Increased platelet serotonin in bipolar depression and hypomania.

Platelet serotonin (5-HT) was higher at 8 a.m. in untreated bipolar depressives than in controls. This high 5-HT was different from unipolar depressives where 5-HT levels were similar to controls but showed altered diurnal rhythmicity. Further differences between unipolar and bipolar patients were found. 5-HT levels and 5-HT uptake into platelets were not correlated in bipolar depressives as found in both unipolar depressives and controls; and 5-HT levels and platelet monoamineoxidase activity tended to be negatively correlated in bipolar but not in unipolar depressives and controls. A longitudinal study of a bipolar II patient throughout three hospitalizations for depression followed by the switch into hypomania, indicated that 5-HT was state independent, remaining constantly high in all clinical conditions and upon remission, reduced only during and after treatment with clomipramine, a 5-HT uptakt dysfunction of indoleamines underlying bipolar illness.