Thyroxine and propylthiouracil-induced changes in the activity of monoamine oxidase in the fetal rat

The activity of monoamine oxidase (MAO) towards tryptamine has been determined in heart, brain and liver of the 21.5 day old rat fetus. The activity was compared between normal, thyroxine and propylthiouracil (PTU) treated animals. Hypothyroidism induced by PTU leads to a decrease in the activity of the three organs, while hyperthyroidism causes an increase in the enzymatic activity. These differences seem to be rather independent of the protein content of the organs. It appears thar rat fetal MAO is under a strong thyroid control.