Determination of structure-activity relationships between fentanyl analogs and human μ-opioid receptors based on active binding site models

Fentanyl is a potent and widely used clinical narcotic analgesic,as well as a highly selective μ-opioid agonist.The present study established a homologous model of the human μ-opioid receptor;an intercomparison of three types of μ-opioid receptor protein sequence homologous rates was made.The secondary receptor structure was predicted,the model reliability was assessed and verified using the Ramachandran plot and ProTab analysis.The predictive ability of the CoMFA model was further validated using an external test set.Using the Surflex-Dock program,a series of fentanyl analog molecules were docked to the receptor,the calculation results from Biopolymer/SiteID showed that the receptor had a deep binding area situated in the extracellular side of the transmembrane domains(TM) among TM3,TM5,TM6,and TM7.Results suggested that there might be 5 active areas in the receptor.The important residues were Asp147,Tyr148,and Tyr149 in TM3,Trp293,and His297 in TM6,and Trp318,His319,Ile322,and Tyr326 in TM7,which were located at the 5 active areas.The best fentanyl docking orientation position was the piperidine ring,which was nearly perpendicular to the membrane surface in the 7 TM domains.Molecular dynamic simulations were applied to evaluate potential relationships between ligand conformation and fentanyl substitution.

δ-Opioid receptor as a potential therapeutic target for ischemic stroke

Ischemic stroke is a global epidemic condition due to an inadequate supply of blood and oxygen to a specific area of brain either by arterial blockage or by narrowing of blood vessels.Despite having advancement in the use of thrombolytic and clot removal medicine,significant numbers of stroke patients are still left out without option for treatment.In this review,we summarize recent research work on the activation ofδ-opioid receptor as a strategy for treating ischemic stroke-caused neuronal injury.Moreover,as activation ofδ-opioid receptor by a non-peptidicδ-opioid receptor agonist also modulates the expression,maturation and processing of amyloid precursor protein andβ-secretase activity,the potential role of these effects on ischemic stroke caused dementia or Alzheimer’s disease are also discussed.

Human μ-opioid receptor overexpressed in Sf9 insect cells functionally coupled to endogenous G_(i/o) proteins

Human μ-opioid receptor （HμOR） with a tag of six consecutive histidines at its carboxyl terminus had been expressed in recombinant baculovirus infected Sf9 insect cells.The maximal binding capacity for the [3H] diprenorphine and [3H]ohmefentanyl （Ohm） were 9.1± 0.7 and 6.52±0.23 nmol/g protein, respectively. The [3H] diprenorphine or [3H] Ohm binding to the receptor expressed in Sf9 cells was strongly inhibited by μ-selective agonists [D-Ala2], N-methylPhe4, glyol5]enkephalin （DAGO）, Ohm, and morphine, but neither by δ nor by K selective agonist. Na+ （100 mM） and GTP （50 μM） could reduce HμOR agonists etorphine and Ohm affinity binding to the overexpressed HμOR. μ-selective agonists DAGO and Ohm effectively stimulated [35S]GTPγS binding (EC50 = 2.7nM and 6.9 nM) and inhibited forskolin- stimulated cAMP accumulation (IC50 = 0.9 nM and 0.3 nM). The agonist-dependent effects could be blocked by opioid antagonist naloxone or by pretreatment of cells with pertussis toxin （PTX）. These results demonstrated that HμOR overexpressed in Sf9 insect cells functionally coupled to endogenous Gi/o proteins.

Expression of mu-opioid receptors in human chronic inflamed knee joint synovium tissue

Objective: To examine the changes of mu-opioid receptors (MORs) expression in human chronic inflamed knee joint synovium tissue. Methods:Knee joint synovium tissues were taken from 21 patients with chronic arthritis (inflamed group) and 6 fresh bodies with normal knee joints (control group). And the expression of MORs was detected by using immunohistochemistry. flow cytometry(FCM) and reverse-transcription polymerase chain reaction (RT-PCR). Results: The expression of MORs in the inflamed group was significantly higher than that in the normal group by using the 3 techniques(P<0. 05). Conclusion: Chronic inflammation enhances the up-regulation of MORs in human knee joint synovium tissue.

Molecular characterization and functional expression of opioid receptor-libe_1 receptor

The opioid receptor-libel receptor (ORL), an orphan receptor whose human and murine complementary DNAs,has been characterized recently. ORL transcripts are particularly abundant in the central nervous system. We demonstrated that ORL expressed in human neuroblastoma SK-N-SH and SH-SY5Y cell lines by radioligand binding assay, reverse transcription polymerase chain reaction (RT-PCR) and Northern analysis in the present study. Stimulation with ORL1 specific agonist, nociceptin/orphanin Fo, increased [34S]GTPrγS binding to SK-N-SH cell membranes (EC50 = 14 ±0.45 nM), and attenuated forskolin-stimulated accumulation of cellular cAMP (EC50= 0.80 ±0.45 nM, indicative that activation of ORL1 activates G proteins and inhibits adenylyl cyclase. Activation of ORL1 receptor was also accessed using CHO:hORL1 cell line by microphysiometer. Treatment of nociceptin/orphanin FQ increased extracellular acidification rate significantly.

Low-dose morphine elicits ventilatory excitant and depressant responses in conscious rats: Role of peripheral <i>µ</i>-opioid receptors

The systemic administration of morphine affects ventilation via a mixture of central and peripheral actions. The aims of this study were to characterize the ventilatory responses elicited by a low dose of morphine in conscious rats;to determine whether tolerance develops to these responses;and to determine the potential roles of peripheral μ-opioid receptors (μ-ORs) in these responses. Ventilatory parameters were monitored via unrestrained whole-body plethysmography. Conscious male Sprague-Dawley rats received an intravenous injection of vehicle or the peripherally-restricted μ-OR antagonist, naloxone methiodide (NLXmi), and then three successive injections of morphine (1 mg/kg) given 30 min apart. The first injection of morphine in vehicle-treated rats elicited an array of ventilatory excitant (i.e., increases in frequency of breathing, minute volume, respiratory drive, peak inspiratory and expiratory flows, accompanied by decreases in inspiratory time and end inspiratory pause) and inhibitory (i.e., a decrease in tidal volume and an increase in expiratory time) responses. Subsequent injections of morphine elicited progressively and substantially smaller responses. The pattern of ventilatory responses elicited by the first injection of morphine was substantially affected by pretreatment with NLXmi whereas NLXmi minimally affected the development of tolerance to these responses. Low-dose morphine elicits an array of ventilatory excitant and depressant effects in conscious rats that are subject to the development of tolerance. Many of these initial actions of morphine appear to involve activation of peripheral μ-ORs whereas the development of tolerance to these responses does not.

Changes of mu and kappa opioid receptors in cathartic colon of rat

Objective: To oberve the changes of mu and kappa opioid receptors in the cathartic colon of rat, and to clarify that whether opioid receptors accounts for the occurrence of slow trait constipation (STC). Methods: The cathartiic colon model of rat was made by feeding with laxatives. The activity of mu and kappa opioid receptors in the cathartic colon of rat was measured by radio-ligand binding assay. Results: Compared with the control group, the maximal binding capacity (Bmax) and affinity(Kd) of mu opioid receptor in cathartic colon group were significantly increased (207. 00 ± 22. 90 fmol/mg·p vs 82. 00 ± 14.23 fmol/mg· p, P ＜ 0.01 ;3.30 ± 0.45 mmol/L vs 2.40 ± 0.57 mmol/L, P ＜ 0.05). The maximal binding capacity of kappa opioid receptor also showed a great increase (957. 00 ± 102. 41 fmol/mg· p vs 459.00 ± 52.41 fmol/mg·p, P ＜ 0.01 ), but no significant difference of affinity was found between the two groups. Conclsion: The mu and kappa opioid receptors may be involved in the functional disorders of cathartic colon.

Immunohistochemical identification of dynorphin A and Kappa opioid receptor-1 in the digestive system of scallop Chlamys farreri

Little is known about the roles of dynorphin and Kappa opioid receptor(KOR) in mollusks. In this study, we aim to determine the distribution of dynorphin A and KOR-1 in the digestive system of the scallop Chlamys farreri. Using immunohistochemical staining, we confirmed the expression of dynorphin A and KOR-1 in the digestive system of C. farreri. Dynorphin A immunopositive cells were identified in intestine and hepatopancreas. In intestine, small and spherical dynorphin A immunopositive cells(4–9 μm in diameter) were scattered among the long columnar epithelial cells(ECs). In hepatopancreas, cells containing masses(5–14 μm in diameter) of dynorphin A immunopositive products were observed in epithelium of acinis. These immunopositive cells may be synthetic and/or secretory cells of dynorphin A. Dynorphin A immunoreactive products were commonly observed in epithelium and connective tissue(CT) of labial palps, mouth labia and stomach, which presented in forms of grains, fibers or flakes. KOR-1 immunoreactive material was observed in ECs and CTs of labial palps, mouth labia and stomach, intestine and hepatopancreas. The distribution of both dynorphin A and KOR-1 in the digestive organs suggests an involvement of dynorphin via KOR-1 in the functional regulation of the digestive system of C. farreri.

Thienorphine induces analgesia by binding κ -and δ-, or by partially binding μ-opioid receptor,thus further regulating cAMP-PKA activity

OBJECTIVE Thienorphine,a new oripavine derivative,has shown to possess stronger antinociceptive effects and better oral bioavailability compared to buprenorphine.The present study examines the effect of thienorphine on c AMP-dependent protein kinase A(PKA) activity in CHO cells expressing μ-,κ-,δ-and ORL1 receptors.In addition,we further examined its analgesic effect in vivo.METHODS The effect of thienorphine on cA MP-dependent PKA redistribution and cA MP inhibition were analyzed in CHO-PKAcatEGFP cells.PKA redistribution assays in CHO-PKAcatEGFP cells stably expressing μ-,κ-,δ-and ORL1 receptors were analyzed by high-throughput screening system to elucidate the efficacy of agonists or antagonists on opioid receptors.Moroever,the antinociceptive effects of thienorphine in vivo were examined using hot plate test.RESULTS Briefly,the maximum inhibition of thienorphine on PKA activity was about 36%,100%,100%and 12% in CHO-μ/κ/δ/ORL1-PKAcatE GFP cel s,respectively.In addition,thienorphine concentrationdependently inhibited the PKA activity with EC50 value of(22.7±18.1) nmol·L^(-1) in CHO-κ-PKAcatE GFP cels and(12.4±7.7) nmol·L^(-1) in CHO-δ-PKAcatE GFP cells.Thienorphine induced approximately 50%antinociceptive effect in mice lacking μ receptors compared to their wild-type controls(P<0.05).Also,the κ and δ selective antagonist nor-binaltorphimine,naltrindole decreased approximately 50%-60% in % MPE of theinorphine in μ-KO mice,respectively.The ORL1 receptor selective antagonist J113397 had no effect in %MPE of theinorphine in μ-KO mice.CONCLUSION Thienorphine induces analgesia through bindingκ-and δ-,or by partially binding μ-opioid receptor,thus further regulating the cAMP-PKA activity.Therefore,thienorphine may be used in acute or chronic pain with minimal addictive potential.

YFa and analogs:Investigation of opioid receptors in smooth muscle contraction

AIM:To study the pharmacological profile and inhibition of smooth muscle contraction by YFa and its analogs in conjunction with their receptor selectivity. METHODS:The effects of YFa and its analogs (D-Ala2) YFa, Y (D-Ala2) GFMKKKFMRF amide and Des-Phe-YGGFMKKKFMR amide in guinea pig ileum (GPI) and mouse vas deferens (MVD) motility were studied using an isolated tissue organ bath system, and morphine and DynA (1-13) served as controls. Acetylcholine was used for muscle stimulation. The observations were validated by specific antagonist pretreatment experiments using naloxonazine, naltrindole and norbinaltor-phimine norBNI. RESULTS:YFa did not demonstrate significant inhibition of GPI muscle contraction as compared with mor-phine (15% vs 62%, P = 0.0002), but moderate inhibition of MVD muscle contraction, indicating the role of κ opioid receptors in the contraction. A moderate inhibition of GPI muscles by (Des-Phe) YFa revealed the role of anti-opiate receptors in the smooth muscle contraction. (D-Ala-2) YFa showed significant inhibition of smooth muscle contraction, indicating the involvement of mainly δ receptors in MVD contraction. These results were supported by specific antagonist pretreatment assays. CONCLUSION:YFa revealed its side-effect-free analgesic properties with regard to arrest of gastroin-testinal transit. The study provides evidences for the involvement of κ and anti-opioid receptors in smooth muscle contraction.

Functional expression of opioid receptor-like receptorand its endogenous specific agonist nociceptin/orphanin FQ during mouse embryogenesis

Expression of opioid receptor-like receptor (ORL1)and its endogenous peptide agonist nociceptin/orphaninFo (N/OFQ) during mouse embryogenesis have been investigated. Transcripts of ORL1 and N/OFQ were detected by RT-PCR in mouse brain of day 8 embryo (E8)and the expression continued afterwards. Northern blotanalysis revealed abundant expression of ORL1 at postnatal day 1 (P1) and N/OFQ at E17 and P1 in the brain butnone was detected in other embryonic tissues. The presence of functional ORL1 in mouse embryonic brain wasalso confirmed by specific binding of [3H] N/OFQ (kd=1.3±0.5 nM and Bmax = 72±9 fmol/mg protein) as wellas by N/OFQ-stimulated G protein activation.

Subtype of Opioid Receptor in Airway SmoothMuscle and the Role of the Receptor in Asthmatic Attacks

In order to elucidate the behavior of opioid receptor in the airway smooth muscle (ASM) and potential role of the receptor in asthmatic attacks electrical field stimulation (EFS) was used to evaluate the effects of different narcotics and naloxine (Nal) on isolated rabbit ASM and biochemical methods were used to assay the influences of morphine (Mor) and pethidine(Pet) on the activities of adenylcyclase (AAC) and phosphodiesterase(APDE) in homogenate derived from rabbit ASM.Nal was used to treat the bronchospasm during anesthesia. It shows that Mor increased the rabbit ASM contraction and Nal reversed this effect, while Nal itself did not affect ASM. Fentanyl(Fen) decreased the contraction and Pet not only decreased the contraction but relaxed the ASM. Mor decreased the AAC in the rabbit ASM but didn’t affect the APDE in the rabbit ASM. Pet had no influence on both the AAC and the APDE. Nal effectively relieved the bronchospasm which failed to the traditional treatment during anesthsia. These indicate that the opioid receptor in the ASM is a new subtype one.Mor exhibits satuable binding the subtype receptor and exerts strong agonistic activity to induce bronchospasm, while Nal antagonizes this effect. Yet Fen and Pet don’s bind this subtype receptor. Endogenous opioid-like peptides may also bind this subtype receptor. In patients with airway hyperreactivity (PAHR) Mor is contraindicated, Fen and Pet may be used. and the latter may be the best choice.Asthma or bronchospasm may be treated with Nal.

Effects of Peripherally Acting Opioid Ligands on Central Opioid Receptors and <i>β</i>-Endorphin Release in Stressed Rats

Using the radioreceptor binding assay, μ-opioid receptor (MOR) affinity in the midbrain of stressed rats was higher than in naive controls. MOR density in the rat frontal cortex was reduced after stress. Intragastric administration of the MOR antagonist naloxone methiodide was followed by an increase in the number of MORs in the frontal cortex. However, the MOR agonist loperamide significantly decreased the density of MORs in the frontal cortex and midbrain of naive animals. Loperamide and naloxone methiodide were shown to prevent an increase in MOR affinity and a decrease in MOR density in the midbrain of rats after restraint stress. The restraint stress was accompanied by an increase in the release of β-endorphin (BE) in the ventral tegmental area (VTA) of control rats. After administration, loperamide slightly decreased the release of BE, naloxone methiodide significantly increased the release of BE in the cingulate cortex (CC) of untreated animals, while drugs had no effect on the release of BE in the VTA. The drugs significantly increased the extracellular level of BE in the CC of stressed animals. Loperamide abolished the increase in the stress-induced release of BE in the VTA. By contrast, naloxone methiodide significantly increased the release of BE in the VTA of stressed rats. Our data indicated that activation of peripheral MORs induces depression of the central part of the μ-opioid system, but suppression of peripheral MOR activity induces activation of the central μ-opioid system, the interaction of which can be modulated by stress.

EFFECT OF ELECTROACUPUNCTURE ON THE IMMUNOREACTIVITY OF FOCAL CUTANEOUS μ-OPIOID RECEPTOR IMMUNOREACTION-POSITIVE FIBERS IN ADJUVANT ARTHRITIC RATS

客观：学习 -opioid 的效果在煽动性的皮肤织物和 electroacupuncture (EA ) 的效果的受体(粗腐殖质) 表示在在助手的粗腐殖质积极纤维的热门免疫反应(红外) 关节炎(AA ) 老鼠。方法：48 只 SD 老鼠的一个总数被使随机化进控制(n =8 ) ，模型(n= 10 ) ， focus-side-EA (n = 10 ) ， non-acupoint-EA (n = 10 ) ，并且 healthy-side-EA (n = 10 ) 组。 AA 模型被完全的 Freund 的助手的下的注射建立( CFA ， 50 L )进左后部的 paw.EA ( 4-16 Hz ， 0.5-1.5 V )为 30 min 在焦点或健康方面和 non-acupoints 上被用于“ Huantiao ”( GB 30 )和“ Yanglingquan ”( GB 34 )。使用的 Non-acupoints 是二个地点 5 公里到健康方面上的 GB 30 和 GB 34。在焦点的真皮、下的纸巾的热门粗腐殖质红外积极的纤维与 immunohistochemical 方法被染色。疼痛的严厉被把测试弄弯的脚(anklejoint ) 检测。结果：与模型组相比，“弯曲脚的测试” 20 在 第9 和 第11 天( P0.05 )并且在 non-acupoint-EA 组在 focus-side-EA 组显著地减少了在上 第8 ， 第9 并且在 CFA ( P 0.05 )的注射以后的 11thd ，都显示双边的 GB 30 和 GB 34 和 non-acupoints 的那 EA 能减轻疼痛。从第 13 天在上，没有重要差别在 3 个 EA 组和模型组(P0.05 ) 之中在“弯曲脚的测试”被发现分数。与控制组比较，在焦点织物的粗腐殖质红外积极的神经纤维的区域价值在在在模型的焦点的粗腐殖质红外积极的神经纤维的 .The 区域价值组织的 3 个 EA 组( P0.05 )是显著地更高的， 3 个 EA 组在控制组( P0.05 )比那高重要。与模型组相比，区域在组和 healthy-side-EA 组显著地增加了的 focus-side-EA (P 0.05 ) 粗腐殖质红外积极的纤维珍视；当在 non-acupoint-EA 组和 healthy-side-EA 组的粗腐殖质红外积极的纤维的那些在 focus-side-EA 是比那显著地低的时，组( P 0.05 )，和没有重要差别在粗腐殖质红外积极的纤维( P0.05 )的区域在模型组， healthy-side-EA 组和 non-acupoint-EA 组之中被发现，在焦点方面上显示 acupoints 的 EA 的更强壮的效果。结论：GB 30 和 GB 34 的 EA 能减轻煽动性的疼痛和起来调整在在 AA 老鼠的焦点的皮纸巾的粗腐殖质红外积极的纤维的表示，施加反煽动性、止痛的效果。

Corelation Between Single Nucleotide Polymorphisms in Mu Opioid Receptor Exon 2 and Stereotypic Behaviour in Sows

Three breeds of sows were observed to investigate the relationship between Single Nucleotide Polymorphisms(SNPs) in Mu Opioid Receptor(MOR)and stereotypic behaviour,such as,sham-chewing,bar biting and standing still in order to better understand the mechanism of stereotypic development of the animals in restrained conditions.MOR exon 2 partial sequences were amplified to analyze single nucleotide polymorphisms by PCR-SSCP.One SNP,a silence mutant was found.A significant difference (P<0.01)was found in the frequency of genotypes in these 3 breeds where only the BB genotype,which was identical to that published in GenBank,was found in the Duroc breed,while no AA genotype was found in Landrace,3 genotypes AA,BB and AB were found in Yorkshire.The result also indicated that the individuals with AA and AB genotypes tended to be more active in sham-chewing than those with the BB genotype(P<0.05).The overall results of this study suggested that sham-chewing of sows may be subjected to both genetic control and environmental conditions,but activity level was more likely to be affected by their environment.We can putatively draw the conclusion that MOR gene has effect on the sham-chewing behavioral traits of sow.

Endorphinergic Attenuation of Distress by Concomitantly Enhancing Endogenous Opioid Release and Switching Opioid Receptor Signaling from an Excessively Excitatory to a Normal Inhibitory Mode

The endogenous opioid system plays a significant role in the modulation of distress in many psychiatric, neurologic, and neurodevelopmental disorders. Many clinical distress symptoms show similarities to the excitatory autonomic withdrawal effects in chronic opioid-dependent animals and humans, as well as to the “quasi-morphine withdrawal syndrome” evoked in naive rodents shortly after acute systemic injection of cyclic AMP-phosphodiesterase (cAMP-PDE) inhibitors. These symptoms result from excessive excitatory opioid receptor signaling and increased endorphin release. Pharmacologic analyses of the remarkably plastic bimodal (excitatory/inhibitory) signaling functions of opioid receptors have utilized microelectrode recordings from opioid-sensitive neurons in tissue cultures of mouse sensory ganglia and hot-water tail-flick assays in mice. These studies led to development of specific chemical formulations that switch opioid receptor signaling from an excessively excitatory to a normal inhibitory mode. Critical combinations of cAMP-PDE inhibitors that release endorphins plus specific agents that switch opioid receptors from excitatory Gs-coupled to inhibitory Gi/Go-coupled signaling were shown to attenuate hyperalgesia and distress evoked by diverse chemical stressors in mouse tail-flick assays. Both the “quasi-morphine withdrawal syndrome” in naive rodents as well as the excitatory withdrawal effects in chronic, opioid-dependent animals and humans may be manifestations of a common Endorphinergic Distress Syndrome (EDS). We suggest that many distress symptoms are caused by EDS, a dysfunctional imbalance in the endogenous opioid system, consisting of abnormal endorphin levels, together with opioid receptors predominately in their excitatory mode. Therefore, concomitantly enhancing endogenous opioid release and switching excessive excitatory opioid receptor signaling to inhibitory signaling can attenuate these distress symptoms. Trials of a critically formulated oral preparation, containing both endorphin enhancers and opioid receptor switchers, have resulted in long-term anxiolytic efficacy and enhanced calm and mental clarity in large numbers of individuals with distress symptoms. These endorphinergic formulations may provide treatment for the emotional and physical distress associated with many psychiatric, neurologic, and neurodevelopmental disorders.

Role of opioid receptor heterodimerization in pain modulation and tolerance development

Protein to protein interactions leading to homo/heteromerization of receptor is well documented in literature. These interactions leading to dimeric/oligomers formation of receptors are known to modulate their function, particularly in case of G-protein coupled receptors. The opioid receptor heteromers having changed pharmacological properties than the constituent protomers provides preferences for novel drug targets that could lead to potential analgesicactivity devoid of tolerance and physical dependence. Heterodimerization of opioid receptors appears to generate novel binding properties with improved specificity and lack of side effects. Further the molecules which can interact simultaneously to both the protomers of the heteromer, or to both the binding sites(orthosteric and allosteric) of a receptor protein could be potential therapeutic molecules. This review highlights the recent advancements in exploring the plausible role of heteromerization of opioid receptors in induction of tolerance free antinociception.

Dopamine D₁- and D₂-Receptors in Immunostimulation under Activation of Mu-Opioid Receptors in Mice with Different Psychoemotional States

The purpose of the present study was to analyze the effect of activation of mu-opioid receptors (mu-OR) on the immune response under blockade of postsynaptic D1-and D2-receptors in mice of the C57BL/6J strain displaying either aggressive or depressive-like behaviors in the social conflict model. It is shown that activation of activation of mu-OR with a highly selective agonist DAGO (100 μg/kg) increased significantly IgM-immune response not only in C57BL/6J mice with an unchanged psychoemotional state but also in mice displaying aggressive or depressive-like behaviors in the social stress model (10 days of agonistic confrontations). Selective blockade of DA receptors of the D1-type with SCH-23390 (1.0 mg/kg with DAGO administration) caused a more pronounced elevation of IgM-immune response than DAGO alone while DAGO effect was completely blocked by prior administration of D2-receptor antagonist haloperidol (1.0 mg/kg). At the same time, both SCH-23390 and haloperidol prevented the immune response increase induced by DAGO injection in mice engaged in aggressive or depressive-like behaviors. Thus, in animals not subjected to social stress DAGO-induced immunostimulation is provided only by D2-receptors, whereas in animals with altered psychoemotional state mu-opioid immunostimulation is mediated by both types of DA receptors—D1 and D2. These data provide evidence for different impacts of the main subtypes of DA receptors in the mediation of immunomodulating effects of mu-opioid system under normal and stressful conditions.