Photoreceptor physiology in the rat is governed by the light environment

Abstract

Albino rats were born and raised in one of three cyclic (12L : 12D) lighting conditions: (1) 5 lux for 9 weeks, (2) 800 lux for 9 weeks, or (3) 800 lux for 9 weeks, followed by 5 lux for 3 weeks (800:5). After the treatment, the following were determined: (i) retinal function as measured by the electroretinogram (ERG); (ii) retinal morphology, including rod outer segment (ROS) length and outer nuclear layer area; (iii) rhodopsin levels in whole retina and ROS preparations; (iv) fatty acid profile of ROS membranes and (v) retinal antioxidant levels. After 9 weeks, rats raised in 800 lux sustained an irreversible loss of photoreceptors which could not be reversed by then placing them in 5 lux. However, these rats displayed significant alterations in all other parameters measured after the 3 weeks in dim cyclic light. ERGs showed a 60% increase of b-wave maximum amplitude in 800:5 rats at 12 weeks over the value at the time of their change to a dim environment, while a-wave amplitude in 800:5 rats increased more than 2·5 times. This increase can be explained by a combination of increased ROS length and increased ROS membrane concentration of rhodopsin during the three weeks in 5 lux. Polyunsaturated fatty acids predominated in the ROS of 5 lux rats and 800:5 rats, but not in 800 lux animals. Measurements of retinal glutathione enzyme activity and vitamin E and C levels were relatively low in 800:5 rats. Some rats from the 800:5 group were exposed to 2000 lux for 24 hr. Retinas of these rats sustained 50% loss of photoreceptors from this exposure. Comparisons are made to previous studies concerning the effect of cyclic light environments on the retinas of albino rats.