Involvement of mitogen-activated protein kinase in agonist-induced phosphorylation of the mu-opioid receptor in HEK 293 cells

Aniquinazoline B, a Fungal Natural Product, Activates the μ-Opioid Receptor

The development of new μ-opioid receptor (MOR) agonists without the undesirable side effects, such as addiction or respiratory depression, has been a difficult challenge over the years. In the search for new compounds, we screened our chemical database of over 40.000 substances and further assessed the best 100 through molecular docking. We selected the top 10 compounds and evaluated them for their biological activity and potential to influence cyclic adenosine monophosphate (cAMP) levels. From the tested compounds, compound 7, called aniquinazoline B, belonging to the quinazolinone alkaloids class and isolated from the marine fungus Aspergillus nidulans, showed promising results, by inhibiting cAMP levels and in vitro binding to MOR, verified through microscale thermophoresis. Transcriptomic data investigation profiled the genes affected by compound 7 and discovered activation of different pathways compared to opioids. The western blot analysis revealed compound 7 as a balanced ligand, activating both p-ERK1/2 and β-arrestin1/2 pathways, showing this is a favorable candidate to be further tested.

Redistribution of ASIC1a channels triggered by IL-6: Potential role of ASIC1a in neuroinflammation

Cytokines, particularly IL-6, play a crucial role in modulating immune responses in the central nervous system (CNS). Elevated IL-6 levels have been observed in neuroinflammatory conditions, as well as in the sera and brains of patients with neurodegenerative diseases such as Parkinson's, Huntington's, Multiple Sclerosis, and Alzheimer's. Additionally, alterations in regional brain pH have been noted in these conditions. Acid-sensing ion channels (ASICs), including ASIC1a, activated by low pH levels, are highly abundant in the CNS and have recently been associated with various neurological disorders. Our study examined the impact of IL-6 on ASIC1a channels in cell cultures, demonstrating IL-6-induced the redistribution of cytosolic ASIC1a channels to the cell membrane. This redistribution was accompanied by increased ASIC1a current amplitude upon activation, as well as elevated levels of phosphorylated CaMKII and ERK kinases. Additionally, we observed posttranslational modifications on the ASIC1a channel itself. These findings provide insight into a potential link between inflammatory processes and neurodegenerative mechanisms, highlighting ASIC1a channels as promising therapeutic targets in these conditions.

Inhibition of Hsp90 in the spinal cord enhances the antinociceptive effects of morphine by activating an ERK-RSK pathway

Morphine and other opioids are commonly used to treat pain despite their numerous adverse side effects. Modulating μ-opioid receptor (MOR) signaling is one way to potentially improve opioid therapy. In mice, the chaperone protein Hsp90 mediates MOR signaling within the brain. Here, we found that inhibiting Hsp90 specifically in the spinal cord enhanced the antinociceptive effects of morphine in mice. Intrathecal, but not systemic, administration of the Hsp90 inhibitors 17-AAG or KU-32 amplified the effects of morphine in suppressing sensitivity to both thermal and mechanical stimuli in mice. Hsp90 inhibition enabled opioid-induced phosphorylation of the kinase ERK and increased abundance of the kinase RSK in the dorsal horns of the spinal cord, which are heavily populated with primary afferent sensory neurons. The additive effects of Hsp90 inhibition were abolished upon intrathecal inhibition of ERK, RSK, or protein synthesis. This mechanism downstream of MOR, localized to the spinal cord and repressed by Hsp90, may potentially be used to enhance the efficacy and presumably decrease the side effects of opioid therapy.

The HECT ubiquitin E3 ligase Smurf2 degrades μ-opioid receptor 1 in the ubiquitin-proteasome system in lung epithelial cells

Opioids are widely used for relieving clinical acute or chronic pain. The biological effects of opioids are through activating μ-opioid receptor 1 (MOR1). Most studies have focused on the consequences of agonist-induced MOR1 phosphorylation, ubiquitination, and internalization. Agonist-mediated MOR1 degradation, which is crucial for receptor stability and responsiveness, has not been well studied. E3 ubiquitin-protein ligase SMURF2 (Smurf2), a homolog to E6AP carboxy terminus (HECT) ubiquitin E3 ligase, has been shown to regulate MOR1 ubiquitination and internalization; however, its role in MOR1 degradation has not been studied. Here, we demonstrate that Smurf2 mediates [d-Ala²,N-MePhe⁴,Gly⁵-ol]-enkephalin (DAMGO, an agonist of MOR1)-induced MOR1 ubiquitination and degradation. DAMGO decreased MOR1 levels in the ubiquitin-proteasome system. MOR1 was modified by a Lys48-linked polyubiquitin chain. Overexpression of Smurf2 induced MOR1 ubiquitination and accelerated DAMGO-induced MOR1 degradation, whereas downregulation of Smurf2 attenuated MOR1 degradation. Furthermore, DAMGO increased lung epithelial cell migration and proliferation, and the effect was attenuated by overexpressing Smurf2. Collectively, these data unveil that Smurf2 negatively regulates MOR1 activity by reducing its stability. We also demonstrate an unrevealed biological function of MOR1 in lung epithelial cells. DAMGO-MOR1 promote cell migration and proliferation in lung epithelial cells, suggesting a potential effect of DAMGO in lung repair and remodeling after lung injury.

Heroin-based crack induces hyperalgesia through β-arrestin 2 redistribution and phosphorylation of Erk1/2 and JNK in the periaqueductal gray area

Continuous use of crack induces hyperalgesia which is related to drug tolerance. Despite cumulative evidence based on the growth rate of crack abuse, no serious study has been focused on the mechanisms of crack-induced hyperalgesia. This study aimed to elucidate whether extracellular signal-regulated kinases (Erk1/2)/β-arrestin pathways are involved in the crack-induced hyperalgesia. Fifty adult male Wistar rats were randomly divided into five groups: normal saline (NS), crack (0.9 mg/kg/day), heroin (1 mg/kg/day), crack + barbadin (100 μM), and heroin + barbadin groups, which received their intraperitoneal (i.p) treatments for four weeks. The thermal sensitivity was assessed using the hot-plate test. Moreover, phosphorylation of the Erk1/2 and JNK, as well as expression of protein kinase C-alpha (PKC-α), Mu-receptor (MOR), and β-arrestin 2 were determined in the whole lysate and membrane fraction using immunoblotting assay in the periaqueductal gray (PAG) area. The results demonstrated that chronic administration of crack and heroin significantly decreased hind-paw withdrawal latency compared to the NS group. Furthermore, crack as well as heroin administration increased phosphorylated Erk1/2 and JNK in the PAG. In addition, membrane β-arrestin 2 and PKC-α were significantly increased in the crack and heroin-received groups, while membrane MOR expression was decreased in the PAG. Nevertheless, co-administration of barbadin, an inhibitor of β-arrestin, and crack or heroin reversed all these changes. Our findings may partially confirm the role of β-arrestin 2 and PKC rearrangements, Erk1/2 and JNK phosphorylation in crack-induced hyperalgesia and provide potential therapeutic targets to attenuate crack-induced hyperalgesia.

STEP signaling pathway mediates psychomotor stimulation and morphine withdrawal symptoms, but not for reward, analgesia and tolerance

Striatal-enriched protein tyrosine phosphatase (STEP) is abundantly expressed in the striatum, which strongly expresses dopamine and opioid receptors and mediates the effects of many drugs of abuse. However, little is known about the role of STEP in opioid receptor function. In the present study, we generated STEP-targeted mice carrying a nonsense mutation (C230X) in the kinase interaction domain of STEP by screening the N-ethyl-N-nitrosourea (ENU)-driven mutant mouse genomic DNA library and subsequent in vitro fertilization. It was confirmed that the C230X nonsense mutation completely abolished functional STEP protein expression in the brain. STEP(C230X-/-) mice showed attenuated acute morphine-induced psychomotor activity and withdrawal symptoms, whereas morphine-induced analgesia, tolerance and reward behaviors were unaffected. STEP(C230X-/-) mice displayed reduced hyperlocomotion in response to intrastriatal injection of the μ-opioid receptor agonist DAMGO, but the behavioral responses to δ- and κ-opioid receptor agonists remained intact. These results suggest that STEP has a key role in the regulation of psychomotor action and physical dependency to morphine. These data suggest that STEP inhibition may be a critical target for the treatment of withdrawal symptoms associated with morphine.

PD98059 Influences Immune Factors and Enhances Opioid Analgesia in Model of Neuropathy

Neuropathic pain treatment remains challenging due to ineffective therapy and resistance to opioid analgesia. Mitogen-activated protein kinase kinase (MAPKK) have been identified as the crucial regulators of pro- and antinociceptive factors. We used PD98059, an inhibitor of the MAPKK family members MEK1/2. The aim of study was to examine the influence of single and/or repeated PD98059 on nociception and opioid effectiveness in neuropathy. Moreover, we examined how PD98059 influences selected members of cellular pathways and cytokines. The PD98059 (2.5 mcg) was intrathecally preemptively administered before chronic constriction injury (CCI), and then once daily for 7 days. Additionally, at day 7 after CCI the PD98059-treated rats received a single injection of opioids. Using Western blot and qRT-PCR techniques in PD98059-treated rats we analyzed the mRNA and/or protein level of p38, ERK1/2, JNK, NF-kappaB, IL-1beta, IL-6, iNOS and IL-10 in the lumbar spinal cord. Our results indicate that PD98059 has an analgesic effects and potentiates morphine and/or buprenorphine analgesia. Parallel we observed that PD98059 inhibit upregulation of the CCI-elevated p38, ERK1/2, JNK and NF-kappaB protein levels. Moreover, PD98059 also prevented increase of pro- (IL-1beta, IL-6, and iNOS) but enhances anti-nociceptive (IL-10) factors. Summing up, PD98059 diminished pain and increased the effectiveness of opioids in neuropathy. The inhibition of MEKs might inactivate a variety of cell signaling pathways that are implicated in nociception.

Design, synthesis, and pharmacological characterization of novel endomorphin-1 analogues as extremely potent μ-opioid agonists

Recently we reported the synthesis and structure-activity study of endomorphin-1 (EM-1) analogues containing novel, unnatural α-methylene-β-aminopropanoic acids (Map). In the present study, we describe new EM-1 analogues containing Dmt(1), (R/S)-βPro(2), and (ph)Map(4)/(2-furyl)Map(4). All of the analogues showed a high affinity for the μ-opioid receptor (MOR) and increased stability in mouse brain homogenates. Of the new compounds, Dmt(1)-(R)-βPro(2)-Trp(3)-(2-furyl)Map(4) (analogue 12) displayed the highest affinity toward MOR, in the picomolar range (Ki(μ) = 3.72 pM). Forskolin-induced cAMP accumulation assays indicated that this analogue displayed an extremely high agonistic potency, in the subpicomolar range (EC50 = 0.0421 pM, Emax = 99.5%). This compound also displayed stronger in vivo antinociceptive activity after iv administration when compared to morphine in the tail-flick test, which indicates that this analogue was able to cross the blood-brain barrier.

Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance

Morphine and related µ-opioid receptor (MOR) agonists remain among the most effective drugs known for acute relief of severe pain. A major problem in treating painful conditions is that tolerance limits the long-term utility of opioid agonists. Considerable effort has been expended on developing an understanding of the molecular and cellular processes that underlie acute MOR signaling, short-term receptor regulation, and the progression of events that lead to tolerance for different MOR agonists. Although great progress has been made in the past decade, many points of contention and controversy cloud the realization of this progress. This review attempts to clarify some confusion by clearly defining terms, such as desensitization and tolerance, and addressing optimal pharmacological analyses for discerning relative importance of these cellular mechanisms. Cellular and molecular mechanisms regulating MOR function by phosphorylation relative to receptor desensitization and endocytosis are comprehensively reviewed, with an emphasis on agonist-biased regulation and areas where knowledge is lacking or controversial. The implications of these mechanisms for understanding the substantial contribution of MOR signaling to opioid tolerance are then considered in detail. While some functional MOR regulatory mechanisms contributing to tolerance are clearly understood, there are large gaps in understanding the molecular processes responsible for loss of MOR function after chronic exposure to opioids. Further elucidation of the cellular mechanisms that are regulated by opioids will be necessary for the successful development of MOR-based approaches to new pain therapeutics that limit the development of tolerance.

MicroRNA 339 down-regulates μ-opioid receptor at the post-transcriptional level in response to opioid treatment

μ-Opioid receptor (MOR) level is directly related to the function of opioid drugs, such as morphine and fentanyl. Although agonist treatment generally does not affect transcription of mor, previous studies suggest that morphine can affect the translation efficiency of MOR transcript via microRNAs (miRNAs). On the basis of miRNA microarray analyses of the hippocampal total RNA isolated from mice chronically treated with μ-opioid agonists, we found a miRNA (miR-339-3p) that was consistently and specifically increased by morphine (2-fold) and by fentanyl (3.8-fold). miR-339-3p bound to the MOR 3'-UTR and specifically suppressed reporter activity. Suppression was blunted by adding miR-339-3p inhibitor or mutating the miR-339-3p target site. In cells endogenously expressing MOR, miR-339-3p inhibited the production of MOR protein by destabilizing MOR mRNA. Up-regulation of miR-339-3p by fentanyl (EC(50)=0.75 nM) resulted from an increase in primary miRNA transcript. Mapping of the miR-339-3p primary RNA and its promoter revealed that the primary miR-339-3p was embedded in a noncoding 3'-UTR region of an unknown host gene and was coregulated by the host promoter. The identified promoter was activated by opioid agonist treatment (10 nM fentanyl or 10 μM morphine), a specific effect blocked by the opioid antagonist naloxone (10 μM). Taken together, these results suggest that miR-339-3p may serve as a negative feedback modulator of MOR signals by regulating intracellular MOR biosynthesis.

Mechanisms of rapid opioid receptor desensitization, resensitization and tolerance in brain neurons

Agonists acting on µ-opioid receptors (MOR) are very effective analgesics but cause tolerance during long-term or repeated exposure. Intensive efforts have been made to find novel opioid agonists that are efficacious analgesics but can elude the signalling events that cause tolerance. µ-Opioid agonists differentially couple to downstream signalling mechanisms. Some agonists, such as enkephalins, D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), methadone and sufentanyl are efficacious at mediating G-protein and effector coupling, as well as triggering MOR regulatory events that include MOR phosphorylation, β-arrestin binding, receptor endocytosis and recycling. By contrast, morphine and closely related alkaloids can mediate efficacious MOR-effector coupling but poorly trigger receptor regulation. Several models have been proposed to relate differential MOR regulation by different opioids with their propensity to cause tolerance. Most are based on dogma that β-arrestin-2 (βarr-2) binding causes MOR desensitization and/or that MOR endocytosis and recycling are required for receptor resensitization. This review will examine some of these notions in light of recent evidence establishing that MOR dephosphorylation and resensitization do not require endocytosis. Recent evidence from opioid-treated animals also suggests that impaired MOR-effector coupling is driven, at least in part, by enhanced desensitization, as well as impaired resensitization that appears to be βarr-2 dependent. Better understanding of how chronic exposure to opioids alters receptor regulatory mechanisms may facilitate the development of effective analgesics that produce limited tolerance.

A new class of highly potent and selective endomorphin-1 analogues containing α-methylene-β-aminopropanoic acids (map)

A new class of endomorphin-1 (EM-1) analogues were synthesized by introduction of novel unnatural α-methylene-β-amino acids (Map) at position 3 or/and position 4. Their binding and functional activity, metabolic stability, and antinociceptive activity were determined and compared. Most of these analogues showed high affinities for the μ-opioid receptor and an increased stability in mouse brain homogenates compared with EM-1. Examination of cAMP accumulation and ERK1/2 phosphorylation in HEK293 cells confirmed the agonist properties of these analogues. Among these new analogues, H-Tyr-Pro-Trp-(2-furyl)Map-NH(2) (analogue 12) exhibited the highest binding potency (K(i)(μ) = 0.221 nM) and efficacy (EC(50) = 0.0334 nM, E(max) = 97.14%). This analogue also displayed enhanced antinociceptive activity in vivo in comparison to EM-1. Molecular modeling approaches were then carried out to demonstrate the interaction pattern of these analogues with the opioid receptors. We found that, compared to EM-1, the incorporation of our synthesized Map at position 4 would bring the analogue to a closer binding mode with the μ-opioid receptor.

NO-released zinc supports the simultaneous binding of Raf-1 and PKCγ cysteine-rich domains to HINT1 protein at the mu-opioid receptor

In the brain, the mu-opioid receptor (MOR) activates neural nitric oxide synthase (nNOS) through the PI3K/Akt pathway. The resulting nitric oxide (NO) enhances the function of the glutamate N-methyl-d-aspartate receptor (NMDAR)/calcium and calmodulin-dependent serine/threonine kinase (CaMKII), which subsequently diminishes MOR signaling strength. Because the ERK1/2 cascade is implicated in opioid tolerance, we analyzed the role of morphine-generated NO in this negative regulation. We found that NO-released endogenous zinc ions recruit the Ras/Raf-1/ERK1/2 cassette to histidine triad nucleotide-binding protein 1 (HINT1). A-Raf and B-Raf showed little or no MOR association. The zinc ions bridge the Raf-1 cysteine-rich domain (CRD) with HINT1 at the MOR C-terminus. Morphine also recruits PKCγ via NO/zinc to the MOR-HINT1 complex. Both Raf-1 and PKCγ CRDs bind simultaneously to HINT1, enabling PKCγ to enhance Raf-1 function to intensify MEK/ERK1/2 activation. Thus, through attached HINT1, the MOR facilitates the cross-talk of two NO- and zinc-regulated signal-transduction pathways, PKC/Src and Raf-1/ERK1/2, implicated in the negative control of morphine effects. This study reveals new aspects of ERK1/2 regulation by the MOR without requiring the transactivation of a receptor tyrosine kinase.

Dissecting how receptor tyrosine kinases modulate G protein-coupled receptor function

Receptor tyrosine kinases and G protein-coupled receptors modulate physiological processes and are also involved in the pathogenesis of some diseases. These receptors have intense bidirectional crosstalks leading to interactions in their signaling pathways and also modulation of the receptors themselves. In some cases, the receptor tyrosine kinases phosphorylate G protein-coupled receptors whereas in others phosphoinositide 3-kinase, protein kinase B and protein kinase C are key elements in these crosstalks. Two paracrine/ autocrine processes also participate, i.e., epidermal growth factor transactivation and sphingosine 1-phosphate generation and signaling. G proteins seem to mediate actions of receptor tyrosine kinases, but how this takes place is far from completely understood; some models are presented. Recent data indicate that the mitogen activated protein kinase cascade also mediate crosstalks. In the present perspective these processes are outlined using information from receptors that have been intensively studied, and important gaps in our knowledge are indicated.

Single-nucleotide polymorphism (A118G) in exon 1 of OPRM1 gene causes alteration in downstream signaling by mu-opioid receptor and may contribute to the genetic risk for addiction

The opioid receptor mu1 (OPRM1) mediates the action of morphine. Although genetic background plays an important role in the susceptibility toward abuse of drugs as evident from familial, adoption and twin studies, association of specific single-nucleotide polymorphisms of OPRM1 gene with narcotic addiction is to be established. Here, we demonstrate the involvement of A118G polymorphism of exon1 of human OPRM1 gene (hOPRM1), with heroin and alcohol addiction, in a population in eastern India. Statistical analysis exhibited a significant association of G allele with both heroin and alcohol addiction with a risk factor of P(trend) < 0.05. The functional significance of G allele in A118G single-nucleotide polymorphisms was evaluated by studying the regulation of protein kinase A (PKA), pCREB, and pERK1/2 by morphine in Neuro 2A cells, stably transfected with either wild type or A118G mutant hOPRM1. Unlike acute morphine treatment, both chronic morphine exposure and withdrawal precipitated by naloxone were differentially regulated by A118 and G118 receptor isoforms when both PKA and pERK1/2 activities were compared. Results suggest that the association of A118G polymorphism to heroin and alcohol addiction may be because of the altered regulation of PKA and pERK1/2 during opioid and alcohol exposures.

Extracellular signal-regulated kinase 1/2 activation counteracts morphine tolerance in the periaqueductal gray of the rat

Repeated administration of opioids produces long-lasting changes in micro-opioid receptor (MOR) signaling that underlie behavioral changes such as tolerance. Mitogen-activated protein kinase (MAPK) pathways, including MAPK extracellular signal-regulated kinases (ERK1/2), are modulated by opioids and are known to produce long-lasting changes in cell signaling. Thus, we tested the hypothesis that ERK1/2 activation contributes to the development and/or expression of morphine tolerance mediated by the periaqueductal gray (PAG). Changes in phosphorylated ERK1/2 expression were assessed with confocal microscopy and compared to behavioral measures of tolerance to the antinociceptive effects of chronic morphine administration. Repeated microinjection of morphine into the PAG produced tolerance and caused a significant increase in ERK1/2 phosphorylation, an effect not evident with acute morphine microinjection. Microinjection of the MAPK/ERK kinase inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene ethanolate (U0126), into the PAG had no effect on antinociception produced by acute morphine administration. However, repeated coadministration of U0126 and morphine into the PAG blocked ERK1/2 phosphorylation and enhanced the development of morphine tolerance. Coadministration of U0126 with morphine only on the test day also enhanced the expression of morphine tolerance. Administration of the irreversible opioid receptor antagonist beta-chlornaltrexamine blocked the activation of ERK1/2 caused by repeated morphine microinjections, demonstrating that ERK1/2 activation was a MOR-mediated event. In summary, these studies show that chronic morphine administration alters ERK1/2 signaling and that disruption of ERK1/2 signaling enhances both the development and expression of morphine tolerance. Contrary to expectations, these data indicate that ERK1/2 activation opposes the development of morphine tolerance.

Detection of the endogenous mu opioid receptor (mopr) in brain

In general, it has been difficult to obtain antibodies which are useful for immunoblotting of endogenous seven-transmembrane receptors (7TMRs) despite the claims made by many companies on commercially available antibodies. In this review, we will use the mu opioid receptor (MOPR) in brain as an example to underscore the importance of using knock-out (K/O) mice and multiple independent approaches (ligand affinity-labeling, receptor phosphorylation and immunoblotting) in identifying 7TMRs following sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE). The rigor and convergence of pharmacological and biochemical data provide confidence on the unequivocal identification of MOPR. The distinct relative molecular masses (Mr's) and band patterns are largely due to variations in the extent of N-glycosylation in different cell lines, brain regions and species.

Opioid-induced proliferation of vascular endothelial cells

Angiogenesis is an important issue in cancer research and opioids are often used to treat pain in cancer patients. Therefore it is important to know if the use of opioids is associated with an aberrant stimulation of tumor growth triggered by the stimulation of angiogenesis in cancer patients. Some studies in the literature have suggested the presence of the μ3 opioid receptor, known as the receptor for many opioids, on endothelial cells, which are key players in the process of angiogenesis. In this study we used endothelial cells known to express the μ3 opioid receptor (MOR3), to evaluate the effects of morphine on angiogenesis. We first investigated the effect of morphine on the proliferation of endothelial cells. We showed that morphine is able to stimulate vascular endothelial cell proliferation in vitro. This effect of morphine is mediated by the mitogen-activated protein kinase (MAPK) pathway as pre-treatment with PD98059 inhibited this excessive proliferation. Because previous studies indicated nitric oxide (NO) as a downstream messenger we investigated the role of NO in the aberrant proliferation of endothelial cells. Our data could not confirm these findings using intracellular NO measurements and quantitative fluorescence microscopy. The potential use and pitfalls of opioids in cancer patients is discussed in light of these negative findings.

Two distinct mechanisms mediate acute mu-opioid receptor desensitization in native neurons

Sustained stimulation of G-protein coupled receptors (GPCRs) leads to rapid loss of receptor function (acute desensitization). For many GPCRs including the mu-opioid receptor (MOR), an accepted mechanism for acute desensitization is through G-protein coupled receptor kinase (GRKs) mediated phosphorylation of the receptor, which facilitates the binding of beta-arrestins (betaarrs) to the receptor and then promotes endocytosis. However, the mechanism(s) that mediate acute desensitization have not yet been well defined in native neurons. This study used whole-cell patch clamp recording of G-protein coupled inward-rectifying potassium (GIRK) currents to assay MOR function and identify mechanisms of acute MOR desensitization in locus ceruleus (LC) neurons. The rate and extent of MOR desensitization were unaffected by beta(arr)-2 knock-out. Disruption of GRK2 function via inhibitory peptide introduced directly into neurons also failed to affect desensitization in wild type or beta(arr)-2 knock-outs. Inhibition of ERK1/2 activation alone had little effect on acute desensitization. However, when both GRK2-beta(arr)-2 and ERK1/2 functions were disrupted simultaneously, desensitization of MOR was nearly abolished. Together, these results suggest that acute desensitization of MOR in native LC neurons is determined by at least two molecular pathways, one involving GRK2 and beta(arr)2, and a parallel pathway mediated by activated ERK1/2.

Long-term morphine treatment decreases the association of mu-opioid receptor (MOR1) mRNA with polysomes through miRNA23b

Mu-opioid receptor (MOR) mediates most of the pharmacological effects of opioid drugs. The expression of MOR is temporarily and spatially regulated at both the transcriptional and post-transcriptional levels. Long-term morphine treatment that induces tolerance does not alter MOR mRNA expression, suggesting no direct link between agonist treatment and MOR gene transcription. We previously identified the 3'-untranslated region (3'-UTR) of the major transcript of mu-opioid receptor (MOR1) and revealed a novel trans-acting factor, miRNA23b, that binds to the K box motif in the 3'-UTR. The interaction between miRNA23b with the MOR1 3'-UTR suppressed receptor translation by inhibiting polysome-mRNA association. In this report, we demonstrate that long-term morphine treatment increases miRNA23b expression in a dose- and time-dependent manner and represses the association of MOR1 mRNA with polysomes through the MOR1 3'-UTR. The translational luciferase reporter assay shows a suppression effect of morphine on reporter activity that requires the MOR1 3'-UTR. This suggests a potential link between MOR expression and morphine treatment at the post-transcriptional level in which a specific miRNA, miRNA23b, is involved.

Effect of inhibitors of mitogen-activated protein kinase kinase on alpha(1B)-adrenoceptor phosphorylation

1 Mitogen-activated protein kinases mediate hormone/neurotransmitter action on proliferation and differentiation and participate in receptor regulation. The effect of inhibitors of mitogen-activated kinase kinase (MEK) on alpha(1B)-adrenoceptor phosphorylation state and function was studied using different cell lines. It was observed that at nanomolar concentrations the MEK inhibitors, PD98059 (2'-amino-3'-methoxyflavone) and UO126 [1,4-(diamino-2,3-dicyano/1,4-bis-(2-aminophenylthio)-butadiene], increased alpha(1B)-adrenoceptor phosphorylation and diminished the functional response of this receptor to noradrenaline. These agents did not alter the action of lysophosphatidic acid. 2 Staurosporine (IC(50) approximately 0.8 nm) (a general protein kinase inhibitor) and bis-indolyl-maleimide I (IC(50) approximately 200 nm) (a selective protein kinase C inhibitor) inhibited PD98059-induced alpha(1B)-adrenoceptor phosphorylation. In contrast, neither wortmannin (phosphoinositide 3-kinase inhibitor) nor genistein (protein tyrosine kinase inhibitor) had any effect. The data suggest the possibility that MEK might exert control on the activity of the enzymes that regulate receptor phosphorylation, such as G-protein-coupled receptor kinases, protein kinase C or serine/threonine protein phosphatases. 3 Coimmunoprecipitation studies showed a constant association of total extracellular signal-regulated kinase 2 (ERK2) with alpha(1B)-adrenoceptors. Association of phospho-ERK 1/2 to alpha(1B)-adrenoceptors increased not only in response to agonist but also in response to agents that increase alpha(1B)-adrenoceptor and ERK1/2 phosphorylation [such as endothelin-1, phorbol 12-myristate-13-acetate (PMA) and epidermal growth factor (EGF)]; not surprisingly, PD98059 decreased this effect. 4 Our data show that blockade of MEK activity results in increased alpha(1B)-adrenoceptor phosphorylation, diminished adrenoceptor function and perturbation of receptor-ERK1/2 interaction.

Regulation of ERK1/2 phosphorylation by acute and chronic morphine - implications for the role of cAMP-responsive element binding factor (CREB)-dependent and Ets-like protein-1 (Elk-1)-dependent transcription; small interfering RNA-based strategy

Extracellular signal-regulated kinases (ERKs) have been shown to be activated by opioids and functionally linked to addiction. Morphine-associated changes in ERK activity seem to be the characteristic features of opioid action. In this study, we observed a rapid and severe increase in ERK1/2 activity after a 5 min morphine treatment of HEK-MOR cells (transfected with the rat mu-opioid receptor MOR1) expressing mu-opioid receptor. Cellular adaptations to chronic (72 h) morphine treatment were manifested by a slight and sustained increase in ERK1/2 activity. Withdrawal caused by an opioid receptor antagonist - naloxone - attenuated phosphorylation of ERK1/2. Little information is available on the precise mechanism of ERK activity regulation. Using RNA interference technology, we generated stably transfected cells with silenced expression of cAMP-responsive element binding factor (CREB) and Ets-like protein-1 (Elk-1) transcription factors, which are known targets for activated ERK1/2. In these cells, ERK1/2 activity regulation was altered. Silencing of CREB or Elk-1 significantly increased ERK activation observed after 5 min of morphine stimulation. The initial level of activated ERKs in these cells was also augmented. Moreover, the cellular response to withdrawal signals and chronic opioid treatment was diminished. These differences suggest that both CREB-dependent and Elk-1-dependent transcription contribute to the expression of proteins regulating morphine-induced ERK activity (particular phosphatases, upstream kinases or their activatory proteins).

Modified Ca(v)1.4 expression in the Cacna1f(nob2) mouse due to alternative splicing of an ETn inserted in exon 2

The Cacna1f^nob2 mouse is reported to be a naturally occurring null mutation for the Ca_v1.4 calcium channel gene and the phenotype of this mouse is not identical to that of the targeted gene knockout model. We found two mRNA species in the Cacna1f^nob2 mouse: approximately 90% of the mRNA represents a transcript with an in-frame stop codon within exon 2 of CACNA1F, while approximately 10% of the mRNA represents a transcript in which alternative splicing within the ETn element has removed the stop codon. This latter mRNA codes for full length Ca_v1.4 protein, detectable by Western blot analysis that is predicted to differ from wild type Ca_v1.4 protein in a region of approximately 22 amino acids in the N-terminal portion of the protein. Electrophysiological analysis with either mouse Ca_v1.4^wt or Ca_v1.4^nob2 cDNA revealed that the alternatively spliced protein does not differ from wild type with respect to activation and inactivation characteristics; however, while the wild type N-terminus interacted with filamin proteins in a biochemical pull-down experiment, the alternatively spliced N-terminus did not. The Cacna1f^nob2 mouse electroretinogram displayed reduced b-wave and oscillatory potential amplitudes, and the retina was morphologically disorganized, with substantial reduction in thickness of the outer plexiform layer and sprouting of bipolar cell dendrites ectopically into the outer nuclear layer. Nevertheless, the spatial contrast sensitivity (optokinetic response) of Cacna1f^nob2 mice was generally similar to that of wild type mice. These results suggest the Cacna1f^nob2 mouse is not a CACNA1F knockout model. Rather, alternative splicing within the ETn element can lead to full-length Ca_v1.4 protein, albeit at reduced levels, and the functional Ca_v1.4 mutant may be incapable of interacting with cytoskeletal filamin proteins. These changes, do not alter the ability of the Cacna1f^nob2 mouse to detect and follow moving sine-wave gratings compared to their wild type counterparts.

Morphine-induced mu-opioid receptor rapid desensitization is independent of receptor phosphorylation and beta-arrestins

Receptor desensitization involving receptor phosphorylation and subsequent betaArrestin (betaArr) recruitment has been implicated in the tolerance development mediated by mu-opioid receptor (OPRM1). However, the roles of receptor phosphorylation and betaArr on morphine-induced OPRM1 desensitization remain to be demonstrated. Using OPRM1-induced intracellular Ca(2+) ([Ca(2+)](i))release to monitor receptor activation, as predicted, [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO), induced OPRM1 desensitization in a receptor phosphorylation- and betaArr-dependent manner. The DAMGO-induced OPRM1 desensitization was attenuated significantly when phosphorylation deficient OPRM1 mutants or Mouse Embryonic Fibroblast (MEF) cells from betaArr1 and 2 knockout mice were used in the studies. Specifically, DAMGO-induced desensitization was blunted in HEK293 cells expressing the OPRM1S375A mutant and was eliminated in MEF cells isolated from betaArr2 knockout mice expressing the wild type OPRM1. However, although morphine also could induce a rapid desensitization on [Ca(2+)](i) release to a greater extent than that of DAMGO and could induce the phosphorylation of Ser(375) residue, morphine-induced desensitization was not influenced by mutating the phosphorylation sites or in MEF cells lacking betaArr1 and 2. Hence, morphine could induce OPRM1 desensitization via pathway independent of betaArr, thus suggesting the in vivo tolerance development to morphine can occur in the absence of betaArr.

Filamin A mutant lacking actin-binding domain restores mu opioid receptor regulation in melanoma cells

We have previously reported that the protein filamin A (FLA) binds to the carboxyl tail of the mu opioid receptor (MOPr). Using human melanoma cells, which do not express filamin A, we showed that receptor down-regulation, functional desensitization and trafficking are deficient in the absence of FLA (Onoprishvili et al. Mol Pharmacol 64:1092-1100, 2003). Since FLA has a binding domain for actin and is a member of the family of actin cytoskeleton proteins, it is usually assumed that FLA functions via the actin cytoskeleton. We decided to test this hypothesis by preparing cDNA coding for mutant FLA lacking the actin binding domain (FLA-ABD) and expressing FLA-ABD in the human melanoma cell line M2 (M2-ABD cell line). We report here that this mutant is capable of restoring almost as well as full length FLA the down-regulation of the human MOPr. It is similarly very effective in restoring functional desensitization of MOPr, as assessed by the decrease in G-protein activation after chronic exposure of M2-ABD cells to the mu agonist DAMGO. We also found that A7 cells, expressing wild type FLA, exhibit rapid activation of the MAP kinases, ERK 1 and 2, by DAMGO, as shown by a rise in the level of phospho-ERK 1 and 2. This is followed by rapid dephosphorylation (inactivation), which reaches basal level between 30 and 60 min after DAMGO treatment. M2 cells show normal activation of ERK 1 and 2 in the presence of DAMGO, but very slow inactivation. The rapid rate of MAPK inactivation is partially restored by FLA-ABD. We conclude that some functions of FLA do not act via the actin cytoskeleton. It is likely that other functions, not studied here, may require functional binding of the MOPr-FLA complex to actin.

Agonist-specific down regulation of mu-opioid receptors: Different cellular pathways are activated by different opioid agonists

Opioid agonists are known to induce down regulation of opioid receptors through the classical pathway that involves phosphorylation, clathrin-dependent endocytosis and lysosomal/endosomal degradation of the internalized receptors. As expected, exposure of mu-opioid receptor (MOR)-transfected HEK-293 cells to either DAMGO (a specific mu-opioid agonist) or etorphine (a wide spectrum opioid agonist) resulted in down regulation of the receptors that was blocked by the kinase inhibitor staurosporine, by hypertonic sucrose and by the lysosomal and proteasomal inhibitors chloroquine and lactacystin. High concentration of etorphine, but not of DAMGO, induced an additional process of down regulation that was resistant to staurosporine, to hypertonic sucrose and to chloroquine-lactacystin. Etorphine, but not DAMGO, also induced down regulation of mu-opioid receptors in isolated membranes of HEK cells. This membrane-delimited down regulation was blocked by selective inhibitors of protease enzymes, suggesting the involvement of membranous serine- and amino-peptidases. This membranous down regulation of opioid receptors was dependent on the concentration of etorphine and was blocked by the opioid antagonist naloxone. Etorphine induced similar down regulation in membranes of HEK-293 cells transfected with delta-opioid receptors (DOR) as well in membranes of cells that endogenously express opioid receptors. This agonist-specific membrane-delimited regulatory process appears to be physiologically relevant and should be taken into account when studying long term effects of opioid drugs.

Role of receptor internalization in opioid tolerance and dependence

Agonist-induced mu-opioid receptor (MOPr) internalization has long been suggested to contribute directly to functional receptor desensitization and opioid tolerance. In contrast, recent evidence suggests that opioid receptor internalization could in fact reduce opioid tolerance in vivo, but the mechanisms that are responsible for the internalization-mediated protection against opioid tolerance are controversely discussed. One prevailing hypothesis is, that receptor internalization leads to decreased receptor signaling and therefore to reduced associated compensatory changes in downstream signaling systems that are involved in the development of opioid tolerance. However, numerous studies have demonstrated that desensitized and internalized mu-opioid receptors are rapidly recycled to the cell surface in a reactivated state, thus counteracting receptor desensitization and opioid tolerance. Further studies revealed agonist-selective differences in the ability to induce opioid receptor internalization. Recently it has been demonstrated that the endocytotic efficacies of opioids are negatively correlated to the induced opioid tolerance. Thus, clearer understanding of the role of opioid receptor trafficking in the regulation of opioid tolerance and dependence will help in the treatment of patients suffering from chronic pain or drug dependence.

Modulation of extracellular signal-regulated kinase (ERK) by opioid and cannabinoid receptors that are expressed in the same cell

In the present study we investigated the signal transduction pathways leading to the activation of extracellular signal-regulated kinase (ERK) by opioid or cannabinoid drugs, when their receptors are coexpressed in the same cell-type. In N18TG2 neuroblastoma cells, the opioid agonist etorphine and the cannabinoid agonist CP-55940 induced the phosphorylation of ERK by a similar mechanism that involved activation of delta-opioid receptors or CB1 cannabinoid receptors coupled to Gi/Go proteins, matrix metalloproteases, vascular endothelial growth factor (VEGF) receptors and MAPK/ERK kinase (MEK). In HEK-293 cells, these two drugs induced the phosphorylation of ERK by separate mechanisms. While CP-55940 activated ERK by transactivation of VEGFRs, similar to its effect in N18TG2 cells, the opioid agonist etorphine activated ERK by a mechanism that did not involve transactivation of a receptor tyrosine kinase. Interestingly, the activation of ERK by etorphine was resistant to the inhibition of MEK, suggesting the possible existence of a novel, undescribed yet mechanism for the activation of ERK by opioids. This mechanism was found to be specific to etorphine, as activation of ERK by the micro-opioid receptor (MOR) agonist DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol] enkephalin) was mediated by MEK in these cells, suggesting that etorphine and DAMGO activate distinct, ligand-specific, conformations of MOR. The characterization of cannabinoid- and opioid-induced ERK activation in these two cell-lines enables future studies into possible interactions between these two groups of drugs at the level of MAPK signaling.

RGS9-2 is a negative modulator of mu-opioid receptor function

Regulators of G-protein signaling (RGS) 9-2 is a striatal enriched protein that controls G protein coupled receptor signaling duration by accelerating Galpha subunit guanosine triphosphate hydrolysis. We have previously demonstrated that mice lacking the RGS9 gene show enhanced morphine analgesia and delayed development of tolerance. Here we extend these studies to understand the mechanism via which RGS9-2 modulates opiate actions. Our data suggest that RGS9-2 prevents several events triggered by mu-opioid receptor (MOR) activation. In transiently transfected PC12 cells, RGS9-2 delays agonist induced internalization of epitope HA-tagged mu-opioid receptor. This action of RGS9-2 requires localization of the protein near the cell membrane. Co-immunoprecipitation studies reveal that RGS9-2 interacts with HA-tagged mu-opioid receptor, and that this interaction is enhanced by morphine treatment. In addition, morphine promotes the association of RGS9-2 with another essential component of MOR desensitization, beta-arrestin-2. We also show that over-expression of RGS9-2 prevents opiate-induced extracellular signal-regulated kinase phosphorylation. Our data indicate that RGS9-2 plays an essential role in opiate actions, by negatively modulating MOR downstream signaling as well as the rate of MOR endocytosis.

Comparative analysis of mu-opioid receptor expression in immune and neuronal cells

Morphine modulates neuronal and immune cell functions via mu-opioid receptors. In primary and Jurkat T cells, and Raji B cells mu-opioid receptor transcripts were detected only after stimulation of the cells with IL-4 or TNF-alpha. Moreover, the amount of the induced mu-opioid receptor mRNA in the immune cells was 15 to 200 times less than those in primary cortical and SH SY5Y neuronal cells. Nevertheless, mu-opioid receptor mRNA in immune cells is processed to functional receptors, as demonstrated by morphine-mediated phosphorylation of mitogen activated protein kinase, morphine-mediated up-regulation of IL-4 mRNA and coupling to adenylyl cyclase in Jurkat cells.

The extracellular signal-regulated kinase signaling pathway is involved in the modulation of morphine-induced reward by mPer1

Although there are clear interactions between circadian rhythms and drug addiction, mechanisms for such interactions remain unknown. Studies have shown that the circadian clock gene Period in Drosophila melanogaster could influence behavioral responses to cocaine, and the mouse homologues, mPer1 and mPer2, modulate cocaine sensitization and reward. In the present study, we applied DNAzyme targeting mPer1 to interfere the expression of mPer1 in CNS in mice, and studied its effects on morphine-induced reward and its molecular mechanism. The results demonstrated that the DNAzyme could attenuate the expression of mPer1 in CNS in mice and downregulate the increased extracellular signal-regulated kinase (ERK) activity induced by morphine in whole brain and the nucleus accumbens, the key region of drug addiction. Mice treated with morphine and injected intracerebroventricularly with DNAzyme did not show preference to the morphine-trained side. These results indicate that drug dependence seems to be influenced at least partially by mPer1 and its mechanism may involve the ERK signal pathway.

Opioids

Opioids are the most effective and widely used drugs in the treatment of severe pain. They act through G protein-coupled receptors. Four families of endogenous ligands (opioid peptides) are known. The standard exogenous opioid analgesic is morphine. Opioid agonists can activate central and peripheral opioid receptors. Three classes of opioid receptors (mu, delta, kappa) have been identified. Multiple pathways ofopioid receptor signaling (e.g., G(i/o) coupling, cAMP inhibition, Ca++ channel inhibition) have been described. The differential regulation of effectors, preclinical pharmacology, clinical applications, and side effects will be reviewed in this chapter.

Tracking the opioid receptors on the way of desensitization

Opioid receptors belong to the super family of G-protein coupled receptors (GPCRs) and are the targets of numerous opioid analgesic drugs. Prolonged use of these drugs results in a reduction of their effectiveness in pain relief also called tolerance, a phenomenon well known by physicians. Opioid receptor desensitization is thought to play a major role in tolerance and a lot of work has been dedicated to elucidate the molecular basis of desensitization. As described for most of GPCRs, opioid receptor desensitization involves their phosphorylation by kinases and their uncoupling from G-proteins realized by arrestins. More recently, opioid receptor trafficking was shown to contribute to desensitization. In this review, our knowledge on the molecular mechanisms of desensitization and recent progress on the role of opioid receptor internalization, recycling or degradation in desensitization will be reported. A better understanding of these regulatory mechanisms would be helpful to develop new analgesic drugs or new strategies for pain treatment by limiting opioid receptor desensitization and tolerance.

ERK-dependent modulation of cerebellar synaptic plasticity after chronic Delta9-tetrahydrocannabinol exposure

Chronic exposure to Delta9-tetrahydrocannabinol (THC) induces tolerance to cannabinoid-induced locomotor effects, which are mediated by cannabinoid receptors (CB1Rs) located in motor control regions, including the cerebellum. There is substantial evidence of cerebellar CB1R molecular adaptation and modifications in receptor signaling after prolonged cannabinoid exposure. However, very little is known about the effects of chronic cannabinoid administration on cerebellar synaptic plasticity, which may contribute to the development of cannabinoid behavioral tolerance. In the cerebellar cortex, activation of CB1R inhibits excitatory synaptic transmission at parallel fiber (PF)-Purkinje cell (PC) synapses by decreasing neurotransmitter release. Our study aimed to investigate the neurophysiological adaptive responses occurring at cerebellar PF-PC cell synapses after repeated THC exposure. In THC-tolerant mice, an increase of the basal release probability was found at PF-PC synapses, in parallel with a facilitation of slow mGluR1 (metabotropic glutamate receptor type 1)-mediated excitatory postsynaptic currents and a reduced sensitivity to the inhibitory effects of the CB1R agonist CP55,940 [(-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol]. Additionally, after repeated THC exposures, presynaptic PF-PC long-term potentiation was blocked by A1R (adenosine receptor-1) activation. Inhibition of the extracellular signal regulated kinase (ERK) pathway prevented these alterations of cerebellar synaptic transmission and plasticity. In summary, we provide evidence for ERK-dependent modulatory mechanisms at PF-PC synapses after chronic THC administration. This contributes to generation of forms of pathological synaptic plasticity that might play a role in cannabinoid dependence.

Morphine activates Arc expression in the mouse striatum and in mouse neuroblastoma Neuro2A MOR1A cells expressing mu-opioid receptors

Activity-regulated cytoskeleton-associated protein (Arc) is an effector immediate early gene product implicated in long-term potentiation and other forms of neuroplasticity. Earlier studies demonstrated Arc induction in discrete brain regions by several psychoactive substances, including drugs of abuse. In the present experiments, the influence of morphine on Arc expression was assessed by quantitative reverse transcription real-time PCR and Western blotting in vivo in the mouse striatum/nucleus accumbens and, in vitro, in the mouse Neuro2A MOR1A cell line, expressing mu-opioid receptor. An acute administration of morphine produced a marked increase in Arc mRNA and protein level in the mouse striatum/nucleus accumbens complex. After prolonged opiate treatment, tolerance to the stimulatory effect of morphine on Arc expression developed. No changes in the striatal Arc mRNA levels were observed during spontaneous or opioid antagonist-precipitated morphine withdrawal. In Neuro2A MOR1A cells, acute, but not prolonged, morphine treatment elevated Arc mRNA level by activation of mu-opioid receptor. This was accompanied by a corresponding increase in Arc protein level. Inhibition experiments revealed that morphine induced Arc expression in Neuro2A MOR1A cells via intracellular signaling pathways involving mitogen-activated protein (MAP) kinases and protein kinase C. These results lend further support to the notion that stimulation of opioid receptors may exert an activating influence on some intracellular pathways and leads to induction of immediate early genes. They also demonstrate that Arc is induced in the brain in vivo after morphine administration and thus may play a role in neuroadaptations produced by the drug.

Cannabinoid receptor type 2 agonists induce transcription of the mu-opioid receptor gene in Jurkat T cells

Opioids and cannabinoids are both associated with analgetic, psychotropic, and immunomodulatory effects. It has been suggested that both systems interact on multiple levels. We hypothesized that cannabinoids induce opioid receptors and investigated cannabinoid-dependent expression of the mu-opioid receptor subtype in a human T cell model. We report that activation of the peripheral cannabinoid receptor type 2 leads to a de novo induction of mu-opioid receptor transcription in Jurkat E6.1 cells. We show that interleukin-4 is transcriptionally induced in response to cannabinoids and that an interleukin-4 receptor antagonist blocks cannabinoid-dependent induction of mu-opioid receptors, indicating that induced expression of interleukin-4 is required in this process. Induction of interleukin-4 is blocked by decoy oligonucleotides directed against STAT5, indicating the requirement of this transcription factor. In addition, we show cannabinoid-dependent phosphorylation of STAT5. Further experiments demonstrate that interleukin-4 then induces phosphorylation of STAT6, which directly transactivates the mu-opioid receptor gene. In addition, STAT6 induces expression of the transcription factor GATA3, which also contributes to mu-opioid receptor gene transcription. The responsive promoter region of the human mu-opioid receptor gene with the binding sites for both factors was mapped to nt -1001 to -950. To demonstrate functional mu-opioid receptor proteins, morphine-mediated phosphorylation of mitogen-activated protein kinase was investigated. We show that phosphorylation of mitogen-activated protein kinase occurs only in cannabinoid-prestimulated Jurkat E6.1 cells and that it is blocked by the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2. In summary, these findings provide a first example for cannabinoid-opioid-interactions in cells of the immune system.

Activation of p38 mitogen-activated protein kinase in spinal microglia mediates morphine antinociceptive tolerance

Compelling evidence has suggested that spinal glial cells were activated by chronic morphine treatment and involved in the development of morphine tolerance. However, the mechanisms of glial activation were still largely unknown in morphine tolerance. In present study, we investigated the role of p38 mitogen-activated protein kinase (p38 MAPK) in the spinal cord in the development of chronic morphine antinociceptive tolerance. We found that intrathecal administration of morphine (15 microg) daily for 7 consecutive days significantly induced an increase in number of phospho-p38 (p-p38) immunoreactive cells in the spinal cord compared with chronic saline or acute morphine treated rats. Double immunofluorescence staining revealed that p-p38 immunoreactivity was exclusively restricted in the activated spinal microglia, not in astrocytes or neurons. Repeated intrathecal administration of 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580) (10 microg or 2 microg), a specific p38 inhibitor, 30 min before each morphine injection for 7 consecutive days significantly attenuated tolerance to morphine analgesia assessed by tail flick test. However, a single intrathecal administration of SB203580 (10 microg) did not antagonize the established tolerance to morphine analgesia. Taken together, these findings suggested that p38 MAPK activation in the spinal microglia was involved in the development of morphine antinociceptive tolerance. Inhibition of p38 MAPK by SB203580 in the spinal cord attenuated but not reversed the tolerance to morphine analgesia. The present study provides the first evidence that p38 activation in spinal microglia played an important role in the development of tolerance to morphine analgesia.

Morphine-induced chemotaxis and brain-derived neurotrophic factor expression in microglia

The addition of morphine at 1 mum induced morphological changes of cultured microglia such that they changed from having globular or bipolar rod-like shapes to being flat and lamellipodial, with membrane ruffling at the edge, which was stained with phalloidin. The membrane ruffling was clearly colocalized with Rac. Morphine also induced chemotaxis in Boyden chamber analysis at concentrations of 1 mum or more in microglia and the microglial cell line EOC 2. All of these changes were abolishable by naloxone, antisense oligodeoxynucleotide for mu-opioid receptor (MOR), pertussis toxin (PTx), and wortmannin, but not genistein or 1,10-phenanthroline. The addition of morphine to microglia stimulated the gene expression of brain-derived neurotrophic factor (BDNF) as early as the 1 hr point, and this lasted for >12 hr. Morphine induced BDNF gene expression and ERK1/2 (extracellular signal-regulated kinase 1/2) phosphorylation, and these were abolishable by naloxone, wortmannin, PD98059, genistein, and 1,10-phenanthroline. The addition of conditioned medium derived from the culture of morphine-treated microglia also increased the phosphorylation of ERK1/2. All of these findings suggest that morphine induces significant changes in both morphology and gene expression at relatively high concentrations, but the underlying signaling pathways downstream of MOR and G(i/o) appear to be different from each other. Phosphoinositide 3-kinase gamma activation and Rac activation are involved in chemotaxis, whereas indirect pathways through ERK1/2 phosphorylation induced by unknown growth factors generated through an MOR-mediated metalloprotease activation are linked to the enhanced BDNF gene expression.

Ras/ERK signalling in cannabinoid tolerance: from behaviour to cellular aspects

We investigated the role of the Ras/extracellular-regulated kinase (ERK) pathway in the development of tolerance to Delta(9)-tetrahydrocannabinol (THC)-induced reduction in spontaneous locomotor activity by a genetic (Ras-specific guanine nucleotide exchange factor (Ras-GRF1) knock-out mice) and pharmacological approach. Pre-treatment of wild-type mice with SL327 (50 mg/kg i.p.), a specific inhibitor of mitogen-activated protein kinase kinase (MEK), the upstream kinase of ERK, fully prevented the development of tolerance to THC-induced hypolocomotion. We investigated the impact of the inhibition of ERK activation on the biological processes involved in cannabinoid tolerance (receptor down-regulation and desensitization), by autoradiographic cannabinoid CB1 receptor and cannabinoid-stimulated [(35)S]GTPgammaS binding studies in subchronically treated mice (THC, 10 mg/kg s.c., twice a day for 5 days). In the caudate putamen and cerebellum of Ras-GRF1 knock-out mice and SL327 pre-treated wild-type mice, CB1 receptor down-regulation and desensitization did not occur, suggesting that ERK activation might account for CB1 receptor plasticity involved in the development of tolerance to THC hypolocomotor effect. In contrast, the hippocampus and prefrontal cortex showed CB1 receptor adaptations regardless of the genetic or pharmacological inhibition of the ERK pathway, suggesting regional variability in the cellular events underlying the altered CB1 receptor function. These findings suggest that at least in the caudate putamen and cerebellum, the Ras/ERK pathway is essential for triggering the alteration in CB1 receptor function responsible for tolerance to THC-induced hypomotility.

Opioid abuse and brain gene expression

Opiate addiction is a central nervous system disorder of unknown mechanism. Neuronal basis of positive reinforcement, which is essential to the action of opioids, relies on activation of dopaminergic neurons resulting in an increased dopamine release in the mesolimbic brain structures. Certain aspects of opioid dependence and withdrawal syndrome are also related to the activity of noradrenergic and serotonergic systems, as well as to both excitatory and inhibitory amino acid and peptidergic systems. The latter pathways have been recently proven to be involved both in the development of dependence and in counteracting the states related to relapse. An important role in neurochemical mechanisms of opioid reward, dependence and vulnerability to addiction has been ascribed to endogenous opioid peptides, particularly those acting via the mu- and kappa-opioid receptors. Opiate abuse leads to adaptive reactions in the nervous system which occur at the cellular and molecular levels. Recent research indicates that intracellular mechanisms of signal transmission-from the receptor, through G proteins, cyclic AMP, MAP kinases to transcription factors--also play an important role in opioid tolerance and dependence. The latter link in this chain of reactions may modify synthesis of target genes and in this manner, it may be responsible for opiate-induced long-lasting neural plasticity.

Opioid control of MAP kinase cascade

Activation of G protein-coupled receptors (GPCRs) may result in phosphorylation of extracellular signal-regulated kinases 1/2 (ERK 1/2). The signaling pathway involves ectodomain shedding, generating epidermal growth factor (EGF)-like ligands, which in turn stimulate the mitogen-activated protein kinase (MAPK) via EGF receptors. The present study investigates into the control of MAPKs by opioidergic GPCRs in human embryonic kidney cells (HEK 293). Experiments were conducted with cells expressing opioid receptors, G protein-coupled receptor kinases, and ERKs. The outcome of our studies let us suggest that EGF-like ligands released by opioid receptor stimulation utilize different EGF receptors to phosphorylate ERKs, while EGF utilizes type 1 receptors. Differences between multiple opioid receptors are apparent with respect to the activation of ERKs. EGF rapidly triggers internalization of the fluorescent EGF receptor type 1, but we failed to observe any sequestration of this receptor type upon exposure of cells to an opioid, since opioids most likely trigger stimulation of a different EGF receptor type. In conclusion, G protein-coupled opioid receptors control the MAPK cascade in a similar fashion as described for non-opioid GPCRs, although distinct differences exist between mu-, delta- and kappa-receptors. EGF-induced ERK activation is mediated by EGF receptor type 1 while opioid receptor activation seems to brings about stimulation via EGF receptor type.

Effector antagonism by the regulators of G protein signalling (RGS) proteins causes desensitization of mu-opioid receptors in the CNS

RATIONALE

In cell culture systems, agonists can promote the phosphorylation and internalization of receptors coupled to G proteins (GPCR), leading to their desensitization. However, in the CNS opioid agonists promote a profound desensitization of their analgesic effects without diminishing the presence of their receptors in the neuronal membrane. Recent studies have indicated that CNS proteins of the RGS family, specific regulators of G protein signalling, may be involved in mu-opioid receptor desensitization in vivo.

OBJECTIVE

In this work we review the role played by RGS proteins in the intensity and duration of the effects of mu-opioid receptor agonists, and how they influence the delayed tolerance that develops in response to specific doses of opioids.

RESULTS

RGS proteins are GTPase-activating proteins (GAP) that accelerate the hydrolysis of GalphaGTP to terminate signalling at effectors. The GAP activity of RGS-R4 and RGS-Rz proteins restricts the amplitude of opioid analgesia, and the efficient deactivation of GalphazGTP subunits by RGS-Rz proteins prevents mu receptor desensitization. However, RGS-R7 proteins antagonize effectors by binding to and sequestering mu receptor-activated Galphai/o/z subunits. Thus, they reduce the pool of receptor-regulated G proteins and hence, the effects of agonists. The delayed tolerance observed following morphine administration correlates with the transfer of Galpha subunits from mu receptors to RGS-R7 proteins and the subsequent stabilization of this association.

CONCLUSION

In the CNS, the RGS proteins control the activity of mu opioid receptors through GAP-dependent (RGS-R4 and RGS-Rz) as well as by GAP-independent mechanisms (RGS-R7). As a result, they can both antagonize effectors and desensitize receptors under certain circumstances.

Activation of μ-Opioid Receptors Transfers Control of Gα Subunits to the Regulator of G-protein Signaling RGS9-2

In mouse periaqueductal gray matter (PAG) membranes, the mu-opioid receptor (MOR) coprecipitated the alpha-subunits of the Gi/o/z/q/11 proteins, the Gbeta1/2 subunits, and the regulator of G-protein signaling RGS9-2 and its partner protein Gbeta5. RGS7 and RGS11 present in this neural structure showed no association with MOR. In vivo intracerebroventricular injection of morphine did not alter MOR immunoreactivity, but 30 min and 3 h after administration, the coprecipitation of Galpha subunits with MORs was reduced by up to 50%. Furthermore, the association between Galpha subunits and RGS9-2 proteins was increased. Twenty-four hours after receiving intracerebroventricular morphine, the Galpha subunits left the RGS9-2 proteins and re-associated with the MORs. However, doses of the opioid able to induce tolerance promoted the stable transfer of Galpha subunits to the RGS9-2 control. This was accompanied by Ser phosphorylation of RGS9-2 proteins, which increased their co-precipitation with 14-3-3 proteins. In the PAG membranes of morphine-desensitized mice, the capacity of the opioid to stimulate G-protein-related guanosine 5'-O-(3-[35S]thiotriphosphate) binding as well as low Km GTPase activity was attenuated. The in vivo knockdown of RGS9-2 expression prevented morphine from altering the association between MORs and G-proteins, and tolerance did not develop. In PAG membranes from RGS9-2 knockdown mice, morphine showed full capacity to activate G-proteins. Thus, the tolerance that develops following an adequate dose of morphine is caused by the stabilization and retention of MOR-activated Galpha subunits by RGS9-2 proteins. This multistep process is initiated by the morphine-induced transfer of MOR-associated Galpha subunits to the RGS9-2 proteins, followed by Ser phosphorylation of the latter and their binding to 14-3-3 proteins. This regulatory mechanism probably precedes the loss of MORs from the cell membrane, which has been observed with other opioid agonists.

Opioids as modulators of cell death and survival--unraveling mechanisms and revealing new indications

Opioids are powerful analgesics but also drugs of abuse. Because opioid addicts are susceptible to certain infections, opioids have been suspected to suppress the immune response. This was supported by the finding that various immune-competent cells express opioid receptors and undergo apoptosis when treated with opioid alkaloids. Recent evidence suggests that opioids may also effect neuronal survival and proliferation or migrating properties of tumor cells. A multitude of signaling pathways has been suggested to be involved in these extra-analgesic effects of opioids. Growth-promoting effects were found to be mediated through Akt and Erk signaling cascades. Death-promoting effects have been ascribed to inhibition of nuclear factor-kappaB, increase of Fas expression, p53 stabilization, cytokine and chemokine release, and activation of nitric oxide synthase, p38, and c-Jun-N-terminal kinase. Some of the observed effects were inhibited with opioid receptor antagonists or pertussis toxin; others were unaffected. It is still unclear whether these properties are mediated through typical opioid receptor activation and inhibitory G-protein-signaling. The present review tries to unravel controversial findings and provides a hypothesis that may help to integrate diverse results.

Endogenous regulator of g protein signaling proteins reduce {mu}-opioid receptor desensitization and down-regulation and adenylyl cyclase tolerance in C6 cells

Chronic exposure of cells to mu-opioid agonists leads to tolerance which can be measured by a reduced ability to activate signaling pathways in the cell. Cell signaling through inhibitory G proteins is negatively regulated by RGS (regulator of G protein signaling) proteins. Here we examine the hypothesis that the GTPase accelerating activity of RGS proteins, by altering the lifetime of Galpha and Gbetagamma, plays a role in the development of cellular tolerance to mu-opioids. C6 glioma cells were stably transfected with mu-opioid receptor and pertussis toxin (PTX)-insensitive Galpha(o) that was either sensitive or insensitive to endogenous RGS proteins. Cells were treated with PTX to uncouple endogenous Galpha proteins followed by exposure to the mu-opioid agonists [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) or morphine. Receptor desensitization as measured by agonist-stimulated [(35)S]GTPgammaS binding and receptor down-regulation as measured by [(3)H]diprenorphine binding were increased in cells expressing RGS-insensitive Galpha(o). Exposure to high concentrations of morphine or the peptidic mu-opioid agonist DAMGO led to a tolerance to inhibit adenylyl cyclase activity in both cell types with a rapid (30 min) and a slower component. Using a submaximal concentration of DAMGO to induce a reduced level of tolerance, a shift in the concentration-effect curve for DAMGO to inhibit adenylyl cyclase activity was seen in the cells expressing RGS-insensitive Galpha(o), but not in the cells expressing RGS-sensitive Galpha(o), which can be partly explained by an increased supersensitization of the adenylyl cyclase response. The results show that RGS proteins endogenously expressed in C6 cells reduce agonist-induced mu-opioid receptor desensitization, down-regulation, and sensitivity to tolerance to inhibit adenylyl cyclase activity.

Exogenous nerve growth factor attenuates opioid-induced inhibition of voltage-activated Ba2+ currents in rat sensory neurons

Nerve growth factor (NGF) promotes the survival of embryonic sensory neurons and maintains the phenotypic characteristics of primary nociceptive neurons postnatally. NGF also contributes to nociceptor activation and hyperalgesia during inflammatory pain states. The purpose of this study was to determine whether NGF might have an additional pronociceptive action by interfering with opioid-mediated analgesia in primary nociceptive neurons. Sensory neurons were isolated from the dorsal root ganglia of weanling rats and kept in standard culture conditions either with or without exogenous NGF (50 ng/ml). Currents through voltage-gated calcium channels were recorded from individual neurons using the whole cell patch clamp technique with Ba(2+) as the charge carrier (I(Ba)). The micro-opioid agonist fentanyl (1 microM) and the GABA(B) agonist baclofen (50 microM) were used to test G protein-dependent inhibition of I(Ba). Fentanyl inhibited I(Ba) by an average of 38+/-4% in untreated cells vs. 25+/-2% in NGF-treated cells (P<0.01). NGF had no effect on I(Ba) current magnitude or kinetics. The NGF-induced attenuation of opioid action was observed as early as 4 h after exposure, but was not seen when NGF was applied by bath perfusion for up to 40 min, suggesting that the effect was not mediated by a rapid phosphorylation event. The effect of NGF was prevented by K-252a (100 nM), an inhibitor of TrkA autophosphorylation. Baclofen-induced inhibition of I(Ba), on the other hand, was not affected by NGF treatment, suggesting that NGF modulation of opioid-mediated inhibition occurred upstream from the G protein. This was supported by the finding that GTP-gamma-S, an agonist independent G protein activator, inhibited I(Ba) similarly in both untreated and NGF treated cells. The results show that NGF selectively attenuated opioid-mediated inhibition of I(Ba) via TrkA receptor activation, possibly by altering opioid receptor function.

Unusual Chemokine Receptor Antagonism Involving a Mitogen-Activated Protein Kinase Pathway

Antagonism of chemokines on chemokine receptors constitutes a new regulatory principle in inflammation. Eotaxin (CCL11), an agonist for CCR3 and an attractant of eosinophils, basophils, and Th2 lymphocytes, was shown to act as an antagonist for CCR2, which is widely expressed on leukocytes and is essential for inflammatory responses. In this report we provide direct evidence for a novel mechanism how chemokine receptor function can be arrested by endogenous ligands. We show that binding of eotaxin to CCR2 stimulates the mitogen-activated protein kinases extracellular signal-regulated kinase 1/2 (ERK1/2). Activation of the mitogen-activated protein kinase kinase 1/2-ERK pathway is indispensable for eotaxin-mediated attenuation of CCR2 function, as inhibition of ERK phosphorylation abolishes the arresting effect. ERK is also activated by CCR2 agonists, e.g., monocyte chemoattractant protein-1 (CCL2). However, the involved pathways are different, although in either case coupling of CCR2 to pertussis toxin-sensitive heterotrimeric G proteins is necessary. The results are in agreement with the view that CCR2 could assume different activation states depending on the ligand it encounters. With respect to actin polymerization and calcium mobilization, the different activation states lead to agonistic and antagonistic responses. It is conceivable that the intracellular signal transduction pathway that is activated by eotaxin could cause an attenuation of proinflammatory responses mediated by CCR2.