Developmental changes in the inward current of the action potential of Rohon‐Beard neurones

Abstract

Rohon-Beard cells in the spinal cord of Xenopus laevis tadpoles were studied in animals from early neural tube to free-swimming larval stages. The onset and further development of electrical excitability of these neurones was investigated in different ionic environments, to determine the ionic species carrying the inward current of the action potential. The cells were inexcitable at early stages (Nieuwkoop and Faber stages 18-20) and did not give action potentials to depolarizing current pulses. The action potential was first recorded at stage 20. The inward current was carried by Ca2+ at stages 20-25, since it was blocked by mM quantities of La3+, Co2+ or Mn2+ and was unaffected by removal of Na+ or the addition of tetrodotoxin (TTX). The action potential was an elevated plateau of long duration (mean 190 ms at stages 20-22). The duration decreased exponentially with repetitive stimulation. The specific Ca2+ conductance (gCa) at the onset of the plateau of the action potential was 2.6 .times. 10-4 mho[reciprocal of ohm]/cm2. A single action potential rose [Ca2+]i [intracellular Ca concentration] by more than 100-fold. At later times (stages 25-40), the inward current of the action potential was carried by both Na+ and Ca2+: the action potential had 2 components, an initial spike which is blocked by removal of Na+ or addition of TTX, followed by a plateau which is blocked by La3+, Co2+ or Mn2+. Finally (stages 40-51), the inward current was primarily carried by Na+, since the action potential was blocked only by removal of Na+ or addition of TTX and the overshoot agreed with the prediction of the Nernst equation for a Na-selective membrane. When the outward current channel was blocked and cells exposed to Na-free solutions, 67% of cells at the latest stages studied were incapable of producing action potentials in which the inward current was carried by divalent cations. The duration of the action potential decreased from a maximum of about 1000 ms to about 1 ms during development. The maximum input resistance decreased from 1000-100 m.OMEGA.. The Ca action potential may play a role in the development of excitability and the growth of the neurons.