Critical dependence of cAMP response element-binding protein phosphorylation on L-type calcium channels supports a selective response to EPSPs in preference to action potentials

Activity-dependent gene expression in neurons shows a remarkable ability to differentiate between different types of stimulation: orthodromic inputs t hat engage synaptic transmission are much more effective than antidromic st imuli that do not. We have studied the basis of such selectivity in culture d hippocampal neurons in which nuclear cAMP response element-binding protei n (CREB) phosphorylation is induced by synaptic activity but not by action potential (AP) stimulation in the absence of EPSPs, although spikes by them selves generate large elevations in intracellular Ca2+. Previous work has s hown that Ca2+ entry through L-type Ca2+ channels plays a dominant role in triggering calmodulin mobilization and activation of calmodulin-dependent k inases that phosphorylate CREB, raising the possibility that L-type channel s contribute to the selective response to EPSPs rather than APs. Accordingl y, we performed voltage-clamp experiments to compare the currents carried b y L-type channels during depolarizing waveforms that approximated APs or de ndritic EPSPs. The integrated current generated by L-type channels was sign ificantly less after mock APs than with EPSP-like depolarizations. The diff erence was traced to two distinct factors. Compared with other channels, L- type channels activated at relatively negative potentials, favoring their o pening with EPSP stimulation; they also exhibited relatively slow activatio n kinetics, weighing against their contribution during an AP. The relative ineffectiveness of APs as a stimulus for CREB phosphorylation could be over come by exposure to the agonist Bay K8644, which potentiated the AP-induced influx through L-type channels by similar to 10-fold. Under normal conditi ons, the unique biophysical properties of L-type channels allow them to act as a kinetic filter to support spike-EPSP discrimination.