Pituitary-Ovarian Function in Rats Exposed to Constant Light: A Chronological Study

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Abstract
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 [luteinizing hormone] 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-pituitary-ovarian 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 [follicle-stimulating hormone] 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: 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[long dash]8.3 [mu]g (7.1[long dash]9.6); PE rats in LL for 61-120 days showed chronic vaginal cornification with no "breakthrough" ovulation, and pituitary LH content was low[long dash]3.8 [mu]g (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.