Ontogeny of testosterone-inducible brain aromatase activity.

The effects of testosterone (T) treatment on androgen-metabolizing enzymes and especially on aromatase were examined in the developing hypothalamus of quail using an in vitro microassay. Testosterone propionate (TP) injected into the pectoral muscles of one-day-old chicks had no effect on formation of 5 alpha-dihydrotestosterone (5 alpha-DHT), 5 beta-dihydrotestosterone (5 beta-DHT) and 5 beta-androstane-3 alpha, 17 beta-diol (5 beta,3 alpha-diol) in the hypothalamus. By contrast, aromatase activity was significantly enhanced in the anterior (PA) and in the posterior (HT) parts of the hypothalamus of these chicks 18-23 h after a single injection of TP. This increase was site specific as it did not occur in control non-target areas, neostriatum intermediale and hyperstriatum ventrale (VN). The increase in hypothalamic aromatase activity was accompanied by T-dependent changes in behavior and somatic development. Although aromatase activity is present as early as day 10 of embryonic life in PA and HT, it becomes inducible by circulating T only after this stage of development. There is also an important increase in basal aromatase activity during embryonic development. At day 10, when the brain is particularly sensitive to the differentiating effects of gonadal steroids, formation of 5 alpha-DHT and 5 beta-DHT in the hypothalamus is higher in females than in males. We conclude that although all important pathways of T are already present on day 10 of embryonic life, the brain metabolism of androgens continues maturing during the whole embryonic life.