Involvement of histamine H3 receptors in scratching behaviour in mast cell-deficient mice

Efficacy of the Multi-Target Compound E153 in Relieving Pain and Pruritus of Different Origins

Itch and pain are closely related but distinct sensations that share largely overlapping mediators and receptors. We hypothesized that the novel, multi-target compound E153 has the potential to attenuate pain and pruritus of different origins. After the evaluation of sigma receptor affinity and pharmacokinetic studies, we tested the compound using different procedures and models of pain and pruritus. Additionally, we used pharmacological tools, such as PRE-084, RAMH, JNJ 5207852, and S1RA, to precisely determine the role of histamine H3 and sigma 1 receptors in the analgesic and antipruritic effects of the compound. In vitro studies revealed that the test compound had potent affinity for sigma 1 and sigma 2 receptors, moderate affinity for opioid kappa receptors, and no affinity for delta or μ receptors. Pharmacokinetic studies showed that after intraperitoneal administration, the compound was present at high concentrations in both the peripheral tissues and the central nervous system. The blood-brain barrier-penetrating properties indicate its ability to act centrally at the levels of the brain and spinal cord. Furthermore, the test compound attenuated different types of pain, including acute, inflammatory, and neuropathic. It also showed a broad spectrum of antipruritic activity, attenuating histamine-dependent and histamine-independent itching. Finally, we proved that antagonism of both sigma 1 and histamine H3 receptors is involved in the analgesic activity of the compound, while the antipruritic effect to a greater extent depends on sigma 1 antagonism.

Mechanisms of unconventional CD8 Tc2 lymphocyte induction in allergic contact dermatitis: Role of H3/H4 histamine receptors

Histamine (HA) is a potent mediator that plays a central role in inflammation and allergy, acting through four G-protein-coupled receptors (i.e. H₁–H₄). HA is an accepted promoter of type 2 immunity in CD4⁺ T cells during hypersensitivity. Previously, we demonstrated that HA can promote antigen cross-presentation, inducing the activation of antigen-specific CD8⁺ T cells in an asthmatic murine model. Non-classical CD8+ T-cell profiles, such as Tc2 or Tc17, are associated with allergic disease persistence and chronicity. In this paper, we focus on the role of the H₃ receptor (H₃R) and the H₄ receptor (H₄R) in the development of allergic contact dermatitis. We were able to show that induction of the type 2 profiles associated with interleukin 13 production, both by CD4 and CD8 lymphocytes, depend on the interaction of HA with H₃R and H₄R. Blocking both receptors using the selective H₃/H₄ receptor antagonist thioperamide or the selective H₄R ligand JNJ777120 reduces the inflammatory response, inducing an immunosuppressive profile associated with the increased proportion of FOXp3⁺ regulatory T lymphocytes and CD11b⁺Gr-1⁺ myeloid suppressor cells. Interestingly, in dendritic cells, only H₄R blockade, and not H₃R blockade, is capable of modulating most of the inflammatory effects observed in our model.

Dissecting the precise nature of itch-evoked scratching

Itch is a discrete and irritating sensation tightly coupled to a drive to scratch. Acute scratching developed evolutionarily as an adaptive defense against skin irritants, pathogens, or parasites. In contrast, the itch-scratch cycle in chronic itch is harmful, inducing escalating itch and skin damage. Clinically and preclinically, scratching incidence is currently evaluated as a unidimensional motor parameter and believed to reflect itch severity. We propose that scratching, when appreciated as a complex, multidimensional motor behavior, will yield greater insight into the nature of itch and the organization of neural circuits driving repetitive motor patterns. We outline the limitations of standard measurements of scratching in rodent models and present new approaches to observe and quantify itch-evoked scratching. We argue that accurate quantitative measurements of scratching are critical for dissecting the molecular, cellular, and circuit mechanisms underlying itch and for preclinical development of therapeutic interventions for acute and chronic itch disorders.

Histamine and Histamine Receptors in Allergic Dermatitis

In this chapter we will first introduce the pathophysiological process of several skin diseases including allergic dermatitis, a common skin disease, including chronic allergic contact dermatitis (CACD), and atopic dermatitis (AD). In CACD and AD patients, repeated skin exposure to antigens contributes to the development of chronic eczematous lesions. Repeated application of haptens on mice allows emulation of the development of CACD in humans. Further, we will focus on H1, H2, and H4 histamine receptors and their effects on CACD and AD. Histamine-deficient mice, with a knockout histidine decarboxylase (HDC) gene, were used to investigate the role of histamine in CACD and AD. Histamine induces infiltration of inflammatory cells, including mast cells and eosinophils, and elevates Th2 cytokine levels in CACD. Histamine promotes the development of eczematous lesions, elevates IgE serum levels, and induces scratching behavior in CACD. The administration of H1 or H4 receptor antagonists was effective to ameliorate these symptoms in murine CACD models. The combination of H1 and H4 receptor antagonists is a potential therapeutic target for chronic inflammatory skin diseases such as CACD and AD, since combined therapy proved to be more effective than monotherapy.

Antihistamines and itch

Histamine is one of the best-characterized pruritogens in humans. It is known to play a role in pruritus associated with urticaria as well as ocular and nasal allergic reactions. Histamine mediates its effect via four receptors. Antihistamines that block the activation of the histamine H₁receptor, H₁R, have been shown to be effective therapeutics for the treatment of pruritus associated with urticaria, allergic rhinitis, and allergic conjunctivitis. However, their efficacy in other pruritic diseases such as atopic dermatitis and psoriasis is limited. The other histamine receptors may also play a role in pruritus, with the exception of the histamine H₂receptor, H₂R. Preclinical evidence indicates that local antagonism of the histamine H₃receptor, H₃R, can induce scratching perhaps via blocking inhibitory neuronal signals. The histamine H₄receptor, H₄R, has received a significant amount of attention as to its role in mediating pruritic signals. Indeed, it has now been shown that a selective H₄R antagonist can inhibit histamine-induced itch in humans. This clinical result, in conjunction with efficacy in various preclinical pruritus models, points to the therapeutic potential of H₄R antagonists for the treatment of pruritus not controlled by antihistamines that target the H₁R.

Investigational drugs for pruritus

INTRODUCTION

Chronic pruritus (CP), defined as itch lasting for > 6 weeks, is a burdensome symptom of several different diseases, dermatological and systemic, with a high negative impact on the quality of life of patients. Given the manifold aetiologies of CP, therapy is often difficult. In recent years, however, novel substances have been developed for treatment of certain CP entities and identified targets.

AREAS COVERED

In this review, the authors present a survey of targets currently believed to be promising (H4R, IL-31, MOR, KOR, GRPR, NGF, NK-1R, TRP channels) and related investigational drugs that are in the preclinical or clinical stage of development. Some substances have already undergone clinical testing, but only one of them (nalfurafine) has been licensed so far. Many of them are most likely to exert their effects on the skin and interfere there with the cutaneous neurobiology of CP.

EXPERT OPINION

Currently, the most promising candidates for new therapeutic agents in CP are neurokinin-1 receptor antagonists and substances targeting the kappa- or mu-opioid receptor, or both. They have the potential to target the neuronal pathway of CP and are thus of interest for several CP entities. The goal for the coming years is to validate these concepts and move forward in developing new drugs for the therapy of CP.

Role of spinal neurotransmitter receptors in itch: new insights into therapies and drug development

Targets for antipruritic therapies are now expanding from the skin to the central nervous system. Recent studies demonstrate that various neuronal receptors in the spinal cord are involved in pruritus. The spinal opioid receptor is one of the best-known examples. Spinal administration of morphine is frequently accompanied by segmental pruritus. In addition to μ-opioid receptor antagonists, κ-opioid receptor agonists have recently come into usage as novel antipruritic drugs, and are expected to suppress certain subtypes of itch such as hemodialysis- and cholestasis-associated itch that are difficult to treat with antihistamines. The gastrin-releasing peptide receptor in the superficial dorsal horn of the spinal cord has also received recent attention as a novel pathway of itch-selective neural transmission. The NMDA glutamate receptor appears to be another potential target for the treatment of itch, especially in terms of central sensitization. The development of NMDA receptor antagonists with less undesirable side effects on the central nervous system might be beneficial for antipruritic therapies. Drugs suppressing presynaptic glutamate-release such as gabapentin and pregabalin also reportedly inhibit certain subtypes of itch such as brachioradial pruritus. Spinal receptors of other neuromediators such as bradykinin, substance P, serotonin, and histamine may also be potential targets for antipruritic therapies, given that most of these molecules interfere not only with pain, but also with itch transmission or regulation. Thus, the identification of itch-specific receptors and understanding itch-related circuits in the spinal cord may be innovative strategies for the development of novel antipruritic drugs.

Histamine H1, H3 and H4 receptors are involved in pruritus

Histamine has long been recognised as a classical inducer of pruritus. However, the specific mechanism of histamine-induced itch has still not been fully understood. The H1 and H4 receptor appear to be key components in the induction of itch. The specific role of the H3 receptor in histamine-induced itch remains unclear. The aim of our study was to investigate the role of the four known histamine receptors (H1-4) in acute itch in mice. Intradermal injection of the selective H3R inverse agonist pitolisant induced strong itch in mice. Pitolisant (50 nmol/injection)-induced pruritus could be completely blocked by a combined treatment with the H1R antagonist cetirizine (15 mg/kg) and the H4R antagonist JNJ 7777120 (15 mg/kg), whereas the H2R antagonist ranitidine (15 mg/kg) failed to inhibit the scratch response. Next, expression and function of histamine receptors on sensory neurons isolated from dorsal root ganglia of mice were investigated. As the itch sensation results from the excitation of sensory nerves in the skin, we further focused on skin specific sensory neurons. Therefore, neurons were retrograde labelled from the skin by means of a fluorescent tracer. Expression of H1R, H3R and H4R on skin innervating sensory neurons was detected. By single-cell calcium imaging, it was demonstrated that histamine induces a calcium increase in a subset of (skin-specific) sensory neurons via activation of the H1R and H4R as well as inhibition of the H3R. It is assumed that the decreased threshold in response to H3R antagonism activates H1R and H4R on sensory neurons, which in turn results in the excitation of histamine-sensitive afferents and therefore elicits the sensation of itch.

Basic mechanisms of itch

Chronic itch represents a burdensome clinical problem that can originate from a variety of aetiologies. Pruriceptive itch originates following the activation of peripheral sensory nerve endings following damage or exposure to inflammatory mediators and ascends to the brain through the spinal thalamic tract. Much insight has been gained into the understanding of the mechanisms underlying pruriceptive itch through studies using humans and experimental animals. More than one sensory nerve subtype is thought to subserve pruriceptive itch which includes both unmyelinated C-fibres and thinly myelinated Aδ nerve fibres. There are a myriad of mediators capable of stimulating these afferent nerves leading to itch, including biogenic amines, proteases, cytokines, and peptides. Some of these mediators can also evoke sensations of pain and the sensory processing underlying both sensations overlaps in complex ways. Studies have demonstrated that both peripheral and central sensitization to pruritogenic stimuli occur during chronic itch.

Targeted treatment of pruritus: a look into the future

Recent advances in pruritus research have elucidated mediators and neuronal pathways involved in itch transmission, and this fast emerging knowledge may possibly be translated into new therapies in the near future. In the skin and peripheral nerves, potential mediator and receptor therapeutic targets include the H4 histamine receptor, protease-activated receptor 2, serine proteases, cathepsin S, peripheral mu- and kappa-opioid receptors, interleukin-31, transient receptor potential vanilloid 1 and 3, fatty acid amide hydrolase, nerve growth factor and its receptor, acetylcholine, and the Mas-related G protein-coupled receptors. In the spinal cord, gastrin-related peptide and its receptor, as well as substance P and its receptor neurokinin receptor-1 serve as potential therapeutic targets. In the brain, reduction of itch perception and modulation of emotions may possibly be achieved through drugs acting on the anterior cingulate cortex. Clinically, management of pruritus should be instituted early and should address the skin pathology, peripheral neuropathy, central sensitization, and the cognito-affective aspects of the disease.

Targeted treatment of pruritus: a look into the future

Recent advances in pruritus research have elucidated mediators and neuronal pathways involved in itch transmission, and this fast emerging knowledge may possibly be translated into new therapies in the near future. In the skin and peripheral nerves, potential mediator and receptor therapeutic targets include the H4 histamine receptor, protease-activated receptor 2, serine proteases, cathepsin S, peripheral mu- and kappa-opioid receptors, interleukin-31, transient receptor potential vanilloid 1 and 3, fatty acid amide hydrolase, nerve growth factor and its receptor, acetylcholine, and the Mas-related G protein-coupled receptors. In the spinal cord, gastrin-related peptide and its receptor, as well as substance P and its receptor neurokinin receptor-1 serve as potential therapeutic targets. In the brain, reduction of itch perception and modulation of emotions may possibly be achieved through drugs acting on the anterior cingulate cortex. Clinically, management of pruritus should be instituted early and should address the skin pathology, peripheral neuropathy, central sensitization, and the cognito-affective aspects of the disease.

Topical cholecystokinin depresses itch-associated scratching behavior in mice

Cholecystokinin (CCK) serves as a gastrointestinal hormone and also functions as a neuropeptide in the central nervous system (CNS). CCK may be a downregulator in the CNS, as represented by its anti-opioid properties. The existence of CCK in the peripheral nervous system has also been reported. We investigated the suppressive effects of various CCKs on peripheral pruritus in mice. The clipped backs of ICR mice were painted with CCK synthetic peptides and injected intradermally with substance P (SP). The frequency of SP-induced scratching was reduced significantly by topical application of sulfated CCK8 (CCK8S) and CCK7 (CCK7S), but not by nonsulfated CCK8, CCK7, or CCK6. Dermal injection of CCK8S also suppressed the scratching frequency, suggesting that dermal cells as well as epidermal keratinocytes (KCs) are the targets of CCKs. As determined using real-time PCR, mRNA for CCK2R, one of the two types of CCK receptors, was expressed highly in mouse fetal skin-derived mast cells (FSMCs) and moderately in ICR mouse KCs. CCK8S decreased in vitro compound 48/80-promoted degranulation of FSMCs with a transient elevation of the intracellular calcium concentration. These findings suggest that CCK may exert an antipruritic effect via mast cells and that topical CCK may be clinically useful for pruritic skin disorders.

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.

Localization of histamine H4 receptors in the central nervous system of human and rat

Existing data on the expression of H(4) histamine receptor in the CNS are conflicting and inconclusive. In this report, we present the results of experiments that were conducted in order to elucidate H(4) receptor expression and localization in the brain, spinal cord, and dorsal root ganglia (DRG). Here we show that transcripts of H(4) receptor are present in all analyzed regions of the human CNS, including spinal cord, hippocampus, cortex, thalamus and amygdala, with the highest levels of H(4) mRNA detected in the spinal cord. In rat, H(4) mRNA was detected in cortex, cerebellum, brainstem, amygdala, thalamus and striatum. Very low levels of H(4) mRNA were detected in hypothalamus, and no H(4) signal was detected in the rat hippocampus. Fairly low levels of H(4) mRNA were detected in examined peripheral tissues including spleen and liver. Interestingly, strong expression of H(4) mRNA was detected in the rat DRG and spinal cord. Immunohistochemical analysis revealed expression of H(4) receptors on neurons in the rat lumbar DRG and in the lumbar spinal cord. Our observations provide evidence of the H(4) presence in both human and rodent CNS and offer some insight into possible role of H(4) in itch and pain.

Participation of histamine H3 receptors in experimental allergic rhinitis of mice

The present study was performed to study the participation of histamine H(3) receptors in nasal symptoms using Sch 50971, a potent and selective agonist of the H(3) receptor. Repeated topical application of antigen caused an increase in sneezing and nasal rubbing in sensitized mice. Oral administration of Sch 50971 and imetit, specific H(3)-receptor agonists, resulted in an inhibition of nasal symptoms induced by an antigen similar to an H(1)-receptor antagonist, cetirizine. Furthermore, simultaneous use of H(3)-receptor agonists, Sch 50971 or imetit, and an H(1)-receptor antagonist, cetirizine, caused a significant inhibitory effect on nasal symptoms at doses that showed no effect when used separately. The number of eosinophils in the nasal mucosa of mice sensitized with antigen was significantly decreased by cetirizine; however, Sch 50971 and imetit had no effect on eosinophil infiltration. These results clearly indicate that H(3) receptors are involved in the etiology of nasal allergy, and the stimulation of H(3) receptors may be useful as a novel therapeutic approach in nasal allergy.

Histamine-induced itch and its relationship with pain

Itch is one of the major complications of skin diseases. Although there are various substances that induce itch or pruritus, it is evident that histamine is the best known endogenous agent that evokes itch. Even though histamine-induced itch has been studied for some time, the underlying mechanism of itch is just beginning to emerge. Although various downstream signaling pathways of histamine receptors have been revealed, more studies are required to determine the cause of histamine-induced itch. It appears that itch and pain involve different neuronal pathways. Pain generally inhibits itch, which indicates an inter-communication between the two. Complex interactions between itch and pain may be expected based on reports on disease states and opioids. In this review, we discuss the molecular mechanism and the pharmacological aspects of histamine-induced itch. Especially, the underlying mechanism of TRPV1 (an anti-pruritus target) has been determined to some extent.

Immunohistochemical localization of histamine H3 receptors in rodent skin, dorsal root ganglia, superior cervical ganglia, and spinal cord: potential antinociceptive targets

Activation of histamine H3 receptors (H3Rs) reduces inflammation and nociception, but the existence of H3Rs on peripheral innervation has never been demonstrated. Here we use antibodies to locate H3Rs in whisker pads, hairy and glabrous hind paw skin, dorsal root ganglia (DRGs), and spinal cords of rats, wild type mice, and H3R knockout (H3KO) mice. Although H3Rs have been hypothesized to be on C and sympathetic fibers, H3R-like immunoreactivity (H3R-LI) was only detected on presumptive periarterial A delta fibers and on A beta fibers that terminated in Meissner's corpuscles and as lanceolate endings around hair follicles. The H3R-positive periarterial fibers were thin-caliber and coexpressed immunoreactivity for calcitonin gene-related peptide (CGRP), substance P, acid sensing ion channel 3, and 200 kDa neurofilament protein (NF). H3R-LI was also detected on epidermal keratinocytes and Merkel cells, but not on Merkel endings, C fibers, any other A delta fibers, or sympathetic fibers. In DRGs, H3R-LI was preponderantly on medium to large neurons coexpressing NF-LI and mostly CGRP-LI. In dorsal horn, CGRP-positive fibers with and without H3R-LI ramified extensively in lamina II; many of the former formed a plexus in lamina V. Low levels of H3R-LI were also present on A beta fibers penetrating superficial and into deeper laminae. The distribution of H3R-LI was similar in rats and wild type mice, but was eliminated or strongly reduced in A delta fibers and A beta fibers, respectively, in H3KO mice. Taken with recently published behavioral results, the present findings suggest that periarterial, peptidergic, H3R-containing A delta fibers may be sources of high threshold mechanical nociception.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.

Neurophysiological, neuroimmunological, and neuroendocrine basis of pruritus

Pruritus (itch) can be defined as an unpleasant cutaneous sensation associated with the immediate desire to scratch. Recent findings have identified potential classes of endogenous "itch mediators" and establish a modern concept for the pathophysiology of pruritus. First, there in no universal peripheral itch mediator, but disease-specific sets of involved mediators. Second, numerous mediators of skin cells can activate and sensitize pruritic nerve endings, and even modulate their growth. Our knowledge of itch processing in the spinal cord and the involved centers in the central nervous system is rapidly growing. This review summarizes the current information about the significance of neuron-skin interactions, ion channels, neuropeptides, proteases, cannabinoids, opioids, kinins, cytokines, biogenic amines, neurotransmitters, and their receptors in the pathobiology of pruritus. A deeper understanding of these circuits is required for the development of novel antipruritic strategies.

Role of Substance P on Histamine H3 Antagonist-Induced Scratching Behavior in Mice

The purpose of the present study was to investigate the involvement of chemical mediators, other than histamine, in the scratching behavior induced by H(3) antagonists. Scratching behavior was induced by the histamine H(3) antagonists iodophenpropit and clobenpropit (10 nmol/site) when they were injected intradermally into the rostral part of the back of mast-cell-deficient (WBB6F1 W/W(v)) and wild-type (WBB6F1 +/+) mice. Subsequently, the effect of spantide, a tachykinin NK(1) antagonist, was measured for 60 min. The effects of the H(3) antagonists on in vitro histamine release from rat peritoneal mast cells were also investigated. When spantide was injected intradermally at a dose of 0.5 nmol/site, it significantly inhibited the response. Furthermore, iodophenpropit and clobenpropit (10(-6)-10(-8) M) did not induce histamine release in isolated rat peritoneal mast cells. Our results indicate that substance P is involved in the skin responses elicited by the histamine H(3) antagonists. Moreover, the fact that these histamine H(3) antagonists did not induce significant increases in the histamine release from rat peritoneal mast cells suggests that the histamine H(3) receptor may not be present in the peripheral cells considered in this study.

Effect of loratadine on mouse models of atopic dermatitis associated pruritus

To confirm the effectiveness of loratadine for relieving pruritus in atopic dermatitis, we examined the effect of this drug using animal models of atopic dermatitis associated pruritus in ICR and hairless mice. As for the results, in ICR mice, single oral administration of loratadine at a dose of 5 or 10 mg/kg significantly inhibited the dorsal scratching behavior induced by histamine or an antigen, and the effect of loratadine was more potent than that of fexofenadine and chlorpheniramine. In hairless mice, oral administration of loratadine at a dose of 10 mg/kg for 6 days significantly inhibited the facial scratching behavior induced by the feeding of a low magnesium diet. Furthermore, oral administration of loratadine at a dose of 10 mg/kg for 7 days also significantly inhibited the histamine-induced scratching behavior in the same animals. These results indicate that loratadine may be effective in preventing pruritus associated with atopic dermatitis.

Roles of histamine and its receptors in allergic and inflammatory bowel diseases

Mast cell has a long history of being recognized as an important mediator-secreting cell in allergic diseases, and has been discovered to be involved in IBD in last two decades. Histamine is a major mediator in allergic diseases, and has multiple effects that are mediated by specific surface receptors on target cells. Four types of histamine receptors have now been recognized pharmacologically and the first three are located in the gut. The ability of histamine receptor antagonists to inhibit mast cell degranulation suggests that they might be developed as a group of mast cell stabilizers. Recently, a series of experiments with dispersed colon mast cells suggested that there should be at least two pathways in man for mast cells to amplify their own activation-degranulation signals in an autocrine or paracrine manner. In a word, histamine is an important mediator in allergic diseases and IBD, its antagonists may be developed as a group of mast cell stabilizers to treat these diseases.

Histamine helps development of eczematous lesions in experimental contact dermatitis in mice

Histamine is released from mast cells in the skin, causing urticaria and itching. However, little is known about the roles of histamine in development of eczematous lesions in contact dermatitis. Effects of histamine on development of eczematous lesions in contact dermatitis were assessed using histamine-deficient mice in which contact dermatitis was developed by repeated application of diphenylcyclopropenone. Development of eczematous lesions in contact dermatitis was suppressed in histamine-deficient mice compared to wild-type mice. H(1) agonist ((6-12-(4-imidazol)ethylamino)-N-(4-trifluoro- methylphenyl)hepatanecarboxamide) promoted development of eczematous lesions in histamine-deficient mice. H(1) receptor antagonist (loratadine) suppressed development of eczematous lesions in wild-type mice, whereas H(2) agonist (dimaprit) and receptor antagonist (cimetidine) were ineffective. These results suggest that histamine facilitates the development of eczematous lesions in a murine model of contact dermatitis via H(1) receptors.