POTASSIUM CURRENT EXPRESSION DURING PRENATAL CORTICOGENESIS IN THE RAT

Using in situ patch-clamp techniques, we have studied K current expres sion in rat telencephalon from embryonic day 12 to 21. For cells recor ded in the ventricular zone, the K current consisted of a delayed rect ifier and a large-conductance calcium-activated component, and display ed little variation from embryonic day 12 to 21. Cells recorded in pia l regions could be separated into two classes: radially oriented, puta tively migrating cells, and cells tangentially oriented in layer I, wh ich were assumed to be Cajal-Retzius cells. When using a voltage-clamp protocol that included a prepulse to -120 mV, Cajal-Retzius cells dis played a larger density of total K current than radial cells, and both types revealed an inactivating component (I-A). The proportion of thi s component increased from embryonic day 18 to 21 in both cell types, although the amplitude of total K current, in the respective cell type , did not vary. This suggested a concomitant decrease in delayed recti fier current, which was verified directly with an appropriate protocol . The activation rate of the delayed rectifier current was slower for ventricular zone cells than for radial or Cajal-Retzius cells. I-A was studied in Cajal-Retzius cells and displayed a strikingly negative (s imilar to -100 mV) voltage of half-maximal steady-state inactivation. Tetraethylammonium ions only blocked the non-inactivating component(s) of K current whereas 4-aminopyridine appeared to decrease both inacti vating and non-inactivating components. The quantitative changes in K current expression are likely to underlie the overall increase in exci tability of differentiating cells. On the other hand, the observation of qualitative differences among channel properties opens an interesti ng area of investigation into their physiological significance. (C) 19 97 IBRO. Published by Elsevier Science Ltd.