Inhibition of brain histamine metabolism by metoprine

To study the extent to which histamine methylation accounts for the biosynthesis of histamine metabolites in brain, the effects of the histamine methyltransferase (HMT) inhibitor metoprine were determined on the whole brain levels of tele-methylhistamine (t-MH), its oxidative metabolite tele-methylimidazoleacetic acid (t-MIAA), and brain HMT activity in albino rats. Metoprine (5-30 mg/kg) reduced brain t-MH levels by about 75% and caused a dose-dependent reduction (70-90%) in HMT activity 4 hr after administration. Furthermore, the levels of t-MH remaining in each brain after metoprine treatment were significantly positively correlated with the remaining HMT activity of that brain after all doses of drug. Although brain t-MIAA levels were reduced by only 30% 4 hr after metoprine administration, the levels were reduced by about 75% 12 hr after the drug, similar to the reduction in t-MH levels. These findings support previous suggestions that t-MH and t-MIAA in brain arise from brain histamine metabolism, and that brain t-MH synthesis is equivalent to histamine methylation.