Intracellular aggregation of hyperphosphorylated tau in neurofibrillary tangles (NFTs) is a major neuropathological hallmark of taupathies such as Alzheimer's disease. Okadaic acid (OKA) is a potent inhibitor of PP2A, leading to abnormal tau phosphorylation. Brain-derived neurotrophic factor (BDNF) is a neurotrophin that is selectively downregulated in AD. In this study, we investigated the effects of OKA induced tau hyperphosphorylation on secreted and cellular levels of BDNF in primary cortical neurons that were treated with 25 nM OKA. Tau phosphorylation at threonine 231 (Thr231) sites was assessed by Western blot using antibodies against phospho-Thr231. Non-phosphorylated tau protein was detected with the Tau-1 antibody. Levels of BDNF secreted to the culture medium were determined by ELISA at the 8th and 24th hours of treatment. Cellular localization and protein expression of BDNF and tau were assessed by immunofluorescent labeling and fluorescent intensity measurements at 24 hours of treatment. Tau hyperphosphorylation was confirmed with increase in Thr231 and the decrease in Tau-1 signals after 8 h of OKA treatment, compared with the control groups, secreted BDNF levels in the OKA-treated group were significantly lower after 24 h of treatment but were not significantly different at 8 h of treatment. BDNF immunoreactivity was seen in cytoplasm and neurites of the neurons in control group. BDNF immunoreactivity significantly decreased in the OKA treated group and this attenuation was significant especially at neurites. Our results suggest that the decrease in BDNF secretion and the BDNF expression might depend on the disruption of microtubule structure caused by tau hyperphosphorylation. (C) 2016 Elsevier B.V. All rights reserved.