There is increasing evidence that Brain-Derived Neurotrophic Factor (BDNF) may be involved in the pathophysiology of depression and the actions of antidepressants.

The aim of the present study was to analyze the relationship between the BDNF plasma levels and clinical variables in unipolar versus bipolar depressed patients.

Thirty-three outpatients (17 with unipolar and 16 with bipolar depression) and 15 healthy controls were included in the study. BDNF concentrations were measured with an ELISA assay method. Severity of depression was assessed by means of the 21-item Hamilton Rating Scale for Depression (HRSD) and the Clinical Global Impressions (CGI) Severity of Illness scale.

The BDNF plasma levels were significantly reduced in patients with unipolar or bipolar depression, with no significant difference between the two groups. Significant and negative correlations were found in the total sample between BDNF levels and the HRSD total scores, the retardation factor scores and CGI “severity of illness” scores. When the same analyses were repeated in each group separately, these findings were confirmed only in patients with bipolar depression.

Our results show that lower BDNF levels may be related to both severity of depression and retardation symptoms in bipolar depression. Further studies need to ascertain whether and how the BDNF levels may be associated with any psychopathological dimensions of the depressive state and be used as a biological marker to differentiate bipolar from unipolar depression.