibition of glutamatergic neurotransmission. These effects were observed in identified POMC neurons, including eight that projected to the MPN. Taken together, these data reveal that estradiol attenuates the pleiotropic inhibitory actions of OFQ/N on POMC neurons: presynaptically through reducing the OFQ/N inhibition of glutamate release and postsynaptically by reducing ORL1 signaling through GIRK channels. As such, they impart critical insight into a mechanism for estradiol to increase the activity of POMC neurons that inhibit sexual receptivity.

The nociceptin/orphanin FQ-like opioid peptide in nervous periesophageal ganglia of land snail Helix aspersa

The neuropeptide nociceptin/orphanin FQ (N/OFQ) and its receptor are members of the endogenous opioid peptide family. In mammals N/OFQ modulates a variety of biological functions such as nociception, food intake, endocrine, control of neurotransmitter release, among others. In the molluscs Cepea nemoralis and Helix aspersa the administration of N/OFQ produces a thermopronociceptive effect. However, little is known about its existence and anatomic distribution in invertebrates. The aim of this study was to provide a detailed anatomical distribution of N/OFQ like peptide immunoreactivity (N/OFQ-IL), to quantify the tissue content of this peptide, as well as to demostrate molecular evidence of N/OFQ mRNA in the nervous tissue of periesophageal ganglia of the land snail H. aspersa. Immunohistochemical, immunocytochemical, radioimmunoanalysis (RIA) and reverse transcription-polymerase chain reaction (RT-PCR) techniques were used. With regard to RT-PCR, the primers to detect expression of mRNA transcripts from H. aspersa were derived from the rat N/OFQ opioid peptide. We show a wide distribution of N/OFQ-IL in neurons and fibers in all perioesophageal ganglia, fibers of the neuropile, nerves, periganglionar connective tissue, aortic wall and neurohemal sinuses. The total amount of N/OFQ-IL in the perioesophageal ganglia (7.75 ± 1.75 pmol/g of tissue) quantified by RIA was similar to that found in mouse hypothalamus (10.1 ± 1.6 pmol/g of tissue). In this study, we present molecular evidence of N/OFQ mRNA expression. Some N/OFQ-IL neurons have been identified as neuroendocrine or involved in olfaction, hydro-electrolyte regulation, feeding, and thermonociception. Therefore, we suggest that N/OFQ may participate in these snail functions.

Nocistatin and nociceptin modulate c-Fos expression in the mice thalamus

Nocistatin and nociceptin/orphanin FQ (N/OFQ) are two neuropeptides which may have opposite effects in several biological functions but their neuro-anatomical sites of interaction are not fully clear. We investigated interaction between the effect of intracerebroventricular (i.c.v.) injection of nocistatin and N/OFQ, on c-Fos expression in the mouse thalamus, using c-Fos immunohistochemistry. We found that co-injection of nocistatin with N/OFQ significantly modulates c-Fos expression in the thalamus. The present study strongly suggests that "Nocistatin-Nociceptin" interaction system in the thalamus may be the promising neuromodulatory sites in the investigation of unlocking their possible therapeutic circuit in nociception, memory and anxiety.

Epidemiologic and Molecular Pathophysiology of Chronic Opioid Dependence and the Place of Naltrexone Extended-Release Formulations in its Clinical Management

Naltrexone implants and depot injections (NI) are a novel form of treatment for opiate dependence (OD). Major questions relate to their absolute and relative efficacy and safety. Opportunely, six recent clinical trial data from several continents have uniformly provided dramatic evidence of the potent, dose-related and highly significant efficacy of NI, with minimal or manageable accompanying toxicity and safety concerns. The opiate-free lifestyle is attained significantly more often with NI adjusted O.R. = 6.00 (95% C.I. 3.86–9.50), P < 10⁻¹⁰. Other drug use and drug craving are also rapidly reduced. The optimum manner in which to commence NI remains to be established. Of particular relevance is the relative safety of NI compared to the chronic opiate agonists (COA) usually employed, as the long-term toxicity of COA is only just being elucidated. Large population-based studies have found elevated rates of cardiovascular disease, six cancers, liver and respiratory disease, and all-cause mortality in COA. Whilst opiates have been shown to trigger numerous molecular pathways, the most interesting is the demonstration that the opiate morphinan’s nucleus binds to the endotoxin groove of the TLR4-MD2 heterodimer. This has the effect of triggering a low grade endotoxaemic-like state, which over time may account for these protean clinical findings, an effect which is reversed by opiate antagonists. This emerging evidence suggests an exciting new treatment paradigm for OD and a corresponding increase in the role of NI in treatment.

Nociceptin/orphanin FQ suppresses the excitability of neurons in the ventromedial nucleus of the hypothalamus

Nociceptin or orphanin FQ (N/OFQ) stimulates food intake when injected into the ventromedial nucleus of the hypothalamus (VMN). The VMN negatively regulates energy balance in part by tonically activating proopiomelanocortin arcuate neurons, thereby suppressing food intake. However, it is not clear how orexigenic neurotransmission within the VMN can stimulate food intake. We tested the hypothesis that the orexigenic action of N/OFQ results from its inhibition of anorexigenic VMN neurons. We studied the effects of N/OFQ on the electrical properties of anorexigenic VMN neurons in acute brain slices. Ionic mechanisms underlying the actions of N/OFQ were studied using whole cell patch-clamp recordings from VMN neurons expressing the anorexigenic leptin receptor (LepRb). Bath application of N/OFQ to LepRb-expressing VMN neurons elicited a robust, reversible membrane hyperpolarization that suppressed neuronal excitability by raising the action potential firing threshold and cell rheobase. N/OFQ activated a postsynaptic, G-protein coupled, inwardly rectifying potassium (GIRK) current that was sensitive to G-protein inactivation, blocked by the GIRK blocker SCH23390, and occluded by the GABAB agonist and potent GIRK activator, baclofen. Application of the selective N/OFQ receptor antagonist SB-612111 blocked the inhibitory effects of N/OFQ. We concluded that N/OFQ directly inhibited VMN neurons by activating a GIRK. These results implicate the site-specific contributions of orexigenic neuropeptides at VMN neurons to suppress anorexigenic output. This study thus advances our understanding regarding the contributions of the VMN to hypothalamic regulation of energy balance.

Hypothalamic opioid-melanocortin appetitive balance and addictive craving

Whilst the parallels between drug and food craving are receiving increasing attention, the recently elucidated complex physiology of the hypothalamic appetite regulatory centres has been largely overlooked in the efforts to understand drug craving which is one of the most refractory and problematic aspects of drug and behavioural addictions. Important conceptual gains could be made by researchers from both appetite and addiction neuroscience if they were to have an improved understanding of each others' disciplines. It is well known in addiction medicine that the use of many substances is elevated in opiate dependency. There is voluminous evidence of very high rates of drug use in opiate agonist maintained patients, and the real possibility exists that opiate agonist therapy therefore increases drug craving. Conversely, opiate antagonist therapy with naloxone or naltrexone has been shown to reduce most chemical and behavioural addictions, and naltrexone is now being developed together with bupropion as the anti-obesity drug "Contrave". Hypothalamic melanocortins, particularly α-MSH, are known to constitute the main brake to consumptive behaviour of food. There is a well described antagonism between melanocortins and opioids at many loci including the hypothalamus. Administration of exogenous opiates is known to both suppress α-MSH and to stimulate hedonic food consumption. Opiate maintenance programs are associated with weight gain. As monoamines, opioids and cannabinoids are known to be involved in appetite regulation, and as endorphin opioids are known to be perturbed in other addictions, further exploration of the hypothalamic appetite regulatory centre would appear to be an obvious, albeit presently largely overlooked, locus in which to study drug and other craving mechanisms.

Opioids as facilitators of feeding: can any food be rewarding?

Palatability is one of the most rewarding aspects of consummatory behavior. Opioids, potent facilitators of intake of sweet and fat tastants, are thought to mediate hedonics of feeding. However, the rewarding context of consumption is not limited to palatability, and gratification can be achieved through other means, e.g., eating to satisfy hunger. The current review discusses the role of opioid peptides in food intake regulation by incorporating this expanded concept of feeding reward. We present evidence that, aside from increasing sugar/fat consumption, opioids propel the intake of diets whose gustatory value is low but are nonetheless consumed under circumstances allowing feeding gratification to occur. Opioids enhance reward-driven consumption by acting within the classical reward circuitry and also by signaling reward at sites that regulate other aspects of food intake, such as satiety and aversion. We conclude that, due to the complexity of neural and functional interactions, opioids are capable of enhancing pleasure of eating any food--palatable, bland or even aversive--making any meal into a more rewarding experience, despite possible consequences.

The role of the NOP receptor in regulating food intake, meal pattern, and the excitability of proopiomelanocortin neurons

We evaluated the role of the nociceptin/orphanin FQ (NOP) receptor in regulating food intake, meal pattern and the activity of hypothalamic arcuate (ARC) neurons. The microstructural analysis of food intake and meal pattern was performed under both food-deprived and ad libitum conditions. Whole-cell patch clamp recordings were obtained using the in vitro hypothalamic slice preparation and biocytin-filled electrodes. NOP receptor knockout mice exhibited significantly reduced body weight. Fasting-induced hyperphagia was diminished for the first 2h of a 6-h re-feeding period, and was associated with decreased meal duration and size, as well as a biphasic effect on meal frequency. The genotype effect observed under ad libitum conditions was comparatively unremarkable. Orphanin FQ/nociceptin (OFQ/N) was able to decrease evoked excitatory postsynaptic current amplitude, increase the S(2):S(1) ratio via the paired-pulse paradigm, and decrease miniature excitatory postsynaptic current frequency in ARC neurons from wild type animals but not NOP receptor knockouts. In addition OFQ/N activated a reversible outward current that was antagonized by the G-protein activated, inwardly-rectifying K(+) (GIRK) channel blocker tertiapin in wild type but not NOP knockout animals. Both the presynaptic and postsynaptic actions of OFQ/N were observed in ARC neurons subsequently determined to be immunopositive for characteristic phenotypic markers of anorexigenic proopiomelanocortin (POMC) neurons. Taken together, these results demonstrate the contribution of the NOP receptor in controlling food intake and meal pattern, as well as glutamate release and GIRK1 channel activity at POMC synapses.

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.

Nutritional state influences Nociceptin/Orphanin FQ peptide receptor expression in the dorsal raphe nucleus

Agonists of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor stimulate food intake. Concordantly, neuroanatomical localization of NOP receptor mRNA has revealed it to be highly expressed in brain regions associated with the regulation of energy balance. However, the specific mechanisms and neurochemical pathways through which physiological N/OFQ influences appetite are not well understood. To investigate this, we examined nutritional state-associated changes in NOP receptor mRNA levels throughout the rostrocaudal extent of the rat brain using in situ hybridization histochemistry (ISHH) and quantitative densitometry analysis. We observed a significant downregulation of NOP receptor mRNA in the dorsal raphe nucleus (DRN) of fasted rats compared to free-feeding rats. In contrast, no difference in NOP receptor mRNA expression was observed in the supraoptic, parventricular, ventromedial, arcuate or dorsomedial nuclei of the hypothalamus, the red nucleus, the locus coeruleus or the hypoglossal nucleus in the fasted or fed state. These data suggest that the endogenous N/OFQ system is responsive to changes in energy balance and that NOP receptors specifically within the DRN may be physiologically relevant to N/OFQ's effects on appetite.

Effect of nociceptin/orphanin FQ on feeding behavior and hypothalamic neuropeptide expression in layer-type chicks

Nociceptin/orphanin FQ (N/OFQ) was identified in 1995 as the endogenous ligand for the orphan G(i)/G(o)-coupled opioid receptor-like 1 receptor (NOP(1)). Exogenous N/OFQ increases food intake in mammals, but its effect and mode of action in chicks are not fully known. We report herein that N/OFQ (5.0 nmol) has a stimulatory effect on food intake in layer-type chicks over a 2-h period after intracerebroventricular (icv) injection. Thirty minutes after central injection of N/OFQ (5.0 nmol) the concentration of agouti-related protein (AGRP) mRNA in the diencephalon increased, while cocaine- and amphetamine-regulated transcript (CART) mRNA decreased. However, concentrations of neuropeptide Y, proopiomelanocortin and glutamate decarboxylase mRNAs, and of catecholamines and excitatory amino acids were not affected. Simultaneous administration of alpha-melanocyte stimulating hormone (alpha-MSH: 1.0 pmol), a competitor of AGRP, completely blocked the orexigenic effect of N/OFQ (5.0 nmol). These data suggest that N/OFQ functions in layer chicks as an orexigenic peptide in the central nervous system, and that the AGRP and the CART neurons may mediate this function, as in mammals.

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.

Amygdalar opioids modulate hypothalamic melanocortin-induced anorexia

We wanted to assess the possibility that opioid activity in the central amygdala (CeA) could modulate the feeding inhibition of melanocortin stimulation of the paraventricular hypothalamus (PVN). The melanocortin system is important in both the acute regulation of satiety and feeding behavior and in the integration of long-term appetite signals. Melanotan II (MTII) is a synthetic MC3R and MC4R agonist which reduces food intake when given intracerebroventricularly (ICV) and into the PVN. Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO), a micro-opioid receptor agonist, increases food intake, while opioid antagonists, like naltrexone (NTX), inhibit food intake after injection into many brain sites involved in appetite regulation, including the CeA. In food-deprived male Sprague-Dawley rats, co-injected intra-PVN MTII partially blocked the orexigenic effect of co-injected intra-CeA DAMGO. Intra-CeA NTX co-injected with intra-PVN MTII reduced food intake significantly more than either alone. NTX administered intra-CeA reduced c-Fos-immunoreactivity (IR) in nucleus accumbens neurons significantly compared to the intra-PVN MTII treated animals, animals co-injected intra-PVN with MTII and intra-CeA with NTX animals, and control animals. Intra-PVN MTII induced c-Fos-IR in significantly more PVN neurons than observed in control animals. Intra-CeA NTX co-injected with intra-PVN MTII induced c-Fos-IR significantly in PVN neurons relative to control and intra-CeA NTX animals. Such data support the significance of opioid action within the CeA as a modulator of the feeding regulation action of melanocortins within the PVN, occurring within the context of a larger appetitive network.

Distinctive role of central histamine H3 receptor in various orexigenic pathways

Despite the well-established role of histamine as an anorexigenic neurotransmitter, the role of histamine H(3) receptors in feeding behavior is controversial. Herein we investigated the role of histamine H(3) receptor on several orexigenic agents in mice. Thioperamide (histamine H(3) receptor inverse agonist) inhibited neuropeptide Y- and nociceptin-induced hyperphagia but had no effect on U-50488 (opioid kappa-receptor agonist)-induced hyperphagia. In contrast, imetit (histamine H(3) receptor agonist) inhibited U-50488-induced hyperphagia but augmented neuropeptide Y-induced hyperphagia while it did not alter nociceptin-induced hyperphagia. These results indicate distinctive roles of histamine H(3) receptors in various orexigenic pathways.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 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 neurological 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).

Alpha-melanocyte stimulating hormone and ghrelin: central interaction in feeding control

Alpha-melanocyte stimulating hormone (alpha-MSH) and ghrelin play significant yet opposite roles in the regulation of feeding: alpha-MSH inhibits, whereas ghrelin stimulates consumption. The two peptidergic systems may interact in the process of food intake control. A single report published thus far has shown that a synthetic agonist of the melanocortin receptors, MTII, injected in the hypothalamic paraventricular nucleus (PVN) decreases feeding generated by ghrelin. We found that very low doses of alpha-MSH and MTII administered ICV significantly reduced ghrelin-dependent hyperphagia. However, an endogenous molecule, alpha-MSH, infused in the PVN did not exert an inhibitory effect on ghrelin-induced consumption, whereas the effective dose of PVN MTII exceeded that necessary to decrease short-term deprivation-induced feeding. We conclude that it is likely that in feeding regulation alpha-MSH and ghrelin "interact" at the central nervous system level, but the involvement of the PVN in this interaction appears questionable.

Involvement of cocaine-amphetamine regulated transcript in the differential feeding responses to nociceptin/orphanin FQ in dark agouti and Wistar Ottawa Karlsburg W rats

Wistar Ottawa Karlsburg W (WOKW) rats and their controls, dark agouti (DA), present different features: in particular, DA rats are lean, while the WOKW are obese and present symptoms of hypertension, dyslipidemia, hyperinsulinemia, and impaired glucose tolerance. The present study tested the hypothesis that these two strains would demonstrate different sensitivity to nociceptin/orphanin FQ (N/OFQ). N/OFQ was injected into the lateral brain ventricle (LBV) of sated DA and WOKW rats, and corticosterone levels in both strains were measured after LBV injection of N/OFQ. LBV N/OFQ injections dose-dependently produced a significant increase in food intake (4 h) in DA rats, but not in WOKW. However, corticosterone levels were increased by N/OFQ to a greater degree in WOKW than in DA rats. Gene sequencing and gene expression of ORL1 receptor and cocaine-amphetamine regulated transcript (Cart) peptide were evaluated to study the difference in N/OFQ-induced feeding behavior in the two strains. WOKW rats had a different amino acid sequence of Cart peptide and a significantly higher expression of Cart in the hypothalamus. The present data show that DA and WOKW rats demonstrate different sensitivity to N/OFQ, and suggest that Cart peptide might be the underlying mechanism of this difference.

Nocistatin attenuated the nociceptin induced c-Fos expression in the mouse hippocampus

Nocistatin and nociceptin/orphaninFQ (N/OFQ) are the two new peptides which may have roles in nociception, memory, anxiety, and other biological functions. Nocistatin acts as a functional antagonist to N/OFQ in several functions, but their neuro-anatomical sites of interaction are unknown. We investigated the effect of combined intracerebroventricular (i.c.v.) injection of nocistatin with N/OFQ, on N/OFQ induced c-Fos expression in the mouse hippocampus, using c-Fos immunohistochemistry. We found that co-injection of nocistatin with N/OFQ significantly attenuated N/OFQ induced c-Fos expression in the hippocampus.

Identification of central nervous system sites involved in the water diuresis response elicited by central microinjection of nociceptin/ Orphanin FQ in conscious rats via c-Fos and inducible cAMP early repressor immunocytochemistry

Intracerebroventricular (i.c.v.) administration of the opioid-like peptide, nociceptin/Orphanin (nociceptin), in conscious rats produces diuretic and antinatriuretic effects. The present study utilised changes in Fos and inducible cAMP early repressor (ICER) immunocytochemistry expression to examine the central nervous (CNS) sites activated or inhibited, respectively, by central administration of nociceptin. Urine samples were collected during control (15 min) and after i.c.v. vehicle (5 microl, n = 12) or nociceptin (10 microg/5 microl; n = 12). Four additional urine samples (15-min) were collected after the i.c.v. injection. The brain was processed for Fos using a commercially available antibody (Oncogene AB-5) and for ICER using a polyclonal anti-ICER antibody raised in rabbits. In vehicle-injected conscious rats, renal excretion of water or sodium was not altered. However, nociceptin produced a rapid and marked increase in urine flow (V) and a decrease in urinary sodium excretion rate. In addition, i.c.v. nociceptin produced a significant increase in Fos staining in the dorsomedial nucleus of the hypothalamus, the perinuclear zone of the supraoptic nucleus, the organum vasculosum of the lamina terminalis (OVLT), the lateral preoptic area and the lateral hypothalamic area compared to control. By contrast, Fos expression decreased in the area postrema and locus coeruleus compared to controls. Furthermore, ICER staining was significantly increased in the perinuclear zone of the supraoptic nucleus, supraoptic nucleus, median preoptic nucleus, OVLT, medial preoptic area, central nucleus of the amygdala, and medial nucleus of the solitary tract. Together, central opioid receptor-like type 1 activation in these CNS regions may participate in the neural pathways involved in the diuretic and antinatriuretic effects of nociceptin.

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.