Membrane currents in small cultured rat hippocampal neurons: a voltage-clamp study.

1. The currents underlying the graded impulses in small cultured hippocampal neurons from rat embryos were analysed under voltage-clamp conditions with the tight-seal whole-cell recording technique. 2. The leak and capacitative currents induced by a potential step were linearly related to the potential in the range studied (-60 to -100 mV). 3. With steps to potentials more positive than -40 mV, at least two different potential-activated currents were detected: an initial transient current and a delayed sustained one. In addition, 40% of the cells studied showed a delayed transient current. 4. The initial transient current showed sigmoid activation and roughly exponential inactivation. Its reversal potential depended on the Na+ concentration and was close to the Na+ equilibrium potential. Further, it was blocked by 3.0 microM-tetrodotoxin, and was abolished when choline was substituted for Na+ in the extracellular solution. We concluded that this current was carried mainly by Na+ ions. 5. The delayed sustained current showed sigmoid activation and almost no inactivation within 40 ms. The reversal potential was close to the K+ equilibrium potential. We concluded that this current was carried mainly by K+ ions. 6. The delayed transient current was outward in the potential range studied (-50 to +120 mV) and did not depend on the pipette Cl- concentration. It was assumed that this current was carried mainly by K+ ions. 7. A quantitative description of the initial transient and the delayed sustained currents was developed on the basis of earlier descriptions of excitable membranes.