Induction of precocious sexual maturation in female rats by electrochemical stimulation of the brain

Abstract

Precocious activation of the pituitary-gonadal axis was induced, in immature female rats, by electrochemical stimulation of the hypothalamus on day 23 of life. The interval between brain stimulation and the occurrence of vaginal opening, the degree of uterine hypertrophy, changes in ovarian steroid content and the incidence of first ovulation were assessed. Based on these criteria, 3 main types of response, each having a distinct time course, could be distinguished. In rats in which the induced increase in gonadal steroid content (estrogen and progestin) was most pronounced, the interval between brain stimulation and vaginal opening was shortest (about 72 h) and the incidence of ovulation was highest. In an intermediate group, vaginal opening occurred about 96 h after brain stimulation ( i.e. 24 h later), and the incidence of ovulation was less. In rats in which estrogen production appeared to lag and ovarian progestin content remained low, vaginal opening was delayed by another 24 h, and ovulation did not occur. The results suggest that brain stimulation, like the administration of pregnant mare serum or of estrogen, leads to a premature increase in ovarian steroidogenesis and that these ovarian hormones cause precocious sexual maturation through the activation of brain areas which are sensitive to the stimulatory feedback action of gonadal steroids and which control the cyclic (ovulatory) release of gonadotrophins.