Histamine and the nasal vasculature: the influence of H1 and H2-histamine receptor antagonism

Combination of H1 and H2 Histamine Receptor Antagonists: Current Knowledge and Perspectives of a Classic Treatment Strategy

Histamine receptor antagonists, which can bind to specific histamine receptors on target cells, exhibit substantial therapeutic efficacy in managing a variety of histamine-mediated disorders. Notably, histamine H1 and H2 receptor antagonists have been extensively investigated and universally acknowledged as recommended treatment agents for numerous allergic diseases and acid-related disorders, respectively. Historically, the combination of H1 and H2 receptor antagonists has been considered a classic treatment strategy, demonstrating relatively superior efficacy compared with single-drug therapies in the treatment of diverse histamine-mediated diseases. The latest emerging studies have additionally suggested the underlying roles of histamine and H1R and H2R in the development of anxiety disorders, arthritic diseases, and postexercise hypotension. Nevertheless, there is still a lack of systematic reviews on the clinical efficacy of combination therapy, greatly limiting our understanding of its clinical application. Here, we present a comprehensive overview of the current knowledge and perspectives regarding the combination of H1 and H2 histamine receptor antagonists in various histamine-mediated disorders. Furthermore, we critically analyze the adverse effects and limitations associated with combination therapy while suggesting potential solutions. Our review can offer a systematic summary and promising insights into the in-depth and effective application of the combination of H1 and H2 receptor antagonists.

A systematic review and meta-analysis of loratadine combined with montelukast for the treatment of allergic rhinitis

Background: Loratadine and montelukast are clinical first-line drugs in the treatment of allergic rhinitis (AR). However, there is no clear evidence of the efficacy of loratadine combined with montelukast in the treatment of AR. This study aimed to evaluate the efficacy and safety of the loratadine-montelukast combination on AR. Methods: In this meta-analysis, searches were conducted on PubMed, Embase, the Cochrane Central Register of Controlled Trials, Web of Science, and China National Knowledge Infrastructure (CNKI). The search terms included loratadine, montelukast, allergic rhinitis, and clinical trials. Meta-analyses were conducted using Rev Man 5.3 and Stata 15 statistical software. Results: A total of 23 studies with 4,902 participants were enrolled. For the primary outcome, pooled results showed that loratadine-montelukast can significantly reduce total nasal symptom scores (TNSS), when compared with loratadine (SMD, -1.00; 95% CI, -1.35 to -0.65, p < 0.00001), montelukast (SMD, -0.46; 95% CI, -0.68 to -0.25, p < 0.0001), or placebo (SMD, -0.93; 95% CI, -1.37 to -0.49, p < 0.00001). For secondary outcomes, pooled results showed that compared with loratadine, loratadine-montelukast can significantly improve nasal congestion, nasal itching, nasal sneezing, nasal rhinorrhea, and rhinoconjunctivitis quality of life questionnaires (RQLQ). Compared with montelukast, loratadine-montelukast can significantly improve nasal itching, and nasal sneezing. Compared with placebo, loratadine-montelukast can significantly improve nasal congestion, and RQLQ. Conclusion: Loratadine-montelukast combination is superior to loratadine monotherapy, montelukast monotherapy, or placebo in improving AR symptoms. Therefore, loratadine-montelukast combination can be an option for patients with moderate-severe AR or poorly response to monotherapy. Systematic review registration number: clinicaltrials.gov, identifier CRD42023397519.

Common Allergens and Immune Responses Associated with Allergic Rhinitis in China

Allergic rhinitis (AR) is a chronic allergic disease of the upper respiratory system that affects approximately 10-40% of the global population. Due to the large number of plant pollen allergens with obvious seasonal variations, AR is common in China. AR is primarily caused by the abnormal regulation of the immune system. Its pathophysiological mechanism involves a series of immune cells and immune mediators, including cytokines. The present review summarizes the common allergens in China and the complex pathophysiological mechanism of AR. Additionally, host allergen contact, signal transduction, immune cell activation, cytokine release, and a series of inflammatory reactions are described according to their sequence of occurrence.

A Synopsis of Guidance for Allergic Rhinitis Diagnosis and Management From ICAR 2023

An updated edition of the International Consensus Statement on Allergy and Rhinology: Allergic Rhinitis (ICAR:AR) has recently been published. This consensus document, which included the participation of 87 primary authors and 40 additional consultant authors, who critically appraised evidence on 144 individual topics concerning allergic rhinitis, provides guidance for health care providers using the evidence-based review with recommendations (EBRR) methodology. This synopsis highlights topical areas including pathophysiology, epidemiology, disease burden, risk and protective factors, evaluation and diagnosis, aeroallergen avoidance and environmental controls, single and combination pharmacotherapy options, allergen immunotherapy (subcutaneous, sublingual, rush, cluster), pediatric considerations, alternative and emerging therapies, and unmet needs. Based on the EBRR methodology, ICAR:AR includes strong recommendations for the treatment of allergic rhinitis: (1) for the use of newer generation antihistamines compared with first-generation alternatives, intranasal corticosteroid, intranasal saline, combination therapy with intranasal corticosteroid plus intranasal antihistamine for patients not responding to monotherapy, and subcutaneous immunotherapy and sublingual tablet immunotherapy in properly selected patients; (2) against the use of oral decongestant monotherapy and routine use of oral corticosteroids.

15-hydroxy eicosadienoic acid is an exacerbating factor for nasal congestion in mice

Allergic rhinitis (AR) is one of the most common allergic inflammatory diseases worldwide. In AR, increased blood flow and vascular permeability in nasal mucosa cause rhinorrhea and nasal congestion. We investigated the role of an 11Z,14Z-eicosadienoic acid-derived metabolite, 15-hydroxy-11Z,13Z-eicosadienoic acid (15-HEDE), in functional changes in vasculature and nasal congestion in AR. Repeated intranasal administration of Ovalbumin (OVA) caused AR symptoms, such as sneezing and nasal congestion, in mice. OVA administration increased the level of 15-HEDE in nasal lavage fluid, which reached approximately 0.6 ng/ml after ten OVA treatments. Upon measuring vascular contraction, treatment with 0.1-3 μM 15-HEDE did not cause contraction in mouse aortae, while it dilated aortae that were pre-contracted by thromboxane receptor stimulation. Pretreatment with the voltage-gated K⁺ (K_V ) channel inhibitor 4-aminopyridine significantly inhibited the 15-HEDE-induced vascular relaxation. Intravital imaging showed that administration of 1 μg 15-HEDE dilated blood vessels, and Mile's assay demonstrated that this administration also caused dye leakage, indicating vascular hyperpermeability in mouse ears. Computed tomography scanning and morphological study revealed that administration of 3 μg 15-HEDE narrowed nasal passages and thickened nasal mucosa in mice. Finally, we confirmed that treating mice with 3 μg 15-HEDE caused rhinitis symptoms, such as abdominal breathing, and reduced respiratory frequency, suggesting nasal congestion. 15-HEDE caused vasodilation by activating K_V channels and increased vascular permeability, which may lead to nasal congestion. Furthermore, 15-HEDE might be a new lipid mediator that exacerbates nasal congestion in AR.

Modulation of Allergic Inflammation in the Nasal Mucosa of Allergic Rhinitis Sufferers With Topical Pharmaceutical Agents

Allergic rhinitis (AR) is a chronic upper respiratory disease estimated to affect between 10 and 40% of the worldwide population. The mechanisms underlying AR are highly complex and involve multiple immune cells, mediators, and cytokines. As such, the development of a single drug to treat allergic inflammation and/or symptoms is confounded by the complexity of the disease pathophysiology. Complete avoidance of allergens that trigger AR symptoms is not possible and without a cure, the available therapeutic options are typically focused on achieving symptomatic relief. Topical therapies offer many advantages over oral therapies, such as delivering greater concentrations of drugs to the receptor sites at the source of the allergic inflammation and the reduced risk of systemic side effects. This review describes the complex pathophysiology of AR and identifies the mechanism(s) of action of topical treatments including antihistamines, steroids, anticholinergics, decongestants and chromones in relation to AR pathophysiology. Following the literature review a discussion on the future therapeutic strategies for AR treatment is provided.

Medical Treatment of Nasal Airway Obstruction

Nasal obstruction is a common, and potentially debilitating, problem. It is caused by a combination of structural factors and/or mucosal swelling/inflammation. The medical treatment of nasal obstruction is aimed at decreasing mucosal inflammation and edema and is generally guided by the underlying cause. Several different drug classes are commonly used in the treatment of nasal obstruction, each with different indications, and pros and cons to their use. This article discusses the most commonly used therapies for nasal obstruction. Current evidence on the efficacy and side effect profile of each therapy is reviewed.

International Consensus Statement on Allergy and Rhinology: Allergic Rhinitis

BACKGROUND

Critical examination of the quality and validity of available allergic rhinitis (AR) literature is necessary to improve understanding and to appropriately translate this knowledge to clinical care of the AR patient. To evaluate the existing AR literature, international multidisciplinary experts with an interest in AR have produced the International Consensus statement on Allergy and Rhinology: Allergic Rhinitis (ICAR:AR).

METHODS

Using previously described methodology, specific topics were developed relating to AR. Each topic was assigned a literature review, evidence-based review (EBR), or evidence-based review with recommendations (EBRR) format as dictated by available evidence and purpose within the ICAR:AR document. Following iterative reviews of each topic, the ICAR:AR document was synthesized and reviewed by all authors for consensus.

RESULTS

The ICAR:AR document addresses over 100 individual topics related to AR, including diagnosis, pathophysiology, epidemiology, disease burden, risk factors for the development of AR, allergy testing modalities, treatment, and other conditions/comorbidities associated with AR.

CONCLUSION

This critical review of the AR literature has identified several strengths; providers can be confident that treatment decisions are supported by rigorous studies. However, there are also substantial gaps in the AR literature. These knowledge gaps should be viewed as opportunities for improvement, as often the things that we teach and the medicine that we practice are not based on the best quality evidence. This document aims to highlight the strengths and weaknesses of the AR literature to identify areas for future AR research and improved understanding.

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.

Pharmacological characterization of GSK1004723, a novel, long-acting antagonist at histamine H(1) and H(3) receptors

BACKGROUND AND PURPOSE

Preclinical pharmacological characterization of GSK1004723, a novel, dual histamine H(1) and H(3) receptor antagonist.

EXPERIMENTAL APPROACH

GSK1004723 was characterized in vitro and in vivo using methods that included radioligand binding, intracellular calcium mobilization, cAMP production, GTPγS binding, superfused human bronchus and guinea pig whole body plethysmography.

KEY RESULTS

In cell membranes over-expressing human recombinant H(1) and H(3) receptors, GSK1004723 displayed high affinity, competitive binding (H(1) pKi = 10.2; H(3) pKi = 10.6). In addition, GSK1004723 demonstrated slow dissociation from both receptors with a t(1/2) of 1.2 and 1.5 h for H(1) and H(3) respectively. GSK1004723 specifically antagonized H(1) receptor mediated increases in intracellular calcium and H(3) receptor mediated increases in GTPγS binding. The antagonism exerted was retained after cell washing, consistent with slow dissociation from H(1) and H(3) receptors. Duration of action was further evaluated using superfused human bronchus preparations. GSK1004723 (100 nmol·L(-1) ) reversed an established contractile response to histamine. When GSK1004723 was removed from the perfusate, only 20% recovery of the histamine response was observed over 10 h. Moreover, 21 h post-exposure to GSK1004723 there remained almost complete antagonism of responses to histamine. In vivo pharmacology was studied in conscious guinea pigs in which nasal congestion induced by intranasal histamine was measured indirectly (plethysmography). GSK1004723 (0.1 and 1 mg·mL(-1) intranasal) antagonized the histamine-induced response with a duration of up to 72 h.

CONCLUSIONS AND IMPLICATIONS

GSK1004723 is a potent and selective histamine H(1) and H(3) receptor antagonist with a long duration of action and represents a potential novel therapy for allergic rhinitis.

Histamine in allergic rhinitis

Histamine plays a major role in allergic rhinitis. In susceptible individuals, allergen induces nasal mast cell degranulation and the release of histamine into the nasal mucosa. Histamine has been detected after controlled challenges with allergen and, when administered into the nasal cavity, elicits signs and symptoms similar to those elicited by allergen. All four histamine receptors have been demonstrated in the nasal mucosa. The role of the four histamine receptors in the pathophysiology of allergic rhinitis are discussed.

The role of antihistamines in the treatment of vasomotor rhinitis

Background

The pathogenesis of vasomotor rhinitis is not understood. It is unlikely that antihistamines, based on their H1 antagonist activity alone, would be effective in this disorder.

Methods

Nonetheless, at least one double-blind, placebo-controlled multicenter trial has found that intranasal azelastine relieves symptoms of this disorder better than placebo. The mechanism responsible for its beneficial effect in nonallergic rhinitis is unclear but probably relates to "anti-inflammatory/antiallergic" activities.

Results

Such mechanisms have been demonstrated for a number of different oral antihistamines, but often the concentrations required in vitro are higher than those that are normally achieved in vivo using recommended dosing. It has been postulated that intranasal administration, which can achieve high local levels, might be a factor responsible for enhancing the "anti-inflammatory/antiallergic" properties.

Conclusions

Interpreting this information allows one to conclude that antihistamines may be potentially effective agents in vasomotor rhinitis, and are more likely to be so when administered intranasally, despite the fact that data documenting this beneficial effect are sparse.

Insights into the mechanisms of histamine-induced inflammation in the nasal mucosa

Histamine is a major autacoid released during allergic reactions. Histamine, when administered to the nasal mucosa, causes symptoms that mimic allergic rhinitis, including nasal blockage, sneezing, pruritus and rhinorrhea. This article provides an overview of the contribution of H1, H2 and H3 receptors to histamine-induced symptom generation, in particular focusing on nasal blockage.

Pathophysiology of nasal obstruction and meta-analysis of early and late effects of levocetirizine

Nasal obstruction, also referred to as congestion, blockage or stuffiness, is a crucial symptom in allergic rhinitis (AR) and may affect sleep as well as quality of life. Early- and late-phase-allergic reactions both contribute to nasal obstruction, although it primarily represents a major symptom in the chronic allergic reaction. A complex network of inflammatory and neurogenic phenomena relates to chronic nasal obstruction, including the subepithelial accumulation of inflammatory cells, particularly mast cells and eosinophils, and the release of neuropeptides. Nasal obstruction is a difficult-to-treat symptom. Vasoconstrictors (decongestants) and intranasal corticosteroids, due to their anti-inflammatory properties, have mainly been used for relieving the nasal passages from the congested mucosa. However, there is accumulating evidence recently that the latest-generation potent antihistamines have decongestant properties in AR. This paper aims to review the pathophysiologic background of nasal obstruction and the evidence for an antihistamine, levocetirizine, in relieving nasal congestion. A meta-analysis on the early and late effects of levocetirizine on nasal obstruction under artificial and natural allergen exposure conditions is presented, demonstrating convincingly that levocetirizine shows a consistent effect on nasal obstruction as early as over the first 2 h and sustained over 6 weeks.

Histamine receptors that influence blockage of the normal human nasal airway

1. The aim of this study was to investigate the mechanisms by which histamine causes nasal blockage. Histamine, 40-800 microg, intranasally into each nostril, induced significant blockage of the nasal airway in normal human subjects, as measured by acoustic rhinometry. 2. Oral pretreatment with cetirizine, 5-30 mg, the H1 antagonist, failed to reverse completely the nasal blockage induced by histamine, 400 microg. 3. Dimaprit, 50-200 microg, the H2 agonist, intranasally, caused nasal blockage, which was reversed by oral pretreatment with ranitidine, 75 mg, the H2 antagonist. 4. A combination of cetirizine, 20 mg, and ranitidine, 75 mg, caused greater inhibition of the nasal blockage caused by histamine, 400 microg, than cetirizine alone. In the presence of both antagonists, there was residual histamine-induced nasal blockage. 5. R-alpha-methylhistamine (R-alpha-MeH), 100-600 microg, the H3 agonist, intranasally, caused nasal blockage, which was not inhibited by either cetirizine or ranitidine. 6. Thioperamide, 700 microg, the H3 antagonist, intranasally, reversed the R-alpha-MeH-induced nasal blockage. Thioperamide alone had no significant action on the nasal blockage induced by histamine, 400 and 1000 microg, but, in the presence of cetirizine, 20 mg, thioperamide further reduced the histamine-induced nasal blockage. 7. Corynanthine, 2 mg, the alpha1-adrenoceptor antagonist, administered intranasally, caused nasal blockage. 8. Corynanthine produced a greater increase in nasal blockage when in combination with bradykinin compared to its combination with R-alpha-MeH. 9. There appears to be a contribution of H1, H2 and H3 receptors to histamine-induced nasal blockage in normal human subjects. The sympathetic nervous system actively maintains nasal patency and we suggest that activation of nasal H3 receptors may downregulate sympathetic activity.

Second-Generation Antihistamines

Antihistamines are useful medications for the treatment of a variety of allergic disorders. Second-generation antihistamines avidly and selectively bind to peripheral histamine H1 receptors and, consequently, provide gratifying relief of histamine-mediated symptoms in a majority of atopic patients. This tight receptor specificity additionally leads to few effects on other neuronal or hormonal systems, with the result that adverse effects associated with these medications, with the exception of noticeable sedation in about 10% of cetirizine-treated patients, resemble those of placebo overall. Similarly, serious adverse drug reactions and interactions are uncommon with these medicines. Therapeutic interchange to one of the available second-generation antihistamines is a reasonable approach to limiting an institutional formulary, and adoption of such a policy has proven capable of creating substantial cost savings. Differences in overall efficacy and safety between available second-generation antihistamines, when administered in equivalent dosages, are not large. However, among the antihistamines presently available, fexofenadine may offer the best overall balance of effectiveness and safety, and this agent is an appropriate selection for initial or switch therapy for most patients with mild or moderate allergic symptoms. Cetirizine is the most potent antihistamine available and has been subjected to more clinical study than any other. This agent is appropriate for patients proven unresponsive to other antihistamines and for those with the most severe symptoms who might benefit from antihistamine treatment of the highest potency that can be dose-titrated up to maximal intensity.

H1-receptors: localization and role in airway physiology and in immune functions

Histamine H(1)-receptors are involved in the pathologic processes of allergy. Clinical trials of H(1)-receptor antagonists have demonstrated the efficacy of these agents in reducing the sneezing, pruritus, and rhinorrhea associated with allergic rhinitis. In the lung, H(1)-receptors mediate the bronchoconstrictive effects of histamine and increase vascular permeability, which lead to plasma exudation. H(1)-receptors are present on T cells, B cells, monocytes, and lymphocytes, and stimulation of these receptors induces pro-inflammatory effects. It has been suggested that a signal from the H(1)-receptor contributes to the antigen receptor-mediated signaling pathways that induce proliferative responses and lead to the production of cytokines and antibodies by T cells and B cells, respectively. It would appear, therefore, that the H(1)-receptor has a wider role in inflammatory processes than simply mediating the actions of histamine.

Epidermal growth factor receptor - but not histamine receptor - is upregulated in seasonal allergic rhinitis

BACKGROUND

We were interested in exploring the molecular mechanisms underlying the observed difference in histamine (H) responsiveness between seasonal allergic rhinitic (SAR) and nonrhinitic (NR) subjects. We hypothesized that SAR subjects express higher nasal mucosal histamine receptor 1 (H1R) and 2 (H2R) levels than do NR subjects. In addition, we examined expression of genes involved in regulating the glandular response, including epidermal growth factor (EGF), EGF receptor (EGFR), and mucins (Muc5Ac and Muc5B).

METHODS

Fourteen subjects, seven SAR and seven NR, were provoked during pollen season with doubling doses of H (0.125-8.0 mg/ml). Nasal airway resistance (NAR) was measured by active posterior rhinomanometry. Provocation was halted when NAR exceeded 150% of baseline. Prior to provocation, nasal scrapings were obtained and mRNA quantified using two-step real-time PCR.

RESULTS

The mean PD50 (concentration of H producing a 50% increase in NAR) was significantly lower in the SAR than NR group (0.36 vs 1.32 mg/ml; P < 0.05). The ratio of relative gene copy numbers between the SAR and NR groups were as follows: H1R, 0.85 (P = 0.52); H2R, 0.67 (P = 0.35); EGF, 1.02 (P = 0.93), and EGFR, 103.5 (P < 0.05).

CONCLUSIONS

There were no significant differences in H1R or H2R mRNA levels between SAR and NR subjects in-season, despite observed differences in H reactivity. SAR subjects, however, did show a significant elevation in EGFR expression, consistent with the observation of mucus hypersecretion in allergic rhinitis.

Canine model of nasal congestion and allergic rhinitis

The ragweed- and histamine-induced decreases in nasal patency in cohorts of ragweed-sensitized and nonsensitized dogs were assessed. The volume of nasal airways (V(NA)) was assessed by acoustic rhinometry and resistance to airflow (R(NA)) by anterior rhinomanometry. Histamine delivered to the nasal passages of five dogs caused a rapid and prolonged increase in R(NA) (0.75 +/- 0.26 to 3.56 +/- 0.50 cmH(2)O. l(-1). min), an effect that was reversed by intranasal delivery of aerosolized phenylephrine. Ragweed challenge in five ragweed-sensitized dogs increased R(NA) from 0.16 +/- 0.02 to 0.53 +/- 0.07 cmH(2)O. l(-1). min and decreased V(NA) from 12.5 +/- 1.9 to 3.9 +/- 0.3 cm(3), whereas administration of saline aerosol neither increased R(NA) nor decreased V(NA). Prior administration of d-pseudoephedrine (30 mg po) attenuated the ragweed-induced increase in R(NA) and decrease in V(NA). Ragweed challenge changed neither R(NA) nor V(NA) in four nonsensitized dogs. Mediator-induced nasal congestion and allergen-induced allergic rhinitis in ragweed-sensitized dogs, which exhibit symptoms similar to human disease, can be used in the evaluation of safety and efficacy of antiallergic activity of potential drugs.

Histamine receptor type coupled to nitric oxide-induced relaxation of guinea-pig nasal mucosa

1 The aim of this study was to characterize the histamine receptor type mediating relaxation of the vascular bed of the nasal mucosa from the guinea-pig, and to determine the role of cyclo-oxygenase products and nitric oxide in this relaxant response to histamine. These studies were performed in isolated nasal mucosae examined in vitro to obtain potencies of histamine receptor-type selective agonists in causing vasorelaxation and to determine affinities of histamine receptor antagonists for inhibiting histamine-induced relaxation. 2 After contraction of nasal mucosae with noradrenaline, histamine caused a maximal relaxation response that was 75 +/- 6% of the contraction caused by noradrenaline with a mean EC50 value of 4.3 +/- 0.5 microM. Neither dimaprit (H2-receptor selective) nor R-alpha-methylhistamine (H3-receptor selective) caused significant relaxation of nasal mucosae. In contrast, betahistine (H1-receptor selective) caused an 81 +/- 7% relaxation of noradrenaline-induced tone with an EC50 value of 15 +/- 1 microM. 3 pA2 experiments were performed to obtain KB values of chlorpheniramine (H1-receptor selective) and diphenhydramine (H1-receptor selective) for blocking histamine-stimulated relaxation of nasal mucosae. KB values for chlorpheniramine (0.87 nM) and diphenhydramine (7.4 nM) were consistent with their interaction at the H1-receptor type. Additionally, neither 10 microM cimetidine (H2-receptor selective) nor 1 microM thioperamide (H3-receptor selective) had any effect on the relaxation curve for histamine. 4 In the presence of 10 microM indomethacin (cyclo-oxygenase inhibitor), histamine caused a maximal relaxation response of 73 +/- 5% of the noradrenaline-induced tone with an EC50 value of 2.9 +/- 0.2 microM, which was not different from control values (EC50 = 5.0 +/- 0.4 microM; maximal relaxation = 71 +/- 6%). In contrast, 200 microM NG-nitro-L-arginine (nitric oxide synthase inhibitor) completely inhibited histamine-induced relaxation of nasal mucosae. 5 In conclusion, data from the present study suggest only the H1-receptor type mediates relaxation of nasal mucosal blood vessels to histamine, and histamine-induced relaxation of nasal mucosae is entirely dependent on nitric oxide production.

Allergic rhinitis: not purely a histamine-related disease

Allergic rhinitis is an inflammatory disorder of the nasal mucosa typified by the symptoms of nasal itch, sneeze, anterior nasal secretions, and nasal blockage. These symptoms arise from the interaction between mediators and neural, vascular, and glandular structures within the nose. Nasal itch, sneezes, and rhinorrhoea are predominantly neural in origin, while nasal obstruction is predominantly vascular. Nasal biopsy studies show accumulation of eosinophils within the lamina propria and epithelium and an increase in tissue and cell surface basophils in both seasonal and perennial allergic rhinitis. These cells are in an activated state. Within the epithelium, increased numbers of mast cells, T cells and Langerhans' cells, which induce T-cell activation, are found. The accumulation of these cells can be linked to chemokine and cytokine generation by the epithelial cells themselves. Thus, the tissue cell recruitment is orchestrated by activated mast cells, T cells, and epithelial cells, with the recruited tissue eosinophils also contributing to their persistence at this site through autocrine mechanisms. Mast cells generate an array of mediators including histamine, tryptase, leukotrienes, and prostaglandins. Histamine is also generated by basophils. Eosinophils and basophils contribute to the leukotriene synthesis within the tissue. Histamine nasal insufflation induces nasal itch, sneeze, and rhinorrhoea as well as nasal blockage, thereby reproducing all the symptoms of allergic rhinitis. These effects are primarily mediated by H1-receptors, and H1-receptor antagonists are a prominent treatment. Antagonism of histamine at these receptors reduces symptoms by about 40-50%, with the greatest effect on the neurally mediated responses. Thus, histamine is a major mediator of allergic rhinitis, but not the sole contributor. Nasal insufflation with leukotrienes, prostaglandins, or kinins is associated with the development of nasal blockage. These mediators act primarily on the nasal vasculature and, in this respect, leukotrienes are potent mediators. Leukotrienes also induce plasma protein exudation, which contributes to the anterior nasal secretions. Studies with combination products have suggested that modifying the effects of both leukotrienes and histamine has complementary effects in relieving nasal symptoms, indicating that both these mediators are relevant to disease expression.