Development of outward potassium currents in inner and outer hair cells from the embryonic mouse cochlea

We recorded K(+) currents in inner (IHCs) and outer (OHCs) hair cells from mice at embryonic days 16 and 18 and on the day of birth (PO) to characterize their early physiological differentiation. In both cell types, outward currents increased in size during late embryonic development, in cells situated in both the apical and basal coils of the cochlea. Currents increased up to six-fold, with current density increasing four-fold. Currents in basal cells were generally larger than in the apex, and currents in IHCs were larger than in OHCs at any given stage. In OHCs, they were initially non-inactivating but gained the partial inactivation characteristic of the K(+) current of neonatal mouse cochlear hair cells, I(K,neo), by day 18 in the base and by P0 in the apex. In IHCs, there was little change, other than in amplitude, with partial inactivation already evident in the base by embryonic day 16. These results suggest that changes in the channel complement of OHCs occur within a few days of terminal mitosis, whereas in IHCs any such development would occur earlier. The progressive development of K(+) currents correlates with a developmental delay of around 2 days from the base to the apex of the cochlea.