Depression: new evidence in support of biological differentiation.

The authors studied monoamine metabolism in patients with bipolar (manic-depressive) and unipolar depressive illness and in normal control subjects under strict dietary conditions before and during administration of carbidopa, a peripheral decarboxylase inhibitor. They found that unipolar depressed patients excreted higher amounts of 3-methoxy-4-hydroxyphenylglycol (MHPG) and 5-hydroxyindoleacetic acid (5-HIAA) in the drug-free period and while receiving carbidopa and significantly less tryptamine only after carbidopa administration. Plasma tryptophan levels differed in the three groups. The authors state that their research design reveals changes in serotonin and nonrepinephrine metabolism in unipolar depressed subjects that distinguish this group from normal and bipolar depressed subjects, suggesting a reduced CNS uptake of tryptophan in unipolar depression.