Potassium currents in isolated CA1 neurons of the rat after kindling epileptogenesis

Abstract

Daily tetanic stimulation of the Schaffer collaterals generates an epileptogenic focus in area CA1 of the rat hippocampus, ultimately leading to generalized tonic-clonic convulsions (kindling). Potassium currents were measured under voltage-clamp conditions in pyramidal neurons, acutely dissociated from the focus of fully kindled rats, one day and six weeks after the last generalized seizure. Their amplitude, kinetics, voltage dependence and calcium dependence were compared with controls. With Ca2+ influx blocked by 0.5 mM Ni2+, the sustained current (delayed rectifier) and the transient current (A-current) were not different after kindling. Calcium influx evoked an additional fast transient current component. This transient calcium-dependent current component was increased by 154%, but only immediately after the seizure. A second, slow calcium-dependent potassium current component was dependent on the intracellular calcium level, set by the pipette as well as on calcium influx. The peak amplitude of this slow calcium-dependent current was under optimal calcium conditions not different after kindling, but we found indications that either calcium homeostasis or the calcium sensitivity of the potassium channels was affected by the kindling process. In contrast to the previously described enhancement of calcium current, kindling epileptogenesis did not change the total potassium current amplitude. The minor changes that were observed can be related either to changes in calcium current or to changes in intracellular calcium homeostasis.