SODIUM CURRENT EXPRESSION DURING POSTNATAL-DEVELOPMENT OF RAT OUTER HAIR-CELLS

Outer hair cells of the cultured organ of Corti from newborn rats (0-1 1 days after birth) were studied in the whole-cell patch-clamp configu ration. A voltage-activated sodium current was detected in 97% (n = 10 9) of the cells at 0-9 days after birth. The properties of this curren t were: (1) its activation and inactivation kinetics were fast and vol tage-dependent, (2) the voltage at half-maximum activation was 35.0 mV (3) its steady-state inactivation was temperature-sensitive (the half -inactivating voltage was -92.6 mV at 23 degrees C and -84.8 mV at 37 degrees C), (4) the reversal potential(80 mV) was close to the sodium equilibrium potential and currents could be abolished by the removal o f extracellular sodium, and (5) tetrodotoxin blocked the current with a K-d of 474 nmol/l. Current amplitudes were up to 1.7 nA at room temp erature. Mean current amplitudes showed a developmental time course wi th a maximum at postnatal days 3 and 7 for outer hair cells from the b asal and apical part of the cochlea, respectively. In current-clamp mo de cells had membrane potentials of -59.7 +/- 11.7 mV (n = 9). When ce lls were hyperpolarized by constant current injection, depolarizing cu rrents were able to trigger action potentials. At 18 days after birth, sodium currents were greatly reduced and barely detectable. The resul ts show that, unlike adult outer hair cells, immature outer hair cells regularly express voltage-gated sodium channels. However, due to mism atching of the sodium current inactivation range and membrane potentia l in vitro, a physiological function appears questionable.