Several delta-opioid receptor ligands display no subtype selectivity to the human delta-opioid receptor

IUPHAR Review - Bivalent and bifunctional opioid receptor ligands as novel analgesics

Though efficacious in managing chronic, severe pain, opioid analgesics are accompanied by significant adverse effects including constipation, tolerance, dependence, and respiratory depression. The life-threatening risks associated with µ opioid receptor agonist-based analgesics challenges their use in clinic. A rational approach to combatting these adverse effects is to develop agents that incorporate activity at a second pharmacologic target in addition to µ opioid receptor activation. The promise of such bivalent or bifunctional ligands is the development of an analgesic with an improved side effect profile. In this review, we highlight ongoing efforts in the development of bivalent and bifunctional analgesics that combine µ agonism with efficacy at κ and δ opioid receptors, the nociceptin opioid peptide (NOP) receptor, σ receptors, and cannabinoid receptors. Several examples of bifunctional analgesics in preclinical and clinical development are highlighted, as are strategies being employed toward the rational design of novel agents.

FBNTI, a DOR-Selective Antagonist That Allosterically Activates MOR within a MOR-DOR Heteromer

This report describes the unique pharmacological profile of FBNTI, a potent DOR antagonist that acts as a MOR agonist via an allosteric mechanism. Binding of FBNTI to opioid receptors expressed in HEK 293 cells revealed a 190-fold greater affinity for DOR (K_i = 0.84 nM) over MOR (K_i = 160 nM). In mice, intrathecal FBNTI produced potent antinociception (ED₅₀ = 46.9 pmol/mouse), which was antagonized by selective MOR antagonists (CTOP, β-FNA). Autoantagonism of the MOR agonism by FBNTI was observed above the ED₇₅ dose, suggesting antagonism of activated MOR. That FBNTI is devoid of agonism in DOR knockout mice is consistent with allosteric activation of the MOR protomer via FBNTI bound to within a MOR-DOR heteromer. This proposed mechanism is supported by calcium mobilization assays, which indicate that FBNTI selectively activates the MOR-DOR heteromer and functionally antagonizes the MOR protomer at >ED₇₅. The unprecedented mode of MOR activation by FBNTI may be responsible for the lack of tolerance after intrathecal (i.t.) administration. FBNTI was highly effective upon topical administration to the ipsolateral hind paw in the Hargreaves assay (EC₅₀ = 0.17 ± 0.08 μM) and without significant contralateral activity, suggesting a lack of systemic exposure.

Bifunctional opioid receptor ligands as novel analgesics

Prolonged treatment of chronic severe pain with opioid analgesics is frought with problematic adverse effects including tolerance, dependence, and life-threatening respiratory depression. Though these effects are mediated predominately through preferential activation of μ opioid peptide (μOP) receptors, there is an emerging appreciation that actions at κOP and δOP receptors contribute to the observed pharmacologic and behavioral profile of μOP receptor agonists and may be targeted simultaneously to afford improved analgesic effects. Recent developments have also identified the related nociceptin opioid peptide (NOP) receptor as a key modulator of the effects of μOP receptor signaling. We review here the available literature describing OP neurotransmitter systems and highlight recent drug and probe design strategies.

Structural Requirements for CNS Active Opioid Glycopeptides

Glycopeptides related to β-endorphin penetrate the blood-brain barrier (BBB) of mice to produce antinociception. Two series of glycopeptides were assessed for opioid receptor binding affinity. Attempts to alter the mu-selectivity of [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO)-related glycopeptides by altering the charged residues of the amphipathic helical address were unsuccessful. A series of pan-agonists was evaluated for antinociceptive activity (55 °C tail flick) in mice. A flexible linker was required to maintain antinociceptive activity. Circular dichroism (CD) in H2O, trifluoroethanol (TFE), and SDS micelles confirmed the importance of the amphipathic helices (11s → 11sG → 11) for antinociception. The glycosylated analogues showed only nascent helices and random coil conformations in H2O. Chemical shift indices (CSI) and nuclear Overhauser effects (NOE) with 600 MHz NMR and CD confirmed helical structures in micelles, which were rationalized by molecular dynamics calculations. Antinociceptive studies with mice confirm that these glycosylated endorphin analogues are potential drug candidates that penetrate the BBB to produce potent central effects.

Can amphipathic helices influence the CNS antinociceptive activity of glycopeptides related to β-endorphin?

Glycosylated β-endorphin analogues of various amphipathicity were studied in vitro and in vivo in mice. Opioid binding affinities of the O-linked glycopeptides (mono- or disaccharides) and unglycosylated peptide controls were measured in human receptors expressed in CHO cells. All were pan-agonists, binding to μ-, δ-, or κ-opioid receptors in the low nanomolar range (2.2-35 nM K(i)'s). The glycoside moiety was required for intravenous (i.v.) but not for intracerebroventricular (i.c.v.) activity. Circular dichroism and NMR indicated the degree of helicity in H2O, aqueous trifluoroethanol, or micelles. Glycosylation was essential for activity after i.v. administration. It was possible to manipulate the degree of helicity by the alteration of only two amino acid residues in the helical address region of the β-endorphin analogues without destroying μ-, δ-, or κ-agonism, but the antinociceptive activity after i.v. administration could not be directly correlated to the degree of helicity in micelles.

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

Delta opioid receptors (DORs) have been considered as a potential target to relieve pain as well as treat depression and anxiety disorders and are known to modulate other physiological responses, including ethanol and food consumption. A small number of DOR-selective drugs are in clinical trials, but no DOR-selective drugs have been approved by the Federal Drug Administration and some candidates have failed in phase II clinical trials, highlighting current difficulties producing effective delta opioid-based therapies. Recent studies have provided new insights into the pharmacology of the DOR, which is often complex and at times paradoxical.

OBJECTIVE

This review will discuss the existing literature focusing on four aspects: (1) Two DOR subtypes have been postulated based on differences in pharmacological effects of existing DOR-selective ligands. (2) DORs are expressed ubiquitously throughout the body and central nervous system and are, thus, positioned to play a role in a multitude of diseases. (3) DOR expression is often dynamic, with many reports of increased expression during exposure to chronic stimuli, such as stress, inflammation, neuropathy, morphine, or changes in endogenous opioid tone. (4) A large structural variety in DOR ligands implies potential different mechanisms of activating the receptor.

CONCLUSION

The reviewed features of DOR pharmacology illustrate the potential benefit of designing tailored or biased DOR ligands.

Modulation of gastrointestinal function by MuDelta, a mixed µ opioid receptor agonist/ µ opioid receptor antagonist

BACKGROUND & PURPOSE

Loperamide is a selective µ opioid receptor agonist acting locally in the gastrointestinal (GI) tract as an effective anti-diarrhoeal but can cause constipation. We tested whether modulating µ opioid receptor agonism with δ opioid receptor antagonism, by combining reference compounds or using a novel compound ('MuDelta'), could normalize GI motility without constipation.

EXPERIMENTAL APPROACH

MuDelta was characterized in vitro as a potent µ opioid receptor agonist and high-affinity δ opioid receptor antagonist. Reference compounds, MuDelta and loperamide were assessed in the following ex vivo and in vivo experiments: guinea pig intestinal smooth muscle contractility, mouse intestinal epithelial ion transport and upper GI tract transit, entire GI transit or faecal output in novel environment stressed mice, or four weeks after intracolonic mustard oil (post-inflammatory). Colonic δ opioid receptor immunoreactivity was quantified.

KEY RESULTS

δ Opioid receptor antagonism opposed µ opioid receptor agonist inhibition of intestinal contractility and motility. MuDelta reduced intestinal contractility and inhibited neurogenically-mediated secretion. Very low plasma levels of MuDelta were detected after oral administration. Stress up-regulated δ opioid receptor expression in colonic epithelial cells. In stressed mice, MuDelta normalized GI transit and faecal output to control levels over a wide dose range, whereas loperamide had a narrow dose range. MuDelta and loperamide reduced upper GI transit in the post-inflammatory model.

CONCLUSIONS AND IMPLICATIONS

MuDelta normalizes, but does not prevent, perturbed GI transit over a wide dose-range in mice. These data support the subsequent assessment of MuDelta in a clinical phase II trial in patients with diarrhoea-predominant irritable bowel syndrome.

The delta opioid receptor antagonist, SoRI-9409, decreases yohimbine stress-induced reinstatement of ethanol-seeking

A major problem in treating alcohol use disorders (AUDs) is the high rate of relapse due to stress and re-exposure to cues or an environment previously associated with alcohol use. Stressors can induce relapse to alcohol-seeking in humans or reinstatement in rodents. Delta opioid peptide receptors (DOP-Rs) play a role in cue-induced reinstatement of ethanol-seeking; however, their role in stress-induced reinstatement of ethanol-seeking is not known. The objective of this study was to determine the role of DOP-Rs in yohimbine-stress-induced reinstatement of ethanol-seeking. Male, Long-Evans rats were trained to self-administer 10% ethanol in daily 30-minute operant self-administration sessions using a FR3 schedule of reinforcement, followed by extinction training. Once extinction criteria were met, we examined the effects of the DOP-R antagonist, SoRI-9409 (0-5 mg/kg, i.p.) on yohimbine (2 mg/kg, i.p.) stress-induced reinstatement. Additionally, DOP-R-stimulated [(35) S]GTPγS binding was measured in brain membranes and plasma levels of corticosterone (CORT) were determined. Pre-treatment with SoRI-9409 decreased yohimbine stress-induced reinstatement of ethanol-seeking but did not affect yohimbine-induced increases in plasma CORT levels. Additionally, yohimbine increased DOP-R-stimulated (35) [S]GTPγS binding in brain membranes of ethanol-trained rats, an effect that was inhibited by SoRI-9409. This suggests that the DOP-R plays an important role in yohimbine-stress-induced reinstatement of ethanol-seeking behavior, and DOP-R antagonists may be promising candidates for further development as a treatment for AUDs.

Emergence of functional spinal delta opioid receptors after chronic ethanol exposure

BACKGROUND

The delta opioid receptor (DOR) is a promising target to treat multiple indications, including alcoholism, anxiety, and nonmalignant pain. The potential of the DORs has been underappreciated, in part, due to relatively low functional expression of these receptors in naïve states. However, chronic exposure to stress, opioids, and inflammation can induce a redistribution of DORs to the cell surface where they can be activated. Previously, DORs were shown to be selectively/exclusively present in spinal cord circuits mediating mechanical sensitivity but not those mediating thermal nociception under naïve conditions.

METHODS

We spinally administered DOR and mu opioid receptor (MOR) selective agonists ([D-Pen2,D-Pen5]-Enkephalin, deltorphin II, SNC80, and DAMGO) and antagonists (naltriben and CTAP) and determined thermal antinociception and mechanical sensitivity in wild-type mice or mice with a genetic disruption of DOR or MOR. Thermal antinociception was measured using a radiant heat tail-flick assay; mechanical sensitivity was measured using von Frey filaments. Dose response curves were generated in naïve mice and mice exposed to ethanol in a model of voluntary consumption.

RESULTS

We show that prolonged exposure to ethanol can promote an upregulation of functional DORs in the spinal cord in thermal pain-mediating circuits but not in those mediating mechanical sensitivity. The upregulated DORs either modulate MOR-mediated analgesia through convergence of circuits or signal transduction pathways and/or interact directly with MORs to form a new functional (heteromeric) unit.

CONCLUSIONS

Our findings suggest that DORs could be a novel target in conditions in which DORs are redistributed.

Phosphorylation of enkephalins: NMR and CD studies in aqueous and membrane-mimicking environments

Phosphorylation of l-serine-containing enkephalin analogs has been explored as an alternative to glycosylation in an effort to increase blood-brain barrier permeability and CNS bioavailability of peptide pharmacophores. Two enkephalin-based peptides were modified for these studies, a set related to DTLES, a mixed μ/δ-agonist, and one related to DAMGO, a highly selective μ-agonist. Each unglycosylated peptide was compared to its phosphate, its mono-benzylphosphate ester, and its β-d-glucoside. Binding was characterized in membrane preparations from Chinese hamster ovary cells expressing human μ, δ and κ-opiate receptors. Antinociception was measured in mice using the 55 °C tail-flick assay. To estimate bioavailability, the antinociceptive effect of each opioid agonist was evaluated after intracerebroventricular (i.c.v.) or intravenous administration (i.v.) of the peptides. Circular dichroism methods and high-field nuclear magnetic resonance were used in the presence and absence of sodium dodecylsulfate to understand how the presence of a membrane might influence the peptide conformations.

Brain P450 epoxygenase activity is required for the antinociceptive effects of improgan, a nonopioid analgesic

The search for the mechanism of action of improgan (a nonopioid analgesic) led to the recent discovery of CC12, a compound that blocks improgan antinociception. Because CC12 is a cytochrome P450 inhibitor, and brain P450 mechanisms were recently shown to be required in opioid analgesic signaling, pharmacological and transgenic studies were performed in rodents to test the hypothesis that improgan antinociception requires brain P450 epoxygenase activity. Intracerebroventricular (i.c.v.) administration of the P450 inhibitors miconazole and fluconazole, and the arachidonic acid (AA) epoxygenase inhibitor N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH) potently inhibited improgan antinociception in rats at doses that were inactive alone. MW06-25, a new P450 inhibitor that combines chemical features of CC12 and miconazole, also potently blocked improgan antinociception. Although miconazole and CC12 were weakly active at opioid and histamine H(3) receptors, MW06-25 showed no activity at these sites, yet retained potent P450-inhibiting properties. The P450 hypothesis was also tested in Cpr(low) mice, a viable knock-in model with dramatically reduced brain P450 activity. Improgan (145 nmol, i.c.v.) antinociception was reduced by 37% to 59% in Cpr(low) mice, as compared with control mice. Moreover, CC12 pretreatment (200 nmol, i.c.v.) abolished improgan action (70% to 91%) in control mice, but had no significant effect in Cpr(low) mice. Thus, improgan's activation of bulbospinal nonopioid analgesic circuits requires brain P450 epoxygenase activity. A model is proposed in which (1) improgan activates an unknown receptor to trigger downstream P450 activity, and (2) brainstem epoxygenase activity is a point of convergence for opioid and nonopioid analgesic signaling.

The novel δ opioid receptor agonist KNT-127 produces antidepressant-like and antinociceptive effects in mice without producing convulsions

We previously reported that the δ opioid receptor (DOP) agonists SNC80 and TAN-67 produce potent antidepressant-like and antinociceptive effects in rodents. However, SNC80 produced convulsive effects. Recently, we succeeded in synthesizing a novel DOP agonist called KNT-127. The present study examined the convulsive, antidepressant-like, and antinociceptive effects of KNT-127 in mice. In contrast to SNC80, KNT-127 produced no convulsions at doses of up to 100mg/kg. In mice subjected to the forced swim test, a screening model for antidepressants, KNT-127 (1mg/kg, s.c.) significantly decreased the duration of immobility and increased the duration of swimming without influencing spontaneous locomotor activity. These behavioral changes were similar to that observed for the tricyclic antidepressant imipramine (6mg/kg). The antidepressant-like effect of KNT-127 in mice was antagonized by pretreatment with naltrindole (NTI), a selective DOP antagonist, or naltriben, a putative DOP(2) subtype antagonist. In addition, KNT-127 (3mg/kg, s.c.) significantly reduced the number of acetic acid-induced abdominal constrictions and the duration of licking time, respectively, in mice subjected to a writhing test and a formalin test. These antinociceptive effects were antagonized by pretreatment with either NTI or 7-benzylidenenaltrexone, a putative DOP(1) subtype antagonist. We propose that KNT-127 should be considered as a candidate compound for the development of DOP-based antidepressants and/or analgesics that lack convulsive effects.

Univalent and bivalent ligands of butorphan: characteristics of the linking chain determine the affinity and potency of such opioid ligands

Bivalent morphinan compounds containing ester linkers were synthesized and their binding affinities at the mu, delta, and kappa opioid receptors determined. Addition of methyl groups adjacent to the hydrolytically labile ester linkage increased stability while only partially affecting binding affinity. The resulting bivalent ligands with optimized spacer length and structure show potent binding profiles with the most potent compound (4b), having K(i) values of 0.47 nM for both the mu and kappa opioid receptors, and 4a, having K(i) values of 0.95 and 0.62 nM for the mu and kappa receptors, respectively. Both 4a and 4b were partial agonists at the kappa and micro receptors in the [(35)S]GTPgammaS binding assay.

The delta(1) opioid receptor is a heterodimer that opposes the actions of the delta(2) receptor on alcohol intake

BACKGROUND

Opioid receptors are clinically important targets for both pain and alcohol abuse. Three opioid receptors have been cloned: mu, delta, and kappa, all of which effect alcohol consumption in animal models. Naltrexone is a nonselective opioid antagonist used for alcoholism, the clinical utility of which is limited by poor efficacy and adverse side effects. Here, we demonstrate that the therapeutic limitations of naltrexone may reflect its poor selectivity. Despite decades of research, several mysteries surround the pharmacology of these receptors. For example, two pharmacologically defined subtypes of delta receptors exist in vivo.

METHODS

Effects of delta subtype-selective ligands (naltrindole, naltriben, tan-67, 7-benzylidene naltrexone) were measured on ethanol consumption in C57BL/6 wildtype and opioid receptor knockout mice using a limited access two-bottle choice paradigm. Affinity and efficacy of naltriben, 7-benzylidenenaltrexone and tan-67 was measured in vitro using radioligand binding and Ca(2+)-mobilizationa assays.

RESULTS

We show that the subtypes of the delta receptor, delta(1) and delta(2), have opposing effects on ethanol consumption. We find that these effects are synergistic; thereby suggesting that delta(1) and delta(2) receptors are distinct molecular targets. Indeed, we provide both in vitro as well as in vivo evidence that the delta(1) subtype is a micro-delta heterodimer and that the delta(2) subtype is most likely a delta homomer.

CONCLUSIONS

Together these data provide insight into the limited actions of the clinically important drug naltrexone and identify a novel target with improved specificity and efficacy for the development of new therapeutics for the treatment of alcoholism.

Extracellular biotransformation of beta-endorphin in rat striatum and cerebrospinal fluid

Numerous studies have investigated the behavioural effects of beta-endorphin, both endogenous and exogenously applied. However, the potential for biotransformation of beta-endorphin in the extracellular space of the brain has not been previously directly addressed in vivo. Utilising microinfusion/microdialysis and matrix-assisted laser desorption/ionisation mass spectrometry, we investigated beta-endorphin biotransformation in the striatum of rats. We infused 1.0 nmol beta-endorphin into the striatum of adult male Fischer rats and observed rapid cleavage resulting in beta-endorphin 1-18, as well as several fragments resulting from further N-terminal degradation. In vitro studies with incubation of full-length beta-endorphin, with and without protease inhibitors, in the incubation fluid of isolated striatal slices indicate that beta-endorphin is initially cleaved predominantly at the Phe(18)-Lys(19), position, as well as at the Leu(17)-Phe(18) position. Investigations of cerebrospinal fluid revealed similar enzymatic cleavage of beta-endorphin. The observed pattern of cleavage sites (Phe(18)-Lys(19) and Leu(17)-Phe(18)) is consistent with published in vitro studies of purified insulin-degrading enzyme cleavage of beta-endorphin. The binding affinities of full-length beta-endorphin, as well as previously identified beta-endorphin fragments alpha-endorphin (beta-endorphin 1-16) and gamma-endorphin (beta-endorphin 1-17), and the fragment identified in the present study, beta-endorphin 1-18, at heterologously expressed mu, delta and kappa-opioid receptors, respectively, were determined; the affinity of the truncation fragments is reduced at each of the receptors compared to the affinity of full length beta-endorphin.

?-Opioid Receptor Ligands Lack Receptor Subtype Selectivity in the Aequorin Luminescence-based Calcium Assay

The aim of the present study was to characterize the binding selectivity of the mu-opioid receptor ligands, endomorphin-1, endomorphin-2, and DAMGO, in the in vitro functional assay, based on the changes in intracellular calcium levels. For the experiments Chinese hamster ovary cells, stably expressing human mu-receptor, were used. The mu-agonist-induced calcium responses were significantly inhibited by naloxone, an opioid antagonist with high preference for the mu-opioid receptors. Naloxonazine, a mu1-non-peptide antagonist, inhibited the effect of all tested mu-agonists. However, there was no significant difference in the antagonist effect of naloxonazine on the calcium response induced by mu1- (endomorphin-2) and mu2-agonists (endomorphin-1, DAMGO). [D-Pro2]endomorphin-1 and [D-Pro2]endomorphin-2, putative peptide mu2- and mu1-antagonists, respectively, which had been shown in vivo to inhibit the antinociception induced by mu-agonists, produced no inhibitory effect in our in vitro experiments. Our results demonstrated that there is only one population of the mu-opioid receptors expressed in the Chinese hamster ovary cells. We suggest that the mu-opioid receptors form a homogenous population in the in vitro systems. However, the existence of mu-receptor subtypes in vivo is still pharmacologically possible.

Synthesis and Biological Evaluation of N-Substituted Quinolinimides, as Potential Ligands for in Vivo Imaging Studies of δ-Opioid Receptors

We report here the syntheses of N-substituted quinolinimide derivatives displaying sufficient affinity and high selectivity for delta-opioid receptors. Among 9-subsituted derivatives, one showed much higher selectivity for the delta receptor in binding assays than the delta antagonist methylnaltrindole (6: Ki = 42 nM; micro/delta and kappa/delta > 238 on rat brain membranes) and antagonist properties. This compound was labeled with carbon-11 (t1/2 = 20.4 min) as a potential radioligand for the noninvasive assessment of delta opioid receptors in vivo with positron emission tomography (PET). A high yielding radiosynthesis of [11C]-6, based on the [11C]methyl introduction on the pyridine moiety by a Stille reaction, was described (radiochemical yield = 60 +/- 10%, specific activities = 0.8 to 1.5 Ci/micromol). The in vivo pharmacological profile in rats indicated that the radiotracer crossed the blood-brain barrier but was not stable and underwent rapid degradation in both plasma and brain. No specific binding was consequently revealed.

Opioid receptors and signaling on cells from the immune system

This review discusses the criteria for determining whether a binding site or functional response is directly mediated by either the mu, delta, or kappa opioid receptors. In 1988, Sibinga and Goldstein published the first review that addressed whether cells from the immune system express opioid receptors. The criteria that they used, namely, structure-activity relationships, stereoselectivity, dose- and concentration-dependence, and saturability are still relevant criteria today for determining if an immunological response is mediated by either the mu, delta or kappa opioid receptors. Radioligand receptor binding studies and functional studies that clearly show the presence of an opioid receptor on immunocytes are presented. Selective agonists and antagonists for the mu, delta, and kappa opioid receptors are discussed, and the need for their use in experiments is emphasized. Conditions used in functional assays are very important. Receptor desensitization and downregulation occur within minutes after the application of an agonist. However, many immunological assays are applying an agonist for days before measuring an immunological effect. The results obtained may reflect changes that are results of receptor desensitization and/or downregulation instead of changes that are observed with acute activation of the receptor. The future of receptor pharmacology lies in the crosstalk and dimerization of G protein-coupled receptors. In transfected systems, opioid receptors have been shown to dimerize with chemokine and cannabinoid receptors, resulting in crosstalk between different types of receptors.

Alkylation of opioid receptors by 5′-naltrindole-isothiocyanate injected into the nucleus accumbens of rats: Receptor selectivity and anatomical diffusion

Subtypes of the delta opioid receptor (Oprd1) have been suggested based on pharmacology studies. However, these subtypes have not been confirmed biochemically using either receptor binding assays or molecular cloning. Naltrindole-5'-isothiocyanate (5'-NTII) is an irreversible opioid antagonist that appears to selectively inhibit the actions of a subset of delta opioid agonists in vivo, referred to as putative delta-2 agonists. The biochemical and anatomical selectivity of wash-resistant inhibition of binding of [(3)H]DAMGO (Oprm1), [(3)H]DPDPE (Oprd1, putative subtype 1 agonist), or [(3)H]deltorphin II (Oprd1, putative subytpe 2 agonist) in coronal sections was assessed using quantitative in vitro autoradiography following injection of 5'-NTII into the nucleus accumbens in rats. 5'-NTII decreased [(3)H]deltorphin II to a greater extent than the binding of the other two radioligands following administration of 0.05-2.5 nmol. The effects of 5'-NTII were largely confined to the nucleus accumbens; however, some loss in the ventral caudate was also noted. In contrast, administration of the nonselective opioid receptor alkylating antagonist beta-chlornaltexamine (beta-CNA) over a similar range of doses was found to be nonselective for either delta radioligand, and produced greater inhibition of Oprm1 relative to Oprd1 binding, consistent with the nonselective pharmacological activity of this antagonist. Although 5'-NTII inhibited [(3)H]deltorphin II binding to a greater extent, the binding of the other two radioligands was decreased over a similar range of doses. Absolute conclusions regarding the involvement of delta-2 opioid receptors in pharmacological or physiological effects based on studies with 5'-NTII should therefore be tempered, and for site-directed studies it would be best to employ doses of 0.5 nmol or lower.

The human herpesvirus 6 G protein-coupled receptor homolog U51 positively regulates virus replication and enhances cell-cell fusion in vitro

Human herpesvirus 6 (HHV-6) is a ubiquitous T-lymphotropic betaherpesvirus that encodes two G protein-coupled receptor homologs, U12 and U51. HHV-6A U51 has been reported to bind to CC chemokines including RANTES, but the biological function of U51 remains uncertain. In this report, we stably expressed short interfering RNAs (siRNAs) specific for U51 in human T cells and then infected these cells with HHV-6. Viral DNA replication was reduced 50-fold by the U51 siRNA, and virally induced cytopathic effects were also inhibited. In contrast, viral replication and syncytium formation were unaltered in cells that expressed a scrambled derivative of the siRNA or an irrelevant siRNA and were restored to normal when a human codon-optimized derivative of U51 was introduced into cells containing the U51 siRNA. To examine the mechanism whereby U51 might contribute to viral replication, we explored the signaling characteristics of U51. None of the chemokines and opioids tested was able to induce G protein coupling by U51, and no evidence for opioid ligand binding by U51 was obtained. The effect of U51 on cell-cell fusion was also evaluated; these studies showed that U51 enhanced cell fusion mediated by the G protein of vesicular stomatitis virus. However, a U51-specific antiserum had no virus-neutralizing activity, suggesting that U51 may not be involved in the initial interaction between the virus particle and host cell. Overall, these studies suggest that HHV-6 U51 is a positive regulator of virus replication in vitro, perhaps because it may promote membrane fusion and facilitates cell-cell spread of this highly cell-associated virus.

Comparison between delta-opioid receptor functional response and autoradiographic labeling in rat brain and spinal cord

The distribution of delta-opioid receptors (DORs) in the rat central nervous system has been previously characterized by radioligand binding and immunohistochemistry. However, the functional neuroanatomy of DORs has not been mapped in any detail; this is potentially important, because these receptors appear to be primarily cytosolic. Opioid receptors can couple to G(i/o) G proteins, a process that is detected by agonist-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) binding. The purpose of this study was therefore to determine the distribution of functional DORs, as assessed by [35S]GTPgammaS autoradiographic labeling in response to the DOR agonist deltorphin II. For comparison, adjacent sections were labeled with [125I]deltorphin II or the DOR antagonist [125I]AR-M100613. In all three assays, mu-opioid receptors were blocked pharmacologically. The distributions of [125I]deltorphin II and [125I]AR-M100613 were highly correlated but not identical. Deltorphin II increased [35S]GTPgammaS binding in a concentration-dependent and naltrindole-sensitive manner. The regional [35S]GTPgammaS response to deltorphin II was only moderately predicted by agonist or antagonist radioligand binding (r = 0.67 and 0.50, respectively). [35S]GTPgammaS responses to deltorphin II were strongest in the extended striatum (caudate putamen, nucleus accumbens, olfactory tubercle) and cerebral cortex. In contrast, some areas reported to mediate DOR analgesia (brainstem, spinal cord) possessed a much lower [35S]GTPgammaS response. These findings demonstrate the existence of a partial mismatch between DOR radioligand binding and [35S]GTPgammaS response. This divergence possibly reflects regional heterogeneity in G-protein receptor coupling, or in the subcellular localization of DOR.

Comparison of immunoblotted delta opioid receptor proteins expressed in the adult rat brain and their regulation by growth hormone

It has previously been suggested that exogenous growth hormone (GH) affect quality of life and higher brain functions through the endogenous opioid system. Recently, we showed that GH down-regulate 72 and 48 kDa delta opioid receptor (DOR) proteins in the adult rat cerebral cortex and cerebellum. In the present study, we found that an antiserum raised against the N-terminus of the DOR also recognizes a 36 kDa protein, not recognized by a C-terminus-directed antiserum. We aimed to investigate the identity of the 72, 48 and 36 kDa proteins and to further study the effects of GH on their expression in different brain regions. The expression was studied in hypophysectomized (Hx) and untreated normal female rats. One subgroup of Hx rats received GH as a daily subcutaneous injection for 19 days. Our data show that treatment with GH in Hx rats normalized the expression of the 72 kDa protein in the cerebral cortex, whereas no significant effect were observed for the 48 or 36 kDa proteins. However, GH significantly reduced the ratio between the 72 and 36 kDa proteins in different brain regions of Hx rats. Our data suggest that GH reduces the levels of a 72 kDa DOR that likely represents a dimeric form of a 36 kDa DOR post-translationally truncated at the C-terminus, and that altered receptor dimerization may be involved in GH induced effects in the central nervous system.

Pharmacological characterization of a 7-benzylidenenaltrexone-preferring opioid receptor in porcine ileal submucosa

In the intestine, opioids produce antidiarrhoeal and constipating actions that are mediated by enteric neurones. Through interactions with opioid receptors (ORs) on submucosal neurones, opioids suppress active ion transport evoked by transmural electrical stimulation (TES) in mucosa-submucosa sheets from the porcine ileum. In this study, we examined the pharmacological characteristics of the previously described OR, which is sensitive to the delta1-OR antagonist 7-benzylidenenaltrexone and modulates neurogenic transepithelial ion transport in this tissue preparation. Increases in short-circuit current (Isc, a measure of active anion transport) evoked by TES in ileal mucosa-submucosa sheets were inhibited by opioid agonists possessing high selectivity for either delta- or micro-ORs including [d-Pen2,5]enkephalin (DPDPE), [d-Ala2, Glu4]deltorphin II, and [d-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (DAMGO). As determined by the Schild analysis, the actions of these agonists were competitively inhibited by 7-benzylidenenaltrexone. The nonequilibrium micro-OR antagonist beta-funaltrexamine inhibited the actions of DAMGO only at a high concentration (1 microm) but did not alter DPDPE or deltorphin II action. At concentrations up to 10 microm, the nonequilibrium delta-OR antagonist naltrindole 5'-isothiocyanate did not alter the actions of delta- or micro-OR agonists. Radioligand binding analyses of neuronal homogenates from the ileal submucosa revealed that the nonselective OR ligand [3H]diprenorphine bound to two populations of specific binding sites. One of these sites possessed binding characteristics similar to the delta-OR. In summary, neurogenic ion transport in the porcine intestine is modulated by an OR which shares pharmacological characteristics of both micro- and delta-ORs and may represent a novel receptor entity.

Phylogenetic changes in the expression of delta opioid receptors in spinal cord and dorsal root ganglia

To assess the validity of rodent models for investigating the role of delta opioid receptors (DOR) in analgesia, the distribution of DOR binding and mRNA were compared between rodent and primate spinal cord and dorsal root ganglia (DRG), using receptor autoradiography and in situ hybridization, respectively. In mouse and rat spinal cord, [(125)I]-deltorphin-labeled DOR binding sites were detected throughout the gray matter. In contrast, in primate and particularly in human spinal cord, DOR binding was mainly present in laminae I-II, with little to no binding in deeper layers. Accordingly, in rodent spinal cord, DOR mRNA was expressed by a large number of neurons distributed throughout the ventral and dorsal horns, whereas in the primate, DOR expression was significantly lower, as evidenced by a moderate number of labeled cells throughout the gray matter in monkey and by only few labeled cells in human, mainly in Clarke's column and lamina IX. Major species differences in DOR expression were also observed in primary afferent cells bodies. In rat DRG, intense DOR mRNA hybridization was primarily observed over large ganglion cells immunopositive for neurofilament 200. In contrast, in monkey and human DRG, DOR mRNA was primarily detected over small and medium-sized ganglion cells. These results demonstrate major differences in the expression and distribution of DOR in the spinal cord and DRG between mammalian species. Specifically, they point to a progressive specialization of DOR toward the regulation of primary somatosensory, namely nociceptive, inputs during phylogeny and suggest that the effects of DOR agonists in rodents may not be entirely predictive of their action in humans.