Objective Concentration of extracellular calcium （[Ca2＋]o） in the central nervous system decreases substantially in different conditions. It results in facilitating neuronal excitability. The goal of this study is to examine the mechanisms of enhanced neuronal excitation in low [Ca2＋]o in order to provide new clues to treat the hyperexcitability diseases in clinic. Methods Whole-cell patch-clamp technique and neuron culture were used in the study. Results The firing threshold of cultured hippocampal neurons decreased markedly in low [Ca2＋]o saline. Unexpectedly, apamine and isoprenaline, antagonists of medium afterhyperpolarization （mAHP） and slow AHP （sAHP） respectively, had no statistic significant effect on excitability of neurons. TTX at a low concentration was sufficient to inhibit/Nap, which blocked the increase of firing frequency in low [Ca2＋]o. It also reduced the number of spikes in normal [Ca2＋]o. Conclusion These results suggest that in cultured hippocampal neurons, modulation of spiking threshold but not AHP may cause the increased excitability in low [Ca2＋]o.
