Figures 3A and
3B show the ON-bipolar cell P2 amplitude plotted as a function of luminous energy for the young (open symbols) and old (filled symbols) LE and SD rats, respectively. The data were modeled by using two hyperbolic functions (
Equation 2) to describe rod and cone bipolar cell responses.
Figure 3C shows that the rod ON-bipolar cell P2 was smaller in the SD rats than in the LE rats (
F 1,47 = 63.3;
P < 0.0001) and that the difference continued in older age (
F 1,47 = 6.5;
P < 0.05) in both the LE (young, 1935 ± 79 μV; old, 1781 ± 79 μV; 8% decrease) and the SD rats (young, 1384 ± 56 μV; old, 1164 ± 47 μV, 16% decrease). There was no age and strain interaction (
F 1,1 = 0.2;
P = 0.65), indicating a similar ageing effect in both strains.
Figure 3D shows that similar to the rod P2, the cone-mediated P2 was smaller in the SD rats than in the LE animals (
F 1,47 = 45.3,
P < 0.001). Moreover, the cone P2 was reduced with age (
F 1,47 = 18.5,
P < 0.0001) in both the LE (young, 718 ± 48 μV; old, 629 ± 13 μV; 13% decrease) and the SD rats (young, 546 ± 16 μV; old, 343 ± 17 μV, 37% decrease). There was no age and strain interaction (
F 1,1 = 2.8;
P = 0.10).
As shown in
Figure 3E, the rod bipolar cell semisaturation constant (
k) had a significant strain and age interaction (
F 1,1 = 15.1;
P < 0.001). The Bonferroni post hoc test revealed that rod P2 was more sensitive in the older SD rats than in the younger ones (young, −3.24 ± 0.02 log cd · s · m
−2; old, −3.45 ± 0.03 log cd · s · m
−2), but there was no age effect in the LE rats (young, −3.36 ± 0.03 log cd · s · m
−2; old, −3.32 ± 0.02 log cd · s · m
−2).
Figure 3F shows a significant strain and age interaction (F
1,1 = 24.5;
P < 0.001) for cone P2 semisaturation. Again, post hoc testing revealed that the cone P2 semisaturation constant was reduced by age in the SD rats (young, −0.64 ± 0.03 log cd · s · m
−2; old, −0.91 ± 0.04 log cd · s · m
−2), but not in the LE rats (young, −0.85 ± 0.03 log cd · s · m
−2; old, −0.73 ± 0.04 log cd · s · m
−2).