The mammalian target of rapamycin (mTOR)/p70S6 kinase (S6K) pathway plays an important role in brain-derived neurotrophic factor (BDNF)-mediated protein synthesis and neuroplasticity. Although many aspects of neuronal function are regulated by intracellular calcium ([Ca(2+)](i)) and calmodulin (CaM), their functions in BDNF-induced phosphorylation of p70S6K and protein synthesis are largely unknown. Here, we report that BDNF, via TrkB-dependent activation of mTOR, induces sustained phosphorylation of p70S6K at Thr389 and Thr421/Ser424. BDNF-induced phosphorylation at Thr389 was dependent on PI3 kinase but independent of ERK-MAPK. The previously identified MAPK phosphorylation site at Thr421/Ser424 required both PI3K and MAPK in BDNF-stimulated neurons. Furthermore, we found that the reduction in [Ca(2+)](i), but not extracellular calcium, blocked the BDNF-induced phosphorylation of p70S6K at both sites. Inhibition of CaM by W13 also blocked p70S6K phosphorylation. In correlation, W13 inhibited BDNF-induced local dendritic protein synthesis. Interestingly, sustained elevation of [Ca(2+)](i) by membrane depolarization antagonized the BDNF-induced p70S6K phosphorylation. Finally, the BDNF-induced p70S6K phosphorylation did not require the increase of calcium level through either extracellular influx or PLC-mediated intracellular calcium release. Collectively, these results indicate that the basal level of intracellular calcium gates BDNF-induced activation of p70S6K and protein synthesis through CaM. (c) 2009 Wiley-Liss, Inc.