Activation Of Mu-opioid Receptors Research Articles

Prenatal ethanol exposure and enkephalinergic neurotransmission.

Endogenous opioids (enkephalins, endorphins and dynorphins) are small peptides that play a main role in pain perception and analgesia, as well as in alcohol (ethanol) reinforcement and reward. Alcohol reinforcement involves the ethanol-induced activation of the endogenous opioid system, a process that may augment the hedonic value and the reinforcing properties of the drug, which in turn increases substance consumption. Changes in opioidergic transmission may contribute to alcohol intoxication and to the neuroadaptive responses produced by the long-lasting exposure to ethanol. Opioidergic transmission may be altered by ethanol at distinct levels, including the expression of precursor mRNAs, biosynthesis and release of opioid peptides, as well as ligand binding to opioid receptors. In adult rats, β-endorphinergic and enkephalinergic transmission, through activation of mu and delta opioid receptors, mediate ethanol reinforcement and high alcohol drinking behavior. Prenatal ethanol exposure (PEE) selectively modifies Methionine-enkephalin (Met-enk) content in several brain regions of infant and adolescent rats, particularly those of the reward circuits. In preweanling rats, Met-enk content is decreased in the ventral tegmental area but is increased in the prefrontal cortex and the nucleus accumbens and other brain areas, as a consequence of a short and moderate ethanol exposure during late gestation. PEE also increases Met-enk levels in the prefrontal cortex and other brain regions of 30-day-old adolescent rats. These findings suggest that mesocorticolimbic enkephalins are essential in ethanol reinforcement in offspring, as previously reported in adult rats.

Deformation-based Morphometry MRI Reveals Brain Structural Modifications in Living Mu Opioid Receptor Knockout Mice.

Mu opioid receptor (MOR) activation facilitates reward processing and reduces pain, and brain networks underlying these effects are under intense investigation. Mice lacking the MOR gene (MOR KO mice) show lower drug and social reward, enhanced pain sensitivity and altered emotional responses. Our previous neuroimaging analysis using Resting-state (Rs) functional Magnetic Resonance Imaging (fMRI) showed significant alterations of functional connectivity (FC) within reward/aversion networks in these mice, in agreement with their behavioral deficits. Here we further used a structural MRI approach to determine whether volumetric alterations also occur in MOR KO mice. We acquired anatomical images using a 7-Tesla MRI scanner and measured deformation-based morphometry (DBM) for each voxel in subjects from MOR KO and control groups. Our analysis shows marked anatomical differences in mutant animals. We observed both local volumetric contraction (striatum, nucleus accumbens, bed nucleus of the stria terminalis, hippocampus, hypothalamus and periacqueducal gray) and expansion (prefrontal cortex, amygdala, habenula, and periacqueducal gray) at voxel level. Volumetric modifications occurred mainly in MOR-enriched regions and across reward/aversion centers, consistent with our prior FC findings. Specifically, several regions with volume differences corresponded to components showing highest FC changes in our previous Rs-fMRI study, suggesting a possible function-structure relationship in MOR KO-related brain differences. In conclusion, both Rs-fMRI and volumetric MRI in live MOR KO mice concur to disclose functional and structural whole-brain level mechanisms that likely drive MOR-controlled behaviors in animals, and may translate to MOR-associated endophenotypes or disease in humans.

Why mu-opioid agonists have less analgesic efficacy in neuropathic pain?

Injury to peripheral nerves often leads to abnormal pain states (hyperalgesia, allodynia and spontaneous pain), which can remain long after the injury heals. Although opioid agonists remain the gold standard for the treatment of moderate to severe pain, they show reduced efficacy against neuropathic pain. In addition to analgesia, opioid use is also associated with hyperalgesia and analgesia tolerance, whose underlying mechanisms share some commonalities with nerve injury-induced hypersensitivity. Here, we reviewed up-to-day research exploring the contribution of mu-opioid receptor (MOR) on the pathophysiology of neuropathic pain and on analgesic opioid actions under these conditions. We focused on the specific contributions of MOR populations at peripheral, spinal and supraspinal level. Moreover, evidences of neuroplastic changes that may underlie the low efficacy of MOR agonists under neuropathic pain conditions are reviewed and discussed. Sensitization processes leading to pain hypersensitivity, molecular changes in signalling pathways triggered by MOR and glial activation are some of these mechanisms elicited by both nerve injury and opioid exposure. Nerve injury-induced pain hypersensitivity might be masking the initial analgesic effects of opioid agonists, and alternatively, sustained opioid treatment to individuals already suffering from neuropathic pain could aggravate their pathophysiological state. Finally, some combined therapies that can increase opioid analgesic effectiveness in neuropathic pain treatment are highlighted. SIGNIFICANCE: This review provides evidence of the low benefit of opioid monotherapy in neuropathic pain and analyses the reasons of this reduced effectiveness. Opioid agonists along with drugs targeted to block the sensitization processes induced by MOR stimulation might result in a better management of neuropathic pain.

Co-localization of mu-opioid and dopamine D1 receptors in the medial preoptic area and bed nucleus of the stria terminalis across seasonal states in male European starlings

In seasonally breeding animals, changes in photoperiod and sex-steroid hormones may modify sexual behavior in part by altering the activity of neuromodulators, including opioids and dopamine. In rats and birds, activation of mu-opioid receptors (MOR) and dopamine D1 receptors in the medial preoptic area (mPOA) often have opposing effects on sexual behavior, yet mechanisms by which the mPOA integrates these opposing effects to modulate behavior remain unknown. Here, we used male European starlings (Sturnus vulgaris) to provide insight into the hypothesis that MOR and D1 receptors modify sexual behavior seasonally by altering activity in the same neurons in the mPOA. To do this, using fluorescent immunohistochemistry, we examined the extent to which MOR and D1 receptors co-localize in mPOA neurons and the degree to which photoperiod and the sex-steroid hormone testosterone alter co-localization. We found that MOR and D1 receptors co-localize throughout the mPOA and the bed nucleus of the stria terminalis, a region also implicated in the control of sexual behavior. Numbers of single and co-labeled MOR and D1 receptor labeled cells were higher in the rostral mPOA in photosensitive males (a condition observed just prior to the breeding season) compared to photosensitive males treated with testosterone (breeding season condition). In the caudal mPOA co-localization of MOR and D1 receptors was highest in photosensitive males compared to photorefractory males (a post-breeding season condition). Seasonal shifts in the degree to which neurons in the mPOA integrate signaling from opioids and dopamine may underlie seasonal changes in the production of sexual behavior.

Heteromerization Modulates mu Opioid Receptor Functional Properties in vivo.

Mu opioid receptors modulate a large number of physiological functions. They are in particular involved in the control of pain perception and reward properties. They are also the primary molecular target of opioid drugs and mediate their beneficial analgesic effects, euphoric properties as well as negative side effects such as tolerance and physical dependence. Importantly, mu opioid receptors can physically associate with another receptor to form a novel entity called heteromer that exhibits specific ligand binding, signaling, and trafficking properties. As reviewed here, in vivo physical proximity has now been evidenced for several receptor pairs, subsequent impact of heteromerization on native mu opioid receptor signaling and trafficking identified and a link to behavioral changes established. Selective targeting of heteromers as a tool to modulate mu opioid receptor activity is therefore attracting growing interest and raises hopes for innovative therapeutic strategies.

OPRM1 A118G and serum β-endorphin interact with sex and digit ratio (2D:4D) to influence risk and course of alcohol dependence

Activation of mesolimbic mu-opioid receptors by their endogenous ligand, β-endorphin, can mediate the rewarding effects of alcohol. However, there is conflicting evidence on the relationship between the mu-opioid receptor (OPRM1) A118G single nucleotide polymorphism (SNP) and alcohol dependence risk. Preclinical evidence suggests that sex and sex hormone-dependent prenatal brain organization may interact with the opioid system to influence alcohol drinking behavior. We genotyped 200 alcohol-dependent patients and 240 healthy individuals for the OPRM1 A118G SNP and measured serum β-endorphin level at recruitment and after acute withdrawal. We then determined the association between these factors and alcohol dependence risk and 24-month outcome in the context of both sex and second-to-fourth digit lengths ratio (2D:4D) – a biomarker of prenatal sex hormone levels. The OPRM1 A118G AA genotype associated with elevated risk of alcohol-related hospital readmission, more readmissions, and fewer days until first readmission in male patients only. After normalizing patient 2D:4D against control 2D:4D, we found that normalized 2D:4D ratios were lower in male 118G patients than male AA patients, suggesting prenatal androgens interact with OPRM1 to influence alcohol dependence risk. In addition, β-endorphin levels after acute withdrawal correlated negatively with withdrawal severity in females but not in males, which may indicate β-endorphin protects against withdrawal-induced stress in a sex-specific manner.

Structure-Activity Relationship Studies of 6α- and 6β-Indolylacetamidonaltrexamine Derivatives as Bitopic Mu Opioid Receptor Modulators and Elaboration of the "Message-Address Concept" To Comprehend Their Functional Conversion.

Structure-activity relationship (SAR) studies of numerous opioid ligands have shown that introduction of a methyl or ethyl group on the tertiary amino group at position 17 of the epoxymorphinan skeleton generally results in a mu opioid receptor (MOR) agonist while introduction of a cyclopropylmethyl group typically leads to an antagonist. Furthermore, it has been shown that introduction of heterocyclic ring systems at position 6 can favor antagonism. However, it was reported that 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(2'-indolyl)acetamido]morphinan (INTA), which bears a cyclopropylmethyl group at position 17 and an indole ring at position 6, acted as a MOR agonist. We herein report a SAR study on INTA with a series of its complementary derivatives to understand how introduction of an indole moiety with α or β linkage at position 6 of the epoxymorphinan skeleton may influence ligand function. Interestingly, one of INTA derivatives, compound 15 (NAN) was identified as a MOR antagonist both in vitro and in vivo. Molecular modeling studies revealed that INTA and NAN may interact with different domains of the MOR allosteric binding site. In addition, INTA may interact with W293 and N150 residues found in the orthosteric site to stabilize MOR activation conformation while NAN does not. These results suggest that INTA and NAN may be bitopic ligands and the type of allosteric interactions with the MOR influence their functional activity. These insights along with our enriched comprehension of the "message-address" concept will to benefit future ligand design.

Morphine effects within the rodent anterior cingulate cortex and rostral ventromedial medulla reveal separable modulation of affective and sensory qualities of acute or chronic pain.

Modulation of pain may result from engagement of opioid receptors in multiple brain regions. Whether sensory and affective qualities of pain are differentially affected by brain opioid receptor circuits remains unclear. We previously reported that opioid actions within the rostral anterior cingulate cortex (ACC) produce selective modulation of affective qualities of neuropathic pain in rodents, but whether such effects may occur in other areas of the ACC is not known. Here, morphine was microinjected into 3 regions of the ACC or into the rostral ventromedial medulla (RVM), and pain behaviors in naive, sham, or spinal nerve ligated (SNL) rats were evaluated. In naive animals, the tail-flick response was inhibited by RVM, but not ACC, morphine. Anterior cingulate cortex morphine did not affect tactile allodynia (the von Frey test) or mechanical (Randall-Selitto) or thermal (Hargreaves) hyperalgesia in spinal nerve ligated rats. In contrary, RVM morphine reduced tactile allodynia and produced both antihyperalgesic and analgesic effects against mechanical and thermal stimuli as well as conditioned place preference selectively in nerve-injured rats. Within the RVM, opioids inhibit nociceptive transmission reflected in both withdrawal thresholds and affective pain behaviors. Activation of mu opioid receptors within specific rostral ACC circuits, however, selectively modulates affective dimensions of ongoing pain without altering withdrawal behaviors. These data suggest that RVM and ACC opioid circuits differentially modulate sensory and affective qualities of pain, allowing for optimal behaviors that promote escape and survival. Targeting specific ACC opioid circuits may allow for treatment of chronic pain while preserving the physiological function of acute pain.

A Novel Approach to Safer Analgesics: Mu Opioid Receptor Agonist & Delta Opioid Receptor Antagonist Peptidomimetics

Opioid analgesics exert their therapeutic and unwanted effects by activation of the mu‐opioid receptor (MOR). Repeated administration of opioid analgesics and subsequent chronic MOR activation promotes opioid tolerance and physical dependence. It has been proposed that activation of MOR with concurrent antagonism of the delta‐opioid receptor (DOR) can attenuate the development of opioid tolerance and dependence. To further investigate the utilization of concurrent MOR agonism and DOR antagonism we designed, synthesized, and characterized a library of “peptidomimetics” that have elements of opioid peptides vital for activity but retain small molecule‐like features necessary for blood‐brain‐barrier penetration. Compounds were evaluated in membrane preparations of cloned cell lines expressing either MOR, DOR, or kappa‐opioid receptors (KOR). [35S]‐GTPγS binding to measure G protein‐coupled activation was used. Full agonists at each opioid receptor (DAMGO, DPDPE, and U69,593, respectively) were employed as standards. Maximum binding and potency (EC50) values were determined from concentration‐effect curves. To measure affinity (Ki) values, competition binding using [3H]‐diprenorphine was performed. Peptidomimetics that demonstrated promising bi‐functional activity were tested for antinociceptive activity in vivo. Compared to repeated morphine‐treated mice, repeated peptidomimetic treated mice developed less analgesic tolerance or physical dependence. Analysis of in vitro and in vivo data has permitted structure‐activity relationships to guide further chemical synthesis and the discovery of opioid analgesics that may have improved clinical utility.Support or Funding InformationSupported by DA‐03910This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

EFFECTS OF ZAMZAM WATER AND METHADONE ON THE EXPRESSION OF MU-OPIOID RECEPTOR-1 GENE IN MORPHINE-DEPENDENT RATS AFTER CHRONIC MORPHINE ADMINISTRATION

Background: Sodium ion is an essential ion that is implicated in many physiological functions. Recently, sodium ion was reported to facilitate the activation of Mu-Opioid Receptor (MOR) by binding at the allosteric site of the MOR. Zamzam water is water originated from Mecca. Couple of studies proved that Zamzam water has the therapeutic effect owing to its mineral. In this study, we want to determine the ion concentration of Zamzam water and then to investigate the effects of Zamzam water and co-treatment with methadone on the regulation of MOR-1gene after chronic morphine administration.
 Materials and Methods: Zamzam water, tap water and normal mineral water were analyzed using Ion chromatography. Meanwhile, in animal study, 50 male Sprague Dawley rats were randomly divided into five groups. All group of rat were made dependence on morphine using intraperitoneal injection except for normal group. Morphine dependent rats then were treated with methadone, Zamzam water and co-treatment methadone with Zamzam water for thirty days, respectively. The Ventral Tegmental Area (VTA) of rat’s brain was dissected and subjected to real-time quantitative RT-PCR to determine the regulation of MOR-1 gene expression. The obtained data were analyzed using SPSS v.11 software, and one-way ANOVA followed by Tukey’s Post-test.
 Results: The data obtained showed that Zamzam water is significantly high in ion concentration compared to tap water and normal mineral water. Besides, the result from gene expression analysis showed co-treatment Zamzam water and methadone significantly prevented the downregulation of MOR as compared to methadone and Zamzam water treatment alone (P

In silico studies of conformational dynamics of Mu opioid receptor performed using gaussian accelerated molecular dynamics

G-protein-coupled receptors play a crucial role in various signaling pathways and function as targets for treating a wide spectrum of diseases. Since the twentieth century, extensive research has been conducted on the Mu opioid receptor (MOR) as a drug target. We examined the MOR inactivation and activation processes using an enhanced sampling method (Gaussian accelerated molecular dynamics), the binding pocket site area method, the root mean square deviation method, and the free energy (potential of mean force) method. This study revealed two important intermediate MOR structures (intermediate and intermediate inactive), and the results suggest that the intermediate MOR structure is responsible for the selectivity of opioids.

Lithium reverses mechanical allodynia through a mu opioid-dependent mechanism.

BackgroundLithium is widely used to treat bipolar disorders and displays mood stabilizing properties. In addition, lithium relieves painful cluster headaches and has a strong analgesic effect in neuropathic pain rat models.ObjectivesTo investigate the analgesic effect of lithium on the cuff model of neuropathic pain.MethodsWe used behavioral and pharmacological approaches to study the analgesic effect of a single injection of lithium in wild-type and mu opioid receptor (MOR) null cuffed neuropathic mice. Mass spectrometry and enzyme-linked immunosorbent assay allowed to measure the levels of endogenous MOR agonist beta-endorphin as well as monoamines in brain and plasma samples 4 h after lithium administration.ResultsA single injection of lithium chloride (100 mg/kg, ip) alleviated mechanical allodynia for 24 h, and this effect was absent in MOR null neuropathic mice. Biochemical analyses highlight a significant increase in beta-endorphin levels by 30% in the brain of lithium-treated mice compared to controls. No variation of beta-endorphin was detected in the blood.ConclusionsTogether, our results provide evidence that lithium induces a long-lasting analgesia in neuropathic mice presumably through elevated brain levels of beta-endorphin and the activation of MORs.

Dichotomous Effects of Mu Opioid Receptor Activation on Striatal Low-Threshold Spike Interneurons.

Striatal low-threshold spike interneurons (LTSIs) are tonically active neurons that express GABA and nitric oxide synthase and are involved in information processing as well as neurovascular coupling. While mu opioid receptors (MORs) and their ligand encephalin are prominent in the striatum, their action on LTSIs has not been investigated. We addressed this issue carrying out whole-cell recordings in transgenic mice in which the NPY-expressing neurons are marked with green fluorescent protein (GFP). The MOR agonist (D-Ala(2), N-MePhe(4), Gly-ol)-enkephalin (DAMGO) produced dual effects on subpopulations of LTSIs. DAMGO caused inhibitory effects, accompanied by decreases of spontaneous firing, in 62% of LTSIs, while depolarizing effects (accompanied by an increase in spontaneous firing) were observed in 23% of LTSIs tested. The dual effects of DAMGO persisted in the presence of tetrodotoxin (TTX), a sodium channel blocker or in the presence of the nicotinic acetylcholine receptor antagonist mecamylamine. However, in the presence of either the GABAA receptor antagonist picrotoxin or the muscarinic cholinergic receptor antagonist atropine, DAMGO only elicited inhibitory effects on LTSIs. Furthermore, we found that DAMGO decreased the amplitude and frequency of spontaneous GABAergic events. Unexpectedly, these effects of DAMGO on spontaneous GABAergic events disappeared after blocking of the muscarinic and nicotinic cholinergic blockers, showing that GABA inputs to LTSIs are not directly modulated by presynaptic MORs. These finding suggest that activation of MORs affect LTSIs both directly and indirectly, through modulation of GABAergic and cholinergic tones. The complex balance between direct and indirect effects determines the net effect of DAMGO on LTSIs.

Enhanced mu opioid receptor-dependent opioidergic modulation of striatal cholinergic transmission in DYT1 dystonia.

Mu opioid receptor activation modulates acetylcholine release in the dorsal striatum, an area deeply involved in motor function, habit formation, and reinforcement learning as well as in the pathophysiology of different movement disorders, such as dystonia. Although the role of opioids in drug reward and addiction is well established, their involvement in motor dysfunction remains largely unexplored. We used a multidisciplinary approach to investigate the responses to mu activation in 2 mouse models of DYT1 dystonia (Tor1a+/Δgag mice, Tor1a+/- torsinA null mice, and their respective wild-types). We performed electrophysiological recordings to characterize the pharmacological effects of receptor activation in cholinergic interneurons as well as the underlying ionic currents. In addition, an analysis of the receptor expression was performed both at the protein and mRNA level. In mutant mice, selective mu receptor activation caused a stronger G-protein-dependent, dose-dependent inhibition of firing activity in cholinergic interneurons when compared with controls. In Tor1a+/- mice, our electrophysiological analysis showed an abnormal involvement of calcium-activated potassium channels. Moreover, in both models we found increased levels of mu receptor protein. In addition, both total mRNA and the mu opioid receptor splice variant 1S (MOR-1S) splice variant of the mu receptor gene transcript, specifically enriched in striatum, were selectively upregulated. Mice with the DYT1 dystonia mutation exhibit an enhanced response to mu receptor activation, dependent on selective receptor gene upregulation. Our data suggest a novel role for striatal opioid signaling in motor control, and more important, identify mu opioid receptors as potential targets for pharmacological intervention in dystonia. © 2017 International Parkinson and Movement Disorder Society.

Cebranopadol, a Mixed Opioid Agonist, Reduces Cocaine Self-administration through Nociceptin Opioid and Mu Opioid Receptors.

Cocaine addiction is a widespread psychiatric condition still waiting for approved efficacious medications. Previous studies suggested that simultaneous activation of nociceptin opioid (NOP) and mu opioid (MOP) receptors could be a successful strategy to treat cocaine addiction, but the paucity of molecules co-activating both receptors with comparable potency has hampered this line of research. Cebranopadol is a non-selective opioid agonist that at nanomolar concentration activates both NOP and MOP receptors and that recently reached phase-III clinical trials for cancer pain treatment. Here, we tested the effect of cebranopadol on cocaine self-administration (SA) in the rat. We found that under a fixed-ratio-5 schedule of reinforcement, cebranopadol (25 and 50 µg/kg) decreased cocaine but not saccharin SA, indicating a specific inhibition of psychostimulant consumption. In addition, cebranopadol (50 µg/kg) decreased the motivation for cocaine as detected by reduction of the break point measured in a progressive-ratio paradigm. Next, we found that cebranopadol retains its effect on cocaine consumption throughout a 7-day chronic treatment, suggesting a lack of tolerance development toward its effect. Finally, we found that only simultaneous blockade of NOP and MOP receptors by concomitant administration of the NOP antagonist SB-612111 (30 mg/kg) and naltrexone (2.5 mg/kg) reversed cebranopadol-induced decrease of cocaine SA, demonstrating that cebranopadol activates both NOP and classical opioid receptors to exert its effect. Our data, together with the fairly advanced clinical development of cebranopadol and its good tolerability profile in humans, indicate that cebranopadol is an appealing candidate for cocaine addiction treatment.

Maintenance on naltrexone + amphetamine decreases cocaine-vs.-food choice in male rhesus monkeys

BackgroundCocaine use disorder remains a significant public health issue for which there are no FDA-approved pharmacotherapies. Amphetamine maintenance reduces cocaine use in preclinical and clinical studies, but the mechanism of this effect is unknown. Previous studies indicate a role for endogenous opioid release and subsequent opioid receptor activation in some amphetamine effects; therefore, the current study examined the role of mu-opioid receptor activation in d-amphetamine treatment effects in an assay of cocaine-vs-food choice. MethodsAdult male rhesus monkeys with double-lumen intravenous catheters responded for concurrently available food pellets and cocaine injections (0–0.1mg/kg/injection) during daily sessions. Cocaine choice and overall reinforcement rates were evaluated during 7-day treatments with saline or test drugs. ResultsDuring saline treatment, cocaine maintained a dose-dependent increase in cocaine-vs.-food choice. The mu-opioid receptor agonist morphine (0.032–0.32mg/kg/h) dose-dependently increased cocaine choice and decreased rates of reinforcement. A dose of the mu-selective opioid receptor antagonist naltrexone (0.0032mg/kg/h) that completely blocked morphine effects had no effect on cocaine choice when it was administered alone, but it enhanced the effectiveness of a threshold dose of 0.032mg/kg/h amphetamine to decrease cocaine choice without also enhancing nonselective behavioral disruption by this dose of amphetamine. Conversely, the kappa-selective opioid antagonist norbinalorphimine did not enhance amphetamine effects on cocaine choice. ConclusionsThese results suggest that amphetamine maintenance produces mu opioid-receptor mediated effects that oppose its anti-cocaine effects. Co-administration of naltrexone may selectively enhance amphetamine potency to decrease cocaine choice without increasing amphetamine potency to produce general behavioral disruption.

Morphine-induced hyperalgesia involves mu opioid receptors and the metabolite morphine-3-glucuronide

Opiates are potent analgesics but their clinical use is limited by side effects including analgesic tolerance and opioid-induced hyperalgesia (OIH). The Opiates produce analgesia and other adverse effects through activation of the mu opioid receptor (MOR) encoded by the Oprm1 gene. However, MOR and morphine metabolism involvement in OIH have been little explored. Hence, we examined MOR contribution to OIH by comparing morphine-induced hyperalgesia in wild type (WT) and MOR knockout (KO) mice. We found that repeated morphine administration led to analgesic tolerance and hyperalgesia in WT mice but not in MOR KO mice. The absence of OIH in MOR KO mice was found in both sexes, in two KO global mutant lines, and for mechanical, heat and cold pain modalities. In addition, the morphine metabolite morphine-3beta-D-glucuronide (M3G) elicited hyperalgesia in WT but not in MOR KO animals, as well as in both MOR flox and MOR-Nav1.8 sensory neuron conditional KO mice. M3G displayed significant binding to MOR and G-protein activation when using membranes from MOR-transfected cells or WT mice but not from MOR KO mice. Collectively our results show that MOR is involved in hyperalgesia induced by chronic morphine and its metabolite M3G.

Morphine activation of mu opioid receptors causes disinhibition of neurons in the ventral tegmental area mediated by \u03b2-arrestin2 and c-Src

The tyrosine kinase, c-Src, participates in mu opioid receptor (MOP) mediated inhibition in sensory neurons in which β-arrestin2 (β-arr2) is implicated in its recruitment. Mice lacking β-arr2 exhibit increased sensitivity to morphine reinforcement; however, whether β-arr2 and/or c-Src participate in the actions of opioids in neurons within the reward pathway is unknown. It is also unclear whether morphine acts exclusively through MOPs, or involves delta opioid receptors (DOPs). We examined the involvement of MOPs, DOPs, β-arr2 and c-Src in the inhibition by morphine of GABAergic inhibitory postsynaptic currents (IPSCs) recorded from neurons in the mouse ventral tegmental area. Morphine inhibited spontaneous IPSC frequency, mainly through MOPs, with only a negligible effect remaining in MOP−/− neurons. However, a reduction in the inhibition by morphine for DOP−/− c.f. WT neurons and a DPDPE-induced decrease of IPSC frequency revealed a role for DOPs. The application of the c-Src inhibitor, PP2, to WT neurons also reduced inhibition by morphine, while the inactive PP3, and the MEK inhibitor, SL327, had no effect. Inhibition of IPSC frequency by morphine was also reduced in β-arr2−/− neurons in which PP2 caused no further reduction. These data suggest that inhibition of IPSCs by morphine involves a β-arr2/c-Src mediated mechanism.

Casein kinase 1-epsilon deletion increases mu opioid receptor-dependent behaviors and binge eating1.

Genetic and pharmacological studies indicate that casein kinase 1 epsilon (Csnk1e) contributes to psychostimulant, opioid, and ethanol motivated behaviors. We previously used pharmacological inhibition to demonstrate that Csnk1e negatively regulates the locomotor stimulant properties of opioids and psychostimulants. Here, we tested the hypothesis that Csnk1e negatively regulates opioid and psychostimulant reward using genetic inhibition and the conditioned place preference assay in Csnk1e knockout mice. Similar to pharmacological inhibition, Csnk1e knockout mice showed enhanced opioid-induced locomotor activity with the mu opioid receptor agonist fentanyl (0.2 mg/kg i.p.) as well as enhanced sensitivity to low-dose fentanyl reward (0.05 mg/kg). Interestingly, female knockout mice also showed a markedly greater escalation in consumption of sweetened palatable food-a behavioral pattern consistent with binge eating that also depends on mu opioid receptor activation. No difference was observed in fentanyl analgesia in the 52.5°C hot plate assay (0-0.4 mg/kg), naloxone conditioned place aversion (4 mg/kg), or methamphetamine conditioned place preference (0-4 mg/kg). To identify molecular adaptations associated with increased drug and food behaviors in knockout mice, we completed transcriptome analysis via mRNA sequencing of the striatum. Enrichment analysis identified terms associated with myelination and axon guidance and pathway analysis identified a differentially expressed gene set predicted to be regulated by the Wnt signaling transcription factor, Tcf7l2. To summarize, Csnk1e deletion increased mu opioid receptor-dependent behaviors, supporting previous studies indicating an endogenous negative regulatory role of Csnk1e in opioid behavior.

Why do Pain Physicians Not Routinely Use Mixed Opioids for the Prevention of Neuraxial Opioid-induced Pruritus?

Background: Pruritus is a very disturbing secondary effect that appears after epidural or intrathecal administration of opioid drugs, especially in the management of postoperative pain. It is induced by the activation of mu opioid receptors and it can often be even more unpleasant than the pain being treated. Objective: A wide variety of drugs with different mechanisms of action have been used, aiming at the prevention of pruritus, with varying results. The aim of this comprehensive review letter is to summarize the current evidence of the available pharmacological options to either treat or prevent pruritus induced by spinal opioids. Method: The articles used in the review were found through a search in Medline, PubMed and Cochrane Library up to December 2016, using the keywords “Neuraxial opioids”, “Intrathecal morphine”, “Pruritus”, “Naloxone”, “Nalbuphine” and “Butorphanol”. Results: The most useful drugs act on the mu and kappa opioid receptors. They are either mu opioid antagonists, like intravenous naloxone, or mixed opioids mu antagonists/kappa agonists, such as intravenous nalbuphine and intravenous or epidural butorphanol, the latter being able also for maintaining the analgesia. Conclusion: Both pruritus prevention and treatment remain a challenge in the treatment of patients receiving spinal opioids for postoperative pain. Recent findings suggest that mixed opioids must be added to evidence-based clinical guidelines for the management of pruritus induced by spinal opioids.