Further studies on platelet serotonin transporter binding in depression

Objective: There is an impressive literature implicating abnormalities in serotonergic neural systems in depression. Many investigators, but not all, have reported low numbers of platelet and brain serotonin (5-HT) transporter sites in drug-free depressed patients. In the present study the authors sought to determine whether the low platelet 5-HT transporter binding in depressed patients is due to previous antidepressant drug exposure. In addition, the binding of both [3H]imipramine and the more specific ligand [3H]paroxetine to the platelet 5-HT transporter was compared in drug-free depressed patients and age- and sex-matched normal comparison subjects. Method: In the first experiment blood samples were obtained from 12 depressed patients who had never received antidepressant drugs and 12 normal comparison subjects, and platelet 5-HT transporter binding was measured by using [3H]imipramine. In the second experiment blood samples were obtained from 28 drug-free depressed patients and 28 age- and sex-matched comparison subjects, and platelet 5-HT transporter binding was assessed by using both [3H]imipramine and [3H]paroxetine. Results: In the first experiment the never-medicated depressed patients exhibited fewer platelet [3H]imipramine binding sites than did the comparison subjects. In the second experiment the drug-free depressed patients had fewer platelet binding sites for both [3H]imipramine and [3H]paroxetine than did the comparison subjects. Conclusions: The low number of platelet [3H]imipramine binding sites does not appear to be due to prior antidepressant drug exposure. The [B.sub.max] of platelet binding sites for both [3H]imipramine and [3H]paroxetine, ligands used to measure 5-HT transporter binding, is abnormally low in depressed patients.