Specific distribution of sodium channels in axons of rat embryo spinal motoneurones

1. The distribution of Na+ channels and development of excitability were in vestigated in vitro in purified spinal motoneurones obtained from rat embry os at E14, using electrophysiological, immunocytochemical and autoradiograp hical methods. 2. One hour after plating the motoneurones (DIV0), only somas were present. They expressed a robust delayed rectifier K+ current (I-DR) and a fast-ina ctivating A-type K+ current (I-A). The rapid neuritic outgrowth was paralle led by the emergence of a fast-activating TTX-sensitive sodium current (I-N a), and by an increase in both K+ currents. 3. The change in the three currents was measured daily, up to DIV8. The lar ge increase in I-Na observed after DIV2 was accompanied by the onset of exc itability Spontaneous activity was observed as from DIV6. 4. The occurrence of axonal differentiation was confirmed by the fact that (i) only one neurite per motoneurone generated antidromic action potentials ; and (ii) I-125-alpha-scorpion toxin binding, a specific marker of Na+ cha nnels, labelled only one neurite and the greatest density was observed in t he initial segment. Na+ channels therefore selectively targeted the axon an d were absent from the dendrites and somas. 5. The specific distribution of Na+ channels was detectable as soon as the neurites began to grow. When the neuritic outgrowth was blocked by nocodazo le, no I-Na developed. 6. It was concluded that, in spinal embryonic motoneurone in cell culture, Na+ channels, the expression of which starts with neuritic differentiation, are selectively addressed to the axonal process, whereas K+ channels are p resent in the soma prior to the neuritic outgrowth.