Effect of loratadine on mouse models of atopic dermatitis associated pruritus

A Diterpenoid of the Medicinal Plant Andrographis paniculata Targets Cutaneous TRPV3 Channel and Relieves Itch

Transient receptor potential vanilloid subtype 3 (TRPV3) is an ion channel implicated in skin physiology and itch. TRPV3 inhibitors can present a novel strategy for combating debilitating itch conditions, and medicinal plants are a natural pool of such compounds. Here, we report the isolation of a TRPV3-inhibiting compound from Andrographis paniculata, a medicinal plant with anti-inflammatory properties whose bioactive components are poorly characterized in terms of molecular targets. Using 1H and 13C NMR and high-resolution mass spectrometry, the compound was identified as a labdane-type diterpenoid, 14-deoxy-11,12-didehydroandrographolide (ddA). The activity of the compound was evaluated by fluorescent calcium assay and manual whole-cell patch-clamp technique. ddA inhibited human TRPV3 in stably expressing CHO and HaCaT keratinocytes, acting selectively among other TRP channels implicated in itch and inflammation and not showing toxicity to HaCaT cells. Antipruritic effects of the compound were evaluated in scratching behavior models on ICR mice. ddA suppressed itch induced by the TRPV3 activator carvacrol. Additionally, ddA potently suppressed histamine-induced itch with efficacy comparable to loratadine, a clinically used antihistamine drug. These results suggest the potential of ddA as a possible safe and efficacious alternative for antipruritic therapy.

Immune sensing of food allergens promotes avoidance behaviour

In addition to its canonical function of protection from pathogens, the immune system can also alter behaviour1,2. The scope and mechanisms of behavioural modifications by the immune system are not yet well understood. Here, using mouse models of food allergy, we show that allergic sensitization drives antigen-specific avoidance behaviour. Allergen ingestion activates brain areas involved in the response to aversive stimuli, including the nucleus of tractus solitarius, parabrachial nucleus and central amygdala. Allergen avoidance requires immunoglobulin E (IgE) antibodies and mast cells but precedes the development of gut allergic inflammation. The ability of allergen-specific IgE and mast cells to promote avoidance requires cysteinyl leukotrienes and growth and differentiation factor 15. Finally, a comparison of C57BL/6 and BALB/c mouse strains revealed a strong effect of the genetic background on the avoidance behaviour. These findings thus point to antigen-specific behavioural modifications that probably evolved to promote niche selection to avoid unfavourable environments.

The histamine H4 receptor mediates inflammation and pruritus in Th2-dependent dermal inflammation

The role of histamine H(4) receptor (H(4)R) was investigated in a T-helper type 2 (Th2)-cell-mediated mouse skin inflammation model that mimics several of the features of atopic dermatitis. Treatment with two specific H(4)R antagonists before challenge with FITC led to a significant reduction in ear edema, inflammation, mast cell, and eosinophil infiltration. This was accompanied by a reduction in the levels of several cytokines and chemokines in the ear tissue. Upon ex vivo antigen stimulation of lymph nodes, H(4)R antagonism reduced lymphocyte proliferation and IL-4, IL-5, and IL-17 levels. One explanation for this finding is that lymph nodes from animals dosed with the H(4)R antagonist, JNJ 7777120, contained a lower number of FITC-positive dendritic cells. The effect of H(4)R antagonism on dendritic cell migration in vivo may be an indirect result of the reduction in tissue cytokines and chemokines or a direct effect on chemotaxis. In addition to anti-inflammatory effects, JNJ 7777120 also significantly inhibited the pruritus shown in the model. Therefore, the dual effects of H(4)R antagonists on pruritus and Th2-cell-mediated inflammation point to their therapeutic potential for the treatment of Th2-mediated skin disorders, including atopic dermatitis.

The role of histamine H1 and H4 receptors in allergic inflammation: the search for new antihistamines

Histamine has a key role in allergic inflammatory conditions. The inflammatory responses resulting from the liberation of histamine have long been thought to be mediated by the histamine H1 receptor, and H1-receptor antagonists--commonly known as antihistamines--have been used to treat allergies for many years. However, the importance of histamine in the pathology of conditions such as asthma and chronic pruritus may have been underestimated. Here, we review accumulating evidence suggesting that histamine indeed has roles in inflammation and immune function modulation in such diseases. In particular, the discovery of a fourth histamine receptor (H4) and its expression on numerous immune and inflammatory cells has prompted a re-evaluation of the actions of histamine, suggesting a new potential for H4-receptor antagonists and a possible synergy between H1 and H4-receptor antagonists in targeting various inflammatory conditions.

Association of a Mutation in TRPV3 with Defective Hair Growth in Rodents

DS-Nh mice and WBN/Kob-Ht rats are spontaneous hairless mutant rodent strains. These animals develop spontaneous dermatitis under normal conditions. The non-hair Nh and Ht phenotypes are inherited in an autosomal dominant fashion, and the Nh mutation possesses a high potency for penetration. We previously reported that genes involved in dermatitis and hairlessness did not segregate from each other. Here, we carried out genetic analysis to identify the genes responsible for these hairless mutations. An amino-acid substitution at the same position in one gene was detected in DS-Nh mice and WBN/Kob-Ht rats: Gly573 to Ser (Nh mutation) or Gly573 to Cys (Ht mutation), located in the transient receptor potential (TRP) cation channel subfamily V member 3 (TRPV3) gene. Mutated TRPV3 was expressed in skin keratinocytes of DS-Nh mice. Histopathological analyses revealed that mast cells in skin lesions were increased in both rodents compared to their age-matched parent strains, and that this may partially be due to hairlessness and dermatitis. We concluded that TRPV3 was the gene responsible for Nh and Ht mutations, and that mutation in TRPV3 possibly correlated with increased mast cell numbers.

Spontaneous scratching behaviour in DS-Nh mice as a possible model for pruritus in atopic dermatitis

Itching is one of the major clinical symptoms in atopic dermatitis (AD) and complicates the management of this pathological condition. An animal model of AD-like pruritus would contribute to a better understanding of AD and could lead to the development of safe and effective antipruritic agents. DS non-hair (DS-Nh) mice raised under conventional conditions spontaneously develop pruritus, which is associated with a dermatitis similar to human AD. There is a significant positive correlation between disease severity and the period of scratching behaviour in DS-Nh mice. In the present study, we found that levels of histamine and nerve growth factor (NGF) in serum and/or skin tissue were higher in DS-Nh mice with AD-like dermatitis than in age-matched mice without dermatitis. The histopathological data indicated that nerve fibres extend into and mast cells infiltrate the surrounding area of the skin lesion. NGF production by XB-2 cells, which was derived from mouse keratinocytes, was enhanced by histamine via the H1 receptor. We also found that prolonged treatment with an H1-antagonist was effective against pruritus through depression of the production of NGF, which is thought to be generated by keratinocytes. We conclude that DS-Nh mice can serve as a suitable model for gaining a better understanding of pruritus in AD, and that prolonged treatment with an H1-antagonist may be beneficial in patients with AD-associated pruritus.