Ca2+ signal summation and NFATc1 nuclear translocation in sympathetic ganglion neurons during repetitive action potentials.

NFATc-mediated gene expression constitutes a critical step during neuronal development and synaptic plasticity. Although considerable information is available regarding the activation and functionality of specific NFATc isoforms, in neurons little is known about how sensitive NFAT nuclear translocation is to specific patterns of electrical activity. Here we used high-speed fluo-4 confocal imaging to monitor action potential (AP)-induced cytosolic Ca2+ transients in rat sympathetic neurons. We have recorded phasic and repetitive AP patterns, and corresponding Ca2+ transients initiated by either long (100-800 ms) current-clamp pulses, or single brief (2 ms) electrical field stimulation. We address the functional consequences of these AP and Ca2+ transient patterns, by using an adenoviral construct to express NFATc1-CFP and evaluate NFATc1-CFP nuclear translocation in response to specific patterns of electrical activity. Ten Hertz trains stimulation induced nuclear translocation of NFATc1, whereas 1 Hz trains did not. However, 1 Hz train stimulation did result in NFATc1 translocation in the presence of 2 mM Ba2+, which inhibits M-currents and promotes repetitive firing and the accompanying small (approximately 0.6 DeltaF/F0) repetitive and summating Ca2+ transients. Our results demonstrate that M-current inhibition-mediated spike frequency facilitation enhances cytosolic Ca2+ signals and NFATc1 nuclear translocation during trains of low frequency electrical stimulation.