背根神经节神经元阿片受体和离子通道的研究进展

阿片及阿片受体与外周神经系统镇痛机制的研究,随着分子生物学技术的发展,已在受体的分子结构、形态学、分子药理学、离子通道和细胞内信号转导系统等方面取得了显著进展。μ、δ、κ阿片受体分子结构上的部分差异决定了它们各自的功能特征。三种受体在初级感觉神经元分布的比例不同,但都能介导细胞Ｃａ＾２＋通道的抑制和Ｋ＾＋电流增加及减少。阿片受体和通道之间由多种第二信使系统偶联。分子药理学研究表明它们还存在亚型受体     

Opioid and nociceptin receptors regulate cytokine and cytokine receptor expression

Opioids were originally discovered because of their ability to induce analgesia, but further investigation has shown that the opioids regulate the function of cells involved in the immune response. We suggest that the regulation of cytokine, chemokine, and cytokine receptor expression is a critical component of the immunomodulatory activity of the opioids. In this paper we review the literature dealing with the regulation of cytokine and cytokine receptor expression by agonists for the three major opioid receptor types (mu, kappa, and delta), and nociceptin, the natural agonist for the orphanin FQ/nociceptin receptor. Although the opioid receptors share a high degree of sequence homology, opposing roles between the kappa opioid receptor (KOR) and the mu opioid receptor (MOR) have become apparent. We suggest that activation of the KOR induces an anti-inflammatory response through the down-regulation of cytokine, chemokine and chemokine receptor expression, while activation of the MOR favors a pro-inflammatory response. Investigation into the opioid receptor-like (ORL1)/nociceptin system also suggests a role for this receptor as a down-regulator of immune function. These effects suggest a broad role for opioids in the modulation of the function of the immune system, and suggest possible targets for the development of new therapeutics for inflammatory and infectious diseases.     

Opioid peptides, opioid receptors and mechanism of down regulation

Biogenesis of various endogenous opioid peptides, anatomical distribution and the characteristics of multiple receptors with which they interact provides an opportunity for understanding the role of opioid systems and mechanism of opioid tolerance. Cellular and anatomical distribution of opioid receptor and their function is important for identification of neuronal systems and local network involved in initiation of drug action and subsequent development of adaptations resulting from repeated drug use. The details concerning discovery and progress in endogenous opioid peptide research and their distribution in brain have been described in this review. This review also describes opioid receptors, their distribution and mechanism of down regulation, which may be one of the causes for tolerance to opioids. Agonist induced down regulation and recent evidence for involvement of ubiquitin/proteasome system in this process has been discussed.     

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.     

Signaling diversity of mu- and delta- opioid receptor ligands: Re-evaluating the benefits of β-arrestin/G protein signaling bias

Opioid analgesics are elective for treating moderate to severe pain but their use is restricted by severe side effects. Signaling bias has been proposed as a viable means for improving this situation. To exploit this opportunity, continuous efforts are devoted to understand how ligand-specific modulations of receptor functions could mediate the different in vivo effects of opioids. Advances in the field have led to the development of biased agonists based on hypotheses that allocated desired and undesired effects to specific signaling pathways. However, the prevalent hypothesis associating β-arrestin to opioid side effects was recently challenged and multiple of the newly developed biased drugs may not display the superior side effects profile that was sought. Moreover, biased agonism at opioid receptors is now known to be time- and cell-dependent, which adds a new layer of complexity for bias estimation. Here, we first review the signaling mechanisms underlying desired and undesired effects of opioids. We then describe biased agonism at opioid receptors and discuss the different perspectives that support the desired and undesired effects of opioids in view of exploiting biased signaling for therapeutic purposes. Finally, we explore how signaling kinetics and cellular background can influence the magnitude and directionality of bias at those receptors.     

Dimerization of morphine and orphanin FQ/nociceptin receptors: generation of a novel opioid receptor subtype

Although orphanin FQ/nociceptin (OFQ/N) receptors are a member of the opioid receptor family of receptors, they bind traditional opioids with very poor affinity. We now demonstrate that mu opioid receptors can physically associate with OFQ/N receptors, resulting in a complex with a unique binding selectivity profile. Immunoprecipitation of epitope-tagged OFQ/N receptors co-precipitates mu receptors. When the two receptors were co-expressed in CHO cells, [3H]OFQ/N retained its high binding affinity for its receptor. However, co-expression of the two receptors increased by up to 250-fold the affinity of a series of opioids in [3H]OFQ/N binding assays. This enhanced affinity was limited to agonists with high affinity for mu receptors. Selective kappa(1) and delta opioids did not lower binding. Despite the dramatic increase in affinity for the opioid agonists in co-expressing cells, the opioid antagonists naloxone and diprenorphine failed to compete [3H]OFQ/N binding.     

Opioid receptor interactions: local and nonlocal,symmetric and asymmetric,physical and functional

The pharmacological effects of opioid drugs and endogenous opioid peptides are mediated by several kinds of receptors, the major ones being mu, delta and kappa. Though classically it has been thought that a particular effect mediated by a drug or other ligand results from its interaction with a single type of receptor, accumulating evidence demonstrates that interactions between receptors play a major role in opioid actions. These interactions may be either local, involving receptors within the same tissue, or nonlocal, between receptors located in different tissues. Nonlocal interactions always involve intercellular mechanisms, whereas local interactions may involve either intercellular or intracellular interactions, the latter including physical association of receptors. Both local and nonlocal interactions, moreover, may be either symmetric, with ligand interaction at one receptor affecting interaction at the other, or asymmetric; and either potentiating or inhibitory. In this article we discuss major examples of these kinds of interactions, and their role in the acute and chronic effects of opioids. Knowledge of these interactions may have important implications for the design of opioids with more specific actions, and for eliminating the addictive liability of these drugs.     

From inhibition to excitation: functional effects of interaction between opioid receptors

Opioids have excitatory effects in multiple regions of the nervous system. Excitation by opioids is generally attributed to inhibition of inhibitory pathways (disinhibition). However, recent studies indicate that opioids can directly excite individual cells. These effects may occur when opioid receptors interact with other G protein coupled receptors, when different subtypes of opioid receptors interact, or when opioids transactivate other receptors such as receptor tyrosine kinases. Changes in the relative level of expression of different receptors in an individual cell may therefore determine its functional response to a given ligand. This phenomenon could represent an adaptive mechanism involved in tolerance, dependence and subsequent withdrawal.     

Opioids modulate periodicity rather than efficacy of peristaltic waves in the guinea pig ileum in vitro

The influence of opioid receptor blockade by naloxone and opioid receptor activation by opioids on peristalsis was studied in isolated segments of the guinea pig ileum.1. (-)Naloxone, but not (+)naloxone, increased the mean number of peristaltic waves per min within periods of elevated intraluminal pressure. Naloxone tended to modify intermittent peristalsis into ongoing peristalsis, whereas opioids worked in an opposite fashion. 2. Maximum amplitudes of luminal volume displacement during single peristaltic waves were not decreased by opioids. (-)Naloxone, however, applied to non-pretreated segments, decreased transitorily the efficacy of single peristaltic waves to a small, but statistically significant degree 3. Enhancement of peristalsis by naloxone decreased over time, although enough naloxone was present to occupy all opioid receptors. This suggests that opioid receptor blockade induces some compensatory mechanism.     

State of the opioid system in rats with different sensitivity to ethanol

In this work we investigated the content of opioids in plasma blood and the changes caused by ethanol (1.5 g/kg, i.p.) in midbrain opioid receptors of rats with different sensitivity to ethanol, as determined according to the duration of ethanol-induced sleep (DEIS). A receptor binding technique with selective delta-DADLE and mu-DAGO radioligands were used. Ethanol even at low dose produced changes in the midbrain opioid receptors which appeared after 6 hours. The response of the receptors and the content of opioids in plasma blood of rats with different DEIS were not identical.     

Morphine regulates gene expression of alpha- and beta-chemokines and their receptors on astroglial cells via the opioid mu receptor

The brain is a target organ for recreational drugs and HIV-1. Epidemiological data demonstrate that opioid abuse is a risk factor for HIV-1 infection and progression to AIDS. Chemokines and their receptors have been implicated in the neuropathogenesis of HIV-1 infections. However, little is known about the effects of opioids on the expression of chemokines and their receptors (the latter also are HIV-1 coreceptors) by cells of the CNS. Herein we describe the effects of morphine on gene expression of the alpha- and beta-chemokines and their receptors by the astrocytoma cell line U87 and by primary normal human astrocyte (NHA) cultures. U87 cells treated with morphine showed significant down-regulation of IL-8 gene expression, whereas expression of the IL-8 receptor CXCR2 was reciprocally up-regulated as detected by RT-PCR. Treatment of NHAs with morphine suppressed IL-8 and macrophage-inflammatory protein-1beta gene expression, whereas expression of their receptor genes, CCR3 and CCR5, was simultaneously enhanced. These morphine-induced effects on U87 and NHA cells were reversed by the opioid mu receptor antagonist beta-funaltrexamine. Morphine also enhanced the constitutive expression of the opioid mu receptor on astroglial cells. Our results support the hypothesis that opioids play a significant role in the susceptibility of the CNS to HIV-1 infection and subsequent encephalopathy by inhibiting local production of HIV-1-protective chemokines (IL-8 and macrophage-inflammatory protein-1beta) and enhancing expression of HIV-1 entry coreceptor genes (CCR3, CCR5, and CXCR2) within the CNS. These effects of opioids appear to be mediated through the opioid mu receptor that we demonstrated on astroglial cells.     

Multiple opioid ligands and receptors in the control of nociception

This paper summarizes the results of recent data characterizing the role of endogenous opioid peptides and opioid receptors in nociception. In addition, evidence is given that antinociception induced by intracerebroventricular injection of opioids into mouse brain is mediated by receptors resembling those mediating the inhibitory action of these substances on the rat vas deferens (putative epsilon-receptors). The endogenous ligands for these receptor are beta-endorphin and the peptides deriving from proenkephalin A.     

Further studies on opioids and hibernation: delta opioid receptor ligand selectively induced hibernation in summer-active ground squirrels

To examine the possible involvement of multiple opioid receptors in animal hibernation, we infused opioids selective for mu, kappa, and delta opioid receptors into summer-active ground squirrels (Citellus tridecemlineatus). The effects of those opioid treatments on the hibernation induced by HIT (Hibernation Induction Trigger) were also examined. Mu opioids morphine (1.50 mg/kg/day) and morphiceptin (0.82 mg/kg/day) and kappa opioid peptide dynorphin A (0.82 mg/kg/day) did not induce hibernation. On the contrary, morphine, morphiceptin and dynorphin A antagonized HIT-induced hibernation in summer-active ground squirrels. Infusion of delta opioid DADLE (D-Ala2-D-Leu5 enkephalin; 1.50 mg/kg/day), however, induced summer hibernation in a manner comparable to that induced by HIT. It is concluded therefore that delta opioid receptor and its ligand may be intimately involved in animal hibernation. In view of the fact that HIT was obtained from winter hibernating animals and might therefore be responsible for natural hibernation, our results also suggest that naturally occurring mu and kappa opioids may play an important role in the arousal state of hibernation.     

Distribution Pattern of Metorphamide Compared with Other Opioid Peptides from Proenkephalin and Prodynorphin in the Bovine Brain

Metorphamide is a [Met]-enkephalin-containing opioid octapeptide with a C-terminal alpha-amide group. It is derived from proenkephalin and is, so far, the only endogenous opioid peptide with a particularly high affinity for mu opioid (morphine) receptors, a somewhat lesser affinity for kappa opioid receptors, and a relatively low affinity for delta opioid receptors. The concentrations of metorphamide in the bovine caudate nucleus, the hypothalamus, the spinal cord, and the neurointermediate pituitary were determined by radioimmunoassay and chromatography separation procedures. Metorphamide concentrations were compared with the concentrations of eight other opioid peptides from proenkephalin and prodynorphin in identical extracts. The other opioid peptides were [Met]-enkephalyl-Arg6-Phe7 and [Met]-enkephalyl-Arg6-Gly7-Leu8 from proenkephalin; alpha-neoendorphin, beta-neoendorphin, dynorphin A(1-8), dynorphin A(1-17), and dynorphin B from prodynorphin; and [Leu]-enkephalin, which can be derived from either precursor. All opioid peptides were present in all four bovine neural tissues investigated. Metorphamide concentrations were lower than the concentrations of the other proenkephalin-derived opioid peptides. They were, however, similar to the concentrations of the prodynorphin-derived opioid peptides in the same tissues. Marked differences in the relative ratios of the opioids derived from prodynorphin across brain regions were observed, a finding suggesting differential posttranslational processing. Differences in the ratios of the proenkephalin-derived opioids across brain regions were less pronounced. The results from this study together with previous findings on metorphamide's mu opioid receptor binding and bioactivities suggest that the amounts of metorphamide in the bovine brain are sufficient to make this peptide a candidate for a physiologically significant endogenous mu opioid receptor ligand.     

Pharmacological Characterization of an Opioid Receptor in the Ciliate Tetrahymena

A pharmacological characterization has been performed of the opioid receptor involved in modulation of phagocytosis in the protozoan ciliate Tetrahymena. Studies on inhibition of phagocytosis by mammalian prototypic opioid agonists revealed that morphine and β-endorphin have the highest intrinsic activity, whereas all the other opioids tested can only be considered partial agonists. However, morphine (a mu-receptor agonist) is twice as potent as β-endorphin (a delta-receptor agonist). Furthermore, the sensitivity for the opioid antagonist naloxone, determined in the presence of morphine and β-endorphin, is very similar to the sensitivity exhibited by mammalian tissues rich in mu-opioid receptors. We suggest that the opioid receptor coupled to phagocytosis in Tetrahymena is mulike in some of its pharmacological characteristics and may serve as a model system for studies on opioid receptor function and evolution.     

Implication of the First and Third Extracellular Loops of the μ-Opioid Receptor in the Formation of the Ligand Binding Site: A Study Using Chimeric μ-Opioid/Angiotensin Receptors

Abstract: Recent studies on chimeric μ/δ-, μ/κ- and δ/κ-opioid receptors have suggested that extracellular loops of the receptors were involved in the discriminatory binding of selective ligands by controlling their entry into the transmembrane binding site. Since homochimeric opioid receptors are mostly informative in terms of selectivity, the role of extracellular loops was examined here by studying heterochimeric μ receptors where the totality or parts of extracellular loops were replaced by the corresponding regions of the receptor for angiotensin II. Chimeric μ receptors with extracellular loop EL1 or EL3 originating from the angiotensin receptor had 100-fold decreased affinities for opioids; the length of the first extracellular loop, which is one residue longer in angiotensin than μ receptors, was shown to be responsible for this situation. Substitution of the μ receptor second extracellular loop by that of the angiotensin receptor diminished by ∼10-fold the affinities for opioids. Since all chimeras had altered affinities for selective and nonselective ligands, we propose that extracellular domains of the μ receptor, particularly the first and third loops, constrain the relative positioning of the connected transmembrane domains where selective as well as nonselective contact points form the opioid binding site.     

Diversity and complexity of the mu opioid receptor gene: alternative pre-mRNA splicing and promoters

Mu opioid receptors play an important role in mediating the actions of a class of opioids including morphine and heroin. Binding and pharmacological studies have proposed several mu opioid receptor subtypes: mu(1), mu(2), and morphine-6beta-glucuronide (M6G). The cloning of a mu opioid receptor, MOR-1, has provided an invaluable tool to explore pharmacological and physiological functions of mu opioid receptors at the molecular level. However, only one mu opioid receptor (Oprm) gene has been isolated. Alternative pre-mRNA splicing has been proposed as a molecular explanation for the existence of pharmacologically identified subtypes. In recent years, we have extensively investigated alternative splicing of the Oprm gene, particularly of the mouse Oprm gene. So far we have identified 25 splice variants from the mouse Oprm gene, which are controlled by two diverse promoters, eight splice variants from the rat Oprm gene, and 11 splice variants from the human Oprm gene. Diversity and complexity of the Oprm gene was further demonstrated by functional differences in agonist-induced G protein activation, adenylyl cyclase activity, and receptor internalization among carboxyl terminal variants. This review summarizes these recent results and provides a new perspective on understanding and exploring complex opioid actions in animals and humans.     

Lung opioid receptors: pharmacology and possible target for nebulized morphine in dyspnea

Opioid receptors are located throughout the respiratory tract. Yet, these have received relatively scant attention compared to other opioid receptors. The most abundant sites within the respiratory tract appear localized within the alveolar walls, other sites appear to line the smooth muscle within the trachea and main bronchi near the lumen. There is about 100-times greater [3H]morphine binding density within the bronchioles and lobes than in the main bronchi or trachea. In addition to the usual mu, delta and kappa types of opioid receptors, 'non-conventional' opioid binding sites have been suggested, although the function of these or of the other opioid receptors in the pulmonary tract is not known. However, they might explain the otherwise counterintuitive apparent utility of morphine treatment of dyspnea. Dyspnea is a common and distressing symptom in terminally-ill cancer patients and patients with chronic lung disease. It results from multiple causes, is difficult to treat and is a significant precipitating factor for late-stage hospital or hospice admissions. Nebulized morphine or other opioids have been reported to have beneficial effect, but the mechanism by which opioids might produce this seemingly contradictory effect is not clear. We review here lung opioid receptor distribution, pharmacology and possible clinical relevance in the treatment of dyspnea.     

Heligmosomoides polygyrus: opioid peptides are involved in immune regulation of the histotropic phase of infection

We investigated the potential effect of agonists of opioid receptors in the experimental model of Heligmosomoides polygyrus primary infection. BALB/c mice infected with H. polygyrus were treated with naltrexone, a non-specific antagonist of all three types of opioid receptors (mu, delta, kappa) prior to and during infection. The blockade of opioid receptors affected the pattern and level of immune response induced by H. polygyrus in the histotropic phase of infection, which suggests that an opioid receptor-linked mechanism is involved in the immune response of mice during this phase of infection. Down-regulation of the inflammatory response against fourth-stage larvae appeared to be by endogenous opioids influenced cytokines and nitric oxide production by macrophages in the peritoneum and in the gut as well as migration of leukocytes towards the antigens. Down-regulation of these mechanisms by opioid receptor agonists in vivo might account for the decreased resistance to H. polygyrus infection.