Opioid receptor-mediated suppression of humoral immune response in vivo and in vitro: involvement of kappa opioid receptors

Evidence for a dynorphin-mediated inner ear immune/inflammatory response and glutamate-induced neural excitotoxicity: an updated analysis

Acoustic overstimulation (AOS) is defined as the stressful overexposure to high-intensity sounds. AOS is a precipitating factor that leads to a glutamate (GLU)-induced Type I auditory neural excitotoxicity and an activation of an immune/inflammatory/oxidative stress response within the inner ear, often resulting in cochlear hearing loss. The dendrites of the Type I auditory neural neurons that innervate the inner hair cells (IHCs), and respond to the IHC release of the excitatory neurotransmitter GLU, are themselves directly innervated by the dynorphin (DYN)-bearing axon terminals of the descending brain stem lateral olivocochlear (LOC) system. DYNs are known to increase GLU availability, potentiate GLU excitotoxicity, and induce superoxide production. DYNs also increase the production of proinflammatory cytokines by modulating immune/inflammatory signal transduction pathways. Evidence is provided supporting the possibility that the GLU-mediated Type I auditory neural dendritic swelling, inflammation, excitotoxicity, and cochlear hearing loss that follow AOS may be part of a brain stem-activated, DYN-mediated cascade of inflammatory events subsequent to a LOC release of DYNs into the cochlea. In support of a DYN-mediated cascade of events are established investigations linking DYNs to the immune/inflammatory/excitotoxic response in other neural systems.

Dynorphin 1-17 and Its N-Terminal Biotransformation Fragments Modulate Lipopolysaccharide-Stimulated Nuclear Factor-kappa B Nuclear Translocation, Interleukin-1beta and Tumor Necrosis Factor-alpha in Differentiated THP-1 Cells

Dynorphin 1–17, (DYN 1–17) opioid peptide produces antinociception following binding to the kappa-opioid peptide (KOP) receptor. Upon synthesis and release in inflamed tissues by immune cells, DYN 1–17 undergoes rapid biotransformation and yields a unique set of opioid and non-opioid fragments. Some of these major fragments possess a role in immunomodulation, suggesting that opioid-targeted therapeutics may be effective in diminishing the severity of inflammatory disorders. This study aimed to examine the immunomodulatory effects of DYN 1–17 and major N-terminal fragments found in the inflammatory environment on nuclear factor-kappaB/p65 (NF-κB/p65) nuclear translocation and the release of interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α) from lipopolysaccharide (LPS)-stimulated, differentiated THP-1 cells. The results demonstrate that NF-κB/p65 nuclear translocation was significantly attenuated following treatment with DYN 1–17 and a specific range of fragments, with the greatest reduction observed with DYN 1–7 at a low concentration (10 nM). Antagonism with a selective KOP receptor antagonist, ML-190, significantly reversed the inhibitory effects of DYN 1–17, DYN 1–6, DYN 1–7 and DYN 1–9, but not other DYN 1–17 N-terminal fragments (DYN 1–10 and 1–11) on NF-κB/p65 nuclear translocation. DYN 1–17 and selected fragments demonstrated differential modulation on the release of IL-1β and TNF-α with significant inhibition observed with DYN 1–7 at low concentrations (1 nM and 10 pM). These effects were blocked by ML-190, suggesting a KOP receptor-mediated pathway. The results demonstrate that DYN 1–17 and certain N-terminal fragments, produced in an inflamed environment, play an anti-inflammatory role by inhibiting NF-κB/p65 translocation and the subsequent cytokine release through KOP receptor-dependent and independent pathways.

Crucial role of peripheral kappa-opioid receptors in a model of periodontal disease in rats

BACKGROUND AND OBJECTIVE

Periodontal disease is a chronic inflammatory condition of the tooth supporting tissues, the periodontium. Opioids have been shown to account for the relief of various chronic and acute inflammatory conditions. The aim of the present study was to investigate the participation of peripheral opioid receptors in development of periodontal disease.

MATERIAL AND METHODS

Morphine and selective agonists and antagonists of opioid receptors were used in an experimental model of ligature-induced periodontal disease in rats. To evaluate the development of disease, the loss of fiber attachment, alveolar bone and number of cells in periodontal tissues were assessed. Measurements of these indicators were obtained by morphometric analysis of histological sections of periodontal-diseased tissues stained with hematoxylin and eosin.

RESULTS

Local administration of either morphine or a selective kappa-opioid agonist for three consecutive days from the onset of periodontal disease reduced the loss of periodontal tissues, without changing the number of leukocytes in inflamed periodontium. Nor-binaltorphimine, a selective kappa-antagonist, reversed the beneficial effects of both morphine and the compound U-50,488 in this model. The use of either an agonist or an antagonist of delta-opioid receptors, however, did not affect disease progression.

CONCLUSION

Our results showed that the beneficial effect of opioids in periodontal disease depended mainly on the activation of specific kappa-opioid receptors located in the periphery. Activation of such receptors could be considered in the management of periodontal disease, since it would not present the classical central side-effects associated with opioid use.

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.

Effects of mu, kappa or delta opioids administered by pellet or pump on oral Salmonella infection and gastrointestinal transit

Our laboratory has shown previously that subcutaneously implanted, slow-release morphine pellets markedly enhanced susceptibility to oral infection with Salmonella typhimurium. Further, morphine, kappa and delta opioid receptor agonists infused via osmotic minipumps were immunosuppressive. The present study compared morphine pellets to morphine pumps and also examined the differential effects of morphine versus U50,488H (kappa agonist), deltorphin II (delta2 agonist), and (D-Pen2, D-Pen5)-enkephalin (DPDPE, delta1 agonist), administered via Alzet minipumps, on oral Salmonella infection and on gastrointestinal transit. The results show that all morphine-pelleted mice (26/26) had a marked increase in Salmonella burden in the Peyer's Patches, mesenteric lymph nodes and spleen. In comparison, only 8/20 mice receiving morphine by minipump at doses ranging from 1 to 25 mg/kg/day had any culturable Salmonella in their organs and the number of bacteria was very low. The level of Salmonella colonization correlated with blood morphine levels and gut transit measured using an intragastric charcoal meal. Morphine pellets inhibited gut transit by 38%, while mice receiving morphine by minipump at doses of 1 to 25 mg/kg/day showed only a dose-dependent 7% to 17% inhibition. Mice receiving various doses of U50,488H or DPDPE had no culturable Salmonella in the three sites. Deltorphin II given by minipump resulted in a moderate level of Salmonella in the spleen. Deltorphin II and U50,488H (0.1 to 10 mg/kg/day) did not suppress gut transit. The present studies indicate that a predominantly mu opioid receptor agonist, morphine, given by slow-release pellet, potentiated Salmonella infection and inhibited gastrointestinal transit. In contrast, morphine in pumps slightly inhibited intestinal transit, but did not sensitize to Salmonella infection. A delta1 opioid receptor agonist did not sensitize to infection, and a delta2 and a kappa opioid receptor agonist had minimal effects on either parameter.

Opioid therapy and immunosuppression: a review

The idea that opioids modulate the immune system is not new. By the late 19th century, Cantacuzene, used morphine to suppress cellular immunity and lower the resistance of guinea pigs to bacterial infection. While exogenous opioids mediate immunosuppression, endogenous opiates exert opposite actions. Acute and chronic opioid administration is known to have inhibitory effects on humoral and cellular immune responses including antibody production, natural killer cell activity, cytokine expression, and phagocytic activity. Opiates behave like cytokines, modulating the immune response by interaction with their receptors in the central nervous system and in the periphery. Potential mechanisms by which central opiates modulate peripheral immune functions may involve both the hypothalamic-pituitary-adrenal axis and the autonomic nervous system. The presence of opioid receptors outside the central nervous system is increasingly recognized. Those receptors have been identified not only in peripheral nerves but also in immune inflammatory cells. The immunosuppression mediated by opiates may explain the increased incidence of infection in heroin addicts. Opiates may also promote immunodeficiency virus infection by decreasing the secretion of alpha and beta chemokines (important inhibitory cytokines for the expression of HIV) and at the same time increasing the expression of chemoreceptors CCR5 and CCR3, coreceptors for the virus. The fact that peripheral immunosupression is mediated at least in part by opioid receptors located in the central nervous system and that intrathecally administered opioids do not exert the same immunosuppressive effects may have important clinical implications for those patients receiving long-term opioid therapy for malignant and nonmalignant pain.

Opioid-induced immunosuppression: is it centrally mediated or peripherally mediated?

Opioid compounds are commonly used pain medications. However, their administration is associated with a number of side-effects. Among them, opioid-induced immunosuppression is a significant medical problem, which is evidenced by a strong association between the use of opioids and exacerbated infections, including AIDS. Research data have demonstrated the effects of opioids to be suppressive on phagocytic, natural killer (NK), B and T cells. However, these immunosuppressive effects may be mediated by mechanisms different from those for antinociceptive actions. This article reviews possible central and peripheral mechanisms of opioid-induced immunosuppression. To the extent that peripherally mediated immunosuppressive effects play a significant role in opioid-induced immunosuppression, novel peripheral opioid antagonists may have a therapeutic role in attenuating opioid-induced immunosuppression without affecting analgesia.

Enhanced humoral response in kappa-opioid receptor knockout mice

Opiates are major analgesics and addictive drugs described also as immunomodulators. Here, we investigated the contribution of kappa-opioid receptor (KOR) activity in immunity in vivo by studying immune responses in KOR knockout mice. These animals displayed a modest reduction in thymus cellularity and CD4(+) cell ratio, parallel to a slight increase in immature CD4(+)CD8(+) lymphocytes. In spleen, KOR null animals showed augmented cell number with no change in cell distribution. T and B lymphocyte proliferative capabilities in vitro, Natural Killer activity and steady-state Ig levels were unchanged in KOR-/- mice. We immunized the mice with the antigen keyhole limpet hemocyanin (KLH). Compared to wild-type (WT) mice, KOR-/- animals produced significant higher levels of antigen-specific total Ig, IgM, IgG1 and IgG2a antibodies. This enhancement of humoral activity was not observed in mu-opioid receptor and delta-opioid receptor knockout animals. These results show that endogenous activation of kappa-opioid receptors may exert a tonic inhibition of antibody (Ab) response.

Exploring the opioid system by gene knockout

The endogenous opioid system consists of three opioid peptide precursor genes encoding enkephalins (preproenkephalin, Penk), dynorphins (preprodynorphin, Pdyn) and beta-endorphin (betaend), proopiomelanocortin (POMC) and three receptor genes encoding mu-opiod receptor (MOR), delta-opiod receptor (DOR) and kappa-opiod receptor (KOR). In the past years, all six genes have been inactivated in mice by homologous recombination. The analysis of spontaneous behavior in mutant mice has demonstrated significant and distinct roles of each gene in modulating locomotion, pain perception and emotional behaviors. The observation of opposing phenotypes of MOR- and DOR-deficient mice in several behaviors highlights unexpected roles for DOR to be further explored genetically and using more specific delta compounds. The analysis of responses of mutant mice to exogenous opiates has definitely clarified the essential role of MOR in both morphine analgesia and addiction, and demonstrated that DOR and KOR remain promising targets for pain treatment. These studies also show that prototypic DOR agonists partially require MOR for their biological activity and provide some support for the postulated mu-delta interactions in vivo. Finally, data confirm and define a role for several genes of the opioid system in responses to other drugs of abuse, and the triple opioid receptor knockout mutant allows exploring non-classical opioid pharmacology. In summary, the study of null mutant mice has extended our previous knowledge of the opioid system by identifying the molecular players in opioid pharmacology and physiology. Future studies should involve parallel behavioral analysis of mice lacking receptors and peptides and will benefit from more sophisticated gene targeting approaches, including site-directed and anatomically-restricted mutations.

The kappa-opioid agonist, asimadoline, alters cytokine gene expression in adjuvant arthritis

OBJECTIVE

We have previously found that the kappa-opioid agonist, asimadoline, attenuates adjuvant arthritis in a dose-dependent, antagonist-reversible manner. To elucidate possible mechanisms, we investigated the effects of asimadoline (5 mg/kg/day i.p.) or vehicle on in vivo cytokine expression and T-cell recruitment in adjuvant arthritis.

METHODS

Arthritis severity was assessed every 3-4 days for 21 days. Rats were killed on days 0, 13 and 21 post-induction and synovial membrane and inguinal lymph nodes were removed for mRNA extraction. Changes in cytokine mRNA expression were measured using reverse transcription-polymerase chain reaction (RT-PCR) and densitometry. T cells in joints were quantified by immunohistochemistry.

RESULTS

Asimadoline significantly decreased arthritis severity at day 13, with a concomitant decrease in synovial membrane expression of cytokines interleukin-17 and transforming growth factor-beta (TGF-beta) mRNA at day 13, and no change in T cell numbers in the joints of arthritic rats. By contrast, in the inguinal lymph nodes, expression of tumour necrosis factor was increased at day 13 and TGF-beta mRNA was increased throughout.

CONCLUSION

An altered balance, therefore, in the pro- and anti-inflammatory effects of TGF-beta by asimadoline might explain its striking anti-arthritic actions.

Chronic opioid treatment of the mouse thymoma cell lines R1.G1 and R1EGO leads to down-regulation of the kappa opioid receptor without desensitization of adenylyl cyclase activity

Kappa opioid agonists alter some immune functions of macrophages, and T- and B-lymphocytes. The mouse thymoma cell lines R1.G1 and R1EGO express only kappa-opioid receptors and these kappa-opioid receptors are coupled to an inhibitory GTP-binding regulatory protein. Binding of kappa-opioid agonists to the opioid receptor leads to the inhibition of adenylyl cyclase activity in these cells. In this study, an acute (15 min) and chronic (24 h) treatment of R1.G1 and R1EGO cell with a potent kappa-opioid agonist (-)U50,488 (100 nM) was studied to determine if a kappa-opioid agonist altered receptor number and/or desensitization of adenylyl cyclase activity in these two cell lines. Chronic treatment of both R1.G1 and R1EGO cells with (-)U50,488 lead to down-regulation of the kappa-opioid receptor, measured as a decrease of approximately 50% in the Bmax value for the binding of [3H]U69,593. The binding affinity (Kd value) was not affected after chronic treatment either in R1.G1 or R1EGO cells. There was no difference in the magnitude of inhibition of adenylyl cyclase activity by (-)U50,488 between the acute (15 min) and chronic (24-h) treatment in both cell lines R1.G1 and R1EGO. This study indicates that chronic opioid treatment of mouse thymoma R1.G1 and R1EGO cell lines leads to down-regulation of the receptor, without desensitization. This phenomenon was observed in R1.1 parent mouse thymoma cell line and recently in CHO cells expressing kappa-opioid receptor. This study demonstrates that unlike some neuronal preparations, chronic opioid treatment of the thymoma cell lines resulted in receptor down-regulation without desensitization.

Modulation of humoral immune responses in the rat by centrally applied Met-Enk and opioid receptor antagonists: functional interactions of brain OP1, OP2 and OP3 receptors

We have previously demonstrated that central application of leucine-enkephalin (Leu-Enk) elicits potentiation and suppression of humoral immune responses through OP(1) (delta) and OP(2) (kappa) receptors, respectively. Interestingly, both effects were found to be additionally dependent on OP(3) (mu) receptor function. In the present study, we have further investigated whether opioid receptor interactions underlie the immunomodulatory effects of endogenous opioids as well as exogenously applied methionine-enkephalin (Met-Enk). For that purpose, the plaque-forming cell (PFC) response was determined in rats injected intracerebroventricularly (i.c.v.) with opioid receptor-selective antagonists and Met-Enk. Application of the OP(1) antagonist ICI 174864, but not naltrindole, resulted in suppression of the PFC response. In contrast, i.c.v. injection of the OP(2) selective antagonist nor-binaltorphimine (nor-BNI) significantly potentiated the PFC response. Both effects, presumably mediated by endogenous opioid peptides, were antagonized by the OP(3) receptor antagonist beta-funaltrexamine (beta-FNA) at a dose that was devoid of immunomodulatory activity. The immunopotentiation of the PFC response induced by Met-Enk was reversed by OP(1) receptor antagonists, naltrindole and ICI 174864, but not by beta-FNA or nor-BNI. On the basis of these and previous findings, it may be concluded that central OP(3) receptors are permissive for the central immunomodulatory action of endogenous opioid peptides and Leu-Enk. In contrast, the central immunoenhancing effect of Met-Enk appears to be mediated through OP(3)-independent OP(1) receptors.

Kappa-opioid regulation of thymocyte IL-7 receptor and C-C chemokine receptor 2 expression

Endogenous and exogenous kappa-opioid agonists have been widely reported to modulate the immune response. We have published results that show that the superantigen-induced proliferative response of thymocytes is inhibited by the selective kappa-opioid agonist trans-3, 4-dichloro-N-methyl-N-[2-(1-pyrolidinyl)cyclohexyl] benzeneaceamide methanesulfonate (U50,488H). Previous work has established that the kappa-opioid receptor is widely expressed within the thymus; however, little is known about the role of the kappa-opioid receptor in the function of thymocytes. In the present report, we have examined the impact of U50,488H administration on the expression of cytokines in superantigen-stimulated thymocytes by RNase protection analysis. We have measured detectable levels of the cytokines IL-2, IL-4, IL-5, IL-13, and IFN-gamma, and the chemokines lymphotactin and RANTES, in stimulated thymocyte cultures; however, addition of U50,488H did not alter the expression of these cytokines. Examination of cytokine receptor expression by these thymocytes revealed a significant inhibition in the expression of the transcript for the IL-7 receptor alpha-chain (IL-7Ralpha), and these results were confirmed by flow cytometry. Surprisingly, the expression of several other cytokine receptor chains including the common gamma-chain, IL-2Rbeta, or the IL-2Ralpha, IL-4Ralpha, and IL-15Ralpha chains, was not altered. In contrast to these results, a significant elevation in the expression of the chemokine receptor CCR2 was observed in U50,488H-treated cultures. These results suggest that the kappa-opioid receptor may function to promote cellular migration at the expense of the sensitivity to the growth-promoting/maturation activity of IL-7.

Endomorphin-1 potentiates HIV-1 expression in human brain cell cultures: implication of an atypical mu-opioid receptor

Endogneous delta and kappa opioid peptides possess a variety of immunomodulatory properties, and kappa-opioid receptor ligands recently were shown to suppress the expression of human immunodeficiency virus type 1 (HIV-1) in microglial cells, the resident macrophages of the brain. To determine whether the newly discovered endogenous mu-opioid receptor ligands endomorphin-1 and -2 would affect HIV-1 replication, these peptides were added to acutely infected brain cell cultures. Endomorphin-1 potentiated viral expression, in a bell-shaped dose-response manner with maximal enhancement approximately equal to 35% at 10(-10) M, in both mixed glial/neuronal cell and purified microglial cell cultures. Endomorphin-1's amplifying effect was blocked by pretreatment of brain cells with either the mu-opioid receptor selective antagonist beta-funaltrexamine or the G protein inhibitor pertussis toxin. However, the classical mu receptor agonists morphine and DAMGO (Tyr-d-Ala-Gly-N-Me-Phe-Gly-ol) had no effect on viral expression or on endomorphin-1's amplifying effect. Taken together, these findings suggest that in this in vitro model of HIV-1 brain infection, endomorphin-1 potentiates viral expression via activation of an atypical mu-selective opioid receptor. They also provide evidence, for the first time, that an endogenous mu-opioid peptide has neuroimmunomodulatory activity.

Brain NPY Y1 receptors rapidly mediate the behavioral response to novelty and a compartment-specific modulation of granulocyte function in blood and spleen

Neuropeptide Y (NPY) alters behavioral activity and innate immune functions of rats within minutes of intracerebroventricular (i.c.v.) application. Using combinations of the Y1-5a,b(6) agonist NPY, the Y1,3,5 agonist [Leu31-Pro34]NPY (LP-NPY), and the selective Y1 antagonist BIBP3226 (BIBP), we investigated whether the NPY-Y1 receptor (Y1R) subtype regulates NPY-induced behavioral and immunological effects at 15 min after i.c.v. application. Administration of both NPY and LP-NPY decreased rearing activity in the open field and suppressed granulocyte function in the blood. These effects were blocked by BIBP pre-treatment. In contrast to the blood, NPY and BIBP+NPY treatments stimulated granulocyte function within the splenic compartment. In addition, a blood leukophilia composed of granulocytes and NK cells was induced by NPY only. We conclude that the tested early effects of NPY are mediated by either the Y1R (rearing, blood granulocyte function), or a non-Y1R (splenic granulocyte function), or by a combined receptor activation (leukocyte mobilization). Furthermore, the immunological effects of NPY demonstrate compartment specificity.

Modulation of innate immune functions by intracerebroventricularly applied neuropeptide Y: dose and time dependent effects

Centrally applied neuropeptide Y (NPY) interacts with the autonomic nervous system and the hypothalamo-pituitary-adrenal (HPA) axis activity. Since these physiological systems have been shown to modulate innate immune functions, the effects of intracerebroventricular (i.c.v.) NPY administration on leukocyte subsets in the blood, spleen and intravascular pool of the lung, blood granulocyte chemiluminescence response, and splenic natural killer (NK) cell-mediated lysis were studied in Lewis rats. Concentration-dependent NPY effects were tested at 15 min and 24 h post i.c.v. injection at dosages of 10(-6) M, 10(-9) M, and 10(-12) M. Time dependent effects were investigated at 15 min, 1 h and 24 h after i.c.v. administration of 10(-9) M NPY. Compared to saline controls, an increased number of granulocytes and NK cells in the blood, associated with a decreased granulocyte function and NK cytotoxicity was observed 15 min following NPY infusion. This initial immunosuppression was followed by long lasting stimulatory effects of NPY on the functional capacity of both cell populations when tested at 1 h and 24 h. The dosage of i.c.v. 10(-6) M NPY produced no changes, whilst 10(-9) M produced maximal, and 10(-12) M still significant effects. Results provide evidence that centrally applied NPY influences innate immunity in a dose and time dependent fashion. Cell mobilization from the vascular marginal pool is likely to be an underlying mechanism for the initial immunosuppression.

Opioid modulation of immune responses: effects on phagocyte and lymphoid cell populations

The literature describing effects of morphine on cells of the immune system points to the clear conclusion that morphine given in vivo suppresses a variety of immune responses that involve the major cell types in the immune system, including natural killer (NK) cells, T cells, B cells, macrophages and polymorphonuclear leukocytes (PMNs). Depression of NK cell activity has been reported in humans, monkeys and rodents. Similarly, responses of T cells are depressed by morphine, as assessed by inhibition of induction of delayed-type hypersensitivity reactions and cytotoxic T-cell activity, modulation of T-cell antigen expression, and depression of responses to T-cell mitogens. Effects on T cells have been reported in humans, monkeys and rodents. Effects of morphine on B-cell activity have mainly been tested in rodents using assays of antibody formation, which also require macrophages and T cells, preventing a conclusion as to the cell type being affected. Consistent effects on phagocytic cell function have been reported in rodents given morphine. In contrast, studies on immunomodulatory effects of morphine added to cells of the immune system in vitro have shown robust effects on some of these cell types, but not others. There is a rich literature demonstrating downregulation of phagocytic cell function by morphine, particularly for human peripheral blood mononuclear cells (PBMCs) and PMNs. Phagocytosis, chemotactic responses, interleukin production, and generation of activated oxygen intermediates and arachidonic acid products have all been reported to be inhibited. On the contrary, the literature does not support direct effects of morphine on NK cell function, is inconclusive concerning effects on B cells, and provides limited evidence for effects on T cells. The divergence between the in vivo and in vitro data suggests that effects on some cells in the immune system observed after in vivo morphine are probably not direct, but mediated. In aggregate, the literature supports the existence of an in vivo neural-immune circuit through which morphine acts to depress the function of all cells of the immune system. Further, there is strong evidence that morphine can directly depress the function of macrophages and PMNs, and modulate expression of one type of T-cell surface marker. There is, however, little evidence for direct effects of morphine on NK cells and B cells. A further complication emerges from reports of immunopotentiation of immune function in in vitro assays using endogenous opioids. The possibility of different receptors for endogenous and exogenous opioids or of interactions among the activated opioid receptors may account for these opposing effects.

Evidence for central opioid receptors in the immunomodulatory effects of morphine: review of potential mechanism(s) of action

This review will discuss studies demonstrating that activation of opioid receptors within the central nervous system alters various immune system parameters. Specifically, natural killer cell cytolytic activity and lymphocyte proliferative responses to mitogen appear to be modulated predominantly, if not exclusively, through central opioid receptors. The potential mechanisms by which central opioid receptors appear to modulate these peripheral immune functions will be examined by evaluating the role of both the hypothalamic-pituitary-adrenal (HPA) axis and the autonomic nervous system. The studies discussed below indicate that acute administration of morphine or related compounds appears to primarily alter peripheral immune function through the sympathetic nervous system, while more prolonged exposure to opioids alter the immune system predominantly by activation of the HPA axis. Finally, the potential clinical relevance of these observations are discussed in relationship to both the therapeutic use, as well as the abuse of opioid compounds.

Opioid peptides mediate heat stress-induced immunosuppression during pregnancy

To clarify the involvement of the opioid system in enhanced immunosuppression induced by heat stress during pregnancy, we examined the effects of heat exposure and intraperitoneal administration of opioid receptor antagonist naloxone on beta-endorphin (beta-EP) in blood, pituitary lobes, and placenta as well as splenic natural killer cell activity (NKCA) and placental steroids in pregnant rats at 15-16 days gestation. Two-way analysis of variance revealed significant increases in blood beta-EP induced by heat and naloxone and a significant interaction between heat and naloxone on blood beta-EP and progesterone (P). Whereas heat reduced NKCA, intraperitoneal administration of naloxone reversed it. Significant increases in blood and placental beta-EP induced by both heat and naloxone administration and a significant interaction on blood and placental beta-EP was observed. These results suggest that immunosuppression produced by heat stress during pregnancy is mediated by the opioid system. A positive correlation between beta-EP in blood and placenta during heat and naloxone administration suggests that increased placental beta-EP during heat results in hypersecretion of beta-EP into blood. P increased by heat during pregnancy may be involved in the immunosuppression.

Opioid peptides inhibit the estradiol-induced proliferation of cultured rat uterine cells

The effect of opioid peptides on estradiol-induced cell proliferation in adult rat uterine primary cell cultures was studied. Estradiol increased cell density by 40%. This estradiol-induced stimulation of cell proliferation was decreased to control values by [D-Met2,Pro5]enkephalinamide. The opioid-induced inhibition of uterine cell proliferation was blocked completely by the specific opiate antagonist naloxone, while naloxone did not have any effect on its own. The inhibition of cell proliferation by enkephalinamide was apparent at each stimulatory estradiol concentration examined. This opioid effect was mediated mainly by the mu opiate receptor. The observed effects occurred within the physiological nanomolar concentration range. Enkephalinamide did not have any effect on the basal proliferation rate of adult rat uterine cells. However, enkephalinamide inhibited the basal rate of cell proliferation in cell cultures prepared from 7-day-old immature rats. In summary, here we present evidence of novel physiological direct cross-talk between the opioid and estrogenic signaling systems in the regulation of normal uterine growth.

Suppression of adjuvant arthritis by kappa-opioid receptor agonist: effect of route of administration and strain differences

It is well established that kappa-opioid receptor agonists exert antiinflammatory and antihyperalgesic effects during nonspecific inflammation as well as suppressive effects on the development of humoral and cell-mediated immune responses to foreign antigens. The aim of this study was to investigate the ability of the kappa-opioid receptor agonist MR 2034 to modulate adjuvant arthritis in the rat. In the first series of experiments, treatments of Wistar rats were performed using several routes of drug administration: intraperitoneal (ip), intracaudal (ic), intracerebroventricular (icv) and intraplantar (ipl). MR 2034 significantly suppressed joint swelling after ip and ic treatment, slightly reduced inflammation after ipl treatment, and did not produce any effect after icv treatment. In the second series of experiments, the suppressive effect of ip injected MR 2034 was investigated using Wistar, Dark August (DA) and Lewis rats. In Wistar rats, MR 2034 significantly decreased the incidence of adjuvant arthritis, and suppressed mean joint score and aggregate joint score. Similarly, in DA rats treated with MR 2034, mean arthritic score was significantly suppressed, but other clinical parameters were not affected. In Lewis rats, however, ip treatment with MR 2034 failed to produce any suppressive effect on joint disease and even potentiated the initial development of arthritis. These data suggest that immunosuppressive and antiinflammatory action of MR 2034 markedly depend on the route of drug administration and strain susceptibility to opioids.

Mirfentanil antagonizes morphine-induced suppression of splenic NK activity in mice

Mirfentanil [N-(2-pyrazinyl)-N-(1-phenethyl-4-piperidinyl)-2-furamide] was studied for its antinociceptive and immunomodulatory effects in mice Mirfentanil (1.0-32.0 mg/kg) increased tail-flick latency to a thermal stimulus and this effect was antagonized (94 +/- 2%) by naltrexone (10.0 mg/kg). Unlike naltrexone, the delta opioid selective antagonist naltrindole (20.0 mg/kg) had no effect on mirfentanil-induced analgesia. In a dose-dependent fashion, the mu-selective antagonists beta-funaltrexamine (1.0-40.0 mg/kg) and naloxonazine (1.0-35.0 mg/kg) blocked mirfentanil (10.0 mg/kg)-induced analgesia up to 75% of the maximum analgesic effect. Norbinaltorphimine (10.0 mg/kg) partially blocked (35%) the maximum analgesic effect following mirfentanil (10.0 mg/kg) administration. Single doses of mirfentanil (0.1-32.0 mg/kg) had no effect on splenic NK activity. However, preadministration of mirfentanil (1.0-10.0 mg/kg) blocked morphine-induced suppression of splenic NK activity. Collectively, the results suggest that mirfentanil is a novel opioid that induces antinociception predominately through mu opioid receptors but, unlike morphine or fentanyl, does not suppress splenic NK activity.

kappa opioid receptors in human microglia downregulate human immunodeficiency virus 1 expression

Microglial cells, the resident macrophages of the brain, play an important role in the neuropathogenesis of human immunodeficiency virus type 1 (HIV-1), and recent studies suggest that opioid peptides regulate the function of macrophages from somatic tissues. We report herein the presence of kappa opioid receptors (KORs) in human fetal microglia and inhibition of HIV-1 expression in acutely infected microglial cell cultures treated with KOR ligands. Using reverse transcriptase-polymerase chain reaction and sequencing analyses, we found that mRNA for the KOR was constitutively expressed in microglia and determined that the nucleotide sequence of the open reading frame was identical to that of the human brain KOR gene. The expression of KOR in microglial cells was confirmed by membrane binding of [3H]U69,593, a kappa-selective ligand, and by indirect immunofluorescence. Treatment of microglial cell cultures with U50,488 or U69,593 resulted in a dose-dependent inhibition of expression of the monocytotropic HIV-1 SF162 strain. This antiviral effect of the kappa ligands was blocked by the specific KOR antagonist, nor-binaltrophimine. These findings suggest that kappa opioid agonists have immunomodulatory activity in the brain, and that these compounds could have potential in the treatment of HIV-1-associated encephalopathy.

Modulation of humoral immune response by central administration of leucine-enkephalin: effects of mu, delta and kappa opioid receptor antagonists

The effect of leucine-enkephalin (Leu-Enk) on primary humoral immune response was investigated following intracerebroventricular (i.c.v.) administration of the peptide in the rat. Leu-Enk stimulated plaque-forming cell (PFC) response in rats i.c.v. injected with 0.1 and 1 micrograms/kg, whereas does of 20 and 50 micrograms/kg exerted immunosuppressive effects. I.c.v. treatment of rats with delta opioid receptor antagonist ICI 174,864 and kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI) blocked stimulation and suppression of PFC response induced by Leu-Enk, respectively. The mu opioid receptor antagonist beta-funaltrexamine (beta-FNA) reversed both immunomodulatory effects produced by Leu-Enk. Since beta-FNA alone had no effect on PFC response (unlike ICI 174,864 and nor-BNI), these data showed that central effects of Leu-Enk on PFC response were mediated by brain mu opioid receptors, and suggested a possible involvement of delta and kappa opioid receptors.

Differential expression of opioid receptor genes in human lymphoid cell lines and peripheral blood lymphocytes

The existence of receptors for opioid compounds on cells of the immune system has long been hypothesized, but has been very difficult to demonstrate unequivocally. We have used reverse-transcription polymerase chain reaction to obtain cDNA clones from the human MOLT-4 and CEM-3 T-leukemic cell lines which are nearly identical to portions of the delta and kappa opioid receptor cDNAs recently isolated from human brain and placenta, respectively. Northern analyses with riboprobes derived from the delta and kappa opioid receptor clones indicate these sequences are expressed at low levels in human peripheral blood lymphocytes and in several human lymphoid cell lines. Sequences corresponding to the mu opioid receptor cDNA were not detected in this study. The results suggest that delta and kappa opioid receptors may be responsible for mediating some direct effects of opioids in immune cells.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. The specific topics covered this year include stress: tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Sequence of kappa-opioid receptor cDNA in the R1.1 thymoma cell line

To our knowledge, this is the first demonstration of kappa-opioid receptor mRNA in cells of the immune system. While the presence of opioid receptors on cells of the immune system has been controversial, cell-binding analysis has indicated that the kappa-opioid receptor is expressed by the immature T cell line R1.1. We have developed a reverse transcriptase-polymerase chain reaction protocol to amplify the mRNA extracted from R1.1 cells with primers derived from the cDNA sequence of the mouse kappa-opioid receptor. Nucleotide sequences of the amplified products were examined and two populations of cDNA were detected which differ in the 5' region upstream of the ATG start codon. Comparison of these sequences to the previously published kappa-opioid receptor cDNA sequence suggests the presence of an intron-exon junction in the 5' non-coding region.

Effect of Met-enkephalin and opioid antagonists on rat macrophages

Effects of Met-enkephalin (Met-ENK) and opioid antagonists on H2O2 release by peritoneal macrophages from DA and AO rats were investigated. Met-ENK increased and decreased H2O2 production by macrophages of DA and AO rats, respectively. These effects were antagonized by low, but not high, concentrations of naloxone and ICI 174864. High concentrations of both antagonists directly modulated H2O2 release and retained the strain-related differences seen with Met-ENK. The results showed direct, strain- and dose-dependent, effects of Met-ENK, naloxone, and ICI 174864 on rat macrophage function.