The generation of calcium signals following synaptic activity is a fundamental property of neurons that controls many neuronal processes including cell survival, learning and memory. In cultured neurons, increases in nuclear calcium concentrations are known to be critical for the activation of gene expression mediated by the transcription factor CREB. CREB has been implicated in transcription-dependent plasticity (late phase LTP). It is still unclear whether nuclear calcium signals are also responsible for CREB mediated gene transcription in vivo. The goal of the study was to visualise nuclear calcium signals in vivo. Investigating the importance of nuclear Ca2+ signalling in the intact brain is impeded by the restrictions and complexities of in vivo experimentation. However, D. melanogaster provides an excellent system to explore the role nuclear Ca2+ in an intact behaving animal. Since Ca2+ signalling of the nucleus is of particular interest transgenic flies expressing the nuclear localised Ca2+ indicator UAS GCaMP NLS to visualise nuclear Ca2+ signals and the nuclear Ca2+/CaM inhibitor UAS 2xM13 myc to interfere with nuclear Ca2+ signalling were generated. Using these flies, it could be shown that nuclear Ca2+ signalling could play a role in LTM. Further, using rAAV-mediated gene transfer the nuclear Ca2+ indicator rAAV GCaMP 2.0 NLS was expressed in CA1 pyramidal neurons of juvenile rats two weeks after in vivo injection. In acute slices from these juvenile rats an increase in nuclear Ca2+ concentration was visualised evoked by LTP inducing electrical stimulation. Using this method a correlation between LTP and increase in nuclear Ca2+ concentration could be shown. It is still unclear whether nuclear Ca2+ signals are necessary to induce LTP in acute slice. The major challenge of Ca2+ imaging in vivo either in flies or in rodents based on difficulties to detect signals through the intransparent skulls. The freshwater polyp H. vulgaris is completely transparent. Therefore, Ca2+ imaging of transgenic hydras expressing the Ca2+-indicators hyGCaMP or hyGCaMP NLS was done. Increase in nuclear Ca2+ signals was observed in ectodermal cells expressing hyGCaMP NLS. At the moment, the role of Ca2+ signals, especially of nuclear Ca2+ signals in hydra is unknown
