Peripheral signaling pathways contributing to non-histaminergic itch in humans.

BACKGROUND: Chronic itch (chronic pruritus) is a major therapeutic challenge that remains poorly understood despite the extensive recent analysis of human pruriceptors. It is unclear how the peripheral nervous system differentiates the signaling of non-histaminergic itch and pain. METHODS: Here we used psychophysical analysis and microneurography (single nerve fiber recordings) in healthy human volunteers to explore the distinct signaling mechanisms of itch, using the pruritogens ß-alanine, BAM 8-22 and cowhage extract. RESULTS: The mode of application (injection or focal application using inactivated cowhage spicules) influenced the itch/pain ratio in sensations induced by BAM 8-22 and cowhage but not ß-alanine. We found that sensitizing pre-injections of prostaglandin E2 increased the pain component of BAM 8-22 but not the other pruritogens. A-fibers contributed only to itch induced by ß-alanine. TRPV1 and TRPA1 were necessary for itch signaling induced by all three pruritogens. In single-fiber recordings, we found that BAM 8-22 and ß-alanine injection activated nearly all CM-fibers (to different extents) but not CMi-fibers, whereas cowhage extract injection activated only 56% of CM-fibers but also 25% of CMi-fibers. A "slow bursting discharge pattern" was evoked in 25% of CM-fibers by ß-alanine, in 35% by BAM 8-22, but in only 10% by cowhage extract. CONCLUSION: Our results indicate that no labeled line exists for these pruritogens in humans. A combination of different mechanisms, specific for each pruritogen, leads to itching sensations rather than pain. Notably, non-receptor-based mechanisms such as spatial contrast or discharge pattern coding seem to be important processes. These findings will facilitate the discovery of therapeutic targets for chronic pruritus, which are unlikely to be treated effectively by single receptor blockade.

Spontaneous activity of specific C-nociceptor subtypes from diabetic patients and mice: Involvement of reactive dicarbonyl compounds and (sensitized) transient receptor potential channel A1.

BACKGROUND: Diabetic metabolism causes changes of the chemical milieu including accumulation of reactive carbonyl species, for example, methylglyoxal (MGO). MGO activates chemosensitive TRPA1 on nociceptors, but the contribution to neuronal pathophysiology causing pain and hyperalgesia in diabetic neuropathy is not fully understood. METHODS: We employed single-nerve-fiber recordings in type 2 diabetes patients with (spDN) and without cutaneous pain (DN) and in streptozotocin-diabetic and healthy mice. In mice, we measured Ca++ transients in cultured DRG neurons and stimulated CGRP release from hairy skin. RESULTS: In diabetic patients, we recorded a large proportion of pathologically altered nerve C-fibers (79%). In spDN patients we found a higher percentage (72%) of spontaneously active C-nociceptors than in DN patients (15%). The proportion of spontaneous activity was highest among pathological fibers with mechanoinsensitive fiber properties which are particularly sensitive to MGO in contrast to mechanosensitive fibers. Mouse polymodal nociceptors, in contrast to purely mechanosensitive C-fibers, showed highest prevalence of TRPA1-related chemosensitivity. In diabetic mice about 37% of polymodal nociceptors developed spontaneous activity and exhibited significantly greater MGO responses, indicating sensitized TRPA1 receptors. Low-threshold mechanosensitive Aδ-fibers were vigorously activated by MGO but independently of TRPA1 activation. INTERPRETATION: Our translational findings suggest that TRPA1-expressing C-nociceptors, which in human correspond to mechanoinsensitive and in mice to polymodal nociceptors, are especially vulnerable to develop spontaneous activity. Those two different nociceptor classes might share the functional role as dicarbonyl-sensitive chemosensors and represent the critical nociceptor population that support the development of pain and hyperalgesia in diabetic neuropathy.

Evaluation and Management of Pruritus in Primary Biliary Cholangitis.

Chronic pruritus is a classic symptom in patients with primary biliary cholangitis. It affects up to two-thirds of patients in the course of the disease. Efficient therapy consists of topical treatment combined with systemic options such as anion exchangers, rifampicin, bezafibrate, µ-opioid receptor antagonists, selective-serotonin receptor uptake inhibitors, and gabapentinoids. Future therapeutic approaches may contain the selective blockade of the enterohepatic cycle by inhibiting the ileal bile acid transporter, the agonism at κ-opioid receptors, and antagonism of the mas-related G protein-coupled receptor X4. As nondrug treatment, ultraviolet B therapy, albumin dialysis, and biliary drainage are available at specialized centers.

Lysophosphatidic acid activates nociceptors and causes pain or itch depending on the application mode in human skin.

ABSTRACT: Lysophosphatidic acid (LPA) is involved in the pathophysiology of cholestatic pruritus and neuropathic pain. Slowly conducting peripheral afferent C-nerve fibers are crucial in the sensations of itch and pain. In animal studies, specialized neurons ("pruriceptors") have been described, expressing specific receptors, eg, from the Mas-related G-protein-coupled receptor family. Human nerve fibers involved in pain signaling ("nociceptors") can elicit itch if activated by focalized stimuli such as cowhage spicules. In this study, we scrutinized the effects of LPA in humans by 2 different application modes on the level of psychophysics and single nerve fiber recordings (microneurography). In healthy human subjects, intracutaneous LPA microinjections elicited burning pain, whereas LPA application through inactivated cowhage spicules evoked a moderate itch sensation. Lysophosphatidic acid microinjections induced heat hyperalgesia and hypersensitivity to higher electrical stimulus frequencies. Pharmacological blockade of transient receptor potential channel A1 or transient receptor potential channel vanilloid 1 reduced heat hyperalgesia, but not acute chemical pain. Microneurography revealed an application mode-dependent differential activation of mechanosensitive (CM) and mechanoinsensitive C (CMi) fibers. Lysophosphatidic acid microinjections activated a greater proportion of CMi fibers and more strongly than CM fibers; spicule application of LPA activated CM and CMi fibers to a similar extent but excited CM fibers more and CMi fibers less intensely than microinjections. In conclusion, we show for the first time in humans that LPA can cause pain as well as itch dependent on the mode of application and activates afferent human C fibers. Itch may arise from focal activation of few nerve fibers with distinct spatial contrast to unexcited surrounding afferents and a specific combination of activated fiber subclasses might contribute.

Endogenous Opioid Levels Do Not Correlate With Itch Intensity and Therapeutic Interventions in Hepatic Pruritus.

Background: Chronic pruritus affects up to 70% of patients with immune-mediated hepatobiliary disorders. Antagonists of the µ-opioid receptor (MOR) and agonists of the κ-opioid receptor (KOR) are used to treat hepatic itch, albeit with limited success. An imbalance between ligands of MOR and KOR receptors has recently been suggested as a potential mechanism of hepatic pruritus. In this study, we therefore investigated systemic levels of important endogenous opioids such as ß-endorphin, dynorphin A, Leu- and Met-enkephalin in plasma of a large cohort of well-characterized patients with immune-mediated cholestatic disorders, including patients with liver cirrhosis, and during effective anti-pruritic therapy. Methods: Plasma samples and clinical data were prospectively collected from well-characterized patients with primary/secondary sclerosing cholangitis (PSC/SSC), primary biliary cholangitis (PBC) and overlap syndromes suffering from pruritus (n = 29) and age-, gender- and disease-matched controls without pruritus (n = 27) as well as healthy controls (n = 20). General laboratory testing for hepatobiliary and renal function was performed. Levels of ß-endorphin, dynorphin A, Leu- and Met-enkephalin were quantified in plasma by ELISA. Intensity of pruritus over the last week was evaluated using a visual analog scale (VAS, 0-10). Results: PBC and PSC patients with or without pruritus did neither differ in disease entity, disease stage, nor in the presence of cirrhosis. While both dynorphin A and ß-endorphin concentrations were lower in pruritic patients compared to those without pruritus and healthy controls, the MOR/KOR ligand ratio was unaltered. No significant differences were observed for Leu- and Met-enkephalin concentrations. Opioid levels correlated with neither itch intensity nor stage of disease. Cirrhotic patients displayed higher concentrations of MOR agonist Leu-enkephalin and KOR agonist dynorphin A. Endogenous opioid levels remained largely unchanged after successful treatment with the potent anti-pruritic drugs rifampicin and bezafibrate. Conclusions: Endogenous opioid levels and the MOR/KOR ligand ratio neither correlate with itch intensity nor differentiate pruritic from non-pruritic patients with immune-mediated liver diseases. Thus, endogenous opioids may modulate signaling pathways involved in hepatic pruritus, but are unlikely to represent the major pruritogens in liver disease.

Mas-related G protein-coupled receptor X2 and its activators in dermatologic allergies.

The Mas-related G protein-coupled receptor X2 (MRGPRX2) is a multiligand receptor responding to various exogenous and endogenous stimuli. Being highly expressed on skin mast cells, MRGPRX2 triggers their degranulation and release of proinflammatory mediators, and it promotes multicellular signaling cascades, such as itch induction and transmission in sensory neurons. The expression of MRGPRX2 by skin mast cells and the levels of the MRGPRX2 agonists (eg, substance P, major basic protein, eosinophil peroxidase) are upregulated in the serum and/or skin of patients with inflammatory and pruritic skin diseases, such as chronic spontaneous urticaria or atopic dermatitis. Therefore, MRGPRX2 and its agonists might be potential biomarkers for the progression of cutaneous inflammatory diseases and the response to treatment. In addition, they may represent promising targets for prevention and treatment of signs and symptoms in patients with skin diseases or drug reactions. To assess this possibility, this review explores the role and relevance of MRGPRX2 and its activators in cutaneous inflammatory disorders and chronic pruritus.

Treatment of Pruritus Secondary to Liver Disease.

PURPOSE OF REVIEW: Pruritus is a common extrahepatic symptom in various liver disorders, in particularly those with cholestatic features. This review summarizes epidemiology, pathophysiology, evidence-based therapeutic recommendations and currently investigated drugs for pruritus in hepatobiliary disorders. RECENT FINDINGS: Recent epidemiological data suggest pruritus as a common and relevant symptom in immune-mediated liver diseases, i.e., primary biliary cholangitis (PBC) with over 70% affected patients, up to 56% suffering from chronic pruritus. The better pathophysiological understanding of hepatic pruritus has led to the identification of novel therapeutic targets, addressed in drug trials using KOR agonists, PPAR agonists, and ileal bile acid transporter inhibitors. Hepatic itch remains among the most agonizing symptoms for affected patients and a clinical challenge for physicians. Therapeutic recommendations include a guideline-based stepwise approach starting with cholestyramine, followed by rifampicin, naltrexone, and sertraline. Bezafibrate and ileal bile acid transporter inhibitors represent promising future anti-pruritic treatment options.

Methylglyoxal causes pain and hyperalgesia in human through C-fiber activation.

The endogenous metabolite methylglyoxal (MG) accumulates in diabetic patients with neuropathic pain. Methylglyoxal could be a mediator of diabetes-induced neuropathic pain through TRPA1 activation and sensitization of the voltage-gated sodium channel subtype 1.8. In this study, we tested the algogenic and sensitizing effect of MG in healthy human subjects using intracutaneous microinjections. The involvement of C fibers was assessed through selective A-fiber nerve block, axon-reflex-erythema, and through single nerve fiber recordings in humans (microneurography). Involvement of the transduction channels TRPA1 and TRPV1 in MG-induced pain sensation was investigated with specific ion channel blockers. We showed for the first time in healthy humans that MG induces pain, axon-reflex-erythema, and long-lasting hyperalgesia through the activation of C nociceptors. Predominantly, the subclass of mechano-insensitive C fibers is activated by MG. A fibers contribute only negligibly to the burning pain sensation. Selective pharmacological blockade of TRPA1 or TRPV1 showed that TRPA1 is crucially involved in MG-induced chemical pain sensation and heat hyperalgesia. In conclusion, the actions of MG through TRPA1 activation on predominantly mechano-insensitive C fibers might be involved in spontaneously perceived pain in diabetic neuropathy and hyperalgesia as well as allodynia.

Management of Chronic Hepatic Itch.

Hepatic itch remains among the most agonizing symptoms for affected patients and a major clinical challenge for physicians. Pruritus may occur in almost all liver diseases, particularly those with cholestatic features. Hepatic itch arises irrespective of the severity of the underlying liver disease or extent of cholestasis. Antihistamines are ineffective in hepatic itch. Therapeutic recommendations consist of a guideline-based stepwise approach, starting with the anion exchange resin cholestyramine, followed by rifampicin, naltrexone, and sertraline. Bezafibrate and ileal bile acid transporter inhibitors are promising future treatment options. Experimental and invasive procedures should be reserved for refractory pruritus.