Biochemical characterization and seasonal changes in the concentration of testosterone-metabolizing enzymes in the European great tit (Parus major) brain

Homogenates of diencephala obtained from brains of European great tits (Parus major) were incubated in the presence of tritiated testosterone (T) as precursor, and four metabolites produced from this steroid were formally identified and quantified. Conversion into 5β-dihydrotestosterone (5β-DHT) constituted the major metabolic pathway of T. Smaller amounts of 5α-dihydrotestosterone (5α-DHT), 5β-androstane-3α,17β-diol (5β-DIOL), and estradiol (E2) were also produced. The metabolism of T was time-dependent, and it varied as a function of the initial precursor concentration. The kinetics of 5β- and 5α-reductases, as well as aromatase, followed the Michaelis-Menten model. It was found that 5β-reductase has a low apparent affinity for T, but is present in large concentrations. In contrast, the apparent affinity for T and the concentration of aromatase were approximately 3.9 times higher and 130 times smaller, respectively, than those of 5β-reductase. Intermediate values were found for 5α-reductase. The validated assay was used to measure seasonal changes in the in vitro metabolism of T in the anterior (AH) and posterior (PH) hypothalamus and the cerebellum (CER) of free-living juvenile and adult male great tits. The production of 5β-DHT was low during the winter period in the PH of adult males, whereas the 5β-DIOL production was low in both parts of the hypothalamus at this time of the year. During autumn the production of these metabolites showed a transitory decrease in both parts of the hypothalamus of the juveniles. The production of 5β-reduced metabolites by the CER was high at all times of the year. In juveniles, the CER production of 5β-DHT did not change seasonally, whereas 5β-DIOL production peaked during summer. In the CER of adults, maximum production of both metabolites occurred during summer. Generally, less T was converted into 5β-reduced metabolites by the PH than by either the AH or the CER. E2 production was observed only in the AH and PH. With one exception (summer; AH), E2 production was high in both parts of the hypothalamus of adults throughout the year. In both AH and PH of juveniles, E2 production was low during summer. In these birds, it increased between summer and autumn in both parts of the hypothalamus, and also between autumn and winter in the PH. The production of 5α-DHT did not change as a function of the season, the age of the birds, or the brain region. In the autumn, birds living in different habitats showed differences in the production of 5β-reduced metabolites, but not in the production of 5α-DHT or of E2. These results are discussed in relation to seasonal changes in behavior and plasma levels of T. Finally, an extensive conversion of T into 5β-reducted metabolites occurred in the liver.