Endomorphin 1 and endomorphin 2 suppress in vitro antibody formation at ultra-low concentrations: anti-peptide antibodies but not opioid antagonists block the activity

Interactions Between Endogenous Opioids and the Immune System

The endogenous opioid system, which consists of opioid receptors and their ligands, is widely expressed in the nervous system and also found in the immune system. As a part of the body's defense machinery, the immune system is heavily regulated by endogenous opioid peptides. Many types of immune cells, including macrophages, dendritic cells, neutrophils, and lymphocytes are influenced by endogenous opioids, which affect cell activation, differentiation, proliferation, apoptosis, phagocytosis, and cytokine production. Additionally, immune cells also synthesize and secrete endogenous opioid peptides and participate peripheral analgesia. This chapter is structured into two sections. Part one focuses on immunoregulatory functions of central endogenous opioids; and part two describes how opioid peptide-containing immune cells participate in local analgesia.

[Endomorphins: structure, localization, immunoregulatory activity]

Endomorphins – endogenous tetrapeptides with the highest affinity for the µ-opioid receptor. Currently, two tetrapeptides that differ in one amino acid residue have been isolated and characterized. The structure of endomorphins differs from the structure of members of three main families of opioid peptides: endorphins, enkephalins, and dynorphins, which contain the same N-terminal sequence. In the central nervous system, endomorphins are distributed everywhere, where they are primarily responsible for antinociception. Distribution of endomorphins in the immune system, similar to that of other opioid peptides, has allowed to suggest their active participation in the processes of immune regulation. This review summarizes modern views on the structure of endomorphins, their localization, possible intracellular mechanisms of signal transmission and their effects on the processes of activation, proliferation and differentiation of cells of innate and adaptive immunity. Endomorphins actively modulate the functions of the cells of the immune system. Peptides predominantly suppress adaptive immunity reactions. There effects on the functions of innate immunity cells (granulocytes, macrophages, monocytes, dendritic cells) depending on the conditions and can have either an inhibitory or stimulating orientation. Thus, endomorphins can be promising compounds that can effectively regulate both nociceptive signals and processes in the immune system.

Mexneurin is a novel precursor of peptides in the central nervous system of rodents

Endomorphins (EMs) have been proposed as the endogenous ligand agonists of the μ-opioid receptor; however, no propeptide precursor protein for EMs has been identified. Here, to identify the presumed precursor of EMs, we designed an immunoscreening assay using specific affinity-purified rabbit antisera raised against synthetic EMs in a whole-mouse brain cDNA library. Following this approach, we identify a DNA sequence encoding a protein precursor, which we name proMexneurin, that contains three different peptide sequences: Mexneurin-1 (an EM-like peptide), Mexneurin-2, and Mexneurin-3, a peptide which appears to be unrelated to EMs. RT-PCR analysis and in situ hybridization reveal a widespread distribution of proMexneurin mRNA throughout the mouse brain. Both Mexneurin-1 and Mexneurin-3 peptides display biological activities in the mouse CNS.

Inducible expression of endomorphins in murine dendritic cells

Bone marrow precursor cells were extracted from C57BL/6J mice aged 7-8 weeks, and dendritic cells were purified using anti-CD11c (a specific marker for dendritic cells) antibody-coated magnetic beads. Immunofluorescence staining revealed that the expression levels of endomorphin-1 and endomorphin-2 were upregulated in dendritic cells activated by lipopolysaccharide. An enzyme immunoassay showed that lipopolysaccharide and other Toll-like receptor ligands promoted the secretion of endomorphin-1 and endomorphin-2 from activated dendritic cells. [(3)H]-thymidine incorporation demonstrated that endomorphin-1 and endomorphin-2 both inhibited the proliferation of T lymphocyte induced by activated dendritic cells. Furthermore, this immunosuppressive effect was blocked by CTOP, a specific antagonist of µ-opioid receptors. Our experimental findings indicate that activated dendritic cells can induce the expression and secretion of endomorphins, and that endomorphins suppress T lymphocyte proliferation through activation of µ-opioid receptors.

Endomorphins in rheumatoid arthritis, osteoarthritis, and experimental arthritis

The opioid tetrapeptides endomorphins (EM)-1 and EM-2 are widely expressed in central nervous system and immune tissues of rats and humans. Their analgesic properties are well characterized but they also have anti-inflammatory properties. EM-1 significantly attenuated the onset of hindpaw inflammation in adjuvant-induced arthritis in rats. Immunohistochemical staining demonstrated the presence of EMs in T cells, macrophages, and fibroblasts in synovial tissues from patients with osteo- or rheumatoid arthritis (RA). In an ex vivo superfusion system, EM-1 potently inhibited the release of proinflammatory cytokines interleukin (IL)-6 and IL-8 from synovial tissues from patients with osteo- or RA. These results demonstrate that EMs are endogenously synthesized within human immune cells and have the potential to act as potent therapeutic agents in the treatment of chronic inflammatory disease. We discuss the clinical potential for EM analogues chemically modified to resist proteolytic degradation and identify modified protease-resistant analogues with enhanced bioactivity.

Nociceptin/orphanin FQ suppresses adaptive immune responses in vivo and at picomolar levels in vitro

Nociceptin/orphanin FQ (N/OFQ), added in vitro to murine spleen cells in the picomolar range, suppressed antibody formation to sheep red blood cells in a primary and a secondary plaque-forming cell assay. The activity of the peptide was maximal at 10(-12) M, with an asymmetric U-shaped dose-response curve that extended activity to 10(-14) M. Suppression was not blocked by pretreatment with naloxone. Specificity of the suppressive response was shown using affinity-purified rabbit antibodies against two N/OFQ peptides and with a pharmacological antagonist. Antisera against both peptides were active, in a dose-related manner, in neutralizing N/OFQ-mediated immunosuppression, when the peptide was used at concentrations from 10(-12.3) to 10(-11.6) M. In addition, nociceptin given in vivo by osmotic pump for 48 h suppressed the capacity of spleen cells placed ex vivo to make an anti-sheep red blood cell response. These studies show that nociceptin directly inhibits an adaptive immune response, i.e., antibody formation, both in vitro and in vivo.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

NSA9, a human prothrombin kringle-2-derived peptide, acts as an inhibitor of kringle-2-induced activation in EOC2 microglia

In neurodegenerative diseases, such as Alzheimer's and Parkinson's, microglial cell activation is thought to contribute to CNS injury by producing neurotoxic compounds. Prothrombin and kringle-2 increase levels of NO and the mRNA expression of iNOS, IL-1beta, and TNF-alpha in microglial cells. In contrast, the human prothrombin kringle-2 derived peptide NSA9 inhibits NO release and the production of pro-inflammatory cytokines such as IL-1beta, TNF-alpha, and IL-6 in LPS-activated EOC2 microglia. In this study, we investigated the anti-inflammatory effects of NSA9 in human prothrombin- and kringle-2-stimulated EOC2 microglia. Treatment with 20-100 muM of NSA9 attenuated both prothrombin- and kringle-2-induced microglial activation. NO production induced by MAPKs and NF-kappaB was similarly reduced by inhibitors of ERK (PD98059), p38 (SB203580), NF-kappaB (N-acetylcysteine), and NSA9. These results suggest that NSA9 acts independently as an inhibitor of microglial activation and that its effects in EOC2 microglia are not influenced by the presence of kringle-2.

Anti-inflammatory neuropeptides: a new class of endogenous immunoregulatory agents

Resolution of inflammation and induction of immune tolerance are essential to stabilize immune homeostasis and to limit the occurrence of exacerbated inflammatory and autoimmune conditions. Multiple mechanisms act together to ensure the re-establishment of immune homeostasis and maintenance of tolerance. The identification of endogenous factors that regulate these processes is crucial for the development of new therapies for inflammatory/autoimmune conditions. Neuropeptides produced during an ongoing inflammatory response emerged as endogenous anti-inflammatory agents that participate in processes leading to the resolution of inflammation and maintenance of tolerance. Anti-inflammatory neuropeptides and hormones such as vasoactive intestinal peptide, urocortin, adrenomedullin, melanocyte stimulating hormone, ghrelin, and cortistatin have beneficial effects in a variety of experimental inflammatory and autoimmune models. Their therapeutic effect has been attributed to their capacity to downregulate innate immunity, to inhibit antigen-specific T(H)1-driven responses, and to generate regulatory T cells. Finally, some of these neuropeptides have been identified as mediators of innate defense acting as natural antimicrobial peptides. Here we present the research findings in the neuropeptide immunoregulatory field, and examine possible therapies based on anti-inflammatory neuropeptides and hormones as a new pharmacologic platform.