Effect of lamotrigine on the Ca2+-sensing cation current in cultured hippocampal neurons

Concentrations of extracellular calcium ([Ca2+](e)) in the CNS decrease sub stantially during seizure activity. We have demonstrated previously that de creases in [Ca2+](e) activate a novel calcium-sensing nonselective cation ( csNSC) channel in hippocampal neurons. Activation of csNSC channels is resp onsible for a sustained membrane depolarization and increased neuronal exci tability. Our study has suggested that the csNSC channel is likely involved in generating and maintaining seizure activities. In the present study, th e effects of anti-epileptic agent lamotrigine (LTG) on csNSC channels were studied in cultured mouse hippocampal neurons using patch-clamp techniques. At a holding potential of -60 mV, a slow inward current through csNSC chan nels was activated by a step reduction of [Ca2+](e) from 1.5 to 0.2 mM. LTG decreased the amplitude of csNSC currents dose dependently with an IC50 of 171 +/- 25.8 (SE) muM. The effect of LTG was independent of membrane poten tial. In the presence of 300 muM LTG, the amplitude of csNSC current was de creased by 31 +/- 3% at -60 mV and 29 +/- 2.9% at +40 mV (P > 0.05). LTG de pressed csNSC current without affecting the potency of Ca2+ block of the cu rrent (IC50 for Ca2+ block of csNSC currents in the absence of LTG: 145 +/- 18 muM; in the presence of 300 muM LTG: 136 +/- 10 muM. n = 5, P > 0.05). In current-clamp recordings, activation of csNSC channel by reducing the [C a2+](e) caused a sustained membrane depolarization and an increase in the f requency of spontaneous firing of action potentials. LTG (300 muM) signific antly inhibited csNSC channel-mediated membrane depolarization and the exci tation of neurons. Fura-2 ratiometric Ca2+ imaging experiment showed that L TG also inhibited the increase in intracellular Ca2+ concentration induced by csNSC channel activation. The effect of LTG on csNSC channels may partia lly contribute to its broad spectrum of anti-epileptic actions.