Prolactin, ovarian and adrenal steroid hormone in relation to the vaginal opening in immature female rats

Abstract

The serum prolactin levels in human girls are thought to be increased gradually throughout puberty, but some authors have reported that prolactin decreases transitorily before menarche. Prolactin has not been shown to play a significant role on the initiation of human pubertal development. However, in immature female rats, it is thought that prolactin is one of the most important hormones during sexual maturation. In experiment I, the serum concentrations of prolactin, estradiol (E2) and dehydroepiandrosterone-sulfate (DHA-S) in immature female rats were measured. Prolactin increases gradually before vaginal opening and tends to decrease after vaginal opening, while E2 increases throughout sexual maturation. No significant changes were noticed in DHA-S levels. In experiment II, sulpiride, which raises serum prolactin level, was injected daily in to 4 groups of immature female rats starting from 14, 21, 28, and 35 days of age till the day before vaginal opening. Physiological saline solution was injected daily in to age-matched controls. On the day of vaginal opening, serum levels of prolactin, E2 and DHA-S were studied, weights of bodies and internal genital organs were measured, and corpora lutea of the ovaries were inspected. Vaginal opening was accelerated in the group of rats injected with sulpiride 5mg every morning from 14 days of age (p less than 0.05). The weights of their bodies and internal organs were less than those of the control rats (p less than 0.05). Percent of ovaries which contained corpora lutea were lower than the controls. Although their serum levels of prolactin tended to be suppressed, three hormone levels were not different from those of control rats on the day of vaginal opening. However, when mean prolactin levels were depicted on the day of vaginal opening in both groups, a peak was noticed at 37 days of life, indicating that the secretion pattern of this hormone has been programmed according to the day of life. There was a close relation between prolactin and DHA-S levels (p less than 0.05). No significant relation between prolactin and E2 levels was noticed. The possible role of prolactin and DHA-S on the regulation of sexual development is suggested. To our knowledge, this is the first demonstration of a significant relation between prolactin and DHA-S in immature female rats.