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

Abstract The experiments have shown that the introduction of cyclic adenosine monophosphate, adenosine 5'-triphosphate and Mg 2+ ions into dialysed isolated dorsal root ganglion neurons of 5–8 day-old rats not only prevents the rapid decline of calcium inward currents during the course of dialysis but restores to a considerable extent the calcium conductance. Introduction of adenosine 5'-triphosphate and Mg 2+ has a much weaker stabilizing effect. This finding made it possible to separate and to investigate in detail the calcium current (I Ca ) in the somatic membrane of all investigated neurons. The maximal amplitude of I Ca was proportional to the concentration of Ca 2+ ions in the extracellular solution between 2 and 14mM; with higher concentration a saturation effect was observed. Replacement of Ca 2+ by Ba 2+ caused about a two-fold increase in the maximal amplitude of inward currents. Addition of Co 2+ , Mn 2+ , verapamil and related substances blocked the calcium current. The activation kinetics of I Ca could be approximated by a modified Hodgkin-Huxley equation using a square power of the m-variable. The activation time constant τ m changed in the range from 16 to 1.8 ms with testing potential change from −40 to +20 mV. The inactivation of I Ca was extremely slow; the half value of steady-state inactivation was observed at holding potential about −60 mV. The potential-dependent and kinetic characteristics of the calcium currents obtained on several neurons from adult rats were similar to those for neurons of new-born ones. It is concluded that the somatic membrane of the rat neurons has a system of electrically-operated selective calcium channels, the normal functioning of which is dependent on intracellular cyclic nucleotide metabolism.