Influence of Monoamine Oxidase Inhibitors on 5-Hydroxytryptamine Synthesis in the Brain.

Two hydrazine (βenylisopro-pylhydrazine and iproniazid) and 2 non-hydrazine (trans-2 phenyclyclolamine and pargyline) monoamine oxidase inhibitors were tested for their influence on the conversion of tryptophan and 5-hydroxytrypto-phan to 5-hydroxytryptamine. It was found that the conversion of 5-hydroxytryptophan was influenced by both types of monoamine oxidase inhibitors but the non-hydrazine inhibitors were most effective. There was an increase in the amount of 5-hydroxytryptamine formed, especially during the first few hours after the inhibitors were administered. Tryptophan conversion to 5-hydroxytryptamine was not significantly influenced by the monoamine oxidase inhibitors. Excitatory responses seen with monoamine oxidase inhibitors and 5-hydroxytryptamine precursors were also observed and appeared to be related to the degree of enzyme inhibition more than to the amount of 5-hydroxytryptamine.