Effects of menthol and cold on histamine-induced itch and skin reactions in man

Adjunctive Management of Itch in Atopic Dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin condition that can be difficult to treat due to a complex etiology and diverse clinical presentations. Itch is the most common symptom associated with AD with profound negative impact on quality of life. Thus, the adjunctive management of itch in patients with AD is needed to control and reduce disease burden. Supplemental treatment options are continuously emerging and undergoing testing in clinical trials. This article summarizes the latest data on topical and systemic adjunctive therapies for AD safety and efficacy in reducing itch.

Quantitative sensory testing in notalgia paresthetica reveals small fiber-type-specific differences in non-pruritic sensitivity: a pilot study

Introduction

Notalgia paresthetica (NP) is a chronic condition characterized by pruritus and other unpleasant dysesthetic sensations unilaterally on the subscapular back. Its specific underlying mechanisms are largely unknown, though hypothesized to be neuropathic. Determination of possible somatosensory contributors to the condition could pave the way for novel treatments.

Objectives

Given the potential involvement of non-pruritic mechanisms in NP, our objective was to broadly characterize the somatosensory function in NP-affected and unaffected skin using methods that have been standardized in pain-free controls and painful neuropathic disorders. We hypothesized that if NP is caused by neuropathic mechanisms not targeted directly to pruritoceptors in the skin, somatosensory abnormalities would not be itchspecific. Second, given the lack of symptoms on the contralateral side of the back, we hypothesized that this region would be normally sensitive.

Methods

In this study, quantitative sensory testing (QST) was used to comprehensively assess the somatosensory function in 15 adult patients with NP. Standardized QST metrics were performed in the NP-affected region and compared with the contralateral asymptomatic skin and itch-free individuals using an age, gender, and site-matched reference data set.

Results

There were no significant differences in sensitivity between symptomatic and asymptomatic skin, except for increased mechanical-evoked itch on the itchy side. However, reference data set comparisons revealed bilateral hyposensitivity to innocuous cold and noxious pinprick and higher temporal summation of pain in patients with NP. In addition, compared with reference data, patients with NP demonstrated decreased sensitivity to cold and pinprick, presence of paradoxical heat sensations, and increased wind-up of pain.

Conclusion

These results suggest a role for Aδ fiber pathways and central sensitization in NP-associated itch. More research is needed to determine whether sensory differences extend beyond the NP-affected dermatomal level and what might cause neuropathy specifically targeting Aδ fibers.

Management of Skin Toxicities in Cancer Treatment: An Australian/New Zealand Perspective

Cancer systemic therapeutics and radiotherapy are often associated with dermatological toxicities that may reduce patients' quality of life and impact their course of cancer treatment. These toxicities cover a wide range of conditions that can be complex to manage with increasing severity. This review provides details on twelve common dermatological toxicities encountered during cancer treatment and offers measures for their prevention and management, particularly in the Australian/New Zealand context where skincare requirements may differ to other regions due to higher cumulative sun damage caused by high ambient ultraviolet (UV) light exposure. Given the frequency of these dermatological toxicities, a proactive phase is envisaged where patients can actively try to prevent skin toxicities.

Interventions for postburn pruritus

BACKGROUND

Postburn pruritus (itch) is a common and distressing symptom experienced on healing or healed burn or donor site wounds. Topical, systemic, and physical treatments are available to control postburn pruritus; however, it remains unclear how effective these are.

OBJECTIVES

To assess the effects of interventions for treating postburn pruritus in any care setting.

SEARCH METHODS

In September 2022, we searched the Cochrane Wounds Specialised Register, the Cochrane Central Register of Controlled Trials (CENTRAL), Ovid MEDLINE (including In-Process & Other Non-Indexed Citations), Ovid Embase, and EBSCO CINAHL Plus. We also searched clinical trials registries and scanned references of relevant publications to identify eligible trials. There were no restrictions with respect to language, publication date, or study setting.

SELECTION CRITERIA

Randomised controlled trials (RCTs) that enrolled people with postburn pruritus to compare an intervention for postburn pruritus with any other intervention, placebo or sham intervention, or no intervention.

DATA COLLECTION AND ANALYSIS

We used the standard methodological procedures expected by Cochrane. We used GRADE to assess the certainty of the evidence.

MAIN RESULTS

We included 25 RCTs assessing 21 interventions with 1166 randomised participants. These 21 interventions can be grouped into six categories: neuromodulatory agents (such as doxepin, gabapentin, pregabalin, ondansetron), topical therapies (such as CQ-01 hydrogel, silicone gel, enalapril ointment, Provase moisturiser, beeswax and herbal oil cream), physical modalities (such as massage therapy, therapeutic touch, extracorporeal shock wave therapy, enhanced education about silicone gel sheeting), laser scar revision (pulsed dye laser, pulsed high-intensity laser, fractional CO2 laser), electrical stimulation (transcutaneous electrical nerve stimulation, transcranial direct current stimulation), and other therapies (cetirizine/cimetidine combination, lemon balm tea). Most RCTs were conducted at academic hospitals and were at a high risk of performance, attrition, and detection bias. While 24 out of 25 included studies reported change in burn-related pruritus, secondary outcomes such as cost-effectiveness, pain, patient perception, wound healing, and participant health-related quality of life were not reported or were reported incompletely. Neuromodulatory agents versus antihistamines or placebo There is low-certainty evidence that doxepin cream may reduce burn-related pruritus compared with oral antihistamine (mean difference (MD) -2.60 on a 0 to 10 visual analogue scale (VAS), 95% confidence interval (CI) -3.79 to -1.42; 2 studies, 49 participants). A change of 2 points represents a minimal clinically important difference (MCID). Due to very low-certainty evidence, it is uncertain whether doxepin cream impacts the incidence of somnolence as an adverse event compared to oral antihistamine (risk ratio (RR) 0.64, 95% CI 0.32 to 1.25; 1 study, 24 participants). No data were reported on pain in the included study. There is low-certainty evidence that gabapentin may reduce burn-related pruritus compared with cetirizine (MD -2.40 VAS, 95% CI -4.14 to -0.66; 1 study, 40 participants). A change of 2 points represents a MCID. There is low-certainty evidence that gabapentin reduces the incidence of somnolence compared to cetirizine (RR 0.02, 95% CI 0.00 to 0.38; 1 study, 40 participants). No data were reported on pain in the included study. There is low-certainty evidence that pregabalin may result in a reduction in burn-related pruritus intensity compared with cetirizine with pheniramine maleate (MD -0.80 VAS, 95% CI -1.24 to -0.36; 1 study, 40 participants). A change of 2 points represents a MCID. There is low-certainty evidence that pregabalin reduces the incidence of somnolence compared to cetirizine (RR 0.04, 95% CI 0.00 to 0.69; 1 study, 40 participants). No data were reported on pain in the included study. There is moderate-certainty evidence that ondansetron probably results in a reduction in burn-related pruritus intensity compared with diphenhydramine (MD -0.76 on a 0 to 10 numeric analogue scale (NAS), 95% CI -1.50 to -0.02; 1 study, 38 participants). A change of 2 points represents a MCID. No data were reported on pain and adverse events in the included study. Topical therapies versus relevant comparators There is moderate-certainty evidence that enalapril ointment probably decreases mean burn-related pruritus compared with placebo control (MD -0.70 on a 0 to 4 scoring table for itching, 95% CI -1.04 to -0.36; 1 study, 60 participants). No data were reported on pain and adverse events in the included study. Physical modalities versus relevant comparators Compared with standard care, there is low-certainty evidence that massage may reduce burn-related pruritus (standardised mean difference (SMD) -0.86, 95% CI -1.45 to -0.27; 2 studies, 166 participants) and pain (SMD -1.32, 95% CI -1.66 to -0.98). These SMDs equate to a 4.60-point reduction in pruritus and a 3.74-point reduction in pain on a 10-point VAS. A change of 2 VAS points in itch represents a MCID. No data were reported on adverse events in the included studies. There is low-certainty evidence that extracorporeal shock wave therapy (ESWT) may reduce burn-related pruritus compared with sham stimulation (SMD -1.20, 95% CI -1.65 to -0.75; 2 studies, 91 participants). This equates to a 5.93-point reduction in pruritus on a 22-point 12-item Pruritus Severity Scale. There is low-certainty evidence that ESWT may reduce pain compared with sham stimulation (MD 2.96 on a 0 to 25 pressure pain threshold (PPT), 95% CI 1.76 to 4.16; 1 study, 45 participants). No data were reported on adverse events in the included studies. Laser scar revision versus untreated or placebo controls There is moderate-certainty evidence that pulsed high-intensity laser probably results in a reduction in burn-related pruritus intensity compared with placebo laser (MD -0.51 on a 0 to 1 Itch Severity Scale (ISS), 95% CI -0.64 to -0.38; 1 study, 49 participants). There is moderate-certainty evidence that pulsed high-intensity laser probably reduces pain compared with placebo laser (MD -3.23 VAS, 95% CI -5.41 to -1.05; 1 study, 49 participants). No data were reported on adverse events in the included studies.

AUTHORS' CONCLUSIONS

There is moderate to low-certainty evidence on the effects of 21 interventions. Most studies were small and at a high risk of bias related to blinding and incomplete outcome data. Where there is moderate-certainty evidence, practitioners should consider the applicability of the evidence for their patients.

Histamine- and pruritogen-induced itch is inhibited by a TRPM8 agonist: a randomized vehicle-controlled human trial

BACKGROUND

Allergies often present challenges in managing itch and the effects of histamine. Cooling agents that act via transient receptor potential melastatin 8 (TRPM8) agonism have shown potential in itch management. However, animal studies on itch have limitations, as animals cannot communicate subjective events and their fur-coated skin differs from that of humans. Human studies offer more direct and reliable information.

OBJECTIVES

To investigate the effects of a specific TRPM8 agonist gel (cryosim-1) on itch induced by various pruritogens in human skin.

METHODS

Calcium imaging experiments determined the binding of cryosim-1 and histamine to their respective receptors. Thirty healthy volunteers underwent skin prick tests with pruritogens and a control vehicle. Itch and pain intensity were measured using a numerical rating scale (NRS) across 10 min. Participants were randomly assigned to pretreatments with vehicle or TRPM8 agonist gel. Tests were repeated at a later date, and skin moisture, transepidermal water loss and mechanical sensitivity were measured.

RESULTS

The in vitro study confirmed that histamine is not a TRPM8 agonist and cryosim-1 does not act as an agonist or antagonist on the human histamine 1 receptor. The TRPM8 agonist gel significantly reduced the itch intensity for all pruritogens compared with the vehicle-only gel. It also reduced itch NRS and the integrated itch score. Mechanical sensitivity was also reduced.

CONCLUSIONS

The specific TRPM8 agonist gel effectively suppressed human skin itch induced by various pruritogens. These versatile actions suggest that cooling agents may be promising treatments for multiple forms of itch stimuli.

A randomized, double-blinded, vehicle-controlled clinical trial of topical cryosim-1, a synthetic TRPM8 agonist, in prurigo nodularis

BACKGROUND

Prurigo nodularis (PN) is an intensively pruritic skin disease that negatively influences quality of life. Cryosim-1 (Intrinsic IB Spot) is a synthetic, selective transient receptor potential melastatin 8 agonist.

AIMS

To investigate the efficacy and safety of cryosim-1 in PN patients.

PATIENTS/METHODS

A randomized, double-blinded, placebo-controlled clinical trial including 30 patients was conducted. The numerical rating scale (NRS) of pruritus was evaluated before and 2 h after cryosim-1 application at every visit.

RESULTS

At week 8, the mean pruritus NRS before serum application (4.7 ± 0.4 treatment, 6.1 ± 0.5 placebo; p = 0.045) and 2 h after serum application (2.8 ± 0.4 treatment, 4.3 ± 0.5 placebo; p = 0.031) were significantly lower in the treatment group, and the mean NRS for sleep disorder was significantly lower in the treatment group (2.2 ± 0.5 treatment, 4.2 ± 0.8 placebo; p = 0.031). The mean satisfaction scales for pruritus improvement were significantly higher in the treatment group (7.2 ± 0.6) than in the placebo group (4.0 ± 0.9; p = 0.005). There was no difference in TEWL between the two groups, and no adverse reactions were reported.

CONCLUSIONS

Cryosim-1 is a safe and effective topical treatment for PN patients.

Mechanisms and treatment of opioid-induced pruritus: Peripheral and central pathways

BACKGROUND AND OBJECTIVE

Pruritus (also known as itch) is defined as an unpleasant and irritating sensation of the skin that provokes an urge to scratch or rub. It is well known that opioid administration can cause pruritus, which is paradoxical as itch and pain share overlapping sensory pathways. Because opioids inhibit pain but can cause itching. Significant progress has been made to improve our understanding of the fundamental neurobiology of itch; however, much remains unknown about the mechanisms of opioid-induced pruritus. The prevention and treatment of opioid-induced pruritus remains a challenge in the field of pain management. The objective of this narrative review is to present and discuss the current body of literature and summarize the current understanding of the mechanisms underlying opioid-induced pruritus, and its relationship to analgesia, and possible treatment options.

RESULTS

The incidence of opioid-induced pruritus differs with different opioids and routes of administration, and the various mechanisms can be broadly divided into peripheral and central. Especially central mechanisms are intricate, even at the level of the spinal dorsal horn. There is evidence that opioid receptor antagonists and mixed agonist and antagonists, especially μ-opioid antagonists and κ-opioid agonists, are effective in relieving opioid-induced pruritus. Various treatments have been used for opioid-induced pruritus; however, most of them are controversial and have conflicting results.

CONCLUSION

The use of a multimodal analgesic treatment regimen combined with a mixed antagonist and κ agonists, especially μ-opioid antagonists, and κ-opioid agonists, seems to be the current best treatment modality for the management of opioid-induced pruritus and pain.

SIGNIFICANCE

Opioids remain the gold standard for the treatment of moderate to severe acute pain as well as cancer pain. It is well known that opioid-induced pruritus often does not respond to regular antipruritic treatment, thereby posing a challenge to clinicians in the field of pain management. We believe that our review makes a significant contribution to the literature, as studies on the mechanisms of opioid-induced pruritus and effective management strategies are crucial for the management of these patients.

TRP (transient receptor potential) ion channel family: structures, biological functions and therapeutic interventions for diseases

Transient receptor potential (TRP) channels are sensors for a variety of cellular and environmental signals. Mammals express a total of 28 different TRP channel proteins, which can be divided into seven subfamilies based on amino acid sequence homology: TRPA (Ankyrin), TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipin), TRPN (NO-mechano-potential, NOMP), TRPP (Polycystin), TRPV (Vanilloid). They are a class of ion channels found in numerous tissues and cell types and are permeable to a wide range of cations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, and others. TRP channels are responsible for various sensory responses including heat, cold, pain, stress, vision and taste and can be activated by a number of stimuli. Their predominantly location on the cell surface, their interaction with numerous physiological signaling pathways, and the unique crystal structure of TRP channels make TRPs attractive drug targets and implicate them in the treatment of a wide range of diseases. Here, we review the history of TRP channel discovery, summarize the structures and functions of the TRP ion channel family, and highlight the current understanding of the role of TRP channels in the pathogenesis of human disease. Most importantly, we describe TRP channel-related drug discovery, therapeutic interventions for diseases and the limitations of targeting TRP channels in potential clinical applications.

Soothing Effect of a Cosmetic Product on Skin Discomforts Induced by a Chemical Irritant (Capsaicin) and UV-Radiation, and after Mosquito Bites and Sunburn in a Real-World Setting

Irritated and itchy skin is a common skin condition. Consumers tend to opt for natural ingredients for irritated skin (e.g., after insect bites or sun exposure). We tested a cosmetic product with 94% of its ingredients being of natural origin, each with its beneficial properties, e.g., nourishing shea butter, cooling menthol, and soothing bisabolol. Skin discomfort was induced either by a chemical irritant (capsaicin) or UV radiation by a solar simulator. In this clinical, prospective, and controlled experimental study, we investigated the soothing effect of the tested product. We observed a soothing effect on the capsaicin-induced itching and stinging sensation with a statistically significant decrease in the discomfort sensations one minute after a single application. The tested product also showed a significant reduction in the UV-induced skin erythema (UVA+B exposure). In a real-world study, these results can be correlated with a decrease of itching and irritation after sunburn or after insect bites.

The distinctive role of menthol in pain and analgesia: Mechanisms, practices, and advances

Menthol is an important flavoring additive that triggers a cooling sensation. Under physiological condition, low to moderate concentrations of menthol activate transient receptor potential cation channel subfamily M member 8 (TRPM8) in the primary nociceptors, such as dorsal root ganglion (DRG) and trigeminal ganglion, generating a cooling sensation, whereas menthol at higher concentration could induce cold allodynia, and cold hyperalgesia mediated by TRPM8 sensitization. In addition, the paradoxical irritating properties of high concentrations of menthol is associated with its activation of transient receptor potential cation channel subfamily A member 1 (TRPA1). Under pathological situation, menthol activates TRPM8 to attenuate mechanical allodynia and thermal hyperalgesia following nerve injury or chemical stimuli. Recent reports have recapitulated the requirement of central group II/III metabotropic glutamate receptors (mGluR) with endogenous κ-opioid signaling pathways for menthol analgesia. Additionally, blockage of sodium channels and calcium influx is a determinant step after menthol exposure, suggesting the possibility of menthol for pain management. In this review, we will also discuss and summarize the advances in menthol-related drugs for pathological pain treatment in clinical trials, especially in neuropathic pain, musculoskeletal pain, cancer pain and postoperative pain, with the aim to find the promising therapeutic candidates for the resolution of pain to better manage patients with pain in clinics.

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.

A neuropeptide code for itch

Itch is one of the most primal sensations, being both ubiquitous and important for the well-being of animals. For more than a century, a desire to understand how itch is encoded by the nervous system has prompted the advancement of many theories. Within the past 15 years, our understanding of the molecular and neural mechanisms of itch has undergone a major transformation, and this remarkable progress continues today without any sign of abating. Here I describe accumulating evidence that indicates that itch is distinguished from pain through the actions of itch-specific neuropeptides that relay itch information to the spinal cord. According to this model, classical neurotransmitters transmit, inhibit and modulate itch information in a context-, space- and time-dependent manner but do not encode itch specificity. Gastrin-releasing peptide (GRP) is proposed to be a key itch-specific neuropeptide, with spinal neurons expressing GRP receptor (GRPR) functioning as a key part of a convergent circuit for the conveyance of peripheral itch information to the brain.

Pharmacological interventions for treating intrahepatic cholestasis of pregnancy

BACKGROUND

Intrahepatic cholestasis of pregnancy (ICP) is a liver disorder that can develop in pregnancy. It occurs when there is a build-up of bile acids in the maternal blood. It has been linked to adverse maternal and fetal/neonatal outcomes. As the pathophysiology is poorly understood, therapies have been largely empiric. As ICP is an uncommon condition (incidence less than 2% a year), many trials have been small. Synthesis, including recent larger trials, will provide more evidence to guide clinical practice. This review is an update of a review first published in 2001 and last updated in 2013.

OBJECTIVES

To assess the effects of pharmacological interventions to treat women with intrahepatic cholestasis of pregnancy, on maternal, fetal and neonatal outcomes.

SEARCH METHODS

For this update, we searched Cochrane Pregnancy and Childbirth's Trials Register, ClinicalTrials.gov, the WHO International Clinical Trials Registry Platform (ICTRP) (13 December 2019), and reference lists of retrieved studies.

SELECTION CRITERIA

Randomised or quasi-randomised controlled trials, including cluster-randomised trials and trials published in abstract form only, that compared any drug with placebo or no treatment, or two drug intervention strategies, for women with a clinical diagnosis of intrahepatic cholestasis of pregnancy.

DATA COLLECTION AND ANALYSIS

The review authors independently assessed trials for eligibility and risks of bias. We independently extracted data and checked these for accuracy. We assessed the certainty of the evidence using the GRADE approach.

MAIN RESULTS

We included 26 trials involving 2007 women. They were mostly at unclear to high risk of bias. They assessed nine different pharmacological interventions, resulting in 14 different comparisons. We judged two placebo-controlled trials of ursodeoxycholic acid (UDCA) in 715 women to be at low risk of bias. The ten different pharmacological interventions were: agents believed to detoxify bile acids (UCDA) and S-adenosylmethionine (SAMe); agents used to bind bile acids in the intestine (activated charcoal, guar gum, cholestyramine); Chinese herbal medicines (yinchenghao decoction (YCHD), salvia, Yiganling and Danxioling pill (DXLP)), and agents aimed to reduce bile acid production (dexamethasone) Compared with placebo, UDCA probably results in a small improvement in pruritus score measured on a 100 mm visual analogue scale (VAS) (mean difference (MD) -7.64 points, 95% confidence interval (CI) -9.69 to -5.60 points; 2 trials, 715 women; GRADE moderate certainty), where a score of zero indicates no itch and a score of 100 indicates severe itching. The evidence for fetal distress and stillbirth were uncertain, due to serious limitations in study design and imprecision (risk ratio (RR) 0.70, 95% CI 0.35 to 1.40; 6 trials, 944 women; RR 0.33, 95% CI 0.08 to 1.37; 6 trials, 955 women; GRADE very low certainty). We found very few differences for the other comparisons included in this review. There is insufficient evidence to indicate if SAMe, guar gum, activated charcoal, dexamethasone, cholestyramine, Salvia, Yinchenghao decoction, Danxioling and Yiganling, or Yiganling alone or in combination are effective in treating women with intrahepatic cholestasis of pregnancy.

AUTHORS' CONCLUSIONS

When compared with placebo, UDCA administered to women with ICP probably shows a reduction in pruritus. However the size of the effect is small and for most pregnant women and clinicians, the reduction may fall below the minimum clinically worthwhile effect. The evidence was unclear for other adverse fetal outcomes, due to very low-certainty evidence. There is insufficient evidence to indicate that SAMe, guar gum, activated charcoal, dexamethasone, cholestyramine, YCHD, DXLP, Salvia, Yiganling alone or in combination are effective in treating women with cholestasis of pregnancy. There are no trials of the efficacy of topical emollients. Further high-quality trials of other interventions are needed in order to identify effective treatments for maternal itching and preventing adverse perinatal outcomes. It would also be helpful to identify those women who are mostly likely to respond to UDCA (for example, whether bile acid concentrations affect how women with ICP respond to treatment with UDCA).

Interventions for chronic pruritus of unknown origin

BACKGROUND

Pruritus is a sensation that leads to the desire to scratch; its origin is unknown in 8% to 15% of affected patients. The prevalence of chronic pruritus of unknown origin (CPUO) in individuals with generalised pruritus ranges from 3.6% to 44.5%, with highest prevalence among the elderly. When the origin of pruritus is known, its management may be straightforward if an effective treatment for the causal disease is available. Treatment of CPUO is particularly difficult due to its unknown pathophysiology.

OBJECTIVES

To assess the effects of interventions for CPUO in adults and children.

SEARCH METHODS

We searched the following up to July 2019: Cochrane Skin Group Specialised Register, CENTRAL, MEDLINE, Embase, and trials registries. We checked the reference lists of included studies for additional references to relevant trials.

SELECTION CRITERIA

We sought to include randomised controlled trials and quasi-randomised controlled trials that assessed interventions for CPUO, as defined in category VI ('Other pruritus of undetermined origin, or chronic pruritus of unknown origin') of the International Forum for the Study of Itch (IFSI) classification, in children and adults. Eligible interventions were non-pharmacological or topical or systemic pharmacological interventions, and eligible comparators were another active treatment, placebo, sham procedures, or no treatment or equivalent (e.g. waiting list).

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures expected by Cochrane. Our primary outcomes were 'Patient- or parent-reported pruritus intensity' and 'Adverse events'. Our secondary outcomes were 'Health-related quality of life', 'Sleep disturbances', 'Depression', and 'Patient satisfaction'. We used GRADE to assess the certainty of evidence.

MAIN RESULTS

We found there was an absence of evidence for the main interventions of interest: emollient creams, cooling lotions, topical corticosteroids, topical antidepressants, systemic antihistamines, systemic antidepressants, systemic anticonvulsants, and phototherapy. We included one study with 257 randomised (253 analysed) participants, aged 18 to 65 years; 60.6% were female. This study investigated the safety and efficacy of three different doses of oral serlopitant (5 mg, 1 mg, and 0.25 mg, once daily for six weeks) compared to placebo for severe chronic pruritus; 25 US centres participated (clinical research centres and universities). All outcomes were measured at the end of treatment (six weeks from baseline), except adverse events, which were monitored throughout. A pharmaceutical company funded this study. Fifty-five per cent of participants suffered from CPUO, and approximately 45% presented a dermatological diagnosis (atopic dermatitis/eczema 37.3%, psoriasis 6.7%, acne 3.6%, among other diagnoses). We unsuccessfully attempted to retrieve outcome data from study authors for the subgroup of participants with CPUO. Participants had pruritus for six weeks or longer. Total study duration was 10 weeks. Participants who received serlopitant 5 mg may have a greater rate of relief of patient-reported pruritus intensity as measured by the visual analogue scale (VAS; a reduction in VAS score indicates improvement) compared to placebo (126 participants, risk ratio (RR) 2.06, 95% confidence interval (CI) 1.27 to 3.35; low-certainty evidence). We are uncertain of the effects of serlopitant 5 mg compared to placebo on the following outcomes due to very low-certainty evidence: adverse events (127 participants; RR 1.48, 95% CI 0.87 to 2.50); health-related quality of life (as measured by the Dermatology Life Quality Index (DLQI); a higher score indicates greater impairment; 127 participants; mean difference (MD) -4.20, 95% CI -11.68 to 3.28); and sleep disturbances (people with insomnia measured by the Pittsburgh Sleep Symptom Questionnaire-Insomnia (PSSQ-I), a dichotomous measure; 128 participants; RR 0.49, 95% CI 0.24 to 1.01). Participants who received serlopitant 1 mg may have a greater rate of relief of patient-reported pruritus intensity as measured by VAS compared to placebo; however, the 95% CI indicates that there may also be little to no difference between groups (126 participants; RR 1.50, 95% CI 0.89 to 2.54; low-certainty evidence). We are uncertain of the effects of serlopitant 1 mg compared to placebo on the following outcomes due to very low-certainty evidence: adverse events (128 participants; RR 1.45, 95% CI 0.86 to 2.47); health-related quality of life (DLQI; 128 participants; MD -6.90, 95% CI -14.38 to 0.58); and sleep disturbances (PSSQ-I; 128 participants; RR 0.38, 95% CI 0.17 to 0.84). Participants who received serlopitant 0.25 mg may have a greater rate of relief of patient-reported pruritus intensity as measured by VAS compared to placebo; however, the 95% CI indicates that there may also be little to no difference between groups (127 participants; RR 1.66, 95% CI 1.00 to 2.77; low-certainty evidence). We are uncertain of the effects of serlopitant 0.25 mg compared to placebo on the following outcomes due to very low-certainty evidence: adverse events (127 participants; RR 1.29, 95% CI 0.75 to 2.24); health-related quality of life (DLQI; 127 participants; MD -5.70, 95% CI -13.18 to 1.78); and sleep disturbances (PSSQ-I; 127 participants; RR 0.60, 95% CI 0.31 to 1.17). The most commonly reported adverse events were somnolence, diarrhoea, headache, and nasopharyngitis, among others. Our included study did not measure depression or patient satisfaction. We downgraded the certainty of evidence for all outcomes due to indirectness (only 55% of study participants had CPUO) and imprecision. We downgraded outcomes other than patient-reported pruritus intensity a further level due to concerns regarding risk of bias in selection of the reported result and some concerns with risk of bias due to missing outcome data (sleep disturbances only). We deemed risk of bias to be generally low.

AUTHORS' CONCLUSIONS

We found lack of evidence to address our review question: for most of our interventions of interest, we found no eligible studies. The neurokinin 1 receptor (NK1R) antagonist serlopitant was the only intervention that we could assess. One study provided low-certainty evidence suggesting that serlopitant may reduce pruritus intensity when compared with placebo. We are uncertain of the effects of serlopitant on other outcomes, as certainty of the evidence is very low. More studies with larger sample sizes, focused on patients with CPUO, are needed. Healthcare professionals, patients, and other stakeholders may have to rely on indirect evidence related to other forms of chronic pruritus when deciding between the main interventions currently used for this condition.

Pain Inhibits GRPR Neurons via GABAergic Signaling in the Spinal Cord

It has been known that algogens and cooling could inhibit itch sensation; however, the underlying molecular and neural mechanisms remain poorly understood. Here, we show that the spinal neurons expressing gastrin releasing peptide receptor (GRPR) primarily comprise excitatory interneurons that receive direct and indirect inputs from C and Aδ fibers and form contacts with projection neurons expressing the neurokinin 1 receptor (NK1R). Importantly, we show that noxious or cooling agents inhibit the activity of GRPR neurons via GABAergic signaling. By contrast, capsaicin, which evokes a mix of itch and pain sensations, enhances both excitatory and inhibitory spontaneous synaptic transmission onto GRPR neurons. These data strengthen the role of GRPR neurons as a key circuit for itch transmission and illustrate a spinal mechanism whereby pain inhibits itch by suppressing the function of GRPR neurons.

TRP Channels as Drug Targets to Relieve Itch

Although acute itch has a protective role by removing irritants to avoid further damage, chronic itch is debilitating, significantly impacting quality of life. Over the past two decades, a considerable amount of stimulating research has been carried out to delineate mechanisms of itch at the molecular, cellular, and circuit levels. There is growing evidence that transient receptor potential (TRP) channels play important roles in itch signaling. The purpose of this review is to summarize our current knowledge about the role of TRP channels in the generation of itch under both physiological and pathological conditions, thereby identifying them as potential drug targets for effective anti-itch therapies.

Menthol, a unique urinary volatile compound, is associated with chronic inflammation in interstitial cystitis

Chronic inflammation is a potential systemic risk factor for many bladder dysfunctions, including interstitial cystitis (IC). However, the underlying mechanism through which a healthy bladder protects itself from inflammatory triggers remains unknown. In this study, we identified odor compounds in urine obtained from IC patients and healthy controls. Using comprehensive solid-phase microextraction-gas chromatography-time-of-flight-mass spectrometry (SPME-GC-TOF-MS) profiling and bioinformatics, we found that levels of urinary volatile metabolites, such as menthol, were significantly reduced in IC patients, compared to healthy controls. In an attempt to understand the mechanistic meaning of our volatile metabolites data and the role of menthol in the immune system, we performed two independent experiments: (a) cytokine profiling, and (b) DNA microarray. Our findings suggest that lipopolysaccharide (LPS)-stimulated inflammatory events, such as the production and secretion of inflammatory cytokines (e.g., TNF-α, IL-6, and IL-1β) and the activation of NF-κB and associated proteins within a large signaling network (e.g., Akt, TLR1, TNFAIP3, and NF-κB), are suppressed by the presence of menthol. These findings broaden our knowledge on the role of urinary menthol in suppressing inflammatory events and provide potential new strategies for alleviating both the odor and inflammation associated with IC.

Cooling the Itch via TRPM8

Cooling is an effective temporary remedy for itch, bringing welcome relief to itchy insect bites, nettle stings, poison ivy, atopic dermatitis, and psoriasis. Menthol, causing a cooling sensation, has similar itch-relieving effects. Palkar et al. demonstrate that TRPM8, a menthol- and cold-activated ion channel, is essential for cooling to relieve itch, suggesting that pharmacologic TRPM8 activation should be explored further as an antipruritic strategy.

Natural product modulators of human sensations and mood: molecular mechanisms and therapeutic potential

Humans perceive physical information about the surrounding environment through their senses. This physical information is registered by a collection of highly evolved and finely tuned molecular sensory receptors. A multitude of bioactive, structurally diverse ligands have evolved in nature that bind these molecular receptors. The complex, dynamic interactions between the ligands and the receptors lead to changes in our sensory perception or mood. Here, we review our current knowledge of natural products and their derived analogues that interact specifically with human G protein-coupled receptors, ion channels, and nuclear hormone receptors to modulate the sensations of taste, smell, temperature, pain, and itch, as well as mood and its associated behaviour. We discuss the molecular and structural mechanisms underlying such interactions and highlight cases where subtle differences in natural product chemistry produce drastic changes in functional outcome. We also discuss cases where a single compound triggers complex sensory or behavioural changes in humans through multiple mechanistic targets. Finally, we comment on the therapeutic potential of the reviewed area of research and draw attention to recent technological developments in genomics, metabolomics, and metabolic engineering that allow us to tap the medicinal properties of natural product chemistry without taxing nature.

Pilot evaluation of the antipruritic efficacy of a topical transient receptor potential melastatin subfamily 8 (TRPM8) agonist in dogs with atopic dermatitis and pedal pruritus

BACKGROUND

Atopic dermatitis (AD) requires a multimodal therapy and there is a need for effective adjunctive interventions. TRPM8 agonists are known to alleviate pruritus by inducing cooling.

OBJECTIVES

To evaluate the efficacy of a novel TRPM8 agonist, 1-diisopropylphosphorylheptane (Cryosim-1), in atopic dogs.

ANIMALS

Nine client owned dogs with moderate to severe pedal pruritus associated with nonseasonal AD.

METHODS

This was a prospective, randomized, double-blinded, intraindividual, placebo-controlled study. A 2% Cryosim-1 or placebo-vehicle cream was applied to each forepaw twice daily for seven days. The owner rated the pruritus manifestations once daily using a pedal Pruritus Visual Analog Scale (PVAS) and provided a Owner's Global Assessment of Treatment Efficacy (OGATE) at study end.

RESULTS

After seven days, the numbers of dogs with a pedal PVAS <2.0 for Cryosim-1 and placebo were three and five of nine, respectively; likewise, OGATE scores of good-to-excellent were two and five of eight, respectively - these proportions were not significant between treatment groups (P = 0.32 and 0.16, respectively). Furthermore, there was no significant difference between Cryosim-1 and placebo in the median percentage change from baseline PVAS (47% versus 75%; P = 0.15) and in the number of dogs with a ≥50% or a ≥90% reduction from baseline pedal PVAS (four of nine versus five of nine, P = 0.50; two of nine versus two of nine, P = 0.72).

CONCLUSIONS

In this pilot trial with a TRPM8 agonist in atopic dogs with pedal pruritus, the twice daily application of a 2% Cryosim-1 cream did not have an antipruritic effect superior to that of its vehicle.

Adjunctive Management of Itch in Atopic Dermatitis

Itch, or pruritus, is a hallmark feature of atopic dermatitis (AD). The impact of AD-related pruritus can range from mildly distressing or distracting to completely disabling. Traditionally, management of itch in AD patients has focused on restoring the altered skin barrier with topical emollients and/or reducing inflammation. A growing emphasis has been placed on directly targeting the neural transmission pathways that mediate itch signaling. Off-label use of neuromodulatory agents has helped reduce this aggravating symptom in atopic patients. This article reviews the current literature on the use of neuromodulatory agents and nonpharmacologic alternative therapies used to treat AD-related pruritus.

GC Method Validation for the Analysis of Menthol in Suppository Pharmaceutical Dosage Form

Menthol is widely used as a fragrance and flavor in the food and cosmetic industries. It is also used in the medical and pharmaceutical fields for its various biological effects. Gas chromatography (GC) is considered to be a sensitive method for the analysis of menthol. GC chromatographic separation was developed using capillary column (VF-624) and a flame ionization detector (FID). The method was validated as per ICH guidelines for various parameters such as precision, linearity, accuracy, solution stability, robustness, limit of detection, and quantification. The tested validation parameters were found to be within acceptable limits. The method was successfully applied for the quantification of menthol in suppositories formulations. Quality control departments and official pharmacopeias can use our developed method in the analysis of menthol in pharmaceutical dosage formulation and raw material.

TRP Channels in Skin Biology and Pathophysiology

Ion channels of the Transient Receptor Potential (TRP) family mediate the influx of monovalent and/or divalent cations into cells in response to a host of chemical or physical stimuli. In the skin, TRP channels are expressed in many cell types, including keratinocytes, sensory neurons, melanocytes, and immune/inflammatory cells. Within these diverse cell types, TRP channels participate in physiological processes ranging from sensation to skin homeostasis. In addition, there is a growing body of evidence implicating abnormal TRP channel function, as a product of excessive or deficient channel activity, in pathological skin conditions such as chronic pain and itch, dermatitis, vitiligo, alopecia, wound healing, skin carcinogenesis, and skin barrier compromise. These diverse functions, coupled with the fact that many TRP channels possess pharmacologically accessible sites, make this family of proteins appealing therapeutic targets for skin disorders.

High-Concentration L-Menthol Exhibits Counter-Irritancy to Neurogenic Inflammation, Thermal and Mechanical Hyperalgesia Caused by Trans-cinnamaldehyde

The transient receptor potential cation channel subfamily M 8 (TRPM8) agonist L-menthol has been used traditionally for its topical counterirritant properties. Although the use of topical L-menthol for pain is casuistically established, evidence regarding its efficacy is negligible. This study aimed to characterize the effect of L-menthol as a counterirritant on cutaneous pain and hyperalgesia provoked by topical application of the transient receptor potential cation channel, subfamily A, member 1 (TRPA1) agonist trans-cinnamaldehyde (CA). In a randomized, double-blinded study CA was applied to a 3 × 3-cm area of the volar forearm evoking neurogenic inflammation, pain, mechanical, and thermal hyperalgesia in 14 healthy volunteers. In different sessions, 10% CA alone or 40% L-menthol applied simultaneously with 10% CA were administered for 20 minutes, throughout which the subjects rated the pain intensity on a visual analogue scale of 0 to 10. Extensive quantitative sensory testing was conducted and superficial blood flow (neurogenic inflammation) was recorded. Administration of CA evoked spontaneous pain, neurogenic inflammation, thermal hyperalgesia, and primary and secondary mechanical hyperalgesia. Coadministration of topical L-menthol reduced spontaneous pain intensity (P < .01), neurogenic inflammation (P < .01), primary mechanical hyperalgesia (P < .05), secondary mechanical hyperalgesia (P < .05), and heat hyperalgesia (P < .05), but not cold hyperalgesia. L-menthol exhibited inhibitory effects on simultaneously established pain, hypersensitivity, and neurogenic inflammation in a human TRPA1-induced pain model. Potent TRPM8 agonists could be useful as topical antihyperalgesics. The study and the trial protocol is registered and approved by the local research ethics committee under the jurisdiction of the Danish Medicines Agency number N-20130005. The protocol also is registered at Clinicaltrials.gov under NCT02653703.

PERSPECTIVE

Drugs interacting with transient receptor potential channels are of great therapeutic potential. In the present study we established cutaneous pain and hyperalgesia using the TRPA1 agonist CA. Subsequently, we showed that the frequently used topical counterirritant and TRPM8 agonist, L-menthol, decreased evoked pain, hyperalgesia, and inflammation, indicating direct and indirect antinociceptive mechanisms.

Trp channels and itch

Itch is a unique sensation associated with the scratch reflex. Although the scratch reflex plays a protective role in daily life by removing irritants, chronic itch remains a clinical challenge. Despite urgent clinical need, itch has received relatively little research attention and its mechanisms have remained poorly understood until recently. The goal of the present review is to summarize our current understanding of the mechanisms of acute as well as chronic itch and classifications of the primary itch populations in relationship to transient receptor potential (Trp) channels, which play pivotal roles in multiple somatosensations. The convergent involvement of Trp channels in diverse itch signaling pathways suggests that Trp channels may serve as promising targets for chronic itch treatments.

Assessment of Itch and Pain in Animal Models and Human Subjects

For the past century, scientists have developed a variety of methods to evaluate itch and pain in both animal models and human subjects to throw light on some of the most important pathways mediating these unpleasant sensations. Discoveries in the mechanisms underlying itch and pain in both physiological and pathological conditions relied greatly upon these studies and may eventually lead to the discovery of new therapeutics. However, it was a much more complicated job to access itch and pain in animal models than in human subjects due to the subjective nature of these sensations. The results could be contradictory or even misleading when applying different methodologies in animal models, especially under pathological conditions with a mixed sensation of itch and pain. This chapter introduces and evaluates some of the classical and newly designed methodologies to access the sensation of itch and pain in animal models as well as human subjects.

Effects of scratching and other counterstimuli on responses of trigeminothalamic tract neurons to itch-inducing stimuli in rats

Counterstimuli such as scratching, pinching, noxious heat and cold, and innocuous cooling and warming have been shown to inhibit itch in humans. In the present study, the effects of each of these counterstimuli were determined on baseline firing rates and on sustained pruriceptive responses of rat trigeminothalamic tract neurons. We found that scratching had little, if any, effect on baseline firing levels but greatly reduced mean pruriceptive firing following scratching for nearly 1 min. None of the other noxious or innocuous counterstimuli significantly inhibited pruriceptive responses. Our results indicate that scratching, but not other counterstimuli, significantly reduces itch-induced responses of trigeminothalamic tract neurons.

Molecular and cellular mechanisms that initiate pain and itch

Somatosensory neurons mediate our sense of touch. They are critically involved in transducing pain and itch sensations under physiological and pathological conditions, along with other skin-resident cells. Tissue damage and inflammation can produce a localized or systemic sensitization of our senses of pain and itch, which can facilitate our detection of threats in the environment. Although acute pain and itch protect us from further damage, persistent pain and itch are debilitating. Recent exciting discoveries have significantly advanced our knowledge of the roles of membrane-bound G protein-coupled receptors and ion channels in the encoding of information leading to pain and itch sensations. This review focuses on molecular and cellular events that are important in early stages of the biological processing that culminates in our senses of pain and itch.

Psychophysical measurements of itch and nociceptive sensations in an experimental model of allergic contact dermatitis

Allergic contact dermatitis (ACD) is a common condition that can significantly affect the quality of life. Contact with allergens results in delayed hypersensitivity reactions involving T lymphocytes, with associated skin inflammation and spontaneous itch and nociceptive sensations. However, psychophysical studies of these sensations are lacking. In the present study, we sensitized 8 healthy volunteers to squaric acid dibutyl ester (SADBE). Two weeks later, 1 volar forearm was challenged with SADBE, and the other with acetone vehicle control. Subsequently, participants rated the maximal perceived intensity of spontaneous itch, pricking/stinging, and burning every 6 to 12 hours for 1 week, using the generalized Labeled Magnitude Scale. In the laboratory, they judged stimulus-evoked sensations within and outside the chemically treated area. The SADBE- but not the acetone-treated skin resulted in 1) localized inflammation, with spontaneous itch and nociceptive sensations peaking at 24 to 48 hours after challenge, 2) alloknesis, hyperknesis, and hyperalgesia to mechanical stimuli that were reduced or eliminated by anesthetic cooling of the SADBE-treated area and restored on rewarming, suggesting that sensations and dysesthesias are dependent on ongoing peripheral neural activity, and 3) enhanced itch to intradermal injection of histamine, BAM8-22, or β-alanine. This experimental model of T-cell-mediated inflammation may prove useful in evaluating potential treatments of itch from ACD.

PERSPECTIVE

In a model of allergic contact dermatitis, experimentally applied in humans, psychophysical measurements were obtained of persistent, spontaneous itch and enhanced stimulus-evoked itch and pain sensations. These sensory measurements will be useful in the identification of the neural mechanisms underlying inflammatory itch and pain.

Itch and pain differences and commonalities

Pain and itch are generally regarded antagonistic as painful stimuli such as scratching suppresses itch. Moreover, inhibition of pain processing by opioids generates itch further supporting their opposing role. Separate specific pathways for itch and pain processing have been uncovered, and several molecular markers have been established in mice that identify neurons involved in the processing of histaminergic and non-histaminergic itch on primary afferent and spinal level. These results are in agreement with the specificity theory for itch and might suggest that pain and itch should be investigated separately on the level of neurons, mediators, and mechanisms. However, in addition to broadly overlapping mediators of itch and pain, there is also evidence for overlapping functions in primary afferents: nociceptive primary afferents can provoke itch when activated very locally in the epidermis, and sensitization of both nociceptors and pruriceptors has been found following local nerve growth factor application in volunteers. Thus, also mechanisms that underlie the development of chronic itch and pain including spontaneous activity and sensitization of primary afferents as well as spinal cord sensitization may well overlap to a great extent. Rather than separating itch and pain, research concepts should therefore address the common mechanisms. Such an approach appears most appropriate for clinical conditions of neuropathic itch and pain and also chronic inflammatory conditions. While itch researchers can benefit from the large body of information of the pain field, pain researchers will find behavioral readouts of spontaneous itch much simpler than those for spontaneous pain in animals and the skin as source of the pruritic activity much more accessible even in patients.

Transmission of pruriceptive signals

In this chapter we discuss the many recent discoveries of the mechanisms by which itch is transmitted: the neurotransmitters and the responses they trigger, the mechanisms by which specific neuronal targets are activated, and the specificity of the pathways. Current data reveal that DRG neurons and spinal cord cells use a remarkably selective set of transmitters to convey pruritic information from the periphery to the brain: glutamate and Nppb are released from primary itch-sensory cells; these molecules activate secondary spinal cord pruriceptive-specific neurons, which in turn utilize Grp to activate tertiary pruriceptive-selective neurons. Intersecting this basic linear excitatory pathway, inhibitory input from dynorphin and neurons that express the somatostatin receptor modify itch sensation. Cumulatively, these studies paint an elegantly simple picture of how itch signals are transformed and integrated in the spinal cord and open new avenues for research efforts aimed at understanding and better treating itch.

Noradrenergic modulation of itch transmission in the spinal cord

Inhibition of both itching and scratching is important in the treatment of chronic pruritic diseases, because itching has a negative impact on quality of life and vigorous scratching worsens skin conditions. Pharmacological modulation of itch transmission in the dorsal horn is an effective way to inhibit both itching and scratching in pruritic diseases. Pruriceptive transmission in the spinal dorsal horn undergoes inhibitory modulation by the descending noradrenergic system. The noradrenergic inhibition is mediated by excitatory α₁-adrenoceptors located on inhibitory interneurons and inhibitory α₂-adrenoceptors located on central terminals of primary sensory neurons. The descending noradrenergic system and α-adrenoceptors in the dorsal horn are potential targets for antipruritic drugs.

Transient receptor potential channels and itch: how deep should we scratch?

Over the past 30 years, transient receptor potential (TRP) channels have evolved from a somewhat obscure observation on how fruit flies detect light to become the center of drug discovery efforts, triggering a heated debate about their potential as targets for therapeutic applications in humans. In this review, we describe our current understanding of the diverse mechanism of action of TRP channels in the itch pathway from the skin to the brain with focus on the peripheral detection of stimuli that elicit the desire to scratch and spinal itch processing and sensitization. We predict that the compelling basic research findings on TRP channels and pruritus will be translated into the development of novel, clinically useful itch medications.

A randomized, investigator-blinded efficacy assessment study of stand-alone emollient use in mild to moderately severe atopic dermatitis flares

BACKGROUND

Whereas emollients are integral to the long-term management of atopic dermatitis (AD), the evidence for their efficacy in disease flares is limited.

OBJECTIVE

We aimed to investigate the stand-alone efficacy of an emollient formulation with regard to improvement of the clinical symptoms, skin barrier function and reduction of pathogenic bacterial colonization in acute stage of AD.

MATERIALS AND METHODS

Twenty AD volunteers aged 12-65 years with symmetric, mild to moderately severe inflammatory lesions on the forearms/arms were recruited for the study. At inclusion, the forearms/arms of each volunteer were randomized to receive for 1 week either an o/w formulation containing licochalcone A (Glycyrrhiza Inflata root extract), decanediol, menthoxypropanediol and ω-6-fatty acids (emollient arm) or 1% hydrocortisone (HC arm); after 1 week, the application of the emollient and HC were discontinued and the volunteers applied a w/o emollient containing licochalcone A and ω-6-fatty acids on both arms for further 3 weeks. The outcomes included reduction of the clinical and itch severity, decrease in S.aureus colonization, improvement of the barrier function, skin hydration and skin tolerability assessed after 1 week (D7) and after 4 weeks (D28) respectively.

RESULTS

In both arms, there was a significant decrease in the severity score, itch intensity, erythema and TEWL on D7 and D28 compared to baseline. In addition, emollient use resulted in pronounced decrease in S.aureus colonization and significant increase of skin hydration on D7. The comparison of the outcomes, based on percentage change from baseline, showed no significant differences between the emollient and HC arm at any time point.

CONCLUSIONS

The results of the study indicate that the 1-week stand-alone application of an emollient, tailored to target inflammation, pruritus, compromised barrier function and pathogenic bacterial colonization may offer benefit for the improvement of mild to moderately severe localized flares of AD.

Dynorphin acts as a neuromodulator to inhibit itch in the dorsal horn of the spinal cord

Menthol and other counterstimuli relieve itch, resulting in an antipruritic state that persists for minutes to hours. However, the neural basis for this effect is unclear, and the underlying neuromodulatory mechanisms are unknown. Previous studies revealed that Bhlhb5^−/− mice, which lack a specific population of spinal inhibitory interneurons (B5-I neurons), develop pathological itch. Here we characterize B5-I neurons and show that they belong to a neurochemically distinct subset. We provide cause-and-effect evidence that B5-I neurons inhibit itch and show that dynorphin, which is released from B5-I neurons, is a key neuromodulator of pruritus. Finally, we show that B5-I neurons are innervated by menthol-, capsaicin-, and mustard oil-responsive sensory neurons and are required for the inhibition of itch by menthol. These findings provide a cellular basis for the inhibition of itch by chemical counterstimuli and suggest that kappa opioids may be a broadly effective therapy for pathological itch.

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Spinal B5-I interneurons function to inhibit itch

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B5-I neurons release the kappa opioid dynorphin

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Kappa opioid signaling bidirectionally modulates itch within the spinal cord

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Spinal B5-I interneurons mediate the inhibition of itch by menthol