Figure 4. Physiological responses of PDE6C++ and rd1/+ rods. A. Light-evoked responses of rd1/+ and PDE6C++ rods in darkness (DA) collected in whole-cell voltage clamp (Vm = −40 mV). Flash responses of a representative rd1/+ rod that (black) yielded 3.4, 6.7, 14, 28, 55, 110, and 220 R*/rod; a representative PDE6C++ rod (red) that yielded 3.8, 7.6, 15, 29, 59, 120, and 240 R*/rod; and a representative rd1/+ rod exposed to 8.4 R*/rod/s (blue) that yielded 8.1, 16, 32, 65, 130, and 260 R*/rod. The timing of the flash is denoted by an arrow. Rod responses were sampled at 1 kHz and further low-pass filtered at 30 Hz. B, Average normalized dim flash response (<25% Rmax) of rd1/+ (black) and PDE6C++ rods (red) and of rd1/+ rods exposed to a background light yielding 8.4 R*/rod/s (blue). The average response was produced by 3.4 R* in rd1/+ rods (n = 37 responses), 3.8 R* in PDE6C++ rods (n = 26 responses), and 4.2 R* in rd1/+ rods exposed to 8.4 R*/rod/s (n = 15 responses). C, Normalized average response-intensity relationship for rd1/+ (n = 9; mean ± SEM) and PDE6C++ rods (n = 10; mean ± SEM) in darkness, and rd1/+ rods exposed to 8.4 R*/rod/s background light (n = 5; mean ± SEM). Each dataset was fit with an exponential saturation function (Lamb et al., 1981) from which the I1/2 value was extracted. The I1/2 values for this fit were 17 R*, 24 R*, and 28 R* for rd1/+, PDE6C++, and rd1/+ rods exposed to 8.4 R*/rod/s, respectively. Dashed horizontal line reflects I1/2 to allow a comparison of sensitivity across genotypes and conditions (see also Table 1).