Opioid Receptor-like Receptor Research Articles

A Visible‐Light‐Mediated Radical Smiles Rearrangement and its Application to the Synthesis of a Difluoro‐Substituted Spirocyclic ORL‐1 Antagonist

AbstractA visible‐light‐mediated radical Smiles rearrangement has been developed to address the challenging synthesis of the gem‐difluoro group present in an opioid receptor‐like 1 (ORL‐1) antagonist that is currently in development for the treatment of depression and/or obesity. This method enables the direct and efficient introduction of the difluoroethanol motif into a range of aryl and heteroaryl systems, representing a new disconnection for the synthesis of this versatile moiety. When applied to the target compound, the photochemical step could be conducted on 15 g scale using industrially relevant [Ru(bpy)3Cl2] catalyst loadings of 0.01 mol %. This transformation is part of an overall five‐step route to the antagonist that compares favorably to the current synthetic sequence and demonstrates, in this specific case, a clear strategic benefit of photocatalysis.

A Visible-Light-Mediated Radical Smiles Rearrangement and its Application to the Synthesis of a Difluoro-Substituted Spirocyclic ORL-1 Antagonist.

A visible-light-mediated radical Smiles rearrangement has been developed to address the challenging synthesis of the gem-difluoro group present in an opioid receptor-like 1 (ORL-1) antagonist that is currently in development for the treatment of depression and/or obesity. This method enables the direct and efficient introduction of the difluoroethanol motif into a range of aryl and heteroaryl systems, representing a new disconnection for the synthesis of this versatile moiety. When applied to the target compound, the photochemical step could be conducted on 15 g scale using industrially relevant [Ru(bpy)3Cl2] catalyst loadings of 0.01 mol %. This transformation is part of an overall five-step route to the antagonist that compares favorably to the current synthetic sequence and demonstrates, in this specific case, a clear strategic benefit of photocatalysis.

Orphanin FQ-ORL-1 regulation of reproduction and reproductive behavior in the female.

Orphanin FQ (OFQ/N) and its receptor, opioid receptor-like receptor-1 (ORL-1), are expressed throughout steroid-responsive limbic and hypothalamic circuits that regulate female ovarian hormone feedback and reproductive behavior circuits. The arcuate nucleus of the hypothalamus (ARH) is a brain region that expresses OFQ/N and ORL-1 important for both sexual behavior and modulating estradiol feedback loops. Within the ARH, the activation of the OFQ/N-ORL-1 system facilitates sexual receptivity (lordosis) through the inhibition of β-endorphin neuronal activity. Estradiol initially activates ARH β-endorphin neurons to inhibit lordosis. Simultaneously, estradiol upregulates coexpression of OFQ/N and progesterone receptors and ORL-1 in ARH β-endorphin neurons. Ovarian hormones regulate pre- and postsynaptic coupling of ORL-1 to its G protein-coupled signaling pathways. When the steroid-primed rat is nonreceptive, estradiol acts pre- and postsynaptically to decrease the ability of the OFQ/N-ORL-1 system to inhibit ARH β-endorphin neurotransmission. Conversely, when sexually receptive, ORL-1 signaling is restored to inhibit β-endorphin neurotransmission. Although steroid signaling that facilitates lordosis converges to deactivate ARH β-endorphin neurons, estradiol-only facilitation of lordosis requires the activation of ORL-1, but estradiol+progesterone does not, indicating that multiple circuits mediate ovarian hormone signaling to deactivate ARH β-endorphin neurons. Research on the role of OFQ/N-ORL-1 in ovarian hormone feedback loops is just beginning. In the rat, OFQ/N may act to terminate gonadotropin-releasing hormone and luteinizing hormone release under positive and negative feedbacks. In the ewe, it appears to directly inhibit gonadotropin-releasing hormone release to mediate progesterone-negative feedback. As a whole, the localization and actions of OFQ/N-ORL-1 system indicate that it may mediate the actions of estradiol and progesterone to synchronize reproductive behavior and ovarian hormone feedback loops.

Nociceptin effect on intestinal motility depends on opioid-receptor like-1 receptors and nitric oxide synthase co-localization.

To study the effect of the opioid-receptor like-1 (ORL1) agonist nociceptin on gastrointestinal (GI) myenteric neurotransmission and motility. Reverse transcriptase - polymerase chain reaction and immunohistochemistry were used to localize nociceptin and ORL1 in mouse tissues. Intracellular electrophysiological recordings of excitatory and inhibitory junction potentials (EJP, IJP) were made in a chambered organ bath. Intestinal motility was measured in vivo. Nociceptin accelerated whole and upper GI transit, but slowed colonic expulsion in vivo in an ORL1-dependent manner, as shown using [Nphe(1)]NOC and AS ODN pretreatment. ORL1 and nociceptin immunoreactivity were found on enteric neurons. Nociceptin reduced the EJP and the nitric oxide-sensitive slow IJP in an ORL1-dependent manner, whereas the fast IJP was unchanged. Nociceptin further reduced the spatial spreading of the EJP up to 2 cm. Compounds acting at ORL1 are good candidates for the future treatment of disorders associated with increased colonic transit, such as diarrhea or diarrhea-predominant irritable bowel syndrome.

Nocistatin: milestone of one decade of research.

A neuropeptide nociceptin or orphanin FQ (N/OFQ) is an endogenous ligand for the orphan opioid receptor-like receptor. During studies on the analysis of the precursor of N/OFQ, we identified a novel neuropeptide produced from the same precursor and named it "nocistatin (NST)". Intrathecal (i.t.) administration of N/OFQ induces pain responses including touch-evoked allodynia and thermal hyperalgesia, and simultaneous administration of NST blocks the allodynia and hyperalgesia induced by N/OFQ. In the years since these discoveries, N/OFQ has been shown to be involved in a wide range of pharmacological activities, such as relaying pain perception in peripheral tissues, to the central nervous system, and NST was shown to have opposite effects on various central functions evoked by N/OFQ. Pharmacological characterization using various neurotransmitter agents, agonists, antagonists and knockout mice in vivo; electrophysiological and immunohistological analysis ex vivo; and molecular cloning using affinity chromatography of high-performance affinity nanobeads; and protein processing measurement using bioluminescence resonance energy transfer (BRET) in vitro have generated new insights into pain transmission regulated by NST and N/OFQ. This review focuses on the molecular and cellular mechanisms of pain transmission regulated by NST.

A selective small molecule NOP (ORL-1 receptor) partial agonist for the treatment of anxiety

Small molecule (1) has been identified as a selective partial agonist of Opioid Receptor Like-1 (ORL-1) with potential utility for the treatment of anxiety and other disorders. Nociceptin (orphanin FQ) is an endogenous peptide ligand that binds to ORL-1, however it does not bind the classical δ, μ and κ opioid receptors with high affinity. The synthesis of 1 involved using a molecular diversity approach, to rapidly advance a library of compounds for biological testing. A lead selective potent partial agonist (35-fold ORL-1/Mu) progressed to ORL-1 (NOP or OP4) proof of concept testing in advanced studies. The synthetic approach and biological data for the related chemical series will be presented.

Tritium-labelled isovaleryl-RYYRIK-NH2 as potential antagonist probe for ORL1 nociceptin receptor

IsoVa-RYYRIK-NH2 is a highly specific antagonist ligand of the opioid receptor-like 1 (ORL1) receptor, an endogenous ligand of which is 17-mer peptide nociceptin. ORL1 antagonists have potential for clinical use as analgesic and antineuropathic drugs, and thus information on the receptor-binding characteristics of antagonists is very important for rational drug design. In the present study, we prepared tritium-labelled isova-RYYRIK-NH2 from its precursor with the 3-methylcrotonyl (CH3)2CCHCO group by a catalytic reduction using tritium gas. The resulting [3H]isoVa-RYYRIK-NH2 was evaluated in a saturation binding assay using the COS-7 cell membrane preparations of transiently expressed ORL1. It exhibited more than 90% specific binding with a dissociation constant of 1.21±0.03nM. From the mutual heterologous binding assays using [3H]isoVa-RYYRIK-NH2 and [3H]nociceptin, isoVa-RYYRIK-NH2 and nociceptin were found to share the receptor-binding site, but each also had a separate specific binding site of its own. They differentiated the two different binding states or conformations of ORL1, which might represent the agonist-active and antagonist-inactive conformations of ORL1. [3H]isoVa-RYYRIK-NH2 is thus a key tracer to uncover the amino acid residues important for receptor inactivation.

Rewarding or aversive effects of buprenorphine/naloxone combination (Suboxone) depend on conditioning trial duration

Buprenorphine is used as a sublingual medication in the treatment of opioid dependence. However, its misuse by i.v. injection may limit its acceptability and dissemination. A buprenorphine/naloxone (ratio 4:1) combination has been developed to reduce diversion and abuse. So far, the relevance of this combination has not been investigated in the animal models traditionally used to study the reinforcing effects of drugs of abuse. The aim of this study was to compare the rewarding effects, assessed by conditioned place preference (CPP), of buprenorphine and buprenorphine/naloxone combination following i.v. administration in mice. Animals were treated with different doses of buprenorphine or buprenorphine/naloxone combination (ratio 4:1), and CPP conditioning trial duration was 5 or 30 min. At the longest trial duration, a bell-shaped dose-response curve was obtained with buprenorphine, which was shifted significantly to the right with naloxone combination. At the shortest trial duration, an aversive effect was observed with the buprenorphine/naloxone combination in animals, involving opioid receptor-like 1 (ORL1). These findings may explain the discrepancies reported in the literature as some authors have shown a reduced buprenorphine/naloxone misuse compared to buprenorphine in opioid abusers, while others have not.

Activation of membrane estrogen receptors attenuates opioid receptor-like1 receptor-mediated antinociception via an ERK-dependent non-genomic mechanism

To our knowledge, the present data are the first to demonstrate that activation of membrane estrogen receptors (mERs) abolishes opioid receptor-like 1 (ORL1) receptor-mediated analgesia via extracellular signal-regulated kinase (ERK)-dependent non-genomic mechanisms. Estrogen was shown previously to both attenuate ORL1-mediated antinociception and down-regulate the ORL1 gene expression. The present study investigated whether non-genomic mechanisms contribute to estrogen-induced attenuation of ORL1-mediated antinociception by the mERs GPR30, Gq-coupled mER, ERα, and ERβ. E2BSA [β-estradiol-6-(O-carboxymethyl)oxime: bovine serum albumin] (0.5mM), a membrane impermeant analog of estradiol, injected intrathecally immediately prior to orphanin FQ (OFQ;10nmol), the endogenous ligand for the ORL1 receptor, abolished OFQ’s antinociceptive effect in both male and ovariectomized (OVX) female rats, assessed using the heat-induced tail-flick assay. This effect was not altered by protein synthesis inhibitor, anisomycin (125μg), given intrathecally 15min prior to E2BSA and OFQ. Intrathecal application of selective receptor agonists permitted the relative contributions of various estrogen receptors in mediating this blockade of the antinociceptive response of OFQ. Activation of GPR30, Gq-mER, ERα, but not ERβ abolished ORL1-mediated antinociception in males and OVX females. E2BSA produced a parallel and significant increase in the phosphorylation of ERK 2 only in OVX females, and pre-treatment with MEK/ERK 1/2 inhibitor, U0126 (10μg), blocked the mER-mediated abolition of ORL1-mediated antinociception in OVX females. Taken together, the data are consistent with the interpretations that mER activation attenuates ORL1-mediated antinociception through a non-genomic, ERK 2-dependent mechanism in females.

Selective κ Opioid Antagonists nor-BNI, GNTI and JDTic Have Low Affinities for Non-Opioid Receptors and Transporters

BackgroundNor-BNI, GNTI and JDTic induce selective κ opioid antagonism that is delayed and extremely prolonged, but some other effects are of rapid onset and brief duration. The transient effects of these compounds differ, suggesting that some of them may be mediated by other targets.ResultsIn binding assays, the three antagonists showed no detectable affinity (K i≥10 µM) for most non-opioid receptors and transporters (26 of 43 tested). There was no non-opioid target for which all three compounds shared detectable affinity, or for which any two shared sub-micromolar affinity. All three compounds showed low nanomolar affinity for κ opioid receptors, with moderate selectivity over μ and δ (3 to 44-fold). Nor-BNI bound weakly to the α2C-adrenoceptor (K i = 630 nM). GNTI enhanced calcium mobilization by noradrenaline at the α1A-adrenoceptor (EC50 = 41 nM), but did not activate the receptor, displace radioligands, or enhance PI hydrolysis. This suggests that it is a functionally-selective allosteric enhancer. GNTI was also a weak M1 receptor antagonist (K B = 3.7 µM). JDTic bound to the noradrenaline transporter (K i = 54 nM), but only weakly inhibited transport (IC50 = 1.1 µM). JDTic also bound to the opioid-like receptor NOP (K i = 12 nM), but gave little antagonism even at 30 µM. All three compounds exhibited rapid permeation and active efflux across Caco-2 cell monolayers.ConclusionsAcross 43 non-opioid CNS targets, only GNTI exhibited a potent functional effect (allosteric enhancement of α1A-adrenoceptors). This may contribute to GNTI's severe transient effects. Plasma concentrations of nor-BNI and GNTI may be high enough to affect some peripheral non-opioid targets. Nonetheless, κ opioid antagonism persists for weeks or months after these transient effects dissipate. With an adequate pre-administration interval, our results therefore strengthen the evidence that nor-BNI, GNTI and JDTic are highly selective κ opioid antagonists.

Estradiol Negatively Modulates the Pleiotropic Actions of Orphanin FQ/Nociceptin at Proopiomelanocortin Synapses

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of proopiomelanocortin (POMC) neurons located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behavior and energy balance. We tested the hypothesis that estradiol modulates the ability of OFQ/N to pre- and postsynaptically decrease the excitability of these cells. To this end, whole-cell patch-clamp recordings were performed in hypothalamic slices prepared from ovariectomized rats, including some that were injected with the retrograde tracer Fluorogold in the medial preoptic nucleus (MPN) to label the POMC neurons regulating sexual receptivity. OFQ/N (1 µ<smlcap>M</smlcap>) evoked a robust outward current in ARH neurons from vehicle-treated animals that was blocked by the opioid receptor-like (ORL)1 receptor antagonist UFP-101 (100 n<smlcap>M</smlcap>) and the G protein-gated, inwardly rectifying K⁺ (GIRK-1) channel blocker tertiapin (10 n<smlcap>M</smlcap>). OFQ/N also produced a decrease in the frequency of glutamatergic, miniature excitatory postsynaptic currents (mEPSCs), which was also antagonized by UFP-101. Estradiol benzoate (2 µg) increased basal mEPSC frequency and markedly diminished both the OFQ/N-induced activation of postsynaptic GIRK-1 channel currents and the presynaptic inhibition of glutamatergic neurotransmission. These effects were observed in identified POMC neurons, including eight that projected to the MPN. Taken together, these data reveal that estradiol attenuates the pleiotropic inhibitory actions of OFQ/N on POMC neurons: presynaptically through reducing the OFQ/N inhibition of glutamate release and postsynaptically by reducing ORL1 signaling through GIRK channels. As such, they impart critical insight into a mechanism for estradiol to increase the activity of POMC neurons that inhibit sexual receptivity.

Evaluation of [18F]MK-0911, a positron emission tomography (PET) tracer for opioid receptor-like 1 (ORL1), in rhesus monkey and human

Antagonism of the central opioid receptor like-1 receptor (ORL1) has been implicated in cognition, and has been a focus of drug discovery efforts to ameliorate the cognitive deficits that remain during the stable treatment of schizophrenia with current antipsychotics. In order to facilitate dose selection for phase II clinical testing an ORL1-specific PET tracer was developed to determine drug plasma concentration versus occupancy relationships in order to ensure that the doses selected and the degree of target engagement were sufficient to ensure adequate proof of concept testing. MK-0911 is a selective, high affinity antagonist for the ORL1 receptor radiolabeled with high specific activity (18)F for positron emission tomography (PET) studies. Evaluation of [(18)F]MK-0911 in rhesus monkey PET studies showed a pattern of brain uptake which was consistent with the known distribution of ORL1. In vitro autoradiography with [(18)F]MK-0911 in rhesus monkey and human brain tissue slices showed a regional distribution that was consistent with in vivo imaging results in monkey. Pre-treatment of rhesus monkeys with high doses of structurally diverse ORL1 antagonists MK-0584, MK-0337, or MK-5757 achieved blockade of [(18)F]MK-0911 in all gray matter regions. Baseline PET studies with [(18)F]MK-0911 in healthy human subjects showed tracer distribution and kinetics similar to that observed in rhesus monkey. Quantification of [(18)F]MK-0911 uptake in repeat human baseline PET studies showed a test-retest variability in volume of distribution (V(T)) averaging 3% across brain regions. Humans dosed orally with MK-5757 showed reduced [(18)F]MK-0911 tracer concentration in brain proportional with MK-5757 dose and plasma level. [(18)F]MK-0911 was useful for determining MK-5757-induced receptor occupancy of ORL1 to guide MK-5757 dose-selection for clinical proof-of-concept studies. Additionally, [(18)F]MK-0911 may be a useful tool for studying the pharmacology of ORL1 in various human populations and disease states.

Case Study. Transdermal buprenorphine controls central neuropathic pain

A 53-year-old male with peripheral sensorimotor neuropathy suffered an intracerebral hemorrhage resulting in right hemiparesis and hemisensory loss. Three months later, he developed constant and burning pain within the entire right side of his body. He was diagnosed with central pain syndrome and treated with antiepileptics and tricyclic antidepressants. Minimal analgesia was achieved, which was limited by intractable sedation and drowsiness. Patient was then treated with oral opioids (morphine and hydrocodone with acetaminophen) in escalating doses that produced cognitive impairment. After an opioid rotation was attempted, by switching morphine to transdermal fentanyl, there was no pain reduction or improved quality of life. A trial of buprenorphine was initiated, by administering transdermal patches in escalating doses in weekly intervals. Patient’s pain was eventually successfully controlled with buprenorphine patch 60 µg/h every 7 days. His self-reported Visual Analogue Scale pain scores decreased from an average of 8/10 to 2/10 or less. Patient’s overall function and participation in home activities increased. Buprenorphine is a partial µ-receptor and a ?-? receptor antagonist known to block NMDA receptors and reduce hyperalgesia secondary to central sensitization.1 Buprenorphine is also a partial agonist at the opioid receptor-like (ORL-1) receptor, which is found to be analgesic and antinociceptive at the level of the spinal cord.1,2 The difference in analgesic responses between buprenorphine and other opioids may be due to different receptor G protein interactions and/or selective activation of neuronal KATP channels by buprenorphine. 3 Deficient opening of KATP channels has been shown to mediate neuropathic pain4; therefore, activation of these channels by buprenorphine may contribute to its analgesic effect in neuropathic pain states wherein other opioids fail. More recently, there have been two case reports in which patients with neuropathic pain of different central etiology were successfully treated with buprenorphine. 5 Despite advances in understanding the pathology related to central pain, effective treatment options are limited. Buprenorphine may be an analgesic option for central pain management when opioids fail to reduce hypersensitivity or when patients exhibit intolerable side effects to other medications. Keywords: buprenorphine, central pain, neuropathic pain, transdermal, analgesia, opioid DOI:10.5055/jom.2012.0141

Case Study. Transdermal buprenorphine controls central neuropathic pain

A 53-year-old male with peripheral sensorimotor neuropathy suffered an intracerebral hemorrhage resulting in right hemiparesis and hemisensory loss. Three months later, he developed constant and burning pain within the entire right side of his body. He was diagnosed with central pain syndrome and treated with antiepileptics and tricyclic antidepressants. Minimal analgesia was achieved, which was limited by intractable sedation and drowsiness. Patient was then treated with oral opioids (morphine and hydrocodone with acetaminophen) in escalating doses that produced cognitive impairment. After an opioid rotation was attempted, by switching morphine to transdermal fentanyl, there was no pain reduction or improved quality of life. A trial of buprenorphine was initiated, by administering transdermal patches in escalating doses in weekly intervals. Patient's pain was eventually successfully controlled with buprenorphine patch 60 μg/h every 7 days. His self-reported Visual Analogue Scale pain scores decreased from an average of 8/10 to 2/10 or less. Patient's overall function and participation in home activities increased. Buprenorphine is a partial μ-receptor and a κ-δ receptor antagonist known to block NMDA receptors and reduce hyperalgesia secondary to central sensitization.(1) Buprenorphine is also a partial agonist at the opioid receptor-like (ORL-1) receptor, which is found to be analgesic and antinociceptive at the level of the spinal cord.(1,2) The difference in analgesic responses between buprenorphine and other opioids may be due to different receptor G protein interactions and/or selective activation of neuronal K(ATP) channels by buprenorphine.(3) Deficient opening of K(ATP) channels has been shown to mediate neuropathic pain(4); therefore, activation of these channels by buprenorphine may contribute to its analgesic effect in neuropathic pain states wherein other opioids fail. More recently, there have been two case reports in which patients with neuropathic pain of different central etiology were successfully treated with buprenorphine.(5) Despite advances in understanding the pathology related to central pain, effective treatment options are limited. Buprenorphine may be an analgesic option for central pain management when opioids fail to reduce hypersensitivity or when patients exhibit intolerable side effects to other medications.

Homology modeling and molecular dynamics simulations of the active state of the nociceptin receptor reveal new insights intoagonist binding and activation

The opioid receptor-like receptor, also known as the nociceptin receptor (NOP), is a class A G protein-coupled receptor (GPCR) in the opioid receptor family. Although NOP shares a significant homology with the other opioid receptors, it does not bind known opioid ligands and has been shown to have a distinct mechanism of activation compared to the closely related opioid receptors mu, delta, and kappa. Previously reported homology models of the NOP receptor, based on the inactive-state GPCR crystal structures, give limited information on the activation and selectivity features of this fourth member of the opioid receptor family. We report here the first active-state homology model of the NOP receptor based on the opsin GPCR crystal structure. An inactive-state homology model of NOP was also built using a multiple template approach. Molecular dynamics simulation of the active-state NOP model and comparison to the inactive-state model suggest that NOP activation involves movements of transmembrane (TM)3 and TM6 and several activation microswitches, consistent with GPCR activation. Docking of the selective nonpeptidic NOP agonist ligand Ro 64-6198 into the active-state model reveals active-site residues in NOP that play a role in the high selectivity of this ligand for NOP over the other opioid receptors. Docking the shortest active fragment of endogenous agonist nociceptin/orphaninFQ (residues 1-13) shows that the NOP extracellular loop 2 (EL2) loop interacts with the positively charged residues (8-13) of N/OFQ. Both agonists show extensive polar interactions with residues at the extracellular end of the TM domain and EL2 loop, suggesting agonist-induced reorganization of polar networks, during receptor activation.

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

BackgroundBuprenorphine is an opioid receptor ligand whose mechanism of action is incompletely understood. MethodsUsing Ca2+ imaging, we assessed the effects of buprenorphine, β-endorphin, and morphine on cytosolic Ca2+ concentration [Ca2+]i, in rat striatal neurons. ResultsBuprenorphine (0.01–1μM) increased [Ca2+]i in a dose-dependent manner in a subpopulation of rat striatal neurons. The effect of buprenorphine was largely reduced by naloxone, a non-selective opioid receptor antagonist, but not by μ, κ, δ or NOP-selective antagonists. β-Endorphin (0.1μM) increased [Ca2+]i with a lower amplitude and slower time course than buprenorphine. Similar to buprenorphine, the effect of β-endorphin was markedly decreased by naloxone, but not by opioid-selective antagonists. Morphine (0.1–10μM), did not affect [Ca2+]i in striatal neurons. ConclusionsOur results suggest that buprenorphine and β-endorphin act on a distinct type/subtype of plasmalemmal opioid receptors or activate intracellular opioid-like receptor(s) in rat striatal neurons.

Molecular Mechanisms of Opioid Receptor-dependent Signaling and Behavior

Opioid receptors have been targeted for the treatment of pain and related disorders for thousands of years and remain the most widely used analgesics in the clinic. Mu (μ), kappa (κ), and delta (δ) opioid receptors represent the originally classified receptor subtypes, with opioid receptor like-1 (ORL1) being the least characterized. All four receptors are G-protein coupled and activate inhibitory G proteins. These receptors form homo- and heterodimeric complexes and signal to kinase cascades and scaffold a variety of proteins.The authors discuss classic mechanisms and developments in understanding opioid tolerance and opioid receptor signaling and highlight advances in opioid molecular pharmacology, behavioral pharmacology, and human genetics. The authors put into context how opioid receptor signaling leads to the modulation of behavior with the potential for therapeutic intervention. Finally, the authors conclude there is a continued need for more translational work on opioid receptors in vivo.

Orphanin FQ in the mediobasal hypothalamus facilitates sexual receptivity through the deactivation of medial preoptic nucleus mu-opioid receptors

Sexual receptivity, lordosis, can be induced by sequential estradiol and progesterone or extended exposure to high levels of estradiol in the female rat. In both cases estradiol initially inhibits lordosis through activation of β-endorphin (β-END) neurons of the arcuate nucleus of the hypothalamus (ARH) that activate μ-opioid receptors (MOP) in the medial preoptic nucleus (MPN). Subsequent progesterone or extended estradiol exposure deactivates MPN MOP to facilitate lordosis. Opioid receptor-like receptor-1 (ORL-1) is expressed in ARH and ventromedial hypothalamus (VMH). Infusions of its endogenous ligand, orphanin FQ (OFQ/N, aka nociceptin), into VMH–ARH region facilitate lordosis. Whether OFQ/N acts in ARH and/or VMH and whether OFQ/N is necessary for steroid facilitation of lordosis are unclear. In Exp I, OFQ/N infusions in VMH and ARH that facilitated lordosis also deactivated MPN MOP indicating that OFQ/N facilitation of lordosis requires deactivation of ascending ARH-MPN projections by directly inhibiting ARH β-END neurons and/or through inhibition of excitatory VMH–ARH pathways to proopiomelanocortin neurons. It is unclear whether OFQ/N activates the VMH output motor pathways directly or via the deactivation of MPN MOP. In Exp II we tested whether ORL-1 activation is necessary for estradiol-only or estradiol+progesterone lordosis facilitation. Blocking ORL-1 with UFP-101 inhibited estradiol-only lordosis and MPN MOP deactivation but had no effect on estradiol+progesterone facilitation of lordosis and MOP deactivation. In conclusion, steroid facilitation of lordosis inhibits ARH β-END neurons to deactivate MPN MOP, but estradiol-only and estradiol+progesterone treatments appear to use different neurotransmitter systems to inhibit ARH-MPN signaling.

TH-030418: a potent long-acting opioid analgesic with low dependence liability

Numerous efforts have been made on the chemical modification of opioid compounds, with the ultimate goal of developing new opioid analgesics that is highly potent and low/non-addictive. In a search for such compounds, TH-030418 [7α-[(R)-1-hydroxy-1-methyl-3-(thien-3-yl)-propyl]-6,14-endo-ethanotetrahydrooripavine] was synthesized. Here, we evaluated the pharmacological activities of TH-030418, in comparison with morphine, the prototype opioid analgesic. In radioligand binding assays, TH-030418 bound potently and nonselectively to μ-, δ-, κ-, and ORL1 (opioid receptor-like 1) receptors stably expressed in CHO (Chinese hamster ovary) cells with K (i) values of 0.56, 0.73, 0.60, and 1.55nM, respectively. When administered subcutaneously, TH-030418 was much more potent than morphine in analgesia, with the ED(50) values of 1.37μg/kg and 1.70μg/kg in hot plate and acetic acid writhing tests, respectively. The opioid antagonist naloxone blocked the antinociceptive effect of TH-030418, indicating that the action of TH-030418 was mediated by opioid receptors. The antinociceptive effect of s.c. TH-030418 in hot plate test lasted for more than 12h, which is much longer than those of morphine (2.5h) and dihydroetorphine (1.5h). In addition, naloxone did not precipitate withdrawal syndrome in the mice treated with TH-030418 previously. Most importantly, TH-030418 did not induce conditioned place preference in mice after chronic treatment. These results indicate that TH-030418 is a potent long-acting opioid analgesic with low dependence liability and may be of some value in the development of new analgesics.

The role of nociceptin and dynorphin in chronic pain: Implications of neuro–glial interaction

Nociceptin-opioid peptide (NOP) receptor, also known as opioid receptor like-1 (ORL1), was identified following the cloning of the kappa-opioid peptide (KOP) receptor, and the characterization of these receptors revealed high homology. The endogenous ligand of NOP, nociceptin (NOC), which shares high homology to dynorphin (DYN), was discovered shortly thereafter, and since then, it has been the subject of several investigations. Despite the many advances in our understanding of the involvement of NOC and DYN systems in pain, tolerance and withdrawal, the precise function of these systems has not been fully characterized. Here, we review the recent literature concerning the distribution of the NOC and DYN systems in the central nervous system and the involvement of these systems in nociceptive transmission, especially under chronic pain conditions. We discuss the use of endogenous and exogenous ligands of NOP and KOP receptors in pain perception, as well as the potential utility of NOP ligands in clinical practice for pain management. We also discuss the modulation of opioid effects by NOC and DYN. We emphasize the important role of neuro-glial interactions in the effects of NOC and DYN, focusing on their presence in neuronal and non-neuronal cells and the changes associated with chronic pain conditions. We also present the dynamics of immune and glial regulation of neuronal functions and the importance of this regulation in the roles of NOC and DYN under conditions of neuropathic pain and in the use of drugs that alter these systems for better control of neuropathic pain.