Effect of lamotrigine on the Ca(2+)-sensing cation current in cultured hippocampal neurons

Concentrations of extracellular calcium ([Ca(2+)](e)) in the CNS decrease substantially during seizure activity. We have demonstrated previously that decreases in [Ca(2+)](e) activate a novel calcium-sensing nonselective cation (csNSC) channel in hippocampal neurons. Activation of csNSC channels is responsible for a sustained membrane depolarization and increased neuronal excitability. Our study has suggested that the csNSC channel is likely involved in generating and maintaining seizure activities. In the present study, the 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 channels was activated by a step reduction of [Ca(2+)](e) from 1.5 to 0.2 mM. LTG decreased the amplitude of csNSC currents dose dependently with an IC(50) of 171 +/- 25.8 (SE) microM. The effect of LTG was independent of membrane potential. In the presence of 300 microM LTG, the amplitude of csNSC current was decreased by 31 +/- 3% at -60 mV and 29 +/- 2.9% at +40 mV (P > 0.05). LTG depressed csNSC current without affecting the potency of Ca(2+) block of the current (IC(50) for Ca(2+) block of csNSC currents in the absence of LTG: 145 +/- 18 microM; in the presence of 300 microM LTG: 136 +/- 10 microM. n = 5, P > 0.05). In current-clamp recordings, activation of csNSC channel by reducing the [Ca(2+)](e) caused a sustained membrane depolarization and an increase in the frequency of spontaneous firing of action potentials. LTG (300 microM) significantly inhibited csNSC channel-mediated membrane depolarization and the excitation of neurons. Fura-2 ratiometric Ca(2+) imaging experiment showed that LTG also inhibited the increase in intracellular Ca(2+) concentration induced by csNSC channel activation. The effect of LTG on csNSC channels may partially contribute to its broad spectrum of anti-epileptic actions.