Plasma progesterone concentrations and length of the first spontaneous oestrous cycle in pubertal rats.

The length of the first spontaneous oestrous cycle in pubertal Wistar-Imamichi strain rats determined by vaginal smears varied from 5 to 18 days. The variation was ascribed to the period (3-16 days) of the stage of vaginal smears consisting of leucocytic cells (L stage). Plasma progesterone concentration and the decidual reaction in the uterus were used as indicators of the function of the corpus luteum and the L stage period was categorized as short, lasting for 3-6 days (average 4 days) with non-functional corpora lutea, or long, lasting 9-16 days (average 12 days) and with functional corpora lutea. Rats with the long L stage showed nocturnal and diurnal prolactin surges, but no daily changes in prolactin values were observed in rats with a short L stage. Daily changes in prolactin concentrations were maintained by the administration of progesterone in rats ovariectomized on Day 6 of the L stage. Plasma progesterone values on Day 6 of the L stage decreased with ergocornine treatment on Days 4 and 5 of the L stage and administration of bovine prolactin restored the level. These results indicate that the L stage observed in the first oestrous cycle is maintained by a positive feedback relation between progesterone and prolactin secretions.