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 test osterone (T) concentrations and reproductive behaviours. Many T-dependent b ehaviours are regulated by the activity of androgenic and oestrogenic metab olites within specific brain regions. Male European starlings breed in spri ng 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-sens itive nuclei in the telencephalon and diencephalon. Outside the breeding se ason, T is undetectable, however, males continue to sing at high levels. Th is suggests that singing outside of the breeding season might not be T-depe ndent as it appears to be in the spring. Alternatively, singing when T is l ow 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 med iated by changes in intracellular T metabolism leading to increased product ion of active or decreased production of inactive metabolites. To explore t he relationship between T-metabolism and reproductive behaviour, we analyse d seasonal changes in the activity of four brain T-metabolizing enzymes: ar omatase, 17 beta -hydroxysteroid dehydrogenase (17 beta -HSDH), 5 alpha -re ductase (all three convert T into active metabolites) and 5 beta -reductase (converts T into an inactive metabolite) in the diencephalon and telenceph alon. In the anterior and posterior diencephalon, the highest aromatase was observed in spring when this region is critical for courtship and copulati on. In the telencephalon, aromatase was highest and 5 beta -reductase was l owest throughout the winter months well prior to the reproductive season an d these enzymes presumably maximize T-activity within this region. Although these data do not indicate whether the metabolic changes occur specificall y within song nuclei, these findings are compatible with the idea that sing ing in male starlings outside the breeding season may be regulated by stero ids despite the presence of low serum T concentrations. Overall, seasonal c hanges in T-metabolizing enzymes appear to play a significant role in seaso nal changes in behaviour and reproductive physiology.