Progesterone: Regulation of Estrous Cycle, Ovulation and Estrous Behavior in the Golden Hamster

Abstract

Three-month-old, intact, virgin, female golden hamsters were treated with progesterone given as a single subcutaneous injection using either 0.05 ml propylene glycol (PG) or sesame oil (SO) as an injection vehicle. The animals were then observed for estrous behavior and time of ovulation. One hundred, 50 and 25 but not 12.5 μg progesterone given on the second day of diestrus at 1900 induces estrous behavior: this is followed by a delay of ovulation and its associated estrous period. The duration of the delay and its manifestation are dependent on dose of progesterone and type of injection vehicle. One hundred μg of progesterone given at various times on D2 and the day of proestrus (P) is also able to induce estrous behavior and delay ovulation, but the duration of delays and the exhibition of a delay depend on the type of injection vehicle and the time of progesterone administration. When SO is the injection vehicle the fraction showing delay and the length of delay are increased in comparison to hormone injected in PG. Following ovulation, estrous behavior is not induced by progesterone administered before or at 1100 on D2 but is induced following injection at or after 1900 on D2. Progesterone does not delay ovulation or estrous behavior unless injected at or before 0030 on P. Whenever delays of ovulation and estrous behavior are observed, they are integral multiples of 24 hr, with ovulation occurring between 0100 and 0400 in the morning. We suggest progesterone may be suppressing an event which occurs just prior to and during the “critical period,” thus effectively abolishing the neural activation of the hypothalamus. (Endocrinology86: 1287, 1970)