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Studies that may clarify the effects of H1/ 2 blockers:

Histamine is biogenic amine that exerts the numerous important biological functions. Alcohol affect histamine action because those have common metabolizing enzymes--aldehyde dehydrogenase and aldehyde oxidase. Acetaldehyde can compete with aldehydes derived from histamine metabolism. Increased blood acetaldehyde resulting from abnormalities of alcohol dehydrogenase genotype in the Orientals population can release histamine from mast cells and basophiles, which induces the hypersensitivity reactions (flushing). These reactions may be blocked by antihistamine drugs. H2-receptor antagonists influence on the ethanol metabolism by the inhibition of the activity of alcohol metabolizing enzymes in the stomach and liver. Decreased activity of stomach alcohol dehydrogenase results in an increase in the blood ethanol concentrations, which may impairs the psychomotor skills and exceeds legal limits of driving. There are same evidences that ethanol affects the brain histamine level by the changes in the activity of enzymes involved in the synthesis and metabolism of histamine.

Alcohol and histamine metabolic pathways in the body have the common enzymes aldehyde dehydrogenase and aldehyde oxidase. The metabolite of ethanol, acetaldehyde, can effectively compete with the metabolites of histamine, methylimidazole acetaldehyde, and imidazole acetaldehyde. At the periphery, alcohol and acetaldehyde liberate histamine from its store in mast cells and depress histamine elimination by inhibiting diamine oxidase, resulting in elevated histamine levels in tissues. Histamine mediates alcohol-induced gastric and intestinal damage and bronchial asthma as well as flushing in Orientals. On the other hand, alcohol provokes food-induced histaminosis and histamine intolerance, which is an epidemiological problem. There are many controversial reports concerning the effect of H2 receptor antagonists on ethanol metabolism and the activity of alcohol dehydrogenase in the stomach. In addition, alcohol affects histamine levels in the brain by modulating histamine synthesis, release, and turnover. Histamine receptor antagonists can affect ethanol metabolism and change the sensitivity of animals to the hypnotic effects of alcohol. In contrast to other neurotransmitters, the involvement of the brain histamine system in the mechanisms of the central actions of alcohol and in the pathogenesis of alcoholism is poorly studied and understood.

Alcohol-induced bronchoconstriction is due to high blood concentrations of acetaldehyde, a metabolic product of ethanol, which lead to the release of histamine from basophils and mast cells. OBJECTIVE: We examined the inhibitory effects of azelastine hydrochloride, which inhibits histamine release and blocks H1 receptors, in alcohol-induced asthma. METHODS: Subjects were 13 Japanese asthmatic patients. We measured the change in FEV1 after ingestion of 30 g of pure ethanol. Blood ethanol, acetaldehyde, histamine, leukotriene C4 (LTC4), and thromboxane B2 (TXB2) concentrations were also measured. Alcohol challenge test was repeated in responders after administration of azelastine for 1 week at 4 mg/day. RESULTS: Of 13 asthmatic patients, five (38.5%) tested positive during an ethanol challenge test, represented by a fall more than 20% in FEV1. The responders had a high blood ethanol, and showed a rise in blood acetaldehyde and histamine concentrations, but not in LTC4 or TXB2. After azelastine treatment, there was no significant fall in FEV1 among responders. Neither the rise in blood ethanol nor blood acetaldehyde levels were blunted by treatment with azelastine, but the rise in blood histamine was blunted by this treatment. CONCLUSION: Our results suggest that antihistamine agents may be effective against alcohol-induced asthma by both blocking H1 receptors and inhibiting histamine release.