micro-Opioid receptor activation prevents acute hepatic inflammation and cell death

Fentanyl overdose: Temporal effects and prognostic factors in SKH1 mice

Fentanyl exposure and overdose are growing concerns in public health and occupational safety. This study aimed to establish parameters of fentanyl lethality in SKH1 mice for future overdose research. Lethality was determined using the up-down procedure, with subjects monitored post-administration using pulse oximetry (5 min) and then whole-body plethysmography (40 min). Following the determination of subcutaneous dose-response, [18F]Fluorodeoxyglucose positron emission tomography (18 F-FDG PET) was performed after LD10 fentanyl at 40 min, 6 h, 24 h or 7 days post-dose. LD10 and LD50 were observed to be 110 and 135 mg/kg, respectively, and consistent with four-parameter logistic fit values of 111.2 and 134.6 mg/kg (r2  = 0.9996). Overdose (LD10 or greater) yielded three distinct cardiovascular groups: survival, non-survival with blood oxygen saturation (SpO2) minimum ≥37% and non-survival with SpO2 <37%. Breaths per minute, minute volume and inspiratory quotient were significantly different between surviving and non-surviving animals for up to 40 min post-injection. 18 F-FDG PET revealed decreased glucose uptake in the heart, lungs and brain for up to 24 h. These findings provide critical insights into fentanyl lethality in SKH1 mice, including non-invasive respiratory effects and organ-specific impacts that are invaluable for future translational studies investigating the temporal effects of fentanyl overdose.

Hepatic transcript signatures predict atherosclerotic lesion burden prior to a 2-year high cholesterol, high fat diet challenge

The purpose of this study was to identify molecular mechanisms by which the liver influences total lesion burden in a nonhuman primate model (NHP) of cardiovascular disease with acute and chronic feeding of a high cholesterol, high fat (HCHF) diet. Baboons (47 females, 64 males) were fed a HCHF diet for 2 years (y); liver biopsies were collected at baseline, 7 weeks (w) and 2y, and lesions were quantified in aortic arch, descending aorta, and common iliac at 2y. Unbiased weighted gene co-expression network analysis (WGCNA) revealed several modules of hepatic genes correlated with lesions at different time points of dietary challenge. Pathway and network analyses were performed to study the roles of hepatic module genes. More significant pathways were observed in males than females. In males, we found modules enriched for genes in oxidative phosphorylation at baseline, opioid signaling at 7w, and EIF2 signaling and HNF1A and HNF4A networks at baseline and 2y. One module enriched for fatty acid β oxidation pathway genes was found in males and females at 2y. To our knowledge, this is the first study of a large NHP cohort to identify hepatic genes that correlate with lesion burden. Correlations of baseline and 7w module genes with lesions at 2y were observed in males but not in females. Pathway analyses of baseline and 7w module genes indicate EIF2 signaling, oxidative phosphorylation, and μ-opioid signaling are possible mechanisms that predict lesion formation induced by HCHF diet consumption in males. Our findings of coordinated hepatic transcriptional response in male baboons but not female baboons indicate underlying molecular mechanisms differ between female and male primate atherosclerosis.

Hepatotoxic effect of tramadol and O-desmethyltramadol in HepG2 cells and potential role of PI3K/AKT/mTOR

1. The aim of this study was to compare the in vitro cytotoxic effect of tramadol and M1 metabolite in HepG2 cell line, the underlying mechanism, and PI3K/AKT/mTOR as potential target.2. Concentrations representing therapeutic level for tramadol (2 µM) and M1 metabolite (0.5 µM) were used. In addition, other increasing concentrations representing higher toxic levels were used (6, 10 µM for tramadol and 1.5, 2.5 µM for M1 metabolites). Cytotoxicity was assessed at 24, 48 and 72 h.3. Both tramadol and M1 metabolites were able to produce cytotoxicity in a dose and time dependent manner. Insignificant difference was detected between cells exposed to tramadol and M1 metabolite at therapeutic concentrations. Tramadol-induced apoptotic and autophagic cell death while M1 metabolite-induced apoptosis only. For PI3K/AKT/mTOR pathway, the therapeutic concentration of tramadol was only able to increase phosphorylation of AKT while higher toxic concentrations were able to increase phosphorylation of whole pathway; Meanwhile, M1 metabolite was able to increase the phosphorylation of the whole pathway significantly in therapeutic and toxic concentrations.4. In conclusion, both tramadol and M1 are equally cytotoxic. Apoptosis and autophagy both mediate hepatic cell death. PI3K/AKT pathway is involved in apoptosis induction while autophagy is regulated through mTOR independent pathway.

The synthetic opioid fentanyl enhances viral replication in vitro

The US is in the midst of a major drug epidemic fueled in large part by the widespread recreational use of synthetic opioids such as fentanyl. Persons with opioid use disorder are at significant risk for transmission of injection-associated infections such as hepatitis B virus (HBV) and hepatitis C virus (HCV). Commonly abused substances may antagonize immune responses and promote viral replication. However, the impact of synthetic opioids on virus replication has not been well explored. Thus, we evaluated the impact of fentanyl and carfentanil using in vitro systems that replicate infectious viruses. Fentanyl was used in cell lines replicating HBV or HCV at concentrations of 1 ng, 100 ng, and 10 ug. Viral protein synthesis was quantified by ELISA, while apoptosis and cell death were measured by M30 or MTT assays, respectively. HCV replicative fitness was evaluated in a luciferase-based system. RNAseq was performed to evaluate cellular gene regulation in the presence of fentanyl. Low dose fentanyl had no impact on HCV replication in Huh7.5JFH1 hepatocytes; however, higher doses significantly enhanced HCV replication. Similarly, a dose-dependent increase in HCV replicative fitness was observed in the presence of fentanyl. In the HepG2.2.15 hepatocyte cell line, fentanyl caused a dose-dependent increase in HBV replication, although only a higher doses than for HCV. Addition of fentanyl resulted in significant apoptosis in both hepatocyte cell lines. Cell death was minimal at low drug concentrations. RNAseq identified a number of hepatocyte genes that were differentially regulated by fentanyl, including those related to apoptosis, the antiviral / interferon response, chemokine signaling, and NFκB signaling. Collectively, these data suggest that synthetic opioids promote viral replication but may have distinct effects depending on the drug dose and the viral target. As higher viral loads are associated with pathogenesis and virus transmission, additional research is essential to an enhanced understanding of opioid-virus pathogenesis and for the development of new and optimized treatment strategies.

The paradoxical role of methionine enkephalin in tumor responses

Methionine enkephalin (MENK) is an opioid peptide composed of five amino acids with multiple biological activities. Since its discovery, MENK has become prominent in neuroregulation and immunoregulation. Tumors have increasingly been a spotlight because of their terrible trends and refractory characteristic. The therapeutic potential of MENK was investigated on a large scale, and there are numerous evidences that MENK exerts anti-tumor effects via two mechanisms. The first mechanism explains the enhanced anti-tumor immune effects of MENK. The second mechanism shows that MENK directly inhibits tumor cell proliferation. However, numerous reports have clarified the pro-tumor role of MENK by inhibiting T and B cell proliferation, promoting tumor cell growth by binding to opioid receptors, leading to desensitization of lymphocytes, and inducing tolerance. It is particularly intriguing that dual reactions are triggered when MENK combines with its opioid receptors; thus, anti-tumor response of the whole body is influenced. This review will expound the dual roles of MENK in tumor responses based on immune cells, cytokines, and tumor cells to provide better suggestions for its application in tumor treatment.

Topical Loperamide-Encapsulated Liposomal Gel Increases the Severity of Inflammation and Accelerates Disease Progression in the Adjuvant-Induced Model of Experimental Rheumatoid Arthritis

This study evaluates the prophylactic effect of the peripherally-selective mu-opioid receptor agonist, loperamide, administered topically in a liposomal gel formulation on pain, inflammation, and disease progression in the adjuvant-induced model of experimental rheumatoid arthritis in female Lewis rats. In a randomized, blinded and controlled animal trial, AIA rats were divided into six groups consisting of eleven rats per group based on the following treatments: loperamide liposomal gel, free loperamide gel, empty liposomal gel, diclofenac gel (Voltaren®), no treatment, and naive control. Topical formulations were applied daily for a maximum of 17 days-starting from day 0 at the same time as immunization. The time course of the effect of the treatments on antinocieption and inflammation was assessed using a paw pressure analgesiometer and plethysmometer, respectively. Arthritis progression was scored daily using an established scoring protocol. At the end of the study, hind paws were processed for histological analysis. Administration of loperamide liposomal gel daily across the duration of the study produced significant peripheral antinociception as expected; however, increased the severity of inflammation and accelerated arthritis progression. This was indicated by an increase in paw volume, behavioral and observational scoring, and histological analysis compared to the control groups. In particular, histology results showed an increase in pannus formation and synovial inflammation, as well as an upregulation of markers of inflammation and angiogenesis. These findings may have implications for the use of loperamide and other opioids in arthritis and potentially other chronic inflammatory diseases.

The effects of tramadol on hepatic ischemia/reperfusion injury in rats

Objectives:

Tramadol is a centrally acting synthetic analgesic. It has a cardioprotective effect against myocardial ischemia-reperfusion (I/R) injury in isolated rat heart. We hypothesized that tramadol may exert a similar protective effect on hepatic I/R injury. Hence, the current investigation was designed to study the possible protective effects of tramadol on experimentally-induced hepatic I/R injury in rats.

Materials and Methods:

Tramadol was administered 30 min before ischemia following which the rats were subjected to 45 min of ischemia followed by 1 h of reperfusion.

Results:

Tramadol attenuated hepatic injury induced by I/R as evidenced by the reduction of transaminases, structural changes, and apoptotic cell death. It decreased the level of inflammatory markers such as tumor necrosis factor-alpha (TNF-α), TNF-α/interleukin-10 (IL-10) ratio, and nuclear factor-κB gene expression. It also increased the anti-inflammatory cytokine, IL-10 levels in hepatic tissues. Furthermore, it reduced oxidative stress parameters except manganese superoxide dismutase activity.

Conclusion:

The results suggest that tramadol has hepatoprotective effects against hepatic I/R injury via anti-inflammatory, antiapoptotic, and antioxidant effects.

Involvement of Mu Opioid Receptor Signaling in the Protective Effect of Opioid against 6-Hydroxydopamine-Induced SH-SY5Y Human Neuroblastoma Cells Apoptosis

INTRODUCTION

The neuroprotective role of opioid morphine against 6-hydroxydopamine-induced cell death has been demonstrated. However, the exact mechanism(s) underlying such neuroprotection, especially the role of subtype receptors, has not yet been fully clarified.

METHODS

Here, we investigated the effects of different opioid agonists on 6-OHDA-induced neurotoxicity in human neuroblastoma SH-SY5Y cell line as an in vitro model of Parkinson's disease. Cell damage was induced by 150 μM 6-OHDA and the cells viability was examined by MTT assay. Intracellular calcium, reactive oxygen species and mitochondrial membrane potential were assessed by fluorescence spectrophotometry method. Immunoblot technique was used to evaluate cytochrome-c and activated caspase-3 as biochemical markers of apoptosis induction.

RESULTS

The data showed that 6-OHDA caused significant cell damage, loss of mitochondrial membrane potential and increase in intracellular reactive oxygen species and calcium levels as well as activated caspase-3 and cytochrome-c release. Incubation of SH-SY5Y cells with μ-opioid agonists, morphine and DAMGO, but not with δ-opioid agonist, DADLE, elicited protective effect and reduced biochemical markers of cell damage and death.

DISCUSSION

The results suggest that μ-opioid receptors signaling participate in the opioid neuroprotective effects against 6-OHDA-induced neurotoxicity.

Activation of μ opioid receptors modulates inflammation in acute experimental colitis

BACKGROUND

μ opioid receptors (μORs) are expressed by neurons and inflammatory cells, and mediate immune response. We tested whether activation of peripheral μORs ameliorates the acute and delayed phase of colitis.

METHODS

C57BL/6J mice were treated with 3% dextran sodium sulfate (DSS) in water, 5 days with or without the peripherally acting μOR agonist, [D-Ala2, N-Me-Phe4, Gly5-ol]-Enkephalin (DAMGO) or with DAMGO+μOR antagonist at day 2-5, then euthanized. Other mice received DSS followed by water for 4 weeks, or DSS with DAMGO starting at day 2 of DSS for 2 or 3 weeks followed by water, then euthanized at 4 weeks. Disease activity index (DAI), histological damage, and myeloperoxidase assay (MPO), as index of neutrophil infiltration, were evaluated. Cytokines and μOR mRNAs were measured with RT-PCR, and nuclear factor-kB (NF-kB), the antiapoptotic factor Bcl-xL, and caspase 3 and 7 with Western blot.

KEY RESULTS

DSS induced acute colitis with elevated DAI, tissue damage, apoptosis and increased MPO, cytokines, μOR mRNA, and NF-kB. DAMGO significantly reduced DAI, inflammatory indexes, cytokines, caspases, and NF-kB, and upregulated Bcl-xL, effects prevented by μOR antagonist. In DSS mice plus 4 weeks of water, DAI, NF-kB, and μOR were normal, whereas MPO, histological damage, and cytokines were still elevated; DAMGO did not reduce inflammation, and did not upregulate Bcl-xL.

CONCLUSIONS & INFERENCES

μOR activation ameliorated the acute but not the delayed phase of DSS colitis by reducing cytokines, likely through activation of the antiapoptotic factor, Bcl-xL, and suppression of NF-kB, a potentiator of inflammation.

Targeting inflammation and wound healing by opioids

Opioid receptors are expressed on peripheral sensory nerve endings, cutaneous cells, and immune cells; and local application of opioids is used for the treatment of inflammatory pain in arthritis, burns, skin grafts, and chronic wounds. However, peripherally active opioids can also directly modulate the inflammatory process and wound healing. Here, we discuss the underlying mechanisms of opioid action and the conceivable therapeutic approaches for opioid treatment, as investigated in experimental and clinical studies. A large number of in vitro experiments and animal model investigations have produced evidence that peripherally active opioids can reduce plasma extravasation, vasodilation, proinflammatory neuropeptides, immune mediators, and tissue destruction. In contrast to currently available anti-inflammatory agents, opioids have not demonstrated organ toxicity, thus making them interesting candidates for drug development. Few clinical studies have tapped into this potential to date.

Effect of remifentanil on mitochondrial oxygen consumption of cultured human hepatocytes

During sepsis, liver dysfunction is common, and failure of mitochondria to effectively couple oxygen consumption with energy production has been described. In addition to sepsis, pharmacological agents used to treat septic patients may contribute to mitochondrial dysfunction. This study addressed the hypothesis that remifentanil interacts with hepatic mitochondrial oxygen consumption. The human hepatoma cell line HepG2 and their isolated mitochondria were exposed to remifentanil, with or without further exposure to tumor necrosis factor-α (TNF-α). Mitochondrial oxygen consumption was measured by high-resolution respirometry, Caspase-3 protein levels by Western blotting, and cytokine levels by ELISA. Inhibitory κBα (IκBα) phosphorylation, measurement of the cellular ATP content and mitochondrial membrane potential in intact cells were analysed using commercial ELISA kits. Maximal cellular respiration increased after one hour of incubation with remifentanil, and phosphorylation of IκBα occurred, denoting stimulation of nuclear factor κB (NF-κB). The effect on cellular respiration was not present at 2, 4, 8 or 16 hours of incubation. Remifentanil increased the isolated mitochondrial respiratory control ratio of complex-I-dependent respiration without interfering with maximal respiration. Preincubation with the opioid receptor antagonist naloxone prevented a remifentanil-induced increase in cellular respiration. Remifentanil at 10× higher concentrations than therapeutic reduced mitochondrial membrane potential and ATP content without uncoupling oxygen consumption and basal respiration levels. TNF-α exposure reduced respiration of complex-I, -II and -IV, an effect which was prevented by prior remifentanil incubation. Furthermore, prior remifentanil incubation prevented TNF-α-induced IL-6 release of HepG2 cells, and attenuated fragmentation of pro-caspase-3 into cleaved active caspase 3 (an early marker of apoptosis). Our data suggest that remifentanil increases cellular respiration of human hepatocytes and prevents TNF-α-induced mitochondrial dysfunction. The results were not explained by uncoupling of mitochondrial respiration.

Soymorphin-5, a soy-derived μ-opioid peptide, decreases glucose and triglyceride levels through activating adiponectin and PPARα systems in diabetic KKAy mice

Soymorphin-5 (YPFVV) derived from soybean β-conglycinin β-subunit is a μ-opioid agonist peptide having anxiolytic-like activity. Here, we show that soymorphin-5 improves glucose and lipid metabolism after long-term oral administration to KKAy mice, a type 2 diabetes model animal. Soymorphin-5 inhibited hyperglycemia without an increase in plasma insulin levels in KKAy mice. Soymorphin-5 also decreased plasma and liver triglyceride (TG) levels and liver weight, suggesting that soymorphin-5 improved lipid metabolism. Soymorphin-5 increased plasma adiponectin concentration and liver mRNA expression of AdipoR2, a subtype of adiponectin receptor that is involved in stimulating the peroxisome proliferator-activated receptor (PPAR)α pathway and fatty acid β-oxidation. The expressions of the mRNA of PPARα and its target genes acyl-CoA oxidase, carnitine palmitoyltransferase 1 A, and uncoupling protein-2, in the liver were also increased after oral administration of soymorphin-5. Furthermore, des-Tyr-soymorphin-5 (PFVV) without μ-opioid and anxiolytic-like activities did not decrease blood glucose levels in KKAy mice. These results suggest that μ-opioid peptide soymorphin-5 improves glucose and lipid metabolism via activation of the adiponectin and PPARα system and subsequent increases of β-oxidation and energy expenditure in KKAy mice.

Opioid receptors and opioid peptide-producing leukocytes in inflammatory pain--basic and therapeutic aspects

This review summarizes recent findings on neuro-immune mechanisms underlying opioid-mediated inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms by immune cell-derived opioid peptides. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generators of impulses relaying nociceptive information towards the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. This is in part achieved by endogenously released immune cell-derived opioid peptides within inflamed tissue. In addition, exogenous opioid receptor ligands that selectively modulate primary afferent function and do not cross the blood-brain barrier, avoid centrally mediated untoward side effects of conventional analgesics (e.g., opioids, anticonvulsants). This article discusses peripheral opioid receptors and their signaling pathways, opioid peptide-producing/secreting inflammatory cells and arising therapeutic perspectives.

Evaluation of the adjuvant activity of naloxone, an opioid receptor antagonist, in combination with heat-killed Listeria monocytogenes vaccine

We have previously demonstrated the adjuvant activity of naloxone (NLX), a general opioid antagonist, using a DNA vaccine for herpes simplex virus type 1. Here, the adjuvant activity of NLX has been evaluated using a heat-killed Listeria monocytogenes (HKLM) vaccine as a model for general immunization against intracellular bacteria. BALB/c mice were divided into three groups: the Vac group received the HKLM vaccine alone; the NLX-Vac group received the HKLM vaccine in combination with the adjuvant NLX; and the control group received phosphate buffered saline (PBS). Our results indicate that the administration of NLX as an adjuvant enhances the ability of the HKLM vaccine to increase lymphocyte proliferation, delayed type hypersensitivity, and skewing of the immune response toward a T-helper 1 (Th1) pattern. Additionally, combination of NLX with the HKLM vaccine improves protective immunity against L. monocytogenes. In conclusion, administration of NLX as an adjuvant for the HKLM vaccine can enhance cell-mediated immunity and shift the immune response to Th1.

A novel adjuvant, the general opioid antagonist naloxone, elicits a robust cellular immune response for a DNA vaccine

While many adjuvants have been discovered and used in research, only a few adjuvants have been permitted for use with human vaccination. We have previously shown that the administration of naloxone (NLX), a general opioid antagonist, during infection with a non-virulent strain of herpes simplex virus type 1 (HSV-1) could enhance protection against HSV-1 challenge. Here, the adjuvant activity of NLX has been evaluated using a DNA vaccine for HSV-1 as a model. BALB/c mice were divided into four groups; for experimental groups, mice received the glycoprotein D1 (gD1) DNA vaccine alone or in combination with the adjuvant NLX. A positive control group received the KOS strain of HSV-1, and a negative control group received PBS. All mice were immunized three times on days 0, 21 and 42. Three weeks after the last immunization, immune responses against HSV-1 were assessed. Our results indicate that the administration of NLX as an adjuvant increased the ability of the gD1 DNA vaccine to enhance cytolytic T lymphocyte activity, lymphocyte proliferation, delayed-type hypersensitivity and shifting the immune response toward a T helper (Th)1 pattern and improved protective immunity against HSV-1. NLX also increased the IgG2a/IgG1 ratio, though it did not affect the production of HSV-1 antiserum. In conclusion, administration of NLX as an adjuvant in combination with the gD1 DNA vaccine can enhance cell-mediated immunity and shift the immune responses to Th1.

Opioids and sensory nerves

This chapter reviews the expression and regulation of opioid receptors in sensory neurons and the interactions of these receptors with endogenous and exogenous opioid ligands. Inflammation of peripheral tissues leads to increased synthesis and axonal transport of opioid receptors in dorsal root ganglion neurons. This results in opioid receptor upregulation and enhanced G protein coupling at peripheral sensory nerve terminals. These events are dependent on neuronal electrical activity, and on production of proinflammatory cytokines and nerve growth factor within the inflamed tissue. Together with the disruption of the perineurial barrier, these factors lead to an enhanced analgesic efficacy of peripherally active opioids. The major local source of endogenous opioid ligands (e.g. beta-endorphin) is leukocytes. These cells contain and upregulate signal-sequence-encoding messenger RNA of the beta-endorphin precursor proopiomelanocortin and the entire enzymatic machinery necessary for its processing into the functionally active peptide. Opioid-containing immune cells extravasate using adhesion molecules and chemokines to accumulate in inflamed tissues. Upon stressful stimuli or in response to releasing agents such as corticotropin-releasing factor, cytokines, chemokines, and catecholamines, leukocytes secrete opioids. Depending on the cell type, this release is contingent on extracellular Ca(2+) or on inositol triphosphate receptor triggered release of Ca(2+) from endoplasmic reticulum. Once secreted, opioid peptides activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of proinflammatory neuropeptides. These effects occur without central untoward side effects such as depression of breathing, clouding of consciousness, or addiction. Future aims include the development of peripherally restricted opioid agonists, selective targeting of opioid-containing leukocytes to sites of painful injury, and the augmentation of peripheral opioid peptide and receptor synthesis.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Increased adiposity on normal diet, but decreased susceptibility to diet-induced obesity in mu-opioid receptor-deficient mice

The mu-opioid receptor encoded by the Oprm1 gene plays a crucial role in the mediation of food reward and drug-induced positive reinforcement, but its genetic deletion has been shown to provide food intake-independent, partial protection from diet-induced obesity. We hypothesized that mu-opioid receptor-deficient mice would show an even greater, intake-dependent, resistance to high-fat diet-induced obesity if the diet comprises a sweet component. We generated an F2 population by crossing the heterozygous offspring of homozygous female Oprm1(-/-) mice (on a mixed C57BL/6 and BALB/c genetic background) with male inbred C57BL/6 mice. Groups of genotyped wild-type (WT) and homozygous mutant (KO) males and females were fed either control chow or a high caloric palatable diet consisting of sweet, liquid chocolate-flavored Ensure together with a solid high-fat diet. Food intake, body weight, and body composition was measured over a period of 16 weeks. Unexpectedly, male, and to a lesser extent female, KO mice fed chow for the entire period showed progressively increased body weight and adiposity while eating significantly more chow. In contrast, when exposed to the sweet plus high-fat diet, male, and to a lesser extent female, KO mice gained significantly less body weight and fat mass compared to WT mice when using chow fed counterparts for reference values. Male KO mice consumed 33% less of the sweet liquid diet but increased intake of high-fat pellets, so that total calorie intake was not different from WT animals. These results demonstrate a dissociation of the role of mu-opioid receptors in the control of adiposity for different diets and sex. On a bland diet, normal receptor function appears to confer a slightly catabolic predisposition, but on a highly palatable diet, it confers an anabolic metabolic profile, favoring fat accretion. Because of the complexity of mu-opioid gene regulation and tissue distribution, more selective and targeted approaches will be necessary to fully understand the underlying mechanisms.