Morphine inhibits human microglial cell production of, and migration towards, RANTES

Opioids Alleviate Oxidative Stress via the Nrf2/HO-1 Pathway in LPS-Stimulated Microglia

Opioids are known to have antioxidant effects and to modulate microglial function under certain conditions. It has been previously shown that opioid ligands can effectively inhibit the release of proinflammatory cytokines when stimulated with lipopolysaccharide (LPS) and convert microglia to an anti-inflammatory polarization state. Here, we used C8-B4 cells, the mouse microglial cell line activated by LPS as a model to investigate the anti-inflammatory/antioxidant potential of selected opioid receptor agonists (DAMGO, DADLE, and U-50488). We found that all of these ligands could exert cytoprotective effects through the mechanism affecting LPS-induced ROS production, NADPH synthesis, and glucose uptake. Interestingly, opioids elevated the level of reduced glutathione, increased ATP content, and enhanced mitochondrial respiration in microglial cells exposed to LPS. These beneficial effects were associated with the upregulation of the Nrf2/HO-1 pathway. The present results indicate that activation of opioid signaling supports the preservation of mitochondrial function with concomitant elimination of ROS in microglia and suggest that an Nrf2/HO-1 signaling pathway-dependent mechanism is involved in the antioxidant efficacy of opioids. Opioid receptor agonists may therefore be considered as agents to suppress oxidative stress and inflammatory responses of microglia.

Crosstalk between Mu-Opioid receptors and neuroinflammation: Consequences for drug addiction and pain

Mu-Opioid Receptors (MORs) are well-known for participating in analgesia, sedation, drug addiction, and other physiological functions. Although MORs have been related to neuroinflammation their biological mechanism remains unclear. It is suggested that MORs work alongside Toll-Like Receptors to enhance the release of pro-inflammatory mediators and cytokines during pathological conditions. Some cytokines, including TNF-α, IL-1β and IL-6, have been postulated to regulate MORs levels by both avoiding MOR recycling and enhancing its production. In addition, Neurokinin-1 Receptor, also affected during neuroinflammation, could be regulating MOR trafficking. Therefore, inflammation in the central nervous system seems to be associated with altered/increased MORs expression, which might regulate harmful processes, such as drug addiction and pain. Here, we provide a critical evaluation on MORs' role during neuroinflammation and its implication for these conditions. Understanding MORs' functioning, their regulation and implications on drug addiction and pain may help elucidate their potential therapeutic use against these pathological conditions and associated disorders.

Morphine suppresses peripheral responses and transforms brain myeloid gene expression to favor neuropathogenesis in SIV infection

The twin pandemics of opioid abuse and HIV infection can have devastating effects on physiological systems, including on the brain. Our previous work found that morphine increased the viral reservoir in the brains of treated SIV-infected macaques. In this study, we investigated the interaction of morphine and SIV to identify novel host-specific targets using a multimodal approach. We probed systemic parameters and performed single-cell examination of the targets for infection in the brain, microglia and macrophages. Morphine treatment created an immunosuppressive environment, blunting initial responses to infection, which persisted during antiretroviral treatment. Antiretroviral drug concentrations and penetration into the cerebrospinal fluid and brain were unchanged by morphine treatment. Interestingly, the transcriptional signature of both microglia and brain macrophages was transformed to one of a neurodegenerative phenotype. Notably, the expression of osteopontin, a pleiotropic cytokine, was significantly elevated in microglia. This was especially notable in the white matter, which is also dually affected by HIV and opioids. Increased osteopontin expression was linked to numerous HIV neuropathogenic mechanisms, including those that can maintain a viral reservoir. The opioid morphine is detrimental to SIV/HIV infection, especially in the brain.

Opioid receptor activation suppresses the neuroinflammatory response by promoting microglial M2 polarization

Activation of microglia is considered the most important component of neuroinflammation. Microglia can adopt a pro-inflammatory (M1) or anti-inflammatory (M2) phenotype. Opioid receptors (ORs) have been shown to control neurotransmission of various peptidergic neurons, but their potential role in regulating microglial function is largely unknown. Here, we aimed to investigate the effect of the OR agonists DAMGO, DADLE and U-50488 on the polarization of C8-B4 microglial cells. We observed that opioids suppressed lipopolysaccharide (LPS)-triggered M1 polarization and promoted M2 polarization. This was reflected in lower phagocytic activity, lower production of NO, lower expression of TNF-α, IL-1β, IL-6, IL-86 and IL-12 beta p40 together with higher migration rate, and increased expression of IL-4, IL-10, arginase 1 and CD 206 in microglia, compared to cells affected by LPS. We demonstrated that the effect of opioids on microglial polarization is mediated by the TREM2/NF-κB signaling pathway. These results provide new insights into the anti-inflammatory and neuroprotective effects of opioids and highlight their potential in combating neurodegenerative diseases.

Adolescent opioid abuse: Role of glial and neuroimmune mechanisms

Opioids are widely prescribed for pain management, and prescription opioid misuse in adolescents has become a major epidemic in the United States and worldwide. Emerging data indicate that adolescence represents a critical period of brain development, and exposure to opioids during adolescence may increase the risk of addiction in adulthood. There is growing evidence that disruptions in brain glial function may be implicated in numerous chronic neuropathologies. Evidence suggests that glial mechanisms have an important role in the development and maintenance of opioid abuse and the risk for addiction. This review will describe glial and neuroimmune mechanisms involved in opioid use disorders during adolescence, which may increase substance use disorder liability later in life. Moreover, this review will identify some important neuro-glial targets, involved in opioid abuse and addiction, to develop future preventions and treatment strategies.

Oxytocin, Dopamine, and Opioid Interactions Underlying Pair Bonding: Highlighting a Potential Role for Microglia

Pair bonds represent some of the strongest attachments we form as humans. These relationships positively modulate health and well-being. Conversely, the loss of a spouse is an emotionally painful event that leads to numerous deleterious physiological effects, including increased risk for cardiac dysfunction and mental illness. Much of our understanding of the neuroendocrine basis of pair bonding has come from studies of monogamous prairie voles (Microtus ochrogaster), laboratory-amenable rodents that, unlike laboratory mice and rats, form lifelong pair bonds. Specifically, research using prairie voles has delineated a role for multiple neuromodulatory and neuroendocrine systems in the formation and maintenance of pair bonds, including the oxytocinergic, dopaminergic, and opioidergic systems. However, while these studies have contributed to our understanding of selective attachment, few studies have examined how interactions among these 3 systems may be essential for expression of complex social behaviors, such as pair bonding. Therefore, in this review, we focus on how the social neuropeptide, oxytocin, interacts with classical reward system modulators, including dopamine and endogenous opioids, during bond formation and maintenance. We argue that an understanding of these interactions has important clinical implications and is required to understand the evolution and encoding of complex social behaviors more generally. Finally, we provide a brief consideration of future directions, including a discussion of the possible roles that glia, specifically microglia, may have in modulating social behavior by acting as a functional regulator of these 3 neuromodulatory systems.

The Role of Opioid Receptors in Immune System Function

Research on the effects of opioids on immune responses was stimulated in the 1980s by the intersection of use of intravenous heroin and HIV infection, to determine if opioids were enhancing HIV progression. The majority of experiments administering opioid alkaloids (morphine and heroin) in vivo, or adding these drugs to cell cultures in vitro, showed that they were immunosuppressive. Immunosuppression was reported as down-regulation: of Natural Killer cell activity; of responses of T and B cells to mitogens; of antibody formation in vivo and in vitro; of depression of phagocytic and microbicidal activity of neutrophils and macrophages; of cytokine and chemokine production by macrophages, microglia, and astrocytes; by sensitization to various infections using animal models; and by enhanced replication of HIV in vitro. The specificity of the receptor involved in the immunosuppression was shown to be the mu opioid receptor (MOR) by using pharmacological antagonists and mice genetically deficient in MOR. Beginning with a paper published in 2005, evidence was presented that morphine is immune-stimulating via binding to MD2, a molecule associated with Toll-like Receptor 4 (TLR4), the receptor for bacterial lipopolysaccharide (LPS). This concept was pursued to implicate inflammation as a mechanism for the psychoactive effects of the opioid. This review considers the validity of this hypothesis and concludes that it is hard to sustain. The experiments demonstrating immunosuppression were carried out in vivo in rodent strains with normal levels of TLR4, or involved use of cells taken from animals that were wild-type for expression of TLR4. Since engagement of TLR4 is universally accepted to result in immune activation by up-regulation of NF-κB, if morphine were binding to TLR4, it would be predicted that opioids would have been found to be pro-inflammatory, which they were not. Further, morphine is immunosuppressive in mice with a defective TLR4 receptor. Morphine and morphine withdrawal have been shown to permit leakage of Gram-negative bacteria and LPS from the intestinal lumen. LPS is the major ligand for TLR4. It is proposed that an occult variable in experiments where morphine is being proposed to activate TLR4 is actually underlying sepsis induced by the opioid.

Glial neuroimmune signaling in opioid reward

The opioid epidemic is a growing public concern affecting millions of people worldwide. Opioid-induced reward is the initial and key process leading to opioid abuse and addiction. Therefore, a better understanding of opioid reward may be helpful in developing a treatment for opioid addiction. Emerging evidence suggests that glial cells, particularly microglia and astrocytes, play an essential role in modulating opioid reward. Indeed, glial cells and their associated immune signaling actively regulate neural activity and plasticity, and directly modulate opioid-induced rewarding behaviors. In this review, we describe the neuroimmune mechanisms of how glial cells affect synaptic transmission and plasticity as well as how opioids can activate glial cells affecting the glial-neuronal interaction. Last, we summarize current attempts of applying glial modulators in treating opioid reward.

Proteomic Analysis of Cyclic Ketamine Compounds Ability to Induce Neural Differentiation in Human Adult Mesenchymal Stem Cells

Neural regeneration is of great interest due to its potential to treat traumatic brain injuries and diseases that impact quality of life. Growth factor mediated differentiation can take up to several weeks to months to produce the cell of interest whereas chemical stimulation may be as minimal as a few hours. The smaller time scale is of great clinical relevance. Adipose derived stem cells (ADSCs) were treated for up to 24 h with a novel differentiation media containing the cyclic ketamine compounds to direct neurogenic induction. The extent of differentiation was investigated by proteome changes occurring during the process. The treatments indicated the ADSCs responded favorably to the neurogenic induction media by presenting a number of morphological cues of neuronal phenotype previously seen and a higher cell population post induction compared to previous studies. Furthermore, approximately 3500 proteins were analyzed and identified by mass spectrometric iTRAQ analyses. The bioinformatics analyses revealed hundreds of proteins whose expression level changes were statistically significant and biologically relevant to neurogenesis and annotated as being involved in neurogenic development. Complementing this, the Bioplex cytokine assay profiles present evidence of decreased panel of stress response cytokines and a relative increase in those involved in neurogenesis.

Morphine-Mediated Brain Region-Specific Astrocytosis Involves the ER Stress-Autophagy Axis

A recent study from our lab has revealed a link between morphine-mediated autophagy and synaptic impairment. The current study was aimed at investigating whether morphine-mediated activation of astrocytes involved the ER stress/autophagy axis. Our in vitro findings demonstrated upregulation of GFAP indicating astrocyte activation with a concomitant increase in the production of proinflammatory cytokines in morphine-exposed human astrocytes. Using both pharmacological and gene-silencing approaches, it was demonstrated that morphine-mediated defective autophagy involved upstream activation of ER stress with subsequent downstream astrocyte activation via the μ-opioid receptor (MOR). In vivo validation demonstrated preferential activation of ER stress/autophagy axis in the areas of the brain not associated with pain such as the basal ganglia, frontal cortex, occipital cortex, and the cerebellum of morphine-dependent rhesus macaques, and this correlated with increased astrocyte activation and neuroinflammation. Interventions aimed at blocking either the MOR or ER stress could thus likely be developed as promising therapeutic targets for abrogating morphine-mediated astrocytosis.

Morphine increases hippocampal viral load and suppresses frontal lobe CCL5 expression in the LP-BM5 AIDS model

Chronic opiate abuse accelerates the development of cognitive deficits in human immunodeficiency virus (HIV)-1 patients. To investigate morphine's effects on viral infection of the central nervous system, we applied chronic morphine treatment to the LP-BM5 murine acquired immunodeficiency syndrome (MAIDS) model. LP-BM5 infection induces proinflammatory cytokine/chemokine production, correlating to increased blood-brain barrier permeability. Morphine treatment significantly increased LP-BM5 viral load in the hippocampus, but not in the frontal lobe. Morphine reduced the chemokine CCL5 to non-infected levels in the frontal lobe, but not in the hippocampus. These data indicate a region-specific mechanism for morphine's effects on virally-induced neurocognitive deficits.

Effect of chronic morphine administration on circulating T cell population dynamics in rhesus macaques

Opioid receptor agonists modulate both innate and adaptive immune responses. In this study, we examined the impact of long-term chronic morphine administration on the circulating T cell population dynamics in rhesus macaques. We found that the numbers of circulating Treg cells, and the functional activity of Th17 cells, were significantly increased with chronic morphine exposure. Our results also show that T cell populations with surface markers characteristic of gut-homing (CD161 and CCR6) and HIV-1 susceptibility (CCR5 and β7 integrin) were increased. These results represent the first detailed report of the impact of chronic morphine administration on circulating T cell dynamics.

Implications of central immune signaling caused by drugs of abuse: mechanisms, mediators and new therapeutic approaches for prediction and treatment of drug dependence

In the past two decades a trickle of manuscripts examining the non-neuronal central nervous system immune consequences of the drugs of abuse has now swollen to a significant body of work. Initially, these studies reported associative evidence of central nervous system proinflammation resulting from exposure to the drugs of abuse demonstrating key implications for neurotoxicity and disease progression associated with, for example, HIV infection. However, more recently this drug-induced activation of central immune signaling is now understood to contribute substantially to the pharmacodynamic actions of the drugs of abuse, by enhancing the engagement of classical mesolimbic dopamine reward pathways and withdrawal centers. This review will highlight the key in vivo animal, human, biological and molecular evidence of these central immune signaling actions of opioids, alcohol, cocaine, methamphetamine, and 3,4-methylenedioxymethamphetamine (MDMA). Excitingly, this new appreciation of central immune signaling activity of drugs of abuse provides novel therapeutic interventions and opportunities to identify 'at risk' individuals through the use of immunogenetics. Discussion will also cover the evidence of modulation of this signaling by existing clinical and pre-clinical drug candidates, and novel pharmacological targets. Finally, following examination of the breadth of central immune signaling actions of the drugs of abuse highlighted here, the current known common immune signaling components will be outlined and their impact on established addiction neurocircuitry discussed, thereby synthesizing a common neuroimmune hypothesis of addiction.

Opioid drug abuse and modulation of immune function: consequences in the susceptibility to opportunistic infections

Infection rate among intravenous drug users (IDU) is higher than the general public, and is the major cause of morbidity and hospitalization in the IDU population. Epidemiologic studies provide data on increased prevalence of opportunistic bacterial infections such as TB and pneumonia, and viral infections such as HIV-1 and hepatitis in the IDU population. An important component in the intravenous drug abuse population and in patients receiving medically indicated chronic opioid treatment is opioid withdrawal. Data on bacterial virulence in the context of opioid withdrawal suggest that mice undergoing withdrawal had shortened survival and increased bacterial load in response to Salmonella infection. As the body of evidence in support of opioid dependency and its immunosuppressive effects is growing, it is imperative to understand the mechanisms by which opioids exert these effects and identify the populations at risk that would benefit the most from the interventions to counteract opioid immunosuppressive effects. Thus, it is important to refine the existing animal model to closely match human conditions and to cross-validate these findings through carefully controlled human studies. Better understanding of the mechanisms will facilitate the search for new therapeutic modalities to counteract adverse effects including increased infection rates. This review will summarize the effects of morphine on innate and adaptive immunity, identify the role of the mu opioid receptor in these functions and the signal transduction activated in the process. The role of opioid withdrawal in immunosuppression and the clinical relevance of these findings will also be discussed.

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.

Protein kinase Czeta mediates micro-opioid receptor-induced cross-desensitization of chemokine receptor CCR5

We have previously shown that the μ-opioid receptor (MOR) is capable of mediating cross-desensitization of several chemokine receptors including CCR5, but the biochemical mechanism of this process has not been fully elucidated. We have carried out a series of functional and biochemical studies and found that the mechanism of MOR-induced cross-desensitization of CCR5 involves the activation of PKCζ. Inhibition of PKCζ by its pseudosubstrate inhibitor, or its siRNA, or dominant negative mutants suppresses the cross-desensitization of CCR5. Our results further indicate that the activation of PKCζ is mediated through a pathway involving phosphoinositol-dependent kinase-1 (PDK1). In addition, activation of MOR elevates the phosphorylation level and kinase activity of PKCζ. The phosphorylation of PKCζ can be suppressed by a dominant negative mutant of PDK1. We observed that following MOR activation, the interaction between PKCζ and PDK1 is immediately increased based on the analysis of fluorescent resonance energy transfer in cells with the expression of PKCζ-YFP and PDK1-CFP. In addition, cells expressing PKCζ kinase motif mutants (Lys-281, Thr-410, Thr-560) fail to exhibit full MOR-induced desensitization of CCR5 activity. Taken together, we propose that upon DAMGO treatment, MOR activates PKCζ through a PDK1-dependent signaling pathway to induce CCR5 phosphorylation and desensitization. Because CCR5 is a highly proinflammatory receptor, and a critical coreceptor for HIV-1, these results may provide a novel approach for the development of specific therapeutic agents to treat patients with certain inflammatory diseases or AIDS.

Changes in the NMR metabolic profile of human microglial cells exposed to lipopolysaccharide or morphine

Microglial cells play a major role in host defense of the central nervous system. Once activated, several functional properties are up-regulated including migration, phagocytosis, and secretion of inflammatory mediators such as cytokines and chemokines. Little, if anything, is known about the metabolic changes that occur during the activation process. High-resolution (1)H nuclear magnetic resonance spectra obtained from perchloric acid extracts of human microglial cell cultures exposed to lipopolysaccharide (LPS) or morphine were used to both identify and quantify the metabolites. We found that human microglia exposed to LPS had increased concentrations of glutamate and lactate, whereas the cells exposed to morphine had decreased concentrations in creatinine, taurine, and thymine. Glutamate and creatinine were the key metabolites differentiating between the two stimuli. These results are discussed in terms of activation and differences in the inflammatory response of human microglial cells to LPS and morphine.

Morphine enhances microglial migration through modulation of P2X4 receptor signaling

Opioids, although fundamental to the treatment of pain, are limited in efficacy by side effects including tolerance and hyperalgesia. Using an in vitro culture system, we report that morphine increased microglial migration via a novel interaction between mu-opioid and P2X(4) receptors, which is dependent upon PI3K/Akt pathway activation. Morphine at 100 nm enhanced migration of primary microglial cells toward adenosine diphosphate by 257, 247, 301, 394, and 345% following 2, 6, 12, 24, and 48 h of stimulation, respectively. This opioid-dependent migration effect was inhibited by naloxone and confirmed to be mu-opioid receptor-dependent through the use of selective agonists and antagonists. PPADS [pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid)], a P2X(1-3,5-7) antagonist, had no effect on microglial migration; however, TNP-ATP [2',3'-O-(2,4,6-trinitrophenyl)-ATP], a P2X(1-7) antagonist, inhibited morphine-induced migration, suggesting a P2X(4) receptor-mediated effect. The PI3K inhibitors wortmannin and LY294002 decreased morphine-induced microglial migration. Iba1 protein, a microglial marker, and P2X(4) receptor expression were significantly increased after 6, 12, 24, and 48 h of morphine stimulation. Together, these results provide evidence for two phases of morphine effects on microglia. The initial phase takes place in minutes, involves PI3K/Akt pathway activation and leads to acutely enhanced migration. The longer-term phase occurs on the order of hours and involves increased expression of Iba1 and P2X(4) receptor protein, which imparts a promigratory phenotype and is correlated with even greater migration. These data provide the first necessary step in supporting microglial migration as an attractive target for the prevention or attenuation of morphine-induced side effects including tolerance and hyperalgesia.

Joint capsule treatment with enkephalin-encoding HSV-1 recombinant vector reduces inflammatory damage and behavioural sequelae in rat CFA monoarthritis

This study assessed enkephalin expression induced by intra-articular application of recombinant, enkephalin-encoding herpes virus (HSV-1) and the impact of expression on nociceptive behaviours and synovial lining inflammation in arthritic rats. Replication-conditional HSV-1 recombinant vectors with cDNA encoding preproenkephalin (HSV-ENK), or control transgene beta-galactosidase cDNA (HSV-beta-gal; control) were injected into knee joints with complete Freund's adjuvant (CFA). Joint temperatures, circumferences and nociceptive behaviours were monitored on days 0, 7, 14 and 21 post CFA and vector treatments. Lumbar (L4-6) dorsal root ganglia (DRG) and spinal cords were immunostained for met-enkephalin (met-ENK), beta-gal, HSV-1 proteins and Fos. Joint tissues were immunostained for met-ENK, HSV-1 proteins, and inflammatory mediators Regulated on Activation, Normal T-cell Expressed and Secreted (RANTES) and cyclo-oxygenase-2, or stained with haematoxylin and eosin for histopathology. Compared to exuberant synovial hypertrophy and inflammatory cell infiltration seen in arthritic rats treated with CFA only or CFA and HSV-beta-gal, the CFA- and HSV-ENK-treated arthritic rats had: (i) striking preservation of synovial membrane cytoarchitecture with minimal inflammatory cell infiltrates; (ii) significantly improved nociceptive behavioural responses to mechanical and thermal stimuli; (iii) normalized Fos staining in lumbar dorsal horn; and (iv) significantly increased met-ENK staining in ipsilateral synovial tissue, lumbar DRG and spinal cord. The HSV-1 and transgene product expression were confined to ipsilateral lumbar DRG (HSV-1, met-ENK, beta-gal). Only transgene product (met-ENK and beta-gal) was seen in lumbar spinal cord sections. Targeted delivery of enkephalin-encoding HSV-1 vector generated safe, sustained opioid-induced analgesia with protective anti-inflammatory blunting in rat inflammatory arthritis.

Methamphetamine enhances HIV infection of macrophages

Epidemiological studies have demonstrated that the use of methamphetamine (meth), a sympathomimetic stimulant, is particularly common among patients infected with HIV. However, there is a lack of direct evidence that meth promotes HIV infection of target cells. This study examined whether meth is able to enhance HIV infection of macrophages, the primary target site for the virus. Meth treatment resulted in a significant and dose-dependent increase of HIV reverse transcriptase activity in human blood monocyte-derived macrophages. Dopamine D1 receptor antagonists (SCH23390 and SKF83566) blocked this meth-mediated increase in the HIV infectivity of macrophages. Investigation of the underlying mechanisms of meth action showed that meth up-regulated the expression of the HIV entry co-receptor CCR5 on macrophages. Additionally, meth inhibited the expression of endogenous interferon-alpha and signal transducer and activator of transcription-1 in macrophages. These findings provide direct in vitro evidence to support the possibility that meth may function as a cofactor in the immunopathogenesis of HIV infection and may lead to the future development of innate immunity-based intervention for meth users with HIV infection.

Opioid and nociceptin receptors regulate cytokine and cytokine receptor expression

Opioids were originally discovered because of their ability to induce analgesia, but further investigation has shown that the opioids regulate the function of cells involved in the immune response. We suggest that the regulation of cytokine, chemokine, and cytokine receptor expression is a critical component of the immunomodulatory activity of the opioids. In this paper we review the literature dealing with the regulation of cytokine and cytokine receptor expression by agonists for the three major opioid receptor types (mu, kappa, and delta), and nociceptin, the natural agonist for the orphanin FQ/nociceptin receptor. Although the opioid receptors share a high degree of sequence homology, opposing roles between the kappa opioid receptor (KOR) and the mu opioid receptor (MOR) have become apparent. We suggest that activation of the KOR induces an anti-inflammatory response through the down-regulation of cytokine, chemokine and chemokine receptor expression, while activation of the MOR favors a pro-inflammatory response. Investigation into the opioid receptor-like (ORL1)/nociceptin system also suggests a role for this receptor as a down-regulator of immune function. These effects suggest a broad role for opioids in the modulation of the function of the immune system, and suggest possible targets for the development of new therapeutics for inflammatory and infectious diseases.

Enkephalin-encoding herpes simplex virus-1 decreases inflammation and hotplate sensitivity in a chronic pancreatitis model

Background

A chronic pancreatitis model was developed in young male Lewis rats fed a high-fat and alcohol liquid diet beginning at three weeks. The model was used to assess time course and efficacy of a replication defective herpes simplex virus type 1 vector construct delivering human cDNA encoding preproenkephalin (HSV-ENK).

Results

Most surprising was the relative lack of inflammation and tissue disruption after HSV-ENK treatment compared to the histopathology consistent with pancreatitis (inflammatory cell infiltration, edema, acinar cell hypertrophy, fibrosis) present as a result of the high-fat and alcohol diet in controls. The HSV-ENK vector delivered to the pancreatic surface at week 3 reversed pancreatitis-associated hotplate hypersensitive responses for 4–6 weeks, while control virus encoding β-galactosidase cDNA (HSV-β-gal) had no effect. Increased Fos expression seen bilaterally in pain processing regions in control animals with pancreatitis was absent in HSV-ENK-treated animals. Increased met-enkephalin staining was evident in pancreas and lower thoracic spinal cord laminae I–II in the HSV-ENK-treated rats.

Conclusion

Thus, clear evidence is provided that site specific HSV-mediated transgene delivery of human cDNA encoding preproenkephalin ameliorates pancreatic inflammation and significantly reduces hypersensitive hotplate responses for an extended time consistent with HSV mediated overexpression, without tolerance or evidence of other opiate related side effects.

Opioids and migration, chemotaxis, invasion, and adhesion of human cancer cells

This study was designed to examine the role of opioids on cell migration, chemotaxis, invasion, and adhesion, with an emphasis on whether the opioid growth factor (OGF, [Met(5)]-enkephalin) or the opioid antagonist naltrexone (NTX) impacts any or all of these processes. Drug concentrations of OGF and NTX known to depress or stimulate, respectively, cell proliferation and growth were analyzed. Three different human cancers (pancreatic, colon, and squamous cell carcinoma of the head and neck), represented by seven different cancer cell lines (PANC-1, MIA PaCa-2, BxPC-3, CAL-27, SCC-1, HCT-116, and HT-29), were evaluated. In addition, the influence of a variety of other natural and synthetic opioids on cell motility, invasion, and adhesion was assessed. Positive and negative controls were included for comparison. OGF and NTX at concentrations of 10(-4) to 10(-6)M, and dynorphin A1-8, beta-endorphin, endomorphin-1, endomorphin-2, leucine enkephalin, [D-Pen(2,5)]-enkephalin (DPDPE), [D-Ala(2), MePhe(4), Glycol(5)]-enkephalin (DAMGO), morphine, and U69,593 at concentrations of 10(-6)M, did not alter cell migration, chemotaxis, or invasion of any cancer cell line. OGF and NTX at a concentration of 10(-6)M, and incubation for 24 or 72h, did not change adhesion of these cancer cells to collagen I, collagen IV, fibronectin, laminin, or vitronectin. Moreover, all other opioids tested at 10(-6)M concentrations and for 24h had no effect on adhesion. These results indicate that the inhibitory or stimulatory actions of OGF and NTX, respectively, on cell replication and growth are independent of cell migration, chemotaxis, invasion, and adhesive properties. Moreover, a variety of other exogenous and endogenous opioids, many specific for the micro, delta, or kappa opioid receptors, also did not alter these biological processes, consonant with previous observations of a lack of effects of these compounds and their receptors on the biology of cancer cells.

Neurotransmitter receptors on microglia

Microglia are the intrinsic immune cells of the brain and express chemokine and cytokine receptors that interact with the peripheral immune cells. Recent studies have indicated that microglia also respond to the brain's classical signalling substances, the neurotransmitters. Here, we review the evidence for the expression of neurotransmitter receptors on microglia and the consequences of this receptor activation for microglial behaviour. It is evident that neurotransmitters instruct microglia to perform distinct types of responses, such as triggering an inflammatory cascade or acquiring a neuroprotective phenotype. Understanding how microglia respond to different neurotransmitters will thus have important implications for controlling the reactivity of these cells in acute injury, as well as for treating chronic neurodegenerative diseases.

Treatment of inflamed pancreas with enkephalin encoding HSV-1 recombinant vector reduces inflammatory damage and behavioral sequelae

This study assessed the efficacy of pancreatic surface delivered enkephalin (ENK)-encoding herpes simplex virus type 1 (HSV-1) on spontaneous behaviors and spinal cord and pancreatic enkephalin expression in an experimental pancreatitis model. Replication-defective HSV-1 with proenkephalin complementary DNA (cDNA) (HSV-ENK) or control beta-galactosidase cDNA (HSV-beta-gal), or media vehicle (Veh) was applied to the pancreatic surface of rats with dibutyltin dichloride (DBTC)-induced pancreatitis. Spontaneous exploratory behavioral activity was monitored on days 0 and 6 post DBTC and vector treatments. The pancreas, thoracic dorsal root ganglia (DRG, T9-10), and spinal cord (T9-10) were immunostained for met-enkephalin (met-ENK), beta-gal, and HSV-1 proteins. Spinal cord was also immunostained for c-Fos, and pancreas was stained for the inflammatory marker regulated on activation, normal T-cells expressed and secreted (RANTES), mu-opioid receptor, and hemotoxylin/eosin. On day 6, compared to pancreatitis and vector controls, the DBTC/HSV-ENK treated rats had significantly improved spontaneous exploratory activities, increased met-ENK staining in the pancreas and spinal cord, and normalized c-Fos staining in the dorsal horn. Histopathology of pancreas in DBTC/HSV-ENK treated rats showed preservation of acinar cells and cytoarchitecture with minimal inflammatory cell infiltrates, compared to severe inflammation and acinar cell loss seen in DBTC/HSV-beta-gal and DBTC/Veh treated rats. Targeted transgene delivery and met-ENK expression successfully produced decreased inflammation in experimental pancreatitis.

Effect of μ and κ opioids on injury-induced microglial accumulation in leech CNS: Involvement of the nitric oxide pathway

Damage to the leech or mammalian CNS increases nitric oxide (NO) production and causes accumulation of phagocytic microglial cells at the injury site. Opioids have been postulated to modulate various parameters of the immune response. Morphine and leech morphine-like substance are shown to release NO and suppress microglial activation. Regarding the known immuno-modulatory effects of selective mu and kappa ligands, we have assessed the effect of these agents on accumulation of microglia at the site of injury in leech CNS. Leech nerve cords were dissected, crushed with fine forceps and maintained in different concentrations of opiates in culture medium for 3 h and then fixed and double stained with Hoechst 33258 and monoclonal antibody to endothelial nitric oxide synthase (NOS). Morphine and naloxone (> or =10(-3) M) but not selective mu agonist, DAMGO [d-Ala2, N-Me-Phe-Gly5(ol)-enkephalin] and antagonist, CTAP [D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2] inhibited the microglial accumulation. The effect of morphine was abrogated by pre-treatment with naloxone and also non-selective NOS inhibitor, l-NAME [N(omega)-nitro-l-arginine-methyl-ester; 10(-3) M] implying an NO-dependent and mu-mediated mechanism. These results are similar to properties of recently found mu-3 receptor in leech, which is sensitive to alkaloids but not peptides. Both selective kappa agonist, U50,488 [3,4-dichloro-N-methyl-N-(2-(1-pyrrolidinyl)cyclohexyl)-benzeneacetamide; > or =10(-3) M], and antagonist, nor-binaltorphimine (nor-BNI; > or =10(-3) M), inhibited the accumulation. The effect of nor-BNI was reversed by l-NAME. Immunohistochemistry showed decreased endothelial NOS expression in naloxone and U50,488-treated cords. Since, NO production at the injury site is hypothesized to act as a stop signal for microglias, opioid agents may exert their effect via changing of NO gradient along the cord resulting in disruption of accumulation. These results suggest an immuno-modulatory role for mu and kappa opioid receptors on injury-induced microglial accumulation which may be mediated via NO.

CCL5 evokes calcium signals in microglia through a kinase-, phosphoinositide-, and nucleotide-dependent mechanism

Microglia, the resident macrophages of the CNS, are responsible for the innate immune response in the brain and participate in the pathogenesis of certain neurodegenerative disorders. Chemokines initiate activation and migration of microglia. The beta-chemokine CCL5 induces an elevation in intracellular calcium concentration ([Ca(2+)](i)) in human microglia. Here, we examined the signal transduction pathway linking activation of chemokine receptor CCR5 to an elevation in [Ca(2+)](i) in cultured microglia by using pharmacological approaches in combination with Fura-2-based digital imaging. The CCL5-induced response required Janus kinase (Jak) activity and the stimulation of an inhibitory G protein. Multiple downstream signaling pathways were involved, including phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk), and phospholipase C (PLC)-mediated release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores. Activation of both the kinase and the lipase pathways was required for eliciting the Ca(2+) response. However, the majority of the [Ca(2+)](i) increase was derived from sources activated by NAD metabolites. Cyclic ADP-ribose (cADPR) evoked Ca(2+) release from intracellular stores, and ADPR evoked Ca(2+) influx via a nimodipine-sensitive channel. Thus, a multistep cascade couples CCR5 activation to Ca(2+) increases in human microglia. Because changes in [Ca(2+)](i) affect chemotaxis, secretion, and gene expression, pharmacologic modulation of this pathway may alter inflammatory and degenerative processes in the CNS.

Protection from neuronal damage evoked by a motivational excitation is a driving force of intentional actions

Motivation may be understood as an organism's subjective attitude to its current physiological state, which somehow modulates generation of actions until the organism attains an optimal state. How does this subjective attitude arise and how does it modulate generation of actions? Diverse lines of evidence suggest that elemental motivational states (hunger, thirst, fear, drug-dependence, etc.) arise as the result of metabolic disturbances and are related to transient injury, while rewards (food, water, avoidance, drugs, etc.) are associated with the recovery of specific neurons. Just as motivation and the very life of an organism depend on homeostasis, i.e., maintenance of optimum performance, so a neuron's behavior depends on neuronal (i.e., ion) homeostasis. During motivational excitation, the conventional properties of a neuron, such as maintenance of membrane potential and spike generation, are disturbed. Instrumental actions may originate as a consequence of the compensational recovery of neuronal excitability after the excitotoxic damage induced by a motivation. When the extent of neuronal actions is proportional to a metabolic disturbance, the neuron theoretically may choose a beneficial behavior even, if at each instant, it acts by chance. Homeostasis supposedly may be directed to anticipating compensation of the factors that lead to a disturbance of the homeostasis and, as a result, participates in the plasticity of motivational behavior. Following this line of thought, I suggest that voluntary actions arise from the interaction between endogenous compensational mechanisms and excitotoxic damage of specific neurons, and thus anticipate the exogenous compensation evoked by a reward.

Opioids as modulators of cell death and survival--unraveling mechanisms and revealing new indications

Opioids are powerful analgesics but also drugs of abuse. Because opioid addicts are susceptible to certain infections, opioids have been suspected to suppress the immune response. This was supported by the finding that various immune-competent cells express opioid receptors and undergo apoptosis when treated with opioid alkaloids. Recent evidence suggests that opioids may also effect neuronal survival and proliferation or migrating properties of tumor cells. A multitude of signaling pathways has been suggested to be involved in these extra-analgesic effects of opioids. Growth-promoting effects were found to be mediated through Akt and Erk signaling cascades. Death-promoting effects have been ascribed to inhibition of nuclear factor-kappaB, increase of Fas expression, p53 stabilization, cytokine and chemokine release, and activation of nitric oxide synthase, p38, and c-Jun-N-terminal kinase. Some of the observed effects were inhibited with opioid receptor antagonists or pertussis toxin; others were unaffected. It is still unclear whether these properties are mediated through typical opioid receptor activation and inhibitory G-protein-signaling. The present review tries to unravel controversial findings and provides a hypothesis that may help to integrate diverse results.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.

Morphine enhances HIV infection of human blood mononuclear phagocytes through modulation of beta-chemokines and CCR5 receptor

BACKGROUND

Injection drug use remains a significant risk for acquiring HIV infection. The mechanisms by which morphine enhances HIV infection of human immune cells are largely unknown.

OBJECTIVE

In this study, we sought to determine the possible mechanisms by which morphine upregulates HIV infection of human blood monocyte-derived macrophages (MDM).

METHODS

In this study, MDM were infected with the R5, X4, and R5X4 HIV strains. HIV replication was determined by performing reverse transcriptase activity assays. HIV receptors were determined by performing reverse transcriptase polymerase chain reactions and flow cytometry assays. beta-chemokines were analyzed by performing enzyme-linked immunosorbent assays. In addition, HIV R5 strain and murine leukemia virus envelope-pseudotyped HIV infection was performed to determine whether morphine affects HIV infection of macrophages at entry level.

RESULTS

Morphine significantly enhanced HIV R5 strain infection of MDM but had little effect on X4 strain infection. The macrophage-tropic R5 strain envelope-pseudotyped HIV infection was markedly increased by morphine, whereas murine leukemia virus envelope-pseudotyped HIV infection was not significantly affected. Furthermore, morphine significantly upregulated CCR5 receptor expression and inhibited the endogenous production of beta-chemokines in MDM. The opioid receptor antagonist naltrexone blocked the effects of morphine on the production of beta-chemokines.

CONCLUSION

Opiates enhance HIV R5 strain infection of macrophages through the downregulation of beta-chemokine production and upregulation of CCR5 receptor expression and may have an important role in HIV immunopathogenesis.

Regulating factors for microglial activation

Microglia, residential macrophages in the central nervous system, can release a variety of factors including cytokines, chemokines, etc. to regulate the communication among neuronal and other types of glial cells. Microglia play immunological roles in mechanisms underlying the phagocytosis of invading microorganisms and removal of dead or damaged cells. When microglia are hyperactivated due to a certain pathological imbalance, they may cause neuronal degeneration. Pathological activation of microglia has been reported in a wide range of conditions such as cerebral ischemia, Alzheimer's disease, prion diseases, multiple sclerosis, AIDS dementia, and others. Nearly 5000 papers on microglia can be retrieved on the Web site PubMed at present (November 2001) and half of them were published within the past 5 years. Although it is not possible to read each paper in detail, as many factors as possible affecting microglial functions in in vitro culture systems are presented in this review. The factors are separated into "activators" and "inhibitors," although it is difficult to classify many of them. An overview on these factors may help in the development of a new strategy for the treatment of various neurodegenerative diseases.

Vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide inhibit chemokine production in activated microglia

Microglia react to even minor disturbances in CNS homeostasis and function as critical regulators of CNS inflammation. Activated microglia secrete inflammatory mediators such as cytokines and chemokines, which contribute to the pathophysiological changes associated with several neuroimmunologic disorders. Microglia-derived inflammatory chemokines recruit various populations of immune cells, which initiate and maintain the inflammatory response against foreign antigens. Entry and retention of activated immune cells in the CNS is a common denominator in a variety of traumatic, ischemic, and degenerative diseases. Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are two structurally related neuropeptides that function as potent anti-inflammatory factors in the periphery. Here we investigated the effects of VIP and PACAP on chemokine production by activated microglia. VIP and PACAP inhibit the expression of the microglia-derived CXC chemokines MIP-2 and KC, and of the CC chemokines MIP-1alpha, -1beta, MCP-1, and RANTES. The inhibition of chemokine gene expression correlates with an inhibitory effect of VIP/PACAP on NFkB binding. The VIP/PACAP inhibition of both chemokine production and of NFkB binding is mediated through the specific receptor VPAC1 and involves a cAMP-dependent intracellular pathway. Of biological significance is the fact that the inhibition of chemokine production by VIP/PACAP leads to a significant reduction in the chemotactic activity generated by activated microglia for peripheral leukocytes, i.e., neutrophils, macrophages, and lymphocytes. Because reduction in the number and activation of infiltrating leukocytes represents an important factor in the control of inflammation in the CNS, VIP and/or PACAP released by neurons during an inflammatory response could serve as neuronal survival factors by limiting the inflammatory process.

Endomorphins 1 and 2 modulate chemotaxis, phagocytosis and superoxide anion production by microglia

We evaluate the role of endomorphins 1 and 2 on microglial functions. Endomorphins 1 and 2 blocked phagocytosis of Escherichia coli. In addition, both markedly inhibited chemotaxis toward zymosan-activated serum. In contrast, when microglia was preincubated with these endomorphins, followed by incubation with LPS before stimulation with phorbol 12-myristate 13-acetate (PMA) at 200 nM, they potentiated superoxide anion production. Furthermore, when microglia was preincubated with these endomorphins together with PMA at 20 nM, followed by stimulation with PMA at 200 nM, superoxide anion production was potentiated. These results suggest that endomorphins 1 and 2 modulate phagocytosis, chemotaxis and superoxide anion production by microglia.

Chemokines and their receptors in the central nervous system

Chemokines are a family of proteins associated with the trafficking of leukocytes in physiological immune surveillance and inflammatory cell recruitment in host defence. They are classified into four classes based on the positions of key cystiene residues: C, CC, CXC, and CX3C. Chemokines act through both specific and shared receptors that all belong to the superfamily of G-protein-coupled receptors. Besides their well-established role in the immune system, several recent reports have demonstrated that these proteins also play a role in the central nervous system (CNS). In the CNS, chemokines are constitutively expressed by microglial cells, astrocytes, and neurons, and their expression can be increased after induction with inflammatory mediators. Constitutive expression of chemokines and chemokine receptors has been observed in both developing and adult brains, and the role played by these proteins in the normal brain is the object of intense study by many research groups. Chemokines are involved in brain development and in the maintenance of normal brain homeostasis; these proteins play a role in the migration, differentiation, and proliferation of glial and neuronal cells. The chemokine stromal cell-derived factor 1 and its receptor, CXCR4, are essential for life during development, and this ligand-receptor pair has been shown to have a fundamental role in neuron migration during cerebellar formation. Chemokine and chemokine receptor expression can be increased by inflammatory mediators, and this has in turn been associated with several acute and chronic inflammatory conditions. In the CNS, chemokines play an essential role in neuroinflammation as mediators of leukocyte infiltration. Their overexpression has been implicated in different neurological disorders, such as multiple sclerosis, trauma, stroke, Alzheimer's disease, tumor progression, and acquired immunodeficiency syndrome-associated dementia. An emerging area of interest for chemokine action is represented by the communication between the neuroendocrine and the immune system. Chemokines have hormone-like actions, specifically regulating the key host physiopathological responses of fever and appetite. It is now evident that chemokines and their receptors represent a plurifunctional family of proteins whose actions on the CNS are not restricted to neuroinflammation. These molecules constitute crucial regulators of cellular communication in physiological and developmental processes.