Phosphorylation-dependent differences in the activation properties of distal and proximal dendritic Na+ channels in rat CA1 hippocampal neurons

At distal dendritic locations, the threshold for action potential generation is higher and the amplitude of back-propagating spikes is decreased. To study whether these characteristics depend upon Na+ channels, their voltage-dependent properties at proximal and distal dendritic locations were compared in CA1 hippocampal neurons. Distal Na+ channels activated at more hyperpolarized voltages than proximal (half-activation voltages were -20.4 +/- 2.4 mV vs. -12.0 +/- 1.7 mV for distal and proximal patches, respectively, n = 16, P < 0.01), while inactivation curves were not significantly different. The resting membrane potential of distal regions also appeared to be slightly but consistently more hyperpolarized than their proximal counterpart. Staurosporine, a nonselective protein kinase inhibitor, shifted the activation curves for both proximal and distal Na+ channels to the left so that they overlapped and also caused the resting potentials to be comparable. Staurosporine affected neither the inactivation kinetics of Na+ currents nor the reversal potential for Na+. These results suggest that the difference in the voltage dependence of activation of distal and proximal Na+ channels can be attributed to a different phosphorylation state at the two locations.