Immune cell-derived beta-endorphin. Production, release, and control of inflammatory pain in rats

Hormones in the immune system and their possible role. A critical review

Immune cells synthesize, store and secrete hormones, which are identical with the hormones of the endocrine glands. These are: the POMC hormones (ACTH, endorphin), the thyroid system hormones (TRH, TSH, T3), growth hormone (GH), prolactin, melatonin, histamine, serotonin, catecholamines, GnRH, LHRH, hCG, renin, VIP, ANG II. This means that the immune cells contain all of the hormones, which were searched at all and they also have receptors for these hormones. From this point of view the immune cells are similar to the unicells (Tetrahymena), so it can be supposed that these cells retained the properties characteristic at a low level of phylogeny while other cells during the evolution accumulated to form endocrine glands. In contrast to the glandular endocrine cells, immune cells are polyproducers and polyreceivers. As they are mobile cells, they are able to transport the stored hormone to different places (packed transport) or attracted by local factors, accumulate in the neighborhood of the target, synthesizing and secreting hormones locally. This is taking place, e.g. in the case of endorphin, where the accumulating immune cells calms pain caused by the inflammation. The targeted packed transport is more economical than the hormone-pouring to the blood circulation of glandular endocrines and the targeting also cares the other receptor-bearing cells timely not needed the effect. Mostly the immune-effects of immune-cell derived hormones were studied (except endorphin), however, it is not exactly cleared, while the system could have scarcely studied important roles in other cases. The evolutionary aspects and the known as well, as possible roles of immune-endocrine system and their hormones are listed and discussed.

Demethylating drugs as novel analgesics for cancer pain

PURPOSE

In this study, we evaluated the analgesic potential of demethylating drugs on oral cancer pain. Although demethylating drugs could affect expression of many genes, we focused on the mu-opioid receptor (OPRM1) gene pathway, because of its role in pain processing. We determined the antinociceptive effect of OPRM1 re-expression in a mouse oral cancer model.

EXPERIMENTAL DESIGN

Using a mouse oral cancer model, we determined whether demethylating drugs produced antinociception through re-expression of OPRM1. We then re-expressed OPRM1 with adenoviral transduction and determined if, and by what mechanism, OPRM1 re-expression produced antinociception. To determine the clinical significance of OPRM1 on cancer pain, we quantified OPRM1 methylation in painful cancer tissues and nonpainful contralateral normal tissues of patients with oral cancer, and nonpainful dysplastic tissues of patients with oral dysplasia.

RESULTS

We demonstrated that OPRM1 was methylated in cancer tissue, but not normal tissue, of patients with oral cancer, and not in dysplastic tissues from patients with oral dysplasia. Treatment with demethylating drugs resulted in mechanical and thermal antinociception in the mouse cancer model. This behavioral change correlated with OPRM1 re-expression in the cancer and associated neurons. Similarly, adenoviral-mediated OPRM1 re-expression on cancer cells resulted in naloxone-reversible antinociception. OPRM1 re-expression on oral cancer cells in vitro increased β-endorphin secretion from the cancer, and decreased activation of neurons that were treated with cancer supernatant.

CONCLUSION

Our study establishes the regulatory role of methylation in cancer pain. OPRM1 re-expression in cancer cells produces antinociception through cancer-mediated endogenous opioid secretion. Demethylating drugs have an analgesic effect that involves OPRM1.

Beta-endorphin 1-31 biotransformation and cAMP modulation in inflammation

A large body of evidence now exists for the immune cell expression, production, and the release of beta-endorphin (BE 1–31) within inflamed tissue. The inflammatory milieu is characterised by increased acidity, temperature and metabolic activity. Within these harsh conditions BE 1–31 is even more susceptible to increased enzymatic degradation over that of plasma or other non-injured tissue. To elucidate the biotransformation pathways of BE 1–31 and provide an insight to the impact of inflamed tissue environments, BE 1–31 and three of its major N-terminal fragments (BE 1–11, BE 1–13 and BE 1–17) were incubated in inflamed tissue homogenates at pH 5.5 for 2 hrs. In addition, the potency of BE 1–31 and five main N – terminal fragments (BE 1–9, BE 1–11, BE 1–13, BE 1–17, BE 1–20) was assessed at mu-opioid receptors (MOR), delta-opioid receptors (DOR), and kappa-opioid receptors (KOR). Opioid receptor potency was investigated by examining the modulation of forskolin induced cAMP accumulation. The majority of the N-terminal fragment of BE 1–31 had similar efficacy to BE 1–31 at MOR. The shortest of the major N-terminal fragments (BE 1–9), had partial agonist activity at MOR but possessed the highest potency of all tested peptides at DOR. There was limited effect for BE 1–31 and the biotransformed peptides at KOR. Major N-terminal fragments produced within inflamed tissue have increased presence within inflamed tissue over that of the parent molecule BE 1–31 and may therefore contribute to BE 1–31 efficacy within disease states that involve inflammation.

Peripheral interactions between cannabinoid and opioid systems contribute to the antinociceptive effect of crotalphine

BACKGROUND AND PURPOSE

Crotalphine is an antinociceptive peptide that, despite its opioid-like activity, does not induce some of the characteristic side effects of opioids, and its amino acid sequence has no homology to any known opioid peptide. Here, we evaluated the involvement of the peripheral cannabinoid system in the crotalphine effect and its interaction with the opioid system.

EXPERIMENTAL APPROACH

Hyperalgesia was evaluated using the rat paw pressure test. Involvement of the cannabinoid system was determined using a selective cannabinoid receptor antagonist. Cannabinoid and opioid receptor activation were evaluated in paw slices by immunofluorescence assays using conformation state-sensitive antibodies. The release of endogenous opioid peptides from skin tissue was measured using a commercial enzyme immunoassay (EIA).

KEY RESULTS

Both p.o. (0.008-1.0 μg·kg(-1) ) and intraplantar (0.0006 μg per paw) administration of crotalphine induced antinociception in PGE2 -induced hyperalgesia. Antinociception by p.o. crotalphine (1 μg·kg(-1) ) was blocked by AM630 (50 μg per paw), a CB2 receptor antagonist, and by antiserum anti-dynorphin A (1 μg per paw). Immunoassay studies confirmed that crotalphine increased the activation of both κ-opioid (51.7%) and CB2 (28.5%) receptors in paw tissue. The local release of dynorphin A from paw skin was confirmed by in vitro EIA and blocked by AM630.

CONCLUSIONS AND IMPLICATIONS

Crotalphine-induced antinociception involves peripheral CB2 cannabinoid receptors and local release of dynorphin A, which is dependent on CB2 receptor activation. These results enhance our understanding of the mechanisms involved in the peripheral effect of crotalphine, as well as the interaction between the opioid and cannabinoid systems.

Mechanisms of acupuncture-electroacupuncture on persistent pain

In the last decade, preclinical investigations of electroacupuncture mechanisms on persistent tissue injury (inflammatory), nerve injury (neuropathic), cancer, and visceral pain have increased. These studies show that electroacupuncture activates the nervous system differently in health than in pain conditions, alleviates both sensory and affective inflammatory pain, and inhibits inflammatory and neuropathic pain more effectively at 2 to 10 Hz than at 100 Hz. Electroacupuncture blocks pain by activating a variety of bioactive chemicals through peripheral, spinal, and supraspinal mechanisms. These include opioids, which desensitize peripheral nociceptors and reduce proinflammatory cytokines peripherally and in the spinal cord, and serotonin and norepinephrine, which decrease spinal N-methyl-D-aspartate receptor subunit GluN1 phosphorylation. Additional studies suggest that electroacupuncture, when combined with low dosages of conventional analgesics, provides effective pain management which can forestall the side effects of often-debilitating pharmaceuticals.

IL-4, JAK-STAT signaling, and pain

During inflammation, several mediators directly or indirectly induce pain including pro-inflammatory cytokines and there is evidence that the JAK-STAT pathway is involved in the formation of pronociceptive cytokines. The same pathway, however, is also of importance for anti-inflammatory cytokines such as IL-4 to counteract the inflammatory reaction and-as it seems based on the current literature-nociceptive symptoms. Current therapeutic approaches targeting molecules of the JAK-STAT signaling cascade are auspicious but as this review demonstrates, more experimental and clinical studies are required to decipher the specific contribution of this pathway in the modulation of pain.

Physiology, signaling, and pharmacology of opioid receptors and their ligands in the gastrointestinal tract: current concepts and future perspectives

Opioid receptors are widely distributed in the human body and are crucially involved in numerous physiological processes. These include pain signaling in the central and the peripheral nervous system, reproduction, growth, respiration, and immunological response. Opioid receptors additionally play a major role in the gastrointestinal (GI) tract in physiological and pathophysiological conditions. This review discusses the physiology and pharmacology of the opioid system in the GI tract. We additionally focus on GI disorders and malfunctions, where pathophysiology involves the endogenous opioid system, such as opioid-induced bowel dysfunction, opioid-induced constipation or abdominal pain. Based on recent reports in the field of pharmacology and medicinal chemistry, we will also discuss the opportunities of targeting the opioid system, suggesting future treatment options for functional disorders and inflammatory states of the GI tract.

Targeting peripheral opioid receptors to promote analgesic and anti-inflammatory actions

Mechanisms of endogenous pain control are significant. Increasing studies have clearly produced evidence for the clinical usefulness of opioids in peripheral analgesia. The immune system uses mechanisms of cell migration not only to fight pathogens but also to control pain and inflammation within injured tissue. It has been demonstrated that peripheral inflammatory pain can be effectively controlled by an interaction of immune cell-derived opioid peptides with opioid receptors on peripheral sensory nerve terminals. Experimental and clinical studies have clearly shown that activation of peripheral opioid receptors with exogenous opioid agonists and endogenous opioid peptides are able to produce significant analgesic and anti-inflammatory effects, without central opioid mediated side effects (e.g., respiratory depression, sedation, tolerance, dependence). This article will focus on the role of opioids in peripheral inflammatory conditions and the clinical implications of targeting peripheral opioid receptors.

A Study on the Mechanism of Cinobufagin in the Treatment of Paw Cancer Pain by Modulating Local β -Endorphin Expression In Vivo

Background. Cinobufagin has been widely used in the treatment of carcinoma and plays an important role in the relief of cancer pain. But the involved mechanism remains unknown. Aim. To investigate the changes in thermal and mechanical hyperalgesia in paw cancer pain in mice and the action mechanism of cinobufagin using a paw cancer pain model. Methods. 60 female mice were randomly divided into 5 groups: control group, model group, cinobufagin group, cinobufagin +NAL-M group, and morphine group; except ones in control group, mice were inoculated with H22 hepatoma cells in the right hind paw. From the 9th day after inoculation, mice were administrated drug once daily lasting for 8 days. The pain behavior was determined on the 2nd, 4th, 6th, and 8th days before and after administration. On the last day, they were sacrificed. The levels of β -END, CRF, and IL-1 β were analyzed by ELISA; immunohistochemistry was performed to detect the expressions of β -END, POMC, and μ -OR in the tumor and adjacent tissue. Results. The thresholds of thermal pain and mechanical pain were significantly increased by cinobufagin. Moreover, the expressions of β -END, CRF, POMC, and μ -OR were significantly upregulated by cinobufagin. The analgesic effect of cinobufagin was blocked by the peripheral opioid receptor antagonist NAL-M. Conclusions. Cinobufagin significantly relieved cancer pain in mice and raised their pain threshold, mainly upregulating the expression levels of β -END and μ -OR in the hind paw tumor and adjacent tissue.

JAK-STAT1/3-induced expression of signal sequence-encoding proopiomelanocortin mRNA in lymphocytes reduces inflammatory pain in rats

BACKGROUND

Proopiomelanocortin (POMC)-derived beta-endorphin1-31 from immune cells can inhibit inflammatory pain. Here we investigated cytokine signaling pathways regulating POMC gene expression and beta-endorphin production in lymphocytes to augment such analgesic effects.

RESULTS

Interleukin-4 dose-dependently elevated POMC mRNA expression in naïve lymph node-derived cells in vitro, as determined by real-time PCR. This effect was neutralized by janus kinase (JAK) inhibitors. Transfection of Signal Transducer and Activator of Transcription (STAT) 1/3 but not of STAT6 decoy oligonucleotides abolished interleukin-4 induced POMC gene expression. STAT3 was phosphorylated in in vitro interleukin-4 stimulated lymphocytes and in lymph nodes draining inflamed paws in vivo. Cellular beta-endorphin increased after combined stimulation with interleukin-4 and concanavalin A. Consistently, in vivo reduction of inflammatory pain by passively transferred T cells improved significantly when donor cells were pretreated with interleukin-4 plus concanavalin A. This effect was blocked by naloxone-methiodide.

CONCLUSION

Interleukin-4 can amplify endogenous opioid peptide expression mediated by JAK-STAT1/3 activation in mitogen-activated lymphocytes. Transfer of these cells leads to inhibition of inflammatory pain via activation of peripheral opioid receptors.

Early systemic granulocyte-colony stimulating factor treatment attenuates neuropathic pain after peripheral nerve injury

Recent studies have shown that opioid treatment can reduce pro-inflammatory cytokine production and counteract various neuropathic pain syndromes. Granulocyte colony-stimulating factor (G-CSF) can promote immune cell differentiation by increasing leukocytes (mainly opioid-containing polymorphonuclear (PMN) cells), suggesting a potential beneficial role in treating chronic pain. This study shows the effectiveness of exogenous G-CSF treatment (200 µg/kg) for alleviating thermal hyperalgesia and mechanical allodynia in rats with chronic constriction injury (CCI), during post-operative days 1-25, compared to that of vehicle treatment. G-CSF also increases the recruitment of opioid-containing PMN cells into the injured nerve. After CCI, single administration of G-CSF on days 0, 1, and 2, but not on day 3, relieved thermal hyperalgesia, which indicated that its effect on neuropathic pain had a therapeutic window of 0-48 h after nerve injury. CCI led to an increase in the levels of interleukin-6 (IL-6) mRNA and tumor necrosis factor-α (TNF-α) protein in the dorsal root ganglia (DRG). These high levels of IL-6 mRNA and TNF-α were suppressed by a single administration of G-CSF 48-144 h and 72-144 h after CCI, respectively. Furthermore, G-CSF administered 72-144 h after CCI suppressed the CCI-induced upregulation of microglial activation in the ipsilateral spinal dorsal horn, which is essential for sensing neuropathic pain. Moreover, the opioid receptor antagonist naloxone methiodide (NLXM) reversed G-CSF-induced antinociception 3 days after CCI, suggesting that G-CSF alleviates hyperalgesia via opioid/opioid receptor interactions. These results suggest that an early single systemic injection of G-CSF alleviates neuropathic pain via activation of PMN cell-derived endogenous opioid secretion to activate opioid receptors in the injured nerve, downregulate IL-6 and TNF-α inflammatory cytokines, and attenuate microglial activation in the spinal dorsal horn. This indicates that G-CSF treatment can suppress early inflammation and prevent the subsequent development of neuropathic pain.

Involvement of cytokines, chemokines and adhesion molecules in opioid analgesia

Tissue destruction is accompanied by an inflammatory reaction. The inflammatory reaction leads to activation of nociceptors and the sensation of pain. Several mediators are responsible for pain and hyperalgesia in inflammation including cytokines, chemokines, nerve growth factor as well as bradykinin, prostaglandins and ATP. Simulatenously however, analgesic mediators are secreted: opioid peptides, somatostatin, endocannabinoids and certain cytokines. Opioid peptides secreted from immune cells are so far the best studied peptides in peripheral inflammatory pain control. This system is hampered for example by anti-adhesion molecule treatment. Novel immunosuppressive drugs for treatment of autoimmune disease targetting cytokines, chemokines or adhesion molecules should therefore be evaluated for potential harmful effects on pain.

Study of beta endorphin metabolism in inflamed tissue, serum and trypsin solution by liquid chromatography-tandem mass spectrometric analysis

Beta endorphin (β-END) is recognised as one of the most significant endogenous neuropeptides, responsible for a wide range of biological activities in the body. However, within the body β-END is exposed to hydrolysis by a variety of enzymes. In this study, we investigated the metabolism and fragmentation pattern of β-END in rat inflamed tissue, in rat serum and in trypsin solution. β-END (1-31)-rat was incubated at 37 °C in each matrix for different incubation times. The resultant fragments were separated using a C4 column and detected by mass spectrometry using total ion current mode. Structural information for the fragments was elucidated using tandem mass spectrometry. Incubation of β-END (1-31)-rat in trypsin solution and in rat serum resulted in 8 and 13 fragments, respectively. Incubation in inflamed rat paw tissue resulted in 22 fragments at pH 7.4 and 26 fragments at pH 5.5. Some of these fragments were common to both pH values. The degradation of β-END (1-31)-rat in inflamed tissue at pH 5.5 was faster than that at pH 7.4. Secondary fragmentation of some larger primary fragments was also observed in this study.

Immune conditions associated with CD4+ T effector-induced opioid release and analgesia

Effector CD4(+) T lymphocytes generated in response to antigens produce endogenous opioids. Thus, in addition to their critical role in host defenses against pathogens, effector CD4(+) T lymphocytes contribute to relieving inflammatory pain. In this study, we investigated mechanisms of opioid release by antigen-experienced effector CD4(+) T cells that leave draining lymph nodes and come back into the inflammatory site. Effector antigen-primed CD4(+) T lymphocytes generated in vitro were intravenously injected into nude mice previously immunized with either cognate or irrelevant antigens in complete Freund adjuvant (CFA). CFA-induced mechanical hyperalgesia was only reduced in mice immunized with cognate antigen. Thus, antinociceptive activity of effector CD4(+) T cells requires the presence of the antigen for which they are specific within the inflammatory site. Accordingly, analgesia was inhibited by neutralizing cognate T cell receptor-mediated interaction between effector CD4(+) T lymphocytes and antigen-presenting cells at the site of inflammation. Analgesia was observed by transferring effector CD4(+) T lymphocytes with Th1 or Th2 phenotype, suggesting that antinociceptive activity is a fundamental property of effector CD4(+) T lymphocytes irrespective of their effector functions. Based on the use of agonists and antagonists selective for each of the opioid receptor subclasses, we showed that analgesia induced by T cell-derived opioids is elicited via activation of δ-type opioid receptors in the periphery. Thus, the antinociceptive activity is a fundamental property associated with the effector phase of adaptive immunity, which is driven by recognition of the cognate antigen by effector CD4(+) T lymphocytes at the inflammatory site.

Cannabinoid CB2 receptors contribute to upregulation of β-endorphin in inflamed skin tissues by electroacupuncture

BACKGROUND

Electroacupuncture (EA) can produce analgesia by increasing the β-endorphin level and activation of peripheral μ-opioid receptors in inflamed tissues. Endogenous cannabinoids and peripheral cannabinoid CB2 receptors (CB2Rs) are also involved in the antinociceptive effect of EA on inflammatory pain. However, little is known about how peripheral CB2Rs interact with the endogenous opioid system at the inflammatory site and how this interaction contributes to the antinociceptive effect of EA on inflammatory pain. In this study, we determined the role of peripheral CB2Rs in the effects of EA on the expression of β-endorphin in inflamed skin tissues and inflammatory pain.

RESULTS

Inflammatory pain was induced by injection of complete Freund's adjuvant into the left hindpaw of rats. Thermal hyperalgesia was tested with a radiant heat stimulus, and mechanical allodynia was quantified using von Frey filaments. The mRNA level of POMC and protein level of β-endorphin were quantified by real-time PCR and Western blotting, respectively. The β-endorphin-containing keratinocytes and immune cells in the inflamed skin tissues were detected by double-immunofluorescence labeling. The CB2R agonist AM1241 or EA significantly reduced thermal hyperalgesia and mechanical allodynia, whereas the selective μ-opioid receptor antagonist β-funaltrexamine significantly attenuated the antinociceptive effect produced by them. AM1241 or EA significantly increased the mRNA level of POMC and the protein level of β-endorphin in inflamed skin tissues, and these effects were significantly attenuated by pretreatment with the CB2R antagonist AM630. AM1241 or EA also significantly increased the percentage of β-endorphin-immunoreactive keratinocytes, macrophages, and T-lymphocytes in inflamed skin tissues, and these effects were blocked by AM630.

CONCLUSIONS

EA and CB2R stimulation reduce inflammatory pain through activation of μ-opioid receptors. EA increases endogenous opioid expression in keratinocytes and infiltrating immune cells at the inflammatory site through CB2R activation.

Role of the mu-opioid receptor in opioid modulation of immune function

Endogenous opioids are synthesized in vivo to modulate pain mechanisms and inflammatory pathways. Endogenous and exogenous opioids mediate analgesia in response to painful stimuli by binding to opioid receptors on neuronal cells. However, wide distribution of opioid receptors on tissues and organ systems outside the CNS, such as the cells of the immune system, indicate that opioids are capable of exerting additional effects in the periphery, such as immunomodulation. The increased prevalence of infections in opioid abuser-based epidemiological studies further highlights the immunosuppressive effects of opioids. In spite of their many debilitating side effects, prescription opioids remain a gold standard for treatment of chronic pain. Therefore, given the prevalence of opioid use and abuse, opioid-mediated immune suppression presents a serious concern in our society today. It is imperative to understand the mechanisms by which exogenous opioids modulate immune processes. In this review, we will discuss the role of opioid receptors and their ligands in mediating immune-suppressive functions. We will summarize recent studies on direct and indirect opioid modulation of the cells of the immune system, as well as the role of opioids in exacerbation of certain disease states.

Modulation of peripheral sensory neurons by the immune system: implications for pain therapy

The concept that the immune system can communicate with peripheral sensory neurons to modulate pain is based mostly on documented interactions between opioid ligands and receptors. Such findings may have broad implications for the development of safer pain medication. Innovative strategies take into account that analgesics should be particularly active in pathological states rather than producing a general suppression of the central nervous system, as with conventional morphine- or cannabinoid-like drugs. Inflammation of peripheral tissue leads to increased functionality of opioid receptors on peripheral sensory neurons and to local production of endogenous opioid peptides. In addition, endocannabinoids were detected in leukocytes, but their role in pain modulation has yet to be addressed. Future aims include the development of peripherally restricted opioid agonists, selective targeting of opioid-containing immune cells to sites of painful injury, and the augmentation of peripheral ligand and receptor synthesis (e.g., by gene therapy). Similar approaches may be pursued for cannabinoids. The ultimate goal is to avoid detrimental side effects of currently available analgesics such as respiratory depression, cognitive impairment, addiction, gastrointestinal bleeding, and thromboembolic complications.

Nonopioid effect of β-endorphin

This review presents the generalized literature data and the results of our own research of the nonopioid effect of β-endorphin, an opioid neuropeptide interacting not only with opioid but also with nonopioid (insensitive to the opioid antagonist naloxone) receptors. The roles of the hormone and its receptors in regulation of the immune, nervous, and endocrine systems are discussed. The effect of neuromediator on the immune system mediated by both opioid and nonopioid receptors is considered in detail. The data on distribution and function of the nonopioid β-endorphin receptor in human and animal organisms are presented. All available data on the characteristics of the nonopioid β-endorphin receptor obtained by means of radioligand analysis are given. The discussed information is supposed to extend our conceptions of the role of β-endorphin in mammals and to be of extensive use in medicine and pharmacology.

Morphine and tumor growth and metastasis

Morphine is an analgesic widely used to alleviate cancer pain. In addition, the perioperative management of pain in cancer surgery patients most often includes opioids. However, there are reports that these drugs may alter cancer recurrence or metastasis. Several mechanisms have been proposed, such as the modulation of the immune response or cellular pathways that control the survival and migratory behavior of cancer cells. The published literature, however, presents some discrepancies, with reports suggesting that opioids may either promote or prevent the spread of cancer. It is of great importance to determine whether opioids, in particular the most widely used, morphine, may increase the risk of metastasis when used in cancer surgery. This review examines the available data on the effects of morphine which influence cancer metastasis or recurrence, including immunomodulation, tumor cell aggressiveness, and angiogenesis, with special emphasis on recently published clinical and laboratory based studies. We further discuss the parameters that may explain the difference between reports on the effects of morphine on cancer.

[The immuno-endocrine system. A new endocrine theory: the problem of the packed transport]

Since the eighties of the last century hormone content was justified in immune cells (lymphocytes, granulocytes, monocytes, macrophages and mast cells), which produce, store and secrete these hormones. Although the amount of these materials in immune cells is relatively small, the mass of the producers (immune cells) is so large, that the phenomenon must be considered from endocrinological point of view, underlying the important differences between the "classical" and immuno-endocrine systems. Cells of the classic (built-in) endocrine system are mono-producers, while immune cells can synthesize many types of hormones (polyproducers). In addition, these cells can transport the whole hormone-producing machinery to the site of need, producing a local effect. This can be observed, for example, in the case of endorphin producing immune cells during inflammation and during early pregnancy around the chorionic villi. Hormone producing immune cells also have receptors for many hormones, so that they are poly-receivers. Via hormone producing and receiving capacity there is a bidirectional connection between the neuro-endocrine and immuno-endocrine systems. In addition, there is a network inside the immuno-endocrine system. The packed transport theory attempts to explain the mechanism and importance of the immuno-endocrine system.

The influence of μ-opioid receptor agonist and antagonist peptides on peripheral blood mononuclear cells (PBMCs)

Milk is one of the main source of biologically-active peptides that may function as regulatory substances called food hormones. After passing the gut-blood barrier, the μ-opioid receptor agonist and antagonist peptides may become the new factors influencing various functions of the human organism. The aim of the conducted research was to determine the influence of μ-opioid receptor agonist peptides: human and bovine β-casomorphin-7 (h/bBCM-7) and antagonistic peptides: casoxin-6 and- D (CXN-6/D) on proliferation and cytokine secretion of human peripheral blood mononuclear cells (PBMCs). The PBMCs proliferation was measured by the use of the BrdU test, which assesses the DNA synthesis activity and the WST-1 test which assesses the activity of mitochondrial dehydrogenase enzymes. The influence of all the investigated peptides on secretion of IL-4, IL-8, IL-13 and IFN-γ was determined by the use of the ELISA tests. Incubating the cells with the peptides has not caused any changes to their enzymatic activity, which has been proved by a WST-1 test. When using a BrdU test, however, it has been observed that there appear changes to proliferation of PBMCs correlated to amounts of bromodeoxyuridine incorporated into the cellular DNA. Moreover, changes to secretion of IL-4 and IL-13 by the cells under the influence of agonists were detected, as well as changes to secretion of IFN-gamma under the influence of all the examined substances. The obtained results provide information on immunomodulatory effects of food-derived opioid peptides, which may be of clinical significance especially in the case of allergic diseases in newborns.

Self-injurious behavior in neurodevelopmental disorders: relevance of nociceptive and immune mechanisms

Self-injurious behavior (SIB) among individuals with intellectual and related neurodevelopmental disorders (IDD) is a clinical challenge and scientific puzzle. The physiological mechanisms regulating the sensory components of SIB remain a mystery with no clear understanding of the underlying pathophysiology. The central dogma regarding sensory processing in general and pain in particular among individuals with IDD and chronic SIB is that sensory processing is reduced and pain is absent or blunted. In this paper, recent findings challenging some of the conventional wisdom regarding pain and sensory function among individuals with IDD and SIB are reviewed. It seems that at least a subgroup of individuals with IDD and chronic SIB may be in a physiological state similar to neuropathic pain in which hyperalgesia is mediated by plasticity mechanisms regulating inflammatory, immune, and nociceptive systems. In response to repeated tissue damage associated with chronic self-injury, innate immune cells may be producing pro-inflammatory and pro-nociceptive cytokines that act on the brain to cause sickness-like behavior and sensitize primary sensory nerve afferents contributing to pain hypersensitivity (i.e., hyperalgesia).

Antagonism of 5-HT(2A) receptors inhibits the expression of pronociceptive mediator and enhances endogenous opioid mechanism in carrageenan-induced inflammation in rats

We have recently reported that treatment with the 5-HT(2A) receptor antagonist ketanserin in the inflamed paw raises the nociceptive threshold above normal level (hypoalgesia) and this response is naloxone-reversible. The present study aimed to investigate neurochemical changes at the site of inflammation and in dorsal root ganglia (DRG) and the spinal cord following the blockade of 5-HT(2A) receptors. Intraplantar injection of ketanserin (20 μg) inhibited carrageenan-induced increase in CGRP immunoreactivity-positive neurons in DRG. On the other hand, administration of ketanserin (20 μg) and 5-HT (10 μg), but not vehicle, enhanced and inhibited recruitment of β-endorphin-expressing immune cells, respectively, in subcutaneous loci of inflamed hindpaw. Moreover, the treatment with ketanserin increased the number of endomorphine-containing cells in the inflamed paw and μ-opioid receptor-expressing neurons in DRG at L4-5 but reduced the expression of endomorphine in superficial layers of the lumbar spinal cord. The present study provided evidence at the cellular level showing that the blockade of 5-HT(2A) receptors inhibited inflammation-associated increase in pronociceptive mediator, and that the pronociceptive property of 5-HT is mediated by the suppression of inflammation-activated opioid mechanism. Therefore, targeting the 5-HT(2A) receptors in the site of inflammation may be a promising approach to inhibit inflammatory pain.

Involvement of the peripheral sensory and sympathetic nervous system in the vascular endothelial expression of ICAM-1 and the recruitment of opioid-containing immune cells to inhibit inflammatory pain

Endogenous opioids are known to be released within certain brain areas following stressful stimuli. Recently, it was shown that also leukocytes are a potential source of endogenously released opioid peptides following stress. They activate sensory neuron opioid receptors and result in the inhibition of local inflammatory pain. An important prerequisite for the recruitment of such leukocytes is the expression of intracellular adhesion molecule-1 (ICAM-1) in blood vessels of inflamed tissue. Here, we investigated the contribution of peripheral sensory and/or sympathetic nerves to the enhanced expression of ICAM-1 simultaneously with the increased recruitment of opioid peptide-containing leukocytes to promote the inhibition of inflammatory pain. Selective degeneration of either peripheral sensory or sympathetic nerve fibers by their respective neurotoxins, capsaicin or 6-hydroxydopamime, significantly reduced the subcutaneous immigration of β-endorphin- (END-) and met-enkephalin- (ENK-)-containing polymorphonuclear leukocytes (PMN) (in the early phase) and mononuclear cells (in the late phase) during painful Freund's complete adjuvant (FCA) rat hind paw inflammation. In contrast, this treatment did not alter the percentage of opioid peptide-containing leukocytes in the circulation. Calcitonin gene-related peptide- (CGRP-) and tyrosine hydroxylase- (TH-) immunoreactive (IR) nerve fibers were in close contact to ICAM-1 IR blood vessels within inflamed subcutaneous tissue. The selective degeneration of sensory or sympathetic nerve fibers attenuated the enhanced expression of vascular endothelial ICAM-1 after intraplantar (i.pl.) FCA and abolished endogenous opioid peptide-mediated peripheral analgesia. Our results suggest that, during localized inflammatory pain, peripheral sensory and sympathetic nerve fibers augment the expression of vascular endothelial ICAM-1 simultaneously with the increased recruitment of opioid peptide-containing leukocytes which consequently promotes the endogenous opioid peptide-mediated inhibition of inflammatory pain. They support existing evidence about a close link between the nervous and the immune system.

T lymphocytes containing β-endorphin ameliorate mechanical hypersensitivity following nerve injury

Neuropathic pain is a debilitating consequence of nerve injuries and is frequently resistant to classical therapies. T lymphocytes mediate adaptive immune responses and have been suggested to generate neuropathic pain. In contrast, in this study we investigated T cells as a source of opioidergic analgesic β-endorphin for the control of augmented tactile sensitivity following neuropathy. We employed in vivo nociceptive (von Frey) testing, flow cytometry and immunofluorescence in wild-type and mice with severe combined immunodeficiency (SCID) subjected to a chronic constriction injury of the sciatic nerve. In wild-type mice, T lymphocytes constituted approximately 11% of all immune cells infiltrating the injury site, and they expressed β-endorphin and receptors for corticotropin-releasing factor (CRF), an agent releasing opioids from leukocytes. CRF applied at the nerve injury site fully reversed neuropathy-induced mechanical hypersensitivity in wild-type animals. In SCID mice, T cells expressing β-endorphin and CRF receptors were absent at the damaged nerve. Consequently, these animals had substantially reduced CRF-mediated antinociception. Importantly, the decreased antinociception was fully restored by transfer of wild-type mice-derived T lymphocytes in SCID mice. The re-established CRF antinociception could be reversed by co-injection of an antibody against β-endorphin or an opioid receptor antagonist with limited access to the central nervous system. We propose that, in response to CRF stimulation, T lymphocytes accumulating at the injured nerves utilize β-endorphin for activation of local neuronal opioid receptors to reduce neuropathy-induced mechanical hypersensitivity. Our findings reveal β-endorphin-containing T cells as a crucial component of beneficial adaptive immune responses associated with painful peripheral nerve injuries.

Involvement of peripheral opioid receptors in electroacupuncture analgesia for carrageenan-induced hyperalgesia

Acupuncture is widely used to relieve pain; however, the mechanism underlying electroacupuncture analgesia (EAA) during inflammatory pain is unclear. We investigated whether endogenous peripheral opioid receptors participated in EAA during hyperalgesia elicited by carrageenan-induced inflammation. Moreover, we investigated which subtype of opioid receptor was involved in EAA. Carrageenan was subcutaneously administered by intraplanter (i.pl.) injection into the left hind paw. Nociceptive thresholds were measured using the paw pressure threshold (PPT). Rats received 3Hz electroacupuncture (EA) for 1h after carrageenan injection. The nonselective peripheral opioid receptor antagonist, naloxone methiodide, was administered by i.pl. injection of the inflamed paw 5min before EA. Also, animals received i.pl. or intravenous (i.v.) injection of selective antagonists against μ(D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-ThrNH2, CTOP), δ(naltrindole, NTI), or κ (nor-Binaltorphimine, nor-BNI) opioid receptors 1h before EA. PPT decreased significantly 3h after carrageenan injection. EA resulted in significant increases of PPT, moreover, PPT elevations persisted for 9h after carrageenan injection. PPT elevations produced by EA were antagonized by local i.pl. injection of naloxone methiodide at 3 and 5h after cessation of EA. NTI, nor-BNI and CTOP blocked EAA from immediately, 1h, and 3h after EA cessation, respectively. The EAA in the inflamed paw could not be blocked by i.v. injection of NTI, nor-BNI and CTOP. These findings suggest that peripheral μ, δ and κ receptors on peripheral nerve terminals are activated by EA, although there is a time difference among these activations.

Opioid receptors and opioid peptide-producing leukocytes in inflammatory pain--basic and therapeutic aspects

This review summarizes recent findings on neuro-immune mechanisms underlying opioid-mediated inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms by immune cell-derived opioid peptides. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generators of impulses relaying nociceptive information towards the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. This is in part achieved by endogenously released immune cell-derived opioid peptides within inflamed tissue. In addition, exogenous opioid receptor ligands that selectively modulate primary afferent function and do not cross the blood-brain barrier, avoid centrally mediated untoward side effects of conventional analgesics (e.g., opioids, anticonvulsants). This article discusses peripheral opioid receptors and their signaling pathways, opioid peptide-producing/secreting inflammatory cells and arising therapeutic perspectives.

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.

Peripheral endothelin B receptor agonist-induced antinociception involves endogenous opioids in mice

Endothelin-1 (ET-1) produced by various cancers is known to be responsible for inducing pain. While ET-1 binding to ETAR on peripheral nerves clearly mediates nociception, effects from binding to ETBR are less clear. The present study assessed the effects of ETBR activation and the role of endogenous opioid analgesia in carcinoma pain using an orthotopic cancer pain mouse model. mRNA expression analysis showed that ET-1 was nearly doubled while ETBR was significantly down-regulated in a human oral SCC cell line compared to normal oral keratinocytes (NOK). Squamous cell carcinoma (SCC) cell culture treated with an ETBR agonist (10(-4)M, 10(-5)M, and 10(-6) M BQ-3020) significantly increased the production of beta-endorphin without any effects on leu-enkephalin or dynorphin. Cancer inoculated in the hind paw of athymic mice with SCC induced significant pain, as indicated by reduction of paw withdrawal thresholds in response to mechanical stimulation, compared to sham-injected and NOK-injected groups. Intratumor administration of 3mg/kg BQ-3020 attenuated cancer pain by approximately 50% up to 3h post-injection compared to PBS-vehicle and contralateral injection, while intratumor ETBR antagonist BQ-788 treatment (100 and 300microg/kg and 3mg/kg) had no effects. Local naloxone methiodide (500microg/kg) or selective mu-opioid receptor antagonist (CTOP, 500microg/kg) injection reversed ETBR agonist-induced antinociception in cancer animals. We propose that these results demonstrate that peripheral ETBR agonism attenuates carcinoma pain by modulating beta-endorphins released from the SCC to act on peripheral opioid receptors found in the cancer microenvironment.

Blockade of intra-articular adrenergic receptors increases analgesic demands for pain relief after knee surgery

Activation of opioid receptors on peripheral sensory nerve terminals by opioid peptides that are produced and released from immune cells can result in inhibition of inflammatory pain. This study tests the hypothesis that postoperative pain is attenuated endogenously through a local sympathetic neurotransmitter-activated release of opioids in patients undergoing knee surgery. We examined the expression of opioid peptides and adrenergic receptors in cells infiltrating inflamed synovial tissue and we hypothesized that intra-articular (i.a.) administration of the adrenergic receptor antagonist labetalol will increase postoperative analgesic consumption and/or pain intensity in these patients. In a double-blind, randomized manner, 75 patients undergoing therapeutic knee arthroscopy received i.a. placebo (20 ml saline) or labetalol (2.5 or 5 mg in 20 ml saline) at the end of surgery. Postoperative pain intensity was assessed by visual analog and verbal rating scales at rest and on exertion, and by the consumption of morphine via patient-controlled analgesia. Synovial biopsies were taken during the operation for double-immunofluorescence confocal microscopy studies. Alpha(1)- and beta(2)-adrenergic receptors were co-expressed in opioid peptide-containing cells. No significant difference was seen in pain scores, but patients receiving 2.5 mg labetalol requested significantly higher amounts of morphine. These findings are consistent with the notion that surgical stress induces sympathetically activated release of endogenous opioids from inflammatory cells and subsequent analgesia via activation of peripheral opioid receptors.

Endothelin-A receptor antagonism attenuates carcinoma-induced pain through opioids in mice

We previously reported that endothelin A (ET-A) receptor antagonism attenuates carcinoma-induced pain in a cancer pain mouse model. In this study, we investigated the mechanism of ET-A receptor-mediated antinociception and evaluated the role of endogenous opioid analgesia. Squamous cell carcinoma (SCC) cell culture treated with the ET-A receptor antagonist (BQ-123) at 10(-6) M and 10(-5) M significantly increased production and secretion of beta-endorphin and leu-enkephalin, respectively. Behavioral studies were performed by inducing tumors in the hind paw of female nude mice with local injection of cells derived from a human oral SCC. Significant pain, as indicated by reduction in withdrawal thresholds in response to mechanical stimulation, began at 4 days after SCC inoculation and lasted to 18 days, the last day of measurement. Local administration of either naloxone methiodide (500 microg/kg), selective antagonists for mu-opioid receptor (CTOP, 500 microg/kg), or delta-opioid receptor (naltrindole, 11 mg/kg) but not kappa-opioid receptor (nor-BNI, 2.5 mg/kg) significantly reversed antinociception observed from ET-A receptor antagonism (BQ-123, 92 mg/kg) in cancer animals. These results demonstrate that antagonism of peripheral ET-A receptor attenuates carcinoma pain by modulating release of endogenous opioids to act on opioid receptors in the cancer microenvironment.

PERSPECTIVE

This article proposes a novel mechanism for ET-A receptor antagonist drugs in managing cancer-induced pain. An improved understanding of the role of innate opioid analgesia in ET-A receptor-mediated antinociception might provide novel alternatives to morphine therapy for the treatment of cancer pain.

Peripheral non-viral MIDGE vector-driven delivery of beta-endorphin in inflammatory pain

Background

Leukocytes infiltrating inflamed tissue produce and release opioid peptides such as β-endorphin, which activate opioid receptors on peripheral terminals of sensory nerves resulting in analgesia. Gene therapy is an attractive strategy to enhance continuous production of endogenous opioids. However, classical viral and plasmid vectors for gene delivery are hampered by immunogenicity, recombination, oncogene activation, anti-bacterial antibody production or changes in physiological gene expression. Non-viral, non-plasmid minimalistic, immunologically defined gene expression (MIDGE) vectors may overcome these problems as they carry only elements needed for gene transfer. Here, we investigated the effects of a nuclear localization sequence (NLS)-coupled MIDGE encoding the β-endorphin precursor proopiomelanocortin (POMC) on complete Freund's adjuvant-induced inflammatory pain in rats.

Results

POMC-MIDGE-NLS injected into inflamed paws appeared to be taken up by leukocytes resulting in higher concentrations of β-endorphin in these cells. POMC-MIDGE-NLS treatment reversed enhanced mechanical sensitivity compared with control MIDGE-NLS. However, both effects were moderate, not always statistically significant or directly correlated with each other. Also, the anti-hyperalgesic actions could not be increased by enhancing β-endorphin secretion or by modifying POMC-MIDGE-NLS to code for multiple copies of β-endorphin.

Conclusion

Although MIDGE vectors circumvent side-effects associated with classical viral and plasmid vectors, the current POMC-MIDGE-NLS did not result in reliable analgesic effectiveness in our pain model. This was possibly associated with insufficient and variable efficacy in transfection and/or β-endorphin production. Our data point at the importance of the reproducibility of gene therapy strategies for the control of chronic pain.

Modification of endothelial biology by acute and chronic stress hormones

OBJECTIVE

An increasing number of studies have examined the role of emotional stress and coronary heart disease; the underlying pathophysiology is still poorly understood. The present study was designed to evaluate the relationship between acute (epi- and norepinephrine) and chronic stress hormones (dexamethasone, beta-endorphin, corticotropin releasing hormone) and endothelial dysfunction.

METHODS

Human microvascular endothelial cells were incubated with stress hormones for 6 and 24 h. ET-1 release and ADMA were quantified via ELISA, NO release by using cell permeable 4.5-diaminofluorescein diacetate (DAF2-DA), oxidative stress fluometrically by the ROS-sensitive carboxy-H2-DCFDA method, mitochondrial metabolic activity by using the colorimetric assay WST-1, ET-1 receptor type A (ET(A)R) protein expression by Western blot, and cell proliferation activity was assessed by the colorimetric assay BrdU.

RESULTS

With respect to analysed acute and chronic stress hormones, ET-1 release was significantly increased. Likewise, protein expression was enhanced after long term incubation (24 h) with norepinephrine and dexamethasone. In contrast, endothelial NO-levels were only influenced by short term stimulation of dexamethasone (upregulation of NO release) and norepinephrine (downregulation of NO release), whereas modified NO concentration mimics altered mitochondrial metabolic activity. Unexpectedly, both oxidative stress and cell proliferation were not modified by stress hormones.

CONCLUSION

Results suggest that acute and chronic stress hormones induce a significant ET-1 release whereas NO release remained mainly unchanged. The imbalance of pro- and antiatherosclerotic factors may play a pivotal role in the initiation of stress-related endothelial dysfunction up to myocardial infarction.

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.

Mycobacteria attenuate nociceptive responses by formyl peptide receptor triggered opioid peptide release from neutrophils

In inflammation, pain is regulated by a balance of pro- and analgesic mediators. Analgesic mediators include opioid peptides which are secreted by neutrophils at the site of inflammation, leading to activation of opioid receptors on peripheral sensory neurons. In humans, local opioids and opioid peptides significantly downregulate postoperative as well as arthritic pain. In rats, inflammatory pain is induced by intraplantar injection of heat inactivated Mycobacterium butyricum, a component of complete Freund's adjuvant. We hypothesized that mycobacterially derived formyl peptide receptor (FPR) and/or toll like receptor (TLR) agonists could activate neutrophils, leading to opioid peptide release and inhibition of inflammatory pain. In complete Freund's adjuvant-induced inflammation, thermal and mechanical nociceptive thresholds of the paw were quantified (Hargreaves and Randall-Selitto methods, respectively). Withdrawal time to heat was decreased following systemic neutrophil depletion as well as local injection of opioid receptor antagonists or anti-opioid peptide (i.e. Met-enkephalin, beta-endorphin) antibodies indicating an increase in pain. In vitro, opioid peptide release from human and rat neutrophils was measured by radioimmunoassay. Met-enkephalin release was triggered by Mycobacterium butyricum and formyl peptides but not by TLR-2 or TLR-4 agonists. Mycobacterium butyricum induced a rise in intracellular calcium as determined by FURA loading and calcium imaging. Opioid peptide release was blocked by intracellular calcium chelation as well as phosphoinositol-3-kinase inhibition. The FPR antagonists Boc-FLFLF and cyclosporine H reduced opioid peptide release in vitro and increased inflammatory pain in vivo while TLR 2/4 did not appear to be involved. In summary, mycobacteria activate FPR on neutrophils, resulting in tonic secretion of opioid peptides from neutrophils and in a decrease in inflammatory pain. Future therapeutic strategies may aim at selective FPR agonists to boost endogenous analgesia.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

Immunology, signal transduction, and behavior in hypothalamic-pituitary-adrenal axis-related genetic mouse models

A classical view of the neuroendocrine-immune network assumes bidirectional interactions where pro-inflammatory cytokines influence hypothalamic-pituitary-adrenal (HPA) axis-derived hormones that subsequently affect cytokines in a permanently servo-controlled circle. Nevertheless, this picture has been continuously evolving over the last years as a result of the discovery of redundant expression and extended functions of many of the molecules implicated. Thus, cytokines are not only expressed in cells of the immune system but also in the central nervous system, and many hormones present at hypothalamic-pituitary level are also functionally expressed in the brain as well as in other peripheral organs, including immune cells. Because of this intermingled network of molecules redundantly expressed, the elucidation of the unique roles of HPA axis-related molecules at every level of complexity is one of the major challenges in the field. Genetic engineering in the mouse offers the most convincing method for dissecting in vivo the specific roles of distinct molecules acting in complex networks. Thus, various immunological, behavioral, and signal transduction studies performed with different HPA axis-related mutant mouse lines to delineate the roles of beta-endorphin, the type 1 receptor of corticotropin-releasing hormone (CRHR1), and its ligand CRH will be discussed here.

Immune cell-derived opioids protect against neuropathic pain in mice

The analgesic effects of leukocyte-derived opioids have been exclusively demonstrated for somatic inflammatory pain, for example, the pain associated with surgery and arthritis. Neuropathic pain results from injury to nerves, is often resistant to current treatments, and can seriously impair a patient's quality of life. Although it has been recognized that neuronal damage can involve inflammation, it is generally assumed that immune cells act predominately as generators of neuropathic pain. However, in this study we have demonstrated that leukocytes containing opioids are essential regulators of pain in a mouse model of neuropathy. About 30%-40% of immune cells that accumulated at injured nerves expressed opioid peptides such as beta-endorphin, Met-enkephalin, and dynorphin A. Selective stimulation of these cells by local application of corticotropin-releasing factor led to opioid peptide-mediated activation of opioid receptors in damaged nerves. This ultimately abolished tactile allodynia, a highly debilitating heightened response to normally innocuous mechanical stimuli, which is symptomatic of neuropathy. Our findings suggest that selective targeting of opioid-containing immune cells promotes endogenous pain control and offers novel opportunities for management of painful neuropathies.

Opioids and sensory nerves

This chapter reviews the expression and regulation of opioid receptors in sensory neurons and the interactions of these receptors with endogenous and exogenous opioid ligands. Inflammation of peripheral tissues leads to increased synthesis and axonal transport of opioid receptors in dorsal root ganglion neurons. This results in opioid receptor upregulation and enhanced G protein coupling at peripheral sensory nerve terminals. These events are dependent on neuronal electrical activity, and on production of proinflammatory cytokines and nerve growth factor within the inflamed tissue. Together with the disruption of the perineurial barrier, these factors lead to an enhanced analgesic efficacy of peripherally active opioids. The major local source of endogenous opioid ligands (e.g. beta-endorphin) is leukocytes. These cells contain and upregulate signal-sequence-encoding messenger RNA of the beta-endorphin precursor proopiomelanocortin and the entire enzymatic machinery necessary for its processing into the functionally active peptide. Opioid-containing immune cells extravasate using adhesion molecules and chemokines to accumulate in inflamed tissues. Upon stressful stimuli or in response to releasing agents such as corticotropin-releasing factor, cytokines, chemokines, and catecholamines, leukocytes secrete opioids. Depending on the cell type, this release is contingent on extracellular Ca(2+) or on inositol triphosphate receptor triggered release of Ca(2+) from endoplasmic reticulum. Once secreted, opioid peptides activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of proinflammatory neuropeptides. These effects occur without central untoward side effects such as depression of breathing, clouding of consciousness, or addiction. Future aims include the development of peripherally restricted opioid agonists, selective targeting of opioid-containing leukocytes to sites of painful injury, and the augmentation of peripheral opioid peptide and receptor synthesis.