Abstract
Purpose :
Considerable evidence indicates that rods are more sensitive than cones from critical physiological differences, including (1) a larger gain per Rh*, and (2) slower response decay. Using transgenic mice we decreased the gain of activation and sped response decay. We sought to create a rod photoreceptor that avoids saturation and remains responsive in background lights significantly brighter than wildtype rods.
Methods :
We used a transgenic mouse model in which the native rod transducin was ablated and replaced with about 10% of normal expression (TrU). We accelerated the rate of rod response decay by increasing the expression of GAP proteins by about 6-fold with transgenic R9AP95 mice. By crossing these two animals and generating TrU;R9AP95 rods, we have combined the two factors affecting the sensitivity of the rods in an attempt to make these cells behave more closely to cones.
Using suction-electrode-recording we measured the responses of single rods to increasing flash strengths up to response saturation. This procedure was repeated under increasing background intensities until increment saturation was reached. (SF) was plotted as a function of background intensity and a Weber-Fechner fit to the data was used to extrapolate a value for I0 (the background intensity required to reduce sensitivity by one-half).
Results :
Lowering the expression of transducin resulted in an increase in SF from 21 photons μm-2 in wild-type (n=12) to 120 in TrU (n=8) animals. Over-expression of R9AP95 also resulted in an increase of SF to 180 photons μm-2 (n=6), and a combined effect resulted in rod photoreceptors with an SF of 470 photons μm-2, nearly 25-fold less than a WT rod. After taking into account the difference in collecting area, this value is nearly that of a WT cone. The value for I0 was also changed from 110 in wildtype (n=6) to 2700 photons μm-2 s-1 in TrU;R9AP95 rods (n=7). The time constant of recovery of a decay function was 250 ms in wildtype cells and 180 ms in TrU;R9AP95 rods, which was not as rapid as in a WT cone.
Conclusions :
By reducing the gain of the phototransduction cascade as well as speeding the kinetics of the response recovery, we have been able to shift the sensitivity of rods towards higher intensities. The cells responded with faster kinetics and remained functional in the presence of background intensities that would normally saturate a rod response in wildtype mice.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.