AGE-DEPENDENT VARIATIONS IN POTASSIUM SENSITIVITY OF A-CURRENTS IN RAT HIPPOCAMPAL-NEURONS

Hippocampal pyramidal neurons were either cultured from prenatal rats or acutely isolated from the brain of newborn and juvenile rats. The i nfluence of lowering the concentration of the extracellular potassium concentration ([K+](0)) on isolated fast transient outward K+ currents (I-A) was studied in these neurons using the patch clamp technique in the whole cell configuration. With respect to the response of I-A to lowering [K+](0), three types of cells were observed. The first subpop ulation of neurons was characterized by a complete suppression of I-A over the whole voltage range under potassium-free solutions (type A ne urons). A second proportion of cells showed an increase of I-A at test pulses below -0 mV and a decrease of I-A at voltages above -0 mV (typ e B neurons). In a third group of neurons, amplitudes of I-A increased al all potentials tested during omission of potassium ions from the e xtracellular superfusate (type C neurons). Whereas type A and type B n eurons were preferentially found in freshly plated cultures and newbor n rats, the majority of type C cells was detected in long-term culture s and in animals of older ages. Thus, hippocampal A-currents lose thei r sensitivity to extracellular potassium ions during early ontogenesis .