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

Reduced spontaneous itch in mouse models of cholestasis

Pruritus is one of the most distressing symptoms in cholestatic patients. Plasma autotaxin (ATX) activity correlates with the severity of pruritus in cholestatic patients, but the pathophysiology is unclear. To study pruritus in mice, we measured scratch activity in cholestatic Atp8b1 mutant mice, a model for Progressive Familial Intrahepatic Cholestasis type 1, and wild type mice (WT) with alpha-naphthylisothiocyanate (ANIT)-induced cholestasis. To induce cholestasis, Atp8b1 mutant mice received a diet containing 0.1% cholic acid (CA) and WT mice were treated with ANIT. In these mice ATX was also overexpressed by transduction with AAV-ATX. Scratch activity was measured using an unbiased, electronic assay. Marked cholestasis was accomplished in both Atp8b1 mutant mice on a CA-supplemented diet and in ANIT-treatment in WT mice, but scratch activity was decreased rather than increased while plasma ATX activity was increased. Plasma ATX activity was further increased up to fivefold with AAV-ATX, but this did not induce scratch activity. In contrast to several reports two cholestatic mouse models did not display increased scratch activity as a measure of itch perception. Increasing plasma ATX activity by overexpression also did not lead to increased scratch activity in mice. This questions whether mice are suitable to study cholestatic itch.

Naltrexone for cholestatic itch: a systematic review

BACKGROUND

Cholestatic itch is caused by intrahepatic liver diseases, such as primary biliary cirrhosis and extrahepatic obstruction of the biliary tree, often caused by tumours. The pathophysiology of cholestatic itch is complex and no single treatment has proved definitive. Naltrexone is an opioid receptor antagonist, which reduces central opioidergic tone, believed to be raised in patients with cholestatic pruritus.

AIM

To review and assess the efficacy of oral naltrexone for the treatment of cholestatic itch.

METHODS

Search of electronic databases, grey literature, clinical trials registries and handsearching for studies including naltrexone for cholestatic itch. Full papers were obtained if relevant and studies graded.

RESULTS

Thirteen papers were included in the analysis, including three randomised controlled trials, one controlled clinical trial, one open-label pilot study, seven case reports and one retrospective notes review. All studies found naltrexone to be effective in relieving pruritus. In all five studies performing statistical analysis, naltrexone significantly reduced pruritus compared with baseline. 37% of patients reported side effects, notably opioid withdrawal-type reactions and recurrence of previous pain, from all pathologies.

CONCLUSIONS

Oral naltrexone therapy helps relieve cholestatic itch and although it should be used with caution in patients using exogenous opioids for analgesia, it should be considered when treating refractory pruritus in patients with end-stage liver disease.

Recent advances in understanding the molecular mechanisms of cholestatic pruritus: A review

Cholestasis, a condition characterized by an abnormal decrease in bile flow, is accompanied by various symptoms such as pruritus. Although cholestatic pruritus is a prominent condition, its precise mechanisms have largely been elusive. Recently, advancements have been made for understanding the etiology and pathogenesis of cholestatic pruritus. The current review therefore focuses on summarizing the overall progress made in the elucidation of its molecular mechanisms. We have reviewed the available animal models on cholestasis to compare the differences between them, characterized potential pruritogens involved in cholestatic pruritus, and have summarized the receptor and ion channels implicated in the condition. Finally, we have discussed the available treatment options for alleviation of cholestatic pruritus. As our understanding of the mechanisms of cholestatic pruritus deepens, novel strategies to cure this condition are awaited.

Possible involvement of the opioidergic system in the modulation of body temperature, jumping behavior and memory process in cholestatic and addicted mice

Cholestasis is related to an increased plasma level of endogenous opioid levels. Naloxone-induced withdrawal syndrome has been reported in a mouse model of cholestasis. Moreover, studies revealed that the memory process is affected by cholestasis. Thus, we aimed at determining whether pharmacological manipulation of the opioidergic system is involved in signs of cholestasis disease such as hypothermia and withdrawal behaviors such as jumping behavior as well as memory process in mice. Cholestasis was induced by bile duct resection in mice and physical dependence was induced by administration of morphine and/or tramadol three times daily (8, 12 and 16 h) at the doses of 25, 50 and 75 mg/kg during three consecutive days. The memory process was assessed by a step-down passive avoidance test. Our results indicated that cholestatic mice showed hypothermia whereas cholestatic- and drug dependent mice indicated hyperthermia. Moreover, administration of morphine (50 mg/kg) and/or tramadol (50 mg/kg) on the 4th day, 2 h before naloxone injection significantly decreased latency to first jumping but increased the number of jumping and rearing behavior as well as locomotor activity in BDL-vs. sham-operated mice. In addition, the latency time of the step-down test decreased in BDL-vs. sham-operated group, showing impairment of memory in BDL mice. The results of this study support the evidence that (1) the opioidergic system involved in thermoregulation of cholestasis mice, (2) μ-opioid receptors play an important role in withdrawal behaviors, and (3) memory process is affected by cholestasis and addiction in mice.

Itch induced by peripheral mu opioid receptors is dependent on TRPV1-expressing neurons and alleviated by channel activation

Opioids remain the gold standard for the treatment of moderate to severe pain. However, their analgesic properties come with important side effects, including pruritus, which occurs frequently after systemic or neuraxial administration. Although part of the opioid-induced itch is mediated centrally, recent evidence shows that the opioid receptor system in the skin also modulates itch. The goal of our study was to identify the peripherally located transducer mechanisms involved in opioid-induced pruritus. Scratching behaviors in response to an intradermal injection of the mu-opioid receptor (MOR) agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) was quantified in mast cell-, PAR2- and TRPV1-deficient mice or following ablation of TRPV1+ sensory neurons. We found that mast cells−/−, PAR-2−/−, or TRPV1−/− mice still exhibit DAMGO-induced itch responses. However, we show that ablation of TRPV1+ neurons or acute TRPV1 activation by capsaicin abolishes DAMGO-induced itch. Overall, our work shows that peripheral DAMGO-induced itch is dependent on the presence of TRPV1-expressing pruriceptors, but not the TRPV1 channel itself. Activation of these fibers by capsaicin prevents the opioid-induced itch.

Altered Expression of Differential Genes in Thoracic Spinal Cord Involved in Experimental Cholestatic Itch Mouse Model

The spinal origin of cholestatic itch in experimental obstructive jaundice mouse model remains poorly understood. In this study, the jaundice model was established by bile duct ligation (BDL) in mice, and differential gene expression patterns were analyzed in the lower thoracic spinal cord involved in cholestatic pruritus after BDL operation using high-throughput RNA sequencing. At 21st day after BDL, the expression levels of ENSRNOG00000060523, ENSRNOG00000058405 and ENSRNOG00000055193 mRNA were significantly up-regulated, and those of ENSRNOG00000042197, ENSRNOG00000008478, ENSRNOGOOOOOO19607, ENSRNOG00000020647, ENSRNOG00000046289, Gemin8, Serpina3n and Trim63 mRNA were significantly down-regulated in BDL group. The RNAseq data of selected mRNAs were validated by RT-qPCR. The expression levels of ENSRNOG00000042197, ENSRNOG00000008478, ENSRNOGOOOOOO 19607, ENSRNOG00000020647, ENSRNOG00000046289 and Serpina3n mRNA were significantly down-regulated in BDL group. This study suggested that cholestatic pruritus in experimental obstructive jaundice mouse model is related with in the changes of gene expression profiles in spinal cord.

Altered expression of differential gene and lncRNA in the lower thoracic spinal cord on different time courses of experimental obstructive jaundice model accompanied with altered peripheral nociception in rats

The spinal origin of jaundice-induced altered peripheral nociceptive response poorly understood. In the current study, we aimed to first validate rats with bile duct ligation (BDL) as a jaundice model accompanied by altered peripheral nociceptive response, and then to analyze differential gene and lncRNA expression patterns in the lower thoracic spinal cord on different time courses after BDL operation by using high-throughput RNA sequencing. The differentially expressed genes (DEGs) identified using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis, followed by clustering analysis, Gene Ontology analysis and pathway analysis. As a result, a total of 2033 lncRNAs were differentially expressed 28d after BDL, in which 1545 probe sets were up-regulated and 488 probe sets were down-regulated, whereas a total of 2800 mRNAs were differentially expressed, in which 1548 probe sets were up-regulated and 1252 probe sets were down-regulated. The RNAseq data of select mRNAs and lncRNAs was validated by RT-qPCR. 28d after BDL, the expressions of lncRNA NONRATT002335 and NONRATT018085 were significantly up-regulated whereas the expression of lncRNA NONRATT025415, NONRATT025388 and NONRATT025409 was significantly down-regulated. 14d after BDL, the expressions of lncRNA NONRATT002335 and NONRATT018085 were significantly up-regulated; the expression of lncRNA NONRATT025415, NONRATT025388 and NONRATT025409 was significantly down-regulated. In conclusion, the present study showed that jaundice accompanied with decreased peripheral nociception involved in the changes of gene and lncRNA expression profiles in spinal cord. These findings extend current understanding of spinal mechanism for obstructive jaundice accompanied by decreased peripheral nociception.

Peripheral and spinal 5-HT receptors participate in cholestatic itch and antinociception induced by bile duct ligation in rats

Although 5-HT has been implicated in cholestatic itch and antinociception, two common phenomena in patients with cholestatic disease, the roles of 5-HT receptor subtypes are unclear. Herein, we investigated the roles of 5-HT receptors in itch and antinociception associated with cholestasis, which was induced by common bile duct ligation (BDL) in rats. 5-HT-induced enhanced scratching and antinociception to mechanical and heat stimuli were demonstrated in BDL rats. 5-HT level in the skin and spinal cord was significantly increased in BDL rats. Quantitative RT-PCR analysis showed 5-HT_1B, 5-HT_1D, 5-HT_2A, 5-HT_3A, 5-HT_5B, 5-HT₆, and 5-HT₇ were up-regulated in peripheral nervous system and 5-HT_1A, 5-HT_1F, 5-HT_2B, and 5-HT_3A were down-regulated in the spinal cord of BDL rats. Intradermal 5-HT₂, 5-HT₃, and 5-HT₇ receptor agonists induced scratching in BDL rats, whereas 5-HT₃ agonist did not induce scratching in sham rats. 5-HT_1A, 5-HT₂, 5-HT₃, and 5-HT₇ agonists or antagonists suppressed itch in BDL rats. 5-HT_1A agonist attenuated, but 5-HT_1A antagonist enhanced antinociception in BDL rats. 5-HT₂ and 5-HT₃ agonists or antagonists attenuated antinociception in BDL rats. Our data suggested peripheral and central 5-HT system dynamically participated in itch and antinociception under cholestasis condition and targeting 5-HT receptors may be an effective treatment for cholestatic itch.

GPBA: a GPCR for bile acids and an emerging therapeutic target for disorders of digestion and sensation

Bile acids (BAs) are digestive secretions that are necessary for the emulsification and absorption of dietary fats. Given the episodic nature of BA secretion and intestinal re-absorption, the circulating and tissue levels of BAs, like those of the gut hormones, fluctuate in fasting and fed states, and BA levels and forms are markedly affected by disease. BAs exert widespread hormonal-like effects by activating receptors in the nucleus and at the plasma membrane. The nuclear steroid receptors mediate the genomic actions of BAs on BA, glucose and lipid homeostasis. GPBA (TGR5) is a G-protein coupled plasma membrane receptor for BAs that mediates many of the rapid, non-genomic actions of BAs. GPBA has been implicated in the control of glucose homeostasis, inflammation and liver functions. Recent observations have revealed an unexpected role for GPBA in the nervous system. GPBA is expressed by enteric neurons and enterochromaffin cells that control peristalsis, and GPBA mediates the prokinetic actions of BAs in the colon that have been known for millennia. GPBA is also present on primary spinal afferent and spinal neurons that are necessary for sensory transduction. BA-induced activation of GPBA in the sensory nervous system promotes scratching behaviours and analgesia, which may contribute to the pruritus and painless jaundice that are observed in some patients with chronic cholestatic disease, where circulating BA concentrations are markedly increased. Thus, GPBA has emerged as an intriguing target for diverse metabolic, inflammatory, digestive and sensory disorders, where agonists and antagonists may be of value.

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.

Advances in pathogenesis and treatment of pruritus

The pathogenesis of itch during cholestasis is largely unknown and treatment options are limited. Lysophosphatidate, female steroid hormones, and endogenous opioids are among the agents discussed as potential pruritogens in cholestasis. The itch-alleviating action of guideline-based therapeutic interventions with anion exchanger resins, rifampicin, opioid antagonists, and serotonin reuptake inhibitors are studied to unravel the molecular pathogenesis of itch. Still, a considerable part of the patients is in need of alternative experimental therapeutic approaches (eg, UV-B phototherapy, extracorporeal albumin dialysis, nasobiliary drainage), providing additional information about the enigmatic pathophysiology of cholestatic pruritus.

The TGR5 receptor mediates bile acid-induced itch and analgesia

Patients with cholestatic disease exhibit pruritus and analgesia, but the mechanisms underlying these symptoms are unknown. We report that bile acids, which are elevated in the circulation and tissues during cholestasis, cause itch and analgesia by activating the GPCR TGR5. TGR5 was detected in peptidergic neurons of mouse dorsal root ganglia and spinal cord that transmit itch and pain, and in dermal macrophages that contain opioids. Bile acids and a TGR5-selective agonist induced hyperexcitability of dorsal root ganglia neurons and stimulated the release of the itch and analgesia transmitters gastrin-releasing peptide and leucine-enkephalin. Intradermal injection of bile acids and a TGR5-selective agonist stimulated scratching behavior by gastrin-releasing peptide- and opioid-dependent mechanisms in mice. Scratching was attenuated in Tgr5-KO mice but exacerbated in Tgr5-Tg mice (overexpressing mouse TGR5), which exhibited spontaneous pruritus. Intraplantar and intrathecal injection of bile acids caused analgesia to mechanical stimulation of the paw by an opioid-dependent mechanism. Both peripheral and central mechanisms of analgesia were absent from Tgr5-KO mice. Thus, bile acids activate TGR5 on sensory nerves, stimulating the release of neuropeptides in the spinal cord that transmit itch and analgesia. These mechanisms could contribute to pruritus and painless jaundice that occur during cholestatic liver diseases.

Potentiation of the transient receptor potential vanilloid 1 channel contributes to pruritogenesis in a rat model of liver disease

Persistent pruritus is a common disabling dermatologic symptom associated with different etiologic factors. These include primary skin conditions, as well as neuropathic, psychogenic, or systemic disorders like chronic liver disease. Defective clearance of potential pruritogenic substances that activate itch-specific neurons innervating the skin is thought to contribute to cholestatic pruritus. However, because the underlying disease-specific pruritogens and itch-specific neuronal pathways and mechanism(s) are unknown, symptomatic therapeutic intervention often leads to no or only limited success. In the current study, we aimed to first validate rats with bile duct ligation (BDL) as a model for hepatic pruritus and then to evaluate the contribution of inflammation, peripheral neuronal sensitization, and specific signaling pathways and subpopulations of itch-responsive neurons to scratching behavior and thermal hypersensitivity. Chronic BDL rats displayed enhanced scratching behavior and thermal hyperalgesia indicative of peripheral neuroinflammation. BDL-induced itch and hypersensitivity involved a minor contribution of histaminergic/serotonergic receptors, but significant activation of protein-activated receptor 2 (PAR2) receptors, prostaglandin PGE2 formation, and potentiation of transient receptor potential vanilloid 1 (TRPV1) channel activity. The sensitization of dorsal root ganglion nociceptors in BDL rats was associated with increased surface expression of PAR2 and TRPV1 proteins and an increase in the number of PAR2- and TRPV1-expressing peptidergic neurons together with a shift of TRPV1 receptor expression to medium sized dorsal root ganglion neurons. These results suggest that pruritus and hyperalgesia in chronic cholestatic BDL rats are associated with neuroinflammation and involve PAR2-induced TRPV1 sensitization. Thus, pharmacological modulation of PAR2 and/or TRPV1 may be a valuable therapeutic approach for patients with chronic liver pruritus refractory to conventional treatments.

[Cholestatic pruritus : new insights into pathophysiology and current treatment]

Pruritus is a common symptom of hepatobiliary disorders and may considerably diminish quality of life. Cholestatic pruritus exerts a circadian rhythm and is typically most severe in the evening hours and early at night. Itching is reported often to be most intense at the palms and the soles, but may also be generalized. The pathophysiological mechanisms of cholestatic pruritus have not been completely clarified. In the past, bile salts, histamine, progesterone metabolites and opioids have been discussed as potential causal substances; a correlation with itch intensity could never be proven. The enzyme autotaxin, which releases lysophosphatidic acid, has recently been identified as potential cholestatic pruritogen. Treatment aims to bind pruritogens in the gut lumen by resins such as cholestyramine, to modulate pruritogen metabolism by rifampicin and to influence central itch signaling by µ-opioid antagonists and selective serotonin re-uptake inhibitors. In cases of refractory pruritus experimental treatment options such as UV-therapy, extracorporeal albumin dialysis and nasobiliary drainage may be considered.

Inhibition of sensory afferents activation and visceral pain by a brominated algal diterpene

BACKGROUND

In the search of new therapeutic options for the treatment of pain, isolation, and testing of secondary metabolites from plant extracts has raised significant attention. We have investigated the effects of the brominated diterpene O(11) 15- cyclo-14-bromo-14,15-dihydrorogiol-3,11-diol (that we have named VLC5), extracted from the Mediterranean red algae Laurencia glandulifera.

METHODS

The pure extract was tested on primary afferent calcium signals induced by high concentration of KCl, transcient receptor potential vanilloid (TRPV)1 (capsaicin) or TRPV4 agonists, histamine, or protease-activated receptor-2 (PAR(2) ) agonist. It was also tested in mice in a model of mustard oil-induced colonic hypersensitivity.

KEY RESULTS

VLC5 was inhibited PAR(2) agonist or histamine-induced calcium mobilization in mouse primary afferents, but did not modify calcium signals induced by high concentrations of KCl, TRPV1 or TRPV4 agonists. The effect of VLC5 on histamine-induced calcium signal in primary afferent was inhibited by pertussis toxin pretreatment and was dependent on the activation of mu- or kappa-opioid receptor agonists, as it was inhibited by selective antagonists of those two receptors, but not by selective antagonist of the delta-opioid receptor. Intraperitoneal treatment of mice with VLC5 (10 mg kg(-1)) significantly reduced visceral pain behaviors induced by the intracolonic administration of mustard oil, in an opioid receptor-dependent manner.

CONCLUSIONS & INFERENCES

We have demonstrated significant analgesic properties for the algal metabolite VLC5, which is able to signal directly to primary afferents, through a mechanism dependent on the activation of opioid receptors. This identifies a new natural compound capable of activating peripheral opioidergic systems, exerting analgesic properties.

Pruritus in chronic cholestatic liver disease

Pruritus is a troublesome complication in patients with cholestatic liver disease. Several links to its pathogenesis have been proposed, including the role of bile acids, endogenous opioid and serotonins, and lysophosphatidic acid. The management of pruritus in cholestasis is challenging. Medical treatment of the underlying cholestatic condition may provide benefit. Extracorporeal albumin dialysis can be pursued for those who have a poor quality of life and failed the various therapeutic interventions, while awaiting liver transplantation. Experimental interventions, and the management of pruritus in certain conditions such as intrahepatic cholestasis of pregnancy and benign recurrent intrahepatic cholestasis, are also briefly reviewed.

Mouse mammary tumor virus in anti-mitochondrial antibody producing mouse models

BACKGROUND & AIMS

A human betaretrovirus resembling the mouse mammary tumor virus (MMTV) has been characterized in primary biliary cirrhosis (PBC) and associated with aberrant pyruvate dehydrogenase complex (PDC)-E2-like expression. As MMTV is prevalent in mice as either an exogenous or endogenous infection, we tested the hypothesis that MMTV is linked with anti-mitochondrial antibody (AMA) production in models with severe immune dysfunction.

METHODS

Evidence for MMTV was assessed in the liver and spleen of mice by PCR and immunochemistry and PDC-E2-like protein by immunochemistry. ELISA and Western blot were used to investigate AMA and anti-MMTV antibody production.

RESULTS

Increased MMTV gag or env expression was detected in the livers of AMA producing mice including NOD.c3c4, CD4 directed dominant negative TGF-β receptor II and IL-2 receptor α knockout mice as well as the NOD parental strain when compared to healthy strains and biliary disease control mice. The NOD.c3c4 mice expressed MMTV surface and capsid proteins and aberrant PDC-E2-like protein in the bile ducts, whereas IL-2 receptor α knockout mice, NOD.c3c4 and the NOD mice expressed MMTV proteins and aberrant PDC-E2-like protein in the spleen. A significant correlation between anti-MMTV antibody production and AMA production was observed in the sera of NOD and NOD.c3c4 mice (p<0.0001).

CONCLUSIONS

The association of betaretroviral protein production and aberrant PDC-E2-like protein expression in the NOD.c3c4, NOD, and the IL-2 receptor α knockout mice is comparable to observations in patients with PBC. The correlation of AMA and anti-MMTV suggests the hypothesis that MMTV infection may trigger the production of AMA.

Pathophysiology and current management of pruritus in liver disease

Pruritus is frequently reported by patients with cholestatic hepatobiliary diseases such as primary biliary cirrhosis, primary sclerosing cholangitis, intrahepatic cholestasis of pregnancy and hereditary cholestatic syndromes, but may accompany almost any other liver disease. Increased concentrations of bile salts, histamine, progesterone metabolites or endogenous opioids have been controversially discussed as potential pruritogens in cholestasis in the past. Most recently, novel insights unravelled lysophosphatidic acid (LPA), a potent neuronal activator, as a potential pruritogen in pruritus of cholestasis. Nevertheless, the pathogenesis of pruritus in cholestasis is still not clearly defined and current antipruritic treatment strategies provide relief only in a part of the affected patients. Based on recent experimental and clinical findings, this review outlines the actual insight in pathogenesis of pruritus in cholestasis and summarizes evidence-based and experimental therapeutic interventions for cholestatic patients suffering from itch.

Two histamine H2 receptor antagonists, zolantidine and cimetidine, modulate nociception in cholestatic rats

Cholestasis is associated with analgesia. The histamine H(2) receptors control pain perception. The involvement of histamine H(2) receptors on modulation of nociception in a model of elevated endogenous opioid tone, cholestasis, was investigated in this study using zolantidine and cimetidine as two H(2) receptor antagonists and dimaprit as a selective H(2) receptor agonist. Cholestasis was induced by ligation of the main bile duct using two ligatures and transsection of the duct at the midpoint between them. A significant increase in tail-flick latencies was observed in cholestatic rats compared to non-cholestatic rats. Administration of zolantidine (10, 20 and 40 mg/kg) and cimetidine (25, 50 and 100 mg/kg) in the cholestatic group significantly increased tail-flick latencies while dimaprit (10 and 20 mg/kg) injection in the cholestatic group decreased tail-flick latencies compared to the saline treated cholestatic group. Antinociception produced by injection of zolantidine and cimetidine in cholestatic rats was attenuated by co-administration of naloxone. Drug injection in non-cholestatic rats did not alter tail-flick latencies compared to the saline treated rats at any of the doses. At the doses used here, none of the drugs impaired motor coordination as revealed by the rota rod test. These data show that the histamine H(2) receptor system may be involved in the regulation of nociception during cholestasis. According to the hypothesis that increasing the nociception threshold in cholestasis may lead to a decrease in the perception of pruritus, the provision of the drugs that increase the threshold to nociception may be a novel approach to the treatment of cholestatic pruritus.

Thrombin receptor: An endogenous inhibitor of inflammatory pain, activating opioid pathways

Serine proteases such as thrombin, trypsin and mast cell tryptase can act on different cell types through protease-activated receptors (PARs). These receptors have been shown to be implicated in several phenomena such as inflammation, platelet activation, immune response and atherosclerosis. Several studies recently reported PARs expression on neurons and some of them demonstrated that these receptors could interfere with nociception. The contribution of PAR(1) to inflammatory pain and the mechanism involved in this phenomenon were investigated. Intraplantar injection of PAR(1) agonist increased withdrawal latency and reduced response frequency to von Frey filaments, thus inhibiting nociceptive response to both mechanical and thermal stimuli in mice. PAR(1) agonist also reduced carrageenan-induced inflammatory hyperalgesia. The anti-nociceptive effects of PAR(1) agonist were mediated by endogenous opioids, as this effect was inhibited by local injection of naloxone methiodide, and because intraplantar injection of PAR(1) agonist increased mRNA expression of the endogenous opioid precursor proenkephalin. However, PAR(1) agonist was not able to inhibit calcium signals in isolated sensory neurons exposed to pro-nociceptive agents. Finally, despite similar inflammatory parameters, PAR(1)-deficient mice showed a strong potentiation of inflammatory hyperalgesia induced by the intraplantar injection of either formalin or carrageenan, or in the chronic model of collagen-induced arthritis, compared to wild-type mice. This study highlights a previously unknown endogenous mechanism of analgesia, showing a central role for the thrombin receptor PAR(1) in the regulation of inflammatory pain and as an activator of opioid pathways.

A potent and selective inhibitor of endocannabinoid uptake, UCM707, potentiates antinociception induced by cholestasis

Cholestasis is associated with changes including analgesia. The endocannabinoid system can reduce pain sensitivity. Considering the interaction between the endogenous opioid and endocannabinoid systems in nociception processing, we studied the effect of UCM707 as a potent and selective inhibitor of endocannabinoid uptake on modulation of nociception in a model of elevated endogenous opioid tone, cholestasis. Cholestasis was induced in male Sprague-Dawley rats by ligation of the main bile duct using two ligatures and transecting the duct at the midpoint between them. Seven days later, tail-flick latencies were measured 10 min after injection of UCM707 (0.1, 1 and 10 mg/kg, i.p.) alone or with co-administration of a CB(1) receptor antagonist, AM251 (1 mg/kg, i.p.), with UCM707 (10 mg/kg, i.p.) in experimental groups. A significant increase (P < 0.01) in tail-flick latency was observed in cholestatic rats compared with rats belonging to unoperated and sham groups. Administration of UCM707 (1 and 10 mg/kg) to cholestatic animals significantly increased tail-flick latency compared with the vehicle-treated cholestatic group (P < 0.05 and P < 0.001, respectively). UCM707 injection in unoperated and sham groups did not alter baseline tail-flick latency compared with vehicle-treated groups. The effect of UCM707 in the cholestatic group was blocked by co-administration of AM251 (1 mg/kg, i.p.) with UCM707. These data showed that the endocannabinoid system is involved in nociception processing during cholestasis and that the effects of UCM707 on the pain threshold in cholestatic rats may be a result of CB(1) receptor activation by the increased extracellular levels of endocannabinoids.

Pathogenesis and treatment of pruritus in cholestasis

Pruritus is an enigmatic, seriously disabling symptom accompanying cholestatic liver diseases and a broad range of other disorders. Most recently, novel itch-specific neuronal pathways, itch mediators and their relevant receptors have been identified. In addition, new antipruritic therapeutic strategies have been developed and/or are under evaluation. This review highlights recent experimental and clinical findings focusing on the pathogenesis and actual treatment of pruritus in cholestatic liver disease. Evidence-based therapeutic recommendations, including the use of anion exchange resins cholestyramine, colestipol and colesevelam, the microsomal enzyme inducer rifampicin, the opioid receptor antagonists naltrexone and naloxone, and the serotonin reuptake inhibitor sertraline, are provided.

Experimental obstructive cholestasis: the wound-like inflammatory liver response

Obstructive cholestasis causes hepatic cirrhosis and portal hypertension. The pathophysiological mechanisms involved in the development of liver disease are multiple and linked. We propose grouping these mechanisms according to the three phenotypes mainly expressed in the interstitial space in order to integrate them.Experimental extrahepatic cholestasis is the model most frequently used to study obstructive cholestasis. The early liver interstitial alterations described in these experimental models would produce an ischemia/reperfusion phenotype with oxidative and nitrosative stress. Then, the hyperexpression of a leukocytic phenotype, in which Kupffer cells and neutrophils participate, would induce enzymatic stress. And finally, an angiogenic phenotype, responsible for peribiliary plexus development with sinusoidal arterialization, occurs. In addition, an intense cholangiocyte proliferation, which acquires neuroendocrine abilities, stands out. This histopathological finding is also associated with fibrosis.It is proposed that the sequence of these inflammatory phenotypes, perhaps with a trophic meaning, ultimately produces a benign tumoral biliary process - although it poses severe hepatocytic insufficiency. Moreover, the persistence of this benign tumor disease would induce a higher degree of dedifferentiation and autonomy and, therefore, its malign degeneration.

Effects of URB597 as an inhibitor of fatty acid amide hydrolase on modulation of nociception in a rat model of cholestasis

Cholestasis is associated with increased activity of the endogenous opioid system that results in analgesia. Endocannabinoid system can reduce pain sensitivity. The use of inhibitors of endocannabinoid metabolism is a novel means of pharmacologically increasing endocannabinoid levels. Considering the interaction that has been shown between the endogenous opioid and endocannabinoid systems in nociception processing, we studied the effects of URB597, a selective inhibitor of FAAH (fatty acid amide hydrolase), on modulation of nociception in a model of elevated endogenous opioid tone, cholestasis. Cholestasis was induced by ligation of the main bile duct using two ligatures and then transection of the duct at the midpoint between them. Seven days after surgery, tail-flick latencies were measured at 60 min after drug administration. A significant increase (P<0.001) in nociception threshold was observed in cholestatic rats compared to unoperated and sham groups. Administration of URB597 (0.3 mg/kg, i.p.) in cholestatic animals significantly (P<0.001) increased tail-flick latency compared to the vehicle treated cholestatic group. URB597 injection to unoperated and sham groups caused a significant (P<0.05, P<0.05) increase in tail-flick latency compared to their respective vehicle treated groups. The antinociceptive effect of URB597 was blocked by coadministration of a cannabinoid CB(1) receptor antagonist, AM251 (1 mg/kg, i.p.) but not by a cannabinoid CB(2) receptor antagonist, SR144528 (1 mg/kg, i.p.) with URB597. These data showed that URB597 as a FAAH inhibitor potentiates antinociception induced by cholestasis in tail-flick test and that the inhibitory effects of URB597 in this model are mediated by cannabinoid CB(1) and not CB(2) receptors.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 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 neurological 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).

Modulation of cholestasis-induced antinociception in rats by two NMDA receptor antagonists: MK-801 and magnesium sulfate

Acute cholestasis is associated with increased activity of the endogenous opioid system that results to changes including analgesia. N-methyl-d-aspartate (NMDA) receptors are involved in the nociceptive pathway and play a major role in the development of morphine induced analgesia. The magnesium acts as a non-competitive NMDA receptor antagonist by blocking the NMDA receptor channel. Considering the reported antinociceptive effect of magnesium sulfate as a NMDA receptor antagonist and the existence of close functional links between NMDA receptor antagonists and magnesium with the opioid system, we studied the effect of acute and chronic administration of MK-801 as a NMDA antagonist and magnesium sulfate on modulation of nociception in an experimental model of elevated endogenous opioid tone, acute cholestasis, using the tail-flick paradigm. Cholestasis was induced by ligation of the main bile duct using two ligatures and then transsection of the duct at the midpoint between them. A significant increase (P<0.001) in nociception threshold was observed in bile duct ligated rats compared to unoperated and sham-operated animals. In acute treatment, MK-801 (0.1 mg/kg, b.i.d), but not magnesium (150 mg/kg magnesium sulfate, i.e. 30 mg/kg of Mg(+2), i.p., b.i.d.) increased antinociception in cholestatic rats compared to saline treated cholestatics (P<0.05). In chronic treatment, administration of MK-801 or magnesium sulfate for 7 consecutive days, increased tail-flick latency (P<0.05, P<0.01) in cholestatic animals compared to saline treated cholestatics. These data showed that NMDA receptor pathway is involved in modulation of cholestasis-induced antinociception in rats and that repeated dosages of magnesium sulfate similar to MK-801 is able to modulate nociception in cholestasis.