β-Arrestin-2 knockout prevents development of cellular μ-opioid receptor tolerance but does not affect opioid-withdrawal-related adaptations in single PAG neurons

Epigenetic regulation in opioid induced hyperalgesia

About 25 million American adults experience pain daily and one of the most commonly prescribed drugs to treat pain are opioids. Prolonged opioid usage and dose escalations can cause a paradoxical response where patients experience enhanced pain sensitivity. This opioid induced hyperalgesia (OIH) is a major hurdle when treating pain in the clinic because its underlying mechanisms are still not fully understood. OIH is also commonly overlooked and lacks guidelines to prevent its onset. Research on pain disorders and opioid usage have recognized potential epigenetic drivers of disease including DNA methylation, histone modifications, miRNA regulation, but their involvement in OIH has not been well studied. This article discusses epigenetic changes that may contribute to pathogenesis, with an emphasis on miRNA alterations in OIH. There is a crucial gap in knowledge including how multiple epigenetic modulators contribute to OIH. Elucidating the epigenetic changes underlying OIH and the crosstalk among these mechanisms could lead to the development of novel targets for the prevention and treatment of this painful phenomena.

Central metabolism as a potential origin of sex differences in morphine antinociception but not induction of antinociceptive tolerance in mice

BACKGROUND AND PURPOSE

In rodents, morphine antinociception is influenced by sex. However, conflicting results have been reported regarding the interaction between sex and morphine antinociceptive tolerance. Morphine is metabolised in the liver and brain into morphine-3-glucuronide (M3G). Sex differences in morphine metabolism and differential metabolic adaptations during tolerance development might contribute to behavioural discrepancies. This article investigates the differences in peripheral and central morphine metabolism after acute and chronic morphine treatment in male and female mice.

EXPERIMENTAL APPROACH

Sex differences in morphine antinociception and tolerance were assessed using the tail-immersion test. After acute and chronic morphine treatment, morphine and M3G metabolic kinetics in the blood were evaluated using LC-MS/MS. They were also quantified in several CNS regions. Finally, the blood-brain barrier (BBB) permeability of M3G was assessed in male and female mice.

KEY RESULTS

This study demonstrated that female mice showed weaker morphine antinociception and faster induction of tolerance than males. Additionally, female mice showed higher levels of M3G in the blood and in several pain-related CNS regions than male mice, whereas lower levels of morphine were observed in these regions. M3G brain/blood ratios after injection of M3G indicated no sex differences in M3G BBB permeability, and these ratios were lower than those obtained after injection of morphine.

CONCLUSION

These differences are attributable mainly to morphine central metabolism, which differed between males and females in pain-related CNS regions, consistent with weaker morphine antinociceptive effects in females. However, the role of morphine metabolism in antinociceptive tolerance seemed limited.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Key differences in regulation of opioid receptors localized to presynaptic terminals compared to somas: Relevance for novel therapeutics

Opioid receptors are G protein-coupled receptors (GPCRs) that regulate activity within peripheral, subcortical and cortical circuits involved in pain, reward, and aversion processing. Opioid receptors are expressed in both presynaptic terminals where they inhibit neurotransmitter release and postsynaptic locations where they act to hyperpolarize neurons and reduce activity. Agonist activation of postsynaptic receptors at the plasma membrane signal via ion channels or cytoplasmic second messengers. Agonist binding initiates regulatory processes that include phosphorylation by G protein receptor kinases (GRKs) and recruitment of beta-arrestins that desensitize and internalize the receptors. Opioid receptors also couple to effectors from endosomes activating intracellular enzymes and kinases. In contrast to postsynaptic opioid receptors, receptors localized to presynaptic terminals are resistant to desensitization such that there is no loss of signaling in the continuous presence of opioids over the same time scale. Thus, the balance of opioid signaling in circuits expressing pre- and postsynaptic opioid receptors is shifted toward inhibition of presynaptic neurotransmitter release during continuous opioid exposure. The functional implication of this shift is not often acknowledged in behavioral studies. This review covers what is currently understood about regulation of opioid/nociceptin receptors, with an emphasis on opioid receptor signaling in pain and reward circuits. Importantly, the review covers regulation of presynaptic receptors and the critical gaps in understanding this area, as well as the opportunities to further understand opioid signaling in brain circuits. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Cellular Tolerance Induced by Chronic Opioids in the Central Nervous System

Opioids are powerful analgesics that elicit acute antinociceptive effects through their action the mu opioid receptor (MOR). However opioids are ineffective for chronic pain management, in part because continuous activation of MORs induces adaptive changes at the receptor level and downstream signaling molecules. These adaptations include a decrease in receptor-effector coupling and changes to second messenger systems that can counteract the persistent activation of MORs by opioid agonists. Homeostatic regulation of MORs and downstream signaling cascades are viewed as precursors to developing tolerance. However, despite numerous studies identifying crucial mechanisms that contribute to opioid tolerance, no single regulatory mechanism that governs tolerance in at the cellular and systems level has been identified. Opioid tolerance is a multifaceted process that involves both individual neurons that contain MORs and neuronal circuits that undergo adaptations following continuous MOR activation. The most proximal event is the agonist/receptor interaction leading to acute cellular actions. This review discusses our understanding of mechanisms that mediate cellular tolerance after chronic opioid treatment that, in part, is mediated by agonist/receptor interaction acutely.

G protein signaling-biased mu opioid receptor agonists that produce sustained G protein activation are noncompetitive agonists

The ability of a ligand to preferentially promote engagement of one signaling pathway over another downstream of GPCR activation has been referred to as signaling bias, functional selectivity, and biased agonism. The presentation of ligand bias reflects selectivity between active states of the receptor, which may result in the display of preferential engagement with one signaling pathway over another. In this study, we provide evidence that the G protein-biased mu opioid receptor (MOR) agonists SR-17018 and SR-14968 stabilize the MOR in a wash-resistant yet antagonist-reversible G protein-signaling state. Furthermore, we demonstrate that these structurally related biased agonists are noncompetitive for radiolabeled MOR antagonist binding, and while they stimulate G protein signaling in mouse brains, partial agonists of this class do not compete with full agonist activation. Importantly, opioid antagonists can readily reverse their effects in vivo. Given that chronic treatment with SR-17018 does not lead to tolerance in several mouse pain models, this feature may be desirable for the development of long-lasting opioid analgesics that remain sensitive to antagonist reversal of respiratory suppression.

Evaluating oliceridine as a treatment option for moderate to severe acute post-operative pain in adults

INTRODUCTION

Despite the advances in regional anesthesia and non-opioid systemic analgesia, opioids remain the primary rescue analgesic for moderate to severe pain. However, the risks and side effects of opioid medications are well documented. Oliceridine is a novel opioid receptor agonist which is thought to have less risk of adverse events, such as postoperative nausea and vomiting (PONV) and respiratory depression.

AREAS COVERED

In this review, the authors discuss the limitations of the current opioid and non-opioid analgesic options. They also review the pharmacokinetics of oliceridine, its analgesic efficacy, and risk of adverse events; and its added clinical value in managing moderate to severe pain.

EXPERT OPINION

Despite the advances in regional anesthesia and multimodal systemic analgesia, opioid free analgesia is only feasible in selected procedures and patients. Oliceridine is effective in the management of moderate to severe pain and appears to be associated with lower risk of nausea and vomiting. The risk of sedation and respiratory depression associated with oliceridine will require further study. The availability of an opioid agonist with a better side effect profile could potentially change the current paradigm of opioid avoidance in postoperative pain management.

Akt and its phosphorylation in nucleus accumbens mediate heroin-seeking behavior induced by cues in rats

Akt is initially identified as one of the downstream targets of phosphatidylinositol-3 kinase (PI3K) and is involved in morphine reward and tolerance. However, whether phospholyration of Akt (p-Akt) mediates heroin relapse remains unclear. Here, we aimed to explore the role of p-Akt in the nucleus accumbens (NAc) in cue-induced heroin-seeking behaviors after withdrawal. First, rats were trained to self-administer heroin for 14 days, after which we assessed heroin-seeking behaviors induced by a context cue (CC) or by discrete conditioned cues (CS) after 1 day or 14 days of withdrawal. We found that the active responses induced by CC or CS after 14 days of withdrawal were higher than those after 1 day of withdrawal. Meanwhile, the expression of p-Akt in the NAc was also greatest when rats were exposed to the CS after 14 days of withdrawal. Additionally, a microinjection of LY294002, an inhibitor of PI3K, into the NAc inhibited the CS-induced heroin-seeking behaviors after 14 days of withdrawal, paralleling the decreased levels of p-Akt in the NAc. Finally, Akt1 or β-arrestin 2 was downregulated via a lentiviral injection to assess the effect on heroin seeking after 14 days of withdrawal. CS-induced heroin-seeking behavior was inhibited by downregulation of Akt1, but not β-arrestin 2, in the NAc. These data demonstrate that Akt phosphorylation in the NAc may play an important role in the incubation of heroin-seeking behavior, suggesting that the PI3K/Akt pathways may be involved in the process of heroin relapse and addiction.

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Discovery, Structure-Activity Relationship, and Mechanistic Studies of 1-((3R,4S)-3-((Dimethylamino)methyl)-4-hydroxy-4-(3-methoxyphenyl)piperidin-1-yl)-2-(2,4,5-trifluorophenyl)ethan-1-one as a Novel Potent Analgesic

Management of moderate to severe pain relies heavily on opioid analgesics such as morphine, oxycodone, and fentanyl in clinics. However, their prolonged use was associated with undesirable side effects. Many new strategies to reduce side effects have been proposed, but not without disadvantages. Using a hot plate model as a phenotypic screening method, our studies identified (3R,4S)-9d with a new scaffold as a potent analgesic with ED₅₀ values of 0.54 mg/kg and 0.021 mg/kg in hot plate and antiwrithing models, respectively. Mechanistic studies showed that it elicited its analgesic effect via the active metabolite (3R,4S)-10a. The mechanism of (3R,4S)-10a-induced activation of the μ opioid receptor (MOR) was proposed by means of molecular dynamics (MD) simulation.

Recent Progress in Opioid Research from an Electrophysiological Perspective

Electrophysiological approaches provide powerful tools to further our understanding of how different opioids affect signaling through opioid receptors; how opioid receptors modulate circuitry involved in processes such as pain, respiration, addiction, and feeding; and how receptor signaling and circuits are altered by physiologic challenges, such as injury, stress, and chronic opioid treatment. The use of genetic manipulations to alter or remove μ-opioid receptors (MORs) with anatomic and cell type specificity and the ability to activate or inhibit specific circuits through opto- or chemogenetic approaches are being used in combination with electrophysiological, pharmacological, and systems-level physiology experiments to expand our understanding of the beneficial and maladaptive roles of opioids and opioid receptor signaling. New approaches for studying endogenous opioid peptide signaling and release and the dynamics of these systems in response to chronic opioid use, pain, and stress will add another layer to our understanding of the intricacies of opioid modulation of brain circuits. This understanding may lead to new targets or approaches for drug development or treatment regimens that may affect both acute and long-term effects of manipulating the activity of circuits involved in opioid-mediated physiology and behaviors. This review will discuss recent advancements in our understanding of the role of phosphorylation in regulating MOR signaling, as well as our understanding of circuits and signaling pathways mediating physiologic behaviors such as respiratory control, and discuss how electrophysiological tools combined with new technologies have and will continue to advance the field of opioid research.

Discovery of 3-((dimethylamino)methyl)-4-hydroxy-4-(3-methoxyphenyl)-N-phenylpiperidine-1-carboxamide as novel potent analgesic

Management of moderate to severe pain by clinically used opioid analgesics is associated with a plethora of side effects. Despite many efforts have been dedicated to reduce undesirable side effects, moderate progress has been made. In this work, starting from Tramadol, a series of 3-((dimethylamino)methyl)-4-hydroxy-4-(3-methoxyphenyl)piperidine-1-carboxamide derivatives were designed and synthesized, and their in vitro and in vivo activities were evaluated. Our campaign afforded selective μ opioid receptor (MOR) ligand 2a (K_iMOR: 7.3 ± 0.5 nM; K_iDOR: 849.4 ± 96.6 nM; K_iKOR: 49.1 ± 6.9 nM) as potent analgesic with ED₅₀ of 3.1 mg/kg in 55 °C hot plate model. Its antinociception effect was blocked by opioid antagonist naloxone. High binding affinity toward MOR of compound 2a was associated with water bridge, salt bridge, hydrogen bond and hydrophobic interaction with MOR. The high selectivity of compound 2a for MOR over δ opioid receptor (DOR) and κ opioid receptor (KOR) was due to steric hindrance of compound 2a with DOR and KOR. 2a, a compound with novel scaffold, could serve as a lead for the development of novel opioid ligands.

Improving the therapeutic window of conventional opioids: novel differential signaling modulators

Conventional opioids are widely used for acute pain management in the postoperative setting. However, a primary concern with conventional opioids is their therapeutic window-the range between doses that produce the desired therapeutic effect (analgesia) and doses that produce unwanted opioid-related adverse events (ORAEs). Conventional µ receptor opioids have a narrow therapeutic window in part because of their mechanism of action (MoA): they bind to µ receptors and non-selectively activate two intracellular signaling pathways, leading to analgesia and to ORAEs. This review explores the clinical potential of µ receptor ligands with differential signaling. Agents with a 'differential signaling" MoA represent an innovative approach that may enhance the therapeutic window. These agents modulate µ receptor activity to selectively engage downstream signaling pathways associated with analgesia while limiting activity in downstream signaling pathways that lead to ORAEs. Differential signaling may fulfill an unmet need in the management of postoperative pain.

Opioid-Induced Tolerance and Hyperalgesia

Opioids are very potent and efficacious drugs, traditionally used for both acute and chronic pain conditions. However, the use of opioids is frequently associated with the occurrence of adverse effects or clinical problems. Other than adverse effects and dependence, the development of tolerance is a significant problem, as it requires increased opioid drug doses to achieve the same effect. Mechanisms of opioid tolerance include drug-induced adaptations or allostatic changes at the cellular, circuitry, and system levels. Dose escalation in long-term opioid therapy might cause opioid-induced hyperalgesia (OIH), which is a state of hypersensitivity to painful stimuli associated with opioid therapy, resulting in exacerbation of pain sensation rather than relief of pain. Various strategies may provide extra-opioid analgesia. There are drugs that may produce independent analgesic effects. A tailored treatment provided by skilled personnel, in accordance with the individual condition, is mandatory. Any treatment aimed at reducing opioid consumption may be indicated in these circumstances. Interventional techniques able to decrease the pain input may allow a decrease in the opioid dose, thus reverting the mechanisms producing tolerance of OIH. Intrathecal therapy with local anesthetics and a sympathetic block are the most common techniques utilized in these circumstances.

PatcherBot: a single-cell electrophysiology robot for adherent cells and brain slices

OBJECTIVE

Intracellular patch-clamp electrophysiology, one of the most ubiquitous, high-fidelity techniques in biophysics, remains laborious and low-throughput. While previous efforts have succeeded at automating some steps of the technique, here we demonstrate a robotic 'PatcherBot' system that can perform many patch-clamp recordings sequentially, fully unattended.

APPROACH

Comprehensive automation is accomplished by outfitting the robot with machine vision, and cleaning pipettes instead of manually exchanging them.

MAIN RESULTS

the PatcherBot can obtain data at a rate of 16 cells per hour and work with no human intervention for up to 3 h. We demonstrate the broad applicability and scalability of this system by performing hundreds of recordings in tissue culture cells and mouse brain slices with no human supervision. Using the PatcherBot, we also discovered that pipette cleaning can be improved by a factor of three.

SIGNIFICANCE

The system is potentially transformative for applications that depend on many high-quality measurements of single cells, such as drug screening, protein functional characterization, and multimodal cell type investigations.

Implications of Chronic Opioid Therapy on Perioperative Complications and Long-Term Surgical Recovery

With chronic opioid use becoming an increasingly common occurrenceamong the general population, perioperative specialties must adapt to the physiologic changes caused by long-term opioids. However, data on the clinicalanesthetics implications of long-term opioid use is scarce. This review intends to survey the literature addressing the molecular mechanisms of long-term opioid use as well as their interaction with various organ systems.

The Contribution of the Descending Pain Modulatory Pathway in Opioid Tolerance

Opioids remain among the most effective pain-relieving therapeutics. However, their long-term use is limited due to the development of tolerance and potential for addiction. For many years, researchers have explored the underlying mechanisms that lead to this decreased effectiveness of opioids after repeated use, and numerous theories have been proposed to explain these changes. The most widely studied theories involve alterations in receptor trafficking and intracellular signaling. Other possible mechanisms include the recruitment of new structural neuronal and microglia networks. While many of these theories have been developed using molecular and cellular techniques, more recent behavioral data also supports these findings. In this review, we focus on the mechanisms that underlie tolerance within the descending pain modulatory pathway, including alterations in intracellular signaling, neural-glial interactions, and neurotransmission following opioid exposure. Developing a better understanding of the relationship between these various mechanisms, within different parts of this pathway, is vital for the identification of more efficacious, novel therapeutics to treat chronic pain.

Endogenous and Exogenous Opioids in Pain

Opioids are the most commonly used and effective analgesic treatments for severe pain, but they have recently come under scrutiny owing to epidemic levels of abuse and overdose. These compounds act on the endogenous opioid system, which comprises four G protein-coupled receptors (mu, delta, kappa, and nociceptin) and four major peptide families (β-endorphin, enkephalins, dynorphins, and nociceptin/orphanin FQ). In this review, we first describe the functional organization and pharmacology of the endogenous opioid system. We then summarize current knowledge on the signaling mechanisms by which opioids regulate neuronal function and neurotransmission. Finally, we discuss the loci of opioid analgesic action along peripheral and central pain pathways, emphasizing the pain-relieving properties of opioids against the affective dimension of the pain experience.

Cellular tolerance at the µ-opioid receptor is phosphorylation dependent

Phosphorylation of the μ-opioid receptor (MOR) is known as a key step in desensitization and internalization but the role in the development of long-term tolerance at the cellular level is not known. Viral expression of wild type (exWT) and mutant MORs, where all phosphorylation sites on the C-terminus (Total Phosphorylation Deficient (TPD)) were mutated to alanine, were examined in locus coeruleus neurons in a MOR knockout rat. Both receptors activated potassium conductance similar to endogenous receptors in wild type animals. The exWT receptors, like endogenous receptors, acutely desensitized, internalized and, after chronic morphine treatment, displayed signs of tolerance. However, TPD receptors did not desensitize or internalize with agonist treatment. In addition the TPD receptors did not develop cellular tolerance following chronic morphine treatment. Thus C-terminal phosphorylation is necessary for the expression of acute desensitization, trafficking and one sign of long-term tolerance to morphine at the cellular level.

Chronic morphine reduces the readily releasable pool of GABA, a presynaptic mechanism of opioid tolerance

KEY POINTS

Chronic treatment with opioids, such as morphine, leads to analgesic tolerance. While postsynaptic opioid tolerance is well documented, the involvement of presynaptic mechanisms remains unclear. We show that chronic morphine reduces the ability of periaqueductal grey (PAG) neurons to maintain GABAergic transmission. This depression of GABAergic transmission was due to a reduction in the effective size of the readily releasable pool. This also led to a reduction in opioid presynaptic inhibition; these presynaptic adaptations need to be considered in the development of strategies to reduce opioid tolerance.

ABSTRACT

The midbrain periaqueductal grey (PAG) plays a critical role in tolerance to the analgesic actions of opioids such as morphine. While numerous studies have identified the postsynaptic adaptations induced by chronic morphine treatment in this and other brain regions, the presence of presynaptic adaptations remains uncertain. We examined GABAergic synaptic transmission within rat PAG brain slices from animals which underwent a low dose morphine treatment protocol which produces tolerance, but not withdrawal. Evoked GABAergic IPSCs (inhibitory postsynaptic currents) were less in morphine compared to control saline treated animals. Postsynaptic GABA_A receptor mediated currents and desensitization, presynaptic release probability (P_r ), and inhibition by endogenous neurotransmitters were similar in morphine and saline treated animals. By contrast, the effective size of the readily releasable pool (RRP) was smaller in morphine treated animals. While the μ-opioid agonist DAMGO produced a reduction in P_r and RRP size in saline treated animals, it only reduced P_r in morphine treated animals. Consequently, DAMGO-induced inhibition of evoked IPSCs during short burst stimulation was less in morphine, compared to saline treated animals. These results indicate that low dose chronic morphine treatment reduces presynaptic μ-opioid inhibition by reducing the size of the pool of vesicles available for action potential dependent release. This novel presynaptic adaptation may provide important insights into the development of efficacious pain therapies that can circumvent the development of opioid tolerance.

Cannabinoid CB1 and CB2 Receptor Signaling and Bias

An agonist that acts through a single receptor can activate numerous signaling pathways. Recent studies have suggested that different ligands can differentially activate these pathways by stabilizing a limited range of receptor conformations, which in turn preferentially drive different downstream signaling cascades. This concept, termed "biased signaling" represents an exciting therapeutic opportunity to target specific pathways that elicit only desired effects, while avoiding undesired effects mediated by different signaling cascades. The cannabinoid receptors CB1 and CB2 each activate multiple pathways, and evidence is emerging for bias within these pathways. This review will summarize the current evidence for biased signaling through cannabinoid receptor subtypes CB1 and CB2.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Allostatic Mechanisms of Opioid Tolerance Beyond Desensitization and Downregulation

Mechanisms of opioid tolerance have focused on adaptive modifications within cells containing opioid receptors, defined here as cellular allostasis, emphasizing regulation of the opioid receptor signalosome. We review additional regulatory and opponent processes involved in behavioral tolerance, and include mechanistic differences both between agonists (agonist bias), and between μ- and δ-opioid receptors. In a process we will refer to as pass-forward allostasis, cells modified directly by opioid drugs impute allostatic changes to downstream circuitry. Because of the broad distribution of opioid systems, every brain cell may be touched by pass-forward allostasis in the opioid-dependent/tolerant state. We will implicate neurons and microglia as interactive contributors to the cumulative allostatic processes creating analgesic and hedonic tolerance to opioid drugs.

Agonist-Specific Recruitment of Arrestin Isoforms Differentially Modify Delta Opioid Receptor Function

Ligand-specific recruitment of arrestins facilitates functional selectivity of G-protein-coupled receptor signaling. Here, we describe agonist-selective recruitment of different arrestin isoforms to the delta opioid receptor in mice. A high-internalizing delta opioid receptor agonist (SNC80) preferentially recruited arrestin 2 and, in arrestin 2 knock-outs (KOs), we observed a significant increase in the potency of SNC80 to inhibit mechanical hyperalgesia and decreased acute tolerance. In contrast, the low-internalizing delta agonists (ARM390, JNJ20788560) preferentially recruited arrestin 3 with unaltered behavioral effects in arrestin 2 KOs. Surprisingly, arrestin 3 KO revealed an acute tolerance to these low-internalizing agonists, an effect never observed in wild-type animals. Furthermore, we examined delta opioid receptor-Ca(2+)channel coupling in dorsal root ganglia desensitized by ARM390 and the rate of resensitization was correspondingly decreased in arrestin 3 KOs. Live-cell imaging in HEK293 cells revealed that delta opioid receptors are in pre-engaged complexes with arrestin 3 at the cell membrane and that ARM390 strengthens this membrane interaction. The disruption of these complexes in arrestin 3 KOs likely accounts for the altered responses to low-internalizing agonists. Together, our results show agonist-selective recruitment of arrestin isoforms and reveal a novel endogenous role of arrestin 3 as a facilitator of resensitization and an inhibitor of tolerance mechanisms.

SIGNIFICANCE STATEMENT

Agonists that bind to the same receptor can produce highly distinct signaling events and arrestins are a major mediator of this ligand bias. Here, we demonstrate that delta opioid receptor agonists differentially recruit arrestin isoforms. We found that the high-internalizing agonist SNC80 preferentially recruits arrestin 2 and knock-out (KO) of this protein results in increased efficacy of SNC80. In contrast, low-internalizing agonists (ARM390 and JNJ20788560) preferentially recruit arrestin 3 and, surprisingly, KO of arrestin 3 produces acute tolerance and impaired receptor resensitization to these agonists. Arrestin 3 is in pre-engaged complexes with the delta opioid receptor at the cell membrane and low-internalizing agonists promote this interaction. This study reveals a novel role for arrestin 3 as a facilitator of receptor resensitization.

Methadone Reverses Analgesic Tolerance Induced by Morphine Pretreatment

BACKGROUND

Opiates such as morphine are the most powerful analgesics, but their protracted use is restrained by the development of tolerance to analgesic effects. Recent works suggest that tolerance to morphine might be due to its inability to promote mu opioid receptor endocytosis, and the co-injection of morphine with a mu opioid receptor internalizing agonist like [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]enkephalin reduces tolerance to morphine. So far, no studies have been conducted to evaluate the ability of methadone to reduce morphine tolerance in morphine-pretreated animals, a treatment sequence that could be encountered in opiate rotation protocol. We investigated the ability of methadone (a mu opioid receptor internalizing agonist used in therapy) to reverse morphine tolerance and the associated cellular mechanisms in the periaqueductal gray matter, a region involved in pain control.

METHODS

We measured analgesic response following a challenge dose of morphine in the hot plate test and investigated regulation of mu opioid receptor (coupling and endocytosis) and some cellular mechanisms involved in tolerance such as adenylate cyclase superactivation and changes in N-methyl-d-aspartate receptor subunits expression and phosphorylation state.

RESULTS

A chronic treatment with morphine promoted tolerance to its analgesic effects and was associated with a lack of mu opioid receptor endocytosis, adenylate cyclase overshoot, NR2A and NR2B downregulation, and phosphorylation of NR1. We reported that a methadone treatment in morphine-treated mice reversed morphine tolerance to analgesia by promoting mu opioid receptor endocytosis and blocking cellular mechanisms of tolerance.

CONCLUSIONS

Our data might lead to rational strategies to tackle opiate tolerance in the frame of opiate rotation.

Nociceptin/Orphanin FQ Receptor Structure, Signaling, Ligands, Functions, and Interactions with Opioid Systems

The NOP receptor (nociceptin/orphanin FQ opioid peptide receptor) is the most recently discovered member of the opioid receptor family and, together with its endogenous ligand, N/OFQ, make up the fourth members of the opioid receptor and opioid peptide family. Because of its more recent discovery, an understanding of the cellular and behavioral actions induced by NOP receptor activation are less well developed than for the other members of the opioid receptor family. All of these factors are important because NOP receptor activation has a clear modulatory role on mu opioid receptor-mediated actions and thereby affects opioid analgesia, tolerance development, and reward. In addition to opioid modulatory actions, NOP receptor activation has important effects on motor function and other physiologic processes. This review discusses how NOP pharmacology intersects, contrasts, and interacts with the mu opioid receptor in terms of tertiary structure and mechanism of receptor activation; location of receptors in the central nervous system; mechanisms of desensitization and downregulation; cellular actions; intracellular signal transduction pathways; and behavioral actions with respect to analgesia, tolerance, dependence, and reward. This is followed by a discussion of the agonists and antagonists that have most contributed to our current knowledge. Because NOP receptors are highly expressed in brain and spinal cord and NOP receptor activation sometimes synergizes with mu receptor-mediated actions and sometimes opposes them, an understanding of NOP receptor pharmacology in the context of these interactions with the opioid receptors will be crucial to the development of novel therapeutics that engage the NOP receptor.