Cs⁺ loading reveals Na⁺-dependent persistent inward current and negative slope resistance region in Aplysia giant neurons

The giant, nonbursting neurons R2 and LP1 of Aplysia californica were loaded iontophoretically with Cs+ to study the slow inward current and its ionic sensitivity, in isolation from K+ outward currents. In the relative absence of the normally large K+ outward currents, a persistent net inward current and comcomitant negative slope resistance region of the current-voltage relatonship was shown to be enhanced, if already present, or revealed, if not present prior to Cs+ loading. The slow inward current and negative slope resistance region of the CS+-loaded neurons showed a marked, concentration-dependent sensitivity to external Na+ concentration. Increases in external Ca2+ reduced the negative slope resistance region and slow inward current. However, both of these were insensitive to decreasing Ca2+ or even replacement of Ca2+ with Co2+. An increase in external Mg2+ had an effect similar to an increase in external Ca2+ on the slow inward current and negative slope resistance region. We conclude that a major portion of the slow inward current and negative slope resistance regions of these nonbursting neurons is caused by a slowly inactivating membrane conductance to Na+. The persistence of this current makes it part of the slow membrane current system affecting spike frequency during prolonged depolarizations.

Duke Scholars

Author Darrell Vincent Lewis Jr. Pediatrics, Neurology