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 mu M tetrodotoxin (TTX), 50 mu M 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 Ca2+-dependent K+ current (I-C), and 50 mu M carbachol, which depresses the I-M current. These cut-rents 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 deinactivation 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 mu M), which indicates the occurrence of an I-D-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,hr-Ar-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 mu M BAPTA-aminoethoxy, revealing a large-amplitude, rapidly inactivating outward current. This residual component is insensitive to 50 mu M 4-aminopyridine and may include a current more related to the I-A-type. Our data provide evidence that, in cultured cortical neurons from rat, the expression of an I-D-like K+ current is highly dependent on internal Ca2+ concentration.