Ovarian changes in Sprague-Dawley rats produced by nocturnal exposure to low intensity light

This study was performed to investigate the potential effects of nocturnal, low-intensity light upon ovarian morphology of female Sprague-Dawley rats and to investigate the cause for ovarian changes which had been observed in an earlier study following transient exposure of control female Sprague-Dawley rats to indirect light of minimal intensity during the nocturnal 12-h dark cycle. Twenty female Sprague-Dawley CD rats (initial age: 5 weeks) were kept for 8 weeks under our standard laboratory conditions including a daily 12-h light cycle (light intensity: approximately 200 lux) followed by a 12-h dark cycle with exposure to an indirect light source of low intensity (approximately 30 lux). Ten female control rats of comparable age from a concurrent toxicology study housed in an adjacent animal room under our standard 12 h light/dark cycle served as controls. At the end of the study the rats were sacrificed, necropsied and the ovaries were evaluated histopathologically. In 5 of the 20 animals we found ovarian atrophy consisting of decreased number and size of corpora lutea and increased number of tertiary follicles and/or follicular cysts. Most corpora lutea present in these ovaries were old, indicating the absence of recent ovulations. In contrast, the incidence of ovarian changes in the control group was 0/10. In conclusion, nocturnal exposure of female Sprague-Dawley rats to light of minimal intensity produced a substantial incidence of ovarian changes and suggests that the incidence of ovarian atrophy observed in a previous study may have been due to transient exposure to indirect nocturnal light of minimal intensity.