The delta opioid receptor agonist KNT-127 relieves innate anxiety-like behavior in mice by suppressing transmission from the prelimbic cortex to basolateral amygdala

AIM

Excitatory projections from the prelimbic cortex (PL) to the basolateral nucleus of the amygdala (BLA) are implicated in the regulation of anxiety-like behaviors, and we previously demonstrated that anxiolytic-like effects of the selective delta-opioid receptor (DOP) agonist KNT-127 is involved in suppressing glutamate neurotransmission in the PL. Here, we investigated the mechanisms underlying the anxiolytic-like effect of KNT-127 in mice by combining optogenetic stimulation of the PL-BLA pathway with behavioral analyses.

METHODS

Four-week-old male C57BL/6J mice received bilateral administration of adeno-associated virus (AAV)2-CaMKIIa-hChR2(H134R)-enhanced yellow fluorescent protein (EYFP) into the PL to induce expression of the light-activated excitatory ionic channel ChR2. Subsequently, an optic fiber cannula connected to a wireless photo-stimulator was implanted into the BLA for optogenetic PL-BLA pathway stimulation. We evaluated innate anxiety using the elevated plus maze (EPM) and open field (OF) tests as well as learned anxiety using the contextual fear conditioning (CFC) test.

RESULTS

Optogenetic activation of the PL-BLA pathway enhanced anxiety-like behaviors in the EPM and OF, while prior subcutaneous administration of KNT-127 (10 mg/kg) reduced this anxiogenic effect. In contrast, optogenetic activation of the PL-BLA pathway had no significant effect on conditioned fear.

CONCLUSION

Our findings indicate that the PL-BLA circuit contributes to innate anxiety and that the anxiolytic-like effects of KNT-127 are mediated at least in part by suppression of PL-BLA transmission. The PL delta-opioid receptor may thus be an effective therapeutic target for anxiety disorders.

Identification and Pharmacological Characterization of a Low-Liability Antinociceptive Bifunctional MOR/DOR Cyclic Peptide

Peptide-based opioid ligands are important candidates for the development of novel, safer, and more effective analgesics to treat pain. To develop peptide-based safer analgesics, we synthesized a mixture-based cyclic pentapeptide library containing a total of 24,624 pentapeptides and screened the mixture-based library samples using a 55 °C warm water tail-withdrawal assay. Using this phenotypic screening approach, we deconvoluted the mixture-based samples to identify a novel cyclic peptide Tyr-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt), which produced dose- and time-dependent antinociception with an ED50 (and 95% confidence interval) of 0.70 (0.52-0.97) mg/kg i.p. mediated by the mu-opioid receptor (MOR). Additionally, higher doses (≥3 mg/kg, i.p.) of CycloAnt antagonized delta-opioid receptors (DOR) for at least 3 h. Pharmacological characterization of CycloAnt showed the cyclic peptide did not reduce breathing rate in mice at doses up to 15 times the analgesic ED50 value, and produced dramatically less hyperlocomotion than the MOR agonist, morphine. While chronic administration of CycloAnt resulted in antinociceptive tolerance, it was without opioid-induced hyperalgesia and with significantly reduced signs of naloxone-precipitated withdrawal, which suggested reduced physical dependence compared to morphine. Collectively, the results suggest this dual MOR/DOR multifunctional ligand is an excellent lead for the development of peptide-based safer analgesics.

Receptor expression and signaling properties in the brain, and structural ligand motifs that contribute to delta opioid receptor agonist-induced seizures

The δ opioid receptor (δOR) is a therapeutic target for the treatment of various neurological disorders, such as migraines, chronic pain, alcohol use, and mood disorders. Relative to μ opioid receptor agonists, δOR agonists show lower abuse liability and may be potentially safer analgesic alternatives. However, currently no δOR agonists are approved for clinical use. A small number of δOR agonists reached Phase II trials, but ultimately failed to progress due to lack of efficacy. One side effect of δOR agonism that remains poorly understood is the ability of δOR agonists to produce seizures. The lack of a clear mechanism of action is partly driven by the fact that δOR agonists range in their propensity to induce seizure behavior, with multiple δOR agonists reportedly not causing seizures. There is a significant gap in our current understanding of why certain δOR agonists are more likely to induce seizures, and what signal-transduction pathway and/or brain area is engaged to produce these seizures. In this review we provide a comprehensive overview of the current state of knowledge of δOR agonist-mediated seizures. The review was structured to highlight which agonists produce seizures, which brain regions have been implicated and which signaling mediators have been examined in this behavior. Our hope is that this review will spur future studies that are carefully designed and aimed to solve the question why certain δOR agonists are seizurogenic. Obtaining such insight may expedite the development of novel δOR clinical candidates without the risk of inducing seizures. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

CYX-5, a G-protein biassed MOP receptor agonist, DOP receptor antagonist and KOP receptor agonist, evokes constipation but not respiratory depression relative to morphine in rats

BACKGROUND

Strong opioid analgesics such as morphine alleviate moderate to severe acute nociceptive pain (e.g. post-surgical or post-trauma pain) as well as chronic cancer pain. However, they evoke many adverse effects and so there is an unmet need for opioid analgesics with improved tolerability. Recently, a prominent hypothesis has been that opioid-related adverse effects are mediated by β-arrestin2 recruitment at the µ-opioid (MOP) receptor and this stimulated research on discovery of G-protein biassed opioid analgesics. In other efforts, opioids with MOP agonist and δ-opioid (DOP) receptor antagonist profiles are promising for reducing side effects c.f. morphine. Herein, we report on the in vivo pharmacology of a novel opioid peptide (CYX-5) that is a G-protein biassed MOP receptor agonist, DOP receptor antagonist and kappa opioid (KOP) receptor agonist.

METHODS

Male Sprague-Dawley received intracerebroventricular bolus doses of CYX-5 (3, 10, 20 nmol), morphine (100 nmol) or vehicle, and antinociception (tail flick) was assessed relative to constipation (charcoal meal and castor oil-induced diarrhoea tests) and respiratory depression (whole body plethysmography).

RESULTS

CYX-5 evoked naloxone-sensitive, moderate antinociception, at the highest dose tested. Although CYX-5 did not inhibit gastrointestinal motility, it reduced stool output markedly in the castor oil-induced diarrhoea test. In contrast to morphine that evoked respiratory depression, CYX-5 increased tidal volume, thereby stimulating respiration.

CONCLUSION

Despite its lack of recruitment of β-arrestin2 at MOP, DOP and KOP receptors, CYX-5 evoked constipation, implicating a mechanism other than β-arrestin2 recruitment at MOP, DOP and KOP receptors, mediating constipation evoked by CYX-5 and potentially other opioid ligands.

The Delta-Opioid Receptor Bidirectionally Modulates Itch

Opioid signaling has been shown to be critically important in the neuromodulation of sensory circuits in the superficial spinal cord. Agonists of the mu-opioid receptor (MOR) elicit itch, whereas agonists of the kappa-opioid receptor (KOR) have been shown to inhibit itch. Despite the clear roles of MOR and KOR for the modulation itch, whether the delta-opioid receptor (DOR) is involved in the regulation of itch remained unknown. Here, we show that intrathecal administration of DOR agonists suppresses chemical itch and that intrathecal application of DOR antagonists is sufficient to evoke itch. We identify that spinal enkephalin neurons co-express neuropeptide Y (NPY), a peptide previously implicated in the inhibition of itch. In the spinal cord, DOR overlapped with both the NPY receptor (NPY1R) and KOR, suggesting that DOR neurons represent a site for convergent itch information in the dorsal horn. Lastly, we found that neurons co-expressing DOR and KOR showed significant Fos induction following pruritogen-evoked itch. These results uncover a role for DOR in the modulation of itch in the superficial dorsal horn. PERSPECTIVE: This article reveals the role of the delta-opioid receptor in itch. Intrathecal administration of delta agonists suppresses itch whereas the administration of delta antagonists is sufficient to induce itch. These studies highlight the importance of delta-opioid signaling for the modulation of itch behaviors, which may represent new targets for the management of itch disorders.

Morphinan derivatives with an oxabicyclo[3.2.1]octane structure as dual agonists toward δ and κ opioid receptors

The κ opioid receptor (KOR) is one of the promising targets to develop analgesics lacking morphine like side effects. To seek a novel KOR agonist we designed 6-amide derivatives with an oxabicyclo[3.2.1]octane structure based on a proposed active conformation of a selective KOR agonist nalfurafine. All the synthesized compounds strongly bound to the KOR and some compound showed KOR selectivities. 6R-Amides were more potent and efficacious KOR agonists than the corresponding 6S-isomers. However, most 6-amide derivatives were partial KOR agonist. Conformational analyses of 6R- and 6S-amide derivatives and nalfurafine well accounted for the difference of KOR agonistic activities between two diastereomers. Surprisingly, the tested N-H amides were full δ opioid receptor (DOR) agonists. Among the tested compounds 7a with benzamide moiety was the most potent dual DOR/KOR agonist. On the other hand, 6S-phenylacetamide 8b was potent full DOR agonist with less efficacious agonist activity for the μ receptor and KOR. 6-Amide derivatives with an oxabicyclo[3.2.1]octane structure were expected to be a promising fundamental skeleton for the dual DOR/KOR agonists and/or selective DOR agonists.

[Neural mechanism underlying the regulation of emotional behavior via δ-opioid receptors]

The delta opioid receptor (DOP) belongs to the G protein-coupled receptor family and is abundant in the limbic system. In recent years, consistent with their distribution, they have been suggested to be involved in the regulation of emotional behavior. In particular, DOP agonists have been shown to exhibit antidepressant and anxiolytic-like effects, and clinical trials are underway as targets for the development of new psychotropic drugs with mechanisms of action different from those of existing monoamine drugs. In this article, we review the roles and mechanisms of DOP in emotion regulation that are being elucidated in basic studies using rodents, and also introduce the current status of its clinical application.

Characterization of the Synergistic Effect between Ligands of Opioid and Free Fatty Acid Receptors in the Mouse Model of Colitis

Background: Recent studies suggest that lipids, including free fatty acids (FFAs), are necessary for proper μ opioid receptor (MOR) binding and that activation of opioid receptors (ORs) improves intestinal inflammation. The objective of the study was to investigate a possible interaction between the ORs and FFA receptors (FFARs) ligands in the colitis. Methods: The potential synergistic effect of ORs and FFARs ligands was evaluated using mouse model of acute colitis induced by dextran sulfate sodium (DSS, 4%). Compounds were injected intraperitoneally (i.p.) once or twice daily at the doses of 0.01 or 0.02 mg/kg body weight (BW) (DAMGO—an MOR agonist), 0.3 mg/kg BW (DPDPE—a δ OR (DOR) agonist) and 1 mg/kg BW (naloxone—a non-selective OR antagonist, GLPG 0974—a FFAR2 antagonist, GSK 137647—a FFAR4 agonist and AH 7614—a FFAR4 antagonist) for 4 days. Results: Myeloperoxidase (MPO) activity was significantly decreased after DAMGO (0.02 mg/kg BW) and GSK 137647 (1 mg/kg BW) administration and co-administration as compared to DSS group. Conclusions: Treatment with ligands of ORs and FFARs may affect the immune cells in the inflammation; however, no significant influence on the severity of colitis and no synergistic effect were observed.

In Vitro Analyses of Spinach-Derived Opioid Peptides, Rubiscolins: Receptor Selectivity and Intracellular Activities through G Protein- and β-Arrestin-Mediated Pathways

Activated opioid receptors transmit internal signals through two major pathways: the G-protein-mediated pathway, which exerts analgesia, and the β-arrestin-mediated pathway, which leads to unfavorable side effects. Hence, G-protein-biased opioid agonists are preferable as opioid analgesics. Rubiscolins, the spinach-derived naturally occurring opioid peptides, are selective δ opioid receptor agonists, and their p.o. administration exhibits antinociceptive effects. Although the potency and effect of rubiscolins as G-protein-biased molecules are partially confirmed, their in vitro profiles remain unclear. We, therefore, evaluated the properties of rubiscolins, in detail, through several analyses, including the CellKey^TM assay, cADDis^® cAMP assay, and PathHunter^® β-arrestin recruitment assay, using cells stably expressing µ, δ, κ, or µ/δ heteromer opioid receptors. In the CellKey^TM assay, rubiscolins showed selective agonistic effects for δ opioid receptor and little agonistic or antagonistic effects for µ and κ opioid receptors. Furthermore, rubiscolins were found to be G-protein-biased δ opioid receptor agonists based on the results obtained in cADDis^® cAMP and PathHunter^® β-arrestin recruitment assays. Finally, we found, for the first time, that they are also partially agonistic for the µ/δ dimers. In conclusion, rubiscolins could serve as attractive seeds, as δ opioid receptor-specific agonists, for the development of novel opioid analgesics with reduced side effects.

Delta Opioid Receptor Agonists Ameliorate Colonic Inflammation by Modulating Immune Responses

The opioid receptors play important roles in the regulation of sense and emotions. Although it is recently revealed that opioid receptors are also expressed in various cells, but not restricted in the central nervous system, the effects of opioids on peripheral immune cells are largely unknown. In the current study, we evaluated the effect of opioids on immune system by using selective agonists for δ opioid receptor. Systemic administration of KNT-127 or intraperitoneal injection of YNT-2715 (a KNT-127-related compound that cannot pass through the blood-brain barrier) significantly alleviated the pathology of dextran sodium sulfate-induced colitis. In KNT-127-treated mice, the levels of an inflammatory cytokine IL-6 in the serum, and macrophages in the mesenteric lymph nodes (MLNs) were decreased in the progression stage, and those of regulatory T cells (Tregs) in the MLN were increased in the recovery stage. In vitro experiments revealed that KNT-127 inhibited the release of IL-6 and another inflammatory cytokine TNF-α from macrophages and accelerated the development of Tregs. Our study suggests that δ opioid agonists act directly on immune cells to improve the pathology of the colitis and can be candidates of immunomodulatory drugs.

Kratom Alkaloids, Natural and Semi-Synthetic, Show Less Physical Dependence and Ameliorate Opioid Withdrawal

Chronic administration of opioids produces physical dependence and opioid-induced hyperalgesia. Users claim the Thai traditional tea "kratom" and component alkaloid mitragynine ameliorate opioid withdrawal without increased sensitivity to pain. Testing these claims, we assessed the combined kratom alkaloid extract (KAE) and two individual alkaloids, mitragynine (MG) and the analog mitragynine pseudoindoxyl (MP), evaluating their ability to produce physical dependence and induce hyperalgesia after chronic administration, and as treatments for withdrawal in morphine-dependent subjects. C57BL/6J mice (n = 10/drug) were administered repeated saline, or graded, escalating doses of morphine (intraperitoneal; i.p.), kratom alkaloid extract (orally, p.o.), mitragynine (p.o.), or MP (subcutaneously, s.c.) for 5 days. Mice treated chronically with morphine, KAE, or mitragynine demonstrated significant drug-induced hyperalgesia by day 5 in a 48 °C warm-water tail-withdrawal test. Mice were then administered naloxone (10 mg/kg, s.c.) and tested for opioid withdrawal signs. Kratom alkaloid extract and the two individual alkaloids demonstrated significantly fewer naloxone-precipitated withdrawal signs than morphine-treated mice. Additional C57BL/6J mice made physically dependent on morphine were then used to test the therapeutic potential of combined KAE, mitragynine, or MP given twice daily over the next 3 days at either a fixed dose or in graded, tapering descending doses. When administered naloxone, mice treated with KAE, mitragynine, or MP under either regimen demonstrated significantly fewer signs of precipitated withdrawal than control mice that continued to receive morphine. In conclusion, while retaining some liabilities, kratom, mitragynine, and mitragynine pseudoindoxyl produced significantly less physical dependence and ameliorated precipitated withdrawal in morphine-dependent animals, suggesting some clinical value.

Modulation of glutamatergic synaptic transmission and neuronal excitability in the prelimbic medial prefrontal cortex via delta-opioid receptors in mice

The medial prefrontal cortex (mPFC) plays a vital role in the processing of emotional events. It has been shown that activation of the glutamatergic transmission in prelimbic subregion of the mPFC (PL-PFC) evoked anxiety-like behavior in rodents. We previously reported that local perfusion of a selective agonist to delta-opioid receptor (DOP), KNT-127, attenuated the veratrine-induced elevation of extracellular glutamate in the PL-PFC and anxiety-like behavior in mice. These results suggested the possibility that KNT-127 suppresses glutamate release from the presynaptic site in the PL-PFC. To examine this possibility directly, we performed whole-cell patch-clamp recording from principal neurons in the PL-PFC and examined the spontaneous and electrically-evoked excitatory postsynaptic currents (EPSC)s. We found that bath application of KNT-127 significantly decreased the frequency of spontaneous and miniature EPSCs. Conversely, amplitude, rise time, and decay time of spontaneous and miniature EPSCs were not affected by bath application of KNT-127. Also, KNT-127 increased paired-pulse ratios of electrically-evoked EPSCs in the PL-PFC principal neurons tested. Further, we analyzed the firing properties of pyramidal neurons in the PL-PFC and found that KNT-127 treatment significantly reduced the number of action potentials and firing threshold. These results suggested that KNT-127 suppresses glutamatergic synaptic transmission by inhibiting glutamate release from the presynaptic site and reduces neuronal excitability in the mouse PL-PFC. We propose the possibility that these suppressing effects of KNT-127 on PL-PFC activity are part of the underlying mechanisms of its anxiolytic-like effects.

The mixed kappa and delta opioid receptor agonist, MP1104, attenuates chemotherapy-induced neuropathic pain

Effective treatments for chronic pain without abuse liability are urgently needed. One in 5 adults suffer chronic pain and half of these patients report inefficient treatment. Mu opioid receptor agonists (MOP), including oxycodone, tramadol and morphine, are often prescribed to treat chronic pain, however, use of drugs targeting MOP can lead to drug dependency, tolerance and overdose deaths. Kappa opioid receptor (KOP) agonists have antinociceptive effects without abuse potential; however, they have not been utilised clinically due to dysphoria and sedation. We hypothesise that mixed opioid receptor agonists targeting the KOP and delta opioid receptor (DOP) would have a wider therapeutic index, with the rewarding effects of DOP negating the negative effects of KOP. MP1104, an analogue of 3-Iodobenzoyl naltrexamine, is a novel mixed opioid receptor agonist with potent antinociceptive effects mediated via KOP and DOP in mice without rewarding or aversive effects. In this study, we show MP1104 has potent, long-acting antinociceptive effects in the warm-water tail-withdrawal assay in male and female mice and rats; and is longer acting than morphine. In the paclitaxel-induced neuropathic pain model in mice, MP1104 reduced both mechanical and cold allodynia and unlike morphine, did not produce tolerance when administered daily for 23 days. Moreover, MP1104 did not induce sedative effects in the open-field locomotor activity test, respiratory depression in mice using whole-body plethysmography, or have cross-tolerance with morphine. This data supports the therapeutic development of mixed opioid receptor agonists, particularly mixed KOP/DOP agonists, as non-addictive pain medications with reduced tolerance.

A non-convulsant delta-opioid receptor agonist, KNT-127, reduces cortical spreading depression and nitroglycerin-induced allodynia

OBJECTIVE

The aim of this study was to determine if the non-convulsant delta-opioid receptor (DOR) agonist, KNT-127, could inhibit migraine-associated endpoints.

BACKGROUND

The DOR has been identified as a therapeutic target for migraine. However, the development of delta agonists is limited as some ligands have seizurogenic properties, which may be related to their ability to induce receptor internalization. While both pro- and non-convulsant delta agonists can reduce migraine-associated allodynia, only the proconvulsant agonist, SNC80, has been shown to decrease cortical spreading depression (CSD). It is unclear if the ability of delta agonists to modulate cortical activity is related to the same signaling mechanisms that produce proconvulsant effects.

METHODS

The effects of the non-convulsant delta agonist, KNT-127, were examined. Repetitive CSD was induced in female C57BL6/J (n = 6/group) mice by continuous application of KCl and the effect of KNT-127/vehicle (Veh) on both local field potentials and optical intrinsic signals was determined. To assess the effect of KNT-127 on established chronic migraine-associated pain, male and female C57BL6/J mice were treated with nitroglycerin (NTG; 10 mg/kg, ip) every other day for 9 days and tested with KNT-127 (5 mg/kg, sc) or Veh on day 10 (n = 6/group). DOR-enhanced green fluorescent protein mice (n = 4/group) were used to confirm the internalization properties of KNT-127 in the trigeminal ganglia, trigeminal nucleus caudalis, and somatosensory cortex.

RESULTS

KNT-127 inhibited CSD events (t(10)  = 3.570, p = 0.0051). In addition, this delta agonist also reversed established cephalic allodynia in the NTG model of chronic migraine (F(1, 20)  = 12.80, p < 0.01). Furthermore, KNT-127 caused limited internalization of DOR in key migraine processing regions.

CONCLUSIONS

This study shows that the antimigraine effects of DOR agonists can be separated from their proconvulsant effects. This data provides valuable information for the continued development of delta agonists for the treatment of migraine.

The Delta-Specific Opioid Glycopeptide BBI-11008: CNS Penetration and Behavioral Analysis in a Preclinical Model of Levodopa-Induced Dyskinesia

In previous work we evaluated an opioid glycopeptide with mixed μ/δ-opioid receptor agonism that was a congener of leu-enkephalin, MMP-2200. The glycopeptide analogue showed penetration of the blood-brain barrier (BBB) after systemic administration to rats, as well as profound central effects in models of Parkinson's disease (PD) and levodopa (L-DOPA)-induced dyskinesia (LID). In the present study, we tested the glycopeptide BBI-11008 with selective δ-opioid receptor agonism, an analogue of deltorphin, a peptide secreted from the skin of frogs (genus Phyllomedusa). We tested BBI-11008 for BBB-penetration after intraperitoneal (i.p.) injection and evaluated effects in LID rats. BBI-11008 (10 mg/kg) demonstrated good CNS-penetrance as shown by microdialysis and mass spectrometric analysis, with peak concentration levels of 150 pM in the striatum. While BBI-11008 at both 10 and 20 mg/kg produced no effect on levodopa-induced limb, axial and oral (LAO) abnormal involuntary movements (AIMs), it reduced the levodopa-induced locomotor AIMs by 50% after systemic injection. The N-methyl-D-aspartate receptor antagonist MK-801 reduced levodopa-induced LAO AIMs, but worsened PD symptoms in this model. Co-administration of MMP-2200 had been shown prior to block the MK-801-induced pro-Parkinsonian activity. Interestingly, BBI-11008 was not able to block the pro-Parkinsonian effect of MK-801 in the LID model, further indicating that a balance of mu- and delta-opioid agonism is required for this modulation. In summary, this study illustrates another example of meaningful BBB-penetration of a glycopeptide analogue of a peptide to achieve a central behavioral effect, providing additional evidence for the glycosylation technique as a method to harness therapeutic potential of peptides.

Optimized Opioid-Neurotensin Multitarget Peptides: From Design to Structure-Activity Relationship Studies

Fusion of nonopioid pharmacophores, such as neurotensin, with opioid ligands represents an attractive approach for pain treatment. Herein, the μ-/δ-opioid agonist tetrapeptide H-Dmt-d-Arg-Aba-β-Ala-NH2 (KGOP01) was fused to NT(8-13) analogues. Since the NTS1 receptor has been linked to adverse effects, selective MOR-NTS2 ligands are preferred. Modifications were introduced within the native NT sequence, particularly a β3-homo amino acid in position 8 and Tyr11 substitutions. Combination of β3hArg and Dmt led to peptide 7, a MOR agonist, showing the highest NTS2 affinity described to date (Ki = 3 pM) and good NTS1 affinity (Ki = 4 nM), providing a >1300-fold NTS2 selectivity. The (6-OH)Tic-containing analogue 9 also exhibited high NTS2 affinity (Ki = 1.7 nM), with low NTS1 affinity (Ki = 4.7 μM), resulting in an excellent NTS2 selectivity (>2700). In mice, hybrid 7 produced significant and prolonged antinociception (up to 8 h), as compared to the KGOP01 opioid parent compound.

δ-opioid receptor activation protects against Parkinson's disease-related mitochondrial dysfunction by enhancing PINK1/Parkin-dependent mitophagy

Our previous studies have shown that the δ-opioid receptor (DOR) is an important neuroprotector via the regulation of PTEN-induced kinase 1 (PINK1), a mitochondria-related molecule, under hypoxic and MPP⁺ insults. Since mitochondrial dysfunctions are observed in both hypoxia and MPP⁺ insults, this study further investigated whether DOR is cytoprotective against these insults by targeting mitochondria. Through comparing DOR-induced responses to hypoxia versus MPP⁺-induced parkinsonian insult in PC12 cells, we found that both hypoxia and MPP⁺ caused a collapse of mitochondrial membrane potential and severe mitochondrial dysfunction. In sharp contrast to its inappreciable effect on mitochondria in hypoxic conditions, DOR activation with UFP-512, a specific agonist, significantly attenuated the MPP⁺-induced mitochondrial injury. Mechanistically, DOR activation effectively upregulated PINK1 expression and promoted Parkin's mitochondrial translocation and modification, thus enhancing the PINK1-Parkin mediated mitophagy. Either PINK1 knockdown or DOR knockdown largely interfered with the DOR-mediated mitoprotection in MPP⁺ conditions. Moreover, there was a major difference between hypoxia versus MPP⁺ in terms of the regulation of mitophagy with hypoxia-induced mitophagy being independent from DOR-PINK1 signaling. Taken together, our novel data suggest that DOR activation is neuroprotective against parkinsonian injury by specifically promoting mitophagy in a PINK1-dependent pathway and thus attenuating mitochondrial damage.

Forebrain delta opioid receptors regulate the response of delta agonist in models of migraine and opioid-induced hyperalgesia

Delta opioid receptor (DOR) agonists have been identified as a promising novel therapy for headache disorders. DORs are broadly expressed in several peripheral and central regions important for pain processing and mood regulation; and it is unclear which receptors regulate headache associated symptoms. In a model of chronic migraine-associated pain using the human migraine trigger, nitroglycerin, we observed increased expression of DOR in cortex, hippocampus, and striatum; suggesting a role for these forebrain regions in the regulation of migraine. To test this hypothesis, we used conditional knockout mice with DORs deleted from forebrain GABAergic neurons (Dlx-DOR), and investigated the outcome of this knockout on the effectiveness of the DOR agonist SNC80 in multiple headache models. In DOR loxP controls SNC80 blocked the development of acute and chronic cephalic allodynia in the chronic nitroglycerin model, an effect that was lost in Dlx-DOR mice. In addition, the anti-allodynic effects of SNC80 were lost in a model of opioid induced hyperalgesia/medication overuse headache in Dlx-DOR conditional knockouts. In a model reflecting negative affect associated with migraine, SNC80 was only effective in loxP controls and not Dlx-DOR mice. Similarly, SNC80 was ineffective in the cortical spreading depression model of migraine aura in conditional knockout mice. Taken together, these data indicate that forebrain DORs are necessary for the action of DOR agonists in relieving headache-related symptoms and suggest that forebrain regions may play an important role in migraine modulation.

Ligand-specific recycling profiles determine distinct potential for chronic analgesic tolerance of delta-opioid receptor (DOPr) agonists

δ-opioid receptor (DOPr) agonists have analgesic efficacy in chronic pain models but development of tolerance limits their use for long-term pain management. Although agonist potential for inducing acute analgesic tolerance has been associated with distinct patterns of DOPr internalization, the association between trafficking and chronic tolerance remains ill-defined. In a rat model of streptozotocin (STZ)-induced diabetic neuropathy, deltorphin II and TIPP produced sustained analgesia  following daily (intrathecal) i.t. injections over six days, whereas similar treatment with SNC-80 or SB235863 led to progressive tolerance and loss of the analgesic response. Trafficking assays in murine neuron cultures showed no association between the magnitude of ligand-induced sequestration and development of chronic tolerance. Instead, ligands that supported DOPr recycling were also the ones producing sustained analgesia over 6-day treatment. Moreover, endosomal endothelin-converting enzyme 2 (ECE2) blocker 663444 prevented DOPr recycling by deltorphin II and TIPP and precipitated tolerance by these ligands. In conclusion, agonists, which support DOPr recycling, avoid development of analgesic tolerance over repeated administration.

Discovery of δ opioid receptor full agonists lacking a basic nitrogen atom and their antidepressant-like effects

We have recently reported that the elaboration of the N-substituent in the δ opioid receptor (DOR) antagonist naltrindole (NTI) enabled the regulation of the DOR activities from full inverse agonists to weak partial agonists. The investigations of amide-type NTI derivatives revealed that N-phenylacetyl and N-dihydrocinnamoyl derivatives 3a and 3b were DOR full agonists. The same transformations were applied to a DOR agonist KNT-127 to provide the more potent DOR agonists 6a and 6b. Among the tested compounds, the most efficacious compound 6a showed dose-dependent antidepressant-like effects in the mouse forced swim test. The antidepressant-like effects by 6a seemed to be more potent than those of KNT-127, which is a more potent DOR agonist in in vitro assays. The amide-type compound like 6a may more fully penetrate into the central nervous system.

The antinociceptive effects of a dual kappa-delta opioid receptor agonist in the mouse formalin test

Pain management is a challenging and unmet medical need. Despite their demonstrated efficacy, currently used opioid drugs and nonsteroidal anti-inflammatory drugs are frequently associated with several adverse events. The identification of new and safe analgesics is therefore needed. MP1104, an analogue of 3'-iodobenzoyl naltrexamine, is a potent nonselective full agonist at mu (MOR), kappa (KOR), and delta (DOR) opioid receptors, respectively. It was shown to possess potent antinociceptive effects in acute thermal pain assays without aversion in mice. In this study, we investigated MP1104 in the formalin test, a model of tonic pain. MP1104 (0.05, 0.1, and 1.0 mg/kg) reduced pain-like behaviors in phases I and II of the formalin test in male and female ICR mice. Pretreatment with KOR antagonist (norbinaltorphimine 10 mg/kg) and DOR antagonist (naltrindole 10 mg/kg) abolished the antinociceptive effects of MP1104 in the formalin test. These findings support the development of MP1104 for further testing in other pain models.

Behavioral pharmacology of the mixed-action delta-selective opioid receptor agonist BBI-11008: studies on acute, inflammatory and neuropathic pain, respiration, and drug self-administration

RATIONALE AND OBJECTIVES

The present study characterized the behavioral pharmacology of a novel, mixed-action delta-selective (78:1) opioid receptor agonist, BBI-11008. This glycopeptide drug candidate was tested in assays assessing antinociception (acute, inflammatory, and neuropathic pain-like conditions) and side-effect endpoints (respiratory depression and drug self-administration).

RESULTS

BBI-11008 had a 78-fold greater affinity for the delta opioid receptor than the mu receptor, and there was no binding to the kappa opioid receptor. BBI-11008 (3.2-100; 10-32 mg kg-1, i.v.) and morphine (1-10; 1-3.2 mg kg-1, i.v.) produced antinociceptive and anti-allodynic effects in assays of acute thermal nociception and complete Freund's adjuvant (CFA)-induced inflammatory pain, with BBI-11008 being less potent than morphine in both assays. BBI-11008 (1-18 mg kg-1, i.v.) had similar efficacy to gabapentin (10-56 mg kg-1, i.v.) in a spinal nerve ligation (SNL) model of neuropathic pain. In the respiration assay, with increasing %CO2 exposure, BBI-11008 produced an initial increase (32 mg kg-1, s.c.) and then decrease (56 mg kg-1, s.c.) in minute volume (MV) whereas morphine (3.2-32 mg kg-1, s.c.) produced dose-dependent decreases in MV. In the drug self-administration procedure, BBI-11008 did not maintain self-administration at any dose tested.

CONCLUSIONS

These results suggest that the glycopeptide drug candidate possesses broad-spectrum antinociceptive and anti-allodynic activity across a range of pain-like conditions. Relative to morphine or fentanyl, the profile for BBI-11008 in the respiration and drug self-administration assays suggests that BBI-11008 may have less pronounced deleterious side effects. Continued assessment of this compound is warranted.

Opioids differentially modulate two synapses important for pain processing in the amygdala

BACKGROUND AND PURPOSE

Pain is a subjective experience involving sensory discriminative and emotionally aversive components. Consistent with its role in pain processing and emotions, the amygdala modulates the aversive component of pain. The laterocapsular region of the central nucleus of the amygdala (CeLC) receives nociceptive information from the parabrachial nucleus (PB) and polymodal, including nociceptive, inputs from the basolateral nucleus of the amygdala (BLA). Opioids are strong analgesics and reduce both the sensory discriminative and the affective component of pain. However, it is unknown whether opioids regulate activity at the two nociceptive inputs to the amygdala.

EXPERIMENTAL APPROACH

Using whole-cell electrophysiology, optogenetics, and immunohistochemistry, we investigated whether opioids inhibit the rat PB-CeLC and BLA-CeLC synapses.

KEY RESULTS

Opioids inhibited glutamate release at the PB-CeLC and BLA-CeLC synapses. Opioid inhibition is via the μ-receptor at the PB-CeLC synapse, while at the BLA-CeLC synapse, inhibition is via μ-receptors in all neurons and via δ-receptors and κ-receptors in a subset of neurons.

CONCLUSIONS AND IMPLICATIONS

Agonists of μ-receptors inhibited two of the synaptic inputs carrying nociceptive information into the laterocapsular amygdala. Therefore, μ-receptor agonists, such as morphine, will inhibit glutamate release from PB and BLA in the CeLC, and this could serve as a mechanism through which opioids reduce the affective component of pain and pain-induced associative learning. The lower than expected regulation of BLA synaptic outputs by δ-receptors does not support the proposal that opioid receptor subtypes segregate into subnuclei of brain regions.

The Meta-Position of Phe4 in Leu-Enkephalin Regulates Potency, Selectivity, Functional Activity, and Signaling Bias at the Delta and Mu Opioid Receptors

As tool compounds to study cardiac ischemia, the endogenous δ-opioid receptors (δOR) agonist Leu⁵-enkephalin and the more metabolically stable synthetic peptide (d-Ala², d-Leu⁵)-enkephalin are frequently employed. However, both peptides have similar pharmacological profiles that restrict detailed investigation of the cellular mechanism of the δOR’s protective role during ischemic events. Thus, a need remains for δOR peptides with improved selectivity and unique signaling properties for investigating the specific roles for δOR signaling in cardiac ischemia. To this end, we explored substitution at the Phe⁴ position of Leu⁵-enkephalin for its ability to modulate receptor function and selectivity. Peptides were assessed for their affinity to bind to δORs and µ-opioid receptors (µORs) and potency to inhibit cAMP signaling and to recruit β-arrestin 2. Additionally, peptide stability was measured in rat plasma. Substitution of the meta-position of Phe⁴ of Leu⁵-enkephalin provided high-affinity ligands with varying levels of selectivity and bias at both the δOR and µOR and improved peptide stability, while substitution with picoline derivatives produced lower-affinity ligands with G protein biases at both receptors. Overall, these favorable substitutions at the meta-position of Phe⁴ may be combined with other modifications to Leu⁵-enkephalin to deliver improved agonists with finely tuned potency, selectivity, bias and drug-like properties.

Mu and Delta Opioid Receptors Are Coexpressed and Functionally Interact in the Enteric Nervous System of the Mouse Colon

BACKGROUND & AIMS

Functional interactions between the mu opioid receptor (MOR) and delta opioid receptor (DOR) represent a potential target for novel analgesics and may drive the effects of the clinically approved drug eluxadoline for the treatment of diarrhea-predominant irritable bowel syndrome. Although the enteric nervous system (ENS) is a likely site of action, the coexpression and potential interaction between MOR and DOR in the ENS are largely undefined. In the present study, we have characterized the distribution of MOR in the mouse ENS and examined MOR-DOR interactions by using pharmacologic and cell biology techniques.

METHODS

MOR and DOR expression was defined by using MORmCherry and MORmCherry-DOR-eGFP knockin mice. MOR-DOR interactions were assessed by using DOR-eGFP internalization assays and by pharmacologic analysis of neurogenic contractions of the colon.

RESULTS

Although MOR was expressed by approximately half of all myenteric neurons, MOR-positive submucosal neurons were rarely observed. There was extensive overlap between MOR and DOR in both excitatory and inhibitory pathways involved in the coordination of intestinal motility. MOR and DOR can functionally interact, as shown through heterologous desensitization of MOR-dependent responses by DOR agonists. Functional evidence suggests that MOR and DOR may not exist as heteromers in the ENS. Pharmacologic studies show no evidence of cooperativity between MOR and DOR. DOR internalizes independently of MOR in myenteric neurons, and MOR-evoked contractions are unaffected by the sequestration of DOR.

CONCLUSIONS

Collectively, these findings demonstrate that although MOR and DOR are coexpressed in the ENS and functionally interact, they are unlikely to exist as heteromers under physiological conditions.

Elucidating the active δ-opioid receptor crystal structure with peptide and small-molecule agonists

Selective activation of the δ-opioid receptor (DOP) has great potential for the treatment of chronic pain, benefitting from ancillary anxiolytic and antidepressant-like effects. Moreover, DOP agonists show reduced adverse effects as compared to μ-opioid receptor (MOP) agonists that are in the spotlight of the current "opioid crisis." Here, we report the first crystal structures of the DOP in an activated state, in complex with two relevant and structurally diverse agonists: the potent opioid agonist peptide KGCHM07 and the small-molecule agonist DPI-287 at 2.8 and 3.3 Å resolution, respectively. Our study identifies key determinants for agonist recognition, receptor activation, and DOP selectivity, revealing crucial differences between both agonist scaffolds. Our findings provide the first investigation into atomic-scale agonist binding at the DOP, supported by site-directed mutagenesis and pharmacological characterization. These structures will underpin the future structure-based development of DOP agonists for an improved pain treatment with fewer adverse effects.

Reduced nucleus accumbens enkephalins underlie vulnerability to social defeat stress

Enkephalins, endogenous ligands for delta opioid receptors (DORs), are highly enriched in the nucleus accumbens (NAc). They are implicated in depression but their role in the NAc, a critical brain region for motivated behavior, is not fully investigated. To provide insight into enkephalin function we used a chronic social defeat stress paradigm, where animals are either categorized as susceptible or resilient to stress based on their performance in a social interaction test. Compared to controls, susceptible animals showed reduced enkephalin levels in the NAc. Such decrease in enkephalin levels is not due to a change in mRNA of its precursor protein, proenkephalin, in susceptible mice but is consistent with increased mRNA levels of enkephalinases in the NAc of susceptible animals. Systemic administration of enkephalinase inhibitors or NAc infusion of the DOR agonist, SNC80, caused a resilient outcome to CSDS. Both treatments increased phosphorylation of ERK, which was downregulated by social defeat stress. To further validate these results, we also used Q175 knock-in mice, an animal model of Huntington's disease in which enkephalin levels are reduced in striatum and comorbidity with mood disorders is common. Consistent with data in wild-type mice, Q175 animals showed reduced enkephalin levels in the NAc and enhanced susceptibility to a social defeat stress. Overall, our data implicate that depression-like behavior induced by social defeat stress arises from disrupted DOR signaling resulting from lowered levels of enkephalins, which is partly mediated through elevated expression of enkephalinases.

Delta opioid receptor agonists are effective for multiple types of headache disorders

Headaches are highly disabling and are among the most common neurological disorders worldwide. Despite the high prevalence of headache, therapeutic options are limited. We recently identified the delta opioid receptor (DOR) as an emerging therapeutic target for migraine. In this study, we examined the effectiveness of a hallmark DOR agonist, SNC80, in disease models reflecting diverse headache disorders including: chronic migraine, post-traumatic headache (PTH), medication overuse headache by triptans (MOH), and opioid-induced hyperalgesia (OIH). To model chronic migraine C57BL/6J mice received chronic intermittent treatment with the known human migraine trigger, nitroglycerin. PTH was modeled by combining the closed head weight drop model with the nitroglycerin model of chronic migraine. For MOH and OIH, mice were chronically treated with sumatriptan or morphine, respectively. The development of periorbital and peripheral allodynia was observed in all four models; and SNC80 significantly inhibited allodynia in all cases. In addition, we also determined if chronic daily treatment with SNC80 would induce MOH/OIH, and we observed limited hyperalgesia relative to sumatriptan or morphine. Together, our results indicate that DOR agonists could be effective in multiple headache disorders, despite their distinct etiology, thus presenting a novel therapeutic target for headache.

Administration of a delta opioid receptor agonist KNT-127 to the basolateral amygdala has robust anxiolytic-like effects in rats

RATIONALE

We previously reported that systemic administration of a selective delta opioid receptor (DOP) agonist, KNT-127, produced potent anxiolytic-like effects in rats. Interestingly, DOPs are highly distributed in the basolateral region of the amygdala (BLA).

OBJECTIVES

In this study, we investigated the effect of intra-BLA administration of KNT-127 on anxiety-like behaviors in rats.

METHODS AND RESULTS

In the elevated plus maze test, bilateral injection of KNT-127 into the BLA significantly and dose-dependently increased time spent in the open arms. The magnitude of KNT-127 (0.08 μg/0.2 μl)-induced anxiolytic-like effects was similar to muscimol (0.1 μg/0.2 μl), which is a selective agonist for the gamma amino butyric acid type A receptors. Further, anxiolytic-like effects of KNT-127 were abolished by pretreatment with naltrindole, a selective DOP antagonist, suggesting that KNT-127-induced anxiolytic-like effects are mediated by DOPs. These anxiolytic-like effects were confirmed using another innate anxiety model, the open field test. Interestingly, intra-BLA administration of KNT-127 also induced anxiolytic-like effects in the contextual fear conditioning test. Moreover, these effects were also abolished by naltrindole pretreatment. Finally, we demonstrated that intra-BLA administration of KNT-127 facilitates extinction learning of contextual fear in conditioned rats.

CONCLUSIONS

Altogether, our findings clearly demonstrate that intra-BLA administration of KNT-127 in rats has robust anxiolytic-like effects not only in innate anxiety-like behavioral tests but also in the contextual fear conditioning test.

Genetic and pharmacological antagonism of NK1 receptor prevents opiate abuse potential

Development of an efficacious, non-addicting analgesic has been challenging. Discovery of novel mechanisms underlying addiction may present a solution. Here we target the neurokinin system, which is involved in both pain and addiction. Morphine exerts its rewarding actions, at least in part, by inhibiting GABAergic input onto substance P (SP) neurons in the ventral tegmental area (VTA), subsequently increasing SP release onto dopaminergic neurons. Genome editing of the neurokinin 1 receptor (NK1R) in the VTA renders morphine non-rewarding. Complementing our genetic approach, we demonstrate utility of a bivalent pharmacophore with dual activity as a μ/δ opioid agonist and NK1R antagonist in inhibiting nociception in an animal model of acute pain while lacking any positive reinforcement. These data indicate that dual targeting of the dopaminergic reward circuitry and pain pathways with a multifunctional opioid agonist-NK1R antagonist may be an efficacious strategy in developing future analgesics that lack abuse potential.

Chemical space screening around Phe3 in opioid peptides: Modulating µ versus δ agonism by Suzuki-Miyaura cross-couplings

In this study, affinities and activities of derivatized analogues of Dmt-dermorphin[1-4] (i.e. Dmt-d-Ala-Phe-GlyNH2, Dmt = 2',6'-dimethyl-(S)-tyrosine) for the µ opioid receptor (MOP) and δ opioid receptor (DOP) were evaluated using radioligand binding studies, functional cell-based assays and isolated organ bath experiments. By means of solid-phase or solution-phase Suzuki-Miyaura cross-couplings, various substituted regioisomers of the phenylalanine moiety in position 3 of the sequence were prepared. An 18-membered library of opioid tetrapeptides was generated via screening of the chemical space around the Phe3 side chain. These substitutions modulated bioactivity, receptor subtype selectivity and highly effective ligands with subnanomolar binding affinities, contributed to higher functional activities and potent analgesic actions. In search of selective peptidic ligands, we show here that the Suzuki-Miyaura reaction is a versatile and robust tool which could also be deployed elsewhere.

Evaluation of morphine-like effects of the mixed mu/delta agonist morphine-6-O-sulfate in rats: Drug discrimination and physical dependence

Morphine-6-O-sulfate (M6S) is as a mixed-action mu/delta (μ/δ) opioid receptor agonist with high potency and analgesic efficacy. These studies used assays of drug discrimination and schedule-controlled responding to assess abuse-liability, tolerance, and physical dependence as compared to morphine in rats. Attempts to train 0.3 mg/kg (IP) M6S from saline failed, but all rats rapidly acquired the discrimination when the training dose was changed to 3.0 mg/kg morphine, and substitution tests showed that morphine and fentanyl both fully substituted for the training dose, M6S and M3A6S (3-O-acetyl ester of M6S) only partially substituted, and salvinorin A did not elicit morphine-like effects. Tolerance to response rate-decreasing effects was studied in rats administered either 1.0 or 3.0 mg/kg morphine or M6S before food-reinforced operant sessions. At both unit doses, tolerance to M6S-elicited rate suppression developed more slowly than tolerance to morphine-induced reductions in response rates. To assess dependence, rats were maintained on 1.0 mg/kg morphine or 1.0 mg/kg M6S until food-reinforced response rates were stable for at least 5 days. Rats were then administered saline or increasing doses of the opioid antagonist naltrexone (NTX) (0.3, 1.0, 3.0, or 10.0 mg/kg) in order to determine antagonist-precipitated withdrawal. NTX precipitated withdrawal was similar in both morphine-maintained and M6S-maintained rats. In conclusion, the mixed μ/δ agonist activity of M6S failed to completely protect against the development of physical dependence, but delayed tolerance development to behavioral effects and resulted in decreased morphine-like subjective effects, perhaps implying a decreased abuse liability over μ agonists.

Allosterism within δ Opioid-κ Opioid Receptor Heteromers in Peripheral Sensory Neurons: Regulation of κ Opioid Agonist Efficacy

There is abundant evidence for formation of G protein-coupled receptor heteromers in heterologous expression systems, but little is known of the function of heteromers in native systems. Heteromers of δ and κ opioid receptors (DOR-KOR heteromers) have been identified in native systems. We previously reported that activation of DOR-KOR heteromers expressed by rat pain-sensing neurons (nociceptors) produces robust, peripherally mediated antinociception. Moreover, DOR agonist potency and efficacy is regulated by KOR antagonists via allosteric interactions within the DOR-KOR heteromer in a ligand-dependent manner. Here we assessed the reciprocal regulation of KOR agonist function by DOR antagonists in adult rat nociceptors in culture and in a behavioral assay of nociception. Naltrindole enhanced the potency of the KOR agonist 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-pyrrolidin-1-ylethyl]acetamide (ICI-199441) 10- to 20-fold, but did not alter responses to 2-(3,4-dichlorophenyl)-N-methyl-N-[(1R,2R)-2-pyrrolidin-1-ylcyclohexyl]acetamide (U50488). By contrast, the potency of U50488 was enhanced 20-fold by 7-benzylidenenaltrexone. The efficacy of 6'-guanidinonaltrindole (6'-GNTI) to inhibit nociceptors was blocked by small interfering RNA knockdown of DOR or KOR. Replacing 6'-GNTI occupancy of DOR with either naltrindole or 7-benzylidenenaltrexone abolished 6'-GNTI efficacy. Further, peptides derived from DOR transmembrane segment 1 fused to the cell membrane-penetrating HIV transactivator of transcription peptide also blocked 6'-GNTI-mediated responses ex vivo and in vivo, suggesting that 6'-GNTI efficacy in nociceptors is due to its positive allosteric regulation of KOR via occupancy of DOR in a DOR-KOR heteromer. Together, these results provide evidence for the existence of functional DOR-KOR heteromers in rat peripheral sensory neurons and that reciprocal, ligand-dependent allosteric interactions occur between the DOR and KOR protomers.

Multifaceted Effects of Delta Opioid Receptors and DADLE in Diseases of the Nervous System

BACKGROUND

The opioid system is considered a potential therapeutic target in a variety of neurological disorders. Delta opioid receptors (DORs) are broadly expressed in the brain, and their activation protects cells from hypoxic/ischemic insults by counteracting disruptions of ionic homeostasis and initiating neuroprotective pathways. The DOR agonist D-Ala2-D-Leu2-Enkephalin (DADLE) promotes neuronal survival, mitigates apoptotic pathways, and protects neurons and glial cells from ischemia-induced cell death, thus making DADLE a promising therapeutic option for stroke. The significant amount of research regarding DORs and DADLE in the last decades also suggests their potential in treating other neurological disorders.

METHODS

This review compiled relevant literature detailing the role of DORs and agonists in central nervous system function and neuropathologies.

RESULTS

Several studies demonstrate potential mechanisms implicating a key interaction between DORs and DADLE in conferring neuroprotective benefits. A better understanding of DOR function in disease-specific contexts is critical to transitioning DOR agonists into the clinic as a therapy for stroke and other neurological diseases.

CONCLUSION

Evidence-based studies support the potential of the delta-opioid family of receptors and its ligands in developing novel therapeutic strategies for stroke and other brain disorders.

Delta Opioid Receptors and Cardioprotection

The opioid receptor family, with associated endogenous ligands, has numerous roles throughout the body. Moreover, the delta opioid receptor (DORs) has various integrated roles within the physiological systems, including the cardiovascular system. While DORs are important modulators of cardiovascular autonomic balance, they are well-established contributors to cardioprotective mechanisms. Both endogenous and exogenous opioids acting upon DORs have roles in myocardial hibernation and protection against ischaemia-reperfusion (I-R) injury. Downstream signalling mechanisms governing protective responses alternate, depending on the timing and duration of DOR activation. The following review describes models and mechanisms of DOR-mediated cardioprotection, the impact of co-morbidities and challenges for clinical translation.

Synthesis, Pharmacology, and Molecular Docking Studies on 6-Desoxo-N-methylmorphinans as Potent μ-Opioid Receptor Agonists

Position 6 of the morphinan skeleton plays a key role in the μ-opioid receptor (MOR) activity in vitro and in vivo. We describe the consequence of the 6-carbonyl group deletion in N-methylmorphinan-6-ones 1-4 on ligand-MOR interaction, signaling, and antinociception. While 6-desoxo compounds 1a, 2a, and 4a show similar profiles to their 6-keto counterparts, the 6-desoxo-14-benzyloxy substituted 3a displays significantly increased MOR binding and agonist potency and a distinct binding mode compared with its analogue 3.

Modulation of mesenteric collecting lymphatic contractions by σ1-receptor activation and nitric oxide production

Recently, it has been reported that a σ-receptor antagonist could reduce inflammation-induced edema. Lymphatic vessels play an essential role in removing excess interstitial fluid. We tested the hypothesis that activation of σ-receptors would reduce or weaken collecting lymphatic contractions. We used isolated, cannulated rat mesenteric collecting lymphatic vessels to study contractions in response to the σ-receptor agonist afobazole in the absence and presence of different σ-receptor antagonists. We used RT-PCR and Western blot analysis to investigate whether these vessels express the σ1-receptor and immunofluorescence confocal microscopy to examine localization of the σ1-receptor in the collecting lymphatic wall. Using N-nitro-l-arginine methyl ester (l-NAME) pretreatment before afobazole in isolated lymphatics, we tested the role of nitric oxide (NO) signaling. Finally, we used 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate fluorescence as an indicator to test whether afobazole increases NO release in cultured lymphatic endothelial cells. Our results show that afobazole (50-150 µM) elevated end-systolic diameter and generally reduced pump efficiency and that this response could be partially blocked by the σ1-receptor antagonists BD 1047 and BD 1063 but not by the σ2-receptor antagonist SM-21. σ1-Receptor mRNA and protein were detected in lysates from isolated rat mesenteric collecting lymphatics. Confocal images with anti-σ1-receptor antibody labeling suggested localization in the lymphatic endothelium. Blockade of NO synthases with l-NAME inhibited the effects of afobazole. Finally, afobazole elicited increases in NO production from cultured lymphatic endothelial cells. Our findings suggest that the σ1-receptor limits collecting lymphatic pumping through a NO-dependent mechanism.NEW & NOTEWORTHY Relatively little is known about the mechanisms that govern contractions of lymphatic vessels. σ1-Receptor activation has been shown to reduce the fractional pump flow of isolated rat mesenteric collecting lymphatics. The σ1-receptor was localized mainly in the endothelium, and blockade of nitric oxide synthase inhibited the effects of afobazole.

Evaluation of Analgesia, Tolerance, and the Mechanism of Action of Morphine-6-O-Sulfate Across Multiple Pain Modalities in Sprague-Dawley Rats

BACKGROUND

Morphine-6-O-sulfate (M6S) is a mixed μ/δ-opioid receptor (OR) agonist and potential alternative to morphine for treatment of chronic multimodal pain.

METHODS

To provide more support for this hypothesis, the antinociceptive effects of M6S and morphine were compared in tests that access a range of pain modalities, including hot plate threshold (HPT), pinprick sensitivity threshold (PST) and paw pressure threshold tests.

RESULTS

Acutely, M6S was 2- to 3-fold more potent than morphine in HPT and PST tests, specifically, derived from best-fit analysis of dose-response relationships of morphine/M6S half-effective dose (ED50) ratios (lower, upper 95% confidence interval [CI]) were 2.8 (2.0-5.8) in HPT and 2.2 (2.1, 2.4) in PST tests. No differences in analgesic drug potencies were detected in the PPT test (morphine/M6S ED50 ratio 1.2 (95% CI, 0.8-1.4). After 7 to 9 days of chronic treatment, tolerance developed to the antinociceptive effects of morphine, but not to M6S, in all 3 pain tests. Morphine-tolerant rats were not crosstolerant to M6S. The antinociceptive effects of M6S were not sensitive to κ-OR antagonists. However, the δ-OR antagonist, naltrindole, blocked M6S-induced antinociception by 55% ± 4% (95% CI, 39-75) in the HPT test, 94% ± 4% (95% CI, 84-105) in the PST test, and 5% ± 17% (95% CI, -47 to 59) or 51% ± 14% (95% CI, 14-84; 6 rats per each group) in the paw pressure threshold test when examined acutely or after 7 days of chronic treatment, respectively.

CONCLUSIONS

Activity via δ-ORs thus appears to be an important determinant of M6S action. M6S also exhibited favorable antinociceptive and tolerance profiles compared with morphine in 3 different antinociceptive assays, indicating that M6S may serve as a useful alternative for rotation in morphine-tolerant subjects.

The induction of the transcription factor Nrf2 enhances the antinociceptive effects of delta-opioid receptors in diabetic mice

The involvement of heme oxygenase 1 (HO-1) in the modulation of the antinociceptive effects of opioids in type 1 diabetes has been demonstrated but the role played by the transcription factor Nrf2 in the regulation of painful neuropathy and in the effects and expression of δ-opioid receptors (DOR) in type 2 diabetes, has not been studied. In male BKS.Cg-m+/+Leprdb/J (db/db) mice, the anti-allodynic effects produced by a Nrf2 transcription factor activator, sulforaphane (SFN) administered alone and combined with two DOR agonists, [d-Pen(2),d-Pen(5)]-Enkephalin (DPDPE) and (+)-4-[(αR)-α-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N diethylbenzamide (SNC-80), were evaluated. The effects of SFN on glucose levels and body weight as well as on the proteins levels of Nrf2, HO-1, NAD(P)H: quinone oxidoreductase 1 (NQO1), MAPKs (JNK) and DOR in sciatic nerve from db/db mice were also assessed. This study showed that the administration of SFN dose dependently reversed mechanical allodynia, reduced hyperglycemia and body weight gain associated to type 2 diabetes and significantly increased the anti-allodynic effects of DPDPE and SNC-80 in db/db mice. This treatment normalized the down regulation of Nrf2 and NQO1 and enhanced the protein levels of HO-1 in db/db mice. Moreover, the administration of SFN also inhibited the JNK phosphorylation and DOR down-regulation in the sciatic nerve of diabetic mice. Our data indicated that SFN treatment is effective in reversing mechanical allodynia and enhancing DOR antinociceptive effects in db/db mice which effects might be mediated by activating Nrf2 signaling, reducing hyperglycemia, inhibiting JNK phosphorylation and avoiding DOR down-regulation in the sciatic nerve of these animals. These results propose SFN, alone and/or combined with DOR agonists, as interesting approaches for the treatment of painful diabetic neuropathy associated to type 2 diabetes in mice.

Activation of the PI3K-Akt pathway promotes neuroprotection of the δ-opioid receptor agonist against cerebral ischemia-reperfusion injury in rat models

The central objective was to identify the role of the PI3K-Akt activation pathway on the neuroprotection of δ-opioid receptor agonist (DADLE) against cerebral ischemia-reperfusion (I/R) injury in a rat model. Fifty-five male Sprague-Dawley (SD) rats were included to establish a middle cerebral artery occlusion (MCAO) model which were then divided into the sham, MCAO, LY294002 (MCAO+DADLE+LY294002 [inhibitor of PI3K-Akt pathway]), DADLE (MCAO+DADLE) and DMSO (MCAO+DADLE+DMSO [dimethyl sulphoxide]) groups. The cerebral infarction (CI) volume and nerve cell apoptosis was determined using TTC and TUNEL staining. Quantitative real-time polymerase chain reaction (qRT-PCR), western blotting and immunohistochemistry staining were applied for the expressions of Bad, Bax, Bcl-2 and cleaved caspase-3. The MCAO group showed higher CI volume, nerve cell apoptosis and cleaved caspase-3 expressions than the DADLE and DMSO groups, which were also higher in the LY294002 group than the DADLE group. Compared with the MCAO group, the mRNA and protein expressions of PI3K and Bcl-2, and the protein expressions of p-Akt and p-Bad were elevated, while the mRNA and protein expressions of Bax were decreased in the DADLE and DMSO groups. Decreased mRNA and protein expressions of PI3K and Bcl-2, reduced protein expressions of p-Akt and p-Bad and elevated mRNA and protein expressions of Bax exhibited in the LY294002 group than the DADLE group. These results indicate that activation of PI3K-Akt pathway promotes the neuroprotection of DADLE against cerebral I/R injury in a rat model by decreasing nerve cells apoptosis.

'Opioidergic postconditioning' of heart muscle during ischemia/reperfusion injury

BACKGROUND

Ischemic preconditioning and postconditioning are the novel strategies of attaining cardioprotection against ischemia/reperfusion (I/R) injury. Previous studies suggested the role of opioid pathway, however the class of opioid receptors responsible for this effect in humans remains unknown. The aim of the study was to assess the influence of opioids on simulated I/R injury outcomes in the hu-man myocardium.

METHODS

Trabeculae of the human right atrium were electrically driven in organ bath and subjected to simulated I/R injury. Morphine (10-4M, 10-5M, 10-6M) or d-opioid receptor agonist DADLE (10-8M, 10-7M, 10-6M) was used at the time of re-oxygenation. Additional trabecula was subjected to hypoxia protocol only (Control). Contractive force of the myocardium was assessed as the maximal force of a contraction (Amax), the rate of rise of the force of a contraction (Slope L) and relaxation as the rate of decay of the force of a contraction (Slope T).

RESULTS

Application of morphine 10-4M resulted in increase of Amax, Slope L and Slope T during re-oxygenation period as compared to Control (77.99 ± 1.5% vs. 68.8 ± 2.2%, p < 0.05; 45.72 ± 2.9% vs. 34.12 ± 5.1%, p < 0.05; 40.95 ± 2.5% vs. 32.37 ± 4.3%, p < 0.05). Parameters were not significantly different in the lower morphine concentrations. Application of DADLE 10-6M resulted in decrease of Amax and Slope L as compared to Control (68.13 ± 5.5% vs. 76.62 ± 6.6%, p < 0.05; 28.29 ± 2.2 vs. 34.80 ± 3.9%, p < 0.05).

CONCLUSIONS

At re-oxygenation, morphine improves systolic and diastolic function of the human myo-cardium in the dose-dependent manner. Delta-opioid receptor stimulation attenuates systolic function of human heart muscle which remains in contrast to previous reports with animal models of I/R injury. (Cardiol J 2017; 24, 4: 419-425).

Constitutive Desensitization of Opioid Receptors in Peripheral Sensory Neurons

Opioid receptors expressed by peripheral pain-sensing neurons are functionally inactive for antinociceptive signaling under most basal conditions; however, tissue damage or exposure to inflammatory mediators (e.g., bradykinin) converts these receptors from a nonresponsive state to a functionally competent state. Here we tested the hypothesis that the basal, nonresponsive state of the mu- and delta-opioid receptors (MOR and DOR, respectively) is the result of constitutive receptor activity that activates desensitization mechanisms, resulting in MOR and DOR receptor systems that are constitutively desensitized. Consistent with our previous findings, under basal conditions, neither the MOR agonist [d-Ala2,N-MePhe4,Gly-ol5]-enkephalin nor the DOR agonist [d-Pen2,5]-enkephalin, inhibited prostaglandin E2 (PGE2)-stimulated cAMP accumulation in peripheral sensory neurons in culture (ex vivo) or inhibited PGE2-stimulated thermal allodynia in the rat hind paw in vivo. Prolonged treatment with naloxone induced MOR and DOR responsiveness both in vivo and ex vivo to a similar magnitude as that produced by bradykinin. Also similar to bradykinin, the effect of naloxone persisted for 60 minutes after washout of the ligand. By contrast, prolonged treatment with 6β-naltrexol, did not induce functional competence of MOR or DOR but blocked the effect of naloxone. Treatment with siRNA for β-arrestin-2, but not β-arrestin-1, also induced MOR and DOR functional competence in cultured peripheral sensory neurons. These data suggest that the lack of responsiveness of MOR and DOR to agonist for antinociceptive signaling in peripheral sensory neurons is due to constitutive desensitization that is likely mediated by β-arrestin-2.

Enkephalin analogues with N-phenyl-N-(piperidin-2-ylmethyl)propionamide derivatives: Synthesis and biological evaluations

N-Phenyl-N-(piperidin-2-ylmethyl)propionamide based bivalent ligands are unexplored for the design of opioid based ligands. Two series of hybrid molecules bearing N-phenyl-N-(piperidin-2-ylmethyl)propionamide derived small molecules conjugated with an enkephalin analogues with and without a linker (β-alanine) were designed and synthesized. Both bivalent ligand series exhibited remarkable binding affinities from nanomolar to subnanomolar range at both μ and δ opioid receptors and displayed potent agonist activities as well. The replacement of Tyr with Dmt and introduction of a linker between the small molecule and enkephalin analogue resulted in highly potent ligands. Both series of ligands showed excellent binding affinities at both μ (0.6-0.9nM) and δ (0.2-1.2nM) opioid receptors respectively. Similarly, these bivalent ligands exhibited potent agonist activities in both MVD and GPI assays. Ligand 17 was evaluated for in vivo antinociceptive activity in non-injured rats following spinal administration. Ligand 17 was not significantly effective in alleviating acute pain. The most likely explanations for this low intrinsic efficacy in vivo despite high in vitro binding affinity, moderate in vitro activity are (i) low potency suggesting that higher doses are needed; (ii) differences in experimental design (i.e. non-neuronal, high receptor density for in vitro preparations versus CNS site of action in vitro); (iii) pharmacodynamics (i.e. engaging signalling pathways); (iv) pharmacokinetics (i.e. metabolic stability). In summary, our data suggest that further optimisation of this compound 17 is required to enhance intrinsic antinociceptive efficacy.

[Agonists of µ- and δ-Opioid Receptors in the Regulation of IL-2, IL-4 and IFN-γ Production by Peripheral Blood Cells in vitro]

It was found that β-endorphin stimulates the PHA (phytohemagglutinin)-induced production of interleukin-4 and has no affect on the production of interferon-gamma in unfractionated leukocytic suspension. In the culture of purified CD4+ T cells, β-endorphin does not affect the concentration of IL-2, IL-4, and IFN-γ, but stimulates the production of IL-4 and inhibits the production of IFN-γ when adding monocytes to the culture. Selective δ-agonist DADLE enhances the PHA-induced production of IL-4 in unfractionated leukocytic suspension and in CD4+ lymphocytes+monocytes system. The synthesis of IFN-γ by purified CD4+ lymphocytes is not afected by the presence of DADLE, DAGO ad Deltorphin II; but when adding monocytes to the culture, the synthesis rate decreases. β-endorphin and selective μ-agonist DAGO enhance the production of IFN-γ by stimulated neutrophils. The production of IFN-γ in CD8+ lymphocytes is not affected by β-endorphin. Thus, opioid peptides have a predominantly Th2 polarizing effect, which is monocyte-mediated, hindering the development of cell response by inhibiting IFN-γ, and stimulating the production of I L-4 by activating δ-receptor. On the other hand, neutrophils can enhance the production of IFN-γ by stimulating μ-receptor.

Activation of mu or delta opioid receptors in the lumbosacral spinal cord is essential for ejaculatory reflexes in male rats

Ejaculation is controlled by a spinal ejaculation generator located in the lumbosacral spinal cord, consisting in male rats of lumbar spinothalamic (LSt) cells and their inter-spinal projections to autonomic and motor centers. LSt cells co-express several neuropeptides, including gastrin releasing peptide (GRP) and enkephalin. We previously demonstrated in rats that GRP regulates ejaculation by acting within the lumbosacral spinal cord. In the present study, the hypothesis was tested that enkephalin controls ejaculation by acting on mu (MOR) or delta opioid receptors (DOR) in LSt target areas. Adult male rats were anesthetized and spinalized and received intrathecal infusions of vehicle, MOR antagonist CTOP (0.4 or 4 nmol), DOR antagonist (TIPP (0.4, 4 or 40 nmol), MOR agonist DAMGO (0.1 or 10 nmol), or DOR agonist deltorphin II (1.3 or 13 nmol). Ejaculatory reflexes were triggered by stimulation of the dorsal penile nerve (DPN) and seminal vesicle pressure and rhythmic contractions of the bulbocavernosus muscle were analyzed. Intrathecal infusion of MOR or DOR antagonists effectively blocked ejaculatory reflexes induced by DPN stimulation. Intrathecal infusion of DAMGO, but not deltorphin II triggered ejaculation in absence of DPN stimulation. Both MOR and DOR agonists facilitated ejaculatory reflexes induced by subthreshold DPN stimulation in all animals. Overall, these results support the hypothesis that enkephalin plays a critical role in the control of ejaculation in male rats. Activation of either MOR or DOR in LSt target areas is required for ejaculation, while MOR activation is sufficient to trigger ejaculation in the absence of sensory stimulation.

Regulation of nitric oxide production by δ-opioid receptors during glaucomatous injury

To determine the roles of nitric oxide in glaucomatous injury and its regulation by δ-opioid-receptor activation, animals were treated with: 1) a selective inducible nitric oxide synthase (iNOS) inhibitor (aminoguanidine; AG; 25 mg/kg, i.p.); 2) δ-opioid-receptor agonist (SNC-121; 1 mg/kg, i.p.); or 3) with both drugs simultaneously for 7 days, once daily. The loss in retinal ganglion cell (RGC) numbers and their function in glaucomatous eyes were significantly improved in the presence of AG or SNC-121; however, we did not see any significant additive or synergistic effects when animals were treated with both drugs simultaneously. The levels of nitrate-nitrite were significantly increased in the glaucomatous retina when compared with normal retina (normal retina 86±9 vs. glaucomatous retina 174±10 mM/mg protein), which was reduced significantly when animals were treated either with SNC-121 (121±7 mM/mg protein; P<0.05) or AG (128±10 mM/mg protein; P<0.05). Additionally, SNC-121-mediated reduction in nitrate-nitrite levels was not only blocked by naltrindole (a δ-opioid-receptor antagonist), but naltrindole treatment potentiated the nitrate-nitrite production in glaucomatous retina (235±4 mM/mg protein; P<0.001). As expected, naltrindole treatment also fully-blocked SNC-121-mediated retina neuroprotection. The nitrotyrosine level in the glaucomatous retina was also increased, which was significantly reduced in the SNC-121-treated animals. Additionally, the expression level of iNOS was clearly increased over the control levels in the glaucomatous retina and optic nerves, which was also reduced by SNC-121 treatment. In conclusion, our data support the notion that nitric oxide plays a detrimental role during glaucomatous injury and inhibition of nitric oxide production provided RGC neuroprotection. Furthermore, δ-opioid receptor activation regulates the production of nitric oxide via inhibiting the activity of iNOS in the retina and optic nerve.

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.

Delta-opioid receptor analgesia is independent of microglial activation in a rat model of neuropathic pain

The analgesic effect of delta-opioid receptor (DOR) ligands in neuropathic pain is not diminished in contrast to other opioid receptor ligands, which lose their effectiveness as analgesics. In this study, we examine whether this effect is related to nerve injury-induced microglial activation. We therefore investigated the influence of minocycline-induced inhibition of microglial activation on the analgesic effects of opioid receptor agonists: morphine, DAMGO, U50,488H, DPDPE, Deltorphin II and SNC80 after chronic constriction injury (CCI) to the sciatic nerve in rats. Pre-emptive and repeated administration of minocycline (30 mg/kg, i.p.) over 7 days significantly reduced allodynia and hyperalgesia as measured on day 7 after CCI. The antiallodynic and antihyperalgesic effects of intrathecally (i.t.) administered morphine (10–20 µg), DAMGO (1–2 µg) and U50,488H (25–50 µg) were significantly potentiated in rats after minocycline, but no such changes were observed after DPDPE (10–20 µg), deltorphin II (1.5–15 µg) and SNC80 (10–20 µg) administration. Additionally, nerve injury-induced down-regulation of all types of opioid receptors in the spinal cord and dorsal root ganglia was not influenced by minocycline, which indicates that the effects of opioid ligands are dependent on other changes, presumably neuroimmune interactions. Our study of rat primary microglial cell culture using qRT-PCR, Western blotting and immunocytochemistry confirmed the presence of mu-opioid receptors (MOR) and kappa-opioid receptors (KOR), further we provide the first evidence for the lack of DOR on microglial cells. In summary, DOR analgesia is different from analgesia induced by MOR and KOR receptors because it does not dependent on injury-induced microglial activation. DOR agonists appear to be the best candidates for new drugs to treat neuropathic pain.

Blockade of central delta-opioid receptors inhibits salt appetite in sodium-depleted rats

Various studies have investigated the role of central opioid peptides in feeding behavior; however, only a few have addressed the participation of opioids in the control of salt appetite. The present study investigated the effect of intracerebroventricular injections of the δ-opioid antagonist, naltrindole (5, 10 and 20 nmol/rat) and the agonist, deltorphin II (2.5, 5, 10 and 20 nmol/rat) on salt intake. Two protocols for inducing salt intake were used: sodium-depletion and the central injection of angiotensin II. In addition, the effect of a central δ-opioid receptor blockade on locomotor activity, on palatable solution intake (0.1% saccharin) and on blood pressure was also studied. The blockade of central δ-opioid receptors inhibits salt intake in sodium-depleted rats, while the pharmacological stimulation of these receptors increases salt intake in sodium-replete animals. Furthermore, the blockade of central δ-opioid receptors inhibits salt intake induced by central angiotensinergic stimulation. These data suggest that during sodium-depletion activation of the δ-opioid receptors regulates salt appetite to correct the sodium imbalance and it is possible that an interaction between opioidergic and angiotensinergic brain system participates in this control. Under normonatremic conditions, δ-opioid receptors may be necessary to modulate sodium intake, a response that could be mediated by angiotensin II. The decrease in salt intake following central δ-opioid receptors blockade does not appear to be due to a general inhibition of locomotor activity, changes in palatability or in blood pressure.

Pharmacological characterization of endomorphin-2-based cyclic pentapeptides with methylated phenylalanine residues

As part of our continuing studies on the structure-activity relationships of cyclic pentapeptides based on the structure of endomorphin-2, we report here the synthesis and biological activities of a new series of analogs incorporating 2', 3' or 4'-methylphenylalanine (MePhe) residues into positions 3 or 4 of the parent cyclopeptide, Dmt-c[d-Lys-Phe-Phe-Asp]NH2 (Dmt=2',6'-dimethyltyrosine). Analogs with MePhe in position 4 showed a row of magnitude increased μ-opioid receptor (MOP receptor) affinity as compared with a parent compound. The in vitro potencies of the new analogs were determined in calcium mobilization assay performed in Chinese Hamster Ovary (CHO) cells expressing human recombinant opioid receptors and chimeric G proteins. All analogs were strong μ/κ (MOP/KOP) receptor agonists and weak δ (DOP) receptor agonists. In the in vivo hot-plate test in mice, the MePhe(4)-modified peptides showed remarkable antinociceptive activity after intracerebroventricular (i.c.v.) administration which was most likely due to the concomitant activation of more than one opioid receptor type.