Several studies demonstrate that opioids modulate the immune response via opioid receptors expressed directly on the immune cells themselves. Recently, it has been suggested that the kappa opioid system has a modulatory role in various inflammatory diseases including rheumatoid arthritis. This modulation may occur via changes in cytokine secretion by monocyte-derived cells. To further study this opioid-immune relationship, we stimulated P388D1 cells, a mouse monocyte-like cell line, with lipopolysaccharide (LPS) in the presence or absence of the kappa opioid-selective ligand, U50,488. Pretreatment with U50,488 significantly reduced LPS-stimulated interleukin-6 (IL-6) production as measured by ELISA. This effect was mediated by the kappa opioid receptor, because nor-binaltorphimine (nor-BNI), a kappa-selective antagonist, blocked this inhibition. It is likely that this reduction of IL-6 protein by U50,488 treatment is attributed to decreases in IL-6 mRNA. RT-PCR experiments demonstrated that U50,488 treatment significantly reduced the LPS-mediated increase in IL-6 mRNA and that this effect was also blocked by nor-BNI. Understanding the mechanism behind the reduction of proinflammatory cytokine production by opioids may lead to the development of more effective therapeutics for inflammatory diseases.
[Show abstract][Hide abstract] ABSTRACT: This review discusses the criteria for determining whether a binding site or functional response is directly mediated by either the mu, delta, or kappa opioid receptors. In 1988, Sibinga and Goldstein published the first review that addressed whether cells from the immune system express opioid receptors. The criteria that they used, namely, structure-activity relationships, stereoselectivity, dose- and concentration-dependence, and saturability are still relevant criteria today for determining if an immunological response is mediated by either the mu, delta or kappa opioid receptors. Radioligand receptor binding studies and functional studies that clearly show the presence of an opioid receptor on immunocytes are presented. Selective agonists and antagonists for the mu, delta, and kappa opioid receptors are discussed, and the need for their use in experiments is emphasized. Conditions used in functional assays are very important. Receptor desensitization and downregulation occur within minutes after the application of an agonist. However, many immunological assays are applying an agonist for days before measuring an immunological effect. The results obtained may reflect changes that are results of receptor desensitization and/or downregulation instead of changes that are observed with acute activation of the receptor. The future of receptor pharmacology lies in the crosstalk and dimerization of G protein-coupled receptors. In transfected systems, opioid receptors have been shown to dimerize with chemokine and cannabinoid receptors, resulting in crosstalk between different types of receptors.
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