SEX-DIFFERENCES IN THE REGULATION OF EMBRYONIC BRAIN AROMATASE

Oestrogen formed from androgen by aromatization plays a critical role in the sexual differentiation of the male brain and behaviour. A quest ion which has still to be answered is what regulates the gender-specif ic changes in aromatase activity forming oestrogen during sensitive pe riods of brain growth. Using a primary cell culture technique and sexe d embryos, we have shown that in the fetal mouse brain, oestrogen form ation in the male is neuronal rather than glial and aromatase activity is regionally localized, being higher in the hypothalamus than in the cortex. The aromatase activity measured from cells in culture has the same enzyme binding affinity (apparent K-m similar to 40 nM) as intac t brain samples. Neurones developing in the embryonic male brain (embr yonic day (ED) 15) contain higher aromatase activity (V-max, 895 fmol/ h/mg protein) than the female (V-max, 604). Although a sex difference exists at early stages of embryonic development (ED 13), the embryonic aromatase system is regulated by steroids later in fetal development. The developing aromatase-containing neuroblasts probably form process es which connect to other aromatase neurones. Immunoreactive staining with an aromatase polyclonal antibody identifies an increase in number s of aromatase-immunoreactive hypothalamic neuronal cell bodies follow ing testosterone treatment. Testosterone treatment also causes both st imulation of neurite growth and branching as web as functional maturat ion of aromatase neurones. In particular, there is an increase in arom atase activity per neurone as well as a dramatic increase in the numbe r of neurones expressing the enzyme. Both the functional and morpholog ical changes depend on androgen receptor stimulation for several days in vitro. This conclusion is supported by colocalization studies which reveal a high number of fetal hypothalamic aromatase neurones co-expr essing androgen receptor. We conclude that testosterone influences the growth of male hypothalamic neurones containing aromatase at a sensit ive period of brain development. Endogenous steroid inhibitors of arom atase, probably formed within the neuroglia, also play a role in the c ontrol of oestrogen production. An endogenous 5 alpha-reduced metaboli te of testosterone, 5 alpha-androstanedione, is almost as potent in in hibiting neuronal hypothalamic aromatase activity (K-i = 23 nM) as the synthetic non-steroidal inhibitors such as the imidazole, fadrozole, and the triazoles, arimidex and letrozole. It is clear that the oestro gen-forming capacity of the male hypothalamus has the special characte ristics and plasticity of regulation which could affect brain differen tiation at specific steroid-sensitive stages in ontogeny. (C) 1997 Els evier Science Ltd.