Alteration in endogenous opioid systems due to chronic inflammatory pain conditions

Production and characterization of antibodies for the specific determination of the opioid peptide Met5-Enkephalin-Arg6-Phe7

Endogenous opioids serve as modulators of neuroendocrine and immune system processes, the investigation of which calls for high-specificity radioimmunoassays (RIAs). This study focuses on the development and use of a specific radioimmunoassay for the opioid peptide Met5-Enkephalin-Arg6-Phe7 (MEAP), the C-terminus part of proenkephalin A. Antibodies were raised in four rabbits and investigated in terms of titre, avidity and specificity, followed by finding ideal conditions for these antibodies in RIA. MEAP concentrations were determined in crude extracts of rat hypothalamus, dorsal root ganglia, adrenals and ankle using this standardized assay after an oxidizing process. At reverse-phase high-pressure liquid chromatography (HPLC), the position of immunoreactive material from rat hypothalamus eluted as two peaks out of which one was compatible with that of synthetic MEAP. All rabbits exhibited individual differences in relative immune response and time of its onset. The avidity constant was 10 times higher on a molar basis for ab 4108 compared with ab 4182. There was no cross-reactivity for ab 4182 to related peptides, such as enkephalins and dynorphin B, and negligible background values for ab 4108. The relative levels ofimmunoreactive MEAP from the CNS versus peripheral tissues contrasted in accordance with current knowledge. It is suggested that reports with RIA results should include characterization of antibodies, extraction procedures, standard curves and compositions of buffers. Furthermore, the results should preferably be expressed in relation to total protein content.

Pain-induced alterations in the dynorphinergic system within the mesocorticolimbic pathway: Implication for alcohol addiction

Latest studies have revealed that pain negatively impacts on reward processing and motivation leading to negative affective states and stress. These states not only reduce quality of life of patients by increasing the appearance of psychiatric comorbidities, but also have an important impact on vulnerability to drug abuse, including alcohol. In fact, clinical, epidemiological but also preclinical studies have revealed that the presence of pain is closely related to alcohol use disorders (AUDs). All this evidence suggests that pain is a factor that increases the risk of suffering AUD, predicting heavy drinking behavior and relapse drinking in those patients with a previous history of AUD. The negative consequences of chronic pain and its impact on stress and AUD are likely mediated by alterations in the central nervous system, especially in the stress and reward systems. Therefore, pain and stress impact on dopaminergic mesolimbic pathway can lead to an increase in drug abuse liability. In this mini review we analyze the interaction between pain, stress, and alcohol addiction, and how dynamic changes in the kappa opioid system might play a crucial role in the development of compulsive alcohol drinking in chronic pain patients.

Does the kappa opioid receptor system contribute to pain aversion?

The kappa opioid receptor (KOR) and the endogenous peptide-ligand dynorphin have received significant attention due the involvement in mediating a variety of behavioral and neurophysiological responses, including opposing the rewarding properties of drugs of abuse including opioids. Accumulating evidence indicates this system is involved in regulating states of motivation and emotion. Acute activation of the KOR produces an increase in motivational behavior to escape a threat, however, KOR activation associated with chronic stress leads to the expression of symptoms indicative of mood disorders. It is well accepted that KOR can produce analgesia and is engaged in chronic pain states including neuropathic pain. Spinal studies have revealed KOR-induced analgesia in reversing pain hypersensitivities associated with peripheral nerve injury. While systemic administration of KOR agonists attenuates nociceptive sensory transmission, this effect appears to be a stress-induced effect as anxiolytic agents, including delta opioid receptor agonists, mitigate KOR agonist-induced analgesia. Additionally, while the role of KOR and dynorphin in driving the dysphoric and aversive components of stress and drug withdrawal has been well characterized, how this system mediates the negative emotional states associated with chronic pain is relatively unexplored. This review provides evidence that dynorphin and the KOR system contribute to the negative affective component of pain and that this receptor system likely contributes to the high comorbidity of mood disorders associated with chronic neuropathic pain.

Met-enkephalins in patients with inflammatory bowel diseases

PURPOSE

Opioid peptides provide a link between the neuroendocrine and immune systems. They modify the inflammatory process through their effect on the synthesis and secretion of cytokines and on the proliferation of leukocytes to the inflammatory lesion. The evaluation analyzed changes in free met-enkephalin concentration values in the serum and colon mucosal biopsy specimens of patients with inflammatory bowel disease (IBD).

MATERIAL AND METHODS

In serum and colon mucosal biopsy specimens, free met-enkephalin levels were determined in 43 patients with ulcerative colitis (UC) and 38 individuals with Crohn's disease (CD). The evaluation analyzed the effect of disease activity, inflammatory lesions of the colon and laboratory parameters, on the level of the investigated marker. The control group consisted of 45 healthy volunteers.

RESULTS

Serum free met-enkephalin levels were depressed in patients with CD (85.4pg/ml) and UC (101.5pg/ml) as compared to the controls (119.4pg/ml). Met-enkephalin levels in colonic biopsies collected from inflammatory lesions in IBD patients were significantly higher as compared to sections without inflammatory lesions (6.59pg/mg vs. 2.89pg/mg, p < 0.01 in the CD group and 6.12pg/mg vs. 3.47pg/mg, p < 0.05 in the UC group) and their level correlated with disease activity.

CONCLUSIONS

The present investigation is the first study that demonstrates changes in free met-enkephalin levels in IBD that may play a role in the pathogenesis and course of the disease. Further studies are necessary to assess the anti-inflammatory effect of opioid peptides.

Mass spectrometry-based neuropeptidomics of secretory vesicles from human adrenal medullary pheochromocytoma reveals novel peptide products of prohormone processing

Neuropeptides are required for cell-cell communication in the regulation of physiological and pathological processes. While selected neuropeptides of known biological activities have been studied, global analyses of the endogenous profile of human peptide products derived from prohormones by proteolytic processing in vivo are largely unknown. Therefore, this study utilized the global, unbiased approach of mass spectrometry-based neuropeptidomics to define peptide profiles in secretory vesicles, isolated from human adrenal medullary pheochromocytoma of the sympathetic nervous system. The low molecular weight pool of secretory vesicle peptides was subjected to nano-LC-MS/MS with ion trap and QTOF mass spectrometry analyzed by different database search tools (InsPecT and Spectrum Mill). Peptides were generated by processing of prohormones at dibasic cleavage sites as well as at nonbasic residues. Significantly, peptide profiling provided novel insight into newly identified peptide products derived from proenkephalin, pro-NPY, proSAAS, CgA, CgB, and SCG2 prohormones. Previously unidentified intervening peptide domains of prohormones were observed, thus providing new knowledge of human neuropeptidomes generated from precursors. The global peptidomic approach of this study demonstrates the complexity of diverse neuropeptides present in human secretory vesicles for cell-cell communication.

Neonatal injury alters adult pain sensitivity by increasing opioid tone in the periaqueductal gray

Studies in both rodents and humans have shown that acute inflammatory pain experienced during the perinatal period produces long-term decreases in pain sensitivity (hypoalgesia) (Grunau et al., 1994a, 2001; Ren et al., 2004; LaPrairie and Murphy, 2007). To date, the mechanisms underlying these long-term adaptations, however, have yet to be elucidated. The present studies tested the hypothesis that neonatal inflammatory pain induces an upregulation in endogenous opioid tone that is maintained into adulthood, and that this increase in opioid tone provides the underlying mechanism for the observed hypoalgesia. On the day of birth (P0), inflammatory pain was induced in male and female Sprague-Dawley rats by intraplantar administration of carrageenan (CGN; 1%). In adulthood (P60), these animals displayed significantly increased paw withdrawal latencies in response to a noxious thermal stimulus in comparison to controls. Systemic administration of the brain-penetrant opioid receptor antagonist naloxone HCl, but not the peripherally restricted naloxone methiodide, significantly attenuated the injury-induced hypoalgesia. Direct administration of naloxone HCl or antagonists directed at the mu or delta opioid receptors into the midbrain periaqueductal gray (PAG) also significantly reversed the injury-induced hypoalgesia in adult rats. Parallel anatomical studies revealed that inflammatory pain experienced on the day of birth significantly increased beta-endorphin and met/leu-enkephalin protein levels and decreased opioid receptor expression in the PAG of the adult rat. Thus, early noxious insult produces long-lasting alterations in endogenous opioid tone, thereby profoundly impacting nociceptive responsiveness in adulthood.

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.

Delta opioid receptor mRNA expression is changed in the thalamus and brainstem of monoarthritic rats

Changes in the mRNA expression of neurotransmitters receptors under chronic pain conditions have been described in various areas of the central nervous system (CNS). Delta opioid receptors (DORs) have been implicated in pain mechanisms but, although its mRNA expression has been studied in the rat CNS, there are no reports describing its distribution in specific thalamic and brainstem nuclei during chronic inflammatory pain. Here, in situ hybridization for DOR mRNA was performed in brain sections from control and monoarthritic (MA) rats with 2, 4, 7 and 14 days of inflammation. Grain densities were determined bilaterally in the ventrobasal complex (VB), posterior (Po), centromedial/centrolateral (CM/CL) and reticular (Rt) nuclei of the thalamus, and in the dorsal reticular (DRt), lateral reticular (LRt) and parvocellular reticular (PCRt) nuclei of the brainstem. Control animals exhibited weak mRNA expression in the VB, Po and CM/CL, as well as in PCRt, while moderate grain densities were observed in the Rt, DRt and LRt. During MA, DOR mRNA expression was significantly decreased (22%) in the Rt contralateral to the affected joint at both 7 and 14 days of inflammation, as compared to controls. A bilateral reduction (35%) was also observed in the DRt at 14 days of MA, while a contralateral increase was found in the PCRt at 7 days (+39%). No significant changes were observed in the other regions analyzed. Thus, data show changes in the DOR mRNA expression during the development of chronic inflammatory pain, in thalamic and brainstem nuclei implicated in pain processing mechanisms.

Inhibition of inflammatory pain by CRF at peripheral, spinal and supraspinal sites: involvement of areas coexpressing CRF receptors and opioid peptides

There is conflicting evidence on the antinociceptive effects of corticotropin-releasing factor (CRF) along the neuraxis of pain transmission and the responsible anatomical sites of CRF's action at the level of the brain, spinal cord and periphery. In an animal model of tonic pain, that is, Freunds complete adjuvant (FCA) hindpaw inflammation, we systematically investigated CRF's ability to modulate inflammatory pain at those three levels of pain transmission by algesiometry following the intracerebroventricular, intrathecal, and intraplantar application of low, systemically inactive doses of CRF. At each level, CRF elicits potent antinociceptive effects, which are dose dependent and antagonized by local, but not systemic CRF receptor antagonist alpha-helical CRF indicating CRF receptor specificity. Consistently, we have identified by immunohistochemistry multiple brain areas, inhibitory interneurons within the dorsal horn of the spinal cord as well as immune cells within subcutaneous tissue--but not peripheral sensory neurons--that coexpress both CRF receptors and opioid peptides. In line with these anatomical findings, local administration of CRF together with the opioid receptor antagonist naloxone dose-dependently reversed CRF's antinociceptive effects at each of these three levels of pain transmission. Therefore, local application of low, systemically inactive doses of CRF at the level of the brain, spinal cord and periphery inhibits tonic inflammatory pain most likely through an activation of CRF receptors on cells that coexpress opioid peptides which results in opioid-mediated pain inhibition. Future studies have to delineate whether endogenous CRF at these three levels contributes to the body's response to cope with the stressful stimulus pain in an opioid-mediated manner.

Opioid peptides and receptors in joint tissues: study in the rat

Using immunohistochemical and biochemical techniques, the occurrence of endogenous opioid peptides and their receptors in normal rat bone and joint tissues was investigated. Opioid receptors were detected, quantified, and characterized in homogenates from capsule/synovium and periosteum using radioligand binding assays. Receptor binding of the nonselective opioid [3H]naloxone to tissue homogenates was stereospecific and saturable, showing similar characteristics to that of brain tissue, although with lower binding capacities. By immunohistochemistry, the neuronal occurrence of four different enkephalins was demonstrated in synovium, bone marrow, periosteum, and juxta-articular bone, whereas no neuronal dynorphin immunoreactivity was detected. Double-staining studies disclosed that enkephalins coexisted with substance P in primary afferent fibers. The applied techniques can be used to assess changes in the distribution of endogenous opioids and their receptors in joint tissues in conditions associated with pain and inflammation. The endogenous opioid system now demonstrated might be targeted and exploited therapeutically to obtain peripheral control of symptoms in joint disorders.

Persistent pain model reveals sex difference in morphine potency

Central or systemic administration of agonists directed at the mu or delta opiate receptors generally produce a greater degree of analgesia in males than in females. To date, most studies examining sex-based differences in opioid analgesia have used acute noxious stimuli (i.e., tail-flick and hot plate test); thus the potential dimorphic response of centrally acting opiates in the alleviation of persistent inflammatory pain is not well established. In the present study, right hind paw withdrawal latency (PWL) to radiant thermal stimuli was measured in intact male and cycling female Sprague-Dawley rats before and after unilateral hind paw injection of the inflammatory agent complete Freund's adjuvant (CFA). Control animals received intraplantar injection of saline. Twenty four hours after CFA or saline injection, animals received either saline or morphine bisulfate (0.5-15 mg/kg sc). Separate groups of control or inflamed animals were tested on their responsiveness to morphine at 7, 14, and 21 days post-CFA or saline. No sex differences were noted for baseline PWLs, and females displayed slightly less thermal hyperalgesia at 24 h post-CFA. At all morphine doses administered, both the antihyperalgesic effects of morphine in the inflamed animals and the antinociceptive effects of morphine in control animals were significantly greater in males compared with females. Similarly, in males, the antihyperalgesic effects of morphine increased significantly at 7-21 days post-CFA; no significant shift in morphine potency was noted for females. These studies demonstrate sex-based differences in the effects of morphine on thermal hyperalgesia in a model of persistent inflammatory pain.

Anti-inflammatory effects of contralateral administration of the kappa-opioid agonist U-50,488H in rats with unilaterally induced adjuvant arthritis

OBJECTIVE

The effect of repeated contralateral treatment with the kappa-opioid receptor agonist U-50,488H {trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzene acetamide methanesulphonate} was investigated in rats with unilaterally induced adjuvant arthritis.

METHODS

Arthritis was induced by injection of Mycobacterium butyricum into the right hindpaw. Inflammatory parameters, nociceptive behaviour and cartilage turnover were evaluated up to 21 days after induction of arthritis.

RESULTS

Contralateral treatment with 0.3 mg U-50,488H into the left hindpaw twice per week reduced the hindpaw oedema, ankle joint inflammation, pain behaviour to mechanical stimuli and severity score of inflammation in the hindpaws of both sides as well as the systemic spread of inflammation to other areas, e.g. tail and/or forepaws, compared with saline-treated animals. Moreover, a significant decrease in the levels of cartilage oligomeric matrix protein was found in animals treated with U-50,488H, suggesting reduction of cartilage damage. The anti-inflammatory and chondroprotective effects of U-50,488H were abolished by administration of the peripheral opioid receptor antagonist naloxone methiodide.

CONCLUSIONS

This is the first report demonstrating that repeated contralateral administration of a kappa-opioid receptor agonist diminishes the development of a symmetrical joint disorder.

Attention to pain localization and unpleasantness discriminates the functions of the medial and lateral pain systems

Functional imaging studies have identified a matrix of structures in the brain that respond to noxious stimuli. Within this matrix, a division of function between sensory-discriminative and affective responses has so far been demonstrated by manipulating either pain intensity or unpleasantness under hypnosis in two different normal volunteer groups studied on separate occasions. Our study used positron emission tomography (PET) to demonstrate this division of function under more natural conditions in a healthy group of volunteers, using a CO(2) laser to provide nociceptive stimuli that selectively activate A-delta and C-fibres without contamination by touch sensations. We measured the differential cerebral responses to noxious and innocuous laser stimuli during conditions of selective attention to either the unpleasantness or location of the stimuli. Attention to location increased responses in the contralateral (right) primary somatosensory and inferior parietal cortices. This result implies that these components of the lateral pain system are concerned mainly with the localization of pain. In contrast, attention to unpleasantness increased responses in bilateral perigenual cingulate and orbitofrontal cortices, contralateral (right) amygdala, ipsilateral (left) hypothalamus, posterior insula, M1 and frontal pole. These areas comprise key components of the medial pain and neuroendocrine systems and the results suggest that they have a role in the affective response to pain. Our results indicate the importance of attentional effects on the pattern of nociceptive processing in the brain. They also provide the first clear demonstration, within a single experiment, of a major division of function within the neural pain matrix.

A rat model of unilateral hindpaw burn injury: Slowly developing rightwards shift of the morphine dose–response curve

Management of pain after burn injury is an unresolved clinical issue. In a rat model of hindpaw burn injury, we examined the effects of systemic morphine on nociceptive behaviors following injury. Injury was induced by immersing the dorsal part of one hindpaw into a hot water bath (85 degrees C) for 4, 7, or 12 s under pentobarbital anesthesia. Mechanical allodynia to von Frey filament stimulation and thermal hyperalgesia to radiant heat were assessed. Burn injury induced by the 12-s (but not 4-, or 7-s) hot water immersion resulted in reliable and lasting mechanical allodynia and thermal hyperalgesia evident by day 1. In addition, there was an upregulation of protein kinase Cgamma and a progressive downregulation of mu-opioid receptors within the spinal cord dorsal horn ipsilateral to injury as revealed by immunohistochemistry and Western blot. In both injured and sham rats, the anti-nociceptive effects of subcutaneous morphine were examined on post-injury days 7 and 14. While the morphine AD50 dose was comparable on day 7 between burn (1.61 mg/kg) and control (1.7 mg/kg) rats, the morphine dose-response curve was shifted to the right in burn-injured rats (4.6 mg/kg) on post-injury day 14 as compared with both the injured rats on post-injury day 7 and sham rats on day 14 (1.72 mg/kg). These data indicate that hindpaw burn injury reliably produces persistent mechanical allodynia and thermal hyperalgesia and that the reduced efficacy of morphine anti-nociception in chronic burn injury may be in part due to a downregulation of spinal mu-opioid receptors.

Mu and delta opioid-stimulated [35S]GTP gamma S binding in brain and spinal cord of polyarthritic rats

Polyarthritis induced by inoculation with complete Freund's adjuvant alters opioid peptides, but does not affect opioid receptor binding. This study was conducted to measure mu and delta opioid receptor-stimulated G-protein activity in brain and spinal cord of rats 19 days after injection of complete Freund's adjuvant or vehicle. Mu and delta opioid-stimulated [35S]GTPgammaS binding measured autoradiographically in caudate-putamen, medial thalamus and periaqueductal gray was unchanged in polyarthritic rats. Delta opioid-stimulated [35S]GTPgammaS binding was significantly decreased in the spinal cord of polyarthritic rats, whereas mu opioid-stimulated activity was unchanged. These data reveal that the functional activity of delta opioid receptors in the spinal cord is altered in polyarthritis.

Cerebrospinal fluid levels of opioid peptides in fibromyalgia and chronic low back pain

BACKGROUND

The mechanism(s) of nociceptive dysfunction and potential roles of opioid neurotransmitters are unresolved in the chronic pain syndromes of fibromyalgia and chronic low back pain.

METHODS

History and physical examinations, tender point examinations, and questionnaires were used to identify 14 fibromyalgia, 10 chronic low back pain and 6 normal control subjects. Lumbar punctures were performed. Met-enkephalin-Arg6-Phe7 (MEAP) and nociceptin immunoreactive materials were measured in the cerebrospinal fluid by radioimmunoassays.

RESULTS

Fibromyalgia (117.6 pg/ml; 85.9 to 149.4; mean, 95% C.I.; p = 0.009) and low back pain (92.3 pg/ml; 56.9 to 127.7; p = 0.049) groups had significantly higher MEAP than the normal control group (35.7 pg/ml; 15.0 to 56.5). MEAP was inversely correlated to systemic pain thresholds. Nociceptin was not different between groups. Systemic Complaints questionnaire responses were significantly ranked as fibromyalgia > back pain > normal. SF-36 domains demonstrated severe disability for the low back pain group, intermediate results in fibromyalgia, and high function in the normal group.

CONCLUSIONS

Fibromyalgia was distinguished by higher cerebrospinal fluid MEAP, systemic complaints, and manual tender points; intermediate SF-36 scores; and lower pain thresholds compared to the low back pain and normal groups. MEAP and systemic pain thresholds were inversely correlated in low back pain subjects. Central nervous system opioid dysfunction may contribute to pain in fibromyalgia.

Decrease in non-selective, non-sustained attention induced by a chronic visceral inflammatory state as a new pain evaluation in rats

The aim of this study was to develop a new behavioral pain test based on the evaluation of cognitive capacity impairments in rats with colitis and to determine the impact of different acute analgesic treatments. Colitis was induced in rats by an enema containing 2,4,6-trinitrobenzen sulfonic acid. Visual non-selective, non-sustained attentional level was assessed by a new behavioral testing procedure. Animals were familiarized on three consecutive days with an open field containing four small, similar, familiar objects. On the day of testing, one of the objects was randomly replaced by a new one. Attentional level was determined by the ability of the rat to perceive this small modification to its familiar environment. The effect of morphine, acetaminophen, aspirin or ibuprofen treatment was assessed on testing day and compared with that observed during a Von Frey test to assess referred tactile hypersensitivity of the skin of the lower back. Rats with colitis had decreased attentional level but no change in their locomotor activity, interest in the environment or memory encoding. Morphine (1 mg/kg, s.c. and 10 microg/rat, i.t.) and acetaminophen (200 mg/kg, p.o.) had a beneficial effect on attentional level and on referred tactile hypersensitivity. Testing for the latter showed that aspirin and ibuprofen (400 mg/kg, p.o.) were ineffective. The decrease in visual non-selective, non-sustained attention induced by chronic inflammatory painful state can be relieved by effective analgesic treatments. This finding could lead to the development of a new behavioral test to assess spontaneous pain in chronic painful subjects.

Attenuation of mechanical but not thermal hyperalgesia by electroacupuncture with the involvement of opioids in rat model of chronic inflammatory pain

Opioid peptides have been proven effective in reducing the sign of hyperalgesia associated with inflammation. Electroacupuncture (EA) produces antinociception via release of endogenous opioid peptides in normal rats. Moreover, intrathecal injection of dynorphin has antinociceptive effect in rats. The present study was designed to examine whether EA has effect on the thermal and mechanical hyperalgesia in rat model of complete Freund's adjuvant (CFA)-induced inflammatory pain. The results are the following: (1) single session of 100Hz EA (0.5-1.0-1.5 mA, 10 min for each intensity) at both Zusanli (ST 36) and Sanyinjiao acupoints (SP 6) significantly increased mechanical withdrawal threshold determined by von Frey filaments but not with thermal withdrawal latency that is determined by hot plate (52 +/- 0.2 degrees C); (2) 100 Hz EA applied twice a week for 4 weeks and showed a significant decrease in the mechanical hyperalgesia at the third and fourth week, with no effect on thermal hyperalgesia; (3) naloxone (20 mg kg(-1)) had the ability to reverse the inhibition of the mechanical hyperalgesia produced by a single session of EA. In conclusion, the present results indicate that a single or repetitive EA could reduce mechanical hyperalgesia, but not thermal hyperalgesia, in CFA-inflammatory pain rats, and the opioid system might be involved in these effects.

FE200041 (d-Phe-d-Phe-d-Nle-d-Arg-NH2): A Peripheral Efficacious κ Opioid Agonist with Unprecedented Selectivity

The side effects typically associated with the clinical profiles of opioid mu-receptor agonists have driven continuing efforts to identify novel efficacious analgesics, including agonists acting at opioid kappa receptors. Unfortunately, the therapeutic potential of kappa agonists seems limited by significant central nervous system side effects. Kappa opioid agonists, however, exhibit potent peripherally mediated antihyperalgesic and antinociceptive effects, suggesting that a peripherally acting kappa agonist may be efficacious in pain control with a more desirable safety profile than that associated with currently available opioids. Here, we report an all D-amino acid tetrapeptide characterized as a novel, highly selective kappa opioid receptor agonist. FE200041 (D-Phe-D-Phe-D-Nle-D-Arg-NH2) showed selectivity for the human kappa opioid receptor of greater than 30,000- and 68,000-fold versus human mu opioid receptor and human delta-opioid receptor receptors, respectively, and efficacious agonist activity using in vitro tissue assays. FE200041 produced local, peripheral antinociception in the hindpaw ipsilateral, but not contralateral, to injection. Antinociceptive effects of FE200041 in the mouse acetic acid writhing assay lasted over 60 min and were antagonized by naloxone and by selective kappa, but not mu, opioid receptor antagonists. FE200041 significantly inhibited acetic acid writhing and inhibited formalin-induced flinching in rats. FE200041 did not elicit sedation or motor impairment after systemic administration at a dose 10-fold higher than that needed to achieve antinociception. FE200041 is thus a potent peripherally restricted opioid kappa agonist with no demonstrable side effects typical of kappa agonists with central nervous system activity and with unprecedented selectivity for the opioid kappa receptor. The pharmacology of this compound suggests the possibility of therapeutic application.

Characterization of mu opioid receptor binding and G protein coupling in rat hypothalamus, spinal cord, and primary afferent neurons during inflammatory pain

Peripheral analgesic effects of opioids are pronounced under inflammatory conditions, e.g., arthritis; however, little is known about adaptive changes of micro opioid receptor binding and G protein coupling in the peripheral versus central nervous system. The present study investigated the effects of inflammation on mu opioid receptor (MOP receptor) binding and G protein coupling of supraspinal, spinal, and peripheral MOP receptors. In addition, MOP receptors were identified in immunohistochemical experiments in dorsal root ganglia (DRG) of inflamed and noninflamed rats. The number of MOP receptor binding sites decreased from hypothalamus (HT) > spinal cord (SC) > DRG. Unilateral Freund's complete adjuvant inflammation of one hindpaw induced a significant up-regulation of MOP receptor sites only in DRG but not in HT or SC. This up-regulation was time-dependent, restricted to the inflamed side, and showed a peak at 24 h. The full-agonist [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO) induced MOP receptor G protein coupling with decreasing efficacies (E(max)) from HT > SC > DRG. Inflammation resulted in significant increases in MOP receptor G protein coupling only in membranes of DRG, but not in HT, SC, or DRG on the contralateral side of inflammation. This suggests that changes in MOP receptor levels are not related to systemically released mediators. These findings show that inflammation causes changes in MOP receptor binding and G protein coupling after DAMGO stimulation selectively in primary afferent neurons but did not cause any adaptive changes of MOP receptor in HT or SC.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The behavioral and neuronal effects induced by repetitive nociceptive stimulation are affected by gonadal hormones in male rats

The role of gonadal hormones in inducing long-term modifications in response to transient nociceptive stimuli was investigated in adult male rats. Three weeks after gonadectomy or sham surgery, animals were randomly divided into groups to be exposed to sham (only a prick in the dorsal hind paw) or formalin treatment (50 microl, 5% s.c. in the dorsal hind paw) once a week for the following 3 weeks. In gonadectomized animals the formalin-induced responses (licking, flexing and jerking of the injected paw) did not differ from those of intact animals after the first formalin injection. However, their levels were higher after the second or third injections. Indeed, in intact animals the formalin-induced responses progressively decreased, being significantly lower after the third injection than after the first; in gonadectomized animals, the formalin-induced responses did not change with repetition of the formalin treatment. In intact rats, c-Fos expression in the paraventricular nucleus of the thalamus and arcuate nucleus of the hypothalamus remained at control levels or decreased in animals injected two or three times with formalin; in gonadectomized rats, c-Fos expression increased with repetition of the noxious stimulation, reaching the highest levels in animals injected three times with formalin. These results show that male gonadal hormones have an inhibitory, adaptive effect on the behavioral and neuronal responses to repeated nociceptive stimulation.

Adeno-associated viral transfer of opioid receptor gene to primary sensory neurons: a strategy to increase opioid antinociception

To develop a genetic approach for the treatment of pain, we introduced a recombinant adeno-associated viral (rAAV) vector containing the cDNA for the mu-opioid receptor (muOR) into primary afferent neurons in dorsal root ganglia (DRGs) of rats, which resulted in a long-lasting (>6 months) increase in muOR expression in DRG neurons. The increase greatly potentiated the antinociceptive effects of morphine in rAAV-muOR-infected rats with and without inflammation. Perforated patch recordings indicated that the efficacy and potency of opioid inhibition of voltage-dependent Ca(2+) channels were enhanced in infected neurons, which may underlie the increase in opiate efficacy. These data suggest that transfer of opioid receptor genes into DRG cells with rAAV vectors may offer a new therapeutic strategy for pain management.

Increase of nociceptive threshold induced by intrathecal injection of interleukin-1beta in normal and carrageenan inflammatory rat

The present study was to investigate the effect of intrathecal (i.t.) injection of interleukin-1 beta (IL-1 beta) on nociception in normal and inflammatory rats. Peripheral inflammation was induced by intraplantar injection (i.pl.) of carrageenan into unilateral hind paw. The nociceptive threshold to noxious thermal stimulation was measured by the paw withdrawal latency (PWL). Intrathecal injection of IL-1 beta (10 ng, 100 ng) significantly increased PWL in normal rats, the peak occurred at 5 min and the effect lasted for 30 min. Similarly, IL-1 beta (10 ng, 100 ng, i.t.) significantly increased the PWL and lasted for more than 60 min in inflammatory rats. Both in normal and inflammatory rats, the IL-1 beta-induced antinociceptive effect was completely abolished by IL-1ra (50 ng, i.t.), and apparently attenuated by naloxone (10 microg, i.t.) or mianserin (20 microg, i.t.). These results suggest that IL-1 beta produces antinociceptive effect by binding IL-1 receptor at the spinal level, and is related to the activation of opioid and 5-HT systems.