Action potentials and sodium inward currents of developing neurons in Xenopus nerve-muscle cultures.

Action potentials and voltage-gated Na+ inward currents from cultured embryonic neurons of Xenopus laevis were recorded using the patch-clamp technique in the whole cell configuration. Neurons together with muscle cells were dissociated from embryos shortly after completion of gastrulation. Under the voltage-clamp condition the voltage-gated Na+ inward current was isolated from other currents by pharmacological means and by ion substitution. A small Na+ current was observed in round cells without neurites (presumptive neurons). The mean amplitude of the peak Na+ current was 2.5 times larger in neurons with short processes than in presumptive neurons. As they developed further by extending longer processes, the maximum amplitude of the Na+ inward current recorded at the soma decreased. In varicosities, the Na+ inward current density was greater than that at the soma of neurons with extended neurites but kinetic properties and voltage-dependency were similar.