Accumulating evidence proposes BDNF as a candidate molecule in the pathophysiology of affective disorders. Reduced levels of peripheral BDNF have been found in drug-free patients with major depressive disorder (MDD), in drug-treated depressed or manic patients with bipolar disorder type I (BD-I), but not in drug-treated euthymic BD-I individuals. No study explored BDNF serum levels in patients with bipolar disorder type II (BD-II). Our aims were to confirm previous findings on peripheral BDNF in MDD and BD-I patients; to explore circulating BDNF also in patients with BD-II; to exclude the influence of comorbid psychiatric disorders on BDNF levels in affective patients.

We measured serum BDNF concentrations in 85 subjects, including 24 euthymic patients with unipolar depression (UD), 17 euthymic patients with BD-I, 11 euthymic patients with BD-II, 11 UD patients with a current major depressive episode and 22 drug-free healthy controls. At the assessment time, 15 patients were drug-treated; the remaining ones were drug-free for at least 4 weeks.

As compared to healthy controls, serum BDNF concentrations were significantly reduced in all the patient groups with no significant inter-group differences. Drug treatments and comorbid psychiatric disorders had no effect on lowered circulating BDNF levels in affective patients.

Present results confirm previous findings of reduced BDNF in patients with MDD and reveal, for the first time, that serum BDNF levels are decreased also in euthymic patients with UD, BD-I and BD-II, independently from drug treatment status and concomitant Axis I psychiatric disorders.