Effect of constant light and darkness on pituitary and serum gonadotropin and sex hormone levels of parturient rats.

Exposure of rats to continuous darkness throughout gestation produced a threefold increase in their serum prolactin and a moderate one in their serum LH levels on the day preceding delivery. Parallel decreases in pituitary prolactin and LH contents were experienced indicating enhanced release of the hormones. In the animals kept in constant light only prolactin release was increased before delivery, their LH levels were unaffected, although diminished pituitary contents due to decreased synthesis were evident. Following parturition the photo-induced prolactin levels in serum and pituitary disappeared, but the LH values of the rats exposed to both extremes of illumination persisted, in contrast to those of the control animals kept in alternating light (12 L : 12 D) whose values fell abruptly. Serum oestradiol was affected neither by darkness nor light but serum progesterone concentrations were depressed before delivery in the rats kept in darkness even more than in the control animals. Exposure to continuous darkness shortened the duration of gestation. The marked tendency to deliver during the daytime hours was not influenced by exposure to continuous darkness but was completely abolished by constant light. It is suggested that the strongly decreased serum progesterone levels--possibly brought about by the high prolactin concentrations observed before parturition--may be contributing to the earlier onset of delivery in the rats exposed to continuous darkness.