It has been discovered in this laboratory that methylnitrosourea (MNU), administered at low dose to mice on day 16 of gestation, produces a late-onset retinal degeneration in all offspring. Such degeneration is prevented, however, by avoiding exposure of the animals to light. Moreover, the effect of the secondary insult (light) could be prevented by high levels of butylated hydroxytoluene (BHT) in the diet subsequent to MNU administration. The present investigation will characterize this model in more detail using light and electron microscopy and measurement of rod outer segment turnover. Specifically, we will determine precisely the role of secondary light exposure in expressing the MNU retinopathy and the role of the radical scavenger in preventing the effects of this secondary insult. The following specific aims will be addressed: 1) to determine the exposure times, schedules, and spectral characteristics of the offending illumination; 2) to define the dose dependence and the time of administration of the radical scavenger required to counteract the effects of the secondary insult by light; 3) to determine the primary toxic effect of BHT for the retina; and, 4) to explore the possible efficacy of other antioxidants and radical scavengers. These experiments will address two very fundamental and highly significant issues. First, that developmental (or genetic) damage may not be expressed in an individual unless there is the appropriate secondary insult or stimulus from the environment. Second, by understanding the role of a secondary insult, it may be possible to intervene in the sequence and prevent expression of developmental or genetic defects. The use of BHT is based on the premise that radicals are produced in the eye photochemically. That BHT can intervene in this secondary insult by light has been established by our preliminary experiments. However, the limits of light exposure and the dose range for the radical scavenger are yet to be determined. Pregnant mice will be untreated or exposed to a standard schedule of MNU, divided into a series of groups, and maintained from the time of birth on various schedules of dark and/or illumination with filtered or unfiltered light. This protocol will be subdivided further and groups, at least five litters each, will be given BHT according to systematic schedules to determine dose dependence and optimum timing and duration of administration. These experiments to determine the role of secondary risk factors, and the design of intervention strategies is highly pertinent to the consideration of human retinal diseases.
