Androgen-metabolizing enzymes show region-specific changes across the breeding season in the brain of a wild songbird

The Lapland longspur (Calcarius lapponicus) is an arctic-breeding songbird that shows rapid behavioral changes during a short breeding season. Changes in plasma testosterone (T) in the spring are correlated with singing but n ot territorial aggression in males. Also, T treatment increases song but no t aggression in this species. In contrast, in temperate-zone breeders, song and aggression are highly correlated, and both increase after T treatment. We asked whether regional or temporal differences in androgen-metabolizing enzymes in the longspur brain explain hormone-behavior patterns in this sp ecies. We measured the activities of aromatase, 5 alpha-reductase and SP-re ductase in free-living longspur males. Aromatase and Sa-reductase convert T into the active steroids 17 beta-estradiol (E-2) and 5 alpha-dihydrotestos terone (5 alpha-DHT), respectively. 5 beta-Reductase deactivates T via conv ersion to 5 beta-DHT, an inactive steroid. We examined seven brain regions at three stages in the breeding season. Overall, aromatase activity was hig h in the hypothalamus, hippocampus, and ventromedial telencephalon (contain ing nucleus taeniae, the avian homologue to the amygdala). 5 beta-Reductase activity was high throughout the telencephalon. Activities of all three en zymes changed over time in a region-specific manner. In particular, aromata se activity in the rostral hypothalamus was decreased late in the breeding season, which may explain why T treatment at this time does not increase ag gression. Changes in 5 beta-reductase do not explain the effects of plasma T on aggressive behavior. (C) 1999 John Wiley & Sons, Inc.