Activation of Nuclear Calcium Dynamics by Synaptic Stimulation in Cultured Cortical Neurons

: L‐type voltage‐sensitive Ca 2+ channels (VSCCs) are enriched on the neuronal soma and trigger gene expression during synaptic activity. To understand better how these channels regulate somatic and nuclear Ca 2+ dynamics, we have investigated Ca 2+ influx through L‐type VSCCs following synaptic stimulation, using the long‐wavelength Ca 2+ indicator fluo‐3 combined with laser scanning confocal microscopy. Single synaptic stimuli resulted in rapid Ca 2+ transients in somatic cytoplasmic compartments (<5 ms rise time). Nuclear Ca 2+ elevations lagged behind cytoplasmic levels by ~60 ms, consistent with a dependence on diffusion from a cytoplasmic source. Pharmacological experiments indicated that L‐type VSCCs mediated ~50% of the nuclear and somatic (cytoplasmic) Ca 2+ elevation in response to strong synaptic stimulation. In contrast, relatively weak excitatory postsynaptic potentials (EPSPs ; ~15 mV) or single action potentials were much less effective at activating L‐type VSCCs. Antagonist experiments indicated that activation of the NMDA‐type glutamate receptor leads to a long‐lasting somatic depolarization necessary to activate L‐type VSCCs effectively during synaptic stimuli. Simulation of action potential and somatic EPSP depolarization using voltage‐clamp pulses indicated that nuclear Ca 2+ transients mediated by L‐type VSCCs were produced by sustained depolarization positive to ‐25 mV. In the absence of synaptic stimulation, action potential stimulation alone led to elevations in nuclear Ca 2+ mediated by predominantly non‐L‐type VSCCs. Our results suggest that action potentials, in combination with long‐lived synaptic depolarizations, facilitate the activation of L‐type VSCCs. This activity elevates somatic Ca 2+ levels that spread to the nucleus.