Discovery of 7-Azanorbornane-Based Dual Agonists for the Delta and Kappa Opioid Receptors through an In Situ Screening Protocol.

In medicinal chemistry, the copper-catalyzed click reaction is used to prepare ligand candidates. This reaction is so clean that the bioactivities of the products can be determined without purification. Despite the advantages of this in situ screening protocol, the applicability of this method for transmembrane proteins has not been validated due to the incompatibility with copper catalysts. To address this point, we performed ligand screening for the µ, δ, and κ opioid receptors using this protocol. As we had previously reported the 7-azanorbornane skeleton as a privileged scaffold for the G protein-coupled receptors, we performed the click reactions between various 7-substituted 2-ethynyl-7-azanorbornanes and azides. Screening assays were performed without purification using the CellKeyTM system, and the putative hit compounds were re-synthesized and re-evaluated. Although the "hit" compounds for the µ and the δ receptors were totally inactive after purifications, three of the four "hits" for the κ receptor were true agonists for this receptor and also showed activities for the δ receptor. Although false positive/negative results exist as in other screening projects for soluble proteins, this in situ method is effective in identifying novel ligands for transmembrane proteins.

Distinct Profiles of Desensitization of µ-Opioid Receptors Caused by Remifentanil or Fentanyl: In Vitro Assay with Cells and Three-Dimensional Structural Analyses.

Remifentanil (REM) and fentanyl (FEN) are commonly used analgesics that act by activating a µ-opioid receptor (MOR). Although optimal concentrations of REM can be easily maintained during surgery, it is sometimes switched to FEN for optimal pain regulation. However, standards for this switching protocol remain unclear. Opioid anesthetic efficacy is decided in part by MOR desensitization; thus, in this study, we investigated the desensitization profiles of REM and FEN to MOR. The efficacy and potency during the 1st administration of REM or FEN in activating the MOR were almost equal. Similarly, in ß arrestin recruitment, which determines desensitization processes, they showed no significant differences. In contrast, the 2nd administration of FEN resulted in a stronger MOR desensitization potency than that of REM, whereas REM showed a higher internalization potency than FEN. These results suggest that different ß arrestin-mediated signaling caused by FEN or REM led to their distinct desensitization and internalization processes. Our three-dimensional analysis, with in silico binding of REM and FEN to MOR models, highlighted that REM and FEN bound to similar but distinct sites of MOR and led to distinct ß arrestin-mediated profiles, suggesting that distinct binding profiles to MOR may alter ß arrestin activity, which accounts for MOR desensitization and internalization.

Novel Opioid Analgesics for the Development of Transdermal Opioid Patches That Possess Morphine-Like Pharmacological Profiles Rather Than Fentanyl: Possible Opioid Switching Alternatives Among Patch Formula.

BACKGROUND: Transdermal fentanyl is widely used in the treatment of severe pain because of convenience, safety, and stable blood concentrations. Nevertheless, patients often develop tolerance to fentanyl, necessitating the use of other opioids; transdermal buprenorphine patch is widely used as an analgesic agent, though available formulation does not provide comparable analgesic effect as transdermal fentanyl patch. Opioids bind to the opioid receptor (OR) to activate both G protein-mediated and ß-arrestin-mediated pathways. We synthesized morphine-related compounds with high transdermal absorbability (N1 and N2) and evaluated their OR activities pharmacologically in comparison with fentanyl and morphine. METHODS: In cells stably expressing µ-opioid receptor (MOR), δ-opioid receptor (DOR), and κ-opioid receptor (KOR), G protein-mediated pathways were assessed using the CellKey and an intracellular cyclic adenosine monophosphate (cAMP) assay, while ß-arrestin-mediated pathways were analyzed with ß-arrestin recruitment and receptor internalization assays. Furthermore, analgesic effects were evaluated using a tail-flick test in mice, and the analgesic effect on fentanyl-tolerant mice was evaluated. RESULTS: In the CellKey and cAMP assays, both N1 and N2 showed the highest affinity for MOR and acted as full agonists as well as partial agonists for DOR and KOR. In the ß-arrestin and internalization assays, only fentanyl acted as a full agonist; N1 and N2 acted as partial agonists of MOR. In the mouse tail-flick test, N1 and N2 showed analgesic effects equivalent to those of fentanyl and morphine. In fentanyl-tolerant mice, fentanyl showed a diminished analgesic effect, whereas N1 and N2 as well as morphine retained their analgesic effects. CONCLUSIONS: While N1 and N2 have higher transdermal absorbability than fentanyl, they also have analgesic effects comparable to those of morphine, suggesting that they may be attractive compounds for the development of novel opioid patches for transitioning from fentanyl patches.

The Steroidal Alkaloid Tomatidine and Tomatidine-Rich Tomato Leaf Extract Suppress the Human Gastric Cancer-Derived 85As2 Cells In Vitro and In Vivo via Modulation of Interferon-Stimulated Genes.

The steroidal alkaloid tomatidine is an aglycone of α-tomatine, which is abundant in tomato leaves and has several biological activities. Tomatidine has been reported to inhibit the growth of cultured cancer cells in vitro, but its anti-cancer activity in vivo and inhibitory effect against gastric cancer cells remain unknown. We investigated the efficacy of tomatidine using human gastric cancer-derived 85As2 cells and its tumor-bearing mouse model and evaluated the effect of tomatidine-rich tomato leaf extract (TRTLE) obtained from tomato leaves. In the tumor-bearing mouse model, tumor growth was significantly inhibited by feeding a diet containing tomatidine and TRTLE for 3 weeks. Tomatidine and TRTLE also inhibited the proliferation of cultured 85As2 cells. Microarray data of gene expression analysis in mouse tumors revealed that the expression levels of mRNAs belonging to the type I interferon signaling pathway were altered in the mice fed the diet containing tomatidine and TRTLE. Moreover, the knockdown of one of the type I interferon-stimulated genes (ISGs), interferon α-inducible protein 27 (IFI27), inhibited the proliferation of cultured 85As2 cells. This study demonstrates that tomatidine and TRTLE inhibit the tumor growth in vivo and the proliferation of human gastric cancer-derived 85As2 cells in vitro, which could be due to the downregulation of ISG expression.

Ketamine Improves Desensitization of µ-Opioid Receptors Induced by Repeated Treatment with Fentanyl but Not with Morphine.

The issue of tolerance to continuous or repeated administration of opioids should be addressed. The ability of ketamine to improve opioid tolerance has been reported in clinical studies, and its mechanism of tolerance may involve improved desensitization of µ-opioid receptors (MORs). We measured changes in MOR activity and intracellular signaling induced by repeated fentanyl and morphine administration and investigated the effects of ketamine on these changes with human embryonic kidney 293 cells expressing MOR using the CellKey™, cADDis cyclic adenosine monophosphate, and PathHunter® ß-arrestin recruitment assays. Repeated administration of fentanyl or morphine suppressed the second MOR responses. Administration of ketamine before a second application of opioids within clinical concentrations improved acute desensitization and enhanced ß-arrestin recruitment elicited by fentanyl but not by morphine. The effects of ketamine on fentanyl were suppressed by co-treatment with an inhibitor of G-protein-coupled receptor kinase (GRK). Ketamine may potentially reduce fentanyl tolerance but not that of morphine through modulation of GRK-mediated pathways, possibly changing the conformational changes of ß-arrestin to MOR.

Intravenous administration of human mesenchymal stem cells derived from adipose tissue and umbilical cord improves neuropathic pain via suppression of neuronal damage and anti-inflammatory actions in rats.

Mesenchymal stem cells (MSCs), which are isolated from adipose tissue (AD-MSCs), umbilical cord (UC-MSCs), or bone marrow, have therapeutic potential including anti-inflammatory and immunomodulatory activities. It was recently reported that MSCs are also effective as a therapeutic treatment for neuropathic pain, although the underlying mechanisms have yet to be resolved. Therefore, in this study, we investigated the effects of human AD- and UC-MSCs on neuropathic pain and its mechanisms using rat models of partial sciatic nerve ligation (PSNL). AD- or UC-MSCs were intravenously administered 4 days after PSNL. Antinociceptive effects were then evaluated using the von Frey and weight-bearing tests. We found that, 3-9 days after the administration of AD- or UC-MSCs to PSNL-exposed rats, both the mechanical threshold and differences in weight-bearing of the right and left hind paws were significantly improved. To reveal the potential underlying antinociceptive mechanisms of MSCs, the levels of activation transcription factor 3- and ionized calcium-binding adapter molecule 1-positive cells were measured by immunohistochemical analysis. AD- and UC-MSCs significantly decreased the levels of these proteins that were induced by PSNL in the dorsal root ganglia. Additionally, UC-MSC significantly improved the PSNL-induced decrease in the myelin basic protein level in the sciatic nerve, indicating that UC-MSC reversed demyelination of the sciatic nerve produced by PSNL. These data suggest that AD- and UC-MSCs may help in the recovery of neuropathic pain via the different regulation; AD-MSCs exhibited their effects via suppressed neuronal damage and anti-inflammatory actions, while UC-MSCs exhibited their effects via suppressed neuronal damage, anti-inflammatory actions and remyelination.

Oxytocin Is a Positive Allosteric Modulator of κ-Opioid Receptors but Not δ-Opioid Receptors in the G Protein Signaling Pathway.

Oxytocin (OT) influences various physiological functions such as uterine contractions, maternal/social behavior, and analgesia. Opioid signaling pathways are involved in one of the analgesic mechanisms of OT. We previously showed that OT acts as a positive allosteric modulator (PAM) and enhances µ-opioid receptor (MOR) activity. In this study, which focused on other opioid receptor (OR) subtypes, we investigated whether OT influences opioid signaling pathways as a PAM for δ-OR (DOR) or κ-OR (KOR) using human embryonic kidney-293 cells expressing human DOR or KOR, respectively. The CellKeyTM results showed that OT enhanced impedance induced by endogenous/exogenous KOR agonists on KOR-expressing cells. OT did not affect DOR activity induced by endogenous/exogenous DOR agonists. OT potentiated the KOR agonist-induced Gi/o protein-mediated decrease in intracellular cAMP, but did not affect the increase in KOR internalization caused by the KOR agonists dynorphin A and (-)-U-50488 hydrochloride (U50488). OT did not bind to KOR orthosteric binding sites and did not affect the binding affinities of dynorphin A and U50488 for KOR. These results suggest that OT is a PAM of KOR and MOR and enhances G protein signaling without affecting ß-arrestin signaling. Thus, OT has potential as a specific signaling-biased PAM of KOR.

Cancer treatment-related cardiovascular disease: Current status and future research priorities.

With the development of new drugs, such as molecular-targeted drugs, and multidisciplinary therapies, cancer treatment outcomes have improved, and the number of cancer survivors is increasing every year. However, some chemotherapeutic agents cause cardiovascular complications (cancer treatment-related cardiovascular disease, CTRCVD), which affect the life prognosis and quality of life (QOL) of cancer patients. Therefore, it is necessary to select treatment methods that take into account the prognosis and QOL of cancer patients, and to take measures against CTRCVD. The mechanism of cardiotoxicity of high-risk drugs, such as doxorubicin and HER2 inhibitors, are still unclear; genetic factors, and cardiovascular disease risk factors (e.g., hypertension, dyslipidemia, and diabetes) are associated with CTRCVD progression. The establishment of methods for prevention, early diagnosis, and treatment of CTRCVD and the generation of evidence for these methods are needed. It is also necessary to develop screening methods for chemotherapy cardiotoxicity. In this review, we discuss the current status of CTRCVD, its complications, and expected countermeasures.

The Japanese Herbal Medicine Hangeshashinto Induces Oral Keratinocyte Migration by Mediating the Expression of CXCL12 Through the Activation of Extracellular Signal-Regulated Kinase.

Several clinical studies have reported that Japanese herbal medicine Hangeshashinto (HST) has beneficial effects on chemotherapy-induced oral ulcerative mucositis (OUM). Our previous research demonstrated that HST improves chemotherapy-induced OUM through human oral keratinocyte (HOK) migration, which was suppressed by mitogen-activated protein kinase (MAPK) and C-X-C chemokine receptor 4 (CXCR4) inhibitors. However, the association between these molecules and HOK migration was unclear. Here, we examined the effects of HST on the expression of CXCR4/CXCR7 and C-X-C motif chemokine ligands 11 and 12 (CXCL11/CXCL12) in HOKs. Our results indicated that HST upregulated CXCL12, but not CXCR4, CXCR7, nor CXCL11 in HOKs. HST-induced expression of CXCL12 was significantly suppressed by an inhibitor of extracellular signal-regulated kinase (ERK), but not of p38 and c-Jun N-terminal kinase (JNK). In addition, HST induced phosphorylation of ERK in HOKs. These findings suggest that HST enhances HOK migration by upregulating CXCL12 via ERK.

Author Correction: The Japanese herbal medicine Hangeshashinto enhances oral keratinocyte migration to facilitate healing of chemotherapy-induced oral ulcerative mucositis.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

[Cardio-oncology - elucidation of the mechanism of cardiac dysfunction caused by cancer therapy and cancer cachexia].

Cardiovascular disorders in cancer patients with cachexia have recently become a great concern. However, the relationship between cancer cachexia and cardiac dysfunction remains unclear, due to lack of suitable models. We established a novel murine model of cancer cachexia by implantation of 85As2 cells, a cell line derived from human gastric cancer cells, presenting anorexia, weight loss and low fat-free mass similar to those observed in patients. Moreover, cardiac dysfunction is expected in this model, which has not been yet examined. In the present study, we firstly evaluated cardiac functions with the model. Secondly, we investigated effects of voluntary wheel running (VWR) on cachexia-induced cardiac dysfunction using this model, as the exercise is considered to be one of therapies for chronic heart failure. 85As2 cells were transplanted subcutaneously into mice, which observed a symptomatic cachexia; decrease in body, skeletal muscle weight, and food intake. In addition, this cachexia mouse developed severe cardiac atrophy and left ventricular ejection fraction (LVEF) also markedly reduced with cachexia progression. Moreover, VWR suppressed the decrease in food intake and skeletal muscle weight loss in this model, and improved LVEF with suppression of heart weight loss. These results imply that our 85As2-cachexia mice models show cardiac dysfunction and VWR may improve not only cachexia symptoms but also cardiac dysfunction. As exercise therapy is generally introduced for the purpose of improving heart failure symptoms, this study suggests a possible therapeutic effect of exercise on cardiac dysfunction induced by cancer cachexia.

Japanese Herbal Medicine Ninjinyoeito Mediates Its Orexigenic Properties Partially by Activating Orexin 1 Receptors.

Cancer cachexia is highly prevalent in patients with progressive cancer and is characterized by decreased food consumption, and body weight. Japanese herbal medicine Ninjinyoeito (NYT), composed of 12 herbal crude drugs, is prescribed in Asian countries to improve several symptoms such as anorexia and fatigue, which are commonly observed in patients with cancer cachexia. However, the action mechanisms of NYT in improving anorexia or fatigue in patients with cancer are not clear. Therefore, in the present study, we examined the effects of NYT on the activities of several G-protein-coupled receptors (GPCRs), which activate hyperphagia signaling in the central nervous system, using an in vitro assay with the CellKey™ system, which detects the activation of GPCRs as a change in intracellular impedance (ΔZ). NYT increased the ΔZ of human embryonic kidney 293 (HEK293) cells expressing orexin 1 receptor (OX1R) and those expressing neuropeptide Y1 receptor (NPY1R) in a dose-dependent manner. On the contrary, NYT did not significantly increase the ΔZ of HEK293A cells expressing growth hormone secretagogue receptor (GHSR) and those expressing NPY5R. The selective OX1R antagonist SB674042 significantly decreased the NYT-induced increase in ΔZ in OX1R-expressing cells. Contrarily, the selective NPY1R antagonist BIBO3340 failed to inhibit the NPY-induced increase in ΔZ in NPY1R-expressing cells. Additionally, we prepared modified NYT excluding each one of the 12 herbal crude drugs in NYT and investigated the effects on the activity of OX1R. Among the 12 modified NYT formulations, the one without citrus unshiu peel failed to activate OX1R. A screening of each of the 12 herbal crude drugs showed that citrus unshiu peel significantly activated OX1R, which was significantly suppressed by SB674042. These finding suggest that NYT and citrus unshiu peel could increase food intake via activation of orexigenic OX1R-expressing neurons in the hypothalamus. This study provides scientific evidence to support the potential of NYT for cancer patients with anorexia.

The Japanese herbal medicine Hangeshashinto enhances oral keratinocyte migration to facilitate healing of chemotherapy-induced oral ulcerative mucositis.

Chemotherapy often induces oral ulcerative mucositis (OUM) in patients with cancer, characterized by severe painful inflammation. Mouth-washing with the Japanese herbal medicine hangeshashinto (HST) ameliorates chemotherapy-induced OUM in patients with colorectal cancer. Previously, we demonstrated that HST decreased interleukin 1ß-induced prostaglandin E2 production in human oral keratinocytes (HOKs) and OUM-induced mechanical or spontaneous pain in rats. However, HST effects on tissue repair functions in HOKs remain unclear. Here, we examined the effects of HST on scratch-induced wound healing in vitro and in vivo. In vitro, HST enhanced wound healing mainly through scratch-induced HOK migration. Screening of the seven constituent medicinal herbs and their major components revealed that Scutellaria root, processed ginger, and Glycyrrhiza components mainly induced the scratch-induced HOK migration. Pharmacokinetic analyses indicated that the active ingredient concentrations in rat plasma following oral HST administration were below the effective doses for HOK migration, suggesting direct effects of HST in OUM. Mitogen-activated protein kinase and C-X-C chemokine receptor 4 inhibitors significantly suppressed HST-induced HOK migration. Moreover, HST enhanced tissue repair in our OUM rat model. Thus, HST likely enhanced OUM tissue repair through oral keratinocyte migration upon MAPK and CXCR4 activation and may be useful in patients with cancer-associated OUM.

Possible biased analgesic of hydromorphone through the G protein-over ß-arrestin-mediated pathway: cAMP, CellKey™, and receptor internalization analyses.

Morphine, fentanyl, and oxycodone are widely used as analgesics, and recently hydromorphone has been approved in Japan. Although all of these are selective for µ-opioid receptors (MORs) and have similar structures, their analgesic potencies and adverse effects (AEs) are diverse. Recent molecular analyses of MOR signaling revealed that the G protein-mediated signaling pathway causes analgesic effects and the ß-arrestin-mediated signaling pathway is responsible for AEs. We used several cell-based analyses that selectively measure cellular responses activated by either G protein- or ß-arrestin-mediated pathways. GloSensor™ cAMP, CellKey™, and receptor internalization assays were performed with four different types of cells stably expressing differentially labelled MOR. EC50 values measured by cAMP and CellKey™ assays had potencies in the order fentanyl ≤ hydromorphone < morphine ≤ oxycodone, all also exhibiting full agonist responses. However, in the internalization assay, only fentanyl elicited a full agonist response. Hydromorphone had the strongest potency next to fentanyl; however, contribution of the ß-arrestin-mediated pathway was small, suggesting that its effect could be biased toward the G protein-mediated pathway. Based on these properties, hydromorphone could be chosen as an effective analgesic.

Carboplatin Enhances the Activity of Human Transient Receptor Potential Ankyrin 1 through the Cyclic AMP-Protein Kinase A-A-Kinase Anchoring Protein (AKAP) Pathways.

Carboplatin, an anticancer drug, often causes chemotherapy-induced peripheral neuropathy (PN). Transient receptor potential ankyrin 1 (TRPA1), a non-selective cation channel, is a polymodal nociceptor expressed in sensory neurons. TRPA1 is not only involved in pain transmission, but also in allodynia or hyperalgesia development. However, the effects of TRPA1 on carboplatin-induced PN is unclear. We revealed that carboplatin induced mechanical allodynia and cold hyperalgesia, and the pains observed in carboplatin-induced PN models were significantly suppressed by the TRPA1 antagonist HC-030031 without a change in the level of TRPA1 protein. In cells expressing human TRPA, carboplatin had no effects on changes in intracellular Ca2+ concentration ([Ca2+]i); however, carboplatin pretreatment enhanced the increase in [Ca2+]i induced by the TRPA1 agonist, allyl isothiocyanate (AITC). These effects were suppressed by an inhibitor of protein kinase A (PKA). The PKA activator forskolin enhanced AITC-induced increase in [Ca2+]i and carboplatin itself increased intracellular cyclic adenosine monophosphate (cAMP) levels. Moreover, inhibition of A-kinase anchoring protein (AKAP) significantly decreased the carboplatin-induced enhancement of [Ca2+]i induced by AITC and improved carboplatin-induced mechanical allodynia and cold hyperalgesia. These results suggested that carboplatin induced mechanical allodynia and cold hyperalgesia by increasing sensitivity to TRPA1 via the cAMP-PKA-AKAP pathway.

A novel strategy for treatment of cancer cachexia targeting xanthine oxidase in the brain.

Cancer cachexia is a systemic wasting syndrome characterized by anorexia and loss of body weight. The xanthine oxidase (XO) inhibitor febuxostat is one of the promising candidates for cancer cachexia treatment. However, cachexic symptoms were not alleviated by oral administration of febuxostat in our cancer cachexia model. Metabolomic analysis with brains of our cachexic model showed that purine metabolism was activated and XO activity was increased, and thus suggested that febuxostat would not reach the brain. Accordingly, targeting XO in the brain, which controls appetite, may be an effective strategy for treatment of cancer cachexia.

Leukemia inhibitory factor via the Toll-like receptor 5 signaling pathway involves aggravation of cachexia induced by human gastric cancer-derived 85As2 cells in rats.

Cancer cachexia is highly prevalent in gastric cancer patients and characterized by decreased food consumption and body weight. We previously created a rat model of cancer cachexia using MKN45cl85 and 85As2 cells derived from human gastric cancer. The 85As2 cells induced cachexia more potently compared to MKN45cl85 cells. To clarify the mechanism underlying the difference in the cachexia-inducing ability of these cells, we conducted DNA microarray analysis, focusing on cell proliferation and the production of leukemia inhibitory factor (LIF), a cachexia-inducing factor. The plasma human LIF levels of 85As2-induced cachexic rats increased as symptoms worsened, whereas the plasma levels of MKNcl85 were low. 85As2 cells displayed more genetic changes compared to MKN45cl85 cells, which were related to Toll-like receptor (TLR) 4/5 signaling. Stimulation of both cells with TLR4 (lipopolysaccharide) or TLR5 (flagellin) agonists did not affect proliferation. However, in 82As2 cells, LIF production was significantly increased by stimulation with TLR5, which was suppressed by an inhibitor of interleukin-1 receptor-associated kinase-1/4, which are important factors in the TLR5 signaling pathway. The increase in LIF production resulting from activation of the TLR5 signaling pathway may contribute to the cachexia-inducing ability of 85As2 cells.

Modulation of synaptic inputs in magnocellular neurones in a rat model of cancer cachexia.

In cancer cachexia, abnormal metabolism and neuroendocrine dysfunction cause anorexia, tissue damage and atrophy, which can in turn alter body fluid balance. Arginine vasopressin, which regulates fluid homeostasis, is secreted by magnocellular neurosecretory cells (MNCs) of the hypothalamic supraoptic nucleus. Arginine vasopressin secretion by MNCs is regulated by both excitatory and inhibitory synaptic activity, alterations in plasma osmolarity and various peptides, including angiotensin II. In the present study, we used whole-cell patch-clamp recordings of brain slices to determine whether hyperosmotic stimulation and/or angiotensin II potentiate excitatory synaptic input in a rat model of cancer cachexia, similar to their effects in normal (control) rats. Hyperosmotic (15 and 60 mmol L-1   mannitol) stimulation and angiotensin II (0.1 µmol L-1 ) increased the frequency, but not the amplitude, of miniature excitatory postsynaptic currents in normal rats; in model rats, both effects were significantly attenuated. These results suggest that cancer cachexia alters supraoptic MNC sensitivity to osmotic and angiotensin II stimulation.

Neuropeptide oxytocin enhances µ opioid receptor signaling as a positive allosteric modulator.

Oxytocin (OT) is a 9-amine neuropeptide that plays an essential role in mammalian labor, lactation, maternal bonding, and social affiliation. OT has been reported to exert an analgesic effect in both humans and animals, and the results of certain animal experiments have shown that the analgesic effect of OT is partially blocked by opioid receptor antagonists. To investigate the relationship between OT and µ opioid receptor (MOR), we evaluated how OT affects MOR in vitro by performing an electrical impedance-based receptor biosensor assay (CellKey™ assay), an intracellular cAMP assay, and a competitive receptor-binding analysis by using cells stably expressing human MOR and OT receptor. In both the CellKey™ assay and the intracellular cAMP assay, OT alone exerted no direct agonistic effect on human MOR, but treatment with 10-6 M OT markedly enhanced the MOR signaling induced by 10-6 M endomorphin-1, ß-endorphin, morphine, fentanyl, and DAMGO. Moreover, in the competitive receptor-binding assay, 10-6 M OT did not alter the affinity of endomorphin-1 or morphine for MOR. These results suggest that OT could function as a positive allosteric modulator that regulates the efficacy of MOR signaling, and thus OT might represent a previously unrecognized candidate analgesic agent.

Usefulness for the combination of G-protein- and ß-arrestin-biased ligands of µ-opioid receptors: Prevention of antinociceptive tolerance.

Background: µ-Opioid receptor internalization is considered to be critically linked to antinociceptive tolerance. Although µ-opioid receptor agonists have been administered simultaneously with other drugs to control pain, little information is available regarding opioid­opioid interactions. Therefore, the present study was designed to further investigate the utility of a new G protein-biased ligand for µ-opioid receptors, TRV130, which has an antinociceptive effect without ß-arrestin-dependent µ-opioid receptor internalization, and its combination with fentanyl using µ-opioid receptor-expressing cells and mice. Results: In the present study, we confirmed that fentanyl produced a profound increase in ß-arrestin-2 recruitment accompanied by µ-opioid receptor internalization, whereas TRV130 did not induce either the recruitment of ß-arrestin-2 or µ-opioid receptor internalization in µ-opioid receptor-expressing cells. Under these conditions, ß-arrestin-2 recruitment accompanied by µ-opioid receptor internalization induced by fentanyl was abolished by TRV130, whereas TRV130 did not alter the reduction of cyclic adenosine monophosphate formation by fentanyl in µ-opioid receptor-expressing cells. In a behavioral assay, TRV130 exerted an antinociceptive effect in a hot-plate test in mice. In a combination test, the antinociceptive effect of TRV130 was synergistically increased by fentanyl. Fentanyl induced antihyperalgesia and development of its tolerance under a neuropathic pain-like state following sciatic nerve ligation. However, treatment of mice with an antinociceptive dose of TRV130 did not induce the rapid development of tolerance to its antihyperalgesic effect under a neuropathic pain-like state. Furthermore, the rapid development of tolerance to the antihyperalgesic effect induced by fentanyl plus TRV130 in mice with sciatic nerve ligation was not observed, unlike in the case of fentanyl alone. Conclusions: These findings provide evidence that activation of the G protein-biased pathway through µ-opioid receptors can alter signaling in the ß-arrestin-2 pathway linked to the stimulation of µ-opioid receptors. Furthermore, the combination of G protein-biased and ß-arrestin-biased ligands of µ-opioid receptors exerts an ideal antinociceptive effect without the rapid development of antinociceptive tolerance.