Modulation of A-currents by [K+](o) in acutely isolated pyramidal neurones of juvenile rat entorhinal cortex and hippocampus

PYRAMIDAL neurones of the entorhinal cortex and of hippocampal areas CAI and CA3 were acutely isolated from juvenile rats. The effect of lowering the concentration of extracellular potassium ions ([K+](o)) on fast transient A-currents (I-A) was studied using the whole-cell configuration of the patch-clamp technique. Upon lowering [K+](o) from 5.4 to 0 mM amplitudes of I-A in all pyramidal neurones were reduced only at test potentials positive to -20 mV, but were increased at potentials below -20 mV. Omission of magnesium ions from the intracellular perfusate resulted in an increase of I-A in external K+-free solution at any potential tested. Moreover, hen [K+](o) was reduced steady-state activation and inactivation curves of I-A were shifted in the hyperpolarizing direction.