Effect of bromocriptine and progesterone on the length of the ovarian cycle in 4- and 5-day estrous cyclic rats

Abstract

To study the effect of prolactin and progesterone on the length of the reproductive cycle in the rat, rats of different estrous cycle length (four and five days, respectively) were injected daily (09.00 h) with either bromocriptine (1 mg/rat) or 70% ethanol vehicle (0.25 ml) from the day of estrus onward, up to the appearance of the next ovulation.Each group of rats was then (16.00, metestrus) also injected with either progesterone (4 mg/rat) or 0.2 ml of olive oil.The effects of these treatments on the length of the estrous cycle was studied by both the recording of vaginal smears daily and by direct visualization of oocyte-cumulus complexes on the ensuing day of estrus (10.00 h-12.00 h).Bromocriptine treatment shortened the length of the cycle by one day in 5-day but not in 4-day cyclic rats, while progesterone treatment lengthened estrous cycles by one day in both groups of rats.Treatment with both bromocriptine and progesterone had no effect on the estrous cycle length of 5-day cyclic rats, but did prolong in one day the cycle of 4-day cyclic rats.These facts suggest that prolactin regulates the length of the ovarian reproductive cycle in the rat through its action on the secretion of progesterone by the corpus luteum.