Biology of Reproduction, Vol 21, 505-509, Copyright © 1979 by Society for the Study of Reproduction

Ovarian Involvement in the Timing Mechanism that Controls Ovulation in Rats

¹ Department of Anatomy, University of Nebraska Medical Center, Omaha, Nebraska 68105

This study attempted to determine if any circadian variation exists in the ovarian ovulatory response to LH. Four day cyclic rats entrained to daily 14 h photoperiods (lights on 0500-1900 h) were used. Rats were administered phenobarbital (100 mg/kg BW; i.p.) at 1300 h proestrus to block the periovulatory increases in serum gonadotropins. In some of the rats, a second injection was given the next day to block the anticipated 24 h delay in the serum LH surge. It was determined that 20 µg bovine LH/100 g BW was the minimal dose of LH that would consistently cause complete ovulation (8 or more ova) when injected s.c. at 1600 h proestrus in phenobarbital blocked rats. This LH dose was then administered to additional phenobarbital blocked rats at 1600 or 2200 h proestrus or at 0400 or 1000 h the next day. Time of ovulation was determined by a serial laparotomy technique. The time interval between LH injection and ovulation was the same (14-15 h) in all groups. The time interval between the onset of the spontaneous LH surge and ovulation is also 14 h in this strain of rat. Increasing the dose of LH 10-fold to 200 µg/100 g BW did not advance the time of ovulation when the LH was injected at 1600 h proestrus in phenobarbital blocked rats. These results suggest that 1) ovarian follicles destined to ovulate in a particular estrous cycle have ripened fully by 1600 h proestrus in terms of their ability to ovulate as soon as possible in response to LH; 2) neither the addition of FSH nor additional LH after 1600 h proestrus can cause ovulation to occur appreciably sooner than can the minimal dose of LH that will cause complete ovulation in the phenobarbital blocked rat; and 3) no circadian variation exists in the ovarian ovulatory response to LH in rats with fully ripened ovarian follicles.

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ACKNOWLEDGMENTS The authors thank Dr. L. E. Reichert, Jr., Dr. G. D. Niswender, Dr. A. F. Parlow and NIAMDD (Pituitary Hormone Distribution Program) for their gifts of materials used for radioimmunoassay and the biological studies.