The study demonstrates increased sodium channel density at the axon initial segment of cultured spinal cord neurons, providing a basis for the lower threshold for action potential generation in vivo.
Sodium channels in cultured neural cells were localized by light microscopic autoradiography of specifically bound 125I-scorpion toxin. Ninety percent of the cell-bound 125I-scorpion toxin was associated specifically with sodium channels as assessed by the blocking of autoradiographic labeling by unlabeled scorpion toxin and by depolarization. Sodium channels were distributed uniformly in the surface membrane of neurites and cell bodies of both morphologically differentiated and undifferentiated cells of clone N18 of mouse neuroblastoma C1300. Sodium channels were distributed nonuniformly in many cultured spinal cord neurons. Visual observation indicated that 37 +/- 5% of cultured spinal cord neurons had a higher sodium channel density on the initial segment of one or more neurites than on the cell body. For these neurons, the density on one neurite initial segment averaged 7.4 +/- 1.9-fold greater than on the adjacent cell body. This increased sodium channel density may be the basis of the lower threshold for action potential generation at the axon initial segment of motor neurons and some spinal interneurons in vivo.
WA Catterall (Wed,) studied this question.