Phosphorylation of the alpha subunit of sodium channels by protein kinase C (PKC) results in reduced peak current and altered current inactivation kinetics, and causes leftward shift in the steady-state inactivation curve of sodium currents. We studied the PKC activity-dependence of the inhibitory effect of lamotrigine on voltage-gated sodium channels in vitro by using whole-cell patch clamp electrophysiology in cultured spinal cord neurons. Sodium channel inhibitory action of lamotrigine was investigated alone and in the presence of the PKC activator phorbol myristil acetate (PMA). Sodium currents were elicited by 8-ms-long rectangular step-depolarizations to 0 mV from -60 mV at 10-second intervals. Lamotrigine decreased sodium currents in a concentration-dependent manner, with an IC50 of 19.7±7.9 mM. In the presence of PMA (0.2 mM) lamotrigine proved to be ten-times more potent (IC50 was 1.9±0.7 mM). The effect of PMA was prevented by the PKC antagonist chelerythrine (1 mM). PMA did not change significantly the shift of the steady state inactivation curve caused by lamotrigine (300 mM; -16.3±1.1 and -18.0±1.3 mV in the presence and absence of PMA, respectively). Our results suggest that the effectiveness of lamotrigine is highly dependent on PKC-mediated phosphorylation of voltage gated sodium channels. The underlying mechanism probably involves a shift in the balance from resting to inactivated or activated states of the ion channel.