Neurotrophins require distinct extracellular signals to promote the survival of CNS neurons in vitro.
Althoughthe neurotrophins BDNF and NT-3 have been recognized as potent survival factors for distinct neuronal populations in the peripheral nervous system, they seem to have only minor effects on the survival of CNS neurons. In the present study, we provide evidence that BDNF and NT-3 require distinct additional extracellular signals in order to effectively promote the survival of several established populations of target neurons in the CNS. In dissociated cell cultures of the embryonic rat mesencephalon, BDNF promoted dopaminergic cell survival only after a delay of several days. Even after prolonged cultivation, survival promoting effects were completely absent with NT-3. Irrespective of the cultivation time, survival promoting effects of both BDNF and NT-3 on dopaminergic neurons were induced or potentiated upon simultaneous depolarization of cultured mesencephalic cells with NMDA or upon activation of cAMP/PKA-dependent signaling pathways with dibutyryl cAMP. Dibutyryl cAMP (dbcAMP), but not NMDA, also potentiated or induced the survival promoting effects of BDNF and NT-3 on cultured cerebellar granule cells. None of these substances, either alone or in combination, affected the survival of cultured cortical neurons. However, cortical cell survival increased upon depolarization with elevated potassium; an effect known to involve the induction of an autocrine BDNF loop. In both cerebellar and mesencephalic neurons, but not in cortical neurons, dbcAMP also potentiated neurotrophin-induced c-fos response, indicating intimate cross-coupling of signaling pathways activated by these different factors. Together these findings suggest that in the CNS, neurotrophins preferentially promote the survival of functionally active neurons. Our findings further reveal that the neuronal response to neurotrophins is modulated in a brain region-specific manner.