Functional Distribution of Three Types of Na⁺ Channel on Soma and Processes of Dorsal Horn Neurones of Rat Spinal Cord

Abstract

Voltage‐gated Na+ channels and their distribution were studied by the patch‐clamp technique in intact dorsal horn neurones in slices of newborn rat spinal cord and in neurones isolated from the slice by slow withdrawal of the recording pipette. This new method of neurone isolation was further used to study the roles of soma and axon in generation of action potentials. Tetrodotoxin (TTX)‐sensitive Na+ currents in intact neurones consisted of three components. A fast component with an inactivation time constant (τf) of 0.6–2.0 ms formed the major part (80–90%) of the total Na+ current. The remaining parts consisted of a slowly inactivating component (τs of 5–20 ms) and a steady‐state component. Single fast and slow inactivating Na+ channels with conductances of 11.6 and 15.5 pS, respectively, were identified in the soma of intact neurones in the slice. Steady‐state Na+ channels were not found in the soma, suggesting an axonal or dendritic localization of these channels. In the whole‐cell recording mode, the entire soma of a dorsal horn neurone could be isolated from the slice by slow withdrawal of the recording pipette, leaving all or nearly all of its processes in the slice. The isolated structure was classified as: (1) ‘soma’ if it lost all of its processes, (2) ‘soma+axon’ complex if it preserved one process and at least 85% of its original peak Na+ current or (3) ‘soma+dendrite’ complex if it preserved one process but the remaining Na+ current did not exceed those observed in the isolated ‘somata’. The spatial distribution of Na+ channels in the neurone was studied by comparing Na+ currents recorded before and after isolation. The isolated ‘soma’ contained 13.8 ± 1.3% of inactivating Na+ current but no steady‐state Na+ current. ‘Soma+axon’ complexes contained 93.6 ± 1.4% of inactivating and 46% of steady‐state Na+ current. In current‐clamp experiments, the intact neurones and isolated ‘soma+axon’ complexes responded with ‘all‐or‐nothing’ action potentials to current injections. In contrast, isolated ‘somata’ showed only passive or local responses and were unable to generate action potentials. It is concluded that dorsal horn neurones of the spinal cord possess three types of TTX‐sensitive voltage‐gated Na+ channels. The method of entire soma isolation described here shows that the majority of inactivating Na+ channels are localized in the axon hillock and only a small proportion (ca 1/7) are distributed in the soma. Steady‐state Na+ channels are most probably expressed in the axonal and dendritic membranes. The soma itself is not able to generate action potentials. The axon or its initial segment plays a crucial role in the generation of action potentials.

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

“…In this preparation, however, the whole-cell Na¤ current is activated sharply (Takahashi, 1990), which suggests that the somatic Na¤ channel density is far too low to generate an action potential. A similar distribution of Na¤ channels was observed in rat spinal neurones of the dorsal horn, in which axotomy almost completely abolished the Na¤ currents (Safronov et al 1997). This steep somato-axonal Na¤ channel gradient in spinal neurones seems to be specific to mammals, since in Xenopus motoneurones, the Na¤ current was gradually activated, which suggests that Na¤ channels were distributed in both somatic and axonal membranes (O'Dowd et al 1988).…”

Role of TTX-Sensitive and TTX-Resistant Sodium Channels in Aδ- and C-Fiber Conduction and Synaptic Transmission

“…TTX-S component of the C-fiber CAPC was blocked in a concentration range between 1 and 100 nM with IC 50 of 14 -33 nM. These IC 50 values are typical for TTX-S Na ϩ channels in a number of preparations (Ogata and Tatebayashi 1993;Safronov et al 1997;Takahashi 1990). Slightly higher sensitivity obtained in our experiments for myelinated nerves might reflect structural differences in fiber organization.…”

“…When the preparation was positioned in the recording chamber, care was taken to avoid direct shadow imposed by the dorsal root on the cut spinal cord surface. Tight-seal whole cell recordings from spinal dorsal horn neurons were done as previously described Safronov et al 1997).…”

“…TTX-R Na ϩ channels were found in small-diameter dorsal root ganglion (DRG) neurons giving origin to unmyelinated C-and weakly myelinated A␦-fiber axons. More recent studies showed that both TTX-S and TTX-R currents are formed by multiple types of Na ϩ channels with distinct electrophysiological properties (Rush et al 1998;Scholz et al 1998) and gene origin (Akopian et al 1996;Dib-Hajj et al 1998;Sangameswaran et al 1996;Tate et al 1998;Toledo-Aral et al 1997).…”

Integration Time in a Subset of Spinal Lamina I Neurons Is Lengthened by Sodium and Calcium Currents Acting Synergistically to Prolong Subthreshold Depolarization

“…A persistent sodium current has been described in lamina I neurons (Safronov et al, 1997;Safronov, 1999), but its differential distribution among cell types and its functional significance remain unknown. Calcium currents can also amplify synaptic depolarization (Gillessen and Alzheimer, 1997; Urban et al, 1998;Crill, 1999) and are involved in generating plateau potentials in deeper dorsal horn neurons (Morisset and Nagy, 1999).…”