Differential modulation of mu-opioid receptor-mediated antitussive activity by delta-opioid receptor agonists in mice

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

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

OBJECTIVE

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

CONCLUSION

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

Central mechanisms II: pharmacology of brainstem pathways

Following systemic administration, centrally acting antitussive drugs are generally assumed to act in the brainstem to inhibit cough. However, recent work in humans has raised the possibility of suprapontine sites of action for cough suppressants. For drugs that may act in the brainstem, the specific locations, types of neurones affected, and receptor specificities of the compounds represent important issues regarding their cough-suppressant actions. Two medullary areas that have received the most attention regarding the actions of antitussive drugs are the nucleus of the tractus solitarius (NTS) and the caudal ventrolateral respiratory column. Studies that have implicated these two medullary areas have employed both microinjection and in vitro recording methods to control the location of action of the antitussive drugs. Other brainstem regions contain neurones that participate in the production of cough and could represent potential sites of action of antitussive drugs. These regions include the raphe nuclei, pontine nuclei, and rostral ventrolateral medulla. Specific receptor subtypes have been associated with the suppression of cough at central sites, including 5-HT1A, opioid (mu, kappa, and delta), GABA-B, tachykinin neurokinin-1 (NK-1) and neurokinin-2, non-opioid (NOP-1), cannabinoid, dopaminergic, and sigma receptors. Aside from tachykinin NK-1 receptors in the NTS, relatively little is known regarding the receptor specificity of putative antitussive drugs in particular brainstem regions. Our understanding of the mechanisms of action of antitussive drugs would be significantly advanced by further work in this area.

Delta-opioid receptor antagonists as a new concept for central acting antitussive drugs

Our recent findings indicated that mu- and kappa-opioid receptors enhance each other's antitussive processes. However, delta-opioid receptors played an inhibitory role in antitussive processes mediated by the mu- and kappa-opioid receptors. We also concluded that delta(1)-opioid receptors may play an inhibitory role, whereas delta(2)-opioid receptors may play a synergistic role, in antitussive processes mediated by mu-opioid receptors. Furthermore, we clearly demonstrated that delta-opioid receptor antagonists, such as naltrindole and 7-benzylidenenaltrexone, produced potent antitussive effects. These delta-opioid receptor-mediated antitussive effects may be mediated by the antagonism of delta(1)-, but not delta(2)-opioid receptors. In this review, we study the possibility of the delta-opioid receptor antagonist as a new concept for central acting antitussive drugs.

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.

Interaction between the mu-agonist dermorphin and the delta-agonist [D-Ala2, Glu4]deltorphin in supraspinal antinociception and delta-opioid receptor binding

1. In rats, the interaction between the mu-opioid agonist dermorphin and the delta-opioid agonist [D-Ala2, Glu4]deltorphin was studied in binding experiments to delta-opioid receptors and in the antinociceptive test to radiant heat. 2. When injected i.c.v., doses of [D-Ala2, Glu4]deltorphin higher than 20 nmol produced antinociception in the rat tail-flick test to radiant heat. Lower doses were inactive. None of the doses tested elicited the maximum achievable response. This partial antinociception was accomplished with an in vivo occupancy of more than 97% of brain delta-opioid receptors and of 17% of mu-opioid receptors. Naloxone (0.1 mg kg-1, s.c.), and naloxonazine (10 mg kg-1, i.v., 24 h before), but not the selective delta-opioid antagonist naltrindole, antagonized the antinociception. 3. In vitro competitive inhibition studies in rat brain membranes showed that [D-Ala2, Glu4]deltorphin displaced [3H]-naltrindole from two delta-binding sites of high and low affinity. The addition of 100 microM Gpp[NH]p produced a three fold increase in the [D-Ala2, Glu4]deltorphin Ki value for both binding sites. The addition of 10 nM dermorphin increased the Ki value of the delta-agonist for the high affinity site five times. When Gpp[NH]p was added to the incubation medium together with 10 nM dermorphin, the high affinity Ki of the delta-agonist increased 15 times. 4. Co-administration into the rat brain ventricles of subanalgesic doses of dermorphin and [D-Ala2, Glu4]deltorphin resulted in synergistic antinociceptive responses. 5. Pretreatment with naloxone or with the non-equilibrium mu-antagonists naloxonazine and beta-funaltrexamine completely abolished the antinociceptive response of the mu-delta agonist combinations. 6. Pretreatment with the delta-opioid antagonists naltrindole and DALCE reduced the antinociceptive response of the dermorphin-[D-Ala2, Glu4]deltorphin combinations to a value near that observed after the mu-agonist alone. At the dosage used, naltrindole occupied more than 98% of brain delta-opioid receptors without affecting mu-opioid-receptors. 7. These data suggest that in the rat tail-flick test to radiant heat, mu- and delta-opioid agonists co-operate positively in evoking an antinociceptive response. Although interactions between different opioid pathways cannot be excluded, in vitro binding results indicate that this co-operative antinociception is probably mediated by co-activation of the delta-opioid receptors at the cellular level by the mu- and delta-agonist.

Antagonistic effect of buprenorphine on the antitussive effect of morphine is mediated via the activation of mu 1-opioid receptors

The effect of buprenorphine on the antitussive effect of morphine was examined in mice. Buprenorphine at doses of 0.1, 0.3 and 1 mg/kg given i.p. alone have no effects on the % inhibition in the number of capsaicin-induced coughs. However, pretreatment with the same doses of buprenorphine for 2 hr significantly attenuated the antitussive effect of morphine (3 mg/kg, i.p.). Naloxonazine, a selective mu 1-opioid receptor antagonist, had no effect of buprenorphine on antitussive effect of morphine. These results suggest that buprenorphine antagonizes the antitussive effect of morphine via the activation of mu 1-opioid receptors.

Evidence for lack of modulation of mu-opioid agonist action by delta-opioid agonists in the mouse vas deferens and guinea-pig ileum

1. There is evidence from in vivo studies for an interaction of mu- and delta-opioid ligands. In the present work this concept has been investigated using the mouse vas deferens and guinea-pig ileum myenteric plexus-longitudinal preparations. 2. In field stimulated vasa deferentia of the mouse, co-administration of sub-effective concentrations of the delta-opioid agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) and [Met5]- or [Leu5]enkephalin had no effect on the dose-response curves of the mu-agonists [D-Ala2,MePhe4, Gly-ol5]enkephalin (DAMGO) and morphine. Similarly, the delta-opioid agonists did not alter the potency of morphine and DAMGO when added at different times prior to the mu-opioid agonists, or when EC50 concentrations of delta-opioid ligands were co-administered. Compounds with preferred activity for the putative delta 1-(DPDPE) or delta 2-([D-Ala2,Glu4]deltorphin II (Delt II)) opioid receptors were ineffective in this respect. 3. The guinea-pig ileum contains delta-opioid receptors. No function of these receptors in mediating blockage of field-stimulated contractions was observed with ligands having affinity for the putative delta 1 or delta 2 subtypes nor were the agonists able to modulate responses to mu-opioid ligands in this tissue. 4. The results demonstrate the modulation of mu-opioid agonists by delta-opioid agonists does not occur in the isolated peripheral tissues examined. Thus the findings do not support the concept of a functional coupling of opioid receptors, though the results may be explained by differences between opioid systems in the brain and peripheral tissues examined.

Involvement of delta 1-opioid receptor antagonism in the antitussive effect of delta-opioid receptor antagonists

The effects of 7-benzylidenenaltrexone (BNTX), a selective delta 1-opioid receptor antagonist, and naltriben, a selective delta 2-opioid receptor antagonist, on the capsaicin-induced cough reflex were studied in mice. I.p. administration of BNTX in doses from 0.1 to 3.0 mg/kg reduced the number of coughs dose dependently. The antitussive effect of BNTX was antagonized by [D-Pen2,5]enkephalin (DPDPE), a selective delta 1-opioid receptor agonist, while [D-Ala2]deltorphin II, a selective delta 2-opioid receptor agonist, had no effect on the antitussive effect of BNTX. Pretreatment with nor-binaltorphimine, a selective kappa-opioid receptor antagonist, had no significant effect on the antitussive effect of BNTX. I.p. administration of naltriben, in doses of 1 and 3 mg/kg, also significantly decreased the number of coughs. Although the antitussive effect of naltriben was antagonized by nor-binaltorphimine, the antitussive effect of naltriben was not attenuated by either DPDPE or [D-Ala2]deltorphin II. The antitussive effects of neither BNTX nor naltriben were antagonized by beta-funaltrexamine, a selective mu-opioid receptor antagonist. Thus, it seems likely that the delta 1-opioid receptor antagonism may be involved in the antitussive effect of delta-opioid receptor antagonists.

Differential mediation of cold water swim stress-induced antinociception by delta-opioid receptor subtypes in diabetic mice

The involvement of delta-opioid receptor subtypes in cold water swim stress (CWSS)-induced antinociception in diabetic mice was compared with that in non-diabetic mice. Three-minute swim stress produced significant antinociception in both diabetic and non-diabetic mice as determined by the tail-pinch test. However, the extent of CWSS-induced antinociception in diabetic mice was significantly greater than that in non-diabetic mice. Pretreatment with naltriben, a selective delta 2-opioid receptor antagonist, significantly attenuated CWSS-induced antinociception in both non-diabetic and diabetic mice. In contrast, although 7-benzylidenenaltrexone, a selective delta 1-opioid receptor antagonist, significantly attenuated CWSS-induced antinociception in diabetic mice, it had no effect in non-diabetic mice. These results suggest that CWSS-induced antinociception in non-diabetic mice is mediated by delta 2-opioid receptors, whereas CWSS-induced antinociception in diabetic mice is mediated by both delta 1- and delta 2-opioid receptors. Furthermore, the enhanced CWSS-induced antinociception in diabetic mice may be due to the activation of delta 1-opioid receptors.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.