BDNF levels in bipolar may be crucial: serum brain-derived neurotrophic factor is decreased in bipolar disorder during depressive and manic episodes

From the article:

Serum BDNF levels were lower in bipolar disorder patients during both manic and depressive episodes, as compared with euthymic patients and healthy controls. This finding suggests that the effects of pharmacological treatment on BDNF levels may be associated with mood stabilization.