Histamine: A mediator of inflammation

Effect of levocetirizine on the contraction induced by histamine on isolated rabbit bronchioles from precision-cut lung slices

The present experiments were designed to study the effects of levocetirizine (a potent selective histamine H(1) antagonist) on the contraction induced by histamine on isolated rabbit bronchioles using the precision-cut lung slice technology. Histamine induced a concentration-dependent contraction of isolated rabbit bronchioles (pD(2) value of 5.6). Mepyramine (0.01-1 micromol/l) induced a shift to the right without any decrease in the concentration-response curve to histamine (pA(2) value of 8.2). Levocetirizine (0.03-0.1 micromol/l) induced both a shift to the right and a decrease in the maximal amplitude of the concentration-response curve to histamine (pA(2) and pD'(2) values of 7.9 and 7.0, respectively). The difference between both compounds could be explained in terms of the difference in the dissociation rate from the histamine H(1) receptor coupled to a putative low receptor reserve present in the rabbit bronchioles.

Topical H2 antagonist prevents periodontitis in a rabbit model

Cimetidine is a powerful H2 receptor antagonist that eliminates histamine's effects on chemotaxis, phagocytosis, and superoxide anion production by phagocytes. The purpose of this study was to analyze the clinical and histopathological changes associated with experimental periodontitis in rabbits in response to topically applied cimetidine. Experimental periodontitis was induced in 21 New Zealand White rabbits using Porphyromonas gingivalis (10(9) CFU) topically applied three times a week for a 6-week period to previously ligatured teeth. Topical application of cimetidine in a liposome carrier for the prevention of periodontitis was evaluated in four groups of four animals each: 1, 10, and 100 mg/ml and no treatment (positive control). In addition, there was a vehicle group (n = 3) that received liposome preparation (carrier) only, and two animals with ligature application alone served as negative controls. Periodontal disease was quantified by direct visualization and radiographical evaluation of bone loss on defleshed skulls and by histological analyses of sections stained with hematoxylin-eosin and tartrate-resistant acid phosphatase. In the no-treatment (positive control) and liposome (vehicle) groups, direct visualization and radiological measurements revealed statistically significant bone loss compared to the negative control. Application of cimetidine at all concentrations tested inhibited inflammation and bone loss by >90%. Histological findings revealed that ligated sites of the positive control and vehicle groups showed significant reduction in bone level (P < 0.05) compared to the three cimetidine groups, with a marked decrease in inflammation. The findings of this study provide morphological and histological evidence that topically active cimetidine is a potent inhibitor of P. gingivalis-elicited periodontal inflammation, arresting and/or preventing tissue destruction and influencing cell populations present in the inflammatory cell infiltrate.

Antihistamines and their role as antipruritics

Antihistamines that bind to the histamine 1 receptor (H1) serve as important therapeutic agents to counter the effects of histamine in the skin. Two generations of antihistamines exist; however, second-generation agents are more advantageous because they cause less sedation, have a longer half life and are more selective for the H1 receptor. While H1 antihistamines have proven to be effective at reversing the pruritus and cutaneous lesions of chronic urticaria, their ability to treat pruritus associated with other cutaneous and systemic diseases is unproven.

Absence of histamine-induced nitric oxide release in the human radial artery: implications for vasospasm of coronary artery bypass vessels

Radial artery (RA) bypass grafts can develop severe vasospasm. As histamine is known to induce vasospasm, its effect on RA was assessed compared with the classic bypass vessels internal mammary artery (MA) and saphenous vein (SV). The vessels were examined in organ chambers for isometric tension recording. Histamine induced contractions on baseline; the sensitivity was higher in RA and SV than MA. After precontraction with norepinephrine, histamine did not evoke relaxations of RA but induced relaxations of MA and less of SV at lower concentrations; it induced contractions at higher concentrations, reaching similar levels in all three vessels. Indomethacin did not affect the response of MA and RA but potentiated relaxations and reduced contractions of SV. Endothelium removal, Nω-nitro-l-arginine methyl ester (l-NAME), or the H2-receptor blocker cimetidine did not affect the response of RA, but inhibited relaxations and enhanced contractions in MA and inhibited relaxations in SV; in the latter, only l-NAME enhanced contractions. Real-time PCR detected much lower expression of endothelial H2-receptor in RA than MA or SV. Western blots revealed similar endothelial nitric oxide (NO) synthase expression in all three vessels. Relaxations to acetylcholine were identical in RA and MA. Thus histamine releases NO by activating the endothelial H2-receptor, the expression of which is much lower in RA than MA or SV. H2-receptor activation also releases prostaglandins in SV, partially antagonizing NO. The lack of histamine-induced NO production represents a possible mechanism of RA vasospasm.

Anti-inflammatory properties of local anesthetics and their present and potential clinical implications

Development of new local anesthetic agents has been focused on the potency of their nerve-blocking effects, duration of action and safety and has resulted in a substantial number of agents in clinical use. It is well established and well documented that the nerve blocking effects of local anesthetics are secondary to their interaction with the Na+ channels thereby blocking nerve membrane excitability and the generation of action potentials. Accumulating data suggest however that local anesthetics also possess a wide range of anti-inflammatory actions through their effects on cells of the immune system, as well as on other cells, e.g. microorganisms, thrombocytes and erythrocytes. The potent anti-inflammatory properties of local anesthetics, superior in several aspects to traditional anti-inflammatory agents of the NSAID and steroid groups and with fewer side-effects, has prompted clinicians to introduce them in the treatment of various inflammation-related conditions and diseases. They have proved successful in the treatment of burn injuries, interstitial cystitis, ulcerative proctitis, arthritis and herpes simplex infections. The detailed mechanisms of action are not fully understood but seem to involve a reversible interaction with membrane proteins and lipids thus regulating cell metabolic activity, migration, exocytosis and phagocytosis.

IL-9 promotes but is not necessary for systemic anaphylaxis

Anaphylaxis represents an extreme form of allergic reaction, consisting of a sensitization phase during which allergen-specific IgE are produced and an acute effector phase triggered by allergen-induced degranulation of mast cells. We studied the role of IL-9, a Th2 cytokine implicated in asthma, in different models of murine anaphylaxis. Using a passive model of systemic anaphylaxis, in which anti-DNP IgE Abs were administered before challenge with DNP-BSA, we found that IL-9-transgenic mice or wild-type mice treated with IL-9 for 5 days were highly sensitive to fatal anaphylaxis. This effect was reproduced in both anaphylaxis-susceptible and -resistant backgrounds (FVB/N or [FVB/N x BALB/c] F(1) mice, respectively) and correlated with increased serum concentrations of mouse mast cell protease-1 level, a protein released upon mast cells degranulation. By contrast, IL-9 did not increase the susceptibility to passive cutaneous anaphylaxis. IL-9 expression also increased the susceptibility to fatal anaphylaxis when mice were sensitized by immunization against OVA before challenge with the same Ag. In this model, serum from sensitized, IL-9-transgenic mice was more potent in transferring susceptibility to OVA challenge into naive mice, indicating that IL-9 also promotes the sensitization stage. Finally, using IL-9R-deficient mice, we found that despite its anaphylaxis-promoting activity, IL-9 is dispensable for development of both passive and active anaphylaxis, at least in the C57BL/6 mouse background. Taken together, the data reported in this study indicate that IL-9 promotes systemic anaphylaxis reactions, acting at both the sensitization and effector stages, but is not absolutely required for this process.

A Review of the Evidence from Comparative Studies of Levocetirizine and Desloratadine for the Symptoms of Allergic Rhinitis

BACKGROUND

Levocetirizine and desloratadine are newer antihistamines indicated for the treatment of allergic rhinitis and chronic idiopathic urticaria.

OBJECTIVE

This article discusses the pharmacokinetics and pharmacodynamics of levocetirizine and desloratadine and reviews studies that have directly compared the effects of these 2 drugs in allergic rhinitis and urticaria.

METHODS

Relevant articles were identified through a search of MEDLINE from 1999 through 2004 using the main search terms levocetirizine and desloratadine.

RESULTS

Levocetirizine is absorbed rapidly and reaches a steady-state plasma concentration more quickly than does desloratadine. It is also metabolized to a lesser extent than desloratadine, has a lower V(d), and has higher specificity for histamine(1) receptors. Eight well-controlled trials were identified that directly compared the effects of levocetirizine and desloratadine in the skin and nose of healthy individuals and patients with allergic rhinitis. Drug activity was measured in terms of wheal, flare, and itch reactions; nasal symptoms or symptom scores; increases in concentrations of inflammatory markers; or facial thermography. In most of these trials, levocetirizine had a faster onset and greater consistency of effect than desloratadine. The differences in the pharmacokinetic and pharmacodynamic profiles of the 2 drugs may partially explain these clinical findings.

CONCLUSIONS

Levocetirizine may be preferred to desloratadine as a treatment option for allergic rhinitis because of its faster onset of action and greater consistency of effect. Although comparative studies in chronic idiopathic urticaria are not available, data from histamine-induced wheal and flare studies in healthy volunteers suggest that levocetirizine may be more effective in preventing itching than desloratadine.

Histamine in allergic inflammation and immune modulation

Histamine, originally considered as a mediator of acute inflammatory and immediate hypersensitivity responses has also been demonstrated to affect chronic inflammation and regulate several essential events in the immune response. On the other hand, various cytokines control histamine synthesis, release and expression of histamine receptors (HRs). The cells involved in the regulation of immune response and hematopoiesis express HRs and also secrete histamine, which can selectively recruit the major effector cells into tissue sites and affect their maturation, activation, polarization and effector functions leading to chronic inflammation. Histamine, acting through its receptor type 2, positively interferes with the peripheral antigen tolerance induced by T regulatory cells in several pathways. Histamine also regulates antigen-specific Th1 and Th2 cells, as well as related antibody isotype responses. The diverse effects of histamine on immune regulation are due to differential expression and regulation of four HRs and their distinct intracellular signals. In addition, differences in affinities of these receptors are highly decisive on the biological effects of histamine and agents that target HRs. This article highlights the findings leading to a change of perspective in histamine immunobiology.

Histamine-cytokine connection in immunity and hematopoiesis

A number of recent studies have led to a reappraisal of the functional capacities of histamine in immunity and hematopoiesis. This change of perspective was provided by the following findings: (1) the evidence for multiple cellular sources of histamine, differing from mature basophils and mast cells by their ability to newly synthesize and liberate the mediator without prior storage, (2) the discovery of a novel histamine receptor (H4R), preferentially expressed on hematopoietic and immunocompetent cells, (3) the potential intracellular activity of histamine through cytochrome P450 and (4) the demonstration of a histamine-cytokine cross-talk. Indeed, cytokines not only modulate the degranulation process of histamine but also control its neosynthesis by the histamine-forming enzyme, histidine decarboxylase (HDC), at transcriptional and post-transcriptional levels. In turn, histamine intervenes in the intricate cytokine network, regulating cytokine production by immune cells through distinct receptors signaling distinct biological effects. This type of regulation is particularly relevant in the context of TH1/TH2 differentiation, autoimmunity and tumor immunotherapy.