The effect of norbinaltorphimine, beta-funaltrexamine and naltrindole on NPY-induced feeding

A Combination of a Dopamine Receptor 2 Agonist and a Kappa Opioid Receptor Antagonist Synergistically Reduces Weight in Diet-Induced Obese Rodents

As the global obesity rate increases, so does the urgency to find effective anti-obesity drugs. In the search for therapeutic targets, central nervous system (CNS) mechanisms engaged in the regulation of energy expenditure and food intake, such as the opioid and dopamine systems, are crucial. In this study, we examined the effect on body weight of two drugs: bromocriptine (BC), a D2R receptor agonist, and PF-04455242, a selective κ opioid receptor (KOR) antagonist. Using diet-induced obese (DIO) rats, we aimed to ascertain whether the administration of BC and PF-04455242, independently or in combination, could enhance body weight loss. Furthermore, the present work demonstrates that the peripheral coadministration of BC and PF-04455242 enhances the reduction of weight in DIO rats and leads to a decrease in adiposity in a food-intake-independent manner. These effects were based on heightened energy expenditure, particularly through the activation of brown adipose tissue (BAT) thermogenesis. Overall, our findings indicate that the combination of BC and PF-04455242 effectively induces body weight loss through increased energy expenditure by increasing thermogenic activity and highlight the importance of the combined use of drugs to combat obesity.

Central effects of opioidergic system on food intake in birds and mammals: a review

Undoubtedly, the food intake process is one of the most necessary physiological functions for the survival of animals and humans. Although; this operation seems simple on the surface, the regulation of the mechanisms involved in it requires the cooperation of many neurotransmitters, peptides, and hormonal factors in the nervous and endocrine systems. Understanding the signals that regulate energy levels and appetite, may open new approaches to therapeutics and drugs used in obesity-related complications. Improving the quality of animal products and health is also possible due to this research. The present review is aimed to sum up the current findings on central effects of opioids on the food consumption of birds and mammals. Based on the reviewed articles, the opioidergic system appears to be one of the key elements in the birds' and mammals' food intake and is closely related to other systems involved in appetite regulation. According to the findings, it seems that the effects of this system on nutritional mechanisms are often applied via kappa- and mu-opioid receptors. Controversial observations have been made regarding opioid receptors, highlighting the need for further studies, especially at the molecular level. The role of opiates in taste or diet craving also showed the efficacy of this system, especially the mu-opioid receptor, on preferences such as diets containing high sugar and fat. Finally, putting the results of this study together with the findings of human experiments and other primates can lead to a correct comprehension of the appetite regulation processes, especially the role of the opioidergic system.

Behavioral plasticity: Role of neuropeptides in shaping feeding responses

Behavioral plasticity refers to changes occurring due to external influences on an organism, including adaptation, learning, memory and enduring influences from early life experience. There are 2 types of behavioral plasticity: "developmental", which refers to gene/environment interactions affecting a phenotype, and "activational" which refers to innate physiology and can involve structural physiological changes of the body. In this review, we focus on feeding behavior, and studies involving neuropeptides that influence behavioral plasticity - primarily opioids, orexin, neuropeptide Y, and oxytocin. In each section of the review, we include examples of behavioral plasticity as it relates to actions of these neuropeptides. It can be concluded from this review that eating behavior is influenced by a number of external factors, including time of day, type of food available, energy balance state, and stressors. The reviewed work underscores that environmental factors play a critical role in feeding behavior and energy balance, but changes in eating behavior also result from a multitude of non-environmental factors, such that there can be no single mechanism or variable that can explain ingestive behavior.

Kappa Opioid Receptor Antagonists as Potential Therapeutics for Mood and Substance Use Disorders

The kappa opioid receptor (KOR) and its primary cognate ligands, the dynorphin peptides, are involved in diverse physiological processes. Disruptions to the KOR/dynorphin system have been found to likely play a role in multiple neuropsychological disorders, and hence KOR has emerged as a potential therapeutic target. Targeting KOR is complicated by close homology to the mu and delta opioid receptors (MOR and DOR), and many KOR ligands have at least moderate affinity to MOR and/or DOR. Animal models utilizing primarily very long-lasting selective KOR antagonists (>3 weeks following a single dose) have demonstrated that KOR antagonism attenuates certain anxiety-like and depression-like behaviors and blocks stress- and cue-induced reinstatement to drug seeking. Recently, relatively selective KOR antagonists with medication-like pharmacokinetic and pharmacodynamic properties and durations of action have been developed. One of these, JNJ-67953964 (also referred to as CERC-501, LY2456302, OpraKappa or Aticaprant) has been studied in humans, and shown to be safe, relatively KOR selective, and able to substantially attenuate binding of a KOR PET tracer to CNS localized KOR for greater than 24 h. While animal studies have indicated that compounds of this structural class are capable of normalizing withdrawal signs in animal models of cocaine and alcohol dependence and reducing cocaine and alcohol intake/seeking, additional studies are needed to determine the value of these second generation KOR antagonists in treating mood disorders and substance use disorders in humans.

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.

Pharmacological Effects and Regulatory Mechanisms of Tobacco Smoking Effects on Food Intake and Weight Control

Beyond promoting smoking initiation and preventing smokers from quitting, nicotine can reduce food intake and body weight and thus is viewed as desirable by some smokers, especially many women. During the last several decades, the molecular mechanisms underlying the inverse correlation between smoking and body weight have been investigated extensively in both animals and humans. Nicotine's weight effects appear to result especially from the drug's stimulation of α3β4 nicotine acetylcholine receptors (nAChRs), which are located on pro-opiomelanocortin (POMC) neurons in the arcuate nucleus (ARC), leading to activation of the melanocortin circuit, which is associated with body weight. Further, α7- and α4β2-containing nAChRs have been implicated in weight control by nicotine. This review summarizes current understanding of the regulatory effects of nicotine on food intake and body weight according to the findings from pharmacological, molecular genetic, electrophysiological, and feeding studies on these appetite-regulating molecules, such as α3β4, α7, and α4β2 nAChRs; neuropeptide Y (NPY); POMC; melanocortin 4 receptor (MC4R); agouti-related peptide (AgRP); leptin, ghrelin, and protein YY (PYY).

Melanin-Concentrating Hormone acts through hypothalamic kappa opioid system and p70S6K to stimulate acute food intake

Melanin-Concentrating Hormone (MCH) is one of the most relevant orexigenic factors specifically located in the lateral hypothalamic area (LHA), with its physiological relevance demonstrated in studies using several genetically manipulated mice models. However, the central mechanisms controlling MCH-induced hyperphagia remain largely uncharacterized. Here, we show that central injection of MCH in mice deficient for kappa opoid receptor (k-OR) failed to stimulate feeding. To determine the hypothalamic area responsible for this MCH/k-OR interaction, we performed virogenetic studies and found that downregulation of k-OR by adeno-associated viruses (shOprk1-AAV) in LHA, but not in other hypothalamic nuclei, was sufficient to block MCH-induced food intake. Next, we sought to investigate the molecular signaling pathway within the LHA that mediates acute central MCH stimulation of food intake. We found that MCH activates k-OR and that increased levels of phosphorylated extracellular signal regulated kinase (ERK) are associated with downregulation of phospho-S6 Ribosomal Protein. This effect was prevented when a pharmacological inhibitor of k-OR was co-administered with MCH. Finally, the specific activation of the direct upstream regulator of S6 (p70S6K) in the LHA attenuated MCH-stimulated food consumption. Our results reveal that lateral hypothalamic k-OR system modulates the orexigenic action of MCH via the p70S6K/S6 pathway.

Hypothalamic κ-opioid receptor modulates the orexigenic effect of ghrelin

The opioid system is well recognized as an important regulator of appetite and energy balance. We now hypothesized that the hypothalamic opioid system might modulate the orexigenic effect of ghrelin. Using pharmacological and gene silencing approaches, we demonstrate that ghrelin utilizes a hypothalamic κ-opioid receptor (KOR) pathway to increase food intake in rats. Pharmacological blockade of KOR decreases the acute orexigenic effect of ghrelin. Inhibition of KOR expression in the hypothalamic arcuate nucleus is sufficient to blunt ghrelin-induced food intake. By contrast, the specific inhibition of KOR expression in the ventral tegmental area does not affect central ghrelin-induced feeding. This new pathway is independent of ghrelin-induced AMP-activated protein kinase activation, but modulates the levels of the transcription factors and orexigenic neuropeptides triggered by ghrelin to finally stimulate feeding. Our novel data implicate hypothalamic KOR signaling in the orexigenic action of ghrelin.

Mice lacking δ-opioid receptors resist the development of diet-induced obesity

Pharmacological manipulation of opioid receptors alters feeding behavior. However, the individual contributions of each opioid receptor subtype on energy balance remain largely unknown. Herein, we investigated whether genetic disruption of the δ-opioid receptor (DOR) also controls energy homeostasis. Mice lacking DOR and wild-type mice were fed with standard diet and high-energy diet (HED). Mice were analyzed in vivo with the indirect calorimetry system, and tissues were analyzed by real-time PCR and Western blot analysis. DOR-knockout (KO) mice gained less weight (P<0.01) and had lower fat mass (P<0.01) when compared to WT mice fed an HED. Although DOR-KO mice were hyperphagic, they showed higher energy expenditure (P<0.05), which was the result of an increased activation of the thermogenic program in brown adipose tissue. The increased nonshivering thermogenesis involved the stimulation of uncoupling protein 1 (UCP1; P<0.01), peroxisome proliferator-activated receptor γ coactivator (PGC1α; P<0.05), and fibroblast growth factor 21 (FGF21; P<0.01). DOR deficiency also led to an attenuation of triglyceride content in the liver (P<0.05) in response to an HED. These findings reveal a novel role of DOR in the control of thermogenic markers and energy expenditure, and they provide a potential new therapeutic approach for the treatment of obesity.

The opioid system and food intake: homeostatic and hedonic mechanisms

Opioids are important in reward processes leading to addictive behavior such as self-administration of opioids and other drugs of abuse including nicotine and alcohol. Opioids are also involved in a broadly distributed neural network that regulates eating behavior, affecting both homeostatic and hedonic mechanisms. In this sense, opioids are particularly implicated in the modulation of highly palatable foods, and opioid antagonists attenuate both addictive drug taking and appetite for palatable food. Thus, craving for palatable food could be considered as a form of opioid-related addiction. There are three main families of opioid receptors (µ, ĸ, and δ) of which µ-receptors are most strongly implicated in reward. Administration of selective µ-agonists into the NAcc of rodents induces feeding even in satiated animals, while administration of µ-antagonists reduces food intake. Pharmacological studies also suggest a role for ĸ- and δ-opioid receptors. Preliminary data from transgenic knockout models suggest that mice lacking some of these receptors are resistant to high-fat diet-induced obesity.

Involvement of the opioid system in the orexigenic and hedonic effects of melanin-concentrating hormone

Melanin-concentrating hormone (MCH) exerts an orexigenic effect that resembles that of opioids, suggesting that the MCH and opioid systems could interact in controlling the food intake behavior. Three series of experiments were conducted in male Wistar rats: 1) to test the ability of the κ-, μ-, and δ-opioid receptor antagonists binaltorphimine (nor-BNI-κ), β-funaltrexamine (β-FNA-μ), and naltrindole (NTI-δ), respectively, to block the stimulating effects of MCH on food intake; 2) to verify the ability of MCH to induce a positive hedonic response to a sweet stimulus when injected into the nucleus accumbens shell (NAcSh) or right lateral ventricle (LV) of the brain; and 3) to assess the ability of nor-BNI, β-FNA, and NTI to block the effects of MCH on the hedonic response to a sweet stimulus. Nor-BNI, NTI (0, 10 and 40 nmol), and β-FNA (0, 10 and 50 nmol) were administered into the LV prior to injecting MCH (2.0 nmol). To assess the hedonic response, rats were implanted with an intraoral cannula allowing for the infusion of a sweet solution into the oral cavity. Food intake was assessed in sated rats during the first 3 h following the MCH or vehicle (i.e., artificial cerebrospinal fluid) injection. The hedonic response to a sweet stimulus was assessed by examining facial mimics, following the intraoral administration of a sucrose solution. Blockade of each of the three opioid receptors by selective antagonists prevented MCH-induced feeding. Furthermore, MCH-injections into the NAcSh and right LV resulted in enhanced hedonic responses. Finally, antagonism of the three opioid receptors blunted the LV-injected, MCH-induced, facial-liking expressions in response to an intraoral sweet stimulus. Overall, the present study provides evidence to link the MCH and opioid systems in the food intake behavior.

Effects of butorphanol on feeding and neuropeptide Y in the rat

Butorphanol ([BT] an opioid receptor agonist/antagonist) is different from other opioid agonists in that a single dose of BT can elicit up to 12 g of chow intake in a satiated rat whereas most opioid agonists induce a mild feeding response (2-3 g). Here, we first examined whether the effectiveness of BT to elicit feeding was affected by dose, method of infusion and possible tachyphylaxis following administration. Secondly, we examined whether BT administration influenced hypothalamic NPY gene expression and peptide levels. A single dose administration of BT (4 mg/kg) significantly increased food intake at 2, 3 and 6 h after administration. However following repeated injections of BT at 4 mg/kg, the cumulative long-term intake of BT-treated rats did not differ from that of controls, indicating that the animals compensate for the increased feeding following BT injection by decreased feeding at a later time. An ascending dose schedule of repeated BT injections resulted in additional feeding. NPY gene expression in the ARC was influenced by how much food had been consumed, but not by BT. The amount of food consumed and the level of NPY mRNA were inversely correlated. This is consistent with NPY's role in normal feeding. BT treatment did not affect either NPY or leptin RIA levels. We conclude that the feeding produced by BT is sensitive to dose and dosing paradigm. Further, its mechanism of action does not appear to be mediated by NPY or leptin pathways.

Anoretic effects of neuropeptide FF are mediated via central mu and kappa subtypes of opioid receptors and receptor ligands

Recently, we demonstrated that neuropeptide FF (NPFF) causes anorexigenic effects in chicks that were associated with the hypothalamus. The present study was designed to better understand some of the central mechanisms that mediate these effects. Co-injection of NPFF and beta-funaltrexamine (FNA, a mu opioid receptor antagonist) did not suppress food intake more than when NPFF and FNA were injected alone. However, co-injection of NPFF and ICI-174,864 (ICI, a delta opioid receptor antagonist) caused a greater reduction in food intake than when NPFF and ICI were injected alone. Co-injection of NPFF and nor-binaltorphimine (BNI, a kappa opioid receptor antagonist) did not cause a greater suppression of food intake than when NPFF and BNI were injected alone. Hyperphagia induced by neuropeptide Y and beta-endorphin (both ligands of opioid receptors) was reversed by NPFF. These results suggest that NPFF-induced satiety has a relationship with mu and kappa but not delta subtypes of opioid receptors, and since NPFF does not bind opioid receptors itself NPFF-associated satiety is likely mediated by effects on opioid receptor ligands such as NPY and beta-endorphin. Thus, NPFF induced satiety may be mediated via modulation of the chick's innate opioid-associated orexigenic system.

Dynorphin knockout reduces fat mass and increases weight loss during fasting in mice

Endogenous opioids, particularly dynorphins, have been implicated in regulation of energy balance, but it is not known how they mediate this in vivo. We investigated energy homeostasis in dynorphin knockout mice (Dyn(-/-) mice) and probed the interactions between dynorphins and the neuropeptide Y (NPY) system. Dyn(-/-) mice were no different from wild types with regards to body weight and basal and fasting-induced food intake, but fecal output was increased, suggesting decreased nutrient absorption, and they had significantly less white fat and lost more weight during a 24-h fast. The neuroendocrine and thermal responses to fasting were at least as pronounced in Dyn(-/-) as in wild types, and there was no stimulatory effect of dynorphin knockout on 24-h energy expenditure (kilocalories of heat produced) or physical activity. However, Dyn(-/-) mice showed increased circulating concentrations of 3,4-dihydroxyphenlacetic acid and 3,4-dihydroxyphenylglycol, suggesting increased activity of the sympathetic nervous system. The respiratory exchange ratio of male but not female Dyn(-/-) mice was reduced, demonstrating increased fat oxidation. Interestingly, expression of the orexigenic acting NPY in the hypothalamic arcuate nucleus was reduced in Dyn(-/-) mice. However, fasting-induced increases in pre-prodynorphin expression in the arcuate nucleus, the paraventricular nucleus, and the ventromedial hypothalamus but not the lateral hypothalamus were abolished by deletion of Y(1) but not Y(2) receptors. Therefore, ablation of dynorphins results in increases in fatty acid oxidation in male mice, reductions in adiposity, and increased weight loss during fasting, possibly via increases in sympathetic activity, decreases in intestinal nutrient absorption, and interactions with the NPYergic system.

Long-term feed intake regulation in sheep is mediated by opioid receptors

These experiments were conducted to determine if 1) syndyphalin-33 (SD33), a mu-opioid receptor ligand, affects feed intake; 2) SD33 effects on feed intake are mediated by actions on opioid receptors; and 3) its activity can counteract the reduction in feed intake associated with administration of bacterial endotoxin. In Exp. 1, 5 mixed-breed, castrate male sheep were housed indoors in individual pens. Animals had ad libitum access to water and concentrate feed. Saline (SAL; 0.9% NaCl) or SD33 (0.05 or 0.1 micromol/kg of BW) was injected i.v., and feed intake was determined at 2, 4, 6, 8, 24, and 48 h after the i.v. injections. Both doses of SD33 increased (at least P < 0.01) feed intake at 48 h relative to saline. In Exp. 2, SAL + SAL, SAL + SD33 (0.1 micromol/kg of BW), naloxone (NAL; 1 mg/kg of BW) + SAL, and NAL + SD33 were injected i.v. Food intake was determined as in Exp. 1. The SAL + SD33 treatment increased (P = 0.022) feed intake at 48 h relative to SAL + SAL. The NAL + SAL treatment reduced (at least P < 0.01) feed intake at 4, 6, 8, 24, and 48 h, whereas the combination of NAL and SD33 did not reduce feed intake at 24 (P = 0.969) or 48 h (P = 0.076) relative to the saline-treated sheep. In Exp. 3, sheep received 1 of 4 treatments: SAL + SAL, SAL + 0.1 micromol of SD33/kg of BW, 0.1 microg of lipopolysaccharide (LPS)/kg of BW + SAL, or LPS + SD33, and feed intake was monitored as in Exp. 1. Lipopolysaccharide suppressed cumulative feed intake for 48 h (P < 0.01) relative to saline control, but SD33 failed to reverse the reduction in feed intake during this period. These data indicate that SD33 increases feed intake in sheep after i.v. injection, and its effects are mediated via opioid receptors. However, the LPS-induced suppression in feed intake cannot be overcome by the opioid receptor ligand, SD33.

Increased expression of mu opioid receptors in animals susceptible to diet-induced obesity

Stimulation of mu opioid receptors preferentially increases the intake of a high fat diet. In this paper we investigated whether there was a difference in the expression of mu opioid receptors between animals susceptible (Osborne-Mendel) or resistant (S5B/Pl) to obesity induced by eating a high fat diet. Immunohistochemical studies demonstrated that Osborne-Mendel rats eating a chow diet had an increased number of mu opioid receptors in the arcuate nucleus when compared to S5B/Pl rats. These immunohistochemical findings were supported by Real Time-PCR which demonstrated that the mRNA level of mu opioid receptors was also increased in the hypothalamus of Osborne-Mendel rats compared to S5B/Pl rats. Low doses of the mu opioid receptor agonist DAMGO [d-Ala(2)-N-Me-Phe(4)-Glycol(5)]-enkephalin administered to Osborne-Mendel rats caused a significant increase in the preference for a diet high in fat. The same doses of DAMGO switched the diet preference of S5B/Pl rats to high fat but did not significantly increase food intake. The combination of these findings suggests that the increased levels of hypothalamic mu opioid receptors in Osborne-Mendel rats may contribute to their preference for a diet high in fat and increase their susceptibility to becoming obese.

The μ-opioid receptor subtype is required for the anorectic effect of an opioid receptor antagonist

A diaryl ether derivative, (6-(4-{[(3-methylbutyl)amino]methyl}phenoxy)nicotinamide, was prepared and investigated for its biochemical properties at cloned opioid receptors and its pharmacological effects on animal feeding. The compound displaced [(3)H]DAMGO binding of human mu-opioid receptor, [(3)H]U-69593 of human kappa-opioid receptor, and [(3)H]DPDPE of human delta-opioid receptor with IC(50) values of 0.5+/-0.2 nM, 1.4+/-0.2 nM, and 71+/-15 nM, respectively. The compound also potently inhibited [(3)H]DAMGO binding of cloned mouse and rat mu-opioid receptors (IC(50) approximately 1 nM), and acted as a competitive antagonist in a cAMP functional assay using cultured cells expressing human or mouse mu-opioid receptors. Following a single oral administration in diet-induced obese mice (at 10 or 50 mg/kg) or rats (at 1, 3, or 10 mg/kg), the compound caused a dose-dependent suppression of acute food intake and body weight gain in both species. Importantly, the anorectic efficacy of the compound was mostly diminished in mice deficient in the mu-opioid receptor. Our results suggest an important role for the mu-opioid receptor subtype in animal feeding regulation and support the development of mu-selective antagonists as potential agents for treating human obesity.

Resistance to diet-induced obesity in mu-opioid receptor-deficient mice: evidence for a "thrifty gene"

Using pharmacological tools, a role for opioid receptors in the regulation of food intake has been documented. However, the involvement of specific receptor subtypes remains questionable, and little information is available regarding a role for opioid receptors in energy metabolism. Using adult male mice lacking the mu-opioid receptor (MOR) gene (MOR-/-), we show that the MOR is not essential for the maintenance of normal levels of ad libitum food intake but does modulate the efficiency of energy storage during high-fat diets through the regulation of energy partitioning. When fed a regular diet, MOR-/- mice displayed only subtle alterations in energy homeostasis, suggesting a relative overuse of fat as a fuel source in the fed state. When fed a high-fat diet, MOR-/- mice were resistant to obesity and impaired glucose tolerance, despite having similar energy intake to wild-type mice. This resistance to obesity was associated with a strong induction of the expression of key mitochondrial enzymes involved in fatty acid oxidation within skeletal muscle. This metabolic role of the MOR, which is consistent with the properties of a "thrifty gene," suggests that the MOR pathway is a potential target for pharmacological intervention in the treatment of obesity associated with the intake of fatty diets.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

NPY-induced feeding: pharmacological characterization using selective opioid antagonists and antisense probes in rats

The ability of neuropeptide Y to potently stimulate food intake is dependent in part upon the functioning of mu and kappa opioid receptors. The combined use of selective opioid antagonists directed against mu, delta or kappa receptors and antisense probes directed against specific exons of the MOR-1, DOR-1, KOR-1 and KOR-3/ORL-1 opioid receptor genes has been successful in characterizing the precise receptor subpopulations mediating feeding elicited by opioid peptides and agonists as well as homeostatic challenges. The present study examined the dose-dependent (5-80 nmol) cerebroventricular actions of general and selective mu, delta, and kappa1 opioid receptor antagonists together with antisense probes directed against each of the four exons of the MOR-1 opioid receptor gene and each of the three exons of the DOR-1, KOR-1, and KOR-3/ORL-1 opioid receptor genes upon feeding elicited by cerebroventricular NPY (0.47 nmol, 2 ug). NPY-induced feeding was dose-dependently decreased and sometimes eliminated following pretreatment with general, mu, delta, and kappa1 opioid receptor antagonists. Moreover, NPY-induced feeding was significantly and markedly reduced by antisense probes directed against exons 1, 2, and 3 of the MOR-1 gene, exons 1 and 2 of the DOR-1 gene, exons 1, 2, and 3 of the KOR-1 gene, and exon 3 of the KOR-3/ORL-1 gene. Thus, whereas the opioid peptides, beta-endorphin and dynorphin A(1-17) elicit feeding responses that are respectively more dependent upon mu and kappa opioid receptors and their genes, the opioid mediation of NPY-induced feeding appears to involve all three major opioid receptor subtypes in a manner similar to that observed for feeding responses following glucoprivation or lipoprivation.

Response of neuropeptide Y-induced feeding to μ-, δ- and κ-opioid receptor antagonists in the neonatal chick

It is known that opioid antagonists reduce the orexigenic effect of neropeptide Y (NPY) in mammals. We studied the effect of three opioid antagonists on NPY-induced feeding in male broiler chicks. Beta-funaltrexamine (beta-FNA), naloxonazine (NAL), ICI-174,864 (ICI) or nor-binaltorphimine (nor-BNI), antagonists of mu-, mu1-, delta- or kappa-receptors, and NPY were co-injected in chicks. Food intake was measured 30 min after treatment. Co-injection of beta-FNA or NAL was effective in reducing NPY-induced feeding, whereas ICI and nor-BNI had little effect on NPY-induced feeding. These data suggest that the mu-opioid receptor, especially the mu1-opioid has some relation to NPY-induced feeding, and implies that an endogenous ligand, such as beta-endorphin, participates in the orexigenic effect of NPY in neonatal chicks.

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.

Our journey with neuropeptide Y: effects on ingestive behaviors and energy expenditure

Clark and colleagues first described the robust orexigenic effects of neuropeptide Y (NPY) in 1984. Our group as well as Stanley et al. confirmed these effects in the same year. During the next 20 years, we investigated the effects of NPY on diet preferences, opioid-related feeding, distributed neural feeding networks, energy metabolism, motivation and discriminative stimulus effects. These data together with data from other laboratories indicate that NPY increases feeding, even when rats work for food; that NPY decreases energy expenditure, particularly by altering thermogenesis; and that NPY's effects on energy metabolism are mediated by a widely distributed neural network involving other neuroregulators known to be involved in energy regulation.

The kappa-opioid antagonist GNTI reduces U50,488-, DAMGO-, and deprivation-induced feeding, but not butorphanol- and neuropeptide Y-induced feeding in rats

Antagonists selective for either kappa- [e.g. nor-binaltorphimine (nor-BNI)] and mu- (e.g. beta-funaltrexamine) opioid receptors have previously been shown to reduce both kappa- and mu-opioid-induced feeding. In the present studies, the anorectic effects of GNTI, a newly synthesized antagonist selective for kappa-opioid receptors, were studied in rats. GNTI (0.032-0.32 nmol; i.c.v.), administered 15 min prior to food access, reduced feeding induced by the kappa-opioid agonist U50,488 (producing a 70% maximal decrease), the mu-opioid agonist DAMGO (90% maximal decrease), and 24 h acute food deprivation (60% maximal decrease). GNTI did not reduce the orexigenic effects of butorphanol, an agonist that binds to both kappa- and mu-opioid receptors, and neuropeptide Y (NPY). Taken together, these results suggest that GNTI is a potent anorectic agent and opioid antagonist in rats. Like nor-BNI, GNTI reduced feeding induced by both kappa- and mu-opioid agonists. However, unlike nor-BNI, GNTI did not alter the orexigenic effects of butorphanol or NPY. Given the selectivity of GNTI and its effectiveness in several of the present experiments, its potency, and its short duration of action compared to nor-BNI, GNTI may serve to be a useful tool to study behavioral effects mediated by kappa-opioid receptors.

NPY receptors and opioidergic system are involved in NPY-induced feeding in goldfish

The present study evaluated the effects of both intraperitoneal (i.p. ) and intracerebroventricular administration of selective Y(1) [(Leu(31), Pro(34))-NPY] and Y(2) [(Pro(13), Tyr(36))-NPY (13-36)] receptor agonists on food intake in satiated goldfish. Food intake (FI) was significantly increased by central administration of the Y(1) agonist (1 microg), but not by the Y(2) agonist, at 2 h postinjection. The feeding increase induced by (Leu(31), Pro(34))-NPY was in a similar magnitude to that obtained after ICV injection of the neuropeptide Y, and both feeding stimulations were reversed by the NPY (27-36), a general NPY antagonist. The i.p. administration of the agonists either did not significantly modify (Y(2) agonist) or decreased (Y(1) agonist) food intake in goldfish. These data indicate that it is the Y(1)-like (similar to Y(1) and/or Y(5)) receptor, and not Y(2), that is involved in the central modulation of the feeding behavior in goldfish. We also investigated the possible involvement of opioid peptides as mediators of the NPY stimulatory action on food intake in goldfish. The ICV administration of naloxone (10 microg), a general opioid antagonist, blocked the NPY-induced feeding in goldfish, suggesting that the opioidergic system is involved in feeding regulation by NPY.

Effects of neuropeptide Y on the discriminative stimulus and antinociceptive properties of morphine

Previous research indicates that opioid receptor blockade diminishes the effects of neuropeptide Y (NPY) on feeding and memory. Conversely, NPY attenuates naloxone-precipitated morphine withdrawal. The present study evaluated the effects of NPY on the discriminative stimulus and antinociceptive effects produced by the prototypical mu opioid, morphine. Rats were trained to discriminate 5.6 mg/kg morphine (IP) from saline using a standard two-lever, food-reinforced, drug discrimination procedure. Across a range of doses (3.0, 5.0, and 10 microg), intracerebroventricular (ICV) injection of NPY failed to substitute for, antagonize, or potentiate the discriminative stimulus effects of morphine. A warm-water tail-withdrawal procedure was used to examine the antinociceptive effects of morphine and NPY, alone and in combination. NPY (3.0 and 10 microg, ICV) failed to alter tail-withdrawal latencies from 52 degrees and 56 degrees C water, whereas morphine (1.0-30 mg/kg, IP) produced a dose-related increase in latencies at both water temperatures. A 10-microg dose of NPY also failed to alter the antinociceptive effects of morphine. This study does not support the idea that the discriminative stimulus and antinociceptive effects of morphine are dependent on an NPYergic pathway.

Opioid receptor blockade promotes weight loss and improves the display of sexual behaviors in obese Zucker female rats

Obese Zucker female rats are hyperphagic, overweight, infertile, and hyporesponsive to the inductive effects of ovarian steroid hormones on sexual behaviors. It has been postulated that endogenous opioid activity may contribute to their obesity and reproductive dysfunction. To test this hypothesis, ovariectomized, adult obese Zucker rats were treated with the opioid receptor antagonist, naltrexone, or saline prior to measurement of steroid-induced sexual behaviors, food intake, and body weight. In estradiol benzoate (EB)-treated rats, naltrexone injection increased the display of sexual receptivity (lordosis quotient, LQ: saline, 11+/-10%; 5 mg/kg naltrexone, 54+/-15%, p < 0.05) and also elicited proceptivity (PRO), which was never observed after saline injection. In EB plus progesterone-treated animals, naltrexone administration enhanced both sexual receptivity and proceptivity (LQ: saline, 17+/-10%; 5 mg/kg naltrexone, 96+/-3%; p < 0.05; PRO: saline, 3.0+/-2.4 bouts/min; 5 mg/kg naltrexone, 45.3+/-12 bouts/min; p < 0.01). Naltrexone injection also decreased 24-h food intake (saline, 24.2+/-0.7 g; 5 mg/kg naltrexone, 17.6+/-1.2 g; p < 0.05) and weight change (saline, +7.3+/-0.8 g; 5 mg/kg naltrexone, -4.5+/-1.4 g, p < 0.01). Morphine treatment blocked these effects of naltrexone on sexual behaviors, food intake, and body weight. These data suggest that endogenous opioids contribute to hyperphagia, obesity, and behavioral hyporesponsiveness to ovarian steroid hormones in obese Zucker rats.

Divergence of the feeding and thermogenic pathways influenced by NPY in the hypothalamic PVN of the rat

Neuropeptide Y (NPY) injected into the paraventricular nucleus (PVN) increases feeding and decreases brown adipose tissue (BAT) uncoupling protein (UCP) and lipoprotein lipase (LPL) mRNA. Previously we reported that the feeding and BAT effects induced by NPY in the PVN are blocked by 50 microg naltrexone (NTX) in the rostral nucleus of the solitary tract (rNTS). We sought to determine whether the effect of rNTS NTX on PVN NPY-induced alterations in energy metabolism occurred at lower doses of NTX. Male Sprague-Dawley rats were fitted with cannulas into two sites: PVN and rNTS. Feeding response, BAT UCP, and LPL mRNA were measured after injection of 0, 5, 10, and 25 microg NTX in the rNTS +/- 1 microg NPY in the PVN. One-hour feeding response to PVN NPY was significantly and dose dependently decreased by 10 and 25 microg rNTS NTX (-23 and -31%, respectively). However, rNTS NTX did not block the PVN NPY-induced decrease in BAT UCP or LPL mRNA. BAT beta-actin mRNA (as a measure of overall changes in gene expression) was unchanged among treatment groups. These results indicate a possible divergence in the PVN NPY feeding-stimulatory/BAT-inhibitory pathway, such that PVN NPY feeding effects may be routed through the rNTS whereas BAT effects may be due to alterations at another neural site.

Beta-casomorphins stimulate and enterostatin inhibits the intake of dietary fat in rats

The effects of beta-casomorphins 1-7, 1-5 and 1-4 on food intake of rats adapted to either a high fat (HF) or high carbohydrate (HC) diet have been studied and compared to the effects of enterostatin. Intracerebroventricular (icv) beta-casomorphin1-7 (beta-CM1-7) stimulated intake of HF diet in overnight fasted rats, but beta-CM1-5 and beta-CM1-4 were ineffective. Peripheral injection of beta-CM1-7 also increased the intake of a high fat diet, but reduced the intake of HC diet in satiated rats. Intracerebroventricular (ICV) beta-CM1-7 caused a dose-dependent increase in the intake of HF diet, but a dose-dependent inhibition of HC ingestion in satiated rats. Enterostatin (ICV) inhibited the beta-CM1-7 stimulation of HF intake, as did the general opioid antagonist naloxone. Ligand binding studies with [3H-pro] enterostatin identified on low affinity binding site (Kd 100nM) on a crude brain membrane preparation. This binding was displaced by beta-CM1-7, beta-CM1-5 and beta-CM1-4. These data suggest that at high doses enterostatin and beta-CM1-7 may interact with the same low affinity receptor to modulate intake of dietary fat.

Neuroanatomical patterns of Fos-like immunoreactivity induced by naltrexone in food-restricted and ad libitum fed rats

Chronic food restriction produces a variety of adaptive changes in physiology and behavior aimed at the preservation of energy homeostasis. The brain opioid system may be involved in the adaptation to food restriction since regional levels of opioid peptides, precursor mRNA, and receptor binding have previously been observed. In the present study, c-Fos immunohistochemistry was used to localize cells that are released from opioid-mediated inhibition by naltrexone under conditions of food restriction and ad libitum feeding. In the majority of hypothalamic and forebrain areas examined, Fos-like immunoreactivity (FLI) was higher in food-restricted rats regardless of injection treatment. This may reflect the persistent stress of underfeeding or the synchronizing effect of afternoon feeding on spontaneous c-fos mRNA expression in food-restricted rats. In two brain regions, bed nucleus of the stria terminalis (BNST) and central amygdala (CEA), naltrexone increased FLI in ad libitum fed rats, exclusively. This result suggests the presence of tonic opioid secretion under basal conditions that is suppressed by food restriction. Interestingly, work in other laboratories indicates that anorectic agents consistently increase FLI in BNST and CEA. In three brain regions--lateral (LH), dorsomedial (DMH) and arcuate hypothalamus (ARC)--naltrexone increased FLI in food-restricted rats, exclusively. This result suggests the presence of opioid secretion that is unique to the state of food restriction. The hypothalamic pattern of FLI is discussed in terms of NPY-opioid interactions that result from the ARC response to changes in circulating insulin, corticosterone and leptin levels during food restriction.

Chronic food restriction and streptozotocin-induced diabetes differentially alter prodynorphin mRNA levels in rat brain regions

It was previously reported that chronic food restriction and streptozotocin-induced diabetes lead to brain region-specific changes in levels of Prodyn-derived peptides. These changes parallel behavioral adaptations that are reversed by opioid antagonists. In the present study, effects of food restriction and diabetes on Prodyn gene expression were measured in rat brain regions using a quantitative solution hybridization mRNA assay. Picogram amounts of Prodyn mRNA were determined in extracts of five brain regions. The highest density of Prodyn mRNA was observed in extracts of nucleus accumbens (4.68 pg/microg total RNA), bed nucleus of the stria terminalis (4.18 pg/microg), and in caudate nucleus (3.51 pg/microg). Lower levels were observed in the lateral hypothalamus (1.87 pg/microg) and central nucleus of the amygdala (1.22 pg/microg). Food restriction and diabetes both markedly increased the levels of Prodyn mRNA in the central amygdala (163% and 93%, respectively). Levels in the lateral hypothalamus were also increased (35% and 29%, respectively), though only the food-restriction effect was statistically significant. Neither treatment altered prodynorphin mRNA levels in the caudate nucleus, nucleus accumbens or bed nucleus of the stria terminalis. These results suggest that dynorphin neurons in central amygdala and lateral hypothalamus may be involved in behavioral or physiological adaptations to sustained metabolic need.

Effect of enterostatin and kappa-opioids on macronutrient selection and consumption

Enterostatin, the activation peptide of pancreatic procolipase, suppresses consumption of high-fat diets and selectively suppresses fat consumption over carbohydrate consumption. Kappa-opioid subtype agonists stimulate feeding whereas antagonists suppress feeding. We investigated the effects of enterostatin, the kappa-opioid agonist U50488, and the kappa-opioid antagonist nor-binaltorphimine (nor-BNI) on macronutrient selection and food consumption in rats adapted to choose between a high-fat (HF) diet or a low-fat-high-carbohydrate (LF) diet. In fasted rats, lateral cerebro-ventricular injection (LV) of enterostatin selectively suppressed consumption of the HF diet, with no effect on LF diet consumption. Nor-BNI also selectively suppressed consumption of the HF diet without affecting LF diet consumption. Additionally, U50488 prevented the suppression of consumption of the HF diet in response to enterostatin. In food-sated rates, U50488 preferentially increased consumption of the HF diet and had no effect on consumption of the LF diet. Combined infusions of subthreshold doses of enterostatin and nor-BNI also inhibited consumption of the HF but not the LF diet, whereas combined infusions of maximal doses of enterostatin and nor-BNI had no additive effects. Collectively, these data suggest that a kappa-opioid pathway modulates selection and consumption of diets high in fat and that enterostatin modulates consumption of dietary fat by interacting with this pathway.

Opioid receptor subtype control of galanin-induced feeding

This study examined the effects of specific antagonists to kappa- and mu-opioid receptors on the feeding induced by injecting galanin into the lateral cerebral ventricle (LCV). Galanin injected into the lateral cerebral ventricle of sated rats stimulated the consumption of high-fat diet when compared to controls injected with saline vehicle. The mu-opioid receptor antagonist, CTOP, completely abolished galanin-induced feeding in sated rats whereas the kappa-opioid receptor antagonist, nor-BNI, had no effect on galanin-induced feeding. Neither CTOP nor nor-BNI alone produced any change in food consumption in sated rats. In fasted rats, on the other hand, nor-BNI significantly decreased consumption of a high-fat diet (> 83%) when compared to animals treated with the saline vehicle, whereas CTOP had no significant effect. These findings suggest that galanin-induced feeding of a high-fat diet is selectively modulated by a pathway involving mu-opioid receptors whereas feeding induced by fasting is dependent on a pathway mediated by kappa-opioid receptors. These data also suggest that galanin does not mediate the feeding response after fasting.

Effects of streptozotocin-induced diabetes on prodynorphin-derived peptides in rat brain regions

Pharmacological studies suggest that diabetes produces changes in the brain opioid system, affecting several behavioral functions including analgesia, feeding and self-stimulation. Previous investigations of opioid receptor binding have failed to explain the unusual opioid pharmacology of the diabetic animal. In the present study, the effects of streptozotocin-induced diabetes on levels of three immunoreactive (ir)-prodynorphin-derived peptides, ir-dynorphin A1-17 (A1-17), ir-dynorphin A1-8 (A1-8) and ir-dynorphin B1-13 (B1-13), were determined in eleven brain regions known to be involved in appetite, taste and reward. Diabetes was found to increase levels of A1-17 in the ventromedial and dorsomedial hypothalamic nuclei (+60% and +25%, respectively) and levels of A1-8 in the dorsomedial and lateral hypothalamus (+45% and +35%, respectively). The possible significance of these results is discussed in relation to (i) diabetic hyperphagia, (ii) medial hypothalamic transduction of circulating insulin levels, and (iii) the potentiation of reward by metabolic need states.

Effects of opioid antagonists naloxone and naltrexone on neuropeptide Y-induced feeding and brown fat thermogenesis in the rat. Neural site of action

Neuropeptide Y administered intracerebroventricularly and into the paraventricular nucleus of the hypothalamus stimulates feeding and decreases brown adipose tissue thermogenesis. Although specific neuropeptide Y antagonists are not yet available, previous studies had shown that the opioid antagonist naloxone blocked neuropeptide Y-induced feeding when both drugs were injected intracerebroventricularly. We wanted to find out if naloxone injected into specific brain sites would block neuropeptide Y effects on feeding and brown fat thermogenesis. Rats were double injected in specific brain sites with neuropeptide Y and either naloxone or naltrexone (a congener of naloxone). Food intake and brown fat measures were assessed. Naloxone or naltrexone in the paraventricular nucleus weakly decreased paraventricular nucleus neuropeptide Y-induced feeding and did not affect neuropeptide Y-induced reductions in brown fat activity. Peripheral naloxone blocked intracerebroventricular neuropeptide Y-induced feeding and brown fat alterations. Fourth ventricular naloxone decreased paraventricular nucleus neuropeptide Y-induced feeding, and naltrexone given into the nucleus of the solitary tract blocked paraventricular nucleus neuropeptide Y-induced alterations in feeding and brown fat. These data indicate that neuropeptide Y in the paraventricular nucleus may act on feeding and brown fat thermogenesis through opioidergic pathways in the nucleus of the solitary tract.

Effects of neuropeptide Y on short-term memory

Neuropeptide Y (NPY) is a ubiquitous neuropeptide which may modulate several behavioral and physiological phenomena. Among other behavioral effects, NPY has been shown to enhance memory processes in mice. The current study employed a delayed conditional discrimination procedure to evaluate the effects of intracerebroventricular injections of NPY on short-term working memory. This conditional discrimination procedure assesses appropriate responding, based on a previously presented stimulus, after various delays have been imposed between the stimulus and the opportunity for a response. Delay values ranged from 0.01 s to 30 s. NPY decreased accuracy across delay values in a dose-dependent manner. The two highest doses of NPY (3.0 and 10.0 micrograms) significantly decreased accuracy. Doses lower than those used in the current study have shown facilitation of memory processes under avoidance procedures in mice. Intraperitoneal naloxone (3.0 mg/kg), an opioid antagonist, completely blocked NPY's memory degrading effects. Procedural differences may account for NPY-induced degradation of short-term working memory under delayed conditional discrimination and previous reports of NPY's enhancement of retention under shock avoidance procedures.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.