Effects of Ca ions on action potentials in immature cultured neurons from chick cerebral cortex

Action potentials evoked by depolarizing pulses were studied in immature cultured cerebral cortical neurons from chick embryos. The majority of action potentials were rather small, and they were still elicited in the presence of 10(-7) gm/ml tetrodotoxin (TTX), but were almost completely abolished in NA+-free solution or by 10(-5) gm/ml TTX in Tyrode's solution. The elevation of external Ca2+ concentration not only increased the maximum rates of rise of action potentials in normal Tyrode's solution with and without low (10(-7) gm/ml) TTX but also regenerated action potentials in high (10(-5) gm/ml) TTX-containing Tyrode's solution or in Na+-free solution. These high Ca2+ effects were blocked by Mn2+ or Co2+. These results suggest that action potentials, which were predominantly Na-dependent, are partially contributed by Ca ions in immature chick cerebral cortical neurons.