17-β-Estradiol increases neuronal excitability through MAP kinase-induced calpain activation

17-beta-Estradiol (E2) is a steroid hormone involved in numerous brain functions. E2 regulates synaptic plasticity in part by enhancing NMDA receptor function and spine density in the hippocampus, resulting in increased long-term potentiation and facilitation of learning and memory. As the calcium-dependent neutral protease, calpain, is also involved in these processes, we tested whether E2 could activate calpain and examined the functional consequences of E2-mediated calpain activation in hippocampus. Calpain activity was analyzed by a fluorescence resonance energy transfer (FRET)-based assay that allows both quantitative determination and spatial resolution. E2 rapidly activated calpain in cultured cortical and hippocampal neurons, prominently in dendrites and dendritic spines. E2-induced calpain activation was mediated through mitogen-activated protein kinase (MAPK), as it was completely blocked by MEK inhibitors. It was also calcium-independent, as it was still evident in presence of the calcium chelator, BAPTA-AM. Activation of ER alpha and ER beta receptors by specific agonists stimulated calpain activity. Finally, the rapid E2-mediated increase in excitability in acute hippocampal slices was prevented by a membrane-permeable calpain inhibitor. Furthermore, E2 treatment of acute hippocampal slices resulted in increased actin polymerization and membrane levels of GluR1 but not GluR2/3 subunits of AMPA receptors; both effects were also blocked by a calpain inhibitor. Our results indicate that E2 rapidly stimulates calpain activity through MAP kinase-mediated phosphorylation, resulting in increased membrane levels of AMPA receptors. These effects could be responsible for E2-mediated increase in neuronal excitability and facilitation of cognitive processes.