Delta receptors are required for full inhibitory coupling of mu-receptors to voltage-dependent Ca(2+) channels in dorsal root ganglion neurons

Therapeutic potential of opioid receptor heteromers in chronic pain and associated comorbidities

Chronic pain affects 20% to 45% of the global population and is often associated with the development of anxio-depressive disorders. Treatment of this debilitating condition remains particularly challenging with opioids prescribed to alleviate moderate to severe pain. However, despite strong antinociceptive properties, numerous adverse effects limit opioid use in the clinic. Moreover, opioid misuse and abuse have become a major health concern worldwide. This prompted efforts to design original strategies that would efficiently and safely relieve pain. Targeting of opioid receptor heteromers is one of these. This review summarizes our current knowledge on the role of heteromers involving opioid receptors in the context of chronic pain and anxio-depressive comorbidities. It also examines how heteromerization in native tissue affects ligand binding, receptor signalling and trafficking properties. Finally, the therapeutic potential of ligands designed to specifically target opioid receptor heteromers is considered. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Opioid Analgesia and Opioid-Induced Adverse Effects: A Review

Opioids are widely used as therapeutic agents against moderate to severe acute and chronic pain. Still, these classes of analgesic drugs have many potential limitations as they induce analgesic tolerance, addiction and numerous behavioural adverse effects that often result in patient non-compliance. As opium and opioids have been traditionally used as painkillers, the exact mechanisms of their adverse reactions over repeated use are multifactorial and not fully understood. Older adults suffer from cancer and non-cancer chronic pain more than younger adults, due to the physiological changes related to ageing and their reduced metabolic capabilities and thus show an increased number of adverse reactions to opioid drugs. All clinically used opioids are μ-opioid receptor agonists, and the major adverse effects are directly or potentially connected to this receptor. Multifunctional opioid ligands or peripherally restricted opioids may elicit fewer adverse effects, as shown in preclinical studies, but these results need reproducibility from further extensive clinical trials. The current review aims to overview various mechanisms involved in the adverse effects induced by opioids, to provide a better understanding of the underlying pathophysiology and, ultimately, to help develop an effective therapeutic strategy to better manage pain.

Functional independence of endogenous µ- and δ-opioid receptors co-expressed in cholinergic interneurons

Class A G-protein-coupled receptors (GPCRs) normally function as monomers, although evidence from heterologous expression systems suggests that they may sometimes form homodimers and/or heterodimers. This study aims to evaluate possible functional interplay of endogenous µ- and δ-opioid receptors (MORs and DORs) in mouse neurons. Detecting GPCR dimers in native tissues, however, has been challenging. Previously, MORs and DORs co-expressed in transfected cells have been reported to form heterodimers, and their possible co-localization in neurons has been studied in knock-in mice expressing genetically engineered receptors fused to fluorescent proteins. Here, we find that single cholinergic neurons in the mouse striatum endogenously express both MORs and DORs. The receptors on neurons from live brain slices were fluorescently labeled with a ligand-directed labeling reagent, NAI-A594. The selective activation of MORs and DORs, with DAMGO (µ-agonist) and deltorphin (δ-agonist) inhibited spontaneous firing in all cells examined. In the continued presence of agonist, the firing rate returned to baseline as the result of receptor desensitization with the application of deltorphin but was less observed with the application of DAMGO. In addition, agonist-induced internalization of DORs but not MORs was detected. When MORs and DORs were activated simultaneously with [Met⁵]-enkephalin, desensitization of MORs was facilitated but internalization was not increased. Together, these results indicate that while MORs and DORs are expressed in single striatal cholinergic interneurons, the two receptors function independently.

Mu Opioid Receptor Heterodimers Emerge as Novel Therapeutic Targets: Recent Progress and Future Perspective

Opioids are the most effective analgesics used in the clinical management of cancer pain or non-cancer pain. However, chronic opioids therapy can cause many side effects including respiratory depression, nausea, sedation, itch, constipation, analgesic tolerance, hyperalgesia, high addictive potential, and abuse liability. Opioids exert their effects through binding to the opioid receptors belonging to the G-protein coupled receptors (GPCRs) family, including mu opioid receptor (MOR), delta opioid receptor (DOR), and kappa opioid receptor (KOR). Among them, MOR is essential for opioid-induced analgesia and also responsible for adverse effects of opioids. Importantly, MOR can form heterodimers with other opioid receptors and non-opioid receptors in vitro and in vivo, and has distinct pharmacological properties, different binding affinities for ligands, downstream signaling, and receptor trafficking. This mini review summarized recent progress on the function of Mu opioid receptor heterodimers, and we proposed that targeting mu opioid receptor heterodimers may represent an opportunity to develop new therapeutics, especially for chronic pain treatment.

Dynamic Opioid Receptor Regulation in the Periphery

Opioids serve a vital role in the current analgesic array of treatment options. They are useful in acute instances involving severe pain associated with trauma, surgery, and terminal diseases such as cancer. In the past three decades, multiple receptor isoforms and conformations have been reported throughout literature. Most of these studies conducted systemic analyses of opioid receptor function, often generalizing findings from receptor systems in central nervous tissue or exogenously expressing immortalized cell lines as common mechanisms throughout physiology. However, a culmination of innovative experimental data indicates that opioid receptor systems are differentially modulated depending on their anatomic expression profile. Importantly, opioid receptors expressed in the peripheral nervous system undergo regulation uncommon to similar receptors expressed in central nervous system tissues. This distinctive characteristic begs one to question whether peripheral opioid receptors maintain anatomically unique roles, and whether they may serve an analgesic advantage in providing pain relief without promoting addiction.

Heteromerization Modulates mu Opioid Receptor Functional Properties in vivo

Mu opioid receptors modulate a large number of physiological functions. They are in particular involved in the control of pain perception and reward properties. They are also the primary molecular target of opioid drugs and mediate their beneficial analgesic effects, euphoric properties as well as negative side effects such as tolerance and physical dependence. Importantly, mu opioid receptors can physically associate with another receptor to form a novel entity called heteromer that exhibits specific ligand binding, signaling, and trafficking properties. As reviewed here, in vivo physical proximity has now been evidenced for several receptor pairs, subsequent impact of heteromerization on native mu opioid receptor signaling and trafficking identified and a link to behavioral changes established. Selective targeting of heteromers as a tool to modulate mu opioid receptor activity is therefore attracting growing interest and raises hopes for innovative therapeutic strategies.

Morphine activation of mu opioid receptors causes disinhibition of neurons in the ventral tegmental area mediated by β-arrestin2 and c-Src

The tyrosine kinase, c-Src, participates in mu opioid receptor (MOP) mediated inhibition in sensory neurons in which β-arrestin2 (β-arr2) is implicated in its recruitment. Mice lacking β-arr2 exhibit increased sensitivity to morphine reinforcement; however, whether β-arr2 and/or c-Src participate in the actions of opioids in neurons within the reward pathway is unknown. It is also unclear whether morphine acts exclusively through MOPs, or involves delta opioid receptors (DOPs). We examined the involvement of MOPs, DOPs, β-arr2 and c-Src in the inhibition by morphine of GABAergic inhibitory postsynaptic currents (IPSCs) recorded from neurons in the mouse ventral tegmental area. Morphine inhibited spontaneous IPSC frequency, mainly through MOPs, with only a negligible effect remaining in MOP−/− neurons. However, a reduction in the inhibition by morphine for DOP−/− c.f. WT neurons and a DPDPE-induced decrease of IPSC frequency revealed a role for DOPs. The application of the c-Src inhibitor, PP2, to WT neurons also reduced inhibition by morphine, while the inactive PP3, and the MEK inhibitor, SL327, had no effect. Inhibition of IPSC frequency by morphine was also reduced in β-arr2−/− neurons in which PP2 caused no further reduction. These data suggest that inhibition of IPSCs by morphine involves a β-arr2/c-Src mediated mechanism.

One cell model establishment to inhibit CaMKIIγ mRNA expression in the dorsal root ganglion neuron by RNA interfere

CaMKII_γ in dorsal root ganglion neurons is closely related to the neuropathic pain, neuron injury induced by local anesthetics. To get great insight into the function of CaMKII_γ in dorsal root ganglion neurons, we need one cell model to specially inhibit the CaMKII_γ mRNA expression. The present study was aimed to establish one cell model to specially inhibit the CaMKII_γ mRNA expression. We designed the CaMKII_γ shRNA sequence and connected with pYr-1.1 plasmid. The ligation product of the CaMKII_γshRNA and pYr-1.1 plasmid was recombined with pAd/PL-DEST vector into pAD-CaMKIIγ-shRNA. adenovirus vector. pAD-CaMKIIγ-shRNA. adenovirus vector infected the dorsal root ganglion neuron to inhibit the CaMKII_γ mRNA expression in vitro. The pAD-CaMKIIγ-shRNA adenovirus vector was verified to be correct by the digestion, sequence. And pAD-CaMKIIγ-shRNA. adenovirus vector can infect the DRG cells to inhibit the CaMKII_γ mRNA or protein expression by the real-time polymerase chain reaction (PCR) or western blotting. Those results showed that we successfully constructed one adenovirus vector that can infect the dorsal root ganglion neuron to inhibit the CaMKII_γ mRNA and protein expression. That will supply with one cell model for the CaMKII_γ function study.

Opioid Receptors

Opioids are the oldest and most potent drugs for the treatment of severe pain. Their clinical application is undisputed in acute (e.g., postoperative) and cancer pain, but their long-term use in chronic pain has met increasing scrutiny. This article reviews mechanisms underlying opioid analgesia and other opioid actions. It discusses the structure, function, and plasticity of opioid receptors; the central and peripheral sites of analgesic actions and side effects; endogenous and exogenous opioid receptor ligands; and conventional and novel opioid compounds. Challenging clinical situations, such as the tension between chronic pain and addiction, are also illustrated.

μ opioid receptor activation hyperpolarizes respiratory-controlling Kölliker-Fuse neurons and suppresses post-inspiratory drive

KEY POINTS

In addition to reductions in respiratory rate, opioids also cause aspiration and difficulty swallowing, indicating impairment of the upper airways. The Kölliker-Fuse (KF) maintains upper airway patency and a normal respiratory pattern. In this study, activation of μ opioid receptors in the KF reduced respiratory frequency and tidal volume in anaesthetized rats. Nerve recordings in an in situ preparation showed that activation of μ opioid receptors in the KF eliminated the post-inspiration phase of the respiratory cycle. In brain slices, μ opioid agonists hyperpolarized a distinct population (61%) of KF neurons by activation of an inwardly rectifying potassium conductance. These results suggest that KF neurons that are hyperpolarized by opioids could contribute to opioid-induced respiratory disturbances, particularly the impairment of upper airways.

ABSTRACT

Opioid-induced respiratory effects include aspiration and difficulty swallowing, suggesting impairment of the upper airways. The pontine Kölliker-Fuse nucleus (KF) controls upper airway patency and regulates respiration, in particular the inspiratory/expiratory phase transition. Given the importance of the KF in coordinating respiratory pattern, the mechanisms of μ opioid receptor activation in this nucleus were investigated at the systems and cellular level. In anaesthetized, vagi-intact rats, injection of opioid agonists DAMGO or [Met(5) ]enkephalin (ME) into the KF reduced respiratory frequency and amplitude. The μ opioid agonist DAMGO applied directly into the KF of the in situ arterially perfused working heart-brainstem preparation of rat resulted in robust apneusis (lengthened low amplitude inspiration due to loss of post-inspiratory drive) that was rapidly reversed by the opioid antagonist naloxone. In brain slice preparations, activation of μ opioid receptors on KF neurons hyperpolarized a distinct population (61%) of neurons. As expected, the opioid-induced hyperpolarization reduced the excitability of the neuron in response to either current injection or local application of glutamate. In voltage-clamp recordings the outward current produced by the opioid agonist ME was concentration dependent, reversed at the potassium equilibrium potential and was blocked by BaCl2 , characteristics of a G protein-coupled inwardly rectifying potassium (GIRK) conductance. The clinically used drug morphine produced an outward current in KF neurons with similar potency to morphine-mediated currents in locus coeruleus brain slice preparations. Thus, the population of KF neurons that are hyperpolarized by μ opioid agonists are likely mediators of the opioid-induced loss of post-inspiration and induction of apneusis.

Alteration of the mu opioid receptor: Ca2+ channel signaling pathway in a subset of rat sensory neurons following chronic femoral artery occlusion

The exercise pressor reflex, a crucial component of the cardiovascular response under physiological and pathophysiological states, is activated via metabolic and mechanical mediators that originate from contracting muscles and stimulate group III and IV afferents. We reported previously that stimulation of mu opioid receptors (MOR), expressed in both afferents, led to a significant attenuation of the reflex in rats whose femoral arteries had been occluded for 72 h. The present study examined the effect of arterial occlusion on the signaling components involved in the opioid-mediated modulation of Ca(2+) channels in rat dorsal root ganglion neurons innervating the triceps surae muscles. We focused on neurons that were transfected with cDNA coding for enhanced green fluorescent protein whose expression is driven by the voltage-gated Na(+) channel 1.8 (Na(V)1.8) promoter region, a channel expressed primarily in nociceptive neurons. With the use of a small interference RNA approach, our results show that the pertussis toxin-sensitive Gα(i3) subunit couples MOR with Ca(2+) channels. We observed a significant leftward shift of the MOR agonist [D-Ala2-N-Me-Phe4-Glycol5]-enkephalin concentration-response relationship in neurons isolated from rats with occluded arteries compared with those that were perfused freely. Femoral occlusion did not affect Ca(2+) channel density or the fraction of the main Ca(2+) channel subtype. Furthermore, Western blotting analysis indicated that the leftward shift did not result from either increased Gα(i3) or MOR expression. Finally, all neurons from both groups exhibited an inward current following exposure of the transient potential receptor vanilloid 1 (TRPV1) agonist, 8-methyl-N-vanillyl-6-nonenamide. These findings suggest that sensory neurons mediating the exercise pressor reflex express Na(V)1.8 and TRPV1 channels, and femoral occlusion alters the MOR pharmacological profile.

Prolonged morphine treatment alters δ opioid receptor post-internalization trafficking

BACKGROUND AND PURPOSE

The δ opioid receptor (DOP receptor) undergoes internalization both constitutively and in response to agonists. Previous work has shown that DOP receptors traffic from intracellular compartments to neuronal cell membranes following prolonged morphine treatment. Here, we examined the effects of prolonged morphine treatment on the post-internalization trafficking of DOP receptors.

EXPERIMENTAL APPROACH

Using primary cultures of dorsal root ganglia neurons, we measured the co-localization of endogenous DOP receptors with post-endocytic compartments following both prolonged and acute agonist treatments.

KEY RESULTS

A departure from the constitutive trafficking pathway was observed following acute DOP receptor agonist-induced internalization by deltorphin II. That is, the DOP receptor underwent distinct agonist-induced post-endocytic sorting. Following prolonged morphine treatment, constitutive DOP receptor trafficking was augmented. SNC80 following prolonged morphine treatment also caused non-constitutive DOP receptor agonist-induced post-endocytic sorting. The μ opioid receptor (MOP receptor) agonist DAMGO induced DOP receptor internalization and trafficking following prolonged morphine treatment. Finally, all of the alterations to DOP receptor trafficking induced by both DOP and MOP receptor agonists were inhibited or absent when those agonists were co-administered with a DOP receptor antagonist, SDM-25N.

CONCLUSIONS AND IMPLICATIONS

The results support the hypothesis that prolonged morphine treatment induces the formation of MOP–DOP receptor interactions and subsequent augmentation of the available cell surface DOP receptors, at least some of which are in the form of a MOP/DOP receptor species. The pharmacology and trafficking of this species appear to be unique compared to those of its individual constituents.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2

In vivo opioid receptor heteromerization: where do we stand?

Opioid receptors are highly homologous GPCRs that modulate brain function at all levels of neural integration, including autonomous, sensory, emotional and cognitive processing. Opioid receptors functionally interact in vivo, but the underlying mechanisms involving direct receptor-receptor interactions, affecting signalling pathways or engaging different neuronal circuits, remain unsolved. Heteromer formation through direct physical interaction between two opioid receptors or between an opioid receptor and a non-opioid one has been postulated and can be characterized by specific ligand binding, receptor signalling and trafficking properties. However, despite numerous studies in heterologous systems, evidence for physical proximity in vivo is only available for a limited number of opioid heteromers, and their physiopathological implication remains largely unknown mostly due to the lack of appropriate tools. Nonetheless, data collected so far using endogenous receptors point to a crucial role for opioid heteromers as a molecular entity that could underlie human pathologies such as alcoholism, acute or chronic pain as well as psychiatric disorders. Opioid heteromers therefore stand as new therapeutic targets for the drug discovery field.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Recent advances on the δ opioid receptor: from trafficking to function

Within the opioid family of receptors, δ (DOPrs) and μ opioid receptors (MOPrs) are typical GPCRs that activate canonical second-messenger signalling cascades to influence diverse cellular functions in neuronal and non-neuronal cell types. These receptors activate well-known pathways to influence ion channel function and pathways such as the map kinase cascade, AC and PI3K. In addition new information regarding opioid receptor-interacting proteins, downstream signalling pathways and resultant functional effects has recently come to light. In this review, we will examine these novel findings focusing on the DOPr and, in doing so, will contrast and compare DOPrs with MOPrs in terms of differences and similarities in function, signalling pathways, distribution and interactions. We will also discuss and clarify issues that have recently surfaced regarding the expression and function of DOPrs in different cell types and analgesia.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Dopamine D3 receptor dysfunction prevents anti-nociceptive effects of morphine in the spinal cord

Dopamine (DA) modulates spinal reflexes, including nociceptive reflexes, in part via the D3 receptor subtype. We have previously shown that mice lacking the functional D3 receptor (D3KO) exhibit decreased paw withdrawal latencies from painful thermal stimuli. Altering the DA system in the CNS, including D1 and D3 receptor systems, reduces the ability of opioids to provide analgesia. Here, we tested if the increased pain sensitivity in D3KO might result from a modified μ-opioid receptor (MOR) function at the spinal cord level. As D1 and D3 receptor subtypes have competing cellular effects and can form heterodimers, we tested if the changes in MOR function may be mediated in D3KO through the functionally intact D1 receptor system. We assessed thermal paw withdrawal latencies in D3KO and wild type (WT) mice before and after systemic treatment with morphine, determined MOR and phosphorylated MOR (p-MOR) protein expression levels in lumbar spinal cords, and tested the functional effects of DA and MOR receptor agonists in the isolated spinal cord. In vivo, a single morphine administration (2 mg/kg) increased withdrawal latencies in WT but not D3KO, and these differential effects were mimicked in vitro, where morphine modulated spinal reflex amplitudes (SRAs) in WT but not D3KO. Total MOR protein expression levels were similar between WT and D3KO, but the ratio of pMOR/total MOR was higher in D3KO. Blocking D3 receptors in the isolated WT cord precluded morphine's inhibitory effects observed under control conditions. Lastly, we observed an increase in D1 receptor protein expression in the lumbar spinal cord of D3KO. Our data suggest that the D3 receptor modulates the MOR system in the spinal cord, and that a dysfunction of the D3 receptor can induce a morphine-resistant state. We propose that the D3KO mouse may serve as a model to study the onset of morphine resistance at the spinal cord level, the primary processing site of the nociceptive pathway.

Molecular Perspectives for mu/delta Opioid Receptor Heteromers as Distinct, Functional Receptors

Opioid receptors are the sites of action for morphine and the other opioid drugs. Abundant evidence now demonstrates that different opioid receptor types can physically associate to form heteromers. Understandings of the nature, behavior, and role of these opioid receptor heteromers are developing. Owing to their constituent monomers’ involvement in analgesia, mu/delta opioid receptor (M/DOR) heteromers have been a particular focus of attention. There is now considerable evidence demonstrating M/DOR to be an extant and physiologically relevant receptor species. Participating in the cellular environment as a distinct receptor type, M/DOR availability is complexly regulated and M/DOR exhibits unique pharmacology from that of other opioid receptors (ORs), including its constituents. M/DOR appears to have a range of actions that vary in a ligand- (or ligands-) dependent manner. These actions can meaningfully affect the clinical effects of opioid drugs: strategies targeting M/DOR may be therapeutically useful. This review presents and discusses developments in these understandings with a focus on the molecular nature and activity of M/DOR in the context of therapeutic potentials.

Select G-protein-coupled receptors modulate agonist-induced signaling via a ROCK, LIMK, and β-arrestin 1 pathway

G-protein-coupled receptors (GPCRs) are typically present in a basal, inactive state but, when bound to an agonist, activate downstream signaling cascades. In studying arrestin regulation of opioid receptors in dorsal root ganglia (DRG) neurons, we find that agonists of delta opioid receptors (δORs) activate cofilin through Rho-associated coiled-coil-containing protein kinase (ROCK), LIM domain kinase (LIMK), and β-arrestin 1 (β-arr1) to regulate actin polymerization. This controls receptor function, as assessed by agonist-induced inhibition of voltage-dependent Ca(2+) channels in DRGs. Agonists of opioid-receptor-like receptors (ORL1) similarly influence the function of this receptor through ROCK, LIMK, and β-arr1. Functional evidence of this cascade was demonstrated in vivo, where the behavioral effects of δOR or ORL1 agonists were enhanced in the absence of β-arr1 or prevented by inhibiting ROCK. This pathway allows δOR and ORL1 agonists to rapidly regulate receptor function.

Pharmacological Profiles of Oligomerized μ-Opioid Receptors

Opioids are widely prescribed pain relievers with multiple side effects and potential complications. They produce analgesia via G-protein-protein coupled receptors: μ-, δ-, κ-opioid and opioid receptor-like 1 receptors. Bivalent ligands targeted to the oligomerized opioid receptors might be the key to developing analgesics without undesired side effects and obtaining effective treatment for opioid addicts. In this review we will update the biological effects of μ-opioids on homo- or hetero-oligomerized μ-opioid receptor and discuss potential mechanisms through which bivalent ligands exert beneficial effects, including adenylate cyclase regulation and receptor-mediated signaling pathways.

Chronic inflammatory injury results in increased coupling of delta opioid receptors to voltage-gated Ca2+ channels

Background

Opioid receptors regulate a diverse array of physiological functions. Mu opioid receptor agonists are well-known analgesics for treating acute pain. In contrast, animal models suggest that chronic pain is more effectively relieved by delta opioid receptor agonists. A number of studies have shown that chronic pain results in increased function of delta opioid receptors. This is proposed to result from enhanced trafficking of the delta opioid receptor to the cell membrane induced by persistent tissue injury. However, recent studies have questioned this mechanism, which has resulted in some uncertainty as to whether delta opioid receptors are indeed upregulated in chronic pain states. To clarify this question, we have examined the effect of chronic inflammatory pain over time using both an ex vivo measure of delta function: receptor-Ca²⁺ channel coupling, and an in vivo measure; the relief of chronic pain by a delta opioid receptor agonist. In addition, as beta-arrestin 2 can regulate delta opioid receptor trafficking and signaling, we have further examined whether deleting this scaffolding and signal transduction molecule alters delta opioid receptor function.

Results

We used the Complete Freund’s Adjuvant model of inflammatory pain, and examined the effectiveness of the delta agonist, SNC80, to both inhibit Ca²⁺ channels in primary afferent neurons and to attenuate mechanical allodynia. In naïve beta-arrestin 2 wildtype and knockout mice, SNC80 neither significantly inhibited voltage-dependent Ca²⁺ currents nor produced antinociception. However, following inflammatory pain, both measures showed a significant and long-lasting enhancement of delta opioid receptor function that persisted for up to 14 days post-injury regardless of genotype. Furthermore, although this pain model did not alter Ca²⁺ current density, the contribution of N-type Ca²⁺ channels to the total current appeared to be regulated by the presence of beta-arrestin 2.

Conclusions

Our results indicate that there is an upregulation of delta opioid receptor function following chronic pain. This gain of function is reflected in the increased efficacy of a delta agonist in both behavioral and electrophysiological measures. Overall, this work confirms that delta opioid receptors can be enhanced following tissue injury associated with chronic pain.

Spinal interaction between the highly selective μ agonist DAMGO and several δ opioid receptor ligands in naive and morphine-tolerant mice

Since the discovery of opioid receptor dimers their possible roles in opioid actions were intensively investigated. Here we suggest a mechanism that may involve the μ-δ opioid heterodimers. The exact role of δ opioid receptors in antinociception and in the development of opioid tolerance is still unclear. While receptor up-regulation can be observed during the development of opioid tolerance no μ receptor down-regulation could be detected within five days. In our present work we investigated how the selective δ opioid receptor agonists and antagonists influence the antinociceptive effect of the selective μ receptor agonist DAMGO in naïve and morphine-tolerant mice. We treated male NMRI mice with 200 μmol/kg subcutaneous (s.c.) morphine twice daily for three days. On the fourth day we measured the antinociceptive effect of DAMGO alone and combined with delta ligands: DPDPE, deltorphin II (agonists), TIPP and TICPψ (antagonists), respectively, administered intrathecally (i.t.) in mouse tail-flick test. In naive control mice none of the δ ligands caused significant changes in the antinociceptive action of DAMGO. The treatment with s.c. morphine resulted in approximately four-fold tolerance to i.t. DAMGO, i.e. the ED₅₀ value of DAMGO was four times as high as in naive mice. 500 and 1000 pmol/mouse of the δ₁ selective agonist DPDPE enhanced the tolerance to DAMGO while 1000 pmol/mouse of the δ₂ selective agonist deltorphin II did not influence the degree of tolerance. However, both δ antagonists TIPP and TICPψ potentiated the antinociceptive effect of i.t. DAMGO, thus they restored the potency of DAMGO to the control level. The inhibitory action of DPDPE against the antinociceptive effect of DAMGO could be antagonized by TIPP and TICPψ. We hypothesize that during the development of morphine tolerance the formation of μδ heterodimers may contribute to the spinal opioid tolerance. δ ligands may affect the dimer formation differently. Those, like DPDPE may facilitate the dimer formation hence inhibit the antinociceptive effect of DAMGO by causing virtual μ receptor down-regulation. Ligands that do not affect the dimer formation do not influence antinociception either but ligands with the presumed capability of disconnecting the dimers may decrease the spinal tolerance to DAMGO.

Opioid receptor heteromers in analgesia

Opiates such as morphine and fentanyl, a major class of analgesics used in the clinical management of pain, exert their effects through the activation of opioid receptors. Opioids are among the most commonly prescribed and frequently abused drugs in the USA; however, the prolonged use of opiates often leads to the development of tolerance and addiction. Although blockade of opioid receptors with antagonists such as naltrexone and naloxone can lessen addictive impulses and facilitate recovery from overdose, systemic disruption of endogenous opioid receptor signalling through the use of these antagonistic drugs can have severe side effects. In the light of these challenges, current efforts have focused on identifying new therapeutic targets that selectively and specifically modulate opioid receptor signalling and function so as to achieve analgesia without the adverse effects associated with chronic opiate use. We have previously reported that opioid receptors interact with each other to form heteromeric complexes and that these interactions affect morphine signalling. Since chronic morphine administration leads to an enhanced level of these heteromers, these opioid receptor heteromeric complexes represent novel therapeutic targets for the treatment of pain and opiate addiction. In this review, we discuss the role of heteromeric opioid receptor complexes with a focus on mu opioid receptor (MOR) and delta opioid receptor (DOR) heteromers. We also highlight the evidence for altered pharmacological properties of opioid ligands and changes in ligand function resulting from the heteromer formation.

Pharmacological characterization of the bifunctional opioid ligand H-Dmt-Tic-Gly-NH-Bzl (UFP-505)

BACKGROUND

While producing good-quality analgesia, µ-opioid (MOP) receptor activation produces a number of side-effects including tolerance. Simultaneous blockade of δ-opioid (DOP) receptors has been shown to reduce tolerance to morphine. Here, we characterize a prototype bifunctional opioid H-Dmt-Tic-Gly-NH-Bzl (UFP-505).

METHODS

We measured receptor binding affinity in Chinese hamster ovary (CHO) cells expressing recombinant human MOP, DOP, k-opioid (KOP), nociceptin/orphanin (NOP) receptors. For activation, we measured the binding of GTPγ(35)S to membranes from CHO(hMOP), CHO(hDOP), rat cerebrocortex, and rat spinal cord. In addition, we assessed 'end organ' responses in the guinea pig ileum and mouse vas deferens.

RESULTS

UFP-505 bound to CHO(hMOP) and CHO(hDOP) with (binding affinity) pK(i) values of 7.79 and 9.82, respectively. There was a weak interaction at KOP and NOP (pK(i) 6.29 and 5.86). At CHO(hMOP), UFP-505 stimulated GTPγ(35)S binding with potency (pEC(50)) of 6.37 and in CHO(hDOP) reversed the effects of a DOP agonist with affinity (pK(b)) of 9.81 (in agreement with pK(i) at DOP). UFP-505 also stimulated GTPγ(35)S binding in rat cerebrocortex and spinal cord with pEC(50) values of 6.11-6.53. In the guinea pig ileum (MOP-rich preparation), UFP-505 inhibited contractility with pEC(50) of 7.50 and in the vas deferens (DOP-rich preparation) reversed the effects of a DOP agonist with an affinity (pA(2)) of 9.15.

CONCLUSIONS

We have shown in a range of preparations and assays that UFP-505 behaves as a potent MOP agonist and DOP antagonist; a MOP/DOP bifunctional opioid. Further studies in dual expression systems and whole animals with this prototype are warranted.

Differential pharmacological actions of methadone and buprenorphine in human embryonic kidney 293 cells coexpressing human μ-opioid and opioid receptor-like 1 receptors

Methadone and buprenorphine are used in maintenance therapy for heroin addicts. In this study, we compared their effects on adenylate cyclase (AC) activity in human embryonic kidney (HEK) 293 cells stably overexpressing human μ-opioid receptor (MOR) and nociceptin/opioid receptor-like 1 receptor (ORL1) simultaneously. After acute exposure, methadone inhibited AC activity; however, buprenorphine induced compromised AC inhibition. When naloxone was introduced after 30 min incubation with methadone, the AC activity was enhanced. This was not observed in the case of buprenorphine. Enhancement of the AC activity was more significant when the incubation lasted for 4 h, and prolonged exposure to buprenorphine elevated the AC activity as well. The removal of methadone and buprenorphine by washing also obtained similar AC superactivation as that revealed by naloxone challenge. The study demonstrated that methadone and buprenorphine exert initially different yet eventually convergent adaptive changes of AC activity in cells coexpressing human MOR and ORL1 receptors.

Preclinical pharmacology of AZD2327: a highly selective agonist of the δ-opioid receptor

In the present article, we summarize the preclinical pharmacology of 4-{(R)-(3-aminophenyl)[4-(4-fluorobenzyl)-piperazin-1-yl]methyl}-N,N-diethylbenzamide (AZD2327), a highly potent and selective agonist of the δ-opioid receptor. AZD2327 binds with sub-nanomolar affinity to the human opioid receptor (K(i) = 0.49 and 0.75 nM at the C27 and F27 isoforms, respectively) and is highly selective (>1000-fold) over the human μ- and κ-opioid receptor subtypes as well as >130 other receptors and channels. In functional assays, AZD2327 shows full agonism at human δ-opioid receptors ([(35)S]GTPγ EC(50) = 24 and 9.2 nM at C27 and F27 isoforms, respectively) and also at the rat and mouse δ-opioid receptors. AZD2327 is active in a wide range of models predictive of anxiolytic activity, including a modified Geller-Seifter conflict test and social interaction test, as well as in antidepressant models, including learned helplessness. In animals implanted with microdialysis probes and then given an acute stressor by pairing electric shock delivery with a flashing light, there is an increase in norepinephrine release into the prefrontal cortex associated with this acute anxiety state. Both the benzodiazepine anxiolytic standard diazepam and AZD2327 blocked this norepinephrine release equally well, and there was no evidence of tolerance to these effects of AZD2327. Overall, these data support the role of the δ-opioid receptor in the regulation of mood, and data suggest that AZD2327 may possess unique antidepressant and anxiolytic activities that could make a novel contribution to the pharmacotherapy of psychiatric disorders.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Mu and delta opioid receptors on nociceptors attenuate mechanical hyperalgesia in rat

Sensitization to mechanical stimuli is important in most pain syndromes. We evaluated the populations of nociceptors mediating mechanical hyperalgesia and those mediating mu-opioid receptor (MOR) and delta-opioid receptor (DOR) agonist-induced inhibition of hyperalgesia, in the rat. We found that: (1) intradermal injection of both the endogenous ligand for the Ret receptor, glia-derived growth factor (GDNF), and the ligand for the tropomyosin receptor kinase A (TrkA) receptor, nerve growth factor (NGF)-which are present on distinct populations of nociceptors-both produce mechanical hyperalgesia; (2) DOR agonist 4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)methyl]-N,N-diethylbenzamide (SNC) but not MOR agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) inhibit GDNF-induced hyperalgesia; (3) both DAMGO and SNC inhibit NGF hyperalgesia, even in rats pretreated with isolectin B4 (IB4)-saporin, a toxin that destroys IB4-binding neurons; (4) co-administration of low doses of DAMGO and SNC produce enhanced analgesia, and; (5) repeated administration of DAMGO produces cross-tolerance to the analgesic effect of SNC. These findings demonstrate that, most nociceptors have a role in mechanical hyperalgesia, only the DOR agonist inhibits GDNF hyperalgesia, and MOR and DOR are co-localized on a functionally important population of TrkA-positive nociceptors.