Effects of Aging on Photoreceptors and Implications for Visual Performance

Visual performance depends in the initial stages on the properties of retinal photoreceptors. In this context we consider the effects of rod outer segment (OS) renewal and OS response gradient on performance. The OS renews itself by forming new membrane near the cilium, at the base of the OS. This membrane then advances towards the tip where it is shed and phagocytosed in the pigment epithelium. Thus the OS is older at the tip than the base, and it has been proposed that it is due to this aging that the observed light responses are smaller and slower at the tip than at the base.

We have tested this hypothesis by exposing Xenopus laevis in a controlled manner to altered temperature and lighting conditions which are known either to slow down or to accelerate OS renewal. We have recorded the light responses of the OS base and tip and found that they are, indeed, correlated with the age differences. In addition to OS membrane age we have also investigated the effects of animal age on the response gradient along the OS. Here we found that during development the response differences decrease and then stabilise at maturity. Concurrently with the decrease of the gradient the response kinetics are slowed down. There is thus a changing trade-off between response gradient and kinetics up to maturity in this animal.

These results have important implications for visual performance. Visual reaction times depend on photoreceptor kinetics; and since photons can be absorbed anywhere along the OS, the response gradient, especially at low light levels, directly affects the reliability of detecting stimulus intensity. We conclude that visual performance in Xenopus laevis is related to the rates of OS renewal and the resulting response gradient in an age-dependent manner.