Membrane phospholipids were measured in platelets of seven medication-
free patients in the manic phase of bipolar affective disorder and seven
healthy comparison subjects. The relative percentage of platelet membrane
phosphatidylinositol-4,5-bisphosphate was significantly higher in the manic
patients than in the comparison subjects. These results are consistent with
an enhanced neuronal second messenger response after 5-hydroxytryptamine
receptor stimulation followed by neurotransmitter release.