Testosterone metabolism and sexual behavior in the chick

A series of experiments was performed to study the behavioral and physiological activity of testosterone (T) metabolites which are produced by neural tissues of male chicks, i.e., mainly 5α- and 5β-dihydrotestosterone (5α-, 5β-DHT), 5α- and 5β-androstan-3α, 17β-diol (5α-,5β-diol), and 4-androstene-3,17-dione (Δ4). It was found that 5β-reduced androgens alone or in combination with estradiol stimulate juvenile copulation in the chick while they have no detectable effect on all somatic variables which are recorded (testicular weight, plasma LH) with the exception of comb size. On the contrary, comb size was increased by T, Δ4, 5α-DHT, and 5α-diol while testis growth was prevented by T and Δ4 only. There is a good correlation between the anatomical localization of the enzymatic activities which metabolize T and the hormonal dependence of the biological responses: The comb converts T into 5α-reduced compounds which affect comb growth. 5β-Reduction is high in the hypothalamus, a fact which can be related to the sensitivity of sexual behavior to 5β-reduced androgens. This suggests that T metabolism is a very important step in the expression of this hormone's physiological effects.