Effect of amodiaquine on histamine level and histamine-methyltransferase activity in the rat brain

Amodiaquine, 10(-5) M, totally inhibited the activity of histamine-methyltransferase (HMT) in the rat brain. Both peripheral (60 mg/kg i.p.) and central (50 and 100 microgram i.vt.) administration of the drug substantially reduced the HMT activity. Amodiaquine given i.p. or i.vt., in various doses, did not change the endogenous histamine (HI) level in the rat brain at any time studied (1-24 h). Amodiaquine (60 or 120 mg/kg i.p.) administered together with an inhibitor of diamine oxidase, i.e. aminoguanidine (either peripherally or centrally) had no effect on the endogenous HI content. Possible involvement of mechanisms other than methylation or oxidation in regulation of the endogenous HI level in the rat brain is discussed.