Prolactin and the control of cycle length in the female rat

Bromocriptine (BRC) was injected on oestrus afternoon at 16.00 h into 5-day cyclic female rats thus causing reduced cycle length to 4-days. When injected on dioestrus 1 at 13.00 h the drug remained without effects. Blood progesterone concentration on dioestrus 1 afternoon and on dioestrus 2 morning was shown to be lower in 5-day cyclic rats given BRC than in their untreated counterparts. No changes in the rate of follicular growth were observed on dioestrus 2 afternoon in BRC injected 5-day cyclic rats. The pattern of prolactin secretion on oestrus afternoon was compared in 4- and 5-day cyclic female rats. High prolactin values were observed on early afternoon in the former and on late afternoon in the latter. The decline in progesterone secretion during the dioestrous period of the cycle was supposed to be responsible for cycle shortening in 5-day cyclic rats given BRC. It was concluded that prolactin might play a critical role in the establishment of a 5-day rhythm in the rat by controlling progesterone secretion during the dioestrous period of the cycle.