Brain-derived neurotrophic factor causes cAMP response element-binding protein phosphorylation in absence of calcium increases in slices and culturedneurons from rat visual cortex

Neurotrophins play a crucial role in the developmental plasticity of the vi sual cortex, but very little is known about the cellular mechanisms involve d in their action. In many models of synaptic plasticity, increases in cyto solic calcium concentration and activation of the transcription factor cAMP response element-binding protein (CREB) are crucial factors for the induct ion and maintenance of long-lasting changes of synaptic efficacy. Whether B DNF modulates intracellular calcium levels in visual cortical neurons and t he significance of this action for BDNF signal transduction is still contro versial. We investigated whether CREB phosphorylation and calcium changes a re elicited by acute BDNF presentation in postnatal visual cortical slices and cultures. We found that BDNF did not cause any calcium increase, but it induced robust CREB phosphorylation in neurons from both preparations. We further analyzed signal transduction and its dependency on calcium changes in cultured neurons. CREB phosphorylation required trkB activation because treatment with the trk inhibitor k252a completely blocked CREB phosphorylat ion. In agreement with the imaging experiments, we verified that calcium ch anges were not necessary for CREB activation because preincubation with BAP TA-AM did not diminish the level of CREB phosphorylation induced by BDNF st imulation. CREB phosphorylation was accompanied by gene expression, because we observed the upregulation of c-fos expression, which was also not affec ted by preincubation with BAPTA-AM. Finally, BDNF caused phosphorylation of mitogen-activated protein kinase (MAPK), and because the treatment with th e MAPK inhibitor U0126 completely abolished CREB activation and c-fos upreg ulation, it is likely that both processes depend mainly on the MAP kinase p athway. These results indicate that MAPK and CREB, but not intracellular ca lcium, are important mediators of neurotrophin actions in the visual cortex .