Effects of duration, intensity, and spectrum of light exposure on sexual maturation time of female rats.

Abstract

The present study was undertaken to evaluate the effects of daily duration, intensity, and spectral composition of light exposure on the sexual maturation of female rats. Pregnant rats (Holtzman) delivered their young in a controlled environment with various illuminations of defined duration, intensity, and spectral composition. Pups were weaned at 22 days of age and separated by sex. Only females were used in the present study. They were checked for vaginal opening twice daily starting at 29 days of age. A group of 5–6 rats was sacrificed from each experimental condition 2–3 days prior to and after the expected date of vaginal opening. Results indicate that exposure to either 22 or 24 h of light/day significantly advanced vaginal opening when compared to 14-h/day exposure. No difference was obtained between animals in 6- and 14-h/day exposure. Light intensity was a significant variable at 14-h/day exposure, but not at 24 h/day. Animals exposed to 14 h of light at 100 lm/m2 matured faster than those at 30 or 600 lm/m2, but not significantly earlier than those at 2400 lm/m2. Animals in blue light exhibited vaginal opening significantly earlier than in red light if exposure was provided for 14 h/day. In continuous exposure, red light was more stimulating. No significant differences between yellow and green light were detected either with 14- or 24-h/day exposure, except for a small population of rats in continuous green light in which vaginal opening was significantly delayed. A significant seasonal variation was also detected in animals raised in white fluorescent light of 14-h/day exposure. Rats raised in the summer months showed vaginal opening significantly earlier than those in the winter. Examination of ovarian and uterine weights prior to vaginal opening revealed no significant differences with increased daily duration but equal intensity of illumination. Increasing intensity between 30 and 600 lm/m2, on the other hand, produced significantly heavier weights prior to vaginal opening. Uterine weights were heaviest at 100 lm/m2. Animals exposed to blue light had significantly heavier ovarian weights before vaginal opening than those in red light, regardless of the duration of exposure. No other differences were detected with various spectral compositions. After vaginal opening, ovarian weights were greater in 22-h/day light exposure than in either 6 or 14 h/day. However, one of two experiments revealed significantly smaller ovaries in rats kept in continuous light than in rats kept in 14 h of daily light exposure. No differences in uterine weights were noted. No differences in ovarian weights were observed following vaginal opening in animals raised in various light intensities. A significant difference in uterine weights was detected between animals raised in 30 and 100 lm/m2 of light intensity at continuous exposure. Differences in spectral composition did not result in significant differences in either ovarian or uterine weights following vaginal opening. Results are interpreted to indicate, first, that variations in the daily duration, intensity, and spectral composition of light exposure can significantly influence the age at which vaginal opening occurs and, therefore, suggest that light can play a significant role in the timing of sexual maturation and should be carefully controlled in studying the role of photoperiods in reproductive function. Secondly, the present data suggest that while in long exposures, intensity may not play a significant role, in 14-h/day-exposure intensity can be a highly significant factor. Finally, our data give no evidence for a chronic stimulation of gonadotropin release as a result of increased photoperiods, but, in the case of increased light intensity, early maturation may have resulted from increased FSH and/or LH release, as evidenced by increased ovarian and uterine weights.