Oligomerization of mu- and delta-opioid receptors. Generation of novel functional properties

Synthesis, modeling, and pharmacological evaluation of UMB 425, a mixed μ agonist/δ antagonist opioid analgesic with reduced tolerance liabilities

Opioid narcotics are used for the treatment of moderate-to-severe pain and primarily exert their analgesic effects through μ receptors. Although traditional μ agonists can cause undesired side effects, including tolerance, addition of δ antagonists can attenuate said side effects. Herein, we report 4a,9-dihydroxy-7a-(hydroxymethyl)-3-methyl-2,3,4,4a,5,6-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7(7aH)-one (UMB 425) a 5,14-bridged morphinan-based orvinol precursor synthesized from thebaine. Although UMB 425 lacks δ-specific motifs, conformationally sampled pharmacophore models for μ and δ receptors predict it to have efficacy similar to morphine at μ receptors and similar to naltrexone at δ receptors, due to the compound sampling conformations in which the hydroxyl moiety interacts with the receptors similar to orvinols. As predicted, UMB 425 exhibits a mixed μ agonist/δ antagonist profile as determined in receptor binding and [(35)S]GTPγS functional assays in CHO cells. In vivo studies in mice show that UMB 425 displays potent antinociception in the hot plate and tail-flick assays. The antinociceptive effects of UMB 425 are blocked by naloxone, but not by the κ-selective antagonist norbinaltorphimine. During a 6-day tolerance paradigm, UMB 425 maintains significantly greater antinociception compared to morphine. These studies thus indicate that, even in the absence of δ-specific motifs fused to the C-ring, UMB 425 has mixed μ agonist/δ antagonist properties in vitro that translate to reduced tolerance liabilities in vivo.

Morphine prevents lipopolysaccharide-induced TNF secretion in mast cells blocking IκB kinase activation and SNAP-23 phosphorylation: correlation with the formation of a β-arrestin/TRAF6 complex

We have previously shown that morphine pretreatment inhibits mast cell-dependent TNF production after LPS injection in the murine peritoneal cavity. In this study, we used bone marrow-derived mast cells (BMMCs) to investigate the molecular mechanisms of that inhibition. We found that morphine prevented LPS-induced TNF secretion in these cells. The observed inhibition was not due to morphine-induced TLR4 internalization and it was related to the blockage of preformed TNF secretion. LPS-induced TNF exocytosis in BMMCs was dependent on tetanus toxin-insensitive vesicle-associated membrane proteins and calcium mobilization, as well as PI3K, MAPK, and IκB kinase (IKK) activation. TNF secretion was also associated to the phosphorylation of synaptosomal-associated protein 23 (SNAP-23), which was found forming a complex with IKK in LPS-activated BMMCs. Morphine pretreatment prevented TLR4-dependent ERK and IKK phosphorylation. Analyzing the signaling events upstream of IKK activation, we found diminished TGF-β-activated kinase 1 (TAK1) phosphorylation and TNFR-associated factor (TRAF) 6 ubiquitination in BMMCs pretreated with morphine and stimulated with LPS. Morphine pretreatment provoked a marked increase in the formation of a molecular complex composed of TRAF6 and β-arrestin-2. Naloxone and a combination of μ and δ opioid receptor antagonists prevented morphine inhibitory actions. In conclusion, our results show that activation of μ and δ opioid receptors with morphine suppresses TLR4-induced TNF release in mast cells, preventing the IKK-dependent phosphorylation of SNAP-23, which is necessary for TNF exocytosis, and this inhibition correlates with the formation of a β-arrestin-2/TRAF6 complex. To our knowledge, these findings constitute the first evidence of molecular crosstalk between opioid receptors and the TLR4 signal transduction system in mast cells.

An immunocytochemical-derived correlate for evaluating the bridging of heteromeric mu-delta opioid protomers by bivalent ligands

Bivalent ligands that contain two pharmacophores linked by a spacer are promising tools to investigate the pharmacology of opioid receptor heteromers. Evidence for occupation of neighboring protomers by two phamacophores of a single bivalent ligand (bridging) has relied mainly on pharmacological data. In the present study, we have employed an immunocytochemical correlate to support in vivo biological studies that are consistent with bridging. We show that a bivalent mu agonist/delta antagonist (MDAN-21) that is devoid of tolerance due to possible bridging of mu and delta protomers prevents endocytosis of the heteromeric receptors in HEK-293 cells. Conversely, a bivalent ligand (MDAN-16) with a short spacer or monovalent mu agonist give rise to robust internalization. The data suggest that the immobilization of proximal mu and delta protomers is due to bridging by MDAN-21. The finding that MDAN-21 and its shorter spacer homologue MDAN-16 possess equivalent activity in HEK-293 cells, but produce dramatically divergent internalization of mu-delta heteromer, is relevant to the role of internalization and tolerance.

On the g-protein-coupled receptor heteromers and their allosteric receptor-receptor interactions in the central nervous system: focus on their role in pain modulation

The modulatory role of allosteric receptor-receptor interactions in the pain pathways of the Central Nervous System and the peripheral nociceptors has become of increasing interest. As integrators of nociceptive and antinociceptive wiring and volume transmission signals, with a major role for the opioid receptor heteromers, they likely have an important role in the pain circuits and may be involved in acupuncture. The delta opioid receptor (DOR) exerts an antagonistic allosteric influence on the mu opioid receptor (MOR) function in a MOR-DOR heteromer. This heteromer contributes to morphine-induced tolerance and dependence, since it becomes abundant and develops a reduced G-protein-coupling with reduced signaling mainly operating via β-arrestin2 upon chronic morphine treatment. A DOR antagonist causes a return of the Gi/o binding and coupling to the heteromer and the biological actions of morphine. The gender- and ovarian steroid-dependent recruitment of spinal cord MOR/kappa opioid receptor (KOR) heterodimers enhances antinociceptive functions and if impaired could contribute to chronic pain states in women. MOR1D heterodimerizes with gastrin-releasing peptide receptor (GRPR) in the spinal cord, mediating morphine induced itch. Other mechanism for the antinociceptive actions of acupuncture along meridians may be that it enhances the cross-desensitization of the TRPA1 (chemical nociceptor)-TRPV1 (capsaicin receptor) heteromeric channel complexes within the nociceptor terminals located along these meridians. Selective ionotropic cannabinoids may also produce cross-desensitization of the TRPA1-TRPV1 heteromeric nociceptor channels by being negative allosteric modulators of these channels leading to antinociception and antihyperalgesia.

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

Delta opioid receptors (DORs) have been considered as a potential target to relieve pain as well as treat depression and anxiety disorders and are known to modulate other physiological responses, including ethanol and food consumption. A small number of DOR-selective drugs are in clinical trials, but no DOR-selective drugs have been approved by the Federal Drug Administration and some candidates have failed in phase II clinical trials, highlighting current difficulties producing effective delta opioid-based therapies. Recent studies have provided new insights into the pharmacology of the DOR, which is often complex and at times paradoxical.

OBJECTIVE

This review will discuss the existing literature focusing on four aspects: (1) Two DOR subtypes have been postulated based on differences in pharmacological effects of existing DOR-selective ligands. (2) DORs are expressed ubiquitously throughout the body and central nervous system and are, thus, positioned to play a role in a multitude of diseases. (3) DOR expression is often dynamic, with many reports of increased expression during exposure to chronic stimuli, such as stress, inflammation, neuropathy, morphine, or changes in endogenous opioid tone. (4) A large structural variety in DOR ligands implies potential different mechanisms of activating the receptor.

CONCLUSION

The reviewed features of DOR pharmacology illustrate the potential benefit of designing tailored or biased DOR ligands.

Stabilization of the μ-opioid receptor by truncated single transmembrane splice variants through a chaperone-like action

The μ-opioid receptor gene, OPRM1, undergoes extensive alternative pre-mRNA splicing, as illustrated by the identification of an array of splice variants generated by both 5' and 3' alternative splicing. The current study reports the identification of another set of splice variants conserved across species that are generated through exon skipping or insertion that encodes proteins containing only a single transmembrane (TM) domain. Using a Tet-Off system, we demonstrated that the truncated single TM variants can dimerize with the full-length 7-TM μ-opioid receptor (MOR-1) in the endoplasmic reticulum, leading to increased expression of MOR-1 at the protein level by a chaperone-like function that minimizes endoplasmic reticulum-associated degradation. In vivo antisense studies suggested that the single TM variants play an important role in morphine analgesia, presumably through modulation of receptor expression levels. Our studies suggest the functional roles of truncated receptors in other G protein-coupled receptor families.

The arrestin-selective angiotensin AT1 receptor agonist [Sar1,Ile4,Ile8]-AngII negatively regulates bradykinin B2 receptor signaling via AT1-B2 receptor heterodimers

The renin-angiotensin and kallikrein-kinin systems are key regulators of vascular tone and inflammation. Angiotensin II, the principal effector of the renin-angiotensin system, promotes vasoconstriction by activating angiotensin AT1 receptors. The opposing effects of the kallikrein-kinin system are mediated by bradykinin acting on B1 and B2 bradykinin receptors. The renin-angiotensin and kallikrein-kinin systems engage in cross-talk at multiple levels, including the formation of AT1-B2 receptor heterodimers. In primary vascular smooth muscle cells, we find that the arrestin pathway-selective AT1 agonist, [Sar(1),Ile(4),Ile(8)]-AngII, but not the neutral AT1 antagonist, losartan, inhibits endogenous B2 receptor signaling. In a transfected HEK293 cell model that recapitulates this effect, we find that the actions of [Sar(1),Ile(4), Ile(8)]-AngII require the AT1 receptor and result from arrestin-dependent co-internalization of AT1-B2 heterodimers. BRET50 measurements indicate that AT1 and B2 receptors efficiently heterodimerize. In cells expressing both receptors, pretreatment with [Sar(1),Ile(4),Ile(8)]-AngII blunts B2 receptor activation of Gq/11-dependent intracellular calcium influx and Gi/o-dependent inhibition of adenylyl cyclase. In contrast, [Sar(1),Ile(4),Ile(8)]-AngII has no effect on B2 receptor ligand affinity or bradykinin-induced arrestin3 recruitment. Both radioligand binding assays and quantitative microscopy-based analysis demonstrate that [Sar(1),Ile(4),Ile(8)]-AngII promotes internalization of AT1-B2 heterodimers. Thus, [Sar(1),Ile(4),Ile(8)]-AngII exerts lateral allosteric modulation of B2 receptor signaling by binding to the orthosteric ligand binding site of the AT1 receptor and promoting co-sequestration of AT1-B2 heterodimers. Given the opposing roles of the renin-angiotensin and kallikrein-kinin systems in vivo, the distinct properties of arrestin pathway-selective and neutral AT1 receptor ligands may translate into different pharmacologic actions.

Anti-analgesic effect of the mu/delta opioid receptor heteromer revealed by ligand-biased antagonism

Delta (DOR) and mu opioid receptors (MOR) can complex as heteromers, conferring functional properties in agonist binding, signaling and trafficking that can differ markedly from their homomeric counterparts. Because of these differences, DOR/MOR heteromers may be a novel therapeutic target in the treatment of pain. However, there are currently no ligands selective for DOR/MOR heteromers, and, consequently, their role in nociception remains unknown. In this study, we used a pharmacological opioid cocktail that selectively activates and stabilizes the DOR/MOR heteromer at the cell surface by blocking its endocytosis to assess its role in antinociception. We found that mice treated chronically with this drug cocktail showed a significant right shift in the ED50 for opioid-mediated analgesia, while mice treated with a drug that promotes degradation of the heteromer did not. Furthermore, promoting degradation of the DOR/MOR heteromer after the right shift in the ED50 had occurred, or blocking signal transduction from the stabilized DOR/MOR heteromer, shifted the ED50 for analgesia back to the left. Taken together, these data suggest an anti-analgesic role for the DOR/MOR heteromer in pain. In conclusion, antagonists selective for DOR/MOR heteromer could provide an avenue for alleviating reduced analgesic response during chronic pain treatment.

Pharmacological modulation of geranylgeranyltransferase and farnesyltransferase attenuates opioid withdrawal in vivo and in vitro

Geranylgeranyltransferase and farnesyltransferase I, are noted to mediate a number of signal transduction cascades which are known to be involved in the causation of opioid withdrawal syndrome. GGTI-2133 and FTI-276 are selective modulators of geranylgeranyltransferase and farnesyltransferase subtype 1 respectively. Therefore, the present study investigated the effect of GGTI-2133 and FTI-276 on propagation of morphine dependence and resultant withdrawal signs in vivo, in sub-chronic morphine mouse model, and in vitro, in isolated rat ileum. Morphine was administered twice daily for 5 days following which a single day 6 injection of naloxone (8 mg/kg, i.p.) precipitated opioid withdrawal syndrome in mice. Withdrawal syndrome was quantitatively assessed in terms of withdrawal severity score and the frequency of jumping, rearing, fore paw licking & circling. Naloxone induced contraction in morphine withdrawn isolated rat ileum was employed as an in vitro model of opioid withdrawal syndrome. An isobolographic study design was employed to assess a potential synergistic activity between GGTI-2133 and FTI-276. GGTI-2133 and FTI-276 dose dependently attenuated naloxone induced morphine withdrawal syndrome both in vivo and in vitro. GGTI-2133 was also observed to exert a synergistic interaction with FTI-276. It is concluded that GGTI-2133 and FTI-276 attenuate the propagation of morphine dependence and reduce withdrawal signs possibly by a geranylgeranyl transferase; farnesyltransferase activation pathway linked mechanisms potentially in an interdependent manner.

Agonist-induced receptor internalization in Chinese hamster ovary cells stably co-expressing β(1)- and β(2)-adrenergic receptors

β(1)- and β(2)-Adrenergic receptors (β(1)-AR and β(2)-AR) are co-expressed in numerous tissues, for example, heart and bladder. They play a very important role in the responses of a variety of organs to sympathetic nerve stimulation. Recent studies suggest that many G protein-coupled receptors, such as β(1)-AR, β(2)-AR, μ opioid receptor and δ opioid receptor, can form homo- and heterooligomers. Previous studies demonstrated that the β(1)-AR and β(2)-AR formed dimers in living HEK 293 cells. The aim of the present study is to investigate whether such heterooligomerization affect the agonist-induced receptor internalization in the CHO-K1 cells stably co-expressing β(1)-AR and β(2)-AR. Using co-immunoprecipitation, we confirmed that β(1)-AR and β(2)-AR formed heterooligomers in the CHO-K1 cells. In cells co-expressing β(1)-AR and β(2)-AR, 30% of β(1)-AR was internalized by isoproterenol, whereas only 20% of β(1)-AR was internalized in cells expressing the β(1)-AR alone. Heterooligomerization did not affect the ratio of internalized β(2)-AR. Salmeterol, a specific β(2)-AR agonist, broke β(1)-AR/β(2)-AR heterooligomers, and induced β(2)-AR-specific internalization in cells co-expressing β(1)-AR and β(2)-AR. The present study demonstrated that heterooligomerization between β(1)-AR and β(2)-AR accelerates the isoproterenol-promoted internalization of the β(1)-AR, and that salmeterol induces β(2)-AR-specific internalization in Chinese hamster ovary (CHO) cells stably co-expressing β(1)-AR and β(2)-AR.

Experimental strategies for studying G protein-coupled receptor homo- and heteromerization with radioligand binding and signal transduction methods

Before the molecular biology era, functional experiments on isolated organs and radioligand binding and biochemical experiments on animal tissues were widely used to characterize G protein-coupled receptors (GPCRs). The introduction of recombinant cell lines expressing a single GPCR type has been a big step forward for studying both drug-receptor interactions and signal transduction. Before the introduction of the concept of receptor oligomerization, all data generated were attributed to the interaction of drugs with receptor monomers. Now, considerable data must be reinterpreted in light of receptor homo- and heteromerization. In this chapter, we will review some of the methods used to study radioligand binding and signal transduction modifications induced by GPCR homo- and heteromerization.

The cannabinoid receptor CB1 modulates the signaling properties of the lysophosphatidylinositol receptor GPR55

The G protein-coupled receptor (GPCR) 55 (GPR55) and the cannabinoid receptor 1 (CB1R) are co-expressed in many tissues, predominantly in the central nervous system. Seven transmembrane spanning (7TM) receptors/GPCRs can form homo- and heteromers and initiate distinct signaling pathways. Recently, several synthetic CB1 receptor inverse agonists/antagonists, such as SR141716A, AM251, and AM281, were reported to activate GPR55. Of these, SR141716A was marketed as a promising anti-obesity drug, but was withdrawn from the market because of severe side effects. Here, we tested whether GPR55 and CB1 receptors are capable of (i) forming heteromers and (ii) whether such heteromers could exhibit novel signaling patterns. We show that GPR55 and CB1 receptors alter each others signaling properties in human embryonic kidney (HEK293) cells. We demonstrate that the co-expression of FLAG-CB1 receptors in cells stably expressing HA-GPR55 specifically inhibits GPR55-mediated transcription factor activation, such as nuclear factor of activated T-cells and serum response element, as well as extracellular signal-regulated kinases (ERK1/2) activation. GPR55 and CB1 receptors can form heteromers, but the internalization of both receptors is not affected. In addition, we observe that the presence of GPR55 enhances CB1R-mediated ERK1/2 and nuclear factor of activated T-cell activation. Our data provide the first evidence that GPR55 can form heteromers with another 7TM/GPCR and that this interaction with the CB1 receptor has functional consequences in vitro. The GPR55-CB1R heteromer may play an important physiological and/or pathophysiological role in tissues endogenously co-expressing both receptors.

Disruption of δ-opioid receptor phosphorylation at threonine 161 attenuates morphine tolerance in rats with CFA-induced inflammatory hypersensitivity

OBJECTIVE

Our previous study identified Threonine 161 (Thr-161), located in the second intracellular loop of the δ-opioid receptor (DOR), as the only consensus phosphorylation site for cyclin-dependent kinase 5 (Cdk5). The aim of this study was to assess the function of DOR phosphorylation by Cdk5 in complete Freund's adjuvant (CFA)-induced inflammatory pain and morphine tolerance.

METHODS

Dorsal root ganglion (DRG) neurons of rats with CFA-induced inflammatory pain were acutely dissociated and the biotinylation method was used to explore the membrane localization of phosphorylated DOR at Thr-161 (pThr-161-DOR), and paw withdrawal latency was measured after intrathecal delivery of drugs or Tat-peptide, using a radiant heat stimulator in rats with CFA-induced inflammatory pain.

RESULTS

Both the total amount and the surface localization of pThr-161-DOR were significantly enhanced in the ipsilateral DRG following CFA injection. Intrathecal delivery of the engineered Tat fusion-interefering peptide corresponding to the second intracellular loop of DOR (Tat-DOR-2L) increased inflammatory hypersensitivity, and inhibited DOR- but not µ-opioid receptor-mediated spinal analgesia in CFA-treated rats. However, intrathecal delivery of Tat-DOR-2L postponed morphine antinociceptive tolerance in rats with CFA-induced inflammatory pain.

CONCLUSION

Phosphorylation of DOR at Thr-161 by Cdk5 attenuates hypersensitivity and potentiates morphine tolerance in rats with CFA-induced inflammatory pain, while disruption of the phosphorylation of DOR at Thr-161 attenuates morphine tolerance.

Interaction and regulatory functions of μ- and δ-opioid receptors in nociceptive afferent neurons

μ-opioid receptor (MOR) agonists such as morphine are powerful analgesics used for pain therapy. However, the use of these drugs is limited by their side-effects, which include antinociceptive tolerance and dependence. Earlier studies reported that MOR analgesic tolerance is reduced by blockade of δ-opioid receptors (DORs) that interact with MORs. Recent studies show that the MOR/DOR interaction in nociceptive afferent neurons in the dorsal root ganglion may contribute to morphine analgesic tolerance. Further analysis of the mechanisms for regulating the trafficking of receptors, ion channels and signaling molecules in nociceptive afferent neurons would help to understand the nociceptive mechanisms and improve pain therapy.

Potentiation of dopamine D1-like receptor signaling by concomitant activation of δ- and μ-opioid receptors in mouse medial prefrontal cortex

Opioid receptors located in the ventral tegmental area are known to regulate dopamine (DA) release from mesocortical afferents to medial prefrontal cortex (mPFC) but little is known on whether in this cortical region activation of opioid receptors affect DA receptor signaling. In the present study we show that in mouse mPFC concomitant activation of either δ- or μ-opioid receptors, but not κ-opioid receptors, potentiated DA D1-like receptor-induced stimulation of adenylyl cyclase activity through a G protein βγ subunit-dependent mechanism. In tissue slices of mPFC, the combined addition of the opioid agonist leu-enkephalin and the DA D1-like receptor agonist SKF 81297 produced more than additive increase in the phosphorylation state of AMPA and NMDA receptor subunits GluR1 and NR1, respectively. Moreover, in primary cultures of mouse frontal cortex neurons, DA D1-like receptor-induced Ser133 phosphorylation of the transcription factor cyclic AMP responsive element binding protein was potentiated by concurrent stimulation of opioid receptors. Double immunofluorescence analysis of cultured cortical cells indicated that a large percentage of DA D1 receptor positive cells expressed either δ- or μ-opioid receptor immunoreactivity. These data indicate that in mouse mPFC activation of μ- and δ-opioid receptors enhances DA D1-like receptor signaling likely through converging regulatory inputs on βγ-stimulated adenylyl cyclase isoforms. This previously unrecognized synergistic interaction may selectively affect DA D1 transmission at specific postsynaptic sites where the receptors are co-localized and may play a role in prefrontal DA D1 regulation of opioid addiction.

Clinically employed opioid analgesics produce antinociception via μ-δ opioid receptor heteromers in Rhesus monkeys

Morphine and related drugs are widely employed as analgesics despite the side effects associated with their use. Although morphine is thought to mediate analgesia through mu opioid receptors, delta opioid receptors have been implicated in mediating some side effects such as tolerance and dependence. Here we present evidence in rhesus monkeys that morphine, fentanyl, and possibly methadone selectively activate mu-delta heteromers to produce antinociception that is potently antagonized by the delta opioid receptor antagonist, naltrindole (NTI). Studies with HEK293 cells expressing mu-delta heteromeric opioid receptors exhibit a similar antagonism profile of receptor activation in the presence of NTI. In mice, morphine was potently inhibited by naltrindole when administered intrathecally, but not intracerebroventricularly, suggesting the possible involvement of mu-delta heteromers in the spinal cord of rodents. Taken together, these results strongly suggest that, in primates, mu-delta heteromers are allosterically coupled and mediate the antinociceptive effects of three clinically employed opioid analgesics that have been traditionally viewed as mu-selective. Given the known involvement of delta receptors in morphine tolerance and dependence, our results implicate mu-delta heteromers in mediating both antinociception and these side effects in primates. These results open the door for further investigation in humans.

Spinal interaction between the highly selective μ agonist DAMGO and several δ opioid receptor ligands in naive and morphine-tolerant mice

Since the discovery of opioid receptor dimers their possible roles in opioid actions were intensively investigated. Here we suggest a mechanism that may involve the μ-δ opioid heterodimers. The exact role of δ opioid receptors in antinociception and in the development of opioid tolerance is still unclear. While receptor up-regulation can be observed during the development of opioid tolerance no μ receptor down-regulation could be detected within five days. In our present work we investigated how the selective δ opioid receptor agonists and antagonists influence the antinociceptive effect of the selective μ receptor agonist DAMGO in naïve and morphine-tolerant mice. We treated male NMRI mice with 200 μmol/kg subcutaneous (s.c.) morphine twice daily for three days. On the fourth day we measured the antinociceptive effect of DAMGO alone and combined with delta ligands: DPDPE, deltorphin II (agonists), TIPP and TICPψ (antagonists), respectively, administered intrathecally (i.t.) in mouse tail-flick test. In naive control mice none of the δ ligands caused significant changes in the antinociceptive action of DAMGO. The treatment with s.c. morphine resulted in approximately four-fold tolerance to i.t. DAMGO, i.e. the ED₅₀ value of DAMGO was four times as high as in naive mice. 500 and 1000 pmol/mouse of the δ₁ selective agonist DPDPE enhanced the tolerance to DAMGO while 1000 pmol/mouse of the δ₂ selective agonist deltorphin II did not influence the degree of tolerance. However, both δ antagonists TIPP and TICPψ potentiated the antinociceptive effect of i.t. DAMGO, thus they restored the potency of DAMGO to the control level. The inhibitory action of DPDPE against the antinociceptive effect of DAMGO could be antagonized by TIPP and TICPψ. We hypothesize that during the development of morphine tolerance the formation of μδ heterodimers may contribute to the spinal opioid tolerance. δ ligands may affect the dimer formation differently. Those, like DPDPE may facilitate the dimer formation hence inhibit the antinociceptive effect of DAMGO by causing virtual μ receptor down-regulation. Ligands that do not affect the dimer formation do not influence antinociception either but ligands with the presumed capability of disconnecting the dimers may decrease the spinal tolerance to DAMGO.

Enhanced brain delivery of the opioid peptide DAMGO in glutathione pegylated liposomes: a microdialysis study

Glutathione PEGylated (GSH-PEG) liposomes were evaluated for their ability to enhance and prolong blood-to-brain drug delivery of the opioid peptide DAMGO (H-Tyr-d-Ala-Gly-MePhe-Gly-ol). An intravenous loading dose of DAMGO followed by a 2 h constant rate infusion was administered to rats, and after a washout period of 1 h, GSH-PEG liposomal DAMGO was administered using a similar dosing regimen. DAMGO and GSH-PEG liposomal DAMGO were also administered as a 10 min infusion to compare the disposition of the two formulations. Microdialysis made it possible to determine free DAMGO in brain and plasma, while the GSH-PEG liposomal encapsulated DAMGO was measured with regular plasma sampling. The antinociceptive effect of DAMGO was determined with the tail-flick method. All samples were analyzed using liquid chromatography-tandem mass spectrometry. The short infusion of DAMGO resulted in a fast decline of the peptide concentration in plasma with a half-life of 9.2 ± 2.1 min. Encapsulation in GSH-PEG liposomes prolonged the half-life to 6.9 ± 2.3 h. Free DAMGO entered the brain to a limited extent with a steady state ratio between unbound drug concentrations in brain interstitial fluid and in blood (Kp,uu) of 0.09 ± 0.04. GSH-PEG liposomes significantly increased the brain exposure of DAMGO to a Kp,uu of 0.21 ± 0.17 (p < 0.05). By monitoring the released, active substance in both blood and brain interstitial fluid over time, we were able to demonstrate that GSH-PEG liposomes offer a promising platform for enhancing and prolonging the delivery of drugs to the brain.

Sequential inter- and intrasubunit rearrangements during activation of dimeric metabotropic glutamate receptor 1

The metabotropic glutamate receptor 1 (mGluR1), a class C member of the heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor family, is a constitutive dimer that regulates excitatory neurotransmission. We investigated the role of homodimer formation in mGluR1 activation by examining activation-dependent inter- and intrasubunit conformational changes by fluorescence resonance energy transfer (FRET). We inserted yellow and cyan fluorescent proteins in the second intracellular loop and at the carboxyl terminus of mGluR1 to act as FRET sensors and expressed these proteins in human embryonic kidney 293 cells. Agonist-dependent activation of these mGluR1 chimeras rapidly increased the intersubunit FRET, suggesting rapid movement of the subunits relative to each other. After intersubunit movement, the intrasubunit FRET decreased, reflecting conformational changes within a subunit. Cotransfection of chimeric receptor subunits that were capable or incapable of G protein coupling revealed that only a single subunit assumes an active state in an mGluR1 receptor dimer.

Circadian-related heteromerization of adrenergic and dopamine D₄ receptors modulates melatonin synthesis and release in the pineal gland

Dopamine and adrenergic receptor complexes form under a circadian-regulated cycle and directly modulate melatonin synthesis and release from the pineal gland.

The role of the pineal gland is to translate the rhythmic cycles of night and day encoded by the retina into hormonal signals that are transmitted to the rest of the neuronal system in the form of serotonin and melatonin synthesis and release. Here we describe that the production of both melatonin and serotonin by the pineal gland is regulated by a circadian-related heteromerization of adrenergic and dopamine D₄ receptors. Through α_1B-D₄ and β₁-D₄ receptor heteromers dopamine inhibits adrenergic receptor signaling and blocks the synthesis of melatonin induced by adrenergic receptor ligands. This inhibition was not observed at hours of the day when D₄ was not expressed. These data provide a new perspective on dopamine function and constitute the first example of a circadian-controlled receptor heteromer. The unanticipated heteromerization between adrenergic and dopamine D₄ receptors provides a feedback mechanism for the neuronal hormone system in the form of dopamine to control circadian inputs.

Animals respond to cycles of light and dark with patterns in sleeping, feeding, body temperature alterations, and other biological functions. The pineal gland translates these light signals received from the retina into a language understandable to the rest of the body through the rhythmic synthesis and release of melatonin in response to the light and dark cycle. This process is controlled by adrenergic receptors. One impressive and mysterious aspect of the system is the rapid ability of rhythmic melatonin production and/or degradation to respond to changes in the cycle. In this study, we demonstrate that part of this response is due to the formation of receptor-receptor complexes (heteromers) between the adrenergic receptors α_1B or β₁ and the D₄ dopamine receptor. Using both biochemical and biophysical methods in transfected cells and in ex vivo tissue we show that dopamine, a neurotransmitter, inhibits adrenergic receptor signaling through these heteromers. This inhibition causes a dramatic decrease in melatonin production of the pineal gland. We postulate that these heteromers provide a rapid feedback mechanism for the neuronal hormone system to modulate circadian-controlled outputs.

The Pseudo signal peptide of the corticotropin-releasing factor receptor type 2A prevents receptor oligomerization

N-terminal signal peptides mediate the interaction of native proteins with the translocon complex of the endoplasmic reticulum membrane and are cleaved off during early protein biogenesis. The corticotropin-releasing factor receptor type 2a (CRF(2(a))R) possesses an N-terminal pseudo signal peptide, which represents a so far unique domain within the large protein family of G protein-coupled receptors (GPCRs). In contrast to a conventional signal peptide, the pseudo signal peptide remains uncleaved and consequently forms a hydrophobic extension at the N terminus of the receptor. The functional consequence of the presence of the pseudo signal peptide is not understood. Here, we have analyzed the significance of this domain for receptor dimerization/oligomerization in detail. To this end, we took the CRF(2(a))R and the homologous corticotropin-releasing factor receptor type 1 (CRF(1)R) possessing a conventional cleaved signal peptide and conducted signal peptide exchange experiments. Using single cell and single molecule imaging methods (fluorescence resonance energy transfer and fluorescence cross-correlation spectroscopy, respectively) as well as biochemical experiments, we obtained two novel findings; we could show that (i) the CRF(2(a))R is expressed exclusively as a monomer, and (ii) the presence of the pseudo signal peptide prevents its oligomerization. Thus, we have identified a novel functional domain within the GPCR protein family, which plays a role in receptor oligomerization and which may be useful to study the functional significance of this process in general.

Structure of the δ-opioid receptor bound to naltrindole

The opioid receptor family comprises three members, the µ-, δ- and κ-opioid receptors, which respond to classical opioid alkaloids such as morphine and heroin as well as to endogenous peptide ligands like endorphins. They belong to the G-protein-coupled receptor (GPCR) superfamily, and are excellent therapeutic targets for pain control. The δ-opioid receptor (δ-OR) has a role in analgesia, as well as in other neurological functions that remain poorly understood. The structures of the µ-OR and κ-OR have recently been solved. Here we report the crystal structure of the mouse δ-OR, bound to the subtype-selective antagonist naltrindole. Together with the structures of the µ-OR and κ-OR, the δ-OR structure provides insights into conserved elements of opioid ligand recognition while also revealing structural features associated with ligand-subtype selectivity. The binding pocket of opioid receptors can be divided into two distinct regions. Whereas the lower part of this pocket is highly conserved among opioid receptors, the upper part contains divergent residues that confer subtype selectivity. This provides a structural explanation and validation for the 'message-address' model of opioid receptor pharmacology, in which distinct 'message' (efficacy) and 'address' (selectivity) determinants are contained within a single ligand. Comparison of the address region of the δ-OR with other GPCRs reveals that this structural organization may be a more general phenomenon, extending to other GPCR families as well.

Disruption of the mu-delta opioid receptor heteromer

The crystal structure of the mu and kappa opioid receptors has revealed dimeric structural arrangements. Mu-delta receptors heteromers also exist and we have identified discrete cytoplasmic regions in each receptor required for oligomer formation. In the carboxyl tail of the delta receptor we identified three glycine residues (-GGG), substitution of any of these residues prevented heteromer formation. In intracellular loop 3 of both mu and delta receptors we identified three residues (-SVR), substitution of any of these residues prevented heteromer formation.

Pathophysiology of GPCR Homo- and Heterodimerization: Special Emphasis on Somatostatin Receptors

G-protein coupled receptors (GPCRs) are cell surface proteins responsible for translating >80% of extracellular reception to intracellular signals. The extracellular information in the form of neurotransmitters, peptides, ions, odorants etc is converted to intracellular signals via a wide variety of effector molecules activating distinct downstream signaling pathways. All GPCRs share common structural features including an extracellular N-terminal, seven-transmembrane domains (TMs) linked by extracellular/intracellular loops and the C-terminal tail. Recent studies have shown that most GPCRs function as dimers (homo- and/or heterodimers) or even higher order of oligomers. Protein-protein interaction among GPCRs and other receptor proteins play a critical role in the modulation of receptor pharmacology and functions. Although ~50% of the current drugs available in the market target GPCRs, still many GPCRs remain unexplored as potential therapeutic targets, opening immense possibility to discover the role of GPCRs in pathophysiological conditions. This review explores the existing information and future possibilities of GPCRs as tools in clinical pharmacology and is specifically focused for the role of somatostatin receptors (SSTRs) in pathophysiology of diseases and as the potential candidate for drug discovery.

Opioid receptor heteromers in analgesia

Opiates such as morphine and fentanyl, a major class of analgesics used in the clinical management of pain, exert their effects through the activation of opioid receptors. Opioids are among the most commonly prescribed and frequently abused drugs in the USA; however, the prolonged use of opiates often leads to the development of tolerance and addiction. Although blockade of opioid receptors with antagonists such as naltrexone and naloxone can lessen addictive impulses and facilitate recovery from overdose, systemic disruption of endogenous opioid receptor signalling through the use of these antagonistic drugs can have severe side effects. In the light of these challenges, current efforts have focused on identifying new therapeutic targets that selectively and specifically modulate opioid receptor signalling and function so as to achieve analgesia without the adverse effects associated with chronic opiate use. We have previously reported that opioid receptors interact with each other to form heteromeric complexes and that these interactions affect morphine signalling. Since chronic morphine administration leads to an enhanced level of these heteromers, these opioid receptor heteromeric complexes represent novel therapeutic targets for the treatment of pain and opiate addiction. In this review, we discuss the role of heteromeric opioid receptor complexes with a focus on mu opioid receptor (MOR) and delta opioid receptor (DOR) heteromers. We also highlight the evidence for altered pharmacological properties of opioid ligands and changes in ligand function resulting from the heteromer formation.

Opiate antagonist prevents μ- and δ-opiate receptor dimerization to facilitate ability of agonist to control ethanol-altered natural killer cell functions and mammary tumor growth

In the natural killer (NK) cells, δ-opiate receptor (DOR) and μ-opioid receptor (MOR) interact in a feedback manner to regulate cytolytic function with an unknown mechanism. Using RNK16 cells, a rat NK cell line, we show that MOR and DOR monomer and dimer proteins existed in these cells and that chronic treatment with a receptor antagonist reduced protein levels of the targeted receptor but increased levels of opposing receptor monomer and homodimer. The opposing receptor-enhancing effects of MOR and DOR antagonists were abolished following receptor gene knockdown by siRNA. Ethanol treatment increased MOR and DOR heterodimers while it decreased the cellular levels of MOR and DOR monomers and homodimers. The opioid receptor homodimerization was associated with an increased receptor binding, and heterodimerization was associated with a decreased receptor binding and the production of cytotoxic factors. Similarly, in vivo, opioid receptor dimerization, ligand binding of receptors, and cell function in immune cells were promoted by chronic treatment with an opiate antagonist but suppressed by chronic ethanol feeding. Additionally, a combined treatment of an MOR antagonist and a DOR agonist was able to reverse the immune suppressive effect of ethanol and reduce the growth and progression of mammary tumors in rats. These data identify a role of receptor dimerization in the mechanism of DOR and MOR feedback interaction in NK cells, and they further elucidate the potential for the use of a combined opioid antagonist and agonist therapy for the treatment of immune incompetence and cancer and alcohol-related diseases.

Crystal structure of the µ-opioid receptor bound to a morphinan antagonist

Opium is one of the world's oldest drugs, and its derivatives morphine and codeine are among the most used clinical drugs to relieve severe pain. These prototypical opioids produce analgesia as well as many undesirable side effects (sedation, apnoea and dependence) by binding to and activating the G-protein-coupled µ-opioid receptor (µ-OR) in the central nervous system. Here we describe the 2.8 Å crystal structure of the mouse µ-OR in complex with an irreversible morphinan antagonist. Compared to the buried binding pocket observed in most G-protein-coupled receptors published so far, the morphinan ligand binds deeply within a large solvent-exposed pocket. Of particular interest, the µ-OR crystallizes as a two-fold symmetrical dimer through a four-helix bundle motif formed by transmembrane segments 5 and 6. These high-resolution insights into opioid receptor structure will enable the application of structure-based approaches to develop better drugs for the management of pain and addiction.

Functional relevance of μ-δ opioid receptor heteromerization: a role in novel signaling and implications for the treatment of addiction disorders: from a symposium on new concepts in mu-opioid pharmacology

Morphine and other opiates are among the most widely prescribed and clinically useful medications for the treatment of chronic pain. However, the applicability of these compounds has been severely hampered by the rapid development of tolerance and physical dependence that typically accompanies their repeated use. A growing body of evidence has implicated the regulated functioning of μ-δ opioid receptor heteromers in both the modulation of morphine-mediated antinociception, and in the limitation of undesirable side effects resulting from chronic opiate exposure. Moreover, μ-δ heteromers exhibit unique ligand binding characteristics and signaling properties, indicating that pharmacological targeting of the μ-δ heteromer may represent a novel therapeutic approach for the management of chronic pain and addiction disorders. Therefore, the present review will attempt to summarize the latest relevant findings regarding the regulation and functional characteristics of the μ-δ heteromer both in vitro and in vivo.

Receptor and channel heteromers as pain targets

Recent discoveries indicate that many G-protein coupled receptors (GPCRs) and channels involved in pain modulation are able to form receptor heteromers. Receptor and channel heteromers often display distinct signaling characteristics, pharmacological properties and physiological function in comparison to monomer/homomer receptor or ion channel counterparts. It may be possible to capitalize on such unique properties to augment therapeutic efficacy while minimizing side effects. For example, drugs specifically targeting heteromers may have greater tissue specificity and analgesic efficacy. This review will focus on current progress in our understanding of roles of heteromeric GPCRs and channels in pain pathways as well as strategies for controlling pain pathways via targeting heteromeric receptors and channels. This approach may be instrumental in the discovery of novel classes of drugs and expand our repertoire of targets for pain pharmacotherapy.

Differential assembly of GPCR signaling complexes determines signaling specificity

Recent proteomic and biochemical evidence indicates that cellular -signaling is organized in protein modules. G protein-coupled receptors (GPCRs) are privileged entry points for extracellular signals that are transmitted through the plasma membrane into the cell. The adequate cellular response and signaling specificity is regulated by GPCR-associated protein modules. The composition of these modules is dynamic and might depend on receptor stimulation, the proteome of a given cellular context, the subcellular localization of receptor-associated modules, the formation of GPCR oligomers and the variation of expression levels of components of these modules under physiological, for example circadian rhythm, or pathological conditions. The current article will highlight the importance of GPCR-associated protein modules as a biochemical basis for signaling specificity.

Cys-27 variant of human δ-opioid receptor modulates maturation and cell surface delivery of Phe-27 variant via heteromerization

The important role of G protein-coupled receptor homo/heteromerization in receptor folding, maturation, trafficking, and cell surface expression has become increasingly evident. Here we investigated whether the human δ-opioid receptor (hδOR) Cys-27 variant that shows inherent compromised maturation has an effect on the behavior of the more common Phe-27 variant in the early secretory pathway. We demonstrate that hδOR-Cys-27 acts in a dominant negative manner and impairs cell surface delivery of the co-expressed hδOR-Phe-27 and impairs conversion of precursors to the mature form. This was demonstrated by metabolic labeling, Western blotting, flow cytometry, and confocal microscopy in HEK293 and human SH-SY5Y neuroblastoma cells using differentially epitope-tagged variants. The hδOR-Phe-27 precursors that were redirected to the endoplasmic reticulum-associated degradation were, however, rescued by a pharmacological chaperone, the opioid antagonist naltrexone. Co-immunoprecipitation of metabolically labeled variants revealed that both endoplasmic reticulum-localized precursors and mature receptors exist as homo/heteromers. The existence of homo/heteromers was confirmed in living cells by bioluminescence resonance energy transfer measurements, showing that the variants have a similar propensity to form homo/heteromers. By forming both homomers and heteromers, the hδOR-Cys-27 variant may thus regulate the levels of receptors at the cell surface, possibly leading to altered responsiveness to opioid ligands in individuals carrying the Cys-27 variant.

Contribution of mu and delta opioid receptors to the pharmacological profile of kappa opioid receptor subtypes

Molecular cloning has identified three opioid receptors: mu (MOR), delta (DOR) and kappa (KOR). Yet, cloning of these receptor types has offered little clarification to the diverse pharmacological profiles seen within the growing number of novel opioid ligands, which has led to the proposal of multiple subtypes. In the present study, utilizing in vitro and in vivo methods including the use of opioid receptor knockout mice, we find that certain antinociceptive effects of the KOR-1 and KOR-2 subtype-selective ligands (+)-(5α,7α,8β)-N-Methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzene-acetamide (U69, 593) and 4-[(3,4-Dichlorophenyl)acetyl]-3-(1-pyrrolidinylmethyl)-1-piperazine-carboxylic acid methyl ester fumarate (GR89, 696), respectively, are potentiated by antagonism of MOR and DOR receptors. We believe that our findings can be best explained by the existence of KOR-DOR and KOR-MOR heteromers. We only find evidence for the existence of these heteromers in neurons mediating mechanical nociception, but not thermal nociception. These findings have important clinical ramifications as they reveal new drug targets that may provide avenues for more effective pain therapies.

Unidirectional cross-activation of GRPR by MOR1D uncouples itch and analgesia induced by opioids

Spinal opioid-induced itch, a prevalent side effect of pain management, has been proposed to result from pain inhibition. We now report that the μ-opioid receptor (MOR) isoform MOR1D is essential for morphine-induced scratching (MIS), whereas the isoform MOR1 is required only for morphine-induced analgesia (MIA). MOR1D heterodimerizes with gastrin-releasing peptide receptor (GRPR) in the spinal cord, relaying itch information. We show that morphine triggers internalization of both GRPR and MOR1D, whereas GRP specifically triggers GRPR internalization and morphine-independent scratching. Providing potential insight into opioid-induced itch prevention, we demonstrate that molecular and pharmacologic inhibition of PLCβ3 and IP3R3, downstream effectors of GRPR, specifically block MIS but not MIA. In addition, blocking MOR1D-GRPR association attenuates MIS but not MIA. Together, these data suggest that opioid-induced itch is an active process concomitant with but independent of opioid analgesia, occurring via the unidirectional cross-activation of GRPR signaling by MOR1D heterodimerization.

Activation of spinal mu- and delta-opioid receptors potently inhibits substance P release induced by peripheral noxious stimuli

Over the past few years, δ-opioid receptors (DOPRs) and μ-opioid receptors (MOPRs) have been shown to interact with each other. We have previously seen that expression of MOPR is essential for morphine and inflammation to potentiate the analgesic properties of selective DOPR agonists. In vivo, it is not clear whether MOPRs and DOPRs are expressed in the same neurons. Indeed, it was recently proposed that these receptors are segregated in different populations of nociceptors, with MOPRs and DOPRs expressed by peptidergic and nonpeptidergic fibers, respectively. In the present study, the role and the effects of DOPR- and MOPR-selective agonists in two different pain models were compared. Using preprotachykinin A knock-out mice, we first confirmed that substance P partly mediates intraplantar formalin- and capsaicin-induced pain behaviors. These mice had a significant reduction in pain behavior compared with wild-type mice. We then measured the effects of intrathecal deltorphin II (DOPR agonist) and DAMGO (MOPR agonist) on pain-like behavior, neuronal activation, and substance P release following formalin and capsaicin injection. We found that both agonists were able to decrease formalin- and capsaicin-induced pain, an effect that was correlated with a reduction in the number of c-fos-positive neurons in the superficial laminae of the lumbar spinal cord. Finally, visualization of NK(1) (neurokinin 1) receptor internalization revealed that DOPR and MOPR activation strongly reduced formalin- and capsaicin-induced substance P release via direct action on primary afferent fibers. Together, our results indicate that functional MOPRs and DOPRs are both expressed by peptidergic nociceptors.

Truncated G protein-coupled mu opioid receptor MOR-1 splice variants are targets for highly potent opioid analgesics lacking side effects

Pain remains a pervasive problem throughout medicine, transcending all specialty boundaries. Despite the extraordinary insights into pain and its mechanisms over the past few decades, few advances have been made with analgesics. Most pain remains treated by opiates, which have significant side effects that limit their utility. We now describe a potent opiate analgesic lacking the traditional side effects associated with classical opiates, including respiratory depression, significant constipation, physical dependence, and, perhaps most important, reinforcing behavior, demonstrating that it is possible to dissociate side effects from analgesia. Evidence indicates that this agent acts through a truncated, six-transmembrane variant of the G protein-coupled mu opioid receptor MOR-1. Although truncated splice variants have been reported for a number of G protein-coupled receptors, their functional relevance has been unclear. Our evidence now suggests that truncated variants can be physiologically important through heterodimerization, even when inactive alone, and can comprise new therapeutic targets, as illustrated by our unique opioid analgesics with a vastly improved pharmacological profile.

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

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

Effects of neuropeptide FF system on CB₁ and CB₂ receptors mediated antinociception in mice

It has been demonstrated that opioid and cannabinoid receptor systems can produce similar signal transduction and behavioural effects. Neuropeptide FF (NPFF) belongs to an opioid-modulating peptide family. NPFF has been reported to play important roles in control of pain and analgesia through interactions with the opioid system. We were interested in whether the central and peripheral antinociception of cannabinoids could be influenced by supraspinal NPFF system. The present study examined the effects of NPFF and related peptides on the antinociceptive activities induced by the non-selective cannabinoid receptors agonist WIN55,212-2, given by supraspinal and intraplantar routes. In mice, the central and peripheral antinociception of WIN55,212-2 are mediated by cannabinoid CB(1) and CB(2) receptors, respectively. Interestingly, central administration of NPFF significantly reduced central and peripheral analgesia of cannabinoids in dose-dependent manners. In contrast, dNPA and NPVF (i.c.v.), two highly selective agonists for NPFF(2) and NPFF(1) receptors, dose-dependently augmented the antinociception caused by intracerebroventricular and intraplantar injection of WIN55,212-2. Additionally, pretreatment with the NPFF receptors selective antagonist RF9 (i.c.v.) markedly reduced the cannabinoid-modulating activities of NPFF and related peptides in nociceptive assays. These data provide the first evidence for a functional interaction between NPFF and cannabinoid systems, indicating that activation of central NPFF receptors interferes with cannabinoid-mediated central and peripheral antinociception. Intriguingly, the present work may pave the way for a new strategy of using combination treatment of cannabinoid and NPFF agonists for pain management. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.