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

1. I have studied the electrical excitability of outgrowing processes of individual neurones in cultures made from dissociated neural plates of embryos of Xenopus laevis prior to the time of neurite outgrowth in vivo. 2. 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. 3. The ionic basis of the excitability of neurites was tested by recording from cells while changing the composition of the salines perfusing the cultures. 4. In cultures less than 10 hr old, all neurites tested made responses which depended on Ca2+. The action potentials of the cell bodies were also Ca2+-dependent at these times. 5. Between 10 and 12 hr in culture, a time at which the cell bodies still made Ca2+-dependent action potentials, neurites acquired the ability to make Na+-dependent responses. At these times, two-thirds of neurites tested retained the ability to produce divalent cation-dependent action potentials when perfused with solutions of isotonic Ba2+. 6. After 12 hr 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 hr in culture. Thus neurites sent out at different times in culture differed in their development of excitability. 7. Cell bodies making exclusively Ca2+-dependent action potentials could be found until about 15 hr in culture, after which time a Na+-dependent component appeared. Cell bodies could then be observed to make action potentials which depended on both 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). 8. The possibility that changes in the ionic basis of action potentials reflected the death of one 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. Such observations showed that the majority of cells in each culture were surviving throughout the period of study. 9. Thus the membranes of the neurites and cell bodies of neurones in these cultures appeared to undergo independently timed changes in the ionic basis of their action potentials.