Sex- and age-related differences in the activity of testosterone-metabolizing enzymes in microdissected nuclei of the zebra finch brain.

Many effects of testosterone (T) in the zebra finch (Taeniopygia guttata) can be mimicked by T-metabolites, mainly estradiol and 5 alpha-dihydrotestosterone. We have therefore studied the neuroanatomical distribution of testosterone-metabolizing enzymes by means of the Palkovits punch technique combined with radioenzyme assay in the brain of adult and young male and female zebra finches. The activity of these enzymes was studied by a one-point assay in 5 nuclei of the song system (X, MAN, HVc, RA, ICo), 2 nuclei of the visual system (ectostriatum, nucleus rotundus) and in limbic and hypothalamic areas. Very noticeable was the presence of a very high aromatase activity in the hippocampal and parahippocampal region and in the nucleus taeniae and the absence of this enzyme in ICo. We found a higher aromatase activity in female than male HVc and RA and a higher 5 alpha-reductase activity in MAN, HVc, RA and ICo of males compared to females. The 5 alpha-reductase was more active in the preoptic area of females. A few sex-related differences in the activity of the 5 beta-reductase were also observed (higher activity in females than in males for area X and RA, but difference in the opposite direction for the ectostriatum). The statistical significance of these differences depended, to some extent, on the statistical technique used to demonstrate them, with the sex differences in RA being by far the most robust ones. Many age-related metabolic differences were also detected but these do not have a clear interpretation since the Km of these enzymes also changes with age. Extremely low levels of 5 beta-reductase activity were found in the nuclei of the visual system in adult birds while this enzymatic activity was very high in young birds. The biological significance of this change with age remains obscure. Correlations are thus observed between the neuroanatomical distribution of T-metabolizing enzymes and of androgen and estrogen receptors with the important exception of ICo which has no aromatase but contains high concentrations of estrogen receptors. Testosterone-metabolizing enzymes are however also present in areas which are not known as steroid targets.