Role of spacer and address components in peptidomimetic delta opioid receptor antagonists related to naltrindole

δ Opioid Receptor Inverse Agonists and their In Vivo Pharmacological Effects

The discovery of δ opioid receptor inverse agonist activity induced by ICI-174,864, which was previously reported as an δ opioid receptor antagonist, opened the door for the investigation of inverse agonism/constitutive activity of the receptors. Various peptidic or non-peptidic δ opioid receptor inverse agonists have since been developed. Compared with the reports dealing with in vitro inverse agonist activities of novel compounds or known compounds as antagonists, there have been almost no publications describing the in vivo pharmacological effects induced by a δ opioid receptor inverse agonist. After the observation of anorectic effects with the δ opioid receptor antagonism was discussed in the early 2000s, the short-term memory improving effects and antitussive effects have been very recently reported as possible pharmacological effects induced by a δ opioid receptor inverse agonist. In this review, we will survey the developed δ opioid receptor inverse agonists and summarize the possible in vivo pharmacological effects by δ opioid receptor inverse agonists. Moreover, we will discuss important issues involved in the investigation of the in vivo pharmacological effects produced by a δ opioid receptor inverse agonist.

Development of Novel δ Opioid Receptor Inverse Agonists without a Basic Nitrogen Atom and Their Antitussive Effects in Mice

Our previous results showed that naltrindole (NTI) derivatives with certain types of electron-withdrawing groups as an N-substituent showed δ opioid receptor (DOR) inverse agonistic activities. We therefore synthesized N-acylated NTI derivatives 3a-e and observed that N-benzoyl and N-cyclopropanecarbonyl derivatives SYK-736 (3b) and SYK-623 (3c) were DOR full inverse agonists and the N-acryloyl derivative 3d was a DOR partial inverse agonist. SKY-623 was over 110-fold more potent than the reference compound ICI-174,864. Both naltriben (NTB) and 7-benzylidenenaltrexone (BNTX) derivatives with N-benzoyl and N-cyclopropanecarbonyl groups were also DOR full inverse agonists. These N-acylated inverse agonists are interesting compounds because they have no basic nitrogen atom, which has been demonstrated to be an important pharmacophore. NTI and BNTX-type DOR inverse agonists SYK-623 and SYK-723 (12c) showed dose-dependent antitussive effects in a mouse cough model induced by citric acid exposure. The antitussive effects by SYK-623 and SYK-723 were significantly attenuated by pretreatment with DOR agonist SNC80.

Discovery of δ Opioid Receptor Full Inverse Agonists and Their Effects on Restraint Stress-Induced Cognitive Impairment in Mice

The cyclopropylmethyl group in classical δ opioid receptor (DOR) antagonist NTI, BNTX, and NTB was replaced with various electron-withdrawing groups to develop DOR inverse agonists. N-Benzyl NTB derivative SYK-657 was a potent DOR full inverse agonist and its potency was over 10-fold potent than that of a reference compound ICI-174,864. Intraperitoneal administration of SYK-657 induced the short-term memory improving effect in mice without abnormal behaviors.

Two delta opioid receptor subtypes are functional in single ventral tegmental area neurons, and can interact with the mu opioid receptor

The mu and delta opioid receptors (MOR and DOR) are highly homologous members of the opioid family of GPCRs. There is evidence that MOR and DOR interact, however the extent to which these interactions occur in vivo and affect synaptic function is unknown. There are two stable DOR subtypes: DPDPE sensitive (DOR1) and deltorphin II sensitive (DOR2); both agonists are blocked by DOR selective antagonists. Robust motivational effects are produced by local actions of both MOR and DOR ligands in the ventral tegmental area (VTA). Here we demonstrate that a majority of both dopaminergic and non-dopaminergic VTA neurons express combinations of functional DOR1, DOR2, and/or MOR, and that within a single VTA neuron, DOR1, DOR2, and MOR agonists can differentially couple to downstream signaling pathways. As reported for the MOR agonist DAMGO, DPDPE and deltorphin II produced either a predominant K⁺ dependent hyperpolarization or a Ca_v2.1 mediated depolarization in different neurons. In some neurons DPDPE and deltorphin II produced opposite responses. Excitation, inhibition, or no effect by DAMGO did not predict the response to DPDPE or deltorphin II, arguing against a MOR-DOR interaction generating DOR subtypes. However, in a subset of VTA neurons the DOR antagonist TIPP-Ψ augmented DAMGO responses; we also observed DPDPE or deltorphin II responses augmented by the MOR selective antagonist CTAP. These findings directly support the existence of two independent, stable forms of the DOR, and show that MOR and DOR can interact in some neurons to alter downstream signaling.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Putative kappa opioid heteromers as targets for developing analgesics free of adverse effects

It is now generally recognized that upon activation by an agonist, β-arrestin associates with G protein-coupled receptors and acts as a scaffold in creating a diverse signaling network that could lead to adverse effects. As an approach to reducing side effects associated with κ opioid agonists, a series of β-naltrexamides 3-10 was synthesized in an effort to selectively target putative κ opioid heteromers without recruiting β-arrestin upon activation. The most potent derivative 3 (INTA) strongly activated KOR-DOR and KOR-MOR heteromers in HEK293 cells. In vivo studies revealed 3 to produce potent antinociception, which, when taken together with antagonism data, was consistent with the activation of both heteromers. 3 was devoid of tolerance, dependence, and showed no aversive effect in the conditioned place preference assay. As immunofluorescence studies indicated no recruitment of β-arrestin2 to membranes in coexpressed KOR-DOR cells, this study suggests that targeting of specific putative heteromers has the potential to identify leads for analgesics devoid of adverse effects.

Twin and triplet drugs in opioid research

Twin and triplet drugs are defined as compounds that contain respectively two and three pharmacophore components exerting pharmacological effects in a molecule. The twin drug bearing the same pharmacophores is a "symmetrical twin drug", whereas that possessing different pharmacophores is a "nonsymmetrical twin drug." In general, the symmetrical twin drug is expected to produce more potent and/or selective pharmacological effects, whereas the nonsymmetrical twin drug is anticipated to show both pharmacological activities stemming from the individual pharmacophores (dual action). On the other hand, nonsymmetrical triplet drugs, which have two of the same pharmacophores and one different moiety, are expected to elicit both increased pharmacological action and dual action. The two identical portions could bind the same receptor sites simultaneously while the third portion could bind a different receptor site or enzyme. This review will mainly focus on the twin and triplet drugs with an evaluation of their in vivo pharmacological effects, and will also include a description of their pharmacology and synthesis.

Synthesis of 14-alkoxymorphinan derivatives and their pharmacological actions

Among opioids, morphinans play an important role as therapeutically valuable drugs. They include pain relieving agents such as naturally occurring alkaloids (e.g. morphine, codeine), semisynthetic derivatives (e.g. oxycodone, oxymorphone, buprenorphine), and synthetic analogs (e.g. levorphanol). Currently used opioid analgesics also share a number of severe side effects, limiting their clinical usefulness. The antagonist morphinans, naloxone and naltrexone are used to treat opioid overdose, opioid dependence, and alcoholism. All these opioid drugs produce their biological actions through three receptor types, mu, delta, and kappa, belonging to the G-protein-coupled receptor family. Considerable effort has been put forward to understand the appropriate use of opioid analgesics, while medicinal chemistry and opioid pharmacology have been continuously engaged in the search for safer, more efficacious and nonaddicting opioid compounds, with the final goal to reduce complications and to improve patient compliance. Toward this goal, recent advances in chemistry, ligand-based structure activity relationships and pharmacology of 14-alkoxymorphinans are reviewed in this chapter. Current developments of different structural patterns of 14-alkoxymorphinans as research tools and their potential therapeutic opportunities are also summarized.

Novel synthesis of a 1,3,5-trioxazatriquinane skeleton using a nitrogen clamp

An alpha-hydroxyaldehyde derived from naltrexone was converted to an oxazoline dimer with ammonium chloride and sodium acetate in MeOH under reflux. The resulting dimer was treated with dl-camphorsulfonic acid in CHCl(3) to give the trimer. The method for trimer synthesis was also applied to general alpha-hydroxyaldehydes to afford trimers in good yield.

Pharmacological studies with a nonpeptidic, delta-opioid (-)-(1R,5R,9R)-5,9-dimethyl-2'-hydroxy-2-(6-hydroxyhexyl)-6,7-benzomorphan hydrochloride ((-)-NIH 11082)

In the search for a selective delta-opioid receptor agonist, (-)-(1R,5R,9R)-5,9-dimethyl-2'-hydroxy-2-(6-hydroxyhexyl)-6,7-benzomorphan hydrochloride ((-)-NIH 11082) and the (+)-enantiomer were synthesized and tested. (-)-NIH 11082 displayed antinociceptive activity in the paraphenylquinone test (PPQ test) in male ICR mice [ED50=1.9 (0.7-5.3) mg/kg, s.c.] and showed little, if any, activity in the tail-flick and hot-plate assays. The (+)-enantiomer was essentially inactive indicating stereoselectivity. Opioid receptor subtype characterization studies indicated that naltrindole, a delta-opioid receptor antagonist, was potent versus the ED80 of (-)-NIH 11082 in the PPQ test [AD50=0.75 (0.26-2.20) mg/kg, s.c]. beta-Funaltrexamine and nor-binaltorphimine, selective mu- and kappa-receptor antagonists, respectively, were inactive versus the ED80 of (-)-NIH 11082. In rats with inflammation-induced pain, (-)-NIH 11082 produced antihyperalgesic effects that were attenuated by naltrindole. In morphine-dependent rhesus monkeys of both sexes, (-)-NIH 11082 neither substituted for morphine nor exacerbated withdrawal signs in the dose range of 4.0 to 32.0 mg/kg, s.c. Neither convulsions nor other overt behavioral signs were observed in any of the species tested. The results indicate that (-)-NIH 11082 has delta-opioid receptor properties.

delta-Opioid receptor antagonists inhibit GIRK channel currents in acutely dissociated brainstem neurons of rat

In this study, we investigated the effects of delta-opioid receptor antagonists on the G protein-coupled inwardly rectifying potassium (GIRK) channel currents induced by serotonin (5-HT) and noradrenaline (NAd) in the dorsal raphe and the locus coeruleus neurons, respectively. Perforated patch and conventional whole-cell patch clamp recording techniques were used for the study. Neurons were acutely dissociated from neonatal rats. Both naltrindole (NTI) and naltriben (NTB), which are selective delta-antagonists possessing antitussive activity in in vivo animal studies, reversibly inhibited the 5-HT-induced GIRK channel currents (I(5-HT)) in dorsal raphe neurons. This inhibition was concentration-dependent and voltage-independent. The half-maximum inhibitory concentration (IC(50)) on I(5-HT) was 9.84x10(-5) M for NTI and 1.28x10(-5) M for NTB. The inhibition was not reversed by 10(-5) M DPDPE, a selective delta-opioid receptor agonist. NTI did not affect 50% effective concentration (EC(50)) on the concentration-response relationship for 5-HT but inhibited the maximum response. In neurons internally perfused with GTPgammaS, both NTI and NTB also inhibited the GIRK channel currents irreversibly activated by 5-HT. Furthermore, these antagonists concentration dependently inhibited 10(-6) M NAd-induced currents (I(NAd)) in locus coeruleus neurons. The IC(50) of NTI on I(NAd) was 8.44x10(-5) M, which was close to that on I(5-HT). The results suggest that NTI and NTB, which are delta-opioid receptor antagonists possessing antitussive activity, may inhibit GIRK channel currents through a non-opioid action, and give further support to our idea previously proposed that centrally acting non-narcotic antitussives have a common characteristic of the inhibitory action on GIRK channels.

Differential effects of CB1 and opioid agonists on two populations of adult rat dorsal root ganglion neurons

Inhibition of primary afferent neurons contributes to the antihyperalgesic effects of opioid and CB1 receptor agonists. Two bioassays were used to compare the effects of the CB1 receptor agonist CP 55,940 and morphine on dissociated adult rat DRG neurons. Both agonists inhibited the increase in free intracellular Ca2+ concentration evoked by depolarization; however, effects of CP 55,940 occurred primarily in large neurons (cell area, >800 microm2), whereas morphine inhibited the response in smaller neurons. Cotreatment with selective blockers of L-, N-, and P/Q-type voltage-dependent Ca2+ channels indicated that CB1 receptors on DRG neurons couple solely with N-type channels but opioid receptors couple with multiple subtypes. Experiments with selective agonists and antagonists of opioid receptors indicated that mu and delta, but not kappa, receptors contributed to the inhibitory effect of morphine on voltage-dependent Ca2+ influx. Because Ca2+ channels underlie release of transmitters from neurons, the effects of opioid agonists and CP 55,940 on depolarization-evoked release of calcitonin gene-related peptide (CGRP) were compared. Morphine inhibited release through delta receptors but CP 55,940 had no effect. Colocalization of CGRP with delta-opioid but not mu-opioid or CB1 receptor immunoreactivity in superficial laminae of the dorsal horn of the spinal cord was consistent with the data for agonist inhibition of peptide release. Therefore, CB1 and opioid agonists couple with different voltage-dependent Ca2+ channels in different populations of DRG neurons. Furthermore, differences occur in the distribution of receptors between the cell body and terminals of DRG neurons. The complementary action of CB1 and opioid receptor agonists on populations of DRG neurons provides a rationale for their combined use in modulation of somatosensory input to the spinal cord.

Novel ligands for the opioid receptors: synthesis and structure-activity relationships among 5'-aryl and 5'-heteroaryl 17-cyclopropylmethyl-4,5 alpha-epoxypyrido[2',3':6,7]morphinans

A series of pyridomorphinans possessing an aryl (10a-s) or heteroaryl (11a-h) substituent at the 5'-position of the pyridine ring of 17-cyclopropylmethyl-4,5 alpha-epoxypyrido[2',3':6,7]morphinan was synthesized and evaluated for binding and functional activity at the opioid delta, mu, and kappa receptors. All of these pyridomorphinans bound with higher affinity at the delta site than at mu or kappa sites. The binding data on isomeric compounds revealed that there exists greater bulk tolerance for substituents placed at the o-position of the phenyl ring than at m- or p-positions. Among the ligands examined, the 2-chlorophenyl (10l), 2-nitrophenyl (10n), 2-pyridyl (11a), and 4-quinolinyl (11g) compounds bound to the delta receptor with subnanomolar affinity. Compound 10c with the p-tolyl substituent displayed the highest mu/delta selectivity (ratio=42) whereas compound 10l with the 2-chlorophenyl substituent displayed the highest kappa/delta selectivity (ratio=23). At 10 microM concentration, the in vitro functional activity determined using [(35)S]GTP-gamma-S binding assays showed that all of the compounds were antagonists devoid of any significant agonist activity at the delta, mu, and kappa receptors. Antagonist potency determinations of three selected ligands revealed that the p-tolyl compound 10c is a potent delta selective antagonist. In the [(35)S]GTP-gamma-S assays this compound had a functional antagonist K(i) value of 0.2, 4.52, and 7.62 nM at the delta, mu, and kappa receptors, respectively. In the smooth muscle assays 10c displayed delta antagonist potency with a K(e) value of 0.88 nM. As an antagonist, it was 70-fold more potent at the delta receptors in the MVD than at the mu receptors in the GPI. The in vitro delta antagonist profile of this pyridomorphinan 10c resembles that of the widely used delta selective antagonist ligand naltrindole.

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

Pharmacological studies performed in vivo suggested that the delta-opioid receptor could exist as two distinct subtypes, delta(1) and delta(2), while in vitro studies are inconclusive. Therefore, we measured the binding and functional selectivity of several putative delta(1)- and delta(2)-opioid receptor-selective compounds in membranes from Chinese hamster ovary cells stably expressing the human delta-opioid receptor. The compounds characterized were the agonists [D-Pen(2),D-Pen(5)]enkephalin (DPDPE, delta(1)) and deltorphin II (delta(2)), and the antagonists 7-benzylidenenaltrexone (BNTX, delta(1)), naltriben (delta(2)), naltrindole 5'-isothiocyanate (delta(2)), and naltrindole (delta(1) and delta(2)). In competition binding assays, all compounds tested showed no preference for the [3H]DPDPE, [3H]deltorphin II, or [3H]naltrindole binding sites. BNTX also showed no selectivity for the delta-opioid receptor over the mu-opioid receptor. In functional assays, the stimulation of [35S]GTPgammaS binding induced by either DPDPE or deltorphin II was potently inhibited by both delta(1)- and delta(2)-opioid receptor-selective antagonists. Together, these results indicate that these compounds are not selective for either the delta(1)- or delta(2)-opioid receptor binding sites in binding or functional assays.

Recent advances in selective opioid receptor agonists and antagonists

Opioid analgesics provide outstanding benefits for relief of severe pain. The mechanisms of the analgesia accompanied with some side effects have been investigated by many scientists to shed light on the complex biological processes at the molecular level. New opioid drugs and therapies with more desirable properties can be developed on the bases of accurate insight of the opioid ligand-receptor interaction and clear knowledge of the pharmacological behavior of opioid receptors and the associated proteins. Toward this goal, recent advances in selective opioid receptor agonists and antagonists including opioid ligand-receptor interactions are summarized in this review article.

Derivatives of 17-(2-methylallyl)-substituted noroxymorphone: variation of the delta address and its effects on affinity and selectivity for the delta opioid receptor

In an effort to establish the importance of the N-(2-methylallyl) substituent in the noroxymorphone series, several derivatives have been synthesized, retaining that N-substituent and modifying the delta address moiety. A few compounds showed moderate binding affinity and selectivity for the delta receptor; none displayed a pharmacological profile as exceptional as N-(2-methylallyl)noroxymorphindole. A second study showed that 3-O-methylation of all derivatives decreases binding affinity. The present results indicate that only a combination of the N-(2-methylallyl) group and an indole delta address provided high selectivity for the delta receptor.

Opioidergic contribution to conditioned place preference induced by corn oil in mice

We previously reported that voluntary intake of corn oil in the light box showed place preference in the conditioned place preference (CPP) test in mice. In the present study, we investigated the contribution of opioidergic systems to the corn oil-induced CPP in mice. Acquisition of the place preference by corn oil intake was blocked by i.p. injections of an opioid mu antagonist, naloxone (0.1 and 0.3 mg/kg), and delta antagonists, 7-benzylidenenaltrexone (0.5 mg/kg) and naltriben (0.5 mg/kg) 15 min before conditioning. The opioid kappa agonist U-50488H (1 and 3 mg/kg i.p.) also blocked corn oil-induced CPP. Naloxone (1 mg/kg, i.p.) and naltriben (0.5 mg/kg, i.p.) did not affect corn oil intake in the home cage. However, 7-benzylidenenaltrexone (0.5 mg/kg, i.p.) and U-50488H (1 mg/kg i.p.) decreased and increased the corn oil intake, respectively. These results suggested that the rewarding effects of corn oil in the CPP test are at least partially mediated via opioidergic systems through mu and delta receptors. Further, we showed that an opioid kappa agonist reduced the rewarding effects of corn oil in the CPP test in mice, although it increased corn oil intake.

Opioid pseudopeptides containing heteroaromatic or heteroaliphatic nuclei

In lieu of H-Dmt-Tic-OH, H-Dmt-analogues included 2-amino-3(1H-benzoimidazol-2-yl)-propionic acid, N(Bzl)Gly, L-octahydroindole-2-carboxylic acid, [3S-(3alpha,4abeta, 8abeta)]-decahydro-3-isoquinoline carboxylic acid, benzimidazole-, pyridoindole- or spiroinden-derivatives, or C-terminally modified. L- or D-Ala, Sar, or Pro were spacers between aromatic nuclei. Only H-Dmt-(Xaa-)-pyridoindole exhibited high affinities with delta and mu antagonism. The peptides competed equally against [3H]DPDPE (delta agonist) or [3H]N,N(CH3)2-Dmt-Tic-OH (delta antagonist) signaling a single delta binding site. The data confirm the importance of Tic for delta affinity and antagonism, while heterocyclic or heteroaliphatic nuclei, or spacer exert effects on mu- and delta-receptor properties.

Fentanyl increases dopamine release in rat nucleus accumbens: involvement of mesolimbic mu- and delta-2-opioid receptors

The effects of the mu-receptor agonist fentanyl on extracellular levels of dopamine in rat nucleus accumbens were studied in awake animals by in vivo brain microdialysis. Fentanyl dose-dependently increased the levels of dopamine when given intravenously (microg/kg) or via a microdialysis probe placed into the ventral tegmental area or the nucleus accumbens (nmol). The effect of fentanyl given into the nucleus accumbens was blocked by systemic administration of the non-selective opioid receptor antagonist naloxone and by accumbens administration of D-Phe-Cys-Tyr-D-Trp-Om-Thr-Phe-Thr-NH2 (nmol), a mu-opioid receptor antagonist, and naltrindole (nmol), a non-selective delta-opioid receptor antagonist, in a dose-dependent manner. The delta2-opioid receptor antagonist, naltriben (nmol), also blocked the effects of fentanyl, whereas the delta1-opioid receptor antagonist, (E)-7-benzylidenenaltrexone (nmol), was ineffective. When marginally effective doses of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH2 and naltriben were given simultaneously, the effect of fentanyl was nearly fully blocked; the pretreatment itself had no effect. Administration of the mu-opioid receptor agonist [D-Ala2, N-Me-Phe4,Gly5-ol]-enkephalin (nmol), the delta1-opioid receptor agonist [D-Pen2,5]-enkephalin (nmol) or the delta2-opioid receptor agonist [D-Ala2,Glu4]-deltorphin (nmol) into the nucleus accumbens enhanced the amount of accumbal dopamine. This study provides evidence that not only activation of delta1- and delta2-opioid receptors, but also activation of mu-opioid receptors in the nucleus accumbens increases the release of accumbal dopamine in freely moving rats. We suggest that the effect of intra-accumbens administration of fentanyl upon accumbal release of dopamine is either due to the simultaneous activation of mu-opioid receptors and delta2-opioid receptors or due to activation of mu-opioid receptors that interact with delta2-opioid receptors in a complex manner.

Synthesis and pharmacological activity of thebaine-derived mu-opioid receptor agonists

Thebaine-derived mu-opioid agonists were synthesized through the reaction of thebaine with N-aryl maleimide and tested for opioid activity. Morphine was used as reference compound. Our results show that an attachment of aryl succinimide group to thebaine produced series of compounds with mu-opioid agonist activity. The most active compound in smooth muscle preparation was compound 6 with an IC50 ratio of delta/mu = 248.69 and was as potent as morphine with ED50 value 26.65 mg kg-1 i.p. in hot-plate test and showed good antinociceptive activity.

Slide-binding characterization and autoradiographic localization of delta opioid receptors in rat and mouse brains with the tetrapeptide antagonist [3H]TIPP

Slide-binding and autoradiographic studies were performed on cryostat sections from brains of adult Sprague-Dawley rats and BALB C mice to describe the binding characteristics of the tetrapeptide [3H]TIPP, an antagonist with high specificity and affinity for the delta opioid receptors. Steady-state binding of [3H]TIPP to cryostat sections of brain paste was reached in 120-180 min of incubation. Specific [3H]TIPP binding resulted in maximal numbers of binding sites (Bmax) of 15.59 and 23.91 fmol/mg protein, and dissociation constants (Kd) of 0.46 and 0.85 nM for rat and mouse brain paste sections, respectively. TIPP displayed the highest affinity for delta opioid receptors in inhibiting specific [3H]TIPP binding, with IC50 values of 0.82 nM and 0.14 nM in rat and mouse brain sections, respectively. While DPDPE was also effective in displacing the specific binding of [3H]TIPP (IC50 = 3.18 +/- 0.53 nM and 0.63 +/- 0.42 nM in rat and mouse brain paste sections, respectively), other subclass-selective or nonopioid ligands were much less effective, or ineffective. Autoradiographic localization of [3H]TIPP binding revealed the characteristic distribution of delta opioid receptors in both species. In consequence of its antagonistic nature, and of its unnatural amino acid residue, which makes this ligand more resistant to biodegradation, [3H]TIPP is a superior ligand for evaluation of the binding characteristics and autoradiogaphic distribution of the delta opioid receptors.

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.

14-Desoxy analogues of naltrindole and 7-spiroindanyloxymorphone: the role of the 14-hydroxy group at delta opioid receptors

The 14-hydroxy group is known to increase the antagonist potency of mu-selective opioid ligands. To investigate the role of this group at the delta opioid receptor, the 14-desoxy analogues (7 and 9) of the delta-selective ligands, naltrindole (1, NTI) and spiroindanyloxymorphone (2, SIOM), have been synthesized and tested. The in vitro pharmacologic activities of 7 and 9 suggest that the 14-hydroxy group plays an important role in determining the delta selectivity and potency of NTI and SIOM.

Selective in vivo binding of [3H]naltriben to delta-opioid receptors in mouse brain

Naltriben (NTB) is a selective antagonist for the putative delta2-opioid receptor. We have determined the regional kinetics and pharmacological profile of [3H]naltriben in vivo in mouse brain. After i.v. administration to CD1 mice, [3H]naltriben uptake and retention were high in striatum, cortical regions and olfactory tubercles, and low in superior colliculi and cerebellum. Robust rank order correlation was found between [3H]naltriben uptake in discrete brain regions and prior delta-opioid receptor binding determinations in vitro and in vivo. [3H]Naltriben binding in vivo was saturable, and was blocked by the delta-opioid receptor antagonist naltrindole, but not by the mu-opioid receptor antagonist cyprodime or the K-opioid receptor agonist (trans)-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]ben zeneacetamide mesylate (U50,488H). (E)-7-Benzylidenenaltrexone (BNTX), a selective antagonist for the putative delta1-opioid receptor, was 9.6- to 12.9-fold less potent than naltriben as an inhibitor of [3H]naltriben binding. Thus, the sites labeled by [3H]naltriben in vivo may correspond to the delta2-opioid receptor subtype. Such assignment is not definitive, particularly considering the 4-fold higher brain uptake of naltriben as compared to (E)-7-benzylidenenaltrexone. Moreover, the regional distribution of [3H]naltriben in brains from CXB-7/BY (CXBK) mice, a strain that shows supraspinal delta1- but not delta2-opioid receptor agonist effects, was quite similar to that found for CD1 mice.

The role of delta-opioid receptors in the discriminative stimulus properties of a low dose of methamphetamine

The effects of selective mu-, delta- and kappa-opioid receptor agonists and antagonists on the discriminative stimulus properties of methamphetamine were examined in rats that had been trained to discriminate between methamphetamine (0.4 mg/kg) and saline. Methamphetamine produced a dose-related increase in methamphetamine-appropriate responses in all of the rats. In generalization tests, neither morphine (a mu-opioid receptor agonist: 0.3-10 mg/kg) nor 3,4-dichloro-N-[2-(1-pyrrolidinyl)cyclohexo]benzeneacetamide (U50,488H: a kappa-opioid receptor agonist: 1.0-8.0 mg/kg) generalized to the discriminative stimulus properties of methamphetamine. A newly synthesized non-peptide selective delta-opioid receptor agonist 2-methyl-4aalpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aalpha- octahydroquinolino(2,3,3,-g)isoquinoline (TAN-67: 32 mg/kg) partially generalized (70% methamphetamine-appropriate responses) to the discriminative stimulus properties of methamphetamine. In combination tests, pretreatment with the mu- and kappa-opioid receptor antagonists, beta-funaltrexamine (9.0 mg/kg) and nor-binaltorphimine (10 mg/kg), respectively, had little or no influence on the discriminative stimulus properties of methamphetamine. In contrast, pretreatment with naltrindole (a non-selective delta-opioid receptor antagonist: 3.0 mg/kg) or naltriben (a selective delta2-opioid receptor antagonist: 1.0 mg/kg), but not with 7-benzylidenenaltrexone (a selective delta1-opioid receptor antagonist: 0.5 and 1.0 mg/kg), significantly attenuated the discriminative stimulus properties of methamphetamine. However, naltrindole (3.0 mg/kg) did not significantly attenuate the discriminative stimulus properties of methamphetamine at a higher training dose (1.0 mg/kg). Our findings may have some bearing on the relative importance of the role of delta-opioid (especially delta2-opioid) receptors in the discriminative stimulus properties of a low dose of methamphetamine.

Syntheses of novel pyridazinomorphinans by inverse electron demand cycloaddition and their binding to mu and kappa receptors

A number of novel pyridazinomorphinans have been synthesized by the inverse electron demand Diels-Alder reaction of various 3,6-disubstituted 1,2,4,5-tetrazines with enamines derived from dihydrocodeinone and with codeinone. Reduction of some of the pyridazinomorphinans did not furnish the expected pyrroloepoxymorphinans; in all cases investigated reductive cleavage of the epoxybridge was observed to yield dihydropyridazino- or pyrrolomorphinans. The structures of all new compounds were assigned by the spectral data, that of the cycloadduct of codeinone was additionally verified by X-ray crystallography. Compounds 5a, 8, 11a and 16 have been evaluated for their affinity at mu and kappa opioid receptors in radioligand binding assays. Their ability to inhibit [3H]DAMGO binding at mu and [3H]U 69.593 binding at kappa receptors, respectively as compared to codeine has been found to be lower.

Involvement of delta 1 and delta 2 opioid receptor subtypes in the development of physical dependence on morphine in mice

The effects of the highly selective delta opioid receptor antagonists naltrindole (NTI) for delta 1 and delta 2 naltriben (NTB) and naltrindole 5'-isothiocyanate (5'-NTII) for delta 2 and 7-benzylidenenaltrexone (BNTX) for delta 1 on the development of physical dependence on morphine were investigated in mice. Neither NTI (3 mg/kg, sc), NTB (0.5 mg/kg, sc), 5'-NTII (0.5 mg/kg, sc) nor BNTX (0.5 mg/kg, sc) suppressed the antinociception induced by morphine (10 mg/kg, sc). Pretreatment with NTI (3 mg/kg, sc), NTB (0.5, 1.0 mg/kg, sc) or 5'-NTII (0.5, 1.0 mg/kg, sc) during chronic treatment with morphine for 5 days significantly suppressed naloxone-induced body-weight loss in morphine-dependent mice. The incidence of jumping and body shakes in morphine-dependent mice that were pretreated with NTI. NTB or 5'-NTII were significantly lower than with morphine alone. Pretreatment with BNTX (0.5, 1.0 mg/kg, sc) during chronic treatment with morphine also significantly suppressed naloxone-induced body-weight loss in morphine-dependent mice, but this suppression was weaker than that by the antagonists. In contrast to mice that had been pretreated with NTI, NTB or 5'-NTII, the incidence of several withdrawal signs, such a jumping and body shakes, was not significantly affected in morphine-dependent mice that were pretreated with BNTX. These findings suggest that both delta 2 and delta 1 opioid receptors may play important roles in modulating the development of physical dependence on morphine.

Opioid diketopiperazines: synthesis and activity of a prototypic class of opioid antagonists

Discovery of high affinity and ultraselective delta opioid dipeptide antagonists composed of 2',6'-dimethyl-L-tyrosyl (Dmt) and 1,2,3,4-tetrahydroisoquinoline-3-carboxylic (Tic) served as the basis for the conformationally restricted diketopiperazine cyclo(Dmt-Tic) and related open chain analogues. These peptides primarily bind to delta opioid receptors: c(Dmt-Tic) displayed 30- to 50-fold higher delta affinity (Ki delta) than its diastereo-isomeric analogues and more than 4000-fold greater than its Tyr cognate; all of the c(Tyr-Tic) analogues were essentially inactive; c[(N-methyl)Dmt-Tic] lost 5-fold in Ki delta, while Ki mu increased 10-fold to yield a nonselective peptide; and the c(Dmt-Phe) series exhibited considerably reduced binding which indicated a synergism between Dmt and Tic in the binding mechanism. Whereas acetyl-Dmt-Tic linear peptides weakly interacted with opioid receptors, Ac-Dmt-Tic-NH2, exhibited better delta antagonist activity than c(Dmt-Tic) and greater delta receptor selectivity (Ki mu/Ki delta = 570). A three point attachment hypothesis for the interaction between c(Dmt-Tic) and the delta receptor was proposed: hydrophobicity imparted by the aromatic rings and the methyl groups of Dmt, hydrogen bonding through the tyramine hydroxyl group, and cation-pi interactions were suggested as contributing factors in binding the diketopiperazine in the receptor pocket. Although c(Dmt-Tic) exhibited a weak antagonist activity with mouse vas deferens, this diketopiperazine may provide a scaffolding for the formation of more potent antagonists for potential therapeutic applications.

Opposite role of delta 1- and delta 2-opioid receptors activated by endogenous or exogenous opioid agonists on the endogenous cholecystokinin system: further evidence for delta-opioid receptor heterogeneity

Using the mouse caudate-putamen, where delta-opioid receptor subtypes have been shown to regulate adenylyl cyclase activity, we show in this study that endogenous enkephalins inhibit enzyme activity through activation of delta 1- and delta 2-opioid receptors. Thus, naltriben or 7-benzylidenenaltrexone as well as the delta-selective antagonist naltrindole (mixed delta 1 and delta 2 antagonist) antagonized inhibition of adenylyl cyclase activity induced by methionine- or leucine-enkephalin, while the micro-antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) was without effect. Furthermore, we have previously shown that activation of delta-opioid receptors increases cholecystokinin release in the central nervous system, resulting in a potentiation of micro-opioid antinociceptive responses, and the respective role of delta 1- and delta 2-opioid receptors in this facilitatory effect has now been evaluated. Activation of delta 2-opioid receptors, either by endogenous enkephalins protected from catabolism by the complete enkephalin-degrading enzyme inhibitor N-((R,S)-2-benzyl-3((S)(2-amino-4-methyl-thio) butyldithio)-1-oxopropyl)-L-phenyl-alanine benzyl ester (RB 101), or by the delta 2-selective agonist Tyr-D-Ser(O-tert-butyl)-Gly-Phe-Leu-Thr(O-tert-butyl) (BUBU), potentiated micro-opioid antinociceptive responses in the hot-plate test in mice. This effect was antagonized by a selective cholecystokinin-A antagonist. Activation of delta 1-opioid receptors by endogenous opioid peptides decreased the micro-opioid responses. These results suggest that stimulation of delta 2-opioid receptors potentiates micro-opioid analgesia in the hot-plate test in mice through an increase in endogenous cholecystokinin release, while activation of delta 1-opioid receptors could decrease it. Thus, the pre-existing physiological balance between opioid and cholecystokinin systems seems to be modulated in opposite directions depending on whether delta 1- or delta 2-opioid receptors are selectively activated. This is the first demonstration that endogenous enkephalins, methionine- and leucine-enkephalin, are the natural ligands of delta-opioid receptor subtypes, and that delta 2-opioid receptor activation may facilitate the endogenous cholecystokinin-related modulation of micro-opioid analgesia, while the delta 1-opioid receptors may have an inhibitory role. These results could have important applications for the characterization of opioid delta 1 and delta 2 as subtypes or subsites and in pain alleviation.

Association of enkephalin catabolism inhibitors and CCK-B antagonists: a potential use in the management of pain and opioid addiction

The overlapping distribution of opioid and cholecystokinin (CCK) peptides and their receptors (mu and delta opioid receptors; CCK-A and CCK-B receptors) in the central nervous system have led to a large number of studies aimed at clarifying the functional relationships between these two neuropeptides. Most of the pharmacological studies devoted to the role of CCK and enkephalins have been focused on the control of pain. Recently the existence of regulatory mechanisms between both systems have been proposed, and the physiological antagonism between CCK and endogenous opioid systems has been definitely demonstrated by coadministration of CCK-B selective antagonists with RB 101, a systemically active inhibitor, which fully protects enkephalins from their degradation. Several studies have also been done to investigate the functional relationships between both systems in development of opioid side-effects and in behavioral responses. This article will review the experimental pharmacology of association of enkephalin-degrading enzyme inhibitors and CCK-B antagonists to demonstrate the interest of these molecules in the management of both pain and opioid addiction.

Comparison of cyclic delta-opioid peptides with non-peptide delta-agonist spiroindanyloxymorphone (SIOM) using the message-address concept: a molecular modeling study

Based upon the message-address concept, this molecular modeling study used the delta-selective agonist spiroindanyloxymorphone (SIOM) as a molecular template for a conformational search and analysis of delta-selective opioid peptides. It was assumed that the tyramine moiety plays the same role for delta-opioid receptor recognition in both peptide and non-peptide ligands. Using 20 reported low-energy conformations of Tyr-cyclo[D-Cys-D-Pen]-OH (JOM-13) for comparison, the geometrical relationship of the two aromatic rings present in SIOM was used for the identification of potential active conformations of JOM-13, from which two delta-receptor-binding models (I and II) were constructed. Models I and II differ from each other in the arrangement of the peptide backbones. To evaluate the two models, a conformational search of two other known delta-selective ligands, [D-Pen2,D-Pen5]enkephalin (DPDPE) and [D-Pen2,L-Pen5]enkephalin (DPLPE) was performed, using the geometrical relationship of the two aromatic rings defined in the two receptor-binding models as a molecular template. Among the conformations generated from the molecular simulation, low-energy conformers of DPDPE and DPLPE conforming to models I and II were identified. Unlike model I, conformers of DPDPE and DPLPE that fit model II contain a cis amide bond in the Gly3 residue.

Differences among mouse strains in the regulation by mu, delta 1 and delta 2 opioid receptors of striatal adenylyl cyclases activated by dopamine D1 or adenosine A2a receptors

It is well established that the intensity of the morphine-induced locomotor stimulatory response in rodents is genetically determined. We have evaluated the ability of mu (DAMGO). delta 1 (DPDPE) and delta 2 ([D-Ala2]deltorphin II; DT-II) selective opioid receptor agonists to inhibit receptor-activated adenylyl cyclase activity. Previous studies have shown that dopamine D1 receptors are preferentially expressed in striatonigral neurons, while adenosine A2a receptors are preferentially expressed in striatopallidal neurons. Our results indicate that in striatum of wild-type CD1 mice as in rat. mu-agonists inhibit D1-activated adenylyl cyclase, but were without inhibitory activity against A2a-activated adenylyl cyclase. In all tested mouse strains, the delta 1 selective agonist, DPDPE, inhibited the A2a-activated adenylyl cyclase but not the D1-activated enzyme. In contrast, the delta 2 agonist, DT-II, inhibited both D1 and A2a activated enzyme in all strains except DBA/2J, where DT-II did not inhibit D1-activated adenylyl cyclase. In C57BL/6J mice, which are highly sensitive to morphine-induced motor stimulation, mu agonists were potent inhibitors of A2a-activated adenylyl cyclase but did not inhibit the D1-activated enzyme, while mu opioids did not inhibit A2a-activated enzyme in DBA/2J mice which show little locomotor response to morphine. In CXBK mice, a strain with known deficiencies in mu-receptor expression, both forms of receptor-activated adenylyl cyclase were less sensitive than in CD1 mice to inhibition by DAMGO. The results suggest probable differences in the specific neural locations of opioid receptor subtypes in certain inbred strains of mice with varying sensitivities to the behavioral effects of morphine.

Role of delta-opioid receptors in mediating the aversive stimulus effects of morphine withdrawal in the rat

An unbiased place preference conditioning procedure was used to examine the role of delta-opioid receptors in mediating the aversive effects of opioid withdrawal. Rats were implanted s.c. with two pellets each containing placebo or 75 mg morphine. Single-trial conditioning sessions with saline and the opioid receptor antagonists naloxone (0.001-1.0 mg/kg, s.c.), naltrindole (0.01-3.0 mg/kg, s.c.) or naltriben (0.01-3.0 mg/kg, s.c.) commenced 4 days later. During these conditioning sessions, physical signs of withdrawal were also quantified. Tests of conditioning were conducted on day 5. Naloxone in doses of 0.01-1.0 mg/kg produced significant conditioned place aversions in morphine-implanted animals. A dose of 0.01 mg/kg produced few physical withdrawal signs whereas higher doses resulted in marked wet dog shakes, body weight loss ptosis and diarrhea. No such effects were observed in control (placebo-implanted) animals. Administration of the selective delta-opioid receptor antagonists naltrindole and naltriben produced dose-related place aversions in morphine-implanted animals. The magnitude of these effects did not differ from that observed with naloxone. The minimum effective doses of naltrindole and naltriben were 0.1 mg/kg. Doses of 0.1-1.0 mg/kg produced few, if any, somatic signs of withdrawal whereas higher doses of these antagonists only produced diarrhea and wet-dog shakes. Other withdrawal signs were absent. In contrast to the opioid receptor antagonists tested, the dopamine D1 receptor antagonist SCH23390 failed to produced conditioned place aversions or physical signs of withdrawal in morphine-pelleted animals. These data demonstrate that the selective blockade of either delta- or mu-opioid receptors is sufficient to induce conditioned aversive effects in morphine-dependent animals. They also indicate that physical symptoms associated with precipitated morphine withdrawal differ depending upon the opioid receptor antagonist employed.