Extracellular signal-regulated kinase 1/2 activation counteracts morphine tolerance in the periaqueductal gray of the rat

Key differences in regulation of opioid receptors localized to presynaptic terminals compared to somas: Relevance for novel therapeutics

Opioid receptors are G protein-coupled receptors (GPCRs) that regulate activity within peripheral, subcortical and cortical circuits involved in pain, reward, and aversion processing. Opioid receptors are expressed in both presynaptic terminals where they inhibit neurotransmitter release and postsynaptic locations where they act to hyperpolarize neurons and reduce activity. Agonist activation of postsynaptic receptors at the plasma membrane signal via ion channels or cytoplasmic second messengers. Agonist binding initiates regulatory processes that include phosphorylation by G protein receptor kinases (GRKs) and recruitment of beta-arrestins that desensitize and internalize the receptors. Opioid receptors also couple to effectors from endosomes activating intracellular enzymes and kinases. In contrast to postsynaptic opioid receptors, receptors localized to presynaptic terminals are resistant to desensitization such that there is no loss of signaling in the continuous presence of opioids over the same time scale. Thus, the balance of opioid signaling in circuits expressing pre- and postsynaptic opioid receptors is shifted toward inhibition of presynaptic neurotransmitter release during continuous opioid exposure. The functional implication of this shift is not often acknowledged in behavioral studies. This review covers what is currently understood about regulation of opioid/nociceptin receptors, with an emphasis on opioid receptor signaling in pain and reward circuits. Importantly, the review covers regulation of presynaptic receptors and the critical gaps in understanding this area, as well as the opportunities to further understand opioid signaling in brain circuits. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Mu-opioid receptor and receptor tyrosine kinase crosstalk: Implications in mechanisms of opioid tolerance, reduced analgesia to neuropathic pain, dependence, and reward

Despite the prevalence of opioid misuse, opioids remain the frontline treatment regimen for severe pain. However, opioid safety is hampered by side-effects such as analgesic tolerance, reduced analgesia to neuropathic pain, physical dependence, or reward. These side effects promote development of opioid use disorders and ultimately cause overdose deaths due to opioid-induced respiratory depression. The intertwined nature of signaling via μ-opioid receptors (MOR), the primary target of prescription opioids, with signaling pathways responsible for opioid side-effects presents important challenges. Therefore, a critical objective is to uncouple cellular and molecular mechanisms that selectively modulate analgesia from those that mediate side-effects. One such mechanism could be the transactivation of receptor tyrosine kinases (RTKs) via MOR. Notably, MOR-mediated side-effects can be uncoupled from analgesia signaling via targeting RTK family receptors, highlighting physiological relevance of MOR-RTKs crosstalk. This review focuses on the current state of knowledge surrounding the basic pharmacology of RTKs and bidirectional regulation of MOR signaling, as well as how MOR-RTK signaling may modulate undesirable effects of chronic opioid use, including opioid analgesic tolerance, reduced analgesia to neuropathic pain, physical dependence, and reward. Further research is needed to better understand RTK-MOR transactivation signaling pathways, and to determine if RTKs are a plausible therapeutic target for mitigating opioid side effects.

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Erythroxylum cuneatum prevented cellular adaptation in morphine-induced neuroblastoma cells

BACKGROUND

Chronic morphine stimulates prolonged stimulation of opioid receptors, especially µ-opioid subtype (MOR), which in turn signals cellular adaptation. However, the sudden termination of morphine after chronic intake causes withdrawal syndrome.

OBJECTIVES

Hence, this study was designed to find an alternative treatment for the morphine withdrawal using the alkaloid leaf extract of Erythroxylum cuneatum (E. cuneatum), done on morphine-exposed neuroblastoma cell lines.

METHODS

SK-N-SH, a commercialised neuroblastoma cell line, was used in two separate study designs; the antagonistic and pre-treatment of morphine. The antagonistic treatment was conducted through concurrent exposure of the cells to morphine and E. cuneatum or morphine and methadone for 24 h. The pre-treatment design was carried out by exposing the cells to morphine for 24 h, followed by 24 h exposures to E. cuneatum or methadone. The cytosolic fraction was collected and run for protein expression involved in cellular adaptation; mitogen-activated protein (MAP)/extracellular signal-regulated (ERK) kinase 1/2 (MEK 1/2), extracellular signal-regulated kinase 2 (ERK 2), cAMP-dependent protein kinase (PKA) and protein kinases C (PKC).

RESULTS

The antagonistic treatment showed the normal level of MEK 1/2, ERK 2, PKA and PKC by the combination treatment of morphine and E. cuneatum, comparable to the combination of morphine and methadone. Neuroblastoma cells exposed to morphine pre-treatment expressed a high level of MEK 1/2, ERK 2, PKA and PKC, while the treatments with E. cuneatum and methadone normalised the expression of the cellular adaptation proteins.

CONCLUSION

E. cuneatum exerted anti-addiction properties by lowering the levels of cellular adaptation proteins, and its effects are comparable to that of methadone (an established anti-addiction drug).

Morphine-induced kinase activation and localization in the periaqueductal gray of male and female mice

Morphine is a potent opioid analgesic with high propensity for the development of antinociceptive tolerance. Morphine antinociception and tolerance are partially regulated by the midbrain ventrolateral periaqueductal gray (vlPAG). However, the majority of research evaluating mu-opioid receptor signaling has focused on males. Here, we investigate kinase activation and localization patterns in the vlPAG following acute and chronic morphine treatment in both sexes. Male and female mice developed rapid antinociceptive tolerance to morphine (10 mg/kg i.p.) on the hot plate assay, but tolerance did not develop in males on the tail flick assay. Quantitative fluorescence immunohistochemistry was used to map and evaluate the activation of extracellular signal-regulated kinase 1/2 (ERK 1/2), protein kinase-C (PKC), and protein kinase-A (PKA). We observed significantly greater phosphorylated ERK 1/2 in the vlPAG of chronic morphine-treated animals which co-localized with the endosomal marker, Eea1. We note that pPKC is significantly elevated in the vlPAG of both sexes following chronic morphine treatment. We also observed that although PKA activity is elevated following chronic morphine treatment in both sexes, there is a significant reduction in the nuclear translocation of its phosphorylated substrate. Taken together, this study demonstrates increased activation of ERK 1/2, PKC, and PKA in response to repeated morphine treatment. The study opens avenues to explore the impact of chronic morphine treatment on G-protein signaling and kinase nuclear transport.

Activation of the spinal EGFR signaling pathway in a rat model of cancer-induced bone pain with morphine tolerance

Cancer-induced bone pain (CIBP) is considered to be one of the most difficult pain conditions to treat. Morphine, an analgesic drug, is widely used in clinical practice, and long-term use of morphine can lead to drug tolerance. Recent reports have suggested that inhibitors of epidermal growth factor receptor (EGFR) may have analgesic effects in cancer patients suffering from pain. Therefore, we sought to determine whether EGFR signaling was involved in morphine tolerance (MT) in a rat model of cancer-induced bone pain. In this study, Walker 256 mammary gland carcinoma cells were inoculated into the tibias of rats to provoke cancer-induced bone pain. Then, morphine was intrathecally administered twice daily for seven consecutive days to induce drug tolerance. We observed sustained increased in the protein levels of EGFR, p-EGFR, ERK1/2, and p-ERK1/2 during the development of morphine tolerance in rats with cancer-induced bone pain by western blotting. The EGFR level was significantly increased in the MT and CIBP + MT groups, and EGFR was colocalized with markers of microglia and neurons in the spinal cords of rats. Inhibition of EGFR by a small molecule inhibitor markedly attenuated the degree of morphine tolerance and decreased the number of microglia, and the protein levels of EGFR, p-EGFR, ERK1/2, and p-ERK1/2 were also reduced. In summary, our results suggest that the activation of the EGFR signaling pathway in spinal microglia plays an important modulatory role in the development of morphine tolerance and that inhibition of EGFR may provide a new therapeutic option for cancer-induced bone pain.

Heat shock protein 90 inhibitors block the antinociceptive effects of opioids in mouse chemotherapy-induced neuropathy and cancer bone pain models

Heat shock protein 90 (Hsp90) is a ubiquitous signal transduction regulator, and Hsp90 inhibitors are in clinical development as cancer therapeutics. However, there have been very few studies on the impact of Hsp90 inhibitors on pain or analgesia, a serious concern for cancer patients. We previously found that Hsp90 inhibitors injected into the brain block opioid-induced antinociception in tail flick, paw incision, and HIV neuropathy pain. This study extended from that initial work to test the cancer-related clinical impact of Hsp90 inhibitors on opioid antinociception in cancer-induced bone pain in female BALB/c mice and chemotherapy-induced peripheral neuropathy in male and female CD-1 mice. Mice were treated with Hsp90 inhibitors (17-AAG, KU-32) by the intracerebroventricular, intrathecal, or intraperitoneal routes, and after 24 hours, pain behaviors were evaluated after analgesic drug treatment. Heat shock protein 90 inhibition in the brain or systemically completely blocked morphine and oxymorphone antinociception in chemotherapy-induced peripheral neuropathy; this effect was partly mediated by decreased ERK and JNK MAPK activation and by increased protein translation, was not altered by chronic treatment, and Hsp90 inhibition had no effect on gabapentin antinociception. We also found that the Hsp90 isoform Hsp90α and the cochaperone Cdc37 were responsible for the observed changes in opioid antinociception. By contrast, Hsp90 inhibition in the spinal cord or systemically partially reduced opioid antinociception in cancer-induced bone pain. These results demonstrate that Hsp90 inhibitors block opioid antinociception in cancer-related pain, suggesting that Hsp90 inhibitors for cancer therapy could decrease opioid treatment efficacy.

Daily intermittent fasting in mice enhances morphine-induced antinociception while mitigating reward, tolerance, and constipation

The opioid epidemic has plagued the United States with high levels of abuse and poor quality of life for chronic pain patients requiring continuous use of opioids. New drug discovery efforts have been implemented to mitigate this epidemic; however, new medications are still limited by low efficacy and/or high side effect and abuse potential. Intermittent fasting (IF) has recently been shown to improve a variety of pathological states, including stroke and neuroinflammation. Numerous animal and human studies have shown the benefits of IF in these disease states, but not in pain and opioid treatment. We thus subjected male and female CD-1 mice to 18-hour fasting intervals followed by 6-hour feed periods with standard chow for 1 week. Mice that underwent this diet displayed an enhanced antinociceptive response to morphine both in efficacy and duration using thermal tail-flick and postoperative paw incision pain models. While showing enhanced antinociception, IF mice also demonstrated no morphine reward and reduced tolerance and constipation. Seeking a mechanism for these improvements, we found that the mu-opioid receptor showed enhanced efficacy and reduced tolerance in the spinal cord and periaqueductal gray, respectively, from IF mice using a S-GTPγS coupling assay. These improvements in receptor function were not due to changes in mu-opioid receptor protein expression. These data suggest that a daily IF diet may improve the therapeutic index of acute and chronic opioid therapies for pain patients in the clinic, providing a novel tool to improve patient therapy and reduce potential abuse.

Differences in antinociceptive signalling mechanisms following morphine and fentanyl microinjections into the rat periaqueductal gray

BACKGROUND

Morphine and fentanyl are two of the most commonly used opioids to treat pain. Although both opioids produce antinociception by binding to mu-opioid receptors (MOR), they appear to act via distinct signalling pathways.

OBJECTIVE

This study will reveal whether differences in morphine and fentanyl antinociception are the result of selective activation of G-protein signalling and/or selective activation of pre- or postsynaptic MORs.

METHODS

The contribution of each mechanism to morphine and fentanyl antinociception was assessed by microinjecting drugs to alter G-protein signalling or block potassium channels linked to pre- and postsynaptic MORs in the ventrolateral periaqueductal gray (PAG) of male Sprague-Dawley rats.

RESULTS

Both morphine and fentanyl produced a dose-dependent antinociception when microinjected into the PAG. Enhancement of intracellular G-protein signalling by microinjection of the Regulator of G-protein Signalling 4 antagonist CCG-63802 into the PAG enhanced the antinociceptive potency of morphine, but not fentanyl. Microinjection of α-dendrotoxin into the PAG to block MOR activation of presynaptic K_v ⁺ channels caused a significant rightward shift in the dose-response curve of both morphine and fentanyl. Microinjection of tertiapin-Q to block MOR activation of postsynaptic GIRK channels caused a larger shift in the dose-response curve for fentanyl than morphine antinociception.

CONCLUSIONS

These findings reveal different PAG signalling mechanisms for morphine and fentanyl antinociception. In contrast with fentanyl, the antinociceptive effects of morphine are mediated by G-protein signalling primarily activated by presynaptic MORs.

SIGNIFICANCE

Microinjection of the opioids morphine and fentanyl into the periaqueductal gray (PAG) produce antinociception via mu-opioid receptor signalling. This study reveals differences in the signalling mechanisms underlying morphine and fentanyl antinociception in the PAG. In contrast with fentanyl, morphine antinociception is primarily mediated by presynaptic opioid receptors and is enhanced by blocking RGS proteins.

The Alpha Isoform of Heat Shock Protein 90 and the Co-chaperones p23 and Cdc37 Promote Opioid Anti-nociception in the Brain

Opioid activation of the mu opioid receptor (MOR) promotes signaling cascades that evoke both analgesic responses to pain and side effects like addiction and dependence. Manipulation of these cascades, such as by biased agonism, has great promise to improve opioid therapy. However, the signaling cascades of the MOR are in general poorly understood, providing few targets for drug development. In our earlier work, we identified Heat shock protein 90 (Hsp90) as a novel and crucial regulator of opioid anti-nociception in the brain by promoting ERK MAPK activation. In this study, we sought to identify the molecular isoforms and co-chaperones by which Hsp90 carried out this role, which could provide specific targets for future clinical intervention. We used novel selective small molecule inhibitors as well as CRISPR/Cas9 gene editing constructs delivered by the intracerebroventricular (icv) route to the brains of adult CD-1 mice to target Hsp90 isoforms (Hsp90α/β, Grp94) and co-chaperones (p23, Cdc37, Aha1). We found that inhibition of the isoform Hsp90α fully blocked morphine anti-nociception in a model of post-surgical paw incision pain, while blocking ERK and JNK MAPK activation, suggesting Hsp90α as the main regulator of opioid response in the brain. We further found that inhibition of the co-chaperones p23 and Cdc37 blocked morphine anti-nociception, suggesting that these co-chaperones assist Hsp90α in promoting opioid anti-nociception. Lastly, we used cycloheximide treatment in the brain to demonstrate that rapid protein translation within 30 min of opioid treatment is required for Hsp90 regulation of opioid response. Together these studies provide insight into the molecular mechanisms by which Hsp90 promotes opioid anti-nociception. These findings thus both improve our basic science knowledge of MOR signal transduction and could provide future targets for clinical intervention to improve opioid therapy.

The Mechanisms Involved in Morphine Addiction: An Overview

Opioid use disorder is classified as a chronic recurrent disease of the central nervous system (CNS) which leads to personality disorders, co-morbidities and premature death. It develops as a result of long-term administration of various abused substances, along with morphine. The pharmacological action of morphine is associated with its stimulation of opioid receptors. Opioid receptors are a group of G protein-coupled receptors and activation of these receptors by ligands induces significant molecular changes inside the cell, such as an inhibition of adenylate cyclase activity, activation of potassium channels and reductions of calcium conductance. Recent data indicate that other signalling pathways also may be involved in morphine activity. Among these are phospholipase C, mitogen-activated kinases (MAP kinases) or β-arrestin. The present review focuses on major mechanisms which currently are considered as essential in morphine activity and dependence and may be important for further studies.

Inflammatory mediators of opioid tolerance: Implications for dependency and addiction

Each year, over 50 million Americans suffer from persistent pain, including debilitating headaches, joint pain, and severe back pain. Although morphine is amongst the most effective analgesics available for the management of severe pain, prolonged morphine treatment results in decreased analgesic efficacy (i.e., tolerance). Despite significant headway in the field, the mechanisms underlying the development of morphine tolerance are not well understood. The midbrain ventrolateral periaqueductal gray (vlPAG) is a primary neural substrate for the analgesic effects of morphine, as well as for the development of morphine tolerance. A growing body of literature indicates that activated glia (i.e., microglia and astrocytes) facilitate pain transmission and oppose morphine analgesia, making these cells important potential targets in the treatment of chronic pain. Morphine affects glia by binding to the innate immune receptor toll-like receptor 4 (TLR4), leading to the release of proinflammatory cytokines and opposition of morphine analgesia. Despite the established role of the vlPAG as an integral locus for the development of morphine tolerance, most studies have examined the role of glia activation within the spinal cord. Additionally, the role of TLR4 in the development of tolerance has not been elucidated. This review attempts to summarize what is known regarding the role of vlPAG glia and TLR4 in the development of morphine tolerance. These data, together, provide information about the mechanism by which central nervous system glia regulate morphine tolerance, and identify a potential therapeutic target for the enhancement of analgesic efficacy in the clinical treatment of chronic pain.

Lack of Antinociceptive Cross-Tolerance With Co-Administration of Morphine and Fentanyl Into the Periaqueductal Gray of Male Sprague-Dawley Rats

Tolerance to the antinociceptive effect of mu-opioid receptor agonists, such as morphine and fentanyl, greatly limits their effectiveness for long-term use to treat pain. Clinical studies have shown that combination therapy and opioid rotation can be used to enhance opioid-induced antinociception once tolerance has developed. The mechanism and brain regions involved in these processes are unknown. The purpose of this study was to evaluate the contribution of the ventrolateral periaqueductal gray (vlPAG) to antinociceptive tolerance and cross-tolerance between administration and co-administration of morphine and fentanyl. Tolerance was induced by pretreating rats with morphine or fentanyl or low-dose combination of morphine and fentanyl into the vlPAG followed by an assessment of the cross-tolerance to the other opioid. In addition, tolerance to the combined treatment was assessed. Cross-tolerance did not develop between repeated vlPAG microinjections of morphine and fentanyl. Likewise, there was no evidence of cross-tolerance from morphine or fentanyl to the co-administration of morphine and fentanyl. Co-administration did not cause cross-tolerance to fentanyl. Cross-tolerance was only evident to morphine or morphine and fentanyl combined in rats pretreated with co-administration of low doses of morphine and fentanyl. This finding is consistent with the functionally selective signaling that has been reported for antinociception and tolerance after morphine and fentanyl binding to the mu-opioid receptor. This research supports the notion that combination therapy and opioid rotation may be useful clinical practices to decrease opioid tolerance and other side effects. PERSPECTIVE: This preclinical study shows that there is a decrease in cross-tolerance between morphine and fentanyl within the periaqueductal gray, which is a key brain region in opioid antinociception and tolerance.

Heroin-based crack induces hyperalgesia through β-arrestin 2 redistribution and phosphorylation of Erk1/2 and JNK in the periaqueductal gray area

Continuous use of crack induces hyperalgesia which is related to drug tolerance. Despite cumulative evidence based on the growth rate of crack abuse, no serious study has been focused on the mechanisms of crack-induced hyperalgesia. This study aimed to elucidate whether extracellular signal-regulated kinases (Erk1/2)/β-arrestin pathways are involved in the crack-induced hyperalgesia. Fifty adult male Wistar rats were randomly divided into five groups: normal saline (NS), crack (0.9 mg/kg/day), heroin (1 mg/kg/day), crack + barbadin (100 μM), and heroin + barbadin groups, which received their intraperitoneal (i.p) treatments for four weeks. The thermal sensitivity was assessed using the hot-plate test. Moreover, phosphorylation of the Erk1/2 and JNK, as well as expression of protein kinase C-alpha (PKC-α), Mu-receptor (MOR), and β-arrestin 2 were determined in the whole lysate and membrane fraction using immunoblotting assay in the periaqueductal gray (PAG) area. The results demonstrated that chronic administration of crack and heroin significantly decreased hind-paw withdrawal latency compared to the NS group. Furthermore, crack as well as heroin administration increased phosphorylated Erk1/2 and JNK in the PAG. In addition, membrane β-arrestin 2 and PKC-α were significantly increased in the crack and heroin-received groups, while membrane MOR expression was decreased in the PAG. Nevertheless, co-administration of barbadin, an inhibitor of β-arrestin, and crack or heroin reversed all these changes. Our findings may partially confirm the role of β-arrestin 2 and PKC rearrangements, Erk1/2 and JNK phosphorylation in crack-induced hyperalgesia and provide potential therapeutic targets to attenuate crack-induced hyperalgesia.

The Contribution of the Descending Pain Modulatory Pathway in Opioid Tolerance

Opioids remain among the most effective pain-relieving therapeutics. However, their long-term use is limited due to the development of tolerance and potential for addiction. For many years, researchers have explored the underlying mechanisms that lead to this decreased effectiveness of opioids after repeated use, and numerous theories have been proposed to explain these changes. The most widely studied theories involve alterations in receptor trafficking and intracellular signaling. Other possible mechanisms include the recruitment of new structural neuronal and microglia networks. While many of these theories have been developed using molecular and cellular techniques, more recent behavioral data also supports these findings. In this review, we focus on the mechanisms that underlie tolerance within the descending pain modulatory pathway, including alterations in intracellular signaling, neural-glial interactions, and neurotransmission following opioid exposure. Developing a better understanding of the relationship between these various mechanisms, within different parts of this pathway, is vital for the identification of more efficacious, novel therapeutics to treat chronic pain.

Cyclosporin A attenuating morphine tolerance through inhibiting NO/ERK signaling pathway in human glioblastoma cell line: the involvement of calcineurin

Cyclosporin A (CsA) is known to have an immunosuppressive action. However, it is also attracting attention due to its effects on the nervous system, such as inhibiting the development and expression of morphine-induced tolerance and dependence through unknown mechanisms. It has been shown that CsA modulates the nitric oxide (NO) synthesis and extracellular signal-regulated kinases (ERK) activation, which are potentially involved in signaling pathways in morphine-induced tolerance in cellular models. Therefore, the current study was designed to evaluate the modulatory role of CsA on the MOR tolerance, by targeting the downstream signaling pathway of NO and ERK using an in vitro model. For this purpose, T98G cells were pretreated with CsA, calcineurin autoinhibitory peptide (CAIP), and N^G-nitro-l-arginine methyl ester (L-NAME) 30 min before 18 h exposure to MOR. Then, we analyzed the intracellular cyclic adenosine monophosphate (cAMP) levels and also the expression of phosphorylated ERK and nitric oxide synthase (nNOS) proteins. Our results showed that CsA (1 nM, 10 nM, and 100 nM) and CAIP (50 µM) have significantly reduced cAMP and nitrite levels as compared to MOR-treated (2.5 µM) T98G cells. This clearly revealed the attenuation of MOR tolerance by CsA. The expression of nNOS and p-ERK proteins were down-regulated when the T98G cells were pretreated with CsA (1 nM, 10 nM, and 100 nM), CAIP (50 µM), and L-NAME (0.1 mM) as compared to MOR. In conclusion, the CsA pretreatment had a modulatory role in MOR-induced tolerance, which was possibly mediated through NO/ERK signaling pathway.

Suppression of RGSz1 function optimizes the actions of opioid analgesics by mechanisms that involve the Wnt/β-catenin pathway

Regulator of G protein signaling z1 (RGSz1), a member of the RGS family of proteins, is present in several networks expressing mu opioid receptors (MOPRs). By using genetic mouse models for global or brain region-targeted manipulations of RGSz1 expression, we demonstrated that the suppression of RGSz1 function increases the analgesic efficacy of MOPR agonists in male and female mice and delays the development of morphine tolerance while decreasing the sensitivity to rewarding and locomotor activating effects. Using biochemical assays and next-generation RNA sequencing, we identified a key role of RGSz1 in the periaqueductal gray (PAG) in morphine tolerance. Chronic morphine administration promotes RGSz1 activity in the PAG, which in turn modulates transcription mediated by the Wnt/β-catenin signaling pathway to promote analgesic tolerance to morphine. Conversely, the suppression of RGSz1 function stabilizes Axin2-Gαz complexes near the membrane and promotes β-catenin activation, thereby delaying the development of analgesic tolerance. These data show that the regulation of RGS complexes, particularly those involving RGSz1-Gαz, represents a promising target for optimizing the analgesic actions of opioids without increasing the risk of dependence or addiction.

Heat-shock protein 90 (Hsp90) promotes opioid-induced anti-nociception by an ERK mitogen-activated protein kinase (MAPK) mechanism in mouse brain

Recent advances in developing opioid treatments for pain with reduced side effects have focused on the signaling cascades of the μ-opioid receptor (MOR). However, few such signaling targets have been identified for exploitation. To address this need, we explored the role of heat-shock protein 90 (Hsp90) in opioid-induced MOR signaling and pain, which has only been studied in four previous articles. First, in four cell models of MOR signaling, we found that Hsp90 inhibition for 24 h with the inhibitor 17-N-allylamino-17-demethoxygeldanamycin (17-AAG) had different effects on protein expression and opioid signaling in each line, suggesting that cell models may not be reliable for predicting pharmacology with this protein. We thus developed an in vivo model using CD-1 mice with an intracerebroventricular injection of 17-AAG for 24 h. We found that Hsp90 inhibition strongly blocked morphine-induced anti-nociception in models of post-surgical and HIV neuropathic pain but only slightly blocked anti-nociception in a naive tail-flick model, while enhancing morphine-induced precipitated withdrawal. Seeking a mechanism for these changes, we found that Hsp90 inhibition blocks ERK MAPK activation in the periaqueductal gray and caudal brain stem. We tested these signaling changes by inhibiting ERK in the above-mentioned pain models and found that ERK inhibition could account for all of the changes in anti-nociception induced by Hsp90 inhibition. Taken together, these findings suggest that Hsp90 promotes opioid-induced anti-nociception by an ERK mechanism in mouse brain and that Hsp90 could be a future target for improving the therapeutic index of opioid drugs.

Sequence-Specific Regulation of Endocytic Lifetimes Modulates Arrestin-Mediated Signaling at the µ Opioid Receptor

Functional selectivity at the µ opioid receptor (µR), a prototypical G-protein-coupled receptor that is a physiologically relevant target for endogenous opioid neurotransmitters and analgesics, has been a major focus for drug discovery in the recent past. Functional selectivity is a cumulative effect of the magnitudes of individual signaling pathways, e.g., the Gα_i-mediated and the arrestin-mediated pathways for µR. The present work tested the hypothesis that lifetimes of agonist-induced receptor-arrestin clusters at the cell surface control the magnitude of arrestin signaling, and therefore functional selectivity, at µR. We show that endomorphin-2 (EM2), an arrestin-biased ligand for µR, lengthens surface lifetimes of receptor-arrestin clusters significantly compared with morphine. The lengthening of lifetimes required two specific leucines on the C-terminal tail of µR. Mutation of these leucines to alanines decreased the magnitude of arrestin-mediated signaling by EM2 without affecting G-protein signaling, suggesting that lengthened endocytic lifetimes were required for arrestin-biased signaling by EM2. Lengthening surface lifetimes by pharmacologically slowing endocytosis was sufficient to increase arrestin-mediated signaling by both EM2 and the clinically relevant agonist morphine. Our findings show that distinct ligands can leverage specific sequence elements on µR to regulate receptor endocytic lifetimes and the magnitude of arrestin-mediated signaling, and implicate these sequences as important determinants of functional selectivity in the opioid system.

Loss of dopamine D1 receptors and diminished D1/5 receptor-mediated ERK phosphorylation in the periaqueductal gray after spinal cord lesion

Neuropathic pain resulting from spinal cord injury is often accompanied by maladaptive plasticity of the central nervous system, including the opioid receptor-rich periaqueductal gray (PAG). Evidence suggests that sensory signaling via the PAG is robustly modulated by dopamine D1- and D2-like receptors, but the effect of damage to the spinal cord on D1 and D2 receptor protein expression and function in the PAG has not been examined. Here we show that 21days after a T10 or C6 spinothalamic tract lesion, both mice and rats display a remarkable decline in the expression of D1 receptors in the PAG, revealed by western blot analysis. These changes were associated with a significant reduction in hindpaw withdrawal thresholds in lesioned animals compared to sham-operated controls. We investigated the consequences of diminished D1 receptor levels by quantifying D1-like receptor-mediated phosphorylation of ERK1,2 and CREB, events that have been observed in numerous brain structures. In naïve animals, western blot analysis revealed that ERK1,2, but not CREB phosphorylation was significantly increased in the PAG by the D1-like agonist SKF 81297. Using immunohistochemistry, we found that SKF 81297 increased ERK1,2 phosphorylation in the PAG of sham animals. However, in lesioned animals, basal pERK1,2 levels were elevated and did not significantly increase after exposure to SKF 81297. Our findings provide support for the hypothesis that molecular adaptations resulting in a decrease in D1 receptor expression and signaling in the PAG are a consequence of SCL.

Sex Differences in GABAA Signaling in the Periaqueductal Gray Induced by Persistent Inflammation

The ventrolateral periaqueductal gray (vlPAG) is a key structure in the descending pain modulatory circuit. Activation of the circuit occurs via disinhibition of GABAergic inputs onto vlPAG output neurons. In these studies, we tested the hypothesis that GABAergic inhibition is increased during persistent inflammation, dampening activation of the descending circuit from the vlPAG. Our results indicate that persistent inflammation induced by Complete Freund's adjuvant (CFA) modulates GABA signaling differently in male and female rats. CFA treatment results in increased presynaptic GABA release but decreased high-affinity tonic GABAA currents in female vlPAG neurons. These effects are not observed in males. The tonic currents in the vlPAG are dependent on GABA transporter activity and are modulated by agonists that activate GABAA receptors containing the δ subunit. The GABAA δ agonist THIP (gaboxadol) induced similar amplitude currents in naive and CFA-treated rats. In addition, a positive allosteric modulator of the GABAA δ subunit, DS2 (4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridin-3-yl]benzamide), increased tonic currents. These results indicate that GABAA δ receptors remain on the cell surface but are less active in CFA-treated female rats. In vivo behavior studies showed that morphine induced greater antinociception in CFA-treated females that was reversed with microinjections of DS2 directly into the vlPAG. DS2 did not affect morphine antinociception in naive or CFA-treated male rats. Together, these data indicate that sex-specific adaptations in GABAA receptor signaling modulate opioid analgesia in persistent inflammation. Antagonists of GABAA δ receptors may be a viable strategy for reducing pain associated with persistent inflammation, particularly in females.

SIGNIFICANCE STATEMENT

These studies demonstrate that GABA signaling is modulated in the ventrolateral periaqueductal gray by persistent inflammation differently in female and male rats. Our results indicate that antagonists or negative allosteric modulators of GABAA δ receptors may be an effective strategy to alleviate chronic inflammatory pain and promote opioid antinociception, especially in females.

Ligand-biased activation of extracellular signal-regulated kinase 1/2 leads to differences in opioid induced antinociception and tolerance

Opioids produce antinociception by activation of G protein signaling linked to the mu-opioid receptor (MOPr). However, opioid binding to the MOPr also activates β-arrestin signaling. Opioids such as DAMGO and fentanyl differ in their relative efficacy for activation of these signaling cascades, but the behavioral consequences of this differential signaling are not known. The purpose of this study was to evaluate the behavioral significance of G protein and internalization dependent signaling within ventrolateral periaqueductal gray (vlPAG). Antinociception induced by microinjecting DAMGO into the vlPAG was attenuated by blocking Gαi/o protein signaling with administration of pertussis toxin (PTX), preventing internalization with administration of dynamin dominant-negative inhibitory peptide (dyn-DN) or direct inhibition of ERK1/2 with administration of the MEK inhibitor, U0126. In contrast, the antinociceptive effect of microinjecting fentanyl into the vlPAG was not altered by administration of PTX or U0126, and was enhanced by administration of dyn-DN. Microinjection of DAMGO, but not fentanyl, into the vlPAG induced phosphorylation of ERK1/2, which was blocked by inhibiting receptor internalization with administration of dyn-DN, but not by inhibition of Gαi/o proteins. ERK1/2 inhibition also prevented the development and expression of tolerance to repeated DAMGO microinjections, but had no effect on fentanyl tolerance. These data reveal that ERK1/2 activation following MOPr internalization contributes to the antinociceptive effect of some (e.g., DAMGO), but not all opioids (e.g., fentanyl) despite the known similarities for these agonists to induce β-arrestin recruitment and internalization.

Biased Agonism of Endogenous Opioid Peptides at the μ-Opioid Receptor

Biased agonism is having a major impact on modern drug discovery, and describes the ability of distinct G protein-coupled receptor (GPCR) ligands to activate different cell signaling pathways, and to result in different physiologic outcomes. To date, most studies of biased agonism have focused on synthetic molecules targeting various GPCRs; however, many of these receptors have multiple endogenous ligands, suggesting that "natural" bias may be an unappreciated feature of these GPCRs. The μ-opioid receptor (MOP) is activated by numerous endogenous opioid peptides, remains an attractive therapeutic target for the treatment of pain, and exhibits biased agonism in response to synthetic opiates. The aim of this study was to rigorously assess the potential for biased agonism in the actions of endogenous opioids at the MOP in a common cellular background, and compare these to the effects of the agonist d-Ala2-N-MePhe4-Gly-ol enkephalin (DAMGO). We investigated activation of G proteins, inhibition of cAMP production, extracellular signal-regulated kinase 1 and 2 phosphorylation, β-arrestin 1/2 recruitment, and MOP trafficking, and applied a novel analytical method to quantify biased agonism. Although many endogenous opioids displayed signaling profiles similar to that of DAMGO, α-neoendorphin, Met-enkephalin-Arg-Phe, and the putatively endogenous peptide endomorphin-1 displayed particularly distinct bias profiles. These may represent examples of natural bias if it can be shown that they have different signaling properties and physiologic effects in vivo compared with other endogenous opioids. Understanding how endogenous opioids control physiologic processes through biased agonism can reveal vital information required to enable the design of biased opioids with improved pharmacological profiles and treat diseases involving dysfunction of the endogenous opioid system.

Involvement of neuronal TGF-β activated kinase 1 in the development of tolerance to morphine-induced antinociception in rat spinal cord

BACKGROUND AND PURPOSE

Tolerance induced by morphine and other opiates remains a major unresolved problem in the clinical management of pain. There is now good evidence for the importance of MAPKs in morphine-induced antinociceptive tolerance. A member of the MAPK kinase kinase family, TGF-β activated kinase 1 (TAK1) is the common upstream kinase of MAPKs. Here, we have assessed the involvement of TAK1 in the development of tolerance to morphine-induced analgesia.

EXPERIMENTAL APPROACH

The effects of an antagonist of TAK1 on morphine tolerance were investigated in vivo using the Randall-Selitto test, and the mechanism was investigated using Western blot and immunohistochemistry. The expression of TAK1 after chronic morphine exposure was also evaluated in vitro by immunohistochemistry.

KEY RESULTS

Chronic intrathecal morphine exposure up-regulated protein levels and phosphorylation of spinal TAK1. TAK1 immunoreactivity was co-localized with the neuronal marker NeuN. Intrathecal administration of 5Z-7-oxozeaenol (OZ), a selective TAK1 inhibitor, attenuated the loss of morphine analgesic potency and morphine-induced TAK1 up-regulation. Furthermore, OZ decreased the up-regulated expression of spinal p38 and JNK after repeated morphine exposure. In vitro studies demonstrated that sustained morphine treatment induced TAK1 up-regulation, which was reversed by co-administration of OZ. A bolus injection of OZ showed some reversal of established morphine antinociceptive tolerance.

CONCLUSIONS AND IMPLICATIONS

TAK1 played a pivotal role in the development of morphine-induced antinociceptive tolerance. Modulation of TAK1 activation by the selective inhibitor OZ in the lumbar spinal cord may prove to be an attractive adjuvant therapy to attenuate such tolerance.

Opioid and chemokine receptor crosstalk: a promising target for pain therapy?

Chemokines and opioids are important regulators of immune, inflammatory and neuronal responses in peripheral and central pain pathways. Recent studies have provided insights into the functional interactions between chemokine receptors and opioid receptors, and their role in pain modulation. In this Progress article, we discuss how crosstalk between these two systems might provide a molecular and cellular framework for the development of novel analgesic therapies for the management of acute and/or chronic pain.

Change in functional selectivity of morphine with the development of antinociceptive tolerance

BACKGROUND AND PURPOSE

Opioids, such as morphine, are the most effective treatment for pain but their efficacy is diminished with the development of tolerance following repeated administration. Recently, we found that morphine activated ERK in opioid-tolerant but not in naïve rats, suggesting that morphine activation of μ-opioid receptors is altered following repeated morphine administration. Here, we have tested the hypothesis that μ-opioid receptor activation of ERK in the ventrolateral periaqueductal gray (vlPAG) is dependent on dynamin, a protein implicated in receptor endocytosis.

EXPERIMENTAL APPROACH

Rats were made tolerant to repeated microinjections of morphine into the vlPAG. The effects of dynamin on ERK activation and antinociception were assessed by microinjecting myristoylated dominant-negative dynamin peptide (Dyn-DN) or a scrambled control peptide into the vlPAG. Microinjection of a fluorescent dermorphin analogue (DERM-A594) into the vlPAG was used to monitor μ-opioid receptor internalization.

KEY RESULTS

Morphine did not activate ERK and Dyn-DN administration had no effect on morphine-induced antinociception in saline-pretreated rats. In contrast, morphine-induced ERK activation in morphine-pretreated rats that was blocked by Dyn-DN administration. Dyn-DN also inhibited morphine antinociception. Finally, morphine reduced DERM-A594 internalization only in morphine-tolerant rats indicating that μ-opioid receptors were internalized and unavailable to bind DERM-A594.

CONCLUSIONS AND IMPLICATIONS

Repeated morphine administration increased μ-opioid receptor activation of ERK signalling via a dynamin-dependent mechanism. These results demonstrate that the balance of agonist signalling to G-protein and dynamin-dependent pathways is altered, effectively changing the functional selectivity of the agonist-receptor complex.

LINKED ARTICLES

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

δ-opioid receptor and somatostatin receptor-4 heterodimerization: possible implications in modulation of pain associated signaling

Pain relief is the principal action of opioids. Somatostatin (SST), a growth hormone inhibitory peptide is also known to alleviate pain even in cases when opioids fail. Recent studies have shown that mice are prone to sustained pain and devoid of analgesic effect in the absence of somatostatin receptor 4 (SSTR4). In the present study, using brain slices, cultured neurons and HEK-293 cells, we showed that SSTR4 and δ-Opioid receptor (δOR) exist in a heteromeric complex and function in synergistic manner. SSTR4 and δOR co-expressed in cortical/striatal brain regions and spinal cord. Using cultured neuronal cells, we describe the heterogeneous complex formation of SSTR4 and δOR at neuronal cell body and processes. Cotransfected cells display inhibition of cAMP/PKA and co-activation of SSTR4 and δOR oppose receptor trafficking induced by individual receptor activation. Furthermore, downstream signaling pathways either associated with withdrawal or pain relief are modulated synergistically with a predominant role of SSTR4. Inhibition of cAMP/PKA and activation of ERK1/2 are the possible cellular adaptations to prevent withdrawal induced by chronic morphine use. Our results reveal direct intra-membrane interaction between SSTR4 and δOR and provide insights for the molecular mechanism for the anti-nociceptive property of SST in combination with opioids as a potential therapeutic approach to avoid undesirable withdrawal symptoms.

Chronic inflammatory pain prevents tolerance to the antinociceptive effect of morphine microinjected into the ventrolateral periaqueductal gray of the rat

The ventrolateral periaqueductal gray (vlPAG) contributes to morphine antinociception and tolerance. Chronic inflammatory pain causes changes within the PAG that are expected to enhance morphine tolerance. This hypothesis was tested by assessing antinociception and tolerance following repeated microinjections of morphine into the vlPAG of rats with chronic inflammatory pain. Microinjection of morphine into the vlPAG reversed the allodynia caused by intraplantar administration of complete Freund's adjuvant and produced antinociception on the hot plate test. Although there was a gradual decrease in morphine antinociception with repeated testing, there was no evidence of tolerance when morphine- and saline-treated rats with hind paw inflammation were tested with cumulative doses of morphine. In contrast, repeated morphine injections into the vlPAG caused a rightward shift in the morphine dose-response curve in rats without hind paw inflammation, as would be expected with the development of tolerance. The lack of tolerance in complete Freund's adjuvant-treated rats was evident whether rats were exposed to repeated behavioral testing or not (experiment 2) and whether they were treated with 4 or 8 prior microinjections of morphine into the vlPAG (experiment 3). These data demonstrate that chronic inflammatory pain does not disrupt the antinociceptive effect of microinjecting morphine into the vlPAG, but it does disrupt the development of tolerance.

PERSPECTIVE

The present data show that induction of chronic inflammatory pain does not disrupt the antinociceptive effect of microinjecting morphine into the vlPAG, but it does attenuate the development of tolerance. This finding indicates that tolerance to opioids in rats with inflammatory pain is mediated by structures other than the vlPAG.

Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance

Morphine and related µ-opioid receptor (MOR) agonists remain among the most effective drugs known for acute relief of severe pain. A major problem in treating painful conditions is that tolerance limits the long-term utility of opioid agonists. Considerable effort has been expended on developing an understanding of the molecular and cellular processes that underlie acute MOR signaling, short-term receptor regulation, and the progression of events that lead to tolerance for different MOR agonists. Although great progress has been made in the past decade, many points of contention and controversy cloud the realization of this progress. This review attempts to clarify some confusion by clearly defining terms, such as desensitization and tolerance, and addressing optimal pharmacological analyses for discerning relative importance of these cellular mechanisms. Cellular and molecular mechanisms regulating MOR function by phosphorylation relative to receptor desensitization and endocytosis are comprehensively reviewed, with an emphasis on agonist-biased regulation and areas where knowledge is lacking or controversial. The implications of these mechanisms for understanding the substantial contribution of MOR signaling to opioid tolerance are then considered in detail. While some functional MOR regulatory mechanisms contributing to tolerance are clearly understood, there are large gaps in understanding the molecular processes responsible for loss of MOR function after chronic exposure to opioids. Further elucidation of the cellular mechanisms that are regulated by opioids will be necessary for the successful development of MOR-based approaches to new pain therapeutics that limit the development of tolerance.

MicroRNAs in opioid pharmacology

MicroRNAs (miRNA), a class of ~22-nucleotide RNA molecules, are important gene regulators that bind to the target sites of mRNAs to inhibit the gene expressions either through translational inhibition or mRNA destabilization. There are growing evidences that miRNAs have played several regulatory roles in opioid pharmacology. Like other research fields such as cancer biology, the area where numerous miRNAs are found to be involved in gene regulation, we assume that in opioid studies including research fields of drug additions and opioid receptor regulation, there may be more miRNAs waiting to be discovered. This review will summarize our current knowledge of miRNA functions on opioids biology and briefly describe future research directions of miRNAs related to opioids.

Distribution of CB1 cannabinoid receptors and their relationship with mu-opioid receptors in the rat periaqueductal gray

The periaqueductal gray (PAG) is part of a descending pain modulatory system that, when activated, produces widespread and profound antinociception. Microinjection of either opioids or cannabinoids into the PAG elicits antinociception. Moreover, microinjection of the cannabinoid 1 (CB1) receptor agonist HU-210 into the PAG enhances the antinociceptive effect of subsequent morphine injections, indicating a direct relationship between these two systems. The objective of this study was to characterize the distribution of CB1 receptors in the dorsolateral and ventrolateral PAG in relationship to mu-opioid peptide (MOP) receptors. Immunocytochemical analysis revealed extensive and diffuse CB1 receptor labeling in the PAG, 60% of which was found in somatodendritic profiles. CB1 and MOP receptor immunolabeling were co-localized in 32% of fluorescent Nissl-stained cells that were analyzed. Eight percent (8%) of PAG neurons that were MOP receptor-immunoreactive (-ir) received CB1 receptor-ir appositions. Ultrastructural analysis confirmed the presence of CB1 receptor-ir somata, dendrites and axon terminals in the PAG. These results indicate that behavioral interactions between cannabinoids and opioids may be the result of cellular adaptations within PAG neurons co-expressing CB1 and MOP receptors.

Analysis of opioid efficacy, tolerance, addiction and dependence from cell culture to human

Opioid agonists are the most effective treatment for pain, but their use is limited by side effects, tolerance and fears of addiction and dependence. A major goal of opioid research is to develop agonists that have high analgesic efficacy and a low profile for side effects, tolerance, addiction and dependence. Unfortunately, there is a serious lack of experimental data comparing the degree to which different opioids produce these effects in humans. In contrast, a wide range of experimental techniques from heterologous expression systems to behaviour assessment in whole animals have been developed to study these problems. The objective of this review is to describe and evaluate these techniques as they are used to study opioid efficacy, tolerance, addiction and dependence.

Molecular mechanisms of opioid receptor-dependent signaling and behavior

Opioid receptors have been targeted for the treatment of pain and related disorders for thousands of years and remain the most widely used analgesics in the clinic. Mu (μ), kappa (κ), and delta (δ) opioid receptors represent the originally classified receptor subtypes, with opioid receptor like-1 (ORL1) being the least characterized. All four receptors are G-protein coupled and activate inhibitory G proteins. These receptors form homo- and heterodimeric complexes and signal to kinase cascades and scaffold a variety of proteins.The authors discuss classic mechanisms and developments in understanding opioid tolerance and opioid receptor signaling and highlight advances in opioid molecular pharmacology, behavioral pharmacology, and human genetics. The authors put into context how opioid receptor signaling leads to the modulation of behavior with the potential for therapeutic intervention. Finally, the authors conclude there is a continued need for more translational work on opioid receptors in vivo.

Endogenous opiates and behavior: 2009

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

Tolerance to the antinociceptive effect of morphine in the absence of short-term presynaptic desensitization in rat periaqueductal gray neurons

Opioids activate the descending antinociceptive pathway from the ventrolateral periaqueductal gray (vlPAG) by both pre- and postsynaptic inhibition of tonically active GABAergic neurons (i.e., disinhibition). Previous research has shown that short-term desensitization of postsynaptic μ-opioid receptors (MOPrs) in the vlPAG is increased with the development of opioid tolerance. Given that pre- and postsynaptic MOPrs are coupled to different signaling mechanisms, the present study tested the hypothesis that short-term desensitization of presynaptic MOPrs also contributes to opioid tolerance. Twice-daily injections of morphine (5 mg/kg s.c.) for 2 days caused a rightward shift in the morphine dose-response curve on the hot plate test (D(50) = 9.9 mg/kg) compared with saline-pretreated (5.3 mg/kg) male Sprague-Dawley rats. In vitro whole-cell patch-clamp recordings from vlPAG slices revealed that inhibition of evoked inhibitory postsynaptic currents (eIPSCs) by the MOPr-selective agonist [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin was decreased in morphine-tolerant (EC(50) = 708 nM) compared with saline-pretreated rats (EC(50) = 163 nM). However, short-term desensitization of MOPr inhibition of eIPSCs was not observed in either saline- or morphine-pretreated rats. Reducing the number of available MOPrs with the irreversible opioid receptor antagonist, β-chlornaltrexamine decreased maximal MOPr inhibition with no evidence of desensitization, indicating that the lack of observed desensitization is not caused by receptor reserve. These results demonstrate that tolerance to the antinociceptive effect of morphine is associated with a decrease in presynaptic MOPr sensitivity or coupling to effectors, but this change is independent of short-term MOPr desensitization.

Morphine regulates dopaminergic neuron differentiation via miR-133b

Morphine is one of the analgesics used most to treat chronic pain, although its long-term administration produces tolerance and dependence through neuronal plasticity. The ability of morphine to regulate neuron differentiation in vivo has been reported. However, the detailed mechanisms have not yet been elucidated because of the inability to separate maternal influences from embryonic events. Using zebrafish embryos as the model, we demonstrate that morphine decreases miR-133b expression, hence increasing the expression of its target, Pitx3, a transcription factor that activates tyrosine hydroxylase and dopamine transporter. Using a specific morpholino to knock down the zebrafish μ-opioid receptor (zfMOR) in the embryos and selective mitogen-activated protein kinase inhibitors, we demonstrate that the morphine-induced miR-133b decrease in zebrafish embryos is mediated by zfMOR activation of extracellular signal-regulated kinase 1/2. A parallel morphine-induced down-regulation of miR-133b was observed in the immature but not in mature rat hippocampal neurons. Our results indicate for the first time that zebrafish embryos express a functional μ-opioid receptor and that zebrafish serves as an excellent model to investigate the roles of microRNA in neuronal development affected by long-term morphine exposure.

Opioid receptor internalization contributes to dermorphin-mediated antinociception

Microinjection of opioids into the ventrolateral periaqueductal gray (vlPAG) produces antinociception in part by binding to mu-opioid receptors (MOPrs). Although both high and low efficacy agonists produce antinociception, low efficacy agonists such as morphine produce limited MOPr internalization suggesting that MOPr internalization and signaling leading to antinociception are independent. This hypothesis was tested in awake, behaving rats using DERM-A594, a fluorescently labeled dermorphin analog, and internalization blockers. Microinjection of DERM-A594 into the vlPAG produced both antinociception and internalization of DERM-A594. Administration of the irreversible opioid receptor antagonist beta-chlornaltrexamine (beta-CNA) prior to DERM-A594 microinjection reduced both the antinociceptive effect and the number of DERM-A594 labeled cells demonstrating that both effects are opioid receptor-mediated. Pretreatment with the internalization blockers dynamin dominant-negative inhibitory peptide (dynamin-DN) and concanavalinA (ConA) attenuated both DERM-A594 internalization and antinociception. Microinjection of dynamin-DN and ConA also decreased the antinociceptive potency of the unlabeled opioid agonist dermorphin when microinjected into the vlPAG as demonstrated by rightward shifts in the dose-response curves. In contrast, administration of dynamin-DN had no effect on the antinociceptive effect of microinjecting the GABA(A) receptor antagonist bicuculline into the vlPAG. The finding that dermorphin-induced antinociception is attenuated by blocking receptor internalization indicates that key parts of opioid receptor-mediated signaling depend on internalization.