Calcium-dependent, slowly inactivating potassium currents in cultured neurons of rat neocortex

Slowly inactivating outward currents were examined in neurons from rat anterior cortex dissociated at postnatal day 1 and recorded after 7-48 days in vitro by the use of whole-cell patch-clamp technique, in the presence of 0.5-0.8 microM tetrodotoxin (TTX). 50 microM carbachol and 1-5 mM CsCl2. Experiments were often carried out in the additional presence of 1-5 mM CsCl2, which blocks the anomalous, inwardly rectifying IQ, the fast Ca(2+)-dependent K+ current (IC), and 50 microM carbachol, which depresses the IM current. These currents were evoked by depolarizing steps to -40 +/- 5 mV from a conditioning hyperpolarization to -110 +/- 10 mV. Their sensitivity to elevation from 2.5 to 12.5 mM in extracellular K+ concentration, together with their sensitivity to 5-15 mM tetraethylammonium, suggests that they are mainly carried by K+ ions. Their activation and inactivation curves show that the threshold for activation is -65 mV, that their inactivation is achieved at -75 mV and that potentials more negative than -120 mV are needed to abolish it. The time-dependence of de-inactivation gives a maximal current amplitude for conditioning hyperpolarizations of 2 s and is best described by a monoexponential function with a time constant of 0.7 s. Slow transient K+ currents were depressed by low doses of 4-aminopyridine (30-100 microM), which indicates the occurrence of an ID-type component in the recorded K+ currents. No slowly declining K+ current was expressed when a recording solution containing 10 mM 1,2-bis (2-aminophenoxy)ethane-N,N,N'-N'-tetraacetic acid (BAPTA), instead of 1-5 mM BAPTA, was used. When recorded without Ca2+ chelator in the pipette, slowly declining K+ currents were blocked by bath-applied 40-50 microM BAPTA-aminoethoxy, revealing a large-amplitude, rapidly inactivating outward current. This residual component is insensitive to 50 microM 4-aminopyridine and may include a current more related to the IA-type. Our data provide evidence that, in cultured cortical neurons from rat, the expression of an ID-like K+ current is highly dependent on internal Ca2+ concentration.