Variable Action of Monoamine Oxidase-inhibiting Hydrazines on Serotonin Metabolism

Abstract

RECENTLY, van Praag and Leijnse¹ reported an interesting finding concerning monoamine oxidase-inhibiting hydrazines ‘Marsilid’ (iproniacid) and ‘Marplan’ (isocarboxazid) on serotonin metabolism. They found that during treatment with these drugs the increase in the excretion of 5-hydroxyindolylacetic acid (5-HIAA) in urine after oral feeding of 5-HIAA decreased in the same way as after oral administration of serotonin (5-hydroxytryptamine = 5-HT) (Fig. 1a). They concluded from this result that: (1) Monoamine oxidase-inhibiting hydrazines have a similar and practically synchronous effect on 5-HIAA excretion after loading with serotonin and 5-HIAA. (2) This effect is brought about through the same mechanism of action. (3) The action of the so-called monoamine oxidase-inhibiting hydrazines on the serotonin loading test cannot be a result of the inhibition of monoamine oxidase.