Seasonal variation in androgen-metabolizing enzymes in the diencephalon and telencephalon of the male European starling (Sturnus vulgaris).
In seasonally breeding songbirds, seasonal fluctuations occur in serum testosterone (T) concentrations and reproductive behaviours. Many T-dependent behaviours are regulated by the activity of androgenic and oestrogenic metabolites within specific brain regions. Male European starlings breed in spring when circulating T concentrations peak. T and its metabolites act within portions of the diencephalon to regulate the pituitary-gonadal axis and to activate courtship and copulation. Song in male starlings is critical for mate attraction during the breeding season and is regulated by steroid-sensitive nuclei in the telencephalon and diencephalon. Outside the breeding season, T is undetectable, however, males continue to sing at high levels. This suggests that singing outside of the breeding season might not be T-dependent as it appears to be in the spring. Alternatively, singing when T is low might continue to be regulated by T due to increased sensitivity of the brain to the action of the steroid. This increased sensitivity could be mediated by changes in intracellular T metabolism leading to increased production of active or decreased production of inactive metabolites. To explore the relationship between T-metabolism and reproductive behaviour, we analysed seasonal changes in the activity of four brain T-metabolizing enzymes: aromatase, 17beta-hydroxysteroid dehydrogenase (17beta-HSDH), 5alpha-reductase (all three convert T into active metabolites) and 5beta-reductase (converts T into an inactive metabolite) in the diencephalon and telencephalon. In the anterior and posterior diencephalon, the highest aromatase was observed in spring when this region is critical for courtship and copulation. In the telencephalon, aromatase was highest and 5beta-reductase was lowest throughout the winter months well prior to the reproductive season and these enzymes presumably maximize T-activity within this region. Although these data do not indicate whether the metabolic changes occur specifically within song nuclei, these findings are compatible with the idea that singing in male starlings outside the breeding season may be regulated by steroids despite the presence of low serum T concentrations. Overall, seasonal changes in T-metabolizing enzymes appear to play a significant role in seasonal changes in behaviour and reproductive physiology.