Altered platelet monoamine oxidase activity in affective disorders.

Platelet MAO activity was found to be elevated in primary depressive illness, and the severity to correlate positively with MAO activity. The reactive depression subgroup's mean platelet MAO activity was not significantly different from that of the controls. The endogenous group's unipolar and bipolar subgroups had significantly different platelet MAO activity, respectively high and low. The differences in MAO activity between unipolar and bipolar patients appeared to persist in the well state, but not after lithium carbonate therapy. These differences in MAO activity were apparent with the substrate tyramine but not with benzylamine. Altered MAO activity in patients with affective disorders may be determined through genetic mechanisms.