A chronological study was made of adenohypophyseal, ovarian, uterine, and vaginal function in 2 groups of adult rats exposed to continuous light (LL): (I) those which were still showing vaginal smear cycles, and which were autopsied during proestrus and estrus of cycles 1–5 in LL, and (II) those which had shown 5, 10, 15, 20 or 30 days of persistent vaginal estrus. Ovulation was still occurring in Group I; furthermore, the ovulatory surge of LH was being released at a time of day similar to that in animals exposed to a light-dark (LD) regimen. However, the exposure to LL did appear to lead to some desynchronization of 24-hr rhythms within the hypothalamic-pituitaryovarian axis; a persistent-estrous (PE) state (Group II) eventually developed, characterized by a failure to release an ovulatory surge of LH and by a “steady-state” secretion of ovarian steroids and folliculotrophin (i.e., “tonic” FSH and LH). The PE state, in turn, could be subdivided into 2 separate stages as a function of the number of days the rats were exposed to LL before the PE state developed :(a) PE rats which had been in LL for 60 days or less showed prolonged vaginal cornification, interrupted occasionally by ovulation, and high pituitary LH content—8.3 μg (7.1–9.6); (b) PE rats in LL for 61–120 days showed chronic vaginal cornification with no “breakthrough” ovulation, and pituitary LH content was low—3.8 μg (2.6–6.6). Several hypotheses are presented in an attempt to elucidate the etiology of the deleterious effect of LL on reproductive function in the rat. (Endocrinology81: 497, 1967)
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Lawton et al. (1967) studied this question.