The absence of resurgent sodium current in mouse spinal neurons

Abstract

The Scn8a gene encodes a neuronal, voltage-gated sodium channel, which is highly 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 mutant `motor endplate disease', Nature Genetics 10 (1995) 461–465; K.L. Schaller, D.M. Krzemien, P.J. Yarowsky, B.K. Krueger, J.H. Caldwell, A novel, abundant sodium channel expressed in neurons and glia, J. Neurosci. 15 (1995) 3231–3242]. Sodium channels in Purkinje cells produce an unusual, “resurgent” current when the cells are repolarized to intermediate potentials (−60 to −20 mV) following a strong depolarization that completely inactivates transient sodium current [I.M. Raman, L.K. Sprunger, M.H. Meisler, B.P. Bean, Altered subthreshold sodium currents and disrupted firing patterns in Purkinje neurons of Scn8a mutant mice, Neuron 19 (1997) 881–891; I.M. Raman, B.P. Bean, Resurgent sodium current and action potential formation in dissociated 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 resembling those either of Purkinje cells or of Xenopus oocytes after heterologous expression of Scn8a. We found that P10–P14 Purkinje cells exhibited resurgent 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 13 of 16 cells; −1.2 to −2.3 pA/pF in three of 16 cells). Furthermore, unlike 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 was 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 to total sodium current in both in P10–P14 Purkinje cells (∼40%; [21]) and cultured P7–P8 spinal motoneurons (∼70% [K.D. Garcı́a, 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 density in spinal motoneurons, J. Neurosci. 18 (1998) 5234–5239]). Thus, the presence or absence of resurgent current, and of persistent sodium current, appears to depend on cellular factors other than the mere presence of the Scn8a transcript.