Different Subtypes Of Opioid Receptors Research Articles

Molecular determinants of distinctive opioid receptor subtype affinities.

Despite being targeted by over 30% of the drugs available on the market, G-Protein Coupled Receptors (GPCR) have proven to be notoriously difficult to characterize on an atomistic level. While it is known that helix 6 (TM6) movement is necessary for signal transduction, structural details of activation remain elusive and each receptor is generally studied individually, making comparisons between homologous proteins challenging. Given their homology, any newly discovered drug targeting a specific GPCR may be promiscuous, adding work- and cost-intensive dimensions to the drug discovery process. Some of the GPCRs that seem the most appealing as drug targets are opioid receptors. As opioids are potent pain relievers with widespread use and addictive properties, understanding how drugs bind particular opioid receptor subtypes would allow the design of non-addictive derivatives and thus help alleviate the ongoing opioid crisis - in the United States only, it is estimated that 4 million people use opioids recreationally or are dependent on them, with about half overdose-related deaths being related to synthetic opioids. In this work, we have used enhanced-sampling molecular dynamics simulations to explore the conformational dynamics of different subtypes of opioid receptors and uncovered the molecular determinants of their affinity to various endogenous peptides and synthetic opioids. Using enhanced sampling techniques, we have obtained free energy surfaces (FES) of activation of the μ-, κ-, and δ-subtypes. By comparing the metastable states obtained from the FES, we have pinpointed key structural differences relevant to the activation process. These findings will allow the development of simple and accurate structure-activity relationship, which in turn will guide the design of new, less addictive opioids.

Ipsilesional versus contralesional postural deficits induced by unilateral brain trauma: a side reversal by opioid mechanism.

Unilateral traumatic brain injury and stroke result in asymmetric postural and motor deficits including contralateral hemiplegia and hemiparesis. In animals, a localized unilateral brain injury recapitulates the human upper motor neuron syndrome in the formation of hindlimb postural asymmetry with contralesional limb flexion and the asymmetry of hindlimb nociceptive withdrawal reflexes. The current view is that these effects are developed due to aberrant activity of motor pathways that descend from the brain into the spinal cord. These pathways and their target spinal circuits may be regulated by local neurohormonal systems that may also mediate effects of brain injury. Here, we evaluate if a unilateral traumatic brain injury induces hindlimb postural asymmetry, a model of postural deficits, and if this asymmetry is spinally encoded and mediated by the endogenous opioid system in rats. A unilateral right-sided controlled cortical impact, a model of clinical focal traumatic brain injury was centred over the sensorimotor cortex and was observed to induce hindlimb postural asymmetry with contralateral limb flexion. The asymmetry persisted after complete spinal cord transection, implicating local neurocircuitry in the development of the deficits. Administration of the general opioid antagonist naloxone and μ-antagonist β-funaltrexamine blocked the formation of postural asymmetry. Surprisingly, κ-antagonists nor-binaltorphimine and LY2444296 did not affect the asymmetry magnitude but reversed the flexion side; instead of contralesional (left) hindlimb flexion the ipsilesional (right) limb was flexed. The postural effects of the right-side cortical injury were mimicked in animals with intact brain via intrathecal administration of the opioid κ-agonist (2)-(trans)-3,4-Dichloro-N-methyl-N-[2-(1-pyrrolidiny)-cyclohexyl]benzeneacetamide that induced hindlimb postural asymmetry with left limb flexion. The δ-antagonist naltrindole produced no effect on the contralesional (left) flexion but inhibited the formation of the ipsilesional (right) limb flexion in brain-injured rats that were treated with κ-antagonist. The effects of the antagonists were evident before and after spinal cord transection. We concluded that the focal traumatic brain injury-induced postural asymmetry was encoded at the spinal level, and was blocked or its side was reversed by administration of opioid antagonists. The findings suggest that the balance in activity of the mirror symmetric spinal neural circuits regulating contraction of the left and right hindlimb muscles is controlled by different subtypes of opioid receptors; and that this equilibrium is impaired after unilateral brain trauma through side-specific opioid mechanism.

Design of first in class bitopic ligands targeting the sodium binding pocket in opioid receptors

Opioid agonists have been used in the field of pain management for centuries, but are considered a major public health concern, especially in the US. One of the biggest challenges consists in understanding the ability to target specifically the different subtypes of opioid receptors and to trigger a specific functional response correlating with an in vivo analgesic response devoid of side‐effects.These opioid receptors are part of the GPCRs family, which is known to have common features across different subfamilies. One important feature of these receptors is the presence of allosteric binding pockets, which is progressively gaining momentum as a novel target for current therapeutic strategies. We hypothesized that targeting an allosteric site could potentially allow for receptor functional selectivity, helping to reduce the overall side effects profile. One site, in particular, known as the Na+ binding pocket, is known to play a key role in GPCR function and is thought to be the center of the functional mechanism, with several bias switches characterized in the pocket residues (Katritch et al., Trends Biochem Sci. 2014).Here we report structure‐based rational drug design of bitopic ligands targeting the Na+ pocket of mu (MOR) and kappa opioid receptors (KOR). Our designed bitopics show high affinity, subtype selectivity, G‐protein bias and analgesic actions in mice. Mutations in Na+ binding pocket diminished functional potency validating allosteric site targeting. Together, results suggest the possibility of achieving subtype selectivity and identification of an additional subpocket in both MOR and KOR to reduce βarrestin‐2 recruitment by targeting an allosteric pocket in the opioid receptors. The highly conserved constitution of the Na+ binding pocket opens the opportunity for rational discovery of new GPCR modulators with desired functional, and potentially therapeutic profile for opioids as well as other class A GPCRs.Support or Funding InformationWe gratefully acknowledge the financial support from Faculty Research Incentive Fund, NIDA, NIAAA and US department of defense. We also would like to thank the Philippe Foundation for their financial support.

SYNTHESIS, CHARACTERIZATION AND BIOLOGICAL PROPERTIES OF NEW CODEINE FE(III) COMPLEX

ABSTRACT The reaction of FeCl 3 hexahydrate with C 18 H 20 NO 3 .PO 4 in acetonitrile produced new complex with general formula [Fe(L)]Cl 2 , (L= codeine). Characterization of the ligand and its complex were made by elemental analysis, FT-IR, 1 H-NMR, Raman and UV–visible spectroscopy. This new complex showed antitumor activity against two kinds of cancer cell lines: AGS (Stomach) and MCF-7 (Breast). Cell cytotoxic effects of the ligand and its complex were assayed by MTT and Natural-red tests. The antibacterial activity of ligand and complex against Pseudomonas aeruginosa , Staphylococcus aureus , Streptococcus pyogenes and Escherichia coli were also examined. Keywords: Codeine; Fe(III) complex; Synthesis; Anti-tumor; AGS and MCF-7; Antibacterial. e-mail: mojdeh.golzani@gmail.com INTRODUCTION Natural products play an important role in diseases control and treatment. The plants extracts are used in the treatment of human diseases all the time. From the last century, a scientific interest for phototherapy increased in several medical fields such as immunology, hematology and oncology [1-5].Currently there are three major ways of treating cancer: radiation therapy, surgery and cytotoxic drugs. Application of plants in medicine has affected the identification of natural compounds such as cocaine, morphine, vinblastine, tax land codeine [5-8].Many different coordination compounds and the mechanism of cytotoxic action have been discussed with regard to the development of new antitumor agents [8-15]. Numerous medicinal plants have been screened extensively for their antimicrobial potential worldwide [1, 11]. Morphine and its derivatives have received considerable attention as narcotic and used in lenitive drudge products. Morphine plays an important role in the therapy of acute and chronic pain. Exogenously delivered morphine acts in the body by activating different subtypes of opioid receptors [14].Over the last decades, proposals have accumulated that morphine present in mammalian tissues and fluids comes not only from an exogenous or dietary origin, but may also be biosynthesized de novo. Latest studies have unequivocally proven the existence of a mammalian morphine biosynthetic pathway [15], which consists of at least 19 steps and is different in its primary steps from that occurring in plants [16]. Meanwhile the endogenous morphine biosynthesis takes place in mammals, their cells and tissues are constantly exposed to certain amounts of morphine and its precursors [17].Investigation of lithium effect in opioid receptors on inhibitory avoidance response in mice has been reported. The results suggest that the opioid receptors in the central nervous system may be involved in the retrieval of lithium state dependent learning [18]. The lithium efficacy in controlling behaviors related with dopamine stimulants and manic states has been proposed [19]. Thus, it may be suggested that lithium‘s action may be mediated through dopamine receptors [20].Furthermore interactions between lithium and opioids have been reported. There is evidence that lithium is able to affect some actions of morphine. In particular, it has been shown that lithium inhibits morphine withdrawal signs in morphine-dependent mice [21], reduces the self-stimulation facilitated by morphine and the expression of tolerance to morphine [22] and alters the morphine induced analgesia in mice [23]. Both drugs produce a conditioned taste avoidance response and suppress schedule-induced polydipsia [24]. Another morphine precursor codeine, which stands in the biosynthetic pathway after salutaridinol, had a higher partial agonist efficacy. Their metal chelates are of interest due to their fascinating chemistry as well as antimicrobial and anti-cancer activities. The other natural ligand from plants such as phenolic compound of mono and sesquiterpenoid, flavonoids also can be used in synthesis of metal complex [25].Codeine is generally named codeine phosphate (Cod.P), morphine-3-methyl ether phosphate, methyl-morphine phosphate, or morphine monomethyl ether phosphate. It takes the general formulae of C

Effects of activation of μ-, κ1-, δ1-, δ2-, and ORL1-receptors on heart resistance to the pathogenic action of delayed ischemia and reperfusion

Different subtypes of opioid receptors (OR) were activated in rats in vivo to study the activation effect on the heart’s resistance to ischemia and reperfusion. It has been established that administration of deltorphin II, a selective δ2-OR agonist, lowered the infarct size/area at risk index (IS/AAR) by 23%. Naltrexone, naloxone methiodide (an OR inhibitor not penetrating the blood-brain barrier (BBB)), and naltriben (δ2-antagonist) eliminated the cardioprotective effect of deltorphin II, while BNTX (a δ1-antagonist) produced no effect on the cardioprotective action of the δ2-agonist. The infarct-reducing effect of deltorphin II was eliminated by administration of chelerythrine (a protein kinase C (PKC) inhibitor), glibenclamide (a KATP-channels inhibitor), and 5-hydroxydecanoate (a mitochondrial KATP-channel blocker). Administration of other opioids did not reduce the IS/AAR index. It has been established that all the deltorphins manifest antiarrhythmic potency. Other opioids do not produce any effect on the incidence of arrhythmia occurrences. The antiarrhythmic effect of deltorphin II was eliminated by preliminary administration of naltrexone, naloxone methiodide, and naltriben, but BNTX did not affect the δ2-agonist’s anti-arrhythmic effect. The preliminary administration of chelerythrine, a PKC inhibitor, eliminated the δ2 agonist’s antiarrhythmic action. However, glibenclamide and 5-hydroxydecanoate did not alter the antiarrhythmic effect by deltorphin II. Therefore, activation of the peripheral δ2-ORs reduces the infarct size and prevents the onset of arrhythmias. The antiarrhythmic effect of the δ2-OR stimulation is mediated by activating PKC and opening the mitochondrial KATP-channels. PKC participates in the antiarrhythmic effect of the δ2-OR activation, but this effect does not depend on the condition of KATP-channels.

Roles of different subtypes of opioid receptors in mediating the ventrolateral orbital cortex opioid-induced inhibition of mirror-neuropathic pain in the rat

Previous studies have demonstrated that opioid receptors in the prefrontal ventrolateral orbital cortex (VLO) are involved in anti-nociception. The aim of this current study was to examine whether opioid receptors in the VLO have effects on the hypersensitivity induced by contralateral L5 and L6 spinal nerve ligation (SNL), termed as mirror neuropathic pain (MNP) in the male rat. Morphine (1.0, 2.5, 5.0 μg) microinjected into the VLO contralateral to the SNL depressed the mechanical paw withdrawal assessed by von Frey filaments and the cold plate (4 °C)-induced paw lifting in a dose-dependent manner on the side without SNL. These effects were antagonized by microinjection of the non-selective opioid receptor antagonist naloxone (1.0 μg) into the same VLO site. Microinjection of endomorphin-1 (5.0 μg), a highly selective μ-opioid receptor agonist, and [ d-Ala 2, d-Leu 5]-enkephalin (DADLE, 10 μg), a δ-/μ-receptor agonist, also depressed the MNP. The effects of both drugs were blocked by selective μ-receptor antagonist β-funaltrexamine (β-FNA, 3.75 μg), but the effect of the DADLE was not influenced by the selective δ-receptor antagonist naltrindole (5.0 μg). Microinjection of the κ-opioid receptor agonist spiradoline mesylate salt (U-62066) (100 μg) had no effect on the MNP. These results suggest that the VLO is involved in opioid-induced inhibition of the MNP and the effect is mediated by μ- (but not δ- and κ-) opioid receptors.

μ- but not δ- and κ-opioid receptors in the ventrolateral orbital cortex mediate opioid-induced antiallodynia in a rat neuropathic pain model

Previous studies have indicated that the ventrolateral orbital cortex (VLO) is involved in opioid-mediated antinociception in the tail flick test and formalin test. The aim of the current study was to examine the effect of opioids microinjected into the VLO on allodynia in the rat L5/L6 spinal nerve ligation (SNL) model of neuropathic pain and determine the roles of different subtypes of opioid receptors in this effect. The allodynia was assessed by both mechanical (von Frey filaments) and cold plate (4 °C) stimuli. Morphine (1.0, 2.5, and 5.0 μg) microinjected into the VLO contralateral to the nerve ligation dose-dependently depressed the mechanical and cold allodynia and these effects were reversed by nonselective opioid receptor antagonist naloxone (1.0 μg) administrated into the same site. Microinjection of endomorphin-1 (5.0 μg), a highly selective μ-opioid receptor agonist, and [ d-Ala 2, d-Leu 5]-enkephalin (DADLE, 10 μg), a δ-/μ-opioid receptor agonist, also depressed the allodynia, and the effects of both drugs were blocked by selective μ-receptor antagonist β-funaltrexamine (β-FNA, 3.75 μg), but the effects of DADLE were not influenced by the selective δ-receptor antagonist naltrindole (5.0 μg). Microinjection of U-62066 (100 μg), a κ-opioid receptor agonist, into the VLO had no effect on the allodynia. These results suggest that the VLO is involved in opioid-induced antiallodynia and μ- but not δ- and κ-opioid receptor mediates these effects in the rat with neuropathic pain.

μ- but not δ- and κ-opioid receptor mediates the nucleus submedius interferon-α-evoked antinociception in the rat

Previous studies have indicated that interferon-alpha (IFN-α) can bind to opioid receptors and exerts an antinociceptive effect in both peripheral and central nervous systems. The current study investigated the antinociceptive effect of IFN-α unilaterally microinjected into the thalamic nucleus submedius (Sm) of rats on noxious thermal stimulus, and the roles of different subtypes of opioid receptors in mediating the Sm IFN-α-evoked antinociception. The results indicated that unilateral microinjection of IFN-α (4, 8, 16 pmol) into the Sm dose-dependently increased the hind paw withdrawal latency from the noxious heat stimulus, and this effect was reversed by pretreatment with non-selective opioid receptor antagonist naloxone (200 pmol) and specific μ-opioid receptor antagonist β-FNA (1 nmol) into the same sites, whereas δ-opioid receptor antagonist ICI174,864 (1 nmol) and κ-opioid receptor antagonist nor-BNI (1 nmol) failed to alter the effect of IFN-α. These results suggest that Sm is involved in IFN-α-evoked antinociception and μ- but not δ- and κ-opioid receptor mediates the Sm IFN-α-evoked antinociception.

The roles of different subtypes of opioid receptors in mediating the nucleus submedius opioid-evoked antiallodynia in a neuropathic pain model of rats

Previous studies have indicated that thalamic nucleus submedius is involved in opioid-mediated antinociception in tail flick test and formalin test. The current study examined the effects of opioids microinjected into the thalamic nucleus submedius on the allodynia developed in neuropathic pain model rats, and determined the roles of different subtypes of opioid receptors in the thalamic nucleus submedius opioid-evoked antiallodynia. The allodynic behaviors induced by L5/L6 spinal nerve ligation were assessed by mechanical (von Frey filaments) and cold (4°C plate) stimuli. Morphine (1.0, 2.5, and 5.0μg) microinjected into the thalamic nucleus submedius contralateral to the nerve injury paw produced a dose-dependent inhibition of the mechanical and cold allodynia, and these effects were reversed by microinjection of the non-selective opioid receptor antagonist naloxone (1.0μg) into the same site. Microinjection of endomorphin-1 (5.0μg), a highly selective μ-opioid receptor agonist, and [d-Ala2, d-Leu5]-enkephalin (10μg), a δ-/μ-opioid receptor agonist, also inhibited the allodynic behaviors, and these effects were blocked by selective μ-opioid receptor antagonist β-funaltrexamine hydrochloride (3.75μg). However, the [d-Ala2, d-Leu5]-enkephalin-evoked antiallodynic effects were not influenced by the selective δ-opioid receptor antagonist naltrindole (5.0μg). Microinjection of the selective κ-receptor agonist spiradoline mesylate salt (100μg) into the thalamic nucleus submedius failed to alter the allodynia induced by spinal nerve ligation. These results suggest that the thalamic nucleus submedius is involved in opioid-evoked antiallodynia which is mediated by μ- but not δ- and κ-opioid receptor in the neuropathic pain model rats.

From inhibition to excitation: Functional effects of interaction between opioid receptors

Opioids have excitatory effects in multiple regions of the nervous system. Excitation by opioids is generally attributed to inhibition of inhibitory pathways (disinhibition). However, recent studies indicate that opioids can directly excite individual cells. These effects may occur when opioid receptors interact with other G protein coupled receptors, when different subtypes of opioid receptors interact, or when opioids transactivate other receptors such as receptor tyrosine kinases. Changes in the relative level of expression of different receptors in an individual cell may therefore determine its functional response to a given ligand. This phenomenon could represent an adaptive mechanism involved in tolerance, dependence and subsequent withdrawal.

μ but not δ and κ opioid receptor involvement in ventrolateral orbital cortex opioid-evoked antinociception in formalin test rats

The present study was designed to investigate the roles of different subtypes of opioid receptors in ventrolateral orbital cortex (VLO) opioid-evoked antinociception in formalin test by using an automatic detection system for recording the nociceptive behavior (agitation) and a manual method for detecting the duration of licking the injected paw in the conscious rat. Formalin (5%, 50 μl) s.c. injected into the hindpaw produced a biphasic agitation response or lengthening duration of licking. Morphine (5 μg) microinjected unilaterally into VLO significantly inhibited the agitation response and the licking time, and these effects were blocked by pre-administration of the non-selective opioid receptor antagonist naloxone (1.0 μg) into the same site. Microinjection of endomorphin-1 (5 μg), a selective μ-receptor agonist, and [ d-Ala 2, d-Leu 5]-enkephalin (DADLE, 10 μg), a δ-/μ-receptor agonist also inhibited the nociceptive behaviors, and both the effects were blocked by selective μ-receptor antagonist β-funaltrexamine hydrochloride (β-FNA; 3.75 μg), but the DADLE-evoked inhibition was not influenced by the selective δ-receptor antagonist naltrindole (5 μg). Microinjection of selective κ-receptor agonist (±)- trans-U-50488 methanesulfonate salt (1.5 μg) failed to alter the nociceptive behaviors induced by formalin injection. The β-FNA and naloxone applied into VLO and morphine into the adjacent regions ventral and dorsal to VLO had no effect on the formalin-evoked nociceptive behaviors. These results suggest that μ- but not δ- or κ-opioid receptor is involved in the VLO opioid-evoked antinociception in formalin test rat.

Multiple opioid receptors and GTP-binding proteins

We believed that GTP-binding protein (G-protein)-coupling receptor always transduces stimulatory signals to G-proteins. From our recent experiments using reconstitution techniques, however, it was revealed that some receptors transduce an inhibitory or no signal to G-proteins in specific tissues, despite some interaction between them. Here we discuss the molecular basis of mechanisms of such diverse modes of functional coupling between different subtypes of opioid receptors and G-proteins.

Relative affinities of the quaternary narcotic antagonist, N-methyl levallorphan (SR 58002), for different types of opioid receptors

The relative affinities for different subtypes of opioid receptors (μ, χ and δ) of the peripheral narcotic antagonist N-methyl levallorphan (SR 58002) have been studied in two in vitro smooth muscle systems (guinea-pig ileum and rabbit vas deferens) and by binding studies (guinea-pig brain and cerebellum membranes) using selective tritiated ligands. All the evidence obtained indicates that SR 58002 is a pure antagonist with relative affinity for μ receptors vs χ and δ superior to that of the parent tertiary compound, levallorphan.