Thromboxane A2 induces itch-associated responses through TP receptors in the skin in mice

Inhibitory effects of the methanol extract of Ganoderma lucidum on mosquito allergy-induced itch-associated responses in mice

Recently, we showed that a methanol extract of Ganoderma lucidum inhibits scratching, an itch-related response, induced by intradermal injections of some pruritogens in mice. The present study investigated whether G. lucidum extract would inhibit allergic itch. In mice sensitized with an extract of salivary gland of mosquito (ESGM), an intradermal injection of ESGM elicited scratching, which was suppressed by oral administration of G. lucidum extract (100 and 300 mg/kg). The scratching was inhibited by the H₁ histamine-receptor antagonist azelastine, but not by the peripherally acting H₁-antagonist terfenadine, at the oral dose of 30 mg/kg. In sensitized mice, ESGM increased the activity of cutaneous nerve, which was suppressed by G. lucidum extract (300 mg/kg). Although terfenadine (30 mg/kg) inhibited plasma extravasation induced by ESGM in the sensitized mice, G. lucidum extract (300 mg/kg) was without effect. These results suggest that G. lucidum extract relieves allergic itch through a peripheral action. The results support the idea that mast cells and H₁ histamine receptors are not the primary sites of the antipruritic action of G. lucidum extract.

Therapy of pruritus

IMPORTANCE OF THE FIELD

Pruritus is the predominant symptom of skin disease. Owing to the poorly understood pathophysiology, the development of effective treatment modalities for pruritus has proven to be particularly difficult. At present, there is no universally accepted therapy for itch. The purpose of this review is to provide an update on the treatment of pruritus.

AREAS COVERED IN THIS REVIEW

An overview of current, emerging and possible future therapies for pruritus is provided.

WHAT THE READER WILL GAIN

Insights into possible treatment regimes for pruritus in different clinical scenarios.

TAKE HOME MESSAGE

The therapy of pruritus is challenging and at present takes on an individualistic approach. Recent advancements in the mechanisms that underlie this distressing symptom have identified new targets for future therapy.

Leukotriene B(4) mediates sphingosylphosphorylcholine-induced itch-associated responses in mouse skin

In atopic dermatitis, the concentration in the skin of sphingosylphosphorylcholine (SPC), which is produced from sphingomyelin by sphingomyelin deacylase, is increased. In the present study, we investigated the itch-eliciting activity of SPC and related substances and the mechanisms of SPC action in mice. An intradermal injection of SPC, but not sphingomyelin and sphingosine, induced scratching, an itch-associated response, which was not suppressed by a deficiency in mast cells or the H(1) histamine receptor antagonist terfenadine. The action of SPC was inhibited by the mu-opioid receptor antagonist naltrexone. SPC action also was inhibited by the 5-lipoxygenase inhibitor zileuton and the leukotriene B(4) antagonist ONO-4057, but not by the cyclooxygenase inhibitor indomethacin. Moreover, SPC action was inhibited by the antiallergic agent azelastine, which suppresses the action and production of leukotriene B(4). Administration of SPC to the skin and to primary cultures of keratinocytes increased leukotriene B(4) production. SPC increased intracellular Ca(2+) ion concentration in primary cultures of dorsal root ganglion neurons and keratinocytes. These results suggest that SPC induces itching through a direct action on primary afferents and leukotriene B(4) production of keratinocytes. Sphingomyelin deacylase and SPC receptors may be previously unreported targets for antipruritic drugs.

What causes itch in atopic dermatitis?

Itch, the hallmark of atopic dermatitis, has a significant impact on quality of life for patients with this disease. Various central and peripheral mediators have been suggested to play a role in the pathophysiology of atopic eczema itch. Significant cross-talk occurs among stratum corneum, keratinocytes, immune cells, and nerve fibers, which are in close proximity to one another and induce itch. The impaired barrier function associated with the itch-scratch cycle further augments this vicious cycle. Recent advances in our understanding of itch pathophysiology shed light on peripheral and central neural sensitization of nerve fibers that contribute significantly to itch in atopic dermatitis. Recently, several new mediators have been described as associated with itch in atopic dermatitis, including serine proteases, interleukin 31, and nerve growth factor. This review covers the peripheral and central mechanisms and mediators involved in pathogenesis of itch in atopic dermatitis.

Involvement of the BLT2 receptor in the itch-associated scratching induced by 12-(S)-lipoxygenase products in ICR mice

BACKGROUND AND PURPOSE

Recently, we reported that 12(S)-HPETE (12(S)-hydroperoxyeicosa-5Z,8Z,10E,14Z-tetraenoic acid) induces scratching in ICR mice. We hypothesized that 12(S)-HPETE might act as an agonist of the low-affinity leukotriene B4 receptor BLT2. To confirm the involvement of the BLT2 receptor in 12(S)-HPETE-induced scratching, we studied the scratch response using the BLT2 receptor agonists compound A (4'-[[pentanoyl (phenyl) amino]methyl]-1,1'-biphenyl-2-carboxylic acid) and 12(S)-HETE (12(S)-hydroxyeicosa-5Z,8Z,10E,14Z-tetraenoic acid).

EXPERIMENTAL APPROACH

A video recording was used to determine whether the BLT2 receptor agonists caused itch-associated scratching in ICR mice. Selective antagonists and several chemicals were used.

KEY RESULTS

Both 12(S)-HETE and compound A dose dependently induced scratching in the ICR mice. The dose-response curve for compound A showed peaks at around 0.005-0.015 nmol per site. Compound A- and 12(S)-HETE-induced scratching was suppressed by capsaicin and naltrexon. We examined the suppressive effects of U75302 (6-[6-(3-hydroxy-1E,5Z-undecadienyl)-2-pyridinyl]-1,5-hexanediol, the BLT1 receptor antagonist) and LY255283 (1-[5-ethyl-2-hydroxy-4-[[6-methyl-6-(1H-tetrazol-5-yl)heptyl]oxy]phenyl]-ethanone, the BLT2 receptor antagonist) on the BLT2 agonist-induced scratching. LY255283 suppressed compound A- and 12(S)-HETE-induced scratching, but U75302 did not. LY255283 required a higher dose to suppress the compound A-induced scratching than it did to suppress the 12(S)-HETE-induced scratching. One of the BLT(2) receptor agonists, 12(R)-HETE (12(R)-hydroxyeicosa-5Z,8Z,10E,14Z-tetraenoic acid), also induced scratching in the ICR mice.

CONCLUSIONS AND IMPLICATIONS

Our present results corroborate the hypothesis that the BLT2 receptor is involved in 12(S)-lipoxygenase-product-induced scratching in ICR mice. We also confirmed that this animal model could be a valuable means of evaluating the effects of BLT2 receptor antagonists.

Mediators of pruritus in psoriasis

The pathogenesis of pruritus in psoriasis remains unclear. Many possible mediators were implicated to transmit or modulate this sensation in psoriasis, but none has been clearly proven to be a causative agent of itching. The most often discussed theory mentioned the importance of impaired innervations and neuropeptides imbalance in psoriatic skin. Other possible causes of itching might be increased expression of interleukin 2 or vascular abnormalities. Recent data indicated that pruritus could be also evoked by opioid system, prostanoids, interleukin 31, serotonin, or proteases. Whether these mechanisms are also involved in pruritus accompanying psoriasis requires further investigation. Limited knowledge of pruritus origin in psoriasis is responsible for the lack of the effective antipruritic treatments for psoriatics. Here, we summarize the current knowledge about the pathogenesis of pruritus in psoriasis and point out possible directions of future studies aiming the pathogenesis of this symptom in psoriasis.

Possible involvement of 5-lipoxygenase metabolite in itch-associated response of mosquito allergy in mice

This study investigated endogenous mediators involved in mosquito allergy-associated itching in mice. An intradermal injection of an extract of mosquito salivary gland elicited marked scratching in sensitized mice. The 5-lipoxygenase inhibitor zileuton (100 mg/kg), the 5-lipoxygenase activating peptide inhibitor MK-886 (10 mg/kg), and the glucocorticoid betamethasone 17-valerate (3 mg/kg) inhibited the scratching. The scratching was not affected by the cyclooxygenase inhibitors indomethacin and ketoprofen, the TP prostanoid receptor antagonist SQ-29548, the leukotriene B(4) antagonist ONO-4057, the cysteinyl leukotriene antagonist pranlucast, the leukotriene D(4) antagonist MK-571, the platelet-activating factor antagonist CV-3988, the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester, the H(2) histamine-receptor antagonist cimetidine, the H(1) histamine-receptor antagonist terfenadine plus cimetidine, and cypoheptadine that blocks the 5-HT(1/2) serotonin receptors. Zileuton (100 mg/kg) inhibited the increased activity of the cutaneous nerve branch induced by an intradermal injection of the extract, suggesting the peripheral action. Zileuton and MK-886 (10 and 100 microM) did not affect high K(+)-induced increase in intracellular Ca(2+) concentration in cultured dorsal root ganglion neurons. The results suggest that 5-lipoxygenase metabolite(s) other than leukotriene B(4) and cysteinyl leukotrienes are involved in mosquito allergy-associated itching.

The pruritus receptor unit: a target for novel therapies

Although the role of prostanoids in itch was actively studied a decade ago, interest in this area has waned in recent years. Andoh et al. (2007, this issue) demonstrate that the prostanoid thromboxane A2 elicits scratching through its TP receptor. Identification of new itch mediators and their respective receptors within the skin will undoubtedly be the focus of future drug development for this distressing symptom.