Pain control by immune-derived opioids

Efficacy of 3 Buprenorphine Formulations for the Attenuation of Hypersensitivity after Plantar Incision in Immunodeficient NSG Mice

Buprenorphine is perhaps the most prescribed analgesic for management of postoperative pain in mice. Although various buprenorphine formulations are effective in commonly used immunocompetent mouse strains, a knowledge gap exists regarding its efficacy in immunodeficient mice. Here we used a plantar incision to evaluate the efficacy of 3 buprenorphine formulations for attenuating postoperative mechanical and thermal hypersensitivity in the immunodeficient NSG mouse strain. We also characterized the pharmacokinetics of these formulations over a 72-h period. We hypothesized that all 3 buprenorphine formulations evaluated-the standard preparation and 2 extended-release products (Bup-HCl, Bup-ER, and Bup-XR, respectively)-would attenuate postoperative mechanical and thermal hypersensitivity resulting from a plantar incision in NSG mice. Male and female NSG mice (n = 48) were allocated to 4 treatment groups: saline (0.9% NaCl, 5 mL/kg SC once); Bup-HCl (0.1 mg/kg SC, BID for 2 d); Bup-ER (1.0 mg/kg SC once); and Bup-XR (3.25 mg/kg SC once). Mechani- cal and thermal hypersensitivity assessments were conducted 24 h before surgery and at 4, 8, 24, 48, and 72 h afterward. All groups of mice showed mechanical and thermal hypersensitivity within the first 24 h after surgery. Behavioral pain indicators (guarding, toe-touching [intermittent partial weight bearing], licking the incision, vocalizations) were observed in some mice from each group at every postoperative time point. Plasma buprenorphine was measured in a separate group of mice and concentrations surpassed the suggested therapeutic level (1.0 ng/mL) for less than 4 h for Bup-HCl, for at least 24 h for Bup-ER, and for 72 h for Bup-XR. Our results indicate that at the dosages studied, these buprenorphine formulations do not adequately attenuate postoperative mechanical and thermal hypersensitivity in the plantar incisional model in NSG mice. These findings support the need for strain-specific analgesic protocols for mice used in research.

A Review of the Effects of Pain and Analgesia on Immune System Function and Inflammation: Relevance for Preclinical Studies

One of the most significant challenges facing investigators, laboratory animal veterinarians, and IACUCs, is how to balance appropriate analgesic use, animal welfare, and analgesic impact on experimental results. This is particularly true for in vivo studies on immune system function and inflammatory disease. Often times the effects of analgesic drugs on a particular immune function or model are incomplete or don't exist. Further complicating the picture is evidence of the very tight integration and bidirectional functionality between the immune system and branches of the nervous system involved in nociception and pain. These relationships have advanced the concept of understanding pain as a protective neuroimmune function and recognizing pathologic pain as a neuroimmune disease. This review strives to summarize extant literature on the effects of pain and analgesia on immune system function and inflammation in the context of preclinical in vivo studies. The authors hope this work will help to guide selection of analgesics for preclinical studies of inflammatory disease and immune system function.

Plasma and Cerebrospinal Fluid Beta-Endorphin Levels Show a Strong Association in Children with Cerebral Malaria

BACKGROUND

Beta (β)-endorphins are endogenous neuropeptides found in the plasma and cerebrospinal fluid (CSF) of humans but there have been reports of the relationship between the plasma and CSF β-endorphin levels in different clinical conditions. However, the relationship between β-endorphin levels in the plasma and CSF of children with cerebral malaria (CM) has not been reported.

AIM

To determine the relationship between β-endorphin levels in the CSF and plasma of children with CM.

SETTINGS AND DESIGN

This cross-sectional study involved 40 children, aged between 6 months and 14 years, admitted with a diagnosis of CM at the Obafemi Awolowo University Teaching Hospitals Complex (OAUTHC), Ile-Ife, Nigeria.

MATERIALS AND METHODS

One milliliter (mL) of venous blood and 1mL of CSF obtained from each subject at admission were used to determine the β-endorphin levels using enzyme-linked immunosorbent assay (ELISA) method.

STATISTICAL ANALYSIS

Bivariate linear regression was used to determine the association between plasma and CSF β-endorphin levels using the correlation coefficient (r), coefficient of determination (R 2), and P values.

RESULTS

The plasma β-endorphin levels significantly positively correlated with CSF β-endorphin (r = 0.568, P = 0.001) such that for every unit rise in plasma β-endorphin, CSF β-endorphin rose by 0.252 pmol/L (confidence interval: 0.132-0.371 pmol/L).

CONCLUSION

The finding of positive correlation between plasma and CSF β-endorphin levels in this study suggests a possible direct link between plasma and CSF in CM, probably from the disruption of the blood-brain barrier that has been reported in CM.

Cerebrospinal fluid and plasma β-endorphin levels in children with cerebral malaria

OBJECTIVES

Cerebral malaria (CM) is the most lethal form of malaria, yet its pathogenesis is not fully understood. Cytoadherence, sequestration, alterations in cytokine expression, inflammation, and microvascular obstruction are all hypothesized to be important in the aetio-pathogenesis of coma which characterizes cerebral malaria and the death which sometimes result. Beta (β)-endorphin has been postulated to be involved in the pathogenetic processes of inflammation and cytokine expression, although the exact role is unknown. The aim of this study was to determine the levels of β-endorphin in cerebrospinal fluid (CSF) and plasma of children with CM and compare the levels of β-endorphin in the plasma of children with CM with that of apparently healthy age- and sex-matched controls at Ile-Ife, Nigeria.

MATERIALS AND METHODS

Additional to the standard investigation for CM, CSF and venous blood samples were obtained from the subjects for the determination of β-endorphin levels.

RESULTS

Forty children with CM were studied along with forty age- and sex-matched controls. The mean CSF β-endorphin (± SD) level for the children with CM was 1.8 ± 0.9 pmol/L. The mean plasma β-endorphin levels at admission (3.1 ± 2.0 pmol/L) and discharge (4.1 ± 3.3 pmol/L) were higher in children with CM than in the control subjects (2.7 ± 0.7 pmol/L). However, only the mean plasma β-endorphin levels at discharge was significantly higher than that of controls (p = .012).

CONCLUSION

Children with CM had higher mean plasma β-endorphin levels compared to the controls and there was increased production of β-endorphins in children with CM during the course of the illness.

Opioidergic effects on enteric and sensory nerves in the lower GI tract: basic mechanisms and clinical implications

Opioids are one of the most prescribed drug classes for treating acute pain. However, chronic use is often associated with tolerance as well as debilitating side effects, including nausea and dependence, which are mediated by the central nervous system, as well as constipation emerging from effects on the enteric nervous system. These gastrointestinal (GI) side effects limit the usefulness of opioids in treating pain in many patients. Understanding the mechanism(s) of action of opioids on the nervous system that shows clinical benefit as well as those that have unwanted effects is critical for the improvement of opioid drugs. The opioidergic system comprises three classical receptors (μ, δ, κ) and a nonclassical receptor (nociceptin), and each of these receptors is expressed to varying extents by the enteric and intestinal extrinsic sensory afferent nerves. The purpose of this review is to discuss the role that the opioidergic system has on enteric and extrinsic afferent nerves in the lower GI tract in health and diseases of the lower GI tract, particularly inflammatory bowel disease and irritable bowel syndrome, and the implications of opioid treatment on clinical outcomes. Consideration is also given to emerging developments in our understanding of the immune system as a novel source of endogenous opioids and the mechanisms underlying opioid tolerance, including the potential influence of opioid receptor splice variants and heteromeric complexes.

Proteinase-activated Receptor 1 Contributed to Up-regulation of Enkephalin in Keratinocytes of Patients with Obstructive Jaundice

Background:

Skin synthesis of endogenous opioids such as enkephalin is considered to be increased in cholestatic rodents, which may induce antinociception in cholestatic liver disease. No studies have reported yet the expression of skin enkephalin in patients with cholestasis.

Methods:

Electrical pain threshold, postoperative morphine consumption, and skin enkephalin expression were measured in patients with jaundice (n = 18) and control patients (n = 16). Male Sprague–Dawley rats (n = 52) and human keratinocyte cell line HaCaT were used in vivo and in vitro studies, respectively. Nociceptive thresholds and plasma and skin levels of methionine-enkephalin were compared in protease-activated receptors-1–antagonized and control bile duct–ligated rats. In in vitro study, the effect on thrombin-induced enkephalin expression was examined and the role of extracellular regulated protein kinases 1/2 and p38 was investigated.

Results:

The authors found that: (1) the electrical pain threshold (mean ± SD) was 1.1 ± 0.1 mA in control patients, whereas it was significantly increased in patients with jaundice (1.7 ± 0.3 mA); 48-h postoperative morphine consumption was approximately 50% higher in the control group than that in the group with jaundice; (2) Skin keratinocytes enkephalin expression was increased in the patients with jaundice; (3) Protease-activated receptors-1 antagonist 1 μg·kg−1·day−1 treatment to the bile duct–ligated rats significantly reduced plasma levels of methionine-enkephalin, nociceptive thresholds, and keratinocytes enkephalin expression; and (4) protease-activated receptors-1 activation induced enkephalin expression through phosphorylation of extracellular regulated protein kinases 1/2 and p38 in keratinocytes.

Conclusion:

Protease-activated receptors-1 activation in peripheral keratinocytes may play an important role in the local synthesis of enkephalin during cholestasis.

Determination of Toll-like receptor-induced cytokine profiles in the blood and cerebrospinal fluid of Chikungunya patients

BACKGROUND

Chikungunya infection caused by Chikungunya virus (CHIKV) is an inflammatory disease affecting the joints and may also lead to neurological complications. We investigated a panel of human Toll-like receptor (TLR)-induced cytokines in Chikungunya patients with and without neurological complications.

METHODS

In a case-control study, a panel of 12 cytokines and chemokines, TNF-α, IFN-α, IL-1β, IL-6, IL-12, IL-17A, IL-8, monocyte chemotactic protein (MCP)-1, RANTES, interferon (IFN)-γ-induced protein (IP)-10, monokine induced by IFN-γ (MIG) and thymus and activation-regulated chemokine (TARC), was analysed using a conventional ELISA protocol in the serum samples of Chikungunya patients without neurological complications and in the cerebrospinal fluid (CSF) and paired serum samples of Chikungunya patients with neurological complications.

RESULTS

The levels of 3 cytokines, IL-1β, IL-17A and IL-8, and 4 chemokines, MCP-1, RANTES, IP-10 and TARC, were raised in serum samples of Chikungunya patients without neurological complications, whereas, 4 cytokines, TNF-α, IFN-α, IL-6 and IL-8, and 4 chemokines, MCP-1, RANTES, MIG and TARC, were elevated in CSF samples of Chikungunya patients with neurological complications. Moreover, the levels of IL-6 and IL-8 cytokines were significantly elevated in the CSF compared to paired serum samples in Chikungunya patients with neurological complications.

CONCLUSIONS

In CHIKV infection, multiple cytokines are elevated in serum and CSF. The elevation in IL-6 and IL-8 cytokines in CSF correlates with neurological involvement.

Mechanism of cancer pain

Ongoing and breakthrough pain is a primary concern for the cancer patient. Although the etiology of cancer pain remains unclear, animal models of cancer pain have allowed investigators to unravel some of the cancer-induced neuropathologic processes that occur in the region of tumor growth and in the dorsal horn of the spinal cord. Within the cancer microenvironment, cancer and immune cells produce and secrete mediators that activate and sensitize primary afferent nociceptors. Pursuant to these peripheral changes, nociceptive secondary neurons in spinal cord exhibit increased spontaneous activity and enhanced responsiveness to three modes of noxious stimulation: heat, cold, and mechanical stimuli. As our understanding of the peripheral and central mechanisms that underlie cancer pain improves, targeted analgesics for the cancer patient will likely follow.

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.

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.

Inflammatory pain is enhanced in delta opioid receptor-knockout mice

To examine the involvement of opioid receptors in inflammatory pain, we compared Complete Freund's Adjuvant-induced hyperalgesia in mice lacking mu, delta or kappa receptors under the same experimental conditions. Mechanical allodynia and thermal hyperalgesia were measured using von Frey filaments and the plantar test, respectively. All three receptor-knockout mice, as well as wild-type animals, developed inflammatory hyperalgesia following Complete Freund's Adjuvant administration. Mu-receptor mutants showed similar hyperalgesia to wild-types in the two tests. Kappa-receptor mutants exhibited enhanced mechanical allodynia compared with wild-type mice but similar thermal hyperalgesia. In contrast, mechanical allodynia and thermal hyperalgesia were both markedly augmented in delta-receptor mutants, indicating a role for an endogenous delta-receptor tone in the control of inflammatory pain. Treatment with the delta-selective agonist SNC80 produced antihyperalgesia, and this effect was abolished in the delta-receptor knockout mice. Altogether, these data demonstrate that delta receptors inhibit inflammatory pain when activated either endogenously or exogenously. We have previously shown enhanced neuropathic pain in delta-receptor knockout mice. The delta receptor definitely represents a promising target for treating chronic pain conditions.

Rat brain opioid peptides-circadian rhythm is under control of melatonin

Several experiments have revealed an Endogenous Opioid System (EOS)-circadian rhythm. The brain-borne hormone, melatonin (MEL) has been shown to regulate the organism photoperiodic activity and may be implicated in the EOS-circadian rhythm. To explore this hypothesis, we studied the effect of functional pinealectomy on the EOS-circadian rhythm by measuring the immunoreactive content of Met-Enkephalin, Leu-Enkephalin and Synenkephalin in both hypothalamus and hippocampus of the rat brain, using standard radioimmunoassay procedures. Experimental animals exposed to white fluorescent light (WFL) for 15days (<50lux), displayed a disruption of the EOS-circadian rhythm, showing that absence of MEL induced a significant decrease of tissue content of enkephalin peptides at 01:00h during the dark-phase of the 24-h circadian rhythm, when compared to control rats. Functional pinealectomized rats exposed to 4 or 6h period of darkness (used to revert the effects induced by the absence of melatonin) significantly increased the tissue content of ME-IR and LE-IR, when compared to both controls and non-exposed WFL-treated rats. In addition, subcutaneous administration of exogenous melatonin (10, 100, 150, 300, 600microg/kg), in WFL-treated animals produced significant dose-dependent increases of ME-IR in both brain regions tested. Finally, luzindole (melatonin receptor antagonist) administration, was not able to prevent the enkephalin tissue increase, induced with the MEL administration (150microg/kg). This data suggest that MEL not only regulates the EOS-circadian rhythm, but also appears to modulate their synthesis in the rat brain from their respective neurons.

Involvement of peripheral opioid mechanisms in electroacupuncture analgesia

The involvement of the peripheral opioid system in modulating inflammatory pain has been well documented. This study aimed to investigate the possibility of electroacupuncture (EA)-mediated peripheral opioid release. Rats were injected with complete Freund's adjuvant in one of the hind paws to induce localized inflammatory pain. The pain behavioral changes were measured by paw withdrawal latency (PWL) to a noxious thermal stimulus. At day 5 of inflammation, rats received a second injection of saline or opioid antagonists into the inflamed paw, followed by EA at 30 Hz, 2 mA, and 0.1 ms for 30 minutes. The EA was conducted at acupuncture point GB30. A control was used in which needles were inserted at GB30 but no electrical stimulation was applied. Rats receiving EA showed a significantly longer PWL as compared with the control from 30 minutes to three hours after EA treatment. Intraplantar but not intraperitoneal injection of naloxone methiodide, a peripherally acting opioid receptor antagonist, eliminated the analgesic effect at 30 minutes after EA treatment. Intraplantar injection of an antibody against beta-endorphin and a corticotropin-releasing factor antagonist also produced a reduction in PWL in rats receiving EA. These data strongly suggest that peripheral opioids are released by EA at the inflammatory site.

Endogenous opioid-mediated antinociception in cholestatic mice is peripherally, not centrally, mediated

BACKGROUND/AIMS

Cholestasis is associated with naloxone reversible antinociception and opiate receptor antagonists are used clinically to treat pruritus. Pain and pruritus are closely interrelated and opioids modulate both sensations. Therefore, we undertook a series of experiments to characterize opioid-mediated antinociception in cholestasis and determine if it occurs inside or outside the CNS.

METHODS

Antinociception scores to both thermal and mechanical stimuli were determined in mice with cholestasis due to bile duct resection vs sham controls.

RESULTS

Cholestatic mice demonstrated significant antinociception to both stimuli compared to controls, which was reversible by the opiate receptor antagonist naloxone. The experiments were repeated with a naloxone derivative, which does not cross the blood-brain-barrier (i.e. naloxone methiodide) with similar results, indicating an opioid antinociceptive effect mediated outside of the CNS. Experiments with intraplantar injections of low dose naloxone methiodide confirmed that cholestasis-associated antinociception occurs at the level of cutaneous nerve endings. These findings were supported by findings of increased dermal met-enkephalin expression in cholestatic mice.

CONCLUSIONS

Cholestasis in mice is associated with antinociception due to local effects of endogenous opioids (i.e. met-enkephalin) at the level of sensory nerve endings. These findings may have direct implications in the management of cholestasis associated pruritus.

Opioids in pain management of blood-related malignancies

Opioids are basic analgesics used in the treatment of moderate to severe pain in patients affected by blood-related malignancies. They should be sequentially administered according to the World Health Organisation scale for cancer pain. Initial treatment and titration with opioids should be based on immediate-release preparations, to be administered at appropriate intervals in order to relieve pain and to satisfy the individual opioid requirement. Once a relatively good pain control has been achieved, a slow release formulation at equivalent doses can be given. Most patients can be adequately managed using oral formulation opioids. However, a small group, such as those presenting severe mucositis or requiring a rapid pain relief, should be managed by intravenous continuous infusion and/or by a patient-controlled analgesia system; while for patients in the community, there are distinct advantages to using the subcutaneous route. Other available routes of administration for opioids, can be used in selected circumstances, including rectal, transdermal, epidural, intrathecal and intramuscular. The invasive neuraxial route has a very limited role in patients with haematological malignancies, given the high risk of infection and bleeding. Through a close observation and a careful management, opioid-related side effects can be effectively prevented and treated. This article reviews the principles of opioid therapy and how opioids can be adapted for patients with pain due to haematological malignancies.

Norman Cousins Lecture. The uses and abuses of psychoneuroimmunology: a global overview

Studies of interactions between the nervous and immune systems that effect immunological and behavioral changes are relevant to our understanding biological issues pertinent to evolution, ethology, ecology, and aging, in addition to our understanding the immune and nervous systems per se. Psychoneuroimmunology also relates to homeland security, science education, and the practice of conventional as well as complementary and alternative medicine. This paper will highlight just some of these global implications of psychoneuroimmunology.

Molecular targets of opiate drug abuse in neuroAIDS

Opiate drug abuse, through selective actions at mu-opioid receptors (MOR), exacerbates the pathogenesis of human immunodeficiency virus-1 (HIV-1) in the CNS by disrupting glial homeostasis, increasing inflammation, and decreasing the threshold for pro-apoptotic events in neurons. Neurons are affected directly and indirectly by opiate-HIV interactions. Although most opiates drugs have some affinity for kappa (KOR) and/or delta (DOR) opioid receptors, their neurotoxic effects are largely mediated through MOR. Besides direct actions on the neurons themselves, opiates directly affect MOR-expressing astrocytes and microglia. Because of their broad-reaching actions in glia, opiate abuse causes widespread metabolic derangement, inflammation, and the disruption of neuron-glial relationships, which likely contribute to neuronal dysfunction, death, and HIV encephalitis. In addition to direct actions on neural cells, opioids modulate inflammation and disrupt normal intercellular interactions among immunocytes (macrophages and lymphocytes), which on balance further promote neuronal dysfunction and death. The neural pathways involved in opiate enhancement of HIV-induced inflammation and cell death, appear to involve MOR activation with downstream effects through PI3-kinase/Akt and/or MAPK signaling, which suggests possible targets for therapeutic intervention in neuroAIDS.

The nervous system and innate immunity: the neuropeptide connection

Many neuropeptides and peptide hormones are very similar to antimicrobial peptides in their amino acid composition, amphipathic design, cationic charge and size. Their antimicrobial activities suggest they may also be directly involved in innate defense. Here we discuss recent advances in understanding how the mammalian nervous system, equipped with neuropeptides and peptide hormones with potent antimicrobial properties, might directly defend the host from microbial assault. As examples, we have chosen five well described, locally produced neuropeptides that may serve a previously unrecognized, direct antimicrobial, innate immune function in four anatomical sites.

The immune system as the sixth sense

One of the truly remarkable discoveries in modern biology is the finding that the nervous system and immune system use a common chemical language for intra- and inter-system communication. This review will discuss some of the pivotal results that deciphered this chemical language. Specifically the nervous and immune systems produce a common set of peptide and nonpeptide neurotransmitters and cytokines that act on a common repertoire of receptors in the two systems. The paper will also review more recent studies that have delineated hardwired and humoral pathways for such bidirectional communication. This is discussed in the context of the idea that the sharing of ligands and receptors allows the immune system to serve as the sixth sense that notifies the nervous system of the presence of entities, such as viruses and bacteria, that are imperceptible to the classic senses. Lastly, this review will suggest ways to apply the newfound knowledge of the sixth sense to understand a placebo effect and to treat human disease.

Tissue levels of leu-enkephalin in rats with adjuvant arthritis

To study the level of leu-enkephalin in bone and joint tissues and in the spinal cord of rats with adjuvant arthritis, arthritis was induced in Lewis rats by the injection of Mycobacterium butyricum in Freund's incomplete adjuvant (FIA). Immunoelectron microscopy (IEM) was used to monitor the cellular distribution of leu-enkephalin in control and arthritis groups, and radioimmunoassay (RIA) was used to measure the concentration in the tissues. The results of IEM showed increased levels of leu-enkephalin in the matrix of the sciatic nerve, in nerve fibres in the synovial membrane and periosteum, as well as in fibroblasts and endothelial cells of the periosteum in arthritic groups. In macrophage-like cells of the synovial membrane as well as monocyte and polymorphonuclear lineage cells in the bone marrow, the level of leu-enkephalin was decreased in the arthritic group. The results of RIA showed that the concentration of leu-enkephalin was lower in the ankle and increased in the spinal cord of arthritic animals compared with controls. In conclusion, leu-enkephalin levels were decreased in joints and in bone marrow, but increased in nerve tissues in the group with arthritis. Further studies are needed to show whether leu-enkephalin is involved in a process that serves to limit the effect of immunisation.

Thermodynamic Measurements and Predictions of the Adsorption of Short-Chain Peptides on Nanothin Polymer Films

This contribution describes experimental measurements of submolecular-level interaction energies involved in the process of peptide adsorption on polymer films. The objective of this study was to use surface plasmon resonance (SPR) spectroscopy to measure the Gibbs energy change on adsorption (DeltaG(ad)) for pairs of various homopeptides on highly uniform, nanothin polymer films and to use these data, along with the principle of additivity, to predict DeltaG(ad) for homologous homopeptides, as well as for a mixed-residue peptide. By using a graft polymerization methodology, a nanothin poly(2-vinylpyridine) film was prepared and adsorption energies were measured first for a homologous series of tyrosine (Y) homopeptides on this film to determine submolecular-level interaction energies. By using SPR, adsorption isotherms were measured for YY and YYY peptides; analysis of these isotherms provided DeltaG(ad) data for a midchain tyrosine unit and a set of chain-end tyrosine units; values were -0.75 +/- 0.07 kcal/mol and -2.12 +/- 0.04 kcal/mol, respectively. Combining the thermodynamic contributions for adsorption of individual tyrosine units allowed a predictive estimate of -5.12 +/- 0.32 kcal/mol for the adsorption energy for YYYYYY; this estimate deviated by only 2.3% from its measured value of -5.24 +/- 0.06 kcal/mol. Similarly, adsorption energies were found for phenylalanine, glycine, and tyrosine-leucine peptides. Combining the thermodynamic contributions for adsorption of individual residue units allowed a predictive estimate of -3.24 +/- 0.38 kcal/mol for a pentapeptide, leucine enkephalin; this estimate deviated by only 3.0% from its measured value of -3.34 +/-0.11 kcal/mol.

Host gender in parasitic infections of mammals: an evaluation of the female host supremacy paradigm

A review of current literature on mammalian hosts' sexual dimorphism (SD) in parasitic infections revealed that (1) it is a scarcely and superficially studied biological phenomenon of considerable significance for individual health, behavior, and lifestyles and for the evolution of species; (2) there are many notable exceptions to the rule of a favorable female bias in susceptibility to infection; (3) a complex network of molecular and cellular reactions connecting the host's immuno-neuroendocrine systems with those of the parasite is responsible for the host-parasite relationship rather than just an adaptive immune response and sex hormones; (4) a lack of gender-specific immune profiles in response to different infections; (5) the direct effects of the host hormones on parasite physiology may significantly contribute to SD in parasitism; and (6) the need to enrich the reductionist approach to complex biological issues, like SD, with more penetrating approaches to the study of cause-effect relationships, i.e., network theory. The review concludes by advising against generalization regarding SD and parasitism and by pointing to some of the most promising lines of research.

Cystatin C as a cerebrospinal fluid biomarker for pain in humans

Through a process of subtraction cloning and differential hybridization, we previously identified several new genes whose expression was induced by peripheral inflammation. One of these coded for cystatin C, a secreted cysteine protease inhibitor in the cystatin superfamily. We hypothesized that, concurrent with increased expression in dorsal horn, increased secretion would elevate the cystatin C content in cerebrospinal fluid (CSF) during active pain states. Alterations were assessed by immunoassay and by surface enhanced laser desorption ionization (SELDI) mass spectrometry with either reverse phase or immobilized anti-cystatin C antibody surfaces using CSF from ten age-matched obstetrical patients at term. Five control subjects were scheduled for an elective caesarian section and were not in pain. Another five subjects were in labor for 8.9+/-1h and were in severe pain as assessed with a visual analog scale and the McGill short form questionnaire. The level of cystatin C as measured by immunoassay in the non-pain patients was 2.77+/-0.75 microg/ml and in the pain patients 5.36+/-0.92 microg/ml (P<0.02). The elevation occurred without significant change in total CSF protein or beta-endorphin content. The cystatin C increase also was detectable by SELDI with either raw CSF or after antibody capture. These data are consistent with our previous animal study and the idea that persistent pain induces the synthesis and release of cystatin C in dorsal spinal cord, the surplus of which overflows into the CSF.

Physiology of chronic spinal pain syndromes: from animal models to biomechanics

STUDY DESIGN

The literature and current research related to spinal pain mechanisms were reviewed, as well as animal models related to its study.

OBJECTIVES

To provide a pragmatic discussion of spinal pain that both reviews relevant research and coherently synthesizes the existing body of literature related to pain, nociception, animal modeling, and injury biomechanics.

SUMMARY OF BACKGROUND DATA

A detailed body of literature suggests that spinal pain mechanisms are quite complicated and involve a host of different processes (e.g., genetics, gender, neurophysiology, and biomechanics) that may contribute to clinical manifestations and symptoms.

METHODS

Both a review of the literature and a presentation of current and ongoing laboratory research are presented. Specific findings from the authors' laboratory using a rodent model of lumbar radiculopathy are presented to elucidate the role of local nerve root biomechanics in initiating and maintaining behavioral symptoms of nociception and pain.

RESULTS

For an understanding of chronic pain, a bidirectional-translational approach that incorporates cross-disciplinary methods such as in vivo biomechanical techniques is required. A conceptual model of chronic spine pain is proposed that details the dynamic and integrated roles of injury, biomechanics, and nociceptive physiology.

CONCLUSIONS

Areas of continued research are highlighted that may help guide the management of painful spine symptoms and syndromes.

Distribution of leucine-enkephalin in bone and joint tissues

The distribution and concentration of leu-enkephalin in periosteum, cortical bone, bone marrow and synovial membrane of normal rats were analysed. Periosteum, cortical bone and bone marrow of the rat femurs were collected as well as the ankles. The distribution of leu-enkephalin was analysed by immunoelectron microscopy and the concentration of leu-enkephalin was measured with radioimmunoassay. Immunoelectron microscopy showed that leu-enkephalin is abundant in monocytes of bone marrow, nerve fibers and endothelial cells in the periosteum and also in macrophage-like-cells of the synovial membrane. The concentration of leu-enkephalin measured by RIA showed highest concentration in bone marrow followed by periosteum and cortical bone. The study supports that leu-enkephalin is present and can be quantified in different compartments of bone and joint tissues suggesting that leu-enkephalin may be involved in the physiological regulation of nociception and immunoregulation.

Opioid control of inflammatory pain regulated by intercellular adhesion molecule-1

Pain can be effectively controlled by endogenous mechanisms based on neuroimmune interactions. In inflamed tissue immune cell-derived opioid peptides activate opioid receptors on peripheral sensory nerves leading to potent analgesia. This is brought about by a release of opioids from inflammatory cells after stimulation by stress or corticotropin-releasing hormone (CRH). Immunocytes migrate from the circulation to inflamed tissue in multiple steps, including their rolling, adhesion, and transmigration through the vessel wall. This is orchestrated by adhesion molecules on leukocytes and vascular endothelium. Intercellular adhesion molecule-1 [ICAM-1 (or CD54)] is expressed by endothelium and mediates adhesion and extravasation of leukocytes. The goal of this study was to show that ICAM-1 regulates the homing of opioid-producing cells and the subsequent generation of analgesia within sites of painful inflammation. This was accomplished using immunofluorescence, flow cytometry, and behavioral (paw pressure) testing. We found that ICAM-1 is upregulated on the vascular endothelium, simultaneously with an enhanced immigration of opioid-containing immune cells into inflamed paw tissue. The intravenous administration of a monoclonal antibody against ICAM-1 markedly decreased the migration of opioid-containing leukocytes and of granulocytes, monocytes-macrophages, and T cells to the inflamed tissue. At the same time, circulating immunocytes increased in numbers, and macroscopic inflammation (hyperalgesia, paw volume, and paw temperature) remained primarily unchanged. Most importantly, peripheral opioid analgesia elicited either by cold water swim stress or by intraplantar administration of CRH was dramatically reduced. Together, these findings indicate that ICAM-1 expressed on vascular endothelium recruits immunocytes containing opioids to promote the local control of inflammatory pain.

Clinical pharmacology of opioids for pain

The pharmacological effects of the opioid analgesics are derived from their complex interactions with three opioid receptor types (mu, delta, and kappa; morphine is an agonist at the mu opioid receptor). These receptors are found in the periphery, at presynaptic and postsynaptic sites in the spinal cord dorsal horn, and in the brain stem, thalamus, and cortex, in what constitutes the ascending pain transmission system, as well as structures that comprise a descending inhibitory system that modulates pain at the level of the spinal cord. The cellular effects of opioids include a decrease in presynaptic transmitter release, hyperpolarization of postsynaptic elements, and disinhibition. The endogenous opioid peptides are part of an endogenous pain modulatory system. A number of opioids are available for clinical use, including morphine, hydromorphone, levorphanol, oxymorphone, methadone, meperidine, oxycodone, and fentanyl, and their advantages and disadvantages for the management of pain are discussed. An understanding of the pharmacokinetic properties, as well as issues related to opioid rotation, tolerance, dependence, and addiction are essential aspects of the clinical pharmacology of opioids for pain.

Increased production of inflammatory cytokines in patients with silent myocardial ischemia

OBJECTIVES

The aim of the study was to examine the inflammatory cytokines in patients with myocardial ischemia to evaluate whether silent ischemia patients exhibit any particular cytokine pattern.

BACKGROUND

Silent myocardial ischemia is frequently observed in patients with coronary artery disease. Various endogenous mechanisms control a patient's perceived intensity of pain. Among them, the inflammatory process and the related cytokine production are known to modulate the threshold for activating the primary afferent nociceptors.

METHODS

Seventy-eight patients with reproducible exercise-induced myocardial ischemia were studied: 34 symptomatic patients, with rest and/or stress angina; 44 asymptomatic patients, with no symptoms during daily life activities or during positive exercise stress test. Venous blood samples were taken from all patients to evaluate the expression of CD11b receptors both on neutrophils and monocytes. Frozen plasma samples (at -80 degrees C) were used to quantify the anti-inflammatory (interleukin-4 and -10, transforming growth factor-beta) and the proinflammatory cytokines (tumor necrosis factor-alpha, interferon-gamma, interleukin-1beta and -6).

RESULTS

In asymptomatic patients lower CD11b receptor expression and higher concentration of anti-inflammatory cytokines were observed. Proinflammatory cytokine production was similar in the two groups.

CONCLUSIONS

The data suggest that an "anti-inflammatory pattern" of cytokine production correlates with silent ischemia and that the immune and inflammatory system activation may be crucial for angina symptoms.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

An opioid system in connective tissue: a study of achilles tendon in the rat

The occurrence of endogenous opioids and their receptors in rat achilles tendon was analyzed by immunohistochemistry (IHC), radioimmunoassay (RIA), and in vitro binding assays. The investigation focused on four enkephalins, dynorphin B, and nociceptin/orphanin FQ. Nerve fibers immunoreactive to all enkephalins (Met-enkephalin, Leu-enkephalin, Met-enkephalin-Arg-Gly-Lys, Met-enkephalin-Arg-Phe) were consistently found in the loose connective tissue and the paratenon, whereas dynorphin B and nociceptin/orphanin FQ could not be detected. The majority of enkephalin-positive nerve fibers exhibited varicosities predominantly seen in blood vessel walls. Measurable levels of Met-enkephalin-Arg-Phe and nociceptin/orphanin FQ were found in tendon tissue using RIA, whereas dynorphin B could not be detected. In addition to the endogenous opioids identified, delta-opioid receptors on nerve fibers were also detected by IHC. Binding assays to characterize the opioid binding sites showed that they were specific and saturable for [3H]-naloxone (Kd 7.01 +/- 0.98 nM; Bmax 23.52 +/- 2.23 fmol/mg protein). Our study demonstrates the occurrence of an opioid system in rat achilles tendon, which may be assumed to be present also in other connective tissues of the locomotor apparatus. This system may prove to be a useful target for pharmacological therapy in painful and inflammatory conditions by new drugs acting selectively in the periphery.

Opioids in chronic pain

The advance in our understanding of the biogenesis of various endogenous opioid peptides, their anatomical distribution, and the characteristics of the multiple receptors with which they interact open a new avenue for understanding the role of opioid peptide systems in chronic pain. The main groups of opioid peptides: enkephalins, dynorphins and beta-endorphin derive from proenkephalin, prodynorphin and proopiomelanocortin, respectively. Recently, a novel group of peptides has been discovered in the brain and named endomorphins, endomorphin-1 and -2. They are unique in comparison with other opioid peptides by atypical structure and high selectivity towards the mu-opioid receptor. Another group, which joined the endogenous opioid peptide family in the last few years is the pronociceptin system comprising the peptides derived from this prohormone, acting at ORL1 receptors. Three members of the opioid receptor family were cloned in the early 1990s, beginning with the mouse delta-opioid receptor (DOR1) and followed by cloning of mu-opioid receptor (MOR1) and kappa-opioid receptor (KOR1). These three receptors belong to the family of seven transmembrane G-protein coupled receptors, and share extensive structural homologies. These opioid receptor and peptide systems are significantly implicated in antinociceptive processes. They were found to be represented in the regions involved in nociception and pain. The effects of opioids in animal models of inflammatory pain have been studied in great detail. Inflammation in the periphery influences the central sites and changes the opioid action. Inflammation increased spinal potency of various opioid receptor agonists. In general, the antinociceptive potency of opioids is greater against various noxious stimuli in animals with peripheral inflammation than in control animals. Inflammation-induced enhancement of opioid antinociceptive potency is characteristic predominantly for mu opioid receptors, since morphine elicits a greater increase in spinal potency of mu- than of delta- and kappa-opioid receptor agonists. Enhancement of the potency of mu-opioid receptor agonists during inflammation could arise from the changes occurring in opioid receptors, predominantly in affinity or number of the mu-opioid receptors. Inflammation has been shown to alter the expression of several genes in the spinal cord dorsal horn. Several studies have demonstrated profound alterations in the spinal PDYN system when there is peripheral inflammation or chronic arthritis. Endogenous dynorphin biosynthesis also increases under various conditions associated with neuropathic pain following damage to the spinal cord and injury of peripheral nerves. Interestingly, morphine lacks potent analgesic efficacy in neuropathic pain. A vast body of clinical evidence suggests that neuropathic pain is not opioid-resistant but only that reduced sensitivity to systemic opioids is observed in this condition, and an increase in their dose is necessary in order to obtain adequate analgesia. Reduction of morphine antinociceptive potency was postulated to be due to the fact that nerve injury reduced the activity of spinal opioid receptors or opioid signal transduction. Our recent study with endogenous ligands of the mu-opioid receptor, endomorphins, further complicates the issue, since endomorphins appear to be effective in neuropathic pain. Identification of the involved differences may be of importance to the understanding of the molecular mechanism of opioid action in neuropathic pain, as well as to the development of better and more effective drugs for the treatment of neuropathic pain in humans.