Agonists determine the pattern of G-protein activation in mu-opioid receptor-mediated supraspinal analgesia

Strategies for developing μ opioid receptor agonists with reduced adverse effects

Opioids are currently the most effective and widely used painkillers in the world. Unfortunately, the clinical use of opioid analgesics is limited by serious adverse effects. Many researchers have been working on designing and optimizing structures in search of novel μ opioid receptor(MOR) agonists with improved analgesic activity and reduced incidence of adverse effects. There are many strategies to develop MOR drugs, mainly focusing on new low efficacy agonists (potentially G protein biased agonists), MOR agonists acting on different Gα subtype, targeting opioid receptors in the periphery, acting on multiple opioid receptor, and targeting allosteric sites of opioid receptors, and others. This review summarizes the design methods, clinical applications, and structure-activity relationships of small-molecule agonists for MOR based on these different design strategies, providing ideas for the development of safer novel opioid ligands with therapeutic potential.

G protein-specific mechanisms in the serotonin 5-HT2A receptor regulate psychosis-related effects and memory deficits

G protein-coupled receptors (GPCRs) are sophisticated signaling machines able to simultaneously elicit multiple intracellular signaling pathways upon activation. Complete (in)activation of all pathways can be counterproductive for specific therapeutic applications. This is the case for the serotonin 2 A receptor (5-HT2AR), a prominent target for the treatment of schizophrenia. In this study, we elucidate the complex 5-HT2AR coupling signature in response to different signaling probes, and its physiological consequences by combining computational modeling, in vitro and in vivo experiments with human postmortem brain studies. We show how chemical modification of the endogenous agonist serotonin dramatically impacts the G protein coupling profile of the 5-HT2AR and the associated behavioral responses. Importantly, among these responses, we demonstrate that memory deficits are regulated by Gαq protein activation, whereas psychosis-related behavior is modulated through Gαi1 stimulation. These findings emphasize the complexity of GPCR pharmacology and physiology and open the path to designing improved therapeutics for the treatment of stchizophrenia.

GPCR signaling bias: an emerging framework for opioid drug development

Biased signaling, also known as functional selectivity, has emerged as an important concept in drug development targeting G-protein-coupled receptors (GPCRs). Drugs that provoke biased signaling are expected to offer an opportunity for enhanced therapeutic effectiveness with minimized side effects. Opioid analgesics, whilst exerting potent pain-relieving effects, have become a social problem owing to their serious side effects. For the development of safer pain medications, there has been extensive exploration of agonists with a distinct balance of G-protein and β-arrestin (βarr) signaling. Recently, several approaches based on protein-protein interactions have been developed to precisely evaluate individual signal pathways, paving the way for the comprehensive analysis of biased signals. In this review, we describe an overview of bias signaling in opioid receptors, especially the μ-opioid receptor (MOR), and how to evaluate signaling bias in the GPCR field. We also discuss future directions for rational drug development through the integration of diverse signal datasets.

Molecular basis of opioid receptor signaling

Opioids are used for pain management despite the side effects that contribute to the opioid crisis. The pursuit of non-addictive opioid analgesics remains unattained due to the unresolved intricacies of opioid actions, receptor signaling cascades, and neuronal plasticity. Advancements in structural, molecular, and computational tools illuminate the dynamic interplay between opioids and opioid receptors, as well as the molecular determinants of signaling pathways, which are potentially interlinked with pharmacological responses. Here, we review the molecular basis of opioid receptor signaling with a focus on the structures of opioid receptors bound to endogenous peptides or pharmacological agents. These insights unveil specific interactions that dictate ligand selectivity and likely their distinctive pharmacological profiles. Biochemical analysis further unveils molecular features governing opioid receptor signaling. Simultaneously, the synergy between computational biology and medicinal chemistry continues to expedite the discovery of novel chemotypes with the promise of yielding more efficacious and safer opioid compounds.

IUPHAR themed review: Opioid efficacy, bias, and selectivity

Drugs acting at the opioid receptor family are clinically used to treat chronic and acute pain, though they represent the second line of treatment behind GABA analogs, antidepressants and SSRI's. Within the opioid family mu and kappa opioid receptor are commonly targeted. However, activation of the mu opioid receptor has side effects of constipation, tolerance, dependence, euphoria, and respiratory depression; activation of the kappa opioid receptor leads to dysphoria and sedation. The side effects of mu opioid receptor activation have led to mu receptor drugs being widely abused with great overdose risk. For these reasons, newer safer opioid analgesics are in high demand. For many years a focus within the opioid field was finding drugs that activated the G protein pathway at mu opioid receptor, without activating the β-arrestin pathway, known as biased agonism. Recent advances have shown that this may not be the way forward to develop safer analgesics at mu opioid receptor, though there is still some promise at the kappa opioid receptor. Here we discuss recent novel approaches to develop safer opioid drugs including efficacy vs bias and fine-tuning receptor activation by targeting sub-pockets in the orthosteric site, we explore recent works on the structural basis of bias, and we put forward the suggestion that Gα subtype selectivity may be an exciting new area of interest.

Structure-based design of bitopic ligands for the µ-opioid receptor

Mu-opioid receptor (µOR) agonists such as fentanyl have long been used for pain management, but are considered a major public health concern owing to their adverse side effects, including lethal overdose1. Here, in an effort to design safer therapeutic agents, we report an approach targeting a conserved sodium ion-binding site2 found in µOR3 and many other class A G-protein-coupled receptors with bitopic fentanyl derivatives that are functionalized via a linker with a positively charged guanidino group. Cryo-electron microscopy structures of the most potent bitopic ligands in complex with µOR highlight the key interactions between the guanidine of the ligands and the key Asp2.50 residue in the Na+ site. Two bitopics (C5 and C6 guano) maintain nanomolar potency and high efficacy at Gi subtypes and show strongly reduced arrestin recruitment-one (C6 guano) also shows the lowest Gz efficacy among the panel of µOR agonists, including partial and biased morphinan and fentanyl analogues. In mice, C6 guano displayed µOR-dependent antinociception with attenuated adverse effects, supporting the µOR sodium ion-binding site as a potential target for the design of safer analgesics. In general, our study suggests that bitopic ligands that engage the sodium ion-binding pocket in class A G-protein-coupled receptors can be designed to control their efficacy and functional selectivity profiles for Gi, Go and Gz subtypes and arrestins, thus modulating their in vivo pharmacology.

αN-Acetyl β-Endorphin Is an Endogenous Ligand of σ1Rs That Regulates Mu-Opioid Receptor Signaling by Exchanging G Proteins for σ2Rs in σ1R Oligomers

The opioid peptide β-endorphin coexists in the pituitary and brain in its ^αN-acetylated form, which does not bind to opioid receptors. We now report that these neuropeptides exhibited opposite effects in in vivo paradigms, in which ligands of the sigma type 1 receptor (σ1R) displayed positive effects. Thus, ^αN-acetyl β-Endorphin reduced vascular infarct caused by permanent unilateral middle cerebral artery occlusion and diminished the incidence of N-methyl-D-aspartate acid-promoted convulsive syndrome and mechanical allodynia caused by unilateral chronic constriction of the sciatic nerve. Moreover, ^αN-acetyl β-Endorphin reduced the analgesia of morphine, β-Endorphin and clonidine but enhanced that of DAMGO. All these effects were counteracted by β-Endorphin and absent in σ1R^-/- mice. We observed that σ1Rs negatively regulate mu-opioid receptor (MOR)-mediated morphine analgesia by binding and sequestering G proteins. In this scenario, β-Endorphin promoted the exchange of σ2Rs by G proteins at σ1R oligomers and increased the regulation of G proteins by MORs. The opposite was observed for the ^αN-acetyl derivative, as σ1R oligomerization decreased and σ2R binding was favored, which displaced G proteins; thus, MOR-regulated transduction was reduced. Our findings suggest that the pharmacological β-Endorphin-specific epsilon receptor is a σ1R-regulated MOR and that β-Endorphin and ^αN-acetyl β-Endorphin are endogenous ligands of σ1R.

Evaluation of Buprenorphine Rotation in Patients Receiving Long-term Opioids for Chronic Pain: A Systematic Review

IMPORTANCE

Individuals with chronic pain who use long-term opioid therapy (LTOT) are at risk of opioid use disorder and other harmful outcomes. Rotation to buprenorphine may be considered, but the outcomes of such rotation in this population have not been systematically reviewed.

OBJECTIVE

To synthesize the evidence on rotation to buprenorphine from full μ-opioid receptor agonists among individuals with chronic pain who were receiving LTOT, including the outcomes of precipitated opioid withdrawal, pain intensity, pain interference, treatment success, adverse events or adverse effects, mental health condition, and health care use.

EVIDENCE REVIEW

PubMed, CINAHL, Embase, and PsycInfo were searched from inception through November 3, 2020, for peer-reviewed original English-language research that reported the prespecified outcomes of rotation from prescribed long-term opioids to buprenorphine among individuals with chronic pain. Two independent reviewers extracted data as well as assessed risk of bias and study quality according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) reporting guidelines. Quality of evidence was assessed with the Grading of Recommendations Assessment, Development and Evaluation (GRADE) system.

FINDINGS

A total of 22 studies were analyzed, of which 5 (22.7%) were randomized clinical trials, 7 (31.8%) were case-control or cohort studies, and 10 (45.5%) were uncontrolled pre-post studies, which involved 1616 unique participants (675 female [41.8%] and 941 male [58.2%] individuals). Six of the 22 studies (27.3%) were primary or secondary analyses of a large randomized clinical trial. Participants had diverse pain and opioid use histories. Rationale for buprenorphine rotation included inadequate analgesia, intolerable adverse effects, risky opioid regimens (eg, high dose and/or sedative coprescriptions), and aberrant opioid use. Most protocols were adapted from protocols for initiating treatment in patients with opioid use disorder and used buccal or sublingual buprenorphine. Very low-quality evidence suggested that buprenorphine rotation was associated with maintained or improved analgesia, with a low risk of precipitating opioid withdrawal. Steady-dose buprenorphine was better tolerated than tapered-dose buprenorphine. Adverse effects were manageable, and severe adverse events were rare. Only 2 studies evaluated mental health outcomes, but none evaluated health care use. Limitations included a high risk of bias in most studies.

CONCLUSIONS AND RELEVANCE

In this systematic review, buprenorphine was associated with reduced chronic pain intensity without precipitating opioid withdrawal in individuals with chronic pain who were receiving LTOT. Future studies are necessary to ascertain the ideal starting dose, formulation, and administration frequency of buprenorphine as well as the best approach to buprenorphine rotation.

Signaling diversity of mu- and delta- opioid receptor ligands: Re-evaluating the benefits of β-arrestin/G protein signaling bias

Opioid analgesics are elective for treating moderate to severe pain but their use is restricted by severe side effects. Signaling bias has been proposed as a viable means for improving this situation. To exploit this opportunity, continuous efforts are devoted to understand how ligand-specific modulations of receptor functions could mediate the different in vivo effects of opioids. Advances in the field have led to the development of biased agonists based on hypotheses that allocated desired and undesired effects to specific signaling pathways. However, the prevalent hypothesis associating β-arrestin to opioid side effects was recently challenged and multiple of the newly developed biased drugs may not display the superior side effects profile that was sought. Moreover, biased agonism at opioid receptors is now known to be time- and cell-dependent, which adds a new layer of complexity for bias estimation. Here, we first review the signaling mechanisms underlying desired and undesired effects of opioids. We then describe biased agonism at opioid receptors and discuss the different perspectives that support the desired and undesired effects of opioids in view of exploiting biased signaling for therapeutic purposes. Finally, we explore how signaling kinetics and cellular background can influence the magnitude and directionality of bias at those receptors.

Unique Pharmacological Properties of the Kappa Opioid Receptor Signaling Through Gαz as Shown with Bioluminescence Resonance Energy Tranfer

Opioid receptors (ORs) convert extracellular messages to signaling events by coupling to the heterotrimeric G proteins, Gα•βγ Classic pharmacological methods, such as [³⁵S]GTPγS binding and inhibition of cyclic AMP production, allow for general opioid characterization, but they are subject to the varying endogenous Gα proteins in a given cell type. Bioluminescence resonance energy transfer (BRET) technology offers new insight by allowing the direct observation of Gα subunit-specific effects on opioid pharmacology. Using a Venus-tagged Gβγ and nanoluciferase-tagged truncated G protein receptor kinase 3, an increase in BRET signal correlated with OR activation mediated by a specific Gα protein. The magnitude of the BRET signal was normalized to the maximum response obtained with 10 µM 2-(3,4-dichlorophenyl)-N-methyl-N-[(1R,2R)-2-pyrrolidin-1-ylcyclohexyl]acetamide (U50,488) for the kappa OR (KOR). Opioids reached equilibrium with the KOR, and concentration-response curves were generated. Although the full agonists U50,488, salvinorin A, nalfurafine, and dynorphin peptides were equally efficacious regardless of the Gα subunit present, the concentration-response curves were leftward shifted when the KOR was signaling through Gαz compared with other Gαi/o subunits. In contrast, the Gα subunit distinctly affected both the efficacy and potency of partial kappa agonists, such as the benzomorphans, and the classic mu opioid antagonists, naloxone, naltrexone, and nalmefene. For example, (-)pentazocine had EC₅₀ values of 7.3 and 110 nM and maximal stimulation values of 79% and 35% when the KOR signaled through Gαz and Gαi1, respectively. Together, these observations suggest KOR pharmacology varies based on the specific Gα subunit coupled to the KOR. SIGNIFICANCE STATEMENT: Opioid receptors couple to various heterotrimeric Gαβγ proteins to convert extracellular cues to precise intracellular events. This paper focuses on how the various inhibitory Gα subunits influence the pharmacology of full and partial agonists at the kappa opioid receptor. Using a bioluminescent assay, the efficacy and potency of kappa opioids was determined. Opioid signaling was more potent through Gαz compared with other Gα proteins. These observations suggest that Gαz may impact opioid pharmacology and cellular physiology more than previously thought.

Differences in antinociceptive signalling mechanisms following morphine and fentanyl microinjections into the rat periaqueductal gray

BACKGROUND

Morphine and fentanyl are two of the most commonly used opioids to treat pain. Although both opioids produce antinociception by binding to mu-opioid receptors (MOR), they appear to act via distinct signalling pathways.

OBJECTIVE

This study will reveal whether differences in morphine and fentanyl antinociception are the result of selective activation of G-protein signalling and/or selective activation of pre- or postsynaptic MORs.

METHODS

The contribution of each mechanism to morphine and fentanyl antinociception was assessed by microinjecting drugs to alter G-protein signalling or block potassium channels linked to pre- and postsynaptic MORs in the ventrolateral periaqueductal gray (PAG) of male Sprague-Dawley rats.

RESULTS

Both morphine and fentanyl produced a dose-dependent antinociception when microinjected into the PAG. Enhancement of intracellular G-protein signalling by microinjection of the Regulator of G-protein Signalling 4 antagonist CCG-63802 into the PAG enhanced the antinociceptive potency of morphine, but not fentanyl. Microinjection of α-dendrotoxin into the PAG to block MOR activation of presynaptic K_v ⁺ channels caused a significant rightward shift in the dose-response curve of both morphine and fentanyl. Microinjection of tertiapin-Q to block MOR activation of postsynaptic GIRK channels caused a larger shift in the dose-response curve for fentanyl than morphine antinociception.

CONCLUSIONS

These findings reveal different PAG signalling mechanisms for morphine and fentanyl antinociception. In contrast with fentanyl, the antinociceptive effects of morphine are mediated by G-protein signalling primarily activated by presynaptic MORs.

SIGNIFICANCE

Microinjection of the opioids morphine and fentanyl into the periaqueductal gray (PAG) produce antinociception via mu-opioid receptor signalling. This study reveals differences in the signalling mechanisms underlying morphine and fentanyl antinociception in the PAG. In contrast with fentanyl, morphine antinociception is primarily mediated by presynaptic opioid receptors and is enhanced by blocking RGS proteins.

Morphine-Induced Modulation of Endolysosomal Iron Mediates Upregulation of Ferritin Heavy Chain in Cortical Neurons

HIV-associated neurocognitive disorders (HAND) remain prevalent and are aggravated by µ-opioid use. We have previously shown that morphine and other µ-opioids may contribute to HAND by inhibiting the homeostatic and neuroprotective chemokine receptor CXCR4 in cortical neurons, and this novel mechanism depends on upregulation of the protein ferritin heavy chain (FHC). Here, we examined the cellular events and potential mechanisms involved in morphine-mediated FHC upregulation using rat cortical neurons of either sex in vitro and in vivo. Morphine dose dependently increased FHC protein levels in primary neurons through µ-opioid receptor (µOR) and Gαi-protein signaling. Cytoplasmic FHC levels were significantly elevated, but nuclear FHC levels and FHC gene expression were unchanged. Morphine-treated rats also displayed increased FHC levels in layer 2/3 neurons of the prefrontal cortex. Importantly, both in vitro and in vivo FHC upregulation was accompanied by loss of mature dendritic spines, which was also dependent on µOR and Gαi-protein signaling. Moreover, morphine upregulated ferritin light chain (FLC), a component of the ferritin iron storage complex, suggesting that morphine altered neuronal iron metabolism. Indeed, prior to FHC upregulation, morphine increased cytoplasmic labile iron levels as a function of decreased endolysosomal iron. In line with this, chelation of endolysosomal iron (but not extracellular iron) blocked morphine-induced FHC upregulation and dendritic spine reduction, whereas iron overloading mimicked the effect of morphine on FHC and dendritic spines. Overall, these data demonstrate that iron mediates morphine-induced FHC upregulation and consequent dendritic spine deficits and implicate endolysosomal iron efflux to the cytoplasm in these effects.

Ligands Exert Biased Activity to Regulate Sigma 1 Receptor Interactions With Cationic TRPA1, TRPV1, and TRPM8 Channels

The sigma 1 receptor (σ1R) and the mu-opioid receptor (MOR) regulate the transient receptor potential (TRP) V1 calcium channel. A series of proteins are involved in the cross-regulation between MORs and calcium channels like the glutamate N-methyl-D-aspartate receptor (NMDAR), including the histidine triad nucleotide-binding protein 1 (HINT1), calmodulin (CaM), and the σ1R. Thus, we assessed whether similar mechanisms also apply to the neural TRP ankyrin member 1 (TRPA1), TRP vanilloid member 1 (TRPV1), and TRP melastatin member 8 (TRPM8). Our results indicate that σ1R and CaM bound directly to cytosolic regions of these TRPs, and this binding increased in the presence of calcium. By contrast, the association of HINT1 with these TRPs was moderately dependent on calcium. The σ1R always competed with CaM for binding to the TRPs, except for its binding to the TRPA1 C-terminal where σ1R binding cooperated with that of CaM. However, σ1R dampened HINT1 binding to the TRPA1 N-terminal. When the effect of σ1R ligands was addressed, the σ1R agonists PRE084 and pregnenolone sulfate enhanced the association of the σ1R with the TRPM8 N-terminal and TRPV1 C-terminal in the presence of physiological calcium, as seen for the σ1R-NMDAR interactions. However, these agonists dampened σ1R binding to the TRPA1 and TRPV1 N-terminal domains, and also to the TRPA1 C-terminal, as seen for σ1R-binding immunoglobulin protein (BiP) interactions in the endoplasmic reticulum (ER). By contrast, the σ1R antagonists progesterone and S1RA reduced the association of σ1R with TRPA1 and TRPV1 C-terminal regions, as seen for the σ1R-NMDAR interactions. Conversely, they enhanced the σ1R interaction with the TRPA1 N-terminal, as seen for σ1R-BiP interactions, whereas they barely affected the association of σ1R with the TRPV1 N-terminal. Thus, depending on the calcium channel and the cytosolic region examined, the σ1R agonists pregnenolone sulfate and PRE084 opposed or collaborated with the σ1R antagonists progesterone and S1RA to disrupt or promote such interactions. Through the use of cloned cytosolic regions of selected TRP calcium channels, we were able to demonstrate that σ1R ligands exhibit biased activity to regulate particular σ1R interactions with other proteins. Since σ1Rs are implicated in essential physiological processes, exploiting such ligand biases may represent a means to develop more selective and efficacious pharmacological interventions.

G protein-coupled receptor heteromers are key players in substance use disorder

G protein-coupled receptors (GPCR) represent the largest family of membrane proteins in the human genome. Physical association between two different GPCRs is linked to functional interactions which generates a novel entity, called heteromer, with specific ligand binding and signaling properties. Heteromerization is increasingly recognized to take place in the mesocorticolimbic pathway and to contribute to various aspects related to substance use disorder. This review focuses on heteromers identified in brain areas relevant to drug addiction. We report changes at the molecular and cellular levels that establish specific functional impact and highlight behavioral outcome in preclinical models. Finally, we briefly discuss selective targeting of native heteromers as an innovative therapeutic option.

Opioids Resistance in Chronic Pain Management

Chronic pain management represents a serious healthcare problem worldwide. Chronic pain affects approximately 20% of the adult European population and is more frequent in women and older people. Unfortunately, its management in the community remains generally unsatisfactory and rarely under the control of currently available analgesics. Opioids have been used as analgesics for a long history and are among the most used drugs; however, while there is no debate over their short term use for pain management, limited evidence supports their efficacy of long-term treatment for chronic non-cancer pain. Therapy with opioids is hampered by inter-individual variability and serious side effects and some opioids often result ineffective in the treatment of chronic pain and their use is controversial. Accordingly, for a better control of chronic pain a deeper knowledge of the molecular mechanisms underlying resistance to opiates is mandatory.

Biased mu-opioid receptor ligands: a promising new generation of pain therapeutics

Opioid chemistry and biology occupy a pivotal place in the history of pharmacology and medicine. Morphine offers unmatched efficacy in alleviating acute pain, but is also associated with a host of adverse side effects. The advent of biased agonism at G protein-coupled receptors has expanded our understanding of intracellular signaling and highlighted the concept that certain ligands are able to differentially modulate downstream pathways. The ability to target one pathway over another has allowed for the development of biased ligands with robust clinical efficacy and fewer adverse events. In this review we summarize these concepts with an emphasis on biased mu opioid receptor pharmacology and highlight how far opioid pharmacology has evolved.

The Use of Transdermal Buprenorphine in Complex Regional Pain Syndrome: A Report of Two Cases

Management of complex regional pain syndrome (CRPS) can be challenging. Various pharmacological approaches have produced mixed results. Buprenorphine activates mu-opioid receptors and antagonizes kappa and delta receptors, acts at N-methyl-d-aspartate (NMDA) receptor, and is an orphan-related ligand-1 receptor agonist. It is available in transdermal patches that last for up to 7 days. This report describes two patients with refractory CRPS who were treated with transdermal buprenorphine. The patients experienced approximately 50% reduction in pain intensity scores. Application site rash that occurred was managed with topical steroid spray used before applying the patch.

Selective up-regulation of functional mu-opioid receptor splice variants by chronic opioids

We recently reported (Verzillo, et al. J. Neurochem: 130, 790-796, 2014) that chronic systemic morphine selectively up-regulates mRNA encoding two C-terminal μ-opioid receptor (MOR) splice variants, MOR-1C1 and MOR-1B2 (MOR-1B2/-1C1). Given the known disconnects between changes in levels of mRNA and corresponding protein, it is essential to directly demonstrate that chronic opioid treatment elevates functional MOR-1B2/-1C1 protein prior to inferring relevance of these MOR variants to spinal opioid tolerance mechanisms. Accordingly, we investigated the ability of chronic opioid exposure to up-regulate MOR protein in Chinese hamster ovary cells stably transfected with rat MOR variants MOR-1B2, MOR-1C1, or MOR-1 (considered to be the predominant MOR). Findings revealed that chronic treatment with the clinically relevant opioids morphine, oxycodone and hydrocodone substantially up-regulated membrane MOR-1B2/-1C1 protein. This up-regulation was abolished by the protein synthesis inhibitor cycloheximide, eliminating contributions from receptor redistribution. The increment in MOR-1B2/-1C1 protein was paralleled by a significant increment in opioid agonist-stimulated GTPγS-binding (reflective of increased aggregate MOR G protein coupling) indicating that up-regulated MOR-1B2/-1C1 represented functional receptors. Strikingly, these tolerance-associated adaptations of MOR-1B2/-1C1 differed considerably from those of MOR-1. Antithetical regulation of MOR-1B2/-1C1 and MOR-1 by chronic opioids has significant implications for the design of new therapeutic agents to counteract opioid analgesic tolerance and accompanying addiction. Since chronic opioids induce MOR-1B2/-1C1 up-regulation in spinal cord of males, but not females, elucidating cellular compartments and intracellular pathways mediating MOR-1B2/-1C1 up-regulation and defining their unique signaling attributes would enable a precision medicinal approach to pain management and addiction therapy. In the spinal cord of males, but not females, chronic morphine up-regulates mRNA encoding two mu-opioid receptor (MOR) variants, MOR-1B2 and MOR-1C1 (MOR-1B2/-1C1). We now demonstrate that chronic treatment with the clinically relevant opioids morphine, hydrocodone or oxycodone up-regulates MOR-1B2/-1C1 functional protein, which is dependent on de novo protein synthesis. Findings underscore the importance of unique signaling attributes of MOR variants to sexually dimorphic tolerance mechanisms.

Ligand-biased activation of extracellular signal-regulated kinase 1/2 leads to differences in opioid induced antinociception and tolerance

Opioids produce antinociception by activation of G protein signaling linked to the mu-opioid receptor (MOPr). However, opioid binding to the MOPr also activates β-arrestin signaling. Opioids such as DAMGO and fentanyl differ in their relative efficacy for activation of these signaling cascades, but the behavioral consequences of this differential signaling are not known. The purpose of this study was to evaluate the behavioral significance of G protein and internalization dependent signaling within ventrolateral periaqueductal gray (vlPAG). Antinociception induced by microinjecting DAMGO into the vlPAG was attenuated by blocking Gαi/o protein signaling with administration of pertussis toxin (PTX), preventing internalization with administration of dynamin dominant-negative inhibitory peptide (dyn-DN) or direct inhibition of ERK1/2 with administration of the MEK inhibitor, U0126. In contrast, the antinociceptive effect of microinjecting fentanyl into the vlPAG was not altered by administration of PTX or U0126, and was enhanced by administration of dyn-DN. Microinjection of DAMGO, but not fentanyl, into the vlPAG induced phosphorylation of ERK1/2, which was blocked by inhibiting receptor internalization with administration of dyn-DN, but not by inhibition of Gαi/o proteins. ERK1/2 inhibition also prevented the development and expression of tolerance to repeated DAMGO microinjections, but had no effect on fentanyl tolerance. These data reveal that ERK1/2 activation following MOPr internalization contributes to the antinociceptive effect of some (e.g., DAMGO), but not all opioids (e.g., fentanyl) despite the known similarities for these agonists to induce β-arrestin recruitment and internalization.

Biased Agonism of Endogenous Opioid Peptides at the μ-Opioid Receptor

Biased agonism is having a major impact on modern drug discovery, and describes the ability of distinct G protein-coupled receptor (GPCR) ligands to activate different cell signaling pathways, and to result in different physiologic outcomes. To date, most studies of biased agonism have focused on synthetic molecules targeting various GPCRs; however, many of these receptors have multiple endogenous ligands, suggesting that "natural" bias may be an unappreciated feature of these GPCRs. The μ-opioid receptor (MOP) is activated by numerous endogenous opioid peptides, remains an attractive therapeutic target for the treatment of pain, and exhibits biased agonism in response to synthetic opiates. The aim of this study was to rigorously assess the potential for biased agonism in the actions of endogenous opioids at the MOP in a common cellular background, and compare these to the effects of the agonist d-Ala2-N-MePhe4-Gly-ol enkephalin (DAMGO). We investigated activation of G proteins, inhibition of cAMP production, extracellular signal-regulated kinase 1 and 2 phosphorylation, β-arrestin 1/2 recruitment, and MOP trafficking, and applied a novel analytical method to quantify biased agonism. Although many endogenous opioids displayed signaling profiles similar to that of DAMGO, α-neoendorphin, Met-enkephalin-Arg-Phe, and the putatively endogenous peptide endomorphin-1 displayed particularly distinct bias profiles. These may represent examples of natural bias if it can be shown that they have different signaling properties and physiologic effects in vivo compared with other endogenous opioids. Understanding how endogenous opioids control physiologic processes through biased agonism can reveal vital information required to enable the design of biased opioids with improved pharmacological profiles and treat diseases involving dysfunction of the endogenous opioid system.

Differential contribution of Gαi/o subunits in the response to food deprivation

Behavioral responses to food deprivation are a fundamental aspect of nervous system function in all animals. Several signaling molecules in the mammalian brain act through G proteins of the Gi/o family to mediate response to food restriction. The present study examined whether food intake changes under a condition of little stimulation to eat, such as that elicited by 4h of food deprivation, was altered by Gi/o isoform silencing induced by intracerebroventricular (i.c.v.) administration of antisense oligodeoxynucleotides (aODN) against the α subunit of Gi1, Gi2, Gi3, Go1 and Go2. The effect of aODN pretreatments on food intake was evaluated 15, 30, 45, and 60min after food re-administration. Selective effects were noted on food intake with anti-Giα1 (3.12-25nmol), Giα3 (1.56-12.5nmol) and Goα2 (3.12-25nmol) aODN exerting increase in food consumption, while anti-Giα2 (3.12-25nmol) and Goα1 (3.12-25nmol) aODN exerting decrease in food consumption. We observed the effect of the α-subunit silencing on food consumption as soon as 15min after food readministration, that was still significant after 60min. At the highest effective doses, different for each anti-Gαi/o subunit, any treatment did not impair motor coordination, nor modified spontaneous mobility and exploratory activity. These results indicate a relevant distinction between Gαi/o subunits on feeding behavior, and suggest that Gi/o proteins are critical for the integrative modulation of normal feeding behavior. Changes in Gi/o protein activity may be associated with modifications of feeding.

Molecular mechanism for opioid dichotomy: bidirectional effect of μ-opioid receptors on P2X₃ receptor currents in rat sensory neurones

Here, we describe a molecular switch associated with opioid receptors-linked signalling cascades that provides a dual opioid control over P2X3 purinoceptor in sensory neurones. Leu-enkephalin inhibited P2X3-mediated currents with IC50 ~10 nM in ~25% of small nociceptive rat dorsal root ganglion (DRG) neurones. In contrast, in neurones pretreated with pertussis toxin leu-enkephalin produced stable and significant increase of P2X3 currents. All effects of opioid were abolished by selective μ-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), nonselective inhibitor naloxone, and by PLC inhibitor U73122. Thus, we discovered a dual link between purinoceptors and μ-opioid receptors: the latter exert both inhibitory (pertussis toxin-sensitive) and stimulatory (pertussis toxin-insensitive) actions on P2X3 receptors through phospholipase C (PLC)-dependent pathways. This dual opioid control of P2X3 receptors may provide a molecular explanation for dichotomy of opioid therapy. Pharmacological control of this newly identified facilitation/inhibition switch may open new perspectives for the adequate medical use of opioids, the most powerful pain-killing agents known today.

The contribution of Gi/o protein to opioid antinociception in an oxaliplatin-induced neuropathy rat model

Oxaliplatin is a chemotherapeutic agent that induces chronic refractory neuropathy. To determine whether opioids effectively relieve this chronic neuropathy, we investigated the efficacies of morphine, oxycodone, and fentanyl, and the mechanisms underlying opioid antinociception, in oxaliplatin-induced neuropathy in rats. Rats exhibited significant mechanical allodynia following 2 weeks of chronic oxaliplatin administration. Within the range of doses that did not induce sedation and/or muscle rigidity, morphine (3 mg/kg, subcutaneously, s.c.) and oxycodone (0.3-0.56 mg/kg, s.c.) completely reversed oxaliplatin-induced mechanical allodynia, whereas fentanyl (0.017-0.03 mg/kg, s.c.) showed partial antinociception. The antinociception of the optimal doses of morphine and oxycodone were completely inhibited by pertussis toxin (PTX; 0.5 μg/rat, i.c.v.), a Gi/o protein inhibitor, while the partial effect of fentanyl was not affected in the oxaliplatin model. In the [(35)S]-GTPγS binding assay, activation of μ-opioid receptor by fentanyl, but not by morphine or oxycodone, in the mediodorsal thalamus was significantly reduced in oxaliplatin-treated rats. These results indicate that the lower antinociceptive potency of fentanyl in the oxaliplatin model might in part result from the loss of PTX-sensitive Gi/o protein activation, and the degree of Gi/o protein activation might be related to the potency of antinociception by opioids in this model.

Alteration of the mu opioid receptor: Ca2+ channel signaling pathway in a subset of rat sensory neurons following chronic femoral artery occlusion

The exercise pressor reflex, a crucial component of the cardiovascular response under physiological and pathophysiological states, is activated via metabolic and mechanical mediators that originate from contracting muscles and stimulate group III and IV afferents. We reported previously that stimulation of mu opioid receptors (MOR), expressed in both afferents, led to a significant attenuation of the reflex in rats whose femoral arteries had been occluded for 72 h. The present study examined the effect of arterial occlusion on the signaling components involved in the opioid-mediated modulation of Ca(2+) channels in rat dorsal root ganglion neurons innervating the triceps surae muscles. We focused on neurons that were transfected with cDNA coding for enhanced green fluorescent protein whose expression is driven by the voltage-gated Na(+) channel 1.8 (Na(V)1.8) promoter region, a channel expressed primarily in nociceptive neurons. With the use of a small interference RNA approach, our results show that the pertussis toxin-sensitive Gα(i3) subunit couples MOR with Ca(2+) channels. We observed a significant leftward shift of the MOR agonist [D-Ala2-N-Me-Phe4-Glycol5]-enkephalin concentration-response relationship in neurons isolated from rats with occluded arteries compared with those that were perfused freely. Femoral occlusion did not affect Ca(2+) channel density or the fraction of the main Ca(2+) channel subtype. Furthermore, Western blotting analysis indicated that the leftward shift did not result from either increased Gα(i3) or MOR expression. Finally, all neurons from both groups exhibited an inward current following exposure of the transient potential receptor vanilloid 1 (TRPV1) agonist, 8-methyl-N-vanillyl-6-nonenamide. These findings suggest that sensory neurons mediating the exercise pressor reflex express Na(V)1.8 and TRPV1 channels, and femoral occlusion alters the MOR pharmacological profile.

Acute morphine alters GABAergic transmission in the central amygdala during naloxone-precipitated morphine withdrawal: role of cyclic AMP

The central amygdala (CeA) plays an important role in opioid addiction. Therefore, we examined the effects of naloxone-precipitated morphine withdrawal (WD) on GABAergic transmission in rat CeA neurons using whole-cell recordings with naloxone in the bath. The basal frequency of miniature inhibitory postsynaptic currents (mIPSCs) increased in CeA neurons from WD compared to placebo rats. Acute morphine (10 μ M) had mixed effects (≥20% change from baseline) on mIPSCs in placebo and WD rats. In most CeA neurons (64%) from placebo rats, morphine significantly decreased mIPSC frequency and amplitude. In 32% of placebo neurons, morphine significantly increased mIPSC amplitudes but had no effect on mIPSC frequency. In WD rats, acute morphine significantly increased mIPSC frequency but had no effect on mIPSC amplitude in 41% of CeA neurons. In 45% of cells, acute morphine significantly decreased mIPSC frequency and amplitude. Pre-treatment with the cyclic AMP inhibitor (R)-adenosine, cyclic 3',5'-(hydrogenphosphorothioate) triethylammonium (RP), prevented acute morphine-induced potentiation of mIPSCs. Pre-treatment of slices with the Gi/o G-protein subunit inhibitor pertussis toxin (PTX) did not prevent the acute morphine-induced enhancement or inhibition of mIPSCs. PTX and RP decreased basal mIPSC frequencies and amplitudes only in WD rats. The results suggest that inhibition of GABAergic transmission in the CeA by acute morphine is mediated by PTX-insensitive mechanisms, although PTX-sensitive mechanisms cannot be ruled out for non-morphine responsive cells; by contrast, potentiation of GABAergic transmission is mediated by activated cAMP signaling that also mediates the increased basal GABAergic transmission in WD rats. Our data indicate that during the acute phase of WD, the CeA opioid and GABAergic systems undergo neuroadaptative changes conditioned by a previous chronic morphine exposure and dependence.

Ligand-specific regulation of the endogenous mu-opioid receptor by chronic treatment with mu-opioid peptide agonists

Since the discovery of the endomorphins (EM), the postulated endogenous peptide agonists of the mu-opioid receptors, several analogues have been synthesized to improve their binding and pharmacological profiles. We have shown previously that a new analogue, cis-1S,2R-aminocyclohexanecarboxylic acid²-endomorphin-2 (ACHC-EM2), had elevated mu-receptor affinity, selectivity, and proteolytic stability over the parent compound. In the present work, we have studied its antinociceptive effects and receptor regulatory processes. ACHC-EM2 displayed a somewhat higher (60%) acute antinociceptive response than the parent peptide, EM2 (45%), which peaked at 10 min after intracerebroventricular (icv) administration in the rat tail-flick test. Analgesic tolerance developed to the antinociceptive effect of ACHC-EM2 upon its repeated icv injection that was complete by a 10-day treatment. This was accompanied by attenuated coupling of mu-sites to G-proteins in subcellular fractions of rat brain. Also, the density of mu-receptors was upregulated by about 40% in the light membrane fraction, with no detectable changes in surface binding. Distinct receptor regulatory processes were noted in subcellular fractions of rat brains made tolerant by the prototypic full mu-agonist peptide, DAMGO, and its chloromethyl ketone derivative, DAMCK. These results are discussed in light of the recently discovered phenomenon, that is, the “so-called biased agonism” or “functional selectivity”.

HINT1 protein cooperates with cannabinoid 1 receptor to negatively regulate glutamate NMDA receptor activity

Background

G protein-coupled receptors (GPCRs) are the targets of a large number of drugs currently in therapeutic use. Likewise, the glutamate ionotropic N-methyl-D-aspartate receptor (NMDAR) has been implicated in certain neurological disorders, such as neurodegeration, neuropathic pain and mood disorders, as well as psychosis and schizophrenia. Thus, there is now an important need to characterize the interactions between GPCRs and NMDARs. Indeed, these interactions can produce distinct effects, and whereas the activation of Mu-opioid receptor (MOR) increases the calcium fluxes associated to NMDARs, that of type 1 cannabinoid receptor (CNR1) antagonizes their permeation. Notably, a series of proteins interact with these receptors affecting their responses and interactions, and then emerge as novel therapeutic targets for the aforementioned pathologies.

Results

We found that in the presence of GPCRs, the HINT1 protein influences the activity of NMDARs, whereby NMDAR activation was enhanced in CNR1^+/+/HINT1^-/- cortical neurons and the cannabinoid agonist WIN55,212-2 provided these cells with no protection against a NMDA insult. NMDAR activity was normalized in these cells by the lentiviral expression of HINT1, which also restored the neuroprotection mediated by cannabinoids. NMDAR activity was also enhanced in CNR1^-/-/HINT1^+/+ neurons, although this activity was dampened by the expression of GPCRs like the MOR, CNR1 or serotonin 1A (5HT1AR).

Conclusions

The HINT1 protein plays an essential role in the GPCR-NMDAR connection. In the absence of receptor activation, GPCRs collaborate with HINT1 proteins to negatively control NMDAR activity. When activated, most GPCRs release the control of HINT1 and NMDAR responsiveness is enhanced. However, cannabinoids that act through CNR1 maintain the negative control of HINT1 on NMDAR function and their protection against glutamate excitotoxic insult persists.

Oxycodone combined with opioid receptor antagonists: efficacy and safety

INTRODUCTION

A mu receptor antagonist combined with oxycodone (OXY) may improve pain control, reduce physical tolerance and withdrawal, minimizing opioid-related bowel dysfunction and act as an abuse deterrent.

AREAS COVERED

The authors cover the use of OXY plus ultra-low-dose naltrexone for analgesia and the use of sustained-release OXY plus sustained-release naloxone to reduce the opioid bowel syndrome. The authors briefly describe the use of sustained-release OXY and naltrexone pellets as a drug abuse deterrent formulation. Combinations of ultra-low-dose naltrexone plus OXY have been in separate trials involved in patients with chronic pain from osteoarthritis and idiopathic low back pain. High attrition and marginal differences between ultra-low-dose naltrexone plus OXY and OXY led to discontinuation of development. Prolonged-release (PR) naloxone combined with PR OXY demonstrates a consistent reduction in opioid-related bowel dysfunction in multiple randomized controlled trials. However, gastrointestinal side effects, including diarrhea, were increased in several trials with the combination compared with PR OXY alone. Analgesia appeared to be maintained although non-inferiority to PR OXY is not formally established. There were flaws to trial design and safety monitoring. Naltrexone has been combined with OXY in individual pellets encased in a capsule. This combination has been reported in a Phase II trial and is presently undergoing Phase III studies.

EXPERT OPINION

Due to the lack of efficacy the combination of altered low-dose naltrexone with oxycodone should cease in development. The combination of sustained release oxycodone plus naloxone reduces constipation with a consistent benefit. Safety has been suboptimally evaluated which is a concern. Although the drug is commercially available in several countries, ongoing safety monitoring particularly high doses would be important.

Agonist-selective effects of opioid receptor ligands on cytosolic calcium concentration in rat striatal neurons

BACKGROUND

Buprenorphine is an opioid receptor ligand whose mechanism of action is incompletely understood.

METHODS

Using Ca(2+) imaging, we assessed the effects of buprenorphine, β-endorphin, and morphine on cytosolic Ca(2+) concentration [Ca(2+)](i), in rat striatal neurons.

RESULTS

Buprenorphine (0.01-1 μM) increased [Ca(2+)](i) in a dose-dependent manner in a subpopulation of rat striatal neurons. The effect of buprenorphine was largely reduced by naloxone, a non-selective opioid receptor antagonist, but not by μ, κ, δ or NOP-selective antagonists. β-Endorphin (0.1 μM) increased [Ca(2+)](i) with a lower amplitude and slower time course than buprenorphine. Similar to buprenorphine, the effect of β-endorphin was markedly decreased by naloxone, but not by opioid-selective antagonists. Morphine (0.1-10 μM), did not affect [Ca(2+)](i) in striatal neurons.

CONCLUSIONS

Our results suggest that buprenorphine and β-endorphin act on a distinct type/subtype of plasmalemmal opioid receptors or activate intracellular opioid-like receptor(s) in rat striatal neurons.

μ-Opioid receptor coupling to Gα(o) plays an important role in opioid antinociception

Opioid analgesics elicit their effects via activation of the mu-opioid receptor (MOR), a G protein-coupled receptor known to interact with Gα(i/o)-type G proteins. Work in vitro has suggested that MOR couples preferentially to the abundant brain Gα(i/o) isoform, Gα(o). However, studies in vivo evaluating morphine-mediated antinociception have not supported these findings. The aim of the present work was to evaluate the contribution of Gα(o) to MOR-dependent signaling by measuring both antinociceptive and biochemical endpoints in a Gα(o) null transgenic mouse strain. Male wild-type and Gα(o) heterozygous null (Gα(o) ⁺/⁻) mice were tested for opioid antinociception in the hot plate test or the warm-water tail withdrawal test as measures of supraspinal or spinal antinociception, respectively. Reduction in Gα(o) levels attenuated the supraspinal antinociception produced by morphine, methadone, and nalbuphine, with the magnitude of suppression dependent on agonist efficacy. This was explained by a reduction in both high-affinity MOR expression and MOR agonist-stimulated G protein activation in whole brain homogenates from Gα(o) ⁺/⁻ and Gα(o) homozygous null (Gα(o)⁻/⁻) mice, compared with wild-type littermates. On the other hand, morphine spinal antinociception was not different between Gα(o) ⁺/⁻ and wild-type mice and high-affinity MOR expression was unchanged in spinal cord tissue. However, the action of the partial agonist nalbuphine was compromised, showing that reduction in Gα(o) protein does decrease spinal antinociception, but suggesting a higher Gα(o) protein reserve. These results provide the first in vivo evidence that Gα(o) contributes to maximally efficient MOR signaling and antinociception.

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

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

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.

μ-opioid receptors: correlation of agonist efficacy for signalling with ability to activate internalization

We have compared the ability of a number of μ-opioid receptor (MOPr) ligands to activate G proteins with their abilities to induce MOPr phosphorylation, to promote association of arrestin-3 and to cause MOPr internalization. For a model of G protein-coupled receptor (GPCR) activation where all agonists stabilize a single active conformation of the receptor, a close correlation between signaling outputs might be expected. Our results show that overall there is a very good correlation between efficacy for G protein activation and arrestin-3 recruitment, whereas a few agonists, in particular endomorphins 1 and 2, display apparent bias toward arrestin recruitment. The agonist-induced phosphorylation of MOPr at Ser(375), considered a key step in MOPr regulation, and agonist-induced internalization of MOPr were each found to correlate well with arrestin-3 recruitment. These data indicate that for the majority of MOPr agonists the ability to induce receptor phosphorylation, arrestin-3 recruitment, and internalization can be predicted from their ability as agonists to activate G proteins. For the prototypic MOPr agonist morphine, its relatively weak ability to induce MOPr internalization can be explained by its low agonist efficacy.

Shared mechanisms for opioid tolerance and a transition to chronic pain

Clinical pain conditions may remain responsive to opiate analgesics for extended periods, but such persistent acute pain can undergo a transition to an opiate-resistant chronic pain state that becomes a much more serious clinical problem. To test the hypothesis that cellular mechanisms of chronic pain in the primary afferent also contribute to the development of opiate resistance, we used a recently developed model of the transition of from acute to chronic pain, hyperalgesic priming. Repeated intradermal administration of the potent and highly selective mu-opioid agonist, [d-Ala(2),N-MePhe(4),gly-ol]-enkephalin (DAMGO), to produce tolerance for its inhibition of prostaglandin E(2) hyperalgesia, simultaneously produced hyperalgesic priming. Conversely, injection of an inflammogen, carrageenan, used to produce priming produced DAMGO tolerance. Both effects were prevented by inhibition of protein kinase Cepsilon (PKCepsilon). Carrageenan also induced opioid dependence, manifest as mu-opioid receptor antagonist (d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2))-induced hyperalgesia that, like priming, was PKCepsilon and G(i) dependent. These findings suggest that the transition from acute to chronic pain, and development of mu-opioid receptor tolerance and dependence may be linked by common cellular mechanisms in the primary afferent.

Buprenorphine for Neuropathic Pain—Targeting Hyperalgesia

Opioids are well known to relieve severe, acute, and chronic nociceptive pain, but neuropathic pain shows a relatively poor response to opioids. Buprenorphine, a partial mu and ORL-1-receptor agonist, kappa-delta receptor antagonist, interacts with different G proteins than potent mu agonists and hence is not cross-tolerant to standard opioids. Buprenorphine blocks central sensitization (hyperalgesia) that is commonly found with neuropathic pain. We present a patient with neuropathic pain and tactile allodynia in which buprenorphine alleviated the hyperalgesia to a greater extent than pain severity. We found buprenorphine to be effective in reducing hypersensitivity in neuropathic pain when pure mu agonists fail to produce a response or in individuals who are intolerant to pure mu agonists.

Supraspinal Gbetagamma-dependent stimulation of PLCbeta originating from G inhibitory protein-mu opioid receptor-coupling is necessary for morphine induced acute hyperalgesia

Although alterations in micro-opioid receptor (microOR) signaling mediate excitatory effects of opiates in opioid tolerance, the molecular mechanism for the excitatory effect of acute low dose morphine, as it relates to microOR coupling, is presently unknown. A pronounced coupling of microOR to the alpha subunit of G inhibitory protein emerged in periaqueductal gray (PAG) from mice systemically administered with morphine at a dose producing acute thermal hyperalgesia. This coupling was abolished in presence of the selective microOR antagonist d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) administered at the PAG site, showing that the low dose morphine effect is triggered by microOR activated G inhibitory protein at supraspinal level. When Gbetagamma downstream signalling was blocked by intra-PAG co-administration of 2-(3,4,5-trihydroxy-6-oxoxanthen-9-yl)cyclohexane-1-carboxylic acid, a compound that inhibits Gbetagamma dimer-dependent signaling, a complete prevention of low dose morphine induced acute thermal hyperalgesia was obtained. Phospholipase C beta3, an enzyme necessary to morphine hyperalgesia, was revealed to be associated with Gbetagamma in PAG. Although opioid administration induces a shift in microOR-G protein coupling from Gi to Gs after chronic administration, our data support that this condition is not realized in acute treatment providing evidence that a separate molecular mechanism underlies morphine induced acute excitatory effect.

Identification of an additional supraspinal component to the analgesic mechanism of action of buprenorphine

BACKGROUND AND PURPOSE

Buprenorphine displays attributes of opioids, but also some features distinct from them. We examined spinal and supraspinal signal transduction of buprenorphine-induced anti-nociception in mice compared with morphine and fentanyl.

EXPERIMENTAL APPROACH

The opioid receptor antagonist naloxone, Pertussis toxin (PTX), G(z) protein antisense and nociceptin/orphanin-FQ receptor agonist nociceptin, and antagonist, JTC-801, were injected supraspinally (intracerebroventricular) and spinally (intrathecal). Also the cell-permeable Ser/Thr protein phosphatase inhibitor okadaic acid was given supraspinally.

KEY RESULTS

Spinal naloxone (20 microg) or PTX (1 microg) attenuated morphine, fentanyl and buprenorphine (s.c.) anti-nociception. Supraspinal naloxone or PTX attenuated morphine and fentanyl, but not buprenorphine anti-nociception. Spinal G(z) protein antisense did not alter buprenorphine, morphine or fentanyl anti-nociception and supraspinal G(z)-antisense did not alter morphine or fentanyl anti-nociception. However, supraspinal G(z)-antisense (not random sense) reduced buprenorphine anti-nociception. Peripheral JTC-801 (1 mgxkg(-1), i.p.) enhanced the ascending (3 mgxkg(-1)) and descending (30 mgxkg(-1)) portions of buprenorphine's dose-response curve, but only spinal, not supraspinal, nociceptin (10 nmolxL(-1)) enhanced buprenorphine anti-nociception. Intracereboventricular okadaic acid (0.001-10 pg) produced a biphasic low-dose attenuation, high-dose enhancement of buprenorphine(3 or 30 mgxkg(-1), s.c.) anti-nociception, but did not affect morphine or fentanyl anti-nociception.

CONCLUSIONS AND IMPLICATIONS

Buprenorphine has an opioid component to its supraspinal mechanism of analgesic action. Our present results reveal an additional supraspinal component insensitive to naloxone, PTX and nociceptin/orphanin-FQ, but involving G(z) protein and Ser/Thr protein phosphatase. These data might help explain the unique preclinical and clinical profiles of buprenorphine.

Methadone antinociception is dependent on peripheral opioid receptors

Morphine and methadone are both high-affinity, potent mu-opioid peptide (MOP) receptor analgesics. In this report, we compared the antinociceptive potencies of these 2 drugs when administered subcutaneously (s.c.), intrathecally (i.t.), or intracerebroventricularly (i.c.v.) in both rat and mouse, using the tail-flick assay. We found that both morphine and methadone were potently antinociceptive when the drugs were administered s.c., showing comparable AD50 values in both species. However, the antinociception produced by methadone, when it was administered centrally, was much weaker than that produced by centrally administered morphine. Specifically, the AD50 value for methadone antinociception was more than 30-fold higher at both the i.t. and i.c.v. sites in mouse and not measurable in rat. Naloxone methiodide (NLX-M), a peripherally restricted antagonist, was used to further examine the relative contribution of central versus peripheral sites to morphine and methadone antinociception. NLX-M, when administered s.c., blocked the antinociceptive effect of either systemically or centrally administered methadone but had little effect on the antinociception produced by centrally administered morphine. Furthermore, centrally administered NLX-M significantly blocked antinociception produced by centrally administered morphine but not that produced by centrally administered methadone. Together, these results suggest that methadone antinociception is significantly dependent on an action of the drug at peripheral sites and could provide novel insight into the neural mechanisms that distinguish morphine versus methadone antinociception.

PERSPECTIVE

Methadone is often used as an alternative for pain management. The present study shows that a peripheral action plays a crucial role in methadone antinociception. This finding could have significant clinical relevance for the use of methadone versus morphine for the treatment of certain types of pain.

Gz mediates the long-lasting desensitization of brain CB1 receptors and is essential for cross-tolerance with morphine

Background

Although the systemic administration of cannabinoids produces antinociception, their chronic use leads to analgesic tolerance as well as cross-tolerance to morphine. These effects are mediated by cannabinoids binding to peripheral, spinal and supraspinal CB1 and CB2 receptors, making it difficult to determine the relevance of each receptor type to these phenomena. However, in the brain, the CB1 receptors (CB1Rs) are expressed at high levels in neurons, whereas the expression of CB2Rs is marginal. Thus, CB1Rs mediate the effects of smoked cannabis and are also implicated in emotional behaviors. We have analyzed the production of supraspinal analgesia and the development of tolerance at CB1Rs by the direct injection of a series of cannabinoids into the brain. The influence of the activation of CB1Rs on supraspinal analgesia evoked by morphine was also evaluated.

Results

Intracerebroventricular (icv) administration of cannabinoid receptor agonists, WIN55,212-2, ACEA or methanandamide, generated a dose-dependent analgesia. Notably, a single administration of these compounds brought about profound analgesic tolerance that lasted for more than 14 days. This decrease in the effect of cannabinoid receptor agonists was not mediated by depletion of CB1Rs or the loss of regulated G proteins, but, nevertheless, it was accompanied by reduced morphine analgesia. On the other hand, acute morphine administration produced tolerance that lasted only 3 days and did not affect the CB1R. We found that both neural mu-opioid receptors (MORs) and CB1Rs interact with the HINT1-RGSZ module, thereby regulating pertussis toxin-insensitive Gz proteins. In mice with reduced levels of these Gz proteins, the CB1R agonists produced no such desensitization or morphine cross-tolerance. On the other hand, experimental enhancement of Gz signaling enabled an acute icv administration of morphine to produce a long-lasting tolerance at MORs that persisted for more than 2 weeks, and it also impaired the analgesic effects of cannabinoids.

Conclusion

In the brain, cannabinoids can produce analgesic tolerance that is not associated with the loss of surface CB1Rs or their uncoupling from regulated transduction. Neural specific Gz proteins are essential mediators of the analgesic effects of supraspinal CB1R agonists and morphine. These Gz proteins are also responsible for the long-term analgesic tolerance produced by single doses of these agonists, as well as for the cross-tolerance between CB1Rs and MORs.

Co-expression of mu and delta opioid receptors as receptor-G protein fusions enhances both mu and delta signalling via distinct mechanisms

Mu and delta opioid receptors (MORs and DORs) were co-expressed as fusion proteins between a receptor and a pertussis insensitive mutant Galpha(i/o) protein in human embryonic kidney 293 cells. Signalling efficiency was then monitored following inactivation of endogenous Galpha(i/o) proteins by pertussis toxin. Co-expression resulted in increased delta opioid signalling which was insensitive to the mu specific antagonist d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2. Under these conditions, mu opioid signalling was also increased and insensitive to the delta specific antagonist Tic-deltorphin. In this latter case, however, no G protein activation was observed in the presence of the delta specific inverse agonist N,N(CH3)2-Dmt-Tic-NH2. When a MOR fused to a non-functional Galpha subunit was co-expressed with the DOR-Galpha protein fusion, delta opioid signalling was not affected whereas mu opioid signalling was restored. Altogether our results suggest that increased delta opioid signalling is due to enhanced DOR coupling to its tethered Galpha subunit. On the other hand, our data indicate that increased mu opioid signalling requires an active conformation of the DOR and also results in activation of the Galpha subunit fused the DOR.

Ligand-specific receptor states: Implications for opiate receptor signalling and regulation

Opiate drugs produce their effects by acting upon G protein coupled receptors (GPCRs) and although they are among the most effective analgesics available, their clinical use is restricted by unwanted side effects such as tolerance, physical dependence, respiratory depression, nausea and constipation. As a class, opiates share a common profile of unwanted effects but there are also significant differences in ligand liability for producing these actions. A growing number of studies show that GPCRs may exist in multiple active states that differ in their signalling and regulatory properties and which may distinctively bind different agonists. In this review we summarize evidence supporting the existence of multiple active conformations for MORs and DORs, analyze information favouring the existence of ligand-specific receptor states and assess how ligand-selective efficacy may contribute to the production of longer lasting, better tolerated opiate analgesics.

Differential activation of G-proteins by mu-opioid receptor agonists

We investigated the ability of the activated mu-opioid receptor (MOR) to differentiate between myristoylated G(alphai1) and G(alphaoA) type G(alpha) proteins, and the maximal activity of a range of synthetic and endogenous agonists to activate each G(alpha) protein. Membranes from HEK293 cells stably expressing transfected MOR were chaotrope extracted to denature endogenous G-proteins and reconstituted with specific purified G-proteins. The G(alpha) subunits were generated in bacteria and were demonstrated to be recognised equivalently to bovine brain purified G(alpha) protein by CB(1) cannabinoid receptors. The ability of agonists to catalyse the MOR-dependent GDP/[(35)S]GTP(gamma)S exchange was then compared for G(alphai1) and G(alphaoA). Activation of MOR by DAMGO produced a high-affinity saturable interaction for G(alphaoA) (K(m)=20+/-1 nM) but a low-affinity interaction with G(alphai1) (K(m)=116+/-12 nM). DAMGO, met-enkephalin and leucine-enkephalin displayed maximal G(alpha) activation among the agonists evaluated. Endomorphins 1 and 2, methadone and beta-endorphin activated both G(alpha) to more than 75% of the maximal response, whereas fentanyl partially activated both G-proteins. Buprenorphine and morphine demonstrated a statistically significant difference between the maximal activities between G(alphai1) and G(alphaoA). Interestingly, DAMGO, morphine, endomorphins 1 and 2, displayed significant differences in the potencies for the activation of the two G(alpha). Differences in maximal activity and potency, for G(alphai1) versus G(alphaoA), are both indicative of agonist selective activation of G-proteins in response to MOR activation. These findings may provide a starting point for the design of drugs that demonstrate greater selectivity between these two G-proteins and therefore produce a more limited range of effects.