Hypothalamic GnRH and pituitary gonadotroph relationships during rat fetal life.

The ontogeny of hypothalamic GnRH, of pituitary and gonadal receptors and of pituitary LH and FSH was studied in the fetal and neonatal rat. Hypothalamic, hypophyseal and gonadal primordia were dissected from animals ranging in age from postconceptual day 12 to birth. Immunoreactive GnRH was detectable in the hypothalamus from fetal day 12 onwards at a low level until day 17, whereafter hypothalamic GnRH content and concentration increased until birth. GnRH receptors were reliably detectable in the pituitary anlage from fetal day 16 onwards and increased progressively with advancing age whether expressed as content or concentration. Signs of pituitary LH synthesis were evident as early as fetal day 12 but intrapituitary LH levels remained low until fetal day 17 when levels increased progressively until the end of gestation. Pituitary FSH was undetectable until fetal day 19, thereafter rising dramatically until the end of gestation. GnRH binding to testicular and ovarian tissues was undetectable throughout the period of fetal development. The possible relations among the developmental changes in hypothalamic GnRH, pituitary GnRH receptors and gonadotrophins are discussed.