The Scn8a gene encodes a neuronal, voltage-gated sodium channel, which is h
ighly expressed in both cerebellar Purkinje neurons and spinal motoneurons
[D.L. Burgess, D.C. Kohrman, J. Galt, N.W. Plummer, J.M. Jones, B. Spear, M
.H. Meisler, Mutation of a new sodium channel gene, Scn8a, in the mouse mut
ant 'motor endplate disease', Nature Genetics 10 (1995) 461-365; K.L. Schal
ler, D.M. Krzemien, P.J. Yarowsky, B.K. Krueger, J.H. Caldwell, A novel, ab
undant sodium channel expressed in neurons and glia, J. Neurosci. 15 (1995)
3231-3242]. Sodium channels in Purkinje cells produce an unusual, "resurge
nt" current when the cells are repolarized to intermediate potentials (-60
to -20 mV) following a strong depolarization that completely inactivates tr
ansient sodium current [I.M. Raman, L.K. Sprunger, M.H. Meisler, B.P. Bean,
Altered subthreshold sodium currents and disrupted firing patterns in Purk
inje neurons of Scn8a mutant mice, Neuron 19 (1997) 881-891; I.M. Raman, B.
P. Bean, Resurgent sodium current and action potential formation in dissoci
ated cerebellar Purkinje neurons, J. Neurosci. 17 (1997) 4517-4526]. Here,
we have examined whether large spinal neurons (predominantly motoneurons),
isolated from P6-P8 mice and cultured overnight, produce sodium currents re
sembling those either of Purkinje cells or of Xenopus oocytes after heterol
ogous expression of Scn8a. We found that P10-P14 Purkinje cells exhibited r
esurgent current (ranging from -3.6 to -15.4 pA/pF in 16 cells at -40 mV),
but cultured spinal neurons had little or no such current (< 0.5 pA/pF in 1
3 of 16 cells; -1.2 to -2.3 pA/pF in three of 16 cells). Furthermore, unlik
e Scn8a channels heterologously expressed in Xenopus oocytes [M.R. Smith, R
.D. Smith, N.W. Plummer, M.H. Meisler, A.L. Goldin, Functional analysis of
the mouse Scn8a sodium channel. J. Neurosci. 18 (1998) 6093-6102], there wa
s not a prominent component of persistent sodium current in either Purkinje
neurons or large spinal neurons. Based on analysis of cells from mice with
a Scn8a null mutation, Scn8a channels appear to contribute significantly t
o total sodium current in both in P10-P14 Purkinje cells (similar to 40%; [
21]) and cultured P7-P8 spinal motoneurons (similar to 70% [K.D. Garcia, L.
K. Sprunger, M.H. Meisler, K.G. Beam, The sodium channel Scn8a is the major
contributor to the postnatal developmental increase of sodium current dens
ity in spinal motoneurons, J. Neurosci. 18 (1998) 5234-5239]). Thus, the pr
esence or absence of resurgent current, and of persistent sodium current, a
ppears to depend on cellular factors other than the mere presence of the Sc
n8a transcript. (C) 1999 Elsevier Science B.V. All rights reserved.