Binding characteristics of the novel highly selective delta agonist, [3H]IIe5,6deltorphin II

Biochemical and pharmacological investigation of novel nociceptin/OFQ analogues and N/OFQ-RYYRIK hybrid peptides

The endogenous ligand nociceptin (N/OFQ) and a positively charged synthetic peptide RYYRIK are both selective for the nociceptin opioid receptor (NOPr). Despite their structural dissimilarity, N/OFQ and RYYRIK compete for the same binding site of NOP receptor possessing full and partial agonistic character, respectively. In the view of the message-address concept, hybrid peptide constructs were probed for the NOP receptor combining different regions of N/OFQ and RYYRIK related peptide sequences. Nine novel nociceptin- or Ac-RYYRIK-NH2 peptide variants or hybrid peptides were synthesized and characterized. Peptides P2 and P8 contain fragments of native N/OFQ. The other seven analogues (P1, P3-7, P9) are composed of Ac-RYYRIK-NH2 fragments and parts of the original nociceptin sequence. The analogues were characterized in receptor binding assays and G-protein activation experiments on rat brain membranes, as well as by electrically stimulated mouse vas deferens bioassay. In receptor binding assays ligands P2, P4, P6 (Ki 0.37 nM) and P7 showed higher affinity (Ki 0.65 nM, 0.6 nM, 0.37 nM and 0.44 nM, respectively) for NOP receptor than their parent compounds N/OFQ (Ki 2.8 nM) or Ac-RYYRIK-NH2 (Ki 4.2 nM). In [35S]GTPγS binding experiments P2 and P3 behaved as full agonists. The other variants exhibited partial agonist properties characterized by submaximal stimulatory effects. In mouse vas deferens bioassay only P2 showed agonist activity. P4, P5, P6 inhibited the biological activity of N/OFQ more effectively than the NOP receptor selective antagonist JTC-801. In summary, hybrid peptides P4, P5 and P6 proved to be NOP receptor partial agonists even antagonists, while P2 peptide retained the full agonist property.

14-O-Methylmorphine: A Novel Selective Mu-Opioid Receptor Agonist with High Efficacy and Affinity

14-O-methyl (14-O-Me) group in morphine-6-O-sulfate (M6SU) or oxymorphone has been reported to be essential for enhanced affinity, potency and antinociceptive effect of these opioids. Herein we report on the pharmacological properties (potency, affinity and efficacy) of the new compound, 14-O-methylmorphine (14-O-MeM) in in vitro. Additionally, we also investigated the antinociceptive effect of the novel compound, as well as its inhibitory action on gastrointestinal transit in in vivo. The potency and efficacy of test compound were measured by [³⁵S]GTPγS binding, isolated mouse vas deferens (MVD) and rat vas deferens (RVD) assays. The affinity of 14-O-MeM for opioid receptors was assessed by radioligand binding and MVD assays. The antinociceptive and gastrointestinal effects of the novel compound were evaluated in the rat tail-flick test and charcoal meal test, respectively. Morphine, DAMGO, Ile^5,6 deltorphin II, deltorphin II and U-69593 were used as reference compounds. 14-O-MeM showed higher efficacy (E_max) and potency (EC₅₀) than morphine in MVD, RVD or [³⁵S]GTPγS binding. In addition, 14-O-MeM compared to morphine showed higher affinity for μ-opioid receptor (MOR). In vivo, in rat tail-flick test 14-O-MeM proved to be stronger antinociceptive agent than morphine after peripheral or central administration. Additionally, both compounds inhibited the gastrointestinal peristalsis. However, when the antinociceptive and antitransit doses for each test compound are compared, 14-O-MeM proved to have slightly more favorable pharmacological profile. Our results affirm that 14-O-MeM, an opioid of high efficacy and affinity for MOR can be considered as a novel analgesic agent of potential clinical value.

New opioid receptor antagonist: Naltrexone-14-O-sulfate synthesis and pharmacology

Opioid antagonists, naloxone and naltrexone have long been used in clinical practice and research. In addition to their low selectivity, they easily pass through the blood-brain barrier. Quaternization of the amine group in these molecules, (e.g. methylnaltrexone) results in negligible CNS penetration. In addition, zwitterionic compounds have been reported to have limited CNS access. The current study, for the first time gives report on the synthesis and the in vitro [competition binding, G-protein activation, isolated mouse vas deferens (MVD) and mouse colon assay] pharmacology of the zwitterionic compound, naltrexone-14-O-sulfate. Naltrexone, naloxone, and its 14-O-sulfate analogue were used as reference compounds. In competition binding assays, naltrexone-14-O-sulfate showed lower affinity for µ, δ or κ opioid receptor than the parent molecule, naltrexone. However, the μ/κ opioid receptor selectivity ratio significantly improved, indicating better selectivity. Similar tendency was observed for naloxone-14-O-sulfate when compared to naloxone. Naltrexone-14-O-sulfate failed to activate [³⁵S]GTPγS-binding but inhibit the activation evoked by opioid agonists (DAMGO, Ile^5,6deltorphin II and U69593), similarly to the reference compounds. Schild plot constructed in MVD revealed that naltrexone-14-O-sulfate acts as a competitive antagonist. In mouse colon, naltrexone-14-O-sulfate antagonized the inhibitory effect of morphine with lower affinity compared to naltrexone and higher affinity when compared to naloxone or naloxone-14-O-sulfate. In vivo (mouse tail-flick test), subcutaneously injected naltrexone-14-O-sulfate antagonized morphine's antinociception in a dose-dependent manner, indicating it's CNS penetration, which was unexpected from such zwitter ionic structure. Future studies are needed to evaluate it's pharmacokinetic profile.

Novel cyclic biphalin analogue with improved antinociceptive properties

Two novel opioid analogues have been designed by substituting the native d-Ala residues in position 2,2' of biphalin with two residues of d-penicillamine or l-penicillamine and by forming a disulfide bond between the thiol groups. The so-obtained compound 9 containing d-penicillamines showed excellent μ/δ mixed receptor affinities (K i (δ) = 5.2 nM; K i (μ) = 1.9 nM), together with an efficacious capacity to trigger the second messenger and a very good in vivo antinociceptive activity, whereas product 10 was scarcely active. An explanation of the two different pharmacological behaviors of products 9 and 10 was found by studying their conformational properties.

Micromolar concentrations of rimonabant directly inhibits delta opioid receptor specific ligand binding and agonist-induced G-protein activity

WHAT IS KNOWN

There is a growing number of evidence showing, that the cannabinoid receptor 1 (CB1) antagonist rimonabant has many non-cannabimimetic actions, such as affecting the opioid system. The direct effect of rimonabant on opioid receptors has been studied so far mainly on μ-opioid receptors. However recently the δ-opioid receptor (DOR) receives much more attention as before, due to its potential therapeutic applications, such as nociception or treatment for psychiatric disorders.

OBJECTIVES

To investigate the direct effect of rimonabant on DOR specific ligand binding and on the DOR mediated G-protein activation.

RESULTS

Micromolar concentrations of rimonabant directly inhibited the DOR specific agonist binding in radioligand competition binding experiments using Chinese hamster ovary cells stably transfected with mouse DOR (CHO-mDOR). However the inhibition occurred also in the subnanomolar range during DOR specific antagonist binding in similar experimental conditions. In functional [(35)S]GTPγS binding assays rimonabant significantly decreased the basal receptor activity in CHO-mDOR but also in parental CHO cell membranes. During DOR agonist stimulation, micromolar concentration of rimonabant attenuated the DOR G-protein activation and the potency of the activator ligand in [(35)S]GTPγS binding assays performed in CHO-mDOR, in wild type and also in CB1/CB2 double knock-out mouse forebrain membranes. Yet again this inhibitory action was DOR specific, since it did not occur during other specific GPCR agonist mediated G-protein activation.

CONCLUSION

Rimonabant directly inhibited DOR function in the micromolar concentrations. The inhibitory actions indicate an antagonistic behavior towards DOR which was established by the followings: (i) rimonabant inhibited DOR antagonist binding more effectively than agonist binding, (ii) the inverse agonistic, agonistic effect of the compound can be excluded, and (iii) additionally according to previous findings the allosteric mechanism can also be foreclosed.

Kainic acid-induced seizure activity alters the mRNA expression and G-protein activation of the opioid/nociceptin receptors in the rat brain cortex

The opioid/nociceptin receptors are involved in many neurological disorders such as Alzheimer's disease, Parkinson's disease and epilepsy. Kainic acid (KA) is an analog of the excitatory amino acid transmitter glutamate and the systemic administration of KA induces status epilepticus (SE) in rodents. In this study, we examined the alterations in the G-protein activity and the gene expression levels of mu, kappa, delta opioid and nociceptin receptors (MOPr, KOPr, DOPr and NOPr) as well as PNOC, the precursor polypeptide of nociceptin-OFQ (N/OFQ) in KA-induced seizures in the rat brain cortex. KA was used to create seizures with the dose of 10 mg/kg body weight i.p. Following the KA administration, the rats were observed for 3 h to assess seizure activity. Seizures occurred approximately 45 min after the KA injection. Only rats exhibiting full limbic seizures, forelimb clonus with rearing, were used in this study. All animals were decapitated 4 h after the administration of KA. Our [(35)S]GTPγS binding results showed that there was a significant difference in both the affinity and efficacy particularly one of NOPr stimulation following KA treatment. Slight, but significant increase was observed for MOPr. Moreover PNOC, NOPr and MOPr mRNA levels were increased by KA treatment but there were no significant changes in the levels of DOPr and KOPr mRNAs. These results show that the activities of opioid/nociceptin receptors can be modified by KA-treatment, and MOPr, PNOC and NOPr are the most responsive to KA-induced seizures in the rat brain cortex.

Design, synthesis, pharmacological evaluation, and structure-activity study of novel endomorphin analogues with multiple structural modifications

This study reports on new proteolytically stable, pharmacologically active endomorphin analogues, incorporating Dmt(1), Achc(2), pFPhe(4), or βMePhe(4) unnatural amino acids. Consistent with earlier results, it was found that the analogues carrying Dmt(1) and Achc(2) residues displayed the highest μ-opioid receptor affinities, depending upon the configuration of the incorporated Achc(2). Combination of such derivatives with pFPhe(4) or βMePhe(4) yielded further compounds with variable binding potencies. Combined application of Dmt(1), cis-(1S,2R)Achc(2), and pFPhe(4) (compound 16) resulted in the most potent analogue. Ligand stimulated [(35)S]GTPγS binding assays indicated that the analogues retained their agonist activities and opioid receptor specificities. NMR and molecular modeling studies of the analogues containing βMePhe(4) or pFPhe(4) confirmed the predominance of bent structures, however, it is apparent that bent structures are energetically more favored than random/extended structures for all studied compounds.

Synthesis and pharmacological activities of 6-glycine substituted 14-phenylpropoxymorphinans, a novel class of opioids with high opioid receptor affinities and antinociceptive potencies

The synthesis and the effect of a combination of 6-glycine and 14-phenylpropoxy substitutions in N-methyl- and N-cycloproplymethylmorphinans on biological activities are described. Binding studies revealed that all new 14-phenylpropoxymorphinans (11−18) displayed high affinity to opioid receptors. Replacement of the 14-methoxy group with a phenylpropoxy group led to an enhancement in affinity to all three opioid receptor types, with most pronounced increases in δ and κ activities, hence resulting in a loss of μ receptor selectivity. All compounds (11−18) showed potent and long-lasting antinociceptive effects in the tail-flick test in rats after subcutaneous administration. For the N-methyl derivatives 13 and 14, analgesic potencies were in the range of their 14-methoxy analogues 9 and 10, respectively. Even derivatives 15−18 with an N-cyclopropylmethyl substituent acted as potent antinociceptive agents, being several fold more potent than morphine. Subcutaneous administration of compounds 13 and 14 produced significant and prolonged antinociceptive effects mediated through peripheral opioid mechanisms in carrageenan-induced inflammatory hyperalgesia in rats.

Comparative biochemical and pharmacological characterization of a novel, NOP receptor selective hexapeptide, Ac-RYYRIR-ol

Nociceptin/orphanin FQ (N/OFQ) is an endogenous neuropeptide, which is widely distributed in central and peripheral nervous system. Some N/OFQ sequence unrelated hexapeptides can effectively bind to the N/OFQ peptide (NOP) receptor and they were used as template for structure-activity studies that lead to discovery of the new NOP selective ligands. In the present study, the pharmacological profile of the novel hexapeptide Ac-RYYRIR-ol was investigated using various in vitro assays including receptor binding and G-protein activation in rat brain membranes, mouse and rat vas deferens, guinea pig ileum, mouse colon and Ca(2+) mobilization assay in chinese hamster ovary (CHO) cells co-expressing the human recombinant NOP receptor and the C-terminally modified Galpha(qi5) protein. In rat brain membranes Ac-RYYRIR-ol displaced both [(3)H]nociceptin/OFQ and [(3)H]Ac-RYYRIK-ol with high affinity (pK(i) 9.35 and 8.81, respectively) and stimulated [(35)S]GTPgammaS binding showing however lower maximal effects than N/OFQ (alpha=0.28). The stimulatory effect of Ac-RYYRIR-ol was antagonized by the selective NOP receptor antagonist UFP-101. In the electrically stimulated mouse vas deferens Ac-RYYRIR-ol displayed negligible agonist activity while antagonizing in a competitive manner (pA(2) 7.99) the inhibitory effects of N/OFQ. Similar results were obtained in the rat vas deferens. In the mouse colon Ac-RYYRIR-ol produced concentration dependent contractile effects with similar potency and maximal effects as N/OFQ. Finally, in the Ca(2+) mobilization assay performed with CHO-hNOP-Galpha(qi5) cells Ac-RYYRIR-ol displayed lower potency and maximal effects (alpha=0.87) compared with N/OFQ. In conclusion, the novel NOP receptor selective hexapeptide Ac-RYYRIR-ol has been shown to have fine selectivity, high potency, furthermore agonist and antagonist effects toward the NOP receptors were measured in various assays; this is likely due to its partial agonist pharmacological activity.

Binding studies of novel, non-mammalian enkephalins, structures predicted from frog and lungfish brain cDNA sequences

Leu- and Met-enkephalin were the first endogenous opioid peptides identified in different mammalian species including the human. Comparative biochemical and bioinformatic evidence indicates that enkephalins are not limited to mammals. Various prodynorphin (PDYN) sequences in lower vertebrates revealed the presence of other enkephalin fingerprints in these precursor polypeptides. Among the novel enkephalins Ile-enkephalin (Tyr-Gly-Gly-Phe-Ile) was primarily observed in the African clawed frog (Xenopus laevis) PDYNs, while the structure of Phe-enkephalin (Tyr-Gly-Gly-Phe-Phe) was predicted by analyzing brain cDNA sequences encoding a PDYN of the African lungfish (Protopterus annectens). Ile-enkephalin can also be found in the PDYNs of four other fish species including the eel, bichir, zebrafish and tilapia, but no further occurrence for the Phe-enkephalin motif is available as yet. Based on sequencing data, the biological relevance of Phe- and Ile-enkephalin is suggested, because both of them can arise by regular posttranslational enzymatic processing of the respective neuropeptide precursors. In various receptor binding assays performed on rat brain membrane preparations both of the new peptides turned out to be moderate affinity opioids with a weak preference for the delta-opioid receptor (DOP) sites. Phe-enkephalin of the lungfish displayed rather unexpectedly low affinities toward the mu-opioid receptor (MOP) and DOP, while exhibiting moderate affinity toward the kappa-opioid receptor (KOP). In receptor-mediated G-protein activation assays measured by the stimulation of [(35)S]GTPgammaS binding, Met-enkephalin produced the highest stimulation followed by Leu-enkephalin, Ile-enkephalin and Phe-enkephalin, whereas the least efficacious among these endogenous peptides was still more effective than the prototype opiate agonist morphine in these functional tests.

Capsaicin inhibits the in vitro binding of peptides selective for mu- and kappa-opioid, and nociceptin-receptors

Capsaicin inhibited the equilibrium specific binding of endogenous opioid-like peptide ligands such as endomorphin-1, nociceptin, and dynorphin((1-17)) in rat brain membrane preparations. We studied the in vitro effect of capsaicin (1-10 microM) on homologous and heterologous competitive binding of opioid ligands, using unlabeled synthetic peptides and the following tritiated compounds: [(3)H]endomorphin-1, [(3)H]endomorphin-2, [(3)H]nociceptin((1-17)) and [(3)H]dynorphin((1-17)). Capsaicin-dependent inhibition was also observed in [(35)S]GTPgammaS stimulation assays in the presence of certain opioid peptides. The inhibition of opioid binding was further investigated using other synthetic and natural mu-opioid ligands such as [D-Ala(2),(NMe)Phe(4),Gly(5)-ol]enkephalin (DAMGO), morphine and naloxone. The decrease in opioid ligand affinity upon capsaicin treatments was most apparent with endomorphin-1, followed by nociceptin and dynorphin. The binding of other investigated opioids were not affected in the presence of capsaicin. In [(3)H]endomorphin-1 binding assays, capsazepine antagonized the inhibitory effect of capsaicin in rat brain membranes suggesting the involvement of TRPV1 receptors. In Chinese hamster ovary (CHO) cells stably expressing mu-opioid receptors, but lacking vanilloid receptors, the inhibition by capsaicin on the binding of [(3)H]endomorphin-1 was not present. It is concluded that the inhibitory effect of capsaicin on the receptor binding affinity of endogenous opioid peptides in brain membrane preparations seems not to be a direct effect, it is rather a negative feedback interaction with opioid receptors.

Novel diastereomeric opioid tetrapeptides exhibit differing pharmacological activity profiles

A novel opioid peptide antagonist analogue, [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe, derived from the potent, delta-receptor selective TIPP tetrapeptide (Tyr-Tic-Phe-Phe) series was synthesized and radiolabeled by catalytic tritiation of its iodinated precursor peptide. The purified radioprobe exhibited a specific activity of 2.15 TBq/mmol (58 Ci/mmol). The novelty of this compound is that it contains structurally modified tyrosine residue (2',6'-dimethyltyrosine, Dmt1) replacing tyrosine (Tyr1) at the N-terminus, and beta-methyl substituted phenylalanine (betaMePhe3) at the third position. As the configuration of betaMePhe3 side-chain might be different due to diastereomerism, and accordingly can alter the biological activity, both unlabeled threo (2S,3R and 2R,3S) diastereomeric analogues were also prepared and included in this study. The affinity and selectivity (delta-opioid versus mu-opioid receptor) were evaluated by radioreceptor binding assays. Agonist or antagonist potencies were determined in [35S]GTPgammaS binding experiments using Chinese Hamster Ovary (CHO) cells selectively expressing delta- or mu-opioid receptors. The equilibrium binding of the radiolabeled peptide derivative [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe to rat brain membranes was saturable and the Scatchard analysis indicated a single binding site with a Kd of 0.3 nM and a Bmax of 127 fmol/mg protein. A study of [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe binding displacement by various receptor-type specific opioid ligands showed the rank order of competitor's potency delta > mu > kappa, suggesting selective labeling of opioid delta-sites. In the functional tests, the (2S,3R) and (2R,3S) peptides exhibited partial agonist behaviour by weakly stimulating regulatory G-proteins in CHO cell membranes transfected with different receptors. Both isomers were quite weak partial agonists at the delta-receptor and reasonable partial agonists at the mu-receptor, with a prevalence of (2S,3R) over (2R,3S) for the mu-receptor. Consistent with these observations both stereomers competitively inhibited the stimulation of [35S]GTPgammaS binding induced by the prototype delta-agonist peptide (pClPhe4)-DPDPE in delta(m) CHO cell membranes, and still the (2S,3R) compound exerted more potent delta-antagonist effect. [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe represents a high affinity new radioligand and also constitute further example of the influence of beta-methyl substitution on the potency and selectivity of TIPP analogues, thus becoming a valuable biochemical and pharmacological tool in opioid research.

β-Methyl Substitution of Cyclohexylalanine in Dmt-Tic-Cha-Phe Peptides Results in Highly Potent δ Opioid Antagonists

The opioid peptide TIPP (H-Tyr-Tic-Phe-Phe-OH, Tic:1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) was substituted with Dmt (2',6'-dimethyltyrosine) and a new unnatural amino acid, beta-MeCha (beta-methyl-cyclohexylalanine). This double substitution led to a new series of opioid peptides displaying subnanomolar delta antagonist activity and mu agonist or antagonist properties depending on the configuration of the beta-MeCha residue. The most promising analog, H-Dmt-Tic-(2S,3S)-beta-MeCha-Phe-OH was a very selective delta antagonist both in the mouse vas deferens (MVD) assay (Ke = 0.241 +/- 0.05 nM) and in radioligand binding assay (K i delta = 0.48 +/- 0.05 nM, K i mu/K i delta = 2800). The epimeric peptide H-Dmt-Tic-(2S,3R)-beta-MeCha-Phe-OH and the corresponding peptide amide turned out to be mixed partial mu agonist/delta antagonists in the guinea pig ileum and MVD assays. Our results constitute further examples of the influence of Dmt and beta-methyl substitution as well as C-terminal amidation on the potency, selectivity, and signal transduction properties of TIPP related peptides. Some of these compounds represent valuable pharmacological tools for opioid research.

Opioid receptor binding characteristics and structure-activity studies of novel tetrapeptides in the TIPP (Tyr-Tic-Phe-Phe) series

The development of the prototype synthetic delta-opioid receptor antagonist peptides TIPP [(H-Tyr-Tic-Phe- Phe-OH); Tic: tetrahydroisoquinoline-3-carboxylic acid] and TIPPpsi (H-Tyr-psiTic-Phe-Phe-OH) by Schiller and coworkers was followed by extensive structure-activity relationship studies, leading to the emergence of numerous analogs that are of pharmacological interest. Eight novel diastereomeric compounds in this peptide family were designed, prepared, and tested biologically to gain structure-activity relationship information. The new multisubstituted tetrapeptide analogs contain both a 2',6'-dimethyltyrosine residue at the N-terminus and beta-methyl-cyclohexylalanine at the third position as replacements for the original first tyrosine and the third phenylalanine, respectively. These derivatives wear either free acidic (-COOH) or amidated (-CONH2) C-terminal. The potency and delta- versus mu-opioid receptor selectivity were evaluated by in vitro radioreceptor-binding assays, while the intrinsic G-protein-activating efficacy of these analogs was tested in [35S]GTPgammaS-binding assays using rat brain membranes or Chinese hamster ovary cells stably expressing mu- or delta-opioid receptors. The analogs showed delta-antagonist selectivity with differences regarding their isomeric forms, and these analogs containing a C-terminal carboxamide group displayed a mixed mu-agonist/delta-antagonist profile, thus they are expected to be safer analgesics with a low propensity to produce tolerance and physical dependence. These results constitute further examples of the influence of beta-methyl substitution and C-terminal amidation on potency, selectivity, and signal transduction properties of TIPP-related peptides as well as they represent valuable pharmacological tools for opioid research.

Synthesis and biological studies of nociceptin derivatives containing the DTPA chelating group for further labeling with therapeutic radionuclides

Nociceptin is an endogenous anti-opiate heptadecapeptide primarily interacting with the nociceptin (NOP) receptor. This neuropeptide-receptor system is involved in pain regulation, tolerance to and dependence on opiates as well as many other physiological and pathophysiological events. The role and mechanisms of nociceptin in pathological conditions is not clearly known yet. In an attempt to have a radiopharmaceutical labeled either with 99mTc or (111)In, we incorporated diethylenetriaminepentaacetic acid (DTPA) as chelator into the structure of [Arg14,Lys15]nociceptin(1-17)-NH2 at the epsilon-amino group of Lys15. Such a radiopeptide may be useful in imaging for diagnostical purposes. Preparation of the peptide ligands was carried out by solid phase synthesis. Two peptides containing DTPA were obtained and purified. The products were [Arg14,Lys(DTPA)15]nociceptin(1-17)-NH2 and its cross-linked dimer on the basis of mass spectrometric analysis. In (115)In3+ binding experiments the conjugates exhibited preserved indium ion chelating properties, indicating the potential use of radiolabeled DTPA-nociceptin derivatives as radiopharmaceutical. Biological properties of these compounds were studied in rat brain membrane preparations by radioligand binding, functional biochemical [35S]GTPgammaS binding assays and mouse vas deferens (MVD) bioassay. Besides the similar in vitro binding characteristics to nociceptin receptor, both of the DTPA-chelated compounds were more potent and efficient than nociceptin in functional biochemical and mouse vas deferens bioassays. Our further aim is to radiolabel these compounds in order to get a radiopharmaceutical which can be used diagnostically.

Binding of the new morphiceptin analogs to human MCF-7 breast cancer cells and their effect on growth

In the present study, we reported on the synthesis of two new mu-opioid peptide analogs, [D-1-Nal3]morphiceptin and [D-1-Nal4]-morphiceptin [1-Nal=3-(1-naphthyl)-alanine] which expressed receptor binding affinities at least at the level of the primary opioid ligands. The new analogs also labeled mu-opioid receptors on the cells of human breast cancer MCF-7 cell line with affinity much higher than that of endomorphins and morphiceptin, the well-known mu-selective opioid peptides. However, none of the tested peptides significantly decreased cell proliferation of MCF-7 cells.

Nociceptin antagonism: probing the receptor by N-acetyl oligopeptides

In search for effective antagonist structures for the nociceptin (NOP) receptor, a number of N-acylated oligopeptides, including N-acyl tetra- and pentapeptides selective for the kappa-opioid receptor, as well as N-acyl hexapeptides bearing the Ac-Arg-Tyr-Tyr-Arg-Ile-Lys (Ac-RYYRIK) core sequence originally isolated from combinatorial chemical libraries, were synthesized and studied in radioreceptor binding assays, [(35)S]GTPgammaS functional tests and in mouse vas deferens (MVD) bioassays. The properties of the novel antagonist candidates were compared to known antagonists. A new antagonist structure with a reduced, primer alcohol C-terminus, Ac-Arg-Tyr-Tyr-Arg-Ile-lysinol (Ac-RYYRIK-ol) was described in the mouse vas deferens tests, showing an equilibrium inhibitory constant value (K(e)) of 2.44 nM, and no agonist effect at 10 microM ligand concentration. Schild-analysis indicated a clearly competitive interaction at the NOP receptor, whereas the peptide did not affect the action of the delta-opioid receptor agonist [D-Ala(2),D-Leu(5)]enkephalin. Ac-RYYRIK-ol also exhibited a high affinity in [(3)H]nociceptin-NH(2) binding competition assays using rat brain membranes. Agonist-induced G-protein activation via NOP receptors was studied in [(35)S]GTPgammaS binding stimulation assays by the use of both native brain tissue preparations and membranes from cultured CHO cells expressing recombinant nociceptin receptors. Ac-RYYRIK-ol displayed only weak intrinsic agonist activity, whereas it effectively inhibited the stimulation generated by nociceptin. The results support the high potency and antagonist nature of Ac-RYYRIK-ol and reveal important roles for both the N- and the C-terminal region of the molecule.

In vitro opioid activity profiles of 6-amino acid substituted derivatives of 14-O-methyloxymorphone

A series of 6-amino acid conjugates (glycine, alanine and phenylalanine) of the highly potent opioid analgesic 14-O-methyloxymorphone was developed in an effort to obtain agonists that would have potentially limited ability to cross the blood-brain barrier. Binding studies revealed that all derivatives displayed high affinities (0.77-2.58 nM) at the mu-opioid receptor in rat brain membranes. They were potent agonists in mouse vas deferens preparation (IC(50)=5.52-26.8 nM). While the alpha-amino acid epimers are favoured by mu-opioid receptors, the beta-epimers proved to have increased interaction with delta-sites. Only the beta-phenylalanine conjugate showed some preference for delta- over mu-opioid receptors and delta-opioid receptor agonist activity. The relatively high delta-opioid receptor affinity of this analogue was also predicted by molecular modelling studies. The newly developed ionizable derivatives could find clinical applications as potent analgesics without the adverse actions of centrally acting opioids.

Opioid receptor binding and in vivo antinociceptive activity of position 3-substituted morphiceptin analogs

Analogs of morphiceptin (Tyr-Pro-Phe-Pro-NH2), a mu-selective opioid receptor ligand, with position 3-modifications, including altered size, lipophilicity, and electronic character, while maintaining aromaticity were synthesized. The activity of the new analogs in in vitro assays and in in vivo hot-plate test of analgesia was compared and the results were consistent. [D-1-Nal3]Morphiceptin was the most potent analog of this series with a 26-fold increase in mu-opioid receptor affinity, a 15-fold potency increase in the GPI assay, and a significant potency increase in the hot-plate analgesic test, as compared with morphiceptin. [d-Qal3]Morphiceptin was found to be a weak antagonist in the GPI assay.

Structure−Activity Study on the Phe Side Chain Arrangement of Endomorphins Using Conformationally Constrained Analogues

Endomorphins-1 and -2 were substituted with all the beta-MePhe stereoisomers in their Phe residues to generate a conformationally constrained peptide set. This series of molecules was subjected to biological assays, and for beta-MePhe(4)-endomorphins-2, a conformational analysis was performed. Incorporation of (2S,3S)-beta-MePhe(4) resulted in the most potent analogues of both endomorphins with enhanced enzymatic stability. Their micro opioid affinities were 4-times higher than the parent peptides, they stimulated [(35)S]GTPgammaS binding, and they were found to be full agonists. NMR experiments revealed that C-terminal (2S,3S)-beta-MePhe in endomorphin-2 strongly favored the gauche (-) spatial orientation which implies the presence of the chi(1) = -60 degrees rotamer of Phe(4) in the binding conformer of endomorphins. Our results emphasize that the appropriate orientation of the C-terminal aromatic side chain of endomorphins is substantial for binding to the micro opioid receptor.

Cloning and characterization of Xen-dorphin prohormone from Xenopus laevis: a new opioid-like prohormone distinct from proenkephalin and prodynorphin

Opioid-like peptides mediate analgesia and induce behavioral effects such as tolerance and dependence by ligand-receptor-mediated mechanisms. The classical opioid prohormones can generate several bioactive peptides, and these divergent families of prohormones share a common well conserved ancestral opioid motif (Tyr-Gly-Gly-Phe). Evidence from pharmacological and molecular cloning studies indicates the presence of multiple isoforms of opioid ligands and receptors that are as yet uncharacterized. To identify potential new members we used the opioid motif as an anchor sequence and isolated two distinct isoforms (Xen-dorphins A and B) of an opioid prohormone from Xenopus laevis brain cDNA library. Xen-dorphin prohormones can generate multiple novel opioid ligands distinct from the known members of this family. Both isoforms are present in a wide variety of tissues including the brain. Two potential bioactive peptides, Xen-dorphin-1A and -1B, that were chemically synthesized showed opioid agonist activity in frog and rat brain membranes using a [35S]GTPgammaS assay. Initial radioligand binding experiments demonstrated that Xen-dorphin-1B binds with high affinity to opioid receptor(s) and with potential preference to the kappa-opioid receptor subtype. Cloning of the Xen-dorphin prohormone provides new evidence for the potential presence of other members in the opioid peptide superfamily.

Synthesis and binding characteristics of a novel enkephalin analogue, [3H]Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe

The endogenous opioid heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF) has been shown to interact with multiple opioid as well as non-opioid sites in mammalian brain membranes. To increase the stability and bioavailability of MERF, new synthetic derivatives with D-amino acid substitutions were prepared and studied. One of the new compounds in this series, Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN), had only moderate affinity in competing with [3H]MERF, whereas it displayed the highest potency in producing antinociception following intrathecal administration. DADN was radiolabeled with 41Ci/mmol specific activity. Specific binding of [3H]DADN was saturable, stereoselective and of high affinity. Chemical stability, increased micro-receptor selectivity, and hydrophobicity of the peptide all contribute to the effectiveness observed in biochemical and pharmacological studies.

Kappa-receptor selective binding of opioid ligands with a heterocyclic bicyclo[3.3.1]nonan-9-one structure

Previous pharmacological results have suggested that members of the heterocyclic bicyclo[3.3.1]nonan-9-one-like compounds are potent kappa-opioid receptor specific agonists. One lead molecule of this series. called compound 1 (dimethyl 7-methyl-2,4-di-2-pyridyl-3.7-diazabicyclo[3.3.1]nonan-9-one-1,5-dicarboxylate) exhibited high affinity for [3H]ethylketocyclazocine and [3H]U-69.593 binding sites in guinea pig cerebellar membranes which known to be a good source for kappa1 receptors. It was shown by molecular modelling that heterocyclic bicyclo[3.3.1]nonan-9-ones fit very well with the structure of ketazocine, a prototypic kappa-selective benzomorphan compound; when compared to the arylacetamide structure of U-69.593, a specific kappa1-receptor agonist, a similar geometry was found with a slightly different distribution of the charges. It is postulated, that the essential structural skeleton involved in the opioid activity is an aryl-propyl-amine element distributed along the N7-C6-C5-C4-aryl bonds.

Binding characteristics of [3H]14-methoxymetopon, a high affinity mu-opioid receptor agonist

The highly potent micro -opioid receptor agonist 14-methoxymetopon (4,5alpha-epoxy-3-hydroxy-14beta-methoxy-5beta,17-dimethylmorphinan-6-one) was prepared in tritium labelled form by a catalytic dehalogenation method resulting in a specific radioactivity of 15.9 Ci/mmol. Opioid binding characteristics of [3H]14-methoxymetopon were determined using radioligand binding assay in rat brain membranes. [3H]14-Methoxymetopon specifically labelled a single class of opioid sites with affinity in low subnanomolar range (Ki = 0.43 nm) and maximal number of binding sites of 314 fmol/mg protein. Binding of [3H]14-methoxymetopon was inhibited by ligands selective for the micro -opioid receptor with high potency, while selective kappa-opioids and delta-opioids were weaker inhibitors. 14-Methoxymetopon increased guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTPgammaS) binding with an EC50 of 70.9 nm, thus, providing evidence for the agonist character of this ligand. The increase of [35S]GTPgammaS binding was inhibited by naloxone and selective micro -opioid antagonists, indicating a micro -opioid receptor-mediated action. [3H]14-Methoxymetopon is one of the few nonpeptide mu-opioid receptor agonists available in radiolabelled form up to now. Due to its high affinity and selectivity, high stability and extremely low nonspecific binding (<10%), this radioligand would be an important and useful tool in probing mu-opioid receptor mechanisms, as well as to promote a further understanding of the opioid system at the cellular and molecular level.

Endomorphin-2, deltorphin II and their analogs suppress formalin-induced nociception and c-Fos expression in the rat spinal cord

In this study, we evaluated the effects of intrathecally administered agonists of mu- and delta-opioid receptor and their analogs on the pain-induced behavior and expression of c-Fos immunoreactivity in the spinal cord, elicited by intraplantar injection of 12% formalin to the hindpaw of the rat. Previous report from our laboratory and other author's study indicated that intrathecal administration of mu agonists morphine and endomorphin-2 and delta-opioid agonist deltorphin II produced a dose-dependent antinociceptive effects in acute and inflammatory pain. In this study, intrathecal injection of morphine (10 microg), endomorphin-2 (5 microg) and its analog Dmt-endomorphin-2 (10 microg) significantly decreased the formalin-induced pain behavior, and lowered a number of c-Fos positive neurons in the laminae I, II and III of the spinal cord by about 40%, 30% and 40%, respectively. Significant reduction of formalin-induced behavioral responses was also observed after i.th. administration of deltorphin II (15 microg) and its analog ile-deltorphin II (15 microg). Agonists of delta-opioid receptor significantly reduced a number of c-Fos positive neurons by about 28% and 40%, respectively. Analog of endomorphin-2 and analog of deltorphin II suppressed more potently expression of c-Fos in the dorsal horn of the spinal cord than the parent peptides. Our study indicates that new analogs of mu- and delta-opioid receptor exhibit strong antinociceptive potency similar or even higher than the parent peptides, and that their effect is positively correlated with the inhibition of c-Fos expression.

Functional interactions between delta- and mu-opioid receptors in rat thermoregulation

The selective delta-opioid receptor agonist deltorphin II (25.0-100.0 microg, i.c.v.) produced biphasic effects on core temperature in rats, in which hypothermia was followed by hyperthermia. Pretreatment with the selective delta-opioid receptor antagonist, naltrindole (25.0 microg, i.c.v.), blocked hypothermia produced by deltorphin II and had a tendency to potentiate the hyperthermic effect of deltorphin II. The non-selective opioid receptor antagonist naloxone (1.5 mg kg(-1), s.c.) potentiated hypothermia, and blocked hyperthermia, produced by deltorphin II (100.0 microg). Also, naloxone potentiated hypothermia produced by a lower dose of deltorphin II (25.0 microg), which did not produce hyperthermia. A similar pattern was found for the selective mu-opioid receptor antagonist, beta-funaltrexamine (5.0 microg, i.c.v.), which potentiated and blocked deltorphin II-induced hypo- and hyperthermia, respectively. The selective kappa-opioid receptor antagonist nor-binaltorphimine (20.0 microg, i.c.v.) had no effects on deltorphin II-induced temperature changes. The present results suggest that deltorphin II produces hypothermia through activation of delta-opioid receptors, whereas the hyperthermic effect of deltorphin II involves activation of mu-opioid receptors. This mu-opioid receptor stimulatory effect of deltorphin II is furthermore more pronounced than was anticipated based on the reported in vitro properties of this compound. The biphasic effect of deltorphin II implies a negative interaction between delta- and mu-opioid receptors in thermoregulation in rats.

Alteration in endogenous opioid systems due to chronic inflammatory pain conditions

The influence of chronic arthritic pain on two endogenous opioid peptides, dynorphin B and [Met5]enkephalin-Arg6-Phe7, and multiple opioid receptors in discrete brain, lumbar spinal cord and pituitary pools was investigated. Using radioimmunoassay and receptor binding assay, we examined the changes in regional opioid peptide levels and opioid receptor activity due to chronic inflammation in adjuvant arthritic rats. At 4 weeks post-inoculation, increased levels of immunoreactive dynorphin B and [Met5]enkephalin-Arg6-Phe7 were measured in tissues of arthritic rats compared with controls. No significant changes in mu-, delta- or kappa-opioid receptors were seen after chronic inflammation. Taken together, these results indicate that in chronic arthritis, opioid receptor changes do not follow the peptide alterations of pro-dynorphin and pro-enkephalin systems. Thus, dynamic modification and modulation of nociceptive information takes place during chronic inflammation. This supports the key role of the central nervous system in chronic inflammatory pain conditions.

Deltorphin II analogues with 6-hydroxy-2-aminotetralin-2-carboxylic acid in position 1

Two approaches to the design of very active and highly selective delta opioid peptides were used to obtain new deltorphin analogues with altered hydrophobic and stereoelectronic properties. Deltorphin II analogues were synthesized with the substitution of Ile instead of Val at positions 5 and 6 in the address domain and 6-hydroxy-2-aminotetralin-2-carboxylic acid (Hat) instead of Tyr(1) in the message domain. In the radioreceptor-binding studies, in which type-specific tritiated opioid ligands were used, (R)- and (S)-Hat-deltorphins exhibited similar K(i) values, revealing high delta selectivity. The peptides displayed agonist properties in the in vitro bioassay, with IC(50) values in the subnanomolar range in the mouse vas deferens assay and in the micromolar or higher range in the guinea pig ileum assay, again demonstrating a high selectivity toward delta receptors. The agonist property of the new ligands was confirmed by means of [(35)S]GTPgammaS-binding experiments in membranes of the rat frontal cortex.

Tritiated kappa receptor antagonist norbinal torphimine: synthesis and in vitro binding in three different tissues

Recently a new antagonist with high selectivity for the kappa receptors (norbinaltorphimine) was developed and tested in various systems. This compound was radiolabelled with tritium resulting in high specific radioactivity (47.2 Ci/mmol). [3H]Norbinaltorphimine was characterized by in vitro radioligand binding assays. The radioligand binds to kappa-opioid receptors with a high potency and selectivity in guinea pig, frog and rat brain membranes. Our results suggest the kappa1 specificity of the radioligand.

Specific activation of the mu opioid receptor (MOR) by endomorphin 1 and endomorphin 2

The recently discovered endomorphin 1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin 2 (Tyr-Pro-Phe-Phe-NH2) were investigated with respect to their direct receptor-binding properties, and to their ability to activate G proteins and to inhibit adenylyl cyclase in both cellular and animal models. Both tetrapeptides activated G proteins and inhibited adenylyl cyclase activity in membrane preparations from cells stably expressing the mu opioid receptor, an effect reversed by the mu receptor antagonist CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2), but they had no influence on cells stably expressing the delta opioid receptor. To further establish the selectivity of these peptides for the mu opioid receptor, brain preparations of mice lacking the mu opioid receptor gene were used to study their binding and signalling properties. Endomorphin 2, tritiated by a dehalotritiation method resulting in a specific radioactivity of 1.98 TBq/mmol (53.4 Ci/mmol), labelled the brain membranes of wild-type mice with a Kd value of 1.77 nM and a Bmax of 63.33 fmol/mg protein. In membranes of mice lacking the mu receptor gene, no binding was observed, and both endomorphins failed to stimulate [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding and to inhibit adenylyl cyclase. These data show that endomorphins are capable of activating G proteins and inhibiting adenylyl cyclase activity, and all these effects are mediated by the mu opioid receptors.

New opioid affinity labels containing maleoyl moiety

Opioid receptor binding properties and pharmacological profiles of novel peptides containing maleoyl function were determined in order to develop new affinity labels. Based on the enkephalin structure peptide ligands were synthesized and tested. Both in in vitro receptor binding experiments and pharmacological studies, all ligands showed agonist character with relatively high affinity (Ki values in the nanomolar range) and good to moderate selectivity. Replacement of Gly2 in the enkephalin frame with D-Ala led to higher affinities with a small decrease in selectivity. The longer peptide chains resulted in compounds with high percentage (up to 86%) of irreversible binding. The selectivity pattern of the ligands is in good agreement with the data obtained from the pharmacological assays (guinea pig ileum and mouse vas deferens bioassays). The newly synthesized peptides could be used in further studies in order to determine more detailed characteristics of the ligand-receptor interaction.

Affinity labeling of delta opioid receptors by an enkephalin-derivative alkylating agent, DSLET-Mal

Opioid binding properties of Tyr-D-Ser-Gly-Phe-Leu-Thr-NH-NH-Gly-Mal (DSLET-Mal), a novel enkephalin-framed affinity label, was determined in rat brain membranes. In competition studies the ligand showed high affinity for the delta opioid sites, labelled by [(3)H][Ile(5,6)]deltorphin II (K(i) = 8 nM), whereas its binding to the mu ([(3)H]DAMGO) and kappa ([(3)H]EKC) sites was weaker. Preincubation of the rat brain membranes with DSLET-Mal at micromolar concentrations resulted in a wash-resistant and dose-dependent inhibition of the [(3)H][Ile(5,6)]deltorphin II binding sites (96% blocking at 10 microM concentration). Intracerebroventricular (ICV) administration of DSLET-Mal reduced the density of delta opioid receptors and had no effect on mu and kappa receptors, as determined by saturation binding studies. [Ile(5, 6)]deltorphin II-stimulated [(35)S]GTPgammaS binding was determined in membrane preparations of different brain areas of the ICV-treated animals. In both frontal cortex and hippocampus DSLET-Mal significantly decreased G protein activation by the delta agonist, having no effect on DAMGO stimulated [(35)S]GTPgammaS binding. DSLET-Mal had qualitatively similar effects on both receptor binding and G protein activation. These characteristics of the compound studied suggest that DSLET-Mal can serve as an affinity label for further studies of the delta-opioid receptors.

Characterization of [125I]AR-M100613, a high-affinity radioligand for delta opioid receptors

AR-M100613 ([I]-Dmt-c[-D-Orn-2-Nal-D-Pro-D-Ala-]) is the iodinated analog of a cyclic casomorphin previously shown to be a potent antagonist at the delta opioid receptor. Specific [125I]AR-M100613 binding to rat whole brain membranes was saturable, reversible, and best fit to a one-site model (Kd = 0.080 +/- 0.008 nM, Bmax = 45.2 +/- 4.4 fmol/mg protein). [125I]AR-M100613 binding was displaced with high affinity by the delta opioid receptor ligands SNC-80, Deltorphin II and DPDPE but not the mu or kappa-selective receptor ligands DAMGO and U69593. Residual non-selective binding of [125I]AR-M 100613 to mu opioid receptors is blocked by the addition of CTOP to the assay buffer. [35S]GTPgammaS binding assays indicate that AR-M100613 is a potent, selective, and reversible antagonist for delta opioid receptors in rat brain membranes. The high-affinity, high specific activity, low nonspecific binding and antagonist profile of [125I]AR-M100613 favor its use as a radiochemical probe for delta opioid receptors.

Mu-opioid receptor specific antagonist cyprodime: characterization by in vitro radioligand and [35S]GTPgammaS binding assays

The use of compounds with high selectivity for each opioid receptor (mu, delta and kappa) is crucial for understanding the mechanisms of opioid actions. Until recently non-peptide mu-opioid receptor selective antagonists were not available. However, N-cyclopropylmethyl-4,14-dimethoxy-morphinan-6-one (cyprodime) has shown a very high selectivity for mu-opioid receptor in in vivo bioassays. This compound also exhibited a higher affinity for mu-opioid receptor than for delta- and kappa-opioid receptors in binding assays in brain membranes, although the degree of selectivity was lower than in in vitro bioassays. Cyprodime has recently been radiolabelled with tritium resulting in high specific radioactivity (36.1 Ci/mmol). We found in in vitro binding experiments that this radioligand bound with high affinity (K(d) 3. 8+/-0.18 nM) to membranes of rat brain affording a B(max) of 87. 1+/-4.83 fmol/mg. Competition studies using mu, delta and kappa tritiated specific ligands confirmed the selective labelling of cyprodime to a mu-opioid receptor population. The mu-opioid receptor selective agonist [D-Ala(2),N-MePhe(4),Gly(5)-ol]enkephalin (DAMGO) was readily displaced by cyprodime (K(i) values in the low nanomolar range) while the competition for delta- ([D-Pen(2), D-Pen(5)]enkephalin (DPDPE)) and kappa- (5alpha,7alpha, 8beta-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro(4, 5)dec-8-yl]-benzene-acetamide (U69,593)) opioid receptor selective compounds was several orders of magnitude less. We also found that cyprodime inhibits morphine-stimulated [35S]GTPgammaS binding. The EC(50) value of morphine increased about 500-fold in the presence of 10 microM cyprodime. These findings clearly indicate that cyprodime is a useful selective antagonist for mu-opioid receptor characterization.

The effect of repeated administration of morphine, cocaine and ethanol on mu and delta opioid receptor density in the nucleus accumbens and striatum of the rat

The present study was carried out to evaluate the effect of morphine, cocaine and ethanol on the density of opioid receptors in the nucleus accumbens and striatum of rat brain. The animals were injected i.p. with morphine in a single dose 20 mg/kg, or twice daily for 10 days in increasing doses of 20-100 mg/kg. Cocaine was administered in a dose of 60 mg/kg/day following the "binge" paradigm, every hour for 3 h, one day (single treatment) or five days (chronic treatment). Ethanol was administered in drinking water at increasing concentrations of 1-6% v/v, for one month. As shown by receptor autoradiography, single morphine and cocaine administration did not influence the binding density of the selective ligand of delta2 receptors [3H]Ile5,6deltorphin b, but single administration of cocaine decreased binding density of a highly selective antagonist of delta receptors, [3H]H-Tyr-Tic psi[CH2-NH]Phe-Phe-OH. Repeated morphine administration decreased the receptor density after both ligands of the delta receptor in the nucleus accumbens after 3, 24 and 48 h, and in the striatum after 24 and 48 h. The density of [3H]Ile5,6deltorphin b binding remained unchanged in both structures following repeated cocaine administration. After repeated cocaine administration either no changes (3 h) or a decrease in the binding of [3H]H-Tyr-Tic psi[CH2-NH]Phe-Phe-OH in the nucleus accumbens and striatum were observed after 24 and 48 h. Ethanol did not influence the binding density of [3H]H-Tyr-Tic psi[CH2-NH]Phe-Phe-OH and [3H]Ile5,6deltorphin b in the nucleus accumbens and striatum at any time-point studied. In the nucleus accumbens and striatum, no changes were found in the binding density of [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol following single or repeated morphine administration. At 3 h after single or repeated "binge" cocaine administration, the binding of [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol was not changed in either structure, but after 24 h the density of mu opioid receptors was decreased in both structures. Ethanol given to rats in drinking water decreased the binding of [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol at the time of exposure to ethanol, yet in the nucleus accumbens only. Ethanol withdrawal decreased the density of the mu receptor in both structures after 24, 48 and 96 h. The above data indicate that repeated administration of morphine evokes a long-lasting down-regulation of the density of delta1 and delta2 opioid receptors, whereas cocaine affects in a similar way only the delta1 subtype in the nucleus accumbens, and to a lesser extent in the striatum. A long-term intake of ethanol solution down-regulates mu opioid receptors in both structures, but has no effect on any type of delta receptors. Thus changes in the particular opioid receptor depend on the type of drug used. Furthermore, the most profound changes are observed after late withdrawal, which may play some role in maintaining the state of dependence.

Irreversible labelling of the opioid receptors by a melphalan-substituted [Met5]enkephalin-Arg-Phe derivative

[Met5]enkephalin-Arg-Phe (Tyr-Gly-Gly-Phe-Met-Arg-Phe) was modified with the methyl esther of melphalan (Mel; 4-bis(2-chloroethyl)amino-L-phenylalanine) and the resulting compounds were studied for their opioid binding properties in guinea pig and rat brain membranes. Three new peptides, with a substitution of a single amino acid, were synthesized (Mel-Gly-Gly-Phe-Met-Arg-Phe, Tyr-Gly-Gly-Mel-Met-Arg-Phe and Tyr-Gly-Gly-Phe-Met-Arg-Mel). In the rat brain, none of these ligands displayed any type specificity, whereas in guinea pig brain membranes the C-terminally modified peptide, Tyr-Gly-Gly-Phe-Met-Arg-Mel ([Mel7]peptide), displayed a kappa-binding profile and was a weak kappa-opioid-receptor agonist in isolated guinea pig ileum. The effect of sodium ions on [Mel7]peptide competition against [3H]naloxone binding indicated a weak agonist nature of the compound. When guinea pig brain membranes were preincubated with 1-10 microM of [Mel7]peptide, an apparently irreversible inhibition of [3H]naloxone ligand binding was observed. These results suggest that the heptapeptide containing melphalan at the C-terminus can be used as a relatively high-affinity irreversible label for the kappa-opioid receptor.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.

Synthesis and binding characteristics of [3H] H-Tyr-Ticpsi[CH2-NH] Cha-Phe-OH, a highly specific and stable delta-opioid antagonist

Substitution of the Phe3 aromatic ring in H-Tyr-Ticpsi[CH2-NH]Phe-Phe-OH with cyclohexylalanine (Cha) has been reported to result in a compound, H-Tyr-Ticpsi[CH2-NH]Cha-Phe-OH (TICP[psi]), showing substantially increased delta-opioid antagonist potency and high delta selectivity. TICP[psi] was radiolabeled by catalytic tritiation of its precursor Tyr(3',5'-I2)1TICP[psi]. Binding characteristics of the new tritiated pseudopeptide were determined using the radioligand binding assay in rat brain membranes. On the basis of the results of saturation binding studies performed at 25 degrees C, an equilibrium dissociation constant (Kd) of 0.35 nM and a receptor density (Bmax) of 112 fmol/mg protein were calculated. This new tritiated ligand exhibits high affinity for delta-opioid receptors, whereas its binding to mu and kappa receptors is weak. A study of [H3]TICP[psi] binding displacement by various receptor-selective opioids showed the following rank order of potency: delta > kappa = mu. These receptor binding characteristics of the ligand, together with its high specific radioactivity (41.3 Ci/mmol) and stability, makes it a useful tool for labeling delta-opioid receptors, both in vitro and in vivo.

Antinociceptive effects of isoleucine derivatives of deltorphin I and deltorphin II in rat spinal cord: a search for selectivity of delta receptor subtypes

Deltorphins show a high affinity and selectivity for delta opioid receptors. Analogs of deltorphins with substitution of Val residues with more hydrophobic Ile appear to have a higher in vitro activity and selectivity than parent deltorphins. In our study, changes in the nociceptive threshold after intrathecally injected deltorphin I (DELT I), deltorphin II (DELT II) and their Ile - derivatives (ILE-DELT I and ILE-DELT II, respectively) were investigated in a tail-flick (TF) and a paw pressure (PP) tests. Male Wistars rats (260-350 g) with a chronically implanted catheter in the lumbar enlargement of the spinal cord were used. DELT I and DELT II, injected i.th. in doses of 0.15, 1.5 and 15 microg, increased the TF latency in a dose-dependent manner. The effect of their derivatives was similar, but the action of ILE-DELT II was shorter than that of the parent peptide. In the PP test, the antinociceptive effects of DELT I and their derivative ILE-DELT I were similar, but the effect of a higher dose of ILE-DELT I lasted longer in comparison with the parent peptide. Both DELT II and ILE-DELT II exhibited a low and short-lasting antinociceptive potency in the PP test. The effect of DELT I (1.5 microg) was antagonized by pretreatment with NTI (30 microg), a non-selective delta opioid receptor antagonist, as well as by the delta2 receptor antagonist NTB (3 microg) and the delta1 antagonist BNTX (1 microg) in both those tests used. The antinociceptive effect of DELT II (1.5 microg) was antagonized by pretreatment with NTI (30 microg) and NTB (3 microg) in the TF test, but not in the PP test. In the latter test, the antinociceptive effect of DELT II was potentiated by pretreatment with BNTX (1 microg). The effects of both the derivatives ILE-DELT I and ILE-DELT II were antagonized by NTI (30 microg) in the TF test, and by NTI (30 microg) and NTB (3 microg) in the PP test. Like in the case of the parent peptide, the effect of ILE-DELT II was potentiated by pretreatment with the delta1 antagonist BNTX (1 microg). Summing up, modification of the DELT I and II by substituting Ile for Val residues appears to influence the delta selectivity rather then the potency of the peptides at spinal delta receptors.

In vitro binding and signaling profile of the novel mu opioid receptor agonist endomorphin 2 in rat brain membranes

The recently discovered endogenous mu receptor selective endomorphin 2 was prepared in tritiated form by a catalytic dehalogenation method resulting in a specific radioactivity of 1.98 TBq/mmol (53.4 Ci/mmol), and used for in vitro labelling of rat brain membranes. The binding was saturable, stereospecific and of high affinity (Kd: 0.97 and 1.12 nM based on kinetic and equilibrium binding studies, respectively). The maximal number of binding sites (Bmax) was found to be 114.8 fmol/mg protein. [3H]Endomorphin 2 was displaced by mu-receptor selective specific peptides and heterocyclic compounds with high affinity, whereas kappa and delta receptor specific ligands were much less potent. The Ki values of endomorphin 1 and 2 in inhibiting [3H]naloxone binding increased by 15-fold in the presence of 100 mM NaCl which indicates the agonist property of these peptides. Endomorphins stimulated [35S]GTPgammaS binding and inhibited adenylyl cyclase activity which also provides evidence for the agonist character of endomorphins.

Affinity profiles of novel delta-receptor selective benzofuran derivatives of non-peptide opioids

Highly selective heterocyclic opioid ligands with potent delta-antagonist activity have been developed on the basis of the "message-address" concept. Using this strategy, benzofuran derivatives corresponding to the non-selective opioid antagonist, naloxone, and to the mu-opioid receptor selective agonists, oxymorphone and oxycodone, were synthesized. In vitro opioid receptor binding profiles and agonist/antagonist character of these compounds were determined in rat brain membrane preparations with highly selective radioligands. All three benzofuran derivatives displayed high affinities for the delta-opioid receptor, much less potency toward the mu-binding site, and were the least effective at the kappa-site. The results indicated that the addition of the bezofuran moiety to these fused ring opioids confers delta-receptor selectivity. The Na+ indices suggested a partial agonist character for oxymorphone- and oxycodone-benzofuran, and an antagonist character for naloxone-benzofuran. These compounds were capable of irreversible inhibition of opioid binding sites in a dose-dependent.