FOLLICULAR AND LUTEAL PROGESTERONE SYNERGIZE TO MAINTAIN 5-DAY CYCLICITY IN RATS

The length of the ovarian cycle in rat is determined by the duration o f progesterone secretion from the corpora lutea (CL) during diestrus. The action of progesterone secretion from the preovulatory follicles o n proestrus is also responsible for the cycle length in 4-day cyclic r ats. To study whether follicular and luteal progesterone participate i n the maintenance of 5-day cyclicity, the effects of the antiprogestag en RU486 (5 mg on proestrus or estrus) on estrous cycle length and on the serum concentrations of LH in 5-day cyclic rats and in C-day cycle experimentally induced by the dopamine agonist CB154 (1 mg on estrus) were investigated. Furthermore, serum concentrations of progesterone on the day of ensuing ovulation were measured to see whether activatio n of the CL function after treatment with RU486 had occurred. Both 5-d ay and CB154-injected rats had a 3-day estrous cycle after RU486 on pr oestrus, while RU486 on estrus shortened by 1-day the estrous cycle le ngth in 5-day but not in CB154-injected rats. Basal serum concentratio ns of LH increased and the LH surge decreased after RU486 treatment in both cycle types. Serum concentrations of progesterone rose only in 5 -day rats injected with RU486. These results indicate that the actions of both follicular and luteal progesterone synergize in maintaining t he length of the estrous cycle in 5-day cyclic rats and that functiona lly active CL increase progesterone production only under the action o f a complete surge of prolactin.