Alterations in deprivation, glucoprivic and sucrose intake following general, mu and kappa opioid antagonists in the hypothalamic paraventricular nucleus of rats

Effects of bariatric surgery and dietary interventions for obesity on brain neurotransmitter systems and metabolism: A systematic review of positron emission tomography (PET) and single-photon emission computed tomography (SPECT) studies

This systematic review collates studies of dietary or bariatric surgery interventions for obesity using positron emission tomography and single-photon emission computed tomography. Of 604 publications identified, 22 met inclusion criteria. Twelve studies assessed bariatric surgery (seven gastric bypass, five gastric bypass/sleeve gastrectomy), and ten dietary interventions (six low-calorie diet, three very low-calorie diet, one prolonged fasting). Thirteen studies examined neurotransmitter systems (six used tracers for dopamine DRD2/3 receptors: two each for 11 C-raclopride, 18 F-fallypride, 123 I-IBZM; one for dopamine transporter, 123 I-FP-CIT; one used tracer for serotonin 5-HT2A receptor, 18 F-altanserin; two used tracers for serotonin transporter, 11 C-DASB or 123 I-FP-CIT; two used tracer for μ-opioid receptor, 11 C-carfentanil; one used tracer for noradrenaline transporter, 11 C-MRB); seven studies assessed glucose uptake using 18 F-fluorodeoxyglucose; four studies assessed regional cerebral blood flow using 15 O-H2 O (one study also used arterial spin labeling); and two studies measured fatty acid uptake using 18 F-FTHA and one using 11 C-palmitate. The review summarizes findings and correlations with clinical outcomes, eating behavior, and mechanistic mediators. The small number of studies using each tracer and intervention, lack of dietary intervention control groups in any surgical studies, heterogeneity in time since intervention and degree of weight loss, and small sample sizes hindered the drawing of robust conclusions across studies.

High-calorie diets uncouple hypothalamic oxytocin neurons from a gut-to-brain satiation pathway via κ-opioid signaling

Oxytocin-expressing paraventricular hypothalamic neurons (PVNOT neurons) integrate afferent signals from the gut, including cholecystokinin (CCK), to adjust whole-body energy homeostasis. However, the molecular underpinnings by which PVNOT neurons orchestrate gut-to-brain feeding control remain unclear. Here, we show that mice undergoing selective ablation of PVNOT neurons fail to reduce food intake in response to CCK and develop hyperphagic obesity on a chow diet. Notably, exposing wild-type mice to a high-fat/high-sugar (HFHS) diet recapitulates this insensitivity toward CCK, which is linked to diet-induced transcriptional and electrophysiological aberrations specifically in PVNOT neurons. Restoring OT pathways in diet-induced obese (DIO) mice via chemogenetics or polypharmacology sufficiently re-establishes CCK's anorexigenic effects. Last, by single-cell profiling, we identify a specialized PVNOT neuronal subpopulation with increased κ-opioid signaling under an HFHS diet, which restrains their CCK-evoked activation. In sum, we document a (patho)mechanism by which PVNOT signaling uncouples a gut-brain satiation pathway under obesogenic conditions.

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.

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.

Commentary: 'Unhealthy diet,' nutrient status, and ADHD symptoms: a confounding role for environmental nitrous oxide exposure - reflections on Rijlaarsdam et al. (2016)

Rijlaarsdam et al. (2016) recently published their findings utilizing a longitudinal design showing that prenatal 'unhealthy diet' was positively associated with IGF2 DNA methylation at birth across both youth cohorts. However, only in the EOP youth was prenatal 'unhealthy diet' positively associated with ADHD symptoms presumably through IGF2 DNA hypermethylation. Rijlaarsdam et al.'s () choice to assess high fat and sugar diet with the Food Frequency Questionnaire (FFQ) may offer some indication as to prenatal nutrient status, as the foods identified by the FFQ in their study are relatively low in free choline. It has been shown that gestational choline deficiency in rats leads to hypermethylation of IGF2. Consistent with the literature describing an association between air pollution and cognitive neurodevelopmental impairment, the author of this commentary has previously proposed through empirical investigation that chronic environmental exposure to the trace levels of the pervasive air pollutant, nitrous oxide (N2 O), may facilitate core features of neurodevelopmental disorders, like ADHD. Impaired acetylcholine synthesis in rats exposed to N2 O has been shown, with a 53% reduction in [1-2H2,2-2H2] choline. Low-dose N2 O exposure is also thought to stimulate central release of opioid peptides, like dynorphin, which play a role in significantly increasing food intake behavior and/or modulating sucrose intake. Taken altogether, these studies present a strong confounder to the interpretation made by Rijlaarsdam et al. () that prenatal 'unhealthy diet' may play a role in the onset of ADHD symptoms in youth with EOP conduct problems through induction of IGF2 DNA hypermethylation. While the 'unhealthy diet' may represent possible maternal nutrient deficiencies during gestation, it is also possible that exposure to air pollutants, particularly N2 O, may not only directly reduce fetal cholinergic status thereby enhancing IGF2 DNA hypermethylation but may also significantly modulate maternal food intake behaviors (i.e. sucrose).

Amylin receptor signaling in the nucleus accumbens negatively modulates μ-opioid-driven feeding

Amylin is a peptide co-secreted with insulin that penetrates into the brain, and produces satiation-like effects via actions in the brainstem, hypothalamus, and mesencephalon. Little is known, however, about the effects of amylin in the nucleus accumbens shell (AcbSh), where a circumscribed zone of intense amylin receptor (AMY-R) binding overlaps reported mappings of a 'hotspot' for μ-opioid receptor (μ-OR) amplification of food reward. Here, the ability of intra-AcbSh AMY-R signaling to modulate μ-OR-driven feeding was explored. Amylin (1-30 ng) was administered with the μ-OR agonist, D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) (0.25 μg), directly into the AcbSh of ad libitum-maintained rats. Amylin dose-dependently reversed DAMGO-induced hyperphagia; 3 ng of amylin reduced DAMGO-mediated feeding by nearly 50%. This dose was, however, completely ineffective at altering DAMGO-induced feeding in the anterior dorsal striatum. Intra-AcbSh amylin alone (3-30 ng) modestly suppressed 10% sucrose intake in ad libitum-maintained rats, and chow in food-deprived rats, but only at the 30-ng dose. This result indicates that reversal of AcbSh DAMGO-induced feeding at a 10-fold lower dose was neither due to malaise nor motoric impairment. Finally, intra-AcbSh infusion of the AMY-R antagonist, AC187 (20 μg), significantly attenuated the ability of prefeeding to suppress DAMGO-induced food intake, with no effects in non-prefed rats. Hence, AMY-R signaling negatively modulates μ-OR-mediated appetitive responses at the level of the AcbSh. The results with AC187 indicate that endogenous AMY-R transmission in the AcbSh curtails opioid function in the postprandial period, suggesting a novel pathway for peripheral-central integration in the control of appetitive motivation and opioid reward.

Central kappa opioid receptors modulate salt appetite in rats

The role of the central opioid system in the control of water and salt intake is complex, with both stimulatory and inhibitory effects having been observed. The aim of the present study was to investigate the participation of the central κ-opioid receptors in the control of salt appetite. Male Wistar rats were submitted to two different experimental protocols: sodium deficit produced by the diuretic, furosemide, and brain angiotensinergic stimulation in rats under normal sodium balance. Lateral ventricle (LV) injections of Nor-binaltorphimine (Nor-BNI) at different doses (5, 10 and 20 nmol) inhibited hypertonic saline solution (1.5%) intake in sodium-depleted rats. The salt appetite induced by an LV injection of angiotensin II (Ang II) (10 ng) was also blocked by Nor-BNI injections into the LV, while no significant change was observed in water intake. Furthermore, the decrease in salt intake seems not to have been due to a general inhibition of locomotor activity or to any change in palatability, since central administration of Nor-BNI failed to modify the intake of a 0.1% saccharin solution when the animals were submitted to a "dessert test" or to induce any significant locomotor deficit in the open-field test. Also the central administration of Nor-BNI was unable to modify blood pressure in sodium-depleted animals. The present results suggest that activation of endogenous κ-opioid receptors modulates salt appetite induced by sodium depletion and by central angiotensinergic stimulation in rats.

The relationship between opioid and sugar intake: review of evidence and clinical applications

Opioid dependence poses significant public health risks arising from associated morbidity and mortality caused by accidents, infectious diseases, and social ramifications of crime and unemployment, among other complications. Opioid use, acute and chronic, is also associated with weight gain, glycemic dysregulation, and dental pathology. The literature supporting the connection between opiate use and development of preference for sweet tastes is reviewed, and further association with dental pathology, weight gain, and loss of glycemic control are considered. Additionally, the impact of sweet tastes on the endogenous opioid system, as pertaining to analgesia, is also discussed. The authors discuss the clinical implications in relation to the aforementioned conditions while treating the opiate-dependent patient.

kappa-Opioid receptors control the metabolic response to a high-energy diet in mice

General opioid receptor antagonists reduce food intake and body weight in rodents, but the contributions of specific receptor subtypes are unknown. We examined whether genetic deletion of the kappa-opioid receptor (KOR) in mice alters metabolic physiology. KOR-knockout (KO) and wild-type (WT) mice were fed a high-energy diet (HED) for 16 wk. KO mice had 28% lower body weight and 45% lower fat mass when compared to WT mice fed an HED. No differences in caloric intake were found. An HED reduced energy expenditure in WT mice, but not in KO mice. KOR deficiency led to an attenuation of triglyceride synthesis in the liver. Malonyl CoA levels were also reduced in response to an HED, thereby promoting hepatic beta-oxidation. Glycemic control was also found to be improved in KO mice. These data suggest a key role for KORs in the central nervous system regulation of the metabolic adaptation to an HED, as we were unable to detect expression of KOR in liver, white adipose tissue, or skeletal muscle in WT mice. This study provides the first evidence that KORs play an essential physiological role in the control of hepatic lipid metabolism, and KOR activation is a permissive signal toward fat storage.-Czyzyk, T. A., Nogueiras, R., Lockwood, J. F., McKinzie, J. H., Coskun, T., Pintar, J. E., Hammond, C., Tschöp, M. H., Statnick, M. A. kappa-Opioid receptors control the metabolic response to a high-energy diet in mice.

Changes in mouse mu opioid receptor Exon 7/8-like immunoreactivity following food restriction and food deprivation in rats

Opioid agonists and antagonists respectively increase and decrease food intake. That selective mu opioid antagonists are more effective than antisense probes directed against the mu opioid receptor (MOR-1) gene in reducing deprivation-induced feeding suggests a role for isoforms. Both food restriction and deprivation alter protein and mRNA levels of opioid peptides and receptors. Antisera directed against Exon 4 of the MOR-1-like immunoreactivity (LI) (Exon 4) clone or directed against mouse Exons 7/8 (mE7/8-LI) revealed high levels of immunoreactivity in brain nuclei related to feeding behavior. Therefore, the present study assessed MOR-1LI and mE7/8-LI in hypothalamic and extrahypothalamic sites in rats exposed to ad libitum feeding, food restriction (2, 7, 14 days), or food deprivation (24, 48 h). MOR-1-LI displayed robust reactivity, but was insensitive to food restriction or deprivation. mE7/8-LI, both in terms of cell counts and relative optical density, was significantly and selectively increased in the dorsal and ventral parvocellular subdivisions of the hypothalamic paraventricular nucleus in food-restricted (14 days) rats, but all other restriction or deprivation regimens were ineffective in other hypothalamic nuclei. In contrast, significant and site-specific decreases in relative optical density in the rostral part of the nucleus tractus solitarius (NTS) were observed in food-restricted (2, 7 days) or food-deprived (24, 48 h) animals, but these regimens were ineffective in the other extrahypothalamic sites. This study indicates the sensitivity of this mE7/8-LI probe in the hypothalamic parvocellular paraventricular nucleus and rostral NTS to food restriction and deprivation in rats.

FLUOXETINE ALTERS MU OPIOID RECEPTOR EXPRESSION IN OBESE ZUCKER RAT EXTRAHYPOTHALAMIC REGIONS

The aim of this article was to describe the effects of chronic fluoxetine on mu opioid receptor expression in obese Zucker rat extrahypothalamic regions. Male obese Zucker (fa/fa) rats were administered with fluoxetine (10 mg/kg; i.p.) daily for two weeks. Brain regional immunostaining for mu opioid receptor was carried out. An increase in the numbers of neural cells immunostained for mu opioid receptor in caudatus-putamen, dentate gyrus, lateral septum, amygdala, and frontal, parietal, and piriform cortices was observed. Increased mu opioid receptor expression in the central amygdaloid nuclei suggests a decreased opioidergic tone at this level that could be involved in fluoxetine anorectic action.

Analysis of the network of feeding neuroregulators using the Allen Brain Atlas

The Allen Brain Atlas, the most comprehensive in situ hybridization database, covers over 21,000 genes expressed in the mouse brain. Here we discuss the feasibility to utilize the ABA in research pertaining to the central regulation of feeding and we define advantages and vulnerabilities associated with the use of the atlas as a guidance tool. We searched for 57 feeding-related genes in the ABA, and of those 42 display distribution consistent with that described in previous reports. Detailed analyses of these 42 genes in the nucleus accumbens, ventral tegmental area, nucleus of the solitary tract, lateral hypothalamus, arcuate, paraventricular, ventromedial and dorsomedial nuclei suggests that molecules involved in feeding stimulation and termination are coexpressed in multiple consumption-related sites. Gene systems linked to energy needs, reward or satiation display a remarkably high level of overlap. This conclusion calls into question the classical concept of brain sites viewed as independent hunger or reward "centers" and favors the theory of a widespread feeding network comprising multiple neuroregulators affecting numerous aspects of consumption.

Chronic prevention of mu-opioid receptor (MOR) G-protein coupling in the pontine parabrachial nucleus persistently decreases consumption of standard but not palatable food

RATIONALE

Acute pharmacological studies implicate mu-opioid receptors (MORs) in the parabrachial nucleus (PBN) of the brainstem in modulating eating. The long-term effects of preventing the cellular function of parabrachial MORs on food consumption remain to be elucidated.

OBJECTIVES

To determine whether (1) chronic inhibition of MOR-mediated G-protein coupling in the PBN of rats would persistently reduce eating and (2) food properties dictate the effects of MOR blockade.

MATERIALS AND METHODS

We microinfused the irreversible MOR antagonist, beta-funaltrexamine (beta-FNA) into the lateral PBN and measured the intake of standard and calorically dense palatable chow for 1 week. First, rats were given standard chow for 20 h daily and a calorically dense palatable chow for 4 h during the day. We infused the agonist, [D: -Ala(2), N-Me-Phe(4), Glycinol(5)]-Enkephalin (DAMGO), 1 week after beta-FNA to probe the acute effects of exogenous stimulation of MORs on palatable food intake. [(35)S]GTPgammaS autoradiography quantified regional loss of MOR cellular function. Next, we measured the actions of beta-FNA on food intake in rats given only standard or palatable chow for 1 week.

RESULTS

One infusion of beta-FNA persistently decreased consumption of standard but not palatable chow, regardless of feeding regimen. beta-FNA also blocked DAMGO-stimulated palatable chow intake, prevented DAMGO-stimulated G-protein coupling in the central and external lateral subnuclei of the PBN, and decreased coupling in the medial PBN. beta-FNA did not affect kappa-opioid receptors.

CONCLUSIONS

MORs in the lateral PBN serve a physiological role in stimulating consumption of standard food. Properties of the diet, such as high palatability or caloric density, may override the influence of inhibiting MOR function.

Mu-opioid receptor cellular function in the nucleus accumbens is essential for hedonically driven eating

Acute pharmacological studies have implicated mu-opioid receptors (MORs) in the shell of the nucleus accumbens (NAC) in mediating responses for palatable food and other natural and drug-induced rewards. However, the long-term behavioral effects of inactivating signal transduction via accumbal MORs, as quantified by an anatomically defined loss of cellular activity, have never been analysed. We combined microinfusion of the irreversible MOR antagonist, beta-funaltrexamine (beta-FNA; 8.0 nmol/0.8 microL, n=9; controls, n=6) with mapping by [35S]GTPgammaS autoradiography to demonstrate an anatomically specific loss of the coupling of MORs to their G-proteins in the dorsal caudomedial shell of the NAC in rabbits. beta-FNA did not alter the stimulated coupling of kappa-opioid receptors. This selective blockade of the cellular function of MORs persistently decreased consumption of a palatable sucrose solution by 40% during a daily 4-h test conducted 2, 3 and 4 days after infusion. beta-FNA did not alter body weight or 20-h consumption of standard chow or water. In 10 different rabbits, infusion of the selective, competitive MOR antagonist, CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) into the same locus produced a reversible decrease in sucrose consumption, with normal intakes returning on the next day. Together, these data appear to establish that MORs in this accumbal subregion support responding for orosensory reward. Overall, these results visualize a discrete brain locus where cellular actions of endogenous opioids mediate behaviors involved in self-administration of foods and perhaps other hedonically valued substances, such as ethanol and drugs of abuse.

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.

Evidence for a mu-opioid-opioid connection between the paraventricular nucleus and ventral tegmental area in the rat

The paraventricular nucleus (PVN) and the ventral tegmental area (VTA) have been shown to be involved in opioid mediated feeding behavior. The present study examined whether mu-opioid signalling between the PVN and VTA affected feeding behavior. Male Sprague-Dawley rats were cannulated with one cannula placed in the PVN and two cannulae placed in the VTA, which allowed for co-administration of the mu-opioid receptor agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO) in one site and the opioid antagonist naltrexone (NTX) in the other site. Bilateral administration of DAMGO (1.2, 2.4 and 4.9 nmol) into the VTA stimulated feeding dose dependently at 2.4 and 4.9 nmol (P<0.05). The DAMGO (2.4 nmol)-induced increase of food intake following injection into the PVN was blocked by bilateral injection of NTX (6.6, 13.2 and 26.5 nmol) into the VTA at 2 and 4 h (P<0.05). In the reverse situation, the DAMGO (2.4 nmol)-induced increase of food intake following injection into the VTA was blocked by injection of NTX (13.2 and 26.5 nmol) into the PVN at 2 and 4 h (P<0.05). The present study suggests that a bidirectional mu-opioid-opioid signalling pathway exists between the PVN and the VTA which influences feeding.

Feeding induced by food deprivation is differentially reduced by opioid receptor antisense oligodeoxynucleotide probes in rats

The increases in food intake following 24 h of food deprivation are reduced by systemic and central administration of general opioid antagonists. The use of selective opioid antagonists revealed that mu-selective antagonists were more effective than kappa-selective antagonists in reducing deprivation-induced intake, whereas delta-selective antagonists were minimally effective. Antisense oligodeoxynucleotide (AS ODN) probes directed against different exons of the mu (MOP), delta (DOP), kappa (KOP) and nociceptin (NOP) opioid peptide receptor genes have been able to differentially alter feeding responses elicited by glucoprivation, lipoprivation and by different opioid peptides and receptor agonists. The present study examined whether lateral ventricular administration of AS ODN probes directed against different exons of the MOP, DOP, KOP or NOP opioid receptor genes altered food intake and body weight changes following 24 h of food deprivation in rats. Deprivation-induced feeding was significantly and maximally reduced by an AS ODN probe directed against exon 2, but not exons 1 or 3 of the KOP gene. This response was also significantly though modestly reduced by AS ODN probes directed against exons 2, 3 or 4 of the MOP gene, exon 1 of the DOP gene, or exon 1 of the NOP gene. Recovery of body weight following postdeprivation food reintroduction was significantly reduced by AS ODN probes directed against either exons 2, 3 or 4 of the MOP gene, exons 1 or 2 of the DOP gene, or exons 1, 2 or 3 of the KOP gene. The parallel patterns in the magnitude of alterations in deprivation-induced feeding by delta antagonists and DOP AS ODN probes on one hand, and by kappa antagonists and KOP AS ODN probes on the other, provide converging and complementary evidence for their relative involvement in this response. The modest reductions by MOP AS ODN probes relative to the more potent reductions induced by mu-selective antagonists suggest that the mu receptor-mediated actions upon deprivation-induced feeding may involve recently-identified splice variants or isoforms of the MOP gene.

An orexigenic role for mu-opioid receptors in the lateral parabrachial nucleus

The pontine parabrachial nucleus (PBN) has been implicated in regulating ingestion and contains opioids that promote feeding elsewhere in the brain. We tested the actions of the selective mu-opioid receptor (mu-OR) agonist [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) in the PBN on feeding in male rats with free access to food. Infusing DAMGO (0.5-4.0 nmol/0.5 microl) into the lateral parabrachial region (LPBN) increased food intake. The hyperphagic effect was anatomically specific to infusions within the LPBN, dose and time related, and selective for ingestion of chow compared with (nonnutritive) kaolin. The nonselective opioid antagonist naloxone (0.1-10.0 nmol intra-PBN) antagonized DAMGO-induced feeding, with complete blockade by 1.0 nmol and no effect on baseline. The highly selective mu-opioid antagonist d-Phe-Cys-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 1.0 nmol) also prevented this action of DAMGO, but the kappa-antagonist nor-binaltorphimine did not. Naloxone and CTAP (10.0 nmol) decreased intake during scheduled feeding. Thus stimulating mu-ORs in the LPBN increases feeding, whereas antagonizing these sites inhibits feeding. Together, our results implicate mu-ORs in the LPBN in the normal regulation of food intake.

Hypericum perforatum CO2 Extract and Opioid Receptor Antagonists Act Synergistically to Reduce Ethanol Intake in Alcohol-Preferring Rats

BACKGROUND

Hypericum perforatum extracts attenuate ethanol intake in alcohol-preferring rats. The opioid receptor antagonists, naloxone and naltrexone, reduce ethanol intake in rats and humans. The combination of different agents that reduce ethanol intake has been proposed as an approach to the pharmacotherapy of alcoholism. This study evaluated the effect on ethanol intake of the combined administration of a CO2 H. perforatum extract and naloxone or naltrexone in genetically selected Marchigian Sardinian alcohol-preferring rats.

METHODS

Ten percent (v/v) ethanol intake was offered 2 hr per day at the beginning of the dark phase of the reverse light-dark cycle. H. perforatum CO2 extract was given intragastrically, 1 hr before access to ethanol. Naloxone or naltrexone was given by intraperitoneal injection 10 min before the extract.

RESULTS

H. perforatum CO2 extract reduced ethanol intake at 31 or 125 mg/kg, but not 7 mg/kg. These doses neither modified food or water intake during access to ethanol, nor reduce 0.2% saccharin intake. Naloxone reduced ethanol and food intake at 3 or 5 mg/kg, but not 1 mg/kg. When naloxone 1 mg/kg was combined with the three doses of H. perforatum CO2 extract, the attenuation of ethanol intake was more pronounced than that observed after the administration of the extract alone. Alcohol intake was also significantly reduced by 7 mg/kg of H. perforatum CO2 extract combined with naloxone 1 mg/kg. The combined treatments never modified the rat's locomotor activity nor the simultaneous intake of food, water or 0.2% saccharin. Naltrexone reduced ethanol intake at 1 and 3 mg/kg, but not at 0.5 mg/kg. When naltrexone 0.5 mg/kg was combined with H. perforatum CO2 extract 7 mg/kg, ethanol intake was markedly reduced.

CONCLUSIONS

These findings provide evidence that H. perforatum CO2 extract and opiate receptor antagonists act synergistically to induce a pronounced and selective reduction of voluntary ethanol consumption in alcohol-preferring rats.

Alterations in food intake elicited by GABA and opioid agonists and antagonists administered into the ventral tegmental area region of rats

Food intake is significantly increased following administration of mu-selective opioid agonists into the ventral tegmental area (VTA) region acting through multiple local opioid receptor subtypes. Since GABA receptor agonists in the VTA region are capable of eliciting feeding, the present study investigated whether feeding elicited by the mu-selective opioid agonist [D-Ala(2), NMe(4), Gly-ol(5)]-enkephalin (DAMGO) in the VTA region was altered by pretreatment into the same site with equimolar doses of either GABA(A) (bicuculline) or GABA(B) (saclofen) antagonists, and further, whether pretreatment with either general opioid or selective GABA receptor antagonists decreased feeding elicited by GABA(A) (muscimol) or GABA(B) (baclofen) agonists in the VTA region. DAMGO-induced feeding in the VTA region was dose-dependently decreased following pretreatment with either GABA(A) or GABA(B) antagonists in the absence of significant alterations in food intake by the antagonists per se. However, the presence of short-lived seizures following bicuculline in the VTA region suggests that this ingestive effect was caused by nonspecific actions. In contrast, GABA(B) receptors are involved in the full expression of mu-opioid agonist-induced feeding in this region since saclofen failed to elicit either seizure activity or a conditioned taste aversion. Pretreatment with naltrexone in the VTA region reduced intake elicited by baclofen, but not muscimol. Finally, baclofen-induced feeding was significantly reduced by saclofen, but not bicuculline, pretreatment in the VTA region. Therefore, possible coregulation between GABA(B) and opioid receptors in the VTA region, as suggested by immunocytochemical evidence, is supported by these behavioral effects upon ingestion.

Analysis of opioid receptor subtype antagonist effects upon mu opioid agonist-induced feeding elicited from the ventral tegmental area of rats

The present study examined opioid receptor(s) mediation of feeding elicited by mu opioid agonists in the ventral tegmental area using general or selective opioid antagonist pretreatment. Naltrexone as well as equimolar doses of selective mu and kappa, but not delta opioid antagonists in the ventral tegmental area significantly reduced mu agonist-induced feeding, indicating a pivotal role for these receptor subtypes in the full expression of this response.

Delta(9)-THC stimulates food intake in Lewis rats: effects on chow, high-fat and sweet high-fat diets

Free-feeding adult male Lewis rats were administered intraperitoneal (i.p.) Delta(9)-tetrahydrocannabinol (THC) in doses of 0.5, 1.0 and 2.5 mg/kg, and effects on food intake were measured at 1, 2, 4, 6 and 24 h postinjection. Rats were fed rat chow, a high-fat diet (HF) or a high-fat sweetened (HFS) diet. Small increases in HF and HFS intake following doses of 0.5 or 1.0 mg/kg were seen at 1, 2 and 4 h, but not 6 or 24 h compared to vehicle intake. Increases following 0.5 and 1.0 mg/kg did not differ from each other at any time point and 2.5 mg/kg produced smaller differences at all time points. There was no difference between HF or HFS intake at any time point although larger increases were seen in the HF group compared to both chow and HFS following 0.5 and 1.0 mg/kg. This work confirms previous data in both humans and rats indicating a stimulatory role for cannabinoids in ingestive behavior.

Naltrexone administered to central nucleus of amygdala or PVN: neural dissociation of diet and energy

There is evidence that opioids may affect food consumption through mechanisms as diverse as reward or energy metabolism. However, these hypotheses are derived from studies employing peripheral or, more rarely, intracerebroventricular administration of drugs. Opioid receptors have a wide distribution in the central nervous system and include a number of regions implicated in food intake such as the hypothalamic paraventricular nucleus (PVN) and the central nucleus of the amygdala (ACe). It is not known whether local opioid receptor blockade in either of these regions will produce similar effects on food intake. To examine this issue, a chronic cannula was aimed at either the PVN or ACe of rats that were fed a choice of a high-fat and high-carbohydrate diet, which allows for the measurement of both preference and total energy consumption. Naltrexone influenced preferred and nonpreferred food consumption, depending on the site of administration. Consumption of both preferred and nonpreferred diets was suppressed after PVN naltrexone administration, whereas only preferred diet intake was reduced after ACe injection of naltrexone. The present evidence indicates that direct stimulation of different brain regions with naltrexone may be associated with diverse effects on diet selection, which may be accounted for by manipulation of specific functional neural circuitry.

The hyperphagic effect of nociceptin/orphanin FQ in rats

Nociceptin/orphanin FQ (NC), the endogenous ligand of the opioid receptor-like1 (ORL1) receptor, has been reported to stimulate feeding in rats. The present article reviews the studies so far published on the effect of NC on food intake and reports new findings concerning the sensitivity of brain regions to the hyperphagic effect of NC in rats. The results obtained indicate that the hypothalamic arcuate nucleus is the most sensitive site among the brain regions so far investigated. On the basis of these findings and of the neurochemical and electrophysiological effects of NC, possible mechanisms of action and possible interactions with other neurotransmitter systems affecting feeding are discussed.

Role of lipid type on morphine-stimulated diet selection in rats

Administration of morphine is said to increase fat consumption among rats allowed to self-select nutrients. However, fats represent a diverse group of molecules, differing in metabolic and sensory properties. Despite this, lipid has yet to be manipulated as a variable in drug-stimulated nutrient selection studies. To determine whether lipid source can impact daily and morphine-stimulated (1, 3, and 10 mg/kg) diet intake, rats were provided with a choice between a high-fat and high-carbohydrate diet in three regimens in which the source of fat was varied between vegetable shortening, lard, or corn oil. Daily and morphine-stimulated diet selections were determined under all conditions. Under daily feeding conditions, rats ate more of the high-lipid diet compared with the high-carbohydrate diet when vegetable shortening or lard was the main lipid alternative, but lipid and carbohydrate intake did not differ when corn oil was the main lipid alternative. When rats were stimulated with morphine, the percentage of lipid increased relative to baseline intake only when the lipid diets were the preferred alternatives (i.e., vegetable shortening or lard). When preference between lipid and carbohydrate diets was neutral (i.e., corn oil condition), morphine did not enhance lipid consumption. These results indicate that morphine increases consumption of total energy or preferred diets and not lipid per se.

Opioids and food intake: distributed functional neural pathways?

Agonists of the mu, delta, kappa and ORL(1)opioid receptors increase food intake while opioid receptor blockade decreases food intake. The majority of the collected data related to opioids and feeding has led to the speculation that opioids are involved in meal maintenance and orosensory reward; however, some data suggest that opioids may impact feeding associated with energy needs. Based on the wide distribution of CNS opioid receptors and the presence of other neuropeptides in the vicinity of opioidergic pathways, it seems likely that opioids affect multiple feeding systems. For example, opioids in the hindbrain might be involved in both sensory and metabolic aspects of food intake, those in the amygdala in processing of 'emotional' properties of foods, and those in the hypothalamus in energy needs. In this review we present data which support functional diversity of opioids in feeding behavior.

Effects of mu and kappa opioid receptor antagonists on glucoprivic induction of Fos immunoreactivity in the rat preoptic area and hypothalamus

Interoreceptors in the central nervous system elicit compensatory behavioral and physiological responses to cellular glucopenia. Antagonism of mu and kappa opioid receptors attenuates glucoprivic hyperphagia, findings that implicate these peptidergic receptors in the central processing of metabolic regulatory signals. Several hypothalamic structures of critical importance for the regulation of energy balance exhibit one or both of these receptors. The following studies investigated the role of these opioid receptors in glucoprivic induction of immediate-early gene expression in these brain sites. Male rats were pretreated with beta-funaltrexamine (mu antagonist), Mr-1452 MS (kappa antagonist), or vehicle prior to intraperitoneal injection of the glucose antimetabolite, 2-deoxy-D-glucose (2DG), then sacrificed by transcardial perfusion 2 h later. Nuclear immunolabeling for the transcription factor, Fos, was observed in several preoptic and hypothalamic sites following 2DG administration. Rats pretreated with the mu antagonist exhibited significantly fewer Fos-positive neurons in the medial preoptic area and dorsomedial hypothalamic nucleus in response to 2DG, compared to vehicle-pretreated controls. Blockade of kappa receptors diminished 2DG and induced Fos staining in the paraventricular and supraoptic nuclei. Numbers of Fos-positive cells in the arcuate nucleus and ventrolateral hypothalamic area were not altered by either antagonist. The present data implicate mu and kappa opioid receptors in neural mechanisms underlying glucoprivic induction of the Fos stimulus-transcription pathway by local neurons in discrete hypothalamic sites.

Association between the amygdala and nucleus of the solitary tract in mu-opioid induced feeding in the rat

The central nucleus of the amygdala (CNA) and the nucleus of the solitary tract (NTS) are important in the regulation of ingestive behavior. We evaluated whether opioid-opioid signaling between the CNA and rostral NTS (rNTS) affect feeding behavior. To test this, rats were doubly cannulated with one cannula placed in the rNTS and one cannula in the CNA, allowing for co-administration of an opioid agonist into one site and an opioid antagonist into the other. Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO) (2 nmol) injected into the CNA (CNA DAMGO) increased feeding more than two-fold compared to the vehicle-injected rats. This increase in food intake was blocked when doses of 26.5 and 79 nmol of naltrexone (NTX) were injected into the rNTs. In the reverse situation, rNTS DAMGO increased food intake above control levels, and CNA NTX blocked DAMGO-induced feeding when administrated in doses of 26.5 and 79 nmol. This suggests that a bi-directional opioid-opioid signaling pathway exists between the CNA and the rNTS which influences feeding via mu-opioid receptors.

Antisense mapping of opioid receptor clones: effects upon 2-deoxy-D-glucose-induced hyperphagia

Antisense oligodeoxynucleotides (AS ODNs) directed against exons 1 and 2 of the MOR-1 clone significantly and markedly reduced (81-93%) hyperphagia induced by the anti-metabolic glucose analogue, 2-deoxy-d-glucose (2DG) across a 4 h time course. AS ODNs directed against exons 3 or 4 of the MOR-1 clone had a more limited (1-2 h) duration of action upon 2DG-induced hyperphagia. 2DG-induced hyperphagia was significantly reduced by AS ODNs directed against exon 2 (44-51%), but not exons 1 or 3 of the KOR-1 clone across a 4 h time course. Whereas an AS ODN probe directed against the KOR3/ORL-1 clone produced small (36%), but significant reductions in 2DG-induced hyperphagia, an AS ODN probe directed against the DOR-1 clone was ineffective. These data provide further converging evidence for the roles of primarily mu, but also kappa1 and kappa3 opioid receptors in mediating the hyperphagic effects of glucoprivation.

Effect of peripheral 2-DG on opioid and neuropeptide Y gene expression

It is well known that 2-Deoxy-d-glucose (2-DG) blocks intracellular utilization of glucose and increases food intake. The aim of the present study was to determine whether administration of 2-DG alters gene expression of the orexigenic peptides, neuropeptide Y (NPY) and endogenous opioids, in the arcuate nucleus of the hypothalamus (ARC). Male Sprague-Dawley rats were injected peripherally (i.p.) with 2-DG (200 or 400 mg/kg body weight) and were sacrificed at 2 or 6 h post injection. Half of the animals were given ad libitum access to food whereas the other half of the animals were food-deprived. 2-DG increased food intake fourfold compared to saline injected animals, but did not affect NPY mRNA levels after 2 h. Messenger RNA levels of ProDynorphin (proDYN), but not pro-opiomelanocortin (POMC) nor proEnkephalin (proENK) were significantly decreased 2 h after 2-DG injection. Administration of 400 mg/kg of 2-DG increased mRNA levels of NPY in the arcuate nucleus after six h, but only in those animals not receiving food.

Effects of the opioid antagonist naltrexone on feeding induced by DAMGO in the central nucleus of the amygdala and in the paraventricular nucleus in the rat

The paraventricular nucleus of the hypothalamus (PVN) and the central nucleus of the amygdala (CNA) are two forebrain structures which are important in regulation of ingestive behavior. DAMGO is one of the most reliable and potent mu-selective opioid ligands that increases feeding in both of these brain nuclei. Administration of naloxone, an opioid antagonist, into the CNA prior to DAMGO blocks DAMGO-induced increases in food intake. The effect of this drug combination on food intake has not been evaluated in the PVN. However, intra-PVN injection of naloxone decreases deprivation and NPY-induced feeding. It has been suggested that CNA may modulate activity of midbrain and caudal brainstem centers via the hypothalamus. Based on these data, we evaluated whether an opioid-opioid interaction is present between the CNA and PVN which might affect feeding behavior. To test this, rats were doubly cannulated with 1 cannula placed in the PVN and 1 cannula in the CNA, allowing for co-administration of the opioid agonist into the PVN and the opioid antagonist into the CNA, and vice versa. CNA DAMGO increased feeding more than two-fold as compared to the vehicle-injected rats. When doses of 10, 12.5 and 25 micrograms of naltrexone (NTX) were injected into the PVN, CNA DAMGO no longer increased food intake above control levels. In the reverse situation, PVN DAMGO also increased food intake above control levels. However, when NTX was administrated unilaterally into the CNA at a relatively high dose (25 micrograms) or bilaterally (12.5 micrograms), PVN DAMGO-induced feeding was not altered. This suggests that an opioid-opioid signaling pathway exists from the CNA to the PVN which influences feeding via mu opioid receptors, whereas such a pathway from the PVN to the CNA does not seem to exist.

Evaluation of opioid receptor subtype antagonist effects in the ventral tegmental area upon food intake under deprivation, glucoprivic and palatable conditions

Opioid receptor subtype antagonists differentially alter food intake under deprivation (24 h), glucoprivic (2-deoxy-D-glucose, 500 mg/kg, i.p.) or palatable (10% sucrose) conditions with mu (beta-funaltrexamine) and kappa (nor-binaltorphamine), but not delta1 ([D-Ala2,Leu5,Cys6]enkephalin) opioid antagonists reducing each form of intake following ventricular microinjection. Both mu and kappa opioid antagonists microinjected into either the hypothalamic paraventricular nucleus or the nucleus accumbens reduce intake under deprivation and glucoprivic conditions. Palatable intake is reduced by both antagonists in the paraventricular nucleus, but only mu antagonists are active in the accumbens. Food intake is stimulated by mu and delta, but not kappa, opioid agonists microinjected into the ventral tegmental area. The present study examined whether food intake under either deprivation, glucoprivic or palatable conditions was altered by bilateral administration of general (naltrexone), mu, kappa, delta1 or delta2 (naltrindole isothiocyanate) opioid antagonists into the ventral tegmental area. Deprivation (24 h)-induced feeding was significantly reduced by high (50 microg), but not lower (10-20 microg) doses of naltrexone (21%), and by delta2 (4 microg, 19%) antagonism in the ventral tegmental area. 2-Deoxy-D-glucose (500 mg/kg, i.p.)-induced hyperphagia was significantly reduced by high (50 microg), but not lower (20 microg) doses of naltrexone (64%), and by delta2 (4 microg, 27%) antagonism in the ventral tegmental area. Sucrose (10%) intake was significantly reduced by naltrexone (20-50 microg, 25-39%) and delta2 (4 microg, 25%) antagonism in the ventral tegmental area. Neither mu, kappa nor delta1 antagonists were effective in reducing any form of intake following microinjection into the ventral tegmental area. These data indicate that the ventral tegmental area plays a relatively minor role in the elicitation of these forms of food intake, and that delta2, rather than mu, kappa or delta1 opioid receptors appear responsible for mediation of these forms of intake by this nucleus.

Sucrose consumption in mice: major influence of two genetic loci affecting peripheral sensory responses

Individual variability in sucrose consumption is prominent in humans and other species. To investigate the genetic contribution to this complex behavior, we conducted behavioral, electrophysiological, and genetic studies, using male progeny of two inbred mouse strains (C57BL/6ByJ [B6] and 129/J [129]) and their F2 hybrids. Two loci on Chromosome (Chr) 4 were responsible for over 50% of the genetic variability in sucrose intake. These loci apparently modulated intake by altering peripheral neural responses to sucrose. One locus affected the response threshold, whereas the other affected the response magnitude. These findings suggest that the majority of difference in sucrose intake between male B6 and 129 mice is due to polymorphisms of two genes that influence receptor or peripheral nervous system activity.

Selective interaction of homophtalazine derivatives with morphine

Homophtalazines show specific binding sites in the nigrostriatal system and to find their target of action the interactions between these derivatives, nerisopam and girisopam, and chlorpromazine, chlordiazepoxide and morphine were assessed. The compounds did not influence the chlorpromazine induced decrease in motility and catalepsy, nor did they alter the antiaggressive and anticonvulsive action of chlordiazepoxide. However, nerisopam and girisopam augmented the agonist potency of morphine to induce catalepsy or analgesia; they also altered the opioid antagonist potency of naloxone. The naloxone-induced decrease in sucrose consumption in drinking water was augmented by nerisopam and girisopam. It is suggested that a possible target of action of homophtalazines is the opioid signal transduction.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. The specific topics covered 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; sex, pregnancy, and development; immunological responses; and other behaviors.

General, mu and kappa opioid antagonists in the nucleus accumbens alter food intake under deprivation, glucoprivic and palatable conditions

Ventricular microinjection studies found that whereas mu (beta-funaltrexamine, B-FNA), mu1 (naloxonazine) and kappa (nor-binaltorphamine, Nor-BNI) opioid receptor antagonists, but not delta antagonists, reduce deprivation-induced intake, kappa and mu, but not mu1 or delta antagonists reduce both 2-deoxy-D-glucose (2DG) hyperphagia and sucrose intake. Since opioid agonists stimulate spontaneous food intake in the accumbens, the present study examined whether administration of either naltrexone, B-FNA or Nor-BNI in the accumbens altered intake under deprivation (24 h), glucoprivic (2DG: 500 mg/kg, i.p.) or palatable sucrose (10%) conditions. Naloxonazine's effects in the accumbens were also evaluated for deprivation-induced intake. Deprivation-induced intake was significantly decreased over 4 h by naltrexone (5-20 micrograms, 44%), B-FNA (1-4 micrograms, 55%) and Nor-BNI (4 micrograms, 31%) but not naloxonazine (10 micrograms) in the accumbens. 2DG hyperphagia was significantly decreased by naltrexone (10-20 microgram, 79%), B-FNA (1-4 micrograms, 100%) and NOR-BNI (104 micrograms, 75%) in the accumbens. Sucrose intake was significantly decreased by naltrexone (50 micrograms, 27%) and B-FNA (1-4 micrograms, 37%), but not NOR-BNI in the accumbens. These data suggest that mu receptors, and particularly the mu2 binding site in the accumbens are responsible for the opioid modulation of these forms of intake in this nucleus, and that this control may be acting upon the amount of intake per se.