Expression density and functional characteristics of the outer hair cell motor protein are regulated during postnatal development in rat

1. The non-linear capacitance (Cnon-lin) of postnatal outer hair cells (OHC s) of the rat was measured by a patch-clamp lock-in technique. Cnon-lin is thought to result from a membrane protein that provides the molecular basis for the unique electromotility of OHCs by undergoing conformational change s in response to changes in membrane potential (V-m). Protein conformation is coupled to V-m by a charged voltage sensor, which imposes Cnon-lin on th e OHC. Cnon-lin was investigated in order to characterize the surface expre ssion and voltage dependence of this motor protein during postnatal develop ment. 2. On the day of birth (P0), Cnon-lin was not detected in OHCs of the basal turn of the cochlea, whilst it was 89 fF in apical OHCs. Cnon-lin increase d gradually during postnatal development and reached 2.3 pF (basal turn, P9 ) and 7.5 pF (apical turn, P14) at the oldest developmental stages covered by our measurements. The density of the protein in the plasma membrane, ded uced from non-linear charge movement per membrane area, increased steeply b etween P6 and P11 and reached steady state (4200 e(-) mu m(-2)) at about P1 2. 3. Voltage at peak capacitance (V-1/2) shifted with development from hyperp olarized potentials shortly after birth (-88.3 mV, P2) to the depolarized p otential characteristic of mature OHCs (-40.8 mV, P14). This developmental difference in V-1/2 was also observed in outside-out patches immediately af ter patch excision. During subsequent wash-out V-1/2 shifted towards the de polarized value found in the adult state, suggesting a direct modulation of the molecular motor. 4. Thus, the density of the motor protein in the plasma membrane and also i ts voltage dependence change concomitantly in the postnatal period and reac h adult characteristics right at the onset of hearing.