Protein kinase C activation decreases activity-dependent attenuation of dendritic Na+ current in hippocampal CA1 pyramidal neurons.

Action potentials recorded from the soma of CA1 pyramidal neurons remain relatively uniform in amplitude during repetitive firing. In contrast, the amplitudes of back-propagating action potentials in dendrites decrease progressively during a spike train. This activity-dependent decrease in amplitude is dependent on the frequency of firing during the train and distance from the soma. Previously, we described a property of Na+ channels that provides a plausible mechanism for the activity dependence of the amplitude of the dendritic action potentials: available Na+ current decreases during trains of action potentials through an inactivation, distinct from fast inactivation, that appears rapid in onset, but slow and voltage-dependent in its recovery. In this study we found that activation of protein kinase C by phorbol esters decreased this activity-dependent inactivation of pharmacologically isolated Na+ current in cell-attached dendritic, but not somatic, patches. Similarly in whole cell recordings phorbol esters decreased the attenuation of back-propagating dendritic action potentials during trains. These results indicate a novel effect of protein kinase C on the dendritic Na+ channel and further support the hypothesis that the activity dependence of the dendritic action potentials is derived from the inactivation properties of Na+ channels.