Electrical excitability of outgrowing neurites of embryonic neurones in cultures of dissociated neural plate of Xenopus laevis.

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Abstract
The electrical excitability of outgrowing processes of individual neurons in cultures made from dissociated neural plates of embryos of X. laevis prior to the time of neurite outgrowth in vivo were studied. The electrical excitability of neurites was tested by stimulating them extracellularly and recording responses with an intracellular electrode in their cell bodies; neurites were excitable at all times examined. The ionic basis of the excitability of neurites was recorded from cells while changing the composition of the salines perfusing the cultures. In cultures less than 10 h old, all neurites tested made responses depending on Ca2+. The action potentials of the cell bodies were Ca2+-dependent at these times. Between 10-12 h in culture, when the cell bodies still made Ca2+-dependent action potentials, neurites acquired the ability to make Na+-dependent responses. At these times, 2/3 of neurites tested retained the ability to produce divalent cation-dependent action potentials when perfused with isotonic Ba2+ solutions. After 12 h in culture no neurites were observed to make Ca2+- or Ba2+-dependent responses; only Na+-dependent responses were observed. Cells continued to initiate and elongate new neurites until about 24 h in culture. Neurites sent out at different times in culture differed in their development of excitability. Cell bodies making exclusively Ca2+-dependent action potentials were found until about 15 h in culture, after which time a Na+-dependent component appeared. Cell bodies were then observed to make action potentials depending on Ca2+ and Na+ until about 3 days in culture. After 3 days, most cell bodies made predominately Na+-dependent action potentials. Unlike the neurites, cell bodies retained the ability to make action potentials in isotonic Ba2+ for as long as the cultures were maintained (up to 5 days). The possibility of changes in the ionic basis of action potentials reflecting the death of 1 population of cells and the simultaneous appearance of another population with different properties was eliminated by observing the fate of single cells while changes in the physiological properties were occurring. Most of the cells in each culture survived throughout the study period. The membranes of the neurites and cell bodies of neurons in these cultures appeared to undergo independently timed changes in the ionic basis of their action potentials.