Antipruritic effect of pretreatment with topical capsaicin 8% on histamine- and cowhage-evoked itch in healthy volunteers: a randomized, vehicle-controlled, proof-of-concept trial

From Compression to Itch: Exploring the Link Between Nerve Compression and Neuropathic Pruritus

Neuropathic itch is a type of chronic pruritus resulting from neural dysfunction along the afferent pathway. It is often accompanied by abnormal sensations such as paresthesia, hyperesthesia, or hypoesthesia. This condition, which may involve motor or autonomic neural damage, significantly impacts patients' quality of life, causing severe itch and associated comorbidities such as depression, disrupted sleep, and social strain. Neuropathic itch accounts for 8% of chronic pruritus cases, though this may be underestimated. This comprehensive review focuses on nerve impingement as the primary pathophysiological mechanism for various forms of neuropathic itch including brachioradial pruritus (BRP), notalgia paresthetica (NP), and anogenital itch. BRP, often seen in middle-aged white women, manifests as pruritus in the dorsolateral forearms typically exacerbated by ultraviolet (UV) exposure and related to cervical spine pathology. NP, prevalent in middle-aged women, presents as pruritus in the upper back due to thoracic spine nerve compression. Anogenital pruritus, affecting 1-5% of adults, is often linked to lumbosacral spine issues after ruling out dermatologic conditions such as lichen sclerosus or lichen simplex chronicus. The pathophysiology of neuropathic itch involves both peripheral and central mechanisms, with nerve damage being a key factor. Diagnosis requires a thorough history, physical examination, and potentially imaging studies. Topical agents such as menthol, capsaicin, and lidocaine are used for mild cases, while systemic medications such as gabapentin, pregabalin, and antidepressants are prescribed for moderate to severe cases; however, no US Food and Drug Administration (FDA)-approved therapies currently exist specifically for neuropathic itch. Understanding the underlying neural dysfunction and appropriate therapeutic strategies is crucial for managing neuropathic itch effectively.

Scratching increases epidermal neuronal branching and alters psychophysical testing responses in atopic dermatitis and brachioradial pruritus

Background

Chronic scratching imposes a major stress on the skin and can lead to itch intensity worsening, and consequently, patients may enter an itch-scratch cycle. This repetitive mechanical stress can result in lichenification, worsening of epidermal barrier function, and enhanced cutaneous inflammation. Furthermore, a reduction of intraepidermal nerve fibers was previously described in lichenification.

Aim

The aim of this study was to investigate the influence of chronic scratching on the epidermal neuroanatomy and on sensory changes, in particular the prevalence of hyperknesis and alloknesis in patients after mechanical, chemical, and electrical stimuli.

Methods

Analyses were performed on pruritic lichenified (chronically scratched), pruritic non-lichenified (not chronically scratched), and non-pruritic non-lesional (unaffected) skin areas of patients with inflammatory pruritus, i.e., atopic dermatitis (n = 35), and neuropathic pruritus, i.e., brachioradial pruritus (n = 34) vs. healthy matched controls (n = 64). Our fine-grained spatial skin characterization enabled specifically studying the differential effects of chronic scratching in inflammatory and neuropathic itch.

Results

Analysis of intraepidermal nerve fiber density showed rarefaction of fibers in all three skin areas of patients compared with healthy controls in both diagnoses. Even more, the two pruritic areas had significantly less nerve fibers than the unaffected skin, whereas electrically induced itch was massively increased. Epidermal branching of the remaining nerve fibers in lichenified/chronically scratched skin was increased, particularly in patients with brachioradial pruritus, which may contribute to the pronounced local neuronal sensitivity. Hyperknesis and alloknesis were found to increase independently of lichenification.

Conclusion

Our results indicate that chronic scratching may not affect intraepidermal nerve fiber density but leads to a stronger branching pattern of intraepidermal nerve fibers, which may contribute to local hypersensitivity. The increased sensitivity in the pruritic areas suggests mechanisms of peripheral sensitization, whereas the increased sensation of electrically and chemically induced itch in unaffected skin indicates central sensitization for itch.

Multifaceted Pharmacological Potentials of Curcumin, Genistein, and Tanshinone IIA through Proteomic Approaches: An In-Depth Review

Phytochemicals possess various intriguing pharmacological properties against diverse pathological conditions. Extensive studies are on-going to understand the structural/functional properties of phytochemicals as well as the molecular mechanisms of their therapeutic function against various disease conditions. Phytochemicals such as curcumin (Cur), genistein (Gen), and tanshinone-IIA (Tan IIA) have multifaceted therapeutic potentials and various efforts are in progress to understand the molecular dynamics of their function with different tools and technologies. Cur is an active lipophilic polyphenol with pleiotropic function, and it has been shown to possess various intriguing properties including antioxidant, anti-inflammatory, anti-microbial, anticancer, and anti-genotoxic properties besides others beneficial properties. Similarly, Gen (an isoflavone) exhibits a wide range of vital functions including antioxidant, anti-inflammatory, pro-apoptotic, anti-proliferative, anti-angiogenic activities etc. In addition, Tan IIA, a lipophilic compound, possesses antioxidant, anti-angiogenic, anti-inflammatory, anticancer activities, and so on. Over the last few decades, the field of proteomics has garnered great momentum mainly attributed to the recent advancement in mass spectrometry (MS) techniques. It is envisaged that the proteomics technology has considerably contributed to the biomedical research endeavors lately. Interestingly, they have also been explored as a reliable approach to understand the molecular intricacies related to phytochemical-based therapeutic interventions. The present review provides an overview of the proteomics studies performed to unravel the underlying molecular intricacies of various phytochemicals such as Cur, Gen, and Tan IIA. This in-depth study will help the researchers in better understanding of the pharmacological potential of the phytochemicals at the proteomics level. Certainly, this review will be highly instrumental in catalyzing the translational shift from phytochemical-based biomedical research to clinical practice in the near future.

Topical Capsaicin in Poly(lactic-co-glycolic)acid (PLGA) Nanoparticles Decreases Acute Itch and Heat Pain

BACKGROUND

Capsaicin, the hot pepper agent, produces burning followed by desensitization. To treat localized itch or pain with minimal burning, low capsaicin concentrations can be repeatedly applied. We hypothesized that alternatively controlled release of capsaicin from poly(lactic-co-glycolic acid) (PLGA) nanoparticles desensitizes superficially terminating nociceptors, reducing burning.

METHODS

Capsaicin-loaded PLGA nanoparticles were prepared (single-emulsion solvent evaporation) and characterized (size, morphology, capsaicin loading, encapsulation efficiency, in vitro release profile). Capsaicin-PLGA nanoparticles were applied to murine skin and evaluated in healthy human participants (n = 21) for 4 days under blinded conditions, and itch and nociceptive sensations evoked by mechanical, heat stimuli and pruritogens cowhage, β-alanine, BAM8-22 and histamine were evaluated.

RESULTS

Nanoparticles (loading: 58 µg capsaicin/mg) released in vitro 23% capsaicin within the first hour and had complete release at 72 h. In mice, 24 h post-application Capsaicin-PLGA nanoparticles penetrated the dermis and led to decreased nociceptive behavioral responses to heat and mechanical stimulation (desensitization). Application in humans produced a weak to moderate burning, dissipating after 3 h. A loss of heat pain up to 2 weeks was observed. After capsaicin nanoparticles, itch and nociceptive sensations were reduced in response to pruritogens cowhage, β-alanine or BAM8-22, but were normal to histamine.

CONCLUSIONS

Capsaicin nanoparticles could be useful in reducing pain and itch associated with pruritic diseases that are histamine-independent.

Mild Skin Heating Evokes Warmth Hyperknesis Selectively for Histaminergic and Serotoninergic Itch in Humans

Chronic itch can severely affect quality of life. Patients report that their chronic itch can be exacerbated by exposure to warm conditions (“warmth hyperknesis”). The aim of this mechanistic study was to investigate the effect of mild heating of the skin in humans on various experimental models of itch. A total of 18 healthy subjects were recruited to the study. Itch was provoked by histamine, serotonin, or cowhage in 3 different sessions. The provoked area was heated with an infrared lamp, and the skin temperature was either not altered, or was increased by 4°C or 7°C. Subsequent to induction of itch, the itch intensity was recorded for 10 min while the skin was heated continuously throughout the entire period of itch induction. Heating the skin resulted in a significant increase in itch intensity when provoked by histamine or serotonin. It is possible that thermoception and pruriception interact and selectively produce a higher itch intensity in histaminergic and serotoninergic itch.

Induction and generalization of nocebo effects on itch

Nocebo effects, that is, negative treatment outcomes due to negative expectancies, can increase itch. Moreover, indirect evidence has shown that nocebo hyperknesis can generalize to another itch modality. Knowledge on response generalization can help to prevent and decrease negative effects. The aims of this study were to investigate (1) the efficacy of inducing nocebo effects on cowhage‐evoked itch via verbal suggestions and (2) whether these effects can generalize to (2a) mechanically evoked touch and (2b) mechanically evoked itch. Forty‐four healthy participants watched a video suggesting that a nocebo solution increases cowhage‐evoked itch and that a control solution does not affect itch. Subsequently, cowhage, mechanical itch, and mechanical touch stimuli were applied. Nocebo effects were measured as the difference in both mean and peak of the outcomes itch and urge to scratch between nocebo and control trials. Main analyses revealed significant nocebo effects on mean and peak itch for all stimuli. For urge to scratch, a significant nocebo effect was only observed for mechanical touch (peak). As mechanical stimuli did not induce pure sensations as planned, posthoc sensitivity analyses were run for mechanical stimuli that individually induced either touch or itch at baseline. These analyses showed similar results for generalization to mechanical itch, but generalization to mechanical touch was non‐significant. This study showed that merely verbal suggestion can induce nocebo effects on cowhage‐evoked itch and that these effects can generalize to another itch modality. Future studies may examine how to prevent negative experiences from generalizing to subsequent encounters.

Sensory defunctionalization induced by 8% topical capsaicin treatment in a model of ultraviolet-B-induced cutaneous hyperalgesia

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.

Common and discrete mechanisms underlying chronic pain and itch: peripheral and central sensitization

Normally, an obvious antagonism exists between pain and itch. In normal conditions, painful stimuli suppress itch sensation, whereas pain killers often generate itch. Although pain and itch are mediated by separate pathways under normal conditions, most chemicals are not highly specific to one sensation in chronic pathologic conditions. Notably, in patients with neuropathic pain, histamine primarily induces pain rather than itch, while in patients with atopic dermatitis, bradykinin triggers itch rather than pain. Accordingly, repetitive scratching even enhances itch sensation in chronic itch conditions. Physicians often prescribe pain relievers to patients with chronic itch, suggesting common mechanisms underlying chronic pain and itch, especially peripheral and central sensitization. Rather than separating itch and pain, studies should investigate chronic itch and pain including neuropathic and inflammatory conditions. Here, we reviewed chronic sensitization leading to chronic pain and itch at both peripheral and central levels. Studies investigating the connection between pain and itch facilitate the development of new therapeutics against both chronic dysesthesias based on the underlying pathophysiology.

Effect of Topical Analgesia on Desensitization Following 8% Topical Capsaicin Application

To prevent pain associated with 8% capsaicin application, pretreatment with local anesthetics, such as EMLA (eutectic mixture of lidocaine 2.5% and prilocaine 2.5%), is considered an option. However, there is contradicting evidence regarding the effects of local analgesia on capsaicin-induced desensitization. In session 1, 2 skin areas in each forearm of 24 healthy volunteers were randomized to 2-hour pretreatment with EMLA/placebo cream. After pretreatment, 8% capsaicin patches were applied for 3 hours in 1 placebo and 1 EMLA pretreated area, obtaining the following four areas: Capsaicin + EMLA, Capsaicin + Placebo, EMLA alone, and Placebo. Pain intensity scores were assessed during the 3-hour application of capsaicin. Warmth detection, heat pain sensitivity, and microvascular reactivity were measured after the removal of capsaicin. After 24 hours, in session 2, all tests were repeated followed by histamine application in each area to examine itch intensity and neurogenic flare. Overall, EMLA caused significant reductions in capsaicin-induced pain compared with placebo (P= .007) and enhanced the capsaicin-induced increase in superficial blood perfusion immediately after the 3-hour capsaicin application (P< .01). Regardless of pretreatment, capsaicin induced heat hyperalgesia immediately after the application (P< .001). Twenty-four hours post application, heat pain sensitivity was normalized. However, WDT increased significantly (P< .001). Capsaicin tended to reduce the itch intensity and significantly reduced the neurogenic flare (P< .05) induced by histamine compared with EMLA alone. The findings suggest that pretreatment with topical analgesic cream reduces application site pain without interfering with the 8% topical capsaicin-induced desensitization. PERSPECTIVE: Pretreatment with local anesthetic EMLA cream might be considered a good therapeutic option to reduce the pain associated with 8% capsaicin application currently used for treatment of neuropathic pain syndromes. This study also suggests the existence of a synergistic effect of capsaicin and EMLA on the process of neurogenic inflammation.

The role of itch and pain modulation in the prediction of phototherapy outcomes: a prospective cohort study

OBJECTIVES

Ability to predict which chronic itch patients will benefit from particular treatments is a challenge. Common features in itch and pain in respect to sensory elicitation, and mechanisms of processing including sensitization and inhibition at the peripheral and central levels, may serve to understand variability in treatment outcomes. As such this study aimed to explore whether phototherapy outcomes can be predicted by psychophysical parameters of pain and itch modulatory processing.

METHODS

Prospective cohort study on chronic-itch patients (n = 44) assessed before 20 treatments of NB UVB. Level of itch and pain reduction following painful stimulation (reflecting the 'pain inhibits pain' phenomenon) used to assess the top-down modulation response efficacy. Magnitude of Conditioned Pain Modulation (CPM) for itch (CPM-itch) and for pain (CPM-pain) (reflecting inhibition) and magnitude of temporal summation (TS) of pain (reflecting ascending facilitation pathways) assessed to predict treatment effect.

RESULTS

Higher improvement of itch symptoms following phototherapy was correlated with more efficient CPM-itch (r = 0.62, p < .001), but not magnitude of CPM-pain or level of temporal summation.

DISCUSSION

Findings emphasize the role of descending inhibition pathways in determining phototherapy efficacy in chronic itch patients. Such an evaluation-based approach may contribute to better patient selection for phototherapy improving patients' disease outcomes.

New insights into the mechanisms behind mechanical itch

Gentle tactile stimuli, such as insects crawling on the skin, can cause itching sensation called mechanical itch. Recent studies have begun to shed light on the neural mechanisms of mechanical itch. Interestingly, the neural pathway for mechanical itch is apparently different from that for chemical itch triggered by the activation of pruriceptors with various mediators. Mechanical itch dysesthesia is frequently seen in patients with chronic itch. Mechanisms of this dysesthesia are plausibly involved in central sensitization. In this review, we summarize the current knowledge of mechanical itch under normal and pathological conditions.

Antipruritic effects of transient heat stimulation on histaminergic and nonhistaminergic itch

BACKGROUND

Chronic itch is notoriously difficult to treat. Counterstimuli are able to inhibit itch, but this principle is difficult to apply in clinical practice, and the mechanisms behind counterstimulation-induced itch suppression in humans are unclear.

OBJECTIVES

Firstly, to analyse the stimulus-response effects of transient heat stimuli on histaminergic and nonhistaminergic itch, and secondly, to investigate whether the antipruritic effect depends on homotopic (peripheral mediation) or heterotopic (central mediation) counterstimulation relative to the itch provocation site.

METHODS

Eighteen healthy volunteers participated (eight female, mean age 25·7 ± 0·8 years). Itch was evoked on premarked areas of the volar forearms, by either histamine (1% solution) or cowhage (35-40 spicules). In addition to the itch provocations (experiment 1), 5-s homotopic heat stimuli at 32, 40, 45 or 50 °C were applied. In experiment 2, heat stimuli were applied either homotopically, intrasegmentally (next to the provocation site) or extrasegmentally (dorsal forearm). Itch intensity was evaluated throughout the procedures using a digital visual analogue scale.

RESULTS

Homotopic counterstimuli inhibited histaminergic itch by 41·3% at 45 °C (P < 0·01) and by 76·7% at 50 °C (P < 0·001). Cowhage-induced itch was less prone to counterstimulation and was significantly diminished only at 50 °C, by 43·6% (P = 0·009). Counterstimulations applied heterotopically were not able to inhibit itch significantly.

CONCLUSIONS

Itch pathway-specific effects of counterstimuli were observed between homo- and heterotopic stimulation. Histaminergic itch was robustly inhibited by short-term homotopic noxious heat stimuli for up to 10 min. Nonhistaminergic itch was only weakly inhibited. The inhibitory effects exerted by the short-term heat stimuli only occurred following homotopic counterstimulation.

Psychophysical and vasomotor evidence for interdependency of TRPA1 and TRPV1-evoked nociceptive responses in human skin: an experimental study

The TRPA1 and TRPV1 receptors are important pharmaceutical targets for antipruritic and analgesic therapy. Obtaining further knowledge on their roles and interrelationship in humans is therefore crucial. Preclinical results are contradictory concerning coexpression and functional interdependency of TRPV1 and TRPA1, but no human evidence exists. This human experimental study investigated whether functional responses from the subpopulation of TRPA1 nociceptors could be evoked after defunctionalization of TRPV1 nociceptors by cutaneous application of high-concentration capsaicin. Two quadratic areas on each forearm were randomized to pretreatment with an 8% topical capsaicin patch or vehicle for 24 hours. Subsequently, areas were provoked by transdermal 1% topical capsaicin (TRPV1 agonist) or 10% topical allyl isothiocyanate ("AITC," a TRPA1 agonist), delivered by 12 mm Finn chambers. Evoked pain intensities were recorded during pretreatments and chemical provocations. Quantitative sensory tests were performed before and after provocations to assess changes of heat pain sensitivity. Imaging of vasomotor responses was used to assess neurogenic inflammation after the chemical provocations. In the capsaicin-pretreated areas, both the subsequent 1% capsaicin- and 10% AITC-provoked pain was inhibited by 92.9 ± 2.5% and 86.9 ± 5.0% (both: P < 0.001), respectively. The capsaicin-ablated skin areas showed significant heat hypoalgesia at baseline (P < 0.001) as well as heat antihyperalgesia, and inhibition of neurogenic inflammation evoked by both 1% capsaicin and 10% AITC provocations (both: P < 0.001). Ablation of cutaneous capsaicin-sensitive afferents caused consistent and equal inhibition of both TRPV1- and TRPA1-provoked responses assessed psychophysically and by imaging of vasomotor responses. This study suggests that TRPA1 nociceptive responses in human skin strongly depend on intact capsaicin-sensitive, TRPV1 fibers.

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.

Diagnosis and Treatment of Canine Acral Lick Dermatitis

Acral lick dermatitis is a common, frustrating disease. It is characterized by incessant licking behavior resulting in raised, thickened, ulcerative plaques, most commonly affecting the lower extremities of dogs. Underlying primary diseases, such as allergic disorders (atopic dermatitis, food allergy, flea allergy), orthopedic or neurologic disorders, parasitic diseases (eg, demodicosis), infections (fungal, bacterial), neoplasia or psychogenic disorders (compulsive/obsessive-compulsive disorder), and perpetuating factors (especially secondary infections) drive the itch-lick cycle. Appropriately addressing the primary underlying disorder and secondary infections and addressing any possible behavioral component via behavioral modification and psychopharmacotherapeutics are vital for successful management of this disease.