BDNF has emerged as a potent modulator of activity-dependent synaptic development and plasticity. Dysfunctions in its trafficking and release, as well as in signaling through its receptor TrkB, have been implicated in the etiology of numerous developmental and neurodegenerative brain disorders. The prevailing notion is that Ca2+ release from IP3-sensitive stores is the only mechanism for BDNF to modulate Ca2+ homeostasis. However, direct evidence identifying the specific signaling and the sources of Ca2+ ions mediating those actions is limited and contradictory. In addition, nothing is known about the actions of native BDNF released during neuronal activity, despite the extensive evidence of the effects of exogenously applied BDNF. The long-term goal of this project is to identify the mechanisms by which TrkB activation sets in motion the wide range of BDNF effects on hippocampal neurons and synapses. In this competing renewal we will focus on our observation that BDNF elicits slow and sustained Ca2+ signals associated with membrane currents, reminiscent of capacitative Ca2+ entry and non-selective cationic currents mediated by TRPC channels, respectively. The specific hypothesis is that BDNF triggers TrkB-dependent PLCgamma activation followed by Ca2+ mobilization from IP3-sensitive stores in CA1 pyramidal neurons, leading to the activation of capacitative Ca2+ entry and a sustained inward current mediated by TRPC channels. The first two Aims will identify the elementary actions of exogenously applied BDNF on membrane currents and intracellular Ca2+ levels, while the third Aim will use this knowledge to identify similar responses evoked by native BDNF released during afferent stimulation. Simultaneous Ca2+ imaging and electrophysiological recording, combined with pharmacological inhibitors, function-blocking antibodies, and siRNA-mediated knockdown will be used to identify the components of the signaling pathway. BDNF scavengers will allow determining whether BDNF released by afferent activity evokes similar Ca2+ signals and inward currents. We expect the proposed studies to provide the most comprehensive understating to date of the immediate actions of BDNF on membrane currents and intracellular Ca2+ homeostasis, leading to enduring changes in synaptic function, structure, and plasticity. ? ?
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