Activation of nuclear calcium dynamics by synaptic stimulation in culturedcortical neurons

L-type voltage-sensitive Ca2+ channels (VSCCs) are enriched on the neuronal soma and trigger gene expression during synaptic activity. To understand b etter how these channels regulate somatic and nuclear Ca2+ dynamics, we hav e investigated Ca2+ influx through L-type VSCCs following synaptic stimulat ion, using the long-wavelength Ca2+ indicator fluo-3 combined with laser sc anning confocal microscopy, Single synaptic stimuli resulted in rapid Ca2transients in somatic cytoplasmic compartments (<5 ms rise time). Nuclear C a2+ elevations lagged behind cytoplasmic levels by similar to 60 ms, consis tent with a dependence on diffusion from a cytoplasmic source. Pharmacologi cal experiments indicated that L-type VSCCs mediated similar to 50% of the nuclear and somatic (cytoplasmic) Ca2+ elevation in response to strong syna ptic stimulation. In contrast, relatively weak excitatory postsynaptic pote ntials (EPSPs; similar to 15 mV) or single action potentials were much less effective at activating L-type VSCCs, Antagonist experiments indicated tha t activation of the NMDA-type glutamate receptor leads to a long-lasting so matic depolarization necessary to activate L-type VSCCs effectively during synaptic stimuli. Simulation of action potential and somatic EPSP depolariz ation using voltage-clamp pulses indicated that nuclear Ca2+ transients med iated by L-type VSCCs were produced by sustained depolarization positive to -25 mV, In the absence of synaptic stimulation, action potential stimulati on alone led to elevations in nuclear Ca2+ 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 V SCCs, This activity elevates somatic Ca2+ levels that spread to the nucleus .