DEVELOPMENT OF ACTION POTENTIAL MECHANISM OF AMPHIBIAN NEURONS ISOLATED IN CULTURE

Nerve and muscle cells differentiated morphologically, in cultures of dissociated cells prepared from amphibian neural plate and underlying mesoderm (Xenopus laevis, Nieuwkoop and Faber stage 15). Cultures were grown in a defined medium containing sterile Steinberg''s salt solution and 0.1% bovine serum albumin, and maintained for periods up to 5 days. These neurons were capable of generating action potentials as early as 8 h after explantation. Electrophysiological determination of the ions responsible for the regenerative inward current of the action potential of these cells in vitro showed a temporal sequence similar to that of Rhohon-Beard neurons differentiationg in vivo: in early cultures the inward current of the action potential was carried chiefly by Ca2+; later both Na+ and Ca2+ were involved, and finally most of the inward current was carried by Na+. The composition of the neuronal population was studied by determining birthdates using autoradiography. Previous work identified a group of neurons including Rohon-Beard cells, extramedullary neurons and large ventral neurons, that are born before the end of gastrulation (stage 13). Both labeled and unlabeled neurons were found in cultures taken from embryos at stage 15 which were injected with [3H]thymidine at stage 13. This indicates that in addition to the group of neurons born by stage 13 (unlabeled), others, in S-phase between stages 13-15 (labeled), were included.