Increased immunoreactive dynorphin and leu-enkephalin in posterior pituitary of obese mice (ob/ob) and super-sensitivity to drugs that act at kappa receptors

Social status and demographic effects of the kappa opioid receptor: a PET imaging study with a novel agonist radiotracer in healthy volunteers

Kappa opioid receptors (KORs) have been characterized as an aversive system in the brain and implicated in social behavior in preclinical models. This work investigated the effect of social status on the KOR system in humans using positron emission tomography (PET) imaging with the novel KOR agonist radiotracer [¹¹C]EKAP. Eighteen healthy participants (mean age 41.2 ± 9.3) completed the Barratt Simplified Measure of Social Status (BSMSS), an MRI and an [¹¹C]EKAP PET scan on the High Resolution Research Tomograph. Arterial blood sampling and metabolite analysis were conducted to obtain the input function. Regions of interest were based upon an MR template and included the reward/aversion areas of the brain. The multilinear analysis-1 (MA1) method was applied to the regional time-activity curves (TACs) to calculate [¹¹C]EKAP regional volume of distribution (V_T). Mixed models and Pearson correlation coefficients were used for body mass index (BMI), gender and age, with age being dropped in subsequent analyses because of nonsignificance. An overall effect of primary ROIs (F_7, 112 7.43, p < 0.0001), BSMSS score (F_1, 13 7.45, p = 0.02), BMI (F_1, 13 23.5, p < 0.001), and gender (F_1, 13 23.75, p < 0.001), but not age (F_1, 13 1.12, p = 0.35) was observed. Regional [¹¹C]EKAP V_T and BSMSS were found to be negatively correlated in the amygdala (r = -0.69, p < 0.01), anterior cingulate cortex (r = -0.56, p = 0.02), caudate (r = -0.66, p < 0.01), frontal cortex (r = -0.52, p = 0.04), hippocampus (r = -0.60, p = 0.01), pallidum (r = -0.59, p = 0.02), putamen (r = -0.62, p = 0.01), and ventral striatum (r = -0.66, p < 0.01). In secondary (non-reward) regions, correlations of [¹¹C]EKAP V_T and BSMSS were nonsignificant with the exception of the insula. There was an inverse correlation between social status and KOR levels that was largely specific to the reward/aversion (e.g., saliency) areas of the brain. This finding suggests the KOR system may act as a mediator for the negative effects of social behaviors in humans.

Dynorphin/Kappa Opioid Receptor Signaling in Preclinical Models of Alcohol, Drug, and Food Addiction

The dynorphin/kappa opioid receptor (KOR) system is implicated in the "dark side" of addiction, in which stress exacerbates maladaptive responses to drug and alcohol exposure. For example, acute stress and acute ethanol exposure result in an elevation in dynorphin, the KOR endogenous ligand. Activation of KORs results in modulation of several neurotransmitters; however, this chapter will focus on its regulatory effects on dopamine in mesolimbic areas. Specifically, KOR activation has an inhibitory effect on dopamine release, thereby influencing reward processing. Repeated stimulation of KORs, for example, via chronic drug and/or stress exposure, results in increased function of the dynorphin/KOR system. This augmentation in KOR function shifts the homeostatic balance in favor of an overall reduction in dopamine signaling via either by reducing dopamine release or by increasing dopamine transporter function. This chapter examines the effects of chronic ethanol exposure on KOR function and the downstream effects on dopamine transmission. Additionally, the impact of chronic cocaine exposure and its effects on KOR function will be explored. Further, KORs may also be involved in driving excessive consumption of food, contributing to the risk of developing obesity. While some studies have shown that KOR agonists reduce drug intake, other studies have shown that antagonists reduce addiction-like behaviors, demonstrating therapeutic potential. For example, KOR inhibition reduces ethanol intake in dependent animals, motivation to self-administer cocaine in chronic stress-exposed animals, and food consumption in obese animals. This chapter will delve into the mechanisms by which modulation of the dynorphin/KOR system may be therapeutic.

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.

Chronic administration of nalmefene leads to increased food intake and body weight gain in mice

Nalmefene is an orally available opioid receptor antagonist that has been shown to suppress appetite in humans, but its effects on chronic food intake and body weight remain unclear. Here, we report that chronic (21-day) oral administration of nalmefene at 2 or 10 mg/kg/day in diet-induced obese (DIO) mice led to significant increases (9-11%) in cumulative food intake. Mice in the nalmefene-treated groups also gained body weight at a rate faster than the control. Body composition analysis showed that the extra body weight gains in the treated animals were mostly due to increased fat accumulation. Since acute nalmefene treatment showed a trend toward a decrease rather than an increase in food intake, it is possible that the orexigenic effect of chronic oral administration of nalmefene was caused by pharmacologically active metabolites rather than the drug itself. Our results argue against the potential use of nalmefene for treating human obesity.

Peptides that regulate food intake: antagonism of opioid receptors reduces body fat in obese rats by decreasing food intake and stimulating lipid utilization

Agonists to opioid receptors induce a positive energy balance, whereas antagonists at these receptors reduce food intake and body weight in rodent models of obesity. An analog of 3,4-dimethyl-4-(3-hydroxyphenyl)piperidine, LY255582, is a potent non-morphinan antagonist for mu-, kappa-, and delta-receptors (K(i) of 0.4, 2.0, and 5.2 nM, respectively). In the present study, we examined the effects of oral LY255582 treatment on caloric intake, calorie expenditure, and body composition in dietary-induced obese rats. Acute oral treatment of LY255582 produced a dose-dependent decrease in energy intake and respiratory quotient (RQ), which correlated with the occupancy of central opioid receptors. Animals receiving chronic oral treatment with LY255582 for 14 days maintained a negative energy balance that was sustained by increased lipid use. Analysis of body composition revealed a reduction in fat mass accretion, with no change in lean body mass, in animals treated with LY255582. Therefore, chronic treatment with LY255582 reduces adipose tissue mass by reducing energy intake and stimulating lipid use.

Autoradiographic determination of changes in opioid receptor binding in the limbic system of the Columbian ground squirrel at different hibernation states

To localize and quantify the state-dependent changes in various opioid receptor subtypes in the limbic system of non-hibernating and hibernating Columbian ground squirrels, quantitative receptor-binding autoradiography was used. Compared to the non-hibernating animals, the binding density of [3H]-[D-Pen2,5]-enkephalin (DPDPE) to the delta receptor in the lateral septum, CA3, and the hippocampal fissure of the hippocampal formation was significantly decreased in the hibernating ground squirrels. A significant reduction in the binding density of [3H]-[D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAGO) to mu receptor was also observed in the medial septum and the CA3 region of the hippocampus of the hibernating animals. In contrast, a decrease in [3H]ethylketocyclazocine (EKC) binding to the kappa receptor was only observed in the claustrum and CA3 of the hippocampus during hibernation. The differential changes in binding to various opioid receptors suggest that different opioid subtypes may exert different physiological roles in regulating the specific states (entrance, maintenance and arousal) of a hibernation bout.

Studies on the effects of glucose in vitro and of the glycaemic state in vivo on the binding characteristics of mu, delta and kappa opiate receptors in mouse brain

The effect of glucose on the binding characteristics of opiate receptor subtypes was investigated in brain membranes from normoglycaemic lean Aston (C57BL/6J) mice using [3H][D-Ala2,MePhe4,Gly5-ol]enkephalin (DAMGO), [3H][D-Pen2,D-Pen5]enkephalin (DPDPE) and [3H]U69,593 as selective ligands for mu, delta and kappa opiate receptors respectively. The equilibrium dissociation constants (Kd) and maximal binding capacities (Bmax) of [3H]DAMGO and [3H]DPDPE were unaltered by 20mM glucose in vitro. Similarly, [3H]U69,593 binding was not modified by increasing the concentration of glucose from 0 to 20mM (P between 0.10 and 0.05), or by the presence of 20mM fructose and of 20mM 3-O-me-glucose, a non-metabolisable sugar, in the incubation medium. The nonselective opiate ligand, [3H]diprenorphine, bound with similar affinity and binding capacity to brain membranes prepared from control and streptozotocin-diabetic Swiss (CD1) mice. The addition of 20mM glucose or of 20mM fructose in vitro induced no changes in their binding parameters. The affinity and binding capacity of [3H]U69,593 to STZ-diabetic Swiss mouse brain membranes was not significantly different to that of normoglycaemic controls; 20mM glucose in vitro had no effect on ligand binding to kappa sites in STZ-diabetic mouse brain membranes. We conclude that glucose does not interact directly with the opiate receptor to modfy it in such as way as could explain the altered sensitivity to different opioid agonists seen in obese and hyperglycaemic animal models in vivo.

Reduced hypothalamic neurotensin concentrations in the genetically obese diabetic (ob/ob) mouse: possible relationship to obesity

Hypothalamic tissue levels of nine regulatory peptides (bombesin, calcitonin gene-related peptide [CGRP], galanin, neuromedin B, neuropeptide Y [NPY], neurotensin, somatostatin, substance P, and vasoactive intestinal peptide [VIP]) were compared in Aston obese diabetic (ob/ob) and lean (+/?) mice aged 4, 16, and 28 weeks. Neurotensin concentrations were significantly lower in ob/ob mice than in lean mice, with a 20% reduction (P = .03) in the whole hypothalamus at 4 weeks of age, a 24% reduction (P = .009) in the lateral hypothalamus at 16 weeks, and a 50% reduction (P = .0007) in the central hypothalamus at 28 weeks of age. Apart from a 42% increase in vasoactive intestinal peptide concentrations in the central hypothalamus of ob/ob mice at 28 weeks (P = .02), levels of the other eight peptides examined did not differ significantly between obese and lean groups. Neurotensin is known to cause anorexia and increased energy expenditure when injected into the central hypothalamus. Reduced hypothalamic neurotensin concentrations may reflect reduced neurotensinergic activity, which might contribute to hyperphagia and decreased energy expenditure, two major defects that contribute to obesity and diabetes in the ob/ob syndrome.

Increased beta-endorphin and dynorphin concentrations in discrete hypothalamic regions of genetically obese (ob/ob) mice

Disturbances in hypothalamic beta-endorphin and dynorphin levels were investigated in non-fasted genetically obese (ob/ob) and homozygous lean mice at 14-15 weeks of age. Eight brain regions were microdissected from fresh, unfixed brain slices, and opioid peptide concentrations were determined in tissue micropunches by radioimmunoassay. A two-fold and five-fold increase in beta-endorphin levels in ob/ob versus lean mice were found in the ventromedial and dorsomedial hypothalamic nuclei respectively. Dynorphin levels were comparable between ob/ob and lean mice in the anterior, lateral, ventromedial and paraventricular hypothalamic areas, but a 5-fold increase in dynorphin concentrations was detected in the dorsomedial hypothalamic nucleus of the ob/ob mouse. These results demonstrate that increased concentrations of beta-endorphin and dynorphin occur in discrete hypothalamic nuclei, which are known to influence food intake and glucose homeostasis. This could signify an important central defect contributing to hyperphagia and glucoregulatory dysfunction in obese mice.

The effects of dietary sucrose on opiate receptor binding in genetically obese (ob/ob) and lean mice

Weanling genetically obese (C57BL/6J-ob/ob) and lean (+/?) mice were given access to either a standard laboratory diet or the standard diet plus a 32% sucrose solution. At the end of a 4-week period, animals were sacrificed and opiate receptor binding determined. Both obese and lean mice given access to sucrose consumed approximately 30% more calories per day than animals given access to the standard diet alone. Obese animals consumed significantly more calories from the sucrose solution than the lean animals. Genetically obese animals weighed more than lean littermates throughout the course of the study. Differences in body weight due to sucrose supplementation in both genetically obese and lean mice were significant by day 10 and increased in magnitude until the termination of the study. Whereas there were no significant differences in specific opiate receptor binding (pmol 3H-naloxone bound/mg brain protein) between the genetically obese and lean animals, opiate receptor binding was significantly greater in genetically obese animals given access to sucrose than in obese animals which had access only to the standard diet. These data demonstrate that the sucrose-induced model of obesity functions in mice and that giving ob/ob mice access to sucrose in addition to a standard laboratory diet results in increased opiate receptor binding.

Central mu, delta, and kappa opioid binding sites, and brain and pituitary beta-endorphin and met-enkephalin in genetically obese (ob/ob) and lean mice

The equilibrium dissociation constants and maximal binding capacities of 3H-dihydromorphine (DHM), 3H-D-Ala2-D-leu3-enkephalin (DADL), and 3H-dynorphin A(1-8) for their respective mu, delta, and kappa opiate binding sites were studied in brain membrane preparations from lean and genetically obese-hyperglycaemic (Aston ob/ob) mice. The concentration of kappa binding sites was 2.7 fold higher in obese compared with lean mouse brain (231 +/- 44.6 versus 83.8 +/- 10.3 fmoles 3H-dynorphin/mg protein respectively, mean +/- SEM). The concentration of delta binding sites in obese was 1.6 fold that in lean mouse brain (94.5 +/- 8.6 versus 59.5 +/- 6.5 fmoles 3H-DADL/mg protein). In contrast, the concentration of brain mu receptors was 40% lower in obese compared with lean mice (20.8 +/- 2.19 and 34.8 +/- 3.1 fmoles 3H-DHM/mg protein respectively). Binding affinities of delta and kappa sites for their respective ligands were not significantly different in lean v. obese mice. However, for mu sites, lean mouse binding data showed two affinities, one was not significantly different from obese (0.35 nM) the second was lower (1.18 nM) for DHM. Increases of approximately 5 fold and 3 fold in the brain content of beta-endorphin and met-enkephalin respectively, and no differences in brain dynorphin levels, were demonstrated in obese mice compared with lean controls. In obese mice, pituitary beta-endorphin content was 9 fold higher, met-enkephalin 4 fold higher and dynorphin 12 fold higher than in lean mice. The striking differences in opioid binding-site characteristics and in endogenous opioid peptide levels in obese compared with lean mice may contribute to the hyperphagia and, directly or indirectly, to the development of hyperglycaemia and hyperinsulinaemia in obese mice.

Experimental obesity: a homeostatic failure due to defective nutrient stimulation of the sympathetic nervous system

The basic hypothesis of this review is that studies on models of experimental obesity can provide insight into the control systems regulating body nutrient stores in humans. In this homeostatic or feedback approach to analysis of the nutrient control system, we have examined the afferent feedback signals, the central controller, and the efferent control elements regulating the controlled system of nutrient intake, storage, and oxidation. The mechanisms involved in the beginning and ending of single meals must clearly be related to the long-term changes in fat stores, although this relationship is far from clear. Changes in total nutrient storage in adipose tissue can arise as a consequence of changes in the quantity of nutrients ingested in one form or another or a decrease in the utilization of the ingested nutrients. A change in energy intake can be effected by increased size of individual meals, increased number of meals in a 24-hour period, or a combination of these events. Similarly, a decrease in utilization of these nutrients can develop through changes in resting metabolic energy expenditure which are associated with one of more of the biological cycles such as protein metabolism, triglyceride for glycogen synthesis and breakdown, or maintenance of ionic gradients for Na+ + K+ across cell walls. In addition, differences in energy expenditure related to the thermogenesis of eating or to the level of physical activity may account for differences in nutrient utilization.

Monoamine oxidase inhibitors and weight gain

Weight gain associated with antidepressant therapy is a common problem that often results in noncompliance. Some authors suggest that monoamine oxidase inhibitors (MAOI) are less likely to produce weight gain than tricyclic antidepressants. This paper addresses the relative potential for weight gain with the MAOI. Assessing the potential for antidepressant-induced weight gain necessitates separating the weight changes associated with alterations in mood disorders from those due to drug-induced alterations in appetite control. The mechanisms of appetite control are reviewed briefly followed by proposed mechanisms by which the MAOI may alter this control. A literature review suggests that phenelzine is the MAOI most likely to induce weight gain; reports of isocarboxazid-induced weight gain are less common. There are no cases of tranylcypromine-induced weight gain in the literature that are clearly associated with the drug. The MAOI probably have different effects on the mechanisms of appetite control.

The effects of the kappa agonist PD-117302 on feeding behaviour in obese and lean Zucker rats

It has been suggested that super-sensitivity to, or overactivity of, endogenous opioid systems, particularly the dynorphin system, may be important in the development of obesity in the obese mouse (ob/ob). We have investigated the possibility that an increase in sensitivity to kappa agonists may also play a role in the development of obesity in another mutant rodent, the Zucker rat. The effects of the selective kappa agonist PD-117302 were investigated in both lean and obese Zucker rats. The lean animals appeared to be more sensitive to the initial hyperphagic effects of PD-117302 than their obese littermates, although this initial hyperphagia was followed by a subsequent decrease in food intake so that by the end of the six-hour test period the animals treated with PD-117302 had eaten less than the saline-treated controls. These changes in food intake were paralleled by increases and decreases in the duration of time spent feeding throughout the experiment. At the doses used in this study PD-117302 had no effect on locomotor activity. It is concluded that obese Zucker rats are not more sensitive to the hyperphagic effects of the kappa agonist (PD-117302 than their lean littermates and therefore it seems unlikely that increased sensitivity to an endogenous kappa opioid system plays a major part in the overeating and obesity observed in this strain of rat.

Opioids and behavior: genetic aspects

Three animal models, based on genetic differences in endogenous opioid peptides and opioid receptors, are described. Obese mice and rats, whose pituitary opioid content is elevated, may be used to investigate eating disorders. Recombinant inbred strains of mice, which differ in brain opioid receptors and analgesic responsiveness, can be used for study of opioid- and nonopioid-mediated mechanisms of pain inhibition. Individual reactivity to opioids can be examined in C57BL/6 and DBA/2 inbred strains of mice. A model that combines a variety of opioid effects is offered and suggests the existence of a genetically determined dissociation of opioid effects on locomotor activity and pain inhibition. In addition, stimulatory locomotor responses in the C57BL/6 reaction type are linked to a high risk of drug addiction and facilitatory effects on adaptive processes, while high analgesic potency in the DBA/2 reaction type is accompanied by a low proneness to drug abuse and amnesic properties of opioids.

Nocturnal depletion of hypothalamic dynorphin in anorexic Walker-256 tumor-bearing rats

Rats implanted with the Walker-256 (W-256) tumor exhibit marked anorexia that is most apparent at night. In this model, the hypothalamic kappa opioid system was examined for deficits that might contribute to this tumor-induced anorexia. In anorexic tumor-bearing rats (TBR), nocturnal levels of ir-DYN-8 were significantly reduced in the hypothalamus, but ir-DYN-17 levels were not. Accumulation of 3H-etorphine or 3H-ethylketocyclazocine, a putative ligand for the kappa receptor subtype, was not increased in the hypothalamus of the TBR, as might have been expected if there were less endogenous dynorphin to occupy the opioid receptors in this region. In vitro binding assays with 3H-ethylketocyclazocine indicated that dynorphin depletion in the TBR was not sufficient to increase the numbers of kappa opioid receptors in the hypothalamus. Also, the sensitivity of kappa opioid receptors involved in feeding was not altered in the TBR as indicated by an intact feeding response to ketocyclazocine. In summary, the marginal deficits of hypothalamic dynorphin in W-256 tumor-bearing rats that coincide with the phase of tumor-induced anorexia may contribute to the reduction in food intake.

Effects of food deprivation and high fat diet on immunoreactive dynorphin A(1-8) levels in brain regions of Zucker rats

The levels of immunoreactive dynorphin A(1-8) (ir-DYN8) were measured in discrete brain regions of lean Zucker rats subjected to food deprivation for 72 hr and to a high fat diet, and in fatty Zucker rats after food deprivation for 72 hr. Fatty rats showed higher concentrations of ir-DYN8 in the cortex and midbrain, when compared to lean rats fed a stock diet ad lib. Food deprivation increased ir-DYN8 levels in the cortex of lean rats and fatty rats and in the hippocampus of fatty rats, but decreased its content in the striatum of lean rats and in the midbrain of fatty rats. The high fat diet increased ir-DYN8 levels in the cortex and midbrain of lean rats. These results suggest that ir-DYN8 levels in extrahypothalamic structures of Zucker rats could be differentially modified under conditions of hereditary obesity and dietary manipulations.

Ingestive behavior of meat and egg-type chickens: equal sensitivity to naloxone

The efficacy of the opioid antagonist naloxone in attenuating ingestive behavior in stocks of chickens genetically selected for either meat (Rock-Cornish, RC) or egg production (Single-Comb White Leghorn, SCWL) was investigated. Because the stocks differ markedly in growth rate, two experiments were conducted to compare RC and SCWL cockerels at similar body weights and at the same age. Birds were injected intramuscularly with either isotonic saline or naloxone HCl at a dose of 5 mg/kg body weight. Food and water were offered ad libitum 15 min postinjection. In RC and SCWL stocks of similar body weight, naloxone significantly attenuated cumulative food and water intake through 210 min following the return of food and water. When administered to RC and SCWL stocks of the same age, naloxone significantly attenuated cumulative food intake for 300 min and cumulative water intake at 60 and 120 min. The relatively long-term depressions in cumulative food and water intake were attributable to significant decreases in incremental consumption within early time periods. There was no significant difference in the efficacy of naloxone in attenuating ingestive behavior when the stocks were compared at either similar body weights or at the same age. The results demonstrate that genetic selection for meat or egg production has not significantly altered opioid mechanisms regulating food and water intake in the domestic fowl.

Increased responsiveness to glucoregulatory effect of opiates in obese-diabetic ob/ob mice

Plasma glucose and insulin responses to opiate receptor stimulation and antagonism were determined in 12-14 week old lean and obese-diabetic Aston ob/ob mice. The opiate receptor antagonist naloxone (1 mg/kg intraperitoneally) rapidly and transiently raised glucose and suppressed insulin concentrations in lean mice, and produced qualitatively similar but more protracted response in ob/ob mice. Selective stimulation of mu- and delta-opiate receptors using the enkephalin analogues Tyr-D-Ala-Gly-MePhe-NH(CH2)2OH (1 mg/kg, intraperitoneally) and Tyr-D-Ala-Gly-Phe-D-Leu (10 mg/kg intraperitoneally) respectively, rapidly and transiently increased glucose and insulin concentrations in lean and ob/ob mice. The ob/ob mice exhibited greater glucose and insulin responses to these analogues. The results provide evidence that endogenous opiates participate in the regulation of glucose and insulin homeostasis, and suggest that increased responsiveness to mu- and delta-opiate receptor stimulation may contribute to the hyperglycaemia and hyperinsulinaemia of obese-diabetic mice.

Experimental manipulations of eating: advances in animal models for studying anorectic agents

The material set out in this text has been designed to show the wide range of procedures which have the capacity to modify eating behavior--to produce hyper- or hypophagia, to alter the profile of eating patterns, or to adjust dietary preferences and selection. Accordingly, in investigating anorectic drugs it seems necessary to observe the effects of drug actions in a variety of experimental models. This strategy will provide a more complete description of the effect of a drug, will throw light on the mechanism of action, and will provide a more realistic base for predicting the effects of drugs in man.

Effect of food deprivation on opioid receptor binding in the brain of lean and fatty Zucker rats

The effect of food deprivation on opioid receptor binding was studied in 6 brain regions of lean and fatty Zucker rats; using [3H]dynorphin A. There was no significant difference between lean and fatty rats fed ad libitum in binding parameters for any regions studied. Food deprivation increased Bmax and/or Kd for cortex, midbrain and striatum of lean rats, and the former two regions of fatty rats. These results suggest that food deprivation may influence opioid receptor binding in lean and fatty Zucker rats.

Endorphins and food intake: kappa opioid receptor agonists and hyperphagia

Evidence from studies which utilise either opiate receptor agonists and antagonists strongly indicate a role for endorphinergic mechanisms in the control of feeding responses. Two means by which these compounds may exert an effect on feeding can be singled-out. Firstly, emerging evidence suggests that the process of achieving satiety (terminating a meal, or choice of a commodity) may be accelerated following treatments with opiate receptor antagonists. Secondly, the preference for highly palatable solutions (sweet solutions have received most attention) in two-bottle tests is blocked after injection of opiate receptor antagonists. This finding has been interpreted in terms of the abolition of the reward or incentive quality associated with the particularly attractive flavour. These two mechanisms of action may represent two aspects of a single, fundamental process. Following an introduction to rat urination model of in vivo kappa agonist activity, the consistent effect of several kappa agonists (including the highly selective U-50,488H) to stimulate food consumption is described. Recognising that members of the dynorphin group of endogenous opioid peptides are kappa receptor ligands, some with a high degree of selectivity, and the evidence the dynorphins and neo-endorphins produce hyperphagia in rats is particularly interesting. Such lines of evidence lead to the hypothesis that peptides of the dynorphin group may act endogenously to promote the expression of normal feeding behaviour.

The effect of opiates and naloxone on food intake in virgin and lactating rats

Lactation provides an excellent model of non-obese hyperphagia. There is accumulating evidence that endogenous opioids play a role in the modulation of the hormonal changes that occur during lactation. Because endogenous opioids appear also to play a role in the regulation of feeding, we studied the effects of the opiate agonist, butorphanol tartrate, and an opiate antagonist, naloxone, on food intake in virgin female rats and in rats during early, mid and late lactation and during post-weaning. It has been reported that female rats are less sensitive to the suppressant effects of nalmefene, an opioid antagonist, than male rats. Therefore, we also examined the effect of naloxone, an opioid antagonist, on spontaneous nocturnal feeding and 24 hour food deprivation-induced food intake in virgin female rats. We found that female rats were relatively insensitive to the food suppressant effects of naloxone following 24 hour food deprivation, while male rats tested under similar conditions had a decreased intake in response to naloxone. Despite the marked hyperphagia that occurred during lactation, there were minimal alterations in the response to opiate agonists and antagonists during this time period. Our data suggest that endogenous opioids may not play a pivotal role in the hyperphagia of lactation.

Effects of 2% sodium chloride imbibition on various opiate related hyperphagic conditions

Dynorphin is one of the most potent appetite stimulants among the endogenous opioids. In this study, we describe the anorexic effects of 5 days of forced 2% NaCl drinking in rats, a regimen which depletes vasopressin as well as dynorphin in the neurohypophysis. Feeding induced by direct activation of kappa-opioid receptors with ketocyclazocine was unaffected by the NaCl regimen. However, 2% NaCl imbibition reduced 2-deoxy-D-glucose (2-DG) induced feeding by 65% and spontaneous nocturnal feeding by 38%. Feeding subsequent to 24 hour food deprivation was not decreased. Naloxone-resistant hyperphagia induced by insulin and spontaneous daytime feeding were also not reduced. The combination of naloxone (3.0 mg/kg) and the NaCl regimen produced an additional 50% reduction in 2-DG induced feeding and an extra 40% decrease in nocturnal feeding. Naloxone, given with 2% NaCl to food deprived animals, retained its appetite suppressing activity, indicating that the NaCl regimen did not deplete the endogenous opioid which mediates food deprivation hyperphagia. These results demonstrate that 2% NaCl imbibition suppresses certain opioid mediated hyperphagias. However, the failure of 2% NaCl to affect all of the naloxone-sensitive types of feeding and the independence of naloxone-sensitive and NaCl-sensitive components suggests that NaCl drinking does not deplete dynorphin in the brain areas which mediate opiate-sensitive hyperphagias.

Assessment of dynorphin-A depletion in the anorexia of Walker-256 tumor bearing rats

Rats bearing the Walker-256 (W-256) tumor display an anorexic profile which resembles that of normal animals forced to drink 2% NaCl [2,24], a regimen which depletes neurohypophyseal dynorphin-A (DYN) [3,9]. As expected, the naloxone reversible feeding induced by 2-deoxy-D-glucose (2-DG) was attenuated (36%) in the W-256 tumor bearing rats (TBR). Interestingly, immunoreactive (ir) levels of dynorphin-A 1-17 (DYN-17) and its postulated breakdown product, dynorphin-A 1-18 (DYN-8), were also reduced in the neurohypophysis of W-256 TBR by 42 and 50%, respectively. However, ir-DYN levels were not reduced in TBR in those brain regions which are probably involved in the regulation of appetite (e.g., hypothalamus). 2-DG itself did not consistently affect ir-DYN levels in any tissue for either controls or TBR. The ratio of DYN-8 to DYN-17 did not change in response to any treatment, including the depletion of both peptides from the NIL of TBR. In summary, the present data do not support DYN depletion as being a factor which contributes to the anorexia of the W-256 TBR.

Opioids and consummatory behavior

Since the second decade of this century it has been known that opiates can influence ingestive behaviors. Generally, opioid agents enhance feeding and opioid antagonists decrease feeding. The present paper reviews the responsiveness of different animal species to opiates in relation to ingestive behaviors, the opioid receptors involved in such consummatory behaviors, the site of action of opioid modulation of feeding, the role of glucose in opioid induced feeding, and endocrine effects on opioid feeding systems. We emphasize the finding that more than one opioid receptor is involved in the modulation of feeding. A large body of evidence indicates a major role for the dynorphin/alpha-neo-endorphin kappa opioid receptor as one of the receptors involved in feeding modulation. Opioids appear to exert their effect predominantly within the central nervous system, though peripheral effects on taste and gastrointestinal function may play a role in opioid-induced feeding. Although opioid blockade acutely blocks food intake, chronic administration of opiate antagonists to humans and laboratory animals has not proven to be an effective means of decreasing body weight. Chronic opiate administration decreases body weight and autosensitization of beta-endorphin increases body weight. Thus, although it is clear that opioids can effect food intake, it is not clear what effect chronic administration of opioids has no food intake or body weight.

CNS tissue levels of dynorphin-A immunoreactivity and the anorexia associated with sodium chloride imbibition in the rat

Forced imbibition of increasing concentrations of sodium chloride (NaCl) in rats reduced daytime 2-deoxy-D-glucose (2-DG) induced feeding in a concentration dependent manner. Pituitary neurointermediate lobe (NIL) levels of immunoreactive (ir)-dynorphin-A 1-17 and 1-8 were also decreased by the NaCl regimen in a concentration dependent manner. However, there was no significant association between the reduction of NIL dynorphin levels and the suppression of 2-DG induced feeding on a within-animal basis. NaCl imbibition did not affect levels of either ir-dynorphin-A 1-17 or 1-8 in the hypothalamus, cerebral cortex, hippocampus, medulla/pons or anterior pituitary. Neither the acute changes following 2-DG administration, nor the comparison of ir-dynorphin-A 1-8/1-17 ratios appeared useful for the assessment of dynorphin-A turnover. Thus, the present results did not support the hypothesis that anorexia of NaCl treated animals results from the depletion of dynorphin-A.

Difference in sensitivity of the mu and kappa systems in cafeteria rats

A highly palatable diet (cafeteria diet) provokes an hyperphagia. The effects of Mu and Kappa opiate antagonists (Mu : Naltrexone 0.5 mg/Kg IP; Kappa Mr2266 0.5; 2.5; 10) and agonists (Mu Morphine 0.05; 0.1; 0.5; 2.5; Kappa Mr2033 0.5; 2.5) were studied on the nocturnal food intake of cafeteria rats and chow rats fed with monotonous food. At low doses Mu as well as Kappa antagonists do not modify the food intake of chow rats, but suppress the hyperphagia induced by the cafeteria diet. Kappa agonist provokes a decrease in food intake in chow and cafeteria rats while the Mu agonist at low doses suppresses the hyperphagia induced by cafeteria diet. The involvement of these two opioid systems in this type of hyperphagia is discussed.

The role of opioid receptor sub-types in tifluadom-induced feeding

There is now considerable evidence that opioid agonists and benzodiazepines increase food and water intake in a variety of animal species. The appetitive effects of the novel opioid-benzodiazepine tifluadom have been investigated. (+/-)-Tifluadom significantly increased food intake in freely-feeding rats. This stimulation of appetite was attributable principally to the activity of the (+)-isomer. Furthermore tifluadom-induced feeding was blocked by the opioid antagonists naloxone, naltrexone, Mr 1452 and Mr 2266 but not by the delta-opioid receptor antagonist ICI 154, 129, or by the benzodiazepine antagonist Ro 15-1788. These results suggest that tifluadom exerts its effect on food intake by interaction with opioid as opposed to benzodiazepine receptors and that this activity is mediated by kappa and/or mu- rather than delta-opioid receptor sub-types.

Differential inhibition by propranolol of feeding induced in rats by various stimuli

Opiate receptor blockade, or forced imbibition of 2% NaCl to deplete pituitary dynorphin decreases 2-deoxy-D-glucose (2-DG), but not insulin-induced hyperphagia, indicating a possible role for dynorphin in the eating associated with endogenous opiates. Beta-adrenergic receptor blockade decreases vasopressin release induced by 2-DG but not by insulin. Because vasopressin and dynorphin are sometimes co-localized, it was hypothesized that naloxone-sensitive feeding might be selectively inhibited by beta-adrenergic blockade with propranolol. Propranolol in doses as low as 2.5 mg/kg inhibited 4 hr feeding induced by 2-DG (400 mg/kg). Propranolol did not significantly affect feeding induced by ketocyclazocine administration (3.0 mg/kg) or by 24 hr food deprivation. Feeding stimulated by insulin (10 U/kg) was significantly inhibited by propranolol (2.5 mg/kg) only when the propranolol was reinjected during the period 2 hr after insulin injection, when the induced feeding was greatest. In summary, propranolol inhibited opiate-related (2-DG) as well as opiate-independent (insulin) hyperphagias. It also failed to inhibit food intake resulting from the opiate related stimulus of 24 hr food deprivation. Therefore, naloxone sensitive hyperphagias were not specifically inhibited by beta-adrenergic blockade, indicating that vasopressin-associated dynorphin is not involved in opiate related feeding.

Which opioid receptor mechanism modulates feeding?

There is substantial evidence for the role of endogenous opioid peptides in the regulation of appetite. This communication examines the possible opioid peptide mechanism(s) which are involved in appetite regulation. In the rat, activation of both the dynorphin-kappa opioid receptor and the beta-endorphin-epsilon opioid receptor appear to enhance feeding, most probably acting in different areas of the central nervous system. It also appears that rats may have a mu anorectic system. Too few studies have been undertaken to define whether the delta or sigma receptor systems are also involved in feeding responses. It is becoming apparent that a great deal of species diversity exists in the feeding responses to opiates, making it difficult to extrapolate the results obtained in rats to other species. In humans, studies with naloxone suggest an opioid sensitive feeding system which possibly is specifically involved in the regulation of carbohydrate uptake. In addition, we report here preliminary data suggesting the presence of a mu anorectic system in humans. Thus, analogous to the findings for the role of opioid receptors in analgesia, it appears that multiple opioid receptors may be involved in appetite regulation, each receptor relating to a different aspect of feeding.

The effect of peripherally administered satiety substances on feeding induced by butorphanol tartrate

Endogenous opioid peptides appear to play a role in the initiation of feeding. Butorphanol, an exogenous opiate which preferentially generalizes to the kappa-sigma opiate receptors, is a potent initiator of feeding. In these studies, we examined the effect of peripherally administered putative satiety substances, cholecystokininoctapeptide, somatostatin, bombesin, gastrin-releasing peptide, thyrotropin-releasing hormone, calcitonin and glucagon on butorphanol induced feeding. With the exception of bombesin, all the other putative satiety factors required 2 to 32 times as high a dose to significantly suppress feeding following butorphanol compared to the dosages required to suppress starvation or tail pinch induced feeding. Bombesin appeared to be approximately equipotent in all systems tested. Haloperidol and atropine both suppressed butorphanol induced feeding supporting our previous hypothesis of an integral relationship between acetylcholinergic-dopaminergic and opioid mechanisms in the initiation of feeding. The findings reported here are compatible with an important role for opioid mechanisms in the initiation of feeding.

The effect of the opioid-benzodiazepine, tifluadom, on ingestive behaviors

A large body of evidence has suggested a role for the endogenous opiates and their receptors in the regulation of appetite. In this study, we report on the effects of tifluadom, a noval opiate with a benzodiazepine-like structure and preferential activity at the kappa opiate receptor, on ingestive behaviors. Tifluadom increases food intake in rats without altering water intake. Tifluadom's effect on feeding is more potent than that of morphine or ketocyclazocine and equivalent to that of butorphanol. The effect is partially resistant to naloxone antagonism. Tifluadom is more potent when administered subcutaneously than when given intraperitoneally. These data provide further support for the concept that kappa opiate receptors represent an important component of the natural feeding drive.

Endogenous opiates: 1982

This article is the fifth installment in an annual series of reviews of successive year's research dealing with the endogenous opiate peptides. Due to the continuing massive increase in the number of studies in this field, it has become impossible to continue comprehensive reviews of all aspects of this work. As a result we have decided that beginning this year the coverage will be abbreviated to emphasize non-analgesic and behavioral work. The specific areas discussed include stress, tolerance and dependence, consummatory responses, alcohol consumption, schizophrenia and emotional disorders, learning and memory, cardiovascular responses, respiratory effects, thermoregulatory effects, neurological deficits and other disorders, activity, and other, miscellaneous behaviors. As in previous years, we have attempted a relatively comprehensive review of the subjects covered only for the previous year and have not made an attempt to evaluate their contributions relative to those of past years.

Genetically obese mice: resistance to metastasis of B16 melanoma and enhanced T-lymphocyte mitogenic responses

The metastasis of B16 melanoma cells differed significantly in obese (ob/ob) and lean (+/?) female mice of strain C57BL/6J. When the mice were inoculated subcutaneously with melanoma cells at 10 to 11 months of age, the primary tumor grew more slowly in obese than in lean littermates and the frequency of lung metastasis was greatly reduced. When the mice were injected with the cells at 4 to 7 months, the primary tumor grew at the same rate in obese and lean mice, but the obese mice again showed a significantly reduced frequency of lung metastasis. That this effect was related to an enhanced immunocompetence in obese mice was supported by the finding that splenic lymphocytes of ob/ob mice showed three times the proliferative response to the T-cell mitogen concanavalin A compared with the proliferative response of lean control mice. The ob/ob mouse may provide a model for the study of enhanced immunocompetence in obese individuals.