Recent research findings indicate that depressive disorders may be divided into two groups, A and B, using specific biochemical and pharmacological criteria. It is suggested that in the A group there is a disorder of norepinephrine metabolism or disposition, whereas serotonin and probably dopamine systems are not altered. Further, there is the possibility that B type patients have a disorder of serotonin, but not norepinephrine or dopamine systems. This biochemical heterogeneity of human depression has implications for both investigators and clinicians, and may account for disparate findings in biological studies of patients with affective disorders.
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