Effects of continuous light on the reproductive cycle of the female rat: Induction of ovulation and pituitary gonadotrophins during persistent oestrus

The majority of rats exposed to constant light for approximately 6 weeks ovulated within 24 hr. after an injection of human chorionic gonadotrophin(HCG), but required 24\p=n-\48hr. after a single injection of progesterone. This suggests that HCG acted directly on the ovary but that progesterone acted indirectly by way of the hypothalamo-hypophysial system. Animals in-jected with progesterone after 6 weeks of constant light failed to ovulate after single or spaced injections of progesterone at 90 days of constant light while HCG administration was still effective. Pituitary content and con-centration of luteinizing hormone (LH) in constant-light animals (duration of constant light: 45 days) were below normal pituitary levels during pro-oestrus and were in the range of normal oestrous values. On the other hand, follicle-stimulating hormone (FSH) content and concentration were similar to those in cyclic rats. Single injections of 1 mg. progesterone changed neither LH nor FSH concentration, despite the fact that such treatment induced ovulation. Bilateral ovariectomy increased both LH and FSH con-tent and concentration in constant-light animals to the same extent as in control light\p=m-\darkanimals