Steroid Control and Sexual Differentiation of Brain Aromatase

Brain aromatase (ARO) activity in the quail is markedly enhanced by testosterone (T). This effect only becomes detectable after several hours and reaches its maximum within a few days, which suggests enzymatic induction at the genomic level. This idea is reinforced by the fact that T also increases the ARO protein, as observed by immunocytochemistry (ICC) and the ARO mRNA, as measured by reverse transcriptase-polymerase chain reaction (RT-PCR). These changes can be mimicked by the administration of estrogens and therefore presumably require T aromatization. In our first test, injection of the non-steroidal ARO inhibitor, R76713 (racemic vorozole), unexpectedly revealed an increase in ARO immunoreactivity in the preoptic area (POA) of treated birds. This property of R76713 was shared by another non-steroidal inhibitor, fadrozole, but not by two steroidal inhibitors, androstatrienedione (ATD) and 4-hydroxy-androstenedione (OHA). These last two compounds markedly decreased the concentration of brain ARO as estimated by ICC. In parallel, ATD and OHA decreased ARO mRNA concentration measured by RT-PCR but vorozole and fadrozole had no effect on these concentrations in the POA, and only caused them to decrease slightly in the posterior hypothalamus. Together, these data indicate that the removal of estrogens caused by steroidal inhibitors decreases the synthesis of ARO, presumably at the transcriptional level. Additional regulatory mechanisms apparently take place after the injection of non-steroidal inhibitors and probably include increased half-life of the protein. The induction of ARO activity by steroids appears to be greater in males than in females, but this difference has been difficult to localize and confirm by assay methods. We therefore analysed by ICC the tridimensional distribution of ARO-ir neurons in the POA of males and females that were sexually mature or gonadectomized and treated with T-filled or control empty implants. Localized sex differences and effects of T were detected in this way. In particular, males had more ARO-ir cells than females in the lateral POA but a difference in the opposite direction was evident in the medial part of this area. These sex differences are largely activational (i.e. caused by the higher T levels in males) but they may also reflect organizational effects of neonatal steroids. Castration decreased ARO-ir cell numbers in the lateral POA, but increased it in the periventricular region. This anatomically specialized control by T may be mediated by three potential mechanisms that are discussed and comparatively evaluated: a migration of ARO neurons towards the ventricle after castration; a differential colocalization of ARO with estrogen receptors or a differential modulation of ARO neurons by catecholaminergic inputs.