Brain-derived neurotrophic factor induces excitotoxic sensitivity in cultured embryonic rat spinal motor neurons through activation of the phosphatidylinositol 3-kinase pathway

Neurotrophic factors (NTFs) can protect against or sensitize neurons to exc itotoxicity, We studied the role played by various NTFs in the excitotoxic death of purified embryonic rat motor neurons. Motor neurons cultured in br ain-derived neurotrophic factor, but not neurotrophin 3, glial-derived neur otrophic factor, or cardiotrophin 1, were sensitive to excitotoxic insult. BDNF also induces excitotoxic sensitivity (ES) in motor neurons when BDNF i s combined with these other NTFs, The effect of BDNF depends on de novo pro tein and mRNA synthesis. Reagents that either activate or inhibit the 75-kD a NTF receptor p75(NTR) do not affect BDNF-induced ES, The low EC50 for BDN F-induced survival and ES suggests that TrkB mediates both of these biologi cal activities. BDNF does not alter glutamate-evoked rises of intracellular Ca2+, suggesting BDNF acts downstream. Both wortmannin and LY294002, which specifically block the phosphatidylinositol 3-kinase (P13K) intracellular signaling pathway in motor neurons, inhibit BDNF-induced ES. We confirm thi s finding using a herpes simplex virus (HSV) that expresses the dominant ne gative p85 subunit of P13K, Infecting motor neurons with this HSV, but not a control HSV, blocks activation of the P13K pathway and BDNF-induced ES, T hrough the activation of TrkB and the P13K signaling pathway, BDNF renders developing motor neurons susceptible to glutamate receptor-mediated cell de ath.