Neocortical potassium currents are enhanced by the antiepileptic drug lamotrigine

Purpose: We used field-potential recordings in slices of rat cerebral cortex along with whole-cell patch recordings from rat neocortical cells in culture to test the hypothesis that the antiepileptic drug (AED) lamotrigine (LTG) modulates K+-mediated, hyperpolarizing currents. Methods: Extracellular field-potential recordings were performed in neocortical slices obtained from Wistar rats aged 25-50 days. Rat neocortical neurons in culture were subjected to the whole-cell mode of voltage clamping under experimental conditions designed to study voltage-gated K+ currents. Results: In the in vitro slice preparation, LTG (100-400 muM) reduced and/or abolished epileptiform discharges induced by 4-aminopyridine (4AP, 100 muM; n = 10), at doses that were significantly higher than those required to affect epileptiform activity recorded in Mg2+-free medium (n = 8). We also discovered that in cultured cortical cells, LTG (100-500 muM; n = 13) increased a transient, 4AP-sensitive, outward Current elicited by depolarizing commands in medium containing voltage-gated Ca2+ and Na+ channel antagonists. Moreover, we did not observe any change in a late, tetraethylammonium-sensitive outward Current. Conclusions: Our data indicate that LTG, in addition to the well-known reduction of voltage-gated Na+ currents, potentiates 4AP-sensitive, K+-mediated hyperpolarizing conductances in cortical neurons. This mechanism of action contributes to the anticonvulsant effects exerted by LTG in experimental models of epileptiform discharge, and presumably in clinical practice.