Relative activity of some inhibitors of mono-amine oxidase in potentiating the action of tryptamine in vitro and in vivo

Several known inhibitors of mono-amine oxidase (iproniazid, isocarboxazid, nialamide, phenelzine, pheniprazine and tranylcypromine) were tested for their ability to (i) inhibit the mono-amine oxidase activity of a rat brain mitochondrial preparation in vitro; (ii) potentiate the action of tryptamine on the isolated rat fundal strip preparation; and (iii) potentiate the acute toxicity of tryptamine in mice. There was some correlation between the order of potency of the drugs in the three tests, particularly in inhibiting the Enzyme activity in the Warburg and in the tryptamine toxicity test in mice. Exceptions to this were isocarboxazid which had unexpectedly high activity on the rat fundal strip preparation, and tranylcypromine which was devoid of tryptamine-potentiation action on the rat fundus preparation although it inhibited rat brain mono-amine oxidase in vitro and potentiated the action of tryptamine in vivo. Tranylcypromine was considerably less active in inhibiting the mono-amine oxidase of rat fundus than rat brain tissue in vitro, while iproniazid and isocarboxazid had about the same potency on the Enzyme from the two tissues.