Opioid Receptors and Immune System – Essay Example
Opioid Receptors and Immune System – Essay Example
The receptors are of significance for nociception, reinforcement of behaviour and emotional behaviour. The receptors are significant for the regulation of GI function, respiration and immune function. The receptors contribute an integral role based on the opioid drug used and also play a significant role based on other chemically unrelated medicines in the body. Opioid receptors are classified into mu, delta, kappa and NOP receptors (Gendelman, H.E. & Ikezu, T 2008 p.551) Opioid Receptors and Immune System – Essay Example.
Kappa and delta-opioid receptors affect various immune cells like peripheral blood polymorphonuclear leukocytes, lymphocytes and macrophages. On the other hand, KOP and DOP receptors are found in several immune cell types comprising B and T cells, macrophages, lymphocytes and peripheral blood mononuclear cells. Kappa selective agonists and morphine increase KOP receptor symptom in the human lymphocytic cell. The activation of T cells with anti-CD3-epsilon (CD3-e) paves way for increased symptoms of DOP receptors stimulating the cells for enhanced response to opioids either exogenous or endogenous. On using morphine to treat peripheral blood mononuclear cells, it promotes TH2 cell differentiation by increasing the production of IL-4. The analysis indicates that morphine suppresses the immune system. The outcome of the treatment is the failure of the cell-induced immunity that has an important role in enhanced mortality and morbidity subsequent to major surgery or trauma (Gendelman, H.E. & Ikezu, T 2008 p.553).
Opioid agonists generate analgesia by associating with certain G protein-coupled receptors in the spinal cord and brain region responsible for the modulation and transmission of pain. There are three major kinds of opioid receptors namely mu, kappa and delta found in several tissues and nervous system sites. The three receptors are now cloned and are known as members of G protein combined family of receptors that contain amino acid sequence homologies.
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. Opioid Receptors and Immune System – Essay Example. 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.Opioid Receptors and Immune System – Essay Example.
Introduction
Beginning in 1975, a robust literature has accumulated to support the conclusion that opioids modulate immune responses. Overwhelmingly, these studies have shown that opioids are immunosuppressive. However, with the publication of a paper in 2005, the hypothesis was formulated that opioids are pro-inflammatory, and that the resulting opioid-induced inflammation mediates addiction. This paper will critically review the evidence that opioids affect functioning of the immune system, and whether the result of this interaction is immunosuppressive or immuno-stimulating. The review will also address mechanisms by which opioids alter immune function, specifically whether there is a direct effect of these drugs on cells of the immune system, or whether the effects are mediated through activation of the hypothalamic-pituitary-adrenal (HPA) axis, the sympathetic nervous system, or other pathways. These studies are of relevance to public health, as there was, and still is, a strong intersection between intravenous drug use and HIV incidence. A major question in this arena has been whether opioids, through their immunomodulatory effects alter host defense to the virus. Opioid Receptors and Immune System – Essay Example.
Early Discovery That Opioids Are Immunosuppressive
In 1979, a paper was published in the Journal of Immunology by Wybran et al., which had a major impact on our understanding of the physiological effect of opioids on the immune system (1). It was reported that morphine inhibited the rosetting of human peripheral blood T cells with sheep red blood cells (SRBCs). Prior to wide-spread use of flow cytometry, the fortuitous ability of SRBCs to adhere to human T cells, but not B cells, was used as the technique for distinguishing the numbers of these two cell populations in a blood sample. What Wybran observed was that the morphine-induced inhibition of rosetting could be blocked by pretreatment with naloxone, an antagonist at opioid receptors (Figure 1). Further, it was shown that the endogenous opioid peptide, met-enkephalin, enhanced the red blood cell rosetting, a phenomenon which was also blocked by naloxone. The Wybran paper set the stage for our future understanding of a broad class of natural interactions between the neural and immune systems, as aptly described in this quote from the paper, “The findings of receptors on T lymphocytes for drugs and substances known to bind to nervous cells may provide further insight into the relations between the immune system and the central nervous system (CNS). Such links may be of utmost importance in various disease states like slow virus infection, multiple sclerosis, and perhaps neurotic and psychotic disorders where immune mechanisms may be involved.” On the larger scale, these words have come to be prophetic. Of immediate interest was the implication of these observations, namely that leukocytes express opioid receptors. In 1982 the AIDS (Acquired Immunodeficiency Syndrome) epidemic emerged. It was recognized that patients suffered from a severe depression in T cell numbers, but the viral cause was not identified until 1986. At the beginning of the outbreak, epidemiologic evidence showed that one third of AIDS patients were intravenous drug abusers, mainly of heroin (2). Based on the Wybran paper, the question arose as to whether the opioids were in some way mediating the immune suppression observed in AIDS patients. Opioid Receptors and Immune System – Essay Example. The National Institute on Drug Abuse introduced an initiative to fund research into the effect of opioids on the immune system. The papers relating to effects of opioids on immune responses are summarized in Table 1.
Structures of alkaloid agonists and antagonists.
Table 1
Effects of opioids on immune functions.
| Function | Species | References |
|---|---|---|
| Suppression of natural killer cell activity | Mouse in vivo Rat in vivo Human in vivo | (20)*, (21) (10–18) (22) |
| Suppression of cellular responses to mitogens ex vivo | Mouse Rat Human | (20)*, (23, 25) (26–28) (30) |
| Depression of antibody production | Mouse in vivo | (21, 23, 31, 33–35)**, (36–38, 40) |
| Mouse in vitro | (42)***, (43) | |
| Depression of T cell mediated adaptive immune responses | Mouse in vivo | (44, 45) |
| Depression of cellularity | Mouse in vivo | (21, 23, 47) |
| Induction of apoptosis | Mouse in vivo | (50) |
| Mouse in vitro | (50) | |
| Human in vitro | (48, 49) | |
| Inhibition of cell growth | Mouse in vivo | (15, 53, 55) |
| Mouse in vitro | (20, 51) | |
| Human in vitro | (52, 56) | |
| Monkey in vivo | (54) | |
| Suppression of phagocytosis | Mouse in vivo Mouse in vitro | (58, 59) (60–64) |
| Down-regulation of cytokines and other inflammatory associated mediators | Mouse in vivo Mouse in vitro Rats in vivo Human in vitro | (20, 84–86) (34, 75, 79–81, 83, 88) (73, 74, 78, 89) (67, 69–72, 82) |
Opioid Pharmacology as Background
Opioid receptors in the brain were initially discovered using biochemical techniques that demonstrated binding of radiolabeled opioid ligands. Three major receptors were discovered that were designated mu, kappa and delta (3–5). In 1993, these receptors were cloned (6–8). Currently, they are termed mu opioid receptor (MOR), kappa opioid receptor (KOR), and delta opioid receptor (DOR). The endogenous ligands for these receptors are the neuropeptides: β-endorphin (MOR), dynorphin (KOR), and methionine-enkephalin (DOR), although these proteins have significant cross-affinity for the other receptors. Morphine has greatest affinity for the MOR, but it can bind to a lesser degree to the other receptors. The analgesic and psychoactive effects of morphine, as well as the adverse effects of respiratory depression and inhibition of gastric transit, are mediated through the MOR. Unlike the endogenous peptide ligands for the opioid receptors, morphine is an alkaloid (Figure 1), a natural product extracted from the opium poppy. Heroin is synthetic diacetyl-morphine. Whereas, intravenous drug abusers at the time of the AIDS epidemic were mostly injecting heroin, subsequent laboratory studies have mostly employed morphine as the opioid. It is felt that results from morphine are generalizable to heroin because heroin is metabolized by deacetylation to morphine. Practical considerations also underlay decisions to use morphine in the laboratory because it is less lipophilic than heroin, making it easier to dissolve, and it is a Schedule 2 rather than a Schedule 1 drug, making it easier to obtain. Results from morphine were also judged to have wider applicability, as morphine, but not heroin, is used therapeutically in millions of patients. Since the MOR is the major target of both compounds, results obtained with morphine are believed to be directly translatable to heroin abuse, although there is some evidence for unique biologically active heroin metabolites (9). Naloxone and naltrexone are major antagonists of the opioid receptors (Figure 1). Both synthetic compounds bind to all three opioid receptors. Some investigators have also used β-funaltrexamine (β-FNA), another antagonist, which is selective for MOR. Narcan® is the brand name for commercially marketed naloxone.Opioid Receptors and Immune System – Essay Example. It has greater affinity for the MOR than morphine, so it is able to displace morphine from the MOR without inducing receptor signaling. A difficulty in working with morphine is that after only short exposures animals can develop physical dependence to the drug. As the half-life of the drug in mice and rats, as in humans, is in the range of several hours, when it is desired to test the effect of more than a single, acute exposure to the drug, strategies are needed to administer morphine without inducing withdrawal. Withdrawal has myriad physiological effects which certainly could affect immune responses. Approaches to maintain chronic levels of morphine include multiple injections over the course of a day, use of slow-release morphine pellets, or use of osmotic mini-pumps. Slow-release pellets are formulated using a nitrocellulose matrix which dispenses the drug slowly over 7 days. Placebo pellets containing only the matrix, and naltrexone pellets releasing the antagonist, are also available. Pellets are implanted surgically, subcutaneously in a skin pocket beside the spine in a manner similar that used to implant osmotic pumps. A further complication with using morphine and similar opioids is that continued exposure leads to “tolerance” to the analgesic effects of the drugs. Like immunological tolerance, opioid tolerance means that that animal no longer responds to the drug or has a lesser response to a given dose of the opioid. Interestingly, tolerance does not develop to the constipating effects of opioids. A question which has been investigated is whether there is tolerance to immunomodulatory effects of opioids, and this topic is considered later in this review.
Opioids and Suppression of Natural Killer (NK) Cell Activity
Among the earliest studies testing the effect of opioids on immune function were those that examined NK cell activity, a measure of innate immunity. Shavit et al. showed that morphine given to rats by subcutaneous (s.c.) injection for 4 days suppressed NK cell activity in the spleen, and that N-methylmorphine, which does not pass the blood-brain barrier, was inactive (10–12). The latter observation suggested that the effect of morphine was mediated through the neuronal system, rather than by acting directly on immune cells in the periphery. This conclusion was reinforced by a high-profile paper in Science by Weber and Pert which showed that injecting morphine directly into the periaqueductal gray region of the rat brain (which is involved in pain sensing) depressed the NK cell activity in spleen 3 h later (13). Similar injections into 5 other brain regions were without effect. Naltrexone blocked the NK suppressive activity of morphine in both studies. Evidence for involvement of adrenergic and sympathetic neurotransmitters, of glucocorticoids, and of dopaminergic and Peptide Y signaling as mediators of opioid immunosuppression of NK cell activity have been found (14–18). Franchi et al. found that subcutaneous injection of rats with 2 doses of morphine, but, interestingly, not buprenorphine, spaced 5 h apart, reduced splenic NK cell activity (19). Sacerdote et al. reported that morphine given in vivo inhibited NK cell activity of mouse spleen cells ex vivo (20). Further proof that opioid receptors mediate the suppression of NK cells was provided by Gaveriaux-Ruff who found that MOR knock-out (k/o) mice did not respond to morphine with a decrease in NK cell activity (21). Interestingly, studies have also been carried out in humans to test the effect of morphine on NK cell activity. Yeager et al. administered morphine intravenously for 24 h to normal, non-opioid abusing volunteers in the hospital, and obtained NK cells from peripheral blood by venipuncture before administration of the opioid, and 2 and 24 h later. Morphine administration resulted in a significant depression in NK cell activity at both time points compared to baseline (22).Opioid Receptors and Immune System – Essay Example. The studies cited above support the conclusion that morphine suppresses NK cell activity in rats, mice and humans, and that the mechanism of the immunosuppression is through the MOR. However, for suppression of NK cell cytotoxicity the effect of morphine does not appear to be direct, but rather is mediated by signals from the neural system.
Opioids and Suppression of Responses to Mitogens
An early observation about the effect of opioids on immune responses was published from the laboratory of Holaday showing that morphine pellet implantation inhibited the response of mouse spleen cells ex vivo to the T cell mitogen, Concanavalin A (ConA), and to the B cell mitogen, bacterial lipopolysaccharide (LPS) (23). These effects were not evident in mice treated with RU486, an inhibitor of glucocorticoids, or in adrenalectomized mice (24). Thomas et al. (25) also reported that morphine depressed B cell proliferation stimulated by anti-IgM and IL-4. Bayer’s group reported that peripheral blood T cells, harvested 2 h after a subcutaneous (s.c.) injection of rats with morphine, were markedly suppressed in their response to ConA (26). The immunosuppressive effects were not duplicated by N-methyl-morphine, leading to the conclusion that central opioid pathways were involved (27). In contrast to the findings of Holaday using mouse spleen cells from animals implanted with a slow-release pellet, the immunosuppression of rat peripheral blood cells to ConA, induced by a single, acute injection of morphine, was not abolished by adrenalectomy, hypophysectomy, or administration of the glucocorticoid antagonist, RU486 (28). Chlorisondamine, a ganglionic blocker, did inhibit the immunosuppression (29). Govitrapong et al. tested the responses of T cells to phytohemagglutinin (PHA) in peripheral blood of heroin addicts and in addicts in withdrawal from the opioid. In both cases, T cell responses were depressed for up to 2 years (30). Thus, opioids were shown to suppress mitogen responses of T cells in mice, rats, and humans, and of B cells in mice when drugs were given in vivo. Sacerdote et al. reported that morphine, but not hydromorphone, codeine, or oxycodone inhibited the mitogen responses of normal mouse spleen cells when the drugs were given in vivo and spleen cells were tested ex vivo (20). Opioid Receptors and Immune System – Essay Example.
Opioids and Suppression of Antibody Production
Opioids Given in vivo and Immunosuppression
The first paper showing that morphine inhibited antibody responses by mouse spleen cells to SRBCs as the antigen was published in 1975 (31). High doses of morphine (75 mg/kg) were injected one day before injection of SRBCs and for 3 days thereafter. Splenic cells from treated or placebo animals plated in vitro and incubated with an excess of SRBCs and complement revealed the number of B cells secreting antibody to the SRBCs, which in the presence of complement lysed the SRBCs producing visible plaques in the lawn of red blood cells. This method is called the plaque-forming cell (PFC) assay and measures the number of cells secreting IgM anti-SRBC antibodies (32). Bussiere et al. administered morphine to mice using slow-release pellets and also found that spleen cells placed ex vivo 72 h after pellet implantation had markedly depressed PFC responses compared to placebo pelleted animals (33). Simultaneous implantation of a naltrexone pellet with a morphine pellet blocked the immunosuppressive effect of morphine, and naltrexone alone had no effect. Kinetic experiments showed that after morphine pellet implantation, onset of suppression of the PFC antibody response was gradual, reached a maximum at 48 h and dissipated by 120 h, which was interpreted as development of tolerance to the immunosuppression (34). Bryant et al. had found a similar time course for morphine-induced suppression of mitogen responses (23). Minipumps have also been used to dispense opioids selective for MOR, KOR, or DOR to test the effect of different opioid ligands on PFC responses (35). This mechanism of dispensing the drugs allowed testing of agonists for which there are no pellets, and also for carrying out dose-response studies. It was found that morphine sulfate (primarily a mu agonist), U50,488H (a kappa selective agonist) and deltorphin II (a delta2 selective agonist) dispensed for 48 h, each inhibited the ex vivo PFC responses of spleen cells, yielding U-shaped dose-response curves. Co-implantation of mini-pumps dispensing antagonists that were receptor selective (CTAP vs. morphine; nor-binaltrophimine vs. kappa, and naltriben vs. delta2) blocked the immunosuppressive effects of the agonists (35). Morphine pellet administration has also been shown to inhibit secretory IgA responses to cholera toxin in gastrointestinal lavage fluid or produced by ex vivo ileal organ cultures from morphine treated mice (36, 37). Morphine pellets have also been shown to decrease the serum immune response to tetanus toxoid when this antigen was given 72 h post pellet implantation in mice, as assayed quantitatively by ELISA (38). Morphine pellets also suppressed the murine serum antibody titer to trinitrophenylated bovine serum albumin, which was blocked by naltrexone (39). Intravenous morphine administered to rats immediately after injection of keyhole limpet hemocyanin depressed serum antibody responses to this antigen and the effect was inhibited by naltrexone (40). Opioid Receptors and Immune System – Essay Example. The experiments cited above on antibody responses involved administration of morphine in vivo and assessment of antibody responses in vivo or ex vivo. They all support an immunosuppressive consequence of morphine exposure. The Kieffer laboratory definitively showed that the MOR mediated the immunosuppressive effects of morphine by testing NK cell activity, proliferative responses of lymphocytes to ConA and LPS, and immunoglobulin levels in LPS-stimulated culture supernatants in wild-type (WT) and MOR k/o mice (21). WT mice demonstrated immunosuppression to this panel of immune responses, but MOR k/o mice were not suppressed. A lingering question in the field as these results on opioid suppression of antibody responses emerged was whether the immunosuppression was due to a direct effect of the opioid on cells of the immune system or whether the effects were indirect through opioid-induced activation of other physiological systems. Pruett et al. argued that part of the suppression of the humoral immune response was due to activation of the HPA axis (41). As noted above in the section on opioids and NK cells, the sympathetic nervous system, the adrenergic system, dopaminergic pathways and Neuropeptide Y have all been implicated in mediating the effects of morphine. As a counter to this hypothesis is the definitive evidence that opioids added in vitro to immune cells suppress antibody responses in vitro.
Opioids Applied in vitro and Immunosuppression of Antibody Responses
In these experiments, spleen cells were taken from opioid naïve mice and were placed in culture where experimental wells received an opioid and control wells received medium. Other neurotransmitter systems of the body cannot be involved in immunosuppression observed by adding opioids to purified cells of the immune system in vitro. Taub et al. reported that addition of DAMGO, a synthetic peptide that is a selective agonist at the MOR, or U50,488H, that is a kappa selective agonist, resulted in inhibition of the PFC response of mouse spleen cells to SRBCs (42). The inhibition of antibody induction was blocked with naloxone, and also with a kappa selective antagonist, respectively. Eisenstein et al. reported that morphine also produced a dose-dependent inhibition of the PFC response which was naloxone reversible and was mouse strain dependent (43).
Thus, the evidence indicates that there are both direct effects of opioids on immune cells, as well as mediated, indirect effects.
Opioids and Depression of T Cell Mediated Adaptive Immune Responses
Several laboratories have reported that morphine administration in vivo blocks adaptive T cell responses. These include reports by Bryant and Roudebusch showing that morphine pellets depressed development of contact sensitivity to picryl chloride and inhibited a graft vs. host reaction in mice (44). Dafny found that morphine blocked development of delayed-type hypersensitivity in rats to attenuated mycobacteria (45), and Molitor et al. reported inhibition of sensitization to 2,4-dinitrofluorobenzene in pigs (46). Opioid Receptors and Immune System – Essay Example.
Morphine and Depression of Cellularity, Induction of Apoptosis, and Inhibition of Cell Growth
Early studies using morphine slow-release pellets showed that drug administration resulted in splenic and thymus cell atrophy (23, 47). Later investigation of this phenomenon provided evidence that morphine added to human peripheral blood mononuclear cells (PBMCs) (48) or to human monocytes (49) in vitro induced apoptosis, in the monocytes by induction of nitric oxide. In a study of morphine-induced apoptosis, Yin et al. confirmed that morphine administration to mice in vivo resulted in reduced cellularity in the spleen, and showed that the opioid induced Fas in the spleen, heart and lung (50). In vitro studies showed that morphine induced Fas in a T cell hybridoma and in human peripheral blood lymphocytes. Addition of Fas ligand (FasL) triggered apoptosis (50). In another line of investigation, Roy’s laboratory reported that morphine added to murine bone marrow cultures inhibited formation of macrophage colonies in soft agar from precursors (51). Vassou et al. (52) found that morphine inhibited in vitro proliferation of human multiple myeloma cells (B cells). Several investigators have also examined effects of morphine pellets on lymphocytes. Altered ratios of CD4 and CD8 T cells were found in the spleen and thymus of mice (15, 53), as well as in monkeys that received daily injections of morphine for 2 years (54). Zhang et al. carried out a more detailed study of the effect of morphine pellets on lymphocyte subsets in the spleen and lymph nodes of mice 7 days after pellet implantation and found reduced numbers of B cells and CD4 and CD8 T cells (55). B cells that were most vulnerable to morphine depletion were IgM+IgD−. In the bone marrow, the number of B cell precursors was reduced. Naïve, memory, and effector memory CD4 and CD8 T cells were all depleted by morphine in the spleen and lymph nodes. Roy et al. showed that morphine added in vitro to human PBMCs or to mouse spleen cells stimulated with anti-CD3/anti-CD28 polarized the T cells to a Th2 phenotype, as evidenced by up-regulated production of IL-4 and IL-5 and decreased IL-2 and IFN-γ (56). Morphine treatment also increased NFAT binding to response elements in cellular DNA. Opioid Receptors and Immune System – Essay Example. Sacerdote et al. also reported that morphine down-regulated IL-2 production when added to Con A-stimulated mouse spleen cells in vitro (20). These studies provide evidence that morphine depresses numbers of major classes of lymphocytes when given in vivo or when added to cells in vitro. There is still controversy as to whether the in vivo effects are mediated by corticosteroids (57).
Effects of Opioids on Cell Subsets and Cytokines
Opioids Suppress Phagocytosis and Microbicidal Activity of Phagocytes and Enhance Viral Replication
An essential aspect of innate immunity is the ability of the neutrophils and macrophages to ingest and kill microbes. The literature on this subject overwhelmingly indicates that morphine suppresses phagocytosis and microbicidal activity of phagocytes. The first demonstration of this phenomenon was reported by Tubaro et al. who administered morphine to mice by s.c. injection for 3 days and then harvested elicited peritoneal macrophages that were tested ex vivo for capacity to engulf and kill the fungus, Candida albicans. It was shown that morphine depressed phagocytosis and fungicidal activity of macrophages which correlated with reduced production of superoxide anion (58). These results were confirmed in regard to inhibition of phagocytosis of Candida albicans using slow-release morphine pellets and harvesting unelicited peritoneal macrophages 48 h after pellet implantation which were tested for phagocytic capacity against the fungus (59). Naltrexone blocked the morphine-induced suppression. Importantly, in this study as well as many others, the effect of opioids added to phagocytic cells in vitro was tested, and it was found that phagocytosis and microbicidal activity were directly inhibited (59). Szabo et al. also used Candida albicans as the target and showed that compounds selective for mu, kappa and DORs all blocked ingestion of the fungus by mouse peritoneal macrophages treated with the opioid in vitro. Opioid Receptors and Immune System – Essay Example. Further, antagonists specific for these receptors inhibited the effect, definitively demonstrating that the opioids are directly acting on these phagocytic cell through opioid receptors (60). Wang et al. reported that murine alveolar macrophages obtained by broncho-alveolar lavage and treated with morphine in vitro had depressed phagocytosis and killing of Streptococcus pneumoniae (61). A series of papers from the laboratory of Reynaud demonstrated that morphine added in vitro to mouse peritoneal macrophages inhibited the Fc-mediated phagocytosis of sheep red blood coated with antibody (62). In a collaborative study with Roy and Loh, this system was probed with pharmacological precision carrying out dose response curves with selective mu, delta, and kappa agonists and antagonists (63). It was found that mu and delta2 agonists inhibited phagocytosis in a dose-dependent manner. Ninković and Roy, using the J774.1 macrophage cell line, provided a mechanism by which morphine added in vitro could disrupt phagocytosis by inhibiting actin polymerization through inhibition of Rac1-GTPase and p38 mitogen-activated protein kinase (MAPK) (64). Another aspect of the innate immune response is the ability to control viral infections. Peterson et al. reported that morphine added to human promonocytes cultured from human brain tissue exacerbated replication of HIV with an inverted U-shaped dose response curve (65). Reports from the laboratory of Rogers supported potentiation of HIV replication in monocytes by morphine (66). Ho et al. showed that heroin (67) and methadone (68) added to human macrophages in vitro, enhanced HIV replication, also with an inverted U-shaped dose response curve.
Effect of Opioids on Macrophages, Microglia, and Macrophage/Microglial-Derived and T Cell-Derived Cytokines and Other Inflammatory Associated Mediators
Peterson and colleagues reported that in human PBMCs, morphine blocked production of the reactive oxygen intermediates, superoxide and peroxide, that are involved in microbicidal mechanisms of phagocytes in response to opsonized zymosan (69). Interferon-γ and TNF-α were also depressed by morphine (70, 71). Evidence was presented that the immunosuppressive cytokine, transforming growth factor-beta (TGF-β), produced by lymphocytes, was mediating the morphine-induced down-regulation of reactive oxygen intermediates (72). Roy’s laboratory reported that morphine added to murine bone marrow cultures inhibited formation of macrophage colonies in soft agar from precursors (51). Bussiere et al. explored the mechanism of immunosuppression of ex vivo antibody formation by mouse spleen cells induced by morphine pellet implantation. Using co-cultures of normal spleen cells or fractionated spleen cell populations, they showed that addition of adherent, but not non-adherent spleen cells from normal animals restored the PFC responses of spleen cells taken from morphine-treated animals. Opioid Receptors and Immune System – Essay Example. Further, antibody responses could be restored by addition of cytokines produced by macrophages, IL-1β or IL-6, or by the macrophage-activating cytokine, IFN-γ. These results support the conclusion that morphine depressed either macrophage numbers or production of macrophage pro-inflammatory cytokines (34). Wang et al. reported that heroin added to human macrophage cultures inhibited both IFN-α and IFN-β, which are antiviral molecules (67). Fecho et al. using slow-release morphine pellets in rats showed that spleen cells placed ex vivo were inhibited in responses to ConA (73, 74). They concluded that the immunosuppression occurred by production of nitric oxide. This result is somewhat anomalous as nitric oxide is considered a product of activated macrophages, and, as documented above, morphine pellets result in macrophages that are not activated, but are down-regulated as determined by depressed production of pro-inflammatory cytokines. In support of the Fecho observations, Khabbazi et al. (75) found that morphine blocked the transition of murine primary bone marrow macrophages and the RAW264.7 cell line to the alternatively activated state M2 state induced by IL-4. This inhibition occurred by blocking IL-4 mediated induction of matrix metallopeptidase 9 (MMP-9) and arginase-1, markers of M2 macrophages. Since arginase-1 inhibits nitric oxide production associated with M1 macrophages, the effect of morphine would be to favor this activation pathway. Findings from the Borea laboratory support a role for morphine in amplifying LPS-induced activation of primary murine microglia (76, 77). This group activated the microglia in vitro with LPS, and found that subsequent exposure to morphine increased the LPS-mediated production of IL-1β, TNF-α, IL-6 and nitric oxide, and this occurred via activation of PKCε and the Akt pathway upstream of ERK1/2 and inducible nitric oxide synthase (76). Further, low doses of morphine activated NF-κB via PKCε (77). However, morphine alone (without LPS) has no effect in either assay. Other investigators have shown that an incision in the paw of a rat (78) or a mouse (79) induced pro-inflammatory cytokines whose production was unchanged or depressed by morphine. Limiroli et al. reported that acute doses of morphine in vivo suppressed both IL-12 and IL-10 in thioglycolate-elicited peritoneal macrophages placed in culture and stimulated with LPS, with or without IFN-γ (80). The Sacerdote laboratory also found that peritoneal macrophages obtained 1 h after a single, s.c. injection of morphine had reduced levels of IL-1β, TNF-α, and IL-12 in response to stimulation with LPS (81). Long et al. reported that addition of morphine to human monocytes in culture depressed levels of TNF-α, and increased the anti-inflammatory cytokine, IL-10 (82). An additional complication is the observation by Roy et al. that micromolar doses of morphine added to murine peritoneal macrophages inhibited IL-6 and TNF-α, whereas nanomolar doses of the opioid up-regulated these pro-inflammatory cytokines (83). These opposite dose-dependent results correlated with depressed and activated levels of NF-kB. Infection of mice with Acinetobacter baumannii or Streptococcus pneumoniae has been shown to induce the cytokines IL-17 (produced by T cells) and IL-23 (produced by macrophages and dendritic cells). Opioid Receptors and Immune System – Essay Example. Mice implanted with slow-release morphine pellets have depressed levels of these pro-inflammatory cytokines, which correlates with increased susceptibility to these infections (84–86). Other studies have examined production of cytokines produced by T cells. Jessop and Taplits reported that morphine added to mouse spleen cells stimulated with ConA in vitro had reduced production of IL-2 and IL-4 (87). Similarly, Roy et al. showed that mouse thymocytes placed in culture and stimulated with PHA and IL-1β had a dose-dependent reduction in secretion of IL-2 when treated with morphine, that correlated with down-regulation of the transcription activator fos (88). Lysle et al. administered morphine by s.c. injection to rats and showed a dose-dependent suppression of production of IL-2 and IFN-γ by splenocytes placed ex vivo and stimulated with ConA (89). Almost all of these studies showed that morphine down-regulates cytokine production, both cytokines that are produced by macrophages and those that are produced by T cells.
Opioids, Cell Movement, Chemokines, and Chemokine Receptors
Effects of Opioids on Cell Movement
Morphine slow-release pellets have been shown to depress leukocyte sticking and rolling along blood vessels in response to oxidized low-density lipoprotein, as visualized using intravital fluorescence microscopy of cells in dorsal skin-fold windows implanted in mice (90). Such inhibition is anti-inflammatory. In studies examining the anti-tumor effects of morphine, serum taken from mice treated with morphine was found to inhibit in vitro migration of bovine aortic endothelial cells and mouse mammary breast carcinoma cells, and to reduce the in vitro invasion capacity of these tumor cells (91). These effects were attributed to decreased serum levels of metalloproteinase 9 (MMP-9) and increased levels of tissue inhibitor of metalloproteinases 1 and 3/4. In a further study by this group, morphine was shown to inhibit IL-4 driven differentiation of macrophages into the M2 state, which correlated with reduced levels of MMP-9 (75). As the M2 state is usually associated with tumor progression, theses studies provide a mechanism by which morphine can exert an anti-tumor effect. Morphine has also been shown to suppress growth of Lewis Lung Carcinoma cells in muce that correlated with reduced production of VEGF (vascular endothelial growth factor) and reduced blood vessel density, length and branching (92). The effect was found to be due to suppression of the p38 MAPK pathway which inhibited VEGF transcription and secretion. Morphine has also been shown to delay wound healing by inhibiting VEGF synthesis as well as recruitment of neutrophils and monocytes to the site of injury (93). Opioid Receptors and Immune System – Essay Example.
