Rapid effects of 17β-estradiol on aggressive behavior in songbirds: Environmental and genetic influences.

17β-estradiol (E2 ) has numerous rapid effects on the brain and behavior. This review focuses on the rapid effects of E2 on aggression, an important social behavior, in songbirds. First, we highlight the contributions of studies on song sparrows, which reveal that seasonal changes in the environment profoundly influence the capacity of E2 to rapidly alter aggressive behavior. E2 administration to male song sparrows increases aggression within 20 min in the non-breeding season, but not in the breeding season. Furthermore, E2 rapidly modulates several phosphoproteins in the song sparrow brain. In particular, E2 rapidly affects pCREB in the medial preoptic nucleus, in the non-breeding season only. Second, we describe studies of the white-throated sparrow, which reveal how a genetic polymorphism may influence the rapid effects of E2 on aggression. In this species, a chromosomal rearrangement that includes ESR1, which encodes estrogen receptor α (ERα), affects ERα expression in the brain and the ability of E2 to rapidly promote aggression. Third, we summarize studies showing that aggressive interactions rapidly affect levels of E2 and other steroids, both in the blood and in specific brain regions, and the emerging potential for steroid profiling by liquid chromatography tandem mass spectrometry (LC-MS/MS). Such studies of songbirds demonstrate the value of an ethologically informed approach, in order to reveal how steroids act rapidly on the brain to alter naturally-occurring behavior.