Developmental expression of the potassium current I-K,I-n contributes to maturation of mouse outer hair cells

1. The expression of K+ currents in mouse outer hair cells (OHCs) was inves tigated as a function of developmental age between postnatal day (P) 0 and P26, using whole-cell patch clamp. 2. During the first postnatal week, a slow outward K+ current (I-K,I- neo) was expressed by all OHCs from the apical coil of the cochlea. The amplitud e of this current increased greatly between P0 and P6. Then, at the beginni ng of the second postnatal week, I-K,I- neo decreased. At the same time, fr om P8 onwards, I-K,I- n, a K+ current characteristic of mature OHCs, was ra pidly expressed. 3. The expression of I-K,I- n coincided with the onset of electromotility o f the cell toady of the OHCs, which could also be detected from P8 onwards and increased substantially in size thereafter. 4. I-K,I- n was reversibly blocked by linopirdine, an inhibitor of members of the KCNQ family of K+ channels, with a K-D of 0.7 mu M. In the cochlea, KCNQ4 is only expressed in OHCs and is responsible for a form of non-syndro mic antosomal dominant deafness. Linopirdine had no effect on other OHC Kcurrents at concentrations up to 200 mu M. We conclude that ion channels un derlying I-K,I- n contain the KCNQ4 subunit. 5. In current damp, depolarizing current injections from the resting potent ial triggered action potentials in OHCs during the first postnatal week. Th ereafter, more rapid and graded voltage responses occurred from more negati ve resting potentials. Thus, OHCs mature rapidly from P8 onwards, and I-K,I - n contributes to this maturation.