The metabolism of testosterone was studied in vitro in anterior pituitary, hypothalamic and hyperstriatal tissues taken from male European starlings in the autumn. In all the tissues studied, testosterone was converted into 5α-androstan-17β-ol-3-one (5α-DHT), 5β-androstan-17β-ol-3-one (5β-DHT), 5β-androstane-3α,17β-diol (5β-THT), 5β-androstane-3,-17-dione and androst-4-ene-3,17-dione.
The 5α-DHT was produced in significantly greater amounts by the pituitary gland than by the hypothalamus and hyperstriatum. The amount of 5α-DHT produced, however, was very low in comparison with the amounts of 5β-reduced metabolites. The amount of 5β-reductase was also higher in the pituitary gland than in the two nervous tissues. The ratios between the production of 5β-DHT, 5β-THT and 5β-androstane-3,17-dione were, however, different in the three tissues: 5β-DHT was produced in the greatest quantities by the hyperstriatum, while the production of 5β-THT, 5β-androstane-3,17-dione and androst-4-ene-3,17-dione was greatest in pituitary tissue.
The role of 5α- and 5β-reduced metabolites in the pituitary gland and in the brain of birds is unknown, but some possibilities arising from the present results are discussed.