Molecular mechanisms and regulation of opioid receptor signaling

Diverse roles of the scaffolding protein RanBPM

Ran-binding protein microtubule-organizing center (RanBPM) appears to function as a scaffolding protein in several signal transduction pathways. RanBPM is a crucial component of multiprotein complexes that regulate the cellular function by modulating and/or assembling with a wide range of proteins in different intracellular regions and thereby mediate diverse cellular functions. This suggests a role for RanBPM as a scaffolding protein. In this article, we have summarized the diverse functions of RanBPM and its interacting partners that have been investigated to date. Also, we have categorized the role of RanBPM into four divisions: RanBPM as a modulator/protein stabilizer, regulator of transcription activity, cell cycle and neurological functions.

Ligand-directed functional selectivity at the mu opioid receptor revealed by label-free integrative pharmacology on-target

Development of new opioid drugs that provide analgesia without producing dependence is important for pain treatment. Opioid agonist drugs exert their analgesia effects primarily by acting at the mu opioid receptor (MOR) sites. High-resolution differentiation of opioid ligands is crucial for the development of new lead drug candidates with better tolerance profiles. Here, we use a label-free integrative pharmacology on-target (iPOT) approach to characterize the functional selectivity of a library of known opioid ligands for the MOR. This approach is based on the ability to detect dynamic mass redistribution (DMR) arising from the activation of the MOR in living cells. DMR assays were performed in HEK-MOR cells with and without preconditioning with probe molecules using label-free resonant waveguide grating biosensors, wherein the probe molecules were used to modify the activity of specific signaling proteins downstream the MOR. DMR signals obtained were then translated into high resolution heat maps using similarity analysis based on a numerical matrix of DMR parameters. Our data indicate that the iPOT approach clearly differentiates functional selectivity for distinct MOR signaling pathways among different opioid ligands, thus opening new avenues to discover and quantify the functional selectivity of currently used and novel opioid receptor drugs.

Biologic mechanisms of oral cancer pain and implications for clinical therapy

Cancer pain is an ever-present public health concern. With innovations in treatment, cancer patients are surviving longer, but uncontrollable pain creates a poor quality of life for these patients. Oral cancer is unique in that it causes intense pain at the primary site and significantly impairs speech, swallowing, and masticatory functions. We propose that oral cancer pain has underlying biologic mechanisms that are generated within the cancer microenvironment. A comprehensive understanding of key mediators that control cross-talk between the cancer and peripheral nervous system, and possible interventions, underlies effective cancer pain management. The purpose of this review is to explore the current studies on oral cancer pain and their implications in clinical management for cancer pain in general. Furthermore, we will explore the endogenous opioid systems and novel cancer pain therapeutics that target these systems, which could solve the issue of opiate tolerance and improve quality of life in oral cancer patients.

Cysteine Cathepsins in the secretory vesicle produce active peptides: Cathepsin L generates peptide neurotransmitters and cathepsin B produces beta-amyloid of Alzheimer's disease

Recent new findings indicate significant biological roles of cysteine cathepsin proteases in secretory vesicles for production of biologically active peptides. Notably, cathepsin L in secretory vesicles functions as a key protease for proteolytic processing of proneuropeptides (and prohormones) into active neuropeptides that are released to mediate cell-cell communication in the nervous system for neurotransmission. Moreover, cathepsin B in secretory vesicles has been recently identified as a β-secretase for production of neurotoxic β- amyloid (Aβ) peptides that accumulate in Alzheimer's disease (AD), participating as a notable factor in the severe memory loss in AD. These secretory vesicle functions of cathepsins L and B for production of biologically active peptides contrast with the well-known role of cathepsin proteases in lysosomes for the degradation of proteins to result in their inactivation. The unique secretory vesicle proteome indicates proteins of distinct functional categories that provide the intravesicular environment for support of cysteine cathepsin functions. Features of the secretory vesicle protein systems insure optimized intravesicular conditions that support the proteolytic activity of cathepsins. These new findings of recently discovered biological roles of cathepsins L and B indicate their significance in human health and disease. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Chronic treatment with the opioid antagonist naltrexone favours the coupling of spinal cord μ-opioid receptors to Gαz protein subunits

Sustained administration of opioid antagonists to rodents results in an enhanced antinociceptive response to agonists. We investigated the changes in spinal μ-opioid receptor signalling underlying this phenomenon. Rats received naltrexone (120 μg/h; 7 days) via osmotic minipumps. The antinociceptive response to the μ-agonist sufentanil was tested 24 h after naltrexone withdrawal. In spinal cord samples, we determined the interaction of μ-receptors with Gα proteins (agonist-stimulated [(35)S]GTPγS binding and immunoprecipitation of [(35)S]GTPγS-labelled Gα subunits) as well as μ-opioid receptor-dependent inhibition of the adenylyl cyclase (AC) activity. Chronic naltrexone treatment augmented DAMGO-stimulated [(35)S]GTPγS binding, potentiated the inhibitory effect of DAMGO on the AC/cAMP pathway, and increased the inverse agonist effect of naltrexone on cAMP accumulation. In control rats, the inhibitory effect of DAMGO on cAMP production was antagonized by pertussis toxin (PTX) whereas, after chronic naltrexone, the effect became resistant to the toxin, suggesting a coupling of μ-receptors to PTX-insensitive Gα(z) subunits. Immunoprecipitation assays confirmed the transduction switch from Gα(i/o) to Gα(z) proteins. The consequence was an enhancement of the antinociceptive response to sufentanil that, in consonance with the neurochemical data, was prevented by Gα(z)-antisense oligodeoxyribonucleotides but not by PTX. Such changes in opioid receptor signalling can be a double-edged sword. On the one hand, they may have potential applicability to the optimisation of the analgesic effects of opioid drugs for the control of pain. On the other hand, they represent an important homeostatic dysregulation of the endogenous opioid system that might account for undesirable effects in patients chronically treated with opioid antagonists. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Regulation of G protein-coupled receptor function by Na+/H+ exchange regulatory factors

Many G protein-coupled receptors (GPCR) exert patterns of cell-specific signaling and function. Mounting evidence now supports the view that cytoplasmic adapter proteins contribute critically to this behavior. Adapter proteins recognize highly conserved motifs such as those for Src homology 3 (SH3), phosphotyrosine-binding (PTB), and postsynaptic density 95/discs-large/zona occludens (PDZ) docking sequences in candidate GPCRs. Here we review the behavior of the Na+/H+ exchange regulatory factor (NHERF) family of PDZ adapter proteins on GPCR signalling, trafficking, and function. Structural determinants of NHERF proteins that allow them to recognize targeted GPCRs are considered. NHERF1 and NHERF2 are capable also of modifying the assembled complex of accessory proteins such as β-arrestins, which have been implicated in regulating GPCR signaling. In addition, NHERF1 and NHERF2 modulate GPCR signaling by altering the G protein to which the receptor binds or affect other regulatory proteins that affect GTPase activity, protein kinase A, phospholipase C, or modify downstream signaling events. Small molecules targeting the site of NHERF1-GPCR interaction are being developed and may become important and selective drug candidates.

Mitochondrial ERK plays a key role in δ-opioid receptor neuroprotection against acute mitochondrial dysfunction

It is well established that stimulating delta-opioid receptor (DOR) with its specific agonists elicits neuroprotection against hypoxia/ischemia. Mitochondrial dysfunction plays a key role in hypoxic neuronal injury, but the effects of DOR activation on mitochondrial dysfunction in neurons are poorly elucidated. In this investigation, we studied the effects of [D-Ala2, D-Leu5] enkephalin (DADLE), a potent DOR agonist, on acute mitochondrial dysfunction and ensuing cell damage induced by sodium azide in primary rat cortical neuronal cultures, and explored possible mechanisms underlying. Here, we show that DADLE reverses NaN(3)-induced acute mitochondrial dysfunction by selectively activating DOR, mainly including mitochondrial membrane depolarization, mitochondrial Ca(2+) overload and reactive oxygen species generation. DOR stimulation also inhibits cytochrome c release and caspase-3 activation, and attenuates neuronal death caused by acute NaN(3) insults. Furthermore, DOR activation with DADLE protects neurons from acute NaN(3) insults mainly through PKC-ERK pathway, and mitochondrial ERK activation is especially required for DOR neuroprotection against acute mitochondrial dysfunction.

Control of striatal signaling by g protein regulators

Signaling via heterotrimeric G proteins plays a crucial role in modulating the responses of striatal neurons that ultimately shape core behaviors mediated by the basal ganglia circuitry, such as reward valuation, habit formation, and movement coordination. Activation of G protein-coupled receptors (GPCRs) by extracellular signals activates heterotrimeric G proteins by promoting the binding of GTP to their α subunits. G proteins exert their effects by influencing the activity of key effector proteins in this region, including ion channels, second messenger enzymes, and protein kinases. Striatal neurons express a staggering number of GPCRs whose activation results in the engagement of downstream signaling pathways and cellular responses with unique profiles but common molecular mechanisms. Studies over the last decade have revealed that the extent and duration of GPCR signaling are controlled by a conserved protein family named regulator of G protein signaling (RGS). RGS proteins accelerate GTP hydrolysis by the α subunits of G proteins, thus promoting deactivation of GPCR signaling. In this review, we discuss the progress made in understanding the roles of RGS proteins in controlling striatal G protein signaling and providing integration and selectivity of signal transmission. We review evidence on the formation of a macromolecular complex between RGS proteins and other components of striatal signaling pathways, their molecular regulatory mechanisms and impacts on GPCR signaling in the striatum obtained from biochemical studies and experiments involving genetic mouse models. Special emphasis is placed on RGS9-2, a member of the RGS family that is highly enriched in the striatum and plays critical roles in drug addiction and motor control.

Pharmacological characterization of 2-methyl-N-((2'-(pyrrolidin-1-ylsulfonyl)biphenyl-4-yl)methyl)propan-1-amine (PF-04455242), a high-affinity antagonist selective for κ-opioid receptors

2-Methyl-N-((2'-(pyrrolidin-1-ylsulfonyl)biphenyl-4-yl)methyl)propan-1-amine (PF-04455242) is a novel κ-opioid receptor (KOR) antagonist with high affinity for human (3 nM), rat (21 nM), and mouse (22 nM) KOR, a ∼ 20-fold reduced affinity for human μ-opioid receptors (MORs; K(i) = 64 nM), and negligible affinity for δ-opioid receptors (K(i) > 4 μM). PF-04455242 also showed selectivity for KORs in vivo. In rats, PF-04455242 blocked KOR and MOR agonist-induced analgesia with ID(50) values of 1.5 and 9.8 mg/kg, respectively, and inhibited ex vivo [(3)H](2-(benzofuran-4-yl)-N-methyl-N-((5S,7R,8R)-7-(pyrrolidin-1-yl)-1-oxaspiro[4.5]decan-8-yl)acetamide ([(3)H]CI977) and [(3)H](2S)-2-[[2-[[(2R)-2-[[(2S)-2-amino-3-(4-hydroxyphenyl) propanoyl]amino]propanoyl]amino]acetyl]-methylamino]-N-(2-hydroxyethyl)-3-phenylpropanamide ([(3)H]DAMGO) binding to KOR and MOR receptors with ID(50) values of 2.0 and 8.6 mg/kg, respectively. An in vivo binding assay was developed using (-)-4-[(3)H]methoxycarbonyl-2-[(1-pyrrolidinylmethyl]-1-[(3,4-dichlorophenyl)acetyl]-piperidine ([(3)H]PF-04767135), a tritiated version of the KOR positron emission tomography ligand (-)-4-[(11)C]methoxycarbonyl-2-[(1-pyrrolidinylmethyl]-1-[(3,4-dichlorophenyl)acetyl]-piperidine ([(11)C]GR103545) in which PF-04455242 had an ID(50) of 5.2 mg/kg. PF-04455242 demonstrated antidepressant-like efficacy (mouse forced-swim test), attenuated the behavioral effects of stress (mouse social defeat stress assay), and showed therapeutic potential in treating reinstatement of extinguished cocaine-seeking behavior (mouse conditioned place preference). KOR agonist-induced plasma prolactin was investigated as a translatable mechanism biomarker. Spiradoline (0.32 mg/kg) significantly increased rat plasma prolactin levels from 1.9 ± 0.4 to 41.9 ± 4.9 ng/ml. PF-04455242 dose-dependently reduced the elevation of spiradoline-induced plasma prolactin with an ID(50) of 2.3 ± 0.1 mg/kg, which aligned well with the ED(50) values obtained from the rat in vivo binding and efficacy assays. These data provide further evidence that KOR antagonists have potential for the treatment of depression and addiction disorders.

Role of mu-opioids as cofactors in human immunodeficiency virus type 1 disease progression and neuropathogenesis

About one third of acquired immunodeficiency syndrome cases in the USA have been attributed to the use of injected addictive drugs, frequently involving opioids like heroin and morphine, establishing them as significant predisposing risk factors for contracting human immunodeficiency virus type 1 (HIV-1). Accumulating evidence from in vitro and in vivo experimental systems indicates that opioids act in concert with HIV-1 proteins to exacerbate dysregulation of neural and immune cell function and survival through diverse molecular mechanisms. In contrast, the impact of opioid exposure and withdrawal on the viral life cycle and HIV-1 disease progression itself is unclear, with conflicting reports emerging from the simian immunodeficiency virus and simian-human immunodeficiency virus infection models. However, these studies suggest a potential role of opioids in elevated viral production. Because human microglia, astrocytes, CD4+ T lymphocytes, and monocyte-derived macrophages express opioid receptors, it is likely that intracellular signaling events triggered by morphine facilitate enhancement of HIV-1 infection in these target cell populations. This review highlights the biochemical changes that accompany prolonged exposure to and withdrawal from morphine that synergize with HIV-1 proteins to disrupt normal cellular physiological functions especially within the central nervous system. More importantly, it collates evidence from epidemiological studies, animal models, and heterologous cell systems to propose a mechanistic link between such physiological adaptations and direct modulation of HIV-1 production. Understanding the opioid-HIV-1 interface at the molecular level is vitally important in designing better treatment strategies for HIV-1-infected patients who abuse opioids.

Removal of polysialylated neural cell adhesion molecule increases morphine analgesia and interferes with tolerance in mice

Neurons that express high levels of polysialylated neural cell adhesion molecule (PSA-NCAM) in adult spinal substantia gelatinosa also express the μ-opioid receptor. While PSA removal from NCAM by spinal intrathecal injection of endoneuraminidase-N (endo-N) did not detectably change opioid receptor expression, morphine-induced analgesia was significantly increased. This analgesic strengthening was detected as early as 15 min after endo-N treatment and persisted for at least 7 days. In addition, the tolerance that develops with chronic morphine treatment was overcome in the absence of PSA. Interestingly, the same effects on analgesia and tolerance were also produced by selective deletion of the NCAM-180 isoform.

Mu opioid signaling protects against acute murine intestinal injury in a manner involving Stat3 signaling

Opiates have long been used as analgesics to relieve pain associated with various medical conditions. Here, we evaluated the effect and mechanism of mu opioid signaling on the intestinal wound healing response and assessed downstream pathways known to be protective against intestinal injury. Mice (C57BL/6) were exposed to 3% dextran sodium sulfate (DSS) for 7 days or 4% DSS for 5 days followed by 7 days of water. The mu opioid receptor (MOR)-specific agonist [D-Arg2,Lys4]dermorphin-(1,4)-amide (DALDA) and the antagonist cyprodime were injected s.c. daily for in vivo studies or used for in vitro analysis. We found that MOR activation attenuated DSS-induced histologic and gross intestinal injury and weight loss; diminished Ifng, Tnf, and Il6 mRNA expression; and promoted intestinal healing during recovery. DALDA also enhanced colonocyte proliferation (Ki-67 staining) by 350%. MOR activation increased Stat3 phosphorylation in both DALDA-treated mice and the CMT-93 cell line. Importantly, DALDA-induced colonocyte migration was completely ablated by shStat3 knockdown. Together, this work shows that MOR activation protects against and enhances recovery from DSS-induced intestinal injury. This is associated with an increase in Stat3 activation. Furthermore, Stat3 is required for DALDA-induced colonocyte migration. Consequently, manipulation of MOR signaling may represent a novel means to promote mucosal healing and to maintain intestinal homeostasis after intestinal injury.

Inhibition of morphine-induced cAMP overshoot: a cell-based assay model in a high-throughput format

Opiates are not only potent analgesics but also drugs of abuse mainly because they produce euphoria. Chronic use of opiates results in the development of tolerance and dependence. Dr Marshall Nirenberg's group at the National Institutes of Health (NIH) was the first to use a cellular model system of Neuroblastoma × Glioma hybrid cells (NG108-15) to study morphine addiction. They showed that opiates affect adenylyl cyclase (AC) by two opposing mechanisms mediated by the opiate receptor. Although the cellular mechanisms that cause addiction are not yet completely understood, the most observed correlative biochemical adaptation is the upregulation of AC. This model also provides the opportunity to look for compounds which could dissociate the acute effect of opiates from the delayed response, upregulation of AC, and thus lead to the discovery of non-addictive drugs. To identify small molecule compounds that can inhibit morphine-induced cAMP overshoot, we have validated and optimized a cell-based assay in a high throughput format that measures cellular cAMP production after morphine withdrawal. The assay performed well in the 1536-well plate format. The LOPAC library of 1,280 compounds was screened in this assay on a quantitative high-throughput screening (qHTS) platform. A group of compounds that can inhibit morphine-induced cAMP overshoot were identified. The most potent compounds are eight naloxone-related compounds, including levallorphan tartrate, naloxonazine dihydrochloride, naloxone hydrochloride, naltrexone hydrochloride, and naltriben methanesulfonate. The qHTS approach we used in this study will be useful in identifying novel inhibitors of morphine induced addiction from a larger scale screening.

β-Endorphin inhibits phagocytic activity of lizard splenic phagocytes through μ receptor-coupled adenylate cyclase-protein kinase A signaling pathway

The receptor-coupled intracellular signaling mechanism of endogenous opioid peptide β-endorphin (β-end) is explored for the first time in ectothermic vertebrates using wall lizard as a model. β-End inhibited the percentage phagocytosis and phagocytic index of lizard splenic phagocytes in a dose-dependent manner. The inhibitory effect of β-end on phagocytosis was completely antagonized by non-selective opioid receptor antagonist naltrexone and also by selective μ-receptor antagonist CTAP. However, selective antagonists for other opioid receptors like NTI for δ-receptor and NorBNI for κ-receptor did not alter the effect of β-end on phagocytosis. This suggests that β-end mediated its inhibitory effect on phagocytic activity of splenic phagocytes exclusively through μ opioid receptors. The μ opioid receptor-coupled downstream signaling cascade was subsequently explored using inhibitors of adenylate cyclase (SQ 22536) and protein kinase A (H-89). Both SQ 22536 and H-89 abolished the inhibitory effect of β-end on phagocytosis in a concentration-related manner. Implication of cAMP as second messenger was corroborated by cAMP assay where an increase in intracellular cAMP level was observed in response to β-end treatment. It can be concluded that β-end downregulated the phagocytic activity of lizard splenic phagocytes through μ opioid receptor-coupled adenylate cyclase-cAMP-protein kinase A pathway.

Multiple targets of μ-opioid receptor-mediated presynaptic inhibition at primary afferent Aδ- and C-fibers

Agonists at μ-opioid receptors (MORs) represent the gold standard for the treatment of severe pain. A key element of opioid analgesia is the depression of nociceptive information at the first synaptic relay in spinal pain pathways. The underlying mechanisms are, however, largely unknown. In spinal cord slices with dorsal roots attached prepared from young rats, we determined the inhibitory effect of the selective MOR agonist [d-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) on monosynaptic Aδ- and C-fiber-evoked EPSCs in lamina I neurons. DAMGO depressed presynaptically Aδ- and C-fiber-mediated responses, indicating that MORs are expressed on central terminals of both fiber types. We next addressed the mechanisms of presynaptic inhibition. The effect of DAMGO at both Aδ- and C-fiber terminals was mainly mediated by an inhibition of N-type voltage-dependent Ca(2+) channels (VDCCs), and to a lesser extent of P/Q-type VDCCs. Inhibition by DAMGO was not reduced by K(+) channel blockers. The rate of miniature EPSCs was reduced by DAMGO in a dose-dependent manner. The opioid also reduced Ca(2+)-dependent, ionomycin-induced EPSCs downstream of VDCCs. DAMGO had no effect on the kinetics of vesicle exocytosis in C-fiber terminals, but decreased the rate of unloading of Aδ-fiber boutons moderately, as revealed by two-photon imaging of styryl dye destaining. Together, these results suggest that binding of opioids to MORs reduces nociceptive signal transmission at central Aδ- and C-fiber synapses mainly by inhibition of presynaptic N-type VDCCs. P/Q-type VDCCs and the transmitter release machinery are targets of opioid action as well.

Differential response to morphine of the oligomeric state of μ-opioid in the presence of δ-opioid receptors

Prolonged morphine treatment induces extensive desensitization of the μ-opioid receptor (μOR) which is the G-protein-coupled receptor that primarily mediates the cellular response to morphine. To date, the molecular mechanism underlying this process is unknown. Here, we have used live cell fluorescence imaging to investigate whether prolonged morphine treatment affects the physical environment of μOR, or its coupling with G-proteins, in two neuronal cell lines. We find that chronic morphine treatment does not change the amount of enhanced yellow fluorescence protein (eYFP)-tagged μOR on the plasma membrane, and only slightly decreases its association with G-protein subunits. Additionally, morphine treatment does not have a detectable effect on the diffusion coefficient of eYFP-μOR. However, in the presence of another family member, the δ-opioid receptor (δOR), prolonged morphine exposure results in a significant increase in the diffusion rate of μOR. Number and brightness measurements suggest that μOR exists primarily as a dimer that will oligomerize with δOR into tetramers, and morphine promotes the dissociation of these tetramers. To provide a plausible structural context to these data, we used homology modeling techniques to generate putative configurations of μOR-δOR tetramers. Overall, our studies provide a possible rationale for morphine sensitivity.

Regulation of male fertility by the opioid system

Endogenous opioid peptides are substances involved in cell communication. They are present in various organs and tissues of the male and female reproductive tract, suggesting that they may regulate some of the processes involved in reproductive function. In fact, the opioid system that operates as a multi-messenger system can participate in the regulation of reproductive physiology at multiple levels, for example, at the levels of the central nervous system, at the testes level and at sperm level. A better understanding of the implication of the opioid system in reproductive processes may contribute to clarifying the etiology of many cases of infertility and the effect of opiate abuse on fertility. Indeed, a novel biochemical tool for the diagnosis and treatment of male infertility could be based upon components of the opioid system. The presence of the opioid system in sperm cells also represents a novel opportunity for reproductive management, for either enhancing the probability of fertilization or reducing it through the development of novel targeted contraceptives.

The influence of monovalent cations on trimeric G protein G(i)1α activity in HEK293 cells stably expressing DOR-G(i)1α (Cys(351)-Ile(351)) fusion protein

The effect of monovalent cations on trimeric G protein G(i)1α was measured at equimolar concentration of chloride anion in pertussis-toxin (PTX)-treated HEK293 cells stably expressing PTX-insensitive DOR- G(i)1α (Cys(351)-Ile(351)) fusion protein by high-affinity [(35)S]GTPgammaS binding assay. The high basal level of binding was detected in absence of DOR agonist and monovalent ions and this high level was inhibited with the order of: Na(+) > K(+) > Li(+). The first significant inhibition was detected at 1 mM NaCl. The inhibition by monovalent ions was reversed by increasing concentrations of DOR agonist DADLE. The maximum DADLE response was also highest for sodium and decreased in the order of: Na(+) > K(+) ~ Li(+). Our data indicate i) an inherently high activity of trimeric G protein G(i)1α when expressed within DOR- G(i)1α fusion protein and determined in the absence of monovalent cations, ii) preferential sensitivity of DOR- G(i)1alpha to sodium as far as maximum of agonist response is involved.

[Do opioids induce hyperalgesia?]

Opioids are the most potent drugs for treatment of acute and chronic pain. However, accumulating evidence suggests that opioids may paradoxically also enhance pain, often referred to as opioid-induced hyperalgesia. Opioid-induced hyperalgesia is defined as an increased sensitivity to pain or a decreased pain threshold in response to opioid therapy. Several mechanisms have been proposed to support opioid-induced hyperalgesia. However, it remains unclear whether opioid-induced hyperalgesia develops during continuous chronic application of opioids or on their withdrawal. This review provides a comprehensive summary of clinical research concerning opioid-induced hyperalgesia and the molecular mechanisms of opioid withdrawal and opioid tolerance and other potential mechanisms which might induce hyperalgesia during opioid therapy will be discussed. The status quo of our knowledge will be summarized and the clinical relevance of opioid-induced hyperalgesia will be discussed.

Opioids: a two-faced Janus

BACKGROUND

Long-term pain is a debilitating condition that is costly to treat and has a significant impact on patient quality of life. Classical opioids have been used for the treatment of pain for centuries and are one of the most effective drug classes available for acute severe pain and long-term pain. However, concerns regarding adverse effects, tolerance to analgesic effects and the potential for addiction have resulted in a reluctance to prescribe and use opioids for the management of long-term non-cancer pain. Adverse events, including gastrointestinal side effects such as constipation, nausea and vomiting, and central nervous system side effects such as sedation are responsible for as many as one in five patients discontinuing opioid treatment, often leading to inadequate pain relief and poor patient quality of life. Therefore, new analgesic therapies are needed that are associated with fewer adverse effects, whilst providing sustainable pain relief for patients with long-term pain.

OBJECTIVE AND METHODS

To provide an overview of the historical development, uses, mechanisms of action, receptor affinities and side-effect profiles of classical opioids. In addition, recent developments and novel approaches for long-term, severe pain treatment are also reviewed.

RESULTS

A number of treatment strategies were identified: co-administration with opioid-sparing analgesics to reduce side effects and/or risk of dependence, the use of peripheral opioid antagonists and novel delivery mechanisms to reduce side effects, the development of non-opioid agents that reduce side effects and enhance analgesia such as glial cell modulators, and the development of novel agents with combined µ-receptor and monoaminergic activity within the same molecule.

CONCLUSIONS

Despite these recent advances, there have been very few completely novel drug developments. Hence, there remains a continuing need for innovative therapeutic strategies for the treatment of long-term pain. The most promising alternatives appear to be the use of traditional opioids together with peripheral opioid antagonists, combining opioids with glial cell modulators, and the use of novel agents with µ-receptor agonist and noradrenaline reuptake inhibitor activity within the same molecule (MOR-NRI compounds).

Phe27Cys polymorphism of the human delta opioid receptor predisposes cells to compromised calcium signaling

A quarter of the human population with European background carries at least one allele of the OPRD1 gene that encodes the delta opioid receptor with cysteine at the amino acid position 27 (hδOR(Cys27)) instead of the evolutionary conserved phenylalanine (hδOR(Phe27)). The two variants have indistinguishable pharmacological properties but, importantly, hδOR(Cys27) differs from hδOR(Phe27) in having low maturation efficiency, lower stability at the cell surface and pronounced intracellular location. Both variants were previously shown to interact with the Sarco(endo)plasmic reticulum Ca²+ ATPase (SERCA) 2b in the early phase of their biosynthesis. We analyzed by pulse-chase assays, whether cellular signaling can affect hδOR(Cys27) maturation. Neither activation of the receptor by a δOR-specific agonist Leu-enkephalin, induction of intracellular calcium (Ca²+) release by ATP nor the direct stimulation of SERCA 2b by protein kinase C activation affected receptor maturation in HEK-293 cells. No signaling-mediated regulation of receptor maturation could therefore be demonstrated. Instead, we found by using single cell Ca²+ measurements that over-expression of hδOR(Cys27), but not hδOR(Phe27), compromised ATP-induced intracellular Ca²+-signaling. Furthermore, hδOR(Cys27) precursors showed slower dissociation from SERCA2b and hδOR(Cys27) expression caused down-regulation of the homocysteine-inducible endoplasmic reticulum-resident ubiquitin domain-like member 1 protein (HERP). We suggest that aging individuals with at least one hδOR(Cys27) encoding allele might have lowered threshold for Ca²+ dysregulation in neurons expressing hδOR.

The novel role of cathepsin L for neuropeptide production illustrated by research strategies in chemical biology with protease gene knockout and expression

Neuropeptides are essential for cell-cell communication in the nervous and endocrine systems. Production of active neuropeptides requires proteolytic processing of proneuropeptide precursors in secretory vesicles that produce, store, and release neuropeptides that regulate physiological functions. This review describes research strategies utilizing chemical biology combined with protease gene knockout and expression to demonstrate the key role of cathepsin L for production of neuropeptides in secretory vesicles. Cathepsin L was discovered using activity-based probes and mass spectrometry to identify proenkephalin cleaving activity as cathepsin L. Significantly, in vivo protease gene knockout and expression approaches illustrate the key role of cathepsin L for neuropeptide production. Notably, cathepsin L is colocalized with neuropeptide secretory vesicles, the major site of proteolytic processing of proneuropeptides to generate active neuropeptides. Cathepsin L participates in producing opioid neuropeptides consisting of enkephalin, β-endorphin, and dynorphin, as well as in generating the POMC-derived peptide hormones ACTH and α-MSH. In addition, NPY, CCK, and catestatin neuropeptides utilize cathepsin L for their biosynthesis. The role of cathepsin L for neuropeptide production indicates its unique biological role in secretory vesicles, which contrasts with its role in lysosomes for protein degradation. Interesting evaluations of protease gene knockout studies in mice that lack cathepsin L compared to the PC1/3 and PC2 (PC, prohormone convertase) indicate the significant role of cathepsin L in neuropeptide production. Thus, dual cathepsin L and prohormone convertase protease pathways participate in neuropeptide production. These recent new findings indicate cathepsin L as a novel 'proprotein convertase' for production of neuropeptides that mediate cell-cell communication in health and disease.

The role of ubiquitination in lysosomal trafficking of δ-opioid receptors

The δ-opioid receptor (DOR) undergoes ligand-induced downregulation by endosomal sorting complex required for transport (ESCRT)-dependent endocytic trafficking to lysosomes. In contrast to a number of other signaling receptors, the DOR can downregulate effectively when its ubiquitination is prevented. We explored the membrane trafficking basis of this behavior. First, we show that internalized DORs traverse the canonical multivesicular body (MVB) pathway and localize to intralumenal vesicles (ILVs). Second, we show that DOR ubiquitination stimulates, but is not essential for, receptor transfer to ILVs and proteolysis of the receptor endodomain. Third, we show that receptor ubiquitination plays no detectable role in the early sorting of internalized DORs out of the recycling pathway. Finally, we show that DORs undergo extensive proteolytic fragmentation in the ectodomain, even when receptor ubiquitination is prevented or ILV formation itself is blocked. Together, these results are sufficient to explain why DORs downregulate effectively in the absence of ubiquitination, and they place a discrete molecular sorting operation in the MVB pathway effectively upstream of the ESCRT. More generally, these findings support the hypothesis that mammalian cells can control the cytoplasmic accessibility of internalized signaling receptors independently from their ultimate trafficking fate.

Unique biological function of cathepsin L in secretory vesicles for biosynthesis of neuropeptides

Neuropeptides are essential for cell-cell communication in the nervous and neuroendocrine systems. Production of active neuropeptides requires proteolytic processing of proneuropeptide precursors in secretory vesicles that produce, store, and release neuropeptides that regulate physiological functions. This review describes recent findings indicating the prominent role of cathepsin L in secretory vesicles for production of neuropeptides from their protein precursors. The role of cathepsin L in neuropeptide production was discovered using the strategy of activity-based probes for proenkephalin-cleaving activity for identification of the enzyme protein by mass spectrometry. The novel role of cathepsin L in secretory vesicles for neuropeptide production has been demonstrated in vivo by cathepsin L gene knockout studies, cathepsin L gene expression in neuroendocrine cells, and notably, cathepsin L localization in neuropeptide-containing secretory vesicles. Cathepsin L is involved in producing opioid neuropeptides consisting of enkephalin, β-endorphin, and dynorphin, as well as in generating the POMC-derived peptide hormones ACTH and α-MSH. In addition, NPY, CCK, and catestatin neuropeptides utilize cathepsin L for their biosynthesis. The neuropeptide-synthesizing functions of cathepsin L represent its unique activity in secretory vesicles, which contrasts with its role in lysosomes. Interesting evaluations of protease gene knockout studies in mice that lack cathepsin L compared to those lacking PC1/3 and PC2 (PC, prohormone convertase) indicate the key role of cathepsin L in neuropeptide production. Therefore, dual cathepsin L and prohormone convertase protease pathways participate in neuropeptide production. Significantly, the recent new findings indicate cathepsin L as a novel 'proprotein convertase' for production of neuropeptides that mediate cell-cell communication in health and disease.

Rapid delivery of internalized signaling receptors to the somatodendritic surface by sequence-specific local insertion

The recycling pathway is a major route for delivering signaling receptors to the somatodendritic plasma membrane. We investigated the cell biological basis for the remarkable selectivity and speed of this process. We focused on the mu-opioid neuropeptide receptor and the beta(2)-adrenergic catecholamine receptor, two seven-transmembrane signaling receptors that traverse the recycling pathway efficiently after ligand-induced endocytosis and localize at steady state throughout the postsynaptic surface. Rapid recycling of each receptor in dissociated neuronal cultures was mediated by a receptor-specific cytoplasmic sorting sequence. Total internal reflection fluorescence microscopy imaging revealed that both sequences drive recycling via discrete vesicular fusion events in the cell body and dendritic shaft. Both sequences promoted recycling via "transient"-type events characterized by nearly immediate lateral spread of receptors after vesicular insertion resembling receptor insertion events observed previously in non-neural cells. The sequences differed in their abilities to produce distinct "persistent"-type events at which inserted receptors lingered for a variable time period before lateral spread. Both types of insertion event generated a uniform distribution of receptors in the somatodendritic plasma membrane when imaged over a 1 min interval, but persistent events uniquely generated a punctate surface distribution over a 10 s interval. These results establish sequence-directed recycling of signaling receptors in CNS neurons and show that this mechanism has the ability to generate receptor-specific patterns of local surface distribution on a timescale overlapping that of rapid physiological signaling.

Activation of phosphatidylinositol 3-kinase/Akt-mammalian target of Rapamycin signaling pathway in the hippocampus is essential for the acquisition of morphine-induced place preference in rats

Hippocampus is a critical structure for the acquisition of morphine-induced conditioned place preference (CPP), which is a usual learning paradigm for assessing drug reward. However, the precise mechanisms remain largely unknown. Phosphatidylinositol 3-kinase (PI3K) and its downstream targets, including Akt, mammalian target of Rapamycin (mTOR) and 70-kDa ribosomal S6 kinase (p70S6K), are critical molecules implicated in learning and memory. Here, we tested the role of PI3K/Akt-mTOR-p70S6K signaling pathway in morphine-induced CPP in the hippocampus. Our results showed that the acquisition of morphine CPP increased phosphorylation of Akt in the hippocampal CA3, but not in the nucleus accumbens (NAc), the ventral tegmental area (VTA) or the CA1. Moreover, the phosphorylated Akt exclusively expressed in the CA3 neurons. Likewise, levels of phosphorylated mTOR and p70S6K were significantly enhanced in the CA3 following morphine CPP. The alterations of these phosphorylated proteins are positively correlated with the acquisition of morphine CPP. More importantly, microinjection of PI3K inhibitor (LY294002) or mTOR inhibitor (Rapamycin) into the CA3 prevented the acquisition of CPP and inhibited the activation of PI3K-Akt signaling pathway. In addition, pre-infusion of β-FNA (β-funaltrexamine hydrochloride), a selective irreversible μ opioid receptor antagonist, into CA3 significantly prevented the acquisition of CPP and impaired Akt phosphorylation. All these results strongly implied that the PI3K-Akt signaling pathway activated by μ opioid receptor in hippocampal CA3 plays an important role in acquisition of morphine-induced CPP.

Changes in the gene expression of specific G-protein subunits correlate with morphine insensitivity in streptozotocin-induced diabetic rats

Several animal and human studies have shown a decreased analgesic potency of morphine in diabetic subjects. Since G-protein subunits have an important role in morphine effects at the cellular level and the exact mechanism(s) of diabetes-induced morphine insensitivity has not been fully clarified yet, the present study was designed to determine the changes in the levels of G(alphai), G(alphas), G(beta) mRNAs and proteins involved in this phenomenon. All experiments were carried out on male Wistar rats. The tail-flick test was used to assess the nociceptive threshold. Diabetes was induced by injection of 50 mg/kg (i.p.) streptozotocin. Four weeks after diabetes induction, the dorsal half of the lumbar spinal cord was assayed for the expression of G-protein subunits using semiquantitative RT-PCR and immunoblotting. The antinociceptive effect of intrathecal morphine (5, 10 and 15 microg i.t.) was significantly reduced in diabetic rats and these effects were reversed with insulin replacement. In diabetic animals, a significant increase in the mRNA levels of G(alphai) (23.5%) was observed in the dorsal portion of the lumbar spinal cord. The mRNA level of G(alphas) and G(beta) did not change. Following diabetes a significant decrease in the protein levels of G(alphai) was induced. In contrast, no significant changes were observed in the protein level of G(alphas) and G(beta). In diabetic animals that received insulin, levels of G(alphai) mRNA and protein were close to those in control rats. In conclusion, our results demonstrate that the expression pattern of the cellular components involved in morphine analgesia changes in diabetic animals. This may be, at least partly, responsible for diabetes-induced morphine insensitivity.

Dual regulation of mu opioid receptors in SK-N-SH neuroblastoma cells by morphine and interleukin-1β: evidence for opioid-immune crosstalk

Treatment of SK-N-SH cells with morphine and interleukin-1beta (IL-1β) produced dual regulation of the mRNA for the human mu opioid receptor (MOR) protein. Morphine produced a decrease in the MOR mRNA while IL-1β increased it, as assessed by real-time quantitative PCR. These data were consistent with immunocytochemical studies of treated and untreated cells. Morphine-mediated down-regulation of MOR was blocked by naltrexone and IL-1β-induced up-regulation of MOR was blocked by interleukin-1 receptor type 1 antagonist. Immune-opioid crosstalk was examined by IL-1β and morphine co-treatment. These data are the first to show dual regulation of MOR in neuroblastoma cells.

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.

Disruption of neuronal CXCR4 function by opioids: preliminary evidence of ferritin heavy chain as a potential etiological agent in neuroAIDS

The chemokine CXCL12 and its receptor, CXCR4, regulate neuronal migration, differentiation, and survival. Alterations of CXCL12/CXCR4 signaling are implicated in different neuropathologies, including the neurological complications of HIV infection. Opiates are important co-factors for progression to neuroAIDS and can disrupt the CXCL12/CXCR4 axis in vitro and in vivo. This paper will review recently identified mechanisms of opiate-induced CXCR4 impairment in neurons and introduce results from pilot studies in human brain tissue, which highlight the role of the protein ferritin heavy chain in HIV neuropathology in patients with history of drug abuse.

Opioid modulation of cell proliferation in the ventricular zone of adult zebra finches (Taenopygia guttata)

Besides modulating pain, stress, physiological functions, motivation, and reward, the opioid system has been implicated in developmental and adult mammalian neurogenesis and gliogenesis. In adult male songbirds including zebra finches, neurons generated from the ventricular zone (VZ) of the lateral ventricles are incorporated throughout the telencephalon, including the song control nuclei, HVC, and area X. Although the endogenous opioid met-enkephalin is present in neurons adjacent to the VZ and is upregulated in song control regions during singing, it is not known whether the opioid system can modulate adult neurogenesis/gliogenesis in zebra finches. We used quantitative RT-PCR and in situ hybridization to demonstrate that μ- and δ-opioid receptors are expressed by the VZ of adult male zebra finches. Treating cultured VZ cells from male birds with the opioid antagonist naloxone led to an increase in cell proliferation measured by 5-bromo-2-deoxyuridine incorporation, whereas administering met-enkephalin had the opposite effect, compared with saline-treated cultures. Systemically administering naloxone (2.5 mg/kg body wt) to adult male zebra finches for 4 d also led to a significant increase in cell proliferation in the ventral VZ of these birds, compared with saline-treated controls. Our results show that cell proliferation is augmented by naloxone in the VZ adjacent to the anterior commissure, suggesting that the endogenous opioids modulate adult neurogenesis/gliogenesis by inhibiting cell proliferation in songbirds.

The endogenous opioid system: a common substrate in drug addiction

Drug addiction is a chronic brain disorder leading to complex adaptive changes within the brain reward circuits that involve several neurotransmitters. One of the neurochemical systems that plays a pivotal role in different aspects of addiction is the endogenous opioid system (EOS). Opioid receptors and endogenous opioid peptides are largely distributed in the mesolimbic system and modulate dopaminergic activity within these reward circuits. Chronic exposure to the different prototypical drugs of abuse, including opioids, alcohol, nicotine, psychostimulants and cannabinoids has been reported to produce significant alterations within the EOS, which seem to play an important role in the development of the addictive process. In this review, we will describe the adaptive changes produced by different drugs of abuse on the EOS, and the current knowledge about the contribution of each component of this neurobiological system to their addictive properties.

Regulation of opioid receptors by endocytic membrane traffic: mechanisms and translational implications

Opioid neuropeptide receptors mediate diverse physiological functions and are important targets for both therapeutic and abused drugs. Opioid receptors are highly regulated in intact cells, and there is reason to believe that this regulation controls the clinical effects of opioid drugs. The present review will discuss some of this evidence, focusing specifically on the regulation of opioid receptors by endocytic membrane trafficking mechanisms. First, some basic principles of regulated endocytosis will be reviewed, and the principle of 'molecular sorting' as a means to determine the functional consequences of endocytosis will be introduced, Most of this information has been derived from studies of simplified cell models. Second, present knowledge about the operation of these mechanisms in physiologically relevant CNS neurons will be discussed, focusing on studies of neurons cultured from rodent brain. Third, recent insight into the effects of endocytic trafficking on opioid regulation in vivo will be considered, focusing on results from studies of transgenic mouse models. Much remains to be learned at these pre-clinical levels, and effects of endocytosis on opioid actions in humans remain completely unexplored. Two particular insights, which have emerged from pre-clinical studies, will be proposed for translational consideration.

Multi-component signaling complexes of the delta-opioid receptor with STAT5B and G proteins

Besides mediating opioid responses in the nervous system and the peripheral tissues, opioid receptors are implicated in signaling mechanisms shared by cytokine receptors. Recent observations have shown that the Signal Transducer and Activator of Transcription 5A (STAT5A) interacts with the mu-opioid receptor (mu-OR) and is phosphorylated upon mu-OR stimulation (Mazarakou and Georgoussi, 2005). In the present study we demonstrate that another member of the STAT family, STAT5B, associates constitutively with the C-terminal tail of the delta-opioid receptor (delta-CT). [D-Ser(2), Leu(5), Thr(6)]-enkephalin-exposure of HEK293 cells, expressing stably the delta-opioid receptor (delta-OR), leads to receptor-dependent STAT5B tyrosine phosphorylation and transcriptional activation. This phosphorylation occurs in a G protein-dependent manner and is carried out by a c-Src kinase. Co-immunoprecipitation studies indicate that STAT5B forms pairs with selective Galpha and Gbetagamma subunits of G proteins and activated c-Src kinase in HEK293 cells. These interactions are formed either constitutively, or upon receptor stimulation. We also demonstrate that the delta-CT serves as a platform for the formation of a multi-component signaling complex (signalosome), consisting of STAT5B, c-Src and selective G protein members. We can thus conclude that STAT5B signaling can be modulated by its coupling with a specific subset of G protein subunits, revealing a novel signaling mechanism for the transcriptional regulation of STAT5B-dependent genes.

Up-regulation of the mu-opioid receptor gene is mediated through chromatin remodeling and transcriptional factors in differentiated neuronal cells

The effects of morphine are mediated mainly through the mu opioid receptor (MOR). Expression of the MOR is up-regulated during neuronal differentiation in P19 embryonal carcinoma cells and epigenetic changes play an important role in MOR up-regulation. This study investigates the basis for differentiation-dependent alterations of MOR chromatin by studying the recruitment or dissociation of several factors to the remodeled chromatin locus. Chromatin immunoprecipitation assays were used to demonstrate the recruitment of the transcriptional activator Sp1 and the chromatin remodeling factors Brg1 and BAF155 to this promoter, as well as the dissociation of repressors [histone deacetylases, mSin3A, Brm, and methyl-CpG-binding protein 2 (MeCP2)]. Histone modifications (acetylation, induction of histone H3-lys4 methylation, and reduction of H3-lys9 methylation) were consistently detected on this promoter. Overexpression of Sp1 strongly enhanced MOR promoter activity, and the histone deacetylase inhibitor trichostatin A also increased promoter activity. In vitro DNA CpG-methylation of the promoter partially blocked binding of the Sp1 factor but induced MeCP2 binding. Coimmunoprecipitation studies also found novel evidence of an endogenous MeCP2 interaction with Sp3 but a weaker interaction with Sp1. Overall, the results suggest that during neuronal differentiation, MeCP2 and DNA methylation mediate remodeling of the MOR promoter by chromatin remodeling factors (Brg1 and BAF155) from a compacted state to a conformation allowing access for transcriptional factors. Subsequent recruitment of the activating transcription factor Sp1 to the remodeled promoter results in MOR up-regulation.

Neurochemical properties of enkephalinergic neurons in lumbar spinal dorsal horn revealed by preproenkephalin-green fluorescent protein transgenic mice

Enkephalin (ENK) has been implicated in nociceptive transmission in the spinal cord while its functional role is not clear because of difficulties in ideally visualizing ENKergic neurons. We thus developed preproenkephalin-green fluorescent protein transgenic mice to better identify ENKergic neurons. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) together with immunohistochemistry and in situ hybridization were first employed to confirm the successful transgenic manipulation and its application in showing spinal ENKergic neurons. The proportions of ENKergic neurons in the spinal cord laminae I, II, III and IV-VI among dorsal horn neurons were 15.8 +/- 3.1%, 39.5 +/- 3.3%, 11.8 +/- 1.9% and 10.7 +/- 2.1%, respectively. Double labeling with other molecules was then performed to further clarify the neurochemical properties of spinal ENKergic neurons. GABA was found to exist in 42.9 +/- 2.8% of ENKergic neurons that were mainly located in lamina I-III. The proportions of parvalbumin-, calretinin-, calbindin- and neuronal nitric oxide synthase-positive cells among the ENKergic neurons were 5.2 +/- 0.7%, 42.6 +/- 2.3%, 25.8 +/- 2.2% and 11.1 +/- 1.6%, respectively. Compared with previously findings obtained with ENK antibody labeling, this line of newly generated mice can be a reliable tool for the study of specific spinal ENKergic neuronal population.

Agonist-selective signaling of G protein-coupled receptor: mechanisms and implications

Agonist-selective signaling or ligand-biased signaling of G protein-coupled receptor (GPCR) has become the focus of an increasing number of laboratories. The principle of this concept is that agonist possesses different abilities to activate different signaling pathways. Current review summarizes the observations of agonist-selective signaling of various GPCRs, indicating the significance of agonist-selective signaling in biological processes. In addition, current review also provides an overview on how agonist-selective signaling is initiated. Especially, the relationship between GPCR-G protein interaction and GPCR-beta-arrestin interaction is discussed in depth.

Pharmacogenetics of drug dependence: role of gene variations in susceptibility and treatment

Drug dependency is a highly prevalent mental health disorder that imposes a significant burden on those directly affected, health care systems, and society in general. There is substantial heritability in the susceptibility to drug addiction, which indicates that there are genetic risk factors. Variation in the human genome is abundant and can directly affect drug dependency phenotypes, for example, by altering the function of a gene product or by altering gene expression. Pharmacogenetic studies can assess the effects of genetic variation on the risk for a particular phenotype (e.g., being an alcoholic). In addition, pharmacogenetic variability in treatment efficacy and adverse reactions can be investigated to identify particular genetic variants associated with altered responses. This review highlights examples of genetic variations that are important in the development and maintenance of specific drug dependencies as well as those that affect the response to treatment.

Modulation of opiate-related signaling molecules in morphine-dependent conditioned behavior: conditioned place preference to morphine induces CREB phosphorylation

Opiate addiction is a chronic, relapsing behavioral disorder where learned associations that develop between the abused opiate and the environment in which it is consumed are brought about through Pavlovian (classical) conditioning processes. However, the signaling mechanisms/pathways regulating the mechanisms that underlie the responses to opiate-associated cues or the development of sensitization as a consequence of repeated context-independent administration of opiates are unknown. In this study we examined the phosphorylation levels of various classic signaling molecules in brain regions implicated in addictive behaviors after acute and repeated morphine administration. An unbiased place conditioning protocol was used to examine changes in phosphorylation that are associated with (1) the expression of the rewarding effects of morphine and (2) the sensitization that develops to this effect. We also examined the effects of a delta-receptor antagonist on morphine-induced conditioned behavior and on the phosphorylation of classic signaling molecules in view of data showing that blockade of delta-opioid receptor (deltaOR) prevents the development of sensitization to the rewarding effects of morphine. We find that CREB phosphorylation is specifically induced upon the expression of a sensitized response to morphine-induced conditioned behavior in brain areas related to memory consolidation, such as the hippocampus and cortex. A similar effect is also observed, albeit to a lesser extent, in the case of the GluR1 subunit of AMPA glutamate receptor. These increases in the phosphorylation levels of CREB and pGluR1 are significantly blocked by pretreatment with a deltaOR antagonist. These results indicate a critical role for phospho-CREB, AMPA, and deltaOR activities in mediating the expression of a sensitized response to morphine-dependent conditioned behavior.

Dysbindin promotes the post-endocytic sorting of G protein-coupled receptors to lysosomes

Background

Dysbindin, a cytoplasmic protein long known to function in the biogenesis of specialized lysosome-related organelles (LROs), has been reported to reduce surface expression of D2 dopamine receptors in neurons. Dysbindin is broadly expressed, and dopamine receptors are members of the large family of G protein-coupled receptors (GPCRs) that function in diverse cell types. Thus we asked if dysbindin regulates receptor number in non-neural cells, and further investigated the cellular basis of this regulation.

Methodology/Principal Findings

We used RNA interference to deplete endogenous dysbindin in HEK293 and HeLa cells, then used immunochemical and biochemical methods to assess expression and endocytic trafficking of epitope-tagged GPCRs. Dysbindin knockdown up-regulated surface expression of D2 receptors compared to D1 receptors, as reported previously in neurons. This regulation was not mediated by a change in D2 receptor endocytosis. Instead, dysbindin knockdown specifically reduced the subsequent trafficking of internalized D2 receptors to lysosomes. This distinct post-endocytic sorting function explained the minimal effect of dysbindin depletion on D1 receptors, which recycle efficiently and traverse the lysosomal pathway to only a small degree. Moreover, dysbindin regulated the delta opioid receptor, a more distantly related GPCR that is also sorted to lysosomes after endocytosis. Dysbindin was not required for lysosomal trafficking of all signaling receptors, however, as its depletion did not detectably affect down-regulation of the EGF receptor tyrosine kinase. Dysbindin co-immunoprecipitated with GASP-1 (or GPRASP-1), a cytoplasmic protein shown previously to modulate lysosomal trafficking of D2 dopamine and delta opioid receptors by direct interaction, and with HRS that is a core component of the conserved ESCRT machinery mediating lysosome biogenesis and sorting.

Conclusions/Significance

These results identify a distinct, and potentially widespread function of dysbindin in promoting the sorting of specific GPCRs to lysosomes after endocytosis.

Agonist-dependent mu-opioid receptor signaling can lead to heterologous desensitization

Desensitization of the micro-opioid receptor (MOR) has been implicated as an important regulatory process in the development of tolerance to opiates. Monitoring the release of intracellular Ca(2+) ([Ca(2+)](i)), we reported that [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO)-induced receptor desensitization requires receptor phosphorylation and recruitment of beta-arrestins (betaArrs), while morphine-induced receptor desensitization does not. In current studies, we established that morphine-induced MOR desensitization is protein kinase C (PKC)-dependent. By using RNA interference techniques and subtype specific inhibitors, PKCepsilon was shown to be the PKC subtype activated by morphine and the subtype responsible for morphine-induced desensitization. In contrast, DAMGO did not increase PKCepsilon activity and DAMGO-induced MOR desensitization was not affected by modulating PKCepsilon activity. Among the various proteins within the receptor signaling complex, Galphai2 was phosphorylated by morphine-activated PKCepsilon. Moreover, mutating three putative PKC phosphorylation sites, Ser(44), Ser(144) and Ser(302) on Galphai2 to Ala attenuated morphine-induced, but not DAMGO-induced desensitization. In addition, pretreatment with morphine desensitized cannabinoid receptor CB1 agonist WIN 55212-2-induced [Ca(2+)](i) release, and this desensitization could be reversed by pretreating the cells with PKCepsilon inhibitor or overexpressing Galphai2 with the putative PKC phosphorylation sites mutated. Thus, depending on the agonist, activation of MOR could lead to heterologous desensitization and probable crosstalk between MOR and other Galphai-coupled receptors, such as the CB1.

[The endogenous opioid system and drug addiction]

Drug addiction is a chronic brain disorder leading to complex adaptive changes within the brain reward circuits. Several neurotransmitters, including the endogenous opioid system are involved in these changes. The opioid system plays a pivotal role in different aspects of addiction. Thus, opioid receptors and endogenous opioid peptides are largely distributed in the mesolimbic system and modulate dopaminergic activity within the reward circuits. Opioid receptors and peptides are selectively involved in several components of the addictive processes induced by opioids, cannabinoids, psychostimulants, alcohol and nicotine. This review is focused on the contribution of each component of the endogenous opioid system in the addictive properties of the different drugs of abuse.