The current status of histamine receptors in human skin: therapeutic implications

A chronic model for evaluating the itching associated with allergic conjunctivitis in rats

The present study was performed to develop a new model for evaluating itching associated with allergic conjunctivitis in rats. Repeated topical application of antigen caused an increase in eye scratching behavior in sensitized animals, and a significant difference was observed from days 21 to 42. Almost the same findings were observed in allergic symptoms, hyperemia and edema. Instillation of histamine also resulted in an increase in eye scratching behavior. The sensitivity to histamine in eye scratching behavior was increased by topical antigen application for 42 days after sensitization. In addition, the number of conjunctival eosinophils was significantly increased by repeated topical antigen application from days 21 to 42 in sensitized rats. Some anti-allergic drugs such as olopatadine (H1 antagonist), cetiridine (H1 antagonist) and ramatroban (thromboxane A2 (TXA2) antagonist) caused an inhibition of eye scratching behavior induced by topical sensitization in a dose-related manner. However, zafirlukast (cys-LT antagonist) caused no significant inhibition even at a dose of 30 mg/kg. The findings in present model of itching in allergic conjunctivitis were mainly through histamine H1-activity, and thromboxane A2 receptors were also involved in the response.

The role of platelet activating factor and the efficacy of apafant ophthalmic solution in experimental allergic conjunctivitis

Platelet-activating factor (PAF) may be an important mediator in allergic conjunctivitis. In this study, apafant, a potent PAF antagonist, was evaluated for topical ocular anti-PAF activity in PAF and antigen stimulated conjunctivitis models. PAF, when injected into parpebral conjunctiva, provoked an acute increase, measured as dye leakage, in conjunctival vascular permeability. Apafant inhibited this response in a dose-related manner, and the inhibitory action of 0.1% apafant lasted for at least 6 hours duration. PAF, when instilled into the conjunctival sac, induced itch-scratching behavior and clinical symptoms, such as conjunctival redness and edema. These were inhibited by pretreatment with apafant ophthalmic solution. In a passive conjunctival anaphylaxis model in guinea pigs, significant inhibition of the allergic response was observed following topical ocular administration of apafant 5 and 15 minutes prior to the antigen challenge. We have demonstrated that PAF plays an important role in the development of allergic conjunctivitis. These results clearly indicate that apafant has potential as a topical ocular anti-PAF for treating allergic conjunctivitis.

Histamine antagonists in the treatment of acute allergic reactions

STUDY OBJECTIVE

We compared the efficacies of cimetidine (an H2-receptor antagonist) and diphenhydramine (an H1-receptor antagonist) alone and in combination for alleviation of symptoms of acute allergic reactions. STUDY DESIGN AND INTERVENTIONS: In this prospective, randomized, double-blind study, patients and examiners assessed the severity of symptoms and signs of acute allergic reactions using a visual-analog scale before treatment and 30 minutes after treatment with 300 mg IV cimetidine and placebo, 50 mg IV diphenhydramine and placebo, or diphenhydramine plus cimetidine.

SETTING AND PARTICIPANTS

Thirty-nine patients with acute allergic reactions presenting to two emergency departments of teaching hospitals.

RESULTS

Of the 35 patients with pruritus, all 12 receiving diphenhydramine placebo had clinically significant relief compared with six of ten (60%) receiving cimetidine plus placebo (P = .03). Twelve of 13 (92%) receiving diphenhydramine plus cimetidine had relief, which was not a significant difference from the single drugs. Comparison of mean differences in pretreatment and post-treatment symptom scores (relief scores) among groups of patients with pruritus detected significantly more relief in the group receiving diphenhydramine plus placebo (80.3 +/- 7.4) than in those receiving cimetidine plus placebo (48.8 +/- 13.4) (P = .022). Of the 33 patients with urticaria, five of 11 (46%) receiving diphenhydramine plus placebo had clinically significant relief compared with eight of ten (80%) receiving cimetidine plus placebo (P = .18). Eleven of 12 patients (92%) receiving diphenhydramine plus cimetidine had relief, which is a significant difference from those receiving diphenhydramine plus placebo (P = .027). Comparison of mean relief scores in patients with urticaria detected significantly more relief in the group receiving diphenhydramine plus cimetidine (55.3 +/- 6.5) than in the group receiving diphenhydramine plus placebo (30.7 +/- 6.1) (P = .006).

CONCLUSION

For treatment of pruritus from acute allergic reactions, diphenhydramine is more effective than cimetidine, and the combination offers no additional benefit. For treatment of acute urticaria, the combination of cimetidine and diphenhydramine is more effective than diphenhydramine alone.

Responses of human dermal microvascular endothelial cells to histamine and their modulation by interleukin 1 and substance P

The action of histamine on human dermal microvascular endothelial cells and modulation of its effects by the cytokine interleukin-1 and the vasoactive neuropeptide substance P have been investigated. Histamine (10(-6)-10(-3) M) induces release of prostaglandin E2 in a concentration- and time-dependent manner. Prostaglandin E2 release is facilitated principally by histamine H1 receptors as the H1 receptor antagonist pyrilamine attenuates prostaglandin E2 release whereas the H2 receptor antagonist cimetidine only slightly reduces release. In contrast to other cells, the histamine/receptor interaction is not associated with increased intracellular accumulation of the cyclic nucleotides, cyclic AMP, or cyclic GMP. Interleukin-1 induces a concentration-dependent release of prostaglandin E2 following 24 h incubation. However, substance P does not increase release of prostaglandin E2 above baseline. In cells incubated with 1 U/ml human recombinant interleukin 1 alpha for 24 h prior to stimulation with histamine (10(-5)-10(-3) M) for 30 min, there is a significant potentiation of histamine-induced release of prostaglandin E2 (p less than 0.05). Using a solubilized cell sonicate prepared from human dermal microvascular endothelial cells incubated with 1 U/ml human recombinant interleukin 1 alpha for 24 h, conversion of exogenous arachidonic acid into prostaglandin E2 increased by 60.19 +/- 18.28%. Cycloheximide partially reduces the increased conversion but completely blocks interleukin-1-induced release of prostaglandin E2 from intact cells. Substance P does not potentiate histamine-induced release of prostaglandin E2 or increase arachidonic acid conversion. These results demonstrate that human dermal microvascular endothelial cells are responsive to histamine and that interleukin-1, but not substance P, can potentiate histamine-induced release of prostaglandin E2. Interleukin-1 appears to act, at least in part, by regulating the availability of free arachidonic acid. Interactions between histamine and interleukin-1 may be important in the modulation of inflammatory reactions in skin.

A review of pruritus

This article reviews the neurophysiology of pruritus and presents evidence that itch is a separate modality from pain, rather than a submodality of pain. The numerous suggested pruritogens are reviewed, and evidence supporting each one is critically examined. The systemic causes of generalized pruritus are reviewed, with detailed review of the literature on uremic pruritus, cholestatic pruritus, and the pruritus associated with polycythemia vera. The specific treatments used for each of these conditions are reviewed, as well as the use of antihistamines in generalized pruritus. An approach to the workup of a patient with pruritus is suggested.

A double blind trial of H1 and H2 receptor antagonists in the treatment of atopic dermatitis

Eighteen patients with atopic dermatitis were randomly chosen for a clinical trial to compare the action of the H2 receptor antagonist cimetidine in combination with the H1 receptor antagonist chorpheniramine against the chlorpheniramine alone and against placebo. Each treatment period lasted for 4 weeks. Intensity of puritus was recorded daily by the patient on an analogue scale. Global clinical assessment was graded on a 5-point scale every 2 weeks by the investigators and weekly by the patient himself. Further study parameters were: quantity of the corticosteroid ointment used for 'emergencies'. immunoglobulin E level, and eosinophil count. The data of 16 patients was evaluated. There was no significant difference in any of the study parameters during the three treatment periods. On the basis of this study, the combined administration of H1 and H2 receptor antagonists is of no benefit in the treatment of atopic dermatitis.

The effect of histamine, H1 and H2 agonists, and H1 and H2 antagonists on postprandial pancreatic polypeptide release in dogs

The influence of histamine, H1 and H2 agonists (2-AETD and dimaprit), and H1 and H2 antagonists (mepyramine and cimetidine) on the postprandial release of pancreatic polypeptide (PP) was assessed in five Labrador retrievers. Infusions of histamine, 0.25 mumol kg-1 h-1; 2AETD, 4 mumol kg-1 h-1; and dimaprit, 1 mumol kg-1 h-1, enhanced postprandial release (131-189% of control values). The response to 2-AETD and dimaprit was significantly different from control values (p less than 0.05). An increase was observed even when 50 mg mepyramine and 400 mg cimetidine were injected as intravenous boluses (131-139% of control values (p less than 0.05]. The action of the antagonists may be explained by partial agonism or histamine release induced by these agents. It is concluded that postprandial PP release is stimulated by H1 and H2 receptors.