Ionic currents in the somatic membrane of rat dorsal root ganglion neurons—I. Sodium currents

Abstract

Measurements of sodium transmembrane ionic currents evoked by depolarizing shifts in membrane potential have been performed on isolated dorsal root ganglion neurons of 5–10-day-old rats. Potassium currents were eliminated by dialysing the neurons with potassium-free solutions. In 10–15% of investigated neurons a tetrodotoxin-resistant component has been revealed in the sodium inward current which differs in its potential-dependent and kinetic characteristics from the main tetrodotoxinsensitive one. The activation kinetics of the tetrodotoxin-sensitive sodium current could be described by the Hodgkin-Huxley model using the cubic power of the m-variable, whereas the activation kinetics of the tetrodotoxin-resistant one can be described using only the square power of m. The time constants of activation and inactivation of the tetrodotoxin-resistant current were about ten times longer than those of the tetrodotoxin-sensitive current. The tetrodotoxin-resistant current was highly sensitive to all extracellular agents which are known as effective blockers of calcium channels (Co 2+, Mn 2+, Cd 2+, D-600 and its derivatives). At the same time, the selectivity of the corresponding channels did not differ significantly from the selectivity of the tetrodotoxin-sensitive sodium channels. The sequence of relative permeabilities for univalent cations was P Na: P Li: P hydrazinium:P NH 4 :P hydroxylammonium:P K = 1.0:0.79:0.43:0.33:0.25:0.18 for the tetrodotoxin-sensitive channels and 1.0:0.98:0.47:0.42:0.26:0.26 for the tetrodotoxin-resistant ones. Thus, the tetrodotoxin-resistant sodium channels combine some features of sodium (the selective filter) and calcium (gating mechanism and binding properties) channels.