A cluster of Ser/Thr residues at the C-terminus of mu-opioid receptor is required for G protein-coupled receptor kinase 2-mediated desensitization

Monoacylglycerol Lipase Protects the Presynaptic Cannabinoid 1 Receptor from Desensitization by Endocannabinoids after Persistent Inflammation

Cannabinoid-targeted pain therapies are increasing with the expansion of cannabis legalization, however, their efficacy may be limited by pain-induced adaptations in the cannabinoid system. Cannabinoid receptor subtype 1 (CB1R) inhibition of spontaneous, GABAergic miniature IPSCs (mIPSCs) and evoked IPSCs (eIPSCs) in the ventrolateral periaqueductal gray (vlPAG) were compared in slices from naive and inflamed male and female Sprague Dawley rats. Complete Freund's Adjuvant (CFA) injections into the hindpaw induced persistent inflammation. In naive rats, exogenous cannabinoid agonists robustly reduce both eIPSCs and mIPSCs. After 5-7 d of inflammation, the effects of exogenous cannabinoids are significantly reduced because of CB1R desensitization via GRK2/3, as function is recovered in the presence of the GRK2/3 inhibitor, Compound 101 (Cmp101). Inhibition of GABA release by presynaptic μ-opioid receptors in the vlPAG does not desensitize with persistent inflammation. Unexpectedly, while CB1R desensitization significantly reduces the inhibition produced by exogenous agonists, depolarization-induced suppression of inhibition protocols that promote 2-arachidonoylglycerol (2-AG) synthesis exhibit prolonged CB1R activation after inflammation. 2-AG tone is detected in slices from CFA-treated rats when GRK2/3 is blocked, suggesting an increase in 2-AG synthesis after persistent inflammation. Inhibiting 2-AG degradation with the monoacylglycerol lipase (MAGL) inhibitor JZL184 during inflammation results in the desensitization of CB1Rs by endocannabinoids that is reversed with Cmp101. Collectively, these data indicate that persistent inflammation primes CB1Rs for desensitization, and MAGL degradation of 2-AG protects CB1Rs from desensitization in inflamed rats. These adaptations with inflammation have important implications for the development of cannabinoid-based pain therapeutics targeting MAGL and CB1Rs.SIGNIFICANCE STATEMENT Presynaptic G-protein-coupled receptors are resistant to desensitization. Here we find that persistent inflammation increases endocannabinoid levels, priming presynaptic cannabinoid 1 receptors for desensitization on subsequent addition of exogenous agonists. Despite the reduced efficacy of exogenous agonists, endocannabinoids have prolonged efficacy after persistent inflammation. Endocannabinoids readily induce cannabinoid 1 receptor desensitization if their degradation is blocked, indicating that endocannabinoid concentrations are maintained at subdesensitizing levels and that degradation is critical for maintaining endocannabinoid regulation of presynaptic GABA release in the ventrolateral periaqueductal gray during inflammatory states. These adaptations with inflammation have important implications for the development of cannabinoid-based pain therapies.

Role of Phosphorylation Sites in Desensitization of µ-Opioid Receptor

Phosphorylation of residues in the C-terminal tail of the µ-opioid receptor (MOPr) is thought to be a key step in desensitization and internalization. Phosphorylation of C-terminal S/T residues is required for internalization (Just et al., 2013), but its role in desensitization is unknown. This study examined the influence of C-terminal phosphorylation sites on rapid desensitization of MOPr. Wild-type MOPr, a 3S/T-A mutant (S363A, T370A, S375A) that maintains internalization, 6S/T-A (S363A, T370A, S375A, T376A, T379A, T383A) and 11S/T-A (all C-terminal S/T residues mutated) mutants not internalized by MOPr agonists were stably expressed in AtT20 cells. Perforated patch-clamp recordings of MOPr-mediated activation of G-protein-activated inwardly rectifying potassium channel (Kir3.X) (GIRK) conductance by submaximal concentrations of Met(5)-enkephalin (ME) and somatostatin (SST; coupling to native SST receptor [SSTR]) were used to examine desensitization induced by exposure to ME and morphine for 5 minutes at 37°C. The rates of ME- and morphine-induced desensitization did not correlate with phosphorylation using phosphorylation site-specific antibodies. ME-induced MOPr desensitization and resensitization did not differ from wild-type for 3S/T-A and 6S/T-A but was abolished in 11S/T-A. Morphine-induced desensitization was unaffected in all three mutants, as was heterologous desensitization of SSTR. Morphine-induced desensitization (but not ME) was reduced by protein kinase C inhibition in wild-type MOPr and abolished in the 11S/T-A mutant, as was heterologous desensitization. These findings establish that MOPr desensitization can occur independently of S/T phosphorylation and internalization; however, C-terminal phosphorylation is necessary for some forms of desensitization because mutation of all C-terminal sites (11S/T-A) abolishes desensitization induced by ME.

Agonist Binding and Desensitization of the μ-Opioid Receptor Is Modulated by Phosphorylation of the C-Terminal Tail Domain

Sustained activation of G protein-coupled receptors can lead to a rapid decline in signaling through acute receptor desensitization. In the case of the μ-opioid receptor (MOPr), this desensitization may play a role in the development of analgesic tolerance. It is understood that phosphorylation of MOPr promotes association with β-arrestin proteins, which then facilitates desensitization and receptor internalization. Agonists that induce acute desensitization have been shown to induce a noncanonical high-affinity agonist binding state in MOPr, conferring a persistent memory of prior receptor activation. In the current study, live-cell confocal imaging was used to investigate the role of receptor phosphorylation in agonist binding to MOPr. A phosphorylation cluster in the C-terminal tail of MOPr was identified as a mediator of agonist-induced affinity changes in MOPr. This site is unique from the primary phosphorylation cluster responsible for β-arrestin binding and internalization. Electrophysiologic measurements of receptor function suggest that both phosphorylation clusters may play a parallel role during acute receptor desensitization. Desensitization was unaffected by alanine mutation of either phosphorylation cluster, but was largely eliminated when both clusters were mutated. Overall, this work suggests that there are multiple effects of MOPr phosphorylation that appear to regulate MOPr function: one affecting β-arrestin binding and a second affecting agonist binding.

Opioid receptor desensitization: mechanisms and its link to tolerance

Opioid receptors (OR) are part of the class A of G-protein coupled receptors and the target of the opiates, the most powerful analgesic molecules used in clinic. During a protracted use, a tolerance to analgesic effect develops resulting in a reduction of the effectiveness. So understanding mechanisms of tolerance is a great challenge and may help to find new strategies to tackle this side effect. This review will summarize receptor-related mechanisms that could underlie tolerance especially receptor desensitization. We will focus on the latest data obtained on molecular mechanisms involved in opioid receptor desensitization: phosphorylation, receptor uncoupling, internalization, and post-endocytic fate of the receptor.

Opioid receptor interacting proteins and the control of opioid signaling

Opioid receptors are seven-transmembrane domain receptors that couple to intracellular signaling molecules by activating heterotrimeric G proteins. However, the receptor and G protein do not function in isolation but their activities are modulated by several accessory and scaffolding proteins. Examples include arrestins, kinases, and regulators of G protein signaling proteins. Accessory proteins contribute to the observed potency and efficacy of agonists, but also to the direction of signaling and the phenomenon of biased agonism. This review will present current knowledge of such proteins and how they may provide targets for future drug design.

Structural features of the G-protein/GPCR interactions

BACKGROUND

The details of the functional interaction between G proteins and the G protein coupled receptors (GPCRs) have long been subjected to extensive investigations with structural and functional assays and a large number of computational studies.

SCOPE OF REVIEW

The nature and sites of interaction in the G-protein/GPCR complexes, and the specificities of these interactions selecting coupling partners among the large number of families of GPCRs and G protein forms, are still poorly defined.

MAJOR CONCLUSIONS

Many of the contact sites between the two proteins in specific complexes have been identified, but the three dimensional molecular architecture of a receptor-Gα interface is only known for one pair. Consequently, many fundamental questions regarding this macromolecular assembly and its mechanism remain unanswered.

GENERAL SIGNIFICANCE

In the context of current structural data we review the structural details of the interfaces and recognition sites in complexes of sub-family A GPCRs with cognate G-proteins, with special emphasis on the consequences of activation on GPCR structure, the prevalence of preassembled GPCR/G-protein complexes, the key structural determinants for selective coupling and the possible involvement of GPCR oligomerization in this process.

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.

Identification of phosphorylation sites in the COOH-terminal tail of the μ-opioid receptor

Phosphorylation is considered a key event in the signalling and regulation of the μ opioid receptor (MOPr). Here, we used mass spectroscopy to determine the phosphorylation status of the C-terminal tail of the rat MOPr expressed in human embryonic kidney 293 (HEK-293) cells. Under basal conditions, MOPr is phosphorylated on Ser(363) and Thr(370), while in the presence of morphine or [D-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO), the COOH terminus is phosphorylated at three additional residues, Ser(356) , Thr(357) and Ser(375). Using N-terminal glutathione S transferase (GST) fusion proteins of the cytoplasmic, C-terminal tail of MOPr and point mutations of the same, we show that, in vitro, purified G protein-coupled receptor kinase 2 (GRK2) phosphorylates Ser(375), protein kinase C (PKC) phosphorylates Ser(363), while CaMKII phosphorylates Thr(370). Phosphorylation of the GST fusion protein of the C-terminal tail of MOPr enhanced its ability to bind arrestin-2 and -3. Hence, our study identifies both the basal and agonist-stimulated phospho-acceptor sites in the C-terminal tail of MOPr, and suggests that the receptor is subject to phosphorylation and hence regulation by multiple protein kinases.

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.

The histidine triad nucleotide-binding protein 1 supports mu-opioid receptor-glutamate NMDA receptor cross-regulation

A series of pharmacological and physiological studies have demonstrated the functional cross-regulation between MOR and NMDAR. These receptors coexist at postsynaptic sites in midbrain periaqueductal grey (PAG) neurons, an area implicated in the analgesic effects of opioids like morphine. In this study, we found that the MOR-associated histidine triad nucleotide-binding protein 1 (HINT1) is essential for maintaining the connection between the NMDAR and MOR. Morphine-induced analgesic tolerance is prevented and even rescued by inhibiting PKC or by antagonizing NMDAR. However, in the absence of HINT1, the MOR becomes supersensitive to morphine before suffering a profound and lasting desensitization that is refractory to PKC inhibition or NMDAR antagonism. Thus, HINT1 emerges as a key protein that is critical for sustaining NMDAR-mediated regulation of MOR signaling strength. Thus, HINT1 deficiency may contribute to opioid-intractable pain syndromes by causing long-term MOR desensitization via mechanisms independent of NMDAR.

Functional selectivity at the μ-opioid receptor: implications for understanding opioid analgesia and tolerance

Opioids are the most effective analgesic drugs for the management of moderate or severe pain, yet their clinical use is often limited because of the onset of adverse side effects. Drugs in this class produce most of their physiological effects through activation of the μ opioid receptor; however, an increasing number of studies demonstrate that different opioids, while presumably acting at this single receptor, can activate distinct downstream responses, a phenomenon termed functional selectivity. Functional selectivity of receptor-mediated events can manifest as a function of the drug used, the cellular or neuronal environment examined, or the signaling or behavioral measure recorded. This review summarizes both in vitro and in vivo work demonstrating functional selectivity at the μ opioid receptor in terms of G protein coupling, receptor phosphorylation, interactions with β-arrestins, receptor desensitization, internalization and signaling, and details on how these differences may relate to the progression of analgesic tolerance after their extended use.

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.

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.

High-purity selection and maintenance of gene expression in human neuroblastoma cells stably over-expressing GFP fusion protein. Application for opioid receptors desensitization studies

Chronic use of opiates such as morphine is associated with drug tolerance, which is correlated with the desensitization of opioid receptors. This latter process involves phosphorylation of opioid receptors by G protein-coupled receptors kinases (GRKs) and subsequent uncoupling by beta-arrestins. To explore these molecular mechanisms, neuronal cell lines, endogenously expressing the opioid receptors, provide an ideal cellular model. Unfortunately, there are two major drawbacks: (1) these cells are refractory to cDNA introduction, resulting in low transfection efficiency; (2) continuous culturing of transfected cells invariably leads to phenotypic drift of the cultures even after an antibiotic selection. So, these cells were dropped in favor of heterologous expression systems, which are easier to transfect but whose relevance as adequate cellular model for studying opioid receptor regulation should be questioned, as recently demonstrated by [Haberstock-Debic, H., Kim, K.A.,Yu, Y.J., von Zastrow, M., 2005. Morphine promotes rapid, arrestin-dependent endocytosis of mu-opioid receptors in striatal neurons. J. Neurosci. 25, 7847-7857]. In this work, we describe a method, based on fluorescence-activated cell sorting (FACS), to select and maintain a high proportion of transfected SK-N-BE cells (a neuronal cell line endogenously expressing human Delta-Opioid Receptor (hDOR)), expressing the beta-arrestin1 fused to green fluorescent protein (GFP). While in functional experiments, we were not able to observe a major effect in non-sorted SK-N-BE cells expressing beta-arrestin1-GFP, the enrichment by 18-fold with FACS resulted in a robust increase of beta-arrestin1-GFP expression associated with strong hDOR desensitization. Moreover, this method also allows to counteract the phenotypic drift and to maintain a high-purity selection of SK-N-BE cells expressing beta-arrestin1-GFP. Thus, this approach provides a valuable tool for exploring opioid receptors desensitization in neuronal cells.

Agonist-Selective Mechanisms of μ-Opioid Receptor Desensitization in Human Embryonic Kidney 293 Cells

The ability of two opioid agonists, [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) and morphine, to induce mu-opioid receptor (MOR) phosphorylation, desensitization, and internalization was examined in human embryonic kidney (HEK) 293 cells expressing rat MOR1 as well G protein-coupled inwardly rectifying potassium channel (GIRK) channel subunits. Both DAMGO and morphine activated GIRK currents, but the maximum response to DAMGO was greater than that of morphine, indicating that morphine is a partial agonist. The responses to DAMGO and morphine desensitized rapidly in the presence of either drug. Expression of a dominant negative mutant G protein-coupled receptor kinase 2 (GRK2), GRK2-K220R, markedly attenuated the DAMGO-induced desensitization of MOR1, but it had no effect on morphine-induced MOR1 desensitization. In contrast, inhibition of protein kinase C (PKC) either by the PKC inhibitory peptide PKC (19-31) or staurosporine reduced MOR1 desensitization by morphine but not that induced by DAMGO. Morphine and DAMGO enhanced MOR1 phosphorylation over basal. The PKC inhibitor bisindolylmaleimide 1 (GF109203X) inhibited MOR1 phosphorylation under basal conditions and in the presence of morphine, but it did not inhibit DAMGO-induced phosphorylation. DAMGO induced arrestin-2 translocation to the plasma membrane and considerable MOR1 internalization, whereas morphine did not induce arrestin-2 translocation and induced very little MOR1 internalization. Thus, DAMGO and morphine each induce desensitization of MOR1 signaling in HEK293 cells but by different molecular mechanisms; DAMGO-induced desensitization is GRK2-dependent, whereas morphine-induced desensitization is in part PKC-dependent. MORs desensitized by DAMGO activation are then readily internalized by an arrestin-dependent mechanism, whereas those desensitized by morphine are not. These data suggest that opioid agonists induce different conformations of the MOR that are susceptible to different desensitizing and internalization processes.

Two C-terminal amino acids, Ser(334) and Ser(335), are required for homologous desensitization and agonist-induced phosphorylation of opioid receptor-like 1 receptors

Various cellular signaling pathways induced by nociceptin activation of ORL1 (opioid receptor-like 1 receptor) develop homologous desensitization. Multiple lines of evidence suggest that agonist-induced phosphorylation of serine (Ser)/threonine (Thr) residues at intracellular carboxyl tail leads to homologous desensitization of G protein-coupled receptors. In the present study, we investigated the functional role played by C-terminal Ser/Thr residues in agonist-induced desensitization and phosphorylation of ORL1. In HEK 293 cells expressing wild-type ORL1 and ORL1(CDelta21), which lacks C-terminal 21 amino acids, nociceptin inhibition of adenylate cyclase activity exhibited homologous desensitization after 1 h pretreatment of nociceptin. In contrast, ORL1(CDelta34), which differs with ORL1(CDelta21) by lacking C-terminal Ser(334), Ser(335) and Ser(343) residues, failed to develop agonist-induced desensitization. Point mutant (S343A) ORL1 underwent homologous desensitization after nociceptin pretreatment. Substituting Ser(334) or Ser(335) with alanine greatly impaired nociceptin-induced ORL1 desensitization. In HEK 293 cells expressing double mutant (S334A/S335A) ORL1, nociceptin pretreatment failed to significantly affect the efficacy and potency by which nociceptin inhibits forskolin-stimulated cAMP formation. Mutation of Ser(334) and Ser(335) also greatly reduced nociceptin-induced ORL1 phosphorylation. These results suggest that two C-terminal serine residues, Ser(334) and Ser(335), are required for homologous desensitization and agonist-induced phosphorylation of ORL1.

Heterodimerization of opioid receptor-like 1 and mu-opioid receptors impairs the potency of micro receptor agonist

Nociceptin activation of ORL1 (opioid receptor-like 1 receptor) has been shown to antagonize mu receptor-mediated analgesia at the supraspinal level. ORL1 and mu-opioid receptor (muR) are co-expressed in several subpopulations of CNS neurons involved in regulating pain transmission. The amino acid sequence of ORL1 also shares a high degree of homology with that of mu receptor. Thus, it is hypothesized that ORL1 and muR interact to form the heterodimer and that ORL1/muR heterodimerization may be one molecular basis for ORL1-mediated antiopioid effects in the brain. To test this hypothesis, myc-tagged ORL1 and HA-tagged muR are co-expressed in human embryonic kidney (HEK) 293 cells. Co-immunoprecipitation experiments demonstrate that ORL1 dimerizes with muR and that intracellular C-terminal tails of ORL1 and muR are required for the formation of ORL1/muR heterodimer. Second messenger assays further indicate that formation of ORL1/muR heterodimer selectively induces cross-desensitization of muR and impairs the potency by which [D-Ala(2),N-methyl-Phe(4),Gly-ol(5)]enkephalin (DAMGO) inhibits adenylate cyclase and stimulates p42/p44 mitogen-activated protein kinase (MAPK) phosphorylation. These results provide the evidence that ORL1/muR heterodimerization and the resulting impairment of mu receptor-activated signaling pathways may contribute to ORL1-mediated antiopioid effects in the brain.

Distinct domains of the mu-opioid receptor control uncoupling and internalization

Homologous desensitization of the micro opioid receptor (muOR) can be resolved into distinct processes that include the uncoupling of the muOR from its G-protein effectors and internalization of cell surface receptors. Using electrophysiological recordings of muOR activation of G-protein-coupled K+ channels (Kir3) in Xenopus laevis oocytes and AtT20 cells, confocal microscopy of receptor localization, and radioligand binding of cell surface receptors, we resolved these desensitization mechanisms to determine the domain of muOR important for receptor uncoupling. Activation of muOR by saturating concentrations of [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), methadone, or fentanyl, but not morphine, produced robust internalization of a green fluorescent protein-tagged muOR. A subsaturating concentration of DAMGO (100 nM) did not cause receptor internalization but markedly reduced the subsequent responsiveness of Kir3 by uncoupling muOR. muOR desensitization in AtT20 cells was confirmed to be homologous, because desensitization by 100 nM DAMGO was blocked by dominant-negative forms of either G protein-coupled receptor kinase (GRK) or arrestin, and pretreatment with DAMGO did not affect the Kir3 response to somatostatin receptor activation. Alanine substitution of a single threonine in the second cytoplasmic loop of the muOR (Threonine 180) blocked agonist-dependent receptor uncoupling without affecting receptor internalization. These results suggest that GRK-dependent phosphorylation of muOR required threonine 180 for uncoupling but that a different GRK and arrestin-dependent mechanism controlled muOR internalization in AtT20 cells.

Suppression of the morphine-induced rewarding effect and G-protein activation in the lower midbrain following nerve injury in the mouse: involvement of G-protein-coupled receptor kinase 2

The present study was designed to investigate whether a state of neuropathic pain induced by sciatic nerve ligation could alter the rewarding effect, antinociception, and G-protein activation induced by a prototype of mu-opioid receptor agonist morphine in the mouse. The sciatic nerve ligation caused a long-lasting and profound thermal hyperalgesia. Under this neuropathic pain-like state, an i.c.v. morphine-induced place preference was observed in sham-operated mice but not in sciatic nerve-ligated mice. However, no differences in the antinociceptive effect of i.c.v.-administered morphine were noted between the groups. The increases in the binding of guanosine-5'-o-(3-[(35)S]thio)triphosphate induced by morphine in lower midbrain membranes including the ventral tegmental area, which contributes to the expression of the rewarding effect of opioid, were significantly attenuated in sciatic nerve-ligated mice. On the other hand, there were no differences in the stimulation of guanosine-5'-o-(3-[(35)S]thio)triphosphate binding to pons/medulla membranes, which plays an important role in the antinociception of mu-opioid receptor agonists, between the groups. In addition, no changes in levels of guanosine-5'-o-(3-[(35)S]thio)triphosphate binding by either the selective delta- or kappa-opioid receptor agonists were noted in membrane of the lower midbrain and limbic forebrain membranes obtained from sciatic nerve-ligated mice. Reverse transcription-polymerase chain reaction analysis showed that sciatic nerve ligation did not alter the mRNA product of mu-opioid receptors in the lower midbrain, indicating that a decrease in some mu-opioid receptor functions may result from the uncoupling of mu-opioid receptors from G-proteins. We found a significant increase in protein levels of G-protein-coupled receptor kinase 2, which causes receptor phosphorylation in membranes of the lower midbrain but not in the pons/medulla, obtained from mice with nerve injury, whereas there were no changes in the protein level of phosphorylated-protein kinase C in the lower midbrain. These results suggest that the uncoupling of mu-opioid receptors from G-proteins by G-protein-coupled receptor kinase 2 in the lower midbrain may, at least in part, contribute to the suppression of the rewarding effect of morphine under neuropathic pain.

Induction of G protein-coupled receptor kinases 2 and 3 contributes to the cross-talk between mu and ORL1 receptors following prolonged agonist exposure

The molecular mechanism(s) underlying cross-tolerance between mu and opioid receptor-like 1 (ORL1) receptor agonists were investigated using two human neuroblastoma cell lines endogenously expressing these receptors and G protein-coupled receptor kinases (GRKs). Prolonged (24 h) activation of the mu receptor desensitized both mu and ORL1 receptor-mediated inhibition of forskolin-stimulated cAMP accumulation and upregulated GRK2 levels in SH-SY5Y and BE(2)-C cells. Prolonged ORL1 activation increased GRK2 levels and desensitized both receptors in SH-SY5Y cells. Upregulation of GRK2 correlated with increases in levels of transcription factors Sp1 or AP-2. PD98059, an upstream inhibitor of extracellular signal-regulated kinases 1 and 2 (ERK1/2), reversed all these events. Pretreatment with orphanin FQ/nociceptin (OFQ/N) also upregulated GRK3 levels in both cell lines, and desensitized both receptors in BE(2)-C cells. Protein kinase C (PKC), but not ERK1/2, inhibition blocked OFQ/N-mediated GRK3 induction and mu and ORL1 receptor desensitization in BE(2)-C cells. Antisense DNA treatment confirmed the involvement of GRK2/3 in mu and ORL1 desensitization. Here, we demonstrate for the first time a role for ERK1/2-mediated GRK2 induction in the development of tolerance to mu agonists, as well as cross-tolerance to OFQ/N. We also demonstrate that chronic OFQ/N-mediated desensitization of ORL1 and mu receptors occurs via cell-specific pathways, involving ERK1/2-dependent GRK2, or PKC-dependent and ERK1/2-independent GRK3 induction.

Regulation of mu-opioid receptors, G-protein-coupled receptor kinases and beta-arrestin 2 in the rat brain after chronic opioid receptor antagonism

The aim of this study was to analyse the biochemical and behavioural consequences of chronic treatment with opioid receptor antagonists in rats. We have evaluated the respiratory depressant and antinociceptive effects of the mu-opioid agonist sufentanil, the density of brain mu-opioid receptors, and the expression of G-protein-coupled receptor kinases and beta-arrestin 2 in cerebral cortex and striatum, following sustained opioid receptor blockade. Our results demonstrate that 24 h after interruption of 7 days chronic infusion of naltrexone (120 microg/h), the respiratory depressant potency of the mu-opioid receptor agonist sufentanil was increased to a similar extent as the antinociceptive potency (about three-fold). This was accompanied by mu-opioid receptor up-regulation in several areas of the rat brain associated with opioid control of pain perception and breathing. Moreover, chronic treatment with either naltrexone (120 microg/h) or naloxone (120 microg/h) caused significant increases in the expression levels of G-protein-coupled receptor kinases types 2, 3, and 6, and of beta-arrestin 2 in brain cortex and striatum. Together our data suggest an increased constitutive receptor activity secondary to mu-opioid receptor up-regulation following chronic antagonist treatment.

Identification of two C-terminal amino acids, Ser(355) and Thr(357), required for short-term homologous desensitization of mu-opioid receptors

Our recent study suggests that a cluster of Ser/Thr residues (T(354)S(355)S(356)T(357)) at the intracellular carboxyl tail of rat mu-opioid receptor (MOR1) is required for the development of short-term homologous desensitization. To investigate the functional role played by individual serine or threonine residue of this (TSST) cluster in the agonist-induced mu-opioid receptor desensitization, point mutant (T354A), (S355A), (S356A) and (T357A) mu-opioid receptors were prepared and stably expressed in human embryonic kidney 293 cells (HEK 293 cells). Similar to wild-type mu-opioid receptors, mutant (T354A) and (S356A) mu-opioid receptors stably expressed in HEK 293 cells developed homologous desensitization after 30 min pretreatment of DAMGO ([D-Ala(2),N-methyl-Phe(4),Gly-ol(5)]enkephalin), a specific mu-opioid receptor agonist. Substituting Ser(355)or Thr(357) with alanine resulted in a significant attenuation of agonist-induced mu-opioid receptor desensitization. In HEK 293 cells stably expressing double mutant (S355A/T357A) mu-opioid receptors, DAMGO pretreatment failed to significantly affect the efficacy and potency by which DAMGO inhibits forskolin-stimulated adenylyl cyclase activity. Consistent with the general belief that agonist-induced phosphorylation of guanine nucleotide binding protein (G protein)-coupled receptors is involved in homologous desensitization. Treating HEK 293 cells expressing wild-type mu-opioid receptors with 5 microM DAMGO for 30 min induced the receptor phosphorylation. Mutation of Ser(355) and Thr(357) also greatly impaired DAMGO-induced mu-opioid receptor phosphorylation. These results suggest that two C-terminal amino acids, Ser(355) and Thr(357), are required for short-term homologous desensitization and agonist-induced phosphorylation of mu-opioid receptors expressed in HEK 293 cells.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

G protein-coupled receptor kinase 2 mediates mu-opioid receptor desensitization in GABAergic neurons of the nucleus raphe magnus

Nucleus raphe magnus (NRM) sends the projection to spinal dorsal horn and inhibits nociceptive transmission. Analgesic effect produced by mu-opioid receptor agonists including morphine partially results from activating the NRM-spinal cord pathway. It is generally believed that mu-opioid receptor agonists disinhibit spinally projecting neurons of the NRM and produce analgesia by hyperpolarizing GABAergic interneurons. In the present study, whole-cell patch-clamp recordings combined with single-cell RT-PCR analysis were used to test the hypothesis that DAMGO ([D-Ala(2),N-methyl-Phe(4),Gly-ol(5)]enkephalin), a specific mu-opioid receptor agonist, selectively hyperpolarizes NRM neurons expressing mRNA of glutamate decarboxylase (GAD(67)). Homologous desensitization of mu-opioid receptors in NRM neurons could result in the development of morphine-induced tolerance. G protein-coupled receptor kinase (GRK) is believed to mediate mu-opioid receptor desensitization in vivo. Therefore, we also investigated the involvement of GRK in mediating homologous desensitization of DAMAMGO-induced electrophysiological effects on NRM neurons by using two experimental strategies. First, single-cell RT-PCR assay was used to study the expression of GRK2 and GRK3 mRNAs in individual DAMGO-responsive NRM neurons. Whole-cell recording was also performed with an internal solution containing the synthetic peptide, which corresponds to G(betagamma)-binding domain of GRK and inhibits G(betagamma) activation of GRK. Our results suggest that DAMGO selectively hyperpolarizes NRM GABAergic neurons by opening inwardly rectifying K(+) channels and that GRK2 mediates short-term homologous desensitization of mu-opioid receptors in NRM GABAergic neurons.

Changes in the expression of G protein-coupled receptor kinases and beta-arrestin 2 in rat brain during opioid tolerance and supersensitivity

We previously demonstrated that chronic treatment of rats with the mu-opioid receptor agonist sufentanil induced pharmacological tolerance associated with mu-opioid receptor desensitization and down-regulation. Administration of the calcium channel blocker nimodipine during chronic treatment with sufentanil prevented mu-opioid receptor down-regulation, induced down-stream supersensitization, and produced supersensitivity to the opioid effects. The focus of the present study was to determine a role for G protein-coupled receptor kinases (GRKs) and beta-arrestin 2 in agonist-induced mu-opioid receptor signalling modulation during chronic opioid tolerance and supersensitivity. Tolerance was induced by 7-day chronic infusion of sufentanil (2 microgram/h). Supersensitivity was induced by concurrent infusion of sufentanil (2 microgram/h) and nimodipine (1 microgram/h) for 7 days. Antinociception was evaluated by the tail-flick test. GRK2, GRK3, GRK6 and beta-arrestin 2 immunoreactivity levels were determined by western blot in brain cortices. Acute and chronic treatment with sufentanil induced analgesic tolerance, associated with up-regulation of GRK2, GRK6, and beta-arrestin 2. GRK3 expression only was increased in the acutely treated group. When nimodipine was associated to the chronic opioid treatment, tolerance expression was prevented, and immunoreactivity levels of GRK2, GRK6 and beta-arrestin 2 recovered the control values. These data indicate that GRK2, GRK3, GRK6 and beta-arrestin 2 are involved in the short- and long-term adaptive changes in mu-opioid receptor activity, contributing to tolerance development in living animals. These observations also suggest that GRKs and beta-arrestin 2 could constitute pharmacological targets to prevent opioid tolerance development, and to improve the analgesic efficacy of opioid drugs.

Threonine 180 is required for G-protein-coupled receptor kinase 3- and beta-arrestin 2-mediated desensitization of the mu-opioid receptor in Xenopus oocytes

To determine the sites in the mu-opioid receptor (MOR) critical for agonist-dependent desensitization, we constructed and coexpressed MORs lacking potential phosphorylation sites along with G-protein activated inwardly rectifying potassium channels composed of K(ir)3.1 and K(ir)3.4 subunits in Xenopus oocytes. Activation of MOR by the stable enkephalin analogue, [d-Ala(2),MePhe(4),Glyol(5)]enkephalin, led to homologous MOR desensitization in oocytes coexpressing both G-protein-coupled receptor kinase 3 (GRK3) and beta-arrestin 2 (arr3). Coexpression with either GRK3 or arr3 individually did not significantly enhance desensitization of responses evoked by wild type MOR activation. Mutation of serine or threonine residues to alanines in the putative third cytoplasmic loop and truncation of the C-terminal tail did not block GRK/arr3-mediated desensitization of MOR. Instead, alanine substitution of a single threonine in the second cytoplasmic loop to produce MOR(T180A) was sufficient to block homologous desensitization. The insensitivity of MOR(T180A) might have resulted either from a block of arrestin activation or arrestin binding to MOR. To distinguish between these alternatives, we expressed a dominant positive arrestin, arr2(R169E), that desensitizes G protein-coupled receptors in an agonist-dependent but phosphorylation-independent manner. arr2(R169E) produced robust desensitization of MOR and MOR(T180A) in the absence of GRK3 coexpression. These results demonstrate that the T180A mutation probably blocks GRK3- and arr3-mediated desensitization of MOR by preventing a critical agonist-dependent receptor phosphorylation and suggest a novel GRK3 site of regulation not yet described for other G-protein-coupled receptors.