The μ opiate receptor as a candidate gene for pain: Polymorphisms, variations in expression, nociception, and opiate responses

There are differences between human individuals and between mouse strains in levels of μ opiate receptor (μOR) expression, responses to painful stimuli, and responses to opiate drugs. One of the best candidates for contributing to these differences is variation at the μOR gene locus. Support for this idea comes from analyses of the human and murine μOR genes. Assessments of individual differences in human μOR expression add further support. Studies with mice, including knockout-transgenic, quantitative trait locus, and strain-comparison studies, also strongly support the possibility that μOR gene alleles would be strong candidates for contributing to individual differences in human nociception and opiate drug responses. This paper reviews current analyses of the murine and human μOR genes, their important variants, and correlations between these variants and opiate influences on pain.

The intrinsic antinociceptive effects of oxycodone appear to be kappa-opioid receptor mediated

Our previous studies in the Sprague-Dawley rat showed that the intrinsic antinociceptive effects of oxycodone are naloxone reversible in a manner analogous to morphine but that in contrast to morphine, oxycodone's antinociceptive effects have a rapid onset of maximum effect (approximate to 5-7 min compared to 30-45 min for morphine), comprise one antinociceptive phase (compared to two phases) and are of relatively short duration (approximate to 90 min compared to approximate to 180 min). In the present study, administration of a range of selective opioid receptor antagonists has shown that the intrinsic antinociceptive effects of oxycodone (171 nmol) are not attenuated by i.c.v. administration of (i) naloxonazine, a mu(1)-selective opioid receptor antagonist, or (ii) naltrindole, a delta-selective opioid receptor antagonist, in doses that completely attenuated the intrinsic antinociceptive effects of equipotent doses of the respective mu- and delta-opioid agonists, morphine and enkephalin-[D-Pen(2,5)] (DPDPE). Although beta-funaltrexamine (beta-FNA) attenuated the antinociceptive effects of oxycodone (171 nmol i.c.v.), it also attenuated the antinociceptive effects of morphine and bremazocine (kappa-opioid agonist) indicative of non-selective antagonism. Importantly, the antinociceptive effects of oxycodone (171 nmol i.c.v.) were markedly attenuated by the prior i.c.v. administration of the selective kappa-opioid receptor antagonist, norbinaltorphimine (nor-BNI), in a dose (0.3 nmol) that did not attenuate the antinociceptive effects of an equipotent dose of i.c.v. morphine (78 nmol). Taken together, these data strongly suggest that the intrinsic antinociceptive effects of oxycodone are mediated by K-opioid receptors, in contrast to morphine which interacts primarily with mu-opioid receptors. (C) 1997 International Association for the Study of Pain. Published by Elsevier Science B.V.

The Role of Skin Opioid Receptor System in Itch

Opioid receptors are key players in induction of chronic itch. This could be confirmed using opiate receptor knockout mice experiments and clinical studies on patients with chronic itch. We have induced a dry skin dermatitis as a model for chronic itching on -(MOR) and -(KOR) opioid receptor knockout (KO) mice. MOR KO mice scratched significantly less than wild type (WT). Additionally the epidermal hypertrophy caused by chronic dermatitis and the amount of epidermal nerve endings in MOR KO mice were significantly decreased than in WT mice. KOR KO mice showed similar scratching behavior as MOR KO mice; however the changes were less significant. In addition, we performed a double blind, placebo controlled, cross over study using topically applied opioid receptor antagonist, Naltrexone, on patients with pruritus in atopic dermatitis. The results revealed significant effects of the topical application of Naltrexone in patients with chronic pruritus (45% improvement of pruritus by VAS compared to placebo, n=24), but not in patients with acute pruritus (7%, n=15). These studies establish the clinical relevance of MOR system and the peripheral, epidermal nerve endings in chronic pruritus and warrant further research and therapeutic potential for such research.

Opiate activation suppresses the drinking, pressor and natriuretic responses induced by cholinergic stimulation of the medial septal area

The present study investigates the participation and interaction between cholinergic and opiate receptors of the medial septal area (MSA) in the regulation of Na+, K+ and water excretion, drinking and blood pressure regulation. Male Holtzman rats were implanted with stainless steel cannulae opening into the MSA. Na+, K+ and water excretion, water intake and blood pressure were measured after injection of carbachol (cholinergic agonist), FK-33824 (an opiate agonist) + carbachol or naloxone (an opiate antagonist) + carbachol into MSA. Carbachol (0.5 or 2.0 nmol) induced an increase in Na+ and K+ excretion, water intake and blood pressure and reduced the urinary volume. FK-33824 reduced the urinary volume and Na+ and K+ excretion. Previous injection of FK-33824 (100 ng) into the MSA blocked the increases in Na+ and K+ excretion, water intake and blood pressure induced by carbachol. Naloxone (10 μg) produced no changes in the effect of 2.0 nmol carbachol, but potentiated the natriuretic effect induced by 0.5 nmol dose of carbachol. These data show an inhibitory effect of opiate receptors on the changes in cardiovascular, fluid and electrolyte balance induced by cholinergic stimulation of the MSA in rats. © 1992.

Effect of naloxone on food competition aggression in food-restricted high and low aggression pigeons (Columba livia)

We determined the effect of the opiate receptor antagonist naloxone on aggression, emotion, feeder control, and eating behavior in high and low aggression female pigeons maintained at 80% of their normal weight and exposed to food competition interactions. Pigeons were divided into pairs by previously ranked high aggression (total time spent in offensive aggression exceeding 60 s/5 min; N = 6 pairs) and low aggression females (time spent in offensive aggression less than 10 s/5 min; N = 6 pairs). A pigeon in each pair received an sc dose of naloxone (1 mg kg-1 ml saline-1) and the other animal received the vehicle. Trials (10 min) were performed 30 min after the naloxone/vehicle administration. The naloxone group of high aggression pigeons showed lower scores of total time spent in offensive aggression (control: 98.6 ± 12.0; naloxone: 46.8 ± 6.6 s; P < 0.05) and higher scores of time spent in emotional responses (control: 3.5 ± 0.6; naloxone: 10.8 ± 2.4 s; P < 0.05) than controls. The other behaviors scored, feeder control and eating behavior, were not affected in this group. The naloxone group of low aggression pigeons, however, showed higher scores of offensive aggression than their controls (5.3 ± 1.3; naloxone: 28.7 ± 8.0 s; P < 0.05). The present results suggest that opiate receptor mechanisms are implicated in offensive aggression responses in high and low aggression pigeons. However, as reported for brain 5-hydroxytryptamine manipulation and GABA-A-benzodiazepine receptor manipulation, the effect of the opiate receptor antagonist on food competition aggression in pigeons was related to their pretreatment level of aggression.

Reinforcing properties of fencamfamine: Involvement of dopamine and opioid receptors

Fencamfamine (FCF) is a psychostimulant classified as an indirect dopamine agonist. The conditioning place preference (CPP) paradigm was used to investigate the reinforcing properties of FCF. After initial preferences had been determined, animals were conditioned with FCF (1.75, 3.5, or 7.0 mg/kg; IP). Only at the dose of 3.5 mg/kg FCF produced a significant place preference. Pretreatment with SCH23390 (0.05 mg/kg, SC) or naloxone (1.0 mg/kg SC) 10 min before FCF (3.5 mg/kg; IP) blocked both FCF-induced hyperactivity and CPP. Pretreatment with metoclopramide (10.0 mg/kg; IP) or pimozide (1.0 mg/kg, IP), respectively, 30 min or 4 h before FCF (3.5 mg/kg; IP), which blocked the FCF-induced locomotor activity, failed to influence place conditioning produced by FCF. In conclusion, the present study suggests that dopamine D 1 and opioid receptors are related to FCF reinforcing effect, while dopamine D 2 subtype receptor was ineffective in modifying FCF-induced CPP.

Enhanced nicotine-seeking behavior following pre-exposure to repeated cocaine is accompanied by changes in BDNF in the nucleus accumbens of rats

We investigated the behavioral and molecular interactions between cocaine and nicotine, through evaluating locomotor activity, nicotine intravenous self-administration and gene expression. Locomotor sensitization was induced in male Wistar rats by repeated cocaine (20 mg/kg; i.p.) or saline injections once a day over 7 days. Three days after the last injection, rats were challenged with either saline or cocaine (15 mg/kg; i.p.) and the locomotor activity was measured. The very next day animals received either saline or nicotine (0.4 mg/kg; s.c.) and the locomotor cross-sensitization was tested. Animals were then prepared with intrajugular catheters for nicotine self-administration. Nicotine self-administration patterns were evaluated using fixed or progressive ratio schedules of reinforcement and a 24-h unlimited access binge. Immediately after the binge sessions animals were decapitated, the brains were removed and the nucleus accumbens was dissected. The dynorphin (DYN), μ-opioid receptor (mu opioid), neuropeptide Y (NPY), brain-derived neurotrophic factor (BDNF), tropomyosin-related tyrosine kinase B receptor (TrkB) and corticotropin- releasing factor receptor type 1 (CRF-R1) gene expression were measured by the reverse transcription-polymerase chain reaction (RT-PCR). Pretreatment with cocaine caused sensitization of cocaine motor response and locomotor cross-sensitization with nicotine. In the self-administration experiments repeated cocaine administration caused an increase in the nicotine break point and nicotine intake during a 24 h binge session. © 2013 Elsevier Inc.

Peripheral kappa and delta opioid receptors are involved in the antinociceptive effect of crotalphine in a rat model of cancer pain

Cancer pain is an important clinical problem and may not respond satisfactorily to the current analgesic therapy. We have characterized a novel and potent analgesic peptide, crotalphine, from the venom of the South American rattlesnake Crotalus durissus terrificus. In the present work, the antinociceptive effect of crotalphine was evaluated in a rat model of cancer pain induced by intraplantar injection of Walker 256 carcinoma cells. Intraplantar injection of tumor cells caused the development of hyperalgesia and allodynia, detected on day 5 after tumor cell inoculation. Crotalphine (6 μg/kg), administered p.o., blocked both phenomena. The antinociceptive effect was detected 1 h after treatment and lasted for up to 48 h. Intraplantar injection of nor-binaltorphimine (50 g/paw), a selective antagonist of κ-opioid receptors, antagonized the antinociceptive effect of the peptide, whereas N,N-diallyl-Tyr-Aib-Phe-Leu (ICI 174,864, 10 μg/paw), a selective antagonist of δ-opioid receptors, partially reversed this effect. On the other hand, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr amide (CTOP, 20 g/paw), an antagonist of μ-opioid receptors, did not modify crotalphine-induced antinociception. These data indicate that crotalphine induces a potent and long lasting opioid-mediated antinociception in cancer pain. © 2013 Elsevier Inc.

Topiramate in opiate withdrawal- comparison with clonidine and with carbamazepine/mianserin

There are some rationales for developing anticonvulsants for the treatment of substance abuse. The blockade of the AMPA/kainate subtype of glutamate receptor by topiramate may be of particular interest, as preclinical studies of withdrawal from opioids suggest that whilst AMPA-receptor antagonists may not be able to prevent tolerance or dependence from developing, they may ameliorate both physical and emotional consequences of withdrawal. Methods. Ten consecutively admitted patients treated with topiramate were compared in a retrospective naturalistic drug utilization observation study with 10 consecutively admitted patients treated with clonidine and with 10 consecutively admitted patients treated with a carbamazepine/ mianserin combination. Results. In 9 cases of the clonidine group and in 7 carbamazepine/mianserin treated patients the dose had been reduced, whereas this occurred in only 2 topiramate treated patients (p < 0.01). Patients in the topiramate group received less p.r.n. myorelaxant medication than the two other groups, and there was a significant difference between the three groups with regard to p.r.n. analgesics (p < 0.05), topiramate and clonidine treated patients receiving fewer analgesics than the carbamazepine/mianserin group. Conclusions. Compared to clonidine and carbamazepine/mianserin, a detoxification scheme using high initial and then decreasing doses of topiramate appeared to be appropriate for most patients and as associated with less analgesic and myorelaxant comedication, indicating a more promising efficacy at the used doses

Endogenous opioid peptide-mediated neurotransmission in central and pericentral nuclei of the inferior colliculus recruits mu(1)-opioid receptor to modulate post-ictal antinociception

Background: The aim of the present work was to investigate the involvement of the mu(1)-endogenous opioid peptide receptor-mediated system in post-ictal antinociception. Methods: Antinociceptive responses were determined by the tail-flick test after pre-treatment with the selective mu(1)-opioid receptor antagonist naloxonazine, peripherally or centrally administered at different doses. Results: Peripheral subchronic (24 h) pre-treatment with naloxonazine antagonised the antinociception elicited by tonic-clonic seizures. Acute (10 min) pre-treatment, however, did not have the same effect. In addition, microinjections of naloxonazine into the central, dorsal cortical and external cortical nuclei of the inferior colliculus antagonised tonic-clonic seizure-induced antinociception. Neither acute (10-min) peripheral pre-treatment with naloxonazine nor subchronic intramesencephalic blockade of mu(1)-opioid receptors resulted in consistent statistically significant differences in the severity of tonic-clonic seizures shown by Racine's index (1972), although the intracollicular specific antagonism of mu(1)-opioid receptor decreased the duration of seizures. Conclusion: mu(1)-Opioid receptors and the inferior colliculus have been implicated in several endogenous opioid peptide-mediated responses such as antinociception and convulsion. The present findings suggest the involvement of mu(1)-opiate receptors of central and pericentral nuclei of the inferior colliculus in the modulation of tonic-clonic seizures and in the organisation of post-ictal antinociception. (C) 2011 Elsevier Ltd. All rights reserved.

Abolition of morphine-immunosuppression in mice lacking the μ-opioid receptor gene

Opiates are potent analgesic and addictive compounds. They also act on immune responses, and morphine, the prototypic opiate, has been repeatedly described as an immunosuppressive drug. Pharmacological studies have suggested that the inhibitory action of opiates on immunity is mediated by multiple opioid receptor sites but molecular evidence has remained elusive. Recently, three genes encoding μ- (MOR), δ-, and κ-opioid receptors have been cloned. To investigate whether the μ-opioid receptor is functionally implicated in morphine immunosuppression in vivo, we have examined immune responses of mice with a genetic disruption of the MOR gene. In the absence of drug, there was no difference between wild-type and mutant mice with regard to a large number of immunological endpoints, suggesting that the lack of MOR-encoded protein has little consequence on immune status. Chronic morphine administration induced lymphoid organ atrophy, diminished the ratio of CD4+CD8+ cells in the thymus and strongly reduced natural killer activity in wild-type mice. None of these effects was observed in MOR-deficient mice after morphine treatment. This demonstrates that the MOR gene product represents a major molecular target for morphine action on the immune system. Because our previous studies of MOR-deficient mice have shown that this receptor protein is also responsible for morphine analgesia, reward, and physical dependence, the present results imply that MOR-targeted therapeutic drugs that are developed for the treatment of pain or opiate addiction may concomitantly influence immune responses.

Agonist-selective endocytosis of mu opioid receptor by neurons in vivo.

Opiate alkaloids are potent analgesics that exert multiple pharmacological effects in the nervous system by activating G protein-coupled receptors. Receptor internalization upon stimulation may be important for desensitization and resensitization, which affect cellular responsiveness to ligands. Here, we investigated the agonist-induced internalization of the mu opioid receptor (MOR) in vivo by using the guinea pig ileum as a model system and immunohistochemistry with an affinity-purified antibody to the C terminus of rat MOR. Antibody specificity was confirmed by the positive staining of human embryonic kidney 293 cells transfected with epitope-tagged MOR cDNA, by the lack of staining of cells transfected with the delta or kappa receptor cDNA, and by the abolition of staining when the MOR antibody was preadsorbed with the MOR peptide fragment. Abundant MOR immunoreactivity (MOR-IR) was localized to the cell body, dendrites, and axonal processes of myenteric neurons. Immunostaining was primarily confined to the plasma membrane of cell bodies and processes. Within 15 min of an intraperitoneal injection of the opiate agonist etorphine, intense MOR-IR was present in vesicle-like structures, which were identified as endosomes by confocal microscopy. At 30 min, MOR-IR was throughout the cytoplasm and in perinuclear vesicles. MOR-IR was still internalized at 120 min. Agonist-induced endocytosis was completely inhibited by the opiate antagonist naloxone. Interestingly, morphine, a high-affinity MOR agonist, did not cause detectable internalization, but it partially inhibited the etorphine-induced MOR endocytosis. These results demonstrate the occurrence of agonist-selective MOR endocytosis in neurons naturally expressing this receptor in vivo and suggest the existence of different mechanisms regulating cellular responsiveness to ligands.

Impaired Compensatory Beta-cell Function And Growth In Response To High-fat Diet In Ldl Receptor Knockout Mice.

In this study, we investigated the effect of low density lipoprotein receptor (LDLr) deficiency on gap junctional connexin 36 (Cx36) islet content and on the functional and growth response of pancreatic beta-cells in C57BL/6 mice fed a high-fat (HF) diet. After 60 days on regular or HF diet, the metabolic state and morphometric islet parameters of wild-type (WT) and LDLr-/- mice were assessed. HF diet-fed WT animals became obese and hypercholesterolaemic as well as hyperglycaemic, hyperinsulinaemic, glucose intolerant and insulin resistant, characterizing them as prediabetic. Also they showed a significant decrease in beta-cell secretory response to glucose. Overall, LDLr-/- mice displayed greater susceptibility to HF diet as judged by their marked cholesterolaemia, intolerance to glucose and pronounced decrease in glucose-stimulated insulin secretion. HF diet induced similarly in WT and LDLr-/- mice, a significant decrease in Cx36 beta-cell content as revealed by immunoblotting. Prediabetic WT mice displayed marked increase in beta-cell mass mainly due to beta-cell hypertrophy/replication. Nevertheless, HF diet-fed LDLr-/- mice showed no significant changes in beta-cell mass, but lower islet-duct association (neogenesis) and higher beta-cell apoptosis index were seen as compared to controls. The higher metabolic susceptibility to HF diet of LDLr-/- mice may be explained by a deficiency in insulin secretory response to glucose associated with lack of compensatory beta-cell expansion.

Peripheral P2x7 Receptor-induced Mechanical Hyperalgesia Is Mediated By Bradykinin.

P2X7 receptors play an important role in inflammatory hyperalgesia, but the mechanisms involved in their hyperalgesic role are not completely understood. In this study, we hypothesized that P2X7 receptor activation induces mechanical hyperalgesia via the inflammatory mediators bradykinin, sympathomimetic amines, prostaglandin E2 (PGE2), and pro-inflammatory cytokines and via neutrophil migration in rats. We found that 2'(3')-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate triethylammonium salt (BzATP), the most potent P2X7 receptor agonist available, induced a dose-dependent mechanical hyperalgesia that was blocked by the P2X7 receptor-selective antagonist A-438079 but unaffected by the P2X1,3,2/3 receptor antagonist TNP-ATP. These findings confirm that, although BzATP also acts at both P2X1 and P2X3 receptors, BzATP-induced hyperalgesia was mediated only by P2X7 receptor activation. Co-administration of selective antagonists of bradykinin B1 (Des-Arg(8)-Leu(9)-BK (DALBK)) or B2 receptors (bradyzide), β1 (atenolol) or β2 adrenoceptors (ICI 118,551), or local pre-treatment with the cyclooxygenase inhibitor indomethacin or the nonspecific selectin inhibitor fucoidan each significantly reduced BzATP-induced mechanical hyperalgesia in the rat hind paw. BzATP also induced the release of the pro-inflammatory cytokines tumor necrosis factor α (TNF-α), interleukin (IL)-1β, IL-6 and cytokine-induced neutrophil chemoattractant-1 (CINC-1), an effect that was significantly reduced by A-438079. Co-administration of DALBK or bradyzide with BzATP significantly reduced BzATP-induced IL-1β and CINC-1 release. These results indicate that peripheral P2X7 receptor activation induces mechanical hyperalgesia via inflammatory mediators, especially bradykinin, which may contribute to pro-inflammatory cytokine release. These pro-inflammatory cytokines in turn may mediate the contributions of PGE2, sympathomimetic amines and neutrophil migration to the mechanical hyperalgesia induced by local P2X7 receptor activation.

Icatibant, An Inhibitor Of Bradykinin Receptor 2, For Hereditary Angioedema Attacks: Prospective Experimental Single-cohort Study.

Hereditary angioedema (HAE) with C1 inhibitor deficiency manifests as recurrent episodes of edema involving the skin, upper respiratory tract and gastrointestinal tract. It can be lethal due to asphyxia. The aim here was to evaluate the response to therapy for these attacks using icatibant, an inhibitor of the bradykinin receptor, which was recently introduced into Brazil. Prospective experimental single-cohort study on the efficacy and safety of icatibant for HAE patients. Patients with a confirmed HAE diagnosis were enrolled according to symptoms and regardless of the time since onset of the attack. Icatibant was administered in accordance with the protocol that has been approved in Brazil. Symptom severity was assessed continuously and adverse events were monitored. 24 attacks in 20 HAE patients were treated (female/male 19:1; 19-55 years; median 29 years of age). The symptoms were: subcutaneous edema (22/24); abdominal pain (15/24) and upper airway obstruction (10/24). The time taken until onset of relief was: 5-10 minutes (5/24; 20.8%); 10-20 (5/24; 20.8%); 20-30 (8/24; 33.4%); 30-60 (5/24; 20.8%); and 2 hours (1/24; 4.3%). The time taken for complete resolution of symptoms ranged from 4.3 to 33.4 hours. Adverse effects were only reported at injection sites. Mild to moderate erythema and/or feelings of burning were reported by 15/24 patients, itching by 3 and no adverse effects in 6. HAE type I patients who received icatibant responded promptly; most achieved improved symptom severity within 30 minutes. Local adverse events occurred in 75% of the patients.