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Fiebig A, Leibl V, Oostendorf D, Lukaschek S, Frömbgen J, Masoudi M, Kremer AE, Strupf M, Reeh P, Düll M, Namer B. Peripheral signaling pathways contributing to non-histaminergic itch in humans. J Transl Med 2023; 21:908. [PMID: 38087354 PMCID: PMC10717026 DOI: 10.1186/s12967-023-04698-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2023] [Accepted: 11/04/2023] [Indexed: 12/18/2023] Open
Abstract
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 β-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 β-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 β-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 β-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 β-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.
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Affiliation(s)
- Andrea Fiebig
- Research Group Neuroscience, Interdisciplinary Centre for Clinical Research, Faculty of Medicine, RWTH Aachen University, Aachen, Germany
- Institute of Neurophysiology, Uniklinik RWTH Aachen University, Aachen, Germany
| | - Victoria Leibl
- Institute of Physiology and Pathophysiology, University of Erlangen-Nürnberg, 91054, Erlangen, Germany
| | - David Oostendorf
- Institute of Physiology and Pathophysiology, University of Erlangen-Nürnberg, 91054, Erlangen, Germany
| | - Saskia Lukaschek
- Research Group Neuroscience, Interdisciplinary Centre for Clinical Research, Faculty of Medicine, RWTH Aachen University, Aachen, Germany
- Institute of Neurophysiology, Uniklinik RWTH Aachen University, Aachen, Germany
| | - Jens Frömbgen
- Research Group Neuroscience, Interdisciplinary Centre for Clinical Research, Faculty of Medicine, RWTH Aachen University, Aachen, Germany
- Institute of Neurophysiology, Uniklinik RWTH Aachen University, Aachen, Germany
| | - Maral Masoudi
- Institute of Physiology and Pathophysiology, University of Erlangen-Nürnberg, 91054, Erlangen, Germany
| | - Andreas E Kremer
- Department of Gastroenterology and Hepatology, University Hospital Zürich, University of Zürich, Zurich, Switzerland
| | - Marion Strupf
- Institute of Physiology and Pathophysiology, University of Erlangen-Nürnberg, 91054, Erlangen, Germany
| | - Peter Reeh
- Institute of Physiology and Pathophysiology, University of Erlangen-Nürnberg, 91054, Erlangen, Germany
| | - Miriam Düll
- Institute of Physiology and Pathophysiology, University of Erlangen-Nürnberg, 91054, Erlangen, Germany
- Department of Medicine 1, University Hospital Erlangen and Friedrich-Alexander-University Erlangen-Nürnberg, Erlangen, Germany
| | - Barbara Namer
- Research Group Neuroscience, Interdisciplinary Centre for Clinical Research, Faculty of Medicine, RWTH Aachen University, Aachen, Germany.
- Institute of Neurophysiology, Uniklinik RWTH Aachen University, Aachen, Germany.
- Institute of Physiology and Pathophysiology, University of Erlangen-Nürnberg, 91054, Erlangen, Germany.
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Mießner H, Seidel J, Smith ESJ. In vitro models for investigating itch. Front Mol Neurosci 2022; 15:984126. [PMID: 36385768 PMCID: PMC9644192 DOI: 10.3389/fnmol.2022.984126] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2022] [Accepted: 10/10/2022] [Indexed: 12/04/2022] Open
Abstract
Itch (pruritus) is a sensation that drives a desire to scratch, a behavior observed in many animals. Although generally short-lasting and not causing harm, there are several pathological conditions where chronic itch is a hallmark symptom and in which prolonged scratching can induce damage. Finding medications to counteract the sensation of chronic itch has proven difficult due to the molecular complexity that involves a multitude of triggers, receptors and signaling pathways between skin, immune and nerve cells. While much has been learned about pruritus from in vivo animal models, they have limitations that corroborate the necessity for a transition to more human disease-like models. Also, reducing animal use should be encouraged in research. However, conducting human in vivo experiments can also be ethically challenging. Thus, there is a clear need for surrogate models to be used in pre-clinical investigation of the mechanisms of itch. Most in vitro models used for itch research focus on the use of known pruritogens. For this, sensory neurons and different types of skin and/or immune cells are stimulated in 2D or 3D co-culture, and factors such as neurotransmitter or cytokine release can be measured. There are however limitations of such simplistic in vitro models. For example, not all naturally occurring cell types are present and there is also no connection to the itch-sensing organ, the central nervous system (CNS). Nevertheless, in vitro models offer a chance to investigate otherwise inaccessible specific cell–cell interactions and molecular pathways. In recent years, stem cell-based approaches and human primary cells have emerged as viable alternatives to standard cell lines or animal tissue. As in vitro models have increased in their complexity, further opportunities for more elaborated means of investigating itch have been developed. In this review, we introduce the latest concepts of itch and discuss the advantages and limitations of current in vitro models, which provide valuable contributions to pruritus research and might help to meet the unmet clinical need for more refined anti-pruritic substances.
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Affiliation(s)
- Hendrik Mießner
- Department of Pharmacology, University of Cambridge, Cambridge, United Kingdom
- Dermatological Skin Care, Beiersdorf AG, Hamburg, Germany
| | - Judith Seidel
- Dermatological Skin Care, Beiersdorf AG, Hamburg, Germany
| | - Ewan St. John Smith
- Department of Pharmacology, University of Cambridge, Cambridge, United Kingdom
- *Correspondence: Ewan St. John Smith,
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Aliotta GE, Lo Vecchio S, Elberling J, Arendt-Nielsen L. Evaluation of itch and pain induced by bovine adrenal medulla (BAM)8-22, a new human model of non-histaminergic itch. Exp Dermatol 2022; 31:1402-1410. [PMID: 35587729 DOI: 10.1111/exd.14611] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2022] [Revised: 05/03/2022] [Accepted: 05/16/2022] [Indexed: 11/27/2022]
Abstract
Chronic itch is a socioeconomic burden with limited management options. Non-histaminergic itch, involved in problematic pathological itch conditions, is transmitted by a subgroup of polymodal C-fibers. Cowhage is traditionally used for studying experimentally induced non-histaminergic itch in humans, but encounter some limitations. The present study therefore aims to design a new human, experimental model of non-histaminergic itch based on the application of bovine adrenal medulla (BAM)8-22, an endogenous peptide that activates MrgprX1 receptor. 22 healthy subjects were recruited. Different concentrations (0.5, 1, and 2 mg/ml) of BAM8-22 solution and vehicle, applied by a single skin prick test (SPT), were tested in the first session. In the second session, the BAM8-22 solution (1 mg/ml) was applied by different number of SPTs (1, 5, and 25) and by heat-inactivated cowhage spicules coated with BAM8-22. Provoked itch and pain intensities were monitored for 9 minutes followed by the measurement of superficial blood perfusion (SBP), mechanical and thermal sensitivity. BAM8-22 induced itch at the concentration of 1 mg/ml, 2 mg/ml (p<0.05), and with the significantly highest intensity when applied through BAM8-22 spicules (p<0.001). No concomitant pain sensation nor increased SBP were observed. SBP increased only in the 25 SPTs area probably due to micro-trauma from the multiple skin penetrations. Mechanical and thermal sensitivities were not affected by any of the applications. BAM8-22 applied through heat-inactivated spicules was the most efficient method to induce itch (without pain nor changes in SBP, mechanical and thermal sensitivity) suggesting BAM8-22 as a novel non-histaminergic, human, experimental itch model.
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Affiliation(s)
- Giulia Erica Aliotta
- Center for Neuroplasticity and Pain (CNAP), SMI, Department of Health Science and Technology, Faculty of Medicine, Aalborg University, Denmark
| | - Silvia Lo Vecchio
- Center for Neuroplasticity and Pain (CNAP), SMI, Department of Health Science and Technology, Faculty of Medicine, Aalborg University, Denmark
| | - Jesper Elberling
- Department of Dermatology and Allergy, Herlev and Gentofte Hospital, Denmark.,Department of Clinical Medicine, University of Copenhagen, Copenhagen, Denmark
| | - Lars Arendt-Nielsen
- Center for Neuroplasticity and Pain (CNAP), SMI, Department of Health Science and Technology, Faculty of Medicine, Aalborg University, Denmark.,Department of Medical Gastroenterology, Mech-Sense, Aalborg University Hospital, Aalborg, Denmark
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Microinjection of pruritogens in NGF-sensitized human skin. Sci Rep 2021; 11:21490. [PMID: 34728705 PMCID: PMC8563721 DOI: 10.1038/s41598-021-00935-x] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2021] [Accepted: 10/19/2021] [Indexed: 11/16/2022] Open
Abstract
Single intradermal injections of nerve growth factor (NGF) evoke prolonged but temporally distinct sensitization patterns to somatosensory stimuli. Focal administration of the non-histaminergic pruritogen cowhage but not histamine resulted in elevated itch at day 21 after NGF administration. Here, we injected bovine adrenal medulla peptide 8–22 (BAM8–22), β-alanine (β-ALA) and endothelin-1 (ET-1) into NGF-treated skin of 11 healthy volunteers and investigated the corresponding itch/pain and flare reactions. β-ALA was the weakest pruritogen, while BAM8–22 and ET-1 were equally potent as histamine. NGF did not sensitize itch or flare reactions induced by any compound, but injection and evoked pain were increased at day 21 and 49. The involvement of histamine H1 receptors in itch was explored in eight subjects after oral cetirizine. ET-1-induced itch and flare were significantly reduced. BAM8–22 and β-ALA itch were not affected, but flare responses after BAM8–22 reduced by 50%. The results indicate that a single NGF injection does not sensitize for experimentally induced itch but increases pain upon pruritogen injection. In healthy humans, pruritic and algetic processing appear differentially regulated by NGF. However, in patients suffering chronic itch, prolonged elevation of NGF-levels under inflammatory conditions may contribute to elevated itch.
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Sensory defunctionalization induced by 8% topical capsaicin treatment in a model of ultraviolet-B-induced cutaneous hyperalgesia. Exp Brain Res 2021; 239:2873-2886. [PMID: 34302514 DOI: 10.1007/s00221-021-06170-0] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2020] [Accepted: 07/02/2021] [Indexed: 10/20/2022]
Abstract
Subpopulations of primary nociceptors (C- and Aδ-fibers), express the TRPV1 receptor for heat and capsaicin. During cutaneous inflammation, these afferents may become sensitized, leading to primary hyperalgesia. It is known that TRPV1+ nociceptors are involved in heat hyperalgesia; however, their involvement in mechanical hyperalgesia is unclear. This study explored the contribution of capsaicin-sensitive nociceptors in the development of mechanical and heat hyperalgesia in humans following ultraviolet-B (UVB) irradiation. Skin areas in 18 healthy volunteers were randomized to treatment with 8% capsaicin/vehicle patches for 24 h. After patches removal, one capsaicin-treated area and one vehicle area were irradiated with 2xMED (minimal erythema dose) of UVB. 1, 3 and 7 days post-UVB exposure, tests were performed to evaluate the development of UVB-induced cutaneous hyperalgesia: thermal detection and pain thresholds, pain sensitivity to supra-threshold heat stimuli, mechanical pain threshold and sensitivity, touch pleasantness, trans-epidermal water loss (TEWL), inflammatory response, pigmentation and micro-vascular reactivity. Capsaicin pre-treatment, in the UVB-irradiated area (Capsaicin + UVB area), increased heat pain thresholds (P < 0.05), and decreased supra-threshold heat pain sensitivity (P < 0.05) 1, 3 and 7 days post-UVB irradiation, while mechanical hyperalgesia resulted unchanged (P > 0.2). No effects of capsaicin were reported on touch pleasantness (P = 1), TEWL (P = 0.31), inflammatory response and pigmentation (P > 0.3) or micro-vascular reactivity (P > 0.8) in response to the UVB irradiation. 8% capsaicin ablation predominantly defunctionalizes TRPV1+-expressing cutaneous nociceptors responsible for heat pain transduction, suggesting that sensitization of these fibers is required for development of heat hyperalgesia following cutaneous UVB-induced inflammation but they are likely only partially necessary for the establishment of robust primary mechanical hyperalgesia.
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