Figure 4.
Ca2+ channel currents show a developmental upregulation and downregulation in both outer and inner hair cells in rats and mice, with the current maximum of OHCs preceding that of the IHCs by several days. A, Average peak IBa in rat OHCs as a function of age. Number of OHCs were as follows: P1, 6; P2, 1; P4, 2; P5, 5; P7, 6; P8, 3; P12, 2; P15, 5; P19, 6; P28, 13; and P30, 1. OHCs were from apical or medial cochlear half-turns from mice younger than P9 and from apical half-turns older than that. B, Average peak IBa in rat IHCs (triangles) and the much smaller average IBa for rat OHCs (circles) as a function of age. Number of IHCs were as follows: P1, 1; P3, 2; P5, 14; P9, 3; P11, 7; P15, 13; P17, 10; P19, 3; P21, 5; and P30, 6. C, Rat OHC and IHC average IBa values were normalized to their developmental peaks, respectively, to compare the time course of their IBa downregulation. D, When the OHC normalized IBa curve was shifted by 8 d to the right, it matched that for the IHC IBa development. E, Mouse OHC IBa data from Figure 1F were normalized to their developmental peak. The same was done for mouse IHCs using Ca2+ current data from Marcotti et al. (2003) (their Fig. 2C). F, Shifting the mouse OHC normalized IBa curve by 4 d reveals similar kinetics of developmental downregulation of IBa/ICa in mouse outer and inner hair cells.