The development of action potentials in cultures of explanted cortical neurons from chick embryos.

Action potentials of explanted cortical neurons from 7- to 9-day chick embryos were investigated at different stages during culturing. The maximum rates of rise of action potentials gradually increased and reached a plateau level at about 1 month in culture. Action potentials were resistant to 10(-7) g/ml tetrodotoxin (TTX) at early stages, became sensitive to TTX in an age-dependent manner, and almost all action potentials were blocked by this concentration of TTX after 25 days. However, 10(-5) g/ml TTX suppressed action potentials at early stages. When action potentials were suppressed by TTX, impulses could still be obtained from immature neurons following addition of 10 mM Ca2+ to the saline. This effect was not observed in mature neurons. The application of 10 mM Mn2+ frequently enhanced action potentials in immature neurons. This effect was not blocked by TTX (10(-5) g/ml) or the removal of external Na+. These results suggest that action potentials in immature neurons depend on Ca2+ and Na+, that Mn2+ can pass through Ca channels, that the sensitivity to TTX develops as the contribution by Na+ becomes greater, and that the contribution by Ca2+ diminishes during maturation.