Delta-opioid receptor immunoreactivity on astrocytes is upregulated during mitosis

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

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

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

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

LINKED ARTICLES

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

Exploring the neuroimmunopharmacology of opioids: an integrative review of mechanisms of central immune signaling and their implications for opioid analgesia

Vastly stimulated by the discovery of opioid receptors in the early 1970s, preclinical and clinical research was directed at the study of stereoselective neuronal actions of opioids, especially those played in their crucial analgesic role. However, during the past decade, a new appreciation of the non-neuronal actions of opioids has emerged from preclinical research, with specific appreciation for the nonclassic and nonstereoselective sites of action. Opioid activity at Toll-like receptors, newly recognized innate immune pattern recognition receptors, adds substantially to this unfolding story. It is now apparent from molecular and rodent data that these newly identified signaling events significantly modify the pharmacodynamics of opioids by eliciting proinflammatory reactivity from glia, the immunocompetent cells of the central nervous system. These central immune signaling events, including the release of cytokines and chemokines and the associated disruption of glutamate homeostasis, cause elevated neuronal excitability, which subsequently decreases opioid analgesic efficacy and leads to heightened pain states. This review will examine the current preclinical literature of opioid-induced central immune signaling mediated by classic and nonclassic opioid receptors. A unification of the preclinical pharmacology, neuroscience, and immunology of opioids now provides new insights into common mechanisms of chronic pain, naive tolerance, analgesic tolerance, opioid-induced hyperalgesia, and allodynia. Novel pharmacological targets for future drug development are discussed in the hope that disease-modifying chronic pain treatments arising from the appreciation of opioid-induced central immune signaling may become practical.

Cell-specific actions of HIV-Tat and morphine on opioid receptor expression in glia

HIV-1 patients who abuse opiate-based drugs, including heroin and morphine, are at a higher risk of developing HIV dementia. The effects of opiates are mediated predominantly through opioid receptors, which are expressed on glial cells. As HIV-1 infection in the CNS is restricted to glial cells, experiments were designed to measure the cell-specific effects of HIV Tat and morphine exposure on opioid receptor expression in both astrocytes and microglia. Specifically, the cell-type-specific pattern of mu opioid receptor (MOR), delta opioid receptor (DOR), and kappa opioid receptor (KOR) localization (surface vs. intracellular) and expression of opioid receptor mRNA were determined after exposure to morphine in the presence and the absence of Tat in primary cultured microglia and astrocytes. Data show that morphine treatment caused significantly decreased cell surface expression of opioid receptors in microglia but not in astrocytes. However, morphine treatment in the presence of Tat significantly increased intracellular expression of opioid receptors and prevented morphine-induced cell surface opioid receptor down-regulation in microglia. These findings document that cell surface opioid receptor expression is divergently regulated by morphine in microglia compared with in astrocytes, and further suggest that HIV-Tat could exacerbate opioid receptor signaling in microglia by increasing receptor expression and/or altering ligand-induced trafficking of opioid receptors.

Chronic pain-induced emotional dysfunction is associated with astrogliosis due to cortical delta-opioid receptor dysfunction

It has been widely recognized that chronic pain could cause physiological changes at supraspinal levels. The delta-opioidergic system is involved in antinociception, emotionality, immune response and neuron-glia communication. In this study, we show that mice with chronic pain exhibit anxiety-like behavior and an increase of astrocytes in the cingulate cortex due to the dysfunction of cortical delta-opioid receptor systems. Using neural stem cells cultured from the mouse embryonic forebrain, astrocyte differentiation was clearly observed following long-term exposure to the selective delta-opioid receptor antagonist, naltrindole. We also found that micro-injection of either activated astrocyte or astrocyte-conditioned medium into the cingulate cortex of mice aggravated the expression of anxiety-like behavior. Our results indicate that the chronic pain process promotes astrogliosis in the cingulate cortex through the dysfunction of cortical delta-opioid receptors. This phenomenon may lead to emotional disorders including aggravated anxiety under chronic pain-like state.

Immunocytochemical Characterization of Delta-Opioid and Mu-Opioid Receptor Protein in the Bovine Pineal Gland

Opioidergic innervation has been identified in the mammalian pineal gland. Recently, opioid receptors in bovine pineal glands have been characterized; the activation of these receptors leads to the stimulation of melatonin synthesis. In this study, the precise localization of opioid receptors in bovine pineal glands was determined by an immunohistochemical technique using antibodies raised against delta-opioid and mu-opioid receptors. Immunoreactivity of these two receptors was present at a moderate level in pinealocytes. A double-labeling study has shown that delta-opioid receptors are localized predominantly with mu-opioid receptors in the same pinealocytes. These immunopositive pinealocytes are often located in a group; however, some of them are dispersed individually. In addition, both types of receptors were found in glial cells and processes. A small number of delta-receptor-immunoreactive nerve fibers were observed in the perivascular space and intraparenchyma of the pineal gland. Mu-opioid receptor immunoreactivity was found in a number of nerve fibers throughout the gland, and in terminal-like dots on pinealocytes. There was immunocolocalization between delta-opioid receptors or mu-opioid receptors and leu-enkephalin in some nerve fibers. The results of this study indicate that the modulatory effect of the opioid system on melatonin secretion in pineal glands might act via opioid receptors on pinealocytes, whereas receptors located on nerve fibers might modulate the release of opioid peptides.

Molecular targets of opiate drug abuse in neuroAIDS

Opiate drug abuse, through selective actions at mu-opioid receptors (MOR), exacerbates the pathogenesis of human immunodeficiency virus-1 (HIV-1) in the CNS by disrupting glial homeostasis, increasing inflammation, and decreasing the threshold for pro-apoptotic events in neurons. Neurons are affected directly and indirectly by opiate-HIV interactions. Although most opiates drugs have some affinity for kappa (KOR) and/or delta (DOR) opioid receptors, their neurotoxic effects are largely mediated through MOR. Besides direct actions on the neurons themselves, opiates directly affect MOR-expressing astrocytes and microglia. Because of their broad-reaching actions in glia, opiate abuse causes widespread metabolic derangement, inflammation, and the disruption of neuron-glial relationships, which likely contribute to neuronal dysfunction, death, and HIV encephalitis. In addition to direct actions on neural cells, opioids modulate inflammation and disrupt normal intercellular interactions among immunocytes (macrophages and lymphocytes), which on balance further promote neuronal dysfunction and death. The neural pathways involved in opiate enhancement of HIV-induced inflammation and cell death, appear to involve MOR activation with downstream effects through PI3-kinase/Akt and/or MAPK signaling, which suggests possible targets for therapeutic intervention in neuroAIDS.

cAMP and protein kinase A contribute to the downregulation of spinal glutamate transporters after chronic morphine

Our previous study has shown that spinal glutamate transporters (GTs) are downregulated following chronic morphine administration; however, how spinal GTs are regulated in this process remains unclear. Here we show that the downregulation of spinal GTs (EAAC1 and GLT-1) induced by a 6-day intrathecal morphine (10 microg, twice daily) treatment regimen was prevented by co-administration of morphine with 2',5'-dideoxyadenosine (ddA, 1 microg, a broad adenylyl cyclase inhibitor) or H89 (10 microg, a selective protein kinase A inhibitor). When co-administered with morphine, ddA or H89 also effectively attenuated the development of morphine tolerance in the same rats, while ddA or H89 alone did not affect the baseline nociceptive response. These results indicate that the downregulation of spinal GTs following chronic morphine is at least in part mediated through the intracellular cyclic AMP and protein kinase A pathway, suggesting that this cellular mechanism of GT regulation may be contributory to the development of morphine tolerance in rats.

The opioid receptors in inner ear of different stages of postnatal rats

There is increasing evidence that the opioid system has a role in hearing. To provide further evidence for such a role, the expression of opioid receptor mRNAs and proteins in the inner ear of rats was studied during development from birth (P0) to postnatal day 16 (P16). A semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) was employed to detect changes in the expression of delta- (DOR) kappa- (KOR) and mu- (MOR) opioid receptor mRNAs in rat cochleae at P0, P4, P8 and P16. Expression of DOR mRNA levels steadily increased from P0 to P8 with no further increases by P16. KOR mRNA was expressed at a relatively high level at P0 and P4 followed by a decrease while MOR mRNA was expressed at a low level at P0 and P4 followed by an increase by P8 and P16. Immunocytochemical labelling of inner ear sections revealed unique developmental and distribution patterns of opioid receptors. In the organ of Corti DOR immunoreactivity (DOR-IR) was detected in hair cells from P4. In contrast MOR-IR was present only in supporting cells at P0-P16. In the spiral ganglion all three receptor subtypes were expressed from P0 on nerve cell soma and qualitatively appeared to increase with age. Also DOR-IR and MOR-IR were detected at P8 and P16 in nerve fibers within the spiral ganglion. In the limbus DOR-IR was detected at P8 and P16 on cells proximal to the tectorial membrane while MOR-IR was detected more distally. In general these findings demonstrate that within the inner ear each receptor subtype follows specific temporal and spatial developmental patterns, some of which may be associated to the onset of hearing. The data provide further evidence that the opioid system may play a role in the development and functioning of the inner ear.

Opioid-induced proliferation through the MAPK pathway in cultures of adult hippocampal progenitors

Administration of opioid agonists or antagonists has been reported to regulate proliferation or survival of neural progenitors in vivo. Here we report that beta-endorphin and selective mu-opioid receptor (MOR) and delta-opioid receptor (DOR) agonists stimulate proliferation of isolated rat adult hippocampal progenitors (AHPs). The AHPs were found to express DORs and MORs, but not kappa-opioid receptors. Incubation with beta-endorphin for 48 h increased the number of AHPs found in mitosis, the total DNA content, and the expression of proliferating cell nuclear antigen. This proliferative effect from beta-endorphin on AHPs was antagonized by naloxone. The beta-endorphin-induced proliferation was mediated through phosphorylation of extracellular signal-regulated kinases 1 and 2 and dependent on phosphatidylinositol 3-kinase and both intra- and extracellular calcium. These data suggest a role for the opioid system in the regulation of proliferation in progenitors from the adult hippocampus.

Chronic morphine induces downregulation of spinal glutamate transporters: implications in morphine tolerance and abnormal pain sensitivity

Tolerance to the analgesic effects of an opioid occurs after its chronic administration, a pharmacological phenomenon that has been associated with the development of abnormal pain sensitivity such as hyperalgesia. In the present study, we examined the role of spinal glutamate transporters (GTs) in the development of both morphine tolerance and associated thermal hyperalgesia. Chronic morphine administered through either intrathecal boluses or continuous infusion induced a dose-dependent downregulation of GTs (EAAC1 and GLAST) in the rat's superficial spinal cord dorsal horn. This GT downregulation was mediated through opioid receptors because naloxone blocked such GT changes. Morphine-induced GT downregulation reduced the ability to maintain in vivo glutamate homeostasis at the spinal level, because the hyperalgesic response to exogenous glutamate was enhanced, including an increased magnitude and a prolonged time course, in morphine-treated rats with reduced spinal GTs. Moreover, the downregulation of spinal GTs exhibited a temporal correlation with the development of morphine tolerance and thermal hyperalgesia. Consistently, the GT inhibitor l-trans-pyrrolidine-2-4-dicarboxylate (PDC) potentiated, whereas the positive GT regulator riluzole reduced, the development of both morphine tolerance and thermal hyperalgesia. The effects from regulating spinal GT activity by PDC were at least in part mediated through activation of the NMDA receptor (NMDAR), because the noncompetitive NMDAR antagonist MK-801 blocked both morphine tolerance and thermal hyperalgesia that were potentiated by PDC. These results indicate that spinal GTs may contribute to the neural mechanisms of morphine tolerance and associated abnormal pain sensitivity by means of regulating regional glutamate homeostasis.

Transcriptional regulation of mouse delta-opioid receptor gene: role of Ets-1 in the transcriptional activation of mouse delta-opioid receptor gene

Previously, we identified a minimum core promoter of the mouse delta-opioid receptor (DOR) gene. The DOR promoter contains an E-box that binds upstream stimulatory factor and is crucial for the DOR promoter activity in NS20Y cells, a mouse neuronal cell line that constitutively expresses DOR. In the present study, we further analyzed the DOR promoter in NS20Y cells and have demonstrated that transcription factor Ets-1 binds to an Ets-1-binding site overlapping the E-box and trans-activates the DOR promoter by synergizing with upstream stimulatory factor in specific DNA binding. In addition, the Ets-1 DNA-binding domain is sufficient to play the functional role of Ets-1 in trans-activating the DOR promoter. Furthermore, through in vivo cross-linking assays and Northern blot analyses, we have demonstrated that Ets-1 binds to the DOR promoter in the neonatal mouse brain and that overexpressed Ets-1 can significantly enhance the expression of DOR mRNA in primary neonatal mouse neuronal cells. Collectively, our data suggest that Ets-1 functions as a trans-activator of the DOR promoter in the neonatal mouse brain and thus may contribute to the development of the mouse brain DOR system.

Differential expression of delta opioid receptors and mRNA in proliferating astrocytes during the cell cycle

Previous immunohistochemical and radioligand binding studies have shown a cell cycle-dependent regulation of the delta opioid receptor (DOR). The relationship between DOR expression and mitosis in primary astroglial cultures of rat cerebral cortex was investigated in this study. The cultures were arrested during the G(1)/S transition or during mitosis. The DOR protein level increased twofold (P = 0.009) during mitosis and DOR mRNA level increased threefold (P = 0.002) during the G(1)/S transition compared to nonsynchronized cultures. DOR mRNA was also elevated (1.6-fold, P = 0.008) during the G(1)/S transition compared with mitotic cells. A premitotic increase in DOR mRNA suggests that elevated DOR protein levels during mitosis might be regulated during transcription.

Reduction by naloxone of lipopolysaccharide-induced neurotoxicity in mouse cortical neuron-glia co-cultures

An inflammatory response in the CNS mediated by activation of microglia is a key event in the early stages of the development of neurodegenerative diseases. Using mouse cortical mixed glia cultures, we have previously demonstrated that the bacterial endotoxin lipopolysaccharide induces the activation of microglia and the production of proinflammatory factors. Naloxone, an opioid receptor antagonist, inhibits the lipopolysaccharide-induced activation of microglia and the production of proinflammatory factors. Using neuron-glia co-cultures, we extended our study to determine if naloxone has a neuroprotective effect against lipopolysaccharide-induced neuronal damage and analysed the underlying mechanism of action for its potential neuroprotective effect. Pretreatment of cultures with naloxone (1 microM) followed by treatment with lipopolysaccharide significantly inhibited the lipopolysaccharide-induced production of nitric oxide and the release of tumor necrosis factor-alpha, and significantly reduced the lipopolysaccharide-induced damage to neurons. More importantly, both naloxone and its opioid-receptor ineffective enantiomer (+)-naloxone were equally effective in inhibiting the lipopolysaccharide-induced generation of proinflammatory factors and the activation of microglia, as well as in the protection of neurons. These results indicate that the neuroprotective effect of naloxone is mediated by its inhibition of microglial activity and may be unrelated to its binding to the classical opioid receptors.