Rod outer segment (ROS) renewal as a mechanism for adaptation to a new intensity environment. I. Rhodopsin levels and ROS length

This study outlines the time course of cellular changes which occur within the renewal process of the rod cell during a switch to a new cyclic intensity environment. In the following experiments we demonstrate that by using the renewal process, Sprague-Dawley rats switched to a new cyclic intensity can adjust both the length of the outer segment and the amount of rhodopsin per retina. Previously it was established that the rod outer segment (ROS) length is not constant when animals are exposed to an intensity different from their normal environment. In the present study, we investigated how changes in ROS length were achieved by the rod cell. We noted that soon after a change in intensity, the ROSs were always shortened. This occurred when rats were switched either to a higher or lower light intensity than their rearing level. A final ROS length was achieved within 21 days (approximately two turnover periods). This length change required decreased disk removal in animals switched to a low light to achieve a lengthened ROS. In animals moved to a higher light intensity, however, disk removal rate did not change but ROS length did shorten, suggesting a change in disk addition. It is known that rhodopsin levels are up- and down-regulated with changes in environmental lighting. In this study, rhodopsin levels of animals switched from a low, cyclic intensity of 3 lx into a more intense cyclic light, 200 lx, dropped dramatically within 7 days to the rhodopsin value of an animal reared in the higher intensity. The rhodopsin level for animals moved to a less intense light, 3 lx, gradually began to rise on the third day in the new light and reached a plateau at the level of a low intensity reared animal on day 21. Despite some oscillation, by day 28 all animals switched into a new light had achieved and sustained rhodopsin values expected for the new light intensity. Changes in ROS length did not fully account for the measured changes in rhodopsin levels. Also, the rate of change in ROS length ( μm day −1) for switched animals varied throughout the time in the new intensity. Furthermore, the rate of lengthening the ROS was different from the rate of shortening the ROS. Shedding patterns for animals born and raised in 3 lx or 200 lx were the same, however, patterns did change once an animal was switched. All these data indicate that several aspects of the renewal process are involved in the modification of the rod cell to a new intensity environment.