The dynamic relationship between mu and kappa opioid receptors in body temperature regulation

Previous studies demonstrated that intracerebroventricular (icv) injection of a kappa opioid receptor agonist decreased, and a mu agonist increased, body temperature (Tb) in rats. A dose-response study with the selective kappa antagonist nor-binaltorphimine (nor-BNI) showed that a low dose (1.25 nmol, icv) alone had no effect, although a high dose (25 nmol, icv) increased Tb. It was hypothesized that the hyperthermia induced by nor-BNI was the result of the antagonist blocking the kappa opioid receptor and releasing its inhibition of mu opioid receptor activity. To determine whether the Tb increase caused by nor-BNI was a mu receptor-mediated effect, we administered the selective mu antagonist CTAP (1.25 nmol, icv) 15 min after nor-BNI (25 nmol, icv) and measured rectal Tb in unrestrained rats. CTAP significantly antagonized the Tb increase induced by icv injection of nor-BNI. Injection of 5 or 10 nmol of CTAP alone significantly decreased the Tb, and 1.25 nmol of nor-BNI blocked that effect, indicating that the CTAP-induced hypothermia was kappa-mediated. The findings strongly suggest that mu antagonists, in blocking the basal hyperthermia mediated by mu receptors, can unmask the endogenous kappa receptor-mediated hypothermia, and that there is a tonic balance between mu and kappa opioid receptors that serves as a homeostatic mechanism for maintaining Tb.     

Tramadol reduces the 5-HTP-induced head-twitch response in mice via the activation of mu and kappa opioid receptors

Tramadol, an atypical opioid analgesic, stimulates both opiatergic and serotonergic systems. Here we have investigated the effect of tramadol in mice on 5-hydroxyptrytophan (5-HTP)-induced head twitch response (HTR), which is an animal model for the activation of the CNS 5-HT(2A) receptors in mice. Tramadol attenuated 5-HTP-induced HTR in a dose-dependent manner as morphine. Furthermore, the nonselective opioid receptor antagonists, naloxone and diprenorphine (M5050), reversed the effect of tramadol on 5-HTP-induced HTR dose-dependently. Interestingly, in contrast to the selective delta opioid receptor antagonist NTI, beta-FNA, a selective mu receptor antagonist, and nor-BNI, a selective kappa opioid receptor antagonist, antagonized the attenuation of 5-HTP-induced HTR by tramadol. In conclusion, administration of tramadol systemically inhibits 5-HTP-induced HTR in mice by activating opiatergic system in the CNS. Our findings show that mu and kappa opioid receptors, but not delta opioid receptor, play an important role in the regulation of serotonergic function in the CNS.     

High-affinity naloxone binding to filamin a prevents mu opioid receptor-Gs coupling underlying opioid tolerance and dependence

Ultra-low-dose opioid antagonists enhance opioid analgesia and reduce analgesic tolerance and dependence by preventing a G protein coupling switch (Gi/o to Gs) by the mu opioid receptor (MOR), although the binding site of such ultra-low-dose opioid antagonists was previously unknown. Here we show that with approximately 200-fold higher affinity than for the mu opioid receptor, naloxone binds a pentapeptide segment of the scaffolding protein filamin A, known to interact with the mu opioid receptor, to disrupt its chronic opioid-induced Gs coupling. Naloxone binding to filamin A is demonstrated by the absence of [(3)H]-and FITC-naloxone binding in the melanoma M2 cell line that does not contain filamin or MOR, contrasting with strong [(3)H]naloxone binding to its filamin A-transfected subclone A7 or to immunopurified filamin A. Naloxone binding to A7 cells was displaced by naltrexone but not by morphine, indicating a target distinct from opioid receptors and perhaps unique to naloxone and its analogs. The intracellular location of this binding site was confirmed by FITC-NLX binding in intact A7 cells. Overlapping peptide fragments from c-terminal filamin A revealed filamin A(2561-2565) as the binding site, and an alanine scan of this pentapeptide revealed an essential mid-point lysine. Finally, in organotypic striatal slice cultures, peptide fragments containing filamin A(2561-2565) abolished the prevention by 10 pM naloxone of both the chronic morphine-induced mu opioid receptor-Gs coupling and the downstream cAMP excitatory signal. These results establish filamin A as the target for ultra-low-dose opioid antagonists previously shown to enhance opioid analgesia and to prevent opioid tolerance and dependence.     

八肽胆囊收缩素对抗mu和Kappa型受体介导的镇痛作用

以往的资料表明,八肽胆襄收缩素(CCK-8)能对抗阿片镇痛,本实验进一步分析 CCK-8对抗哪一类型阿片受体激动剂的镇痛作用。给大鼠脊髓蛛网膜下腔(I.T.)注射 CCK-8(剂量4ng到1.0μg)既不产生痛敏也不产生镇痛。I.T.注射特异性的μ受体激动剂 PL01710 ng 或 k 受体激动剂 NDA P500 ng 引起的镇痛作用可被注射 CCK-8 4ng 所对抗。而L.T.注射δ受体激动剂 DPDPE(6.5,13.0和26.Oμg)引起的镇痛作用不能被 CCK-8(4ng,40ng I.T.)所对抗。但 CCK-8对抗 PL017和 NDAP 镇痛的作用可被 I.T.CCK 受体拮抗剂 proglumide(3μg)所翻转。以上结果表明,I.T.注射 CCK-8可有效地对抗μ和 k 受体介导的镇痛,并且这种对抗作用是经 CCK 受体介导而实现的。     

16-Me cyprenorphine (RX 8008M): a potent opioid antagonist with some delta selectivity

16-Me cyprenorphine (RX 8008M) has been investigated in a number of isolated tissue preparations and found to be a pure opioid antagonist with Ke values at the delta, mu and kappa receptors of 0.73, 1.77 and 59.6 nM respectively. Comparisons of the mu, kappa and delta Ke values with a number of other antagonists in the mouse vas deferens have been made and show that the 16-Me substituent results in a marked enhancement of delta activity, making RX 8008M the most selective non-peptide delta antagonist available at the present time.     

Stress responsivity, addiction, and a functional variant of the human mu-opioid receptor gene

Discovery and characterization of the functional A118G mu-(mu)-opioid receptor variant led to hypotheses, now in part proven, about its role in alterations of endogenous human physiology and in responses to opioid antagonist administration. Differences in cellular expression levels, ligand binding, and signal transduction for variant receptors have been documented in vitro. Human genetic studies also indicate that individuals carrying one or two copies of the 118G allele may have increased risk for opiate and alcohol addictions and that this polymorphism may also explain some of the variability in success of opioid antagonist treatment for alcoholism. Future research will further define the role of the A118G variant in addictive diseases and their treatment, in pain perception and opioid analgesia, and for a myriad of other responses mediated by the mu-opioid receptor.     

Analgesic effect of interferon-alpha via mu opioid receptor in the rat

Using the tail-flick induced by electro-stimulation as a pain marker, it was found that pain threshold (PT) was significantly increased after injecting interferon-alpha (IFN alpha) into the lateral ventricle of rats. This effect was dosage-dependent and abolished by monoclonal antibody (McAb) to IFN alpha. Naloxone could inhibit the analgesic effect of IFN alpha, suggesting that the analgesic effect of IFN alpha be related to the opioid receptors. Beta-funaltrexamine (beta-FNA), the mu specific receptor antagonist could completely block the analgesic effect of IFN alpha. The selective delta-opioid receptor antagonist, ICI174,864 and the kappa-opioid receptor antagonist, nor-BNI both failed to prevent the analgesic effect of IFN alpha. IFN alpha could significantly inhibit the production of the cAMP stimulated by forskolin in SK-N-SH cells expressing the mu-opioid receptor, not in NG108-15 cells expressing the delta-opioid receptor uniformly. The results obtained provide further evidence for opioid activity of IFN alpha and suggest that this effect is mediated by central opioid receptors of the mu subtype. The evidence is consistent with the hypothesis that multiple actions of cytokines, such as immunoregulatory and neuroregulatory effects, might be mediated by distinct domains of cytokines interacting with different receptors.     

Analgesic (sub anesthetic) nitrous oxide interacts with the endogenous opioid system: a review of the evidence

The concept that anesthesia and analgesia are distinct states and therefore are possibly mediated by different mechanisms is stressed. Analgesic nitrous oxide is shown to act at specific rather than non specific central nervous system sites, as well as having a large number of actions similar to morphine the classical opioid. This includes the fact that specific opioid antagonists attenuate the effects of both morphine and analgesic nitrous oxide. Evidence is also provided showing that nitrous oxide may be a partial agonist and that it may interact with the endogenous opioid system by the release of endogenous opioids, and/or by direct action at the mu, delta, sigma and kappa receptors.     

Mu and delta receptors: their role in analgesia in the differential effects of opioid peptides on analgesia

Utilizing the mouse tail-flick assay, the rank order of analgesic potency for various opioids (i.c.v.) is beta h-endorphin greater than D-Ala2-D-Leu5-enkephalin greater than morphine greater than D-Ala2-met-enkephalinamide much greater than met-enkephalin much greater than leu-enkephalin. Assuming mu receptor mediation of analgesia, there is an affinity and analgesic potency (ie: D-Ala2-Leu5-enkephalin has 1/7 the affinity of morphine for the mu receptor but is 18X more potent as an analgesic). Additionally, sub-analgesic doses of various opioid peptides have opposite effects on analgesic responses. Leu-enkephalin, D-Ala2-D-Leu5-enkephalin or beta h-endorphin potentiate morphine or D-Ala2-met-enkephalinamide analgesia whereas met-enkephalin or D-Ala2-met-enkephalinamide antagonize opioid-induced analgesia. Using the enkephalins as the prototypic delta ligands (100 fold selective) and based on their effects on analgesia, we suggest that Leu-enkephalin-like peptides interact with the delta receptor as an "agonist" to facilitate and met-enkephalin-like peptides as an "antagonist" to attenuate analgesia. Given the biochemical evidence of a coupling between mu and delta receptors, we suggest that the mechanism of facilitation or attenuation of analgesia by the enkephalins is a direct in vivo consequence of this coupling. Further, the analgesic potencies of various opioid ligands can be better correlated to the combination of their simultaneous occupancy of mu and delta receptors.     

Codeine and 6-Acetylcodeine Analgesia in Mice

Summary 1. Acetylation of morphine at the 6-position changes its pharmacology. To see if similar changes are seen with codeine, we examined the analgesic actions of codeine and 6-acetylcodeine.2. Like codeine, 6-acetylcodeine is an effective analgesic systemically, supraspinally and spinally, with a potency approximately a third that of codeine.3. The sensitivity of 6-acetylcodeine analgesia to the mu-selective antagonists β-FNA and naloxonazine confirmed its classification as a mu opioid. However, it differed from the other mu analgesics in other paradigms.4. Antisense mapping revealed the sensitivity of 6-acetylcodeine to probes targeting exons 1 and 2 of the mu opioid receptor gene (Oprm), a profile distinct from either codeine or morphine. Although heroin analgesia also is sensitive to antisense targeting exons 1 and 2, heroin analgesia also is sensitive to the antagonist 3-O-methylnaltrexone, while 6-acetylcodeine analgesia is not.5. Thus, 6-acetylcodeine is an effective mu opioid analgesic with a distinct pharmacological profile.     

Opioid agonist/antagonist effect of naloxone in modulating rabbit jejunum contractility in vitro.

Opioid peptides are the most effective drugs in controlling pain; their action is elicited by binding to specific membrane receptors. The gastrointestinal tract represents, after the nervous system, the site in which the opioid receptors are expressed at high levels. The opioid agonist morphine has a significant inhibitory effect on intestinal motility, this action is blocked by naloxone an opioid antagonist mainly active at mu and kappa receptors. In this study the presence of mu opioid receptor on rabbit jejunum was investigated by western blot. The effects of beta-endorphin, the endogenous opioid peptide with the highest affinity to the mu opioid receptor and those of naloxone on spontaneous rabbit jejunum contractions were evaluated. Beta-endorphin (10(-6) M) showed a relaxant effect on jejunum contractility while naloxone showed a dual effect inducing an increase of spontaneous contractility at low concentrations (10(-6) M, 10(-7) M, 10(-8) M) and a decrease when high concentrations (10(-3) M, 10(-4) M, 10(-5) M) were utilized. The obtained results demonstrate that mu opioid receptor is expressed in rabbit jejunum and suggest that this receptor may be involved in mediating the effects of both opioid agonist and antagonist on jejunum contractions.     

弓状核中阿片受体在细胞介素所致发热效应中的作用

本研究旨在了解弓状核内的阿片受体在体温调节中的作用。研究使用细胞介素ＩＬ１β做致热源。以自动推进器向ＳＤ雄性大鼠弓状核微量注射１μ１ＩＬ１β。在给药前３０ｍｉｎ分别向弓状核微量注射通常阿片受体拮抗剂纳洛酮（Ｎａｌ）、阿片受体μ、δ和κ各自特异性拮抗剂ＣＴＡＰ、ＮＴＩ和ｎｏｒＢＮＩ做预处理,用生理盐水（Ｓａｌ）做对照。结果表明：ＩＬ１β所致的升体温效应能被Ｎａｌ和ＣＴＡＰ阻断,提示弓状核中的阿片受体（主要是μ受体）参与或介导了ＩＬ１β的致热效应;δ和κ受体特异性拮抗剂阻断ＩＬ１β所致的体温升高效应不明显。提示δ和κ阿片受体参与体温调节的可能性较小。对照ＡＲＨ和ＰＯＡＨ中阿片受体在ＩＬ１β所致发热中的作用可发现：二者作用极为相似,这一结果有力地支持了弓状核是体温调节中枢重要组成部分的观点。     

Novel trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines as mu opioid receptor antagonists with improved opioid receptor selectivity profiles

A series of N-substituted trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines, mu opioid receptor antagonists, analogs of alvimopan, were prepared using solid phase methodology. This study led to the identification of a highly selective mu opioid receptor antagonist, which interacts selectively with mu peripheral receptors.     

High-affinity carbamate analogues of morphinan at opioid receptors

A series of carbamate analogues were synthesized from levorphanol (1a), cyclorphan (2a) or butorphan (3a) and evaluated in vitro for their binding affinity at mu, delta, and kappa opioid receptors. Functional activities of these compounds were measured in the [(35)S]GTPgammaS binding assay. Phenyl carbamate derivatives 2d and 3d showed the highest binding affinity for kappa receptor (K(i)=0.046 and 0.051 nM) and for mu receptor (K(i)=0.11 and 0.12 nM). Compound 1c showed the highest mu selectivity. The preliminary assay for agonist and antagonist properties of these ligands in stimulating [(35)S]GTPgammaS binding mediated by the kappa opioid receptor illustrated that all of these ligands were kappa agonists. At the mu receptor, compounds 1b, 1c, 2b, and 3b were agonists, while compounds 2c-e and 3c-e were mu agonists/antagonists.     

Potentiation of opioid analgesia by cocaine: the role of spinal and supraspinal receptors.

These studies examined the effect of cocaine on the analgesia produced by systemically and centrally administered opioid agonists. Cocaine (50 mg/kg, s.c.) increased the analgesic potency of systemic, ICV and IT morphine; and the ICV and IT analgesic effects of the delta selective peptide, [D-Pen2,D-Pen5]enkephalin (DPDPE). Cocaine also increased the analgesic potency of the mu selective ligand [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO) administered ICV. However, cocaine did not alter the ED50 for IT DAGO. GC-MS studies indicated that brain cocaine concentration was approximately 3.0 micrograms/g wet weight 45 min following s.c. administration. These results suggest that cocaine-induced increases in opioid analgesic potency are mediated at brain mu and delta receptors and spinal mu receptors. Furthermore, there might be functional differences between spinal and supraspinal sites at which DAGO produces analgesia.     

14-O-Heterocyclic-substituted naltrexone derivatives as non-peptide mu opioid receptor selective antagonists: Design,synthesis, and biological studies

Mu opioid receptor antagonists have clinical utility and are important research tools. To develop non-peptide and highly selective mu opioid receptor antagonist, a series of 14-O-heterocyclic-substituted naltrexone derivatives were designed, synthesized, and evaluated. These compounds showed subnanomolar-to-nanomolar binding affinity for the mu opioid receptor. Among them, compound 1 exhibited the highest selectivity for the mu opioid receptor over the delta and kappa receptors. These results implicated an alternative ‘address’ domain in the extracellular loops of the mu opioid receptor.     

Spinal administration of selective opioid antagonists in amphibians: evidence for an opioid unireceptor

In mammals, opioids act by interactions with three distinct types of receptors: mu, delta, or kappa opioid receptors. Using a novel assay of antinociception in the Northern grass frog, Rana pipiens, previous work demonstrated that selective mu, delta, or kappa opioids produced a potent antinociception when administered by the spinal route. The relative potency of this effect was highly correlated to that found in mammals. Present studies employing selective opioid antagonists, beta-FNA, NTI, or nor-BNI demonstrated that, in general, these antagonists were not selective in the amphibian model. These data have implications for the functional evolution of opioid receptors in vertebrates and suggest that the tested mu, delta, and kappa opioids mediate antinociception via a single type of opioid receptor in amphibians, termed the unireceptor.     

Upregulation of opioid receptor subtypes correlates with potency changes of morphine and DADLE

Chronic treatment with opioid antagonists increases the potency of opioid agonists and produces an increase in brain opioid binding sites. In the present study, 8 day treatment with naltrexone blocked morphine and DADLE analgesia for the entire treatment period and increased mu 1, mu 2 and delta opioid receptor binding sites in mouse brain. mu 1 and mu 2 binding were increased by 81 and 67%, respectively, while delta binding was increased by 31%. Consistent with these binding changes, the potency of ICV morphine to produce analgesia was increased by over 3-fold, while the potency of ICV DADLE was increased by only 1.7. These findings indicate that relative increases in opioid receptor subtypes agree with pharmacodynamic studies on potency changes of opioid agonists.     

Different effects of methionine-enkephalin on paw edema in two inbred rat strains

The effect of intraplantarly (i.pl.)-injected methionine-enkephalin (ME) on Concanavalin A (Con A)-induced paw edema in Dark Agouti (DA) and Albino Oxford (AO) rats was investigated. ME suppressed edema in DA rats, which was antagonized with naloxone (non-selective opioid receptor antagonist) and naltrindole (delta opioid receptors antagonist). Potentiating effect of ME in AO rats was blocked by naloxone, nor-binaltorphimine (kappa opioid receptors antagonist) and beta-funaltrexamine (mu opioid receptors antagonist). Dexamethasone suppressed edema in both rat strains. These findings suggest that strain-dependent differences in the effects of ME on inflammation in DA and AO rats could be related to diversity in opioid receptors expression in these strains.