Balthazart J
Laboratory of General and Comparative Biochemistry, University of Liège, Belgium.
J Steroid Biochem Mol Biol. 1991;40(4-6):557-70. doi: 10.1016/0960-0760(91)90277-c.
Many central actions of testosterone (T) require the transformation of T into several metabolites including 5 alpha-dihydrotestosterone (5 alpha-DHT) and estradiol (E2). In birds as in mammals, 5 alpha-DHT and E2, alone or in combination, mimic most behavioral effects of T. The avian brain is, in addition, able to transform T into 5 beta-DHT, a metabolite which seems to be devoid of any behavioral or physiological effects, at least in the context of reproduction. By in vitro product-formation assays, we have analyzed the distribution, sex differences and regulation by steroids of the 3 main T metabolizing enzymes (aromatase, 5 alpha- and 5 beta-reductases) in the brain of the Japanese quail (Coturnix c. japonica) and the zebra finch (Taeniopygia guttata castanotis). In the hypothalamus of quail and finches, aromatase activity is higher in males than in females. It is also decreased by castration and increased by T. The activity of the 5 alpha-reductase is not sexually differentiated nor controlled by T. The 5 beta-reductase activity is often higher in females than in males but this difference disappears in gonadectomized birds and no clear effect of T can be observed at this level. The zebra finch brain also contains a number of steroid-sensitive telencephalic nuclei [e.g. hyperstriatum ventrale, pars caudale (HVc) and robustus archistriatalis (RA)] which play a key role in the control of vocalizations. These nuclei also contain T-metabolizing enzymes but the regulation of their activity is substantially different from what has been observed in the hypothalamus. Aromatase activity is for example higher in females than in males in HVc and RA and the enzyme in these nuclei is not affected by castration nor T treatment. In these nuclei, the 5 alpha-reductase activity is higher in males than in females and the reverse is true for the 5 beta-reductase. These sex differences in activity are not sensitive to gonadectomy and T treatment and might therefore be organized by neonatal steroids. We have been recently able to localize aromatase-immunoreactive (AR-ir) neurons by ICC in the brain of the quail and zebra finch. Positive cells are found in the preoptic area, ventromedial and tuberal hypothalamus. AR-ir material is found in the perikarya of cells and fills the entire cellular processes including axons. At the electron microscope level, immunoreactive material can clearly be observed in the synaptic boutons. This observation raises questions concerning the mode of action of estrogens produced by central aromatization of T.
睾酮（T）的许多中枢作用需要将T转化为几种代谢产物，包括5α-二氢睾酮（5α-DHT）和雌二醇（E2）。与哺乳动物一样，在鸟类中，5α-DHT和E2单独或联合使用时，可模拟T的大多数行为效应。此外，鸟类大脑能够将T转化为5β-DHT，至少在繁殖背景下，这种代谢产物似乎没有任何行为或生理效应。通过体外产物形成分析，我们分析了日本鹌鹑（Coturnix c. japonica）和斑胸草雀（Taeniopygia guttata castanotis）大脑中3种主要的T代谢酶（芳香化酶、5α-和5β-还原酶）的分布、性别差异以及类固醇对它们的调节。在鹌鹑和草雀的下丘脑，雄性的芳香化酶活性高于雌性。阉割会使其降低，而T会使其升高。5α-还原酶的活性没有性别差异，也不受T的控制。5β-还原酶的活性通常雌性高于雄性，但这种差异在去势鸟类中消失，在这个水平上未观察到T有明显作用。斑胸草雀大脑还包含一些对类固醇敏感的端脑核[例如，尾侧腹侧上纹状体（HVc）和原纹状体粗壮核（RA）]，它们在发声控制中起关键作用。这些核也含有T代谢酶，但其活性调节与在下丘脑中观察到的情况有很大不同。例如，在HVc和RA中，雌性的芳香化酶活性高于雄性，这些核中的酶不受阉割和T处理的影响。在这些核中，5α-还原酶活性雄性高于雌性，而5β-还原酶则相反。这些活性的性别差异对去势和T处理不敏感，因此可能是由新生儿期的类固醇组织形成的。最近，我们通过免疫细胞化学（ICC）在鹌鹑和斑胸草雀的大脑中定位了芳香化酶免疫反应性（AR-ir）神经元。在视前区、腹内侧和结节状下丘脑发现了阳性细胞。在细胞周质中发现了AR-ir物质，并充满了包括轴突在内的整个细胞突起。在电子显微镜水平上，在突触小体中可以清楚地观察到免疫反应性物质。这一观察结果引发了关于T中枢芳香化产生的雌激素作用方式的问题。
