双向阿片功能调节剂：胍丁胺

近年研究表明某些药物尽管不能与阿片受体发生相互作用，但能对阿片药理作用产生重要的调节．特别是有些药物能对阿片功能产生双向调节作用，即增强阿片镇痛，对抗阿片耐受和躯体依赖．我们将这些不与阿片受体发生作用，但具有双向调节阿片功能的药物称之为双向阿片功能调节剂(biphasic opioid function modulator，BOFM)．基于我们的研究工作，可以认定胍丁胺就是一个典型的双向阿片功能调节剂．胍丁胺本身有弱的镇痛作用，它能增强吗啡镇痛，对抗吗啡耐受和依赖：胍丁胺产生上述作用的主要机制与抑制阿片长期作用下在阿片受体信号转导系统产生的代偿性适应过程相关．     

A biphasic opioid function modulator: agmatine

Recently it has been revealed that some agents that are not able to interact with opioid receptors play an important role in regulating the pharmacological actions of opioids. Especially, some of them show biphasic modulation on opioid functions, which enhance opioid analgesia, but inhibit tolerance to and substance dependence on opioids. We would like to call these agents which do not interact with opioid receptors, but do have biphasic modulation on opioid functions as biphasic opioid function modulator (BOFM). Mainly based on our results, agmatine is a typical BOFM. Agmatine itself was a weak analgesic which enhanced analgesic action of morphine and inhibited tolerance to and dependence on opioid. The main mechanisms of agmatine were related to inhibition of the adaptation of opioid receptor signal transduction induced by chronic treatment of opioid.     

Receptor endocytosis counteracts the development of opioid tolerance

In contrast to endogenous opioids, the highly addictive drug morphine activates the mu-opioid receptor without causing its rapid endocytosis. It has recently been reported that coapplication of low concentrations of [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) facilitates the ability of morphine to stimulate mu-opioid receptor endocytosis and prevents the development of morphine tolerance in rats. To investigate the clinical relevance of this finding for analgesic therapy, the endocytotic efficacies of a series of clinically used opioids were determined, and the effect of a combination of these drugs with morphine on the mu-opioid receptor endocytosis in receptor-expressing human embryonic kidney (HEK) 293 cells was quantified. The combination of morphine and opioid drugs with high endocytotic efficacies (e.g., DAMGO, etonitazene, sufentanil, beta-endorphin, piritramide, or methadone) did not result in a facilitation of morphine-mediated endocytosis but rather in a decrease of the receptor endocytosis mediated by the tested opioid drugs. These findings demonstrate a partial agonistic effect of morphine on the agonist-induced receptor endocytosis. Moreover, we demonstrated that the endocytotic potencies of opioid drugs are negatively correlated with their ability to cause receptor desensitization and opioid tolerance in HEK 293 cells. These results strongly support the hypothesis that mu-opioid receptor endocytosis counteracts receptor desensitization and opioid tolerance by inducing fast receptor reactivation and recycling. In addition, it is shown that agonist-induced receptor endocytosis facilitates the compensatory up-regulation of the cAMP pathway, a cellular hallmark of opioid withdrawal. Our findings suggest that opioids with high endocytotic efficacies might cause reduced opioid tolerance but can facilitate compensatory mechanisms, resulting in an enhanced opioid dependence.     

Mu-opioid receptor desensitization: is morphine different?

Opioid tolerance and dependence are important phenomena. The contribution of acute mu-opioid receptor regulatory mechanisms to the development of analgesic tolerance or physical dependence are unknown, and even the mechanisms underlying relatively rapid receptor desensitization in single cells are unresolved. To a large degree, the uncertainty surrounding the mechanisms and consequences of short-term regulation of tau-opioid receptors in single cells arises from the limitations in the experimental design in many of the studies that have investigated these events. Receptor overexpression and use of assays in which regulatory mechanisms are likely to blunt control determinations have led to measurements of opioid receptor activity that are likely to be insensitive to receptor uncoupling. Together with uncertainties concerning molecular details of tau-opioid receptor interactions with potential regulatory molecules such as G protein-coupled receptor kinases and arrestins, we are left with an incomplete picture crudely copied from the well-worked-out regulatory schema for beta(2)-adrenoceptors. As a consequence, suggestions that clinically relevant tau-opioid receptor agonists may have different propensities to produce tolerance and dependence that arise from their differential recruitment of regulatory mechanisms are premature, and have not yet been appropriately assessed, nor explained in the context of a thoroughly established regulatory scheme. In this commentary, we outline the experimental limitations that have given rise to conflicting ideas about how mu-opioid receptors are regulated, and identify the issues we feel still need to be addressed before we can understand why morphine promotes receptor trafficking differently to other opioids.     

阿片受体C-末端的功能及与其相互作用蛋白的研究进展

阿片受体C-末端氨基酸在激动剂作用后受体的磷酸化、脱敏及内吞过程中发挥了重要作用,C-末端部分缺失或不同氨基酸位点的突变对受体功能有明显影响,如μ阿片受体C-末端的Thr394、Thr383、Thr357、Ser355,δ受体C末端的Ser363和κ受体C末端的Ser369等。除了公认的参与阿片受体C-末端磷酸化的激酶,近年来,人们寻找到其他一些与C-末端相互作用的蛋白,这些相互作用蛋白对阿片受体功能有不同的影响,如PPL、FilaminA、PLD2、PKCI、GASP和EBP50/NHERF等,但不能很好的解释阿片类物质的依赖、耐受与成瘾。因此,寻找特异性与C-末端相互作用的蛋白,成为阿片成瘾机制研究的一个方向。     

Endocannabinoid system and opioid addiction: behavioural aspects

Cannabinoids produce a variety of pharmacological effects very similar to those elicited by opioids. Direct and indirect interactions with opioid system have been proposed to explain some cannabinoid effects such as analgesia and attenuation of opioid-withdrawal syndrome, and evidence has been provided in support to the notion that rewarding properties of cannabinoids and opioids might be functionally linked. In particular, a growing body of studies points to an important role of the endogenous cannabinoid system in the modulation of opioid rewarding and addictive effects. The current review examines progresses in the past few years in the elucidation of cannabinoid-opioid interactions in drug abuse and dependence, focusing on recent findings from behavioural studies using different animal models of addiction. Specifically, here we review data on the behavioural aspects (i.e., drug abuse, dependence, tolerance, sensitization, relapse and drug vulnerability) of the specific, often reciprocal, cross-talk between cannabinoids and opioids with particular reference to the role of the endocannabinoid system in opioid addiction. The potential biochemical mechanisms involved in these pharmacological interactions are discussed together with possible therapeutic implications in the pharmacotherapy of opioid dependence. However, individuation of the precise anatomical substrates and molecular mechanisms of such interaction still remains a complex and challenging field for future research.     

Acute Opioid Receptor Desensitization And Tolerance: Is There A Link?

1. Morphine, used long-term for the treatment of pain, results in drug tolerance. The therapeutic benefits, as well as side effects, of morphine are mediated predominantly via activation of mu-opioid receptors. Although the underlying mechanisms for opioid tolerance remains unclear, early adaptive processes, such as acute receptor desensitization and receptor downregulation, have been suggested to be crucial to the development of opioid tolerance. 2. Other neuroadaptations resulting from chronic opioid use include upregulation of the cAMP pathway, an increase in the cAMP response element-binding protein and Fos-related antigens. However, the connection between upregulation of these cellular elements and the mechanism behind the behavioural phenomenon remains unclear. 3. Acute receptor desensitization is thought to occur via uncoupling of the receptor and G-protein, which is followed by internalization of the receptor from the cell membrane. This process occurs after a few minutes of agonist exposure. Receptor-G-protein uncoupling is mediated via phosphorylation of putative sites on the intracellular loops of activated receptors. 4. Acute desensitization and downregulation of receptors both result in a reduction of agonist efficacy. These events occur early in the cascade of cellular adaptation; however, it is uncertain whether these processes contribute to the long-term changes in receptor sensitivity that occur after repeated exposure to opioids. 5. Acute desensitization may, in fact, be a protective mechanism whereby cells adapt to avoid the development of physiological drug tolerance by rapidly attenuating receptor-mediated signalling. Those drugs that do not cause receptor internalization, such as morphine, may have higher propensities to develop tolerance.     

G-protein receptor kinase 3 (GRK3) influences opioid analgesic tolerance but not opioid withdrawal

1. Tolerance to opioids frequently follows repeated drug administration and affects the clinical utility of these analgesics. Studies in simple cellular systems have demonstrated that prolonged activation of opioid receptors produces homologous receptor desensitization by G-protein receptor kinase mediated receptor phosphorylation and subsequent beta-arrestin binding. To define the role of this regulatory mechanism in the control of the electrophysiological and behavioral responses to opioids, we used mice having a targeted disruption of the G-protein receptor kinase 3 (GRK3) gene. 2. Mice lacking GRK3 did not differ from wild-type littermates neither in their response latencies to noxious stimuli on the hot-plate test nor in their acute antinociceptive responses to fentanyl or morphine. 3. Tolerance to the electrophysiological response to the opioid fentanyl, measured in vitro in the hippocampus, was blocked by GRK3 deletion. In addition, tolerance to the antinociceptive effects of fentanyl was significantly reduced in GRK3 knockouts compared to wild-type littermate controls. 4. Tolerance to the antinociceptive effects of morphine was not affected by GRK3 deletion although morphine tolerance in hippocampal slices from GRK3 knockout mice was significantly inhibited. Tolerance developed more slowly in vitro to morphine than fentanyl supporting previous work in in vitro systems showing a correlation between agonist efficacy and GRK3-mediated desensitization. 5. The results of these studies suggest that GRK3-mediated mechanisms are important components of both electrophysiologic and behavioral opioid tolerance. Fentanyl, a high efficacy opioid, more effectively produced GRK3-dependent effects than morphine, a low efficacy agonist.     

阿片依赖病理过程中NMDA受体与阿片受体的相互作用及分子机制

阿片依赖的病理生理学基础与机体长期接触阿片类物质后产生的代偿性适应相关。近年来研究发现 ,NMDA受体拮抗剂、抗体和反义核酸能抑制阿片躯体和精神依赖。该文从神经生化和分子生物学角度 ,综述了NMDA受体作用系统与阿片受体作用系统的相互作用以及NMDA受体参与阿片依赖的分子机制 ,并评价了NMDA受体拮抗剂及其他相关物质治疗阿片依赖的应用前景     

75 years of opioid research: the exciting but vain quest for the Holy Grail

Over the 75-year lifetime of the British Pharmacological Society there has been an enormous expansion in our understanding of how opioid drugs act on the nervous system, with much of this effort aimed at developing powerful analgesic drugs devoid of the side effects associated with morphine--the Holy Grail of opioid research. At the molecular and cellular level multiple opioid receptors have been cloned and characterised, their potential for oligomerisation determined, a large family of endogenous opioid agonists has been discovered, multiple second messengers identified and our understanding of the adaptive changes to prolonged exposure to opioid drugs (tolerance and physical dependence) enhanced. In addition, we now have greater understanding of the processes by which opioids produce the euphoria that gives rise to the intense craving for these drugs in opioid addicts. In this article, we review the historical pathway of opioid research that has led to our current state of knowledge.     

The other side of the opioid story: modulation of cell growth and survival signaling

Opioids have been used as pain control medications for thousands of years. Opioids are highly effective analgesics clinically available for controlling moderate and severe pain. Recent genetic knockout and knockin studies have definitively demonstrated that the analgesic effect is mediated through opioid receptors. In addition to their analgesic effect, opioids also have the potential to develop tolerance and physical dependence. Moreover, opioids can modulate cell proliferation and survival. Attempts to design better opioid drugs to eliminate or diminish these undesirable effects for clinical benefits have achieved limited success. In recent years, investigation of the effects of opioid-mediated cell proliferation and survival has been very active, resulting in many publications. However, the molecular targets of such non-analgesic effects are complex. Several important pathways that control cell proliferation, survival, and apoptosis have been reported to be associated with the non-analgesic effects, which may be mediated through both opioid receptor signaling and other non-opioid receptor molecular entity-mediated signaling. This review tries to bring the attention of the medicinal chemistry community to new developments and advances in the research areas of opioid-mediated cell proliferation and survival. Further investigation of the molecular mechanism of these non-analgesic opioid effects may eventually yield useful information such as new drug targets, which may be explored to benefit for clinical treatments such as targeted cancer therapy, cancer pain management, regeneration of neurons, and recovery from drug addiction.     

Tolerance and cross-tolerance to the rate-suppressing effects of opioids in butorphanol-treated rats: influence of maintenance dose and relative efficacy at the mu receptor

 The purpose of the present investigation was to examine the development of tolerance to the rate-suppressing effects of mu and kappa opioids in rats administered either 3.0 (low) or 30 (high) mg/kg per day of butorphanol, an opioid with low relative efficacy at the mu receptor. The mu opioids butorphanol, buprenorphine, morphine, fentanyl and sufentanil, and the kappa opioid U50,488 dose-dependently suppressed responding under all conditions examined. In rats administered the low maintenance dose of butorphanol, tolerance developed to the effects of butorphanol, buprenorphine and morphine, but not to fentanyl and sufentanil. In rats administered the high maintenance dose, tolerance developed to all of the mu opioids examined. In both treatment groups, the degree to which tolerance developed was greater for butorphanol and buprenorphine than for morphine, fentanyl and sufentanil; and the degree to which tolerance developed to these mu opioids was greater in rats administered the high maintenance dose of butorphanol. The tolerance that developed to morphine, fentanyl and sufentanil was not altered when tested at both 23 and 47 h following the previous maintenance dose of butorphanol, suggesting that these changes were not due to any acute pharmacological interactions between butorphanol and the test compound (i.e., antagonism). Tolerance was also conferred to the kappa opioid U50,488 in both groups of rats, and in rats administered the high maintenance dose, this effect was obtained when tested 23 and 47 h following the previous maintenance dose of butorphanol. Physical dependence developed in rats administered the high maintenance dose of butorphanol, as evidenced by the development of enhanced sensitivity to the rate-suppressing effects of naloxone, and the finding that 30 mg/kg naloxone decreased body weight in a time-dependent manner. No physical dependence was apparent in rats administered the low maintenance dose of butorphanol. These data suggest that during chronic treatment with butorphanol, (1) greater degrees of tolerance are conferred to drugs possessing low efficacy at the mu opioid receptor, (2) tolerance is enhanced as the maintenance dose of the toleragen is increased, and (3) mu-opioid tolerance may be observed under conditions that do not produce mu-opioid dependence. Received: 25 November 1997 / Final version: 21 February 1998     

Opioid dependence and cross-dependence in the isolated guinea-pig ileum

The development of opioid dependence and tolerance attributed to selective types of opiate receptors was studied in the isolated ileum of guinea pigs chronically exposed to specific opioids. These investigations were based on reports that in this preparation highly tolerant opiate receptors may coexist with opiate receptors of almost unchanged sensitivity. Thus, the ilea were set up in vitro and tested for tolerance and dependence. Apparently precipitation of the withdrawal contracture, indicating dependence, proved a more sensitive parameter than the phenomenon of tolerance. Maximal dependence was determined at rather low degrees of tolerance (5 to 10 fold). The intensity of the withdrawal contracture failed to increase as opiate tolerance did. Furthermore, the experiments failed to present evidence for the existence of selective dependence at specific opiate receptor types. These findings may suggest multiple adaptational mechanisms upon chronic activation of opiate receptors. One mechanism may be responsible for the development of dependence and a low degree of tolerance, whilst a further increase of tolerance may be associated with changes at the opiate binding site level.     

Opioid ligands with mixed μ/δ opioid receptor interactions: An emerging approach to novel analgesics

Opioids are widely used in the treatment of severe pain. The clinical use of the opioids is limited by serious side effects such as respiratory depression, constipation, development of tolerance, and physical dependence and addiction liabilities. Most of the currently available opioid analgesics exert their analgesic and adverse effects primarily through the opioid mu receptors. A large number of biochemical and pharmacological studies and studies using genetically modified animals have provided convincing evidence regarding the existence of modulatory interactions between opioid mu and delta receptors. Several studies indicate that delta receptor agonists as well as delta receptor antagonists can provide beneficial modulation to the pharmacological effects of mu agonists. For example, delta agonists can enhance the analgesic potency and efficacy of mu agonists, and delta antagonists can prevent or diminish the development of tolerance and physical dependence by mu agonists. On the basis of these observations, the development of new opioid ligands possessing mixed mu agonist/delta agonist profile and mixed mu agonist/delta antagonist profile has emerged as a promising new approach to analgesic drug development. A brief overview of mu-delta interactions and recent developments in identification of ligands possessing mixed mu agonist/delta agonist and mu agonist/delta antagonist activities is provided in this report.     

Experimental Models of Opiate Tolerance and Dependence

Abstract: Since observations in whole animals provide only limited information on the mechanisms underlying tolerance and dependence, less complex isolated tissue, cell culture and single neurone models have been developed, the most widely exploited of which is the guinea pig ileum. Prolonged exposure to morphine leads to tolerance to its acute inhibitory effects. Dependence is indicated by neuronal hyperexcitability when morphine is withdrawn or abruptly displaced from its receptor by naloxone. The mechanism of withdrawal hyperexcitability may be relatively non-selective since responses of both central and peripheral neurones to a variety of excitants are enhanced. Behavioural and biochemical experiments implicate elevated neuronal calcium and adenylate cyclase in the expression of dependence. However the relationship between these observations and the hyperexcitability of single neurones has not been rigorously examined. Moreover the relevance of an analysis of tolerance and physical dependence in experimental models to opioid reward mechanisms is uncertain. Although recent evidence suggests the importance of the ventral tegmental area in the rewarding actions of opiates, the biochemical and electrophysiological effects of exogenous opiates and of endogenous opioids on these neurones remain largely unexplored.     

A new turn of research for morphine dependence]

The World Health Organization (WHO) developed practical guidelines for pain relief in cancer patients in 1986. Although morphine is a standard opioid analgesic with sufficient analgesic potency, it also has undesirable effects such as drug dependence. Considering the significant of the management of patients with chronic cancer pain, it is no exaggeration to say that the investigation of morphine dependence is now most required research for pain relief. Various studies provide arguments to support substantial roles for mu-opioid receptors associated with the mesolimbic dopaminergic pathway and the possible involvement of delta-opioid receptors in the rewarding effect by morphine in animals. By contrast, the activation of kappa-opioid receptors leads to the suppression of this effect of morphine. It is noteworthy that chronic inflammatory nociception enhances endogenous kappa-opioidergic system, leading to the suppression of rewarding effects of morphine. These results obtained from the basic research strongly reflect the clinical results that psychological dependence on morphine is not a major concern when morphine is used to control pain for cancer patients. Another limiting factor in the clinical utilization of opioids is that repeated administration leads to the development of tolerance to opioids. At the cellular level, phosphorylation of opioid receptors by protein kinases, especially G-protein-coupled receptor kinase (GRK) and protein kinase C (PKC), is likely to play a major role in these tolerant and dependent states. We recently found that repeated administration of mu-agonist causes a down regulation of mu-receptor-mediated G-protein activation, which is associated with a specific upregulation of PKC gamma isoform. We therefore propose that PKC gamma may play a critical role in the development of morphine tolerance.     

Analysis of opioid efficacy, tolerance, addiction and dependence from cell culture to human

Opioid agonists are the most effective treatment for pain, but their use is limited by side effects, tolerance and fears of addiction and dependence. A major goal of opioid research is to develop agonists that have high analgesic efficacy and a low profile for side effects, tolerance, addiction and dependence. Unfortunately, there is a serious lack of experimental data comparing the degree to which different opioids produce these effects in humans. In contrast, a wide range of experimental techniques from heterologous expression systems to behaviour assessment in whole animals have been developed to study these problems. The objective of this review is to describe and evaluate these techniques as they are used to study opioid efficacy, tolerance, addiction and dependence.     

Drug dependence and the endogenous opioid system

The discovery of endogenous opioids has markedly influenced the research on the biology of drug dependence. Evidence has been presented that these brain substances are self-administered by laboratory animals. This finding, among others, has led to the hypothesis that endogenous opioids are involved in reinforcing habits, including dependence on drugs of abuse. The course of drug dependence is presented as a continuum from no drug use via controlled use to an actual dependence on the drug. Specific brain opioid systems belonging to four conceptualized brain circuits are described to be involved during the different phases of the drug dependence continuum. More recent research to delineate the role of endogenous opioid systems in drug dependence has focussed on genetic research in humans and animals. Among others, the findings obtained from studies of opioid receptor and opioid peptide precursor knockout mice provided further support for a role of endogenous opioid systems in drug dependence, in agreement with previous pharmacological studies.